Abstract
•N-hydroxymethylacrylamide hydrogel electrolytes were prepared using free radical mechanism.•Hydrogel electrolytes were characterized using FTIR, XRD, and FESEM.•Hydrogel electrolytes containing lithium trifluoromethanesulfonate salt attained higher ionic conductivity.•Hydrogel based supercapacitors exhibited excellent results in terms of specific capacitance, energy density, power density, and cyclic stability.
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.
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