Research On Asymmetric Supercapacitor Based On Nanostructures

Supercapacitors or electrochemical capacitors have attracted much attentionbecause they can instantaneously provide higher power density than batteries, andhigher energy density than conventional dielectric capacitors. However, SCs otfensuffer from a lower energy density (normally <10Wh k"1g) than batteries, whichlimits its application. So, it is necessary to develop higher energy density withoutsacrificing the power density and cycle life. In this connection, a promising way toincrease energy density is to develop asymmetric supercapacitors, which consist of abattery-type Faradaic electrode as energy source and a capacitive electrode as a powersource. And asymmetric systems have been extensively explored by combiningcomplementary potential windows of the positive electrode and negative electrode toincrease the operation voltage, resulting in a notable improvement of the energydensity. Recently, the core-shell nanowires heterostructures with large electroactivesurface area and a short path for charge carriers are especially attractive in energyapplications. Therefore, in this paper, we designed and fabricated core-shellheterostructure electrode, and assembled asymmetric supercapacitor, conductingresearch and exploration.Firstly, porous nickel foam was used as substrate to prepare ultrathin Ni(OH)2nanowalls by a simple hydrothermal. We also have designed a series of controlexperiment. And the temperature and time are parameters. The morphologies ofelectrode mateirals were characterized by SHM. And the cyclic voltammetry (CV),galvanostatic charge-discharge (GCD) were conducted to characterize theperformance of electrode materials. The comparison and analysis of the results of theelectrochemical capacitor properties were studied to investigate the reactiontemperature and time on ultrathin Ni(OH)2nano-wall its impact. As expeirmentalresults shown, with the increase of reaction time, loading mass of the active materialalso increased, its morphology basically remain unchanged. The reaction time was10h, the temperature at100°C, the Ni(OH)2electrode achieved the specific capacitance of1624F g-1at2A g-1of current density. While the reaction temperature was180℃, the morphology of Ni(OH)2was nanoparticles with200nm size, rather than nano-wall. And the performance of Ni(OH)2electrode decreased significantly, with the specific capacitance of240F g-1. In short, compared with the reaction time, the effect of temperature on the morphology and performance is more significant.Secondly, CO3O4nanowires were directly grown on porous nickel foam by a facile hydrothermal method. In order to improve the electrochemical activity and conductivity of CO3O4nanowires, CO3O4nanowires have been hydrogenated by annealing in a hydrogen atmosphere. Through controlling the flow rate of hydrogen, experimental results show that when the flow rate of hydrogen is7.5sccm, the porous hydrogenated CoOx (denoted as H-CoOx) nanowires obtained, compared with the CO3O4nanowires, the specific capacitance increased by2times (816F g-1, at the scan rate of10mV s-1). Moreover, the electrical conductivity was improved (the equivalent series resistance was0.48Ω). Subsequently, H-CoOx was used as the core, supporting ultrathin Ni(OH)2by hydrothermal to obtain H-CoOx@Ni(OH)2core-shell heterogeneous nanomaterial. At2A g-1of current density, the core-shell nanowires electrode achieved the specific capacitance of2196F g-1. The electrode exhibits good stability with94%retention of the initial capacitance after2000cycles.On the negative electrode, the RGO@Fe3O4nanocomposites were prepared by a simple in situ solvothermal method using graphere oxide and FeCl3and FeCl2as the precursor. The structural morphologies and performance of RGO@Fe3O4were characterized by SEM, electrochemical measurements. The experimental results show that the specific capacitance of RGO@Fe3O4was316F g-1at1A g-1of current density. Compared with traditional carbon materials with about200F g-1in the aqueous electrolyte, this result was improved obviously.Finally, an asymmetric supercapacitor device based on H-CoOx@Ni(OH)2core-shell NWs as positive electrode and reduced graphene oxide (RGO) encapsulated Fe3O4nanocomposites as negative electrode was successfully fabricated. The cyclic voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy measurements were conducted. The results show, H-CoOx@Ni(OH)2//RGO@Fe3O4asymmetric supercapacitor achieved high energy density (～45.3Wh kg-1at1010W kg-1), high power density (～7080W kg-1at23.4Wh kg-1). After5000galvanostatic charge-discharge test, it retained91.8%of the initial capacitance. What’more, we have done a demonstration of the asymmetric supercapacitor. After charging for～1min, one such22cm2asymmetric supercapacitors device demonstrated to be able to drive a small windmill (0.8V,0.1W) for20min, which show a broad application prospect.