Heat-Treatment of a Fourteen Membered Macrocyclic Fe Complex for Electrocatalytic Oxygen Reduction

Abstract

For the broad application of polymer electrolyte fuel cells (PEFCs), the development of non-precious-metal (NPM) catalysts for oxygen reduction is extremely important. To date, many NPM catalyst have been synthesized by pyrolyzing Fe-, N-, and C-containing precursors, but they suffer from the low density and uncertain chemical structure of their active sites. On the contrary, our research group has been studying a fourteen-membered macrocyclic Fe complex, which was inspired by the FeNx sites embed in graphene sheets (Fig. 1).In this study, a fourteen-membered macrocyclic Fe complex was loaded on a carbon support and then heat-treated, and found to be quite active as a non-precious metal oxygen reduction catalyst. The oxygen reduction current at 0.8 V (RHE) was 13.3 A gcat -1. The improvement is attributed to a high turnover frequency rather than the number of active sites. X-ray absorption spectroscopy and temperature programmed decomposition for the Fe complex suggested that the counter anions and the protons on the nitrogen atom in the ligand were eliminated by the heat-treatment. This change probably influenced the adsorption energy of the oxygen species during the oxygen reduction and resulted in the improved catalytic activity (Fig. 2). Simulation based-on the density functional theory well support this hypothesis. Figure 1