Abstract: Renewable-electricity-powered electrochemical CO₂ reduction reactions (CO₂RR) to highly value-added multi-carbon (C₂₊) fuels or chemicals have been widely recognized as a promising approach for achieving carbon recycling and thus bringing about sustainable environmental and economic benefits. Cu-based catalysts have been demonstrated as the only candidate metal CO₂RR electrocatalysts that catalyze the C-C coupling. Unfortunately, huge challenges still exist in the highly selective CO₂RR to C₂₊ products due to the higher activation barrier of C-C coupling and complex multi-electron reaction. Key fundamental issues regarding both active species and product formation pathways have not been elucidated by now, but recent developments of advanced strategies and characterization tools allow one to comprehensively understand the Cu-based CO₂RR mechanism. Herein, we review recent advance and perspective of Cu-based CO₂RR catalysts, especially in terms of active phases and product formation pathways. Then, strategies in catalysts design for CO₂RR toward C₂₊ products are also presented. Importantly, we systematically summarized the advanced tools for investigating the CO₂RR mechanism, including in situ/operando spectroscopy techniques, isotope labeling, and theoretical calculations, aiming at unifying the knowledge of active species and product formation pathways. Finally, future challenges and constructive perspectives are discussed, facilitating the accelerated advancement of CO₂RR mechanism research.

Key words: CO₂RR, Raman spectrum, Active phases, Formation pathways, Reaction mechanisms, Copper, DFT calculations