Symmetric All-Solid-State Supercapacitor Operating at 1.5 V Using a Redox-Active Gel Electrolyte

The charge storage characteristics of a symmetric, all-solid-state supercapacitor device composed of redox-active PVA–K 3 Fe(CN) 6 –K 4 Fe(CN) 6 gel electrolyte and carbon nanotube paper electrodes are investigated in this study. Three-electrode tests performed with aqueous K 3 Fe(CN) 6 /K 4 Fe(CN) 6 electrolyte exhibit an area-normalized specific capacitance 5 times larger than conventional aqueous H 3 PO 4 electrolyte due to pseudocapacitive contribution of the redox ions. The all-solid-state device composed of binder-free, carbon nanotube paper electrodes is thin and lightweight and has superior electrochemical performance. It exhibits a high area-normalized capacitance of 390 mF cm –2 at a low current density of 2 mA cm –2, which is twice and four times that of devices composed of conventional PVA–H 3 PO 4 and PVA–KOH gel electrolytes, respectively. Furthermore, cyclic voltammetry tests and long-term stability tests demonstrate a large stability voltage window of 1.5 V, compared to only 1.0 and 0.4 V, respectively, for the PVA–H 3 PO 4 and PVA–KOH devices. High capacitance combined with a wide voltage window leads to a maximum volumetric energy density value of 2.47 mWh cm –3 and maximum power density value of 0.6 W cm –3 . It is expected that this novel, symmetric, lightweight, binder-free, all-solid-state supercapacitor system may provide a scalable strategy towards powering future wearable electronic devices.