Capacitive deionization (CDI) is a promising technology that has gained interest recently for the desalination of brackish water. Hierarchically porous carbons are commonly used for CDI due to their high surface areas and various pore size distributions that maximize ion adsorption capacity and rate.¹ Electrospinning is an effective way of generating nanofibers with high inter-fiber macroporosity that can be converted to carbon fiber and modified to improve surface area, total pore volume, and pore size distribution. This work describes the usage of sacrificial mesopore formers in tandem with a micropore etching technique to induce hierarchical porosity. The combination of mesopores (2-50 nm) and micropores (<2 nm) enables fast ion adsorption rates and capacity.² Mesopores will be formed via the dissolution of silica nanoparticles (10-20 nm) that are introduced into the fibers during the electrospinning step. Mesopore volume will be controlled by changing silica content in the fiber. An optimum mesopore contribution is found, at which point added mesoporosity maximizes ion transport. Further increasing mesoporosity is detrimental to fiber conductivity and surface area and results in decreased performance as reported in the literature. ³ After mesopore formation, micropores are etched on the surface of the fibers through KOH loading and thermal activation. Micropore formation is optimized to maximize specific surface area while maintaining physical integrity of the fibers. This sequential technique creates a hierarchical network of pores from the inherent microporosity of the fiber network, to the mesopores, and finally micropores to maximize usage of surface area. Fiber electrodes fabricated in this method achieve specific surface areas exceeding 1400 m² g^-1, with pore volumes exceeding 1.0 cc g^-1. The pore size distributions are highly controlled, with 80% of total pore volume coming from pores less than 20 nm in radius. In 500 ppm constant voltage CDI tests, these fiber electrodes obtain a salt adsorption capacity of over 14 mg g^-1, at a salt adsorption rate of ~3 mg g^-1 min^-1, showcasing the high capacity matched with high rate of these easily fabricated, inexpensive materials.

References:

1. Liu, T., Serrano, J., Elliott, J., Yang, X., Cathcart, W., Wang, Z., ... Liu, G. (2020). Exceptional capacitive deionization rate and capacity by block copolymer-based porous carbon fibers. Science Advances.
2. Baround, T., Giannelis, E. (2019). Role of Mesopore Structure of Hierarchical Porous Carbons on the Electrosorption Performance of Capacitive Deionization Electrodes. ACS Sustainable Chemistry & Engineering.
3. Pan, H., Yang, J., Wang, S., Xiong, Z., Cai, W., & Liu, J. (2015). Facile fabrication of porous carbon nanofibers by electrospun PAN/dimethyl sulfone for capacitive deionization. Journal of Materials Chemistry A.