| The need for developing energy storage devices with fast charge-discharge rate,long cycle life,and high energy and power densities has motivated tremendous effort in the research of supercapacitors.It has attracted tremendous attention for applications,such as energy back-up systems,portable devices,consumer electronics,hybrid electric vehicles,and biomedical devices.However,the energy density of supercapacitors is still unsatisfactory,especially compared with Li-ion batteries.The energy density is related to the specific capacitance and cell potential.Thus,it is worthwhile to design and fabricate electrodes with high specific capacitance and wide voltage window.Herein,transition metal based compounds were selected as the electrodes due to their high theoretical capacitance.Moreover,the transition metal based compounds were also combined with carbon materials to further improve the specific area,electrical conductivity and cyclic stability,which are important for enhancing the electrochemical performance for supercapacitor materials.The main researches are as follows:(1)The rod-like VSB-5(Ni20[(OH)12(H2O)6][(HPO4)8(PO4)4]·12H2O)with hierarchical structure was obtained by adjusting the adding amount of aqueous ammonia via a hydrothermal method with adding additional template agents.Simultaneously,we proposed a formation mechanism that VSB-5 was transformed from Ni3(PO4)2·8H2O.The diameters of VSB-5 rods are controlled by the morphology of Ni3(PO4)2·8H2O:the diameters of the VSB-5 rods transformed from the parallelepiped Ni3(PO4)2·8H2O are larger than that transformed from amorphous Ni3(PO4)2·8H2O.More importantly,a confirmatory experiment was designed to examine the proposed speculation.During the electrochemical performance tests,VSB-5 was proved exhibiting the most excellent performance among all the obtained nickel phosphate compounds.The rod-like VSB-5 exhibits a high specific capacitance of 1542 F g-1 at a current density of 0.5 A g-1.To fabricate the supercapacitor,rod-like VSB-5 and reduced graphene oxide hydrogel(r GH)were used as the positive and negative electrodes respepctively,which delivers a maximum energy density of 42.4 Wh kg-1.The formation mechanism of VSB-5 was further studied by adjusting the feed ratio of nickel and phosphorus sources and controlling the reaction time.With the increasing of the reaction time,the products were transformed form flower-like NH4Ni PO4·6H2O to trigonal prism Ni3(PO4)2·8H2O,and finally formed the rod-like VSB-5.The capacitance of VSB-5 could reach 1422.1 F g-1,which is higher than that of NH4Ni PO4·6H2O and Ni3(PO4)2·8H2O.(2)We present a simple one-step hydrothermal method to obtain the carbon and nitrogen co-doped nickel hydroxide/nickel sulfide(C/N-Ni(OH)2/NixSy)with sandwich structure by using egg white as precursor.Due to the abundant elements(C,N,and S)and strong ability to associate with metal ions,egg white can not only act as carbon and nitrogen sources for doping,but also offer sulfur source for NixSy.Taking advantages of C/N co-doping and sandwich structure,the as-prepared C/N-Ni(OH)2/NixSy exhibits high capacitance of 1731.2 F g-1 at 0.5A g-1.Besides,the fabricated hybrid supercapacitor C/N-Ni(OH)2/NixSy//reduced graphene oxide hydrogel shows a remarkably energy density(38.98 Wh kg-1)and an excellent cycling stability(10000 cycles,107.6%).Replacing ammonia aqueous with thiourea in the preparation of C/N-Ni(OH)2/NixSy to study the influence of precipitant on the composition,structure and properties of the products.With the presence of thiourea,C/N-Ni S was obtained,which was composed of two crystalline forms of Ni S.The two crystal structures of Ni S show obvious differences in particle size,which are micron and nano-scale respectively.The electrochemical tests show that the maximum capacitance of C/N-Ni S electrode is 570.3 F g-1,and it is more suitable for low current density applications.The maximum energy density of C/N-Ni S//rGH is 14.8 Wh kg-1,and the corresponding power density is 716.8 W kg-1.(3)The flexible CNT film/double side carbon tape/CNT film(CDC)with macro sandwich structure was designed as substrate,and the positive electrode CDC@Mn O2 was obtained with a simple one-step water bath method.Due to the special sandwich structure,the mass loading of Mn O2 could reach 6.6 mg cm-2.The smallest structural unit of Mn O2 is ultrathin nanosheets.Notablely,these ultrathin nanosheets would aggregate into three forms.They are the dense coverages of Mn O2 nanosheets on the surfaces of CNTs and double side carbon tape(DSC),and Mn O2 nanoflowers.The specific capacitance of CDC@Mn O2 could reach 1.1 F cm-2 at a current density of 1 m A cm-2 with a wide voltage window of 1.2 V.Inspired by the superior electrochemical performance of the CDC@Mn O2 positive electrode,a CDC@Fe2O3 film with Fe2O3 mass loading of 5.0 mg cm-2 was fabricated as the negative electrode.Two kinds of morphology of Fe2O3 was observed:microspheres and dense layer.The CDC@Fe2O3 electrode delievers a high area capacitance of 339.1 m F cm-2.Then,a flexible supercapacitor was successfully assembled with CDC@Mn O2 and CDC@Fe2O3 as positive and negative electrodes,respectively.This flexible supercapacitor possesses a large voltage window of 2.1V,a high volume capacitance of 6.8 F cm-3,and a high energy density of 4.1 mWh cm-3 at the power density of 22.3 mW cm-3. |
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