Preparation Of Cobalt-Iron Layered Double Hydroxide Derived Composites And Application In Electrocatalytic Oxidation

Secondary energy sources such as hydrogen energy,and metal-air batteries have received much attention due to energy and environmental problems,and the oxygen evolution reaction（OER）,one of the key reactions,limits the large-scale application of these energy sources.The slow four-electron transfer process of OER severely limits the rate of the reaction and reduces the conversion efficiency,and the development of stable,and high-performance electrocatalysts is of great importance to improving the efficiency of hydrogen production.The reduction of overpotential and the conversion of organics into high-value-added products are achieved by using small molecules of organics（e.g.,urea,glucose,methanol,etc.）to undergo oxidation reactions at the anode instead of OER.Presently,the majority of the main electrocatalysts are made of precious metals,but their high price and limited availability prevent them from being used in large-scale applications.Layered Double Hydroxide（LDH）materials have been widely used and studied in electrocatalysis because of their unique two-dimensional structure,abundant and adjustable cation species,good stability,and low cost.However,their poor electrical conductivity,limited active sites,and low intrinsic catalytic activity limit their practical applications.In this thesis,Cobalt-Iron LDH materials are doped,compounded,and derived to modulate the catalytic activity and effectively enhance their OER and small molecule electrocatalytic oxidation performances,as follows:（1）The ternary CoFe_1-XMn_X-LDH（0<X<1）has been synthesized by simple hydrothermal synthesis to investigate the effect of the introduction ratio of Mn on the crystalline phase,morphology,and electronic structure of CoFe-LDH.CoFe_0.75Mn_0.25-LDH is grown on nickel foam and has a unique nanowire-nanosheet layered nanostructure in addition to having the best possible electronic structure.Further experimental results show that CoFe_0.75Mn_0.25-LDH has excellent OER performance,requiring overpotentials of only 243 and 298m V to reach 20 and 100 m A/cm².In addition,CoFe_0.75Mn_0.25-LDH can be used for the urea oxidation reaction（UOR）,requiring only 1.367 V to reach 10 m A/cm²,and by assembling a solar cell-driven allolytic water demonstration system,a current density of 10 m A/cm² can be obtained at a voltage of 1.58 V.（2）A simple room-temperature electrodeposition method has been used to compound amorphous Ni（OH）₂ on crystalline CoFe-LDH to form CoFe-LDH/Ni（OH）₂ composites,and the effects of deposition time and compounding on the electrocatalytic performance have been investigated.The OER performance under alkaline conditions has been investigated,requiring an overpotential of only 274 m V to achieve a current density of 100 m A/cm² with excellent stability.The same excellent catalytic performance is demonstrated in the glucose oxidation（GOR）test,where a current density of 100 m A/cm² can be achieved at 1.436 V.The composite of Ni（OH）₂ nanowires with CoFe-LDH nanosheets is found to effectively increase the number of exposed active sites and accelerate the charge transfer,while the synergistic interaction between Co-Fe-Ni can effectively adjust the electronic structure and enhance the intrinsic catalytic activity of the catalyst,resulting in the preparation of a bifunctional electrocatalyst with excellent OER and GOR performance.（3）The S-doped Co₃Se₄/Fe₃Se₄ heterostructure has been prepared by a two-step hydrothermal method using CoFe-LDH as the precursor,the original nanosheet array morphology has been successfully preserved,and the effect of S doping amount and selenization on the catalytic performance has been investigated.S-Co₃Se₄/Fe₃Se₄ exhibits excellent OER performance in 1 M KOH,requiring only 255 m V of overpotential to reach a current density of 100 m A/cm² with good stability,which remained good after a 100h stability test.In the methanol oxidation reaction（MOR）test,a current density of 100 m A/cm² is achieved with only 1.353V_RHE.The unique nanowire-nanosheet array structure increases the number of exposed active sites,and the introduction of sulfur tunes the electronic structure of Co while inducing the enhancement of selenium vacancies and enhancing the electrical conductivity.A feasible strategy for performance optimization and electronic structure tuning of layered double hydroxide materials is provided as a practical reference for the development of OER and MOR bifunctional electrocatalysts.