Electrolytic Properties of Tetramethylammonium Compound in Highly Concentrated Solutions and Its Application to Electric Double-Layer Capacitors

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The use of tetramethylammonium ions (TMA⁺) at moderately high concentrations of the electrolytes can increase the number of ions accumulated in pores of an activated carbon electrode and bring about high capacitance of an electric double-layer capacitor (EDLC). The solubility of tetramethylammonium difluoro(oxalato)borate (TMADFOB) in propylene carbonate (PC) was 2.0 M (M = mol dm⁻³) or more. The maximal ionic conductivity of a TMADFOB solution in PC was observed at about 1.6 M at 25°C. The increase in the concentration of the electrolyte resulted in the increase in the gravimetric capacitance. The gravimetric capacitance found for TMADFOB was slightly higher than that for triethylmethylammonium tetrafluoroborate (TEMABF₄).