Synthesis Of Nickel-based Metal-organic Frameworks Derivatives And Their Application In Electro-oxidation Of Urea

Urea oxidation reaction（UOR）has potential applications in the fields of hydrogen production from electrolytic water,direct urea fuel cells,and wastewater treatment.However,UOR involves complex multi-electron reaction paths and slow reaction kinetics,which requires the design and development of high performance catalysts.Nickel-based materials with low cost and excellent catalytic activity have gradually become the focus of researchers.Strategies such as electronic structure control,morphology engineering and atom doping can significantly improve the electrocatalytic performance of nickel-based materials.Metal-organic frameworks（MOFs）are crystalline or amorphous coordination compounds composed of organic ligands and inorganic node coordination compounds.Their instability in alkaline solution gives them the ability to convert into specific morphologies of hydroxides/oxyhydroxides.In this paper,a simple one-step solvothermal method was used as the strategy for the synthesis of MOFs.Firstly,based on the synthesis of high performance Ni（OH）₂ electrocatalysts,the optimal organic ligand of pyromellitic acid（PA）was successfully selected from a series of aromatic polycarboxylic acids.Secondly,a series of bimetallic MOFs and their derivatives were synthesized using PA as the selected ligand,and their electrochemical properties in the electro-oxidation of urea were studied.The specific research contents are as follows:（1）Three nickel-based MOFs were synthesized by solvothermal method and in-situ converted into a series of Ni（OH）₂ self-supporting electrodes to investigate the effects of different aromatic polycarboxylic acids on the morphology,structure and electrochemical behavior of Ni（OH）₂.Electrochemical datas indicate that the Ni（OH）₂ derived from pyromellitic acid MOF exhibits the highest electrocatalytic activity,reaching current density of 50 m A cm^-2 at 1.364 V（vs.RHE）.Its excellent electrochemical properties are attributed to the three-dimensional porous layered structure,resulting in more exposure of active sites and excellent electron/mass transfer properties.In addition,the Ni（OH）₂ derived from pyromellitic acid MOF has excellent long-term operational stability.（2）Using nickel foam as three-dimensional substrate and nickel source simultaneously,nickel foam self-supporting Ni Co MOF precursor was designed and synthesized through pyromellitic acid organic ligand and Co coordination.After alkaline treatment,it converted into a Ni doped Co（OH）₂ three-dimensional aggregate catalyst（Ni Co（OH）₂）.Different Ni/Co ratios can regulate the micromorphology of the derivative Ni Co（OH）₂.The results of scanning electron microscopy（SEM）indicate that Ni Co（OH）₂-2 exhibits three-dimensional spherical shapes composed of uniform hexagonal nanosheets.The unique two-dimensional hexagonal nanosheets assembly hierarchy provide rich side surfaces,highly accessible surface areas,and numerous edge sites.In-situ Raman results showed that the carboxyl groups adsorbed on the surface of the catalyst can accelerate the proton coupled electron transfer pathway to improve the UOR electrocatalytic activity,which drives a current density of 100 m A cm^-2 at only1.368 V（vs.RHE）.（3）NiMn MOF was synthesized using a bimetallic synergetic strategy,and three-dimensional layered NiMn OOH nanosheets electrocatalyst with a large number of Ni OOH active sites was synthesized in-situ using a room temperature alkaline conversion method.The effects of different metal salts on the derivatives were also studied.X-ray photoelectron spectroscopy analysis shows that Cl^-is more favorable for the formation of Ni³⁺species than SO₄^2-and NO₃^-.The synergistic effect between Ni and Mn can reconstruct the electronic structure of nickel centers,promoting the formation of rich Ni OOH catalytic active species.Ni₅Mn₅ only requires 1.34 V（vs.RHE）to provide a current density of 100 m A cm^-2,while maintaining high catalytic stability for 10 hours.