First principles study of cobalt (Hydr)oxides under electrochemical conditions

Density functional theory (DFT) calculations with on-site Coulomb repulsion are carried out to study the relative stabilities of crystalline cobalt oxides and hydroxides - CoO, Co(OH)₂, Co₃O₄, CoO(OH), and CoO₂ - in electrochemical environment. Co(OH)₂ is the thermodynamic ground state under reducing conditions, i.e., at voltages V < 0 relative to the standard hydrogen electrode (SHE) potential in acidic solution, whereas CoO(OH) and CoO₂ are stable under oxidizing conditions, i.e., at external voltage larger than 1.23 eV vs SHE in basic solution. These results, combined with surface structure studies of the (0001) natural cleavage surface of CoO(OH), show that a CoO₂^x- (x = 0-0.5) layer is present when the surface is exposed to solution under oxidizing conditions, in agreement with recent experimental findings. Study of the energetics of water oxidation at regular surface sites of CoO(OH)(0001) indicates however that water deprotonation to form a surface OH species is energetically very costly. Different active sites, e.g. steps, are thus responsible for the observed high activity of crystalline cobalt oxide for electrochemical oxygen evolution.