The Influence of Inorganic Filler Particle Size on Composite Ion

ARTICLE pubs.acs.org/JPCC

The Influence of Inorganic Filler Particle Size on Composite Ion-Exchange Membranes for Desalination Chalida Klaysom,† Seung-Hyeon Moon,‡ Bradley P. Ladewig,†,§ G. Q. Max Lu,*,† and Lianzhou Wang*,† †

ARC Centre of Excellence for Functional Nanomaterials, School of Chemical Engineering and Australian Institute of Bioengineering and Nanotechnology, The University of Queensland, Qld 4072, Australia ‡ Gwangju Institute of Science and Technology, School of Environmental Science and Engineering, Gwangju 500-712, Republic of Korea § Monash University, Department of Chemical Engineering, Vic 3800, Australia

bS Supporting Information ABSTRACT: In this work, we report a new class of organicinorganic nanocomposite ion-exchange membranes containing a sulfonated functionalized polymer (sulfonated polyethersulfone) and sulfonated mesoporous silica (SS). The effect of SS filler size (20 and 100 nm) on membrane structures and properties has been investigated. The results revealed the significant impact of filler sizes on macroscopic properties, such as morphologies, physico-electrochemical performance, and mechanical and thermal stabilities of the resultant composite membranes. The appropriate amount of smaller-sized SS fillers (20 nm diameter) led to better overall properties of the composite membranes with a conductivity up to 2.7 mS cm1, a permselectivity of 98% (around 640% and 16% improvement of conductivity and permselectivity compared with the pristine membranes, respectively), and high thermal and mechanical stability due to intimate polymerinorganic filler interaction. The performance of the composite membranes in the desalination of a NaCl solution was evaluated by a lab-scale electrodialysis cell in comparison with a benchmark membrane, FKE. The results revealed that the optimized composite with 0.5 wt % inorganic SS additive with a smaller particle size (20 nm diameter) exhibited an overall desalination performance comparable to that of FKE. Moreover, the energy consumption of the composite membrane was brought down to nearly a half compared to that of the pristine polymer membrane.

1. INTRODUCTION Ion-exchange membranes (IEMs) have attracted much interest in a variety of applications ranging from the food industry and fuel cells to desalination. The development of new IEMs with good selectivity and high chemical, thermal, and mechanical stability to meet the growing demand of such a versatile application is of significant importance. A number of advanced polymer processes have been developed and applied to the fabrication of new IEMs, including chemical modification of polymer chains and the use of hybrid organicinorganic materials.110 Great effort has been dedicated to developing polymeric membranes with high conductivity and selectivity mainly by chemical modification of the polymer backbone to obtain a high degree of ionexchange capacity.15 However, chemical modification in such a high degree often results in a highly swollen membrane and low mechanical stability.15 Therefore, a fabric support or a certain degree of cross-linkage is required to give the strength to the membranes. Recently, the concept of combining two distinct materials to form composites that retain desirable properties from both components has been intensively studied in the development of membranes, particularly for fuel cell application. For polymer-based composites, the purposes of adding inorganic r 2011 American Chemical Society

additives are mainly to improve thermal, chemical, and mechanical properties of polymer matrix while keeping their flexible and processable properties and chemical functionalities. Many research groups utilize inorganic fillers to improve the water uptake of proton-exchange membranes at high working temperatures.710 The studies revealed that functionalized inorganic fillers could provide extra functional groups for proton exchange and also kept moisture inside the structure to assist proton migration, which consequently improved proton conductivity at high working temperatures in fuel cell applications.7,10 Though the composite concept was also demonstrated to be a promising approach for developing IEMs for various applications, much fewer studies have been reported in important water purification application, such as electrodialysis (ED) desalination.1113 In fact, the ion conductivity of IEMs depends not only on the ion-exchange capacity (IEC) but also on the water content and mobility of the ions, which are closely linked to the structural properties of the membranes.14 Recently, we have developed a group of new Received: December 22, 2010 Revised: June 24, 2011 Published: June 29, 2011 15124

dx.doi.org/10.1021/jp112157z | J. Phys. Chem. C 2011, 115, 15124–15132

The Journal of Physical Chemistry C organicinorganic composite IEMs based on sulfonated polyethersulfone (sPES), a high-performance thermoplastic, and sulfonated mesoporous silica (SS) nanoparticles.15,16 While the polymer (sPES) provides good thermal and mechanical stability, the SS supplies additional functional groups, enhancing IEC, water uptake, conductivity, and selectivity of the composite membranes. It was well known that some key parameters have to be considered when incorporating inorganic fillers in a polymer matrix. Among those parameters, aspect ratio and size of fillers are of considerable importance due to the changes of interfacial properties in the composites.17 In the newly developed sPESSS composite membrane system, we have demonstrated an alternative, easy approach to improve the conductivity and electrochemical properties of the membranes by tuning the membrane structure with surface functionalized inorganic fillers that carry extra ion-exchangeable functional groups. In this work, we further report our new findings on the significant effect of inorganic filler sizes on the structure and performance of composite membranes in desalination application, which has been rarely studied.17 It is evident that an appropriate amount of small-sized SS fillers can result in an overall desalination performance comparable to the commercial membrane FKE. It is expected that this study will lead to a better understanding on how to design new composite membranes with a controllable structure and desirable properties for a niche application.

2. EXPERIMENTAL SECTION 2.1. Preparation of the Sulfonated Mesoporous SiO2. The SS particles with a diameter of 20 nm were prepared by a socalled liquid-phase method with the assistance of a basic amino acid.18 Typically, the solution containing 360 g of DI water, 1.664 g of cetylmethylammonium chloride (C16TMACl, Tokyo Chemical Industry), and 0.209 g of amino acid arginine (Arg, Sigma-Aldrich, 98%) was prepared at room temperature under vigorous stirring, followed by the addition of 2.083 g of tetraethyl orthosilicate (TEOS, Fluka, 98%). The resultant solution was stirred continuously at 60 °C for 24 h. The reaction was then terminated, and the highly dispersed silica was obtained. The silica powders were then collected by solvent evaporation, followed by calcination at 500 °C in air for 10 h to remove the residual reactants. The surface of the as-synthesized silica was functionalized with the sulfonate group via a grafting method.19 More specifically, 0.6 g of calcined silica was dispersed in 100 cm3 of dry toluene. The mixture solution was heated at 110 °C and stirred under reflux. A 0.6 cm3 portion of mercaptopropyl trimethoxysilane (MPTMS, Sigma) was added in the mixture solution, and the reaction continued for 16 h. The grafted powder was collected by centrifugation and washed with acetone several times before it was redispersed in 10 cm3 of H2O2 at room temperature for 48 h. The suspended powders were centrifuged and added into 30 cm3 of 1 mol dm3 H2SO4 under stirring for 2 h. The surface functionalization procedure was completed by filtering the modified particles and drying them at room temperature. The highly ordered mesoporous silica with a size of 100 150 nm was prepared according to our previous studies.9,20 The sulfonated mesoporous silica with two different particle sizes, 20 and 100 nm, was named SS-20 nm and SS-100 nm, respectively. Note that the as-synthesized powders were ground to reduce the agglomeration before being added into the polymer

ARTICLE

matrix. The surface sulfonation of the mesoporous silica led to the decrease of both the specific surface area and the pore volume due to the grafting and pore filling of functional SO3 groups on the inner surface of the mesopores.9 The ion-exchange capacity of the SO3H-functionalized mesoporous SiO2 was measured by back-titration. The structure and morphology of the SS were studied by transmission electron microscopy (TEM, JEOL 1010) and X-ray diffraction (XRD, Rigaku Miniflex, Cu KR, 30 kV and 15 mA). Nitrogen adsorptiondesorption isotherms were conducted at 77 K (Quadrasorb SI). The BET (BrunauerEmmettTeller) specific surface area (ABET) and pore size distribution using the BJH method were calculated from the adsorption and desorption data, respectively. 2.2. Membrane Preparation. The sulfonated polyethersulfone (sPES) was used as the polymer matrix for the composite membranes.20,21 Typically, 25 wt % polymer solutions in dimethylformamide (DMF, Sigma) were first prepared. The SS (02 wt %) was then mixed with the polymer solutions at 60 °C for 4 h under stirring. Sonication with 25% intensity was applied to the mixture solution for 3 min (Branson Digital Sonifier 450, output 400 W) before it was cast onto glass substrates using a doctor blade to control the casting film thickness at ca. 0.40 mm, followed by a two-step phase inversion process, which is a combination process of solvent evaporation and phase immersion technique. The cast film was then dried in a vacuum oven at 60 °C for 10 min and precipitated in a 6070 °C DI water bath. Afterward, the as-prepared membrane sheet with a thickness of 50 ( 10 μm was obtained and kept in DI water. A series of membranes were named referring to the weight percent of SS added in the polymer matrix. For instance, 0.2SS-20 nm is the abbreviation for the composite membranes of sPES containing 0.2 wt % of SS of 20 nm. The resultant membranes were equilibrated in the working solution for at least 6 h before use. 2.3. Membrane Characterization. Morphology and Physicochemical Properties. Scanning electron microscopy (SEM, JEOL 6300) and transmission electron microscopy (TEM, JEOL 1010) were used to observe the morphology and the distribution of inorganic SS fillers in the polymer matrix of the prepared membranes, respectively. Ion-exchange capacities (IECs) were measured by a titration of substituted protons from the sulfonate functional groups of the prepared membranes equilibrated in NaCl solution. The IEC was estimated in terms of equivalent value of the substituted proton per unit weight of dry membranes (mequiv g1). The term “water uptake” refers to the amount of water content per unit weight of dry membranes, while the free volume fraction is the ratio of water volume to the volume of wet membrane equilibrated in water. Electrochemical Properties. The electrochemical properties of the membranes were investigated at room temperature by a twocompartment cell in which each chamber is separated by a membrane sample with an effective area of 1 cm2. The resistance of membranes was measured by impedance spectroscopy (IS, Solarton 225B) in a working solution of 0.5 mol dm3 NaCl with a frequency range of 1106 Hz and an oscillating voltage of 100 mV in amplitude.22 The selectivity of an IEM can be determined by two terms, transport number and permselectivity. The term “transport number” represents the fraction of total current carried by counterions through the IEM, while the term “permselectivity” refers to how easily the counterions migrate through the IEM 15125

dx.doi.org/10.1021/jp112157z |J. Phys. Chem. C 2011, 115, 15124–15132

The Journal of Physical Chemistry C

ARTICLE

Figure 1. Schematic experimental setup of the ED cell.

Figure 2. TEM images of (a) SS-20 nm and (b) SS-100 nm. (c) XRD patterns of the two sulfonated mesoporous silica materials.

compared to the co-ions. The potential difference (Em) across the membrane was measured by a multimeter connecting to Ag/ AgCl reference electrodes immersed in 0.01 mol dm3 NaCl in one compartment and 0.05 mol dm3 NaCl in the other compartment. The transport number, t i, was then calculated by eq 1, and the permselectivity of the membrane was finally estimated according to the resultant t i as defined by eq 2 RT a1 ð2t̅ i 1Þln Em ¼ ð1Þ nF a2

Ps ¼

t̅ i ti 1 ti

ð2Þ

where R is the gas constant, F the Faraday constant, T the absolute temperature, a1 and a2 are the mean activities of electrolyte solutions in the testing cell, n the electrovalence of the counterion (ith), the concentrations of electrolyte solutions in the testing cell, t i the transport number of the counterion in the membrane, and ti is that of the same ion in free solution at the same concentration. The currentvoltage characteristic and chronopotentiometry (by Solartron Multistat 1480) were also used to study the electrochemical behavior of the prepared membranes in 0.025 mol dm3 NaCl.

Thermal and Mechanical Stabilities. Thermal stability of the membranes was investigated using thermogravimetric analysis (TGA) (Mettler Teledo) under a nitrogen flow of 20 cm3 min1 at a heating rate of 10 °C min1. A tensile test by an Instron 5800 was used to measure the mechanical properties of membranes in a wet state at room temperature. 2.4. Desalination by Electrodialysis. The desalination performance of the prepared membranes was investigated using a custom-designed lab-scale ED cell. The cell consists of a fivecompartment chamber made from a Perspex sheet with an O-ring to prevent leakage. Each chamber provides 4 cm2 active areas for membranes. Membranes were arranged into the cell, following the scheme of the ED setup as shown in Figure 1. Specifically, the commercial cation-exchange membranes FKE (FumaTech, Germany) were placed next to the electrode, separating the product solution from the rinsing solution of the electrodes. These electrode rinsing compartments were circulated with 3 wt % Na2SO4 at 30 cm3 min1. The auxiliary commercial anionexchange membrane (FAA, FumaTech) was placed next to the FKE toward the anode side; the investigated membrane was placed next to the one toward the cathode side, creating a compartment of dilute solution in between. The ED was carried out with a constant potential difference at 7 V. The feed solution of NaCl with a starting concentration of 0.2 mol dm3 was circulated at 30 cm3 min1 through each compartment. The salt concentration in each compartment was determined by a conductometer, and the pH of 15126



ARTICLE

the solution from each chamber was measured to check whether the water-splitting phenomena was occurring during the process. The performances of the prepared membranes are compared with the commercial membrane in terms of flux, current efficiency, and energy consumption. All the detailed characterizations and desalination test, including the equations used for the estimation of membrane properties and performance, are provided in our previous studies and the literature.20,23

3. RESULTS AND DISCUSSION 3.1. Property of Inorganic Fillers. Surface functionalized mesoporous silica with diameters of approximately 20 and 100 nm was successfully synthesized (Figure 2a,b). The smallangle XRD pattern of the as-synthesized SS-100 nm showed wellresolved reflections of a 2D hexagonal mesostructure in the particles. The indexes of (100), (110), (200), and (210) confirmed the highly ordered pore structure of the sample. In the case of SS-20 nm, one broad peak at around 1.7° (2θ) of the XRD pattern (Figure 2c) was observed, indicating a less-ordered mesoporous nature of the materials, which is in agreement with the TEM observation. Nitrogen adsorption/desorption isotherms were also conducted to measure the specific surface areas and pore volume of the synthesized particles (Figure S1, Supporting Information). It was apparent that both samples exhibited no characteristic of mesoporosity. This can be attributed to the grafting and pore filling with SO3H groups, which makes gas adsorption measurements very difficult. The details of the BET surface area and pore volume are listed in Table1. Note that a broader pore size distribution between 10 and 25 nm was observed in SS-20 nm, which is likely due to the interparticle

porosity in the sample. It was clear that SS-20 nm has a smaller BET surface area while a larger pore volume, possibly due to its less ordered, but larger pores. Nevertheless, for both mesoporous silicas with different particle sizes, a high degree of surface functionalization with sulfonate groups was successfully obtained (1.58 and 1.80 mequiv g1 for SS-20 nm and SS 100 nm, respectively). 3.2. Morphology of the Membranes and Their Strength. The sulfonated mesoporous silica (SS-20 nm) particles were mixed with the polymer matrix (sPES) to form composite membranes via a so-called two-step phase inversion technique. The phase inversion is a well-known technique for preparing porous membranes through which the structure and porosity of the membranes can be easily adjusted by varying properties of the casting solution. The cross-sectional morphologies of the prepared composites are depicted in Figure 3. It is clearly observed from the SEM images that the porosity of the membrane increased with increasing the amount of SS loading. The inorganic fillers increased the porosity of the membranes via two main ways. First, the addition of inorganic fillers altered the

Table 1. Sizes and Surface Properties of the Synthesized Inorganic Fillers property

SS-20 nm

SS-100 nm

particle size (nm) ABET (m2 g1)

2025 557

100150 942

pore volume (cm3 g1)

1.18

0.51

IEC (mequiv g1)

1.58

1.80

Figure 4. TEM images of composite membranes with (a1, a2) 0.2 wt % SS-100 nm and (b1, b2) 0.2 wt % SS-20 nm inorganic fillers. The arrows indicate the inorganic cluster formation in the polymer matrix.

Figure 3. SEM images of various membranes at different magnifications: (a1, a2) pristine membrane, (b1, b2) 0.2 wt % SS-20 nm, (c1, c2) 0.5 wt % SS20 nm, (d1, d2) 1.0 wt % SS-20 nm, (e1, e2) 2.0 wt % SS-20 nm, and (fi) 0.2, 0.5, 1.0, and 2.0 wt % SS-100 nm, respectively. 15127



ARTICLE

Figure 5. Mechanical and thermal properties of the prepared membranes: (a) tensile stress, (b) tensile strain, (c) Young’s modulus, and (d) TGA curves. Dotted lines in (ac) indicate the average values for pristine IEM without inorganic fillers.

properties of the casting solution, leading to a polymer-lean phase around the inorganic fillers that favors the pore formation in a high exchange rate of solvent/nonsolvent during the phase inversion process. Second, the relatively low organicinorganic adhesion causes the void formation around their interface, which also provides extra porosity to the matrix. At a low degree of inorganic filler loading, the fillers with both smaller and larger sizes can be dispersed well throughout the polymer matrix; however, the smaller filler (SS-20 nm) gave a better polymerfiller interaction and thus resulted in less porosity compared with the bigger filler (SS-100 nm). The distribution and aggregation of inorganic fillers in membranes can be clearly observed by TEM images in Figure 4 (for aggregations, refer to arrows in membranes). Note that, with the same amount of added inorganic fillers, smaller SS-20 nm particles led to smaller pores in the membranes (Figure 4a1,a2 and b1,b2). At a higher loading of smaller SS-20 nm particles, the resultant composites also suffered from an apparent particle agglomeration, which resulted in a larger porous structure (Figure 3d,e, for 1 and 2 wt % of SS-20 nm additives). The mechanical and thermal stability of the membranes were investigated using a tensile test and TGA analysis (Figure 5). As mentioned above, the smaller fillers have a better organicinorganic interaction that can significantly enhance the strength of composite, while the addition of SS-100 nm did not lead to an apparently improved strength (Figure 5a). On the other hand, the presence of inorganic fillers also increased the elastic property of the composites, resulting in a higher elongation capability of the membranes. As a result of the tensile stress and strain changes, the modulus values of composites did not change significantly. All the composite membranes exhibited similar thermal stabilities, which are sufficient for desalination operation (Figure 5d).

3.3. Physicochemical and Electrochemical Properties of Membranes. The physicochemical and electrochemical proper-

ties of the composite membranes containing 20 nm SS fillers were tested and are summarized in Table 2. The properties of the commercial membrane, FKE (containing sulfonated polyether ether ketone (sPEEK)), and the composite with 0.2 wt % SS100 nm (the optimal membranes with large filler sizes) are also included in Table 2 for a comparison. Whereas the IEC slightly declined, the water uptake, conductivity, and selectivity of the membranes were significantly increased. An onset was found at 1.0 wt % loading for the water uptake, volume fraction, and conductivity, and at 0.5 wt % loading for the transport number and permselectivity of the membranes. Though the surface-modified fillers carried extra ion-exchangeable functional groups, the IEC of the composites was not improved as was expected. The slight decrease of the IEC might be attributed to, on one hand, the good interaction between the polymer matrix and inorganic fillers at a low percentage of loading, and on the other hand, the loss of accessible ionexchange functional groups at higher loading is due to inorganic agglomeration formation. However, due to the inherit properties of silica in retaining moisture and the increment of the free volume fraction of the composite enhanced by incorporating SS, the water uptake was improved with increasing the added amount of SS. Therefore, the conductivity of the membranes was also improved consequently. At the onset loading, where the fillers started to agglomerate, the water uptake dropped, possibly due to the reduction of accessible functional groups. The similar trend was found for the ion conductivity of the membranes, since ion migration is promoted by the water channel. It was evident from the 0.2SS-100 nm sample that the water uptake, volume fraction, and the conductivity were higher than those of membranes containing smaller fillers, which may be 15128



ARTICLE

Table 2. Physicochemical and Electrochemical Properties of the Prepared Membranes Compared with a Commercial Membrane, FKE name

water uptake (%)

IEC (mequiv g1)

σ (mS cm1)

transport number

permselectivity (%)

free volume fraction

0 SS

38.67 ( 9.49

0.75 ( 0.02

0.336 ( 0.05

0.90 ( 0.01

83.50

0.433 ( 0.039

0.2SS-20 nm 0.5SS-20 nm

41.81 ( 1.23 77.55 ( 5.01

0.72 ( 0.02 0.69 ( 0.00

0.444 ( 0.07 2.700 ( 0.33

0.99 ( 0.01 0.98 ( 0.04

99.20 96.91

0.439 ( 0.034 0.685 ( 0.030

1.0SS-20 nm

63.53 ( 10.73

0.67 ( 0.05

2.232 ( 1.38

0.96 ( 0.04

94.51

0.661 ( 0.013

2.0SS-20 nm

63.76 ( 3.25

0.65 ( 0.02

2.197 ( 0.96

0.93 ( 0.02

88.50

0.614 ( 0.015 0.740 ( 0.050

0.2SS-100 nm

92.00 ( 10.73

0.83 ( 0.05

5.554 ( 1.00

0.95 ( 0.01

91.81

FKE

48.10 ( 5.00

1.23 ( 0.03

3.834 ( 0.02

0.95 ( 0.01

91.74

NA

Figure 7. Chronopotentiogram (a) and derivative dE/dt (b) of the obtained membranes measured at room temperature in 0.025 mol dm3.

Figure 6. Schematic representative structures of composites: (a) effect of inorganic filler sizes and (b) two-phase model.

associated with the higher porosity of the 0.2SS-100 nm, as described in the previous section. It is well accepted that, besides the IEC and fixed change density of IEMs, the transport number and permselectivity of the IEMs also depend highly on the porosity and structure of the membranes.14,24,25 As mentioned earlier, a higher percentage of inorganic filler loading led to an apparent particle agglomeration, forming large pore sizes and voids, consequently affecting the permselectivity of the membranes. The transport characteristic and behavior of IEMs are normally described by a two-phase model that considers the heterogeneous IEMs’ structure on the microscale consisting of a joint-gel phase (polymer with fixed charged functional groups and their compensated solution combined with an inert phase) and intergel phase (the filling electrolyte in the membrane free volume or pores).2426 It has been reported that the selectivity of IEMs depends on the volume fraction of these two phases. As the volume fraction of

the intergel phase increases, the Donnan exclusion becomes less effective, and consequently, the less selective membranes with lower permselectivity will be obtained (see Figure 6b). The fraction of intergel phase has been proved to have a direct relation with the morphology of the membrane.24 Figure 6 demonstrates the effect of pore size and membrane structure on the selectivity of the membrane in the presence of inorganic fillers using the two-phase model. In the case of the small fillers, better polymerfiller interaction resulted in less interfacial gap, pore size, and porosity, as previously mentioned (see Figure 6a). Thus, lower fraction of the intergel phase and better selectivity properties were expected in comparison with the membrane with bigger fillers. The results from this section revealed the important impact of inorganic filler sizes on membrane structure and porosity. The smaller the fillers, the better organicinorganic interactions, which, in turn, results in better selectivity compared to those composites containing bigger fillers. However, the conductivity of the composite from SS-100 nm was still superior to those composites with SS-20 nm. 3.4. Surface Heterogeneity and Electrochemical Behavior of Membranes. It is known that structure and surface properties of membranes play a vital role in the electrochemical and transport phenomena for IEMs.2730 In this work, the transport behavior and surface heterogeneity of the prepared composite IEMs were investigated using chronopotentiometry. The obtained chronopotentiograms and their characteristic values of the membranes are illustrated in Figure 7 and Table 3, respectively. 15129



ARTICLE

Table 3. Characteristic Values from Chronopotentiograms of the Prepared Membranes and Commercial Membrane, FKE transport number

Eo (volt)

Emax (volt)

ΔE

τa (s)

τb (s)

Δt (s)

ε

0 SS

0.90

3.46

3.97

0.51

32.2

23.5

26

0.855

0.2SS-20 nm

0.99

3.25

3.85

0.60

23.2

21.6

23

0.964

0.5SS-20 nm

0.98

3.22

3.88

0.66

24.0

23.9

24

0.998

1SS-20 nm

0.97

3.19

3.91

0.72

24.8

24.7

24

0.998

2SS-20 nm

0.93

3.15

3.93

0.78

28.7

25.6

25

0.946

FKE

0.95

3.13

3.97

0.85

26.6

26.0

24

0.988

membrane

a

Estimated from eq 1. b Deprived experimentally from chronopotentiogram.

Typical chronopotentiograms consisting of three-step potential changes were obtained. Particularly, once the constant current density was applied through the system, a potential (E0) onset was observed due to the resistance of the membrane and working solution. This initial potential remained constant or gradually increased for a certain period, corresponding to the concentration polarization, because the counterions leave the depleting solution passing through ion-selective membranes and accumulate on the other side of the membrane interface. The schematic diagram discribing concentration polarization phenomena of IEMs is provided in the Supporting Information (Figure S2); revelant information can also be found in the literature.26,3133 When the concentration near the membrane interface of the depleting solution drops near to zero, the potential suddenly increases before reaching the steady value (Emax) at the final stage. The difference between the initial potential and the maximum potential from chronopotentiograms (ΔE) was believed to relate indirectly to the thickness of the concentration gradient developed at the membrane interfacial zone.28 The important characteristic parameter at the sharp increase of potential is called the transition time (τ), which can be defined by the well-known Sand’s equation τ¼

ðC0 zi FÞ2 πD 4i2 ðt̅ i ti Þ2

ð3Þ

where i is the current density, C0 is the concentration of the electrolyte, F is the Faraday constant, and zi is the valence of the ith ion. The deviation between transition times derived experimentally from chronopotentiograms and calculated theoretically by eq 3 can be observed, indicating the existence of a certain proportion of a nonconducting region on the membrane surface. It is worth noting that Sand’s equation was proposed under the assumption of the entire conductive membrane surface. Considering that an IEM consists of conducting and nonconducting regions, the local current density of the conducting regions (i*) will be higher than the superficial current density (i) of the entire surface.27 The current density of conducting regions, i*, can then be defined as a function of the fraction of the conducting region (ε) and i, by the following equation i ¼ i=ε

ð4Þ

Therefore, the fraction of the conducting region can be calculated by substituting eq 4 into eq 3, expressed as derived Sand’s equation as follows: ε¼

2iτ1=2 ðt̅ i ti Þ C0 zi FðπDÞ1=2

ð5Þ

Figure 8. iv curve (a) and derivative dE/di (b) of the obtained membranes measured at room temperature in 0.025 mol dm3.

From the chronopotentiograms, the results clearly showed that inorganic fillers had contributed to the conduction region of the membranes. The surface functionalized inorganic fillers can be considered as an ion-exchange resin. The more it is added, the more homogeneous the membranes are. However, there was a trade-off at a certain loading where the composites suffered from particle agglomeration. In addition, the time taken for the membranes to reach the steady state also reflects the homogeneity of the membranes. Normally, time taken for the membrane to reach steady state increased with increasing surface heterogeneity. This was shown on the shape of the chronopotentiogram, as more homogeneous membranes showed a better defined transition region, indicated by lower Δt values in Figure 8b, where the derivative dE/dt was not equal to zero. The currentpotential curve (iv) is another useful technique for investigating the transport phenomenon and polarization of the membranes. In membrane processes, the parameter that was believed to limit the operating condition and the performance of membranes is concentration polarization.29 In electrodialysis, this limiting point is well known as the limiting current density (LCD), which can be experimentally obtained by the iv curve. The iv curve of the prepared composites is depicted in Figure 8. Typical S-shaped iv curves were obtained. In the first region, the linear relationship between potential and current was observed corresponding to Ohms’ law. The plateau region then occurred due to the concentration polarization near the membrane interface. The potential rises up again after the plateau region, contributed by many phenomena, such as water dissociation, gravitational convection, and electroconvection.33 According to the classical concentration polarization, when the transport number of the membrane increased, the LCD of the 15130



ARTICLE

Table 4. Characteristic Values from iv Curves of the Composite Membranes and Commercial FKE Membrane i (mA cm2)

i* (mA cm2)

ΔE

0 SS

2.19

2.56

1.74

0.2SS-20 nm 0.5SS-20 nm

2.03 2.47

2.10 2.47

1.21 0.80

1SS-20 nm

2.09

2.09

0.53

2SS-20 nm

2.12

2.24

0.53

FKE

2.06

2.08

1.10

membrane

Table 5. Desalination Performance by ED from the Composite Membranes and the Commercial FKE Membrane flux (mol m2 h1)

η

P (kWh kg1 salt)

0SS

4.56

0.56

5.74

0.2SS-20 nm 0.5SS-20 nm

5.61 7.38

0.76 0.98

4.19 2.89

FKE

7.46

0.83

3.92

sample

membrane was expected to be lower, as defined by the given equation. ilim ¼

jzjCFD δðt̅ i ti Þ

ð6Þ

However, as can be noticed from Table 4, the LCD read from the iv curve at the transition of the Ohmic to plateau region did not follow the trend predicted by the classical concentration polarization. This might due to the effect of surface heterogeneity of the composite membranes in the previous discussion. Moreover, it has been reported that the LCD can also be affected by several factors, including the amount of water content. The jump in the LCD value of 0.5 wt % SS-20 nm may be attributed to high water content that brought about an increase in conductivity and diffusion permeability of the membranes.14 In addition, the backward diffusion of the electrolyte solution can also increase the LCD value of the membranes. The plateau length of the iv curve is another important characteristic representing the thickness of the diffusion boundary layer (DBL) and the energy required to destroy the DBL. It was clear that the plateau length depends on the surface heterogeneity of the membranes and the current line distribution. The membrane with more surface heterogeneity has a longer plateau length (ΔE) due to concentration compensation of the low conducting regions that tried to equalize the interfacial concentration among the regions of different conductivities.34 It is worth noting that the same trend of electrochemical behavior for the composites containing SS100 nm fillers was also observed. We can thus draw a clear conclusion from the electrochemical results that a small amount of SS additive not only improved the electrochemical property of the membranes, but also adjusted the surface property of the membranes to be more homogeneous with a higher surface conducting fraction. The peak point was observed at around 0.5 wt % loading. With a higher loading of SS fillers, the properties of the composites became worse due to the significant agglomeration of the fillers. 3.5. NaCl Desalination by ED. The preliminary desalination test by ED was also carried out. Table 5 summarizes the

performance of various membranes. It was clear that the performance of the composite membranes was quite comparable to that of the commercial benchmark FKE membrane. Importantly, a pH change in the desalination process, a signal of water dissociation, was not observed from the prepared membranes. This implies the excellent stability of the composite membrane under the applied potential (see the Supporting Information, Figure S3). In the case of the FKE membrane, a pH change was observed after the experiment commenced for 60 min. This result suggests one of the advantages of the prepared membranes over the commercial one. By adding a small amount of surface functionalized SS, the conductivity and the transport number of the membranes were significantly improved. These two properties have an important influence on the performance of membranes, especially in an electrodriving process, such as ED. As can be clearly seen from Table 5, the current efficiency, flux of salts, and the energy consumption of the composites exhibited significant improvement compared with those of the pristine membrane. In general, the overall performance of the 0.5 wt % SS-20 nm was better than that of the FKE membrane.

4. CONCLUSIONS The surface-functionalized mesoporous silica with different particle sizes was used as the additives to prepare composite membranes containing sulfonated polyethersulfone (sPES) as the polymer matrix. The effect of inorganic filler sizes on the structure and macroscopic properties of the resultant membranes was investigated in detail. It was found that the addition of SS fillers led to a higher degree of porosity in the composite membranes due to (1) the facilated nucleation of the polymer lean phase in the casting solution during the membrane formation by phase inversion and (2) the interfacial pore formation between the polymer and inorganic fillers. It was clearly indicated that the smaller-sized SS particles exhibited better interfacial adhesion with the polymer matrix, resulting in smaller pores and porosity. Such a porosity had a significant impact on the transport properties of the membranes. As a consequence, not only the electrochemical properties but also the thermal and mechanical stability of the membranes were signifciantly improved. Moreover, structural optimization of the membranes was found to be critically important to acquire the best combination of the two components. It was found that a membrane with a 0.5 wt % SS-20 nm loading gave the optimized overall properties of conductivity, permselectivity, and mechanical and thermal stability. This composite showed excellent performance in the desalination of the NaCl solution by ED, which was perceptibly better than the performance of the benchmark membranes. Note that the power consumption during the ED process of the composites was reduced to almost a half compared with that of the pristine membrane. Thus, the strategy demonstrated here may see a facile and more economical way for the designing of new types of IEMs for ED desalination application. ’ ASSOCIATED CONTENT

bS

Supporting Information. Nitrogen adsorptiondesorption isotherms, pore size distributions, schematic diagram of concentration polarization, and pH change of solution in desalination. This material is available free of charge via the Internet at http://pubs.acs.org.

15131



’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT Financial support from the Australian Research Council (through its ARC Centre of Excellence and Discovery programs), CSIRO Advanced Membranes for Water Treatment Cluster Project, and Thai Government (through Higher Educational Strategic Scholarship for Frontier Research Network) is gratefully acknowledged. Additional thanks are due to Richard and Robyn Webb for technical support on the SEM and TEM sample preparation and to Frances Stahr, Abijit Shrotri, and Dr. Roland Marchall for their help on the synthesis of mesoporous silica and surface characterization. ’ REFERENCES (1) Dai, H.; Guan, R.; Li, C.; Liu, J. Solid State Ionics 2007, 178, 339–345. (2) Feng, S.; Shang, Y.; Xie, X.; Wang, Y.; Xu, J. J. Membr. Sci. 2009, 335, 13–20. (3) Johnson, B. C.; Yilgor, I.; Tran, C.; Iqbal, M.; Wightman, J. P.; Lloyd, D. R.; McGrath, J. E. J. Polym. Sci., Polym. Chem. Ed. 1984, 22, 721–737. (4) Kerres, J.; Cui, W.; Disson, R.; Neubrand, W. J. Membr. Sci. 1998, 139, 211–225. (5) Kerres, J.; Cui, W.; Reichle, S. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2421–2438. (6) Sankir, M.; Bhanu, V. A.; Harrison, W. L.; Ghassemi, H.; Wiles, K. B.; Glass, T. E.; Brink, A. E.; Brink, M. H.; McGrath, J. E. J. Appl. Polym. Sci. 2006, 100, 4595–4602. (7) Gomes, D.; Marschall, R.; Nunes, S. P.; Wark, M. J. Membr. Sci. 2008, 322, 406–415. (8) Marschall, R.; Bannat, I.; Caro, J.; Wark, M. Microporous Mesoporous Mater. 2007, 99, 190–196. (9) Marschall, R.; Bannat, I.; Feldhoff, A.; Wang, L.; Lu, G. Q. M.; Wark, M. Small 2009, 5, 854–859. (10) Wilhelm, M.; Jeske, M.; Marschall, R.; Cavalcanti, W. L.; Tolle, P.; Kohler, C.; Koch, D.; Frauenheim, T.; Grathwohl, G.; Caro, J.; Wark, M. J. Membr. Sci. 2008, 316, 164–175. (11) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K.; Rangarajan, R. Macromolecules 2004, 37, 10023–10030. (12) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K.; Rangarajan, R. J. Colloid Interface Sci. 2005, 287, 198–206. (13) Nagarale, R. K.; Shahi, V. K.; Rangarajan, R. J. Membr. Sci. 2005, 284, 37–44. (14) Berezina, N. P.; Kononenko, N. A.; Dyomina, O. A.; Gnusin, N. P. Adv. Colloid Interface Sci. 2008, 139, 3–28. (15) Klaysom, C.; Marschall, R.; Moon, S.-H.; Ladewig, B. P.; Lu, G. Q. M.; Wang, L. J. Mater. Chem. 2010, 21, 7401. (16) Klaysom, C.; Moon, S.-H.; Ladewig, B. P.; Lu, G. Q. M.; Wang, L. J. Membr. Sci. 2011, 371, 37–44. (17) Chazeau, L.; Gauthier, C.; Vigier, G.; Cavaille, J. Y. In Handbook of Organic-Inorganic Hybrid Materials and Nanocomposites; Nalwa, H. S., Ed.; American Scientific Publishers: Stevenson Ranch, CA, 2003; Vol. 2, pp 63111. (18) Yokoi, T.; Karouji, T.; Ohta, S.; Kondo, J. N.; Tatsumi, T. Chem. Mater. 2010, 22, 3900–3908. (19) Tsubokawa, N.; Yoshikawa, S. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 581–586. (20) Klaysom, C.; Marschall, R.; Ladewig, B. P.; Lu, G. Q. M.; Wang, L. J. Mater. Chem. 2010, 20, 4669–4674. (21) Klaysom, C.; Ladewig, B. P.; Lu, G. Q. M.; Wang, L. J. Membr. Sci. 2010, 368, 48–53.

ARTICLE

(22) Nagarale, R. K.; Gohil, G. S.; Shahi, V. K. Adv. Colloid Interface Sci. 2006, 119, 97–130. (23) Shahi, V. K. Solid State Ionics 2007, 117, 3395–3404. (24) Elatar, A.; Elmidaoui, A.; Pismenskaia, N.; Gavach, C.; Pourcelly, G. J. Membr. Sci. 1998, 143, 249–261. (25) Volodin, E.; Pismenskaia, N.; Nikonenko, V.; Larchet, C.; Pourcelly, G. J. Colloid Interface Sci. 2005, 285, 247–258. (26) Larchet, C.; Nouri, S.; Auclair, B.; Dammark, L.; Nikonenko, V. Adv. Colloid Interface Sci. 2008, 139, 45–61. (27) Choi, J.-H.; Kim, S.-H.; Moon, S.-H. J. Colloid Interface Sci. 2001, 241, 120–126. (28) Vyas, P. V.; Ray, P.; Adhikary, S. K.; Shah, B. G.; Rangarajan, R. J. Colloid Interface Sci. 2003, 257, 127–134. (29) Balster, J.; Krupenko, O.; Punt, I.; Stamatialis, D. F.; Wessling, M. J. Membr. Sci. 2005, 263, 137–145. (30) Balster, J.; Yildirim, M. H.; Stamatialis, D. F.; Ibanez, R.; Lammertink, R. G. H.; Jordan, V.; Wessling, M. J. Phys. Chem. B 2007, 111, 2152–2165. (31) Krol, J. J.; Wessling, H.; Strathmann, H. J. Membr. Sci. 1999, 162, 145–154. (32) Tanaka, Y. J. Membr. Sci. 2004, 244, 1–16. (33) Nikonenko, V. V.; Pismenskaia, N. D.; Belova, E. I.; Sistat, P.; Huguet, P.; Pourcelly, G.; Larchet, C. Adv. Colloid Interface Sci. 2010, 160, 101. (34) Pismenskaya, N.; Sistat, P.; Huguet, P.; Nikonenko, V.; Pourcelly, G. J. Membr. Sci. 2004, 228, 65–76.

15132


The Influence of Inorganic Filler Particle Size on Composite Ion

Recommend Documents