The Oretical Study On Electrocatalytic Oxygen Evolution Reaction Over Iron-based LDHs And The Delamination Behavior Of LDHs

Water electrolysis is the focus of electrochemical energy storage and conversion research,and the design and synthesis of electrocatalysts for the oxygen evolution reaction（OER）is the key to improve the energy efficiency of water electrolysis.Iron-based layered double hydroxides（LDHs）are one of the efficient OER electrocatalysts,and their electrocatalytic performance can be effectively improved by tuning their microstructure.The common methods to optimize the microstructure of catalysts are to modify the catalytic interface and to control its thickness and size.In this work,density functional theory（DFT）is used to investigate the structural characteristics,electronic properties,electrocatalysis of iron-based LDHs electrocatalysts containing vacancies and supported by Ir O₂.The mechanism of oxygen evolution reaction have been studied,and the influence of interlayer spacing stretching on the energy and properties of LDHs（different metal species,proportions,and anion species）during the delamination process is also explored.The main contents and conclusions are as follows:（1）A single-layer NiFe-LDHs and CoFe-LDHs models with 8 different vacancy types is established（single vacancy:H,O,Ni,Fe;metal-oxygen double vacancy:Ni/Co-O,Fe-O;metal-oxygen-hydrogen triple vacancies:Ni/Co-OH,Fe-OH）,the effects of different types of vacancies on the catalytic OER performance of Ni/Co Fe-LDHs were studied from the aspects of structure,electronic properties,and mechanism.The results show:that on the single-vacancy Ni/Co Fe-LDHs,the OER potential energy determination step（PDS）is（H₂O（l）+O*→OOH*+H⁺+e^-）;on the di-and tri-vacancy Ni/Co Fe-LDHs,the PDS is（OOH*→*+O₂+H⁺+e^-）.It is found that the difference in adsorption energy(ΔG_OOH*-ΔG_OH*)has a volcanic relationship with the overpotential(η^OER),and the Ni Fe-LDHs containing Fe-O-H vacancies and Co Fe-LDHs containing Fe vacancies are in a“volcanic”relationship.,theirη^OER are 0.388V and 0.817 V,respectively.Thus the Ni Fe-LDHs with three vacancies have better catalytic performance.Further analysis of the partial density of states of the 3d orbitals of the Ni/Co Fe-LDHs containing Fe-O-H/Fe vacancies shows that the introduction of these two vacancies can delocalize electrons near the Fermi level（E_F）,and make the center of the d-band move away from the E_F.The strong adsorption energy of the bulk is weakened to a moderate adsorption energy,which effectively reduces theη^OER and improves the electrocatalytic OER performance.（2）The electronic properties and electrocatalytic OER mechanism of composite oxides（Ni Fe OOH and Co Fe OOH）obtained by using Ni Fe-LDHs and Co Fe-LDHs as precursors before and after IrO₂ supporting were calculated,theoretical screening of the active sites of Ir O₂@Ni/Co Fe OOH was carried out.The results show:the PDS are（H₂O（l）+O*→OOH*+H⁺+e^-）,and the active sites for OER are Fe sites wether Ir O₂ supported on Ni/Co Fe OOH or not.After Ir O₂ supporting,the Gibbs free energy change of the PDS can be effectively reduced,and the overpotential of the supported catalyst is lower than that of its clean surface and the bulk LDHs of the previous chapter.It can be seen from the differential charge map and Hirshfeld charge analysis that Ir O₂ can transfer electrons to the metal active sites,thereby improving the OER catalytic performance.（3）The changes of binding energy and band structure of M^Ⅱ_nM^Ⅲ/ΙV-A-LDHs(M^ΙΙ=Mg²⁺,Co²⁺,Ni²⁺,Zn²⁺;M^ΙΙΙ=Al³⁺,Ga³⁺,Fe³⁺;M^ΙV=Ti⁴⁺;n=2,3;A=Cl^-,NO₃^-,ClO₄^-,SO₄^2-,CO₃^2-,HCOO^-)containing 6 different interlayer guests and 7 different layer hosts under different interlayer spacings were calculated.The results show:for the above 13 kinds of LDHs,the binding energie first increase and then remain unchanged with the increase of the interlayer stretching ratio（n）.When the host-guest interaction is very weak,and delamination will occur.By analyzing the band structure,it is found that for the above LDHs,If the interlayer anion charge is large,or the layer contains transition metals,it will have a small band gap.At the same time,it was found that the band gap of LDHs will be reduced after delamination,which is beneficial to accelerate the electron transport and realize high-efficiency electrocatalytic OER.This work demonstrates the effect of vacancies and supports and thickness on the performance of OER over LDHs,which provides theoretical information and basis for the screening and design of LDHs-based OER electrocatalysts.