Boosting energy-storage capability in carbon-based supercapacitors using low-temperature water-in-salt electrolytes

Supercapacitors (SCs) are high-power energy storage devices with ultra-fast charge/discharge properties. SCs using concentrated aqueous-based electrolytes can work at low temperatures due to their intrinsic properties, such as higher freezing point depression (FPD) and robustness. Besides the traditional organic- and aqueous-based (salt-in-water) electrolytes used in SCs, water-in-salt (WISE) sodium perchlorate electrolytes offer high FPD, non-flammability, and low-toxicity conditions, allowing the fabrication of safer, environmentally friendly, and more robust devices. For the first time, this work reports a comprehensive study regarding WISE system's charge-storage capabilities and physicochemical properties under low-temperature conditions (T < 0 °C) using mesoporous carbon-based electrodes. The effect of temperature reduction on the electrolyte viscosity and electrical properties was investigated using different techniques and the in-situ (or operando) Raman spectroscopy under dynamic polarization conditions. The cell voltage, equivalent series resistance, and specific capacitance were investigated as a function of the temperature. The cell voltage (U) increased ∼ 50%, while the specific capacitance decreased ∼ 20% when the temperature was reduced from 25 °C to −10 °C. As a result, the maximum specific energy (E = CU2/2) increased ∼ 100%. Therefore, low-temperature WISEs are promising candidates to improve the energy-storage characteristics in SCs.