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On the Role of Hydrogen and Peroxyl Radicals in the Degradation of Aromatic Hydrocarbon-Based Proton Exchange Membranes

This study demonstrates that while hydroxyl radicals are the primary agents driving the chemical degradation of aromatic hydrocarbon-based proton exchange membranes, hydrogen and peroxyl radicals contribute negligibly to substrate loss under acidic conditions relevant to fuel cell operation.

Original authors: Lorenz Gubler, Zongyi Han, Yingying Yu, Thomas Nauser, Tamas Nemeth

Published 2026-07-25

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CC BY 4.0

Original authors: Lorenz Gubler, Zongyi Han, Yingying Yu, Thomas Nauser, Tamas Nemeth

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where we can turn sunlight and wind into clean, storable fuel—hydrogen—that powers our cars and heats our homes without polluting the air. To make this happen, we rely on tiny but mighty machines called fuel cells and electrolyzers. At the heart of these machines is a special plastic sheet, a "Proton Exchange Membrane" (PEM), which acts like a bouncer at a club: it lets positive hydrogen ions (protons) pass through to do their work but blocks electrons and gases from mixing and causing a mess. For a long time, scientists used membranes made of fluorine-based plastics (like Teflon cousins) because they were tough. But now, because of concerns about those fluorine chemicals in the environment, researchers are switching to membranes made from aromatic hydrocarbons—basically, rings of carbon atoms that look like a honeycomb. The problem? These new, eco-friendly membranes are a bit fragile. Inside the machine, they are constantly under attack by invisible, hyper-active "chemical assassins" called radicals. These radicals are like tiny, frantic bees that have lost their queen; they zip around, biting and breaking the membrane's structure, eventually causing the machine to fail. The big question for scientists has always been: Which of these chemical assassins is the real villain? Is it the famous hydroxyl radical (HO•), or are the lesser-known hydrogen radicals (H•) and peroxyl radicals (HOO•/ROO•) the ones actually tearing the membrane apart?

In this study, a team of researchers from Switzerland decided to play detective to solve this mystery. They couldn't just look inside a running fuel cell easily, so they built a controlled "crime scene" in a lab. They used a machine that shoots gamma rays at water (a process called radiolysis) to create a steady stream of these chemical radicals, mimicking the chaos inside a real fuel cell. To see who was doing the damage, they used two different "model suspects"—small molecules that look like the building blocks of the new hydrocarbon membranes. They then set up a series of experiments to isolate the radicals, kind of like putting different suspects in separate interrogation rooms.

First, they let all the radicals loose to see how much damage they caused. As expected, the membranes got chewed up, and the damage got worse when oxygen was present or the solution was less acidic. But the real magic happened when they started using a "radical scavenger" (a chemical called tert-butanol) to act as a bodyguard. This bodyguard is very good at catching the famous hydroxyl radical (HO•) but leaves the others alone.

When they removed the hydroxyl radical from the mix, the damage to the membrane models almost completely stopped. This was a huge clue. It turned out that the hydrogen radicals (H•), which some people thought might be dangerous, were actually quite harmless to these carbon rings. In fact, when a hydrogen radical tried to attack the membrane, it just bounced off or restored the molecule to its original state, like a friendly tap on the shoulder that didn't leave a bruise. Similarly, the peroxyl radicals (HOO• and ROO•), which form when oxygen is around, were also found to be mostly ineffective. They were too slow or too bulky to cause any real harm compared to the hydroxyl radical.

The researchers found that the hydroxyl radical (HO•) is the undisputed boss of destruction. It's the only one that really knows how to break the carbon rings apart. The study suggests that if we want to build better, longer-lasting fuel cells, we shouldn't waste too much energy worrying about the hydrogen or peroxyl radicals. Instead, we need to focus our efforts on protecting the membrane from the hydroxyl radical. The key takeaway is that the new hydrocarbon membranes are actually quite tough against most of the chemical chaos; they just need a shield against that one specific, hyper-aggressive hydroxyl attacker. By understanding exactly who the villain is, scientists can now design better "bodyguards" (antioxidants) to keep our green energy machines running smoothly for a long time.

Technical Summary: On the Role of Hydrogen and Peroxyl Radicals in the Degradation of Aromatic Hydrocarbon-Based Proton Exchange Membranes

Problem Statement
Chemical degradation by free radicals is a primary limitation on the durability of aromatic hydrocarbon (HC)-based proton exchange membranes (PEMs) used in fuel cells and water electrolyzers. While the role of the hydroxyl radical (HO•) in degrading these materials is well-established, the specific contributions of hydrogen radicals (H•) and peroxyl radicals (HOO•/ROO•) remain poorly understood. Current mitigation strategies focus heavily on HO•, yet the potential reactivity of H• and HOO•/ROO• toward aromatic ionomers lacks quantitative data. Clarifying the roles of these less reactive species is essential for designing more durable HC membranes and developing targeted antioxidant strategies.

Methodology
The study employs the $\gamma$ -radiolysis of water using a $^{60}$ Co source to generate reactive species at a controlled, steady-state rate, mimicking the radical exposure experienced by ionomers in operating devices. The experimental design utilizes specific scavenging conditions to isolate the effects of individual radical types on two aromatic sulfonate model compounds: 4-(tert-butyl) phenylsulfonate (BPS) and p-diphenylsulfonate (DPS). These models represent structural motifs found in radiation-grafted and polyphenylene-based PEMs, respectively.

The experimental approach involves:

Radical Generation: Water radiolysis produces HO•, hydrated electrons ( $e^-_{aq}$ ), and H•.
Selective Suppression:
- HO• Suppression: Tert-butanol (tBuOH) is used at high concentrations (1 M) to scavenge HO•, converting it into a carbon-centered radical (•tBuOH) that is less reactive toward the aromatic substrates.
- H• Isolation: In $N_2$ -saturated, acidic solutions (pH 0 and 2), hydrated electrons are quantitatively protonated to form H•.
- HOO•/ROO• Isolation: In $O_2$ -containing solutions, H• and •tBuOH react rapidly with oxygen to form hydroperoxyl (HOO•) and organic peroxyl (ROO•) radicals, respectively.
Analysis: Model compounds were exposed to defined cumulative radical doses. Residual concentrations of BPS and DPS were quantified using High-Performance Liquid Chromatography (HPLC) to determine the degree of degradation.

Key Results

Dominance of HO•: In the absence of scavengers, degradation increased with radical dose. The presence of oxygen and higher pH (pH 2 vs. pH 0) enhanced degradation, consistent with established HO•-induced pathways involving the formation of HO-adducts ( $\bullet$ Ar-OH) and their subsequent irreversible reaction with oxygen.
Negligible Impact of H•: When HO• was scavenged by tBuOH in $N_2$ -saturated solutions, the degradation of BPS was almost completely suppressed. For DPS, degradation levels remained similar to the unsuppressed case, which the authors attribute to the reversible addition of the scavenger-derived •tBuOH radical to the less sterically hindered DPS, rather than H• reactivity. This indicates that H• radicals cause negligible direct degradation of the aromatic substrates.
Low Reactivity of Peroxyl Radicals: In $O_2$ -containing solutions with HO• scavenged, degradation was completely suppressed for BPS and moderately suppressed for DPS. The remaining minor degradation in DPS is attributed to the reaction of the scavenger-derived •tBuOH radical, not HOO•. The study concludes that HOO• and organic peroxyl radicals (ROO•) exhibit only minor reactivity toward the aromatic model compounds.
Structural Sensitivity: BPS showed significantly higher degradation than DPS under HO•-rich conditions, likely due to additional degradation pathways (alkyl C-H abstraction and dealkylation) associated with its tert-butyl group.

Significance and Claims
The paper asserts that for aromatic hydrocarbon-based PEMs, the hydroxyl radical (HO•) is the principal agent responsible for chemical degradation, while H• and HOO•/ROO• contribute little to substrate loss. The findings suggest that:

Mechanistic Understanding: The degradation mechanism is governed by the formation of HO-adducts. The reversibility of H• addition and the low reactivity of peroxyl radicals mean they do not initiate the irreversible damage pathways characteristic of HO•.
Design Guidance: Future HC ionomer development should prioritize the use of electron-rich aromatic segments to shift the equilibrium of radical intermediates toward repairable cation radicals ( $Ar^{\bullet+}$ ) rather than vulnerable HO-adducts.
Antioxidant Strategy: Mitigation efforts should focus primarily on minimizing or repairing damage caused by HO• radicals, rather than targeting H• or peroxyl radicals.

The authors note limitations, acknowledging that the dilute aqueous solution used in $\gamma$ -radiolysis does not fully capture confinement effects, reduced mobility, or crosslinking phenomena present in actual polymer membranes. Consequently, while the study provides crucial mechanistic insights, it emphasizes that cell tests in fuel cells and electrolyzers remain indispensable for validating membrane durability under real operating conditions.

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