Study Of High-performance Cobalt-based Hydroxide Electrode Materials And Cobalt Hydroxide-activated Carbon Asymmetric Supercapacitors

The rapid consumption of fossil fuels, the global population growth and the deterioration of the environment problems and other issues are accelerating the demands for high energy density, high power density and long cycle life of energy storage and conversion equipments. Among them, supercapacitor as energy storage equipment has a good power performance, can meet the needs of fast charging and discharging, has been widely concerned in recent years. However, its energy density is limited in practical applications,so improving the energy density of supercapacitor is a major concern of researchers. In this context, due to its greater potential window and higher energy density than the symmetric supercapacitor,ASCs has been widely researched. Based on this point, we obtain Co（OH）₂by potentiostatic deposition, and assemble Co（OH）₂|KOH|AC ASCs.the maximum energy density is 27.7Wh/kg in1A/g. Although the energy density of ASCs has greatly improved compared with the energy density of commercial carbon based EDLC（ <10Wh/kg）, it still can not meet actual needs, so we need to continue to develop test plans to further improve energy density.As is known to all, electrode materials and electrolyte are the main components of the supercapacitor, and they are the main influence factors of the performance of SCs. For electrolyte, in earlier work, people pay attention mainly to improve the voltage window, and in recent years, researchers also proposed to add redox meditors to electrolyte to provide additional pseudocapacitance, thus to greatly improve the energy density of supercapacitors. But the related literature reports mainly focused on the addition of one redox meditor, which often effect specific capacitance of a singer electrode. Therefore, to achieve high energy density of Co（OH）₂-AC asymmetric supercapacitor, we use ion exchange membrane, and add different redox meditors to electrolyte in the positive and negative electrode chamber to improve the charge storage capacity of the positive and negative electrodes. we assembled Co（OH）₂|KOH+K₃Fe（CN）₆| KOH+PPD|AC new asymmetric supercapacitor, whose energy density can reach 72.6Wh/kg, is 2.9 times higher of in KOH electrolyte（25Wh/kg）.Soadding K₃Fe（CN）₆and PPD to electrolyte can improve energy density. But in the cycle stability test, the residual energy density after 5000 s charge and discharge is only 44.1% of the initial energy density.A large decrease in energy density may be related to the permeability of the membrane and the oxidation of PPD in air. In view of this phenomenon, Fe（CN）₆^4-is not through the membrane, and can achieve redox reaction in the negative electrode, we replaced PPD with K₄Fe（CN）₆, and assembled into CO（OH）₂|KOH+K₃Fe（CN）₆| KOH+ K₄Fe（CN）₆|AC asymmetric supercapacitor. After 5000 s charge/discharge,its energy density remains74.2%,whose stability is significantly improved. And its energy density is54.6Wh/kg,which is higher than in KOH and lower than in PPD.The result shows different electrolyte systems have their advantages and disadvantages. In short, adding redox-actived meditors is an effective means to improve energy density.In addition, we can also improve the energy density of supercapacitor by improving the electrode material. In the research of electrode materials, researchers have put forward different methods to improve the electrochemical performance of electrode materials, such as composite electrode materials, surface modification of substrate materials and so on. According to the above ideas. We did the following experiment to improve capacitance and cycle stability of the above cobalt hydroxide electrode: i) by acid etching carbon paper substrate materials, we increase the specific surface area and surface wettability. Under conditions of etching 20 min, the specific capacitance of cobalt hydroxide electrode can reach 1491F/g in 4A/g,which is far higher than without etching electrode material（648F/g）; ii) The difference of the redox potential of cobalt and nickel, the influence of Al on the stability of alpha-Co（OH）₂structure and the synergistic effect of different metal elements in the deposition process,we doped Al and Ni and prepared cobalt-aluminum layered double hydroxide and cobalt-nickel-aluminium ternary hydroxide. After doping Al,the cycle stability of the electrode material has significantly improved.Its energy density can remain 90.9% in 8A/g after 1000 s charge and discharge, which is higher than that of Co（OH）₂electrode（80%）.While the specific capacitance of cobalt-nickel-aluminum ternary hydroxides has greatly improved, specific capacitance can be as high as2829F/g in 4A/g.In short, we have achieved high performance of electrode materials to provide a greater possibility for high energy density of supercapacitor.