UC Berkeley

Membrane-electrode assemblies utilize ionomer-coated electrocatalysts to achieve facile ion transport. Consequently, isolation of intrinsic catalyst kinetics from measured polarization curves is challenging, as the properties of the catalyst and ionomer both affect the measurements. Here, we employ a Pt microelectrode coated by a thin perfluorosulfonic acid (PFSA) layer to measure polarization curves for the hydrogen oxidation reaction/hydrogen evolution reaction (HER/HOR). Intrinsic electrode kinetics are isolated by theoretical analysis of the local catalyst microenvironment, accounting for mass transport and thermodynamics. The observed enhancements in HER and HOR rates with increasing relative humidity (RH) at the working electrode are attributable to two competing factors: the decrease in activity of H+ in the ionomer and the dominant decrease in the water reorganization energy in the Marcus–Hush–Chidsey representation of HER/HOR kinetics. The increase in intrinsic rate with increasing RH is attributed to increased H+ transfer dynamics resulting from reduced confinement of water within the subnanometer water layer between the catalyst and ionomer as RH increases.