Improved Capacitive Deionization By Using Graphene-based Porous Carbon Materials

Wastewater and brackish water treatment is a crucial technology.Particularly,some countries produce potable water by demanding suitable and cost-effective technology.Capacitive deionization（CDI）is an innovative water purification technology,which consume low-energy and no secondary waste are produced.The electrode materials are quite crucial for the deionization performance.Electrosorption capacity is effectively associated with textural features of carbon electrode materials such as exceptional electrical conductivity,large specific surface area,well-defined porous structure,fast response to ion adsorption-desorption,high durability,high resistance to organic fouling and biofouling and robust chemical and electrochemical stability against corrosion.Recently,due to various innovations in everyday applications and fundamental research,graphene-based materials have attracted a lot of attention in CDI electrode materials.However,the commonly used carbon materials suffer from such issues as unsuitable porous structures,low conductivity and wettability,stronginteractions and irreversible agglomerations of graphene layers which limit their practical applications.Currently,a prominent alternative is to develop and design hybrid composite of graphene sphere and porous carbon as electrode materials,which exhibit improved performance compared to simple graphene,due to synergistic effects of graphene sphere and porous carbon.Therefore,3D graphene-like electrode materials are attractive towards desalination because they exhibit extraordinary conductivities and enhanced specific surface area.This thesis presents research on the development of N-doped 3D graphene nanospheres,layered mesoporous carbon sheets and their composites,emerging carbon materials derived from[Ni₂（EDTA）],glucose and urea precursors.So far,production of graphene in bulk reveals issues including agglomeration,restacking,poor exfoliation,structural damage and low electrosorption capacity.However,the high electroactive behavior,moderate conductivity,high chemical stability,and even with moderate exposed surface area makes them potential support materials or excellent candidates for CDI.Two research projects presented emphasis on finding new production methodology,enhancing the specific capacitance through graphene nanospheres intercalation,heteroatom（N-doping）functionality and controlling meso and micropores formation to enhance the electrosorption performance and utility.The first study addresses a straightforward and cost-effective approach to prepare3D intercalated graphene sheet-sphere nanocomposite architectures as high-performance CDI electrodes by using graphene oxide and[Ni₂（EDTA）]as precursors.3D graphene nanospheres between graphene sheets inhibit the restacking of graphene layers.Significantly,the as-prepared 3D mesoporous graphene sheet-sphere nanocomposite architectures possess high specific surface are,suitable pore size distribution,fine wettability,high specific capacitance as well as large discharge time.By using 3D mesoporous graphene sheet-sphere nanocomposite architectures as an electrode,ultrahigh electrosorptive capacity of 22.09 mg g^-1 is achieved in a 500 mg L^-1 Na Cl solution at 1.2 V,and the salt removal percentage of the obtained electrodes is around 90%.Furthermore,the electrodes present good deionization stability and regeneration performance.This work also offers a promising solution to develop highly effective electrode materials in the CDI process for brackish and sea water desalination.The second study addresses a novel and cost-effective strategy to fabricate graphene nanospheres decorated N-doped layered mesoporous carbon sheets（GNNLMCSs）as highly-efficient electrode materials for CDI applications.The effects of unique morphology,chemical composition and N-doping were clarified by conducting controlled experiments.The obtained materials exhibit a layered porous sheet-like structure decorated with graphene nanospheres,large surface area,regular micro and mesopores channels and high-level N-doping（up to 10.56 at%）.The obtained CDI electrode delivers high specific capacitance,fine wettability,fast ion diffusion and excellent charge transfer ability.High salt removal capacity(23.42 mg g^-1)with fast removal rate has been demonstrated at 1.2 V in a 500 mg L^-1Na Cl solution,indicating the synergistic effect of N-doping and special morphological structures on the improvement of CDI performance.Besides,the obtained CDI electrode also present good deionization stability over multi-regeneration cycles and the desalination efficiency is up to 90%.The result presented in this work opens up a new avenue for the development of high-performance CDI electrode materials for desalination of saline water.