Preparation Of Ni-based Hydroxides Based On Metal-organic Framework And Study Of Their Electrochemical Performance

Benefited from high theoretical specific capacity and low cost,Nickel-based hydroxides are rated as a most promising option for supercapacitive material.Nevertheless,unitary nickel hydroxide fails to meet the needs of high-performance energy storage devices in terms of its poor conductivity,humdrum morphology and low electrochemical performance.Introducing other metal elements such as zinc,aluminum,chromium,manganese and cobalt into the Ni(OH)₂ lattice to form a doped nickel-based hydroxide can effectively improve the intrinsic characteristics,thus improving the electrochemical performance.In addition,synergistic effect of various components is also beneficial for further enhancing the electrochemical property during the electrochemical reaction.Besides,developing rational morphology and structure can exert an important effect for enhancing the electrochemical performance of electrode materials.However,it is difficult to synthesize ideal morphology by traditional synthesis methods such as hydrothermal synthesis.Hence,it is extraordinary demand for constructing rational and ideal structure which can promotes the electrochemical property.Metal-organic frameworks(MOFs)can both serve as sacrificial templates and provide metal ions when used to prepare electrode materials due to their low density,large specific surface area and high porosity.Meanwhile,electrode materials via MOFs as templates generally own high specific surface area and adjustable morphology,endowing them with enhanced electrochemical properties.Therefore,utilizing MOFs as templates to fabricate electrode materials exhibits a very broad application prospect in the field of supercapacitors.As is stated above,this paper mainly utilizes MOFs as templates to prepare nickel-based hydroxides with tailored components and rational structure,and makes fully use of their large specific surface area,porosity and the synergistic effects of various components to promote the energy storage capacity of the devices.Via MOFs as templates was used to perform the controllable adjustment of the rational structure of elecrtode material,and the corresponding phase characterization was carried out.In the meantime,the electrochemical properties of electrode material were also discussed.The main research work is shown as follows:1.Hollow porous nickel-cobalt-manganese hydroxide(NiCoMn-OH)polyhedra assembled from ultra-thin nanosheets were fabricated via a simple,environmentally friendly and low-temperature hydrothermal reaction,utilizing ZIF-67 as templates.During the hydrothermal reaction,utilizing the mixed solvent of DMF/ethanol/deionized water combined with the addition of chloride and nitrate can well control the rate of etching ZIF-67 template and co-precipitation of metal ions,so as to prepare high-quality materials with hollow polyhedral structure.The obtained NiCoMn-OH polyhedra possess multi-order hollow structure assembled with ultrathin nanosheets,and the shell of the hollow polyhedral structure has a large number of internal nanopores,which provides abundant active sites and also facilitates the rapid diffusion of ions.The specific surface area of NiCoMn-OH polyhedra is as high as 264.24 m² g^-1 because of their excellent morphology and structure,which guarantees sufficient contact between the active material and the electrolyte in the electrochemical reaction.In addition,the doping of cobalt and manganese into nickel hydroxide not only effectively improve the intrinsic properties of hydroxide,but also enhance its electrochemical properties through strong synergistic effects between components.Given this,the NiCoMn-OH electrode exhibits the excellent performance,featured with excellent capacitance of 661.8 C g^-1 at 1 A g^-1,superior rate performance and high cycling stability.Moreover,NiCoMn-OH//AC also displays a superior energy density of 43.2 Wh kg^-1,and excellent cycling stability of 100%after 10,000 cycles.2.Hollow porous nickel-cobalt-zinc hydroxide(NiCo3Zn1-OH)polyhedra were synthesized by proton etching and chemical deposition,using Co3Zn1-ZIFs as templates.The rate of etching Co3Zn1-ZIFs templates and co-precipitation of metal ions can be well controlled in the subsequent reaction process by tailoring the content of cobalt and zinc in MOFs templates,so as to fabricate the hollow polyhedra with good dispersion and high quality.The ultrathin nanosheets are stacked to form the shell of NiCo3Zn1-OH polyhedra,which has abundant specific surface area and active sites,and promotes the rapid migration of ions.The hollow porous morphology and interconnected nanosheets can serve as ion reservoirs,which significantly reduce the diffusion distance of ions,accelerate the diffusion process of ions,and thus improving the electrochemical performance of the electrode.In addition,the appropriate amount of cobalt and zinc doped into the lattice of nickel hydroxide can not only effectively improve the conductivity,but also provide the synergistic effect for improving the electrochemical performance.When used as a electrode,the NiCo3Zn1-OH electrode displays the excellent electrochemical performance,featured with outstanding capacitance of 692.8 C g^-1 at 1 A g^-1 and good stability(74.5%after 8000 cycles).Besides,the assembled NiCo3Zn1-OH//AC device also exhibits high energy density of 56.4 Wh kg^-1.Simultaneously,NiCo3Zn1-OH//AC device possesses excellent cycle stability of 93.75%after 10,000 cycles.3.Hierarchical core-shell MnO₂@NiCoZn-OH composites were prepared by depositing MOFs templates on MnO₂ nanotubes and further converting the MOFs templates into nickel-based hydroxides via hydrothermal reaction.The content of zinc and cobalt exerts an important influence on the morphology regulation.Furthermore,the outer ultrathin NiCoZn-OH nanosheets are tightly attached to the surface of MnO₂ nanotube to form a heterogeneously hierarchical core-shell structure,which not only provides rich active sites,facilitates the rapid ions migration,but also effectively avoids the agglomeration of NiCoZn-OH materials.Moreover,the excellent dispersibility of MnO₂@NiCoZn-OH can also provide more contact area with the electrolyte,shorten the ion transport path,and facilitate the rapid transport of ions.Meanwhile,the seamless and tight integration between the internal MnO₂and external staggered NiCoZn-OH nanosheets ensures the structural integrity during the redox reaction,endowing the electrode with excellent cycle stability.When utilized as a electrode,the MnO₂@NiCoZn-OH electrode exhibits a excellent capacitance of 627.7 C g^-1 at 1 A g^-1,a superior capacitance retention of 54%at 30 A g^-1 and good stability of 72.7%after 6000 cycles.In addition,we also assemble a hybrid supercapacitor consisted of MnO₂@NiCoZn-OH and active carbon,and the MnO₂@NiCoZn-OH//AC device still possesses excellent electrochemical performance,featured with outstanding energy density of 49.4 Wh kg^-1 at 842.7 W kg^-1 and superior cycle stability(91.3%after 10,000 cycles).4.The sandwich-like NiZn-OH nanosheets aligned on reduced graphene oxide(rGO)is designed and prepared via ZIF-8 as templates.The ultrathin NiZn-OH nanosheets are attached to both sides of rGO to form a sandwich-like construction,endowing the composite(NiZn-OH/rGO)with abundant electrolyte-accessible contact sites.The usage of rGO substrate can not only effectively avoid the agglomeration of NiZn-OH nanosheets,but also improve the conductivity of active material.In addition,Zn doping into Ni(OH)₂ structure can alter intrinsic property,so as to enhance the electronic conductivity and active site.Benefited from the integration of excellent electronic conductivity of rGO and the high capacity of NiZn-OH,the NiZn-OH/rGO exhibits enhanced electrochemical property.When utilized as a electrode,NiZn-OH/rGO delivers a excellent capacitance(615.4 C g^-1 at 1 A g^-1),the superior capacitance retention(62.3%at 30 A g^-1)and outstanding durable stability(87.5%after 8000 cycles).In addition,the assembled NiZn-OH/rGO//AC supercapacitor also shows excellent energy density(the maximum energy density is 53.7 Wh kg^-1)and excellent cycle stability(89.7%after 10000 cycles).