Activated carbon/iron oxide composites were synthesized by the chemical reduction of activated carbon/FeO(OH) using hydrazine hydrate as reducing agents to optimize the hydrazine concentration. Changes in the crystal structures of iron oxide are observed as the concentration of hydrazine is varied. These changes affect the redox reactions and kinetics of iron oxide during charge discharge process, which can significantly affect the supercapacitive performance. The characterization of morphology and microstructure of the as-prepared composites demonstrates that FeO(OH) is successfully formed on the activated carbon surface and reduced to Fe3O4 by hydrazine reduction. The capacitive properties of the as-prepared composites are evaluated using cyclic voltammetry, galvanostatic charge/discharge testing, and electrochemical impedance spectroscopy in a three-electrode experimental setup using a 1 M Na2SO3 aqueous solution as the electrolyte. The capacitive property of the activated carbon/Fe3O4 electrode which is reduced by 5 ml of hydrazine shows excellent electrochemical performance (168.5 F g(-1) at a current density of 2 A g(-1)). It is anticipated that these optimized process to prepare activated carbon/ironoxide composite is a promising fabrication method for supercapacitor electrodes. (C) 2015 Elsevier Ltd. All rights reserved.