Macroporous Carbon Films Support Vs. Conventional Carbon Particles with FeN4 Sites for Oxygen Reduction in Gas-Diffusion-Electrodes

Novel synthetic approaches have been developed to maximize the active site density of atomically dispersed Fe sites coordinated to nitrogen in carbon matrix. (Fe-N-C) (1,2) Nevertheless, the realistic maximum for single iron sites in these catalysts is deduced to be limited to 10 21 sites per gram. (3) Therefore, for the application these Fe-N-C catalysts at the cathode for the oxygen reduction reaction in fuel cells, the catalyst material loading has to be increased to be able compete with the performance of Pt/C catalysts. Due to the resulting increased thickness of Fe-N-C catalyst layers, the oxygen available for the ORR at high current densities is dependent on the insufficient transport of oxygen across the catalyst layer. Freestanding catalyst nanostructured films, whose porosity is controlled across the entire thickness of the catalyst layer are expected to improve the mass transport at high current densities. Carbon nanostructures could provide the support substrate for such freestanding catalyst films. For instance, mesophase pitch-based films have been templated by silica (SiO 2 ) nanoparticles have favourable morphologies for supporting Pt. (4) However, harsh conditions and long reaction times are required to etch away the SiO 2 . In our work, SiO 2 nanoparticles are substituted by polystyrene nanoparticles, which decompose and tracelessly evaporate during the carbonization process. We have fabricated carbonized films of multiple thicknesses. We found 100 µm to be optimal, substantially thicker than Pt based electrodes to accommodate the lower density of sites and cheaper catalyst material. which is a usual thickness for Fe-N-C-based cathodes. Iron phthalocyanine (FePc) as a model active site is adsorbed on the freestanding carbon film. Gas-Diffusion-Electrodes (GDE) enable us to investigate the performance of such freestanding thin film catalysts at high current densities relevant for fuel cells. (5) A comparison of the ORR performance of catalyst layers, based on carbon films support (A) and conventional carbon particle support (B) are presented. We discuss the effect of adjusting the thickness of the two types of catalyst layers on the performance. 1 Barrio J, Pedersen A, Feng J, Sarma S. C., Wang M, Li A. Y. , Luo, H. Ryan M. P., Titirici M.-M., Stephens I. E. L. J. Mater. Chem. A, 2022;10: 6023-30 2 Mehmood A, Pampel J, Ali G, Ha H Y, Ruiz-Zepeda F, Fellinger T P Adv. Energy Mater., 2018; 8: 11649-55 3 Mehmood A, Gong M, Jaouen F, Roy A, Zitolo A, Khan A, Sougrati M.-T., Primbs M., Martinez Bonastre A, Fongalland D, Drazic G, Strasser P, Kucernak A Nat. Cat., 2022;5, 311-23 4 Atwa M, Li X, Wang Z, Dull S, Xu S, Tong X, Tang R, Nishihara H, Prinz F, Birss V 2021;8: 2451-62 5 Inaba M, Jesen A W, Sievers G W, Escudero-Escribano M, Zana A, Arenz M Energy Environ. Sci,2018;11: 988-94