Role of LiCoO₂ surface terminations in oxygen reduction and evolution kinetics

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) activities of LiCoO₂ nanorods with sizes in the range from 9 to 40 nm were studied in alkaline solution. The sides of these nanorods were terminated with low-index surfaces such as (003), while the tips were terminated largely with high-index surfaces such as (104), as revealed by high-resolution transmission electron microscopy. Electron energy loss spectroscopy demonstrated that low-spin Co³⁺ prevailed on the sides, while the tips exhibited predominantly high- or intermediate-spin Co³⁺. We correlated the electronic and atomic structure to higher specific ORR and OER activities at the tips as compared to the sides, which was accompanied by more facile redox of Co^2+/3+ and higher charge transferred per unit area. These findings highlight the critical role of surface terminations and electronic structures of transition-metal oxides on the ORR and OER activity.