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Oxygen Reduction at the Pt/Carbon Black-Polyimide Ionomer Interface Kenji Miyatake, Takuya Omata, Donald A. Tryk, Hiroyuki Uchida, and Masahiro Watanabe* Clean Energy Research Center, and Fuel Cell Nanomaterials Center, UniVersity of Yamanashi, 4 Takeda, Kofu, Yamanashi 400-8510, Japan ReceiVed: October 27, 2008; ReVised Manuscript ReceiVed: February 10, 2009
We have investigated the oxygen reduction reaction (ORR) at the interface of Pt nanoparticles dispersed on carbon black (Pt/CB) with a sulfonated polyimide ionomer layer (SPI-8). The electrochemical experiments were carried out in air-saturated 0.1 M HClO4 aqueous solution at 25 °C via the rotating ring disk electrode technique. Slow-sweep (5 mV s-1) hydrodynamic voltammetry yielded accurate estimates of the activity of the Pt catalysts under steady state conditions. It was found that the ORR was purely kinetically controlled provided that the polyimide ionomer layer covering the Pt/CB catalysts was thinner than 0.05 µm (threshold thickness) for higher potentials. When thicker than 0.05 µm, the oxygen diffusion limitation through the ionomer was non-negligible for potentials lower than 0.8 V vs the reversible hydrogen electrode, RHE. The threshold thickness was approximately half of that for a similar perfluorinated ionomer (Nafion)-coated Pt/ CB (Nafion-Pt/CB) electrode. The hydrogen peroxide yield, P(H2O2), was lower than 0.6% of the overall ORR current in the 0.7-+0.8 V vs RHE range for the SPI-8-Pt/CB electrodes, which was somewhat lower than that for the Nafion-Pt/CB ones. In particular, P(H2O2) was negligibly low ( 0.88 V), the Tafel slopes were ∼-60 mV decade-1 for all electrodes. This value is in good agreement with those reported for single crystal Pt electrodes,42-44 polycrystalline Pt electrodes,45 and Pt/CB catalysts36,44 and is consistent with the ORR taking place on an OH-covered Pt surface.46,47
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Figure 8. Inverse current density plots for the ORR at SPI-8-Pt/CB (0.05 µm SPI-8 layer) electrode at 0.7, 0.76, 0.8, and 0.84 V. Figure 6. Tafel plots for the ORR at SPI-8-Pt/CB electrodes in airsaturated 0.1 M HClO4 solution at 25 °C. The potential sweep rate was 5 mV s-1. Each i∞ value was determined individually from a separate plot of iD-1 vs ω-1/2 (discussed later).
1 ⁄ iD ) 1 ⁄ i k + 1 ⁄ i f + 1 ⁄ i d
(2)
where ik, if, and id are the kinetically controlled current density, the diffusion-limited current density through the SPI-8 ionomer film, and the convective-diffusion-limited current density through the solution, with the measured current being divided by the geometric surface area of the disk Ageo (0.283 cm2). In order to take into account the presence of two Tafel slope regions in the pure kinetically limited behavior, it is possible to separate ik into two components, ik,60 and ik,120:
1 1 1 ) + ik ik,60 ik,120
Figure 7. Hydrodynamic voltammograms for the ORR at SPI-8-Pt/ CB (0.05 µm SPI-8 layer) electrode in air-saturated 0.1 M HClO4 solution at 25 °C. The potential sweep rate was 5 mV s-1; the electrode area was 0.283 cm2.
At lower potentials, i.e., higher overpotentials (E < 0.85 V), the Tafel slopes increased to ∼-120 mV decade-1 for all of the electrodes, most clearly visible with the thin SPI-8 layers (0.04 and 0.05 µm). This value is usually considered to correspond to either the ORR occurring on the OH-free Pt surface,43,44,47,48 or to rate-limiting oxygen mass transport within flooded catalyst agglomerates.49-53 This issue is beyond the scope of the present work and will be reported elsewhere. For all SPI-8 film thicknesses, the Tafel curves deviated from the 120 mV decade-1 line at higher current densities, with larger deviations as the thickness increased. It is evident that mass transport (oxygen diffusion) through the ionomer layer, together with the intrinsic kinetics, were the main contributors to the ORR behavior for the electrodes with SPI-8 layers at higher overpotentials. As discussed later, an additional current-limiting factor was also identified. We discuss the effect of the ionomer layer thickness on the ORR activity in the following section. Effect of SPI-8 Thickness on ORR Activity. In order to evaluate the ORR activity of the SPI-8-Pt/CB electrodes and the effect of SPI-8 thickness, hydrodynamic voltammograms were measured at different rotation rates. As a typical example, Figure 7 shows hydrodynamic voltammograms for the SPI-8Pt/CB (0.05 µm SPI-8 layer) working disk and Pt ring collection electrodes. As expected, currents ID and IR observed at both of the electrodes increased with increasing rotation rate. As first proposed by Lawson et al.,29 the inverse ORR current density (1/iD) can be divided into three terms, as follows:
(3)
With the use of both components, the mass transport-corrected Tafel plots can be simulated with high precision (see Supporting Information). For an ionomer film-coated bulk Pt disk, the mass transport limited current density through the ionomer film if is given by the following expression:29
if ) nFCfDfL-1
(4)
where Cf and Df are the oxygen solubility and diffusion coefficient in the ionomer film, and L is the film thickness; the units of the CfDf product are mol cm-1 s-1. Here, we have not attempted to make use of n values to calculate this product. In Figure 8, plots at a range of potentials (0.7, 0.76, 0.8, and 0.84 V) (often referred to as Koutecky-Levich plots) are shown (current density)-1 vs (rotation rate)-1/2. They all exhibited excellent linearity. The curves were reasonably parallel, consistent with rate-limiting solution phase mass transport. For the highest potential (0.84 V), there was a slight decrease in slope, due to the onset of radial diffusion within the film to catalyst agglomerates. The iD values obtained by extrapolating ω-1/2 to zero, i.e., infinite rotation rate, are henceforth referred to as i∞. As mentioned earlier, we again stress that the current densities are corrected only for the solution phase mass transport.
1 1 1 L 1 + ) + ) i∞ if ik nFCfDf ik
(5)
Later in this work, we will see that it is necessary to add a third mass transport term that is thickness-independent, but only for higher current densities. In any case, at low current densities and thinner films, the film transport term if will be seen later to become relatively small, and the pure kinetic currents are obtainable. This was seen in Figure 6, in which the intrinsic ik values were observed in the potential region above ∼0.88 V,
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Figure 9. (A) Solution phase mass transport-corrected current density, i∞ as a function of the SPI-8 thickness, L, at (a) 0.7, (b) 0.76, (c) 0.8, and (d) 0.84 V vs RHE, based on the ID values in Figure 5. (B) Corresponding results for Nafion-Pt/CB (experimental values shown as solid symbols). Curves calculated from eq 6 are shown as solid lines; pure kinetic values, corrected for film mass transport, are shown at right (see text for details).
with a Tafel slope close to 60 mV decade-1. These kinetic currents can be considered to be intrinsic to the ionomer-catalyst interface. In a separate publication, which will be reported elsewhere, we will present a detailed account of the attempted fitting of the data to eq 5 and the need to modify this equation in order to fully simulate the behavior at higher current densities. Briefly, we found a nonzero intercept for the i∞-1 vs L plot in the plateau current region, i.e, where (ik)-1 would be close to zero. Thus, to properly simulate the behavior, it was necessary to add a third term. The latter is considered to correspond to a limiting current due to lateral diffusion to active catalyst agglomerates. The value was ∼0.03 A cm-2 (geo); thus, it does not have a major effect on most of the behavior reported herein. This type of lateral mass transport has been treated previously in the literature in the context of diffusion of oxygen to isolated active sites on carbon electrodes.54
1 1 1 1 L 1 1 ) + + ) + + i∞ if ik ilat nFCfDf ik ilat
(6)
In the same fashion as reported in our previous work,37 we have plotted the pure solution mass transport-corrected RDE data, i.e., i∞ as a function of L-1 (Figure 9A). When L-1 is smaller than ∼20 µm-1 (or L is larger than 0.05 µm), the i∞ values increase nearly linearly with increasing L-1, consistent with linear diffusion of oxygen in the SPI-8 layer. When L-1 is larger than ∼20 µm-1 (or L is smaller than 0.05 µm), the i∞ values tend toward plateau values for the higher potentials examined. Specifically, the i∞ values approach the purely kinetically controlled current densities ik, since the mass transport within the ionomer if becomes less important for L values that are sufficiently small. On the basis of the parameters obtained from the detailed analysis based on eq 6, we have also plotted the theoretical curves based on eq 6, with both ik terms included explicitly, as in eq 3. At the right of Figure 9A, we show the pure kinetic currents densities that were used for each potential (open symbols, based on the average for the middle
range of film thicknesses); in addition, we show the average and error bars (one standard deviation) for the kinetic values averaged for all film thicknesses. It can be seen that the i∞ values are somewhat lower than the pure kinetic values values at 0.8 V and decrease to about 75% of these values at ∼0.05 µm, which we define as the threshold thickness. Although the currents for the thickest films (0.8 and 1.6 µm) are larger than expected and that for the 0.04 µm film is somewhat lower than expected, the fits to the bulk of the experimental data are reasonably good. We also performed similar measurements for a Nafion film at a single film thickness (0.1 µm) in the present work and found significantly larger i∞ current densities at smaller L-1 values. These points are shown in Figure 9B. The pure kinetic currents are shown as open symbols at right. The latter are seen to be very similar to those for SPI-8, i.e., for SPI-8, ∼0.5 ( 0.1 mA cm-2 (normalized to real Pt surface area) at 0.84 V, 1.3 ( 0.3 mA cm-2 (Pt) at 0.8 V and 3.25 ( 0.75 mA cm-2 (Pt) at 0.76 V; and for Nafion, ∼0.6 mA cm-2 (Pt) at 0.84 V, 1.5 mA cm-2 (Pt) at 0.8 V and 3.5 mA cm-2 (Pt) at 0.76 V. The values at 0.7 V (off scale, ∼11.4 ( 2.6 mA cm-2 (Pt) for SPI-8 and 11.9 mA cm-2 (Pt) for Nafion) were also very similar to each other. The similarity for the kinetic currents shows that for the overall ORR, the interactions of the ionomers with the Pt/C electrocatalyst are nearly the same. While it was not possible to carry out a full analysis of these results based on a single thickness, we can still make a reliable estimate of the slope term (pure diffusion through the film if) which is the only one needed to fit the data, because the kinetic terms can be obtained by direct fitting of the mass transportcorrected Tafel plot (see Supporting Information), and ilat was assumed to be the same as that for SPI-8. The if term for Nafion (1.25 × 10-3 A cm-2 µm) was approximately twice that for SPI-8 (6.2 × 10-4 A cm-2 µm), consistent with our estimate of Lthresh as ∼0.10 µm. This ratio is much less than the 18-fold difference found in one of our previous studies, in which we found an oxygen permeability coefficient;22 5.3 × 10-9 cm3(STD) cm/cm2 s cm Hg for Nafion and 2.9 × 10-10 cm3(STD) cm/cm2 s cm Hg for SPI-8. (Note that the measurement conditions were not the same as those in the present RDE experiments: previously, the oxygen permeability was measured at 80 °C and 90% relative humidity (RH) for membranes ∼50 µm thick.). Another important factor to consider is that the ionomer film morphology and properties might be somewhat dependent on thickness. In any case, the difference in the if terms shows that the oxygen transport properties for SPI-8 were significantly lower than those for Nafion. Thus, the threshold ionomer thickness for SPI-8 is smaller than those for Nafion ionomers to be used in gas diffusion electrodes for ORR. However, for practical applications in PEFCs, the supposed ionomer thickness on the Pt/CB catalysts would be much smaller, since most of the catalyst particles are located in the nanopores of the CB supports (on the order of 50 nm), and thus the gas diffusion limiting behavior in the ionomer layer should not be critical. We should also note that the gas mass transport studied here is necessarily that of the liquid electrolyte-filled ionomer and is intrinsically less efficient than that in the humidified but not flooded ionomer. In Figure 10 is plotted the hydrogen peroxide yield, P(H2O2), as a function of the SPI-8 thickness L at 0.7, 0.76, and 0.8 V. The P(H2O2) is defined as the percentage of hydrogen peroxide formation out of the total ORR current and was calculated as follows:
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Figure 10. Hydrogen peroxide yield, P(H2O2), as a function of the SPI-8 thickness, L.
P(H2O2) ) 2IR ⁄ (N × ID + IR) × 100
(7)
The P(H2O2) was relatively small (
