Observation of electrode potential in electrochemical double layer capacitors with variations in temperature, scan rate, and ion size

Abstract Owing to different atomic sizes, the ionic transport rates of anions and cations are not identical under the same electric field. However, both ions in the electrodes of electrochemical double layer capacitors (EDLCs) should have the same transport rate in order to maintain the charge neutrality of the entire cell. The faster-moving ion requires a lower electric field than its slower-moving counterion in EDLCs. Anions (BF4−) smaller than quaternary ammonium cations require a lower electric field than cations. The relative ratios of ionic transport resistance between anions (positive electrode, R+) and cations (negative electrode, R−) were obtained with 1.0 M tetraethylammonium BF4 in acetonitrile. The value of R+/R− was 0.037 at 25 °C and increased to 0.41 at 55 °C, as the cationic transport rate improved with temperature. Although the positive electrode exhibited narrower potential ranges than the negative electrode, the electrolysis of electrolytes was more active on the positive electrode than on the negative electrode. The use of large cations and high operation rates was identified as a way to narrow the range of the positive electrode, and thus reduce electrolysis.