Chemical Composition Regulation And OER Performance Of NiFe Layered Double Hydroxide

Hydrogen energy is considered to be the most promising new energy source to replace traditional fuels in the new era due to its high calorific value and cleaner.Among many hydrogen production technologies,electrolyzed water has received extremely high attention because of its low production pollution,high product purity and more mature technology.However,in the two half-reactions of water electrolysis,the oxygen evolution reaction（OER）that occurs at the anode requires high barriers,which is a bottleneck restricting the full reaction.Therefore,the development of OER catalysts with excellent performance and economical utility has become a current research hotspot.NiFe LDH is the properest catalyst material to replace precious metals due to its excellent performance and low cost.Considering that during the hydrothermal growth process of NiFe LDH,the nickel-iron ratio inside the material is dynamically changing.So the nickel-iron ratio of NiFe LDH can be controlled by changing the hydrothermal process,and the synthesis of FeOOH/NiFe LDH electrocatalyst with self-doped component elements can be realized.In this paper,the single-step and two-step hydrothermal methods were used to control the chemical composition of NiFe LDH,and the physical characteristics of the product and the OER catalytic performance were tested and analyzed.In the single-step hydrothermal process,a series of NiFe LDH samples with different nickel-iron ratios were obtained by controlling the gradient of the feed nickel-iron ratio of the raw materials.The experimental results show that NiFe LDH can be successfully formed in a wide range of feed the ratio of nickel:iron（1.5:1-3.5:1）,and the crystallinity of LDH is the highest when the ratio of nickel:iron is around 2:1.When the feed ratio of nickel:iron is less than 1:1,the secondary phase FeOOH is formed because NiFe LDH cannot accommodate excessive iron species.The growth of LDH in samples with too high nickel content is relatively slow,and even after a long period of hydrothermal time,its lattice maturity will still be adversely affected.However,if the iron content in the raw material is too high,when the hydrothermal temperature is changed,the initiation temperature of the phase change from NiFe LDH to NiFe₂O₄ drops significantly,which will have a negative impact on the electrocatalytic performance of OER.The overpotential of Ni_2.2Fe₁ LDH with a moderate nickel-iron ratio at 10 m A·cm^-2 is only244.0 m V,and the Tafel slope is only 25.92 m V·dec^-1,showing the most excellent catalytic ability for OER reaction.At the same time,it has high catalytic stability.On this basis,this paper uses a two-step hydrothermal method to further explore the influence of nickel-iron distribution on the OER performance of the product.Firstly,a metal salt solution is pre-hydrothermally treated,and then it is subjected to a second hydrothermal treatment with another metal salt solution to construct a differentiated nickel-iron ratio distribution in LDH.The method can successfully synthesize a self-doped FeOOH/NiFe LDH heterostructure electrocatalyst with a core-shell structure,which has a special three-level assembly structure.Under optimal conditions,at a current density of 10 m A·cm^-2,its overpotential is only 192.0 m V,the Tafel slope is24.21 m V·dec^-1,and the electric double layer capacitor is as high as 15.0 m F·cm^-2.And it can maintain stability for more than 30 h at a current density level of 10 m A·cm^-2.Its excellent OER activity derives from the synergistic effect of the heterostructure of FeOOH and NiFe LDH.