| Supercapacitor,with ultra-high power density,long cycle life and excellent rate performance,is considered as a new type of electrochemical energy storage device.It has broad application prospects in hybrid electric car and the portable electronic products.However,the relatively low energy density limits the large-scale application of supercapacitor.Therefore,improving the energy density of the device is the main challenge in the field of supercapacitor research.According to the energy density formula E = 1/2 CV2,there are two effective ways to enhance the energy density,one is to increase the specific capacitance of the device C,and the other is to broaden the voltage window of the device V.In view of the above problems,in this thesis,by increasing the specific surface area and electrical conductivity of the electrode material,adjusting its morphology and compound optimization,the specific capacity was improved.In addition,symmetrical supercapacitors,asymmetric supercapacitors,and lithium-ion supercapacitors were engaged to increase the voltage window.Furthermore,the energy storage mechanism were studied by chemical kinetic quantitative analysis and first principle analysis.The main research works are as follows:1.A series of single-crystal GaN mesoporous membranes(GaNMM)were fabricated and then applied in supercapacitors for the first time,furthermore,the storage mechanism of the GaN based pseudocapacitor was proposed.The GaNMM materials were prepared by a two-step electrochemical etching method,and the single crystal quality,specific surface area,structural stability and electrical conductivity of the electrode materials were investigated.And the GaNMM-based symmetric supercapacitor exhibited an ultra-high power density(45 mW cm-2,1800 mW cm-3)and an ultra-long cycle life(96%capacitance retention after 50000 cycles).In addition,the energy storage mechanism of the gallium oxynitride layer was proposed:GaN1-xOx +2y H+ +2y e-(?)GaN1-x-yOx-y+y H2O.GaN crystal-based supercapacitors provide motivation for the application of the crystal family in the field of electrochemical energy storage.2.GaN/GP flexible electrodes were obtained by growing GaN nanowires on graphite paper substrates,and explored the application of this material in flexible supercapacitors,the storage mechanism of the gallium oxynitride were investigated by elelctrochemical test and XPS characterization.The GaN NWs were grown on the GP by CVD methods to increase the electrical conductivity of the GaN material while increasing the specific surface area of the GaN material.The electrode exhibited excellent specific capacitance(237 mF cm-2)and outstanding cycling performance.The flexible symmetric supercapacitor also manifested high energy and power densities(0.30 mW h cm-3 and 1,000 mW cm-3).This makes it possible for GaN-based materials to be used in the field of portable electronics.3.The GaN/MnO2/MnON sandwich structure was designed and prepared to further improve the specific capacity of GaN-based materials.And the mechanism of electrochemical performance improvement was explored by chemical kinetic quantitative analysis.The unique hierarchical structure of the material not only provided a highly conductive network,but also will overcame the volume change during charge/discharge processes.Therefore,the obtained GaN/MnO2/MnON nanostructure showed excellent electrochemical performance with a specific capacity up to 2021 mF cm-2.Furthermore,the assembled flexible SC device exhibited exceptional cycling stability(95.5%capacity retention after 10,000 cycles)and also provided an excellent energy density of 0.76 mW h cm-3.In addition,the chemical kinetic analysis proved that the MnON layer had a contribution of pesoducapacitance,and proposed a reaction mechanism:MnO2-xNx +v K+y e(?)MnO2-xNxKy.The electrode design concept can be applied in other metal oxides electrode in energy storage device industries.4.To address the crucial issue of volume expansion of metal oxides,a transition-metal oxynitride layer(TMON,M:Fe,Co,Ni and V)was synthesised on TMO nanowires.FESEM,HRTEM and XPS were used to characterise the TMONs with oxides cores and the corresponding oxynitride shells.The unique oxynitride layer possessed numerous active sites,excellent conductivity and outstanding stability.Specifically,the specific capacity of the TMON electrode(CoON,198 mAh g-1was enhanced by approximately 2-fold relative to that of its corresponding oxide(Co3O4,95 mAh g-1),and the capacitance remained above 94.6%even after 10,000 cycles.First-principles analyses were performed to investigate the mechanism underlying the improved electrochemical performances.Furthermore,the asymmetric supercapacitor assembled by 3D NiON exhibited higher voltage window(1.6 V),and excellent energy density(25.75 mWh cm-2).5.To further improve the energy density of the material,we designed and prepared a bimetallic oxynitride material(NiCoON),and a nitrogen-doped carbon nanowires material(NHC),and assembled these two self-supporting electrodes with a lithium ion supercapacitor device(LICs).NiCo2O4/NiCoON nanowire composites were prepared on the surface of carbon fibers by hydrothermal and nitriding methods.Polypyrrole-derived nitrogen-doped carbon nanowire materials were prepared on carbon cloth using electrochemical deposition and high-temperature carbonization.and these two materials showed excellent elelctrochemical properties.The voltage window of the LICs was up to 4.0 V.the capacity faded only 9.5%after 10000 cycles,and its energy density was as high as 96.2 Wh kg-1.The structural design and assembly strategy of the positive and negative electrodes in the LICs provide a new choice for the fabrication of high-energy storage devices. |
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