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Treatment and Resource Recovery

Unravelling pH Changes in Electrochemical Desalination with Capacitive Deionization
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  • Antony C. Arulrajan
    Antony C. Arulrajan
    Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands
    Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands
    More by Antony C. Arulrajan
    Orcidhttps://orcid.org/0000-0001-9306-449X
  • Jouke E. Dykstra*
    Jouke E. Dykstra
    Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands
    *Email: [email protected]
    More by Jouke E. Dykstra
    Orcidhttps://orcid.org/0000-0002-0377-4779
  • Albert van der Wal
    Albert van der Wal
    Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands
    More by Albert van der Wal
  • Slawomir Porada*
    Slawomir Porada
    Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands
    *Email: [email protected]
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Environmental Science & Technology

Cite this: Environ. Sci. Technol. 2021, 55, 20, 14165–14172
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https://doi.org/10.1021/acs.est.1c04479
Published September 29, 2021

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Abstract

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Membrane capacitive deionization (MCDI) is a water desalination technology employing porous electrodes and ion-exchange membranes. The electrodes are cyclically charged to adsorb ions and discharged to desorb ions. During MCDI operation, a difference in pH between feed and effluent water is observed, changing over time, which can cause the precipitation of hardness ions and consequently affect the long-term stability of electrodes and membranes. These changes can be attributed to different phenomena, which can be divided into two distinct categories: Faradaic and non-Faradaic. In the present work, we show that during long-term operation, as the electrodes age over time, the magnitude and direction of pH changes shift. We studied these changes for two different feed water solutions: a NaCl solution and a tap water solution. Whereas we observe a pH decrease during the regeneration with a NaCl solution, we observe an increase during regeneration with tap water, potentially resulting in the precipitation of hardness ions. We compare our experimental findings with theory and conclude that with aged electrodes, non-Faradaic processes are the prominent cause of pH changes. Furthermore, we find that for desalination with tap water, the adsorption and desorption of HCO3–and CO32– ions affect the pH changes.

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Synopsis

This work provides mechanistic understanding of undesirable processes that limit the long-term stability of electrically driven water desalination technologies.

Introduction

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Increasing demand for surface water and groundwater has resulted in water stress and scarcity in many countries. (1) To overcome water scarcity, technologies can be employed to produce potable water from brackish or saline sources. (2) Capacitive deionization (CDI) is one such technology for water desalination employing porous electrodes, which are cyclically charged and discharged. In conventional CDI, during the charging step, ions are adsorbed in electrical double layers (EDLs) in the micropores of the porous electrodes, and feed water is desalinated. During discharge, ions are desorbed, resulting in a concentrated effluent stream. (3,4)
Although CDI is a promising technology for brackish water desalination, stable long-term operation remains a challenge. (3,5) In addition to capacitive ion storage in EDLs, undesirable Faradaic processes also occur. (6,7) These Faradaic processes are considered to be the main reason for a decrease in desalination performance over time. Furthermore, these processes can cause pH changes during desalination, (8−10) which can result in the precipitation of salt when hardness ions (Mg2+, Ca2+, and Fe2+/Fe3+) and carbonate ions (HCO3– and CO32–) are present in solution and consequently affect the desalination performance and long-term stability of the CDI cell. (8−10)
Several mechanisms cause pH changes in CDI, which can be divided into two categories: Faradaic reactions and non-Faradaic processes. Non-Faradaic processes, such as differences in ionic mobilities, affect individual ion adsorption rates, for example of H+ and OH−, and thereby the local pH. Other processes that must be considered are acid-base reactions between ionic species, and the protonation or deprotonation of chemical surface groups present on the electrode material, which can also result in pH changes during CDI operation. (11) Faradaic reactions that can occur, either at the anode or at the cathode, have also been studied. While at the anode, reactions such as carbon, chloride, and water oxidation are mostly considered, at the cathode, the reduction of dissolved oxygen and water is relevant. (6,7,12,13) Among these reactions, carbon oxidation and dissolved oxygen reduction have been shown to significantly destabilize the desalination performance. At the anode, carbon oxidation results in the formation of oxygen-containing chemical groups. The direct consequence of these oxygenated surface groups is the occurrence of “inversion peaks” during operation which lowers the desalination efficiency by enhanced coion expulsion from the electrodes. (14) At the cathode, oxygen reduction has experimentally been confirmed by measuring the dissolved oxygen and hydrogen peroxide concentration changes in the effluent water during CDI operation. (15) Other Faradaic processes that can occur during operation are chloride oxidation (formation of Cl2 gas) at the anode and water reduction (formation of H2 gas) at the cathode. Formed Cl2 gas can immediately dissociate in water to form HOCl, resulting in a decreased effluent pH. (6) Faradaic reactions related to carbon oxidation occur mainly on pristine electrodes, which are electrodes that have only been used for a limited number of cycles before the study was conducted. (5)
In order to mitigate negative effects of Faradaic processes in CDI, three main strategies have been proposed. First, different operational modes have been investigated to limit the cell voltage, to optimize the operational voltage window, (16−18) or to invert the cell voltage applied during charging and discharging steps. (19,20) Second, alternative CDI cell designs are considered, either by applying different flow patterns (i.e., flow by/flow through) (17,18) or by placing ion-exchange membranes (IEMs) between the flow channels and the porous electrodes, a cell design we refer to as membrane capacitive deionization (MCDI). (21−23) Third, electrode modifications, such as titania deposition on carbon to prevent carbon oxidation, have been proposed. (24,25)

Figure 1

Figure 1. MCDI (a–c) and inverted MCDI (i-MCDI) (d–f) for cyclic water desalination. In MCDI, water is desalinated during charging (b), and the electrodes are regenerated during discharge (c). In i-MCDI, the electrodes are modified with ion-exchange polymers which are located in the carbon pores, resulting in high counterion concentrations in the electrodes in the uncharged state (d). During charging, these ions are expelled from the electrode compartments, and a concentrated salt solution (concentrate) is produced (e). During discharge, water is desalinated (f).

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One of the most effective strategies to keep a high desalination performance is the inclusion of IEMs in the CDI cell, a cell design we refer to as MCDI. These IEMs are placed in front of the electrodes and contribute to maintaining the desalination performance in two ways: the membranes effectively slow down the transport of dissolved oxygen to the electrodes and consequently limit the rate of carbon oxidation and the related pH changes. (26,27) However, during long-term operation, dissolved oxygen will eventually oxidize the electrically positively polarized electrode (anode), and oxygen-containing surface groups will form. (22,28) Whereas these groups would have negatively affected the desalination performance in CDI (without membranes) due to enhanced coion expulsion during desalination, in MCDI, the desalination performance does not decline because IEMs prevent coions from leaving the electrode region. (22,28) Furthermore, Faradaic reactions during MCDI operation were also confirmed in our previous work by microscopic physics-based modeling of pH changes. (11) This study predicted only minor pH changes during desalination in MCDI when non-Faradaic processes, including the effect of different mobilities of various ions on transport rates, combined with acid–base reactions and the presence of chemical surface groups in the micropores are taken into account. In order to match theoretical predictions of pH changes with experimental data, this study concluded that Faradaic reactions in the micropores must be considered. (11)
MCDI has successfully been applied on industrial scale, and systems have been running for many consecutive cycles. (29) In these systems, pH changes are still observed, and although these changes do not directly affect the salt adsorption performance, they can still be problematic.
Experimentally and theoretically, many studies have reported a pH decrease during desorption if the feed water only contains sodium chloride. However, the present work shows that with real brackish water (tap water), which contains a mixture of different ions, a pH increase is observed during desorption. This pH increase during desorption is problematic, especially when we operate at high water recoveries, resulting in high salt concentrations in the flow channel during desorption. These conditions, with high salt concentrations combined with high pH values, can lead to the precipitation of salts such as calcium carbonate (scaling) on membranes or other components of the CDI cell. Furthermore, if the aim is to remove amphoteric ions, which are ions that can protonate and deprotonate based on solution pH, such as phosphate, ammonium, carbonate and boron, the local pH affects the charge of the ion and thus the removal performance.
To improve our understanding of these pH fluctuations, we conducted experiments with aged electrodes. These aged electrodes have been used in MCDI for more than 2000 desalination cycles and are therefore (partly) oxidized. Furthermore, we conduct experiments both with a NaCl solution and with tap water, and we perform experiments with different water recoveries to be close to practical desalination conditions. We study these fluctuations in conventional MCDI operation and in i-MCDI operation. In i-MCDI, electrodes are chemically modified: we synthesized an electrode with positively charged chemical groups and an electrode with negatively charged chemical groups. A cation-exchange membrane (CEM) was placed in front of the electrode modified with negatively charged chemical groups and an anion-exchange membrane (AEM) was placed in front of the electrode modified with positively charged chemical groups (Figure 1). Fritz et al. (2019) demonstrated that this configuration can be successfully employed for water desalination, that water is desalinated during discharge (adsorption), and that a concentrated effluent stream (desorption) is produced during the charging step. (30)
This study reports pH fluctuations with aged electrodes in MCDI and i-MCDI operation at different water recoveries. Furthermore, we compare our experimental findings of the NaCl experiments with theory that only includes non-Faradaic processes, and we find that the theory describes the trends of the experimental data well.

Materials and Methods

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Desalination experiments were conducted using an MCDI and an i-MCDI stack. The MCDI stack consisted of two cells, each with two activated carbon electrodes (PACMM 203, δe ∼ 250 μm, Material Methods, Irvine, CA, USA), two IEMs, and a nylon spacer for an evenly distributed water flow. A CEM (Neosepta CMX, ASTOM Corporation, Tokyo, Japan) was placed in front of one electrode, an AEM (Neosepta AMX, ASTOM Corporation, Tokyo, Japan) was placed in front of the other, and a nylon spacer was placed between the membranes. The i-MCDI stack consisted of two cells with chemically modified electrodes, which were activated carbon electrodes modified with ion-exchange polymers. Anion-exchange polymer-modified electrodes (AEPEs) were prepared by immersing activated carbon electrodes in an ionomer solution [FAS solution, 24 wt % of the polymer dissolved in N-methyl-2-pyrrolidone (NMP); Fumatech, Bietigheim-Bissingen, Germany] that was diluted six times using NMP. Immersed electrodes were then placed inside a vacuum chamber for 30 min and then dried for 24 h at 100 °C. Similarly, the cation-exchange polymer-modified electrodes (CEPEs) were prepared by immersing the activated carbon electrode in a six-time-diluted (using NMP) ionomer solution (FKS solution, 16 wt % of the polymer dissolved in NMP; Fumatech, Bietigheim-Bissingen, Germany). After modification, the electrodes were washed with ultrapure water before placement in the i-MCDI stack. The i-MCDI stack consisted of two cells, each with an AEPE and a CEPE. In front of each CEPE electrode, a CEM was placed, and in front of each AEPE electrode, an AEM was placed, and a nylon spacer was placed between the CEM and AEM. Graphite sheets were used as current collectors to electrically connect the porous carbon electrodes with the external electrical circuit, which connects the stack to a potentiostat (Iviumstat, Ivium Technologies, The Netherlands).
All desalination experiments were conducted using aged electrodes and NaCl solutions prepared using ultrapure water (resistivity of 18.2 Ω cm at 25 °C) or tap water. The electrodes were aged by charging and discharging the MCDI stack for more than 2000 and the i-MCDI stack for more than 3000 desalination cycles using constant current operational mode with a fixed current density (11.1 A/m2), and a 5 mM NaCl solution, which was constantly purged with compressed air, was pumped through the MCDI or i-MCDI stack at a 12.5 mL/min flow rate (effluent pH changes during the aging process of an MCDI stack are shown in Figure S1). After the aging process, to compare experimental data with theoretical calculations, desalination experiments were conducted with 5 and 20 mM NaCl solutions which were purged with N2. Thereafter, further desalination experiments were conducted, either with a 5 mM NaCl solution purged with air or with tap water (not purged with air, neither with N2), for 80–100 desalination cycles. All desalination experiments were conducted in constant current operational mode with the same current density and flow rate used during the aging process. Desalination experiments were conducted for different values of water recovery. To increase water recovery, the flow rate during desorption was reduced. During desalination experiments with NaCl solution, the feed water was pumped from a 50 L tank which was purged with compressed air to the MCDI or i-MCDI stack and the effluent water was recycled to the same tank. In the tap water experiments, the feed water was pumped from a 20 L tank to the MCDI or i-MCDI stacks and the effluent water was not recycled. The tap water feed tank was constantly refilled to maintain the water level. During desalination experiments, the feed water pH was continuously measured using a pH sensor connected to the stack inlet. Similarly, the effluent pH and conductivity were continuously measured.
For the desalination experiments conducted with a NaCl solution, the salt concentration was calculated from the measured conductivity using a calibration curve. For the tap water experiments, in addition to the online conductivity and pH measurements, the effluent water during adsorption and desorption (from the last three cycles of each experiment) was collected separately and the pH was immediately measured after sampling. Also, the concentrations of cations (Na+, K+, Ca2+, and Mg2+) and anions (Cl–, NO3–, PO43–, and SO42–) present in the samples were analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES, Optima 5300 DV, PerkinElmer) and ion chromatography (IC, Dionex Aquion, Thermo Scientific) which are shown in Table S1 in the Supporting Information. The concentration of dissolved inorganic carbon (DIC), i.e., the total concentration of the ionic species H2CO3, HCO3–, and CO32–, and the effluent pH (during adsorption and desorption) were calculated using Visual MINTEQ 3.1 software, as discussed in the next section.

pH and Ionic Speciation Calculations Using Visual MINTEQ

Visual MINTEQ 3.1 is a software tool that uses a chemical equilibrium model to calculate, in water, the ionic speciation, the solubility, and several parameters related to the water chemistry, including pH and DIC, which are calculated based on charge and mass balances of the different (ionic) species in solution. In our study, we used MINTEQ to calculate the pH and DIC of water samples by including the experimentally determined concentrations of all other ions from ICP-OES and IC analysis (Supporting Information, Table S1). Furthermore, we specify the partial CO2 gas pressure in air, which is in equilibrium with the aqueous phase. With these inputs, the pH, the total concentration of DIC, and the speciation thereof (concentrations of H2CO3, HCO3–, and CO32–) were calculated by MINTEQ for tap water and for effluent water during adsorption and desorption.

Results and Discussion

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Comparing Data and Theory: Desalinating a NaCl Solution

Desalination experiments were conducted using an MCDI stack with aged electrodes and a N2-purged NaCl solution for 100 desalination cycles. With aged electrodes, the average concentration reduction during a desalination step, Δc, and charge efficiency were stable for all 100 cycles (see Supporting Information, Figure S2). During the adsorption step, the effluent pH is higher than the feed pH, whereas during desorption, the effluent pH is lower than the feed pH, see Figure 2. We compare experimentally observed and theoretically predicted effluent pH changes by employing the theoretical framework as described in a previous work, (11) which considers the effect of non-Faradaic phenomena, including the effect of different diffusion coefficients of the ions present in solution, and the presence of chemical surface groups. (11) The theoretical pH changes are calculated for a feed solution with a NaCl concentration of 20 mM, whereas the experiments were performed with feed concentrations of 5 and 20 mM. Interestingly, we observe, considering the trend, a good description of the effluent pH changes by theory, i.e., we observe a pH increase during adsorption and a pH decrease during desorption. The dynamics of experimentally observed pH changes with a 5 mM NaCl solution, both for a solution purged with N2 (Figure 2a) and with air (Figure 3a), is similar to the theoretically calculated pH changes for a 20 mM NaCl solution, but the magnitude of the experimentally observed pH changes is larger than that predicted by theory. For the 20 mM NaCl feed solution, the magnitude of experimentally observed pH changes is similar to that of the theoretical predictions for the adsorption step, whereas for the desorption step, the magnitude is slightly larger than that predicted by theory. However, the dynamics is not similar to the predictions, which could possibly be explained by the limited sensitivity of the pH sensor (<0.1 pH unit). Clearly, the theory, which only includes non-Faradaic phenomena, such as the effect of different diffusion coefficients of the ionic species present in solution, describes the data well compared to our previous study. (11) In our previous study, we compared theory with experiments conducted with pristine electrodes, and we concluded that Faradaic phenomena must be considered to explain the experimental data. In the present work, however, we used aged electrodes, and the agreement of theory with data indicates that with aged electrodes, non-Faradaic phenomena are the prominent cause of pH changes.

Figure 2

Figure 2. Experimentally observed effluent pH (a,b) and theoretically calculated effluent pH (c) for MCDI in a N2-purged NaCl solution. The inset in c shows the theoretically calculated zoomed effluent pH change.

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Figure 3

Figure 3. Cell voltage, effluent concentration (or conductivity in the case of tap water), and effluent pH changes of MCDI experiments with a NaCl solution (a) and with tap water (b) and i-MCDI experiments with tap water (c) after reaching dynamic steady state.

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Furthermore, we studied pH changes with an air-purged NaCl-containing solution and with tap water (see Table S1 for the exact composition of the tap water). The pH of the NaCl solution purged with air reduces to 5.6 as CO2 from air dissolves in water, resulting in the formation of carbonic acid (H2CO3) which then dissociates into protons, bicarbonate, and carbonate ions. The pH of tap water is 7.72 due to the presence of carbonate and bicarbonate ions in the solution. The sum of the concentrations of hardness ions, i.e., calcium and magnesium, and of bicarbonate/carbonate ions present in the tap water studied in this work is in the range of moderately hard water. The desalination experiments were done for 80–100 cycles to reach dynamic steady state, which means that the cell voltage, salt adsorption, and effluent pH of a particular cycle are the same as of the previous cycle (see Figure S3 for averaged pH and peak cell voltage for 80–100 cycles). These results are shown in Figure 3a,b. Interestingly, for the MCDI experiments conducted with tap water, the pH changes are reversed compared to the case of NaCl, i.e., the pH value decreases during adsorption and increases during desorption (Figure 3b). As the observed pH increase during desorption with tap water can result in scaling, strategies have to be developed and explored to invert these changes. To that end, tap water desalination experiments were conducted with an i-MCDI stack. Figure 3c shows that the effluent pH changes observed in i-MCDI are, for tap water desalination, similar to those in MCDI: the effluent pH decreased during adsorption and increased during desorption. This finding suggests that the pH changes in tap water are independent of cell operation during adsorption and desorption.
In order to get more insight into pH changes with tap water, after steady state is reached in tap water desalination experiments, the effluent water during adsorption and desorption is collected (after 90 desalination cycles) and the pH and ionic concentrations are measured (composition is reported in Tables S3 and S4 in the Supporting Information). Tables S3 and S4 show that during tap water desalination, the HCO3– ion is the dominantly adsorbed anion, while Na+ is the dominantly adsorbed cation. Since the HCO3– ion is an amphoteric ion (i.e., an ion which can act as both acid and base), which is in local chemical equilibrium with dissolved CO2, any change in the concentration can result in a local shift of acid–base equilibria, which can affect the pH. This finding further supports the argument that for tap water desalination, pH changes are mainly due to the adsorption and desorption of HCO3– ions during operation.
In Figure 4, we compare the experimentally measured relative effluent pH change for tap water desalination during adsorption and desorption with calculated pH values using Visual MINTEQ 3.1. The calculated pH follows a similar trend as the measured pH, i.e., the effluent pH decreases during adsorption and increases during desorption. The calculated values are based on the experimentally determined ionic composition of the water collected during adsorption and desorption, and MINTEQ adjusted the concentration of DIC to maintain the charge and mass balance. Though there is a difference between measured pH and calculated pH under different water recovery conditions, the direction of pH changes is in agreement. This shows that the pH changes observed during tap water desalination are due to a change in the concentration of DIC (mainly HCO3– ions, considering the pH and composition of the analyzed water) during adsorption and desorption. The preferential adsorption of HCO3– ions during tap water desalination could mainly be due their high concentration over other anions present in the tap water.

Figure 4

Figure 4. Comparison between experimentally measured and theoretically calculated effluent pH changes in MCDI and i-MCDI during tap water desalination.

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In order to obtain more insights into the effect of DIC on the effluent pH, we have performed an analysis where we changed the concentration reduction of HCO3– (ΔcHCO3–) during the desalination step, see Figure 5a. For this analysis, we used the ionic composition of the water during adsorption and desorption of the 50% water recovery experiment (Supporting Information, Table S3). In order to maintain the charge balance, we adjusted the concentration of Na+ ions accordingly. Figure 5a shows that the pH changes are dependent on ΔcHCO3– and that an increased removal of HCO3– ions during the adsorption step results in more pronounced pH changes.

Figure 5

Figure 5. (a) Effluent pH prediction as a function of bicarbonate ion removal, ΔcHCO3–. (b) Prediction of effluent pH during desorption for different water recoveries. (c) Prediction of Langelier saturation index (LSI) as a function of water recovery.

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Next, we predicted the scaling potential as a function of water recovery. To do this, we calculated the pH of the water collected during desorption using Visual MINTEQ software for the 50% water recovery experiment based on the measured ionic composition. To calculate the pH of the effluent water during desorption at higher water recoveries, the ionic composition of the solution was calculated by decreasing the volume during desorption while maintaining the total amount of ions desorbed during discharge. Using these calculated ionic compositions, the pH of water during desorption is predicted using MINTEQ, as well as the Langelier Saturation Index (LSI). The LSI is a measure to express the scaling potential based on pH, total dissolved solids (TDS), alkalinity, and the concentration of Ca2+ ions. Figure 5b shows that for water recoveries higher than 70%, we observe a significant scaling potential (LSI > 0.5), and the scaling potential increases with water recovery.
As shown in Figure 5a, during tap water desalination, pH changes are mainly caused by the adsorption and desorption of carbonate species. If carbonate species are not present, however, pH changes are attributed to the effect of the difference in mobilities of Na+, Cl–, H+, and OH– ions on individual ion transport rates. These two different phenomena result in the inversion of the direction of pH changes experimentally observed (Figure 3a,b). To obtain more insights, we have calculated the average effluent pH change for a 20 mM NaCl solution based on the theory as previously described and also for an artificial water solution with a total concentration of salts equal to 20 mM, with equimolar concentrations of NaCl and NaHCO3, using Visual MINTEQ 3.1. Figure 6a shows the calculated pH changes for a NaCl solution as a function of the concentration reduction of NaCl during desalination, ΔcCl−, whereas in Figure 6b, we show pH changes for the case of overall Δc = 3.45 mM, but here the ratio between the adsorption of Cl– and HCO3–/CO32– is varied. Figure 6 clearly shows that the direction of the average pH changes during adsorption and regeneration is exactly opposite for these two cases. Increasing ΔcCl– results in more notable pH changes, with an average pH above the feed water pH during the adsorption step and below during the desorption step. In the case of artificial tap water, pH changes become more notable with the increasing adsorption of carbonate species (HCO3– and CO32–), resulting in a pH decrease during adsorption and increase during desorption. This result highlights the importance to study pH changes under practical desalination conditions and the need to control effluent pH changes by reducing HCO3– adsorption, in order to mitigate scaling problems.

Figure 6

Figure 6. (a) Effluent pH calculated for different values of the concentration reduction of NaCl (i.e., ΔcCl–) and (b) prediction of the effluent pH at fixed concentration reduction (Δc = 3.45 mM) with different concentration reductions of carbonate species (ΔcHCO3–,CO32–) over Cl–.

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In conclusion, we studied pH changes in MCDI and i-MCDI using aged electrodes, with a NaCl solution and with tap water. When dynamic steady state is reached, the observed effluent pH does not significantly change during desalination. The trend of these pH changes with a NaCl feed water purged with N2 is in agreement with our theory that only considers the differences in diffusion coefficients of the ions, which indicates that non-Faradaic processes are the main cause of pH changes with aged electrodes in MCDI. This finding is significantly different from the existing understanding that Faradaic processes are the major cause of pH changes in MCDI. In addition, we showed that pH changes observed for tap water desalination show different dynamics than for NaCl: the effluent pH increases during desorption. We explain this inversion of the direction by the adsorption of bicarbonate ions (HCO3–) during desalination and by the desorption during regeneration. This work also shows that in future studies on pH changes in CDI, experiments with aged electrodes are strongly recommended.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.est.1c04479.

  • Additional information about pH calculations, water compositions, diffusion coefficients used for theoretical calculations, charge efficiency, concentration reduction, pH profiles during aging, and LSI calculation procedure (PDF)

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Author Information

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  • Corresponding Authors
  • Authors
    • Antony C. Arulrajan - Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands;  Wetsus, European Centre of Excellence for Sustainable Water Technology, Oostergoweg 9, 8911 MA Leeuwarden, The Netherlands;  Orcidhttps://orcid.org/0000-0001-9306-449X
    • Albert van der Wal - Environmental Technology, Wageningen University, Bornse Weilanden 9, 6708 WG Wageningen, The Netherlands
  • Author Contributions

    The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was performed in the cooperation framework of Wetsus, European Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is cofunded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the European Union Regional Development Fund, the Province of Fryslân, and the Northern Netherlands Provinces. This work is part of a project that has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 665874. The authors thank the participants of the research theme “Concentrates” for fruitful discussions and financial support.

References

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    Water desalination via capacitive deionization: what is it and what can we expect from it?
    Suss, M. E.; Porada, S.; Sun, X.; Biesheuvel, P. M.; Yoon, J.; Presser, V.
    Energy & Environmental Science (2015), 8 (8), 2296-2319CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Capacitive deionization (CDI) is an emerging technol. for the facile removal of charged ionic species from aq. solns., and is currently being widely explored for water desalination applications. The technol. is based on ion electrosorption at the surface of a pair of elec. charged electrodes, commonly composed of highly porous carbon materials. The CDI community has grown exponentially over the past decade, driving tremendous advances via new cell architectures and system designs, the implementation of ion exchange membranes, and alternative concepts such as flowable carbon electrodes and hybrid systems employing a Faradaic (battery) electrode. Also, vast improvements have been made towards unraveling the complex processes inherent to interfacial electrochem., including the modeling of kinetic and equil. aspects of the desalination process. In our perspective, we critically review and evaluate the current state-of-the-art of CDI technol. and provide definitions and performance metric nomenclature in an effort to unify the fast-growing CDI community. We also provide an outlook on the emerging trends in CDI and propose future research and development directions.
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  4. 4
    Porada, S.; Zhao, R.; van der Wal, A.; Presser, V.; Biesheuvel, P. M. Review on the Science and Technology of Water Desalination by Capacitive Deionization. Prog. Mater. Sci. 2013, 58, 1388– 1442,  DOI: 10.1016/j.pmatsci.2013.03.005
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    4
    Review on the science and technology of water desalination by capacitive deionization
    Porada, S.; Zhao, R.; van der Wal, A.; Presser, V.; Biesheuvel, P. M.
    Progress in Materials Science (2013), 58 (8), 1388-1442CODEN: PRMSAQ; ISSN:0079-6425. (Elsevier Ltd.)
    A review. Porous carbon electrodes have significant potential for energy-efficient water desalination using a promising technol. called Capacitive Deionization (CDI). In CDI, salt ions are removed from brackish water upon applying an elec. voltage difference between two porous electrodes, in which the ions will be temporarily immobilized. These electrodes are made of porous carbons optimized for salt storage capacity and ion and electron transport. We review the science and technol. of CDI and describe the range of possible electrode materials and the various approaches to the testing of materials and devices. We summarize the range of options for CDI-designs and possible operational modes, and describe the various theor.-conceptual approaches to understand the phenomenon of CDI.
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  5. 5
    Uwayid, R.; Seraphim, N. M.; Guyes, E. N.; Eisenberg, D.; Suss, M. E. Characterizing and Mitigating the Degradation of Oxidized Cathodes during Capacitive Deionization Cycling. Carbon 2021, 173, 1105– 1114,  DOI: 10.1016/j.carbon.2020.11.045
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    5
    Characterizing and mitigating the degradation of oxidized cathodes during capacitive deionization cycling
    Uwayid, Rana; Seraphim, Nicola M.; Guyes, Eric N.; Eisenberg, David; Suss, Matthew E.
    Carbon (2021), 173 (), 1105-1114CODEN: CRBNAH; ISSN:0008-6223. (Elsevier Ltd.)
    Capacitive deionization (CDI) is a fast-emerging technol. typically applied to brackish water desalination and ion-selective sepns. In a typical cell, feedwater is desalinated via ion electrosorption into micropore elec. double layers (EDLs) of charging porous carbon electrodes. Several studies have previously demonstrated that oxidizing the cathode via a nitric acid pretreatment enhances the cells salt adorption capacity (SAC). It was recently reported that oxidized cathodes can degrade rapidly during cell cycling, yet the mechanisms and mitigation strategies remain unknown. Here, we exptl. characterize the performance and degrdn. of nitric acid-oxidized com. carbon cloth cathodes. For a full cycle time (FCT) of 100 min and 1 V applied, we obsd. a 42.5% redn. of SAC by the 100th cycle, and measured a redn. in cathode micropore chem. charge concn. at pH = 7 from -1.5 M to -0.25 M after cycling. We further found that cell charging time and electrode mass are major determinants of degrdn. rate, for example, reducing FCT to 30 min and 10 min allows for SAC decay of only ~ 14% and <2%, resp., over 100 cycles. The insights provided here allow us to posit degrdn. mechanisms, and develop long-lasting, high performance CDI cells with oxidized cathodes.
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  6. 6
    He, D.; Wong, C. E.; Tang, W.; Kovalsky, P.; Waite, T. D. Faradaic Reactions in Water Desalination by Batch-Mode Capacitive Deionization. Environ. Sci. Technol. Lett. 2016, 3, 222– 226,  DOI: 10.1021/acs.estlett.6b00124
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    6
    Faradaic Reactions in Water Desalination by Batch-Mode Capacitive Deionization
    He, Di; Wong, Chi Eng; Tang, Wangwang; Kovalsky, Peter; Waite, T. David
    Environmental Science & Technology Letters (2016), 3 (5), 222-226CODEN: ESTLCU; ISSN:2328-8930. (American Chemical Society)
    Non-Faradaic (ion electrosorption) and Faradaic (oxidn.-redn.) effects in a batch-mode capacitive deionization (CDI) system were studied, with results showing that both effects were enhanced with an increase in charging voltage (0.5-1.5 V). Significant concns. of H2O2 were obsd. with the generation of H2O2 initiated by cathodic redn. of O with subsequent consumption occurring as a result of cathodic redn. of H2O2. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concn. of H2O2 generated over a range of CDI operating conditions. Significant pH fluctuations were obsd. at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H2O2 could provide the means of inducing disinfection or trace contaminant degrdn. provided H2O2 could be effectively activated to more powerful oxidants (by, for example, UV irradn.).
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  7. 7
    Lado, J. J.; Pérez-Roa, R. E.; Wouters, J. J.; Isabel Tejedor-Tejedor, M.; Anderson, M. A. Evaluation of Operational Parameters for a Capacitive Deionization Reactor Employing Asymmetric Electrodes. Sep. Purif. Technol. 2014, 133, 236– 245,  DOI: 10.1016/j.seppur.2014.07.004
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    7
    Evaluation of operational parameters for a capacitive deionization reactor employing asymmetric electrodes
    Lado, Julio J.; Perez-Roa, Rodolfo E.; Wouters, Jesse J.; Isabel Tejedor-Tejedor, M.; Anderson, Marc A.
    Separation and Purification Technology (2014), 133 (), 236-245CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
    Capacitive deionization is a potential technol. for water softening. Low-cost high surface area carbons coated with 2 different metal oxides (SiO2 on the cathode and Al2O3 on the anode) were used. CaSO4 removal was studied using a 400-mL reactor in a single-pass mode. The effect of applied voltage and flow rate on ion removal/regeneration, charge efficiency, and energy consumption was detd. High potentials (>1.2 V) led to pH acidification and increased likelihood of Faradaic reactions affecting ion electrosorption and charge efficiency. CaSO4 removal amounted 4.38 mg/g of electrode material after 15 min of cell polarization at 1.2 V. Charge efficiencies of 60% and an energy consumption of 0.12 KWh/mol of salt removed were obtained. Different regeneration modes (open circuit, short-circuit (SC) and reverse voltage (RV)) were studied. SC regeneration resulted in the highest ion regeneration efficiency while short applications of RV increased water recovery values but also increased energy cost. Oxide coatings avoided ion crossover when short circuit or low reverse voltage were used in regeneration. Ca2+ and SO42- adsorbed specifically on SiO2 and Al2O3, resp., with Ca2+ also adsorbing specifically to carbon alone. These chem. affinities directly influence the desorption process.
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  8. 8
    Chen, T.; Neville, A.; Yuan, M. Calcium carbonate scale formation-assessing the initial stages of precipitation and deposition. J. Pet. Sci. Eng. 2005, 46, 185– 194,  DOI: 10.1016/j.petrol.2004.12.004
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    8
    Calcium carbonate scale formation - assessing the initial stages of precipitation and deposition
    Chen, Tao; Neville, Anne; Yuan, Mingdong
    Journal of Petroleum Science & Engineering (2005), 46 (3), 185-194CODEN: JPSEE6; ISSN:0920-4105. (Elsevier B.V.)
    Scale formation is a serious problem encountered in many industries including oil or gas prodn., water transport, power generation and batch pptn. Normally, studies of scale formation have been focused on pptn. processes in the bulk soln. using bulk jar methods where the pptn. tendency rate and inhibitor effectiveness are quantified. Several recent studies have started to focus on scale deposits formed on the surface of metals. Calcareous scale formation was studied both in the bulk soln. and on the metal surface in 3 supersatd. scale formation solns. which represent typical waters encountered in oil and gas prodn. An electrochem. technique, using a rotating disk electrode (RDE), was used to quantify scale formation on the metal surface. With this technique, redn. of O was considered at the surface of a RDE. The rate of O-redn. at the surface of the RDE enables the extent of surface coverage of scale to be assessed. To understand the formation and growth of the surface scale deposit, surface anal. was used in conjunction with this technique. SEM was used for analyzing the microstructure of the scale. At the same time, inductively coupled plasma was used for analyzing the quantity of the ppt. formed in the bulk soln. and scale formed on the metal surface by dissolving the scale. It is demonstrated that bulk pptn. and surface deposition have different dependencies on the index of supersatn. and so to completely understand an industrial scaling system both processes should be studied.
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  9. 9
    Gabrielli, C.; Maurin, G.; Poindessous, G.; Rosset, R. Nucleation and Growth of Calcium Carbonate by an Electrochemical Scaling Process. J. Cryst. Growth 1999, 200, 236– 250,  DOI: 10.1016/S0022-0248(98)01261-5
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    9
    Nucleation and growth of calcium carbonate by an electrochemical scaling process
    Gabrielli, C.; Maurin, G.; Poindessous, G.; Rosset, R.
    Journal of Crystal Growth (1999), 200 (1/2), 236-250CODEN: JCRGAE; ISSN:0022-0248. (Elsevier Science B.V.)
    Calcium carbonate scale was electrochem. deposited from carbonically pure hard waters on gold electrodes of an electrochem. quartz crystal microbalance. Various data concerning the rate of the scaling process were deduced from the chronoelectrogravimetric responses. The nucleation rate of CaCO3 crystals and the surface coverage were evaluated from SEM image anal. The effects of a surface pretreatment, the oxygen concn. and the degree of hardness of the water were studied. TEM was used to identify the crystal structure of small nuclei. Calcite, aragonite or vaterite crystal forms were obtained depending on exptl. conditions. At room temp. calcite was predominantly formed for conditions which were favorable to a fast nucleation rate whereas the vaterite form is systematically obtained for exptl. conditions leading to a slow nucleation rate. The temp. increase favored the formation of either aragonite needles or vaterite crystallites with a different morphol.
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  10. 10
    Wang, T.; Zhang, C.; Bai, L.; Xie, B.; Gan, Z.; Xing, J.; Li, G.; Liang, H. Scaling Behavior of Iron in Capacitive Deionization (CDI) System. Water Res. 2020, 171, 115370,  DOI: 10.1016/j.watres.2019.115370
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    10
    Scaling behavior of iron in capacitive deionization (CDI) system
    Wang, Tianyu; Zhang, Changyong; Bai, Langming; Xie, Binghan; Gan, Zhendong; Xing, Jiajian; Li, Guibai; Liang, Heng
    Water Research (2020), 171 (), 115370CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    This study investigated the fouling and scaling behaviors in a capacitive deionization (CDI) system in the presence of iron and natural org. matter (NOM). It was found that the salt adsorption capacity (SAC) significantly decreased when treating Fe-contg. brackish water, with higher Fe concns. leading to severer SAC redn. Raman spectroscopy, XPS and X-ray diffraction (XRD) anal. demonstrated that Fe2O3 appeared to be the predominant foulant attached on the electrode surface, which was difficult to be removed via backwashing, indicating the irreversible property of the foulant. Further characterizations (e.g., N2 sorption-desorption isotherms, electrochem. impedance spectroscopy and cyclic voltammetry) revealed that the CDI electrodes suffered from obvious deterioration such as sp. surface area loss, resistance increase and capacitance decline with the occurrence of Fe scaling. While the presence of NOM alleviated the Fe scaling through NOM-Fe complexing effects, NOM itself was found to have neg. impacts on CDI desalination performance due to their strong interactions with the carbon electrodes.
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  11. 11
    Dykstra, J. E.; Keesman, K. J.; Biesheuvel, P. M.; van der Wal, A. Theory of PH Changes in Water Desalination by Capacitive Deionization. Water Res. 2017, 119, 178– 186,  DOI: 10.1016/j.watres.2017.04.039
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    11
    Theory of pH changes in water desalination by capacitive deionization
    Dykstra, J. E.; Keesman, K. J.; Biesheuvel, P. M.; van der Wal, A.
    Water Research (2017), 119 (), 178-186CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    In electrochem. water desalination, a large difference in pH can develop between feed and effluent water. These pH changes can affect the long-term stability of membranes and electrodes. Often Faradaic reactions are implicated to explain these pH changes. However, quant. theory has not been developed yet to underpin these considerations. We develop a theory for electrochem. water desalination which includes not only Faradaic reactions but also the fact that all ions in the water have different mobilities (diffusion coeffs.). We quantify the latter effect by microscopic physics-based modeling of pH changes in Membrane Capacitive Deionization (MCDI), a water desalination technol. employing porous carbon electrodes and ion-exchange membranes. We derive a dynamic model and include the following phenomena: (I) different mobilities of various ions, combined with acid-base equil. reactions; (II) chem. surface charge groups in the micropores of the porous carbon electrodes, where elec. double layers are formed; and (III) Faradaic reactions in the micropores. The theory predicts small pH changes during desalination cycles in MCDI if we only consider phenomena (I) and (II), but predicts that these pH changes can be much stronger if we consider phenomenon (III) as well, which is in line with earlier statements in the literature on the relevance of Faradaic reactions to explain pH fluctuations.
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  12. 12
    Lee, J.-H.; Bae, W.-S.; Choi, J.-H. Electrode Reactions and Adsorption/Desorption Performance Related to the Applied Potential in a Capacitive Deionization Process. Desalination 2010, 258, 159– 163,  DOI: 10.1016/j.desal.2010.03.020
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    12
    Electrode reactions and adsorption/desorption performance related to the applied potential in a capacitive deionization process
    Lee, Jae-Hun; Bae, Wi-Sup; Choi, Jae-Hwan
    Desalination (2010), 258 (1-3), 159-163CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Desalination expts. were performed by constructing a capacitive deionization (CDI) unit cell with a carbon electrode prepd. from activated carbon powder (ACP). Through CDI expts., the mechanism of adsorption, desorption and electrode reactions were investigated by measuring cond., effluent pH, and the current passed through the cell under different electrode potentials. The salt-removal efficiency increased with increasing potential at the range of 0.8-1.5 V. Addnl., the pH of the soln. varied significantly with a change in potential. At potentials less than 1.0 V, the pH increased due to the redn. of dissolved oxygen and the pH decreased at potentials over 1.2 V due to oxidn. reactions at the anode. The change in current revealed that adsorbed ions were not completely desorbed and a fraction of ions were retained at the carbon electrode. These accumulated ions were re-adsorbed at the electrode surface when a potential was re-applied, which led to a decrease in the salt-removal efficiency of CDI.
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  13. 13
    Landon, J.; Gao, X.; Omosebi, A.; Liu, K. Emerging Investigator Series: Local PH Effects on Carbon Oxidation in Capacitive Deionization Architectures. Environ. Sci.: Water Res. Technol. 2021, 7, 861– 869,  DOI: 10.1039/D1EW00005E
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    13
    Emerging investigator series: local pH effects on carbon oxidation in capacitive deionization architectures
    Landon, James; Gao, Xin; Omosebi, Ayokunle; Liu, Kunlei
    Environmental Science: Water Research & Technology (2021), 7 (5), 861-869CODEN: ESWRAR; ISSN:2053-1419. (Royal Society of Chemistry)
    In this work, the effect of pH and potential is examd. for the oxidn. of carbon cloth electrodes used in capacitive deionization (CDI) processes. The degree of oxidn. of the electrode surface, examd. using the electrode's potential of zero charge (Epzc) and measured using chronoamperometry and cyclic voltammetry, is found to be strongly correlated to the pH of the soln. at the interface. Local pH measurements are examd. at anodes and cathodes in full CDI and membrane-assisted capacitive deionization (MCDI) cells at cell voltages ranging from 0.3-1.2 V. The cathode is shown to be basic under charging potentials while the anode is found to be acidic. This local pH is found to be highly transient during charging and discharging in CDI cells while the pH is found to be relatively static in the MCDI cells, maintaining a basic pH at the cathode and an acidic pH at the anode even when the cell is discharged. Ion exchange membranes (IEM) are found to have two functions: (1) limiting co-ion expulsion that results from specific ion adsorption and (2) limiting the effects of parasitic Faradaic reactions on the sepn. process by stabilizing the local pH thereby mitigating dissolved oxygen redn. at the cathode and lessening carbon oxidn. at the anode. Performance comparisons including the salt adsorption capacity and charge efficiency are also compared for these systems.
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  14. 14
    Biesheuvel, P. M.; Hamelers, H. V. M.; Suss, M. E. Theory of Water Desalination by Porous Electrodes with Immobile Chemical Charge. Colloids Interfac. Sci. Commun. 2015, 9, 1– 5,  DOI: 10.1016/j.colcom.2015.12.001
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    Zhang, C.; He, D.; Ma, J.; Tang, W.; Waite, T. D. Faradaic Reactions in Capacitive Deionization (CDI) - Problems and Possibilities: A Review. Water Res. 2018, 128, 314– 330,  DOI: 10.1016/j.watres.2017.10.024
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    15
    Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: A review
    Zhang, Changyong; He, Di; Ma, Jinxing; Tang, Wangwang; Waite, T. David
    Water Research (2018), 128 (), 314-330CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Capacitive deionization (CDI) is considered to be one of the most promising technologies for the desalination of brackish water with low to medium salinity. In practical applications, Faradaic redox reactions occurring in CDI may have both neg. and pos. effects on CDI performance. In this review, we present an overview of the types and mechanisms of Faradaic reactions in CDI systems including anodic oxidn. of carbon electrodes, cathodic redn. of oxygen and Faradaic ion storage and identify their apparent neg. and pos. effects on water desalination. A variety of strategies including development of novel electrode materials and use of alternative configurations and/or operational modes are proposed for the purpose of mitigation or elimination of the deterioration of electrodes and the formation of byproducts caused by undesired side Faradaic reactions. It is also recognized that Faradaic reactions facilitate a variety of exciting new applications including i. the incorporation of intercalation electrodes to enhance water desalination or to selectively sep. certain ions through reversible Faradaic reactions and ii. the use of particular anodic oxidn. and cathodic redn. reactions to realize functions such as water disinfection and contaminant removal.
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  16. 16
    Zhao, R.; Satpradit, O.; Rijnaarts, H. H. M.; Biesheuvel, P. M.; van der Wal, A. Optimization of Salt Adsorption Rate in Membrane Capacitive Deionization. Water Res. 2013, 47, 1941– 1952,  DOI: 10.1016/j.watres.2013.01.025
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    16
    Optimization of salt adsorption rate in membrane capacitive deionization
    Zhao, R.; Satpradit, O.; Rijnaarts, H. H. M.; Biesheuvel, P. M.; van der Wal, A.
    Water Research (2013), 47 (5), 1941-1952CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Membrane capacitive deionization (MCDI) is a water desalination technique based on applying a cell voltage between 2 oppositely placed porous electrodes sandwiching a spacer channel that transports the water to be desalinated. In MCDI, ion-exchange membranes are positioned in front of each porous electrode to prevent co-ions from leaving the electrode region during ion adsorption, thereby enhancing the salt adsorption capacity. MCDI can be operated at const. cell voltage (CV), or at a const. elec. current (CC). We present both exptl. and theor. results for desalination capacity and rate in MCDI (both in the CV- and the CC-mode) as function of adsorption/desorption time, salt feed concn., elec. current, and cell voltage. We demonstrate how by varying each parameter individually, it is possible to systematically optimize the parameter settings of a given system to achieve the highest av. salt adsorption rate and water recovery.
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  17. 17
    Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. The Effect of the Flow-Regime, Reversal of Polarization, and Oxygen on the Long Term Stability in Capacitive de-Ionization Processes. Electrochim. Acta 2015, 153, 106– 114,  DOI: 10.1016/j.electacta.2014.12.007
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    17
    The effect of the flow-regime, reversal of polarization, and oxygen on the long term stability in capacitive de-ionization processes
    Cohen, Izaak; Avraham, Eran; Bouhadana, Yaniv; Soffer, Abraham; Aurbach, Doron
    Electrochimica Acta (2015), 153 (), 106-114CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    The demand for potable water is continuously increasing. Therefore energy-efficient H2O desalination methods are the focus of intensive research. Capacitive deionization (CDI) is an energy-efficient H2O desalination technol. This study focuses on solving the problem of electrode oxidn. and degrdn. in CDI cells. The effect of the geometric flow regime was studied. Comparison of flow-through vs. flow-by in CDI cells indicates that geometry has an impact on the electrooxidn. rates of the pos. polarized electrodes. The authors examd. operation with periodic potential (difference) application by alternating the electrodes polarization. While operating in such a way, the life of CDI cells could be pronouncedly extended without any drops in the desalination level. The authors studied the effect of O, which is unavoidably dissolved in the aq. solns., on the stability of the electrodes in CDI processes, with the aid of prolonged expts. under N atm. The inevitably dissolved air in regular brackish H2O significantly impacts the oxidn. rate of the pos. charged electrodes in CDI. Stabilization means for CDI cells are discussed.
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  18. 18
    Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. Long Term Stability of Capacitive De-Ionization Processes for Water Desalination: The Challenge of Positive Electrodes Corrosion. Electrochim. Acta 2013, 106, 91– 100,  DOI: 10.1016/j.electacta.2013.05.029
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    18
    Long term stability of capacitive de-ionization processes for water desalination: The challenge of positive electrodes corrosion
    Cohen, Izaak; Avraham, Eran; Bouhadana, Yaniv; Soffer, Abraham; Aurbach, Doron
    Electrochimica Acta (2013), 106 (), 91-100CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    Corrosion of the pos. electrodes, in capacitive deionization (CDI) cells for water desalination processes, is a major problem that may prevent them from becoming practically important. This paper deals with the consequence of the corrosion of the pos. electrodes in CDI processes on the desalination performance, in terms of capacity and the ratio between adsorption of counter-ions and desorption of co-ions. The detrimental effect of the pos. electrodes oxidn. on the de-ionization efficiency is demonstrated and discussed. The role of the p.d. applied to CDI cells on the electrodes' stability was explored as well. We used for this study CDI cells comprising several pairs of activated carbon electrodes and 3 electrodes cells contg. ref. electrode. The interrelated parameters measured included potential, current, concn. (translated from cond. measurements) and pH vs. time. The present study and the understanding gained herein, will enable the development of durable, long term and effective CDI processes.
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    Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior. Energy Environ. Sci. 2015, 8, 897– 909,  DOI: 10.1039/C4EE03172E
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    19
    Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior
    Gao, Xin; Omosebi, Ayokunle; Landon, James; Liu, Kunlei
    Energy & Environmental Science (2015), 8 (3), 897-909CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    Unsustainable and inefficient capacitive deionization (CDI) performance has been obsd. through CDI operation with carbon xerogel (CX) electrodes for 50 h using a const.-voltage charging method. This behavior is primarily accounted for by changes in the surface chem. for the studied material via oxidn. of the carbon electrodes in an aq. soln. In order to improve performance stability, we have developed a novel CDI system using an anode with net neg. surface charges and a cathode with net pos. surface charges. As a result, salt sepn. in this system is achieved in an opposing manner to the conventional CDI system, e.g., when the system is charged using a power source, cations and anions are desorbed at the anode and cathode, resp. This system is named the inverted capacitive deionization (i-CDI) system. Most importantly, salt sepn. in the i-CDI system was maintained for over 600 h, which is approx. an increase of 530% in lifetime compared to a CDI system operated under similar conditions. This enhanced performance stability is attributed to the use of oxidized anodes in the i-CDI system, which limits the possibility for loss in sepn. performance due to carbon oxidn. in an aq. soln.
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    Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes. Environ. Sci. Technol. 2015, 49, 10920– 10926,  DOI: 10.1021/acs.est.5b02320
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    Enhanced salt removal in an inverted capacitive deionization cell using amine modified microporous carbon cathodes
    Gao, Xin; Omosebi, Ayokunle; Landon, James; Liu, Kunlei
    Environmental Science & Technology (2015), 49 (18), 10920-10926CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
    Microporous SpectraCarb carbon cloth was treated using nitric acid to enhance neg. surface charges of COO- in a neutral soln. This acid-treated carbon was further modified by ethylenediamine to attach -NH2 surface functional groups, resulting in pos. surface charges of -NH3+ via pronation in a neutral soln. Through multiple characterizations, in comparison to pristine SpectraCarb carbon, amine-treated SpectraCarb carbon displays a decreased potential of zero charge but an increased point of zero charge, which is opposed to the effect obtained for acid-treated SpectraCarb carbon. An inverted capacitive deionization cell was constructed using amine-treated cathodes and acid-treated anodes, where the cathode is the neg. polarized electrode and the anode is the pos. polarized electrode. Const.-voltage switching operation using NaCl soln. showed that the salt removal capacity was approx. 5.3 mg g-1 at a max. working voltage of 1.1/0 V, which is an expansion in both the salt capacity and potential window from previous i-CDI results demonstrated for carbon xerogel materials. This improved performance is accounted for by the enlarged cathodic working voltage window through ethylenediamine-derived functional groups, and the enhanced microporosity of the SpectraCarb electrodes for salt adsorption. These results expand the use of i-CDI for efficient desalination applications.
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    Gao, X.; Omosebi, A.; Holubowitch, N.; Liu, A.; Ruh, K.; Landon, J.; Liu, K. Polymer-Coated Composite Anodes for Efficient and Stable Capacitive Deionization. Desalination 2016, 399, 16– 20,  DOI: 10.1016/j.desal.2016.08.006
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    Polymer-coated composite anodes for efficient and stable capacitive deionization
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    Desalination (2016), 399 (), 16-20CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    In the contemporary literature, diminished salt removal in a CDI device is primarily due to carbon oxidn. at the anode in aq. solns. Therefore, an anion exchange polymer is used to prep. a composite carbon as a CDI anode. Results from repetitive CDI testing shows that more efficient and consistent long-term salt removal is achieved when a flow-through CDI stack is configured with composite anodes compared to polymer-free anodes. Anal. of the effluent pH and steady-state current indicates that this performance improvement may be due to the minimization of parasitic reactions by shielding of the carbon electrodes with the selective polymer layer coated at the anode.
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    Biesheuvel, P. M.; Zhao, R.; Porada, S.; van der Wal, A. Theory of Membrane Capacitive Deionization Including the Effect of the Electrode Pore Space. J. Colloid Interface Sci. 2011, 360, 239– 248,  DOI: 10.1016/j.jcis.2011.04.049
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    Theory of membrane capacitive deionization including the effect of the electrode pore space
    Biesheuvel, P. M.; Zhao, R.; Porada, S.; van der Wal, A.
    Journal of Colloid and Interface Science (2011), 360 (1), 239-248CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
    Membrane capacitive deionization (MCDI) is a technol. for water desalination based on applying an elec. field between two oppositely placed porous electrodes. Ions are removed from the water flowing through a channel in between the electrodes and are stored inside the electrodes. Ion-exchange membranes are placed in front of the electrodes allowing for counterion transfer from the channel into the electrode, while retaining the coions inside the electrode structure. We set up an extended theory for MCDI which includes in the description for the porous electrodes not only the electrostatic double layers (EDLs) formed inside the porous (carbon) particles, but also incorporates the role of the transport pathways in the electrode, i.e., the interparticle pore space. Because in MCDI the coions are inhibited from leaving the electrode region, the interparticle porosity becomes available as a reservoir to store salt, thereby increasing the total salt storage capacity of the porous electrode. A second advantage of MCDI is that during ion desorption (ion release) the voltage can be reversed. In that case the interparticle porosity can be depleted of counterions, thereby increasing the salt uptake capacity and rate in the next cycle. In this work, we compare both exptl. and theor. adsorption/desorption cycles of MCDI for desorption at zero voltage as well as for reversed voltage, and compare with results for CDI. To describe the EDL-structure a novel modified Donnan model is proposed valid for small pores relative to the Debye length.
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    Choi, J.-H.; Yoon, D.-J. The Maximum Allowable Charge for Operating Membrane Capacitive Deionization without Electrode Reactions. Sep. Purif. Technol. 2019, 215, 125– 133,  DOI: 10.1016/j.seppur.2019.01.003
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    The maximum allowable charge for operating membrane capacitive deionization without electrode reactions
    Choi, Jae-Hwan; Yoon, Duck-Jin
    Separation and Purification Technology (2019), 215 (), 125-133CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
    We studied a new method to operate a membrane capacitive deionization (MCDI) without electrode reactions by controlling the total charge (TC) supplied to the MCDI cell. The MCDI expts. were carried out by varying the TC (45-90 C/g) at a high cell potential. When ∼59 C/g of charge was supplied to the C electrode, the electrode reaction began. Thus the MAC value, which is the max. charge supplied to the carbon electrode at the point where the electrode reactions begin, was 59 C/g. The C oxidn. reaction occurred when 59-83 C/g of charge was supplied. At 83 C/g or more, the water oxidn. reaction proceeded at a rapid rate, resulting in a drastic decrease in the charge efficiency. No electrode reactions were found during the MCDI operation at a TC value lower than the MAC value. When the MCDI system was operated at a TC higher than the MAC, however, the effluent pH and concn. changed as the adsorption and desorption process was repeated, resulting in deterioration of the desalination performance. It was verified that the electrode reactions can be easily controlled by adjusting the TC supplied to the MCDI cell.
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    Srimuk, P.; Ries, L.; Zeiger, M.; Fleischmann, S.; Jäckel, N.; Tolosa, A.; Krüner, B.; Aslan, M.; Presser, V. High Performance Stability of Titania Decorated Carbon for Desalination with Capacitive Deionization in Oxygenated Water. RSC Adv. 2016, 6, 106081– 106089,  DOI: 10.1039/C6RA22800C
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    High performance stability of titania decorated carbon for desalination with capacitive deionization in oxygenated water
    Srimuk, Pattarachai; Ries, Lucie; Zeiger, Marco; Fleischmann, Simon; Jaeckel, Nicolas; Tolosa, Aura; Kruener, Benjamin; Aslan, Mesut; Presser, Volker
    RSC Advances (2016), 6 (108), 106081-106089CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
    Performance stability in capacitive deionization (CDI) is particularly challenging in systems with a high amt. of dissolved oxygen due to rapid oxidn. of the carbon anode and peroxide formation. For example, carbon electrodes show a fast performance decay, leading to just 15% of the initial performance after 50 CDI cycles in oxygenated saline soln. (5 mM NaCl). We present a novel strategy to overcome this severe limitation by employing nanocarbon particles hybridized with sol-gel-derived titania. In our proof-of-concept study, we demonstrate very stable performance in low molar saline electrolyte (5 mM NaCl) with satd. oxygen for the carbon/metal oxide hybrid (90% of the initial salt adsorption capacity after 100 cycles). The electrochem. anal. using a rotating disk electrode (RDE) confirms the oxygen redn. reaction (ORR) catalytic effect of FW200/TiO2, preventing local peroxide formation by locally modifying the oxygen redn. reaction.
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    Srimuk, P.; Zeiger, M.; Jäckel, N.; Tolosa, A.; Krüner, B.; Fleischmann, S.; Grobelsek, I.; Aslan, M.; Shvartsev, B.; Suss, M. E.; Presser, V. Enhanced Performance Stability of Carbon/Titania Hybrid Electrodes during Capacitive Deionization of Oxygen Saturated Saline Water. Electrochim. Acta 2017, 224, 314– 328,  DOI: 10.1016/j.electacta.2016.12.060
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    Enhanced performance stability of carbon/titania hybrid electrodes during capacitive deionization of oxygen saturated saline water
    Srimuk, Pattarachai; Zeiger, Marco; Jaeckel, Nicolas; Tolosa, Aura; Kruener, Benjamin; Fleischmann, Simon; Grobelsek, Ingrid; Aslan, Mesut; Shvartsev, Boris; Suss, Matthew E.; Presser, Volker
    Electrochimica Acta (2017), 224 (), 314-328CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    Capacitive deionization (CDI) is a promising technol. for the desalination of brackish H2O due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in O-free electrolyte, but by contrast poor stability when O is present due to a gradual oxidn. of the C anode. This oxidn. leads to increased concn. of O-contg. surface functional groups within the micropores of the C anode, increasing parasitic co-ion current and decreasing SAC. Activated C (a.c.) was chem. modified with TiO2 to achieve addnl. catalytic activity for O-redn. reactions on the electrodes, preventing O from participating in C oxidn. Using this approach, the SAC can be increased and the cycling stability prolonged in electrochem. highly demanding O-satd. saline media (5 mM NaCl). The electrochem. O redn. reaction (ORR) occurring in the authors' CDI cell was evaluated by the no. of electron transfers during charging and discharging. Depending on the amt. of TiO2, different ORR pathways take place. A loading of 15% TiO2 presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR.
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    Tang, W.; He, D.; Zhang, C.; Kovalsky, P.; Waite, T. D. Comparison of Faradaic Reactions in Capacitive Deionization (CDI) and Membrane Capacitive Deionization (MCDI) Water Treatment Processes. Water Res. 2017, 120, 229– 237,  DOI: 10.1016/j.watres.2017.05.009
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    Comparison of Faradaic reactions in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) water treatment processes
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    Water Research (2017), 120 (), 229-237CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
    Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) are the most common cell architectures in the use of CDI for water treatment. In this work, the Faradaic reactions occurring in batch-mode CDI and MCDI processes were compared by investigating the variation of H2O2 and dissolved oxygen (DO) concns., pH, cond. and current during charging and discharging under different charging voltages. During charging, the H2O2 concn. in CDI increased rapidly and then decreased while almost no H2O2 was generated in MCDI due to the inability of oxygen to penetrate the ion exchange membrane. Chem. kinetic models were developed to quant. describe the variation of H2O2 concn. and found to present satisfactory descriptions of the exptl. data. The pH drop during charging could be partially explained by Faradaic reactions with proton generation assocd. with oxidn. of the carbon electrodes considered to be the major contributor. The electrode potentials required for the induction of Faradaic reactions were analyzed with this anal. providing robust thermodn. explanations for the occurrence of carbon oxidn. at the anode and H2O2 generation at the cathode during the ion adsorption process. Finally, electrochem.-induced ageing of the carbon electrodes and the resulting performance stability were investigated. The findings in this study contribute to a better understanding of Faradaic reactions in CDI and MCDI and should be of value in optimizing CDI-based technologies for particular practical applications.
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    van Limpt, B.; van der Wal, A. Water and Chemical Savings in Cooling Towers by Using Membrane Capacitive Deionization. Desalination 2014, 342, 148– 155,  DOI: 10.1016/j.desal.2013.12.022
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    Water and chemical savings in cooling towers by using membrane capacitive deionization
    van Limpt, B.; van der Wal, A.
    Desalination (2014), 342 (), 148-155CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
    Membrane capacitive deionization (MCDI) is a water desalination technol. based on applying a voltage difference between two oppositely placed porous carbon electrodes. In front of each electrode, an ion exchange membrane is positioned, and between them, a spacer is situated, which transports the water to be desalinated. In this study we detd. the water and chem. savings that can be achieved in a cooling tower by desalinating the feed water stream with a full-scale MCDI system. By monitoring the water use of the cooling tower, and comparing this to a scenario without MCDI, chem. savings up to 85% could be achieved. Addnl., water savings up to 28%, and waste water savings up to 48% could be achieved. MCDI energy use for desalination of cooling tower feed water was between 0.1 and 0.2 kWh per cubic meter of produced desalinated water. Preferential uptake of chloride and calcium was obsd., which lowers the risk of scaling and corrosion in the cooling tower and allows for further chem. and water savings.
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    Fritz, P. A.; Boom, R. M.; Schroen, K. Polyelectrolyte-Activated Carbon Composite Electrodes for Inverted Membrane Capacitive Deionization (IMCDI). Sep. Purif. Technol. 2019, 220, 145– 151,  DOI: 10.1016/j.seppur.2019.03.053
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    Polyelectrolyte-activated carbon composite electrodes for inverted membrane capacitive deionization (iMCDI)
    Fritz, Pina A.; Boom, R. M.; Schroen, K.
    Separation and Purification Technology (2019), 220 (), 145-151CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
    A new way of desalination using capacitive deionization (CDI) technol. is by inverting the potential profile (inverted capacitive deionization iCDI). This means ions adsorb to the electrodes at 0 V and desorb when biasing the electrodes to larger potential differences. Previously, this operation was achieved by prepg. electrode materials with anionic and cationic surface charges. Here we show, as a novelty, that an inverted CDI operation is also possible with conventional activated carbon electrodes when used in combination with ion exchange membranes (inverted membrane capacitive deionization iMCDI). Further we show that, the salt sepn. could be increased to 5.2 mg/g using 0 V for ion loading and -1.5 V for regeneration of polyelectrolyte-activated carbon composite electrodes. These are made with a water sol. styrene butadiene rubber binder and pos. (poly(diallyldimethyl-ammoniumchloride)) and neg. charged (polystyrene sulfonate) polyelectrolytes and used in combination with ion exchange membranes. This leads to increased sepn. performance, and exergy efficiency, whereas cumulative exergy loss values remain low, indicating promising resource use efficiencies, competitive with conventional membrane capacitive deionization (MCDI).
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  2. Shao-Wei Tsai, Min-Chen Wu, How Yong Ng, Chia-Hung Hou. Advancing the Electrosorption Selectivity of Nitrate Through Fine-Tuning Hydrophobic Ammonium Functional Groups in Anion Exchange Membranes for Membrane Capacitive Deionization. ACS ES&T Water 2024, 4 (12) , 5598-5607. https://doi.org/10.1021/acsestwater.4c00619
  3. Yuquan Li, Renyuan Li, Mengchun Wu, Le Shi, Xiaozhi Wang, Peng Wang, Wenbin Wang. Faradaic Rectification in Electrochemical Deionization and Its Influence on Cyclic Stability. ACS ES&T Engineering 2024, 4 (4) , 956-965. https://doi.org/10.1021/acsestengg.3c00517
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  24. Zhizhao He, Yingnan Li, Yuan Wang, Christopher J. Miller, John Fletcher, Boyue Lian, T. David Waite. Insufficient desorption of ions in constant-current membrane capacitive deionization (MCDI): Problems and solutions. Water Research 2023, 242 , 120273. https://doi.org/10.1016/j.watres.2023.120273
  25. Qifeng Wang, Qinghao Wu, Shujuan Meng, Hongju Liu, Dawei Liang. Selective removal of ammonium ions with transition metal hexacyanoferrate (MHCF) electrodes. Desalination 2023, 558 , 116646. https://doi.org/10.1016/j.desal.2023.116646
  26. Weiqing Kong, Xu Ge, Mengqi Yang, Qingao Zhang, Jingyi Lu, Haokun Wen, Hanyu Wen, Desheng Kong, Meng Zhang, Xiao Zhu, Yuanyuan Feng. Poly-p-phenylene as a novel pseudocapacitive anode or cathode material for hybrid capacitive deionization. Desalination 2023, 553 , 116452. https://doi.org/10.1016/j.desal.2023.116452
  27. Irina Chernyshova, Malin Suup, Caroline Kihlblom, Hanumantha Rao Kota, Kurt Aasly, Sathish Ponnurangam. Green Mining of Mining Water Using Surface E-Precipitation. 2023https://doi.org/10.2139/ssrn.4462994
  28. Antony Cyril Arulrajan, Min-Chen Wu, Slawomir Porada, Jouke Eabele Dykstra, Chia-Hung Hou, Albert van der Wal. Understanding Ph Changes and Mitigation of Mineral Scaling in Membrane Capacitive Deionization. 2023https://doi.org/10.2139/ssrn.4653438
  29. Valentin Goldberg, Tobias Kluge, Fabian Nitschke. Herausforderungen und Chancen für die Lithiumgewinnung aus geothermalen Systemen in Deutschland – Teil 1: Literaturvergleich bestehender Extraktionstechnologien. Grundwasser 2022, 27 (4) , 239-259. https://doi.org/10.1007/s00767-022-00522-5
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  31. Qinghao Wu, Dawei Liang, Shanfu Lu, Haining Wang, Yan Xiang, Doron Aurbach, Eran Avraham, Izaak Cohen. Advances and perspectives in integrated membrane capacitive deionization for water desalination. Desalination 2022, 542 , 116043. https://doi.org/10.1016/j.desal.2022.116043
  32. Wei Long, Qianhong She. A multifunctional and low-energy electrochemical membrane system for chemical-free regulation of solution pH. Water Research 2022, 216 , 118330. https://doi.org/10.1016/j.watres.2022.118330
  33. Tianting Pang, Frank Marken, Dengsong Zhang, Junjie Shen. Investigating the role of dissolved inorganic and organic carbon in fluoride removal by membrane capacitive deionization. Desalination 2022, 528 , 115618. https://doi.org/10.1016/j.desal.2022.115618
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Cite this: Environ. Sci. Technol. 2021, 55, 20, 14165–14172
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https://doi.org/10.1021/acs.est.1c04479
Published September 29, 2021

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  • Abstract

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    Figure 1

    Figure 1. MCDI (a–c) and inverted MCDI (i-MCDI) (d–f) for cyclic water desalination. In MCDI, water is desalinated during charging (b), and the electrodes are regenerated during discharge (c). In i-MCDI, the electrodes are modified with ion-exchange polymers which are located in the carbon pores, resulting in high counterion concentrations in the electrodes in the uncharged state (d). During charging, these ions are expelled from the electrode compartments, and a concentrated salt solution (concentrate) is produced (e). During discharge, water is desalinated (f).

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    Figure 2

    Figure 2. Experimentally observed effluent pH (a,b) and theoretically calculated effluent pH (c) for MCDI in a N2-purged NaCl solution. The inset in c shows the theoretically calculated zoomed effluent pH change.

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    Figure 3

    Figure 3. Cell voltage, effluent concentration (or conductivity in the case of tap water), and effluent pH changes of MCDI experiments with a NaCl solution (a) and with tap water (b) and i-MCDI experiments with tap water (c) after reaching dynamic steady state.

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    Figure 4

    Figure 4. Comparison between experimentally measured and theoretically calculated effluent pH changes in MCDI and i-MCDI during tap water desalination.

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    Figure 5

    Figure 5. (a) Effluent pH prediction as a function of bicarbonate ion removal, ΔcHCO3–. (b) Prediction of effluent pH during desorption for different water recoveries. (c) Prediction of Langelier saturation index (LSI) as a function of water recovery.

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    Figure 6

    Figure 6. (a) Effluent pH calculated for different values of the concentration reduction of NaCl (i.e., ΔcCl–) and (b) prediction of the effluent pH at fixed concentration reduction (Δc = 3.45 mM) with different concentration reductions of carbonate species (ΔcHCO3–,CO32–) over Cl–.

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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BB3MXmtVKjtQ%253D%253D&md5=e9b5163eacbec6dbb0592edc4b32ba44
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      Non-Faradaic (ion electrosorption) and Faradaic (oxidn.-redn.) effects in a batch-mode capacitive deionization (CDI) system were studied, with results showing that both effects were enhanced with an increase in charging voltage (0.5-1.5 V). Significant concns. of H2O2 were obsd. with the generation of H2O2 initiated by cathodic redn. of O with subsequent consumption occurring as a result of cathodic redn. of H2O2. A kinetic model of the Faradaic processes was developed and found to satisfactorily describe the variation in the steady-state concn. of H2O2 generated over a range of CDI operating conditions. Significant pH fluctuations were obsd. at higher charging voltages. While the occurrence of Faradaic reactions may well contribute to pH fluctuations and deterioration of electrode stability and performance, the presence of H2O2 could provide the means of inducing disinfection or trace contaminant degrdn. provided H2O2 could be effectively activated to more powerful oxidants (by, for example, UV irradn.).
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD2MXitVajt7w%253D&md5=5d983f19210dc419326a58c5e03aaf0b
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      Nucleation and growth of calcium carbonate by an electrochemical scaling process
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      Calcium carbonate scale was electrochem. deposited from carbonically pure hard waters on gold electrodes of an electrochem. quartz crystal microbalance. Various data concerning the rate of the scaling process were deduced from the chronoelectrogravimetric responses. The nucleation rate of CaCO3 crystals and the surface coverage were evaluated from SEM image anal. The effects of a surface pretreatment, the oxygen concn. and the degree of hardness of the water were studied. TEM was used to identify the crystal structure of small nuclei. Calcite, aragonite or vaterite crystal forms were obtained depending on exptl. conditions. At room temp. calcite was predominantly formed for conditions which were favorable to a fast nucleation rate whereas the vaterite form is systematically obtained for exptl. conditions leading to a slow nucleation rate. The temp. increase favored the formation of either aragonite needles or vaterite crystallites with a different morphol.
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      Wang, T.; Zhang, C.; Bai, L.; Xie, B.; Gan, Z.; Xing, J.; Li, G.; Liang, H. Scaling Behavior of Iron in Capacitive Deionization (CDI) System. Water Res. 2020, 171, 115370,  DOI: 10.1016/j.watres.2019.115370
      10
      Scaling behavior of iron in capacitive deionization (CDI) system
      Wang, Tianyu; Zhang, Changyong; Bai, Langming; Xie, Binghan; Gan, Zhendong; Xing, Jiajian; Li, Guibai; Liang, Heng
      Water Research (2020), 171 (), 115370CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      This study investigated the fouling and scaling behaviors in a capacitive deionization (CDI) system in the presence of iron and natural org. matter (NOM). It was found that the salt adsorption capacity (SAC) significantly decreased when treating Fe-contg. brackish water, with higher Fe concns. leading to severer SAC redn. Raman spectroscopy, XPS and X-ray diffraction (XRD) anal. demonstrated that Fe2O3 appeared to be the predominant foulant attached on the electrode surface, which was difficult to be removed via backwashing, indicating the irreversible property of the foulant. Further characterizations (e.g., N2 sorption-desorption isotherms, electrochem. impedance spectroscopy and cyclic voltammetry) revealed that the CDI electrodes suffered from obvious deterioration such as sp. surface area loss, resistance increase and capacitance decline with the occurrence of Fe scaling. While the presence of NOM alleviated the Fe scaling through NOM-Fe complexing effects, NOM itself was found to have neg. impacts on CDI desalination performance due to their strong interactions with the carbon electrodes.
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      Dykstra, J. E.; Keesman, K. J.; Biesheuvel, P. M.; van der Wal, A. Theory of PH Changes in Water Desalination by Capacitive Deionization. Water Res. 2017, 119, 178– 186,  DOI: 10.1016/j.watres.2017.04.039
      11
      Theory of pH changes in water desalination by capacitive deionization
      Dykstra, J. E.; Keesman, K. J.; Biesheuvel, P. M.; van der Wal, A.
      Water Research (2017), 119 (), 178-186CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      In electrochem. water desalination, a large difference in pH can develop between feed and effluent water. These pH changes can affect the long-term stability of membranes and electrodes. Often Faradaic reactions are implicated to explain these pH changes. However, quant. theory has not been developed yet to underpin these considerations. We develop a theory for electrochem. water desalination which includes not only Faradaic reactions but also the fact that all ions in the water have different mobilities (diffusion coeffs.). We quantify the latter effect by microscopic physics-based modeling of pH changes in Membrane Capacitive Deionization (MCDI), a water desalination technol. employing porous carbon electrodes and ion-exchange membranes. We derive a dynamic model and include the following phenomena: (I) different mobilities of various ions, combined with acid-base equil. reactions; (II) chem. surface charge groups in the micropores of the porous carbon electrodes, where elec. double layers are formed; and (III) Faradaic reactions in the micropores. The theory predicts small pH changes during desalination cycles in MCDI if we only consider phenomena (I) and (II), but predicts that these pH changes can be much stronger if we consider phenomenon (III) as well, which is in line with earlier statements in the literature on the relevance of Faradaic reactions to explain pH fluctuations.
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    12. 12
      Lee, J.-H.; Bae, W.-S.; Choi, J.-H. Electrode Reactions and Adsorption/Desorption Performance Related to the Applied Potential in a Capacitive Deionization Process. Desalination 2010, 258, 159– 163,  DOI: 10.1016/j.desal.2010.03.020
      12
      Electrode reactions and adsorption/desorption performance related to the applied potential in a capacitive deionization process
      Lee, Jae-Hun; Bae, Wi-Sup; Choi, Jae-Hwan
      Desalination (2010), 258 (1-3), 159-163CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Desalination expts. were performed by constructing a capacitive deionization (CDI) unit cell with a carbon electrode prepd. from activated carbon powder (ACP). Through CDI expts., the mechanism of adsorption, desorption and electrode reactions were investigated by measuring cond., effluent pH, and the current passed through the cell under different electrode potentials. The salt-removal efficiency increased with increasing potential at the range of 0.8-1.5 V. Addnl., the pH of the soln. varied significantly with a change in potential. At potentials less than 1.0 V, the pH increased due to the redn. of dissolved oxygen and the pH decreased at potentials over 1.2 V due to oxidn. reactions at the anode. The change in current revealed that adsorbed ions were not completely desorbed and a fraction of ions were retained at the carbon electrode. These accumulated ions were re-adsorbed at the electrode surface when a potential was re-applied, which led to a decrease in the salt-removal efficiency of CDI.
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    13. 13
      Landon, J.; Gao, X.; Omosebi, A.; Liu, K. Emerging Investigator Series: Local PH Effects on Carbon Oxidation in Capacitive Deionization Architectures. Environ. Sci.: Water Res. Technol. 2021, 7, 861– 869,  DOI: 10.1039/D1EW00005E
      13
      Emerging investigator series: local pH effects on carbon oxidation in capacitive deionization architectures
      Landon, James; Gao, Xin; Omosebi, Ayokunle; Liu, Kunlei
      Environmental Science: Water Research & Technology (2021), 7 (5), 861-869CODEN: ESWRAR; ISSN:2053-1419. (Royal Society of Chemistry)
      In this work, the effect of pH and potential is examd. for the oxidn. of carbon cloth electrodes used in capacitive deionization (CDI) processes. The degree of oxidn. of the electrode surface, examd. using the electrode's potential of zero charge (Epzc) and measured using chronoamperometry and cyclic voltammetry, is found to be strongly correlated to the pH of the soln. at the interface. Local pH measurements are examd. at anodes and cathodes in full CDI and membrane-assisted capacitive deionization (MCDI) cells at cell voltages ranging from 0.3-1.2 V. The cathode is shown to be basic under charging potentials while the anode is found to be acidic. This local pH is found to be highly transient during charging and discharging in CDI cells while the pH is found to be relatively static in the MCDI cells, maintaining a basic pH at the cathode and an acidic pH at the anode even when the cell is discharged. Ion exchange membranes (IEM) are found to have two functions: (1) limiting co-ion expulsion that results from specific ion adsorption and (2) limiting the effects of parasitic Faradaic reactions on the sepn. process by stabilizing the local pH thereby mitigating dissolved oxygen redn. at the cathode and lessening carbon oxidn. at the anode. Performance comparisons including the salt adsorption capacity and charge efficiency are also compared for these systems.
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    14. 14
      Biesheuvel, P. M.; Hamelers, H. V. M.; Suss, M. E. Theory of Water Desalination by Porous Electrodes with Immobile Chemical Charge. Colloids Interfac. Sci. Commun. 2015, 9, 1– 5,  DOI: 10.1016/j.colcom.2015.12.001
      There is no corresponding record for this reference.
    15. 15
      Zhang, C.; He, D.; Ma, J.; Tang, W.; Waite, T. D. Faradaic Reactions in Capacitive Deionization (CDI) - Problems and Possibilities: A Review. Water Res. 2018, 128, 314– 330,  DOI: 10.1016/j.watres.2017.10.024
      15
      Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: A review
      Zhang, Changyong; He, Di; Ma, Jinxing; Tang, Wangwang; Waite, T. David
      Water Research (2018), 128 (), 314-330CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Capacitive deionization (CDI) is considered to be one of the most promising technologies for the desalination of brackish water with low to medium salinity. In practical applications, Faradaic redox reactions occurring in CDI may have both neg. and pos. effects on CDI performance. In this review, we present an overview of the types and mechanisms of Faradaic reactions in CDI systems including anodic oxidn. of carbon electrodes, cathodic redn. of oxygen and Faradaic ion storage and identify their apparent neg. and pos. effects on water desalination. A variety of strategies including development of novel electrode materials and use of alternative configurations and/or operational modes are proposed for the purpose of mitigation or elimination of the deterioration of electrodes and the formation of byproducts caused by undesired side Faradaic reactions. It is also recognized that Faradaic reactions facilitate a variety of exciting new applications including i. the incorporation of intercalation electrodes to enhance water desalination or to selectively sep. certain ions through reversible Faradaic reactions and ii. the use of particular anodic oxidn. and cathodic redn. reactions to realize functions such as water disinfection and contaminant removal.
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    16. 16
      Zhao, R.; Satpradit, O.; Rijnaarts, H. H. M.; Biesheuvel, P. M.; van der Wal, A. Optimization of Salt Adsorption Rate in Membrane Capacitive Deionization. Water Res. 2013, 47, 1941– 1952,  DOI: 10.1016/j.watres.2013.01.025
      16
      Optimization of salt adsorption rate in membrane capacitive deionization
      Zhao, R.; Satpradit, O.; Rijnaarts, H. H. M.; Biesheuvel, P. M.; van der Wal, A.
      Water Research (2013), 47 (5), 1941-1952CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Membrane capacitive deionization (MCDI) is a water desalination technique based on applying a cell voltage between 2 oppositely placed porous electrodes sandwiching a spacer channel that transports the water to be desalinated. In MCDI, ion-exchange membranes are positioned in front of each porous electrode to prevent co-ions from leaving the electrode region during ion adsorption, thereby enhancing the salt adsorption capacity. MCDI can be operated at const. cell voltage (CV), or at a const. elec. current (CC). We present both exptl. and theor. results for desalination capacity and rate in MCDI (both in the CV- and the CC-mode) as function of adsorption/desorption time, salt feed concn., elec. current, and cell voltage. We demonstrate how by varying each parameter individually, it is possible to systematically optimize the parameter settings of a given system to achieve the highest av. salt adsorption rate and water recovery.
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    17. 17
      Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. The Effect of the Flow-Regime, Reversal of Polarization, and Oxygen on the Long Term Stability in Capacitive de-Ionization Processes. Electrochim. Acta 2015, 153, 106– 114,  DOI: 10.1016/j.electacta.2014.12.007
      17
      The effect of the flow-regime, reversal of polarization, and oxygen on the long term stability in capacitive de-ionization processes
      Cohen, Izaak; Avraham, Eran; Bouhadana, Yaniv; Soffer, Abraham; Aurbach, Doron
      Electrochimica Acta (2015), 153 (), 106-114CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      The demand for potable water is continuously increasing. Therefore energy-efficient H2O desalination methods are the focus of intensive research. Capacitive deionization (CDI) is an energy-efficient H2O desalination technol. This study focuses on solving the problem of electrode oxidn. and degrdn. in CDI cells. The effect of the geometric flow regime was studied. Comparison of flow-through vs. flow-by in CDI cells indicates that geometry has an impact on the electrooxidn. rates of the pos. polarized electrodes. The authors examd. operation with periodic potential (difference) application by alternating the electrodes polarization. While operating in such a way, the life of CDI cells could be pronouncedly extended without any drops in the desalination level. The authors studied the effect of O, which is unavoidably dissolved in the aq. solns., on the stability of the electrodes in CDI processes, with the aid of prolonged expts. under N atm. The inevitably dissolved air in regular brackish H2O significantly impacts the oxidn. rate of the pos. charged electrodes in CDI. Stabilization means for CDI cells are discussed.
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    18. 18
      Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. Long Term Stability of Capacitive De-Ionization Processes for Water Desalination: The Challenge of Positive Electrodes Corrosion. Electrochim. Acta 2013, 106, 91– 100,  DOI: 10.1016/j.electacta.2013.05.029
      18
      Long term stability of capacitive de-ionization processes for water desalination: The challenge of positive electrodes corrosion
      Cohen, Izaak; Avraham, Eran; Bouhadana, Yaniv; Soffer, Abraham; Aurbach, Doron
      Electrochimica Acta (2013), 106 (), 91-100CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Corrosion of the pos. electrodes, in capacitive deionization (CDI) cells for water desalination processes, is a major problem that may prevent them from becoming practically important. This paper deals with the consequence of the corrosion of the pos. electrodes in CDI processes on the desalination performance, in terms of capacity and the ratio between adsorption of counter-ions and desorption of co-ions. The detrimental effect of the pos. electrodes oxidn. on the de-ionization efficiency is demonstrated and discussed. The role of the p.d. applied to CDI cells on the electrodes' stability was explored as well. We used for this study CDI cells comprising several pairs of activated carbon electrodes and 3 electrodes cells contg. ref. electrode. The interrelated parameters measured included potential, current, concn. (translated from cond. measurements) and pH vs. time. The present study and the understanding gained herein, will enable the development of durable, long term and effective CDI processes.
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    19. 19
      Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior. Energy Environ. Sci. 2015, 8, 897– 909,  DOI: 10.1039/C4EE03172E
      19
      Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption-desorption behavior
      Gao, Xin; Omosebi, Ayokunle; Landon, James; Liu, Kunlei
      Energy & Environmental Science (2015), 8 (3), 897-909CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      Unsustainable and inefficient capacitive deionization (CDI) performance has been obsd. through CDI operation with carbon xerogel (CX) electrodes for 50 h using a const.-voltage charging method. This behavior is primarily accounted for by changes in the surface chem. for the studied material via oxidn. of the carbon electrodes in an aq. soln. In order to improve performance stability, we have developed a novel CDI system using an anode with net neg. surface charges and a cathode with net pos. surface charges. As a result, salt sepn. in this system is achieved in an opposing manner to the conventional CDI system, e.g., when the system is charged using a power source, cations and anions are desorbed at the anode and cathode, resp. This system is named the inverted capacitive deionization (i-CDI) system. Most importantly, salt sepn. in the i-CDI system was maintained for over 600 h, which is approx. an increase of 530% in lifetime compared to a CDI system operated under similar conditions. This enhanced performance stability is attributed to the use of oxidized anodes in the i-CDI system, which limits the possibility for loss in sepn. performance due to carbon oxidn. in an aq. soln.
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    20. 20
      Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes. Environ. Sci. Technol. 2015, 49, 10920– 10926,  DOI: 10.1021/acs.est.5b02320
      20
      Enhanced salt removal in an inverted capacitive deionization cell using amine modified microporous carbon cathodes
      Gao, Xin; Omosebi, Ayokunle; Landon, James; Liu, Kunlei
      Environmental Science & Technology (2015), 49 (18), 10920-10926CODEN: ESTHAG; ISSN:0013-936X. (American Chemical Society)
      Microporous SpectraCarb carbon cloth was treated using nitric acid to enhance neg. surface charges of COO- in a neutral soln. This acid-treated carbon was further modified by ethylenediamine to attach -NH2 surface functional groups, resulting in pos. surface charges of -NH3+ via pronation in a neutral soln. Through multiple characterizations, in comparison to pristine SpectraCarb carbon, amine-treated SpectraCarb carbon displays a decreased potential of zero charge but an increased point of zero charge, which is opposed to the effect obtained for acid-treated SpectraCarb carbon. An inverted capacitive deionization cell was constructed using amine-treated cathodes and acid-treated anodes, where the cathode is the neg. polarized electrode and the anode is the pos. polarized electrode. Const.-voltage switching operation using NaCl soln. showed that the salt removal capacity was approx. 5.3 mg g-1 at a max. working voltage of 1.1/0 V, which is an expansion in both the salt capacity and potential window from previous i-CDI results demonstrated for carbon xerogel materials. This improved performance is accounted for by the enlarged cathodic working voltage window through ethylenediamine-derived functional groups, and the enhanced microporosity of the SpectraCarb electrodes for salt adsorption. These results expand the use of i-CDI for efficient desalination applications.
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    21. 21
      Gao, X.; Omosebi, A.; Holubowitch, N.; Liu, A.; Ruh, K.; Landon, J.; Liu, K. Polymer-Coated Composite Anodes for Efficient and Stable Capacitive Deionization. Desalination 2016, 399, 16– 20,  DOI: 10.1016/j.desal.2016.08.006
      21
      Polymer-coated composite anodes for efficient and stable capacitive deionization
      Gao, X.; Omosebi, A.; Holubowitch, N.; Liu, A.; Ruh, K.; Landon, J.; Liu, K.
      Desalination (2016), 399 (), 16-20CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      In the contemporary literature, diminished salt removal in a CDI device is primarily due to carbon oxidn. at the anode in aq. solns. Therefore, an anion exchange polymer is used to prep. a composite carbon as a CDI anode. Results from repetitive CDI testing shows that more efficient and consistent long-term salt removal is achieved when a flow-through CDI stack is configured with composite anodes compared to polymer-free anodes. Anal. of the effluent pH and steady-state current indicates that this performance improvement may be due to the minimization of parasitic reactions by shielding of the carbon electrodes with the selective polymer layer coated at the anode.
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    22. 22
      Biesheuvel, P. M.; Zhao, R.; Porada, S.; van der Wal, A. Theory of Membrane Capacitive Deionization Including the Effect of the Electrode Pore Space. J. Colloid Interface Sci. 2011, 360, 239– 248,  DOI: 10.1016/j.jcis.2011.04.049
      22
      Theory of membrane capacitive deionization including the effect of the electrode pore space
      Biesheuvel, P. M.; Zhao, R.; Porada, S.; van der Wal, A.
      Journal of Colloid and Interface Science (2011), 360 (1), 239-248CODEN: JCISA5; ISSN:0021-9797. (Elsevier B.V.)
      Membrane capacitive deionization (MCDI) is a technol. for water desalination based on applying an elec. field between two oppositely placed porous electrodes. Ions are removed from the water flowing through a channel in between the electrodes and are stored inside the electrodes. Ion-exchange membranes are placed in front of the electrodes allowing for counterion transfer from the channel into the electrode, while retaining the coions inside the electrode structure. We set up an extended theory for MCDI which includes in the description for the porous electrodes not only the electrostatic double layers (EDLs) formed inside the porous (carbon) particles, but also incorporates the role of the transport pathways in the electrode, i.e., the interparticle pore space. Because in MCDI the coions are inhibited from leaving the electrode region, the interparticle porosity becomes available as a reservoir to store salt, thereby increasing the total salt storage capacity of the porous electrode. A second advantage of MCDI is that during ion desorption (ion release) the voltage can be reversed. In that case the interparticle porosity can be depleted of counterions, thereby increasing the salt uptake capacity and rate in the next cycle. In this work, we compare both exptl. and theor. adsorption/desorption cycles of MCDI for desorption at zero voltage as well as for reversed voltage, and compare with results for CDI. To describe the EDL-structure a novel modified Donnan model is proposed valid for small pores relative to the Debye length.
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    23. 23
      Choi, J.-H.; Yoon, D.-J. The Maximum Allowable Charge for Operating Membrane Capacitive Deionization without Electrode Reactions. Sep. Purif. Technol. 2019, 215, 125– 133,  DOI: 10.1016/j.seppur.2019.01.003
      23
      The maximum allowable charge for operating membrane capacitive deionization without electrode reactions
      Choi, Jae-Hwan; Yoon, Duck-Jin
      Separation and Purification Technology (2019), 215 (), 125-133CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
      We studied a new method to operate a membrane capacitive deionization (MCDI) without electrode reactions by controlling the total charge (TC) supplied to the MCDI cell. The MCDI expts. were carried out by varying the TC (45-90 C/g) at a high cell potential. When ∼59 C/g of charge was supplied to the C electrode, the electrode reaction began. Thus the MAC value, which is the max. charge supplied to the carbon electrode at the point where the electrode reactions begin, was 59 C/g. The C oxidn. reaction occurred when 59-83 C/g of charge was supplied. At 83 C/g or more, the water oxidn. reaction proceeded at a rapid rate, resulting in a drastic decrease in the charge efficiency. No electrode reactions were found during the MCDI operation at a TC value lower than the MAC value. When the MCDI system was operated at a TC higher than the MAC, however, the effluent pH and concn. changed as the adsorption and desorption process was repeated, resulting in deterioration of the desalination performance. It was verified that the electrode reactions can be easily controlled by adjusting the TC supplied to the MCDI cell.
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    24. 24
      Srimuk, P.; Ries, L.; Zeiger, M.; Fleischmann, S.; Jäckel, N.; Tolosa, A.; Krüner, B.; Aslan, M.; Presser, V. High Performance Stability of Titania Decorated Carbon for Desalination with Capacitive Deionization in Oxygenated Water. RSC Adv. 2016, 6, 106081– 106089,  DOI: 10.1039/C6RA22800C
      24
      High performance stability of titania decorated carbon for desalination with capacitive deionization in oxygenated water
      Srimuk, Pattarachai; Ries, Lucie; Zeiger, Marco; Fleischmann, Simon; Jaeckel, Nicolas; Tolosa, Aura; Kruener, Benjamin; Aslan, Mesut; Presser, Volker
      RSC Advances (2016), 6 (108), 106081-106089CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
      Performance stability in capacitive deionization (CDI) is particularly challenging in systems with a high amt. of dissolved oxygen due to rapid oxidn. of the carbon anode and peroxide formation. For example, carbon electrodes show a fast performance decay, leading to just 15% of the initial performance after 50 CDI cycles in oxygenated saline soln. (5 mM NaCl). We present a novel strategy to overcome this severe limitation by employing nanocarbon particles hybridized with sol-gel-derived titania. In our proof-of-concept study, we demonstrate very stable performance in low molar saline electrolyte (5 mM NaCl) with satd. oxygen for the carbon/metal oxide hybrid (90% of the initial salt adsorption capacity after 100 cycles). The electrochem. anal. using a rotating disk electrode (RDE) confirms the oxygen redn. reaction (ORR) catalytic effect of FW200/TiO2, preventing local peroxide formation by locally modifying the oxygen redn. reaction.
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    25. 25
      Srimuk, P.; Zeiger, M.; Jäckel, N.; Tolosa, A.; Krüner, B.; Fleischmann, S.; Grobelsek, I.; Aslan, M.; Shvartsev, B.; Suss, M. E.; Presser, V. Enhanced Performance Stability of Carbon/Titania Hybrid Electrodes during Capacitive Deionization of Oxygen Saturated Saline Water. Electrochim. Acta 2017, 224, 314– 328,  DOI: 10.1016/j.electacta.2016.12.060
      25
      Enhanced performance stability of carbon/titania hybrid electrodes during capacitive deionization of oxygen saturated saline water
      Srimuk, Pattarachai; Zeiger, Marco; Jaeckel, Nicolas; Tolosa, Aura; Kruener, Benjamin; Fleischmann, Simon; Grobelsek, Ingrid; Aslan, Mesut; Shvartsev, Boris; Suss, Matthew E.; Presser, Volker
      Electrochimica Acta (2017), 224 (), 314-328CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      Capacitive deionization (CDI) is a promising technol. for the desalination of brackish H2O due to its potentially high energy efficiency and its relatively low costs. One of the most challenging issues limiting current CDI cell performance is poor cycling stability. CDI can show highly reproducible salt adsorption capacities (SACs) for hundreds of cycles in O-free electrolyte, but by contrast poor stability when O is present due to a gradual oxidn. of the C anode. This oxidn. leads to increased concn. of O-contg. surface functional groups within the micropores of the C anode, increasing parasitic co-ion current and decreasing SAC. Activated C (a.c.) was chem. modified with TiO2 to achieve addnl. catalytic activity for O-redn. reactions on the electrodes, preventing O from participating in C oxidn. Using this approach, the SAC can be increased and the cycling stability prolonged in electrochem. highly demanding O-satd. saline media (5 mM NaCl). The electrochem. O redn. reaction (ORR) occurring in the authors' CDI cell was evaluated by the no. of electron transfers during charging and discharging. Depending on the amt. of TiO2, different ORR pathways take place. A loading of 15% TiO2 presents the best CDI performance and also demonstrates a favorable three-electron transfer ORR.
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    26. 26
      Tang, W.; He, D.; Zhang, C.; Kovalsky, P.; Waite, T. D. Comparison of Faradaic Reactions in Capacitive Deionization (CDI) and Membrane Capacitive Deionization (MCDI) Water Treatment Processes. Water Res. 2017, 120, 229– 237,  DOI: 10.1016/j.watres.2017.05.009
      26
      Comparison of Faradaic reactions in capacitive deionization (CDI) and membrane capacitive deionization (MCDI) water treatment processes
      Tang, Wangwang; He, Di; Zhang, Changyong; Kovalsky, Peter; Waite, T. David
      Water Research (2017), 120 (), 229-237CODEN: WATRAG; ISSN:0043-1354. (Elsevier Ltd.)
      Capacitive deionization (CDI) and membrane capacitive deionization (MCDI) are the most common cell architectures in the use of CDI for water treatment. In this work, the Faradaic reactions occurring in batch-mode CDI and MCDI processes were compared by investigating the variation of H2O2 and dissolved oxygen (DO) concns., pH, cond. and current during charging and discharging under different charging voltages. During charging, the H2O2 concn. in CDI increased rapidly and then decreased while almost no H2O2 was generated in MCDI due to the inability of oxygen to penetrate the ion exchange membrane. Chem. kinetic models were developed to quant. describe the variation of H2O2 concn. and found to present satisfactory descriptions of the exptl. data. The pH drop during charging could be partially explained by Faradaic reactions with proton generation assocd. with oxidn. of the carbon electrodes considered to be the major contributor. The electrode potentials required for the induction of Faradaic reactions were analyzed with this anal. providing robust thermodn. explanations for the occurrence of carbon oxidn. at the anode and H2O2 generation at the cathode during the ion adsorption process. Finally, electrochem.-induced ageing of the carbon electrodes and the resulting performance stability were investigated. The findings in this study contribute to a better understanding of Faradaic reactions in CDI and MCDI and should be of value in optimizing CDI-based technologies for particular practical applications.
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    27. 27
      Ogumi, Z.; Takehara, Z.; Yoshizawa, S. Gas Permeation in SPE Method: I . Oxygen Permeation Through Nafion and NEOSEPTA. J. Electrochem. Soc. 1984, 131, 769,  DOI: 10.1149/1.2115696
      27
      Gas permeation in solid polymer electrolyte method. I. Oxygen permeation through Nafion and Neosepta
      Ogumi, Z.; Takehara, Z.; Yoshizawa, S.
      Journal of the Electrochemical Society (1984), 131 (4), 769-73CODEN: JESOAN; ISSN:0013-4651.
      The permeation of O at atm. pressure through Nafion 120 (I) [63346-31-6] and Neosepta ACH-45T (II) [59680-51-2] ion-exchange membranes was investigated by an electrochem. monitoring technique, which utilizes the solid polymer electrolyte prepd. by an electroless plating method. The O diffusion coeffs. were almost the same (∼10-7 cm2.s-1) for each material, but the O soly. was much higher in I than in II. The O soly. in II was explained in terms of dissoln. in the aq. component of the membrane, but the O soly. in I was too high for such an explanation, and was postulated to involve the role of the poly(tetrafluoroethylene) backbone.
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    28. 28
      Dykstra, J. E. Desalination with porous electrodes : Mechanisms of ion transport and adsorption. Ph.D. Thesis, Wageningen University, 2018.
      There is no corresponding record for this reference.
    29. 29
      van Limpt, B.; van der Wal, A. Water and Chemical Savings in Cooling Towers by Using Membrane Capacitive Deionization. Desalination 2014, 342, 148– 155,  DOI: 10.1016/j.desal.2013.12.022
      29
      Water and chemical savings in cooling towers by using membrane capacitive deionization
      van Limpt, B.; van der Wal, A.
      Desalination (2014), 342 (), 148-155CODEN: DSLNAH; ISSN:0011-9164. (Elsevier B.V.)
      Membrane capacitive deionization (MCDI) is a water desalination technol. based on applying a voltage difference between two oppositely placed porous carbon electrodes. In front of each electrode, an ion exchange membrane is positioned, and between them, a spacer is situated, which transports the water to be desalinated. In this study we detd. the water and chem. savings that can be achieved in a cooling tower by desalinating the feed water stream with a full-scale MCDI system. By monitoring the water use of the cooling tower, and comparing this to a scenario without MCDI, chem. savings up to 85% could be achieved. Addnl., water savings up to 28%, and waste water savings up to 48% could be achieved. MCDI energy use for desalination of cooling tower feed water was between 0.1 and 0.2 kWh per cubic meter of produced desalinated water. Preferential uptake of chloride and calcium was obsd., which lowers the risk of scaling and corrosion in the cooling tower and allows for further chem. and water savings.
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    30. 30
      Fritz, P. A.; Boom, R. M.; Schroen, K. Polyelectrolyte-Activated Carbon Composite Electrodes for Inverted Membrane Capacitive Deionization (IMCDI). Sep. Purif. Technol. 2019, 220, 145– 151,  DOI: 10.1016/j.seppur.2019.03.053
      30
      Polyelectrolyte-activated carbon composite electrodes for inverted membrane capacitive deionization (iMCDI)
      Fritz, Pina A.; Boom, R. M.; Schroen, K.
      Separation and Purification Technology (2019), 220 (), 145-151CODEN: SPUTFP; ISSN:1383-5866. (Elsevier B.V.)
      A new way of desalination using capacitive deionization (CDI) technol. is by inverting the potential profile (inverted capacitive deionization iCDI). This means ions adsorb to the electrodes at 0 V and desorb when biasing the electrodes to larger potential differences. Previously, this operation was achieved by prepg. electrode materials with anionic and cationic surface charges. Here we show, as a novelty, that an inverted CDI operation is also possible with conventional activated carbon electrodes when used in combination with ion exchange membranes (inverted membrane capacitive deionization iMCDI). Further we show that, the salt sepn. could be increased to 5.2 mg/g using 0 V for ion loading and -1.5 V for regeneration of polyelectrolyte-activated carbon composite electrodes. These are made with a water sol. styrene butadiene rubber binder and pos. (poly(diallyldimethyl-ammoniumchloride)) and neg. charged (polystyrene sulfonate) polyelectrolytes and used in combination with ion exchange membranes. This leads to increased sepn. performance, and exergy efficiency, whereas cumulative exergy loss values remain low, indicating promising resource use efficiencies, competitive with conventional membrane capacitive deionization (MCDI).
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