Increasing the Durability of Polymer Electrolyte Membranes Using Organic Additives.

Herein, we utilize organic radical scavengers to mitigate the chemical degradation of polymer membranes without sacrificing their proton conductivity. Several hydrocarbon composite membranes based on sulfonated poly(arylene ether sulfone) (SPES50, degree of sulfonation = 50%) and containing organic radical scavengers were prepared and characterized in terms of water uptake, ion-exchange capacity, proton conductivity, and oxidative stability, being additionally exposed to hydrogen peroxide for accelerated oxidative stability testing. Precise analysis of the molecular weight and its distribution before and after the above test confirmed that the incorporation of radical scavengers enhanced the chemical durability of membranes while maintaining their proton conductivity. Finally, in an accelerated open circuit voltage durability test, composite membranes showed lifetimes exceeding 1400 h, whereas pristine SPES50 failed after 750 h. On the basis of the above, organic radical scavengers were concluded to be superior to those based on transition-metal compounds, not engaging in any interactions with the sulfonate groups of the membrane polymer and hence not compromising their proton conductivity.

Introduction

Fuel cells, that is, electrochemical devices converting chemical energy into electrical energy, provide one of the most promising alternatives to the use of fossil fuels and are capable of clean energy production. Specifically, proton exchange membrane fuel cells (PEMFCs) have recently attracted increased attention, being characterized by elevated energy efficiency because of the utilization of a non-Carnot cycle for power generation, as opposed to the operation of the internal combustion engine. Moreover, the high power density and lighter system components compared to those of other cells make PEMFCs well suited for automotive and residential applications.[1]

Perfluorosulfonated acid (PFSA) membranes such as Nafion are widely used in PEMFCs but suffer from a number of disadvantages, for example, high cost and insufficient oxidative stability,[2,3] which necessitates the search for better alternatives such as hydrocarbon-based membranes. In particular, sulfonated aromatic polymers exhibit better thermal stability and mechanical strength than their PFSA counterparts, additionally featuring the advantages of low cost and ease of synthesis.[4−10]

Chemical membrane degradation is another important issue hindering the commercialization of PEMFCs, being predominantly caused by the attack of hydroxyl radicals generated during cell operation[11−13] on polymer chains and resulting in cell failure. Reactive chemical species (RCS) such as peroxide and the abovementioned hydroxyl radicals generated during cell operation are known to attack the main and the side chains of PFSA polymers and cause their decomposition, which often occurs under low relative humidity (RH) conditions.[14−16] Moreover, an alternative failure mechanism featuring the scission of polymer chains caused by the decomposition of sulfonic acid groups has been proposed by Endoh et al.[17]

Transition metals such as Ce and Mn can mitigate the degradation of fuel cell membranes by protecting polymers from the attack of hydroxyl radicals and thus acting as radical scavengers. However, despite their beneficial effects on membrane durability, the introduction of these metals causes undesirable side effects, for example, they decrease the proton conductivity of membranes.[18,19] Recently, we have studied hydrocarbon composite membranes containing Ce ions,[19,20] revealing that although the above membranes showed exceptional stability during open circuit voltage (OCV) accelerated stability testing lasting over 2000 h, they suffered from a large decrease in the ionic conductivity with the increasing Ce content, which was ascribed to the formation of ionic cross-linkages between Ce ions and sulfonate groups. Recently, the oxide forms of transition metals and silica-supported radical scavengers have also been suggested to mitigate the degradation of membranes.[21−24] Although the exact mechanism has not been completely elucidated, these radical scavenging properties are generally accepted.

In addition to those of transition metals, the antioxidation effects of organic compounds have been long studied in the fields of biology, food science, and polymer chemistry. RCS are known to induce cellular injuries such as membrane lipid peroxidation, DNA alteration, protein damage, and enzyme inactivation,[25,26] which necessitate the development of organic antioxidants (e.g., phenolic and N-heterocyclic compounds) capable of mitigating these effects.[27−29]

Phenolic compounds were the first group of antioxidants investigated for use in the food and pharmaceutical industries, being capable of deactivating peroxyl radicals as follows[30,31]

As demonstrated above, the phenolic H-atom is abstracted by a peroxyl radical to yield a stable aroxyl radical (ArO•) that is unable to propagate the chain,[32] and the radical scavenging ability of phenols is therefore thought to be correlated with their O–H bond dissociation enthalpy.[29,33] Terephthalic acid was also reported to increase the chemical stability of PFSA.[34] On the other hand, nitrogen-containing heterocyclic compounds (N-heterocycles) can also act as radical scavengers. Specifically, electron-rich nitrogen atoms might absorb free radicals, allowing N-heterocycles to donate a single electron to RCS and remove free radicals owing to resonance delocalization.[35−37] Several other antioxidants are also known, for example, quinones, which are used as free-radical polymerization inhibitors.[38] More recently, α-tocopherol (TOH) was reported to be effective for mitigating the degradation of PFSA membranes owing to its regeneration properties.[39]

Herein, we utilize organic radical scavengers to improve the oxidative stability of polymer membranes because these compounds were not expected to undergo ionic crosslinking observed for transition metals and should thus effectively mitigate the radical attack-induced degradation of membranes without reducing their proton conductivity. Specifically, four representative scavengers were selected, that is, TOH, 2,6-dimethoxy-1,4-benzoquinone (BQ), hydroquinone (HQ), and 2,2′-bipyridine (BPY) (Figure ). The utilized membranes were based on sulfonated poly(arylene ether sulfone) (SPES50) and thus belonged to the hydrocarbon type.

Figure 1

Chemical structures of selected radical scavengers: (a) TOH, (b) BQ, (c) HQ, and (d) BPY.

The effects of organic radical scavengers were screened using a Fenton’s reagent/rhodamine B (RhB) method followed by UV–vis spectroscopy analysis.[40,41] The interactions of organic radical scavengers with sulfonic acid groups were observed by nuclear magnetic resonance (NMR), water uptake, ion exchange capacity (IEC), and proton conductivity measurements. Additionally, the oxidative stability of membranes was tested by exposing them to hydrogen peroxide vapor, which simulated the environment encountered during the fuel cell operation. The effects of radical scavengers were analyzed by gel permeation chromatography (GPC), dynamic light scattering (DLS), and multiangle light scattering (MALS) measurements before and after exposure to hydrogen peroxide vapor. Finally, membranes containing selected radical scavengers were subjected to the OCV hold test under the conditions stipulated by Japan’s New Energy and Industrial Technology Development Organization (NEDO).

Results and Discussion

Investigation of the Radical-Scavenging Activity

The radical-scavenging activity of selected chemicals was probed by using Fenton’s reaction in the presence of RhB. Because the RCS generated from Fenton’s reagent are able to degrade RhB, the time-dependent concentration of this dye in Fenton’s solution was used to monitor the radical-scavenging ability of organic species. The decrease in the RhB concentration by RCS can be explained by eqs –5 as follows

The intensity of RhB absorption at 550 nm decreased with increasing H2O2 concentration (Figure a), which implied that the degradation of RhB in the absence of organic radical scavengers was rapid. Successive additions of 10 μL of H2O2 resulted in the progressive loss of the RhB absorption intensity, which finally decreased to near-zero values. However, this intensity decrease was less pronounced in the presence of organic radical scavengers (Figure b–e). Specifically, upon the addition of 10 μL of H2O2, 91, 90, 94, and 93% of the initial RhB absorbance intensities were preserved in the presence of TOH, BQ, HQ, and BPY, respectively, whereas only 69% of the initial intensity was preserved under scavenger-free conditions (Figure ). For 10 μL of H2O2, the RhB absorbance intensity decreased to 9% of the initial value in the absence of scavengers, whereas the values in the range of 45–73% were observed in their presence. The best performance was observed for TOH and BPY, whereas lower absorbance retention was observed for BQ and HQ. However, even in the latter case, the final absorbance was much higher than that observed for pure RhB, which confirmed that despite a certain variability, the radical-scavenging ability of all investigated species was high enough to effectively hinder the degradation of RhB.

Figure 2

UV–vis spectra of the RhB/Fenton’s solution system recorded for different H2O2 concentrations (a) in the absence of radical scavengers and in the presence of (b) TOH, (c) BQ, (d) HQ, and (e) BPY.

Figure 3

UV absorbance changes of RhB in the presence of different organic radical scavengers.

Characterization of Composite Membranes

The prepared composite membranes were transparent and homogenous (Figure a,b), and the presence/state of organic radical scavengers therein was confirmed/determined by overlaying the 1H NMR spectra of HQ, an SPES50 pristine membrane, and SPES50-HQ (Figure c). Notably, the spectrum of SPES50-HQ featured a prominent HQ peak at 6.57 ppm and unchanged peaks (1–9) of pristine SPES50, which indicated that the above scavenger did not interact with SPES50, particularly with its sulfonic acid groups (Figure c). Similar behavior was observed for other scavengers, as shown in Figure S1. As already mentioned in the Introduction, although metal ions such as Ce or Mn are effective radical scavengers, their strong interaction with the sulfonic acid groups of membrane polymers can hinder proton transport.[19] Thus, the addition of Ce ions shifted the signal of protons adjacent to the sulfonic acid group, with the above shift increasing with the increasing concentration of Ce ions. Thus, in contrast to metal ions, organic radical scavengers were well dispersed throughout the membranes and did not interact with sulfonic acid groups.

Figure 4

Photographs of (a) SPES50 and (b) SPES50-BQ and (c) 1H NMR spectra of HQ, SPES50, and SPES50-HQ.

To clarify, we also measured the Fourier transform infrared spectroscopy (FT-IR) spectra of the composite and SPES50 membranes, as shown in Figure S2. The symmetric stretching at 1055 cm–1 and the antisymmetric stretching at 1220 cm–1 of the sulfonic acid group were observed for all membranes. No shift was observed for the composite membranes, which implies no interaction of organic radical quenchers with sulfonic acid groups.

The water uptakes and IECs of the composite membranes are listed in Table . Specifically, the water uptakes of SPES50-TOH, SPES50-HQ, SPES50-BQ, and SPES50-BPY equaled 72, 73, 70, and 69%, respectively, whereas pristine SPES50 showed a value of 73%. Similarly, the IECs of the composite membranes were almost identical to those of the pristine membrane, that is, the water uptake and IEC were not affected by the incorporation of organic radical scavengers, similarly to the NMR spectra. Conversely, the incorporation of metal ions decreased the water uptake and IEC because of the formation of ionic cross-linkages between metal ions and sulfonic acid groups.[19]

Table 1

Water Uptake and IECs of Composite Membranes

	SPES50	SPES50-TOH	SPES50-BQ	SPES50-HQ	SPES50-BPY
water uptake (%)	73	72	70	73	69
IEC (meq/g)a	1.96	1.94	1.93	1.94	2.00

Determined by titration with NaOH.

The tensile properties of the SPAES50 and composite membranes were measured at room temperature and atmospheric pressure (Figure S3). These results are also summarized in Table S1 along with the calculated Young’s moduli. The SPAES and composite membranes exhibited almost a similar strain and modulus. This result means that the mechanical properties were not affected by organic radical scavengers.

Proton Conductivity

Figure shows the RH-dependent proton conductivity of composite membranes determined at 80 °C, revealing that the proton conductivity of SPES50 decreased with RH, whereas those of SPES50-TOH and SPES50-BPY were slightly reduced but still similar to that of SPES50. Notably, SPES50-BQ and SPES50-HQ showed proton conductivities almost identical to that of SPES50 even at a low RH, which, considering the organic additives as proton transfer interferences, was rather unexpected, that is, even in the absence of any interaction between the organic radical scavenger and sulfonic acid groups, the well-dispersed organic additive was expected to inhibit the aggregation of the sulfonic acid into hydrophilic clusters. In lieu of a better explanation, the retention of the proton conductivity at a low RH was ascribed to the hydrophilic nature of BQ and HQ. As mentioned above, the use of metal ions as typical radical scavengers resulted in the formation of ionic cross-linkages with sulfonic acid groups and a decreased proton conductivity, whereas all selected organic radical scavengers did not exhibit this detrimental effect because they did not interact with sulfonate groups.

Figure 5

RH-dependent proton conductivities of composite membranes determined at 80 °C.

Hydrogen Peroxide Vapor Exposure Test

The oxidative stability of the composite membranes was probed using a hydrogen peroxide vapor exposure test. Notably, the ex-situ Fenton’s test commonly used to determine the oxidative stability was not used because it is performed in the solution state and in an environment different from that encountered during PEMFC operation. Therefore, the hydrogen peroxide vapor exposure test was specifically designed to imitate the PEMFC environment and was conducted under conditions most appropriate for the fast screening of membrane degradation, that is, at 120 °C and an RH of less than 10%. The oxidative stabilities of composite membranes were evaluated by analyzing the number-average molecular weight (Mn) of the constituent polymer before and after the test (Figure and Table ), and the extent of degradation was calculated as the percent change in Mn before and after the test. Figure a and Table reveal that pristine SPES50 exhibited an extent of degradation of 42%, which was accompanied by an increase in the polydispersity index (PDI), whereas the corresponding values for SPES50-TOH, SPES50-BQ, SPES50-HQ, and SPES50-BPY equaled 18, 9, 26, and 15%, respectively. These results show that the incorporation of radical scavengers significantly enhanced the membrane oxidative stability, with the best performance observed for BQ and BPY. Interestingly, the observed order of additive performance was different to that observed for the RhB/Fenton’s reagent test and the hydrogen peroxide exposure experiment. Specifically, the latter test evaluated the performance of composite membranes, whereas the former aimed to determine the pure oxidative performance of organic scavengers. Although BPY was the best-performing additive according to both experiments, a discrepancy was observed for TOH and BQ. The lower performance of TOH in the peroxide exposure experiment might be ascribed to the poor compatibility of this scavenger with the SPES50 polymer because of the different chemical structure of the former, which prohibited scavenging in the polymer matrix.

Figure 6

GPC profiles of (a) SPES50 and (b) SPES50-TOH, (c) SPES50-BQ, (d) SPES50-HQ, and (e) SPES50-BPY membranes before/after oxidative stability testing.

Table 2

Molecular Weights of Pristine SPES50 and Composite Membranes before and after Oxidative Stability Testing

	molecular weight (kDa)
	before			after
sample	Mn	Mw	PDI	Mn	Mw	PDI	extent of degradation (%)
SPES50	74	206	2.80	43	129	3.44	42
SPES50-TOH	72	187	2.60	59	213	3.87	18
SPES50-BQ	70	183	2.64	64	188	3.13	9
SPES50-HQ	74	200	2.68	55	181	3.29	26
SPES50-BPY	71	194	2.74	60	199	3.32	15

Additionally, we also tested the oxidative stability in Fenton’s reagent for all membranes. Even its test conditions are different from actual fuel cell operation; this is a common method for oxidative stability. The residual weight % of pristine SPES50 after the Fenton’s test was only 20%, whereas those for SPES50-TOH, SPES50-BQ, SPES50-HQ, and SPES50-BPY were 90, 97, 95, and 98, respectively.

Figure shows that for some samples, the content of high-molecular-weight species increased after the oxidative stability test. This phenomenon was particularly pronounced for SPES50-TOH, where the increases in both high- and low-molecular-weight polymer contents resulted in a broadened molecular weight distribution, as confirmed by numerous repetitions of this test. In view of the above, we hypothesized that the generated radicals could recombine with the polymer and change its conformation and molecular weight, in line with the report of Mack et al.[42] Specifically, if the polymer chains are attacked by RCS during the durability test, reactive radicals could be transferred to the SPES50 polymer chain and eventually lead to an intermolecular recombination reaction with radicals on other SPES50 polymers. Mack et al. showed that increase in the molecular weights was observed for some polymers subjected to the Fenton’s test, which was ascribed to the washing out of low-molecular-weight polymers during the test and/or to polymer–residual radical recombination. Herein, we tried to identify the cause of this abnormal behavior by subjecting SPES50-TOH to DLS and MALS measurements before and after the test (Figure and Table S2). Figure shows that the Z-average diameter of the polymer increased from 43.2 to 68.5 nm after the oxidative stability test, with the diameter distribution being almost identical to that obtained from the GPC profile and featuring simultaneous increases in the abundances of both low- and high-molecular-weight species.

Figure 7

DLS diagrams obtained before/after oxidative stability testing.

An MALS analysis offered more detailed data for further discussion, as listed in Table S2. The constant [a] of the Mark–Houwink–Sakurada equation was calculated and correlated with the polymer conformation, reflecting a conformation change to spherical from random-coil. The spherical structure was ascribed to polymer vulcanization, which sometimes occurs in rubber and olefin polymers upon UV light exposure. However, further studies are needed to identify the underlying reason more clearly.

This phenomenon might be unfavorable for fuel cell membranes. Although radicals were simply thought to degrade polymers and thus lower the chemical durability of membranes, the mechanical properties of membranes could also be affected, that is, radical-induced crosslinking might decrease the membrane viscoelasticity and result in embrittlement. Thus, the mechanical degradation of membranes during wet–dry cycling can be accelerated by their crosslinking. A new class of radical scavengers can be suggested to mitigate the crosslinking of membranes. For example, additional functional groups or additives for suppressing the chain transfer of radicals might be suitable candidates.

Accelerated OCV Test

On the basis of the conductivity data and the results of the hydrogen peroxide exposure experiment, we identified BQ and BPY as the best-performing radical scavengers and therefore characterized membrane electrode assemblies (MEAs) prepared from SPES50, SPES50-BQ, and SPES50-BPY in-situ by the accelerated OCV hold test (NEDO conditions, 90 °C and 30% RH). As shown in Figure , SPES50 membranes impregnated with organic radical scavengers exhibited stable OCV operation for up to 1400 h, whereas the performance of pristine SPES50 declined after 750 h. Moreover, voltage drops of 184 (after ∼750 h), 42 (1400 h), and 64 (1400 h) μV/h were observed for SPES50, SPES50-BQ, and SPES50-BPY, respectively. However, the OCV of SPES50-BPY started to decrease after 1200 h, whereas that of SPES50-BQ remained stable up to 1400 h. The hydrogen crossover current of the MEAs was also measured by using the linear-sweep voltammogram (LSV) after the test or periodically. The initial hydrogen crossover current densities of all MEAs were almost the same; however, the LSV of the SPES50 MEA could not be measured, even after 730 h of operation because the hydrogen crossover current exceeded the range of our analysis equipment. The LSVs of the SPES50-BQ and SPES50-BPY MEAs gradually increased with the OCV time; however, they were much lower than that of the pristine SPES50 MEA.

Figure 8

Time dependence of the OCV observed during accelerated stability testing at 90 °C and 30% RH (NEDO conditions).

These results were consistent with those of hydrogen peroxide exposure experiments and indicated the successful functioning of organic additives in membranes as radical scavengers. However, the OCVs of SPES50-BQ and SPES50-BPY are decreasing, and their molecular weights were observed to decrease to 53 and 60% of their original values after the test. The IEC values of SPES50-BQ and SPES50-BPY after the OCV were 1.83 and 1.85 meq/g, respectively, which were more than 90% of the original IEC. Our recent post-test analysis of the OCV test has found that the degradation of both sulfonic acid functional groups and ether linkages in the polymer backbone was responsible for the decrease in the molecular weight.[20]

This implies that BQ and BPY might slowly diffuse out of the catalyst layer, resulting in failure of the membrane. More stable and promising radical scavengers seem to be required in the future in terms of the long-term stability.

Conclusions

Herein, we successfully improved the oxidative stability of hydrocarbon-based composite PEMFC membranes by impregnation with organic radical scavengers, which were shown to be superior to transition metals. Specifically, the use of transition metal additives resulted in the formation of ionic cross-linkages with polymer sulfonate groups, and thus decreased the proton conductivity, whereas no such negative effect was observed for organic radical scavengers, which could effectively mitigate radical attack-induced membrane degradation. NMR, water uptake, IEC, and proton conductivity data showed that organic radical scavengers did not interact with the sulfonic acid groups of SPES50, in contrast to metal-based radical scavengers, effectively mitigating the oxidative degradation of membranes, as confirmed by molecular weight measurements before and after the hydrogen peroxide vapor exposure test. However, the unexpected radical crosslinking (as confirmed by DLS and MALS data) could potentially deteriorate the mechanical properties of membranes.

Finally, organic radical scavenger-impregnated membranes performed well in the fuel cell test, with the overall best performance observed for BQ. Thus, the present work provides a new way of enhancing the performance of PEMFCs and is expected to facilitate their commercialization.

Experimental Section

Materials

SPES50 (degree of sulfonation = 50%) was purchased from Yanjin Company (China). Dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) were sourced from Duksan Pure Chemicals (Korea). HQ, BPY, BQ, and RhB were purchased from Sigma-Aldrich (USA).

Prescreening of the Radical Scavenging Activity

Fenton’s reagent and RhB were used to investigate the activity of radical scavengers.[40,41] Typically, 0.2 mmol of an organic radical scavenger was added to 20 mL of deionized water with 6 ppm RhB containing 30 ppm Fenton’s catalyst and 0.25 mmol H2SO4 (for better dissolution of radical scavengers[39]), and the reaction mixture was stirred to obtain a homogeneous solution. Subsequently, 10 μL of aqueous 3 wt % H2O2 was added dropwise, and the mixture was stirred for 10 min to reach equilibrium, which was followed by repeated addition of 10 μL of H2O2. This procedure was repeated six times to reach an H2O2 amount of 60 μL, and UV spectra were recorded at every 10 min equilibrium at a given concentration. All experiments were performed in duplicates, and the obtained results were reported as averages of two measurements.

Preparation of Composite Membranes

SPES50 was dissolved in DMSO to obtain a 10 wt % solution that was subsequently treated with TOH, BQ, HQ, or BPY (a loading of 2.5 mol % per sulfonic acid group of SPES50 was used), and the resulting mixtures were stirred in nitrogen at 25 °C for 8 h to obtain transparent solutions. The above solutions were poured onto a flat glass plate and dried at 70 °C for 8 h to achieve a membrane thickness of 50 μm. The thus prepared membranes were oven-dried at 120 °C for 6 h in vacuum, washed with deionized water to remove residual solvents, and denoted as SPES50-XX, where XX stands for the utilized organic radical scavenger.

Characterization of Composite Membranes

The as-prepared composite membranes were analyzed by 1H NMR (Bruker AVANCE III, 600 MHz) using deuterated DMSO (DMSO-d6) as the solvent. The FT-IR spectra were measured using a Nicolet-5700 FT-IR spectrometer (Thermo Electron Corporation, USA) equipped with a diamond-attenuated total reflection accessory.

To determine the water uptake, composite membranes were oven-dried at 120 °C in vacuum for 24 h and immediately weighed. Importantly, the oven was connected to a nitrogen gas source to minimize the contact with atmospheric moisture. Subsequently, the dried membranes were immersed into deionized water at room temperature for 24 h, and the water uptake was calculated aswhere Wwet and Wdry represent the membrane weights in the wet and dry states, respectively.

IECs were measured by back titration. Typically, membranes were immersed into a large excess of 5 M aqueous NaCl for 24 h, and HCl produced by ion exchange was titrated with 0.01 N NaOH using a titrator (Metrohm 848 Titrino Plus, Swiss). The IEC was calculated aswhere VNaOH and CNaOH represent the consumed volume and concentration of NaOH, respectively.

The proton conductivities were measured in a BeckTech four-probe-type conductivity cell using a Solartron 1260 impedance/gain-phase analyzer and a Solartron 1287 electrochemical interface. The proton conductivity was determined at 80 °C and variable RH aswhere D is the distance between electrodes, L is the membrane width, B is the membrane thickness, and R is the impedance resistance.

The GPC instrument used to estimate the molecular weight was equipped with a UV–visible detector (YL 9120, Young Lin, Korea), and a system featuring three columns (Showdex KD-802.5, KD-804, and KD-805) connected in series was used for accurate molecular weight analysis. HPLC-grade DMF containing 0.05 M LiBr was used as the mobile phase at a flow rate of 1.0 mL/min, and molecular weight calibration was performed using a polystyrene standard. The tensile strengths of the membranes were confirmed using a universal test machine (UTM) (TopTac 2000, Yeonjin Corp., Korea). The stress versus strain curves were obtained for samples that were cut into a dumbbell shape according to the JIS K 6251-7 standard [35 × 6 mm2 (total), 15 × 2 mm2 (test area)]. The measurements were performed at a strain rate of 10 mm/min at room temperature.

Accelerated Oxidative Stability Test

Oxidative stability testing was performed by exposing composite membranes to hydrogen peroxide vapor to imitate fuel cell operation conditions.[43] Typically, 2 × 2 cm2 membrane samples were held in a specially designed chamber at 120 °C for 38 h at RH = 10%. Hydrogen peroxide vapor, generated by heating a reservoir containing 10 wt % aqueous hydrogen peroxide and deionized water, was introduced into the chamber using ultrapure nitrogen gas as the carrier. After the test, membranes were washed with deionized water several times and dried at room temperature. The molecular weights of the composite membranes before and after the test were checked by GPC to estimate the degree of oxidative degradation. DLS (Zetasizer Nano ZS, Malvern Instruments, UK) and MALS (Wyatt MALS system, DAWN 8+, Wyatt Technology Corporation, USA) measurements were used to measure the particle size and molecular weight of the polymer, respectively. The oxidative stability of the membranes was also evaluated by immersing the membrane samples in the Fenton’s reagent (3% H2O2 containing 3 ppm FeSO4) at 30 °C for 24 h. The change in weights before and after the test was measured.

In Situ OCV Accelerated Oxidative Stability Test

For long-term stability tests, MEAs were prepared as follows. A decal transfer film was coated with a carbon-supported Pt catalyst (HISPEC4000, Johnson Matthey Catalysts) and a Nafion ionomer (Dupont, 20 wt %), and electrodes with an area of 9 cm2 were applied to both sides of the membrane upon 20 min of compression at 100 °C with edge sealing. Pt anode and Pt cathode catalysts were used at loadings of 0.24 and 0.21 mg/cm2, respectively. Following the NEDO guidelines, the accelerated oxidative stability test was performed at 90 °C and 30% RH with hydrogen (anode) and air (cathode) supplied to the electrodes at flow rates of 0.20 and 0.42 L/min, respectively. The OCV was continuously monitored using a potentiostat (ZIVE SP2, WonATech, Korea). The LSV measurements were carried out with hydrogen supplied to the anode at a rate of 0.2 L/min and nitrogen supplied to the cathode at a rate of 0.5 L/min to measure the hydrogen crossover current from the anode to the cathode for every one or two hundred hours. The potential was swept from 0.1 to 0.6 V at a scanning rate of 1 mV/s, and the hydrogen crossover current was obtained at a potential of 0.6 V.