The Preparation And Electrochemical Properties Of Metal Oxides/ACF Composite Electrode

Supercapacitor is a new kind of energy storage device between common capacitors and secondary cells. It can supply much higher specific energy than common capacitors, and also, has higher specific power, longer cyclic life and better high-temperature resistance performance than secondary cells. Its many advantages including easy maintenance have made itself to be used widely.The key of supercapacitor is the electrode materials, which are usually made of activated carbons and transitional metal oxides and so on. Generally, carbon electrodes have shown lower specific energy and slow deterioration by oxidation and high internal resistance. Transition-metal oxides are known as excellent electrode materials for supercapacitor with charge-storage mechanisms based predominantly on pseudo-capacitance. If porous carbon and metal oxides are combined, the advantages of the two materials will be used fully, and then, the electrochemical properties of capacitors could be improved greatly.The present paper studied the preparation and electrochemical properties of composite electrode materials of several metal oxides /ACF by the citric acid sol-gel method. Influence of different components, loading amount, preparation conditions of Ni, Mn, W, Fe metal oxides to the charge-discharge performance have been widely investigated. The electrochemical properties of the Ni/Mn, Ni/W, Ni/Mn/Fe, Ni/Mn/W metal oxide with ACF composite systems were tested by means of constant direct current and cyclic voltammetry, using 1.5mol/L KOH as the electrolyte, and 0.9V as the work potential.To Ni/Mn/ACF system, the structure and specific surface area of the composite electrodes were characterized by wide X-ray diffraction(XRD), scanning electron microscope (SEM)and isothermal adsorption.The experimental results show that metal oxides are amorphous and grainy, particle size is from 0.1 to 1μm, and specific surface area is lower than ACF. The composite electrodes have better electrochemical reversible and electrochemical capacitance than Ni/ACF or Mn/ACF system. When the charge-discharge current is 5mA, the specific capacitance of composite electrode can be up to 290.2F/g when the mol ratio of nickel and manganese is 3:1 and heat treatment is at 400℃(The content of NiO/MnO₂ on ACF is 3.9%(wt.)). On the contrary, the specific capacitance of ACF is only 125F/g, and had better cycle stability.To Ni/W/ACF system, the specific capacitance of composite electrode is 239.1F/g at 5mA, when the mol ratio of nickel and tungsten is 3:1 and heat treatment is at 400℃for 1 hour (The content of NiO/WO₃ on ACF is 3.5%(wt.)). Compared to the Ni/ACF and W/ACF system, it has better specific capacitance and cycle stability.On the basis of the two metal oxide with ACF, the preparation and electrochemical properties of NiO/MnO₂/Fe₂O₃/ACF and NiO/MnO₂/WO₃/ACF composite electrode were further studied.To Ni/Mn/Fe/ACF system, under 5 mA charge-discharge current, the specific capacitance of composite electrode is 205.7 F/g when the mol ratio of nickel, manganese and iron is 1:1.818:0.182 and heat treatment is at 550℃for 1 hour (The content of NiO/MnO₂/Fe₂O₃on ACF was 4.3%(wt.)).But the charge-discharge efficiency is very lower, maybe iron electrode is easy passivation and self-discharge serious in the alkaline solution which limits its extensive application in the secondary cells.To Ni/Mn/W/ACF system, the specific capacitance of composite electrode is 205.7 F/g at 5 mA, when the mol ratio of nickel, manganese and tungsten is 3:0.75:025 and heat treatment is at 400℃for 1 hour (The content of NiO/MnO₂/WO₃on ACF is 4.5%(wt.)).The electrode properties of two-oxide composite electrodes with ACF are better than those of three-oxide electrodes. Among of them, composite electrode of NiO/MnO₂ /ACF is the best. Owing to that Ni and Mn are transitional metal elements, and close in the periodic table, their radius of atomic and ion are at same level, so it is possible that Mn can incorporate Ni to form a solid solution and a homogeneous network structure, which is helpful to store and access charge freely, and then enhance specific capacitance and charge-discharge efficiency.As a new type of composite electrode material, Ni/Mn/ACF can realize a balance between material property and cost, has excellent performance of specific capacitance than single electrode material, and will get a broad application prospect.