Synergistic effects of nanoarchitecture and oxygen vacancy in nickel molybdate hollow sphere towards a high-performance hybrid supercapacitor

The facile design and fabrication of nanoarchitectured binary transition metal oxide electrode materials are essentially required for the advancement of high-performance supercapacitors (SCs). Herein, we prepared an oxygen-vacant NiMoO₄ (Ov-NiMoO₄) hollow sphere via a simple hydrothermal approach and subsequent heat treatment under an argon atmosphere. In particular, the oxygen vacancy is confirmed by using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), Raman, and differential reflectance spectroscopy (DRS) UV-Vis spectra studies. Furthermore, the generation of the oxygen vacancy could enhance the electrical conductivity and improve Faradaic redox sites. Significantly, the Ov-NiMoO₄ hollow sphere depicts a larger specific capacity (C_sp) of 496 mA h g⁻¹ at 1 A g⁻¹ than the bare-NiMoO₄ (b-NiMoO₄; 279 mA h g⁻¹) thermally treated under air. Furthermore, the hybrid SC (HSC) is fabricated based on the Ov-NiMoO₄//activated carbon, revealing a high specific capacitance (C_s) of 120 F g⁻¹ and providing a large energy density (ED) of 37.49 W h kg⁻¹ and power density (PD) of 36.61 kW kg⁻¹. Moreover, the HSC shows considerable cyclic stability of ~91.14% over 20 000 cycles. The results divulge that the poor crystallinity and the introduction of oxygen vacancies play a vital role in enhancing the charge-storage capability of the materials.