Preparation Of Self-supporting MOFs Nanosheet Electrodes And Study Of Their Water Electrolysis Performanc

As a green and clean energy,hydrogen energy has the advantages of light weight,high calorific value,and pollution-free combustion,and is considered to be one of the most effective energy carriers to achieve carbon neutrality.Water electrolysis is an efficient and clean hydrogen production technology,and its hydrogen production process is simple and the product purity is high.In recent years,electrolyzed water technology is rapidly moving towards scale and commercialization.Traditional precious metals Pt,Ir,Ru and their composites have high electrocatalytic performance,but due to their scarcity and high cost,they hinder large-scale commercial applications.This has prompted us to explore alternative non-precious metal-based electrocatalysts that are efficient,stable and abundant.Metal-organic frameworks（MOFs）are a class of crystalline porous materials with high specific surface area,high porosity and designable structure,which show great development potential as novel electrocatalysts.However,the inherent low conductivity and low stability of MOFs greatly hinder their further application in the field of electrocatalysis.How to design and synthesize highly stable and highly conductive MOFs is the key to improving electrocatalytic performance.Based on the above ideas,Ni-based metal-organic frameworks（Ni-MOFs）and polyoxometalate-based metal-organic frameworks（POMOF）were selected in this paper,using nickel foam with high conductivity as the base material,using self-sacrificing template methods to grow Ni-MOF nanosheets and POMOF nanopillar arrays stacked from two-dimensional nanosheets,and fabricating self-supporting MOFs nanoarrays to leak more active sites and improve the conductivity of MOFs to optimize the electrocatalytic performance of electrocatalysts.Finally,the performance of electrocatalytic hydrogen evolution（HER）in acidic media and oxygen evolution（OER）in alkaline conditions were studied.The specific content is summarized as follows.（1）Preparation of self-supporting Ni-based metal-organic framework（Ni-MOF）nanosheet electrode and its oxygen evolution performance:3,5-bis（1-imidazole）pyridine and terephthalic acid were selected as ligands,and Ni（NO₃）₂was used as the metal source to construct Ni-based metal-organic framework materials.On 3D porous nickel foam with good electrical conductivity,the 2D ultrathin（about 2 nm）nickel-based MOF nanosheet array was synthesized by self-sacrificing template method.The association between 2D nanosheets and crystal growth preference was studied,and the ligand-assisted controllable growth strategy was adopted,and the thickness of 2D nanosheets changed correspondingly after the corrosion-dissolution-coordination process in the ligand solution through the corrosion-dissolution-coordination process by regulating the amount of ligand terephthalic acid.The ultrathin Ni-MOF nanosheets after high-temperature pyrolysis in a nitrogen atmosphere are transformed into nitrogen-doped carbon nanocomposites,which exhibit satisfactory electrocatalytic OER performance,only 170m V overpotential to reach a current density of 10 m A cm^-2,and have a very low Tafel slope of 41 m V dec^-1,which shows good electrocatalytic stability after stability tests up to20 h.（2）Preparation and hydrogen evolution performance of self-supporting polyoxometalate-based metal-organic frameworks（POMOFs）nanoelectrodes:Two Keggin-based metal-organic framework materials([ε-PMo ₈^VMo ₄^VIO₃₇（OH）₃Zn₄][BPB]（abbreviatedasZn Mo-POMOF,BPB=1,4-bis（p-pyridyl）benzene）and[Co₂（TIB）₂(PMo₁₂O₄₀)]·Cl·4H₂O（abbreviatedasCo Mo-POMOF,TIB=1,3,5-Tris（4-pyridyl）benzene）,were constructed by traditional hydrothermal synthesis method.In these two compounds,POM fragments as nodes are directly linked to organic ligands,forming one-dimensional chains and two-dimensional layers structures.Polyoxometalate-based metal-organic framework（Co Mo-POMOF and Zn Mo-POMO）nanomaterials are grown in situ on conductive substrate nickel foam as self-supporting electrodes by self-sacrificing template method.Due to the strong redox characteristics of POM,the high specific surface area,high porosity of MOF and high conductivity of nickel foam substrate,the prepared Co Mo-POMOF and Zn Mo-POMOF self-supporting electrodes exhibit excellent electrocatalytic performance for HER in acidic media（0.5 M H₂SO₄）,with low overpotentials of 137 m V and 180 m V at current density of 10 m A cm^-2,and Tafel slopes of 59 m V dec^-1and 66 m V dec^-1,respectively,as well as good electrochemical stability.The four different modes of oxygen（μ₃-bridging oxygen（O_a）,μ₃-central oxygen（O_c）,μ₂-bridging oxygen（O_b）and terminal oxygen（O_t））present in the Keggin unit were studied in detail for the Gibbs adsorption free energy of H*(ΔG_H*).The results showed thatμ₃-bridging oxygen Oa site was the optimal adsorption site,and the Gibbs free energy for H*adsorption was lower than that of the other three oxygen active sites.