Letter to the Editor pubs.acs.org/journal/ascecg
Comment on “Ultrahigh Desalinization Performance of Asymmetric Flow-Electrode Capacitive Deionization Device with an Improved Operation Voltage of 1.8 V” Steven Hand and Roland D. Cusick*
Downloaded via 80.82.77.83 on June 26, 2018 at 21:32:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
Department of Civil & Environmental Engineering University of Illinois at Urbana−Champaign, 3217 Newmark Civil Engineering Laboratory, 205 North Mathews Avenue, Urbana, Illinois 61801-2352, United States
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.7b00463 n Xu et al.,1 an asymmetric flow-electrode capacitive deionization system is evaluated. The novelty of this system lies in the use of a faradaic cathode slurry composed of MnO2coated activated carbon (AC). This modification produced via electroless deposition has been explored previously for energy storage applications2 but not desalination. The use of a MnO2 cathode enabled expansion of the working potential from the typical 1.23,4 to 1.8 V. With this operating window, they state their system can remove 78% of the salt in a 100 mM NaCl recirculating batch experiment. A Kim−Yoon plot6 was used to evaluate the deionization rate (mg g−1 min−1) and adsorption capacity (mg g−1) of their system. However, the results present in the Kim−Yoon plot lead one to question the validity of this study. Xu et al. report a maximum electrosorptive capacity of greater than 1 g of NaCl per g of active material for the evaluated system. This is significant considering typical reported values for salt adsorption range from 1 to 15 mg per g of AC for CDI and MCDI.5 Assuming perfect operating conditions (no loss of working capacitance during charge and discharge operation), the maximum electrosorptive capacity, Γ, of a (pseudo)capacitor can be given by the following equation:
I
Γ=
C×U × Λ × εRT × MWNaCl F
Figure 1. Maximum salt adsorption as a function of specific capacitance assuming ideal round-trip energy efficiency. Each line represents operation at given charge efficiency.
electrodialysis, not adsorption. We respectfully request an explanation or correction to the published manuscript. We also request that the authors report observed charge efficiency and electrode solution conductivity to ensure the accuracy of the presented results. Over estimating adsorption capacity can lead your readership to misunderstand the capabilities of capacitive deionization systems.
■
(1)
*E-mail:
[email protected]. Phone: +1 (217) 244-6727.
where C is specific capacitance of the of the active material (F g−1), U is maximum whole cell potential (V), F is Faraday’s constant, Λ is the charge efficiency, εRT is the round-trip efficiency, and MWNaCl is the molecular weight of NaCl. In the Electrochemical Characterization section of Xu et al., the authors present specific capacitances of their AC and AC/ MnO2 flow electrode materials as 122.5 and 104.2 F g−1, respectively. If material is conservatively assumed to have a capacitance of 122.5 F g−1, despite the presence of an AC/ MnO2 cathode and the material balancing described in the Electrochemical Characterization section, and an operating potential of 1.8 V, then the maximum salt capacity as predicted by eq 1 at 100% charge and round-trip efficiencies is 134 mg g−1 (Figure 1). This value is nearly an order of magnitude less than the maximum electrosorptive capacity reported by Xu et al. (1 g g−1). It is possible that operating the cell at 1.8 V led to water electrolysis within the electrode channels. Electrolytic current would drive excess ionic flux across the ion-exchange membranes and into the electrode slurries. If this were the case, the vast majority of reported salt removal would be due to © 2017 American Chemical Society
AUTHOR INFORMATION
Corresponding Author ORCID
Steven Hand: 0000-0002-9002-5070 Notes
The authors declare no competing financial interest.
■
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, 189. (2) Raymundo-Piñero, E.; Khomenko, V.; Frackowiak, E.; Béguin, F. Performance of Manganese Oxide/CNTs Composites as Electrode Materials for Electrochemical Capacitors. J. Electrochem. Soc. 2005, 152 (1), A229−A235. (3) Jeon, S.; Park, H.; Yeo, J.; Yang, S.; Cho, C. H.; Han, M. H.; Kim, D. K. Desalination via a new membrane capacitive deionization
Received: December 5, 2016 Published: February 17, 2017 2035
DOI: 10.1021/acssuschemeng.6b02951 ACS Sustainable Chem. Eng. 2017, 5, 2035−2036
ACS Sustainable Chemistry & Engineering
Letter to the Editor
process utilizing flow-electrodes. Energy Environ. Sci. 2013, 6 (5), 1471−1475. (4) Porada, S.; Weingarth, D.; Hamelers, H. V. M.; Bryjak, M.; Presser, V.; Biesheuvel, P. M. Carbon flow electrodes for continuous operation of capacitive deionization and capacitive mixing energy generation. J. Mater. Chem. A 2014, 2 (24), 9313−9321. (5) Porada, S.; Zhao, R.; van der Wal, A.; Presser, V.; Biesheuvel, P. M. Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58 (8), 1388−1442. (6) Kim, T.; Yoon, J. CDI Ragone plot as a functional tool to evaluate desalination performance in capacitive deionization. RSC Adv. 2015, 5 (2), 1456−1461.
2036
DOI: 10.1021/acssuschemeng.6b02951 ACS Sustainable Chem. Eng. 2017, 5, 2035−2036
