Theory-Guided Design of N-Confused Porphyrinic Covalent Organic Frameworks for Oxygen Reduction Reaction

Covalent organic frameworks (COFs) enable the precise and controlled synthesis of single-atom catalysts with uniformly distributed active metal centers, offering opportunities to elucidate the impact of subtle coordination environment changes on the catalytic performance. Inspired by N-confused porphyrins, we designed 20 metalloporphyrin-based COFs with either M-N3C1 or M-N4 centers, where M refers to 3d transition metals from Sc to Zn, for the electrocatalysis of oxygen reduction reaction (ORR). Density functional theory calculations predicted Co-based COFs to be the best ORR catalysts among the screened catalysts. Hence, Co-N3C1-COF and Co-N4-COF with a high crystallinity were synthesized. The Co-N3C1-COF exhibited improved ORR performance over the parent Co-N4-COF, as it had a reduced overpotential and increased four-electron selectivity, corroborating theoretical predictions. The enhanced performance was ascribed to the increase in electron density on Co as the coordination environment transits from Co-N4 to Co-N3C1. This not only facilitated the adsorption of O2 and critical intermediates but also changed the potential-determining step, which in turn made the ORR free energy profile of Co-N3C1-COF approach equilibrium for all elementary steps, thus leading to a reduced overpotential. This combined theoretical and experimental work exemplified carbon coordination in porphyrin-based COFs as an effective strategy to facilitate the catalytic capability for ORR. A descriptor was also provided to guide the design of coordination-varied Por-COFs catalysts.