Controllable Preparation Of NaTi₂(PO₄)₃/Carbon Composite For Hybrid Capacitive Deionization

Human civilization has been developing at an astonishing rate since the beginning of the 21st century,which not only accelerates the rapid development of industry and agriculture but also brings huge pollution to the environment,especially to the freshwater resources.As is known to all,fresh water only accounts for?2.5%of the global water resources,while most of this freshwater is in the form of solid glaciers that cannot be utilized.On the other hand,seawater,taking up to?97%of the global water resources,could be an effective way to obtain freshwater resources through desalination.However,traditional seawater desalination techniques(i.e.,reverse osmosis,multi-stage flash evaporation,electrodialysis)are mostly plagued by their intrinsic problems like high energy consumption,high cost,and secondary pollution.Capacitive Deionization(CDI),as an emerging desalination technology,enjoys various advantages over traditional desalination technology like low energy consumption,zero secondary pollution,high water use ratio,etc.However,the desalination capacity of conventional carbon-based CDI is limited by its intrinsic double-layer character.Therefore,it is urgent to develop new types of electrode materials with high desalination capacity to meet the needs of multiple scenarios in the future.Ever since 2014,a new type of intercalation faradic electrode material has been developing rapidly,and was thought to be the next generation of CDI electrode materials due to their high theoretical ion storage capacity,high charge efficiency,etc.Among these Faradaic materials,Na Ti₂(PO₄)₃,(NTP)with a sodium superionic conductor(NASICON)structure is known for its extremely high theoretical capacity(133 m Ah g^-1)has attracted great attention in the scientific community.In this thesis,we focus on the development of NTP-based Hybrid Capacitive Deionization(HCDI)electrode materials,through multi-component composition,morphology/structural manipulation,and material modification strategies to enhance the electric conductivity and the structural stability of NTP so as to further improve the desalination capacity and cycling stability of the NTP-based HCDI.The research contents of this thesis are as follows:1.To address the low desalination capacity issue of traditional carbon-based CDI,NTP cube with NASICON structure was synthesized by a simple solvothermal/calcination method using P25 as the precursor.Owing to the excellent sodium ion intercalation/deintercalation ability of NTP,the NTP-based HCDI displays improved desalination performance over conventional carbon-based CDI.The NTP displays good desalination performance with the maximum desalination capacity of 29.8 mg g^-1,the maximum desalination rate of 8.1 mg g^-1 min^-1in 1000 mg L^-1 Na Cl solution when used as the electrode for HCDI.However,the desalination performance of NTP-based HCDI is still far from its theoretical capacity,which is probably due to the poor electrical conductivity of NTP.Moreover,repeated ion intercalation and deintercalation could cause significant volume change,which could damage the structure of NTP,further causing the desalination performance degradation.Therefore,it is urgent to find further solutions to the abovementioned issues so as to meet the requirement of practical application.2.Introducing carbon into Faradaic electrode materials to construct Faradaic/carbon composites should be an effective strategy to effectively improve the conductivity of the electrode material and potentially increase its structural stability.Therefore,we proposed a method to construct a NTP/CNF composite through electrospinning,calcination,and solvent-thermal reaction.Compared with NTP,NTP/CNF has a conductive network structure.The result shows that the NTP/CNF displays a high specific capacitance of 203.1 F g^-1 at a scan rate of 10 m V s^-1.Meanwhile,the NTP/CNF displays a maximum desalination capacity of 48.9 mg g^-1 and a maximum desalination rate of 12.1 mg g^-1 min^-1 in 1000 mg L^-1 Na Cl solution,both of which were better than those of pure NTP.The results vividly demonstrate that our strategy of building NTP/CNF to be effective in improving the desalination performance and stability,which should be ascribed to the enhanced electrical conductivity and better ion transfer during the desalination process.Meanwhile,the nanofiber network structure of the CNF could serve as a“structural buffer”and subsequently improve the long-term cycle stability.Although the desalination performance of NTP/CNF has been greatly improved compared with pristine NTP,the desalination performance of NTP/CNF is still far from being satisfactied,which is probably due to the low loading density of NTP on CNFs and less uniform particle size of the NTP.In addition,the electrospinning approach suffers issues like complex equipment,lower carbon yield,which has limited the practical application of NTP/CNF.3.Metal-Organic Frameworks(MOFs),as a rapidly developing class of coordination polymers,enjoys many advantages such as high specific surface area,highly porous surface,and controllable morphology and pore structure,which could be an excellent candidate as the precursor to prepare NTP/C.Therefore,titanium-containing MOF(MIL-125)was used as the precursor for in-situ synthesis of NTP/C composite.The results show that the NTP/C exhibits both excellent electrochemical performance(213.1 F g^-1,at a scan rate of 10 m V s^-1)and durable desalination performance with a maximum desalination capacity of 61.5 mg g^-1 and desalination rate of 14.6 mg g^-1 min^-1 in 1000 mg L^-1 Na Cl solution.The excellent desalination performance should be due to the carefully designed structure of the MOF-NTP/C,where the carbon particles are closely combined with the NTP particles,while the NTP/C are also closely connected with each other to form a three-dimensional conductive network,which could give rise to excellent electrical conductivity and structural stability and ultimately enhance the desalination capacity and durability.4.During the preparation of MOF-NTP/C,we surprisingly found that the surface morphology of the Ti-MOF could be easily altered by changing the polarity of the solvent,resulting in a simple strategy to realize the synthesis of NTP/C with controlled surface morphology(Cube/Disk(NTP/C(C/D),Cube/Tetragon(NTP/C(C/T),Cube/Sphere(NTP/C(C/S)and Cube/Polyhedron(NTP/C(C/P)),which could be a perfect opportunity to further explore the influence of surface morphology of NTP/C on their desalination performance.Therefore,electrochemical and desalination evaluations were conducted on four MOF-NTP/C samples.The results show that the surface morphology does play a very significant impact on the desalination performance,and the NTP/C(C/S)shows the best desalination performance with the desalination capacity and rate up to 74.6 mg g^-1 and 14.4 mg g^-1 min^-1 in 1000 mg L^-1Na Cl solution,respectively.It can be concluded that the surface morphology of MOF-NTP/C could largely influence the“packing-density”of MOF-NTP/C and further affect its desalination performance.Specifically,the surface morphology of NTP/C(C/S)could trigger the lowest“steric hindrance”,which could form a densely packed structure,and subsequently enhance the electrical conductivity and charge transfer.