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 ∼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.