The Fabrication Of Hollow Structures Derived From Metal-Organic Frameworks And Their Applications In Electrocatalysis

Producing hydrogen by water splitting is a really promising way to alleviate the energy crisis because of its high energy conversion efficiency and environmental friendliness.Water splitting consists of two half reactions,hydrogen evolution reaction(HER)and oxygen evolution reaction(OER).To promote the practical applications of water splitting,efficient electrocatalysts are required to lower the reaction energy barrier of HER and OER,and improve the energy efficiency.Among the many reported electrocatalysts,the hollow structures drived from metal-organic frameworks(MOFs)have received tremendous attention due to their large specific surface area,rich activity sites,and facile mass transport.However,most of the MOF-derived hollow structures were inorganic materials with poor electrical conductivity.The pyrolysis was usually used after the fabrication of hollow structures to obtain carbon materials to improve the electrical conductivity of hollow structures for HER or OER,and the pyrolysis would increase the complexity and energy consumption of synthesis.In this work,we used the instability of MOFs to fabricate the hollow structures.Based on the structure instability of MOFs,the polymerization process could be induced by the organic ligands or metal ions released from the MOFs during the disassembly,and the metallic compounds/polymers composites would be obtained after the reaction.This method could introduce polymers into the MOF-derived hollow structures to improve the electrical conductivity,optimize the activity of metal sites,and improve the performance of the hollow structures.This work provides a new way to design and synthesis efficient electrocatalysts.In chapter 2,the ZIF-67 containing cobalt was used as template.Utilizing the strong coordination between dopamine and cobalt ion,the structure of ZIF-67 was destroyed and the 2-methylimidazole released from ZIF-67 would induce the polymerization of dopamine.The polydopamine(PDA)/cobalt nanocontainers would be obtained after the reaction.Then,PDA/Co HNCs were transformed to novel hierarchical hollow electrocatalyst materials which is made up of CoP hollow nanoparticles(NPs)and hollow N-doped carbon nanocages(CoP@NCHNCs)via a pyrolysis-phosphidation strategy.The sizes of CoP hollow NPs could be adjusted and the PDA offered heteroatom N to facilitate the activity.Because of the special structure,the CoP@NCHNCs exhibited excellent performance for both HER and OER,demonstrating the advantage of hierarchical hollow structures.Then,the sizes and structures of CoP NPs were adjusted by changing the conditions of pyrolysis and phosphidation.The performances for electrocatalysis of different CoP NPs were compared,and the hollow NPs exhibited the best performance.This work provided a new method to design complex hollow materials.In chapter 3,based on the previous work,in order to simplify the synthesis of hollow structures and avoid carbonization or phosphorylation to reduce the energy consumption during synthesis,we obtained MIL-88(Fe)with the core part in a metastable state by adjusting the solvent used in the synthesis of MOF.When MIL-88(Fe)is used as a template,the internal structure of MIL-88(Fe)can be destroyed by using water as etching agent,and pyrrole is added to the reaction solution.The Fe released during the etching process was used to induce the polymerization of pyrrole to obtain the hydrolysis product of MIL-88(Fe)/polypyrrole(PPy)composite spindle nanoparticles.On the basis of this method,the MIL-88(FeNi)is used as a template,and water is also used as the etching agent.Pyrrole is added to the reaction solution,and the etching time is extended so that MIL-88(FeNi)is completely hydrolyzed to obtain hollow nanospindles consist of PPy and Fe-Ni(oxy)hydroxides.This material exhibits excellent activity and stability for OER without any high temperature treatment,and this work provides a new approach to green synthesis of electrocatalysts.In chapter 4,with the experience of the previous work,in order to further explore the potential of hollow structures for practical applications in electrocatalysis,we used MIL-88(FeCoNi)grown directly on nickel foam(NF)as a template,selected tannic acid(TA)as etching agent,and added pyrrole while preparing the hollow structures.A novel TA and PPy modified hollow MIL-88 arrays(HMIL-88@PPy-TA)was obtained and it was applied to the OER in artificial seawater.From the performance tests of OER,we found that after the electrocatalysis,the outer layer of HMIL-88@PPy-TA formed iron and nickel hydroxides with lattice structure,which makes it have outstanding activity for OER,even in alkaline artificial seawater.This work expands the practical application of MOFs-derived hollow structures in electrocatalysis.In conclusion,we constructed a variety of metal compounds/polymers composite hollow structures by exploiting the instability of MOFs.This method not only constructs hollow structures,but also introduces conducting polymers,which would optimize the electrocatalytic performance of the materials.The electrocatalytic splitting of alkaline seawater was elementarily realized,providing the possibility of practical applications of this type of electrocatalyst.By combining performance characterization and theoretical studies,it is confirmed that the doping of conducting polymers can optimize the catalytic activity of metal sites.These works promote the development of MOFs-derived materials in design and practical applications.