The composite hydrogel electrolytes have been developed by in situ free radical polymerization of N, N dimethylacrylamide in the presence of sodium montmorillonite (Na-MMT) as a physical crosslinking agent and magnesium trifluoromethanesulfonate (MgTf2) as an ionic source to improve their conductivity. Moreover, the incorporation of highly conductive poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) enhanced the electrochemical performance and increased the conductivity by providing conduction pathways through the PEDOT:PSS chains. The formation of composite hydrogel electrolytes was confirmed by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) analysis, and thermogravimetric analysis (TGA). The surface morphology was observed using field emission scanning electron microscopy, and energy dispersive X-ray (EDX) analysis was carried out to find the elements present. The ionic conductivity was studied at room temperature and temperature ranging from 303 K to 373 K. PEDOTDMA35 achieved the maximum ionic conductivity at room temperature (8.6 × 10−3 S/cm) and in the entire temperature range i.e., from 303 K to 373 K. Electrochemical performance of the symmetric supercapacitors was carried out at different scan rates (3 to 100 mV/s) and different current densities (100 to 500 mA/g) to calculate the specific capacitance, energy density, and power density. AC/PEDOTDMA35/AC attained the highest specific capacitance of 280 F/g at 3 mV/s and 376.6 F/g at 100 mA/g (energy density ∼52.35 Wh/kg and power density ∼100.08 W/kg). In addition, prototype supercapacitor was fabricated and used to light up the light emitting diode (LED). The self-healing efficiency of the composite hydrogel electrolyte was also investigated. The results indicate that synthesized hydrogel electrolytes have the potential to be used in aqueous flexible and self-healable supercapacitors.

The composite hydrogel electrolytes developed from conducting polymers own superb/excellent conductive, self-healing, and mechanical properties. However, it is still challenging to exploit them in energy storage devices. To address this issue, novel poly(acrylamide)/poly(3,4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT:PSS) composite hydrogel electrolytes were developed through free radical polymerization in which sodium montmorillonite clay was added as a physical crosslinker. The different amounts of PEDOT:PSS were added to the composite hydrogel electrolytes. The structure of the synthesized composite hydrogel electrolytes was investigated by X-ray diffraction (XRD) analysis. The surface morphology of the composite hydrogel electrolyte was examined with the field emission scanning electron microscopy (FESEM) and elemental composition was determined using energy-dispersive X-ray spectroscopy (EDX). The ionic conductivity was measured at ambient temperature. Among the synthesized electrolytes, PEDOTAAM34 achieved the highest ionic conductivity of 13.7 × 10−3 S/cm at room temperature. Furthermore, electrochemical studies were performed by sandwiching the composite hydrogel electrolytes between symmetric carbon-coated graphite electrodes. The fabricated AC/PEDOTAAM34/AC based symmetric supercapacitor attained the highest specific capacitance of 327 F/g at 3 mV/s and 385.4 F/g (energy density of 53.57 Wh/kg at a power density of 100.08 W/kg) at 100 mA/g current density. The self-healable characteristics of symmetric supercapacitors were confirmed by powering up a light-emitting diode (LED).

Flexible, and self-healable supercapacitors possess significant advantages over liquid electrolyte based supercapacitors. Liquid electrolytes are usually organic and not environmentally friendly. Therefore, flexible water-based electrolyte is a reliable choice. Water-based hydrogel electrolyte is a novel approach to overcome liquid electrolyte related drawbacks. In this study, we propose physically crosslinked poly (N, N-dimethylacrylamide) hydrogel using sodium montmorillonite as a crosslinking agent. The hydrogel electrolytes are prepared by adding different weight % (wt.%) of magnesium trifluoromethanesulfonate (MgOTf) salt as a source of ions. The synthesized hydrogel and hydrogel electrolytes are characterized via functional group identification, thermal decomposition, surface morphology, and elemental composition. Fourier transform infrared (FTIR) deconvolution and transference number measurement are conducted to confirm the complexation and ionic species. Based on the surface morphology, FTIR, and transference number measurement, it could be deduced that enhanced ionic conductivity is due to the amorphous nature of hydrogel electrolyte. Furthermore, an ionic conductivity study revealed that hydrogel electrolyte formulated by adding 30 wt.% salt (DMA3) attained maximum ionic conductivity of 6.69 × 10−3 S/cm. The electrochemical characterization of fabricated symmetric supercapacitor described that AC/DMA3/AC achieved the highest specific capacitance of 119 F/g at 3 mV/s and 142 F/g at 50 mA/g along with 98.07 % capacitance retention at 4 A/g after 5000 consecutive charge-discharge cycles. In addition, a flexible prototype supercapacitor device was designed using two symmetrical optimized AC/DMA3/AC cells connected in series, charged with external power supply, and lighted up the light-emitting diode (LED). The self-healability of the prototype was also examined. Results are very promising for the development of compact, self-standing, self-healable, highly flexible, lightweight, and biocompatible supercapacitors.

Hydrogels are gaining more and more attention in the scientific and technological development due to its capacity to hold huge amount of water. This property is feasible due to the presence of hydrophilic polymer strain which leads to extensive use of this material. The soft physical nature makes them to be competitive material to be used in supercapacitors as electrolytes. However, hydrogels as electrolyte in the supercapacitors have essential concerns such as electrochemical performance, stability, and small potential window compared to the supercapacitors fabricated using organic electrolytes. Recent advancement in novel water-in-salt hydrogel electrolyte-based supercapacitor with wide potential window gained potential interest. Fluidic water and salt ions rapidly immobilize within the polymer hydrogel network and become part of the polymer scaffold and improve the overall performance of the supercapacitor. Therefore, in the present work, poly (N-hydroxymethylacrylamide) (NHMA) hydrogel and lithium salt containing hydrogel electrolytes were prepared through free radical mechanism. Ammonium persulfate was used as a free radical initiator while sodium montmorillonite (clay) was used as a crosslinking agent. Lithium trifluoromethanesulfonate (LiTF) salt was added as source of ions because the ionization of the salt would provide free moving ions. The synthesized hydrogel electrolytes were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), and field emission scanning electron microscopy (FESEM). Electrochemical impedance spectroscopy (EIS) was used to study the ionic conductivity of the prepared hydrogel electrolytes. It proved that the hydrogel electrolyte containing 30 wt.% LiTF (NHMA3) has exhibited highest ionic conductivity of 6.6 × 10−3 S/cm and the lowest activation energy (Ea) of 0.085 eV. The hydrogel electrolytes were imbedded in electric double layer capacitor (EDLC) by using activated carbon electrode which was then tested using cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) techniques. These two techniques showed that the hydrogel AC/NHMA3/AC has reached maximum specific capacitance of 165.19 F/g at 5 mV/s and 287.96 F/g with specific energy of 39.63 W h/kg and specific power of 199.16 W/kg at 200 mA/g. In addition, supercapacitor retained 98.5% capacitance after 5000 cycles at a current density of 5 A/g. Hence, it can be said that the synthesized hydrogel electrolytes have significant potential for smart, light weight, and flexible electronic devices.

In this study, silver (Ag) and cobalt oxide (Co3O4) decorated polyaniline (PANI) fibers were prepared by the combination of in-situ aniline oxidative polymerization and the hydrothermal methodology. The morphology of the prepared Ag/Co3O4@PANI ternary nanocomposite was studied by scanning electron microscopy and transmission electron microscopy, while the structural studies were carried out by X-ray diffraction and X-ray photoelectron spectroscopy. The morphological characterization revealed fibrous shaped PANI, coated with Ag and Co3O4 nanograins, while the structural studies revealed high purity, good crystallinity, and slight interactions among the constituents of the Ag/Co3O4@PANI ternary nanocomposite. The electrochemical performance studies revealed the enhanced performance of the Ag/Co3O4@PANI nanocomposite due to the synergistic/additional effect of Ag, Co3O4 and PANI compared to pure PANI and Co3O4@PANI. The addition of the Ag and Co3O4 provided an extended site for faradaic reactions leading to the high specific capacity. The Ag/Co3O4@PANI ternary nanocomposite exhibited an excellent specific capacity of 262.62 C g−1 at a scan rate of 3 mV s−1. The maximum energy and power density were found to be 14.01 Wh kg−1 and 165.00 W kg−1, respectively. The cyclic stability of supercapattery (Ag/Co3O4@PANI//activated carbon) consisting of a battery type electrode demonstrated a gradual increase in specific capacity with a continuous charge–discharge cycle until ~1000 cycles, then remained stable until 2500 cycles and later started decreasing, thereby showing the cyclic stability of 121.03% of its initial value after 3500 cycles.

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.

A series of poly (methylacrylate-co-methylacrylic acid) (P(MMA-co-MAA)) gel polymer electrolytes containing iodide/triiodide (I−/I3−) redox mediator from sodium iodide (NaI) dopant salt was synthesized and studied on their conductivity and power conversion efficiency as applied in dye-sensitized solar cells (DSSC). A relationship of complex permittivity with increasing frequency was established as well as dispersion relation with modulus studies. Temperature dependence study forms an Arrhenius plot and the highest ionic conductivity achieved was 1.07 mS cm−1 at room temperature with activation energy of 0.224 eV for the gel polymer electrolytes (GPE) with 40 wt% NaI. Fourier transform infrared (FTIR) and X-ray diffraction (XRD) spectroscopy was utilized to evaluate the formation of complexes between the copolymer and salt. Again at 40 wt% NaI, the best performance was observed under photovoltaic investigation using DSSC with energy conversion efficiency of 2.34%. To further understand the electrochemical properties of the GPE, steady-state measurement of triiodide diffusion coefficient was done.

The replacement of liquid electrolytes with gel polymer substituents in dye-sensitized solar cells (DSSCs) has become an accepted strategy not only to overcome problems encountered previously but also to increase the shelf life and photovoltaic performance of DSSCs. In that perspective, a gel polymer electrolyte system based on poly(methyl methacrylate-co-methacrylic acid), P(MMA-co-MAA), was prepared with iodine/iodide redox mediator from a sodium salt and the commercially available ionic liquid 1-hexyl-3-methylimidazolium iodide (HMIm+I−) and was studied for its ionic conductivity and energy conversion efficiencies along with structural investigation of the complex by Fourier transform infrared spectroscopy and X-ray diffraction. The highest conductivity achieved was 2.33 mS cm−1 at room temperature for the gel polymer electrolyte with 20 wt% HMIm+I−. With a generic DSSC arrangement of TiO2 sensitized with N719 dye photoanode/electrolyte/Pt-containing counter electrode, the copolymer-based conducting electrolyte displayed power conversion efficiency of 5.30% under illumination of 100 mW cm−2.