Capacitive deionization (CDI) is an electrochemical desalination technology in which low voltage (~1.2 V) is applied between two porous carbon electrodes. Ions in solution migrate under the effect of the electric field and are adsorbed on the polarized electrodes’ surfaces. The electrodes are regenerated through short-circuiting the cell (0.0 V), discharging adsorbed salt to a brine stream. One of the major challenges in the design of CDI electrodes is improving charge efficiency (CE) (amount of ions removed per electrical charge applied). Fixed charge addition through the incorporation of ion-exchange membranes or electrode surface modification are known to achieve this by mitigating co-ion repulsion and providing enhanced ionic selectivity. In this work, we present a method to introduce fixed charge in the macroporous electrode structure by replacing the traditionally used, petrochemically-derived, polyvinylidene fluoride (PVDF) binders with chitosan (CS) and carboxymethyl cellulose (CMC), two biodegradable polysaccharides with fixed positively (CS) and negatively (CMC) charged functional groups. When CS and CMC are utilized as anodic and cathodic electrode binders, respectively, salt adsorption capacity (SAC) and charge efficiency reached peak values of 14.1 mg/g and 91%, compared to 5.1 mg/g and 31% obtained with PVDF binders. Surface-modified carbon electrodes with PVDF binders were also synthesized and used as a means of comparison. Using modified carbons, SAC and CE reached values of 16.2 mg/g and 87%, illustrating that incorporation of charged binders is a viable method for performance improvement. Sensitivity of the binders’ charge density to pH was also assessed, demonstrating the need for an anodic binder with a higher pKa than chitosan to maintain enhanced performance at higher pH. Finally, we propose a mechanism for the enhanced system's operation which takes into consideration the bare carbon’s surface characteristics and the addition of charge in the electrode’s macrostructure.

Enhancing counter-ion adsorption efficiency in capacitive deionization with charged polysaccharide binders

255th National Meeting (2018)

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