Preparation Of Nickel-iron-based Catalyst Material And Its Electrocatalytic Performance For Ethanol

As a clean energy conversion and storage technology,direct ethanol fuel cell(DEFC)has caused widespread research by researchers.Although the precious metal Pt or Pd is currently an ultra-efficient electro-catalyzed ethanol oxidation(EOR)catalyst,these catalysts have the problems of being easily poisoned and expensive,preventing their industrial application.Recent research work shows that NiFe-LDHs containing non-noble metals has extraordinary electrocatalytic EOR performance.However,the slightly insufficient electron transport capacity restricts the electrocatalytic activity of NiFe-LDHs.In order to improve its electrocatalytic ability,the researchers mainly worked on the following two aspects:(1)using LDHs as precursors to prepare metal oxides,nitrides and sulfides;(2)using certain carbon materials such as graphene,Carbon nanotubes,carbon nanodots,etc.as a carrier of LDHs.How to improve the electrical conductivity of LDHs-based electrocatalyst materials has become a key scientific issue in this field.In this paper,based on the improvement of the electrical conductivity of EOR electrocatalyst materials,two types of LDHs-based electrocatalyst materials have been developed.First,NiFe-LDHs was used as a precursor,and the new nano-hydroxide/alloy nanostructured material,NiFe(OH)x/Ni3Fe,was prepared by means of hydrogen reduction treatment.Various physical and chemical characterization methods were selected to analyze the microstructure and chemical composition of the composite nanomaterial.After discussing the changes of the composition and structure of the catalyst at various temperature reduction treatments,the microscopic formation mechanism of the sample material was deduced.At the same time,in this thesis,carbon nanotubes(CNTs)are used as substrates,and NiFe-LDHs/CNT composites with three-dimensional spherical structures are prepared by controlling different crystal growth times.The growth process of NiFe-LDHs on carbon nanotube materials was studied.Finally,this paper uses two types of LDHs-based catalysts in DEFC anode catalysts in the ethanol oxidation reaction(Ethanol oxidation reaction EOR)to explore the structure-activity relationship between the EOR reaction and the catalyst material.The characteristics of the research conclusions and results are shown below:(1)NiFe(OH)x/Ni3Fe composite nanomaterials were synthesized by means of H2 reduction reaction using precursors based on nickel iron hydrotalcite materials.Each characterization indicates that the Ni3Fe alloy with a particle size of about 3nm is confined to the NiFe(OH)x lamellar structure with low crystallinity.The results of further research show that when the reduction temperature is 150-200?,the spacing between the NiFe-LDHs layers gradually decreases,which in turn leads to the destruction of the layered structure and the formation of NiFe(OH)x with a composite hydroxide structure.At the same time,a small amount of Ni3Fe nanoalloy was precipitated by reduction.When the hydrogen reduction temperature was further increased to 250?,Ni3Fe nano-alloy particles became larger and increased,forming a NiFe(OH)x/Ni3Fe structure.When the hydrogen reduction temperature was 350?,a small amount of composite oxide NiFeOx began to appear.When the temperature continued to rise to 450?,Ni3Fe alloy with larger particles was formed.(2)The NiFe(OH)x/Ni3Fe samples obtained at different reduction temperatures were used in the DEFC anode EOR reaction to test its electrocatalytic performance,compared with the original powder samples.The experiment found that the samples obtained by each reduction temperature treatment had higher electrocatalytic performance than the original NiFe-LDHs powder.Among them,the best performance is the sample with a reduction temperature of 250?.This is due to the synergy of NiFe(OH)x and Ni3Fe alloy.We speculate that the NiFe(OH)x phase is the active site of the EOR anode catalytic reaction,which is beneficial to the oxidation activation of Ni(OH)2 to NiOOH.At the same time,the NiFe(OH)x phase provides a large number of hydroxyl groups which can promote the desorption of the acetyl group of the reaction intermediate.The Ni3Fe phase plays a role in promoting the transfer of electrons during the reaction,which may be beneficial to the activation and dissociation of ethanol molecules.(3)NiFe-LDHs/CNT material with three-dimensional spherical structure was synthesized by urea hydrothermal method.The characterization results show that the particle size of the composite material is about 2-4 ?m,and it has a uniform spherical shape.Characterization studies the structural characteristics of NiFe-LDHs/CNT composites with different synthesis reaction time,and speculates the formation process of NiFe-LDHs/CNT structure.As the reaction time increases,the sample gradually grows from an irregular structure into a spherical structure,and eventually grows into a uniform spherical shape.(4)The NiFe-LDHs/CNT samples prepared at different reaction times were applied to the DEFC reaction.By comparison,it was found that the NiFe-LDHs/CNT samples synthesized in 12h had the highest current density(the oxidation current density was 0.8V at 0.8V potential).58mA·cm-2),and has extremely high electrocatalytic long-term stability.This may be due to the electron/space synergy localization between CNT and NiFe-LDHs.