On the causes of non-linearity of galvanostatic charge curves of electrical double layer capacitors

Abstract

In the analysis of electrochemical behavior of electrical double layer capacitors (EDLC), it is generally accepted that the linear form of chronopotentiograms testifies to the ideal polarization of the electrode and any deviations from the linearity are interpreted most often as the appearance of Faraday processes on the electrode surface. However, studies on electrochemical behavior of supercapacitors based on carbon materials should take into account the surface nonequipotentiality caused by the porosity of electrode materials, which complicates the formation of electrical double layer (EDL). The influence of this factor is clearly demonstrated by the results obtained in this work with the use of micro- and mesoporous carbon materials of different origin. To accurately estimate the dependence of linearity of chronopotentiograms on porosity of the electrode material, low current densities were used (from 0.008 to 0.08 mA/cm2). A comparison of charge curves at different charge currents showed that the time dependence of voltage changes considerably with an increase in the current load. A formal comparison of chronopotentiograms linearity obtained at different current loads was made using dimensionless coordinates on the time axis, which were expressed as a percentage of the overall polarization time. Only the charging branch of chronopotentiogram was analyzed here because the anodic branch had similar formal characteristics.