关键词: M-N-C材料, 单位点催化剂, 电子结构, 配位环境, 吉布斯自由能

Abstract: 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.

Key words: M-N-C material, Single-atom catalyst, Electronic structure, Coordination environment, Gibbs free energy