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

Abstract

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, NiGa 2 O 4 , 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 NiGa 2 O 4 nanoparticles. The NiGa 2 O 4 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 NiGa 2 O 4 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 NiGa 2 O 4 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 NiGa 2 O 4 /rGO field emitter delivers a low turn-on field of 4.42 V/μm@1 μA/cm 2 with a long field emission current stability of over 5 h in comparison to pristine NiGa 2 O 4 (turn-on field of 6.56 V/μm@1 μA/cm 2 ), 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 NiGa 2 O 4 compared to pristine NiGa 2 O 4 . The interaction between NiGa 2 O 4 and rGO involves charge transfer from NiGa 2 O 4 to the C 2p orbital. In addition, the reduction in work function for the hybrid structure justifies its improved field emission properties.

Electric-Field Emission Mechanism in Q-Carbon Field Emitters

“…The work function can be defined as the difference of energy of an electron between the vacuum level and the Fermi level. Mathematically, the work function (φ w ) can be expressed as , Here, φ 0 , e , and E F represent the electrostatic potential of the vacuum level, charge of an electron, and Fermi energy, respectively. We found that the work function of the Q-carbon electron field emitter is around 3.62 eV, which is in good agreement with the previous experimental outcomes .…”

Synthesis of different manganese tungstate nanostructures for enhanced charge-storage applications: theoretical support for experimental findings

“…We computed theoretically the quantum capacitance for the (À111) surface of MnWO 4 . Using the density of states, the quantum capacitance could be determined from the following relation: 35,64…”

“…where D(E) denotes the density of states, whereas F G indicates the electrode potential. The thermal broadening function can be written as: 35,64…”

“…We computed theoretically the quantum capacitance for the (−111) surface of MnWO 4 . Using the density of states, the quantum capacitance could be determined from the following relation: 35,64 where D ( E ) denotes the density of states, whereas Φ G indicates the electrode potential. The thermal broadening function can be written as: 35,64 F T = (4 kT ) −1 sech 2 ( E /2 kT )It is clear from Fig.…”

“…Using the density of states, the quantum capacitance could be determined from the following relation: 35,64 where D ( E ) denotes the density of states, whereas Φ G indicates the electrode potential. The thermal broadening function can be written as: 35,64 F T = (4 kT ) −1 sech 2 ( E /2 kT )It is clear from Fig. 14 that the quantum capacitance, which was noteworthy for the lower dimensional system, yielded the maximum value of 6700 μF cm −2 at the potential of −4 V. The experimentally obtained total capacitance can be correlated with the quantum capacitance from the following relation: 35,64 where C EDL indicates the capacitance due to the electrical double-layer.…”