Preparation And Oxygen Evolution Reaction Properties Of Rare Earth Layered Double Hydroxides

In recent years,hydrogen energy has attracted much attention due to its cleanness and high energy density,and electrocatalytic water splitting to produce hydrogen stands out as an efficient and clean technology.The slow four-electron transfer process and high overpotential in electrocatalytic oxygen evolution reaction（OER）make it a bottleneck for electrocatalytic water splitting for hydrogen production.Therefore,it is of great significance to design an efficient,inexpensive and highly stable catalyst.Layered double hydroxide（LDH）is a kind of inorganic functional material with a two-dimensional layered structure.The metal ions in the main layer are substitutable and the ratio of metal ions is tunable.LDH can also be combined with other materials.Combined to form nanocomposites with hierarchical structures,therefore,rational design and preparation of LDH catalysts can improve their catalytic activity.The unique electronic structure（4f layer electronic structure）,coordination number,and high oxygen storage capacity of rare earth elements have important applications in catalysis.Therefore,this paper focuses on the study of LDH.LDH is regulated by rare earth elements to improve the intrinsic catalytic activity of LDH,and then MoS₂ or MoSe₂is introduced to further improve the electrocatalytic activity of the material.The details are as follows:（1）A series of OER catalysts including CoFe LDH,La-CoFe LDH,Ce-CoFe LDH were prepared by coprecipitation method and one-step hydrothermal method.They were analyzed by SEM,TEM,XRD,FT-IR and XPS characterization techniques,and the results showed that the morphology and layered structure of La-or Ce-doped LDH did not change.The results of electrocatalytic OER performance studies show that the doping of La or Ce can enhance the electrocatalytic activity of CoFe LDH.At a current density of 10 m A/cm²,the OER potential of 6.7%La-CoFe LDH is 1.547 V,which is lower than CoFe LDH（1.694 V）.At the same time,6.7%La-CoFe LDH also has a lower Tafel slope（125 mV/dec）,and a lower charge transfer resistance（37.5Ω）.Since La³⁺and Ce³⁺have unique electronic structures and the ionic radius of La³⁺and Ce³⁺are much larger than that of Fe³⁺.After La³⁺or Ce³⁺partially replaces Fe³⁺,there are many defects in the CoFe LDH system,which affects the electronic structure of CoFe LDH.Enhanced electron coordination in the system makes it exhibit excellent electrocatalytic activity.（2）Based on the experimental results of the regulation of CoFe LDH by La or Ce,a CoAl LDH,Ce and MoS₂ doubly doped CoAl LDH（Ce-CoAl LDH@MoS₂）was prepared by replacing the multivalent Fe in the LDH with the monovalent Al.The experimental results show that the double doping of Ce and MoS₂ could significantly improve the catalytic activity of CoAl LDH.In the OER process,when the current density reaches 10 m A/cm²,the OER potential of 5%Ce-CoAl LDH@MoS₂ is1.508 V,which is much lower than CoAl LDH（1.733 V）.The Tafel slope of 5%Ce-CoAl LDH@MoS₂ is 65.7 mV/dec,which is lower than CoAl LDH（197.6 mV/dec）.Mechanistic studies show that the introduction of Ce and MoS₂affects the electronic structure of CoAl LDH,increases the active defect sites of CoAl LDH,enhances its conductivity,and reduces the overpotential of OER.（3）Based on the high electrocatalytic activity of Co-LDH in OER,the paper also studied Ni-LDH.A new type of composite material is designed and prepared by a simple co-precipitation method and hydrothermal method.It is composed of Ce-doped NiFe LDH and MoSe₂（Ce-NiFe LDH@MoSe₂）.The electrochemical test results of OER show that the overpotential of 6.7%Ce-NiFe LDH@MoSe₂ is only 221 mV at 10m A/cm²,which is better than NiFe LDH（409 mV）and Ru O₂（298 mV）.And 6.7%Ce-NiFe LDH@MoSe₂ also has a small Tafel slope（35.8mV/dec）,a small charge transfer resistance（16Ω）and a large electrochemically active surface area（698 cm²）.Mechanistic analysis shows that the excellent OER performance of 6.7%Ce-NiFe LDH@MoSe₂benefits from the increase of defect sites and the coordination of the interface between MoSe₂ and Ce-NiFe LDH.