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. 2022 Sep 7;126(37):15606–15616. doi: 10.1021/acs.jpcc.2c04566

EPR Study on the Oxidative Degradation of Phenyl Sulfonates, Constituents of Aromatic Hydrocarbon-Based Proton-Exchange Fuel Cell Membranes

Tamas Nemeth †,‡,*, Mikhail Agrachev §, Gunnar Jeschke §, Lorenz Gubler †, Thomas Nauser ‡
PMCID: PMC9512017  PMID: 36176316

Abstract

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Sulfonated aromatic hydrocarbon-based ionomers are potential constituents of next-generation polymer electrolyte fuel cells (PEFCs). Widespread application is currently limited due to their susceptibility to radical-initiated oxidative degradation that, among other intermediates, involves the formation of highly reactive aromatic cation radicals. The intermediates undergo chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups, all leading to performance loss and eventual membrane failure. Laser flash photolysis experiments indicated that cation radicals can also be formed via direct electron ejection. We aim to establish the major degradation pathway of proton-exchange membranes (PEMs). To this end, we irradiated aqueous solutions of phenyl sulfonate-type model compounds with a Xe arc lamp, thus generating radicals. The radicals were trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO), and the formed adducts were observed by electron paramagnetic resonance (EPR). The formed DMPO spin adducts were assigned and relative adduct concentrations were quantified by simulation of the experimental EPR spectra. Through the formation of the DMPO/•SO3– adduct, we established that desulfonation dominates for monoaromatic phenyl sulfonates. We observed that diaryl ether sulfonates readily undergo homolytic C–O scission that produces DMPO/•aryl adducts. Our results support the notion that polyphenylene sulfonates are the most stable against oxidative attack and effectively transfer electrons from DMPO, forming DMPO/•OH. Our findings help to identify durable moieties that can be used as building blocks in the development of next-generation PEMs.

Introduction

Climate change and anthropogenic air pollution are currently one of the greatest challenges of humanity.1 By extensive use of fossil fuels in combustion engines, transportation is responsible for nearly one-third of the greenhouse gas emissions worldwide and is therefore a major contributing factor to global warming.2 Renewable energy sources, in combination with electrochemical devices for energy storage, have partially been able to mitigate these problems. It is conceivable to replace traditional combustion engines with environmentally friendly fuel cell-based systems that directly convert the chemical energy stored in various fuels into electricity with high efficiency and zero emission. Although vehicles powered by polymer electrolyte fuel cells (PEFCs) have been on the market for several years, the technology itself has yet to reach maturity. The main causes are the comparatively fast degradation of fuel cell membranes and electrodes, expensive cell constituents, use of noble metal catalysts, and low production volumes entailing high unit costs. Current state-of-the-art membranes are based on perfluoroalkylsulfonic acid (PFSA) ionomers.3 This ionomer class is of high chemical stability, yet its synthesis is costly and involves the formation of hazardous intermediates. Additionally, enhanced gas crossover and material softening preclude high-temperature (≥100 °C) applications.4,5 Non- and partially fluorinated aromatic hydrocarbon-based ionomers offer cost-effective, more environmentally friendly alternatives to PFSA that show superior performance,6−9 albeit with increased susceptibility to radical-induced degradation. Before successful commercialization, this challenge needs to be addressed.10 Oxidizing radical species are formed during the operation of the fuel cell in the presence of humidified gases, H2/O2, and the noble metal catalyst.4 Among the formed radical species, HO• has the highest reactivity. Hence, studies that target ionomer degradation primarily discuss the action of this species.

Generally, oxidative degradation of the aromatic hydrocarbon-based ionomers (Ar) is initiated by the near-diffusion-limited electrophilic attack of HO• on the aromatic ring, forming hydroxycyclohexadienyl radicals (HO-adducts), Scheme 1, reaction (1).5 Attack on alkyl substituents of the membrane is at least an order of magnitude slower and therefore of little relevance.11 The formed HO-adducts have been identified and characterized by electron paramagnetic resonance (EPR)12 and pulse radiolysis methods.13−16

Scheme 1. Follow-Up Reactions of Oxidative Attack Initiated by HO•.

Scheme 1

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At low pH, acid-catalyzed water elimination from HO-adducts results in the formation of cation radicals, reaction (2).12,17 Cation radicals can be one-electron-reduced and thus “repaired” via reaction (3). Therefore, they have been identified as key intermediates in the prolongation of the lifetime of PEMs.16,18

In the absence of a repair reaction, both HO-adducts and cation radicals undergo further degradation. Oxygen as a biradical reacts with HO-adducts to form peroxyl radicals, which undergo HOO• elimination, yielding reactive phenols, reactions (4) and (5).19 Alternatively, intermolecular rearrangement of peroxyl radicals yields bicyclic compounds and eventually ring-opened derivatives.

Chain cleavage (dealkylation/dearylation) and the loss of protogenic sulfonate groups have been proposed as follow-up or parallel degradation reactions, leading to inevitable membrane failure.20−22 Theoretical studies involving DFT calculations hypothesized that desulfonation is the energetically favorable degradation route for polyaromatic model compounds and that moieties containing heteroatoms are possible weak points susceptible to chain scission.23 We recently showed that an increase in electron density of the aromatic ring has a favorable impact on the stability against radical-induced degradation.16

During the operation of the FC, the cleaved degradation products are washed out due to the electroosmotic drag.24 Although “simple” desulfonation leaves the bulk material intact and “only” accounts for performance loss, chain cleavage further damages the polymer and may be the cause of premature mechanical failure through pinhole formation.25

In an attempt to discern between dealkylation/dearylation and desulfonation as possible major routes of ionomer degradation, we identified, by EPR, the short-lived degradation products of photoexcited sulfonated aromatic model compounds. Phenyl sulfonates are typical constituents of non- and partially fluorinated hydrocarbon-based PEMs. Selected representatives of this class, potassium phenyl sulfonate (PS), potassium 4-(tert-butyl) phenyl sulfonate (BPS), and potassium 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) were chosen as model structures in this study. Tert-butyl group containing derivatives represent polystyrene-based PEMs traditionally produced by irradiation grafting.26,27 The electron-donating methoxy group of BMPS accounts for changes in the electron density of the parent polymer. Aromatic model compounds, potassium 4,4′-oxydiphenylsulfonate (4SPPS), potassium 4-phenoxybenzenesulfonate (4PPS), and potassium biphenyl-4-sulfonate (4DPS), were selected to account for the structural variety of polyphenylene- and polyether-type membranes (Figure 1). All derivatives contain cleavable alkyl, alkoxy, aryl, aryloxy, and/or sulfonate groups.

Figure 1.

Figure 1

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Structure of the aromatic sulfonates, selected as model compounds of this study.

Due to the transient nature of the generated radical species, they could not be detected directly by CW EPR spectroscopy. Therefore, the nitrone spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) has been employed.

DMPO forms characteristic oxygen-, sulfur- or carbon-centered spin adducts with radicals and therefore has been extensively used to investigate chemical and biological reactions involving free radicals, according to Scheme 2.28,29

Scheme 2. General Mechanism for the Spin Trapping by DMPO.

Scheme 2

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Apart from the assignment of spin adducts in relation to the stability of the model compounds, we also show below that the HO• adduct of DMPO can be generated in the absence of free HO• and that the nature and ratio of formed radical spin adducts depend on the pKa of formed radical intermediates.

Experimental Section

Chemicals

Model compounds potassium phenyl sulfonate (PS), potassium 4-(tert-butyl) phenyl sulfonate (BPS), potassium 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS), potassium 4,4′-oxydiphenylsulfonate (4SPPS), potassium 4-phenoxybenzenesulfonate (4PPS), and potassium biphenyl-4-sulfoante (4DPS) were synthesized as described earlier.16,30,31 Sulfuric acid 95% (Fischer Scientific) was used as received and diluted as necessary. Potassium phosphate buffer (KPi) was prepared from potassium phosphate dibasic and potassium phosphate monobasic. Ultrapure water was provided by a Milli-Q or Evoqua Ultra Clear UV Plus water purification system.

Laser Flash Photolysis

Laser flash photolysis (LFP) experiments were carried out with an Applied Photophysics LKS 50 instrument (Leatherhead, U.K.) equipped with a Quantel Brilliant B Nd:YAG Laser (Les Ulis, France), of which the fourth harmonic was used. A Tektronix DPO 3054 oscilloscope (Beaverton, USA) has been used instead of the original part (LKS 50). Measurements were performed in a 1 cm quartz fluorescence cuvette (Hellma). Solutions were air-saturated. Unless stated otherwise, samples were irradiated only once. Photolysis of 25 μm – 0.5 mM aqueous sample solutions (10 mM H2SO4) was performed.

Electron Paramagnetic Resonance

EPR spectra were recorded at room temperature with a Bruker EMX X-band spectrometer equipped with a standard rectangular Bruker EPR cavity (ER4102ST). Aliquots of 0.05 mL of aqueous sample solutions were introduced into the resonator by means of a 1 mm outer diameter quartz capillary. Solutions were air-or argon-saturated. In the case of argon saturation, samples were evacuated and refilled with argon using a Schlenk-line, then the argon-saturated solutions were transferred into a glovebox, where the EPR capillaries were filled and sealed. All CW EPR spectra were acquired at room temperature with the following spectrometer settings: microwave frequency ≈ 9.8 GHz, center field = 3405 G, sweep width = 150 G, modulation amplitude = 0.5 G, modulation frequency = 100 kHz, microwave power = 2.012 mW, power attenuation = 20 dB, conversion time = 40.96 ms, and time constant = 20.48 ms, while all measured g-factors were offset-corrected against a known standard (i.e., free radical 1,1-diphenyl-2-picrylhydrazyl). The EPR spectra were continuously acquired during the irradiation and separately stored using a 2D acquisition mode, thus enabling a time-resolved monitoring of the process. All of the spectra were simulated using the EasySpin package.32

CW Photolysis

Aqueous solutions of model compounds (50–200 mM) and DMPO (10–100 mM) in 10 mM H2SO4 or 0.1 mM H2SO4 or 100 mM KPi buffer were photoexcited during the EPR measurements with a 75 W Xe arc lamp.

Results

Laser Flash Photolysis

Photochemistry deals with the interaction of matter and low-energy photons resulting in electronic excited states and follow-up photochemical reactions, with the applied energy deposited directly on the chromophore.33 Radiation chemistry, on the other hand, involves the interactions of ionizing radiation with matter in a mass proportional manner. The energy is quantitatively deposited on the solvent, which is then ionized and initiates follow-up reactions with solutes.33 We recently showed the transient absorption spectra of BMPS cation radicals produced in pulse radiolysis experiments, which exhibit an absorbance maximum at 450 nm.16 Under certain conditions, laser flash photolysis (LFP) can be a convenient complementary method to pulse radiolysis to generate radicals.34

When we applied short (5 ns) pulses of 266 nm light with an energy of 10 mJ to the air-saturated solutions of BMPS, we observed the formation of the same absorption spectrum as in pulse radiolysis experiments assigned to BMPS•+ (Figure 2). We assume that single-electron ejection takes place according to reaction (6).

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Figure 2.

Figure 2

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Transient absorption spectra 1 μs (red squares) and 50 μs (black triangles) after the flash, obtained from time-resolved absorbance readings, measured in air-saturated solutions that contained 0.5 mM BMPS and 10 mM H2SO4.

Then we extended the LFP experiments to all structurally related model compounds and recorded the spectra for the electronic excited states (Figure S1). In the case of PS and BPS, we did not observe any absorbance changes in the LFP experiments, which could mean either that the formed products absorb only in the UV region or that the excited state products are very short-lived (see also SI).

BMPS•+ formed via LFP has a lifetime of <10 μs. Analogously, a similarly short lifetime was observed for the products of excitation for 4PPS, 4SPPS, and 4DPS. The quick decay of the formed intermediates hindered us from detecting them directly by CW EPR; we therefore used the nitrone spin trap DMPO in the follow-up experiments.

Electron Paramagnetic Resonance

Phenyl Sulfonates

In this section, we present the EPR spectra of DMPO adducts generated by irradiation of aqueous solutions of monoaromatic phenyl sulfonates. Reported rates of spin trapping by DMPO strongly depend on the trapped radical, with both steric and electronic effects being at play. Therefore, we used high spin trap concentrations to maximize the trapping rate and, consequently, the EPR signal. In the cases of both PS and BPS, we did not observe the formation of DMPO adducts, while photolysis of the methoxy group containing BMPS resulted in the formation of DMPO radical adducts.

First, we irradiated an aqueous solution of 100 mM BMPS and 10 mM DMPO in 10 mM sulfuric acid (Figure 3). No EPR signal was detected in control experiments in the absence of BMPS or in the absence of DMPO in accordance with the comparatively short lifetime of BMPS•+. We assume that under continuous irradiation in the first step, BMPS•+ is formed that reacts with DMPO or undergoes further degradation. The spectra represented in Figure 3 are a result of prolonged irradiation, approximately 12.5–16 min (average of scans 30–39), until the maximum intensity was reached. During the buildup (Figure 4), the relative yield of radicals did not change, and the experimental spectrum could be simulated as a sum of two components (Figure 3, left). The dominating adduct, 90% of the total intensity, was simulated with aN1 = 14.7 G, aHβ1 = 15.9 G, and assigned to the DMPO/•SO3– adduct.29 The minor component, 10% of the total intensity, was simulated with aN2 = 15.0 G, aHβ2 = 15.0 G, and could be assigned to the DMPO/•OH adduct.29 From a first-order fit to the data set, pseudo-first-order rate constants of kobs,buildup = 3 × 10–3 s–1 and kobs,decay = 6 × 10–4 s–1 were obtained for the consecutive buildup and decay reactions, Figure 4. In the follow-up experiments, we used a higher DMPO concentration of 100 mM, which improved the signal-to-noise ratio.

Figure 3.

Figure 3

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Left: (A) Formation of radical adducts during the irradiation of BMPS (100 mM) and DMPO (10 mM) in 10 mM sulfuric acid (black). Average of scans 19–25 is shown. Computer simulation of the DMPO/•SO3– (aN = 14.7 G, aHβ = 15.9 G) and DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) radical adducts (red). (B, C) Simulation of the separated components of the simulated spectrum. Right: (A) Formation of radical adducts during the irradiation of BMPS (100 mM) and DMPO (10 mM) in 10 mM sulfuric acid, pH 2 (black). Average of scans 30–39 is shown. Computer simulation of the DMPO/•SO3–, DMPO/•OH, and DMPO/•alkyl (aN = 16.4 G, aHβ = 22.7 G) radical adducts (red). (B–D) Simulation of the separated components of the simulated spectrum.

Figure 4.

Figure 4

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The buildup and decay of EPR signal intensity (black circles) as a function of time recorded during the irradiation of BMPS (100 mM) and DMPO (10 mM) in 10 mM sulfuric acid; apparent rate constants obtained from the fit (red line): kobs, buildup = 3 × 10–3 s–1, kobs, decay = 6 × 10–4 s–1.

In the course of the decay (Figure 3, right), at the expense of the DMPO/•SO3– adduct, a new component appeared and was assigned to a DMPO/•alkyl adduct (aN3 = 16.4 G and aHβ3 = 22.7). Simulation showed a composite spectrum of DMPO/•SO3– (75%), DMPO/•alkyl adduct (15%), and DMPO/•OH (10%).35 The mechanism of adduct formation is discussed in detail in the Discussion section of the manuscript.

Reactant concentration and changes in pH can have an apparent effect on the kinetics of spin trapping and on the nature of formed spin adducts.36 Therefore, we systematically changed the concentration of BMPS from 50 to 100 mM and to 200 mM. Figure 5 displays how this affects the estimated component ratio. Figure 6 shows the changes brought about by a pH increase from acidic to neutral. In all cases, the initially formed adducts were analyzed as the average of the spectra recorded during the buildup.

Figure 5.

Figure 5

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Effect of concentration: (A, D, G) Formation of radical adducts during the irradiation of 50 mM (A) or 100 mM (D) or 200 mM (G) BMPS and DMPO (100 mM) in 10 mM sulfuric acid, pH 2 (black). Average of scans 3–6 is shown. Computer simulation of the DMPO/•SO3– (aN = 14.7 G, aHβ = 15.9 G) and DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) radical adducts (red). (B, C, E, F, H, I) Simulation of the separated components of the simulated spectra (A, D, G), respectively.

Figure 6.

Figure 6

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Effect of pH. Left: (A) Formation of radical adducts during the irradiation of BMPS (100 mM) and DMPO (100 mM) in 0.1 mM sulfuric acid (black). Average of scans 3–6 is shown. Computer simulation of the DMPO/•SO3– (aN = 14.7 G, aHβ = 15.9 G) and DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) radical adducts (red). (B, C) Simulation of the separated components of the simulated spectra (A). Right: (D) Formation of radical adducts during the irradiation of BMPS (100 mM) and DMPO (100 mM) in 100 mM KPi buffer (black). Computer simulation of the DMPO/•SO3– (aN = 14.7 G, aHβ = 15.9 G) and DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) radical adducts (red). (E, F) Simulation of the separated components of spectra (D).

Derivatives of Biphenyl and Diphenyl Ether

Irradiation of an aqueous solution of 100 mM 4PPS and 100 mM DMPO in 10 mM sulfuric acid resulted in the formation of a two-component spectrum (Figure 7). Analysis of the initially formed adducts showed that both DMPO/•OH (60%) and DMPO/•aryl (40%, aN4 = 15.8 G, aHβ4 = 24.4 G) adducts are present.37 Prolonged irradiation did not change the component ratio. Analogously, photolysis of a 100 mM 4PPS and 100 mM DMPO in 100 mM KPi buffer (pH = 7) solution resulted in the formation of an identical component ratio, albeit with higher absolute intensity.

Figure 7.

Figure 7

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Left: (A) Formation of radical adducts during the irradiation of 4PPS (100 mM) and DMPO (100 mM) in 10 mM sulfuric acid (black). Average of scans 3–6 is shown. Computer simulation of the DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) and DMPO/•aryl (40%, aN4 = 15.8 G, aHβ4 = 24.4 G) radical adducts (red). (B, C) Simulation of the separated components of the simulated spectra (A). Right: (D) Formation of radical adducts during the irradiation of 4PPS (100 mM) and DMPO (100 mM) in 100 mM KPi buffer (black). Results of computer simulation (red) and the separated components of spectra (E, F).

Irradiation of a 100 mM 4DPS and 100 mM DMPO in 10 mM sulfuric acid gave rise to a single product, DMPO/•OH. No byproducts were formed, even after prolonged irradiation. At neutral pH, a more intense single-component spectrum was recorded (Figure 8).

Figure 8.

Figure 8

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Left: Formation of radical adducts during the irradiation of 4DPS (100 mM) and DMPO (100 mM) in 10 mM sulfuric acid (black). Average of scans 3–6 is shown. Computer simulation of the DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) radical adducts (red). Right: Formation of radical adducts during the irradiation of DPS (50 mM) and DMPO (100 mM) in 100 mM KPi buffer (black). Computer simulation of the DMPO/•OH (aN = 15.0 G, aHβ = 15.0 G) radical adduct (red).

Discussion

DMPO/•OH Formed in the Absence of HO•

In the presence of both BMPS and DMPO, we observed the formation of DMPO/•OH, DMPO/•SO3–, and DMPO/•alkyl spin adducts (Figure 3) during continuous photoexcitation. In the case of 4PPS, we could identify DMPO/•OH and DMPO/•aryl spin adducts (Figure 7). Meanwhile, for 4DPS, DMPO/•OH dominated the spectrum (Figure 8).

While it is common to trap HO• as DMPO/•OH adduct, HO• itself was not readily formed in our experiments: based on the structures of BMPS, 4PPS, and 4DPS, Figure 1, the formation of HO• cannot be expected by their photolysis. Therefore, a different reaction is responsible for the presence of DMPO/•OH. Apart from the direct spin trapping reaction involving DMPO, Scheme 2, two alternate reactions yield the same product—inverted spin trapping and the Forrester–Hepburn reaction.29,38

Both consist of consecutive one-electron oxidation and nucleophilic addition steps in reversed order (Schemes 3 and 4) and have often been responsible for misleading artifacts in biology.

Scheme 3. Inverted Spin Trapping.

Scheme 3

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Scheme 4. Forrester–Hepburn Reaction.

Scheme 4

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Since addition of OH– to DMPO is not feasible under our reaction conditions (Scheme 4), the formation of DMPO/•OH can only be explained in terms of the inverted spin trapping reaction (Scheme 3). Cation radicals, formed via direct photo ejection in reaction (6), are electron-deficient species and have high electrode potentials,39 capable of oxidizing DMPO despite its relatively high electrode potential of E°((DMPO)•+/DMPO) = 1.63 V,40reaction (7). The formed DMPO•+ is highly reactive toward nucleophiles (Scheme 3), such as water, and yields DMPO/•OH, according to reaction (8).

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We did not observe the formation of DMPO/•OH in control experiments performed in the absence of model compounds.

Fate of Hydrated Electrons

If photo-irradiation results in electron ejection, cation radicals and hydrated electrons are formed according to reaction (6). Although several reactions are described in the literature as possible competing reactions for hydrated electrons (see Table S1 in the SI), due to the significantly higher concentration of both DMPO and the respective phenyl sulfonate, follow-up reactions involving them must dominate at neutral pH (Table 1). Therefore, the formation of DMPO and phenyl sulfonate anion radicals may be assumed, reactions (9) and (10).41,42 Under acidic conditions, established by 10 mM H2SO4, protons will effectively compete for hydrated electrons, reaction (11).11 Pseudo-first-order rate constants (k′) are calculated by multiplying the reported rate constants (k) by the solute concentration, and the obtained k′ can be used for direct comparison and to calculate the expected product distribution (yield) under different conditions.

Table 1. Competing Reactions for the Hydrated Electron at Different pH.

reaction k (M–1 s–1) conc (mM) k′ (s–1) yield (%)
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3 × 109 100 3 × 108 43a, 33b
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4 × 109c 100 4 × 108 57a, 44b
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2 × 1010 10 2 × 108 –a, 22b
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a

Calculated yield at pH = 7.

b

Calculated yield at pH = 2.

c

The rate constant reported for benzene sulfonic acid was used.42

DMPO•– and Ar•–, formed via reactions (9) and (10), rapidly protonate, even at neutral pH, and produce DMPO/•H and cyclohexadienyl radicals (H-adducts), reactions (12) and (13). Alternatively, aromatic radical anions with suitable leaving groups are highly reactive.42 Accordingly, they may dissociate into an anion and a carbon-centered aromatic radical, reaction (14).43

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DMPO/•H and phenyl sulfonate H-adducts are also formed via the addition of H•, reactions (15) and (16).44 The latter may react with oxygen to form peroxyl radical intermediates that release hydroperoxyl radicals in a consecutive step, reaction (17), analogously to HO-adducts of phenyl sulfonates, (reaction (3)).

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We did not observe the formation of DMPO/•H or DMPO/•OOH in our experiments. Similarly, DMPO/•H was absent when aqueous H2O2-swollen SPEEK-type ionomers were UV-irradiated.45

The Absence of DMPO/•H and DMPO/•OOH Adducts

We established above that the presence of DMPO/•OH in our experiments is not related to HO• formation. Therefore, we do not expect that the formation of DMPO/•OOH is suppressed by the several orders of magnitude faster spin trapping of HO•.44 The radical HOO• may undergo disproportionation to H2O2 and O246 and decompose to HO•,47 while DMPO/•OOH can be reduced or decomposed to DMPO/•OH. To explain the absence of DMPO/•OOH, these reactions would need to be very fast, given the high DMPO concentration used and the fact that we observe adducts of larger radical species. We therefore performed control experiments in the absence of air, under argon saturation, to probe the significance of the HOO• pathway. Results were identical to the ones observed under air saturation. Therefore, we speculate that the HOO•-forming reactions (5) and (17) are of little relevance here.

We may rationalize the absence of DMPO/•H by speculating that the rate of reaction (15) is overestimated. In an analogy, Madden and Taniguchi reported a more modest rate constant for the reaction of DMPO with the hydrated electron than what was estimated previously and showed that the yield of reaction (15) was only 45% due to the formation of an undetectable side product.41

Typically, one or two orders of magnitude lower rate constants are reported for reaction (16), the addition of H•, than for reaction (1), the addition of HO•. This is peculiar considering that both HO• and H• are strongly electrophilic in nature.48 Addition of HO• or H• is a two-step process. First, a π-complex is formed, which transforms into a stable σ-bonded radical in a consecutive step.15 Although we accept a factor of five higher rate constants for reaction (1), we may assume that the reported rate constants for H-addition are systematically underestimated.

If these explanations hold, the formation of H-adducts, reactions (13) and (16), dominates under both neutral and acidic conditions. These carbon-centered radicals are short-lived, decay in less than 50 μs,49 and based on the absence of DMPO/•Ar(-H) in our experiments, do not form stable adducts with DMPO.

If light irradiation produces cation radicals with a low quantum yield, the highly oxidizing and transient excited state aromatics may react with the bulk, according to reaction (18).

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The fate of the formed cation and anion radicals has been discussed in detail above. Reaction (18) does not involve the formation of hydrated electrons. Therefore, it may provide an alternative explanation for the absence of DMPO/•H, reactions (12) and (15), in our experiments.

Formation of DMPO/•SO3–, DMPO/•Alkyl, and DMPO/•Aryl Adducts

We established that if cation radicals are formed, they are in a dynamic equilibrium with HO-adducts through reaction (2).16 Both HO-adducts and cation radicals decay via desulfonation or dealkylation/dearylation. Additionally, HO-adducts may react with O2 to form phenolic products and hydroperoxyl radicals, reaction (3).

We observed the formation of DMPO/•SO3– in the case of BMPS and 4SPPS (Figures 3 and S2): homolytic cleavage of the sulfonate group yields •SO3–, a nucleophilic species that reacts with DMPO at lower rates than HO•.28 In contrast to DMPO/•OH, DMPO/•SO3– is not readily formed through the inverted spin trapping mechanism: if DMPO is oxidized to its cation radical, it will predominantly react with water, the only nucleophile present in abundance. Therefore, DMPO/•SO3– can only be a product of direct spin trapping by DMPO, reaction (19). Similarly, dealkylation of irradiated BMPS gives rise to methyl and/or tert-butyl radicals, also trapped by DMPO, reactions (20) and (21), and the formed alkyl spin adducts are practically indistinguishable.35

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Alkyl adducts could only be detected after prolonged irradiation, suggesting that they are not primary degradation products of BMPS photoexcitation.

In contrast, aryl adducts were readily formed for aryl ethers 4PPS and 4SPPS (Figures 7 and S2) in reaction (22), in line with the reported propensity of such compounds for ether bond scission.45

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Aryl-oxygen bond cleavage produces both phenyl and aryloxy radicals (Scheme 5), yet we did not observe DMPO/•OAr. The EPR study on SPEEK-type ionomers by Pinteala and Schlick describes that the choice of solvent can be crucial: in DMSO, DMPO/•OAr was the dominating species, while in aqueous solutions, DMPO/•Ar was the dominating one.45 This might explain the absence of DMPO/•OAr adducts in our experiments, even in control experiments performed at low temperature (70 K).

Scheme 5. C–O Bond Scission for 4PPS.

Scheme 5

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Effect of Concentration

Increasing BMPS concentration affected the ratio of [DMPO/•OH] / [DMPO/•SO3–] (Figure 5). Simulations showed that increasing the concentration of BMPS from 50 mM via 100 mM to 200 mM while keeping the DMPO concentration at a constant 100 mM leads to a decrease in the ratio from 4 to 3 and eventually to 2.33, suggesting that at higher phenyl sulfonate concentration, desulfonation prevails. In pulse radiolysis experiments on oligomers of α-methylstyrene sulfonates, we recently observed that both the buildup and decay of radical intermediates are significantly accelerated at high ionic strength.50 Our present results also suggest that a change in ionic strength can facilitate degradation and offer an explanation for the change in product distribution. The ratio of DMPO radical adducts did not show any changes with concentration for model compounds DPS, 4PPS, and 4SPPS.

Effect of pH

In the case of BMPS, an increase in the pH has a profound effect on the ratio of [DMPO/•OH] / [DMPO/•SO3–]. This ratio changes from 4 at pH 2 to 0.18 at pH 7 (Figure 6). This finding is in direct support of the notion that equilibrium (2) is pH-dependent. At high pH, it is shifted toward the less stable HO-adduct, in accordance with its reported pKa,Ar•+ ≈ 2–3.16 As a result, fewer cation radicals are available for DMPO oxidation, reaction (7), directly affecting the ratio. We estimate that at pH 7, the EPR signal is approximately twice as strong as the one at pH 4. Higher EPR intensities observed at pH 7 also suggest the formation of more radical species, indicating decreased stability of the formed HO-adducts, in line with our earlier results.16

The product ratio for the other model compounds, 4PPS and 4DPS, remained unchanged upon increasing the pH from 2 to 7 (Figures 7 and 8), in line with their lower pKa,Ar•+ than that of BMPS.

The stability of DMPO spin adducts depends on pH.51 However, at the time scale of our experiments (≪120 s/recorded spectrum), this has little bearing on the recorded intensities. The observably higher EPR intensities at pH 7 support the general notion of decreased radical stability for aryl sulfonates at elevated pH.

Implications for FC Membranes

It was established earlier that for PEMs, heteroatoms in the polymer chain are susceptible to radical-induced degradation.45 In accordance, we observed the formation of an appreciable amount of DMPO/•Ar adducts for the aryl ether-type model compounds 4PPS and 4SPPS originating from the cleavage of the C–O bond.

We recently reported on the effect of electron density of aromatic compounds on their radical-induced degradation.16 We found that electron-rich derivatives are more stable because they have a higher pKa,Ar•+; therefore, the equilibrium between HO-adducts and cation radicals, equilibrium (2), lies on the side of the more protected cation radical. We also showed that cation radicals could be one-electron-reduced, “repaired” (reaction (3)).

In the present study, we speculate that the DMPO/•OH adduct is formed through one-electron oxidation of DMPO by Ar•+ followed by nucleophilic addition of water, reactions (7) and (8). The implications are twofold: first, DMPO/•OH was observed only in the case of the relatively electron-rich model compounds (BMPS, 4PPS, and 4DPS), where cation radicals are sufficiently long-lived. Second, electron transfer from DMPO to Ar•+ can be considered a repair reaction of cation radicals. The apparent effect of the electron density of aromatics on stability can be implemented during the rational design of PEMs. Although we did not observe any product formation for PS and BPS, based on our earlier observations, we expect these electron-poor derivatives to be less stable than BMPS, 4PPS, and 4DPS.16

Presently, membrane development is increasingly focused on the synthesis of polyphenylene-type ionomers, partly due to their inherently high durability in FC tests.10,52,53 For biphenyl-type model compound 4DPS, a structural analogous compound of polyphenylenes, we did not observe the formation of any degradation products. This suggests a good stability of this compound against desulfonation or dearylation, confirming its applicability as a building block of PEMs.

Parrondo et al. reported on the reactive oxygen species-mediated degradation of anion exchange membrane-based FCs (AEMFC). Based on 31P NMR results that indicated the formation of both HO• and HOO• adducts of the nitrone spin trap 5-diisopropoxy-phosphoryl-5-methyl-1-pyrroline N-oxide (DIPPMPO), the authors suggested a mechanism involving the attack of dioxygen and HO– on the membrane constituents.54 In a related paper, in situ EPR measurements were performed on a micro-AEMFC inserted into an EPR resonator; the authors concluded that HO•, HOO•, and H• are formed in AEMFCs, in support of the original mechanism.55 In a recent comment, Meyerstein advises caution with interpreting the detection of spin trap/•OH adduct as evidence for HO• formation.56 Herein, our results provide further evidence for a possible erroneous assignment by showing that indeed DMPO/•OH can be formed by DMPO oxidation and subsequent hydrolysis under conditions where HO• is not expected to be present.

Conclusions

We have presented the detection of radicals generated by photoexcitation of PEM model compounds using the nitrone spin trap DMPO. Dependent on the studied model compound, spin adducts DMPO/•OH, DMPO/•SO3–, DMPO/•alkyl, and DMPO/•aryl were readily formed.

DMPO/•OH originates from the inverted spin trapping reaction, while the other adducts are the products of direct spin trapping. Our results suggest that desulfonation is preferred to dealkylation for simple phenyl sulfonates, aryl ethers readily undergo homolytic C–O scission, and polyphenylenes are less susceptible to oxidative degradation. Through the use of simple model compounds, our findings attempt to give an account of PEM aging and may be applied for the development of more durable next-generation PEMs.

Acknowledgments

The authors would like to thank Mr. Tym de Wild (Paul Scherrer Institute) and Dr. Reinhard Kissner (ETH Zurich) for helpful discussions.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.2c04566.

  • The Supporting Information file contains the results of the laser flash photolysis experiments, EPR study on 4SPPS model compound, and considerations of possible competing reactions (PDF)

Author Contributions

The manuscript was written through contributions of all authors. T.N. synthesized, purified, and characterized the model compounds, and was responsible for the curation and analysis of data. M.A. was also responsible for the curation and analysis of data. G.J. and T.N. conceptualized this study and, together with T.N. and M.A., were responsible for the design of the experiments and interpretation of the data. L.G. was essential for the conceptualization of the project, securing funds, and project supervision. All authors have given approval to the final version of the manuscript.

The authors wish to thank the Swiss National Science Foundation (SNSF) for project funding (Grant No. 175493).

The authors declare no competing financial interest.

Supplementary Material

jp2c04566_si_001.pdf (384.3KB, pdf)

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