Capacitive water desalination with hierarchical porous electrodes [electronic resource]
Keywords: Mechanical Engineering.

Water purification is the rendering of non-potable water into water fit for human consumption and use, and may involve many processes including desalination, disinfection and decontamination of the water. The step of removing dissolved salts (the desalination step) is typically the most expensive of the complete process, and so this step has been the focus of intense engineering and scientific research for the past several decades. Capacitive desalination (CD) is a promising emerging water desalination technique as, relative to the established desalination technology reverse osmosis (RO), it requires no membrane components, can operate at low (sub-osmotic) pressures, and can potentially utilize less energy for brackish water desalination. In a typical CD cell, the feed water flows through the separator layer between two electrically charged, nanoporous carbon electrodes. This architecture results in significant performance limitations, including an inability to easily (in a single charge) desalinate moderate brackish water feeds and slow, diffusion-limited desalination. We here describe an alternative architecture, where the feed flows directly through electrodes along the primary electric field direction, which we term flow-through electrode (FTE) capacitive desalination. Using macroscopic porous electrode theory, we show that FTE CD enables significant reductions in desalination time and can desalinate higher salinity feeds per charge. We then demonstrate these benefits using a custom-built FTE CD cell containing novel hierarchical carbon aerogel monoliths as an electrode material. The pore structure of our electrodes includes both micron-scale and sub-10 nm pores, allowing our electrodes to exhibit both low flow resistance and very high specific capacitance (> 100 F g−1). Our cell demonstrates feed concentration reductions of up to 70 mM NaCl per charge and a mean sorption rate of nearly 1 mg NaCl per g aerogel per min, 4 to 10 times higher than that demonstrated by the typical CD cell architecture. We also show that, as predicted by our model, our cell desalinates the feed at the cell's RC timescale rather than the significantly longer diffusive timescale characteristic of typical CD cells. We also present a combined theoretical (linear circuit model) and experimental (electrochemical impedance spectroscopy) study of hierarchical porous carbon electrode capacitors which integrate nanoscale pores into a micron-scale porous network. Our experiments are performed on a set of custom-fabricated hierarchical carbon aerogel electrodes with varying pore structure, including electrodes with sub-nanometer (sub-nm) pores. Our combined theory and experimental approach allows us to demonstrate the utility of our model, perform detailed characterizations of our electrodes, study the effects of pore structure variations on impedance, and propose hierarchical electrode design and characterization guidelines. Further, we demonstrate that our approach is promising towards the detailed study of ion storage mechanisms in hierarchical electrodes with sub-nanometer pores. We further demonstrate a simple and novel experimental system for in situ measurements of spatially and temporally resolved ion concentration between charging electrodes in a CD cell. To our knowledge, our system is the first to demonstrate such measurements. Importantly, as opposed to the commonly-used technique of effluent conductivity measurements for studying CD cells, our system enables the study of salt ion and desalination dynamics in a controlled environment without flow. Thus, our approach enables detailed diagnostics of transport-related phenomena in CD cells, and provides data which we use to validate modeling efforts.