performance of silver nanoparticles in the catalysis of the oxygen … · 2019-05-02 ·...

Performance of silver nanoparticles in the catalysis of the oxygen reduction reaction in neutral media Efficiency limitation due to hydrogen peroxide escape

Christopher C M Neumann Eduardo Laborda Kristina Tschulik Kristopher R Ward and

Richard G Compton ()

Department of Chemistry Physical and Theoretical Chemistry Laboratory Oxford University South Parks Road Oxford OX1 3QZUnited Kingdom

Received 4 May 2013 Accepted 9 May 2013 copy Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2013 KEYWORDS oxygen reduction reaction silver nanoparticles nanoelectrocatalysis hydrogen peroxide escape fuel cells

ABSTRACT The electrocatalytic activity for oxygen reduction reaction (ORR) at neutral pHof citrate-capped silver nanoparticles (diameter = 18 nm) supported on glassy carbon (GC) is investigated voltammetrically Novelly the modification of thesubstrate by nanoparticles sticking to form a random nanoparticle array and thevoltammetric experiments are carried out simultaneously by immersion of the GC electrode in an air-saturated 01 M NaClO4 solution (pH = 58) containing chemically-synthesized nanoparticles

The experimental voltammograms of the resulting nanoparticle array aresimulated with homemade programs according to the two-proton two-electron reduction of oxygen to hydrogen peroxide where the first electron transfer is ratedetermining In the case of silver electrodes the hydrogen peroxide generated is partially further reduced to water via heterogeneous decomposition

Comparison of the results obtained on a silver macroelectrode and silvernanoparticles indicates that for the silver nanoparticles and particle coverages(0035ndash0457) employed in this study the ORR electrode kinetics is slower andthe production of hydrogen peroxide larger on the glassy carbon-supported nanoparticles than on bulk silver

1 Introduction

The field of nanocatalysis is of major interest in various research fields and novel applications have emerged in areas such as biosensing [1] chemical sensing [2 3] and material sciences [4] The changed physical

and chemical properties of nanosized materials are extensively investigated in order to develop new catalysts for important processes such as the oxygen reduction reaction (ORR) This is a key process in biological systems and in energy transformations in particular being frequently identified as a major

Nano Research 2013 6(7) 511ndash524 DOI 101007s12274-013-0328-4

Address correspondence to richardcomptonchemoxacuk

| wwweditorialmanagercomnaredefaultasp

512 Nano Res 2013 6(7) 511ndash524

efficiency-limiting process of fuel cells [5] Consequently much effort is made in the design of new cathodes and cell conditions to speed up the reaction and promote the full reduction of oxygen to water

The number of studies reported reflects not only the difficulty to drive the reduction of oxygen but also the intricate complexity of its mechanism The latter depends on the electrode material and electrolyte and it can involve different simultaneous pathways and intermediates Although the details of the mechanism are still not fully understood there are some features generally accepted for most electrode materials in aqueous solution Thus it has been proposed that oxygen (O2) can be reduced either by a direct path to water [6 7]

2 2 2O 4 H O 4e 2H O+ 4OH (1)

or a series 2+2 process in which there are two electrons transferred at a time with hydrogen peroxide (H2O2) as an intermediate[8ndash10]

2 2 2 2

2 2 2 2

O 2 H O 2e H O +2OHH O 2 H O 2e 2H O +2OH

(2)

The first electron transfer to form the superoxide ion (

2O ) is reported to be rate determining for a variety of metals such as platinum [11 12] palladium [11 12] and silver [13]

2 2O e O (3)

Moreover hydrogen peroxide has been detected as an ORR intermediate in aqueous solution for several electrode materials including silver [14] and carbon electrodes [15] This suggests that at least partially on these cathodes ORR follows a 2+2 mechanism where the H2O2 electrogenerated can either diffuse away from the electrode surface or be reduced to water Whether the latter goes via an electrochemical step (Eq (2)) or homogeneousheterogeneous disproportionation [16 17] (Eq (4)) is unclear

2 2 2 21H O O H O2

(4)

The production of hydrogen peroxide in fuel cells however is highly undesirable as this can degrade

the membranendashelectrode assembly [18 19] reduces the cathodic current and most importantly it makes the cell voltage drop limiting the overall electrode efficiency [20] Therefore more efficient energy devices build on favouring the full reduction of oxygen to water using many different metal nanoparticles in this context Thus previous studies have reported that after cathode modification with platinum gold and palladium nanoparticles [21ndash26] the ORR overpotential and H2O2 production decrease as the amount of nano-particles is increased This reflects the faster ORR kinetics on the metal against that on the substrate material but it does not necessarily imply that the kinetics on the nanoparticles is different from that of the bulk metal

In order to gain a better understanding the rigorous examination of the electrocatalytic activity of nano-particle arrays requires suitable simulations of the ORR mechanism and of the diffusion of species to and away from the electrode surface A kinetic study of this complex system will be outlined in this paper focusing on the ORR on silver nanoparticles in nearly neutral aqueous solution ( pH 58 ) Within the search for cheaper non-platinum catalysts a lot of attention has been drawn to silver [6 27 28] which shows relatively high catalytic activity and thermodynamic stability over a wide pH range [29ndash31] The catalysis of ORR at neutral pH is of particular interest in the development of microbial fuel cells (MFCs) that require more efficient catalysts in neutral media

An entirely new experimental approach is designed in this work that provides an easy way for simul-taneous modification of the electrode and the study of ORR voltammetry with different nanoparticle loads Thus the substrate (glassy carbon electrode GCE) is immersed in an air-saturated solution containing chemically-synthesized silver nanoparticles that collide with and stick to the surface Cyclic voltammograms in the ORR potential region are recorded at different times corresponding to different nanoparticle coverage The latter can be worked out afterwards from the charge associated to the exhaustive oxidation of the par-ticles at the end of the experiment This methodology also enables us to obtain a uniform distribution of nanoparticles low agglomeration and a number of different coverages more easily than would be the case

wwwtheNanoResearchcom∣wwwSpringercomjournal12274 | Nano Research

513Nano Res 2013 6(7) 511ndash524

for drop casting methods [32] The experimental voltammograms are analyzed by

making use of simulations developed in our group [33ndash35] for electrochemical experiments on nanoparticle arrays when both the substrate and particles are electroactive [36] First the mechanism for ORR in neutral solution is investigated on glassy carbon and silver electrodes Next this is applied to the fitting of the voltammetry on the modified electrode with the kinetic parameters on the nanoparticles as adjustable variables The results obtained provide insight into the ORR mechanism the catalytic activity of silver nanoparticles and the optimum conditions for their performance in fuel cells

2 Experimental

21 Chemicals

The chemicals used in this study were of analytical grade and purchased from Sigma Aldrich unless stated otherwise and solutions were obtained by dissolving the respective chemical in ultrapure water (Millipore resistivity ca 182 MΩcm at 25 degC)

22 Silver nanoparticles Synthesis and charac-terisation

The synthesis of citrate-capped silver nanoparticles was performed as described in Ref [32] The obtained silver nanoparticle suspension was centrifuged (Centrifuge 5702 Eppendorf Hamburg Germany) for 30 min at a rotation speed of 4400 rpm and the resulting supernatant was used as the silver nanoparticle stock suspension in this study

The nanoparticle size was analysed by Nanoparticle Tracking Analysis (Nanosight LM 10 NanoSight Ltd Amesbury UK) yielding a modal radius of r = 9 nm (see Fig 1 black squares) Further characterisation by UVndashvis spectroscopy (U-2001 Hitachi Tokyo Japan) was performed in the range of 600 nm to 300 nm using a tungsten iodide and a deuterium light source (scan rate = 100 nmmiddotminndash1) A single absorption peak was detected at 410 nm (see inset in Fig 1) whichmdashin agreement with the Nanoparticle Tracking Analysis datamdashsuggests an estimated radius of silver nano-particles of about 10 nm [37]

Figure 1 Size distribution of the silver nanoparticles used as determined by Nanoparticle Tracking Analysis Inset UVndashvis absorption spectrum of silver nanoparticle suspension showing a peak maximum at 410 nm

23 Electrochemical analyses

All electrochemical experiments were performed in a conventional three electrode setup using a microAutolabⅡ potentiostat (Metrohm-Autolab BV Utrecht Netherlands) The temperature of the electrolyte was set and held at 25 degC using a thermostated water bath A carbon rod was used as counter electrode and a saturated calomel electrode (SCE) served as a reference electrode (potential E = 244 mV vs standard hydrogen electrode all potentials stated in this article refer to values against SCE) Macro glassy carbon (GC r = 155 mm) macro silver (r = 025 mm) and silver nanoparticle-modified glassy carbon electrodes were employed as working electrodes (WE)

24 Oxygen reduction reaction on macroelectrodes

Cyclic voltammetry at various scan rates was per-formed to measure the kinetics of oxygen reduction on a macro glassy carbon and a macro silver WE The experiments were conducted in freshly prepared air- saturated 01 M NaClO4 (sodium perchlorate) solutions The analysed potential range was 0 mV to ndash1000 mV for the glassy carbon electrode and 0 to ndash700 mV for the silver macroelectrode Additionally cyclic voltam-mograms (CVs) at various scan rates were recorded for both electrodes in a solution that had been deoxygenated by purging N2 for 15 min prior to the experiments These scans were used as ldquoblanksrdquo to


514 Nano Res 2013 6(7) 511ndash524

subtract the background signal from the respective measurement data in presence of oxygen and the obtained background-corrected CVs were used for all further analysis

25 Nanoparticle sticking experiments

A new experimental strategy has been employed in this work for the study of the electron transfer kinetics on nanoparticles supported on a substrate This method is based on the immersion of the substrate in the working solution containing the electroactive species (oxygen in this case) and the nanoparticles By virtue of Brownian motion the particles collide with the substrate and can stick to it forming an array of ldquonanoelectrodesrdquo (see Fig 2) Voltammetric experi-ments at different immersion times provide results for a range of nanoparticle loads very easily In addition this approach offers a more uniform distribution of particles and the reduction of aggregation with respect to drop casting methods

Figure 2 Scheme of the glassy carbon electrode modified by silver nanoparticles in solution that stick to its surface ((Ⅰ) and (Ⅲ)) and catalyze the oxygen reduction reaction in the voltammetric experiments ((Ⅱ) and (Ⅳ))

Therefore to 8425 mL of a 01 M NaClO4 electrolyte solution 1575 mL of the silver nanoparticle stock suspension was added and a glassy carbon electrode (r = 155 mm) was immersed in this solution for various sticking times During this time the WE was held at a standby potential of ndash250 mV which has been reported to yield a linear increase of the amount of silver nanoparticles adsorbed on the electrode surface [38] Oxygen reduction on the WE with accordingly increasing silver nanoparticle surface coverage was characterised by running a CV every 150 s up to a sticking time of 1950 s During each CV the potential was cycled once between 0 mV to ndash1000 mV vs SCE at a scan rate of 100 mVmiddotsndash1 Before and after running the CVs the WE was held at the standby potential of ndash250 mV to allow for more nanoparticles to adhere to the surface

To quantify the load of silver nanoparticles adherent to the glassy carbon electrode the potential was swept to anodic potentials after a time period of 3400 s to strip the silver nanoparticles from the carbon surface Therefore the potential was shifted from 150 mV to 500 mV in the silver nanoparticle containing 01 M NaClO4 at a scan rate of 50 mVmiddotsndash1 This treatment has been previously demonstrated to allow for quantitative stripping of silver from a carbon surface [39] Assuming a linear increase of the amount of silver nanoparticles sticking to the surface (as found in Ref [38]) thus the surface coverage of silver for each of the oxygen reduction CVs has been calculated

3 Results and discussion

The quantitative study of the voltammetry on the silver nanoparticle-modified electrode requires the identification of a suitable reaction mechanism as well as the determination of the kinetics on the glassy carbon substrate

The cyclic voltammograms obtained for oxygen reduction on glassy carbon and silver electrodes are displayed in Figs 3(a) and 3(b) for scan rates varying from 25 mVmiddotsndash1 to 1200 mVmiddotsndash1 For both systems a single reductive peak is observed which increases in height with increasing scan rate The oxygen reduction occurs at lower potentials on the silver macroelectrode (ca ndash150 mV vs SCE) than on the glassy carbon


515Nano Res 2013 6(7) 511ndash524

Figure 3 CVs showing the oxygen reduction in air-saturated 01 M NaClO4 for different scan rates at (a) a macro glassy carbon electrode and (b) a silver macroelectrode CVs were blank corrected by subtracting CVs measured in N2-purged electrolyte

electrode (ca ndash400 mV vs SCE) This significantly reduced overpotential for oxygen reduction displays the higher catalytic activity of silver for ORR Based on these results a mechanistic and kinetic study of ORR on glassy carbon and silver is carried out in the next sections

31 ORR on glassy carbon macroelectrode

In spite of the extensive investigations on the ORR the multistep mechanism is still unclear due to its complexity Depending on the cathode material and experimental conditions (solvent pH species in solution hellip) the ORR can follow several pathways with different intermediates and efficiency [8 40] The

latter can be quantified in function of the effective number of electrons transferred per O2 molecule ( eff 1 4n ) and the amount of hydrogen peroxide formed The production of hydrogen peroxide has been confirmed in the case of carbon electrodes [15] such that ORR follows at least in part the following serial mechanism

0

0 2 2O e Ofk E

rate determining (5)

2 2 2O H O e HO OH very fast (6)

2 2 2 2HO H O H O OH (7)

where 0k and 0fE are the standard rate constant

transfer coefficient and formal potential respectively of the first electron transfer This has usually been iden-tified as the rate determining step [21] Subsequently a proton-coupled electron transfer takes place instantaneously given the much more positive formal potential of the redox couple

2 2HO HO (ca +780 mV with respect to

2 2O O [41]) Considering the acid dissociation constant of 2 2H O ( apK 116 ) the

2HO ion will protonate at the pH of the present

study ( pH 58 ) The simulation of the ORR cyclic voltammetry at the

glassy carbon electrode has been carried out according to the above EEC mechanism (where ldquoErdquo refers to an electrochemical process and ldquoCrdquo to a homogeneous chemical reaction) with the commercial software DigiSimreg The second electron transfer (6) is set as fully-driven and so the electrochemical response is qualitatively defined by the kinetics of the first electron transfer This is modelled with the Butlerndash Volmer formalism [42 43] that taking into account that the superoxide (

2O ) is electroreduced immediately such that its concentration

2O ( 0) 0c x (where x is the distance from the surface of the WE) establishes the following relationship for the surface flux of oxygen

0

2

2 2

O0O O

0

e 0fF E E

RT

x

cD k c x

x (8)

Therefore the current response corresponds to a two-electron fully-irreversible cathodic process that can be characterized by the transfer coefficient of


516 Nano Res 2013 6(7) 511ndash524

the first step and the combined parameter

0

0 exp fE Fk

RT

Table 1 includes the data corresponding to the best fitting of the experimental voltammograms recorded in the range of scan rates 25ndash1200 mVmiddotsndash1 Literature values for oxygen concentration (

2

bulkOc = 025 mM [44])

and diffusion coefficient (2OD = 5 2 1196 10 cm s [45])

have been used assuming the same D-value for all the participating species The value of the transfer coefficient has been determined from the best fit of the peak current in the whole range of 25ndash1200 mVmiddotsndash1 (note that for the mechanism considered the 0k value does not affect the peak height) and once is known

0k is extracted from the fitting of the peak potential at the different scan rates As can be seen in Fig 4 a satisfactory agreement between experiments and simulation is obtained which supports the suitability of the mechanism employed for the parameterization of the process The variation of the peak current ( pI ) with the scan rate ( ) follows the square root de-pendence predicted by the RandlesndashŠevčiacutek equation that in the case of fully-irreversible processes has the form [42 43]

2

2

Obulk 1 2eff O0496p

FDI n FAc

RT (9)

where A is the area of the electrode eff 2n given that the second electron transfer is very fast and F R and T have the usual meanings The slope of the plot

1 2 vs pI is 276 times 10ndash5 A s12middotVndash12 from which a value for the transfer coefficient of 031 is extracted which compares well with that obtained from the fitting of the voltammograms (see Table 1)

In conclusion the results obtained on glassy carbon are compatible with the electroreduction of oxygen to hydrogen peroxide without significant decomposi-tion of H2O2 occurring within the time scale of the measurements

32 ORR on silver macroelectrode

The mechanism proposed for glassy carbon (Eqs (5)ndash (7)) was initially used for the study of ORR on silver macroelectrode given that the production of 2 2H O has

Figure 4 Experimental (black) and simulated (red) ORR vol-tammetry on glassy carbon macroelectrode in air-saturated 01 M NaClO4 at different scan rates (indicated on the graph) The simulated curves are obtained with the EEC mechanism discussed in section 31 and the values shown in Table 1 CVs were blank corrected by subtracting CVs measured in N2 purged electrolyte The error bars correspond to the upper and lower limit values of k0 in Table 1


517Nano Res 2013 6(7) 511ndash524

also been confirmed when the silver macroelectrodes are employed [14] Nevertheless the fitting of the experimental results was not possible The shape of the voltammograms could be described satisfactorily with 07 but the experimental current exceeds ca 17 times the theoretical one (not shown) This indicates that the number of electrons transferred per O2 molecule has an effective value of eff 33n as reported previously in the Ref [28]

Given that the results obtained on glassy carbon suggest that the decomposition of 2 2H O in solution is not significant for the time scale of the experiments the greater current recorded must be related to heterogeneous processes taking place on the silver surface but not on glassy carbon These can be assigned to the electroreduction of hydrogen peroxide to water that has been proposed to start predominantly with a chemical dissociation step [21]

hetdis

2 2 Ag

2 2

H O 2OH

2OH 2e 2H O 2H O + 2OH

k

(10)

where the two-proton two-electron step is being fully driven [41] and khetdis is the heterogeneous rate constant of the dissociation reaction Additionally it is also well known that silver catalyzes the decomposition of hydrogen peroxide following a first-order kinetics with respect to hydrogen peroxide at low 2 2H O concentrations [46 47]

hetdec2 2 2 2Ag

1H O H O O2

k (11)

Both mechanisms hetEECC EE (given by Eqs (5)ndash(7) (10)) and hetEECC (given by Eqs (5)ndash(7) (11)) predict a gradual transition from hydrogen peroxide gene-ration (two-electron process) to water production (four-electron reduction) as hetk increases (see the green curve in Fig 11) Thus in (10) the product species of the chemical dissociation is rapidly reduced to water whereas in (11) each 2 2H O molecule yields

ldquo 21 O2

rdquo that can potentially transfer two more electrons

within the catalytic cycle established by (5)ndash(7)(11) with water as the final product Consequently both mechanisms were considered for the fitting of the voltammetry on the silver macroelectrode with home-made programs (see the Appendix for more details) assuming that all the electron transfers except for the first one are fully driven and the protonation reaction is diffusion-controlled The results corresponding to the best fitting of the experimental voltammograms in the range of 25ndash1200 mVmiddotsndash1 together with the varia-tions of the peak current and potential with the scan rate are shown in Figs 5 and 6 The and hetk values are determined from the value of the peak current and its variation with the scan rate Subsequently the standard electrochemical rate constant is obtained from the fitting of the peak potential The parameters obtained are included in Table 1 Note that in this case the variation of the peak current with the scan rate deviates from the linear RandlesndashŠevčiacutek relationship as a result of the more complex mechanism of ORR on silver

Table 1 Parameters employed in the simulations corresponding to the best fitting of the experimental ORR voltammograms for the different electrodes considered Glassy carbon (GC) silver (Ag) and silver-nanoparticle-modified glassy carbon (Ag NPGC) electrodes ldquoErdquo refers to an electrochemical process ldquoCrdquo to a homogeneous chemical reaction and ldquoChetrdquo to a heterogeneous chemical reaction

GC Ag Ag NPGC Parameter

EEC hetEECC EE hetEECC hetEECC

0SCE

0exp f FEk

RT

(cmmiddotsndash1) 76 7 a647 1 10 46 6 a

2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027

knet (cmmiddotsndash1) mdash 55 times 10ndash3 13 times 10ndash2 13 times 10ndash2

2

bulk

Oc (mM) 025

2OD (cm2middotsndash1) 196 times 10ndash5

a Values corresponding to the best fit of the peak potential in the range of scan rates 25ndash1200 mVmiddotsndash1 with the corresponding upper and lower limits b Value corresponding to the best fit of the peak potential in the range of nanoparticle coverage 0035ndash0457 with the corresponding upper and lower limits


518 Nano Res 2013 6(7) 511ndash524

Figure 5 Experimental (black) and simulated (red) ORR voltam-metry on silver macroelectrode in air-saturated 01 M NaClO4 at different scan rates (indicated on the graph) The simulated curves are obtained with the hetEECC EE mechanism discussed in section 32 and the values shown in Table 1 CVs were blank corrected by subtracting CVs measured in N2-purged electrolyte The error bars correspond to the upper and lower limit values of k0 in Table 1

As can be seen the two above reaction schemes account for the values of the experimental peak current and provide a suitable description of the variations of the peak current and potential with scan rate However the hetEECC EE scheme predicts a crossing in the cyclic voltammograms that is not observed experimentally (see Fig 5) This crossing is related to the rate of transformation of hydrogen peroxide in the heterogeneous process Obviously the latter also takes place during the reverse scan yielding a species that is reduced much more rapidly than oxygen in the forward scan Consequently at the foot of the wave a greater reduction current is predicted for intermediate hetdisk values where a significant 2 2H O amount exists around the electrode surface during the reverse scan For smaller hetdisk values the hetero-geneous reaction does not take place to a significant extent whereas for large hetdisk 2 2H O dissociates rapidly such that it does not accumulate around the electrode An equivalent behaviour has been reported experimentally for the oxidation of 2-phenylnorbornene that follows an ECE-like mechanism where C corres-ponds to a homogeneous reaction [48]

As can be seen in Fig 6 the hetEECC mechanism does not predict the above crossing and so it provides a better description of the electrochemical response Moreover the fitting of the voltammograms yields

2 1hetdec 13 10 cm sk which compares very well

with the value reported for the decomposition rate constant of hydrogen peroxide on silver 10 13

2 110 cm s [17 49]

33 ORR on silver nanoparticle-modified glassy carbon macroelectrode

331 Electrode modification and cyclic voltammetry

To study the oxygen reduction kinetics of silver nanoparticle-modified carbon surfaces nanoparticle sticking experiments were performed A glassy carbon macroelectrode was immersed into a silver nanoparticle-containing solution and a holding potential of ndash250 mV (vs SCE) was applied which has been reported to result in a linear increase of the number of silver nanoparticles adhering on this surface [38] Thus the longer the sticking time t is the higher the silver nanoparticle coverage The advantage of this


519Nano Res 2013 6(7) 511ndash524

Figure 6 Experimental (black) and simulated (red) ORR voltam-metry on silver macroelectrode in air-saturated 01 M NaClO4 at different scan rates (indicated on the graph) The simulated curves are obtained with the hetEECC mechanism discussed in section 32 and the values shown in Table 1 CVs were blank corrected by subtracting CVs measured in N2 purged electrolyte The error bars correspond to the upper and lower limit values of k0 in Table 1

modification method over drop casting methods is that nanoparticle agglomeration is strongly reduced and a more homogeneous distribution of nanoparticles on the surface is obtained [32]

The CVs recorded at 1100 mV s for various sticking times ie various silver nanoparticle coverages are shown in Fig 7 With increasing sticking time a tran-sition from the CV obtained for a bare glassy carbon surface (t = 0) towards those to the catalytically more active silver macroelectrode is observed Indeed the onset of the oxygen reduction peak is shifted to less cathodic values with increasing silver nanoparticle coverage

To quantify the coverage of the glassy carbon electrode with silver nanoparticles anodic stripping voltammetry was used to quantify the amount of silver adhering to the carbon surface after a sticking time of 3400 s (see Fig 8) Assuming a linear increase of the amount of silver nanoparticles sticking to the surface as found previously [38] thus yields a time dependent sticking of silver nanoparticles according to

9 1C 199 10 C s sQ t t (12)

Taking the modal size of silver nanoparticles rNP = 9 nm as determined by Nanoparticle Tracking Analysis (see Section 22) and considering that one electron is consumed during the oxidation of Ag to

Figure 7 CVs recorded during silver nanoparticle sticking experi-ments showing the increasing catalytic activity of the surface with increasing sticking time ie with increasing coverage of the GC electrode with silver nanoparticles scan rate = 01 Vmiddotsndash1


520 Nano Res 2013 6(7) 511ndash524

Figure 8 Oxidative stripping of the silver nanoparticles adherent to a glassy carbon electrode after 3400 s of silver nanoparticle sticking experiments in 01 M NaClO4 standby potential = ndash250 mV scan rate = 50 mVmiddotsndash1

Ag+ the charge related to dissolution of a single nanoparticle can be estimated

3NP

NPr

43

F rQM

(13)

where ρ is the density of silver (ρ (Ag) = 1049 gmiddotcmndash3) and Mr the molar mass of silver atoms (Mr(Ag) = 10787 gmiddotmolndash1) The derived charge per silver nano-particle QNP is 29 times 10ndash14 C and thus Eq (14) can be used to yield the number of silver nanoparticles sticking to the glassy carbon macroelectrode at each time

4 1696 10 s sN t t (14)

332 ORR kinetics on silver nanoparticles

The catalytic activity of silver nanoparticles for ORR has been quantified through the analysis of the experimental voltammetry recorded on a glassy carbon electrode gradually modified with silver nanoparticles For this it is necessary to consider the particular characteristics of the mass transport towards or away from an assembly of electroactive nanoparticles The simulation method developed in our group for hemispherical particles on an electroactive surface has proven to provide satisfactory description of these systems a detailed account of which is presented in the Appendix and Ref [36] In brief the nanoparticles are considered to have the same size and be evenly

distributed on the substrate surface The kinetics of the electron transfer reactions occurring at the particles and the substrate are modelled with the ButlerndashVolmer model (see Appendix) [42 43] According to the ldquodiffusion domain approximationrdquo [50 51] identical cylindrical domains are assigned to all the particles and the two-dimensional problem of one of them is solved numerically following an Alternating Direction Implicit (ADI) method [52] the result being scaled by the total number of particles

According to the above for the simulation of the voltammetry we need to input the radius and coverage of nanoparticles The former is set at the modal value determined by Nanoparticle Tracking Analysis (ie rNP = 9 nm) that corresponds to the most representative case in terms of diffusion domain Regarding the surface coverage of the particles this is defined as

2

NPN rA

(15)

where N is the number of particles (calculated as detailed in Section 331) and A the area of the glassy carbon substrate (A = 0075 cm2)

The above computational strategy together with the hetEECC mechanism has been employed for the study of the ORR voltammetry on a glassy carbon macroelectrode that is gradually modified by immersion in a silver nanoparticles solution In the simulations the kinetics of ORR on glassy carbon and that of hydrogen peroxide decomposition on silver are set according to the values obtained in Sections 31 and 32 whereas the kinetic and mechanistic parameters for ORR on the silver nanoparticles are adjusted in order to achieve a satisfactory fitting of the voltammetric results

The experimental and simulated voltammograms at different sticking times are shown in Fig 9 the parameters corresponding to the best-fit of the peak current and potential being shown in Table 1 As discussed previously the oxygen reduction over-potential decreases progressively as time proceeds and the number of silver nanoparticles on the glassy carbon surface increases leading to a greater con-tribution to the overall electrochemical response On the other hand the peak height does not vary signi-ficantly in the range of nanoparticle coverage studied 0035ndash0457 Therefore whereas the overpotential


521Nano Res 2013 6(7) 511ndash524

Figure 9 Experimental (black) and simulated (red) ORR voltam-metry on silver nanoparticle-modified glassy carbon macroelectrode in air-saturated 01 M NaClO4 at 100 mVmiddotsndash1 and different sticking times (indicated on the graph) The simulated curves are obtained with the hetEECC mechanism discussed in Section 32 and the values shown in Table 1 CVs were blank corrected by subtracting CVs measured in N2 purged electrolyte The error bars correspond to the upper and lower limit values of k0 in Table 1

for the reduction of oxygen to hydrogen peroxide is decreased with the electrode modification the production of hydrogen peroxide increases since this does not decompose quantitatively on the nanoparticle surface

With respect to the ORR kinetics this has been quantified by fitting the variation of the peak current and peak potential with the sticking time (and so with the nanoparticle coverage) with the kinetic para-meters of step (5) as adjustable variables The results (shown in Table 1) show a very significant decrease of the cathodic transfer coefficient when moving from the silver macroelectrode to citrate-capped silver nanoparticles For better comparison of the catalytic activity for ORR of the different materials investigated here the corresponding reduction rate constants of the rate-determining step (5) are plotted in Fig 10

0

(5)red 0 e

fE E F

RTk k (16)

As can be seen the ORR is slower on the nano-particles employed in this work than on bulk silver as a result of the dramatic decrease of the transfer coefficient This may be related to differences in the adsorptivity of reactants and intermediate species

Regarding the lack of hydrogen peroxide decom-position detected on the modified electrodes this

Figure 10 Reduction rate constant of ORR (Eq (16)) on glassy carbon macroelectrode (GC) silver macroelectrode (Ag) and silver nanoparticles supported on GC (AgNPGC) calculated from the kinetic parameters obtained experimentally (Table 1) and 0E f

298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524

is connected to the ldquo 2 2H O escaperdquo towards the bulk solution To study this phenomenon Fig 11 shows the variation of the apparent number of electrons per O2 molecule with hetdeck at different coverage The curves have been calculated from the peak current of the simulated voltammograms of the array of silver nanoparticles at 100 mVmiddotsndash1 with the hetEECC mech-anisms assigning the value eff 2n to the peak current when 1

hetdec 0 cm sk As can be observed for the range of coverage of the present study (le 0457) and

2 1hetdec 10 cm sk the value predicted is eff 2n

that is hydrogen peroxide is not further reduced to water This agrees with our experimental results where the peak height does not increase with the sticking time This behaviour is explained by the large particle interdistance (more than 30 times the nanoparticle radius) such that the contribution of radial diffusion is important Consequently unless the hetk value is very fast the electrogenerated 2 2H O diffuses away from the electrode very rapidly and it does not decompose on the silver surface When the nanoparticle coverage is increased the radial diffusion to individual nanoparticles turns into linear diffusion to the entire nanoparticle array and the ldquo 2 2H O escaperdquo is slowed down

Figure 11 Variation of the effective number of electrons trans-ferred per O2 molecule with the rate constant of the hydrogen peroxide heterogeneous decomposition (11) on a nanoparticle- modified glassy carbon macroelectrode at different coverage (values indicated on the graph) neff is calculated from the magnitude of the peak current of the cyclic voltammograms at 100 mVmiddotsndash1 simulated using the kinetics for glassy carbon macrosilver and nanosilver reported in Table 1

4 Conclusions

The kinetics of ORR on citrate-capped silver nano-particles (diameter = 18 nm) supported on macro glassy carbon (GC) have been investigated following a new experimental approach This is based on the immersion of the GC substrate in the air-saturated solution containing nanoparticles and it enables us to perform the modification of the substrate (with homo-geneous particle distribution and low agglomeration) and the voltammetric experiments simultaneously

The voltammetry of oxygen reduction on the glassy carbon macroelectrode could be described satisfactorily with a two-proton two-electron process yielding hydrogen peroxide On the silver macroelectrode hydrogen peroxide has been found to decompose on the surface such that a significant amount is further reduced to water The application of this mechanism to the quantitative study of the ORR response on silver nanoparticles supported on GC at low coverage (0035ndash0457) points out a significant change in the kinetics on the nanoparticles such that quantitative ORR occurs at larger overpotentials Moreover for such low coverage hydrogen peroxide is not further reduced to water due to fast diffusion away from the electrode surface

Acknowledgements

E L thanks the Fundacioacuten Seneca de la Region de Murcia (Spain) for financial support

Electronic Supplementary Material Supplementary material about the modeling of the cyclic voltam-metry of oxygen reduction at the different electrodes is available in the online version of this article at httpdxdoiorg101007s12274-013-0328-4

References

[1] Jiang S Win K Y Liu S H Teng C P Zheng Y G

Han M Y Surface-functionalized nanoparticles for bio-

sensing and imaging-guided therapeutics Nanoscale 2013

5 3127ndash3148

[2] Santos A Kumeria T Losic D Nanoporous anodic

aluminum oxide for chemical sensing and biosensors Trac-

Trends Anal Chem 2013 44 25ndash38


523Nano Res 2013 6(7) 511ndash524

[3] Campbell F W Compton R G The use of nanoparticles

in electroanalysis An updated review Anal Bioanal Chem

2010 396 241ndash259

[4] Majeed K Jawaid M Hassan A Abu Bakar A Abdul

Khalil H P S Salema A A Inuwa I Potential materials

for food packaging from nanoclaynatural fibres filled hybrid

composites Mater Des 2013 46 391ndash410

[5] Zhang L Zhang J J Wilkinson D P Wang H J

Progress in preparation of non-noble electrocatalysts for PEM

fuel cell reactions J Power Sources 2006 156 171ndash182

[6] Tammeveski L Erikson H Sarapuu A Kozlova J

Ritslaid P Sammelselg V Tammeveski K Electrocatalytic

oxygen reduction on silver nanoparticlemulti-walled carbon

nanotube modified glassy carbon electrodes in alkaline

solution Electrochem Commun 2012 20 15ndash18

[7] Han J J Li N Zhang T Y AgC nanoparticles as an

cathode catalyst for a zinc-air battery with a flowing alkaline

electrolyte J Power Sources 2009 193 885ndash889

[8] Yeager E Electrocatalysts for O2 reduction Electrochim

Acta 1984 29 1527ndash1537

[9] Lim D H Wilcox J Mechanisms of the oxygen reduction

reaction on defective graphene-supported Pt nanoparticles from

first-principles J Phys Chem C 2012 116 3653ndash3660

[10] Yashtulov N A Revina A A Flid V R The mechanism

of oxygen catalytic reduction in the presence of platinum and

silver nanoparticles Russ Chem Bull 2010 59 1488ndash1494

[11] Spendelow J S Wieckowski A Electrocatalysis of oxygen

reduction and small alcohol oxidation in alkaline media

Phys Chem Chem Phys 2007 9 2654ndash2675

[12] Zhang J Mo Y Vukmirovic M B Klie R Sasaki K

Adzic R R Platinum monolayer electrocatalysts for O2

reduction Pt monolayer on Pd(111) and on carbon-supported

Pd nanoparticles J Phys Chem B 2004 108 10955ndash10964

[13] Appleby A J Oxygen reduction and corrosion kinetics

on phase-oxide-free palladium and silver electrodes as a

function of temperature in 85 orthophosphoric acid J

Electrochem Soc 1970 117 1373ndash1378

[14] Saacutenchez-Saacutenchez C M Bard A J Hydrogen peroxide

production in the oxygen reduction reaction at different

electrocatalysts as quantified by scanning electrochemical

microscopy Anal Chem 2009 81 8094ndash8100

[15] Horrocks B R Schmidtke D Heller A Bard A J

Scanning electrochemical microscopy 24 Enzyme ultramicro-

electrodes for the measurement of hydrogen peroxide at

surfaces Anal Chem 1993 65 3605ndash3614

[16] Chatenet M Genies-Bultel L Aurousseau M Durand R

Andolfatto F Oxygen reduction on silver catalysts in solutions

containing various concentrations of sodium hydroxidemdash

comparison with platinum J Appl Electrochem 2002 32

1131ndash1140

[17] Adanuvor P K White R E Oxygen reduction on silver

in 65M caustic soda solution J Electrochem Soc 1988

135 2509ndash2517

[18] Fuller T Gasteiger H A Cleghorn S Ramani V Zhao

T Nguyen T V Haug A Bock C Lamy C Ota K

Proton Exchange Membrane Fuel Cells 7 The Electroche-

mical Society Pennington 2007

[19] Sethuraman V A Weidner J W Haug A T Pemberton

M Protsailo L V Importance of catalyst stability vis-agrave-vis

hydrogen peroxide formation rates in PEM fuel cell electrodes

Electrochim Acta 2009 54 5571ndash5582

[20] Seidel Y E Schneider A Jusys Z Wickman B

Kasemo B Behm R J Mesoscopic mass transport effects

in electrocatalytic processes Faraday Discuss 2009 140

167ndash184

[21] Ruvinskiy P S Bonnefont A Pham-Huu C Savinova

E R Using ordered carbon nanomaterials for shedding light

on the mechanism of the cathodic oxygen reduction reaction

Langmuir 2011 27 9018ndash9027

[22] Zhang Y R Asahina S Yoshihara S Shirakashi T

Oxygen reduction on Au nanoparticle deposited boron-doped

diamond films Electrochim Acta 2003 48 741ndash747

[23] Uchida H Yano H Wakisaka M Watanabe M

Electrocatalysis of the oxygen reduction reaction at Pt and

Pt-alloys Electrochemistry 2011 79 303ndash311

[24] Jiang L Hsu A Chu D Chen R Size-dependent activity

of palladium nanoparticles for oxygen electroreduction in

alkaline solutions J Electrochem Soc 2009 156 B643ndash

B649

[25] Chen S L Kucernak A Electrocatalysis under conditions

of high mass transport rate Oxygen reduction on single

submicrometer-sized Pt particles supported on carbon J

Phys Chem B 2004 108 3262ndash3276

[26] Antoine O Bultel Y Durand R Oxygen reduction

reaction kinetics and mechanism on platinum nanoparticles

inside Nafionreg J Electroanal Chem 2001 499 85ndash94

[27] Lim E J Choi S M Seo M H Kim Y Lee S Kim

W B Highly dispersed Ag nanoparticles on nanosheets of

reduced graphene oxide for oxygen reduction reaction in

alkaline media Electrochem Commun 2013 28 100ndash103

[28] Singh P Buttry D A Comparison of oxygen reduction

reaction at silver nanoparticles and polycrystalline silver

electrodes in alkaline solution J Phys Chem C 2012 116

10656ndash10663

[29] Garcia A C Gasparotto L H S Gomes J F Tremiliosi-

Filho G Straightforward synthesis of carbon-supported Ag

nanoparticles and their application for the oxygen reduction

reaction Electrocatal 2012 3 147ndash152

[30] Demarconnay L Coutanceau C Leacuteger J M Electro-

reduction of dioxygen (ORR) in alkaline medium on AgC


524 Nano Res 2013 6(7) 511ndash524

and PtC nanostructured catalystsmdasheffect of the presence of

methanol Electrochim Acta 2004 49 4513ndash4521

[31] Alia S M Duong K Liu T Jensen K Yan Y

Supportless silver nanowires as oxygen reduction reaction

catalysts for hydroxide-exchange membrane fuel cells

ChemSusChem 2012 5 1619ndash1624

[32] Toh H S Batchelor-McAuley C Tschulik K Uhlemann

M Crossley A Compton R G The anodic stripping

voltammetry of nanoparticles Electrochemical evidence for

the surface agglomeration of silver nanoparticles Nanoscale

in press DOI 101039C3NR00898C

[33] Davies T J Compton R G The cyclic and linear sweep

voltammetry of regular and random arrays of microdisc

electrodes Theory J Electroanal Chem 2005 585 63ndash82

[34] Ward K R Lawrence N S Hartshorne R S Compton

R G Cyclic voltammetry of the ECprime mechanism at

hemispherical particles and their arrays The split wave J

Phys Chem C 2011 115 11204ndash11215


R G The theory of cyclic voltammetry of electrochemically

heterogeneous surfaces Comparison of different models for

surface geometry and applications to highly ordered pyrolytic

graphite Phys Chem Chem Phys 2012 14 7264ndash7275

[36] Wang Y Ward K R Laborda E Salter C Crossley A

Jacobs R M J Compton R G A joint experimental and

computational search for authentic nano-electrocatalytic

effects Electrooxidation of nitrite and L-ascorbate on gold

nanoparticle-modified glassy carbon electrodes Small 2013

9 478ndash486

[37] Augustine R Rajarathinam K Synthesis and charac-

terization of silver nanoparticles and its immobilization on

alginate coated sutures for the prevention of surgical wound

infections and the in vitro release studies Int J Nano Dim

2012 2 205ndash212

[38] Zhou Y G Rees N V Compton R G Electrodendash

nanoparticle collisions The measurement of the sticking

coefficient of silver nanoparticles on a glassy carbon electrode

Chem Phys Lett 2011 514 291ndash293

[39] Ward Jones S E Campbell F W Baron R Xiao L

Compton R G Particle size and surface coverage effects in

the stripping voltammetry of silver nanoparticles Theory and

experiment J Phys Chem C 2008 112 17820ndash17827

[40] Divišek J Kastening B Electrochemical generation

and reactivity of the superoxide ion in aqueous solutions

J Electroanal Chem Interfacial Electrochem 1975 65

603ndash621

[41] Sawyer D T Electrochemistry for Chemists 2nd Ed

Wiley-Interscience New York 1995

[42] Bard A J Faulkner L R Electrochemical Methods

Fundamentals and Applications 2nd Ed John Wiley amp

Sons Inc New York 2001

[43] Compton R G Banks C E Understanding Voltammetry

2nd Ed World Scientific London 2011

[44] Millero F J Huang F Graham T B Solubility of oxygen

in some 1-1 2-1 1-2 and 2-2 electrolytes as a function of

concentration at 25 J Solution Chem 2003 32 473ndash487

[45] Han P Bartels D M Temperature dependence of oxygen

diffusion in H2O and D2O J Phys Chem 1996 100 5597ndash

5602

[46] Hitt D L Zakrzwski C M Thomas M A MEMS-based

satellite micropropulsion via catalyzed hydrogen peroxide

decomposition Smart Mater Struct 2001 10 1163ndash1175

[47] Goszner K Koumlrner D Hite R On the catalytic activity

of silver I activity poisoning and regeneration during the

decomposition of hydrogen peroxide J Catal 1972 25

245ndash253

[48] Fox M A Akaba R Curve crossing in the cyclic

voltammetric oxidation of 2-phenylnorbornene Evidence

for an ECE reaction pathway J Am Chem Soc 1983 105

3460ndash3463

[49] Merkulova N D Zhutaeva G V Shumilova N A

Bagotzky V S Reactions of hydrogen peroxide on a silver

electrode in alkaline solution Electrochim Acta 1973 18

169ndash174

[50] Amatore C Saveacuteant J M Tessier D Charge transfer

at partially blocked surfaces A model for the case of

microscopic active and inactive sites J Electroanal Chem

Interfacial Electrochem 1983 147 39ndash51

[51] Reller H Kirowa-Eisner F Gileadi E Ensembles of

microelectrodes A digital- simulation J Electroanal Chem


[52] Press W H Teukolsky S A Vetterling W T Flannery

B P Numerical Recipes The Art of Scientific Computing

3rd Ed Cambridge University Press Cambridge 2007

[53] Wang Y Laborda E Ward K R Compton R G Kinetic

study of oxygen reduction reaction on electrodeposited gold

nanoparticles of diameter 17 nm and 40 nm in 05 M sulfuric

acid Submitt 2013


512 Nano Res 2013 6(7) 511ndash524

efficiency-limiting process of fuel cells [5] Consequently much effort is made in the design of new cathodes and cell conditions to speed up the reaction and promote the full reduction of oxygen to water

The number of studies reported reflects not only the difficulty to drive the reduction of oxygen but also the intricate complexity of its mechanism The latter depends on the electrode material and electrolyte and it can involve different simultaneous pathways and intermediates Although the details of the mechanism are still not fully understood there are some features generally accepted for most electrode materials in aqueous solution Thus it has been proposed that oxygen (O2) can be reduced either by a direct path to water [6 7]

2 2 2O 4 H O 4e 2H O+ 4OH (1)

or a series 2+2 process in which there are two electrons transferred at a time with hydrogen peroxide (H2O2) as an intermediate[8ndash10]

2 2 2 2

2 2 2 2

O 2 H O 2e H O +2OHH O 2 H O 2e 2H O +2OH

(2)

The first electron transfer to form the superoxide ion (

2O ) is reported to be rate determining for a variety of metals such as platinum [11 12] palladium [11 12] and silver [13]

2 2O e O (3)

Moreover hydrogen peroxide has been detected as an ORR intermediate in aqueous solution for several electrode materials including silver [14] and carbon electrodes [15] This suggests that at least partially on these cathodes ORR follows a 2+2 mechanism where the H2O2 electrogenerated can either diffuse away from the electrode surface or be reduced to water Whether the latter goes via an electrochemical step (Eq (2)) or homogeneousheterogeneous disproportionation [16 17] (Eq (4)) is unclear

2 2 2 21H O O H O2

(4)

The production of hydrogen peroxide in fuel cells however is highly undesirable as this can degrade

the membranendashelectrode assembly [18 19] reduces the cathodic current and most importantly it makes the cell voltage drop limiting the overall electrode efficiency [20] Therefore more efficient energy devices build on favouring the full reduction of oxygen to water using many different metal nanoparticles in this context Thus previous studies have reported that after cathode modification with platinum gold and palladium nanoparticles [21ndash26] the ORR overpotential and H2O2 production decrease as the amount of nano-particles is increased This reflects the faster ORR kinetics on the metal against that on the substrate material but it does not necessarily imply that the kinetics on the nanoparticles is different from that of the bulk metal

In order to gain a better understanding the rigorous examination of the electrocatalytic activity of nano-particle arrays requires suitable simulations of the ORR mechanism and of the diffusion of species to and away from the electrode surface A kinetic study of this complex system will be outlined in this paper focusing on the ORR on silver nanoparticles in nearly neutral aqueous solution ( pH 58 ) Within the search for cheaper non-platinum catalysts a lot of attention has been drawn to silver [6 27 28] which shows relatively high catalytic activity and thermodynamic stability over a wide pH range [29ndash31] The catalysis of ORR at neutral pH is of particular interest in the development of microbial fuel cells (MFCs) that require more efficient catalysts in neutral media

An entirely new experimental approach is designed in this work that provides an easy way for simul-taneous modification of the electrode and the study of ORR voltammetry with different nanoparticle loads Thus the substrate (glassy carbon electrode GCE) is immersed in an air-saturated solution containing chemically-synthesized silver nanoparticles that collide with and stick to the surface Cyclic voltammograms in the ORR potential region are recorded at different times corresponding to different nanoparticle coverage The latter can be worked out afterwards from the charge associated to the exhaustive oxidation of the par-ticles at the end of the experiment This methodology also enables us to obtain a uniform distribution of nanoparticles low agglomeration and a number of different coverages more easily than would be the case


513Nano Res 2013 6(7) 511ndash524



2 Experimental

21 Chemicals











514 Nano Res 2013 6(7) 511ndash524











515Nano Res 2013 6(7) 511ndash524






0

0 2 2O e Ofk E



2 2 2 2HO H O H O OH (7)










0

2

2 2

O0O O

0

e 0fF E E

RT

x

cD k c x

x (8)



516 Nano Res 2013 6(7) 511ndash524


0

0 exp fE Fk

RT


2

bulkOc = 025 mM [44])




2

2

Obulk 1 2eff O0496p

FDI n FAc

RT (9)








517Nano Res 2013 6(7) 511ndash524



hetdis

2 2 Ag

2 2

H O 2OH


k

(10)


hetdec2 2 2 2Ag

1H O H O O2

k (11)


ldquo 21 O2






0SCE

0exp f FEk

RT


2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027


2

bulk

Oc (mM) 025




518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


513Nano Res 2013 6(7) 511ndash524



2 Experimental

21 Chemicals











514 Nano Res 2013 6(7) 511ndash524











515Nano Res 2013 6(7) 511ndash524






0

0 2 2O e Ofk E



2 2 2 2HO H O H O OH (7)










0

2

2 2

O0O O

0

e 0fF E E

RT

x

cD k c x

x (8)



516 Nano Res 2013 6(7) 511ndash524


0

0 exp fE Fk

RT


2

bulkOc = 025 mM [44])




2

2

Obulk 1 2eff O0496p

FDI n FAc

RT (9)








517Nano Res 2013 6(7) 511ndash524



hetdis

2 2 Ag

2 2

H O 2OH


k

(10)


hetdec2 2 2 2Ag

1H O H O O2

k (11)


ldquo 21 O2






0SCE

0exp f FEk

RT


2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027


2

bulk

Oc (mM) 025




518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


514 Nano Res 2013 6(7) 511ndash524











515Nano Res 2013 6(7) 511ndash524






0

0 2 2O e Ofk E



2 2 2 2HO H O H O OH (7)










0

2

2 2

O0O O

0

e 0fF E E

RT

x

cD k c x

x (8)



516 Nano Res 2013 6(7) 511ndash524


0

0 exp fE Fk

RT


2

bulkOc = 025 mM [44])




2

2

Obulk 1 2eff O0496p

FDI n FAc

RT (9)








517Nano Res 2013 6(7) 511ndash524



hetdis

2 2 Ag

2 2

H O 2OH


k

(10)


hetdec2 2 2 2Ag

1H O H O O2

k (11)


ldquo 21 O2






0SCE

0exp f FEk

RT


2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027


2

bulk

Oc (mM) 025




518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


515Nano Res 2013 6(7) 511ndash524






0

0 2 2O e Ofk E



2 2 2 2HO H O H O OH (7)










0

2

2 2

O0O O

0

e 0fF E E

RT

x

cD k c x

x (8)



516 Nano Res 2013 6(7) 511ndash524


0

0 exp fE Fk

RT


2

bulkOc = 025 mM [44])




2

2

Obulk 1 2eff O0496p

FDI n FAc

RT (9)








517Nano Res 2013 6(7) 511ndash524



hetdis

2 2 Ag

2 2

H O 2OH


k

(10)


hetdec2 2 2 2Ag

1H O H O O2

k (11)


ldquo 21 O2






0SCE

0exp f FEk

RT


2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027


2

bulk

Oc (mM) 025




518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


516 Nano Res 2013 6(7) 511ndash524


0

0 exp fE Fk

RT


2

bulkOc = 025 mM [44])




2

2

Obulk 1 2eff O0496p

FDI n FAc

RT (9)








517Nano Res 2013 6(7) 511ndash524



hetdis

2 2 Ag

2 2

H O 2OH


k

(10)


hetdec2 2 2 2Ag

1H O H O O2

k (11)


ldquo 21 O2






0SCE

0exp f FEk

RT


2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027


2

bulk

Oc (mM) 025




518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


517Nano Res 2013 6(7) 511ndash524



hetdis

2 2 Ag

2 2

H O 2OH


k

(10)


hetdec2 2 2 2Ag

1H O H O O2

k (11)


ldquo 21 O2






0SCE

0exp f FEk

RT


2838 10 69 6 a3452 10 27 3 b

1221 10

033 071 070 027


2

bulk

Oc (mM) 025




518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


518 Nano Res 2013 6(7) 511ndash524






2 110 cm s [17 49]





519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


519Nano Res 2013 6(7) 511ndash524





9 1C 199 10 C s sQ t t (12)




520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


520 Nano Res 2013 6(7) 511ndash524



3NP

NPr

43

F rQM

(13)


4 1696 10 s sN t t (14)





2

NPN rA

(15)





521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


521Nano Res 2013 6(7) 511ndash524




0

(5)red 0 e

fE E F

RTk k (16)




298 mV (vs SCE) [53]


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


522 Nano Res 2013 6(7) 511ndash524






4 Conclusions



Acknowledgements



References




5 3127ndash3148





523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


523Nano Res 2013 6(7) 511ndash524



2010 396 241ndash259

















Acta 1984 29 1527ndash1537






























1131ndash1140



135 2509ndash2517












167ndash184














B649















10656ndash10663








524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013


524 Nano Res 2013 6(7) 511ndash524


















Phys Chem C 2011 115 11204ndash11215











9 478ndash486





2012 2 205ndash212












603ndash621













5602







245ndash253




3460ndash3463




169ndash174














acid Submitt 2013

performance of silver nanoparticles in the catalysis of the oxygen … · 2019-05-02 ·...

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