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Sep 9, 2013 - This paper reports on using this concept, for optimizing energy efficiency, as well as to improve upon the electro-adsorption of ions an...
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Optimizing the Energy Efficiency of Capacitive Deionization Reactors Working under Real-World Conditions Enrique García-Quismondo,*,† Cleis Santos,† Julio Lado,‡ Jesús Palma,† and Marc. A. Anderson†,§ †

Electrochemical Processes Unit, IMDEA Energy Institute, Ave. Ramón de la Sagra 3, Mostoles Technology Park E28935, Mostoles, Spain ‡ IMDEA Water Institute, Scientific Technology Park − Alcalá University, E-28805, Alcalá de Henares, Spain § Environmental Chemistry and Technology Program, University of Wisconsin-Madison, Madison, Wisconsin, 53706, United States S Supporting Information *

ABSTRACT: Capacitive deionization (CDI) is a rapidly emerging desalination technology that promises to deliver clean water while storing energy in the electrical double layer (EDL) near a charged surface in a capacitive format. Whereas most research in this subject area has been devoted to using CDI for removing salts, little attention has been paid to the energy storage aspect of the technology. However, it is energy storage that would allow this technology to compete with other desalination processes if this energy could be stored and reused efficiently. This requires that the operational aspects of CDI be optimized with respect to energy used both during the removal of ions as well as during the regeneration cycle. This translates into the fact that currents applied during deionization (charging the EDL) will be different from those used in regeneration (discharge). This paper provides a mechanistic analysis of CDI in terms of energy consumption and energy efficiencies during the charging and discharging of the system under several scenarios. In a previous study, we proposed an operational buffer mode in which an effective separation of deionization and regeneration steps would allow one to better define the energy balance of this CDI process. This paper reports on using this concept, for optimizing energy efficiency, as well as to improve upon the electro-adsorption of ions and system lifetime. Results obtained indicate that real-world operational modes of running CDI systems promote the development of new and unexpected behavior not previously found, mainly associated with the inhomogeneous distribution of ions across the structure of the electrodes.

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INTRODUCTION Capacitive deionization (CDI) is an electrochemical water treatment technique that promotes the adsorption of ions in the electrochemical double layer (EDL) of a charged electrode surface by applying an electric potential, thus storing energy in the form of a capacitor1−10 and producing deionized water. Adsorbed ions are desorbed from the surface of the electrodes by eliminating the electric field (or possibly reversing it although not reported here), resulting in recovering a part of the energy used previously and producing a regenerated solution.11−14 A schematic of this electro-sorption/desorption process is shown in Figure 1. Figure 1 (left) shows that when the electrodes are charged by applying a potential to saline water containing positive and negative charged ions, these ions are adsorbed on the electrodes, thereby producing deionized water at the outlet. In the regeneration step, Figure 1 (right), a wash solution is circulated while the electrodes are depolarized, so that ions are desorbed from the electrodes and pass into the bulk of the solution, resulting in a stream of higher concentration. These two stages are essentially the same as charging and discharging an electrochemical double layer capacitor. Deion© 2013 American Chemical Society

ization (charging) and regeneration (discharging) involves reversible electro-adsorption15 with electrochemical response behaving in a purely capacitive fashion without any Faradaic (redox reactions) contribution. This allows one to recover a portion of the energy during regeneration, much as in a doublelayer capacitor during discharge. Due to its energy storage capabilities, CDI has the potential to be an attractive alternative to Reverse Osmosis (RO). Energy consumption in this process has ranged from less than 0.6 kWh·m−3 for total dissolved solids (TDS) removal from brackish water16 to 1.37−1.67 kWh·m−3 for arsenic removal from groundwater.17 For the above-mentioned reasons, CDI has been explored already for removing a variety of inorganic compounds from different types of water, i.e. seawater, groundwater, industrial process water, and wastewater.16−20 However, as will be discussed subsequently, energy optimization in CDI systems will likely not be the same as for EDL capacitors. In the real world, CDI involves circulating Received: Revised: Accepted: Published: 11866

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phenomenon is much more intense when different currents are applied for each of the charging and discharging cycles and can lead to errors when analyzing the results with respect to energy and ion removal efficiencies.28 We have noted this phenomenon in a previous paper and showed the results of experimental testing with a CDI unit operating at different current intensities during charge and discharge.28 In this paper, we proposed a new method of analysis that allows one to examine energy efficiencies of both cycles (deionization and regeneration) by implementing an intermediate step (named decoupling) reducing the effect of factors coming from the previous cycle. We expand on this concept by discussing the influence of different current intensities during charge−discharge cycling, in terms of capacitance, Equivalent Series Resistance (ESR), and energy efficiencies, as well as the influence of deionization performance on other critical variables such as electrolyte concentration.

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Figure 1. Schematic diagram of capacitive deionization showing deionization and regeneration steps.

MATERIALS AND METHODS Electrodes. The electrodes employed in our CDI system consisted of porous conducting carbons coated with a nanoporous film of nonconducting oxides. This system is asymmetric, with thin-film coatings covering each porous conducting carbon electrode; one is coated with an acidic SiO2 nanoporous thin-film (negative surface charge) and the other is coated with a basic Al2O3 nanoporous film (positive surface charge) aimed at preventing oppositely charged ions from being readsorbed during regeneration.29,30 The BET surface areas of the supported acidic SiO2 and basic Al2O3 were 18 and 10 m2·g−1, respectively.31 These electrode materials were supplied by the University of Wisconsin-Madison Environmental Chemistry and Technology Program. CDI Stack. Experimental details of the CDI stack have been outlined before28 and will briefly be summarized here. The stack design consisted of fifteen plates with an exposed area of 318 cm2 each, spaced 5 mm apart, and fitted between two stainless steel sheets forming a frame around each electrode of 5 mm (see Figure S1 right in the Supporting Information (SI)). The flow circuit comprised a 50 cm3 vessel, the reactor, and a peristaltic pump (Masterflex model 77521-47) fitted with a pump head (Masterflex type 7518-00), interconnected with Viton tubing. The tubing was attached to the cell entries and exits with PTFE connectors (see Figure S1 left, SI). The separate compartments were arranged hydraulically in series, while the electrical connection of the cells was in parallel. The layout of the CDI experiments is based in a single-pass method as described by Biesheuvel and co-workers.27 Reagents. NaHCO3 reagent (98% purity, Sigma-Aldrich) was used as received. Aqueous solutions at different salt concentrations (200−20 000 mg·L−1) were freshly prepared with ultrapure water (18 MΩ·cm resistivity) from a Milli-Q Integral water purification system water purification system. This system (total reservoir 0.05 L), did not allow for changes in the NaHCO3 concentrations over the full range of experiments on a realistic time scale. Electrochemical Experiments. Chronopotentiometry and electrochemical impedance spectroscopy (EIS) measurements were performed for the systematic study of capacitive deionization, on an electrochemical workstation (AUTOLAB potentiostat/galvanostat Model PGSTAT 302N) with a testing procedure based on a “decoupled” constant current (CC) profile. This testing configuration was described in detail in an earlier paper.28 In this paper, we have demonstrated that in

water solutions instead of an ideal electrolyte, making the analogy between EDL capacitors and CDI processes difficult. Unlike conventional ultracapacitors, in CDI, the behavior of the system upon deionization and regeneration depends strongly on the characteristics of the media in two ways. First, the use of a non- ideal electrolyte (generally brackish water, or wastewater effluents) affects deionization performance in terms of ohmic losses (as compared to ultracapacitors that use ideal electrolytes), and may therefore show certain anomalies, particularly with respect to recovering this energy efficiently. Second, operating conditions can be significantly different depending on whether the liquid flowing through the electrodes is an untreated water (i.e., wastewater), that usually contains a low concentration of ions and therefore is of high electrical resistivity, or alternatively if the fluid is a concentrated solution, such as the typical wash solution for regeneration, which would have good electrical conductivity for effective ion desorption. This feature is particularly important when these two stages are consecutive, as in CDI. Therefore, if one wishes to optimize the CDI process, it must be recognized that deionization should be forced to work at low currents followed by a regeneration step at higher currents, keeping in mind that the higher the ion concentrations, the higher the electrical conductance. We believe that these operational aspects are relevant to the lifetime of a CDI system operating under consecutive cycling conditions in real world scenarios. Another important feature of this charge−discharge profile has to do with renewable energy applications. To utilize as much of this energy as possible, CDI must operate using currents of differing intensities during deionization and regeneration. Therefore, a mechanistic analysis of the operational aspects of CDI operating under differing currents should allow one to optimize this system. Unfortunately, few studies have focused on these operational conditions with respect to the efficiency of ion removal and energy consumption.21−28 The type of test procedure of applying charge and discharge at different rates, while quite usual for other electrochemical energy storage systems such as rechargeable batteries and supercapacitors, is not common for CDI and its effect on the energy efficiency and on aging mechanisms has not been previously studied. In CDI, deionization and regeneration cycles are not symmetric. This means that one cycle affects the other. This 11867

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energy that is recovered, Er, compared to the energy stored, as shown in the following equations:

order to understand the behavior of adsorption and desorption stages independently it is necessary to implement a constant voltage (CV) step between the removal and regeneration stage. This allows one to reduce the study phase (i.e., charge) on the influence of the previous cycle current (i.e., discharge) and vice versa. This method provides more reliable information on the electrochemical behavior of the charging and discharging processes separately, with no interference from secondary phenomena. Results of other test procedures based on “coupled” experiments at different current intensities for charge and discharge provide only limited information concerning the cycles with respect to the total energy saved or consumed during operation. Tests of the CDI stack in this work included consecutive charge−discharge experiments conducted with the same electrolyte under different current rates (0.125−0.55 A). First, a constant current charge was applied until the charge voltage achieved a preset limit (1.5 V), at which point the voltage was fixed and current began to decrease exponentially until the “limiting current from charge”, Il,c, was obtained. Subsequently, a constant current discharge was applied until the voltage reached 0 where, once again, voltage was maintained and current increased exponentially until the “limiting current from discharge”, Il,d, was reached. The next cycle repeated this procedure. The measurements were performed several times until a “dynamic equilibrium”, where both adsorption and desorption processes are balanced. A schematic representation of the profile configuration can be seen in Figure S2 (SI). The main figures of merit for this CDI system have been extensively described in the earlier paper.28 From the above detailed charge/discharge test profiles, capacitance and ESR during the charge and discharge steps can be determined. Capacitance can be calculated from the slope of the charge or discharge steps.32 ESR values are calculated from the ohmic drop at the beginning of each step, defined here as ΔVohm,c and ΔVohm,d for ΔV in charge and discharge, respectively, taking into account not only the current for actual step but also the rate corresponding to the previous process (“limiting current”), as described in Figure S2 (SI). In this manner, when different rates are applied, it is possible to evaluate charge−discharge stages separately.28 As noted above, these CDI systems are performing as supercapacitors and therefore, they are also storing energy. Indeed, this paper rigorously defines CDI performance in terms of the current efficiency of deionization and regeneration processes as well as current efficiency of the overall process. Figure S3 (SI) summarizes the main energy components evaluated in these charge/discharge tests. According to the description of the energy components, the entire energy charged, Ec, is the area under the curve during charge, while the net energy consumption (named here energy stored, Est) is the work used to effectively remove the dissolved ions from the solution. The difference (named here as “energy not stored”, Ens) corresponds to the electrical charge consumed by the desorption of the co-ions having the same sign of charge as that of the electrical charge in the solid phase and is a factor that contributes to limiting current efficiency in charging the CDI system as has been reported by other authors.33 The energy recovered, Er, is the area under the curve during discharge. Current efficiency during charging scales directly with the area of the electrical charge stored, Est, and the energy charged, Ec. Meanwhile, current efficiency during discharge is the ratio of

ξCharge =

Est Ec

ξDischarge =

(1)

Er Est

(2)

Current efficiency of charge−discharge cycle is:

ξCycle = ξCharge·ξDischarge

(3)

Note that the intermediate constant voltage step between charge and discharge is only a tool used to evaluate charge− discharge stages separately. Therefore, even though it is an energy-consuming step, for figures of merit calculations, the energy associated with this region has been ignored. It is important to note here that the reactor used herein is not designed for evaluations of absolute amount of ions adsorbed, as salt concentrations are not obtained by these measurements but only changes in the electrical charge are required for calculating the current efficiency of the CDI processes. During testing, conductivity, pH, and temperatures were measured periodically (HANNA Instruments HI 2550 pH/ORP and EC/ TDS/NaCl meter). Finally, EIS measurements were performed with 10 mV AC perturbation in the 4 kHz to 10 mHz frequency range at open circuit potential.

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RESULTS AND DISCUSSION Under real operating conditions, CDI would ideally run with such flexibility as to simulate a supercapacitor working under conditions of peak power, experiencing therefore, charge (or deionization) at high current followed by a discharge (or regeneration) at low current at consecutive cycling, and vice versa. Based on these premises, we show the results of CDI unit cycling measurements studying the influence of different discharge rates with the same Icharge, and similarly testing different charging rates over constant Idischarge. Testing conditions are summarized in Table 1. Table 1. Testing Profiles: (1) Varying the Current of Charging and (2) Varying the Current of Discharge test

cycle: charge−discharge

key test

1

(0.125 A)−(0.55 A) (0.20 A)−(0.55 A) (0.30 A)−(0.55 A) (0.45 A)−(0.55 A) (0.55 A)−(0.55 A) (0.20 A)−(0.125 A) (0.20 A)−(0.20 A) (0.20 A)−(0.30 A) (0.20 A)− (0.45 A) (0.20 A)−(0.55 A)

discharge current = constant

2

charge current = constant

A complete analysis of the CDI performance has been made focused on the ESR and capacitance values in deionization and regeneration, as well as on current efficiencies and EIS measurements. Charge−Discharge Experiments. To determine the effect of current intensity on the ion adsorption−desorption processes, capacitance has been measured during cycling tests. Figure 2 shows a comparison of capacitance values at different 11868

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porous structure have difficulty being desorbed when the current applied is high. Regarding capacitance obtained under different electrolyte concentrations, it can be seen that the higher the NaHCO3 concentration (higher conductivity), the higher the capacitance. This increase in capacitance is explained by the higher concentration of ions, and is consistent with ESR values (see Tables S1 and S2 in the SI) and conductivity measurements (see Table S3 in the SI). The conductivity of salt water was measured during adsorption and desorption stages although no significant differences were found within the same range of salt concentration. This is consistent with the volume of reservoir employed that does not allow changes in concentration of the electrolyte during operation. For the same salt concentration employed, ESR values (shown for different salt concentrations and for different testing profiles in Tables S1 and S2 in the SI) are independent of the current applied maintaining a constant value. This is typical of ohmic capacitors where the contribution of the electrical contacts and the electrode to the total resistance is constant. Therefore, if only the electrolyte is changed and the experimental conditions are fixed, the differences in the ESR are only related to the electrolyte conductivity. This agrees with conductivity measurements (Table S3 in SI). Concerning the pH, substantial differences were not found in the measurements during these experiments (around 7.90−8.50). This is coherent with the type of experiments performed here in which the electrolyte concentrations (and therefore ionic strength) remain virtually constant during the cycle. With regard to the energy requirements in deionization and regeneration processes, Figure 4 shows charging current versus

Figure 2. Charge capacitance of the CDI unit tested according to Test 1 profile. Discharge-rate constant (0.55 A).

charging rates at a fixed Idischarge (Test 1). An analysis of capacitance shows that is clearly dependent on the current rate. This effect is expected as some authors claim that this dependence is due to the porous structure of the electrode34−37 and they have attributed this effect to slow ion diffusion into the pores of the electrode. The depth to which ions penetrate the electrode media seems to be dependent on the rate of charge, as well as on the surface area of the electrode. Macroporosity plays an important role for the transport of ionic species to and from the reaction sites within the interior of the electrode, while surface area (microporosity) provides sites for adsorption−desorption. Therefore, based on porosity function, for the same electrodes (i.e., same characteristics of porosity), we can assume that the extent to which ions can penetrate is determined by Icharge. Under a low rate of charging, one can expect that ions migrate without difficulty into the pores, the area available for ion adsorption is greater, and capacitance is higher. In contrast, at a high rate of charge, the diffusion of ions into the pores is hindered, and under such conditions, the adsorption cannot proceed into the interior but stops at the surface of the electrode, decreasing the capacitance, as shown schematically in Table of Contents artwork (TOC). The same phenomena can be seen concerning the desorption mechanism in Test 2 when different rates of Idischarge are applied for the same charging current (Figure 3). Here, one can see a decrease in discharge capacitance. Therefore, as can be expected, those ions that were within the interior of the

Figure 4. Charge, discharge, and cycle current efficiency in CDI unit. Test 1 profile: 20 000 mg·L−1 NaHCO3. Discharge-rate constant (0.55 A).

the current efficiency of the process for charge and discharge steps as well as for the complete cycle, according to Test 1 (20 000 mg·L−1 salt concentration). To explain the increase in current efficiency in discharge when charging rates are higher (shown on the right part of Figure 4), we evaluate the trends of the energy stored with respect to current efficiency in charging. Therefore, as illustrated in Figure 4, if only Icharge is changed, and Idischarge remains constant, variations in the efficiency of the discharge step are dependent on the efficiency of the charging process. By increasing the charging rate, as mentioned above, ion migration to the inner part of the electrode is obstructed by

Figure 3. Discharge capacitance of the CDI unit tested according to Test 2 profile. Charge-rate constant (0.20 A). 11869

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Therefore, if the number of ions adsorbed within the electrodes is similar (same charging currents), when discharge rates are increased, ion migration from the inner part of the electrode through the three-dimensional structure of the porous material will be hindered, decreasing the energy delivered by the system similar to that occurring during the discharge of a supercapacitor. These results are in good agreement with the mechanism proposed in this work with respect to the influence of current intensity on capacitance values (see TOC). Having described the electro-sorption/desorption mechanism, it is now appropriate to consider an operational mode of replacing the electrolyte when deionization finishes and regeneration begins. This is defined by a deionization stage where the liquid flowing through the system is an untreated water with low conductivity, and therefore its ability to conduct electric current low (i.e., low Icharge). This stage has to be followed by a regeneration step where the fluid is a concentrated solution (“wash water” shown schematically in Figure 1) showing good electrical conductivity allowing high Idischarge. This asymmetric operational procedure for CDI could be useful in real-world conditions such as those integrated with renewable energy systems in order to mitigate the effects of power fluctuations caused by the intermittent nature of clean energy solutions (i.e., wind and solar power generation). In situations like this, a CDI system suitable for this application should be capable of being flexible with respect to the ability to change rates of current applied since fast power modulation and continuous operation are required. Other potential applications of this flexible operational mode can be implemented in water treatment plants, where two CDI units can operate in a parallel configuration with the first unit in the deionization cycle using the energy stored in the second unit which is in the regeneration cycle discharging the adsorbed ions to the rejected solution while supplying DC power to the first unit. In such a case, in order to match the two CDI units, one in regeneration and the other in a deionization cycle, an asymmetric profile in which different currents are applied for each of the deionization and regeneration cycles should be utilized. As discussed above, these manners of operation (close to real life scenarios) may cause an irregular distribution of ions across the structure of the porous electrode leading to an inefficient deionization−regeneration cycle. However, this effect can be minimized with some operational strategies that we consider an important and essential part of the CDI process. An increase in the net current efficiency of the complete system is expected when low current rates are used, solutions with high salinity are fed to the system, and deionization/regeneration currents ratio are in the range 0.7−1.6. Based on these results, the most optimum operating conditions were obtained at the highest electrolyte concentration (20 000 mg·L−1), at low-rate of charge (0.20 A), and discharge current at 0.125−0.30 A. Under these conditions, total capacitance is around 60−70 F and ESR is close to 0.22 Ω on an average. In terms of an energy strategy, at selected operating conditions, current efficiencies higher than 90% for the charging process and about 70% for discharge were obtained, thus producing 65% of a balance between the energy supplied during deionization and the energy recovered during regeneration. These results in energetic performance are in the range of previously published data on capacitive deionization39 and reveal that, under real-world modes of operation, the mechanism of electrostatic adsorption exerts an electrical role in the functioning of regenerating the system and shows

the three-dimensional structure of the electrode. Thus, decreasing current efficiency in charging causes ions to be adsorbed mainly on the electrode surface. Under such conditions, those ions adsorbed on the surface are readily available during the discharge process to be desorbed. Under these circumstances, when Idischarge is maintained constant, the discharge process is more efficient even when the previous charging process has shown poor behavior. At higher charging rates (0.55 A), this effect is more remarkable, where discharge is even more efficient than in charging the system. It is important to remark that for the same Idischarge, the current efficiency of the cycle increases at higher Icharge from 45% to 57% due essentially to an inefficient deionization. However, it should be noted that even though this charge/ discharge configuration (high charging rates−high round trip efficiency) allows one to increase energetically the efficiency of the global process, this manner of operation provokes a low activity in the inner part of the electrode that is used only partially, while the surface works in excess. Under these conditions, the net effective surface area for adsorption− desorption processes is strongly reduced. CDI performance at different discharge-rates and fixed Icharge (Test 2) is shown in Figure 5. Current efficiency values in

Figure 5. Charge, discharge, and cycle current efficiency in CDI unit. Test 2 profile: 20 000 mg·L−1 NaHCO3. Charge-rate constant (0.20 A).

charging do not vary substantially for the different discharge rates, and are around 93%. This behavior is expected, since under this testing profile Icharge is maintained constant, and suggests that by using our decoupled testing method with an intermediate step we obtain expected results that are explainable according to test conditions. Clearly, for a given charging rate, desorption experiences diffusional difficulties when submitted to high current. This has already been experienced by Długołec̨ ki et al. under similar profile testing (fixed Icharge and different Idischarge) over a Membrane Capacitive Deionization system,38 and it is due to the internal accumulation of ions on the porous electrode structure obtained in the previous stage (deionization) resulting in a loss of discharge capacitance, and as a consequence, a decrease in current efficiency in discharge. This is because, for the same charging conditions (characterized by low current density where ions are adsorbed quite homogeneously on the whole surface area of the electrodes), the extent to which ions sequestered in the inner part of the electrodes can be desorbed is determined by Idischarge. 11870

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the importance of adequately selecting the operational aspects and the composition of the electrolyte in CDI systems. Electrochemical Impedance Spectroscopy Tests. Finally, electrochemical impedance spectroscopy measurements were performed to investigate the rate capability of these oxidecoated carbon electrodes. Impedance spectra have been recorded at open circuit potential and compared using different solution concentrations. Results are shown in Figure 6 where the Nyquist plots for 200, 2000, and 20 000 mg·L−1 NaHCO3 concentrations are represented.

Article

ASSOCIATED CONTENT

S Supporting Information *

Figure S1. Photographs showing capacitive deionization system. By courtesy of Proingesa. Figure S2. Charge/discharge configuration. Equivalent series resistance graphical estimation is included. Figure S3. Energy components in CDI systems according to decoupled CC operational mode. Table S1. ESR in Charge/Discharge Curves. Test 1. Table S2. ESR in Charge/ Discharge Curves. Test 2. Table S3. Electrolyte conductivity. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +34 91 737 11 32; fax: +34 91 737 1 1 40; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Environmental Chemistry & Technology Program, University of Wisconsin-Madison for electrodes supply, and to the company Proingesa for CDI reactor provision. Financial support from the Ministry of Science and Innovation through INNPACTO Program (IPT-2011-1450310000 (ADECAR project)) and the cooperation of its participants (Isolux Ingenieria,́ S.A., Proingesa, Córdoba University, and Nanoquimia, S.L.) is greatly acknowledged.

Figure 6. Nyquist plot of CDI unit for different solution concentration.

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Impedance spectra from all electrolytes exhibit common features, the characteristic shape associated with an ideal super capacitor behavior; a semicircle at high frequency (namely polarization resistance Rp) due to contact resistances and affected basically by the quality of cell assembly, uncompensated solution resistance Rs from the first intercept point of the ReZ axis which is directly related to electrolyte conductivity, and a diffusion controlled process at low frequency (Warburg impedance). As can be deduced from Figure 6, solution resistance decreases clearly with salt concentration. Here, the values were found be 0.060, 0.150, and 0.913 Ω for 20 000, 2000, and 200 mg·L−1 NaHCO3 solution, respectively. The polarization resistance (small capacitive loops at frequencies between 4 kHz and 1 Hz) represents the capacitive ideal behavior at the electrode−electrolyte interface, which provides some indication as to how much the double layer is polarized (accumulated ions). From the Nyquist plot, it can be seen that this value varies slightly in magnitude due to ion concentration at the electrode−electrolyte interface. The diffusional part of the signal is not changed drastically by modifying concentrations but a small variation was observed in 200 mg·L−1 where Warburg impedance appears at lower frequencies. The imaginary part of the impedance (-ImZ axis) decreases slightly with an increase in solution concentration that implies that higher concentrations have a positive tendency for a higher rate of diffusion (small capacitance improvement). The plot tends to a straight-line (a spike) characteristic of a porous capacitor. These results are consistent with those detailed above in which it was claimed that performance of the CDI system at high electrolyte concentrations will be advantageous in terms of net energy consumption.

ABBREVIATIONS CDI capacitive deionization EDL electrochemical double layer ESR equivalent series resistance PTFE polytetrafluoroethylene CC constant current CV constant voltage EIS electrochemical impedance spectroscopy BET Brunauer, Emmett, and Teller

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REFERENCES

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dx.doi.org/10.1021/es4021603 | Environ. Sci. Technol. 2013, 47, 11866−11872