Electrochemical Synthesis And Electrocatalytic Behavior Of NiFe-based Hydroxides For Oxygen Evolution

Hydrogen is considered a promising substitute for energy and fossil fuels because of its high mass specific energy density.Electrochemical water splitting provides a promising way for hydrogen production.However,the slow kinetics of the multielectron oxygen evolution reaction（OER）in water splitting is the greatest obstacle to hydrogen production.At present,the anode materials for water splitting still rely too much on highly active precious metals,such as iridium or ruthenium oxide.While,the high price and low reserves of these noble metal oxides limit their large-scale application.Enhance,It is urgent to develop a new type of cheap and efficient OER electrocatalyst to improve the economy of hydrogen production from water splitting.Transition metal-based hydroxides electrocatalysts are promising OER electrocatalysts due to their special electronic structure,s Table chemical properties and high intrinsic activity.Among them,Ni Fe-based hydroxide catalyst is considered to have more outstanding catalytic performance for OER,but it has the disadvantages of poor conductivity and low intrinsic catalytic activity.To further improve the electrocatalytic activity and stability of Ni Fe-based hydroxide for oxygen evolution,this paper is carried out from two aspects:"improving the electronic conductivity of Ni-Fe-based hydroxides"and"promoting the intrinsic catalytic activity of Ni-Fe-based hydroxides".Thus,a series of composite modified Ni Fe-based hydroxide electrocatalysts were prepared on conductive substrate nickel foam via electrochemical synthesis combined with template method,hydrothermal method,dissolution method and corrosion doping method,respectively.The rational design of electrocatalyst and the optimization of catalytic performance for oxygen evolution an be realized by increasing catalyst adhesion,conductivity,element doping,increasing catalytic activity area and creating catalytic defect sites.The specific research contents of this thesis are divided into the following four parts:（1）Ni Fe layered double hydroxide（Ni Fe-LDHs）is considered to be a promising substitute for noble metal electrocatalysts in oxygen evolution reaction.In this work,a Ni Fe-LDH/Ni/NF nanowire electrocatalyst was designed and prepared on nickel foam by means of a polystyrene microsphere template and two-step in situ electrodeposition method.The electrocatalyst had a high specific surface area,an increased number of electrocatalytic active sites and an improved electrochemical stability.In Ni Fe-LDHs/Ni/NF,the porous nickel interlayer is closely bound to the nickel foam matrix and the catalytic layer Ni Fe-LDHs nanowires,respectively.This structure not only improves the electron transfer ability of the catalyst nanowires to the matrix,but also increases the apparent active area of the catalyst,which is more favorable for catalytic OER.Compared with Ni/NF and Ni Fe-LDHs/NF,Ni Fe-LDHs/Ni/NF shows lower overpotential 247 m V(10 m A cm^-2),lower Tafel slope 35.52 m V dec^-1and longer catalytic stability(24 h@10 m A cm^-2).In the assembled overall water splitting system（+）Ni Fe-LDHs/Ni/NF||Ni/NF（-）,a current density of 10 m A cm^-2can be obtained at a low potential of 1.55V.（2）Amorphous Fe OOH nanosheets are electrodeposited onto NF-supported Ni Co₂S₄nanotube arrays to form a three-dimensional Fe OOH@Ni Co₂S₄/NF heterostructure,is a bifunctional electrocatalyst for overall water splitting.Thanks to the 3D structure and heterointerface,the Fe OOH@Ni Co₂S₄/NF catalyst shows good catalytic activity and stability for OER in 1 M KOH solution.It only needs 228 m V overpotential to reach 10m A cm^-2current density,Tafel slope as low as 44.03 m V dec^-1,and good OER catalytic stability(50 h@10 m A cm^-2).At the same time,it also shows good catalytic activity for hydrogen evolution reaction with only 112 m V overpotential to achieve 10 m A cm^-2current density.In addition,a low voltage of 1.56 V is required for Fe OOH@Ni Co₂S₄/NF to reach a current density of 10 cm^-2in the assembled overall water splitting system.Detailed research shows that the 3D core-shell structure with a large electrolyte contact interface,good electronic conductivity,large surface exposure of active sites by the Ni Co₂S₄nanotube arrays,and strong electron interaction at the heterogeneous interface between Fe OOH and Ni Co₂S₄,are the key factors for the excellent performance.（3）The amorphous Ni Fe Al-LDHs electrocatalysts were prepared by electrodeposition with nickel foam as the support,and the D-Ni Fe Al-LDHs electrocatalyst with defect sites was then obtained by alkali etching.The introduction of trivalent aluminum ions will change the coordination environment of Ni and Fe atoms in Ni Fe Al-LDHs.In addition,etching Al species of Ni Fe Al-LDHs in strong alkaline solution can greatly increase the number of active sites.The electrocatalytic activity of D-Ni Fe Al-LDHs is higher than that of Ni Fe-LDHs due to the introduction of iron and nickel defects in D-Ni Fe Al-LDHs nanosheets,which effectively adjusts the surface electronic structure and accelerates the adsorption of OH^–,thus increasing the catalytic rate of OER.It only needs a low overpotential of 262 m V with the current density of 10m A cm^–2,and the Tafel slope as low as 41.67 m V dec^–1.（4）A promisingly nano-porous W-doped oxygen vacancies-containing layered double hydroxides（Ni Fe W-LDHs）in situ growth on Ni foam via electrodeposition combined with chemical corrosion engineering strategy.By doping appropriate amount of W into Ni Fe-LDHs,the electronic structures of Ni and Fe are modulated through changing the local environment,and the oxygen vacancies are generated,resulting in abundant OER catalytic active centers on the catalyst surface.Due to excellent electronic conductivity and three-dimensional nano-porous configuration,the representative Ni Fe W₃-LDHs exhibits remarkable electrocatalytic activity for OER with a low overpotential(η₁₀=211 m V)and a small Tafel slope(36.44 m V dec^–1).Ni Fe W₃-LDHs can also maintain stability for at least 120 h at 10 m A cm^–2current density.Additionally,XPS and electron paramagnetic spectra indicated that W doping could significantly increase the oxygen vacancy concentration of Ni Fe-LDHs.Moreover,theoretical calculation also demonstrated the oxygen vacancy introduced by W doping could effectively tune the intrinsic electronic structure of Ni Fe-LDHs and optimize the adsorption energy of intermediates（*OOH）,thus accelerating the kinetics of OER.