Role of Surface Hydroxyls in Atomic-Scale Copper Restructuring during CO Electroreduction

The nanoscale structure of electrocatalyst surfaces governs the selectivity and kinetics of reactions including CO₍₂₎ electroreduction (CO₍₂₎R). Yet, their evolution under reaction conditions remains elusive, and the roles of surface hydroxyls (OH_ad) and the interfacial microenvironment in surface restructuring are poorly understood. Combining electrochemical atomic force microscopy, Raman spectroscopy, and grand canonical modeling, we reveal that OH_ad acts synergistically with CO_ad to restructure copper (Cu) electrocatalysts during COR. Mixed OH_ad/CO_ad coverage promotes lifting of surface atoms into metastable states, generating Cu adatoms and nanoclusters at mild cathodic potentials, which aggregate or dissolve at more negative potentials. This restructuring into low-coordinated Cu sites is accompanied by disordering of the interfacial water network. Nanocluster stability depends critically on CO partial pressure, while hydroxyls remain kinetically trapped on the roughened Cu surface. These findings underscore the importance of surface kinetics and interfacial microenvironments in atomic-scale surface restructuring, urging a reassessment of catalytic surface states under realistic conditions.