Hierarchical Porosity in Electrospun Capacitive Deionization Electrodes

Abstract

Capacitive deionization (CDI) has gathered significant interest in recent years for the desalination of brackish water. Hierarchically porous carbon nanofibers offer high surface area paired with various pore size distributions that enable high ion adsorption capacity and adsorption 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. Balancing the contribution of micropores (<2 nm) and mesopores (2-50 nm) is vital to ensure the optimization of adsorption capacity and rate, as micropores contribute more to the specific surface area while mesopores contribute more to ion transport.2, 4 In this work, we explore the role of various pore size distributions on the performance of carbon fiber electrodes generated through electrospinning in CDI through novel material development and characterization paired with pore-scale modeling. The relationship between micropore/mesopore volume ratio and its effect on capacitance and rate capabilities of CDI electrodes will be discussed. Electrospun fibers are modified by including varying amounts of sacrificial pore formers during the electrospinning process, which are then distributed throughout the final fiber. These sacrificial pore formers are removed via solvent or heat treatments post-electrospinning for tuning mesopores.3, 4 Different size pore formers are combined to create bimodal mesopore size distributions on the carbon fiber surface. To further develop the pore structure, thermal and chemical surface treatments are employed to etch micropores into the porous fibers. This creates a hierarchical pathway from macro-, to meso-, to micropore for effective salt transport and to maximize the usage of surface area. Pore-scale models of the computationally reconstructed carbon fibers have been developed to understand the deionization process in the hierarchical pores.