Creating coordinatively unsaturated metal sites in metal-organic-frameworks as efficient electrocatalysts for the oxygen evolution reaction: Insights into the active centers

Metal-organic-frameworks (MOFs), in which metal ions are single-atomically dispersed, are regarded as one of the most promising single-atom doped catalysts. Co-based species have been considered as a potential candidate to replace the precious RuO2 to electrocatalyze oxygen evolution reaction (OER). Zeolitic imidazolate frameworks-67 (ZIF-67), a Co-containing MOF, may be an excellent precursor for single-atom OER electrocatalysts due to its rich and uniform distribution of cobalt species. In principle, Co ions are fully coordinated (except for those on the surfaces) in ZIF-67 without accessible sites for electrocatalysis. One way to utilize this single-atom material as electrocatalysts is to remove some of the ligands attached to Co atoms to create coordinately unsaturated metal sites (CUMSs) as the catalytic centers for OER. Herein, we, for the first time, have created CUMSs in ZIF-67 through dielectric barrier discharge (DBD) plasma etching. The CUMSs act as excellent catalytic centers for OER with a promising electrocatalytic activity, even comparable to the precious RuO2. Interestingly, the OER activity of the CUMSs is reversible by supplementing the missing ligands. Our density-functional theory calculations also demonstrated the contribution of the unsaturated metal sites to the high catalytic activity for OER.