This study investigates the structural evolution of Cu nanocubes under CO₂ reduction reaction (CO₂RR) conditions and identifies a previously unexplored yet critical pathway for Cu catalyst reconstruction: alkali cation-induced cathodic corrosion. The research combines identical location transmission electron microscopy (IL-TEM), cyclic voltammetry (CV), in situ X-ray absorption fine structure (XAFS) spectroscopy, and ab initio molecular dynamics (AIMD) simulations. The results show that Cu catalysts undergo surface reconstructions when the electrode potential is more negative than an onset value (e.g., -0.4 V_RHE with 0.1 M KHCO₃). This process is critical for the formation of smaller Cu particles and dynamic catalyst morphologies. However, it may limit long-term selectivity and activity enhancement by controlling the morphology of Cu pre-catalysts. Operating Cu catalysts at less negative potentials can prevent this corrosion, allowing Cu nanocubes to maintain a stable selectivity advantage over spherical Cu nanoparticles. The study also highlights the importance of alkali cations in the electrolyte for this corrosion process. The findings suggest that cathodic corrosion is a key factor in Cu catalyst behavior during CO₂RR, and that controlling this process could improve the stability and performance of Cu catalysts in electrochemical reactions. The research provides insights into the mechanisms of Cu catalyst reconstruction and the role of alkali cations in this process, which could inform the design of more efficient and stable Cu-based electrocatalysts for CO₂RR and other reactions.This study investigates the structural evolution of Cu nanocubes under CO₂ reduction reaction (CO₂RR) conditions and identifies a previously unexplored yet critical pathway for Cu catalyst reconstruction: alkali cation-induced cathodic corrosion. The research combines identical location transmission electron microscopy (IL-TEM), cyclic voltammetry (CV), in situ X-ray absorption fine structure (XAFS) spectroscopy, and ab initio molecular dynamics (AIMD) simulations. The results show that Cu catalysts undergo surface reconstructions when the electrode potential is more negative than an onset value (e.g., -0.4 V_RHE with 0.1 M KHCO₃). This process is critical for the formation of smaller Cu particles and dynamic catalyst morphologies. However, it may limit long-term selectivity and activity enhancement by controlling the morphology of Cu pre-catalysts. Operating Cu catalysts at less negative potentials can prevent this corrosion, allowing Cu nanocubes to maintain a stable selectivity advantage over spherical Cu nanoparticles. The study also highlights the importance of alkali cations in the electrolyte for this corrosion process. The findings suggest that cathodic corrosion is a key factor in Cu catalyst behavior during CO₂RR, and that controlling this process could improve the stability and performance of Cu catalysts in electrochemical reactions. The research provides insights into the mechanisms of Cu catalyst reconstruction and the role of alkali cations in this process, which could inform the design of more efficient and stable Cu-based electrocatalysts for CO₂RR and other reactions.