Construction And Energy Storage Mechanism Of The Nickel Based Bimetallic Compound For High Performance Supercapacitor Electrode

Supercapacitors have been attracted extensive attention owing to their high power density,long cycle life,fast charge/discharge time and low price.At present,the main research goal of supercapacitors is to improve their energy density under the condition of ensuring high power density and meet the demand of flexible wearable electronic devices.The electrode material plays a key role in influencing the performance of supercapacitors.Nickel based bimetallic compound electrode material has been widely studied due to its high theoretical specific capacitance,excellent electrochemical activity and low cost.In addition,compared with the corresponding monometallic compound,the nickel based bimetallic compound possesses richer redox states,which could present excellent electrochemical performance benefiting from the synergistic effect between different metal ions.However,the electrical conductivity of single nickel based bimetallic compound is lower than carbon material.Meanwhile,the structure of electrode material is easy to agglomerate in the preparation process,which could result in the few electroactive sites and low utilization rate of electroactive material.Thus,it is difficult to realize the high theoretical capacitance in the actual test.In addition,the structure of nickel based bimetallic compound is easy to collapse in the large current and repeated charge/discharge processes,which results in the poor rate property and cycle performance.Therefore,single nickel based bimetallic compound is inadequate to fulfil the actual requirement for supercapacitors with high energy density and power density.In this paper,three modification strategies are put forward to enhance the electrical conductivity and structural stability of nickel based bimetallic compound electrode material:1)Compounding the nickel based bimetallic compound with carbon materials.2)Constructing the electrode material with self-supporting hierarchical core-shell nanoarchitectures.3)Doping foreign metal ions into the nickel based bimetallic compound.Then the inherent mechanism with improved performance of different modification methods is deeply analyzed.This research would provide the practical experimental scheme for the modification study of nickel based bimetallic compound,and offer research basis for its practical application in supercapacitors.The primary contents of this paper are as follows:（1）The agglomeration problem of nickel based bimetallic compound is resolved by the modification measure for compounding nickel based bimetallic compound with carbon material.The three-dimensional Ni Co₂O₄ nanospheres and two-dimensional nitrogen doped graphene（NG）nanosheets have been compounded by the solvothermal method,and the electrochemical performance of single Ni Co₂O₄,compound Ni Co₂O₄/NG and Ni Co₂O₄/G electrodes is compared.The Ni Co₂O₄ nanospheres with high theoretical specific capacitance could occur richer redox reactions.Nitrogen element provides more carriers for graphene and improves the wettability of graphene.NG nanosheets could improve efficiently the agglomeration problem of Ni Co₂O₄ nanospheres,which further enhances the utilization rate of electrode material and endows composite electrode with more excellent conductivity.Therefore,the combination of three-dimensional Ni Co₂O₄nanospheres and two-dimensional NG nanosheets could play a great synergistic role.The specific capacitance of Ni Co₂O₄/NG composite electrode is 1.90 and 1.35 times higher than those of Ni Co₂O₄ and Ni Co₂O₄/G electrodes,respectively,and the assembled asymmetric supercapacitors also achieve excellent energy density（30.56 Wh/kg）and power density（9750.46 W/kg）.（2）The volume expansion problem of nickel based bimetallic compound is improved via constructing the composite material（nickel based bimetallic compound@carbon）with abundantly porous structure.The hierarchical core-shell nanocomposite with hollow mesoporous carbon nanospheres（HMCS）as template enwrapped by prussian blue analogs（PBA）-derived Ni-Fe-P nanocubes（Ni-Fe-P@HMCS）has been successfully constructed through the coprecipitation and phosphorization methods.The Ni-Fe-P material with typical metal character could present the excellent electrochemical activity.Meanwhile,the addition of HMCS can effectively improve the dispersibility of metal compounds and endow composite electrode with superior conductivity.Most importantly,the composite material with porous structure could efficiently relieve the volume swell caused by the electrochemical reaction stress in the large current and multiple charge/discharge process.So,the cyclic stability of Ni-Fe-P@HMCS electrode exceeds 100%after 5000 cycles,and assembled Ni-Fe-P@HMCS//HMCS hybrid supercapacitor realizes the high energy density and excellent flexibility.（3）The structural stability of nickel based bimetallic compound is improved by constructing self-supporting hierarchical core-shell electrode materials.The novel hierarchical core-shell nanocolumn array with Ni Co₂S₄ hollow nanowires as core and Ni Co P nanosheets as shell,has been directly synthesized on nickel foam（Ni Co₂S₄@Ni Co P/NF）via hydrothermal,vulcanized and phosphatized processes,and the electrochemical properties of Ni Co₂S₄@Ni Co P/NF,single Ni Co₂S₄/NF,and Ni Co P/NF electrodes are comparatively studied.The Ni Co₂S₄@Ni Co P/NF electrode with hierarchical core-shell structure possesses excellent structural stability and large specific surface area,so can provide rich electroactive sites and realize adequate electrochemical reaction.The specific capacitance of Ni Co₂S₄@Ni Co P/NF electrode is 3.50 and 1.72 times higher than those of Ni Co₂S₄/NF and Ni Co P/NF electrodes.Meanwhile,the Ni Co₂S₄@Ni Co P/NF electrode realizes the 70.29%of rate capability and 92.94%of cyclic stability.（4）The conductivity of nickel based bimetallic compound is improved through the modification measure of metal ion doping.A series of Fe doped Ni Co P hierarchical nanoarrays with different doping concentrations on nickel foam（Fe-Ni Co P/NF-x%,x=0,4,6.25,12.5 and 25）have been synthesized by hydrothermal and phosphatized methods.The structures,particle sizes and electrochemical properties of electrode materials with different doping concentrations are comparatively investigated.The theoretical calculation result indicates that the conductivity of electrode material enhances after Fe doped.Meanwhile,the structure of electrode materials gets stable and the particle size decreases with the increase of doping concentration.Thus,the specific capacitance of optimal Fe-Ni Co P/NF-12.5%is 2.27 times higher than that of undoped Ni Co P/NF electrode,and the cyclic stability of Fe-Ni Co P/NF-12.5%is 95.72%.