Preparation Of Electrospun Phenolic Resin-based Carbon Fiber Mats And Their Capacitive Deionization Performance

Capacitive deionization (CDI) is a newly developed desalination method byadsorbing charged ions from aqueous solution onto the surface of porous electrodematerials under external power supply, which has many advantages like energy-saving,regenerative, pollution-free and etc. According to literatures, the choice of flexible andfree-standing electrode materials with high specific surface area is the critical issue forthe success of CDI.Compared to conventional phenolic resin-based carbon fibers, electrospun phenolicresin-based carbon fibers not only retain the high carbon yield and high specific surfacearea, but also will improve the mechanical properties due to smaller fiber diameter. Butup to now, electrospun phenolic resin-based carbon fibers reported in the literatures stillhave some disadvantages like poor mechanical properties, complex procedures, and etc.Thus it is necessary to optimize the preparation method of electrospun phenolicresin-based carbon fibers. Free-standing phenolic resin-based carbon mats with highflexibility and low cost could be competitive electrode materials for CDI.In this work, we prepared uniform and continuous phenolic resin fibers byelectrospinning solutions of thermosetting phenolic resin (resol), or thermoplasticphenolic resin (novolac), or their mixture. All of them are commercially available, thushave low cost. It was found that as the ratio of resol increased, electrospun phenolicresin fibers could be cured by immersing in curing solutions, or by stepped-heating atlow temperature in air. After immersed in curing solutions, the fibers closely stackedtogether, resulting in poor flexibility of the mats. But for stepped-heating, the fibermats can completely cross-linked while maintaining a loose network structure, and thushas excellent flexibility.In-depth study of electrospinning process of resol solutions showed that by addinglittle amount of polyvinyl alcohol butyral (PVB), its spinnability was effectivelyimproved. While the solution concentration is less than35wt%, and the viscosity isbetween15-30mpa s, uniform and continuous beads-free phenolic resin ribbons couldbe obtained. Further study indicates that the non-linear molecular structure of resol andits relatively high solution conductivity is in favor of the formation of ribbons, while the solvent with too high or too low evaporation rate is not conducive to the formation ofribbons.After being stepped-heated, the cured electrospun phenolic resin fiber mats werefurther carbonized and activated. All of the carbonized samples show great flexibility,and the maximum specific surface area858m2/g was obtained at1000℃. For steamactivation, while activated at900℃for30min, carbon mats with good mechanicalproperties and high specific surface area of1343m2/g was obtained.Capacitive deionization characterization shows that, while the initial concentrationof NaCl solution is100mg/L and the applied voltage is1.2V, the physical adsorptioncapacity of electrospun phenolic resin-based activated carbon fibers is very high, whichmay reduce the reversible adsorption capacity. But the larger pore width, higher specificsurface area, flexible and free-standing structure, still lead to faster adsorption rate andhigher reversible adsorption capacity, which reaches7.6mg/g，higher than that of othermaterials tested under similar conditions.