Preparation Of Transition Metal MOFs-Based Catalysts For Electrocatalytic Water Splitting Under AC Magnetic Field

Hydrogen（H₂）is considered to be an ideal energy source for alleviating energy crisis and environmental problems because of its high energy density,pollution-free,and renewable.Therefore,hydrogen production through water electrolysis due to clean and pollution-free shows great development prospects.However,the anodic oxygen evolution reaction（OER）in the water electrolysis process involves four-electron transfer,leading to a slow kinetics.Also,the current commercial electrocatalysts are dominated by scarce precious metals,which greatly increases the cost of hydrogen production.Hence,the key to large-scale industrialized hydrogen production lies in the efficient and low-cost water electrolysis technology.Recently,metal-organic frameworks（MOFs）and their derivatives have received extensive attention in the field of water electrolysis due to their high specific surface area,tunable pore structure,and diversification of central metals.In addition,the significant enhancement effect of the magnetic field on the water electrolysis process shows excellent application potential in the field of water electrolysis.Therefore,water electrolysis using MOFs based electrocatalysts under a magnetic field offers an attractive solution strategy and broad prospects for large-scale hydrogen production.However,the research in the above direction is still in its infancy and faces many problems:（1）The structure-activity relationship between MOFs and their HER or/and OER intrinsic activity needs to be improved;（2）The effect mechanism of the AC magnetic field on the water electrolysis system is still unclear;（3）The design strategies for electrocatalysts using in the magnetic field and electric field coupling system need to be proposed.Therefore,exploring the composition and microstructure design principles of efficient and stable electrocatalysts suitable for AC magnetic field and clarifying the influence mechanism of AC magnetic field on the OER performance of electrocatalysts are the keys to the large-scale application of magnetic field-assisted water electrolysis using MOFs-based electrocatalysts.In view of this,this paper analyzes the intrinsic relationship between the composition,morphology of MOFs and their derivatives and their water electrolysis performance through the composition design and morphology control of MOFs and their derivatives.Furthermore,combined with the linear sweep voltammetry curves and time-current curves,the water electrolysis behaviors of MOFs and their derivatives under AC magnetic field were explored,and the relationship model between AC magnetic field,electrocatalyst composition and structure and their water electrolysis performance was established.And the design strategies for high-performance electrocatalysts under AC magnetic field were proposed.The detailed research results are as follows:A series of three-dimensional（3D）spherical superstructures Ni Fe₂O₄@Mg-MOF-74were successfully constructed by a simple solvothermal method with citric acid as additive.The morphology of obtained superstructure can be precisely regulated by controlling the ratio of ferrite to magnesium salt,and the formation mechanism of the superstructure was analyzed.Furthermore,the ferromagnetic 3D superstructure Fe_0.64Ni_0.36@Fe-Ni-Mg O@C was obtained by carbonization at high temperature.And the overpotential of the ferromagnetic electrocatalyst at 10 m A cm^-2 was 403 m V,which was 34.5%lower than that before carbonization,which was attributed to enhanced charge transport properties due to the formation of graphitized carbon and Fe_0.64Ni_0.36alloys,and the hierarchical pore structure providing additional channels for diffusion and mass transfer.After introducing an AC magnetic field,the HER overpotential of the derivative at 10 m A cm^-2 in alkaline conditions first decreased and then increased with the increase of AC magnetic field strength.In particular,the significant promotion effect was obtained at 2.312 m T,which reduces the overpotential by 7.7%than that without AC magnetic field,resulting from the result of the combined effect of the induced electromotive force and the changes of magnetic and thermal Gibbs free energy caused by AC magnetic field.A series of multi-component spherical superstructure Ni Co Fe-MOF-74 with controllable composition were designed and fabricated based on multi-element doping strategy.The unique spherical superstructure of the electrocatalysts can increase accessible active sites and provide additional diffusion channels for ion transport.In addition,the multi-element synergy can effectively optimize the adsorption energy of the electrocatalysts for the reaction intermediates,thereby significantly improving OER activities.Among them,Ni Co Fe-MOF-74 with the best performance achieved an overpotential as low as 273 m V at 10 m A cm^-2,and can work stably for more than 20h.Moreover,the OER behavior of Ni Co Fe-MOF-74 under AC magnetic field was carefully investigated.And the two-sided effect of AC magnetic field on OER process was discovered for the first time,that is,AC magnetic field could significantly promote the OER performance of electrocatalysts at low current densities(η₁₀ of Ni Co Fe-MOF-74 decreased by 36.7%),but inhibit OER process at high current densities(η₄₀ of Ni Co Fe-MOF-74 increased by 20.9%),originating from the combined effect of magnetohydrodynamic（MHD）convection-accelerated mass transfer,the optimized reaction path of the spin polarization effect,and the confinement of ion migration by the electromagnetic induction effect caused by AC magnetic field.Three electrocatalysts Co,Co O,and Co₃O₄ with different magnetic properties were obtained using single-metal MOFs（ZIF-67）as the precursor by adjusting the heat treatment process.Then,the intrinsic relationship between the AC magnetic field,the magnetic properties of electrocatalysts,and OER behavior was further investigated.It was found that ferromagnetic ordered electrocatalyst Co achieved the best OER promotion effect under AC magnetic field,where the overpotential of 10 m A cm^-2without i R correction at 4.320 m T was 36.6%lower than that without AC magnetic field,which was due to the simultaneous existence of the magnetic heating effect and the spin polarization effect of the ferromagnetic ordered electrocatalyst Co under an AC magnetic field.On the one hand,the excellent magnetic heating effect of ferromagnetic ordered catalysts under AC magnetic field was used to activate the local high temperature reaction.On the other hand,spin polarization of paramagnetic/ferromagnetic interface was occurred under AC magnetic field through the spin-pinning effect,thereby promoting the generation of triplet oxygen.Nitrogen-doped Zn Co₂O_4-δwith tunable oxygen vacancy was prepared by controlling the decomposition behavior of the precursor Zn Co-ZIF synthesized at room temperature.The presence of oxygen vacancies and doping of nitrogen elements could optimize the adsorption energy of electrocatalysts for reactive intermediates,thereby enhancing the OER performance of the electrocatalysts.And the electrocatalyst with abundant oxygen vacancies,Zn Co₂O_4-δ-350,exhibited the best OER activity with an overpotential of 406 m V at 10 m A cm^-2 in alkaline electrolyte as well as with no obvious performance degradation after 27 h of continuous electrocatalysis.In addition,it was found that electrocatalysts with high oxygen vacancy concentration obtained more significant promotion effect at low current densities when an AC magnetic field was introduced into the water electrolysis system,where electrocatalyst Zn Co₂O_4-δ-350 obtained a rather low non-i R-corrected overpotential(@10 m A cm^-2)of 243 m V at 4.752 m T,which was 44.5%lower than that without AC magnetic field.The excellent OER enhancement of AC magnetic field on the electrocatalysts with abundant oxygen vacancies was closely related to the efficient mass transport of MHD（local）convection at oxygen vacancies and the ferromagnetism generated by the double exchange between Co²⁺-O^2--Co³⁺.