Design Of Hybrid Capacitive Deionization Electrode And Its Desalination Performance

With increasing population and environmental contamination,the scarcity of fresh water has become one of the most challenging issues in the foreseeable future.Considering that saline water is abundant on the earth,the conversion of seawater into fresh water through desalination technologies has become the most effective way to resolve the issues.Capacitive deionization(CDI),which is cost effective,low energy consumption,and environmental compatibility in comparison with other conventional desalination techniques,has drawn great attention from the researchers worldwide in recent years.Electrode materials are the key factor of CDI and greatly affect the desalination performance.According to the inherent properties of electrode materials,the mechanism of CDI can be divided into the electric double layer(EDL)and faradaic reaction.At present,most of the electrode materials suffer from low salt adsorption capacity(SAC)and poor cycle regeneration ability,which largely limits the practical process of CDI.In this study,we focus on the design of high-performance CDI key electrode materials.With this aim in mind,to configure novel boron carbon nitride(BCN)nanosheets(EDL principle)and carbon/MnO2 composites materials(Faradaic reaction principle)to improve the SAC,charge efficiency and cyclic stability by reasonably optimizing the composition,morphology,surface characteristics,electrical conductivity of electrochemieally active materials and combining with hybrid capacitive deionization(HCDI).The main contents and conclusions of the dissertation are as follows:(1)Using g-C3N4 as both the template and the nitrogen source,a thin amorphous carbon was coated on surface the g-C3N4 via a hydrothermal method with glucose as the carbon source.Subsequently,BCN nanosheet with high specific surface area rich in pore structure was prepared by co-heating with boric acid and investigated as CDI electrode materials for the first time.The as-prepared BCN nanosheets show high B and N atom content,with a potential synergistic effect between them,results in fast ion diffusion and good charge transfer ability.When applied in CDI,BCN nanosheets demonstrate a high salt adsorption capacity of 13.6 mg g-1 at 1.4 V applied voltage when the initial NaCl concentration is 500 mg L-1 and long-life cyclability over 15 loops with 96.1%capacity retention at 0.8 V.(2)The NaMnO2(It should be noted that in the whole paper,NaMnO2 is only an abbreviation,rather than the chemical formula.)nanosheets with open sandwich structure was constructed via electrostatic coprecipitation and layer-by-layer self-assembly of colloidal MnO2 nanosheet.The interlayer spacing of NaMnO2 was enlarged by the insertion of Na+ ions,which facilitates the rapid diffusion and storage of salt ions in layers of NaMnO2.Simultaneously,carbon nanotubes(CNTs)are used as conductive bridges and network crosslinkers to fonn 3D CNT/NaMnO2 with high conductivity and open pore structure.When the two layered MnO2-based nanomaterials were evaluated as HCDI redox-active electrodes,and assembled with AC into HCDI,show superior desalination performance.Both material-based cells demonstrated an excellent SAC of 28.3 and 40.0 mg g-1 for NaMnO2//AC and 32.7 and 42.6 mg g-1 for CNT/NaMnO2//AC in 500 and 20000 mg L-1 Nael solution,respectively.Moreover,after 100 adsorption-desorption cycles,the SAC retention rate of NaMnO2//AC and CNT/NaMnO2//AC can be as high as 92.1%and 93%,respectively.(3)A HC@MnO2 composite with pore structure was synthesized by in-situ growth strategy and using hollow porous carbon spheres(HC)with high surface area and conductivity as the starting material,and then used as the cathode of HCDI.The unique structure of HC@MnO2 not only has high conductivity,but also facilitates the penetration and diffusion of salt ions,thereby enhancing the synergistic effect of the electric double layer and Faraday storage.Meanwhile,a hollow carbon sphere with net positive surface charges(PHC)was modified by poly(diallyldimethylammonium chloride)(PDDA)and used as the anode of HCDI.The obtained HC@MnO2 and PHC electrodes were assembled into a membrane-free HCDI system,which exhibited outstanding CDI performance.PHC//HC@MnO2 shows a high SAC and maximum removal rate of 30.7 mg g-1 and 7.8 mg g-1 min-1 at 1.2 V applied voltage when the initial NaCl concentration is 500 mg L-1 and long-life cyclability over 50 loops with 90.1%capacity retention at 0.8 V.In addition,PHC//HC@MnO2 not only has higher SAC and ion removal rate,but also has lower cost production advantage in comparison with membrane capacitive deionization(MCDI,namely conventional HCDI).