Engineering macroporous carbon film support for freestanding Fe-N-C cathode 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 is usually neglected at early stages of catalyst development. In this work, the reverse approach is taken: A carbon film support with an interconnected macropore network is designed for improved mass transport. 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 and combined with a model FeN4 active site. 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. The mass transport properties of film supports with thickness of 30 µm and 70 µm are analysed and compared to a conventional powder catalyst layer based on Vulcan XC 72 powder support. The average overpotentials for powder and film supports at a high current density of 2 A/cm2 are centred in the narrow range of 0.52 V ± 0.03 V, which highlights the competitive mass transport performance of the macroporous film support.