(807g) Development of An Electrochemical Deionization System Based On Pseudo-Capacitors

Authors: 
Achilleos, D. S., Massachusetts Institute of Technology
Mao, X., Massachusetts Institute of Technology
Hatton, T. A., Massachusetts Institute of Technology



One of the most pervasive problems in deionization systems is their high energy consumption which is often accompanied with low ion removal efficiency.[1] The development of Pseudo-capacitive deionization technology, an electrochemical separation process which combines the low energy consumption of Capacitive Deionization Technology (CDI)[2] and the high capacitance of pseudo-capacitors, is of great importance. This new technology may constitute a future milestone on the road towards a robust, low-cost and environmentally friendly deionization method. The development of proper electrodes is crucial for the establishment of an efficient electrochemical separation process. Electrodes that exhibit increased capacitance and thus store a high amount of charge show increased deionization capacitance. Enrichment of the traditional electrical double-layer capacitors (EDLCs)[3] with electro-active species which exhibit fast and reversible redox reactions, enables increased capacitance and ion removal efficiency.

In order to address these problems, we are exploring the pseudo-capacitive deionization of solutions, based on an asymmetric two-electrode configuration which can separate ions from polar organic phases. The electrodes, which consist of novel carbon-based pseudo-capacitors, carry electro-responsive poly(vinyl ferrocene) (PVF) and poly(anthraquinone) (PAQ) chains. The high loading of the electrodes with electro-active species and thus their improved separation efficiency is achieved due to the surface-modification of the electrodes with the electro-active polymers. Upon applying potential to the system, the separation process is activated in a controlled and remote manner; the electrodes possess a substantial amount of opposite charge and thus interact electrostatically and withdraw ions from the organic phase. In particular, upon electrical stimulation, the ferrocene units of the PVF-based anode undergo one electron oxidation to form the ferrocenium cations. These cationic species interact electrostatically with anions and remove them in a selective manner as shown by UV/vis spectroscopy. The selectivity of the electrode depends on the geometry of the anions and is not disturbed by the organic counter-ions in solution. More specifically, the electrode selectivity is higher towards anions with trigonal planar geometry and lower for spherical anions as proved by square wave voltammetry (SWV). On the other hand, the AQ moieties of the cathode upon electrical stimulation form the AQ radical anion and the AQ dianion. Thus the negatively charged AQ species interact electrostatically and remove cations from the organic phase as shown by X-ray photoelectron spectroscopy (XPS). The AQ-cation interactions are proved by SWV to be time-dependent and are affected by the valence and the radius of the cation. We envisage that our deionization device can be useful in pharmaceutical industrial processes, especially when the removal of low concentration of ions is required to obtain highly pure products.

References

[1]           a) R. F. Service, Science 2006, 313, 1088; b) R. Semiat, Environmental Science and Technology 2008, 42, 8193.

[2]           a) T. J. Welgemoed, C. F. Schutte, Desalination 2005, 183, 327; b) Y. Oren, Desalination 2008, 228, 10.

[3]           a) Y. Zhai, Y. Dou, D. Zhao, P. F. Fulvio, R. T. Mayes, S. Dai, Advanced Materials 2011, 23, 4828; b) P. Simon, Y. Gogotsi, Nat Mater 2008, 7, 845.