Preparation Of Ni-Based Metal-Organic Framework Self-Supporting Electrodes And Their Oxygen Evolution Performances

As a green energy technology that can convert intermittent electrical energy into hydrogen energy,water electrolysis for hydrogen production with zero-carbon emissions and simple operation,which can alleviate the problems of environmental pollution and climate warming caused by fossil fuels combustion.The key to this technology is to develop efficient and low-cost Hydrogen Evolution Reaction（HER）and Oxygen Evolution Reaction（OER）electrocatalysts.However,multi-step proton-coupled electron transfer involved in OER seriously affects the efficiency of hydrogen production.On the other hand,although traditional noble metals Ir O₂,Ru O₂and their composites have high OER catalytic performance,their scarcity and high cost seriously restrict their large-scale commercialization to satisfy future demand.Therefore,this prompted us to explore alternative efficient,stable and abundant non-noble metal-based catalysts,such as Ni-based materials.Metal-Organic frameworks（MOFs）materials are considered as one of the most promising electrocatalysts for OER due to their permanent porosity,rich structural,uniformly dispersed metal centers and high specific surface area.In this paper,Ni-based MOF materials were selected to optimize their electrocatalytic performance and deepen the knowledge and understanding of related mechanisms via two mainly strategies:the first strategy is to tune the intrinsic activity of MOFs by constructing intermetallic synergistic effects or heterostructures;the second strategy is to fabricate MOF self-supporting electrodes by secondary growth or in situ growth method to expose more active sites and improve the conductivity of MOF.The specific research work is as follows:（1）Preparation of Fe Co Ni-btz/NF self-supporting electrode by secondary growth method and its OER property:neutral azole-based compound of(1,4-bis（4H-1,2,4-triazol-4-yl）benzene was selected as ligands,and Ni Cl₂was used as the metal source to design and synthesize two ionic MOFs（Ni-btz）,as well as their unique crystal structures were investigated in detail.Then,nickel foam（NF）with a 3D macroporous structure and good electrical conductivity were selected as the substrate,the continuous dense monometallic Ni-btz/NF electrode and trimetallic Fe Co Ni-btz/NF electrode were prepared using the secondary growth（crystallization）method.The characterization results show that Fe Co Ni-btz/NF electrode exhibits the most excellent electrocatalytic OER performance and good electrocatalytic stability compared to monometallic Ni-btz/NF electrode and directly synthesized MOFs electrode,requiring only the overpotential of 263 m V to achieve a current density of 10 m V cm^-2with a small Tafel slope of 64 m V dec^-1.The post-catalytic characterization revealed that Ni-btz/NF and Fe Co Ni-btz/NF surface were restructured during the OER process to generate metal oxyhydroxide,which was generally considered as the active centers of OER,and accordingly we speculated on the OER reaction mechanism in this system.（2）Preparation of Ni Fe-btz/NF-OH self-supporting electrode by semi-sacrificial template method and its OER property:a Ni（OH）₂film by acid-base treatment was grown on the surface of NF substrate as a semi-sacrificial template,and then continuous and dense 3D Ni Fe-btz/NF was prepared by a hydrothermal solvothermal method.Benefiting from the ionic framework and Ni-Fe bimetallic synergistic effect of MOFs,the as-prepared Ni Fe-btz/NF-OH electrode exhibits excellent OER performance under alkaline conditions with a low overpotential of 239 m V at 10 m A cm^-2.The electronic state changes of atoms and reaction intermediates during the OER reaction were investigated using X-ray Photoelectron Spectroscopy（XPS）and In-Situ Raman measurements（In-Situ Raman）.The OER reaction mechanism,electronic states density and charge transfer in the Ni Fe-btz system were also fully explored by Density Functional Theory calculations（DFT）.Notably,Partial Density of States（PDOS）data for Ni Fe-btz indicate that Cl 2p is the main contributor to the electron density near the Fermi level（E_F）,suggesting the azole-based ligands have a favorable effect of on the enhancement of MOFs conductivity.（3）Preparation of Ni-MOF/Ni（OH）₂/NF heterostructure self-supporting electrode by lattice matching method and its OER property:Ni（OH）₂with layered structure was grown on the surface of NF by hydrothermal synthesis,and then Ni-dobpdc（2,5-dihydroxybiphenyldicarboxylic acid as ligand）matched with Ni（OH）₂lattice was selected as the target MOF electrocatalyst.Uniform and continuous composite electrodes were prepared under hydrothermal and solvent thermal conditions.The growth mechanism of the composite electrodes was investigated by changing the reaction time,and the OER performance of these composite MOF film was probed.Among them,Ni-MOF/Ni（OH）₂/NF-24 h（thickness of about 3.56μm）has the most excellent OER activity,requiring only the low overpotential of255 m V to reach a current density of 10 m A cm^-2,which is superior to the vast majority of Ni-based materials.We took the intermediates obtained after different cyclic voltammetry tests as the entry point and deeply investigated the reason of gradual increase of Ni-MOF/Ni（OH）₂/NF activity.Meanwhile,monodispersed Fe atoms were introduced into the Ni-dobpdc framework,and the obtained electrode of OER performance was further improved.It is required only 225 m V overpotential to achieve a current density of 10 m A cm^-2,and the tested current density was able to meet industrial application standards of 500 m V cm^-2.