A room temperature ionic liquid is immobilized on the surfaces of layered double hydroxides to form a solvent-free fluid via a silylation reaction. A series of solid polymer electrolyte membranes based on quaternized polysulfone and the solvent-free fluid are fabricated by solution casting method, which are tough and flexible and display moderate KOH aqueous solution uptakes. A symmetric electrical double-layer capacitor is assembled by using an optimized quaternized polysulfone-based solid polymer electrolyte membrane with KOH solution and two activated carbon electrodes. The single carbon electrode delivers a specific capacitance of 114.0 F g(-1) at a current density of 1 A g(-1) and outstanding cycling stability, indicating that this novel solid polymer electrolyte membrane may be appropriate for electrical double-layer capacitors. (C) 2018 Elsevier Ltd. All rights reserved.