Preparation And Electrocatalytic Ethanol Performance Of Confined Structure Nickel-iron-based Nanocomposite Catalysts

Direct ethanol fuel cells(DEFC)can be mass produced from biomass feedstocks,is non-toxic,has a high boiling point,and has a higher energy density than methanol.In addition,its fuel supply system is compatible with existing gas stations.Therefore,DEFC has shown promising development prospects in small portable electronic devices and electric vehicles.At present,Pt or Pd-based electrocatalysts for alcohol fuel cells have problems such as being easily poisoned,having poor durability,and being expensive,and the development of non-precious metal catalyst materials for high-performance DEFC is called a hot spot of research.In this paper,nickel-iron hydrotalcite(NiFe-LDHs;Ni/Fe = 3:1)was used as a precursor material,and hydrogen-reducing treatment was performed at a certain temperature to prepare a confined structure metal alloy/hydroxide nanocomposite:Ni3Fe/NiFe(OH)x.A variety of characterization techniques were used to study the composition,microstructure,and valence state of nanocomposites.The effects of different reduction temperatures on the structure of the materials were investigated.The formation mechanism of Ni3Fe/NiFe(OH)x nanocomposites was inferred.On the basis of this,nickel-iron hydrazine hydrotalcite(NiFeRu-LDHs;Ni/Fe/Ru = 3:1:0.01)was used as the precursor material,and the confined structure metal alloy/oxide nanocomposite was prepared by hydrogen reduction treatment.Material:Ni3FeRu/NiFeOx.The effect of Ru addition in the bulk laminate on the reduction process of NiFeRu-LDHs was studied.Finally,the prepared samples were used as electrocatalyst materials for ethanol oxidation reaction,and the structure-activity relationship between the catalyst structure and the electrocatalytic performance of ethanol was explored.Main research contents and results are as follows:(1)Using NiFe-LDHs as precursor materials,Ni3Fe/NiFe(OH)x nanocomposites were prepared by hydrogen reduction.The characterization results show that Ni3Fe alloy nanoparticles are limited to the weakly crystalline NiFe(OH)x matrix.The Ni3Fe particle size is approximately 3 nm.The influence of different reduction temperatures on the structure of the material was studied in depth.It was found that when the hydrogen reduction temperature is lower than 200 ?,the collapse of the NiFe-LDHs structure leads to the formation of the NiFe(OH)x nickel-iron hydroxide composite;at this time,the Ni3Fe alloy phase is reduced and precipitated,which is consistent with the "topological transformation" mechanism.With the reduction temperature increased to 250 ? Ni3Fe alloy particles gradually grow up,resulting in Ni3Fe/NiFe(OH)x nanocomposites.When the reduction temperature reached 350?,a small amount of NiFeOx was formed in the sample matrix.As the reduction temperature continues to increase to 450 ?,the NiFeOx in the sample begins to be reduced,eventually producing Ni3Fe alloy particles.(2)The samples prepared at different reduction temperatures were applied to ethanol oxidation reaction and compared with NiFe-LDHs raw powder samples.It was found that the samples prepared at different reduction temperatures had higher electrocatalytic activity than the NiFe-LDHs original powder samples;the Ni3Fe/NiFe(OH)x samples prepared at the reduction temperature of 250? exhibited the best electrocatalytic activity.(The current density is 7.103mA·cm-2).This is due to the synergy between the Ni3Fe phase and the NiFe(OH)x phase in the catalyst sample.By further testing the electrocatalytic properties of the sample,the structure-activity relationship between the catalyst structure and the electrocatalytic performance of ethanol was explored.It is speculated that the NiFe(OH)x phase acts as an electrocatalytic active site for ethanol,favoring the generation of active Ni2+/Ni3+ redox couples;at the same time,its abundant structure hydroxyl groups can promote the desorption of acetyl groups.However,the Ni3Fe phase may contribute to the dissociation activation of the ethanol molecule and at the same time promote the electron transfer during the catalytic process.(3)Using NiFeRu-LDHs as precursor materials,NiFeOx@Ni3FeRu nanocomposites with core-shell structure were prepared by hydrogen reduction.The characterization results show that the Ni3FeRu alloy particle core size is about 50nm,while the NiFeOx shell is about 5nm thick.It was found that the addition of Ru in the bulk laminate accelerated the reduction of LDHs.It is speculated that Ru is first reduced by hydrogen in the sample;due to the "hydrogen overflow" effect,Ni3Fe particles are generated.At this point,the sample was a confined structure metal alloy/hydroxide nanocomposite Ni3FeRu/NiFe(OH)x.With the reduction temperature increased to 400 ?,the Ni3FeRu particles in the sample grow up,while the NiFe(OH)x phase gradually transforms into NiFeOx,forming a core-shell NiFeOx@Ni3FeRu nanocomposite.When the reduction temperature rises to 450 ?,Ni3FeRu alloy particles are produced.(4)The samples prepared at different reduction temperatures were applied to ethanol oxidation reaction and compared with NiFe-LDHs original powder samples.The core-shell NiFeOx@Ni3FeRu samples prepared at a reduction temperature of 400 ?,exhibited the best electrocatalytic activity(current density is 8.789 mA·cm-2).This may be due to the interfacial effect between the metal alloy phase and the oxide phase,further improving the catalytic activity of ethanol.On the contrary,Ru particles in the alloy particles may contribute to the dissociation of the Ni3Fe phase to the activated ethanol molecules.