Controlled Etching Strategy To Prepare MIL-88A-derived High-performance Electrocatalytic Seawater Catalyst

Electrocatalytic water splitting technology,powered by sustainable energy sources such as solar and wind,provide a green path of hydrogen energy,which is expected to replace traditional fossil energy sources and promote the development of the sustainable society.However,during the large-scale industrialization process,the consumption of fresh water and the high cost of catalysts are the problems that should be solved,which limits the application of electrolytic water splitting technology.Seawater,accounting for 96.5%of global water resources,is a kind of natural resource almost inexhaustible.Therefore,the development of electrocatalytic water splitting technology can avoid the consumption of fresh water and effectively lower the cost of H₂production.Meanwhile,the seawater electrolysis process can simultaneously produce two chemical products（chlorine and Na OH）with high added,which is an ideal strategy for hydrogen production from electrolytic water.However,the complex ionic environment of seawater may block and venerate the active center of the catalyst,dramatically reducing the lifetime of the catalyst.Therefore,the design and development of efficient and stable non-precious metal seawater electrolysis catalysts has become a hot research topic in this field,today.In this thesis,using Fe-based metal-organic framework（MIL-88A）as precursors,a series of hollow catalysts are constructed by controlled etching strategy,and the intrinsic activity of these catalysts are optimized by elemental doping,defect engineering,and heterostructure construction strategies.Therefore,excellent electrolytic seawater performances are achieved.The specific research contents are shown as follows:1.Fabrication of Zn-doped Ni Fe layered double hydroxide nanocages and the study of their performance in electrolytic seawater.In this study,Zn-doped MIL-88A（MIL stands for Material of Institute Lavoisier）is prepared as a presursor,and then Zn-Ni Fe trimetallic hydroxide hollow double-shell nanocages（ZNF LDH）is synthesized under the etching effect of urea and the appearance of Ni source.The unique hollow double-shell is constructed by ZNF LDH nanosheets supported by each other,which provides ultra-high active specific surface area and sufficient electrolyte storage space for the electrocatalytic process.Meanwhile,the self-support interlayers can enhance the mechanical stability of the hollow double-shell nanocages.The ZNF LDH double-shell nanocages present highly efficient in the oxygen evolution reaction（OER）at high currents,with an overpotential(η₁₀₀)of 300 m V required to reach a current density of 100 m A cm^-2in a 1 M KOH solution.In seawater conditions,ZNF LDH still showed excellent activity and stability,which display aη₁₀₀in 1 M KOH seawater solution requiring only 323 m V and only 4.6%degradation at a constant current density of 50 m A cm^-2for 20 h.2.Controlled preparation of carbon quantum dots doped Fe/Co/Ni phosphide open nanotubes and study of their performance in electrolytic seawater.In this work,a carbon quantum dot-doped trimetallic phosphide（FCNP@CQDs）with a unique open nanotube structure is first prepared by selective etching and phosphorylation processes using carbon quantum dot-doped MIL-88A（MIL-88A@CQDs）as a precursor.Due to the introduced carbon quantum dots and dop Co/Ni elements,there are rich defects are formed inside the phosphide,which could expose more highly active catalytic site.Meanwhile,the open nanotube structure of FCNP@CQDs can promote the diffusion of electrolyte and the emission of the generated gas.Therefore,the FCNP@CQDs exhibit excellent bifunctional catalytic performance.In alkaline seawater solution,the OER of FCNP@CQDs required a low overpotential(η₂₀)of 268 m V to reach 20 m A cm^-2current density,while theη₂₀of HER is 150 m V.Furthermore,a FCNP@CQDs//FCNP@CQDs electrolyzer is then assembled using both FCNP@CQDs as cathode and anode,which required only 1.61V to achieve a current density of 10 m A cm^-2in alkaline seawater.The construction of rich-defect open hollow structures provides a promising way for the design and preparation of bifunctional seawater splitting catalysts.3.The construction of dumbbell@tube open structure by anisotropic etching strategy and the study of their electrocatalytic chlorine evolution reaction properties.In this work,through an NH₃-driven anisotropic etching strategy,a part of crystalline plane of carbon quantum dots-functioned MIL-88A nanorod（MIL-88A-CQDs）are selectively etched to prepare a dumbbell@tube open structure（MIL-88A@LDH-CQDs）.The etching reaction will prior along the（011）crystalline plane and the released Fe³⁺will co-precipitate with Co²⁺and Ni²⁺on the interface to generate the LDH shell,which produces a unique dumbbell@tube structure.Then,a carbon quantum dots-functioned tri-metal phosphide with a dumbbell@tube open structure（Co/Ni-Fe P-CQDs D@T）is obtained by a morphology-maintained phosphating process.The optimized Co/Ni-Fe P-CQDs D@T displays a CER（Chlorine evolution reaction）overpotential of 85 m V to achieve a current density of 10 m A cm^-2in 5.0 M Na Cl（p H=2）.The Co/Ni-Fe P-CQDs D@T also showed excellent CER selectivity,such as the selectivity of 93%by electrochemical test methods and 97.2%selectivity by iodine titration at 5.0 M Na Cl（p H=2）.Furthermore,the chlorination mechanism of Co/Ni-Fe P-CQDs D@T is also explored.It shows that at high Na Cl concentration（2.0-5.0 M,p H=2）,the Co/Ni-Fe P-CQDs D@T delivers a Volmer-Tafel/Heyrovsky mechanism with Tafel/Heyrovsky as rate-determining step（RDS）and the Tafel:Heyrovsky ratio gradually decreases with the potential increasing.The unique structural design and the excellent CER performance of the phosphide provide a new design path for the preparation of next-generation CER electrocatalysts.