Methanol on Teflon and Hydrocarbon Proton

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Sorption of Water/Methanol on Teflon and Hydrocarbon Proton Exchange Membranes Chenfeng He,† Frej Mighri,† Michael D. Guiver,*,‡,§ and Serge Kaliaguine*,† †

Department of Chemical Engineering, Laval University, Quebec, Quebec G1V 0A6, Canada State Key Laboratory of Engines, Department of Mechanical Engineering, Tianjin University, Tianjin 300072, China § Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China ‡

S Supporting Information *

ABSTRACT: The sorption of water and methanol droplets on Teflon films, as well as on various representative classes of hydrocarbon-based proton exchange membranes (PEMs) was investigated using contact angle measurement (drop shape method) during wetting under ambient open-air conditions. Teflon films exhibited constant hydrophobic surfaces when contacted with water, but a significant sorption of methanol. The PEMs showed slow sorption of water, and a significant sorption of methanol. The differences in sorption of water and methanol on Teflon and PEMs arose from the match/compatibility in the surface free energies as well as polarities between a liquid and a membrane. The significant discrepancies in surface free energies and polarities between water (72.0 mJ m−2 and 70.1%, respectively) and Teflon film (14.0 mJ m−2 and 4.9%, respectively) lead to a highly hydrophobic surface and no discernible sorption of water on Teflon films, while the relative similarity or minor discrepancy in surface free energies and polarities between methanol (22.5 mJ m−2 and 17.0%, respectively) and Teflon film (14.0 mJ m−2 and 4.9%, respectively) results in a significant sorption of methanol on Teflon. The surface free energies of PEMs were calculated using the harmonic-mean approach, based on contact angle measurements using both water and diiodomethane as probes. The results show that PEMs have initial surface free energies ranging from 44.1 to 54.0 mJ m−2 along with polarities in the range of 20.8 to 29.1%, for a selection of typical sulfonated polymers. The surface free energies of ionomers were principally contributed to by the nonpolar component, but the presence of polar groups in the polymer increased the polar component, leading to an increase in surface free energy. Of the PEMs investigated, sulfonated poly(aryl ether ether nitrile) has a higher surface energy than those of other ionomers with similar sulfonate contents. The compatibility between water/methanol and PEMs was investigated on the aspect of surface free energies. The present study provides a plausible strategy to prescreen potential PEMs and optimize membrane electrode assembly (MEA) fabrication. KEYWORDS: Teflon, capillary, methanol, absorption, affinity/repellency, hydrocarbon proton exchange membranes (PEMs), surface/interface, surface free energy (SFE), interfacial compatibility/matching, membrane electrode assembly (MEA)

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interfaces,8 dimensional swelling and adhesive compatibility between the Nafion ionomer−catalyst matrix and the hydrocarbon PEM,7 although previous studies investigated the reorientation of sulfonic groups on Nafion film upon wetting/drying9−12 and sorption of water on Nafion film,13 and the structure and properties of PEMFC at interfaces.14 Despite the development of numerous hydrocarbon PEMs, the interfacial matching/compatibility between a PEM and the other layers in a MEA is less understood. We present an investigation on sorption of water/methanol on PTFE which is used in gas diffusion layer in PEMFC, as well as the surface free energies (SFEs) of a representative selection of hydrocarbonbased proton exchange membranes (PEMs): copolymerized

INTRODUCTION

The proton exchange membrane (PEM) functions as a catalyst support, a separator between the fuel and air, and an electrolyte for proton transport from the anode to the cathode in a PEMFC.1,2 Membrane electrode assemblies (MEAs) are typically fabricated by bonding carbon-supported platinum catalyst electrodes onto the PEM electrolyte, using a Nafiontype ionomer as a catalyst support, regardless of the PEM employed. Consequently, fuel cells (FCs) are highly integrated systems of heterogeneous materials comprising gas, liquid, and solid. The structure and activity at the various interfaces play key roles on the fuel cell overall performance as vital as the individual components. Much effort has been given to investigate the synthesis, architecture, and morphology of sulfonated PEMs,1−6 as well as FC performance.1,7 Relatively few systematic studies have however been made on sulfonated PEM surfaces and © XXXX American Chemical Society

Received: February 29, 2016 Accepted: April 21, 2016

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DOI: 10.1021/acsami.6b02543 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. Evolution of water (a, ∼10 μL) and methanol (b, ∼6 μL) droplets of contact angle (CA, black open square), base area (BA, red solid square), and droplet volume (DV, blue filled circle) on Teflon films under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). Membranes were used as received.

among other PEMs, catalyst layers, and gas diffusion layers in a MEA could be developed.

sulfonated poly(ether ether ketone) (SPEEK-HQ), sulfophenylated poly(aryl ether ether ketone) (Ph-SPEEK), sulfophenylated poly(aryl ether ether ketone ketone) (Ph-m-SPEEKK), and sulfonated poly(aryl ether ether nitrile) (SPAEEN-B). In a previous work, we reported the surface orientation of sulfonic groups in SPEEK-HQ membranes.8 It was shown that they exhibit hydrophobicity in water vapor, and conversely, hydrophilicity in liquid water. This change in hydrophilicity/ hydrophobicity of a membrane resulted from the migration and reorientation of sulfonic acid groups to the surface from the bulk in a few seconds upon wetting. The surface characteristics of SPEEK-HQ membranes were dependent on the sulfonate moiety content as well as on the membrane fabrication procedure. In a more recent work,15 we reported the effect of annealing and acidification on the surface hydrophilicity of various representative classes of hydrocarbon-based PEMs. In all cases, a more hydrophilic membrane surface develops after heat annealing under vacuum at elevated temperatures, resulting from ionic cluster decomposition. The annealing time also had some influence, but in different ways depending on the class of PEM. This opposite trend for different PEMs might be attributed to the competition between the cluster decomposition and the sulfonic acid group migration to the subsurface upon annealing. Membrane acidification leads to more hydrophilic surfaces by elimination of the hydrogen bonding that exists between strongly bound residual solvent such as dimethylacetamide (DMAc) and PEM sulfonic acid groups. Our above-mentioned work focused on the surface behavior of PEMs. In the present study, we investigate the interface issues according to the aspect of surface free energies of PEMs. The objectives of this study are to (1) investigate the water and methanol sorption on Teflon, as well as hydrocarbon-based PEMs; (2) measure the surface free energies of PEMs; (3) establish the matching/compatibility between a PEM and the liquid; and (4) provide possible approaches to alleviate methanol crossover in direct methanol fuel cell (DMFC). Although we merely investigated the water/methanol sorption, as well as surface free energies of PTFE and a representative selection of hydrocarbon-based PEMs in the present study, we believe that this strategy can also be extended as an approach to investigate other ionomer membranes PEMs and liquids. Applying this approach, the matching/compatibility

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EXPERIMENTAL SECTION

Materials. Chemicals specification and purification were described in our previous work.15 Detailed information is presented in the Supporting Information (SI). Synthesis and Characterization of Polymers. Sulfonated aromatic polymers: SPEEK-HQ, Ph-SPEEK, Ph-m-SPEEKK, and SPAEEN-B were synthesized according to our previous work,8,16−18 and the detailed synthesis and characterization of polymers were reported previously.8,15 Figure S1 shows the structures of the polymers used in the present study. Membrane Fabrication. Hydrocarbon-based PEMs in proton form were prepared following protocols which involved acid treatment, described elsewhere.15 Detailed information is presented in the SI. Methods of Measurement. Wetting behaviors of water, methanol, CH2I2, and mixtures of methanol and water on Teflon and PEM membrane surfaces were investigated by the contact angle method using a goniometer. Surface free energies of polymers were determined using the contact angle method. Measurements are described in detail in the SI.

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RESULTS AND DISCUSSION Nafion PEM derives its high conductivity and low dimensional swelling from its well-defined phase-separated morphology between the highly flexible hydrophobic phase (tetrafluoroethylene, TFE moiety) and the flexible superacidic hydrophilic phase. Opposite to the excellent performance of Nafion in hydrogen fuel cells (HFCs), the inherently high methanol crossover through Nafion PEM limits its performance in DMFC by lowering fuel utilization as well as adversely affecting the oxygen reducing cathode.19,20 Given the presence of some liquid water and methanol in PEMs, as well as air being also present in PEMFC applications, we initially investigated the wetting behavior of water and methanol on Teflon film, which is used in the diffusion layer21 in DMFC and has the same backbone as that of Nafion. Water and Methanol Behavior on Teflon Films. Figure 1a shows that the Teflon film has a hydrophobic (initial contact angle (ICA) of ∼129°) and nonwetting surface; the decrease in BA and DV of water droplets resulted from water evaporation during wetting. Oppositely, Figure 1b shows that Teflon film was significantly wetted by methanol, and rapid adsorption of B


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Figure 2. Schematic of water droplet (a) and methanol droplet (b) on Teflon films; as well as water repellency (c) and methanol affinity (d) in a single capillary on Teflon films.

to smaller pores ranging 0.2 to 0.3 μm, or bigger pores ranging 5 to 30 μm27−30 depending on the fabrication procedure. These pore structures in Teflon interconnect with each other, thus, forming microfibrous capillaries with diameters ranging from several to tens of microns.27 Capillaries and low surface free energy of Teflon (19.5 mJ m−2)31 result in Teflon material being employed as a common organic liquid filter material since Teflon can be wetted spontaneously by organic liquids that have surface tension lower than 27 mJ m−2.32 Due to the high surface tension of water (72.0 mJ m−2) but low surface tension of methanol (22.5 mJ m−2) (Table S1), water and methanol exhibit opposite capillary actions in Teflon. In a sufficiently narrow capillary, capillary action is the ability of a liquid to flow in narrow spaces without the assistance of external forces, originating from intermolecular forces between the liquid and surrounding solid surfaces in capillaries. The pressure difference across the interface between the liquid and the air can be expressed by eq 3.

methanol, i.e., the ICA (∼23°), BA (∼20 mm2), and DV (∼6 μL) of the methanol droplet decreased to values of ∼0 in ∼10 s, resulting from the sorption of methanol in Teflon film. Similar to our work, Benziger’s group13 reported methanol behavior on Teflon film with no further interpretation. γsv = γsl + γlv cos θ

(1)

Figure 2 depicts the physical definition of contact angle (θ) that is a tangential angle for the liquid droplet on the flat solid surface at the equilibrium state created by three interfacial surface tensions. Young’s equation (eq 1) describes the surface behavior of a liquid on the surface of any object/solid dependent on the competitions among contact angle and three vectors of interfacial surface tensions, i.e., the primary surface tensions/surface free energies of the liquid (γlv), the object/solid (γsv), and the interfacial tension between the solid and the liquid (γsl) in a solid−liquid−vapor system.22 For lowenergy surface systems, γsv and γlv can be related to surface tension/surface free energy of the solid (γs) and surface tension of liquid (γl), respectively, since the equilibrium film pressures could be neglected.23 Thus, Young’s equation is simplified to the following: γs = γsl + γl cos θ

Δp =

2γ cos θ a

(3)

where Δp refers to the pressure difference across the interface between the liquid and the air, γ refers to the surface tension of a liquid, a refers to the diameter of a capillary, and θ refers to the contact angle of a liquid on a solid surface. For liquids having contact angle smaller than 90°, θ refers to the contact angle of the liquid on the wall of the capillary as well. The difference in capillary pressure drives the motion of a liquid in a capillary, in a direction depending on the contact angle of the liquid on the capillary wall. By applying the methanol ICA of Teflon at the value of ∼23°, the positive value of cos θ results in a positive capillary pressure difference (eq 3) across the interface between methanol and air (Figure 2b,d). With the pair of methanol and Teflon, the Young’s equation (eq 1) shows that the intermolecular forces between Teflon and methanol (γsl) is lower than the surface free energy of Teflon (γsv or γs), i.e., γsl is lower than γsv. Consequently, the higher surface tension of Teflon pulls methanol molecules through the

(2)

In the case of interactions among phases, the spreading/ wetting/contracting of one liquid surface over another liquid/ solid surface reaches an equilibrium state which is determined by the minimization of surface energy.24,25 Spreading, wetting, and sorption are facilitated by low interfacial free energy, high solid surface free energy, and low liquid surface free energy/ surface tension.26 Figures 1a and b illustrate the nonwetting (ICA ≈ 129°) and almost no-sorption of water, but strong wetting (ICA ≈ 23°) and significant sorption of methanol on Teflon film, respectively. We attribute these differences in sorption to water-repellency (hydrophobic) capillaries, but methanol-affinity capillaries on Teflon films. Due to its nonsolubility and high melting temperature (342 °C),27 Teflon films/fibers are prepared by extrusion and pressing,27,28 leading C


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Figure 3. Evolution of water droplet (∼10 μL) of contact angle (CA, black open square), base area (BA, red solid square), and droplet volume (DV, blue filled circle) on methanol-coated Teflon films under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH) (a), as well as schematic of water affinity in a single capillary on methanol-coated Teflon films (b).

Figure 4. Evolution of contact angle (a) and base area (b) of mixture droplets (∼6−10 μL) on Teflon films under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). The methanol−water mixture (v:v) at the ratio of 0:1 (square), 5:5 (triangle), 7:3 (open circle), 9:1 (filled circle), and 1:0 (star).

methanol-coated Teflon films is ∼86°, which is significantly lower than the initial water contact angle on Teflon film of ∼129° (Figure 1a). We attribute this decrease in contact angle to the initial methanol sorption in the capillaries of Teflon (Figure 2b, d). According to the discussion in the previous section, the polarity and the surface tension of a methanolcoated Teflon film were increased by methanol sorbed on Teflon. The strong hydrogen bonding between water and methanol molecules results in an interfacial attraction between water and methanol, leading to wetting and sorption of water on the methanol-coated Teflon surface (Figure 3b). Figure 3a indicates that there are three stages of water behavior on methanol-coated Teflon films. (Noticeable data points from Figure 3 are summarized in Table S2 for convenient reference.) Stage I refers to the sorption of water on methanol-coated Teflon films for the first ∼100 s. At that stage, the CA (∼86°), BA (∼9.0 mm2), and DV(∼9.0 μL) decrease to ∼69.0°, 7.2 mm2, and 4.4 μL, respectively. Stage II refers to a relatively stable stage with the equilibrium between sorption and evaporation of water for ∼100 to 230 s. At this stage, CA, BA, and DV have relatively constant values within the standard deviation. Meanwhile, CA (θ), surface tension of methanol-coated Teflon (γs), surface tension of water (γl), and the interfacial tension between methanol-coated Teflon and water (γsl) can be described by Young’s equation (eq 2).

capillaries (Figure 2b,d), driving the methanol motion in the capillaries of Teflon. This effect is designated as the capillary action. On the basis of the special capillary action of methanol in porous Teflon, Liang and co-workers proposed and demonstrated a prototype of laboratory-made DMFC.33 However, the water contact angle of ∼129° yields a negative value of cos θ, thus, the Teflon−water interface has a higher surface tension/energy (γsl) than the Teflon-air interface (γsv, eq 1). Consequently, water is repelled by the Teflon surface, leading to nonwetting and nonsorption of water in capillaries of Teflon (Figure 2a, c). In the sections above, we reported that Teflon film exhibits a hydrophobic surface, but absorbs methanol rapidly. We attribute this difference in the sorption of water and methanol to the difference in surface tensions/surface energies of water and methanol, as well as capillary action in Teflon. To further support this result, we investigated the sorption of various liquids on Teflon and methanol-coated Teflon films. Sorption of Water on Methanol-Coated Teflon Films. In an attempt to verify the analysis above, we investigated water behavior on methanol-coated Teflon films. To this end, methanol was applied on the Teflon film and the excess removed using blotting paper, followed by applying a water droplet on the methanol-coated Teflon films immediately. Figure 3a shows that the initial water contact angle on D


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Figure 5. Evolution of contact angle (a, b) and base area (c, d) of methanol droplets on the top surfaces of various hydrocarbon proton exchange membranes (PEMs) under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). PEMs are in proton form and were annealed at 120 °C for 24 h prior to measurements.

Stage III refers to the stage of dominant evaporation over sorption of water on methanol-coated Teflon film starting at ∼230 s. During this stage, BA (∼7.2 mm2) and DV (∼4.4 μL) decreased to ∼0, but CA increased gradually from ∼70.0° to ∼96.0° which is much higher than the ICA of ∼86°. We attribute the increase in CA to water evaporation, leading to the triple line motion in the system.34 Due to the evaporation of liquid, the droplet temperature decreases to counterpoise the latent heat of the evaporation. Given the fact that surface tension depends on temperature which is described by the “Eötvös rule”, the surface tensions increase once the reduction of temperature becomes significant. These changes in surface tensions of the various interfaces in a system disturb the equilibrium given by Young’s equation (eq 2). In order to progress toward stability, the droplet deforms to minimize the energy of the system, leading to the motion of the triple line of a droplet on methanol-coated Teflon surface, resulting in the increase in CA in our case. Liu et al. recently reported the motion of methanol droplet on Teflon films,35 and the methanol CA increase during the evolution of methanol CA vs time. They attributed the increase in contact angle of methanol to methanol evaporation, and reported the methanol droplet surface temperature under ambient open-air initially decreases to a minimum value, and later increases to ambient temperature. Sorption of Methanol−Water Solution on Teflon Films. To investigate the surface tension effect in sorption of liquids, we measured the evolution of CA and BA of various methanol−water solutions on Teflon films under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). Figure 4a shows that initial CAs of droplets decreased with the increase in

methanol to water volume ratio in a methanol-aqueous solution. (Noticeable data points from Figure 4 is summarized in Table S3 for convenient reference.) Compared to a water droplet with an initial CA of ∼129° (Figure 1), Figure 4a shows that methanol−water solutions with volume ratios of 5:5, 7:3, and 9:1 (methanol: water) display an ICA of ∼103.0°, 80.0°, and 70.7°, respectively, with methanol having a minimum ICA value of ∼24.1° (compare with Figure 1b). Figure 4a shows that the water droplet remains stable for 1300 s, whereas the methanol droplet was absorbed in ∼10 s. The methanol−water solution was absorbed, and the BA of droplet decreased for ∼210 s, irrespective of the ratio of methanol to water. Figure 4a shows that CA decreased to the values of ∼90.2°, 51.3°, and 28.4° in ∼210 s, respectively. Due to the high surface tension of water (72.75 mJ m−2 at 20 °C),36 but low surface tension of methanol (22.95 mJ m−2 at 20 °C),36 the surface tension of a methanol−water solution decreases with increasing methanol to water ratio.36 As described in the previous sections, the significant difference in surface tensions between water and methanol, as well as the low surface energy of Teflon films (20.0 mJ m−2 at 20 °C),36 result in the water droplet remaining stable on Teflon surface (Figure 1a), while the methanol droplet was absorbed rapidly (Figures 4 and 1b). Wetting and sorption are promoted by the low interfacial free energy between the solid and the liquid, high solid surface free energy, and low liquid surface free energy/low liquid surface tension.26 Figure 4 shows that CA of methanol− water with volume ratio of 9:1, 7:3, and 5:5 (methanol: water) decreased from ∼70.7° to 28.4°, 80.0° to 51.3°, and ∼102.9° to 90.2°, respectively, showing that the methanol−water solution with higher methanol ratio has a more rapid droplet sorption. E


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SPAEEN-B59 and SPAEEN-B56, respectively, a mere ∼5.5% difference in SCs of SPAEEN-B59 and SPAEEN-B56 resulted in approximately three times faster methanol sorption in SPAEEN-B59 than in SPAEEN-B56. The reason for this dramatic difference in methanol sorption between SPAEENB59 and SPAEEN-B56 is not clear. It may be related to the “percolation threshold” at which the membrane morphology changes from a “closed” structure to a much more “open” one.38 We infer that the percolation threshold of SPAEEN-B might be located between SC values of 54.3 and 59.8%. Figure 5 shows that all PEMs in the present study have slower methanol sorption than Teflon (Figure 1b). We conclude that the high methanol affinity of capillaries in Teflon films results in a faster sorption of methanol than in the PEMs. Meanwhile, all PEMs show stronger absorption of methanol (Figure 5) than that of water (Figure S2 and Figure 6). Compared with the higher surface tension of water (72.8 mJ m−2 in Table S1), the lower surface tension of methanol (22.5 mJ m−2 in Table S1) results in a faster methanol sorption in PEMs. To support this interpretation, we determined the surface free energies of PEMs. Analysis of Surface Free Energy. The solid surface free energy plays a key role in the wetting and adsorption behavior of a liquid, as well as the adhesion between two phases. To evaluate the compatibility/matching between PEMs, liquid and other layers in a MEA, we determined the surface free energies of PEMs and their polar components. Nevertheless, SFE cannot be directly measured. Among several theoretical approaches developed to determine the surface free energy, the contact angle method has been widely employed due to its simple procedure.26,39−41 Advancing contact angle can characterize the hydrophilicity/ hydrophobicity of solid surfaces,42 and its values are employed for calculation of surface free energy of solids.43 The value of initial contact angle is close to that of the advancing contact angle44 due to the short time gap between the formation of a liquid droplet on the membrane and the beginning of its image recording during measurement. Given this time gap being controlled by the goniometer (it is less than 0.025 s in the present study), surface energies of ionomers were calculated using ICA instead of advancing contact angle in the present study. The contact angle method is initially based on Young’s equation (eq 2), where γ refers to the individual surface tension/surface free energy, whereas l and s indicate the probe liquid surface and solid surface, respectively. Fowkes39 proposed that the surface tension or surface energy can be defined as the sum of contributions of dispersion interaction (dispersive component, γd) and nondispersive interaction. The latter is also defined as the polar component (γp). Thus, eq 4 describes the addition of surface energy components.

Figure 4a shows that CAs of methanol−water solutions start increasing after ∼210 s, irrespective of the ratio of methanol to water (Figure 4b). The CAs increase from ∼90.2°, 51.3°, and 28.4°, to ∼103.7°, 77.8°, and 56.9° for methanol−water solution with volume ratio of 5:5, 7:3, and 9:1, respectively. This increase in CA resulted from the droplet motion due to both sorption and evaporation of methanol−water solution. In the case of interactions among phases, the spreading/wetting/ contracting of one liquid surface over another liquid/solid surface reaches an equilibrium state which is dominated by the minimization of surface energy.24,25 During sorption and evaporation of a solution, the droplet of methanol−water solution tends to contract, and the droplet shape deforms to minimize the energy of the system, leading to the motion of the triple-line of a droplet on the Teflon surface, thus finally leading to the observed increase in CA. Figure 4 shows that there is a decreasing trend for contact angle for methanol/water = 7:3 and 9:1, from 400 to 1200 s. This decreasing trend in contact angle results from the absorption of liquid in porous materials which is described in detail in ref 37. Sorption of Methanol and Water on Various Sulfonated PEMs. In DMFCs, the presence of methanol at the cathode, which occurs as a result of methanol crossover, lowers fuel utilization efficiency, polarizes the cathode, and poisons the catalyst sites for oxygen reduction, thus, leading directly to significant performance losses.19 In the present study, in addition to the above study of Teflon membranes, we also investigated the evolution of methanol droplets on various hydrocarbon PEMs to illustrate the various behaviors in methanol sorption. Figure 5 shows the evolution of methanol droplets (∼6 μL) on various PEMs until the droplets disappeared under ambient open-air. (Noticeable data points from Figure 5 are summarized in Table S4 for convenient reference.) Compared with the sorption of methanol on Teflon in ∼10 s (Figure 1b and Table S4), Figure 5 shows a slower sorption of methanol on these PEMs. More precisely, Figure 5a shows the similar methanol-wetting behavior of SPEEK-HQ-80, Ph-SPEEK, and Ph-m-SPEEK surfaces. Corresponding to their ICAs of ∼20.4°, 18.0°, and 14.9°, the CA initially minimizes to ∼8.4°, 9.7° and 5.1°, respectively, in ∼24, 24, and 15 s, followed by an increase in CA to a maximum of ∼15.9°, 19.7°, and 8.1°, respectively. Later, CA decreased again until the methanol droplet disappeared in ∼86, 81, and 100 s, respectively. As described in the preceding sections, we attribute the initial decrease in CA to the sorption of methanol on PEMs, and the increase in CA to the evaporation of methanol. After reaching the maximum, the methanol drop was further absorbed on PEM, and CA decreased until the methanol droplet vanished. Figure 5b shows the similar methanol sorption behavior of SPAEEN-B59 and SPAEEN-B56 films. Here 59/56 refers to the target sulfonate content (SC) that is the molar ratio percentage of potassium 2,5-dihydroxybenzenesulfonate or sulfonated hydroquinone (SHQ) to 2,6-difluorobenzonitrile (2,6-DFBN) for SPAEEN-BXX. This evolution is however different from that of SPEEK-HQ-80, Ph-SPEEK and Ph-mSPEEK surfaces in Figure 5a. Figure 5b shows that SPAEENB59 and SPAEEN-B56 have similar ICAs of ∼19.4° and 18.3°, respectively. Both CAs start decreasing to a similar value, but over a significantly different time. Figure 5b shows the faster sorption of methanol in SPAEEN-B59 compared to SPAEENB56 (∼20 and 80 s, respectively). With the same structural repeat units (Figure S1), as well as similar sulfonic acid group contents (SCs) of 59.8 and 54.3%,15 corresponding to

γi = γid + γip

(4)

On the basis of Young’s equation (eq 2) and the additivity of surface tensions (eq 4), various methods were developed to calculate the surface free energy using contact angle measurements of two probe liquids, and the two most widely used probes are CH2I2 and water. Using the “harmonic mean” method, Wu45 developed eq 5 to calculate the surface free energy of polymers: F


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ity,χps ,%) can be obtained using eqs 6 and 7, respectively. Correspondingly, the distribution of dispersive component (dispersity, χds , %) can be obtained using eq 8. Table S5 shows the values of surface free energy, the dispersive and polar components. Figure 7a,b shows the initial surface free energies and final surface free energy of PEMs along with corresponding polarity, using values of ICA and final contact angle (FCA, after wetting for 300 s), respectively. Similar processes were employed to determine the SFE of Teflon, but using the “geometric-mean method” (eq 9).26,46 Thus, the surface free energy and the polarity of Teflon films were also calculated. Applying values of ICA and FCA, the initial SFE (Figure 7a) and final SFE (Figure 7b) along with corresponding polarity were obtained (Table S5), respectively. γs = γsd + γsp χsp (%) =

γsp γs

(6)

× 100 (7)

χsd (%) = 1 − χsp

(8)

(1 + cos θ l)(γld + γlp) = 2( γldγsd +

(9)

Figure 7 shows in most cases a higher value of the dispersive (nonpolar) component than that of the polar component in PEMs, and the polarity increased upon wetting for 300 s (Figure 7b). With similar IECs of 2.20 and 2.23 mmol g−1 for SPEEK-HQ-80 and Ph-SPEEK,15 respectively, Ph-SPEEK exhibits a higher SFE than that of SPEEK-HQ-80. Figure 7 also shows that Ph-m-SPEEKK has a higher SFE than SPEEKHQ-80 even though the former possesses a lower IEC of 1.77 mmol g−1.15 It is worth noting that SPAEEN-B59 and SPAEEN-B56 with relatively lower SCs possess the highest SFEs among all PEMs examined in the present study. We attribute this higher SFE to the increased numbers of polar groups in the polymer chains. In addition to SO3H groups, CN groups on the polymer chains of SPAEEN-B increase its SFE and polar component compared to those ionomers solely containing SO3H groups. This higher SFE facilitates maintaining polar molecules within a membrane. Additionally, Figure 7b shows that the final SFE, referring to the surface energy of membrane upon wetting for 300 s, is higher for all membranes than the initial SFE (Figure 7a). This increase in SFE obviously results from wetting, and the higher polarity of the final surface energy suggests the reorientation of hydrophilic groups during wetting, by which hydrophilic groups being attracted by the water molecules migrate to the surface from the bulk. This result is consistent with Figure S2 and Figure 6, as well as with our previous work.8 We report that Teflon has a SFE of 14.0 mJ m−2 with a polar component of 4.0% (Figure 7a). This value of SFE is low compared with those reported in literature, such as 19.5 and 24 mJ m−2,48 20 mJ m−2,47 as well as 19.0 mJ m−2 with the polarity of 4.6%.45 These discrepancies in surface free energy of Teflon arise from various calculation methods, assumptions, as well as liquid−probe pairs used in measurements. Moreover, the SFE of a polymer is dependent on its morphology as well. Figure 7 shows that the dispersive term accounts for the major part of the SFE, however, the polar force plays a role in another way in a system, that is, the interfacial tension is mainly

Figure 6. Evolution of contact angle (a), base area (b), and drop volume (c) of ∼10 μL deionized water droplet on the top (solid symbol) and bottom (open symbol) surfaces of Ph-SPEEK (star), Phm-SPEEKK (circle), SPAEEN-B59 (triangle), and SPAEEN-B56 (diamond) membranes under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). All membranes are in proton form and were annealed at 120 °C for 24 h prior to contact angle measurements. The identical standard deviation on a given normalized parameter was selectively labeled for sake of clarity.

⎛ γ dγ p γ dγ p ⎞ (1 + cos θl)(γld + γlp) = 4⎜⎜ d l s d + p l s p ⎟⎟ γl + γs ⎠ ⎝ γl + γs

γlpγsp )

(5)

where θl refers to the contact angle of liquid l. This approach could be applied accurately between polymers, or between a polymer and a liquid.45 By inputting values of water and CH2I2 contact angles, respectively, and the corresponding surface tension components of the probe liquids (CH2I2 and water in Table S1) in eq 5, two simultaneous equations are obtained. Thus, the polar component (γps ) and dispersion component (γsd) of the solid are obtained by resolving these two simultaneous equations. Furthermore, the solid surface free energy (γs) and the distribution of polar component (polarG


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Figure 7. Initial free surface energies (a) and final surface free energies (b) of films. The surface free energies (left axis) of the top surface (dashed column) and the bottom surface (gray solid column) along with corresponding polarity (blue solid ball, right axis). Error bars of surface free energy were obtained from the standard deviation of contact angle measurements. PEMs are in the proton form and were annealed at 120 °C for 24 h prior to measurements. Teflon was measured as received, and its surface free energy (white column) was obtained by the geometric mean method. The surface free energy of PEEK (white column) is from ref 47.

Figure 8. Evolution of contact angle (a and c) and base area (b and d) of CH2I2 droplet (∼2 μL) on the top surface of various membranes under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). PEMs are in proton form and were annealed at 120 °C for 24 h prior to measurements. Teflon was measured as received.

determined by the disparity in the polarities of the two phases.45 A high interfacial tension results in the mutual nonspreading and nonsorption of the system, thus, whether spreading/wetting/sorption will occur in a system could be estimated by the magnitude of the interfacial tension related to the “matching” or “mismatching” of the polarity of the two phases and the packing density at the interface. It could be expected that spreading/wetting/sorption is favored by

matching the polarity and molecular geometry of the two phases.31 For instance, the polarity is 35% in poly(viny1 acetate); 0% in polyethylene. The interfacial tension between the two at 140 °C is 11.3 dyn/cm. If poly(viny1 acetate) was completely nonpolar, then the interfacial tension against polyethylene would be approximately zero according to both Wu’s work45 and Fowkes’ equation.49 It is evident that spreading/wetting/sorption is favored by low interfacial free H



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CONCLUSIONS We report that the ionomers under study in this work have much lower methanol contact angles than those of water, and there is a significant sorption of methanol molecules in the PEMs. This observation could be related to the crossover of methanol in the membranes made of these ionomers. The significant differences in methanol and water behavior on ionomer surfaces show that spreading/wetting/sorption is facilitated by a low interfacial free energy, high solid surface free energy and low liquid surface free energy. Meanwhile, the sorption of a liquid (methanol/water) is dependent on the compatibility of polarities between the liquid and the PEM, i.e., the matching in polarities between the liquid and PEM results in the wetting/adhesion of the liquid on the PEM. A PEM possesses a higher surface energy in liquid water than in water vapor. The surface free energies of ionomers were principally contributed by the nonpolar component, while the surface energy was increased by the introduction of polar groups. The nitrile groups in SPAEEN-B resulted in SPAEENB ionomers having higher surface energies than those of the other ionomers with similar sulfonate contents. The present study provides a plausible strategy to optimize MEA by evaluating the compatibility between various layers in a MEA. According to the aspect of surface free energy on the various layers in a MEA, the prescreening investigations can be initially conducted on potential PEMs to evaluate the compatibility and adhesion between a PEM, catalyst layers, and gas diffusion layers in a MEA. Moreover, the methanol crossover of a PEM could probably be alleviated by increasing the surface tension of methanol fuel feed or by decreasing the surface free energy of a PEM in DMFC. The former can be realized by adding a methanolmiscible liquid with higher surface tension (such as water) into methanol fuel.51,52 The latter can be realized by surface enrichment of hydrophobic functional groups on PEM by surface modification via surface fluorination.53 This strategy is practical for the design and optimization of PEM surface for MEA fabrication in FC applications, as well as other integrated systems comprising Teflon or other porous materials.

energy, high solid surface free energy and low liquid surface free energy.26 Compatibility Analysis between Membrane and Liquid. To evaluate the compatibility/matching of two phases, we further investigated the wetting behavior of CH2I2 droplets on Teflon as well as PEMs under ambient open-air conditions (21 ± 0.5 °C at 65−70% RH). Figure 8 shows that CA and BA of CH2I2 on Teflon and the various PEMs remain stable for 300 s, and exhibit the absence of wetting and sorption of CH2I2. With similar surface energies/tensions of CH2I2 (50.8 mJ m−2 in Table S1) and PEMs (45−60 mJ m−2 in Figure 7a), the significant mismatch in the polarities of surface free energies in CH2I2 (3.5% in Table S1) and PEMs (21.9−29.2% in Figure 7), results in a higher interfacial tension between CH2I2 and a PEM, leading to the nonwetting and nonsorption of CH2I2 on PEMs.50 More precisely, the high SFE of CH2I2 (50.8 mJ m−2 in Table S1) compared to the SFE of SPEEK-HQ-80 (45.3 mJ m−2 in Figure 7a) results in the higher initial CA of CH2I2 on SPEEKHQ-80 (Figure 8a) compared to ICA of CH2I2 on Ph-SPEEK, Ph-m-SPEEKK, SPAEEN-B59, and SPAEEN-B56 (Figure 8). The lower ICA of CH2I2 on the other PEMs results from the higher SFE of Ph-SPEEK (54.9 mJ m−2), Ph-m-SPEEKK (53.5 mJ m−2), SPAEEN-B59 (52.8 mJ m−2), and SPAEEN-B56 (51.2 mJ m−2). With the increase of SFE of PEMs, the attractive interaction between CH2I2 and PEMs increases, leading to a lower contact angle of CH2I2 on PEMs (Figure 8). Teflon and CH2I2 have similar polarities of 4.0 (Figure 7) and 3.5% (Table S1), respectively. The significantly lower SFE of Teflon (14.0 mJ m−2 in Figure 7a) compared to much higher surface tension of CH2I2 (50.8 mJ m−2 in Table S1) results in the absence of spreading and wetting of CH2I2 on Teflon since a solid having a lower surface free energy cannot be wetted by a liquid having a higher surface tension, even if the solid and the liquid have similar polarities. The greatest difference between the surface tensions of CH2I2 (50.8 mJ m−2 in Table S1) and Teflon (14.0 mJ m−2 in Figure 7a) results in the largest initial contact angle of CH2I2 of 92° on Teflon film measured in the present study. The high polarities of both water and PEMs, corresponding to 70.1% (Table S1) and 21.9−29.2% (Figure 7a), respectively, result in a lower interfacial tension between water and a PEM. Consequently, the compatibility between water and a PEM leads to strong spreading, wetting, and sorption of water on PEMs even though water possesses a much higher surface tension of 72.8 mJ m−2 (Table S1) than initial SFEs of PEMs (lower than 55.0 mJ m−2 in Figure 7a). Figure S2 shows the spreading, wetting, and sorption of water on SPEEK-HQ membranes, and the increase in the spreading and wetting of water with the increase in sulfonate content. Compared with SPEEK-HQ-60 membrane, the higher increase in base area (Normalized BA) of SPEEK-HQ-45 originated from NMP as solvent in SPEEK-HQ-45 casting,8 while DMAc was used in any other PEM casting. Our previous work shows that NMP leads to a higher increase in surface area during wetting.8 Figure 6 shows that the spreading, wetting and sorption of water on Ph-SPEEK, Ph-m-SPEEKK, SPAEEN-B59, and SPAEEN-B56 films, and that SPAEEN-B membranes have the lowest spreading and sorption of water among these PEMs. It is concluded that not only the difference in polarity and the surface energies are important, but also whether the probe has the highest/lowest polarity and/or surface energy compared with the solid polymer.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02543. Nomenclature, membrane fabrication, chemicals specification and purification, as well as methods of measurement (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(M.D.G.) Tel +86-22-2740-4479; e-mail michael.guiver@ outlook.com; [email protected]. *(S.K.) Tel +1 418-656-2708; e-mail serge.kaliaguine@gch. ulaval.ca. Notes

The authors declare no competing financial interest.

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Methanol on Teflon and Hydrocarbon Proton

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