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The research of energy-storage systems has been encouraged in the last ten years by the rapid development of portable electronic gadgets. Hybrid-ion capacitors are a novel kind of capacitor-battery hybrid energy storage device that has earned a lot of interest because of their high power density while maintaining energy density and a long lifecycle. Mostly, lithium-based energy storage technology is now being studied for use in electric grid storage. But the price increment and intermittent availability of lithium reserves make lithium-based commercialization unstable. Therefore, sodium-based technologies have been proposed as potential substitutes for lithium-based technologies. Sodium-ion capacitors (SICs) are acknowledged as potential innovative energy storage technologies which have lower standard electrode potentials and lower costs than lithium-ion capacitors. However, the large radius of the sodium ion also contributes to unfavorable reaction kinetics, low energy density, and brief lifespan of SICs. Recently, transition metal oxide (TMO)-based candidates have been considered potential due to the large theoretical capacity, environmental friendliness, and low cost for SICs. This brief study summarizes current advancements in research of TMOs and sodium-based TMOs as electrode candidates for SIC applications. Also, we have covered in detail the state of the exploration and upcoming prospects of TMOs for SICs.

Here, nickel cobalt oxide micro-flowers (NCO-F) have been prepared by the hydrothermal method for their application as electrode material in supercapacitors. The electrochemical performance of the prepared material has been studied in three different aqueous electrolytes (1 M Na2SO4, 1 M KOH, and 1 M Na2SO4 with hydroquinone (Hq) and benzoquinone (Bq) as redox additives). The NCO-F shows maximum specific capacitance in 1 M KOH (470 Fg−1) followed by 1 M Na2SO4 with Hq and Bq additives (173 Fg−1) and 1 M Na2SO4 (101 Fg−1) at the scan rate of 5 mVs−1. However, the higher value of energy density and power density is obtained in 1 M Na2SO4 (35.80 Whkg−1 at 424 Wkg−1). This is because of the higher potential window possessed by NCO-F in 1 M Na2SO4. The results illustrate that the role of the potential window is important which in turn is governed by the type of electrolyte.

In the present work, thin films of nickel cobalt oxide and polyvinyl alcohol nanocomposite (NCO-PVA) are prepared by chemical methods. The as-prepared NCO-PVA films have been structurally studied by X-ray diffraction and Fourier transform infrared spectroscopy. The morphology of the NCO-PVA films has been studied by Field emission scanning electron microscopy and High-resolution transmission electron microscopy. X-ray diffraction analysis has been used to study its crystallinity and phase. The optical properties of NCO-PVA nanocomposite films have also been studied. The surface area and pore size measurements have also been done. The UV-Vis spectrum analysis reveals that the films possess two direct band gaps (2.91 eV and 3.48 eV). The red shift of Photoluminescence emission has been observed for nanocomposite films with increasing excitation intensity. The AC electrical conductivity of NCO-PVA films has been studied as the high value of AC conductivity is essential for getting high power and high energy density in supercapacitors. Also, the high value of the dielectric constant of nanocomposite at small frequencies shows its supercapacitive nature. A short relaxation time (~6.37 ≤ 10−7 s) reveals a very small delay in molecular polarization along with changing the electric field, suggesting fewer energy losses. Electrochemical performance is checked by cyclic voltammetry, galvanostatic charge-discharge, and impedance spectroscopy measurements using 1 M Na2SO4neutral aqueous electrolytic solution. Also, the symmetric supercapacitor has been fabricated, using NCO-PVA films as the electrodes, which exhibits the specific capacitance value of50 F g−1and energy density of 18.9 W h kg−1. The results reveal that the prepared nanocomposite films have enough potential for supercapacitor device fabrication, especially the flexible supercapacitors.

We have prepared nanocomposite films of reduced graphene oxide (rGO) and nickel cobalt oxide quantum dots (NCO-QDs) by chemical method, and these are represented by NCO-rGO. Different nanocomposite films have been formed by varying the ratio of NCO-QDs and rGO. The obtained value of the specific capacitance of the nanocomposite (NCO5-rGO1.5) is 265 F g−1 (260 C g−1) at 0.73 A g−1 which is much higher than rGO alone (96 F g−1 at 0.33 A g−1). As the asymmetric/hybrid configuration also enhances the potential window, so in this work, we have fabricated an asymmetric supercapacitor with NCO5-rGO1.5 as the positive electrode and rGO as a negative electrode using 1 M Na2SO4, neutral aqueous solution, as the electrolyte. The assembled supercapacitor delivered the specific capacitance value of 131 F g−1 (197 C g−1) at an energy density of 47 W h kg−1 and a power density of 631 W kg−1 within a wide potential window of 1.6 V. The potential window of the device can be extended up to 2.4 V, but in that case, its coulombic efficiency decreases. The device retains a specific capacitance of 69% after 1000 cycles of charge-discharge. Also, a solid-state device has been fabricated, which displays a potential of 3.4 V after charging 20 s. All these results may prove fruitful for the design of high energy density supercapacitors in the future.

The synthesis of reduced graphene oxide (rGO) involves the reduction of graphene oxide (GO). GO is usually synthesized by Hummer’s method that includes extensive use of concentrated acid and specific reaction conditions. To omit this, we developed a greener and safer alternative for rGO synthesis. Electrochemical etching of carbon rods was done in the lemon juice solution. We prepared four different samples to see the effect of different operating voltages on the quality of rGO. Raman analysis and elemental analysis were carried out to investigate the variation in the synthesized rGO samples because of different applied voltages. X-ray diffraction spectra were analyzed by the Debye-Scherrer equation to perceive crystallite size variations and d-spacing. Electron microscopy displayed rGO sheets with folded edges. UV–Visible absorption spectra confirmed the rGO formation and upsurge in the number of layers with an increase in voltage. Comparative studies confirmed the best quality rGO produced at 10 ​V bias voltage. This process may replace other rGO production techniques for its chemical-free and hassle less synthesis approach with the possibility of further improvements.

Quantum Dot Sensitized Solar Cells are a likely replacement for Silicon-based solar cells. Counter electrodes are a fundamental aspect of QDSSC’s performance. NiS being a less expensive material is a decent choice for the purpose. In this paper, we have discussed the synthesis of NiS by Successive Ionic Layer Adsorption Reaction. Optical, Crystallographic and Electrical studies have been presented. Electrical studies of the device with NiS counter electrode is compared with characteristics of the device with CNTs as the counter electrode. SILAR method is easy and less time to consume than chemical bath deposition or any other method. Results show the success of NiS synthesized by SILAR method as the counter electrode.

Binary metal oxides have great potential as electrode materials for supercapacitors. Three-dimensional flower-like nickel-cobalt oxide (NiCo2O4) nanostructures have been obtained by a simple and cost-effective hydrothermal synthesis followed by annealing at distinct temperatures. The effect of calcination temperature, on the structure and the electrochemical behaviour of the nanostructures, has also been investigated. Highly porous, hollow and continuous structures of NiCo2O4 quantum dots (NCO-QDs), formed at the calcination temperature of 300 °C, delivered a specific capacitance of 362 F g−1 at a current density of 0.5 A g−1. The good electrochemical behaviour of these nanostructures may be related to their unique, highly porous, integrated nanostructures that provide a good electroactive surface for faradaic redox reactions to occur. Also, an asymmetric-supercapacitor with NCO-QDs as positive electrode and reduced graphene oxide (rGO) as negative electrode has been fabricated which exhibits a good charge storage capacity (81 F g−1 at the scan rate of 5 mV s−1) along with very good cycling stability (86% capacitance retention after 1000 cycles). The energy density and power density of the supercapacitor have been checked at a potential window of 2.5 V after 1000 charge-discharge cycles. The device has shown a very good energy density of 69.5 W h kg−1 along with high power density of 2.22 kW kg−1 at a current density of 1.5 A g −1. So, this study suggests that the prepared NCO-QDs are prospective candidates for fabricating supercapacitor electrodes with good electrochemical performance in an extended potential window.