LAUSR

Abstract Tetramethylammonium (TMA) ion is the smallest quaternary ammonium ion (QAI) that can be used in electrochemical double layer capacitor (EDLC) applications. Small ions improve the capacitance of EDLCs by increasing the adsorbed ionic density. Herein, we first report the utilization of TMA BF 4 in EDLCs and the properties of the smallest QAI are investigated. However, because of the poor solubility of TMA BF 4 , it must be combined with another salt, forming a binary-salt electrolyte. In this study, the maximum TMA BF 4 concentration used was 5 mol%. These binary salt mixtures contain a conventional electrolyte salt such as tetraethylammonium BF 4 , trimethylethylammonium BF 4 , or spiro-bipyrrolidinium BF 4 . In addition, 4–5 mol% TMA BF 4 was added to the conventional salt solution and a binary-salt electrolyte was obtained, leading to 12–13% increase in the capacitance compared to that of a 100 mol% conventional electrolyte at 10 A g −1 . This work proposes a new method to improve the performance of EDLCs by using binary-salt electrolytes.