A Trade‐Off Between Thermodynamics and Kinetics of O 2 Binding for Highly Active and Selective Electrocatalytic Oxygen Reduction Reaction

ABSTRACT O 2 binding to metal ions is an essential step in the oxygen reduction reaction (ORR). Tailoring O 2 binding is anticipated to realize highly active and selective four‐electron ORR. Herein, we report on fine‐tuning the molecular pocket of Co porphyrins to get a trade‐off between thermodynamics and kinetics of O 2 binding for ORR with high activity and selectivity. Three Co tetra (2‐amidophenyl)porphyrins with an αααα structure but different steric hindrance at the pocket bottleneck are synthesized. The pocket thermodynamically favors O 2 binding by stabilizing O 2 adducts and thus improves the 4e selectivity, but the sterically hindered pocket entrance may obstruct the access of O 2 to Co. By tuning the pocket entrance, we tailored the rate constant and equilibrium constant of O 2 binding, and consequently, we achieved efficient ORR with a half‐wave potential of 0.83 V versus RHE and a transferred electron number of 3.78. This performance is remarkable among reported mononuclear Co porphyrins.