The capacitance mechanisms of magnetite (Fe3O4) electrochemical capacitor in Na2SO3, Na2SO4, and KOH aqueous solutions have been investigated by electrochemical quartz-crystal microbalance analysis, along with cyclic voltammetry and X-ray photoelectron spectroscopy. The oxide thin-film electrode was prepared by an electroplating method, and exhibits a capacitance of similar to 170, 25, and 3 F/g in 1.0 M Na2SO3 (aq), Na2SO4 (aq), and KOH (aq), respectively. Strong specific adsorption of the anion species was evidenced in all solutions. Experimental results indicate that, in Na2SO3 (aq), the capacitive current of magnetite electrode originates from the combination of electric double-layer capacitance (EDLC) and the pseudocapacitance that involves successive reduction of the specifically adsorbed sulfite anions, from SO32- through, e. g., S2-, and vice versa. In Na2SO4 (aq), the current is due entirely to EDLC. Furthermore, due to the specific adsorption behavior, magnetite exhibits high EDLC, > 30 mu F/cm(2), in both Na2SO3 and Na2SO4 solutions. The lowest capacitance of magnetite was observed in KOH, which is attributed to the formation of an insulating layer on the magnetite surface. (c) 2005 The Electrochemical Society. [DOI: 10.1149/1.2131820] All rights reserved.

Operating characteristics, including capacitance, leakage current, operating potential range, cycling stability and open-circuit self-discharge behaviours, of the magnetite (Fe 3 O 4 ) supercapacitor, containing 10 wt % carbon black as conductive additive, in aqueous electrolytes of Na 2 SO 3 , KOH and Na 2 SO 4 were investigated. Although the capacitance of the oxide was found to depend heavily on electrolyte composition, the self-discharge mechanism in these electrolytes appeared to be the same. Reduction in the dissolved oxygen content (DOC) of the electrolyte reduced the leakage current and profoundly improved the cycling stability. In particular, Na 2 SO 3(aq) gives the highest capacitance, nearly 30 F (g-Fe 3 O 4 ) −1 or 80 μ F cm −2 of actual surface area, with an operation range of 1.1 V based on a leakage current less than 0.1 mA F −1 , and the electrode showed no deterioration after 10 4 cycles under a DOC < 0.1 ppm.

We report here the electrochemical capacitive behaviors of both a solution-oxidized iron oxide thin-film and magnetite particulate electrodes. The thin-film electrode, which was similar to2 PM thick and comprised mainly Fe3O4 (magnetite), exhibited capacitances greater than 0.2 F/cm(2) of geometric area of electrode in aqueous electrolytes including solutions of alkali sulfates, sulfites, and chlorides. The magnetite particulate electrodes, on the other hand, showed a specific capacitance from a few tens to 510 F/g-Fe3O4, depending on the state of dispersion of the oxide crystallites, in Na2SO3(aq) with an operation voltage of 1.2 V. Studies on potential-determining reaction confirmed pseudocapacitance mechanism driven by SO3-2.