Improved performance and long-term stability of activated carbon doped with nitrogen for capacitive deionization

Desalination 481 (2020) 114362

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Improved performance and long-term stability of activated carbon doped with nitrogen for capacitive deionization Chun-Chia Hsu, Yi-Heng Tu, Yu-Hsiang Yang, Jeng-An Wang, Chi-Chang Hu

T

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Laboratory of Electrochemistry and Advanced Materials, Department of Chemical Engineering, National Tsing Hua University, Hsin-Chu 30013, Taiwan

G R A P H I C A L A B S T R A C T

A R T I C LE I N FO

A B S T R A C T

Keywords: Capacitive deionization Activated carbon Nitrogen doping Acid pre-treatment Long-term stability

Nitrogen-doped activated carbon (NAC) is prepared by a combination of acid pre-treatment and thermal nitrogen doping for the positive electrode of asymmetric capacitive deionization (a-CDI) cells. The oxygen content in AC controlled by the acid pre-treatment significantly affects the doping amount of N atoms from melamine, which enhances the surface negative charge in NACs to promote the salt adsorption capacity (SAC). Here NAC with 30% HNO3 pre-treatment (NAC30) possesses a highly negatively charged surface to exhibit a fast ion desorption rate during discharging. The asymmetrical NAC30//AC cell shows the maximum reversible SAC of 24.7 ± 1.6 mg g−1. In addition, the negative surface charge of NAC30 is further promoted and the reversible SAC of NAC30//AC are greatly enhanced to ca. 55 mg g−1 when pH of the 8 mM NaCl solution is adjusted from 5.4 to 7.5. In the long-term stability test, NAC30//AC remains 40% of its maximum reversible SAC after 100 charging-discharging cycles, indicating that our nitrogen doping is able to effectively reduce the oxidation of activated carbon, confirmed by the electrochemical impedance analysis.

1. Introduction Capacitive deionization (CDI) becomes a promising desalination technology for the brackish water treatment nowadays because of the increasing demand for fresh water around the world [1,2]. By applying

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a voltage over the electrodes of a CDI cell (i.e., charging), the electrical double layer formed at the electrode/solution interface enables the adsorption of ions in water. When the voltage is removed (i.e., open circuit state), these ions are released back to the solution to regenerate the electrodes. Because CDI is membrane-free and does not need extra

Corresponding author at: Department of Chemical Engineering, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsin-Chu 30013, Taiwan. E-mail address: [email protected] (C.-C. Hu). URL: http://mx.nthu.edu.tw/~cchu/ (C.-C. Hu).

https://doi.org/10.1016/j.desal.2020.114362 Received 19 November 2019; Received in revised form 2 February 2020; Accepted 2 February 2020 0011-9164/ © 2020 Elsevier B.V. All rights reserved.

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thermal energy, it circumvents the problems of traditional desalination technologies with its low cost, low energy consumption, and high desalination efficiency [3,4]. Numerous materials have been employed as the active materials in the CDI systems, such as metal oxides, conductive polymers [5], polyoxometalates [6] and metal-organic frameworks [7]. These new materials open up the potential and variety of the CDI applications. In addition, various concepts of CDI systems, such as the usage of CDI as the CO2 capture system [8] and the flow type capacitive deionization (FCDI) system [9], have been developed in recent years. Although there are several breakthroughs and new applications in the CDI field, the traditional CDI systems employing porous carbon materials still play an important role on the basis of their abundance and low cost. Since the adsorption/desorption of ions in such CDI systems is based on the nonfaradaic process, the properties of carbon materials, such as specific surface area, pore size [10], electrical conductivity [11,12], and surface functional groups/charges [13,14], are the key factors affecting the CDI performance. Graphene [15], carbon aerogel [16], carbon nanotube (CNT) [17,18], and activated carbon (AC) [10,19–21] are the commonly used materials for CDI. Among them, AC possesses the advantages of high specific surface area, easy accessibility and low cost, making it be an ideal candidate for the CDI electrodes [1]. However, its hydrophobic property results in the low utilization of pores, and the oxidation-prone property of carbon on the positive electrode causes the decay of CDI performance. The carbon oxidation is attributable to the faradaic reactions in water [22], leading to the deterioration of carbon structure as well as the shifting of potential at the point of zero charge (EPZC) [23,24]. To solve this issue, more and more studies commit to diminish the oxidation of carbon-based electrodes, such as reducing the applied voltage, adjusting the applied voltage range [25], or periodically alternating the voltage [26]. For example, in our previous study [27], an AC//AC device with an optimized potential window of 1.4 V demonstrates the high cycling stability (18 cycles in 6 h) and the high desalination capacity of 12 mg g−1. However, these methods focus on adjusting the operation conditions rather than altering the material properties since changes in material properties, such as a more hydrophilic surface or an improved electrical conductivity, usually enhance the salt adsorption capacity (SAC) of AC. Consequently, the CDI performance of AC-based cells can be maximized to compensate its drawback of short lifetime. Introducing heteroatoms into carbon structures is an effective way to alter its properties. Nitrogen doping was found to improve the electrical conductivity [28,29] as well as the hydrophilicity [30,31], which facilitates the ion transport and enhances the utilization of pores. Furthermore, nitrogen doping also generates defects to create more accessible surface area for ion adsorption and the accumulation of charges [32–34], further promoting the SAC of resultant materials and increasing the stability of carbons [35]. Various methods have been proposed for doping heteroatoms onto carbons [36–41], which can be classified into two strategies: (1) direct carbonization of heteroatom-containing precursors and (2) chemical doping onto carbons. In 2015, Pan et al. [40] first reported the usage of N-doped graphene for the CDI application and then, several researchers started to explore the new-family N-doped carbons in the CDI systems [38,39]. For example, Xu et al. utilized the N-doped mesostructured carbon nanocrystals to point out that pyridinic nitrogen is more favorable to the electrosorption of sodium ions in comparison with pyrrolic nitrogen [41]. For the second approach, Mykola Seredycha et al. [42] proposed that the formation of quaternary-N and pyridine-N-oxide during nitrogen doping is enhanced by the increase in the oxygencontaining functional groups on carbon surface, which are helpful to improve electronic conductivity. In this work, the hydrophilicity of AC is improved by the acid pre-treatment to increase the density of Ocontaining functional groups (o-AC) meanwhile N doping is conducted by thermal annealing of the o-AC-melamine mixture under the N2

atmosphere to obtain N-doped activated carbons (NACs). These NACs not only improve the hydrophilicity of AC via the formation of Ocontaining functional groups but also optimize the electron transport through the formation of quaternary-N and pyridine-N-oxide. Moreover, NACs exhibit high ion adsorption ability in the 8 mM NaCl solution and inhibit its oxidation in a long-term operation test, demonstrating their promising application potential in CDI systems. 2. Materials and approaches 2.1. Materials The commercial AC powders (ACS25) were purchased from the China Steel Chemical Corporation, Taiwan. Nitric acid (HNO3) and melamine were used for the acid pre-treatment and nitrogen doping of AC, respectively. Polyvinylidene difluoride powders (PVDF, 534,000 MW) were used as a binder and 1-methyl-2-pyrrolidone (NMP) was used as the solvent in the preparation of carbon-PVDF coating slurries. 2.2. Preparation of N-doped activated carbon (NAC) AC powders were pre-treated in 5% or 30% nitric acid solutions at 60 °C for 1 h. After this acid pre-treatment, they were washed with DI water until the solution pH approached 7, then they were filtered and dried in an oven at 80 °C overnight [43]. The acid-treated AC powders were impregnated with an aqueous solution of 5.6 mM melamine and stirred at 60 °C for 1 h. Afterwards, the water in the solution was evaporated in an oven at 80 °C overnight to obtain the dried sample. The melamine/AC powders were heated at 900 °C for 1 h under the N2 atmosphere. The sample was washed with 60 °C DI water for 10 min and dried in an oven at 80 °C overnight to obtain the nitrogen-doped AC (NAC). The as-prepared NAC powders were used as the positive electrodes and were named as NACX, where X represents the HNO3 concentration in the acid treatment process. NAC0 indicates the N-doped AC without the acid treatment process. 2.3. Characterization The specific surface areas were estimated by the Brunauer-EmmettTeller (BET) method and the pore size distributions were analyzed by the non-linear density function theory (NLDFT) model from the N2 adsorption/desorption isotherms. The chemical environment of nitrogen doped onto AC was examined by the X-ray photoelectron spectroscopy (XPS, ULVAC-PHI, PHI Quantera SXM). The surface charge was measured by a particle size/zeta potential analyzer (Malvern, Zetasizer Nano ZS90). The surface hydrophilicity was measured by a contact angle instrument (First Ten Angstrom, FTA125). To measure the electrochemical properties of AC and NACs, a threeelectrode system was employed and a platinum wire and an Ag/AgCl wire were used as the counter and reference electrodes, respectively. Electrochemical impedance spectroscopy (EIS) was measured at the open-circuit potential (EOCP) under the three-electrode mode in the 8 mM NaCl solution on a CHI 6273e electrochemical workstation (Ch Instrument). 2.4. CDI tests The CDI cells were consisted of two carbon-coated electrodes with equal mass of active materials (3 mg cm−2). The active material (NACs and AC powders) was coated on the titanium plate (ca. 0.3 g) with a geometric area of 3 cm × 1 cm. The coating slurry of active materials was a mixture of the active materials and PVDF (in a weight ratio of 9:1) dispersed in 100 μL 1-Methyl-2-pyrrolidone (NMP). After coating, all electrodes were dried in a vacuum oven at 80 °C overnight. The CDI tests were conducted in a 70-mL solution of 8 mM NaCl 2

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with a parallel flow between two electrodes (see Fig. S1) at a flow rate of 30 mL min−1. In our preliminary study, the SAC of our system was not significantly changed by the increase of the flow rate from 30 to 35 and 40 mL min−1. The active materials on the positive and negative electrodes were NACs and AC, respectively, which is denoted as NACX//AC although an AC//AC cell was employed for the comparison purpose. The CDI measurements were charged/discharged at 1.2/0 V for 30/30 min for comparing the salt adsorption capacity (SAC) of the CDI cells employing various NACs. The discharge time was changed from 0 to 30 min to examine its effect on the SAC, while the charge time was fixed to be 10 min in this study. The pH of the 8 mM NaCl solution was adjusted from 5.4 to 4.1 and 7.5 to demonstrate the influence of surface charge on the CDI performance. Conductivity change was measured by a conductivity meter and transformed into the salt concentration from a calibration curve. The SAC (Γ, mg g−1) was estimated by Eq. (1):

Γ=

(C1 − C0 ) × V × MW m

which provides intimate interactions between the oxygen-functional groups of AC and melamine during the impregnation process [46]. Therefore, melamine molecules are able to be dispersed uniformly into the whole carbon structure rather than at the surface. Consequently, higher N-doping contents are obtained for the AC with the acid pretreatment. Note that the oxygen-containing functional groups formed on AC after the acid pre-treatment generally include CeO, C]O, and OC=O which can be identified by the XPS analysis (see Fig. S4 in the Supporting Information). N-6: pyridinic-N, N-5: pyrrolic-N, N-Q: quaternary-N, N-X: pyridinic-oxide-N. The EIS results of AC and various NACs are shown as a negative Nyquist plot in Fig. 2. The high real-part resistance (ca. 53–55 Ω) of all EIS spectra at the high-frequency end on the x-axis is reasonably attributed to a low concentration of NaCl in the testing solution (8 mM) and the relatively long distance between the working electrode and the Ag/AgCl reference electrode. From a comparison of all EIS spectra in Fig. 2, all N-doped ACs show smaller arcs in the high-middle-frequency region than the as-received AC. The presence of such a distorted arc in the high-middle-frequency region has been described by a porous layer model expressed in terms of the electronic impedance of the porous solid and the ionic impedance from the pores filled with electrolytes. The real-part resistance in this arc obviously decreases with the application of the thermal N doping, attributable to the significant reduction in both electronic and ionic impedances of NACs. From Fig. 1 and Table 1, the formation of quaternary-N and pyridine-N-oxide in NACs can enhance the electronic conductivity of carbon materials to reduce the resistance and interfacial contact impedance among carbon particles [28]. The acid pre-treatment and nitrogen functional groups can improve the hydrophilicity of NACs to lower the carbon/electrolyte interface impedance and promote the utilization of electric doublelayer [31,47,48]. All these effects make the low arc impedance of NACcoated electrodes. To further investigate the hydrophilic property of carbon materials, the contact angle analyses are shown in Fig. 3 which reveals the variation in water contact angles in 60 s. The faster decreases in water contact angles of NAC5 and NAC30 indicate their more hydrophilic surface, which is caused by the more nitrogen and oxygen functional groups on their surface. The zeta potential patterns of AC and NACs measured in the 8 mM NaCl solution with various pH values are shown in Fig. 4 to investigate the effect of surface charges which significantly affect the ion transport through the electrostatic force of expulsion or attraction. From an examination of Fig. 4, the surfaces of AC and NACs are positively charged in the acidic solutions (pH < 4) and all surfaces become negatively charged at pH > 5. In addition, the pH values reaching the point of zero charge (PZC) are 3.5 for NAC30 and NAC5, 4.5 for NAC0, and 5.0 for AC, respectively. Moreover, at pH > 5, the order of carbon materials with respect to decreasing the zeta potential is: AC > NAC0 > NAC5 > NAC30, revealing that the acid pre-treatment and N-doping can shift the surface charge to a more negative value; i.e., more negative charges anchored on the surfaces of NACs. Since the zeta potential of NAC30 is more negative than that of NAC5 when pH is higher than 4.5, more functional groups with negative charges have been introduced into the carbon structure by the acid pretreatment in a more concentrated HNO3 solution. Since pH of the 8 mM NaCl solution in the CDI test is about 5.4–5.8 in this work, NAC30 provides the most negative zeta potential among all NACs (due to its high content of oxygen and pyridinic-oxide-N functional groups). The highly negative surface charges of NAC30 may exhibit a strong impact on the ion transport and thus, affect the ion adsorption/desorption performance in the CDI measurements.

(1)

where C1 and C0 represent the salt concentrations after and before ion removal; V is the total volume of the brackish water (70 mL); MW is 58.44 g mol−1 for the molecular weight of NaCl; m (g) is the total mass of active materials on two electrodes (18 mg without PVDF in this work). Since the ratio of AC and PVDF is 9:1, the total mass of the two coatings onto the two Ti substrates in a single cell is 20 mg. The longterm stability of symmetric and asymmetric cells was respectively tested at 1.2/0 V for 47 and 100 cycles, where the charge and discharge steps respectively lasted for 30 min. The SAC retention (RSAC, see Eq. (2)) is proposed to be an index for the long-term stability:

RSAC =

SACi SACmax

(2) −1

where SACi (mg g ) is the salt adsorption capacity at the ith cycle and SACmax (mg g−1) is the maximum salt adsorption capacity during the long-term stability test. 3. Results and discussion 3.1. Materials characterization Fig. S2(a) compares the N2 adsorption-desorption isotherms and pore size distributions of various NAC samples and the as-received AC. All these samples show the typical type I isotherms, indicating their micropore-dominated structures [44]. However, with the nitrogen doping, the specific surface area (SSA) of AC decreases from 2421 m2 g−1 to 2089 m2 g−1 (NAC0), 1970 m2 g−1 (NAC5), and 1887 m2 g−1 (NAC30), respectively. This is attributable to the formation of oxygen- and nitrogen-functional groups, resulting in the minor damage of pore walls and/or blocking of micropores [43,45]. As seen in Fig. S2(b), the as-received AC exhibits the highest micropore contents, especially at the pore diameter < 1 nm. The above idea for the minor damage of AC after the acid pre-treatment and thermal N doping is supported by the SEM images shown in Fig. S3 in the Supporting Information since there is no significant morphology difference among AC, NAC0 and NAC30. Here, the XPS results reveal the successful doping of nitrogen atoms onto AC via the thermal doping process. In addition, Fig. 1 shows the deconvolution of XPS N1s core-level spectra of various NAC samples, where the peaks of pyridinic-N, pyrrolic-N, quaternary-N and pyridinicoxide-N are respectively centered at 398.7, 400.3, 401.4, and 402.8 eV. The relative contents of these N-containing functional groups are listed in Table 1. From this table, the nitrogen contents in NAC5 and NAC30 are higher than that in NAC0, indicating that the acid pre-treatment can introduce more oxygen-containing functional groups and favors the doping of nitrogen atoms into the carbon structure. This is due to the improved hydrophilic surface resulting from the acid pre-treatment,

3.2. The CDI performance Fig. 5(a)–(d) shows the results of SAC against charge-discharge time 3

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Fig. 1. (a) N 1s XPS spectra of (a) NAC0, (b) NAC5 and (c) NAC30.

which is never reported before. The great enhancements in SAC0 for cells 3 and 4 are reasonably due to the improved hydrophilicity and the high ion adsorption capability of N-functional groups [49] of NAC5 and NAC30. The former factor enables ions to reach the inner pore surface to improve the adsorption capability. Since the reversible SAC in cells 3 and 4 is only 20–25 mg g−1, a lot of ions are kept in the electrodes, resulting in the lower practical SAC in comparison with their inherent adsorption capabilities (i.e., SAC0). From Fig. S5, the solution conductivity gradually came back to the initial value in the above 7-h CDI measurement when cell 1 was employed. This phenomenon suggests the gradual release of adsorbed ions in the initial charge half cycle but the reversible SAC does not increase with the gradual release of ions. The above phenomenon is not very obvious for cells 3 and 4 although cell 2 using the NAC0 positive electrode shows the CDI behavior between cells 1 and 3. Accordingly, the above differences in the CDI performance are mainly attributed to the presence of the N surface functional groups. Of course, the unbalanced adsorption and desorption of ions, because of differences in ion adsorption and desorption rates as well as in charge and discharge times, during the repeated CDI cycling program may also result in the extra ion releasing. This idea is supported by the more obvious phenomenon when the discharge time is 30 min from an examination of Figs. 5(b)–(d), one of the purposes for our proposed CDI program. From a comparison of Fig. 5(b)–(d), several features need to be mentioned. First, the reversible SACs of cells 3 and 4 are obviously higher than those of cells 1 and 2, indicating the advantage of combining the acid pre-treatment and N doping. Second, the decay in reversible SAC for cell 3 is faster than that for cell 4 although the SAC0 of the former cell is somewhat higher than that of the latter, indicating the HNO3 concentration effect. Third, the ion adsorption rate for cell 4 is faster than its corresponding ion desorption rate but this phenomenon is not obvious for cells 1–3, also indicating the HNO3 concentration effect. All the above differences in the CDI program for these cells suggest the complicated properties of N-doped ACs obtained by the combination of the acid pre-treatment and N doping. Since the reversible SACs of cells 3 and 4 are obviously increased with prolonging the charge-discharge time from an examination of Fig. 5, the discharge time is prolonged from 10 min to 15, 20 and 30 min to desorb residual ions for promoting the SAC values of all CDI cells while the charge time is fixed to be 10 min. Fig. 6(a) reveals that the reversible SAC values of cells 1–4 linearly increase with prolonging the discharge time from 10 to 30 min, implying the continuous desorption of ions within the micropores of AC and NACs. In addition, the order of cells with respect to decreasing the slope of the SAC vs. discharge time plots is: cell 4 > cell 3 ≫ cell 1 ≈ cell 2. This effect may be correlated to the hydrophilic characteristics of AC and NACs since the order of carbon with respect to increasing the 60-s water contact angle follows the same trend (see Fig. 3). Based on this idea, ions adsorption can go deeply into the pore structure of the hydrophilic NAC5 and NAC30 and these ions requires long time to be completely desorbed

Table 1 The summarized specific surface area (SSA) and XPS results for AC, NAC0, NAC5 and NAC30. SSA (m2 g−1)

AC NAC0 NAC5 NAC30

2421 2089 1970 1887

XPS analysis O (at.%)

N (at.%)

N-6

N-5

N-Q

N-X

4.3 3.9 5.4 5.4

0.2 1.0 2.2 2.1

– 0.43 0.48 0.43

– 0.27 0.26 0.27

– 0.23 0.17 0.17

– 0.07 0.09 0.13

Fig. 2. The negative Nyquist plots of (1) AC, (2) NAC0, (3) NAC5 and (4) NAC30.

for four CDI cells consisting of AC//AC (cell 1), NAC0//AC (cell 2), NAC5//AC (cell 3), and NAC30//AC (cell 4) for the 1st, 9th, 6th, and 13th cycles, respectively. These data were extracted from Fig. S5 in the Supporting Information, which were evaluated by a CDI program where these four cells were charged/discharged at 1.2/0 V and 10/10 min in the initial 3 cycles, following at 1.2/0 V and 30/30 min for 3 cycles, then at 1.2/0 V and 10/10 min for 5 cycles, and at 1.2/0 V and 10/ 30 min for the final 2 cycles. This CDI program is employed to evaluate the effect of positive electrode materials on the CDI characteristics. From Fig. 5a, all cells show very large ion adsorption capacities during the first charging half cycles (denoted as SAC0) while lots of ions cannot be released out in the following discharge cycles. Moreover, the SAC0 values (ca. 87 mg g−1) of cells 3 and 4 (i.e., using the NAC5 and NAC30 positive electrodes) are about three times of that of cell 1 (28 mg g−1) although the reversible SAC in cells 3 and 4 is only 15–25 mg g−1 (depending on the charge-discharge time). This phenomenon reveals the synergistic effect between acid pre-treatment and thermal N doping, 4

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Fig. 3. Variations in water contact angle with time for AC, NAC0, NAC5 and NAC30.

ascribed to the low utilization of micropores because of the hydrophobic surface properties of AC and NAC0. Hence, the effects of discharge time on the reversible SAC are not obvious. Since the slope of the SAC vs. discharge time plot for cell 4 is larger than that for cell 3, the rate of ion desorption within NAC30 is believed to be faster than that within NAC5 since both NACs provide very similar SAC0 values (see Fig. 5(a)). Based on the ion-adsorption site concept [49], the reversible SAC should strongly depend on the total amount of reversible adsorption/desorption sites within AC and NACs as well as the rate of ion desorption. Accordingly, the difference in the reversible SAC at various discharge times between NAC5 and NAC30 may also result from their different ion desorption rates. To clarify this opinion and to eliminate the influence of concentration gradient on the ion desorption rate, two different charging times were applied to the two CDI cells with their positive electrodes of NAC5 and NAC30, respectively so that both cells adsorbed equal amounts of ions from the change in SACs. The above two cells were then discharged and the SAC against the discharge time curves are shown in Fig. 6(b). From Fig. 6(b), the cell utilizing the NAC30 positive electrode exhibits a faster ion desorption rate than the one using the NAC5 positive electrode. The exact reasons responsible for the above difference are unclear but probably due to the higher negative surface charge density and/or the higher density of pyridinic-oxide‑nitrogen (see Table 1) on NAC30 in comparison with NAC5. Hence, at a fixed

Fig. 4. Zeta potential against pH plots of AC, NAC0, NAC5 and NAC30.

from the microporous structure during the discharge process. Similarly, the small increase in the reversible SACs for cells 1 and 2 may be 5

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Fig. 5. SAC of the 8 mM solution using the CDI cells containing a negative AC electrode and a positive (1) AC, (2) NAC0, (3) NAC5, and (4) NAC30 charged and discharged at 1.2 V and 0 V, respectively for the (a) 1st, (b) 9th, (c) 6th, and (d) 13th cycle.

enhanced when both acid pre-treatment and nitrogen doping are conducted since the saturated SAC values of cells 1–4 are 11.6 ± 0.8, 12.8 ± 0.4, 19.4 ± 1.0, and 24.7 ± 1.6 mg g−1, respectively. The increases in both oxygen and nitrogen contents in NAC5 and NAC30 make the carbon surface be more hydrophilic, facilitating the ion transport and improving the utilization of pores. On the other hand, cell 2 containing NAC0 only exhibits a slight increase in the SAC, probably due to its low nitrogen content and less negatively charged surface in comparison with NAC5 and NAC30. Here, the instantaneous salt adsorption rate, ri-SAC, a time-dependent index, is defined as follow:

discharge time, the faster desorption rate of NAC is, the more sites are left for the ion adsorption in the next cycle, leading to the higher SAC value. Fig. 7 shows the dependence of reversible SAC of four CDI cells consisting of AC//AC (cell 1), NAC0//AC (cell 2), NAC5//AC (cell 3), and NAC30//AC (cell 4) on the CDI discharge time where the charge time of CDI is fixed to be 10 min. Again, the variation in the SAC of these cells is attributable to the change in the positive electrode material because of the same AC negative electrodes. From an examination of these four curves, the SAC of these four cells linearly increases with the discharge time during the initial 5 min, gradually increases with prolonging the following 10 min, and approaches saturated when the discharge time is above 15 min. Clearly, the SAC of AC is slightly improved by the thermal nitrogen doping process only but is greatly

ri − SAC =

(SAC2 − SAC1 ) (t2 − t1 )

(3)

Fig. 6. (a) The effect of discharge time on the SAC in the next cycle of cells 1–4 and (b) the SAC during the discharge process for the cells using (1) NAC5 and (2) NAC30 positive electrodes after equal adsorption amounts in the charge process. 6

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promising strategy of combining the acid pre-treatment and thermal nitrogen doping processes to enhance the SAC. Since the surface charges of AC and NACs are pH-dependent meanwhile the cell of NAC30//AC shows the highest SAC value, the pH effect of the NaCl solution on the SAC of NAC30//AC is examined here and the corresponding results for charged/discharged at 1.2/0 V for 30/ 30 min are shown in Fig. 8. From Fig. 8(a), when pH decreases from 5.4 for the as-prepared 8 mM NaCl solution to 4.1 by the addition of 0.01 M HCl, the surface charges of positive and negative electrodes are negative and positive, respectively (see Fig. 8(b)). The SAC sharply decreases from 23.5 (the 3rd cycle) to 5.7 mg g−1 (the 4th–6th cycles) because the negative charges on the positive electrode and the positive charges on the negative electrode may lead to the inversed-CDI responses within a certain cell voltage range, probably reducing the CDI performance. On the other hand, when the solution pH was increased to 7.5 by the addition of 0.01 M NaOH, the SAC dramatically increased to 103 mg g−1 (the 7th cycle) although a lot of ions adsorbed by this cell are not released back to the solution. In fact, the reversible SAC becomes about 55 mg g−1 in the following 2 cycles and the SAC of 103 mg g−1 obtained in the 7th cycle can be considered to be a new SAC0 in this pH = 7.5 solution. In other words, the more negative charges on both NAC and AC electrodes in the pH = 7.5 solution in comparison with those in the pH = 5.4 solution sharply promote their SAC0 and reversible SAC values. These results demonstrate the importance of negative surface charges of NAC and AC on the CDI performance. A similar phenomenon that the adsorption capacities of cadmium ions on N-rich carbons were greatly enhanced with higher pH has been found [49], supporting our results, although the exact reasons responsible the phenomenon are unclear. Obviously, how to enhance the ratio between reversible SAC and SAC0 is an important topic in the usage of NAC materials for the CDI applications. In addition, the solution pH may be one of the key factors determining the ratio between reversible SAC and SAC0. Accordingly, SAC0 is meaningful in the material sciences of modified ACs and the field of CDI.

Fig. 7. The reversible SAC against the discharge time plots for the CDI cells containing a negative AC electrode and a positive (1) AC, (2) NAC0, (3) NAC5, and (4) NAC30 charged at 1.2 V for 10 min and discharged at 0 V for various times.

where SAC1 and SAC2 are the salt adsorption capacities at the discharge time = t1 and t2, respectively. From curve 4 in Fig. 7, the ri-SAC value in the 1st min discharge time is equal to (2.4–0)/ (1–0) = 2.4 mg g−1·min−1 and ri-SAC from 1 to 2.5 min is (7.2–2.4)/ (2.5–1) = 3.2 mg g−1·min−1. These ri-SAC data against their discharge times are shown Fig. S6 to find the suitable discharge time since the NAC30//AC cell exhibits the largest reversible SAC among the 4 cells employed in this work. Clearly, ri-SAC reaches the maximal value of ca. 3.5 mg g−1·min−1 during the initial 5 min, which quickly drops to a low value < 0.3 mg g−1·min−1 at the discharge times ≥15 min. This phenomenon is mainly attributable to the constant voltage operation of the CDI system employed in this work. When 1.2 V was applied to the cell, a very large current flow was found at the beginning, leading to a high ri-SAC value at the same time without considering the fact that the amount of ions adsorbed by the electrode is far from its saturation capacity. With prolonging the CDI discharge time, both positive and negative electrodes reach their quasi-steady state potentials (i.e., iR becomes small because of the decay in i with time), reducing the salt adsorption rate. Based on the results in Figs. 7 and S6, a discharge time of 15 min seems to be long enough for the practical CDI application. Moreover, a higher ri-SAC may result from a higher reversible SAC at the same CDI time, meanwhile ri-SAC should be affected by the operation method and the intrinsic characteristics of active materials. The detailed properties and CDI performances of our NACs//AC cells and other CDI systems containing N-doped carbon materials are compared in Table 2. Clearly, the SAC values of cells 3 and 4 are obviously higher than all the reference data shown in this table. Of course, the high SAC values in cells 3 and 4 may be also attributable to the high specific surface areas of AC and NACs employed in this work while a comparison of the SAC values for cells 1–4 in Fig. 7 reveals the

3.3. Long-term stability The long-term stability of the AC//AC and NAC30//AC cells has been charged and discharged at 1.2/0 V for 30/30 min in the 8 mM NaCl solution for many cycles and curves 1 and 2 in Fig. 9 show their respective results. On both curves, a gradual rise in SAC during the initial 10–20 cycles is found. This phenomenon may be attributed to the sufficient pore wetting of both AC and NAC after the multiple chargedischarge cycling. This effect improves the utilization of the surface area of electrode materials, promoting the SAC. Unfortunately, the AC//AC cell starts to show an obvious decay in SAC at the 26-28th cycles and the remained SAC suddenly drops to 0.51 of its maximum SAC in the following a few cycles. The decay rate of SAC is very fast although the charging cell voltage is only 1.2 V. Finally, the SAC retention is only around 10% of its maximum SAC in the 47-cycle test. After this 47-cycle test, this AC//AC cell shows the inversed CDI (denoted as i-CDI [54]) behavior which desorbs ions during the charging process, revealing the failure of this CDI system. The above

Table 2 Microporous properties and SAC comparisons of CDI cells using various N-doped carbons. N-doped carbon

SSA (m2 g−1)

Applied voltage (V)

N-AC NC-800 AN-CFs

N/A 798 905.3

1.2 1.2 1.2

NDC-Cs-900 NAC0//AC NAC5//AC NAC30//AC

850 2089//2421 1970//2421 1887//2421

1.2 1.2

Initial NaCl concentration −1

50 mg L 1 mM 500 mg L−1 1000 mg L−1 5 mM 8 mM

7

SAC (mg g−1)

Ref.

2.85 8.52 12.32 16.56 15.0 12.8 ± 0.4 19.4 ± 1.0 24.7 ± 1.6

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Fig. 8. (a) SAC of NAC30//AC vs. charge-discharge time in 8 mM NaCl with pH = (1) 4.1, (2) 5.4, and (3) 7.5 and (b) the corresponding zeta potentials of (1) AC and (2) NAC30 in 8 mM NaCl with pH = 4.1, 5.4 and 7.5.

increase in the real-part resistances in the high-middle-frequency arcs (increasing ca. 75–80 Ω) was found for both positive and negative electrodes in the AC//AC cell after the long-term stability test. From an examination of Fig. 10(a) and (b), there are two impedance arcs on the EIS spectra of the two AC electrodes in the AC//AC cell after the longterm stability test. The small arcs in the high-frequency part for the above two electrodes before and after the long-term stability test are overlapped. These impedance arcs in the relatively high-frequency part are reasonably attributed to the contact impedance at the coating/ substrate interface, which should be not significantly affected by the long-term stability test. However, a significant increase in the real part resistance for the second semicircles in the high-to-middle-frequency part for these electrodes after the long-term stability test is observed. This semicircle is accordingly attributed to the contact impedance among AC particles within the coating since the surface of AC is gradually oxidized during the repeated cycling test, leading to the increase in the real part resistance. Hence, the electrical conductivity of AC became worse and the contact impedance was enlarged by the damaged oxidation of AC. On the other hand, the real-part resistance for the second semicircle in the high-to-middle-frequency part on the NAC electrode only increased about 15 Ω after the 100-cycle test. This obvious difference between NAC and AC reveals that AC has been stabilized by the combination of acid pre-treatment and thermal nitrogen doping. Third, the real-part resistance for the second semicircle in the high-to-middle-frequency part on the negative AC electrode in the NAC30//AC cell did not change with the application of the 100-cycle test. However, the resistance for the second semicircle in the high-tomiddle-frequency part on the negative AC electrode in the AC//AC cell increased after the 47-cycle CDI test. This result should be due to the positive shift in the working potential windows of both positive and negative electrodes in AC//AC during the 47-cycle CDI test because of significant oxidation of the positive AC electrode and consequent oxidation of the negative AC electrode. The XPS was used to identify the damaged oxidation on AC and NAC after the long-term stability test. Table 3 reveals that the oxygen contents of positive and negative electrodes in AC//AC significantly increased from 4.3% to 10.8 and 7.9%, respectively after the long-term stability test. The increase in oxygen-functional groups is believed to cause the shift in Epzc resulting from the deterioration of the carbon structure, leading to the decay in the SAC. The Epzc values of both AC electrodes in the AC//AC cell were positively shifted from 0.1 to 0.5 V after the long-term stability test (see Fig. S7 in the Supporting Information) since the SAC retention of the AC//AC cell is only around 10% in the 47-cycle test. On the other hand, the XPS analysis shows that when the positive electrode is replaced with NAC30, the degree of oxidation is alleviated for both electrodes after the 100-cycle test since the oxygen contents of NAC30 and AC slightly increase from 5.4% to

Fig. 9. The ratio of SAC from the ith cycle to the maximum SAC (denoted as SACi/SACmax) of (1) NAC30//AC and (2) AC//AC charged/discharged at 1.2/ 0 V for 30/30 min in the long-term stability test.

phenomenon has been attributed to the damaged oxidation of AC on the positive electrode shifting its potential of zero charge (Epzc) [23,24]. Accordingly, we did not construct the AC//AC cell with its positive electrode coated with the AC pre-treated with 5 or 30% HNO3 in this work because in this system, a cell voltage of 1.2 V is too high for the CDI application [55]. On the other hand, when the positive electrode is replaced with NAC30, the SAC decay rate of this asymmetric NAC30// AC cell is obviously slower than that of AC//AC and the i-CDI behavior is not visible during the 100-cycle long-term test. Moreover, after this 100-cycle test, the NAC30//AC still remains 40% of its maximum SAC. These results indicate that the combination of acid pre-treatment and thermal nitrogen doping effectively improves the long-term stability of AC in the repeated CDI cycling test. To examine the changes in electrochemical properties of electrode materials after the long-term stability test, the EIS spectra of both electrodes in AC//AC and NAC30//AC cells were measured and the results are shown in Fig. 10. From an examination of Fig. 10, several features have to be mentioned. First, the real-part resistances in the high-middle-frequency arcs for the three as-prepared AC electrodes were similar (ca. 100–130 Ω). This result indicates the acceptable reliability in preparing the AC electrodes in this work. The presence of such an impedance arc is usually due to the significant contact impedance among AC particles within the coating and the contact impedance at the coating/substrate interface [56,57]. Second, an obvious

8

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Fig. 10. EIS spectra of (a, c) positive and (b, d) negative electrodes in (a, b) AC//AC and (c, d) NAC30//AC before and after the long-term stability test.

conductivity and hydrophilic property were significantly improved. The NAC30//AC cell exhibited an excellent ion adsorption ability of 24.7 ± 1.6 mg g−1 under 1.2 V/0 V, much higher than 11.6 ± 0.8 mg g−1 for AC//AC. The very negative surface charges made the ions desorb fast from the microporous structure of NACs. The reversible SAC of NAC30//AC even reached up to 55 mg g−1 when both electrodes were of the negatively charged surfaces which manifest the positive effect of the negative surface charge on the CDI performance. More importantly, the positive NAC30 electrode showed very high resistance to the damaged oxidation in the 100-cycle CDI test, revealing the promising strategy of combining acid pre-treatment and nitrogen doping.

Table 3 XPS analyses of AC//AC and NAC30//AC before and after the long-term stability test. Positive//negative electrode

AC//AC NAC30//AC

XPS analysis (at.%) C

O

N

89.0/91.7 90.4/94.0

10.8/7.9 7.8/5.7

0.2/0.3 1.8/0.3

7.8% and from 4.3% to 5.7%, respectively. Such increases in the oxygen content also moved the Epzc of NAC30 and AC from 0.25 to 0.6 V and from 0.1 to 0.5 V, respectively (see Fig. S7 in the Supporting information). The above results are in a good agreement with the EIS results, indicating the minor damage of the positive electrode. The deconvolution spectrum of N 1s (see Fig. S8) does not show obvious changes in the nitrogen-functional groups, revealing the excellent stability of NAC30 in the CDI test. Moreover, from the XPS O1s core level spectra shown in Fig. S9 (see the Supporting Information), only the positive AC electrode in the AC//AC cell after the 100-cycle test shows a significant increase in C]O, indicating a higher oxidation degree in comparison with the others two negative AC and positive NAC30 electrodes. Hence, the poor SAC retention of the AC//AC cell in the long-term stability test may be due to not only the oxygen content but also types of functional groups. From all the above results and discussion, we provide a new perspective on improving the long-term stability of AC through the combination of acid pre-treatment and nitrogen doping.

CRediT authorship contribution statement Chun-Chia Hsu: Project administration, Methodology, Formal analysis, Writing - original draft. Yi-Heng Tu: Formal analysis, Validation, Writing - review & editing. Yu-Hsiang Yang: Formal analysis, Validation, Writing - review & editing. Jeng-An Wang: Formal analysis, Validation, Writing - review & editing. Chi-Chang Hu: Supervision, Conceptualization. Declaration of competing interest We declared that we have no conflicts of interest to this work. We also declared that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work.

4. Conclusions Acknowledgments In summary, we provided a new perspective on improving the longterm stability of AC through the combination of acid pre-treatment and nitrogen doping. The acid pre-treatment favored the introduction of nitrogen atoms into the carbon structure, and the electrical

This research was funded by MOST-Taiwan under contract numbers MOST 105-2221-E-007-127-MY3, 106-2221-E-007-089-MY3 and 1082923-E-007-004-MY3, which are gratefully acknowledged. 9

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Appendix A. Supplementary data [28]

Supplementary data to this article can be found online at https:// doi.org/10.1016/j.desal.2020.114362.

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References

[30]

[1] S. Porada, R. Zhao, A. van der Wal, V. Presser, P.M. Biesheuvel, Review on the science and technology of water desalination by capacitive deionization, Prog. Mater. Sci. 58 (2013) 1388–1442. [2] M.E. Suss, S. Porada, X. Sun, P.M. Biesheuvel, J. Yoon, V. Presser, Water desalination via capacitive deionization: what is it and what can we expect from it? Energy Environ. Sci. 8 (2015) 2296–2319. [3] M.A. Anderson, A.L. Cudero, J. Palma, Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: will it compete? Electrochim. Acta 55 (2010) 3845–3856. [4] J. Choi, P. Dorji, H.K. Shon, S. Hong, Applications of capacitive deionization: desalination, softening, selective removal, and energy efficiency, Desalination 449 (2019) 118–130. [5] Y.-H. Tu, C.-F. Liu, J.-A. Wang, C.-C. Hu, Construction of an inverted-capacitive deionization system utilizing pseudocapacitive materials, Electrochem. Commun. 106486 (2019). [6] H.-Y. Chen, R. Al-Oweini, J. Friedl, C.Y. Lee, L. Li, U. Kortz, U. Stimming, M. Srinivasan, A novel SWCNT-polyoxometalate nanohybrid material as an electrode for electrochemical supercapacitors, Nanoscale 7 (2015) 7934–7941. [7] M. Wang, X. Xu, Y. Liu, Y. Li, T. Lu, L. Pan, From metal-organic frameworks to porous carbons: A promising strategy to prepare high-performance electrode materials for capacitive deionization, Carbon 108 (2016) 433–439. [8] L. Legrand, O. Schaetzle, R.C.F. de Kler, H.V.M. Hamelers, Solvent-free CO2 capture using membrane capacitive deionization, Environmental Science & Technology 52 (2018) 9478–9485. [9] J. Chang, F. Duan, H. Cao, K. Tang, C. Su, Y. Li, Superiority of a novel flow-electrode capacitive deionization (FCDI) based on a battery material at high applied voltage, Desalination 468 (2019) 114080. [10] C.-L. Yeh, H.-C. Hsi, K.-C. Li, C.-H. Hou, Improved performance in capacitive deionization of activated carbon electrodes with a tunable mesopore and micropore ratio, Desalination 367 (2015) 60–68. [11] B. Lee, N. Park, K.S. Kang, H.J. Ryu, S.H. Hong, Enhanced capacitive deionization by dispersion of CNTs in activated carbon electrode, ACS Sustain. Chem. Eng. 6 (2018) 1572–1579. [12] T. Alencherry, N. A.R, S. Ghosh, J. Daniel, V. R, Effect of increasing electrical conductivity and hydrophilicity on the electrosorption capacity of activated carbon electrodes for capacitive deionization, Desalination 415 (2017) 14–19. [13] W. Huang, Y. Zhang, S. Bao, R. Cruz, S. Song, Desalination by capacitive deionization process using nitric acid-modified activated carbon as the electrodes, Desalination 340 (2014) 67–72. [14] T. Yan, B. Xu, J. Zhang, L. Shi, D. Zhang, Ion-selective asymmetric carbon electrodes for enhanced capacitive deionization, RSC Adv. 8 (2018) 2490–2497. [15] P. Liu, T. Yan, L. Shi, H.S. Park, X. Chen, Z. Zhao, D. Zhang, Graphene-based materials for capacitive deionization, J. Mater. Chem. A 5 (2017) 13907–13943. [16] P. Xu, J.E. Drewes, D. Heil, G. Wang, Treatment of brackish produced water using carbon aerogel-based capacitive deionization technology, Water Res. 42 (2008) 2605–2617. [17] S. Zhang, Y. Wang, X. Han, Y. Cai, S. Xu, Optimizing the fabrication of carbon nanotube electrode for effective capacitive deionization via electrophoretic deposition strategy, Progress in Natural Science: Materials International 28 (2018) 251–257. [18] H. Li, S. Liang, M. Gao, G. Li, J. Li, L. He, The study of capacitive deionization behavior of a carbon nanotube electrode from the perspective of charge efficiency, Water Sci. Technol. 71 (2014) 83–88. [19] G. Wang, B. Qian, Q. Dong, J. Yang, Z. Zhao, J. Qiu, Highly mesoporous activated carbon electrode for capacitive deionization, Sep. Purif. Technol. 103 (2013) 216–221. [20] Z. Chen, H. Zhang, C. Wu, Y. Wang, W. Li, A study of electrosorption selectivity of anions by activated carbon electrodes in capacitive deionization, Desalination 369 (2015) 46–50. [21] C.-H. Hou, C.-Y. Huang, A comparative study of electrosorption selectivity of ions by activated carbon electrodes in capacitive deionization, Desalination 314 (2013) 124–129. [22] C. Zhang, D. He, J. Ma, W. Tang, T.D. Waite, Faradaic reactions in capacitive deionization (CDI) - problems and possibilities: a review, Water Res. 128 (2018) 314–330. [23] I. Cohen, E. Avraham, Y. Bouhadana, A. Soffer, D. Aurbach, Long term stability of capacitive de-ionization processes for water desalination: the challenge of positive electrodes corrosion, Electrochim. Acta 106 (2013) 91–100. [24] X. Gao, A. Omosebi, J. Landon, K. Liu, Dependence of the capacitive deionization performance on potential of zero charge shifting of carbon xerogel electrodes during long-term operation, J. Electrochem. Soc. 161 (2014) E159–E166. [25] D. Lu, W. Cai, Y. Wang, Optimization of the voltage window for long-term capacitive deionization stability, Desalination 424 (2017) 53–61. [26] H. Zhang, P. Liang, Y. Bian, X. Sun, J. Ma, Y. Jiang, X. Huang, Enhancement of salt removal in capacitive deionization cell through periodically alternated oxidation of electrodes, Sep. Purif. Technol. 194 (2018) 451–456. [27] Y.-J. Chen, C.-F. Liu, C.-C. Hsu, C.-C. Hu, An integrated strategy for improving the

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

[42]

[43]

[44] [45]

[46]

[47]

[48]

[49] [50]

[51]

[52]

[53]

10

desalination performances of activated carbon-based capacitive deionization systems, Electrochim. Acta 302 (2019) 277–285. Z.R. Ismagilov, A.E. Shalagina, O.Y. Podyacheva, A.V. Ischenko, L.S. Kibis, A.I. Boronin, Y.A. Chesalov, D.I. Kochubey, A.I. Romanenko, O.B. Anikeeva, T.I. Buryakov, E.N. Tkachev, Structure and electrical conductivity of nitrogendoped carbon nanofibers, Carbon 47 (2009) 1922–1929. T. Chen, L. Pan, T.A.J. Loh, D.H.C. Chua, Y. Yao, Q. Chen, D. Li, W. Qin, Z. Sun, Porous nitrogen-doped carbon microspheres as anode materials for lithium ion batteries, Dalton Trans. 43 (2014) 14931–14935. Y. Li, L. Liu, Y. Wu, T. Wu, H. Wu, Q. Cai, Y. Xu, B. Zeng, C. Yuan, L. Dai, Facile synthesis of nitrogen-doped carbon materials with hierarchical porous structures for high-performance supercapacitors in both acidic and alkaline electrolytes, J. Mater. Chem. A 7 (2019) 13154–13163. S.L. Candelaria, B.B. Garcia, D. Liu, G. Cao, Nitrogen modification of highly porous carbon for improved supercapacitor performance, J. Mater. Chem. 22 (2012) 9884–9889. Z. Wen, X. Wang, S. Mao, Z. Bo, H. Kim, S. Cui, G. Lu, X. Feng, J. Chen, Crumpled nitrogen-doped graphene nanosheets with ultrahigh pore volume for high-performance Supercapacitor, Adv. Mater. 24 (2012) 5610–5616. L.-F. Chen, X.-D. Zhang, H.-W. Liang, M. Kong, Q.-F. Guan, P. Chen, Z.-Y. Wu, S.H. Yu, Synthesis of nitrogen-doped porous carbon nanofibers as an efficient electrode material for supercapacitors, ACS Nano 6 (2012) 7092–7102. J. Tang, R.R. Salunkhe, J. Liu, N.L. Torad, M. Imura, S. Furukawa, Y. Yamauchi, Thermal conversion of core–shell metal–organic frameworks: a new method for selectively functionalized nanoporous hybrid carbon, J. Am. Chem. Soc. 137 (2015) 1572–1580. J. Luo, D. Tian, Z. Ding, T. Lu, X. Xu, L. Pan, Enhanced cycling stability of capacitive deionization via effectively inhibiting H2O2 formation: the role of nitrogen dopants, J. Electroanal. Chem. 855 (2019) 113488. J.-S.M. Lee, M.E. Briggs, C.-C. Hu, A.I. Cooper, Controlling electric double-layer capacitance and pseudocapacitance in heteroatom-doped carbons derived from hypercrosslinked microporous polymers, Nano Energy 46 (2018) 277–289. S.-M. Li, S.-Y. Yang, Y.-S. Wang, H.-P. Tsai, H.-W. Tien, S.-T. Hsiao, W.-H. Liao, C.L. Chang, C.-C.M. Ma, C.-C. Hu, N-doped structures and surface functional groups of reduced graphene oxide and their effect on the electrochemical performance of supercapacitor with organic electrolyte, J. Power Sources 278 (2015) 218–229. P. Shi, C. Wang, J. Sun, P. Lin, X. Xu, T. Yang, Thermal conversion of polypyrrole nanotubes to nitrogen-doped carbon nanotubes for efficient water desalination using membrane capacitive deionization, Sep. Purif. Technol. 235 (2020) 116196. M. Wang, X. Xu, J. Tang, S. Hou, M.S.A. Hossain, L. Pan, Y. Yamauchi, High performance capacitive deionization electrodes based on ultrathin nitrogen-doped carbon/graphene nano-sandwiches, Chem. Commun. 53 (2017) 10784–10787. Y. Liu, T. Chen, T. Lu, Z. Sun, D.H. Chua, L. Pan, Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization, Electrochim. Acta 158 (2015) 403–409. X. Xu, A.E. Allah, C. Wang, H. Tan, A.A. Farghali, M.H. Khedr, V. Malgras, T. Yang, Y. Yamauchi, Capacitive deionization using nitrogen-doped mesostructured carbons for highly efficient brackish water desalination, Chem. Eng. J. 362 (2019) 887–896. M. Seredych, D. Hulicova-Jurcakova, G.Q. Lu, T.J. Bandosz, Surface functional groups of carbons and the effects of their chemical character, density and accessibility to ions on electrochemical performance, Carbon 46 (2008) 1475–1488. D. Hulicova-Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Combined effect of nitrogen- and oxygen-containing functional groups of microporous activated carbon on its electrochemical performance in supercapacitors, Adv. Funct. Mater. 19 (2009) 438–447. S.J. Gregg, K.S.W. Sing, H. Salzberg, Adsorption surface area and porosity, J. Electrochem. Soc. 114 (1967) 279C. Z. Wang, Y. Liu, K. Li, D. Liu, T. Yang, J. Wang, J. Lu, The influence and mechanism of different acid treatment to activated carbon used as air-breathing cathode catalyst of microbial fuel cell, Electrochim. Acta 246 (2017) 830–840. C.-C. Hu, J.-H. Su, T.-C. Wen, Modification of multi-walled carbon nanotubes for electric double-layer capacitors: tube opening and surface functionalization, J. Phys. Chem. Solids 68 (2007) 2353–2362. Z. Zhao, Z. Yang, Y. Hu, J. Li, X. Fan, Multiple functionalization of multi-walled carbon nanotubes with carboxyl and amino groups, Appl. Surf. Sci. 276 (2013) 476–481. P. Biswas, R. Bandyopadhyaya, Effect of acid treatment on activated carbon: preferential attachment and stronger binding of metal nanoparticles on external carbon surface, with higher metal ion release, for superior water disinfection, Ind. Eng. Chem. Res. 58 (2019) 7467–7477. Y. Jia, B. Xiao, K. Thomas, Adsorption of metal ions on nitrogen surface functional groups in activated carbons, Langmuir 18 (2002) 470–478. A.S. Yasin, J. Jeong, I.M.A. Mohamed, C.H. Park, C.S. Kim, Fabrication of N-doped &SnO2-incorporated activated carbon to enhance desalination and bio-decontamination performance for capacitive deionization, J. Alloys Compd. 729 (2017) 764–775. N.-L. Liu, S. Dutta, R.R. Salunkhe, T. Ahamad, S.M. Alshehri, Y. Yamauchi, C.H. Hou, K.C.W. Wu, ZIF-8 derived, nitrogen-doped porous electrodes of carbon polyhedron particles for high-performance electrosorption of salt ions, Sci. Rep. 6 (2016) 28847. L. Zhang, Y. Liu, T. Lu, L. Pan, Cocoon derived nitrogen enriched activated carbon fiber networks for capacitive deionization, J. Electroanal. Chem. 804 (2017) 179–184. S. Porada, F. Schipper, M. Aslan, M. Antonietti, V. Presser, T.P. Fellinger, Capacitive deionization using biomass-based microporous salt-templated heteroatom-doped carbons, ChemSusChem 8 (2015) 1867–1874.

Desalination 481 (2020) 114362

C.-C. Hsu, et al.

[56] T.-C. Chou, C.-H. Huang, R.-A. Doong, C.-C. Hu, Architectural design of hierarchically ordered porous carbons for high-rate electrochemical capacitors, J. Mater. Chem. A 1 (2013) 2886–2895. [57] H.-H. Shen, C.-C. Hu, Determination of the upper and lower potential limits of the activated carbon/propylene carbonate system for electrical double-layer capacitors, J. Electroanal. Chem. 779 (2016) 161–168.

[54] X. Gao, A. Omosebi, J. Landon, K. Liu, Surface charge enhanced carbon electrodes for stable and efficient capacitive deionization using inverted adsorption–desorption behavior, Energy Environ. Sci. 8 (2015) 897–909. [55] C.-C. Hu, C.-F. Hsieh, Y.-J. Chen, C.-F. Liu, How to achieve the optimal performance of capacitive deionization and inverted-capacitive deionization, Desalination 442 (2018) 89–98.

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