Construction Of Hybrid Supercapacitors In Aquous Electrolytes And Their Performance Optimizations

Supercapacitors (SC) with attractive characteristics such as high power density, low equivalent series resistance (ESR) and permanent charge-discharge life are considered as promising energy storage devices that perfectly fill the gap between dielectric capacitors and traditional batteries. In the light of green chemistry and cost reduction, SCs based on aqueous solutions with high energy density would be a hot issue. An asymmetric configuration, where the relative electrodes are constituted from MnO2 and porous carbon, has recently been proved to be a green energy resource for lower cost, lower toxic and lower pollution. Utilizing the complement of opposite over-voltage, a MnO2-carbon based capacitor owns the stable voltage window of 2 V in aqueous system According to energy formula:E=CV2/2, increasing the capacitance will be an efficient strategy for the enhancement of energy density. The present paper focuses on the charge capabilities of supercapacitor from two different angles:the improvement of electrode materials and the optimization of hybrid structure.Firstly, a novel hybrid supercapacitor (AC-MnO2 & AC) with MnO2 & AC as positive electrode and AC as negative electrode is fabricated, which integrates approximate symmetry and asymmetry behaviors in the distinct parts of 2 V operating windows. The comparative result demonstrated dual functions of activated carbon (AC) as conductive agent and active substance of positive electrode. Thereinto, in range of 0-1.1 V, both AC in the positive and negative electrode assemble as a symmetry structure via parallel connection which offer more capacitance and less internal resistance. After 2500 cycles, maximum energy density and power density are 18.2 Wh kg-1 as 4% loss and 10.1 W kg-1 as 8% loss compared to the initial cycle.Secondly, porous carbons with high micro-pore volume were made by the phosphoric activation on rich nitric polymer. Both concentration and content of phosphoric acid strongly influence the texture structure of porous carbon. A promising capacitance of 240 F g-1 can reach using the 60% mass concentration and 1.3 (multiplying the mass of precursor) content of phosphoric. Also, the existents of nitric functional groups can improve the ionic conduction in pore structure and multiplying power.Finally, we synthesize MnO2 and discussed the relationship between electrochemical behaviors and crystal structure. Both liquid precipitation and hydro-thermal synthesis were used in this paper. According to the liquid precipitation method, on purpose of increasing the amount of 2×2 crystal channel, we raise the density of alkali metal ion in the solvent and obtained increasing pseudocapacitance activated sites successfully. Furthermore, by heat treatment under 350℃,α-MnO2 with good crystal structure were made. Cyclic voltammetry curve ofα-MnO2 can keep rectangle feature even under the scan rate of 100 mVs"1 which indicated a perfect multiplying characteristic. Within the hydro-thermal system, the MnO2 with different crystalline phase were made, and the comparative capacitance isα-MnO2>δ-MnO2 >γ-MnO2>β-MnO2.In summary, we synthesized electrode materials with high specific capacitances and good multiplying characteristics at the angle of raising pore volume and increasing pseudocapacitance activated sites. Furthermore, a novel hybrid supercapacitor (AC-MnO2 & AC) is established which explicit the role of carbon in positive electrode and offer a new platform for carbon-based composite material. In this paper, we developed an aqueous supercapacitor with high power density by the improvement of electrode material and optimization of hybrid structure.