Design, Fabrication And Properties Of Monolithic Activated Carbon Nanofiber Composites For Capacitive Deionization

Water is one of the indispensable natural resources for life, and for the economic activities of human beings across the world. Capacitive deionization (CDI) technology, based on the electrochemical principle of electric double layers, is a promising technique for removel of salt ions from concentrated aqueous solutions, via the adsorption of the excessive charged ions into the electrical double layer region at the interfaces between the porous electrodes and the salt solution when an external cell potential is supplied in two porous electrodes. The CDI technology is environmentally friendly, energy-efficient, simple and easy operation compared with traditional desalination techniques such as membrane desalination, electrodialysis and thermal process currently available. Up to now, the controlled preparation of high performance porous electrodes with tuned pore structure, surface properties and conductivity is one of the bottle-neck problems faced by the CDI technology. Electrospinning is a convenient, efficient, and environmentally friendly method to prepare flexible and macroscopic monolithic web-like materials. The objective of this paper is to fabricate non-woven monolithic acticated carbon nanofiber composites for CDI electrode materials with high specific surface area and high conductivity, which has been done by the addition of conductive carbon black (CB, zero dimentional material), carbon nanotubes (CNT, one dimentional material) and reduced oxidized graphene (two dimentional material) by a simple and versatile electrospinning technology. The reduced oxidized graphene/activated carbon fiber composites for CDI electrode materials were also synthesized by spray-assisted electrospinning technique. In the last chapter, CDI cells were constructed by making use of a commercial activated carbon with well-developed mesopore structure as electrode material, with an aim of working out the optimized operation conditions for desalination and cycle regeneration in terms of the cell potential, the adsorption time, the initial concentration, etc. The main work and relevant conclusions are summarized below.(1) The CB particles with well-developed mesoporous structure can be dispersed and embedded inside the electrospun ACF webs by electrospinning, followed by subsequent CO2 activation, which helps to tailor the hierarchical pore distribution and improve the conductivity, leading to electrode materials with high salt removal capacity. The as-made free-standing electrospun CB/ACF900 electrode material exhibits an electrosorption capacity as high as 6.5 mg g-1 and good cycle stability as the CDI electrode due to the distinctive hierarchical structure.(2) The composites with a "line-in-line" structure made of 1D CNTs with high length to diameter ratio that are embedded in activated carbon nanofiber (ACF) have been prepared by a direct co-electrospinning and subsequent CO2 activation strategy. The introduction of CNTs can greatly improve the conductivity of the composites while the CO2 activation can render the composites with high porosity. The hybrid structure can provide an excellent storage space and pathways for ion adsorption and transportion. As electrode materials for CDI, the as-made CNT/ACF composites with higher electrical conductivity and well-developed mesoporous structure have showed an excellent electrosorption capacity of 6.4 mg g-1 and good regeneration performance in comparison with ACF in the absence of CNTs.(3) Graphene with 2D plannar structure has been used to fabricate composites for CDI desalination. The composites made of reduced graphene oxide and activated carbon nanofiber (RGO/ACF) with a "line-plane-line" structure for CDI have been produced by co-electrospinning the RGO (reduced graphene oxide) and ACF (activated carbon nanofiber), followed by CO2 activation. In the electrospun RGO/ACF composites, the nanofibers function to immobilize graphene and to avoid the agglomeration of graphene, while the graphene helps to increase the conductivity as a conductive agent and to improve the pore size distribution. It has been found that the RGO/ACF-10 has an excellent electrosorption capacity as high as 7.2 mg g-1 at 400 mg L-1 of the initial salt concentration.(4) The composites made of reduced graphene oxide and activated carbon nanofiber (S-RGO/ACF) are prepared by in situ electrospinning the polymeric nanofibers with simultaneous spraying graphene oxide. The free-standing carbon nanofiber webs act as the framework for sustaining graphene that helps to prevent the agglomeration of graphene and to provide a high conductivity for the efficient charge transfer in the composite electrodes. Moreover, the individual fibers decorated by graphene flakes have a rougher surface and a pore size distribution that is demanded for the efficient charged adsorption onto the surface. The as-made S-RGO/ACF electrode with a "line-in-line"structure is favored to the storage and removal of salt ions, evidenced by the good desalination performance with a desalination capacity of 9.2 mg g-1 and a current efficiency of 34%.(5) The capacitive deionization performance of the stacked CDI cells contructed by making use of a commercial activited carbon electrodes with high surface area and high mesopore ratio has been examined in detail. The effects of the operation conditions such as the cell potential, the adsorption time, the initial concentration on the desalination and cycle refeneration performance have been addressed in terms of the current efficiency, the adsorption capacity and the specific energy consumption in large-scale.