A direct comparison of flow-by and flow-through capacitive deionization

Capacitive deionization (CDI) is an electrochemical ion removal technique that could offer a lower energy or more selective alternative to reverse osmosis for water treatment applications. Over a dozen CDI cell architectures have been developed, but few studies have directly compared these designs for performance metrics such as ion adsorption rate and energy efficiency. Two of the most popular cell architectures include flow-by CDI, in which a feed stream flows parallel to two charged porous electrodes, and flow-through electrode CDI, in which the feed stream flows perpendicular to the electrodes. In this study, flow-by and flow-through CDI architectures were compared using commercially available electrode materials. Experimentally observed salt adsorption capacity (SAC), average salt adsorption rate (ASAR), and charge efficiency (Λ) was then compared to theoretical models over a range of voltage conditions (0.2–1.2 V), charge cycle times (1–60 min), and flow rates (10–30 mL min^{- 1}). Cell architecture affected the SAC, ASAR, Λ, electrode stability, and oxidation rates for a given electrode material. Flow-by CDI tended to have higher SAC and better charge efficiency than flow-through CDI, yet flow-through CDI demonstrated a higher ASAR, particularly for shorter half-cycle times.