Unprecedented Effect of Impurity Cations on the Oxygen Reduction

assembly was installed in the cell, with the saturated calomel reference electrode (SCE) ... where jk(film) is the kinetic current corresponding to th...
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Langmuir 1999, 15, 8490-8496

Unprecedented Effect of Impurity Cations on the Oxygen Reduction Kinetics at Platinum Electrodes Covered with Perfluorinated Ionomer Tatsuhiro Okada,*,† Jørgen Dale,‡ Yuusuke Ayato,† Odd Andreas Asbjørnsen,‡ Makoto Yuasa,§ and Isao Sekine§ National Institute of Materials and Chemical Research, Higashi 1-1, Tsukuba, Ibaraki 305-8565, Japan, Department of Thermal Energy and Hydropower, Norwegian University of Technology and Science, N-7034 Trondheim, Norway, and Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Chiba 278-8510, Japan Received May 24, 1999. In Final Form: July 28, 1999

Platinum electrodes covered with a perfluoro-sulfonated ionomer membrane show unique behaviors in comparison with bare platinum immersed in aqueous solutions. The electrochemical interface between the metal and the polymer can be different from the interface between the metal and the solution phase. In this context, platinum electrodes covered with such a polymer membrane are an interesting system, and deserve detailed study. Here the oxygen reduction reaction at the platinum surface covered with a perfluorosulfonated ionomer membrane is investigated kinetically in 0.1 N H2SO4 in the presence of 0.001 N Na+, K+, or Ca2+ ions, using rotating disk electrodes. It is discovered that the impurity ions, even in small amounts, hinder enormously the rate of the charge transfer step of oxygen reduction at the platinum covered with perfluoro-sulfonated ionomer. Especially the effect of Na+ in the membrane is very serious considering the fact that there exists only 2% Na+ of the exchange site in the membrane phase at this condition. Platinum covered with perfluoro-sulfonated ionomer membrane has historically attracted much research interest, based on the fact that oxygen concentration and H+ ion concentration in the membrane are both larger than those in normal acidic solutions, and could show larger catalytic activity than in bare platinum (Gottesfeld, S.; et al. J. Electrochem. Soc. 1987, 134, 1455. Lawson, D. R.; et al. J. Electrochem. Soc. 1988, 135, 2247). However, this expectation encountered disappointing failure (Zecevic, S. K.; et al. J. Electrochem. Soc. 1997, 144, 2973). Results here indicate that such a paradox could be accounted for by the effect of the metal-polymer interface that alters the reaction conditions of oxygen reduction.

1. Introduction The oxygen reduction reaction is of basic and practical interest in many such fields as fuel cells,4,5 oxygen sensors,6 respiratory chain,7 solar energy conversion, etc. Since the reaction involves a four-electron reaction path, the process needs a high energy barrier for its occurrence, and is difficult to control. This slow process might be influenced in many ways, if the passage of electrons at the electrode surface is manipulated by some intermediates and neighboring species that would exist in the vicinity of the electrode, e.g., in the electric double layer.8 * Corresponding author. Tel.: +81 298 54 4437. Fax: +81 298 54 4678. E-mail: [email protected]. † National Institute of Materials and Chemical Research. ‡ Norwegian University of Science and Technology. § Science University of Tokyo. (1) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S. J. Electrochem. Soc. 1987, 134, 1455. (2) Lawson, D. R.; Whiteley, L. D.; Martin, C. R.; Szentirmay, M. N.; Song, J. I. J. Electrochem. Soc. 1988, 135, 2247. (3) Zecevic, S. K.; Wainright, J. S.; Litt, M. H.; Gojkovic, S. L.; Savinell, R. F. J. Electrochem. Soc. 1997, 144, 2973. (4) Bockris, J. O’M.; Srinivasan, S. Fuel Cells: Their Electrochemistry; McGraw-Hill: New York, 1969. (5) Hoar, J. P. The Oxygen Electrode on Noble Metals. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Ed.; Interscience Pub.: New York, 1967; Vol. 6. (6) Madou, M. J.; Morrison, S. R. Chemical Sensing with Solid State Devices; Academic Press: Boston, 1989; Chapter 5. (7) Stryer, L. Biochemistry; W. H. Freeman & Co.: San Francisco, 1975; Chapter 14. (8) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980.

A platinum electrode covered with perfluoro-sulfonated ionomer is often utilized as a catalyst for low-temperature fuel cells, water electrolysis, etc. The oxygen reduction reaction on this catalyst is of practical importance with the expectation that both oxygen and H+ ion are enriched in the ionomer as compared with the outer solution phase, and therefore, the kinetics will be enhanced here.1-3 In relevance to this, the effect of foreign impurity ions on the catalytic performance of electrodes covered by ionomer membranes has not been studied before, and will give an interesting subject, if the effect is not trivial. 2. Experimental Section 2.1 Materials. 5% Nafion solution dissolved in water and aliphatic alcohol was purchased from Aldrich. Dimethylformamide (DMF 99.5%, Wako Pure Chemical Industries, Ltd.) was used as cosolvent to cast the Nafion film from solution. Sulfuric acid was of reagent grade (95%, Wako). Sodium sulfate (99%, Wako), potassium sulfate (99.5%, Wako), and calcium sulfate (99%, Wako) were used without further purification. Water was first deionized (specific resistance 107 Ω cm) by Millipore Milli-Q II, and then doubly distilled using a quartz glass distillation flask. Oxygen gas and nitrogen gas were of 99.9% purity. 2.2 Preparation of Electrodes Covered with Ionomer. The rotating disk electrode (RDE) was made of a platinum disk (diameter 4 mm) and a surrounding cylindrical Teflon holder. The electrode surface was roughened with #400 emery paper. The electrode was cleaned with ethanol, then soaked in a 50/50 H2SO4/HNO3 mixture for 15 min, rinsed with hot pure water (about 100 °C) for 2 min, and finally rinsed in pure water for 15 min under sonication. The electrode was subjected to a potential cycling between -0.32 and 1.18 V vs SCE at 0.1 V s-1 for 30 min

10.1021/la990625e CCC: $18.00 © 1999 American Chemical Society Published on Web 09/11/1999

Impurity Cation Effect on Perfluorinated Ionomer Pt Electrodes in 0.1 N H2SO4 saturated with nitrogen gas. After drying, the electrode was mounted on a spin coater with the surface upward, and rotated at a rate of 1000 rpm; 25 µL of a mixture of 10 parts commercial 5% Nafion solution and 1 part DMF was dropped on the surface and spin-coated for 5 min.9 New layers were subsequently added until the desired thickness (9-15 µm) was attained. The film was dried for 2 h at 80 °C and then annealed for 10 h at 135 °C in a vacuum. 2.3 Measurement of Oxygen Reduction on Film Covered Platinum. The electrochemical cell was a double jacket glass cell of volume 100 cm3, and it enabled temperature control at 25 ( 0.5 °C by water circulation through the jacket. A three-electrode assembly was installed in the cell, with the saturated calomel reference electrode (SCE), the Pt wire spiral counter electrode, and the RDE working electrode. The electric rotator for RDE was an EG&G PARC model 616 RDE. The electrochemical control and measurements were done with a Nikko Keisoku model DPSG-1 potentio-galvanostat, Hokuto Denko model HB-103 linear scanner, and Riken Denshi model F-35 X-Y recorder. The Nafion film covered platinum RDE was soaked in 0.1 N H2SO4 saturated with oxygen gas, and the electrochemical measurements for oxygen reduction were first carried out for noncontaminated film. In the case of the contaminated film, the Nafion film covered platinum RDE was soaked in 0.1 N H2SO4 with 0.001 N Na2SO4, 0.001 N K2SO4, or 0.001 N CaSO4 saturated with oxygen gas. The kinetic measurements were done for three different soaking times, 3 h, 1 day, and 3 days. The electrode was then soaked in fresh 0.1 N H2SO4 overnight and subjected to potential cycling between -0.32 and 1.18 V vs SCE at 0.1 V s-1 for 1 h in 0.1 N H2SO4 and finally soaked in fresh 0.1 N H2SO4 1 day more. The recovery of the electrode to the noncontaminated film covered condition was confirmed before repeating the measurement in a solution containing the next contaminant ion. Before the measurement the RDE was cycled between -0.32 and 1.18 V vs SCE at 0.1 V s-1 in nitrogen saturated 0.1 N H2SO4 until steady state was attained (about 2 h). The electrolyte was then bubbled with oxygen gas for at least 30 min. During the electrochemical measurements, oxygen gas was fed to the top air volume but only a small amount to the solution, to keep the oxygen content in the electrolyte constant. For recording the oxygen reduction current on RDE, the potential was scanned from 0.88 V to -0.22 V vs SCE and back at a scanning rate of 0.01 V s-1, at several rotation speeds, between 200 and 1600 rpm. A cyclic voltammogram (CV) was recorded for 15 min between -0.32 and 1.18 V vs SCE at 0.1 V s-1 in the same 0.1 N H2SO4 at quiescence. Linear scanning voltammetry (LSV) was carried out with the same electrode. The potential was cycled between 0 and 0.9 V vs SCE at a scan rate of 0.1 V s-1 for 2 min and then held at 0.9 V for 2 min. The LSV was recorded for the potential range 0.9 V to 0 V vs SCE, at different scanning rates between 0.01 and 0.2 V s-1, at electrolyte quiescence.

3. Calculations

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jf,l ) nFDf

C/f δf

(3)

and jL is the diffusion limiting current in the solution.

jL ) nFDsC/s/δh ) 1.554nFDs2/3v-1/6C/s w1/2

(4)

Here n is the number of electrons for the O2 reduction reaction, F is Faraday’s constant, kf(E) is the rate constant as a function of the electrode potential, C/f and Df are the maximum concentration and the diffusion coefficient of O2 in the Nafion film, δf is the thickness of the film, C/s and Ds are the concentration and diffusion coefficient of O2 in the solution, δh is the thickness of the Nernst diffusion layer, v is the kinematic viscosity of the solution and w is the rotation speed (Hz) of RDE. It is assumed that there is a proportional partition between the O2 concentration in the film Cf(δf) and that in the solution Cs(δf) at the film-solution interface at any time (independent of the current density and the rotation speed)

Cf(δf) ) ξCs(δf)

(5)

It is easily noted that in the absence of the film, eq 1 becomes

1 1 1 ) + j jk(bare) jL

(6)

and the kinetic current in this case becomes

jk(bare) ) nFkf(E)C/s ) jk(film)/ξ

(7)

By plotting the inverse of the current density against the inverse of the square root of w (Koutecky-Levich plots), one can observe from the intercept of the plots jk(bare)-1 and jk(film)-1 + jf,l-1 for bare and filmed RDE, respectively. Knowing the value of ξ, jf,l can be calculated from which the film thickness δf is obtained using eq 3. From the slope of the Koutecky-Levich plot, one can determine n from eq 4, if other parameters are known. If the current density for filmed Pt-RDE is measured for either pure or contaminated Nafion film, and plotted in a Koutecky-Levich graph, then the difference in the intercept gives

1 1 1 1 1 1 - ) + jc jr jk,c(film) jk,r(film) jf,l,c jf,l,r

3.1 Kinetic Current of Oxygen Reduction on Nafion Covered Pt. In the RDE experiment, the measured current density j (A cm-2) in the mixed control regime of charge transfer, diffusion of oxygen gas in the film and in the solution is expressed as follows,2,3 if all the processes are linear with respect to O2 concentration

It can be assumed that in the limiting current region of oxygen reduction (high polarization), the kinetic current grows very high so that

1 1 1 1 + + ) j jk(film) jf,l jL

1 1 98 0 jk,c(film) jk,r(film) ηf-∞

(1)

where jk(film) is the kinetic current corresponding to the maximum O2 concentration in the film,

jk(film) ) nFkf(E)C/f jf,l is the diffusion limiting current in the film, (9) Moore, R. B.; Martin, C. R. Anal. Chem. 1986, 58, 2569.

(2)

(8)

(9)

jc and jr at this condition are denoted as jl,c and jl,r, respectively, so that

1 1 1 1 1 1 + ) - jk,c(film) jk,r(film) jc jr jl,c jl,r

(10)

If jk(bare) is known from the Koutecky-Levich plot of bare Pt-RDE, then jk,r(film) is calculated from eq 7, and likewise jk,c(film) is calculated from eq 10. All the kinetic

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currents are to be scaled by the active surface area obtained from the hydrogen adsorption-desorption peaks in CVs. 3.2 Transport Parameters of Oxygen in the Nafion Film. Assuming that the film thickness does not change by the presence of impurity ions, the ratio of eq 3 for contaminated film to pure film gives

C/f,cDf,c C/f Df

)

jf,l,c jf,l,r

(11)

In the LSV experiment, the peak current for a totally irreversible system is expressed as follows8

jp ) 2.99 × 105n(Rna)1/2C*D1/2E˙ 1/2

(12)

where R is transfer coefficient, and na is the number of electrons involved in the rate-determining step and E˙ is the potential scan rate (V s-1). The peak potential Ep is

Ep ) E0′ -

[

( ) (

)]

RnaFE˙ RT D1/2 0.780 + ln 0 + ln RnaF RT k

1/2

(13) 0′

0

where E is the formal potential and k is the standard heterogeneous rate constant.8 Equations 12 and 13 are valid either for filmed or bare Pt electrodes, because the time lapse of measurements is very short and the thickness of the diffusion layer is within the phase (film or solution) contacting the Pt surface. From the scan rate dependence of the peak potential, one obtains

dEp 2.3RT )d log E˙ 2RnaF

(14)

Also from the difference in peak potentials for filmed and bare Pt, one obtains the ratio of diffusion coefficients from eq 13, if E0′, k0 and Rna are the same for filmed and bare Pt

[

2RnaF Df (Ep,b - Ep,f) ) exp Ds RT

]

(15)

The ratio of concentration of O2 in the film and in the solution is calculated from eq 12 under the same assumption

C/f C/s

)

()

Sf Ds Sb Df

1/2

(16)

where S ) djp/dE˙ 1/2 and the subscripts f and b stand for filmed and bare electrodes, respectively. From the literature values of Ds and C/s , Df and C/f are calculated from eqs 15 and 16. In the case of Pt covered with contaminated film, the reaction kinetics may differ from Pt covered with pure film, and eq 12 yields the ratio of transport parameters in the contaminated film against the pure film

C/f,cDf,c1/2 C/f Df1/2

)

( )

Sf,c Rna Sf (Rna)c

1/2

(17)

Both eqs 11 and 17 can be used to compare C/f and Df in contaminated and pure films.

Figure 1. Cyclic voltammograms of a platinum disk covered with Nafion film at a scan rate of 0.1 V s-1 in various solutions saturated with oxygen gas. Solution (a) 0.1 N H2SO4, (b) 0.1 N H2SO4 + 0.001 N Na2SO4, (c) 0.1 N H2SO4 + 0.001 N K2SO4, (d) 0.1 N H2SO4 + 0.001 N CaSO4.

4. Results 4.1 Cyclic Voltammetry. Figure 1a shows the steadystate CV on a Nafion film-covered platinum disk electrode in 0.1 N H2SO4 saturated with oxygen gas. In comparison with the CV of bare platinum, the hydrogen adsorptiondesorption peaks were slightly rounded, but other features were very similar between two electrodes. Table 1 shows the active (true) surface area as calculated by the hydrogen wave in CV, the bare platinum being chosen as the reference of unit surface area. For the bare electrode, the active surface area decreased about 19% as compared with the freshly prepared surface in a few days after preparation, but this active surface area did not change largely between bare and the filmed electrodes. The kinetic current is scaled by this active surface area. Figure 1b-d shows CVs for platinum covered with Nafion films after the electrodes were soaked in 0.1 N H2SO4 together with various kinds of 0.001 N impurity ions. No apparent change was observed for electrodes filmed with Nafion containing Na+, but those with K+ and Ca2+ impurity ions show slight changes both in the hydrogen adsorption-desorption region and in the platinum oxide formation-reduction region. Note that the oxygen reduction potential overlaps with the platinum oxide formation-reduction region, and the oxide layer that is affected by the presence of impurity ions might alter the oxygen reduction kinetics. This possibility will be studied further in the following sections. 4.2 Kinetic Current of Oxygen Reduction. Figures 2a-c show polarization curves of oxygen reduction at bare and Nafion film covered platinum electrodes in oxygen gas saturated 0.1 N H2SO4. The polarization behavior of oxygen reduction was characterized by the appearance of a hysteresis, and above 0.45 V vs SCE, the current was smaller for the potential sweep of negative direction from that of the positive direction. This was ascribed to the presence of an oxide layer on platinum surfaces that are oxidized at potentials above 0.6 V and reduced at potentials

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Table 1. True Surface Area Shown as a Ratio of the Hydrogen Adsorption-Desorption Peak Area of CVs of the Nafion Film Covered Platinum Electrodes, in Comparison with that of Bare Platinum in 0.1 N H2SO4 Nafion film covered Pt electrode solution

bare Pt 0.1 N H2SO4

0.1 N H2SO4

0.1 N H2SO4 + 0.001 N Na2SO4

0.1 N H2SO4 + 0.001 N K2SO4

0.1 N H2SO4 + 0.001 N CaSO4

run 1 run 2 run 3

1.00 1.10 0.96

1.11 1.01 1.03

1.06 0.97 1.01

1.03 1.15 1.06

0.93 1.05 1.02

Figure 2. Polarization curves of oxygen reduction at the platinum RDE. (a) Bare platinum at 400 rpm in 0.1 N H2SO4, (b) Nafion film covered platinum at 400 rpm in 0.1 N H2SO4, (c) Nafion film covered platinum at 200 rpm in 0.1 N H2SO4, (d) Nafion film covered platinum at 200 rpm in 0.1 N H2SO4 + 0.001 N Na2SO4 after 67 h of immersion. The curves (a), (c), and (d) are shifted by +0.3 V, -0.3 V, and -0.6 V, respectively, along the potential axis in reference to curve (b).

less than 0.1 V.1,3 This implies that oxygen reduction on platinum is occurring on partially oxide covered surfaces. Figure 2d shows the polarization curve at the Nafion film covered electrode measured in 0.1 N H2SO4 + 0.001 N Na2SO4 saturated with oxygen gas. In the presence of impurity ions, accumulation of impurity ions at the platinum surface might have caused a less apparent increase in the oxygen reduction current at the anodic scan. Thus the condition at the platinum-polymer interface would play an important role in the whole kinetic processes. Figure 3 depicts the Koutecky-Levich plots for oxygen reduction current on bare platinum and the Nafion film covered platinum electrodes that are immersed in pure 0.1 N H2SO4 and in 0.1 N H2SO4 + 0.001 N K2SO4 at various soaking times. It is seen that the intercept of the plots changes between the bare and the filmed electrodes, the latter being also affected by the presence of impurity ions depending on the soaking time. On the other hand, the bare platinum immersed in 0.1 N H2SO4 containing impurity ions at as high a level as 0.01 N reveals itself to be unaffected by oxygen reduction. The kinetic current jk,c(film) of the oxygen reduction is calculated using eq 10. In Figure 4, jk,c(film)/jk,r(film) is plotted as a function of soaking time, i.e., the kinetic current for the film covered platinum contacting the solution containing various kinds of impurity ions, in comparison with that for the same electrodes measured in no-impurity conditions. An unambiguous drop in the kinetic current is observed in all cases of impurity ions, and this drop increased with the increase in the soaking time. Note that in the case of platinum covered with contaminated film, the hysteresis that appeared in the forward and backward scans of a polarization curve of oxygen reduction diminished along with the soaking time, indicating the change in the platinum oxide barrier. Similar changes are observed in

Figure 3. Koutecky-Levich plots of oxygen reduction currents at 0.20 V vs SCE on bare and the Nafion film covered platinum in various solutions. (O) bare platinum in 0.1 N H2SO4,a (b) bare platinum soaked in 0.1 N H2SO4 + 0.01 N K2SO4,a (×) Nafion film covered platinum in 0.1 N H2SO4, (4) Nafion film covered platinum soaked in 0.1 N H2SO4 + 0.001 N K2SO4 for 3 h, (0) Nafion film covered platinum soaked in 0.1 N H2SO4 + 0.001 N K2SO4 for 29 h, (]) Nafion film covered platinum soaked in 0.1 N H2SO4 + 0.001 N K2SO4 for 80.5 h. a No noticeable changes in the plots were observed for bare platinum soaked in pure or for impurity containing 0.1 N H2SO4 for 2-50 h.

Figure 4. Kinetic current of oxygen reduction on platinum covered with Nafion film in 0.1 N H2SO4 solution containing 0.001 N Na2SO4(0), 0.001 N K2SO4(O), or 0.001 N CaSO4(4), saturated with oxygen gas. The kinetic current jk,c(film) for contaminated film normalized by the kinetic current jk,r(film) for noncontaminated film is plotted as a function of soaking time. Open symbols: 0.15 V, half-closed symbols: 0.20 V, closed symbols: 0.25 V vs SCE.

the oxide formation-reduction peaks in the CVs, and it is anticipated that electron tunneling across the oxide layer and/or the electrical double layer might have been altered by the presence of impurity ions. Seeing the polarization curves in positive and negative potential scans, the change in the oxide layer appears to be not large enough to cause this significant change in the chargetransfer kinetics of oxygen reduction.

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Figure 5. Peak current in LSV plotted against E˙ 1/2. (O) bare platinum in 0.1 N H2SO4, (4) Nafion film covered platinum in 0.1 N H2SO4, (0) bare platinum in 0.1 N H2SO4 + 0.001 N Na2SO4, (]) Nafion film covered platinum in 0.1 N H2SO4 + 0.001 N Na2SO4. Table 2. Ratios of the Maximum Concentration and the Diffusion Coefficients of Oxygen in Nafion Film Immersed in Various Kinds of Solutions, in Comparison with Those in the Solution at 25 °C 0.1 N H2SO4 C/f (δf)/C/s Df/Ds C/f (δf)/ 10-6 mol cm-3 Df/10-5 cm2 s-1

1.08a 1.04b 0.90a 0.92b 1.62a 1.55b

0.2 N 1.0 N 0.7 N 14.7 N H2SO4c H2SO4c H3PO4c H3PO4c 1.5 0.4

2.7 0.2

2.8 0.1

18 0.07

1.80a 1.84b

a

Film thickness 8.6 µm (this work). b Film thickness 14.6 µm (this work). c Data from ref 3.

4.3 Oxygen Transport in the Film. Figure 5 shows the peak current measured in LSV plotted against the square root of the scanning rate for bare platinum and for the Nafion film covered platinum in 0.1 N H2SO4 and in 0.1 N H2SO4 containing 0.001 N Na2SO4. Good linearity in the plot supports the rationale of the present analysis, and transport parameters are evaluated using the methodology in the calculation section. Calculated values of oxygen concentration and the diffusion coefficients based on eqs 15 and 16 are compared with reported values in Table 2.3 A good conformity is seen between this work and reported results, showing the reliability of the present measurements. Also it is to be noted that oxygen concentration in the film as compared to the solution is larger and the diffusion coefficient smaller when the concentration of the acid solution contacting the membrane increases. Figure 6 shows the calculated parameters C/f,cDf,c and / Cf,cDf,c1/2 vs soaking time, based on eqs 11 and 17 for Nafion films contaminated with various kinds of impurity ions in comparison to pure films. In the Nafion film containing impurity cations, both parameters decrease with soaking time, but the reduction is smaller compared to the reduction for kinetic currents. A possible mechanism for this reduction of transport parameters in the contaminated film is that impurity ions changed the structure of the polymer and its flexibility by, for example, the shrinkage of hydrophilic domains and/or the cross-linking of cation exchange sites,10,11 thus introducing a barrier for the oxygen flux in the polymer network. (10) Okada, T.; Mφller-Holst, S.; Gorseth, O.; Kjelstrup, S. J. Electroanal. Chem. 1998, 442, 137. (11) Okada; T., Nakamura; N.; Yuasa, M.; Sekine, I. J. Electrochem. Soc. 1997, 144, 2744.

Figure 6. The parameters C/f,cDf,c and C/f,cDf,c1/2 as expressed by the ratio of these values for the contaminated film against those for the noncontaminated film are shown in the time course. Symbols are the same as in Figure 4.

5. Discussion The surprising fact is that the oxygen reduction is strongly influenced by small amounts of impurity ions. The amounts of Na+ and Ca2+ in the membrane were measured to be 2% and 10% of the cation exchange site, respectively.10,11 Note that this effect is only seen when the platinum surface is covered with ionomer films; a bare platinum electrode immersed in 0.1 N H2SO4 containing as high as a 0.01 N impurity ion level did not demonstrate this effect (Figure 3). It is also interesting to note that when the electrode covered with contaminated film was brought in contact with pure H2SO4 for 1-2 days, the film recovered to the original noncontaminated condition, and the kinetic current as well as the transport parameters were characteristic of platinum covered with pure film. There are some possibilities to explain these phenomena. The mechanism of deterioration might be different between the kinetic current and the transport parameters, and these are discussed separately. As can be seen from eq 1, jk(film) and jf,l are mutually separable parameters. First about the deterioration of the kinetic current, one may assume that the presence of impurity ions in the film would give rise to some blockage of active platinum surfaces by under-potential deposition of metal ions or by a covering action of the polymer chain, and hinder the charge-transfer process. However, this blockage mechanism can be ruled out after the result of CVs for filmcovered platinum in pure form or with impurity ions containing H2SO4. The active surface area in the hydrogen adsorption-desorption region did not reveal significant changes by the presence of ionomer films and/or by impurity ions. Second, one could suspect that some change of mechanism in the charge-transfer process occurred by the presence of impurity ions. Figure 7 shows the peak potential as a function of the logarithm of the potential sweep rate, in the LSV for bare platinum electrode and a platinum electrode covered with Nafion film. The linearity between Ep against log E˙ strongly indicates that the kinetics of oxygen reduction is totally irreversible8 under all conditions studied here. Multiplying the transfer

Impurity Cation Effect on Perfluorinated Ionomer Pt Electrodes

Figure 7. Peak potential in LSV plotted against log E˙ . (O) bare platinum in 0.1 N H2SO4, (4) Nafion film covered platinum in 0.1 N H2SO4, (0) bare platinum in 0.1 N H2SO4 + 0.001 N Na2SO4.

Figure 8. Transfer coefficient times the number of electrons involved in the rate-determining step, Rna, as expressed by the ratio of this value for the contaminated film against that for the noncontaminated film, is shown in the time course. Symbols are the same as in Figure 4.

coefficient R times the number of electrons in the rate determining step na on platinum covered with contaminated Nafion film is calculated in reference to that for pure film, and is plotted in Figure 8 as a function of soaking time. The results show no significant change in Rna, indicating that the reaction mechanism does not change by the presence of impurity ions in the film at this impurity level. The result that there is no significant change in the shape of the LSV curves before and after the impurity penetration in the film also supports this conclusion. Third, one could consider that O2 and/or H+ ion in the polymer might be depleted at the platinum-polymer interface by the accumulation of impurity ions at this interface. The oxygen reduction proceeds with the scheme: O2 + 4H+ + 4e f 2H2O, and depletion of either O2 or H+ would seriously affect the reaction kinetics, see eq 2. It is reported that the electroactivity of Cu(bpy)22+ at the Nafion film coated platinum is severely inhibited by a drastic decrease in the diffusion rate of the complex.12 Seeing the result of Figure 6, a slightly decreasing trend is observed for the O2 concentration by impurity ion penetration, but it is not enough to account for the large decrease in jk,c(film). A decrease in H+ might also occur by the presence of impurity ions, but considering the fact that concentration of H+ in Nafion polymer is of the order of 1 mol dm-3, this possibility is rather small. (12) Rong, D.; Anson, F. C. J. Electroanal. Chem. 1996, 404, 171.

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The remaining reason might be the change in the platinum oxide structure, or that of the electric double layer structure at the interface where charge transfer occurs, or both. CVs and polarization curves indicated some change in the platinum oxide layer, which might have altered the electron-tunneling rate across this layer, but this possibility is expected to be rather small. Another possibility is the change in the structure of the electric double layer by specific adsorption of the impurity ionpolymer complex at the platinum surface. In the platinum-ionomer interface, the anion and cation distribution in the electric double layer would be no more like the distribution in platinum-solution interface.13,14 Considering the significant effect observed here, this kind of system will open a new research field in the sense of a constrained electric double layer at the solid electrode-polymer electrolyte interface. As for the change in the transport parameters, the most probable reason might be the change in flexibility of the polymer network. Note the fact that both Df and C/f are very sensitive to the environment, e.g., the concentration of the contacting acid solutions, as can be seen in Table 2. Therefore, the presence of impurity ions in the film could well affect these parameters. If oxygen gas would permeate preferentially through hydrophilic domains in the polymer, then the structure and size of the hydrophilic channel, which are influenced by impurity ions, would play a major role in the transport process of oxygen. The results of the present work are of serious practical implications in such cases as the cathode catalyst layer utilizing platinum covered with ionomer films in, e.g., polymer electrolyte fuel cells,15 where small amount of contaminant ions might cause enormous performance degradation.16,17 Considering the fact that the fuel cell involves a similar configuration as in the present electrode system, and that the cathode chamber is fed by an air stream, this effect should be accounted for very carefully. Further work is in progress to clarify the mechanism for the deterioration of the oxygen reduction kinetics on the Nafion film covered electrode. 6. Conclusions An unprecedented effect of impurity ions on the oxygen reduction kinetics on ionomer film covered platinum is first reported in this study. The deterioration of oxygen reduction kinetics on perfluorinated ionomer-filmed electrodes by the presence of impurity ions is discovered, and this effect turns out to be very serious even in the case of only small amount of impurity ions in the solution phase. This effect is also of significant practical implications in such cases as the cathode catalyst layer in polymer electrolyte fuel cells.16-19 The degradation mechanism is studied in two aspects: one is the charge transfer of oxygen reduction, and the other is the transport of oxygen gas through the polymer layer. The kinetic current of charge transfer dropped about 50% after 1-3 days for platinum electrodes covered with contaminated film as compared with pure film. No evidence was observed for a change in the reaction (13) Wang, J. X.; Adzˇic, R. R. J. Electroanal. Chem. 1998, 448, 205. (14) Cha, C.; Zu, Y. Langmuir 1998, 14, 6280. (15) Ticianelli, E. A.; Derouin, C. R.; Redondo, A.; Srinivasan, S. J. Electrochem. Soc. 1988, 135, 2209. (16) Mitsuda, K. NEDO Report, FY 1996; NEDO: Tokyo, 1997; p 90. (17) Wakizoe, M. NEDO Report, FY 1997; NEDO: Tokyo, 1998; p 51. (18) Okada, T.; Ayato, Y.; Yuasa, M.; Sekine, I. J. Phys. Chem. B 1999, 103, 3315. (19) Okada, T. J. Electroanal. Chem. 1999, 465, 1; 18.

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mechanism of the oxygen reduction in contaminated conditions. The reduced kinetic current was attributed to the change of electron tunneling, either through the platinum oxide or the electric double layer of the platinum surface.

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The oxygen concentration and diffusion coefficients also declined for contaminated film. This was ascribed to the changes in polymer flexibility. LA990625E