Preparation And Catalytic Properties Of Fe/Ni/Mo Based Composite Catalysts For Water Splitting

With its high energy density,zero pollution and renewable nature,hydrogen is one of the ideal alternatives to traditional fossil energy sources.The use of renewable energy sources such as wind,solar and tidal energy to produce hydrogen by electrolysis of water can effectively improve energy efficiency and help to achieve the goal of“double carbon”development.At present,there are problems with high overpotential,short lifespan,and high cost in electrode oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）in industrial electrolysis of water for hydrogen production.Developing high-performance catalysts,improving the reaction kinetics and performance of electrodes,and reducing the activation energy and overpotential of electrolysis of water are the key to reducing costs.Precious metal-based catalysts（Ru O₂,Pt,Ir O₂）show advantages in solving the problem of high overpotential and low energy conversion efficiency,but their small reserves and high cost limit large-scale industrial applications.Therefore,the development of non-precious metal catalysts with abundant earth reserves,low cost and excellent catalytic performance is of great significance for the large-scale industrial application of hydrogen production by water electrolysis.In this thesis,non-precious metal（Fe,Ni,Mo）electrolytic water catalysts were synthesized using in situ growth methods to investigate their catalytic mechanisms and relationship between structure and performance.The specific content includes the following aspects:（1）Based on the theory of atomic diffusion and crystallization,the nano-catalyst Fe OOH@Ni₃（NO₃）₂（OH）₄/Nickle foam（NF）was prepared by in situ growth method and applied as an anode for oxygen evolution reaction（OER）under alkaline conditions.Flower cluster-like 3D nanosheets with high specific surface area were formed at 160°C,Fe OOH@Ni₃（NO₃）₂（OH）₄/NF has a significant catalytic performance for oxygen evolution reaction(100 m A·cm^-2@η=248 m V,where theηvalue corresponds to the overpotential at the corresponding current density).In situ Raman testing reveals that the catalyst underwent a phase transition from Ni₃（NO₃）₂（OH）₄ to Ni（OH）₂ and then to Ni OOH during the OER process,confirming that Fe OOH and Ni OOH are the main active intermediates at high potentials.After analysis of the combined XRD,Raman,XPS,SEM and TEM characterization results,we conclude that the excellent catalytic activity of the catalyst can be attributed to the nanoflake structure,which fully exposes the active center;The self-supporting integrated structure can reduce contact resistance and improve electron conduction speed.（2）Highly active and more stable catalysts（Fe OOH@Fe₂O₃@Ni（OH）₂/NF）were prepared using the introduction of a reductive atmosphere and a staged multi-frequency annealing method.The catalysts were annealed at multiple frequencies under a reductive Ar/H₂atmosphere and were prepared with a three-dimensional folded star-shaped nanosheet morphology.In addition,the catalysts exhibit a large number of amorphous and heterogeneous interfacial features.Fe OOH@Ni₃（NO₃）₂（OH）₄/NF shows improved catalytic stability for about90 h.A combination of XPS and in situ Raman tests revealed that partial electron transfer from Ni to Fe in the multicomponent heterogeneous interface and charge redistribution improved the chemisorption strength of the oxygen intermediate.In addition,the high potential oxidation of Ni³⁺with an e_g occupancy close to half-full optimizes the binding energy of the oxygen intermediate,which improves the intrinsic kinetics of the reaction and hence the catalyst performance.（3）A combination of electrochemical deposition and gas templating methods was used to prepare NiFe-LDH@NiFe₂O₄/NF catalysts with both activity and stability under high current density.Changing the deposition source and deposition time and adjusting the ratios will change the deposition morphology and the amount of deposition.The research results indicate that moderate and uniform deposition can improve catalyst activity,while excessive deposition can lead to surface cracks and agglomeration of nanosheets on the catalyst,resulting in a decrease in catalytic performance.Prepared catalyst NiFe-LDH@NiFe₂O₄/NF showed good OER catalytic activity and stability under 1.0 M KOH electrolyte(100 m A·cm^-2@η=253 m V,400m A·cm^-2@η=376 m V),and showed stable operation for 167 hours under high current density of 400 m A·cm^-2.The excellent OER catalytic activity and stability of NiFe-LDH@NiFe₂O₄ at industrial current density are mainly due to the high active phaseβ-Ni OOH and phase defects of the catalyst;The one-piece structure of the catalyst and the collector facilitates the increase of the electron transfer rate;The gas template facilitates the escape of bubbles and maintains the structural integrity of the catalyst electrode.The relationship between active phase change and catalytic activity at high current density was analyzed by Raman characterization.The catalyst Ni-O bond elongation during OER and a partial phase transition fromγ-Ni OOH toβ-Ni OOH occurred,with increased disorder and defects on the catalyst surface phase.The transformation of theβ-Ni OOH phase with a lower overpotential at high potentials and the reduction of the catalyst surface reorganization energy barrier favors the increased catalytic activity.（4）Amorphous carbon supported Mo₂C/NF hydrogen evolution catalysts were prepared by solid-state synthesis at different temperatures.It was found that amorphous carbon-loaded Mo₂C/NF-600 exhibited excellent catalytic performance when used as a HER electrocatalyst in alkaline solutions,with an overpotential of 58 m V at a current density of 10 m A·cm^-2.The particle size of nano Mo₂C in the catalyst is about 5nm,and the fine and evenly dispersed nano particles can expose more active site.The amorphous carbon carrier network enriches the electronic conduction path and improves the conduction speed,which improves the hydrogen evolution performance of the catalyst.The results of electrolytic water tests with Mo₂C/NF-600and NiFe-LDH@NiFe₂O₄/NF showed that a current density of 10 m A·cm^-2 requires a voltage of 1.563 V.The dual electrode system can operate stably for 160 hours under a high current density of 400 m A·cm^-2.This thesis aims to prepare the iron/nickel/molybdenum based composite catalyst by in-situ synthesis method,which exhibits excellent catalytic activity and stability for electrolysis of water under alkaline conditions.The integrated structure of catalyst and current collector,the increased phase disorder and defects during the catalytic process,the transformation of active phases,the elongation of metal oxygen bonds,and changes in electronic configuration jointly enhance the activity and stability of the catalyst,providing a reference and theoretical basis for the rational design and preparation of efficient electrolytic water catalysts.