A Trade-Off Between Thermodynamics and Kinetics of O2 Binding for Highly Active and Selective Electrocatalytic Oxygen Reduction Reaction

O2 binding to metal ions is an essential step in the oxygen reduction reaction (ORR). Tailoring O2 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 O2 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 O2 binding by stabilizing O2 adducts and thus improves the 4e selectivity, but the sterically hindered pocket entrance may obstruct the access of O2 to Co. By tuning the pocket entrance, we tailored the rate constant and equilibrium constant of O2 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.