Preparation Of Nickel Hexacyanoferrate Composites For Hybrid Capacitive Deionization

The shortage of water resources,especially the shortage of fresh water resources,is one of the most serious problems in recent years,which has seriously affected human life and production.As a new desalination method,capacitive deionization(CDI)has attracted extensive attention.Due to its simple operation,low cost,low energy consumption and environmental friendliness,it plays a great role in seawater desalination,resource extraction and heavy metal removal.However,due to the influence of materials and devices,the traditional CDI faces the dilemma of insufficient cycle stability and low desalination efficiency in practical application,which limits its application for large-scale commercialization.Developing new electrode materials with high-performance and stablility is an important way to improve CDI technology.It is found that the side reactions,overlapping effect and co-ion exclusion on the traditional CDI electrode seriously affect the improvement of CDI cycle stability and desalination capacity.In recent years,Faradaic electrode materials have been successfully introduced into CDI and a hybrid capacitive deionization(HCDI)was constructed,which achieved excellent desalination performance.Different from traditional CDI electrodes,Faradaic electrode materials can absorb specific charges through Faradaic reaction and store charges by intercalting ions into lattice/interlayer or transformation.Therefore,they are less affected by side reactions and salt concentration.Recently,various Faradaic electrodes have been applied into desalination devices.Among them,Prussian blue analogues(PBAs),as a kind of electrode materials with stable structure and excellent performance,has attracted extensive attention of researchers.However,PBAs has several drawbacks including poor electronic and ionic conductivity,easy agglomeration in the preparation process and serious volume expansion,resulting in its unsatisfied desalination performance.Therefore,in this paper,1D,2D and 3D high conductive materials were incorporated into nickel hexacyanoferrate(Ni HCF)to construct Ni HCF composites.The unique conductive network structures could promote the transport of charges and ions,increase the specific surface area to provide more active sites,reduce the agglomeration of Ni HCF material and buffer the volume expansion,resulting in the improved the desalination capacity and cycle stability.The main research contents are as follows:1.Ni HCF/CNT composites were prepared by introducing carbon nanotubes(CNTs)with 1D structure to connect Ni HCF particles in series.The introduction of CNT shortens the transport path of electrons and ions,reduces the charge transfer resistance,weakens the impact of particle agglomeration,and is conducive to the improvement of desalination capacity.By adjusting the ratio of CNT to Ni HCF,the Ni HCF/CNT composits showed a specific surface area of 205 m~2 g^-1 and charge transfer impedance(R_ct)of 0.81?.In cyclic voltammetry(CV)test,the Ni HCF/CNT composite electrode showed a specific capacitance of 249 F g^-1 at a scan rate of 10 m V s^-1.The HCDI constructed by Ni HCF/CNT composites and AC showed a desalination capacity of 29.1 mg g^-1 and a desalination rate of 7.2 mg g^-1 min^-1.After 30 cycles,the desalination capacity retention rate was 62%.However,CNT in the composites can not effectively alleviate the volume expansion of Ni HCF particles,resulting in insufficient desalination stability.Therefore,further improvement is needed.2.In order to alleviate the volume expansion,the composites of MXene and Ni HCF(Ni HCF/MX)was prepared by introducing Ti₃C₂T_x MXene with 2D structure and high conductivity.The surface of Ti₃C₂T_x is rich in functional groups,such as-F,-OH,etc.,which can form multi-point connection with Ni HCF particles,increase the contact and thus improve the electrochemical performance.Moreover,the layered structure of Ti₃C₂T_x can buffer the volume expansion of Ni HCF and effectively improve the stability.By adjusting the ratio of MXene to Ni HCF,Ni HCF/MX composite displayed a specific surface area of 381 m~2 g^-1,a R_ct value of 1.43?and a specific capacitance of 217 F g^-1 obtained from CV test(10 m V s^-1).The HCDI constructed by Ni HCF/MX composite and AC showed a desalination capacity of 26.3 mg g^-1 and a desalination rate of 9.4 mg g^-1 min^-1.After 30 cycles,84%of the desalination capacity was maintained.Compared with Ni HCF/CNT,the cycle stability of Ni HCF/MX was greatly improved.However,the specific surface area of the prepared Ni HCF/MX composite is not high,due to its bulk structure,which is not beneficial to the electrochemical activity Therefore,further improvement is needed.3.In order to improve the electrochemical desalination capacity,2D graphene(r GO)with high specific surface area was introduced to prepare Ni HCF/r GO composites.The high specific surface area of r GO is conducive to the electrochemical activity.High conductivity and high ductility of r GO can effectively improve the charge transfer of the composites.By adjusting the ratio of r GO to Ni HCF,Ni HCF/r GO composite showed a specific surface area of 724 m~2 g^-1,with an R_ct value of 1.13?and specific capacity of 271 F g^-1(10 m V s^-1).The HCDI constructed by Ni HCF/r GO composite and AC showed a desalination capacity of 31.8 mg g^-1 and a desalination rate of 9.8 mg g^-1 min^-1.After 30 cycles,the desalination capacity retention rate reached79%.Although the desalination capacity of Ni HCF/r GO is higher than that of Ni HCF/MX,its charge transfer impedance is still high,which is not conducive to improving the desalination performance.4.In order to effectively reduce the charge transfer resistance of the electrode,Ni HCF/ZC composites were prepared by dispersing Ni HCF particles on the surface of Zi F-8 derived carbon(ZC)polyhedron with 3D structure.The polyhedral structure of ZC can provide rich charge transfer channels in multiple dimensions by forming effective connections with Ni HCF particles.By adjusting the ratio of ZC to Ni HCF,the Ni HCF/ZC composite showed a specific surface area of 610 m~2 g^-1,a charge transfer impedance of 0.76?and the specific capacitance reached 223 F g^-1 at a scan rate of 10m V s^-1.The HCDI constructed by Ni HCF/ZC composite and AC showed a desalination capacity of 33.5 mg g^-1,which was higher than those of the HCDI based on Ni HCF/CNT,Ni HCF/MX and Ni HCF/r GO,and a desalination rate of 9.7 mg g^-1 min^-1.However,after 30 cycles,its desalination capacity retention rate was only 66%.