Asymmetric Capacitive Behaviors Between The Positive And Negative Electrode Of Carbon-based Supercapacitors

Supercapacitors were extensively studied due to their promising properties in terms of energy storage and power supply, which could fill the gap between the secondary batteries and dielectric capacitors. Supercapacitors could be reversibly charged/discharged at higher rates as compared to batteries. Current major research efforts focused on improving their deliverable energy. Many attempts have been made to increase the energy density of supercapacitors, and these could be categorized into two approaches. The one was to improve the specific capacitance of electrode materials by tailoring the pore structure and modifying surface chemistry environment. The other was to broaden the working voltage of electrolyte. However, less attention had been paid on the integrated performance between electrode materials and electrolyte. In this work, matching relationships between both electrodes with electrolytes were investigated by using a modified two-electrode and three-electrode configurations combined with the artful-designed test program and unique analysis methods.(1) Oxygen functional groups exhibited asymmetric capacitance behaviors in KOH electrolyte. The negative pseudo-capacitances were much larger than the positive ones owing to redox occurred on the negative region and K+ intercalation. Model materials were employed to prove the mechanism of K+ intercalation. Compared with the identical electrochemical response in H2SO4 electrolyte, huge difference in the capacitance values of both electrodes in KOH electrolyte would give a less contribution to the cell capacitance. To optimize the cell performance, a suitably increasing the weigh of active materials in the positive, especially for activated carbon with high proportion of oxygen-containing functional groups, was recommended.(2) Acidic treatment of activated carbon was helpful to introduce some nitrogen species and improve their content in the carbon matrix. The concentration of nitrogen was greatly determined by SiO2 particle size, carbonization temperature and resin content. The existence of SiO2 enhanced the nitrogen content of carbon aerogels, but was harmful to transform from N-6,N-5 to N-Q. Nitrogen functional groups had much higher contribution to the capacitance of the negative electrode than the positive one in both KOH and H2SO4 electrolyte.(3) The correlations between adsorbed ion size and pore size on the positive and negative electrodes were investigated. The asymmetric capacitance distribution of 7 F/g in the negative electrode and 113 F/g in the positive one occurred for electrode materials with less developed porosity (PAC-1). This could be ascribed to the surface saturation of the negative electrode by cation, limiting the overall capacitance and working voltage of the device. However, very developed porosity (PAC-3) could not profit from the sufficient unitization of surface area owing to a weakened interaction between ions and pore wall. Specific area capacitance of PAC-3 (6.7μF/cm2) was less than that of PAC-1 (11.3μF/cm). The inferior capacitance of one electrode affected negatively not only the overall properties of the cell but also the performance of the other electrode. A new asymmetric supercapacitor composed of PAC-1 and PAC-3 was explored, which could take advantage of merit of the each material. The optimal matching between pores size and ion dimension with respect to each electrode should be considered for the maximum capacitance value of the capacitor unit.(4) Solvated effects on the capacitance performance were investigated by using solvent-free ionic liquid (ILs). Results showed that the capacitance of ILs would be improved when ILs were solvated at the room temperature. Converserly, Solvent would lead to the deterioration of capacitance for some ILs at 60℃. Effects of solvent were determined by the working temperature, ion type and pore architectures of electrode materials.(5) The effects of microcrystalline, cell voltage, electrode potential, ion dimensions and solvent types on the electrochemical activation (EA) for the cell as well as each electrode were examined systematically for the first time. EA was a potential-driven process, which was initiated only above a certain onset potential no matter which electrode was polarized. The EA extent increased with increasing applied voltages. The material with an expanded graphitic structure was more favorable for EA process. Meanwhile, both electrodes demonstrated an asymmetrical capacitance-potential relationship. The cations owned the superior intercalation capability to anions. The bulkier the ions, the harder the EA. After EA process, the materials exhibited impure double-layer characteristics and some extra energy would be consumed in maintaining the ion-accessible surface areas.(6) A series of pyrolized graphite oxides (PGO) with doo2 from 0.3976 to 0.3504 nm were prepared by heat-treatment of graphite oxides. PGO were employed as model materials to investigate the EA. The correlation between graphitic structure, ions type, electrode potential, and capacitance was systematically and quantitatively examined. The EA was a graphitic structure-dependent behavior. With the interlayer distance reducing of graphitic electrode, electrochemical behaviors occurred on it would transform from an electric double-layer process to EA then to an intercalation-deintercalation behavior. This work was meaningful to exploit the synergy effects between capacitance and voltage of microcrystalline electrode.