Investigation on the pseudocapacitive characteristics of the electrochemically modified graphite electrode in electrolytes with different pH

The pseudocapacitive properties of an electrochemically modified graphite electrode (MGE) in electrolytes with different pH are investigated, and the transformation of the corresponding reaction mechanism and contribution of pseudocapacitance to the overall capacitance are analyzed in detail by cyclic voltammetry, electrochemical impedance spectroscopy and self-discharge tests. At low pH, the pseudocapacitance is attributed to the redox reaction among hydroxyl, carbonyl and carboxyl groups, which takes up nearly 61% of the overall capacitance. As the pH increases from 0 to 2, the carboxyl groups begin to ionize, the proportion of the redox reaction between carbonyl and carboxyl groups is decreased, and the contribution of pseudocapacitance to the overall capacitance decreases too. With the pH further increasing, the carboxyl groups are almost completely ionized and the redox reaction occurs only between hydroxyl and carbonyl groups, the contribution of pseudocapacitance drops to 33% and the double layer capacitance begins to play a dominate role. Despite of a lower specific capacitance for MGE in neutral solution, a broader potential window is obtained and the energy density would be increased nearly 1.33 times as large as that in acidic solution. The results indicate that the electrochemical system of MGE with the neutral electrolyte may be a promising candidate for a high energy density supercapacitor.