Unraveling Oxygen Reduction Reaction Mechanisms on Carbon-Supported Fe-Phthalocyanine and Co-Phthalocyanine Catalysts in Alkaline Solutions

In this work, we combine electrochemical measurements, such as the rotating disk electrode (RDE) and the rotating ring-disk electrode (RRDE) techniques, and density functional theory (DFT) calculations to elucidate the mechanisms of the oxygen reduction reaction (ORR) on carbon-supported Fe-phthalocyanine (FePc/C) and Co-phthalocyanine (CoPc/C) catalysts in 0.1 M NaOH solutions. The onset potential for ORR on FePc/C catalyst is found to be around 0.05 V vs. Hg/HgO in 0.1 M NaOH solutions, which is 100 mV more positive than that on CoPc/C. RDE and RRDE measurements show that the ORR mechanism is via a 4e− pathway on the FePc/C while it is through a 2e− pathway on the CoPc/C catalyst. The catalyst stability tests reveal that FePc is much less stable than CoPc under fuel cell cathode working conditions. Moreover, DFT calculations were performed to study the adsorption of O2, H2O, OH, HOOH, and H2OO molecules on FePc and CoPc molecule catalysts. We conclude the following from our theoretical and experimental results for the ORR on FePc/C and CoPc/C catalysts: (1) the lower the O2 adsorption energy, the higher the kinetics of the ORR will be; (2) the ORR pathways, 2e− or 4e−, are mainly determined by the H2O2 adsorption configurations; and (3) OH adsorption on the catalysts is considered to be an important factor to affect the catalyst stability.