The Micro-Structures And Capacitance Of The CO/NI/ACF, CO/MN/ACF Electrods

As a new kind of energy storage devices, supercapacitors (SCs) have a lot of advantages, including fast charge and discharge, high efficiency, long lifetime and so on. SCs are widely used in electronics, communications, electric vehicles, new energy resources and other fields. However, SCs have an inherent disadvantage, lower storage capacity. Electrode materials play important roles in SCs. There are two kinds of common electrode materials:porous carbons which store electric charges in the form of electric double electrode layers, and transition-metal oxides which generate pseudo-capacitance by faraday reactions. Generally, carbon electrodes have low specific capacitance and good charge-discharge performance. Transition-metal oxides are just the opposite. In this paper, these two materials were properly combined with physicochemical methods for full use of their advantages. Compared with single electrodes, the specific capacitance of composite electrodes was2-5times of single ones. The detailed results are shown as follows:The composite electrodes of activated carbon fiber (ACF) and Co/Ni or Co/Mn metal oxides were prepared by hydrothermal method and sol-gel method. Effects of key technical factors, such as precursor concentration, ratio of oxides, heat treatment temperature and others on morphological structures and electrochemical performances of metal oxides have been investigated systematically.Firstly, composites of Co/Ni binary metal oxides and ACF were prepared by hydrothermal method. Under the condition of lower reaction concentration, Co/Ni binary metal oxides grow in the form of nanowires or needles to form three-dimensional net structures on the surface of ACF. The higher the concentration is, the thicker the nanowires are packed on ACF, and even the more nanospheres are formed. When the concentration of Co2+and Ni+in the precursor is3mM, the specific capacitance of composite electrodes can reach the highest value (668.7F/g), and when a5mA continuous current is used to charge and discharge, the specific capacitance can reach668.7F/g, which is more than5times of the specific capacitance of the ACF’s(125F/g).In order to deeply explore micro-structures and electrochemical performance of composites, Co/Ni and Co/Mn binary metal oxides were respectively combined with ACF by sol-gel method. All of the composites have shown significant porous structures. For Co/Ni/ACF composite electrodes, when the molar ratio of Co/Ni is1:0.5and the charge-discharge current is5mA, the specific capacitance can reach360.2F/g, which is2-3times of the specific capacitance of Ni/ACF (157.9F/g) or Co/ACF (129.6F/g) electrodes. For Co/Mn/ACF composite electrodes, when molar ratio of Co and Mn is1:2, the specific capacitance is298.1F/g, the value is obviously higher than that of single metal/ACF electrodes. These results indicate that there are synergistic effects between different metallic oxides.