Improving the Mechanical Durability of Short-Side-Chain Perfluorinated Polymer Electrolyte Membranes by Annealing and Physical Reinforcement.

Physically reinforced short-side-chain perfluorinated sulfonic acid electrolyte membranes were fabricated by annealing and using a porous support. Five types of solution-cast membranes were produced from commercial perfluorinated ionomers (3M and Aquivion (AQ)) with different equivalent weights, annealed at different temperatures, and characterized in terms of ion conductivity, water uptake, and in-plane/through-plane swelling, while the effect of annealing on physical structure of membranes was evaluated by small-angle X-ray scattering and dynamic mechanical analysis. To create a reinforced composite membrane (RCM), we impregnated a polytetrafluoroethylene porous support with 3M 729 and AQ 720 electrolytes exhibiting excellent proton conductivity and water uptake. The electrolyte impregnation stability for the porous support was evaluated using a solvent resistance test, and the best performance was observed for the 3M 729 RCM annealed at 200 °C. Both annealed and nonannealed 3M 729 RCMs were used to produce membrane electrode assemblies, the durability of which was evaluated by open-circuit voltage combined wet-dry cycling tests. The nonannealed 3M 729 RCM survived 5800 cycles, while the 3M 729 RCM annealed at 200 °C survived 16 600 cycles and thus exhibited improved mechanical durability.

Introduction

The environmentally friendly nature and high energy efficiency of proton-exchange membrane fuel cells (PEMFCs) make them promising power sources for transportation.[1] In particular, perfluorosulfonic acid (PFSA) electrolyte membranes feature the advantages of high proton conductivity and physicochemical stability, and have therefore become representative cation exchange membranes used in PEMFCs.[2] The PFSA polymer comprises a hydrophobic poly(tetrafluoroethylene) (PTFE) main chain and hydrophilic side chains ending with sulfonic acid groups. The PTFE backbone provides mechanical strength by engaging in crystalline structure formation, while the side-chain sulfonic acid groups facilitate proton transport by engaging in the formation of ion transport pathways.[3]

Ion conductivity and durability are the most important indicators of PFSA membrane performance. Specifically, ion conductivity is the most important factor determining the ohmic resistance of the membrane electrode assembly (MEA) in PEMFCs and is mainly influenced by the concentration of sulfonic acid groups (often equivalent weight (EW) or ion exchange capacity (IEC)) in the PFSA polymer. EW is defined as the molecular weight of the PFSA ionomer per mmol of sulfonic acid groups, while IEC refers to the ion exchange capacity determined by the concentration of sulfonic acid groups (Figure ). As the EW and IEC of the same polymer structure directly affect the concentration of the ion transport carrier, they are important indices determining the conductivity of the electrolyte membrane.

Figure 1

Chemical structures of (a) Nafion, (b) 3M, and (c) Aquivion ionomers.

Nafion, first developed by DuPont in 1960 and long studied as an ionomer binder in polymer electrolyte membranes and catalyst layers, features several limitations due to its long-side-chain (LSC) form and an EW of ≥1100 g/meq. 3 In contrast, the recently developed Aquivion (Solvay) and PFSA (3M) ionomers have the advantage of lowered EW (<1100) and short side chains (SSCs).[4,5] Compared to long-side-chain PFSA (LSC-PFSA), SSC-PFSA exhibits increased durability due to the lower risk of deterioration by the shorter side chains and the reduced numbers of ether groups and tertiary carbon atoms.[6,7] In particular, the decreased side chain length can increase the crystallinity of the main chain (which affects the mechanical properties of the electrolyte membrane) and hence elevate glass transition temperature (Tg).[4,7−9] SSC membranes have lower EWs (almost to 700 g/meq) than those of LSC membranes and allow fuel cells to be operated at higher proton conductivities and temperatures. For this reason, much research is currently performed on SSC-PFSA as potential ionomers to replace LSC-PFSA.[10]

PFSA durability and proton conductivity should be improved for PEMFC commercialization. In general, the failure mechanisms of PFSA electrolyte membranes involve chemical and physical degradation.[11−14] Chemical degradation is caused by the attack of hydroxyl radicals generated by (i) side reactions during fuel cell operation or (ii) crossover of H2 and O2 gas fuel on the electrolyte membrane.[15,16] Physical deterioration is caused by the repeated swelling and drying of the membrane during the start/stop cycling of the fuel cell and results in membrane failure.[17]

Chemical deterioration can be prevented through the addition of external antioxidants. Metal (e.g., Ce) ions were used as antioxidants in early studies,[18,19] while in later investigations, the scope was expanded to metal oxides,[20−24] metal nanoscale composites,[25−29] transition metal–ligand complexes,[30] transition metals immobilized on supports,[31,32] bimetallic antioxidants,[33] organic antioxidants,[34,35] and polymeric antioxidants.[36,37] In addition, we have recently reported a problem of Ce3+ usage, revealing that although this species improves the long-term durability of polymer membranes, it concomitantly reduces proton conductivity.[19] To mitigate this problem and demonstrate that oxidative stability can be improved without sacrificing proton conductivity, hydrocarbon composite membranes and perfluorinated composite membranes impregnated with organic antioxidants were developed.[34,35]

Annealing and physical reinforcement through the use of porous supports are well-known methods of improving the mechanical durability of electrolyte membranes. To realize thin and strong electrolyte membranes expanded poly(tetrafluoroethylene) (ePTFE) porous supports can be impregnated with the PFSA ionomer, which has already been commercialized by Gore.[38,39] On the other hand, the annealing-induced improvement of physical properties is ascribed to the concomitant increase in the polymer crystallization degree and the decrease in the free volume between polymer chains.[40−42] Therefore, several studies have been conducted on Nafion (i.e., LSC-PFSA) to confirm the effects of annealing conditions such as temperature, time, and humidification on membrane physical structure changes and electrochemical properties.[42−51]

Despite the abovementioned advantages of SSC-PFSA EW and crystallinity, the effects of annealing on SSC-PFSA performance have been underexplored. In particular, no systematic research has been conducted on SSC-reinforced composite membranes used in real fuel cells. Therefore, research to optimize the annealing process for 3M and Aquivion is needed to improve the performance of SSC-PFSA-based PEMFCs.

Herein, a systematic investigation was conducted to improve the durability of SSC-PFSA electrolyte membranes. Membranes were produced for each EW using a commercial PFSA electrolyte, annealed under various conditions, and evaluated in terms of ion conductivity, water uptake, and swelling ratio before/after annealing. The effect of annealing on the physical structure of PFSA was probed by small-angle X-ray scattering (SAXS) and dynamic mechanical analysis (DMA). The porous ePTFE support was impregnated with the PFSA ionomer with the best proton conductivity and water uptake properties to produce a reinforced composite membrane (RCM). To determine the optimum annealing temperature, the RCM was annealed within a wide temperature range. Finally, the electrolyte membrane with the best performance and durability was used for MEA fabrication and subjected to an open-circuit voltage (OCV) combined wet–dry cycling test, which was conducted according to the protocol newly proposed by the U.S. Department of Energy (DOE) in 2016 to evaluate both chemical and mechanical durability at the same time.

Results and Discussion

Effects of Annealing on Solution-Cast Membranes

Conductivity and Water Uptake

As shown in Figure , solution-cast membranes were produced using 3M ionomers (EW 729 and 825) and AQ ionomers (EW 720, 790, and 830), and named as 3M 729, 3M 825, AQ 720, AQ 790, and AQ 830. All five types of membranes were fabricated to have similar thicknesses of 55–60 μm to minimize the effect of thickness variation. Figure a compares the effects of annealing temperature on proton conductivity, revealing that the proton conductivity of the nonannealed membrane increased with decreasing EW and was higher for AQ membranes than for 3M membranes in the case of similar EW. The conductivity of all types of membranes decreased with increasing annealing temperature, as did their water uptake (Figure b). This finding was ascribed to changes in polymer morphology due to the formation of crystalline regions in the main chain of polymers during annealing. This increased crystallinity increased the suppressive force for the polymer membrane swelling ratio and hence decreased the water uptake and proton conductivity.

Figure 2

(a) Proton conductivities and (b) water uptakes of 3M 729, 3M 825, AQ 720, AQ 790, and AQ 830 membranes annealed at 140, 160, and 180 °C.

Figure shows the relative humidity (RH)-dependent proton conductivity of 3M 729 and AQ 720 membranes before and after annealing at 180 °C, revealing that the proton conductivity of both membranes decreased after annealing for all RH conditions. Prior to this, the concern about the decomposition of sulfonic acid during the annealing process was verified by measuring IECs. As listed in Table S1, the IEC of 3M 729 decreased from 1.36 to 1.30 meq/g upon annealing; that of AQ 720 also decreased from 1.41 to 1.31 meq/g. For both membranes, IEC values were slightly decreased by annealing, but it was not significant to affect proton conductivity. Moreover, the difference in conductivity before and after the annealing at low RH region became larger, whereas the difference at high RH seems to be not significant. In the case of 3M 729, the conductivity at 100% RH decreased by 18% upon annealing (from 128 to 106 mS/cm), and a more significant decrease by 32% was observed at 30% RH (from 1.7 to 1.1 mS/cm). Similarly, the conductivity of the AQ 720 membrane showed a small reduction rate by 14% (from 130 to 112 mS/cm) at 100% RH and a high reduction rate by 29% (from 2.4 to 1.7 mS/cm) at 30% RH. It might be ascribed to the high sensitivity of conductivity on the structural and morphological changes at low humidity.

Figure 3

RH-dependent proton conductivities of 3M 729 and AQ 790 membranes at 80 °C before and after the annealing process.

Dimensional Stability

In general, fuel cells repeatedly start and stop during operation. The electrolyte membrane reaches a dry condition in the stationary state and swells during operation because of the generation of water in the cathode. This wet–dry cycling reduces the physical durability of the electrolyte membrane. Therefore, to be physically durable, the membrane is required to have excellent dimensional stability. To investigate the annealing-induced improvement in the dimensional stability of the SSC-PFSA electrolyte membrane, we measured and compared in-plane and through-plane swelling ratios (Figure ), revealing that similarly to the water uptake, they decreased with increasing annealing temperature. This behavior is expected to enhance durability but may negatively affect fuel cell performance by decreasing proton conductivity during fuel cell operation. Additionally, annealing temperature did not have any effect on membrane anisotropy, probably because the reduction rates of in-plane and through-plane swelling ratios in the two membranes were almost identical.

Figure 4

(a) In-plane and (b) through-plane swelling ratios of 3M 729, 3M 825, AQ 720, AQ 790, and AQ 830 membranes annealed at 140, 160, and 180 °C.

SAXS

SAXS patterns were recorded to observe the effects of annealing on membrane nanomorphology and structure. In general, we observed two peaks centered at q ≈ 1.9 and 0.5 nm–1 that were ascribed to the hydrophilic ionomer and the hydrophobic PTFE matrix, respectively.[53−55] The former peak represents the distance between hydrophilic group clusters, while the matrix peak represents the long-range correlation of the lamellar structure of the main chain PTFE. As shown in Figure S1, peaks corresponding to two regions of the matrix and the ionomer were observed.

The y-axis was amplified to Iq (where I is a scattered intensity and q is scattering vector) to observe the ionomer peak change more clearly, and the result was compared to Figure a, with d-spacings (d = 2π/q) calculated using q values corresponding to peak maxima shown in Figure b. For the 3M 729 membrane, the intensity of the ionomer peak increased with increasing annealing temperature, while an intensity increase up to 160 °C followed by a decrease at higher temperature was observed for AQ 720. Figure b reveals that d-spacing increased with increasing temperature. In particular, annealing increased ionomer peak intensity compared to that of the nonannealed sample. Moreover, the matrix knee pattern of Figure S1 shows that even long-range clustering in the hydrophobic region increased with increasing annealing temperature. Considering these results, the extent of phase separation in the hydrophobic matrix and the hydrophilic ionomer was concluded to increase with increasing annealing temperature.

Figure 5

(a) Maximum intensities and (b) d-spacings of ionomer peaks obtained for 3M 729 and AQ 720 annealed at 140, 160, and 180 °C.

On the other hand, water uptake and proton conductivity decreased with increasing annealing temperature for both 3M and AQ membranes, which, considering only the ionomer d-spacing, was unusual. Increasing ionomer d-spacing increases the extent of phase separation between hydrophilic and hydrophobic domains, which is expected to result in elevated proton conductivity and water uptake. However, the physical interaction between the hydrophobic matrices also increased after annealing and thereby acted as a factor reducing the water uptake.

DMA

Storage modulus, loss modulus, and tan δ were probed by DMA to gain further insights into the effects of annealing. Figure compares the above parameters of 3M 729 and AQ 720 membranes with relatively high proton conductivity before and after annealing at 180 °C. For both membranes, annealing reduced the storage modulus gradient. The initial storage modulus of the 3M 729 membrane increased by 23% upon annealing (from 643 to 830 MPa), while that of the AQ 720 membrane increased by 18% (from 485 to 571 MPa).

Figure 6

DMA results for (a) nonannealed 3M 729, (b) 3M 729 annealed at 180 °C, (c) nonannealed AQ 720, and (d) AQ 720 annealed at 180 °C.

Comparison of tan δ curves showed that the Tg of 3M 729 increased from 134 to 143 °C after annealing, while that of AQ 720 increased from 131 to 139 °C. Thus, annealing increased the membrane crystallization degree and, hence, Tg and mechanical durability,[43] in agreement with the aforementioned water uptake behavior. Interestingly, annealing also increased the Tg of 3M 720, with a detailed explanation provided below.

PFSA Chemical Structure Considerations

Figure a,b shows the absolute and relative contents of the individual components of the PFSA ionomer, enabling an accurate comparison of 3M and AQ membranes with similar EWs. Theoretically, the 3M membrane features a lower main chain content and a higher side chain content than the AQ membrane (Figure b), given the same EW (i.e., the same ratio of sulfonic acid end groups), and the two membranes are therefore expected to have different crystallization degrees (Figure c).

Figure 7

(a) Absolute and (b) relative weight fractions of the three domains for various PFSA ionomers; (c) schematic representations of similar EW 3M and Aquivion models.

In general, branched polymers with long side chains should have a low degree of main chain crystallization, as has been reported in many studies comparing LSC-PFSAs with SSC-PFSAs.[9,16,56,57] However, the results we obtained when investigating SSC-PFSA 3M and AQ were unexpected. As shown in Figure , AQs with very short side chains were expected to show higher Tgs and improved phase separation (based on SAXS and DMA results); however, their Tgs and d-spacings were lower than those of 3M PFSA. These findings suggest that the longer side chain acts positively to phase separation among SSC-PFSAs. In other words, the longer side chain (hydrophilic domain) of 3M PFSA showed increased mobility and was therefore expected to play an important role in the formation of hydrophilic region clusters. For this reason, the d-spacing could be higher, which further increased the phase separation with the hydrophobic region connected to the same polymer chain. These results indicate that 3M PFSA showed a more distinct phase separation even in the matrix knee of the hydrophobic region, as shown in Figure S1.

Effects of Annealing on RCMs

Conductivity and Water Uptake

In the preceding experiment, 3M 729 and AQ 720 ionomers with excellent proton conductivity and water uptake were used to produce RCMs with the impregnated ePTFE porous support to further enhance mechanical durability. The thus produced composite membranes had a thickness of 25–30 μm (average = 27 μm). As shown in Figure S2, Fourier transform infrared (FT-IR) spectroscopy was used to confirm that the produced samples did not undergo any chemical degradation in the temperature range used for annealing.

Figure shows that both proton conductivity and water uptake decreased with increasing annealing temperature, which was ascribed to the concomitant increase of crystallinity. The behavior of RCMs on both characteristics was similar to that of solution-cast membranes. In comparison to solution-cast membranes, the conductivity of nonannealed 3M 729 and AQ 720 membranes was 146 and 149 mS/cm, respectively, while those of RCM_3M 729 and RCM_AQ 720 presented 105 and 109 mS/cm. RCM showed slightly lower conductivity than the nonreinforced one by introducing ePTFE support. The ePTFE support has a porosity of 75% with about 10 μm thickness. Thus, the volume of ePTFE support in RCM (27 μm thick) might occupy around 9.3% contrast to nonreinforced parts. Since the nonconducting region by ePTFE support caused the small loss of water absorption, water uptake of solution-cast membranes, 3M 729 (37%) and AQ 720 (39%), was almost 10% higher than those of reinforced membranes, RCM_3M 729 (27%) and RCM_AQ 720 (28%). Meanwhile, the conductivity of 3M 729 and AQ 720 membranes annealed at 180 °C was decreased to 134 and 136 mS/m, respectively. The conductivity of RCM_3M 729 and RCM_AQ 720 annealed at 200 °C was also reduced to 87 and 90 mS/cm, respectively. As mentioned in Section , it is considered that the crystallinity of ionomers gradually affects to suppress the water uptake of the membrane with increasing annealing temperature.

Figure 8

(a) Proton conductivities and (b) water uptakes of reinforced 3M 729 and AQ 720 membranes annealed at 140, 170, and 200 °C.

To confirm the increase of the dimensional stability of SSC-PFSA RCMs upon annealing, we measured the corresponding in-plane and through-plane swelling ratios (Figure ), revealing that they decreased with increasing annealing temperature. The in-plane swelling ratio of RCM_3M 729 decreased from 24 to 16%, while that of RCM_AQ 720 decreased from 26 to 18%. Moreover, the through-plane swelling ratio of RCM_3M 729 decreased from 16 to 10%, and that of RCM_AQ 720 decreased from 17 to 11%, which reflected the concomitantly improved dimensional stability. In comparison to the in-plane swelling ratio of nonreinforced membranes, the ratios of 3M 729 and AQ 720 membranes annealed at 180 °C were 28 and 32%, respectively, while those of RCM_3M 729 and RCM_AQ 720 annealed at 170 °C presented 18 and 20%. RCM showed a much lower swelling ratio than nonreinforced one even at the lower annealing temperature. The ePTFE support with a porosity of 75% plays an important role as holding the electrolyte within RCMs, thereby suppressing the swelling ratio of RCM contrast to the nonreinforced one. Therefore, the introduction of the porous support greatly improved dimensional stability, similarly to annealing.

Figure 9

(a) In-plane and (b) through-plane swelling ratios of reinforced 3M 729 and AQ 720 membranes annealed at 140, 170, and 200 °C.

Solvent Resistance Test

The electrolyte impregnation stability with respect to the RCM porous support was evaluated by the accelerated solvent resistance test. RCMs were immersed into ethanol for 3 days to observe the extent of electrolyte leakage and the electrolyte present in the support pores through scanning electron microscopy (SEM) imaging. As shown in Figure , the originally transparent composite membranes turned white during the test, and the electrolyte impregnated into the support escaped. Figure b,d shows that the amount of escaped electrolyte decreased with increasing annealing temperature, and membranes annealed at 200 °C maintained a wider transparency region. As this behavior was in good agreement with that of solution-cast membrane water uptake, the increased polymer crystallization degree was ascribed to the increase in the extent of PFSA phase separation with increasing annealing temperature or to the concomitant decrease of water uptake.

Figure 10

Photographs and cross-sectional SEM images of reinforced 3M 729 and AQ 720 membranes acquired (a, c) before and (b, d) after the solvent resistance test.

After the solvent resistance test, the cross-section of the RCM annealed at 200 °C was observed by SEM to visually confirm the degree of electrolyte loss (Figure ). In Figure a,c, the red dotted lines show the enlarged support area of RCM_3M 729 and RCM_AQ 720. Initially, the support fibrils were thickened by the impregnated electrolyte, and the electrolyte-filled pores were easily identifiable. However, after the solvent resistance test, the support fibrils became tapered because of electrolyte leakage (Figure b,d). The region of electrolyte leakage from the support was partially observed in the large-scaled SEM images (Figure S3). On the other hand, RCM_AQ 720 suffered more extreme electrolyte leakage than RCM_3M 729, and the support of the former was torn into several layers. Therefore, the electrolyte impregnation stability was concluded to be higher in RCM_3M 729 than that in RCM_AQ 720.

OCV Combined Wet–Dry Cycling Test of RCMs with the 3M 729 Ionomer

Among the two RCMs with similar water uptakes and proton conductivities, the 3M 729 membrane showed higher Tg and solvent resistance, and was therefore used for MEA fabrication and evaluated using the OCV combined wet–dry cycling test. This new accelerated degradation assessment (AST) protocol was proposed by DOE to simultaneously assess the mechanical and chemical durability of the MEA in a short time.[58]

The effect of annealing on RCM durability was investigated by comparing samples before and after annealing. Figure shows the OCV change of the RCM-containing MEA during cycling. The nonannealed membrane exhibited a dramatic performance decrease after 4000 cycles, i.e., the OCV was maintained only for 120 h, and the termination occurred after 5800 cycles. In contrast, the membrane annealed at 200 °C survived 16 600 cycles while stably maintaining OCV. On the other hand, Mukundan et al. reported that an MEA including the Nafion XL membrane, which is chemically and mechanically reinforced using Ce ions and ePTFE, was stable until around 440 h by the OCV combined wet–dry cycling test.[59] Compared to this result, it is considered that the annealed RCM-MEA considerably endured against the new AST in this study. Furthermore, the OCV of the nonannealed membrane decreased at a rate of 1.41 mV/h, whereas a much smaller rate of 0.52 mV/h was observed for the annealed-membrane MEA. The decomposition of the nonannealed electrolyte membrane was rapid even under the conditions when its high-frequency resistance (HFR) was checked every 1000 h. These results suggest that annealing can greatly improve the chemical and mechanical durability of polymer fuel cell RCMs and enhance the performance of the SSC-PFSA electrolyte membranes by affecting the polymer structure.

Figure 11

OCV and HFR changes in single cells assembled with nonannealed and annealed reinforced 3M 729 membranes, as determined by the OCV combined wet–dry test at 90 °C with a cycle duration of 30/45 s.

Conclusions

The mechanical stability of electrolyte membranes comprising short-side-chain perfluorinated sulfonic acid polymers increased upon annealing, while the use of a porous support allowed for physical reinforcement. The decrease of proton conductivity, water uptake, and in-/through-plane swelling ratios of 3M and Aquivion membranes upon annealing suggests that membrane electrochemical characteristics can be changed by heat-induced alteration of crystallinity and ion channel size. SAXS and DMA showed that annealing enhances the phase separation of the electrolyte membrane and thus increases its d-spacing and Tg. The effect of annealing was different for each electrolyte membrane, being most pronounced for the 3M one, which suggests that the use of slightly longer side chains improves the aggregation of sulfonate group clusters and hence increases the crystallinity of the hydrophobic region, the distance between clusters, and Tg.

The ePTFE porous support was impregnated with the selected 3M 729 and AQ 720 electrolytes to produce RCMs that were annealed at 140, 170, or 200 °C. As in the case of the solution-cast membrane, proton conductivity, water uptake, and swelling ratio of RCM decreased with increasing annealing temperature. The stability of the polymer electrolyte in the porous support was evaluated by the solvent resistance test, and the highest stability was observed for the RCM_3M 729 membrane annealed at 200 °C. As a result, annealing was concluded to affect solution-cast and reinforced composite membranes in a similar way. Single cells assembled with nonannealed and annealed RCMs survived 5800 and 16 600 cycles, respectively, and were concluded to exhibit markedly different chemical and physical durabilities. Thus, annealing under optimal conditions allowed the fabrication of high-performance SSC-PFSA membranes with enhanced durabilities and proton conductivities.

Experimental Section

Materials and Chemicals

3M PFSA powder-type ionomers with EWs of 729 (E-21669D) and 825 (E-21669A) were purchased from 3M Co. Aquivion PFSA dispersion-type ionomers with EWs of 720 (AQ D72-25BS), 790 (AQ D79-24BS), and 830 (AQ D83-25BS) were purchased from Solvay Solexis. N-methyl-2-pyrrolidone (NMP; 872-50-4, anhydrous, 99%) and ethanol (459836, 99.5%, extra pure) were obtained from Sigma-Aldrich. The porous ePTFE substrate (Product code: #1326, Tetatex 0.07 μm PTFE membrane) was procured from Donaldson Company, Inc., and used as received.

Preparation and Annealing of PFSA Membranes

Solution-Cast Membranes

3M and Aquivion PFSA solutions (10 wt %) were prepared in NMP. In the former case, the ionomer powder was simply dissolved in NMP, whereas in the latter case, the original aqueous solution was subjected to solvent substitution. Both PFSA solutions were filtered using a Whatman 0.45 μm PTFE syringe filter, cast on a glass plate (Figure a), and dried overnight on a hot plate at 70 °C. The thus produced solution-cast membranes were further dried in a vacuum oven at 80 °C for 2 days to remove the residual solvent, placed into 3 M aqueous HCl at 80 °C for 6 h, washed with distilled water, and dried at room temperature for 2–3 days. Annealing process of solution-cast membranes was carried out at various temperatures of 140, 160, and 180 °C for 1 h in a vacuum condition. After that, the annealed membranes were quenched to room temperature.

Figure 12

Preparation of (a) solution-cast membranes and (b) RCMs.

RCMs

As shown in Figure b, RCMs were fabricated in four steps as follows: (i) a PFSA solution was cast on a glass plate using a doctor blade, (ii) a porous ePTFE substrate was placed on top of the bottom PFSA layer, (iii) the top layer of PFSA was formed by a second PFSA solution casting, (iv) the layered membranes were dried overnight on the hot plate at 70 °C. As in the case of solution-cast membranes, RCMs were further dried in a vacuum oven at 80 °C for 2 days, treated with 3 M HCl at 80 °C for 6 h to eliminate impurities and residual NMP, rinsed with distilled water, and dried at room temperature overnight. Finally, RCMs were annealed at different temperatures 140, 170, and 200 °C for 1 h in a vacuum oven.

Characterization of PFSA membranes

Proton Conductivity

Proton conductivity (σ) was measured using a four-electrode conductivity cell (MCC, WonAtech, South Korea) equipped with a Solartron 1260 impedance/gain-phase analyzer and a Solartron 1287 electrochemical interface. The conductivity in a fully swollen state in water was determined at 25 °C in distilled water. Meanwhile, RH-dependent conductivity was measured at 80 °C with 30, 50, 70, 90, and 100% RHs.where D is the distance between electrodes, L is the membrane width, T is the membrane thickness, and R is the ohmic resistance obtained from an impedance plot in the frequency range of 10–1–105 Hz.

Water Uptake and Swelling Ratio

For water uptake and swelling ratio measurements, samples with dimensions of 2 × 2 cm2 were vacuum-oven-dried at 100 °C for 24 h and immediately weighed on an analytical balance (EM 120-HR, PRECISA, Switzerland) at 100 °C. The sample size and thickness were checked using vernier calipers and a micrometer (Mitutoyo, CD-15CPX, Japan), respectively. To achieve the fully water-absorbed state, samples were soaked in distilled water at room temperature for 1 day. Excess water on the sample surface was gently removed by a paper tissue, and sample weight, size, and thickness were remeasured. Water uptake was calculated by eq 2, where Wwet and Wdry represent the wet and dry membrane weights, respectively. In-plane and through-plane swelling ratios were calculated by eqs 3 and 4, respectively. In these equations, Awet and Adry stand for wet and dry areas, respectively, and Twet and Tdry are wet and dry thicknesses, respectively.

IEC

IEC change of membrane upon annealing was measured by back titration. The membrane samples were soaked into an excess of a 5 M NaCl solution for 24 h, and then HCl coming out from the membrane by ion exchange was titrated with 0.01 N NaOH using a titrator (Metrohm 848 Titrino Plus, Swiss), and IEC was determined by eq .where VNaOH and CNaOH are the consumed volume and concentration of NaOH, respectively.

Fourier Transform Infrared (FT-IR) Spectroscopy Analysis

Annealing-induced changes of membrane chemical structure were investigated by FT-IR spectroscopy (NICOLET 5700 FT-IR) in attenuated total reflectance mode (Smart MIRacle accessory, Diamond, PIKE technologies). Prior to analysis, membrane samples were dried overnight in a desiccator at room temperature. Spectra were recorded at a resolution of 4 cm–1 in the range of 700–4000 cm–1 and normalized with respect to the highest-absorbance peak of CF2, which is a major unit of the PTFE backbone, to investigate the variation of peak intensity for two ethers in the side chain.

SAXS Measurements

The nanostructure of PFSA membranes was analyzed by SAXS. The corresponding instrument (SAXSpace, Anton Paar, Austria) was operated by focusing a line-shaped beam of Cu Kα radiation (λ = 0.154 nm) onto the detector plane. The high-resolution mode was chosen to detect a minimum scattering vector, qmin, of 0.03 nm–1 (q = (4π/λ)sin θ, where 2θ is the scattering angle). The prepared samples, stored in a vacuum desiccator overnight before the experiment, were fixed with a beam adjustment slit at the spring holder and placed into a chamber with a vacuum of 1.9–2.0 mbar. The scattering wave vector (q) and intensity curve were depicted using a one-dimensional intensity profile produced by azimuthal integration of the collected two-dimensional scattering patterns. The d-spacing of the hydrophilic domain was calculated using Bragg’s law (eq ).

The mechanical properties of solution-cast membranes were probed by DMA (Q800, TA Instruments) for specimens dried overnight in a desiccator at room temperature. Membrane samples with dimensions of 15 × 15 mm2 and a thickness of 50 μm were tested in multi-frequency strain mode at a constant frequency of 10 Hz using an initial static force of 0.05 N, a force track of 125.0%, and an oscillation amplitude of 10 μm. Temperature sweeping was conducted in the range of 25–225 °C at a heating rate of 5 °C/min. The collected stress–temperature data were used to plot graphs of storage modulus, loss modulus, and tan δ, and thus determine Tg.

The stability of reinforced PFSA membranes was evaluated by a solvent resistance test. Membrane samples (2 × 2 cm2) were dried in a vacuum oven at 70 °C overnight and immersed in ethanol for 3 days. The change of membrane thickness was checked by a micrometer, and cross-sectional morphology was investigated by scanning electron microscopy (SEM; Hitachi SEM S-4800, Japan).

OCV Combined Wet–Dry Cycling Test

A 9 cm2 MEA was fabricated by a decal transfer method with an anode/cathode Pt loading of 0.25–0.30 mgPt/cm2.[52] Carbon-supported Pt (TEC10F50E, Tanaka Kikinzoku Kogyo K. K., Japan) and an Aquivion ionomer (D83-24B) were used as the metal catalyst and binder for the electrode layer, respectively. OCV combined wet–dry cycling was performed using the protocol proposed by the U.S. DOE.[58]

Testing was performed at 90 °C and 0–100% RH with 40 sccm/cm2 H2 and high-purity air supplied to the anode and cathode, respectively. The OCV was continuously monitored by a potentiostat (ZIVE SP2, WonATech, Korea). In the wet–dry cycle test, the cell was tested at 90 °C for 30 s dry (0%)/45 s wet (100%) conditions as one cycle (denoted as 30/45 s). High-frequency resistance (HFR) was measured at 90 °C and 100% RH using a potentiostat (HCP-803, Bio-Logic Science Instruments, France) at 1–20 000 Hz every 1000 cycles until 10 000 cycles and subsequently every 2000 cycles. Cycling was terminated when the OCV dropped to less than 80% of the initial value.