Investigation Of The Supercapacitor Properties Of Cobalt Oxide/3D Graphene Composite Materials

With the rapid development of economy, the rising of energy consumption,especially the depletion of the non-renewable fossil fuels, has brought serious issuesof society and environment. Development of new electrochemical energy storagedevices with high energy density, high power density and long cycle life hasbecoming the urgent project to be solved, which also get more and more attention ofresearchers.Among the new green energy storage devices, Supercapacitor has get thefavour of many researchers due to their advantages, such as fast charging anddischarging, high power, environmental friendly and long-term cyclingstability.However, the low energy density of the supercapacitor would bring thedisadvantages in quality and volume and limit their application fields, such asemerging intelligent electronic equipment, transportation, military equipment andweapons (such as tanks and missile), fieldwork, detrimental air vehicle, etc.How toimprove the energy density, long-term cycling stability and power density and lowerproduction cost of the novel energy storage device, has become highlight projects tobe overcome.As known, the features of the supercapacitor are larger the power density, longercycle stability and low energy density. Therefore, the key of solving this problem is todevelop the energy density with no loss of power density.Based on the formula ofenergy density (E=12CV2), there are two routes to improve the energy density:improving the specific capacitance and broadening the working potential window.Thekey of improving the specific capacitance and broadening the potential window aredeveloping the electrode material and electrolyte, respectively.Considering with ourexperimental conditions and background, we choose the development of the specificcapacitance to be the key route, and research the Co3O4electrode system in this thesis.Cobalt oxide is a very competitive electrode material because of its low cost andhigh theoretical specific capacitance (3560F/g). However, the most of experimentalspecific capacitance is lower than1000F/g as report. Graphene is a carbon material with high electrical conductivity, good chemical stability, excellent mechanicalstrength and large specific surface area. As reported, the combination of transitionmetal oxide and graphene would improve their properties.In this thesis, in order to improve the electrochemical properties (specificcapacitance and long-term cycling stability) of Co3O4-based supercapacitor,3Dgraphene nanosheets were produced by plasma-enhanced chemical vapor deposition(PECVD), and used as substrate for combination of Co3O4. Improving the electrolytecan be used as another route to increase the specific capacitance. The main contents ofthis thesis are summarized as following:1. The cobalt oxide/nickel foam (Co3O4/Ni) electrode material prepared byhydrothermal-thermal decomposition method shows high specific capacitance, andlong-term cycling stability under high current density.When the current density were5Ag-1、10Ag-1、20Ag-1and40Ag-1，the specific capacitance were1373.8F/g、1162.9F/g、899.5F/g�'�675.5F/g. With the current density of20Ag-1and charge anddischarge cycle of1000times, specific capacitance is still maintain68.7%of theinitial specific capacitance, which shows good supercapacitor performance.2. Although the cobalt oxide/nickel foam (Co3O4/Ni form) electrode materialshows good electrochemical performance, their narrow working potential windowlimit the high energy density.Based on the idea of electrolyte contribution to thespecific capacitance， we choose the re-oxidation materials into the potassiumhydroxide electrolyte. The electrode materials and electrolyte both make acontribution to specific capacitance.After the test, specific capacitance can reachabove7000F/g in the solution of1M KOH with0.08M K3Fe(CN)6.After the1000constant current charge-discharge test, specific capacitance can maintain82.1%of theinitial specific capacitance in1m KOH electrolyte with0.04K3Fe(CN)6and currentdensity of40A/g. It indicates the new supercapacitor shows high current density andhigh cycle stability.3. Although the theoretical value of the specific capacitance of Co3O4electrodematerials is very high, the experimental data is very difficult to get high specific capacitance. How to improve the specific capacitance of Co3O4is still a hot topic. Wefirstly prepared the cobalt oxide/graphene nanosheets (GNS) composite electrodematerials by the combination of plasma enhanced chemical vapor deposition (PECVD)technique and hydrothermal-thermal decomposition method.The experimental resultindicates that the specific capacitance is more than2400F/g. with the test of20A/gcurrent density and constant current charge and discharge1000times, the specificcapacitance is maintain78%of the initial specific capacitance and shows goodelectrochemical performance.4、Considering the above work, we designed a new system of constraintcomposite electrode capacitance(Co3O4/GNS-KOH/K3Fe(CN)6) to improve thespecific capacitance. The experimental data reveals that the supercapacitor systemshows high specific capacitance, above10000F/g, in1M KOH solution with0.08MK3Fe (CN)6.