platinum electro-dissolution in acidic media upon potential cycling

Platinum Electro-dissolution in Acidic Mediaupon Potential Cycling

Liyan Xing & M. Akhtar Hossain & Min Tian &

Diane Beauchemin & Kev T. Adjemian &

Gregory Jerkiewicz

Published online: 7 November 2013# Springer Science+Business Media New York 2013

Abstract Electro-dissolution of Pt(poly) electrodes isexamined under potential cycling conditions in relation tothe applied lower and upper potential limits (EL, EU), thepotential cycling range (ΔE ), the number of potential cycles(n ), the exposure time (t exposure), and the electrolytetemperature (T ); the amount of electro-dissolved Pt (mPt)present in the electrolyte is analyzed using inductivelycoupled plasma mass spectrometry. Monitoring the potentialof working and counter electrodes (EWE, ECE) reveals that inmany instances ECE is higher than EWE, indicating that asurface oxide also develops on CE and a considerable amountof electro-dissolved Pt originates fromCE. Thus, in the case ofresearch employing a two-compartment cell, the amount ofelectro-dissolved Pt corresponds to the species originatingfrom both WE and CE. The application of a three-compartment cell, with a Nafion membrane used to separatethe WE and CE compartments, allows the quantification ofelectro-dissolved Pt originating only from WE or CE. Theamount of electro-dissolved Pt is much greater in the two-compartment cell than in the three-compartment one, whenECE covers a broad potential range that includes the regions ofPt oxide formation and reduction; this provides clear evidencethat CE makes a major contribution to total amount of electro-dissolved Pt. The value of mPt depends on EL and EU thatdefine ΔE . In the case of 0.10≤ΔE ≤0.20 V, there is noelectro-dissolution of Pt; in the case of ΔE =0.30, there isslight electro-dissolution of Pt; and in the case of 0.40≤ΔE ≤

0.70 V, there is significant electro-dissolution of Pt. The valueofmPt increases with the magnitude ofΔE . The greatest valueof mPt is observed when ΔE =0.50 V and when ΔE coversthe potential range of Pt oxide formation and reduction, andthe region of interfacial place exchange (1.10≤E ≤1.20 V).Temperature variation has a slight impact on the electro-dissolution of Pt and for given ΔE and n the increase inT slightly decreases mPt. An analysis of the impact of s onthe electro-dissolution of Pt reveals that the process is onlyslightly greater at s =25 mV s–1 than at s =50, 100, 200, or500mV s–1. For a given t exposure, the value ofmPt is greater fors =500 mV s–1 than for lower values of s because a high valueof s translates into a larger number of oxide formation-reduction events. The quantity of electro-dissolved Pt within5,000 potential cycles varies from 0.12 to 4.91 monolayers(MLs) of Pt, depending on EL, EU, andΔE . Eleven reactionscan be used to explain anodic and cathodic electro-dissolutionas well as chemical dissolution of Pt. Yet, it is impossible toexplain the cathodic dissolution of Pt without making anarbitrary assumption that anodic polarization of Pt in the0.85≤E ≤1.40 V range generates PtO2, instead of PtO asreported in earlier literature.

Keywords Platinum electrocatalysts . Platinum oxide .

Platinum electro-dissolution . Potential cycling . Inductivelycoupled plasmamass spectrometry

Introduction

Polymer electrolyte membrane fuel cells (PEMFCs) utilizinghydrogen gas as a fuel attract a lot of interest due to theirability to generate electrical energy without producing carbondioxide. The key electrochemical reactions, hydrogenoxidation reaction (HOR) and oxygen reduction reaction(ORR), take place at platinum nanoparticle (Pt-NP)

L. Xing :M. A. Hossain :M. Tian :D. Beauchemin :G. Jerkiewicz (*)Department of Chemistry, Queen’s University, 90 Bader Lane,Kingston, ON K7L 3N6, Canadae-mail: [email protected]

K. T. AdjemianNissan Technical Center North America, 39001 Sunrise Drive,Farmington Hills, MI 48331, USA

Electrocatalysis (2014) 5:96–112DOI 10.1007/s12678-013-0167-9

electrocatalysts. Platinum is an excellent electrocatalyst forPEMFCs due to its mechanical stability, corrosion resistancein aqueous media of various pH values, ability to dissociateand chemisorb H2(g) and O2(g), and unique electronicproperties that facilitate the adsorption and surface diffusionof reaction intermediates.

Membrane electrode assemblies (MEAs) made fromNafion, carbon black, and Pt-NPs are the most expensivecomponent of fuel cell stacks, and Pt-NPs account for amajority of their cost. Polymer electrolyte membrane fuel cellscan become a viable means of generating electrical energy,provided that their cost is reduced and long-term performanceincreased. The cost reduction can be accomplished bydecreasing the Pt loading through the use of particles thatare a few nanometers in size, increasing the dispersion andminimizing the agglomeration of Pt-NPs, and incorporatingless-expensive catalytic metals, such as Ni, Fe, Co, Pd, etc.[1–4]. The lifetime of MEAs is strongly related to undesiredreactions, such as Pt oxide formation, Pt dissolution, andcarbon support corrosion through oxidation reactions.

The oxidation and dissolution of Pt-NPs are key phenomenaresponsible for their degradation and irreversible material loss.Clearly, as the amount of Pt at which HOR and ORR take placedecreases, the power that PEMFCs can deliver becomes reduced.Reduction in the size of Pt electrocatalysts improves theirutilization but at the same time increases their surface Gibbsenergy (surface tension); the latter gives rise to greater reactivitytowards most electrochemical reactions, including surfaceoxidation and dissolution. Thus, the Pt-NP size, reactivity, andstability are closely interrelated. In order to enhance thedurability and to extend the lifetime of MEAs, it is vital tounderstand the phenomena responsible for their degradation[5–9]. In the case of bulk, polycrystalline Pt (abbreviated asPt(poly)) in an aqueous H2SO4 or other acidic solutions, itssurface electro-oxidation commences at a potential ofE=0.85 V and the higher the applied potential and the longerthe oxidation time, the thicker the surface oxide [10]. Theapplication of E ≤1.7 V results in oxide layers comprising onlyone species, namely PtO, while the application of E ≥1.8 Vresults in oxide layers containing PtO and PtO2. In a typicalexperiment in which an oxide layer is formed underpotentiostatic conditions (E =const) and then reduced usingcyclic voltammetry (CV), the reduction of PtO gives rise to acathodic peak in the 0.6–1.1Vrange and the reduction of PtO2 tothree cathodic peaks in the 0.0–0.6 V range; the PtO reductionpeak shifts towards lower potentials and grows in size as theoxide charge density increases [11]. The species PtO grows to alimiting thickness of twomonolayers (MLs) and afterwards PtO2

starts to develop on top of PtO without reaching any limitingthickness. In addition, PtO is reduced at higher potentials thanPtO2 resulting in Pt covered with a layer of PtO2 [12]. The latterbehavior is surprising and intriguing because one would expectPt4+ to be reduced to Pt2+ prior to its reduction to Pt0.

In an aqueous acidic solution having an activity of H+ equalto one (a Hþð Þ ¼ 1:00), ORR commences at E =1.229 Vandits current density (j ) increases as E decreases; thus, thepotential ranges of PtO formation and reduction overlap thepotential range of ORR. Consequently, in the case of Pt(poly)covered with a layer of PtO, the reduction of PtO to Pt canoccur concurrently with ORR affecting the reactionmechanism and kinetics. Therefore, it is of vital importanceto gain an insight into the behavior of Pt in this potential rangein order to identify and quantify phenomena responsible for Ptdegradation and loss in PEMFCs.

Here, we report on a systematic study of Pt electro-dissolution in 0.5 M aqueous H2SO4 solution upon potentialcycling. The process is examined in relation to the appliedpotential limits, the potential scan range, the number ofpotential cycles, the exposure time, and the electrolytetemperature. The amount of electro-dissolved Pt present inthe electrolyte is analyzed using inductively coupled plasmamass spectrometry.

Experimental

Electrodes and Electrochemical Cells

Cyclic-voltammetry measurements were conducted using aBio-Logic potentiostat (model SP 150) controlled byproprietary Potentiostat/Galvanostat/EIS-Monitoring Software(Bio-Logic). The working (WE) and counter (CE) electrodeswere made of Pt foil; they were spot-welded to Pt wires thatwere sealed in glass tubes. The geometric dimensions of WEand CE employed using a two-compartment cell were1.00 cm×1.00 cm and using a three-compartment cell1.55 cm×1.65 cm; their respective geometric surface areaswere the same to within 2–3 %. The electrochemically activesurface areas (Aecsa) of WE and CE were determined on thebasis of the under-potential deposition of hydrogen chargedensity [13, 14]. They were Aecsa=3.45±0.07 cm2 in the caseof the two-compartment cell and Aecsa=8.08±0.23 cm2 in thecase of the three-compartment cell. The reference electrode(RE) was a Pt mesh/Pt black electrode placed in a separatecompartment through which H2(g) (99.999 % in purity,Praxair) was bubbled. It was immersed in the same electrolyteas that in the WE compartment and served as a reversiblehydrogen electrode (RHE); all potentials are given with respectto RHE. Two types of electrochemical cells were used for thepotential-cycling experiments: a two-compartment cell and athree-compartment cell. In the two-compartment cell, WE andCE were placed in one compartment, while RE was placed in aseparate compartment that was connected to the maincompartment using a Luggin capillary. The opening of theLuggin capillary was reduced during potential cycling to

Electrocatalysis (2014) 5:96–112 97

minimize the migration of Pt ions to the RE compartment. Inthe three-compartment cell, WE and CE were placed in twodifferent compartments electrolytically connected by a meansof Nafion 211 membrane. For temperature-dependent studies,the WE and CE compartments were equipped with waterjackets that were used to circulate water at a predeterminedtemperature (T ), namely T =293, 313, and 333 K. Theelectrolyte temperature in each compartment was monitoredusing a thermometer or a thermocouple; it agreed with thepredetermined temperature to ±0.5 K. The RE assembly wasthe same as in the case of the two-compartment cell. Because Ptcan undergo electro-dissolution from both WE and CE, theapplication of three-compartment cell allowed determiningthe amount of electro-dissolved Pt originating solely fromWE (see “Results and Discussion”).

Cleaning Procedures and Electrolyte Solutions

The two-compartment and three-compartment cells werecleaned using well-established and widely acceptedprocedures [15]. After each experiment, the cells were rinsedwith de-ionized water and soaked in a mixture of concentratedHNO3 and concentrated H2SO4 (1:1 volume ratio) for over24 h. Prior to use, the acid mixture was drained and the cellswere rinsed repetitively with ample amounts of ultra-highpurity water (18.2 MΩ cm; MilliPore, Milli-Q3). Each freshlyprepared electrode was cleaned by the following steps: (1)degreasing in acetone under refluxed for 3 h, (2) rinsing withultra-high purity water, (3) soaking in a mixture ofconcentrated HNO3 and concentrated H2SO4 (1:1 volumeratio) for 3 h, (4) rinsing with ultra-high purity water, (5)electrochemical annealing by repetitive potential cycling in0.5 M aqueous H2SO4 in the 0.05–1.40 V potential range tomake sure that a CV profile characteristic of a clean systemwas obtained [11, 16], and (6) storing in concentrated H2SO4.Prior to use, the electrodes were rinsed with ultra-high puritywater and subsequently with electrolyte solution. Theelectrolyte solution (0.5 M aqueous H2SO4) was made fromFluke TraceSelect H2SO4 and ultra-high purity water. Nafion211 membrane (2.5 cm×2.5 cm) was soaked in the electrolytefor over 24 h prior to use; a newNafionmembranewas used ineach experiment. The standard solutions for ICP-MSmeasurement were prepared by dilution of Pt stock solution(10,000 μg/mL in 10 % HCl; SCP Science) in 0.1 M aqueousH2SO4 [17].

Potential Cycling Experiments and Electrolyte SolutionCollection for Analysis

After assembling the experimental setup, the WE and CEcompartments were purged with ultra-high purity Ar(g)(99.999 % in purity, Praxair) for ca. 30 min; the electrolytesolution in the WE compartment was agitated using a

magnetic stirrer; ultra-high purity H2(g) (99.999 % in purity;Praxair) was bubbled through the RE compartment.Afterwards, WE was cycled 10–20 times in the 0.05–1.40 Vpotential range at a potential scan rate of s =50 mV s–1. Then,an aliquot of ca. 1.0 mL of electrolyte was withdrawn from theWE compartment (see “Results and Discussion”); it served asa blank. Potential cycling at s =50 mV s–1 was startedimmediately afterwards; the electrolyte in the WEcompartment was continuously agitated using a magneticstirrer. A separate series of experiments was conducted inorder to evaluate the impact of the potential scan rate on Ptelectro-dissolution; in this case, the experimental work wasconducted at five different values of s , namely s =25, 50, 100,200, and 500 mV s–1. An aliquot of ca. 1.0 mL of electrolytewas taken at the following cycles (the exact cycle number mayvary slightly in some experiments): 20th, 50th, 100th, 200th,500th, 1,000th, 2,000th, 3,000th, 4,000th, and 5,000th. Theelectrolyte solution volume was maintained constant byadding ca. 1.0 mL of fresh electrolyte after each sampling.The electrolyte samples were carefully weighed andaccurately diluted fivefold prior to submission for ICP-MSmeasurements [17].

ICP-MS Analysis

Inductively coupled plasma mass spectrometry analysis wasperformed using a Varian 820-MS instrument with a detectionlimit of 10 ppt. External calibration was used to determine thePt concentration in the electrolyte samples. A blank solutionof 0.1 M aqueous H2SO4 was measured followed by thestandard solutions with ascending Pt concentration. Thesample introduction system was then flushed with the blanksolution until the counts returned to the level comparable tothat prior to measurements. The samples were then analyzedin the sequence of collection [17].

Data Treatment and Uncertainty Analysis

A calibration curve was generated using the data for thestandard solutions by linear regression in order to correlatethe counts to the concentration of Pt in the sample solutions.The final concentrations were obtained by taking into accountthe dilution factor. The total mass of electro-dissolved Pt wasdetermined by multiplying by the volume of the electrolyte.The mass of electro-dissolved Pt originating from the Ptelectrodes is expressed per 1.00 cm2 of A ecsa. Theexperimental uncertainty of the amount of electro-dissolvedPt is ca. 5 %. The total uncertainty is the sum of severalexperimental uncertainties related to (1) the preparation ofsquare- and rectangle-shaped Pt(poly) electrodes out of Ptsheet that serve as WE and CE (it is practically impossibleto prepare two electrodes having exactly the samedimensions), (2) the determination of A ecsa of WE and CE,

98 Electrocatalysis (2014) 5:96–112

(3) the withdrawal of 1.0 mL electrolyte aliquots for ICP-MSmeasurements and their subsequent tenfold dissolution, (4)the calibration of ICP-MS, and (5) the determination ofmPt onthe basis of ICP-MS measurements.

Results and Discussion

General Remarks

The lower and upper potential limits (EL, EU) in the cyclingexperiments define the size of the potential range (ΔE) and itslocation on the potential scale in relation to the onset of Ptoxide formation and reduction as well as the overpotential (η )of ORR. Figure 1 presents a set of 30 potential rangesemployed in the course of research. The bottom, left cornercell refers to the widest potential range, namely,ΔE =0.70 V,and the subsequent diagonal color-coded cells to ΔEgradually decreasing to 0.10 V with an interval of 0.10 V.The 0.60–0.95 V range represents typical potential variationsof a PEMFC cathode under operating conditions. The 0.05–1.20Vrange covers the oxide formation and reduction regionsemployed in the course of research and the region of theunder-potential deposition of H (UPD H); the cyclic-voltammetry features in the UPD H region are used to (1)examine the electrode cleanliness, (2) determine A ecsa, and (3)examine if the initial polycrystalline surface undergoes anystructural changes to a preferentially oriented surface. BecauseORR commences at E =1.229 V and because the interfacialplace exchange accompanying the oxide formation on Pt(poly)occurs in the 1.10–1.20 V range, potential cycling in the 0.05–1.20 V region is very important because it sheds light on theimpact of the interfacial place exchange on the electro-dissolution of Pt [10]. The amount of electro-dissolved Ptdepends not only on the potential limits but also on otherexperimental variables and conditions; thus, we examined theamount of electro-dissolved Pt as a function of the followingparameters: (1) the potential limits (EL, EU), (2) the potential

range (ΔE) that is related to EL and EU via ΔE =EU–EL, (3)the number of potential cycles (n), (4) the potential scan rate (s),(5) the exposure time (texposure) that is related to the scan rate,and (6) the experimental temperature (T). It is important to addthat the experiments were conducted under ideal conditions,meaning that Ar(g) was bubbled through the WE and CEcompartments, WE and CE were made of high-purity bulk Pt,and there was no carbon support. However, this contributionpossibly presents the most comprehensive and systematic studyof Pt electro-dissolution as a function of the above-listedparameters. It is important to investigate the process under idealconditions prior to increasing the complexity of experimentalsetup or conditions. As explained in the “Experimental” section,in the course of research we employed two experimental setups,30 potential ranges, typically 5,000 potential cycles perexperiment, collected 11 analytes per experiment, and carriedexperiments at three temperatures and at five potential scanrates. If all combinations of the experimental parameters wereto be employed, then we would have to conduct 900experiments and collect 4,500,000 CV profiles. However, byrecognizing trends and identifying conditions that resulted in noor negligible Pt electro-dissolution, we were able to narrowdown the number of most important experiments to 120 andcollected ca. 600,000 CV profiles.

Impact of the Potential Window Sizeon the Electro-dissolution of Pt and the Variation of ECE

during Potential Cycling Experiments

Table 1 presents the amount of electro-dissolved Pt (mPt, ng cm–2)

upon potential cycling in 0.5 M aqueous H2SO4 at s=50 mV s–1

and T = 293 K in two-compartment and three-compartment cells for different values of EL, EU and ΔEafter n =1,000, 2,000, and 5,000 potential cycles. The resultsare color-coded to indicate no or negligible (mPt≤100 ng cm–2;black print), slight (100<mPt<500 ng cm

–2; mauve print), andsignificant (mPt≥500 ng cm–2; red print) electro-dissolution ofPt. Initially, we employed a two-compartment cell and as theexperimental work progressed, we started monitoring thepotential of counter electrode (ECE) using a digital multimeter;the ECE values reported in Table 1 are not corrected for the IRdrop and the average experimental uncertainty is ca. 0.020 V.These measurements indicated that as the potential of workingelectrode (EWE) reached its low range, ECE adopted thepotential range corresponding to Pt oxide formation andreduction; in other words, WE and CE experienced differentpolarization conditions. The ECE values reported in Table 1show that in many instances ECE was found to be even higherthan EWE suggesting that a surface oxide also developed on CEand a substantial amount of electro-dissolved Pt could originatefrom CE. Therefore, we performed a new set of experimentsand employed a three-compartment cell with the objective to(1) quantify the amount of electro-dissolved Pt originating from

Potential Cycling Range (EL-EU) in V vs. RHE

0.05-1.200.60-0.95

0.50-0.600.50-0.70 0.60-0.700.50-0.80 0.60-0.80 0.70-0.800.50-0.90 0.60-0.90 0.70-0.90 0.80-0.900.50-1.00 0.60-1.00 0.70-1.00 0.80-1.00 0.90-1.000.50-1.10 0.60-1.10 0.70-1.10 0.80-1.10 0.90-1.10 1.00-1.100.50-1.20 0.60-1.20 0.70-1.20 0.80-1.20 0.90-1.20 1.00-1.20 1.10-1.20

Potential Cycling Range (ΔE) in V vs. RHE0.70 0.60 0.50 0.40 0.30 0.20 0.10

Fig. 1 Thirty potential cycling ranges showing the lower and upperpotential limits (EL, EU) employed in potential cycling experiments.Diagonal, color-coded cells refer to the same value ofΔE . The potentialscale refers to RHE


Table 1 Amount of electro-dissolved Pt (mPt) (ng cm–2) upon potential cycling in 0.5 M aqueous H2SO4 at s =50 mV s–1 and T=293 K in two-compartment and three-compartment cells for different EL, EU, and ΔE values

Two-Compartment Cell Three-Compartment Cell

EWE (V) 0.002 V

ECE (V) 0.020 V

mPt (ng cm–2) ECE (V)0.020 V

mPt (ng cm–2)

1000 2000 5000 1000 2000 5000

E = 0.10 V

0.50-0.60 1.65-1.70 -25 1.2 -45 2.2 -76 3.8

0.60-0.70 1.30-1.65 1.50.08

-11.70.58

-29 1.4

0.70-0.80 0.87-1.00 -1.40.07

-0.40.02

2.60.13

0.80-0.90 1.42-1.45 1.9 0.09 -1.50.08

-5.30.26

0.90-1.00 0.85-0.89 -1.80.09

-3.90.20

-6.50.32

1.00-1.10 0.84-0.95 2.1 0.10 -3.60.18

-4.90.24

1.10-1.20 0.80-0.94 -0.80.04

-3.90.20

-3.90.20

E = 0.20 V

0.50-0.70 1.56-1.85 9.4 0.47 -25 1.2 -80 4.1 1.53-1.68 9.70.48

9.80.49

24 1.2

0.60-0.80 1.55-1.68 0.6 0.03 -24 1.2-193

9.6 1.42-1.62 30 1.5 24 1.2 33 1.6

0.70-0.90 1.40-1.60 10.70.54

2.0 0.10 -11.9

0.601.33-1.56 -1.8

0.09-3.40.17

5.60.28

0.80-1.00 1.0-1.50 -2.80.14

-12.60.63

-28 1.4

0.90-1.10 0.77-0.87 10.40.52

12.50.62

15.30.76

1.00-1.20 0.70-0.82 -1.30.06

-3.60.18

-7.60.38

E = 0.30 V

0.50-0.80 1.60-1.70 15.80.80

24 1.2 28 1.4 1.29-1.60 1 0.05 18 0.90 50 2.5

0.60-0.90 1.40-1.60 -20 1.0 -30 1.5 -51 2.5 0.82-1.50 8.00.40

18.10.90

34 1.7

0.70-1.00 1.20-1.50 180 9.0 170 8.4 155 7.5 0.81-1.24 7.40.37

13.20.66

52 2.6

0.80-1.10 0.81-1.25 245 12.2 320 16.0 490 24 0.77-1.10 79 3.9 167 8.425512.8

0.90-1.20 0.81-1.03 47 2.4 57 2.9 54 2.7 98 4.9 135 6.8135

6.8


WE, (2) quantify the amount of electro-dissolved Pt originatingfrom CE, and (3) perform a comparative analysis of the resultsobtained using the two types of cells. In order to betterunderstand the variations of ECE in relation to EWE, wemonitored ECE in a two-compartment cell as a function of time(t), whileEWEwas scanned between two limits at s =50mV s–1

and T =293 K. Figure 2a through d presents E versust transients for WE (black lines) and CE (red lines) for fourpotential scan ranges: (1) 0.60–0.95 V (Fig. 2a), (2) 0.50–1.20 V (Fig. 2b), (3) 0.05–1.20 V (Fig. 2c), and (4) 0.05–1.40 V (Fig. 2d). An analysis of these potential transients leadsto the following observations: (1) the ECE versus t profiles arenon-linear and have different shapes in all four instances; (2)when EWE reaches the highest value, ECE adopts the lowestvalue and vice versa; (3) in the case of potential cycling in the0.05–1.20 and 0.05–1.40 V ranges, EWE and ECE cover thesame potential ranges; (4) in the case of potential cycling in the0.60–0.95 V range, ECE adopts values that are up to 0.05 V

lower than the values of EWE (the shape of ECE versus ttransient indicates that CE spends a longer time in the potentialregion of Pt oxide formation than WE); and (5) in the case ofpotential cycling in the 0.50–1.20 V range, ECE adopts valuesthat are 0.3–0.4 V lower than the values of EWE. Figure 3athrough d presents I versus E profiles for WE (they are regularCV profiles) and CE (they may not be referred to as CVprofiles because in the case of CE the value of ECE does notchange linearly with time) for four potential scan ranges ofWE: (1) 0.60–0.95 V (Fig. 3a), (2) 0.50–1.20 V (Fig. 3b),(3) 0.05–1.20 V (Fig. 3c), and (4) 0.05–1.40 V (Fig. 3d). TheCV profiles for WE are color-coded and refer to seven regions(regions I through VII); not all regions are observed in the Iversus E transients for WE and CE. Regions I through VIIcorrespond to the following surface electrochemicalphenomena taking place at WE: (1) region I, the anodicdesorption of HUPD; (2) region II, the anodic double-layercharging; (3) region III, the anodic Pt oxide formation; (4)

Table 1 (continued)

Two-Compartment Cell Three-Compartment Cell

EWE (V) 0.002 V

ECE (V) 0.020 V

mPt (ng cm–2) ECE (V)0.020 V

mPt (ng cm–2)

1000 2000 5000 1000 2000 5000

E = 0.35 V (0.95 – 0.60 V)

0.60-0.95 0.90-1.60 69 3.5 49 2.5 31 1.6 0.94-1.54 104 5.2 126 6.3158

7.9

E = 0.40 V

0.50-0.90 1.46-1.70 -18.00.90

-46 2.3 -73 3.7 0.90-1.55 -16.60.83

-17.50.80

-1.20.06

0.60-1.00 0.85-1.50 335 17 330 17 280 14 0.78-1.45 10.10.50

38 1.9 143

7.2

0.70-1.10 0.75-1.50 1240 62 2040

102 3960

198 0.76-1.47 121 6.0

25012.5

555 28

0.80-1.20 0.80-1.22 380 19 660 33 1890

94 0.80-1.22 565 28 980 49

1800 90

E = 0.50 V

0.50-1.00 1.00-1.55 240 12.0 224 11.020010.0

0.75-1.53 5.40.27

29 1.5 104

5.2

0.60-1.10 0.75-1.52 2470124

3920196

6620330

0.75-1.43 124 6.2 225 11.2

48024

0.70-1.20 0.75-1.52 3570 178

7160 360

12520625

0.72-1.47 635 331110

56 2180

109

E = 0.60 V

0.50-1.10 0.75-1.52 3020 150

4400220

6670334

0.73-1.48 77 3.8 159 7.9 320

16

0.60-1.20 0.75-1.50 4000200

6840 340

10800540

0.71-1.43 585 29 1135

57 2080

104

E = 0.70 V

0.50-1.20 0.75-1.54 4070 203

6440 322

9820491

0.71-1.48 537 27 780 39 990

50


region IV, the anodic-to-cathodic scan reversal; (5) region V,the cathodic Pt oxide reduction; (6) region VI, the cathodicdouble layer charging; and (7) region VII, the cathodicadsorption of HUPD. The same colors are employed to marksections of I versus E profile for CE that correspond to color-coded sections of CV profiles for WE. In the case of WE,the I versus E profiles (CV profiles) can be readilyconverted to I versus t profiles in order to determine thecharge (Q ) values associated with specific surfaceelectrochemical processes because the potential scan rateof WE is constant (sWE=∂EWE/∂t=const). However, in thecase of CE, the I versus E profiles cannot be easilyconverted to I versus t profiles in order to determine theQ values for specific surface electrochemical processesbecause the potential scan rate of CE is not constant(sCE=∂ECE/∂t ≠const). However, the conversion of Iversus ECE profiles into I versus t profiles can be

achieved using the experimentally determined ECE versust profiles (see Fig. 2). Then, the I versus t profiles for CEcan be integrated to determine the Q values for specificsurface electrochemical processes occurring at CE.

The results presented in Table 1 show that the amount ofelectro-dissolved Pt is much greater in the two-compartmentcell than in the three-compartment one, when ECE covers abroad potential range that includes the regions of Pt oxideformation and reduction. In this case, CE makes a majorcontribution to the experimentally measuredmPt and the resultsreported for the two-compartment cell substantiallyoverestimate the amount of electro-dissolved Pt originatingfrom WE (the amount of electro-dissolved Pt in the two-compartment cell is given in nanograms per square centimeterof WE because initially we were unaware of the fact that itcould also originate from CE). It is important to emphasize thatin the case of experimental work conducted using a two-

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EWE

ECE

Fig. 2 E versus t transients for Ptworking and counter electrodes(EWE in black and ECE in red) forfour potential scan ranges: a0.60–0.95 V, b 0.50–1.20 V, c0.05–1.20 V, and d 0.05–1.40 V.The potential cycling is carriedout in 0.5 M aqueous H2SO4 ats =50 mV s–1 and T=293 K

a b

dcWE

CE

WE

CE

WE

CE

CE

WE

Fig. 3 Cyclic-voltammetryprofiles for Pt working electrodeand I versus E profiles for Ptcounter electrode for four potentialscan ranges: a 0.60–0.95 V, b0.50–1.20 V, c 0.05–1.20 V, and d0.05–1.40 V. The potential cyclingis carried out in 0.5 M aqueousH2SO4 at s=50 mV s–1 andT=293 K. The CV profiles arecolor coded and refer up to sevenregions (region I through regionVII) corresponding to differentsurface electrochemicalphenomena


compartment cell, there is no obvious method of separatingcontributions to Pt electro-dissolution from WE and CE.Consequently, a three-compartment cell needs to be employed.

Table 1 shows some small, negative values of m Pt,especially for experiments in which ΔE was narrow, namely0.10 or 0.20 V; the negative values of mPt point to removal ofelectro-dissolved Pt from electrolyte. At first, this behaviorseems counterintuitive but can be explained bearing in mindthat prior to the commencement a new series of potentialcycling experiments WE was cycled several times between0.05 and 1.40 V to verify that the system was impurity-free;we refer to this step as cleanliness verification. Afterwards, analiquot of electrolyte solution was collected and the amount ofelectro-dissolved Pt was analyzed. The negative values ofmPt

indicate that the amount of electro-dissolved Pt after cycling ina narrow potential range was lower than that after the

cleanliness verification. The decrease inmPt could be assignedto the following phenomena: (1) Pt adsorption on the cellwalls, (2) Pt adsorption on or trapping in the Nafionmembrane, and (3) Pt electrodeposition [5, 7]. In order toverify which of the three phenomena was responsible for thedecrease in mPt, we performed two new experiments. In thefirst experiment, following the cleanliness verification, theelectrolyte was removed, the cell rinsed with high-puritywater, and then filled with fresh electrolyte solution for severalhours and analyzed for Pt content; the analysis revealed noelectro-dissolved Pt. In another experiment, following thecleanliness verification, the Nafion membrane was removed,rinsedwith high-purity water, soaked for several hours in freshelectrolyte solution, and then analyzed for Pt content; theanalysis also revealed no electro-dissolved Pt. On the basisof these experiments, we concluded that the decrease in mPt

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t(n

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0.7 V0.8 V0.9 V1.0 V1.1 V1.2 V

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0.7 V0.8 V0.9 V1.0 V1.1 V1.2 V

Fig. 5 TwomPt versus n and twomPt versus texposure graphs forEL=0.50 Vand several values ofEU, namely EU=0.70, 0.80, 0.90,1.00, 1.10, and 1.20 V. The uppergraphs show the entire set of mPt

versus n and mPt versus texposureplots; the bottom graphs focus onthe initial n =1,000 potentialcycles and texposure=500 min,respectively. The potentialcycling is carried out in 0.5 Maqueous H2SO4 at s=50 mV s–1

and T=293 K

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0100200300400500600700800900

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texposure(min)

Fig. 4 TwomPt versus n and twomPt versus texposure graphs forEU=1.20 Vand several values ofEL, namely EL=0.05, 0.50, 0.60,0.70, 0.80, 0.90, 1.00, and 1.10 V.The upper graphs show the entireset ofmPt versus n andmPt versustexposure plots; the bottom graphsfocus on the initial n =1,000potential cycles and the initialexposure time of texposure=500min,respectively. The potential cyclingis carried out in 0.5 M aqueousH2SO4 at s=50 mV s–1 andT=293 K


could be assigned to Pt electrodeposition during potentialcycling over a narrow potential range. Although we did notanalyze the kinetics of Pt electrodeposition because theelectro-dissolved Pt was present in several forms [17] andtheir concentrations were very low, it is reasonable to proposethat the amount of Pt that is electrodeposited depends on thepotential limits and solution stirring (hydrodynamic effect).

Impact of the Lower Potential Limit on the Electro-dissolutionof Pt

We analyzed the impact of EL variation with EU=const on theamount of electro-dissolved Pt as a function of the number ofpotential cycles (n ≤5,000) and the exposure time (t exposure);all experiments were conducted at s =50 mV s–1 andT =293 K. Figure 4 presents two mPt versus n and two mPt

versus t exposure graphs for EU=1.20 V and several values ofEL, namely EL=0.05, 0.50, 0.60, 0.70, 0.80, 0.90, 1.00, and1.10 V; the upper graphs show the entire set of mPt versus nand mPt versus t exposure plots, while the bottom graphs focuson the initial n =1,000 potential cycles and t exposure=500 min,respectively. The amount of electro-dissolved Pt is cumulativeand refers to the total amount detected in the electrolytesolution for a given n or t exposure. Potential cycling in the0.90–1.20 V range results in the smallest amount of electro-dissolved Pt and the m Pt versus n plot levels off after500 cycles. Further increase of n (or t exposure) had no impacton the amount of electro-dissolved Pt. In the case of EL=0.05,0.50, 0.60, 0.70, or 0.80 V, the electro-dissolution of Ptcontinues and the amount of Pt-containing species in theelectrolyte increases as n (or t exposure) is extended. However,the amount of electro-dissolved Pt is not the same in the caseof these five EL values. Potential cycling with EL=0.60, 0.70,or 0.80 V produces similar trends and similar amounts of

electro-dissolved Pt; potential cycling with EL=0.50 Vgenerates less and with EL=0.05 V the least of electro-dissolved Pt. Practically no significant electro-dissolution of Ptwas observed in the case of potential cycling in the 1.00–1.20and 1.10–1.20 V ranges (the results are not shown).

Impact of the Upper Potential Limit on the Electro-dissolutionof Pt

We analyzed the impact of EU variation with EL=const on theamount of electro-dissolved Pt as a function of the number ofpotential cycles (n ≤5,000) and the exposure time; allexperiments were conducted at s =50 mV s–1 and T =293 K.Figure 5 presents two mPt versus n and two mPt versust exposure graphs for EL=0.50 V and several values of EU,namely EU=0.70, 0.80, 0.90, 1.00, 1.10, and 1.20 V; theupper graphs show the entire set of mPt versus n and mPt

versus t exposure plots, while the bottom graphs focus on theinitial n =1,000 potential cycles and t exposure=500 min,respectively. The amount of electro-dissolved Pt is cumulativeand refer to the total amount detected in the electrolytesolution for a given n or t exposure. Potential cycling up toEU=0.70, 0.80, and 0.90 V results in negligible electro-dissolution of Pt, and up to EU=1.00 V in slight Pt electro-dissolution. For these four values of EU (0.70–1.00 V), furtherincrease of n (or texposure) had no major impact on the amountof electro-dissolved Pt. An increase of EU up to 1.10 and1.20 V leads to substantial Pt electro-dissolution; an increaseof EU from 1.10 to 1.20 V triples the amount of electro-dissolved Pt. Elsewhere [10], it was demonstrated that in thecase of Pt(poly) an interfacial place exchange between Ptsurface atoms (Ptsurf) and electro-adsorbed O species (Oads)takes place in the 1.10–1.20 V potential range. Thus, theconsiderable increase in the amount of electro-dissolved Pt

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Fig. 6 TwomPt versus n and twomPt versus texposure graphs forT=313K. The upper graphs showthe entire set of mPt versus n andmPt versus texposure plots; thebottom graphs focus on the initialn =1,000 potential cycles andtexposure=500 min, respectively.EU=1.20 Vand EL=0.50, 0.60,0.70, 0.80, and 0.90 V. Thepotential cycling is carried outin 0.5 M aqueous H2SO4 ats =50 mV s–1


as EU is raised from 1.10 to 1.20 V can be associated with theinterfacial place exchange. In the case of EU=1.10 and1.20 V, the electro-dissolution of Pt continues and theamount of Pt-containing species in the electrolyte increasesas n (or t exposure) is extended. Finally, practically noelectro-dissolution of Pt was observed in the case of potentialcycling in the 0.50–0.60 V range (the results are not shown).

Impact of Temperature on the Electro-dissolution of Pt

In a separate series of experiments, we examined the impact ofT variation (T =293, 313, and 333 K) on the electro-dissolution of Pt. On the basis of initial experiments conductedat T =293 K, we selected conditions that facilitated slight andsignificant electro-dissolution of Pt; thus, one value of

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Fig. 7 TwomPt versus n and twomPt versus texposure graphs forT=333K. The upper graphs showthe entire set of mPt versus n andmPt versus texposure plots; thebottom graphs focus on the initialn =1,000 potential cycles andtexposure=500 min, respectively.EU=1.20 Vand EL=0.50, 0.60,0.70, 0.80, and 0.90 V. Thepotential cycling is carried out in0.5 M aqueous H2SO4 ats =50 mV s–1

Table 2 Amount of electro-dissolved Pt (mPt) (ng cm–2) upon potential cycling in 0.5 M aqueous H2SO4 in three-compartment cell at s =50 mV s–1 for

different EL, EU, and ΔE values, and for T=293, 313, and 333 K

EWE (V) 0.002 V

Three-Compartment Cell T = 298 K



1000th 2000th 5000th 1000th 2000th 5000th 1000th 2000th 5000th

1.10-1.200.20.01

0.90.01

3.00.15

14.80.74

18.10.90

26.61.3

1.00-1.204.80.24

11.60.58

6.30.31

21.41.1

31.01.6 44 2.2

0.90-1.20984.9

1366.8

1376.8

462.3 59 2.9 57 2.8 70 3.5 79 3.9

1055.2

0.80-1.20565

28 980 491800

90 650

32 1150

58 1360

68 380

19606

30 900

45

0.70-1.20640

32 1110

56 2180

109 763

381260

63 1980

99 485

24 740

37 1120

56

0.60-1.20585

29 1135

57 2080

104 698

351200

60 1680

84 596

30 890

44 1210

60

0.50-1.20537

27 780

39 990

50 657

33 1085

54 1670

84 411

21 477

24 866

43

0.50-1.005.40.27

29.41.5

104 5.2

26.71.3

74.93.7

254 12.7

5.50.27

52.22.6

238 12

0.60-1.10124

6.2 225 11.2

481 24

20410 402 20

765 38

204 10.2

21911

217 11

0.50-1.1076.9

3.8 159

7.9 320

16 193

9.2 440 22 721

36 127

6.4 142

7.1 363

18

0.60-0.95 68.7

3.4 48.7

2.4 31 1.6 –6.2 0.31

–4.4 0.22

0.10 0.01

12.3 0.62

28.7 1.4 60 3.0


EU=1.20 V (a potential beyond the onset potential of theinterfacial place exchange) and five values of EL, namelyE L=0.50, 0.60, 0.70, 0.80, and 0.90 V (potentialscorresponding to different stages of PtO reduction); allexperiments were conducted at s =50 mV s–1 and thenumber of potential cycles was between n =5,000 and 9,000, depending on EL. Figure 6 presents two mPt versusn and two mPt versus t exposure graphs for T =313 K; theupper graphs show the entire set of mPt versus n and mPt

versus t exposure plots, while the bottom graphs focus on theinitial n =1,000 potential cycles and t exposure=500 min,respectively. Figure 7 presents analogous results forT =333 K. Table 2 lists representative values of mPt

for T =293, 313, and 333 K, selected E L and EU

values, and after n =1,000, 2,000, and 5,000 potentialcycles, respectively. They demonstrate that for a givenpotential range and a given number of potential cycles,the increase in T slightly decreases the amount ofelectro-dissolved Pt. In order to better understand theimpact of T variation on Pt electro-dissolution, we

recorded CV profiles in the 0.05≤E ≤1.40 V range atdifferent temperatures in the 298≤T ≤333 K range (25≤T ≤60 °C) with an interval of ΔT =5 K (Fig. 8). Theresults show that an increase of T increases the amount of PtOthat is formed but does not affect the onset potential of PtOformation. On the other hand, the increase of T shifts the oxidereduction peak towards higher potentials indicating that in thecase of elevated temperatures complete PtO reduction can beaccomplished at higher potentials than those required toreduce a similar amount of PtO at lower temperatures. Thefact that a smaller amount of electro-dissolved Pt is observedat elevated temperatures (for given EL, EU, and n values) andalso that a smaller amount of electro-dissolved Pt isobserved upon EL decrease (for given EU, T, and n values;see the section “Impact of the Lower Potential Limit on theElectro-dissolution of Pt”) suggests that as the amount ofsurface PtO that is reduced increases, the amount of electro-dissolved Pt decreases.

Impact of Potential Scan Rate on the Electro-dissolutionof Pt

In a separate series of experiments we examined the impact ofpotential scan rate, namely s =25, 50, 100, 200, and500 mV s–1, on the electro-dissolution of Pt upon cycling inthe 0.60≤E ≤1.20 V range at T =293 K. We selected thispotential range because the results presented in Table 1demonstrate that in the case of EL=0.60 V and EU=1.20 Vthe electro-dissolution of Pt is substantial. The number ofpotential cycles varied between n =4,000 for s =25 mV s–1

and n =18,000 for s =500 mV s–1. Figure 9 presents two mPt

versus n and twomPt versus t exposure graphs for the five valuesof potential scan rate; the upper graphs show the entire set ofmPt versus n and mPt versus t exposure plots, while the bottomgraphs focus on the initial n =2,000 potential cycles andt exposure=1,000 min, respectively. An analysis of the results

-0.20

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0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

j(m

A c

m-2

)

E (V vs. RHE)25°C

30°C

35°C

40°C

45°C

50°C

55°C

60°C

Fig. 8 CV profiles for Pt working electrode in the 0.05≤E ≤1.40 V rangeat different temperatures in the 298≤T ≤333 K range (25≤T ≤60 °C) withan interval of ΔT=5 K. The potential cycling is carried out in 0.5 Maqueous H2SO4 at s=50 mV s–1

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Fig. 9 TwomPt versus n and twomPt versus texposure graphs for thefive values of potential scan rate(s=25, 50, 100, 200, and500 mV s–1). The upper graphsshow the entire set ofmPt versus nand mPt versus texposure plots; thebottom graphs focus on the initialn =2,000 potential cycles andtexposure=1,000 min, respectively.The potential cycling is carriedout between EL=0.60 V andEU=1.20 V in 0.5 M aqueousH2SO4 at T=293 K


E = 1.00 – 0.70 = 0.30 V

E = 1.10 – 0.80 = 0.30 V E = 1.20 – 0.80 = 0.40 V

E = 1.00 – 0.60 = 0.40 V

E = 1.00 – 0.50 = 0.50 V

E = 1.20 – 0.70 = 0.50 V E = 1.20 – 0.50 = 0.70 V

E = 1.20 – 0.60 = 0.60 V

ΔΔ Δ

Δ Δ

ΔΔ

ΔΔ

Fig. 10 CVprofiles for Pt working electrode forΔE in the 0.30–0.70Vrange obtained in 0.5M aqueous H2SO4 at T=293K and s=50mV s–1. The CVprofile numbers are color-coded as indicated in the legend


indicates that for the same number of cycles, the electro-dissolution of Pt is only slightly greater at s =25 mV s–1 thanat higher scan rates; the difference in the amount of electro-dissolved Pt is small for s =50, 100, 200, and 500 mV s–1. Forthe same t exposure, the amount of electro-dissolved Pt isgreater for s =500 mV s–1 than for lower values ofpotential scan rate. This behavior is expected because fort exposure=const a high scan rate translates into a largernumber of oxide formation-reduction events that stimulatePt electro-dissolution.

Cyclic-Voltammetry Behavior during Potential CyclingExperiments

The results presented in Figs. 4, 5, 6, 7, and 9, and Tables 1and 2 clearly demonstrate that Pt undergoes electro-dissolution and that the extent of the process depends on severalexperimental parameters, namely EL, EU, n , T, and s . In orderto better understand the process and to identify trends, werecorded CV profiles for different ΔE =EU–EL ranges in0.5 M aqueous H2SO4 solution at s =50 mV s–1 and T =293 K. Figure 10 presents selected sets of CV profiles forΔE in the 0.30–0.70 V range; the CV profile numbers arecolor-coded. We do not show CV profiles for ΔE =0.10 and0.20 V because such potential cycling conditions resulted in noelectro-dissolution of Pt. In the case of ΔE =0.30 V, there isslight electro-dissolution of Pt and mPt reaches a valueof mPt=52 ng cm–2 after 5,000 potential cycles for EL=0.70 Vand EU=1.00 V, and a value of mPt=255 ng cm–2

for EL=0.80 V and EU=1.10 V (Table 1; the data for

the three-compartment cell). We assign the low degree ofelectro-dissolution of Pt to a small amount of Pt oxide andthe lack of interfacial place exchange. In the case of ΔE =0.40 V, there is slight electro-dissolution of Pt for EL=0.60and EU=1.00 Vand significant electro-dissolution of Pt forEL=0.80 and EU=1.20 V; in both instances, the values ofmPt gradually increase with n although at different rates(Table 1; the data for the three-compartment cell). Themuch higher values of mPt for EL=0.80 V and EU=1.20 V as compared to EL=0.60 V and EU=1.00 V areassigned to a large amount of Pt oxide and the interfacialplace exchange that occurs in the 1.10≤E ≤1.20 V range. Inthe case ofΔE =0.50V, there is slight electro-dissolution of Ptfor E L=0.50 and EU=1.00 V, and significant electro-dissolution of Pt for EL=0.70 and EU=1.20 V. In bothcases, the values of m Pt gradually increase with nalthough at different rates (Table 1; the data for the three-compartment cell). The much greater values of mPt for EL=0.70 V and EU=1.20 V as compared to EL=0.50 V and EU=1.00 V are assigned to a large amount of Pt oxide and theinterfacial place exchange. In the case ofΔE =0.60 V, there issignificant electro-dissolution of Pt and the values of mPt

gradually increase with n (Table 1; the data for the three-compartment cell). The values of mPt after 1,000, 2,000, and5,000 potential cycles are similar to those for ΔE =0.40 V(E L=0.80 V and E U=1.20 V) and for ΔE =0.50 V(EL=0.70 V and EU=1.20 V). The significant electro-dissolution of Pt is assigned to a large amount of Pt oxideand the interfacial place exchange. Finally, in the case ofΔE =0.70 V there is significant electro-dissolution of Ptand the values of mPt gradually increase with n (Table 1;the data for the three-compartment cell). Again, the significantelectro-dissolution of Pt is again assigned to a large amount ofPt oxide and the interfacial place exchange. However, thevalue of mPt after 5,000 potential cycles is about half of thosefor ΔE =0.40 V (EL=0.80 V and EU=1.20 V), ΔE =0.50 V(EL=0.70 V and EU=1.20 V), and ΔE =0.60 V (EL=0.60 Vand EU=1.20 V). This behavior suggests that some of theinitially electro-dissolved Pt undergoes electrodeposition.

The CV profiles presented in Fig. 10 for ΔE =0.30 and0.40 V show a substantial decrease in the current density (j )as the number of potential cycles rises. It is tempting tointerpret this behavior as passivity development. However,under these potential conditions the amount of PtO that isformed is very small. In addition, these are very longexperiments and the electrolyte solution is never impurity-free. Consequently, uncontrolled and poorly definedimpurities (present in trace amounts) adsorb on the Ptelectrode surface giving rise to the decrease of j andsuppression of CV features. This behavior is not observedin the case of higher ΔE values (and also higher EU valuesdespite ΔE being small) because the upper potential limit is

Table 3 Amount of electro-dissolved Pt (mPt) (ng cm–2), number ofelectro-dissolved Pt atoms (σPt) per square centimeter, number ofelectro-dissolved monolayers of Pt (nML,Pt) within 5,000 potential cycles,and percentage of one monolayer of Pt electro-dissolved per potentialcycle (pML,Pt); potential cycling in 0.5M aqueous H2SO4 at s=50mV s–1

and T=293 K in three-compartment cell for different EL, EU, and ΔEvalues

EWE (V)±0.002 V mPt (ng cm–2) σPt (cm–2) nML,Pt (–) pML,Pt (%)

0.70–1.00 52 1.60×1014 0.12 0.0023

0.90–1.20 135 4.17×1014 0.304 0.0061

0.60–0.95 155 4.78×1014 0.349 0.0070

0.80–1.10 255 7.87×1014 0.574 0.0115

0.50–1.10 320 9.88×1014 0.721 0.0144

0.60–1.10 480 14.8×1014 1.08 0.0216

0.70–1.10 555 17.1×1014 1.25 0.0250

0.50–1.20 990 30.6×1014 2.23 0.0447

0.80–1.20 1,800 55.5×1014 4.05 0.0810

0.60–1.20 2,080 64.2×1014 4.69 0.0937

0.70–1.20 2,180 67.3×1014 4.91 0.0982


high enough to oxidize adsorbed impurities (a small amountof the anodic charge goes into the oxidative desorption ofimpurities).

Quantity of Electro-dissolved Pt within 5,000 Potential Cyclesand per Potential Cycle

The results presented in Table 1 for the three-compartment cellfacilitate an examination of the quantity of electro-dissolvedPt within 5,000 potential cycles expressed as a number of Ptatoms per square centimeter of WE (σPt) and a number ofmonolayers of Pt (nML,Pt); they also allow an analysis ofelectro-dissolved Pt per potential cycle expressed aspercentage of one monolayer of Pt surface atoms (pML,Pt).Table 3 presentsmPt, σPt, nML,Pt, and pML,Pt values for severalEL, EU, andΔE values, and for s =50 mV s–1 and T =293 K.The determination of the number of monolayers of electro-dissolved Pt within 5,000 potential cycles and per potentialcycle is done bearing in mind that the surface density of atomsin polycrystalline Pt is σPt(poly)=1.37×10

15 cm–2. The resultspresented in Table 3 demonstrate that the quantity of electro-dissolved Pt within 5,000 potential cycles can be as little asnML,Pt=0.12 and as large as 4.91, while the amount of electro-dissolved Pt per potential cycle varies from pML,Pt=0.0023 to0.0982 % of one monolayer (ML) of Pt surface atoms.Although the quantities of electro-dissolved Pt seem rathersmall, the loss of five MLs of Pt from a spherical nanoparticlecan translate into a considerable decrease of its size (loss of 10MLs in an overall diameter). Here, we present results forbulk, polycrystalline Pt, while research on the electro-dissolution of Pt nanoparticles is under way. Due to thelarge contribution of surface Gibbs energy to the overallGibbs energy of Pt nanoparticles and the enhancedreactivity of Pt nanoparticles as compared to bulk Pt, it

is expected that Pt nanoparticles will undergo fasterelectro-dissolution than bulk Pt.

Mechanism of Electro-dissolution of Pt

Elsewhere [18], six possible electrochemical and chemicalreactions of Pt and PtO2 dissolution were proposed anddiscussed. Topalov et al. [19] provided very convincingevidence for the existence of both anodic and cathodicelectro-dissolution of Pt; they also observed that the amountof anodically dissolved Pt does not depend on EU, while theamount of cathodically dissolved Pt increases as EU isgradually raised towards higher values. Our results of ion-exchange chromatography coupled to inductively coupledplasma mass spectrometry measurements (IEC–ICP-MS)demonstrate that potential cycling results in both Pt2+(aq)and Pt4+(aq) species, with at least 80 % of electro-dissolvedPt being present as Pt2+ complexes [17]. On the basis ofexisting results in the literature and the above-presentedresults and discussion, we list the following possible pathwaysof electro-dissolution (anodic and cathodic) and chemicaldissolution of Pt, PtO, and PtO2 in aqueous acidic solutionin the absence of reactive gases (e.g., O2 or H2) andsubsequently discuss their feasibility.

AnodicElectro-dissolution of Pt to Pt2+(aq) with a reaction rate of

ν1:

Pt sð Þ→ν1 Pt2þ aqð Þþ2 e− ð1Þ

Electro-dissolution of Pt to Pt4+(aq) with a reaction rate ofν2:

Pt sð Þ→ν2 Pt4þ aqð Þþ 4e− ð2Þ

0.8 1.2 1.6E [VSHE]

0.85-1.60 VPtO formation

E 1.60 V PtO2 formation

E (H+,O2/H2O) = 1.229 V

0.50-1.10 V PtO reduction

E (H+/H2) = 0.000 V

E range of H2(g) oxidation

E range of O2(g) reduction

E range of H2(g) generation

E range of O2(g)generation

0.0 0.4

0.10-0.40 VPtO2 reduction

UPD H

anodic Pt /PtO electro-dissolution

E (Pt2+/ Pt) = 1.188 V

cathodic Pt/PtO electro-dissolution

AN Ads

o

o oFig. 11 Graphical representationof the main Faradaic and non-Faradaic reactions taking placeat Pt electrodes. The graphrefers to an aqueous acidicelectrolyte solution having anactivity of proton equal to1.00 (aðHþðaqÞ ¼ 1:00)


Electro-dissolution of PtO to Pt4+(aq) prior to the interfacialplace exchange with a reaction rate of ν3:

Ptδþ−Oδ−chem þ 2Hþ aqð Þ→ν3 Pt4þ aqð Þ þ H2O lð Þ þ 2e−

ð3ÞElectro-dissolution of PtO to Pt4+(aq) after the interfacial

place exchange with a reaction rate of ν4:

Pt2þ−O2−quasi�3D lattice þ 2Hþ aqð Þ→ν4 Pt4þ aqð Þ þ H2O lð Þ þ 2e−

ð4ÞCathodicElectro-dissolution of PtO to Pt2+(aq) prior to the interfacial


Ptδþ−Oδ−þ 4Hþ aqð Þ þ 2e−→ν5 Pt2þ aqð Þ þ H2O lð Þ þ H2 gð Þ

ð5ÞElectro-dissolution of PtO to Pt2+(aq) after the interfacial


Pt2þ−O2−quasi�3D lattice þ 4Hþ aqð Þ þ 2 e−→

ν6Pt2þ aqð Þ

þ H2O lð Þ þ H2 gð Þð6Þ

Electro-dissolution of PtO to Pt0(aq) with a reaction rate ofν7:

Pt2þ−O2−quasi�3D lattice þ 2Hþ aqð Þ þ 2e−→

ν7Pt0 aqð Þ þ H2O lð Þ

ð7ÞElectro-dissolution of PtO2 to Pt

2+(aq) with a reaction rateof ν8:

PtO2 þ 4Hþ aqð Þ þ 2 e−→ν8 Pt2þ aqð Þ þ 2H2O lð Þ ð8Þ

ChemicalDissolution of PtO to Pt2+(aq) prior to the interfacial place

exchange with a reaction rate of ν9:

Ptδþ−Oδ−chem þ 2Hþ aqð Þ→ν9 Pt2þ aqð Þ þ H2O lð Þ ð9Þ

Dissolution of PtO to Pt2+(aq) after the interfacial placeexchange with a reaction rate of ν10:

Pt2þ−O2−quasi�3D lattice þ 2Hþ aqð Þ→ν10

Pt2þ aqð Þ þ H2O lð Þ ð10Þ

Dissolution of PtO2 to Pt0 with a reaction rate of ν11:

PtO2 þ 4Hþ aqð Þ→ν11 Pt4þ aqð Þ þ 2H2O lð Þ ð11ÞRecent measurements of Topalov et al. [19] clearly point to

the existence of electro-dissolved Pt species in anodic andcathodic scans. The amount of electro-dissolved Pt in theanodic scan was reported to be relatively constant andindependent of the upper potential limit. On the other hand,

the amount of electro-dissolved Pt in the cathodic scan wasreported to increase with an increase of the upper potentiallimit. Our IEC–ICP-MS measurements demonstrate thatpotential cycling results in the formation of both Pt2+(aq)and Pt4+(aq) containing species [17]. Because Pt0 speciescannot be detected using ICP-MS, the applicability of thereaction 7 cannot be tested and an alternative experimentalapproach is required. Our results also indicate that the electro-dissolution of Pt is facilitated by the development of Pt oxideand the interfacial place exchange. Mitsushima et al. [20]reported on the existence direct chemical dissolution ofPtO2(s) but not of un-oxidized Pt. Thus, it is unlikely thatdirect anodic dissolution of un-oxidized Pt (reactions 1 and 2)takes place. To the best of our knowledge, potential cyclingand anodic polarization up to E =1.20 V generates only Pt2+

surface species. Consequently, the reactions 8 and 11, whichimply the existence of PtO2(s) species undergoing electro-dissolution, are unlikely to occur. The reactions 5 and 6stipulate simultaneous generation of Pt2+(aq) and H2(g).Because the standard potential of the Pt2+(aq)/Pt0(s)redox couple is E °(Pt2+/Pt)=1.188 V [21] and is higherthan the standard potential of the H+(aq)/H2(g) redox couple,E°(H+/H2)=0.00 V, the reactions 5 and 6 are unlikely to takeplace or could be followed by a second step in which Pt2+(aq)is reduced to Pt(s)0 and H2(g) is oxidized to H(aq)+. Thereactions 3 and 4 can explain the formation of Pt4+(aq) speciesthat is observed experimentally. The reactions 9 and 10 canexplain the existence of the species Pt2+(aq), but the chemicaldissolution cannot be assigned either to the anodic or cathodicscan. One could propose that anodically formed PtO (eitherprior to or after the place interfacial exchange) undergoessubsequent chemical dissolution at a slow rate that is potentialindependent (slower than the rate of PtO formation). If thisreaction were to take place, then one should be able to observechemical dissolution of pre-oxidized Pt in an aqueous acidicsolution without applying any potential. However, the amountchemically dissolved Pt would be very tiny and the processwould cease as soon as the PtO layer were dissolved. In thecase of potential cycling experiments, the dissolved PtO layercan be reformed in a subsequent anodic scan and then thechemical dissolution of PtO can continue leading to aprogressively increasing amount of Pt2+(aq) species in theelectrolyte. The results of Mitsushima et al. [20] provideunambiguous evidence for the existence of PtO2(s)dissolution, but at the present time there is no evidence forthe existence of chemical dissolution of PtO(s). Consequently,more research on the subject is urgently needed.

This discussion of possible mechanistic pathways ofelectrochemical and chemical dissolution of Pt indicates thatat the present time we are able to explain the formation ofPt4+(aq) species through anodic dissolution of PtO or theformation of Pt2+(aq) species through chemical dissolutionof PtO. However, we are unable to explain the cathodic


formation of electro-dissolved Pt species without making anarbitrary assumption that anodic polarization at or potentialcycling up to 1.20 V generates PtO2. Such an assumptionwould contradict the results of our previous study of theelectro-oxidation of Pt and would call for additional researchon the electro-oxidation of Pt in the 0.85–1.40 V range [10,22–24]. It is important to add that the existence of two electro-dissolved Pt species, namely Pt2+(aq) and Pt4+(aq) present ascomplexes, observed using IEC–ICP-MS [17] could be theresult of an equilibrium that establishes in the bulk ofelectrolyte following the generation of one species as a resultof anodic, cathodic, or chemical dissolution of Pt or Pt oxide.

Research on interfacial electrochemical phenomenaattracts enormous attention due to the importance of Pt tothe PEMFC technology. In Fig. 11, we attempt to graphicallysummarize the main Faradaic and non-Faradaic phenomenathat can take place at Pt electrodes. The graph refers to anaqueous acidic electrolyte solution having an activity ofproton equal to 1.00 (aðHþðaqÞ ¼ 1:00 ). Because thepotential range of the adsorption of HUPD and anions dependson the electrode surface geometry, the potential rangesreferring to these and other processes are approximate.However, the diagram effectively reveals the complexity ofthe Pt/aqueous electrolyte system.

Comment on the Selection of Counter Electrode Material

The selection of a suitable material for the counter electrodeand its electrochemically active surface are of greatimportance when studying the electro-dissolution of Pt.Specifically, when a given potential measured with respectto RE is applied toWE (EWE), a current density (j ) determinedby the Butler–Volmer equation passes through it. Thepotential of CE (ECE) adjusts spontaneously so that theabsolute value of the current passing throughWE (IWE) equalsthe absolute value of the current passing through CE (ICE);∣IWE∣=∣I CE∣ and ∣jWE∣×A ecsa,WE=∣j CE∣×A ecsa,CE

(where Aecsa,WE and Aecsa,CE are the electrochemically activesurface areas of WE and CE, respectively). Kinetics of theelectrode process taking place at WE can be examined using athree-electrode configuration on the condition that the currentpassing through the circuit is not limited by the kinetics of theprocess taking place at CE. Thus, the electrochemically activesurface area of CE needs to be at least an order of magnitudelarger than that of WE and the kinetics of the process takingplace at CE needs to faster than the kinetics of the processtaking place at WE. If this condition is not fulfilled, then therate of the process occurring at WE might be limited by therate of the reaction taking place at CE. In the case of CE madeof Pt, our monitoring of ECE (Fig. 2) indicates that theelectrode processes occurring at CE are the electro-adsorption of HUPD, double-layer charging, and Pt electro-

oxidation and electro-dissolution. Because UPD H anddouble-layer charging are very fast processes, they do notlimit the electro-oxidation and electro-dissolution of Pt thatare very slow processes. Thus, the selection of Pt as a counterelectrode in the research on electro-dissolution of Pt is suitableon the condition that a proton-exchange-membrane (e.g.,Nafion or similar) is employed to separate the WE and CEcompartments. The application of high cathodic potentials toWE results in spontaneous adoption of high anodic potentialsby CE made of Pt and its substantial electro-dissolution. It isinteresting to add that controlled electro-dissolution of Pt canbe successfully employed to prepare in situ micro- and nano-structured Pt deposits on WE as demonstrated elsewhere [25,26].

Comment on the Usage of Aecsa as a Means of Evaluatingthe Degradation of Pt-Based Electrocatalysts

Analysis of the evolution of Aecsa of pure Pt or Pt-containingelectrocatalysts upon prolonged potential cycling is oftenemployed to identify and to quantify degradation phenomena,which include (1) electro-dissolution of Pt-NPs, (2)agglomeration of small Pt-NPs into large ones, (3) Ostwaldripening, (4) corrosion of the carbon support and associated withit loss of electric conduct between Pt-NPs and the carbon supportthat acts as a current collector, and (5) physical detachment of Pt-NPs from the carbon support. The results presented in Fig. 10clearly reveal gradual suppression of CV features characteristicof Pt by uncontrolled impurities always present even in high-purity aqueous electrolytes. The loss of these features becomesevident if the upper potential limit in potential cycling orpotential stepping experiments is insufficiently high and theduration of such experiments long. The loss of these features issimply due to blocking of surface sites by adsorbed impuritiesthrough physisorption or chemisorption and may not beassigned to any of the above-listed phenomena.Many impuritiesadsorbed on Pt can be easily removed through oxidativedesorption provided that the upper potential limit in potentialcycling or potential stepping experiments is high enough,typically 1.20 Vor higher. Thus, in prolonged potential cyclingor potential stepping experiments, the disappearance of CVfeatures characteristic of impurity-free Pt may not beunambiguously assigned to any of the above-listed degradationphenomena. Consequently, there exists a need for distinctionbetween (1) a true material loss, (2) passivity development, and(3) Aecsa loss due to surface blockage by impurities.

Conclusions

Electro-dissolution of Pt working and counter electrodesunder potential cycling conditions in aqueous acidic solutions


can be studied using electrochemical methods and inductivelycoupled plasma mass spectrometry. Electro-dissolved Ptoriginates from both the working and counter electrodes. Ananodic process occurring at the working electrode drives acathodic process taking place at the counter electrode and viceversa. Depending on the potential applied to the workingelectrode, the counter electrode can experience either similaror different polarization conditions. In some instances, thecounter electrode experiences higher anodic potentials thanthe working electrode. Consequently, the amount of electro-dissolved Pt originating from the counter electrode issubstantially higher than the amount of electro-dissolved Ptoriginating from the working electrode. In order todifferentiate between electro-dissolved Pt originating fromonly the working electrode, a three-compartment cell needsto be employed. Several electrochemical and chemicalreactions can explain the formation of Pt2+(aq) and Pt4+(aq)species that are observed experimentally. However, at thepresent time it is impossible to explain the cathodic dissolutionof Pt without making an arbitrary assumption that anodicpolarization of Pt in the 0.85≤E ≤1.40 V range generatesPtO2 instead of PtO. Thus, further research on the electro-oxidation of Pt is urgently needed.

Acknowledgments The authors gratefully acknowledge financialsupport toward this project from the Nissan Motor Company throughthe Nissan Technical Center North America. They also acknowledgeinfrastructure support from the Natural Sciences and EngineeringResearch Council of Canada, Canada Foundation for Innovation, andQueen’s University. We acknowledge stimulating discussion withmembers of the Electrocatalysis Group of the Max Planck Society forthe Advancement of Science in Düsseldorf led by K.J.J. Mayrhofer.

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platinum electro-dissolution in acidic media upon potential cycling

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