Preparation And Study Of Symmetric Supercapacitor Based On Cobalt Hydroxide Thin Film Electrodes

With the development of economy, more and more natural resources are needed, the problem of destruction of the natural environment is getting more and more serious. This makes the earth’s rich natural resources are increasingly scarce, such as the haze, global warming and so on. Supercapacitor as a green environment-friendly energy storage devices hasattracted more and more attention.According to the positive and negative electrode materials, the supercapacitor can be divided into symmetric supercapacitor and asymmetric supercapacitor. Compared to the asymmetric supercapacitor, the production of symmetricsupercapacitor is simple, and the cost is low. And with respect to the supercapacitor of the organic system, the aqueous electrolyte supercapacitor is more green, safe and environmental friendly. But the working window of the aqueous electrolyte is lowerabout 1.2 V. Therefore, if you want to improve the energy density of the capacitorit is necessary to improve thecapacitance of the capacitor. The specific capacitance of the capacitor is derived from the electrode and the electrolyte. Bothelectrode and electrolyte can storage and release charge through the oxidation reduction reaction independently at the same time, and do not affect each other which is the most effective means to enhance the performance of chemical power source device.Amongall these electrode materials, the transition metal compounds cobalt hydroxide（Co（OH）₂） is widely studied and used, especially theα-Co（OH）₂ as the electrode material hasa higher capacitance and good reversibility. Adding an electrochemically active substance to the electrolyte can also increase the capacitance of the capacitor. Eelectrochemically active substance such as potassium ferricyanide（K₃[Fe（CN）₆]）, potassium hexacyanoferrate（K₄[Fe（CN）₆]）, p-phenylenediamine（PPD） as the representatives of the inorganic ions and neutral particles caused the interest of many scientists.The purpose of this paper isto use α-Co（OH）₂as the electrode material of thesymmetric supercapacitor,and to join electrochemically active substance in the traditional electrolyte（KOH） while use the ion exchange membrane to get negative systerm and positive systerm separated in order to form symmetric electrode and electrolyte asymmetric system to improve energy density. There has been no report about s symmetric supercapacitor used α-Co（OH）₂ as electrode meterial. It is also found that he the specific capacitance and energy density of the supercapacitor can be greatly improved by improving the concentration of the electrolyte. The experimental results based on the above ideas are as follows:First,we use electrochemical deposition method to produce α-Co（OH）₂ grown perpendicularlyon the carbon paper（CFP） witha nano flower microstructure.The mass of the α-Co（OH）₂ is about 0.96 mg, which specific capacitance can reach to 742 F/g in KOH electrolyte.The capacitance can reach to 2126 F/g, when the concentration of K₃[Fe（CN）₆] increased to 0.05 mol /L. By adding PPD into KOH,a pair of redox peaks appears in the low potential and the test potential window expanded from-0.1 0.45 V to-1.0⁰.45 V.Secondly,when using 1mol / L KOH as electrolyte,Co（OH）₂|KOH|Co（OH） 2 symmetric supercapacitor’senergy density can reach to 4.44 Wh / kg, the operating voltage window can reach to 1.5 V, and also hada good cycle life（1000 cycles, 87.6%）. And the charge storage mechanism of the supercapacitor is the oxygen reduction reaction of each electrodes which is very different from the previously reported symmetric capacitor with β- Co（OH） 2 as the electrode material, KOH as electrolyte whose storage mechanism is one electrode is pseudocapacitance type and another is double electric layer type.Thirdly,when using 1mol / L KOH + 0.05 mol / L K₃[Fe（CN）₆] and 1mol / L KOH + 0.05 mol / L K₄[Fe（CN）₆] as the positive and negative electrolyte of the capacitor, Co（OH）₂|KOH+K₃[Fe（CN）₆]|| K₄[Fe（CN）₆]+KOH|Co（OH）₂ capacitors energy density can reach to 7.75 Wh / kg, when the concentration ofredox material K₃[Fe（CN）₆] and K₄[Fe（CN）₆] were 0.35 mol / L, the capacitor capacitance is about 30.5 F/g, energy density is about 9.5 Wh / kg, corresponding to the power density of 145.7 W / kg. After 1000 cycles, the cycling stability maintained at 89.2%. It is found that the energy density of the capacitor can be improved by the addition of redox substances, and the stability of the capacitor is not affected, and the electrode and the electrolyte generate capacitance independently.Finally,When using 1 mol / L KOH+0.02 mol / L K₃[Fe（CN）₆] and 1mol / L KOH + 0.01 mol / L PPD as the positive and negative electrolyte of the capacitor, the energy density is up to to 8.31 Wh / kg.When the concentration of K₃[Fe（CN）₆] and PPD increase respectively to 0.36 and 0.18 mol / L, the energy density is 49.5 Wh / kg, corresponding to a power density of 197 W/ kg. Due to the potential difference between PPD and K₃[Fe（CN）₆],the the energy density of the capacitorincreased greatly, but due to the mutual influence between PPD the electrode and the fact that PPD will go through the exchange membrane, the stability of the capacitor is not good（1000 cycles, 40.4%）.This problem is to be solved.