Effect of ion charges on the electric double layer capacitance of activated carbon in aqueous electrolyte systems

Carbon based electrochemical double layer capacitors (EDLCs) are known for high power density, but their energy density is limited due to surface characteristics of the electrode materials as well as the size and charge of the ions used in the electrolyte. Therefore, considering the current demand for enhanced energy density devices, we investigated the use of multivalent electrolytes to increase the capacitance of activated carbon (AC) based EDLCs. As part of these studies, we examined the effect of the charge of the multivalent ions on the capacitive behavior of microporous AC electrodes and compared with the univalent Li+ system. We performed impedance and cyclic voltammetry measurements on AC electrodes in a symmetric two electrode configuration to determine the impedance and capacitance with respect to varying charge and concentration of the ions in the aqueous nitrate electrolytes. These studies clearly demonstrated an increased capacitance with Mg2+ and Al3+ implying the possible effects of ion mobility and electrolyte conductivity in addition to the multivalent charge. These preliminary observations clearly point to the importance of selection of electrolyte ions with more charge, conductivity, and suitable size with respect to the pore size of the electrodes in order to increase the capacitance of EDLCs.