a Schematic illustration of aqueous electrolyte supercapacitor and quasi-solid-state supercapacitor shows different charge storage sites. The black dash line in the aqueous electrolyte device represents a separator. b Cyclic voltammograms for symmetric supercapacitors using 1 M H2SO4 aqueous electrolyte and poly(vinyl alcohol)/H2SO4 gel electrolyte. c Galvanostatic charge–discharge profiles for quasi-solid-state device at current densities ranging between 0.5 and 25 mA cm−2; curves at 1, 5, 10, and 25 mA cm−2 are horizontally expanded 2×, 10×, 15×, and 20×, respectively. Inset shows IR drop at current densities of 5, 10, and 25 mA cm−2. d Quasi-solid-state supercapacitor charge-discharge curves after 10,000 cycles at 5 and 25 mA cm−2 exhibit 87% and 90% capacitance retention, respectively (coulombic efficiency is ~100%). e Cyclic voltammograms for a single quasi-solid-state supercapacitor are collected at voltage windows of 1 and 1.2 V; a tandem device (comprised of three supercapacitors connected in series) withstands a 3.6 V window. The tandem device is waterproof after coated by epoxy and exhibits a stable cyclic voltammogram. f Photograph shows a supercapacitor module lighting up a green light-emitting diode. This tandem device (4 cm × 3 cm × 1 cm) contains three supercapacitors (4 cm × 1 cm × 1 cm) connected in series; the core–shell structure of an electrode is also shown.