Design and implementation of an automated electrochemical
flow system coupled with mass spectrometry for investigation
of the dissolution behavior of platinum
Dissertation
zur
Erlangung des Grades
“Doktor der Naturwissenschaften”
an der Fakultät für Chemie und Biochemie
der Ruhr-Universität Bochum
vorgelegt von
Angel Angelov Topalov
aus Ruse, Bulgarien
(geboren in Sankt-Petersburg, Russland)
Bochum 2014
1. Gutachter: Prof. Dr. Martin Stratmann
2. Gutachter: Prof. Dr. Wolfgang Schuhmann
Tag der Einreichnung: 24.01.2014
Tag der Disputation: 04.03.2014
Gewidmet meiner lieben Familie
(��������� �� �� ����������� ��)
i
Acknowledgement The work described in this thesis was carried out at the Max-Planck-Institute für Eisenforschung
GmbH in Düsseldorf with financial support of Center for Electrochemical Sciences, Bochum.
I would like to express my deepest gratitude to Prof. Dr. Martin Stratmann for supervising
my thesis and his scientific contribution during the annual gatherings of the department. His
critics have strongly motivated me and helped me to find and improve crucial aspects. I
furthermore thank Prof. Dr. Wolfgang Schuhmann for kindly accepting to be second
reviewer and for the time he invested. I appreciate his mentor position during my membership
in CES.
I would like to acknowledge Dr. Karl Mayrhofer, first of all for choosing me as one of the
first PhD students in his starting group and introducing me to the basic concepts of
electrochemistry, the time and patience he spent in several discussions. Especially for the
freedom and trust he provided me over those great three years in the daily work, tinkering in
the labs on the weekends and in many general aspects. I’m grateful for this opportunity, where
he supported me to develop myself as well as in professional and in social fields. Thank you
Karl!
I would like to thank “the core” of the Electrocatalysis Group, Josef Meier, Sebastian
Klemm and Ioannis Katsounaros for a nice working time, their support in building up the
labs of the group in the beginning from ground zero, and for the many nice evenings we spend
together at the Greek restaurant. It is my pleasure to work with you guys!
I would like to thank Andrea Mingers and Jörg Puszcz for taking care of general technical
issues and troubleshooting of ICP-MS, for a smooth daily schedule. I would like to thank also
all members of the Mechanical Workshop for their quick and precise work. A big
acknowledgement is dedicated to all formal and current members of the Electrocatalysis Group
for the nice working atmosphere. Special thanks to Aleksandar Zeradjanin, for helping me to
understand many fundamental issues and being a constructive discussion partner. I really
appreciate the time and ideas he shared with me. X���� �� ��� ���!�!
I would like to thanks several people for their help in different experimental issues: Serhij
Cherevko – assisting in the reconfiguration of SFC system for temperature dependent
measurements; Ashokanand Vimalanandan - SKP measurements; Adnan Sarfraz &
Andreas Erbe – in situ Raman and in-situ Elipsometry measurements; Sergiy Borodin & Julia
Klemm – XPS measurements and data evaluation.
ii
Last but not least. My family, I’m proud of having such great parents Angel and Ludmila
Topalovi that provided me wonderful childhood and a lot of possibilities in my life, and
showed me that there are always alternative views. My wife Slaveya Topalova and our little
son Angel, who supported me all the way to the end of this work! Everything that I achieve in
my life wouldn’t be possible without the strong support of my lovely family. Thank you!
Translation of the last paragraph in Bulgarian:
“Последно, но не на последно място. Искам да блогадаря на семейството ми, аз съм
горд да имам такива невероятни родители Ангел и Людмила Топалови, които ми
предоставиха прекрасно детство и много възможности в живота ми и ми показа, че
винаги има алтернативна страна на живота. Същото така искам да благодаря на
съпругарата ми Славея Топалова и нашият син Ангел, които ме подкрепяха през
цялото време до края на тази дисертация! Всичко, което постигнам в живота ми не би
било възможно без силната подкрепа на прекрасното ми семейство. Благодаря ви!”
iii
iv
Abstract The main subject of this thesis is the investigation of the dissolution behavior of
polycrystalline platinum during an electrochemical treatment by utilizing simultaneous online
elemental analysis of the electrolyte. For this purpose, a novel micro-electrochemical scanning
flow cell (SFC) based on the concept of a channel electrode combined with inductively coupled
plasma – mass spectrometer (ICP-MS) has been designed and developed. The developed
system is completely automated to enable high-throughput and combinatorial measurements.
This has been achieved by creating a universal-modular software approach that allows parallel
asynchronous control and acquisition within a single controlling program. A proof of concept
of the new combined system is presented, as well as a long-term performance test showcasing
the reliability of the approach. The created software architecture has been adopted for the
automation of up to now five rotating disc electrode setups and provides a solid backbone
solution for the further development of currently seven SFC systems.
The advantages provided by parallel monitoring of the dissolved species by post analysis
under automated control are used for the systematic investigation of the influence of several
experimental parameters on the dissolution of polycrystalline Pt. The performance of the
coupled SFC/ICP-MS is first optimized to achieve very low effective detection limits below
10 ppt, which enables a precise description of the dissolved amounts in the sub-monolayer
region. Thus for the first time a time resolved Pt concentration profile within a single cyclic
voltammogram is presented. The extensive experimental data explicitly shows that dissolution
appears during the sub-surface oxide formation at ca. 1.1 VRHE as well as during the reduction
of the oxide layer, where irreversible roughening of the surface acts as a necessary precursor to
trigger dissolution. These two different cases of dissolution have been specified as (i) “anodic”
dissolution route with typical material loss in order of ca. 1 ng/cm2 (ca. 0.25% of a single
monolayer of platinum) almost independent of the overpotential for the oxide formation and
the time scale of the experiments, and (ii) “cathodic” dissolution route that represents the main
contribution to the overall dissolution amounts and shows a strong variation with pH, amount
of formed oxide, the experimental time scale and temperature. Within the framework of the
newly gained knowledge, the existing thermodynamic models of Pt dissolution are scrutinized
and new mechanism proposed.
Overall the work summarized in this thesis presents novel methodological developments
and scientific information, and has already been a solid basis for further studies especially of
dissolution processes. So the developed experimental approach has not only been applied for
v
platinum, which is the focus of this work, but has been also applied in several parallel
investigations on stability issues of other noble metals and multi component systems during
this thesis. The major result has been a deep insight into metal dissolution in general, often
even clarifying the mechanism, as well as broad and reliable quantitative datasets of dissolution
rates that provide valuable guidelines for the engineering limits of catalyst materials, such as for
the application of Pt-based materials in fuel cells.
vi
Content Acknowledgement ..................................................................................................... i�Abstract ..................................................................................................................... iv�Glossary ................................................................................................................. viii�1� Motivation – Platinum and its role in the renewable energy concept ................. 1�
1.1� Electrochemical conversion of energy from renewable sources ............................... 2�1.2� Stability – the challenge for Pt catalysts in electrochemical energy conversion ...... 3�
2� Importance, aim and general approach of the study ........................................... 5�2.1� Political-economical aspects and perspectives ............................................................. 5�2.2� Brief overview on platinum dissolution and open questions ..................................... 7�
3� Theoretical background and literature review ..................................................... 9�3.1� Electrode reactions ........................................................................................................... 9�3.2� Anodic corrosion of metals ........................................................................................... 10�3.3� Electrochemistry of platinum ....................................................................................... 12�
3.3.1� Oxide layer ............................................................................................................... 13�3.4� Dissolution of platinum ................................................................................................. 17�
3.4.1� Polycrystalline surfaces ........................................................................................... 17�3.4.2� High surface area Pt catalysts ................................................................................ 20�3.4.3� Structural changes ................................................................................................... 22�3.4.4� Theoretical model ................................................................................................... 23�
4� Technical background ....................................................................................... 25�4.1� Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) .................................. 25�4.2� X-ray Photoelectron Spectroscopy .............................................................................. 27�4.3� Scanning Kelvin Probe microscopy ............................................................................. 27�
5� Automated system development – from design to implementation ................. 29�5.1� Hardware – between commercial and custom solutions .......................................... 30�
5.1.1� Rotating Disc Electrode (RDE) ............................................................................ 30�5.1.2� Scanning Flow Cell system (SFC) ......................................................................... 31�5.1.3� Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) .......................... 34�
5.2� Software development .................................................................................................... 35�5.2.1� Software architecture – modular approach ......................................................... 35�5.2.2� User interface – friendly and efficient design ..................................................... 37�
vii
5.3� Data management ........................................................................................................... 39�5.3.1� Data storage .............................................................................................................. 39�5.3.2� Evaluation – quick data processing ...................................................................... 40�
6� System validation – proof of functionality and limits in application ................ 42�6.1� Automation - proof of concept .................................................................................... 42�6.2� Performance of electrochemical systems – RDE vs. SFC ........................................ 44�6.3� Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) and coupling with the
SFC 46�7� Results and discussion ....................................................................................... 52�
7.1� Electrochemistry of platinum in acidic media ............................................................ 52�7.2� Influence of the overpotential for oxide formation and reduction on platinum
dissolution ........................................................................................................................................ 53�7.3� Steady state dissolution during chronoamperometry ................................................ 56�7.4� Decoupling the influence of the time scale of experiment and the amount of formed
oxide on the dissolution rates ....................................................................................................... 59�7.5� Influence of the concentration of the protons ........................................................... 62�7.6� Effect of the reactive gases on Pt dissolution ............................................................ 65�7.7� Enhancement of Pt dissolution in the presence of chlorides .................................. 68�7.8� Temperature dependence of the dissolution .............................................................. 70�7.9� Ex-situ XPS and SKP investigation of platinum oxide ............................................. 72�
8� Comprehensive discussion ................................................................................ 77�9� Summary and outlook ........................................................................................ 85�References ............................................................................................................... 87�
Appendix .................................................................................................................................. 100�Publications list: .................................................................................................................. 100�Oral presentations: ............................................................................................................. 101�Poster presentations ........................................................................................................... 102�
Curriculum Vitae – Angel A. Topalov ................................................................................. 104�
viii
Glossary Abbreviations
AFC Alkaline fuel cell
ARXPS Angle-resolved X-ray photoelectron spectroscopy
a.u. Arbitrary units
ddl Dynamic link library
ppb Particles per billion
ppm Particles per million
ppt Particles per trillion
ca. Circa (Latin: around)
CAD Computer aided design
CE Counter electrode
CER Chlorine evolution reaction
COM Component object model (Serial port)
CV Cyclic voltammogram
DAQ Digital acquisition
DMFC Direct methanol fuel cell
EEG “Erneuerbare Energie Gesetz” (renewable energy act)
EIS Electrochemical impedance spectroscopy
e.g. exempli gratia (Latin: for example)
et al. et alii (Latin: and others)
etc. et cetera (Latin: so on)
FC Fuel cell
FIFO First-in-first-out
FWHM Full width at half maximum
HFM High field model
i.e. id est (Latin: that is)
ICP-MS Inductively coupled plasma – mass spectroscopy
ICP-OES Inductively coupled plasma – optical emission spectroscopy
LSM Lanthanum Strontium Manganite
LSV Linear sweep voltammetry
MCFC Molten carbonate fuel cell
ix
MO Molecular orbital
n.a. Not available
NASA National Aeronautics and Space Administration
NGM Nucleation and growth mechanism
OCP Open circuit potential
OER Oxygen evolution reaction
ORR Oxygen reduction reaction
PAFC Phosphoric acid fuel cell
PC Personal computer
PDM Point defect model
PEM Place exchange mechanism
PEMFC Proton exchange membrane fuel cell
PGM Platinum group metals
RDE Rotating disc electrode
RHE Reversible hydrogen electrode
RE Reference electrode
RF Radio frequency
RRDE Rotating ring disc electrode
rpm Revolutions per minute
SA Specific activity
SCE Saturated calomel electrode
SECM Scanning electrochemical microscopy
SERS Surface-enhanced Raman spectroscopy
SFC Scanning flow cell
SHE Standard hydrogen electrode
SKP Scanning Kelvin Probe
SOFC Solid oxide fuel cell
UI User interface
UPD Under potential deposition
USB Universal serial bus
VI Virtual instrument
vs. versus
WE Working electrode
XPS X-ray photoelectron spectroscopy
x
YSZ Yttrium stabilized zirconia
Formula abbreviations
Ageo Geometric electrode surface area
Areal Real platinum surface area
C Concentration
D Diffusion coefficient
E Potential
Ea Activation energy
E� Standard potential
Eeq Equilibrium potential
F Faraday constant
G Gibbs energy
im Measured current
idiss Dissolution current
idl Diffusion limited current
ik Kinetic current
j Current density
k Reaction rate
m Mass
M Molar mass
n Number of electrons
QCO Charge determined for CO-stripping
QH Charge determined from HUPD
R Ohmic resistance
t Time
� Scan rate
V Volume velocity
Greek letters
� Charge transfer coefficient
Diffusion layer thickness
� Potential
� Overpotential
xi
� Coverage
� Work function
xii
Chapter 1: Motivation –Platinum and its role in the renewable energy concept
1
1 Motivation – Platinum and its role in the renewable energy concept
The noble metals have fascinated mankind throughout its entire known history. The
ancient cultures like Thracians or/and Mayan were utilizing gold ore for utensils and jewelers
hundreds of years B.C., with the main role to emphasize a social status. The other metals in the
platinum group were rarely present by the ancient cultures [1]. This can be attributed to their
extremely low abundance in terms of amount and geographic distribution and the fact that they
usually appear in nature as alloys. Over the last decades with the development of natural
sciences and industry, the focus of the usage has significantly shifted from esthetic to practical
one, determined predominantly by their physical/chemical properties. In the 17th century, when
platinum was re-discovered by the Spanish conquerors in Colombia, it was considered as a
parasitic product of the gold mining. It acquires the name platinum from the Spanish word
Platina i.e. little silver. The high melting temperature and corrosion resistance make it a very
interesting material for the glass industry. The first real breakthrough in recognizing the
significance of platinum was due to its catalytic properties, e.g. for reforming of crude
petroleum. This was possible due to the separation methodology of platinum ore developed by
the Russian scientist P. G. Sobolevsky, which was later on adapted on large scale [1]. This
scientific/industrial achievement opened a new page in the history of platinum mining and
created the base for the current commercial purification technologies in metallurgy of noble
metals.
Another important historical episode in recognition of the catalytic properties of platinum
was the ability of platinum to promote hydrogen oxidation. This discovery was attributed to the
German scientist J. W. Döbereiner, who showed the spontaneous ignition of gas mixture of
hydrogen and air in presence of platinum [2]. Further investigation of the British scientist
W. R. Grove emphasized the potential of platinum in a so-called gas voltaic battery [3],
nowadays known as a fuel cell. Sir Grove separated the process into two half-cell reactions,
where the electrons are diverted through an external electric circuit. In this way, he performed
energy conversion, i.e. from chemical to electric energy, and showed the basic principles of the
operation of modern hydrogen-based fuel cells.
Chapter 1: Motivation
2
1.1 Electrochemical conversion of energy from renewable sources
The 21th century is marked by the new way of usage of resources especially electrical energy.
Germany as the seventh biggest producer and consumer of electricity in the world (and leader
in the European Union) has a pioneer role in the integration of alternative energy sources [4].
In the year 2000, a new national regulation for renewable energies sources of electricity (EEG)
with the target of replacing/reducing the power supply from the nuclear plants and combustion
of fossil fuels by the year 2050 was approved. This political act led to a rush in the deployment
of renewable energy sources, mainly solar and wind parks that covered more than half of the
overall market of renewable sources by 2012. The industrial upscaling of those technologies
proved their capabilities in supplying hundreds to thousands of megawatts. Installed wind
power sources in Germany by 2012 reached above 31 GW what could provide in theory the
minimum energy consumption [5] (for comparison the biggest German nuclear power plant
Asar-2 has ca. 1.4 GW power). Nevertheless only ca. 8.1 GW of the wind power was effectively
used in 2012 on average [5]. Unfortunately, the integration of the ‘green’ energy sources into
the national power grids and maintaining constant supply turned out to be a serious challenge,
due to the volatility caused by the unavoidable fluctuating meteorological conditions. This is an
issue that can be alleviated only with intermediate buffering of the overproduction and reuse in
case of shortage. Energy storage in chemical bonds is proven to be a concept that can provide
the required capacity on different scales of magnitude. A perspective option is the usage of
hydrogen as an energy carrier with theoretical capacity up to 120 MJ/kg, produced for instance
by water electrolysis [6]. The backwards conversion to electrical energy, can be performed for
instance by separated anodic hydrogen oxidation and cathodic oxygen reaction reactions in
proton exchange membrane fuel cells (PEMFC).
The fuel cell technology is known as a concept from the beginning of the 19th century,
nevertheless the first publically known application is recorded one century later in the space
programs of NASA [3,7]. Records on the usage of FCs in Soviet Union are not publically
available due to their possible military application in submarines. The family of the FCs is
broad with respect to the nature of the charge carrier, operation temperature, the type of
electrolyte etc. In the last decades, the FCs technologies were boosted in several directions.
Table 1.1 represents only some of the commercially available systems. One of the most
Chapter 1: Motivation –Platinum and its role in the renewable energy concept
3
promising technologies is PEMFC. Due to its potential to replace combustion engine in mobile
application, a lot of research was focus in the optimization of the performance.
PEMFC DMFC PAFC AFC MCFC SOFC
Fuel H2 CH3OH H2 H2 H2 H2
Electrolyte
Hydrated Sulfonated
Organic Polymer
Hydrated Sulfonated
Organic Polymer
Phosphoric acid
Potassium Hydroxide
Molten Lithium/ Potassium Carbonate
Yttria- stabilized Zirconia
Charge
carrier H+ H+ H+ OH- CO3
2- O2-
Cathode Pt/FeNxCy PtxRuy Pt NiO NiO LSM
Anode Pt PtxRuy Pt Ni/Precious
metals NiO Ni/YSZ
Operation
temperature 70-100°C 90°C 160-220°C
25-75°C
100-250°C 660°C
800-
1000°C
Table 1.1 Summary of the differences of some of the fuel cells (data adopted from ref. [7])
1.2 Stability – the challenge for Pt catalysts in electrochemical energy conversion
Noble metals, and especially platinum play a key role in the design of electrocatalysts for
energy conversion. The main drawback for the deployment of PEMFC on a large scale is the
high cost of the systems, where one of the main contributors is the price of the catalyst. In
order to compensate the relatively high costs related to catalysis, significant improvements in
performance become necessary. Many research studies have shown ways of improving the
catalyst activity without sufficient consideration of stability, which lead to the development of
many catalyst concepts without industrial relevance. Attempts to replace the noble materials, in
particular platinum, with cheaper alternatives are still ongoing research topics [8–10]. While a
significant improvement has been achieved in tuning the activity of non-noble catalysts, the
long term performance still remains a challenge [11,12]. The state-of-the-art materials used in
commercial PEMFC stacks are therefore platinum-based, which themself suffer from
Chapter 1: Motivation
4
degradation over the extended operation times under applied conditions [13–15]. In order to
render PEMFC technology in electric vehicles viable, an understanding of the underlying
degradation mechanism becomes necessary and these stability issues have to be considered in
the design and development stage of the catalyst.
Recently several groups illustrated the complexity of this problem utilizing in situ electron
microscopy with electrochemical techniques [16–19]. Degradation tests on carbon supported
platinum nano-particles, the typical catalyst for the ORR in low-temperature fuel cells, showed
that different mechanism lead to the loss of active surface area, as summarized in figure 1.1. A
smart design approach has been developed over the last years to inhibit the mechanical
detachment, prevent migration of particles by incorporation in mesoporous structures and also
to reduce the degradation rate of the carbon support by high graphitization [A.10, A.17]. One
of important issues remaining is the dissolution of the catalyst during operation, which appears
to be unavoidable. This leads to the questions: a) what interfacial processes really describe the
Pt dissolution mechanism and b) is it possible to protect Pt during relevant operational
conditions.
Figure 1.1 Illustration of the four major degradation mechanisms of nano-particulated
platinum catalyst (Modified from its original version in ACS Catalysis from ref. [A.5]).
Detachment Support corrosion Dissolution Coalescence
Chapter 2: Importance, aim and general approach of the study
5
2 Importance, aim and general approach of the study
2.1 Political-economical aspects and perspectives
The importance of noble materials is determined mainly by their industrial applications. In
particular, platinum can be found in several fields [20] despite being an extremely rare metal
with crustal abundance of only ca. 3 ppb. Mining sources with sufficient amounts of platinum,
relevant for commercial exploitation, can be found only in two geographic regions in the world,
namely in the border of Zimbabwe/South Africa and several locations in Russia, held
respectively by the Anglo American Holding and Norilsk Nickel [21]. More than 38% of the overall
produced platinum in 2012 was used in the automotive industry in catalytic convertors, with an
average amount of 3-7 g/car [22]. This huge market for platinum was secured by two political
events ‘Clean Air Act’ in the US and the consequent EU derivatives 91/441/EEC &
93/59/EEC for emission standards for vehicles, both triggered officially by environmental
issues. Similar political decisions were made subsequently by a number of countries (Russia,
China, India etc.), which increased the international demand drastically. Moreover platinum
finds also application in several other fields like a) various industrial processes such as the
refinement of petroleum to obtain high octane gasoline or primary feedstocks for polymers,
glass production, hard disc manufacturing, production of nitric acid, fertilizers etc. b) medical
and high tech applications: antitumor drugs, dental restorations sensors etc. c) investment and
jewelry [20]. All this insured the stable market of platinum with the consumption of
approximately 228 tons per year over the last decade. In figure 2.1 are illustrated produced and
consumed amounts of platinum only in 2012, since the demand over the last 6 years is
maintained practically on the same level.
Chapter 2: Importance, aim and general approach of the study
6
Figure 2.1 a) Demand of platinum by application in different industrial branches; b) platinum
supply by producing countries; the data is adapted from Johnson & Matthey annual report
2013 [20].
In contrast to the 19th century, where the economical aspects were dominating
developments, nowadays, sustainability is of crucial importance. It means that symbiosis
between the economical, social and environmental criteria is required to prevent disposable
usage of critical resources and ecological disasters (like Chernobyl, Fukoshima etc.). This trend
can be observed even from figure 2.1, where a significant gap of ca. 63.22 tons per year
between supply and demand is filled by the recycling sector, utilizing old jewelry and car
exhaust catalysts. Figure 2.2 shows the price development of platinum over the last 20 years.
The general tendency in the market indicates a rising price with casual fluctuation, mostly due
to political and economical decisions. Considering the continuously growing price of all PGM
materials, only smart usage can justify their application in future.
Figure 2.2 Chart of the platinum market price over the last 20 years, data adopted from
Johnson and Matthey [20].
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Chapter 2: Importance, aim and general approach of the study
7
The unique catalytic properties and the promising application in PEMFCs will have a strong
influence on the overall Pt market. Thus, resolving the issues related with the dissolution
behavior of Pt is of crucial importance for the future of PEMFC technology, system design and
implementation. Moreover, stability limits of platinum will be one of the key parameters in
defining the framework of application also in other technologies. An improvement of the
performance of Pt demands an improved fundamental understanding of the ongoing
degradation phenomena. In particular, the comprehension of the dissolution phenomena on
model system like polycrystalline platinum is an important step in creating an in-depth picture
about the stability of real supported catalysts.
2.2 Brief overview on platinum dissolution and open questions
In 1905 Tafel et al. observed (re)deposition of platinum on the cathode during caffeine
reduction, concluding that platinum is a bad choice for the anode [23]. This was one of the first
reports on the instability of platinum during electrochemical treatment, and in fact implicitly
suggested an anodic route of the dissolution. Later on, in the search for materials for oxygen
evolution reaction during water electrolysis, Kolotkin and Chemodanov developed a
radiotracer method for the investigation of low dissolution rates of PGM elements [24], and
showed for the first time that the dissolution of platinum is determined by the applied anodic
potential. Other studies have shown that dissolution can appear during reduction of the Pt (–
oxide) surface, considering a cathodic route as main contribution for the degradation [25]. Over
the last decades, the opinions on the mechanism diverged, whether Pt dissolution is: a) a
competitive reaction to oxide formation b) intermediate product in the reduction of the oxide,
c) or predominantly a chemical process. Even for the “simplest” system, polycrystalline Pt,
there is no unified point of view on the mechanism.
Slightly better resolved is the quantification of Pt dissolution, however literature reports are
highly diverging and often not trustful due to missing experimental details. The analysis of the
representative literature shows that the expected dissolution amounts are in the range of some
ng/cm2. A simple order estimation for the expected charge balance, based on the assumption
of a valence of 2 [26] for the dissolved species gives us:
������ ���� �
�����������
� ���������� ������� � ������� � �������� ��� � ���� ��
���
Chapter 2: Importance, aim and general approach of the study
8
Considering that only the formation charge of a single monolayer of oxide is in the order of
hundreds of �C/cm2, it is obvious that dissolution cannot be resolved purely by measuring
currents at a working electrode during potentiodynamic experiments. Moreover, despite being
highly sensitive, other electrochemical approaches based on double electrodes suffer from
overlapping parasitic processes and are of limited use when multi-element analysis is demanded.
Thus other, complementary techniques with high sensitivity and a capability to analyze various
elements in parallel are required to quantify Pt and Pt-alloy dissolution. Probing amounts as
low as 1012 atoms/cm2 can be achieved for instance with multi-elemental analysis using
inductively coupled plasma mass spectrometer (ICP-MS). As there is no commercially available
system with the required capabilities for time-resolved dissolution monitoring, the first step
within this thesis is the design and development of an electrochemical system that can be
combined online with mass spectrometry. At the second stage of this work, a combinatorial
screening over the experimental parameters should be performed; where the dissolution
behavior of polycrystalline Pt will be revisited in time resolved in situ experiments. The
influence of the experimental parameters in correlation with the surface state will be discussed
so that a tentative dissolution mechanism can be proposed. Additionally, a quantitative
description of the dissolution rates is provided for several operation conditions, which is of
high relevance for engineers working with platinum based materials and can help to predict the
lifetime of catalyst/electrode materials on an industrial scale.
Chapter 3: Theoretical background and literature review
9
3 Theoretical background and literature review
This chapter is targeted to introduce the general aspects of electrochemistry on platinum
and to provide a framework of the existing knowledge on its stability. Platinum has been
intensively studied over the last century, thus the literature review is restricted to the most
relevant publications for this work.
3.1 Electrode reactions Electrochemical processes appearing on the electrode/electrolyte interface with transfer of
charged species can be described as oxidation and reduction reactions as shown in the
schematic energy diagram in figure 3.1. In order to trigger an electrochemical reaction, the
energy level � of the electrons at the electrode has to be higher than the lowest unoccupied
molecular orbital (LUMO) of the electro-active species in the solution for a cathodic reaction,
or to be lower than the highest occupied molecular orbital (HOMO) for anodic one [27].
Figure 3.1 Illustration of energy levels at the electrode/electrolyte interface and changes during
the anodic and cathodic polarizations in red and blue respectively.
This is not always the case for electrode immersed in solution at open circuit condition.
Thus its energy state is typically modified by applied external polarization using a second
electrode, where the potential difference is defined as:
�� � �� � �
That is further related to change of Gibb’s free energy of the system:
Chapter 3: Theoretical background and literature review
10
�� � ����
where � is number of electrons, � is Faraday’s constant (96485 As/mol). In this way, different
reactions can be driven to proceed by variation of potential �. Thermodynamically favorable
conditions do not always initiate a reaction due to so called activation barrier of the process.
Arrhenius related experimentally the rate constant k and the activation energy �� as [28]:
� � �������
where � is pre-exponential (frequency) factor, � is temperature. The rate of electrochemical
reaction is proportional to the flowing current that can be directly measured. Applying
overpotential to promote a reaction, affects its rates on different extend.
� � ��� � �
This is attributed to kinetic hindering described by Buttler-Volmer equation [27]:
� � �� ��� ��� ������ � � ��� ����
�� �
The overall current response j is given as a function of the charge transfer coefficient �,
exchange current density �� and overpotential � � � � ��� . The described Faradaic processes
can be separated in to two types: (i) continuous and (ii) adsorption processes. In first case, the
reaction can theoretically proceed infinitely, whereas the second one adsorption of the
electrochemically active species causes a blockage of electrode surface and consequent drop of
the reaction rate. The coverage � can be expressed by Langmuir isotherm:
� � ������������
or ���� � �����
using the Nernst equation, the partial pressure �� of the adsorbed species is related to the
equilibrium potential.
��� � �� � ��� ����
Consequently, the coverage can be expressed as:
��� � � ��� ���
��� ���� � �
��
where ��� is the adsorption constant and ��is standard potential.
3.2 Anodic corrosion of metals Dissolution of metals is a result of an electrochemical process, in which the metallic atoms
are released from the lattice of the substrate in form of ions. This phenomenon is part of the
broad field of corrosion. It is commonly accepted that such anodic processes involve
Chapter 3: Theoretical background and literature review
11
intermediate surface state between the metal and the electrolyte. There are several possible
pathways that describes it [29]:
(i) Aquo-ligand mechanism:
� � ����� � �� (4.1)
����� � ����� � �� (4.2)
(ii) Hydroxo-ligand mechanism:
� � ��� � ������ � �� � �� (4.3)
������ � ������ � �� (4.4)
(iii) Aniono-ligand mechanism
� � �� � ����� � �� (4.5)
����� � ����� � �� (4.6)
Typically the active anodic corrosion takes a place without significant obstacles due to minor
interaction with passivation film. Thus the dissolution rate is usually determined by the counter
balancing cathodic reaction like hydrogen evolution reaction (HER) or oxygen reduction
reaction (ORR):
�� � ���� � ��� � ���� (4.7)
��� � ��� � �� (4.8)
The metal dissolution does not necessary proceed as a continuous process. Formation of
MOHads has a dual role, namely as part of the dissolution reaction pathway (reaction 4.3 and
4.4), but also as a precursor for the passivation of the surface. The MOHads can react further by
de-protonation of the water forming M(OH)2 in a pre-passivation step [29,30]. The following
oxide formation usually gives a dense layer with limited ionic conductivity that significantly
hinders high initial dissolution rates as illustrated in figure 4.2 [31]. Typically the current
response in the passive region is related to the variation of the oxide dimensions and
independent of the applied potential. However, the thickness of the passive film spreads over a
broad range from a few nanometers (e.g. Pt) to several micrometers (e.g. Al). In this region,
metal dissolution is controlled by the chemical dissolution of the oxide film.
Further positive extension of the potential in the region of the OER causes breakdown of
the passivity and the metal dissolution can rise again. This is closely related with the initiation
of the oxygen evolution, especially when the oxide layer is actively participating in the reaction
[32,33].
Chapter 3: Theoretical background and literature review
12
Figure 4.2 Schematic representation of the current density-plot of dissolving metal divided in
to three parts, starting from active dissolution over the passivation and finishing with
transpassive region, where the main current response is result of the OER (image redrawn
from ref. [30]).
3.3 Electrochemistry of platinum Following the thermodynamic predictions for the potential-pH behavior in aqueous
environment, platinum is relatively inert over a broad region as shown in figure 3.1 [34]. The
blue parallelogram indicates the thermodynamic stability window of water, where Pt is mainly
present in metallic form. However, an increase of the potential leads to transition to Pt(II) and
Pt(IV) as illustrated by line 1 and 2 respectively. Platinum in this region is expected in soluble
form (Pt2+) only at highly acidic pH below 0, where the dissolution can proceed over direct
oxidation of the bulk material or chemical dissolution of hydroxide film (Pt(OH)2). For all
other pH values the formation of PtO2 is thermodynamically expected, and dissolution should
only proceed over intermediate oxygenated species of Pt(IV).
��� ����
���
��
�������������
��������
������������
�����
������
��� �
����
�������������
�����������������
�����
Chapter 3: Theoretical background and literature review
13
Figure 3.1 Pourbaix diagram of the platinum-water system at 25°C, adopted from ref. [34].
3.3.1 Oxide layer Experimentally, cyclic voltammetry is an excellent approach for an initial study of electrode
surfaces. Platinum is one of the best-known catalysts for HER [35,36], however the potential
region below 0 VRHE is not of interest in this study. A potential excursion starting slightly more
positive than the hydrogen evolution reaction (HER) shows an anodic current response
between 0.05 and ca. 0.4 VRHE (figure 3.2a). Those are characteristic features of the hydrogen
desorption of weakly and strongly bond hydrogen on platinum surfaces with different
crystallographic orientations, known as HUPD (under-potential deposited) region [37]. It can be
used for estimation of the roughness of the electrode based on a ratio between the active
surface area and the geometric one when the double-layer charge is subtracted from the overall
current [38]. Note, that Frumkin et al. showed that a linear correction leads to underestimation
of the real HUPD charge in sulfuric acid. With respect to the nature of the electrolyte, linear
correction can lead also to overestimation like in the concentrated HCl and HBr [39]. Overall
the linear interpolation gives a systematic error in the order of 15-20% that has be taken in to
account.
Chapter 3: Theoretical background and literature review
14
Figure 3.2 a) Linear sweep voltammetry on polycrystalline platinum in 1 N H2SO4 represented
as total capacities (line 1) and true double layer capacities (CDL) (line 2). The dashed line 3 is the
difference between 1 and 2. Line 4 corresponds to formal double layer correction based on
linear interpolation from the intermediate region between hydrogen and oxygen adsorption
regions) [39]. b) Plot of a cyclic voltammogram of Pt in 0.5M H2SO4 including the
corresponding ratio between the anodically consumed charge in the surface oxidation vs. HUPD
charge (Reproduced by permission of Elsevier form ref. [40]).
The flat region between ca. 0.4 and 0.8 VRHE is related to the double layer charging, where no
Faraday reaction is observed in inert media. Further positive increase of the potential above
ca. 0.8 VRHE causes oxidation of the surface. Conway and co-worker described this region as a
formation of 2-dimesional O/OH overlay array on the Pt surface by formation of structures as
Pt4/OH, Pt2/OH and Pt/OH represented by three overlapping anodic peaks OA1, OA2 and OA3
(figure 3.2b) [40]. Around 1.1 VRHE, a full coverage with oxygenated species is reached and the
successive oxide formation continues over interfacial reorganization of the platinum and
absorbed O/OH transforming into a quasi-3D layer. This phenomena is known as place-
exchange and causes irreversible roughening of the surface, as described in the model
introduced by Vetter and Schulze (PEM) [41,42]. Consequent oxide growth follows a
logarithmic relation in time that can be also predicted with various models: PEM, point defect
model (PDM) [43,44], high-field model (HFM) [45,46] or nucleation and growth mechanism
(NGM) [47]. However, only the PDM incorporates a possible dissolution as a parallel reaction
to the oxide formation. Shibata made an important suggestion that the oxide is comprised from
two layers, where a thick oxide is growing between the metal substrate and the outmost
Chapter 3: Theoretical background and literature review
15
monolayer [48,49]. Later works showed that the thin layer is present between the metallic phase
and the outer layer, and was classified into two types [50–52]:
(i) -oxide film (or barrier layer): a monolayer of compact anhydrous metal oxide
containing mainly (e.g. PtO or PtO2).
(ii) �-oxide film (or outer layer): hydrous metal oxide (e.g. PtO(OH)2 or Pt(OH)2.2H2O)
with microscopically porous structure [52,53].
Various experimental techniques have been employed in the characterization of the nature
and the composition of the oxide layer. Studies utilizing XPS analysis showed that platinum is
present mainly in two oxidation states within the oxide layer, namely Pt(II) and Pt(IV), where
different stoichiometry has been postulated like PtO [41,54–57], Pt(OH)2 [58], PtO2 [59–61],
PtO(OH)2 [51] and Pt(OH)4 [59,62–65]. Using ellipsometry, Bockris et al. found that the
thickness of the oxide is the linearly proportional to the applied potential, with scaling factor of
0.95nm/V in the range of 1.0-1.6 VNHE [66]. The authors consider PEM as reasonable to
describe the oxide growth with following reaction path:
�� � ��� � ���� � �� � �� (3.1)
���� �������������� ���� (3.2)
���� � ��� � �� � �� (3.3)
Macdonald et al. present diverting values of 2.0-2.5nm/V using electrochemical impedance
spectroscopy (EIS) with same linear relation (figure 3.3b) [67,68]. Despite the discrepancy in
the exact values, it is generally accepted that only a few monolayers of oxide are formed even at
highly positive potentials.
Chapter 3: Theoretical background and literature review
16
Figure 3.3 Dependency of platinum oxide film thickness on the applied potential a) in
0.1N H2SO4 (Reproduced by permission of AIP Publishing LLC from ref. [66]) and b)
0.1M KOH (Reproduced by permission of The Electrochemical Society form ref. [67])
Ross and Wagner concluded lack of crystallographic order in the oxide layer, due to an
absence of reflectivity in low-energy electron diffraction (LEED) experiments on
electrochemically formed film at 1.7 VRHE [69,70]. The amorphous structure can be attributed
to existence of hydroxide outer layer (i.e. �-oxide) that incorporates water molecules [51,53,71].
However, an attenuated LEED reflectivity was observed in the range between 1.2 and 1.7 VRHE,
the authors related this fact to partial oxide coverage. Alternatively, it can also be interpreted as
a growing thicker outer layer over the inner one.
Figure 3.4 Illustration of the ongoing processes that lead to formation of bilayer oxide
structure on metal following the point defect model (PDM) (Reproduced by permission of The
Electrochemical Society form ref. [44]).
Using surface-enhanced Raman spectroscopy (SERS) on thin-film platinum deposited on a
gold substrate, Weaver et al. investigated the oxidation process in ambient environment [72].
The authors envisaged that initial subsurface oxide formation proceed over diffusion of the
oxygen into the platinum lattice via vacancies/defect, rather than “concerted” platinum-oxygen
place-exchange. Macdonald and co-workers also expressed severe doubt about the place-
exchange mechanism, considering that it does not predict a steady-state thickness of the barrier
oxide layer and proposed the point defect model (PDM). In their recent ex-situ investigation
Chapter 3: Theoretical background and literature review
17
utilizing angle-resolved X-ray photoelectron spectroscopy (ARXPS) the bilayer model was
confirmed, by showing that electrochemically formed Pt oxide has a highly defected structure
with an inner part consisting of n-type Pt(II) barrier oxide layer [68,73,74]. Following the PDM,
the authors postulated that formation of the inner oxide (-oxide) is a result of generation and
annihilation of oxygen vacancies at the metal/barrier interface and considered this layer as
passive. The outer oxide is formed by hydrolysis of transmitted cations through the primary
oxide film as illustrated in figure 3.4, where a competition between precipitation of oxygenated
species on the �-oxide/electrolyte interface and possible dissolution occurs.
3.4 Dissolution of platinum 3.4.1 Polycrystalline surfaces
Platinum dissolution was detected for the first time in the beginning of the twentieth
century [23], indirectly as a side reaction occurring on the counter electrode. Furthermore, loses
of platinum electrode material in industrial production of hydrogen peroxide, attracted the
interest of the scientific community towards the dissolution phenomena. Chemodanov and
Kolotkin addressed this issue using Pt foil irradiated in a nuclear reactor, where the dissolution
rates were monitored as a function of the radiation of the electrolyte samples [24]. Figure 3.5
shows a representative measurement performed in 3M HClO4 for different anodic polarization,
where the dissolution current is in a range of ca. 10-5–10-8 A/cm2 (e.g. 10-0.1 ng/cm2s), several
orders lower than the overall current [24,75]. An important finding was the interrelation of the
kinetics between two formally independent processes, namely metal dissolution and the oxygen
evolution reaction (OER). Similar parallelism was observed also by Bockris and co-workers
[76,77], suggesting that both reaction paths proceed over common intermediate surface
oxide(s) [78]. Additionally, the authors reported the appearance of Pt traces during the negative
polarization into the HUPD region with rates of ca. 10-8 A/cm2, which drop after a few minutes
below the detection limit. It was suggested that the oxygenated species on the surface have a
dual impact, namely they are involved in the dissolution process and also act as an inhibitor of
the corrosion at relatively high potentials [79]. It was proposed that the transport of water
molecules, as a main source of oxygen, to the electrode interface has a key role. Moreover, it
was concluded that the direct electrochemical dissolution ��� � ���� � ���� is
thermodynamically very unfavourable. Ota et al. came to a similar conclusion measuring the
platinum losses by a semi-microbalance. After several hundreds of hours of
chronopotentiometry at elevated temperatures, it was shown that the Pt corrosion rate is
Chapter 3: Theoretical background and literature review
18
proportional to the OER current and the activation energy for the dissolution process
remained the same (18 kJ/mol) at 1 and 2 A/cm2 showing the enhanced dissolution with
elevated temperature [80].
Figure 3.5 Comparison of measured dissolution and overall current in relation with the applied
potential at three different temperatures (�)57°C, (�)25°C and (�)-18°C (adopted from ref.
[24]).
Later on Rand and Woods performed electrochemical experiments based on repetitive
cycling between 0.41 and 1.46 VRHE in 1M H2SO4 at 25°C. They reported an average dissolved
amount of 4.8 ng/cm2cycle, where the contribution of Pt(II) vs. Pt(IV) was estimated by post
analysis to be 3:10 [81]. Using a charge imbalance �� � ��� � ��� between the anodic and
cathodic potential sweep (in the range for oxidation and reduction of the surface) as illustrated
in figure 3.6a, the authors concluded that platinum is dissolving anodically arguing with results
obtained by Chemodanov et al. for the region of OER [24]. This is surprising considering that
none of those published results is performed at OER potentials and the proposed mechanism
responds rather to a chemical process. Moreover, while �� was reported for several upper
potential limits, the dissolution amount was determined only for one positive vertex potential
by post analysis, namely for 1.46 VRHE. The dissolution in this case, corresponds to
��=10.6 �C/cm2, that is smaller by a factor of three in comparison to the charge imbalance
measured electrochemically for the same potential window as shown in figure 3.6b.
Chapter 3: Theoretical background and literature review
19
Figure 3.6 a) Cyclic voltammogram for platinum at potential sweep rate of 40 mV/s; b)
Relation of the charge imbalance �� with the upper potential limit (Reproduced by permission
of Elsevier from ref. [81]).
Johnson et al. made a controversial study using a rotating ring disc electrode (RRDE), where
a reductive current on the gold ring disc was interpreted as redeposition of dissolved platinum
during the oxide reduction [25]. Traces of Pt(II) were found in 0.1M HClO4 with average
amount of 3.3 ng/cm2cycle after potential cycling between 0.2-1.2 VSCE with a scan rate of
0.5 V/min. The increase in the reductive current at the ring was observed around 0.8 VSCE in
the cathodic sweep (ca. 1.05 VSHE), thus the authors considered stripping of the oxide layer as a
descriptor for the dissolution process, given with following reaction:
���� � ��� � ��� � ���� � ��� (3.4)
Tsuru and co-workers adopted a similar sample-collector approach using a channel flow
system and confirming the previous results of Johnson et al. [25,82–84]. However, the datasets
should be interpreted cautiously due to the fact that residual oxygen in the electrolyte obviously
causes overlapping reductive currents on the collector electrode. Nevertheless, the post analysis
showed unambiguously the presence of platinum traces in the solution and qualitatively
confirmed the dissolution during the reduction of the oxide layer [85].
A similar observation indicating the cathodic dissolution route during the chlorine evolution
reaction (CER) was made by Roberts et al. utilizing a platinum-coated electrochemical quartz
crystal microbalance (EQCM) [86]. Once the oxide layer is not subjected to reduction, no
significant dissolution can be observed. Massive enhancement in dissolved amounts up to more
than half of a monolayer per cycle (ca. 272.7 ng/cm2cycle) is monitored in 0.1M HCl+0.9M
NaCl for a potential window from -0.2 to 1.05 VSCE. This was expected, when taking into
account that Pt forms a water soluble complex with chloride ions, which can promote the
dissolution by increasing the removal of Pt ions from the vicinity of the electrode and prevent a
Chapter 3: Theoretical background and literature review
20
possible redeposition [87–89]. Several consequent EQCM studies have been performed,
providing large datasets on Pt dissolution predominantly in acidic media, with typical amounts
in the order of few ng/cm2cycle and with a general trend of enhanced dissolution with
increasing acidity and temperature [90–94]. As a consequence, there is no common opinion for
the exact dissolution mechanism, and several reaction pathways have been proposed in
literature:
o Direct electrochemical dissolution:
�� � ���� � ��� (3.5)
�� � ���� � ��� (3.6)
o Chemical dissolution:
���� � ������ � ������ � �� � �� �� �� (3.7)
�� �� � � ��� � ���� � ��� (3.8)
��� � ��� � ����� � �� � �� �� �� (3.9)
��� � ��� � ���� � ��� (3.10)
���� � ��� � �� � �� �� �� (3.11)
The reductive dissolution can be found also as a competition between dissolution and
reduction following an overall reaction [91]:
���� � ��� � � �� � �� � ����� � �� � �� � ���� (3.12)
An important unresolved issue is the actual oxidation state of the dissolved species. Recently
Xing et al. addressed this question using ion exchange chromatography, where the platinum
traces were analyzed after continuous cycling in sulfuric acid. Both, the presence of Pt(II) as
well as Pt(IV) ions were reported with a ration of 4:1 for most cases [26].
3.4.2 High surface area Pt catalysts Besides fundamental studies on polycrystalline Pt, the stability of platinum was additionally
investigated on nano-particulate catalysts. However, in this case the long-term stability is
determined not only by dissolution but also other degradation processes [14,95,96] which
constitute overall catalyst degradation: (i) detachment of the catalyst, (ii) coalescence of the
particles, (iii) corrosion of the support and (iv) platinum dissolution. A fifth degradation
process, namely Ostwald ripening, is often mentioned additionally, but can be also considered
as a consequence of primary dissolution. In this case dissolution of the Pt and deposition on
neighboring particles results in a change of the particle size distribution and overall surface
Chapter 3: Theoretical background and literature review
21
morphology [96]. The overlap of several degradation processes makes resolving the dissolution
mechanism on nanoparticles very difficult.
Figure 3.7 a) Measured equilibrium concentration of platinum as a function of the applied
potential in 0.57M HClO4 (Reproduced by permission of The Electrochemical Society form
ref. [97]); b) Extrapolated dissolution rate after polarization for several minutes in 1M HClO4 at
elevated temperature (Reproduced by permission of The Electrochemical Society form
ref. [92]).
Myers et al. showed that the extension of the anodic potential limit during potential cycling
has a significant effect on the degradation of Pt/C catalyst [97]. The trend of increasing
dissolution was confirmed using EQCM for thin film of platinum by Dam and Bruijn after
performing chronoamperometric experiments from 0.45 VRHE to different upper potential
limits (figure 3.7b). An important observation by Myers et al. was that an increase of the
dissolution of polycrystalline Pt in the region between 0.9-1.1 VRHE gives a slope of
92 mV/decade, which is significantly higher than 29.5 mV/decade predicted for two electron
direct electrochemical dissolution as illustrated with reaction 3.2. [34,97]. The maximum
equilibrium concentration was obtained at 1.1 VRHE, where the platinum surface state is
distorted by sub-surface oxide formation [98]. Further positive extension of the applied
potential caused a drop of the dissolved amounts associated to the formation of a passive oxide
layer that prevents the dissolution. Interestingly, the obtained concentrations of dissolved Pt
are very close to the thermodynamic expected equilibrium value of 29 nM for chemical
dissolution governed by reaction 3.10.
Chapter 3: Theoretical background and literature review
22
3.4.3 Structural changes Biegler et al. showed the structural alternation of platinum surface under potential cycling
conditions, concluding the weakening of Pt-Pt interaction for oxygen coverage above 1 with
respect to the Pt atoms [99,100]. The authors also recognized the reduction of the oxide layer
as an important process for the irreversible roughening, where the large amount of adsorbed
oxygen atoms provide the required energy to promote the redistribution of Pt surface atoms
and their possible dissolution. Theoretical investigations of Eikerling et al. confirmed that the
formation of Pt-O leads to a drop of cohesive energy, which is a subject of chemical
dissolution (reaction 3.11) rather than to a direct electrochemical process (reaction 3.5 or 3.6)
[101,102]. STM studies of Itaya and co-workers experimentally proved the increased surface
mobility of platinum atoms even at relatively low potentials, consequently leading to structural
reorganization and formation of diatomic steps [103,104]. In a later work Komanicky et al.
showed the same phenomenon on different crystallographic orientations below the potential of
sub-surface oxide formation [105]. A recent in-situ AFM study, Hoshi related the change of the
surface morphology to the dissolution of low coordinated sites on cubic and tetrahedral Pt
nanoparticles under potential cycling between 0.6 and 1.1 VRHE [106]. It is suggested that
dissolution phenomena, which appear at lower potentials than for polycrystalline surfaces may
be due to the surface defects and their energetics [14].
Figure 3.8 STM images of Pt(111) after electrochemical treatment in 0.1 M HClO4 saturated
with CO between 0.07 and 0.95 VRHE (Reproduced by permission of Journal of the American
Chemical Society from ref. [107]).
Recently Watanabe et al. showed that during CO bulk oxidation under continuous cycling,
the structural reorganization occurs in the form of a movement of adatoms and leads to a
general smoothening of the surface as shown in figure 3.8 [107]. The authors proposed an early
Chapter 3: Theoretical background and literature review
23
stage formation of oxide on the low coordinated sites and their later dissolution in the negative
potential sweep [108]. Promoted mobility of surface Pt atoms is observed also for whole
particles of Pt on carbon support forming circular patterns in the nanometer range [109]. This
was interpreted as indirect effect of the reactive gases on the degradation in general. Indeed
some studies also propose an enhanced dissolution in oxygen saturated solution, and relate this
to early stage formation of subsurface oxide [110,111].
3.4.4 Theoretical model The oxide formation causes a structural alternation of the surface state. Based on the
assumption that a partial coverage of oxygenated species on the platinum surface leads to
dissolution, Darling and Meyer model this phenomenon for spherical nanoparticles using
kinetic analysis of three possible states Pt, Pt2+ and PtO [112]. The authors neglect the chemical
dissolution of PtO justifying this with a rather low, but unknown rate constant, although
several experimental studies on extended platinum surfaces suggested rather an equilibrium
chemical reaction as the key process. Moreover, a certain potential shift is introduced to adjust
the missing initial step of OH adsorption in the oxide formation. Figure 3.9 shows the
predicted amounts of dissolved species with respect to the potential scale, where the change of
coverage starts around 0.8 VSHE and the surface is fully covered at ca. 1.25 VSHE. Following this
theoretical model, the dissolution should vanish with reaching a full surface coverage and
should not be enhanced with increase of the potential above ca. 1.25 VSHE. Certainly, this is
contradictory to the trend observed in their experimental study as depicted in figure 3.9b.
Furthermore the dissolution is described as a direct electrochemical process (reaction 3.5) that
appears in parallel to the surface passivation due to oxide formation, however experimentally
the first dissolution traces (figure 3.9a) are present 300 mV more negative than the
thermodynamically expected for Pt/Pt2+ reaction.
Chapter 3: Theoretical background and literature review
24
Figure 3.9 a) Simulated oxide coverage in correlation with soluble platinum concentration
during potential cycling with 10 mV/s; b) Predicted dissolution amount versus the upper
potential limit for potential sweep experiments (Reproduced by permission of The
Electrochemical Society form ref. [112]).
In summary, major discrepancies on the topic of Pt dissolution exist in the literature, and
even for polycrystalline platinum the exact dissolution mechanism has not been resolved to
date.
Chapter 4: Theoretical background
25
4 Technical background The aim of this section is to provide a brief introduction into the technical details of the
experimental methodology and specification of the used equipment.
4.1 Inductively Coupled Plasma - Mass Spectrometry (ICP-MS)
Inductively coupled plasma - mass spectrometry is a multi-elemental analytical technique
that allows qualitative and quantitative description of species in a concentration range from
ppm to ppt. Small portions of a liquid sample are pumped into the introduction system that is
comprised of a nebulizer and a cyclone chamber [113]. The resulting aerosol is introduced
through an injector tube into the ICP torch, where it undergoes number of physical changes.
Initially, the finely dispersed droplets are losing the solvation shell forming small solid particles.
The consequent propagation in the plasma leads to sublimation and transformation in to
ground state atoms. The final conversion to ions takes place in the so-called normal analytic
zone at ca. 6000°K, as illustrated in figure 4.1. The available energy is ca. 15.8 eV, which is
sufficient to cover only the first ionization energy of most of the elements of the Mendeleev
table [114]. This insures a transfer of predominantly single charged ions in to vacuum chamber.
Figure 4.1 Schematic representation of the ICP torch consisting of three concentric quartz
tubes. The finely dispersed aerosol is introduced over an injector tube in the plasma, where it
undergoes several “dissociation” steps as illustrated on top. The resulting charged ion stream is
directed toward a triplet interface directly into the vacuum chamber.
Chapter 4: Theoretical background
26
After the ions pass the interface, the stream is focused by ion optics and forwarded to the
mass analyzer. Nowadays most commercially used systems are based on quadrupole mass
spectrometers, as is the ICP-MS adopted in this study (NexION 300x, Perkin Elmer, US).
Their principle of operation is briefly illustrated in figure 4.2. Using a combination of oscillating
and constant magnetic field, conditions at which only ions with a certain mass-to-charge ratio
have a stable flight path are created. In this way, the isotopes of interest are filtered and arrive
to the detector. The residual portion of the initial ion stream that passes the selection criteria of
the mass filter hits the analyzer. In order to cover six to seven orders of magnitude in
concentrations, there is a long list of technical solutions. NexION 300x is equipped with an
electron multiplier with dual operation mode (digital and analog) that allows measurements in a
broad range by reducing the sensitivity at high ion load. The resulting information is presented
as intensity in arbitrary units (i.e. count) that can be quantified using an initial calibration of the
device (described in more details in chapter 6).
Figure 4.2 a) Schematic illustration of flight path of ions with different masses; b) Simplified
Mathieu stability diagram of a quadrupole, showing the stability conditions for two isotopes
and the influence of modifying the DC and RF components [114].
It is worth to mention that the performance of the mass filter is a compromise between the
mass selectivity and element sensitivity. Namely, increasing the precision in mass determination
leads to a decrease of the effective detection limit and vise versa. Using an ICP-MS several
elements of the periodic table can be analyzed. While real-time parallel measurements are not
possible because only one mass is acquired at a time, the short dwell time during switching
between masses (50-100 ms) allows for efficient sequential investigations.
Chapter 4: Theoretical background
27
4.2 X-ray Photoelectron Spectroscopy XPS is an experimental method based on the photoelectric effect utilizing energy dispersive
analysis of the emitted photoelectrons, which enables the determination of sample composition
and the electronic state in the vicinity of the surface. An X-ray beam is generated in an
excitation source (aluminum anode, K=1486.6 eV, PHI2000) with a typical energy range of
200-1200 eV. The X-ray can penetrate several micrometers of a solid, nevertheless XPS is still a
surface sensitive technique. This is determined by the fact that the maximum depth of escaping
electrons is only some nanometers, for example in case of platinum the mean free path is
ca. 11 nm [115]. The electrons ejected from the sample with a specific energy are filtered by a
hemispherical analyzer. The binding energy is estimated from the analysis of the kinetic energy
by the following relation:
�������� � �� � ��������� � �� where � is Planck’s constant (6.626�10-24 J/s), � is the frequency of the excitation X-rays,
�������� is the measured kinetic energy of photoelectrons, and�� is the work function of the
spectrometer [116]. The binding energy spectrum is processed with a library adjusted to C1s
Carbon peak position for PHI2000 at 284.1 eV using CasaXPS software.
4.3 Scanning Kelvin Probe microscopy The Kelvin probe technique was introduced to measure the work function of different
materials by Lord Kelvin [117]. Utilizing a lateral oscillation of the probe, a current flow is
induced between the tip and the sample. It is compensated by applying an external bias that
results in a shift of the Fermi energy of the tip. The substrate is usually grounded to prevent
change of the energy level. This so called compensation method enables an estimation of the
work function by measurement of the applied potential. The exact value is calculated using
following relation [27]:
������� �� � ������ � ��������� ���� � ���
In its modern version, the sample is placed on translational stage with a backside contact to
a reference tip, thus enabling the scanning of a certain area of the sample.
Chapter 4: Theoretical background
28
Figure 4.5 a) Schematic setup of the SKP system; b) Energy levels of the sample and tip
before approaching the surface; c) Illustration of equilibration of the energy levels between the
tip and the sample by applying a potential difference of �U.
Chapter 5: Automated system development – from design to implementation
29
5 Automated system development – from design to implementation
The discovery of new fundamental phenomena and development of improved materials is
often achieved by adopting smart design of advanced experimental methods and combination
of different, complementary techniques. Also modern electrochemistry requires adequate
approaches to increase the information depth on electrode reactions and in particular handle
the large amount of parameters that come into play. Several electrochemical and analytical
methods with high-throughput screening capabilities have therefore been developed during the
previous decades, as for instance the scanning micropipette contact method (SMCM) [118],
flow type-scanning droplet cell (f-SDC) [119,120], scanning electrochemical microscopy
(SECM) [121–123], multi working electrode array [124] etc., using the benefits of computational
control over the hardware. All these systems have something in common, namely the self-made
components running under a single software application.
The investigation of the stability during electrochemical treatment requires an approach
based on the coupling of electrochemical methods online with a spectrometer. As many of the
functionality criteria were not strictly defined (or not defined at all) at the beginning of the
project, the design of the following experimental equipment was done following the spiral
model of software/system development, namely to code the development with maximum
flexibility with respect to the changing or unknown, upcoming requirements [125]. The
electrochemical experiments are usually performed using a potentiostat that allows current
and/or potential control. In this project the Reference 600 from Gamry Intruments was chosen.
Its compatibility with several programming languages is very beneficial for the implementation
of custom measuring procedures. The final source code was implemented in graphical
programming environment LabVIEW using embedded ActiveX object. The controlling
programs were distributed in a compiled form as executable on the laboratory computers.
Chapter 5: Automated system development – from design to implementation
30
5.1 Hardware – between commercial and custom solutions
Appropriate choice of the equipment is crucial for the accurate and efficient high
throughput system. An important criterion is the interface of the hardware, which has to allow
external control. All devices have digital and/or analog input. The communication with
hardware components was integrated with self-made routines directly in LabVIEW, or in some
cases indirectly over dynamic link libraries (ddl).
5.1.1 Rotating Disc Electrode (RDE) The rotating disc electrode is a well-established technique for electrochemical investigations.
It founds often applications in the fundamental studies mainly due to the precisely defined
mass transport regime [126,127], nevertheless no commercial vendor provides a completely
automated solution for usage of RDE, due to the number of components that have to work in
parallel and different requirements in laboratories. The developed RDE system within this
work can be separated into three main sections:
(i) Electrochemical part – The potentiostat (Reference 600, Gamry) is connected to the
working, counter and reference electrodes placed into in-house made Teflon® three
compartment cell.
(ii) Rotator part – The working electrode is fixed on a motor (EDI101), where the
rotation rate is controlled by Speed Controlling Unit (CVT101 from Radiometer
Analytical). The digital access is made by a DAQ card (NI USB 6009), where 1 mV
of output voltage corresponds to approximately 1 rpm. Each RDE system has its
own correction factor for the precise speed control. The communication is
performed without feedback, due to limitations of the Speed Controlling Unit.
(iii) Gas system – The self-made two channels gas system is assembled from Swagelok
parts as illustrated in figure 5.1. The gases can be exchanged between argon,
oxygen, hydrogen, carbon monoxide and carbon dioxide using six magnet valves
(6011, Bu�rkert) controlled by a customized unit containing ICP-DAS i-7520-CR and
i-7055D-CR cards. Additionally, the argon stream can be mixed with one of the
listed gases in precisely defined ratio. The flow rates are adjusted by two
commercial mass flow controllers (Bronkhorst, Series:EL-FLOW) in the range of 0-
200 ml/min connected via a FlowBUS network and accessible over a converter
Chapter 5: Automated system development – from design to implementation
31
unit.
Important to notice is the separate interface of the hardware components in each section.
This makes possible their asynchronous operation without causing race condition and allows
quick replacement in case of defect or general change of the hardware. Figure 5.1 illustrates a
sketch of the hardware. For simplicity, several physical connections are removed (like the
cables between the potentiostat and the electrodes, power supplies, valve connections,
FlowBUS network etc.).
Figure 5.1 Schematic representations of the hardware components in the rotating disc
electrode setup and their communication interface.
5.1.2 Scanning Flow Cell system (SFC) The SFC system is based on the concept of a channel electrode, where the electrolyte is
flowing over the working electrode. In contrast to its classical version, the sample (i.e. working
electrode) is not integrated in to the walls, but it is externally introduced on a three-dimensional
translational stage. For this purpose a completely new design of the flow cell was made in this
work. Figure 5.2a-b depicts only two of the developed CAD models of electrochemical cells in
the SFC system, which were in-house manufactured using a milling machine (CAM 4-02
IMPRESSION, VHF camfacture AG) from polyacrylate or polycarbonate. The control over the
cell geometry during fabrication with a �m-precision insures the reproducibility of the
measurements with different cells. The contact area with the sample is restricted by silicon
gasket (RTV 118Q, Momentive), so that the cells with 0.4 and 1 mm channels have defined
Chapter 5: Automated system development – from design to implementation
32
contact areas of 0.25 and 1.1 mm2, respectively. The illustrated silicone sealing is not gas tight,
thus to prevent possible diffusion of ambient gases (especially oxygen) into the electrolyte, two
outer purging channels with argon are employed (figure 5.2). In this manner, the spacing
between the sample and the main body of the SFC is continuously filled with inert gas that
shields the gasket from air.
Figure 5.2 a) CAD model of the SFC cell with 1mm electrolyte channels and external purging
modification, b) alternative SFC cell geometry with 0.4 mm inner diameter of the channels;
Magnified image of SFC tip: c) in non-contact mode and d) in contact with the sample. [A.11]
The SFC cell can be positioned at various points in the XY-plane allowing investigation of
local properties of the working electrode. The counter electrode is introduced in the electrolyte-
supplying channel. In this way a possible redeposition of the dissolved species from the sample
on the counter electrode is prevented. The reference electrode is positioned in the opposite
channel to the counter electrode (i.e. in the outlet), in order to reduce the IR drop by acting like
classical Luggin capillary and also to prevent possible chloride contaminations from leaking of
the commercial electrode (Ag/AgCl/3M KCl, Metrohm). This electrode configuration is used
for the measurements in the following chapters. Note, that the cell design is only one of the
bottlenecks in the development of the whole SFC. The system itself is made out of a number
of different devices that increases the complexity significantly in comparison to the RDE
system, as illustrated in figure 5.3.
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"��"�����
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Chapter 5: Automated system development – from design to implementation
33
Figure 5.3 Simplified schematic representation of the hardware components in the scanning
flow cell (SFC) with their communication interface and connections. [A.18]
Similar to the RDE system, the hardware can be classified in three main sections:
(i) Electrochemical part – The potentiostat (Reference 600, Gamry) is connected to the
electrodes over a shielded extension cable box. The reference and counter are
placed stationary with respect to the SFC tip. The working electrode (WE) is
contacted by a tungsten needle and can be moved over three axes; Additionally the
WE is placed on a thermal conductive platform that is incorporated in the
temperature controlled water circuit of the main electrolyte vessel maintained by a
bath circulator (Fisherbrand FBC 735, Fisher Scientific);
(ii) Positioning part – The sample is fixed on a platform on top of a three-dimensional
translational stage (M403.6DG motors, step precision of 0.1 �m) with independent
axis control over a PCI card (C843.4, Physik Instrumente) integrated in the controlling
laboratory PC. Optional optical adjustment of the position can be done using two
Chapter 5: Automated system development – from design to implementation
34
USB cameras with 10x and 70x magnification (SMX-M83C, Sumix). The contact
force between SFC cell and the sample is monitored with bending force sensor
(KD45-2N, ME-Meßsysteme) over a self-made acquisition module (ICP-DAS i-7520-
CR and i-7016-CR cards). Typical values in contact mode of the SFC are between
50 and 500 mN;
(iii) Gas system – Similar configuration as described in chapter 5.1.1-(iii), with minor
modification to the outlet. The channel 1 is used for outer purging of the SFC tip
and the second one is used for the saturation of the electrolyte in the main vessel,
as shown on figure 5.3;
The electrolyte flow is driven through the cell by an integrated MP2 peristaltic pump,
incorporated directly in the ICP-MS. In order to decrease the time delay and improve the
lateral velocity between the SFC and the ICP-MS, tubing with reduced dimensions with respect
to the supply channels and the one in the SFC tip is used. In non-contact mode a continuous
electrolyte flow is sustained (figure 5.2c). Several technical modifications are incorporated to
provide stable system performance in non-contact mode like precise adjustment of the
curvature of the meniscus. Maintaining constant electrolyte level in the main vessel is achieved
by continuous supply by a second peristaltic pump that compensates the outflowing amount.
5.1.3 Inductively Coupled Plasma - Mass
Spectrometry (ICP-MS) In conventional applications the ICP-MS is used for multi elemental analysis of species with
constant concentrations in the region down to ppb/ppt. The samples are usually prepared in
acidic media and mixed addition of a reference element, later on used as an internal standard.
In case of the coupling with SFC, to prevent possible interference of the internal standard with
the electrochemical measurements, both solutions are mixed after the electrochemical cell and
shortly before the introduction system of the ICP-MS, as shown on figure 5.4. Using tubing
with different inner diameter (typically 0.38 or 1 mm) allows the user to vary the mixing ratios
(4:1, 1:1 and 1:4). This is used additionally to increase/decrease the effective sensitivity in some
cases. Afterwards, the mixed solution is dispersed in to the spray chamber by the nebulizer
(Meinhard) with argon stream of ca. 0.9 L/min. The operation range of the nebulizer is the
determining component for the electrolyte flow in the combined system and thus has major
impact on the system design. For example, a SFC cell configuration with 1 mm channel
Chapter 5: Automated system development – from design to implementation
35
diameter operates with an average volume velocity of ca. 0.2 mL/min (through the cell) at a 1:1
mixing ratio. The aerosol is introduced in the ICP torch, where it is decomposed, dissociated
and ionized. After that the ion stream passes through the triple cone interface in to the vacuum
chamber, where the isotopes of interest are ‘filtered’. As mentioned before, the quadrupole
provides a stable ‘flight’ path only for a single charged isotope over a short period of time (i.e.
dwell time of 50-100 ms). Thus a set of isotopes is measured in sequential order (i.e. method)
by several repetitions (i.e. sweeps) in accumulative mode, which gives the final acquisition value
of one method. In case of platinum, a method covering the two most abundant isotopes (194Pt
and 195Pt) and the internal standard of 187Re gives an effective acquisition rate of ca. 1 Hz for
5 sweeps/reading.
Figure 5.4 Schematic representation of the modified introduction system of the ICP-MS and
the hardware components within the device.
5.2 Software development 5.2.1 Software architecture – modular approach
The hardware-controlling program represents the core of the development. It has to
combine different devices with several kinds of communication in terms of interface and
timing under the roof of single software. This requires maximum flexibility in the software
architecture in the run-time procedure. Nevertheless, the general structure follows a sequential
Chapter 5: Automated system development – from design to implementation
36
logic as illustrated in the flowchart on figure 5.5. First, the input parameters for the header of
the data are requested, and then the main program is initialized. In the next step, the
communication with the hardware components is validated. After successful start-up of the
equipment, the program goes in stand-by for the run-time mode, which can be terminated and
followed by safety shut down of the hardware in case of closing the software. As a final step,
the main program closes with all running processes in the background and creates an event log
for the operation time in a single ASCII file.
Figure 5.5 Flowchart of the programming logic of the main controlling application; Steps 1
and 2 are performed in parallel during real application, in order to reduce the boot time. Step 3
initialize and check the hardware with possible transition to step 5 in case of error in the
hardware functionality. Step 4 is described in details in figure 5.6;
The challenging part is to create the source code architecture for step (4) of the flowchart
from figure 5.5. It requires several parallel functionalities like: visualization, data storage,
communication (acquisition/control) etc., where all those processes have different execution
timing. The first step is to create a straightforward separation of the programming logic and
user inputs from single hardware functionalities, or to generate a so-called abstraction level
[125,128]. That will allow at a later stage an integration of smart algorithms in between, like
automatic execution under predefined conditions [129]. This is achieved by extending the
classical consumer/producer design pattern [125]. In its original version, it has one producer
Chapter 5: Automated system development – from design to implementation
37
loop usually for the user interface (in pink in figure 5.6) and only one programming loop for
the operations (in blue in figure 5.6). That limits the execution frequency to a single time cycle
of the consumer loop, which can be overcome by introducing a separate structure for each
hardware component. The operation commands to control the different processes are
generated centralized in the “producer loop” and distributed over the “System Queue” to the
consumer. Here appears a problem in distinguishing the destinations (e.g. consumer loop) of a
single task. In addition, many continuously running parallel processes will cause overuse of the
laboratory computer resources (most likely crash) even with integrated timeout. Those issues
are resolved by the development of universal algorithm that can be inserted and configured
individually for each consumer loop. This enables continuous, standby or task-oriented
execution, just by a simple modification of the input constants. The block diagram of the
source code is illustrated in the zoom in of the “CheckVI” in figure 5.6.
Figure 5.6 Schematic illustration of the developed modular design pattern in LabVIEW for the
run-time procedures. The pink represents the UI loop with event handler and the blue is used
for the single hardware components or individual programming functionalities (e.g. data save or
graphical visualization). (Reproduced by permission of AIP Publishing LLC from ref. [A.3])
It is crucial to emphasize that data acquisition is separated from data processing procedures
on purpose. The information is buffered on the principle of first-in-first-out (FIFO) in the
“Data Queue” (fig.5.6). In this way, no data is lost, even in case of slow performance of the
laboratory computer or redistribution of the resources on other tasks.
5.2.2 User interface – friendly and efficient design The programming functionality is the heart of the software, and the user interface (UI) is its
face. The UI has to be intuitive and restrictive for the operator in order to be used efficiently.
This is achieved by following the general guidelines for UI, by separating it into three main
Chapter 5: Automated system development – from design to implementation
38
fields: status indicators, controlling parameters and visualization [130]. Figure 5.7 depicts the
UIs of the RDE and SFC controlling programs. Their similarity enables operators of one
system to become familiar with the other in a short period of time. The UI has on top the first
field (in red box) containing status indicators for running processes and the predefined worklist
for the automatic mode. The second field includes controlling parameters for all hardware,
which can be changed online for the manual mode or can be used in creating/modifying the
worklist for the automatic one. In the last part (in blue box), a simultaneous graphical data
representation is made for the last running method in different forms (like current vs. potential,
or potential/current vs. time, etc.). Furthermore, different functions are activated or disabled
with respect to the operation mode or the status of the program. It can be easily recognized
that, for example, the mode cannot be changed during a running measurement.
Figure 5.7 User interface of the controlling software application of the (a) SFC and (b) RDE
system. The Front Panel is divided into three fields. (i) Status indicators and worklist in the red
box; (ii) Controlling parameters and settings in the green and (iii) online data display in the
blue;
Chapter 5: Automated system development – from design to implementation
39
5.3 Data management 5.3.1 Data storage
An automated approach implies also automated storage of a large amount of data. As shown
in figure 5.6, saving and visualization of the data is separated from the acquisition process,
which allows different data treatments to be performed independent on the acquisition
procedure. All folders and data files are created in the background without any interaction of
the user, reducing the operation effort and possible errors. The data storage is made in defined
structures in separate folders for each worklist in the automatic mode or in a temp folder for
the manual execution as shown on figure 5.8.
Figure 5.8 Schematic tree structure of the storage folders and the data files; Example of
consumed HDD space and amount of data files after a single degradation test on commercial
catalyst Pt/C over ca. 10h;
An additional security algorithm is implemented to prevent loss of information in case of
sudden system collapse or failure of the main power supply. The acquired values for the
running experiment are buffered in the file last-measurement.txt in real time. Even in case of
shortage of the main power supply of the equipment, the dataset is saved automatically. An
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Chapter 5: Automated system development – from design to implementation
40
important aspect is the reverse data tracking, meaning that not only the general structure is
strictly defined, but also the file formats. All files are saved in ASCII format (figure 5.9), which
it is not that space-saving as binary format, but on the other hand doesn’t restrict the evaluation
procedure only to the developed programs in LabVIEW. The data file can thus be accessed
with any text editing application.
Figure 5.9 File format of the descriptor file in a) and a selected single measurement file in b).
Both are separated in three section: (i) Information about used hardware including user
comments, (ii) Header of the content and (iii) Data or descriptive table.
5.3.2 Evaluation – quick data processing Treatment of a large amount of data as produced by automated experimental equipment is
typically a time consuming procedure. Loading and visualization of data in most of the cases,
follows several rather routine steps, and analysis of the experimental results by the user is thus
often a rather demanding process. The data overflow is unavoidable during high throughput
experiments, but the efficiency of the evaluation methodology can be improved. For that
purpose, a set of supporting programs has been created. Figure 5.10 depicts two of them: a)
processing of electrochemical data and in b) combined evaluation of ICP-MS data with the
electrochemical one from the SFC. Operations used on a daily basis (like determination of
active surface area from CO-stripping/Hupd region, calculation of number of exchanged
Chapter 5: Automated system development – from design to implementation
41
electrons, Levich-Koutecky plot, Tafel etc.) are completed in a few mouse clicks. The evaluation
protocol can be saved and loaded in a binary TDMS format. The architecture of the software
follows the so-called state machine design pattern with integrated event handler. That allows
simple extension without affecting the existing algorithms, just by introducing new cases in the
existing source code and accessing the data from a shift register.
Figure 5.10 User interface of the data evaluation programs: a) for the electrochemical dataset;
b) for a combined evaluation of the datasets from the ICP-MS and SFC system;
Chapter 6: System validation – proof of functionality and limits in application
42
6 System validation – proof of functionality and limits in application
As described in the previous chapter, a variety of different hardware and software
algorithms have been combined in order to achieve fully automated experimental setups. The
increasing complexity of the system inevitably leads to a higher probability of hidden errors.
The programming logic of individual components was proven in several separate tests. Here
will be shown only the reliability of the computer control during real operation conditions and
verified of the full automation of the coupling between the electrochemical and spectrometric
systems.
6.1 Automation - proof of concept The system test is performed using a degradation study of a 3 nm commercial carbon-
support platinum catalyst (TKK, Japan) with one of the RDE setups. Different glassy carbon
RDE tips are prepared with the same loading of 20 �gPt/cm2 dried under nitrogen atmosphere.
The long-term performance is validated under automatic mode of operation using an
accelerated degradation protocol simulating harsh start-stop conditions of operation [131]. The
experimental procedure is illustrated in the flowchart in figure 6.1a. Initially, the potential of the
reference electrode versus RHE is evaluated in hydrogen atmosphere by measuring the open
circuit potential (OCP), and then a short pretreatment is applied to insure the adhesion of the
catalyst layer to the RDE tip. In the next step, the specific activity (SA) for the oxygen
reduction reaction (ORR) is determined at four rotation rates (400, 900, 1600 and 2500 rpm)
using the equation [132]:
��������� ���������� � ��
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� ����
���������������������
� ����� �����������
where��� is the measured current at 0.9 VRHE, ��� is the diffusion-limited current at the same
rotation rate, ��� is the oxidation charge from the first CO-stripping procedure in the region
between 0.7 and 1 VRHE (figure 6.1b) and � correspond to the charge density of polycrystalline
platinum (ca. 0.195 mC/cm2) [133]. The variation of the SA value is based on experimental
Chapter 6: System validation – proof of functionality and limits in application
43
results from five different samples. The following degradation procedure leads to a decrease in
the active surface area over time, as measured at different stages in between the degradation
protocol by CO stripping method. After 7200 degradation cycles, more than 60% of the active
surface area is lost and/or not accessible (figure 6.1c). The maximum relative standard
deviation in the determined area at the end of the degradation protocol is less than 8% as
determined from five different samples, and is attributed to the precision of the preparation
methodology of thin films on the RDE tips.
Figure 6.1 a) Schematic illustration of the applied experimental sequence; b) Time evolution of
the active surface area during the degradation procedure, measured by CO stripping (inset).
The error bar is based on the deviation in the results of five samples on five measuring days. c)
The arrows indicate the evolution of the cyclic voltammograms between 0.4 and 1.4 VRHE at a
scan rate of 1 V/s (black shows the forward scan and in red the backward one) (Reproduced by
permission of AIP Publishing LLC from ref. [A.3]).
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Chapter 6: System validation – proof of functionality and limits in application
44
The automated degradation test showed long term operation of the developed system over
several hours without any user interaction. As the software architecture of the RDE and SFC
follows similar algorithms, no separate validation for the SFC is performed. Moreover the
reliability of the SFC system(s) can be observed in the many experimental series shown in the
next chapter.
6.2 Performance of electrochemical systems – RDE vs. SFC
The electrochemical response of both systems is evaluated using the same polycrystalline
platinum sample in freshly prepared 0.1 M perchloric acid. Figure 6.2 shows the cyclic
voltammograms (CVs) in argon-saturated electrolyte for the RDE and SFC setups in a) and b),
respectively. The CVs in both systems are comparable and reproducible. The HUPD region in
case of the SFC system is slightly shifted downward to negative currents. This is an indicator
for residual oxygen in the solution that can be reduced at these potentials (i.e. causes a reductive
current response). The partial leaking of oxygen under the silicon gasket remains an issue. It
has to be mentioned that despite the fact that the SFC configuration has its drawbacks, it still
provides a powerful tool for comparative analysis, online combinations of electrochemistry
with complementary techniques and in comparison with RDE it has negligible IR-drop due to
the typically low working currents. The electrochemical measurements with the SFC are limited
in the region where there is no massive gas evolution on the counter and/or working electrode,
thus the potential windows are chosen precisely to prevent blocking of the channels by bubbles.
In the next step, the electrolyte is saturated with oxygen and the specific activity of platinum
for the ORR is determined. The kinetic current is extracted from the overall current (figure
6.2.c-d), which is corrected for diffusional effects in the positive-going potential sweep
following the Levich-Koutecky equation. The Tafel plot has been acquired using the evaluation
software from chapter 5. The experimental results from the RDE system show coinciding
curves for the different rotation rates, which is not the case for the SFC. An increase in the
flow velocity in the channels leads to a positive shift in the kinetic current response. This can
be attributed to the change of the flow profile distribution over the surface with different flow
velocities, e.g. increase of the velocity leading to shrinking of the hydrodynamic layer, Later is
linearly proportional to the diffusion layer thickness and leads in combination with an
Chapter 6: System validation – proof of functionality and limits in application
45
inhomogeneous flow profile to an inhomogeneous current density distribution on the surface
in the kinetically controlled region.
Figure 6.2 Electrochemical measurements in 0.1M HClO4 on polycrystalline platinum: Cyclic
voltammograms made in argon are shown in a) for the SFC and in b) for the RDE; ORR
measurements and the corresponding Tafel plots in c,e) and d,f) for the SFC and RDE system
respectively; The insets in c) and d) shows the change in the limiting current with square root
of the angular velocity for the RDE and third root of the volume velocity for the SFC;
Chapter 6: System validation – proof of functionality and limits in application
46
The non-uniform distribution of the flow profile is critical and does not allow a direct
analytical solution of the diffusion limited current with respect to the flow rate at this statge.
Nevertheless the extracted mass transport limited current follows the third root relation
predicted for the channels electrode [134]. Furthermore, the order of the diffusion layer
thickness can be estimated using the general relation for the measured currents [27]:
� � �����
where � is the number of exchanged electrodes (for ORR on platinum is assumed to be 4), � is
the Faraday constant (96485 As/mol), C is the bulk oxygen concentration in 0.1M HClO4
(1.2 mM) and k is the mass transfer coefficient that can be further expressed in case of a
diffusion limited reaction as the ratio between the diffusion coefficient of oxygen (D=1.67�10-
5 cm2/s) and the diffusion layer thcikness. From the limited current in region within 0.2-
0.5 VRHE (Fig. 6.2d), the average thickness of the diffusion layer can be estimated to be in the
range of ca. 78-157 �m. A single precise value cannot be considered for certain flow velocity
due to the inhomogeneous distribution of the flow profile as illustrated in figure 6.3. Exact
evaluation of the SFC system will require numerical simulation in future.
Figure 6.3 Illustration of the distribution of the flow profile lines for: a) scanning flow cell, b)
channel electrode and c) rotating disc electrode;
6.3 Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) and coupling with the SFC
The ICP-MS is particularly interesting for the investigation of low dissolution rates and
provides its results in relative units (i.e. counts) for the isotope(s) of interest. The quantification
of the signal is made on the basis of a calibration made before the actual measurements. For
that purpose, electrolytes with known concentration are prepared and measured. Typically, the
intensities of the investigated isotopes are normalized to the relative constant intensity of the
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Chapter 6: System validation – proof of functionality and limits in application
47
internal standard. Figure 6.4 depicts an example of calibration curves for some of the noble
metals in sulfuric acid.
Figure 6.4 Calibration curves for some of the platinum group metals in concentration range of
0.5-5 �g/L, performed by the method with two groups for 193Ir, 195Pt, 197Au measured against
5 �/L of an internal 187Re standard, and for 102Ru, 103Rh, 106Pd versus 89Y as an internal standard
(5 �g/L). The equation for the linear fitting is presented for each isotope in the legend. Mixing
ratio of the internal standard with the analyte is 1:1.
For the purpose of conventional quantitative analysis, the solution of interest is
continuously introduced into the mass spectrometer until a constant response is achieved,
which is further measured several times (i.e. replicates). The final concentration is calculated
from the normalized intensity using a calibration curve (figure 6.4) for the corresponding
electrolyte matrix. In the case of coupled SFC and ICP-MS, the content of the species in the
electrolyte is changing with time. The transient signals are recorded based on the same principle
as the stationary one, however, can be measured only once (replicate(s)=1). In order to evaluate
the performance of quantification in this mode and the correlation between the spectrometric
and electrochemical signal, on the dissolution of Cu from a bulk copper sample is investigated
by applying a chronopotentiometric sequence. Figure 6.5 shows the resulting spectrometric
signal calculated as “dissolution current” using Faraday’s law:
����� � � � � � � � � �� �
� � � ��� � ���� ��
��� � ��������������� �
������������ ����� �� ��
�
Chapter 6: System validation – proof of functionality and limits in application
48
where � is the valence of the ions, � is the calculated concentration using the corresponding
calibration curve, � is the volume velocity, � is Faraday’s constant and � is the molar mass.
Initially, the ICP-MS signal is recorded with the SFC not in contact with Cu sample, which
provides the background signal as shown in the first seconds of the inset in figure 6.5. Once
the contact with the sample is achieved, the native layer of oxide is dissolved in a chemical
process and results in the initial peak in the Cu signal. Applying anodic currents causes an
enhancement in the dissolution rate and thus in the measured spectrometric signal. A good
correlation between the applied current and the dissolution rate from the ICP-MS is achieved.
Note, however, with the increase in the applied currents a substantial mismatch in the
magnitude of the measured spectrometric signal appears. This is related to the fact that the
dissolution current is calculated for monovalent copper ions based on the thermodynamic
prediction of the Pourbaix diagram [34]. Under the chosen experimental conditions, copper
can however also be dissolved partially in a bivalent state, which tends to be dependent on the
current density. This phenomenon has been previously reported in other studies [135,136].
Figure 6.5 On top the experimental procedure for the measurements is illustrated. The
measurement started in non-contact mode, the Cu working electrode surface is then
Chapter 6: System validation – proof of functionality and limits in application
49
approached and the sequence with modulated current is applied once contact has been
established. Finally the cell is lifted up (i.e. away from the sample). Below the spectrometric
signal (Idiss) and the applied current (Iapplied) are plotted. The inset shows a magnification of the
initial dissolution peak after the contact with the sample. Electrolyte: 1 mM HCl. [A.4]
A straightforward correlation of the electrochemical and spectrometric signals is achieved by
modifying the experimental conditions. Increasing the acidity of the electrolyte leads to
dissolution of copper practically only in Cu+ form. Figure 6.6 shows the nice overlap of both
datasets, where the ICP-MS signal closely follows the electrochemistry.
Figure 6.6 Correlation between the spectrometric and electrochemical signal in 10 mM HCl
electrolyte. The applied experimental sequence is similar to the one used in figure 6.5 [A.4].
Synchronization between electrochemical and spectrometric signal is another important
issue that has to be addressed. Figure 6.7a illustrates an experimental sequence of applied
anodic current on a copper sample, while the corrosion rate is recorded in parallel. Two main
parameters are extracted from the results, namely the time shift between SFC and ICP-MS
response and the peak broadening with respect to the flow rate of the electrolyte. Once the
volume velocity through the SFC is above 50 �L/min only minor changes are detected in both
parameters, as shown in figure 6.7c. The optimal flow rate for the following experiments is
determined also from a combination of two further criteria: (i) stable electrolyte flow in the
Chapter 6: System validation – proof of functionality and limits in application
50
SFC during non-contact mode and (ii) optimal nebulizer performance of the ICP-MS, where
the second one is dominating in order to achieve high sensitivity. The experimental results
reveal a volume flow of ca. 51 and ca. 193 �L/min as optimal for the 0.4 and 1 mm SFC cell
configurations, respectively, with mixing ratios of 1:4 and 1:1 between the electrolyte and the
internal standard solution.
Figure 6.7 Schematic representations of the applied current signal of 200 nA on a copper
sample in a) and illustration of the measured signal by the ICP-MS in b); The analyte and the
internal standard are mixed in ratio 1:4. The full width at half maximum (FWHM) and time
delay are graphically shown in c) for different pump velocities in 0.1M H2SO4. for the cell with
0.4 mm diameter channels.
The time delay has two contributing parts, as shown in figure 6.8: (i) time required to escape
from the diffusion layer and (ii) time for the lateral transition of the dissolved species to the
detector of the ICP-MS.
������ � ����� � ����� ����� �
���������������
����
������
����� ��
�����
�����
������ �
�
����� ��
���
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�����
� ���������
�!�
�!�
$���
#""����
"����
����"�"%��� ��#�
�!�
Chapter 6: System validation – proof of functionality and limits in application
51
The main impact originates from the convectional part, although the diffusion time can also
cause certain variations with the thickness of the diffusion layer, as plotted in figure 6.7b. The
shift in the time scale is determined experimentally on each measuring day, and lies typically in
the range of 16-20 seconds. The peak spreading is an unavoidable aspect of the experimental
system. It originates from different lateral velocities in convective flow in the tubing (figure6.3)
and the inhomogeneous profile of the diffusion layer. Species dissolution at the same moment
will thus be introduced at different time in the convective stream and the transport in the
tubing to the ICP-MS is accompanied with additional variation of the time delay before
reaching the detector. In the case of copper, five seconds of active dissolution result in peaks
with a FWHM of three to seven times broader than the electrochemical signal (figure 6.7c).
Figure 6.8 a) Schematic representation of the factors influencing the overall delay time; b)
theoretical prediction for the required time to diffuse through certain distance for diffusion
coefficient of 5�10-6cm2/s.
Chapter 7: Results and discussion
52
7 Results and discussion 7.1 Electrochemistry of platinum in acidic media
Polycrystalline platinum is a well-studied catalytic material, often used as a model system.
Figure 7.1 depicts exemplary the cyclic voltammogram of the polycrystalline surface measured
with SFC cell (1 mm diameter of the channels). In the potential window of 0-0.4 VRHE, the
typical current response for hydrogen adsorption and desorption can be observed (so called
HUPD region). It is often used for the “active” surface determination based on the works of
Conway and co-workers under the assumption of 210 �C/cm2 (1e-/Pt atom) for a single
monolayer formation [40,59]. Unfortunately minor variation of the lower potential limit cause a
significant change in the integrated value and can lead to random errors and several studies are
questioning the reliability of this method [132,137]. Additionally, the different time scale
contributes also to the variation of the HUPD charge, as shown in figure 1b. It has to be
mentioned that the correction of the double layer charging is performed with a simplified linear
interpolation, which introduces an additional uncertainty as already discussed by Frumkin and
co-workers [138]. An alternative method of active surface area determination is CO-stripping,
which however could affect the dissolution behavior of platinum. As the roughness can change
during these experiments and its influence on the dissolution is unknown, in a first approach all
measured values in the further work are represented as current as well as dissolution rate
normalized to the geometric area of the electrode. The second region between 0.4 and 0.8 VRHE
is related to the double layer charging, where the platinum shows rather chemically inert
behavior. Above ca. 0.8 VRHE an increase in the current is observed. The region between 0.8 and
1.0 VRHE has different interpretations in the literature. Conway and co-worker suggested an
initial water de-protonation and OH adsorbed on the surface [139]. An alternative statement is
made by Jerkiwiecz et al. who assume the surface oxidation to proceed directly with
chemisorbed oxygen [140]. Nevertheless most of the authors agree on the formation of
subsurface oxide starting around 1.1 VRHE [42,98]. Further extension of the anodic potential
promotes the oxide growth and above ca. 1.6 VRHE the oxygen evolution reaction (OER) begins.
The negative going sweep contains a reductive peak (1.0-0.5 VRHE) that can be used for
quantification of the amount of oxide formed in the anodic scan.
Chapter 7: Results and discussion
53
Figure 7.1 a) Cyclic voltammogram measured with SFC-1mm between 0.05 and 1.5 VRHE with
a scan rate of 0.2 V/s in 0.1M HClO4 electrolyte continuously purged with an argon stream.
The volume flow rate is 194 �L/min that corresponds to average lateral velocities of ca. 6 and
1 mm/s for 0.4 and 1 mm channels, respectively; b) Integrated charge in region between 0.05
and 0.4 VRHE with subtraction of the capacitive response for seven different scan rates.
7.2 Influence of the overpotential for oxide formation and reduction on platinum dissolution
As it becomes clear from Chapter 3, it is not resolved in literature what exactly triggers Pt
dissolution and how much Pt dissolves under different conditions. Most of the representative
studies assume that the oxide plays a key role in the process of dissolution [25,81,112].
Figure 7.2 shows an experimental sequence of applied cyclic voltammograms, where the
positive potential limit is extended stepwise from 1.0 to 1.85 VRHE, thus modifying the
overpotential for the oxide formation. The recorded online ICP-MS signal shows no detectable
dissolution as long as the potential cycling is performed within the HUPD and double-layer
region, even up to initial Pt oxidation at 1.0 VRHE. Note that the typical detection limit of the
ICP-MS is less than 10 ppt (equal to ca. 0.7 �g/cm2L or ca. 3 pg/cm2s or ca.
9.3�109 atoms/cm2s), based on three times the standard deviation of the blank signal [113,141].
For further simplicity in the following text is used the statement “no dissolution” is used,
which should be considered correctly as no significant dissolution for the given conditions with
respect to the detection limit for our analysis. Once the upper potential is increased above
ca. 1.1 VRHE first traces of platinum are observed. With further extension of the anodic limit, the
amounts of dissolved platinum per cycle become more pronounced, and the two peaks can be
Chapter 7: Results and discussion
54
distinguished during one CV as shown in figure 7.2c. A relatively small peak is observed in all
positive going sweeps, which on a first view tends to be constant with respect to the upper
vertex potential. The second one appears in the cathodic scan around 1.0 VRHE and is
significantly larger. This finding is in good agreement with the results obtained by Johnson et al.
who consider the cathodic route of dissolution as the main one [25].
Figure 7.2 a) The applied experimental sequence in 0.1 M HClO4/Ar consists of 2 cyclic
voltammograms with a scan rate of 0.01 V/s for each potential window; always starting from
0.1 VRHE to an upper potential limit between 1.0 and 1.8 VRHE and raised in steps of 0.05 VRHE
b) corresponding time-resolved dissolution profile of Pt presented on the same time axis as a).
c) The inset provides a magnification of the region around 6000 s. d) Amount of dissolved Pt
normalized per cycle plotted versus the upper vertex potential for cyclic voltammograms from
a) and additional measurement with SFC-0.4mm, including the three representative literature
references from Johnson et al. [25], Darling et al. [112] and Woods et al. [81]. [A.11]
The dissolution profile in Figure 7.2b can be furthermore quantified by numerical
integration of the peaks with a certain time region, using the following relation (all data is
processed by the developed software from chapter 5.3.2):
� � � � �� � � �
� �������
��
where � is the volume velocity of the electrolyte, � is the concentration profile measured
within the time period between �� and �� . The dissolution amounts in each cycle are
interpreted versus the upper potential values in figure 7.2d. The experimental sequence is
performed with two different configurations of the SFC systems with 0.4 and 1 mm channels.
The results from both experiments are nicely overlapping, what shows the negligible influence
Chapter 7: Results and discussion
55
of the cell geometry on the platinum dissolution. The integrated values change from “bellow
the detection limit” (for potentials less than 1.1 VRHE) to almost 10 ng/cm2cycle (at ca. 1.8 VRHE).
Considering an average surface density of 1.3�1015 atom per cm2 for a polycrystalline platinum
sample (equal to ca. 420 ng/cm2) [139], the dissolution amount can be represented in
percentages of the single monolayer. From figure 7.2d it becomes obvious that the amounts do
not exceed 2.4% of a monolayer per cycle in this series and addition a rough comparison with
the few existing values of the three most representative studies is given. Despite the slightly
different experimental conditions (scan rate, pH of electrode etc.), the normalized dissolution
amounts are quite similar for the corresponding upper potential limit.
The platinum dissolution is strongly influenced by the applied potential profile, especially in
the region of oxide formation/reduction as shown in figure 7.2c. In order to investigate the
effect of the overpotential for the oxide reduction on Pt dissolution, a sequence of cyclic
voltammograms is used, with fixed upper vertex limit at 1.55 VRHE and variable lower one as
plotted in figure 7.3a. Significant dissolution is detected when the operational potentials spread
over the region that allows oxidation and follow-up reduction. As soon as the potential window
is narrowed to potentials that are not sufficient to reduce the surface, the dissolution amount
drops beneath the detection limit. Thus, if the lower vertex potential stays above ca. 1.05 VRHE,
the spectrometric signal remains below the detection limit. This can be again easily recognized
from the integrated amounts in figure 7.3c. In the region of oxide formation below the onset of
the oxygen evolution reaction platinum surface is passivated. Once the lower potential limit is
shifted back to reductive values dissolution is observed again. Thus the transition between the
oxidized surface and reduced state is a necessary condition for the significant platinum
dissolution at room temperature and potentials not exceeding 1.55 VRHE. Two “stability”
windows can be defined for the electrochemical treatment in 0.1M HClO4 (i) between 0.05 and
ca. 1.05 VRHE and (ii) between ca. 1.05 and 1.55 VRHE, where platinum dissolution can also be
neglected.
Chapter 7: Results and discussion
56
Figure 7.3 a) Applied potential sequence in 0.1M HClO4/Ar at room temperature cyclic
voltammograms with a scan rate of 0.01 V/s and an upper potential limit of 1.55 VRHE; the
lower potential limit is gradually changed from 0.05 VRHE until 1.05 VRHE and down again in
steps of 0.05 V; b) Corresponding platinum dissolution profile plotted on the same time axis as
in (a), the dashed horizontal line shows the detection limit for the measurement; c) Integrated
amount of dissolved Pt for each single cycle to different lower potential limits of the cyclic
voltammograms from (b). (Reproduced from ref. [A.15] with permission from The Royal
Society of Chemistry)
7.3 Steady state dissolution during chronoamperometry
From the experiments above it is clear that a perturbation of the potential signal in time is
affecting the dissolution and can accelerate or suppress its rate. Maintaining constant potential
at relatively high anodic value was also already reported as a promoter of the dissolution [24,79]
in contrast to the dissolution behavior within the stability window of the water that is rather
unclear. Figure 7.4ab illustrates one representative experimental sequence where initially the
sample is polarized at 0.15 VRHE for 200 s, than oxidized at 1.6 VRHE for 300 s and further
reduced in the consequent steps. For each variation of the potential the dissolution profile
increases sharply and afterwards declines over the time of ca. 200 s. Again an increase of the
platinum signal is detected only in a case of change in the surface state. Modulation of the
applied potential within one of the emphasized “stability” windows does not cause dissolution.
A further, more refined chronoamperometric series shows again that transition in the region
around 1.1 VRHE triggering the dissolution during the oxide formation (figure 7.4cd) as well as
during the reduction, while a clear drop in the dissolution signal occurs during extended
Chapter 7: Results and discussion
57
polarization times. Thus it can be concluded that steady-state dissolution within the stability
window of water plays a minor role compared to the transient dissolution
Figure 7.4 Representative chronoamperometric experimental sequences that demonstrates
typical behavior of Pt dissolution under steady-state conditions in 0.1M HClO4/Ar. The graph
a) shows the applied potential sequence (potential holds at 0.15, 1.6, 1.0 and 0.15 VRHE) with
the corresponding dissolution profile in b). [A.11] c) Applied potential profile, starting from
OCP followed by 30 sec potential hold at 0.12 VRHE, then increasing the potential value from
0.17 VRHE up to 1.47 VRHE with steps of 0.1 V and backwards where in seven of the
chronoamperometric steps the duration time was extended to 300 sec as indicated. The vertical
red dashed line illustrates the transition from 1.07 VRHE to 1.17 VRHE step during the oxidation.
d) corresponding dissolution profile to c) with integrated amounts for the peaks. The dotted
line indicates the detection limit as determined from the daily calibration.
In order to investigate platinum dissolution in the region of oxygen evolution, an additional
experimental series is conducted. As long-term chronoamperometric measurements at high
anodic potentials e with the SFC are critical due to technical limitations related with blockage
of the tubing by gas bubbles, a linear sweep voltammetry with a low scan rate of 2 mV/s is
applied. This allows short excursions to relatively high anodic potentials for a reasonable period
of time and avoids extensive gas evolution in the SFC system. Note, that an alternative
approach of using a conventional three electrode cell and performing post-analysis is
accompanied with several pitfalls like possible deposition of the dissolved species on the
counter electrode and a large volume of electrolyte that leads to even lower effective
concentration to analyze. Again the onset of the dissolution can be clearly observed around
1.1 VRHE (figure 7.5). Considering the data acquired in the region of oxide formation (figure 7.4)
it is expected that the dissolution rate should drop after certain period of time (ca. 200 s).
Chapter 7: Results and discussion
58
Nevertheless, exceeding 1.6 VRHE leads to a rather small, but still detectable dissolution rate
during the stationary polarization in the potential region of oxygen evolution reaction (OER).
A similar phenomena was observed during electrolysis in chloride containing solutions after
tens to hundreds of hours of operation using platinum as an anode [79,142]. Nevertheless, Pt
seems to be highly corrosion resistant in comparison with other noble materials at such anodic
potentials [77]. This can be attributed to the formation of a chemically stable Pt-oxide layer,
and to the so-called electrolyte route [50,76] for the oxygen evolution reaction on platinum,
which does not involve the oxide in the reaction pathway.
Figure 7.5 a) Platinum dissolution profile during anodic polarization from 0.05 to 1.85 VRHE
with scan rate of 2 mV/s plotted versus the applied potential in 0.01M, 0.1M, 1M and 5M
HClO4 saturated with argon solutions. The dashed line indicates the detection limit for the
measurement; b) Tafel plot of the dissolution current (calculated with valence 2) for the anodic
and cathodic polarization in 0.1M HClO4 from a).
Note on this occasion, that due to the small currents in the system, the experiments are
conducted without IR compensation. The typical operation range of the potentiostat is
between 6 and 60 �A and the measured resistance is around 100-200 Ohm. This results in an
IR drop in the range of 0.6 to 12 mV. The current exceeds to 6 �A usually only above 1.6 VRHE,
which keeps the uncertainty of the potential axis less than 1.2 mV for the low potential values.
The dissolution rate can be interpreted as a current response and presented in Tafel plot
(figure 7.5b). The extrapolated slopes for the anodic and cathodic potential sweeps are ca. 120
and ca. 305 mV/decade, the values are significantly higher than the previously reported
92 mV/decade by Myer et al. [97]. This can be attributed to the fact that the literature values are
extracted from transient measurements and are based on the assumption that the overall
dissolution amount per cycle is directly proportional to reaction rate without separating the
“anodic” and “cathodic” contribution. Interestingly, the dissolution current in the cathodic
Chapter 7: Results and discussion
59
potential sweep practically remains with a constant slope over more than 600 mV, this rather
indicates a reaction independent on the potential like possible chemical dissolution of some
intermediate oxide.
7.4 Decoupling the influence of the time scale of experiment and the amount of formed oxide on the dissolution rates
It is important to have in mind that a variation of the potential window has the additional
effect of extending/reducing the effective time of reactions(s) in certain potential regions, like
for example in the region of oxide formation. Thus it is also important to investigate the
influence of the scan rate during transient experiments. Figure 7.6ab shows an experimental
series in a fixed potential window with an increasing scan rate from 0.005 to 0.5 V/s and the
online detected platinum, respectively. The corresponding dissolution amounts are decreasing
from ca. 5.9 to 1 ng/cm2cycle with increasing scan rate. A detailed investigation of the
electrochemical signal reveals that the amount of formed oxide also varies significantly with the
scan rate, as evaluated by the integration of the oxide reduction peak in the region from 1 to
0.5 VRHE with subtraction of the capacitive response. The reductive charge is calculated as
following from the cyclic voltammograms:
� � � ������� � � �����
��
��
Assuming that reduction charge of a single monolayer (ML) oxide is equal to 420 �C/cm2
[92], it is observed that no more than 1.5 ML are formed for a calculated roughness of the
electrode of ca. 1.3. The correlation of the oxide reduction charge with the dissolution amount
is plotted in figure 7.6c. It is important to mention again that the dissolution charge is 2-3
orders of magnitude smaller than the calculated charges from the electrochemical signal, as
predicted in the chapter 2 (1 ng/cm2~0.99 �C/cm2).
Chapter 7: Results and discussion
60
Figure 7.6 a) Applied experimental series in 0.1M HClO4/Ar; a cyclic voltammogram between
0.05 and 1.5 VRHE with a scan rate of 0.005, 0.01, 0.025, 0.05, 0.1, 0.2 and 0.5 V/s; b)
Corresponding platinum dissolution profile plotted on the same time axis as in (a), the dashed
line shows the detection limit for the measurement; c) The amount of dissolved Pt and
dissolution rate for each cycle versus the integrated charge for Pt oxide reduction recorded
during the same cycle whereas on top is indicated the corresponding scan rate. The dissolution
charge is calculated for an assumed valence of two. (Reproduced from ref. [A.15] with
permission from The Royal Society of Chemistry)
The results suggest that (figure 7.6c) the dissolution amounts are directly proportional to the
amount of oxide formed, which is itself inversely related to the scan rate. Unfortunately, the
contribution of anodic and cathodic dissolution can only be explicitly separated for scan rates
lower than 25 mV/s; at higher scan rates the two peaks overlap. Moreover, the influence of the
scan rate cannot be resolved unambiguously, as it changes both, the amount of formed oxide
and the time for its reduction. Due to the fact that steady-state dissolution within 0.0-1.6 VRHE
diminishes during constant polarization (figure 7.4), an advanced experimental protocol was
applied to split the influence of the timescale of the experiment from the influence of the
amount of formed oxide on dissolved Pt amount, while also resolving the anodic and cathodic
dissolution. An exemplary potential profile is presented in figure 7.7ab, where the dissolution is
clearly separated for the positive and negative going potential sweep. The effect of
overpotential of oxide formation is also investigated by changing the upper potential limit from
1.25 to 1.65 VRHE, while always using the same extended amount of time during constant
polarization. Identical experimental series are performed with six different scan rates of the
LSV steps. The data for the cathodically and anodically dissolved amount determined from
Chapter 7: Results and discussion
61
these experiments is plotted in figure 7.7c (in green color) and figure 7.7d (in red color),
respectively.
Figure 7.7 a) Applied experimental sequence in 0.1M HClO4/Ar, potential hold at 0.05 VRHE
followed by a sweep with a scan rate of 0.01 V/s to the corresponding upper potential limit,
hold for 300 seconds, and then a sweep back to 0.05 VRHE and successive hold; b) Platinum
dissolution profile plotted on the same time axis as in (a); Amount of dissolved platinum per
sweep for the same experiments as on (a-b) for scan rates of 0.01, 0.2, 0.5, 1, 2 and 4 V/s for
the positive sweep in (d) and negative in (c); (e) cathodically dissolved amount plotted versus
the reductive charge in the negative sweep. (Reproduced from ref. [A.15] with permission from
The Royal Society of Chemistry)
An increase of the upper potential limit has only a minor influence on the anodic dissolution
that coincides with results from figure 7.2b where similar reproducible peaks are observed
above ca. 1.3 VRHE (in the positive potential sweep). Surprisingly, a variation of the scan rate
from 0.01 to 4 V/s has also no distinguishable effect. It appears to be practically constant value
around 0.8 ��0.3 ng/cm2cycle (i.e. amount within ca. 0.2% of a single monolayer). Neither the
amount of the formed oxide nor the experimental time affects the anodic dissolution. It seems
that the anodic dissolution is triggered by the subsurface oxide formation and is rather related
Chapter 7: Results and discussion
62
to the initial state of platinum surface or some unstable intermediate species like a “transition”
oxide.
A different behavior is observed during the reductive scan (figure 7.7c). An increase of the
overpotential for the oxide formation leads to an enhancement of dissolved amounts for all
scan rates. From the work of Bockris et al., it is known that in the region between 1 and
1.6 VSHE an increase of the overpotential leads to a proportional increase in the amount of
formed oxide [66]. This relation is considered to be valid for thickness of less than a monolayer.
Similar to experiments in figure 7.6c, the reductive charge is evaluated from the negative going
potential sweep and compared with the cathodically dissolved amount (figure 7.7e). Thus the
previously found lower total platinum dissolution can be understood from the similar behavior
of the dominating cathodic dissolution. Interestingly, while the qualitative behavior that
dissolved platinum scales almost linearly with the oxide reductive charge is true for each scan
rate, the quantitative results are clearly diverting. Moreover, similar reductive charges do not
lead to identical dissolution amounts. In other words, it is not only important how much oxide
is formed, but also on what timescale it is reduced. The amount of dissolved platinum is
decreasing with high scan rates, becoming comparable with the anodic case and almost
independent from the amount of formed oxide when the surface is nearly instantaneously
reduced. It seems that additionally diffusion of the dissolved species from the surface into the
bulk electrolyte plays a role and has to be considered in the discussion of the mechanism at a
later stage.
7.5 Influence of the concentration of the protons
The electrolyte composition and especially the proton concentration are expected to have a
significant contribution to the dissolution of platinum. Several reports showed the
enhancement in overall dissolution in case of platinum-based nano-catalysts with higher acidity
[83,143]. Figure 7.8 shows a summary of dissolution amounts derived from cyclic
voltammograms in sulfuric and perchloric acids for several potential regions and different
electrolyte concentrations. Independent on the electrolyte acidity, the onset of platinum
dissolution coincides for all measurements around 1.1 VRHE. This suggests a proton dependent
reaction to be the rate-determining step. The onset in a case of sulfuric acid is thereby slightly
shifted positive. This might be related to higher adsorption strength of sulfates/bisulfates in
comparison with perchlorates that results in a shift in the onset for platinum oxidation [139].
Furthermore, the dissolution amounts can be investigated above ca. 1.3 VRHE separately for the
Chapter 7: Results and discussion
63
anodic and cathodic branches. Similar to the previous findings, the platinum dissolved in the
anodic sweep shows a minor increase with extending the upper potential limit. The values are
rather scattered around 1 ± 0.3 ng/cm2 cycle, showing the minor increase (almost independent)
with the pH change on the anodic dissolution for concentrations up to 1M. A clear
enhancement is detected only in the extreme case of 5M HClO4. Nevertheless, it still stays with
factor lower to amount in the cathodic scan that remains the main contributor to the overall
dissolution for all electrolytes. The amounts in the cathodic case, scale with the increased
acidity of the electrolyte. This is expected considering that the oxide reduction proceeds via
protonation. Surprisingly, the quantities for the dissolved platinum are different for almost
identical conditions in perchloric and sulfuric acids. It can be assumed that both acids have
similar pH for the same concentration, since the second dissociation constant of sulfuric acid is
1010 times smaller than the first one [144]. It has to be mentioned that the variation of the
concentration changes the pH value as well as the amount of the spectator species present in
the electrolyte. The nature of the electrolyte can therefore additionally affect the quantitative
behavior of the dissolution, while the qualitative behavior follows same trends in both acids.
Figure 7.8 The amount of dissolved platinum during potential cycling at a scan rate of
0.01 V/s between 0.05 VRHE and the corresponding upper potential in a) HClO4 and b) H2SO4
with different concentrations. The red and green color codes show the dissolved amount
during the positive and negative sweep for the corresponding concentration (�) 5M; (�) 1M;
(�) 0.1M and (�) 0.01M. All electrolytes are continuously purged with argon. (Reproduced
from ref. [A.15] with permission from The Royal Society of Chemistry)
Taking into account that protons are influencing the dissolution, a logical question appears
about the behavior in basic solutions. Figure 7.9 shows the dissolution amounts in 1 mM
sodium hydroxide, which is at the limit of the technical capabilities for the ICP-MS regarding
Chapter 7: Results and discussion
64
alkaline pH. Again the dissolution is obtained in the anodic and cathodic sweeps around
potentials for formation and reduction of the oxide respectively. The onset of the dissolution is
similar as in the case of acidic electrolytes at ca. 1.1 VRHE. The integrated amounts follow the
general trend of decreasing dissolution amount with increasing pH. The quantitative
description has to be considered criticality, however, since the dissolution profile is only slightly
above the quantification limit. Nevertheless the overall behavior is qualitatively not different
from the acidic conditions, the predominant dissolution occurs during the reduction of the
oxide. The amounts detected in the anodic sweep are again almost identical for different upper
potential limits. Another aspect that can affect the dissolution amounts is the solubility of the
platinum in electrolytes with different pH value. It is known that transport of Pt ions in acidic
media proceeds over complexing with anions and forming water-soluble product (e.g. [PtCl6]2-),
where in basic solutions hydroxylated complexes (e.g. PtOH+ and Pt(OH)2(aq)) play a major
role in transport within the electrolyte [26,89,145]. The solubility of the products in the acid
environment is significantly higher than in the basic one. However, the maximum values in the
dissolution profile remian three orders of magnitude lower than the solubility of Pt2+ in 1mM
NaOH (i.e. ca. 152 �g/L) [146]. This shows that the obtained results are not an artifact of the
experiment, but a real effect of change of the pH.
Figure 7.9 Overall dissolution amount during potential cycling from 0.05 VRHE to the
corresponding upper potential limit with scan rate of 0.01 V/s in 1mM NaOH/Ar. The black
color represent the amount for a single cycle, where the red and green color present the
amounts detected in the positive and negative potential sweep respectively.
Chapter 7: Results and discussion
65
7.6 Effect of the reactive gases on Pt dissolution
Platinum is typically used as a catalytic material and should ideally be stable during certain
operational conditions. Especially, the stability issues in the presence of reactive gases like
oxygen, hydrogen and carbon monoxide are of high relevance for the application in fuel cells.
A recent study of Kongkanand et al. suggested a certain influence of oxygen as a promoter for
the dissolution of platinum-based nanocatalyst during potential treatment using post analysis
[111]. This phenomenon is addressed in this work by performing identical experiments in
perchloric acid saturated with different gases. The comparison of the dissolution profiles for Ar,
O2, H2 and CO-saturated electrolyte is presented in figure 7.10. No quantitative or qualitative
difference can be observed in the dissolution profiles using electrolyte saturated with oxygen,
hydrogen and argon (i.e. ca. 7 and 8 ng/cm2cycle for the corresponding CVs up to 1.55 and
1.65 VRHE). Further variation of the potential window in oxygen-saturated solution showed the
same results as found in the case of argon (not shown). Therefore it can be concluded that
previously reported results in literature on Pt/C degradation are rather due to structural
changes than to dissolution of active material. Distinctively different is the dissolution behavior
during bulk CO oxidation, where a massive increase in the platinum amount is detected (ca. 25
and 37 ng/cm2cycle). Surprisingly, almost no anodic dissolution peak is observed and the
cathodic one appears ca. 100 mV more negative than in case of argon.
Figure 7.10 a) Applied experimental sequence of two cyclic voltammograms from 0.1 up to
1.55 and 1.65 VRHE with a scan rate of 0.01 V/s followed by open circuit potential
measurement; b) The corresponding dissolution profiles in 0.1M HClO4 saturated and
continuously purged with Ar, H2, O2 or CO on the same time scale. (Reproduced by
permission of The Electrochemical Society form ref. [A.16])
Chapter 7: Results and discussion
66
Further, the influence of the overpotential for the bulk CO oxidation is correlated with the
platinum dissolution signal. Figure 7.11 depicts a series of electrochemical CO oxidation, where
the upper potential limit is varied from 0.87 VRHE upwards. Once the potential window is
extended above ca. 1.1 VRHE platinum starts dissolving considerably, similar to the dissolution in
inert argon atmosphere, which indicates the important contribution of sub-surface oxide to the
overall dissolution also in this case. Again almost no dissolution signal is observed in the
positive going sweep for all potential ranges similar to the results in figure 7.10. This is
surprising considering that the CO is smoothening the platinum surface even at low potentials
below 0.8 VRHE by removing adislands [132,147]. Further increase of the overpotential causes
the increase in the dissolution amounts. This can be related to the fact that the anodic
extension of the potential window, leads to formation of more Pt-oxides, which can be partially
dissolved during subsequent reductive scan. The exact amount of oxide(s) cannot be extracted
from the electrochemical signal, due to the overlapping with the CO oxidation response. For all
potential windows, the onset of the dissolution in the cathodic potential sweep is observed at
ca. 0.85 VRHE what is very similar to the value for the initial CO oxidation.
Chapter 7: Results and discussion
67
Figure 7.11 a) Cyclic voltammograms from 0.15 VRHE to different vertex potential with a scan
rate of 0.01 V/s; b) The dissolution amounts per cycle vs. the corresponding upper potential; c)
Dissolution profile in the negative going potential sweep. (Reproduced by permission of The
Electrochemical Society form ref. [A.16])
CO oxidation is often used for the estimation of active surface area of platinum-based
catalyst, a method known as CO Stripping [132]. Figure 7.12 shows the expected platinum
losses for potential treatment during change of the used gases. Initially, the electrochemical
treatment in presence of argon shows identical behavior as previously discussed. A change of
the used gas to CO under potential control does not trigger any dissolution. A reference cyclic
voltammogram for CO bulk oxidation results in the same dissolved amount as in figure 7.11.
Consequently, the gas supply was switched back to argon and the electrolyte is purged for an
hour to remove the CO from the solution. The following CVs showed smaller dissolved
Chapter 7: Results and discussion
68
amounts than expected. In the first cycle only a cathodic dissolution peak is detected, mainly
due to the remaining, pre-adsorbed CO on the surface that is already removed in the negative
potential sweep. The second cycle has an increased dissolution and the typical dissolution
features reappear as for example in figure 7.2c. A close look at the electrochemical signal
reveals the origin for the reduced amount, namely residual CO is present in the electrolyte. A
complete removal of the CO from the electrolyte remains an issue for this SFC experimental
setup. Thus, a different experiment is performed, where the platinum sample is pretreated in a
bulk electrochemical cell with CO-saturated solution. The surface is then covered with CO by
holding the potential at 0.05 VRHE for 5 minutes. Afterwards the sample is taken out and
contacted with the SFC under potential control and three cyclic voltammograms are performed
as shown on figure 7.12cd. Surprisingly, the dissolution during the CO stripping procedure is
significantly lower than expected for CO bulk oxidation or under Ar atmosphere and in a good
agreement with the results in figure 7.12ab.
Figure 7.12 a) and c) Illustrates the applied potential with the applied gas purging sequence; b)
and d) Corresponding dissolution profile and integrated amounts for the detected peaks for an
investigation of a polycrystalline Pt sample pre-treated in a separate, CO-purged cell with
potential hold for 5 min at 0,05 VRHE, before investigation in the SFC under Ar. (Reproduced
by permission of The Electrochemical Society form ref. [A.16])
7.7 Enhancement of Pt dissolution in the presence of chlorides
Usage of platinum in chloride-containing electrolytes introduces additional complexity to
the process of dissolution. A few literature reports showed the quantification of the detected
dissolution rates under chronoamperometric conditions after several hour of operation [24,142].
Chapter 7: Results and discussion
69
It is known that chlorides promote platinum dissolution, but the mechanism of promotion is
still unclear. A series of experiments were performed with the SFC in perchloric acid with the
addition of different concentrations of chlorides to address this question. It is important to
mention that despite all experiments are conducted with Suprapure® chemicals from Merck, the
produced HClO4 is always accompanied with minimal amounts of Cl-. The original 70%
(i.e. 11.6 M) solution has less than 1 ppm Cl- by quality certificate, which means a “pure”
0.1M HClO4 as used before has no more than ca. 0.4 �M of Cl-. Figure 7.13 shows the
dissolution profile for various potential windows and the integrated amounts of dissolved Pt.
The general behavior seems to be unchanged with addition of Cl-. The dissolution is detected
again above ca. 1.1 VRHE in the positive and negative potential sweep direction, where the peaks
become only more emphasized with increasing Cl- content. This suggests that the chlorides
don’t change the dissolution pathway. As mentioned above, platinum forms a water-soluble
complex with Cl- ions, which can facilitate the removal of platinum ions from the vicinity of
the electrode. In this way a possible redeposition could be partially inhibited. Quantitatively, the
addition of small amounts of HCl in the order of a few 10-6 moles do not induce any
alternation from the dissolution in 0.1M HClO4, only for concentrations above 10 �M the
difference becomes significant. Note that the dissolution indicates a drop in the overall
amounts after a certain potential. This can be considered as an artifact of the measurement
possibly related to blocking of the surface. All dissolution amounts are normalized to the
geometric area, not to the effective/accessible one. For example in the case of addition of
0.1mM HCl, chlorine evolution reaction is observed in the cyclic voltammograms above
1.45 VRHE that can result in bubble formation and therefore decrease the accessible surface area.
Figure 7.13 a) Dissolution amounts of the platinum are plotted for cyclic voltammograms
from 0.05 VRHE to the various corresponding upper potential limits. 0.1M HClO4 was used with
addition of HCl and the electrolyte was continuously purged with argon. The black color
Chapter 7: Results and discussion
70
corresponds to the total dissolution in a single cycle, red color for the anodic sweep and the
green for the cathodic one.
7.8 Temperature dependence of the dissolution
Large-scale industrial applications of platinum as a catalyst are usually not at room
temperature. For example, a PEMFC stack or low-temperature electrolyzers are operated at
approximately 85°C [148]. All experiments performed in the previous sections are made in
temperature-controlled laboratories (at ca. 20°C). Figure 7.14 shows cyclo-voltammetric
measurements made at five different temperatures within a fixed potential window using a scan
rate of 0.01 V/s. The increase in the temperature is achieved using a circulator bath, which
heats up the electrolyte vessel as well as the sample itself within the same water circuit. An
intermediate period of 5 min is used to achieve a thermal equilibrium of the system before each
CV is recorded using sweep rate of 0.01 V/s. Surprisingly, the overall dissolved amounts are
decreasing with increasing temperature. While the dissolution peak during the oxide formation
is increasing with temperature and appears to be for all five cycles exactly at 1.1 VRHE as shown
in figure 7.14ab, an opposite behavior is observed for the negative potential sweep. Variation of
the temperature can affect not only the dissolution, but also the oxide formation. Thus the
amount of formed oxide is estimated from the voltammograms. A correlation between the
reductive charges from the negative going potential sweeps with the dissolved amount is
presented in figure 7.15.
Figure 7.14 a) Applied potential profile consisting of cycles between 0.05 and 1.6 VRHE in
0.1M H2SO4 for 25, 35, 45, 55 and 65 degree Celsius with intermediate potential hold at
0.5 VRHE for 300 sec to equilibrate the temperature; b) Corresponding spectrometric profile
Chapter 7: Results and discussion
71
with indicated dissolution onset; c) dissolution amounts for the positive and negative potential
sweeps, and the overall one within a single cycle plotted for different temperatures. [A.18]
For the same potential window, an increase in temperature results in an increased oxide
formation, but for a decrease in the overall dissolution per cycle. The distinction originates
from the unusual temperature dependence of dissolution in the cathodic sweep. In contrast to
the previous results, formation of more oxide at elevated temperatures leads to diminished
amount of dissolve platinum in the reductive scan. The difference in dissolution behavior
during positive and negative potential sweeps also explains the trend observed by several
groups for which during the potential treatment an increase in the dissolution amount was
detected whilst raising the temperature, suggesting the predominant contribution of the anodic
dissolution at high scan rates [91,92,149].
Figure 7.15 a) Relation between the reduction charge and the dissolution amount for different
temperatures (data from figure 7.14); b) Logarithm of the dissolution amounts from the anodic
and cathodic potential sweep plotted versus one over the temperature.
Furthermore, from the reaction rate law and Faraday’s law stems that the dissolution
amount should be linearly proportional to the rate e.g. the slope of the log(m(Pt)) vs. 1/T
should be the same. Namely, the rate constant could be expressed as the product of the
dissolved amount and proportionality factor that will affect only the offset in figure 7.15.
However, this is only under the assumption of presence of steady-state dissolution. In the case
of “anodically” dissolved platinum, an Arrhenius behavior can be observed and the activation
energy of the process can be estimated from the slope using following relation:
� � � ��� � ���� or ��� � � � ��
��� � �������
Chapter 7: Results and discussion
72
where � is the rate constant of the process, �� is the activation energy of the process, � is the
universal gas constant (8.314 J/(K.mol)), � is the temperature in Kelvin and � is pre-
exponential factor. The activation energy for the dissolution in the anodic sweep is calculated
ca. 13 kJ/mol. This value is very close to the calculated 18 kJ/mol for Pt dissolution during
OER at high anodic potential by Ota et al. [80].
In the cathodic case is the observed non-Arrhenius behavior, which does not allow a direct
extraction of Ea (“negative” activation energy) and rather indicates a more complex pathway.
The proposed approach of Eyring–Polanyi can be helpful in treating complex reactions, where
the reaction rate is defined as [150]:
� � ���� ���� ����
where �� is the Boltzmann constant (1.38�10-23 J/K), � is the Planck’s constant (6.63�10-34
J/s) and �� is the Gibbs energy. The rate law can be further modified using the definition of
�� � �� � ��� as:
� � ���� ���� ��
� ���� � ���� � or �� � � �� ���
� � ��� �
���� �
From figure 7.15b, the dissolution in the reductive scan has a positive slope that means the
partial derivative of the �� � with respect ����should be bigger than zero:
� � � �� ������� �
������� �� � � ���� �
��� � ��� ��� � ���� � �
This results that enthalpy of the reaction �� � ��� , where at room temperature
�� � ���������. The negative sign is indicating for the “cathodic” dissolution to be an
exothermic process.
Note, that the following evaluation is made under the assumption that dissolution proceed
for the “anodic” and “cathodic” route within the same time frame under steady-state condition,
where the detected amount should be linearly propositional to the rate. However the previous
results indicate rather a transient nature of the dissolution. Thus the value of the calculated
activation energy and the trend Arrhenius/non-Arrhenius behavior of the dissolution processes,
have to be considered critical.
7.9 Ex-situ XPS and SKP investigation of platinum oxide
Understanding the nature of the formed platinum oxides has an essential role for resolving
the dissolution mechanism. Although platinum is an extensively studied material, there is still a
Chapter 7: Results and discussion
73
lot of uncertainty concerning the chemical structure of the electrochemically formed oxide layer.
The opinions spread from hydroxide species to different combinations of hydrous oxides
containing various oxidation states [139]. Pure electrochemical investigations have their
limitations in clarifying the oxide and thus require complementary techniques. For this purpose,
an oxide layer has been electrochemically prepared by first cleaning a Pt foil in 0.1M HClO4 by
several cycles between 0.05 and 1.6 VRHE. Afterwards, it was polarized at 0.27 VRHE for 10 min.
Under potential control, the foil is partially pulled out of the solution and then the potential is
changed to 1.27 VRHE for 10 min. The same procedure is done once again with polarization at
1.97 VRHE for 10 min. Finally, the sample is taken completely out of the electrolyte, dried in a
nitrogen stream and introduced into the translation chamber of the X-ray photoelectron
spectrometer (XPS).
The reference measurement performed on a polycrystalline platinum surface gives the
typical double peak response at 71.1 eV and 74.4 eV for Pt4f as shown in figure 7.16a, which is
in good agreement with literature values [151]. The second area polarized at relatively low
overpotential for the oxide formation (ca. 1.27 VRHE), shows the characteristic fingerprint of
PtO2 at 75.1 eV and 78.4 eV, in line with previous reports [56–58,152]. The formation of a
Pt(IV) state at this “early” stage of oxidation is not directly intuitive, but thermodynamically
possible [34]. This can be correlated to observed passivity in section 7.2, where potential
cycling within the stability window of water does not lead to significant dissolution. In the third
case, the sample is anodically polarized in the region of intensive oxygen evolution reaction. No
clear stoichiometry for PtO or PtO2 can be derived from the XPS spectra. The fitted data
indicates a mixed oxide layer that consists of both oxidation states, +2 and +4. Unexpectedly,
the intermediate oxidation state of PtO appears in the region of OER, where transpassive
dissolution starts to appear again [24,79].
Chapter 7: Results and discussion
74
Figure 7.16 Pt-4f X-ray photoelectron spectra measured at location initially polarized for
10 min at: a) 0.27 VRHE; b) 1.27 VRHE and c) 1.97 VRHE; The red curve represents the measured
data, other color represent the fittings generated by CASA software.
Probing the surface of the Pt/PtxOy substrate is of crucial importance for recognizing
surface states involved in the dissolution mechanism proceeding on the electrode-electrolyte
interface. XPS spectra provide information about the average composition over the top layers.
Note, however, that these results have to be considered with caution for the discussion, as the
oxidation states might change firstly upon drying after the polarization, and secondly upon
introduction into the vacuum chamber for analysis.
Chapter 7: Results and discussion
75
In addition to XPS, Scanning Kelvin Probe (SKP) was utilized to investigate the change of
the average electron energy after different polarizations (see Figure 7.17). The work function
can be evaluated in different ways [153], in this work is used the following relation:
� �� � ����� � � ���� � ���
that was extracted from the absolute potential scale introduced in Bard et al. [27] (pp.54). The
initial surface scan over the reduced area gives almost an identical value of 5.51 eV that is
relatively close to the previously reported 5.4 eV by Trasatti [153]. A slight variation in the
comparison with literature is predominantly related to the use of different equations.
Polarization at 1.1 VRHE leads to a significant drop of the work function related to the change
of the dipole orientation of Pt-O on the surface during the place-exchange mechanism. Further
positive extension of the applied potential again increases the value of �, which is expected for
oxide growth. The same reduced platinum sample was introduced under CO stream and
consequently measured by SKP, a significant drop is observed to �������������� in
comparison to the “free” surface (5.51 eV). It is known from measurements in UHV systems
that the adsorbed CO can lead to a decrease up to ca. 1.1 eV [154].
Figure 7.17 Scanning Kelvin Probe measurements made on a polycrystalline platinum surface
pretreated in 0.1M HClO4/Ar at indicated potential values for 10 min in dried nitrogen
atmosphere.
While the electrochemically formed oxides are typically formed quite fast, all the SKP
measurements are made ex-situ after long-term polarization. Thus a one-to-one correlation to
the dissolution results in the previous chapters has to be considered critically. Moreover, the
tests performed in humidified oxygen atmosphere in the SKP showed a fluctuating response,
which are expected considering the adjustment of the oxide layer to the different environment.
Chapter 7: Results and discussion
76
Finally, after a certain time the work function approaches the one corresponding to pure
platinum, related to instability of the oxide layer. This remains as an issue of ex-situ
measurements.
Chapter 8: Comprehensive discussion
77
8 Comprehensive discussion Platinum dissolution is closely related to the electrochemical formation and reduction of the
oxide layer as shown in several experimental series in chapter 7. It can be described as a
transient process taking place during a potential change with time around ca. 1.1 VRHE, the
critical potential for subsurface oxide formation. The results furthermore indicate that
dissolution is detected during positive and negative polarization, where the second one prevails
mainly with respect to the overall amount per cycle. While the “cathodic” dissolution indicated
a strong influence on the applied potential window, time scale, pH and anion concentration,
the “anodic” case follows a rather moderate dependence on the experimental parameters. To
grasp the meaning beyond the presented phenomenological relations, first of all, the most
important finding are summarized in table 8.1.
PPaarraammeetteerrss ““AAnnooddii cc”” ddii ssssoo lluutt iioonn ““CCaatthhooddii cc”” ddii ssssoo lluutt iioonn
Time dependence transient process transient process
Anodic limit minor influence significant increase with
more positive anodic limit
Scan rate minor influence significant increase with
decreasing scan rate
Cathodic limit minor influence significant increase with
more negative cathodic limit
pH increase with drop in pH significant increase with
decreasing in pH
Temperature increase with higher
temperatures
decrease with higher
temperatures
Table 8.1. Distinction behavior between “anodic” and “cathodic” dissolution behavior in
relation to the experimental parameters.
Due to the possible interrelation between oxide layer formation and the dissolution process,
it is important to revisit some of the existing concepts for Pt oxidation. During anodic
polarization, initial oxidation of the Pt surface in the region between 0.8-0.9 VRHE is described
as OH adsorption or alternatively as direct adsorption of O2-. Based on the analysis of Conway
Chapter 8: Comprehensive discussion
78
and co-workers, it can be assumed that a full coverage of one with oxygenated species is
achieved at about 1.1 VRHE [40]. However, Bockris et al. considered a maximal surface coverage
with oxygen containing species to be around 0.25 of a full monolayer at equilibrium conditions
[155]. Below the critical coverage, the enthalpy of formation of the chemisorbed phase is
higher than the enthalpy of formation of the oxide. As the oxygen coverage approaches the
critical value, the repulsive interactions between the oxygenated species gradually reduce the
enthalpy of chemisorption until, at the critical coverage, it becomes equal to the enthalpy of
formation of the oxide film. Beyond the critical coverage, repulsive interactions in the densely
packed electronegative O adlayer induce surface reconstruction into a more energetically
favorable configuration by occupation of the sub-surface sites, known as place exchange
[41,98,156]. Of essential importance is to notice that the transition between the O chemisorbed
phase and the appearance of an oxide film is a thermodynamically, and not a kinetically, driven
process. Taking into consideration the observations of Conway et al. that around 1.05-1.1 VRHE
the amount of adsorbed species corresponds to the full monolayer and that a critical coverage
according to Bockris et al. is around 0.25 of the full monolayer, it seems that place exchange
starts after reaching the critical coverage, while additional adsorption until a full monolayer
occurs via sub-surface oxide formation [139,155]. It is important to mention, that independent
of the definition of full coverage, it is generally accepted that formation of sub-surface oxide
initiates at ca. 1.1 VRHE on polycrystalline platinum.
During the online trace analysis, no dissolved platinum was detected between 0.8-0.9 VRHE,
in the region in which the adsorption of oxygenated species on the surface begins. The
determined onset potential for platinum dissolution was ca. 1.1 VRHE. This means that
dissolution is triggered in the moment when the critical coverage is reached. Above 1.1 VRHE,
three processes are possible, adsorption of oxygen containing species, oxide growth and Pt
dissolution. Anodic polarization causes a downshift of the Fermi level and induces an increase
in the work function. To equilibrate, the system tends to reduce the work function by injecting
of electrons from adsorbed oxygen containing species towards the electrode and by inversing
the direction of the Pt-O dipoles by further place exchange between Pt atoms (Ptex) in the
lattice and oxygen containing species on top, as shown experimentally in figure 7.17. As a
consequence further oxidation causes an irreversible roughening of the surface. Platinum atoms
being pushed up/away from their original position in the lattice structure and O/OH moving
below the platinum atoms results in Pt atoms weakly bond to the bulk platinum lattice [157],
which makes them susceptible to dissolution. The existence of an equilibrium between the
process of adsorption of oxygen containing species and the process of oxide growth is a
Chapter 8: Comprehensive discussion
79
potential explanation why the typical feature of the cyclic voltammograms of Pt in acidic media,
between 1.05 and 1.55 VRHE is a plateau with clear absence of an increase of a faradaic current
(figure 7.1). Thermodynamically the oxidation of the platinum surface can be expressed by the
following equations:
�� � ��� � ��� � ��� � ��� �� � ������ �������� (8.1)
��� � ��� � ���� � ��� � ��� �� � ������ �������� (8.2)
���� � ��� � ���� � ��� � ��� �� � ������ �������� (8.3)
The experimentally applied anodic potentials in this work are predominantly below 1.8 VRHE
thus the formation of ���� is disregarded. Two states are expected, namely ��� and ����,
where the second one should be present over a broader potential range. However the
experimental investigations of the oxide layer have shown a complex bilayer structure, as
discussed in chapter 3. In a similar potential range direct electrochemical dissolution is expected
to proceed in parallel to the chemical dissolution:
�� � ���� � ��� �� � ������ ���������������� (8.4)
��� � ��� � ���� � ��� ��� ���� � ������ ��� (8.5)
A list of models has been proposed in the past to describe the oxide growth on platinum,
while only a few of them incorporate the dissolution phenomena. One of the most famous
models for the platinum dissolution is proposed by Darling and Meyer [112]. It considers the
dissolution as competition between dissolution (reaction 8.4) and formation of chemically
stable oxide (reaction 8.1). Namely, as long as the surface is not fully covered with passive
oxide species, platinum can be dissolved into the electrolyte by direct oxidation. Figure 8.3
illustrates schematically the change of the oxide coverage during cyclic voltammetry as
suggested by this partial oxide coverage model. During partial coverage the dissolution should
reach a maximum, once full coverage is achieved, no dissolution should be observed anymore.
There are, however, several incoherencies with the experimental results. First, experimentally
dissolution is only observed after full oxide coverage has been reached. Secondly, an increase of
the upper potential limit (beyond 1.1 VRHE) leads to a higher dissolved amount, which cannot
be explained from this model as shown schematically in figure 8.1. Furthermore, variation of
the scan rate should affect the dissolved amount in the negative as well as in the positive
potential sweep, which is however different from the experimental fact (table 8.1) that only the
“cathodic” route is influenced.
Chapter 8: Comprehensive discussion
80
Figure 8.1 Illustration of change of oxide coverage (limit by 1 with respected to the model of
Darling and Meyer [112]) during potential cycling within different potential windows. The
green rectangles represent the regions of dissolution.
Platinum forms only a few monolayers of oxide film consisting of two different kinds of
composition: a thin inner one consists of platinum in the second oxidation state (also called -
oxide) and a thicker outer one (�-oxide) (figure 8.2). An alternative approach describing the
oxide growth in the framework of a possible dissolution is the point defect model (PDM)
suggested by Macdonald et al. [43,68]. It proceeds over initial formation of a compact passive
film (-oxide on figure 8.2), where the consequent oxide formation undergoes over
precipitation of hydroxide species on the outer oxide/electrolyte interface. A disturbance of the
oxide layer from its equilibrium state leads to a depassivation of the surface. Following the
point defect model, this will induce movement of metal ions across the oxide layer, and cause a
partial dissolution. Within the PDM, a positive linear polarization will cause more or less
continuous dissolution once the outer oxide layer has been formed. However, the
chronoamperometric series performed in the region of oxide formation did not lead to
dissolution, which speaks against the beliefs of the PDM. Also a variation of the time scale
should influence the “anodically” dissolved amount, which is contradicted by the results in this
work.
��
��
��������
���
�����
����
������������
�����
���
���
���������� �����������������
��������
��������
Chapter 8: Comprehensive discussion
81
Figure 8.2 Illustration of change of the surface state during anodic polarization 1-4 and the
cathodic one 4-6, including the intermediate steps during the oxide formation in 3-4.
The experimental results indicate 1.1 VRHE as an initial potential for the dissolution for all
used acidic solutions as well as for 1 mM NaOH, suggesting a proton dependent reaction to be
involved. The direct electrochemical dissolution (8.4) should proceed 88 mV more positive, so
that ca. 1.04 mM Pt2+ (equal to ca. 202.8 mg/L) are required in the vicinity of the electrode to
shift the equilibrium potential to 1.1. The effectively measured concentrations are several
orders of magnitude smaller, typically in the lower range of �g/L. Nevertheless, the equilibrium
potential of reaction 8.4 could be different for lower coordinated sites. Alternatively, a chemical
dissolution of stoichiometric ��� could cause the platinum loss, as shown in reaction 8.5.
Nevertheless, no constant dissolution could be explicitly detected in region 1.1 to 1.2 VRHE or
above, as the spectrometric signal decreases for chronoamperometric measurements below the
detection limit.
Considering all the results it is clear that the underlying oxide formation and reduction is
determining the dissolution processes of Pt. However, thermodynamic considerations fail to
explain the mechanism in detail. Thus a kinetic model becomes necessary to describe both, the
anodic and cathodic dissolution. During anodic polarization a change in the electrode potential
causes a disturbance of the equilibrium between oxide formation and adsorption of O/OH
species. The downshift of the Fermi level (i.e. increase of the work function) will be manifested
instantly by an increase of the coverage and a re-establishment of equilibrium by new place
exchange. At these positive potentials, the Ptex atoms/ions formed due to place-exchange can
be easily passivated by adsorption of additional O/OH species. However, minor amounts of
the formed Ptex sites are also prone to dissolution. In particular, it can be assumed that Pt sites
that are already low-coordinated before oxidation are more susceptible than Pt sites from facets.
����
���������������
� ��
�������������
�"��
������������� ������
�#��
��������������������
�$���!��
������������������ �� ���������������
���
�������
�����������
��
����������
�����������
�����������
����������
�����������
�����������
Chapter 8: Comprehensive discussion
82
This could explain on the one hand that the amount of Pt dissolved during the anodic
dissolution is rather constant and independent from pH, potential limits and scan rate, as the
amount of low-coordinated sites in the initial state does not vary extensively. Of course, more
extended single-crystal studies will be necessary to confirm this in future. Moreover, the drop
of the dissolution rates to values below the detection limit during constant polarization can be
explained based on this. Once the Ptex sites from the outer surface of the Pt-oxide are removed
and a quasi steady-state is reached, no further place-exchange takes place and the remaining Ptex
are stabilized in the surface oxide. It could be expressed as chemical reaction:
����� � ��� � ���� � ��� (8.6)
In contrast to the “anodic” dissolution, the dominating “cathodic” dissolution is
qualitatively different. It appears below ca. 1.05 VRHE during the reduction of the surface after
potential excursion above 1.1 VRHE, and strongly depends on several experimental parameters.
An almost linear relation was obtained between the dissolved Pt and the amount of formed
oxide, with a slope depending on the time scale of the experiment. The surface state at
potentials above 1.3 VRHE can be assumed with simplified ���� stoichiometry, taking into
account the XPS data from figure 7.16. A negative sweep will lead to protonation of the oxide
layer and changes in the passivity of the layer. The processes can be expressed with following
reactions:
���� � ��� �������������� ���� � ��� (8.7)
���� � ��� � ��� � ���� � ��� (8.8)
���� � ��� � ��� � ����� ��� (8.9)
Chemical dissolution of Pt/Pt4+ is unlikely, considering that no evidence was found in acidic
media and also most of the literature reports Pt2+ as the soluble product in the electrolyte [26].
The electrochemical reductive dissolution (8.8) is expected thermodynamically to proceed at:
�� � ������ ������� � ���������������� An order estimation of the effective measured Pt concentration gives 10-8 molar (ca. 2 �g/L),
thus the equilibrium potential can be written as:
�� � ������ �������
A shift of the potential with 118 mV with each pH unit will give a much lower value than
experimentally observed (ca. 1.05 VRHE), but is certainly thermodynamically possible in very
acidic environments (above 1M). Moreover, the reaction 8.8 requires two electrons and four
protons or alternatively it can be split into two consecutive steps. Initial reduction of ���� to
��� (8.2) followed by chemical dissolution of ��� (8.5). A possible dissolution pathway in this
case is illustrated in figure 8.3.
Chapter 8: Comprehensive discussion
83
Figure 8.3 Illustration of reductive reaction pathway/s including solvable intermediate.
Taking into consideration that there is no significant cathodic dissolution before reaching
1.05 VRHE it can be assumed that the predominant form of surface oxide above 1.05 VRHE is
���� (before region of intense OER). It seems that ��� (or �����) is a crucial intermediate
and the chemical dissolution is an unavoidable part of the overall picture. It has to be
mentioned that reaction 8.5 (or 8.6) is based on protonation of ��� and is independent on the
electrode potential. Comparing the oxide reduction and the dissolution charges (figure 7.7e),
the predominant fraction of the formed ��� is reduced to ��, while only a much smaller
portion is chemically dissolved. The low amount of dissolved platinum can be related to
possible re-deposition of the dissolved species in the vicinity of the electrode, which is
reasonable considering the low potentials during the reduction process. Moreover, the
influence of re-deposition on the cathodic dissolution can also explain the decreasing amount
of Pt with increasing scan rate, as it depends on the effective diffusion from the surface into
the bulk electrolyte.
Important additional hints to the dissolution mechanism are provided by the CO stripping
and CO bulk oxidation experiments. It is known that CO smoothens the platinum surface by
increasing the surface diffusion and therefore reducing the amount of defect sites and adatoms
[107]. A novel finding is that anodic dissolution is vanishing during CO bulk oxidation and
almost completely disappears for CO stripping. This confirms the influence of low coordinated
sites or their lack in case of anodic dissolution (like Ptex). In addition, the adsorbed CO can also
physically block the surface from possible redeposition in the cathodic sweep and promote in
this way also the dissolution amount during CO bulk oxidation, whereas the absence of CO in
the solution give smaller dissolved amount for the stripping experiment. Once the surface is
smoothened, the cathodically dissolved Pt diminishes even less than in case of an Ar saturated
solution for the same conditions (figure 7.12d). Further potential cycling increases the
roughness of the surface already in the following cycle, and the expected dissolution is soon
reestablished.
PtO2� PtO� Pt�
Pt4+�
+2H++2e-�
Pt2+�
+2H++2e-�
12
3
4
5
6
Chapter 8: Comprehensive discussion
84
In summary, the oxide formation leads to lattice distortion of the platinum interface due to
the place-exchange mechanism, creating higher amounts of Ptex that can chemically dissolve. At
anodic polarization above 1.1 VRHE, the high driving force for the adsorption of O/OH leads
to a quick passivation of the surface followed by a transition to a higher oxidation state. During
this transition some limited amount of dissolved platinum originating from low coordinated
sites occurs, i.e. anodic dissolution, which is almost independent from the experimental
parameters. A more or less opposite picture appears during the reduction of the oxide layer
where the transition between different oxidation states and low overpotential for O/OH
adsorption leads to de-passivation of the surface, where anodically formed ����� can be
dissolved before a full reduction of the surface. However the exact surface state of the
platinum oxide layer during electrochemical treatments remains under discussion and will be
the key to completely resolve the picture on the dissolution mechanism.
Chapter 10: References
85
9 Summary and outlook This work presented describes a comprehensive study on platinum dissolution mostly in
acidic media: from design and implementation of new experimental equipment through
extended investigation of the dissolution behavior to finally a tentative model on the
mechanism. In the initial stage a universal modular software approach based on an extended
consumer/producer design pattern with asynchronous control and acquisition over different
hardware components was developed. The software architecture incorporates smart algorithms
for automatic execution of electrochemical high-throughput and combinatorial experimental
series. This technical achievement was adopted in the development of five automated Rotating
Disc Electrode systems (as black box solution) and seven Scanning Flow Cell backbones (as a
basic platform for further development). The developed software with its modular approach
provides a solid base for further modifications and extensions of the functionality by including
new procedures and/or incorporation of other hardware equipment without affecting the main
programming logic or architecture. The source code and the implemented logic are
documented and can be reused by the successors in follow-up studies.
The innovative design and functionality of the Scanning Flow Cell enables straightforward
integration of analytical methods for online-monitoring of the electrolyte constituents during
electrochemical measurements. Especially the coupling with inductively coupled plasma mass
spectrometer (ICP-MS) and the further optimization of the system performance allow online
detection of dissolved species in the sub-monolayer region. The reliability of the developed
software/system was verified in several individual steps and the long-term performance was
evaluated over 5 days of degradation test on Pt/C catalysts. Additionally, a proof-of-concept
was achieved for the combined SFC/ICP-MS technique on a copper bulk material, used as a
model system, where the direct correlation between the applied galvanostatic sequence and
measured spectrometric signal was shown. Thus, not only a qualitative but also a quantitative
description can be presented for an estimation of corrosion rates.
In the second stage of this project, a targeted screening of the experimental parameters was
performed to determine which of them have a crucial influence on the platinum dissolution
and to which extent. It was found that a transition between the reduced and oxidized surface
state around the potential of sub-surface oxide formation (e.g. 1.1 VRHE) is triggering significant
platinum loss. Clear separation of the dissolution during the anodic and cathodic polarization
was demonstrated in correlation with the amount of formed oxide, time scale of the
Chapter 10: References
86
experiment, pH and temperature. Two potential windows of electrochemical treatment have
been emphasized within which the dissolution rate is negligible. The possible dissolution
pathways were revisited and discussed intensively in the previous chapter 8 in relation to the
new findings. The effect of the reactive gases on the dissolution of polycrystalline Pt was
presented for the first time. Improved understanding of the dissolution phenomena of
platinum is of high importance from the fundamental point of view as well as from applied
perspectives. The gained knowledge about stability issues of Pt is determining also its
application limits, and provides guidelines for optimization of operation conditions of
platinum-based materials. The extended quantitative description of the dissolution rate can be
further used for engineering purposes to predict Pt losses during various experimental
conditions investigated in this work.
For the complete resolving of the dissolution mechanism of platinum, however, a missing
puzzle part remains the surface state during the electrochemical treatment. Only with in-situ
experiments in a comparable electrochemical environment, it will be possible to answer this
question in future works.
During this work, also several parallel studies on noble metals and bimetallic compositions
were performed and published that explicitly show broad range of application of the developed
methodology. The novel electrochemical scanning flow system (SFC) can be further extended
with alternative analytical tool. One example is an ongoing project on the investigation of the
activity and selectivity of electrocatalytic materials with a combined SFC and DEMS system.
Overall, the ability of parallel monitoring over several parameters like stability, activity and
selectivity provides a powerful tool for systematic investigation of real systems like fuel cell
catalyst and electrode materials.
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Appendix
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Appendix Publications list: 20. Cherevko S.; Zeradjanin, A.R.; Topalov, A. A.; Kulik, N.; Katsounaros, I.; Mayrhofer, K.J.J.; Stability-Activity Interrelation for the Electrocatalysis of Acidic Oxygen Evolution on Noble Metals, (2014) --- under revision --- 19. Zeradjanin, A.R.; Topalov, A. A.; Overmeere, Q.v.; Cherevko, S.; Chen X.X.; Ventosa, E.; Schuhmann W.; Mayrhofer, K.J.J.; Rational design of the electrode morphology for oxygen evolution – enhancing the performance for catalytic water oxidation, (2014) ---in press--- 18. Cherevko S.; Topalov, A. A.; Zeradjanin, A.R.; Keeley, G.P.; Mayrhofer, K.J.J.; Temperature-Dependent Dissolution of Platinum in Acidic Media, (2014) Electrocatalysis --- in press --- 17. Meier, J.C.; Galeano, C.; Katsounaros, I.; Witte, J.; Bongard, H.J.; Topalov A. A.; Baldizzone, C.; Mezzavilla, S.; Schüth, F.; Mayrhofer, K.J.J.; Design criteria for active and stable Pt/C fuel cell catalysts, 5 (2014) 44-67 Beilstein Journal of Nanotechnology, doi: 10.3762/bjnano.5.5 16. Topalov, A. A.; Zeradjanin, A.R.; Cherevko, S.; Mayrhofer, K.J.J.; The impact of dissolved reactive gases on platinum dissolution in acidic media, 40 (2014) 49-53 Electrochemistry Communications, doi: 10.1016/j.elecom.2013.12.021 15. Topalov, A. A.; Cherevko, S.; Zeradjanin, A. R.; Meier, J. C.; Katsounaros, I.; Mayrhofer, K. J.J.; Towards a comprehensive understanding of platinum dissolution in acidic media, 5 (2), (2014) 631-638 Chemical Science, doi: 10.1039/C3SC52411F 14. Cherevko, S.; Topalov, A. A.; Zeradjanin, A. R.; Katsounaros, I.; Mayrhofer, K.J.J.; Gold Dissolution: Towards Understanding of Noble Metal Corrosion, 3 (37), (2013) 16516 - 16527 RSC Advances, doi: 10.1039/C3RA42684J 13. Scuppert, A. K.; Topalov, A. A.; Savan, A.; Ludwig, A.; Mayrhofer, K.J.J.; Composition-Dependent Oxygen Reduction Activity and Stability of Pt–Cu Thin Films, 1 (2013) ChemElectroChem, doi: 10.1002/celc.201300078 12. Cherevko, S.; Topalov, A. A.; Katsounaros, I.; Mayrhofer, K.J.J.; Electrochemical dissolution of gold in acidic medium, 28 (2013) 44-46 Electrochemistry Communications, doi: 10.1016/j.elecom.2012.11.040 11.a) Topalov, A. A.; Katsounaros, I; Auinger, M.; Cherevko, S.; Meier, J.C.; Klemm, S.O.; Mayrhofer, K.J.J.; Dissolution of platinum – limits for the deployment of electrochemical energy conversion? , 51 (50), (2012) 12613-12615 Angewandte Chemie International Edition VIP Status, doi: 10.1002/anie.201207256 11.b) Topalov, A. A.; Katsounaros, I; Auinger, M.; Cherevko, S.; Meier, J.C.; Klemm, S.O.; Mayrhofer, K.J.J.; Die Auflösung von Platin – Grenzen für den Einsatz zur elektrochemischen Energieumwandlung? , 124 (50), (2012) 12782-12785 Angewandte Chemie VIP Status, doi: 10.1002/ange.201207256 10. Galeano, C.; Meier, J.C.; Peinecke, V.; Bongard, H.J.; Katsounaros, I.; Topalov, A. A.; Lu, A.H.; Mayrhofer, K.J.J.; Schueth, F.; Towards Highly Stable Electrocatalysts via Nanoparticle Pore Confinement 134 (50), 20457-20465, JACS (2012) doi: 10.1021/ja308570c 9. Ankah, G. N.; Pareek, A.; Cherevko S.; Topalov, A. A.; Rohwerder, M.; Renner, F. U.; The influence of halides on the initial selective dissolution of Cu3Au (111), Electrochimica Acta, 85 (2012) 384-392 doi:10.1016/j.electacta.2012.08.059 8. Meier, J.C.; Katsounaros, I; Galeano, C.; Bongard, H.; Topalov, A.A.; Kostka, A.; Karschin, A.; Stability investigations of electrocatalysts on the nanoscale, Energy & Environment Science 5 (11), (2012), 9319-9330, doi:10.1039/C2EE22550F 7. Schuppert, A.K.; Topalov, A. A.; Katsounaros, I.; Klemm, S. O.; Mayrhofer, K. J. J., A Scanning Flow Cell System for Fully Automated Screening of Electrocatalyst Materials, Journal of The Electrochemical Society 11 (2012) 159 (2012) doi: 10.1149/2.009211jes
Appendix
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6. Klemm, S. O.; Karschin, A.; Topalov, A. A.; Mingers, A. M.; Katsounaros, I.; Mayrhofer, K. J. J., Time and potential resolved dissolution analysis of rhodium using a microelectrochemical flow cell coupled to an ICP-MS, Journal of Electroanalytical Chemistry 677–680 (2012), 50–55, doi:10.1016/j.jelechem.2012.05.006 5. Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Kostka, A.; Schu �th, F.; Mayrhofer, K. J. J., Degradation Mechanisms of Pt/C Fuel Cell Catalysts under Simulated Start-Stop Conditions, ACS Catal. 2 (2012), 832-843, doi:10.1021/cs300024h 4. Klemm, S. O.; Topalov, A. A.; Laska, C. A.; Mayrhofer, K. J. J., Coupling of a high throughput microelectrochemical cell with online multielemental trace analysis by ICP-MS, Electrochemistry Communications 13 (2011), 1533–1535, doi:10.1016/j.elecom.2011.10.017 3. Topalov, A. A.; Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Mayrhofer, K. J. J., Development and integration of a LabVIEW-based modular architecture for automated execution of electrochemical catalyst testing, Review of Scientific Instruments 82 (2011), no. 11, 114103-1-114103-5,doi:10.1063/1.3660814 2. Auinger, M.; Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Biedermann, P. U.; Topalov, A. A.; Rohwerder, M.; Mayrhofer, K. J. J., Near-surface ion distribution and buffer effects during electrochemical reactions, Phys. Chem. Chem. Phys. 13 (2011), 16384–16394, doi:10.1039/c1cp21717h 1. Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Topalov, A. A.; Biedermann, P. U.; Auinger, M.; Mayrhofer, K. J. J., The effective surface pH during reactions at the solid–liquid interface, Electrochemistry Communications 13 (2011), no. 6, 634-637, doi:10.1016/j.elecom.2011.03.032
Oral presentations: 13. Schuppert, A. K.; Topalov, A. A.; Savan, A.; Ludwig, A.; Mayrhofer, K. J. J., Pt-Cu Alloys as Catalysts for the Oxygen Reduction Reaction – A Thin-Film Study of Activity and Stability, 2013. 224th ECS Meeting, San Francisco, CA, USA, 2013-10-27 to 2013-11-01
12. Topalov, A. A.; Zeradjanin, A. R.; Mayrhofer, K. J. J., (Elektro-)chemische Energieumwandlung - essentieller Baustein der Energiewende, 2013. 26. Spektrometertagung, Friedrichshafen, Germany, 2013-09-10 to 2013-09-11 (invited talk)
11. Topalov, A. A.; Cherevko, S.; Zeradjanin, A. R.; Meier, J. C.; Katsounaros, I.; Mayrhofer, K. J. J., Dissolution of Pt during oxygen reduction in acidic media, 2013. GDCh-Wissenschaftsforum Chemie 2013, Darmstadt, Germany, 2013-09-01 to 2013-09-04 (Förderpreis der GDCh-Fachgruppe Angewandte Elektrochemie 2013)
10. Topalov, A. A.; Cherevko, S.; Zeradjanin, A. R.; Mayrhofer, K. J. J., Stability of Electrocatalyst Materials – a Limiting Factor for the Deployment of Electrochemical Energy Conversion?, 2013. Third Russian-German Seminar on Catalysis “Bridging the Gap between Model and Real Catalysis. Energy-Related Catalysis”, Burduguz, Lake Baikal, Russia, 2013-06-24 to 2013-06-27, (Award for an excellent oral presentation of young scientist)
9. Zeradjanin, A. R.; Topalov, A. A.; Cherevko, S.; Schuhmann, W.; Mayrhofer, K. J. J., Rational design of morphological pattern for efficient electrocatalytic gas evolution, 2013. 4th Regional Symposium on Electrochemistry South-East Europe, Ljubljana, Slovenia, 2013-05-26 to 2013-05-30
8. Topalov, A. A.; Zeradjanin, A. R.; Cherevko, S.; Mayrhofer, K. J. J., Investigation of platinum stability by in-situ mass spectrometry, 2013. 4th Regional Symposium on Electrochemistry South-East Europe, Ljubljana, Slovenia, 2013-05-26 to 2013-05-30
7. Topalov, A. A., Wissenschaftler-/in, was ist das?, 2012. Berufs- und Studieninformationstag, Lore-Lorentz Schule, Düsseldorf, Germany, 2012-12-04
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6. Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Schüth, F.; Mayrhofer, K. J. J., Electrode Materials for Electrochemical Energy Conversion, 2012. Electrochemistry 2012, Fundamental and Engineering Needs for Sustainable Development, Munich, Germany, 2012-09-17 to 2012-09-19
5. Topalov, A. A.; Mayrhofer, K. J. J., Pt dissolution monitored by ICP-MS, 2012. Workshop "Challenges in Energy Research", MPI-BAC, Mülheim, Germany, 2012-03-12 to 2012-03-12
4. Cherevko., S; Topalov, A. A.; Mayrhofer, K. J. J., Effect of Cathodic Polarization on the Electrochemistry of Gold Surfaces, 2012. 63rd Annual Meeting of the International Society of Electrochemistry, Prague, Czech Republic, 2012-09-19 to 2012-09-24
3. Topalov, A. A.; Mayrhofer, K. J. J., Kopplung ICP-MS mit Elektrochemie: Online Untersuchung von Materialkorrosion sowie Stabilität von Brennstoffzellenkatalysatoren, 2012. Anorganica 2012, Hilden, Germany, 2012-09-13 to 2012-09-13 (invited talk)
2. Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Schüth, F.; Mayrhofer, K. J. J., Role of Carbon Support for Activity and Stability of Fuel Cell Catalysts, 2011. 15th Annual Green Chemistry & Engineering Conference, Washingtion D.C., USA, 2011-06-21 to 2011-06-23
1. Katsounaros, I.; Meier, J. C.; Topalov, A. A.; Klemm, S. O.; Hodnik, N.; Mayrhofer, K. J. J., Oxygen reduction reaction: Surface coverage vs. activity, 2011. 8th Greek Conference of Chemical Engineering, Thessaloniki, Greece, 2011-05-26 to 2011-05-28
Poster presentations 11. Topalov, A. A.; Zeradjanin, A. R.; Cherevko, S.; Mayrhofer, K. J. J., Investigation of electro-chemical dissolution of platinum under the influence of reactive gases by in-situ mass spectrometry, /2013. São Paulo School of Advanced Sciences on Electrochemistry, Energy Conversion and Storage - SPASECS, São Paulo, Brazil, 2013-12-07 to 2013-12-14 (Poster Award) 10. Schuppert, A. K.; Topalov, A. A.; Savan, A.; Klemm, S. O.; Ludwig, A.; Mayrhofer, K. J. J., Fast Screening of PEMFC-Catalysts with a Scanning Flow Cell System, /2012. Electrochemistry 2012, Munich, Germany, 2012-09-17 to 2012-09-19 9. Topalov, A. A.; Klemm, S. O.; Meier, J. C.; Katsounaros, I.; Mayrhofer, K. J. J., Investigation of the anodic and cathodic dissolution of platinum in acidic media, /2012. GDCh meeting – Electrochemistry 2012, Munich, Germany, 2010-09-17 to 2010-09-19 8. Mayrhofer, K. J. J.; Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Schüth, F., Activity and stability of Pt/HGS catalysts for application in fuel cells, /2012. GDCh meeting – Electrochemistry 2012, Munich, Germany, 2010-09-17 to 2010-09-19 7. Klemm, S. O.; Karschin, A.; Schuppert, A. K.; Topalov, A. A.; Katsounaros, I.; Mayrhofer, K. J. J., Rhodium electrode dissolution in sulfuric acid during electrochemical treatment investigated with a scanning flow cell coupled to an ICP-MS, /2012. 63rd Annual Meeting of the International Society of Electrochemistry, Prague, Czech Republic, 2012-09-19 to 2012-09-24 6. Schuppert, A. K.; Topalov, A. A.; Savan, A.; Klemm, S. O.; Ludwig, A.; Mayrhofer, K. J. J., Fast Screening of PEMFC-Catalysts with a Scanning Flow Cell System, /2012. 63rd Annual Meeting of the International Society of Electrochemistry, Prague, Czech Republic, 2012-09-19 to 2012-09-24 5. Topalov, A. A.; Meier, J. C.; Katsounaros, I.; Klemm, S. O.; Mayrhofer, K. J. J., Online Monitoring of the Dissolution of Platinum during Electrochemical Experiments by Coupled ICP-MS, /2012. 63rd Annual Meeting of the International Society of Electrochemistry, Prague, Czech Republic, 2012-09-19 to 2012-09-24
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4. Auinger, M.; Katsounaros, I.; Meier, J. C.; Biedermann, P. U.; Topalov, A. A.; Klemm, S. O.; Rohwerder, M.; Mayrhofer, K. J. J., The Effective Surface pH during Reactions at the Solid/Liquid Interface , /2012. 63rd Annual Meeting of the International Society of Electrochemistry, Prague, Czech Republic, 2012-08-19 to 2012-08-24 3. Meier, J. C.; Galeano, C.; Katsounaros, I.; Topalov, A. A.; Schüth, F.; Mayrhofer, K. J. J.;, IL-TEM and IL-Tomography Stability Investigations of Fuel Cell Catalysts, /2012. 63rd Annual Meeting of the International Society of Electrochemistry, Prague, Czech Republic, 2012-09-19 to 2012-09-24, (Poster Award) 2. Auinger, M.; Katsounaros, I.; Meier, J. C.; Klemm, S. O.; Biedermann, P. U.; Topalov, A. A.; Rohwerder, M.; Mayrhofer, K. J. J., Near Surface Ion Distribution and Buffer Effects during Electrochemical Reactions, /2011. 14th Austrian Chemistry Days, Linz, Austria, 2011-09-26 to 2011-09-29 1. Katsounaros, I.; Topalov, A. A.; Mayrhofer, K. J. J., Electrochemical reduction of CO2 to fuels: directions and perspectives, /2010. Electrochemistry 2010: From Microscopic Understanding to Global Impact, Bochum, Germany, 2010-09-13 to 2010-09-15
Appendix
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Curriculum Vitae – Angel A. Topalov PERSONAL INFORMATION
� Birthday: 11.02.1985, Sankt-Petersburg � Nationality: Bulgarian, German � Martial status: married � e-mail: [email protected]
PHD STUDIES
05/2010 - present Max-Planck-Institut für Eisenforschung, Düsseldorf
� PhD student in the Electrocatalysis Group of Dr. Karl J.J. Mayrhofer in the Department for Interface Chemistry and Surface Engineering of Prof. Martin Stratmann in close cooperation with Prof. Wolfgang Schuhmann
� PhD thesis about “Design and implementation of automated system based on coupling of electrochemical flow cell with mass spectrometry for investigation of dissolution behavior of platinum”
� PhD courses at the Ruhr-Universität Bochum (RUB): - Intellectual Property Rights - Advanced Electroanalytical Methods
� Soft skill courses of the International Max-Planck Research School: (Presentation Skills, Scientific Writing & Publishing, Leadership Skills, Project & Self Management)
� Certified LabVIEW Developer (6/2011-5/2013) � Training and supervision of new PhD students and interns � Supervisor in DAAD RISE program 2011 � Building up the laboratories of the Mayrhofer group
- System design of Scanning Flow Cell (SFC) - Software development for RDE and SFC setups - Coupling of ICP-MS with Scanning Flow Cell
UNIVERSITY
2004 - 2009 University of Leipzig, Physics Department
� M.Sc. degree in physics, International Physics Study Program 2003 - 2004 Sofioter University “Sv. Kliment Ohridski”, Physics Department
AWARDS
12/2013 São Paulo School of Advanced Sciences on Electrochemistry, Energy
Conversion and Storage - SPASECS 2013, Sao Paulo, Brazil
� Energy & Environmental Science Poster Prize
09/2013 GDCh-Wissenschaftsforum Chemie 2013, Darmstadt, Germany
� Award of German Chemical Society (GDCh) for Applied Electrochemistry 2013
Appendix
105
06/2012 3rd Russian-German Seminar on Catalysis, Burduguz, Russia
� Best oral presentation of young scientist
SCIENTIFIC RECORD
Peer-reviewed articles published till Januar 2014 18 Total number of citations (according to google scholar) 180 H-index (according to google scholar) 8
WORK EXPERIENCE
10/2012 – present Max-Planck-Institut für Eisenforschung GmbH (TVöD E13)
05/2010 – 09/2012 Center for Electrochemical Sciences, RUB (TV-L E13)
Multiple student jobs (e.g.):
11/2008 – 03/2010 CAD support, Ingenieurbüro für Bauabrechnung – Reißmüller
09/2009 – 12/2009 Technical support, Malberg EDV-Systemberatung GmbH
04/2008 – 05/2008 HIWI, University of Leipzig, Physics Department
SCHOOL
1998 - 2003 Secondary Mathematical School “Baba Tonka”, Ruse, Bulgaria
1991 - 1998 Elementary School “Paisii Hilendaski”, Ruse, Bulgaria
LANGUAGE SKILLS
English fluent
German fluent
Bulgarian Mother tongue
Russian Mother tongue