# Total oxidation stress protection of thiourea-based hybrid fuel cell membranes

Claire Tougne, Meriem Daoudi, Evelise Ferri, V.H. Mareau, H. Mendil-Jakani, V. Dufaud, Catherine C Santini, E. Espuche, O. Lottin, J.C. Perrin, et al.

► To cite this version:

Claire Tougne, Meriem Daoudi, Evelise Ferri, V.H. Mareau, H. Mendil-Jakani, et al. Total oxidation stress protection of thiourea-based hybrid fuel cell membranes. Electrochimica Acta, 2025, 541, pp.147338. 10.1016/j.electacta.2025.147338. hal-05394314

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# Total oxidation stress protection of thiourea-based hybrid fuel cell membranes

C. Tougne<sup>a</sup>, M. Daoudi<sup>b</sup>, E. Ferri<sup>c,d</sup>, V.H. Mareau<sup>a, D</sup>, H. Mendil-Jakani<sup>a</sup>, V. Dufaud<sup>c, D</sup>, C. Santini<sup>c</sup>, E. Espuche<sup>d, D</sup>, O. Lottin<sup>b, D</sup>, J.C. Perrin<sup>b, D</sup>, J. Dillet<sup>b</sup>, A. El Kaddouri<sup>b, D</sup>, L. Gonon<sup>a, *, D</sup>

## ARTICLE INFO

Keywords:

PEMFC

ionomer

hybrid membranes

aging

antioxidant

## ABSTRACT

To date, the durability of fuel cells remains highly dependent on the intrinsic stability of the polymer electrolyte at the heart of the device. Despite numerous efforts to develop new electrolytes, the chemical stabilisation strategy appears to be the most promising route to ensure long working life. In order to prevent the elution of incorporated stabilisers by the water produced during fuel cell operation, the sol-gel (SG) route represents a promising approach as it allows for the creation of a 3D SG phase (containing radical-inhibiting groups) in the electrolyte bulk. In this study, SG precursors containing thiourea groups were used as stabilisers against chemical degradation of sulfonated Poly (Ether Ketone) membranes (sPEEK). The results demonstrate that this SG phase initially provides complete protection to the sPEEK but for a limited time (sacrificial protection). After this full protection, a secondary protection is provided, resulting in a significant reduction in the rate of oxidation of the sPEEK phase when compared to unprotected sPEEK. Spectroscopic analyses indicate that the duration of full protection is not dependent on the SG content. Finally, fuel cell tests demonstrate the operability of hybrid membranes and the effectiveness of the SG phase in enhancing the membrane's durability.

## 1. Introduction

The Proton Exchange Membrane Fuel Cell (PEMFC) represents a promising, environmentally friendly technology for the production of electricity from hydrogen and oxygen. One of the key components of a PEMFC is the proton exchange membrane (PEM) which serves as both a gas separator and proton conductor, situated at the core of the device. To date, no membrane has demonstrated the required characteristics of the fuel cell (high proton conduction, low gas permeability, excellent thermomechanical properties) under the conditions of operation desired for heavy-transport application: operation at high temperatures (100–150 °C) using dry gases and for periods of up to 30,000 h (2030 targets) [1, 2]. PEM deterioration under such challenging and/or prolonged operational conditions is highly detrimental and ultimately results in system failure [3–12]. Even the perfluorinated membranes (i.e. PFSA-type) at the heart of today's devices are unable to meet the targeted

specifications due to a loss of their thermomechanical properties and proton conductivity [13]. This technological bottleneck is therefore impeding the development of this PEMFC technology. It is therefore essential to improve the membrane's performance and durability.

A number of strategies have been proposed for the development of more durable membranes that perform better at high temperatures and low relative humidity (RH) [3,4,8–11,14–16], among which the most promising are the development of interpenetrated networks [16,17] (IPN) (one network for mechanical strength, cross-linked or not, the other for proton conduction) and/or the design of new block-copolymers [18–20] or polyaromatic membranes like sPEEK.

Nevertheless, to date, none of these strategies has resulted in the development of a membrane that exhibits all the desired characteristics. In fact, the design of more durable membranes that operate at high temperatures and low relative humidity is extremely challenging, as one key property (conductivity) is generally improved at the expense of

<footnote><sup>a</sup> Université Grenoble Alpes, CEA, CNRS, IRIG-SyMMES 38000 Grenoble, France</footnote>

<footnote><sup>b</sup> Université de Lorraine, CNRS, LEMTA 54000 Nancy, France</footnote>

<footnote><sup>c</sup> Université Lyon 1, CPE Lyon, CNRS, UMR 5128, Catalysis, Polymerization, processes and Materials (CP2M), 43 Bd du 11 novembre 1918 F-69616 Villeurbanne Cedex, France</footnote>

<footnote><sup>d</sup> Université Claude Bernard Lyon 1, CNRS, UMR 5223, Ingénierie des Matériaux Polymères, INSA Lyon, Université Jean Monnet F-69622 Villeurbanne Cedex, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: laurent.gonon@univ-grenoble-alpes.fr (L. Gonon).</footnote>
others (service life, mechanical properties, etc.). Since the early 2000s, an alternative has been trialled: the incorporation of fillers and reinforcements to enhance the physical properties of the membrane. More recently, in 2008, the addition of chemical stabilisers [3,11,12,14, 21-60] was employed to mitigate oxidative stress.

Two routes are being explored for chemical stabilization: 1- the addition of transition metal ions [3,11,12,14,21-44,55-58,61] (or nanoparticles of their oxides) acting as radical reducers and 2- the addition of sacrificial antioxidants [3,11,12,14,45-54,57,59,60] such as phenolic compounds or H₂O₂ reducers (sulphide, phosphite, phosphoric acid...). Due to their infinite lifetime, redox systems are the most widely studied for stabilising PEMs. While the chemical ageing of perfluorinated electrolytes remains a limitation on the lifetime of the device, this represents a stronger barrier for the alternative membranes, such as sPEEK, whose lifetime is only a few hundred hours compared with a few thousands of hours for perfluorinated electrolytes. Hydroxyl radicals are known to react readily with aromatic compounds [30,37]. As a result, the lifetime of hydroxyl radicals is considerably shorter in membranes which are comprised substantially by of aromatic units than that seen for PFSA [38] membranes. This short lifetime may not be sufficient for redox stabilisers to be effective against hydroxyl radicals in polyaromatic membranes. A stabilisation strategy combining a sacrificial antioxidant and redox systems could be a general solution for PEMs. In addition, for both routes, questions arise concerning the method of introduction of these additives/fillers, their impact on the functional properties of the membranes (proton conductivity, mechanical properties and gas permeability), the quality of their dispersion and their possible elution during cell operation.

The introduction of stabilising functions (redox or sacrificial) by the sol-gel route appears to be a promising approach [8,12,31,55,57,62]. Indeed, silicate sol-gel precursors of the general formula (R'O)(1-α)Si(Rα) can be chemically modified so that their organofunctional group R provides chemical stabilisation (redox or sacrificial) to the host membrane. The hybridisation of the membrane can be achieved by grafting the SG precursor onto the surface of nanoparticles such as SiO₂ and then dispersing them in the polymer electrolyte solution [13,55,62]. In addition to the difficulties inherent to this technique (homogeneous dispersion of nanofillers in the membrane), the modification of the nanostructure of the membrane (quality of phase separation between hydrophilic and hydrophobic domains) or traces of solvent residues (such as NMP) can have a significant impact on the durability of the hybrid membrane [55,62,63]. Finally, these nanofillers are generally eluted by the water produced during fuel cell operation [21]. An interesting alternative to the dispersion of nanofillers is the technique of impregnation/condensation of the SG precursors directly within the membrane in the swollen state [8,62]. The fact that the host ionomer is not dissolved allows for the preservation of its initial nanostructure and to create a 2D or 3D SG phase at the heart of the membrane. This hybridisation technique is attractive because it can be adapted to all available ionomers.

In a recent development, E. Ferry et al. [64,65] have created a new generation of hybrid membranes based on sPEEK and a SG phase containing thiourea-type organofunctional groups. The thiourea function is one of the sacrificial antioxidants that is highly effective against hydrogen peroxide and a large number of oxidising agents, leading to the formation of disulphide and urea functions [66-68]. By adjusting the number of hydrolysable sites in the SG precursor (3 or 6), E. Ferry et al. [65] have succeeded in developing a range of new stabilised polymer electrolytes with SG uptakes ranging from a few weight percent to 30 wt % while maintaining optimised thermomechanical properties.

The objective of this study is to investigate the durability of thiourea hybrid membranes by carrying out accelerated ex-situ ageing in the presence of hydrogen peroxide. In contrast to the ex situ tests that are typically conducted (with Fenton's reagent), the chemical degradation will only be thermo-activated, i.e. carried out without the addition of ferrous ions. The addition of ferrous ions inevitably leads to a partial

exchange of acid sites and therefore to partial chemical cross-linking of the membrane (divalent ions). This results in a change in the membrane's swelling ability, which consequently affects the accessibility of oxidising species. The stability of these membranes was evaluated in H₂O₂ solutions with concentrations ranging from 0.025 to 1 wt% (7.4 mM to 294 mM) to achieve a limited degradation in order to study the degradation process.

The impact of the SG phase on the durability of the membrane was quantified by precise measurement of the degradation products formed (UV and IR spectroscopies). The study was concluded with an accelerated ageing test in a volatile organic compound (VOC) cell. The results will demonstrate that a thiourea-type SG phase provides full protection for the sPEEK phase up to a threshold H₂O₂ concentration. Above this threshold, the secondary protection remains effective, resulting in a notable reduction in the rate of degradation when compared to unprotected sPEEK.

## 2. Experimental

### 2.1. Materials

The hybrid membranes studied consist of a sPEEK polymer phase hybridized by a sol-gel phase. The sPEEK host membrane used was a FUMAPEM from Fumatech with a thickness of 30 µm and an ion exchange capacity (IEC) of 1.4 meq·g⁻¹. The hybrid membranes were prepared in accordance with the protocol described by E. Ferry et al. [65] from two commercial sol-gel precursors: N-Propyltriethoxysilane (PTES, CAS number: 2550-02-9 from Sigma-Aldrich) and N,N'bis [3-(triethoxysilyl)propyl] thiourea (HTU, CAS number: 69,952-89-2 from Gelest) and a precursor synthesised on demand according to the procedure described by E. Ferry et al. [64,65] N-phenyl,N'-[3-(triethoxysilyl)propyl]thiourea (TTU).

In a typical procedure, a sPEEK membrane was first re-acidified (1 M HCl solution, 30 °C, 4 h), then nanostructured (liquid water, 80 °C for 72 h) and finally swollen (in a 64/36 water/ ethanol mixture at pH = 3 for 24 h) to condition the membrane prior to SG impregnation. Pre-hydrolysis of SG precursors was performed separately at a concentration of 0.1 mol·L⁻¹ during 2 h at 30 °C in the same solvent mixture used to swell the membranes. The sPEEK membrane was then immersed into this solution for a period of 25 h at 30 °C allowing the reactive sol-gel phase to grow *in situ* directly into the core of the membrane. The resulting membrane underwent an additional post-condensation step (24 h at 80 °C under nitrogen atmosphere) to reach complete condensation of the SG network. Finally, an additional hydrothermal post-treatment step (72 h in water at 80 °C) followed by a second acidification step were performed to respectively elute uncondensed SG oligomers (if any) and ensure that the hybrid membrane was in its acid form. Hybrid membranes with varied SG uptakes ranging from 10 to 30 % were prepared to evaluate the influence of the sol-gel uptake on the membrane properties and performance.

The chemical stability of the hybrid membranes was compared with the chemical stability of a reference sPEEK membrane (referred to as "sPEEK") which had undergone the same treatments as those used to prepare the hybrid membranes, but without the addition of sol-gel precursors [65].

The PTES hybrid membrane should permit the investigation of the influence of an inert SG phase (no organofunctional group with antioxidant properties) on the durability of the membrane (potential decrease of the swelling of the membrane and therefore lower access of oxidising species into the membrane).

The other two SG precursors used have a thiourea-type organofunctional group known for its sacrificial antioxidant properties [66-68]. It can be reasonably assumed that these two precursors will exhibit similar antioxidant properties. However, as HTU has 6 hydrolysable groups compared to only 3 for TTU, the cross-linking density of these two SG phases should be very different. For identical SG uptake,
the impact of these two precursors on membrane swelling will differ.

All the compositions tested in this study are presented in Table 1. These chemical compositions were chosen to introduce the greatest amount of antioxidant phase while maintaining sufficient ionic conductivity to enable their use in fuel cell.

## 2.2. Physical and chemical properties of membranes

### 2.2.1. Water uptake

The water uptake ($W_{Upt}$) of the membranes was measured at room temperature after the membranes had been swollen for 72 hours in liquid water at 80 °C. Excess water on the surfaces was removed using absorbent paper (KimTech) immediately prior to weighing:

$$W_{Upt} = \frac{mHyb_{wet} - mHyb_{dry}}{mHyb_{dry}} \times 100 \% \quad (1)$$

$mHyb_{dry}$ and $mHyb_{wet}$ are the mass of hybrid membrane in the dry and the wet state. These masses were measured after a complete drying under nitrogen flow at room temperature (24 h).

### 2.2.2. Proton conductivity

In-plane resistance was measured at room temperature by voltammetry, using a linear voltage sweep with an Essential VSP potentiostat (BioLogic Science Instrument) and a BT-110 conductivity cell (Scribner Inc., USA) according to four-electrode method [69]. The membranes were immersed in water at 80 °C for 72 hours and re-acidified in 1 M HCl solution at room temperature for 4 hours, then rinsed three times with pure water before measurements. The slope of the voltage vs. current response was used as the resistance (R in Ω) characteristic of in-plane conductivity. The conductivity $\sigma$ (S.cm$^{-1}$) of the membrane was calculated according to Eq. (2):

$$\sigma = \frac{L}{W \times e \times R} \quad (2)$$

where L is the distance between the electrodes (cm), W is the width of the membrane (cm) and e is the thickness of the membrane (cm).

### 2.2.3. Thermogravimetric analysis (TGA)

The thermal stability of the membranes was assessed by thermogravimetric analysis (TGA) using a Metler Toledo TGA/DSC device to obtain the temperatures of desorption ($T_{désorption}$), desulfonation ($T_{désulfonation}$) and degradation ($T_{dégradation}$). Five milligrams of membrane were exposed to temperatures ranging from 25 °C to 700 °C with a ramp of 10 °C/min under air flow (30 mL/min).

### 2.2.4. Gas transport properties

Hydrogen permeability measurements were performed at 23 °C. The membranes were then inserted into the permeation cell and desorbed under secondary vacuum overnight. Subsequently, a gas pressure of 2 bar was introduced in the upstream compartment of the permeation cell, and the pressure into the downstream part of the cell was recorded as a function of time with a Datametrics pressure sensor (10 Torr). The downstream pressure versus time recording enabled the calculation of the permeability coefficient $P(H_2)$ from the slope of the steady-state linear part of the curve. $P(H_2)$ was expressed in barrer ($1 \text{ barrer} = 10^{-10} \text{ cm} \cdot \text{STP} \text{ cm} \cdot \text{cm}^{-2} \text{ s}^{-1} \text{ cm} \cdot \text{Hg}^{-1}$)

<table><caption>**Table 1**

Chemical compositions of the membranes.</caption><thead><tr><th>Acronym</th><th>sPEEK</th><th>PTES 7 %SG</th><th>HTU 7 %SG</th><th>TTU 7 %SG</th><th>TTU 18 %SG</th></tr></thead><tbody><tr><td>SG uptake</td><td>0 %</td><td>7 % of a PTES phase</td><td>7 % of a HTU phase</td><td>7 % of a TTU phase</td><td>18 % of a TTU phase</td></tr><tr><td>SG phase</td><td>–</td><td></td><td></td><td></td><td></td></tr></tbody></table>

## 2.3. Ex situ accelerated ageing

### 2.3.1. Ageing test

In order to assess the effectiveness of the SG phase in inhibiting or slowing down oxidative attack, accelerated ageing tests were carried out in $H_2O_2$ solutions. The solutions were prepared by diluting a concentrated solution of $H_2O_2$ (30 wt%) in pure water without the addition of ferrous ions. This method avoids any risk of physical cross-linking of the membrane via its ionic groups, which would limit its water uptake and therefore its exposure to hydrogen peroxide.

The chemical degradation was activated thermally at 80 °C for 24 hours. Given that the decomposition kinetics of hydrogen peroxide are slow, it can be assumed that the $H_2O_2$ concentration remains constant throughout the 24-hour exposure of the membranes [70]. Concentration of $H_2O_2$ ranging from 0.025 to 1 wt% (i.e. 7.4 mM to 294 mM) were used to vary the degradation extent.

Each ageing experiment was conducted by immersing the membranes in a fixed volume of hydrogen peroxide solution (2 mL) per mg of dry hybrid membrane. Previous studies [71] have demonstrated that this concentration range is particularly well suited to studying the chemical stability of sPEEK membranes. The high oxidizability of sPEEK makes it a particularly interesting material for studying the effectiveness of the chemical stabilisation.

### 2.3.2. Ageing quantification

The ageing of sPEEK results in numerous chain scissions leading to the formation of additional hydrophilic functions and the elution of oligomers [72]. The impact of ageing has been quantified by various techniques.

* **Mass monitoring of the membrane**

After 24 h at 80 °C in $H_2O_2$ followed by quenching $H_2O_2$ oxidation by immersion in pure water, membranes were weighed in order to monitor the evolution of the water uptake with aging (eq. (1)).

m aged $Hyb_{wet}$ and m aged $Hyb_{dry}$ were measured after aging as described above (2.2.1).

Additionally, monitoring the dry weight loss of the membrane enables the quantification of the loss of material following the elution of oligomers (chain scissions) and oxidation products.

$$Dry weight loss (wt\%) = \frac{|m \text{ aged } Hyb_{dry} - mHyb_{dry}|}{mHyb_{dry}} \times 100 \% \quad (3)$$

* **Quantification of degradation products by UV-Visible spectroscopy**

The quantity of degradation products eluted in the oxidising medium was determined by UV-Visible absorption spectroscopy from 200 to 800 nm using a ThermoFisher Nanodrop 2000c.

* **Quantification of degradation products trapped in the membrane**

The quantity of oxidation products trapped in the membrane was quantified by transmission infrared spectroscopy using a Thermofisher Nicolet iS50 FTIR spectrometer. The spectra were recorded in 32 scans with a resolution of 4 cm$^{-1}$.

The oxidation products characteristic of the oxidation of the sPEEK phase absorb at a wavenumber of 1735 cm$^{-1}$, which corresponds to carboxylic species formed by sPEEK oxidation. The absorbance at this wavelength was measured after deconvolution of overlapping bands between 1760 and 1680 cm$^{-1}$. The absorbance was then normalised
relative to the thickness of the membrane in the dry state before ageing and the amount of sPEEK phase (before ageing).

## 2.4. Membrane operability in fuel cells and accelerated stress tests

### 2.4.1. Membrane electrodes assembly fabrication

The Membrane Electrode Assemblies (MEA) were prepared by hot pressing (135 °C for 3 min and 30 s at 4600 N, i.e. 6,4 Mpa), using Sigracet 28 BCE Gas Diffusion Electrodes (GDE) with a platinum loading of 0.3 mg/cm² and a Vulcan-type carbon support with a 40 % Pt/C ratio. To achieve good adhesion of the electrodes to the membranes, it was necessary to add a 0.3 mg/cm² Nafion overcoat to the GDE. This overcoat was applied by the GDE supplier. Further details of the MEA assembly method and the preliminary studies required to calibrate the thickness of the Nafion overcoat can be found in a previous work [73]. A performance comparison between sPEEK and Nafion membrane of similar thickness and assembled in the conditions with identical GDE are also available in that report [73].

To assess the quality of the membrane electrode assemblies (MEAs), we measured polarization curves, a standard benchmark for evaluating the overall performance of PEMFC. Polarization curves illustrate the evolution of cell voltage as a function of current density, providing insight into performance losses arising from activation overvoltage, ohmic resistance, and, possibly, concentration overvoltage. In this study, the polarization curves were evaluated between 1 V and 0.4 V by imposing current density steps of 50 s and measuring the corresponding voltage. We used a small 1.9 cm × 3.8 cm (7.22 cm²) single cell supplied with constant flow rates (12.5 L·h⁻¹ for oxygen and 6.3 L·h⁻¹ for hydrogen). This single cell was made of two identical gold-coated stainless-steel plates on the anode and cathode sides, with single serpentine channel (19 passes) of 0.7 mm in depth and 1 mm in width. Although several sets of operating conditions were tested, for comparison purposes, the performance of all MEAs was assessed at 70 °C with 80 % Relative Humidity (RH) gas inlet on the anode and cathode sides. The flow field plate temperature was controlled via the water circulation loop. The polarization curves were measured following a two-hour break-in stage consisting of the repetition of potentiostatic steps at OCV (30 s), 0.6 V (45 s) and 0.3 V (60 s).

### 2.4.2. Accelerated stress test

The Accelerated Stress Test (AST) was used to assess the durability of membranes in fuel cells, combining OCV (Open Circuit Voltage) hold and load-driven humidity cycling. In practice, membrane drying is induced by keeping the gas flow rates unchanged (i.e. equal to the values required to reach oxygen and hydrogen stoichiometry of 1.5 at high current density) during OCV hold, while their inlet RH is set to relatively low values (typically 50 % RH) during the aging test. Membrane drying is evidenced by the observation of a brief voltage drop when the current density is increased at the end of the OCV hold step (see Fig. 8b in Section 3.3). The main advantage of such multi-stressor AST is that they can be adjusted to target either the whole MEA (using load – and thus potential – cycling, OCV hold and humidity cycling) or one of the MEA components [74,75]. In contrast to previous studies [74,76,77] the total duration of the OCV steps was increased to 33s/60 s in order to induce faster membrane degradation. The cell temperature was also set at 10 °C higher than during the measurement of the polarization curve (i.e. 80 °C).

The AST sequence (Fig. 8a) was repeated during 24 hours. Subsequently, a characterization stage was performed to assess the membrane degradation rate. That characterization stage is fully described in our previous works [74,77]: it lasted about one hour, during which the ElectroChemical Surface Area (ECSA), the hydrogen permeation current, and the FC impedance spectra were measured [73]. In this work, impedance spectra were used only to monitor the FC high frequency resistance, which did not vary significantly. Therefore, it should be noted that the hydrogen permeation current was the main parameter

used to monitor the durability of the membrane. Therefore, it was systematically measured in two situations, with the hydrogen and nitrogen flows alternated: i.e. first by flushing the cathode compartment with 10 slph of nitrogen and the anode compartment with 10 slph of hydrogen, then the opposite. In both cases, a voltage of 0.6 V was applied to the cell. The raw value of the hydrogen permeation current is not easy to interpret without varying the measurement parameters [78]: for that reason, only their evolution during the AST was used to detect membrane failure. The hydrogen permeation current was frequently equal to zero, or even negative at MEA beginning of life. The AST was stopped when the hydrogen permeation current reached values higher than 50 mA/cm²: for this threshold to be reached, these two steps, i.e. 24 AST followed by the characterization stage had to be repeated 10 to 20 times, depending on the membrane tested.

## 3. Results and discussion

### 3.1. Physico-chemical properties of membranes

The physico-chemical properties of the sPEEK host membrane and the hybrid membranes prior to ex-situ ageing tests are presented in Table 2.

It should be stressed here that the SG phase was found to be very well distributed within the HTU, TTU and MPTES hybrid membranes. This was observed through the membranes cross-sections (opened by cryo-ultramicrotomy) by scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), but also by transmission electron microscopy (TEM) and atomic force microscopy (AFM) on both topographic and mechanical images. No pure SG phase could be identified down to a nanometric scale (resolution limit of these techniques). Illustrative TEM and AFM images are presented in the supporting information SI-1). The final thickness of the hybrid membranes remains of a similar magnitude to that of the initial sPEEK host membrane. The observed fluctuations can be attributed to the SG uptake. With the exception of the PTES membrane, the water uptake of the hybrid membranes is lower than that of the sPEEK host membrane. It was found that an increase in the number of hydrolysable sites carried by the SG precursor (HTU vs TTU) and an increase in SG uptake (from 7 to 18 wt%) led to a decrease in water uptake resulting in a drop in the proton conductivity of the membranes. The SG phases studied did not significantly modify the thermal stability of the membranes (as studied by TGA and reported as the temperatures of desorption (T<sub>desorption</sub>), desulfonation (T<sub>desulfonation</sub>) and degradation (T<sub>degradation</sub>). The temperature range over which water desorption occurs (30–150 °C) is consistent with that

<caption>Table 2</caption>

Physico-chemical properties of the host sPEEK membrane and hybrid membranes: thickness in the dry (overnight N₂ drying at room temperature) and hydrated states (72 h at 80 °C in water) (e<sub>dry</sub> and e<sub>wet</sub>), thickness swelling rate in water (G<sub>thickness</sub>) water uptake (W<sub>upt</sub>), proton conductivity at the hydrated state (σ<sub>H+</sub>), the permeability coefficient P(H₂) and thermal stability of the membranes obtained by TGA: temperatures of desorption (T<sub>desorption</sub>), desulfonation (T<sub>desulfonation</sub>) and degradation (T<sub>degradation</sub>).

<table><thead><tr><th></th><th>sPEEK</th><th>PTES 7 %<br>SG</th><th>HTU 7 %<br>SG</th><th>TTU 7 %<br>SG</th><th>TTU 18 %<br>SG</th></tr></thead><tbody><tr><td>e<sub>dry</sub> (μm)</td><td>25</td><td>30</td><td>28</td><td>20</td><td>22</td></tr><tr><td>e<sub>wet</sub> (μm)</td><td>39</td><td>43</td><td>40</td><td></td><td>32</td></tr><tr><td>G<sub>thickness</sub> (%)</td><td>56 %</td><td>43 %</td><td>43 %</td><td>100 %</td><td>45 %</td></tr><tr><td>W<sub>upt</sub> (%)</td><td>200 ± 10</td><td>270 ± 12</td><td>130 ± 20</td><td>180 ± 22</td><td>160 ± 25</td></tr><tr><td>σ<sub>H+</sub> (mS/cm)</td><td>70</td><td>59</td><td>~52–54</td><td></td><td>53</td></tr><tr><td>P(H₂) (barrier)</td><td>3,7</td><td>N/A</td><td>2,7</td><td>2,5</td><td>3,3</td></tr><tr><td>T<sub>desorption</sub> (°C)</td><td>33–150</td><td>33–140</td><td>33–145</td><td>33–145</td><td>34–150</td></tr><tr><td>T<sub>desulfonation</sub> (°C)</td><td>260 °C</td><td>250 °C</td><td>245 °C</td><td>265 °C</td><td>260 °C</td></tr><tr><td>T<sub>degradation</sub> (°C)</td><td>415 °C</td><td>405 °C</td><td>410 °C</td><td>410 °C</td><td>410 °C</td></tr></tbody></table>
of the host sPEEK, as is the temperature at which desulfonation begins (approximately 250 °C) and the onset of degradation of the polymer phase (410 °C).

The hybrid membranes exhibit a slightly lower hydrogen permeability coefficient in comparison with the sPEEK membrane. In all cases, the permeability of the membranes studied remains lower than the permeability of a Nafion-like perfluorinated membrane (5 barrier), which exhibits superior gas barrier properties.

It can be concluded that all the membranes studied possess the required characteristics to operate in fuel cell.

$$G_{\text{thickness}} (\%) = \frac{e_{\text{wet}}}{e_{\text{dry}}} \times 100 \% \quad (4)$$

The membrane thickness was measured using a micrometer (palmer).

## 3.2. Ex situ accelerated ageing

This section presents the evolution of the macroscopic properties of the PTES, HTU and TTU hybrid membranes with 7 wt%SG, and the TTU with 18 wt%SG, as well as those of the reference sPEEK membrane as a function of the concentration of hydrogen peroxide to which they were exposed.

### 3.2.1. Water uptake as a function of ageing

Increase in water uptake of aged membranes with respect to unaged ones can provide insights about: i/ an increase in membrane polarity/ hydrophilicity ii/ a decrease in the average length of the chains, and therefore their entanglements iii/ the generation of porosity or micro/ nano defects [79–81].

The evolution of water uptake of aged membranes as a function of the hydrogen peroxide concentration in the ageing medium is presented in Fig. 1a. In the case of the sPEEK membrane, a significant increase in water uptake is observed above a H₂O₂ threshold concentration of 0.5 % to reach a water uptake of >600 wt% for a H₂O₂ concentration of 0.6 wt %.

The water uptake of the PTES membrane is not modified for H₂O₂ concentrations up to 0.4 wt% and highly increases above this threshold value.

This result suggests that the PTES SG phase is ineffective in inhibiting oxidative stress, as expected. The lower threshold concentration at

which membrane swelling, and therefore ageing, occurs is likely due to the increased water sorption ability of the unaged PTES membrane (270 wt% vs. 200 wt% for the sPEEK membrane, see Table 2) which facilitates the diffusion of oxidizing species within the membrane. The PTES sol-gel phase, which is considered to be inert due to the absence of stabilising functions, has no significant impact on the chemical stability of the sPEEK phase. Indeed one can notice a similar increase of water uptake for sPEEK and PTES membranes as high H₂O₂ concentration.

For hybrid membranes with thiourea functions, i.e. HTU, TTU 7 wt% SG and the TTU 18 wt%SG hybrid membrane, no significant increase in water uptake was observed during ex situ ageing, whatever the hydrogen peroxide concentration. These hybrid membranes therefore appear to be stabilised by the reactive SG phase, although it should be noted that these membranes are necessarily less exposed to H₂O₂ due to their lower water uptake (water uptake values in the range of 130–180 % (Table 2)).

As already mentioned, increase in water uptake for aged state can be assigned to different membrane modifications. The next step is to analyse the dry mass loss of these membranes as a function of the hydrogen peroxide concentration of the ageing solution.

### 3.2.2. Dry weight loss as a function of ageing

Fig. 1b shows a significant mass loss (of approximately 5 wt%) for the sPEEK membrane at concentrations from 0.4 wt%H₂O₂. For the PTES hybrid membrane, a loss of mass of a similar magnitude appears from 0.2 wt % H₂O₂. This mass loss accelerates sharply from 0.7 wt% H₂O₂, reaching 25 wt% for a sPEEK membrane and up to 40 wt% for a PTES hybrid membrane at a concentration of 0.8 wt% H₂O₂. This macroscopic measurement confirms that the addition of a non-reactive SG phase does not protect the membrane against oxidation. In fact, it has a slight pro-oxidising effect, as evidenced by the greater mass loss observed for the PTES hybrid membrane compared to the sPEEK membrane. This is probably related to the increased water uptakes described above (270 wt % for PTES vs. 200 wt% for the sPEEK membrane) and therefore the increased amount of H₂O₂ in the PTES hybrid membrane. In the case of thiourea hybrid membranes containing 7 wt%SG, the appearance of a significant loss of mass (of the order of 5 wt%) is only visible from 0.7 wt % H₂O₂. At 0.8 wt% H₂O₂ (the maximum concentration supported by the sPEEK and PTES membranes before fragmentation) the mass loss for 7 wt%SG thiourea hybrid membranes is 5 times less than for the sPEEK membrane and 8 times less than for the PTES hybrid membrane.

<figure><img src="image_4.png" /><figcaption>Fig. 1. (a) -Water uptake and (b) -Dry weight loss of membranes: sPEEK membrane (●), 7 wt%SG hybrids PTES (○), HTU (■), TTU (▲)and 18 wt%SG TTU (△) as a function of the hydrogen peroxide concentration of the ageing solution.</figcaption></figure>
These macroscopic measurements show that the thiourea SG phase can protect the sPEEK phase of the hybrid membranes in addition to limiting the initial water absorption of the hybrid membrane.

We also observed that an increase in the SG uptake from 7 to 18 wt% for a TTU membrane resulted in a significantly greater mass loss from 0.7 wt% H₂O₂. We can therefore conclude that there is an optimum for SG uptake and that greater uptake of stabilising functions is not necessarily beneficial. These macroscopic measurements suggest that the two SG thiourea phases HTU and TTU effectively protect the sPEEK membrane from oxidation (no significant change in mass loss and water uptake) up to 0.6 wt% H₂O₂ (vs. 0.3 wt% for the unprotected sPEEK membrane).

To deepen our analysis we will now use UV-vis spectrometry of the ageing solutions to track the apparition of sPEEK degradation products, and FTIR to follow the chemical evolution of the membranes.

### 3.2.3. Degradation products in ageing solutions

The raw absorption spectra are plotted in Fig. 2a for a 0.15 wt% H₂O₂ reference solution (after 24 h at 80 °C) and a 0.15 wt% H₂O₂ solution in which an sPEEK membrane was immersed during ageing (24 h at 80 °C). It can be observed that the absorbance at 295 nm of the sPEEK membrane ageing solution (π conjugated oxidized products and sPEEK oligomers) is much higher than that measured for the reference solution. This marked increase in absorbance reflects the presence of degradation products characteristic of sPEEK that are eluted in the H₂O₂ solution⁷⁰. A calibration curve was used to determine that an absorbance of 0.1 at 295 nm corresponds to the elution of 1.6 × 10⁻³ mg/mL of degradation products, i.e. <0.3 wt% of the initial membrane mass (ageing is done with 0.5 mg of dry membrane per mL of H₂O₂ solution). Such subtle loss of mass cannot be quantified reliably by weighing (see Fig. 1b) due to the weighing uncertainty. UV spectroscopy is therefore the only way to detect the early stages of degradation by precise quantification of the degradation products eluted in the ageing solution. The evolution of the UV absorption spectra of the membrane ageing solutions are shown in Fig. 2b for different H₂O₂ concentrations after subtraction of the contribution of residual H₂O₂ (UV spectrum of the reference solution).

The change in absorbance at 295 nm as a function of H₂O₂ concentration is shown in Fig. 3a.

We observed a linear increase in absorbance at 295 nm as a function of H₂O₂ concentration for the sPEEK membrane and a similar trend for the TTU hybrid membrane. However, no degradation products were detected for the HTU hybrid membrane, up to a concentration of 0.1 wt % hydrogen peroxide. The detection of degradation products observed by UV for the TTU membranes are related to the oxidation mechanism of

the thiourea function (aniline release) presented in Fig. 4.

Reactions involving the thiourea functional group are comprehensively reviewed in the article by S. Sahu et al. [68]. The primary oxidation product of a thiourea function is a urea function. In the presence of water and in an acidic medium (sulphonic acid functions of the sPEEK phase) this urea function can hydrolyse to form an amine function and a terminal carbamic acid function [82]. For the TTU SG phase, chain break N°2 seems more likely than N°1 because of the possible conjugation between the released amine function and the aromatic ring located alpha to the nitrogen atom (conjugation that stabilises the released molecule). This chain break results in the release of an aniline molecule which absorbs at 295 nm. UV spectroscopy is therefore unable to quantify the degradation products from the TTU SG phase separately from those from the host sPEEK phase.

In the case of a hybrid HTU membrane, this hydrolysis leads to an intramolecular chain scission within the sol-gel phase. As the two chain ends created are chemically bound to the sol-gel network (hexa-functional SG phase), the only water-soluble product that can be eluted is carbonic acid (hydrolysis of the carbamic acid function), which cannot be detected by UV. For this membrane, the absorbance measured at 295 nm remains zero up to a H₂O₂ concentration of 0.1 wt%. Assuming that the reactivities of the SG HTU and TTU phases are the same, it is highly likely that the products detected at 0.1 wt% H₂O₂ for the TTU membrane are indeed from the SG phase (aniline) and not from the sPEEK.

### 3.2.4. Degradation products in the membranes

As degradation products can leach out of the membranes but can also remain trapped inside, it is important to characterize their possible presence in the aged membranes.

The IR spectra recorded by transmission for sPEEK membranes (with and without oxidative ageing) are shown in Fig. 5. Oxidation of the sPEEK phase results in the appearance of a fairly broad group of overlapping bands between 1760 and 1680 cm⁻¹ characteristic of the various oxidation products formed. Their detection in the aged membrane and the absence of significant mass loss at low concentrations of hydrogen peroxide confirm that the elution of oligomers resulting from these chain scissions remains very limited. At high concentrations of hydrogen peroxide (above 0.3 wt% for sPEEK), the rate of chain breaks becomes sufficient to allow their strong elution and the detection of significant mass loss. While analysis of the spectrum in the 1760 - 1680 cm⁻¹ region allows quantification of ageing products, the region between 1680 and 800 cm⁻¹ cannot be used as it is saturated due to the thickness of the membrane.

The absorption band for the main oxidation products characteristic

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** (a) UV spectra of ageing solution from a sPEEK membrane exposed to 0.15 wt% H₂O₂ and of the corresponding H₂O₂ 0.15 wt% ageing solution alone after 24 h @ 80 °C and (b) absorbance of ageing solution from a sPEEK membrane exposed to different concentrations of H₂O₂ after subtraction of the residual absorbance due to the presence of H₂O₂.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** (a) UV quantification at 295 nm of sPEEK oxidation products eluted into ageing solution as a function of H₂O₂ concentration. (b) FTIR spectroscopy quantification of non-eluted sPEEK degradation products as a function of H₂O₂ concentration. The membranes studied were: sPEEK membrane (●) and 7 wt% SG hybrids HTU (■), TTU (▲).</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Semi-developed representation of the oxidation and hydrolysis mechanisms of sol-gel phases with a thiourea sacrificial function.</figcaption></figure>

of oxidation of the sPEEK phase is at 1735 cm⁻¹. After deconvolution of the group of overlapping bands (procedure described in the supporting information SI-2), the change in absorbance at 1735 cm⁻¹ as a function of the H₂O₂ concentration can be used to evaluate the membrane oxidation 'kinetics' (analogy between the number of reactions induced over time and, the number of reactions induced by an increase in H₂O₂ concentration) as presented in Fig. 3b

Fig. 3b shows that in the case of a sPEEK membrane, oxidation products are detectable even under very mild ageing conditions (typically at an H₂O₂ concentration of 0.025 wt%).

These analyses confirm the very low intrinsic chemical stability of sPEEK. Fig. 6 shows the FTIR spectra recorded for the aged hybrid membranes in the region of interest 1760–1680 cm⁻¹. A gradual increase in absorbance is observed at 1700 cm⁻¹ then, beyond a critical hydrogen peroxide concentration of 0.1 wt%, the first oxidation products of the sPEEK phase are detected at 1735 cm⁻¹. The absorption band at 1700 cm⁻¹ is associated with the formation of carbamic acid functions resulting from the oxidation of thiourea functions to urea and their hydrolysis to aniline (TTU) or primary amine (HTU) and carbamic acid. Carbamic acids are covalently bound to the sol-gel network formed in the membrane. They are probably stabilised by an intramolecular mesomeric effect (NH/CO), by dimer formation (a process favoured by their confinement in the membrane but also by possible ionic exchange with the host sPEEK matrix). This absence of oxidation products

characteristic of sPEEK (1735 cm⁻¹) at low concentrations of H₂O₂ (< 0.1 wt%) confirms that the degradation products detected by UV for the TTU membrane ageing solutions originate from the oxidation of the SG phase (release of aniline) and not from the sPEEK phase.

These SG phase oxidation products (1700 cm⁻¹), which precede the formation of sPEEK phase oxidation products, continue to accumulate in parallel with the formation of oxidation products characteristic of the sPEEK phase at 1735 cm⁻¹ (hydrogen peroxide concentrations greater than 0.15 wt%).

The evolution of the absorbance at 1735 cm⁻¹ as a function of hydrogen peroxide concentration is shown in Fig. 3b Although no oxidation products of sPEEK are detected up to 0.15 wt% H₂O₂, above this concentration the apparent oxidation rate of the sPEEK phase of the hybrid membranes is lower than that measured for the non-stabilised sPEEK membrane: approximately 0.45 A.[H₂O₂]⁻¹ vs 0.74 A.[H₂O₂]⁻¹.

These analyses indicate that the SG phase has two modes of action. Up to a hydrogen peroxide concentration of 0.15 wt%, the SG phase provides total protection for the sPEEK phase, generally referred to as the induction period (“oxidation induction time” before degradation occurs). During this induction period, the sPEEK phase is completely protected from oxidation. This is why the absorbance measured at 1735 cm⁻¹, which is characteristic of sPEEK oxidation products, remains zero. The oxidizing species generated are entirely consumed by the thiourea groups of the sol-gel (SG) phase, which oxidize to form carbamic acid
<figure><img src="image_8.png" /><figcaption>Fig. 5. FTIR transmission spectra of a sPEEK membrane after 24 h at 80 °C in water (-) or submitted to a 0.3 wt% H₂O₂ ageing solution at 80 °C (-). Inset: a broadening of the 1800–1600 cm⁻¹ zone characteristic of oxidation products.</figcaption></figure>

functions (absorption band at 1700 cm⁻¹). Beyond this induction period (at H₂O₂ concentrations above 0.15 wt%), an increase in absorbance at 1735 cm⁻¹ is observed (Fig. 3b). However, the kinetics of degradation product formation are slower than those observed in an sPEEK membrane without the SG phase, demonstrating that the SG phase continues to limit the degradation of the sPEEK phase. This secondary protection is likely provided by thiourea functions still available, particularly in the inter-bundle regions of sPEEK, which are less accessible than those located in the ionic domains of sPEEK.

### 3.2.5. Influence of the SG uptake on ageing

We will now study the impact of the SG uptake on the durability of the membrane. This study was only carried out for the TTU membranes, for which the SG uptake has little impact on the membrane conductivity (Table 2).

As mentioned above, the analysis of the ageing solution by UV spectroscopy is not very informative as we cannot distinguish the products resulting from the oxidation of the sPEEK phase from those

resulting from the oxidation of the TTU SG phase. Nevertheless, we can see from Fig. 7a that at low concentrations of H₂O₂ (<0.1 wt%), the absorbance of the ageing solution from the 18 wt% SG membrane is higher than that measured for the 7 wt% SG membrane. The phenomenon seems to be reversed at higher H₂O₂ concentrations. If this variation in absorbance is indeed related to the release of aniline, it means that either this elution is facilitated by a higher SG phase content, or that the SG phase becomes more reactive and therefore the thiourea functions are converted to acid at a higher rate for the same hydrogen peroxide concentration. Quantification of the oxidation products remaining in the membrane by IR spectroscopy should provide answers. Fig. 7b shows the quantification of oxidation products in the sPEEK phase as a function of H₂O₂ concentration. As previously observed, this is a two-stage process. There is an initial period of complete protection during which the absorbance at 1735 cm⁻¹ is zero, indicating no degradation of the sPEEK phase. This is followed by an increase in absorbance at this wavenumber characteristic of oxidation of the sPEEK. Unexpectedly, the threshold concentration = induction time (i.e. the period of complete protection) is halved as the SG phase uptake increases (end of this first period for a concentration of 0.75 wt% H₂O₂ in the case of a 18 wt% SG membrane vs 1.5 wt% H₂O₂ for a 7 wt% SG membrane).

However, an increase in SG uptake is associated with a thinning of the membrane. For example, the thickness of a 18 wt% TTU membrane in the swollen state is 30 µm compared to 42 µm for the 7 wt% membrane for similar water contents (180 ± 22 wt% and 160 ± 25 wt% for 7 and 18 wt% SG, respectively). The ability of the SG phase to inhibit any oxidation depends on the intrinsic reactivity of the reactive function but also on the diffusion rate of the oxidising species to the heart of the membrane.

Assuming that H₂O₂ diffusion follows Fick's law and that the diffusion coefficient changes little over the SG range studied (7 – 18 wt%), the 'kinetics' (equivalence between an increase in H₂O₂ concentration and an increase in ageing time) should be normalised by the square of the thickness (Fig. 7b).

Fig. 7b shows the standardised 'kinetics' for these two membranes. It can be seen that the threshold concentration at which membrane ageing is observed is the same for both membranes (7 and 18 wt%). Beyond this period of complete protection, the oxidation rate of the 18 wt% membrane is reduced by a factor of 3 compared to the 7 wt% membrane. This result therefore tends to show that for both membranes, the amount of SG phase that fully protects the sPEEK phase is the same regardless of the SG uptake. On the other hand, the higher the SG uptake, the more effective the secondary protection (lower slopes after induction time in Fig 7).

<figure><img src="image_9.png" /><figcaption>Fig. 6. FTIR spectra of HTU (a) and TTU (b) 7 wt% hybrid membranes after exposure to different H₂O₂ concentrations.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 7. (a)** - UV quantification at 295 nm of the degradation products eluted from sPEEK and from the TTU sol-gel phase, into ageing solution and (b) - IR quantification of the non-eluted products of the membranes and oxidation characteristics of the sPEEK phase (after normalisation by the square of the thickness) as a function of the hydrogen peroxide concentration. The membranes studied were: sPEEK membrane (●) and hybrids TTU 7 wt% (▲) and TTU 18 wt%SG (Δ).</figcaption></figure>

### 3.3. Membrane durability in fuel cells

Overall, fourteen different membrane formulations were tested in FC and the main points of the polarization curves measured with the three formulations that gave the best performance are shown in Table 3 below (extensive data is presented in the supporting information SI-3), together with the sPEEK membrane. As can be seen in Table 3, the best results were obtained with the sPEEK membrane, TTU hybrid membranes with a SG uptake of 6.5 and 18.7 wt%-and a HTU hybrid membrane with a SG uptake of 5.6 wt%. Three points are worth mentioning:

* None of the SG hybrid membranes achieves performances higher than those of the sPEEK membrane but those of the TTU hybrid membrane with a SG uptake of 6.5 wt% were equivalent.

* The performance observed would probably have been much better if the membrane/electrode interface had been of better quality: we are currently working on improving the drying process to obtain smoother and flatter membrane surface.

* Anode and cathode ECSA (26 ± 2 m²g⁻¹) measurements showed only small variations between the different MEAs, indicating that the quality of the membrane electrode interface did not have significant impact on the electrode operation.

The accelerated ageing protocol described in Section 2.4 makes it possible to monitor various parameters that can help to understand the degradation mechanisms of MEA. However, their evolutions are not always easy to interpret, especially when they are related to several FC components or materials: this is the case of the polarisation curves (in the supporting information SI-4), which can be impacted by the operation of the two electrodes, the membrane, and by the interface between them. Nevertheless, the evolution of the polarisation curves clearly showed that the MEAs prepared with the hybrid membranes did not degrade at a similar rate. Note that two membranes of each type were tested to check for the repeatability of the results.

<caption>**Table 3**</caption>

Power densities for different voltages extracted from polarisation curves for sPEEK membrane and hybrid HTU/TTU membranes.

<table><thead><tr><th></th><th>OCV</th><th>1.20 A/cm²</th><th>2.64 A/cm²</th></tr></thead><tbody><tr><td>sPEEK</td><td>0.99 V</td><td>0.53 V</td><td>0.29 V</td></tr><tr><td>HTU 5.6 %SG</td><td>1.00V</td><td>0.30 V</td><td>-</td></tr><tr><td>TTU 7 %SG</td><td>0.99 V</td><td>0.54 V</td><td>0.30 V</td></tr><tr><td>TTU 19 %SG</td><td>0.99 V</td><td>0.49 V</td><td>-</td></tr></tbody></table>

The main parameters that can be used as indicators of the membrane state of health are the FC Open Circuit Voltage (OCV) and the hydrogen permeation currents, the latter being more sensitive to possible degradation. Both are plotted in Fig. 8.

One can see in Fig. 8, that the TTU 18 wt%SG hybrid membrane has a lower durability than the sPEEK membrane: its permeation increases sharply after the first 24 h. Although, the hybrid TTU 7 wt% has a higher durability than the sPEEK membrane: permeation starts after 192 h and increases rapidly. This result is surprising given the good chemical stability of both TTU 18 wt% and TTU 7 wt% hybrid membranes in ex-situ degradation tests.

On the other hand, hybrid HTU 5.6 wt% proved to be twice as durable as the sPEEK membrane, with hydrogen permeation occurring after 288 h, compared to 144 h for the reference. This makes the HTU 5.6 wt% hybrid membrane the most durable of all the hybrid and pure sPEEK membranes tested. Unfortunately, as discussed above its FC performance was rather disappointing. These results show that the SG phase can improve the durability of the sPEEK membranes, except in the case of the TTU hybrid membranes with 18 % SG uptake that presented more significant performance losses. One possible contribution to this behavior was the decrease in the gas barrier property observed for TTU hybrid membranes with such a high SG uptake (Table 2).

To summarize the results presented in this section, hybridization of sPEEK membranes can actually improve their durability in FC compared to pure sPEEK. However, the impact of hybridization is highly dependent on the nature of the precursor and of the sol-gel content.

### 4. Conclusion

Chemical stabilisation of polymer electrolytes is now essential to achieve lifetimes approaching 30,000 hours for commercial devices used in transport applications. Among the various possible approaches, we show that chemical stabilisation by the sol-gel route is particularly interesting. It allows the post-modification of a polymer electrolyte with very limited impact on its functional properties (proton conduction and gas permeability). In this work, we demonstrate that a SG phase containing thiourea-type organofunctional groups significantly reduces the rate of chemical degradation of a sPEEK membrane, despite the fact that sPEEK is very sensitive to oxidative stress. This SG phase provides complete initial protection to the sPEEK phase. Above a threshold concentration of H₂O₂, the SG phase significantly reduces the rate of oxidation of the sPEEK phase (secondary protection). In the range of SG investigated (7 – 18 wt%), the threshold H₂O₂ concentration above which degradation of the sPEEK phase is observed does not appear to depend on the SG phase uptake and therefore on the quantity of
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Accelerated stress test. (a) Current density and voltage profiles during the humidity and load cycling AST. (b) AST for sPEEK membrane (●), hybrid TTU 18 wt% (●), hybrid TTU 7 wt% (■) and HTU 5.6 wt% (■) in permeation (■■) and OCV (■■■).</figcaption></figure>

stabilising chemical functions. Stack tests carried out at the OCV confirm the effectiveness of these treatments.

### CRediT authorship contribution statement

**C. Tougne:** Writing – review & editing, Writing – original draft,

Investigation, Formal analysis, Data curation. **M. Daoudi:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation. **E. Ferri:** Writing – review & editing, Writing – original draft, Investigation, Formal analysis, Data curation. **V.H. Mareau:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Investigation, Funding acquisition, Formal
analysis, Data curation, Conceptualization. **H. Mendil-Jakani:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **V. Dufaud:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. **C. Santini:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Investigation, Funding acquisition, Conceptualization. **E. Espuche:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **O. Lottin:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **J.C. Perrin:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **J. Dillet:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **A. El Kaddouri:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Methodology, Investigation, Funding acquisition, Data curation, Conceptualization. **L. Gonon:** Writing – review & editing, Writing – original draft, Validation, Supervision, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

This work was supported by the French National Research Agency (project ANR-18-CE05-0027 MULTISTABLE).

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.electacta.2025.147338.

## Data availability

Data will be made available on request.

## References

[1] Clean hydrogen partnership, strategic research and innovation agenda 2021-2027, 2022.

[2] J. Marcinkoski, R. Vijayagopal, J. Adams, B. James, J. Kopasz, R. Ahluwalia, DOE Advanced Truck Technologies subsection of the electrified powertrain roadmap technical targets for hydrogen-fueled long-haul tractor-trailer trucks, (2019).

[3] T. Kwon, Y. Lim, J. Cho, R. Lawler, B.J. Min, W.A. Goddard, S.S. Jang, J.Y. Kim, Antioxidant technology for durability enhancement in polymer electrolyte membranes for fuel cell applications, Mater. Today 58 (2022) 135-163, https://doi.org/10.1016/j.MATTOD.2022.06.021.

[4] L. Dubau, L. Castanheira, F. Maillard, M. Chatenet, O. Lottin, G. Maranzana, J. Dillet, A. Lamibrac, J.C. Perrin, E. Moukheiber, A. Elkaddouri, G. De Moor, C. Bas, L. Flandin, N. Caqué, A review of PEM fuel cell durability: materials degradation, local heterogeneities of aging and possible mitigation strategies, Wiley. Interdiscip. Rev. Energy. Env 3 (2014) 540-560, https://doi.org/10.1002/WENE.113.

[5] T. Jahnke, G. Futter, A. Latz, T. Malkow, G. Papakonstantinou, G. Tsotridis, P. Schott, M. Gérard, M. Quinaud, M. Quiroga, A.A. Franco, K. Malek, F. Calle-Vallejo, R. Ferreira De Morais, T. Kerber, P. Sautet, D. Loffreda, S. Strahl, M. Serra, P. Polverino, C. Pianese, M. Mayur, W.G. Bessler, C. Kompis, Performance and degradation of Proton Exchange membrane fuel cells: state of the art in modeling from atomistic to system scale, J. Power. Sourc. 304 (2016) 207-233, https://doi.org/10.1016/j.JPOWSOUR.2015.11.041.

[6] A. Kraytsberg, Y. Ein-Eli, Review of advanced materials for proton exchange membrane fuel cells, Energy. Fuel. 28 (2014) 7303-7330, https://doi.org/10.1021/EF501977K/ASSET/IMAGES/LARGE/EF-2014-01977K_0025.

[7] J.W. Lim, D. Lee, M. Kim, J. Choe, S. Nam, D.G. Lee, Composite structures for proton exchange membrane fuel cells (PEMFC) and energy storage systems (ESS): review, Compos. Struct. 134 (2015) 927-949, https://doi.org/10.1016/J.COMSTRUCT.2015.08.121.

[8] C.M. Branco, S. Sharma, M.M. De Camargo Forte, R. Steinberger-Wilckens, New approaches towards novel composite and multilayer membranes for intermediate temperature-polymer electrolyte fuel cells and direct methanol fuel cells, J. Power. Sourc. 316 (2016) 139-159, https://doi.org/10.1016/j.JPOWSOUR.2016.03.052.

[9] E. Quartarone, S. Angioni, P. Mustarelli, Polymer and composite membranes for proton-conducting, high-temperature fuel cells: a critical review, Materials 10 (2017) 687, https://doi.org/10.3390/MA10070687, 2017, Vol. 10, Page 687.

[10] A. Chandan, M. Hattenberger, A. El-Kharouf, S. Du, A. Dhir, V. Self, B.G. Pollet, A. Ingram, W. Bujalski, High temperature (HT) polymer electrolyte membrane fuel cells (PEMFC) – A review, J. Power. Sourc. 231 (2013) 264-278, https://doi.org/10.1016/j.JPOWSOUR.2012.11.126.

[11] Z. Rui, J. Liu, Understanding of free radical scavengers used in highly durable proton exchange membranes, Prog. Nat. Sci. 30 (2020) 732-742, https://doi.org/10.1016/j.PNSC.2020.08.013.

[12] X. Yu, S. Bai, Q. Li, Z. Zhao, Q. Sun, S. Cao, H. Cui, M. Liu, Q. Xu, C.C. Hou, Performance optimization by antioxidant strategies for proton exchange membrane fuel cells: recent progress and future, EnergyChem 7 (2025) 100142, https://doi.org/10.1016/j.enchem.2024.100142.

[13] S.J. Osborn, M.K. Hassan, G.M. Divoux, D.W. Rhoades, K.A. Mauritz, R.B. Moore, Glass transition temperature of perfluorosulfonic acid ionomers, Macromolecules 40 (2007) 3886-3890.

[14] H. Huang, X. Zeng, X. Zhang, J. Fan, H. Li, Proton exchange membrane based on interpenetrating polymer network structure for excellent cell performance and chemical stability, J. Power. Sourc. 558 (2023) 232602, https://doi.org/10.1016/J.JPOWSOUR.2022.232602.

[15] H. Tang, K. Geng, Y. Hu, N. Li, Synthesis and properties of phosphonated polysulfones for durable high-temperature proton exchange membranes fuel cell, J. Memb. Sci. 605 (2020) 118107, https://doi.org/10.1016/j.memsci.2020.118107.

[16] E. Qu, M. Xiao, D. Han, S. Huang, Z. Huang, W. Liu, S. Wang, Y. Meng, A novel high temperature fuel cell proton exchange membrane with nanoscale phase separation structure based on crosslinked polybenzimidazole with poly(vinylbenzyl chloride), Nanomaterials 13 (2023), https://doi.org/10.3390/nano13020266.

[17] V. Delhorbe, S.R. Reijerkerk, C. Cailleteau, M. Bathfield, L. Chikh, F. Gouanve, L. Ogier, E. Espuche, B. Ameduri, S. Vidal, G. Gebel, A. Morin, O. Fichet, Polyelectrolyte/fluorinated polymer interpenetrating polymer networks as fuel cell membrane, J. Memb. Sci. 429 (2013) 168-180, https://doi.org/10.1016/J.MEMSCI.2012.11.032.

[18] A. Pokprasert, P. Theato, S. Chirachanchai, Proton donor/acceptor copolymer brushes on sulfonated poly(ether ether ketone) membrane: an approach to construct efficient proton transfer pathway in polymer electrolyte membrane fuel cell, Polymer 240 (2022) 124523, https://doi.org/10.1016/J.POLYMER.2022.124523.

[19] F. Wijaya, S. Woo, H. Lee, A.F. Nugraha, D. Shin, B. Bae, Sulfonated poly (phenylene-co-arylene ether sulfone) multiblock membranes for application in high-performance fuel cells, J. Memb. Sci. 645 (2022) 120203, https://doi.org/10.1016/j.MEMSCI.2021.120203.

[20] S. Swaby, N. Ureña, M. Teresa Pérez-Prior, C. Del Río, A. Vázquez, J.Y. Sanchez, C. Iojoiu, B. Levenfeld, Proton conducting sulfonated polysulfone and polyphenylsulfone multiblock copolymers with improved performances for fuel cell applications, (2023), https://doi.org/10.1016/j.jiec.2023.02.037.

[21] M. Zaton, J. Rozière, D.J. Jones, Mitigation of PFSA membrane chemical degradation using composite cerium oxide-PFSA nanofibres, J. Mater. Chem. Mater. 5 (2017) 5390-5401, https://doi.org/10.1039/C6TA01977B.

[22] A.M. Baker, L. Wang, W.B. Johnson, A.K. Prasad, S.G. Advani, Nafion membranes reinforced with ceria-coated multiwall carbon nanotubes for improved mechanical and chemical durability in polymer electrolyte membrane fuel cells, J. Phys. Chem. C 118 (2014) 26796-26802, https://doi.org/10.1021/JP5078399/ASSET/IMAGES/LARGE/JP-2014-078399_0008.JPEG.

[23] D. Banham, S. Ye, T. Cheng, S. Knights, S.M. Stewart, M. Wilson, F. Garzon, Effect of CeOx crystallite size on the chemical stability of CeOx nanoparticles, J. Electrochem. Soc. 161 (2014) F1075-F1080, https://doi.org/10.1149/2.0931410JES/XML.

[24] M.T. Taghizadeh, M. Vatanparast, Preparation and evaluation of Nafion/SnO2 nanocomposite for improving the chemical durability of proton exchange membranes in fuel cells, RSC. Adv. 6 (2016) 56819-56826, https://doi.org/10.1039/C6RA07849D.

[25] L. Liu, Y. Xing, Y. Li, Z. Fu, Z. Li, H. Li, Double-layer expanded polytetrafluoroethylene reinforced membranes with cerium oxide radical scavengers for highly stable proton exchange membrane fuel cells, ACS. Appl. Energy. Mater 5 (2022) 8743-8755, https://doi.org/10.1021/ACSAEM.2C01263/ASSET/IMAGES/MEDIUM/AE2C01263_M018.GIF.

[26] T. Kwon, S.H. Park, B.J. Min, S. Park, S. Ramadhani, Y. Lim, S.S. Jang, H. Jeong, H. J. Son, J.Y. Kim, Ce(III)-based coordination-complex-based efficient radical scavenger for exceptional durability enhancement of polymer application in proton-exchange membrane fuel cells and organic photovoltaics, Adv. Energy. Sustain. Res. 3 (2022) 2200011, https://doi.org/10.1002/AESR.202200011.
[27] Y.S. Kang, S. Jang, E. Choi, S. Jo, S.M. Kim, S.J. Yoo, Sandwich-like Nafion composite membrane with ultrathin ceria barriers for durable fuel cells, Int. J. Energy. Res. 46 (2022) 6457-6470, https://doi.org/10.1021/ER7582.

[28] K. Xu, S. Pei, W. Zhang, Z. Han, P. Guan, L. Wang, Y. Zou, H. Ding, X. Ma, C. Xu, P. Liu, Y. Zhang, Ce (III)-terephthalic acid metal-organic frameworks as highly efficient -OH radical scavengers for fuel cells and investigation of its antioxidation mechanism, Mater. Today. Energ. 31 (2023) 101195, https://doi.org/10.1016/J.MTENER.2022.101195.

[29] T. De Wild, T. Nemeth, P. Becker, D. Günther, T. Nauser, T.J. Schmidt, L. Gubler, Repair of aromatic hydrocarbon-based membranes tested under accelerated fuel cell conditions, J. Power. Sourc. 560 (2023), https://doi.org/10.1016/j.jpowsour.2022.232525.

[30] P.A. Yurova, V.R. Malakhova, E.V. Gerasimova, I.A. Stenina, A.B. Yaroslavtsev, Nafion/surface modified ceria hybrid membranes for proton exchange fuel cell application, Polymers 13 (2021) 2513, https://doi.org/10.3390/POLYM13152513/S1.

[31] X. Shen, X. Liang, Y. Xu, W. Yu, Q. Li, X. Ge, L. Wu, T. Xu, In-situ growth of PPy/MnOx radical quenching layer for durability enhancement of proton exchange membrane in PEMFCs, J. Memb. Sci. 675 (2023) 121556, https://doi.org/10.1016/J.MEMSCI.2023.121556.

[32] J. Zhang, Z. Xie, J. Zhang, Y. Tang, C. Song, T. Navessin, Z. Shi, D. Song, H. Wang, D.P. Wilkinson, Z.S. Liu, S. Holdcroft, High temperature PEM fuel cells, J. Power. Sourc. 160 (2006) 872-891, https://doi.org/10.1016/j.JPOWSOUR.2006.05.034.

[33] J. Wang, Y. Dai, R. Wan, W. Wei, S. Xu, F. Zhai, R. He, Grafting free radical scavengers onto polyarylethersulfone backbones for superior chemical stability of high temperature polymer membrane electrolytes, Chem. Eng. J. 413 (2021) 127541, https://doi.org/10.1016/j.CEJ.2020.127541.

[34] S.H. Yook, H.Y. Kim, S.J. Kim, S. Choi, T. Kwon, H. Cho, J.M. Kim, K.R. Yoon, S. Jo, S.Y. Lee, H.J. Kim, H.J. Son, K.H. Chae, J. Kim, K.Y. Lee, J.Y. Kim, Boosting antioxidation efficiency of nonstoichiometric CeOx nanoparticles via surface passivation toward robust polymer electrolyte membrane fuel cells, Chem. Eng. J. 432 (2022) 134419, https://doi.org/10.1016/j.JPOWSOUR.2022.134419.

[35] H. Huang, X. Zhang, J. Fan, H. Li, H. Wang, Expanded polytetrafluoroethylene functionalized with free radical scavengers and hydrophilic groups for superior chemical stability of proton exchange membranes, Int. J. Hydrog. Energy 47 (2022) 18109-18121, https://doi.org/10.1016/J.IJHYDENE.2022.03.272.

[36] T. Agarwal, I. Matanovic, S. Adhikari, E.J. Park, S. Komini Babu, Y.S. Kim, D. Tian, C. Bae, O. Morales-Collazo, J.F. Brennecke, A.K. Prasad, S.G. Advani, A. Sievert, T. Hopkins, A. Park, R. Borup, Mitigating cerium migration for perfluorosulfonic acid membranes using organic ligands, J. Power. Sourc. 554 (2023) 232320, https://doi.org/10.1016/j.JPOWSOUR.2023.232320.

[37] S. Wang, M. Yan, Y. Li, C. Vinado, J. Yang, Separating electronic and ionic conductivity in mix-conducting layered lithium transition metal oxides, J. Power. Source. 393 (2018) 75-82, https://doi.org/10.1016/j.JPOWSOUR.2018.05.005.

[38] D. Shin, M. Han, Y.G. Shul, H. Lee, B. Bae, Analysis of cerium-composite polymer-electrolyte membranes during and after accelerated oxidative-stability test, J. Power. Sources 378 (2018) 468-474, https://doi.org/10.1016/J.JPOWSOUR.2017.12.074.

[39] X. Liu, Y. Li, M. Li, N. Xie, J. Zhang, Y. Qin, Y. Yin, M.D. Guiver, Durability enhancement of proton exchange membrane fuel cells by ferrocyanide or ferricyanide additives, J. Memb. Sci. 629 (2021) 119282, https://doi.org/10.1016/J.MEMSCI.2021.119282.

[40] P.P. Sharma, D.A. Kim, P.P. Sharma, D. Kim, A facile and sustainable enhancement of anti-oxidation stability of nafion membrane, Membrane 12 (2022) 521, https://doi.org/10.3390/MEMBRANES12050521, 2022, Vol. 12, Page 521.

[41] L. Liu, Y. Li, R. Qiao, Y. Xing, H. Li, Reinforced composite membranes based on expanded polytetrafluoroethylene skeletons modified by a surface sol-gel process for fuel cell applications, (2021). https://doi.org/10.1021/acs.energyfuels.1c01205.

[42] H. Huang, S. Xu, J. Zhou, F. Luo, J. Fan, H. Li, Mitigation of chemical degradation in perfluorosulfonic acid proton exchange membrane using regenerable hindered amine functionalized carbon quantum dots, J. Memb. Sci. 636 (2021) 119164, https://doi.org/10.1016/J.MEMSCI.2021.119164.

[43] D.C.T. Vo, M.D.T. Nguyen, D. Kim, Dual sulfonated poly(arylene ether ketone) membrane grafted with 15-crown-5-ether for enhanced proton conductivity and anti-oxidation stability, Mol. Syst. Eng. 4 (2019) 901-911, https://doi.org/10.1039/C9ME00009G.

[44] F. Bu, Y. Zhang, L. Hong, W. Zhao, D. Li, J. Li, H. Na, C. Zhao, 1,2,4-Triazole functionalized poly(arylene ether ketone) for high temperature proton exchange membrane with enhanced oxidative stability, J. Memb. Sci 545 (2018) 167-175, https://doi.org/10.1016/J.MEMSCI.2017.09.072.

[45] Y. Yao, J. Liu, W. Liu, M. Zhao, B. Wu, J. Gu, Z. Zou, Vitamin E assisted polymer electrolyte fuel cells, Energy. Env. Sci. 7 (2014) 3362-3370, https://doi.org/10.1039/C4EE01774A.

[46] Y. Buchmüller, A. Wokaun, L. Gubler, Polymer-bound antioxidants in grafted membranes for fuel cells, J. Mater. Chem. Mater. 2 (2014) 5870-5882, https://doi.org/10.1039/C3TA15321E.

[47] Y. Buchmüller, Z. Zhang, A. Wokaun, L. Gubler, Antioxidants in non-perfluorinated fuel cell membranes: prospects and limitations, RSC. Adv. 4 (2014) 51911-51915, https://doi.org/10.1039/C4RA09792K.

[48] M. Tanaka, Y. Takeda, T. Wakiya, Y. Wakamoto, K. Harigaya, T. Ito, T. Tarao, H. Kawakami, Acid-doped polymer nanofiber framework: three-dimensional proton conductive network for high-performance fuel cells, J. Power. Sourc. 342 (2017) 125-134, https://doi.org/10.1016/j.JPOWSOUR.2016.12.018.

[49] S.H. Shin, A. Kodir, D. Shin, S.H. Park, B. Bae, Perfluorinated composite membranes with organic antioxidants for chemically durable fuel cells,

Electrochim. Acta 298 (2019) 901-909, https://doi.org/10.1016/J.ELECTACTA.2018.12.150.

[50] K. Ro Yoon, K. Ah Lee, S. Jo, S. Ho Yook, K. Young Lee, I.D. Kim, J. Young Kim, K. R. Yoon, K.A. Lee, S. Jo, S.H. Yook, J.Y. Kim, K.Y. Lee, I. Kim, Mussel-inspired polydopamine-treated reinforced composite membranes with self-supported CeOx radical scavengers for highly stable PEM fuel cells, Adv. Funct. Mater. 29 (2019) 1806929, https://doi.org/10.1002/ADFM.201806929.

[51] A. Kodir, S.H. Shin, S. Park, M.R. Arbi, T.H. Yang, H. Lee, D. Shin, B. Bae, Macromolecular antioxidants for chemically durable polymer electrolyte fuel cell membranes, Int. J. Energy. Res. 46 (2022) 7186-7200, https://doi.org/10.1002/ER.7607.

[52] S.H. R. Wang, T. Tian, H. Liu, H. Zhang, H. Tang, Self-assembly-cooperating in situ construction of MXene-CeO2 as hybrid membrane coating for durable and high-performance proton exchange membrane fuel cell, (2022). https://doi.org/10.1021/acsschemeng.2c00087.

[53] Y. Huo, Q. Li, Z. Rui, R. Ding, J. Liu, J. Li, J. Liu, A highly stable reinforced PEM assisted by resveratrol and polydopamine-treated PTFE, J. Memb. Sci. 635 (2021) 119453, https://doi.org/10.1016/j.MEMSCI.2021.119453.

[54] T. Agarwal, S. Adhikari, Y.S. Kim, S. Komini Babu, D. Tian, C. Bae, N.N.T. Pham, S. G. Lee, A.K. Prasad, S.G. Advani, A. Sievert, W.P. Rasika Liyanage, T.E. Hopkins, A. Park, R. Borup, Fluoroalkyl phosphonic acid radical scavengers for proton exchange membrane fuel cells, J. Mater. Chem. Mater. 11 (2023) 9748-9754, https://doi.org/10.1039/D2TA09421E.

[55] C. Zheng, N. Xie, X. Liu, L. Wang, W. Zhu, Y. Pei, R. Yue, H. Liu, S. Yin, J. Yao, J. Zhang, Y. Yin, M.D. Guiver, Durability improvement of proton exchange membrane fuel cells by doping silica-ferrocyanide antioxidant, J. Memb. Sci. 690 (2024), https://doi.org/10.1016/j.memsci.2023.122195.

[56] S.M. Andersen, C.F. Nørgaard, M.J. Larsen, E. Skou, Tin dioxide as an effective antioxidant for proton exchange membrane fuel cells, J. Power. Sourc. 273 (2015) 158-161, https://doi.org/10.1016/j.jpowsour.2014.09.051.

[57] S. Zhao, Y. Liao, R. Wang, G. Liu, H. Zhang, H. Tang, Polyphenol synergistic cerium oxide surface engineering constructed core-shell nanostructures as antioxidants for durable and high-performance proton exchange membrane fuel cells, Chem. Eng. J. 472 (2023) 144804, https://doi.org/10.1016/j.cei.2023.144804.

[58] S. Zhao, Y. Liao, R. Wang, Y. Li, J. Shuai, L. Wang, B. Liu, R. Chen, T. Tian, H. Zhang, H. Tang, Phosphate-grafted polyethyleneimine-induced multifunctional cerium oxide as an antioxidant for simultaneously enhancing the proton conductivity and durability of proton exchange membranes, Adv. Compos. Hybrid. Mater. 7 (2024) 1-20, https://doi.org/10.1007/s42114-024-00883-w.

[59] H. Kim, S.H. Yook, H.Y. Kim, Y. Choi, Y. Lim, Y. Hwang, J. Kim, K.Y. Lee, S.S. Jang, J. Park, J.Y. Kim, Tailor-made charged catechol-based polymeric ligands to build robust fuel cells containing antioxidative nanoparticles, Adv. Electron. Mater 8 (2022) 2200171, https://doi.org/10.1002/AELM.202200171.

[60] B. Barik, B. Singh, Y. Namgung, M.S. Islam, M.K. Han, J.Y. Park, S.J. Song, 2-Mercaptoethanesulfonic acid as an effective free-radical scavenger in perfluorosulfonic acid membrane-based polymer electrolyte membrane fuel cells, J. Electrochem. Soc. 171 (2024) 074501, https://doi.org/10.1149/1945-7111/ad5a3d.

[61] F.D. Coms, H. Liu, J.E. Owejan, Mitigation of perfluorosulfonic acid membrane chemical degradation using cerium and manganese ions, ECS. Trans. 16 (2008) 1735-1747, https://doi.org/10.1149/1.2982015/XML.

[62] G. Monin, Stabilisation chimique des électrolytes polymères pour pile à combustible, (2012).

[63] M. Strukelj, J.C. Hedrick, J.L. Hedrick, R.J. Twiege, Solvent effects on the preparation of novel amorphous poly(aryl ether benzils), Macromolecules 27 (1994) 6277-6285, https://doi.org/10.1021/MA00100A008/ASSET/MA00100A008.FP.PNG_V03.

[64] E. Machado Ferri da Silva, Multi-stabilized Hybrid Membranes For Fuel Cell and Electrolyzer, Université Claude Bernard - Lyon 1, 2023.

[65] E. Ferri, C. Tougne, M. Daoudi, F. Gouanvé, O. Gain, L. Gonon, V. Mareau, H. Mendil-Jakani, A. El Kaddouri, J.C. Perrin, J. Dillet, O. Lottin, C.C. Santini, E. Espuche, V. Dufaud, Towards more durable fuel cell: elaboration of antioxidant thiourea-based hybrid sPEEK membranes using a Sol-Gel approach, (2024).

[66] M. Hoffmann, J.O. Edwards, Kinetics and mechanism of the oxidation of thiourea and N,N'-dialkylthioureas by hydrogen peroxide, Inorg. Chem. 16 (1977) 3333-3338, https://doi.org/10.1021/IC50178A069/ASSET/IC50178A069.FP.PNG_V03.

[67] M. Arifoglu, W.N. Marmer, R.L. Dudley, Reaction of Thiourea with hydrogen peroxide: 13C NMR studies of an oxidative/reductive bleaching process, 62 (1992) 94-100. https://doi.org/10.1177/004051759206200206.

[68] S. Sahu, P. Rani Sahoo, S. Patel, B.K. Mishra, Oxidation of thiourea and substituted thioureas: a review, J. Sulfur. Chem. 32 (2011) 171-197, https://doi.org/10.1080/17415993.2010.550294.

[69] R.S. Waremra, P. Betaubun, Analysis of electrical properties using the four point probe method, (n.d.). https://doi.org/10.1051/e3sconf/201873.

[70] C.C. Lin, F.R. Smith, N. Ichikawa, T. Baba, M. Itow, Decomposition of hydrogen peroxide in aqueous solutions at elevated temperatures, Int. J. Chem. Kinet. 23 (1991) 971-987, https://doi.org/10.1002/KIN.550231103.

[71] N. Huynh, J.P.C. Fernandes, P.A. Bayle, M. Bardet, E. Espuche, J. Dillet, J.C. Perrin, A. El Kaddouri, O. Lottin, V.H. Mareau, H. Mendil-Jakani, L. Gonon, Sol-gel route: an original strategy to chemically stabilize proton exchange membranes for fuel cell, J. Power. Sourc. 462 (2020) 228164, https://doi.org/10.1016/j.JPOWSOUR.2020.228164.

[72] C. Perrot, Mécanismes de dégradation des membranes polyaromatiques sulfonées en pile à combustible, http://Www.Theses.Fr (2006).
[73] M. Daoudi, E. Ferri, C. Tougne, A. El Kaddouri, J.C. Perrin, J. Dillet, L. Gonon, V. Mareau, H. Mendil-Jakani, V. Dufaud, E. Espuche, O. Lottin, Impact of sulfonated poly(ether ether ketone) membranes pretreatments on their physicochemical properties and fuel cell performances, J. Polym. Sci. 61 (2023) 761–776, https://doi.org/10.1002/POL.20220649.

[74] S. Touhami, L. Dubau, J. Mainka, J. Dillet, M. Chatenet, O. Lottin, Anode aging in polymer electrolyte membrane fuel cells: anode monitoring by ElectroChemical impedance spectroscopy, J. Power. Sourc. 481 (2021) 228908, https://doi.org/10.1016/j.JPOWSOUR.2020.228908.

[75] C.S. Gittleman, Y.H. Lai, F.D. Coms, S. Suzuki, T. Tanahashi, T. Doi, al -, J.D. Fairweather, C.S. Gittleman -, Accelerated stress testing of fuel cell membranes subjected to combined mechanical/chemical stressors and cerium migration you may also like (Invited) durability of polymer electrolyte membranes for automotive applications highly Accelerated stress testing of proton exchange fuel cell membranes for heavy duty transportation applications, (n.d.). https://doi.org/10.1149/2.0241806jes.

[76] R. Sgarbi, W.A. Idir, Q. Labarde, M. Mermoux, P. Wu, J. Mainka, J. Dillet, C. Marty, F. Micoud, O. Lottin, M. Chatenet, Does the platinum-loading in proton-exchange membrane fuel cell cathodes influence the durability of the membrane-electrode assembly? Ind. Chem. Mater. 1 (2023) 501–515, https://doi.org/10.1039/D3IM00059A.

[77] S. Touhami, M. Crouillere, J. Mainka, J. Dillet, C. Nayoze-Coynel, C. Bas, L. Dubau, A. El Kaddouri, F. Dubelley, F. Micoud, M. Chatenet, Y. Bultel, O. Lottin, Anode defects’ propagation in polymer electrolyte membrane fuel cells, J. Power. Sourc. 520 (2022) 230880, https://doi.org/10.1016/j.JPOWSOUR.2021.230880.

[78] P. Pei, Z. Wu, Y. Li, X. Jia, D. Chen, S. Huang, Improved methods to measure hydrogen crossover current in proton exchange membrane fuel cell, Appl. Energ. 215 (2018) 338–347, https://doi.org/10.1016/j.apenergy.2018.02.002.

[79] S. Naudy, F. Collette, F. Thominette, G. Gebel, E. Espuche, Influence of hygrothermal aging on the gas and water transport properties of Nafion® membranes, J. Memb. Sci. 451 (2014) 293–304, https://doi.org/10.1016/j.MEMSCI.2013.10.013.

[80] C. Damian, E. Espuche, M. Escoubes, Influence of three ageing types (thermal oxidation, radiochemical and hydrolytic ageing) on the structure and gas transport properties of epoxy–amine networks, Polym. Degrad. Stab. 72 (2001) 447–458, https://doi.org/10.1016/S0141-3910(01)00045-3.

[81] C. Jost, M. Lacuve, S. Haller, E. Espuche, X. Colin, Influence of thermo-oxidative aging on the structure and the water transport properties of sulfur-crosslinked EPDM, Polym. Degrad. Stab. 234 (2025) 111210, https://doi.org/10.1016/j.POLYMDEGRADSTAB.2025.111210.

[82] Ł. Berlicki, M. Bochno, A. Grabowiecka, A. Białas, P. Kosikowska, P. Kafarski, N-substituted aminomethanephosphonic and aminomethane-P-methylphosphinic acids as inhibitors of ureases, Amino. Acids. 42 (2012) 1937–1945, https://doi.org/10.1007/S00726-011-0920-4/FIGURES/4.