Performance Enhancement of Capacitive Deionization By Using Ionomer-Combined Carbon Electrodes and Pore-Filled Ionomer Membranes

Abstract

Ion selective polymers (ionomers) are currently widely used in water treatment applications such as the diffusion dialysis, electrodialysis, and capacitive deionization for the desalination of brackish water, production of table salt, recovery of valuable metals from the effluents of metal-plating industry, and recoveries of acid and base from the spent leaching solutions. The functional ionomers are also utilized in various energy devices such as polymer electrolyte membrane fuel cells, redox flow batteries and so on. Especially, capacitive deionization (CDI) is thought to be one of the attractive environmentally-friendly desalination processes. It consumes relatively small energy for operation compared with other competing processes such as reverse osmosis. In addition, it does not produce any secondary regeneration wastes for a re-use. Comparing to the conventional CDI, membrane-CDI (MCDI) and ionomer coated-CDI have gained great interests due to the higher ion selectivity and removal efficiency. In this work, therefore, we have synthesized and characterized functional ionomers for the successful CDI application. The anion-exchange polymer was made from poly(2,6-dimethyl-1,4-phenylene oxide) through the bromination and consecutive amination processes. Meanwhile, the cation-exchange polymer was prepared via the sulfonation of polyetheretherketon. The functionalized ionomers exhibited excellent electrochemical properties of both the low electrical resistance and high ion selectivity. As a result, the functionalized ionomers which were coated on porous carbon electrodes largely improved the current and salt removal efficiencies of the CDI process. We have also developed a pore-filled IEMs (PFIEMs) for the applications to cost and energy efficient CDI processes. The PFIEMs composed of a highly inert and tough porous substrate and a polymer with functional groups that fills the pores can provide both high ion conductivity and excellent mechanical properties. In addition, the PFIEMs could be produced in a cheaper way by a roll-to-roll continuous process. The PFIEMs with a film thickness below 50 μm have been successfully prepared and characterized via various electrochemical analyses. As a result, they were shown to possess excellent electrochemical and mechanical properties. Their electrochemical characteristics have also been optimized for the successful applications to MCDI processes. (This work was supported by the Human Resource Training Program for Regional Innovation and Creativity through the Ministry of Education and National Research Foundation of Korea (NRF-2015H1C1A1034436) and also by the Technology Innovation Program funded by the Ministry of Trade, industry & Energy (MOTIE) (No. 10047796).)