New Operational Modes to Increase Energy Efficiency in Capacitive

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New Operational Modes to Increase Energy Efficiency in Capacitive Deionization Systems Enrique Garcia - Quismondo, Cleis Santos, Jorge Soria, Jesús Palma, and Marc A. Anderson Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b05379 • Publication Date (Web): 11 May 2016 Downloaded from http://pubs.acs.org on May 17, 2016

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Environmental Science & Technology

New Operational Modes to Increase Energy Efficiency in Capacitive Deionization Systems Enrique García-Quismondoa*, Cleis Santosa, Jorge Soriaa, Jesús Palmaa, Marc. A. Andersona,b a

Electrochemical Processes Unit, IMDEA Energy Institute, Ave. Ramón de la Sagra 3, Mostoles Technology Park E28935, Mostoles, Spain. Fax: +34 917 371 140; Tel: +34 917 371 132.

b

Civil and Environmental Engineering · 1415 Engineering Drive, Madison, WI 53706, University of WisconsinMadison, WI, USA 53706. Tel: +1-608-4468160. * Corresponding author. Tel.: +34 917 371 132; fax: +34 917 371 140. E-mail address: [email protected]

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ABSTRACT: In order for Capacitive Deionization (CDI) as a water treatment technology to achieve commercial success, substantial improvements in the operational aspects of the system should be improved in order to efficiently recover the energy stored during the deionization step. In the present work, to increase the energy efficiency of the adsorption−desorption processes, we propose a new operational procedure that utilizes a concentrated brine stream as a washing solution during regeneration. Using this approach, we demonstrate that by replacing the electrolyte during regeneration for a solution with higher conductivity, it is possible to substantially increase round-trip energy efficiency. This procedure was experimentally verified in a flow cell reactor using a pair of carbon electrodes (102cm geometric area) and NaCl solutions having concentrations between 50 and 350 mmol·L-1. According to experimental data, this new operational mode allows for a better utilization of the three-dimensional structure of the porous material. This increases the energetic efficiency of the global CDI process to above 80% when deionization/regeneration currents ratio are optimized for brackish water treatment.

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KEYWORDS: Capacitive Deionization; electrochemical energy storage systems, water treatment; operational procedures.

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Capacitive Deionization (CDI) is a technology that allows one to produce deionized water by adsorbing ions in the electrochemical double layer (EDL) of a charged electrode surface by applying an electric field, as shown schematically in Table of Contents artwork (TOC). This process is reversible, so adsorbed ions can be desorbed from the surface of the electrodes by applying reverse current or short-circuit. This allows one to recover a part of the energy used previously while producing an effluent with higher concentrations similar to the retentate of a Reverse Osmosis (RO) process 1–3. This cycle, the same as charging and discharging a double-layer capacitor 4–8, makes it a straightforward, non-energy intensive and low environmental impact technology in which one of the main features is the recovery of energy stored in the double-layer during ion desorption.

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This is an important feature of CDI because, provided that the energy stored in the deionization system can be easily recovered in the regeneration step, the net energy consumption is the difference between the energy supplied during deionization and the energy recovered during regeneration. In principle, one could expect that this would make the entire process more energetically favorable, especially compared with other energy intensive water treatment techniques such as RO and electrodialysis (ED). However, some aspects such as inefficiencies and irreversibility during the

INTRODUCTION

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charging/discharging cycle must also be considered 9,10. Unfortunately, until now, there has been little effort to recover the energy efficiently even though it is a key selling point for the CDI process 11.

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Electrochemical Supercapacitors can typically show round-trip efficiencies over 90-95% 12,13. According to theoretical calculations based on the minimum thermodynamic work required to remove salts from a solution containing 35 g/L to produce a 0.3 g/L solution, if a CDI system was able to reach round-trip efficiencies of 85% it could be a competitive technology for seawater desalination 4.

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In one of our previous studies 14, a CDI reactor with porous conducting carbons as electrodes was evaluated under an unconventional testing configuration that allowed us to interpret the behavior of both the adsorption and desorption steps independently. In that study, by adjusting the deionization/regeneration current density ratios, we were able to improve the round-trip energy efficiencies to a maximum of 65%.

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However, we believe that further improvements in energy efficiency are restricted due to limitations imposed by the regeneration step. For this reason, we have continued to study the operational aspects of the CDI process. In this work, we optimize the desorption process by using an innovative operational method. In this case, we replace the liquid in the reactor at the end of its deionization cycle (or charge) by a concentrated brine solution and perform regeneration (or discharge) using this brine. We refer to this mode of operation as a Highly Efficient Operating Procedure for CDI, and to the best of our knowledge, this is the first report in which charge/discharge curves based on this procedure are shown.

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MATERIALS AND METHODS Electrodes and Reagents. The electrodes were prepared by coating a titanium foil with a printing slurry composed of activated carbon (Picactif BP 10), polyvinylidene fluoride (PVDF), Vulcan XC-72R and n-methyl pyrrolidone (NMP), followed by annealing at 140°C (2ºC/min ramp) for 4 h. In this way, we obtained electrodes with controllable carbon thickness (50 micr0ns) and mass loading properties (2 mg/cm2). Different NaCl aqueous solutions were used as the electrolyte. NaCl reagent (98% purity, Sigma-Aldrich) was used as received. Aqueous solutions at different concentrations were freshly prepared with ultra-pure water (18MΩ·cm resistivity) from a Milli-Q® Integral water purification system. Typically, experiments were performed at a temperature of 298 K and employed 70 cm3 of electrolyte. A peristaltic pump was calibrated to provide a mean linear flow rate of 1 cm/s through the interelectrode gap of the 4 mm. In addition, unless otherwise stated, each experiment employed a new pair of electrodes. This system contained a sufficient volume of electrolyte so as to not allow for changes in the NaCl concentrations over the full range of experiments conducted over a realistic timescale.

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Reactor. The cell design utilizes two identical polypropylene plates (95 mm × 45 mm × 10 mm). Two holes towards the top and bottom of the plates act as entries and exits for the electrolyte. The interelectrode gap was controlled with thin Viton gaskets and a polypropylene electrolyte flow compartment that had a 30 mm wide, 80 mm long, 2 mm thickness channel machined at the center. On each side of the electrodes, active material was scraped away to provide electrical contact with the titanium back contact. These edges fit under the gasket so that they were not exposed to the electrolyte solution and a 31.6 mm×31.6 mm area of active electrode protruded into the electrolyte flow channel. The cell was held together with a small clamp consisting of two steel plates and six bolts that allowed for dismantling and reconstruction of the cell. The flow circuit consisted of the cell and a Masterflex Model 77521-47 pump fitted with a Masterfex pump head, Type 7518-00, interconnected with Viton tubing. The tubing was connected to the cell entries and exits with polytetrafluoroethylene (PTFE) connectors. The CDI system is shown in Figure 1.

Here, we present experimental results for charge, discharge and round-trip energy efficiencies in a CDI flow cell both in a conventional operational mode and in this new procedure as function of salt concentration and electrical current. We demonstrate that by replacing the electrolyte in regeneration for a solution with high conductivity, it is possible to systematically optimize the parameter settings to achieve the highest round-trip energy efficiency. In particular, we investigate the effect of increasing the current density in discharge. We are careful to note that all characterization results are obtained from electrical parameters. However, salt adsorption rates should be expected to show similar trends.

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FIG 1

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Electrochemical Characterization. Chronopotentiometry measurements were performed for the systematic study of capacitive deionization. We used an electrochemical workstation (Biologic VMP3 multichannel potentiostat–galvanostat coupled with EC-Lab v10.18 software) and a testing procedure based on an asymmetric constant current (CC) profile. This testing configuration was described in detail in an earlier paper 14.


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Tests of the CDI cell included consecutive charge – discharge experiments conducted under both the same current rates during charge and discharge or an asymmetric profile with different charge and discharge rates (based on different charge-discharge current density ratios) with a maximum cell voltage of 1.5 V and current densities ranging from 2.5 to 50 A·m-2. Secondly, a constant current discharge was applied until the voltage reached zero. The next cycle repeats this procedure. Between the deionization and regeneration stage, we implemented a constant voltage (CV) step to avoid energy efficiencies being interfered by the current in the preceding charge or discharge stage of the process 14. We note that this period would not be used running CDI systems in real-world operational conditions (or if used, the step would be shorter). This constant voltage step is only used to evaluate charge−discharge stages separately when different currents are applied for each of the charging and discharging cycles. Therefore, even though experimentally it is an energyconsuming step (about a quarter of the energy consumed in deionization), for figures of merit calculations, the energy associated with this region has been ignored.

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In the Highly Efficient Operating Procedure for CDI here presented, this CV step serves also to allow for the replacement of electrolyte preventing desorption of adsorbed ions. In this case, this step will necessarily last the minimum time required to empty the reactor content. The impact of this stage in energy efficiency will be discussed later.

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Test experiments under this new operational mode were conducted under the same charge and discharging currents. However, asymmetric experiments based on different charge-discharge current density ratios were tested as well. The measurements were performed several times until a ‘dynamic equilibrium’, where both adsorption and desorption processes were balanced 5.

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It is important to remark that here we present results in terms of current efficiency for charge and discharge steps. These efficiencies are always less than that of coulombic efficiency (ions removed / coulombs supplied) because this method takes into account ohmic losses while the coulombic efficiencies do not. As mentioned above, parameters such as the absolute amount of ions adsorbed or salt concentrations were not obtained by these measurements. The reactor used herein is not designed for such an evaluation as only changes in the electrical charge are required for calculating the energy efficiency of the CDI processes. In this paper, the total energy used for charging is calculated from integrating the voltage vs. time plot, including the energy consumed in processes that are not due to ion adsorption, the charge current efficiency is thus extracted with the area of the electrical charge stored and the total energy charged. Meanwhile, discharge efficiency is the ratio of energy that is recovered, compared to the energy stored previously. A more detailed procedure and the energy components calculations (Current efficiencies in charge and discharge) can be found in an earlier paper 15.

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During testing, conductivity, pH and temperatures were measured periodically (HANNA Instruments HI 2550 pH/ORP & EC/TDS/NaCl Meter). The CDI cell was cycled up to 5 times with the same electrodes to confirm the reproducibility of the results. All experiments exhibited excellent stability and reproducibility of the charge – discharge cycles showing a relative error less than 1.2%.

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RESULTS AND DISCUSSION

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Results show that the efficiency of ion removal is very dependent not only of the current density, as noted previously, but of the solution salinity as well. Judging from these results, and assuming a minimum of 65% efficiency as acceptable, the charge current efficiency appeared to be limited to a 20 A·m-2current density for solutions having a salt content between 100 and 350 mmol·L-1, and to 15 and 4 A·m-2, for electrolytes with 50 and 10 mmol·L-1 concentrations, respectively.

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The different electrolyte resistivity of these solutions, which is directly related to their salt content, can reasonably explain such a loss in performance under energy demanding conditions, as it relates to ion mobility. In Table 1, the conductivity of the different solutions can be seen and Table 2 shows the Equivalent Series Resistance (ESR) values obtained from the voltage difference at the beginning of the charging and discharging stages at different electrolyte concentrations (calculated from the average of the different current densities).

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FIG 2

Charge and Discharge Current Efficiencies and the Round-trip Efficiency of a CDI flow cell. Since electrodes composed of activated carbon have high surface areas and also many types of surface sites, ions may be preferentially adsorbed on certain sites. However, if a high current density is applied, ions are adsorbed less efficiently because the tortuosity paths cause steric difficulties for adsorption. This can be seen in Figure 2, where energy current efficiency in charge is plotted at different current densities. The flow cell was fed with different salt concentrations (10, 50, 100 and 350 mmol·L-1) and was charged and discharged at the same current density for adsorption and desorption steps over a range of 2.5 – 20 A·m-2.



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The conductivity of salt water was within the same range of salt concentration during adsorption and desorption stages since no significant differences were expected using these small electrodes.

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Table 1. Electrolyte Conductivity. -1 [NaCl] (mmol·L ) Conductivity (mS/cm) 10 1.14±0.09 50 5.95±0.11 100 11.03±1.23 350 38.67±0.56

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Table 2. ESR in Charge/Discharge Curves. Test 1. -1

[NaCl] (mmol·L ) 10 50 100 350

ESR Charge 2 (mohm·m ) 81.7±4.4 31.6±13.2 17.0±0.5 13.8±1.8

ESR Discharge 2 (mohm·m ) 71.7±3.4 26.4±8.4 15.3±2.0 13.8±1.1

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The opposite effect was observed with the current efficiency during the regeneration step (Figure 3) where values increased with higher current densities and higher discharge current efficiencies were obtained when salinity content was lower.

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This effect, as explained in our previous work 15, is due to the fact that the intensity of the electrical current applied in deionization and consequently the manner in which ions accumulate on the electrode surfaces play a significant role in the desorption current efficiency during regeneration. This occurs in such a way that less favorable deionization conditions (low electrolyte conductivity and high applied electrical currents), mean less favorable transport conditions such that ions cannot proceed into the interior of the electrode. This results in a more favorable and effective regeneration process. This behaviour seems to be confirmed as shown in Figure 2. The higher charge current efficiencies are mainly associated with higher salt concentrations (100 and 350 mmol·L-1) that is coherent with high electrical conductivity. Meanwhile, in Figure 3 discharge current efficiency values decrease significantly when higher salt concentrations with good electrical conductivity are employed.

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In order to evaluate the global efficiency of the technique, round-trip efficiencies for each experiment were calculated for each salt concentration. Results are displayed in Figure 4. In this figure, two trends can be seen relating to the two slopes found in each of the curves. There appears to be an initial increase in the round-trip efficiency in all experiments when there is an increase in the applied electrical current. This is caused to some extent by a remarkable improvement in the discharge current efficiency (Figure 3). However, inefficiencies in charge become so large at higher energy demand (Figure 2) that a smoothing effect on the round trip efficiency is obtained. The round-trip efficiency thus remains stable around 54% for 100 and 350 mmol·L-1 concentrations and at 66% for a 50 mmol·L-1 concentration. Unfortunately, this is quite far from the targeted results of around 80-85% 4.

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FIG 3

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FIG 4

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Proposal for the Highly Efficient Operating Procedure. This procedure consists on replacing the water treated solution by a concentrated brine stream as a washing solution during regeneration. The main purpose of this strategy is to exploit those features observed in the previous analysis for different salinity solutions. This analysis showed good electrical conductivity, such as ion mobility and electrolyte access to the pores, which could facilitate desorption of those ions that have previously been adsorbed in the inner part of the electrodes porous structure.

From these results, it seems that to further improve the potential for desalination and energy recovery, it is crucial to enhance the desorption process. In this paper, we propose an innovative operational mode in which a solution with very high salt content (therefore extremely high ion mobility) is used as a regeneration electrolyte.


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The deionization (removal of ions) stage is performed with NaCl solution to remove salt ions (Step 1 as described schematically in Figure 5). For this removal stage, it can be postulated that if a moderate current density is applied to polarize the electrodes one should experience a uniform adsorption of ions throughout the three-dimensional structure of the porous material 15. Once the maximum potential is reached, we maintain a constant voltage step meanwhile the reactor is being emptied (Step 2 in Figure 5). When the reactor is drained, a brine water solution (3.5 mol·L-1) is fed into the reactor. Once the system is completely refilled, a negative current (discharge) is applied to start the ion desorption process under conditions that presumably favor access to the porous structure of the electrode (Step 3 in Figure 5). This procedure is expected to improve performance with respect to removing ions from the electrode as well as to conduct electrons to surface sites. Finally, we keep a constant voltage step meanwhile the reactor is being drained again (Step 4 in Figure 5) and a fresh NaCl solution is fed into the reactor to proceed with the next cycle.

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Voltage and current evolution during Highly Efficient Operating Procedure cycling is shown in Figure 5 for an experiment with brackish water (50 mmol·L-1) in the deionization step and brine water (3.5 mol·L-1) as a washing solution in regeneration. This figure also includes schematic graphs describing the process. The curves show a linear voltage response with a positive slope during the charging step (ions are adsorbed into the electrodes), and once the maximum voltage is reached, an exponential decay in current can be seen. At this point, noise associated with the reactor emptying and subsequent brackish water filling the reactor is evident. Finally, on discharge, a linear negative slope is obtained (ion desorption).

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The sharp peaks in current intensity obtained when filling the reactor (rating around 25 A·m-2), observed in all tests, are particularly interesting as they may be associated with capacitive double layer expansion. This phenomena has already been reported 16–18 based on the rise of voltage between the two electrodes due to the variation in the thickness of the electric double layer that forms on the internal surface of the porous electrodes, taking place when the solution concentration is altered.

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We believe that our innovative operational mode conditions favor energy efficiency (and likely as well the amount of ions adsorbed) due to the increase in entropy that occurs inside the pores when there is a difference of concentration between the solution with which the reactor is filled and the concentration of ions in the pores of the electrodes. This capacitive double layer expansion has previously been reported by D. Brogioli et al. 19–21 and although it has not been extensively evaluated in this work, it could presumably allow greater energy recovery in CDI systems.

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Note that for this paper, this intermediate constant voltage step between charge and discharge is only a tool used to evaluate the efficiency of the process during the replacement of the electrolyte. Therefore, even though it may be a promising strategy to extract energy, for energy efficiency calculations, the energy associated with this region (estimated experimentally as a quarter of the energy consumed in deionization) has been considered outside the scope of this paper. Nevertheless, the authors are aware of the importance of this step and special attention will have to be paid in designing the hydraulic system of the cell to reduce the duration of this controlled-voltage step in order to minimize efficiency losses in any real operation.

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FIG 5

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Table 3. Highly Efficient Operating Procedure for CDI Testing Conditions at Different Discharge-Rates and Fixed Icharge

This Highly Efficient Operating Procedure -CDI cycling has been experimentally evaluated with 4 different tests based on using a low charging rate (5 A·m-2) and brackish water (50 mmol·L-1) in the deionization step followed by a discharge using brine water (3.5 mol·L-1) as a washing solution and 8, 10, 20 and 50 A·m-2 current density pulses, respectively. Testing conditions are summarized in Table 3.

Test [NaCl]Charge [NaCl]Discharge ICharge IDischarge -1 -1 -2 -2 (mmol·L ) (mol·L ) (A·m ) (A·m ) 1 50 3.5 5 8 2 50 3.5 5 10 3 50 3.5 5 20 4 50 3.5 5 50

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Charge, discharge and round-trip current efficiency results at such different discharge-rates and fixed Icharge are shown in Figure 6. Charge current efficiency values in charging do not vary substantially for the different discharge rates, and are around 90-95% consistent with those obtained under the conventional operation method at 5 A·m-2, shown in Figure 2.

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Clearly, for a given charging rate, desorption experiences diffusional difficulties when performed at high current. This has already been experienced by Długołęcki et al. under similar profile testing (fixed Icharge and different Idischarge) 6,15 and is



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explained due to the ion migration from the inner part of the electrode throughout the three-dimensional structure of the porous material. However, here the extent of this decay in efficiency is significantly smoothed by using brine water (3.5 mol·L-1) during regeneration.

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In the case of round-trip energy efficiency, we can see that the desorption mechanism depends on discharge rate. From these figures, one can surmise that by using this new operational mode during discharge, ion diffusion out of the porous structure is relatively rapid even when Charge / Discharge current density ratio is low (slow charge rate vs fast discharge rate, for example 5:50 A·m-2). This allows one to achieve round-trip energy efficiencies close to 79-82% with a (Icharge/Idischarge) ratio between 0.5 and 0.6 (5:8 and 5:10 A·m-2 charge:discharge ratios). This value is considerably higher than the value obtained by using a conventional operating method, with brackish water in both charge and discharge steps, that being 66% at 10:10 A·m-2.

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Similarly, this operational mode cycling has been studied at different charge-rates and fixed Idischarge using relatively a high rate of discharge (20 A·m-2) and brine water (3.5 mol·L-1) as a washing solution, and a deionization step with brackish water (50 mmol·L-1) at 5, 7.5, 10 and 20 A·m-2 current density pulses, respectively. Testing conditions are summarized in Table 4 and results of charge, discharge and round-trip current efficiency are compiled in Figure 7.

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Table 4. Highly Efficient Operating Procedure for CDI Testing Conditions at Different Charge-Rates and Fixed Idischarge

FIG 6

Test [NaCl]Charge [NaCl]Discharge ICharge IDischarge -1 -1 -2 -2 (mmol·L ) (mol·L ) (A·m ) (A·m ) 1 50 3.5 5 20 2 50 3.5 7.5 20 3 50 3.5 10 20 4 50 3.5 20 20

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An analysis of charge current efficiency shows that ion migration to the inner part of the electrode is clearly dependent on the current rate as already reported by some authors 22–24, that attribute this phenomena to slow ion diffusion into the three-dimensional structure of the electrode. Even more importantly, the tests performed by this Highly Efficient Operating Procedure -CDI method show that over a wide range of charging rates (from 5 to 20 A·m-2), it is possible to maintain a balance between an homogeneous ions adsorption in the whole electrode structure, and a complete desorption step in which most of these ions are recovered (73-77% discharge current efficiency). This allows one to reach round-trip efficiencies around 75%.

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It is important to remark that even though a wide range of charge:discharge currents are applied under both profile testing; fixed Icharge and different Idischarge and vice versa, the round-trip current efficiency remains in all cases above 67% (Figures 6 and 7). This shows the flexibility of this innovative process.

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This methodology of reusing the water spent in each regeneration step as a washing solution is an excellent strategy in which to optimize the CDI system as it allows a significant reduction on the effluent volume (and therefore it may increase the water recovery factor). Furthermore, the salt concentration of the regenerated water can be adjusted accurately by limiting the recycle to a certain point (i.e a concentrated brine of 3.5 mol·L-1). This procedure could thus have a significant impact on the plant balance improvement if compared with RO.

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With the intent to extend this innovative procedure to the treatment of water having a higher salt content, similar experiments were performed with saline water (350 mmol·L-1) in deionization. Under these conditions a better use of the electrode structure would be expected upon exposure to the 350 mmol·L-1 NaCl solution that is of low electrical resistivity and therefore of effective ion adsorption. Another predictable effect is that the current rate applied during charge could be higher.

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Based on these premises, in Figures 8 and 9 we show the results of 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 5.

FIG 7


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FIG 8

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FIG 9

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According to these figures and in comparison with former experiments, a substantial improvement is observed applying this new operational mode. This is both with respect to efficiency but it also to indirectly enhancing the deionization capacity since it increases the number of adsorption sites by performing regeneration with brine. Ions, even those adsorbed in the inner parts of the porous electrode structure, are accessible to be efficiently desorbed in this process.

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Moreover, this new operation procedure is able to treat saline water tolerating higher regeneration current densities. This allows one to perform shorter cycles, thus allowing for a greater number of cycles per day and faster deionization as measured in terms of kilograms per time.

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We note that this new operating method needs further investigation, particularly in regard to the phenomena occurring during emptying and filling of the reactor. However, according to the results shown one can adapt charge - discharge current rates to increase the round-trip energy efficiency to approach values around 80% for both brackish and saline water treatment making this technology competitive as compared to RO over a wide range of salinity4.

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In order to estimate the impact of this energy efficiency improvement in the functioning of CDI, we have calculated the energy work required to produce 1 m3 of clean solution (5 mmol·L-1 of NaCl) as a function of the salt concentration at the inlet with an operating cell voltage of 1.2V. The range of salt concentration has been enlarged to beyond the experimental work performed in this study and RO expected consumption (green line) has been added for comparative purposes. This has been plotted in Figure 10. The net energy consumption in CDI is based on the ratio between the work recovered during the regeneration to the work applied for ions adsorption. If a deionizer were able to operate recovering the maximum possible energy in the discharge (thermodynamics plot), the application of CDI could be largely extended to sea water treatment. However, as CDI systems are still far from ideal, for comparative purposes round trip efficiencies ranging from very high (95%) to moderate efficiencies (75%) for solutions of different concentrations are also shown. Results indicate that, under the selected conditions in this work (purple dots) and at 50 mmol·L-1 concentration CDI operating under Highly Efficient Operating Procedure this process could be a competitive technology as compared with RO, and at 350 mmol·L-1 concentration CDI reaches around 75% efficiency very close to 85%, where CDI could become a serious competitor with RO.

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To conclude, this study has identified a new strategy for operating CDI systems based on replacing the electrolyte when deionization finishes and regeneration begins. The procedure is designed to have a deionization stage where the liquid flowing through the system is an untreated water, and is followed by a regeneration step in which the fluid is a brine water solution showing good electrical conductivity thus allowing high Idischarge, to comply with the demanded power of removing ions from the porous electrode structure.

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From the results of tests performed, this new operational mode allows for a high utilization of the three-dimensional structure of the porous material, which allows better energy efficiency.

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According to experimental data for brackish water and saline water deionization, one can increase the energetic efficiency of the global CDI process to above 80% when deionization/regeneration currents ratio are optimized.

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Overall, this Highly Efficient Operating Procedure for CDI that utilizes a concentrated brine stream as a washing solution during regeneration provides for a further understanding of the operational aspects of CDI and could be useful in realworld conditions with respect to factors such as the ability to improve adsorption-desorption cycle rate and to remarkably increase the water recovery.

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Corresponding Author

Table 5

FIG 10

* Phone: +34 91 737 11 32. Fax: +34 91 737 1 1 40. E-mail: [email protected]

Funding Sources



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The work described in this paper was jointly supported by the Ministry of Science and Innovation through INNPACTO Program (IPT-2011-1450-310000 (ADECAR project)).

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ACKNOWLEDGMENT

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We thank the Ministry of Science and Innovation through INNPACTO Program (IPT-2011-1450-310000 (ADECAR project)) and the cooperation of its participants (Isolux Ingenieria, S.A., Proingesa, Córdoba University, and Nanoquimia, S.L.) is greatly acknowledged.

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ABBREVIATIONS

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REFERENCES

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(1)

Ying, T.-Y.; Yang, K.-L.; Yiacoumi, S.; Tsouris, C. Electrosorption of Ions from Aqueous Solutions by Nanostructured Carbon Aerogel. J. Colloid Interface Sci. 2002, 250 (1), 18–27.

(2)

Zou, L.; Li, L.; Song, H.; Morris, G. Using mesoporous carbon electrodes for brackish water desalination. Water Res. 2008, 42 (8-9), 2340–2348.

(3)

Zou, L.; Morris, G.; Qi, D. Using activated carbon electrode in electrosorptive deionisation of brackish water. Desalination 2008, 225 (1-3), 329–340.

(4)

Anderson, M. a.; Cudero, A. L.; Palma, J. Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete? Electrochim. Acta 2010, 55 (12), 3845–3856.

(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)

Długołęcki, P.; van der Wal, A. Energy recovery in membrane capacitive deionization. Environ. Sci. Technol. 2013, 47 (9), 4904–4910.

(7)

Porada, S.; Bryjak, M.; Wal, A. Van Der; Biesheuvel, P. M. Electrochimica Acta Effect of electrode thickness variation on operation of capacitive deionization. Electrochim. Acta 2012, 75, 148–156.

(8)

Biesheuvel, P. M.; Zhao, R.; Porada, S.; van der Wal, a. Theory of membrane capacitive deionization including the effect of the electrode pore space. J. Colloid Interface Sci. 2011, 360 (1), 239–248.

(9)

Biesheuvel, P. M. Thermodynamic cycle analysis for capacitive deionization. J. Colloid Interface Sci. 2009, 332 (1), 258– 264.

(10)

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.

(11)

Oren, Y. Capacitive deionization (CDI) for desalination and water treatment — past, present and future (a review). Desalination 2008, 228 (1-3), 10–29.

(12)

Miller, J. R.; Burke, A. F. Electrochemical capacitors: Challenges and opportunities for real-world applications. Electrochem. Soc. Interface 2008, 17 (1), 53–57.

(13)

Kötz, R.; Carlen, M. Principles and applications of electrochemical capacitors. Electrochim. Acta 2000, 45 (15-16), 2483– 2498.

(14)

García-Quismondo, E.; Gómez, R.; Vaquero, F.; Cudero, A. L.; Palma, J.; Anderson, M. New testing procedures of a capacitive deionization reactor. Phys. Chem. Chem. Phys. 2013, 15 (20), 7648–7656.

(15)

García-Quismondo, E.; Santos, C.; Lado, J.; Palma, J.; Anderson, M. a. Optimizing the energy efficiency of capacitive deionization reactors working under real-world conditions. Environ. Sci. Technol. 2013, 47 (20), 11866–11872.

(16)

Rica, R.; Ziano, R.; Salerno, D.; Mantegazza, F.; van Roij, R.; Brogioli, D. Capacitive Mixing for Harvesting the Free Energy of Solutions at Different Concentrations. Entropy 2013, 15 (4), 1388–1407.

(17)

Rica, R. a.; Ziano, R.; Salerno, D.; Mantegazza, F.; Bazant, M. Z.; Brogioli, D. Electro-diffusion of ions in porous electrodes for capacitive extraction of renewable energy from salinity differences. Electrochim. Acta 2013, 92, 304–314.

(18)

Rica, R. A.; Brogioli, D.; Ziano, R.; Salerno, D.; Mantegazza, F. Ions Transport and Adsorption Mechanisms in Porous Electrodes During Capacitive-Mixing Double Layer Expansion (CDLE). J. Phys. Chem. C. Nanomater. Interfaces 2012, 116

CDI, Capacitive deionization; EDL, electrochemical double layer; RO, reverse osmosis; ED, electrodyalisis; PVDF, polyvinylidene fluoride; NMP, n-methyl pyrrolidone; PTFE, Polytetrafluoroethylene; CC, constant current; CV, constant voltage; ESR, equivalent series resistance.


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Environmental Science & Technology (32), 16934–16938.

(19)

Brogioli, D.; Zhao, R.; Biesheuvel, P. M. A prototype cell for extracting energy from a water salinity difference by means of double layer expansion in nanoporous carbon electrodes. Energy Environ. Sci. 2011, 4 (3), 772.

(20)

Brogioli, D.; Ziano, R.; Rica, R. a.; Salerno, D.; Kozynchenko, O.; Hamelers, H. V. M.; Mantegazza, F. Exploiting the spontaneous potential of the electrodes used in the capacitive mixing technique for the extraction of energy from salinity difference. Energy Environ. Sci. 2012, 5 (12), 9870.

(21)

Marino, M.; Misuri, L.; Jiménez, M. L.; Ahualli, S.; Kozynchenko, O.; Tennison, S.; Bryjak, M.; Brogioli, D. Journal of Colloid and Interface Science Modification of the surface of activated carbon electrodes for capacitive mixing energy extraction from salinity differences. J. Colloid Interface Sci. 2014, 436, 146–153.

(22)

Shi, H. Activated carbons and double layer capacitance. Electrochim. Acta 1996, 41, 1633–1639.

(23)

Toupin, M.; Brousse, T.; Bélanger, D. Influence of microstucture on the charge storage properties of chemically synthesized manganese dioxide. Chem. Mater. 2002, 14, 3946–3952.

(24)

Reddy, R. N.; Reddy, R. G. Synthesis and electrochemical characterization of amorphous MnO2 electrochemical capacitor electrode material. J. Power Sources 2004, 132 (1-2), 315–320.

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List of Figures Figure 1 (Left) Schematic of the CDI Cell configuration employed in experiments (Right) Picture of the CDI Cell. Figure 2 Charge current efficiency for different NaCl solution concentration. Test profile: same current rates during charge and discharge. Figure 3. Discharge current efficiency for different NaCl solution concentrations. Test profile: same current rates during charge and discharge. Figure 4. Round-trip efficiency for different NaCl solution concentrations. Test profile: same current rates during charge and discharge. Figure 5. Charge–discharge configuration according to Highly Efficient Operating Procedure for CDI. Brackish water (50 mmol·L-1) in charge step and brine water (3.5 mol·L-1) as washing solution in discharge. Figure 6. Charge, discharge and round-trip current efficiencies for a 50 mmol·L-1 NaCl solution concentration. Test profile: different discharge-rates and fixed Icharge. Figure 7. Charge, discharge and round-trip current efficiencies for a 50 mmol·L-1 NaCl solution concentration. Test profile: different charge-rates and fixed Idischarge. Figure 8. Charge, discharge and round-trip current efficiencies for a 350 mmol·L-1 NaCl solution concentration. Test profile: different discharge-rates and fixed Icharge. Figure 9. Charge, discharge and round-trip current efficiencies for a 350 mmol·L-1 NaCl solution concentration. Test profile: different charge-rates and fixed Idischarge. Figure 10. Electrical work to produce 1m3 of a solution containing 5 mmol·L-1 of NaCl from solutions of different concentrations using capacitive deionization (CDI). The operating cell voltage = 1.2 V, round trip efficiencies 70–95%. Adapted from Anderson at el.


10

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FIG 1

FIG 2



FIG 3

FIG 4


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FIG 5



FIG 6

FIG 7

FIG 8


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FIG 9

FIG 10



254x190mm (150 x 150 DPI)


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New Operational Modes to Increase Energy Efficiency in Capacitive

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