Understanding Capacitive Deionization Performance By Comparing Its Electrical Response with an Electrochemical Supercapacitor

Capacitive deionization (CDI) is an electrochemical water treatment process that allows one to purify saline water by removing ionic species while storing energy simultaneously. The key to CDI is the adsorption of charged particles (ions) in the electrical double layer (EDL) of an electrode upon polarization by a direct current (DC) power source, the same as charging an electrochemical supercapacitor [1][2]. However, there are some differentiating aspects between CDI and supercapacitors that must be considered before this technology can be commercially competitive [3]. 

When a supercapacitor is charged, energy used is influenced by different parameters such as the current intensity, the electrode capacitance, and also by the current intensity of the previous step (discharge). If the capacitor operates under ideal conditions (i.e, high conductor electrolyte, low resistance connections), the influence of these parameters is practically negligible. However, if the system works under non-ideal conditions (greater electrolyte resistance) as in a CDI system, the influence of these parameters in subsequent stages may be relevant when the current intensity during charging is different than that used in discharge. 

Thus, a better understanding of the operational problems arising from the use of Supercapacitors is significantly important to aid their application as CDI in real-world devices including desalination, municipal drinking water, wastewater treatment and water-reuse projects [4]. 

In this work, a 310 Farads – 2.5 Volts supercapacitor cell and a CDI stack (70 Farads – 1.5 Volts) are tested under new operational cycling modes based on successively apply discharge rates with the same charge intensity, and similarly different charging rates over constant discharge intensity [5]. This approach allow to analyses comparatively the impact of applying charge and discharge at different rates on electrical performance (capacitance, resistance and round-trip energy efficiencies) of both technologies; supercapacitors and CDI. 

References 

[1] P.M. Biesheuvel, J. Colloid Interface Sci. 2009,332, 258-264. 

[2] P.M. Biesheuvel, B. van Limpt, A. van der Wal, J. Phys. Chem. C 2009, 113, 5636-5640. 

[3] M.A. Anderson, A.L. Cudero, J. Palma, Electrochimica Acta 2010, 55, 3845-3856. 

[4] E. García-Quismondo, C. Santos, Jesús Palma, M. A. Anderson, Desalination and Water Treatment 2014, DOI: 10.1080/19443994.2014.984929. Accepted

[5] E. García-Quismondo, R. Gómez, F. Vaquero, A. López Cudero, J. Palma, M.A. Anderson,, Phys. Chem. Chem. Phys. 2013, 15 (20), 7648-7656.