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

Abstract

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 O 2, H 2 O, OH, HOOH, and H 2 OO 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 O 2 adsorption energy, the higher the kinetics of the ORR will be; (2) the ORR pathways, 2e − or 4e −, are mainly determined by the H 2 O 2 adsorption configurations; and (3) OH adsorption on the catalysts is considered to be an important factor to affect the catalyst stability.