Engineering macroporous carbon film supports for freestanding Fe–N–C cathodes at high current densities†

As the oxygen reduction reaction (ORR) kinetics account for the largest share of performance losses for fuel cells, most research in platinum group metal (PGM)-free catalysts prioritize on improving the activity of catalysts by maximizing the active site density and by engineering of the local coordination environment of the active sites to meet the activity targets. Thereby, the mass-transport capabilities of the catalyst are usually neglected at early stages of catalyst development. In this work, the reverse approach is taken: a freestanding carbon film support with an interconnected macropore network is prepared to improve the mass and charge transport of conventional PGM-free particle based catalyst layers. Carbon precursors mesophase pitch and polyvinylalcohol (PVA) are combined with the macropore template polystyrene (PS) spheres in a ball-milling process to form a slurry for casting the film which is subsequently carbonized in different atmospheres to tune the micropore volume. The macroporous films are thoroughly characterized by means of SEM, N₂ sorption, XPS and Mercury Intrusion Porosimetry (MIP) and tested as ORR catalyst support for a model FeN₄ active site in a gas-diffusion-electrode (GDE) half-cell, which can operate at high current density conditions. This study is the first to compare two support classes, a new in-house synthesized carbon film support and commercial Vulcan XC 72 particle support, in cathode catalyst layers by keeping the molecular catalyst (FePc) on both supports the same. Special focus is directed to the mass transport performances of both type of supports, which are compared at high current densities at 2 A cm⁻² in the GDE cell.