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Increasing the energy efficiency of Capacitive Deionization Systems

Supercapacitors (SC), also known as electrochemical double layer capacitors (EDLC), is a mature energy storage technology that achieves high energy efficiencies. Those values are usually in the range of 95% when comparing the energy delivered in the discharge phase with the energy stored in the charging step. At the beginning of XXI century, Capacitive DeIonization (CDI), an emerging technology based on the same principles, although being used in this case for water treatment, attracted the attention of the scientific community. In this technology the ions are removed from the solution and stored in the electrochemical double layer formed on the electrode surface. In a subsequent step the ions can be released forming a brine. In this way water can be treated while storing energy.  However, one of the main differences is that the energy efficiency of the CDI system is relatively low compared to SC. This mainly due to in commercial EDLCs, an ideal organic/aqueous electrolyte can be chosen and designed to minimize resistance losses. In contrast, CDI systems employ only aqueous ionic solutions (i.e. mainly NaCl solutions have been studied in literature) with different salt concentrations depending on the application.

In the work published in Electrochimica Acta*, the electrical response of an ideal EDLC’s and a CDI system based on a parallel plate electrochemical flow reactor were compared. different CDI operational modes were studied with the aim of improving the energy efficiency. The experimental findings suggest that by simply replacing the solution used during deionization by a more concentrated solution (brine water) during regeneration, round-trip efficiency of CDI can be increased from ≈40% to 70%, approaching those of EDLC’s.

The research also shows that, under this operational mode, a wider range of deionization/regeneration current density ratios can be used while still maintaining high efficiency values. This implies that CDI might operate with a higher degree of flexibility by modifying the power applied depending on the feed composition.

IMDEA Energy Institute is a leader in Spain in developing energy efficient water treatment processes. Research lines are developed from the fundamental perspective at laboratory level to the industrial point of view including the design and fabrication of prototypes for advanced applications

(*) Santos, C., García-Quismondo, E., Palma, J., Anderson, M.A., Lado, J.J.
Understanding capacitive deionization performance by comparing its electrical response with an electrochemical supercapacitor: Strategies to boost round-trip efficiency
(2020) Electrochimica Acta, 330, art. no. 135216. https://doi.org/10.1016/j.electacta.2019.135216 

More Information: Julio J. Lado, Postdoctoral Researcher, Electrochemical Processes Unit Julio.lado@imdea.org

Event Date: 
Monday, July 13, 2020