Electrocatalysis in direct methanolfuel cells:in-situprobing of PtRu anode catalyst surfaces

摘要

Stripping voltammetry of CO adsorbed at unsupported, highly dispersed PtRu anode catalysts, is demonstrated here as an in-situ probe of surface composition for this type of direct methanol fuel cell (DMFC) catalyst. Reported data [1] for stripping voltammetry of CO adsorbed at bulk PtRu alloy electrode surfaces served as the basis for this in-situ surface diagnostic. CO stripping data collected in this work for three dispersed, unsupported PtRu catalyst samples — all of significant oxygen content as received — strongly suggest that surface metal alloy domains of atomic ratio Ru:Pt near 1:1 are key for higher DMFC anode activity. The PtRu catalyst sample of highest DMFC anode activity displays highest CO stripping peak currents and charges per unit (BET) surface area under a given scan rate and temperature. This indicates higher population of metal alloy sites, free of blocking by oxide components of the sample. Combined data from CO stripping and XRD measurements indicate that a higher degree of overall alloying, as brought about by more extensive sample reduction, results also in a more reduced catalyst surface. We conclude that the activity of unsupported, dispersed PtRu catalysts in anodic oxidation of methanol depends on two major factors: maximized catalyst surface area and a catalyst surface with maximum number of metal alloy sites of atomic ratio close to 1:1. The best DMFC anode catalyst sample examined by us has a BET surface area (as received) of 70 m2 g — and the CO stripping data suggest that, in the fuel cell at the relevant anode potential, 50% of that BET surface area is metal PtRu alloy of composition near 1:1.