Electrochemical capacitor of magnetite in aqueous electrolytes

The capacitive behaviours of magnetite nanocrystallites, along with conductive carbon black additive, in aqueous electrolytes, including sodium sulphite, sulphate, chloride, and phosphate, and potassium hydroxide, have been characterised by means of cyclic voltammetry and chrono-potentiometry. The behaviour can be categorised into three groups. Sodium sulphate and chloride electrolytes give capacitances (similar to5 F/g-Fe3O4) close to the space-charge capacitance of the oxide. Potassium hydroxide and sodium phosphate gave rise to a higher capacitance (similar to7 F/g) which can in part be attributed to a surface redox reaction which involves hydroxide ions. Sodium sulphite results in the highest capacitances, which depend heavily on the dispersion of magnetite crystallites on the conductive matrix and range from similar to30 to 510 F/g-Fe3O4 with an operation voltage range of 1.2 V. Correlation between rest potential and anion-concentration gives strong indication of potential-determining process which involve either OH under extremely alkaline conditions (pOH < 3) or SO32- in the sulphite electrolyte. (C) 2002 Elsevier Science B.V. All rights reserved.