Preparation Of An Electrochemically Modified Graphite Electrode And Its Electrochemical Performance For Pseudo-capacitors

Electrochemical capacitor is a new device for electric energy storage. It has much more energy density than conventional capacitors, and much higher power density than batteries. As one of the most important components of electrochemical capacitor, electrode material is a key factor influencing the performance of the electrochemical capacitor. Therefore, more research and development have been focused on the electrode materials in recent years.In this dissertation, the preparation, characterization and pseudo-capacitive performance of the modified graphite electrode (MGE) were systematically studied. The pseudo-capacitive and the electrocatalytic property were studied in different electrolytes. Furthermore, a composite electrode of PANi/MGE was prepared and a preliminary analysis was conducted. The surface structure and properties of MGE were investigated by many methods, such as scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy, N₂ adsorption, Fourier transform infrared spectroscopy (FT-IR), thermogravimetry, Boehm's titration, X-ray photoelectron spectroscopy (XPS). The capacitive performance of MGE and PANi/MGE were tested by electrochemical measurements such as constant current charge-discharge, cyclic voltammogram (CV), electrochemical impedance spectrum (EIS) etc. The effect of oxygen-containing functional groups on the pseudo-capacitive characteristic of MGE was further investigated. In this paper, the main results were as following:GE modified by recurrent galvanic pulses technique showed a perfect pseudo-capacitive character. The optimal technological parameters were determined as anodic current of 200 mA (300 s) and cathodic current of -120 mA (100 s) with 6 cycles.After electrochemical activation, the smooth surface of GE was changed into an obviously porous, rough and three-dimensional active layer with turbostratic and multi-layered microcrystalline flake structure, and the number of active sites corresponding to the oxygen-containing functional groups significantly increased. The material showed a high specific capacitance of 179.7 F g^-1 with significant pseudo-capacitive character, as well as a good stability. The effect of oxygen-containing functional groups on pseudo-capacitive characteristics of MGE was investigated through quantitative analysis. It was found that the pseudo-capacitance took up nearly seventy percent of the overall capacitance according to the CV curve, and such a high specific capacitance was mainly attributed to the continuous redox reactions between the hydroxyl, carbonyl and carboxyl groups.The pseudo-capacitive properties of MGE in electrolytes with different pH were investigated, and the reaction mechanism and the contribution of pseudo-capacitance were analyzed in detail by CV, EIS and self-discharge tests. At low pH, the pseudo-capacitance was attributed to the redox reaction among hydroxyl, carbonyl and carboxyl groups, and the pseudo-capacitance took up nearly 61% of the overall capacitance. At pH 1², the carboxyl groups were ionized, the proportion of the redox reaction between carbonyl and carboxyl groups was decreased, and the overall pseudo-capacitance decreased too. With the pH further increasing, the carboxyl groups were almost completely ionized and the redox reaction occurred only between hydroxyl and carbonyl groups, the contribution of pseudo-capacitance drop to 33% and the double layer capacitance began to play a dominate role.Pseudo-capacitive characteristics in acidic and neutral electrolytes and electrocatalytic property for Cl-, NO3– and Fe³⁺/Fe²⁺ of MGE were evaluated by cyclic voltammetry (CV) tests. It was found that MGE exhibited a considerable pseudo-capacitance (the specific capacitance was high up to 1.730 F cm^-2 ) in H2SO4, and an excellent pseudo-capacitive characteristic in HCl except a narrow potential window due to the electrocatalytic activity of MGE for chlorine evolution reaction (the onset potential of chlorine evolution was negatively shifted up to 238 mV). However, MGE in HNO3 indicated no pseudo-capacitive characteristic, but its electrocatalytic activity for the reduction of the nitric acid was noticeably presented. In addition, MGE presented a perfect electrocatalytic activity for the redox reaction of Fe³⁺/Fe²⁺, this reversible redox couple also could be used for energy charge/discharge. Compared with pseudo-capacitive characteristic in acidic solution, the potential window of MGE in neutral solution was substantially broadened; its energy density was greatly improved eventually even though the corresponding peak current density was decreased.A composite electrode of PANi/MGE was prepared by electrochemical deposition. Its pseudo-capacitive characteristic was preliminarily evaluated and it was found that a synergistic effect arose between the PANi and MGE. The apparent specific capacitance was high up to 2.34 F cm^-2 and indicative of a huge application potential.