- Email: [email protected]
Nafion-AC-based asymmetric capacitive deionization
Accepted Manuscript Title: Nafion-AC-based asymmetric capacitive deionization Author: Wenshu Cai Junbin Yan Taimoor Hussin Jianyun Liu PII: DOI: Reference:
S0013-4686(16)32615-9 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.069 EA 28541
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
6-10-2016 6-12-2016 10-12-2016
Please cite this article as: Wenshu Cai, Junbin Yan, Taimoor Hussin, Jianyun Liu, Nafion-AC-based asymmetric capacitive deionization, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nafion-AC-based asymmetric capacitive deionization Wenshu Cai, Junbin Yan, Taimoor Hussin, Jianyun Liu * College of Environmental Science and Engineering, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, Donghua University, Shanghai 201620, China * E-mail: [email protected]
1
Abstract: Activated carbon (AC) has been widely used to prepare electrode sheet for capacitive deionization (CDI) owing to its high surface area for ionic adsorption. However, further modification of the AC electrode is still necessary to improve the CDI performance by enhancing the ionic transportation inside the electrode. Here, an asymmetry CDI cell was prepared using Nafion-AC composite with low PTFE content (3.5%) as the negative electrode and AC as the positive electrode. Nafion functions not only as a binder to improve the adhesion of electrode sheet, but also as a mediator to provide an ion conductive pathway for counterions. The morphology and elemental analysis of Nafion-AC were investigated via scanning electron microscopy (SEM) and elemental mapping. The electrochemical capacitance property of the Nafion-AC electrode was studied by cyclic voltammetry (CV). The desalination capacity of the asymmetric CDI cell arrived at 10.8 mg/g, compared with the symmetric AC-based cell (6.9 mg/g). Contribution of Nafion and the corresponding mechanism were analyzed by differential capacitance tests and inverse charging. It demonstrates that on the Nafion-AC composite containing low PTFE content, the co-ion repulsion was reduced and the electrode adhesion was improved. The electrode performance has been conveniently evaluated by monitoring the variation of the charging efficiency and desalination amount with time under an appropriate charge input, which is much helpful to guide the operation process in practical CDI applications. 2
Keywords: Asymmetric capacitive deionization; Charge efficiency; Nafion; Co-ion repulsion; controlling charge input.
3
Introduction Capacitive deionization (CDI), has been considered as an energy-efficient and eco-friendly desalination technology due to its merits such as low energy consumption, non-secondary pollution and easy regeneration in comparison to conventional technologies, which are operated either under high pressure (such as reverse osmosis [1, 2]) or under high voltage ( such as electrodeionization [3, 4]). A typical CDI is usually operated by controlling the charge-discharge process of the cell under an appropriate voltage (usually less than 1.2 Volts), and meanwhile the adsorption /desorption process of ions can be monitored. During charging process, ions are attracted towards the opposite charged electrode and form an electric double layer (EDL) by electrostatic attraction. After that, ions are released from the electrodes to the bulk solution by short circuit or reversed charging [5-8]. Therefore, the electrolysis of water does not occur in CDI process, but it is inevitable in electrodeionization which often causes secondary pollution due to the production of acid and alkali [4]. The porous carbon is commonly used as CDI electrode material due to its high specific surface area and abundant pores. Currently, various porous carbon materials have been investigated in CDI process such as activated carbon powder (AC) [9, 10], carbon nanofiber (CNF) [11, 12], ordered mesoporous carbon (OMC) [13], carbon xerogel (CX) [14], carbon nanotubes (CNTs) [15-17] and graphene [18, 19]. In all of
4
these, activated carbon stands out as one of the most common commercially available and cost-efficient carbon materials for scaling-up application [20, 21]. When the CDI cell is charged, the counter-ions are preferably adsorbed in the electrode, but the co-ions are repelled into the solution simultaneously, which is called co-ions repulsion effect. It results in reduced desalination performance [9, 22-24]. Currently, the major strategy to mitigate the co-ion repulsion effect is to enhance the charge-selectivity of the CDI cell by implementing ion exchange membrane [9, 15, 25], introducing the surface chemical charges on the electrodes [14, 26-29], or increasing discharge voltage of the cell [24]. However, the macroscopic ion transport in between particles or pore-particles interphase is tardy due to the depletion of active ions upon applied voltage [30], which may result in the decrease of the ionic conductivity. Therefore, efforts are still required to improve the ionic diffusion and transportation in the macroporous region. In deionization process, the charged particles have been doped in the matrix to facilitate the ionic transport due to the variation of the pore volume inside the matrix and to enhance the charge selectivity of the matrix [31, 32] Nafion, a perfluorinated sulfonic acid copolymer, possesses an excellent charge-selectivity property due to rich sulfonic groups [33, 34]. In addition, it has been widely used as the binding agent to enhance the stability of the electrode [35, 36]. Therefore, Nafion can be used not only as a binder partially replacing the hydrophobic polytetrafluoroethylene (PTFE) to improve the wettability of electrodes, but also as a mediator providing an
5
ion-conducting pathway in the macropores to facilitate the ionic transportation across the electrode. And thus, with Nafion particles filling up the macropores inside the AC electrode, the ionic tranportation resistance can be decreased [30]. The schematic inner structure of the AC sheet in the presence of Nafion has been drawn in Fig. 1. In addition to the electrode material, the operating conditions of CDI cell affect the desalination performance to a large extent [20, 37-39]. It is necessary to understand the relation between operating parameters and desalination performance. Currently most of the CDI cells work under constant current by ending up with a particular cell voltage [22, 40, 41]. However, the initial cell potential (as called IR drop) is restricted by the cell resistance and applied current density [22]. Higher current density results in bigger IR drop. Therefore, it is unreasonable to evaluate the desalination performance by just controlling the terminal voltage. In this study, the Nafion-AC composite was prepared by simply mixing Nafion solution with AC particles with low PTFE binder content. The Nafion-AC asymmetric CDI cell was assembled with Nafion-AC as negative electrode and AC as positive electrode. Under constant charge input conditions, the desalination performance and effect of applied current density was investigated. This research is much beneficial to practical engineering applications.
2. Experimentals 2.1 Materials Activated carbon (AC) was purchased from Yihuan Carbon Co., Ltd. Nafion
6
solution (5 wt％) and Polytetrafluroethylene (PTFE, 60 wt％) were obtained from Du Pont Co., Ltd. All other chemicals were bought from Sinopharm Chemical Reagent Co., Ltd. All the reagents were used without further purification. All the aqueous solutions were prepared using ultrapure water (18.2 MΩ cm, Nanopure, Barnstead, USA).
2.2 Fabrication of Nafion-AC composite electrode To fabricate the carbon electrode, carbon slurry was prepared by mixing AC, PTFE and Nafion solution in a certain mass ratio. Subsequently, the mixture was stirred into uniform carbon slurry and dried in an oven at 95℃ for 12 h. And then it was rolled into a thin sheet (~0.3 mm). Finally the obtained sheet was cut into a 60 mm×30 mm square as electrode for CDI cell assembly. The Nafion-AC composites with the Nafion:AC mass ratio of 2.5:100, 5:100, 7.5:100 and 10:100 were denoted as Nafion(2.5)-AC, Nafion(5)-AC, Nafion(7.5)-AC and Nafion(10)-AC, respectively. PTFE content in these composites is 3.5 wt% unless mentioned elsewhere. As a control, AC electrode was fabricated with the above similar method but without Nafion.
2.3 Characterization The surface morphology and elemental composition of the electrodes were examined by scanning electron microscopy with energy-dispersed analysis of X-ray (SEM /EDX, Hitachi, S-4700, Japan). The tensile strength of electrode sheets was studied by H5KS universal testing machine (Tinius Olsen, US). The wettability was determined by water contact angle test using JGW-360B (Xihuayi Technology Co. 7
Ltd., Beijing, China). Nitrogen adsorption-desorption isotherms were conducted at 77 K with an ASAP 2060 machine (Micrometitics, Norcross, GA). The electrochemical performance of the electrode was investigated with a two-electrode cell in 58.5 g dm-3 sodium chloride (NaCl) solution by cyclic voltammetry (CV) on the CHI760D workstation (Shanghai Chenhua instrument Co., Ltd). The specific capacitance of the electrode was obtained from CV curves in two-electrode cell system based on the following equation (1) [42,43]: Cs =
4 ∫ 𝐼𝑑𝑉
(1)
𝑣∆𝑉𝑚
Where the Cs (F/g) is the specific capacitance; I (A) is the current; 𝑣 (V/s) is the scan rate; ∆𝑉 (V) is the applied voltage window and m (g) is the total mass of two electrodes. The electrochemical impedance spectroscopy (EIS) (in 58.5 g dm-3 NaCl) and differential capacitance measurements (in 500 mg dm-3 NaCl) were carried out using a μAUTOLAB-III potentiostat instrument (Metrohm, Switzerland) controlled by FRA software with three-electrode cell system. The Nafion-AC composite electrode with a size of 10mm×10mm was used as the working electrode, an Ag/AgCl (3M KCl) electrode was used as the reference electrode, and a coiled platinum wire as the counter electrode. The NaCl solution used for electrochemical characterization was purged with high purity nitrogen for at least 30 min to remove the dissolved oxygen before use.
2.4 Capacitive deionization experiments An asymmetric CDI cell was prepared for desalination experiments. Unless 8
mentioned otherwise, Nafion-AC electrode was used as negative and AC electrode as positive electrode in the experiment, and the cell was denoted as -Nafion-AC||AC, unless. +Nafion-AC||AC represents the reverse CDI cell with Nafion-AC as positive and AC as negative electrode. In addition, a symmetric CDI cell with two pieces of AC electrodes was used as a control. The CDI cell was operated in a closed-loop mode in both adsorption and desorption, and an influent solution containing 500 mg dm-3 NaCl was fed continuously to it at a flow rate of 10 mL/min using a peristaltic pump. The cells were equilibrated with NaCl solution overnight before measurements. The capacitive deionization test was carried out in 50 mL NaCl solution using a computer controlled battery test system (LANHE, CT2001A, WuHan, China). The electrosorption test was conducted by a constant current for a certain time. Then the cell was discharged to 0 V by short-circuit self-discharging mode. The applied current and corresponding voltage were recorded using a computer. The conductivity of the saline water was monitored continuously by a conductivity meter (Mettler Toledo, S230), and the average desalination amount was calculated from 3 adsorption/desorption cycles. The relationship between conductivity and NaCl concentration was obtained according to a calibration curve made prior to the experiment. The desalination amount and the corresponding desalination rate were defined as follows [23, 42]: Г=
(𝑐0 −𝑐𝑒 )×𝑉
(2)
𝑚 Г
𝑣𝐷 = 𝑡
(3) 9
Where Г (mg/g) is the desalination amount; 𝑐0 (mg dm-3) and 𝑐𝑒 (mg dm-3) are the initial and final NaCl concentration, respectively; V (L) is the solution volume; m (g) is the total mass of the two electrodes; vD (mg/g/min) is the desalination rate and t (min) is the charge time. Charge efficiency, a vital parameter to evaluate CDI performance, is defined as the ratio of the equivalent charge of adsorbed salt to the total charge input to the cell, as described according to the following equation: ᴧ=
(𝑐0 −𝑐𝑒 )×𝑉×𝐹
(4)
𝑄×𝑀
Where ᴧ is the charge efficiency, F is the Faraday constant (96,485 C/mol), M (58.5 g /mol) is the molar mass of sodium chloride and Q (charge, C) is charge input.
3 Results and discussion 3.1 Optimization of electrode preparation The CDI electrode must withstand the impact force from water circulation. Therefore, the AC electrode with good mechanical property in CDI cell is very important to lower the electrode resistance and to retain the desalination performance. In a common electrode preparation recipe, 5% ~10% PTFE binder [44, 45] has been used to achieve a good mechanical strength of the electrode with high mass loading, but high PTFE amount will lead to high electrode resistance due to its hydrophobic property. It has been reported that Nafion is a good binding agent for coating [35, 36]. Therefore, Nafion was added into the electrode in order to reduce the amount of PTFE and enhance the ionic transportation owing to its negative charge property inside the electrode. Here, the Nafion-AC electrode with 7.5% Nafion addition and AC electrode 10
without Nafion (both electrodes contain 3.5% PTFE) were prepared in order to investigate the effect of Nafion on mechanical property of the electrode. The mechanical strength of both electrodes with same sample size was measured by universal testing machine. Nafion-AC electrode shows a tensile strength of 0.43 MPa, which is 2.3 times higher than that of AC electrode. It is much comparable to the calendaring AC electrode sheet reported previously [46]. It was also found that the AC electrode was much prone to break with some particles falling off. Therefore, Nafion in the composite enhances the adhesion between AC particles and improves the strength and stiffness of the electrode sheet, which renders the electrode flexible and easy handling even with low PTFE content. As mentioned above, since AC electrode with 3.5% PTFE content has poor mechanical property, it will not be used for further study any more. Three carbon sheets including AC with 7% PTFE (AC-7%PTFE), Nafion-AC with 7.5% Nafion and 7% PTFE (Nafion(7.5)-AC-7%PTFE) and Nafion-AC with 7.5% Nafion and 3.5% PTFE (Nafion(7.5)-AC-3.5%PTFE) were fabricated and their characteristics were studied by EIS and water contact angle, respectively. Fig.2 exhibits the Nyquist plots of the AC-7%PTFE, Nafion(7.5)-AC-7%PTFE and Nafion(7.5)-AC-3.5%PTFE electrodes in 58.5 g dm-3 NaCl in the frequency range from 1 mHz to 100 kHz. The semicircle in high frequency region reveals the interfacial charge transfer resistance (Rct).
The
impedance
line
in
the
low-frequency
region
represents
a
diffusion-controlled process, and the line to the imaginary axis of the impedance spectra, in all cases, approaching to a vertical indicates a desirable capacitive behavior 11
[47]. It is noted that in the presence of Nafion, the Rct of the electrode decreases obviously, and the straight lines at low frequency are inclined steeply to the imaginary part, compared to AC-7%PTFE electrode. In addition, Nafion(7.5)-AC-3.5%PTFE electrode, due to low PTFE content, possesses a lower Rct value, and the ionic diffusion and migration on/inside the electrode sheet become faster, implying a better electrochemical double-layer capacitance behavior than Nafion(7.5)-AC-7%PTFE electrode. However, the EIS straight line of AC-7%PTFE electrode at lower frequency region is far away from imaginary axis, indicating a slow ion diffusion velocity. This result confirms the effect of Nafion in the electrode (as shown in Fig.1) The water contact angle tests of the AC-7%PTFE, Nafion(7.5)-AC-7%PTFE and Nafion(7.5)-AC-3.5%PTFE electrode sheets were done to investigate the wettability of the composite electrode (figure not shown). It was found that AC-7%PTFE is hydrophobic with a contact angle of 130. With Nafion addition, the Nafion(7.5)-AC-7%PTFE electrode has the contact angle of 115. By reducing the PTFE amount to 3.5%, the hydrophilicity of the resultant Nafion(7.5)-AC-3.5%PTFE electrode has obvious improvement with the corresponding contact angle of 85. Therefore, the partial replacement of hydrophobic PTFE with Nafion can effectively improve the wettability of electrodes due to the sulfonic groups from Nafion. The enhancement of wettability facilitates the migration of water and ions on/inside the electrode. The result is well consistent with that from EIS. Moreover, Nafion, as a cation exchanger, improves both the ionic conductivity and the capacitance performance. In the following experiments, 3.5 wt% PTFE was added for all 12
Nafion-AC electrodes.
3.2 Characterization of Nafion-AC composite electrode The distribution of Nafion in the Nafion-AC composite was studied by SEM. Fig. 3A-D shows the SEM images of the Nafion(2.5)-AC, Nafion(5)-AC, Nafion(7.5)-AC, Nafion(10)-AC surfaces, respectively. At low Nafion content, only a small quantity of Nafion particles are distributed on the surface of AC particles, as marked in a white circle (Fig. 3A). With the increase of Nafion content in the composite electrode, more and more particles were deposited on carbon surface with a uniform distribution. But high Nafion amount results in obvious agglomeration, which may lead to the blockage of the pores on the carbon particle surface and poor electronic conductivity of the composite. The EDX elemental maps in the SEM scan range of Nafion(7.5)-AC electrode were depicted in Fig. 4, including overlay map of all elements (Fig. 4A) and individual maps of S (Fig. 4B) and F (Fig. 4C) elements, respectively. They are all distributed uniformly over the composite. The presence of S confirms the distribution of Nafion around AC particles, which facilitates the diffusion of hydrated ions and favors the electrosorption [41, 48]. It is reported that the presence of the macropores in the pore-particles interphase will result in the transient decrease of ion concentration and thus the reduction of ionic conductivity inside porous electrode due to the depletion of active ions during adsorption [30]. Nafion coating around particles possibly reduces the macropores by filling Nafion in between pore-particles inside the electrode. Here, the N2 adsorption-desorption
isotherm
was 13
conducted
to
investigate
the
Bruanuer-Emmeet-Teller (BET) specific surface area and pore size distribution of Nafion-AC composites. The results with different Nafion contents were collected in Table 1. With the addition of Nafion, Nafion-AC composites show larger BET specific surface area and more micropore volume than AC. It is possible that Nafion molecules/particles are well filled in the macropores, contributing some micropores. The change of the pore distribution in the presence of Nafion will, to some extent, be beneficial to the steady ionic transportation during the CDI process [30], as described in Fig. 1. However, too high Nafion amount results in the decrease of the specific surface area, due to the blockage of the pores by Nafion coating as confirmed by SEM image.
3.3 CV analysis CV technique is an effective way to evaluate the specific capacitance of the electrode and electrosorption ability of the electrodes. The CV measurement was carried out in a two-electrode cell. Fig. 5A shows the CV curves of AC, Nafion(2.5)-AC, Nafion(5)-AC, Nafion(7.5)-AC, Nafion(10)-AC electrodes in the presence of 58.5 g dm-3 NaCl solution, respectively. These curves exhibit ideal rectangular shape without any redox peaks in the wide voltage window, indicating a typical electrochemical double layer behavior. The specific capacitances were calculated from the CV curves based on Eq. (1), and the corresponding values were collected in Table 2. Increasing Nafion content from 0 ~ 7.5% leads to the increase of specific capacitance Cs from 119 to 148 F/g. But too high Nafion content results in the decrease of Cs. It is reasonable that a moderate amount of Nafion in the electrode 14
can enhance the ionic conductivity, but overload of Nafion will reduce the electronic conductivity, resulting in the decrease of Cs. These results are in line with that of SEM characterization. Fig. 5B displays the capacitance retention ratio of various electrodes calculated from CV curves at different scan rates to evaluate the ion transport behavior with different Nafion ratio. The reduced inner-pore ion transport rate leads to the decline of capacitance at high scan rate [49]. Nafion(10)-AC shows a more steep descent, while Nafion(7.5)-AC exhibits a large capacitance retention ratio than others at high scan rates, indicating better ion transfer behavior. The capacitance performance of different electrodes is ranked as: Nafion-(7.5)-AC > Nafion-(5)-AC > Nafion-(2.5)-AC > AC > Nafion-(10)-AC. Therefore, Nafion(7.5)-AC is desirable for CDI applications.
3.4 Desalination performance CDI test was conducted in NaCl solution with an initial conductivity of approximately 1050 μS/cm. The cell was cycled in NaCl solution overnight to reach the physical adsorption balance and then the fresh NaCl solution was used during CDI charge-discharge process. The asymmetric cells were assembled with Nafion-AC electrode as a negative electrode and AC electrode as positive electrode. The ionic adsorption property during charging stage
was
investigated
using the
AC||AC,
-Nafion(2.5)-AC||AC,
-Nafion(5)-AC||AC, -Nafion(7.5)-AC||AC and -Nafion(10)-AC||AC cells. Fig. 6 shows the adsorption equilibrium curves on different electrodes by charging at a current density of 30 mA/g. Obviously, in the presence of Nafion, the electrode easily 15
arrives at the adsorption equilibration, compared with AC electrode. With the increase of the Nafion amount, the adsorption rate and the maximum adsorption amount increases gradually. The Nafion(7.5)-AC||AC has the fastest desalination rate, and its adsorption amount arrives at the maximum in 25 min, and then it levels off with further charging, indicating the equilibration of co-ion desorption and counter-ion adsorption. The desalination capacity of 10.8 mg/g has been achieved in the Nafion(7.5)-AC||AC cell, compared with the AC||AC cell of 6.9 mg/g. The desalination capacity, desalination rate and the corresponding charge efficiency for different electrodes were summarized in Table 2. It confirms that Nafion, as a binder and ionic mediator in between AC particles, facilitates the ionic transportation, and thus effectively enhances the desalination capacity. However, too high amount of Nafion in the electrode leads to a reduction of desalination amount due to blockage of pores and increase of the resistance, as demonstrated previously by CV and N2 adsorption-desorption isotherm tests.
3.5 Charge selectivity of Nafion-AC electrode in the asymmetric CDI cell The charge selectivity of the electrodes, using two identical Nafion(7.5)-AC||AC cells, was investigated via applying different connection to power supply, one with Nafion(7.5)-AC
as
negative
electrode
(-Nafion-AC||AC),
the
other
with
Nafion(7.5)-AC as positive electrode (+Nafion-AC||AC). In a constant-current (60 mA/g) mode with the charge input of 4 mAh, the voltage/ current curves (A, C) and the corresponding solution conductivity (B, D) in the -Nafion-AC||AC and 16
+Nafion-AC||AC were illustrated in Fig. 7, respectively. For -Nafion-AC||AC cell, the conductivity of the solution decreases during the charge process and goes up to the initial level during the discharge process. It indicates that anions are adsorbed onto the positive electrode whereas cations are adsorbed onto the negative electrode. They were desorbed into the solution when discharging. It is noteworthy, however, that when applying a reverse current during the charge process, the resultant +Nafion-AC||AC cell exhibits a reverse conductivity variation. This similar inverted CDI behavior was found by Landon [50, 51]. It is known that the removal amount of ions during the charging process is the difference between the counter-ions adsorbed on the electrodes and the co-ions desorbed from the electrodes [52]. Usually, the symmetric AC cell shows the same desalination trends when reversing the polarity of the cell. However, Nafion-AC electrode surface possesses lots of sulfonic groups, which make Cl- difficult to be adsorbed due to the charge repulsion when used as positive electrode. Impact of the surface charge renders more co-ions desorbed out of the electrodes than counter-ions adsorbed on the electrode. It finally causes the reverted conductivity variation. The result indicates Nafion-AC has a good cation-selectivity. Therefore, the desalination performance can be improved effectively only when Nafion-AC is used as the negative electrode. This strong ionic repulsion from the electrode will certainly lead to the enhancement of the charge efficiency owing to the attenuation of the co-ion effect. This big difference between Nafion-AC positive and negative electrode can be further explained by measuring the potential of zero charge (Epzc) of the electrodes 17
through analyzing the potential-differential capacitance curves. The differential capacitance (CD), defined as the dependence of the surface charge on the electrode potential is very suitable for analyzing the property of electric double layer on the porous carbon electrode. CD varies with electrode potential, being related to the electronic charges of carbon surface, the mobile ionic charges and immobile chemical charges in the micropore, according to the dynamic EDL model [26]. The potential differential capacitance curves were obtained by scanning the different electrodes at a frequency of 1 Hz in 500 mg dm-3 NaCl solution. At the minimum of the differential capacitance, the surface is non-charged, with most loose diffusion layer and low capacitance. Therefore, the minimum of the camel curves corresponds to the Epzc value. The Epzc of different electrodes were collected in Table 2. Unmodified AC electrode possesses the Epzc of about 69 mV, indicating the neutral charge state of the surface. While, the Epzc values of Nafion-AC composite electrodes increase gradually from 236 mV (Nafion(2.5)-AC) to 715 mV (Nafion(10)-AC). It indicates that more negatively charged groups exist on the electrode surface with the Nafion addition [23, 53, 54], which undoubtedly enhance the cation-selectivity and reduce the co-ion repulsion effect.
3.6 The influence of charge input In the previous study, the constant-current charging mode has been applied by controlling the cutoff voltage of around 1.2 V [41, 55]. Thus the charge input is much related to current density due to IR drop. Moreover, it is difficult to predict optimal desalination amount and charge efficiency simultaneously. These can be illustrated via 18
comparing the desalination process between controlling the terminal voltage of 1.2 V (Fig. 8A) and controlling the constant charge input (Fig. 8B). At the terminal voltage of 1.2 V, there is a significant difference of the deionization efficacy at various current density, rising from higher IR drop at higher current density and pretty low charge input. The high cell resistance (including the solution resistance and contact resistance) results in the reduction of the factual charging potential window. However, the cutoff voltage varies between 0.88 V and 2.07 V (IR drop was subtracted) when the same charge input was supplied with the charging current varying from 40 to 200 mA/g, respectively. And there is a reasonable conductivity variation. The desired current density can be conveniently adjusted based on EDL voltage windows in CVs and the charge efficiency requirement. The dynamic curves of charge efficiency and desalination amount with charge input at the Nafion(7.5)-AC were depicted in Fig. 9. Clearly, both the desalination amount and charge efficiency grow with charge input at the initial charging period. Then the charge efficiency descends and the desalination value levels off when the charge input increases continuously. Therefore, the charge efficiency is pretty low near the equilibrium desalination stage as shown in Table 2. It suggests that excessive charge input causes high energy consumption due to water splitting or electrolysis. However the less charge input results in very low desalination amount, and thus low availability of electrode material. Interestingly, the cross-point between desalination amount and charge efficiency appears at the same charge quantity at one electrode even though the different constant currents were applied. Therefore, it is very 19
convenient to define the optimal charge input of the electrode through this dynamic curve, where the acceptable desalination amount and charge efficiency can be found at the same time.
3.7 The influence of the current density Fig.10A shows the conductivity variation of the CDI cell at the charge input value of 3.5mAh with different current densities. And the desalination amount and desalination rate were calculated according to Eq.(2) and Eq.(3), respectively, and shown in Fig. 10B. Under the same charge input condition, the desalination amount is strongly related to current density. As the current density increases, the desalination amount increased sharply, but descends quickly at high current density. The maximum desalination amount was obtained over a narrow current density range of 40 ~ 60 mA/g, and the charge efficiency has the same variation trend with the maximum of 67% (data unshown). Charge efficiency and desalination amount decrease gradually at higher current density. It is because severe polarization could happen at high current density. On the other hand, the desalination rate becomes very low with decreasing the current density due to slow ionic transferring velocity. Therefore, the appropriate current density should be adjusted carefully to obtain the optimal desalination amount. Meanwhile, an acceptable desalination rate should be considered for practical CDI application.
3.8 Stability of the CDI cell The stability and regeneration of the electrode materials were investigated in 500 mg dm-3 NaCl solution via a constant-current mode (50 mA /g) with the charge input 20
of 4 mAh. The voltage and conductivity variation were continuously monitored during charge-discharge process. In the more than 100 cycles, there is no serious going-up of the voltage, and the conductivity fluctuation is only 2.3% at the early several cycles. This steady voltage and conductivity variation indicates an excellent stability and regeneration ability of the electrode materials for desalination.
Conclusions In the present study, the Nafion-AC electrode has been studied with enhanced mechanical property, and improved ionic conductivity and capacitance for CDI. The strong cation-selectivity of the Nafion-AC electrode has been demonstrated by the opposite desalination characteristic of -Nafion-AC/AC and +Nafion-AC/AC. The dynamic curves of desalination amount and charge efficiency are very convenient to discover the optimal charge input of one electrode giving consideration to both desalination amount and charge efficiency at the same time, irrespective of the current density. Controlling charge input is more reasonable to investigate the parameter effect on desalination performance than controlling the terminal voltage in the charging process. Care should be taken to adjust the current density in order to optimize the operation parameters in CDI device. Under the constant charge input mode, the present Nafion-AC electrode is stable in CDI multi-cycling. The present study is of great use for the practice operation of the CDI device and engineering applications.
Acknowledgements The authors gratefully acknowledge the financial support of this research by the 21
National Natural Science Foundation of China (Grants 21476047 and 21105009), and the State Key Laboratory of Electro analytical Chemistry (SKLEAC201205).
22
References: [1] G.D. Kang, Y.M. Cao, Development of antifouling reverse osmosis membranes for water treatment: A review, Water Res, 46 (2012) 584-600. [2] A. Perez-Gonzalez, A.M. Urtiaga, R. Ibanez, I. Ortiz, State of the art and review on the treatment technologies of water reverse osmosis concentrates, Water Res, 46 (2012) 267-283. [3] L. Alvarado, A. Chen, Electrodeionization: principles, strategies and applications, Electrochimica Acta, 132 (2014) 583-597. [4] Ö. Arar, Ü. Yüksel, N. Kabay, M. Yüksel, Various applications of electrodeionization (EDI) method for water treatment—A short review, Desalination, 342 (2014) 16-22. [5] 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. [6] 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, Progress in Materials Science, 58 (2013) 1388-1442. [7] F.A. AlMarzooqi, A.A. Al Ghaferi, I. Saadat, N. Hilal, Application of Capacitive Deionisation in water desalination: A review, Desalination, 342 (2014) 3-15. [8] Y. Oren, Capacitive deionization (CDI) for desalination and water treatment — past, present and future (a review), Desalination, 228 (2008) 10-29. [9] Y. Bian, X. Yang, P. Liang, Y. Jiang, C. Zhang, X. Huang, Enhanced desalination performance of membrane capacitive deionization cells by packing the flow chamber with granular activated carbon, Water Res, 85 (2015) 371-376. [10] J.-H. Choi, Fabrication of a carbon electrode using activated carbon powder and application to the capacitive deionization process, Separation and Purification Technology, 70 (2010) 362-366. [11] Q. Dong, G. Wang, T. Wu, S. Peng, J. Qiu, Enhancing capacitive deionization performance of electrospun activated carbon nanofibers by coupling with carbon nanotubes, Journal of colloid and interface science, 446 (2015) 373-378. [12] G. Wang, C. Pan, L. Wang, Q. Dong, C. Yu, Z. Zhao, J. Qiu, Activated carbon nanofiber webs made by electrospinning for capacitive deionization, Electrochimica Acta, 69 (2012) 65-70. [13] S. Nadakatti, M. Tendulkar, M. Kadam, Use of mesoporous conductive carbon black to enhance performance of activated carbon electrodes in capacitive deionization technology, Desalination, 268 (2011) 182-188. [14] X. Gao, J. Landon, J.K. Neathery, K. Liu, Modification of Carbon Xerogel Electrodes for More Efficient Asymmetric Capacitive Deionization, Journal of the Electrochemical Society, 160 (2013) E106-E112. [15] H. Li, L. Zou, Ion-exchange membrane capacitive deionization: A new strategy for brackish water desalination, Desalination, 275 (2011) 62-66. [16] L. Yang, Z. Shi, W. Yang, Enhanced capacitive deionization of lead ions using air-plasma treated carbon nanotube electrode, Surface and Coatings Technology, 251 (2014) 122-127. [17] C. Yan, L. Zou, R. Short, Single-walled carbon nanotubes and polyaniline composites for capacitive deionization, Desalination, 290 (2012) 125-129. 23
[18] A.G. El-Deen, J.-H. Choi, C.S. Kim, K.A. Khalil, A.A. Almajid, N.A.M. Barakat, TiO2 nanorod-intercalated reduced graphene oxide as high performance electrode material for membrane capacitive deionization, Desalination, 361 (2015) 53-64. [19] L. Liu, L. Liao, Q. Meng, B. Cao, High performance graphene composite microsphere electrodes for capacitive deionisation, Carbon, 90 (2015) 75-84. [20] D. Liu, K. Huang, L. Xie, H.L. Tang, Relation between operating parameters and desalination performance of capacitive deionization with activated carbon electrodes, Environ. Sci.: Water Res. Technol., 1 (2015) 516-522. [21] C. Yan, L. Zou, R. Short, Polyaniline-modified activated carbon electrodes for capacitive deionisation, Desalination, 333 (2014) 101-106. [22] J.-H. Choi, Determination of the electrode potential causing Faradaic reactions in membrane capacitive deionization, Desalination, 347 (2014) 224-229. [23] T. Wu, G. Wang, Q. Dong, B. Qian, Y. Meng, J. Qiu, Asymmetric capacitive deionization utilizing nitric acid treated activated carbon fiber as the cathode, Electrochimica Acta, 176 (2015) 426-433. [24] T. Kim, J.E. Dykstra, S. Porada, A. van der Wal, J. Yoon, P.M. Biesheuvel, Enhanced charge efficiency and reduced energy use in capacitive deionization by increasing the discharge voltage, Journal of colloid and interface science, 446 (2015) 317-326. [25] Y.-J. Kim, J.-H. Choi, Enhanced desalination efficiency in capacitive deionization with an ion-selective membrane, Separation and Purification Technology, 71 (2010) 70-75. [26] P.M. Biesheuvel, H.V.M. Hamelers, M.E. Suss, Theory of Water Desalination by Porous Electrodes with Immobile Chemical Charge, Colloids and Interface Science Communications, 9 (2015) 1-5. [27] X. Gao, S. Porada, A. Omosebi, K.L. Liu, P.M. Biesheuvel, J. Landon, Complementary surface charge for enhanced capacitive deionization, Water Res, 92 (2016) 275-282. [28] X. Gao, A. Omosebi, N. Holubowitch, A. Liu, K. Ruh, J. Landon, K. Liu, Polymer-coated composite anodes for efficient and stable capacitive deionization, Desalination, 399 (2016) 16-20. [29] Mare D. Andelman, Worcester, MA, Polarized electrode for flow-through cpacitive deionization, US patent, 2014/0346046 A1. [30] P.M. Biesheuvel, Y. Fu, M.Z. Bazant, Diffuse charge and Faradaic reactions in porous electrodes, Physical review. E, Statistical, nonlinear, and soft matter physics, 83 (2011) 061507. [31] Y. Dzyazko, L. Rozhdestveskaya, Y. Zmievskii, Y. Volfkovich, V. Sosenkin, N. Nikolskaya, S. Vasilyuk, V. Myronchuk, V. Belyakov, Heterogeneous Membranes Modified with Nanoparticles of Inorganic Ion-Exchangers for Whey Demineralization, Materials Today: Proceedings, 2 (2015) 3864-3873. [32] M.A. Khan, M. Kumar, Z.A. Alothman, Preparation and characterization of organic–inorganic hybrid anion-exchange membranes for electrodialysis, Journal of Industrial and Engineering Chemistry, 21 (2015) 723-730. [33] N. White, M. Misovich, A. Yaroshchuk, M.L. Bruening, Coating of Nafion membranes with polyelectrolyte multilayers to achieve high monovalent/divalent cation electrodialysis selectivities, ACS applied materials & interfaces, 7 (2015) 6620-6628. [34] X.H. Yan, R. Wu, J.B. Xu, Z. Luo, T.S. Zhao, A monolayer graphene – Nafion sandwich membrane for direct methanol fuel cells, Journal of Power Sources, 311 (2016) 188-194. [35] A.S. Razavian, S.M. Ghoreishi, A.S. Esmaeily, M. Behpour, L.M.A. Monzon, J.M.D. Coey, Simultaneous sensing of L-tyrosine and epinephrine using a glassy carbon electrode modified with 24
nafion and CeO2 nanoparticles, Microchimica Acta, 181 (2014) 1947-1955. [36] Y. Wu, S. Zhai, K. Lu, L. Gao, Electrochemical oxidation behavior of 8-azaguanine at graphene-Nafion composite film-modified glassy carbon electrode, Journal of Solid State Electrochemistry, 18 (2014) 1593-1600. [37] Y.-J. Kim, J.-H. Kim, J.-H. Choi, Selective removal of nitrate ions by controlling the applied current in membrane capacitive deionization (MCDI), Journal of Membrane Science, 429 (2013) 52-57. [38] C. Huyskens, J. Helsen, A.B. de Haan, Capacitive deionization for water treatment: Screening of key performance parameters and comparison of performance for different ions, Desalination, 328 (2013) 8-16. [39] S. Porada, M. Bryjak, A. van der Wal, P.M. Biesheuvel, Effect of electrode thickness variation on operation of capacitive deionization, Electrochimica Acta, 75 (2012) 148-156. [40] H. Pan, J. Yang, S. Wang, Z. Xiong, W. Cai, J. Liu, Facile fabrication of porous carbon nanofibers by electrospun PAN/dimethyl sulfone for capacitive deionization, J. Mater. Chem. A, 3 (2015) 13827-13834. [41] J. Liu, M. Lu, J. Yang, J. Cheng, W. Cai, Capacitive desalination of ZnO/activated carbon asymmetric capacitor and mechanism analysis, Electrochimica Acta, 151 (2015) 312-318. [42] Y. Liu, T. Chen, T. Lu, Z. Sun, D.H.C. Chua, L. Pan, Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization, Electrochimica Acta, 158 (2015) 403-409. [43] M.D. Stoller, R.S. Ruoff, Best practice methods for determining an electrode material's performance for ultracapacitors, Energy Environ. Sci., 3 (2010) 1294-1301. [44] B. Wei, J.C. Tokash, G. Chen, M.A. Hickner, B.E. Logan, Development and evaluation of carbon and binder loading in low-cost activated carbon cathodes for air-cathode microbial fuel cells, RSC Adv., 2 (2012) 12751-12758. [45] M. Aslan, D. Weingarth, N. Jäckel, J.S. Atchison, I. Grobelsek, V. Presser, Polyvinylpyrrolidone as binder for castable supercapacitor electrodes with high electrochemical performance in organic electrolytes, Journal of Power Sources, 266 (2014) 374-383. [46] J. Liu, W. Cai, H. Yang, L. Ping, R. Xiong, Method for Preparing Composition, Sheet Composing the Composition and Electrode Comprsing the Sheet, US Patent, 201101752A1. [47] A.K. Mishra, S. Ramaprabhu, Functionalized Graphene-Based Nanocomposites for Supercapacitor Application, The Journal of Physical Chemistry C, 115 (2011) 14006-14013. [48] Y. Liu, W. Ma, Z. Cheng, J. Xu, R. Wang, X. Gang, Preparing CNTs/Ca-Selective zeolite composite electrode to remove calcium ions by capacitive deionization, Desalination, 326 (2013) 109-114. [49] D. Wang, F. Li, M. Liu, G. Lu, H. Cheng. Mesopore-Aspect-Ratio Dependence of Ion Transport in Rodtype Ordered Mesoporous Carbon, J. Phys. Chem. C, 112 (2008), 9950–9955. [50] X. Gao, A. Omosebi, J. Landon, K. Liu, Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes, Environmental science & technology, 49 (2015) 10920-10926. [51] 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. [52] E. Avraham, M. Noked, Y. Bouhadana, A. Soffer, D. Aurbach, Limitations of charge efficiency in capacitive deionization processes III: The behavior of surface oxidized activated carbon electrodes, 25
Electrochimica Acta, 56 (2010) 441-447. [53] A. Omosebi, X. Gao, J. Landon, K. Liu, Asymmetric electrode configuration for enhanced membrane capacitive deionization, ACS applied materials & interfaces, 6 (2014) 12640-12649. [54] V. Lockett, M. Horne, R. Sedev, T. Rodopoulos, J. Ralston, Differential capacitance of the double layer at the electrode/ionic liquids interface, Physical chemistry chemical physics : PCCP, 12 (2010) 12499-12512. [55] J. Liu, S. Wang, J. Yang, J. Liao, M. Lu, H. Pan, L. An, ZnCl2 activated electrospun carbon nanofiber for capacitive desalination, Desalination, 344 (2014) 446-453.
26
Figure 1
Fig 1. The schemetric illustration of the innerstructure of the Nafion-AC composite as compared to AC sheet.
27
Figure 2
800 40
-z" ()
700 600
-z" ()
500
20
0 0
20
40
60
80
100
z' ()
400 300 200 100 0 0
100
200
300
400
500
600
700
800
z' ()
Fig.2 EIS of AC-7 % PTFE (square), Nafion(7.5)-AC-7%PTFE (cycle) and Nafion(7.5)-AC-3.5% PTFE (triangle) electrodes, insert is the magnification in the high frequency region.
28
Figure 3
(A)
(B)
(C)
(D)
Fig.3 SEM images of Nafion(2.5)-AC (A), Nafion(5)-AC (B), Nafion(7.5)-AC (C) and Nafion(10)-AC (D). Nafion particles were marked with white circles.
29
Figure 4 (A)
(B)
(C)
Fig.4 EDX elemental overlap maps of various elements on the Nafion(7.5)-AC electrode (A); The individual elemental maps of S (B), and F (C).
30
Figure 5
AC Nafion(2.5)-AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
(A)
80
(B) Capacitance Retention Ratio (%)
Current (mA/g)
160
0
-80
AC Nafion(2.5)-AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
100
80
60
40
20
0
0.0
0.4
0.8
0
1.2
5
10
15
20
Scan rate (mV/s)
Voltage (V)
Fig.5 (A) CVs of the electrodes containing different ratio of Nafion; Scan rate: 1 mV/s; (B) Capacitance retention ratio of the electrodes containing different ratio of Nafion as a function of scan rates.
31
Figure 6
120
C0 - Ce (mg/L)
90
60
AC Nafion(2.5)-AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
30
0 0
10
20
30
40
t (min)
Fig.6 Desalination amount of different electrodes in 500 mg dm-3 NaCl solution during charging process. Current density: 30mA/g.
32
Figure 7
0.8
40
0.4
30
0.0
20
-0.4
10
(B) 1050
-0.8 0
Conductivity (μS/cm)
Voltage (V)
1100 50
Current (mA)
(A)
1.2
1000
950
900
850
-1.2 -10 0
40
80
120
160
800
200
0
50
100
t (min)
50 40
0.4
30
0.0
20
-0.4
10
-0.8
Conductivity (μS/cm)
0.8
1300
Current (mA)
(C)
1.2
Voltage (V)
150
200
t (min)
(D)
1250
1200
1150
0
1100
-10
1050
-1.2 0
50
100
150
0
200
t (min)
50
100
150
t (min)
Fig.7 Cell voltage/ current curves (A, C) and conductivity variation (B, D) on -Nafion-AC||AC (A, B) and +Nafion-AC||AC (C, D) during the continuous charge-discharge process.
33
200
Figure 8
(A)
(B)
1050
1000
Conductivity (μS/cm)
Conductivity (μS/cm)
1050
200 mA/g 950
40 mA/g 900
850
200 mA/g
1000
40 mA/g
950
900
850
0
1
2
3
4
Charge quantity (mAh)
0
1
2
3
Charge quantity (mAh)
Fig.8 Conductivity variation of Nafion-based CDI cell (A) with a terminal voltage of 1.2V and (B) with a terminal charge quantity of 3.5 mAh. The charging current from bottom to top corresponds to 40, 80, 120, 160 and 200 mA/g, respectively.
34
4
Figure 9
Charge efficiency (%)
8 60 6 40 4 40mA/g 80mA/g 120mA/g
20
2
0
Desalination amount (mg/g)
10
80
0 0
1
2
3
4
5
Charge quantity (mAh)
Fig.9 Dynamic variation in desalination amount and charge efficiency of Nafion-AC-based CDI cell during charging with the current density of 40, 80 and 120 mA/g.
35
Figure 10 1100
9
2.0
(A)
(B)
1000
950
900
850
200 mA/g
20 mA/g
800 0
10
20
30
40
50
t (min)
8
1.5
7
1.0
6
0.5
5
0.0
40
80
120
160
200
Current density (mA/g)
Fig.10 (A) Conductivity variation of Nafion-AC-based CDI cell over time at different current densities; (B) Desalination amount and desalination rate calculated from (A) versus the current density.
36
Desalination rate (mg/g/min)
Desalination amount (mg/g)
Conductivity (μS/cm)
1050
Tables Table 1 Specific surface area, pore volume and average pore size of AC and Nafion-AC-based composite
AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
BET (m2/g)
Vtotal (cm3/g)
1668 2247 2276 2202
0.82 1.04 1.06 1.03
37
Vmicro (cm3/g)
Average pore size (nm)
0.61 0.94 0.94 0.91
1.98 1.85 1.87 1.87
Table 2 Specific capacitance, Epzc and desalination performance of Nafion-AC negative electrodes with various Nafion amount
AC Nafion-(2.5)-AC Nafion-(5)-AC Nafion-(7.5)-AC Nafion-(10)-AC
Г (mg/g)
ᴧ (%)
𝑣𝐷 (mg/g/min) Cs (F/g)
E pzc (mV)
6.9 7.3 9.5 10.8 7.8
24 29 40 45 31
0.19 0.24 0.35 0.44 0.27
69 236 352 554 715
38
119 135 138 148 109
S0013-4686(16)32615-9 http://dx.doi.org/doi:10.1016/j.electacta.2016.12.069 EA 28541
To appear in:
Electrochimica Acta
Received date: Revised date: Accepted date:
6-10-2016 6-12-2016 10-12-2016
Please cite this article as: Wenshu Cai, Junbin Yan, Taimoor Hussin, Jianyun Liu, Nafion-AC-based asymmetric capacitive deionization, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.12.069 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nafion-AC-based asymmetric capacitive deionization Wenshu Cai, Junbin Yan, Taimoor Hussin, Jianyun Liu * College of Environmental Science and Engineering, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, Donghua University, Shanghai 201620, China * E-mail: [email protected]
1
Abstract: Activated carbon (AC) has been widely used to prepare electrode sheet for capacitive deionization (CDI) owing to its high surface area for ionic adsorption. However, further modification of the AC electrode is still necessary to improve the CDI performance by enhancing the ionic transportation inside the electrode. Here, an asymmetry CDI cell was prepared using Nafion-AC composite with low PTFE content (3.5%) as the negative electrode and AC as the positive electrode. Nafion functions not only as a binder to improve the adhesion of electrode sheet, but also as a mediator to provide an ion conductive pathway for counterions. The morphology and elemental analysis of Nafion-AC were investigated via scanning electron microscopy (SEM) and elemental mapping. The electrochemical capacitance property of the Nafion-AC electrode was studied by cyclic voltammetry (CV). The desalination capacity of the asymmetric CDI cell arrived at 10.8 mg/g, compared with the symmetric AC-based cell (6.9 mg/g). Contribution of Nafion and the corresponding mechanism were analyzed by differential capacitance tests and inverse charging. It demonstrates that on the Nafion-AC composite containing low PTFE content, the co-ion repulsion was reduced and the electrode adhesion was improved. The electrode performance has been conveniently evaluated by monitoring the variation of the charging efficiency and desalination amount with time under an appropriate charge input, which is much helpful to guide the operation process in practical CDI applications. 2
Keywords: Asymmetric capacitive deionization; Charge efficiency; Nafion; Co-ion repulsion; controlling charge input.
3
Introduction Capacitive deionization (CDI), has been considered as an energy-efficient and eco-friendly desalination technology due to its merits such as low energy consumption, non-secondary pollution and easy regeneration in comparison to conventional technologies, which are operated either under high pressure (such as reverse osmosis [1, 2]) or under high voltage ( such as electrodeionization [3, 4]). A typical CDI is usually operated by controlling the charge-discharge process of the cell under an appropriate voltage (usually less than 1.2 Volts), and meanwhile the adsorption /desorption process of ions can be monitored. During charging process, ions are attracted towards the opposite charged electrode and form an electric double layer (EDL) by electrostatic attraction. After that, ions are released from the electrodes to the bulk solution by short circuit or reversed charging [5-8]. Therefore, the electrolysis of water does not occur in CDI process, but it is inevitable in electrodeionization which often causes secondary pollution due to the production of acid and alkali [4]. The porous carbon is commonly used as CDI electrode material due to its high specific surface area and abundant pores. Currently, various porous carbon materials have been investigated in CDI process such as activated carbon powder (AC) [9, 10], carbon nanofiber (CNF) [11, 12], ordered mesoporous carbon (OMC) [13], carbon xerogel (CX) [14], carbon nanotubes (CNTs) [15-17] and graphene [18, 19]. In all of
4
these, activated carbon stands out as one of the most common commercially available and cost-efficient carbon materials for scaling-up application [20, 21]. When the CDI cell is charged, the counter-ions are preferably adsorbed in the electrode, but the co-ions are repelled into the solution simultaneously, which is called co-ions repulsion effect. It results in reduced desalination performance [9, 22-24]. Currently, the major strategy to mitigate the co-ion repulsion effect is to enhance the charge-selectivity of the CDI cell by implementing ion exchange membrane [9, 15, 25], introducing the surface chemical charges on the electrodes [14, 26-29], or increasing discharge voltage of the cell [24]. However, the macroscopic ion transport in between particles or pore-particles interphase is tardy due to the depletion of active ions upon applied voltage [30], which may result in the decrease of the ionic conductivity. Therefore, efforts are still required to improve the ionic diffusion and transportation in the macroporous region. In deionization process, the charged particles have been doped in the matrix to facilitate the ionic transport due to the variation of the pore volume inside the matrix and to enhance the charge selectivity of the matrix [31, 32] Nafion, a perfluorinated sulfonic acid copolymer, possesses an excellent charge-selectivity property due to rich sulfonic groups [33, 34]. In addition, it has been widely used as the binding agent to enhance the stability of the electrode [35, 36]. Therefore, Nafion can be used not only as a binder partially replacing the hydrophobic polytetrafluoroethylene (PTFE) to improve the wettability of electrodes, but also as a mediator providing an
5
ion-conducting pathway in the macropores to facilitate the ionic transportation across the electrode. And thus, with Nafion particles filling up the macropores inside the AC electrode, the ionic tranportation resistance can be decreased [30]. The schematic inner structure of the AC sheet in the presence of Nafion has been drawn in Fig. 1. In addition to the electrode material, the operating conditions of CDI cell affect the desalination performance to a large extent [20, 37-39]. It is necessary to understand the relation between operating parameters and desalination performance. Currently most of the CDI cells work under constant current by ending up with a particular cell voltage [22, 40, 41]. However, the initial cell potential (as called IR drop) is restricted by the cell resistance and applied current density [22]. Higher current density results in bigger IR drop. Therefore, it is unreasonable to evaluate the desalination performance by just controlling the terminal voltage. In this study, the Nafion-AC composite was prepared by simply mixing Nafion solution with AC particles with low PTFE binder content. The Nafion-AC asymmetric CDI cell was assembled with Nafion-AC as negative electrode and AC as positive electrode. Under constant charge input conditions, the desalination performance and effect of applied current density was investigated. This research is much beneficial to practical engineering applications.
2. Experimentals 2.1 Materials Activated carbon (AC) was purchased from Yihuan Carbon Co., Ltd. Nafion
6
solution (5 wt％) and Polytetrafluroethylene (PTFE, 60 wt％) were obtained from Du Pont Co., Ltd. All other chemicals were bought from Sinopharm Chemical Reagent Co., Ltd. All the reagents were used without further purification. All the aqueous solutions were prepared using ultrapure water (18.2 MΩ cm, Nanopure, Barnstead, USA).
2.2 Fabrication of Nafion-AC composite electrode To fabricate the carbon electrode, carbon slurry was prepared by mixing AC, PTFE and Nafion solution in a certain mass ratio. Subsequently, the mixture was stirred into uniform carbon slurry and dried in an oven at 95℃ for 12 h. And then it was rolled into a thin sheet (~0.3 mm). Finally the obtained sheet was cut into a 60 mm×30 mm square as electrode for CDI cell assembly. The Nafion-AC composites with the Nafion:AC mass ratio of 2.5:100, 5:100, 7.5:100 and 10:100 were denoted as Nafion(2.5)-AC, Nafion(5)-AC, Nafion(7.5)-AC and Nafion(10)-AC, respectively. PTFE content in these composites is 3.5 wt% unless mentioned elsewhere. As a control, AC electrode was fabricated with the above similar method but without Nafion.
2.3 Characterization The surface morphology and elemental composition of the electrodes were examined by scanning electron microscopy with energy-dispersed analysis of X-ray (SEM /EDX, Hitachi, S-4700, Japan). The tensile strength of electrode sheets was studied by H5KS universal testing machine (Tinius Olsen, US). The wettability was determined by water contact angle test using JGW-360B (Xihuayi Technology Co. 7
Ltd., Beijing, China). Nitrogen adsorption-desorption isotherms were conducted at 77 K with an ASAP 2060 machine (Micrometitics, Norcross, GA). The electrochemical performance of the electrode was investigated with a two-electrode cell in 58.5 g dm-3 sodium chloride (NaCl) solution by cyclic voltammetry (CV) on the CHI760D workstation (Shanghai Chenhua instrument Co., Ltd). The specific capacitance of the electrode was obtained from CV curves in two-electrode cell system based on the following equation (1) [42,43]: Cs =
4 ∫ 𝐼𝑑𝑉
(1)
𝑣∆𝑉𝑚
Where the Cs (F/g) is the specific capacitance; I (A) is the current; 𝑣 (V/s) is the scan rate; ∆𝑉 (V) is the applied voltage window and m (g) is the total mass of two electrodes. The electrochemical impedance spectroscopy (EIS) (in 58.5 g dm-3 NaCl) and differential capacitance measurements (in 500 mg dm-3 NaCl) were carried out using a μAUTOLAB-III potentiostat instrument (Metrohm, Switzerland) controlled by FRA software with three-electrode cell system. The Nafion-AC composite electrode with a size of 10mm×10mm was used as the working electrode, an Ag/AgCl (3M KCl) electrode was used as the reference electrode, and a coiled platinum wire as the counter electrode. The NaCl solution used for electrochemical characterization was purged with high purity nitrogen for at least 30 min to remove the dissolved oxygen before use.
2.4 Capacitive deionization experiments An asymmetric CDI cell was prepared for desalination experiments. Unless 8
mentioned otherwise, Nafion-AC electrode was used as negative and AC electrode as positive electrode in the experiment, and the cell was denoted as -Nafion-AC||AC, unless. +Nafion-AC||AC represents the reverse CDI cell with Nafion-AC as positive and AC as negative electrode. In addition, a symmetric CDI cell with two pieces of AC electrodes was used as a control. The CDI cell was operated in a closed-loop mode in both adsorption and desorption, and an influent solution containing 500 mg dm-3 NaCl was fed continuously to it at a flow rate of 10 mL/min using a peristaltic pump. The cells were equilibrated with NaCl solution overnight before measurements. The capacitive deionization test was carried out in 50 mL NaCl solution using a computer controlled battery test system (LANHE, CT2001A, WuHan, China). The electrosorption test was conducted by a constant current for a certain time. Then the cell was discharged to 0 V by short-circuit self-discharging mode. The applied current and corresponding voltage were recorded using a computer. The conductivity of the saline water was monitored continuously by a conductivity meter (Mettler Toledo, S230), and the average desalination amount was calculated from 3 adsorption/desorption cycles. The relationship between conductivity and NaCl concentration was obtained according to a calibration curve made prior to the experiment. The desalination amount and the corresponding desalination rate were defined as follows [23, 42]: Г=
(𝑐0 −𝑐𝑒 )×𝑉
(2)
𝑚 Г
𝑣𝐷 = 𝑡
(3) 9
Where Г (mg/g) is the desalination amount; 𝑐0 (mg dm-3) and 𝑐𝑒 (mg dm-3) are the initial and final NaCl concentration, respectively; V (L) is the solution volume; m (g) is the total mass of the two electrodes; vD (mg/g/min) is the desalination rate and t (min) is the charge time. Charge efficiency, a vital parameter to evaluate CDI performance, is defined as the ratio of the equivalent charge of adsorbed salt to the total charge input to the cell, as described according to the following equation: ᴧ=
(𝑐0 −𝑐𝑒 )×𝑉×𝐹
(4)
𝑄×𝑀
Where ᴧ is the charge efficiency, F is the Faraday constant (96,485 C/mol), M (58.5 g /mol) is the molar mass of sodium chloride and Q (charge, C) is charge input.
3 Results and discussion 3.1 Optimization of electrode preparation The CDI electrode must withstand the impact force from water circulation. Therefore, the AC electrode with good mechanical property in CDI cell is very important to lower the electrode resistance and to retain the desalination performance. In a common electrode preparation recipe, 5% ~10% PTFE binder [44, 45] has been used to achieve a good mechanical strength of the electrode with high mass loading, but high PTFE amount will lead to high electrode resistance due to its hydrophobic property. It has been reported that Nafion is a good binding agent for coating [35, 36]. Therefore, Nafion was added into the electrode in order to reduce the amount of PTFE and enhance the ionic transportation owing to its negative charge property inside the electrode. Here, the Nafion-AC electrode with 7.5% Nafion addition and AC electrode 10
without Nafion (both electrodes contain 3.5% PTFE) were prepared in order to investigate the effect of Nafion on mechanical property of the electrode. The mechanical strength of both electrodes with same sample size was measured by universal testing machine. Nafion-AC electrode shows a tensile strength of 0.43 MPa, which is 2.3 times higher than that of AC electrode. It is much comparable to the calendaring AC electrode sheet reported previously [46]. It was also found that the AC electrode was much prone to break with some particles falling off. Therefore, Nafion in the composite enhances the adhesion between AC particles and improves the strength and stiffness of the electrode sheet, which renders the electrode flexible and easy handling even with low PTFE content. As mentioned above, since AC electrode with 3.5% PTFE content has poor mechanical property, it will not be used for further study any more. Three carbon sheets including AC with 7% PTFE (AC-7%PTFE), Nafion-AC with 7.5% Nafion and 7% PTFE (Nafion(7.5)-AC-7%PTFE) and Nafion-AC with 7.5% Nafion and 3.5% PTFE (Nafion(7.5)-AC-3.5%PTFE) were fabricated and their characteristics were studied by EIS and water contact angle, respectively. Fig.2 exhibits the Nyquist plots of the AC-7%PTFE, Nafion(7.5)-AC-7%PTFE and Nafion(7.5)-AC-3.5%PTFE electrodes in 58.5 g dm-3 NaCl in the frequency range from 1 mHz to 100 kHz. The semicircle in high frequency region reveals the interfacial charge transfer resistance (Rct).
The
impedance
line
in
the
low-frequency
region
represents
a
diffusion-controlled process, and the line to the imaginary axis of the impedance spectra, in all cases, approaching to a vertical indicates a desirable capacitive behavior 11
[47]. It is noted that in the presence of Nafion, the Rct of the electrode decreases obviously, and the straight lines at low frequency are inclined steeply to the imaginary part, compared to AC-7%PTFE electrode. In addition, Nafion(7.5)-AC-3.5%PTFE electrode, due to low PTFE content, possesses a lower Rct value, and the ionic diffusion and migration on/inside the electrode sheet become faster, implying a better electrochemical double-layer capacitance behavior than Nafion(7.5)-AC-7%PTFE electrode. However, the EIS straight line of AC-7%PTFE electrode at lower frequency region is far away from imaginary axis, indicating a slow ion diffusion velocity. This result confirms the effect of Nafion in the electrode (as shown in Fig.1) The water contact angle tests of the AC-7%PTFE, Nafion(7.5)-AC-7%PTFE and Nafion(7.5)-AC-3.5%PTFE electrode sheets were done to investigate the wettability of the composite electrode (figure not shown). It was found that AC-7%PTFE is hydrophobic with a contact angle of 130. With Nafion addition, the Nafion(7.5)-AC-7%PTFE electrode has the contact angle of 115. By reducing the PTFE amount to 3.5%, the hydrophilicity of the resultant Nafion(7.5)-AC-3.5%PTFE electrode has obvious improvement with the corresponding contact angle of 85. Therefore, the partial replacement of hydrophobic PTFE with Nafion can effectively improve the wettability of electrodes due to the sulfonic groups from Nafion. The enhancement of wettability facilitates the migration of water and ions on/inside the electrode. The result is well consistent with that from EIS. Moreover, Nafion, as a cation exchanger, improves both the ionic conductivity and the capacitance performance. In the following experiments, 3.5 wt% PTFE was added for all 12
Nafion-AC electrodes.
3.2 Characterization of Nafion-AC composite electrode The distribution of Nafion in the Nafion-AC composite was studied by SEM. Fig. 3A-D shows the SEM images of the Nafion(2.5)-AC, Nafion(5)-AC, Nafion(7.5)-AC, Nafion(10)-AC surfaces, respectively. At low Nafion content, only a small quantity of Nafion particles are distributed on the surface of AC particles, as marked in a white circle (Fig. 3A). With the increase of Nafion content in the composite electrode, more and more particles were deposited on carbon surface with a uniform distribution. But high Nafion amount results in obvious agglomeration, which may lead to the blockage of the pores on the carbon particle surface and poor electronic conductivity of the composite. The EDX elemental maps in the SEM scan range of Nafion(7.5)-AC electrode were depicted in Fig. 4, including overlay map of all elements (Fig. 4A) and individual maps of S (Fig. 4B) and F (Fig. 4C) elements, respectively. They are all distributed uniformly over the composite. The presence of S confirms the distribution of Nafion around AC particles, which facilitates the diffusion of hydrated ions and favors the electrosorption [41, 48]. It is reported that the presence of the macropores in the pore-particles interphase will result in the transient decrease of ion concentration and thus the reduction of ionic conductivity inside porous electrode due to the depletion of active ions during adsorption [30]. Nafion coating around particles possibly reduces the macropores by filling Nafion in between pore-particles inside the electrode. Here, the N2 adsorption-desorption
isotherm
was 13
conducted
to
investigate
the
Bruanuer-Emmeet-Teller (BET) specific surface area and pore size distribution of Nafion-AC composites. The results with different Nafion contents were collected in Table 1. With the addition of Nafion, Nafion-AC composites show larger BET specific surface area and more micropore volume than AC. It is possible that Nafion molecules/particles are well filled in the macropores, contributing some micropores. The change of the pore distribution in the presence of Nafion will, to some extent, be beneficial to the steady ionic transportation during the CDI process [30], as described in Fig. 1. However, too high Nafion amount results in the decrease of the specific surface area, due to the blockage of the pores by Nafion coating as confirmed by SEM image.
3.3 CV analysis CV technique is an effective way to evaluate the specific capacitance of the electrode and electrosorption ability of the electrodes. The CV measurement was carried out in a two-electrode cell. Fig. 5A shows the CV curves of AC, Nafion(2.5)-AC, Nafion(5)-AC, Nafion(7.5)-AC, Nafion(10)-AC electrodes in the presence of 58.5 g dm-3 NaCl solution, respectively. These curves exhibit ideal rectangular shape without any redox peaks in the wide voltage window, indicating a typical electrochemical double layer behavior. The specific capacitances were calculated from the CV curves based on Eq. (1), and the corresponding values were collected in Table 2. Increasing Nafion content from 0 ~ 7.5% leads to the increase of specific capacitance Cs from 119 to 148 F/g. But too high Nafion content results in the decrease of Cs. It is reasonable that a moderate amount of Nafion in the electrode 14
can enhance the ionic conductivity, but overload of Nafion will reduce the electronic conductivity, resulting in the decrease of Cs. These results are in line with that of SEM characterization. Fig. 5B displays the capacitance retention ratio of various electrodes calculated from CV curves at different scan rates to evaluate the ion transport behavior with different Nafion ratio. The reduced inner-pore ion transport rate leads to the decline of capacitance at high scan rate [49]. Nafion(10)-AC shows a more steep descent, while Nafion(7.5)-AC exhibits a large capacitance retention ratio than others at high scan rates, indicating better ion transfer behavior. The capacitance performance of different electrodes is ranked as: Nafion-(7.5)-AC > Nafion-(5)-AC > Nafion-(2.5)-AC > AC > Nafion-(10)-AC. Therefore, Nafion(7.5)-AC is desirable for CDI applications.
3.4 Desalination performance CDI test was conducted in NaCl solution with an initial conductivity of approximately 1050 μS/cm. The cell was cycled in NaCl solution overnight to reach the physical adsorption balance and then the fresh NaCl solution was used during CDI charge-discharge process. The asymmetric cells were assembled with Nafion-AC electrode as a negative electrode and AC electrode as positive electrode. The ionic adsorption property during charging stage
was
investigated
using the
AC||AC,
-Nafion(2.5)-AC||AC,
-Nafion(5)-AC||AC, -Nafion(7.5)-AC||AC and -Nafion(10)-AC||AC cells. Fig. 6 shows the adsorption equilibrium curves on different electrodes by charging at a current density of 30 mA/g. Obviously, in the presence of Nafion, the electrode easily 15
arrives at the adsorption equilibration, compared with AC electrode. With the increase of the Nafion amount, the adsorption rate and the maximum adsorption amount increases gradually. The Nafion(7.5)-AC||AC has the fastest desalination rate, and its adsorption amount arrives at the maximum in 25 min, and then it levels off with further charging, indicating the equilibration of co-ion desorption and counter-ion adsorption. The desalination capacity of 10.8 mg/g has been achieved in the Nafion(7.5)-AC||AC cell, compared with the AC||AC cell of 6.9 mg/g. The desalination capacity, desalination rate and the corresponding charge efficiency for different electrodes were summarized in Table 2. It confirms that Nafion, as a binder and ionic mediator in between AC particles, facilitates the ionic transportation, and thus effectively enhances the desalination capacity. However, too high amount of Nafion in the electrode leads to a reduction of desalination amount due to blockage of pores and increase of the resistance, as demonstrated previously by CV and N2 adsorption-desorption isotherm tests.
3.5 Charge selectivity of Nafion-AC electrode in the asymmetric CDI cell The charge selectivity of the electrodes, using two identical Nafion(7.5)-AC||AC cells, was investigated via applying different connection to power supply, one with Nafion(7.5)-AC
as
negative
electrode
(-Nafion-AC||AC),
the
other
with
Nafion(7.5)-AC as positive electrode (+Nafion-AC||AC). In a constant-current (60 mA/g) mode with the charge input of 4 mAh, the voltage/ current curves (A, C) and the corresponding solution conductivity (B, D) in the -Nafion-AC||AC and 16
+Nafion-AC||AC were illustrated in Fig. 7, respectively. For -Nafion-AC||AC cell, the conductivity of the solution decreases during the charge process and goes up to the initial level during the discharge process. It indicates that anions are adsorbed onto the positive electrode whereas cations are adsorbed onto the negative electrode. They were desorbed into the solution when discharging. It is noteworthy, however, that when applying a reverse current during the charge process, the resultant +Nafion-AC||AC cell exhibits a reverse conductivity variation. This similar inverted CDI behavior was found by Landon [50, 51]. It is known that the removal amount of ions during the charging process is the difference between the counter-ions adsorbed on the electrodes and the co-ions desorbed from the electrodes [52]. Usually, the symmetric AC cell shows the same desalination trends when reversing the polarity of the cell. However, Nafion-AC electrode surface possesses lots of sulfonic groups, which make Cl- difficult to be adsorbed due to the charge repulsion when used as positive electrode. Impact of the surface charge renders more co-ions desorbed out of the electrodes than counter-ions adsorbed on the electrode. It finally causes the reverted conductivity variation. The result indicates Nafion-AC has a good cation-selectivity. Therefore, the desalination performance can be improved effectively only when Nafion-AC is used as the negative electrode. This strong ionic repulsion from the electrode will certainly lead to the enhancement of the charge efficiency owing to the attenuation of the co-ion effect. This big difference between Nafion-AC positive and negative electrode can be further explained by measuring the potential of zero charge (Epzc) of the electrodes 17
through analyzing the potential-differential capacitance curves. The differential capacitance (CD), defined as the dependence of the surface charge on the electrode potential is very suitable for analyzing the property of electric double layer on the porous carbon electrode. CD varies with electrode potential, being related to the electronic charges of carbon surface, the mobile ionic charges and immobile chemical charges in the micropore, according to the dynamic EDL model [26]. The potential differential capacitance curves were obtained by scanning the different electrodes at a frequency of 1 Hz in 500 mg dm-3 NaCl solution. At the minimum of the differential capacitance, the surface is non-charged, with most loose diffusion layer and low capacitance. Therefore, the minimum of the camel curves corresponds to the Epzc value. The Epzc of different electrodes were collected in Table 2. Unmodified AC electrode possesses the Epzc of about 69 mV, indicating the neutral charge state of the surface. While, the Epzc values of Nafion-AC composite electrodes increase gradually from 236 mV (Nafion(2.5)-AC) to 715 mV (Nafion(10)-AC). It indicates that more negatively charged groups exist on the electrode surface with the Nafion addition [23, 53, 54], which undoubtedly enhance the cation-selectivity and reduce the co-ion repulsion effect.
3.6 The influence of charge input In the previous study, the constant-current charging mode has been applied by controlling the cutoff voltage of around 1.2 V [41, 55]. Thus the charge input is much related to current density due to IR drop. Moreover, it is difficult to predict optimal desalination amount and charge efficiency simultaneously. These can be illustrated via 18
comparing the desalination process between controlling the terminal voltage of 1.2 V (Fig. 8A) and controlling the constant charge input (Fig. 8B). At the terminal voltage of 1.2 V, there is a significant difference of the deionization efficacy at various current density, rising from higher IR drop at higher current density and pretty low charge input. The high cell resistance (including the solution resistance and contact resistance) results in the reduction of the factual charging potential window. However, the cutoff voltage varies between 0.88 V and 2.07 V (IR drop was subtracted) when the same charge input was supplied with the charging current varying from 40 to 200 mA/g, respectively. And there is a reasonable conductivity variation. The desired current density can be conveniently adjusted based on EDL voltage windows in CVs and the charge efficiency requirement. The dynamic curves of charge efficiency and desalination amount with charge input at the Nafion(7.5)-AC were depicted in Fig. 9. Clearly, both the desalination amount and charge efficiency grow with charge input at the initial charging period. Then the charge efficiency descends and the desalination value levels off when the charge input increases continuously. Therefore, the charge efficiency is pretty low near the equilibrium desalination stage as shown in Table 2. It suggests that excessive charge input causes high energy consumption due to water splitting or electrolysis. However the less charge input results in very low desalination amount, and thus low availability of electrode material. Interestingly, the cross-point between desalination amount and charge efficiency appears at the same charge quantity at one electrode even though the different constant currents were applied. Therefore, it is very 19
convenient to define the optimal charge input of the electrode through this dynamic curve, where the acceptable desalination amount and charge efficiency can be found at the same time.
3.7 The influence of the current density Fig.10A shows the conductivity variation of the CDI cell at the charge input value of 3.5mAh with different current densities. And the desalination amount and desalination rate were calculated according to Eq.(2) and Eq.(3), respectively, and shown in Fig. 10B. Under the same charge input condition, the desalination amount is strongly related to current density. As the current density increases, the desalination amount increased sharply, but descends quickly at high current density. The maximum desalination amount was obtained over a narrow current density range of 40 ~ 60 mA/g, and the charge efficiency has the same variation trend with the maximum of 67% (data unshown). Charge efficiency and desalination amount decrease gradually at higher current density. It is because severe polarization could happen at high current density. On the other hand, the desalination rate becomes very low with decreasing the current density due to slow ionic transferring velocity. Therefore, the appropriate current density should be adjusted carefully to obtain the optimal desalination amount. Meanwhile, an acceptable desalination rate should be considered for practical CDI application.
3.8 Stability of the CDI cell The stability and regeneration of the electrode materials were investigated in 500 mg dm-3 NaCl solution via a constant-current mode (50 mA /g) with the charge input 20
of 4 mAh. The voltage and conductivity variation were continuously monitored during charge-discharge process. In the more than 100 cycles, there is no serious going-up of the voltage, and the conductivity fluctuation is only 2.3% at the early several cycles. This steady voltage and conductivity variation indicates an excellent stability and regeneration ability of the electrode materials for desalination.
Conclusions In the present study, the Nafion-AC electrode has been studied with enhanced mechanical property, and improved ionic conductivity and capacitance for CDI. The strong cation-selectivity of the Nafion-AC electrode has been demonstrated by the opposite desalination characteristic of -Nafion-AC/AC and +Nafion-AC/AC. The dynamic curves of desalination amount and charge efficiency are very convenient to discover the optimal charge input of one electrode giving consideration to both desalination amount and charge efficiency at the same time, irrespective of the current density. Controlling charge input is more reasonable to investigate the parameter effect on desalination performance than controlling the terminal voltage in the charging process. Care should be taken to adjust the current density in order to optimize the operation parameters in CDI device. Under the constant charge input mode, the present Nafion-AC electrode is stable in CDI multi-cycling. The present study is of great use for the practice operation of the CDI device and engineering applications.
Acknowledgements The authors gratefully acknowledge the financial support of this research by the 21
National Natural Science Foundation of China (Grants 21476047 and 21105009), and the State Key Laboratory of Electro analytical Chemistry (SKLEAC201205).
22
References: [1] G.D. Kang, Y.M. Cao, Development of antifouling reverse osmosis membranes for water treatment: A review, Water Res, 46 (2012) 584-600. [2] A. Perez-Gonzalez, A.M. Urtiaga, R. Ibanez, I. Ortiz, State of the art and review on the treatment technologies of water reverse osmosis concentrates, Water Res, 46 (2012) 267-283. [3] L. Alvarado, A. Chen, Electrodeionization: principles, strategies and applications, Electrochimica Acta, 132 (2014) 583-597. [4] Ö. Arar, Ü. Yüksel, N. Kabay, M. Yüksel, Various applications of electrodeionization (EDI) method for water treatment—A short review, Desalination, 342 (2014) 16-22. [5] 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. [6] 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, Progress in Materials Science, 58 (2013) 1388-1442. [7] F.A. AlMarzooqi, A.A. Al Ghaferi, I. Saadat, N. Hilal, Application of Capacitive Deionisation in water desalination: A review, Desalination, 342 (2014) 3-15. [8] Y. Oren, Capacitive deionization (CDI) for desalination and water treatment — past, present and future (a review), Desalination, 228 (2008) 10-29. [9] Y. Bian, X. Yang, P. Liang, Y. Jiang, C. Zhang, X. Huang, Enhanced desalination performance of membrane capacitive deionization cells by packing the flow chamber with granular activated carbon, Water Res, 85 (2015) 371-376. [10] J.-H. Choi, Fabrication of a carbon electrode using activated carbon powder and application to the capacitive deionization process, Separation and Purification Technology, 70 (2010) 362-366. [11] Q. Dong, G. Wang, T. Wu, S. Peng, J. Qiu, Enhancing capacitive deionization performance of electrospun activated carbon nanofibers by coupling with carbon nanotubes, Journal of colloid and interface science, 446 (2015) 373-378. [12] G. Wang, C. Pan, L. Wang, Q. Dong, C. Yu, Z. Zhao, J. Qiu, Activated carbon nanofiber webs made by electrospinning for capacitive deionization, Electrochimica Acta, 69 (2012) 65-70. [13] S. Nadakatti, M. Tendulkar, M. Kadam, Use of mesoporous conductive carbon black to enhance performance of activated carbon electrodes in capacitive deionization technology, Desalination, 268 (2011) 182-188. [14] X. Gao, J. Landon, J.K. Neathery, K. Liu, Modification of Carbon Xerogel Electrodes for More Efficient Asymmetric Capacitive Deionization, Journal of the Electrochemical Society, 160 (2013) E106-E112. [15] H. Li, L. Zou, Ion-exchange membrane capacitive deionization: A new strategy for brackish water desalination, Desalination, 275 (2011) 62-66. [16] L. Yang, Z. Shi, W. Yang, Enhanced capacitive deionization of lead ions using air-plasma treated carbon nanotube electrode, Surface and Coatings Technology, 251 (2014) 122-127. [17] C. Yan, L. Zou, R. Short, Single-walled carbon nanotubes and polyaniline composites for capacitive deionization, Desalination, 290 (2012) 125-129. 23
[18] A.G. El-Deen, J.-H. Choi, C.S. Kim, K.A. Khalil, A.A. Almajid, N.A.M. Barakat, TiO2 nanorod-intercalated reduced graphene oxide as high performance electrode material for membrane capacitive deionization, Desalination, 361 (2015) 53-64. [19] L. Liu, L. Liao, Q. Meng, B. Cao, High performance graphene composite microsphere electrodes for capacitive deionisation, Carbon, 90 (2015) 75-84. [20] D. Liu, K. Huang, L. Xie, H.L. Tang, Relation between operating parameters and desalination performance of capacitive deionization with activated carbon electrodes, Environ. Sci.: Water Res. Technol., 1 (2015) 516-522. [21] C. Yan, L. Zou, R. Short, Polyaniline-modified activated carbon electrodes for capacitive deionisation, Desalination, 333 (2014) 101-106. [22] J.-H. Choi, Determination of the electrode potential causing Faradaic reactions in membrane capacitive deionization, Desalination, 347 (2014) 224-229. [23] T. Wu, G. Wang, Q. Dong, B. Qian, Y. Meng, J. Qiu, Asymmetric capacitive deionization utilizing nitric acid treated activated carbon fiber as the cathode, Electrochimica Acta, 176 (2015) 426-433. [24] T. Kim, J.E. Dykstra, S. Porada, A. van der Wal, J. Yoon, P.M. Biesheuvel, Enhanced charge efficiency and reduced energy use in capacitive deionization by increasing the discharge voltage, Journal of colloid and interface science, 446 (2015) 317-326. [25] Y.-J. Kim, J.-H. Choi, Enhanced desalination efficiency in capacitive deionization with an ion-selective membrane, Separation and Purification Technology, 71 (2010) 70-75. [26] P.M. Biesheuvel, H.V.M. Hamelers, M.E. Suss, Theory of Water Desalination by Porous Electrodes with Immobile Chemical Charge, Colloids and Interface Science Communications, 9 (2015) 1-5. [27] X. Gao, S. Porada, A. Omosebi, K.L. Liu, P.M. Biesheuvel, J. Landon, Complementary surface charge for enhanced capacitive deionization, Water Res, 92 (2016) 275-282. [28] X. Gao, A. Omosebi, N. Holubowitch, A. Liu, K. Ruh, J. Landon, K. Liu, Polymer-coated composite anodes for efficient and stable capacitive deionization, Desalination, 399 (2016) 16-20. [29] Mare D. Andelman, Worcester, MA, Polarized electrode for flow-through cpacitive deionization, US patent, 2014/0346046 A1. [30] P.M. Biesheuvel, Y. Fu, M.Z. Bazant, Diffuse charge and Faradaic reactions in porous electrodes, Physical review. E, Statistical, nonlinear, and soft matter physics, 83 (2011) 061507. [31] Y. Dzyazko, L. Rozhdestveskaya, Y. Zmievskii, Y. Volfkovich, V. Sosenkin, N. Nikolskaya, S. Vasilyuk, V. Myronchuk, V. Belyakov, Heterogeneous Membranes Modified with Nanoparticles of Inorganic Ion-Exchangers for Whey Demineralization, Materials Today: Proceedings, 2 (2015) 3864-3873. [32] M.A. Khan, M. Kumar, Z.A. Alothman, Preparation and characterization of organic–inorganic hybrid anion-exchange membranes for electrodialysis, Journal of Industrial and Engineering Chemistry, 21 (2015) 723-730. [33] N. White, M. Misovich, A. Yaroshchuk, M.L. Bruening, Coating of Nafion membranes with polyelectrolyte multilayers to achieve high monovalent/divalent cation electrodialysis selectivities, ACS applied materials & interfaces, 7 (2015) 6620-6628. [34] X.H. Yan, R. Wu, J.B. Xu, Z. Luo, T.S. Zhao, A monolayer graphene – Nafion sandwich membrane for direct methanol fuel cells, Journal of Power Sources, 311 (2016) 188-194. [35] A.S. Razavian, S.M. Ghoreishi, A.S. Esmaeily, M. Behpour, L.M.A. Monzon, J.M.D. Coey, Simultaneous sensing of L-tyrosine and epinephrine using a glassy carbon electrode modified with 24
nafion and CeO2 nanoparticles, Microchimica Acta, 181 (2014) 1947-1955. [36] Y. Wu, S. Zhai, K. Lu, L. Gao, Electrochemical oxidation behavior of 8-azaguanine at graphene-Nafion composite film-modified glassy carbon electrode, Journal of Solid State Electrochemistry, 18 (2014) 1593-1600. [37] Y.-J. Kim, J.-H. Kim, J.-H. Choi, Selective removal of nitrate ions by controlling the applied current in membrane capacitive deionization (MCDI), Journal of Membrane Science, 429 (2013) 52-57. [38] C. Huyskens, J. Helsen, A.B. de Haan, Capacitive deionization for water treatment: Screening of key performance parameters and comparison of performance for different ions, Desalination, 328 (2013) 8-16. [39] S. Porada, M. Bryjak, A. van der Wal, P.M. Biesheuvel, Effect of electrode thickness variation on operation of capacitive deionization, Electrochimica Acta, 75 (2012) 148-156. [40] H. Pan, J. Yang, S. Wang, Z. Xiong, W. Cai, J. Liu, Facile fabrication of porous carbon nanofibers by electrospun PAN/dimethyl sulfone for capacitive deionization, J. Mater. Chem. A, 3 (2015) 13827-13834. [41] J. Liu, M. Lu, J. Yang, J. Cheng, W. Cai, Capacitive desalination of ZnO/activated carbon asymmetric capacitor and mechanism analysis, Electrochimica Acta, 151 (2015) 312-318. [42] Y. Liu, T. Chen, T. Lu, Z. Sun, D.H.C. Chua, L. Pan, Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization, Electrochimica Acta, 158 (2015) 403-409. [43] M.D. Stoller, R.S. Ruoff, Best practice methods for determining an electrode material's performance for ultracapacitors, Energy Environ. Sci., 3 (2010) 1294-1301. [44] B. Wei, J.C. Tokash, G. Chen, M.A. Hickner, B.E. Logan, Development and evaluation of carbon and binder loading in low-cost activated carbon cathodes for air-cathode microbial fuel cells, RSC Adv., 2 (2012) 12751-12758. [45] M. Aslan, D. Weingarth, N. Jäckel, J.S. Atchison, I. Grobelsek, V. Presser, Polyvinylpyrrolidone as binder for castable supercapacitor electrodes with high electrochemical performance in organic electrolytes, Journal of Power Sources, 266 (2014) 374-383. [46] J. Liu, W. Cai, H. Yang, L. Ping, R. Xiong, Method for Preparing Composition, Sheet Composing the Composition and Electrode Comprsing the Sheet, US Patent, 201101752A1. [47] A.K. Mishra, S. Ramaprabhu, Functionalized Graphene-Based Nanocomposites for Supercapacitor Application, The Journal of Physical Chemistry C, 115 (2011) 14006-14013. [48] Y. Liu, W. Ma, Z. Cheng, J. Xu, R. Wang, X. Gang, Preparing CNTs/Ca-Selective zeolite composite electrode to remove calcium ions by capacitive deionization, Desalination, 326 (2013) 109-114. [49] D. Wang, F. Li, M. Liu, G. Lu, H. Cheng. Mesopore-Aspect-Ratio Dependence of Ion Transport in Rodtype Ordered Mesoporous Carbon, J. Phys. Chem. C, 112 (2008), 9950–9955. [50] X. Gao, A. Omosebi, J. Landon, K. Liu, Enhanced Salt Removal in an Inverted Capacitive Deionization Cell Using Amine Modified Microporous Carbon Cathodes, Environmental science & technology, 49 (2015) 10920-10926. [51] 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. [52] E. Avraham, M. Noked, Y. Bouhadana, A. Soffer, D. Aurbach, Limitations of charge efficiency in capacitive deionization processes III: The behavior of surface oxidized activated carbon electrodes, 25
Electrochimica Acta, 56 (2010) 441-447. [53] A. Omosebi, X. Gao, J. Landon, K. Liu, Asymmetric electrode configuration for enhanced membrane capacitive deionization, ACS applied materials & interfaces, 6 (2014) 12640-12649. [54] V. Lockett, M. Horne, R. Sedev, T. Rodopoulos, J. Ralston, Differential capacitance of the double layer at the electrode/ionic liquids interface, Physical chemistry chemical physics : PCCP, 12 (2010) 12499-12512. [55] J. Liu, S. Wang, J. Yang, J. Liao, M. Lu, H. Pan, L. An, ZnCl2 activated electrospun carbon nanofiber for capacitive desalination, Desalination, 344 (2014) 446-453.
26
Figure 1
Fig 1. The schemetric illustration of the innerstructure of the Nafion-AC composite as compared to AC sheet.
27
Figure 2
800 40
-z" ()
700 600
-z" ()
500
20
0 0
20
40
60
80
100
z' ()
400 300 200 100 0 0
100
200
300
400
500
600
700
800
z' ()
Fig.2 EIS of AC-7 % PTFE (square), Nafion(7.5)-AC-7%PTFE (cycle) and Nafion(7.5)-AC-3.5% PTFE (triangle) electrodes, insert is the magnification in the high frequency region.
28
Figure 3
(A)
(B)
(C)
(D)
Fig.3 SEM images of Nafion(2.5)-AC (A), Nafion(5)-AC (B), Nafion(7.5)-AC (C) and Nafion(10)-AC (D). Nafion particles were marked with white circles.
29
Figure 4 (A)
(B)
(C)
Fig.4 EDX elemental overlap maps of various elements on the Nafion(7.5)-AC electrode (A); The individual elemental maps of S (B), and F (C).
30
Figure 5
AC Nafion(2.5)-AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
(A)
80
(B) Capacitance Retention Ratio (%)
Current (mA/g)
160
0
-80
AC Nafion(2.5)-AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
100
80
60
40
20
0
0.0
0.4
0.8
0
1.2
5
10
15
20
Scan rate (mV/s)
Voltage (V)
Fig.5 (A) CVs of the electrodes containing different ratio of Nafion; Scan rate: 1 mV/s; (B) Capacitance retention ratio of the electrodes containing different ratio of Nafion as a function of scan rates.
31
Figure 6
120
C0 - Ce (mg/L)
90
60
AC Nafion(2.5)-AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
30
0 0
10
20
30
40
t (min)
Fig.6 Desalination amount of different electrodes in 500 mg dm-3 NaCl solution during charging process. Current density: 30mA/g.
32
Figure 7
0.8
40
0.4
30
0.0
20
-0.4
10
(B) 1050
-0.8 0
Conductivity (μS/cm)
Voltage (V)
1100 50
Current (mA)
(A)
1.2
1000
950
900
850
-1.2 -10 0
40
80
120
160
800
200
0
50
100
t (min)
50 40
0.4
30
0.0
20
-0.4
10
-0.8
Conductivity (μS/cm)
0.8
1300
Current (mA)
(C)
1.2
Voltage (V)
150
200
t (min)
(D)
1250
1200
1150
0
1100
-10
1050
-1.2 0
50
100
150
0
200
t (min)
50
100
150
t (min)
Fig.7 Cell voltage/ current curves (A, C) and conductivity variation (B, D) on -Nafion-AC||AC (A, B) and +Nafion-AC||AC (C, D) during the continuous charge-discharge process.
33
200
Figure 8
(A)
(B)
1050
1000
Conductivity (μS/cm)
Conductivity (μS/cm)
1050
200 mA/g 950
40 mA/g 900
850
200 mA/g
1000
40 mA/g
950
900
850
0
1
2
3
4
Charge quantity (mAh)
0
1
2
3
Charge quantity (mAh)
Fig.8 Conductivity variation of Nafion-based CDI cell (A) with a terminal voltage of 1.2V and (B) with a terminal charge quantity of 3.5 mAh. The charging current from bottom to top corresponds to 40, 80, 120, 160 and 200 mA/g, respectively.
34
4
Figure 9
Charge efficiency (%)
8 60 6 40 4 40mA/g 80mA/g 120mA/g
20
2
0
Desalination amount (mg/g)
10
80
0 0
1
2
3
4
5
Charge quantity (mAh)
Fig.9 Dynamic variation in desalination amount and charge efficiency of Nafion-AC-based CDI cell during charging with the current density of 40, 80 and 120 mA/g.
35
Figure 10 1100
9
2.0
(A)
(B)
1000
950
900
850
200 mA/g
20 mA/g
800 0
10
20
30
40
50
t (min)
8
1.5
7
1.0
6
0.5
5
0.0
40
80
120
160
200
Current density (mA/g)
Fig.10 (A) Conductivity variation of Nafion-AC-based CDI cell over time at different current densities; (B) Desalination amount and desalination rate calculated from (A) versus the current density.
36
Desalination rate (mg/g/min)
Desalination amount (mg/g)
Conductivity (μS/cm)
1050
Tables Table 1 Specific surface area, pore volume and average pore size of AC and Nafion-AC-based composite
AC Nafion(5)-AC Nafion(7.5)-AC Nafion(10)-AC
BET (m2/g)
Vtotal (cm3/g)
1668 2247 2276 2202
0.82 1.04 1.06 1.03
37
Vmicro (cm3/g)
Average pore size (nm)
0.61 0.94 0.94 0.91
1.98 1.85 1.87 1.87
Table 2 Specific capacitance, Epzc and desalination performance of Nafion-AC negative electrodes with various Nafion amount
AC Nafion-(2.5)-AC Nafion-(5)-AC Nafion-(7.5)-AC Nafion-(10)-AC
Г (mg/g)
ᴧ (%)
𝑣𝐷 (mg/g/min) Cs (F/g)
E pzc (mV)
6.9 7.3 9.5 10.8 7.8
24 29 40 45 31
0.19 0.24 0.35 0.44 0.27
69 236 352 554 715
38
119 135 138 148 109