Structural Design Of The Nickel-based Metal-Organic Frameworks And Its Electrocatalytic Performance

The development of various energy storage and conversion technologies can effectively improve the composition of the energy mix,among which the development of water electrolysis for green hydrogen production using renewable electrical energy and the electrosynthesis of hydrogen peroxide are key electrochemical technologies to drive the energy transition revolution.However,the slow kinetics and low selectivity of electrocatalytic reactions lead to the high energy conversion efficiency and cost of these technologies.Therefore,there is an urgent need to develop non-precious metal-based electrocatalytic materials with low cost and high activity to drive their further development.Metal-organic framework(MOF)materials have a wide range of applications in energy electrocatalysis due to their advantages of dispersed metal active sites,tunable coordination environment and rich porous structure.However,MOF-based materials have the problems of low intrinsic activity,poor electrical conductivity and unclear electrocatalytic reaction mechanism,which limit the electrocatalytic activity of MOF-based materials.In this thesis,we aim to develop highly active Ni-based MOF materials and structurally design Ni-based MOF materials through doping modulation and interfacial coupling strategies to modulate their material morphology and microscopic electronic structure,enhance the electrocatalytic activity of Ni-based MOF materials,and reveal the conformational relationships between active site composition and electronic structure and electrocatalytic performance.To address the problem of low intrinsic catalytic activity of Ni-based MOF materials,a non-metallic S-doped Ni-based MOF nanosheet array electrode material(S-NiBDC)was developed by morphology design and doping modulation strategy.Due to the macroscopically ordered nanosheet array structure and the modulation of the electronic structure by non-metallic S doping,the overpotential of the electrocatalytic alkaline hydrogen evolution reaction is only 310 mV at a current density of 1.0 A cm-2,and the Tafel slope is 75 mV dec-1.The stability of electrocatalytic hydrogen evolution at high current density can exceed 150 h.The experimental and theoretical studies demonstrate that the S doping can regulate the electronic structure and local coordination environment of Ni-based MOF materials,and generate the "Ni2-S1" ternary active region in the framework structure,which can not only enhance the structural stability of the MOF materials,but also enhance the ability of the catalytic materials for water activation and improve the electrocatalytic hydrogen evolution activity.To address the problem of unclear catalytic reaction mechanism of Ni-based MOF materials,a rare-earth metal Ce-doped Ni-based MOF nanosheet array electrode material(Ce-NiBDC/OG)was developed by in situ hydrothermal doping strategy.Thanks to the superhydrophilic/superhydrophobic properties of the electrode material surface and the modulation effect of the Ce doping,the Ce-NiBDC/OG exhibited excellent performance in alkaline electrolytic oxygen evolution reaction with an overpotential of 265 mV and a Tafel slope of 46 mV dec-1 at a current density of 10 mA cm-2.The experimental characterization combined with calculations show that Ce-NiBDC/OG can be derived to Ce-doped NiOOH nanocrystals(Ce-NiOOH)during the electrocatalytic oxygen evolution in alkaline media,and the in situ derived Ce-NiOOH is the active component in oxygen evolution reaction of this MOF-based materials.The doping of Ce enhances the adsorption of OH-and significantly reduces the reaction energy barrier in the oxygen evolution reaction.To address the slow reaction kinetics of Ni-based MOF materials for hydrogen evolution reaction,a nickel phosphide coupled Ni-based MOF nanorod array electrode material(Ni-MOF/Ni2P@EG)was developed by a template-vapor conversion coupled with a controlled phosphorization strategy.Thanks to the enhanced hydrophilicity and charge transport capability of the material,Ni-MOF/Ni2P@EG exhibits good performance in alkaline electrolytic hydrogen evolution reaction with an overpotential of 132 mV and a Tafel slope of 59 mV dec-1 at a current density of 10 mA cm-2.The strong interfacial coupling between Ni2P and Ni-MOF promotes the transfer of electrons from Ni2P to Ni-MOF,which increases the Ni valence state in Ni-MOF/Ni2P@EG materials and facilitates the adsorption of water molecules in the alkaline electrolytic hydrogen evolution reaction,thus accelerating the water dissociation kinetics and improving the performance of the electrolytic hydrogen evolution reaction.To address the low selectivity of Ni-based MOF materials for the preparation of hydrogen peroxide by electroreduction of oxygen,a nickel hydroxide coupled with non-metallic B-doped Ni-based MOF nanosheet material(Ni(OH)2/B-NiBDC)was developed through a ligand doping strategy combined with in situ hydrolysis process.The selectivity of Ni(OH)2/B-NiBDC in the electrocatalytic two-electron oxygen reduction reaction to hydrogen peroxide in alkaline media is excellent with a selectivity of>95%over a wide potential range.The characterizations combined with experiments showed that the doping of non-metallic B and interfacial coupling could simultaneously modulate the electronic structure of the catalytic material,and the boron doping significantly inhibited the further electrocatalytic reduction of hydrogen peroxide.The catalytic current density of 1000 mA cm-2 was further tested in a gas diffusion cell,while the Faraday efficiency could be maintained at about 90%and the electrocatalytic stability could reach 24 h.In summary,a series of Ni-based MOF catalytic materials were prepared by metal/nonmetal doping modulation and interfacial coupling strategy in this thesis,and showed excellent activities for hydrogen production from water electrolysis and hydrogen peroxide production from electroreduction of oxygen,which provided research ideas and theoretical guidance for the rational design and development of Ni-based MOF catalytic materials.