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.