Preparation Of A New Type Of Two-dimensional Amorphous Metal Organic Framework And Its Oxygen Evolution Properties

Due to the serious environmental problems caused by the excessive use of fossil energy sources,the development of clean renewable energy become particularly important.Hydrogen energy is considered to be an ideal alternative for fossil fuels,and the production of hydrogen by electrolysis water provides an effetive and environmentally friendly method for large-scale hydrogen production.However,the anodic oxygen evolution reaction?OER?in the water electrolysis process involves a multi-step reaction process that the transfer of four electrons needs to overcome a huge reaction energy barrier and leding to slowly kinetic process,which is the key to limit the entire electrolysis water process.The introduction of high-efficiency OER catalysts can significantly reduce the potential energy of the reaction and improve the efficiency of the catalytic reaction.Therefore,the development of efficient OER catalysts is the focus of current research.Metal-organic frameworks?MOFs?have high specific surface area and controllable graded pores,a high degree of element selectivity and structural adjustability,and have shown great flexibility in material design,becoming our first choice.At the same time,we found that the surface of the layered two-dimensional material will expose a large number of coordination unsaturated surface atoms,which has higher reactivity and more catalytic reaction active sites.However,the existing two-dimensional MOFs synthesis methods?layered MOFs block stripping method,solvothermal synthesis,and surfactant-assisted methods,etc.?usually have the disadvantages of complex processes,low yields,and easiness to destroy structures.The thermal stability of MOFs is relatively low,which causes them to partially or completely collapse during the pyrolysis conversion process,making it difficult to maintain the two-dimensional morphology.In addition,in the existing synthesis of MOFs,imidazole and its derivatives with a polyhedral structure are generally used as ligands,and the coordination mode is single,which greatly limits the designability of the MOFs structure.In view of the unique ligand advantages of hexamethylenetetramine?HMT?,a series of transition metal-based MOFs were synthesized using hexamethylenetetramine as the ligand,which could be used directly as an electrolytic water anode oxygen evolution catalyst.XRD,XPS,TEM,SEM,FT-IR,UV-Vis,electrochemical testing,etc.were used to characterize the structure and test its performance.The main study contents are as follows:1.?Co,x Ni?-HMT was synthesized by a simple one-step low-temperature water bath method,which can be used directly as a catalyst without pyrolysis and has a good catalytic effect for OER semi-reaction.When the electrolyte was 1 M KOH solution,the current density overpotential of 10 mA·cm^-2 is only 330 mV,and the slope of Tafel is only 66 mV·dec^-1.It still has good stability after 20 hours,and the voltage fluctuation is only 0.0269 V.Its excellent performance is mainly benefit from the bimetallic synergistic effect of Co and Ni,and there are a large number of uncoordinated hydroxyl groups on the surface of the sheet.2.During the test,we found that the solvent has a certain effect on the dispersion of the catalyst,so we explored,on a certain basis,the effect of the solvent ratio and the change in the amount of Nafion on the performance of the catalyst.The existence of optimized Nafion content can form stable and efficient catalyst layers CL.Adding too much Nafion ionomer can block the interface mass transfer and active site.At the same time,if the amount of Nafion is too small,it will affect the growth rate of current density.Our research indicates that a well-organized ionomer network with 10%CL of Nafion can show excellent electrochemical performance.At this ratio,Nafion,catalyst particles,and electrolyte solution form an effective interaction,thereby ensuring good connectivity of the charge conduction path without inhibiting gas diffusion.3.The slow kinetics of the oxygen evolution reaction?OER?greatly limits the decomposition of water.Highly oxidized metals are needed as favorable active centers in OER.We synthesized the?Co,xFe?-HMT electrocatalyst by using the same synthesis method as 1.When the electrolyte was 1 M KOH solution,the current density overpotential reached 10mA·cm^-2 as low as 287 mV,corresponding to a Tafel slope of only 52 mV?dec^-1.After 20 hours of stability testing,it still has good stability,and the performance tends to become better.Its excellent performance mainly depends on the Fe element that can make Co into a higher o intermediates xidation level of Co³⁺.Its binding energy is lower than Co²⁺and it is easier to form CoOOH,which reduces the OER reaction barrier and thus enhances OER activity.