The NaClO 4-water eutectic electrolyte for environmentally friendly electrical double-layer capacitors operating at low temperature

A Parejo-Tovar and F Béguin, ENERGY STORAGE MATERIALS, 69, 103387 (2024).

DOI: 10.1016/j.ensm.2024.103387

Herein, owing to its remarkable ionic conductivity, the 8.84 mol kg -1 NaClO 4 water -in -salt (WIS) eutectic electrolyte is suggested for the low temperature operation of electrical double -layer capacitors down to -35 degrees C. Molecular dynamic simulations reveal that the local structure of the solution remains relatively stable from RT to -35 degrees 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 degrees C to -35 degrees 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 ClO 4 - 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 NaClO 4 -water electrolyte demonstrate low values of their resistive components down to -30 degrees C, while the devices can operate with rated voltage of 2.0 V and 1.7 V at -30 degrees C and RT, respectively. At -30 degrees C and under discharge power as high as 10 kW kg - 1 , the AC//AC capacitor in the NaClO 4 -water electrolyte outperforms the specific output energy of a traditional AC//AC cell in the 1 M TEABF 4 /ACN electrolyte. Hence, for sub -ambient temperatures, the presented environmentally friendly EDLC holds promise to compete with traditional devices implementing an organic electrolyte.

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