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Triblock copolymer ionomer membranes
Journal of Membrane Science 217 (2003) 227–242
Triblock copolymer ionomer membranes Part I. Methanol and proton transport Yossef A. Elabd a,∗ , Eugene Napadensky a , James M. Sloan a , Dawn M. Crawford a , Charles W. Walker b a
US Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD 21005-5069, USA b US Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783-1197, USA Received 20 June 2002; received in revised form 7 February 2003; accepted 28 February 2003
Abstract In this study, the transport properties (i.e. methanol and proton transport) of a triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), at various ion contents were investigated for its application to the direct methanol fuel cell (DMFC). Methanol permeabilities in S-SIBS were more than an order of magnitude lower than Nafion 117, but conductivities were only three-fold less at ambient conditions. S-SIBS was approximately 5–10 times more selective (i.e. proton conductivity/methanol permeability) than Nafion 117 over an ion content range of 0.5–1.0 mmol/g. Also, the transport results follow a power law dependent percolation model, where the critical exponent for diffusion was much lower than those reported in other studies, suggesting a more ordered nanostructure. In addition, transport at higher temperatures (80 ◦ C) was examined. At 80 ◦ C, S-SIBS was only 1.3 times more selective than Nafion 117 at an ion content of 1.0 mmol/g, but was 39 times more selective at an ion content of 0.5 mmol/g. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Direct methanol fuel cell; Polymer electrolyte membranes; Barrier membranes; Electrochemistry; Pervaporation
1. Introduction Ion-containing polymers (i.e. ionomers) or polymer electrolyte membranes (PEMs) have been used in a variety of electrochemical applications, such as sensors, batteries, and low-temperature fuel cells. One application in particular, the direct methanol fuel cell (DMFC), has shown promise as an alternative energy source for the future [1]. In the DMFC, the PEM serves as both a cell separator, separating the anode from the cathode, and an electrolyte, conduct∗ Corresponding author. Tel.: +1-410-306-1285; fax: +1-410-306-0676. E-mail address: [email protected] (Y.A. Elabd).
ing protons from the anode to the cathode. Currently, the most frequently used PEM in DMFCs is Nafion® (DuPont), a perfluorosulfonate ionomer. One of the critical problems that has hindered the progress of the DMFC is high methanol permeation rates in Nafion [2]. When methanol diffuses across the PEM and is present at high levels at the cathode (i.e. methanol crossover), there is a reduction in cathode potential that results in decreased cell efficiency [3]. In the future, a more selective (i.e. good proton conductor and methanol barrier) membrane will be required for the DMFC to become a viable option for alternative energy. Recently, several alternative PEMs have been investigated for fuel cell applications, such as sulfonated
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00127-3
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poly(styrene) [4], sulfonated poly[bis(3-methylphenoxy) phosphazene] [5], acid-doped poly(benzimidazole) [6], sulfonated poly(phenylene oxide) [7], radiation-grafted polymers [8], sol–gel hybrids containing heteropolyacids [9], and zeolite-polymer composites [10]. In addition to these membranes, another polymer electrolyte of interest is the block copolymer ionomer. Block copolymer ionomers are a highly ordered sequence of both ionic and non-ionic blocks unlike random ionomers, in which ionic groups are randomly arranged along the polymer chain. Several block copolymer ionomers, such as sulfonated poly(4vinylpyridinium-styrene-4-vinylpyridinium) [11], sulfonated poly(styrene-(ethylene-co-butylene)-styrene) [12,13], methacrylic-based block ionomers [14], and sulfonated poly(styrene-isobutylene-styrene) (S-SIBS) [15], have been synthesized and characterized. Although there is limited information regarding the transport properties of block copolymer ionomers [39,40], they show potential promise as an alternative PEM for several reasons. First, the non-ionic block can be designed to be a barrier for methanol, and second, block copolymer ionomers have the ability to self assemble into unique nanostructured morphologies [12,13] that may lead to different transport properties compared to random ionomers. The objective of this study was to examine the transport properties (i.e. proton conductivity and methanol permeability) of a triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene), to evaluate its potential as a viable PEM for the DMFC.
2. Theory Experimentally, the transport of protons and methanol are the two primary diffusing solutes of interest. Proton transport can be described by the Nernst–Plank equation [16]: ∇Cp F ∇ψ −jp = Dp Cp + zp (1) Cp RT where jp is the proton flux, and Dp , Cp , and zp are the diffusion coefficient, concentration, and charge, respectively, for protons. Also, in Eq. (1), F is the Faraday’s constant, R the gas constant, T the temperature, and ψ the electrostatic potential. For a constant concentration of protons in the membrane, Cp , or fixed
concentration of ion sites in the polymer, and a constant membrane thickness, L, Eq. (1) can be simplified to [17]: Dp Cp F ψ (2) jp = L RT where protons are defined by a charge, zp , of (+1). Proton conductivity can be defined: Dp C p F 2 (3) RT For methanol, diffusion can be described by Fick’s law as σp =
−jm = Dm ∇Cm
(4)
where Dm and Cm are the diffusion coefficient and concentration, respectively, for methanol. When the concentration of methanol on the donor side (anode) of the membrane, Cmo , is constant and there is a zero-sink boundary condition for concentration on the receptor side (cathode), then Eq. (4) can be represented as Dm Km Cmo (5) jm = L where Km is the partition coefficient (the ratio of methanol concentration inside the membrane to that in the adjacent solution) and the product Dm Km is the methanol permeability, Pm . Membrane selectivity for protons and methanol can be defined as the ratio of fluxes [17]: jp σp ψ β= (6) = jm Pm FCmo Selectivity can also be expressed as the ratio of proton conductivity to methanol permeability: σp α= (7) Pm since the quantity (ψ/FCmo ) depends on the conditions chosen for membrane separation, which are parameters that will be held constant in this study. Note that both conductivity and permeability are the two quantities that will be experimentally measured in this study and are both proportional to their respective diffusion coefficients. For enhanced DMFC performance, a high selectivity is desired (i.e. a high value for α in Eq. (7)). Aside from temperature effects, the driving factors that affect selectivity are the diffusion coefficient of protons, Dp , the concentration of protons in
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the membrane or concentration of fixed ion sites in the membrane, Cp , the diffusion coefficient of methanol, Dm , and the partition coefficient of methanol in the membrane, Km . Increasing membrane selectivity is a multifaceted problem since several of these factors are interdependent and also a function of several other parameters. For instance, Dm is a function of methanol concentration in the membrane, but also a function of molecular size and polymer free volume, and Km is a function of Cp and also a function of the solubility parameter of the polymer. Also, Dp is a function of Cp , and also hydration (water content) and membrane structure. Investigations on several ionomers (e.g. Nafion, sulfonated poly(styrene), poly-2-acrylamido-2-methylpropane sulfonic acid, sulfonated poly(ethylene)) have shown that proton conductivity increases with ionomer hydration, whereby protons are transported via hydrolyzed acidic sites [18,19]. Additionally, conductivity experiments on ionomers suggest that transport properties are dependent on ionomer morphology. Although there are diverse opinions regarding the detailed morphology of ion-containing polymers, particularly Nafion, there is an agreement that phase segregation occurs in ionomers. Aggregates of ions form due to the electrostatic interactions between ion pairs, leading to the formation of two phases: ion-rich domains and ion-poor domains. The ion-rich domains (aggregates of ions) are referred to as ion clusters, while ion-poor domains are mostly hydrocarbon polymer [20]. In particular, X-ray analysis on Nafion by Gierke et al. [21] suggests that ion clusters approximately 5 nm in size are interconnected by small narrow ionic channels on the order of 1 nm. Whether the transport of protons are facilitated through these ionic channels or through ion clusters connected in some other form, is still unclear, but it is evident from several investigations that the diffusion of protons does obey a percolation model [22–26]. Fig. 1 illustrates the concept of percolation theory, similarly depicted by Mohanty et al. [27], where a minority phase (1) is interspersed within a majority phase (2). At low volume fractions of the minority phase (e.g. ion clusters), shown in Fig. 1a, there are isolated clusters (I). As the minority volume fraction increases, some of the clusters become interconnected or accessible, so there is a combination of both isolated (I) and accessible (A) regions (Fig. 1b). The critical con-
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Fig. 1. Illustration depicting a phase-segregated system, where the minority phase (1) is interspersed within the majority phase (2) (shaded region). (a) All of the minority phase is isolated (I); (b) the minority phase contains both parts that are isolated (I) and accessible (A); (c) all of the minority phase is accessible (A). Transport occurs through the interconnected accessible pathways.
centration at which the isolated clusters become interconnected is usually referred to as the “percolation threshold.” No transport occurs below the percolation threshold, since transport can only occur through interconnected or accessible clusters. The total volume fraction of the minority phase can be defined as a combination of the isolated and accessible regions: φ1T = φ1I + φ1A
(8)
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where φ1T is the total volume fraction of the minority phase, φ1I the volume fraction of isolated clusters, and φ1A the volume fraction of interconnected or accessible regions. As the volume fraction is increased even further, then all of the interspersion becomes accessible (Fig. 1c). Kirkpatrick [28] defines a diffusion volume fraction, φ1D , that is equal to or less than the accessible volume fraction:
ideal or ordered. In this study, the experimental transport results will be evaluated with the percolation model described above (Eq. (12)), in order to probe the possible effects of membrane structure on transport properties.
φ1D ≤ φ1A
3.1. Materials
(9)
This is where diffusion actually occurs, and this is usually less than the accessible volume fraction due to tortuosity and inactive dead ends. The diffusion volume fraction only becomes equal to the accessible volume fraction when the accessible volume is arranged in parallel layers aligned with the driving force [28]. According to percolation theory, the accessible and diffusion volume fractions can be related to the excess volume fraction near the percolation threshold through a power law dependency: φ1A ∝ (φ1 − φ1,c )γA
(10)
φ1D ∝ (φ1 − φ1,c )γD
(11)
where γ A and γ D are the critical exponents for the accessible and diffusion regions, respectively, φ1,c is the critical volume fraction, where the percolation threshold occurs (i.e. where isolated regions become interconnected and diffusion begins to occur), and (φ1 − φ1,c ) is the excess volume fraction. Eq. (11) can be related to conductivity or diffusivity through this expression: σ D ∝ ∝ (φ1 − φ1,c )γD σo Do
(12)
where σ and D are the observed conductivity and diffusivity, respectively, and σ o and Do are the inherent conductivity and diffusivity, respectively, in the diffusing phase. Kirkpatrick [28] calculated values of 0.3–0.4 and 1.6–1.7 for γ A and γ D , respectively, using a three-dimensional lattice model simulation with a random distribution of the minority phase. A more recent simulation [29] has shown a value of 2.0 for γ D . Values for γ D give an indication of the non-ideality or randomness of the ionomer structure. As this value approaches lower values than the ones reported here for random simulations, then the system becomes more
3. Experimental
The unmodified poly(styrene-isobutylene-styrene) triblock copolymer was provided by Kuraray Co. Ltd., Tsukuba research laboratories (sample name-TS-3000S, lot. no. 990215) with the reported properties: 30.84 wt.% styrene, 0.95 specific gravity, Mw = 71,920 g/mol, Mn = 48,850 g/mol, and PDI = 1.47. Nafion® 117 was obtained from C.G. Processing Inc. (1100 equivalent weight). The Nafion membranes were pretreated to their acid form, similar to a procedure reported elsewhere [30], by refluxing the membrane in a HCl/HNO3 (50/50 (v/v)) solution, then leaching out the excess acid by refluxing in deionized water three times, and drying in a vacuum oven at 125 ◦ C. HPLC grade water (J.T. Baker) and methanol (EM Scientific) were used for all transport experiments. Other chemicals used are as follows: sodium hydroxide (Aldrich, ACS grade, Assay +97%), tetrahydrofuran (THF; Burdick & Jackson, HPLC grade, Assay 99.9%), glacial acetic acid (J.T. Baker, Assay 100%), thymol blue (J.T. Baker), toluene (VWR), hexanol (J.T. Baker), methylene chloride (EM Science, HPLC grade), sulfuric acid (Fisher Scientific, Assay 95.4%), and acetic anhydride (Mallinckrodt). 3.2. Membrane preparation The chemical structure of SIBS before and after sulfonation is shown in Fig. 2. Sulfonation of SIBS was performed similar to a procedure described elsewhere [12]. An example of the sulfonation procedure used in this study is as follows: 25 g of the triblock copolymer was dissolved in 300 ml of methylene chloride. This solution was refluxed, while a specified amount of a solution of acetyl sulfate in methylene chloride was added. The reaction was terminated after 5 h by adding approximately 30–50 ml of methanol. The
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Table 1 Ion-exchange capacities and densities Polymer name
IEC (mmol/g)a
IEC (mmol/g)b
ρ (g/cm3 )
Nafion 117 S-SIBS-0 S-SIBS-0.1 S-SIBS-0.4 S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0
NA NA 0.11 0.39 0.45 0.62 0.72 0.94 0.96
0.91c NA 0.12 0.36 0.47 0.63 0.71 0.94 0.97
2.27 0.95 0.95 0.96 0.97 1.00 1.05 1.06 1.04
± ± ± ± ± ± ±
0.01 0.01 0.02 0.04 0.01 0.01 0.01
± ± ± ± ± ± ± ± ±
0.12 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
NA: not applicable. a Determined by titration. b Determined by elemental analysis. c Properties provided by C.G. Processing Inc. (1100 equivalent weight).
Fig. 2. Chemical structure of poly(styrene-isobutylene-styrene) (SIBS) and sulfonated poly(styrene-isobutylene-styrene) (S-SIBS). Sulfonation of styrene units occurs randomly (R ≡ only H for 0 mol% sulfonation, R ≡ only SO3 − for 100 mol% sulfonation, and R ≡ combination of H and SO3 − for a sulfonation percent >0 and <100 mol%).
reacted polymer solution was then precipitated with warm water. The precipitate was washed several times with hot water and acetone, dried in a vacuum oven at 45 ◦ C for 24 h, washed in cold water two times, and then dried again at 45 ◦ C for 24 h. This procedure was repeated with different amounts of acetyl sulfate to produce several sulfonated polymers with various sulfonic acid concentrations (i.e. ion contents), see Table 1. The ion-exchange capacity (IEC) (mmol of sulfonic acid/g of polymer) of each polymer was determined by both titration and elemental analysis (EA). Titration was accomplished by dissolving 0.5 g of polymer in 30 ml of tetrahydrofuran and then titrating with 0.02N sodium hydroxide in methanol to the phenolphthalein endpoint with a thymol blue indicator. The sodium hydroxide solution was standardized against glacial acetic acid. The accuracy of each titration was confirmed by EA, which was conducted by Atlantic Microlab Inc. in Norcross, Georgia. The results from both procedures are listed in Table 1. The IECs determined from EA are within the standard de-
viation of those determined from titration, where the values listed in Table 1 for titration are the average of multiple experiments and the standard deviation is the error between these experiments. Hereafter, the sulfonated triblocks produced will be referred to as S-SIBS-#, where S-SIBS represents sulfonated poly(styrene-isobutylene-styrene) and the succeeding number, #, refers to the IEC. After sulfonation and evaluation, the S-SIBS samples were then dissolved in a mixed solvent of toluene/hexanol (85/15 (w/w)) at 5% (w/v) and cast from solution in open Teflon Petri dishes for approximately 3–4 days at ambient conditions. Solvent-cast membranes were then annealed in a vacuum oven at 50 ◦ C for an additional 2 weeks to remove any residual solvent. All membrane thicknesses were measured with a digital micrometer (±1 m accuracy), and these thicknesses were accounted for in both methanol and proton transport calculations. Thicknesses measured for Nafion 117 and the S-SIBS membranes were 187 and 200–300 m, respectively. 3.3. Permeability The methanol permeability of each membrane was measured using a side-by-side glass diffusion cell equipped with a thermal jacket, shown in Fig. 3. The diffusion cell was purchased from PermGear Inc. of Hallentown, PA. Prior to all experiments, membranes were hydrated in HPLC grade water for at least 48 h. Then for each experiment, a membrane was clamped between well-stirred donor (A) and receptor (B)
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Fig. 3. Schematic diagram of side-by-side glass diffusion cell with flow-through in-line FTIR-ATR spectroscopic detector.
compartments with a membrane cross-sectional area of 0.636 cm2 exposed to the solutions in both compartments. The receptor compartment (VB = 3.4 ml capacity) was initially filled with water, while the donor compartment (VA = 3.4 ml capacity) was charged with 2.0 M methanol (8 vol.%), a standard concentration used in current DMFCs [2]. The concentration of methanol that permeates through the membrane was measured in the flow-through receptor side with the use of a low-flow pump (Watson–Marlow) and a real-time in-line Fourier transform infrared, attenuated total reflectance (FTIR-ATR) spectrometer for detection. The FTIR spectrometer (Nicolet Magna 560 Series) used was equipped with a flow-through horizontal ATR cell (Specac Inc.) containing its own thermal jacket. A zinc selenide ATR crystal (Specac Inc.) with a refractive index of 2.4 and entry and exit faces beveled at a 45◦ angle was used. Infrared spectra were continuously recorded throughout each experiment at 67 s intervals using 128 scans and 4 cm−1 resolution for each collected spectrum. In all experi-
ments, both the side-by-side diffusion cell and ATR cell were temperature controlled with the same circulating water bath. The additional liquid volume in the receptor side due to the in-line sampling technique was calculated for each experiment by measuring the weight change in water added. The added volume was approximately 9 ml. Permeability of methanol can be determined by using an approximate solution of the continuity equation for diffusion in plane sheet geometry at early times [31]: PCA A L2 CB (t) = t− (13) VB L 6D for the boundary conditions: CA CB , where CA and CB are the concentration of methanol in the donor and receptor compartments, respectively. In Eq. (13), L is the membrane thickness, VB the volume of the receptor compartment, A the cross-sectional area of the membrane (0.636 cm2 ), and P the permeability coefficient for methanol. The methanol permeability, P,
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is defined here as the product DK, where D is the methanol diffusion coefficient, and K the partition coefficient. In Eq. (13), the permeability, P, can be determined from the slope of the methanol concentration in the receptor compartment versus time. By rearranging Eq. (13) to CB (t)VB L L2 =P t− (14) CA A 6D the permeability coefficient is directly equal to the slope of [(CB (t)VB L)/(CA A)] versus time. Eq. (14) allows for the comparison of membranes of different thicknesses on the same plot. 3.4. Conductivity
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3.6. Density The density of each sample was measured using a helium pycnometer (Quantachrome Ultrapycnometer 1000) and the results are listed in Table 1. This is a non-invasive procedure using purified helium as the displaced medium. After calibration, the specimens were placed in the measurement cell and purged with helium for 5–10 min. At least three samples were taken for each data point and the values in Table 1 reflect the averages and standard deviations of these values. 4. Results and discussion 4.1. Permeability
The proton conductivity of each polymer membrane was determined by AC impedance spectroscopy. The measurements were taken between 10 Hz and 100 kHz using a Solartron AC impedance system (1260 impedance analyzer, 1287 electrochemical interface, Zplot software). A four-point probe method was employed using a test fixture designed and constructed at Case Western Reserve University [32]. Membrane samples measuring 3 cm by 0.5 cm were first hydrated in deionized water before being placed into the test fixture assembly. The conductivity cell (i.e. test fixture assembly) was placed in a sealable container with electrical feedthroughs and a small amount of water to maintain an atmosphere of 100% relative humidity. Impedance measurements were taken at each temperature after equilibrating for a minimum of 2 h, where temperature was controlled by placing the sealable container in a Tenney chamber. 3.5. Solubility The solubility of each polymer was determined using a Hi-Res TGA 2950 Thermogravimetric Analyzer (TA Instruments). Samples weighing between 5 and 15 mg were pre-saturated in either water or 2.0 M methanol for a minimum of 1 week. Then the weight loss of solvent was measured after heating the samples to 250 ◦ C at 1 ◦ C/min. No weight loss was observed on dry samples and no weight loss was observed over 100 ◦ C on the pre-saturated samples.
In order to measure the concentration of methanol in the receptor side of the diffusion cell using the in-line non-invasive infrared technique, the infrared absorbance spectrum for methanol in water was calibrated with respect to concentration. Fig. 4 shows infrared spectra of five reference solutions ranging from 0.2 to 1.0 vol.% of methanol in water, where the absorbance peak at 1016 cm−1 represents the C–O stretching vibration of methanol. These concentrations are in the range of those studied for the permeation experiments. Fig. 5 shows the resultant calibration from Fig. 4, where the peak height of each absorbance spectra is plotted versus methanol concentration. Fig. 5 shows a linear relationship between absorbance and concentration with a slope equal to 0.03. Using the calibration in Fig. 5, the concentration of methanol can be plotted versus time, shown in Fig. 6. Methanol permeability can be determined from the slope of this data using Eq. (13). Results from this experiment show that the methanol permeability of Nafion 117 is 1.98 × 10−6 cm2 /s at a temperature of 25 ◦ C and a feed concentration of 2.0 M methanol. The methanol permeability determined from this study is similar to values previously reported in literature: 2.3 × 10−6 cm2 /s at ambient temperature and 1.0 M methanol [17] and (1–1.5) × 10−6 cm2 /s at 22 ◦ C and 2.0 M methanol [33]. Fig. 7 shows three different permeation experiments at 25 ◦ C on the same plot (Nafion 117, S-SIBS-0.6, and S-SIBS-1.0), where [(CB (t)VB L)/(CA A)] is plotted versus time.
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Fig. 4. Infrared absorbance spectra representing the C–O stretching vibration (peak maximum at 1016 cm−1 ) of methanol at different methanol/water concentrations (1.0, 0.8, 0.6, 0.4, 0.2 vol.% of methanol in water).
The methanol permeabilities determined directly from the slope of the data in Fig. 7 using Eq. (14), reveals that the S-SIBS membrane is more than an order of magnitude lower in methanol permeability compared to Nafion 117 at a similar IEC. Methanol permeabilities for all the membranes studied are shown in Fig. 8 as a function of IEC. The methanol permeability for the membranes: S-SIBS-0, S-SIBS-0.1, and S-SIBS-0.4, could not be measured.
At an IEC of approximately 0.46 mmol/g the onset of the percolation threshold is observed, whereby connectivity between ionic domains provide transport pathways and permeability can occur. Above the percolation threshold, the methanol permeability for S-SIBS-0.5 is approximately 217 times lower than Nafion 117, while S-SIBS-1.0 (similar IEC to Nafion 117) is 15 times lower. The solid line in Fig. 8 represents a regression to the percolation model,
Fig. 5. Calibration: infrared absorbance of C–O stretch (peak height) of methanol vs. methanol concentration (slope = 0.03).
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Fig. 6. Receptor methanol concentration as a function of time for the diffusion of methanol through Nafion 117 at 25 ◦ C. The permeability was determined from the slope of the line (P = 1.98 × 10−6 cm2 /s).
which will be discussed in more detail later in this paper. 4.2. Conductivity and selectivity Proton conductivity experiments, measured with AC impedance spectroscopy, are also shown in Fig. 8 as a function as IEC along with the methanol permeability results. The conductivity experiments in Fig. 8 were conducted at ambient conditions (21–22 ◦ C).
In Fig. 8, Nafion 117 is approximately three times more conductive than S-SIBS-1.0 and approximately 21 times more conductive than S-SIBS-0.5. The conductivity for Nafion 117 determined in this study at ambient temperatures, 0.067 S/cm, is similar to values reported in the literature with similar techniques and at similar temperatures: 0.067 S/cm [35], 0.061 S/cm [36], and 0.054–0.082 S/cm [33]. Conductivity also follows the percolation model with a similar percolation threshold to that of the permeability data and
Fig. 7. Permeability data for three different membranes at 25 ◦ C: Nafion 117 (䉫), S-SIBS-1.0 (䊊), and S-SIBS-0.6 ( ). The permeabilities are directly equal to the slopes of the lines, see Eq. (14).
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Fig. 8. Methanol permeability (25 ◦ C) and proton conductivity (21–22 ◦ C) vs. IEC for Nafion 117 (䉫, 䉬, respectively) and S-SIBS membranes (䊊, 䊉, respectively). The solid lines represent a regression to the percolation model, where the percolation threshold is approximately 0.46 mmol/g.
these results will be discussed in more detail in the next section. Fig. 9 shows the selectivity of the membranes studied, where selectivity is defined by Eq. (7). The selectivity of the S-SIBS membranes is approximately 5–10 times more selective than Nafion 117 with selectivities of 10.4, 4.3, 9.3, 5.5, and 5.3 times greater than Nafion 117 with increasing IEC. In this study, there is no apparent trend in selectivities in relation to IEC. Further experiments are underway on S-SIBS membranes with
a variety of IECs higher than Nafion to understand the cause for the increase in selectivity and the variation that is observed here. This increase in selectivity may be due to the low methanol and water solubility of polyisobutylene (PIB), where PIB is the major component of the triblock copolymer ionomer, S-SIBS (70 wt.% of the unmodified block copolymer) [37]. Low solubility of water and 2.0 M methanol (<1 wt.% uptake) in the unsulfonated block copolymer (70 wt.% PIB) is also demonstrated in this study (Fig. 10).
Fig. 9. Selectivity (conductivity/permeability) vs. IEC at 21–25 ◦ C for Nafion 117 (solid bar) and S-SIBS membranes (open bars).
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Fig. 10. Solubility data for all membranes studied: volume fraction of water and 2.0 M methanol in Nafion 117 (䊐, 䉫, respectively) and S-SIBS membranes ( , 䊊, respectively) as a function of IEC.
4.3. Percolation Membrane structure may be another reason for increased selectivity. The transport data in Fig. 8 for the S-SIBS membranes were fit to the percolation model with respect to IEC, but the percolation model in Eq. (12) relates diffusivity or conductivity to the volume fraction of the minority phase. In order to determine the volume fraction of the minority phase
or solvent in the polymer, the solubility of 2.0 M methanol and water was determined using gravimetric analysis at each ion-exchange capacity. Fig. 10 shows the results of these experiments, where the volume fraction was determined from the weight fraction of solvent in the polymer and the density of the solvent and polymer (see Table 1). Weight fraction is defined here as the ratio of solvent uptake or loss to the wet weight of the polymer (weight of polymer + weight
Fig. 11. Determination of percolation values: log–log plot of methanol permeability (䊊) and proton conductivity (䊉) vs. the excess volume fraction of solvent (2.0 M methanol and water for permeability and conductivity, respectively) in S-SIBS membranes.
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of solvent uptake). The weight fraction of water uptake in Nafion 117 determined in this study was approximately 0.25, similar to another study that reports a value of 0.26 [19]. In Fig. 10, there is a linear relationship between the volume fraction of both water and 2.0 M methanol in the polymer and IEC in the ion content range studied above the percolation threshold, represented by the solid lines. Fig. 11 shows a log–log plot of both proton conductivity and methanol permeability as a function of excess volume fraction in the polymer (water for conductivity and 2.0 M methanol for permeability), where the critical volume fraction, φ1,c , was 0.077 and 0.095 for water and 2.0 M methanol, respectively. The slope of the data is equivalent to the critical exponent for diffusion, γ D , and was 0.76 and 1.15 for conductivity and permeability, respectively. The critical exponent for conductivity in S-SIBS is significantly lower than those reported in the literature for other random copolymer ionomers and Nafion [22–26], shown in Table 2. This result alludes to possible structural differences in this block copolymer ionomer compared to that of other random ionomers. Also, the value of 0.76 for the critical exponent determined in this study is closer to Kirpatrick’s [28] value for the accessible critical exponent (γA = 0.3–0.4) than that of the diffusion critical exponent (γD = 1.6–1.7), suggesting that this block copolymer ionomer possesses a more ordered structure than other random ionomers.
Table 2 Percolation values Polymer name
γD
φ1,c
Reference
Nafion Poly(methyl methacrylateco-methacrylic acid) Poly(styrene-comethacrylic acid) Sulfonated poly(phenylene oxide) Sulfonated poly[bis(3methylphenoxy) phosphazene] S-SIBS
1.5 ± 0.02 1.35
0.10 0.26
[22] [23]
1.7
0.165
[24]
1.5
0.16
[25]
1.26
0.175–0.25
[26]
0.76
0.077
This work
4.4. Temperature effects In addition to results at low temperatures, transport properties at higher temperatures were studied. Typically temperatures up to 80 ◦ C have been investigated in DMFC operation [2,3]. Beyond 80 ◦ C, dehydration of Nafion occurs and leads to a reduction in proton conductivity. Fig. 12 shows the effects of temperature on methanol permeability for Nafion 117 and two different S-SIBS membranes (0.5 and 1.0 mmol/g). The
Fig. 12. Methanol permeability as a function of temperature for three different membranes: Nafion 117 (䉫), S-SIBS-1.0 (䊊), and S-SIBS-0.5 (+).
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Table 3 Methanol permeabilities and proton conductivities Polymer name
Nafion 117 S-SIBS-0 S-SIBS-0.1 S-SIBS-0.4 S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0 a b
Methanol permeability (cm2 /s)
Proton conductivity (S/cm)
25 ◦ C
80 ◦ C
21–22 ◦ C
80 ◦ C
2.01 × 10−6
3.53 × 10−6
6.70 × 10−2
1.60 × 10−1
a
a
a
a
a
a
a
a
a
a
a
9.24 6.03 6.10 1.37 1.39
× × × × ×
10−9 10−8 10−8 10−7 10−7
4.29 × 10−8 b b b
1.40 × 10−6
3.20 8.70 1.90 2.53 2.47
a
× × × × ×
10−3 10−3 10−2 10−2 10−2
7.65 6.27 5.90 8.09 8.30
× × × × ×
10−2 10−2 10−2 10−2 10−2
Not measurable. Not measured.
permeability decreases exponentially with the inverse of temperature following an Arrhenius behavior. Although the methanol permeability of S-SIBS-1.0 is 15 times lower than Nafion 117 and S-SIBS-0.5 is 217 times lower when measured at 25 ◦ C, the methanol permeability for S-SIBS-1.0 and S-SIBS-0.5 is only 2.5 and 82 times lower, respectively, than Nafion 117 at 80 ◦ C. Permeabilities at 25 and 80 ◦ C are listed in Table 3, where the values at 25 ◦ C are the average of the all experiments for each membrane shown in Fig. 8. Activation energies can be calculated from the slope of the data in Fig. 12 (see Table 4). The activation energy for permeability is almost four times greater for S-SIBS-1.0 compared to Nafion 117 and approximately three times greater for S-SIBS-0.5. The activation energy for Nafion 117 in this study, 10 kJ/mol, is similar to other studies, 12 kJ/mol [34] and 18 kJ/mol Table 4 Activation energies Polymer name
Nafion 117 S-SIBS-0 S-SIBS-0.1 S-SIBS-0.4 S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0
Activation energy (kJ/mol) Methanol permeability
Proton conductivity
9.90 NA NA NA 30.62 NA NA NA 37.10
11.53 NA NA NA 41.24 25.00 18.83 17.36 18.11
NA: not applicable.
[33]. One possible explanation for the significant difference in activation energies may be caused by the difference in glass transition temperatures between Nafion 117 and S-SIBS [15,38]. Fig. 13 shows the effects of temperature on proton conductivity, where the solid lines represent a regression to an Arrhenius model. From 21–22 to 80 ◦ C, the conductivity of the S-SIBS membranes, ranging from 0.7 to 1.0 mmol/g, all increase by approximately 4-fold, S-SIBS-0.6 increases 7-fold, and S-SIBS-0.5 increases 24-fold, while Nafion 117 only increases by a factor of 2. Interestingly, at 80 ◦ C, all the S-SIBS membranes studied are approximately only one-half as conductive compared to Nafion 117. Similar to permeability, the activation energies of conductivity for the S-SIBS membranes are higher than Nafion 117 (see Table 4), but unlike permeability, the activation energies for conductivity for S-SIBS decrease with increasing IEC. Temperature may affect the block copolymer ionomer morphology, which may in turn affect proton mobility. Experiments are currently underway to determine the polymer structure and its effects on transport properties. Additionally, the activation energy determined for Nafion 117 in this study, 11.5 kJ/mol, compares well to a prior study, 11 kJ/mol [33]. Fig. 14 shows the effects of temperature on selectivity. At higher temperatures (80 ◦ C) the selectivity for S-SIBS-1.0 is reduced (1.3 times more selective than Nafion 117 compared to 5 times more selective at 21–25 ◦ C), however S-SIBS-0.5 increases in selectivity at higher temperatures (39 times more selective than Nafion 117 compared to 10 times more selective
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Fig. 13. Proton conductivity as a function of temperature for Nafion 117 (䉬), S-SIBS-1.0 (䊊), S-SIBS-0.9 (䊐), S-SIBS-0.7 ( ), S-SIBS-0.6 (×), and S-SIBS-0.5 (+).
Fig. 14. Selectivity vs. IEC at 80 ◦ C and 21–25 ◦ C, comparing Nafion 117 (solid bars) to S-SIBS membranes (1.0 and 0.5 mmol/g, open bars).
at 21–25 ◦ C). Although conductivities are much lower for S-SIBS close to the percolation threshold (S-SIBS-0.5) compared to Nafion 117, this membrane is still interesting due to its low methanol permeability.
5. Conclusions In this study, the transport properties of a triblock copolymer ionomer show that it is a viable alternative
to Nafion for its application to the DMFC. S-SIBS is slightly less conductive than Nafion, but has a methanol permeability more than an order of magnitude lower than Nafion allowing for enhanced fuel cell efficiency. The increased selectivity may be due to low methanol and water solubility in polyisobutylene, the major component of the triblock copolymer ionomer examined in this study. Also, the percolation values calculated in this study suggest that the membrane possesses a more ordered structure compared
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to other ionomers. This ordered structure may also greatly affect the transport properties. An experimental investigation using small-angle X-ray scattering is underway to determine the actual morphology of S-SIBS in order to understand its effect on the transport properties. In addition, the fabrication of membrane electrode assemblies (anode/membrane/cathode composite) using S-SIBS and subsequent fuel cell testing with S-SIBS as the PEM is underway.
Acknowledgements This work was performed while the author (Y.A. Elabd) held a National Research Council Research Associateship Award at the US Army Research Laboratory. The authors gratefully acknowledge Alexis Hillock for assistance with thermogravimetric experiments.
Nomenclature A CA CB CB (t)
Cm Cmo Cp D Dm Do Dp F jm jp Km L
cross-sectional area of the membrane (0.636 cm2 ) concentration of methanol in the donor compartment concentration of methanol in the receptor compartment concentration of methanol measured in the receptor compartment as a function of time methanol concentration concentration of methanol on the donor side (anode) of the membrane concentration of protons or concentration of ion sites in the polymer observed diffusivity methanol diffusion coefficient inherent diffusivity proton diffusion coefficient Faraday’s constant methanol flux proton flux methanol partition coefficient membrane thickness
Pm R t T VB zp
241
methanol permeability gas constant time temperature volume of the receptor compartment proton charge
Greek letters α selectivity (ratio of proton conductivity to methanol permeability) β selectivity (ratio of fluxes) γ A critical exponents for the accessible regions γ D critical exponents for the diffusion regions σ observed conductivity σo inherent conductivity σp proton conductivity φ1A volume fraction of interconnected or accessible regions φ1,c critical volume fraction φ1D diffusion volume fraction φ1I volume fraction of isolated clusters φ1T total volume fraction of the minority phase ψ electrostatic potential
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Triblock copolymer ionomer membranes Part I. Methanol and proton transport Yossef A. Elabd a,∗ , Eugene Napadensky a , James M. Sloan a , Dawn M. Crawford a , Charles W. Walker b a
US Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, MD 21005-5069, USA b US Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783-1197, USA Received 20 June 2002; received in revised form 7 February 2003; accepted 28 February 2003
Abstract In this study, the transport properties (i.e. methanol and proton transport) of a triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene) (S-SIBS), at various ion contents were investigated for its application to the direct methanol fuel cell (DMFC). Methanol permeabilities in S-SIBS were more than an order of magnitude lower than Nafion 117, but conductivities were only three-fold less at ambient conditions. S-SIBS was approximately 5–10 times more selective (i.e. proton conductivity/methanol permeability) than Nafion 117 over an ion content range of 0.5–1.0 mmol/g. Also, the transport results follow a power law dependent percolation model, where the critical exponent for diffusion was much lower than those reported in other studies, suggesting a more ordered nanostructure. In addition, transport at higher temperatures (80 ◦ C) was examined. At 80 ◦ C, S-SIBS was only 1.3 times more selective than Nafion 117 at an ion content of 1.0 mmol/g, but was 39 times more selective at an ion content of 0.5 mmol/g. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Direct methanol fuel cell; Polymer electrolyte membranes; Barrier membranes; Electrochemistry; Pervaporation
1. Introduction Ion-containing polymers (i.e. ionomers) or polymer electrolyte membranes (PEMs) have been used in a variety of electrochemical applications, such as sensors, batteries, and low-temperature fuel cells. One application in particular, the direct methanol fuel cell (DMFC), has shown promise as an alternative energy source for the future [1]. In the DMFC, the PEM serves as both a cell separator, separating the anode from the cathode, and an electrolyte, conduct∗ Corresponding author. Tel.: +1-410-306-1285; fax: +1-410-306-0676. E-mail address: [email protected] (Y.A. Elabd).
ing protons from the anode to the cathode. Currently, the most frequently used PEM in DMFCs is Nafion® (DuPont), a perfluorosulfonate ionomer. One of the critical problems that has hindered the progress of the DMFC is high methanol permeation rates in Nafion [2]. When methanol diffuses across the PEM and is present at high levels at the cathode (i.e. methanol crossover), there is a reduction in cathode potential that results in decreased cell efficiency [3]. In the future, a more selective (i.e. good proton conductor and methanol barrier) membrane will be required for the DMFC to become a viable option for alternative energy. Recently, several alternative PEMs have been investigated for fuel cell applications, such as sulfonated
0376-7388/03/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00127-3
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poly(styrene) [4], sulfonated poly[bis(3-methylphenoxy) phosphazene] [5], acid-doped poly(benzimidazole) [6], sulfonated poly(phenylene oxide) [7], radiation-grafted polymers [8], sol–gel hybrids containing heteropolyacids [9], and zeolite-polymer composites [10]. In addition to these membranes, another polymer electrolyte of interest is the block copolymer ionomer. Block copolymer ionomers are a highly ordered sequence of both ionic and non-ionic blocks unlike random ionomers, in which ionic groups are randomly arranged along the polymer chain. Several block copolymer ionomers, such as sulfonated poly(4vinylpyridinium-styrene-4-vinylpyridinium) [11], sulfonated poly(styrene-(ethylene-co-butylene)-styrene) [12,13], methacrylic-based block ionomers [14], and sulfonated poly(styrene-isobutylene-styrene) (S-SIBS) [15], have been synthesized and characterized. Although there is limited information regarding the transport properties of block copolymer ionomers [39,40], they show potential promise as an alternative PEM for several reasons. First, the non-ionic block can be designed to be a barrier for methanol, and second, block copolymer ionomers have the ability to self assemble into unique nanostructured morphologies [12,13] that may lead to different transport properties compared to random ionomers. The objective of this study was to examine the transport properties (i.e. proton conductivity and methanol permeability) of a triblock copolymer ionomer, sulfonated poly(styrene-isobutylene-styrene), to evaluate its potential as a viable PEM for the DMFC.
2. Theory Experimentally, the transport of protons and methanol are the two primary diffusing solutes of interest. Proton transport can be described by the Nernst–Plank equation [16]: ∇Cp F ∇ψ −jp = Dp Cp + zp (1) Cp RT where jp is the proton flux, and Dp , Cp , and zp are the diffusion coefficient, concentration, and charge, respectively, for protons. Also, in Eq. (1), F is the Faraday’s constant, R the gas constant, T the temperature, and ψ the electrostatic potential. For a constant concentration of protons in the membrane, Cp , or fixed
concentration of ion sites in the polymer, and a constant membrane thickness, L, Eq. (1) can be simplified to [17]: Dp Cp F ψ (2) jp = L RT where protons are defined by a charge, zp , of (+1). Proton conductivity can be defined: Dp C p F 2 (3) RT For methanol, diffusion can be described by Fick’s law as σp =
−jm = Dm ∇Cm
(4)
where Dm and Cm are the diffusion coefficient and concentration, respectively, for methanol. When the concentration of methanol on the donor side (anode) of the membrane, Cmo , is constant and there is a zero-sink boundary condition for concentration on the receptor side (cathode), then Eq. (4) can be represented as Dm Km Cmo (5) jm = L where Km is the partition coefficient (the ratio of methanol concentration inside the membrane to that in the adjacent solution) and the product Dm Km is the methanol permeability, Pm . Membrane selectivity for protons and methanol can be defined as the ratio of fluxes [17]: jp σp ψ β= (6) = jm Pm FCmo Selectivity can also be expressed as the ratio of proton conductivity to methanol permeability: σp α= (7) Pm since the quantity (ψ/FCmo ) depends on the conditions chosen for membrane separation, which are parameters that will be held constant in this study. Note that both conductivity and permeability are the two quantities that will be experimentally measured in this study and are both proportional to their respective diffusion coefficients. For enhanced DMFC performance, a high selectivity is desired (i.e. a high value for α in Eq. (7)). Aside from temperature effects, the driving factors that affect selectivity are the diffusion coefficient of protons, Dp , the concentration of protons in
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the membrane or concentration of fixed ion sites in the membrane, Cp , the diffusion coefficient of methanol, Dm , and the partition coefficient of methanol in the membrane, Km . Increasing membrane selectivity is a multifaceted problem since several of these factors are interdependent and also a function of several other parameters. For instance, Dm is a function of methanol concentration in the membrane, but also a function of molecular size and polymer free volume, and Km is a function of Cp and also a function of the solubility parameter of the polymer. Also, Dp is a function of Cp , and also hydration (water content) and membrane structure. Investigations on several ionomers (e.g. Nafion, sulfonated poly(styrene), poly-2-acrylamido-2-methylpropane sulfonic acid, sulfonated poly(ethylene)) have shown that proton conductivity increases with ionomer hydration, whereby protons are transported via hydrolyzed acidic sites [18,19]. Additionally, conductivity experiments on ionomers suggest that transport properties are dependent on ionomer morphology. Although there are diverse opinions regarding the detailed morphology of ion-containing polymers, particularly Nafion, there is an agreement that phase segregation occurs in ionomers. Aggregates of ions form due to the electrostatic interactions between ion pairs, leading to the formation of two phases: ion-rich domains and ion-poor domains. The ion-rich domains (aggregates of ions) are referred to as ion clusters, while ion-poor domains are mostly hydrocarbon polymer [20]. In particular, X-ray analysis on Nafion by Gierke et al. [21] suggests that ion clusters approximately 5 nm in size are interconnected by small narrow ionic channels on the order of 1 nm. Whether the transport of protons are facilitated through these ionic channels or through ion clusters connected in some other form, is still unclear, but it is evident from several investigations that the diffusion of protons does obey a percolation model [22–26]. Fig. 1 illustrates the concept of percolation theory, similarly depicted by Mohanty et al. [27], where a minority phase (1) is interspersed within a majority phase (2). At low volume fractions of the minority phase (e.g. ion clusters), shown in Fig. 1a, there are isolated clusters (I). As the minority volume fraction increases, some of the clusters become interconnected or accessible, so there is a combination of both isolated (I) and accessible (A) regions (Fig. 1b). The critical con-
229
Fig. 1. Illustration depicting a phase-segregated system, where the minority phase (1) is interspersed within the majority phase (2) (shaded region). (a) All of the minority phase is isolated (I); (b) the minority phase contains both parts that are isolated (I) and accessible (A); (c) all of the minority phase is accessible (A). Transport occurs through the interconnected accessible pathways.
centration at which the isolated clusters become interconnected is usually referred to as the “percolation threshold.” No transport occurs below the percolation threshold, since transport can only occur through interconnected or accessible clusters. The total volume fraction of the minority phase can be defined as a combination of the isolated and accessible regions: φ1T = φ1I + φ1A
(8)
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where φ1T is the total volume fraction of the minority phase, φ1I the volume fraction of isolated clusters, and φ1A the volume fraction of interconnected or accessible regions. As the volume fraction is increased even further, then all of the interspersion becomes accessible (Fig. 1c). Kirkpatrick [28] defines a diffusion volume fraction, φ1D , that is equal to or less than the accessible volume fraction:
ideal or ordered. In this study, the experimental transport results will be evaluated with the percolation model described above (Eq. (12)), in order to probe the possible effects of membrane structure on transport properties.
φ1D ≤ φ1A
3.1. Materials
(9)
This is where diffusion actually occurs, and this is usually less than the accessible volume fraction due to tortuosity and inactive dead ends. The diffusion volume fraction only becomes equal to the accessible volume fraction when the accessible volume is arranged in parallel layers aligned with the driving force [28]. According to percolation theory, the accessible and diffusion volume fractions can be related to the excess volume fraction near the percolation threshold through a power law dependency: φ1A ∝ (φ1 − φ1,c )γA
(10)
φ1D ∝ (φ1 − φ1,c )γD
(11)
where γ A and γ D are the critical exponents for the accessible and diffusion regions, respectively, φ1,c is the critical volume fraction, where the percolation threshold occurs (i.e. where isolated regions become interconnected and diffusion begins to occur), and (φ1 − φ1,c ) is the excess volume fraction. Eq. (11) can be related to conductivity or diffusivity through this expression: σ D ∝ ∝ (φ1 − φ1,c )γD σo Do
(12)
where σ and D are the observed conductivity and diffusivity, respectively, and σ o and Do are the inherent conductivity and diffusivity, respectively, in the diffusing phase. Kirkpatrick [28] calculated values of 0.3–0.4 and 1.6–1.7 for γ A and γ D , respectively, using a three-dimensional lattice model simulation with a random distribution of the minority phase. A more recent simulation [29] has shown a value of 2.0 for γ D . Values for γ D give an indication of the non-ideality or randomness of the ionomer structure. As this value approaches lower values than the ones reported here for random simulations, then the system becomes more
3. Experimental
The unmodified poly(styrene-isobutylene-styrene) triblock copolymer was provided by Kuraray Co. Ltd., Tsukuba research laboratories (sample name-TS-3000S, lot. no. 990215) with the reported properties: 30.84 wt.% styrene, 0.95 specific gravity, Mw = 71,920 g/mol, Mn = 48,850 g/mol, and PDI = 1.47. Nafion® 117 was obtained from C.G. Processing Inc. (1100 equivalent weight). The Nafion membranes were pretreated to their acid form, similar to a procedure reported elsewhere [30], by refluxing the membrane in a HCl/HNO3 (50/50 (v/v)) solution, then leaching out the excess acid by refluxing in deionized water three times, and drying in a vacuum oven at 125 ◦ C. HPLC grade water (J.T. Baker) and methanol (EM Scientific) were used for all transport experiments. Other chemicals used are as follows: sodium hydroxide (Aldrich, ACS grade, Assay +97%), tetrahydrofuran (THF; Burdick & Jackson, HPLC grade, Assay 99.9%), glacial acetic acid (J.T. Baker, Assay 100%), thymol blue (J.T. Baker), toluene (VWR), hexanol (J.T. Baker), methylene chloride (EM Science, HPLC grade), sulfuric acid (Fisher Scientific, Assay 95.4%), and acetic anhydride (Mallinckrodt). 3.2. Membrane preparation The chemical structure of SIBS before and after sulfonation is shown in Fig. 2. Sulfonation of SIBS was performed similar to a procedure described elsewhere [12]. An example of the sulfonation procedure used in this study is as follows: 25 g of the triblock copolymer was dissolved in 300 ml of methylene chloride. This solution was refluxed, while a specified amount of a solution of acetyl sulfate in methylene chloride was added. The reaction was terminated after 5 h by adding approximately 30–50 ml of methanol. The
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Table 1 Ion-exchange capacities and densities Polymer name
IEC (mmol/g)a
IEC (mmol/g)b
ρ (g/cm3 )
Nafion 117 S-SIBS-0 S-SIBS-0.1 S-SIBS-0.4 S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0
NA NA 0.11 0.39 0.45 0.62 0.72 0.94 0.96
0.91c NA 0.12 0.36 0.47 0.63 0.71 0.94 0.97
2.27 0.95 0.95 0.96 0.97 1.00 1.05 1.06 1.04
± ± ± ± ± ± ±
0.01 0.01 0.02 0.04 0.01 0.01 0.01
± ± ± ± ± ± ± ± ±
0.12 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
NA: not applicable. a Determined by titration. b Determined by elemental analysis. c Properties provided by C.G. Processing Inc. (1100 equivalent weight).
Fig. 2. Chemical structure of poly(styrene-isobutylene-styrene) (SIBS) and sulfonated poly(styrene-isobutylene-styrene) (S-SIBS). Sulfonation of styrene units occurs randomly (R ≡ only H for 0 mol% sulfonation, R ≡ only SO3 − for 100 mol% sulfonation, and R ≡ combination of H and SO3 − for a sulfonation percent >0 and <100 mol%).
reacted polymer solution was then precipitated with warm water. The precipitate was washed several times with hot water and acetone, dried in a vacuum oven at 45 ◦ C for 24 h, washed in cold water two times, and then dried again at 45 ◦ C for 24 h. This procedure was repeated with different amounts of acetyl sulfate to produce several sulfonated polymers with various sulfonic acid concentrations (i.e. ion contents), see Table 1. The ion-exchange capacity (IEC) (mmol of sulfonic acid/g of polymer) of each polymer was determined by both titration and elemental analysis (EA). Titration was accomplished by dissolving 0.5 g of polymer in 30 ml of tetrahydrofuran and then titrating with 0.02N sodium hydroxide in methanol to the phenolphthalein endpoint with a thymol blue indicator. The sodium hydroxide solution was standardized against glacial acetic acid. The accuracy of each titration was confirmed by EA, which was conducted by Atlantic Microlab Inc. in Norcross, Georgia. The results from both procedures are listed in Table 1. The IECs determined from EA are within the standard de-
viation of those determined from titration, where the values listed in Table 1 for titration are the average of multiple experiments and the standard deviation is the error between these experiments. Hereafter, the sulfonated triblocks produced will be referred to as S-SIBS-#, where S-SIBS represents sulfonated poly(styrene-isobutylene-styrene) and the succeeding number, #, refers to the IEC. After sulfonation and evaluation, the S-SIBS samples were then dissolved in a mixed solvent of toluene/hexanol (85/15 (w/w)) at 5% (w/v) and cast from solution in open Teflon Petri dishes for approximately 3–4 days at ambient conditions. Solvent-cast membranes were then annealed in a vacuum oven at 50 ◦ C for an additional 2 weeks to remove any residual solvent. All membrane thicknesses were measured with a digital micrometer (±1 m accuracy), and these thicknesses were accounted for in both methanol and proton transport calculations. Thicknesses measured for Nafion 117 and the S-SIBS membranes were 187 and 200–300 m, respectively. 3.3. Permeability The methanol permeability of each membrane was measured using a side-by-side glass diffusion cell equipped with a thermal jacket, shown in Fig. 3. The diffusion cell was purchased from PermGear Inc. of Hallentown, PA. Prior to all experiments, membranes were hydrated in HPLC grade water for at least 48 h. Then for each experiment, a membrane was clamped between well-stirred donor (A) and receptor (B)
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Fig. 3. Schematic diagram of side-by-side glass diffusion cell with flow-through in-line FTIR-ATR spectroscopic detector.
compartments with a membrane cross-sectional area of 0.636 cm2 exposed to the solutions in both compartments. The receptor compartment (VB = 3.4 ml capacity) was initially filled with water, while the donor compartment (VA = 3.4 ml capacity) was charged with 2.0 M methanol (8 vol.%), a standard concentration used in current DMFCs [2]. The concentration of methanol that permeates through the membrane was measured in the flow-through receptor side with the use of a low-flow pump (Watson–Marlow) and a real-time in-line Fourier transform infrared, attenuated total reflectance (FTIR-ATR) spectrometer for detection. The FTIR spectrometer (Nicolet Magna 560 Series) used was equipped with a flow-through horizontal ATR cell (Specac Inc.) containing its own thermal jacket. A zinc selenide ATR crystal (Specac Inc.) with a refractive index of 2.4 and entry and exit faces beveled at a 45◦ angle was used. Infrared spectra were continuously recorded throughout each experiment at 67 s intervals using 128 scans and 4 cm−1 resolution for each collected spectrum. In all experi-
ments, both the side-by-side diffusion cell and ATR cell were temperature controlled with the same circulating water bath. The additional liquid volume in the receptor side due to the in-line sampling technique was calculated for each experiment by measuring the weight change in water added. The added volume was approximately 9 ml. Permeability of methanol can be determined by using an approximate solution of the continuity equation for diffusion in plane sheet geometry at early times [31]: PCA A L2 CB (t) = t− (13) VB L 6D for the boundary conditions: CA CB , where CA and CB are the concentration of methanol in the donor and receptor compartments, respectively. In Eq. (13), L is the membrane thickness, VB the volume of the receptor compartment, A the cross-sectional area of the membrane (0.636 cm2 ), and P the permeability coefficient for methanol. The methanol permeability, P,
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is defined here as the product DK, where D is the methanol diffusion coefficient, and K the partition coefficient. In Eq. (13), the permeability, P, can be determined from the slope of the methanol concentration in the receptor compartment versus time. By rearranging Eq. (13) to CB (t)VB L L2 =P t− (14) CA A 6D the permeability coefficient is directly equal to the slope of [(CB (t)VB L)/(CA A)] versus time. Eq. (14) allows for the comparison of membranes of different thicknesses on the same plot. 3.4. Conductivity
233
3.6. Density The density of each sample was measured using a helium pycnometer (Quantachrome Ultrapycnometer 1000) and the results are listed in Table 1. This is a non-invasive procedure using purified helium as the displaced medium. After calibration, the specimens were placed in the measurement cell and purged with helium for 5–10 min. At least three samples were taken for each data point and the values in Table 1 reflect the averages and standard deviations of these values. 4. Results and discussion 4.1. Permeability
The proton conductivity of each polymer membrane was determined by AC impedance spectroscopy. The measurements were taken between 10 Hz and 100 kHz using a Solartron AC impedance system (1260 impedance analyzer, 1287 electrochemical interface, Zplot software). A four-point probe method was employed using a test fixture designed and constructed at Case Western Reserve University [32]. Membrane samples measuring 3 cm by 0.5 cm were first hydrated in deionized water before being placed into the test fixture assembly. The conductivity cell (i.e. test fixture assembly) was placed in a sealable container with electrical feedthroughs and a small amount of water to maintain an atmosphere of 100% relative humidity. Impedance measurements were taken at each temperature after equilibrating for a minimum of 2 h, where temperature was controlled by placing the sealable container in a Tenney chamber. 3.5. Solubility The solubility of each polymer was determined using a Hi-Res TGA 2950 Thermogravimetric Analyzer (TA Instruments). Samples weighing between 5 and 15 mg were pre-saturated in either water or 2.0 M methanol for a minimum of 1 week. Then the weight loss of solvent was measured after heating the samples to 250 ◦ C at 1 ◦ C/min. No weight loss was observed on dry samples and no weight loss was observed over 100 ◦ C on the pre-saturated samples.
In order to measure the concentration of methanol in the receptor side of the diffusion cell using the in-line non-invasive infrared technique, the infrared absorbance spectrum for methanol in water was calibrated with respect to concentration. Fig. 4 shows infrared spectra of five reference solutions ranging from 0.2 to 1.0 vol.% of methanol in water, where the absorbance peak at 1016 cm−1 represents the C–O stretching vibration of methanol. These concentrations are in the range of those studied for the permeation experiments. Fig. 5 shows the resultant calibration from Fig. 4, where the peak height of each absorbance spectra is plotted versus methanol concentration. Fig. 5 shows a linear relationship between absorbance and concentration with a slope equal to 0.03. Using the calibration in Fig. 5, the concentration of methanol can be plotted versus time, shown in Fig. 6. Methanol permeability can be determined from the slope of this data using Eq. (13). Results from this experiment show that the methanol permeability of Nafion 117 is 1.98 × 10−6 cm2 /s at a temperature of 25 ◦ C and a feed concentration of 2.0 M methanol. The methanol permeability determined from this study is similar to values previously reported in literature: 2.3 × 10−6 cm2 /s at ambient temperature and 1.0 M methanol [17] and (1–1.5) × 10−6 cm2 /s at 22 ◦ C and 2.0 M methanol [33]. Fig. 7 shows three different permeation experiments at 25 ◦ C on the same plot (Nafion 117, S-SIBS-0.6, and S-SIBS-1.0), where [(CB (t)VB L)/(CA A)] is plotted versus time.
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Fig. 4. Infrared absorbance spectra representing the C–O stretching vibration (peak maximum at 1016 cm−1 ) of methanol at different methanol/water concentrations (1.0, 0.8, 0.6, 0.4, 0.2 vol.% of methanol in water).
The methanol permeabilities determined directly from the slope of the data in Fig. 7 using Eq. (14), reveals that the S-SIBS membrane is more than an order of magnitude lower in methanol permeability compared to Nafion 117 at a similar IEC. Methanol permeabilities for all the membranes studied are shown in Fig. 8 as a function of IEC. The methanol permeability for the membranes: S-SIBS-0, S-SIBS-0.1, and S-SIBS-0.4, could not be measured.
At an IEC of approximately 0.46 mmol/g the onset of the percolation threshold is observed, whereby connectivity between ionic domains provide transport pathways and permeability can occur. Above the percolation threshold, the methanol permeability for S-SIBS-0.5 is approximately 217 times lower than Nafion 117, while S-SIBS-1.0 (similar IEC to Nafion 117) is 15 times lower. The solid line in Fig. 8 represents a regression to the percolation model,
Fig. 5. Calibration: infrared absorbance of C–O stretch (peak height) of methanol vs. methanol concentration (slope = 0.03).
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Fig. 6. Receptor methanol concentration as a function of time for the diffusion of methanol through Nafion 117 at 25 ◦ C. The permeability was determined from the slope of the line (P = 1.98 × 10−6 cm2 /s).
which will be discussed in more detail later in this paper. 4.2. Conductivity and selectivity Proton conductivity experiments, measured with AC impedance spectroscopy, are also shown in Fig. 8 as a function as IEC along with the methanol permeability results. The conductivity experiments in Fig. 8 were conducted at ambient conditions (21–22 ◦ C).
In Fig. 8, Nafion 117 is approximately three times more conductive than S-SIBS-1.0 and approximately 21 times more conductive than S-SIBS-0.5. The conductivity for Nafion 117 determined in this study at ambient temperatures, 0.067 S/cm, is similar to values reported in the literature with similar techniques and at similar temperatures: 0.067 S/cm [35], 0.061 S/cm [36], and 0.054–0.082 S/cm [33]. Conductivity also follows the percolation model with a similar percolation threshold to that of the permeability data and
Fig. 7. Permeability data for three different membranes at 25 ◦ C: Nafion 117 (䉫), S-SIBS-1.0 (䊊), and S-SIBS-0.6 ( ). The permeabilities are directly equal to the slopes of the lines, see Eq. (14).
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Fig. 8. Methanol permeability (25 ◦ C) and proton conductivity (21–22 ◦ C) vs. IEC for Nafion 117 (䉫, 䉬, respectively) and S-SIBS membranes (䊊, 䊉, respectively). The solid lines represent a regression to the percolation model, where the percolation threshold is approximately 0.46 mmol/g.
these results will be discussed in more detail in the next section. Fig. 9 shows the selectivity of the membranes studied, where selectivity is defined by Eq. (7). The selectivity of the S-SIBS membranes is approximately 5–10 times more selective than Nafion 117 with selectivities of 10.4, 4.3, 9.3, 5.5, and 5.3 times greater than Nafion 117 with increasing IEC. In this study, there is no apparent trend in selectivities in relation to IEC. Further experiments are underway on S-SIBS membranes with
a variety of IECs higher than Nafion to understand the cause for the increase in selectivity and the variation that is observed here. This increase in selectivity may be due to the low methanol and water solubility of polyisobutylene (PIB), where PIB is the major component of the triblock copolymer ionomer, S-SIBS (70 wt.% of the unmodified block copolymer) [37]. Low solubility of water and 2.0 M methanol (<1 wt.% uptake) in the unsulfonated block copolymer (70 wt.% PIB) is also demonstrated in this study (Fig. 10).
Fig. 9. Selectivity (conductivity/permeability) vs. IEC at 21–25 ◦ C for Nafion 117 (solid bar) and S-SIBS membranes (open bars).
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237
Fig. 10. Solubility data for all membranes studied: volume fraction of water and 2.0 M methanol in Nafion 117 (䊐, 䉫, respectively) and S-SIBS membranes ( , 䊊, respectively) as a function of IEC.
4.3. Percolation Membrane structure may be another reason for increased selectivity. The transport data in Fig. 8 for the S-SIBS membranes were fit to the percolation model with respect to IEC, but the percolation model in Eq. (12) relates diffusivity or conductivity to the volume fraction of the minority phase. In order to determine the volume fraction of the minority phase
or solvent in the polymer, the solubility of 2.0 M methanol and water was determined using gravimetric analysis at each ion-exchange capacity. Fig. 10 shows the results of these experiments, where the volume fraction was determined from the weight fraction of solvent in the polymer and the density of the solvent and polymer (see Table 1). Weight fraction is defined here as the ratio of solvent uptake or loss to the wet weight of the polymer (weight of polymer + weight
Fig. 11. Determination of percolation values: log–log plot of methanol permeability (䊊) and proton conductivity (䊉) vs. the excess volume fraction of solvent (2.0 M methanol and water for permeability and conductivity, respectively) in S-SIBS membranes.
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of solvent uptake). The weight fraction of water uptake in Nafion 117 determined in this study was approximately 0.25, similar to another study that reports a value of 0.26 [19]. In Fig. 10, there is a linear relationship between the volume fraction of both water and 2.0 M methanol in the polymer and IEC in the ion content range studied above the percolation threshold, represented by the solid lines. Fig. 11 shows a log–log plot of both proton conductivity and methanol permeability as a function of excess volume fraction in the polymer (water for conductivity and 2.0 M methanol for permeability), where the critical volume fraction, φ1,c , was 0.077 and 0.095 for water and 2.0 M methanol, respectively. The slope of the data is equivalent to the critical exponent for diffusion, γ D , and was 0.76 and 1.15 for conductivity and permeability, respectively. The critical exponent for conductivity in S-SIBS is significantly lower than those reported in the literature for other random copolymer ionomers and Nafion [22–26], shown in Table 2. This result alludes to possible structural differences in this block copolymer ionomer compared to that of other random ionomers. Also, the value of 0.76 for the critical exponent determined in this study is closer to Kirpatrick’s [28] value for the accessible critical exponent (γA = 0.3–0.4) than that of the diffusion critical exponent (γD = 1.6–1.7), suggesting that this block copolymer ionomer possesses a more ordered structure than other random ionomers.
Table 2 Percolation values Polymer name
γD
φ1,c
Reference
Nafion Poly(methyl methacrylateco-methacrylic acid) Poly(styrene-comethacrylic acid) Sulfonated poly(phenylene oxide) Sulfonated poly[bis(3methylphenoxy) phosphazene] S-SIBS
1.5 ± 0.02 1.35
0.10 0.26
[22] [23]
1.7
0.165
[24]
1.5
0.16
[25]
1.26
0.175–0.25
[26]
0.76
0.077
This work
4.4. Temperature effects In addition to results at low temperatures, transport properties at higher temperatures were studied. Typically temperatures up to 80 ◦ C have been investigated in DMFC operation [2,3]. Beyond 80 ◦ C, dehydration of Nafion occurs and leads to a reduction in proton conductivity. Fig. 12 shows the effects of temperature on methanol permeability for Nafion 117 and two different S-SIBS membranes (0.5 and 1.0 mmol/g). The
Fig. 12. Methanol permeability as a function of temperature for three different membranes: Nafion 117 (䉫), S-SIBS-1.0 (䊊), and S-SIBS-0.5 (+).
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Table 3 Methanol permeabilities and proton conductivities Polymer name
Nafion 117 S-SIBS-0 S-SIBS-0.1 S-SIBS-0.4 S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0 a b
Methanol permeability (cm2 /s)
Proton conductivity (S/cm)
25 ◦ C
80 ◦ C
21–22 ◦ C
80 ◦ C
2.01 × 10−6
3.53 × 10−6
6.70 × 10−2
1.60 × 10−1
a
a
a
a
a
a
a
a
a
a
a
9.24 6.03 6.10 1.37 1.39
× × × × ×
10−9 10−8 10−8 10−7 10−7
4.29 × 10−8 b b b
1.40 × 10−6
3.20 8.70 1.90 2.53 2.47
a
× × × × ×
10−3 10−3 10−2 10−2 10−2
7.65 6.27 5.90 8.09 8.30
× × × × ×
10−2 10−2 10−2 10−2 10−2
Not measurable. Not measured.
permeability decreases exponentially with the inverse of temperature following an Arrhenius behavior. Although the methanol permeability of S-SIBS-1.0 is 15 times lower than Nafion 117 and S-SIBS-0.5 is 217 times lower when measured at 25 ◦ C, the methanol permeability for S-SIBS-1.0 and S-SIBS-0.5 is only 2.5 and 82 times lower, respectively, than Nafion 117 at 80 ◦ C. Permeabilities at 25 and 80 ◦ C are listed in Table 3, where the values at 25 ◦ C are the average of the all experiments for each membrane shown in Fig. 8. Activation energies can be calculated from the slope of the data in Fig. 12 (see Table 4). The activation energy for permeability is almost four times greater for S-SIBS-1.0 compared to Nafion 117 and approximately three times greater for S-SIBS-0.5. The activation energy for Nafion 117 in this study, 10 kJ/mol, is similar to other studies, 12 kJ/mol [34] and 18 kJ/mol Table 4 Activation energies Polymer name
Nafion 117 S-SIBS-0 S-SIBS-0.1 S-SIBS-0.4 S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0
Activation energy (kJ/mol) Methanol permeability
Proton conductivity
9.90 NA NA NA 30.62 NA NA NA 37.10
11.53 NA NA NA 41.24 25.00 18.83 17.36 18.11
NA: not applicable.
[33]. One possible explanation for the significant difference in activation energies may be caused by the difference in glass transition temperatures between Nafion 117 and S-SIBS [15,38]. Fig. 13 shows the effects of temperature on proton conductivity, where the solid lines represent a regression to an Arrhenius model. From 21–22 to 80 ◦ C, the conductivity of the S-SIBS membranes, ranging from 0.7 to 1.0 mmol/g, all increase by approximately 4-fold, S-SIBS-0.6 increases 7-fold, and S-SIBS-0.5 increases 24-fold, while Nafion 117 only increases by a factor of 2. Interestingly, at 80 ◦ C, all the S-SIBS membranes studied are approximately only one-half as conductive compared to Nafion 117. Similar to permeability, the activation energies of conductivity for the S-SIBS membranes are higher than Nafion 117 (see Table 4), but unlike permeability, the activation energies for conductivity for S-SIBS decrease with increasing IEC. Temperature may affect the block copolymer ionomer morphology, which may in turn affect proton mobility. Experiments are currently underway to determine the polymer structure and its effects on transport properties. Additionally, the activation energy determined for Nafion 117 in this study, 11.5 kJ/mol, compares well to a prior study, 11 kJ/mol [33]. Fig. 14 shows the effects of temperature on selectivity. At higher temperatures (80 ◦ C) the selectivity for S-SIBS-1.0 is reduced (1.3 times more selective than Nafion 117 compared to 5 times more selective at 21–25 ◦ C), however S-SIBS-0.5 increases in selectivity at higher temperatures (39 times more selective than Nafion 117 compared to 10 times more selective
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Fig. 13. Proton conductivity as a function of temperature for Nafion 117 (䉬), S-SIBS-1.0 (䊊), S-SIBS-0.9 (䊐), S-SIBS-0.7 ( ), S-SIBS-0.6 (×), and S-SIBS-0.5 (+).
Fig. 14. Selectivity vs. IEC at 80 ◦ C and 21–25 ◦ C, comparing Nafion 117 (solid bars) to S-SIBS membranes (1.0 and 0.5 mmol/g, open bars).
at 21–25 ◦ C). Although conductivities are much lower for S-SIBS close to the percolation threshold (S-SIBS-0.5) compared to Nafion 117, this membrane is still interesting due to its low methanol permeability.
5. Conclusions In this study, the transport properties of a triblock copolymer ionomer show that it is a viable alternative
to Nafion for its application to the DMFC. S-SIBS is slightly less conductive than Nafion, but has a methanol permeability more than an order of magnitude lower than Nafion allowing for enhanced fuel cell efficiency. The increased selectivity may be due to low methanol and water solubility in polyisobutylene, the major component of the triblock copolymer ionomer examined in this study. Also, the percolation values calculated in this study suggest that the membrane possesses a more ordered structure compared
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to other ionomers. This ordered structure may also greatly affect the transport properties. An experimental investigation using small-angle X-ray scattering is underway to determine the actual morphology of S-SIBS in order to understand its effect on the transport properties. In addition, the fabrication of membrane electrode assemblies (anode/membrane/cathode composite) using S-SIBS and subsequent fuel cell testing with S-SIBS as the PEM is underway.
Acknowledgements This work was performed while the author (Y.A. Elabd) held a National Research Council Research Associateship Award at the US Army Research Laboratory. The authors gratefully acknowledge Alexis Hillock for assistance with thermogravimetric experiments.
Nomenclature A CA CB CB (t)
Cm Cmo Cp D Dm Do Dp F jm jp Km L
cross-sectional area of the membrane (0.636 cm2 ) concentration of methanol in the donor compartment concentration of methanol in the receptor compartment concentration of methanol measured in the receptor compartment as a function of time methanol concentration concentration of methanol on the donor side (anode) of the membrane concentration of protons or concentration of ion sites in the polymer observed diffusivity methanol diffusion coefficient inherent diffusivity proton diffusion coefficient Faraday’s constant methanol flux proton flux methanol partition coefficient membrane thickness
Pm R t T VB zp
241
methanol permeability gas constant time temperature volume of the receptor compartment proton charge
Greek letters α selectivity (ratio of proton conductivity to methanol permeability) β selectivity (ratio of fluxes) γ A critical exponents for the accessible regions γ D critical exponents for the diffusion regions σ observed conductivity σo inherent conductivity σp proton conductivity φ1A volume fraction of interconnected or accessible regions φ1,c critical volume fraction φ1D diffusion volume fraction φ1I volume fraction of isolated clusters φ1T total volume fraction of the minority phase ψ electrostatic potential
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