Highly Active, CO-Tolerant and Robust Hydrogen Anode Catalysts: Pt-M (M=Fe, Co, and Ni) Alloys with Stabilized Pt Skin

Abstract

For residential co-generation polymer electrolyte fuel cell (PEFC) systems operated with reformates, the mass activity for the hydrogen oxidation reaction (HOR) at the state-of-the-art commercial Pt-Ru/C anode catalyst is insufficient, especially when operated under high CO concentration (> 500 ppm) in a fuel processing system simplified for cost reduction. Recently, we have developed a novel CO-tolerant catalyst by forming two uniform atomic layers of stabilized Pt skin on Pt-Co alloy nanoparticles (Pt2AL–PtCo/C) and found its superlative CO-tolerant HOR activity together with high robustness with respect to air exposure.1 In the present research, to establish a clear strategy for designing CO-tolerant catalysts, we have examined the effect of the nonprecious metal species M (M = Fe, Co, Ni) in Pt2AL–Pt-M/C on the CO-tolerance and the robustness. The Pt2AL–PtFe/C, Pt2AL–PtCo/C, and Pt2AL–PtNi/C were prepared by the use of a high-surface-area carbon black support (specific surface area = 780 m2 g−1) in the same manner as that described previously.2, 3 Two commercial catalysts, c-Pt2Ru3/C and c-Pt/C, were used for comparison. The experimental procedure, in which the channel flow electrode cell (CFE) was used, was the same as that described in ref. 1. Each catalyst was uniformly dispersed on an Au substrate as the working electrode with a constant loading of carbon support of 11 µg cm−2, which corresponds to approximately two monolayers in height of the carbon black particles. Nafion film was coated on the catalyst layer with an average thickness of 0.075 µm. All electrode potentials were referred to the reversible hydrogen electrode (RHE). Figure 1 shows the apparent mass activities, MA app, at 20 mV vs. RHE for the HOR at various catalysts in H2-purged 0.1 M HClO4 solution at 70°C and 90°C without (t ad = 0, CO-free) and with adsorbed CO (t ad = 90 min at E = 50 mV in 1000 ppm CO/H2 saturated solution). For the HOR activity on the CO-free surface of the catalysts (t ad = 0), the Pt2AL–Pt-M/C was found to exhibit higher MA app than those of c-Pt/C and c-Pt2Ru3/C. The largest MA app was seen at Pt2AL–PtFe/C, 330 A gmetal −1 at 70°C, which was about 1.8 times higher than that of c-Pt/C and 2.5 times higher than that of c-Pt2Ru3/C. All of the catalysts showed a decrease in MA app at 90°C compared to that at 70°C, which is ascribed mainly to the decreased H2 concentration in the solution at high temperature, but the trend of the MA appvalues was unchanged. After 90-min CO poisoning (t ad = 90 min) at 70°C, as shown by the black bar for the c-Pt/C electrode, MA app decreased greatly, due to a decrease in the hydrogen adsorption sites blocked by strongly adsorbed CO. In contrast, the Pt2AL–Pt-M/C and c-Pt2Ru3/C catalysts exhibited excellent CO-tolerance, suggesting that the HOR active sites were not so rigidly blocked by CO, due to its enhanced mobility. Specifically, the CO tolerance increased in the order Pt2AL–PtNi/C < c-Pt2Ru3/C < Pt2AL–PtCo/C < Pt2AL–PtFe/C. At 90°C, the CO tolerance for all the catalysts was improved. The Pt2AL–Pt-M/C still showed higher MA app than those of c-Pt/C and c-Pt2Ru3/C, e.g., the MA app at Pt2AL–PtFe/C was 3.4 times higher than that of c-Pt/C and 2.8 time higher than that of c-Pt2Ru3/C. Thus, in the practical temperature range, the highest CO-tolerant HOR activity was demonstrated for Pt2AL–PtFe/C. The effect of particle size and the optimization of preparation methods are in progress in our laboratory. This work was supported by funds for the “SPer-FC” project from NEDO of Japan.