Study On Preparation And Performance Of Nickel-based Composite Catalyst Materials For Direct Ethanol Fuel Cell

Direct ethanol fuel cell(DEFC)is an effective technical way to realize clean energy storage and conversion.More and more research groups are focusing on the controlled preparation and performance control of ethanol electrocatalytic oxidation(EOR)catalyst materials.The traditional precious metal Pt and Pd-based catalysts have high catalytic activity,but the limitation of reserve and price force the research of non-precious metal EOR catalysts to become a trend.Recent studies have shown that nickel-iron-based hydrotalcites(NiFe-LDHs)materials have good catalytic activity in the EOR reaction,but there are disadvantages such as poor electron transport ability and easy aggregation of samples,which limit its further application.This work is based on improving the electron transport capacity of NiFeLDHs electrocatalyst materials,using Ni-based LDHs as precursors.Two DEFC catalyst materials are prepared by alkaline etching and carbon nanotube(CNT)loading strategies:β-Ni(OH)2/NiFe2O4 heterostructure composite material and NiFe-LDHs/CNT three-dimensional spherical composite material.The morphology,composition and structure of the samples are analyzed through a variety of characterization techniques,the formation mechanism of the two composite materials is speculated,and the structure-activity relationship is studied.The main research contents and results are as follows:(1)β-Ni(OH)2/NiFe2O4 heterostructure composite material is prepared in KOH strong alkaline solution using NiFeAl-LDHs as the precursor.Alkalinity obviously affects the composition and morphology of the sample.The mechanism of the sample formation process shows that the formation of βNi(OH)2/NiFe2O4 is mainly divided into three steps:First,the removal of a small amount of unstable aluminum ions from the LDHs laminate.Then the lattice rearrangement process occurs:after a small amount of Al3+ is selectively removed,the remaining atoms migrate and rearrange into a relatively stable structure.In addition,the dissolution of Al3+leads to a decrease in the positive charge of the main laminate,and the balanced anion between the LDHs layers diffuses outward,and α-Ni(OH)2 is gradually dehydrated in concentrated alkali to prepare for the subsequent crystal transformation.The final step is the topological transformation process:the remaining NiFe-LDHs components after aluminum removal are topologically transformed into NiFe2O4 spontaneously.At the same time,α-Ni(OH)2 also grows slowly and irreversibly to become thermodynamically more favorable β-Ni(OH)2 nanosheets through the topological transformation mechanism,and this thermodynamically driven process can realize spontaneous delaminating and redeposition.Electrocatalytic test research shows that β-Ni(OH)2/NiFe2O4 has higher catalytic activity,which can be attributed to the synergistic catalytic effect of the two components of βNi(OH)2 and NiFe2O4.β-Ni(OH)2 as the main active component provides more active hydroxyl groups for the reaction;NiFe2O4 has super high conductivity due to the valence jump,thereby achieving higher electrocatalysis through the synergistic effect of the two performance.(2)NiFe-LDHs/CNT three-dimensional spherical composite samples are prepared through loading NiFe-LDHs on CNT by hydrothermal method.The study of the hydrothermal treatment process shows that the CNT aggregate into woven clusters to form a conductive network during the reaction for 0.5 h.Ni2+and Fe3+ ions are adsorbed on the surface of the CNT to form amorphous iron hydroxide precipitation.The in-situ phase transition from amorphous iron hydroxide to α-FeOOH/Fe(OH)3 nanosheets occurs during the reaction for 1 h.At the same time,Ni2+ adsorbed on the CNT surface is doped into αFeOOH/Fe(OH)3 nanosheets,and CO32-is inserted between the layers to achieve charge balance,stacking of nanosheets to form a layered structure and then form a 2 μm spherical prototype.After 6 hours of reaction,the Ni2+ in the adsorption solution is gradually transformed into NiFe-LDHs nanosheets.After 12 hours of reaction,NiFe-LDHs grow uniformly on the surface of CNT,forming 4 μm three-dimensional spherical NiFe-LDHs/CNT composite material.Electrochemical tests have found that the EOR catalytic performance of NiFe-LDHs/CNT composite far exceeds that of hydrotalcite-based and nickel-based electrocatalyst materials.