All-temperature electrolytes for aqueous electrochemical capacitors operating from −60 to 140 °C

Electrolyte is the determining factor for whether electrochemical energy-storage systems using aqueous electrolytes can operate in all-temperature environments. In this contribution, we present a type of H 3 PO 4 –ethylene glycol–H 2 O proton conducting electrolyte that enables aqueous electrochemical capacitors to operate within a temperature range of −60 to 140 °C, representing the widest operating temperature window of aqueous electrolytes to date. The physicochemical properties of the electrolytes are characterized. Using the designed electrolytes, the electrochemical properties of symmetric electrochemical capacitors based on GaN and graphite electrodes are evaluated at full temperature (−60–140 °C), respectively. The volume ratio of ethylene glycol to water is optimized, and the conduction mechanism of protons is discussed. Compared with the base electrolyte (52 wt% H 3 PO 4 aqueous solution), the addition of ethylene glycol improves the freezing resistance, electrothermal stability, and wettability of the electrolyte, thereby reducing the equivalent series resistance and improving the electrode kinetics and rate performance of the electrochemical capacitors. The improvement of electrochemical performance of electrochemical capacitors by the designed electrolytes is verified through theoretical calculations.