| The rapid consumption of fossil fuels and the increasing severity of environmental problems have prompted people to realize the importance of research and development of green renewable clean energy and related energy storage equipment.As a high energy density,clean and pollution-free renewable energy source,hydrogen has become one of the most attractive alternative energy sources for fossil fuels.Electrocatalytic water splitting to produce hydrogen has attracted widespread attention due to its sustainable characteristics such as high efficiency and low cost.Because of the fast charge and discharge capacity,high charge and discharge efficiency,long cycle life,and high power density,supercapacitors have shown great application value and advantages in various fields such as portable electronic equipment,new energy vehicles,military and aerospace fields.The best electrocatalytic water splitting materials are precious metal materials.However,due to the problem of price and reserves,they can not be used on a large scale,so non-precious metal materials have become the current research focus.For electrocatalytic hydrogen evolution(HER)materials,their intrinsic catalytic activity and the number of catalytic active sites are the two main factors affecting their catalytic performance.For electrocatalytic hydrogen evolution,in accordance with the basic principles of catalytic reactions,take appropriate measures to increase the number of active sites of the material,while optimizing the adsorption behavior of material to water molecules and active hydrogen,improving the charge transfer ability to reduce the reaction energy barrier,speed up the reaction kinetics rate is the key to improve the performance of electrocatalytic hydrogen evolution materials.In other words,improving the intrinsic catalytic activity and the number of active sites of the material simultaneously is the key to improving the performance of the electrocatalytic hydrogen evolution material.For supercapacitor materials,according to the mechanism of the capacitance process,increasing the number of active sites of the material,improving the charge transfer ability of the material itself and the interface between the material and the solution at the same time to improve the specific capacitance,rate performance and cycle stability of the material is the key to the current development of supercapacitor materials.Nickel based materials are often used in the field of catalysis and energy storage due to their abundant reserves,low cost,and active chemical properties.Layered double hydroxides(LDHs)materials are very suitable as precursors for synthetic materials because their metals are distributed atomically in the laminate and the composition structure is adjustable.When selecting appropriate metal components and proportions to prepare corresponding LDHs with special morphology(ultrathin/hollow structure,etc.),and using them as precursors through specific post-treatment methods,materials with the following characteristics can be obtained:can maintain the ultrathin/hollow structure of the precursor,and can make full use of the advantages of the multi-metal component of the LDHs precursors to form related metal element doping or heterojunction.Therefore,the materials prepared with LDHs as precursors not only have abundant reactive sites,but also can have improved intrinsic performances through the formation of doping/heterojunctions.Therefore,in view of the main factors affecting the performance of catalysis and capacitance,this paper takes nickel based LDHs as precursors,through reasonable design the composition and structure of the precursor materials,and through appropriate post-treatment methods,while maintaining the macroscopic morphology of the precursors,regulating the microstructure of the materials,and taking advantage of the polymetallic properties of LDHs to obtain related metal element doping and multi-component heterojunction materials.The above methods can optimize the material’s composition and structure,electrical behavior,and the adsorption free energy to the reaction intermediates,etc.to regulate the active site of the material,the conductivity,reduce the energy barrier of the catalytic reaction,and ultimately improve the catalytic and capacitive performance of the materialThe specific research contents of this thesis are as follows:In chapter 1,the compositional structurer,main properties and main synthesis methods of LDHs materials are briefly described.Then,the classification,advantages,related synthesis methods,and specific applications of materials synthesized with LDHs as precursors are introduced systematically,followed by an overview of the basic mechanisms of electrocatalytic water splitting and supercapacitors as well as the application of materials synthesized with LDHs as precursors in these fields are described.Finally,based on the basic principles of catalytic and capacitive reactions and the advantages of LDHs as the precursors,the significance of this topic and the main research contents are described.In chapter 2,in view of the main limiting factors of electrocatalytic hydrogen evolution,LDH was used as precursor to synthesize highly dispersed Ni3N@VN heterojunction material and studied the influence of introduction of traditional capacitance material VN on the particle size and electrocatalytic hydrogen evolution performance of the Ni3N material.NiV-LDH was used as a precursor to perform nitriding treatment to obtain a uniformly dispersed Ni3N@VN heterojunction material.Through SEM,ECSA and other testing and characterizing methods,it is proved that NiV-LDH is used as the precursor to perform nitriding treatment,and the limitation effect of VN during post-processing can reduce the particle size of the material and increase the number of active sites of the material.Combined with LSV,Tafel,EIS,TOF,exchange current density,CO2 adsorption test and other tests and characterizations,it has been proved that the introduction of VN can promote the charge transfer of the system,accelerate the dissociation of water molecules and the desorption of hydroxyl groups to improve the intrinsic catalytic ability of the material and enhance the electrocatalytic performance of the material.The Ni3N@VN//NiFe-LDH water splitting system using Ni3N@VN heterojunction material as the cathode material and the electrodeposited NiFe-LDH as the anode material also exhibits good electrocatalytic water splitting performance.In Chapter 3,based on the previous chapter,aiming at the problems of poor hydrophilic and low efficiency of dissociating water molecules of metallic Ni,NiV-LDH was annealed to introduce V-O that plays an important role in OER into the HER system to obtain uniformly dispersed Ni/V2O3 heterojunction materials.According to SEM,TEM and ECSA test results,it is shown that due to the confined effect of V2O3 during post-treatment process,a uniformly dispersed and fine-grained metallic Ni material can be obtained,which proves the advantage of NiV-LDH as a precursor.Combining a series of electrochemical tests,XPS tests,TPR tests,CO2 adsorption tests,contact angle tests and DFT theoretical calculations etc.,the reasons for the introduction of V2O3 to improve the electrocatalytic HER performance of metallic Ni is that:promote the electron transfer between Ni and V2O3 interface,optimize the adsorption and dissociation ability to water molecules,promote the desorption of hydrogen and accelerate the bubble separation,reduce the ohmic resistance,and then increase the number of active sites and the intrinsic catalytic performance of the material at the same time.In Chapter 4,the results of the above research show that the introduction of appropriate V-based materials can promote the electrocatalytic performance of some Ni-based materials.Based on this result,the effect of the introduction of V species on the electrocatalytic performance of other Ni-based materials is further studied.Ni0.2Mo0.8N,a bimetal material with rich electronic properties was selected.In order to solve the problem of high overpotential of the material in the electrocatalytic water splitting,the electronic state density of the material was adjusted by the strategy of V-doping to optimize the adsorb and bond ability of the material to water molecules and reaction intermediates,thereby enhancing its water-splitting performance.Combined with XRD,SEM,TEM,XPS,CO2 adsorption,ECSA,TOF,and DFT calculation and other testing and characterization methods,it was proved that V doping causes electron transfer between different elements in the material and enhances the adsorption dissociation ability of material to water molecules in the HER process and promotes the desorption of the product hydrogen and accelerates the charge transfer.In addition,the doped V element and the O element generated after the structural reorganization in the OER process optimize the adsorption of material to oxygen-containing intermediates,reduce the energy barrier of the reaction,and thus improving the OER performance of the material.In Chapter 5,aiming at the main factors limiting the performance of supercapacitors,novel ultra-thin NiV-S material was synthesized using LDH as precursor.The effects of the introduction of ultra-thin structure and S species on the supercapacitance performance of materials were mainly studied.The ultra-thin NiV-LDH material was designed and prepared as a new type of supercapacitor electrode material to study its capacitance performance.Aiming at the problem of high specific capacitance but insufficient stability when using NiV-LDH material as the electrode material of supercapacitor,S2-was introduction by sulfuration treatment.Through SEM,TEM and AFM,and a series of electrochemical tests,it was proved that the material after sulfuration retains the original ultra-thin structure of the LDH precursor,thus ensuring a rich reactive site and a short ion diffusion distance.And the introduction of S species greatly improves the conductivity of the material,so the material after sulfuration shows higher supercapacitance performance than NiV-LDH.In addition,the introduction of S species also increased the number of covalent bonds of the material to slow down the volume expansion and structural strain in the reaction process,thereby improving the stability of the material.In addition,NiV-S//AC supercapacitor assembled with NiV-S as the positive electrode and activated carbon as the negative electrode also showed good capacitance performance.In Chapter 6,based on the previous chapter,a Co-doped Ni3S2 material was synthesized using LDH as the precursor and the influence of the introduction of Co on the composition structure and capacitance performance of traditional Ni3S2 material was studied.Through a series of electrochemical tests,it was found that the introduction of Co in Ni3S2 to regulate the composition and structure of the material makes the material have more reactive sites and form a three-dimensional hierarchical structure,which shortens the distance of ion diffusion and promotes charge transfer and then greatly enhances the specific capacitance value and rate performance of the material.The Co-Ni3S2//AC supercapacitor was assembled with Co-Ni3S2 as the positive electrode and activated carbon as the negative electrode also showed good performance.In Chapter 7,the research contents of this thesis are summarized,the innovation points are summarized,the deficiencies are pointed out,and the future work is prospected. |
