Theory-Guided Design of N-Confused Porphyrinic Covalent Organic Frameworks for Oxygen Reduction Reaction
Pengpeng Shao, Zhixin Ren, Bo Zhao, Xiao Wang, Jie Li, Jing Xie, Bo Wang, Xiao Feng
Abstract
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–N 3 C 1 or M–N 4 centers, where M refers to 3 d 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–N 3 C 1 –COF and Co–N 4 –COF with a high crystallinity were synthesized. The Co–N 3 C 1 –COF exhibited improved ORR performance over the parent Co–N 4 –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–N 4 to Co–N 3 C 1 . This not only facilitated the adsorption of O 2 and critical intermediates but also changed the potential-determining step, which in turn made the ORR free energy profile of Co–N 3 C 1 –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.