A Nature-inspired Antioxidant Strategy based on Porphyrin for Aromatic Hydrocarbon Containing Fuel Cell Membranes

<jats:p id="p1">The use of hydrocarbon-based proton conducting membranes in fuel cells is currently hampered by the insufficient durability of the material in the device. Membrane aging is triggered by the presence of reactive intermediates, such as HO·, which attack the polymer and eventually lead to chain breakdown and membrane failure. An adequate antioxidant strategy tailored towards hydrocarbon-based ionomers is therefore imperative to improve membrane lifetime. In this work, studies on reaction kinetics using pulse radiolysis and γ-radiolysis as well as fuel cell experiments are performed. The feasibility of increasing the stability of hydrocarbon-based membranes against oxidative attack by implementing a nature-inspired antioxidant strategy is demonstrated. It is found that metalated-porphyrins are suitable for damage transfer and can be used in the fuel cell membrane to reduce membrane aging with no impact on fuel cell performance.</jats:p>

section_content: Renewable energy sources in combination with electrochemical energy storage and conversion devices are necessary to mitigate global warming and anthropogenic air pollution. In this context, fuel cells (FCs) have received a lot of attention as next generation power sources that can convert chemical energy into clean electricity with high efficiency. [1] Proton exchange membranes (PEM) are used in the hydrogen fuel cell as a solid electrolyte. Commonly used PEM materials are based on commercially available perfluorosulfonic acid (PFSA) ionomers. They are typically produced by free radical co-polymerization of two monomers: perfluorinated vinyl ether sulfonyl fluoride and tetrafluoroethylene. The material has, however, inherent shortcomings. [5] Secondly, the material has a high permeability of reactant gases. [6] Thirdly, the precursors are very toxic and can be inherently expensive to handle and use in production. [6, 7] inally, there are increased environmental concerns for fluorinated super acids that can leak into soil and water streams and may eventually end up in the food chain. [8, 9] With the current goals of the EU, USA, and Japan to invest more into hydrogen solutions to combat global warming, the production of these materials is expected to increase drastically. Alternative membrane chemistries without fluorine, commonly referred to as hydrocarbon-based membranes, are a viable approach to reduce the ecological risks associated with PFSA-type membrane materials. Furthermore, the increased flexibility with respect to synthetic strategies allows for a more precise finetuning of essential membrane properties, e. g., conductivity, reactant permeability. [10, 11] uring fuel cell operation harmful radical species, such as the highly oxidizing HO * and the reductive H *, are produced, which damage/react with the membrane constituents. [12] Membrane durability may be enhanced by (a) choice of a more redox-resistant PEM and (b) incorporation of antioxidants to reduce damage. The concept of antioxidants stems from biology, where certain compounds alleviate oxidative damage. 

In PFSA ionomers, radical scavengers, such as Ce(III), are added to the membrane to quench a large fraction of HO * and thereby protect the polymer against radical attack. [13, 14] This approach is ineffective in hydrocarbon-based ionomers. The fast reaction of HO * with aromatic units is practically unavoidable, [15] therefore, complementary to radical scavenging, other approaches, such as repair of damaged constituents, are required. 

We have previously shown that the complex mechanism of radical attack on aromatic compounds results in the formation of cation radicals, relatively long-lived (> μs) intermediates that can undergo one-electron reduction via a repair cycle (Scheme 1). [16] n biological systems, several families of enzymes that handle or produce strong oxidants contain chlorophyll or heme as the active site, i. e., iron containing porphyrins. Some examples of prominent enzyme families are; catalase, which "detoxifies" hydrogen peroxide, myeloperoxidase, which produces bursts of hypochlorite for antimicrobial defense, and cytochrome P450, which oxidizes xenobiotics. As hemes are used in biology to handle highly oxidizing species, they are required to be particularly resistant to permanent oxidative damage. It occurred to us that the corresponding general class of chemicals, porphyrins, might be a suitable candidate to handle the powerful oxidants formed during fuel cell operation. In addition to their resistance to oxidative damage, a suitable coordinated transition metal would be chemically ideal to mediate one-electron transfer processes, as required for antioxidant defense of radicals. The heme of cytochrome P450, for example, can have four different oxidation states. [17] Biology usually operates porphyrins around pH 7, where these tetradentate, aromatic nitrogen ligands form very strong transitionmetal complexes. PEMFCs operate at around pH 0, where protonation of the coordinated nitrogens may compete with the metal binding. We surmised that for the concept to work in fuel cells, an optimal combination of porphyrin and metal needs to be found. Nevertheless, we could not escape the appeal of using a known, evolutionary proven, oxidation-resistant class of redox-catalysts as antioxidants for fuel cells. 

"Porphyrin class" molecules are annealed aromatic moieties, known as macrocyles, with a central cavity containing four nitrogen coordination sites (Figure 1 ). The free base porphyrin, without a metal center, contains two amine nitrogenÀ hydrogen bonds. These are replaced by nitrogenÀ metal bonds in the metalated structure. In principle, all four nitrogens can be protonated, which competes with the metal binding. Therefore, under acidic conditions the metal binding is weakened. A porphyrin-based antioxidant for a fuel cell requires an optimally matched central transition metal that is stably bound even under strongly acidic conditions. Firstly, the size of the metal ion needs to match the cavity size; secondly, the metal should form strong metalÀ nitrogen bonds. Additionally, for antioxidant action with a radical, the porphyrin complex needs to facilitate one-electron transfer reactions. The heme of cytochrome P450 can have four oxidation states: Fe(II), Fe(III), Fe(IV) and Fe(IV) + porphyrin cation radical. Importantly, even with the highest oxidation state as part of the catalytic cycle, these enzymes are stable. [17] We, therefore, hypothesized that "porphyrin complex" cation radicals may be stable enough for our purposes -which was later supported by our results. 

[20] Porphyrin derivatives are therefore "ideal electron-buffers" that have found application either in the catalyst layer [21] or in the polymer electrolyte membrane [22] of fuel cells. 

Herein we explore the possibility of damage mitigation of aromatic polymers using porphyrins as catalytic repair agents. First, we undertook pulse-radiolysis kinetics experiments with a small-molecule model compound for aromatic hydrocarbonbased PEMs, potassium 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) and a water-soluble porphyrin-derivative, [5,10,15,20tetraphenylporphinato] copper(II) 4,4',4'',4'''-tetrasulfonic acid tetrasodium salt (CuÀ TPPS). We then measured the corresponding reaction rate and established that BMPS could be repaired by CuÀ TPPS. We further substantiated the results with degradation tests where radicals were produced continuously at low concentration via γ-radiolysis of water. Subsequently, proof-ofprinciple fuel cell experiments were carried out with poly(αmethylstyrene sulfonate) (PAMSS) based PEMs that contained covalently attached porphyrins 2 or Cu-2 as antioxidants (Figure 1 ). As will be shown below, porphyrins can indeed mitigate radical-induced degradation and that this effect is not based on simple radical scavenging.
section_name: Introduction

section_content: The first step of the electrophilic attack of HO * on aromatic hydrocarbon-based PEMs (abbreviated as Ar) is predominantly by oxidative addition to the ring. This leads to the formation of hydroxycyclohexadienyl radicals (HO-adducts), Reaction (1). This reaction proceeds with diffusion-limited rates for aromatic sulfonates, specifically, a rate constant (at room temperature) of 4. 7×10 9 M À 1 s À 1 is reported for phenylsulfonate (pH 7) [23] and 9. 1×10 8 M À 1 s À 1 for 4-tert-butyl phenylsulfonate (pH 3. 6). [24] nder acidic conditions, typical for PEMFCs, HO-adducts are in a rapid acid-base equilibrium with water and form cation radicals, Reaction (2). [16] )

We recently showed that cation radicals, Ar * +, are the only intermediates that are sufficiently long-lived (> μs) for a repair reaction to operate. [25] Repair, Reaction (3a), is possible even with the poor reductants Ce(III) and Mn(II), as aromatic cation radicals are typically potent oxidants, E°(Ar * + /Ar) = ~2 V. [26] If the products, here Ce(IV) and Mn(III), are reduced back by a side product of fuel-cell operation, in this case H 2 O 2, a catalytic process is established. However, such inorganic additives can induce a significant performance penalty in the FC as they replace the conductive protons of the sulfonate groups as counter-ions. 

Here we explore the scenario of repair by porphyrins. As they are electroneutral, they should not affect FC performance. The repair mechanism is expected to be applicable to other aromatic hydrocarbon ionomers.
section_name: Results and Discussion

section_content: We used BMPS, a monomer analogous model compound for PAMSS-based membranes, as kinetics experiments are generally performed in dilute aqueous media. 

We have recently shown that for electron rich compounds such as BMPS, equilibrium (2) lies to the right, favoring the formation of cation radicals. [27] Also, the characteristic absorbance bands are clearly separated for the free radicals (HOadducts and cation radicals) stemming from pulse-radiolysis of BMPS, allowing unobscured observations. 

As a reaction partner we selected the Cu(II) complex of meso-tetra-(4-sulfonatophenyl) porphyrin (CuÀ TPPS) with known standard potential [E 1/2 (Cu(II)À TPPS * + /Cu(II)À TPPS) = + 0. 77 V]. [18] Metalated porphyrins are applicable as the acidic operation conditions of a fuel cell lead to protonation of non-metalated free-base porphyrins, which decreases their reactivity to act as electron donors in Reaction (3a). From the various metal centers considered [Zn(II), Mn(III), Cu(II), Ni(II) and Fe(III)], we selected Cu(II) as it is known to form kinetically inert inplane complexes with porphyrin hosts. [28] e studied the kinetics of electron transfer from the cation radical of BMPS to the metalated porphyrin CuÀ TPPS, Reaction (3b) via pulse radiolysis. 

Pulse radiolysis is a convenient method to study oneelectron transfer reactions of aromatic sulfonates [27] or porphyrins. [29] Radiolysis of water by ionizing radiation produces primary radicals, HO * (45 %), * H (10 %) and e aq À (45 %), with established yields: G(HO * ) = G(e aq À ) = 0. 28 μmol J À 1 and G(H * ) = 0. 06 μmol J À 1, Reaction (4). [30] 2 O?HO

Porphyrins can be one-electron reduced by hydrated electrons to yield porphyrin anion radicals. Similarly, suitable oxidants may one-electron oxidize them to cation radicals. [18] e were interested in the latter, and therefore established appropriate reactions conditions where the yield of porphyrin cation radicals can be maximized. In neutral solutions, under N 2 O-saturation (24. 8 mM at 20 °C) solvated electrons, e aq À, can be quantitatively converted into HO *, [30] Reaction (5), which avoids direct reaction of e aq À with porphyrins. We mimicked the acidic environment of PEMs by adding sulfuric acid. However, dependent on pH, Reaction (6) may effectively compete with Reaction (5). Therefore, we performed all experiments in 1 mM H 2 SO 4 solutions where Reaction (5) still dominates and ~85 % of hydrated electrons will be converted to HO *. For dilute aqueous solutions the yield of HO * amounts to G(HO * ) = 0. 28 μmol J À 1 + 0. 24 μmol J À 1 = 0. 52 μmol J À 1 and G(H * ) = 0. 06 μmol J À 1 + 0. 04 μmol J À 1 = 0. 10 μmol J À 1. High concentrations of solutes may cause changes in the radiochemical yields. [31]

When we pulse irradiated N 2 O-saturated 50 μM solutions of CuÀ TPPS and 100 mM KBr in 1 mM H 2 SO 4 formed HO * reacts with Br À to form Br 2 *À in two steps, Reactions (7a) and (7b). [32] ere, due to the presence of 100 mM KBr, the radiochemical yield is increased to G(Br 2 *À )~0. 6 μmol J À 1. [32] r 2 *À is a potent one-electron oxidant, E°(Br 2 *À /Br À ) = + 1. 63 V, [33] which rapidly oxidizes CuÀ TPPS, Reaction (8). Consequently, Figure 2 depicts the transient absorbance spectrum of CuÀ TPPS * +. 

The obtained spectrum of CuÀ TPPS * + is very similar to the one reported for its Zn(II) analogue, [29] the observed absorbance maximum ~650 nm is characteristic of π-cation radicals. [34] In the next step we evaluated the feasibility of electron transfer involving BMPS * +, Reaction (3b). We selected reaction conditions similar to our previous work to maximize the radiochemical yield of BMPS * + under slightly acidic conditions. [27] riefly, we pulse irradiated a N 2 O-saturated solution that contained 2 mM BMPS, 50 μM CuÀ TPPS, 100 mM NaClO 4 and 1 mM H 2 SO 4. The ionic strength has an impact on reaction kinetics; [25] therefore, we adjusted it to match the experiments involving KBr. The fortyfold excess of BMPS over porphyrin ensures that HO * predominantly reacts with BMPS, Reaction (1), while the direct reaction of HO * and CuÀ TPPS is negligible. 

In absence of KBr, the yield of BMPS * + corresponds to G(HO * ) = G(BMPS * + ) = 0. 52 μmol J À 1. Under the assumption that Reaction (3b) is operative, formed BMPS * + will react with CuÀ TPPS to form CuÀ TPPS * +. We perceive that the transient spectra obtained in both experiments are dominated by the same absorbing species, CuÀ TPPS * + (Figure 2 ). Additionally, we estimated similar rate constants of formation, ~1×10 9 M À 1 s À 1, at 650 nm in both experiments (Figure S2 ). These findings confirm the feasibility of damage transfer from one electron oxidized BMPS to the CuÀ TPPS antioxidant, with a nearly diffusion limited rate constant.
section_name: The mechanistic model

section_content: We showed above that damage can be transferred from the one-electron oxidized BMPS to CuÀ TPPS. To further substantiate our results, we compared the stability of BMPS against radical attack in the presence and absence of CuÀ TPPS. Oxidizing HO * were produced by radiolysis of water using a 60 Co γ-radiation source. In contrast to pulse-radiolysis, where the focus is on time resolution to understand reaction cascades, continuous exposure to γ-irradiation allows for a constant production rate of HO * (r HO * � 31 μM•h À 1 ). The latter technique better represents continuous radical formation in an operating fuel cell (see Supporting Information) and is therefore better suited for product analysis. Air saturated solutions of 1 mM BMPS at pH = 0 (1 M H 2 SO 4 ) with or without 0. 01 mM CuÀ TPPS were irradiated. In contrast to the pulse radiolysis experiments, the solutions were air-saturated and, therefore contained approximately 0. 2 mM O 2. 

The degree of degradation (DoD) of BMPS was quantified by high performance liquid chromatography with ultra-violet light detector (HPLC-UV). In Figure 3 the DoD is shown as a function of formed HO *, where x = y represents a stoichiometry of one (i. e., equimolar degradation). [25] DoD increases with [HO * ] total formed by radiolysis, and in the case of BMPS lies below the diagonal indicating sub-equimolar damage. For samples containing 10 μM (1 mol%) of CuÀ TPPS we observed an overall lower DoD. Additionally, DoD decreases linearly with increasing [CuÀ TPPS] (Figure S3 ).
section_name: Porphyrins mitigate radical-induced degradation

section_content: The observed decrease in DoD by the addition of CuÀ TPPS (Figure 3 ) cannot be caused by simple HO * scavenging, Reaction (9). For this mechanism to be effective, radicals need to react predominantly with the porphyrin antioxidant instead of BMPS. This contradicts known kinetics as diffusion-limited rate constants are reported for the reaction of HO * with aromatic sulfonates, [23, 24] Reaction (1), and porphyrins, Reaction (9). Assuming similar rate constants and establishing a 100-fold excess of BMPS over CuÀ TPPS, we ensured that formed HO * react primarily (> 90 %) with BMPS. 

We found in the steady-state γ-radiolysis experiments, that the addition of CuÀ TPPS resulted in an overstoichiometric decrease in DoD, namely, there was a higher decrease in the degradation than added porphyrin (Figure 3 ). Disproportionation of the metalated porphyrin cation radical is reported to be its major deactivation pathway resulting in the formation of the two-electron oxidized dication and regeneration of the metalloporphyrin, Reaction (10). [35] This would enable the repair of up to 2 mol of BMPS * + by 1 mol of CuÀ TPPS. 

This, however, cannot explain the observations: 10 μM CuÀ TPPS decreases the observed damage to BMPS by 36 μM (~35 %) at 214 μM HO * formed, whereas at the high dose of 536 μM HO * formed the decrease is 76 μM (~30 %), both well above a stoichiometric value of two (Figure 3 ). All of these results suggest that the repair agent goes through several cycles and therefore implies a "catalytic" behavior. Although the underlying mechanism is currently unknown, we speculate that reduction of the radical cation is feasible, Reaction (11), with H 2, H *, HOO * or H 2 O 2 as possible reductants.
section_name: Regeneration of the porphyrin antioxidant

section_content: In the following experiments, we implemented these findings in the fuel cell.
section_name: Cu-TPPS

section_name: Fuel cell experiments

section_content: CuÀ TPPS is highly water-soluble, and for a dedicated repair strategy to work in a FC we needed a membrane with a porphyrin that would not be washed out of the membrane during operation. We therefore opted for a membrane with a covalently bound porphyrin. Accordingly, we first co-grafted the monomers α-methylstyrene (AMS), 2-methyleneglutaronitrile (MGN) and 4-vinylbenzyl chloride (VBC) onto electron-beam irradiated ethylene tetrafluoroethylene (ETFE) sheets (Scheme 2). [36] Where the chloromethyl group of VBC was later used as an anchor point to covalently attach the porphyrin repair agent (Scheme 3). In principle, the protonation of a nonmetalated free base porphyrin could decrease its efficacy to act as a repair agent. We intended to study this in more detail, therefore, we synthesized two porphyrin derivatives, a non-metalated porphyrin and its Cu(II)-complexing analogue attached to the VBC linker. 

Three different membranes were prepared: first, a pristine grafted membrane with VBC but without the additional modification of porphyrin attachment, called AMV; second, a modified membrane with non-metalated porphyrins, called AMVÀ Por; and finally, a modified membrane that contains the porphyrin with the Cu(II) metal centre, called AMVÀ PorÀ Cu II. 

As can be seen in Figure 4, the pristine membrane has a light brown transparent colour, the porphyrin membrane is green when protonated, and the Cu(II) containing porphyrin membrane is wine-red. This is due to the way that porphyrin absorbs visible light dependent on its protonation state. This allowed us to visually confirm that the attachment reactions were successful. The porphyrin-modified membranes were left in 1 M H 2 SO 4 for 4 weeks and UV-vis analyses of the respective acidic solutions showed no detectable porphyrin leach-out. The porphyrin membrane without Cu(II) could be deprotonated by placing it inside an alkaline solution, e. g., 1 M KOH, whereupon the colour instantaneously changed from green to red. We compiled the measured physical properties of the radiationgrafted membranes prepared in this study in Table 1. The graft level and thickness were consistent as the membranes were prepared from the same batch of grafted films. The ion exchange capacity (IEC) is slightly lower for the modified membranes. This is not surprising, as the mass increases due to the addition of porphyrin molecules, whereas the number of proton conducting groups remains the same. This dilution effect is also observed in the in-plane proton conductivity, which decreases slightly after modification. The proton conductivity at room temperature of the pristine membrane is similar to that of Nafion 212 (70 � 1 mS cm À 1 ), which has a comparable thickness of ~50 μm in the water-swollen state.
section_name: Membrane preparation and characterization

section_content: Before starting the accelerated stress tests, the performance of the membranes was evaluated by measuring the polarization curve galvanostatically (Figure 5 ). For the membranes prepared in this work we recorded several curves, and we present the averages, as well as the standard error, for membranes without and with Cu(II)À porphyrin. 

Under the measurement conditions, we could operate the cell up to 2 A cm À 2 with the prepared membranes. The performance of non-modified membranes was slightly better than for membranes containing Cu(II)À porphyrin. This is in line with observed IEC and conductivity values. However, compared to the sample-to-sample variation this small difference seems negligible. The fact that non-modified and modified membranes both give satisfying and reproducible fuel cell performance indicates that this stabilization strategy does not significantly impair cell performance.
section_name: Fuel cell performance

section_content: We performed two types of degradation studies; in a short test we fixed the fuel cell operation time to 50 h, which allowed us to quantitatively compare different membrane properties before and after the test. In extended tests, membranes were operated until the cell failure criterion was reached, in this case an open circuit cell voltage below 800 mV. Table 1. Physical properties of irradiation grafted membranes prepared in this work. The gravimetric grafting level for the films was 53 � 3 %.
section_name: Accelerated stress test

section_content: Short name Thickness [a] ]

[μm]

Proton conductivity [c] [mS cm
section_name: Type

section_content: Fuel cell membrane chemical stability was investigated using accelerated stress tests at open circuit voltage (OCV), as described in the experimental section. It is important to point out, in brief, that testing conditions can significantly affect the observed rate of degradation. In our work, we use an accelerated protocol with a high O 2 partial pressure of p(O 2 ) � 200 kPa, which is significantly higher than in the commonly used testing protocol proposed by the US Department of Energy (90 °C, 30 % RH, air, 1. 5 bar) with p(O 2 ) � 27 kPa. [37] We purposefully chose not to use RH cycling or dynamic operation protocols, which would introduce additional stressors, e. g., mechanical stress. We tested the three types of membranes described in the previous section. In Figure 6, the open circuit cell voltage, high frequency resistance (HFR), 1 st derivative of HFR, and H 2 crossover are shown as in-situ state-of-health metrics for the membrane during the accelerated stress test. It can be seen that the OCV decreases with time, where the AMV membrane shows the fastest decrease. The absence of an abrupt decrease in cell voltage indicates that the membranes stay intact during the test and no pinholes are formed. The HFR is a measure mainly consisting of the ohmic resistance of the membrane. An increase in HFR, when under constant temperature and R. H, is therefore related to a decrease in conductivity caused by a loss of proton conductive groups due to radicalinduced chemical-degradation. Figure 6b, shows that HFR increase is largest for AMV, then AMVÀ Por, and finally AMVÀ PorÀ Cu II. Furthermore, differences are clearer when the first derivative of the HFR, Figure 6c, is plotted. Here, we see that the HFR' increases linearly, with AMV increasing the fastest. 

For quantification, we compare the acceleration of degradation (the second derivative), which is an order of magnitude higher for AMV (30

Finally, the H 2 crossover current is measured at the beginning, middle, and end of test. It can be seen in Figure 6d that the porphyrin has a positive effect on the gas barrier properties as the initial gas crossover is lower by a factor of two. This may be related to porphyrins changing the ionic domain size as measured by Carbone et al. using small-angle Xray scattering (SAXS). [22] The increase in gas crossover, can also be taken as a metric for the rate of membrane degradation, as the gas crossover is expected to increase due to radical induced membrane thinning. For the porphyrin-free AMV membrane, the crossover increases by 97. 8 � 0. 2 μA cm À 2 h À 1 whereas for the H + -and Cu(II)À porphyrin membranes the change is 5 � 2 and À 0. 4 � 5 μA cm À 2 h À 1, respectively, clearly indicting the faster degradation in the absence of porphyrins. 

These results show that porphyrins have a positive effect on the chemical stability of aromatic hydrocarbon-based membranes and qualitatively support the findings of the pulse radiolysis study and continuous gamma radiolysis experiments of CuÀ TPPS and BMPS. Furthermore, we measured IEC before and after testing, see Table 2 for the summarized results. The results show that the non-modified membrane is the least stable against radical-induced degradation, as these membranes had an IEC loss of approximately 60 % whereas for the membranes containing porphyrin or Cu(II)À porphyrin the loss in IEC is much lower, ~13 % and ~12 %, respectively. The better retention of IEC of the porphyrin containing membranes will allow for a longer operation compared to the non-modified membranes.
section_name: Short-term OCV hold experiments

section_content: Long-term OCV hold experiments were performed on membranes in duplicate or triplicate. The cell was kept under OCV conditions, with intermittent H 2 crossover measurements, until the cell voltage dropped below 0. 8 V. Similarly to the previous section, the durability plot is shown in Figure 7 containing the in-situ fuel cell data. 

Figure 7a shows the open cell circuit voltage as a function of time, we can see that for both membranes the OCV starts at around 1 V and slowly decays over time. The voltage decay rate for the AMVÀ Por membrane seems to be slightly faster than that of AMVÀ PorÀ Cu(II). The absence of a sudden voltage drop means no large pinholes were formed during the degradation protocol, indicating that the ETFE backbone stayed intact, as was observed in previous studies to high degrees of membrane degradation. [38] Furthermore, the change of HFR and its first derivative as a function of time is evaluated. As shown in Figure 7b and 7c, within the first 50 hours of the experiment, there does not seem to be a significant increase in HFR for either type of membrane. This is in line with the previous observations in the short-term OCV hold experiments. However, over the full duration of the experiment, we observe an exponential increase in the HFR and HFR' for the AMVÀ Por membrane, which is not the case for the Cu(II)À porphyrin (AMVÀ PorÀ Cu II ) membrane. This behavior seems to indicate a self-accelerating mechanism that leads to rapid degradation after the first 100 hours of OCV hold. This could be due to irreversible damage, e. g., oxidation, of the non-metalated porphyrin, preventing it from acting as a repair agent. The accumulated damage can then manifest as a local weak point in the membrane, leading to increased gas crossover, which in turn further accelerates degradation. In contrast, for the membrane containing Cu(II)À porphyrins, we do not observe this accelerated degradation. It is unclear whether this selfacceleration is completely absent or simply shifted to longer operational times. In none of our experiments with AMVÀ PorÀ Cu(II) did we see this exponential increase before the minimum cell voltage was reached. 

In Figure 7d, the hydrogen crossover current is shown, which was measured intermittently during the OCV hold experiment. For both types of membrane, the crossover current increased linearly over time until membrane failure occurred (Figure 7d ). After the cell shutdown criteria were reached, crossover currents of 30 and 69 mA cm À 2 were obtained for AMVÀ PorÀ Cu II and AMVÀ Por, respectively, implying pinhole formation. This was further confirmed through visual inspection of the delaminated membrane electrode assemblies (MEAs). Expectedly, despite the lower starting crossover current of the AMVÀ Por membrane, the degradation is faster. After ~100 hours the hydrogen crossover of AMVÀ Por becomes larger than that of AMVÀ PorÀ Cu(II). The hydrogen crossover measurements do not seem to support the theory of a self-accelerating mechanism. Despite the longer lifetime and lower degradation of the AMVÀ PorÀ Cu(II) compared to the AMVÀ Por membranes, the monitored parameters in the fuel cell still indicate that some degradation occurs. This is further supported by the 70 % loss of IEC, shown in Table 3. The experiments involving continuous exposure to γ-irradiation also indicated that some degradation of the model compound is inevitable, Figures 3 and S3. These are most likely originating from reactions that involve oxygen.
section_name: Long-term OCV hold experiments

section_content: As shown in previous sections, the presence of a metal center, in this case Cu(II), is important to further stabilize the membranes. By measuring the Cu-content, we investigated how stable this metalÀ porphyrin complex is in the local acidic and oxidative environment of the proton exchange membrane during accelerated fuel cell tests. 

Two different membrane areas were analyzed, the active area (AA) and non-active area (NAA), as indicated in Figure 8. Both parts of the membrane are located inside the fuel cell and will thus be exposed to similar temperature and humidity conditions. However, the AA is the only part of the membrane that is in contact with the gas diffusion electrodes and, thus, electrochemically active. The AA is thus the part of the membrane that undergoes radical-induced degradation during fuel cell testing. The NAA is outside of the fuel cell electrode area and is protected by a 25 μm polyethylene napthalate (PEN) sub-gasket and, therefore, does not undergo radical-induced degradation. We observed a change in colour in the AA after accelerated FC testing. This indicates that during fuel cell testing the porphyrin chromophore is undergoing chemical modification. A structural change could affect the stability of the Cu(II)-complex, which may result in Cu(II) loss. Therefore, we attempted to measure the relative Cu-content in the membrane by using two complementary methods, X-ray fluorescence spectroscopy (XRF) and laser ablation inductively coupled plasma time-of-flight mass spectrometry (LA-ICP-MS). XRF is generally used to measure metal content in PEMs, [39] while we recently used LA-ICP-MS in a study to determine Ce-content in PEMs. [36] Since no certified polymer reference materials exist, strong matrix effects were expected for the quantification. Therefore, both methods represent semi-quantitative approaches, and the concentrations can only be compared relatively and not as absolutes. We compared the Cu-content in AA and NAA of two membranes that were tested in the fuel cell for 50 and 336 hours. For both membranes, there is no significant difference in the Cu-content (Table S1 ). This indicates that no quantifiable amounts of Cu(II) are being removed from the membrane active area during testing.
section_name: Analysis of the Cu-content of the membrane

section_content: Long-term catalytic repair relies on the ability of the repair agent to restore itself. By using data from the fuel cell membrane accelerated tests, we estimated the turnover number (TON, i. e., number of repair actions per molecule) of the Cu(II)À porphyrin-type antioxidant. The TON was estimated based on the Cu-content, measured by XRF and LA-ICP-MS, and loss of IEC during ASTs (see SI for calculations). We estimated a TON of 35 � 6 (LA-ICP-MS) to 130 � 30 (XRF). A high TON indicates catalytic repair. In comparison to our previous work, [40] where a TON of ~25 was estimated for the repair by Ce(III), the comparatively higher TON found in the present study suggests a higher efficiency for the Cu(II)À porphyrin as an antioxidant. The higher TON and improved stability, i. e., better retention of the antioxidant during FC operation, are apparent merits of the porphyrin-based approach. A possible drawback is the multistep synthesis of the metalated-porphyrin compared to the commercially available crown ethers.
section_name: Catalytic repair mechanism

section_content: The fuel cell experiments reported here were carried out under accelerated chemical degradation conditions, for which we previously estimated a rate of HO * formation of 2. 6 • 10 À 6 M • s À 1. [40] Under nominal fuel cell operating conditions, the HO * formation rate is estimated to be 10 À 8 to 10 À 7 M • s À 1 (see Supporting Information). If we target a membrane-lifetime of 25'000 h, [41] and assume a concentration of the repair agent of 1 %, i. e., ~0. 01 M, then a TON of 80 to 800 would be required. In comparison, industrial catalysts reach TON values of 10 6 to 10 7. [42] A viable antioxidant strategy for aromatic hydrocarbonbased fuel cell membranes is therefore certainly conceivable and within reach.
section_name: Implications for fuel cell membrane development

section_content: The use of porphyrins as antioxidants for aromatic hydrocarbon based fuel cell membranes was studied. Pulse radiolysis measurements performed on the aromatic model compound potassium 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) indicate that radical induced damage can be transferred to the porphyrin-type antioxidant, Cu(II) complex of meso-tetra-(4sulfonatophenyl) porphyrin (CuÀ TPPS). In γ-radiolysis experiments substantial radical induced degradation of BMPS was observed under acidic conditions. Addition of 1 mol% CuÀ TPPS resulted in an overstoichiometric decrease in the degree of degradation of BMPS, indicating catalytic repair by the antioxidant. Cu(II)À porphyrin was covalently attached to an aromatic hydrocarbon-based radiation grafted proton exchange membrane. In the H 2 fuel cell, we found no significant performance differences between membranes with or without attached porphyrin. Chemical stability of the membranes in the fuel cell significantly improved with the porphyrin antioxidant present. Furthermore, CuÀ porphyrin yielded the largest improvement in terms of chemical stability, indicated by the increase in lifetime of the membrane in accelerated degradation tests, even with a Cu(II) loading below 0. 1 wt%. With a calculated turnover number larger than 50 for the Cu(II)À porphyrin, the repair mechanism is very likely to be catalytic. The improved chemical stability under accelerated conditions is expected to translate to a substantial increase in membrane-lifetime under regular fuel cell conditions, where the oxidative conditions are less harsh. The ability of Cu(II)À porphyrins to reduce oxidative membrane aging is likely applicable to other aromatic hydrocarbon-based ionomers.
section_name: Conclusions

section_content: Materials α-methylstyrene (AMS, 99 %, Sigma Aldrich), 2-methyleneglutaronitrile (MGN, 95 %, ABCR), 4-vinylbenzyl chloride (VBC, IUPAC name: 1-(chloromethyl)-4-ethenylbenzene, 99 %, Merck), chlorosulfonic acid (Sigma Aldrich),NaClO 4, KBr, nitric acid (Fisher Scientific) and sulfuric acid 95 % (Fischer Scientific) were all used as received. Ultra-pure water was provided by a Milli-Q or Evoqua Ultra Clear UV Plus water purification system.
section_name: Experimental Section

section_content: The literature procedure was followed to obtain the monomer analogous model compound, potassium 4-(tert-butyl)-2-methoxyphenylsulfonate (BMPS) that was used in the pulse radiolysis and continuous γ-radiolysis experiments. [36] First, its free acid was synthesized and then was neutralized with 0. 05 M KOH to yield the desired potassium salt. Purity of the compound was evaluated using an Agilent 1200 series HPLC system (Agilent, USA) and was found to be > 99 %. 10 μL of BMPS was injected into a Waters μBondapak C18 Column (125 Å, 10 μm, 3. 9 mm×300 mm, Part No. WAT027324) using a gradient elusion method and an eluent solution of AcN/H 2 O containing 0. 025 % HCOOH. The same system was used to evaluate the degree of degradation following the continuous irradiation experiments.
section_name: Synthesis of BMPS

section_content: The water-soluble porphyrin derivative, [5,10,15,20-Tetraphenylporphinato]copper(II) 4,4',4'',4'''-tetrasulfonic acid tetrasodium salt (CuÀ TPPS), used as a model compound in the pulse radiolysis and continuous γ-radiolysis experiments, was synthesized according to the literature. [43] The porphyrin derivatives, 5-(4aminophenyl)-10,15,20-triphenylporphyrin (2) and its copper(II) complex (Cu-2), were synthesized as described in the literature [44, 45] and then were covalently bound to the sulfonated membranes.
section_name: Porphyrin synthesis

section_content: Pulse radiolysis experiments were carried out with the 2 MeV Febetron 705 accelerator at ETH Zurich, that has a pulse-width of approximately 50 ns. [46] Absorbed doses were determined by KSCN dosimetry, based on G = 0. 635 μmol J À 1 and ɛ 472 = 7580 M À 1 cm À 1. The dose is measured in Gray, where 1 Gy = 1 J kg À 1 or in water � 1 J L À 1, a 10 % uncertainty is associated with the dose.
section_name: Pulse radiolysis experiments

section_content: During continuous γ-radiolysis, aqueous samples were exposed to 60 Co γ-radiation with a dose rate of ~110 Gy/h at the irradiation facility of PSI (Gammacell 220, Atomic Energy of Canada Limited). The accumulated dose was controlled by varying the exposure time. All samples were air saturated.
section_name: Continuous γ-radiolysis experiments and subsequent product analysis

section_content: ETFE sheets (DuPont) with a thickness of 25 μm were washed with ethanol and dried before they were e-beam irradiated with a total dose of 100 kGy. The irradiated films were stored at À 80 °C before grafting. The activated sheets were rolled and placed in a 60 mL cylindrical reactor containing a 80 vol % monomer mixture of AMS: MGN:VBC (4 : 1:0. 15 molar ratio) in dichloromethane (DCM), and were bubbled with N 2 for one hour to remove dissolved O 2. Afterwards the reactor was placed in a pre-heated water bath at 55 °C to start the grafting reaction and was left to proceed for 72 hours. Then, the grafted films were removed from the monomer solution and washed with acetone to remove any excess reactants. The final films were then dried in vacuo at 60 °C overnight to obtain the dry mass. 

Dry films were immersed in 10 vol % of chlorosulfonic acid in DCM to convert styrene to styrene sulfonate groups. The reaction was carried out at room temperature for 4-6 hours. After completion of the reaction, the films were taken out of the solution and placed in water to quench any residual chlorosulfonic acid, followed by three washing steps with water. The films were then placed in a beaker containing ultra-pure water and left stirring overnight at 80 °C. The overall reaction is shown below in Scheme 3.
section_name: Synthesis of sulfonated AMS/MGN/VBC co-grafted membranes

section_content: The VBC amount was measured for AMS/MGN/VBC grafted membranes in our previous work with elemental analysis to be (AMS/MGN/ VBC = 2. 7 : 2. 9 : 1). [36] A membrane sheet (~800 mg) typically contained around 0. 065 mmol of the VBC co-monomer. In a 60 mL glass reactor porphyrin 2 (72 mg, 0. 114 mmol, 1. 75 eq. ) or Cu-2 (74 mg, 0. 107 mmol, 1. 65 eq. ) and Et 3 N (35 mg, 48 uL, 0. 346 mmol, 5. 25 eq. ) were dissolved at room temperature in a mixture of DMF and H 2 O (8 : 1, 60 mL in total). Then the sulfonated membrane was immersed, the reactor was placed under N 2 and stirred at room temperature for 15 min. Finally, the reactor was stirred at 65 °C for 18 h. After the selected porphyrin was covalently bound to the membrane the solution was cooled to room temperature. The membrane was successively washed with DMF, acetone and water and then placed for 1 h in a 1 M H 2 SO 4 solution. After this protonation step the membrane was washed three times with water.
section_name: Covalent attachment of porphyrins 2 and Cu-2 to the membrane

section_content: Ion exchange capacities were determined by acid-base titration. First the membrane was exchanged overnight in 1 M KCl solution, to yield the potassium form and release protons into solution. The free protons in solution were determined by a single endpoint titration using a 702 SM Titrino (Metrohm AG, Switzerland) titrating unit with 0. 05 M KOH as a titrant. Afterwards the membrane was taken out and rinsed three times with ultra-pure water to remove any excess salt. The membrane was dried under vacuum at 80 °C to obtain the dry mass which was corrected for the difference in molecular mass of the counter ion, Equation (1). 

Where m is the mass in g and M is the molar mass in g mol À 1 for potassium or proton and n is the number of protons in mol. Three to five smaller pieces were cut from the same membrane and measured, or a single piece was measured in triplex to obtain the average IEC according to Equation (2).
section_name: Ion exchange capacity (IEC) determination

section_content: Proton conductivities were measured in-plane with respect to the membrane. The measurement was conducted using a Bekktech BT-112 conductivity cell (Scribner, USA), which consists of a 4-point probe setup with Pt-wires. The cell was connected to a LCR meter (GWInstek LCR-6100) and the resistance, R, was measured at a frequency of 1 kHz and a perturbation amplitude of 100 mV. Throughout the measurement, the membranes were fully hydrated by immersion in ultra-pure water (18. 2 MΩ). The wet thickness of the membrane was measured using a MT1281 Heidenhain thickness gauge and averaging over three different measurement spots. The conductivity was calculated using Equation (3). 

σ is the conductivity in S/cm, l is the distance of the voltage sense probes in mm, d is the sample width in cm, R is the resistance at 1 kHz in kΩ, and t is the membrane thickness in μm.
section_name: Proton conductivity measurements

section_content: Prior to fuel cell testing selected membranes were fabricated into membrane electrode assemblies. The procedure was as follows. 

Wet membrane was taken out and dried briefly between two pieces of dust-free paper. The membrane was then placed in-between two 25 μm thick PEN subgaskets and two commercial Pt-coated gas diffusion electrodes (Johnson Matthey ELE 0244, 0. 4 mg Pt/cm 2 ) and hot-pressed using a hydraulic lab press (Carver) at 110 °C and 4 MPa for 180 seconds. The final MEAs were cut to size and directly assembled into the cell. 

The fuel cell used for the experiments is designed in-house and consists of two graphite plates with a parallel flow field, two Auplated copper current collectors, and two aluminium compression plates that have 6 mm Swagelok connections. The active area is 16 cm 2 with the subgasket. For all experiments a co-flow config-uration was used. The compression of the MEA was measured to be roughly 2. 5 MPa using Fuji Film pressure sensitive film.
section_name: Membrane electrode assembly (MEA) preparation and fuel cell assembly

section_content: The fuel cells were operated and tested on a fuel cell test bench that was designed and built in-house. LabVIEW software was used to monitor and control the different devices and parameters in the test bench. Gas flow rates were controlled using a mass flow controller (Brooks Instrument, the Netherlands), water flow was controlled using a liquid flow controller (Bronkhorst, the Netherlands). The gas and water flows were mixed using a controlled evaporator mixer (CEM, Bronkhorst, the Netherlands) to control the gas relative humidity. The high frequency resistance (HFR) was measured using a Hioki 3561-01 AC Milliohm Hi-Tester/Battery Tester. A load bank (Agilent system DC electronic load, N3300A) was used to draw current from the cell. For H 2 crossover measurements a DC power supply was used (Agilent, E3633A). The cell temperature was measured using a K-type thermocouple that was inserted into the middle of the cathode flow field. 

For every newly assembled cell a hydrogen leak test was performed to make sure the cell was leak tight. After passing the leak test, the procedure consisted of: 1) conditioning protocol, 2) polarization curves, 3) hydrogen crossover measurement, 4) accelerated stress test.
section_name: Fuel cell testing

section_content: Every newly assembled cell was conditioned before any measurements. Fully humidified H 2 and O 2, with a flow rate of 200 ccm, were supplied to the anode and cathode, respectively. The cell was kept at 80 °C with a backpressure of 2. 5 bar a and a current density of 0. 5 A cm À 2 for 16 hours. During the conditioning period the cell voltage increased and the HFR decreased, and after 16 hours no significant changes occurred.
section_name: Cell conditioning procedure

section_content: Polarization curves were measured galvanostatically, directly after the break-in procedure. The current was increased from 0 to 0. 25 A cm À 2 in steps of 0. 0625 A cm À 2 and from 0. 25 to 2 A cm À 2 in steps of 0. 25 A cm À 2. H 2 and O 2 were fed to the anode and cathode throughout, with a stoichiometry of 1. 5 and a minimum flow rate of 200 sccm. The cell was maintained at 80 °C with a backpressure of 2. 5 bar a with 100 % R. H. at the inlet.
section_name: Polarization curve

section_content: The hydrogen gas crossover current was measured using staircase voltammetry. The counter and reference electrode were placed under H 2, whereas the working electrode (WE) was placed under N 2 and left to equilibrate to reduce all PtÀ O to Pt on the WE. The cell potential was set to 0. 8 V and decreased stepwise to 0. 2 V in steps of 0. 1 V, and afterwards increased stepwise back to 0. 8 V in steps of 0. 2 V. The duration of each step was 60 seconds to ensure a stable current was obtained. The average current of the last 15 seconds was recorded and plotted as a function of cell potential. The limiting crossover current was determined by extrapolating the current to the OCV, which is around 0. 1 V for an MEA with an intact membrane.
section_name: Staircase voltammetry

section_content: Accelerated stress tests were performed in a similar matter to our previous work at OCV with fully humidified H 2 and O 2 and a back pressure of 2. 5 bar a to ensure a high oxygen partial pressure. [36] The cell temperature and gas humidification was kept constant at 80 °C and 100 % R. H., respectively.
section_name: Accelerated stress tests

section_content: After the fuel cell testing protocol, the cell was purged with dry N 2 and cooled down to room temperature. The cell was disassembled, and the MEA was removed. To remove the GDEs the MEA was immersed in a mixture of 50 vol % water and ethanol, which caused the MEA to delaminate. The active area of the membrane was cut out manually around 2 mm from the edge. Pieces of non-active area were also tested for comparison. A small piece of 1×1 cm was cut out of the active area to be analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS).
section_name: Membrane post-test analysis

section_content: The elemental concentration determination of Cu was carried out using LA-ICP-TOFMS. A single pulse analysis approach was chosen to perform a depth profiling of the samples in order to determine potential heterogeneity in the elemental distribution. [47] ArF excimer laser (193 nm, GeoLas C, Lambda Physik, Goettingen) equipped with a modified parallel flow ablation cell (PFAC) [48] was used to carry out the experiments. The ablation cell was operated using a mixture of He (99. 999 %, PanGas AG, Dagmersellen, Switzerland) and Ar (99. 999 %, PanGas AG, Dagmersellen). The ICP-TOFMS (icpTOF2R, TOFWERK AG, Thun, Switzerland) was operated with a H 2 -reaction cell (99. 9999 %, PanGas AG, Dagmersellen, Switzerland) to suppress plasma species and prevent detector saturation. Operating settings are described in Table 4.
section_name: Laser ablation inductively coupled mass spectrometry (LA-ICP-MS)

section_content: Small pieces of membranes were cut into rectangles and attached to double sided tape with a layer of aluminium foil in between. The aluminium foil allowed to determine when the sample was fully consumed, by observing the emerging 27 Al + signal alongside a decrease in 32 S +. The double sided tape was then attached to a microscope slide and placed into the ablation cell and onto a 3D-XYZ stage. Additionally, the standard reference material NIST SRM 610 was used as an external standard and was placed onto the stage at an equal height to the samples. 

A gas blank measurement was performed at the beginning and end of every measurement series. A measurement series consisted of six measurements of the external standard, followed by four measurements each of three separate samples, followed by another six measurements of the external standard. Each measurement consisted of 200 consecutive laser pulses. For the membrane samples, total consumption was achieved after 10 to 100 laser pulses, varying between samples.
section_name: Analysis procedure

section_content: All data evaluation was performed using R in the RStudio IDE. [49, 50] The transient signal of each individual laser pulse was integrated separately for each element. Linear regressions of the external standard measurements were performed to determine the instrumental signal drift during a measurement series. A background subtraction was performed, and the sensitivities were calculated for the analytes at the time of measurements. Based on signal integrals, the concentrations could be calculated for each individual laser pulse. However, this approach did not allow to differentiate between irregular ablation behavior and changes in concentrations. The method by Longerich et al. [51] for ablation rate corrections could not be applied, as the main elements of the unknown samples were either below limit of detection (LOD) within the external standard or could not be analyzed by use of an ICP-TOFMS. As such, a semi-quantitative approach was chosen, where the 32 S + signal was normalized to the average 32 S + signal for each respective sample, which allowed to correct for variations in ablation rate within samples. As the sulfur content within a sample can be assumed to be homogenous, any variation in the 32 S + signal can be assumed to be a change in ablation rate. The signal intensities of other elements were then divided by the resulting value as a correction for differing ablation rates. Using the corrected signal intensities, the concentrations of the analytes were then calculated from their corresponding sensitivities. Concentrations of individual laser pulses were averaged for each individual sample. It is important to note that such a procedure allows only for relative comparisons between the samples as long as they all consist of the same matrix.
section_name: Data analysis

section_content: XRF was performed with an EDAX Orbis PC XRF. Membrane samples were vacuum dried at 80 °C before analysis. Analysis was performed at ambient conditions using an accelerating voltage of 35 kV and a current of 500 μA. Copper and sulfur were quantified from the area of the respective emission bands which are 8. 1 and 8. 9 eV for Cu K α and K β emission, respectively, and 2. 31 eV for S K α emission. The S intensity was used to normalize the data. The equipment was calibrated with high purity known elements.
section_name: X-ray fluorescence spectroscopy (XRF)

section_name: Acknowledgements

section_content: The authors gratefully acknowledge the financial support of the Swiss National Science Foundation (grant no. 175493 ). The help of Dr. Ekaterina Pomjakushina and Dr. Dariusz Jakub Gawryluk with the XRF experiments is greatly appreciated. The authors are grateful to Dr. Gunnar Schwarz and Ms. Chiara Fabbretti for the digestion ICP-MS study. Open Access funding provided by ETH-Bereich Forschungsanstalten.
section_name: Acknowledgements

section_content: The data that support the findings of this study are available from the corresponding author upon reasonable request.

section_name: Data Availability Statement

section_name: Supporting Information

section_content: The authors have cited additional references within the Supporting Information. [40, 52, 53]
section_name: Supporting Information

section_content: T. de Wild: data curation, formal analysis, investigation, validation, visualization, writing of original draft (equal). J. Wurm: preliminary data curation (supporting). P. Becker: ICP-MS data curation & validation (supporting). D. Günther: ICP-MS validation (supporting). T. Nauser: conceptualization, writing of original draft (supporting). T. J. Schmidt: supervision (supporting). L. Gubler: funding acquisition, project administration, writing of original draft (supporting). T. Nemeth: validation, data curation, formal analysis, writing of original draft (equal).
section_name: Author Contributions

section_content: The authors declare no conflict of interest.
section_name: Conflict of Interests