Outstanding Low‐Temperature Capacitance of an AC–AC Supercapacitor in Acid Redox Electrolyte

The capacitive behavior of AC–AC supercapacitor (AC=activated carbon) at −18 °C are greatly improved by using an acid redox electrolyte comprising p-phenylenediamine. By introducing p-phenylenediamine into 38 % H2SO4 solution, the energy density at −18 °C is remarkably elevated to the level achievable at 23 °C in 38 % H2SO4.