Unexpected Presence of Alkali Metal Cation Impurities, Stabilizing Cu+ in Oxide-Derived Cu for CO2 Electrolysis

Electrocatalysts often undergo dynamic phase transitions during electrochemical operation, which introduce structural and chemical complexities that obscure the fundamental origins of their catalytic performance. Oxide-derived Cu (OD-Cu), produced by the electrochemical reduction of Cu oxide, exemplifies this challenge and continues to prompt debate regarding the nature of its high activity in CO ₂ electrolysis (CO ₂ RR). Herein, using cryogenic atom probe tomography, we show the formation of unexpected nanoscale Na ⁺ -containing microstructures within OD-Cu, originating from phase transitions of Cu oxide in a NaHCO ₃ electrolyte. Comparative studies with Na ⁺ -free OD-Cu and pulsed electrolysis identify a strong correlation between Na ⁺ impurity incorporation and enhanced CO ₂ RR activity. Complementary in situ Raman spectroscopy studies further confirm that these foreign impurities stabilize labile yet catalytically active Cu ⁺ species. Overall, our findings elucidate the pivotal role of Na ⁺ impurities and provide mechanistic insights to guide the rational design of synthetic and operational strategies for more efficient CO ₂ valorization.