Cu-Based Tandem Catalysts for Electrochemical CO<sub>2</sub> Reduction

Through the combustion of fossil fuels and other human activities, large amounts of CO2 gas have been emitted into the atmosphere, causing many environmental problems, such as the greenhouse effect and global warming.Thus, developing and utilizing renewable clean energy is crucial to reduce CO2 emission and achieve carbon neutrality.The electrochemical CO2 reduction reaction (CO2RR) has been considered as an effective approach to obtain high value-added chemicals and fuels, which can store intermittent renewable energy and achieve the artificial carbon cycle.In addition, due to its multiple advantages, such as mild reaction conditions, tunable products, and simple implementation, electrochemical CO2RR has attracted extensive attention.Electrochemical CO2RR involves multiple electron-proton transfer steps to obtain multitudinous products, such as C1 products (CO, HCOOH, CH4, etc.) and C2 products (C2H4, C2H5OH, etc.).The intermediates, among which * CO is usually identified as the key intermediate, and reaction pathways of different products intersect, resulting in an extremely complex reaction mechanism.Currently, copper has been widely proven to be the only metal catalyst that can efficiently reduce CO2 to hydrocarbons and oxygenates due to its suitable adsorption energy for * CO.However, the low product selectivity, poor stability, and high overpotential of pure Cu hinder its use for the production of industrial-grade multi-carbon products.Tandem catalysts with multiple types of active sites can sequentially reduce CO2 molecules into desired products.When loaded onto a co-catalyst that can efficiently convert CO2 to * CO (such as Au and Ag), Cu acts as an electron donor owing to its high electrochemical potential.* CO species generated from the substrate can spillover onto the surface of electron-poor Cu due to the stronger adsorption and be further reduced to C2+ products.The use of Cu-based tandem catalysts for electrochemical CO2RR is a promising strategy for improving the performance of CO2RR and thus, has become a research hotspot in recent years.In this review, we first introduce the reaction routes and tandem mechanisms of electrochemical CO2RR.Then, we systematically summarize the recent research progress of Cu-based tandem catalysts for electrochemical CO2RR, including Cu-based metallic materials (alloys, heterojunction, and core-shell structures) as well as Cu-based framework materials, carbon materials, and polymer-modified materials.Importantly, the preparation methods of various Cu-based tandem catalysts and their structure-activity relationship in CO2RR are discussed and analyzed in detail.Finally, the challenges and opportunities of the rational design and controllable synthesis of advanced tandem catalysts for electrochemical CO2RR are proposed.