Fundamental Aspects of ad-Metal Dissolution and Contamination in Low and Medium Temperature Fuel Cell Electrocatalysis: A Cu Based Case Study Using In Situ Electrochemical X-ray Absorption Spectroscopy

Electrochemical methods (cyclic voltammetry and rotating ring disk electrode) and in situ X-ray absorption spectroscopy (XAS) were used in conjunction to study copper adsorption onto carbon-supported platinum nanoparticles over the operating potential range of proton exchange membrane fuel cell (PEMFC) cathodes and anodes (similar to 0.0-1.0 V vs the reference hydrogen electrode, RHE). Our purpose was to better understand the detrimental effects of Cu leaching from high-activity dealloyed PtCux electrocatalysts. These studies were conducted in CuSO4-doped 0.1 N solutions of HCO4 and H2SO4 under both inert and oxygenated conditions Over the anode potential range (similar to 0.0-03 V vs RHE), concentrations of Cu2+ as low as 10 mu M were found to coat the active Pt surfaces, thereby drastically inhibiting the hydrogen oxidation reaction. Over the Cu underpotential deposition region (0.35-0.70 V vs RHE), Cu2+ concentrations >= 0.05 mM resulted in Cu deposition onto Pt. This was found to lower the oxygen reduction reaction activity of Pt by skewing the reaction mechanism toward the two-electron pathway (peroxide production) away from the desired four-electron pathway (water). Electrochemical methods were inconclusive as to the effects of Cu2+ at potentials greater than 0.84 V. While in situ Pt L-3 edge extended X-ray absorption fine structure (EXAFS) revealed definitive Pt-Cu scattering paths below 0.84 V, Cu was not observed at higher potentials. The Delta mu analysis of X-ray absorption near-edge structure (XANES) revealed that Cu(0) coadsorbs under high Cu2+ concentrations in HClO4, and that H2SO4 results in Cu(0) coadsorption at lower concentrations. Extending the Delta mu analysis to lower potentials revealed the interplay of Cu2+, O(H), and H+ coadsorption with respect to potential, Cu2+ concentration, and sparging environment (inert or oxygenated). These studies verify that Cu leaching from PtCux-alloy electrocatalysts can have detrimental effects on both the anode and cathode sides of a PEMFC, and similar experiments can be extended to probe the adsorption effects of other transition metals from PtMx alloys.