Investigation of Transport Mechanisms for Sulfonated Silica-Based Fuel Cell Electrode Structures

Abstract

Ceramic carbon electrodes (CCEs) have demonstrated their ability to function as proton exchange membrane fuel cell electrodes under low relative humidity (RH) conditions. Small quantities of sulfonated silane in the catalyst layer produced electrodes with high surface area, porosity, and water retention which improved catalytic activity and proton conductivity. The purpose of this work was to investigate the mechanisms that facilitate enhanced performance of CCE electrodes under different RH conditions. Differences in transport phenomena related to membrane, electrode, and reactant concentration components were measured and compared for standard Nafion-based and CCE cathode catalyst layers using oxygen, air, helox (21% O2 in He), and 4% O2 in N2. Membrane electrode assemblies were characterized via cyclic voltammetry and electrochemical impedance spectroscopy. CCE cathodes displayed decreased resistance related protonic and electronic transport when relative humidity was lowered, and both types of electrodes suffered limitations due to oxygen transport losses where the oxygen also undergoes reduction in the catalyst layer. Remarkably, at 20% RH there was no change in performance at lower oxygen concentrations or mass transport loss observed for the CCE cathodes, indicating that the overall oxygen transport (through the gas diffusion layer and ionomer) is enhanced using this type of electrode structure.