Korean J. Chem. Eng., 38(1), 1-7 (2021) pISSN: 0256-1115 DOI: 10.1007/s11814-020-0677-0 eISSN: 1975-7220 REVIEW PAPER INVITED REVIEW PAPER

A brief review on the recent achievements in flow-electrode capacitive deionization

Seyed Nezameddin Ashrafizadeh†, Ardalan Ganjizade, and Amin Navapour

Research Lab for Advanced Separation Processes, Department of Chemical Engineering, Iran University of Science and Technology, Narmak, Tehran 16846-13114, Iran (Received 23 June 2020 • Revised 7 September 2020 • Accepted 11 September 2020)

AbstractThe recent studies, achievements and advances in Flow-Electrode Capacitive Deionization (FCDI), known as a relatively new method, are briefly reviewed. First, the geometry and configuration of membranes and electrodes, and their effects on the performance of FCDI are studied. The effect of feeding procedure of slurries on the operation of FCDI is then reviewed. The mode of applying the electric field to the system as another factor influencing the FCDI performance is also discussed. The effect of slurries and the membrane properties on desalination is another subject which is reviewed. Along with all the above mentioned investigations, it is attempted to introduce the new pat- ents related to FCDI and to show an outlook for such a desalination process. It is finally concluded that, despite some shortcomings, due to the high number of studies dedicated to the FCDI and according to recent advancements in this field, not only can FCDI become a mature and applicable desalination process in the future but also play an important role in the other engineering fields. Keywords: Capacitive Deionization, Flow-electrode, Electrochemical Separation, Desalination

INTRODUCTION lower input energy needed for performing desalination by means of MCDI, this process may be more advantageous than CDI. How- As the accessible volume of fresh potable water is limited, and ever, the fact that the desalination of water cannot be performed production of sweet water is crucial for various industrial activi- continuously (due to saturation of the electrodes) is still a draw- ties, desalination of water has become an inevitable challenge. Besides back of MCDI. Jeon et al. [3] suggested a modified form of MCDI, the traditional methods engaged in heat transfer and/or membrane named as FCDI, wherein two aqueous carbon slurries, known as barriers, capacitive deionization (CDI) [1], membrane capacitive flow electrodes (FEs), are pumped in the spaces between the cur- deionization (MCDI) [2] and flow-electrode capacitive deioniza- rent collectors and membranes. In such systems, carbon particles tion (FCDI) [3] have recently attracted researchers’ attention. It is play the same role as the fixed porous electrodes in MCDI (Fig. said that due to their energy efficiency, environmental friendliness 1(c)). The inserted electric field between current collectors thus and ion removal efficiency, such processes are better alternatives makes the ions transport through the membranes () for the common existing methods like , multistage and become adsorbed on the EDLs of moving suspended carbon flash, and multi-effect desalination. particles (capacitive mechanism) [5]. Despite CDI and MCDI, in In CDI, a low DC electric field is applied across two fixed porous some modes of operations, FCDI is capable of simultaneously remov- carbon electrodes between which the water to be treated is pumped. ing the salts and regenerating the FEs, and consequently desalinating In such a process, water desalination occurs by the electrically- the water continuously [4,6,7]. The superiority of FCDI desalina- driven migration of ions and formation of electric double layers tion over the two other named processes is depicted in Fig. 1(d). (EDL) in the vicinity of the electrodes (Fig. 1(a)). However, ion On one hand, new patents, somehow related to FCDI, are emerg- migration and desalination terminate as the electrodes become satu- ing one after another (Fig. 1(e)) [8-12]. On the other hand, although rated. Hence, for CDI, one must consider a step in which a reverse there are some published researches generally reviewing the recent or zero potential is applied to remove the adsorbed ions from the studies dealing with capacitive deionization methods [13,14], to electrodes and regenerate them. Thus, CDI must be performed in the best of our knowledge, no study has deeply focused on FCDI. charge-discharge cycles. As a result, it can be said that CDI cannot Here, we are not only reviewing the latest studies on FCDI, but desalinate water continuously. The effort to improve the efficiency also briefly reporting the recent advances in this field and present- and capacity of CDI resulted in the introduction of MCDI. MCDI ing an outlook for this novel method. (Fig. 1(b)) is different from CDI only in the cation and anion ex- change membranes placed next to the cathode and anode elec- AN OVERVIEW ON THE ADVANCES OF FCDI trodes, respectively [2,4]. As the membranes do not let the co-ions leave the electrodes during the adsorption process, and due to the Thanks to recent developments, FCDI has found, or will find, its way into different fields of science, including chemical engineer- †To whom correspondence should be addressed. ing [15], environmental sciences [3], and energy [16]. However, E-mail: [email protected] there still exist some shortcomings with such a process, such as the Copyright by The Korean Institute of Chemical Engineers. relatively high energy consumption [17], inefficient charge transport

1 2 S. N. Ashrafizadeh et al.

Fig. 1. (a), (b), (c) schematic of CDI, MCDI and FCDI, respectively, (d) a comparison between their performances, (e) title of the patents somehow dealing with FCDI [8-12]. between the current collectors [18], inevitable clogging of electrode volumes of brine with recovery of about 50% [4]. In addition to channels, membrane scaling and membrane abrasion by the FEs regeneration of the FEs, separation of the brine from the elec- carbon particles [4,17]. In addition to the attempts to produce pre- trodes became guaranteed by integration of the FCDI with micro- dictive, robust and reliable models describing FCDI [19,20], the filtration (i.e., by means of FCDI/MF system). FCDI/MF system can investigations on this process mainly focus on optimization of con- separate brackish water into a potable stream and a brine stream figurations, modes of operation, and the membranes and FEs prop- continuously with high water recovery rates (up to 97%) [22]. Mem- erties to overcome the named problems. brane-less FCDI designs lowered the process costs [23]. Stack design 1. Configuration of FCDI/Geometry of the Membranes and of FCDIs increased the capacity of desalinated water production Spacers [24,25]. Such a design (with five unit cells) demonstrated the same The primary conventional FCDI systems were made up of two separation efficiency, electrical charge efficiency and five-times higher FEs and two IEMs. The later introduced, enhanced configura- electrical current compared to a single cell design [24]. Higher salt tions improved the performance of FCDI. Single module [7] (Fig. removal could be achieved by combination of FCDI and neutral- 2(a)), two-module [4] (Fig. 2(b)) and two-step regeneration with ization dialysis (FCND) (Fig. 2(d)). At the same condition, both energy recovery (Fig. 2(c)) versions of FCDI [21], regenerating the the average salt adsorption rate and the salt removal efficiency of FEs, made it possible to perform desalination continuously. For the FCND were found to be 1.68-times higher than that of FCDI [26]. single module system, a desalination rate of 70% was achieved for Besides, tunable salt rejection, higher energy efficiency and more water recovery of 80% with current efficiency of 93% (1 gNaCl/l water recovery were reported to be gained using hybrid FCDI-nano- in the feed water) [7]. By means of two module FCDI, one can filtration systems. The energy consumption of such a hybrid sys- gain a desalination rate of 99% for recovery of as high as 90% tem is reported to be 80-84% of a brackish water reverse osmosis (1 gNaCl/l in the feed water); this is accounted as one of the advan- system for the same feed at water recovery of 70% [27]. Honey- tages of such a method over reverse osmosis, producing higher comb-shaped lattice design of FCDI (Fig. 2(e)) [16] and mem-

January, 2021 A brief review on the recent achievements in flow-electrode capacitive deionization 3

Fig. 2. Schematics of (a) single module FCDI, adapted with permission from Rommerskirchen et al. [7], © 2015 Elsevier Ltd, (b) two-mod- ule FCDI, adapted with permission from Gendel et al. [4], © 2014 Elsevier Ltd, (c) two step regeneration, adapted with permission from Rommerskirchen et al. [21], © 2018 American Chemical Society, (d) FCND, adapted with permission from Wang et al. [26], © 2016 Elsevier Ltd, (e) Honeycomb-shaped lattice design of FCDI, adapted from ref. [16], with permission from The Royal Society of Chemistry, (f) Membrane-spacer assembly of FCDI, adapted with permission from Lee et al. [28], © 2018 Elsevier Ltd, (g) the two chamber device suggested by Ma et al. [30] for energy recovery, adapted with permission from Ma et al. [30], © 2019 Elsevier Ltd, (h) the novel FCDI design with cylindrical IEMs acting as the electrode flow channels [31], published by The Royal Society of Chemistry, (i) FCDI cells arranged in series for continuous desalination and energy production, adapted with permission from Hatzell et al. [32], © 2014 American Chemical Society, (j) the FCDI potential may increase up to 1.8 V by using asymmetric FEs, adapted with permis- sion from Xu et al. [35], © 2017 American Chemical Society, (k) effect of carbon black additives on the resistance, adapted with per- mission from Liang et al. [36], © 2017 Elsevier Ltd.

Korean J. Chem. Eng.(Vol. 38, No. 1) 4 S. N. Ashrafizadeh et al.

Fig. 2. Continued. brane-spacer assembly of FCDI (Fig. 2(f)) [28], brought about not constant voltage or constant current modes. In most studies, FCDI only independency to free-standing IEMs, but also great potential is conducted under a constant voltage mode. In such processes, as for scaling up and commercializing of the process. Employment of long as the slurry particles are not saturated, the salinity of the prod- titanium mesh membrane assembly, instead of conventional plate uct decreases. As the slurry particles saturate, the salinity of the shape current collectors, optimized the charge transfer in the FEs product starts to increase. Hence, in such cases the salinity of the and as a result increased the FCDI’s salt removal rate. The average product experiences a minimum. However, by continuously intro- salt removal rate of such a system was 76% higher than the con- ducing fresh slurries to the system, or regenerating FEs, one can trol FCDI system benefiting from conventional graphite plate cur- keep the salinity of the product at its minimum. On the other hand, rent collectors [29]. Utilization of the two chamber device, shown during the FCDI process under constant current mode, unless the in Fig. 2(g), made the recovery of the energy consumed by FCDI FEs get regenerated, the cell voltage increases up to voltages even possible [30]. Benefiting from cylindrical IEMs acting as the FE higher than 1.23 V, wherein unwanted faradaic reactions (like elec- channels, the novel FCDI design introduced by Porada et al. [31], trolysis of water, oxygen reduction, oxidation of carbon [33], and used differences in water salinity, and differences in gas phase CO2 evolution of hydrogen and oxygen [34]) occur in a more acceler- concentration, for energy production (Fig. 2(h)). The flexible design, ated manner, resulting in higher energy consumption and lower in which the FEs, passing through four FCDI systems arranged in charge efficiency of the system. Despite the few studies wherein series, continuously getting charged and discharged, made it possi- cell voltages as high as 1.8 [35] and 2.4 V [36,37] have been used, ble to continuously harvest energy through desalination (charge) FCDI is usually conducted at cell voltage lower than 1.23 V [3, and regeneration (discharge) steps, respectively (Fig. 2(i)). Such a 16,28] to prevent the mentioned side reactions. However, by using design, for which a total power density up to 50.9 mW/m2 was re- some techniques like improving the slurry particles, as will be dis- ported, can be accounted as an emerging energy storage system cussed later, one can suppress such faradaic reactions and increase like batteries and super capacitors [32]. the voltage limit [35]. 2. Modes of Operation 2-2. Feeding Modes: Single Pass vs Batch 2-1. Electricity Exertion Modes: Constant Voltage vs Constant FCDI can be performed either in batch or single pass mode. If Current the feed is introduced just once to the system and the outlet is ac- Similar to CDI and MCDI, FCDI can be performed under either counted as the final product, the mode is single pass. While, if the

January, 2021 A brief review on the recent achievements in flow-electrode capacitive deionization 5 outlet is recycled to the system again and again, the mode is known diffusion mechanisms [48]. However, one should not neglect the as batch. In the single pass mode, although the concentration of possibility of water osmosis in this process [39]. The way we choose FCDI outlet rapidly declines as the constant voltage is inserted, the the salinity of the slurries relative to the feed will determine the saturation of the electrode particles leads to a slight increase in the intensity of each of the named possible mass transfer mechanisms. product salinity. As the electrode particles get fully saturated, no Although, increasing the salt content of the slurries increases the desalination takes place and the salinity of the product becomes electrode capacitance and lowers both the equivalent series resis- equal to that of the feed. However, by regenerating the slurry or tance (ESR) and the FEs viscosity [49]; in the cases that the salinity permanently using a fresh slurry, one can postpone the saturation of the slurries is far more than that of the feed, there will develop and carry on the desalination process for a long time. On the other not only an osmotic pressure leading to water transport from the hand, the outlet of the FCDIs working in batch mode is recycled feed to the slurries, but also the diffusion of salt ions from slurries and its salinity steadily decreases during the process, if the FEs get into the feed [48]. To compensate these unfavorable mass transfers, regenerated. one must apply higher cell voltage and consequently consume more 2-3. Flow-electrodes Introduction Modes: OC vs SCC vs ICC energy [36]. On the other hand, choosing the slurry salinity to be One can consider three modes of slurry introduction into the far less than that of the feed makes the regeneration of the elec- FCDI system. The processes wherein the FCDI systems are perma- trode flows and energy recovery very difficult. Using dimensionless nently fed by fresh slurries are known to be working in open cycle ------diffusion flux numbers like DMR  , discussed By Moreno et al. operation (OC) mode. In the short-circuited closed cycle opera- migration flux tion (SCC) mode [6,15,38-40], the slurries from the anode and the [48], may help work out the mechanism of separation in FCDI. It cathode cells are recycled to a single container and mixed before is reported that by conducting FCDI with DMR of less than 5, one re-introduction into the anode and cathode chambers. In contrast can mitigate the diffusion flux [48]. In addition, matched electric to SCC, in isolated closed cycle operation (ICC) mode [6,41], the strength of the feed and FEs is reported to guarantee the stability slurry flows from the anode and the cathode cells are recycled into of the SCC mode of FCDI. two separate containers and separately re-introduced to their cor- The kinetics of adsorption is an important parameter determin- responding anode and cathode chambers, respectively. ing the desalination efficiency of capacitive deionization. For a typ- In OC mode, as each slurry particle is introduced to the sys- ical capacitive deionization process, it has been proven that the tems just once; one can consider the capacity of the electrode par- electrodes should have high surface areas, high EDL capacitance, ticles to be infinite. However, because the slurries are not recycled, high conductivity and high chemical stability [50]. The FEs in the such a mode may not be economically feasible. On the other FCDI must also benefit from such properties. Recent studies have hand, the mixing of the slurries in SCC mode results in regenera- proven that higher porosity of carbon particles [51] or higher sur- tion of the electrode particles and postpones the saturation of the face area of carbons results in higher desalination capacity of FCDI. electrode slurries. It has been shown that SCC is more energy effi- However, using FEs of high carbon content is challenging. High cient than ICC for softening of brackish water [42]. However, in content of carbon particles (more than 25%) increases the pump- terms of average salt removal rate and charge efficiency, SCC/sin- ing energy demand and leads not only to more intensive agglom- gle pass mode and ICC/single pass mode are revealed to be supe- eration but also higher viscosity of the slurries [52]. rior to other modes of operation [43]. To modify electrode performance of CDI, various porous car- Following the novel strategy, named switch cycle operation, in bon materials such as carbon aerogels, carbon black, powdered acti- which the saturated FEs of each FCDI step are introduced to the vated carbon and fiber cloth have been studied electrode channels of the opposite signs in the next sequential FCDI [50]. Combining the activated carbon with other materials like ion desalination step, one can increase the salt removal efficiency of exchange resins, MnO2 and titania, applying surface coating, dop- the FCDI process by 1.7-times that of SCC mode [44]. ing with foreign elements and depositing ion-selective membranes, 3. Materials water soluble binders and other materials on the surface of the car- Most of the investigations performed to recognize the effect of bon electrodes are among the other remedies practiced to modify materials properties on FCDI performance have focused on the the performance of the CDI’s fixed electrodes [53]. Similar attempts FEs. FEs, used in different processes like novel flow electrode reverse have been done for FCDI. According to Hatzell et al. [54], using electrodialysis (FE-RED) [45], flow batteries [46] and electrochemi- slurries containing oxidized carbon particles could also lead to cal flow capacitors (EFCs) [47], are suspensions made of small higher desalination capacity. By means of such a remedy, Hatzell active capacitive particles with high surface areas (typically acti- succeeded in increasing the mass loading of the FE and the FE gravi- vated carbon (AC)), as well as ion-containing electrolytes and con- metric capacitance by 40% and 25%, respectively, without sacrific- ductive additives. NaCl-containing and acidic electrolytes are usually ing flowability [54]. Besides, functionalization of the activated carbon preferred for desalination and energy production processes, respec- particles of the anode and the cathode slurries by adding trimeth- tively [17]. The shape, size and porosity of the carbon particles, as ylammonium (AC-N) and sulfonization (AC-S), respectively, prac- well as the salinity and the electrochemical features of the electro- ticed by Park et al. [55], made it possible to use more carbon particle lyte determine the FEs properties. However, adsorption of the ions content in the FEs (up to 35%). Such a functionalization also leads into the EDLs may change the FEs properties during the process, to a rise in the FE conductivity and an increase in desalination effi- which must be considered in FEs design [5]. ciency from 8.2% to 27.7%. It must be said that the charge on the FCDI removes the feed’s ions mainly by electric migration and FE’s functionalized particles at a given pH and voltage is import-

Korean J. Chem. Eng.(Vol. 38, No. 1) 6 S. N. Ashrafizadeh et al. ant in dictating the efficiency achieved. Furthermore, it has been technology has also put a step further into the energy fields. As a shown that by coating the particles with polymers and induction result, one can say that FCDI is a multi-task process which will be of higher hydrophilicity into their surfaces, one can have slurries of prevalent in a wider range of processes in the future. In this regard, lower viscosity. On the other hand, investigations on the systems future studies are expected to focus on making energy consump- with asymmetric flow-electrodes, wherein the cathode and the anode tion more competitive, lowering the cost of materials, solving the slurries contained AC and AC/MnO2, respectively, revealed that in problem of electrode channel clogging and improving slurry con- such systems the potential window may increase up to 1.8V (Fig. ductivity. Furthermore, finding a way to make the use of slurries 2(j)) [35]. In high voltages (2, 2.4 and 2.8 V), FCDIs benefiting from with higher content of carbon particles possible is a vital issue. It is copper hexacyanoferrate-AC as flow electrode pair, are also shown predicted that studies on tailoring the surface properties of carbon to work better than FCDIs with AC-AC electrode pair in terms of particles will lead to a rise in adsorption capacity, elimination of salt removal rate and charge efficiency [56]. In addition, it has unwanted faradaic reactions and increase in the potential window been reported that unlike the ohmic resistance of the FEs, the width. Finally, the pressure loss and the high pumping energy charge efficiency of FCDI rises by the addition of the additives like demand are the challenges expected to be solved in the future. carbon black (Fig. 2(k)) [36] or conductive plate-shaped graphite particles [40] to the FEs. 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