Enhancement of Pseudocapacitive Behavior, Cyclic Performance, and Field Emission Characteristics of Reduced Graphene Oxide Reinforced NiGa2O4 Nanostructured Electrode: A First Principles Calculation to Correlate with Experimental Observation

The rational construction and design of supercapacitors (SCs) with electrochemically favorable structural configuration and appreciable cyclic performances are highly demanding. Herein, a novel Ga-based transition metal oxide, NiGa2O4, was synthesized via sol–gel self-ignition techniques to design an advanced electrode for supercapacitor and field emitter applications. Reitveld refinement of XRD, FTIR, and Raman spectra reveals a larger cubic parameter due to the incorporation of Ga over NiO, which promotes faster charge kinetics and various metal oxide (Ni–O and Ga–O) stretching vibrations in the molecular fingerprint. Reduced graphene oxide (r-GO) with an enlarged surface area has been utilized as a conductive substrate to prevent the aggregation of NiGa2O4 nanoparticles. The NiGa2O4 electrodes exhibit a specific capacitance of 415 F g–1 at a current density of 1.5 A g–1 with capacitance retention of 74.2% at a current density of 20 A g–1, in addition to outstanding cyclic performance over 3000 cycles (capacitance retention = 96.2%). R-GO-reinforced NiGa2O4 electrodes (10:1) possess a higher specific capacitance of 643 F g–1 at a current density of 1.5 A g–1 in comparison to pristine NiGa2O4 with 77.66% retentivity at a current density of 20 A g–1, together with upgraded cyclic stability of 99.1% retentivity over 3000 cycles. The NiGa2O4/rGO field emitter delivers a low turn-on field of 4.42 V/μm@1 μA/cm2 with a long field emission current stability of over 5 h in comparison to pristine NiGa2O4 (turn-on field of 6.56 V/μm@1 μA/cm2), which recommends it as a potential field emitter (FE) for vacuum micro- and nanoelectronics. Various FE parameters with field enhancement factor (β) are compared with those of other efficient field emitters. Experimental data were supported through theoretical insight from density functional theory (DFT) simulations. Enhanced quantum capacitance and increased density of states near the Fermi level contribute toward superior charge storage performance in r-GO-reinforced NiGa2O4 compared to pristine NiGa2O4. The interaction between NiGa2O4 and rGO involves charge transfer from NiGa2O4 to the C 2p orbital. In addition, the reduction in work function for the hybrid structure justifies its improved field emission properties.