Alkaline hydrogel electrolyte from biosourced chitosan to enhance the rate capability and energy density of carbon-based supercapacitors

Abstract

This paper reports the development of a safe carbon-based supercapacitor, which is based on a green biodegradable hydrogel electrolyte that is prepared from chitosan biopolymer and KOH as the electrolyte source. The impact of electrolyte solution ageing time on electrolyte gel formation is investigated. A critical time of 2 days is necessary to obtain gel electrolytes mechanically exploitable. This is associated with the gel structural modification, as observed by FTIR and ¹H/¹³C NMR. Between 2 and 4 days, the capacitance increases from 76 to 95 F g⁻¹ and remains stable up to 21 days. Good rate handling is achieved (62%) with a capacitance of 59 F g⁻¹ at 10 A g⁻¹. Remarkably, the developed gel exhibits good stability when the cell voltage is increased from 0.8 V to 1.3 V. The voltage window extension allows to obtain for the C–C device, a high energy density (5.1 W h kg⁻¹) at a power density of 32.5 W kg⁻¹, which is almost 3 times higher than that delivered by liquid 2 M KOH at 0.8 V. The gel electrolyte could be used with pseudocapacitive materials, C/Co₃O₄ and voltage window extension is achieved along with significant increase in energy density from 1.66 to 6.31 W h kg⁻¹. Better capacitance retention is obtained by the chitosan–KOH gel electrolyte than by liquid KOH. Advantageously, the gel electrolyte prevents the electrode degradation and positive current collector from undergoing corrosion.