Single atom-based catalysts for electrochemical CO2 reduction

Electrochemical CO2 reduction reaction (CO2RR), powered by renewable energy, emerges as a promising approach against environmental issues and energy crisis by converting CO2 into value-added chemicals. Single atom catalysts (SACs) with isolated metal atoms dispersed on supports exhibit outstanding performance for CO2 electroreduction, because of their strong single atom-support interactions, maximum metal utilization and excellent catalytic activity. However, SACs suffer from agglomeration of particles, low metal loading, and difficulty in large-scale production. In addition, molecular catalysts as another single atom-based catalyst, consisting of ligands molecules connected to metal ions, exhibited similar metal-nitrogen (M-N) active centers as that in metal-nitrogen-carbon (M-N-C) SACs, which were highly active to CO2 reduction due to their well-defined active sites and tunability over the steric and electronic properties of the active sites. Nonetheless, molecular catalysts are challenged by generally moderate activity, selectivity and stability, poor conductivity and aggregation. Many works have been devoted to overcoming these issues of SACs and molecular catalysts for efficient CO2RR, but only limited reviews for systematic summary of their fabrication, application, and characterizations, which were highlighted in this review. Firstly, we summarize recent advanced strategies in preparing SACs for CO2RR, including wet-chemistry approaches (defect engineering, spatial confinement, and coordination design), other synthetic methods and large-scale production of SACs. Besides, electrochemical applications of SACs and molecular catalysts on CO2RR are discussed, which involved the faradaic efficiency and partial current density of the desired product as well as the catalyst stability. In addition, ex-situ and in-situ/operando characterization techniques are briefly assessed, benefiting probing the active sites and understanding the CO2RR catalytic mechanisms. Finally, future directions for the development of single atom-based catalysts (SACs, molecular catalysts) are pointed out.