Dependence of Co/Fe ratios in Co-Fe layered double hydroxides on the structure and capacitive properties

Co-Fe layered double hydroxides (LDHs) with different Co/(Fe2+, Fe3+) atomic ratios are fabricated via coprecipitation followed by an I-2 partial oxidation process. The initial Co/Fe ratio in solution is changed in a wide range and has a significant influence on the phase and structure of the product. Structural analysis confirms that Co-Fe LDHs with four Co/Fe molar ratios (2.35, 1.07, 0.58 and 0.24) shows a typical hydrotalcite-like structure and layered plate-like morphology. A high Fe content will destroy the hydrotalcite structure and lead to the formation of magnetite particles. Electrochemical data demonstrate that the specific capacitance of Co-Fe LDHs is strongly dependent on Co/Fe ratio. The Co-Fe LDHs with a high Co/Fe ratio of 2.35 can deliver the highest capacitance of 728 F g(-1) at 1 A g(-1) among them and when assembled into asymmetric supercapacitor with activate carbon as negative material, it can exhibit an energy density of 27.3 Wh kg(-1) at a power density of 823.5 W kg(-1). The incorporation of Fe not only prevents the formation of beta-Co(OH)(2) but also delivers a high capacitance. But, too much Fe is not preferred because it will lead to unstable structures and poor electrochemical performance due to the generation of iron compounds. (C) 2016 Elsevier Ltd. All rights reserved.