Improving Electrosorption Performance in Membrane Assisted Capacitive Deionization Cells Using Asymmetric Electrodes Configuration

Capacitive deionization (CDI) is an emerging salt treatment technology that functions by concentrating ionized salt into porous electrodes with the aid of an applied electric field. Unlike some de-salting technologies like distillation or reverse osmosis that separate water from salt content with the aid of added heat or pressure, CDI electrostatically targets the salt content instead, and it is particularly attractive for the treatment of mid (brackish water) to low (tap water) concentration salt streams [1]. In addition, CDI offers the possibility of energy recovery in the convenient form of electricity which can be used to directly power subsequent CDI units, or other auxiliary devices. Membrane-assisted capacitive deionization (MCDI) is an improvement to conventional CDI whereby ion-exchange membranes (IEMs) are placed next to the porous carbon electrodes to increase desalination performance by mitigating losses that result from converting electronic to ionic charge at the electrode-electrolyte double layer.

During MCDI operation, electrode polarization results in the attraction of counter- ions (opposite in polarity to the charged surface), while co-ions (similar in polarity to the charged surface) are repelled away from the polarized surface [2]. The membranes used for the operation hinder the repulsion of co-ions back into the bulk, leading to an increased flux of counter-ions to balance the co-ions contained in the macropore space resulting in typically higher charge efficiencies over the CDI only system. In a departure from the traditional architecture which utilizes the same electrodes to form both the anode and cathode, MCDI systems can instead be asymmetrically assembled with ion-specific electrodes to increase specific charge excesses, and whereby any such specificity can be quantified via potential of zero charge (PZC) measurements.

In order to demonstrate benefits to deionization performance, we will present the electrosorption performance and charging characteristics of MCDI cells configured with pristine and nitric acid-treated Zorflex activated carbon electrodes with PZCs of -0.2 and 0.2 V vs SCE, respectively, based on differential capacitance measurements. Results from the asymmetric configuration will be compared to those from pristine anode-pristine cathode configurations to quantify the extent of variation in performance.

Acknowledgements

The authors are grateful to the U.S. - China Clean Energy Research Center, U.S. Department of Energy for project funding (No. DE-PI0000017), and thankful for the support of the State of Wyoming Advanced Conversion Technologies Task Force for their support.

References

[1]        Y. Oren, Desalination 228 (2008) 10.

[2]        E. Avraham, Y. Bouhadana, A. Soffer, D. Aurbach, J. Electrochem. Soc. 156 (2009) P95.