Investigation on capacitance mechanisms of Fe3O4 electrochemical capacitors

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.