Surface phases of Cu₂O(111) under CO₂ electrochemical reduction conditions

Density functional theory (DFT) calculations were performed to examine the relative stability of Cu- and O-terminated Cu₂O(111) surfaces as well as possible surface phases of Cu₂O(111) under applied potentials relevant for carbon dioxide (CO₂) electrochemical reduction. The Cu-terminated surface is found to be more favored than the O-terminated surface at potentials less than - 0.52 V-SHE at a pH = 7. Adsorption stabilities of H*, OH*, O*, and H₂O* were examined by calculating the formation free energy of the adsorbate as a function of electrode potential. A H* covered surface is the most favored surface under reduction conditions. At pH values of 7 and 13, formation of an O-vacancy requires potentials of 0.61 and 0.25 V-SHE, respectively. At more negative potentials relevant to reduction of CO₂, formation of an O-vacancy is thermodynamically favored, indicating a facile reduction of the Cu₂O surface as observed experimentally.