Comparison of cobalt complex catalysts having a fourteen-membered ring structure and conventional sixteen-ones for oxygen reduction and hydrogen evolution reactions

Abstract

Developing efficient and durable non-platinum group metal (non-PGM) catalysts is critical for advancing the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) in acidic conditions. Recently, we presented the enhanced performance of carbon supported cobalt complex catalysts with a 14-membered ring hexaazamacrocyclic ligand (Co-14MR/C) catalysts than carbon supported Co phthalocyanine (CoPc/C) catalysts having 16-membered ring (16MR) for the acidic ORR and HER in terms of catalytic activity and durability to propose Co-14MR structures as promising design basis of non-PGM catalyst. In the present study, we extend the comparative analysis to include Co tetraphenylporphyrin (CoTPP), another widely used Co-16MR complex. Although the performance of CoTPP/C catalysts surpasses those of CoPc/C catalysts, Co-14MR/C catalysts show higher activity and durability than CoTPP/C catalysts for both reactions. Therefore, it is indicated that Co-14MR/C catalysts are more effective for the ORR and HER compared to Co-16MR/C catalysts. In addition, based on structural analysis using extended X-ray absorption fine structure (EXAFS) spectroscopy, it is confirmed that the shorter Co-N bond length Co-14MR/C catalysts than Co-16MR/C catalysts is one of the key structural factors to long-term durability. These findings highlight the unique advantages of the 14MR ligand in enhancing both the activity and durability of CoN₄ electrocatalysts.