Enhanced Salt Removal in an Inverted Capacitive ... - ACS Publications

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Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes Xin Gao,† Ayokunle Omosebi,† James Landon,*,† and Kunlei Liu*,†,‡ †

Center for Applied Energy Research, University of Kentucky, Lexington, Kentucky 40511, United States Department of Mechanical Engineering, University of Kentucky, Lexington, Kentucky 40506, United States

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Downloaded by RUTGERS UNIV on August 29, 2015 | http://pubs.acs.org Publication Date (Web): August 27, 2015 | doi: 10.1021/acs.est.5b02320

S Supporting Information *

ABSTRACT: Microporous SpectraCarb carbon cloth was treated using nitric acid to enhance negative surface charges of COO− in a neutral solution. This acid-treated carbon was further modified by ethylenediamine to attach −NH2 surface functional groups, resulting in positive surface charges of −NH3+ via pronation in a neutral solution. Through multiple characterizations, in comparison to pristine SpectraCarb carbon, amine-treated SpectraCarb carbon displays a decreased potential of zero charge but an increased point of zero charge, which is opposed to the effect obtained for acid-treated SpectraCarb carbon. An inverted capacitive deionization cell was constructed using aminetreated cathodes and acid-treated anodes, where the cathode is the negatively polarized electrode and the anode is the positively polarized electrode. Constant-voltage switching operation using NaCl solution showed that the salt removal capacity was approximately 5.3 mg g−1 at a maximum working voltage of 1.1/0 V, which is an expansion in both the salt capacity and potential window from previous i-CDI results demonstrated for carbon xerogel materials. This improved performance is accounted for by the enlarged cathodic working voltage window through ethylenediamine-derived functional groups, and the enhanced microporosity of the SpectraCarb electrodes for salt adsorption. These results expand the use of i-CDI for efficient desalination applications.

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INTRODUCTION Even though two-thirds of our world’s surface is covered by water, only about 3% of the water can be directly used for agriculture, consumption, and industry.1 With growing population and industrialization, water scarcity and quality issues are becoming two of the most severe global challenges of our time.2,3 The United Nations predicts that 2−7 billon people will be threatened by a lack of access to clean water by the middle of the century.4 In addition, the U.S. Environmental Protection Agency will require more stringent regulations on the effluent streams from steam-generating power plants, which limits the arsenic, selenium, mercury, and total dissolved solids in these streams.5 As a consequence, efficient, cost-effective, and environmentally friendly methods for desalination need to be pursued for a sustainable future. Capacitive deionization (CDI) is an emerging and fast-growing technology for desalination, which is an alternative to reverse osmosis, electrodialysis, and distillation. In contrast to these membrane and thermal technologies, CDI is an electrochemical method to remove salts by applying an electrical potential across porous carbon electrodes primarily for the desalination of brackish water.6−8 In addition, this method offers the advantages of lowpressure operation, minimized operating and maintenance costs, and possibly higher energy efficiency, all of which can help reduce the environmental impact of this water treatment process.6 © XXXX American Chemical Society

Over the past few decades, CDI technology has rapidly progressed through various architectures that include conventional CDI, membrane-assisted CDI, hybrid CDI, and flowelectrodes CDI cells. 7,9−15 In these conventional CDI processes, salts are adsorbed when a voltage is applied to charge the cell and are desorbed when the applied voltage is reduced to discharge the cell. However, in recent literature, this voltage-induced adsorption has been shown to diminish with time for continuous charging and discharging of CDI cells formed with activated carbon cloth and carbon xerogel electrodes, where repulsion (or desorption) peaks were established when these cells were charged when concentration (or conductivity) as a function of time was plotted.16−18 This inverted behavior is due to carbon oxidation in aqueous solutions, which leads to increased negative surface charge on the anode.16,18 By leveraging the impact of surface charge properties for carbon electrodes on ion adsorption and desorption,19 inverted CDI (i-CDI) was recently proposed, developed, and demonstrated,20 which embarks upon new routes to improve salt removal performance through carbon functionalization.6 It has Received: May 9, 2015 Revised: August 18, 2015 Accepted: August 20, 2015

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DOI: 10.1021/acs.est.5b02320 Environ. Sci. Technol. XXXX, XXX, XXX−XXX

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Environmental Science & Technology

cathodes and acid-treated anodes. New insight is provided on the effect of applied voltages on i-CDI salt removal performance via knowledge of the location of the EPZC with respect to the potential distribution. The combination of proper surface chemistries will be shown to result in a highly efficient, stable, and environmentally friendly salt separation process.

been reported that i-CDI features excellent operation longevity but lacked substantial salt removal capacity (or salt adsorption capacity).20 Therefore, in this work, we expand the effective working voltage window and use microporous carbon electrodes, leading to improved salt removal capacity of an i-CDI cell to aid in the practical use of this new separation process for efficient desalination applications. The i-CDI working voltage window is regulated by the potential difference between the potential of zero charge (EPZC) of the cathode and anode.20 Therefore, to enlarge this potential difference and increase the salt adsorption capacity, the cathode needs a more negative EPZC whereas the anode requires a more positive EPZC. Earlier studies illustrated that when a carbon electrode was oxidized and/or coated with silica to possess negative surface charges such as −COO−, a positively shifted EPZC was observed for this carbon in a neutral salt solution.21−24 Following on from this understanding as well as the theory introduced by McCafferty,25 it is postulated that positive surface charges on a carbon electrode will lead to a negatively shifted EPZC, facilitating the further expansion of the working voltage window for an i-CDI cell. It has been demonstrated that carbon treated with amine solutions or gaseous ammonia results in the attachment of amine (−NH2) surface functional groups.26−37 The resulting −NH2 can be protonated to −NH3+ in a neutral solution such as NaCl,36−39 correspondingly creating positive surface charges on the carbon.7,36 This carbon with a positive charge enhanced surface can be used as a cathode material in a salt separation cell such as that shown in Figure 1. In addition, CDI, MCDI, and i-CDI

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MATERIALS AND METHODS

Carbon Preparations. Pristine SpectraCarb (SC) (2225 activated carbon fabric, Caplinq) was first washed with acetone to remove impurities. To enhance the negative and positive surface charges, pristine SC was treated using as-received nitric acid (70%, Sigma-Aldrich) and ethylenediamine (≥99%, SigmaAldrich) solutions. As shown in Figure 2, a pristine SC was immersed into a container filled with 400 mL of nitric acid at room temperature for ∼24 h. After immersion, to remove any residual acid, the SC was washed with a significant amount of deionized water followed by drying at 280 °C in air for ∼120 h. The resulting carbon at this stage is denoted as the N-SC, meaning the negative surface charge enhanced SC electrode. This electrode was used as the anode material for i-CDI cells in this work. Following on from the N-SC preparation, another glass container with 400 mL of N2 purged ethylenediamine was heated to around 120 °C in a silicone oil bath to treat the N-SC sample. This heating step was continued until all of the ethylenediamine solution evaporated. The carbon was further cleaned with deionized water, and then dried at 105 °C under N2 atmosphere in a convection oven. The resulting carbon is herein referred to as the P-SC and was used as the cathode material in the present i-CDI work because its surface charge was positively enhanced. The same procedure was adopted to treat the carbon xerogel (CX) materials, and their characterizations are depicted in Figure S3. In addition, it should be mentioned that a similar procedure has been reported to modify carbon nanotubes forming −SO3− and −NH3+ for conventional CDI applications.42 Carbon Characterizations. A scanning electron microscope (SEM) instrument (S-4800, Hitachi), a porosity analyzer (ASAP2020, Micrometrics), and a Fourier-transform infrared (FTIR) spectrometer (Nicolet 6700, Thermo Scientific) were used to characterize the surface morphologies, porosities, and surface species, respectively, for the SC, N-SC, and P-SC. N2 adsorption−desorption isotherms were recorded at 77 K using approximately 0.15 g of sample degassed at 160 °C for at least 4 h. FTIR samples were prepared with around 0.07 g of a mixture of carbon/KBr at a ratio of 0.3 wt % (SC vs KBr), and FTIR spectra were collected by coadding 256 scans at 4 cm−1 resolution. To estimate the point of zero charge (pHPZC), about 0.05 g of the sample (SC, N-SC, or P-SC) was placed in NaCl-based solutions at various pH values for ∼120 h. To adjust the pH values, X g of 4.3 mM NaCl solution was mixed with Y g of either 4.3 mM NaOH or HCl solution, where X + Y = 10 g. The pH values measured before and after sample immersion were plotted, and the pHPZC values were estimated from the plateaus in these plots.43 To estimate the potential of zero charge (EPZC), a three-electrode cell was constructed, consisting of approximately 0.36 cm2 of a sample working electrode, a saturated calomel reference electrode (SCE), and a counter electrode of about 3.6 cm2 of SC electrode. Cyclic voltammetry was performed at 0.5 mV s−1 in 4.3 mM deaerated NaCl

Figure 1. Salt separation i-CDI cell for environmental remediation and desalination applications. This cell is composed of porous carbon electrodes with completely different surface charge properties. In this representation, salt is desorbed when a voltage is applied to charge the cell whereas salt is adsorbed when the applied voltage is reduced.

operation with positively shifted EPZC values due to anode oxidation has been sufficiently addressed in the literature,15,16,18,23,40,41 but no information exists for negative EPZC shifting in these applications. In this study, we present multiple characterizations of aminetreated SpectraCarb and carbon xerogel materials, accompanied by cycling tests of i-CDI cells configured with amine-treated B


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Figure 2. Schematic of carbon treatments using nitric acid and ethylenediamine solutions, resulting in the negative surface charge enhanced (N-SC) and postive surface charge enhanced (P-SC) carbon electrodes for i-CDI applications.

Figure 3. (a) Working principle of an i-CDI cell based upon a cycling mode between applied voltage and short-circuit. In i-CDI, salts are desorbed when a voltage is applied to charge the cell while salts are adsorbed when the applied voltage is reduced to discharge the cell. In the sketch, E+ and E− represent the potentials at the anode and cathode, respectively. For this case, when the cell is shorted during discharging, Eo is equivalent to E+ and E−, i.e., Eo = E+ = E−. (b) Use of the Donnan model to demonstrate the ionic concentration in micropores as a function of potential at an electrode.44

solution. The “V-shape” in the resulting voltammogram aided in assessing the EPZC.19,20 Cycling Tests and Performance Evaluation. To characterize the salt removal and current response, a flow system was constructed, which is composed of an i-CDI cell, an in-line conductivity sensor (19500-45, Cole-Parmer), a peristaltic pump (Masterflex L/S, Cole-Parmer), and a polyethylene tank. The i-CDI cell was assembled with 16 pairs of electrodes separated by 0.15 cm thick silicon rubber spacers. Experimental automation and timing were facilitated with a computer-controlled relay box (Denkovi Assembly Electronics Ltd.). Before collecting conductivity and current data, 31 L of 4.3 mM deaerated NaCl solution was circulated at 20 mL min−1 for about 12 h at a short-circuit voltage. (The NaCl solution was prepared using approximately 7.9 g of NaCl salt dissolved into 31 L of deionized water.) Because of the large volume of salt solution used, the steady-state concen-

tration (cst) after the adsorption and desorption profiles is effectively equivalent to the concentration for the initial bulk solution. Therefore, the salt removal capacity (Γ) was calculated by the volumetric flow rate (Φ) multiplied by the integration of the concentration curve with time (t) taken for both the adsorption and desorption steps (eq 1). Γ = (M / m )Φ

∫ (c(t ) − cst)dt

(1) −1

where M is the molecular weight of NaCl (58.44 g mol ), and m is the total mass of the dried cathodes and anodes used in the present tests (12.5 g). Similarly, the charge passed density (Q) at both the charging and discharging steps was calculated by integration of the current density curves. Finally, the charge efficiency (Λ) is the ratio of Γ (converted to equivalent charge using Faraday’s constant (F)) to the charge passed density (eq 2). C


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Environmental Science & Technology Λ = (ΓF /M )/(Q )

(2)


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RESULTS AND DISCUSSION Summary of Material Characterizations. Physical and chemical characterizations of the pristine and treated carbon electrodes have been detailed in the Supporting Information (Figures S1−3). Through these characterizations, it is evident that compared to the pristine SC electrode, (1) the pore volume of the N-SC was mostly preserved but the pore volume of the P-SC was significantly reduced, (2) the pHPZC value of the N-SC decreased but the pHPZC value of the P-SC increased, and (3) the EPZC value of the N-SC was positively shifted but the EPZC value of the P-SC was negatively shifted. Furthermore, by surface species identification, it is confirmed that functional groups such as −COOH and −NH2 have been attached on the surface of the N-SC and P-SC, respectively. This means that the N-SC and P-SC surfaces will possess native negative surface charges (COO−) and positive surface charges (NH3+), respectively, in a neutral solution such as a dilute NaCl solution. Working Principle of i-CDI. In an i-CDI cell, salts are desorbed and adsorbed during charging and discharging, respectively, which is opposed to the functionality of a conventional CDI cell.20 This difference can be achieved by using a positive surface charge enhanced cathode (e.g., P-SC) and negative surface charge enhanced anode (e.g., N-SC) to construct an i-CDI cell. According to Figure 3a, we envision that in a cyclical charging and discharging test, when an i-CDI cell is charged for ion desorption after a previous discharging step (e.g., the cell was shorted) for ion adsorption, an applied potential (Eapplied) is distributed to the potential at the cathode (E−) and anode (E+) from the Eo voltage, resulting in electrostatic forces being built at the cathode (Eo − E−) and anode (E+ − Eo). These electrostatic forces lead to salts being desorbed in the i-CDI cell as the E+ and E− approach their respective EPZC values, least ion adsorption regions.19 When the cell is discharged again at the Eo voltage shown in Figure 3a, salts are adsorbed in the i-CDI cell even though electrostatic forces are reduced. This behavior is because when E+ and E− are far from their respective EPZC values, the electrodes substantially facilitate ion adsorption, which can be demonstrated in Figure 3b using the Donnan model for the electrochemical double layer for CDI applications.19,44−46 Therefore, it is expected that the maximum salt adsorbed in an i-CDI cell during discharging will occur after salt was fully desorbed using the entire working voltage window (E+,PZC − E−,PZC) during charging. In the following section, the surface charge enhanced SC electrodes (shown in Figures S1−2) will be applied to an i-CDI cell to demonstrate their salt removal capacity and charge efficiency. Cycling Performance. The functionality of the i-CDI cell is experimentally demonstrated in Figure 4, in which the i-CDI cell was charged at various voltages and discharged at the shortcircuit condition in 31 L of ∼4.3 mM deaerated NaCl solution at 20 mL min−1. As shown in the plots, salt is desorbed when the cell is charged, and adsorbed when the cell is discharged. By increasing the applied voltage, this reversible process becomes significant, resulting in enlarged areas under the concentration and current plots, which is very similar to the behavior observed in a conventional CDI cell.47 In the same plots, the i-CDI cell operated at 1.25/0 V displays a diminished net salt removal but a boosted current response. For instance, as indicated by the arrow, the

Figure 4. Selected cycles when an i-CDI cell was configured with 16 pieces of P-SC cathodes and 16 pieces of N-SC anodes. These tests were performed at different charging voltages for salt desorption and a short-circuit voltage for salt adsorption (X/0 V, where X = 0.15−1.25) in ∼31 L of ∼4.3 mM deaerated NaCl solution at 20 mL min−1. Each charging and discharging half-cycle took 4000 s. The full plots can be found in Figure S5.

desorption peaks indicate that a small amount of salt was desorbed instead of being adsorbed at the onset of discharging. Similarly, the adsorption troughs suggest that a small amount of salt was adsorbed rather than being desorbed during the initial moments of charging. It should be noted that the diminished net salt removal in adsorption−desorption behavior only occurs when higher voltages are applied to the i-CDI cell. This performance is accounted for by the i-CDI working voltage regulated by the difference between E+,PZC and E−,PZC. As demonstrated in Figure 3a, if E+ and E− are greater than the respective E+,PZC and E−,PZC, the additional electrostatic forces created by (E+ − E+,PZC) and (E− − E−,PZC) will be used for undesirable ion adsorption−desorption during i-CDI operation. Under such conditions, a certain amount of cations and anions is adsorbed during charging but desorbed during short-circuit, which is exactly reflected in the i-CDI case operated at 1.25/0 V. In addition, because the current (or charge passed) is still enhanced at 1.25/0 V in Figure 4, we understand that this diminished net salt removal will lead to the i-CDI cell being less efficient. Salt Removal Capacity and Charge Efficiency. Following on from the cycling tests shown in Figures 4, S4, and S5, salt adsorption−desorption cycles were evaluated in terms of salt removal capacity, charge passed, and charge efficiency to characterize the i-CDI cell configured with N-SC (-CX) anodes and P-SC (-CX) cathodes. As depicted in Figure 5a,b,d,e, an increase in the voltage applied gradually improves both the salt removal capacity and charge passed. This improved performance can be attributed to enlarged electrostatic forces produced at the cathodes and anodes. It is also found that the salt removal capacities during charging and discharging are equivalent. However, this similarity is not observed for the charge passed. The greater charged passed during charging suggests that electrical energy is consumed by not only salt D


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cathodes and N-CX anodes (solid triangles). In fact, performance shown in Figure 5 can be fully predicted and modeled using the Donnan model shown in Figure 3b, where the minimum in adsorption defines salt removal performance.44 Additionally, we postulate that microporosity in a carbon electrode may aid in the salt removal capacity,49 but may limit the rate of salt transport, causing the cell to become somewhat less efficient. Efficiency of these separation systems depends on a multitude of factors including charging time, concentration of solution, and level of salt removed, among others, but salt transport and its contribution to the overall charge efficiency should also be considered when evaluating the resulting system. Performance Improvement. In this study, the improved salt removal capacity was mainly attributed to the use of new treatment routes to expand further the i-CDI working voltage window and the use of microporous electrodes to enhance the available adsorption sites. As shown in Figure 5a,d, a prior i-CDI cell configured with pristine and negative surface charge functionalized mesoporous CX electrodes offered a salt removal capacity of ∼1.7 mg g−1 at its maximum working voltage of ∼0.8/0 V (open triangles).20 In comparison to the current CX results (solid triangles) in the same plots, the use of the ethylenediamine treatment can effectively expand the working voltage window up to ∼1.03 V for the i-CDI cell. This enhancement is reflected by an increase in the salt removal capacities until the applied voltage is greater than 1.1 V. Thus, it is understood that the higher salt removal capacity achieved in this work can be attributed to the working voltage window expanded by further shifting of the cathode EPZC. In addition, according to the same plots, very similar conclusions can be drawn when comparing the salt removal capacities for the i-CDI cell configured with the N-SC anodes paired with the P-SC (solid squares) and pristine SC (open squares) cathodes. Finally, because it has been reported that the salt removal capacity mainly depends on the microporosity of a carbon electrode,49 we further used microporous SC electrodes to aid in boosting the salt removal capacities. Therefore, in this study, we report that the current i-CDI cell with the modified SC electrodes offers a salt removal capacity of ∼5.3 mg g−1 with an effective working voltage of ∼1.1/0 V (solid squares). This advancement for i-CDI systems has demonstrated that through proper surface chemistries and knowledge of the underlying electrochemical properties, a highly efficient system can be created for use in various desalination processes in both remote and industrial applications.

Figure 5. Performance evaluations of (a−c) charging step for salt desorption and (d−f) short-circuit for salt adsorption for i-CDI cells configured with surface charge enhanced SC and CX electrodes according to Figures 4, S4, and S5 (solid symbols). In the same plots, i-CDI data using 4.3 mM deaerated NaCl from ref 20 and Figure S7 using oxidized CX and SC anodes paired with pristine CX* and SC* cathodes, respectively, are also overlaid for comparison (open symbols) in order to specify the effect of the cathode’s EPZC shifting on salt removal performance for an i-CDI cell.

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transport but also parasitic reactions such as carbon oxidation and/or water splitting. Therefore, as shown in Figure 5c,f, the charge efficiency during charging is always lower than discharging. Interestingly, Figure 5c,f depicts that an increase in the applied voltage leads to decreased charge efficiency for the iCDI cell, as opposed to a conventional CDI cell tested under similar experimental conditions.47,48 As shown in previous CDI literature, the charge efficiency of a CDI cell will be improved if the distributed electrode potentials (E+ and E−) are placed far beyond their respective EPZCs.23,40 Following on from this conclusion and incorporating with the working principle in Figure 3a, in an i-CDI cell, smaller applied voltages always lead to the E+ and E− being far from their respective EPZC values, leading to higher charge efficiencies. This conclusion is confirmed by another i-CDI cell configured with P-CX

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.5b02320. Characterizations of acid- and amine-treated SC and CX electrodes (Figures S1−3 and Table S1), cycling tests and performance evaluation of an i-CDI cell using the treated CX electrodes (Figure S4), cycling tests and performance evaluation of an i-CDI cell using the treated SC electrodes (Figure S5), performance stability of an iCDI cell using the treated SC electrodes (Figure S6), cycling tests of an i-CDI cell with the pristine SC cathodes and acid-treated anodes (Figure S7), and effect of the EPZC locations on salt removal performance (Figures S8−9) (PDF). E


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desalination: The challenge of positive electrodes corrosion. Electrochim. Acta 2013, 106, 91−100. (17) Bouhadana, Y.; Avraham, E.; Noked, M.; Ben-Tzion, M.; Soffer, A.; Aurbach, D. Capacitive deionization of NaCl solutions at nonsteady-state conditions: inversion functionality of the carbon electrodes. J. Phys. Chem. C 2011, 115 (33), 16567−16573. (18) Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Dependence of the Capacitive Deionization Performance on Potential of Zero Charge Shifting of Carbon Xerogel Electrodes during Long-Term Operation. J. Electrochem. Soc. 2014, 161 (12), E159−E166. (19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. (20) Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior. Energy Environ. Sci. 2015, 8 (3), 897−909. (21) Bayram, E.; Ayranci, E. A systematic study on the changes in properties of an activated carbon cloth upon polarization. Electrochim. Acta 2011, 56 (5), 2184−2189. (22) Tobias, H.; Soffer, A. The immersion potential of high surface electrodes. J. Electroanal. Chem. Interfacial Electrochem. 1983, 148 (2), 221−232. (23) Avraham, E.; Noked, M.; Cohen, I.; Soffer, A.; Aurbach, D. The Dependence of the Desalination Performance in Capacitive Deionization Processes on the Electrodes PZC. J. Electrochem. Soc. 2011, 158 (12), P168−P173. (24) Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Enhancement of charge efficiency for a capacitive deionization cell using carbon xerogel with modified potential of zero charge. Electrochem. Commun. 2014, 39 (0), 22−25. (25) McCafferty, E. Relationship between the isoelectric point (pHpzc) and the potential of zero charge (Epzc) for passive metals. Electrochim. Acta 2010, 55 (5), 1630−1637. (26) Ramanathan, T.; Fisher, F. T.; Ruoff, R. S.; Brinson, L. C. Amino-Functionalized Carbon Nanotubes for Binding to Polymers and Biological Systems. Chem. Mater. 2005, 17 (6), 1290−1295. (27) Zhu, N.; Chen, X.; Zhang, T.; Wu, P.; Li, P.; Wu, J. Improved performance of membrane free single-chamber air-cathode microbial fuel cells with nitric acid and ethylenediamine surface modified activated carbon fiber felt anodes. Bioresour. Technol. 2011, 102 (1), 422−426. (28) Shen, J.; Huang, W.; Wu, L.; Hu, Y.; Ye, M. Study on aminofunctionalized multiwalled carbon nanotubes. Mater. Sci. Eng., A 2007, 464 (1−2), 151−156. (29) Hsieh, C.-T.; Teng, H.; Chen, W.-Y.; Cheng, Y.-S. Synthesis, characterization, and electrochemical capacitance of amino-functionalized carbon nanotube/carbon paper electrodes. Carbon 2010, 48 (15), 4219−4229. (30) Chen, X.; Wang, J.; Lin, M.; Zhong, W.; Feng, T.; Chen, X.; Chen, J.; Xue, F. Mechanical and thermal properties of epoxy nanocomposites reinforced with amino-functionalized multi-walled carbon nanotubes. Mater. Sci. Eng., A 2008, 492 (1−2), 236−242. (31) Murugesan, S.; Myers, K.; Subramanian, V. Amino-functionalized and acid treated multi-walled carbon nanotubes as supports for electrochemical oxidation of formic acid. Appl. Catal., B 2011, 103 (3− 4), 266−274. (32) Vuković, G.; Marinković, A.; Obradović, M.; Radmilović, V.; Č olić, M.; Aleksić, R.; Uskoković, P. S. Synthesis, characterization and cytotoxicity of surface amino-functionalized water-dispersible multiwalled carbon nanotubes. Appl. Surf. Sci. 2009, 255 (18), 8067−8075. (33) Vuković, G. D.; Marinković, A. D.; Č olić, M.; Ristić, M. Đ.; Aleksić, R.; Perić-Grujić, A. A.; Uskoković, P. S. Removal of cadmium from aqueous solutions by oxidized and ethylenediamine-functionalized multi-walled carbon nanotubes. Chem. Eng. J. 2010, 157 (1), 238−248. ́ tkowski, A. The (34) Biniak, S.; Szymański, G.; Siedlewski, J.; Swia̧ characterization of activated carbons with oxygen and nitrogen surface groups. Carbon 1997, 35 (12), 1799−1810.

AUTHOR INFORMATION

Corresponding Authors

*James Landon. E-mail: [email protected]. Fax: +1 859 257 0302. Tel: +1 859 257 0349. *Kunlei Liu. E-mail: [email protected]. Fax: +1 859 257 0302. Tel: +1 859 257 0293. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the U.S.−China Clean Energy Research Center, U.S. Department of Energy for project funding (No. DE-PI0000017). The authors also thank Mr. R. Perrone for help in designing and constructing the CDI cells.


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REFERENCES

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