[ECS 214th ECS Meeting - Honolulu, HI (October 12 - October 17, 2008)] ECS Transactions - The Chemistry of Fuel Cell Membrane Chemical Degradation

The Chemistry of Fuel Cell Membrane Chemical Degradation

Frank D. Coms

General Motors Corporation, Fuel Cell Research Labs, 10 Carriage Street, Honeoye Falls, NY 14472, USA

Abstract

A detailed thermochemical and kinetic analysis of PFSA ionomers and the reactive oxygen species formed in an operating fuel cell is reported. The analysis reveals that hydroxyl radical is the only oxygen species capable of abstracting a hydrogen atom from carboxylic acid intermediates, thereby propagating the PFSA main chain unzipping process. Two chemically specific, main chain scission mechanisms are proposed. The first involves formation of sulfonyl radicals under dry conditions and the second invokes the action of hydrogen atoms formed from hydrogen gas and hydroxyl radical. The thermochemical analysis provides a strong basis from which to rationalize the results from a broad range of in-situ and ex-situ fuel cell degradation studies.

Introduction

Proton exchange membrane (PEM) fuel cells offer considerable promise for

future portable power applications due to their low operating temperatures, high power densities and tolerance to start up and shut down transients. One significant challenge to the widespread commercialization of this technology is membrane durability (1-3). Over the past five years, numerous studies of chemical degradation of perfluorosulfonic acid (PFSA) membranes have been performed but, surprisingly, little attention has been given to the thermochemical details of proposed degradation pathways. A thermochemical analysis can be used to exclude hypotheses that are clearly unreasonable on a kinetic or thermodynamic basis. Furthermore, a comprehensive thermochemical analysis can provide strong supporting evidence for proposed mechanisms and, at best, can lead to the generation of new hypotheses of degradation that can be subjected to further experimental scrutiny. In this report, a detailed thermochemical and kinetic analysis of potential fuel cell degradation mechanisms is provided. This analysis begins with thermochemical evaluations of proton exchange membranes (PEMs) and the oxidizing species formed during fuel cell operation. The analysis is based on the best available experimental thermochemical data and, where experimental data is not available, density functional theory (DFT) calculations are employed to provide reasonable estimates of the required parameters. This study emphasizes the properties of PFSA-based PEMs, but some of the major thermochemical and kinetic properties of hydrocarbon-based material will also be explored. With the thermochemical and kinetics properties of the fuel cell components as a foundation, the observed membrane chemical degradation in fuel cell and ex-situ tests will be reviewed compared to current degradation theories. Where inconsistencies are found among experiment, thermochemistry and mechanism, alternative, consistent mechanisms are proposed.

ECS Transactions, 16 (2) 235-255 (2008)10.1149/1.2981859 © The Electrochemical Society

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Thermochemistry of PEM Materials

Perfluorosulfonic Acids

Perfluorosulfonic acid (PFSA) based membranes such as Nafion® were hailed as a great leap forward for hydrogen fuel cell technology owing to their high chemical stability relative to early generation membrane materials based on sulfonated polystyrenes (2). Indeed, Nafion® membranes allowed the useful constant-current lifetimes of PEMs to be extended from a few hundred hours, in the case of sulfonated polystyrenes, to tens of thousands of hours, depending on membrane thickness and operating conditions (3). Nafion® is the random copolymer of tetrafluoroethylene and a sulfonated perfluorovinyl ether. The chemical stability of PFSA membranes is generally attributed to the very strong and chemically inert C-F bonds. Note that Nafion® and PFSA ionomers in general have several types of C-F bonds. For example, there are primary, secondary and tertiary C-F bonds which have different thermodynamic stabilities. In addition, the carbon-fluorine bond types can be further classified into those adjacent to oxygen or sulfur. The thermodynamic stability of each C-F bond subtype will be assessed by evaluating its experimental bond enthalpy ( H298) or computed bond dissociation energy (D0).

Table I shows the experimental and calculated bond enthalpies of a variety of C-F bonds representative of the types found in PFSAs. Note that only two C-F bond enthalpies have been determined experimentally; perfluoromethane and perfluoroethane (4). Due to the lack of experimental values we will utilize theoretical calculations to provide estimates of the respective C-F, C-O and C-S bond strengths found in PFSAs. The B3LYP/6-31G(d) density functional theoretical model is primarily used throughout this work to calculate the homolytic bond dissociation energies as it represents a very good compromise between speed and accuracy (5). In some cases, the M05-2X theoretical model is used with either 6-31G(d) or 6-311++G(d,p) basis sets (6). All computed structures correspond to potential energy minima on their respective potential energy surfaces as determined by frequency calculations. All computed bond energies (D0) are corrected for zero point energy differences using values determined from B3LYP/6-31G(d) frequency calculations. Carbon-fluorine covalent bonds are, in general, very strong (105 -130 kcal/mol) owing to the high ionic character of the covalent bond. As shown in Table I, the calculated C-F bond strengths of CF4 and C2F6 are only 2-4 kcal/mol below the experimental values at the B3LYP/6-31G(d) level of theory. This level of accuracy is considered a quite acceptable given the uncertainties in the experimental values (ca. ± 2 kcal/mol). Table I. Calculated and Experimental C-F Bond Strengths Reaction

B3LYP/6-31G* Calculated D0 (kcal/mol)

Experimental H298 (kcal/mol)

CF4 3 128.5 130.8 C2F6 2F5 123.2 126.9 C3F8 n- 3F7 122.0 n.a. C3F8 i- 3F7 114.5 n.a. CF3OCF3 CF3OCF2 124.3 n.a. (CF3)3CF (CF3)3 104.8 n.a. (CF3)2(CF3O)CF (CF3)2(CF3O)C 109.3 n.a. CF3SO3H 2SO3 120.7 n.a. CF3CO2H 2CO2 114.5 n.a.

ECS Transactions, 16 (2) 235-255 (2008)



The calculated primary C-F bond strengths of perfluoroethane and propane are very similar as expected and only about 5-6 kcal/mol weaker than the C-F bonds of CF4. The secondary C-F bond of perfluoropropane is about 8 kcal/mol weaker than the primary C-F bond, following a similar trend observed in the analogous hydrocarbons (4). A tertiary C-F bond is calculated to be about 105 kcal/mol, or about 10 kcal/mol weaker than a secondary C-F bond of perfluoropropane. The impact of the ether function can be seen in entries 5 and 7 of Table I where presence of an oxygen atom adjacent to the C-F bond of interest leads to a stronger C-F bond relative to a perfluorinated carbon neighbor. For example, relative to the primary C-F bond of perfluoropropane, the analogous bond of perfluorodimethylether is actually 10 kcal/mol stronger while the tertiary C-F bond of perfluoroisobutane is strengthened by about 4 kcal/mol when adjacent to an ether oxygen. Finally, the impacts of sulfonic and carboxylic acid groups on C-F bond strengths are shown in the last two entries of Table I. Relative to perfluoroethane, the sulfonic acid and carboxylic group decrease the strengths of the respective primary C-F bonds by 2 and 9 kcal/mol, respectively.

A summary of the impact of structure on C-F bond strengths follows. There is a fairly strong dependence of C-F bond strength on substitution where the order is primary (~125 kcal/mol) > secondary (~115 kcal/mol) > tertiary (~105 kcal/mol). This can be compared to analogous hydrocarbons where the secondary and tertiary C-H bonds are weaker than the primary bonds of ethane by 3.0 and 5.0 kcal/mol, respectively. Substitution of a perfluorocarbon group with a perfluoroether leads to a general C-F bond strengthening of 4-10 kcal/mol while substitutions with acid groups leads to C-F bond strength weakening. It is important to note that while there are small to moderate bond strength changes relative to perfluoroethane; all C-F bonds are quite strong and thermodynamically stable.

Table II shows the calculated bond enthalpies of backbone C-C and C-O bonds in both hydrocarbon and perfluorocarbon systems. Compared to hydrocarbon systems, both C-C and C-O bonds of the perfluorocarbon are significantly stronger, particularly the ether linkage which is almost 20 kcal/mol stronger for perfluorodimethylether than its hydrocarbon analog, dimethylether. The high calculated thermodynamic stability of the C-C backbone is in agreement with previous calculations of perfluoroethylene by Dixon (7). Summarizing, computational analysis of the PFSA backbone bonds reveals that these bonds are even stronger than the analogous hydrocarbon systems and thus are not anticipated to be kinetically labile. Table II. Calculated and Experimental Bond Strengths Bond Energy D (kcal/mol) Bond Energy D (kcal/mol) Bond B3LYP/6-31G* Expa Bond B3LYP/6-31G* Expa Dixonb H3C-H 103.2 104.3 F3C-F 128.5 130.8 n.a. H5C2-H 98.3 100.3 F5C2-F 123.2 126.9 129.4 H3C-CH3 86.8 88.8 F3C-CF3 92.7 98.8 96.4 H5C2-CH3 83.7 87.1 F5C2-CF3 88.1 n.a. 102.2 H3CO-CH3 76.9 82.6 F3CO-CF3 99.6 101.2 a. Ref 4. b. Ref 7.

In this section we turn our attention to the thermochemical properties of the sulfonic acid groups of PFSAs in addition to those of perfluorocarboxylic acids. The carboxylic acid groups are sometimes present as impurities in virgin PFSA membrane (8) and are




also formed during PFSA chemical degradation (8-10). A study of these functional groups reveals some interesting differences between the hydrocarbon and fluorocarbon classes. Table III shows the experimental bond strengths of the C-S, C-C and O-H bonds of the acidic functional groups. The C-S bonds to the sulfonic acid group are clearly the weakest bonds in both the hydrocarbon and fluorocarbon families. Significantly, the C-S bond of trifluoromethanesulfonic acid is calculated to be about 9 kcal/mol weaker than the corresponding bond of methanesulfonic acid. The weak C-S bond is certainly related to the relatively long bond length (Table IV), which is calculated to be 0.078 Å (M05-2X/6-311++G(d,p)) longer than the corresponding bond in methanesulfonic acid. This substantial calculated bond length difference is also mirrored by available experimental values of the sulfonic acids and related materials (11-13). As shown in Table IV, the electron diffraction C-S bond length of trifluoromethanesulfonic acid is 1.834 Å (11), in good agreement with the calculated value. While no experimental structure is available for methanesulfonic acid, experimental structures are available for acid derivatives methanesulfonyl chloride and methanesulfonyl fluoride which have C-S bond lengths of 1.763 and 1.759 Å (13), respectively. Based on these results, it is reasonable to conclude that the C-S bond of methanesulfonic acid is very near 1.760 Å or 0.074 Å shorter than that found for trifluoromethanesulfonic acid in excellent agreement with the theoretical structure. Interestingly, as shown in Table IV, the C-S bonds of the analogous hydrocarbon and perfluorocarbon dimethyl ethers are of very similar length. These results suggest that the bond weakness of the C-S bond of trifluoromethanesulfonic acid is related to the combined effects of two electropositive atoms involved in the bonding pair. In other words, the strong electron withdrawing effect of the three fluorines of the CF3 group in combination with the strong electron withdrawal of the oxygen atoms bound to the sulfur of the SO3 group serves to make the C-S bond of the perfluoro acid rather electron poor resulting in a weak, long bond. Whatever the origin of the weakness of this bond, this enhanced lability may play a significant role in the overall fuel cell chemical stability of PFSA membranes such as Nafion®. This is particularly significant because perfluorosulfonic acid structural motif is present in all members of the class and is also responsible for the extremely high acidities of fluorosulfonic acids. Table III. Calculated and Experimental Bond Strengths for Acids and End Groups Bond Energy D0 (kcal/mol) Bond Energy D0 (kcal/mol) Bond B3LYP/6-31G* Exp Bond B3LYP/6-31G* Exp H3C-SO3H 65.4 n.a F3C-SO3H 56.4 n.a. H3CSO3-H 95.6 n.a. F3CSO3-H 97.0 n.a. H3C-CO2H 89.4 86.5 F3C-CO2H 85.1 83.2 H3CCO2-H 97.9 105.8 F3CCO2-H 101.9 n.a. H3C-H 103.2 104.3 F3C-H 101.2 107.4 Table IV. Calculated and Experimental C-S or C-C Bond Lengths (Å) of Alkyl and Perfluoroalkyl Sulfonic and Carboxylic acids.

Bond Theory Experiment CH3-SCH3 1.812a 1.802 CF3-SCF3 1.820 a - CH3-SO3H 1.771 a - CF3-SO3H 1.849 a 1.834

CH3-SO2CH3 1.785 a 1.770

CF3-SO2CF3 1.880 a - CH3-CO2H 1.497b 1.480 CF3-CO2H 1.538 b 1.526

a. M05-2X/6-311++G(d,p). b. B3LYP/6-31G(d)




As noted above, the perfluorocarboxylic acid end group, while not a structural element of an idealized PFSA, can be found in virgin PFSA membranes as a synthetic impurity and it is certainly formed during chemical degradation reactions (8-10). First, as seen for the sulfonic acid pairs, the C-C bond of the perfluorocarboxylic acid is both weaker and longer than the C-C bond of the hydrocarbon analog although the trend is not as pronounced as found in the sulfonic acids (Tables III and IV).

Because the homolytic O-H bond strengths of perfluorocarboxylic and sulfonic acids are not known, their values were calculated (Table III). The theoretical model used in these computations is anticipated to give consistently weaker than experimental bond strengths owing to the highly polar nature of the acidic O-H bonds. This offset is a well known deficiency of this limited basis set (14). As shown in Table V, larger, triple-zeta basis sets including diffuse functions and hydrogen polarization functions more accurately reproduce the experimental values of O-H bonds. While the absolute bond strengths obtained with the 6-31G(d) basis set are not correct, the trends are meaningful. First, all calculated O-H bond strengths are less than the calculated O-H bond strength of water at the same level of theory. Second, the calculated bond strengths of the sulfonic acids are 2-5 kcal/mol less than the analogous carboxylic acids. In general terms, the calculated O-H bond strengths of the fluoroacids are 1-4 kcal/mol stronger than their hydrocarbon analogs. Table V. Calculated and Experimental Bond Strengths of Fuel Cell Oxidants Reaction Experimental H298

(kcal/mol) Calculated D0 (kcal/mol)

B3LYP/6-31G* B3LYP/6-311++G** H2O 118.8 109.0 113.6 HF 136.4 122.6 131.9 H2O2 87.5 76.3 81.2 H2O2 50.3 49.7 42.5 OOH 2 49.2 43.0 47.1 FOH 51.4 49.7 42.0

Like carboxylic acids, C-H bonds can also be found in impure or chemically degraded PFSAs in the form of CF2H (Table III). These bonds (ca. 107 kcal/mol) are stronger than typical aliphatic hydrocarbon C-H bonds (90-98 kcal/mol) but, as discussed below, they are nevertheless chemically reactive in the aggressive environment of an operating fuel cell as will be discussed in subsequent sections. Fuel Cell Oxidants

It is well established that hydrogen peroxide is generated in an operating fuel cell and has been accordingly implicated in membrane degradation processes for over forty years. Hydrogen peroxide can be formed either via an electrochemical reaction at the anode or by chemically by a crossover mechanism (15, 16). While the concentration of hydrogen peroxide within an operating fuel cell membrane can vary with operating conditions, in-situ electrochemical detection methods (17) indicate that the levels are near 10 ppm (ca. 3 x10-4 M). In addition to hydrogen peroxide, the very aggressive hydroxyl

-situ ESR spin trapping techniques (18-19). Due to the facile reaction betwee 2O2 (eq 1be present. While all three reactive oxygen species (ROS) can participate in the degradation of PFSA ionomers, the respective chemistries and the roles played by each




ROS in the degradation process are profoundly different. The aggressiveness or chemical reactivity of the various oxidants can be, in part, assessed by determining the thermochemical properties of the reactive species. One viewpoint of the chemical potencies of these species is reflected in their respective reduction potentials, shown in Table VI (20). The reduction potentials shown in Table VI clearly illustrate the extremely high oxidizing power of the hydroxyl radial relative to hydroperoxyl radical. H2O2 2O k = 2.7 x107 M-1 s-1 [1] Table VI Reduction Potentials of Reactive Oxygen Species Half Cell Reaction Reduction Potential at pH = 0 (SHE) OH + H+ + e- H2O 2.59 OOH + H+ + e- H2O2 1.48

H2O2 + 2H+ + 2e- 2 H2O 1.74

The reactivity differences of the ROS can also be appreciated from the perspective of the experimental bond strength data shown in Table V. First, consider the chemical nature of the hydroxyl radical. The first entry of Table V shows that the oxygen atom of

bond to hydrogen through abstraction and form H2O. Because almost all covalent bonds to hydrogen are weaker than that of H2O, such abstraction reactions will usually be exothermic or nearly thermoneutral. This includes the acidic hydrogen atoms of sulfonic and carboxylic acids. Hydrogen fluoride (HF), found in the effluent of degrading fuel cells, is exceptional in this regard. Because the H-F bond strength (136 kcal/mol) is greater than that of H2O, it is not susceptible to abstraction by hydroxyl radical. Hydrogen peroxide, like H2O, possesses two O-H bonds although the O-H bond energies are considerably weaker than those of water (21). Specifically, the first bond dissociation energy is more than 30 kcal/mol weaker than that of H2O. Thus, in strongly oxidizing environments, H2O2 accordingly functions as a reducing agent (eq. 1). This hydrogen atom donating reaction is probably the fastest reaction of hydrogen peroxide in an operating fuel cell. In sharp contrast to OH, the hydroperoxyl radical does not readily abstract hydrogen atoms because most

covalent bonds to hydrogen are thermodynamically more stable than the O-H bond of H2O2. Indeed, the chemical literature indicates (22) that hydroperoxyl radical is a rather feeble with regard to hydrogen atom abstraction (Figure 1). While a hydrogen abstraction reaction by hydroperoxyl radical occurs at the doubly allylic site of linoleic acid (BDE = 76 kcal/mol), no reaction is observed with the allylic hydrogens of oleic acid (BDE = 88 kcal/mol). Given that the covalent bonds to hydrogen of an impure or degrading PFSA are 15-20 kcal/mol stronger than that of oleic acid (Table III); it is very unlikely that hydroperoxyl will participate in any kind of hydrogen abstraction reaction in PFSAs. Further evidence of the relative reactivity of OH is given by noting that its reaction rate constant with linoleic and oleic acid (23) are diffusion controlled (k ~ 1010M-1s-1). Hydroperoxyl radical itself is most likely to function as a hydrogen atom donor and, in this sense; it is far more potent than H2O2 due to its extremely weak O-H bond (49 kcal/mol). Thus, in the highly oxidizing environment of an operating PEM fuel cell, both H2O2 and OOH most often function as reducing agents. Finally, another significant feature of H2O2, and all peroxides in general, is its weak O-O bond (50 kcal/mol) which is prone to homolytic cleavage, particularly when catalyzed by metals or UV irradiation.




Figure 1. Hydrogen atom abstraction of weak C-H bonds by hydroperoxyl radical. PFSA Chemical Degradation

With the basic thermochemical parameters of PEM materials and potential degrading species as a foundation, the possible degrading chemistries can be evaluated in a rigorous manner. As mentioned previously, hydroxyl radical has a strong propensity to abstract hydrogen atoms. Table VII summarizes the experimental and computed enthalpies of reaction for a variety of potential hydrogen atom abstractions. As anticipated, virtually all hydrogen atom abstraction reactions by OH are exothermic with the lone exception of HF. It is recognized that reaction exothermicity does not necessarily imply facile kinetics, however there exists a large bodies of gas (24) and solution phase (23) experimental work that have shown that such exothermic reactions are quite facile. In addition, aqueous solution studies indicate that room temperature hydrogen abstraction reactions by OH have rate constants between 106 1010 M-1s-1. The absolute rates are largely governed by the strength of the breaking bond and hence, the rates correlate with reaction exotsolution and are anticipated to occur during fuel cell operation, Table VIII shows that abstractions of fluorine by OH are, as pointed out by others (25), clearly unreasonable on thermodynamic grounds no matter the strength of the C-F bond. The reason for this is quite simple; the energy cost of breaking a C-F bond (105 -130 kcal/mol) far exceeds the energy gain via the formation of the new O-F bond of HOF (51 kcal/mol). Table VII. Experimental and Calculated Fuel Cell Reaction Enthalpies Reaction Experimental H298

(kcal/mol) Calculated Erxn (kcal/mol)

B3LYP/6-31G* CF3 3 + H2O -11.4 -7.8

2O +16.5 +13.6 H2O2 2O -29.6 -32.7 CH3CO2 CH3CO2 2O -15.1 -11.1 CF3CO2 CF3CO2 2O n.a -7.0 CF3SO3 CF3SO3 2O n.a. -12.0 Table VIII. Calculated Reaction Enthalpies of Fluorine abstraction by OH Reaction Calculated Erxn B3LYP/6-31G*

(kcal/mol) CF4 3 + HOF 78.8 C2F6 2F5 + HOF 73.5 C3F8 n- 3F7 + HOF 72.3 C3F8 i- 3F7+ HOF 64.8 (CF3)3 (CF3)3 55.1 (CF3)2(CF3 (CF3)2(CF3O)C + HOF 59.6

(CH2)6CO2HCH3(CH2)6

H H H H

(CH2)6CO2H

H HCH3(CH2)4

(CH2)6CO2HCH3(CH2)6

H H H

(CH2)6CO2H

HCH3(CH2)4

XNo Reaction

k = 1.4 x 103 M-1s-1BDE = 76 kcal/mol

BDE = 88 kcal/mol




The PFSA chemical degradation mechanism proposed by Curtin et al (Figure 2) is consistent with the thermochemical analysis that has been presented to this point. In this mechanism, the degradation initiates by hydroxyl radical abstraction of a hydrogen atom from a so-called weak end group consisting of a carboxylic acid or perhaps a C-H bond. The carboxyl radical immediately liberates CO2 to give a fluorocarbon radical which subsequently forms a fluoroalcohol via reaction with hydroxyl radical or perhaps a functional equivalent species present in higher concentration. Fluoroalcohols readily eliminate HF to give an acylfluoride that hydrolyzes to give another carboxylic acid along with an additional equivalent of HF. The chain degradation, sometimes called unzipping, is further propagated by abstraction of the acidic hydrogen by another hydroxyl radical, thus initiation and propagation steps are identical. There are three elements of this mechanism that deserve comment. First, PFSA degradation requires the presence of unstable or hydrogen-bearing end groups to initiate the process; therefore, the

would not degrade. This, of course, is consistent with the thermochemical arguments surrounding the inertness of C- Second, the hydrogen atom abstraction

(k~ 106M-1s-1) and very likely the rate determining step under most operating conditions (26). The low rate constant for this abstraction is, no doubt, due to the strength of the O-H bond of the perfluorocarboxylic acid. Third, the mechanism does not account for any main chain scissions that would increase the number of ends groups with time. According to this mechanism, with the exception of cleaved side chains, the number of end groups would not increase and the rate of degradation would remain fairly constant.

Figure 2. PFSA main chain unzipping process as proposed by Curtin et. al.

Fuel Cell Chemical Degradation

At this point, it is important to examine the behavior of a PFSA-based membrane electrode assembly (MEA) during chemical degradation testing. Typical fuel cell testing conditions that aim to isolate chemical degradation as the primary source of failure involve high temperatures, low relative humidities of the inlet gas streams and open circuit voltage (OCV) conditions. Figure 3 shows the voltage degradation and fluoride release rates of a Nafion®-based MEA (NRE212, 50 µm) run under OCV conditions at 95 °C, and 50% RH of the air and hydrogen streams. The MEA was prepared by applying standard Pt/C electrodes (0.4 mg/cm2 Pt) by decal transfer to both anode and cathode. Over the course of the 200 hour test, the voltage dropped by about 170mV (ca. 850 µV/h) and the fluoride release rate rose by about one order of magnitude before plateauing at a

CO2

(CF2CF2)n CF2 CF2

(CF2CF2)n CF2 CF2H

(CF2CF2)n CF2 CF2 CO2H (CF2CF2)n CF2 CF2OH

HF

(CF2CF2)n CF2 CF

O(CF2CF2)n CF2 C

OH

O

HF

H2O(CF2CF2)n CF2

CO2

Unzipping Continues




value of 3 x10-5 the original fluoride inventory, thinned significantly from its original 55 µm to 35 µm and developed a large hydrogen crossover current. The increase in the fluoride release rate during the test is consistently observed when unmitigated PFSA MEAs are tested in this manner. This result, which has also been observed by others (9, 10, 27), strongly suggests that new end groups are generated by some kind of chain scission process. As noted above, the Curtin mechanism of PFSA degradation offers no pathway of chain scission.

Figure 3. Experimental plot of NRE 212CS based MEA under hot, dry OCV (95 °C, 50% RH) conditions showing both voltage decay and fluoride release rate (FRR) profiles.

on the well-known reaction between ferrous iron and hydrogen peroxide to generate hydroxyl radicals. During the course of the reaction, iron cycles between the ferrous and ferric oxidation states as shown in equations 2-4 (28). Depending on the stabilities of the tested materials, the degradation tests can run for hundreds of hours requiring the

test, a large fraction of the hydroxyl radicals will be deactivated via its reaction with hydrogen peroxide. As peroxide is consumed the rate of hydroxyl radical generation will decrease but its lifetime will increase due to the decreased reaction rate with hydrogen peroxide. While the concentration of hydroperoxyl radicals will exceed that of hydroxyl

accomplished only model fluorocarbons have shown that for degradation to occur, one or more hydrogen bearing end groups must be present. The hydrogen atom, in a sense, provides a foothold for hydroxyl radical attack and the thermodynamic driving force for formation of the strong O-H bond of H2O. Substrates that possess only a sulfonic acid end group, as would

Note that the sulfonic acid groups exist exclusively in the sulfonate anion form in




aqueous solution. The model compound studies have noted general increased reactivity of branched model compounds suggesting that branched materials are more fragile than their straight chain counterparts (29). This conclusion is unlikely; with a given end group, the rate differences observed between branched and straight compounds should be very similar. The observed differences are more likely related to differences in the water solubility, and hence concentration, of the respesolution. Fe2+ + H2O2 + H+ Fe3+ 2O [2] Fe3+ + H2O2 Fe2+ + [3] Fe3+ Fe2+ + O2 + H+ [4]

In an effort to improve the chemical durability of PEMs, PFSA manufacturers have successfully reduced the number of hydrogen bearing end groups through post fluorination methods. Consistent with the thermochemical arguments presented here,

hydrogen-bearing end groups and the membrane degradation rate as determined by fluoride release rate (27). Materials with a greater concentration of hydrogen bearing end groups degrade faster that those with fewer in these solution tests. Again, if a perfect PFSA material existed, there would be little or test solution. Much to the consternation of the fuel cell community and PFSA manufacturers, the post fluorination of the ionomers has not lead to significant improvements in membrane chemical durability during fuel cell operation. For example,

show no difference in degradation rate under OCV tests (27). These findings strongly suggest that in addition to the end group degradation pathway, other pathways must be operative in a fuel cell. Furthermore, based on the details of increasing fluoride release rates during fuel cell OCV tests, chain scission pathways must be operative. Finally, these as yet undefined chain scission pathways of degradation are clearly not operative in

Chain Scission Mechanisms

As noted above, there is considerable experimental evidence indicating that chain scission reactions are occurring in fuel cell tests conducted under high temperature OCV testing. While there is general acknowledgement of chain scission processes there has been little discussion addressing chemically specific mechanisms of chain scission. Most of the discussion has been phenomenological in nature. For example, chain scission processes have been ascribed to all of the non-PTFE functional groups of PFSAs including the ether functionality, the tertiary fluorine groups or the sulfonic acid groups without addressing specific chemical reactions. A fundamental chemical understanding of the PFSA degradation mechanism can lead to the development of rational approaches to mitigation and, perhaps more importantly, accurately define the inherent chemical and physical limitations of a variety of current and future material sets. Here, we explore novel degradation initiation mechanisms that can potentially lead to main chain cleavage. For this analysis, we employ both experimental and computed thermochemical and kinetic data as the basis for developing mechanisms that are consistent with an array of in-situ and ex-situ fuel cell degradation tests.




Sulfonyl Radical Mechanism

Recently, two groups have reported results from ex-situ vapor phase hydrogen peroxide tests (9, 10, 30). Both groups report that hydrogen peroxide vapor is very aggressive toward PFSA membranes and, in fact, is able to generate scissions in the main chain and thereby produce very high overall degradation rates. Furthermore, one group reports that these chain scission (9, 10). These findings are in sharp contrast to solution phase hydrogen peroxide experiments where, in

2+, high concentrations of hydrogen peroxide produce only low concentrations of fluoride (31-33). The sharp distinction in hydrogen peroxide induced degradation rates of PFSAs subjected to wet (solution) and dry (vapor) conditions interestingly mirrors the observed fuel cell degradation behavior where dry conditions have long been recognized as far more damaging than wet (2, 15). A common explanation accounting for these rate differences at different RH values involves increased hydrogen peroxide concentration under the drier conditions (15). While there some evidence that peroxide concentration levels are somewhat elevated under dry conditions (34), it seems doubtful that peroxide concentration alone could account for the dramatic changes in peroxide aggressiveness that is observed in the vapor phase tests. A very plausible rationale for these observations can be found by considering the chemistries of hydrogen peroxide and the functional group at the very heart of PEMs: the sulfonic acid group.

Perfluorosulfonic acids, such as trifluoromethanesulfonic acid are sometimes referred to as super acids because of their extremely low pKa values (ca. -13). Thus, in a wet PFSA membrane, the sulfonic acid group is 100% deprotonated with acidic proton residing on water forming H3O+, the primary proton carrier in a PEM fuel cell. Thus, under wet conditions, the sulfonic acid group exists as the rather chemically inert sulfonate salt form SO3

-. We now consider the changes that occur as the membrane dries. As the level of hydration decreases, the pKa value of the sulfonic acid will increase due to the diminished solvation of the charged products of ionization. In the dry limit, the acidic proton must reside someplace other than on a water molecule. Given the lack of charge stabilization by H2O in the dry state, the proton will reside on the sulfonic acid group as SO3H. At 20% RH, PFSA membranes are hydrated to the extent of about two water molecules per SO3H group ( =2) (35). Modeling studies of PFSA ionization indicate that at least three water molecules per SO3H group are required to support significant ionization (36, 37). It is therefore reasonable to conclude that as the membrane becomes drier the proton will tend to increase its residence time on the sulfonic acid group. As discussed below, the location of the proton has a significant impact on the chemistry in the dry membrane of a fuel cell.

If the acidic proton resides on the -SO3 group as SO3to form the -SO3 VII, Figure 4). Second, in another mode of reactivity, the protonated acid, not the sulfonate anion, can participate in a reaction with hydrogen peroxide, a very strong alpha-effect nucleophile (38), to form a peroxy acid. Due to the difunctional nature of hydrogen peroxide and the accepted nanostructure of Nafion which places acidic sites in close proximity, it is quite plausible that a bissulfonyl peroxide could form. Bissulfonyl peroxides are known materials (39) and a review of their chemistry is relevant to decomposition of PEM materials. Alkyl- and phenyl-bissulfonyl peroxides are generally stable at room temperature but are known to decompose via O-O




bond homolysis at temperatures above 40 °C to give sulfonyl radicals which can initiate radical polymerization reactions (40) or abstract hydrogen atoms from the solvent to form the corresponding sulfonic acids. Significantly, bis(trifluoromethylsulfonyl) peroxide has also been prepared and shown to have very different chemistry (41). Specifically, bis(trifluoromethylsulfonyl) peroxide, prepared at -23 °C, was observed to explode

trioxide and trifluoromethyl trifluoromethane sulfonate (Figure 4). As noted for the alkyl and aryl derivatives, the decomposition reaction of perfluorinated bissulfonyl peroxide involves the formation of the sulfonyl radical but in this case, the sulfonyl radical cleaves to trifluoromethyl radical and sulfur trioxide faster than hydrogen abstraction. The cleavage reaction of the respective sulfonyl radicals is discussed below.

Figure 4. Formation of Bissulfonyl peroxides and sulfonyl radicals.

Figure 5 shows the calculated C-S fragmentation energies of three classes of sulfonic acids and the corresponding sulfonyl radicals (42). Recall that the C-S bonds are the weakest bonds of perfluorosulfonic acids and that the C-S bond of trifluoromethane-sulfonic acid is about 11 kcal/mol weaker than its hydrocarbon analog. It is clear that with regard to C-S bond cleavage, the SO3 substantial bond weakening effect. The fragmentation rates of radicals are generally quite rapid because there is no

RfCF2 SO

O

O HH2O2 H2ORfCF2 S

OO

O OH

RfCF2 SO

O

O OHCF2RfS

OO

OH

RfCF2 SO

O

O OS CF2Rf

O O

H2O

R SO

O

O OS R

O O

+ +

+ +

R SO

O

O

Sulfonyl Peroxide

Bissulfonyl Peroxide

240-60 °C

CF3 SO

O

O OS CF3

O O

10 °CCF3CF3 CF3 S

OO

OCF3

SO3+ +

CF3 SO

O

OCF3 SO3+

R= alkyl or aryl Sulfonyl Radical

explosive!

OH RfCF2 SO

O

O




net change in the number for radical species, unlike the fragmentation of a closed shell species. The bond weakening effect is quite constant across all three classes of sulfonic acids being about 55 kcal/mol. As found for the closed shell sulfonic acids, the perfluoro sulfonyl radical has both the longest and weakest bond. Thus, the factors which contribute to the relative weakness of the C-S bond of the acid persist in the sulfonyl radical. The C-S bond of the trifluoromethanesulfonyl radical is very weak (ca. 10 kcal/mol), having an estimated half-life of only about 0.1 µs at 95 °C (Table IX). There are two important points to emphasize with regard to degradation mechanisms involving sulfonyl radicals. First, from a thermochemical perspective, there is comparatively little difference in the propensity to form the sulfonyl radicals from perfluorocarbon, hydrocarbon or aromatic substrates under dry conditions. The calculated O-H and O-O bonds strengths of the acids and peroxides are similar and thus rates of radical formation are estimated to be well within two orders of magnitude of one another. Second, once the sulfonyl radicals are formed, there are profound differences in the fragmentation rates of the respective families. For example, the long lifetime of an aryl sulfonyl radical at 95 °C (ca. 105 s) virtually guarantees that it will be deactivated by reaction with a hydrogen atom donor, such as hydrogen peroxide, and not fragment to form SO3 and an aryl radical. This is supported by experimental evidence (39, 40). In contrast, the very weak C-S bond of the perfluorosulfonyl radical will fragment to form the perfluororadical on a time scale faster than or competitive with hydrogen atom transfer. In PFSAs, this C-S bond cleavage will afford a side chain terminal fluororadical that has previously been reported in ESR studies (43). The terminal perfluororadical, once formed, will follow the degradation pathway outlined in Figure 6, ultimately forming an oxyradical at the junction with the main chain of the PFSA backbone. This oxyradical can fragment to give an acyl fluoride and another fluorocarbon radical. This is a specific and highly probable mechanism of PFSA main chain scission that has been sought. It is important to note that only a relatively small number of these side chain radicals are required to generate additional chain breaks that lead to degradation rates observed during chemical degradation tests (9, 27). Furthermore, by this mechanism, the side chain damage can be initiated only during very dry conditions although the damage can be propagated via the unzipping process during relatively wet conditions. After great efforts to prevent chemical degradation through creating a pure PFSA, it is rather ironic that the inherent weakness or Achilles heel of PFSA ionomers could potentially be based on the highly acidic, proton conducting group itself. Table IX. Estimated Fragmentation Rates of Sulfonyl Radicals

Radical M05-2X/6-31G(d) Calc Frag Energy

(kcal/mol)

Rounded Fragmentation

Energy (kcal/mol)

Calculated Fragmentation

368 K Rate (s-1)a

Relative Rate

CF3SO3 10.5 10 107 1 CH3SO3 21.8 20 101 10-6 C6H5SO3 34.7 30 10-5 10-12

a. Calculated using Arrhenius expression, k = Aexp(Ea/RT), Assumed pre-exponential term value is 1013 s-1




Figure 5 Calculated (M05-2X/6-31G(d))fragmentation energies of various sulfonic acids and sulfonyl radicals

Figure 6. Propagation of side chain fluororadical to induce main chain scission.

The sulfonyl radical initiation mechanism described above is consistent with a number of other experimental observations. First, this mechanism indirectly provides a rationale for inert nature of perfluorosulfonic acid model compounds in the solution

Fragmentation Energies (kcal/mol)

34.7

10.5

21.8

66.6

76.2

M05-2X/6-31G(d)

1.846

1.771

+ SO3

+ SO3

+ SO3

H5C6

CF3

CH3

H5C6 SO

O

O

H5C6 SO

O

O H

F3C SO

O

O

H3C SO

O

O

F3C SO

O

O H

H3C SO

O

O H+ SO3HCH3

+ SO3HCF3

+ SO3HH5C6 87.1

54.4

56.1

52.4

1.761

1.770

1.849

1.771

CF2 CF

CF2

OCF2CFO CF2CF2SO3

CF3

PP CF2 CF

CF2

OCF2CFO CF2CF2

CF3

PP+SO3

CF2 CF

CF2

OPP

9 HF, 5 CO2

CF2 CF

OP CF2 P+

Main Chain Scission




(29, 44). Because the model compounds are in aqueous solution, the sulfonic acid groups are fully ionized and thus unable to form a sulfonyl radical via reaction with hydroxyl radical or reaction with hydrogen peroxide. In a fuel cell experiment, Fenton and coworkers reported that a PSFA membrane in which all protons were exchanged for alkali metal ions were was stable to OCV conditions (45). Presumably, even though this membrane cannot conduct protons, the reactive species (hydroxyl radical/hydrogen peroxide) are still generated but unable to induce damage because the acid group is deprotonated. The Hydrogen Radical Mechanism

While the sulfonyl radical initiation mechanism described here provides a plausible explanation for chemical degradation involving main chain scissions and also provides a rationale for accelerated degradation under dry conditions, we have also entertained another plausible mechanism of initiation and main chain scission. As stated previously,

When considering the reactivoperating fuel cell, it is necessary to systematically evaluate all possible reactions based on the known chemical composition of the system. In the previous discussion, the hydrogen atom abstraction from hydrogen peroxide to form hydroperoxyl radical and

known reactivity toward this good hydrogen atom donor. There is, however, another good hydrogen atom donor present in an operating fuel cell that has not yet been considered: hydrogen gas. produce H2O and the hydrogen atom (eq. 5) (23, 24). This reaction occurs in the atmosphere and has a second order rate constant of reaction for H2 slightly greater than that with H2O2. Furthermore, hydrogen gas is present in a similar concentration range as hydrogen peroxide therefore the rates of hydroxyl radical with these two species should be comparable under most conditions. H2 2O k = 4.2 x107 M-1 s-1 [5]

2O2, which significantly attenuates or buffers the 2 produces

ery reactive species capable of inducing damage to PFSAs. While not as aggressive as OH, hydrogen radical is very reactive having a standard reduction potential of 2.3 V vs. SHE (46). Unlike OH, the hydrogen atom could be very aggressive toward the C-F bonds of PFSAs. The reactivity of H toward abstraction of fluorine atoms for the C-F bonds of PFSAs is thermodynamically driven by the formation of the very strong H-F bond (136 kcal/mol). Table X summarizes computed and, where available, experimental, enthalpies of reaction for hydrogen atom abstraction of fluorine from various fluorocarbon models. Inspection of the values for perfluoromethane and perfluoroethane reveals that the computed reaction enthalpy values are about 10 kcal/mol higher than experimental values. This is, again, largely due to the systematic error of the theoretical model which underestimates the bond strength of the H-F bond by about 14 kcal/mol (Table V). Recall from earlier discussion that there is a significant decrease in C-F bond strength as substitution increases (Table I). In a PFSA ionomer, the most abundant type by far is a secondary C-F bond. A reasonable estimate for the bond enthalpy is about 115 kcal/mol. If the empirical correction factor of 10 kcal/mol is




applied to the calculated reaction enthalpy for the secondary fluorine atom abstraction reaction by hydrogen atom, the estimated reaction enthalpy is -18 kcal/mol. The weakest C-F bonds of the ionomer occupy the tertiary ether positions. Applying the same empirical correction factor gives an estimated reaction enthalpy of -23 kcal/mol. These estimated values of reaction enthalpy for the secondary and tertiary C-F bonds can be compared to experimental values of perfluoromethane and perfluoroethane of -5 and -9 kcal/mol, respectively. There are no experimental activation energies available for fluorine abstraction reaction by hydrogen for secondary or tertiary C-F bonds but data are available for perfluoromethane and perfluoroethane which are reported to be 44 and 30 kcal/mol, respectively (47). These activation energies are too high to produce kinetically significant rates under fuel cell operating conditions and thus fluorine atom abstraction from primary C-F bonds does not occur. However, given the significant increase in reaction exothermicities with increasing substitution, it is quite reasonable to conclude that the activation energies of the more highly substituted bonds would be considerably lower and correspondingly afford kinetically significant rates under fuel cell operating conditions. We have performed preliminary calculations indicating that abstractions of secondary and tertiary C-F bonds could be kinetically feasible. A kinetically reasonable reaction mechanism of main chain scission via F atom abstraction by hydrogen is shown in Figure 7. As noted during discussion of the sulfonyl radical mechanism, the acceleration of fuel cell chemical degradation does not require large numbers of chain scissions. Table X. Calculated and Experimental Reaction Enthalpies for Fluorine Atom Abstraction by Hydrogen Radical Reaction

B3LYP/6-31G* Calculated BDE

(kcal/mol)

Experimental H298 (kcal/mol)

CF4 3 + HF 5.9 -5.0 C2F6 2F5 + HF 0.6 -8.9 C3F8 n- 3F7 + HF -0.5 n.a. C3F8 i- 3F7+ HF -8.1 n.a. (CF3)3 (CF3)3 -17.8 n.a. (CF3)2(CF3 (CF3)2(CF3O)C + HF -13.2 n.a.

Like all chemical mechanisms, the hydrogen atom abstraction mechanism proposed here cannot be proven but there are number of experimental observations that are consistent with such a pathway. First, selective anode side degradation of PFSA membranes subjected to OCV testing has recently been reported by two groups (10, 48). In one of these studies, the extent of degradation was monitored by analyzing cross sections using FTIR to monitor the number of carboxyl groups which were clearly much higher on the anode side of the MEA (10). In these studies the high degradation is associated with the portion of the MEA with the highest H2 gas concentration which

2. There are, of course, other potential explanations for the anode side degradation including selective generation of H2O2 on the anode due to O2 crossover from the cathode. In another set of experiments, the degradation rates were carefully probed as a function of gas partial pressure (32). In these studies, it was determined that the degradation rate has a nearly second order dependence on hydrogen partial pressure and only a first order dependence on oxygen partial pressure. It is possible that the greater sensitivity of degradation rate to hydrogen concentration signifies another kinetic role for hydrogen gas other than generation of an




oxygenated reactive species at the catalytic surface. Finally, recent in-situ fuel cell ESR spin trapping studies have detected the presence of the hydrogen atom (49, 50). Further experimentation is required to verify that the source of the spin adduct is indeed the

2. While all of the degradation studies summarized above have a number of potential causes, none of them is inconsistent with the operation of a hydrogen radical mechanism. Furthermore, given that the reaction

2 gas is facile and the concentrations of these species are well documented and significant under fuel cell operating conditions, this mode of reactivity must be considered at a potential degradation pathway.

Figure 7. Proposed main chain scission process initiated by fluorine atom abstraction by hydrogen radical. Refinement and Modifications of Curtin Mechanism

As stated earlier, the PFSA degradation mechanism proposed by Curtin is consistent with the observed products of chemical degradation and does not invoke thermodynamically unreasonable intermediates. The mechanism, as commonly presented, is quite linear and direct but, based on the thermochemical analysis discussed here; it does not capture the true complexity of the chemistry of H2O2 even though this reactive intermediate is present in quite significant amounts in an operating fuel cell. Here, a more complete PFSA decomposition mechanism is proposed which is based almost entirely on known thermochemical and kinetic parameters.

It is reasonable to start the discussion with the chemistry of perfluorocarboxylic acid because of it importance as an impurity in early generations of PFSA materials and as an experimentally observed intermediate in the degradation process. Perfluorocarboxylic acids have pKa values near 0 and thus are approximately 50% protonated form in the low pH (pH ~ 0) environment of an operating fuel cell. As discussed above, the degradation is

2O and the very unstable carboxyl radical which rapidly decomposes to give CO2 and the perfluororadical (Figure 8). The linear Curtin mechanism (Figure 2) indicates that the fluororadical reacts

CF2 C CF2

OCF2CFO CF2CF2SO3H

CF3

PPCF2 CF

CF2

OCF2CFO CF2CF2SO3H

CF3

PP

HF

CF2 C CF2

OCF2CFO CF2CF2SO3H

CF3

PPO

CF2 COCF2CFO CF2CF2SO3H

CF3

PO

CF2 P

H2O2

+CF2 COH

PO

HOCF2CFO CF2CF2SO3HCF3

+

342 Fragment

2 HFH2O

HO2CCFO CF2CF2SO3HCF3




likelihood is an intermediate in PFSA degradation, this step proposes the combination reaction of two highly reactive species that are present in very low concentration. Undoubtedly, these two radicals will separately react with other species long before they find each other in the reactive fuel cell matrix. The most likely reaction for both of these radicals is with H2O2 functioning as a hydrogen atom donor. Thus, as shown in equation 1 2O2 react to give a hydrofluorocarbon end group. The hydrofluorocarbon is not a dead end in the degradation mechanism but rather a detour because it is converted back to the

quencher is hydrogen gas which will form the potentially damaging hydrogen radical as the by product. Both of the hydrogen transfer reactions of the fluororadical with H2 and H2O2 are known facile reactions (47). Regarding the pathway from the fluororadical to fluoroalcohol, a new reaction is proposed involving hydrogen peroxide wherein the weak O-O bond of H2O2 is cleaved. In addition to the fluoroalcohol, an equivalent of hydroxyl

3, this reaction is highly exothermic (-68 kcal/mol) and can be rationalized by the formation of a strong C-O bond while cleaving a weak O-O bond. While this reaction is considerably slower than the hydrogen transfer detour reactions, it serves as the siphon the fluororadical to chain shortening degradation. In addition, this reaction provides a pathway to generate OH within the membrane, spatially well removed from a catalyst layer. We have computed the activation energy for this process using DFT and find that the barrier is less than 20 kcal/mol.

Figure 8. Modified main chain unzipping mechanism involving H2O2 and H2 gas.

Conclusion

A comprehensive thermochemical analysis of PFSA ionomers and the reactive oxygen species formed in an operating fuel cell reveals that hydroxyl radical is the only oxidant capable of abstracting a hydrogen atom from a carboxylic acid intermediate and thereby propagating the PFSA main chain unzipping process. Chemical degradation studies of PFSA-based MEAs exhibit accelerating fluoride release rates with time and are consistent with PFSA main chain scissions. Accordingly, two chemically specific, and thermochemically sound, main chain scission mechanisms are proposed. The first involves formation of a sulfonyl radical under dry membrane conditions via either H-atom abstraction from the protonated acid or reaction of the protonated acid with H2O2 to form sulfonyl peroxides. The PFSA-based sulfonyl radical possesses an extremely weak

H2O2+

H2H2O

(CF2CF2)n CF2H

H2OH2O2

(CF2CF2)n CF2 CO2H (CF2CF2)n CF2

(CF2CF2)n CF2H

(CF2CF2)n CF2OH

H2O, CO2

Potentially Damaging

H2O 2 HF

(CF2CF2)n CO2H

H-atom Donor

Slow H-atom Abstractionk ~ 1x106 M-1s-1




C-S bond and rapidly cleaves forming SO3 and a fluororadical. Significantly, the analogous C-S bonds of hydrocarbon and aromatic sulfonyl radicals are much stronger and therefore do not undergo C-S bond cleavage. The second main chain scission mechanism involves hydrogen atoms formed from reaction between hydrogen gas and hydroxyl radical. Hydrogen radical has a high affinity for fluorine bonds and could abstract backbone fluorine atoms at a kinetically relevant rates and form main chain radicals resulting in scission. Based on results of the thermochemical analysis, modifications to the main chain unzipping mechanism were proposed involving hydrogen peroxide and hydrogen gas. Finally, considering the central roles played by OH and H2O2 outlined in this report, an ideal chemical mitigation system must either deactivate these species before they induce damage or prevent their formation altogether.

Acknowledgements

I would like to thank Dr. Han Liu and Dr. Cortney Mittelsteadt of Giner Electrochemical Systems LLC for helpful and insightful technical discussions.

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[ECS 214th ECS Meeting - Honolulu, HI (October 12 - October 17, 2008)] ECS Transactions - The Chemistry of Fuel Cell Membrane Chemical Degradation