Construction Of Efficient NiFe-based Oxygen And Hydrogen Evolution Electrocatalysts And Catalytic Activity Study

Hydrogen production from alkaline water electrolysis is a promising new industrial technology,which can replace fossil fuels for hydrogen production.In the process of alkaline water electrolysis,the energy conversion efficiency of oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）is affected due to the slow kinetics.The transition metal（such as Ni,Fe,Co,etc.）based electrocatalysts have attracted extensive attention due to the advantages of low cost and corrosion resistance.Compared with single metal catalysts,bimetallic catalysts（especially Ni Fe-based catalysts）have higher catalytic activity in alkaline medium.Meanwhile,doping heteroatoms（such as P,S,N,etc.）in transition metal base catalyst can regulate its electronic structure,promote the adsorption of reaction intermediates on the surface,and improve the catalytic activity of OER/HER.Based on this,in this paper,Ni Fe LDH was prepared by electroplating,combined with the chemical vapor deposition strategy to phosphorylate and sulfurate treatment based on Ni Fe LDH,respectively,constructed Ni_xFe_1-xP and Ni_xFe_1-xS electrocatalysts.In 1.0 M KOH electrolyte,their OER/HER performance were systematically studied.The active sites of the catalyst were revealed by X-ray photoelectron spectroscopy（XPS）,in-situ Raman and density functional theory（DFT）calculations.The main research contents are as follows:（1）The OER performance and active sites of the synthesized Ni Fe LDH,Ni_xFe_1-xP and Ni_xFe_1-xS catalysts were studied.In 1.0 M KOH,the Ni_xFe_1-xS shows an ultra-low overpotential of 122 m V at 10 m A cm^-2 compared with the other two catalysts.Ni_xFe_1-xP and Ni Fe LDH have similar OER activity and both require the 598 m V overpotential to reach 300m A cm^-2.In the OER process,it found that Fe in Ni Fe LDH and Ni_xFe_1-xP partially dissolves into the electrolyte and both are oxidized to Ni OOH by XPS and in-situ Raman,which well explains the reason for their similar activity.However,Fe dissolution is significantly less for Ni_xFe_1-xS,and partial oxidation forms Fe₂O₃/Fe OOH species.Density functional theory shows that Fe₂O₃/Fe OOH has a lower energy barrier,which can promote OER dynamics and enhance the electron localization function（ELF）of Fe-O bond,thus improving the OER activity of Ni_xFe_1-xS.（2）The HER catalytic performance and active sites of on Ni_xFe_1-xP and Ni_xFe_1-xS catalysts are investigated.In the process of HER,Ni_xFe_1-xP catalyst have better HER performance than Ni_xFe_1-xS catalyst.Ni_xFe_1-xP only provides 101 m V overpotential to drive the current density of 10 m A cm^-2,far lower than Ni_xFe_1-xS（144 m V）.It is revealed that the dissolution of S in Ni_xFe_1-xS and P in Ni_xFe_1-xP are accompanied with the dynamic evolution of the catalyst surface through XPS and in-situ Raman.Specifically,the dissolution of P in Ni_xFe_1-xP leads to the in-situ formation of defectiveα-Fe OOH species,while Ni_xFe_1-xS is found partial in situ conversion to theγ-Fe OOH phase.Density functional calculations shows that the defectiveα-Fe OOH promotes kinetics compared withγ-Fe OOH andα-Fe OOH,thus improving HER activity of Ni_xFe_1-xP.In this study,the combination of experiment and theory found that partial dissolution of Ni Fe-based catalysts was accompanied by the in-situ transformation of theirs surface active components during the alkaline electrolyte OER/HER process,thus affecting their catalytic activity in OER/HER,providing reasonable guidance for comprehensive understanding of the intrinsic activity of Ni Fe-based compounds and optimization design of efficient electrocatalysts.