Journal of Membrane Science 469 (2014) 478–487
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Stable and efficient composite anion-exchange membranes based on silica modified poly(ethyleneimine)–poly(vinyl alcohol) for electrodialysis Ravi P. Pandey a,b, Amit K. Thakur a, Vinod K. Shahi a,b,n a Electro-Membrane Processes Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific & Industrial Research (CSIR), Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India b Academy of Scientific and Innovative Research, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific & Industrial Research (CSIR), Bhavnagar 364002, Gujarat, India
art ic l e i nf o
a b s t r a c t
Article history: Received 6 February 2014 Received in revised form 21 June 2014 Accepted 23 June 2014 Available online 15 July 2014
Anion exchange membranes (AEMs) have found numerous electrochemical applications because of their good conductivity and permselectivity. Herein, we are reporting a method to prepare silica modified poly (ethyleneimine) (SMPEI) with 3-Glycidoxypropyl-trimethoxysilan (GPTMS) by epoxide ring opening reaction. Stable AEMs of different compositions were prepared with SMPEI and a plasticizer poly(vinyl alcohol) (PVA) by acid catalyzed sol–gel followed by formal cross-linking. The reported method is simple and a green alternative for the preparation of AEM without the use of hazardous chemicals. Suitability of prepared AEMs for electrodialytic application was assessed by analyzing their physicochemical properties, stabilities under operating conditions, conductivity, electro-osmotic and chronopotentiometry studies. A highly suitable membrane, SMPEI/PVA-40, exhibited 55.32 mS cm 1 conductivity (in equilibration with 0.04 N NaCl solution at 30 1C), ion-exchange capacity (1.31 meq g 1) and permselectivity (0.79) with good electrodialytic performance. Preparation protocols and properties of the reported composite AEM represent a promising starting point for architecting highly conducting and stable AEMs, but we have to study the trade-off between properties, stabilities and electro-osmotic mass drag before its commercial exploitation. & 2014 Elsevier B.V. All rights reserved.
Keywords: Anion exchange membrane Chronopotentiometry Permselectivity Membrane conductivity Electrodialysis
1. Introduction Positively charged quaternary ammonium tethered polymers have found numerous successful applications in fuel cells, flow batteries, water electrolyzers, electrodialysis (ED), and reverse electrodialysis [1–8]. Current state of knowledge, concerning structure–morphology– performance relationships of anion exchange membranes (AEMs), Abbreviations: AEM, anion exchange membrane; IEM, ion exchange membrane; SMPEI, silica modified poly(ethyleneimine); ED, electrodialysis; CMME, chloromethyl methyl ether; DC, diluted comportment; CC, concentrated comportment; PEM, proton exchange membrane; IEC, ion exchange capacity; CEM, cation exchange membrane; PVA, poly(vinyl alcohol); TGA, thermo-gravimetrical analysis; φw, volume fraction of water in the membrane matrix; χm, surface charge concentration; I, applied current density across the IEM; τ, transition time; Ps, membrane permselectivity; κm, ionic conductivity; A, surface area of membrane; DSC, differential scanning calorimetry; DMA, dynamic mechanical analyzer; SEM, scanning electron microscopy; E, energy consumption; CE, current efficiency n Corresponding author at: Electro-Membrane Processes Division, CSIR-Central Salt and Marine Chemicals Research Institute (CSIR-CSMCRI), Council of Scientific & Industrial Research (CSIR), Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India. Fax: þ 91 0278 2566970. E-mail addresses:
[email protected],
[email protected] (V.K. Shahi). http://dx.doi.org/10.1016/j.memsci.2014.06.046 0376-7388/& 2014 Elsevier B.V. All rights reserved.
is much less in comparison with that about proton exchange membranes (PEMs) [9,10]. Several AEMs based on different polymers, such as poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), copolymer of chloromethylstyrene and divinylbenzene, PVDF-vinylbenzyl chloride, and poly(vinyl alcohol)-poly(1,3-diethyl-1-1-vinyl imidazolium bromide), are reported in the literature [11–15]. These AEMs were prepared via chloromethylation of polysulfone, poly(ether imide), and Cardo poly(ether sulfone)-based polymers using chloromethyl methyl ether (CMME) followed by amination with a tertiary amine [16,17]. In addition, different cationic groups such as imidazolium [18], phosphonium [19], guanidinium [20], sulfonium [21], and pyridinium moieties [22] were also screened for developing stable and efficient AEM. In general, preparation procedures of AEMs, especially chloromethylation and quaternary amination, are complicated and require potentially harmful chemicals such as chloromethyl styrene, different amines and pyridine, etc. [7,23,24] Thus, usage of an expensive and hazardous chemical dramatically increases the manufacturing cost of the AEM. However, the promising strategies for preparing efficient AEM, effect of hydrophobic and ionic clusters on membrane conductivity, and different experimental protocols to assess the suitability of AEM for electro-driven separation technologies are not yet clear.
R.P. Pandey et al. / Journal of Membrane Science 469 (2014) 478–487
Hydrophobic and ionic clustering in membrane morphology is a key factor for enhanced ion conductivity, which was validated in the case of cation-exchange membranes such as Nafion and sulphonated polymers [25]. Ionic clustering contributes to the formation of hydrophilic channels for the fast diffusion of water and conduction of ions through the membrane, while hydrophobic clusters contribute toward phase separation and stabilities of ion-exchange membranes [10,26]. To facilitate the formation of ionic and hydrophobic clusters, different strategies such as grafting of polymer chain, usages functionalized monomer or block copolymers were adopted [3,7,27]. To solve these problems, attention was rendered for developing organic–inorganic composite membrane forming materials because they combine attractive properties or organics and inorganics [7,27–29]. Therefore, to design stable and efficient membranes by eco-friendly preparation protocols with suitable hydrophobic and ionic clustering responsible for good conductivity is extremely important for next-generation materials.
479
For developing high performance AEMs with superior morphology, conductivity, stability and scalable synthetic steps, herein we disclose a novel route for producing silica-modified poly (ethylenimine) (SMPEI) and membrane preparation methodology by acid catalyzed sol–gel using poly(vinyl alcohol) (PVA) as a plasticizer. Detailed physicochemical and electrochemical properties of SMPEI/PVA membranes were analyzed to assess their suitability for ED.
2. Experimental 2.1. Materials Poly(ethyleneimine) (PEI, 50 wt% solution in water), and 3-Glycidoxypropyl-trimethoxysilan (GPTMS) were obtained from Sigma Aldrich Chemicals. Poly(vinyl alcohol) (PVA, Mw: 125,000; degree of polymerization: 1700, degree of hydrolysis: 88%), methyl
Scheme 1. Schematic reaction route for the preparation of SMPEI/PVA composite AEMs.
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iodide, methanol, dimethyl sulphoxide (DMSO) formaldehyde, Na2SO4, H2SO4, HCl, NaOH, and NaCl etc., of AR grade, were obtained from SD fine chemicals and used without further purification. For all the experiments, double distilled water was used. 2.2. Membrane preparation PEI was modified by GPTMS through epoxide ring opening reaction. In a typical reaction, PEI (7.75 mmol) and GPTMS (4.23 mmol) were added to a single-necked round-bottomed flask containing 10 ml DMSO and stirred for 6 h at 80 1C to obtain a transparent solution of silica-modified PEI (SMPEI) [30]. The structure of SMPEI was assessed by FTIR spectrum. For the preparation of SMPEI/PVA composite membrane, 10 wt% of PVA solution was prepared in water and the desired quantity of SMPEI was added under stirred conditions for 12 h at room temperature. The obtained clear solution was transformed into gel by the sol– gel process under acidic medium (pH: 2.0). The highly viscous gel was then transformed into thin film on a cleaned glass plate using a knife of desired thickness and dried in a vacuum oven at 60 1C (24 h). Chemical cross-linking of completely dried thin film was achieved by its treatment with HCHO þH2SO4 mixture solution at 60 1C for 3 h in closed vessels. Cross-linked membranes were placed in 10% CH3I solution in methanol at room temperature for 24 h for quaternization (Scheme 1). The obtained quaternized membranes were equilibrated with acid and base to eliminate any type of impurities present in the matrix, and the conditioned membranes were stored in deionized water. These membranes were designated as SMPEI/PVA-X, where X refers to wt% of SMPEI (varying between 20% and 40%). 2.3. Instrumental and physicochemical characterization of membranes Detailed instrumental and physicochemical characterization of SMPEI/PVA membranes has been included in Section S1, ESI. Membrane water uptake and ion-exchange capacity values were used with advantage for the estimation of membrane surface charge density (χm) (Section S2, ESI).
30 (EcoChemie, B.V. Utrecht, The Netherlands)). Electrolyte solutions of known concentrations (50 cm3) were continuously recirculated in two compartments with the help of peristaltic pumps. A constant current density was applied through electrodes, and saturated calomel electrodes in each compartment were used to measure the variation in potential difference with time across the ion exchange membrane under static conditions. The solutions of both the compartments were vigorously recirculated between two successive experiments to ensure the return of equilibrium conditions in two solution–membrane interfacial zones. Ionic conductivity of SMPEI/PVA membranes was measured in equilibration with NaCl solutions of different concentrations using potentiostat/galvanostat frequency response analyzer (Auto Lab, model PGSTAT 30). The membranes were sandwiched between two 4 cm2 stainless steel circular electrodes. Direct current (dc) and sinusoidal alternating currents (ac) were supplied to the respective electrodes for recording the frequency at a scanning rate of 1 μA/s within a frequency range of 106 to 1 Hz. The membrane resistances (R) were obtained from Nyquist plots. The ionic conductivity (κm) was calculated by
κ m ðS=cmÞ ¼
L ðcmÞ ½R ðΩÞ A ðcm2 Þ
ð1Þ
where L is the distance between the electrodes used to measure the potential and A is the surface area of the membrane. A two-compartment membrane cell (20.0 cm2 effective membrane area) was employed for measuring the electro-osmotic permeability of composite membranes in equilibration with 0.01 M NaCl solution. Both the compartments were kept under constantly stirred conditions. A known potential was applied across the membrane using Ag/AgCl electrodes, and subsequently volume flux was measured by observing the movement of the liquid in a horizontally fixed capillary tube of known radius. The current of the system was recorded with the help of a digital multimeter. Several experiments were performed to obtain reproducible values.
2.6. Electrodialytic performance of composite AEMs 2.4. Oxidative and hydrolytic stability The oxidative stability of the reported membrane was examined in Fenton's reagent (3% H2O2 aqueous solution containing 2 ppm FeSO4) at 80 1C for 8 h. For hydrolytic stability test, a small piece of membrane was boiled in water for 24 h at 140 1C in a pressurized closed vial. Nucleophilic stability of AEMs was examined in 4 M NaOH solution at 80 1C for 2 h. Oxidative, nucleophilic and hydrolytic stabilities of the composite membranes were evaluated by measuring their weight, IEC and conductivity loss after treatment. Oxidative, hydrolytic and alkaline stability of commercial AEM (obtained from Hangzhou Iontech Environmental Technology Co., Ltd., China, IONSEP™ low water permeation special separation membrane model CN standard (CNS)) was also measured for comparison. 2.5. Chronopotentiometric, ionic conductivity and electro-osmotic permeability studies The chronopotentiometric responses for SMPEI/PVA membranes were recorded in equilibrium with NaCl solutions Perspex cell as reported earlier [31]. The cell contained two compartments separated by the ion exchange membrane (25.0 cm2). A constant current was applied across the membrane using two dimensionally stable titanium electrodes coated with precious metal oxide, with the help of potentiostat/galvanostat (Auto Lab, Model PGSTAT
Electrodialytic performance for water desalination of SMPEI/ PVA membranes was assessed by an in-house prepared ED cell [7]. A commercially sourced cation-exchange membrane (CEM) (CMV obtained from SELMION) was used in the ED experiments. An ED cell was packed with six cell pairs of CEM and AEM (SMPEI/PVA membrane) (effective area: 6.6 10 3 m2). The flow pattern of the ED cell was based on parallel-cum-series and it consisted of electrode wash (EW) (cathode and anode), diluted compartment (DC), and concentrated compartment (CC) (Fig. S1, ESI). The feed flow velocity of each stream was kept constant (0.006 m3/h) using peristaltic pumps. Na2SO4 solution (0.02 M) was recirculated in both electrode wash (EW) compartments, separately. Initially, NaCl solution (0.2 M) was fed into the DC and CC in a recirculation mode. Precious metal oxide-coated titanium sheets (TiO2 sheet coated with a triple precious metal oxide (titanium–ruthenium– platinum) of 6.0 μm thickness, and 6.6 10 3 m2 effective area) obtained from Titanium Tantalum Products (TITAN, Chennai, India) were used as the cathode and anode. Constant voltage across the electrodes was applied by DC power supply (Aplab India, model L1285), and the resulting current was recorded with the help of digital multimeter in series. The whole setup was placed at ambient condition (303 K) without any additional temperature control. Changes in conductivities and pH of DC and CC output were regularly monitored by placing the conductivity and pH electrodes in the respective containers during all the experiments.
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481
SMPEI
822
1455
%T
1037
1651
2816 2932
763
3434
4000
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
400
cm-1 Fig. 1. FT-IR spectrum of SMPEI.
3. Results and discussion 3.1. Structural and morphological characterization of SMPEI/PVA composite AEM SMPEI was synthesized via epoxide ring opening reaction, and the FTIR spectrum are presented in Fig. 1. The broad absorption band at 3434 cm 1 arises due to the presence of –OH and –NH groups in SMPEI. Other absorption bands at 2932, 2816, and 1455 cm 1 arise due to the C–H stretch of methylene groups, C–H stretch of –OCH3 groups, and methylene C–H bend, respectively. The absorption bands at 1651 and 1037 cm 1 arise due to the N–H stretch of secondary amine and Si–O–C (organic siloxane) stretch, respectively. SMPEI/PVA composite membranes of varied compositions were prepared by the sol–gel method (Scheme 1). The cross-linked structure of SMPEI/PVA membrane was confirmed by the resonance peaks at 93.86 and 87.58 ppm in 13CNMR spectra (Fig. 2(a)). The resonance peaks at 9.1 ppm were assigned to C–Si bond (–CH2–Si). Quaternization of amino groups were confirmed by resonance peaks at 63.24 and 67.51 ppm due to the carbon of quaternary ammonium groups. Fig. 2(b) illustrates the 29Si-NMR spectra of SMPEI/PVA composite membrane. The resonance peaks at 67.54 ppm are assigned to the tri-functional nature of silica. FTIR spectra of uncross-linked, cross-linked, and quaternized membranes (Fig. 3) showed an absorption band at 3370–3325 cm 1 (–OH stretching vibration). The absorption bands at 2929–2919 cm 1 were observed due to –CH2 stretching vibration, while the weak absorption band at 1468–1441 cm 1 showed deformation and wagging vibration of –OCH2. The crosslinked structure of the membrane was also confirmed by cyclodiether (–C–O–C–) at 1031–1018 cm 1. Chemical cross-linking with formaldehyde is a two-step process: (i) formation of hemiacetal because of reaction between formaldehyde and –OH groups (PVA) and (ii) further in reaction of hemiacetal with another –OH group and formation of acetal group. Cross-linked membranes were quarternized with CH3I in methanol at ambient temperature [32]. The absorption bands at 1102–1096 cm 1 (characteristic Si– O–Si asymmetrical stretching and Si–O–C) confirmed molecular level of hybridization organic and inorganic segments. Under acidic conditions, C–O–C (1367 cm 1) and Si–O–C groups were formed because of a reaction between PVA and formaldehyde (cross-linking agent) or silanol groups. The bands at 1257– 1243 cm 1 were assigned due to the C–N stretching of the tertiary amine, while peaks in the region of 1634–2158 cm 1 and 3238 cm 1 indicate the presence of a quaternary ammonium group and quaternary ammonium salt. Elemental (CHNS) analyses of SMPEI/PVA membranes also confirmed increase in nitrogen content with SMPEI content in the membrane matrix Table 1.
Fig. 2. (a) Solid state 13C-NMR spectrum of SMPEI/PVA-40 composite AEM and (b) solid state 29Si-NMR spectrum of SMPEI/PVA-40 composite AEM.
SEM micrographs (surface and cross-section) of SMPEI/PVA composite membranes revealed their dense and homogeneous nature without cracks, holes, and phase separations (Fig. 4), while EDX data support the membrane composition as observed in the afore-mentioned discussions (Fig. 4(e)). Also, Fig. 4(d) shows the
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2356 2158
2082
(3)SMPEI/PVA-40 3325
3194
1727
2920
1634
1445
1243 1102 1018
923
789
752
3238
(2)SMPEI/PVA-40
%T 3349
1441
3172 2919 2758 2464 3464 3042 2612
1382
1247 1031
836
753
(1)SMPEI/PVA-40 3105 3370
3252
2853
1468
1367
1257 852
2929
752
1092
4000
3600
3200
2800
2400
2000
1800
1600
1400
1200
1050
1000
800 700
cm-1 Fig. 3. FTIR spectra of SMPEI/PVA-40 composite AEM: (1) uncross-linked; (2) cross-linked; and (3) quaternized with CH3I.
Table 1 Composition of SMPEI/PVA composite AEMs. Membrane
SMPEI/PVA-20 SMPEI/PVA-30 SMPEI/PVA-40
SMPEI (%)/wt
20 30 40
PVA (%)/wt
80 70 60
Elemental analysis C (wt%)
H (wt%)
N (wt%)
61.2 56.32 51.68
8.62 9.01 9.86
5.89 7.03 8.61
optical image of composite membranes and reveals their transparent nature. 3.2. Thermal, mechanical, oxidative and hydrolytic stabilities Thermal stability of SMPEI/PVA composite membranes was investigated by TGA curves (Fig. S2, ESI), which shows two-step weight loss.
In the first step, the membrane lost absorbed water, while in the second step, degradation of quaternary ammonium group along with membrane matrix occurred. SMPEI/PVA-40 membrane exhibited comparatively slow decomposition rate due to the formation of silica cluster and left about 32.6% chars. Functionalization at inorganic part (silica) improved thermal stabilities of the membranes, and crosslinking stimulated membrane stabilities. The DSC thermograms for SMPEI/PVA-30 and SMPEI/PVA-40 membranes (Fig. S3, ESI) showed first glass transition (Tg) temperature at 86 and 95 1C, respectively. The Tg values of the composite membranes increased with SMPEI content in the membrane matrix. This may be attributed to restrictive segmental motion of cross-linked polymer chains and ionic interactions within functional groups [33]. The mechanical stability of composite membranes was evaluated via the dynamic mechanical analyzer (Fig. S4, ESI), and good mechanical stability was observed under experimental conditions. SMPEI/PVA-40 membrane exhibited 3135 MPa storage modulus at 30 1C. At low PVA composition
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Fig. 4. SEM images of SMPEI/PVA-40 composite AEM: (a) cross-linked membrane; (b) after methylation; (c) cross-section of methylated membrane (d) optical image (e) EDX data and table for elemental analysis of methylated membrane.
(o60%, w/w), SMPEI–PVA membrane turned brittle and lost the mechanical stability. Here, it is interesting to record that an appreciable content of PVA was required to achieve significant degree of cross-linking and thus a flexible and stable membrane. The effect of silica content on the crystallinity of the composite membrane was investigated by WXRD (Fig. S5, ESI), and typical peaks at 18–251 and confirmed semi-crystalline nature of the membrane. Peak broadness further confirmed homogeneity and compatibility between organic (PVA) and inorganic phases (SMPEI). Oxidative, hydrolytic and alkaline stability for different composite membranes and commercial AEM were assessed in terms of loss in membrane weight, IEC, and conductivity (Table S1, ESI). Composite membranes showed about 9.12–14.96% loss in IEC and 7.08–12.46% loss in membrane conductivity during treatment under strong oxidative conditions. Further, marginal loss in IEC and conductivity under hydrolytic conditions confirmed their stable nature. This was attributed to the cross-linked structure,
while loss in IEC and conductivity slightly enhanced with SMPEI content may be due to an increase in functional group density. 3.3. Physicochemical properties of composite membranes The presence of water molecules plays an important role during ionic transmission across the ion exchange membrane. Water uptake values for the composite membrane have been expressed as volume fraction of water in the membrane phase (φw), and increased with SMPEI content in the membrane matrix (Table 2). This was attributed to the increase in functional group density in the membrane matrix. IEC may be defined in terms of equivalent ionic functional groups per unit of dry membrane weight, and is an important parameter to assess the ionic charge nature of the membrane matrix responsible for conductivity. The IEC values of composite membranes increased with the SMPEI content in the membrane matrix, which may be due to the increase in the concentration of
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Table 2 Volume fraction of water in the membrane matrix (φw), ion-exchange capacity (IEC), surface charge concentration (χm), applied current density across the IEM (I), transition time (τ), membrane permselectivity (Ps), and ionic conductivity (κm) values for SMPEI/PVA composite AEMs. Membrane
SMPEI/PVA-20 SMPEI/PVA-30 SMPEI/PVA-40
φw
0.295 0.344 0.397
Physicochemical properties
Electrochemical properties
IECa (meq g 1)
IECb (meq g 1)
χm (mmoldm 3)
Iτ1/2c (mA cm 2 s)
Psc
κmd (mS cm 1)
0.75 1.04 1.31
0.77 1.16 1.55
0.63 0.88 1.14
7.10 6.78 6.40
0.71 0.75 0.79
42.18 46.99 55.32
a
Measured value. Calculated value. c Estimated from chronopotentimetric curves in equilibration with 0.01 M NaCl solutions. d Measured in 0.04 N NaCl solutions. b
available ionic sites (Table 2). Theoretical values for IEC are also included in Table 2, which are comparable to the experimental IEC values. Further, experimental IEC and water uptake (φw) values increased linearly with SMPEI content in the membrane matrix and may be used with advantage to determine the surface charge concentration (χm) of the membranes in units of (moles of sites)/ (unit volume of wet membrane) (Section S2, ESI). The χm value of composite membranes also increased with SMPEI content in the membrane matrix, which further evidenced the increase in ionizable functional groups (Table 2). 3.4. Membrane ionic conductivity Membrane ionic conductivity (κm) was measured in equilibration with NaCl solutions (0.005–0.04 M), and increased with the molarity of equilibrating NaCl solutions (Fig. 5). Membrane conductivity was attributed to the presence of electrolyte solution in the pore/void of the membrane matrix [34]. At low electrolytic molarity, conductivity increased rapidly, while at high concentration, increment was relatively small. Membrane conductivity depends on the concentration of counter-ions and fixed exchangeable groups in the matrix. Thus, κm values also increased with SMPEI content in the membrane matrix or alternative increase in surface charge concentration values (Table 2). An increase in κm may be attributed to increase in: (i) IEC; (ii) water uptake; and (iii) hydrophilic charged functional group concentration. All these above-mentioned reasons were responsible for the observed variation of membrane conductivity with SMPEI content in the membrane matrix. The effect of pH on membrane conductivity was also investigated in the 2.0–8.0 pH range (Fig. S6, ESI) in equilibration with 0.01 M NaCl solution. κm increased with pH of equilibrating solution and showed maximum dissociation of quaternary ammonium groups in pH range of 6–8. Thus, reported SMPEI membrane assessed to be suitable for electrodialytic application under neutral solutions. Some salient features (IEC, membrane conductivity and burst strength) of commercial membranes are compared with prepared SMPEI/PVA-40 membrane, in Table 3, under similar experimental conditions [35,36]. These data reveal suitability of SMPEI/PVA-40 membrane for different electro-membrane applications.
solutions of concentration Ci, under boundary conditions, at the transition time τ and applied current density I across the IEM, counter-ion transport number (t m i ) may be expressed as [37–39]
3.5. Chronopotentiometric studies
I τ1=2 ¼
Chronopotentiometric study is a rapid technique to observe the variation in concentration of electro-active species with time on membrane–solution interfacial zone. Chronopotentiometric observations may also be employed to estimate the transport parameters of ion exchange membrane (IEM), directly or indirectly associated with the ion transfer process [37,38]. This technique is also useful for studying the concentration polarization properties of an IEM. Consider an IEM separating two identical electrolytic
where D is the diffusion coefficient of the electrolyte, τ is the transition time, and ti is the counter-ion transport number in the solution phase. Diffusion coefficient of the electrolyte (D) at a given electrolyte concentration was estimated by ionic diffusion coefficients D1 and D2 using relationship D¼ [D1D2(z1 z2)/ (z1D1 z2D2)]. The D1 and D2 values used for the estimation of D were obtained from ionic conductance data [40] at a given concentration using the Debye–Huckel–Onsager equation [41].
Fig. 5. Variation of membrane conductivity with concentration of NaCl solutions (0.005–0.04 M).
Table 3 Comparison of SMPEI/PVA-40 membrane with reported AEMs. Membrane SMPEI/PVA-40 AMX AMI-7001 ACS AEM-2 AEM-70 a b
IEC (meq g 1) 1.31 1.2–1.7 1.3 7 0.1 1.2–1.4 1.30 1.18
κm (mS cm 1) a
55.32 06.40b 01.15b 07.50b 02.22 3.45
(Ps)
CE (%)
Reference
0.79 – 0.90 –
95.4 – – – 70 92.6
This study 35 36 35 37 38
0.90
Measured in 0.04 N NaCl. Measured in 0.5 N NaCl.
Z i FðD π Þ1=2 Ci 2ðt m i t i Þ
ð2Þ
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concentration polarization ability. All three prepared membranes exhibited identical inflection with different τ values (transition time) and two potential zones. Iτ1/2 values were fairly constant with applied current density (I) at given electrolyte concentration (Fig. 6(a) (inset)). In electro-driven ionic transport, Iτ1/2 value may be inversely proportional to the ionic mobility or more precisely counter-ion transport numbers across the membrane (t m i ). Furthermore, ion transport phenomena across the composite AEMs are nearly identical in nature with different t m values. i Relatively low Iτ1/2 value for SMPEI/PVA-40 membrane in given concentration of NaCl (Table 2) confirmed its rapid polarization is due to easy transport and greater mobility of the counter-ion in the membrane phase. The t m i values were used with advantage to estimate the membrane permselectivity using the following equation: Ps ¼
tm i ti 1 ti
ð3Þ
The membrane permselectivity (Ps) data in a given electrolyte solution deduced empirically by employing the above equation revealed that composite AEMs behaved as good ion selective. The membrane selectivity increased with SMPEI content in the membrane matrix and SMPEI/PVA-40 membrane exhibited about 0.79 permselectivity (Table 2). The permselectivity arises due to the membrane charged nature and its ability to discriminate between counter-ions and co-ions. Thus, chronopotentiometric studies revealed that among the prepared AEM, SMPEI/PVA-40 membrane appeared to be suitable for electrodialytic applications.
3.6. Electro-osmotic permeability
Fig. 6. Chronopotentiometric curves: (a) for SMPEI/PVA-40 membrane in equilibration with 0.01 M NaCl solution at different current densities, It1/2 vs I (inset); (b) of different membranes at 2.6 mA cm 2 current density in equilibration with 0.01 M NaCl solution.
Knowledge of electro-osmotic drag of mass (solvent) across AEM is essential for intelligent designing of an efficient ionselective membrane. Electro-osmotic flux across the AEM was observed due to: (i) the presence of ionic charged sites in the membrane matrix and (ii) the existence of an electrical potential at the membrane solution interface called zeta potential. Slope of straight lines for electro-osmotic flux vs. coulombs passed (Fig. S7, ESI) was used to estimate β values, which implies that 1.0 C of electricity will exert a drag effect sufficient to carry β cm3 of water across 1.0 cm2 membrane area. Equivalent pore radius (r) for the different composite AEM was estimated with the help of β, using the Katchalsky and Curran approach [42]:
r¼ The chronopotentiometric response of the SMPEI/PVA-40 membrane in contact with 0.01 M solution of NaCl was recorded as a function of applied current density (2.2–2.6 mA cm 2) (Fig. 6(a)). Typical chronopotentiograms showed initial potential jump due to the uncompensated Ohmic resistance, the potential difference reaches a plateau after about 6–8 s (depending upon applied current densities). A subsequent increase in current density altered the potential at the initial stage across the membrane and shifted the inflection point close to y-axis. This results in the quick polarization of the membrane at the commencement of an optimum value. For the ion-exchange membrane, transport property depends on the interfacial character of the membrane and electrolytic environment, viz., concentration and pH of electrolyte solutions in which the membrane is operated. The chronopotentiograms for different composite AEMs in equilibration with 0.01 M NaCl solutions (Fig. 6(b)) were also recorded to assess their electrochemical properties and
8ηF β 0
f 1w
!1=2 ð4Þ
where F denotes the Faraday constant; η denotes the coefficient of viscosity of the permeate. Frictional coefficient between counter0 0 ion and water (f 1w ) may be defined as f 1w ¼ RT=Di (where Di is the diffusion coefficient of the single ion i in the free solution, R is the gas constant, and T is the absolute temperature). The ionic diffusion coefficient (Di) at a given electrolyte concentration was obtained from ionic conductance data. Equivalent pore radius (r) values for different composite membranes (Fig. 7) increased with SMPEI content in the membrane matrix. Incorporation of SMPEI in the membrane matrix enhanced surface charge density and thus equivalent pore radius of the membrane. Increased pore radius may enhance the mass transfer (solvent) by electro-osmotic drag across the membrane. Furthermore, 3.83–4.75 Å equivalent pore radius of composite membranes suggested their quite dense nature and usefulness for electrodialysis.
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Table 4 Electrodialysis (ED) performance for SMPEI/PVA composite AEMs at 4.0 V applied potential (feed solution: 0.2 M, and treated solution: 0.001 M; with 6 cell pairs and membrane area: 6.6 10 3 m2).
4.9
r (Å)
4.7 4.5
Membrane
W (k Whkg 1)
CE (%)
4.3
SMPEI/PVA-20 SMPEI/PVA-30 SMPEI/PVA-40
0.391 0.329 0.308
62.7 79.5 95.4
4.1 3.9 3.7 3.5
15
20
25
30
35
40
45
SMPEI content (%) Fig. 7. Variation of equivalent pore radius (r) with SMPEI content in the membrane phase in equilibration with 0.01 M NaCl solution.
SMPEI/PVA-20 SMPEI/PVA-30 SMPEI/PVA-40
200
Current (A)
0.4
150
0.3
100
0.2 50 0.1
Concentration of DC (mol m-3 )
SMPEI/PVA-20 SMPEI/PVA-30 SMPEI/PVA-40
0.5
0 0.0
0
2
4
6
8
10
12
14
Time x 103 (s) Fig. 8. Variation of current and concentration (dilute compartment (DC)) with time for composite AEMs at constant applied voltage (4.0 V) during ED (feed of DC: 0.2 M NaCl solution).
3.7. ED performance of different composite AEMs for water desalination Water desalination by ED was performed using six cell pairs of composite AEM and commercial CEM (schematic diagram depicted in Fig. S1, ESI) for desalination of NaCl solution (0.2 M). The suitability of developed AEMs for electrodialysis was assessed in terms of estimating current efficiency and energy consumption for the salt removal. During the ED experiments employing different AEMs, current (at 4.0 V constant applied voltages) initially increased with time, and after reaching maxima, it decreased progressively (Fig. 8). Initially, CC offered high electrical resistance because distilled water was used as feed. With the onset of the experiment, Na þ and Cl ions were electro-migrated in the opposite direction from DC to CC, which simultaneously reduced the resistance of CC stream. Consequently, solution concentration of DC progressively reduced (Fig. 8). Among the three composite AEMs, SMPEI/PVA-40 exhibited the highest current under similar experimental condition and rate of concentration depletion for NaCl. Similarly, rate of ionic transport (flux) increased linearly with time (Fig. S8, ESI). This information for electrodialytic desalination ability of composite AEMs confirmed the trend: SMPEI/PVA404 SMPEI/PVA-30 4 SMPEI/PVA-20. Further, suitability of an
AEM for electrodialysis applications may be assessed by fixed charge concentration, IEC, conductivity, permselectivity and flux of ions under ED operating conditions. In addition, energy consumption (W) and current efficiency (CE) values included in Table 4 are also equally important parameters to assess the suitability of the membrane. The method adopted for the estimation of W and CE has been included in (Section S3, ESI). For composite AEM under similar electrodialytic conditions, energy consumption decreased, while CE increased with increase in SMPEI content in the membrane matrix. Incorporation of SMPEI in the membrane matrix contributes toward extent of membrane functionalization and thus permselectivity, conductivity, and fixed charge concentration. These parameters are essential for an efficient AEM. Also, under similar conditions, SMPEI/PVA-40 membrane exhibited 95.4% current efficiency, which showed the potential of the developed AEM for electrochemical devices.
4. Conclusions PEI was modified by GPTMS through epoxide ring opening reaction to synthesize SMPEI and composite AEMs by the sol–gel process in aqueous media using PVA as plasticizer. The reported method for the preparation of composite AEM avoids any solvent residue or hazardous chemicals. The structure of the cross-linked membranes and the presence of quaternary ammonium groups were confirmed by FTIR and NMR. These AEMs were characterized by measuring physicochemical and electrochemical properties to assess their suitability for electrodialytic salt separation. SMPEI/ PVA-40 membrane showed 1.31 meq g 1 IEC and 55.32 mS cm 1 conductivity, 0.79 permselectivity and good chronopotentiometric response in equilibration with NaCl solution. Electro-osmotic study revealed that mass drag across these membranes and their equivalent pore radius increased with SMPEI content in the membrane matrix. Prepared stable and flexible membranes with controllable properties assessed to be a suitable AEM candidate. Electrodialytic studies confirmed efficient nature of these composite AEMs, where rate of ionic electro-transport and energy consumption during the desalination process depended on the SMPEI content in the membrane matrix, under similar experimental conditions. Preparation protocols and properties of reported composite AEM represent promising starting point for architecting highly conducting and stable AEMs. In particular, recent advances in membrane-based separation technologies also encourage the development of a variety of composite anionexchange membranes for diversified applications.
Notes Electronic supplementary information (ESI): Instrumental and physicochemical characterization of membranes, water uptake, ion-exchange capacity (IEC) measurements, estimation of energy consumption (E), current efficiency (CE), schematic diagram of electrodialysis (ED) cell, TGA, DSC, and DMA thermograms of
R.P. Pandey et al. / Journal of Membrane Science 469 (2014) 478–487
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