Synthesis,Structure And Electrocatalytic Properties Of Layered Double Hydroxides And Their Derivatives As Self-Supported Electrodes For Water Electrolysis

Water splitting has been regarded as an ideal strategy to produce high-purity hydrogen due to its advantages of environmental friendliness and abundant resources.It has been known that water splitting process includes hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.However,due to the slow kinetics of two reactions,the efficiency of electrolytic water is not satisfactory.Therefore,it is very necessary to design high-performance electrocatalysts to reduce the reaction barrier of OER and HER.At present,noble metal-based materials are regarded as the best electrocatalysts for OER and HER.However,the practical application of noble metal-based materials is limited due to their scarcity and high price.Therefore,it is of great significance to develop transition metal catalysts with low price and excellent performance.Layered double hydroxides（LDH）have become one of the potential non-noble metal catalysts because of their unique layered structure and easy modulation of compositions.However,the low intrinsic conductivity and poor stability of LDH are not conducive to the transfer of electrons during electrocatalysis process,and thus their electrocatalytic performance needs to be further improved.To address the above problems,three kinds of LDH-based（or LDH derivatives）catalysts with unique micromorphology and composition have been constructed by the strategies of heteroatom doping and reaction condition optimization,and the relationship of the composition,structure and catalytic properties of the materials have been systematically studies.The main contents of this work are as follows:（1）Sulfur-doped LDH-based nanosheet arrays（S-CoFe-LDH）were successfully synthesized on nickel foam by the combined process of hydrothermal reaction and room-temperature sulfurization and the structure,micromorphology and OER properties of the materials were investigated in detail.The improved electrocatalytic performance can be ascribed to nanosheet array and sulphur doping,which endow the S-CoFe-LDH electrode abundant active sites and excellent electrical conductivity.The S-CoFe-LDH-10h electrode shows excellent OER activity,only requiring as low as the overpotential of 258 m V to drive 100 m A cm^-2,as well as exhibiting superb stability of85 h at the current density of 100 m A cm^-2in alkaline electrolyte.（2）The CoFe-LDH nanoarrays were grown on nickel foam by one-step hydrothermal method and the effects of hydrothermal reaction temperature and time on the structure,micromorphology and OER properties of the materials were studied systematically.The hydrothermal reaction temperature plays a decisive role in optimizing the micromorphology of the materials,while the hydrothermal reaction time has little effect on the micromorphology of the electrocatalysts.The CoFe-LDH electrocatalysts with the morphologies of nanosheet,goose feather and nanosheet-nanowire can be synthesized by adjusting the hydrothermal reaction temperature.The CoFe-LDH material with nanosheet-nanowire morphology provides more active sites and facilitates faster electron transfer,and thus exhibites the optimum OER properties:the low overpotential of 242 m V at 100 m A cm^-2,as well as an ultra-long durability of 97 h.（3）The P-doped Ni O/Fe₂O₃ nanoflowers were successfully synthesized as electrocatalysts for overall water splitting by the combination process of hydrothermal reaction and low-temperature phosphating using NiFe-LDH as precursor,and the structure,morphology and catalytic properties of the materials were investigated systematically.After P doping,the morphology of the material is gradually changed from nanosheets to nanoflowers,which increases the specific surface area and exposes more active sites;on the other hand,the doping of P regulates the electronic structure of the material.As a resutt,P_0.1-Ni O/Fe₂O₃ and P_0.15-Ni O/Fe₂O₃ show the superb catalytic performance for OER(236 m V@200 m A cm^-2)and HER(147 m V@20m A cm^-2)in 1 M KOH electrolyte,respectively.In addition,the current density of 20 m A cm^-2 is reached under the low voltage of 1.65 V,and an excellent durability of 100 h is observed for overall water splitting when the P_0.1-Ni O/Fe₂O₃ and the P_0.15-Ni O/Fe₂O₃are used as the anode and the cathode,respectively.