Diffusion-Mediated Morphological Transformation in Bifunctional Mn2O3/CuO-(VO)3(PO4)2middot6H2O for Enhanced Electrochemical Water Splitting

A strategical approach for morphological trans-formation 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 (Mn2O3) resulted in a morphological transformation from solid spheres to hollow spherical structures due to the ion-exchange diffusion (Kirkendall effect) of Cu ions into Mn2O3 particles. This hollowness resulted in the advancement of the bifunctional electrocatalytic behavior of Mn2O3/CuO (overpotential (ri10) of 280 mV for an OER and 310 mV for an HER at a current density of 10 mA/cm2) by virtue of increased exposed surface active sites aiding the adsorption of water molecules on the surface. The increased electrochemical active surface area (ECSA/Cdl = 34 mF/cm2) and reduced charge transfer resistance resulted in the formation of Mn2O3/CuO hollow spheres to achieve an approximately threefold enhancement in the turnover frequency (TOF) compared to the bare Mn2O3. The electrocatalytic efficiency of Mn2O3/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 (ri10) 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 Mn2O3 and CuO based on their work function (phi) 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 similar to 10-fold increment in TOF values compared to the bare counterpart.

Automated summary

A strategical approach involving morphological transformation and heterojunction formation is used to address the limitations of uni-metal oxide electrocatalysts and enhance their bifunctionality. The in situ generation of CuO over Mn2O3 leads to a morphological transformation from solid spheres to hollow spherical structures, which improves the electrocatalytic performance by increasing exposed surface active sites. Deposition of 2D VOP sheets further enhances the electrocatalytic efficiency of Mn2O3/CuO. The formation of an n-p heterojunction between Mn2O3 and CuO, along with the effect of the VOP overlayer, significantly increases the turnover frequency compared to the bare counterpart.