Influence of the Working and Counter Electrode Surface Area Ratios

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Influence of the Working and Counter Electrode Surface Areas Ratio on the Dissolution of Platinum under Electrochemical Conditions Min Tian, Christine Cousins, Diane Beauchemin, Yoshihisa Furuya, Atsushi Ohma, and Gregory Jerkiewicz ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00200 • Publication Date (Web): 22 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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Influence of the Working and Counter Electrode Surface Areas Ratio on the Dissolution of Platinum under Electrochemical Conditions

Min Tian1, Christine Cousins1, Diane Beauchemin1, Yoshihisa Furuya2, Atsushi Ohma2, Gregory Jerkiewicz* 1 2

Department of Chemistry, Queen’s University, Kingston, ON, K7L 3N6, Canada

Nissan Research Center, Nissan Motor Company, 1-Natsushima Cho, Yokosuka, Kanagawa 237-8523, Japan

* Corresponding Author E-mail: [email protected]. Tel.: (613) 533-6413

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ABSTRACT:

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The potential variation of Pt counter electrode (CE) in a three-electrode

configuration is monitored as the potential of Pt working electrode (EWE) follows a triangleshaped program in 0.5 M aqueous H2SO4. The spontaneously adopted CE potential (ECE) is reported for different values of the ratio of geometric surface areas of WE and CE (Ageom,WE/Ageom,CE).

The ECE versus time (t) transients are non-linear and resemble

charging/discharging curves. In the case of Ageom,WE > Ageom,CE, ECE adopts higher values than EWE, and vice versa. In the case of Ageom,WE/Ageom,CE = 10:1, the values of ECE are 0.3 – 0.4 V higher than the highest values of EWE. The high ECE values give rise to the development of a thick surface oxide that undergoes subsequent dissolution.

A novel three-compartment

electrochemical cell is employed to examine simultaneously the dissolution of WE and CE, and to monitor their potentials; the amount of dissolved Pt is quantitatively analyzed using inductively coupled plasma mass spectrometry. The magnitude of the Ageom,WE/Ageom,CE ratio has a significant impact on the CE oxidation and dissolution. The oxidation and dissolution of CE depend on the lower potential limit of WE; the amount of surface oxide and the quantity of dissolved Pt significantly increase as the WE potential limit is reduced from 0.50 V to 0.05 V because CE adopts a high potential. The presence of dissolved O2 also affects the dissolution of CE but to a lesser extent than the Ageom,WE/Ageom,CE ratio or the lower potential limit of WE. Field emission scanning electron microscopy analysis of the CE morphology following prolonged potential cycling in the presence of dissolved O2 reveals a thick surface oxide that has dry mudlike structure. The slightly higher dissolution of CE under these conditions is attributed to physical detachment of some of the cracked surface oxide.

This research advances the

understanding of Pt dissolution with some of the new knowledge being applicable to fuel cells.


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Keywords: Platinum; platinum oxide; platinum dissolution; inductively-coupled plasma-mass spectrometry; counter electrode; potential monitoring


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INTRODUCTION Platinum is a well-known and very active catalytic material that finds application in different areas of heterogeneous catalysis. In electrochemistry, it is a very important electrocatalyst that is employed in the form of carbon-supported nanoparticles in polymer electrolyte membrane fuel cells (PEMFCs) to facilitate the hydrogen oxidation and oxygen reduction reactions (HOR and ORR, respectively).1 Platinum-based electrocatalysts can also be employed in electro-oxidation of small organic compounds such as methanol or formic acid for electrical energy production.2 However, its high cost and slow but unavoidable chemical and electrochemical degradation significantly impede efforts the objectives of which is to implement PEMFCs in personal transportation vehicles.3 Application of nanostructured Pt materials, such as Pt nanoparticles (PtNPs), reduces the cost of catalysts and improves their utilization.4-11

Yet, nanostructured

materials, including Pt nanoparticles, possess high surface Gibbs energy that enhances their activity towards both desired as well as undesired reaction. These interrelated characteristics give rise to gradual degradation of Pt-NPs through chemical and electrochemical reactions.3 A loss of catalytic material translates into a loss of the electrochemically active surface area available for HOR and ORR.12-15 In operating PEMFCs (accelerating, idling, start-up and shutdown), Pt electrocatalysts can experience high anodic potentials and, consequently, undergo oxidation forming a surface oxide that can dissolve.16

Chemical and electrochemical Pt

degradation is a complex process that currently attracts considerable attention.

It was

investigated in relation to conditions that Pt electrocatalysts can experience,17-21 such as such as electrolyte composition and pH,22-25 temperature,3,26-28 presence of neutral or reactive gases,29-31 potential scan rate,3,22,24 and the upper and lower potential limits in potential cycling or stepping measurements.3,23,26,32-39 Platinum oxide is known to protect the underlying Pt by forming a


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compact passive layer but can also enhance Pt dissolution.24,33,36,37 Mitsushima et al. studied Pt dissolution in 1.0 M aqueous H2SO4 by employing various potential versus time programs (symmetric, fast-cathodic and slow cathodic triangle), and observed enhanced Pt loss rate for a triangle-shaped signal with a slow cathodic sweep.34 In a set-up where the working electrode (WE) and counter electrode (CE) are made of Pt, both electrodes can undergo dissolution.3,40 As the dissolution behavior can be accompanied by subsequent electrodeposition under favorable potential conditions, the two phenomena affect the Pt mass balance between WE and CE if they are not placed in individual compartments separated by a suitable membrane. In laboratory-type experimental research, this complication can be avoided by using Au or carbon counter electrodes.41,42 However, Au also undergoes dissolution and if the WE and CE compartments are not separated by means of a suitable membrane that can block the transport of Ptz+ and Auz+ cations (most likely present in the form of aqua complexes) from one compartment to another, dissolved Pt can electrodeposit on Au and vice versa. Carbon counter electrodes are suitable materials because they do not generate compounds that can undergo subsequent electrodeposition.43 However, it has to be recognized that they can undergo irreversible oxidative degradation with the generation of mainly CO2.

Such concurrently

occurring processes can detrimentally impact research on Pt dissolution and electrocatalytic activity evaluation in long-term experiments.

To the best of our knowledge, there are no

quantitative data for the dissolution of Pt counter electrode in relation to its size as compared to the working electrode.

Such measurements require carefully planned experiments and a

purpose-designed three-compartment electrochemical cell with a proton-conducting membrane as well as means of simultaneously collecting electrolyte aliquots from the counter electrode and working electrode compartments.


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In this contribution, we examine the dissolution behavior of Pt working and counter electrodes in 0.5 M aqueous H2SO4. This electrolyte was selected because a vast majority of our previous research on Pt oxidation and dissolution behavior was conducted in this electrolyte. We report potential values of the counter electrode for various values of the ratio of the working electrode and counter electrode surface areas (AWE/ACE). We also report the amounts of dissolved Pt originating from these electrodes as determined using inductively coupled plasma mass spectrometry. The dissolution of the counter electrode is examined in relation to AWE/ACE ratio values in the presence of dissolved oxygen. The morphology of the counter electrode surface is analyzed by field emission scanning electron microscopy (FE-SEM). The outcome of this research generated new knowledge about the dissolution of Pt materials and benefits the science and technology of fuel cells.

EXPERIMENTAL SECTION Electrochemical Cell, Electrodes, and Electrolytes. Monitoring the potential values

of Pt working and counter electrodes as well as examining their dissolution requires a suitable electrochemical cell and a proton conducting membrane separating WE and CE compartments. Figure 1 shows the three-compartment cell employed in this research. It consists of the WE and CE compartments that have the same dimensions, thus the same volumes, and a reference electrode (RE) compartment that is connected to the main cell body by means of a Luggin capillary. The WE and CE compartments are separated by means of a Nafion 211 membrane that facilitates electrolytic contact while preventing the respective electrolyte solutions from intermixing; a fresh Nafion membrane was used in each experiment. The placement of the Luggin capillary is deliberately unconventional in the sense that it is located close to the Nafion membrane, thus halfway between WE and CE. By using this arrangement we were able to use 6 ACS Paragon Plus Environment

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the same RE to monitor the potential of the WE and CE (the potential values reported in the paper are IR-drop corrected; in one instance the IR drop of ca. 2 Ω is due to the electrolyte and in the other instance the ID drop of ca. 5 Ω is due to the electrolyte and the Nafion membrane). The cell design facilitates evaluation of the potential and dissolution of the working and counter electrodes.

In addition, the placement of WE and CE can be exchanged if there exists a

possibility that some dissolved Pt can originate from RE (e.g. detachment of Pt black from the bulk Pt substrate followed by its dissolution). The regular and exchanged placements of WE and CE are shown in Figure 1 using red and blue fonts, respectively. The platinum WE and CE were made of Pt foil (99.995% in purity, Alfa-Aesar) and RE of platinum mesh (99.995% in purity, Alfa-Aesar) on which Pt black was electrodeposited. manufacture followed well-established procedures.44

Details of the electrode design and Scanning electron microscopy (SEM)

operating in the secondary electron mode was employed to acquire Pt electrode surface images prior to and after surface oxide formation. The 0.5 M aqueous H2SO4 solution was made from high purity sulfuric acid (TraceSelect, Fluke) and ultra-high purity water (Milli-Q, Millipore; it resistivity was ρ ≥ 18.2 MΩ cm). Because the concentrated H2SO4 was of high purity, it was used without any additional purification.


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Figure 1. Three-compartment electrochemical cell employed in Pt dissolution studies that has identical working electrode (WE) and counter electrode (CE) compartments, which are separated by a Nafion membrane. The Luggin capillary of the reference electrode (RE) is placed close to the membrane facilitating monitoring of the potential of both the WE and CE.

Electrochemical Measurements and Electrolyte Solution Sampling for Analysis.

Electrochemical experiments were performed using the above-described electrochemical cell and WE and CE whose geometric surface areas (Ageom) were 1.0, 2.0, 5.0 and 10.0 cm2, respectively. Since both electrodes were made of exactly the same batch of Pt wire and Pt foil, and were pretreated using exactly the same approach, they had the same values of the surface roughness factor that was R = 1.6 ± 0.10 (R = Aecsa/Ageom, where Aecsa is the electrochemically active surface area and Ageom is the geometric surface area).44 Consequently, the ratio of the electrochemically active surface areas equaled the ratio of the geometric surface areas (Aecsa,WE/Aecsa,CE = Ageom,WE/Ageom,CE). The WE and CE electrodes were designed and manufactured in such a way so that the ratios of their surface areas (Aecsa,WE/Aecsa,CE) were 10:1, 2:1, 1:2, and 1:10. Results for WE and CE having the same dimensions, thus for the ratio Aecsa,WE/Aecsa,CE = 1:1, were reported elsewhere and are not discussed here.3 The platinum WE and CE were degreased in acetone under reflux followed by rinsing with ultra-high purity ethanol. Then, they were soaked in concentrated H2SO4 for 24 hours followed by repetitive rinsing (at least ten times) with ultrahigh purity water (Milli-Q, Millipore; it resistivity was ρ ≥ 18.2 MΩ cm). The Pt black/Pt RE was placed in a separate compartment through which high-purity H2(g) at a pressure of 1.0 atm was bubbled; it served as a reversible hydrogen electrode (RHE). All potential values are reported with respect to RHE. Potential cycling experiments in 0.5 M aqueous H2SO4 solution were performed over extended periods of time at room temperature (T = 293 ± 1 K). Unless


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stated otherwise, ultra-high purity N2(g) was bubbled prior to and during potential cycling experiments to expel any reactive gasses. Dissolved Pt species present in the Nafion membrane could modify its conductivity, thus the IR drop. As in our previous work, the used Nafion membranes were placed in ultra-high water for several hours and then analyzed for Pt content using inductively couple plasma mass spectrometry (see below); the analysis revealed no dissolved Pt most likely due to the large solution volume/Nafion surface area ratio.3 Inductively Coupled Plasma Mass Spectrometry Measurements. Prior to each series

of Pt dissolution experiments, WE was cycled repetitively between 0.05 and 1.40 V at a potential scan rate of s = 50 mV s–1.

This approach generated cyclic-voltammetry (CV) profiles

characteristic of impurity-free electrodes consistent with literature standards.44 Figure 2 presents CV profiles for Pt working electrodes having various geometric surface areas in the 1.0 – 10 cm2 range; they were acquired in 0.5 M aqueous H2SO4 solution at s = 50 mV s–1 and a temperature of T = 293 K. They reveal all the usual features corresponding to the electro-adsorption and electro-desorption of under-potential deposited H (HUPD) as well as the formation and reduction of Pt surface oxide. The volumes of electrolyte solution in the working electrode and counter electrode compartments were constant and 40 ± 1.0 mL. After repetitive potential cycling (typically ca. ten cycles), an aliquot of 1.0 mL of electrolyte was withdrawn from both compartments for inductively coupled plasma mass spectrometry (ICP-MS) measurements; it served as a blank. An aliquot of 1.0 mL of electrolyte solution was collected from the working electrode and counter electrode compartments at the following cycles: 20th, 50th, 100th, 200th, 500th, 1000th, 2000th, 3000th, 4000th, and 5000th. In each instance, 1.0 mL of fresh electrolyte was added to each electrode compartment to maintain the electrolyte volume constant throughout the entire series of measurements.

The gradual dilution of the dissolved Pt remaining in the


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electrolyte solution was taken into account when reporting the amount of dissolved Pt originating from WE and CE. The solutions of Pt used to prepare a calibration curve were made with standard solution from SCP Science (10,000 µg mL–1 in 10% aqueous HCl). The Pt standard solutions of 1, 5, 10, 25, 50, 75 ppb were prepared by a series of dilution of the Pt standard solution using 0.1 M aqueous H2SO4 solution. The concentration of Pt in the electrolyte was analyzed using a Varian 820-MS ICP-MS instrument equipped with regular Ni cones; the Pt detection limit was 10 ppt.3,18 After ICP-MS measurements, the sample introduction system was flushed with the blank solution until the counts reached the level comparable to that prior to these measurements.

1.0

0.5

0.0

I (mA)

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-0.5 2

A geom = 10.0 cm -1.0

2

A geom = 5.0 cm

A geom = 2.0 cm2 -1.5

A geom = 1.0 cm2

-2.0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

E (V, RHE)

Figure 2. CV profiles for Pt foil working electrode having different geometric surface area (AWE = 1.0 – 10 cm2); ACE = 1.0 cm2; electrolyte: 0.5M aqueous H2SO4; potential scan rate: s = 50 mV s–1; and temperature: T = 293 K.

Figure 3A presents a number of counts versus time plot for the

195

Pt isotope for the above-

mentioned six standard solutions. The areas under the respective peaks were integrated and a calibration curve in the form of area versus 195Pt concentration is presented in Figure 3B. Fitting of the experimental data using a linear regression shows that they follow a straight line with a 10 ACS Paragon Plus Environment

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correlation coefficient being 0.9999. The dissolved Pt samples collected during potential cycling experiments were diluted five times and analyzed in the sequence of their collection. The amount of dissolved Pt, thus Pt concentration that is then converted to Pt mass, was determined using the calibration curve (Figure 3B).18

Figure 3. (A) Number of counts versus time plot for the 195Pt isotope obtained using an inductively coupled plasma-mass spectrometer for six Pt standard solutions of 1, 5, 10, 25, 50, 75 ppb. (B) Calibration line prepared by fitting of the experimental data using a linear regression.

RESULTS AND DISSCUSSION Electrochemical Behavior of Working and Counter Electrodes. In order to examine

the potential of counter electrode (ECE) in relation to the triangle-shaped programmed potential


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applied to the working electrode (EWE), we digitally recorded ECE as a function of time (t), while EWE was scanned between 0.05 V and 1.40 V at 50 mV s–1 and T = 293 K in 0.5 M aqueous H2SO4 solution. Figure 4A shows E versus t transients for WE (the blue tringle-shaped transient) and CE for two extreme values of the Ageom,WE/Ageom,CE ratio, namely 10:1 (the black transient) and 1:10 (the red transient). Because the two ECE versus t transients have different shapes, they are discussed separately. In the case of Ageom,WE/Ageom,CE = 10:1, the following features are observed in the CE potential transient: (i) the ECE versus t profile is non-linear and significantly differs from the EWE versus t profile; (ii) when EWE reaches the highest value, ECE adopts the lowest value and vice versa; (iii) ECE increases rapidly when EWE is scanned in the cathodic direction; the ECE versus t transient forms a distorted plateau and adopts values in the 1.73 – 1.85 V range; these ECE values are between 0.3 and 0.4 V higher than the values of EWE; and (iv) CE spends a significantly longer time (ca. 10 s in the case of s = 50 mV s–1) in the potential region of Pt oxide formation than WE, thus it can develop a thicker Pt oxide layer than WE. In the case of Ageom,WE/Ageom,CE = 1:10, the following characteristics are observed in the potential transient: (i) the ECE versus t profile is non-linear and significantly differs from the EWE versus t profile; (ii) when EWE reaches the highest value, ECE adopts the lowest value and vice versa; however, while EWE is scanned between 0.05 and 1.40 V, the values of ECE are between 0.10 and 0.62 V; (iii) ECE increases rapidly to 0.62 V when EWE approaches its lowest limit of 0.05 V; and (iv) CE never reaches potential values corresponding to the formation of Pt oxide; thus CE is never covered with an oxide layer.

We also examined the ECE versus t behaviour for the

Ageom,WE/Ageom,CE ratio being 2:1 and 1:2. Figure 4B presents E versus t transients for WE (a blue triangle-shaped transient) and CE for Ageom,WE/Ageom,CE = 2:1 (a black transient) and 1:2 (a red transient). The two ECE versus t profiles have a similar shape but as expected adopt different


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potential values. In the case of Ageom,WE/Ageom,CE = 2:1, when EWE reaches the highest value (1.40 V), ECE adopts the lowest value of 0.05 V. As EWE linearly decreases to 0.05 V, ECE gradually increases to ca. 1.16 V and remains in this potential range for ca. 7 s and then increases further to 1.60 V, which is 0.20 V greater than the highest potential value of WE. During a complete transient of WE from 0.05 to 1.40 and back to 0.05 V which takes 54 s, WE spends 22 ± 1 s and CE 30 s in the potential region of Pt oxide formation (the oxide formation on Pt(poly) commences at 0.85 V).45 The longer oxidation time translates into a slightly thicker oxide layer developed on CE that can undergo subsequent dissolution. In the case of Ageom,WE/Ageom,CE = 1:2, the highest value of ECE is 1.15 V and the lowest is 0.05 V, while the time spent in the potential range of oxide formation is 17 ± 1 s. The shorter time spent in the oxide formation regions gives rise to a thinner oxide layer that can undergo subsequent dissolution. The ECE versus t transients also reveal plateau-like features in the descending and ascending sections; in the descending section the average plateau potential values are 0.80 and 0.85 V, respectively, and in the ascending section 1.16 and 0.89 V, respectively. It is interesting to notice that ECE versus t transients exhibit a typical charge/discharge behavior. We do not present ECE versus t transients for Ageom,WE/Ageom,CE = 1:1 because they are shown elsewhere.3 The observation that the potential of CE adopts different values when the ratio Ageom,WE/Ageom,CE is changed and that different amounts of surface oxide develop, suggests that CE should exhibit a different dissolution behavior in each case.


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Figure 4. (A) Potential (E) versus time (t) transients for the working electrode (WE; the blue triangle-shaped transient) and the counter electrode (CE) for two extreme values of the Ageom,WE/Ageom,CE ratio, 10:1 (the black transient) and 1:10 (the red transient). (B) E versus t transients for WE (the blue triangle-shaped transient) and CE for Ageom,WE/Ageom,CE = 2:1 (the black transient) and 1:2 (the red transient). The potential scan range: 0.05 – 1.40 V; electrolyte: 0.5 M aqueous H2SO4; potential scan rate: s = 50 mV s–1; and temperature: T = 293 K.

At any given time, the current of the counter electrode (ICE) equals the negative of the current of the working electrode (IWE), IWE = –ICE. The E versus t transients for CE presented in Figure 4 allow us to prepare I versus E plots. Importantly, these plots are not CV profiles because the potential of CE does not change in a linear manner as in the case of WE. Figure 5A shows an I versus E profile for WE, which is a CV profile; the inset demonstrates the corresponding E versus t program and shows that the initial E value is 0.50 V and increases to 1.40 V, then the scan direction is changed and E decreases to 0.05 V followed by another direction change and a scan to a final value of 0.50 V. The beginning and the end of the transient are marked with a small cross. The CV profile in Figure 5A is divided into seven regions (Region I through VII in the legend) that are colored coded and correspond to the following interfacial electrochemical phenomena: Region I (0.05–0.40 V), the anodic desorption of HUPD;


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Region II (0.40–0.80 V), the anodic double-layer charging; Region III (0.80–0.1.40 V), the anodic Pt oxide formation; Region IV (1.40–1.00 V), the anodic-to-cathodic scan reversal; Region V (1.00–0.60 V), the cathodic Pt oxide reduction; Region VI (0.60–0.40 V), the cathodic double layer charging; and Region VII (0.40–0.05 V), the cathodic adsorption of HUPD.3 Figures 5B through 5E show I versus E transients CE but in each case the Ageom,WE/Ageom,CE ratio has a different value, namely 10:1 (Figure 5B), 2:1 (Figure 5C), 1:2 (Figure 5D), and 1:10 (Figure 5E). For clarity of the presentation, the same colors are used to indicate the potential values that CE adopts as the potential of WE is scanned as indicated in Figure 5A. Also, the beginning and the end of the transient are marked with a small cross. It is important to note that the shape of the I versus E transients for CE is dramatically different from the CV profile for WE. If the potential of CE adopts values corresponding to the electro-adsorption and electro-desorption of HUPD, then the process occurs but it does not give rise to the characteristic features observed in typical CV profiles. The same observation applies to potential regions corresponding to the Pt surface oxide formation and its reduction.


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Figure 5. (A) I versus E profile, a cyclic-voltammetry profile, for the working electrode (WE); the inset demonstrates that the transient commences at 0.50 V and increases to 1.40 V, then its direction is changed and scanned to 0.05 V followed by another direction change and a scan to a final potential of 0.50 V. The CV profile is divided into seven regions (Region I through VII) that are colored coded. Graphs (B) through (E) present I versus E transients for the counter electrode (CE) for different values of the Ageom,WE/Ageom,CE ratio: (B) 10:1; (C) 2:1; (D) 1:2; and (E) 1:10; electrolyte: 0.5 M aqueous H2SO4; potential scan rate: s = 50 mV s–1; and temperature: T = 293 K. The same colors indicate the potential values that CE spontaneously adopts.

Dissolution of Platinum Working and Counter Electrodes. In the experiments reported in

this contribution, EWE follows a triangle-shaped programmed potential (a cyclic-voltammetry profile), while ECE adjust spontaneously so that IWE = –ICE. However, the results presented in Figure 4 demonstrated that depending on the Ageom,WE/Ageom,CE ratio value, ECE can adopt values as high as 1.85 V. Consequently, CE experiences significantly higher anodic potentials than WE and develops a thicker surface oxide layer than that formed on WE. Because the potential values adopted by CE are different from those of WE, the dissolution behavior of CE is also expected to be different. In a separate series of experiments, we examined the dissolution behavior of CE and WE in 0.5 M aqueous H2SO4 upon potential cycling in the 0.50 – 1.20 V range at s = 50 mV s–1 and T = 293 K as a function of the number of potential transients. This potential range was selected because it corresponds to typical potential values of ORR taking place at Pt materials in PEM fuel cells. The three-compartment experimental setup allowed us to analyze the dissolution of CE as a function of the number of potential transients (n) and the Ageom,WE/Ageom,CE ratio. As explained in the Experimental section, aliquots of electrolyte were collected from the working and counter electrode compartments and the amount of dissolved Pt, expressed as Pt mass (mPt) per 1.00 cm2 of Ageom was determined using ICP-MS (the conversion of the amount of dissolved


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Pt per 1.00 cm2 of Aecsa is accomplished by diving mPt by the roughness factor here being R = 1.6). The graphs in Figure 6A show the mass of dissolved Pt originating from CE versus the potential transient number (mPt,CE versus n) for various values of the Ageom,WE/Ageom,CE ratio, namely 10:1, 2:1, 1:1, 1:2, and 1:10. The amount of Pt is cumulative and refers to the total amount of dissolved Pt up to the nth potential transient. In all instances, the values of mPt,CE rise with increasing n, but this trend is pronounced the most for Ageom,WE/Ageom,CE = 10:1. In the case of Ageom,WE/Ageom,CE = 1:10, the values of mPt,CE do not exceed 105 ng cm–2 and in the case of Ageom,WE/Ageom,CE = 1:2 they do not exceed the value of 510 ng cm–2. Under these conditions the dissolution of CE is practically negligible and slight only for n = 5000.3

In the case of

Ageom,WE/Ageom,CE = 1:1 the highest value of mPt is 2560 ng cm–2 and in the case of Ageom,WE/Ageom,CE = 2:1 it is 2210 ng cm–2. The amount of dissolved Pt after n = 5000 potential cycles is significant and in both cases corresponds to ca. 5 surface monolayers of Pt. In the case of Ageom,WE/Ageom,CE = 10:1 the amount of dissolved Pt is very large because the highest value of mPt is 12690 ng cm–2; it corresponds to ca. 28.5 surface monolayers of Pt. These results demonstrate that the amount of dissolved Pt originating from CE strongly depends on the Ageom,WE/Ageom,CE ratio and increases by more than two orders of magnitude when the value of Ageom,WE/Ageom,CE changes from 1:10 to 10:1. The significant increase in the amount of dissolved Pt originating from CE as the Ageom,WE/Ageom,CE ratio rises from 1:10 to 10:1 can be associated with the thicker Pt surface oxide layer that develops and can undergo subsequent chemical and electrochemical dissolution (see below the section Phenomena and Conditions Enhancing Platinum Dissolution). In addition, the significant increase in mPt,CE can be associated with the structure of Pt surface oxide that during formation and reduction involves interfacial structural transformation (the so-called interfacial place exchange) that enhances Pt dissolution.3,17,22,24


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These observations are consistent with the results of Mitsushima et al. who reported significant cathodic dissolution of Pt oxide at high anodic potentials within the vicinity of 1.8 V.31 Significant Pt dissolution was also observed during the formation of very thick oxide layers on Pt(poly) composed mainly of PtO2 through the application of high anodic potentials between 1.8 and 2.3 V.46 It is important to add that in the latter case PtO2 is the outer layer being in contact with the electrolyte solution, while PtO is sandwiched between Pt and PtO2. Thus, it is not obvious whether PtO can undergo chemical or electrochemical dissolution unless the outer PtO2 layer is not compact and exposes the underlying PtO. The concurrently performed ICP-MS measurements allowed us to examine the dissolution behavior of WE as a function of the Ageom,WE/Ageom,CE ratio. Figure 6B shows the mass of dissolved Pt originating from WE versus the potential cycle number (mPt,WE versus n) graphs for the Ageom,WE/Ageom,CE ratio having the values of 10:1, 2:1, 1:1, 1:2, and 1:10. As in the case of CE, the amount of Pt is cumulative and refers to the total amount of dissolved Pt up to the nth potential transient. As in the case of CE, we observe that the values of mPt,WE rise with increasing n for all values of the Ageom,WE/Ageom,CE ratio but this trend is pronounced the most in the case of Ageom,WE/Ageom,CE = 1:1 with the highest value of mPt,WE being 6110 ng cm–2, which is about half of the maximum mPt,CE value. In the case of Ageom,WE/Ageom,CE = 10:1, the amount of dissolved Pt is the smallest and equals 520 ng cm–2; this value is twelve times smaller than the above-reported value for WE and CE having the same surface areas (Ageom,WE/Ageom,CE = 1:1). The dependence of the value of mPt,WE on the Ageom,WE/Ageom,CE ratio is very surprising and unexpected but the results are reproducible and reliable. Because the same trend is reported for ten values of n, the behavior may not be assigned to experimental uncertainty. At the present time we are unable to offer a reasonable explanation. However, bearing in mind the growing


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interest in Pt dissolution, we believe that it is important to report these unexpected results even if we are unable to offer any plausible justification.

Figure 6. (A) Mass of dissolved Pt originating from the counter electrode (CE) versus the potential cycle number (mPt,CE versus n) for various values of the Ageom,WE/Ageom,CE ratio, namely 10:1, 2:1, 1:1, 1:2, and 1:10. (B) Mass of dissolved Pt originating from the working electrode (WE) versus the potential cycle number (mPt,WE versus n) graphs for the Ageom,WE/Ageom,CE ratio having the values of 10:1, 2:1, 1:1, 1:2, and 1:10. Electrolyte: 0.5 M aqueous H2SO4; potential scan rate: s = 50 mV s–1; potential cycling range: 0.50 – 1.20 V; and temperature: T = 293 K.

Influence of the Lower Potential Limit of the Working Electrode and the Presence of Dissolved Oxygen on the Counter Electrode Dissolution.

Existing results,

including identical location transmission electron microscopy data, demonstrate that the Pt dissolution behavior and morphology depend on the applied potential and the presence of dissolved oxygen.3,29-31,47-49 Our results presented in Figure 4 clearly show that CE adopts high anodic potential values in the case of Ageom,WE/Ageom,CE = 10:1. In order to further examine this behavior, we conducted a separate series of potential cycling experiments in which the upper potential limit of WE was fixed at 1.20 V and the lower potential of WE was either 0.05 V or 0.50 V. The 0.05 V limit was selected because in this case the highest potential adopted by CE is high enough to facilitate the formation of PtO (Figure 4A). On the other hand the 0.50 V limit


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was selected because various stability tests of Pt-based catalysts used in fuel cell are typically conducted in the 0.50 – 1.20 V potential range.3,24 The experimental research on the dissolution behavior of CE was conducted both in the absence and presence of dissolved O2 in order to examine whether the dissolved gas (required for ORR) has any influence on the process. Figure 7A shows mPt,CE versus n graphs for the three sets of experimental data, namely for the 0.05 – 1.20 V range and for the 0.50 – 1.20 V range in the absence and presence of O2; the potential limits refer to WE while the amount of dissolved Pt to CE. As expected, the values of mPt,CE are significantly higher when the lower potential limit of WE is 0.05 V. In all cases we observe an increase in mPt,CE with rising n. In the case of the 0.05 V limit, the highest value of mPt,CE is 22570 ng cm–2 and in the case of the 0.50 V limit it is mPt,CE = 12690 ng cm–2 (see Figure 6A). The value of mPt,CE = 22570 ng cm–2 corresponds to the dissolution of ca. 50.7 surface monolayers of Pt. The plots also reveal that the values of mPt,CE exceed 2200 ng cm–2 (ca. 4.9 surface monolayers of Pt) already after n = 500 cycles. In a separate series of experiments we examined if dissolved O2 present in both the WE and CE compartments has any impact on the dissolution of CE, while WE is repetitively cycled in the 0.50 – 1.20 V range. It is important to emphasize that under these conditions the main electrochemical process taking place at WE is ORR and the current associated with this process is higher than that due to the oxide formation or the oxide reduction. Due the employment of a stationary electrode and the bubbling of O2(g) through the CE compartment, it was difficult to obtain meaningful potential versus time transients for CE. The results presented in Figure 7A demonstrate that the presence of O2 has a small yet not negligible impact on the CE dissolution. In the case of n < 1500 cycles, the amount of dissolved Pt in the presence of O2 is higher than in its absence. On the other hand, in the case for n > 1500 the amount of dissolved Pt is lower than


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in its presence; a crossover point of these two transients is in the vicinity of n = 1500. Although the impact of O2 on the dissolution and morphology of Pt was examined by others, those data refer to the dissolution of WE while we analyze the dissolution of CE.26,29,47 However, it is reasonable to suggest that our results support the proposal that the presence of O2 influences Pt dissolution by enhancing the transport of dissolved Pt species. In addition, the process (Pt dissolution) can also be affected by the concurrently occurring ORR. The lower potential limit that CE adopts is below the potential of PtO reduction, therefore a passive state does not develop.3 Although in this contribution we focus on the dissolution behavior of Pt and do not examine the mechanism of ORR, it is possible that some reaction intermediates and byproducts accompanying ORR, and especially the formation of H2O2, could have an impact on the amount of dissolved Pt.

Figure 7. (A) Mass of dissolved Pt originating from the counter electrode (CE) versus the potential cycle number (mPt,CE versus n) for Ageom,WE/Ageom,CE = 10:1 in the 0.05 – 1.20 V range and in the 0.50 – 1.20 V range in the absence and presence of O2. (B) Mass of dissolved Pt originating from the counter electrode (CE) versus exposure time (mPt,CE versus texposure) for the same conditions as in the Figure A. Electrolyte: 0.5 M aqueous H2SO4; potential scan rate: s = 50 mV s–1; and temperature: T = 293 K.


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Rate of Platinum Dissolution. Platinum dissolution was a subject of several experimental studies. Johnson et al. examined the process in aqueous 1 M aqueous H2SO4 and 0.1 M aqueous HClO4 solutions by applying potential cycling and reported dissolution rates of 71.9 ng cm–2 h–1 and 53.6 ng cm–2 h–1, respectively.50 Fayette et al. investigated Pt dissolution in two different electrolytes, namely 0.1 M aqueous HClO4 and 2 M aqueous HCOOH + 0.1 M HClO4 solutions, upon repetitive potential cycling.20 The respective Pt dissolution rates of 61 ± 8 ng cm–2 h–1 and 272 ± 30 ng cm–2 h–1 clearly demonstrate that a low pH value and the presence of formate anion significantly enhance the process.

Topalov et al. conducted experiments using a unique

electrochemical scanning flow cell directly couple with ICP-MS for real-time measurements.17 Conversion of their values of mass of dissolved Pt per CV transient (10 ng cm–2 cycle–1) using the scan rate (10 mV s–1) and the lower and upper potential limits (0.10 – 1.80 V) yields a dissolution rate of 100 ng cm–2 h–1. We convert the mPt,CE versus n plots (Figure 7A) to mPt,CE versus exposure time (texposure) plots by taking into account the potential limits and the scan rate (Figure 7B). In the case of potential cycling in the 0.05 – 1.20 V range the same number of cycles corresponds to longer texposure values. The inset in Figure 7B show the respective mPt,CE versus texposure plots for the initial texposure = 5,000 s and reveals that these plots are almost linear. The slopes of these plots multiplied by 3600 s allow us to determine the dissolution rate values, which are as follows: (i) 755 ng cm–2 h–1 for the 0.50 – 1.20 V range; (ii) 1990 ng cm–2 h–1 for the 0.50 – 1.20 V range in the presence of O2; and (iii) 3090 ng cm–2 h–1 for the 0.05 – 1.20 V range. We wish to emphasize that these plots refer to Ageom,WE/Ageom,CE = 10:1. The three Pt dissolution rates are significantly higher than those cited above even though in one case of the reported data the process is facilitated by the low value of pH and the presence of formate in high concentration.


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Phenomena and Conditions Enhancing Platinum Dissolution. The results presented in Figure 7 show that dissolved O2 affects the dissolution behaviour of Pt counter electrode. In order to better understand the role of O2 in the process and any possible morphological changes, we acquired SEM images of the platinum CE prior to and after the formation of an oxide layer. The image presented in Figure 8A demonstrates that the Pt foil has a uniform morphology with features such as scratches, small cracks and pits originating from the manufacturing that involves rolling. Figure 8B shows clearly identifiable flakes of Pt oxide residing on top of the underlying Pt substrate; the oxide has dry mud (dry cracked mud) structure. The voids between the Pt oxide flakes suggest that either the Pt oxide develops in isolated regions creating a non-uniform layer or that the oxide layer initially develops uniformly over the entire surface but some flakes separate from the surface because the volume of a unit cell of Pt oxide is greater than that of Pt. Depending on their size, Pt oxide flakes can either precipitate at the vessel’s bottom or remain suspended within the electrolyte solution. Regardless of their final location, the detached Pt oxide can undergo chemical dissolution (electrochemical dissolution is impossible due to the lost electrical contact). We favor the second proposal according to which some Pt oxide flakes detach from the underlying substrate, although at the present time we have no direct evidence to support this proposal, because there is no reason for the Pt oxide to develop only at some arbitrary locations.


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Figure 8. SEM images of counter electrode prior to (A) and after (B) electrochemical dissolution in the presence of oxygen. Electrolyte: 0.5 M aqueous H2SO4; potential scan rate: s = 50 mV s–1; potential cycling range: 0.50 – 1.20 V; and temperature: T = 293 K.

The observation that in the case of Ageom,WE/Ageom,CE = 10:1 and 2:1 the counter electrode adopts high anodic potential (up to 1.85 V and 1.60 V, respectively) suggests that PtO2 can also develop.3 Such a formed PtO2 layer can undergo subsequent cathodic and chemical dissolution according to eqs 1 and 2:

PtO 2 (s) + 4 H + (aq ) + 2 e − → Pt 2 + (aq) + 2 H 2 O(l)

(1)

PtO 2 (s) + 4 H + (aq ) → Pt 4 + (aq) + 2 H 2 O(l)

(2)

Elsewhere,3 we reported that potential cycling generates both Pt2+ and Pt4+ ions (with at least 80% of dissolved Pt being present as Pt2+), and discussed that both PtO and PtO2 can undergo chemical dissolution and that the process requires an acidic medium. Although the nature of the complex compounds that form remains unknown, they are most likely aqua complexes. However, small amount of anions such as chloride or formate could alter the nature of these complexes and as reported elsewhere can also accelerate the Pt dissolution.20 Our proposal that


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PtO2 develops and undergoes subsequent chemical and electrochemical dissolution is supported by the results of Mitsushima et al.34 and Guilminot et al.51, who reported the formation of PtO2. Recent results of Topalov et al.17, that report important data pointing to the existence of both anodic and cathodic dissolution of Pt, also support the proposal that PtO2 forms and subsequently dissolves. Thus, our new results on the dissolution behavior of the Pt counter electrodes are in agreement with general trends observed for Pt working electrodes. To summarize our new results reported here and possible Pt dissolution mechanisms discussed elsewhere, it is reasonable to say that in the case of metallic (unoxidized) Pt materials direct anodic dissolution takes place and gives rise to a small value of mPt.3 In the case of Pt materials covered with a layer of anodically formed PtO, the species can undergo both chemical and electrochemical dissolution giving rise to an intermediate or large value of mPt. Finally, in the case of Pt materials covered with a layer of PtO2, the species can undergo both cathodic and chemical dissolution giving rise to an intermediate or large value of mPt. We mention in the Introduction that the experimental work was designed and conducted in aqueous H2SO4 solution because a majority of our previous research on Pt oxidation and dissolution was performed in this medium.

When employing our results to improve the

understanding of phenomena occurring in fuel cells, one need to bear in mind that the (bi)sulfate is a strongly adsorbing species, while trifluoromethanesulfonic acid (CF3SO3H) is the smallest fluorinated sulfonic acid and can serve as a suitable molecular model of a strong, fluorinated acid. Thus, additional work on Pt dissolution in relation to the AWE/ACE ratio should be conducted in this medium. However, the highest purity CF3SO3H commercially available does not match the purity of commercial H2SO4 or HClO4, thus new purification methods need to be developed or


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the impact of unavoidable contaminants on Pt dissolution behavior needs to be carefully examined. It has to be emphasized that in this research WE is placed a compartment outgassed with N2(g) and CE in a compartment either outgassed with N2(g) or deliberately saturated with O2(g). The conditions encountered in hydrogen fuel cells are different and involve H2(g) in the anode compartment and O2(g) in the cathode compartment. The activity H+(aq) and the fugacity of H2(g) control the anode potential and make Pt dissolution impossible. The dissolution of CE placed in electrolyte saturated with O2(g) can be of relevance to the dissolution of the fuel cell cathodes under certain conditions.

CONCLUSIONS A detailed analysis of the potential behavior and dissolution of platinum counter electrode in 0.5 M aqueous H2SO4 solution in relation to the ratio of surface areas of working and counter electrodes (Ageom,WE/Ageom,CE) was performed for the first time. As the potential of the working electrode (WE) versus time program follows a triangle shape in the 0.05 – 1.40 V range, the potential versus time transients for the counter electrode (CE) are non-linear and resemble charging/discharging curves. The experimental work was conducted for the Ageom,WE/Ageom,CE ratio being 10:1, 2:1, 1:2, and 1:10. The lower potential limit of CE is not affected by the Ageom,WE/Ageom,CE ratio but the upper limit is. In the case of Ageom,WE < Ageom,CE, the highest value of the potential of CE (ECE) is lower than the highest potential of WE (EWE) that is 1.40 V. However, in the case of Ageom,WE > Ageom,CE, the highest value of ECE is in the 1.73 – 1.85 V range and such high anodic potentials facilitate the growth of a thick surface oxide that can undergo subsequent dissolution. The dissolution behavior of both WE and CE was simultaneously examined in relation to the Ageom,WE/Ageom,CE ratio using inductively coupled plasma mass 26 ACS Paragon Plus Environment

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spectrometry and a purpose-designed three-compartment electrochemical cell. It was found the Ageom,WE/Ageom,CE ratio has a profound impact on the CE dissolution.

In the case of

Ageom,WE/Ageom,CE = 10:1, the amount of dissolved Pt is ca. six times greater than that in the case of Ageom,WE/Ageom,CE = 1:1. The surface oxidation and dissolution of the Pt CE was also examined in the H2SO4 electrolyte saturated with O2 and for Ageom,WE/Ageom,CE = 10:1. The presence of O2 has little impact on the Pt dissolution behavior, but the Pt surface oxide is thick and possess a dry mud-like structure; the latter is not observed in the case of the electrolyte deaerated with N2. The unique morphology of the thick Pt surface oxide can give rise to its physical detachment and consequently the higher value of dissolved Pt. The results reported in this contribution call for additional research to be conducted in high-purity trifluoromethanesulfonic acid (CF3SO3H) of gradually increasing concentrations. In addition, research should be conducted under conditions that mimic some operating conditions encountered in hydrogen fuel cells, such as anode and cathode placed in compartments saturated with H2(g) and O2(g), respectively, and operating at elevated temperatures.

ACKNOWLEDGEMENTS The authors gratefully acknowledge financial support toward this project from the Nissan Motor Company through the Nissan Research Center. They also acknowledge infrastructure support from the Natural Sciences and Engineering Research Council of Canada, Canada Foundation for Innovation, and Queen’s University.

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45. Jerkiewicz, G.; Vatankhah, G.; Lessard, J.; Soriaga, M. P.; Park, Y. S. Electrochim. Acta 2004, 49, 2451-2459. 46. Tremiliosi-Filho, G.; Jerkiewicz, G.; Conway, B. E. Langmuir 1992, 8, 658-667. 47. Dubau, L,; Castanheira, L.; Berthomé, G.; Maillard, F. Electrochim. Acta 2013, 110, 273– 281. 48. Nikkuni, F.; Ticianelli, E.; Dubau, L.; Chatenet, M. Electrocatalysis 2013, 4, 104-116. 49. Nikkuni, F. R.; Dubau, L.; Ticianelli, E. A.; Chatenet, M. Appl. Catal. B: Environmental 2015, 176-177, 486-499. 50. Johnson, D. C.; Napp, D. T.; Bruckenstein, S. Electrochim. Acta 1970, 15, 1493-1509. 51. Guilminot, E.; Corcella, A.; Charlot, F.; Maillard, F.; Chatenet, M. J. Electrochem. Soc. 2007, 154, B96-B105.


ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents Graphic


Page 32 of 48

Page 33 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Figure 1

ACS Catalysis

CE compartment WE compartment WE compartment CE compartment

RE compartment

Membrane

Luggin capillary

ACS Paragon Plus Environment

ACS Catalysis

Figure 2

1.0

0.5

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 34 of 48

0.0

-0.5 2

Ageom = 10.0 cm -1.0

2

Ageom = 5.0 cm

2

Ageom = 2.0 cm -1.5

2

Ageom = 1.0 cm

-2.0 0.0

0.2

0.4

0.6

0.8

E (V, RHE)


1.0

1.2

1.4

Page 35 of 48

2.5

Count (AU×106)

A

75 ppb

2.0 50 ppb

1.5

1.0 25 ppb 0.5

10 ppb 1ppb

5 ppb

0.0 0

200

400

600

t (s) 3

B Area (AU×107)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58

ACS Catalysis

Experimental Fitting (R2=0.9999)

2

1

0 0

20

40

Pt Conc (ppb)

Figure 3


60

80

ACS Catalysis

2.0

2.0

A

Figure 4

B

1.5

E (V, RHE)

E (V, RHE)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 36 of 48

1.0

0.5

1.5

1.0

0.5

0.0

0.0 0

40

80

120

160

0

40

80

t (s)

t (s)


120

160

Page 37 of 48

Figure 5A

1.5 1.0

A

0.5 +

0.0 -0.5

1.6

E (RHE, V)

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

ACS Catalysis

-1.0 -1.5

1.2 0.8 0.4 0.0 0

20

40

60

t (s)

-2.0 0.0

0.4

0.8

E (V, RHE)


1.2

1.6

ACS Catalysis

Figure 5B

2.0 1.5

B

1.0

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 38 of 48

0.5 0.0

+

-0.5 -1.0 -1.5 0.0

0.4

0.8

1.2

E (V, RHE)


1.6

Page 39 of 48

Figure 5C

0.4

C 0.2

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

ACS Catalysis

+

0.0

-0.2 0.0

0.4

0.8

1.2

E (V, RHE)


1.6

ACS Catalysis

Figure 5D

D 0.2

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 40 of 48

0.0

+

-0.2 0.0

0.5

1.0

E (V, RHE)


1.5

Page 41 of 48

Figure 5E

E 0.2

I (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

ACS Catalysis

0.0

+

-0.2 0.0

0.5

1.0

E (V, RHE)


1.5

0.0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Figure 5 Legend

0.2

0.4

ACS Catalysis

0.6

0.8

1.0

E (V, RHE) Region I Region II Region III Region IV Region V Region VI Region VII


1.2

Page 42 of 48 1.4

Page 43 of 48

Figure 6A

14000

A

m Pt, CE (ng cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

ACS Catalysis

12000 10000 8000

1:10 1:2 1:1 2:1 10:1

6000 4000 2000 0 0

1000

2000

3000

n (-)


4000

5000

ACS Catalysis

Figure 6B

7500

m Pt, WE (ng cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

B

6000

4500

Page 44 of 48

1:10 1:2 1:1 2:1 10:1

3000

1500

0 0

1000

2000

3000

n (-)


4000

5000

6000

Page 45 of 48

Figure 7A

25000

A

m Pt,CE (ng cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

ACS Catalysis

20000

0.50 - 1.20 V 0.05 - 1.20 V 0.50 - 1.20 V, O2

15000

10000

5000

0 0

1000

2000

3000

n (-)


4000

5000

ACS Catalysis

Figure 7B

30000

B 25000

0.50-1.20 V 0.05-1.20 V 0.50-1.20 V, O2

20000 5000

mPt, CE (ng cm-2 )

m Pt, CE (ng cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Page 46 of 48

15000 10000 5000

4000 3000 2000 1000 0 0

1000

2000

3000

4000

5000

texposure (s)

0 0

1000

2000

3000

4000

texposure (min)


5000

6000

Page 47 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

ACS Catalysis

B

A

Figure 8

10 m


10 m

ACS Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Graphic TOC


Page 48 of 48

Influence of the Working and Counter Electrode Surface Area Ratios

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