Proton Conduction on Ionomer-Free Pt Surfaces

Abstract

AC impedance was measured for PTFE-bound, unbound, and ionomer-bound Pt-black electrodes, as well as for a conventional Pt/Vulcan electrode. Adsorbed water contributes to the proton conduction on ionomer-free Pt black surfaces, which display a strong dependence on relative humidity. The conductivity was 1-2 orders of magnitude lower than that of ionomer-containing electrodes. While non-dimensional scaling of the impedance data indicates a single conduction mechanism occurring in each electrode type, activation energies suggest that the mechanisms were different between ionomer-free and ionomer-bound electrodes. Ionomer-bound electrodes show an activation energy that strongly depends upon RH, while that of ionomer-free Pt black does not. Remarkably, conductivity on Pt black is 2-3 orders of magnitude greater than that of bulk water, suggesting an enhanced conduction mechanism occurring at the Pt surface.