Supercapacitors have gained a lot of attention due to their unique features like high power, long cycle life and environment-friendly nature. They act as a link for energy-power difference between a traditional capacitor (having high power) and fuel cells/batteries (having high energy storage). In this perspective, a worldwide research has been reported to address this and rapid progress has been achieved in the advancement of fundamental as well as the applied aspects of supercapacitors. Here, a concise description of technologies and working principles of different materials utilized for supercapacitors has been provided. The main focus has been on materials like carbon-based nanomaterials, metal oxides, conducting polymers and their nanocomposites along with some novel materials like metal-organic frameworks, MXenes, metal nitrides, covalent organic frameworks and black phosphorus. The performance of nanocomposites has been analysed by parameters like energy, capacitance, power, cyclic performance and rate capability. Some of the latest supercapacitors such as electrochromic supercapacitor, battery-supercapacitor hybrid device, electrochemical flow capacitor, alternating current line filtering capacitor, micro-supercapacitor, photo-supercapacitor, thermally chargeable supercapacitor, self-healing supercapacitor, piezoelectric and shape memory supercapacitor have also been discussed. This review covers the up-to-date progress achieved in novel materials for supercapacitor electrodes. The latest fabricated symmetric/asymmetric supercapacitors have also been reported.

Hydrothermal synthesis technique is used to synthesize nanosized NiCo2O4 particles. The as-synthesized nanosized particles are structurally characterized by Raman spectroscopy. The nanostructure and morphology of the prepared nanoparticles are studied by Field Effect Scanning Electron Microscopy and Raman spectroscopy. DC electrical conductivity has been measured in the temperature range 303–402 K and the activation energy has been calculated. The absorbance of the nanoparticles in the UV–Visible range is recorded. The absorption spectrum analysis shows that the material possesses two direct band gaps, one corresponding to 1.74 eV and the other corresponding to 2.2 eV. The changes in AC conductivity and dielectric constant with a variation of frequency are studied. The AC conductivity exhibits Jonscher’s power law. From the dielectric studies, it is seen that with increasing frequency, the dielectric constant reduces. Also, the dielectric constant value of the nanoparticles is very high which indicates its supercapacitive behaviour. The small dielectric loss (0.32 at 3.5 kHz) shows a very small delay (relaxation time ∼4.54 × 10−5 s) in molecular polarization along with the varying electric field which reveals that the energy losses are less. The AC impedance spectroscopy also exhibits that the synthesized material has the potential as a good supercapacitive material.