The capacitance mechanisms of magnetite ͑Fe 3 O 4 ͒ electrochemical capacitor in Na 2 SO 3 , Na 2 SO 4 , 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 ϳ170, 25, and 3 F/g in 1.0 M Na 2 SO 3 ͑aq͒, Na 2 SO 4 ͑aq͒, and KOH͑aq͒, respectively. Strong specific adsorption of the anion species was evidenced in all solutions. Experimental results indicate that, in Na 2 SO 3 ͑aq͒, the capacitive current of magnetite electrode originates from the combination of electric double-layer capacitance ͑EDLC͒ and the pseudocapacitance that involves successive reduction of the specifically adsorbed sulfite anions, from SO 3 2− through, e.g., S 2− , and vice versa. In Na 2 SO 4 ͑aq͒, the current is due entirely to EDLC. Furthermore, due to the specific adsorption behavior, magnetite exhibits high EDLC, Ͼ30 F/cm 2 , in both Na 2 SO 3 and Na 2 SO 4 solutions. The lowest capacitance of magnetite was observed in KOH, which is attributed to the formation of an insulating layer on the magnetite surface.Electrical double-layer capacitance ͑EDLC͒ arises from the potential dependence of the surface density of charges stored electrostatically ͑i.e., nonfaradaically͒ at the interfaces of capacitor electrodes. 1-4 EDLC electrochemical capacitors are complemented by capacitors based on the so-called pseudocapacitance, which involves faradaic reactions but behaves like a capacitor rather than a galvanic cell. [5][6][7][8][9][10][11][12][13][14][15] While EDLC typically has a specific capacitance in the order of 10 F/cm 2 of true surface area of the electrode material, pseudocapacitance often has a value that is 10 to 100 times greater.The most widely studied pseudocapacitive material is hydrous RuO 2 . 5-9 The pseudocapacitance of this material is known to arise from successive multielectron transfer at Ru cation sites, from, e.g., Ru 2+ to Ru 3+ and then to Ru 4+ , balanced by conversion of the OH − site to the O 2− sites in the oxide structure by proton transfer. There is a continuously variable degree of oxidation/reduction, leading to the capacitor behavior. Because RuO 2 is very expensive, searching for cheaper pseudocapacitive electrode materials has been a major subject in the research on electrochemical capacitors. Goodenough et al. 10,11 reported in 1999 the observation of pseudocapacitance on hydrous MnO 2 . Its mechanism was suggested to involve multielectron transfer at Mn cation sites, balanced by intercalation/extraction of cations within the oxide structure.An aqueous Fe 3 O 4 ͑magnetite͒ electrochemical capacitor is another emerging inexpensive system. 12-16 Large capacitances have been reported in alkali sulfites and sulfate solutions. In particular, the capacitance of the oxide was found to be sensitive to the anion species but not to either alkaline cations or electrolyte pH ͑ഛ11͒. These behaviors sugge...