Redox-active polymer hydrogel electrolyte in biowaste-derived microporous carbon-based high capacitance and energy density ultracapacitors

High capacitance and energy density in ultracapacitors can be achieved by suitable match between electrode material and electrolyte. In this paper, the development of biowaste-derived microporous carbon electrode material combined with cost-effective redox mediator incorporated polymer hydrogel electrolyte is described for high performance ultracapacitor applications. A high specific capacitance of ~200 F g^-1 at a current density of 1 A g^-1 is attained for ultracapacitor based on Terminalia Catappa fruit shell-derived microporous carbon (TCFSMC) electrode material and PVA/H₂SO₄/FeCl₃.6H₂O (PHF) based redox-active polymer hydrogel electrolyte. At the same time, an excellent coulombic efficiency and cyclic stability of 99.7% and 64% respectively are maintained up to long 90000 charge-discharge cycles. Further, different combinations of 2, 3, 4 and 5 ultracapacitors in series are assembled to realize high energy density with wide voltage window. Maximum energy and power density of ~120 W h kg^-1 and 43 kW kg^-1 respectively are realized for 5 ultracapacitors assembled in series with a 5 V voltage window. Moreover, a charged device of 2 ultracapacitors connected in series is able to light up a commercial red-light emitting diode (LED) for more than 150 s, demonstrating its promising potential in practical applications. Such electrode material and electrolyte match provide opportunities for the development of next generation low-cost ultracapacitors with improved capacitance as well as energy density.