Abstract
To address the problems (high price, insufficient total activity and deactivation) related to usage of pure Pt as cathode catalysts for oxygen reduction reactions (ORR) in polymer electrolyte fuel cells (PEFC), carbon-supported Pt-Sn(oxidized shell)/C and Pt-Sn(reduced)/C were used as membrane electrode assembly (MEA) cathode catalysts in PEFC. I-V-load curves in the initial load-cycles showed an increase of the performance for Pt-Sn(oxidized shell)/C, but no increase in the performance was observed with Pt-Sn(reduced)/C and standard Pt/C catalysts. The observed increase in the performance for the Pt-Sn(oxidized shell)/C catalyst may be due to an synergy effect of Pt-nanoparticle core and SnOx-cluster shell formed on the Pt nanoparticle surface. In-situ XAFS at Pt LIII-edge and Sn K-edge under the PEFC operating conditions showed the absence of electron transfer from Sn to Pt and also no changes in the Sn state, indicating its stability. The in situ XAFS for the Pt-Sn(oxidized shell)/C also revealed that Pt nanoparticles made Pt-O bondings at higher voltages than 1.0 V vs. RHE, but the coordination number of Pt-O bonds was much smaller than that for Pt/C. Thus, the Pt-Sn(oxidized shell)/C is a promising candidate as a MEA cathode catalyst in a PEFC system.
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