Membrane Assisted Capacitive Deionization with Binder-Free Carbon Xerogel Electrodes
Authors: Omosebi Ayokunle 
Published: February 2020

Ayokunle Omosebi, Xin Gao, James Landon, and Kunlei Liu

Center for Applied Energy Research

University of Kentucky, Lexington, KY, 40511, USA

Growing concerns regarding the global production and sustainability of affordable clean water requires prompt attention. Presently, reverse osmosis (RO) is extensively employed for water purification, but significant energy consumption by the separation process is becoming an obstacle [1]. Capacitive de-ionization (CDI) is an alternative technique for clean water production from brackish sources whereby counter-ions from the bulk solution are transported and stored in porous electrodes under the influence of an applied electric field. Upon relaxation of the applied potential, the energy used for ion adsorption can be recovered. However, because the electric field attracts counter-ions in solution as well as repels co-ions, this results in reduced adsorption efficiency [2,3]. Membrane capacitive deionization (MCDI) is an improvement to the conventional CDI process by affixing ion-selective membranes to the electrodes to prevent the repulsion of co-ions during an adsorption event for a more efficient process. While several variations of highly porous carbon materials from activated carbon to carbon aerogels have been demonstrated for MCDI applications, unlike these materials that require auxiliary operations like supercritical drying, plasma etching, cryogenic drying, or the incorporation of binder materials, mesoporous carbon xerogel (CX) electrodes (Figure 1 top) may possess the attributes necessary for large scale production of carbon electrodes for CDI applications. This work examines the use of a binder-free CX infiltrated carbon cloth electrode coupled with anionic and cationic membranes for improved desalination.

Deionization experiments were conducted by continuously recirculating 400 ml of a 580 uS/cm (~ 5mM) NaCl stream through the CDI cell at a rate of ~32 ml/min. The test system was equipped with an inline conductivity probe for continuous monitoring of concentration. A total of 1.3 g of CX was used as electrode material, and each electrode was separated by a 5 mm Teflon spacer. Neosepta AEM and CEM were used as anionic and cationic membranes for the MCDI tests, respectively. Performance testing was done in a constant voltage mode by cycling between 1.2 V and 0 V (short circuit) every 45 minutes. As shown in Figure 1 (bottom), clearly the MCDI outperforms the conventional system. During the desalination tests, the conductivity drop observed for the MCDI system was ~2 times the drop for the CDI system. Future studies will demonstrate the effect of membrane incorporation on the potential of zero charge as this can significantly affect CDI performance.

Acknowledgements

The authors are thankful for the support of the State of Wyoming Advanced Conversion Technologies Task Force for supporting this research.

References

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3. J. Landon, X. Gao, B. Kulengowski, J. K. Neathery, and K. Liu, J. Electrochem. Soc. 159 (11), A1861 (2012).