Capacitive matching of pore size and ion size in the negative and positive electrodes for supercapacitors

The capacitive behavior of activated carbons with different pore structures in two kinds of electrolytes, Bu4NBF4 and Et4NBF4 in propylene carbonate (PC) was studied using three kinds of cell configuration. The correlations between adsorbed ion size and pore size on the positive and negative electrodes were investigated. The matching of pore size and cation size was more predominant in the capacitor unit, especially for the electrode materials with less developed porosity. The asymmetric capacitance distribution of 7 F g−1 in the negative electrode and 113 F g−1 in the positive electrode occurred for electrode materials with less developed porosity. This could be ascribed to the surface saturation of the negative electrode by electrolyte ions, limiting the overall capacitance and working voltage of device. However, very developed porosity could not profit from the sufficient unitization of surface area, due to a weakened interaction between ions and pores wall. The specific area capacitance experienced a significant decrease from 11.3 to 6.7 μF cm−2 with the pore volume increasing. Since the different pore sizes were required for different electrolytes ion on the negative and positive electrodes, the optimal matching between pores size and ions dimension with respect to each electrode should be considered for the maximum capacitance value of the capacitor unit.