Enhancing the Electrochemical Performance of Electric Double-Layer Capacitors by Applying Mesoporous Carbon Gel RFCX to the Electrode
Authors: Cheng Zairan , Oda Hirokazu , Nakagawa Kiyoharu 
Published: February 2020

Electric double-layer capacitor (EDLC) has been required high capacitance and high-rate performance. The capacitance of EDLC comes from the electrostatic charge accumulation at the interface between electrode and electrolyte. So the capacitance is highly dependent on the surface area of elecrode material. For this reason, microporous activated carbon had been used for many years as EDLC electrode materials. However, recent research use organic electrolyte to enhance the energy density of EDLC. In this case, microporous electrode materials are not good at ion transport and reduce rate perfermance because of the large ion diameter of organic electrolyte. Therefore, an optimal electrode material must provide an appropriate pore size for ion transport. Recently, mesoporous materials have been reported to be more effective in increasing capacitance at high charge/discharge rates.

Based on the recent reports, we focused on the study of novel mesoporous materials. We prepared several kinds of mesoporous materials carbon gel RFCX (resorcinol formaldehyde carbon xerogel) with different pore size distribution. RFCX is a kind of mesoporous carbon material with a 3D interconnected network structure and under the control of the pore structure by varying certain conditions of the sol-gel process. We controled the concentration of catalyst and developed three types of mesoporous materials carbon gel RFCX with a large number of mesopores around 10 nm, 25 nm and 40 nm. For comparison, we used two kinds of commercial activated carbon products M (surface area 900 m2g-1) and Y (surface area 1500 m2g-1). Both of them are high-surface-area microporous activated carbon. And we examined each sample’s electrochemical behavior as the electrode for electric double-layer capacitors in organic electrolyte of tetra ethyl ammonium tetra fluoroborate in propylene carbonate (TEABF4/PC). Ion diameter of electrolyte ion TEA+ in solvation is 1.96 nm. And ion diameter of electrolyte ion BF4
- in solvation is 1.71 nm. Diameter was caculated by Car-Parrinello method.

Carbon gel RFCX enhanced the electrolyte ion transport by the large pore diameter (2 ~ 50 nm). RFCXs showed higher surface areal capacitance of 4.0 ~ 6.2 μF cm−2 than the commercial product M (0.5 ~1.5 μF cm−2) in organic electrolyte. Though they are the similar surface area (about 800 m2g-1 and 900 m2g-1). Moreover, the RFCXs also showed higher surface areal capacitance than the commercial product Y (2.2 ~ 3.5 μF cm−2). Though surface area of RFCX is only a half of Y (1500 m2g-1). And RFCXs showed high-rate performance of 70% owing to the large mesopore volume. These results indicated that capacitance of EDLC in organic electrolyte indepandent of surface area of electrode material. Thus, we considered that the volume rate of mesopore of electrode material is one of the most important determinant of rate performance.

In summary, EDLC based on the mesoporous carbon RFCX exhibited better electrochemical performance than microporous activated carbon in organic electrolyte. It is considered that RFCX has potential for high performance energy-related applications.