Preparation And Electrocatalytic Water Splitting Properities Of ZIF-67 And Its Derived Sulfides

Due to the depletion of fossil fuels and the deterioration of environment,it is vital to develop green and recyclable clean energy.Electrocatalytic water splitting has been expected to be a breakthrough to solve this problem.In order to reduce the energy consumption and lower the overpotential during water splitting process,it is significant to develop non-noble metal-based catalysts with excellent activity.During recent years,metal organic frameworks（MOFs）-derived transition metal materials attracted lots of attention,which could inherite the open structural frameworks,large specific surface area and flexible structures of MOFs.However,the research investigating second metal effects of pristine MOFs catalysts systematically and the controllable modification of MOFs-derived materials needs to be further expanded.Based on the situation,this thesis starts with exploring the electrocatalytic properties of pristine ZIF-67 nanoarrays loaded on carbon cloth（CC）substrates,and then Co-based sulfides and their composites with high activity were prepared by functionalizing ZIF-67.Also,the correlation between the electronic structures of Co atoms and the catalytic activity of water splitting was investigated to reveal the scientific question of high catalytic activity through theoretical calculations.Transition metals（Ni,Cu,Zn）were introduced into ZIF-67 by co-deposition method under room temperature using carbon cloth as conductive substrate.Specifically,the trend of oxygen evolution reaction（OER）performance on these Co（M）ZIFs is Co Zn ZIFs>Co Ni ZIFs>Co Cu ZIFs>ZIF-67,among which Co Zn ZIFs exhibits the most excellent OER performance with the overpotential of 287 m V to deliver a current density of10 m A cm^-2.X-ray photoelectron spectroscopy（XPS）indicates that the different OER activity can be attributed to the different electronic structures of the Co active sites causing by the doping of Ni,Cu and Zn atoms.Theoretical calculation results confirm that the d-orbit center and the adsorption of intermediates of OER process can be improved greatly after the introduction of Zn atoms into ZIF-67,which is beneficial to reduce the energy barrier of rate-determining step（RDS）and improve the OER performance.Co（Zn）S₂nanoarrays catalysts were synthesized by functional treatment of Co Zn ZIFs through chemical vapor deposition method.Co（Zn）S₂nanoarrays exhibit excellent OER activity,with an overpotential of only 248 m V at a current density of 10 m A cm^-2.XPS and in-situ Raman spectroscopy indicate that the Co（Zn）S₂precatalysts undergo phase transform process into Co（Zn）OOH absorbed with SO₄^2-during OER process as active phase.Theoretical calculations show that the doping of Zn atoms can affect the electronic structures of the Co active sites,resulting in more charge transfer between Co and O atoms,which optimizes the charge transfer efficiency and improves the OER activity.This research shows that increasing the oxidation number of the Co active center appropriately is one of the effective ways to enhance OER activity.CoS₂-V₂O₅composite materials,in which Co atoms were replaced by Zn atoms partially,were synthesized by anion exchange and chemical vapor deposition methods,developing a new strategy to reduce surface oxidation of sulfides and thereby enhance electrocatalytic hydrogen evolution（HER）performance.By designing the oxidation control experiment and operating the XPS characterizations,it could be concluded that the higher the surface oxidation degree of Co S₂,the worse HER performance,and the introduction of V₂O₅can reduce the degree of surface oxidation of Co S₂,which could optimize the HER activity,the overpotential of Co S₂-V₂O₅is reduced by 144 m V to only 128 m V when the current density is 10 m A cm^-2.Density functional theory（DFT）calculations show that the surface oxidation of Co S₂increases the Gibbs free energy of the water splitting step which served as RDS,thus hindering the HER process.Therefore,reducing the surface oxidation of CoS₂by introducing V₂O₅is an effective strategy to enhance HER performance.