Fabrication Of Cobalt-nickel Based Metal Oxide Nanoarrays And Their Applications In Supercapacitors

As a new type of green energy storage devices,supercapacitors（also known as electrochemical capacitors）,have attracted much attention because of their advantages such as fast charging and discharging,long cycle life,high power density and high reliability.The electrode material has a crucial influence on the energy storage performance of supercapacitors.Therefore,the research on improving electrochemical performances of the electrode materials is the key point in the field of supercapacitors.Because of the high specific capacities and good electrochemical activities,transition metal compounds have great potential for development and research.Both the electrochemical activities of materials and charge transport channels in electrodes are very crucial to achieving high performance supercapacitor.For conventional electrodes,the addition of polymeric binder and conductive agent can result in complex manufacturing process,and the disorder of the electrode structure greatly limits the charge transport efficiency in the energy storage process.In view of the above problems,in this paper,we take cobalt nickel based metal oxides as the research objects,and a variety of cobalt nickel based metal oxides（including binary metal oxides）were grown directly on current collectors to achieve highly porous and self-supporting array structures.We systematically studied the effects of different preparation parameters on cobalt nickel oxide properties and structure,and thus affect the electrochemical properties of the electrodes.And the main contents are as follows:The preparation and electrochemical properties of Co₃O₄ were systematically studied,and Co₃O₄ nanowires were synthesized on Ni foam by a hydrothermal method.By changing the experimental parameters such as precursor concentration,reaction temperature and annealing temperature,the structures of electrode materials were optimized,and Co₃O₄ nanowire arrays with good arrangement and high porosity were obtained.The results show that the concentration of precursor has an important effect on the density of the nanowire arrays,and thus affects the charge transfer efficiency in electrode.High concentration can lead to serious agglomeration and adhesion phenomenon of nanowires,but when the concentration is too low,nanowires cannot achieve good self-support.The reaction temperature has an important effect on the crystal structure of the material.The product obtained at the reaction temperature of 90 °C is CoO,and with the increase of the reaction temperature,the electrode material changes to Co₃O₄.The annealing temperature has a great influence on the crystallinities and morphologies of the electrode materials.With the increase of the annealing temperature,the structure of the electrode material changes from the nanowire to nanosheet.The obtained Co₃O₄ nanowire arrays can deliver a specific capacitance of 842 F/g at a current density of 1 A/g,and the capacitance retention is about 70.3% when the current density increases 15 times,implying good rate capability.Moreover,the capacitance retention is about 81.4% after 5000 cycling tests.Binary metal oxides nanomaterials were also prepared as electrode materials,including CoMoO4 nanosheet arrays,NiCo₂O₄ nanowire and nanosheet arrays,and ultrathin NiMoO4 nanosheets.The morphologies and electrochemical properties of CoMoO4 nanosheet arrays were optimized by adjusting the concentration of precursor,reaction temperature and surfactant.The obtained electrode can deliver a specific capacitance of 912 F/g at a current density of 1 A/g,and a capacitance retention of 74.3% at 15 A/g,which is higher than Co₃O₄ electrode.For NiCo₂O₄ materials,two types of nanoarrays（nanowire arrays and nanosheet arrays）were prepared by changing the surfactant,and the charge transfer difference between the two electrodes was investigated.Results show that both NiCo₂O₄ NWAs and NiCo₂O₄ NSAs electrodes exhibit good electrochemical properties,and obtained high specific capacitances of 898 and 923 F/g,and areal capacitances of 1.62 and 1.47 F/cm2 at 2 mA/cm2,respectively.The two electrodes also show good rate capabilities of 76.8% and 85.7% at 15 A/g,and cycling stabilities of 83.4% and 88.5% after 5000 cycles,respectively.The uninterrupted network formed by tight interconnection between NiCo₂O₄ nanosheets is beneficial to electron transfer and structure stability of electrode,thus exhibiting better electrochemical performance.Additionally,ultrathin NiMoO4 porous nanosheets were synthesized,while the rate capability is just 60.7% due to the poor arrangement of nanostructure and need further improvement.Improving the utilization efficiency of the internal space in the electrode is an effective way to boost the electrochemical performance of electrode materials.In this work,we designed and fabricated three kinds of core-shell structure: Co₃O₄-NWA@NiMoO4,NiCo₂O₄-NWA@NiMoO4,and NiCo₂O₄-NSA@NiMoO4 hybrid electrodes,in which the pristine nanoarrays serve as highway for electron transfer.The uniform coating of ultrathin NiMoO4 nanosheets can improve the utilization efficiency without sacrificing the porosity of electrode,thus greatly enhancing the electrochemical properties of electrodes.At a current density of 2 mA/cm2,NiCo₂O₄-NSA@NiMoO4 hybrid electrodes achieve a high specific capacitance of 1941 F/g and areal capacitance of 7.29 F/cm2,which can still maintain 84.1% at 60 mA/cm2,exhibiting remarkable rate capability.A type of asymmetric supercapacitors based on NiCo₂O₄-UNSA@NiMoO4 electrodes shows a maximum energy density of 52.6 Wh/kg,suggesting its great potential for energy storage applications.Besides,this research would give a general reference for the rational design of other active materials for high-performance supercapacitors.