Ionic Conductivity over Metal/Water Interfaces in Ionomer‐Free Fuel Cell Electrodes

Abstract Ion conduction and oxygen reduction reaction (ORR) kinetics at metal/water interfaces are important in determining the performance of proton exchange membrane fuel cells (PEMFCs). However, the coupled ion transport and electrochemical reactions in ionomer‐free domains of catalyst layers are still unclear. In this work, we deconvolute the ion transport and oxygen reduction reaction (ORR) by studying metal/water interfaces of platinum (Pt) and gold (Au) ionomer‐free porous cathodes. In our experiments, a conventional membrane electrode assembly (MEA) was used for electrochemical characterization of ionomer‐free electrodes. Cyclic voltammetry (CV) and polarization curve measurements at different relative humidity (RH) conditions show that active surface area and ORR current density increases with RH. We also use microstructured electrode scaffold (MES) apparatus for measurement of the ionic conductivity in the cathode. The results show that the ionic conductivity increases with the RH and water coverage. The potential dependence of ionic conductivity for Au approximately follows the classical Guoy‐Chapman electrical double layer (EDL) behavior, while the ionic conductivity of the Pt electrode did not show a strong potential dependence, indicating that it could also be affected by other mechanisms, such as formation of surface oxide and diffusion of adsorbed hydrogen on its surface.