Long-Term Evaluation of Modified Activated Carbon Electrodes for Capacitive Deionization
Authors: Serrano Mora Adrian , Wilkinson David P. , Mohseni Madjid 
Published: February 2020

Capacitive deionization (CDI) is an emerging electrochemical technology for the desalination of brackish waters (1000 – 15 000 mg L-1 TDS). It offers higher water recoveries and it is less energy intensive than reverse osmosis and thermal distillation, the dominant desalination technologies. CDI relies on the electroadsorption of ions on the electrical double layers formed at the electrode surfaces when a potential is applied. As a consequence, a diluted, purified stream of water is produced. Once the surface is saturated, the potential is removed or reversed, and ions are released back into the solution. This process is performed cyclically, and ideally, it could be carried out indefinitely. However, there are secondary reactions that prevent this from being a reality, and the desalination performance will decay over time. In particular, carbon oxidation modifies the electrode surface over time, thus negatively affecting the electrochemical properties such as resistivity, potential of zero charge (PZC), and surface area [1]–[4]. In addition, the oxygen reduction reaction (ORR) consumes charge that otherwise would have been used to adsorb ions.

Despite recent significant advances in materials and cell configurations for CDI, the long-term stability of most materials developed has not been studied. As a consequence, there are a number of materials and design approaches reported in literature with enhanced initial salt adsorption capacities but lack any long-term evaluation of the CDI performance.

In this research, the utilization of activated carbon based electrodes for long-term capacitive deionization is explored. Specifically, the effect of electrode additives on the improvement of the cycling stability is investigated. The function of these additional materials is to promote oxygen reduction in order to reduce the availability of oxygen that can be incorporated (i.e., by oxidation) onto the electrode surface, and therefore increase the long-term performance [5]–[7]. It is presumed that, if these secondary reactions can be controlled, the process of oxidation will be delayed to some extent, therefore extending the life of the electrodes and offering acceptable desalination for a longer period of time.

Non-precious additives which are able to promote the ORR are incorporated into a baseline electrode at different loadings, and their electrochemical properties including their ORR potential are monitored during prolonged cycling in a representative TDS solutions, i.e., as NaCl. Moreover, the disruption of the porous network by these additives is assessed by scanning electron microscopy (SEM) and other characterization techniques. Results from this study will provide insight into the feasibility of using activated carbons for long-term capacitive deionization. Furthermore, the inclusion of additives could offer a relatively simple and cheap solution to the challenge of carbon oxidation, and result in a more robust and efficient desalination technology.

References

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[6] A. G. El-Deen, N. A. M. Barakat, K. A. Khalil, M. Motlak, and H. Yong Kim, “Graphene/SnO2 nanocomposite as an effective electrode material for saline water desalination using capacitive deionization,” Ceram. Int., vol. 40, no. 9 PART B, pp. 14627–14634, Nov. 2014.

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