Activation and characterization of Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells

Abstract

Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells (PEFCs) have been prepared from a precursor containing 10 wt% Fe as Fe(OH) 2 adsorbed on carbon black (Fe(OH) 2/C). Activation of the precursor was performed in two steps: (i) H 2 reduction at 600°C; (ii) pyrolysis in acetonitrile (AN) vapor at various temperatures (400, 600, 800, 1000°C). The electrocatalytic properties of all catalysts were tested in rotating disk electrode (RDE) experiments and in single H 2/O 2 gas diffusion electrode (GDE) assemblies. Catalysts for O 2 reduction were obtained for Fe(OH) 2/C pyrolyzed at 600°C and higher. Stable currents in the fuel cell assembly were observed for the catalysts prepared at 800°C and higher. Leaching the excess iron from the catalysts by exposure to an H 2SO 4 solution increased their catalytic activities. Exposure of the acid leached catalyst prepared at 1000°C to Cl 2 at 650°C removed additional quantities of excess iron and increased the catalyst activity even further. The catalyst obtained after these treatments retained an iron content of 3.3 wt%. The survival of the catalytic activity in that material even after the Cl 2 treatment suggests that either Fe is present in the active site in a high oxidation state unleachable under the form of FeCl 3, or that Fe is not a constituent of the active site. In this case, its role would be limited to catalyzing the formation of the carbon and nitrogen based active site. The XRD, XPS, ToF-SIMS and TEM analyses performed on the catalysts prepared during this study were inconclusive in resolving this issue as they all were dominated by the presence of inactive iron particles and AN pyrolysis products.