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Production Of Nickel Hydroxide-based Composite Electrode Materials And Study Of Their Electrochemical Properties

Posted on:2016-04-14Degree:MasterType:Thesis
Country:ChinaCandidate:J Y LiuFull Text:PDF
GTID:2181330467999983Subject:Materials Physics and Chemistry
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Development of green energy and seriously environmental problems are makingsupercapacitor, a new type of energy conversion and storage device, attract more andmore attention. Its capacity of charge storage is much higher than that of physicalcapacitor, and charge-discharge rate and efficiency better than that of primary orsecondary battery. Modern scientists and engineers have focused on the research anddevelopment of supercapacitors with both high energy density and power density.Electrode materials, electrolytes and separators are the key factors to the highperformace of supercapacitors.Pseudocapacitance, charge storage and release achieved by highly reversibleredox reaction on the surface electrode, compared with electrochemical double-layercapacitance, exhibits higher energy density and power density of pseudocapacitor.Pseudocapacitive metal oxide/metal hydroxide and conducting polymer, especially theoptimal electrode material RuO2are the used for pseudocapacitors. Recent years, dueto the demand of lightweight, low cost and high performance of energy storage devices,especially the fast development of portable electronic device and electric vehicles, anintensive and world-wide research on pseuodocapacitive metal oxide/metal hydroxidehas been carried out. Among them, Ni(OH)2is an attractive material in view of its hightheoretical specific capacitance, well-defined redox transitions and low cost. However,poor electronic conductivity and low cycling stability during charge-discharge are thetwo major barriers for its broad applications. Graphene, having high conductivity,thermal conductivity, and excellent chemical stability, has been found an ideal carbonelectrode material for supercapacitors with an ideal capacitance value of up to500F/gwhen the entire specific surface area is fully used (~3000m2/g). Combined withgraphene, electronic active Ni(OH)2electrode material could achieve high performanceof supercapacitor due to their synergistic effect. What’s more, we can also improve thestability as well as increase the utilization, reduce the dosage and enhance the specificcapacitance of the single active hydroxide electrode material by adding other kinds ofhydroxides/oxides. So it’s helpful to establish a hybrid electrode system that makes the electrode material itself keep high capacitance performance and show good stability atthe same time.Ni foam-supported (NF) horizontally aligned graphenes (HG) and verticallyaligned graphenes (VG, also called carbon nanowalls) were fabricated by plasmaenhanced chemical vapor deposition (PECVD) method and that have been used assubstrates for electrodeposition of Ni(OH)2nanosheets (marked as Ni(OH)2/H-GNs/NFand Ni(OH)2/V-GNs/NF, respectively). Although the Ni(OH)2/H-GNs/NF exhibitsexcellent cycling stability, its specific capacitance is less than2000F/g. While thespecific capacitance of the Ni(OH)2/V-GNs/Ni is up to2539F/g (at the current densityof23.1A/g), but it exhibits poor cycling stability. Therefore, this work choseNi(OH)2/VG/NF for investigation to obtain electrode materials with both highelectrochemical properties and cycling stability. The concrete content of this study wassummarized as follows:Firstly, vertically aligned grapheme nanosheets have been synthesized usingplasma enhanced chemical vapor deposition (PECVD) on Ni foam (the deposition timeis30min) and that have been used as substrate for electrodeposition of Ni(OH)2nanosheets. We try to vary the time of electrodeposition to find out the optimum massratio of Ni(OH)2and carbon nanowalls. It turned out that when the electrochemicaltime is set at12min (the quality of the Ni(OH)2is about0.4mg), the active material isβ-Ni(OH)2. Its specific capacitance is about1534F/g and the electrode maintains46%value of the initial specific capacitance after2000cycles at a current density of23.1A/g.Secondly, we synthesized cobalt-doped composite electrode on the basis of theabove experiments and a degassing pretreatment of VG/NF was processed before theelectrodeposition. The results showed that (1) when the mole ratio of Co2+/Ni2+comesto5%, the specific capacitance reaches the maximum,2905F/g and the electrodemaintains69%value of the initial specific capacitance after2000cycles at a currentdensity of23.1A/g;(2) after degassing pretreatment of carbon nanowalls, the specificcapacitance of nanocomposite is relatively low at first, but after activation in theprocess of electrochemical charging and discharging, both the specific capacitance and cycling stability improved largely.
Keywords/Search Tags:Supercapacitor, Plasma enhanced chemical vapor deposition (PECVD), Verticaloriented graphene (carbon nanowalls), Ni(OH)2, Co(OH)2, Pretreatment
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