determination of the active sites on platinum/carbon ... focus of chemical engineering is to design...
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The material presented here is based upon work supported by the National Science Foundation under Award No. EEC-0813570.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
Introduction
Methods
Synthesis
1. Incipient wetness impregnation with Pt salt/Water/Ethanol
solution and a chosen carbon nanotube followed by air drying
2. Reduction of the catalysts in a tube furnace under hydrogen
at 200oC for 30min shown in figure 3
3. Catalyst dispersion in 0.5wt% nafion solution by sonication
4. Placement 100μL of catalyst on glassy carbon electrode
followed by air drying as shown in figure 4
Testing
1. A standard three electrode set up was constructed with a
Ag/AgCl reference electrode, Pt wire counter electrode, and
glassy carbon working electrode shown in figure 5
2. All experiments were conducted in 0.5M H2SO4
3. Pre-treatment run using voltages as shown in figure 6
4. CO stripping technique performed
5. Surface area integration determined
Determination of the Active Sites on Platinum/Carbon Electrocatalysts Shelly Vanyo, Summer 2013 RET Project
Project Advisors: John Matthiesen & Dr. Jean-Philippe Tessonier Center for Biorenewable Chemicals, Iowa State University, Ames, IA 50010
Abstract
As the global human population increases, so does our per capita energy demands. Yet, our society still relies heavily on petroleum-based chemical energy. One focus of Chemical Engineering is to design catalysts, which are materials that increase the rate of a reaction without being appreciably consumed.1
However, a limiting factor of identifying ideal catalysts is the ability to be highly active, stable, and selective towards their reaction.1 In the following study electrocatalysts were synthesized to improve the hydrogen evolution reaction and oxygen evolution reaction, shown in figure 1.
Figure 1: Hydrogen Evolution Reaction & Oxygen Evolution Reaction2
Acknowledgements
When comparing electrocatalysts, the quality of the material needs to be
referenced to a defined quantity of the catalyst. Electrocatalyts can be
referenced to the mass of the electrocatalysts, geometric surface area of
an electrode, geometric surface area of the electrocatalyst, or to the
electrochemical active surface area of the material. As a researcher
approaches the end of the previous list, the quality of comparing
electrocatalysts increases as well as the difficulty of obtaining the value.3
The electrochemical active surface area (ECSA) is determined utilizing
two different techniques. This first technique is an integration under the
hydrogen desorption region correcting for the double layer. The values
calculated are then referenced towards 210μC/cm2, the charge of
stripping a monolayer of hydrogen on platinum.4 Carbon Monoxide (CO)
stripping is then used as the second technique to corroborate the
surface area determined by the previous integration. The integration in
the CO oxidation region is compared to a value of 420μC/cm2, the
charge of stripping a monolayer of CO on platinum.4
The final objective of this project is to create active, stable
electrocatalysts for the hydrogen evolution reaction and the oxygen
evolution reaction. Platinum surface atomic concentrations for a series of
5, 10, 20wt% Platinum/Carbon (Pt/C) electrocatalysts were explored
using cyclic voltammetry. However, only the 20wt% Pt/C sample is
shown. Figure 2 shows a representative SEM image of Pt supported on
high temperature annealed carbon nanotubes (here 5 wt% Pt/C).
National Science Foundation, sponsored by award No. EEC-0813570
Iowa State University
Dr. Adah Leshem, Program Director
Figure 2
SEM of 5wt % Pt/C
Future Research Determination of the electrochemical surface area on other
high temperature annealed carbon supports
Results
Works Cited 1 Satterfield, C. N. Heterogeneous Catalysis in Industrial Practice. 2nd Ed. (McGraw-Hill, Inc., 1991). 2 Greeley, J. & Markovic, N. The road from animal electricity to green energy: combining experiment and
theory in electrocatalysis. Energy & environmental science 5, 9246-9256 (2012). 3 Jaramillo, T. Energy Tutorial: Electrocatalysis 101. (Stanford, 2012). 4 Chen, D. et al. Determining the Active Surface Area for Various Platinum Electrodes. Electrocatalysis 2,
207-219 (2011).
Pretreatment-H2SO4
1. CV: 200mV/s
2. CA: 5min @ -0.3V vs. Ref
3. CV: 200mV/s
4. CA: 5min @ -0.3V vs. Ref
5. CV: 200mV/s figure 7
CO Stripping-H2SO4
1. CA: 1 hr 25 min at -0.094 V vs. Ref
2. CV: 10mV/s figure 8
Figure 4: Glassy Carbon
electrode with catalyst
Figure 5: Electrochemical
setup
Figure 6: Pre-Treatment and CO
stripping parameters
UPD-H Area
Apt = 20.05cm2
CO-Oxidation Area
Apt = 18.26cm2
UPD-H Area
Apt = 19.13cm2
Figure 7: Cyclic Voltammetry curve of step five
in the catalyst pretreatment
Figure 8: Cyclic Voltammetry curve of
step 2 in CO stripping
Figure 3: Tube furnace
Stripping of the under potential deposition region of hydrogen
(UPD-H) occurs around 0.0V as shown in figures 7 and 8. This is
indicative of how much hydrogen was adsorbed. By comparing this
value to the charge of the formed monolayer, 210 C/cm2, the
surface area can be calculated.4 This change is affected by the
surface characteristics and is necessary to ensure an effective
monolayer.
CO is easily adsorbed to the electrode surface and displaces pre-
adsorbed hydrogen. During CO stripping, CO is desorbed from the
electrocatalyst surface. The value is then compared to desorption
value of a monolayer of CO, which is 420 C/cm2.4 It is important to
ensure oxygen has been removed during these measurements.
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