Co(II)-Based Molecular Complexes for Photochemical CO2 Reduction

Abstract

Nowadays, more than 85% of the energy is generated by fossil fuels. The excessive utilization of finite fossil fuels has resulted in the crises of energy shortage and global warming caused by greenhouse gas emissions. Researchers have conceived several means for trying to solve these problems, among which the sunlight-driven CO2 reduction is viewed as a sustainable process that utilizes CO2 as the raw material to produce chemical fuels, including CO, formate, and CH4; this method not only realizes the conversion and storage of intermittent solar energy, but also decreases the CO2 concentration in the atmosphere and alleviates global warming. However, photochemical CO2 reduction usually undergoes a sluggish process due to the inertness of CO2. Moreover, the selectivity of the CO2 reduction reaction is also challenged by the hydrogen evolution reaction, which exhibits faster reaction kinetics. In this context, the rational design and synthesis of efficient and selective catalysts for photochemical CO2 reduction are major challenges. Recently, non-noble metal Co(II) complexes as molecular catalysts have shown excellent catalytic performances in photocatalytic CO2 reduction. During the past several decades, significant progress has been achieved in improving the applicability of Co(II) complexes for photocatalytic CO2 reduction. In this review, we systematically report the latest research progress on the use of Co(II) complexes in photocatalytic CO2 reduction. To describe the progress of this research, we characterized the Co(II)-based molecular catalysts into four categories according to ligand types, namely: (1) macrocyclic ligands, (2) polypyridine ligands, (3) porphyrin and porphyrin-like ligands, and (4) nonplanar N4 ligands. The progress of the research on the heterogeneity of Co(II) molecular complexes used for photochemical CO2 reduction was also introduced and discussed. Furthermore, the effects of catalyst structures on catalytic efficiency, selectivity, and stability were particularly summarized and discussed, with the aim of revealing and building the relationship between catalyst structures and catalytic performances to guide the future design and synthesis of Co(II) molecular complexes with excellent catalytic performance. Finally, the current challenges and problems in photocatalytic CO2 reduction were summarized, and several suggestions for designing efficient Co(II)-based molecular catalysts for photocatalytic CO2 reduction were put forward. The trends for the future development of Co(II) complex molecular catalysts were also investigated with regard to photocatalytic CO2 reduction.