The Effect of Carbon Support Surface Functionalization on PEM Fuel Cell Performance, Durability, and Ionomer Coverage in the Catalyst Layer

Surface functionalization of nitrogen-containing species on carbon support has been shown to affect electrocatalytic activity and carbon-ionomer interaction in the fuel cell electrodes, but studies of the functionalization on mass transport are limited. We reported two schemes for grafting positively (nitrogen groups) charged species and a scheme for negatively (sulfonates) charged species on the surfaces of three carbon materials. The functionalization was characterized with BET, XPS, contact angle measurements, and zeta potentials. In full-cell tests, improvements over high current densities were observed in samples reacted with para-phenylenediamine or ammonia, whereas the performance decreased after functionalization with sulfonate groups. The improvement at high current densities exceeded the mass-activity improvement and was attributed to reduced mass-transfer polarizations. Furthermore, a statistical approach was developed to examine the changes in ionomer surface coverage using STEM images with fluorine EDS maps. Durability studies followed the DOE’s protocol of potential cycling between 1.0 V and 1.5 V. Faster rates of carbon loss were found to occur after all three surface functionalization schemes due to preferential oxidation of the weaker covalently bonded functional groups, but despite the increased rates of loss, after amination the most durable carbon support showed end-of-life performance comparable to the initial performance.