The Pt loading amount for both the Pt/Nb-SnO2 cathode catalyst and the Pt/CB anode catalyst was 0.1 mg cm-2. The cell performances were evaluated under H2/O2 at 80 °C and 100% RH before and after the durability evaluation. Figure 1a shows the protocol for the durability evaluation involving OCV and load holding times at a low current density of 0.33 A cm-2 (sample A1), at a high current density of 0.55 A cm-2 (sample A2), and, in the case of Fig. 1b, the load cycles between the upper cell voltage at 0.94 V and the lower cell voltage at 0.6 V were operated by a potentiostat under H2/N2 (sample A3). A1, A2 and A3 conditions were performed with H2; utilizations of the reactant gases were H2 (70%)/air (40%), H2 (70%)/O2 (20%) and H2 (100 mL min-1)/N2 (100 mL min-1) at 80 °C and 80% RH in Fig. 2. These durability evaluations can be characterized as follows: A1, A2, with the OCV/load holding times of 60 s/3 s; and A3, with upper/lower cell voltage holding times of 60 s/3 s; the sweep rate of 165 mV s-1 from the lower to upper cell voltage, simulates the durability test of A1.
Figures 3 and 4 show the cell performances and mass activities under H2/O2 at 80 °C and 100% RH before and after the durability evaluation. After the durability evaluation, the cell performances for both A1 (H2/air condition) and A2 (H2/O2 condition) were nearly the same as their initial performances, but that for A3 (H2/N2 condition) decreased. The mass activity change decreased with the higher O2 partial pressure in the cathode after the durability evaluation. The mass activity of A2 (H2/O2 condition) was the highest of all conditions after the durability evaluation, in spite of the upper cell voltage being the highest. The behavior was in contrast to the case of a carbon-supported Pt catalyst (not shown here). These results suggest that the durability of the Pt/Nb-SnO2 cathode catalyst could correlate with the thickness of the electron depletion layer between the surface of the Nb-SnO2 and Pt. We reported that the electron depletion layer was induced by adsorption of oxygen species on the surface of SnO2 [4]. Therefore, we consider that the degradation from Pt dissolution in the electrochemical oxidation reaction (Pt → Pt2+ + 2e-) could be suppressed by increasing the thickness of the electron depletion layer due to an increase in the amount of adsorbed oxygen species as a function of O2 partial pressure. We will discuss in detail the degradation mechanisms of the Pt/Nb-SnO2 cathode catalyst in comparison with Pt/GCB during the load cycle durability evaluation.
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