Defect-Engineered Ultrathin δ-MnO2 Nanosheet Arrays as Bifunctional Electrodes for Efficient Overall Water Splitting

Recently, defect engineering has been used to intruduce half-metallicity into selected semiconductors, thereby significantly enhancing their electrical conductivity and catalytic/electrocatalytic performance. Taking inspiration from this, we developed a novel bifunctional electrode consisting of two monolayer thick manganese dioxide (delta-MnO2) nanosheet arrays on a nickel foam, using a novel in-situ method. The bifunctional electrode exposes numerous active sites for electrocatalytic rections and displays excellent electrical conductivity, resulting in strong performance for both HER and OER. Based on detailed structure analysis and density functional theory (DFT) calculations, the remarkably OER and HER activity of the bifunctional electrode can be attributed to the ultrathin d-MnO2 nanosheets containing abundant oxygen vacancies lead to the formation od Mn3+ active sites, which give rise to halfmetallicity properties and strong H2O adsorption. This synthetic strategy introduced here represents a new method for the development of non-precious metal Mn-based electrocatalysts for eddicient energy conversion.