Anti-freezing supercapacitors using novel choline phosphate aqueous electrolytes

Conventional aqueous and organic electrolytes often suffer from toxicity, flammability, poor performance at low temperatures, or limited electrochemical stability, motivating the search for environmentally friendly alternatives. Herein, we report a comprehensive study on aqueous choline - phosphate electrolytes based on choline dihydrogen phosphate (CDHP) and mixed choline dihydrogen phosphate and choline hydrogen phosphate (CDHP + CHP) systems as green electrolytes for low-temperature energy storage devices. Both salts were synthesized via neutralization of choline hydroxide with phosphoric acid. Physicochemical analysis revealed that pure CDHP solutions exhibit acidic pH, which limits the electrochemical stability window (ESW) due to hydrogen evolution. However, the introduction of CHP effectively tunes the electrolyte pH towards near-neutral values while simultaneously enhancing ionic conductivity. The mixed CDHP + CHP electrolyte achieved improved ESW, maintaining high conductivity across a concentration range from 1 to 5 mol $\cdot$ kg^-1 and down to -20°C. Electrochemical investigations on carbon/carbon symmetric capacitors using these electrolytes reveal electric double-layer (EDL) performance without redox activity in the low voltage range. However, mixed redox processes and EDL charging were observed at higher voltage, which have been evaluated with temperature-dependent electrochemical impedance spectroscopy. By modeling the Nyquist plot, absolute ion diffusion coefficients and desolvation rates have been calculated, which support the analysis of the Arrhenius plots showing that the mixed electrolyte lowers the bulk ion diffusion energy barrier for pure CDHP, driving its superior sub-zero performance.