Electrocatalytic Oxidation Based On Nickel-based Metal-organic Frameworks

Due to the non-renewable and environmental pollution problems of fossil fuels,human society urgently needs to develop sustainable and clean energy.Due to its high calorific value and zero carbon emissions,hydrogen has become one of the ideal clean energy sources.Hydrogen production by water electrolysis is considered to be an effective way to produce hydrogen on a large scale.Water electrolysis consists of two half-reactions:hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.OER is the key to restricting the efficiency of water electrolysis due to its slow kinetics.The electrochemical oxidation of small organic molecules（such as urea,5-hydroxymethylfurfural,hydrazine hydrate,methanol,etc.）can form high value-added chemicals and deal with environmental pollution problems,which has a lower overpotential than OER and can reduce energy consumption.For the catalytic oxidation of different small organic molecules,the development of efficient catalysts is still the key.Although noble metal-based materials such as ruthenium dioxide（RuO2）and iridium dioxide（IrO2）are efficient anode catalysts,their large-scale applications are limited by high cost.Metal-organic frameworks（MOFs）are hybrid materials with intramolecular pores formed by the self-assembly of organic ligands and metal ions.They are widely used in the field of electrocatalysis,but their poor electrical conductivity and stability limit further applications.In this thesis,in order to improve the catalytic performance of MOF and control its electronic structure,ion doping,graphene quantum dots control and other methods are mainly used.Specifically,ferric iron（Fe3+）,carboxylated graphene quantum dots（GQD）,and aminated graphene quantum dots（N-GQD）were introduced into Ni-MOF.The morphology of the material was investigated by scanning transmission electron microscopy.The electronic structure was analyzed by X-ray diffraction（XRD）and X-ray photoelectron spectroscopy（XPS）,and its electrocatalytic oxidation performance was tested.Its main contents are as follows:（1）In this thesis,iron-doped nickel-based metal-organic framework nanosheet arrays（FeNi-MOF NSs）were synthesized by a one-step solvothermal method,which applied to urea electrocatalytic oxidation（UOR）assisted total water splitting,achieving 10 and 100 mA cm-2 current densities only need overpotentials of 131 and 155 mV.Combined with systematic experimental characterization methods and theoretical computational studies,the excellent electrocatalytic performance of FeNi-MOF NSs for urea electrocatalytic oxidation was attributed to the electronic structure reconfiguration of the nickel center in the metal-organic framework induced by high-valent iron doping,resulting in the formation of abundant high-valent nickel as active center for catalytic oxidation.In addition,additional electronic states was formed between the energy levels,which improved the conductivity and optimized the adsorption energies of reactants,intermediates and product molecules on the catalyst surface,there by accelerating the UOR kinetics.Based on the excellent urea electrocatalytic oxidation performance and catalytic hydrogen evolution capability of FeNi-MOF NSs,we further assembled a urea electrolyzer,which obtained a current density of 10 mA cm-2 at 1.43 V and excellent stability during long-term electrolysis.（2）Then,carboxylated graphene quantum dots（CGQDs）were added to nickel-iron bimetallic MOF nanosheet arrays（NiFe-MOF NSs）,and the structural similarities and differences between CGQDs and ligand terephthalic acid（BDC）were used to adjust Periodic and electronic structures.It was found that the addition of CGQDs formed a 2D/2D nanoarray structure（NiFe@GQD）.By further changing the ratio of CGQDs,the morphology and size of the secondary sheet-like structures can be regulated.Among them,the optimized NiFe9@GQD4 can achieve a current density of 100 mA cm-2 with only an overpotential of 298 mV in the electrocatalytic oxygen evolution reaction.The crystal form of NiO obtained by further calcining NiFe9@GQD4 in air shown more excellent performance in electrocatalytic oxygen evolution,and only needed an overpotential of 256 mV to reach a current density of 100 mA cm-2.In view of the excellent OER catalytic activity after calcination,the electrocatalytic activity towards 5-hydroxymethylfurfural（HMF）was further investigated.Experiments shown that HMF was directionally converted to 2,5-furandicarboxylic acid（FDCA）at lower overpotentials with higher yields and conversions.（3）Further,the electronic structure of nickel-iron bimetallic MOFs was regulated by doped amine-graphene quantum dots（N-GQDs）.The coordination environment and electronic structure of metal centers in MOFs can be regulated by utilizing the difference in coordination ability between amino groups in N-GQDs and carboxyl groups in MOF ligands with metal ions.It was found that MOF materials with rhombic nanosheet array structure were obtained after the introduction of N-GQDs,and exhibited excellent catalytic performance in electrocatalytic oxygen evolution and urea oxidation reactions,achieving OER and UOR current densities of 100 mA cm-2 at 284 mV and 170 mV overpotentials,respectively.The catalytic activity was maintained during constant voltage electrolysis for 20 h.The enhanced electrocatalytic oxidation performance mechanism induced by N-GQDs was further explored:（1）the introduction of N-GQDs induced the periodic structure changes in the periodic structure of MOFs,providing more active sites;（2）The coordination of amino groups in N-GQDs modulates the electronic structure of the nickel-iron center,which is beneficial to electron transport.