Water-in-Acid Gel Polymer Electrolyte Realized through a Phosphoric Acid-Enriched Polyelectrolyte Matrix toward Solid-State Supercapacitors

A new concept of water-in-acid gel polymer electrolytes (GPEs) is introduced. The simple and scalable UV-light-assisted synthesis of a copolymer matrix possessing polyelectrolyte behavior, followed by swelling in minimally diluted H₃PO₄ (15.1 M/88 wt % aqueous solution), effects formation of a high proton-conducting, self-standing, and mechanically stable polyelectrolyte GPE (PGPE). Retention of high mechanical stability despite the presence of a large amount of liquid species makes it a promising candidate for replacing conventional GPEs. The high proton conductivity (9.8 × 10^–2 S cm^–1) of the PGPE at an ambient temperature of 303 K is attributed to the high concentration of the conducting species present in the polymer matrix. The PGPE-based polyaniline (PANI) supercapacitor device (PANI-1) with a mass loading of 1 mg cm^–2 exhibits a high specific gravimetric capacitance of 385 F g^–1 at a current density of 0.25 mA cm^–2. At the same current density, the PANI-5 device retains high gravimetric and areal capacitance values of 258 F g^–1 and 1288 mF cm^–2, respectively. The low equivalent series resistance value of 0.78 Ω (for the PANI-5 device) further proves the excellent electrode–electrolyte interface formed by the water-in-acid GPE. A 100% capacitance retention even after 9000 continuous charge–discharge cycles strongly indicates the feasibility of adopting water-in-acid GPEs in future supercapacitors.