From Graphene To 3D Graphene:The Design Of Highly Efficient Capacitive Deionization Electrode

Capacitive deionization(CDI)is known as an emerging desalination technology working on the principle of electrical-double layer(EDL)theory.In recent years with the increase of population and the development of industrialization,water shortage have been a key issue in limiting the development of human society.Seeking promising water-treatment technology has attracted enormous attentions amongst the world.In this background,CDI technologies based on EDL theory have been rising.Generally,porous carbons with high specific surface area and good electrical conductivity have been considered as ideal CDI electrode materials.Despite current progresses achieved for porous carbons,the obtained CDI performances still cannot meet the demand of practical applications.Therefore,exploring novel porous carbons plays a key role in the development of CDI technology.Graphene(GE),as a new two-dimensional porous carbon,has been receiving extensive attentions in applications of CDI due to its enormous theoretical specific surface area of 2630 m2 g-1 and high electrical conductivity.Although some progresses have been made up to date,the the common shortcomings in GE,such as poor desalination capacity,the natural restack tendency of GE nanosheets,low specific surface area,are still to be overcome perfectly;thus limit its applications in CDI.To address these issues,here we have prepared a series of GE and 3D GE materials through the structural design and surface modification,and systematically studied the relationship between their structural characteristics,morphology and CDI performance.By optimizing the preparation processes,the superior CDI electrode materials could be obtained.Detailed contents in this work are summarized as follows:1)By using graphene oxide(GO)as precursors,nitrogen-doped GE,mesoporous GE and ultrathin nitrogen-doped carbon/GE nanosandwiches were successfully prepared via heat treatment in NH3,etching treatment by KMnO4 and in situ growth of metal-organic frameworks layer,respectively.Sequently,their structural characteristics,morphologies and CDI performances were investigated and compared with pristine GE.The results show that i)through nitrogen doping,the obtained nitrogen-doped GE shows high nitrogen content,large surface area and good electrical conductivity,thus showing high desalination capacity(12.8 mg g-1);ii)through the structural design,the obtained mesoporous GE exhibits large accessible surface area and short ion diffusion path,thus showing fast electrosorption/desorption rate and high desalination capacity(14.2 mg g-1);iii)through covering ultrathin nitrogen-doped carbon layers on the surfaces of GE,the obtained nitrogen-doped carbon/GE nanosandwiches exhibit large surface area,high nitrogen content and superior desalination capacity(17.5 mg g-1).2)3dGE was prepared via directly freeze drying GO solution with a subsequent thermal-treatment process.The structure,morphology,and CDI performances of 3dGE materials were investigated and compared with pristine GE.The results show that the obtained 3dGE materials show a novel three-dimensional porous structure which is beneficial for alleviating the aggregation of GE nanosheets,thus improving the specific surface areas and pore volumes of 3dGE materials.Furthermore,3dGE shows higher desalination capacity(14.9 mg g-1).The 3dGE was further subject to NH3 treatment and nitrogen doped 3dGE was achieved.The effect of nitrogen doping on the specific surface area,electrochemical and CDI performance were investigated.The result shows that nitrogen doping is beneficial for improving the wettability and enhancing electrical conductivity.The obtained nitrogen-doped 3dGE exhibits ultrhigh desalination capacity(21.0 mg g-1).3)Carbon nanotubes(CNTs)/3dGE hybirds were prepared via directly freeze drying GO/CNTs mixed solution with a subsequent thermal-treatment process.The structure,morphology,and CDI performances of CNTs/3dGE hybrids were investigated.The results show that i)CNTs serve as the spacers between GE nanosheets,which endows the obtained CNTs/3dGE hybrids with high surface areas and large pore volume.When the mass ratio of CNTs is 20 wt%,the obtained CNTs/3dGE hybrid exhibits the largest surface area and pore volume;ii)the increase of CNTs ratio in CNTs/3dGE hybrids promotes the electrical conductivity of CNTs/3dGE hybrids;iii)CNTs/3dGE hybrid with 20 wt%CNTs exhibits the highest desalination capacity(18.7 mg g-1).4)Carbon spheres(CSs)/3dGE hybrids were prepared via directly freeze drying GO/CSs mixed solution with a subsequent thermal-treatment process.The structure,morphology,and CDI performances of CSs/3dGE hybrids were investigated.The results show that i)microporous CSs serve as the spacers between GE nanosheets,endowing the obtained CSs/3dGE hybrids with hierarchically macro/meso/microporous structures,high surface areas and large pore volume;ii)when the mass ratio of CSs is 10 wt%,the obtained CSs/3dGE hybrid exhibits the largest surface area and pore volume;iii)the CSs/3dGE hybrid with 10 wt%CSs exhibits the highest desalination capacity(21.0 mg g-1).