Design Of Novel Carbon-based Composites And Research On Performance For Capacitive Deionization

Fresh water is a key factor for humankind to sustain life.In recent decades,the demand for water resources is growing at an alarming rate due to the rapid population growth,urbanization,and industrialization.Less than 3%of all water on earth is fresh water,and only 1.2%of all fresh water is surface water which can be directly utilized.The availability of fresh water is further limited by water pollution.Among various pollutants in the water,hazardous anions and heavy metal ions become a major concern due to their adverse effects on human health.Therefore,the shortage of clean water and water pollution has become global issues.Capacitive deionization(CDI),as an electro-adsorption water purification technology,has gained increasing attention due to the merits of low cost,low energy consumption,high efficieny,rapid regeneration,and environmental benignity.When applying a low potential on the two electrodes,the ions in solution can be quickly adsorbed and harvested in the oppositely-charged electrodes.Thus,the clean water is obtained.The ion removal capacity of CDI has a close relation with electrode materials.Various of carbon materials have been extensively explored as CDI electrode materials due to the advantages of porous structure,high surface area,and electrochemical stability.Unfortunately,the major limitation of these carbon materials is the ion removal capacity are far from satisfactory.Based on the above research situation and drawbacks,we novelly designed and prepared some modified carbon-based composites,investigated their capacitive deionization performance,and verified ion removal mechanisms.The main work is described as follows:(1)Trace Fe uniformly dispersed in the nitrogen-doped graphene(Fe-N-C)material was synthesized by a simple assembly and pyrolysis method using graphene as carbon precursor,g-C₃N₄ as nitrogen precursur,and water-soluable F127 as dispersant.The tight interfacial interaction between Fe atoms and surrounding nitrogen atoms favored the generation of plentiful active sites in the carbon matrix during the pyrolysis treatment and could tremendously enhance the electro-adsorption activity.The obtained two-dimensional ultrathin graphene nanosheets possessed relatively high specific surface area and abundant pores,which were beneficial to expose more active sites and promote the adsoption of salt ions.Profiting from the N doping and trace Fe introduction,Fe-N-C demonstrated splendid ion adsorption properties of 36.35 mg/g in a 500 mg/L saline solution at 1.2 V.Additionally,the electrode material also had superb stability and regeneration properties.In situ Raman and ex situ XPS measurements unraveled the removal mechanism of ions from saline water,and the reinforced adsorption of ions was due to the introduction of trace Fe boosting electron transfer of electro-adsorption sites during the CDI process.(2)Trace Fe uniformly dispersed in the nitrogen-doped carbon matrix through in situ growth of Fe doped ZIF8@g-C₃N₄ with the subsequent pyrolysis treatment using the precursors of g-C₃N₄,Zn salt,Fe salt and 2-Methylimidazole.Due to the synergistic effects of the heterostructure,polymorphous materials with diverse Fe-N species were successfully prepared through modulating the Fe concentration in the synthetic process.Among these materials,the Fe-N-CPNS,with the morphology of carbon nanosheet coated hollow polyhedral carbon skeleton,had the optimal Fe-N species.The formation evolution of the unique hollow structure was due to the influence of the Kirkendall effect.Benefiting from the structural advantages and homogeneity of active sites,the Fe-N-CPNS showed superb CDI performance with ion adsorption capacity of 34.38mg/g in a 500 mg/L NaCl solution at 1.2 V,as well as regeneration stability.In addition,it also showed high charge efficiency,high energy efficiency and low specific energy consumption.(3)Nb₂O₅ nanoparticles tightly anchored on the nitrogen-doped graphene sheets(Nb₂O₅@N-C)composite was synthesized via an assembly approach followed by pyrolysis treatment using the precursors of g-C₃N₄,graphene,Nb salt and 2-Methylimidazole.Nb₂O₅,as a representative Na⁺ions capture material with intercalation-conversion type,had a larger interplanar spacing for Na⁺ions diffusion.The synergetic effects of structure optimization(nanostructure)and surface engineering(nitrogen-doped carbon coating)were beneficial to enhancing electrical conductivity and offering fast Na⁺ions diffusion pathways.Impressively,the Nb₂O₅@N-C composite demonstrated an excellent ion adsorption capacity of 35.4 mg/g in a 500mg/L NaCl solution at 1.2 V,fast ion adsorption rate,high charge effiency,low energy loss and stable regeneration ability.In situ Raman and ex situ XPS measurements unraveled that the adsorption-intercalation-conversion mechanism of ions removal from water was the reversible redox reaction of Nb₂O₅ and electro-adsorption of N-C substrate.(4)Hollow porous nitrogen-doped carbon framework(PNC)was first synthesized through in situ growth of ZIF8 on polystyrene nanospheres followed by calcination treatment.After that,uniformly dispersed Al₂O₃ coating on the surface of PNC(CAO-PNC)composites were successfully prepared via atomic layer deposition(ALD).Al₂O₃had been used as a potential defluorination agent due to its moderate hydrophilicity,high adsorption capacity,and a selectivity of reversible chemical reaction toward F^-ions and carbon materials have high surface area and abundant pores for F^-ions adsorption.Benefiting from the synergistic effects of Al₂O₃ deposition and porous nitrogen-doped carbon matrix,the obtained electrode demonstrated exceptional F^-ions removal efficiency at 1.2 V(95.8%and 92.9%in 5 and 10 mg/L F^-solutions,respectively),high charge efficieny,low energy loss,and good regeneration ability after20 consecutive defluorination cycles.The removal mechanisms are the reversible conversion of Al-F species and the insertion of F^-ions into carbon skeleton,which were verified by in situ Raman,in situ XRD and ex situ XPS measurements.