Design of a ZnMoO4 porous nanosheet with oxygen vacancies as a better performance electrode material for supercapacitors

Abstract

A ZnMoO₄ porous nanosheet with oxygen vacancies (ZnMoO₄-OV) was synthesized by hydrothermal synthesis and the hydrogenation reduction method. The ZnMoO₄-OV porous nanosheet delivers a higher specific surface area together with a more diverse pore size distribution compared to the ZnMoO₄ nanosheet. The density functional theory calculation results exhibit that ZnMoO₄-OV has lower energy band gap (3.019 eV) than ZnMoO₄ (1.92 eV). The electronic density of states plots reveal that ZnMoO₄-OV possesses higher electron state distribution at the Fermi energy level than ZnMoO₄. ZnMoO₄-OV porous sheet achieves higher specific capacitance (1673 g⁻¹) than ZnMoO₄ (797 F g⁻¹) at 2 mA cm⁻². It also achieves superior capacitance retention rate (82.9%) to ZnMoO₄ (48.6%) with the current density increasing from 2 to 20 mA cm⁻². The introduction of oxygen vacancies can increase the carrier density, accelerate the electron transfer, promote the electrical conductivity, and accordingly strengthen the redox reactivity of ZnMoO₄-OV. An asymmetric supercapacitor is also constructed by using ZnMoO₄-OV as the positive electrode and activated carbon as the negative electrode. It achieves a high energy density of 60.1 W h kg⁻¹ at a power density of 800 W kg⁻¹, together with a good cycle life. Both experimental measurements and theorical calculations are applied to prove the promotive role of oxygen vacancies to achieve the supercapacitive performance of ZnMoO₄-OV.