The NaClO4-water eutectic electrolyte for environmentally friendly electrical double-layer capacitors operating at low temperature

Herein, owing to its remarkable ionic conductivity, the 8.84 mol kg−1 NaClO4 water-in-salt (WIS) eutectic electrolyte is suggested for the low temperature operation of electrical double-layer capacitors down to -35 °C. Molecular dynamic simulations reveal that the local structure of the solution remains relatively stable from RT to -35 °C, with distinct channel-like domains formed by hydrogen-bonded water molecules, which facilitate the ionic diffusion within the bulk of the electrolyte. Interestingly, on activated carbon (AC) electrodes, the electrolyte demonstrates an electrochemical stability window (ESW) increasing from 1.8 V to 2.1 V as temperature decreases from 25 °C to -35 °C. The high ESW of this electrolyte is due to i) pH close to neutrality enabling high overpotential of water reduction on porous AC electrodes; ii) WIS characteristics with a reduced amount of free water in the Stern layer under positive polarization of AC; iii) higher oxidation potential at low temperature ascribed to the enhanced immobilization of the ClO4− anions in the Stern layer as well as reduced diffusion of water molecules in the AC electrode porosity. The electrochemical investigations on laminate AC//AC cells in the NaClO4-water electrolyte demonstrate low values of their resistive components down to -30 °C, while the devices can operate with rated voltage of 2.0 V and 1.7 V at -30 °C and RT, respectively. At -30 °C and under discharge power as high as 10 kW kg−1, the AC//AC capacitor in the NaClO4-water electrolyte outperforms the specific output energy of a traditional AC//AC cell in the 1 M TEABF4/ACN electrolyte. Hence, for sub-ambient temperatures, the presented environmentally friendly EDLC holds promise to compete with traditional devices implementing an organic electrolyte.