Optimal conditions for efficient flow-electrode capacitive deionization

Journal Pre-proofs Optimal Conditions for Efficient Flow-Electrode Capacitive Deionization Kexin Tang, Sotira Yiacoumi, Yuping Li, Jorge Gabitto, Costas Tsouris PII: DOI: Reference:

S1383-5866(19)35219-0 https://doi.org/10.1016/j.seppur.2020.116626 SEPPUR 116626

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

13 November 2019 25 January 2020 25 January 2020

Please cite this article as: K. Tang, S. Yiacoumi, Y. Li, J. Gabitto, C. Tsouris, Optimal Conditions for Efficient Flow-Electrode Capacitive Deionization, Separation and Purification Technology (2020), doi: https://doi.org/ 10.1016/j.seppur.2020.116626

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Optimal Conditions for Efficient Flow-Electrode Capacitive Deionization Kexin Tang,a,b,c Sotira Yiacoumi,a Yuping Li,b,*Jorge Gabitto,d Costas Tsouris a,e,* a

School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0373, USA

b

Institute of Process Engineering, Division of Environment Technology and Engineering, Beijing Engineering Research Center of Process Pollution Control, Chinese Academy of Sciences, Beijing 100190, PR China

c

School of Chemical Engineering and Technology, National Engineering Research Center for Distillation Technology, Tianjin University, Tianjin 300072, PR China

d

Department of Chemical Engineering, Prairie View A&M University, Prairie View, Texas 774460397, USA

e

Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6181, USA *Corresponding author: Email: [email protected] (Dr. Costas Tsouris); Telephone: +1 865-241-3246 Email: [email protected] (Professor Yuping Li); Telephone: +86 10-82544844-810

Submitted for publication in Separation and Purification Technology November 2019 Revised: January 2020

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Abstract One of the current barriers to achieving fast and stable performance for flow-electrode capacitive deionization (FCDI) is determining optimal operating parameters. To date, however, no consensus has been reached for universal conditions for FCDI. Through experimental and modeling approaches in this study, we systematically evaluated the influence of applied potential (V = 1.2-2.4 V) and electrolyte concentration (C0 = 0.05-0.5 M) on the FCDI and electrodialysis (ED) desalination processes. Evaluation indicators include the concentration decrease in the desalinated solution, salt removal rates, pH fluctuations, charge efficiency, and energy consumption. Results demonstrate that the dynamic curves of concentration decrease at 2.0 V nearly overlap with the response at 1.6 V at certain electrolyte concentrations, while the salt removal rates at 0.2 M salt concentration were the best among all concentrations tested at a range of applied potential. It was thus concluded that the optimum conditions for FCDI operation are 1.6 V applied potential and 0.2 M initial salt concentration, under which Faradaic reactions are not being triggered, and concentration polarization does not significantly affect ion transfer. Furthermore, a comparative study between FCDI and ED indicated that ED has a different dependence on the electrolyte concentration and applied potential, in which the desalination can be linearly enhanced with increasing potential but greatly limited at high concentrations. Due to the presence of carbon particles in FCDI, the enhanced charge/ion transfer is probably the main reason for the different desalination performance of FCDI and ED. The optimal operating parameters obtained in this work could be used as basic test conditions for further development of new carbon-based materials for FCDI.

Keywords: Flow-electrode; Capacitive deionization; Optimization; FCDI modeling; Activated carbon

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1. Introduction Humans are currently facing a serious water crisis due to the explosive demand for water and the simultaneous depletion of water sources [1]. Seawater desalination could effectively solve the worldwide problem of water shortage as seawater reserves are abundant [2]. Compared to conventional thermal and pressure-driven desalination technologies (e.g., multistage flash distillation and reverse osmosis), capacitive deionization (CDI) could be used to desalt water in a more energy-efficient and environmentally friendly way [3, 4]. During the last two decades, CDI technologies have been intensively advanced in the fields of configuration design, operating conditions, and electrode materials [4-6]. With respect to the development of electrode materials for instance, by introducing new porous carbon frameworks [7, 8], performing surface modifications [9, 10], and forming composites/hybrids with conductive carbon, polymers [7, 11], and metal compounds [12-15], researchers have greatly increased the salt removal capacity (SRC) and rate (SRR) of CDI electrode materials [4]. However, difficulties still remain in continuously treating high salinity water because of the limited SRC of stationary electrodes and the high correlation between energy consumption and salt concentration [4, 16]. Recently, flow-electrode CDI (FCDI) has gained particular attention because of its fast demineralization, continuous operation, easy-to-load characteristics [17-21] and its wide applications [22, 23]. The flow-electrodes (FEs) consist of carbon powder slurries, which are neutralized by direct contact of positive and negative FEs outside the deionization unit. Upon neutralization, ions adsorbed by the carbon particles are released into the electrolyte [19, 21], and the particles are recycled. FCDI is considered an energy-efficient desalination technique as the energy input during desalination can be partially recovered by the discharge of FEs in another FCDI unit [24, 25].

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In general, ions migrate faster in a stronger electric field. Due to the composition of FE (i.e., water, inorganic salt, and carbon), however, the applied potential for an FCDI device is limited by electrochemical reactions inside FEs including water splitting, chlorine ions oxidation, oxygen reduction, carbon oxidation and reduction, etc. [26-28]. Though the electrochemical reactions can be weakened by enhancing the charge transfer in the FE (higher carbon loading or carbon black addition) [19, 27], there still exists a threshold of the applied potential that can trigger the above electrochemical reactions. As evidence, in an FCDI system constructed by Liang et al. [27] (carbon content: 20 wt.% activated carbon), the conductivity decrease for different applied voltages (0.6-4.8 V) demonstrated different kinetics. From 0.6 to 1.5 V, the conductivity almost linearly decreased with the applied potential. In the range of 1.5 to 1.8 V, however, the conductivity at t = 15 min only slightly decreased, whereas the corresponding charge efficiency dropped sharply, indicating that a potential in the range of 1.5 and 1.8 V triggered the electrochemical reaction(s) in FCDI. A similar potential threshold was still observed in FEs with higher salt concentrations and with carbon black added. In contrast, Waite et al. [26] recently reported a linear relationship between the applied potential (1.2 to 2.0 V) and the average SRR (ASRR) at various carbon loadings (0 to 10 wt.%). The authors suggested that a high dynamic charge efficiency could be achieved at a high voltage (e.g., 2.0 V) under the condition that the coupling of charge and counterions is sustained in the electrical double layers (EDLs). It is interesting to observe this contradictory effect of the applied potential in these two similar FCDI systems. Our previous study also showed that, in a stationary electrode, the SRC increased linearly within the potential range of 1.2 to 2.4 V [29]. Therefore, it is critical to further investigate the effect of applied potential on the desalination performance of FCDI and the corresponding effects on the stationary and flowable electrodes. In addition to the enhancement of desalination by electric field intensity, the charge/ion transfer in an electrochemical device is strongly linked to the electrode properties. In an FCDI

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system, a consensus for fast and efficient charge/ion transfer is using FEs with excellent conductivity and large SRC. Effective ways to increase the desalination performance of FEs include adjustment of physical properties of carbon particles [19, 25, 30-32], the introduction of additives [27, 33], and variation of electrolyte properties [18, 34]. It is reported that the use of high carbon loading [19, 20, 26] or surface-modified carbon particles [30, 31] can enhance the salt removal rate and reduce energy consumption. Additives like redox couples [33] and carbon black [27] can be used as charge mediators between ions and carbon particles to facilitate desalination. Forming a conductive network in the flow channel can facilitate the charge transfer between the flow-electrodes and the ion exchange membranes (IEMs) [35, 36]. Furthermore, high electrolyte concentration is reported to compensate for the reduction in the performance of the FEs, which attributed to the resistance of water used as the electrolyte [18], but Moreno and Hatzell [34] have demonstrated that the concentration difference between FE and feed solution (FS) should be remained low to avoid the back diffusion of ions. In the actual operation of an FCDI apparatus, however, there is inevitably a concentration difference between FE and FS. Moreover, the use of electrolyte at an extremely high concentration could reduce its salt adsorption ability. Therefore, there should be a threshold of the electrolyte concentration, as well as a maximum concentration difference between FE and FS, that provides a stable and efficient operation of FCDI. Extensive studies have proven that electrodialysis (ED) contributes to or competes with the capacitive desalination of FCDI [17, 26, 37]. There are few reports, however, on the optimal operating conditions for the simplified ED device because a typical ED device generally contains several pairs of alternately arranged anion and cation exchange membranes, and the potential or current density applied across a membrane will be assigned evenly at a high applied voltage (e.g., 20 V) [38]. Therefore, along with investigating the optimal conditions for FCDI,

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studying the effects of operating conditions on the simplified ED process can help us better understand the dynamic contribution or impact of ED on FCDI. In this study, FEs were fabricated by dispersing activated carbon in sodium chloride solution with a carbon loading of 7.41 wt.% and a varying salt concentration (0.05, 0.1, 0.2, or 0.5 M). A salt solution of the same concentration and volume was used in FEs and FS for each FCDI run. Desalination experiments and simulations were carried out with the aim of obtaining (1) the optimal applied potential and electrolyte concentration, (2) the effects of the applied potential and electrolyte concentration on the desalination performance of FCDI and ED processes, and (3) theoretical insights of the different mechanisms involved in desalination by FCDI and ED. 2. Experimental section 2.1 Preparation of flow-electrodes Steam-activated, acid-washed carbon powders (AC, 8 g, Alfa Aesar) were uniformly dispersed in sodium chloride solution (NaCl, 100 mL) via sonication (24 h, 47 kHz, Model 2210R-MT, BRANONIC® ultrasonic cleaner) and subsequent magnetic stirring (350 rpm, 48 h) at various NaCl concentrations (C0= 0.05, 0.1, 0.2, or 0.5 M). NaCl solutions were prepared by dissolving sodium chloride crystals (>99.9%, Sigma-Aldrich) in deionized water (Milli-Q® Integral) using a volumetric flask (1000 mL). Four different flow-electrodes (FEs) were prepared with a carbon loading of approximately 7.41 wt.%. 2.2 Flow-electrode capacitive deionization experiments The installation and operation of the FCDI process, as well as the experimental arrangement and equipment used in the experiments, are identical to those used in our previous work [35]. Briefly, the prepared FEs suspensions were pumped into and out of the anode and cathode compartments of the FCDI device. Meanwhile, the FS (100 mL), having the same concentration as the electrolyte in FE, was circulated between the flask and the feed

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compartment of the apparatus (thickness: 3 mm, volume: 23.4 mL). The flowrates of FEs and FS were 25 and 10 mL min–1, respectively. The anode/feed compartments and feed/cathode compartments were separated by anion (AR103QDP) and cation (CR61CMP) exchange membranes (Suez Water Technology & Solutions), respectively. The FCDI unit was operated at various potentials (V=1.2, 1.6, 2.0, or 2.4 V) and FS concentrations (C0=0.05, 0.1, 0.2, or 0.5 M), and for each FCDI run, the initial concentrations (C0) of FS and the electrolyte of FEs were the same. FS conductivity and current response were recorded every 1 min, and pH values in FE and FS were measured every 30 min. In addition, the desalination performance of ED, a simplified (carbon-free FEs) FCDI process, was investigated under identical conditions. Note that the prepared flow-electrode suspensions were circulated in two modes for different experimental purposes: (1) when the effects of the applied potential and salt concentration on the desalination performance of FCDI were evaluated, a short-circuited closed-cycle (SCC) operation was adopted, and (2) when the evolution of surface properties of carbon particles in the flow-anode and flow-cathode was investigated, the isolated closed-cycle (ICC) operation was used [39]. 2.3 Characterization During FCDI experiments, the surface properties of the AC particles (e.g., surface charge), may gradually change. This behavior may affect the interactions among particles themselves and between carbon particles and charged ions. A measure of the surface charge of the AC particles may be obtained from surface potential or zeta potential measurements. For such measurements, the particle size must be in the colloidal regime (particle size < 1 µm). Therefore, a planetary micro mill machine (PULVERISETTE 7 premium line, Fritsch, Germany) was used to ball-mill the carbon powders (800 rpm, 2 hours). The particle size analysis after ballmilling was conducted using a laser particle size analyzer (Nano ZS, Malvern Panalytical, UK) and confirmed by field emission scanning electron microscopy (JSM-7001F, JEOL, Japan).

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The average diameter of particles obtained after ball-milling was 250 nm (Fig. S1 of the Supporting Information). An FCDI experiment was performed in ICC operation using the FEs prepared from the ball-milled AC particles to evaluate possible changes in the surface charge of anode and cathode carbon particles. This FCDI system was operated under the optimum conditions determined in Section 2.2, e.g., 1.6-V applied potential, 0.2-M electrolyte concentration, 100mL electrolyte volume, and 25- and 10-mL min–1 flowrates for FE and FS, respectively. A concentration of 0.2 M NaCl solution (100 mL) was used for the FS. Another difference is that the FE suspensions (7.41 wt.%) of the anode and cathode (50 mL each) were cycled separately, from which 0.1 mL samples were taken every 30 min. The zeta potential of diluted suspensions was measured using the Nano ZS Malvern equipment, with a dilution factor of 2,000 and solution conductivity of 30 µS cm−1. Although the dilution changed the ionic strength of the solution, thus affecting the surface potential, it was necessary for the measurements to be performed and consistent for all the sample measurements in this work. In general, a decrease in the ionic strength increases the absolute value of the surface potential. 2.4 Modeling Methods A new model that combines a phenomenological approach with a volume averaging method to simulate the operation of slurry carbon electrodes is summarized here. Details of this model can be found in reference [40]. This ‘hybrid approach’ leads to the basic equations describing the electrosorption process in FEs. This model is applicable to solid porous particles that behave as ideal conductors. In order to simulate the operation of an FCDI half-cell, see Fig. 1, we used the full cell models reported by Rommerskirchen et al. [41] for FCDI and Biesheuvel et al. [42] for MCDI. The operation of the membrane was modeled after Galama et al. [43]. The model also considers the presence of limiting concentration gradients at membrane/solution interfaces, known as concentration polarization (Hoek et al. [44]).

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The slurry electrode presents a macroscale represented by an elementary volume (REVM) of radius RM, comprising the flowing solution, -phase, and porous particles, -phase. The porous particles are represented by an REV of radius Rm comprising microscopic pores, phase, and solid, -phase.

Fig. 1. Schematic of the slurry carbon electrode.

The following assumptions were used in the modeling work: 

Stationary REVM.



The liquid phase is continuous while the solid porous-particles phase is discontinuous.



The solid particles move with a constant v velocity.



The fluid moves with a constant v velocity.



The velocity of both phases is approximately the same (v  v).



The solid particle concentration is constant throughout the domain (Sonin and Issacson, [45]). The system moves like a rigid body. It contains always the same number of particles.



There is no mass transport among the solid particles. They interact only with the liquid phase. 9



The ionic adsorption in the internal surface of the porous particles is instantaneous following a Donnan equilibrium relationship with the solution outside.



The values of the potential () and ionic concentrations (ci,) inside the porous particles are constant.

A two-step volume averaging technique (Whitaker [46]) was used to develop the mass balance equations in both phases: 𝜀𝛾

∂〈𝑐𝑖,𝛾〉𝛾 ∂𝑡

𝛾

= ∇  𝜀𝛾 𝐷𝑖,𝑒𝑓𝑓  ∇〈𝑐𝑖,𝛾〉𝛾 + 𝑧𝑖 𝜀𝛾 ∇  𝑈𝑖,𝑒𝑓𝑓  〈𝑐𝑖,𝛾〉 ∇〈𝛾〉

𝛾

― 𝜀𝛾 𝑣𝛾 ∇〈𝑐𝑖,𝛾〉𝛾 ― 𝑀𝑇𝛾, in -phase ∂〈𝑐𝑖,〉

𝜀

∂𝑡

(1) 

= ∇  𝜀 𝐷𝑖,𝑒𝑓𝑓  ∇〈𝑐𝑖,〉 + 𝑧𝑖 𝜀 ∇  𝑈𝑖,𝑒𝑓𝑓  〈𝑐𝑖,〉 ∇〈〉



― 𝜀 𝑣 ∇〈𝑐𝑖,〉 + 𝑀𝑇𝛾 , in -phase

(2)

Here, 𝑀𝑇𝛾 is the interphase mass transfer term; 〈𝑐𝑖,𝛾〉𝛾and 〈𝑐𝑖,〉 are the specific volume averaged concentrations of the i-species in the - and -phases; 〈𝛾〉𝛾 and 〈〉 are the specific volume averaged potentials in the - and -phases; 𝜀γ and 𝜀 are the volume fractions of both phases in the REV; 𝐷𝑖,𝑒𝑓𝑓 are the effective diffusivity tensors for the i-species in both phases; and 𝑈𝑖,𝑒𝑓𝑓 are the effective mobility tensors for the i-species in both phases. The summation and subtraction of eqs. (1) and (2) in the way reported by Bisheuvel and Bazant [47], assuming that in the -phase the convective term is much higher that the mobility and diffusive ones, leads to the following equations: 𝜀𝛾

∂〈𝑐𝛾〉𝛾 ∂𝑡

= ∇  𝜀𝛾 𝐷𝑒𝑓𝑓  ∇〈𝑐𝛾〉𝛾 ― 𝜀𝛾 𝑣𝛾 ∇〈𝑐𝛾〉𝛾 ― 𝜀 𝑣 ∇〈𝑐〉 ― 𝜀 𝛾

∂𝜌

𝛾

 0 = 𝜀𝛾 ∇  𝑈𝑒𝑓𝑓  〈𝑐𝛾〉 ∇〈𝛾〉 ― 𝜀 𝑣 ∇〈𝑐〉 ― 𝜀 ∂𝑡 , in -phase

In Eq. (3) we defined a total mass parameter, 𝑐𝑗 = parameter, 𝜌𝑗 =

(〈𝑐 +,𝑗〉𝑗 ― 〈𝑐 ―,𝑗〉𝑗) 2

(〈𝑐 +,𝑗〉𝑗 + 〈𝑐 ―,𝑗〉𝑗) 2

∂𝑐 ∂𝑡

, in -phase

(3) (4)

. In Eq. (4) j is a charge

, where j is the phase index, j =  . In eqs. (3) and (4), the 10

assumption of equal average ionic concentrations (〈𝑐 +,𝛾〉𝛾~ 〈𝑐 ―,𝛾〉𝛾) has been used. Eqs. (3) and (4) can be used to simulate the ionic transport processes in the flow electrodes. 3. Results and Discussion 3.1 Optimal conditions for FCDI Salt removal in an FCDI system is sensitive to the applied potential and ionic strength. Therefore, the kinetics of concentration decrease in FCDI at various potentials (1.2 to 2.4 V) and electrolyte concentrations (0.05 to 0.5 M) were investigated. 3.1.1

Applied potential

As shown in Fig. 2A, regardless of the value of C0, the effect of the applied potential on the ion concentration difference is consistent. Increasing the applied potential from 1.2 to 1.6 V led to a significant concentration difference from the initial FS concentration as a function of time; however, when the applied potential increased to 2.0 V, the concentration-difference vs time response almost overlapped with that at 1.6 V for all FCDI experiments. Then, increasing further the applied potential from 2.0 to 2.4 V led again to a significant concentration difference compared to that at 1.6 V. It is worth noting that the concentration difference of FCDI-0.1 M at V= 1.6 V was slightly higher than that at V= 2.0 V, which may be caused by the evolution of system properties (e.g., continuously changing osmotic pressure between the two sides of IEMs, intensified faradaic reactions, and degradation of membranes and carbon particle properties) or experimental variability. In contrast, the concentration difference vs time in the ED process was approximately linear for all potentials and much lower than the corresponding response in the FCDI system (Fig. 2B). Another outstanding result is that at low concentrations (e.g., C0 = 0.05 and 0.1 M), the FCDI device removed nearly all the ions from FS, even at a relatively low potential (V= 1.2 V). At a moderate concentration (C0 = 0.2 M), nearly complete depletion of ions from the FS was achieved under overpotential (2.4 V, Fig.

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2A), even though the concentration difference and osmotic pressure between FE and FS could be as high as 20 g L−1 and 1.6 MPa, respectively (Figs. S2 and S3).

Fig. 2. Time-dependent concentration differences from the initial concentration of the feed solution in FCDI (A) and ED (B) processes under various values of applied potential (V = 1.2, 1.6, 2.0, and 2.4 V) and initial salt concentration (C0 = 0.05, 0.1, 0.2, and 0.5 M).

Theoretically, when the applied potential is increased, the driving force for ion migration increases and, consequently, the SRR value increases, which is verified from the concentration difference vs time responses in the ranges of 1.2-1.6 V and 2.0-2.4 V, as well as from results published in the literature [26, 27]. The abnormal kinetic behavior of the concentration difference in the range of 1.6-2.0 V indicates that an applied potential in this range triggers faradaic reactions in the FCDI cell, leading to a decreased actual ion driving force. By combining results reported by Liang et al. [27], we may predict that the potential at which faradaic reactions appear is between 1.6 and 1.8 V. The optimum value of applied potential may vary slightly with the composition of FEs (e.g., the type and concentration of electrolyte and the particle size and content of carbon powders) and the configuration of the FCDI device (e.g., carbon properties, membrane resistance, and spacer thickness). Overall, the optimum

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applied potential for a stable FCDI operation is in the range of 1.6 V. Beyond this voltage, some of the electrical energy provided to the electrodes is wasted.

3.1.2

Electrolyte concentration

Inspired by the studies of Kim’s [18] and Hatzell’s groups [34], this section aims to identify an optimum electrolyte concentration that ensures a high conductivity of the FEs without causing severe ion back-diffusion. To investigate the effect of electrolyte concentration on the FCDI performance, we plotted the SRR vs time for different electrolyte concentrations in Fig. 3 (see the calculation of SRR in Text S1 of Supporting Information).

Fig. 3. Dynamics of salt removal rate (SRR) in FCDI (A) and ED (B) processes under various values of applied potential (V = 1.2, 1.6, 2.0, and 2.4 V) and initial salt concentration (C0 = 0.05, 0.1, 0.2, and 0.5 M). (To clearly observe the SRR dynamics of the ED process, readers are recommended to visit the online version of this figure.)

Since the ion driving force was weakened by the depletion of ions in the FS, all the SRRs kept decreasing. As depicted in Fig. 3A, for all the applied potentials, FEs exhibited optimum SRR kinetics at C0 = 0.2 M. For instance, at C0 = 0.05, 0.2, and 0.5 M, the ASRR values of FEs were in the ranges of 2.70-4.96, 5.93-11.30, and 6.26-10.91mg s−1 m−2, respectively. The decrease in the ASRR values from C0 = 0.2 to 0.5 M is not significant, suggesting that FCDI 13

may be effective for the full range between 0.2 and 0.5 M electrolyte concentration, even though undesirable phenomena may occur at high ion concentrations including: (i) The ion accommodation capacity of the electrolyte may be limited. Although the solution concentrations on both sides of the IEMs were the same at the beginning of the process, the electrolyte concentration increased significantly with applied potential and gradually approached saturation as more ions were accumulated in the FEs. (ii) Due to the uneven distribution of ion-exchange groups in the IEMs [48], the selective permeability of IEM may deteriorate at high concentrations, leading to adsorption of coions or counter ion leakage caused by the concentration difference of ions across the IEMs [49, 50]. (iii) The energy consumption by any CDI technique, including FCDI, increases with increasing salt concentration [16]. These results and potential negative phenomena at high salt concentrations indicate that a moderate concentration (e.g., 0.2 M) is more suitable as it ensures a high FE conductivity and allows stable operation of the FCDI system. As described by Rommerskirchen et al. [51] and Doornbusch et al. [19], however, during a practical operation of the FCDI process, the concentration difference between the FS and the FE electrolyte would be constant, which differs from the constantly changing concentration difference in this study. Since the concentration of FE will always be higher than FS (back-diffusion of counterions, Fig. S2), it is necessary to more accurately determine the range of optimum electrolyte concentration in practical applications. It is worth noting that the effects of salt concentration (0.05 to 0.5 M) and applied potential (1.2 to 2.4 V) on FCDI desalination are different from the effects of the same variables on overpotential assisted membrane CDI (OP-MCDI). According to our previous studies, the desalination rate of MCDI increased monotonically with increasing electrolyte concentration

14

and applied potential [29, 52]. This behavior could be explained by their different electrosorption mechanisms and device configurations. In OP-MCDI, ions are removed and stored in the EDLs of stationary carbon electrodes, with negligible influence from ED. In FCDI, however, in addition to the EDL capacitive mechanism, the electrodialysis mechanism partially contributes to desalination (Table S1) [17, 26]. Furthermore, since the carbon particles are immersed into electrolyte solution in an open environment, oxygen-promoted carbon degradation may be more severe in FCDI, leading to a different dependence of FCDI performance on applied potential and salt concentration. The results and discussion presented in this section indicate that to avoid triggering faradaic reactions and ensure a high enough conductivity for FEs, FCDI should operate at the optimal conditions of 1.6-V applied potential and 0.2-M electrolyte concentration. Furthermore, since desalination by the electrodialysis mechanism is different in FCDI and OP-MCDI, the influence of operating conditions on ED should also be considered. 3.2 Comparative studies of FCDI and ED desalination FCDI and ED share similarities in equipment assembly and desalination mechanism [17, 26, 37]; therefore, it would be interesting to investigate if the salt concentration and applied potential had similar influences on the ED process (see Figs. 2B, 3B, and S4). According to our results, however, inconsistent effects have been observed for FCDI and ED because of the presence of carbon particles in the FEs of FCDI (Fig. 4). 3.2.1

Influence of applied potential and electrolyte concentration on ED

With regard to the influence of applied potential, unlike the hindered desalination performance at 2.0 V for FCDI, the desalination performance of ED increased almost linearly with increasing applied potential. As shown in Fig. S5A, the ASRR values vs applied potential of ED were linearly fitted with slopes of 0.737, 0.617, 0.493, and 0.411 for C0 = 0.05, 0.1, 0.2, and 0.5 M, respectively.

15

Fig. 4. Desalination performance in FCDI (top) and ED (bottom) as a function of initial salt concentration: (A) average salt removal rate (ASRR), (B) average charge efficiency, and (C) energy consumption. Data points were connected using modified Bezier curves.

Even though the influence of salt concentration on ED was not clearly shown in Fig. 3B, it demonstrated a strong impact on the change of ASRR values for different levels of applied potential (Fig. 4A). For example, at V= 1.2 V, desalination by ED could be enhanced by increasing the salt concentration because faradaic reactions were either not triggered or rather slow at low voltage, while increasing the salt concentration increased also the electrical conductivity of the ED module. When the applied potential was further increased (V= 1.6, 2.0, or 2.4 V), however, the desalination performance of ED deteriorated significantly (ASRR: 0.61 to 0.44 mg s−1 m−2) with increasing salt concentration (C0: 0.05 to 0.5 M), due to intensified faradaic reactions at high voltages and salt concentrations [53]. Respectively, the ED process demonstrated better charge efficiency and lower energy consumption at a higher concentration and a lower potential (e.g., 1.2 V, Figs. 4B and 4C) [54]. For other cases, increasing both the applied potential and salt concentration would dramatically decrease the charge efficiency and increase the energy input. 16

3.2.2

Different desalination performance of FCDI and ED

The preceding results clearly show the different influences of applied potential and electrolyte concentration on FCDI and ED. For FCDI, optimal values of 1.6-V applied potential and 0.2-M electrolyte concentration were determined; however, ED desalination favored high applied potential and low electrolyte concentration. Comparative studies of FCDI and ED also demonstrated that FCDI had much better desalination performance than ED (Figs. 4B, 4C, and S5B). For example, at the optimal conditions determined here (i.e., C0 = 0.2 M, V = 1.6 V), the ASRR values, average charge efficiency, and energy consumption of FCDI were 8.61 mg s−1 cm−2, 91.2%, and 36.9 kT/ion, respectively. The same performance metrics had values of 0.21 mg s−1 cm−2, 73.0% and 43.4 kT/ion for ED under identical operating conditions. Nevertheless, ED still contributes to the desalination performance of FCDI through electrodialytic mechanism [26], and its contribution is stronger at high applied potentials and low salt concentrations (Table S1). By analyzing the configuration differences of these two desalination modes (with and without carbon particles in FEs) and considering the similar stagnant FCDI performance at 1.8 V in the study by Liang et al. [27], in which sodium sulfate solution was used as electrolyte, we determined that the differences were most likely caused by the presence of carbon particles rather than the type of electrolyte. Due to the absence of carbon particles, ion transfer in ED was subject to the driving force and the ionic strength. It is reasonable that the ion transfer in ED is faster at a higher applied potential because of the intensified electric field. Even though high ionic strength can enhance ion transfer, it can also increase the concentration polarization effect in ED. In contrast, since the carbon particles promote charge/ion transfer in the FEs and store ions into their pores, the concentration polarization effect was only observed at a high concentration of 0.5 M in FCDI. For instance, compared to the performance deterioration of

17

ED at moderate concentrations of C0= 0.1 and 0.2 M, FCDI removed almost all the ions from FS under identical conditions. 3.2.3

pH changes in FCDI and ED

Several studies have shown that the addition of carbon particles in FEs can enhance the ion/charge transfer. One evidence for this behavior is that the pH value can be maintained in a relatively-stable and near-neutral range [19, 26, 30]. Since the presence of carbon is the reason behind the differences between the FCDI from ED performance at various values of applied potential and electrolyte concentration, the pH response in these two systems could be an important factor in analyzing the differences.

Fig. 5. Time-dependent pH changes in the feed solution (FS) and flow-electrode (FE) for (A) FCDI and (B) ED systems. From the left panel to the right: C0 = 0.05, 0.1, 0.2, 0.5 M.

As shown in Fig. 5A, due to the presence of carbon particles, the pH values of FEs are in the alkaline range (pH > 7) under all conditions except for C0 = 0.2 M and V = 2.4 V. Meanwhile, 18

the FS became more acidic as the experiments progressed for increasing applied potential and electrolyte concentration. During the 3-6 h duration of the experiments at FS concentrations of C0 = 0.05 and 0.1 M, the increase of pH values was caused by the adsorption of hydrogen ions. These results can be explained by the ongoing faradaic reactions and the decreased charge efficiency of the system (Figs. 4B, S6, and S7). For the alkaline FE, hydrogen and hydroxide ions should be neutralized after particle collision, charge neutralization, and discharge of ions to bring their pH close to neutral. The unbalanced pH values may be caused by (i) uneven adsorption and (ii) unbalanced faradaic reactions of cathode and anode due to the surface properties of carbon [17, 26, 55]. Nativ et al. [17] and Ma et al. [26] respectively reported a higher affinity of carbon particles to sodium and chloride ions. According to their work, when carbon had an affinity to sodium ions, the concentration of hydrogen ions generated in the anode was higher than the concentration of hydroxyl ions generated in the cathode, while the opposite was shown when carbon was more likely to adsorb chloride ions. The fact that FEs were alkaline in this research (i.e., a surplus of hydroxyl over hydrogen ions) indicates that some charge in the cathode was consumed to generate more hydroxyl ions; therefore, the carbon particles employed in the study have a preference of adsorbing chloride ions in the anode. Even so, due to carbon oxidation at the anode, the pH can be decreased to near neutral at an overpotential (e.g., V = 2.4 V), at which point the faradaic reactions at the cathode and anode may be balanced. It should also be noted here that the preference of carbon particles to adsorb sodium or chloride ions should be correlated to their surface charge, i.e., carbon particles of positive surface charge could preferentially adsorb chloride ions, while negatively charged particles should preferentially adsorb sodium ions, and the particle surface charge can be manipulated by surface modification [30, 31]. Due to the absence of carbon particles in ED, the cathode and anode did not have a higher affinity to any ions. Therefore, pH variations could only be caused by faradaic reactions, such

19

as water splitting and oxygen reduction [53, 55]. As shown in Fig. 5B, pH changes in ED showed dependence on the applied potential. This is probably because different faradaic reactions are triggered at different potentials. For example, when V = 1.6 V, oxygen reduction may occur and cause a pH increase [56]. However, at V = 2.4 V, the electrical energy is sufficient to trigger additional faraday reactions in the system; meanwhile, a pH change due to concentration polarization at the FE/membrane interface may also occur.

3.2.4

Stability operated at optimal conditions

In addition to the pH measurements of the system to evaluate the quality of treated water, attention should also be focused on changes in carbon-particle properties that will provide an insight for the long-term operation of the FCDI process with regard to the process stability and desalination performance. Changes of the zeta potential of carbon particles in the cathode and anode of the FCDI system are presented in Fig. 6. It is noted that these measurements could only be made possible after the anodic and cathodic suspensions had been diluted and stored for more than 24 hours, during which ions adsorbed in the EDL diffused into the bulk solution and the EDL expanded and established a diffusion equilibrium [57]. The zeta potential of the same particles at different ionic strengths shows the same isoelectric point, and its absolute value decreases as the ionic strength is increased. Thus, the absolute values of the obtained zeta potential measurements for dilute suspensions should be higher compared to those of the original suspensions in the FEs. This behavior means that the zeta potential values of the concentrated suspensions should be slightly shifted upward in Fig. 6, toward (but not over) the 0-mV potential line.

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Fig. 6. Zeta potential changes of the anode and cathode carbon particles during the operation of the flow-electrode capacitive deionization (FCDI) system.

After one hour of operation, the carbon particles in both the anode and cathode deviated from the initial surface charge (−25.9 mV) by approximately 15 mV. Subsequently, the surface charge of cathodic particles sustained a stable value of −40 mV, whereas that of the anodic particles ranged from −10 to −15 mV. The drifting from −10 to −15 mV should be attributed to the hydroxyl, carbonyl, or carboxyl functional groups generated by carbon oxidation [58]. This result indicates that, even at 1.6 V and cycled in ICC operation, the anode experienced a slow degradation. Since SCC FCDI operation has been reported to be much more energyefficient than ICC FCDI operation [59], and because in SCC operation the carbon particles are constantly redistributed in the anode and cathode FEs, the performance degradation of FEs may slow down in SCC operation [60]. 3.3 Simulation results The model presented in Section 2.4 can be used to qualitatively simulate the experiments performed in this work (Fig. 7). The reason is that the experimental FE operates through flow in channels that are mostly horizontal in shape [35], while the model applies to a half-cell which has the same vertical area for the water channel, membrane, FE, and current collector. The model can also be used to simulate the ED process in the half-cells depicted in Fig. 1. 21

Fig. 7. Simulation results: (A) average water concentration in desalination channel as a function of applied voltage, (B) influence of salt channel initial concentration on water desalination efficiency, and (C) comparison between ED and FCDI. The data used in the simulations are provided in Table S2. In all cases, we calculated a dimensionless average salt value in the water flow channel as representative of all salt removed during the cell operation. The assumption of symmetric FCDI cells was used in all the 22

calculations. In the case of a binary, single charge salt, NaCl for example, the assumption leads to removal of the same number of ions in both half-cells. In Fig. 7A, the influence of the applied potential on salt removal is exhibited. Similar to the experimental results (Fig. 2A), the simulation results show that salt removal increases with applied potential until a critical value is reached (1.5 V); higher potential values do not significantly change the salt removal rate. These results show that water desalination by the FCDI process is controlled by the electromigration of ions from the water channel into the FEs until the critical voltage value is reached. For higher voltage values, the process becomes controlled by concentration polarization, and electrochemical reactions are expected to appear. Calculation of concentration polarization values, not shown here, proved that, in fact, this is the case for voltage values greater than 1.5 V. The effect of initial salt concentration on the overall efficiency of the simulated FCDI process is shown in Fig. 7B. As initial salt concentration increases, the overall ion capture efficiency (% of the initial amount removed) decreases as the EDLs inside the porous particles become smaller. The absolute amount of salt removed increases, however, as more salt is captured in the porous particles. This is the reason why a plot of average SRR shows a reverse trend; i.e., the higher the initial concentration the higher the SRR value. Simulation results, not shown here, confirmed this trend. A comparison between simulated ED and FCDI processes under identical operating conditions indicates that, when the porous particles are present, salt removal increases significantly (Fig. 7C).

4. Conclusions FCDI is an alternative to conventional desalination processes for waters of wide-ranging salinity because of its fast deionization rates, continuous operation, and low capital and operating costs. By using common AC particles to prepare the FEs, determining optimal 23

operating conditions is an important step prior to a practical application; however, no consensus has been reached yet on the range of operating parameters that are most suitable for FCDI. In this study, by systematically comparing the desalination performance of FCDI and ED at various values of applied potential and salt concentration, we have derived several important conclusions: (i) FCDI has the fastest desalting rate, excellent charge efficiency, and low energy consumption when the initial ion concentration (C0) in FS is in the range of 0.2 M (or 10,00015,000 ppm) and V=1.6 V. (ii) ED contributes to the desalination of FCDI, but not significantly. (iii) The applied potential and salt concentration influence differently the desalination performance of FCDI and ED. Instead of showing optimal conditions, the desalination of ED increases linearly with increasing applied potential; however, the influence of the electrolyte concentration shows a strong dependence on the applied potential. (iv) The difference between FCDI and ED is mainly due to the presence of carbon particles in FEs. A preliminary investigation of the stability of FEs at the optimum conditions indicates that the degradation of anodic particles is inevitable but not significant. The theoretical model used in this work only allows for qualitative comparisons. However, a very good qualitative agreement was found between experimental data and simulation results. The model used predicts an increase in salt removal rate as the applied voltage increases. After a critical voltage value is reached, salt removal is controlled by concentration polarization. The predicted critical value (1.5 V) was found to be very close to the experimentally determined value (1.6 V). The SRR increases as the initial concentration in the FS increases; however, the overall salt removal efficiency decreases. The addition of conducting porous particles to the FEs significantly increases the salt removal rate compared to the ED process. Regarding the limiting voltage (or current) of the water-based FEs, future research should be focused on exploring the factors that limit voltage/current and ways to eliminate those limitations, such as the design of a new FCDI module and synthesis of new materials with

24

parallel optimization of the operating conditions, so that new FCDI modules can sustain a higher potential to enable faster ion removal and more efficient water desalination.

Acknowledgments Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DEAC05-00OR22725 with the US Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan). The authors would like to thank the School of Civil and Environmental Engineering of the Georgia Institute of Technology and the National Natural Science Foundation of China (NSFC) for financial support via Grant numbers 51425405 and 21377130. Kexin Tang appreciates the financial support from the China Scholarship Council (CSC, No. 201606250079). Costas Tsouris acknowledges support from the Laboratory Directed Research and Development Program of the Oak Ridge National Laboratory. Special thanks are given to Mr. Yongxia Yang for his help in using the ball-milling machine.

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Graphical abstract

31

Highlights 

Optimal flow-electrode capacitive deionization (FCDI) occurs at 1.6V and 0.2M salt.



FCDI and electrodialysis (ED) have different dependencies on operating conditions.



ED contributes less to desalination by FCDI at higher concentrations.



The difference of FCDI and ED is caused by the presence of carbon particles in FCDI.



Good agreement was found between experimental and simulation results.

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Author Statement Kexin Tang: Methodology, Investigation, Writing – Original draft preparation, Visualization Sotira Yiacoumi: Project administration, Resources Yuping Li: Co-supervision, Funding acquisition Jorge Gabitto: Software, Data Curation Costas Tsouris: Conceptualization, Validation, Supervision, Writing - Reviewing and Editing

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