Preparation And Electrocatalytic Performance Of Layered Double Hydroxides And Their Derivatives

In the era of energy crisis and increasingly serious environmental pollution,searching efficient,clean and sustainable energy has become an urgent issue.In recent years,hydrogen energy has attracted attention as a high-energy-density green energy source.Electrolyzed water technology has become the most interesting method of large-scale hydrogen production due to its simple operation,environmental protection and high efficiency.However,water electrolysis technology has the problems of high reaction overpotential and slow reaction kinetics.How to reduce the required voltage for the electrolysis of water is the key to achieving efficient hydrogen production.Electrocatalytic water splitting consists of two half reactions:the cathode hydrogen evolution reaction(HER)and the anode oxygen evolution reaction(OER).At present,noble metal Pt and Ir/Ru based materials,as the reference catalysts of the cathode and anode,have good catalytic performance in the process of water electrolysis,but their scarcity and high cost hinder the industrial development of water electrolysis.Therefore,it is of great significance for the development of water electrolysis technology to develop efficient,cheap and abundant non-noble metal-based catalysts.Layered double hydroxide(LDH)is a potential two-dimensional layered OER catalyst.The type and proportion of metal cations in the lamellar are adjustable,and the anions between the layers are fluidity and exchangeability.More importantly,the derivatives based on LDH materials can achieve performance enhancement or multifunctional,so LDH materials have attracted the attention of researchers in the field of electrocatalysis.However,their intrinsic conductivity is poor,and the powder materials are prone to crimp and stack of nanosheets during the testing,which is not conducive to the electron transfer and the exposure of active sites in the electrocatalytic process.NiV-LDH as a new type of OER electrocatalyst with excellent performance,has not been reported in the field of HER.To solve the above problems,this paper,based on NiV-LDH,focuses on the morphology design and the regulation of electronic structure of materials.From the two aspects of increasing the number of catalyst active sites and improving the intrinsic activity of catalyst sites,the following five aspects of work are carried out:(1)Binary NiV-LDH/NF and ternary NiCoV-LTH/NF self-supported electrodes have been successfully prepared on nickel foam(NF)by one-step hydrothermal method.Interestingly,the introduction of Co into NiV-LDH can not only induce the formation of porous nanosheets,exposing a large number of active sites,but also change the electron density around Ni and V,which promote the absorption of hydrogen species in the process of hydrogen evolution.Therefore,NiCoV-LTH/NF showed better HER catalytic activity and reaction kinetics than NiV-LDH/NF.(2)A NiCoV-LTH/NF self-supported electrode material in-suit grown on NF was successfully prepared by one-step hydrothermal method.It is found that Ni2+from NF can be controlled into NiCoV-LTH/NF by adjusting the Co/V molar ratio,and the structure conversion from single crystal nanoneedles(S-NiCoV-LTH/NF)to poly crystalline nanosheets(P-NiCoV-LTH/NF)can be realized.Meanwhile,the electronic structure of NiCoV-LTH is optimized.The resulted P-NiCoV-LTH/NF exhibits excellent OER performance in 1 M KOH solution,with overpotential of 340 mV and 373 mV at current density of 500 mA cm-2 and 1000 mA cm-2,respectively,and excellent stability over 180 hours.The work in this chapter provides a new idea for the design and synthesis of industrial water splitting catalysts.(3)A new heterostructure catalyst Ni3S2@NiV-LDH/NF was prepared by a partial anion exchange reaction.The structure is composed of Ni3S2 nanoparticles embedded in ultrathin NiV-LDH nanosheet arrays supported on NF.We found that the length of active edge and surface chemical states of NiV-LDH can be adjusted by regulating the coupling interface,which can expose more catalytic active sites and enhance the electron interaction between NiV-LDH and Ni3S2,thus greatly promoting the dissociation kinetics of water.The optimized Ni3S2@NiV-LDH/NF heterostructure exhibited excellent electrocatalytic activity of HER and OER in alkaline medium,and the overpotential was only 126 mV and 190 mV at the current density of 10 mA cm-2,respectively.More importantly,Ni3S2@NiV-LDH/NF is used as both anode and cathode to assemble an electrolytic cell can achieve a current density of 10 mA cm-2 at a voltage of only 1.53V and has an excellent durability of 160 hours.This work provides a new perspective for the development of high activity heterogeneous catalysts for electrolysis of water.(4)A dendritic V-Ni3S2@NiO core-shell nanoarray self-supporting electrode(V-Ni3S2@NiO/NF)was synthesized on NF by a V-doping triggered self-assembly strategy.The core-shell structure consisted of ultrathin V-doped NiO nanoshells(2～7 nm)and high-crystal Ni3S2 core.The unique hierarchical structure provides multi-dimensional charge transfer channels and abundant catalytic active sites for water splitting reaction,thus improving the kinetics of water electrolysis.Moreover,when anodic oxidation in an alkaline medium,V(IV)on the surface of V-Ni3S2@NiO/NF material will partially precipitate,which can accelerate the surface reconstruction of V-Ni3S2@NiO/NF and promote the formation of abundant highly active species NiOOH on its surface,exposing more active centers and promoting the adsorption of OH-intermediates,thus significantly improving the OER kinetics.In addition,when V-Ni3S2@NiO/NF is assembled into an electrolytic cell,the current density of 10 mA/cm2 can be catalyzed at a voltage of 1.52 V,and the stability can be maintained for at least 55 h.This work provides a new strategy for the development of highly efficient water splitting catalysts.(5)Using NiV-LDH as precursor,an ultra-thin carbon layer composite(Ni/VN@Ni-NC)was prepared by using the high temperature sintering method,which was modified by single atom Ni and uniformly dispersed Ni and VN nanoparticles.By adjusting the Ni/V molar ratio,the size and density of the nanoparticles of the target product can be effectively controlled,and their electronic structure can be optimized.We found that during the transformation of V in NiV-LDH into VN,the surrounding Ni will combine with N on the carbon layer to form a highly active Ni-Nx bond,which improves the intrinsic activity of the material.In addition,the synergistic effect between metallic Ni and VN nanoparticles promotes the adsorption/desorption of H*and H2 on the catalyst during HER process.The optimized Ni/VN@Ni-NC-2 showed excellent HER catalytic activity in both alkaline and neutral electrolytes.When the current density was 10 mA cm-2,its overpotential was only 84 mV and 182 mV,respectively.This work provides a design idea for the development of single atom catalysts and efficient hydrogen evolution catalysts.