Electronic Structure Regulation of Single-Site M-N-C Electrocatalysts for Carbon Dioxide Reduction

The current global population and economy depends on fossil fuel consumption; however, the uncontrolled exploitation of fossil fuels has caused a series of energy crises and environmental problems, such as energy exhaustion, annual temperature rise, climate deterioration, and ocean acidification, which have already threatened the sustainable development of all living organisms. Therefore, finding renewable and reliable energy sources as well as reducing carbon dioxide (CO2) emissions have become the key focus in recent years. During the electrocatalytic CO2 reduction reaction (CO2RR) under relatively mild conditions, CO2 is converted into valuable products, such as C1, C2, and C2+ hydrocarbons, which is an effective strategy towards realizing "carbon neutrality". Electrocatalytic CO2RR is complex as it involves multiple electron/proton transfer processes. The reaction mechanism is also complex and involves many intermediates, which ultimately affects product selectivity. The large-scale application of the CO2RR requires the development of cheap and efficient electrocatalysts. Atomically dispersed metal and nitrogen co-doped carbon (M-N-C) materials, with large surface areas, 100% atomic availability, unsaturated coordination, and relatively uniform active sites, are promising catalysts for the CO2RR. M-N-C materials also have adjustable properties. For example, tuning the coordination environment of the central metal ions changes the electronic properties and atomic structures of the metal ions, which provides a new way for designing catalysts with high CO2RR performances. Therefore, it is of great significance to investigate the effect of regulating the electronic structure of M-N-C materials at the atomic level on catalytic activity and selectivity during the CO2RR. Additionally, the reduction potentials of the half reactions of most CO2RR products are within +/- 0.2 V of the hydrogen evolution reaction (HER), and most catalysts that bind CO2 are rich in electrons and active for the HER. Therefore, it is also necessary to design catalysts that can kinetically inhibit the competitive HER during the CO2RR. In this review, we discuss the synthesis methods of M-N-C materials, the reaction pathways of CO2 reduction to C1, C2, and C2+ hydrocarbons, and the main factors affecting the CO2RR. Specifically, three strategies for regulating the electronic structures and geometric configurations of M-N-C materials are systematically reviewed, namely, the modification of the carbon base surface of M-N-C materials, selection of appropriate central metal ions, and regulation of the coordination environment of the central metal ions. The effects of different active sites on the selectivity towards various products during the catalytic CO2RR are also discussed in detail. Finally, we highlight the current challenges and future development directions of M-N-C materials for the electrocatalytic CO2RR.