Research On Ni,Co-Prussian Blue Analogs Modified Carbon Nanotube Electrodes And Capacitive Deionization

Capacitive deionization（CDI）is a new seawater desalination technology,which has been widely developed due to its advantages of low cost,flexible scale,and no secondary pollutants.The CDI approach removes seawater ions by using the principles of Electrical double-layer（EDL）and pseudocapacitance.The electrode material is the key factor affecting the performance of CDI.To optimize CDI desalination performance,the electrode material should have high specific capacitance,great conductivity,and good cycle stability.Because of their high electrochemical activity and theoretical specific capacity,Prussian blue analogues（PBAs）with open cubic skeleton structures are extensively used in battery,capacitor,and sensor fields.However,the application of PBAs in CDI is restricted by their poor conductivity.In our previous work,we have developed Ni/Fe PBA/graphite tube composites with high electrochemical performance,demonstrating that Ni doping can significantly improve the electrochemical performance of Fe-PBA by activating Fe²⁺/Fe³⁺redox sites,but Ni does not participate in the redox reaction.In this study,Ni,Co-PBA/HCT was obtained by preparing Ni and Co bimetallic doped hollow carbon nanotube（HCT）by adjusting the bimetallic source and then reacting with K₄[Fe（CN）₆]to grow Ni,Co-PBA/HCT.The synergistic redox effect of Ni,Co,and Fe was explored to further enhance the redox activity of PBAs,improve the electrochemical performance and specific capacitance of PBAs electrode materials,and thus improve the deionization capacity of capacitances.The main contents are as follows:（i）At first,metal-doped carbon nanotubes were obtained by high-temperature carbonization using Ni Cl₂,Co Cl_2,and melamine as metal and carbon sources.Under acidic conditions,metal ions precipitated from the surface of carbon nanotubes react with K₄[Fe（CN）₆]to form nucleation center in situ growth of PBA particles.Finally,hollow carbon nanotubes Ni,Co-PBA/HCT supported by Ni,and Co bimetallic PBA were prepared successfully.At the same time,three conditions of metal doping,carbonization temperature,and acidity were changed,and a control experiment was designed to explore the influence of different synthesis conditions on the electrode material preparation process.The materials were characterized by scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,Fourier transform infrared（FTIR）,X-ray photoelectron spectroscopy（XPS）,and X-ray diffraction（XRD）,and the composition and structure of the materials were analyzed.SEM and TEM results show that bimetal-doped HCT has regular and uniform hollow carbon nanotube structure,and the supported PBA cube particles are small（20-50 nm）,and closely adhere to the surface of hollow carbon nanotubes,to achieve rapid electron transport and ion diffusion.XRD results show that PBA has a typical lattice structure and good crystallinity.However,the displacement of characteristic peaks corresponding to different metal doping proves that Ni and Co bimetallic PBA is not a physical mixture of two single metal PBAs,but the result of Ni and Co metals randomly replacing the inner spin sites of PBA.（ii）The electrochemical behavior and characteristics of different materials were analyzed by electrochemical test and desalting test to further explore the relationship between electrochemical performance and morphology of materials.Comparing the electrochemical performance of the composites prepared under different metal doping modes,different carbon nanotube structures,and acidity conditions,it is found that Ni,Co-PBA/HCT has excellent electrochemical performance and high specific capacity.In the three-electrode system,the basic electrochemical performance showed a high specific capacity of 344.68 F g^-1(at a current density of 1 A g^-1).,which is much higher than that of Ni-PBA/HCT and Co-PBA/HCT.Combining with characterization methods such as XPS,the electrochemical mechanism of the materials was analyzed,and it was proved that the synergistic redox effect was produced by the poly metal with the simultaneous valence changes of Co and Fe in the electrochemical process.The combination of bimetallic doped PBAs and highly conductive carbon-based materials can effectively improve the conductivity and electrochemical properties of electrode materials.Based on this,the capacitance deionization performance of different electrode materials was studied.Ni,Co-PBA/HCT showed a high desalination capacity of 73.48 mg g^-1（1000 ppm Na Cl and 1.2 V voltage）and desalination rate of3.12 mg g^-1 min^-1（10 m M Na Cl and 1.2 V voltage）in capacitive deionization test.The successful preparation of high-performance bimetallic PBA composite carbon-based materials provides a feasible idea for the development and application of PBAs-based materials in the field of CDI.