On the Parasitic Surface Adsorption of Pyrrolidinium and Phosphonium-based Ionic Liquids Preventing Accurate Differential Capacitance Measurements

Differential capacitance measurements are known to provide vital information regarding electrical double layer charging as well as interfacial structuring of ionic liquids and ionic liquid-based electrolytes. Several hurdles have prevented these types of measurements from becoming widely used, including the fact that there exists no real consensus as to how the measurement needs to be performed and the results analyzed. To add to the difficulty, some ionic liquids are known to display a hysteresis process, thus inducing measurement variabilities. In this report, we study pyrrolidinium and phosphonium-based ionic liquid electrolytes and show that hysteresis processes indeed exist and that these are mostly the consequence of cationic adsorption on the surface. Atomic force microscopy experiments reveal that pyrrolidinium-based systems display a much denser degree of ionic compaction at the interface, compared to the phosphonium-based systems, a fact that we correlate with the much more intense hysteresis measured in pyrrolidinium-based systems. We further propose a new method for the measurement of differential capacitance and compare it with other methods in use. It is found that the proposed method allows to minimize hysteresis phenomena, thereby leading to better accuracy.