Electron-rich active sites created by fine-tuning the electronic structure of Co(II) porphyrin frameworks for high-performance CO2 electroreduction

Abstract

Electrochemical reduction of CO2 for sustainable production of carbon-based chemicals and fuels is a promising approach to solve environmental and energy problems. However, the development of electrocatalysts with satisfactory catalytic activity and product selectivity is still a considerable challenge. Porphyrin organic frameworks (POFs) are considered one of the most promising models for regulating the structure at the molecular level. In this work, we propose a strategy that integrates nanoscale and molecular scale to synthesize several organic-inorganic composite materials by the template-oriented polymerization of isolated Co(II) porphyrin (CoPor-) on the carbon nanotube (CNT) scaffold. The electronic structure of CoPor-active sites in the POFs for electrocatalytic CO2 reduction was tuned by modification of the substituents. Remarkably, the optimal sample CoPOF-(Me)4@CNT was found to reduce CO2 to CO with a high Faradaic efficiency for CO production (FECO) of 93 % at the overpotential of 600 mV and a remarkable turnover frequency (TOF) value of 9720 h−1 at −1.0 V vs reversible hydrogen electrode (RHE), surpassing the performance of most recently reported CoPOFs in terms of selectivity and efficiency. Moreover, this electrocatalyst is stable for more than 42 h without a loss in reactivity. In-depth experimental research suggests that the electron-rich active sites created by methyl substituents with an electron-donating effect and the synergy between POF nanolayers and CNT scaffold successfully modulate the electronic structure of Co sites in this reticular framework, thereby enhancing the electrocatalytic performance.