Triblock copolymer ionomer membranes

Journal of Membrane Science 231 (2004) 181–188

Triblock copolymer ionomer membranes Part II. Structure characterization and its effects on transport properties and direct methanol fuel cell performance Yossef A. Elabd a,∗ , Charles W. Walker b , Frederick L. Beyer c a

Department of Chemical Engineering, Drexel University, Philadelphia, Pennsylvania, PA 19104, USA US Army Research Laboratory, Sensors and Electron Devices Directorate, Adelphi, MD 20783, USA US Army Research Laboratory, Weapons and Materials Research Directorate, Aberdeen Proving Ground, Aberdeen, MD 21005, USA b

c

Received 5 May 2003; received in revised form 12 November 2003; accepted 20 November 2003

Abstract In this study, the morphology, transport properties, selectivity, and direct methanol fuel cell (DMFC) performance of a triblock copolymer ionomer, sulfonated poly(styrene–isobutylene–styrene) (S-SIBS), were investigated. The structure of S-SIBS membranes, ranging from 0.5 to 1.0 meq/g, was characterized using small-angle X-ray scattering and revealed a lamellar morphology with a preferred orientation in the plane of the membrane. Subsequently, proton conductivity measured normal to the plane of the membrane was over 10 times lower than when measured in plane, compared to only a 2.5 times difference in conductivity between the two techniques in Nafion 117. When proton conductivity was measured normal to the plane, the resulting selectivities (i.e. proton conductivity/methanol permeability) of S-SIBS membranes are similar to Nafion 117. A previous study [J. Membrane Sci. 217 (2003) 227] reports 5–10 times higher selectivities compared to Nafion 117 when conductivities were measured in plane. In addition, the DMFC performance of S-SIBS (1.0 meq/g) was lower than that of Nafion 117. These significant differences in transport properties and fuel cell performance with respect to polymer structure in S-SIBS membranes provide new insights into designing oriented nanostructures in polymer electrolyte membranes for enhanced transport properties for fuel cell applications. © 2003 Elsevier B.V. All rights reserved. Keywords: Direct methanol fuel cell; Polymer electrolyte membranes; Barrier membranes; Electrochemistry; Pervaporation

1. Introduction The direct methanol fuel cell (DMFC) provides an alternative energy source for the future, whereby a polymer electrolyte membrane (PEM) serves as the centerpiece (cell separator and proton conductor). Unfortunately, poor PEM selectivity (i.e. proton conductivity/methanol permeability) has deterred the progress of the DMFC. High methanol permeabilities in PEMs lead to lower cell efficiencies and lifetimes [1]. Over the past decade, several investigators have focused on developing new PEMs in an attempt to increase DMFC efficiencies [2–8].

∗ Corresponding author. Tel.: +1-215-895-0986; fax: +1-215-895-5837. E-mail address: [email protected] (Y.A. Elabd).

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.11.019

Recently, our laboratory [9] has focused on the synthesis (via sulfonation) and characterization of block copolymer ionomers for application to the DMFC as a possible alternative PEM to Nafion® (DuPont), the most frequently used PEM in fuel cells. Block copolymer ionomers conjoin the concepts of two different materials: block copolymers and ionomers. Block copolymers are two distinctively different polymers (i.e. blocks) arranged on the same polymer chain in an ordered sequence. In the solid state, block copolymers microphase separate on a sub-micron or nanometer scale due to the thermodynamic incompatibility between unlike blocks. A variety of self-assembled block copolymer morphologies have been studied, ranging from spheres arranged on a cubic lattice, hexagonally packed cylinders, interpenetrating gyroid morphologies, to alternating lamellae [10]. Ionomers, frequently referred to as PEMs, however, are polymers with ionic pendant groups or pairs arranged either randomly or

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systematically along the polymer backbone. Unlike block copolymers, ionomers phase segregate on a nanometer scale due to electrostatic interactions among ion pairs forming two phases: ion-rich and ion-poor phases. The ion-rich domains are an aggregation of ions, referred to as ion clusters, while the ion-poor domains are mostly non-ionic polymer [11]. One investigation [12], on Nafion in particular, suggests not only phase segregation leading to two separate domains, but also that ion clusters are interconnected by small narrow ionic channels on the order of 1 nm in diameter. When ionic clusters transition from isolated to interconnected domains, this is referred to as an insulator to conductor transition (percolation threshold). Above this threshold, protons transport across the polymer through interconnected ionic pathways. Several investigations [13–17] have experimentally shown that transport in ionomers follows a percolation model, whereby no transport occurs below the percolation threshold and transport above the threshold is a function of the excess volume fraction and can be expressed as σ D ∝ ∝ (φ1 − φ1,c )γD σ0 D0

(1)

where σ and D are the observed conductivity and diffusivity, respectively; σ 0 and D0 the inherent conductivity and diffusivity in the diffusing phase, respectively; γ D the critical exponent for diffusion; φ1 the volume fraction of the diffusing or minority phase; φ1,c 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 ) the excess volume fraction. Values for γ D give an indication of the order of the ionomer structure. At high values (e.g., 1.6–1.7), the structure is non-ideal or random, and at low values (e.g., 0.3–0.4), the structure is more ideal or ordered [18]. When one block in a block copolymer contains ionic groups, this material can be classified as a block copolymer ionomer. In this study, a block copolymer ionomer was synthesized by incorporating ionic groups (sulfonic acid) randomly along the backbone of the styrene blocks (via sulfonation). Although there is limited information regarding the transport properties of block copolymer ionomers [9,19,20], a variety of block copolymer ionomers have been synthesized and characterized [21–25]. In theory, a block copolymer ionomer should self-assemble into a three-phase morphology, whereby different blocks microphase separate due to thermodynamic incompatibilities and another phase segregation will occur within the ionic block due to electrostatic interactions among ion pairs. Using X-ray scattering, Wiess et al. [26] revealed a three-phase morphology exists in sulfonated poly(styrene–(ethylene-co-butylene)–styrene) (<25 mol% sulfonation of the styrene block) with a Bragg spacing of approximately 3–4 nm for the ionic domains and 20–30 nm for the polystyrene domains. Further studies will be required to access whether a three-phase morphology exists in block copolymer ionomers at higher

sulfonation levels (>25 mol% sulfonation of the styrene blocks). In a recent study [9], the transport properties (i.e., methanol and proton transport) of a triblock copolymer ionomer, sulfonated poly(styrene–isobutylene–styrene) (SSIBS), were investigated and compared to Nafion to determine if S-SIBS could serve as a viable alternative PEM for application to the DMFC. S-SIBS membranes were approximately 5–10 times more selective compared to Nafion 117, with methanol permeabilities more than an order of magnitude lower and proton conductivities only three times less. In addition, the transport results followed a power law dependent percolation model, where the critical exponent for diffusion (γ D ) was much lower than those reported in other studies, suggesting a more ordered structure. Results from this study [9] suggest a direct relationship between both structure and transport properties in dense block copolymer ionomers. In order to understand this relationship and its possible effects on enhancing membrane selectivity, the morphologies of S-SIBS membranes were characterized using small-angle X-ray scattering (SAXS). These results were analyzed with respect to the proton conductivity (in the plane and normal to the plane of the membrane), percolation theory, and DMFC performance to understand structure–transport property relationships in block copolymer ionomers.

2. Experimental 2.1. Materials Sulfonation and preparation of S-SIBS membranes have been described in detail elsewhere [9]. The unsulfonated polymer, poly(styrene–isobutylene–styrene) (SIBS) triblock copolymer, was provided by Kuraray Co., Ltd., Tsukuba research laboratories (sample name-TS-3000S, lot. no. 990215) with the reported properties: 19.36 mol% (30.84 wt.%) styrene, 0.95 specific gravity, Mw = 71,920 g/mol, Mn = 48,850 g/mol, and PDI = 1.47. The ion-exchange capacity (IEC) of each S-SIBS membrane used in this study was determined by elemental analysis (EA) and the results are listed in Table 1. 2.2. Small-angle X-ray scattering Small-angle X-ray scattering was performed on a beamline X27C at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, NY. Two-dimensional scattering patterns were collected on a pinhole-collimated system using Fujitsu image plates and read by a Fujitsu BAS 2000 image plate reader. Specialty software available at Brookhaven National Laboratory was used to reduce two-dimensional data to one-dimensional intensity versus scattering vector (q) plots after background subtraction by circular averaging. The X-ray wavelength

Y.A. Elabd et al. / Journal of Membrane Science 231 (2004) 181–188 Table 1 Polymer samples and their ion-exchange capacities Polymer name

IEC (meq/g)a

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

0.91b NAc 0.12 0.36 0.47 0.63 0.71 0.94 0.97

a

From Ref. [9]. Properties provided by C.G. Processing, Inc. (1100 equivalent weight). c Not applicable. b

employed was 1.605 Å. The calibration standard was silver behenate and the sample to detector distance was 210 cm. 2.3. Conductivity The proton conductivity of each membrane was determined by AC impedance spectroscopy between 10 and 100 kHz using test fixtures with two different geometric configurations (see Fig. 1). Measurements in the plane of the membrane were determined with a four-electrode cell, shown in Fig. 1a. The experimental protocol used for the four-electrode cell is described in more detail else-

Membrane

1cm

Current Carrying Electrodes

Voltage Measuring Electrodes

183

where [9]. Proton conductivity measurements normal to the plane of the membrane were determined with the use of a two-electrode cell comprised of 1.26 cm2 stainless steel blocking electrodes, illustrated in Fig. 1b. All membranes were pre-hydrated in deionized water and enclosed in a sealable cell to maintain hydration during impedance measurements. Typically, a four-electrode cell is preferred over a two-electrode cell because of the significant frequency dependence on impedance at low frequencies due to interfacial impedance [35]. However, using only one technique cannot accurately account for the effects that oriented structures in polymer membranes may have on transport properties. In this study, conductivity was measured at the upper limit of the frequency range shown above, where there is only a minor dependency on frequency [35]. 2.4. MEA fabrication and DMFC tests In order to test DMFC performance on the membranes studied, separate membrane electrode assemblies (MEAs) were fabricated with Nafion 117 and S-SIBS-1.0 as the PEMs. A schematic of the DMFC and MEA is illustrated in Fig. 2. For the Nafion MEA, Pt/Ru (50/50 wt.%, Alpha Aesar) and Pt (Alpha Aesar) were deposited by a catalyst ink transfer method on the anode and cathode side of the membrane, respectively. The Pt/Ru ink consisted of 15 wt.% Nafion binder, prepared from a solution of 5 wt.% Nafion (Aldrich) suspended in a water/propanol mixture. The ink was painted onto a Teflon® -coated Fiberglass support (5 cm2 , C.S. Hyde Co.), dried, and subsequent layers applied to obtain a coverage of 4.2 mg/cm2 Pt/Ru. The Pt ink was prepared similarly with 10 wt.% Nafion binder and applied to another Teflon® -coated Fiberglass coupon with a coverage of 4.9 mg/cm2 . A dry Nafion 117 membrane (>5 cm2 ) was sandwiched between these two coupons, with Carbon Cloth Reactant Diffusion Layer

(a)

CO2

H 2O

Membrane

(b)

Graphite Block With Parallel Channel Flow Paths

Solid Blocking Electrodes

Fig. 1. Schematic diagram of cells used to measure membrane proton conductivity via AC impedance spectroscopy: (a) four-electrode cell (in the plane of the membrane) and (b) two-electrode cell (normal to the plane of the membrane).

CH3OH + H 2O

O2

MEA (Anode/PEM/Cathode) Fig. 2. Schematic diagram of the direct methanol fuel cell. The membrane electrode assembly consists of a Pt/Ru anode catalyst/PEM/Pt cathode catalyst composite.

Y.A. Elabd et al. / Journal of Membrane Science 231 (2004) 181–188

the catalyst side of each coupon facing the membrane, and hot pressed at 125 ◦ C between two steel plates at 272 kg/cm2 for 30 s. The Teflon® /Fiberglass coupons were then removed with the catalyst layers remaining on both sides of the membrane. The MEA (anode/PEM/cathode composite) was then sandwiched between two carbon cloth diffusion layers (E-TEK) and bolted into the DMFC single cell test fixture (Fuel Cell Technologies). The MEA with S-SIBS-1.0 as the PEM was prepared similarly to the Nafion MEA with several minor differences. The anode and cathode were prepared with a 15 and 10 wt.% binder of S-SIBS-1.0, respectively, from a solution of 5 wt.% S-SIBS-1.0 dissolved in THF. Catalyst coverage on the transfer coupons was 5.7 mg/cm2 Pt/Ru and 4.9 mg/cm2 Pt. Ink transfer onto the S-SIBS-1.0 membrane was accomplished by pressing between two steel plates at 363 kg/cm2 for 1 min at room temperature. Testing was done with a fuel cell test station (Fuel Cell Technologies) and a liquid chromatograph pump (Shimatzu) to accurately control methanol flow rate. The conditions for the DMFC test with Nafion 117 as a PEM are as follows: 1.0 M methanol with a flow rate of 1 ml/min and 207 MPa back gauge pressure to prevent the vaporization of methanol; air humidified at 80 ◦ C with a flow rate of 400 ml/min and gauge pressure of 207 MPa; and a cell temperature of 80 ◦ C. The conditions for the DMFC test with S-SIBS-1.0 as a PEM are as follows: 1.0 M methanol with a flow rate of 4 ml/min and 138 MPa back gauge pressure; air humidified at 85 ◦ C with a flow rate of 400 ml/min and gauge pressure of 207 MPa; and a cell temperature of 80 ◦ C. Cells had an open circuit potential of approximately 0.6 V, and DMFC tests were conducted by sweeping potential from 0.6 to 0.2 V by increments of 0.02 V every 10 s and recording current.

3. Results and discussion

18

10

Intensity (Arbitrary Units)

184

q* 2q*

16

10

3q* 4q*

14

10

S-SIBS-1.0

12

10

S-SIBS-0.9

10

10

8

10

Increasing IEC

SSIBS-0.7

6

10

S-SIBS-0.6

4

10

100

S-SIBS-0.5

1 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-1

q (nm ) Fig. 3. Small-angle X-ray scattering intensities as a function of scattering vector for S-SIBS membranes ranging in IEC from 0.5 to 1.0 meq/g. Intensity maximums representative of a lamellar morphology are indicated with arrows. The intensity curves are offset here for legibility with lowest to highest IEC samples arranged from bottom to top.

tering vector positions: q∗ , 2q∗ , 3q∗ , 4q∗ , and 5q∗ , for all S-SIBS membranes (0.5–1.0 mmol/g), where q∗ is the first order reflection. This scattering pattern specifically corresponds to a lamellar morphology. In addition, Fig. 4 shows the scattering profiles for two different X-ray scattering orientations, in the plane and normal to the plane of the same membrane (S-SIBS-0.7). Scattering normal to the plane of the membrane shows weak scattering and no evidence of an ordered morphology in this orientation. These results suggest an anisotropic structure, where the lamellar domains are highly oriented in the plane of the membrane, illustrated in the inset of Fig. 4. These patterns where observed in all the membranes examined in Fig. 3 (S-SIBS-0.5 to S-SIBS-1.0). The lamellar morphology is periodic in one dimension and has a single interplanar distance or spacing. The interplanar or Bragg spacing, which has been interpreted as an average

3.1. Membrane structure characterization 5

10

4

normal to the plane Membrane in the plane

Intensity

SAXS experiments were conducted on the S-SIBS membranes to determine polymer structure and their possible effects on transport properties. Since transport is the main interest in this study, the S-SIBS membranes above the percolation threshold (0.5–1.0 meq/g), where transport occurs, was the focus of the structural characterization. Fig. 3 shows the intensity profiles (I versus q) for each S-SIBS membrane, ranging in IEC from 0.5 to 1.0 meq/g, where the samples were characterized in the plane of the film. The scattering vector, q, can be defined:

10

1000 100 10

in the plane normal to the plane

1 0.2

4π sin(θ) q= λ

(2)

where 2θ and λ are the scattering angle and wavelength, respectively. Fig. 3 shows a periodic distribution with distinct reflections in the intensity maximums located at the scat-

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

-1

q (nm ) Fig. 4. SAXS profiles for S-SIBS-0.7 for X-ray directions both in the plane and normal to the plane of the membrane. An illustration with the paths of X-rays and the lamellar morphology in the plane of the membrane is depicted in the inset of the graph.

Y.A. Elabd et al. / Journal of Membrane Science 231 (2004) 181–188

185

Table 2 Bragg spacing and lattice spacing of S-SIBS membranes

Proton Conductivity

Polymer name

dlam (nm)

alam (nm)

S-SIBS-0.5 S-SIBS-0.6 S-SIBS-0.7 S-SIBS-0.9 S-SIBS-1.0

26.27 27.86 29.66 27.87 28.74

26.40 27.85 29.67 27.86 28.75

Methanol Permeability Membrane (a)

domain spacing or size, can be calculated from Bragg’s law [27]: 2π q∗

(3)

The Bragg spacing, dlam , is determined from the maximum in the first order reflection, q∗ , in Fig. 3. In addition, the lamellar domain spacing, alam , a more accurate domain spacing or size, can be calculated from a regression method using all of the observed peaks (i.e. experimental interplanar spacings) [28] using the equations: alam = dh (lam)h,

h = 1, 2, 3, 4, . . .

dh (lam) = mh + b

(4)

Proton Conductivity Methanol Permeability Membrane

(b) Fig. 5. Illustration depicting the transport pathways in the membrane: (a) methanol and proton transport normal to the plane and in the plane of the membrane, respectively and (b) methanol and proton transport both normal to the plane of the membrane.

(5)

where (h) is the order of reflection and the y intercept (b) is ideally zero. The values calculated for both dlam and alam are listed in Table 2 and are similar for each membrane. Both dlam and alam range from 26 to 30 nm with no apparent trend in relation to sulfonation or ion content in the block copolymer ionomers. 3.2. Structure–transport relationships In a previous study [9], the measured selectivity (i.e. proton conductivity/methanol permeability) of S-SIBS membranes were 5–10 times higher than Nafion 117 over an ion content range of 0.5–1.0 meq/g. However, proton and methanol transport were measured in different directions with respect to the plane of the membrane in this previous study. Proton conductivity was measured in the plane of the membrane with a four-electrode technique, shown in Fig. 1a, while methanol permeability was measured normal to the plane of the membrane with a permeation cell (Fig. 5a). In this study, proton conductivity was measured normal to the plane of the membrane using a two-electrode technique (Fig. 1b) and compared to the four-electrode technique, in order to understand the effect of structure on transport properties. The two-electrode technique, measuring impedance in the same direction as the methanol transport studies (Fig. 5b), is the direction of relevance for fuel cells. Fig. 6 shows the measured proton conductivity for S-SIBS and Nafion 117 using both techniques (four- and two-electrode techniques) as a function of IEC. The conductivity values in Fig. 6 at each IEC are an average of multiple experiments, where the average standard deviation was 7%. Comparing the two techniques should account for anisotropy in

the S-SIBS membranes, determined from the SAXS analysis. The data in Fig. 6 shows a 2.5-fold difference in conductivity for Nafion between the two techniques. Both values for Nafion: 0.067 and 0.027 S/cm for the four- and two-electrode techniques, respectively, are similar to values reported in literature for Nafion using these two techniques at similar temperatures: 0.067 S/cm [29], 0.061 S/cm [30], 0.054–0.082 S/cm [31] for the four-electrode technique; and 0.024 S/cm [29], 0.022 S/cm [32] for the two-electrode technique. Gardner and Anantaraman [29] report similar values for proton conductivity to the data taken in this study using both techniques and suggest the differences measured for Nafion are primarily due to the differences in the two techniques. 1

Proton Conductivity (S/cm)

dlam =

0.1 0.01 0.001 0.0001

Percolation Threshold

-5

10

0.2

0.4

0.6

0.8

1

1.2

1.4

IEC (meq/g) Fig. 6. Proton conductivity measured in the plane and normal to the plane of the membrane vs. IEC for Nafion 117 (䉬, 䉫, respectively) and S-SIBS membranes (䊉, , respectively).

186

Y.A. Elabd et al. / Journal of Membrane Science 231 (2004) 181–188 Table 3 Percolation values

0.01 0.001 0.0001 -5

10

0.01

0.1 θ

i

-θ

1

i,c

Fig. 7. Log–log plot of proton conductivity measured in the plane (䊉) and normal to the plane () of the membrane vs. excess volume fraction of the diffusing phase (water uptake) in S-SIBS membranes. The solid lines represent a regression to the percolation model.

In contrast to the conductivity data for Nafion, S-SIBS membranes demonstrate a 9–12 times reduction in conductivity, for IECs of 0.6–1.0 meq/g, when comparing the two-electrode technique to the four-electrode technique (shown in Fig. 6). This difference is 4–5 times larger than the difference in conductivity for Nafion. Specifically for S-SIBS-0.5, the difference in conductivity was 34-fold, which is 14 times larger than the difference in conductivity for Nafion. Recent investigations [33,34] have demonstrated similar relationships between conductivity and structure in self-assembled oriented polymer systems. 3.3. Percolation

γD

φ1,c

Reference

Nafion Poly(methyl methacrylateco-methacrylic acid) Poly(styrene-co-methacrylic acid) Sulfonated poly(phenylene oxide) Sulfonated poly[bis(3methylphenoxy) phosphazene] S-SIBS-1.0 S-SIBS-1.0

1.5 ± 0.02 1.35

0.10 0.26

[13] [14]

1.7

0.165

[15]

1.5

0.16

[16]

1.26

0.175–0.25

[17]

0.76a 1.22b

0.077a 0.077b

[9] This work

a Determined from measuring proton conductivity in the plane of the membrane. b Determined from measuring proton conductivity normal to the plane of the membrane.

lamellar domains). The critical exponent for diffusion for S-SIBS normal to the plane of the membrane (γD = 1.22) is a higher value, similar to other studies (γD = 1.3–1.7) and similar to the critical exponent for methanol permeability (γD = 1.15) [9], indicating a less ordered or random structure (transport is in the opposite direction of the preferred orientation of the lamellar domains). The combination of proton conductivity, SAXS, and the regression to the percolation model reveals that transport in the direction of oriented domains provides less resistance in the membranes and can enhance transport properties. 3.4. Selctivity Fig. 8 shows proton conductivity, measured with the two-electrode technique, as a function of IEC along with methanol permeability results [9]. S-SIBS-1.0 (similar IEC to that of Nafion 117) is 10 times less conductive and 15 times less permeable than Nafion 117, while S-SIBS-0.5 (close to the percolation threshold) is 287 times less conduc-

-5

10

1 0.1

-6

10

0.01 -7

10

0.001 -8

10

Percolation Threshold -9

10

0.2

0.4

0.6

0.8

1

1.2

0.0001 10 1.4

-5

Proton Conductivity (S/cm)

Structure–transport property relationships can be further analyzed with the use of percolation theory. Fig. 7 shows a log–log plot of proton conductivity (measured with both techniques) as a function of excess volume fraction in the polymer, where the excess volume fraction is the difference between volume fraction and critical volume fraction. The volume fraction was calculated from the water solubility and density of the polymer, which were both measured previously [9]. The critical volume fraction, φ1,c (i.e., percolation threshold) was determined from the x-axis intercept of the plot of conductivity versus volume fraction and was 0.077 for both the four- and two-electrode techniques. From Fig. 7, the critical exponent for diffusion, γ D , can be determined from the slope (Eq. (1)) and was 0.76 and 1.22 for the four- and two-electrode technique, respectively. The critical volume fraction and critical exponent determined from this study are listed in Table 3 along with other percolation values determined from previous studies on ionomers [13–17]. The critical exponent for diffusion in S-SIBS for transport in the plane of the membrane (γD = 0.76) indicates a more ordered structure, which was confirmed with SAXS (transport is in the same direction as the preferred orientation of

Polymer name

2

0.1

Methanol Permeability (cm /s)

Proton Conductivity (S/cm)

1

IEC (meq/g) Fig. 8. Methanol permeability and proton conductivity (normal to the plane) vs. IEC for Nafion 117 (䉫, 䉬, respectively) and S-SIBS membranes (䊊, , respectively).

Y.A. Elabd et al. / Journal of Membrane Science 231 (2004) 181–188

nanostructures that facilitate ion diffusion in the proper direction for the electrochemical processes occurring at each electrode.

-4

Selectivity (10 ) (S s/cm3)

5 4 3

4. Conclusions 2 1 0 0.9

0.5

0.6

0.7

0.9

1.0

IEC (meq/g) Fig. 9. Selectivity (conductivity (normal to the plane)/permeability) vs. IEC for Nafion 117 (solid bar) and S-SIBS membranes (open bars).

tive and 217 times less permeable than Nafion 117. Moreover, Fig. 9 shows the selectivity of the membranes studied, where selectivity is defined as proton conductivity/methanol permeability. The selectivity of the S-SIBS membranes (0.5–1.0 meq/g) is similar to Nafion 117 (1.3×104 S s/cm3 ), ranging from (1–3) × 104 S s/cm3 . These values differ from an earlier study [9] (conductivity measured in the plane of the membrane), where selectivities were 5–10 times higher than Nafion 117 with conductivities only 3 times lower.

Fig. 10 shows the DMFC performance (cell voltage versus current density) with both Nafion 117 and S-SIBS-1.0 as PEMs. The current density capability for S-SIBS-1.0 is much lower than Nafion 117 at all cell voltages, as anticipated; since the conductivity of S-SIBS-1.0 measured normal to the plane of the membrane in this study is an order of magnitude lower than Nafion 117. Poor ionic conductivity results in significant ohmic losses and indicates mass transport limitations. This emphasizes the importance of oriented

700 600 500 400 300 200 100 0

50

100

Block copolymer ionomers are intriguing materials because of their ordered ionic structures. When standard block copolymer morphologies exist, such as cylinders or lamellae, conductive nanochannels can be envisioned within a three-phase morphology framework, because the ionic groups are fixed within one of the blocks. In addition, future studies at higher ionic levels may reveal new structures not commonly seen in non-ionic block copolymers. These structural possibilities in block copolymer ionomers and their effects on transport properties are of unique interest. This study demonstrates the effect of polymer nanostructure in block copolymer ionomers, particularly nanodomain orientation, on the transport properties and subsequently DMFC performance. Future studies on block copolymer ionomers with higher IECs or differently oriented nanostructures should create PEMs that support higher current densities in DMFC tests.

Acknowledgements

3.5. DMFC performance

Cell Voltage (mV)

187

150

200

250

300

350

2

Current Density (mA/cm ) Fig. 10. Direct methanol fuel cell performance (cell voltage vs. current density) for Nafion 117 (䉬) and S-SIBS-1.0 (䊉).

This work was performed while Y.A. Elabd held a National Research Council Research Associateship Award at the US Army Research Laboratory.

Nomenclature Alam dlam D D0 h q q∗

lamellar lattice spacing Bragg spacing observed diffusivity inherent diffusivity order of reflection scattering vector first order reflection scattering vector

Greek symbols γD critical exponent for the diffusion θ scattering angle λ wavelength σ observed conductivity σp proton conductivity σ0 inherent conductivity φ1 volume fraction of the diffusing or minority phase φ1,c critical volume fraction (φ1 − φ1,c ) excess volume fraction

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