Diffusion-Mediated Morphological Transformation in Bifunctional MnO/CuO-(VO)(PO)·6HO for Enhanced Electrochemical Water Splitting.

A strategical approach for morphological transformation and heterojunction formation was utilized to suppress the shortcomings of uni-metal oxide electrocatalysts and enhance their bifunctionality. In situ generation of copper oxide (CuO) over the surface of manganese oxide (MnO) resulted in a morphological transformation from solid spheres to hollow spherical structures due to the ion-exchange diffusion (Kirkendall effect) of Cu ions into MnO particles. This hollowness resulted in the advancement of the bifunctional electrocatalytic behavior of MnO/CuO (overpotential (η) of 280 mV for an OER and 310 mV for an HER at a current density of 10 mA/cm) by virtue of increased exposed surface active sites aiding the adsorption of water molecules on the surface. The increased electrochemical active surface area (ECSA/ = 34 mF/cm) and reduced charge transfer resistance resulted in the formation of MnO/CuO hollow spheres to achieve an approximately threefold enhancement in the turnover frequency (TOF) compared to the bare MnO. The electrocatalytic efficiency of MnO/CuO was further enhanced by virtue of the faster charge transfer coefficient of two-dimensional (2D) vanadyl phosphate hexahydrate (VOP) sheets deposited over its surface. This boosted the overall water splitting with attained overpotential (η) values of 190 and 220 mV with Tafel slopes of 60 and 105 mV/decade for an OER and HER, respectively. The morphological transformation and formation of an n-p heterojunction between MnO and CuO based on their work function (φ) values evaluated from the density functional theory (DFT) calculation and the effect of the VOP overlayer for faster reaction kinetics at the electrolyte interface resulted in an ∼10-fold increment in TOF values compared to the bare counterpart.