6980
J. Phys. Chem. B 2001, 105, 6980-6986
The Effect of Impurity Cations on the Oxygen Reduction Kinetics at Platinum Electrodes Covered with Perfluorinated Ionomer Tatsuhiro Okada,* Yuusuke Ayato,† Hiroki Satou,† Makoto Yuasa,† and Isao Sekine† 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: March 5, 2001; In Final Form: May 13, 2001
The mechanism of how impurity cations suppress the kinetics of oxygen reduction reaction on platinum surface covered with perfluoro-sulfonated ionomer film was studied in 0.1 N H2SO4 in the presence of various kinds of impurity ions of several concentrations. Impurity cations tested were Li+, Na+, K+, Ca2+, Fe3+, Ni2+, and Cu2+ with the amount of 0.1%, 1%, and 10% as compared with H+ in the solution. Platinum disk of a rotating disk electrode was spin-coated with Nafion solution, and after drying the Nafion film-covered platinum was obtained. The electrochemical measurements were performed to evaluate both charge transfer and diffusion kinetics of oxygen reduction at the Nafion film-covered electrode. It was discovered that the impurity ions hindered enormously the rate of charge-transfer step at platinum covered with perfluoro-sulfonated ionomer. The suppression started already at 0.1% level of impurity concentration, but did not increase much at over 1% level. No suppression effect for oxygen reduction was observed for a bare platinum in the solution containing impurity ions, indicating that the effect is specific to the metal electrode-ionomer membrane interface. Also both the diffusion coefficient of oxygen and oxygen concentration in the membrane decreased by the presence of impurity cations. It was implied that all the process is related to the reorientation of polymer networks in the membrane, which might bring about the modification of electric double layer at the platinum-ionomer interface.
1. Introduction In the recent works, the effect of impurity cations on the kinetics of oxygen reduction reaction on platinum covered with perfluorinated ionomer membranes was investigated for different cationic species, in relevance to the problems of membrane contamination and performance degradation in polymer electrolyte fuel cells.1,2 Alkali and alkaline earth metal cations1 or transitional metal cations2 showed tendency to suppress the charge-transfer step that occurs at the platinum-ionomer interface. Those works were conducted with the aim of modeling the degradation of cathode performance during the cell operation.3 Impurity ions, even in small amount as low as 1% of H+ concentration in the solution, hindered enormously both the oxygen reduction kinetics and the transport processes of oxygen in the membrane phase. This surprising fact was not observed for platinum-electrolyte solution interface, and therefore, regarded as phenomena specific to the metal-ionomer membrane interface. As one of the possible causes for this phenomenon, “constrained electric double layer”4 at the platinumionomer interface was considered, but the detailed mechanism was not clear. In this study, the effect of impurity ions on the catalytic performance of platinum electrodes covered with ionomer membranes is investigated further for different ions with †
Department of Industrial Chemistry. * Corresponding author. FAX: +81 298 61 4678. E-mail: okada.t@ aist.go.jp.
different concentrations, to look into more detailed mechanisms about this phenomenon occurring at the metal-ionomer membrane interface. It is intended to obtain an insight about the change in kinetic factors at the platinum-ionomer system, in relation to the structural changes of polymer networks that occur as a result of the cation penetration process in the membrane. 2. Experimental Methods and Calculations 2.1. Materials. Nafion solution was 5% polymer dissolved in aliphatic alcohol, as purchased from Aldrich. 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.5,6 Sulfuric acid was of reagent grade, and lithium sulfate (Li2SO4‚H2O), sodium sulfate (Na2SO4), potassium sulfate (K2SO4), calcium sulfate (CaSO4‚2H2O), iron(III) sulfate n-hydrate (Fe2(SO4)3‚nH2O, n ) 6-9), nickel sulfate hexahydrate (NiSO4‚6H2O), and copper sulfate pentahydrate (CuSO4‚5H2O) were reagent grade from Wako, and used without further purification. Water was first deionized by Millipore Milli-Q II, and then doubly distilled using quartz glass 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 #400 emery paper in order to achieve a good platinum-Nafion film
10.1021/jp010822y CCC: $20.00 © 2001 American Chemical Society Published on Web 06/23/2001
Oxygen Reduction Kinetics
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6981
adherence. The electrode was cleaned and subjected to a potential cycling (-0.32 and 1.18 V vs SCE at 0.1 V s-1) in 0.1 N H2SO4.1,2 After 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 was transferred to the surface,5,6 and spin-coated for 5 min. New layers were subsequently added until the desired thickness (4-15 µm) was attained. The film was dried for 1 h at 80 °C and then annealed for 5 h at 135 °C in a vacuum. 2.3. Electrochemical Measurements on Film-Covered Platinum. 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. Rotation speed was controlled by an electric rotator EG&G PARC model 616 RDE. The electrochemical measurements were carried out at 25 ( 0.5 °C with a Toho Technical Research model 2000 potentio-galvanostat, a Toho Technical Research model FG-02 function generator, and a 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 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 0.1 N H2SO4 containing 0.0001, 0.001, or 0.01 N of various kinds of metal sulfates. The kinetic measurements were done for the time course over 3-5 days. The electrode was then soaked in fresh 0.1 N H2SO4 and subjected to potential cycling until the recovery of the electrode to the noncontaminated condition was confirmed, before repeating the measurement in the next contaminant solution. For the measurement, the electrode surface was first cleaned by potential cycling in nitrogen saturated 0.1 N H2SO4. The bubbling gas was then switched to oxygen and oxygen reduction current was recorded for the potential range 0.88 V to -0.22 V vs SCE at a scanning rate of 0.01 V s-1, and at rotation speed of 200, 300, 400, 600, 900, and 1600 rpm. 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 the linear scanning voltammetry (LSV), the current was recorded for the potential 0.9 V 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. In the Fe3+ contaminated film, the reduction current of Fe3+ to Fe2+ occurred, and this current was corrected by measuring the current in nitrogen saturated solution. 2.4. Calculations. Data of CV, RDE, and LSV were analyzed in the same manner as reported before.1,2,7,8 The kinetic (charge transfer) current jk,c(film) of oxygen reduction on Pt surface covered with Nafion film was obtained in two ways. At small overpotential η where the diffusion limitation of oxygen through the Nafion film is small, jk,c was directly obtained from the measured current, i.e.,2
j 98 jk,c(film) ηf0
(1)
On the other hand, at large overpotentials, the rate of the chargetransfer process is large both for the bare and Nafion filmed Pt, so that
1 1 98 0 jk,c(film) jk,r(film) ηf-∞
(2)
where jk,r(film) is the charge-transfer current for Nafion filmed Pt at the noncontaminated condition. The difference in the
intercept of the Koutecky-Levich graphs of oxygen reduction measured for the contaminated and noncontaminated conditions are
1 1 1 1 1 1 - ) + jc jr jk,c(film) jk,r(film) jf,l,c jf,l,r
(3)
where jf,l is the diffusion limiting current in the film,
jf,l ) nFDf
C/f δf
(4)
and C/f and Df are the maximum concentration and the diffusion coefficient of oxygen in the Nafion film, δf being the thickness of the film. From eqs 2 and 3, jk,c(film) is obtained if jk,r(film) for noncontaminated condition is known:1
1 1 1 1 1 1 ) - ) j j j j jk,c(film) jk,r(film) c r l,c l,r
(5)
where jl,c and jl,r correspond to jc and jr at the condition given by eq 2, respectively. Under the assumption that the film thickness does not change largely by the presence of impurity ions, the ratio of eq 4 for contaminated film to the pure film gives
C/f,cDf,c C/f Df
)
jf,l,c jf,l,r
(6)
Here jf,l,r and jf.l.c are calculated from the difference in the Koutecky-Levich plots for filmed RDE in noncontaminated and contaminated conditions, respectively. In the linear sweep voltammetry (LSV), the peak current for a totally irreversible system is expressed as follows:9,10
jp ) 2.99 × 105n(Rna)1/2C*D1/2υ1/2
(7)
where R is transfer coefficient, and na is the number of electrons involved in the rate-determining step and υ is the potential scan rate (V s-1). The peak potential is9
Ep ) Eo′ -
[
( ) (
)]
RnaFυ D1/2 RT 0.780 + ln 0 + ln RnaF RT k
1/2
(8)
where Eo′ is the formal potential and k0 is the standard heterogeneous rate constant. From the scan rate dependence of peak currents for Pt covered with contaminated film and pure film, the following relation yields as the ratio of transport parameters in the contaminated film against the pure film:
C/f,cD1/2 f,c
) 1/2
C/f,rDf,r
( )
Sf,c (Rna)r Sf,r (Rna)c
1/2
(9)
where S ) djp/dυ1/2 and the subscripts r and c stand for filmed electrodes in pure and contaminated conditions, respectively. Both eqs 6 and 9 can be used to compare C/f and Df in the contaminated and pure films.
6982 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Okada et al.
Figure 1. Cyclic voltammograms on platinum covered with Nafion film at a scan rate of 0.1 V s-1. Solution: (a) 0.1 N H2SO4, (b) 0.1 N H2SO4+0.0001 N CaSO4, (c) 0.1 N H2SO4 + 0.001 N CaSO4, (d) 0.1 N H2SO4 + 0.01 N CaSO4,.
Figure 2. Cyclic voltammograms at a scan rate of 0.1 V s-1 on platinum covered with Nafion film in contact with (a) 0.1 N H2SO4+0.0001 N NiSO4, (b) 0.1 N H2SO4 + 0.001 N NiSO4, (c) 0.1 N H2SO4 + 0.01 N NiSO4.
Also from the scan rate dependence of the peak potential, one obtains
dEp 2.3RT )d log υ 2Rna
(10)
3. Experimental Results 3.1. Cyclic Voltammetry. Figure 1 compares the steady-state cyclic voltammograms (CVs) on Nafion film-covered platinum disk electrodes in oxygen gas saturated 0.1 N H2SO4 and in 0.1 N H2SO4 containing Ca2+ of 0.0001, 0.001, and 0.01 N levels. In the CVs of Nafion filmed platinum electrodes in 0.1 N H2SO4 containing impurity ions Na+, K+ or Ca2+, the hydrogen adsorption-desorption peaks and platinum oxide formationreduction peaks are slightly rounded as compared to those for platinum covered with pure films. However, the active surface area as calculated from the hydrogen adsorption-desorption peaks remains the same. The differences between CVs are not significant among Nafion filmed electrodes containing these impurity cations. CVs measured for electrodes filmed with Nafion containing Fe3+, Ni2+, or Cu2+ impurity ions show specific changes both in the hydrogen adsorption-desorption region and in the platinum oxide formation-reduction region. In Ni2+ contaminated case shown in Figure 2, the current due to Ni2+ reduction superimposed on the hydrogen adsorption current, and platinum oxide formation current is also affected at high impurity levels. In the case of Ni2+ as large as 10% level as compared with H+ in the solution, the negative current at -0.3 V vs SCE showed a sharp increase, indicating the onset of H2 evolution reaction at less negative potentials. For the platinum covered with Fe3+ or Cu2+ contaminated film, CVs are largely distorted, as seen in Figure 3 and Figure 4. These are ascribed to a reduction current of Fe3+ to Fe2+ or Cu2+ deposition on platinum surfaces, respectively. It should be noticed that in Figure 4, where the system is measured at various immersion times, the change in CVs occurs in a short time (within less than a few hours after the immersion). The results indicate the presence of impurity
Figure 3. Cyclic voltammograms at a scan rate of 0.1 V s-1 on platinum covered with Nafion film in contact with (a) 0.1 N H2SO4 + 0.0001 N Fe2(SO4)3, (b) 0.1 N H2SO4 + 0.001 N Fe2(SO4)3, (c) 0.1 N H2SO4 + 0.01 N Fe2(SO4)3.
ions at the vicinity of Pt surface from the very early stage. This fact reveals a fast exchange of cations in the membrane, resulting in a fast penetration of cations onto the platinum-ionomer interface. 3.2. Kinetic Current of Oxygen Reduction. The polarization curves of oxygen reduction region measured with platinum RDE covered with Nafion film containing several kinds of impurity ions showed specific changes, depending on the impurity ions. 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.11 This hysteresis decreased for Nafion film covered platinum, in Na+, K+, Ca2+ and Ni2+ contaminated conditions, and this
Oxygen Reduction Kinetics
Figure 4. Cyclic voltammograms at a scan rate of 0.1 V s-1 on platinum covered with Nafion film in contact with 0.1 N H2SO4 + 0.001 N CuSO4 at several immersion times. (a) pure 0.1 N H2SO4, (b) 4.5 h, (c) 54 h, (d) 71 h.
Figure 5. Kinetic current of oxygen reduction on platinum covered with Nafion film in 0.1 N H2SO4 solution containing Na+ with 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 level: 0.1% (0, 9) or 10% (O, b). Calculated using eq 1 (0, O) or eq 5 (9, b).
indicates that the surface condition for the charge transfer, either across the oxide layer or the electrical double layer, has altered by the presence of impurity ions. This is also supported by the change in the oxide formation-reduction peaks in the CV. For platinum covered with Fe3+ or Cu2+ containing films, polarization curves are largely distorted due to the overlapping of metal ion reduction or deposition currents on the oxygen reduction currents. In the latter case, currents are analyzed only for small overpotential (see eq 1) where Cu2+ deposition does not initiate.12 The kinetic current jk,c of oxygen reduction is calculated using eq 1 or eq 5 for Nafion film-covered electrodes immersed in solutions 0.1 N H2SO4 containing various kinds of impurity cations at 0.1, 1 and 10% levels. In Figure 5, the degradation curve of jk,c obtained by eq 1 for smaller overpotentials and by eq 5 for larger overpotentials are compared for the case of Na+ contaminated film. The results obtained by two methods are consistent with each other, indicating the applicability of the present analysis. Also the trend appears to be the same for all the electrode potentials between 0.15 and 0.55 V vs SCE.
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6983
Figure 6. Kinetic current of oxygen reduction on platinum covered with Nafion film in 0.1 N H2SO4 solution containing various kinds of impurity cations with 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 level: 0.1% (0), 1% (4), or 10% (O). (a) Li+, (b) Ca2+, (c) Fe3+, (d) Ni2+.
In Figure 6, jk,c is plotted as a function of soaking time, in comparison with that for the same film-covered electrodes measured in no-contaminant condition jk,r. The kinetic current decreases with the soaking time except for the Li+ contaminated case where the kinetic current did not decrease largely. It is interesting to note that in most cases the degradation of the kinetic current emerges between 0.1 and 1% impurity level, but does not grow further beyond 10% level. This degradation continued until it reached 40 to 60% of the initial level after about 3-5 days. The largest degradation occurred in Ca2+ or Ni2+ contaminated film for the case of 1% level, while for 10% level, the current drop increased further for Fe3+ contaminated film. Another very interesting feature is that in all cases, the jk,r dropped gradually with time, and approached a steady state after 3-5 days. This is in strong contrast to the case of ion penetration, which is observed to occur in a very short time (see Figure 4). 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 10 in reference to that for pure film, and is plotted as a function of soaking time in Figure 7. The results show that Rna stayed about 0.5 ( 0.05 with no significant changes along with time, indicating that the reaction mechanism did not change by the presence of impurity ions in the film. 3.3. Oxygen Transport in the Film. Calculated parameters C/f,cDf,c and C/f,cD1/2 f,c are shown in Figure 8 in the time course, based on eqs 6 and 9 for Nafion films contaminated with impurity ions in comparison to those for pure films. The parameter C/f,cDf,c or C/f,cD1/2 f,c decreased in the similar manner with soaking time, but the extent of decrease is 20 to 40% of the initial level, which is smaller as compared with the decrease in jk.c. It should be noticed that, unlike the case of jk,c, no significant differences are observed in the time change of these transport parameters between film-covered platinum electrodes contaminated with different kinds of impurity cations. The exception was for the Li+ contaminated film, where the degradation of transport parameters was not observed significantly like for the kinetic current.
6984 J. Phys. Chem. B, Vol. 105, No. 29, 2001
Figure 7. 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. Impurity level: 0.1% (+), 1% (b), or 10% (0).
Okada et al. The “blockage” mechanism of active platinum surfaces by metal ions or by hydrophobic part of the polymer chain is one possibility, but this should be ruled out because the active surface area as calculated from the hydrogen wave in CV showed only little differences between pure and contaminated systems, except for the case of Cu2+ contaminated film. In the last system deposition of Cu2+, in monolayer amount, occurs on platinum surface. The change of reaction schemes of oxygen reduction in the presence of impurity ions should also be ruled out, because in all cases of platinum electrodes covered with contaminated films, the parameter Rna remained almost unchanged (Figure 7). Limited rate of reaction of the scheme O2 + 4H+ + 4e f 2H2O due to the decreased concentration of H+ at the platinumionomer interface may, for one possibility, occur by the presence of these contaminant ions. However, the results about the transference number of H+ in the membrane showed a marked decrease only when the amount of impurity cations dominates the membrane cationic site over 50%,13-15 and therefore the interpretation based on lowered H+ concentration should also be ruled out. Very high H+ concentration in the film, over 1 mol dm-3, would also give less support to this mechanism. Slow diffusion rate of oxygen in the contaminated film does not completely account for the hindrance of the scheme O2 + 4H+ + 4e f 2H2O, because the decrease in Df and C/f are rather small as compared with the decrease in jk,c (compare Figures 5, 6 and Figure 8). Also note that jk,c corresponds to the kinetic current at the maximum concentration of oxygen in the film, not the changed concentration at the platinum-Nafion interface. At the membrane-electrolyte interface, Donnan potential ∆φDon arises due to the exclusion of anions in the solution by the sulfonic acid groups in the membrane. For the case of monovalent cations, assuming that anion in the solution is totally rejected from the membrane, ∆φDon is expressed by the following equation:16
∆φDon ) -
( )
RT γ+Bm ln F k+B0
(BdH+ or impurity cation) (11)
here γ+ is activity coefficient of cation in the membrane, and k+ is the hypothetical partition coefficient of cation between solution and membrane phases, in the absence of. sulfonic acid groups. Equating ∆φDon for both H+ and another impurity cations implies Figure 8. The parameters C/f,cDf,c and C/f,cD1/2 f,c as expressed by the ratio of these values for the contaminated film against those for the noncontaminated film are shown in the time course. Impurity level: 0.1% (0), 1% (4), or 10% (O). (a) Na+, (b) Ca2+, (c) Fe3+, (d) Ni2+.
4. Discussion Previous work revealed that the charge transfer kinetics of oxygen reduction as well as the oxygen transport processes in the platinum-ionomer membrane system are strongly influenced by the presence of impurity ions.1,2 On the other hand, bare platinum electrodes immersed in 0.1 N H2SO4 containing impurity ions did not reveal this effect. The results in the present work show that this impurity effect for the platinum-ionomer system generates even at as low level as 0.1% impurity in comparison with H+ in the solution. There may be several causes that could account for this degradation of charge-transfer kinetics at the platinum-Nafion interface.
∆φDon ) -
(
RT CSO3- rc + 1 ln kH F 2cSO42- k+ rc + γ+ γH
)
(12)
where CSO3- is concentration of sulfonic acid groups in the membrane, cSO42- is that of SO42- (analytical) in the solution, and rc is the ratio of concentration of impurity ion as compared with H+ in the solution. The shift of ∆φDon by the presence of impurity ions for rc ) 0.001, 0.01 and 0.1 might cause a change in the potential difference at the platinum-membrane interface, but seeing eq 12, this contribution would be minute. CVs and polarization curves revealed some change of the platinum surface, and this might be due to the change in the structure of the electric double layer or the platinum oxide layer by specific adsorption of impurity ion-polymer complex at the platinum surface.17,18 Note that the oxygen reduction potential coincides with platinum oxide formation-reduction region, and
Oxygen Reduction Kinetics
J. Phys. Chem. B, Vol. 105, No. 29, 2001 6985
TABLE 1: Impurity Levels of Cations (%), Water Content (λ ) nH2O/nSO3-), Density in the Wet State [d(wet) g cm-3] in Nafion Membranes in Equilibrium with 0.1 N H2SO4 Containing Various Kinds of Impurity Cations, and the Degree of Degradation in the Kinetic Current jk,c/jk,r in Contaminated Conditions in Comparison with Noncontaminated Conditions impurity level in Nafion membrane (% of SO3-), λ, d(wet), jk,c/jk,r Li+
Na+
K+
Ca2+
Cu2+
Ni2+
Fe3+
0.1
0.1 (22.1) [1.56]
0.2 (22.1) [1.56]
0.8 (22.1) [1.57] 0.9 6 (22.2) [1.57] 0.8
1 (20.9) [1.64] 0.9 10 (20.9) [1.66] 0.5 66 (18.8) [1.70] 0.5
1.1 (22.1) [1.61]
1.0
0.1 (20.8) [1.64] 0.8 2 (20.6) [1.64] 0.5 14 (20.3) [1.64] 0.5
1.5 (22.1) [1.61] 0.8 16 (22.0) [1.61] 0.4 64 (21.0) [1.64] 0.4
1.6 (22.1) [1.61] 0.9 17 (22.0) [1.62] 0.6 77 (19.8) [1.68] 0.4
impurity level in solution (%)
10
the oxide layer that is affected by the presence of impurity ions might alter the oxygen reduction kinetics. However, seeing the changes of the extent of hysteresis in polarization curves as discussed in section 3.2, the platinum oxide solely does not account for the observed changes in jk,c/jk,r. Table 1 shows the impurity level in the Nafion membrane in contact with the solution containing several levels of impurity cations used in this experiment.13-15,19 For divalent and trivalent cations, the amount of impurities in the membrane is much higher that that in the solution. It should be noticed, however, that the degradation of jk,c/jk,r reaches a saturation state when the impurity level in the membrane becomes about 10%, and does not increase further beyond this level. Table 1 also shows the water content, density in the wet state of membranes containing various kinds of impurity ions with various amounts. For divalent and trivalent impurity cations, a clear tendency is observed that the water content decreases and density increases as the amount of impurity increases. This may indicate that the structure of polymer network shrinks when the impurity ions are incorporated in the membrane. For Na+ and K+, the same tendency is observed although with less extent than observed for divalent or trivalent cations. It is anticipated that for multivalent cations, the affinity of cations to sulfonic acid groups would be higher than that in monovalent cations, and would cause cross-linking of the polymer network when they are incorporated in the membrane. The only exception is the membrane containing Li+, where cationic composition, water content, and density of the membrane showed a little opposite tendencies with increasing the impurity level, as compared with membranes containing other types of impurity cations. Note that degradation of oxygen reduction kinetics was not significant for Li+ containing membranes (Figure 6). The change in the polymer structure and degradation of jk.c appears to be linked, either directly or indirectly, each other. Seeing the changes of jk,c/jk,r as a function of time in Figure 5 and Figure 6, the process appears to need a long time to attain a stationary state. This is against the expectation that ion exchange reaction between the film and the solution would be much faster. Therefore, degradation of jk,c would not be the direct consequence of adsorption of impurity cations at the platinum-Nafion interface, but would be connected to a much slower process. It is seen in Table 1 that the amount of water to be brought into the membrane is different for different cation species. The process of water movement in the membrane would be much slower than the ion exchange process, because the polymer chain should undergo some relaxation processes accompanying this re-distribution of water. It is therefore
3 (21.3) [1.59] 0.5 31 (19.8) [1.62]
17 (22.0) [1.63] 0.6 65 (21.2) [1.65]
anticipated that some reorientation of polymer network should possibly occur at the platinum-polymer interface, after the process of ion exchange takes place. This would inevitably give rise to the change of the charge distribution at the vicinity of the platinum surface, and would cause the occurrence of “constrained electric double layer”.4 The change in the polymer structure is also accounted for by the reduction of transport parameters in the contaminated film. Comparing Figures 5, 6, and Figure 8, it is observed that the degradation of these parameters occurs in the same time span as the degradation of kinetic current. Notice should be directed to the possible structural change of the electric double layer at the platinum-Nafion interface in relation to the absorption of impurity ions in the polymer network. In the case of the metal-electrolyte solution interface, apart from the inner Helmholtz layer, an electric double layer forms as a result of the balance between the electrostatic interaction of ionic species and the surface charge of the electrode, and thermal motion of particles in the solution phase. The thickness of the electric double layer (Gouy-Chapman double layer), 1/κ, is given by16
1 κ
)
x
RT
∑i zi F ci
(13)
2 2
where is the dielectric constant of the solution and zi and ci are the valence and concentration of ionic species in the solution. The rate of electrode reactions is strongly affected by the magnitude of the electric field in this electric double layer. On the other hand, in the case of metal-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 not free in motion. It is also pointed out that the discrete spatial locations available to the charges in the polymer or solid electrolyte would alter the diffuse double layer, which is less evident than in an metal-electrolyte solution interface, or would even show an oscillation behavior in the charge density profile.20,21 The electric field at the vicinity of the platinum would be determined by the binding force of cations to sulfonic acid groups. If H+ ion is replaced by cations that have higher affinity to sulfonic acid groups than H+, the electric double layer would be more constrained than that for pure H+ system, and the thickness of the electric double layer would become larger. Then it will be difficult to maintain a high electric field as realized in the pure H+ system, which gives rise to slower kinetics of the charge transfer in the oxygen reduction reaction.
6986 J. Phys. Chem. B, Vol. 105, No. 29, 2001 When there are impurity cations of multivalence, this would cause thinner electric double layer than for monovalent cations as anticipated from the trend in eq 13, and the electric field would become larger than that for monovalent cations. The tendency given Table 1 appears to be in conformity with this anticipation. The above discussion would need a careful examination before it is to be accepted, but it will be able to expect a new research field of metal-ionomer interface, which would open a possibility to control the structure of the electric double layer and thus the kinetics of desired reaction, by manipulating the polymer structure at the interface. 5. Conclusions The degradation of oxygen reduction kinetics on Nafion filmed electrodes by the presence of impurity ions was studied, and this effect turned out to be very serious even in the case of only small amount of impurity ions (0.1 to 1%) in the solution phase. In the Nafion film the amount of cations was about 0.1 to 10% of the cation exchange sites. The kinetic current for the charge transfer of oxygen reduction on platinum electrodes covered with contaminated film was found to drop about 40-60% as compared with pure film after 1-5 days of immersion in 0.1 H2SO4 containing impurity cations of 0.1 to 1% level as compared with H+. This observation was specific to Nafion film covered platinum. The drop was different for different impurity ions. No evidence was observed for altered reaction mechanism of oxygen reduction, the blockage of platinum surface or reduced H+ ion transport in the film, in contaminated conditions. The reduced kinetic current was ascribed to the change of the electric double layer at the platinum-ionomer interface. For the transport of oxygen gas through the polymer layer, parameters such as oxygen concentration and/or diffusion
Okada et al. coefficients also declined for contaminated film. This drop was to the similar extent among contaminated films with different kinds of cations, but to a lesser extent than the drop of the charge-transfer rate jk,c. This was ascribed to the changes in the polymer flexibility. References and Notes (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) Wakizoe, M. NEDO Report Fy. 1997; 1988; p 51. (4) Wang, J. X.; Adzˇic, R. R. J. Electroanal. Chem. 1998, 448, 205. (5) Moore, R. B.; Martin, C. R. Anal. Chem. 1986, 58, 2569. (6) Zook, L. A.; Leddy, J. Anal. Chem. 1996, 68, 3793. (7) Lawson, D. R.; Whiteley, L. D.; Martin, C. R.; Szentirmay, M. N.; Song, J. I. J. Electrochem. Soc. 1988, 135, 2247. (8) Watanabe, M.; Igarashi, H.; Yosioka, K. Electrochim. Acta, 1995, 40, 329. (9) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980. (10) Gileadi, E. Electrode Kinetics; VCH Publishers: New York, 1993. (11) Gottesfeld, S.; Raistrick, I. D.; Srinivasan, S. J. Electrochem. Soc. 1987, 134, 1455. (12) Abe, T.; Swain, G. M.; Sashikata, K.; Itaya, K. J. Electroanal. Chem. 1995, 382, 73. (13) Okada, T.; Møller-Holst, S.; Gorseth, O.; Kjelstrup, S. J. Electroanal. Chem. 1998, 442, 137. (14) Okada, T.; Nakamura, N.; Yuasa, M.; Sekine, I. J. Electrochem. Soc. 1997, 144, 2744. (15) Okada, T.; Ayato, Y.; Yuasa, M.; Sekine, I. J. Phys. Chem. B 1999, 103, 3315. (16) Vetter, K. J. Electrochemical Kinetics; Academic Press: New York, 1967. (17) Kinoshita, K. Electrochemical Oxygen Technology; John Wiley & Sons: New York, 1992. (18) Cha, C.; Zu Y. Langmuir 1998, 14, 6280. (19) Okada, T.; Satou, H.; Okuno, M.; Yuasa, M. Unpublished data. (20) Yethiraj, A. J. Chem. Phys. 1999, 111, 1797. (21) Horrocks, B. R.; Armstrong, R. D. J. Phys. Chem. B 1999, 103, 11332.
