Tuning electronic properties of cobalt phthalocyanines for oxygen reduction and evolution reactions

Metal-phthalocyanines are a class of catalytically active materials promising in energy conversion and storage fields (e.g., electrocatalysis). However, understanding and controlling the electrochemical properties in metal-phthalocyanine systems is challenging. Herein, we elucidate the electrocatalytic origins of a series of cobalt-phthalocyanine molecular catalysts and fine-tune their electronic properties at the atomic level, both experimentally and computationally. The interactions between the cobalt center and the local coordination environment are regulated by introducing either electron-donating or electron-withdrawing groups on the phthalocyanine ligand, and the spin-orbit splitting of cobalt is increased by similar to 0.15 eV compared with the non-substituted ligand. Specifically, the aminated cobalt phthalocyanine-based electrocatalysts exhibit low free energies in the rate-determining steps of the oxygen reduction (-1.68 eV) and oxygen evolution reactions (0.37 eV). This contributes to the high electrocatalytic activity (e.g., a halfwave potential of 0.84 V and an overpotential of 0.30 V at 10 mA cm(-2)), featuring a high selectivity of a four-electron pathway (i.e., a negligible by-product of hydrogen peroxide). These catalysts also exhibit exceptional kinetic current density (Tafel slope of 100 mV dec(-1)) in oxygen reduction reactions, in addition to a superior power density (158 mW cm(-2)) and a high cycling stability (>1,300 cycles) in Zn-air batteries, outperforming the commercial Pt/C and/or RuO2 counterparts.