The processes of photocatalytic CO 2 reduction (pCO 2 R) and electrochemical CO 2 reduction (ECO 2 R) 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-C 3 N 4 )-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 pCO 2 R and ECO 2 R during the application process. In this review, we focus on Cu as a single metal atom decorated on CN for efficient photoelectrochemical CO 2 reduction (pECO 2 R), where ECO 2 R increases the electrocatalytic active area and promotes electron transfer, while pCO 2 R 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-C 3 N 4 are discussed. Moreover, for a deeper understanding of the history of the development of pCO 2 R and ECO 2 R, the basics of CO 2 reduction, including pCO 2 R and ECO 2 R over g-C 3 N 4 , 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.

Zero or negative emissions of carbon dioxide (CO2) is the need of the times, as inexorable rising and alarming levels of CO2 in the atmosphere lead to global warming and severe climate change. The electrochemical CO2 reduction (eCO2R) to value‐added fuels and chemicals by using renewable electricity provides a cleaner and more sustainable route with economic benefits, in which the key is to develop clean and economical electrocatalysts. Carbon‐based catalyst materials possess desirable properties such as high offset potential for H2 evolution and chemical stability at the negative applied potential. Although it is still challenging to achieve highly efficient carbon‐based catalysts, considerable efforts have been devoted to overcoming the low selectivity, activity, and stability. Here, we summarize and discuss the recent progress in carbon‐based metal‐free catalysts including carbon nanotubes, carbon nanofibers, carbon nanoribbons, graphene, carbon nitride, and diamonds with an emphasis on their activity, product selectivity, and stability. In addition, the key challenges and future potential approaches for efficient eCO2R to low carbon‐based fuels are highlighted. For a good understanding of the whole history of the development of eCO2R, the CO2 reduction reactions, principles, and techniques including the role of electrolytes, electrochemical cell design and evaluation, product selectivity, and structural composition are also discussed. The metal/metal oxides decorated with carbon‐based electrocatalysts are also summarized. We aim to provide insights for further development of carbon‐based metal‐free electrocatalysts for CO2 reduction from the perspective of both fundamental understanding and technological applications in the future.

Back cover image: The electrochemical CO2 reduction (eCO2R) to value‐added fuels and chemicals by using renewable electricity provides a cleaner and more sustainable route with economic benefits. In article number https://doi.org/10.1002/cey2.87, Jinli Qiao et al. review the recent progress of the activity, product selectivity and stability in carbon‐based metal‐free catalysts. The key challenges and future potential approaches for efficient eCO2R to low carbon‐based fuels are highlighted. The role of electrolytes, electrochemical cells design and evaluation, product selectivity and structural composition are also mentioned.

In view of global energy transition and environmental issues, electrochemical conversion of carbon dioxide (CO 2 ) to high value-added chemicals by using clean renewable electricity, as an advanced carbon capture, utilization and storage (CCUS) technology, demonstrates a promising approach to reach the carbon neutrality with additional economic benefits as well. Over the past decade, various new valid catalysts in electrochemical CO 2 reduction (ECO2R) have been designed and intensively investigated. Unfortunately, constructing appropriate ECO2R electrolyzer with high conversion rate and long-term stability to unleash the full potential benefits of electrocatalysts remains a recognized challenge, especially as it has not yet attracted attention. This review summarizes the progress of ECO2R reactor and their corresponding structure characteristics/ electrochemical performance. Besides, the current challenges and bottlenecks of CO2RR reactor are discussed. We aim to introduce the advances in ECO2R electrolyzer in detail to offer enlightenment for large-scale industrial application of ECO2R.

No abstract