Investigation on Capacitance Mechanisms of Fe[sub 3]O[sub 4] Electrochemical Capacitors

The capacitance mechanisms of magnetite ( Fe 3 O 4 ) electrochemical capacitor in Na 2 S O 3 , Na 2 S O 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 ∼ 170 , 25, and 3 F ∕ g in 1.0 M Na 2 SO 3 ( aq ) , Na 2 SO 4 ( aq ) , and KOH ( aq ) , respectively. Strong specific adsorption of the anion species was evidenced in all solutions. Experimental results indicate that, in Na 2 SO 3 ( 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 SO 3 2 − through, e.g., S 2 − , and vice versa. In Na 2 SO 4 ( aq ) , the current is due entirely to EDLC. Furthermore, due to the specific adsorption behavior, magnetite exhibits high EDLC, 30 μ F ∕ cm 2 , in both Na 2 S O 3 and Na 2 S O 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.