Theoretical Investigation of the H2O2-Induced Degradation Mechanism of Hydrated Nafion Membrane via Ether-Linkage Dissociation.

A H2O2-induced degradation mechanism is presented for the hydrated Nafion membrane proceeding through the dissociation of the ether linkages of the side chains. Although the durability of proton-exchange membrane fuel cells clearly depends on the degradation rate of the membrane, typically Nafion, the degradation mechanism still has not been resolved. It has often been assumed that the principal mode of degradation involves OH• radicals; in contrast, we show here that a H2O2-induced degradation mechanism is more likely. On the basis of state-of-the-art theoretical calculations and detailed comparison with experimental results, we present such a mechanism for the hydrated Nafion membrane, proceeding through the dissociation of the ether linkage of the side chains, with a relatively low activation energy. In this mechanism, (H2O)λHO3S-CF2-CF2-O-O-H (λ is the hydration number) is obtained as a key degradation fragment. Possible subsequent decomposition-reaction mechanisms are also elucidated for this fragment. The calculated vibrational spectra for the intermediates and products proposed in these mechanisms were found to be consistent with the experimental IR spectra. Further consideration of this H2O2-mediated degradation mechanism could greatly facilitate the search for ways to combat membrane degradation.

Introduction

Degradation of proton-exchange electrolyte membranes is the most serious problem in fuel-cell operations, determining the durability of proton-exchange membrane fuel cells (PEMFCs).[1,2] Proton-exchange membranes in PEMFCs conduct protons produced from the oxidation of hydrogen gas on the anode to a catalyst composed of, for example, Pt alloy on the cathode for oxygen-reduction reactions (ORR) to proceed. Under PEMFC operating conditions, hydrogen peroxide (H2O2) is produced to some extent and degrades the proton-exchange membrane, deteriorating the proton conductivity of the membrane.[3] Experiments have shown that the H2O2 production of PEMFC results from O2 crossover in proton-exchange membranes.[4−6] That is, oxygen (O2) gas injected at the cathode permeates to the anode and then reacts with hydrogen radicals on the anode to produce H2O2. A rotating ring-disk measurement study clearly shows that H2O2 is formed by the reaction of O2 and H2 adsorbing on Pt surface at low potentials; especially on Pt(111), 100% H2O2 formation is observed.[7] This reaction is so facile that it is useful even as a H2O2 synthesis method.[8] Nevertheless, the OH radical has usually been assumed to be the reactive species in the degradation of proton-exchange membranes. Fenton test studies, however, have raised questions concerning this assumption.[9,10] In the Fenton test, the reaction of H2O2 and Fe2+ ions has been interpreted to provide OH radicals as the active intermediate:[11,12] H2O2 + Fe2+ → OH• + OH– + Fe3+, although several theoretical studies support another hypothesis[13] that FeO2+ ion is the most likely active intermediate: H2O2 + Fe2+ → H2O + FeO2+.[14,15] On the basis of a Fenton test of a Nafion membrane, which is the most widely used proton-exchange membrane, Kinumoto et al. found that the degradation proceeded in 30% H2O2 solution at 80 °C and the decomposition ratios of SO42– groups and C–F bonds were inconsistent with those obtained in the absence of the Fenton reagent in the PEMFC:[9] the ratios were 33 and 68% for SO42– groups and C–F bonds after 9 days in the Fenton test, and 7 and 1% after 5.5 days under the PEMFC operating conditions, respectively. Using gas-phase H2O2 exposure, Hommura et al. found that the rate of COOH formation does not approach zero as Fe2+ concentration decreases and confirmed, on the basis of the chemical kinetics, that Nafion is decomposed by H2O2 alone, that is, without metal ions.[10] IR spectral analyses also showed that after the Fenton test, Nafion exhibited peaks around 900 and 1600 cm–1, which were not found in the IR spectra of the decomposed Nafion under PEMFC operating conditions.[16] The instability of the OH radical in aquo has also been hardly taken into account in conventional studies. The lifetime of OH radicals is 100 ns in aquo, which is too short to cause degradation over a wide region in the proton-conducting channels, where water clusters are present predominantly near the sulfonic acid groups. The large dissociation energy of the OH radical on the Pt surface is also an obstacle for the OH-radical-induced degradation: Under a H2-rich H2/O2/H2O mixed gas, the dissociation of the OH radical from the Pt surface requires more than 40 kcal/mol even at very high temperature (around 1000 K).[17] We note, however, that the degradation can indeed be caused by radicals such as OH• when metal ions are supplied from the end plate of the PEMFC[18] and can be inhibited by adding radical scavengers like cerium dioxide (CeO2) nanoparticles.[19] It is, therefore, reasonable to consider that the degradation of proton-exchange membranes proceeds through two reactions, H2O2-induced and OH-radical-induced reactions, depending on the surrounding environment, such as humidity conditions, the distance from the catalyst layer, and the presence of metal ions.

The degradation mechanism of the Nafion membrane has also been vigorously investigated.[1,3−6,16,20−33] Aoki et al. and other groups revealed that the degradation mainly proceeds near the anode as a result of O2 crossover.[3−6] Endoh et al. and other groups reported that the degradation rate increased as the relative humidity decreased.[21−23] Ghassemzadeh et al. revealed that the degradation proceeds only in the presence of a Pt catalyst in addition to H2 and O2 and that it is hardly affected by the morphology of the Nafion membrane.[24] Various studies have also been reported on the degradation fragment species. Deng et al. showed that hydrogen fluoride (HF)- and CF2-containing fragments were produced.[25] LaConti et al. also reported that the degradation produces F– ion, CO2, and low-molecular-weight perfluorocarbon sulfonic acids.[1] As another possible degradation product, several studies have suggested the perfluoro(3-oxa-5-methyl)pentane-1-sulfonic-5-carboxylic diacid (HOOC–CF(CF2)–O–CF2CF2–SO3H),[26−29] implying the degradation through the ether-linkage dissociation of the side chains, although the identification of this compound is not completely certain, as discussed later. Akiyama et al. also supported the formation of a carboxylic acid by showing that the most remarkable difference in the IR spectra of Nafion before and after the degradation was the appearance of the peaks assigned to the vibrational modes of carboxylic acid groups.[16] There have also been several theoretical studies focusing on the degradation of the Nafion membrane. Assuming that the degradation takes place at the weakest bonds, Okamoto suggested that the C–S bond of the side chain is the most reactive part.[30] Coms calculated decomposition-reaction enthalpies and suggested that only the OH radical is able to abstract a H atom from a carboxylic acid to unzip the main chain of Nafion.[34] Yu et al. have recently explored various radical-induced degradation processes for the end groups of both the main and side chains, assuming that the reactive species are radicals coming from the ORR, such as the OH radical.[31,32] It is important to note, however, that hydration water clusters attached to the sulfonic acid groups have not been included in their calculation models, although they must certainly be present for protons to be conducted within the membranes. Despite many experimental and theoretical studies on this topic, the degradation mechanism of the Nafion membrane has never been confirmed, as far as we know. It is, therefore, meaningful to theoretically reveal the degradation mechanism of the Nafion membrane using realistic calculation models, considering, for example, the hydration structures of the membrane and H2O2.

Proton conduction and hydration structures of Nafion membrane have been investigated from various aspects, both experimentally and theoretically. It has been accepted that the proton conduction in aquo proceeds through the de Grotthuss[35] and vehicle mechanisms.[36] In the de Grotthuss mechanism, protons transfer at high speed using hydrogen-bond networks through the Zundel cation H5O2+ complex, that is, the [H2O··H··OH2]+ complex.[37,38] Because of the very low reaction barrier height of about 2 kcal/mol, the de Grotthuss mechanism has been assumed in past theoretical studies on proton conduction[39] and proton dissociation[40] in the Nafion membrane. Experiments, however, have shown that Nafion contains a number of unconnected hydrogen-bond networks under low-humidity conditions despite its high proton conductivity.[41,42] Protons in low-humidity Nafion are, therefore, considered to transfer not only by the de Grotthuss mechanism but also by the vehicle mechanism. In the vehicle mechanism, protonated water molecules diffuse to deliver protons. This implies that proton conduction requires the dissociation of protonated water molecules at low humidity. Theoretical calculations, however, show that protonated water clusters require very large energies to dissociate from the sulfonic acid groups of Nafion at low humidity (about 100 kcal/mol).[43] As an alternative mechanism, we have suggested a relay mechanism to explain the proton conduction between the unconnected hydrogen-bond networks in the low-humidity Nafion membrane.[43] In the relay mechanism, proton conduction proceeds through the relay of protonated water clusters from one sulfonic acid group to another using the long side chains of Nafion. As this mechanism requires only about 30 kcal/mol to dissociate protonated water clusters, it is considered to proceed even at room temperature. We note that this relay mechanism involves doubly hydrated structures of the Nafion membrane. Through a detailed comparison of experimental and theoretical results, we have recently revealed that the sulfonic acid groups of Nafion are doubly hydrated, regardless of humidity conditions.[44] Comparing the experimental humidity dependence of the IR spectrum of the Nafion membrane under PEMFC operating conditions and the theoretical hydration number dependence of the vibrational spectrum of the hydrated Nafion membrane model, we found that the theoretical spectral dependence is consistent with the experimental one only for a doubly hydrated Nafion model optimized using an initial structure, in which protons are detached from the sulfonic acid groups. This result strongly supports the proton conduction through the relay mechanism under the low-humidity condition and the doubly hydrated Nafion membrane structure, regardless of the humidity.

In this study, we theoretically reveal the degradation mechanism of the hydrated Nafion electrolyte membrane in a PEMFC, focusing on the H2O2-induced degradation and subsequent reactions of the degradation fragment molecule. After detailing the computational methods, we first elucidate the H2O2-induced degradation mechanism of the doubly hydrated Nafion membrane. Then, we verify the decomposed hydrated Nafion membrane and the degradation fragment molecule by comparing the calculated vibrational spectra with the experimental IR spectrum results. We finally investigate the subsequent reaction processes of the degradation fragment molecule.

Computational Details

All calculations have been performed for the doubly hydrated structures of Nafion membrane molecular model, in which water molecules are explicitly hydrated, using the long-range correction[45] of Becke 1988 exchange[46] plus the Lee–Yang–Parr correlation[47] (LC-BLYP) functional (the only parameter μ = 0.47[48]) with the cc-pVDZ basis set.[49,50] We have adopted the double-unit model of hydrated Nafion membrane shown in Figure a. Geometry optimizations of the hydrated Nafion structures have been carried out for several initial structures maximizing the number of the hydrogen bonds. For the calculated hydrated Nafion model, we have used the doubly hydrated structures (Figure ) following our previous study based on the experimental and theoretical IR spectral analyses of hydrated Nafion.[44] In Figure , the optimum geometries of the doubly hydrated double-unit Nafion membrane model with H2O2 molecule are illustrated for the hydration numbers per sulfonic acid group: λ = 1–4, which models the low-humidity conditions near the anode.[51,52] It is important to note that as shown in this figure, protonated water molecules are dissociated only for λ = 3 and 4, whereas they attach the sulfonic acid group for λ = 1 and 2. This is consistent with conventional theoretical results.[43] Using these optimum geometries as the initial structures, we have optimized the geometries of the hydrated Nafion + H2O2 models by extending the C–O ether-linkage distance (Figure ) from 1.2 to 4.0 Å. Protons are assumed to be dissociated to model proton transfers in the relay mechanism.[43,44] The Gaussian 09 suite of program[53] has been used to perform all of the LC-BLYP calculations. All of the optimized structures have been checked to ensure that they yield positive, real frequencies. Transition-state calculations have been performed by the quadratic synchronous transit method.[54,55] The predictor–corrector integrator method[56,57] was used to calculate the intrinsic reaction coordinates of the reactions. The vibrational modes contributing to IR spectra and their assignments were analyzed using GaussView 5.0.8.[58]

Figure 1

Doubly hydrated Nafion electrolyte membrane model: (a) chemical formula and (b) close-up view of ether linkage, which undergoes dissociation, modeled by elongation in successive steps.

Figure 2

Optimized structures of doubly hydrated Nafion membrane with one hydrogen peroxide (H2O2) molecule for various hydration numbers per sulfonic acid group (λ), which are calculated by LC-BLYP/cc-pVDZ method.

Calculated Results and Discussions

H2O2-Induced Degradation Mechanism of Nafion Membrane

Following the experimental studies on the degradation mechanism of hydrated Nafion membrane mentioned above, we imposed the following assumptions to the calculation model:On the basis of these assumptions, we have explored the degradation mechanism of the hydrated Nafion membrane.

The degradation of Nafion membrane takes place in the vicinity of the anode. This is because the anode side of the membrane undergoes severer degradation than the cathode side.[1,24] The hydration numbers of the sulfonic acid groups, therefore, should be relatively small to model the hydration conditions near the anode.[51,52]

The sulfonic acid groups are doubly hydrated in the hydrated Nafion. As mentioned above, our previous IR spectroscopy study proves that Nafion has a doubly hydrated structure, regardless of the humidity conditions.[44]

The degradation initially proceeds with the ether-linkage dissociation of the side chains. Several experimental studies have suggested a carboxylic acid in the degradation fragment species, which is considered to originate from the ether-linkage dissociation.[26−29]

The morphology of the hydrated Nafion membrane has negligible effect on the degradation mechanism, although the morphology depends on the hydration number.[59] This is because an 19F NMR experiment showed that the morphology of the Nafion membrane hardly affects the degradation ratio.[24]

As a result, we succeeded in elucidating a H2O2-induced degradation mechanism of the hydrated Nafion membrane. Figure displays the hydrated Nafion model before and after the H2O2-induced decomposition for the hydration number of λ = 4. As shown in the figure, H2O2, which participates in a hydrogen-bond network with the hydration water cluster of the sulfonic acid groups, decomposes the ether linkages, forming an alcohol group and an −O–OH group. That is, the H2O2-induced decomposition is expressed in the following scheme:

Figure 3

Optimized structures of hydrated Nafion membrane model (a) before and (b) after degradation, with the close-up view of dissociated C–O bond. Geometries are optimized by LC-BLYP/cc-pVDZ method. The dissociated ether C–O bond is indicated by blue dotted circles and H2O2 and the −OH and −O–OH groups after decomposition are indicated by green dotted circles. “Rp” in the chemical formula represents the residual part of the Nafion membrane model.

We should emphasize that this reaction path is obtained by simply extending the ether C–O bond of the hydrated Nafion membrane with the H2O2 molecule. This indicates that the ether linkage is directly cleaved by H2O2 without producing OH radicals. Although this scheme has the appearance of being unlikely because of the unfamiliar −O–OH group produced, it is interesting to note that this scheme is analogous to the double-bond cleavage mechanism by ozone:

Similar mechanisms are also suggested for decompositions after TiO2 photocatalytic reactions.[60] It is, therefore, reasonable to suppose that the −O–O– bond is not uncommon in the process of bond cleavages in gas phase.

Figure illustrates the potential energy curves of the H2O2-induced ether-linkage dissociation of the doubly hydrated Nafion membrane for λ = 1–4 in terms of the C–O bond distance of the ether linkage. The figure shows that the reaction barrier heights are moderate, 32–40 kcal/mol, nearly independent of hydration number. This indicates that the ether-linkage dissociation reaction proceeds irrespective of the humidity of the membrane, without any other reactive species, such as radicals. On the other hand, the reaction energies decrease from λ = 1 to 2 but then gradually increase from λ = 2 to 4: 20.4, 15.1, 19.4, and 25.7 kcal/mol for λ = 1–4, respectively. This result seems inconsistent with the experimental results showing that the degradation ratio increases with decreasing humidity.[21−23] This inconsistency is explained by the decrease of O2 crossover and O2 adsorption as well as the instability of OH radicals in aquo. The increase of humidity leads to the obstruction of the membrane channels by water, decreasing the O2 crossover, which causes the degradation.[3−6] It is also known that water molecules prevent O2 adsorption on the electrode surface. Therefore, the increase of humidity suppresses H2O2 production and consequently slows the degradation. The increasing humidity also destabilizes OH radicals, which are reactive species in other degradation mechanisms; therefore, it may reduce the infiltration of OH radicals into the internal region of the membrane. The calculated potential energy curves suggest that the H2O2-induced degradation reaction of the ether linkages naturally proceeds irrespective of the water volume of the Nafion membrane channels. Because H2O2 is incorporated in the hydrogen-bond networks of water clusters, this result is consistent with the experimental results showing that the degradation progresses deeply into the interior of the membrane.[1]

Figure 4

Potential energy curves of ether-linkage dissociation reaction with reference to the energy of the reactant in terms of the dissociated C–O bond distance for λ = 1–4. Geometries are calculated by LC-BLYP/cc-pVDZ method. Chemical structures of reactant, transition state (TS), and product are also shown.

In Figure , the dependence of the barrier heights on the hydration number, that is, λ = 2 > 1 > 4 > 3, seems unreasonable. However, this dependence can be interpreted by considering the proton-dissociation nature of the Nafion membrane. As shown in Figure , the proton is dissociated from the sulfonic acid group at λ = 3. On the basis of this barrier height dependence, we, therefore, suppose that the presence of the dissociated proton may support the ether-linkage dissociation by decreasing the barrier heights. This support may be attenuated by increasing the hydration number. This hydration number dependence of the barrier heights suggests that this ether-bond dissociation proceeds under low-humidity condition around λ = 3.

How likely is the H2O2-induced degradation compared to the OH-induced degradation? In the OH-induced degradation mechanism, the formation of OH radicals on the electrode catalyst surface has been taken as the rate-determining step. Yu et al. theoretically found the following reaction path for the OH-radical formation on the Pt(111) surface:[31]Because of the lower reaction barrier (18 kcal/mol), this reaction seems to proceed in preference to the H2O2-induced degradation because of its higher reaction barrier (30–40 kcal/mol). We note, however, that reaction is endothermic, with a reaction energy of −17 kcal/mol, and has another similar low-barrier exothermic reaction path (reaction barrier is 10 kcal/mol and reaction energy is 30 kcal/mol)In this reaction, OH radicals need to desorb to become reactive species. The adsorption energy of the OH radical is evaluated as 48 kcal/mol,[31] which is much higher than the reaction barrier of the H2O2-induced degradation. This process is supposed to be the rate-determining step of the OH-radical-induced mechanism. It is important to note, also, that in the vicinity of the electrode surface, there are hydration water clusters, to which adsorbed molecules, such as H2O2, are hydrated. It is theoretically revealed that OH radicals have no stable structures in water and therefore these radicals react spontaneously with each other, with no reaction barrier, when the surrounding water molecules are explicitly incorporated. That is, when there are vicinal water clusters, OH radicals are converted back to H2O2 in reactions and 2. This indicates that OH radicals need to avoid vicinal water clusters to be reactive species. However, we would emphasize that this does not disprove the OH-radical-induced degradation. In the presence of metal ions, such as Fe2+ ions, experimental studies have observed that OH radicals are formed with a very low activation energy, as mentioned above. We, therefore, conclude that the H2O2-induced degradation proceeds together with the OH-induced degradation.

Vibrational Spectra of Decomposed Hydrated Nafion Membrane

Next, let us compare the vibrational spectra of the decomposed hydrated Nafion membrane and degradation fragment molecule, that is, (H2O)λHO3S–CF2–CF2–O–O–H, with the experimental IR spectra of decomposed hydrated Nafion membrane and its degradation fragment species to discuss the reliability of the H2O2-induced degradation mechanism. Akiyama et al. observed the IR spectra of the fragment species and decomposed Nafion membrane in aquo.[16] The experimental IR spectra show that the degradation generates sharp peaks around 1700 cm–1 and wide peaks around 2300 and 3300 cm–1 and that the Fenton test, producing OH radicals, results in various IR peaks around 900 and 1600 cm–1 compared to those of decomposed hydrated Nafion membrane. Comparing the calculated vibrational spectra of the hydrated Nafion before and after degradation with the experimental IR spectra, we can, therefore, test the decomposed Nafion membrane and degradation fragment molecule.

Figure displays the calculated IR spectra of hydrated Nafion before and after degradation for λ = 1–4. As shown in the figure, strong peaks appear around 2000, 2400, and 3000–3700 cm–1 after the degradation. On the other hand, the degradation slightly changes the peaks in the wavelength region below 1700 cm–1. We note that the experimental IR spectra of the degradation fragment species also show strong peaks in the wavelength regions of 1700–2000, 2200–2400, and 3000–3700 cm–1, slightly affecting the peak strengths for the wavelength region below 1700 cm–1.[16] This consistency strongly supports the H2O2-induced ether-linkage dissociation mechanism. This also suggests that the main degradation fragment molecule is (H2O)λHO3S–CF2–CF2–O–O–H, not containing a carboxylic acid group −COOH but a hydroperoxyl group −O–OH. Although this degradation fragment molecule is considered to be converted to the carboxylic acid, it seems chemically stable due to its poor reactivity, as shown above. On the basis of the vibrational spectra, we propose that (H2O)λHO3S–CF2–CF2–O–O–H is one of the main degradation fragment molecules and that several byproducts are derived from the subsequent reactions of this molecule.

Figure 5

Vibrational spectra of decomposed hydrated Nafion membrane model before and after degradation in the wavenumber region of 0–4000 cm–1 for λ = 1–4, which are calculated by LC-BLYP/cc-pVDZ method.

What types of vibrational modes do the peaks appearing after the degradation of hydrated Nafion membrane correspond to? Table displays the theoretical peak assignments of the IR spectra of hydrated Nafion membrane before and after the H2O2-induced ether-linkage dissociation for λ = 4. The table clearly indicates that the strong peaks appearing around 2000, 2400, and 3000–3700 cm–1 after the degradation correspond to the OH stretching modes of the hydroperoxyl and alcohol groups formed after the ether-linkage dissociation and the accompanying OH stretching modes of water molecules around these groups. We note that the peaks corresponding to the vibrations of the hydroperoxyl group appear at 831, 1015, and 1060 cm–1. As shown in Figure , these peaks are hidden behind other peaks in the calculated IR spectra. Interestingly, these peaks are, however, visible in the experimental IR spectra of the decomposed Nafion membrane.[16] It is also interesting to note that the high peaks assigned to the OH-bond stretching of H2O2 appear at 3553 and 3700 cm–1 for the predecomposed Nafion. The IR absorption of these peaks supports the H2O2-induced dissociation because the OH stretching of H2O2 determines the reactivity of this dissociation. The assignment of the vibrational spectral peaks, therefore, also supports the H2O2-induced ether-linkage dissociation.

Table 1

Peak Energies and the Corresponding Peak Assignments of IR Spectra of Predecomposed and Decomposed Doubly Hydrated Nafion Membrane Model for λ = 4 (Eight Hydration Water Molecules in Total)a

predecomposed Nafion + hydrated H2O2		decomposed Nafion
peak energy (cm–1)	assignments	peak energy (cm–1)	assignments
1522, 1606, 1801, 2178, 2532, 2855, 3020, 3173, 3174, 3385, 3433, 3491, 3589, 3597, 3672, 3693, 3721, 3900, 3913	O–H stretching of H2O	2401, 2517, 3032, 3105, 3144, 3168, 3320, 3353, 3431, 3640, 3646, 3660, 3664, 3703, 3763, 3834	O–H stretching of H2O
3553, 3700	O–H stretching of H2O2	3206	O–H stretching of –O–OH
		1598, 1777, 2000, 2035	O–H stretching of −OH and H2O
1424, 1427, 1435, 1444	C–C stretching	1345, 1416, 1427, 1443, 1464	C–C stretching
1420	O–H swinging of H2O2	1502, 1667, 1820	H2O stretching, wagging and rocking
1347, 1351, 1357	SO3– asymmetric stretching + C–F stretching	1350, 1359	SO3– asymmetric stretching + H2O scissoring
1234, 1261, 1265, 1268, 1275, 1284, 1286, 1289, 1297, 1299, 1304, 1306, 1310, 1315, 1322, 1328, 1335, 1336, 1339, 1367, 1379	C–F stretching	1261, 1264, 1268, 1273, 1289, 1295, 1304, 1307, 1311, 1315, 1316, 1321, 1326, 1328, 1333, 1336, 1339, 1345, 1361, 1374	C–F stretching
1209, 1212, 1232	SO3– asymmetric stretching + H3O+ scissoring	1192, 1215, 1220, 1222, 1238	SO3– asymmetric stretching + C–F stretching
1038	O–C–C scissoring	1035, 1164	O–C–C scissoring
1083	SO3– asymmetric stretching + H3O+ rocking	1015, 1060	SO3– asymmetric stretching + O–O stretching of –O–OH + H3O+ rocking
593, 614, 676, 835, 859, 908	H3O+ rocking	546, 693, 717, 726, 761, 777, 887, 925, 1045, 1063, 1131	H3O+ rocking
		831	O–H swinging of –O–OH
507	C–C–C symmetric stretching + H2O rocking	507	C–C–C symmetric stretching

For comparison, H2O2 molecule is hydrated in the hydration water molecules attached to the predecomposed Nafion membrane. Only the peaks with strength higher than 10–38 esu2 cm2 are listed. The peak assignments corresponding to the vibrations of H2O2 in the predecomposed Nafion and −OH and −O–OH groups in the decomposed Nafion are underlined.

The degradation fragment molecule can be explored in further detail on the basis of the vibrational spectra. Akiyama et al. also observed the IR spectrum of the degradation fragment species dried in vacuum. In the experimental IR spectrum, strong peaks appeared around 1700, 2500, and 3500 cm–1 after the degradation. Figure illustrates the calculated vibrational spectra of the hydrated degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–O–H, for various λ values. As shown in the figure, the peaks corresponding to the strong peaks of the experimental IR spectrum are assigned to the vibrational modes of the degradation fragment molecule for different hydration numbers. This result suggests that the degradation fragment molecule can exist simultaneously with different numbers of hydration water molecules. Because the experimental IR spectrum is given for the dried species in vacuum, this also indicates that hydration water molecules strongly bind to the sulfonic acid groups of the Nafion membrane. This result also supports the conclusion of our previous study that the proton conduction proceeds through the relay mechanism in Nafion under low-humidity conditions due to the very high hydration energy of the sulfonic acid groups.[43] We, therefore, propose that the degradation fragment molecule is strongly hydrated, even under dry conditions.

Figure 6

Vibrational spectra of main degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–O–H, in the wavenumber region of 0–4000 cm–1 for λ = 1–4, which are calculated by LC-BLYP/cc-pVDZ method.

Subsequent Decomposition Reactions of the Degradation Fragment Molecule

For comparative verification with the experimental results, it is meaningful to consider the subsequent decomposition reactions of the presumed main degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–O–H. Several past studies have suggested that HO3S–CF2–CF2–O–CF(CF3)–COOH, called “molecule A”, is the main degradation product.[10,29,61] Molecule A is similar to the present degradation fragment molecule in that both of them are derived from the ether-linkage dissociation of the side chain of Nafion membrane. Actually, a similar molecule, HO3S–CF2–CF2–O–CF(CF3)–CF2–O–OH, is considered to be formed by the present decomposition mechanism, although it is different from molecule A, including the end −CF2–O–OH group instead of the −COOH group. Although molecule A was assigned on the basis of 19F NMR and IR spectral analyses,[16,61] this assignment prompts several questions: The 19F NMR spectrum was not observed after a PEMFC degradation but after a Fenton test, and the IR spectrum also supports the present degradation fragment molecule, as mentioned above. What we need to confirm the presumed main degradation fragment molecule is, however, not to rule out the existence of molecule A but to elucidate how experimentally found decomposition byproducts are formed from this molecule. It is, therefore, meaningful to explore the subsequent decomposition-reaction mechanisms of the presumed main degradation fragment molecule.

As a result, we found two probable monomolecular decomposition reactions of the degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–OHThe potential energy curves of these reactions on the intrinsic reaction coordinates are drawn in Figure . We note that despite both these reactions are exothermic as mentioned later, they are considered not to proceed without a catalyst, for example, Pt surface and metal ions, under fuel-cell operating conditions, due to the very high barrier heights.

Figure 7

Reaction potential energy curves of the main degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–O–H, on the intrinsic reaction coordinate with the optimized structures of the reactant, TS, and product, which are calculated by LC-BLYP/cc-pVDZ method: Decompositions to (a) SO2 and (b) FC-O-OH.

As shown in Figure (a), reaction exothermically produces sulfur dioxide (SO2), which is experimentally found as a degradation product,[33] for any hydration number. Although SO2 is a gas at room temperature, it is hydrated to produce sulfurous acid (H2SO3), which is also experimentally found.[33] The degradation fragment molecule of reaction , CF3–CFO––O–OH, appears to be unstable and therefore seems to convert to a carboxylic acid. In many trial calculations, we, however, found that no monomolecular decomposition reaction takes place without artificial bond cleavage for this molecule. We, therefore, presume that this molecule is also included in the residual species of the degradation. The vibrational spectrum of this molecule provides intense peaks for 1000–1700 and 3023 cm–1. By reference to the experimental IR spectrum of dried dissolved species,[16] such peaks were found to appear in these wavenumber regions after the degradation. However, these intense peaks are hidden in the vibrational spectrum of the main degradation fragment molecule in Figure . This fragment molecule is expected to be found in the 19F NMR spectrum because it simply exhibits two peaks with 3:1 peak strengths. However, we consider that this reaction requires a catalyst to proceed due to the high barrier heights, which exceed 60 kcal/mol at least (λ = 1).

The potential energy curves of reaction appear to indicate that it does not proceed due to the very high reaction barrier heights, 137.34, 139.52, 140.14, 140.83, 141.03, and 141.25 kcal/mol for λ = 0–5, respectively, and the endothermic nature, with high, positive reaction energies, 50.74, 52.06, 51.24, 51.27, 48.31, and 46.27 kcal/mol. Following the endothermic nature, we should note that FC–O–OH would be too unstable to be present as a residual species. Figure plots the potential energy curve of the decomposition reaction of FC–O–OH. As shown in the figure, FC–O–OH transforms into the carboxylic acid FCOOH with a very large reaction energy (155.15 kcal/mol) and then it is decomposed to HF and CO2, which are both experimentally found in the degradation species,[1,25] with relatively low activation energies (30.68 kcal/mol at the maximum). Overall therefore, reaction is very exothermic, with a large reaction energy, which totals more than 100 kcal/mol. We should, however, be reminded that this reaction requires a catalyst to proceed due to the very high barrier heights. Let us then assume that a catalyst supports this reaction. The degradation fragment molecule of this reaction is (H2O)λHO3S–CF3, although this molecule has not yet been experimentally assigned, as far as we know. However, it may be included in the low-molecular-weight perfluorocarbon sulfonic acid in the degradation species[1] because the terminal products of this reaction, HF and CO2, are experimentally detected. The vibrational spectra of this molecule depend on the hydration number for the maximum peak wavenumbers: 616, 2768, 2038, and 2526 cm–1 for λ = 0–3, respectively. As shown in Figure , these peaks are also hidden in the vibrational spectrum of the main degradation fragment molecule except for the peak at 2038 cm–1 (λ = 2). Because the experimental IR spectrum of dried dissolved species exhibits no peak around 2038 cm–1, this may suggest that the λ value for (H2O)λHO3S–CF3 is very small (λ = 0 or 1) under dry conditions.

Figure 8

Decomposition-reaction potential energy curve of FC–O–OH to HF + CO2 on the intrinsic reaction coordinate, which is calculated by LC-BLYP/cc-pVDZ method. The intrinsic reaction coordinates are normalized for each reaction step. The optimized structures of local minimum (MIN) and TS points are also shown.

In summary, we propose that CF3–CFO––O–OH and (H2O)λHO3S–CF3 molecules are formed as stable byproducts in addition to the experimentally observed SO2, HF, and CO2 molecules in the thermal decomposition of the main degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–O–H. We expect that these degradation fragment molecules will be experimentally found, for example, in the 19F NMR spectrum analysis.

Conclusions

In this study, we have theoretically investigated the H2O2-induced degradation mechanism of doubly hydrated Nafion membrane in the PEMFC and its subsequent decomposition reactions focusing on the ether-linkage dissociation of the side chain and thermodynamic reactions. As a result, we have revealed the ether-linkage decomposition mechanism by the H2O2 molecule, which is hydrated in the protonated water cluster around the sulfonic acid group. We have also found the decomposition reactions of the degradation fragment molecule derived from the ether-linkage dissociation and have confirmed that the expected products are consistent with conventional experimental results.

On the basis of reasonable assumptions, we have calculated the potential energy curves of the ether-linkage dissociation of the doubly hydrated Nafion membrane model with H2O2 molecule in the hydration-protonated water cluster for various hydration numbers. Consequently, we have found the decomposition-reaction pathways providing relatively low reaction barriers of 32–40 kcal/mol, which are analogous to the C–C double-bond cleavage mechanism by ozone.

We have compared the vibrational spectra of the resulting decomposed Nafion membrane and degradation fragment molecule, (H2O)λHO3S–CF2–CF2–O–O–H. As a result, we have confirmed that the vibrational spectra are consistent with the experimental IR spectra of the decomposed hydrated Nafion membrane and degradation fragment species, despite the fragment molecule having an unfamiliar end group, −CF2–O–OH.

We have also explored the subsequent reactions of the presumed degradation fragment molecule for various hydration numbers. In consequence, we have obtained two possible decomposition-reaction processes giving CF3–CFO––O–OH and (H2O)λHO3S–CF3 molecules as degradation byproducts in addition to the experimentally observed SO2, HF, and CO2. However, we found that these subsequent reactions would require a catalyst, such as Pt surface or metal ions, to proceed, due to the high barrier heights. We have also found that these byproducts are quite stable and would not be expected to undergo subsequent monomolecular reactions and therefore suggested that these byproducts should be contained in the degradation species.

In conclusion, we have proposed a H2O2-induced decomposition mechanism of Nafion membrane, which is likely to proceed simultaneously with conventional OH-radical-induced mechanisms. The schematic diagram of the decomposition mechanism is illustrated in Figure . We expect that elucidating this decomposition mechanism will shed light on ways to develop strategies to combat the degradation of proton-exchange membranes.

Figure 9

Schematic diagram of the H2O2-induced decomposition mechanism of hydrated Nafion membrane. “Rp” represents the residual part of Nafion membrane model.