Revealing the relationship between Cu valence and product selectivity in CO₂ electrolysis

Realizing the evolution of Cu valence state in the electrocatalytic CO₂ reduction reaction (eCO₂RR) is crucial to maintain active sites, optimize reaction conditions and product selectivity and understand reaction mechanism and kinetics. In this context, we investigated the dynamic transition of Cu(II), Cu(I) and Cu(0) under the eCO₂RR conditions and impact of such transition on reaction intermediate species and product selectivity. In situ surface-enhanced Raman spectroscopy and attenuated total reflectance-surface-enhanced infrared absorption spectroscopy results coupled with quantitative data collected from a flow cell show that Cu (I) species contributes to CO₂ activation, CO adsorption, and subsequent C-C coupling processes, while the initial adsorption of Cu (0) in CO₂ playing a crucial role. These findings clarify how the valance of Cu changes with applied cathodic potential and reaction time, thus offering guidelines on optimization of operating conditions during CO₂ electrolysis to maintain the active sites responsible for forming desirable products. These new understandings lay a solid ground for purposely designing Cu-based catalysts for improved performance in CO₂ electrolysis, significantly advancing our knowledge on the behavior of Cu-based electrocatalysts in CO₂ electrolysis.