Structural Regulation Of NiFe-based Oxygen Evolution Electrocatalyst And Research On Structure-activity Relationship

The lack of high-performance catalysts for oxygen evolution reaction（OER）severely limits the hydrogen production efficiency of electrolytic water technology.Compared with other electrocatalysts,nickel-iron（Ni Fe）based transition metal catalysts possess outstanding catalytic activity,good operational durability and abundant reserves,which are the most promising catalysts for realizing low-cost industrial hydrogen production,and further improving the activity of Ni Fe based materials has become the research focus in the field of OER.In the process of exploring high-efficiency Ni Fe based catalysts,researchers have found that the catalytic performance is closely related to its own structure.Therefore,the systematic revelation of the structure-efficiency relationship has important guiding significance for the design of efficient Ni Fe based OER catalysts.In this paper,a series of efficient and durable Ni Fe based OER catalysts were designed and prepared from the angle of morphology,element composition and heterojunction.The relationship between the structure of Ni Fe based catalysts and the OER performance was studied by combining structural characterization,electrochemical test and density functional theory（DFT）calculation.The main research content is divided into the following three aspects:（1）The structure regulation based on morphology,Ni Fe layered double hydroxide（LDH）with different citrate content were prepared by one-step hydrothermal method.The results show that the participation of citrate in the preparation of Ni Fe LDH contributes to the uniform distribution of Ni and Fe elements,and thus better plays the synergistic effect.In addition,using citrate as intercalator can enlarge the layer spacing of Ni Fe LDH crystal structure and induce the partial exfoliation of nanosheet structure,while the size of layer spacing and the degree of exfoliation are regulated by the content of citrate.It was found that the layer spacing determined the catalytic reaction mechanism,and Ni Fe LDH with large layer spacing showed faster reaction kinetics.Meanwhile,the exfoliated nanosheet has an ultrafine structure,which is conducive to the exposure of abundant edge active sites.Benefit from these,the citrate intercalation Ni Fe LDH-4 catalyst exhibited significantly enhanced OER activity,with the required overpotential at 10 m A cm^-2 current density(η₁₀)reduced by about 30 m V compared with the traditional carbonate intercalation Ni Fe LDH-1.（2）Take element composition as the entry point of structural regulation,a series of Ni Fe alloys with different Ni/Fe stoichiometric ratios and surface structures have been prepared.It was found that the symbiotic oxide layer structure of Ni Fe alloy has a significant effect on the OER activity.It is found that the symbiotic oxide layer structure on the surface of Ni Fe alloy has a significant effect on the OER activity.The surface Fe/Ni ratio and oxygen vacancy（O_v）content of Ni Fe alloys are significantly different with the change of Ni/Fe stoichiometric ratio,and the structure of the symbiotic oxide layer can also be changed by adjusting the hydrogenation temperature,thus affecting the OER activity.DFT calculations show that the presence of oxide layer on the surface of Ni Fe alloy constitutes the metal/oxide interface that drives the OER collaboratively,which can change the rate control step of catalyst and reduce the limiting reaction barrier.The O_v in the oxide layer can regulate the adsorption energy of intermediate,and the appropriate content of O_v can effectively improve the OER activity.Ultimately,the Ni₄Fe₁ alloy with the optimal surface structure showed superior OER performance(η₁₀=242 m V),significantly superior to the noble metal Ir O₂ catalyst.（3）Take constructing heterojunction and further enhancing the activity of Ni Fe alloy as a foothold,a series of Ni₃Fe/Mo O_x materials with Mott-Schottky heterojunctions were in-situ synthesized by high pressure hydrogenation.The results show that amorphous Mo O_x can not only confine Ni₃Fe alloy to expose abundant catalytic sites,but also effectively improve the wettability of the catalyst in the electrolyte.Meanwhile,the internal electric field at the Ni₃Fe/Mo O_x heterojunction can accelerate the charge transfer,and the Mo element can effectively regulate the electronic structure of the Ni site,thus accelerating the formation of the active phase Ni OOH.DFT calculations show that the construction of heterojunction can optimize the adsorption energy of Ni Fe alloy for OOH*intermediate and further reduce the limiting reaction barrier.Finally,the structurally optimized Ni₃Fe/Mo O_x-0.5 showed excellent OER performance(η₁₀=224 m V),and the assembled electrolytic cell only needed 1.54 V voltage to produce a current density of 10 m A cm^-2.