Voltage-Based Stabilization of Microporous Carbon Electrodes for

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Voltage-Based Stabilization of Microporous Carbon Electrodes for Inverted Capacitive Deionization Xin Gao, Ayokunlei Omosebi, James Landon, and Kunlei Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b08968 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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The Journal of Physical Chemistry

Voltage-Based Stabilization of Microporous Carbon Electrodes for Inverted Capacitive Deionization X. Gaoa, A. Omosebia, J. Landona,*, and K. Liua,b,* a

Center for Applied Energy Research, University of Kentucky, Lexington, KY 40511

b

Department of Mechanical Engineering, University of Kentucky, Lexington, KY 40506

*

James. [email protected] and [email protected]

1 ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Abstract Inverted capacitive deionization (i-CDI) is examined using microporous SpectracarbTM carbon electrodes in 10 mmol L-1 NaCl solution without deaeration. i-CDI testing shows that using conventional operational methods, i.e., Vch = 0.8 V and Vdis = 0 V (0.8/0 V), cannot stabilize salt separation after approximately 409 hours with an averaged salt adsorption capacity (SAC) of 6.0±0.8 mg g-1. The cycled anode possesses a collapsed cyclic voltammogram due to an increase in the sheet resistance by the formation of a surface oxide layer. This layer eventually suppresses electronic charge utilization in the i-CDI cell causing degraded salt separation. By analysis of potential distributions incorporated with the modified Donnan model, an improved i-CDI operational method is proposed by reducing Vch to 0.4 V and Vdis to -0.4 V (0.4/-0.4 V) while maintaining a voltage window (Vch-Vdis) of 0.8 V. The improved i-CDI testing demonstrates that not only is the separation process stabilized up to approximately 420 hours but the SAC also increases to 7.2±0.3 mg g-1. Additionally, operation at 0.4/-0.4 V possesses more stable pH and dissolved oxygen (DO) responses than that at 0.8/0 V. We believe that such improved performance stems from a reduced Vch mitigating carbon oxidation at the anode and DO reduction at the cathode while the reduced Vdis compensates for salt removal capacity.


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1 Introduction Fresh water scarcity is considered as a major threat to sustaining human activities. As noted, approximately 1.1 billion people worldwide lack access to potable water, and a total of 2.7 billion people find water scarce for at least one month of the year.1 Although two-thirds of the world’s surface is covered by water, less than 1% of that water can be directly used for human comsumption.2 As a result, efficient, cost-effective, and environmentally friendly desalination technologies need to be developed to secure water accessibility. In addition to reverse osmosis, distillation, and electrodialysis, capacitive deionization (CDI) is an alternative to treat water with moderate salt content, e.g., brackish water, which benefits from low-pressure operation, minimized maintenance costs, environmental friendliness, and possibly higher energy efficiency.3

+ -

Salt desorption

Salt adsorption Cation

Anion

H2O

Current collector

Carbon electrode

Positive surface charge

Negative surface charge

Figure 1 Representation of the working mechanism of an inverted capacitive deionization (iCDI) cell. When the i-CDI cell is charged using a power source, cations and anions are desorbed



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at the anode and cathode, respectively. When the i-CDI cell is discharged under a short-circuit condition, cations and anions are adsorbed at the anode and cathode, respectively. In the present work, we discuss and show a modified i-CDI operational method to improve its stability for long-term operation.

CDI desalinates water by electro-adsorbing ionic charges at carbon electrodes,4 and its performance is highly dependent upon the properties of the carbon electrodes. Normally, increasing the microporosity of the carbon electrodes improves salt separation,3 but similar improvements can be alternatively achieved by using carbon electrodes with enhanced surface charges and electroactivities in both Faradaic and non-Faradaic systems.2,5-9 However, diminished salt separation is often reported during repetitive charging and discharging cycles, along with inversion behavior, i.e., desorption peaks at the beginning of charging (or adsorption) steps, which is primarily accounted for by electrochemical oxidation at the carbon anode (or the positive electrode).10-13 Such concern recently provoked several studies focusing on how to retain salt separation while mitigating anode oxidation. Successful methods can be summarized as follows: reduction in the charging voltage,10,14 use of ion-exchange materials,15-16 use of alternating polarization,17-18 operation under inverted CDI (i-CDI) mode (Fig. 1),19-20 and modification of the carbon surface.21-22 Those efforts have facilitated the development of CDI technology so as to alleviate the threat of water crisis. Microporous SpectracarbTM carbon electrodes are used in the present work to study the impact of repetitive charging and discharging on long-term operation of an i-CDI process. Characterization results, especially for the cycled anodes, are accordingly discussed along with the recent i-CDI work using microporous carbon electrodes in ref. (20), emphasizing that 4 ACS Paragon Plus Environment

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ultimately unstable salt separation is mainly due to additional carbon oxidation at the microporous anode, a conclusion similar to that drawn from a conventional CDI cell although over far extended time periods.10 Most importantly, through the modified Donnan (mD) model combined with potential distribution data, we demonstrate for the first time, the stabilization of salt separation for microporous carbon electrodes without membranes or additional electrode treatments by reducing both the charging and discharge voltages to an i-CDI cell.

2 Experimental 2.1 Carbon Preparation Microporous SpectracarbTM 2225 (Pr-SC) carbon cloth sheets were purchased from Engineered Fibers Technology, Connecticut, USA. To enhance the negative surface charge with a positively shifted potential of zero charge (EPZC), 30 g of Pr-SC was immersed into a container with 400 mL of nitric acid (70%, Sigma-Aldrich) at room temperature for 24 hours. The cloth was subsequently washed with a significant amount of deionized water until the pH of the water was ~4.5 followed by drying at 280°C in air overnight. The resulting cloth was termed oxidized SC (Ox-SC) with negatively enhanced surface charge, and its characterization results are already reported in ref (2). Note that, in the present work, to establish i-CDI salt removal, the Ox-SC served as the anode (positive electrode) paired with the Pr-SC at the cathode (negative electrode). 2.2. Electrode Characterization Cyclic voltammetry was carried out at 0.5 mV s-1 in 10 mmol L-1 non-deaerated NaCl using a potentiostat (Reference 600, Gamry Instruments). A three-electrode configuration was



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assembled, consisting of a carbon working electrode with geometric area of about 0.36 cm2, a saturated calomel reference electrode (SCE) (Koslow Scientific), and a titanium mesh counter electrode (Dexmet Cooperation) with geometric area of approximately 18 cm2. Each electrode was tightly placed in the cell in which the distance between the SCE tip and the carbon electrode was about 1 cm. The measured current was converted to specific capacitance (C) by

= ⁄( × )

(1)

where I is the current, s is the potential scan rate, and m is the mass of the dry carbon sample. Potential distributions were measured in 10 mmol L-1 non-deaerated NaCl by configuring two studied carbon electrodes, i.e., the Pr-SC cathode and the Ox-SC anode, and a SCE reference electrode. A similar setup has been sketched and introduced in ref. (11). Each electrode was tightly placed in the setup such that the distance between the pair of studied electrodes was 0.05 cm, the same configuration used in the flow-through cell. The distance between the tip of the SCE reference electrode and the studied electrode was around 1 cm. Each studied carbon electrode had an approximate geometric area of 0.36 cm2. By applying a voltage using a power supply (E3632A, Agilent) across the cathode and anode, the potential across each electrode versus the SCE electrode was recorded using a data logger (GL220, Graphtec Cooperation). Pore structures of the carbon electrodes were characterized using a surface area and porosity analyzer (ASAP2020, Micromeritics). Approximately 100 mg of sample was degassed at 160°C under vacuum for 12 hours, and its isotherm was recorded by N2 adsorption/desorption at 77 K. Since the non-localized density functional theory (NLDFT) and its fitting mechanism inclusively accommodate various pore geometries,23 NLDFT was selected to calculate cumulative pore volumes and pore size distributions of the carbon samples.


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Sheet resistances of the carbon electrodes were measured using a homemade setup containing four contact points. The setup was composed of four spherical spring-loaded contacts mounted co-linearly in a plastic fixture spaced 0.34 cm apart. To ensure a constant spring pressure, a 60 g weight was loaded on the plastic fixture during testing. Voltage drop (V) between the two internal contacts was measured when I was fixed at 4.83 mA passing through the two external contacts, and sheet resistance (Rsheet) was calculated using24

= (⁄ln (2))(/)

(2)

In the present work, the starting and cycled electrodes were ex-situ characterized, where starting and cycled means before and after i-CDI testing, respectively. Note that, after i-CDI testing, the first pair of cycled electrodes at the feed inlet was vacuum-dried overnight and resized for post characterization using the same route as described above. 2.3 i-CDI Testing Inverted salt separation was executed in a flow system as sketched in ref. (18), in which a flowthrough cell with electrodes adjacent to titanium current collectors, an in-line conductivity sensor (Alpha Cond 500, Thermo Scientific), an in-line pH sensor (Alpha pH 500, Thermo Scientific), a polyethylene tank, and a peristaltic pump (Masterflex L/S, Cole-Parmer) were connected via polyethylene and silicone tubing (Cole-Parmer). In the cell, 4 pieces of the Ox-SC anodes were paired with 4 pieces of the Pr-SC cathodes, and the total mass of the dry electrodes was about 2.9 g. Each pair was separated by an approximately 0.02 cm filter paper (Whatman) embedded in a 0.05 cm thick silicon rubber spacer (McMaster-Carr). The geometric area of each electrode directly exposed to NaCl solution was around 3.2×6.5 cm2.



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During testing, 31 L of 10 mmol L-1 NaCl solution without deaeration was continuously circulated through the cell at 25 mL min-1. Prior to each test, the cell was first discharged to adsorb salt for approximately 15 min. Automation and timing were facilitated with a computercontrolled relay box (Denkovi Assembly Electronics). Time taken for each charging and discharging step was 2,000 s, respectively. Conductivity, pH, and current data were recorded every 5 s via a data acquisition logger (GL220, Graphtec Cooperation). For each adsorption/discharging and desorption/charging step, salt removal capacity, i.e., salt adsorption capacity (SAC) and salt desorption capacity (SDC), were calculated by multiplying the volumetric flow rate (Φ) by the integration of the concentration (c) with time (t) via

SAC or SDC = (Φ/) ((t) − " )#$

(3)

where M is the molecular weight of NaCl, m is the total mass of the dry electrodes used in the flow-through cell, and cf is the last data point of salt concentration right before switching the applied voltage. The electronic charge passed (Q) for both the charging and discharging steps was calculated by integrating the current with time and normalizing by electrode mass. Thus, the charge efficiency, Λ, is calculated using

Λ = (SAC &)/(')

(4)

where F is Faraday’s constant. 2.4 Modified Donnan Model for a Carbon Electrode In the present work, the modified Donnan (mD) model was used to facilitate our discussion regarding the merits of our proposed i-CDI operational method in sections 3.2 and 3.3. The mD model with chemical surface charge is able to describe both inverted and conventional capacitive 8 ACS Paragon Plus Environment

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salt separation in the contemporary CDI literature,25 making it particularly useful in the present work along with the measured potential distribution data (Fig. 3(b) and 4(b)). Briefly, in the mD model with chemical surface charge, for an electrode, mobile ionic (σionic) charge due to cations and anions is balanced by both electronic charge (σelec) in the carbon matrix, and immobile chemical surface charge (σchem) in the micropores at the carbon electrode, which is given by

σionic + σelec + σchem = 0

(5)

Following the inclusion of charge balancing, the potential at an electrode, E, is

0 = (Δϕ3 + Δϕ4 )5

(6)

where VT is the thermal voltage, ∆φD is the Donnan potential, and ∆φS is the Stern potential. For a 1:1 salt such as NaCl, at equilibrium,

ΔϕD = − arcsinh(σionic⁄2inf )

(7)

ΔϕS = (σelec &)/(S T )

(8)

where CS is the capacitance of the Stern layer. The total ion concentration in the micropores, cions, is given by 2

2ions = σ2ionic + (2inf )

(9)

To solve the mD model for a single electrode, the values of σchem and CS need to be given. Herein, we assumed the σchem at the anode is -0.5 mmol L-1 while σchem at the cathode is 0.5 mmol L-1. Under such an assignment, the EPZC of the anode sits positively with reference to 9 ACS Paragon Plus Environment


that of the cathode when plotting cions as a function of Eo in Fig. 3(a) and Fig. 4(a). As a result, the EPZC of the Ox-SC with negatively enhanced surface charge should be greater than that of the

0.2

0

Pr-SC (a) 1 10 Pore Size (nm)

0.0

0.6

4

0.4

2

0.2

100

-1

100

EPZC -50

-100

Ox-SC (b)

0

150

0

0.8

6

150

50

Vtot=0.71 mL g-1

1 10 Pore Size (nm)

-1

2

8

Cumulative Pore Volume / mL g

0.4

-1

4

-2

0.6

Pore Size Distribution / 10 mL g nm

6

-1

Vtot=0.79 mL g-1 0.8

Cumulative Pore Volume / mL g

8

Capacitance / F g

-1

-2

-1

Pore Size Distribution / 10 mL g nm

-1

-1

Pr-SC.

Capacitance / F g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0

EPZC

50 0 -50

-100

Pr-SC (c)

-150 -0.50 -0.25 0.00 0.25 0.50 0.75

Potential / V vs SCE

Ox-SC (d)

-150 -0.50 -0.25 0.00 0.25 0.50 0.75


Figure 2 Characterizations of the starting Pr-SC and Ox-SC electrodes. Pore size distributions and cumulative pore volumes for the (a) Pr-SC and (b) Ox-SC. Cyclic voltammograms for the (c) Pr-SC and (d) Ox-SC at 0.5 mV s-1 in 10 mM NaCl without deaeration. The estimated EPZCs are


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highlighted by arrows. In Fig. S2, electrochemical impedance spectroscopy was also used to detect the EPZCs.

3 Results and Discussion 3.1 Pore Structures and Cyclic Voltammograms for Starting Electrodes Through NLDFT, N2 adsorption/desorption isotherms in Fig. S1 are used to study pore structures for both the Pr-SC and Ox-SC electrodes. As depicted in Fig. 2(a), the pore size distribution indicates that the Pr-SC is a microporous carbon material with ~99% of its pore volume attributed to pores with sizes between ~0.4 and ~2.0 nm. Fig. 2(b) shows that the Ox-SC has a pore size distribution similar to the Pr-SC, suggesting that the pore structure of the carbon sample is nearly retained before and after the nitric acid treatment. In addition, the cumulative pore volume yields a total pore volume (Vtot) of ~0.79 and ~0.71 mL g-1 for the Pr-SC and OxSC, respectively. To estimate the location of the EPZC, cyclic voltammetry was carried out at 0.5 mV s-1 in 10 mmol L-1 NaCl without deaeration. As shown in Fig. 2(c), the Pr-SC has a distinct V-shaped ion-swapping region encompassing the estimated EPZC between ~-0.29 and ~-0.06 V vs SCE, in which the electrode has a least salt adsorption capacity.26-27 In contrast, the voltammogram in Fig. 2(d) displays a positively shifted ion-swapping region at ~0.61 V vs SCE for the Ox-SC. This EPZC shift towards a positive potential is due to enhanced negative surface charge resulting from the carbon electrode oxidation by nitric acid,28 which can be demonstrated in Fig. 3(a) by adjusting the σchem in the mD model. Together with additional results in Fig. S2 achieved by electrochemical impedance spectroscopy (EIS), the EPZCs for the Pr-SC and Ox-SC are estimated



as ~-0.16 and ~0.56 V vs SCE, respectively, and these EPZCs will be used to estimate the

15

0.8

Edis,a=Edis,c=Eo=Vdis 10

5

Ech,a=Ea,PZC

Ech,c=Ec,PZC 0

Inverted behavior Vch,max = Ea,PZC-Ec,PZC

-0.5

(a)

0.0

0.5


-1

maximum charging voltage for i-CDI operation.

Salt Adsorption / mg g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Ech,a

0.6

Anode window

0.4

Eo =Edis,a =Edis,c

0.2 0.0

Cathode window

Ech,c

-0.2 -0.4

0.8/0 V (b) 0

Potential / V vs Eo

1

2

3

Time / h

Figure 3 (a) Working principle of inverted capacitive deionization demonstrated using a potential distribution. The V-shaped curves for salt adsorption at the anode (solid line) and cathode (dashed line) are constructed by the mD model for an electrode with NaCl concentration of Cinf = 10 mmol L-1, stern capacitance of CS = 135 F mL-1, thermal voltage of VT = 25.6 mV, chemical surface charge of σchem,a = -0.5 mol L-1 (solid line) and σchem,c = 0.5 mol L-1 (dashed line), and micropore volume of vmic = 0.75 mL g-1. (b) A potential distribution was measured at 0.8/0 V when an Ox-SC anode and a Pr-SC cathode are employed in 10 mmol L-1 NaCl without deaeration. Such an electrode-pair configuration resulted in i-CDI salt separation as shown in Fig. 5. In these plots, E and V mean the potential at an electrode and cell voltage, respectively. The subscripts of a, c, ch, dis, and o denote the anode, cathode, charging, discharge, and shortcircuit, respectively.


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3.2 i-CDI Working Principle and Its Potential Distribution i-CDI salt separation requires anodes and cathodes with substantial negative and positive charges, respectively. Along with the mD model with chemical surface charge for a single carbon electrode, Fig. 3(a) demonstrates the i-CDI working mechanism in terms of a potential distribution, which was previously detailed in ref. (29). Briefly, when an i-CDI cell is charged following a prior discharge at the short-circuit voltage, Vdis = Eo, the charging voltage (Vch) is distributed to the potential at the anode (Ech,a) and cathode (Ech,c). The driving force, Ech,a - Eo and Eo - Ech,c, can result in salt desorption, as both the Ech,a and Ech,c approach their respective electrode’s EPZCs, where the ions strored in carbon micropores is smallest. When the cell is discharged at Eo, salt can be adsorbed, since Edis,a and Edis,c are moved away from the EPZCs (Fig. 3(a)). Furthermore, it is apparent that enhancing chemical surface charge at either electrode can expand the working voltage window of i-CDI, i.e., Vch – Vdis = Ea,PZC - Ec,PZC, thereby resulting in a higher SAC.29 Following from the i-CDI working mechanism demonstrated above, a potential distribution was measured with an Ox-SC anode paired with a Pr-SC cathode in 10 mmol L-1 NaCl without deaeration. Based upon the estimated EPZCs depicted in Fig. 2(c) and (d), the maximum of Vch is estimated to be between 0.7-0.8 V for inverted salt separation. A result obtained from operation at Vch = 0.8 V coupled with Vdis = 0 V (0.8/0 V) is presented versus a SCE reference electrode in Fig. 3(b). It can be seen that the Vch of 0.8 V is split to the Ech,a of ~0.59 and Ech,c of ~-0.21 V vs SCE during the charging steps. During the discharging steps, Edis,a and Edis,c are equivalent to ~0.19 V vs SCE. Under such an operation, at the anode, a comparison of the measured and suggested standard (Eo) potentials indicates possible oxidation via30-32



C + 2H; O ↔ CO; + 4H ? + 4e@ , 0 B = ~0.46 − ~0.66 V vs SCE

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(10)

Eq. (10) generally describes that the carbon matrix can become disrupted by carbon oxidation, thereby reducing its electrical conductivity and porosity as reported previously in ref. (33 20). At the cathode, dissolved oxygen (DO) is likely reduced with a two-electron transfer in acidic (Eq. 11(a)) and alkaline (Eq. 12(a)) aqueous solutions via30, 32, 34-35

O; ,IJBKL I + 2H ? + 2e@ ↔ H; O; , 0 B = 0.45 V vs SCE

(11a)

H; O; + 2H ? + 2e@ ↔ 2H; O, 0 B = 1.52 V vs SCE

(11b)

@ B O; ,IJBKL I + H; O + 2e@ ↔ HO@ ; + OH , 0 = −0.31 V vs SCE

(12a)

@ @ B HO@ ; + H; O + 2e ↔ 3OH , 0 = 0.63 V vs SCE

(12b)

Eq. (11) and (12) may exacerbate carbon oxidation at the anode by asymmetrically redistributing electrode potential due to Faradaic rectification in addition to modifying local pH values.34 Thus, we predict that electrochemical oxidation at an anode and dissolved oxygen reduction at the paired cathode using Vch = 0.8 V will be more substantial compared to Vch < 0.8 V. Furthermore, ref. (20) has reported that i-CDI degradation occurred in 20 hours when 1.4 V was used to charge a cell configured with microporous carbon electrodes. In addition, ref. (10, 30) have shown that decreases in the applied voltages can reduce effluent pH changes attributed to both H2O2 formation via dissolved oxygen reduction at the cathode and carbon oxidation at the anode.


15

Edis,a

Vdis = Edis,a-Edis,c

0 -0.5

Anode window

Inverted behavior

10

5

0.8

Edis,c

Ech,c

Ech,a Vch = Ech,a-Ech,c

(a) 0.0

0.5



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

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0.4

Ech,a

0.0

Edis,a

-0.4

0

1

2

3

0.8

Cathode window 0.4

Edis,c

0.0

Ech,c

-0.4

0.4/-0.4 V (b) 0

Potential / V vs Eo

1

2

3

Time / h

Figure 4 (a) To mitigate carbon oxidation at the anode while simultaneously enhancing salt adsorption, a new cycling method is proposed in the present study, where the charging and discharge voltages are enhanced. (b) Potential distributions were measured at 0.4/-0.4 V in 10 mmol L-1 NaCl without deaeration when an Ox-SC anode and a Pr-SC cathode are employed. (Note that the parameters and subscripts used to construct Fig. 4(a) can be found in Fig. 3.)

3.3 Modified i-CDI Operation To mitigate the detrimental effects discussed accordingly through Eq. (10) - (12), a reduction in the charging voltage is an obvious way to reduce the potential at both the anode and cathode; however, through the i-CDI mechanism in Fig. 3(a), the resulting salt adsorption capacity will be smaller if Vdis equals Eo. To compensate for such a drawback while minimizing carbon oxidation, a modified charging/discharging method is proposed in Fig. 4(a) by means of reducing Vch and Vdis, a method similar to the extended-voltage CDI (eV-CDI) effect that is capable of enhancing 15 ACS Paragon Plus Environment


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the SAC (2, 25). In the following, together with both the mD model and potential distributions, we demonstrate the merits of the modified operation. The modified operation can enhance the salt adsorption of an i-CDI cell. In Fig. 3(a), the shaded area highlights the maximum charging voltage, Vch,max = Ea,PZC - Ec,PZC = 0.72 V, when the mD model employs the parameters depicted in the caption of Fig. 3. Under such conditions for i-CDI operation with a working voltage window between Vch = Vch,max and Vdis = Eo, the maximum SAC is ~7.7 mg (NaCl) g-1 (electrode). As depicted in the shaded area in Fig. 4(a) using the same parameters for the mD model, Vch decreases from 0.72 to 0.36 V coupled with a reduction in Vdis from 0 to -0.36 V. It is found that, even though the total working voltage remains at 0.72 V, the same as it was in Fig. 3(a), this voltage adjustment yields a SAC of ~10.0 mg g-1, a value greater than that calculated for conventional i-CDI operation in Fig. 3(a). This result is consistent with the earlier conclusion drawn in ref. (36-37), in which operating a CDI cell away from the EPZCs of the electrodes can improve separation performance. The modified operation can also stabilize the separation process for an i-CDI cell configured with microporous carbon electrodes. Fig. 4(b) presents the potential distribution measured at Vch = 0.4 V coupled with Vdis = -0.4 V (0.4/-0.4 V) when an Ox-SC anode paired with a Pr-SC cathode was tested in 10 mmol L-1 non-deaerated NaCl solution. As shown, the potentials at the anode and cathode are alternatively switched between ~-0.05 and ~0.35 V vs SCE. Such operation, i.e., reversing electrode polarity for each charging and discharging step, produces stable salt separation, and the related studies have been introduced in ref. (17-18). Furthermore, in comparison to the potentials measured at 0.8/0 V in Fig. 3(b), the reduced Ea and Ec at 0.4/-0.4 V can retard both carbon oxidation at the anode and DO reduction at the cathode, thereby minimizing i-CDI performance degradation as discussed in section 3.2. In summary, we 16 ACS Paragon Plus Environment

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expect that our proposed method can stabilize the separation process while improving the salt removal capacity.


-1

0.8 V

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1 Time / h

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-1

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0V

0.8 V

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10

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161 Time / h

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-1

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0V

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8 370

Salt Concentration / mmol L

0V


14

371 Time / h

372



-1

Salt Concentration / mmol L -1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1


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-0.4 V

0.4 V

12

10

8 0 14

1 Time / h -0.4 V

2

0.4 V

12

10

8 160

14

161 Time / h -0.4 V

162

0.4 V

12

10

8 370

(a)

371 Time / h

372

(b) 18 ACS Paragon Plus Environment

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8

0V

8

0.8 V

0.4 V

-0.4 V

7

7.1

6.8

pH

pH

7

6.4

6.3

6

6

5 0 8

1 Time / h 0V

5 0

2

8

0.8 V

1 Time / h -0.4 V

2

0.4 V

7 6.7

pH

pH

7

6.2

6.0

6

6

5.5

5 160 8

161 Time / h 0V

5 160

162

8

0.8 V

161 Time / h -0.4 V

162

0.4 V

7

7

6.7

pH

pH

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5 370

6.3

6

6

5.8

5.1

371 Time / h

372

5 370

371 Time / h

372

(d)

(c)


-1

Specific Current / mA g

40

0

5.4 mA g-1 -0.5 mA g-1

-20 -40

0V

40 20 2.3 mA g-1

0 -20

-0.9 mA g-1

-40 161 Time / h

20 1.9 mA g-1

0 -1.3 mA g-1

-20 -40 0

162

40

0V

0.8 V

20 1.9 mA g-1

0 -1.2 mA g-1

-20 -40 370

371 Time / h

372

1 Time / h 0.4 V

2

-0.4 V

40 20 1.0 mA g-1

0 -1.2 mA g-1

-20 -40 160

161 Time / h 0.4 V

-1

-1

160

-0.4 V

40

2

0.8 V

Page 20 of 38

0.4 V

-1

1 Time / h


20

-1


0.8 V



0V

0


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

-1


162

-0.4 V

40 20 0.9 mA g-1

0 -1.2 mA g-1

-20 -40 370

371 Time / h

372

(f)

(e)


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5 Selected profiles during repetitive charging and discharging. (a), (c), and (e) show the effluent salt concentration, effluent pH, and current, respectively, for operation at 0.8/0 V. (b), (d), and (f) depict the effluent salt concentration, effluent pH, and current, respectively, for operation at 0.4/-0.4 V. These cells were charged and discharged for about 370 cycles in ~31 L of ~10 mmol L-1 non-deaerated NaCl solution at 25 mL min-1. The full plots can be found in Fig. S3 and S4.

3.4 i-CDI Testing at 0.8/0 V and 0.4/-0.4 V A flow-through cell stack was used with 4-piece of Ox-SC anodes paired with 4-piece of Pr-SC cathodes in order to form i-CDI salt separation using 10 mmol L-1 NaCl without deaeration. As shown in Fig. 5(a), a typical inverted salt separation profile, forming adsorption valleys at 0 V and desorption peaks at 0.8 V, is observed from the onset of operation. As cycling progresses, salt separation gradually improves as depicted by the pronounced adsorption valleys and desorption peaks in the 160th-162nd hours. However, this improvement is not retained, as diminished adsorption valleys and desorption peaks are observed between the 370th and 372nd hours. By comparison, in Fig. 5(b), operation at 0.4/-0.4 V offers very stable salt separation over a similar course of testing. As with the effluent salt separation, the effluent pH is correspondingly modified. Fig. 5(c) depicts that the level of the effluent pH continuously descends during operation at 0.8/0 V, e.g., pH fluctuates between 6.3 and 7.1 during 1st-2nd hours while it decreases to 5.1-5.8 during 370th372nd hours. In contrast, in Fig. 5(d), the level of the effluent pH resulting from operation at 0.4/0.4 V remains nearly stable with consistent pH fluctuations between 6.2 and 6.8. According to Eq. (10)-(12), the decreased level of the effluent pH typically suggests that carbon oxidation at 21 ACS Paragon Plus Environment


Page 22 of 38

the anode is pronounced during operation at 0.8/0 V. Moreover, in Fig. S5 resulting from a separate i-CDI test, operation at 0.4/-0.4 V possesses a more stable DO response than that of 0.8/0 V, which is mainly accounted for by reducing the charging voltage from 0.8 to 0.4 V during salt desorption. Therefore, through such pH and DO comparisons, we can also conclude that carbon oxidation at the anode and DO reduction at the cathode are significantly mitigated during operation at 0.4/-0.4 V. Nonetheless, there are still additional electrochemical responses, e.g., the segmented pH responses, which may be due to both DO reduction and/or native surface charge on the electrode.18, 30, 32, 38-40 Salt separation and electrochemical reactions both relate to the current profile. In general, after switching to the applied voltage, the current exponentially decays at the beginning and then levels off to a steady state. For operation at 0.8/0 V in Fig. 5(e), along with operating time, this profile is modified with peak currents only becoming more substantial in the middle of operation (160th-162nd hours), a behavior similar to the improved salt separation in Fig. 5(a). For operation at 0.4/-0.4 V in Fig. 5(f), the current profile barely changes, giving a distinct contrast to the inconsistent current profile associated with operation at 0.8/0 V. It can also be surmised that the diminished current at steady state during operation at 0.4/-0.4 V suggests more sluggish electrochemical reactions in comparison to 0.8/0 V. Moreover, owing to the decreased peak current in Fig. 5(e), operation at 0.8/0 V depicts an increase in the total resistance of the cell with operating time, and this point will be further discussed in section 3.5 together with the cyclic voltammograms for the cycled electrodes.


6 4 2

0.8 V Desorption 0 0

200

Salt Desorption Capacity / mg g

-1

Salt Desorption Capacity / mg g

8

400

8 6 4 2 0 0

-1

-2 -4 -6 -8 0

0 V Adsorption 200 Cycle

0.4 V Desorption 200

400

Cycle

0

Salt Adsorption Capacity / mg g

-1

Cycle Salt Adsorption Capacity / mg g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

Page 23 of 38

400

0 -2 -4 -6 -8 0

(a)

-0.4 V Adsorption 200 Cycle

400

(b)



20 Charge Passed -1 during Desorption / C g

Charge Passed -1 during Desorption / C g

20 15 10 5 0 0

200 Cycle

400

10 5 0 0

Charge Passed -1 during Adsorption / C g

0

-5

-10

-15 0

15

0.8 V Desorption

0 Charge Passed -1 during Adsorption / C g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 38

400

200

400

Cycle

-5

-10


0.4 V Desorption

-15 0

(c)


400

(d)


1.0

0.8

0.8

Charge Efficiency

1.0

0.6 0.4 0.2

0.6 0.4 0.2

0.4 V Desorption

0.8 V Desorption 0.0 0

200

0.0 0

400

Cycle 1.0

1.0

0.8

0.8

0.6 0.4 0.2 0.0 0

200

400

Cycle

Charge Efficiency

Charge Efficiency

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Charge Efficiency

Page 25 of 38

0.6 0.4 0.2


400

0.0 0

(e)


400

(f)

Figure 6 Performance evaluations according to Fig. 5. (a), (c), and (e) show salt removal capacity, charge passed, and charge efficiency, respectively, for operation at 0.8/0 V. (b), (d), and (f) show salt removal capacity, charge passed, and charge efficiency, respectively, for operation at 0.4/-0.4 V.



Page 26 of 38

Through Eq. (3) and (4), the SAC/SDC, charge passed, and charge efficiency for both salt separations are plotted as a function of cycle number in Fig. 6(a)-(f). As observed, operation at 0.4/-0.4 V clearly offers improved performance over that at 0.8/0 V for long-term operation. For instance, operation at 0.4/-0.4 V results in an initial SAC/SDC of about 6 mg g-1, a value higher than that for operation at 0.8/0 V, and operation at 0.4/-0.4 V offers nearly stable operation, whereas 0.8/0 V operation shows a distinct arch-shaped plot for the SAC/SDC versus cycle number. (Note that we will discuss the formation of the arch-shaped plot in section 3.6.) The increased SAC/SDC has been mathematically interpreted using the mD model in section 3.3 and experimentally validated in Fig. S10. Finally, by using the electronic charge passed during salt adsorption and desorption, the total energy per volume of treated water for operation at 0.8/0 and 0.4/-0.4 V is 35.2±4.3 and 35.3±0.7 J L-1, respectively, and its calculations are illustrated in the supplementary information (section 6). We believe that the stabilized salt separation is attributed to mitigation of carbon oxidation at the anode. To verify carbon oxidation at the anode during long-term i-CDI operation, comparisons of cyclic voltammograms between the starting and cycled electrodes are carried out. These comparisons can further improve our understanding on the effect of operating methods on salt separation, especially for the arch-shaped trend of the SAC/SDC obtained from operation at 0.8/0 V when an i-CDI cell was configured with microporous carbon electrodes.


Page 27 of 38

(a)

Anode 0.8/0 V -1

Capacitance / F g

Capacitance / F g

-1

100

0

-100

-1

-1

Capacitance / F g

0

-0.5

Starting Cycled

0.0 0.5 Potential / V vs SCE

(d)

Cathode 0.8/0 V

100

-100

0

-0.5


(c)

100

(b) Anode 0.4/-0.4 V

-100

Starting Cycled

-0.5

Capacitance / F g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Starting Cycled


Cathode 0.4/-0.4 V

100

0

-100 -0.5

Starting Cycled


Figure 7 Cyclic voltammograms for the starting (dashed line) and cycled (solid line) electrodes, where starting and cycled means the electrode before and after i-CDI testing, respectively. (a) and (c) cycled Ox-SC anode and Pr-SC cathode resulting from operation at 0.8/0 V, respectively. (b) and (d) cycled Ox-SC anode and Pr-SC cathode resulting from operation at 0.4/-0.4 V,



Page 28 of 38

respectively. These experiments were carried out at 0.5 mV s-1 in 10 mM NaCl solution without deaeration with the same solution that was used in the cycling tests.

3.5 Comparison between Starting and Cycled Electrodes Cyclic voltammograms for the cycled electrodes at 0.5 mV s-1 in 10 mM non-deaerated NaCl solution are presented in Fig. 7. Compared to the starting anodes, only the cycled Ox-SC anode from operation at 0.8/0 V in Fig. 7(a) depicts a collapsed voltammogram, and its shape is changed from box-like to spindle-like. This change suggests an increased surface resistance for the cycled Ox-SC anode, which is confirmed by the measured sheet resistance in Table 1 together with a theoretical study on the shape of a cyclic voltammogram in Fig. S6. Furthermore, the increased surface resistance may be due to the formation of a layer containing oxygen, which is indicated by comparing the atomic percentage ratio of oxygen to carbon for all of the cycled anodes through X-ray photoelectron spectroscopy (Fig. S7). In addition, since all of the electrodes were connected in series in the i-CDI cell, total resistance of the cell increases. This point may be reflected by looking at the decreased peak currents from ~36 mA g-1 in 160th-162nd hours to ~23 mA g-1 in 370th-372nd hours in Fig. 5(e). At present, we believe that the main factor in the cause of increased resistance is the continued formation of a surface oxide layer at the cycled anode. This layer suppresses electronic charge transport from the carbon matrix to the surface of the anode (or ionic charge transfer from the bulk to the surface of the anode). As a consequence, it can be imagined that, during operation at 0.8/0 V, salt separation gradually succumbs to a failure of ionic storage at the anode, and this limiting anode leads to both the SAC/SDC and charge passed eventually decreasing in Fig. 6(a) and (c), respectively. For the cycled anode associated with operation at 0.4/-0.4 V in Fig. 7(b), 28 ACS Paragon Plus Environment

Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the nearly preserved voltammogram suggests retained ionic transfer during the charging and discharging steps. This preservation must be due to the reduced Vch from 0.8 to 0.4 V directly reducing the driving force for carbon oxidation at the anode and for DO reduction at the cathode. The latter reduction can affect the driving force for carbon oxidation at the anode as reported in ref. (17, 34). Even though the cycled anode from operation at 0.4/-0.4 V has a preserved voltammogram, its paired cathode in Fig. 7(d) depicts a shifted V-shaped region containing the EPZC towards the positive direction compared to the starting electrode. On the contrary, for the cycled cathode in Fig. 7(c), its voltammogram after operation at 0.8/0 V is similar to what it was before testing. These observations suggest that reducing the discharge voltage from 0 to -0.4 V provides more driving force for carbon oxidation at the cathode as compared by the Edis,c in the potential distributions in Fig. 3(b) and 4(b), which may be supported by changes in the sheet resistances of the cathodes displayed in Table 1. We believe that cathode oxidation due to operation at 0.4/-0.4 V is drastically reduced in comparison to anode oxidation for operation at 0.8/0 V. However, we are also aware that, because of carbon oxidation, the cathode may eventually limit salt separation if the testing period is further extended, and stabilization of the carbon cathode through surface treatments or modified discharge voltages may ultimately be necessary.



Page 30 of 38

Table 1 Comparisons of pore volume and sheet resistances for the starting and cycled electrodes, where starting and cycled mean the electrode before and after i-CDI testing.

sheet resistance / Ω

pore volume / mL g-1

anode

11.8±0.9

0.71

cathode

4.0±0.3

0.79

anode

91.2±1.1

0.58

cathode

4.1±0.2

0.73

anode

29.8±1.2

0.66

cathode

4.5±0.7

0.71

samples starting

0.8/0 V

0.4/-0.4 V

Finally, Table 1 summarizes the additional characterizations of the carbon samples before and after the long-term i-CDI tests. As noted, the cycled anodes possess an increased sheet resistance but reduced pore volume, supporting previous results regarding the use of microporous carbon electrodes in long-term i-CDI and/or CDI cycling.10, 20, 33 Herein, we deduce that, as the anode surface oxidizes, the electrically conducting conjugated π network may be disrupted due to the formation of an oxide layer, thereby diminishing both the electrical conductivity and pore volume of the anode.41-43 By comparison, such changes are much milder for the cycled cathodes.


Page 31 of 38

Cathode -1

100 EPZC

EPZC

0

-100

(a) -0.5

Starting Cycled

0.0 0.5 1.0 Potential / V vs SCE

Capacitance / F g

-1

Anode Capacitance / F g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

0

-100

(b) -0.5

Starting Cycled


Figure 8 Cyclic voltammograms for the (a) Ox-SC anode and (b) Pr-SC cathode resulting from operation at 0.8/0 V for 36 cycles. These experiments were carried out at 0.5 mV s-1 in 10 mM non-deaerated NaCl solution with the same solution that was used in the cycling test.

3.6 Arch-Shaped Trend of SAC/SDC Resulting from Operation at 0.8/0 V Electrochemical oxidation at the anode can temporarily improve inverted salt separation, e.g., at the onset of operation at 0.8/0 and 0.4/-0.4 V in Fig. 6(a) and (b). To facilitate discussion, a new cell was tested at 0.8/0 V but for 36 cycles, and its performance in Fig. S8 is similar to those depicted in Fig. 5. The cycled electrodes were examined by cyclic voltammetry at 0.5 mV s-1 in 10 mM non-deaerated NaCl solution. As shown in Fig. 8(a), compared to the starting anode, the cycled anode retains a box-like voltammogram, but its V-shaped region containing the EPZC is shifted from ~0.56 to ~0.74 V vs SCE. As a result, along with the nearly unmodified EPZC for the cycled cathode in Fig. 8(b), the positively shifted EPZC extends the working voltage window for



Page 32 of 38

i-CDI operation. Under such a condition, the salt removal capacity can be improved; furthermore, we believe that the Ech,a, Ech,c, and Eo are positively shifted along with the potential axis (Fig. S9). However, as cycling is continuously repeated, further electrochemical oxidation will prompt an increase in the surface resistance of the anode, thereby limiting the i-CDI separation process (See discussion in section 3.5 regarding the formation of an electrically resistive surface layer containing oxygen for the Ox-SC anode cycled for ~370 cycles).

3.7 General Methods and Considerations to Achieve i-CDI Operation Operating an i-CDI cell relies primarily upon the EPZC for both cathode and anode, wherein the EPZC of the anode should be more positive than that of the cathode, and the difference between the two EPZCs is the maximum of charging voltage, i.e., Vch,max = Ea,PZC - Ec,PZC (See Fig. 3(a)). Practically, to perform i-CDI operation, an as-received carbon electrode needs to be chemically modified to create carbon electrodes with opposite surface charge. For instance, using nitric acid oxidizes the carbon surface, thereby leading to its EPZC being positively shifted. Conversely, a negatively shifted EPZC can be obtained using amine solutions. Gas-phase treatments can also be used to produce the desired surface functionalities and EPZC shifts. Such modifications for carbon electrodes have been documented in ref. (29, 37, 44-47). Estimation of the EPZC of a porous carbon electrode to define the voltages to be used in an i-CDI system should be conducted via cyclic voltammetry at a low scan rate or electrochemical impedance spectroscopy at a low frequency in a salt solution with low salt concentration. Moreover, EPZC can also be measured by an immersion method but only for a carbon electrode with high porosity as already described in ref. (28, 48). In addition, measurements of point of zero


Page 33 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


charge (pHPZC) for a carbon electrode can be an alternative to estimate EPZC as indicated in ref. (49-50). For performance stabilization (including CDI operation), reduction of the charging voltage is recommended to minimize carbon oxidation at the anode as well as DO reduction at the cathode. To achieve an improved SAC, we generally consider extending the discharge voltage leading to the eV-effect. It should be noted that the increased discharge voltage can cause the (prior) cathode to oxidize if the potential at the (prior) cathode is not properly managed and kept below a defined threshold. While voltage regulation can extend the lifetime of CDI/i-CDI electro-desalination cells, there is still a need for the development of microporous carbon electrodes with sound anti-oxidation capabilities, which will be the focus of ongoing work.

4 Conclusions In the present work, we study long-term operation of an i-CDI flow-through cell configured with microporous carbon electrodes. Conventional i-CDI operation, e.g., Vch = 0.8 V and Vdis = 0 V for this work, results in a degraded separation process within approximately 409 hours with an average SAC value of 6.0±0.8 mg g-1. Characterization results show that operation at 0.8/0 V results in a collapsed voltammogram for the cycled anode, which is attributed to an increase in the resistance of the anode. We believe that the main factor in the cause of increased resistance is the continued formation of a surface oxide layer, whereby this layer eventually suppresses ionic storage in the i-CDI cell causing performance degradation. Through a study on the potential distribution along with the mD model with chemical surface charge, a modified i-CDI operating method is proposed by reducing both Vch and Vdis. It



Page 34 of 38

is found that the reduced Vch mitigates carbon oxidation at the anode while the reduced Vdis compensates for salt adsorption. Testing through about 370 cycles at Vch = 0.4 V and Vdis = -0.4 V demonstrates that not only is the separation process stabilized up to approximately 420 hours but the salt removal capacity is also improved to 7.2±0.3 mg g-1. Characterization results show the voltammogram of the anode for operation at 0.4/-0.4 V is retained. Additionally, operation at 0.4/-0.4 V possesses more stable pH and DO responses than that at 0.8/0 V. However, we are also aware that, because of carbon oxidation, the cathode may eventually limit salt separation if the testing period is further extended. Nevertheless, cathode oxidation stemming from operation at 0.4/-0.4 V is much milder in comparison to anode oxidation for operation at 0.8/0 V.

Supporting Information Description 1. N2 Adsorption and Desorption Isotherms. 2. EPZCs Estimated Using Electrochemical Impedance Spectroscopy. 3. Cycling Performance at 0.8/0 V for 368 Cycles (Approximately 409 Hours). 4. Cycling Performance at 0.4/-0.4 V for 378 Cycles (Approximately 420 Hours). 5. Changes in Effluent Dissolved Oxygen and pH during Operation at 0.4/-0.4 and 0.8/0 V. 6. Calculations of Total Energy Consumption for Treated Water. 7. Effect of Resistance and Capacitance on Cyclic Voltammogram. 8. XPS Spectra for Cycled Anodes. 9. Cycling Performance at 0.8/0 V for 36 Cycles. 10. Expanded i-CDI Working Voltage Window. 11. Calculated and Experimental SAC Values.

Acknowledgment This work was supported by the U.S.-China Clean Energy Research Center, U.S. Department of


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Energy [DE-PI0000017]. These authors would like to thank Mr. R. Perrone for help in designing and constructing the flow-through cell stack.

References 1. https://www.worldwildlife.org/threats/water-scarcity. 2. Gao, X.; Porada, S.; Omosebi, A.; Liu, K. L.; Biesheuvel, P. M.; Landon, J., Complementary Surface Charge for Enhanced Capacitive Deionization. Water Res. 2016, 92, 275-282. 3. Suss, M. E.; Porada, S.; Sun, X.; Biesheuvel, P. M.; Yoon, J.; Presser, V., Water Desalination Via Capacitive Deionization: What Is It and What Can We Expect from It? Energy Environ. Sci. 2015, 8, 2296-2319. 4. Blair, J.; Murphy, G., Electrochemical Demineralization of Water with Porous Electrodes of Large Surface Area. In Saline Water Conversion, American Chemical Society: 1960; Vol. 27, pp 206-223. 5. Ma, D.; Wang, Y.; Han, X.; Xu, S.; Wang, J., Electrode Configuration Optimization of Capacitive Deionization Cells Based on Zero Charge Potential of the Electrodes. Sep. Purif. Technol. 2017. 6. Yang, J.; Zou, L.; Choudhury, N. R., Ion-Selective Carbon Nanotube Electrodes in Capacitive Deionisation. Electrochim. Acta 2013, 91, 11-19. 7. Wu, T.; Wang, G.; Dong, Q.; Qian, B.; Meng, Y.; Qiu, J., Asymmetric Capacitive Deionization Utilizing Nitric Acid Treated Activated Carbon Fiber as the Cathode. Electrochim. Acta 2015, 176, 426-433. 8. Su, X.; Kulik, H. J.; Jamison, T. F.; Hatton, T. A., Anion-Selective Redox Electrodes: Electrochemically Mediated Separation with Heterogeneous Organometallic Interfaces. Adv. Func. Mater. 2016, 26, 3394-3404. 9. Achilleos, D. S.; Hatton, T. A., Selective Molecularly Mediated Pseudocapacitive Separation of Ionic Species in Solution. ACS Appl. Mater. Interfaces 2016, 8, 32743-32753. 10. 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. 11. Gao, X.; Omosebi, A.; Landon, J.; Liu, K., Dependence of the Capacitive Deionization Performance on Potential of Zero Charge Shifting of Carbon Xerogel Electrodes During LongTerm Operation. J. Electrochem. Soc. 2014, 161, E159-E166. 12. Bouhadana, Y.; Ben-Tzion, M.; Soffer, A.; Aurbach, D., A Control System for Operating and Investigating Reactors: The Demonstration of Parasitic Reactions in the Water Desalination by Capacitive De-Ionization. Desalination 2011, 268, 253-261. 13. Bouhadana, Y.; Avraham, E.; Noked, M.; Ben-Tzion, M.; Soffer, A.; Aurbach, D., Capacitive Deionization of Nacl Solutions at Non-Steady-State Conditions: Inversion Functionality of the Carbon Electrodes. J. Phys. Chem. C 2011, 115, 16567-16573.



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-1

15

Edis,a=Edis,c=Eo=Vdis 10

5

Ech,a=Ea,PZC

Ech,c=Ec,PZC 0 -0.5

Inverted behavior Vch,max = Ea,PZC-Ec,PZC 0.0

0.5

Potential / V vs Eo


-1

TOC Graphic Salt Adsorption / mg g

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 38

15

Edis,a

Vdis = Edis,a-Edis,c

Edis,c

Inverted behavior

10

5

Ech,c

Ech,a Vch = Ech,a-Ech,c

0 -0.5

0.0

0.5

Potential / V vs Eo

Conventional i-CDI operation

Proposed i-CDI operation

Vch,max = Ea,PZC – Ec,PZC / Vdis = Edis,a = Edis,c

Vch = Ech,a – Ech,a / Vdis = Edis,a - Edis,c


Voltage-Based Stabilization of Microporous Carbon Electrodes for

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