OH−-Binding Effects on Metallophthalocyanine Catalysts for O2 Reduction Reaction in Anion Exchange Membrane Fuel Cells

Abstract

Electrocatalytic activity of the oxygen reduction reaction (ORR) on carbon-supported metallophthalocyanine (MPc/C, M = Fe, Co, Ni, and Mn) catalysts was studied with a rotating disk electrode (RDE) and a rotating ring-disk electrode (RRDE) in 0.1 M NaOH solutions. FePc/C shows better ORR activity than CoPc, NiPc, and MnPc in 0.1 M NaOH solutions. Density functional theory (DFT) calculations were performed to study the adsorption of O2, H2O, OH, HOOH, and H2OO molecules on FePc, CoPc, NiPc, and MnPc molecule catalysts. Investigations using various MPc/C molecules as the cathode catalyst in anion exchange membrane fuel cells (AEMFCs) revealed that the catalysts, such as FePc/C, with high ORR activities observed with a RDE in 0.1 M NaOH solutions, do not warrant the high performance observed in the AEMFCs. DFT calculation results indicate that the FePc molecules are favorable for the adsorption of OH− rather than O2 or H2O, especially under AEMFC operation conditions. Electrochemical impedance (EIS) spectra obtained while operating the AEMFCs revealed that the resistance of OH− transportation from the cathode to the anode depends on the cell potentials and the nature of the MPc molecules. As predicted by the DFT calculation results, the FePc/C catalyst shows the highest OH− transport resistant at a high current and a low cell voltage region. The bonding strength between OH− and MPc molecules is a critical factor that determines the performance of the MPc molecules in AEMFCs. The fundamental discrepancy between ORR activities observed with an RDE in a standard three-electrode cell and ORR activities observed in an AEMFC is discussed.