Preparation And Modification Of Metal Organic Framework MIL-101(Cr) Electrode And Its Capacitive Deionization Defluorination/Desalination Performance And Mechanism

The impact of fluoride contamination in groundwater and industrial wastewater has received a lot of attention from researchers.In recent years,the emerging capacitive deionization（CDI）technology is considered to be the most promising electrochemical fluoride removal technology due to its low energy consumption and no secondary pollution,and the development of high-efficiency capacitive deionization electrodes has become a hot spot and difficult research area for scholars at home and abroad.Metal-organic framework（MOF）has attracted much attention because of its super high specific surface area and easily adjustable structure,but the mechanism of direct use for CDI fluoride removal is not yet clear.In this study,we proposed a new concept of simple construction of high-performance MOF electrodes for the efficient removal of fluorine pollution in real water bodies,and successfully prepared a variety of MOF-based electrode materials to improve the fluoride removal performance of the electrodes by improving the interfacial electrochemical properties,and investigated the mechanism of fluoride removal.In addition,by introducing carbon nanotubes to change the morphology and size of MOF,and by using disordered defects and high conductivity of carbon nanotubes to give the material anion selectivity,the mechanism of the influence of carbon nanotubes on the morphology and size of MOF and the fluoride removal performance and environmental stability of carbon nanotubes composite MOF materials were investigated,and the main findings are as follows:（1）MIL-101（Cr）（Cr-MOF）with ultra-high specific surface area,high water stability,and open metal sites（OMS）was prepared by hydrothermal method for the defluorination of CDI.Carbon black was used as a"chain"to connect the fluoride ions stored in the pores of MIL-101（Cr）as"blocks"to improve the electrical conductivity and ion storage capacity of the Cr-MOF/carbon black electrode（Cr-MOF electrode）.The constructed"blockchain-like"Cr-MOF electrode exhibits excellent defluorination capacity(39.84 mg _{Na F} g_electrodes^-1),low energy consumption(1.2 k Wh kg^-1_{Na F}),and good stability.Using dissipative electrochemical quartz crystal microbalance and XPS analysis combined with density flooding theory（DFT）calculations,it was revealed that the mechanism of Cr-MOF fluoride removal is mainly the redox reaction of open metal sites coupled with confined water,and thus the Cr-MOF electrode has excellent fluoride removal capacity.Introducing the"block-like chain"effect to enhance the charge transfer capability,provides a new idea to regulate the interfacial charge of poorly conducting materials and thus construct efficient and stable anionic CDI electrodes.（2）Long multi-walled carbon nanotubes with excellent electrical conductivity were introduced into the Cr-MOF synthesis process to guide the growth of pencil-like Cr-MOF（LCMOF）with disordered defect structures.The decomposition temperature of LCMOF was found to be elevated to about 500°C by thermogravimetric analysis（TGA）,which is significantly higher than that of the original Cr-MOF at 370°C.The introduction of long multi-walled carbon nanotubes improved the stability of the composite.The presence of long multi-walled carbon nanotubes with disordered defect structures effectively improves the interfacial electrochemical behavior of LCMOF and enhances the transport of fluorine ions on the LCMOF electrode.It was found that LCMOF has the property of an"anion sieve"for preferential removal of anions,which is due to the fact that LCMOF with disordered defects exposes more Lewis acid sites and enhances the removal ability of Lewis base fluoride ions.The fluoride removal capacity of LCMOF-3 is 34.10 mg _{Na F} g_electrodes^-1 and the energy consumption is 0.99 k Wh kg^-1_{Na F}.LCMOF-3 can adapt to complex environmental conditions and has good fluoride removal cycle stability.The coupling of laboratory experiments and finite element simulation results revealed that high flow rate and high voltage are beneficial to enhance the fluoride removal performance of LCMOF-3 materials with a high relative ratio of mesopores.The fluoride removal mechanism of LCMOF-3 has been analyzed by means of ex-situ XPS,and the ion adsorption is facilitated by the electron transfer between the OMS of LCMOF-3 and F.The fluoride removal performance of LCMOF-3 exceeds that of most of the known fluoride removal electrode materials.This research work provides an important scientific basis for the application of MOF materials with disordered defect structures in CDI technology.（3）The production of large particle size Cr-MOF（SCMOF）was"cooked"by introducing short multi-walled carbon nanotubes into the Cr-MOF synthesis process.It was found that the mechanism of carbon nanotubes affecting the growth of Cr-MOF includes the coupling of disruption of superstructure assembly and Ostwald ripening.In addition,carbon nanotubes are involved in the competition for anisotropy during the growth of Cr-MOF,resulting in the growth of Cr-MOF along the c-axis in a pencil-like morphology.The introduction of short multi-walled carbon nanotubes improved the thermal stability and charge transfer ability of Cr-MOF.The fluoride removal and desalination properties of SCMOF were investigated,and it was found that it did not have the"anion sieve"effect of LCMOF due to the low exposure of Lewis acid sites in SCMOF.Since SCMOF-3 is also a material with a relatively high proportion of mesopores,higher voltage,and flow rate are beneficial to the fluoride removal performance.SCMOF-3 still has some fluoride removal performance for actual natural water bodies,which indicates that SCMOF-3 has good prospects for practical applications.Since SCMOF-3 is similar to the combination of Cr-MOF and LCMOF-3,its fluoride removal mechanism is also similar to the combination of both,i.e.,redox and electron transfer in open metal sites,and the presence of confined water promotes the mass transfer effect of fluoride ions and enhances the fluoride removal performance of SCMOF-3.This research work reveals the mechanism of carbon nanotube influence on the controlled synthesis of MOF morphology and size,which provides an important reference for high-performance fluoride removal electrode materials.In summary,this paper presents a simple method for constructing high-performance defluorination electrode materials,which promotes the application of poorly conductive materials in the field of capacitive deionization and defluorination.In addition,the disordered defect and large-size MOF materials were synthesized by introducing carbon nanotubes,and it was found that the disordered defect structure is beneficial to enhance the electrochemical performance and defluorination performance of the materials,and the mechanism of carbon nanotubes affecting the growth of MOF of specific morphological size was also explored.This study is an important scientific basis and theoretical value for the construction of high-performance and stable CDI electrodes.