Structure Design And Electrocatalytic Performance Of NiFe-based Layered Double Hydroxides For Overall Water Splitting

Fabrication of efficient,stable and cost-effective bifunctional electrocatalysts to achieve overall water splitting reaction（OWS）has become a task of high priority,in view of the urgent need for the promotion of renewable energy-based hydrogen production systems.Transition metal-based layered double hydroxides（LDH）as one of promising bifunctional electrocatalysts have been intensely studied,due to the advantages of unique2D layered structure and outstanding physicochemical properties.This paper summarizes the latest strategies for LDH design modification,and designs a series of research experiments including nano-topography optimization,sheet thinning,defect introduction and element doping on OWS performance improvement and OER mechanism for related LDH electrocatalysts are also detailedly analysed.Finally,the current challenges and future development directions,which will provide ideas for developing better LDH dual-functional electrocatalysts and exploring effective equipment suitable for practical applications are put forward.The main research contents are as follows:（1）The easy preparation method for fabricating 3D porous core-shell Ni nanochains@Ni Fe layered double hydroxide（LDH）nanosheets are reported towards high efficent oxygen evolution reaction（OER）,hydrogen evolution reaction（HER）and OWS.In specific,a simple magnetic field-assisted method is used for the in-situ growth of Ni nanochain cores with large surface areas,allowing the subsequent vertical growth of few-layered Ni Fe LDH nanosheets to form the densely packed shells.Benefiting from the meticulously designed nanoarchitecture,the electrocatalyst possesses rich exposed active sites,plentiful charge transfer channels as well as high porosity for release of gas bubbles.The OER performance(η₁₀=218 m V)and durability of the electrocatalyst are far better than those of both commercial Ru O₂ and Ir O₂,while its HER(η₁₀=92 m V)performance is competitive over the benchmarking Pt/C electrode in alkaline electrolytes.When these catalysts are further employed as anode and cathode electrodes,small cell voltages of1.53 and 1.78 V can be achieved with the current densities of 10 and 100 m A·cm^-2for a long-term OWS reaction.Towards the commercial use,we design an electrolytic overall-water-splitting cell pack,which demonstrates a linear relationship between the numbers of packing cells and the increase of total current under a given voltage.（2）An ultrathin defect rich electrocatalyst（denoted as U-Ni Fe LDH-X）is prepared on Ni foam substrate via electrodeposition and subsequent acid etching.The inversitgations on physical characterization electrochemical analysis and density functional theory calculations reveal together that the acid treatment creates ultrathin LDH nanosheets,high amount of metal and oxygen vacancies which subtly modulates local coordination environments and electronic structures of the iron/nickel cations and finally brings a low energy barrier for OER.The acidic and oxidizing properties of phosphomolybdic acid provide many benefits for the modification of Ni Fe LDH.The U-Ni Fe LDH-PMo12 exhibit the best OER performance,with ultralow overpotentials of206,249 and 305 m V to achieve the current densities of 10,100 and 500 m A·cm^-2,accompanied by good stability under OER conditions.（3）Re-Ni Fe LDH-Ag P doped with different rare earth elements（Re=Ce,La and Nd）is prepared by phosphating nickel foam by chemical vapor deposition（CVD）method and the following electrochemical deposition.The synergistic effect of Re,Ni and Fe is observed,greatly improving OER activity of Re-Ni Fe LDH with onlyη₁₀ of 207 m V.Futher phosphating modification,the HER performance of Re-LDH was greatly improved.Thereinto the best HER performance of Ce-LDH-Ag P Withη₁₀ of 130 m V,which is approximately 95 m V lower than that of LDH(η₁₀=225 m V).Benefitting from the regulation of the chemical environment and electronic configuration of Ni and Fe by the empty d and f orbitals in rare earth elements,successful design of Re-Ni Fe LDH-Ag P in the work sheds light on development of high efficient electrocatalysts.