Letter to the Editor pubs.acs.org/journal/ascecg
Response to Comment on “Ultrahigh Desalinization Performance of Asymmetric Flow-Electrode Capacitive Deionization Device with an Improved Operation Voltage of 1.8 V” Xingtao Xu, Miao Wang, Yong Liu, Ting Lu, and Likun Pan* Shanghai Key Laboratory of Magnetic Resonance, School of Physics and Materials Science, East China Normal University, 3663 N. Zhongshan Rd., Shanghai 200062, China
ACS Sustainable Chem. Eng. 2017, 5 (1), 189−195. DOI: 10.1021/acssuschemeng.6b01212 ACS Sustainable Chem. Eng. 2017, 5 (3). DOI: 10.1021/acssuschemeng.6b02951
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FCDI device not only contains the electrosorption process of active materials but also includes the electrodialytical desalinization mechanism.) As reported by Gendel et al.,2 H+ (anode) and OH− (cathode) can be generated during the FCDI process, thus underlining an additional electrodialytical (EDI) desalinization mechanism within capacitive deionization, which proceeds in parallel to the known electrosorption mechanism. Therefore, there is no doubt that electrodialytical desalinization also plays an important role in our AFCDI device. In addition, in order to have a clear understanding of the desalinization mechanism of FCDI, we further tested the desalinization performance of EDI (FCDI device without electrodes) at 1.2 and 1.8 V (Figure 1a). These results have also been compared with those of FCDI and AFCDI (Figure 1b). It is shown that when operating voltage is 1.2 V, the EDI process shows a desalinization efficiency of 0.38, which is 64% of that of the FCDI process (0.59), indicating the important role of EDI desalinization in the FCDI process. Moreover, when the voltage reaches 1.8 V, the EDI desalinization efficiency reaches a high value of 0.92. Interestingly, this value is higher than that of AFCDI (0.78). This is due to the following reasons: In an EDI process where the operating voltage is above 1.2 V,
irst of all, we are very thankful to Hand and Cusick for the interest in our recent work on the asymmetric flowelectrode capacitive deionization (AFCDI) device1 and their pertinent comments. Hereafter, we refer to Hand and Cusick as “authors”. The authors claim that the electrosorption capacity values shown in the Kim−Yoon plot (Figure 7, original article1) are inconsistent with the results obtained from capacitance. The high operation voltage of 1.8 V in the AFCDI device can cause water electrolysis within the electrode channels, and thus, the high electrolytic current can drive excess ionic flux across the ion-exchange membranes and into the electrode slurries. As a result, the high desalinization performance of the AFCDI device is mainly due to the water electrolysis. We appreciate their comments. In the original article,1 we planned to use the Kim−Yoon plot to analyze and compare the deionization performances of the FCDI and AFCDI devices. (The electrosorption capacity values of the FCDI and asymmetric FCDI devices were calculated by eq 5 in the original article.) In fact, using the Kim−Yoon plot method to analyze the deionization performances of FCDI should be incorrect because Faradaic reactions of water electrolysis cannot be avoided in FCDI devices. (The deionization process of the
Figure 1. (a) Normalized concentration profiles and (b) salt removal efficiencies for FCDI, AFCDI, and EDI (1.2 and 1.8 V) devices. Received: February 13, 2017 Published: February 17, 2017 © 2017 American Chemical Society
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DOI: 10.1021/acssuschemeng.7b00463 ACS Sustainable Chem. Eng. 2017, 5, 2037−2038
ACS Sustainable Chemistry & Engineering
Letter to the Editor
water electrolysis becomes violent, and the high electrolytic current can drive excess ionic flux across the ion-exchange membranes, thus enhancing the desalinization efficiency. However, more gas of O2 or H2 will be generated in this process. Also, in our AFCDI device, the operating voltage of 1.8 V will not generate gas of O2 or H2 because in the AFCDI device, if the MnO2-activated carbon electrode is positively polarized, the generated potential values will be larger than the thermodynamic evolution of O2 due to the faradaic reactions at the electrode/electrolyte interface (Mn (IV) to Mn (III)). On the other hand, when activated carbon is negatively polarized in an aqueous electrolyte, high overpotential values for H2 evolution can be achieved because hydrogen produced is adsorbed in the micropores of the carbon electrode. These reasons have been proved by other groups.3,4 In fact, during our experiments, no obvious phenomenon of water decomposition has been observed. Although the desalinization performance is a little lower than that of the pure EDI process, our AFCDI process can provide a much safer route to obtain high desalinization performance without water decomposition, indicating the potential of AFCDI devices for practical desalinization applications. Considering all of the above reasons, we make the following correction to our original paper: All of the section titled Kim− Yoon Plot Analysis on page 193 of the original article should be considered inaccurate, including eqs 5 and 6 and Figure 7.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +86 21 62234132. Fax: +86 21 62234321. E-mail:
[email protected] (Likun Pan). ORCID
Likun Pan: 0000-0001-9294-1972 Notes
The authors declare no competing financial interest.
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REFERENCES
(1) Xu, X.; Wang, M.; Liu, Y.; Lu, T.; Pan, L. Ultrahigh desalinization performance of asymmetric flow-electrode capacitive deionization device with an improved operation voltage of 1.8 V. ACS Sustainable Chem. Eng. 2017, 5 (1), 189−195. (2) Nativ, P.; Badash, Y.; Gendel, Y. New insights into the mechanism of flow-electrode capacitive deionization. Electrochem. Commun. 2017, 76 (3), 24−28. (3) Hatzell, K. B.; Fan, L.; Beidaghi, M.; Boota, M.; Pomerantseva, E.; Kumbur, E. C.; Gogotsi, Y. Composite manganese oxide percolating networks as a suspension electrode for an asymmetric flow capacitor. ACS Appl. Mater. Interfaces 2014, 6 (11), 8886−8893. (4) Khomenko, V.; Raymundo-Pinero, E.; Béguin, F. Optimisation of an asymmetric manganese oxide/activated carbon capacitor working at 2V in aqueous medium. J. Power Sources 2006, 153 (1), 183−190.
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DOI: 10.1021/acssuschemeng.7b00463 ACS Sustainable Chem. Eng. 2017, 5, 2037−2038
