Preparation Of CoFe Layered Double Hydroxide-based Materials And Their Efficient Oxygen Evolution Performance

With global energy and environmental issues becoming critical,the exploitation and use of renewable and clean energy sources has become essential.With its extremely high mass-energy density,hydrogen is a carrier for energy conversion and utilization.Among the various methods of hydrogen preparation,electrolytic water to hydrogen is among the most exciting and effective use of renewable energy sources due to its simple preparation process,non-polluting preparation products and easy mass production.Electrocatalytic water splitting is made up of two half-reactions occurring at the cathode and the anode,namely hydrogen derivative reaction（HER）with two electron transfers at the cathode and oxygen derivative reaction（OER）with four electron transfers at the anode.In contrast to the two-electron transfer procedure in HER,the OER course deals with the transfer of four electrons,and its higher number of transferred electrons,higher reaction potential barrier and slow kinetic process limit the efficiency of the electrolytic water reaction and greatly hinder the scale up of hydrogen production from electrocatalytic water splitting.Therefore,the design and development of efficient and stable oxygen precipitation catalysts is one of the biggest challenges in the field of hydrogen production from electrocatalytic water splitting.Among various OER catalysts,Co Fe layered double hydroxide（CoFeLDH）materials have the advantages of unique two-dimensional layered structure,rich and variable chemical composition,highly dispersed metal cations,excellent stability and preparation method with simple and low cost,which have wide application prospects in OER reactions.However,the poor electrical conductivity and limited active sites hinder the industrial application of CoFeLDH.In this thesis,the self-grown CoFeLDH material on nickel foam（NF）is used as the basis to improve the disadvantages of poor activity,low conductivity and few active sites of CoFeLDH by means of material compounding,anion conversion and elemental doping in turn.The main research contents and innovation points of this thesis are as follows.（1）The NiO@CoFeLDH/NF composites with sandwich structure were constructed by hydrothermal method by growing CoFeLDH nanosheets in situ on both sides of NiO nanosheets,and the material composite method was used to construct the NiO@CoFeLDH/NF composites with sandwich structure.The in situ growth of NiO@CoFeLDH/NF composites on NiO nanosheets features a large number of interfacial contacts between the two phases,which is beneficial to the electron transfer between the catalyst transfer between the catalyst and the electrode.In addition,the high coupling of the two phases causes the electronic structure of CoFeLDH to change,producing higher valence transition metal(Co³⁺and Fe³⁺)active sites that are more favorable for the OER reaction.The NiO@CoFeLDH/NF was tested to require an overpotential of 224 m V and 303 m V,a Tafel slope of 52 m V dec^-1,a charge transfer resistance of 0.89Ωat current densities of 10 m A·cm^-2 and 100 m A·cm^-2,an electrochemically active surface area of 45 cm² and stability for 20 h at a current density of 10 m A·cm^-2.（2）Based on NiO@CoFeLDH/NF composites,the core-shell structured Ni_0.85Se@CoFeLDH/NF nanosheet arrays were constructed by replacing the O atoms in NiO with Se atoms using the anion transformation method.Thanks to the high conductivity of NF and Ni_0.85Se and the close interfacial contact between Ni_0.85Se and CoFeLDH,the mutual transfer of carriers between the catalyst and the substrate material is accelerated and the OER kinetic process is accelerated.Secondly,the strong electronic interactions generated by the compounding of the two substances lead to an increase in the amount of the real active substances M³⁺（Ni and Co）for the OER reaction.In addition,the thin crystalline/amorphous CoFeLDH nanosheets produced by the electrodeposition preparation method are wrapped on the surface of porous Ni_0.85Se nanosheets to form a porous 3D structure,which not only provides a large number of active sites but also increases the contact area between the electrolyte and the active sites.The Ni_0.85Se@CoFeLDH/NF composites were tested at current densities of 10 m A·cm^-2and 100m A·cm^-2 with an overpotential of 223 m V and 280 m V and a Tafel slope of 53 m V dec^-1.The charge transfer resistance is 0.49Ω,the electrochemically active surface area is 168 cm² and the stability is maintained for 24 h at a current density of 10 m A·cm^-2.（3）The Ni_0.85Se@Ni-CoFeLDH/NF composite catalysts were prepared based on Ni_0.85Se@CoFeLDH/NF composites by partially replacing the Co and Fe atoms in CoFeLDH with Ni atoms by elemental doping method.Thanks to the doping of Ni element,the Ni atom itself can not only serve as a highly active reaction site,but also the defects introduced in the doping process are favorable to the OER reaction,which increases the number of active sites and electrochemical activity of the composite.The Ni_0.85Se@Ni-CoFeLDH/NF composites were tested to require an overpotential of 235 m V and 299 m V at current densities of 10m A·cm^-2and 100 m A·cm^-2,a Tafel slope of 49 m V dec^-1 charge transfer resistance of 0.78Ω,electrochemically active surface area of 234 cm² and stability at a current density of 10m A·cm^-2 for 24 h.