Designed Materials for Enhanced Oxygen Reduction Electrocatalysis in PEM Fuel Cells: Novel Materials and Next Generation Synchrotron Based in Situ

摘要

A systematic RRDE investigation has been conducted in Trifluoromethane Sulfonic Acid (TFMSA) as a function of concentration (in the range 1 to 6 M) which corresponds to a change in mole ratio of water: acid from 50:1 in 1M to 4:1 in 6M TFMSA. This change in relative amount of water in the various concentrations can also be indirectly correlated to the relative humidity in an operating PEM fuel cell. The scope of this effort was to (a) confirm the shift and lowering of water activation on supported PI alloy electrocatalysts relative to Pt at lower concentrations (1 M), (b) compare the inherent activity for ORR on supported Pt and Pt alloy nanoparticles without the effect of oxide formation via activation of water, this was enabled at higher concentrations of TFMSA (6 M), (c) Relate the activation energy values at 1 M for Pt and Pt alloy electrocatalysts for further insight into the nature of the rate determining step in the mechanism. Our results confirm that for fully hydrated systems akin to 1 M concentration the alloys shift the formation and extent of water activation on the Pt alloy surfaces; this has been correlated with in-situ XAS data (changes to Pt electronic states and short range atomic order) as well as via direct EXAFS probe of formation of oxygenated species above 0.75 V (typical potential for initiation of surface oxides on PI). The lowering of oxide formation agrees well with the extent of enhancement of ORR activity. Activation energy determinations at 1 M concentration however revealed no difference between Pt and Pt alloys indicating thereby that the rate limiting step remains unchanged. At lower water activity (6 M) with negligible water activation (and hence surface oxides), Pt surface was found to possess a higher activity for ORR as compared to the alloys.