Enhanced Charge Efficiency in Capacitive Deionization Achieved by

ARTICLE pubs.acs.org/JPCC

Enhanced Charge Efficiency in Capacitive Deionization Achieved by Surface-Treated Electrodes and by Means of a Third Electrode Izaak Cohen,* Eran Avraham, Malachi Noked, Abraham Soffer, and Doron Aurbach Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel ABSTRACT: In this paper, we report on attempts to improve the charge efficiency of electrochemical capacitive deionization (CDI) processes without limiting the range of applied potentials, by using surface-treated (oxidized or reduced) activated carbon fiber (ACF) electrodes, and by means of a third, auxiliary electrode. For oxidizing the ACF electrodes, we etched ACFs for different periods of time with a concentrated nitric acid solution. For reduction of the ACFs, several surface treatments were considered: reaction with hydrogen at high temperatures, removal of oxygen surface groups by heating under vacuum at high temperatures, reaction with a concentrated aqueous solution of a sodium borohydride solution, and reaction with a concentrated sodium borohydride solution after oxidation with a concentrated nitric acid solution. To examine the charge efficiency, we elaborated a special flow-through cell (where the solution flows through the ACF electrode) with a silver/silver chloride mesh reference electrode. The feasibility of using surface-treated carbon electrodes and/or of using a third, auxiliary electrode (with which the potential applied to each electrode can be controlled) for enhancing the charge efficiency is discussed and examined. We were able to demonstrate an increase in the charge efficiency of the CDI process by 30% without the need to reduce the potential range of operation.

1. INTRODUCTION The worldwide request for potable water is continuously increasing due to population growth, global warming, and contamination of fresh-water sources. To avoid a water crisis, seawater and brackish water (BW) containing up to 35 000
40 000 and 2000
10 000 ppm of salts, respectively, have to be desalted and purified. The main water desalination methods include expensive evaporation/distillation processes1,2 that require heat produced directly from burning fossil fuel, electrodialysis,3 electrostatic shielding,4,5 and reverse osmosis.6,7 Capacitive deionization (CDI) can serve as an energy-efficient alternative for the desalination of BW.8 In the CDI method, there are two modes related to the flow of the solution, flow-by and flow-through operations (where, in flow-by operation mode, the solution flows in parallel to the electrodes and, in flow-through mode, the solution flows vertical to the electrodes). The flow regime and mode used determine the type of electrodes that have to be used. Using the flow-by mode,9 there is no problem of friction resistance to solution transport. The migration and diffusion processes of ions to the electrodes occur in parallel to the solution flow. The porosity of the electrodes has to be designed so as to facilitate the very fast migration and diffusion of the ions into them. Using the flow-through regime,10,28,29 the solution has to flow through the macropores (500
200 nm) present in the electrodes. Hence, the electrode has to be designed to allow a facile flow of the solution through them. A suitable electrode material in this case is clothing comprising activated carbon fibers. The electrodes for CDI processes usually consist of r 2011 American Chemical Society

activated carbon fibers (ACFs) or carbon Aerogel,11
14 with a specific surface area of hundreds of m2/g. The removal of the salt in these processes is obtained by the highly reversible electroadsorption of ions upon the application of a potential difference between the two electrodes of the cell. Because the process mainly involves electrostatic, nonchemical interactions,15,16 activated carbon electrodes in CDI cells are expected to undergo an impressive number of charge/discharge cycles.17 Upon the desalination process in CDI cells comprising two oppositely charged electrodes, each electrode adsorbs counterions18,19 and desorbs co-ions. However, the adsorbed counterions, at say one electrode, are, in parallel, desorbed as co-ions from the other electrode. Thus, the electrical charge injected into the desalination cell is exploited not only for the desirable adsorption but also for the desorption of the counterions, which results in a lower charge efficiency and less salt removal per unit of charge.20,27 Recently,21
25 it was suggested that ion-exchange membranes be combined within CDI cells (referred to herein as MCDI). In this way, co-ions can be prevented from leaving the electrode region. (The cation-exchange membrane is placed adjacent to the positive electrode, and the anion-exchange membrane is placed adjacent to the negative electrode.) Thus, the charge efficiency can be high due to the elimination of the desorption of co-ions that can be considered as a parasitic process. One should take into account that Received: July 21, 2011 Revised: September 5, 2011 Published: September 05, 2011 19856

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Figure 1. Scheme of the flow-through cell used in this work. All the components are explained (section 2.1).

including ion-exchange membranes to the CDI cell increases the overall reactor volume, enlarges the resistance for solution flow and ion transport, and increases drastically the cost of the membrane compared to that of the cells. Surface chemical treatments of activated carbon electrodes can affect the transport of ions into the carbon electrode pores and, as a consequence, the desalination performance.29,20 The formation of functional surface groups on the ACF surface in the desalination cell during prolonged CDI processes may change significantly the electrodes’ potential of zero charge (PZC) value, which makes the CDI cell no longer symmetric in terms of equal distribution of potential between the two electrodes in the cell. Moreover, activated carbon electrodes that may be initially fully symmetrical, namely, adsorbing equally anions and cations when positive and negative potentials are applied versus their PZC, may lose their symmetry due to formation of functional groups that may impede adsorption of one type of ions. In turn, it is quite possible that the presence of functional surface groups on activated carbon electrodes that possess wide pores (>0.58 nm29) is not supposed to affect too much electroadsorption of any kind of ions into them. One of the goals of this article and related work is to show that controlled implementation of functional groups on the surface of ACF electrodes can direct them to work in domains where adsorption of counterions dominates remarkably over desorption of co-ions, which results in a very high desalination efficiency. This option is further discussed in detail in the following sections. We intend to demonstrate herein that it is possible to enhance the charge efficiencies in CDI processes by forcing the activated carbon electrodes to operate at working potential domains where the desorption of co-ions is negligible . Another important aspect of the work described herein is the use of a CDI cell containing an auxiliary third electrode. The use of a three-electrode CDI cell enabled the initial charging of one of the electrodes, thus forcing the electrodes in the cell to work in domains where the adsorption of counterions pronouncedly dominates the desorption of co-ions.

2. EXPERIMENTAL SECTION 2.1. Cell Design. The CDI cell used is a “flow-through” type electroadsorber in which the solution is forced to flow through the activated carbon cloth electrodes, as shown in Figure 1. The

electrodes are activated carbon cloth comprising activated carbon fibers (ACFs). They have a flange-type design in which flat circles of cloth are enclosed between two PVC covers with PVC dispenser plates to ensure the homogeneous flow of the solution through the whole circular cross section of the carbon cloth electrode. The electrode material was a commercial carbon cloth (ACC-5092-15) from Nippon Kynol, Japan, with a high surface area of 1440 m2/g (BET) obtained by the carbonization and activation of a phenol
formaldehyde polymer. The diameter of the carbon cloth samples was 54 mm, and the thickness was 0.5 mm. A sheet of polypropylene cloth served as a separator, and a sealing gasket was formed by soaking silicon glue into the rims of the separator. The separator exhibited a fairly low resistance to water flow so that the pressure drop across the cell was no more than 200 mbar. It was thick enough to prevent short circuits via protruding carbon fibers. A poly(tetrafluoroethylene) (PTFE) ring spacer formed a 1 mm gap filled by two ACF sheets, which served as the working electrode. The current collectors were the “Grafoil” brand graphite paper. To ensure radial mixing throughout the unit cell, the grafoil sheets were perforated in such a pattern that the holes in each alternate grafoil sheet did not overlap. A third Ag/AgCl mesh electrode (the electrode was made of silver mesh that was deeply anodized in a NaCl solution to yield a thick, adherent AgCl layer) was placed between the two ACF electrodes (Figure 1). The thickness of the AgCl layer was calculated27 to be equivalent to more than 3 times the charge that can be stored in the electrical double layer of the ACF (assuming 1 V applied between the electrodes). To ensure that there was no change in the carbon properties (e.g., surface oxidation) during the charge
discharge cycling, the individual potential of the positively polarized electrode was measured with respect to a Ag/ AgCl wire (another electrode) that was placed adjacent to the positively polarized electrode.29 Before measurements were performed with the CDI cell, periodic cycles of charge and discharge were applied to the cell in order to bring the system into a steady state. 2.2. The System’s Setup. The system’s setup was described in detail in previous work.27,30 The experimental setup includes the potentiostat, electrochemical cell, a conductivity probe, and a peristaltic pump as the main units. The solution contains (for all measurements) about 0.1 M NaCl (>99% pure, Frutarom, Israel). The solution volume was about 50 mL. The conductivity cell, in series with the main electrochemical cell, was connected to a 19857

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Figure 2. Illustration of the operation of symmetric CDI cells whose electrodes are shorted to zero (a) or to a predetermined potential E0 > 0, in order to obtain a better charge efficiency (b). Plots of counterion adsorption and co-ion desorption as a function of the potential applied to the electrodes are displayed. The maximal amount of counterions adsorbed and the co-ions desorbed by the electrodes at the vortex potentials are marked as bars A and B, respectively; the electrodes’ EPZC and the potential of cell shorting, E0, are marked therein.

conductometer from Metrohm, Inc. (model 714). The flow rate was adjusted to 30 mL/min. The conductivity was recorded on a RS232 computer using Labview (version 10). The sampling rate was 5 s. The electrochemical cell was operated by a potentiostat (PGSTAT Autolab electrochemical measuring system) from Eco Chemie, Inc. (The Netherlands). 2.3. Surface Treatments. 2.3.1. Oxidation. The carbon electrodes were oxidized by etching them in a concentrated HNO3 solution (70%, Bio Lab) at room temperature for 1, 2, and 3 h. After etching the carbon, the carbon samples were washed with a sodium bicarbonate solution (>99.5%, Ofer Chemical Laboratory (OCL)) until the pH was 7. The carbon electrodes were then rinsed with deionized water (18.2 MΩ) and dried in a furnace in air at 80 °C for 2 h. The potential of zero charge (PZC) of the electrodes was measured by the immersion procedure that was described in a previous publication.26 Each carbon electrode was initially measured in a NaCl solution while being exposed to an ambient atmosphere for several hours. In these measurements, the stability of the electrode’s PZC was monitored 2.3.2. Reduction. For reduction of the carbon electrodes, we attempted four procedures: 1. The carbon samples were introduced into 1.32 M NaBH4 solution (Sigma Aldrich, 98%) for 2.5 h. The solution was then diluted with a large excess of deionized water. The carbon electrode was dried under a flow of N2 (99.99%, Oxygen and Argon Center, Rehovoth, Israel), in order to prevent oxidation. 2. Reducing the carbon samples by hydrogen at elevated temperatures (∼1000 °C). 3. Evacuation of the carbon samples at elevated temperature (1000 °C). 4. As a first step, oxidation of the carbon samples with concentrated HNO3 for 2 h. As described in section 2.3.1, the carbon surface etched by nitric acid should be covered with stable oxygen-containing groups. These groups can be considered as reactive sites that should be more vulnerable to further reduction by hydrides and even hydrogen at elevated temperatures. Hence, as a second step, the oxidized carbon can be reduced with NaBH4, as indicated above. At the end of each of the above-described procedures, we measured the electrode’s PZC by an immersion method in a NaCl solution (see ref 26). The stability of the PZC during several hours was also monitored.

3. RESULTS AND DISCUSSION 3.1. Desalination with Pristine Activated Carbon Cloth Electrodes in a CDI Cell. For simplification, we can treat the

desalination cell as a simple equivalent circuit analog of two identical capacitors in series (electrical double-layer (EDL) capacitors) that possess the same capacity (CP and CN, respectively), with a solution resistance in between (Rs). Let us assume that the electrodes are ideally polarized so that only electrostatic interactions take place, when a potential difference is applied within a potential lower than that which causes water electrolysis. Since CP (F/g) = CN (F/g), any charge delivered between the electrodes (dq) is accompanied by an equal symmetrical change at the potential difference, dEP (V) =
dEN (V). (CP and EP refer to the positively polarized electrode and CN and EN to the negatively polarized electrode). Assuming that there is no change in the carbon electrode properties (e.g., oxidation of the carbon surface), we can assume that, during discharge of the CDI cell, the potential of each electrode with respect to a reference electrode (denoted herein as E0) (1) is E0 ðVÞ ¼ ðEPZC P ðVÞ þ EPZC N ðVÞÞ=2

ð1Þ

Since EPZC P = EPZC N in a typical symmetric CDI cell E0 ðVÞ ¼ EPZC P ðVÞ ¼ EPZC N ðVÞ

ð2Þ

where E0 is the potential of each individual electrode when the electrodes are short-circuited with respect to a reference electrode and EPZC is the immersion potential of the electrodes.27 Note that E0 is the individual potential of the electrodes when being discharged, while the potential between the electrodes (EP
EN) when being short-circuited (denoted herein as Edischarge) equals zero. The electroadsorption/electrodesorption behavior of a typical symmetrical cell is illustrated in Figure 2a, which shows a schematic dependence of the amount of counterions adsorbed and co-ions desorbed onto/from the positive and negative electrodes on the electrode’s potential. The bars denoted as A (related to both electrodes) mark the amount of counterions adsorbed on the electrodes at the maximal potential applied, whereas B bars in Figure 2a mark the amount of co-ions desorbed from the electrodes at the maximal potential applied. The charge that flows in the cell is consumed by both adsorption and desorption processes (assuming that there is no parasitic process in the cell), and hence, the total charge involved translated to moles (Ntotal) is equal to the amount of the counterions adsorbed 19858

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Figure 3. Illustration of the ions’ adsorption/desorption behavior of a typical activated carbon electrode before and after oxidation (wide pore carbon, > η ð2aÞ A2bðmol=vÞ þ B2bðmol=vÞ A2aðmol=vÞ A2aðmol=vÞ þ B2aðmol=vÞÞ

ð4Þ

Hence, to avoid working at potentials where the desorption of coions from the electrodes is significant and to obtain higher charge efficiencies, it is recommended that the electrodes be forced to work at potentials where E0 . EPZC (for fully symmetric cells). However, this mode of operation has a significant drawback because the cell consumes a lesser amount of charge. This obviously results in a lower amount of desalted water per charge
discharge cycle.28 This is well demonstrated by comparing the A bars in Figure 2a,b: A2a . A2b. However, it may be possible to achieve, by surface treatments to the carbon electrode as depicted in the following section, a high charge efficiently in CDI processes without limiting the capacity of the process. 3.2. Desalination with Surface-Treated Carbon Electrodes in the CDI Cells. In a previous work, we explored the influence of the presence of oxygen-containing surface groups on carbon electrodes in CDI processes. It was shown that, when using widepore carbon electrodes (>0.58 nm), the only effect measured was the change in the location of the electrode’s PZC (positive shift), while the capacity was not affected and the symmetry of the ionadsorption behavior with respect to the new PZC was preserved,29 namely, Γ+(E + D) = Γ+(E) and Γ
(E + D) = Γ
(E), where Γ+ and Γ
are the moles of cations and anions adsorbed on the negative and positive electrodes, respectively, as a function of the potential E(V), applied to the CDI cell, and D is

2. E0 ðVÞ ¼ ðEPZC P ðVÞ þ EPZC N ðVÞÞ=2 ¼ 0 Figure 4 demonstrates schematically the ideal situation in which the CDI cell comprises a positive electrode whose EPZC was negatively shifted by reduction and a negative electrode whose EPZC was positively shifted by oxidation so that the absolute values of these shifts were the same (conditions 1 and 2 above). When such a cell is loaded with a solution, there is a potential difference between the electrodes (both of their PZC values), which is equal to twice the shift in their PZC values. Hence, when the cell is initially shorted to E0, the negative electrode’s potential is shifted to a potential lower than EPZC N and the positive electrode is shifted to a potential higher than EPZC P. In the symmetrically, ideal situation described in Figure 4, the absolute values of these potential shifts of the electrodes upon initial shorting are equal. As illustrated in the figure, when the CDI cell is polarized, the potential applied (PE) is divided equally between the two electrodes (|EP| = |EN|, EP + EN = E). The electrodes are forced, due to their initial potential shift, to work at potential domains where the adsorption of counterions pronouncedly dominates the desorption of co-ions. Hence, AN . BN; AP . BP (see Figure 4). In the ideal symmetrical situation illustrated in this figure, AN = AP and BN = BP, where BN + AN or BP + AP are equal to the total charge involved (translated to molar values). Note that, if the oxidation and reduction processes do not lead to a symmetrical shift in the PZC of the electrodes, the potential applied to the cell is not divided equally between the electrodes. Hence, while E = EP + EN, EP - EN. In turn, the charges involved in the electrode processes are equal. Hence, EP = EN and AN + BN = AP + BP. For preserving electroneutrality, AP + BN = AP + BP (i.e., the total changes in the amount of positive and negative ions in the solution due to the polarization of the cell have to be the same. 3.3. Symmetric Oxidation and Reduction of Activated Carbon Fibers. The potential of the ACF electrode after immersion (Ei) (with respect to a reference electrode) is given by26 Zσ Ei ¼ E0 þ χs þ 0

1 dσ þ χ Cd

ð5Þ

where Cd is the electrical double-layer differential capacity of the electrode
solution interface, χs is the surface potential that arises from the solvent polarization, and E0 is the sum of potential differences that exist at the interface between the reference electrode and the ACF electrode. χ is the surface potential that arises from 19859

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Figure 4. Illustration of the operation of a nonsymmetric CDI cell whose positive electrode is reduced (its PZC was negatively shifted) and whose negative electrode is oxidized (its PZC was positively shifted). The electrodes are shorted to zero (E0 = 0). Because of the symmetric shift in the PZC of the electrodes to opposite directions, the potential applied to the cells is equally divided between the electrodes, which are forced to work at potential domains in which counterion adsorption is dominant. Thereby, the charge efficiency of the CDI process is very high. The maximal amount of counterions adsorbed and the co-ions desorbed by the electrodes at the vortex potentials are marked as bars A and B, respectively; the electrodes’ EPZC and the potential of cell shorting, E0, are marked therein.

Figure 5. (A) Potential/time profiles of the HNO3-etched activated carbon electrodes, during different periods of etching time versus a Ag/AgCl reference electrode. (B) Typical potential/time profiles of the reduced activated carbon electrode versus a Ag/AgCl reference electrode (see reduction procedures in the text). A 0.1 M NaCl aqueous solution was used.

surface dipoles that may be controlled, like in this work, by surface pretreatments. Assuming that charge transfer across the interface, during immersion, does not occur, the immersion potential is equal to the PZC of the ACF. The immersion potential is first measured and correlated with the degree of surface oxidation (positive shift compared to surface-untreated ACF) and surface reduction (negative shift compared to surface-untreated ACF). Any change of the measured potential between the ACF and the reference electrode on time should be attributed to the instability of the surface groups formed on the carbon, in the ambient atmosphere. Figure 5A shows the immersion potentials (as explained in section2.3.1) of the etched electrodes with respect to a Ag/AgCl reference electrode in 0.1 M NaCl solution in an ambient atmosphere versus time. It can be seen that the positive shift in the PZC is higher (compared to the pristine electrode) as the carbon electrode was etched for a longer period. Moreover, the oxygen surface groups formed on the carbon seem to be stable, judging from the E versus t plots in Figure 5A. The methods mentioned for reducing (shown in section 2.3.2) led to a significant negative shift of the electrode’s PZC. However, when

the reduced ACF was exposed to the ambient atmosphere, a gradual decrease in the potential (with respect to a Ag/AgCl electrode) was obtained (Figure 5B). This means that the reduced carbon electrodes are not stable at the ambient atmosphere. Because we failed to stabilize the reduced carbon electrodes, we turned to another approach to bring the PZC of the positive electrodes to the desirable low value by using a CDI cell composed of a surface-oxidized electrode and a precharged electrode, as described at the next section. 3.4. Improving the Behavior of the CDI Cells Using an Auxiliary Built-In Electrode. It is possible to operate a CDI cell containing one oxidized electrode (which functions as the negative electrode) and a positive electrode that is, in fact, a pristine, untreated activated carbon electrode, where Edischarge = 0, and obtain a high charge efficiency without limiting the applied potential window. This can be achieved by means of a third auxiliary electrode, as follows. Prior to the application of the potential difference between the electrodes in the CDI cell, the positive electrode is negatively polarized, with the assistance of the third electrode. We can assume no change in the carbon electrode’s properties, a negligible self-discharge of the positive electrode after its charging (to a negative potential), and no excess of charge (from prior charging) on the negative electrode. 19860

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The Journal of Physical Chemistry C Figure 6 illustrates schematically the proposed operation. EPZC (the PZC of the negative electrode) was shifted positively by surface oxidation, and it is stable in solution. The positive electrode was negatively charged versus the Ag/AgCl auxiliary electrode to a negative potential that is comparable in its absolute value (e.g., 250 mV) to the positive shift in the PZC of the negative electrodes due to its surface oxidation. When the cell is shorted, as a first step, the positive electrode may return to a potential close to its PZC (confirmed by measurements vs the Ag/AgCl electrode) and the negative electrode is negatively charged, thus being shifted to a potential lower than its PZC (EPZC N). This process forces this electrode to adsorb cations and desorb most of the anions contained in its pores (see the wellexplained illustration in Figure 6). Hence, when the CDI cell is polarized after this initial step, the negative electrode works only in a potential domain where the cation adsorption dominates N

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almost completely over anion desorption. The positive electrode works less effectively (see Figure 6), because its initial potential, which is, in fact, E0, the cell shorting potential, is close to its PZC. Because only the PZC of the negative electrode was shifted (by oxidation), while shifting symmetrically the PZC of the positive electrode by reduction is much harder (reduced activated carbon electrodes are never stable in ambient condition, namely, exposure to atmospheric oxygen), the CDI cell ceases to be symmetrical. Hence, the potential E, applied to the cell, is not divided equally between the electrodes (EN and EP are different, but EN + EP = E). What has to be equal is the charge on the two electrodes; hence, QP = QN and AP + BP = AN + BN, A and B (see the relevant bars in Figure 6) being the moles of ions adsorbed and desorbed (respectively) by the electrodes. Despite the nonsymmetrical operation of the CDI cell using this mode of operation, it increases the charge efficiency of the desalination process by CDI. Figure 7A presents the concentration/time and current/time profiles of a CDI cell, consisting of identical untreated electrodes. The charge
discharge cycles were between 0 and 900 mV. Each cycle lasts 60 min. The charge efficiency (η) was calculated according to eq 6 η¼

Figure 6. Illustration of the operation of a nonsymmetric threeelectrode CDI cell whose negative electrode is oxidized (its PZC was positively shifted) and whose positive electrode comprised untreated (pristine) activated carbon. It contains also a Ag/AgCl auxiliary/ reference electrode. Using this electrode, the positive electrode was initially polarized to a negative potential and the cell was shorted. Because of the shift in the PZC of the negative electrode because it was oxidized, it is forced to work at potential domains in which counterion adsorption is dominant, which leads to a high efficiency of the CDI process. The cell is not symmetrical, and hence, the potential applied to the cell is divided between the electrodes unequally. Only the charges involved in the electrodes are equal (as marked therein). The maximal amount of counterions adsorbed and the co-ions desorbed by the electrodes at the vortex potentials are marked as bars A and B, respectively; the electrodes’ EPZC and the potential of cell shorting, E0, are marked therein.

V ðCi
Cf ÞF Z I dt

ð6Þ

where V is the solution volume, F is the Faraday constant, Ci is the solution concentration at the beginning of the charging step, of the solution at the end of the charging Cf is the concentration R step, and I dt is the amount of the charge passed in the cell upon charging. In these experiments in which untreated carbon electrodes were used, the charge efficiency was only 0.58. At the second stage, we used a CDI cell with the Ag/AgCl auxiliary electrode (placed between the two electrodes) whose negative electrode was oxidized. Its PZC was found to be about +250 mV vs Ag/AgCl, shifted positively from the original PZC of these electrodes (when used as pristine, untreated electrodes) by a similar potential. The positive electrode was an untreated carbon electrode. Prior to the application of potential differences between the electrodes, the pristine electrode was negatively polarized down to about
250 mV with respect to the Ag/AgCl electrode. The cell was shorted, and then potential difference

Figure 7. Periodic concentration/time and current/time (small window) profiles measured of a symmetrical CDI cell (A), operating as illustrated in Figure 2a, and of a nonsymmetrical CDI cell (B), consisting of an auxiliary Ag/AgCl electrode, an oxidized negative electrode, and a precharged untreated positive electrode, operating as illustrated in Figure 6. A 0.1 M NaCl aqueous solution was used. 19861

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The Journal of Physical Chemistry C cycles between 0 and 900 mV were applied to the CDI cell (each cycle lasts 60 min). We limited the experiments to only a few charge
discharge cycles, because the solution circulated through the CDI cell and was open to the air. Under prolonged cycling, the evaporation of water leads to a distortion in the cell’s response. However, the few cycles presented are enough to prove the concept that we intended to demonstrate. Figure 7B presents the concentration/time and current/time profiles of the CDI cell described above. The charge efficiency was found to be about 0.93. This improvement in the charge efficiency is very significant. Moreover, the concentration/time profiles in these experiments were very reproducible, meaning that the excess of charge that was loaded initially on the positive (untreated) electrode was preserved (no self-discharge). It should be noted that the level of desalination in these experiments was not high, due to the relatively low ratio between the total electrode pore volume and the solution volume in the flow system. It is clear that an impressive level of desalination can be realized in optimized “flow-through” cells, when the electrode pore volume/solution volume ratio in the cell is optimized (i.e., big enough). However, the goal of this work was to prove a concept rather than exhibit prototype desalination systems (by CDI).

4. CONCLUSION In this work, attempts were made to improve the charge efficiency of CDI processes without using ion-exchange membranes and without limiting the working potential window. The limitation of a usual CDI process that uses a symmetrical cell comprising identical activated carbon electrodes and the way to increase the charge capacity by limiting the electrodes’ operational voltage domains are well illustrated in Figure 2. The idea was to negatively shift the positive electrode’s PZC by surface reduction and to positively shift the negative electrode’s PZC by surface oxidation of the activated carbon electrodes. When these are achieved, the electrodes are naturally forced to work at potential domains in which adsorption of counterions (the desirable CDI process) pronouncedly dominates over co-ion desorption (to be considered as a parasitic process), while they can be shorted to E0 = 0. Hence, a high charge efficiency can be obtained not on account of effective desalination. This concept is very clearly illustrated in Figure 4. We demonstrate herein that it is possible to positively shift the PZC of the activated carbon electrodes by their controlled oxidation in HNO3 solution. The positive shift in the PZC thus obtained remains very stable in NaCl solutions. In contrast, it was impossible to obtain a stable negative shift in the PZC of these electrodes by any reduction process that we attempted. Surface reduction of these electrodes leads indeed to an initial negative shift in their PZC; however, they are reoxidized while being exposed to an ambient atmosphere (due to reactions with oxygen). Consequently, we developed an unsymmetrical CDI cell containing an auxiliary Ag/AgCl electrode, whose negative electrode was initially oxidized, and its PZC was positively shifted (and remained stable). The positive electrode in the cell was a pristine, untreated activated carbon electrode, which was initially polarized cathodically to a potential lower than its PZC, using the auxiliary electrode as counter and reference electrodes for this process. Shorting the cell after this charging brings the negative electrode to work only at voltage domains were adsorption of counterions is the clearly dominating process. Hence, the cells can be operated at wide potential domains, being shorted to zero. The above concept is clearly illustrated and demonstrated in Figure 6.

ARTICLE

A relevant suitable experimental setup was used in this work, including two- and three-electrode cells. CDI experiments were carried out with both fully symmetrical cells, comprising untreated activated carbon electrodes (operating as demonstrated in Figure 2a) and nonsymmetrical cells. The latter comprised an oxidized negative electrode, a Ag/AgCl auxiliary/reference electrode, and an untreated positive electrode than was initially polarized to a low potential (using the auxiliary electrode). They were operated as illustrated in Figure 6. The work described herein proved indeed the operational concept described in Figure 6. A charge efficiency of 0.93 was thus obtained, not on account of the level of desalination per cycle, compared to a charge efficiency of 0.58 and a lower desalination capacity per cycle, exhibited for CDI processes with symmetrical cells containing untreated, pristine electrodes (operated according to the scheme of Figure 2). The possibility to use the above concepts in practical CDI processes is further examined.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

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