Effect of Concentration of Tetraethylammonium Tetrafluoroborate on the Cell Performance of Electrochemical Double-layer Capacitors

<p>The solubility of tetraethylammonium tetrafluoroborate (TEA BF4) in acetonitrile (AN) is approximately 1.65 M. Solutions ranging from 0.3 to 1.5 M were prepared to investigate the effect of concentration on performance and cycle life of electrochemical double-layer capacitors. The time constant indicates that the rate capability improves with increasing concentration. Higher concentrations result in higher energy densities at the highest power densities. Accelerated durability tests and electrolysis current on the positive electrodes revealed that AN degraded more readily than salt in the electrolyte during cycling. Therefore, higher concentrations, such as 1.2 and 1.5 M, are advantageous for prolonged cycling and high-temperature conditions. The significant degradation of AN and the binder on the positive electrode increases cell resistance, thereby reducing cycle life. Concentrations closer to the solubility limit of the salt were more favorable for increasing the energy density and cycle life compared with lower concentrations. However, concentrations close to solubility make ionic movement very sluggish at low temperatures, which limits use because their performance deteriorates rapidly at low temperatures.</p>