Capacitive deionization (CDI) is increasingly being considered as a promising desalination alternative to reverse osmosis and other well-established technologies, especially when it comes to treating low salinity water, such as groundwater. Lifetime and charge efficiency (CE) are the two metrics which determine the competitiveness of CDI technologies, which is why enhancing design is a major developmental challenge. It is known that ion-exchange membranes (IEMs) and chemical surface modification contribute to this goal by eliminating the effects of co-ion repulsion. Previous research conducted in our lab, showed how the use of charged biodegradable polysaccharide compounds, namely chitosan (CS) and carboxymethylcellulose (CMC), as anodic and cathodic electrode binders respectively, improved charge efficiency and lifetime values of the electrodes in which they were employed, compared to a symmetric cell where electrodes were bound with the traditionally used, petrochemically derived, polyvinylidene fluoride (PVDF). As a means of comparison, surface-modified electrodes were also fabricated and their performance assessed when assembling them in another CDI unit. The specific salt adsorption (SSA) behavior of the CS-CMC bound cell, combined with the cyclic voltammetry (CV) results and SSA at various discharge voltages, suggested the binder was enhancing salt adsorption performance and mitigating co-ion repulsion by modifying the macrostructure, producing a similar effect to an IEM.
In this work, we propose a mechanism for the enhanced salt adsorption and charge efficiency observed with the charged polysaccharide binders, where a hybrid system composed of CDI and MCDI sub-units can illustrate the effects of the improved electrode macrostructure. We fit the untreated carbon data using the Amphoteric Donnan Model (ADM) to consider the asymmetry in acidic and basic groups in the pristine carbon surface, which is the main cause of co-ion repulsion. The SSA value calculated for this system only differs by 5% from that obtained experimentally and charge efficiency shows a similar trend. To simulate impact of charged binders on ionic transport within the electrode macropores, MCDI sub-units were introduced. At a MCDI surface coverage of 7.5% we observe the co-ion repulsion peak disappears as a result of the addition of fixed charge. When decreasing the membrane thickness, thus resembling the polysaccharide coating of the carbon, the diffusion timescale across the selective interface is significantly reduced, and therefore governs the overall cell behavior by modifying the concentration in the flow channel for subsequent units. The model discretizes rapid transport across these selective interfaces and slower transport across traditional CDI sub-units, although in reality the electrode structure is much more complex and thus differences between the simulated results and the actual behavior can find reason in this simplification. Finally, we determine that an anode binder pKa should be one log unit greater than the influent pH to make the system less sensitive to pH fluctuations
