Preparation And Performance Of Nickel Cobalt Layered Double Hydroxide Nanosheet Arrays As Efficient Oxygen Evolution Reaction Electrocatalyst

The development of clean and sustainable energies as alternatives to fossil fuels has become a strong demand of modern society.Oxygen evolution reaction?OER?plays an important role in most energy conversion and storage processes,such as water splitting and metal-air batteries.Therefore,it is urgent to find high-performance electrocatalysts to improve energy conversion efficiency.Noble metal-based catalysts such as IrO₂ and RuO₂ are the most advanced catalysts for OER,but their high cost and scarce limit their large practical applications.Developing non-noble metal based OER catalysts has received much attentions.Ni-based and Co-based especially nickel cobalt layered double hydroxides?NiCo-LDH?demonstrates outstanding advantages in OER,due to their large specific surface area,layered structure,tuneable anions,interlayer distance and easily regulable electronic structure.Moreover,the conductive material such as nickel foam?NF?as the substrate,the active material close coated with NF,thus the active material will not fall off from electrode during the catalytic OER process,which improves the stability of electrocatalysts.In this paper,high activity NiCo-LDH based nanosheets were prepared on NF through hydrothermal method.Moreover,S-NiCo-LDH/NF nanosheets were prepared through one-step hydrothermal method.Firstly,the ternary NiCoFe-LDH/NF nanosheets were prepared by two-step hydrothermal method.X-ray diffraction?XRD?,scanning electron microscopy?SEM?and X-ray photoelectron spectroscopy?XPS?were applied to analyze the structure of as-prepared samples,and applied as an oxygen evolution reaction electrocatalyst.The oxidative etching and cation exchange caused by Fe³⁺-incorporating induce the change of morphology and the formation of Ni³⁺,thus increase the specific surface area and expose more active sites,finally greatly enhance the electrocatalytic OER performance.As a result,the Ni₂CoFe_0.5-LDH/NF electrode exhibits significantly enhanced oxygen evolution reaction performance with a small overpotential of 240 mV at the current density of 10 mA cm^-2,a small Tafel slope of 65 mV dec^-1 as well as an excellent durability in alkaline conditions for 72 h.Especially,the overpotential of Ni₂CoFe_0.5-LDH/NF is only 269 mV and 291 mV at 50 and 100 mA cm^-2,respectively.This work provides a good paradigm for the design and fabrication of ternary NiCoFe-based functional electrocatalysts,and demonstrates the great practical potential of the Ni₂CoFe_0.5-LDH/NF electrocatalyst in oxygen evolution reaction.Secondly,Ag/NiCo-LDH/NF composite was prepared by one-step hydrothermal method,and a series of structural characterization and electrocatalytic OER performance of the samples were conducted.The results indicate that the NiCo LDH nanosheet composite with Ag particles can form thinner nanosheet and more Co²⁺,and create more oxygen vacancies on the catalyst surface.The generation of more oxygen vacancies on NiCo-LDH surface improve the electronic conductivity and create more active sites for OER,thus enhancing the OER performance.Compared with NiCo-LDH/NF,Ag/NiCo-LDH/NF has better catalytic OER activity in alkaline condition.A current density of 10 mA cm^-2 can be achieved with a small overpotential of 261mV.Moreover,Ag/NiCo-LDH/NF only needed low overpotential of 300 mV and 324 mV at higher current density of 50 and 100 mA cm^-2,respectively.Besides,Ag/NiCo-LDH/NF shows outstanding OER stability with 80 h tested in alkaline condition.This work confirmed that LDH composite with Ag particles is an effective strategy to create oxygen vacancies on catalyst surface,which can improve the catalytic activity of materials for OER.Finally,S-doped NiCo-LDH/NF was prepared by one-step hydrothermal method using L-cysteine as the sulfur source.According to the results of XRD,SEM,TEM and XPS,the morphology and electronic structure of the samples can be effectively regulated by adjusting the amount of L-cysteine.When 0.81mmol of L-cysteine was added,S-NiCo-LDH/NF?S3?nanosheet formed porous structure,which helps to accelerate electrolyte penetration and the release of gas bubbles,and promotes the formation of high valence active phase,thus improving the activity of electrocatalytic OER.Among these as-prepared samples,the S3 shows the best OER performance,the overpotential at the current density of 10 mA cm^-2 is only 230 mV,and it has a low Tafel slope of 75mV dec^-1,as well as an excellent durability in alkaline conditions for 72 h.Such superior catalytic performance enables the potential application of S-NiCo-LDH/NF-3 in OER.