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Semi-continuous capacitive deionization using multi-channel flow stream and ion exchange membranes
Desalination 425 (2018) 104–110
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Semi-continuous capacitive deionization using multi-channel flow stream and ion exchange membranes
MARK
Choonsoo Kima, Pattarachai Srimuka,b, Juhan Leea,b, Mesut Aslana, Volker Pressera,b,⁎ a b
INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Capacitive deionization Semi-continuous desalination Ion exchange membrane Multi-channel flow stream
Capacitive deionization (CDI) is a promising desalination process, but conventional static electrode CDI operates by sequentially cycling through charging and discharging to produce fresh and concentrated water, respectively. However, an effective continuous operation is desirable for optimized system operation. Here, we report a semicontinuous desalination process with a novel modified CDI cell architecture using a multi-channel flow stream and ion exchange membranes (MC-MCDI). This MC-MCDI consists of two channels including side and middle channels with a pair of cation and anion ion exchange membranes where the feed streams can be separately distributed without mixing. The MC-MCDI design allows semi-continuous production of clean water since the separated middle and side channels are alternately desalinated and regenerated: one channel is being desalinated while the other channel is regenerated. Therefore, the cell can produce clean water during both charging and discharging, enabling semi-continuous operation. In addition, with the benefit from similar cell configuration with membrane CDI, the MC-MCDI design exhibits a high salt adsorption capacity (SAC) of 22 ± 2 mg/ g and charge efficiency of 90 ± 2% at middle and side channels during charging and discharging with reverse voltage operation (cell voltage of +1.2 V vs. −1.2 V).
1. Introduction Capacitive deionization (CDI) has emerged as a promising desalination technology for brackish water due to its ability to attain high
⁎
energy efficiency [1–5]. A conventional CDI cell employs a pair of porous carbon electrodes (or multiple pairs for scale-up applications) including activated carbon or carbon cloth [6–9]. Typically, feed water flows through a spacer between a pair of carbon electrodes (flow-by
Corresponding author at: INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany. E-mail address: [email protected] (V. Presser).
http://dx.doi.org/10.1016/j.desal.2017.10.012 Received 7 August 2017; Received in revised form 16 September 2017; Accepted 8 October 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.
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Fig. 1. Multi-channel membrane CDI (MC-MCDI) cell designed with flow channels and ion exchange membranes: schematic diagram (A), cell components (B) and exploded view of the cell (C).
regeneration and, thereby, accomplishes more continuous operation. In 2017, Lee et al. introduced a “rocking-chair desalination battery” system introduced using the Prussian blue materials including sodium nickel hexacyanoferrate (NaNiHCF) and sodium iron hexacyanoferrate (NaFeHCF) [26]. This novel concept requires the physical change of the solution in the two flow channel compartments and, thus, allows desalination during the charging and discharging step. Thereby, this concept shares similarities with the desalination battery of Pasta et al. (Ref. [24]) and Capmix systems for energy extraction from salinity gradients (Ref. [28]). Fully continuous CDI operation was enabled by use of carbon suspensions, leading to the emergence of flow electrode CDI (FCDI) [29,30]. The use of carbon suspensions as flowable electrodes had previously been applied to fundamental studies [31], chemical reactors [32], and energy storage [33]. FCDI enables to continuously generate fresh water since desalination and electrode regeneration can be decoupled by using two physically separated cells [34–36]. The continuous supply of uncharged carbon particles further leads to a higher ion removal capacity; therefore, FCDI can even desalinate feed water with higher concentration compared to static electrode CDI [29,37]. The intrinsic limitation of low carbon mass loading in flow electrodes can further be overcome by the use of fluidized beds (FB-CDI) [38]. Although suspension electrodes have shown remarkable innovations and improvements [39], the technology suffers at present from energy cost associated with pumping, possible clogging on long-term operation, and low conductivity compared to conventional carbon electrodes [1,40]. While these issues may be overcome in the future, present-day research still strives for alternative systems that enable fully continuous or improved semi-continuous CDI operation. Ideally, such technologies would be based on a conventional CDI cell design with small
cell) [10]. Commonly, CDI employs cyclic operation with a charging and a discharging step. The charging of the cell accomplishes desalination via ion electro-adsorption and the discharging accomplishes electrode regeneration by electro-desorption [1,2]. During galvanostatic or potentiostatic charging, salt ions are electro-adsorbed by forming an electrical double layer (EDL) at the interface between porous electrode and water, leading to salt removal from the feed stream. The adsorbed ions are released from the electrodes during the discharging step, which is operated with reversed polarity, or short circuit resulting in a concentrated effluent stream [11,12]. Depending on the cell architecture, type of electrode materials, presence of membrane, and operation conditions, there are many types of static electrode architectures including flow-by CDI [13], flow-through CDI [14,15], membrane CDI [16], inverted CDI [17], hybrid CDI [18,19], pseudocapacitive CDI [20–22], cation desalination [23], and desalination batteries [24–26]. Conventional static electrode CDI cells operate by sequential charging and discharging to produce fresh and concentrated water, respectively. From a system engineering point of view, intermittent operation is unfavorable and continuous production of clean water is much more desirable. In addition, the repeated cycling between adsorption to desorption leads to the partial mixing of just-produced fresh water with concentrated water in the effluent stream. Thus, auxiliary devices to control conductivity (e.g., electronic valve and conductivity measurement device) are required, triggering additional installment and maintenance costs [27]. To overcome these limitations, novel concepts have been reported [26,27]. For example, the “merry-goround” operational by the use of moving wire-type capacitive electrodes was reported by Porada et al. in 2012 [27]. The latter concept physically moves the wire electrodes after desalination elsewhere for 105
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The charge efficiency was estimated by Eq. (2):
modifications and by employing conventional CDI electrode materials (i.e., activated carbon) to expedite the further technology development. Motivated by these challenges, this work introduces semi-continuous desalination without mixing of effluent freshwater and concentrated water using a recently-reported CDI cell architecture with a multi-channel flow stream and ion exchange membrane (MC-MCDI) [41]. The MC-MCDI cell consists of three flow channels including two side channels and one middle channel, separated by a pair of ion exchange membranes (one cation and one anion ion exchange membranes). We had initially designed this cell to capitalize on enhanced desalination capacity of the middle channel by increasing the side channel ion concentration [41]. As we show in this work, operation of the MC-MCDI cell also allows semi-continuous production of clean water since the separated middle and side channels are alternately desalinated and regenerated (i.e., one channel is desalinated while the other channel is regenerated). For this, the same saline concentration is used in all channels, whereas our previous work focused on operation with salinity gradients. In addition, the outlets are separated between middle and side channels so that mixing of just-produced freshwater with concentrated water in the effluent is inhibited.
Charge efficiency =
Especific =
V m
∫t
t2
Idt
1
(3)
where I is the applied current, t2 − t1 is the operating time, V is the voltage during operation, and m is the mass of both activated carbon cloth electrodes. Note that the activated carbon cloth did not contain any binder; the active mass is therefore identical with the electrode mass. The overall energy consumption of MC-MCDI was calculated with the summation of energy consumption for middle and side channels. The experiments were performed in triplicated and the results showed reproducibility of the data, with deviations of about 5–10%. 3. Results and discussion
2.1. Cell design
3.1. Characterization of ACC electrode and its conventional CDI performance
The cell configuration of MC-MCDI was first introduced by us in Ref. [41] and is additionally depicted in Fig. 1. The MC-MCDI cell is composed of two side channels (5 × 5 × 0.2 cm3, acrylic glass) and one middle channel (5 × 5 × 0.4 cm3, acrylic glass). The channels are separated by a pair of ion exchange membranes (6 × 6 cm2, Neosepta CMC and AMX, Astom Corporation), and their volumes are 10 mL. A titanium mesh (5 × 5 cm2, H-TEC system GmbH) was employed as current collector. Activated carbon cloth (ACC, Kynol-ACC-507-20) was cut to rectangular shape with 4 × 4 cm2 (total weight of an electrode pair = 280 mg) and was used as electrode. More data on the ACC electrodes is found in our recent paper (Ref. [6]) and in Supplementary information, Fig. S1.
The key properties and performance metrics of activated carbon cloth Kynol-ACC-507-20 were briefly summarized in Supplementary information, Fig. S1 and align with our previous study published in Ref. [6]. As can be seen from scanning electron micrographs found in Supplementary information, Fig. S1A, the carbon cloth consists of yarns composed of several individual fibers (ca. 10 μm). The activated carbon cloth has a very high carbon content (> 95 mass%) and a large specific surface area (ca. 1939 m2/g, average pore size = 0.96 nm). Raman spectroscopy reveals the incompletely graphitic nature of the carbon with peak positions of D- and G-modes at 1340 cm− 1 and 1604 cm− 1, respectively, and a full-width at half maximum (FWHM) around 122 cm− 1 and 53 cm− 1 for D- and G-mode, respectively [43–45]. Electrochemically, the ACC electrode exhibits almost ideal capacitive behavior evidenced by the rectangular CV shape when tested in aqueous 1 M NaCl (Fig. S1E) with a maximum specific capacitance of 123 F/g at 0.1 A/g (Fig. S1F) [46]. The salt adsorption capacity (SAC) and charge efficiency of ACC electrode obtained in a conventional CDI and MCDI setup with the design outlined in Ref. [47] are shown in Supplementary information, Fig. S2. The ACC electrode reveals a typical conductivity profile in CDI operation with an inflow concentration of 5 mM NaCl (inset in Fig. S2). This indicates that ions are adsorbed and desorbed in the way that is expected from a conventional CDI material. The maximum SAC and charge efficiency are 12.5 ± 0.5 mg/g and 77 ± 0.3% on average over 100 cycles, respectively. When adding a pair of ion exchange membranes in a conventional CDI setup, the ACC electrodes exhibit a maximum SAC of 14.6 ± 0.5 mg/g (18.7 ± 0.3 mg/g), a charge efficiency of 84 ± 2% (86 ± 2%) when we use a cell voltage of + 1.2 V vs. 0 V (or: + 1.2 V vs. − 1.2 V). Therefore, our ACC electrodes show a very promising performance in the range expected for a nanoporous carbon with well-developed pore structure [48].
2.2. Desalination performance characterization of MC-MCDI The desalination performance of MC-MCDI was benchmarked in saline media with 5 mM NaCl (dissolved oxygen content: ca. 8 mg/L) supplied by a peristaltic pump (volumetric flow rate = 10 mL/min at middle and side channels). The recycling volume of middle and side channels was 10 L. The conductivity profiles were measured at the effluent of the middle and side channels. For the activation of ACC electrode before desalination test, the electrode conditioning was performed with five polarization cycles from +1.2 V to − 1.2 V cell voltage (half-cycle duration for 30 min). While the first half-cycle was always using a cell voltage of + 1.2 V, we used different cell voltages for the second half-cycle; namely, values from − 1.2 V to 0 V. In addition, the influence of flow rate on the desalination performance was investigated by CDI measurement with various feed flow rates ranging from 0 mL/min to 30 mL/min at middle and side channels with cell voltage of + 1.2 V vs. −1.2 V, respectively. The salt adsorption capacity (SAC) was calculated by Eq. (1): k n=0
(2)
where MNaCl is molecular weight of NaCl, F is the Faradaic constant and I is the current during constant cell voltage operation. The energy consumption for middle and side channels was estimated further by Eq. (3) for constant voltage operation and expressed as energy consumption per removed ion (kT) [42]:
2. Experimental description
∑ (C0 − Cn)·ν·MNaCl·(tn − tn−1)/m
SAC ⎛ ∫ Idt ⎞ /⎜ ⎟ MNacl ⎝ F ⎠
(1)
3.2. Semi-continuous desalination with MC-MCDI
where C0 and Cn are the initial and effluent concentrations converted from conductivities, ν is the volumetric flow rate, MNaCl is the molecular weight, tn − tn − 1 is the time interval for conductivity measurement, and m is the mass of both activated carbon cloth electrodes. Note that the activated carbon cloth electrodes are binder-free and free-standing, so the total electrode mass is equal to the mass of activated carbon.
Fig. 2 depicts the profiles of effluent conductivity and pH of the middle channel (MC) and side channels (SC) obtained with cell voltage of + 1.2 V vs. − 1.2 V, indicating the semi-continuous desalination process of MC-MCDI. As can be seen in Fig. 2A, the conductivity profiles of water effluent from the MC exhibit ion electro-adsorption/electro106
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Fig. 3. Salt adsorption capacity (SAC), charge efficiency, and energy consumption (kT) at middle and side channels (MC and SC) of multi-channel flow stream membrane CDI (MCMCDI) with constant cell voltage operation of + 1.2 V vs. −1.2 V. Desalination was carried out at 5 mM NaCl with 10 mL/min flow rate for 30 min half-cycle duration.
Fig. 2. Representative effluent conductivity (A) and pH (B) profiles at middle and side channels (MC and SC) of multi-channel membrane CDI (MC-MCDI) with constant cell voltage operation of + 1.2 V vs. − 1.2 V. Desalination test was conducted at 5 mM NaCl with 10 mL/min flow rate for 30 min half-cycle duration.
desorption during polarization at + 1.2 V (first half-cycle) and at − 1.2 V (second half-cycle), respectively. When comparing the profiles of MC and SC, we see that both channels depict opposite sorption cycles: the conductivity in MC is rising when the SC is depleted of ions and vice versa. This implies that MC-MCDI is alternately desalinated and concentrated at the separated MC and SC allowing semi-continuous production of clean water. Additionally, the outlets are separated by channels and membranes, and thus, when the effluent can be switched with an auxiliary valve between the two channels, there is no mixing of just-produced fresh water with concentrated water in the effluents of MC and SC. The pH profiles of water effluent from MC and SC are shown in Fig. 2B. The pH increase and decrease in MC (pH decrease and increase in SC) during polarization at + 1.2 V and at − 1.2 V is attributed to the oxygen reduction reaction (ORR) and/or carbon oxidation [49,50]. Overall, the observed pH changes of MC and SC are small (ΔpH < 1) compared to the pH change in conventional CDI (ΔpH > 3–4) [6,49]. Therefore, the use of ion exchange membranes effectively suppresses parasitic Faradaic reactions and this well-known effect has previously been reported for MCDI systems [51]. The desalination performance of MC-MCDI is displayed in Fig. 3. With cell voltage of +1.2 V vs. − 1.2 V, the MC and SC both show an average SAC of 22 ± 2 mg/g and an average charge efficiency of around 90%. The energy consumption per removed ion in MC and SC is 12 ± 0.5 kT per channel and the overall energy consumption of the MC-MCDI cell during one cycle is 24 kT. This excellent performance is accomplished because the use of ion exchange membranes (MCDI) capitalizes on blocking co-ion repulsion and reverse voltage operation
Fig. 4. Schematic diagram for operating mechanism for semi-continuous desalination process of multi-channel flow stream membrane CDI (MC-MCDI) with cell voltage of + 1.2 V vs. − 1.2 V. AEM: anion exchange membrane; CEM: cation exchange membrane; MC: middle channel; SC: side channel.
[41]. The mechanism behind the semi-continuous desalination process is schematically depicted in Fig. 4. During the first half-cycle using a cell voltage of + 1.2 V (Fig. 4A), co-ions are released from pores in the carbon electrodes at the SC. The released co-ions in the SC are blocked by the IEMs and prevented from crossing over to the MC. To accomplish charge neutrality, additional ion transportation of counter-ions is triggered from the MC into the two SCs. Hence, the SC concentration is 107
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Fig. 5. Effect of negative polarization ranging from − 1.2 V to 0 V on conductivity profiles of middle and side channels (A and B) while positive polarization was kept to +1.2 V, salt adsorption capacity (SAC), charge efficiency (C), and energy consumption (kT) (D) of multi-channel flow stream membrane CDI (MC-MCDI). Desalination test was conducted at 5 mM NaCl with 10 mL/min flow rate for 30 min half-cycle duration with constant charging cell voltage of + 1.2 V.
consumption of the SC gradually decreased (no significant difference in energy consumption at MC) with change of the cell voltage during the second half-cycle from − 1.2 V to 0 V. Note that SC accomplishes desalination with a SAC of around 3 mg/g during discharging with cell voltage of 0 V, possibly leading to energy recovery. Thus, by operating at a cell voltage of + 1.2 V vs. 0 V, the overall energy consumption of MC-MCDI can be < 12 kT (the energy recovery ratio was approximately 45%: applied energy of 12 J/g and recovered energy of 5.4 J/g). We further investigated the influence of the feed flow rate on the desalination performance (Fig. 6). In our experiment, the flow rate of the MC was kept constant at 10 mL/min when the SC feed flow was varied (and vice versa: constant SC flow rate of 10 mL/min while varying the MC feed). A more detailed outline of the variation of the desalination performance with different flow rates is found in Supplementary information, Fig. S3. As shown in Fig. 6A–B, variation of the SC flow rate in the range of 0–30 mL/min has only a small effect on the desalination performance of the MC; the latter showed an average SAC of around 21 mg/g with 90% charge efficiency and 12 kT energy consumption. We also found only a small influence of the MC feed flow rate on the desalination performance of the SC effluent stream, with an average SAC of around 20.5 mg/g with 90% charge efficiency and 12 kT energy consumption (Fig. 6C–D). Finally, we characterized the performance stability during continuous cycling (100 cycles; Fig. 7). Over 100 cycles, MC and SC exhibit a highly stable desalination performance with SAC of 22 ± 2 mg/g (SACmax of 23 mg/g; SACmin of 20 mg/g) and charge efficiency over 90 ± 2%. In contrast, a conventional CDI suffers from a poor longterm performance stability in an aerated solution; only in case of deaerated solution, long-term performance in CDI also is stable
increased while that of MC is decreased leading to a high charge efficiency (> 90%). During the second half-cycle at a cell voltage of − 1.2 V (Fig. 4B), the co-ions released from the carbon electrodes migrate towards the MC and in this case, they can pass through IEMs leading to the desalination of SC (increased concentration in the MC). The MC-MCDI cell enables an overall SAC of ca. 44 mg/g of MC and SC per one cycle. This value is much higher than SAC values (13–18 mg/g) obtained in a conventional CDI cell or a conventional MCDI cell with the same material (Supplementary information, Fig. S2). Consequently, adopting a MCDI cell concept but adding two additional flow channels (i.e., SC), the MC-MCDI architecture enables semi-continuous desalination process with excellent desalination performance. In a next step, we evaluated the influence of the cell voltage of the second half-cycle between − 1.2 V and 0 V on the MC-MCDI performance (Fig. 5). From the conductivity profiles at MC and SC in Fig. 5A–B, as the cell voltage during the second half-cycle is changed from − 1.2 V to 0 V, the amplitude of electro-adsorption/desorption peaks seen in the conductivity profiles of water effluent from MC and SC are decreased. This decrease is more pronounced for the SC compared to water effluent from the MC. Accordingly, the SC SAC (Fig. 5C) is reduced from 22 mg/g to 3 mg/g with a decrease of the charge efficiency from 91% to 26%. For comparison, the decrease of SAC and charge efficiency at MC is significantly smaller: reduction of SAC from 22 mg/g to 13 mg/g and charge efficiency from 90% to 80%. The high cell voltage during the second half-cycle (i.e., −1.2 V) leads to a larger number of expelled co-ions at the carbon electrode and flow stream in SC (full ion depletion at SC) than a low cell voltage at the second halfcycle (i.e., 0 V). Overall, the energy consumption per removed ion of MC-MCDI (Fig. 5D) is 12–24 kT in this cell voltage range. The energy 108
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Fig. 6. Effect of feed flow rate at side and middle channel (SC and MC) on conductivity profiles and salt adsorption capacity (SAC), charge efficiency and energy consumption (kT) at MC (A and B) and SC (C and D) of multi-channel membrane CDI (MC-MCDI), respectively. Desalination test was conducted at 5 mM NaCl for 30 min half-cycle duration with constant cell voltage operation (+ 1.2 V vs. − 1.2 V), and the flow rate at MC (or SC) was kept with 10 mL/min during change of flow rate at SC (or MC) ranging from 0 mL/min to 30 mL/min.
MCDI operation (Fig. 2B). 4. Conclusions In this study, we demonstrate the semi-continuous desalination process with MC-MCDI which is designed with multi-channel flow stream and ion exchange membranes. MC-MCDI allows semi-continuous production of clean water with the separated middle and side channels resulting in alternating desalination and regeneration to avoid mixing of just-produced freshwater with concentrated water in the effluent. For a prototype device, installation of simple valves is required to switch the effluent obtained from MC and SC, respectively. Our cell design allows high SAC values of 22 ± 2 mg/g with a charge efficiency of 90 ± 2% for the middle channel and side channel during cell voltage cycling between + 1.2 V and − 1.2 V. In addition, MC-MCDI shows a promising performance stability in long-term operation with benefit of IEMs suppressing the degradation of ACC electrode with limited parasite Faradaic reactions. We believe the semi-continuous desalination process of MC-MCDI can open further opportunities to water desalination via CDI for enhancing effectiveness.
Fig. 7. Salt adsorption capacity (SAC) and charge efficiency at the middle and side channels (MC and SC) of multi-channel membrane CDI (MC-MCDI) over 100 cycles. The long-term operation was performed at 5 mM NaCl with constant cell operation (+ 1.2 V vs. −1.2 V) and 10 mL/min flow rate for 30 min half-cycle duration.
Acknowledgements
(Supplementary information, Fig. S2) due to the parasitic Faradaic processes including ORR and carbon oxidation reactions [49]. The ORR of dissolved oxygen produces hydrogen peroxide leading to significant pH fluctuation, and it is one of the major factors to trigger poor longterm stability [50]. Yet, in MCDI, the IEMs suppress the penetration of dissolved oxygen at the electrode surface so that the production of hydrogen peroxide is inhibited [51]. Therefore, MCDI (and in consequence: MC-MCDI) reveals a better long-term performance than CDI [51]. This aligns with the small pH value variation observed during MC-
S.P. acknowledges financial support by the German Academic Exchange Service (DAAD award number 91579066). The authors thank Eduard Arzt for his continuing support and Hwirim Shim for discussion (both at INM). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// 109
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doi.org/10.1016/j.desal.2017.10.012. [26]
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Desalination journal homepage: www.elsevier.com/locate/desal
Semi-continuous capacitive deionization using multi-channel flow stream and ion exchange membranes
MARK
Choonsoo Kima, Pattarachai Srimuka,b, Juhan Leea,b, Mesut Aslana, Volker Pressera,b,⁎ a b
INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany Department of Materials Science and Engineering, Saarland University, 66123 Saarbrücken, Germany
G RA P H I C A L AB S T R A C T
A R T I C L E I N F O
A B S T R A C T
Keywords: Capacitive deionization Semi-continuous desalination Ion exchange membrane Multi-channel flow stream
Capacitive deionization (CDI) is a promising desalination process, but conventional static electrode CDI operates by sequentially cycling through charging and discharging to produce fresh and concentrated water, respectively. However, an effective continuous operation is desirable for optimized system operation. Here, we report a semicontinuous desalination process with a novel modified CDI cell architecture using a multi-channel flow stream and ion exchange membranes (MC-MCDI). This MC-MCDI consists of two channels including side and middle channels with a pair of cation and anion ion exchange membranes where the feed streams can be separately distributed without mixing. The MC-MCDI design allows semi-continuous production of clean water since the separated middle and side channels are alternately desalinated and regenerated: one channel is being desalinated while the other channel is regenerated. Therefore, the cell can produce clean water during both charging and discharging, enabling semi-continuous operation. In addition, with the benefit from similar cell configuration with membrane CDI, the MC-MCDI design exhibits a high salt adsorption capacity (SAC) of 22 ± 2 mg/ g and charge efficiency of 90 ± 2% at middle and side channels during charging and discharging with reverse voltage operation (cell voltage of +1.2 V vs. −1.2 V).
1. Introduction Capacitive deionization (CDI) has emerged as a promising desalination technology for brackish water due to its ability to attain high
⁎
energy efficiency [1–5]. A conventional CDI cell employs a pair of porous carbon electrodes (or multiple pairs for scale-up applications) including activated carbon or carbon cloth [6–9]. Typically, feed water flows through a spacer between a pair of carbon electrodes (flow-by
Corresponding author at: INM - Leibniz Institute for New Materials, 66123 Saarbrücken, Germany. E-mail address: [email protected] (V. Presser).
http://dx.doi.org/10.1016/j.desal.2017.10.012 Received 7 August 2017; Received in revised form 16 September 2017; Accepted 8 October 2017 0011-9164/ © 2017 Elsevier B.V. All rights reserved.
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Fig. 1. Multi-channel membrane CDI (MC-MCDI) cell designed with flow channels and ion exchange membranes: schematic diagram (A), cell components (B) and exploded view of the cell (C).
regeneration and, thereby, accomplishes more continuous operation. In 2017, Lee et al. introduced a “rocking-chair desalination battery” system introduced using the Prussian blue materials including sodium nickel hexacyanoferrate (NaNiHCF) and sodium iron hexacyanoferrate (NaFeHCF) [26]. This novel concept requires the physical change of the solution in the two flow channel compartments and, thus, allows desalination during the charging and discharging step. Thereby, this concept shares similarities with the desalination battery of Pasta et al. (Ref. [24]) and Capmix systems for energy extraction from salinity gradients (Ref. [28]). Fully continuous CDI operation was enabled by use of carbon suspensions, leading to the emergence of flow electrode CDI (FCDI) [29,30]. The use of carbon suspensions as flowable electrodes had previously been applied to fundamental studies [31], chemical reactors [32], and energy storage [33]. FCDI enables to continuously generate fresh water since desalination and electrode regeneration can be decoupled by using two physically separated cells [34–36]. The continuous supply of uncharged carbon particles further leads to a higher ion removal capacity; therefore, FCDI can even desalinate feed water with higher concentration compared to static electrode CDI [29,37]. The intrinsic limitation of low carbon mass loading in flow electrodes can further be overcome by the use of fluidized beds (FB-CDI) [38]. Although suspension electrodes have shown remarkable innovations and improvements [39], the technology suffers at present from energy cost associated with pumping, possible clogging on long-term operation, and low conductivity compared to conventional carbon electrodes [1,40]. While these issues may be overcome in the future, present-day research still strives for alternative systems that enable fully continuous or improved semi-continuous CDI operation. Ideally, such technologies would be based on a conventional CDI cell design with small
cell) [10]. Commonly, CDI employs cyclic operation with a charging and a discharging step. The charging of the cell accomplishes desalination via ion electro-adsorption and the discharging accomplishes electrode regeneration by electro-desorption [1,2]. During galvanostatic or potentiostatic charging, salt ions are electro-adsorbed by forming an electrical double layer (EDL) at the interface between porous electrode and water, leading to salt removal from the feed stream. The adsorbed ions are released from the electrodes during the discharging step, which is operated with reversed polarity, or short circuit resulting in a concentrated effluent stream [11,12]. Depending on the cell architecture, type of electrode materials, presence of membrane, and operation conditions, there are many types of static electrode architectures including flow-by CDI [13], flow-through CDI [14,15], membrane CDI [16], inverted CDI [17], hybrid CDI [18,19], pseudocapacitive CDI [20–22], cation desalination [23], and desalination batteries [24–26]. Conventional static electrode CDI cells operate by sequential charging and discharging to produce fresh and concentrated water, respectively. From a system engineering point of view, intermittent operation is unfavorable and continuous production of clean water is much more desirable. In addition, the repeated cycling between adsorption to desorption leads to the partial mixing of just-produced fresh water with concentrated water in the effluent stream. Thus, auxiliary devices to control conductivity (e.g., electronic valve and conductivity measurement device) are required, triggering additional installment and maintenance costs [27]. To overcome these limitations, novel concepts have been reported [26,27]. For example, the “merry-goround” operational by the use of moving wire-type capacitive electrodes was reported by Porada et al. in 2012 [27]. The latter concept physically moves the wire electrodes after desalination elsewhere for 105
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The charge efficiency was estimated by Eq. (2):
modifications and by employing conventional CDI electrode materials (i.e., activated carbon) to expedite the further technology development. Motivated by these challenges, this work introduces semi-continuous desalination without mixing of effluent freshwater and concentrated water using a recently-reported CDI cell architecture with a multi-channel flow stream and ion exchange membrane (MC-MCDI) [41]. The MC-MCDI cell consists of three flow channels including two side channels and one middle channel, separated by a pair of ion exchange membranes (one cation and one anion ion exchange membranes). We had initially designed this cell to capitalize on enhanced desalination capacity of the middle channel by increasing the side channel ion concentration [41]. As we show in this work, operation of the MC-MCDI cell also allows semi-continuous production of clean water since the separated middle and side channels are alternately desalinated and regenerated (i.e., one channel is desalinated while the other channel is regenerated). For this, the same saline concentration is used in all channels, whereas our previous work focused on operation with salinity gradients. In addition, the outlets are separated between middle and side channels so that mixing of just-produced freshwater with concentrated water in the effluent is inhibited.
Charge efficiency =
Especific =
V m
∫t
t2
Idt
1
(3)
where I is the applied current, t2 − t1 is the operating time, V is the voltage during operation, and m is the mass of both activated carbon cloth electrodes. Note that the activated carbon cloth did not contain any binder; the active mass is therefore identical with the electrode mass. The overall energy consumption of MC-MCDI was calculated with the summation of energy consumption for middle and side channels. The experiments were performed in triplicated and the results showed reproducibility of the data, with deviations of about 5–10%. 3. Results and discussion
2.1. Cell design
3.1. Characterization of ACC electrode and its conventional CDI performance
The cell configuration of MC-MCDI was first introduced by us in Ref. [41] and is additionally depicted in Fig. 1. The MC-MCDI cell is composed of two side channels (5 × 5 × 0.2 cm3, acrylic glass) and one middle channel (5 × 5 × 0.4 cm3, acrylic glass). The channels are separated by a pair of ion exchange membranes (6 × 6 cm2, Neosepta CMC and AMX, Astom Corporation), and their volumes are 10 mL. A titanium mesh (5 × 5 cm2, H-TEC system GmbH) was employed as current collector. Activated carbon cloth (ACC, Kynol-ACC-507-20) was cut to rectangular shape with 4 × 4 cm2 (total weight of an electrode pair = 280 mg) and was used as electrode. More data on the ACC electrodes is found in our recent paper (Ref. [6]) and in Supplementary information, Fig. S1.
The key properties and performance metrics of activated carbon cloth Kynol-ACC-507-20 were briefly summarized in Supplementary information, Fig. S1 and align with our previous study published in Ref. [6]. As can be seen from scanning electron micrographs found in Supplementary information, Fig. S1A, the carbon cloth consists of yarns composed of several individual fibers (ca. 10 μm). The activated carbon cloth has a very high carbon content (> 95 mass%) and a large specific surface area (ca. 1939 m2/g, average pore size = 0.96 nm). Raman spectroscopy reveals the incompletely graphitic nature of the carbon with peak positions of D- and G-modes at 1340 cm− 1 and 1604 cm− 1, respectively, and a full-width at half maximum (FWHM) around 122 cm− 1 and 53 cm− 1 for D- and G-mode, respectively [43–45]. Electrochemically, the ACC electrode exhibits almost ideal capacitive behavior evidenced by the rectangular CV shape when tested in aqueous 1 M NaCl (Fig. S1E) with a maximum specific capacitance of 123 F/g at 0.1 A/g (Fig. S1F) [46]. The salt adsorption capacity (SAC) and charge efficiency of ACC electrode obtained in a conventional CDI and MCDI setup with the design outlined in Ref. [47] are shown in Supplementary information, Fig. S2. The ACC electrode reveals a typical conductivity profile in CDI operation with an inflow concentration of 5 mM NaCl (inset in Fig. S2). This indicates that ions are adsorbed and desorbed in the way that is expected from a conventional CDI material. The maximum SAC and charge efficiency are 12.5 ± 0.5 mg/g and 77 ± 0.3% on average over 100 cycles, respectively. When adding a pair of ion exchange membranes in a conventional CDI setup, the ACC electrodes exhibit a maximum SAC of 14.6 ± 0.5 mg/g (18.7 ± 0.3 mg/g), a charge efficiency of 84 ± 2% (86 ± 2%) when we use a cell voltage of + 1.2 V vs. 0 V (or: + 1.2 V vs. − 1.2 V). Therefore, our ACC electrodes show a very promising performance in the range expected for a nanoporous carbon with well-developed pore structure [48].
2.2. Desalination performance characterization of MC-MCDI The desalination performance of MC-MCDI was benchmarked in saline media with 5 mM NaCl (dissolved oxygen content: ca. 8 mg/L) supplied by a peristaltic pump (volumetric flow rate = 10 mL/min at middle and side channels). The recycling volume of middle and side channels was 10 L. The conductivity profiles were measured at the effluent of the middle and side channels. For the activation of ACC electrode before desalination test, the electrode conditioning was performed with five polarization cycles from +1.2 V to − 1.2 V cell voltage (half-cycle duration for 30 min). While the first half-cycle was always using a cell voltage of + 1.2 V, we used different cell voltages for the second half-cycle; namely, values from − 1.2 V to 0 V. In addition, the influence of flow rate on the desalination performance was investigated by CDI measurement with various feed flow rates ranging from 0 mL/min to 30 mL/min at middle and side channels with cell voltage of + 1.2 V vs. −1.2 V, respectively. The salt adsorption capacity (SAC) was calculated by Eq. (1): k n=0
(2)
where MNaCl is molecular weight of NaCl, F is the Faradaic constant and I is the current during constant cell voltage operation. The energy consumption for middle and side channels was estimated further by Eq. (3) for constant voltage operation and expressed as energy consumption per removed ion (kT) [42]:
2. Experimental description
∑ (C0 − Cn)·ν·MNaCl·(tn − tn−1)/m
SAC ⎛ ∫ Idt ⎞ /⎜ ⎟ MNacl ⎝ F ⎠
(1)
3.2. Semi-continuous desalination with MC-MCDI
where C0 and Cn are the initial and effluent concentrations converted from conductivities, ν is the volumetric flow rate, MNaCl is the molecular weight, tn − tn − 1 is the time interval for conductivity measurement, and m is the mass of both activated carbon cloth electrodes. Note that the activated carbon cloth electrodes are binder-free and free-standing, so the total electrode mass is equal to the mass of activated carbon.
Fig. 2 depicts the profiles of effluent conductivity and pH of the middle channel (MC) and side channels (SC) obtained with cell voltage of + 1.2 V vs. − 1.2 V, indicating the semi-continuous desalination process of MC-MCDI. As can be seen in Fig. 2A, the conductivity profiles of water effluent from the MC exhibit ion electro-adsorption/electro106
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Fig. 3. Salt adsorption capacity (SAC), charge efficiency, and energy consumption (kT) at middle and side channels (MC and SC) of multi-channel flow stream membrane CDI (MCMCDI) with constant cell voltage operation of + 1.2 V vs. −1.2 V. Desalination was carried out at 5 mM NaCl with 10 mL/min flow rate for 30 min half-cycle duration.
Fig. 2. Representative effluent conductivity (A) and pH (B) profiles at middle and side channels (MC and SC) of multi-channel membrane CDI (MC-MCDI) with constant cell voltage operation of + 1.2 V vs. − 1.2 V. Desalination test was conducted at 5 mM NaCl with 10 mL/min flow rate for 30 min half-cycle duration.
desorption during polarization at + 1.2 V (first half-cycle) and at − 1.2 V (second half-cycle), respectively. When comparing the profiles of MC and SC, we see that both channels depict opposite sorption cycles: the conductivity in MC is rising when the SC is depleted of ions and vice versa. This implies that MC-MCDI is alternately desalinated and concentrated at the separated MC and SC allowing semi-continuous production of clean water. Additionally, the outlets are separated by channels and membranes, and thus, when the effluent can be switched with an auxiliary valve between the two channels, there is no mixing of just-produced fresh water with concentrated water in the effluents of MC and SC. The pH profiles of water effluent from MC and SC are shown in Fig. 2B. The pH increase and decrease in MC (pH decrease and increase in SC) during polarization at + 1.2 V and at − 1.2 V is attributed to the oxygen reduction reaction (ORR) and/or carbon oxidation [49,50]. Overall, the observed pH changes of MC and SC are small (ΔpH < 1) compared to the pH change in conventional CDI (ΔpH > 3–4) [6,49]. Therefore, the use of ion exchange membranes effectively suppresses parasitic Faradaic reactions and this well-known effect has previously been reported for MCDI systems [51]. The desalination performance of MC-MCDI is displayed in Fig. 3. With cell voltage of +1.2 V vs. − 1.2 V, the MC and SC both show an average SAC of 22 ± 2 mg/g and an average charge efficiency of around 90%. The energy consumption per removed ion in MC and SC is 12 ± 0.5 kT per channel and the overall energy consumption of the MC-MCDI cell during one cycle is 24 kT. This excellent performance is accomplished because the use of ion exchange membranes (MCDI) capitalizes on blocking co-ion repulsion and reverse voltage operation
Fig. 4. Schematic diagram for operating mechanism for semi-continuous desalination process of multi-channel flow stream membrane CDI (MC-MCDI) with cell voltage of + 1.2 V vs. − 1.2 V. AEM: anion exchange membrane; CEM: cation exchange membrane; MC: middle channel; SC: side channel.
[41]. The mechanism behind the semi-continuous desalination process is schematically depicted in Fig. 4. During the first half-cycle using a cell voltage of + 1.2 V (Fig. 4A), co-ions are released from pores in the carbon electrodes at the SC. The released co-ions in the SC are blocked by the IEMs and prevented from crossing over to the MC. To accomplish charge neutrality, additional ion transportation of counter-ions is triggered from the MC into the two SCs. Hence, the SC concentration is 107
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Fig. 5. Effect of negative polarization ranging from − 1.2 V to 0 V on conductivity profiles of middle and side channels (A and B) while positive polarization was kept to +1.2 V, salt adsorption capacity (SAC), charge efficiency (C), and energy consumption (kT) (D) of multi-channel flow stream membrane CDI (MC-MCDI). Desalination test was conducted at 5 mM NaCl with 10 mL/min flow rate for 30 min half-cycle duration with constant charging cell voltage of + 1.2 V.
consumption of the SC gradually decreased (no significant difference in energy consumption at MC) with change of the cell voltage during the second half-cycle from − 1.2 V to 0 V. Note that SC accomplishes desalination with a SAC of around 3 mg/g during discharging with cell voltage of 0 V, possibly leading to energy recovery. Thus, by operating at a cell voltage of + 1.2 V vs. 0 V, the overall energy consumption of MC-MCDI can be < 12 kT (the energy recovery ratio was approximately 45%: applied energy of 12 J/g and recovered energy of 5.4 J/g). We further investigated the influence of the feed flow rate on the desalination performance (Fig. 6). In our experiment, the flow rate of the MC was kept constant at 10 mL/min when the SC feed flow was varied (and vice versa: constant SC flow rate of 10 mL/min while varying the MC feed). A more detailed outline of the variation of the desalination performance with different flow rates is found in Supplementary information, Fig. S3. As shown in Fig. 6A–B, variation of the SC flow rate in the range of 0–30 mL/min has only a small effect on the desalination performance of the MC; the latter showed an average SAC of around 21 mg/g with 90% charge efficiency and 12 kT energy consumption. We also found only a small influence of the MC feed flow rate on the desalination performance of the SC effluent stream, with an average SAC of around 20.5 mg/g with 90% charge efficiency and 12 kT energy consumption (Fig. 6C–D). Finally, we characterized the performance stability during continuous cycling (100 cycles; Fig. 7). Over 100 cycles, MC and SC exhibit a highly stable desalination performance with SAC of 22 ± 2 mg/g (SACmax of 23 mg/g; SACmin of 20 mg/g) and charge efficiency over 90 ± 2%. In contrast, a conventional CDI suffers from a poor longterm performance stability in an aerated solution; only in case of deaerated solution, long-term performance in CDI also is stable
increased while that of MC is decreased leading to a high charge efficiency (> 90%). During the second half-cycle at a cell voltage of − 1.2 V (Fig. 4B), the co-ions released from the carbon electrodes migrate towards the MC and in this case, they can pass through IEMs leading to the desalination of SC (increased concentration in the MC). The MC-MCDI cell enables an overall SAC of ca. 44 mg/g of MC and SC per one cycle. This value is much higher than SAC values (13–18 mg/g) obtained in a conventional CDI cell or a conventional MCDI cell with the same material (Supplementary information, Fig. S2). Consequently, adopting a MCDI cell concept but adding two additional flow channels (i.e., SC), the MC-MCDI architecture enables semi-continuous desalination process with excellent desalination performance. In a next step, we evaluated the influence of the cell voltage of the second half-cycle between − 1.2 V and 0 V on the MC-MCDI performance (Fig. 5). From the conductivity profiles at MC and SC in Fig. 5A–B, as the cell voltage during the second half-cycle is changed from − 1.2 V to 0 V, the amplitude of electro-adsorption/desorption peaks seen in the conductivity profiles of water effluent from MC and SC are decreased. This decrease is more pronounced for the SC compared to water effluent from the MC. Accordingly, the SC SAC (Fig. 5C) is reduced from 22 mg/g to 3 mg/g with a decrease of the charge efficiency from 91% to 26%. For comparison, the decrease of SAC and charge efficiency at MC is significantly smaller: reduction of SAC from 22 mg/g to 13 mg/g and charge efficiency from 90% to 80%. The high cell voltage during the second half-cycle (i.e., −1.2 V) leads to a larger number of expelled co-ions at the carbon electrode and flow stream in SC (full ion depletion at SC) than a low cell voltage at the second halfcycle (i.e., 0 V). Overall, the energy consumption per removed ion of MC-MCDI (Fig. 5D) is 12–24 kT in this cell voltage range. The energy 108
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Fig. 6. Effect of feed flow rate at side and middle channel (SC and MC) on conductivity profiles and salt adsorption capacity (SAC), charge efficiency and energy consumption (kT) at MC (A and B) and SC (C and D) of multi-channel membrane CDI (MC-MCDI), respectively. Desalination test was conducted at 5 mM NaCl for 30 min half-cycle duration with constant cell voltage operation (+ 1.2 V vs. − 1.2 V), and the flow rate at MC (or SC) was kept with 10 mL/min during change of flow rate at SC (or MC) ranging from 0 mL/min to 30 mL/min.
MCDI operation (Fig. 2B). 4. Conclusions In this study, we demonstrate the semi-continuous desalination process with MC-MCDI which is designed with multi-channel flow stream and ion exchange membranes. MC-MCDI allows semi-continuous production of clean water with the separated middle and side channels resulting in alternating desalination and regeneration to avoid mixing of just-produced freshwater with concentrated water in the effluent. For a prototype device, installation of simple valves is required to switch the effluent obtained from MC and SC, respectively. Our cell design allows high SAC values of 22 ± 2 mg/g with a charge efficiency of 90 ± 2% for the middle channel and side channel during cell voltage cycling between + 1.2 V and − 1.2 V. In addition, MC-MCDI shows a promising performance stability in long-term operation with benefit of IEMs suppressing the degradation of ACC electrode with limited parasite Faradaic reactions. We believe the semi-continuous desalination process of MC-MCDI can open further opportunities to water desalination via CDI for enhancing effectiveness.
Fig. 7. Salt adsorption capacity (SAC) and charge efficiency at the middle and side channels (MC and SC) of multi-channel membrane CDI (MC-MCDI) over 100 cycles. The long-term operation was performed at 5 mM NaCl with constant cell operation (+ 1.2 V vs. −1.2 V) and 10 mL/min flow rate for 30 min half-cycle duration.
Acknowledgements
(Supplementary information, Fig. S2) due to the parasitic Faradaic processes including ORR and carbon oxidation reactions [49]. The ORR of dissolved oxygen produces hydrogen peroxide leading to significant pH fluctuation, and it is one of the major factors to trigger poor longterm stability [50]. Yet, in MCDI, the IEMs suppress the penetration of dissolved oxygen at the electrode surface so that the production of hydrogen peroxide is inhibited [51]. Therefore, MCDI (and in consequence: MC-MCDI) reveals a better long-term performance than CDI [51]. This aligns with the small pH value variation observed during MC-
S.P. acknowledges financial support by the German Academic Exchange Service (DAAD award number 91579066). The authors thank Eduard Arzt for his continuing support and Hwirim Shim for discussion (both at INM). Appendix A. Supplementary data Supplementary data to this article can be found online at https:// 109
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