Capacitive Deionization Using Composite Carbon Electrodes

Abstract

Brackish water resources may be an attractive option for human consumption, agriculture, and industry if efficient water purification can be implemented. In the past few decades, research and development of various desalination technologies have been carried out, among which distillation, reverse osmosis, and electrodialysis are the most commonly known and widespread.1 Capacitive deionization (CDI) is an alternative, emerging, and energy-efficient technology for water desalination, which employs an electrochemical flow cell configured with polarized porous carbon electrodes to remove ionized salts in a stream with low molar concentration. Briefly, by regulating an external voltage to a CDI cell, ionized salts are electrostatically captured (or released) in the pores of the carbon electrodes, resulting in the stream being deionized (or the electrodes being regenerated).2-4 Recent studies have found that the salt adsorption capacity (SAC) could be substantially improved by using surface modified carbon electrodes resulting from nitric acid and ethylenediamine treatments.5 Combined with the modified Donnan model including a term of chemical surface charge, this improved SAC was accounted for by enhancement of the chemical charges immobilized in the carbon micropores, validating both enhanced CDI (e-CDI) and extended-voltage CDI (eV-CDI) effects in the CDI literature (Fig. 1).6In summary, it is considered that, for the carbon electrodes used in a CDI cell, an increase in the chemical surface charges makes the pores more readily available for salt adsorption under proper applied voltages. In addition to the surface modified carbon electrodes, immobilized chemical charges can be found in ion-exchange materials. For instance, a well-known cation-exchange polymer, Nafion, contains the negative chemical charges, -SO3 -, while an anion-exchange polymer typically holds positive chemical charges, e.g., NR4 + and NR3 +. As a consequence, together with the knowledge gained above, ion-exchange polymers coating were used in our current studies to explore new composite carbon electrodes for CDI cycling tests. As shown in an initial test (Fig. 2), the addition of ion-exchange polymers results in the SAC not only being increased but also being stabilized with operational time when NaCl solution was used. In this presentation, the preparation and characterizations of composite carbon electrodes will be detailed including comparisons to conventional CDI and membrane capacitive deionization cells. Furthermore, these composite electrodes will be configured into a CDI cell to investigate both e-CDI and eV-CDI effects in various salt solutions such as CaCl2, Na2SO4, and NH4NO3. In addition, the relevant charge efficiency and cycling longevity will be reported and discussed. Figure 1. Demonstration of both enhanced CDI (e-CDI) and extended voltage CDI (eV-CDI) effects using the modified Donnan model with the addition of chemical surface charge. The parameters used in the model can be found in ref. (5 and 6). Figure 2. Improved salt adsorption capacity and operational stability using cation- and anion-exchange polymers added to the carbon cathode and anode, respectively, in a CDI cell. The CDI cell was operated using 1 V charging and at 0 V discharging in ~31 L of ~7 mM deaerated NaCl solution.