Biosourced Antioxidants for Chemical Durability Enhancement of Perfluorosulfonic Acid Membrane

The chemical durability of perfluorosulfonic acid (PFSA) membranes is a topic of growing interest to meet Department of Energy (DOE) durability targets for heavy‐duty vehicle (HDV) applications. State‐of‐the‐art membranes like Nafion, rely on the use of cerium, heteropolyacids, and other inorganic additives to increase PFSA chemical durability. A less explored avenue for the oxidative stabilization of PFSA and hydrocarbon membranes is the use of organic antioxidants. No reversible organic antioxidant has been demonstrated to date which can enhance membrane lifetime by factors comparable to cerium. Here, ellagic acid (EA) is demonstrated as a promising radical scavenger for PFSA's. It is found that the incorporation of EA enhances the chemical durability of Nafion by 160%. EA, when incorporated with cerium as an electron donorenhances Nafion durability by at least 80% compared to a membrane incorporated with just cerium in DOE‐defined durability tests. EA is found to be reversible in acidic conditions like those of fuel cells and its reversibility could be further enhanced by the use of suitable co‐antioxidants.


Introduction
Polymer electrolyte membranes (PEM) play a vital role in the performance and durability of fuel cells (FC).The purpose of these membranes is to selectively transport protons from the anode to the cathode while preventing the crossover of H 2 and O 2 gases.However, it is challenging to completely eliminate gas crossover, and some degree of crossover is inevitable.Crossover gases DOI: 10.1002/adfm.202308856react within the catalyst layers to form peroxide, which can dissociate into hydroxyl radicals in the presence of transition metal impurities within the system. [1]Hydroxyl radicals react with the membrane causing the ionomer to degrade which results in the eventual failure of the membrane. [2]erium is traditionally used as a radical scavenger due to its rapid Ce +3 /Ce +4 reversibility within the FC environment, which enables efficient radical scavenging with low cerium loadings. [1]Within fuel cell membranes, cerium loadings as low as 0.6 wt.% have been found to reduce fluoride emissions during accelerated stress testing (AST) by over an order of magnitude and open circuit voltage (OCV) decay by a factor of 20. [1,3] Due to its high radical scavenging efficacy, 2 nd generation Toyota MIRAI vehicles employ membrane electrode assembly (MEA) incorporated with cerium additives. [4]owever, being cationic in nature, cerium has a natural tendency to migrate within an MEA environment much like protons.This mobility compromises the radical scavenging performance and ultimately negates the benefits of its inclusion in FCs. [5,6][8] Alternatively, a promising yet relatively unexplored approach to increasing the oxidative stability of PFSA membranes involves the use of organic antioxidants.Organic antioxidants offer advantages such as ease of incorporation and compatibility with the membrane and have the potential for reduced migration under FC operation.
Various types of antioxidants that have been studied for their potential to improve the oxidative stability of perfluorosulfonic acid (PFSA) membranes include heteropolyacids (HPAs), [9] quinones, [10] phenols, [11] phosphonic acids, [12] and carboxylic acids. [8]While HPAs are known to scavenge radicals and enhance durability, their water solubility is concerning and they may suffer from similar issues as cerium. [9,13]Phosphonic acids are promising radical scavenging alternatives to cerium but platinum poisoning due to phosphonic acid migration remains a concern. [12]Ferrocyanide/ferricyanide redox has been found to be an effective antioxidant strategy for PFSA and has been demonstrated to extend Nafion's durability in OCV test conditions. [14]While Nafion showed a 40% loss in OCV, ferrocyanide and ferricyanide-incorporated PFSA showed only a 2.2% loss in OCV for the same duration. [14]Another widely explored class of radical scavengers are phenolic and heterocyclic derivatives such as -tocopherol, 2,6-dimethoxy-1,4benzoquinone, hydroquinone, and 2,2-bipyridine. [11]Benzoquinone was found to be an effective antioxidant for poly(arylene ether sulfone) membranes and demonstrated the ability to enhance PFSA durability with reduced membrane thinning and fluoride emission rate (FER). [11]Quercetin was also found to double the chemical durability of Nafion in a nine-day OCV test. [10]Carboxylic acids such as cinnamic acid [15] and terephthalic acid [16] have also been studied as radical scavengers, but, they both exhibited much higher FER than a membrane incorporated with cerium.More recently, alizarin was investigated as a radical scavenger.When it was incorporated in Nafion along with cerium, the membrane lifetime surpassed 450 h under the 80 °C OCV hold test. [17]The enhancement was attributed to cerium complexing with alizarin.
Several of the studies mentioned above did not evaluate the organic radical scavengers (ORS) in terms of reversibility and oxidative stability, while others did not employ standard DOE testing protocols to investigate the effect of ORS incorporation on Nafion durability.Additionally, most of the ORS discussed above show relatively low antioxidant activity compared to cerium. [10,11,15]he highest reported OCV lifetime for Nafion incorporated with an ORS alone is 216 h (for alizarin) which increased to 450 h when the organic scavenger was incorporated into the membrane along with cerium.In comparison, our investigations have found that the lifetime for Nafion XL, which is a commercial ceriumincorporated membrane from Chemours (now discontinued), is 500 h in the DOE-recommended OCV hold test.This shows the limitations of the ORS systems investigated in the past.Furthermore, the ORS discussed earlier are limited by their chemical instability under the potential conditions of a fuel cell and their irreversible redox properties.In addition, previous studies have not investigated the migration of ORS out of the membrane dur-ing operation, which could cause their absorption onto platinum active sites resulting in a loss of catalytic performance.
In this work, we have systematically selected and evaluated ORS for FC applications by conducting both in situ and ex situ investigations, which led to the identification of ellagic acid (EA) as a promising candidate.EA's reversibility under various conditions, the effect of its incorporation on Nafion's properties, and mobility issues during operation are investigated.While the use of EA as an ORS presents some remaining challenges, avenues to resolve them are also suggested.

Suitability of Various ORS for PEM
A wide range of common radical scavengers was initially screened by conducting a rhodamine dye (RhB) test.The investigated compounds included methyl indole, melatonin, hydroquinone, uric acid, chitosan, ascorbic acid, melanin, and ellagic acid.The structures of all the investigated ORSs are shown in Figure 1.Details of the RhB experiment are given in the experimental section, and the results are shown in Figure S1, Supporting Information.Briefly, RhB shows distinct absorption at 550 nm in the UV-Vis region.When Fenton's reagent (FR) is added to RhB, its absorbance quickly dies to zero due to the degradation of RhB by the action of hydroxyl radicals.If radical scavengers are added to RhB before the addition of FR, •OH radical attack on RhB can be suppressed or eliminated and its absorbance could be preserved.This property is used to compare the radical scavenging abilities of different antioxidants in this study.
Among the investigated materials, melatonin showed radical scavenging comparable to cerium in the ex situ investigation (Figure S1, Supporting Information).Cerium showed an initial rise in UV-Vis absorbance likely due to complex formation with RhB.To investigate the structural stability of melatonin in the environment of interest, it was soaked in 0.5 M sulfuric Figure 2. A) П localization after C2-OH dissociation in ellagic acid molecule [24] ; B) Regeneration mechanism of ellagic acid in the acidic environment. [25]id solution at 80 °C.Melatonin is insoluble in water at 0.1 M but it decomposed on standing in an acid solution for 24 h and dissolved as shown in Figure S2, Supporting Information.This confirms that melatonin is unstable in acidic conditions.Melatonin degradation under elevated temperature and aerobic conditions has also been discussed elsewhere. [18]We further verified this in situ by preparing a Nafion membrane incorporated with melatonin.Figure S3, Supporting Information shows the results of OCV hold testing where melatonin failed in a similar timeframe as baseline Nafion.The mechanism of hydroxyl scavenging by melatonin includes the formation of cyclic-3 hydroxy melatonin. [19,20]Melatonin could have itself degraded during OCV hold testing by pyrrole ring cleavage forming N-acetyl-N-formyl-5-methoxykynuramine, [19] or the deprotonation of the amide functional group in the indole ring forming secondary degradation products.Melatonin instability in aerobic conditions and high temperatures explains its inability to scavenge hydroxyl radicals during in situ investigations. [21]A more conjugated structure without the sidechain of melatonin might show better pH and temperature stability.
To test this hypothesis, we investigated melanin as an alternative.The structure of melanin is shown in Figure 1.Melanin is prepared from tyrosine by reaction with hydrogen peroxide.Quinone groups of melanin could participate in reversible hydroxyquinone and o-quinone transition resulting in high radical scavenging.The absence of a sidechain like that in melatonin could further enhance the stability of this molecule.To investigate melanin structural stability, it was soaked in 0.5 M sulfuric acid solution at 80 °C for 24 h similar to melatonin.No visual sign of degradation was observed.Therefore, changes in the FTIR spectrum of melanin before and after soaking in acid were used to evaluate degradation (Figure S4, Supporting Information).A clear change in the FTIR spectrum indicates melanin degradation.Nafion membrane incorporated with melanin also degraded rapidly (Figure S5, Supporting Information) confirming the instability of melanin under fuel cell operating conditions.Melanin can oxidize to pyrrolic acid in the subsequent reaction with hydroxyl radicals which could explain the observed degradation. [22]lthough melanin does not have susceptible groups like amide in the sidechain, the formation of hydroxyquinone results in an un-stable structure for the melanin intermediate which might lead to cleavage, and therefore irreversible degradation over time during the OCV hold testing.
Both melanin and melatonin are indoles.Indoles are susceptible to radical attacks forming the indolyl radical. [23]These radical intermediates end up forming benzonitrile and benzenes releasing the radicals CN•, CH 3 •, CH 2 CN•, and C 3 H 3 •. [23]These radicals could explain the increased degradation of the membrane when indoles are incorporated in Nafion.Based on these tests, we concluded that indoles are not suitable antioxidants for Nafion.Therefore, we examined polyphenolic structures similar to the structure of melanin that might not show the instabilities observed with indoles.Ellagic acid appeared to be a promising molecule showing a high conjugation effect on the formation of radical intermediates.It was therefore explored in further detail.
Figure 2 shows the EA structure.EA has a symmetrical structure with two sets of ortho hydroxy substituents on opposing rings.Both C2 or C7 and C3 or C8 can participate in hydroxyl radical scavenging.The heat of formation calculations suggest that the first OH dissociation will occur at C3-OH. [24]However, the dissociation at C2-OH will result in extensive П localization resulting in a more stable phenoxyl radical.Therefore, C2 carbon is the most likely site for OH radical attack for hydrogen abstraction. [24]he pathways by which EA-neutral species can scavenge hydroxyl radicals are hydrogen transfer (HT), radical adduct formation (RAF), single electron transfer (SET), and sequential proton loss electron transfer (SPLET). [26]At pH below 4, the neutral form of EA is abundant and the deprotonated form is absent.Therefore, hydrogen transfer is the dominant mechanism of radical scavenging under these conditions. [25,26] The reaction of EA with radicals becomes even faster in an aqueous medium when the deprotonation of EA to its anion intermediates becomes possible. [25]After scavenging of a radical species, EA forms anion intermediates including H 2 EA −. and H 2 EA 2 − as shown in Figure 2b. [25]Conversion of H 2 EA −. to H 2 EA 2 − is a nearly instantaneous and almost barrierless reaction with a reaction barrier of 0.98 kcal mol −1 .The regeneration of H 2 EA 2 − to H 3 EA − is a strong function of environmental pH as shown below. [25] where the Gibbs free energy for this reaction is ΔG = −20.79+ 2.303RT(pH) kcal mol −1 . [25]The lower the pH of the system, the higher the propensity of this reaction.In the presence of a strongly acidic system like PFSA (pH ≈ 0), [27] this reaction is expected to be facile. [25]Another mechanism that could contribute to the antioxidant activity of EA is the formation of chelating structures with transition metal ions.Chelation is thermodynamically favored for both the neutral and anion species of EA. [25] Chelation with transition metal impurities that are responsible for Fenton's reaction could also reduce the degradation of PFSA.EA shows two oxidation peaks at around 0.6 and 0.8 V in pH 1.5 methanol aqueous solution. [24,28]as shown in Figure S6, Supporting Information.The first peak may be attributed to the oneelectron one-proton transfer of EA to the phenoxyl radical, and the second peak to subsequent the one-electron one-proton transfer to the quinone structure. [24]Two feeble reduction peaks are observed which indicates quasi-irreversibility of the reaction.Additional peaks might appear in CVs resulting from the oxidation of EA by adsorption to the catalyst surface, resulting in compounds that further participate in chemical reactions and do not regenerate.A drop in current density was observed while running multiple CV cycles with EA suggesting that EA forms electroinactive products after adsorption. [24]The potential barrier ob-served for electron transfer to the electroactive species of EA in solution could likely be overcome if strong electron donors are present to prevent intermediates from participating in further reactions.The reversibility of EA appears to be a function of various environmental conditions including pH, potential, mobility, concentration, and other chemical species around it as discussed above.Therefore, there is a need for further investigation and experimental validation.

Investigation of EA Reversibility
Before conducting in situ membrane durability and performance testing, we verified that EA is reversible under representative fuel cell testing conditions.Accordingly, we use aqueous FR to trigger EA degradation.EA is extracted from tree bark and other natural sources using a strongly acidic solution [29] and therefore before any investigation, we soaked the EA in dilute sulfuric acid and filtered and washed it thoroughly with water before recording the baseline EA spectrum (Figure 3A).We then investigated the reversibility of EA in 3 wt% peroxide solution without any added Fenton catalysts.No sign of EA degradation was observed as evident in Figure 3B.We then dispersed EA in a 10 ppm Fenton solution prepared with miliQ water.After 24 h of soaking in the Fenton solution, EA was washed and resoaked in acid solution before recording its FTIR.The original structure of EA was not present as evidenced in Figure 3C.This indicates that EA degraded in the Fenton solution and post-soaking in acid did not ensure reversibility.To test if decreasing the pH could ensure reversibility, EA was soaked in a 10 ppm Fenton solution prepared in 0.5 M sulfuric acid.This time, EA showed no changes in its FTIR (Figure 3D) confirming the reversible antioxidant activity.A slight broadening of the peak around 3000 cm −1 is likely due to hydrogen bonding between the hydroxyl groups of EA with the acid present in the solution.This suggests that in the aqueous environment of ex situ Fenton's testing, low pH is crucial to ensure the reversibility of EA.From Equation (2), both pH 7 and 1 result in negative Gibbs free energy indicating that H 2 EA 2 − to H 3 EA − transition should be facile under both conditions.However, EA showed irreversible degradation in neutral pH.It is likely that at neutral pH, anionic intermediates were formed which participated in subsequent reactions and therefore did not regenerate.Hydrogen transfer is the dominant mechanism at low pH conditions.The stable phenoxyl radical in acidic conditions might have been regenerated by other radical intermediates like O 2 .− causing reversible antioxidant capacity in low pH conditions.

Ex Situ Investigation of the Effect of EA on Nafion Properties
We conducted an ex situ aqueous Fenton test to compare the radical scavenging capabilities of EA and cerium.Fenton's test involves the generation of hydroxyl radicals by the decomposition of hydrogen peroxide in the presence of transition metal impurities as per Haber Weiss's reaction. [30]This is usually used to imitate the radical environment in an operating fuel cell for PFSA membranes.The degradation of the membrane is measured in terms of fluoride emissions which are quantified using chromatographic techniques.
As shown in Figure 4A, FER for the Nafion after 24 h of testing was 0.186 μg F −1 cm −2 h −1 , whereas the cerium-incorporated membrane and the EA-incorporated membrane showed FERs of 0.130 and 0.132 μg F −1 cm −2 h −1 , respectively.Therefore, it appears that EA can scavenge hydroxyl radicals to the same efficacy as cerium.However, the FER for the EA-incorporated membrane was nearly twice that of the cerium-incorporated membrane after 48 h, while still remaining about half of the baseline membrane at 0.047 μg F −1 cm −2 h −1 .The FER trend suggests that the EA quinoidal structure is degrading during Fenton's testing.No acid was added during Fenton's testing and therefore the pH of the solution was close to 7. Anionic intermediates of EA formed on reacting with OH radicals could have participated in side reactions and were therefore not regenerated.
Next, we investigated the effect of EA incorporation on the membrane's mechanical properties.Since EA is not soluble in water and only partially soluble in some organic solvents, [31] its incorporation will likely cause local disruptions in the microstructure of Nafion which might reduce its mechanical properties.As expected, the tensile toughness of the EA-incorporated membrane was reduced by 78% (740 Mpa) compared to the baseline Nafion, 3448 MPa (Figure 4B).Moreover, it showed a 40% drop in toughness compared to the cerium-incorporated membrane, while also experiencing a considerable reduction in the elongation and ultimate strength (Figure 4C).This is likely due to the change in the microstructure of the PFSA which is now disturbed by the incorporation of an incompatible small hydrocarbon molecule making the membrane more brittle.Therefore, even for relatively similar Young's moduli for all the membranes, their toughness differs markedly.
This drop in toughness also implies that any microcrack formation in Nafion is likely to propagate faster in the EAincorporated membrane than in the cerium-incorporated membrane.This means that after microcracks and pinholes start forming during chemical durability testing, the reduction in mechanical properties could exert an influence on the membrane's lifetime, resulting in early failure that is falsely attributed to a lack of chemical durability.
Though EA reduced elongation (Figure 4C), it increased the membrane's water uptake by 5.8% (Figure 4D).This is likely due to the change in the morphology of the membrane due to the incorporation of EA.The EA-incorporated membrane's increased water uptake might be beneficial for its chemical durability when subjected to dry conditions at OCV for extended durations.

In Situ Chemical Durability Testing
To verify the radical scavenging ability of EA observed in Fenton's test, we evaluated Nafion durability enhancement by EA in-corporation using the OCV-hold accelerated stress test (AST) recommended by the DOE FC technical team. [32]The AST involves holding the membrane at OCV under H 2 /Air at 90 °C and 30% RH.Details of the MEA preparation and cell assembly are discussed in the experimental section.Briefly, MEAs were prepared and conditioned at 0.6 V for 24 h at 80 °C and 100% RH in a tripleserpentine cell, following which the membrane was subjected to the OCV-hold test.All membranes tested were 30 μm thick.The membrane was declared failed when its hydrogen crossover current density exceeded 15 mA cm −2 .
Figure 5A,B show the OCV decay and the hydrogen crossover current density, respectively, during the AST.The incorporation of 5 wt% EA increased the durability of the membrane by 160% from the value of 120 h for Nafion to 312 h for EA-incorporated membrane.To our knowledge, this is the highest durability enhancement reported from an organic radical scavenger for a PFSA membrane.
If anion EA species were present during AST testing, low pH conditions would ensure the high reversibility of EA as per the mechanism presented in Equation (2).However, the pH conditions of Nafion suggest that a large fraction of EA would be present in its neutral form.This makes hydrogen transfer the dominant radical scavenging pathway for EA in the PFSA environment.Strong electron donors are conducive to preventing phenoxyl radical intermediates from participating in other side reactions.Therefore, the presence of a co-antioxidant might increase the durability of the EA-incorporated membrane.To verify this hypothesis, we tested a Nafion membrane incorporated with both EA and cerium.To test if cerium could aid EA reversibility, we subjected the membrane to the same AST protocol as discussed above.The cerium-incorporated Nafion showed a durability of 504 h, whereas the EA+Ce-incorporated membrane showed a lifetime of 900 h after which the test was discontinued in the interest of time although the membrane was still viable (Figure 5C).
Figure 5D shows the change in crossover for both membranes over the test duration.Since the EA anion fraction is expected to be small in the non-aqueous acidic PFSA environment, it is unlikely that EA is forming a strong stable complex with cerium.The following equilibrium (Figure 6) is however possible which could contribute to the enhanced durability observed.This has also been suggested in past DFT investigations [25] where the Gibbs free energy of complex formation by EA neutral species with ions is negative.
It is plausible that both EA and cerium are acting as radical scavengers independently, but the 900 h lifetime of EA+Ce incorporated membrane exceeds the sum of the membrane lifetimes observed when cerium and EA are present alone, which are 504 and 312 h, respectively.The rate constant of the EA reaction with OH radicals at biological pH is 1.91 × 10 9 M −1 s −1 , [26] an order of magnitude faster than the cerium reaction with OH radicals, 3 × 10 8 M −1 s −1 . [1]These rate constants suggest that EA should be the primary antioxidant in the system.The lifetimes and rate constants suggest that cerium and EA might be working as a complex as shown in Figure 6.However, further experiments with higher amounts of individual radical scavengers, that is, cerium and EA, and the lifetime comparison in OCV test with Ce+EA would help to validate this hypothesis.
The EA radical scavenging mechanism is very complex for an aqueous system. [26]The analysis is further complicated for a system like Nafion which comprises a non-polar region where one mechanism might dominate, and other regions that imitate bulk water in highly hydrated conditions where several different radical scavenging mechanisms could become active.DFT investigations could help elucidate the radical scavenging mechanism for EA-type additives in PFSA, and provide additional insights beyond experimental observations to motivate the design of more efficient antioxidant systems.
Nevertheless, we have demonstrated that ellagic acid represents a new radical scavenger with the potential to aid the radical scavenging activity of cerium in PFSA, or even eliminate the use of cerium to meet the membrane's chemical durability lifetime targets.It is to be noted that the MEA conditioning protocol for durability testing was chosen such that the components, if mobile, do not migrate from the membrane.If the components were to migrate before commencing durability testing, analysis of durability data would be complicated by the loss of radical scavengers from the membranes which is difficult to quantify, especially for organic additives like EA.Therefore, we conducted investigations on the mobility effect of organic additives and their impact on the performance of fuel cells and catalyst layers separately to deconvolute the two effects.

Mobility of EA under Fuel Cell Operation
Migration of cerium, especially in the presence of water gradients, can be an issue inside operating fuel cells which is why immobilization strategies within the Nafion structure have been pursued using chelating agents and ligands. [6]To investigate possible EA migration, a two-layer membrane with a total thickness of 50 μm was cast with one layer of 10 μm thickness loaded with 20 wt% EA represented by the orange color in the schematic in Figure 7A, and the other layer of 40 μm thickness comprising Nafion with no additive, represented by the grey color.Membrane fabrication details are described in the experimental section.This membrane with a through-thickness gradient of EA was then assembled in the differential cell shown in the experimental section such that the anode is towards the EA side and the cathode is towards the Nafion side.This is to ensure that the proton flux is in the direction from EA to Nafion.This membrane was then subjected to the conditioning protocol for 48 h as described in the experimental section.
Figure 7B shows the change in the FTIR spectrum of the membrane before and after the conditioning for both the EA and Nafion sides of the membrane.As evident from Figure 7B, signatures for EA became weaker after conditioning on the EA side of the membrane.Some catalysts were transferred to the membrane during the conditioning test after the gas diffusion layer was removed from MEA for FTIR measurement.The carbon transferred to the membrane caused the rise in absorbance observed at the end of the test.EA incorporation causes a distinct browning of the membrane which remained intact after conditioning as shown in the EOT images (Figure S7S7, Supporting Information).This indicates that EA is still present in the membrane but significant through-thickness migration of EA occurred during the 48 h conditioning period.On the Nafion side, no signs of EA are observed after conditioning so the migration distance of EA during the test seems to be less than 40 μm.
A second membrane was cast with a gradient of EA in the inplane direction (Figure 7C).The EA side was kept towards the gas inlet (orange) and the Nafion side towards the gas outlet (grey).The same conditioning protocol was run for this cell.Similar to the previous case, FTIR spectra for the EA and Nafion portions were recorded for the membrane before and after conditioning (Figure 7D).No change in the FTIR spectra was observed for either portion of the membrane.This suggests that no in-plane migration of EA occurred during the conditioning protocol.Images of the membrane after conditioning are shown in Figure S7, Supporting Information.Both membranes still show browning, indicating the presence of EA in the membrane.
These tests suggest that EA can migrate in a through-plane direction during fuel cell operation.EA migration in the plane, and therefore its loss to the inactive areas of the membrane is unlikely.Since EA is insoluble in water, the migration in the throughplane direction could result from ellagic acid ionic intermediates forming in the water transport channel and transporting by ionic forces.Water gradients in the MEA from the cathode to the anode along with the electro-osmotic drag would influence the degree of migration of EA in the MEA.Nevertheless, it is clear that ellagic acid migrates during fuel cell operation and therefore strategies to design membranes with EA as a structural motif are desirable to tap into its full radical scavenging potential.
During the AST testing, although the operating conditions are dry, EA could migrate over hundreds of hours of testing.Migra-tion to catalyst layers would result in loss of EA from the membrane, which would result in a drop in achievable radical scavenging performance for a given EA loading.The migration of EA can also affect the performance of the fuel cell.It is therefore important to design membranes with EA tethered to the polymer.It also motivates the design of polymers that are intrinsically radical scavenging in nature.

Performance of EA-Incorporated Membranes
Poisoning of the platinum surface by polyphenols is known in the literature. [33]While EA shows a strong antioxidant ability, it is crucial to understand its effect on catalyst activity and therefore system performance, if any.Therefore, the performance and catalyst poisoning of all the membranes was evaluated by testing them in 5 cm 2 differential cells under H 2 /Air flow after a standard over-humidified conditioning protocol for two days.Details of the testing conditions are provided in the Experimental Section.The high-frequency resistance (HFR) was similar for all the membranes at around 0.05 Ω cm 2 .The performance of various membranes is shown in Figure 8A.The performance is comparable for various membranes in the kinetics and the ohmic resistance regions.Loss in the mass transport region for the EA membrane is likely due to flooding due to higher water uptake shown by EA-incorporated ionomers.
Cyclic voltammograms of various membranes are shown in Figure 8B.A clear reduction in electrochemically active surface area (ECSA) was observed for EA incorporated membrane.EA, being a small molecule could have migrated to the catalyst layers and adsorbed to the platinum active sites causing a drop in the ECSA observed.Even after the drop in ECSA, performance for EA membrane is similar to others suggesting that ellagic acid is making up for the ECSA loss by other mechanisms which is beyond the scope of this study.
Nevertheless, the impact of EA incorporation in the membrane on the performance of the MEA is not significant.
Overall, we showed that EA undergoes migration owing to its small molecular size as well as being untethered to the polymer.The migration could have implications for the long-term performance of the MEA.It is therefore desirable that these additives be integrated into the design of the polymer itself to eliminate association with the catalyst particles.

Conclusion
We investigated organic radical scavengers as potential antioxidants for Nafion due to their ease of integration into the membrane and reduced mobility issues afflicting traditional antioxidants like cerium.We found that indoles are unsuitable as organic radical scavengers for PFSA membranes due to their instability and reactivity under typical fuel cell operating conditions.Among polyphenols, ellagic acid was identified as a potential alternative to cerium.Ellagic acid had radical scavenging activity comparable to cerium in Fenton's test; it showed reversible redox behavior at low pH and was stable in fuel cell operating conditions.
The limitation of most organic antioxidants is their irreversibility.Since ellagic acid phenoxyl and anion intermediates can participate in side reactions depending on the chemical environment, we used cerium as a co-antioxidant to prevent intermediate side reactions of ellagic acid.Accordingly, a Nafion membrane incorporated with a combination of cerium and ellagic acid had a lifetime of at least 900 h.When present alone, ellagic acid increased the lifetime of Nafion by 160%.Ellagic acid together with a strong electron donor like cerium confers lifetime enhancement that greatly exceeds the lifetime enhancement observed from each of them alone, 504 h for cerium and 312 h for ellagic acid.Therefore, we propose the ortho-quinone-type structure as a promising structural motif in organic radical scavengers for PFSA membranes.

Experimental Section
Materials: Nafion 212 and D2020 were purchased from Ion Power.Dimethyl acetamide, cerium nitrate hexahydrate, hydroquinone, chitosan, melatonin, melanin, ellagic acid dihydrate, methyl indole, uric acid, and ascorbic acid were obtained from Sigma Aldrich and used without further purification.Hydrogen peroxide 30 wt%, ferrous sulfate heptahydrate, and 98% sulfuric acid were obtained from Fischer Scientific.
Rhodamine Dye Assay: For the Fenton reaction, the following stock solutions were first prepared: 0.2 mM RhB, 10 mM FeSO 4 in 2 mM H 2 SO 4 , and 20 mM H 2 O 2 .Different organic radical scavengers in 2 mM concentration were added last to a solution containing 0.010 mM RhB, 1.0 mM FeSO 4 , and 2.0 mM H 2 SO 4 .The test solution was then diluted to 10 mL with deionized water.RhB absorption at 550 nm was measured 1 min after the hydrogen peroxide was added.
Membrane Preparation: A 0.4 gm sample of N212 was chopped into small pieces and dissolved in dimethyl acetamide at 90 °C.When the membrane was completely dissolved, 20 mg of ellagic acid dihydrate was added and the mixture was stirred overnight.The dispersion was then poured into a 100 cm 2 casting tray.The membrane was cast at 80 °C and then annealed at 130 °C overnight after all the solvent evaporated.Hot water was then poured into the casting tray to peel the membrane off the casting surface.The membrane was then pressed at 4000 lbf before testing.The cerium-incorporated membrane was cast by adding 7.7 mg cerium to the Nafion dispersion, everything else being the same.Ellagic acid (EA) +Ce membrane was cast by mixing 7.7 mg of Ce and 20 mg of EA with Nafion dispersion.
Membrane with a Through-Thickness Gradient of Ellagic Acid: 3 ml of D2020 solution was mixed with 1 ml of Isopropyl alcohol.The dispersion was stirred for 30 min and then mixed with 20 wt.% of ellagic acid.The mixture was stirred for 2 h.A 10 × 10 cm glass slide was fixed to an inhouse membrane casting machine and the absolute gap between the glass slide and the rod coater was set to 0.1 mm.The D2020 dispersion was then poured onto the glass slide.The rod coater was swiped across the length of the membrane to cast the desired membrane thickness.Another dispersion was prepared using 3 ml of D2020 mixed with 1 ml of IPA with no additives.After the complete drying of the first layer, four layers of the D2020 dispersion were cast on top of the formed membrane in sequential steps.This membrane was held at 80 °C in a hot air oven for 24 h followed by annealing at 130 °C overnight.This membrane was then hot pressed at 130 °C at 4000 lb for 5 min.The thickness of the final membrane after drying and hot pressing was around 50 μm with the top 10 μm loaded with EA and the remaining 40 μm being Nafion (Figure S7, Supporting Information).
Membrane with an In-Plane Gradient of EA: The dispersions were prepared as above.The absolute gap between the glass slide and the rod coater was set to 0.5 mm.The rod coater was swiped across half the length of the glass slide with D2020 dispersion containing EA.After this, the rod coater was stopped and the excess dispersion was wiped.Fresh diluted D2020 dispersion was prepared and then poured on the glass slide and the rod coating resumed till the entire glass slide was swiped.This membrane was held at 80 °C in a hot air oven for 24 h followed by annealing at 130 °C overnight.This membrane was then hot pressed at 130 °C at 4000 lb for 5 min.before further testing.
Aqueous Fenton Reagent Test (FRT): FRT is an ex situ test wherein radicals are generated to attack the Nafion sites and thereby reduce the membrane weight over time.Fluoride anions emitted are used as a measure of the degradation of Nafion.Fenton solution contains 10 ppm Fe 2+ ions along with 3 wt.% of H 2 O 2 .A 12 cm 2 piece of membrane of thickness ≈30 μm was placed in 22 ml of Fenton solution prepared in 0.5 M sulfuric acid solution.The solution was refreshed every 24 h.All Fenton solution samples were collected to evaluate their fluoride concentration later using Dionex ion chromatography (Thermo Scientific).The test was repeated with three membrane samples and the averaged values are reported as the observed FER.Values at discrete time intervals indicated the average fluoride emissions during that time interval.
Mechanical Property Testing: Mechanical properties of the membranes were measured using DMA Q800 from TA instruments.Membranes were cut into 0.6 × 2.5 cm samples.The temperature of the chamber was maintained at 30 °C and the sample was kept at ambient humidity conditions.The sample was clamped in the chamber and then tensile stress was applied on the membrane at a constant rate of 0.5 Mpa min −1 until the membrane yielded.The test was repeated twice to confirm repeatability.The toughness of the membrane was evaluated by calculating the area under the stress-strain curve.
Water Uptake Testing: 1 × 1 cm samples of baseline Nafion, Ce-added, and EA-added membranes were dried in a hot air oven at 100 °C overnight.The weight of the dried membranes was measured quickly by removing the membranes from the oven (DW).These membranes were then soaked in water at 80 °C for 24 h.The membranes were then taken out and the surface water was blotted with Kimwipes and their weight was quickly measured (WW).All the membranes were measured three times and the average of the three measurements was recorded as the water uptake (%) given by WW−DW DW × 100.Fuel Cell Testing and AST Testing: As-synthesized catalysts were incorporated into the MEA by directly spraying a water/n-propanol-based ink onto a Nafion211 membrane.An MEA of 5 cm 2 size was prepared with a Pt loading of ≈0.1 mgPt cm −2 on both the anode and cathode for fuel cell testing.The MEA was sandwiched between two graphite plates with 14 straight parallel flow channels.
The differential cell was operated at 80 °C with 150 kPa absolute pressure using H 2 /air gases at a gas flow rate of 1000/3000 sccm on the anode and cathode side respectively.Seven conditioning cycles were run before recording the performance and the CV for the various membranes.ECSA was obtained by calculating underpotentially-deposited hydrogen (HUPD) charge in CV curves between 0.1-0.4V (0.4-0.45 V background subtracted) assuming a value of 210 μC cm −2 for the adsorption of a hydrogen monolayer on Pt (CV curves were obtained under 150 kPa total pressure under H 2 /N 2 at anode/cathode using 30 °C cell temperature, and 80 °C humidifier temperature at the flow rate of 1000/3000 sccm).
A 5 cm 2 MEA was prepared with a Pt loading of ≈0.1 mgPt cm −2 on both anode and cathode for AST testing under a single serpentine cell with gases flowing in the co-flow direction.The membrane used for AST testing was 30 μm thick.The cell was held at 0.6 V for 24 h at 80 °C, 100% RH, and 500/1000 sccm H 2 /Air at a backpressure of 150 kPa absolute for conditioning before starting the OCV test.OCV test was performed by holding the cell at 90 °C, 30% RH till the crossover exceeded 15 mA cm 2 CV was recorded every 24 h to measure hydrogen crossover by purging the cathode side with nitrogen.
FTIR Measurement: All FTIR measurements were performed in ATR mode using the Thermo Scientific Smart iTR system.

Figure 1 .
Figure 1.Structures of various organic radical scavengers investigated in this study.

Figure 3 .
Figure 3. Structural change in EA after soaking in various solutions: A) Acid at 80 °C for 24 h; B) 3 wt% peroxide solution at 80 °C for 24 h; C) Fenton's solution at 80 °C for 24 h followed by acid soaking; D) Fenton's solution prepared in 0.5 M sulfuric acid at 80 °C for 24 h.

Figure 5 .
Figure 5. AST for the EA-incorporated membrane.A) OCV decay for Nafion and EA incorporated membrane at 90 °C, 30%RH conditions under the flow rate of 500/1000 sccm of H 2 /Air at anode/cathode; B) Hydrogen crossover current density change during AST; C) OCV decay for Nafion incorporated with 1.8 wt% Ce and 1.8 wt% Ce +5 wt% EA; D) Hydrogen crossover current density change for EA+Ce and Ce incorporated membrane over the OCV-hold test.

Figure 6 .
Figure 6.Ellagic acid chelating with cerium during the OCV hold testing.

Figure 7 .
Figure 7. Migration testing for EA under fuel cell operating conditions.A) Schematic of the membrane with a through-thickness gradient of EA (orange), Nafion (grey); B) FTIR for EA and Nafion sides of membrane before and after conditioning; C) Schematic of the membrane with an in-plane gradient of EA; D) FTIR for EA and Nafion sides of membrane before and after conditioning.

Figure 8 .
Figure 8. Fuel cell performance of the Nafion membrane, Ce-membrane, and EA-membrane.A) Differential fuel cell testing under 1000/3000 sccm of H 2 /Air at anode/cathode (total backpressure of 150 kPa under 80 °C and 100% RH); B) Cyclic voltammograms for the three membranes.