Unexpected Presence of Alkali Metal Cation Impurities, Stabilizing Cu + in Oxide-Derived Cu for CO 2 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 CO2 electrolysis (CO2RR). 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 NaHCO3 electrolyte. Comparative studies with Na + -free OD-Cu and pulsed electrolysis identify a strong correlation between Na + impurity incorporation and enhanced CO2RR 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 CO2 valorization.