Copper as a single metal atom based photo-, electro-, and photoelectrochemical catalyst decorated on carbon nitride surface for efficient CO2 reduction: A review

The processes of photocatalytic CO2 reduction (pCO2R) and electrochemical CO2 reduction (ECO2R) have attracted considerable interest owing to their high potential to address many environmental and energy-related issues. In this aspect, a single Cu atom decorated on a carbon nitride (CN) surface (Cu-CN) has gained increasing popularity because of its unique advantages, such as excellent atom utilization and ultrahigh catalytic activity. CN-particularly graphitic CN (g-C3N4)-is a photo-and electrocatalyst and used as an important support material for single Cu atom-based catalysts. These key functions of Cu-CN-based catalysts can improve the catalytic performance and stability in the pCO2R and ECO2R during the application process. In this review, we focus on Cu as a single metal atom decorated on CN for efficient photoelectrochemical CO2 reduction (pECO2R), where ECO2R increases the electrocatalytic active area and promotes electron transfer, while pCO2R enhances the surface redox reaction by efficiently using photogenerated charges and offering integral activity as well as an active interface between Cu and CN. Interactions of single Cu atom-based photo-, electro-, and photoelectrochemical catalysts with g-C3N4 are discussed. Moreover, for a deeper understanding of the history of the development of pCO2R and ECO2R, the basics of CO2 reduction, including pCO2R and ECO2R over g-C3N4, as well as the structural composition, characterization, unique design, and mechanism of a single atom site are reviewed in detail. Finally, some future prospects and key challenges are discussed.