Preparation Of Self-supported Two-dimensional Metal-organic Framework And Its Electrocatalytic Performance

Metal-organic framework materials（MOFs）are a new type of porous crystal materials which are self-assembled by metal ion（cluster）nodes and organic ligands through coordination bonds.Due to its characteristics of high specific surface area,adjustable pore structure,rich and modifiable components,scattered and periodic arrangement of active sites,etc.It is widely used in gas adsorption and separation,drug transmission,optical（electrical）sensing,catalysis,energy conversion and storage.However,the poor conductivity of MOFs materials and the low density of effective active sites limit their application in the field of hydrogen production by electrolytic water.To solve the above problems,in this paper,two-dimensional MOFs were grown in situ using conductive metal foam nickel（NF）as the base,and the micro-morphology,chemical composition and electronic structure of MOFs were regulated by in-situ formation of conductive second phase,organic ligand replacement and heteroatom doping.By improving the intrinsic conductivity,exposing more electrochemical active sites and accelerating the diffusion of electrolyte ions and electron migration,MOFs-based bifunctional water electrolytic catalyst with excellent electrocatalytic performance was obtained.The main research contents are as follows:（1）Nickel sulfide composite Ni Fe-terephthalic acid（TPA）nanosheets with high conductivity were prepared in situ via a two-step hydrothermal method.The size and crystal structure of nickel sulfide nanoparticles formed in situ can be controlled by controlling the concentration of sulfide agent.When the sulfurizing reagent concentration was 0.075 mmol L^-1,Ni S/Ni₃S₂ quantum dots（NSQDs）with a particle size of about 5 nm were formed in situ on the surface of Ni Fe-TPA,which were uniformly distributed on the Ni Fe-TPA nanosheets.The overpotential of the oxygen evolution reaction（OER）of NSQDs@Ni Fe-TPA was only 219 m V at 10 m A cm^-2,and the current density remained 90%at 1.5 V（vs.RHE）for 60 h,showing excellent stability.When the concentration of sulfurizing reagent t was increased to 0.15 mmol L-1,the prepared Ni₃S₂@Ni Fe-TPA showed excellent HER performance.When the current density is 10 m A cm^-2,the overpotential is only 109 m V,and when the current density is 20 m A cm^-2 running for 60 h,the overpotential only attenuates 20 m V.In addition,when the current density of NSQDs@Ni Fe-TPA and Ni₃S₂@Ni Fe-TPA is 10 m A cm^-2,the voltage of the cell is only 1.66 V,and the electrocatalytic performance of the cell is almost not attenuated after 60 h operation.Its excellent electrocatalytic performance is due to its self-supporting three-dimensional structure,which avoids the use of binders and facilitates the release of bubbles.Nickel sulfide and MOFs optimize the structure and electronic interaction,exposing more electrocatalytic sites and enhancing the electronic conductivity of MOFs.（2）Ni-thiophene dicarboxylic acid（Ni-TDC/NF）doped Ru with nickel foam as base by one-pot solvothermal method,and directly optimized the structure of MOF.The experimental results show that the doping amount of Ru significantly affects the morphology and electrocatalytic performance of Ni-TDC.The bifurcation at the top of Ni-TDC/NF becomes more and more obvious with the increase of Ru doping amount.In a series of Ru-doped samples,Ni₅Ru-TDC with a Ni/Ru molar ratio of 5 showed the best electrocatalytic activity.When the prepared Ni₅Ru-TDC was used as OER catalyst and HER catalyst,the overpotential was only235 m V（OER）and 35 m V（HER）when the current density was 10 m A cm^-2.After 60 h stability test,the overpotential of OER almost did not increase,and the current density of HER only attenuated by 2 m A cm^-2.When it is assembled into a symmetrical two-electrode water decomposition system,the current density of 10 m A cm^-2can be reached with only 1.53 V voltage,and the system can run stably for 40 h at this current density.The excellent catalytic activity and stability of Ru doped Ni-TDA are due to the fact that NF as the base increases the conductivity of the catalyst and prevents the agglomeration of Ni₅Ru-TDC during the catalytic process.Its brush-like structure can expose a wealth of active sites,while its three-dimensional porous structure can rapidly transfer mass and generate H₂ and O₂ escape as well as the synergistic interaction of Ru and Ni ions to optimize the electronic structure.（3）Co-MOFs（Co-TDA-1,1’-Fc_x/NF）of 1,1-ferrocene dicarboxylate（1,1-Fc）and thiophene dicarboxylate were prepared by a one-step solvothermal method.The experimental results show that 1,1-Fc ligand replacement can not only improve the conductivity of Co-TDA,but also regulate the microstructure of Co-TDA which can be transformed from nano-strip Co-TDA to nano-needle Co-TDA-1,1’-Fc_x/NF.Because ferrocene group of Fc has electrons,it can be used as a conductive shortcut in the OER process,and one-dimensional acicular Co-TDA-1,1’-Fc_x/NF can fully expose the active site,thus improving the OER performance of Co-TDA-1,1’-Fc_x/NF.The best OER activity was observed when the molar content of 1,1’-Fc was 0.2.The overpotential of Co-TDA-1,1’-Fc_0.2/NF is 236 m V at the current density of 10 A cm^-2,and can run stably for 60 h at the current density of 10 m A cm^-2.Its excellent OER activity is due to the structural changes that allow Co-TDA-1,1’-Fc_0.2/NF to have a larger surface area and be in full contact with the electrolyte during the catalytic process.