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Fabrication of an Anion-Exchange Membrane by Pore-Filling using Catechol–DABCO Coating and its Application to Reverse Electrodialysis Jiyeon Choi, SeungCheol Yang, Namjo Jeong, Hanki Kim, and Won-sik Kim Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01666 • Publication Date (Web): 22 Aug 2018 Downloaded from http://pubs.acs.org on August 25, 2018

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Fabrication of an Anion-Exchange Membrane by Pore-Filling using Catechol–DABCO Coating and its Application to Reverse Electrodialysis Jiyeon Choi*, SeungCheol Yang, Nam-Jo Jeong, Hanki Kim, Won-Sik Kim

Korea Institute of Energy Research, Jeju Global Research Center, 200 Haemajihean-ro, 63357, Republic of Korea

2018. 08. 11

Submitted to Langmuir

*

To whom correspondence should be addressed: Jiyeon Choi, Ph.D., Tel: 064-800-2239, Fax: 064-800-5308 E-mail: [email protected]

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ABSTRACT1 We have successfully exploited Michael-type addition reaction between catechol and DABCO (1,4-diazabicyclo-[2,2,2]octane) molecules under alkaline conditions for the formation of new quaternary ammonium (QA) groups in an anion-exchange membrane. The anion-exchange membranes (AEMs) were prepared using the pore-filling method by addition of

electrolytes

(vinyl

benzyl

trimethylammonium

chloride

(VBTMA),

dopamine

methacrylamide (DMA) bearing a catechol group, and ethylene glycol diacrylate as a crosslinker) to a porous substrate. The formation of new QA groups by the reaction of DABCO with catechol components was confirmed by characterization of new peaks in the FT-IR spectra of the AEMs. The DABCO-bound AEM demonstrated a significant decrease in area resistance (0.4 Ω·cm2) and increase in permselectivity (94%). Furthermore, the electrochemical properties of the AEMs could be controlled by altering the concentrations of VBTMA and DMA, and the formation of new bonds between DMA and DABCO. The calculated theoretical (4.31 W/m2) and practical (1.52 W/m2) power densities during a reverse electrodialysis (RED) process employing the membrane with the best properties (E2C1DMA0.5-DABCO) were by 33% and 18% higher than those of a system utilizing a commercial membrane, Neosepta®AMX (3.25 and 1.29 W/m2). Therefore, the AEM synthesized in this study is a good candidate for use in RED applications.

KEYWORDS pore-filling membrane, anion-exchange membrane, catechol-diamine, DABCO coating, reverse electrodialysis

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Abbreviations: AEM, anion-exchange membrane; ATR, attenuated total reflectance; CD, charge density; CEM, cation-exchange membrane; DABCO, 1,4-diazabicyclo-[2,2,2]octane; DI, deionized; DMA, dopamine methacrylamide; DTG, differential thermogravimetry; EGDA, ethylene glycol diacrylate; IEC, ion-exchange capacity; IS, impedance spectroscopy; PECH, polyepichlorohydrin; QA, quaternary ammonium; RED, reverse electrodialysis; SEM, scanning electron microscopy; SW, swelling degree; TGA, thermogravimetric analysis; VBTMA, vinyl benzyl trimethylammonium chloride. 2


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INTRODUCTION The exhaustion of traditional energy sources (such as fossil fuels), together with environmental problems such as global pollution and global warming, has accelerated the need for the development of renewable and sustainable energy sources.1,2 Among the membrane-based renewable technologies reported to date, reverse electrodialysis (RED), which harvests energy from the difference in the concentration of salt in sea water (i.e., concentrated salt) and river water (i.e., dilute salt), has been receiving considerable attention owing to the ongoing technological improvements of the individual components, such as ionexchange membranes, 3,4 membrane stacking,5 and electrodes.6 Ion-exchange membranes play an important role in RED, since the effective separation of cations and anions between concentrated and dilute solutions is essential to generating a potential difference (called the Donnan potential 7). Once the cations and anions have been directed to the cathode and anode, respectively, they are converted to electrons through redox reactions at both electrode surfaces, and subsequently produce an electric current in the external circuit. 8 Although ion-exchange membranes have been already used successfully in various academic and industrial fields, there have been only few reports on the development of ionexchange membranes suitable for RED until the report by Guler et al..9 The authors synthesized an anion-exchange membrane specifically for a RED system using an elastomer (polyepichlorohydrin, or PECH) and a cross-linker (1,4-diazabicyclo-[2,2,2]octane, or DABCO), without the need for a harmful chloromethylation process. Since then, the authors have also reported the fabrication of micro-structured anion-exchange membranes10 and monovalent-selective membranes.11 Hong et al. 12,13 reported on cation-exchange membranes with nanocomposite structures for power generation, applied in an RED stack. In these membranes, they employed sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) polymer as a base matrix and functionalized iron(III) oxide (Fe2O3-SO4–) nanoparticles. Meanwhile, Kim et al. 14 fabricated ultra-thin pore-filling membranes for an RED stack, demonstrating that the stack had a significantly reduced internal resistance and improved the power density owing to the use of a specific high-open-area spacer. Several studies reported to date have suggested that anion-exchange membranes (AEMs) have several weak points when compared to cation-exchange membranes (CEMs). That is, cationic head-groups containing quaternary ammonium (QA) groups readily degrade 3


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under strong alkaline conditions via Hofmann elimination15. Although the RED system operates under mild conditions, compared to electrodialysis (ED), fuel cell systems, etc., with the increase of cell pairs, the open-circuit voltage (OCV) increased as well. This means that the total voltage generated from the RED stack can exceed the voltage required for water dissociation, i.e. above 1.23 V. Finally, pH changes occurred inside RED stack, causing degradation of QA groups. The general process employed for quaternary amination is very harmful as a result of the necessary halogen methylation step, as most chemicals used are carcinogenic to humans.4 Furthermore, it is difficult to generate dense QA groups in AEMs simply by increasing the ratio of the QA monomer to the crosslinking agent since the mechanical stability and permselectivity are in a trade-off relationship, and the reactivity between the monomer and crosslinker can vary.16 Hence, it is not easy to fabricate an AEM by using directly a monomer bearing a QA group—as a result, significant research efforts have been devoted to controlling the density of QA groups and the mechanical strength of membranes by incorporation of an additional step to the quaternization process after the halogen methylation.4 We fabricated a new AEM with a pore-filling structure using vinylbenzyltrimethyl ammonium chloride (VBTMA) as the main anion-exchange group. Compared to conventional membranes, a small ratio of VBTMA relative to the cross-linker was used to maintain mechanical strength. In addition, we introduced a novel QA process based on a reaction between catechol and an amine that can proceed under mild conditions to generate support functionalities for the QA groups and to overcome the disadvantages associated with using low concentration of the VBTMA monomer. In terms of the membrane structure, we selected the pore-filling membrane that contains a thin layer of porous substrate only, and is thus beneficial for maintaining mechanical stability and reducing ionic resistance.17–19 In RED systems, it is known that low membrane resistance is a crucial factor for increasing the power density, in particular as the improved transport of ions helps to generate electrons at both electrode surfaces via a redox reaction with the rinse solution.19 In addition, we hypothesized that for the formation of new QA groups, bonding between a catechol and amine would be possible via a Michael addition in an alkaline environment. Catechols and their derivatives are abundant in nature (e.g., fruits and vegetables) and are known to function as antioxidants and chelating agents that can coordinate to metal ions.20 In particular, the amino acid found in the adhesive protein of mussels (known as 3,4-dihydroxyphenyl-l4


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alanine, or DOPA) 21,22 is one of the most famous catechol derivatives. Since catechol groups can form strong chemical and physical interactions, including covalent and hydrogen bonds, they have been used for surface modification in various fields, including those of biomedical devices23-25 and supercapacitors.26 The catechol groups can form a stable coating layer on a surface through self-polymerization, although this reactivity is dependent on pH. Unfortunately, the underlying mechanism responsible for this polymer network remains currently unclear. However, it is known that the presence of catechol groups in a polymer chain allows for further modifications with thiol- and nitrogen-containing compounds.20,27,28 To this end, we synthesized dopamine methacrylamide (DMA)29,30, which easily polymerizes during UV irradiation due to the presence of double bond, and examined the possibility of quaternary amination via Michael addition between a catechol and an amine (Scheme (a)), which can proceed under very mild conditions in order to improve the conventional amination. Finally, we selected a bicyclic diamine, DABCO, as the diamine for the formation of new QA groups via reaction with catechol groups owing to its superior structural stability when compared to the stabilities of alternative trimethyl ammonium groups. Specifically, the cationic head groups typically utilized in AEMs can undergo Hofmann elimination, which is a representative reaction and one of several degradation processes that can occur in the presence of OH– nucleophiles.15

These degradation processes, however, require that β-

hydrogens and nitrogen atoms adopt an anti-periplanar conformation, and, since DABCO contains β-hydrogens within a rigid cage structure, the anti-periplanar conformation is inaccessible, which effectively hampers the Hofmann elimination. In this study, we have demonstrated that the reaction between catechol and diamine moieties (Scheme (b)), although not suitable for the creation of the main ion-conducting units, can yield secondary units capable of supporting the functional groups within the membrane. Consequently, we employed DMA to introduce new functionalization into a membrane in the form of new QA groups. Finally, we investigated the impact of the DMA and anion-exchange monomer concentrations on the properties of the resulting membranes, and subsequently confirmed the potential application of the pore-filling AEM with the best properties in an RED system (Scheme (c)).

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Scheme. (a) Possible outcomes of reaction between catechol and DABCO [20] via Michaeltype addition; (b) synthetic procedure leading to AEM formation, and; (c) schematic structure of the pore-filling membrane based on the reaction between DMA (catechol compound) and DABCO (bicyclic diamine) motifs.

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EXPERIMENTAL SECTION Materials. VBTMA, ethylene glycol diacrylate (EGDA), DABCO, and ethanol were purchased from Sigma–Aldrich (USA). Microporous substrate comprised of polyolefin (porosity: 40 vol.%, average pore diameter: 70 nm, substrate thickness: 22 µm) was purchased from Asahi Kasei (Tokyo, Japan). Potassium hexacyanoferrate(II) and potassium hexacyanoferrate(III), both of EP grade, were used for the rinse solution and purchased from Daejung Chemical (South Korea). All chemicals were used without further purification.

Preparation of pore-filling membrane. For the electrolyte solution, DMA, functioning as a cross-linker with a catechol group, was synthesized following a previously reported method.29 DMA, was dissolved along with the other components (VBTMA as an anion-exchanging functional group, and EGDA) in EtOH (see Table 1 for a summary of the component ratios). The photo-initiator 2-hydroxy-2-methylpropiophenone was also added to the electrolyte solution at a concentration of 1 wt.% with respect to the total weight of electrolytes (VBTMA) and cross-linkers (EGDA and DMA). The porous substrate was immersed in the solution and the resultant electrolyte-impregnated substrate was sandwiched between two fluorine-coated polyethylene terephthalate (PET) films and placed in a UV chamber (with an energy density of 0.9 J/cm2) for 60 min. After UV irradiation, the membrane was washed with deionized water to remove any water-soluble compounds from the surface and dried at room temperature. Table 1. Composition ratios of electrolyte monomer (E) and cross-linker (C). Sample

E:C (w/w)

E:C (mmol/mmol)

E

EGDA

DMA

E

EGDA

DMA

E2C1-DMA0.5

2

0.95

0.05

9.45

5.58

0.23

E2C1-DMA1.0

2

0.9

0.1

9.45

5.29

0.45

E2C1-DMA2.0

2

0.8

0.2

9.45

4.70

0.90

E4C1-DMA0.5

4

0.95

0.05

18.9

5.58

0.23

E6C1-DMA0.5

6

0.95

0.05

28.4

5.58

0.23

Treatment of DABCO solution for catechol–amine bonding 7


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The pore-filled membrane was immersed in a solution DABCO (10 mM) in deionized water (DI) for 10 min, washed with deionized water several times, and dried at room temperature.

Membrane Characterization. The surface and cross-sectional images of the prepared membranes were observed by scanning electron microscopy (SEM, FEI Teneo, USA). For the cross-sectional images, the samples were cut manually while they were frozen in liquid nitrogen. The chemical structures of the prepared membranes were analyzed by Fourier transform infrared (FT-IR) spectroscopy. The FTIR spectra of the membranes were obtained with an FTIR-6300 spectrometer (Jasco, Japan) and the measurements were performed in the attenuated total reflectance (ATR) mode. For each sample, 128 scans were collected with a resolution of 4 cm–1 across the range of 3500–800 cm–1. Thermogravimetric analysis (TGA) was carried out using a PerkinElmer TGA (TGA4000, USA) instrument. The samples were heated in ceramic crucibles from 50 ºC to 700 ºC under a nitrogen atmosphere, at a heating rate of 5 ºC/min.

Electrochemical properties of anion-exchange membranes. The IEC of each membrane, which represents the amount of fixed charges in the membrane per unit weight of the membrane, was measured by titration. The Cl– form of the membrane was converted to the OH– form by immersion in 2-M NaOH for 24 h, and subsequent rinsing with DI water to remove any remaining free OH– groups. In the next step, the membrane was immersed in 3M NaCl for 24 h, displacing the OH– ions in the membrane with Cl– anions. The resultant NaCl solution containing the OH– ions released from the membrane was titrated against a 10mM HCl solution. The IEC values were calculated from the following Eq. (1):

=

×

(1)

Where CHCl is the concentration of HCl solution (mol L–1), VHCl is the volume of HCl solution (L) used, and W dry is the weight (g) of the membrane after drying at 60 °C. The SD was determined from the mass difference between the dried and fully hydrated membranes (Eq. 2). The membrane was dried at 80 ºC for 24 h until a constant weight for the dry material was obtained. The membrane was subsequently immersed in 0.5-M NaCl at room temperature for 24 h before being removed, wiped with a paper towel, and quickly 8


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weighed on a microbalance.

=

(2)

In Eq. (2), Wwet and Wdry are the weight of the water-swollen and the dry membrane, respectively. The fixed charge density (CDfix) was determined from the ratio of the IEC divided by the SD of the membrane, according to the Eq. (3):

=

(3)

Area resistance of the membrane was measured following a DC current method. The membrane was soaked in 0.5-M NaCl for 24 h prior to the measurement in order to ensure the membrane was equilibrated in the NaCl solution. The membrane was fixed to a clip cell with flat platinum electrodes and investigated by impedance spectroscopy (IS) with an LCR meter (Delta United Instruments, DU-6011, Taiwan) at a frequency of 1 kHz in 0.5-M NaCl solution at room temperature. The membrane resistance (R) was calculated by subtracting the electrolyte resistance (Rs) from the equilibrated membrane resistance (Rm) in the electrolyte solution (Eq. 4). The effective membrane area (S) was 0.725 cm2.

R = (Rm - Rs) × S

(4)

The membrane potential (Em) was measured in a two-compartment cell, in which an AEM with an effective area of 4 cm2 was positioned between two electrolyte solutions (NaCl) with concentrations of 0.5 M and 0.017 M. The potential difference across the membrane was obtained using a multimeter connected to an Ag/AgCl reference electrode. The t- was calculated from Eq. (5): "#

'

[ /( $ %&'( )],) = .

(5)

where t- is the anion transport number, R is the gas constant (8.314 J/(mol K)), F is the Faraday constant (96,485 C/mol), T is the absolute temperature (K), and a1 and a2 are the activities of the concentrated and the diluted NaCl solutions (mol/L), respectively. The permselectivity (α) was calculated from the membrane potential (Em) recorded across the membrane while it was in contact with the two electrolyte solutions of different 9


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concentration.31

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The permselectivity of an anion-exchange membrane can be defined in

terms of its transport number, shown in Eq. (6):

α=

2 01 01 2 03

(6)

where t is the transport number obtained from the membrane potential, and subscript M and X refer to counter-ions and co-ions in the membrane, respectively. Superscript m and s refer to the membrane and solution phases, respectively. Each experiment was basically repeated 3 times and data are expressed as means ± standard

deviation.

Composition of RED stack. The RED performance of AEMs prepared in this study was investigated in an RED stack. Figure 1a shows the inner structure of RED stack corresponding to one cell pair. A cell pair consisted of an AEM and a CEM in this study, and we used five cell pairs and installed them between two endplates made of acrylic resins. A mesh type spacer with an open area of 81.3% and thickness of 100 µm (DS Mesh, South Korea) was used to prevent contact between the CEM and the AEM, and to secure a channel for electrolyte flow. A gasket was used of PTFE with a thickness of 100 µm (Tommy Hecco, South Korea) . For

comparison,

we

chose

representative

commercial

ion-exchange

membranes,

Neosepta®AMX and Neosepta®CMX (Astom corp., Tokyo, Japan). The electrodes were made of platinum-clad niobium mesh with a diameter of 50 mm, and the current collectors were comprised of titanium. The effective area corresponding to electrode area was 19.6 cm2. Gasket and spacer for electrode also used the same material. An additional CEM, Neosepta®CMX (Astom corp., Tokyo, Japan) as a shield membrane was installed near the electrode to prevent the transfer of negatively charged iron complexes into the stack. The electrode rinse solution (ERS) was composed of K4Fe(CN)6 (0.05 M) and K3Fe(CN)6 (0.05 M) dissolved in DI water.

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Operation of the RED stack. Figure 1b shows schematic representation of RED operation. Artificial seawater as an electrolyte with high concentration of ions (HC, 0.5M NaCl) and river water as the electrolyte with low concentration (LC, 0.017M NaCl), were used as the feed solutions. The electrolytes (HC, LC) were injected into the RED cell at a flow rate of 50 mL/min by 2 peristaltic pumps, respectively. The flow rate of the rinse solution flowing into the electrode surface was set to 50 mL/min by a peristaltic pump (ColeParmer, Masterflex L/S Digital drive, USA) and it was recirculated. The power density in the 2-electrode system was measured using a linear voltage sweep (LSV) in a potentiostat (IviumStat, The Netherlands), with a sweep rate of 40 mV/sec, although the current obtained from an LSV could include non-faradaic values along with the major faradaic value. Each experiment was basically repeated 3 times and the values of practical Pmax summarized in Table 2 are expressed as mean ± standard deviation.

(a) (b)

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Figure 1. (a) Structure of one cell in RED stack and (b) schematic representation of an RED stack: C is a cation-exchange membrane, A is an anion-exchange membrane, HC is the highconcentration electrolyte (0.5 M NaCl), LC is the low-concentration electrolyte (0.017 M NaCl), and ERS is the electrode rinse solution (0.05M).

RESULTS AND DISCUSSION Characterization of Membrane Structure. The surface morphologies and whole membrane images of the pore-filling membranes are shown in Figure 2. The top and cross-sectional images were observed by SEM. PE is a porous polyolefin substrate used as a support material, and is illustrated in Figures 2(a) and (d). Following membrane fabrication, sparse and rough surface changed to dense, smooth (Figures 2(b) and (e)), as well as transparent, as seen in Figure 2(g). This observation can be attributed to the fact that the transparent polyester polymer was immersed, and subsequently crosslinked into the porous substrate. The thickness of the original PE layer was 23 µm, while the membrane after fabrication displayed a thickness of 25 µm. These results confirm that the thickness remained virtually unchanged as a result of the polishing of the polymer layer coated on the PE surface. Subsequent treatment of the membrane with the diamine solution (10 mM) caused the color to change to transparent yellow. This change could be observed in all samples, irrespective of the concentration of the diamine solution used. The surface morphology and thickness were also maintained after the diamine treatment (Figures 2(c) and (f)).

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Figure 2. Surface and cross-sectional images as observed by SEM; (a, d) porous PE substrate, (b, e) E2C1-DMA0.5 membrane, and (c, f) E2C1-DMA0.5-DABCO membrane. (g) the photograph shows PE, E2C1-DMA0.5 and E2C1-DMA0.5-DABCO (left to right).

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Figure 3. ATR-FTIR spectra of (a) PE substrate; E2C1-DMA0.5 before (b) and after (c) treatment with a diamine solution (DABCO).

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In order to confirm the formation of new bonds following the DABCO treatment, we collected the FT-IR spectra in the ATR mode. Figure 3 shows the spectra of the porous substrate PE, membrane E2C1-DMA0.5, and the membrane treated with DABCO solutions of different concentrations. The intense peaks at 2846 and 2913 cm–1 are characteristic of the symmetric and asymmetric stretching of aliphatic –CH2 and –CH3 groups.32 Compared with PE, the membranes exhibited several new peaks. For example, the peaks at 3380 and 1711 cm–1 correspond to the stretching vibration of the O–H bond and the stretching vibration of the –C=O bond, respectively. 33 The phenyl groups corresponding to VBTMA appeared at 1463 and 1613 cm–1,34 whilst the broad peaks observed in the range from 1374 to 1394 cm–1, and at 1337 cm-1, could be ascribed to the secondary and tertiary aromatic amine (–C–N) stretching vibrations.35 These new peaks are diagnostic of the formation of new bonds between the catechol and the diamine.

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Figure 4. (a) TGA curves and (b) DTG of E2C1 membranes before and after DABCO treatment.

Thermal stability of the membranes was confirmed by thermo-gravimetric measurements under a nitrogen atmosphere. Figures 4(a) and (b) show the TGA and differential thermogravimetry (DTG) curves of the AEMs before and after DABCO treatment. In the case of PE, the weight loss occurred in a temperature range from 400 to 500 °C, and was associated with the degradation of the main chain. While the PE porous substrate appeared to undergo one-step degradation, a three-step degradation profile was observed for the AEM (E2C1-DMA0.5) containing trimethylammonium groups derived from VBTMA. The first weight loss stage was observed at temperatures above 200 °C, and corresponded to the degradation of the quaternary ammonium groups; the total weight loss during this step was about 5%. The second peak, observed at 413 °C, arises from the degradation of hydroxyl groups derived from the catechol moieties in the membrane, and represents a total weight loss of about 78%. The third peak at 465 °C arises from the cleavage of the backbone of the PE substrate, and is responsible for a total weight loss of about 9%. It is known that the degradation of quaternary ammonium groups occurs typically in the temperature range from 130 to 350 °C. In the present work, the DABCO-treated membrane started to degrade at a temperature of 150 °C, and this new peak corroborates the formation 16


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of new chemical bonds as a result of the diamine treatment.36 After the diamine treatment, the weight decrease observed in the region around 168 °C represented a weight loss of about 1%. Moreover, the weight loss around 230 °C increased to around 6.5%. In summary, the amount of electrolyte added to the PE porous substrate was 79.8% before the diamine treatment, and 87.5% (at maximum) after the diamine treatment. These results confirm that 8% of the ammonium groups were newly formed during the diamine treatment. Based on these results, we can conclude that the thermal stability of the membrane was maintained, and therefore, this procedure can be considered as suitable for application in an RED system that can be operated under mild process conditions. To compare the effect of the DMA concentration on the ability to bind DABCO more specifically, we summarized the DTG results in Figure 4(b) and determined the differences before and after the DABCO treatment based on the appearance of two new peaks. These new peaks appeared at 168 °C and 208 °C, were well resolved, and corresponded to quaternary ammonium groups formed by the reaction of catechols with DABCO and VBTMA, respectively. We assumed that an increase in the amount of DMA would result in an increase in the number of DABCO binding sites. However, the changes in the electrolyte content before and after DABCO treatment were highest in the DMA0.5 sample. That is, 5% before DABCO treatment (DMA0.5) and 8% after DABCO treatment (DMA0.5-DABCO), showing a 3% increase in the content of anion-exchange functional groups. This result indicates that DABCO’s bicyclic structure may not easily bind the catechol present in DMA as a result of steric hindrance. From the FT-IR and TGA results, we could confirm that reaction between DMA and DABCO resulted in the formation of new QA groups in the AEMs with pore-filling structures; although, it should be noted that it was not confirmed whether DABCO functioned as a cross-linker between DMA and DMA via Michael addition or not.

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The Effect of VBTMA Concentration.

Figure 5. Area resistance (a) and permselectivity (b) relative to the ratio of VBTMA to crosslinker determined for membranes before and after treatment with DABCO solution. The 18


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ratio of DMA was constant at 0.5. Figure 5 details the results of area resistance and permselectivity determined for the membranes synthesized in the present study in order to compare the effect of VBTMA concentration and DABCO treatment. Figure 5(a) indicates that the higher the amount of VBTMA, the lower the area resistance. The same decreasing trend was observed after the DABCO treatment. Specifically, before the DABCO treatment, the area resistance of E2Cl, E4C1, and E6Cl was 0.80, 0.76, and 0.63 Ω·cm2, respectively, and after DABCO treatment, the values decreased to 0.754, 0.498, and 0.44 Ω·cm2, respectively. All values of area resistance were less than 1 Ω·cm2, while the minimum value recorded was 0.44 Ω·cm2 for E6C1-DMA0.5-DABCO. This observation can be explained by the hydrophilic properties of the membrane arising from the presence of the quaternary ammonium groups, which form short pathways for ions in the membrane and reduce consequently the area resistance.14 However, in case of permselectivity, the values corresponding to E4C1 and E6C1 were slightly lower when compared to that of the E2Cl sample. Since the ratios of the cross-linker relative to the monomer in the case of E4C1 and E6C1 were relatively low in comparison to the E2C1 sample, it is possible that the number of cross-linking molecules present was not adequate to allow the formation of a sufficient network between VBTMA and EGDA, and thus, the cross-linking density was low. The results suggested that the amount of VBTMA was solely responsible for decreasing the area resistance, and not the diamine content. In the case of E2C1, the DABCO-treated membrane displayed the highest permselectivity among the samples; by contrast, the permselectivities of E4C1 and E6C1 were not influenced by the DABCO treatment. The permselectivity values were 88.6% (E2C1), 91% (E4C1), and 88% (E6C1) before DABCO treatment, and 94.0%, 90.1%, and 87.8% after DABCO treatment, respectively. Unlike in the case of area resistance, the permselectivity decreased as the number of QA groups increased. This trend can be understood by the fact that an increase in the content of quaternary ammonium groups lead to an increase in the swelling degree of the membrane, thereby decreasing its ion selectivity. The Effect of the DMA to DABCO Ratio. In the case of a bare membrane prior to DABCO treatment, the area resistance decreased according to a corresponding increase in its content of ion-conducting groups such as VBTMA (Figure 5a); however, this change was independent of an increase in DMA ratio (Figure 6). Additionally, Figure 6 also shows the 19


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changes in area resistance and permselectivity observed as a function of the DMA ratio: at lower values of DMA (0.5 wt%) before and after the DABCO treatment, the area resistance values were 0.80 and 0.75 (Ω·cm2), and permselectivity values were 87.9% and 94%, respectively. At higher DMA concentration (1.0 and 2.0 wt%), the DABCO treatment had little effect on both the area resistance and permselectivity. For this reason, the DMA ratio of 0.5 wt% was confirmed to be the most suitable for facilitating the introduction of new QA groups. The results reported by Nijmeijer37 and Komkova38 described similar trends for the amination of chloromethyl groups when the amount of DABCO was limited. It is possible that the limitation imposed by the DMA ratio affects its ability to interact with DABCO is caused by steric hindrance.

Figure 6. Area resistance (square) and permselectivity (circle) observed as a function of the concentration of DMA in the composition of E2C1, before (white) and after (dark) DABCO treatment.

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Figure 7. IEC (square) and CDfix (circle) of E2C1 membranes determined as a function of DMA concentration before (white) and after (dark) DABCO treatment. The IEC values depend directly on the amount of quaternary ammonium groups incorporated in the polymer, and thus, they are indicative of the actual ion-exchange sites available for ionic conduction. Figure 7 shows the IEC and fixed-charge density (CDfix) values observed for the E2C1 membrane as a function of the concentration of DMA, both before and after the DABCO treatment. The IEC value obtained for E2C1-DMA0.5 after DABCO treatment (1.39 meq gdry–1) was much higher than that obtained for the before DABCO treatment (1.24 meq gdry–1). This difference can be rationalized by the increase in the number of charged groups (QA) incorporated in the membrane. However, as shown in Figure 7, further increase in the amount of DMA did not produce a further increase in the value of IEC through its reaction with DABCO. Rather, at high DMA concentrations (above 1.0 wt%), the IEC values remained almost unchanged. Specifically, in case of DMA1.0, the IEC values were 1.37 meq gdry–1 (before DABCO treatment) and 1.26 meq gdry–1 (after DABCO treatment). Similarly, in the case of DMA2.0, the IEC values were 1.36 meq gdry–1 and 1.28 meq gdry–1 before and after the DABCO treatment, respectively. It is well known that charge density plays an important role in determining permselectivity and area resistance. To date, a large number of studies related to RED systems have also tried to elucidate the relationship between IEC and the degree of swelling.8,9 To this end, the 21


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studies have employed the term fixed-charge density (CDfix), which is used to describe the amount of fixed ionic charges in the membrane per weight of its absorbed water. Based on the values of CDfix shown in Figure 7, which reflects the swelling degree, it is apparent that the values did not differ from the trend observed for the IEC values. That is, while E2C1DMA0.5 showed a dramatic change from 5.9 to 10.7 meq gwet–1 as a result of the DABCO treatment, both E2C1-DMA1.0 and DMA2.0 remained almost unchanged. In general, a membrane with a high IEC would undergo full swelling rapidly as a result of high water uptake mediated by the increase in hydrophilicity.37 However, the porous substrates used in this study have high mechanical strength, and consequently, act to increase the density of the electrolyte in the pore network. Therefore, the beneficial characteristics of the pore-filling membrane are demonstrated most accurately by the sample E2C1-DMA0.5-DABCO. Taking the results shown in Figures 6 and 7 together, it is apparent that in order to increase the number of QA groups, a DMA concentration of 0.5 wt% is optimal. Moreover, the present results confirmed that the fixed charge inherent to an ion-exchange membrane can be controlled by a post-treatment using diamine solution, even if only a small amount of the ionexchange monomer (such as VBTMA) was used. Theoretical and Experimental RED Power Density. Herein, the AEMS that possess characteristics advantageous to RED systems are compared to commercially available systems by calculating their theoretical power densities. The maximum power density (Pmax) of an RED unit system can be calculated from individual membrane parameters and properties of model solutions corresponding to river and seawater. In this study, in order to calculate Pmax we used Eq. 7, as suggested by Dlugolecki [8] et al. @AB =

[0 C

) "# %& (BD ⁄B )] #

.(F' ,G ⁄HD ,G ⁄H )

(7)

In this equation, t- is the transport number of anions calculated by equation (1); ac and ad are the mean ionic activities of model sea and river water solutions, respectively; Raem is the ionic resistance of the anion-exchange membrane; d is the spacer thickness between the membranes (0.2 mm), and; kc and kd are the specific ionic conductivities of the model solutions. To compare the characteristics of E2C1 membranes and AMX, the theoretical power densities were calculated by Eq. (7). It was also assumed that the cation exchange membrane was ideal, i.e. the values of area resistance and permselectivity were 0 and 1, 22


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respectively, for the calculation of power density. Area resistance and the transport number are the values determined experimentally by the methods mentioned previously. The Pmax values calculated using Eq. (7) are given in Table 2, where each specific Pmax depends on the transport number, area resistance, and spacer thickness. 39 The specific theoretical Pmax can be obtained up to values of 8 W/m2, if thin spacers with a thickness of 0.1 mm are used. The calculations show that the low-resistance and high-permselectivity sample E2C1-DMA0.5-DABCO yielded 33% and 61% higher power than that obtained with the commercial membrane (Neosepta®AMX), when spacer thicknesses of 0.2 and 0.1 mm, respectively, were used. According to the report by Kim et al., 14

the power density in a RED system is mainly influenced by the IEC and membrane

thickness. Although the thickness of each membrane is the same (25 µm), the E2C1DMA0.5-DABCO sample, with the highest IEC as well as the highest CDfix, demonstrated the highest power density. Figure 8 provides a comparison of practical RED performances of the membranes prepared in this study with and without DABCO treatment and a commercial system in terms of their gross power density curves. In this experiment, we used ERS without a supporting electrolyte such as Na2SO3 and NaCl although it plays a role in maintaining membrane potential difference as well as ionic strength. It was confirmed that there is not significant difference between the cases with and without the supporting electrolyte in the ERS when there are not many cell pairs in the RED stack, as in this study (data not shown). The highest power density of 1.52 W/m2 was observed when 5 pairs of E2Cl-DMA0.5-DABCO and Neosepta®CMX were used; however, the value of power density differed significantly from the theoretical value. It is thought that the internal resistance in the RED stack cell is not correctly reflected when the theoretical value is calculated. Generally speaking, the internal resistance is composed of the ohmic resistance, boundary layer resistance, and bulk layer resistance. 8,14 The ohmic resistance corresponds to the area resistance that is influenced by the material properties of the ion-exchange membrane. By contrast, the boundary layer resistance and bulk layer resistance are non-ohmic resistances, which means that they are strongly influenced by changes in electrolyte concentration inside the RED cell. As shown in Eq. (7), although the value of ohmic resistance is included in the calculation of power density, the non-ohmic resistances are not included. Although there were big differences between the 23


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theoretical and practical values, the results confirmed that the order of power density remained the same (Table 2), that is, E2C1-DMA0.5 > E2C1-DMA1.0 > E2C1-DMA2.0. Table 2. Membrane parameters and calculated specific power densities of RED units employing membranes before and after DABCO treatment. DABCO treatment ®

Neosepta AMX Samples

BEFORE

(commercial membrane)*

AFTER

E2C1-

E2C1-

E2C1-

E2C1-

E2C1-

E2C1-

DMA0.5

DMA1.0

DMA2.0

DMA0.5

DMA1.0

DMA2.0

Area resistance

2.35

0.809

0.801

0.722

0.754

0.764

0.717

(Ωcm2)

±0.53

±0.036

±0.048

±0.016

±0.036

±0.027

±0.009

Permselectivity

90.7

87.9

91.7

91.5

93.8

93.0

91.4

(%)

±2.0

±2.0

±0.8

±0.7

±0.4

±0.4

±0.6

3.25

3.94

4.07

4.08

4.31

4.28

4.21

1.289

1.420

1.335

1.428

1.524

1.485

1.426

±0.007

±0.014

±0.008

±0.002

±0.003

±0.001

±0.001

Calculated Pmax** (W/m2) Practical Pmax 2

(W/m )

* The area resistance and permselectivity of AMX were measured under the same conditions as for the fabricated membrane. ** The values of Pmax were calculated under the assumption that a 0.2 mm-thick spacer was used.

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Figure 8. Comparison of RED performance of anion-exchange membranes and commercial membrane (Neosepta®AMX). Neosepta®CMX employed a cation-exchange membranes.

From these results, it was apparent that the use of DABCO treatment following membrane fabrication enhanced the ion selectivity and area resistance of the membranes. Moreover, it was found that a lower content of DMA was more beneficial in terms of its reaction with DABCO than a higher content. Therefore, these AEMS based on pore-filling structures and featuring DABCO-bonded to catechol unit in DMA show promising performance as materials for use in future RED systems.

CONCLUSIONS In the present work, we reported the fabrication of tailor-made AEMs, which contain QA groups that are formed by the reaction of catechol moieties and DABCO. The AEMs were prepared by pore filling of a porous PE substrate with electrolyte monomers; whilst the formation of new QA groups was induced by immersion in a DABCO solution. The area resistance, permselectivity, swelling degree, and ion-exchange capacity of these membranes were investigated with respect to the DMA concentration, VBTMA concentration, and also, before and after DABCO treatment. We confirmed that the higher the DMA ratio or the VBTMA ratio was, the lower the area resistances were; however, the permselectivities did not depend on the DMA ratio. Comparing with before and after DABCO treatment, the case of E2C1-D0.5-DAB particularly showed dramatic changes. That is, the area resistance decreased to 0.754 Ω·cm2 from 0.809 Ω·cm2, while the permselectivity increased to 93.8 % from 87.9 % due to the reaction of DMA-DABCO. Based on these results, we confirmed that the introduction of new QA groups was realized under mild conditions via Michael-type addition. Assuming that a spacer with 0.1 mm thickness was used, the theoretical RED power density (4.31 W/m2) of E2C1-D0.5-DAB was calculated to be 33 % higher than that of a commercial membrane (AMX) (3.25 W/m2). In the practical RED test, we obtained a power density of 1.524 W/m2 for the combination of Neosepta® CMX and AEM (E2C1-DMA0.5DAB), and this value was 18 % higher than the combination of Neosepta®CMX and Neosepta®AMX (1.289 W/m2) as well. These results demonstrate that the electrochemical properties of AEMs can be controlled by altering the concentration of DMA and by treatment 25


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with DABCO solution. In summary, this study illustrates the significant potential of the new design of ion-exchange membranes for RED system.

ACKNOWLEDGEMENT This research was supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation (NRF) funded by the Ministry of Science, ICT & Future Planning (2017M1A2A2047366, B7-6630).

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Graphical abstract

We successfully fabricated pore-filled anion-exchange membranes, which exploit the reaction between catechol and DABCO molecules for the formation of new quaternary ammonium groups as anion-exchange functional groups; and we demonstrated significantly higher power density (1.524 W/m2) when the membrane was incorporated in a reverse electrodialysis stack than that obtained for a control group (1.29 W/m2).

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Fabrication of an Anion-Exchange Membrane by ... - ACS Publications

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