In the present review, we focused on the fundamental concepts of hydrogels—classification, the polymers involved, synthesis methods, types of hydrogels, properties, and applications of the hydrogel. Hydrogels can be synthesized from natural polymers, synthetic polymers, polymerizable synthetic monomers, and a combination of natural and synthetic polymers. Synthesis of hydrogels involves physical, chemical, and hybrid bonding. The bonding is formed via different routes, such as solution casting, solution mixing, bulk polymerization, free radical mechanism, radiation method, and interpenetrating network formation. The synthesized hydrogels have significant properties, such as mechanical strength, biocompatibility, biodegradability, swellability, and stimuli sensitivity. These properties are substantial for electrochemical and biomedical applications. Furthermore, this review emphasizes flexible and self-healable hydrogels as electrolytes for energy storage and energy conversion applications. Insufficient adhesiveness (less interfacial interaction) between electrodes and electrolytes and mechanical strength pose serious challenges, such as delamination of the supercapacitors, batteries, and solar cells. Owing to smart and aqueous hydrogels, robust mechanical strength, adhesiveness, stretchability, strain sensitivity, and self-healability are the critical factors that can identify the reliability and robustness of the energy storage and conversion devices. These devices are highly efficient and convenient for smart, light-weight, foldable electronics and modern pollution-free transportation in the current decade.

This article presents the effect of different calcination temperatures on the structural, morphological and capacitance of nickel phosphate (Ni3(PO4)2) as an electrode material for supercapacitor applications. Ni3(PO4)2 was synthesized via a sonochemical method followed by calcination at different temperatures (300, 600 and 900 °C, denoted as N300, N600 and N900, respectively). The phase structure and purity of Ni3(PO4)2 were confirmed by X-ray diffraction (XRD) and Fourier transform infrared (FTIR) analysis. The surface morphologies showed that the particle size increased with increasing the calcination temperatures. The electrochemical performance of N300, N600 and N900 were investigated using cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) in a 1 M KOH electrolyte. It was found that N300 exhibited the maximum specific capacity of 620 C g−1 at 0.4 A g−1, which was significantly higher than N600 (46 C g−1) and N900 (14 C g−1). Here, the enhanced electrochemical performance was obtained due to the amorphous structure and augmentation of the redox active sites of the N300 particles. Additionally, the fabricated N300//activated carbon based asymmetric supercapacitor can be cycled reversibly at a cell voltage of 1.45 V. The device exhibited an energy density of 76 W h kg−1 and a power density of 599 W kg−1 with life cycles of 88.5% capacitance retention after 3000 cycles.