Effects of Additives on Oxygen Reduction Kinetics at the Interface

The Journal of Physical Chemistry C 2010 114 (34), 14635-14645. Abstract | Full Text ... V. Tolmachev. Electrochemical and Solid-State Letters 2010 13...
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Langmuir 2003, 19, 2325-2332

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Effects of Additives on Oxygen Reduction Kinetics at the Interface between Platinum and Perfluorinated Ionomer Tatsuhiro Okada,*,† Hiroki Satou,‡ and Makoto Yuasa‡ National Institute of Advanced Industrial Science and Technology, Higashi 1-1-1, Central 5, Tsukuba, Ibaraki 305-8565, Japan, and Department of Industrial Chemistry, Faculty of Science and Technology, Science University of Tokyo, 2641 Yamazaki, Noda, Chiba 278-8510, Japan Received August 26, 2002. In Final Form: November 25, 2002 The oxygen reduction reaction (ORR) at platinum electrodes covered with perfluorosulfonated ionomer film was studied in 0.05 mol dm-3 H2SO4 in the presence of various kinds of ammonium ions. Ammonium ion derivatives (R4N)2SO4 (R ) H, CH3) and (R4N)HSO4 (R ) C2H5, C3H7, and C4H9) were added with various amounts in the solution. The electrochemical measurements of ORR were performed to evaluate both charge transfer and diffusion kinetics of oxygen reduction at the Nafion film covered platinum rotating disk electrode. ORR was affected differently for different molecular weights of ammonium derivative ions. The suppression of ORR started in a short time for ammonium derivative ions of high molecular weight, reflecting the strong adsorption at the platinum surface. It was inferred that these ions strongly distorted the electric field at the platinum|ionomer interface. From the knowledge, a new method to inhibit the ORR degradation at platinum|ionomer interface was proposed, and several kinds of carboxylic acids or amino acids were tested as additives in the ionomer film. It was discovered that these additives successfully inhibit the ORR degradation brought about by impurity cations. After investigating the ORR kinetics, it was found that the additives prevented the degradation of the charge transfer step effectively, but the diffusion process in the film was degraded more by these additives.

1. Introduction In recent works, it was reported that alkali and alkaline earth metal cations or transition metal cations strongly hinder the kinetics of the oxygen reduction reaction (ORR) on platinum covered with perfluorinated ionomer membranes, relating to the membrane contamination and performance degradation in polymer electrolyte fuel cells.1-3 Those cations, even in small amounts as low as 1% H+ concentration in the solution, suppressed the charge transfer step that occurs at the platinum|ionomer interface. Results were connected to the degradation of performance during the fuel cell operation.4 This surprising fact was not observed for the platinum|electrolyte solution interface and, therefore, was thought to be specific to the metal|ionomer membrane interface.2,3 No evidence was observed about the altered reaction mechanisms, reduced H+ ion in the ionomer film, or blockage of the interface by impurity ions, but the structural change of the ionomer appeared to be related to this degradation. In this study the effect of impurity ions on the catalytic performance of platinum electrodes covered with ionomer membranes is investigated further. Ammonium derivative cations are examined, because these ions are hydrophobic as compared to alkali metal cations or other metal ions, are expected to cause structural changes to the ionomer, and affect the metal|ionomer membrane interface differ* Corresponding author. Fax: +81 298 61 4678. E-mail: okada.t@ aist.go.jp. † National Institute of Advanced Industrial Science and Technology. ‡ Science University of Tokyo. (1) Okada, T.; Dale, J.; Ayato, Y.; Asbjørnsen, O. A.; Yuasa, M.; Sekine, I. Langmuir 1999, 15, 8490. (2) Okada, T.; Ayato, Y.; Dale, J.; Yuasa, M.; Sekine, I.; Asbjørnsen, O. A. Phys. Chem., Chem. Phys. 2000, 2, 3255. (3) Okada, T.; Ayato, Y.; Satou, H.; Yuasa, M.; Sekine, I. J. Phys. Chem. B 2001, 105, 6980. (4) Wakizoe, M. NEDO Report, Fy. 1997; NEDO: Tokyo, 1998; p 51.

ently from the latter ions. Recently, it was reported that some tens of parts per million ammonium in the fuel gas badly affects the fuel cell performance,5 and this fact also motivates the present study. It is intended to obtain insight into the mechanism of the kinetic degradation, and from this viewpoint a new method is tested in which some additives that would work at the metal|ionomer membrane interface could alter the degradation scheme of ORR. If the interface between platinum and ionomer plays the major role, such additives might to some extent interfere with the phenomena at the interface. This new method would become especially important in fuel cell technology, when the lifetime and longevity are the issues for practical applications. 2. Experimental Methods 2.1. Materials. Nafion solution was 5% polymer (EW ) 1100) dissolved in aliphatic alcohol, as purchased from Aldrich. N,N′Dimethylformamide (HCON(CH3)2, 99.5%, abbreviated as DMF, Wako Pure Chemical Industries, Ltd.) was used as cosolvent to cast the Nafion film from solution.6,7 Sulfuric acid was of reagent grade (Wako), and ammonium sulfate ((NH4)2SO4, 99.5% Wako), tetramethylammonium sulfate ([(CH3)4N]2SO4, 96.0%, Aldrich), tetraethylammonium bisulfate ([(C2H5)4N]HSO4, 99.0%, Fluka), tetrapropylammonium bisulfate ([(C3H7)4N]HSO4, 99.0%, Fluka), and tetrabutylammonium bisulfate ([(C4H9)4N]HSO4, 99.0%, Fluka) were used as received. As additives maleic acid (cis-C2H2(COOH)2, 99.0%, Wako), fumaric acid (trans-C2H2(COOH)2, 98.0%, Wako), phthalic acid (C6H4(COOH)2, 99.5%, Kanto Chemical Co. Inc.), glycine (H2NCH2COOH, 99.0%, Wako), and D-R-alanine (CH3CH(NH2)COOH, 98.0%, Wako) were used without further purification. Water was first deionized by Millipore Milli-Q II, and then doubly distilled using quartz glass (5) Uribe, F. A.; Zawodzinski, T. A.; Gottesfeld, S. In Proton Conducting Membrane Fuel Cells II; Gottesfeld, S., Fuller, T., Eds.; The Electrochemical Society Proceedings Series PV 98-27; The Electrochemical Society: Pennington, NJ, 1999; p 229. (6) Moore, R. B.; Martin, C. R. Anal. Chem. 1986, 58, 2569. (7) Zook, L. A.; Leddy, J. Anal. Chem. 1996, 68, 3793.

10.1021/la020743t CCC: $25.00 © 2003 American Chemical Society Published on Web 01/30/2003

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distillation flask. Oxygen and nitrogen gases were of 99.9% and 99.9995% purity, respectively. 2.2. Preparation of Platinum Electrodes Covered with Ionomer. A rotating disk electrode (RDE) of platinum disk (diameter 4 mm, apparent area 0.13 cm2) was used as the working electrode. The surface was roughened with no. 600 emery paper to achieve a good platinum|Nafion film adherence. The electrode was first cleaned in 50/50 H2SO4/HNO3 for 15 min, rinsed in hot water for 2 min, and then ultrasonically cleaned in pure water for 15 min. Then the electrode was dipped in H2SO4 for 10 min and finally rinsed by use of the same procedure as above. The electrode was subjected to a potential cycling (-0.32 and 1.18 V vs SCE at 0.1 V s-1 for 30 min) in deaerated 0.05 mol dm-3 H2SO4. After rinsing in pure water and drying, the electrode was mounted on a spin coater with the surface upward, and 25 µL aliquots of a mixture of 10 parts commercial 5% Nafion solution and 1 part DMF were transferred to the surface6,7 and spincoated for 5 min. New layers were subsequently added until the desired thickness (ca. 10 µm) was attained. The film was dried for 30 min at 80 °C and then annealed for 5 h at 135 °C in a vacuum. 2.3. Electrochemical Measurements on Film-Covered Platinum. The electrochemical measurements were carried out at 25 ( 0.5 °C with a Toho Technical Research Model 2000 potentiogalvanostat, a Toho Technical Research Model FG-02 function generator, and a Riken Denshi Model F-35 X-Y recorder. A glass cell with the saturated calomel reference electrode (SCE), the Pt wire counter electrode, and the Nafion film covered platinum RDE working electrode was used for the measurement. The rotation speed of the RDE was controlled by an electric rotator EG&G PARC Model 616 RDE. The Nafion film covered platinum RDE was soaked in 0.05 mol dm-3 H2SO4 saturated with oxygen gas, and the electrochemistry of oxygen reduction was measured first for noncontaminated film. For the case of contaminated film, the Nafion film covered platinum RDE was soaked in oxygen gas saturated 0.05 mol dm-3 H2SO4 containing various kinds of sulfates or bisulfates of ammonium derivatives. The kinetic measurements were done for the time course over 3-5 days. For the measurement, the electrode surface was first cleaned by potential cycling in nitrogen-saturated 0.05 mol dm-3 H2SO4. The bubbling gas was then switched to oxygen and the ORR current was recorded for the potential range 0.88 to -0.22 V vs SCE at a scanning rate of 0.01 V s-1 and at rotation speeds ω of 200, 300, 400, 600, 900, 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 solution at quiescent condition. For linear scanning voltammetry (LSV), the current was recorded for the potential 0.9 through 0 V vs SCE, at 0.01, 0.02, 0.05, 0.1, and 0.2 V s-1, at a quiescent condition of the electrolyte. 2.4. Analyses. Data of CV, RDE, and LSV were analyzed in the same manner as reported before.1-3 From CV, general trends of current-potential characteristics were obtained, and also the active surface for the oxygen reduction reaction was evaluated from the hydrogen adsorption-desorption peaks. For RDE, the measured current density j of oxygen reduction on Nafion film covered Pt in the mixed control regime of charge transfer, diffusion of oxygen gas in the film, and in the solution is expressed as follows:

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

(1)

where jk(film) is the kinetic (charge transfer) current at the interface, jf,l is the diffusion limiting current in the film, and jL is the diffusion limiting current in the solution. The thickness of the film should be thin enough to allow for the steady diffusion layer for oxygen developed inside the film that is thinner than the hydrodynamic boundary layer8 in the solution for RDE condition, but thick enough for the transient diffusion layer to grow only in the vicinity of the platinum surface in the film in (8) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980.

the LSV measurements. Then film thickness was chosen to be around 10 µm. jk,c(film) at the contaminated condition was obtained in two ways. At small overpotential η where the diffusion limitation of oxygen through the Nafion film and through the solution is small, jk,c was directly obtained from the measured current, i.e.

8 j (film) j9 ηf0 k,c

(2)

On the other hand, at large overpotentials, the rate of the charge transfer process jk,c(film) is obtained from the difference in the intercept of the Koutecky-Levich graphs8-10 of oxygen reduction, 1/jc and 1/jr, which are measured for the contaminated and noncontaminated conditions, respectively.

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

(3)

where jk,r(film) is the charge transfer current for Nafion film covered Pt at the noncontaminated condition. jl,c and jl,r correspond to jc and jr at the diffusion limiting current conditions. Here it is assumed that the solution terms in eq 1 cancel out for noncontaminated and contaminated cases. For the diffusion parameters in the Nafion film, the maximum concentration and the diffusion coefficient of oxygen, Cf* and Df, respectively, are evaluated. From RDE measurement, the ratio of Cf*Df for contaminated film in reference to pure film is obtained from the Koutecky-Levich plots:

Cf,c*Df,c jf,l,c ) Cf*Df jf,l,r

(4)

where jf,l,c and jf,l,r are limiting current densities in the Nafion film and are obtained at the condition of ω f ∞ and η f -∞. In linear sweep voltammetry (LSV), the ratio of Cf*Df1/2 is obtained from the scan rate dependence of the peak currents jp for Pt covered with contaminated film against the pure film:8,10

Cf,c*Df,c1/2 Cf,r*Df,r

) 1/2

( )

Sf,c (Rna)r Sf,r (Rna)c

1/2

(5)

where S ) djp/dυ1/2, with υ the potential scan rate (V s-1), R is the transfer coefficient, and na is the number of electrons involved in the rate-determining step. Both eqs 4 and 5 can be used to compare Cf* and Df in the contaminated and pure films. Also from the scan rate dependence of the peak potential, one obtains8

dEp 2.3RT )d log υ 2Rna

(6)

3. Experimental Results: Effect of Ammonium Derivative Ions 3.1. Cyclic Voltammetry. Cyclic voltammograms (CVs) of Nafion film covered platinum electrodes in oxygen gas saturated 0.05 mol dm-3 H2SO4 containing NH4+, (CH3)4N+, and (C2H5)4N+ ions in 10% amount as compared with H+ are shown in parts a, b, and c, respectively, of Figure 1. For these ions, CVs did not show significant changes compared to the system of pure H2SO4, as long as the added amount was not large (1% H+). When the amount of these ions was 10% as compared to H+ in the solution, peaks due to platinum oxide formation and reduction decreased, but the peaks due to hydrogen adsorption and desorption did not change greatly. (9) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S. J. Electrochem. Soc. 1987, 134, 1455. (10) Zecevic, S. K.; Wainright, J. S.; Litt, M. H.; Gojkovic, S. Lj.; Savinell, R. F. J. Electrochem. Soc. 1997, 144, 2973.

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Figure 1. Cyclic voltammograms on Nafion film covered platinum after 4 days in 0.05 mol dm-3 H2SO4, containing ammonium derivative ions at a scan rate of 0.1 V s-1. Impurity ions added: (a) 0.005 mol dm-3 (NH4)2SO4; (b) 0.005 mol dm-3 [(CH3)4N]2SO4; (c) 0.01 mol dm-3 [(C2H5)4N]HSO4; (d) 0.001 mol dm-3 [(C3H7)4N]HSO4; (e) 0.0001 mol dm-3 and (f) 0.001 mol dm-3 [(C4H9)4N]HSO4. Solution was saturated with oxygen gas.

In the case of ammonium derivative ions with higher molecular weight as in (C3H7)4N+ or (C4H9)4N+, different behavior was observed as shown in Figure 1d-f, and hydrogen adsorption-desorption peaks as well as platinum oxide formation-reduction peaks were distorted already in the lower amount of 0.1% as compared to H+ in the solution. It should be noticed that with (C4H9)4N+, when the system is measured at various immersion times, the change in CVs occurred in a short time (within less than a few hours after the immersion). This result indicates that the impurity ion (C4H9)4N+ adsorbs on the Pt surface from the early stage, and blocks the current passage. This fact was not observed in the case of NH4+, (CH3)4N+, and (C2H5)4N+ ions, where cations influence the platinum surface only gradually. Table 1 compares the area of hydrogen adsorptiondesorption peaks as obtained from the steady-state CV on Nafion film covered platinum in nitrogen gas saturated

0.05 mol dm-3 H2SO4 containing different kinds of ammonium derivative ions. The difference in the active surface area of platinum before and after the coating of Nafion was only minute, although the CV curve in the latter case was slightly rounded in the hydrogen adsorption-desorption region. For the system of Nafion film covered platinum containing ammonium derivative ions, different trends were again observed between two groups. For NH4+, (CH3)4N+, and (C2H5)4N+ ions, the active surface area as calculated from the hydrogen adsorption-desorption peaks did not change greatly by addition of these ions. However, for (C3H7)4N+ or (C4H9)4N+, the active surface sites on platinum decreased appreciably even when the amount of these ions was 0.1%. 3.2. Kinetic Current of Oxygen Reduction. The polarization curves of the oxygen reduction region measured with platinum RDE covered with Nafion film containing various kinds of ammonium derivative cations

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Table 1. Ratio of the Hydrogen Adsorption-Desorption Peak Area of CVs of the Nafion Film Covered Pt Electrodes, in the Presence of Impurity Ions, in Comparison with that of Bare Platinum in 0.05 mol dm-3 H2SO4 impurity level in solution (%)

time (h)

NH4+

(CH3)4N+

impurity ion (C2H5)4N+

0.1 0.1 0.1 0.1 0.1

0 2 25 75 100

1.0 1.0 1.0 1.0 1.0

0 2 25 75 100

1.00 0.97 0.95 0.96 1.04

1.00 0.95 0.97 0.97 0.98

1.00 0.93 0.99 0.99 0.96

10 10 10 10 10

0 2 25 75 100

1.00 1.01 1.01 1.00 1.02

1.00 0.93 0.92 0.95 0.96

1.00 0.91 0.90 0.96 0.92

(C3H7)4N+

(C4H9)4N+

1.05 0.99 0.95 0.98 0.95

1.00 0.82 0.82 0.80 0.83

1.03 0.94 0.93 0.91 0.89

0.98 0.76 0.81 0.78 0.82

Figure 2. Polarization curves at a scan rate of 0.01 V s-1 on platinum covered with Nafion film in contact with 0.05 mol dm-3 H2SO4 + 0.005 mol dm-3 [(C2H5)4N]HSO4 (a) or 0.05 mol dm-3 H2SO4 + 0.0005 mol dm-3 [(C4H9)4N]HSO4 (b). Rotation speed of RDE: 400 rpm.

showed specific changes. With no impurity ions, the hysteresis appears between the forward and backward scans of the potential, and this is ascribed to the state of platinum oxide being different at different potential regions.9 This hysteresis appeared differently for Nafion film covered platinum, and in the (C4H9)4N+-containing condition, the current of oxygen reduction in the anodic potential scanning was smaller than that in the cathodic direction (Figure 2). Also, polarization curves are largely distorted in the case of ammonium derivative ions of high molecular weight. These results indicate that the surface condition for the charge transfer, across either the oxide layer or the electrical double layer, has been altered by the presence of these ammonium derivative ions. This is also supported by the change in the oxide formation-reduction peaks in the CV. The kinetic current jk,c of oxygen reduction is calculated using eq 2 or eq 3 for Nafion film covered electrodes immersed in 0.05 mol dm-3 H2SO4 solutions containing various kinds of impurity cations. In Figure 3, the degradation curves of jk,c for various kinds of ammonium derivative ions are plotted as a function of soaking time, in comparison with that for the no-contaminant condition, jk,r. Equations 2 and 3 showed good agreement in the

Figure 3. Kinetic current of oxygen reduction on platinum covered with Nafion film in 0.05 mol dm-3 H2SO4 solution containing (a) NH4+ (×, +), (CH3)4N+ (O, b), or (C2H5)4N+ (], [) and (b) (C3H7)4N+ (4, 2) or (C4H9)4N+ (0, 9) ions of various amounts as compared with H+. 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. Impurity amounts: ×, O, ], 1%; +, b, [, 10%; 4, 0, 0.1%; and 2, 9, 1% as compared with H+ in the solution.

results. Also, the potential at which jk,c was calculated did not affect the calculated results of jk,c/jk,r in the range of 0.15-0.50 V vs SCE, and the curves are given at 0.2 V. Different trends were observed in the degradation curves of jk,c between two groups of ions. For NH4+, (CH3)4N+, and (C2H5)4N+ ions, jk,c decreased slowly with the soaking time, and this trend was larger for larger amount of ammonium derivative ions. Also, the degradation was larger for larger molecular weights of ammonium derivative cations. For 10% cation amount as compared to H+ in the solution, the degradation continued until it reached 60%-70% of the initial level after about 3-5 days. On the other hand, for (C3H7)4N+ or (C4H9)4N+, the degradation occurred rapidly in about several hours, and

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Figure 5. Transfer coefficient times 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 3.

Figure 4. Cf,c*Df,c1/2 for oxygen transport in Nafion film in contact with 0.05 mol dm-3 H2SO4 containing (a) NH4+, (CH3)4N+, or (C2H5)4N+ ions and (b) (C3H7)4N+ or (C4H9)4N+ ions. The ratios 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 3.

even in the case of 1% contamination, the degradation reached to about 70%. A very interesting feature is that specific adsorption appeared to occur for these ammonium derivative ions, with possible blockage of platinum surface being the cause of this rapid effect. 3.3. Oxygen Transport in the Film. Calculated parameters Cf,c*Df,c and Cf,c*Df,c1/2 revealed different trends between the group NH4+, (CH3)4N+, or (C2H5)4N+ ions and the group (C3H7)4N+ or (C4H9)4N+ ions. For the former group, both the parameters Cf,c*Df,c and Cf,c*Df,c1/2 decreased gradually with soaking time in 3-4 days, but the extent of degradation was much larger in Cf,c*Df,c1/2 than in Cf,c*Df,c (see Figure 4a, where the result of eq 5 is shown for NH4+, (CH3)4N+, and (C2H5)4N+ ions). This indicates that the major cause of decline is due to the decrease in Cf* rather than that in Df through the time course. The remarkable difference from the results of Nafion films contaminated with the group (C3H7)4N+ or (C4H9)4N+ as compared to the group NH4+, (CH3)4N+, or (C2H5)4N+ is that in this case the abrupt change occurred in the time course for both Cf,c*Df,c and Cf,c*Df,c1/2 parameters as shown in Figure 4b. From the fact that the decrease in the former parameter (about 80%-90%) is larger than the latter parameter (about 20%-40%), it is deduced that in this case the decline in Df is more significant than that in Cf*. This strong contrast infers different mechanisms in the degradation of oxygen transport between the contaminant ions of the group NH4+, (CH3)4N+, and (C2H5)4N+ and the group (C3H7)4N+ and (C4H9)4N+. 3.4. Change in the Reaction Mechanism in Oxygen Reduction Reaction. The transfer coefficient R times the number of electrons in the rate-determining step na on platinum covered with Nafion film containing impurity ions is calculated using eq 6 in reference to that for pure film, and is plotted as a function of soaking time in Figure 5. The results for NH4+, (CH3)4N+, (C2H5)4N+, and (C3H7)4N+ ions show that Rna stayed about 0.5 ( 0.05

with no significant changes along with time, while for (C4H9)4N+ a negative shift was observed. The last case would indicate that the reaction route of ORR may be affected due to the specific adsorption of the ammonium derivative ions of large molecular size. 4. Experimental Results: Test of Inhibitor Additives in Nafion Film In reference to the polymer mass, 2%-4% additives were incorporated into the Nafion film, and ORR electrochemistry was measured in 0.05 mol dm-3 H2SO4 containing 10% of Na+ or Ca2+ ions (in equivalent concentration as compared with H+). These impurity ions were selected because practically such ions would commonly be found in the fuel cell chamber,11 and moreover, very strong suppression to ORR kinetics was observed in previous works.1,3 Ammonium derivative ions were not added as impurity ions because a different degradation mechanism from Na+ or Ca2+ ions was anticipated. The additives tested were maleic acid, fumaric acid, phthalic acid, glycine, and D-R-alanine. These were added into the 5% Nafion solution with DMF, and were spin-coated on Pt disk as described in the Experimental Section. For the annealing of the polymer film in the vacuum, the temperature was set to 125 or 130 °C to avoid decomposition of the additives. 4.1. Cyclic Voltammetry and Polarization Characteristics of ORR. In all cases of additives incorporated in the Nafion film on Pt disk electrode, there were observed very few changes in CVs in comparison with the bare Pt measured in 0.05 mol dm-3 H2SO4. Also, for Nafion film covered Pt disk electrode with additives, no appreciable differences were found between CVs measured in 0.05 mol dm-3 H2SO4 and those measured in 0.05 mol dm-3 H2SO4 containing 10% of Na+ or Ca2+ ions. The active surface area as calculated form the hydrogen adsorptiondesorption peaks remained the same, indicating that the tested additives did not interfere with Pt or block the Pt surface. These facts may imply that the degradation of ORR by impurity cations as observed in Nafion film covered Pt was more or less diminished by the presence of additives, because with no additives the CVs changed due to the presence of Na+ or Ca2+ ions.1,3 Polarization curves of ORR on Pt disk electrodes covered with Nafion film containing D-R-alanine as the additive is shown in Figure 6. Without the additive, the ORR (11) Mitsuda, K. NEDO Report, Fy. 1996; NEDO: Tokyo, 1997; p 90.

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Figure 6. Polarization curves at a scan rate of 0.01 V s-1 on Nafion film covered platinum without (a) or with D-r-alanine (b), in contact with 0.05 mol dm-3 H2SO4 + 0.005 mol dm-3 CaSO4. Gray curves show the polarization curves on Nafion film covered platinum in 0.05 mol dm-3 H2SO4.

current decreased largely by the presence of 10% Ca2+ ion, but incorporation of D-r-alanine in the film drastically improved this degradation. In some cases the additives in the film caused a decrease in the limiting current (3%10% decrease as compared with Nafion film covered Pt with no additive), but even in such cases the decrease in the current after the impurity ions were added was drastically reduced. Fumaric acid, phthalic acid, and glycine were also good examples of inhibiting the degradation behavior of ORR on Nafion film covered Pt. 4.2. Charge Transfer and Transport Characteristics of ORR. The inhibiting effect of additives against degradation of ORR on Nafion film covered Pt was investigated for both the charge transfer step and the diffusion step in the film. In Figure 7, jk,c/jk,r is plotted as a function of time for various combinations of impurity cations and additives. Without the additives, jk,c/jk,r dropped about 40%-50% from the initial value, after 4-5 days.1,3 Except for maleic acid, the charge transfer kinetics was protected from degradation caused by the presence of impurity cations. Figure 8 shows the time course of Cf,c*Df,c in the presence of 10% Ca2+, as compared with those for the noncontaminated film with addition of fumaric acid. Without the additives, both Cf,c*Df,c and Cf,c*Df,c1/2 parameters decreased about 20%-30% from the initial value, in the presence of 10% Ca2+.3 Decrease of Cf,c*Df,c1/2 was not large (about 10%), but decrease of Cf,c*Df,c was enhanced for the Nafion film containing additives. This means that the diffusion coefficient of oxygen in the film was suppressed by the additives. The Rna parameter was plotted against time for Nafion film covered Pt with various kinds of additives in the presence of Na+ or Ca2+. Rna remained almost constant with time (Rna ) 0.5), and no substantial change in the transition state of the rate-determining step was anticipated.

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Figure 7. Kinetic current of oxygen reduction on platinum covered with Nafion film with additives, in contact with 0.05 mol dm-3 H2SO4 solution containing 10% Na+ or Ca2+ ions as compared with H+. 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. Impurity ions and additives are (a) 10% Na+, fumaric acid; (b) 10% Ca2+, fumaric acid; (c) 10% Ca2+, maleic acid; and (d) 10% Ca2+, glycine.

Figure 8. Cf,c*Df,c in Nafion film containing fumaric acid, in contact with 0.05 mol dm-3 H2SO4 + 0.005 mol dm-3 CaSO4. The ratios of these values for the contaminated film against those for the noncontaminated film are shown in the time course.

5. Discussion The platinum|ionomer membrane system, unlike the platinum|electrolyte solution system, is very special, and the charge transfer kinetics of oxygen reduction O2 + 4H+ + 4e f 2H2O as well as the oxygen transport processes are strongly influenced by the presence of impurity ions.1-3 This impurity effect occurred even at as low a level as 0.1% impurity in comparison with H+ in the solution. Among several causes that may account for the impurity effect on the degradation of charge transfer kinetics of ORR, the electric double layer effect and platinum oxide effect were thought to be most plausible.3 In all cases, the fact that degradation of the charge transfer kinetics occurred more strongly than (about 60%-70% decrease) the degradation of oxygen transport parameters in the Nafion film (about 20%-30%) means that the effect was connected to the platinum|Nafion interface rather than the polymer bulk. In the present experiment the “blockage” of active platinum surfaces by ammonium derivative ions turned

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Table 2. Characteristics of ORR and Film Properties on Nafion Film Covered Pt Electrodes, in the Presence of Various Kinds of Impurity Ions parametera

impurity level in solution (%)

NH4+

(CH3)4N+

impurity ion (C2H5)4N+

l d(wet) κ/κH [jk] [CD] [CD1/2]

0.1 0.1 0.1 0.1 0.1 0.1

λ d(wet) κ/κH [jk] [CD] [CD1/2]

1.0 1.0 1.0 1.0 1.0 1.0

20.5 1.57 0.96 0.87 0.97 0.92

16.1 1.69 0.65 0.74 0.85 0.84

14.7 1.64 0.52 0.84 0.80 0.86

λ d(wet) κ/κH [jk] [CD] [CD1/2]

10 10 10 10 10 10

19.8 1.64 0.80 0.78 0.91 0.89

13.0 1.66 0.49 0.47 0.68 0.46

13.4 1.62 0.38 0.30 0.65 0.46

(C3H7)4N+

(C4H9)4N+

18.5 1.60 0.92 0.93 0.27 0.80

19.0 1.62 0.86 0.75 0.09 0.60

13.8 1.59 0.33 0.36 0.23 0.64

1.4 1.70 0.0 0.29 0.04 0.54

a λ ()n H2O/nSO3-), water content; d(wet), density in the wet state; κ/κH, conductivity ratio with reference to H-form membrane, for Nafion membranes in equilibrium with 0.05 mol dm-3 H2SO4 containing various kinds of ammonium derivative ions. For H-form Nafion membrane, λ ) 20.5 and d(wet) ) 1.61. [jk], [CD], and [CD1/2]: jk,c, Cf,c*Df,c, and Cf,c*Df,c1/2 parameters for Nafion film covered Pt electrodes after 4 days in the presence of impurity ions as compared to no-impurity conditions.

out to be another mechanism, as apparently seen in the change in the active surface area of platinum in CV (Table 1). Polarization curves also revealed specific changes caused by (C3H7)4N+ or (C4H9)4N+ ions, probably due to the blockage of the platinum surface. A secondary effect would be the change in the structure of the electric double layer or the platinum oxide layer by specific adsorption at the platinum surface.12 Note that the platinum|ionomer interface that is affected by the presence of these ions might alter the oxygen reduction kinetics. The hysteresis that occurred in the oxygen reduction current between anodic and cathodic scans would be related to the state of the platinum surface, and the result in Figure 2 indicates significant changes in this respect. In such cases, the transition state in the reaction schemes of oxygen reduction must also be influenced by the adsorption of impurity ions, because the parameter Rna showed diversion from the initial value (Figure 5). Negative shift of Rna infers R < 0.5, and the potential energy barrier in the charge transfer step is distorted so that the path of reactant side (O2) becomes longer than the product side in the reaction coordinate.8 It is interesting to note that, concerning the effect of ammonium derivative ions, there appeared two kinds of schemes about the degradation of ORR. For ammonium derivative ions of lower molecular weight such as NH4+, (CH3)4N+, or (C2H5)4N+ ions, the trends were similar to those observed in alkali or alkaline earth metal cations. For these ions, changes of jk,c as well as of Df and Cf* occurred rather slowly, and needed a long time to attain a stationary state. In this case the degradation of jk,c would not be the direct consequence of adsorption of these cations at the platinum|Nafion interface, but would be connected to a much slower process. It is observed that the degradation of these parameters occurs in the same time span as the rearrangement of polymer network. At the platinum|Nafion interface, the charge distribution in the polymer is constrained, because the sulfonic acid groups are connected to the side chain of the polymer, and are not free in motion.13 The binding force of ammonium ions to sulfonic acid groups would make the (12) Gileadi, E. Electrode Kinetics; VCH Publishers: New York, 1993. (13) Wang, J. X.; Adzic, R. R. J. Electroanal. Chem. 1998, 448, 205.

charge separation smaller than in the H+-sulfonic acid pair. Then it will be difficult to maintain a high electric field as realized in the pure H+ system, and gives rise to slower kinetics of the charge transfer in the oxygen reduction reaction. When the size of ammonium ions becomes larger, this would cause a more diffused electric double layer, and the electric field would become smaller. It was also reported from the study of platinum|Nafion and gold|Nafion interfaces that there might occur a change in potential distribution due to the preferred association of polymer network at the interface.14 On the other hand, for ammonium derivative ions with higher molecular weight such as (C3H7)4N+ or (C4H9)4N+ ions, the decline in the above parameters occurred abruptly that might have connected with the direct interaction of these ions with the platinum surface. It is suspected that the polymer structure was also influenced differently between two groups of ammonium derivative cations, because in the latter kinds of ions bulky ammonium cations would be trapped in the “intermediate zone” of the Nafion polymer among the three-phase structure of polymer chain, side chain, and ionic channel,15 while in the former groups of ions, ionic channels would be preferred to reside. For the group of NH4+, (CH3)4N+, and (C2H5)4N+ ions, the decrease of Cf* occurred, and for the group of (C3H7)4N+ and (C4H9)4N+ ions, the decrease of Df occurred in the polymer film. For ammonium derivative ions of lower molecular weight, the ions will occupy the ionic channel and drive out water molecules there. The oxygen will dissolve more preferentially in the hydrophilic region of the Nafion polymer, and thus Cf* will be decreased, if this domain is occupied by ammonium derivative ions. On the other hand, the ammonium derivative ions of larger size will bring about the irreversible change of the polymer network not only in the ionic channel but also in the polymer backbone. Once this happens, the path for oxygen diffusion will be blocked and Df will fall from the initial stage. Table 2 summarizes changes in ORR and the characteristics of Nafion films in the presence of impurity ions. (14) Chu, D.; Tryk, D.; Gervasio, D.; Yeager, E. B. J. Electroanal. Chem. 1989, 272, 277. (15) Yeager, H. L.; Steck, A. J. Electrochem. Soc. 1981, 128, 1880.

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Langmuir, Vol. 19, No. 6, 2003

It is seen in Table 2 that the amount of water to be brought into the membrane differs very much for different ammonium ion species. For ammonium derivative ions of higher molecular weight, the membrane drying and conductivity decrease are much more serious than those in the membrane containing ammonium derivative ions of lower molecular weight. Especially the membrane drying occurs to an extreme extent, and this might cause almost irreversible changes in the polymer structure. What may be anticipated from this structural change would be the high barrier for electron transfer from platinum metal to oxygen molecule at the platinum|polymer interface. Based on this idea, a new approach has been tested in which some additives that may counteract this structural change at the interface by virtue of the opposite charge from impurity cations are incorporated in the polymer film. Results in Figure 6 reveal that this idea certainly works as if they show the inhibitor function against ORR suppression caused by impurity ions. The reason that brought about this improvement could be made in several ways. Apart from the reasoning expected above, there can be other mechanisms. One would be that the carboxylic acid group makes an ion pair with the impurity cations, and decreased the effective concentration of the latter ions in the Nafion polymer. The fact that CVs of the Nafion film covered platinum disk with additives did not alter significantly from those of no additives supports this idea. Second would be that these additives formed an intermediate layer between platinum and Nafion polymer, and created a liquid electrolyte-like structure where there is no degradation by impurity ions. The unfortunate point was that the inhibitor effect of additives against degradation was observed only for the charge transfer processes at the platinum|Nafion interface, and the oxygen diffusion in the membrane was degraded to the same extent or even more by the presence of additives. Because Df was found to decrease more than Cf*, this could be attributed to the state of polymer network changed by the interaction with the additives. In practical applications to fuel cell technology, the molecular structure and the amount of additives should be carefully optimized so that this phenomenon could be avoided and still maintain the inhibitor function effectiveness.

Okada et al.

The characterization of inhibitor additives would need further study, but a new research field of the metal|ionomer interface is expected, where it is possible to effectively control the interface structure and the kinetics of the desired reaction by manipulating the polymer structure at the interface. 6. Conclusions The degradation of oxygen reduction kinetics on Nafion film covered electrodes by the presence of ammonium derivative ions was studied, and mechanisms based on the change in platinum|ionomer interface conditions were discussed. The charge transfer rate of oxygen reduction on platinum electrodes covered with Nafion film was found to drop about 60%-70% after 3-5 days of immersion in 0.05 mol dm-3 H2SO4 containing impurity ions of 0.1%1% as compared with H+ in the solution. The drop was different for different ammonium derivative ions. Also, the transport parameters of oxygen through the polymer layer, such as oxygen concentration and/or diffusion coefficients, decreased differently, depending on the ammonium derivative ions. For ions with lower molecular weight, degradation was gradual, and it was in the same time span as in the rearrangement of polymer network that occurred after the incorporation of impurity ions. For ions with higher molecular weight, the drop was straightforward, and the mechanism based on the structural change and redistribution of charge at the platinum surface was thought to be the major cause. A new method to prevent degradation of ORR was discovered based on the additives in the polymer film that covers the platinum surface. Among various kinds of additives tested, some carboxylic acids and amino acids were found to be good inhibitors against degradation of charge transfer reaction. However, the degradation of oxygen transport in the polymer was not improved, probably due to the interaction of the additives with the bulk polymer structure. The present investigation gave promising perspectives to cope with the problems of degradation and lifetime of polymer electrolyte fuel cells. LA020743T