Experimental and theoretical realization of an advanced bifunctional 2D δ-MnO2 electrode for supercapacitor and oxygen evolution reaction via defect engineering

Abstract

Modulating the intrinsic physicochemical properties of crystalline 2D materials by dint of defect engineering largely enables multi-functionality. Uniform thin layered nanosheets further self-assembled at micro scale forming embossing structures of δ-MnO2 were fabricated by microwave irradiation technique. The irradiation of UV/O3 impacts incorporation of oxygen vacancy into the pristine system. Furthermore, detailed structural, morphological, surface analytical and electrochemical investigations evidenced outstanding energy storage and conversion activities. The asymmetric device δ-MnO2-UVT//F-MWCNT with an extended potential window 1.5 V, exhibited maximum energy density of 39 Wh/kg at a power density of 468.75 W/kg. The defect structural design exhibited excellent electrocatalytic OER activity with lowest overpotential (η20, 300 mV) and Tafel slope (71 mV/dec). The efficiency and stability of the illuminated material showed outstanding performances. To support our experimental findings, we have presented the electronic structures and quantum capacitance for pristine δ-MnO2 and δ-MnO2 with O vacancy employing Density Functional Theory (DFT) simulations. Presence of O vacancy makes a semi-conducting to metallic transition. The oxygen vacancies delocalize the neighboring electrons around the low coordinated Mn atoms and these delocalized electrons can be easily moved into the conduction band resulting improved conductivity in the material. In addition, the computed quantum capacitance tendency is as follows, δ-MnO2-UVT > δ-MnO2 which associates with the experimental supercapacitance behaviour of these systems.