Effect of Ce-Based Scavengers on Properties and Stability of Recast Aquivion® Membranes as Mitigating Agents of Degradation for PEMFC Application

TL;DR

This study explores how cerium-based scavengers affect the stability and performance of Aquivion® membranes used in fuel cells.

Contribution

A novel Pt/CeO2 scavenger is synthesized and tested for mitigating membrane degradation in PEMFCs.

Findings

The Pt/CeO2 scavenger was synthesized using a mild aqueous method without organic ligands.

The membrane with 1.0 wt.% syn-scavenger showed the lowest degradation and best performance after testing.

The scavenger enhanced radical scavenging activity through Pt-ceria redox interactions.

Abstract

Polymeric electrolyte membranes based on a low equivalent-weight Aquivion® commercial dispersion (D72-25BS; EW = 720 g eq−1, Syensqo) were fabricated using a standardized in-house doctor-blade casting technique for application in proton exchange membrane fuel cells (PEMFCs). The low equivalent-weight (EW) Aquivion® dispersion is a copolymer of tetrafluoroethylene (TFE) and sulfonyl fluoride vinyl ether (SFVE), commonly referred to as a short-side-chain (SSC) ionomer, which exhibits higher ion-exchange capacity (IEC) and proton conductivity than long-side-chain (LSC) perfluorosulfonic membranes. A home-made 30 wt.% Pt/CeO2 radical scavenger (denoted syn-scavenger) was synthesized via a colloidal method and incorporated into the Aquivion® membranes to investigate its mitigating effect on chemical degradation induced by peroxide radicals, a role typically associated with Ce-based scavengers. Particularly, the unique aspects of the Pt/CeO2 scavenger synthesis could be summarized in the following points: (i) the mild aqueous deposition approach enabling highly dispersed Pt species on CeO2 without the use of organic ligands; and (ii) the tailored redox interaction between Pt and ceria that enhances radical scavenging activity. Two Aquivion® membranes (denoted Aqu) containing different syn-scavenger loadings (1.0 and 1.5 wt.%) were prepared and compared with a pristine Aquivion® membrane and a membrane containing commercial CeO2 (1.0 wt.%). Physicochemical characterization of the scavenger was performed using transmission electron microscopy (TEM), BET surface area analysis, and X-ray diffraction (XRD). The membranes were characterized by micro-Raman spectroscopy, water uptake and hydration number (λ), IEC, and proton conductivity measurements. To assess membrane stability, exsitu chemical oxidative degradation tests were conducted using Fenton’s reagent. Overall, the membrane containing 1.0 wt.% syn-scavenger emerged as the most promising candidate, exhibiting favourable chemical–physical properties and the lowest reductions in IEC and proton conductivity following the degradation test.

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1. Introduction

Proton exchange membrane fuel cells (PEMFCs) have emerged as a leading electrochemical energy conversion technology for a wide range of applications (automotive, stationary, marine and aerospace systems, etc.). Their appeal lies in a unique combination of high-power density, near-zero pollutant emissions, rapid start-up capability, operational reliability, compactness and low acoustic signature. Despite significant technological progress, their large-scale commercialization still faces critical challenges related to performance enhancement, energy efficiency, compactness, long-term durability, and overall system integration [1]. In particular, automotive PEMFCs are required to operate under increasingly demanding conditions (T > 100 °C; RH < 60–70%) to accelerate electrode reaction kinetics, improve tolerance to fuel impurities and reduce the complexity, size and energy consumption of thermal and humidification subsystems [2]. Such harsh environments impose severe constraints, particularly on the polymer electrolyte membrane—the core component of the PEMFC—which remains a key limiting factor of the technology. Still today, perfluorosulfonic acid (PFSA) polymers are widely regarded as the benchmark membrane materials for PEMFCs, due to their exceptional chemical inertness, mechanical robustness, and high proton conductivity (PC) under electrochemical operating conditions [3]. However, PFSA membranes are susceptible to chemical degradation during fuel cell operation. This degradation is primarily driven by the formation of highly reactive radical species (e.g., •OH and •OOH), originating from gas crossover, hydrogen peroxide formation, and Fenton-type reactions. These processes trigger detrimental phenomena such as polymer backbone scission, side-chain decomposition, membrane thinning, loss of mechanical integrity, pinhole formation, and, ultimately, deterioration of electrochemical performance [4]. For these reasons, improving their chemical durability and degradation resistance remains a research priority in the PEMFC field. Significant attention has been devoted to PFSA materials with short side chains (SSC-PFSA), such as commercial Aquivion^®^, a perfluorinated ionomer by Syensqo characterized by the shortest side-chain architecture [5,6]. Compared to long-side-chain PFSA (LSC-PFSA) membranes such as Nafion^®^, this distinctive molecular design imparts several advantageous physicochemical properties: a higher glass transition temperature (T_g_), enhanced PC, water mobility, particularly under low-humidity and high-temperature conditions [7,8], mechanical strength, dimensional stability, and thermal resistance. Moreover, SSC-PFSA membranes preserve a significant degree of crystallinity even at EW below 800, exhibiting improved mechanical integrity, enhanced resistance to chemical attack and reduced gas permeability. The determination of the polymeric membrane crystallinity, affecting durability and performance [9], is a fundamental task to address [10,11] because it strongly influences water uptake, dimensional swelling, mechanical reinforcement and the formation of interconnected hydrophilic domains responsible for proton transport [9]. Variations in crystallinity during operation, induced by hydration cycling, thermal stress or chemical degradation, can significantly alter both proton- and water-transport mechanisms [12,13]. Recent operando and in situ characterization studies have highlighted the dynamic evolution of PFSA membrane crystallinity under realistic fuel cell conditions, underscoring the necessity of correlating structural changes with electrochemical performance and degradation pathways [11]. Several studies report that membrane–electrode assemblies (MEAs) incorporating Aquivion^®^ SSC-PFSA membranes outperform their Nafion^®^-based counterparts, particularly in terms of power density, durability and performance retention under harsh conditions [14,15,16]. Current state-of-the-art research is increasingly focused on strategies to mitigate the chemical degradation, preserving or enhancing the membrane crystallinity and transport properties. These approaches include the following: the incorporation of radical scavengers (RSs), reinforcement with inorganic fillers or nano-composites, membrane crosslinking, and the development of advanced PFSA architectures with tailored side-chain length and distribution: the goal is to achieve a balance between PC, mechanical strength and long-term chemical durability. In order to improve the peroxidic degradation resistance, the incorporation of cerium-based radical scavengers (e.g., CeO_2_) [17] in SSC-PFSA membranes is one of the most promising mitigation strategies [18,19]. However, it can modify its internal structure [20] and crystallinity, influencing the membranes’ properties, such as proton and water transport [12,13]. Cerium is a well-known material, able to react more quickly with oxidant radicals with respect to -SO_3_H groups of the polymer and protect the PFSA chain. Cerium can cycle between Ce^3+^/Ce^4+^ redox states, efficiently quenching radicals before they attack the polymer backbone [21] by converting radicals into water through successive redox interactions [22]:

$$[eqn]$$ $$[eqn]$$ $$[eqn]$$ $$[eqn]$$

Enhancing redox cycling and overall scavenging efficiency involves increasing the Ce^3+^ content, which can be achieved through various approaches such as elemental doping, forming composites with other materials and modifying the material’s morphology, structure or composition [23].

The strong metal–support interaction (SMSI) phenomenon arises when noble metal catalysts such as Pt, Pd, or Rh are combined with reducible metal oxides like CeO_2_, SnO_2_, TiO_2_, MnO_2_, WO_3_, Nb_2_O_5_, or V_2_O_5_ [24]. This interaction promotes electron transfer from the noble metal to the oxide support, while reactive oxygen species simultaneously migrate from the oxide surface to the metal, enhancing catalytic behaviour. Several studies have reported the incorporation of metallic nanoparticles supported on metal oxides into polymer electrolyte membranes (PEMs) [25]. Watanabe and co-workers proposed composite membranes containing highly dispersed Pt nanoparticles and metal oxides to inhibit gas crossover via catalytic recombination of H_2_ and O_2_. Furthermore, hygroscopic oxides were incorporated to improve water retention and enable operation at low humidity. In PEFC tests, these membranes outperformed conventional ones due to lower gas crossover, even without humidification [26].

Following this approach, recast Aquivion^®^ membranes containing dispersed CeO_2_-based scavenger particles—either as pure oxide or combined with metals—have emerged as an important research pathway toward improving PEMFC lifetime. For example, D’Amato et al. [17] investigated Aquivion^®^ membranes incorporating polydopamine-coated CeO_2_ nanoparticles (PDA-CeO_2_), where the PDA coating ensured a stable peroxide decomposition capacity under high-temperature and low-humidity conditions.

Moreover, the radical scavenging activity remained effective up to approximately 4 wt.% filler loading. In general, the developed composite membranes exhibited significantly improved resistance in Fenton’s tests and OCV-accelerated durability tests. Siracusano et al. [21] investigated PEM electrolysis using Aquivion^®^ membranes containing Ce-based scavengers under real operating conditions. After approximately 3500–3800 h of operation, the Ce-containing membrane demonstrated improved voltage efficiency and reduced H_2_ crossover compared to the reference membrane. Furthermore, the incorporation of Ce-based scavengers enhanced long-term stability and resistance to gas crossover, owing to the increased membrane tortuosity, under harsh electrolyzer operating conditions. These findings strongly support their potential application in PEMFCs, as well. D’Urso et al. [27] prepared reinforced Aquivion^®^ membranes loaded with mono- and bimetallic (Ce and Ce–Cr) scavengers supported on silica. The monometallic Ce-based scavenger achieved the best overall performance and durability, with Ce-containing membranes improving MEA lifetime by up to sevenfold compared with the baseline membrane. Although the bimetallic Ce–Cr systems require further optimization, the reported evidence clearly demonstrates that Ce-based scavengers significantly mitigate chemical degradation in Aquivion^®^ membranes for PEMFC applications.

In this study, a 30 wt.% Pt/CeO_2_ scavenger [28] was synthesized via a colloidal method and incorporated into Aquivion^®^ membranes to investigate its ability to mitigate chemical degradation induced by peroxide radical—a function typically associated with Ce-based scavengers. The unique aspects of the Pt/CeO_2_ scavenger synthesis can be summarized as follows: (i) a mild aqueous deposition approach enabling highly dispersed Pt species on CeO_2_ without the use of organic ligands, and (ii) a tailored redox interaction between Pt and ceria that enhances radical scavenging activity. Furthermore, the preparation method for recast membranes based on an SSC–Aquivion^®^ ionomer with low equivalent weight (EW) was refined [29]. Specifically, a commercial EW720 Aquivion^®^ dispersion (D72-25BS, Syensqo) was used to cast membranes incorporating different amounts (1.0–1.5 wt.%) of the synthesized scavenger [30]. The aim was to evaluate the well-known mitigating effect of CeO_2_-based radical scavengers on membrane degradation and to assess the resulting improvements in structural, chemical–physical and conductivity properties for PEFC applications. The performance of the scavenger-containing membranes was evaluated both before and after an ex situ chemical oxidative test in Fenton’s reagent solution (denoted Fenton’s test, FT) and compared with a pristine membrane and a membrane containing commercial CeO_2_, all prepared using the same casting method.

2. Materials and Methods

2.1. Materials Used for 30%-Pt/CeO2 Scavengers’ Synthesis

Commercial ceria (CeO_2_) (University of Cape Town, South Africa), chloroplatinic acid or hexachloroplatinic acid (H_2_PtCl_6_) (Engelhard, Rome, Italy), sodium carbonate (Na_2_CO_3_) (Sigma-Aldrich, St. Louis, MO, USA), sodium metabisulfite (Na_2_S_2_O_5_) (Carlo Erba Reagents, Milan, Italy), sulfuric acid (H_2_SO_4_) (Sigma-Aldrich, St. Louis, MO, USA), 20% sodium hydroxide (NaOH) solution (Sigma-Aldrich, St. Louis, MO, USA) and hydrogen peroxide (H_2_O_2_) (Carlo Erba Reagents, Milan, Italy) were purchased with a purity of 99.99% and used without additional purification.

2.2. Synthesis of Commercial 30% Pt/CeO2-Based Scavengers

A 30 wt.% of Pt supported on commercial CeO_2_ was prepared using a colloidal technique employing the sulphite complex route, which consisted of impregnating the CeO_2_ with a Pt sulphite precursor, as shown in Scheme 1. To synthesize the sulphite complex (Na_6_Pt(SO_3_)4), at first, an aqueous solution of hexachloroplatinic acid (H_2_PtCl_6_) is prepared. Then, the temperature (T) is raised to 65 °C. Slowly, sodium carbonate (Na_2_CO_3_) is added to set the pH to 7, stirring for 1 h. Temperature is raised to 75 °C, and sodium metabisulfite (Na_2_S_2_O_5_) was added until the solution’s yellow colour changed to milky white. After filtration, the solid was oven-dried overnight at 100 °C. For 30% wt.% Pt/CeO_2_, two solutions were prepared: at first, the Pt precursor solution was prepared by dissolving Na_6_Pt(SO_3_)4 in water (0.004 M, pH 6.9, 25 °C). An appropriate amount of CeO_2_ (30 wt.%) was dispersed in H_2_O and ultrasonicated for 1 h before use. The Pt solution was acidified to pH 2.8 with concentrated H_2_SO_4_ before the addition of the CeO_2_ suspension. After mixing, the system was stirred for 30 min. and the pH was adjusted to 5.3 using 20 wt.% NaOH. The suspension was further stirred for 2.5 h, followed by the gradual addition of a molar excess of 40 wt.% H_2_O_2_. The pH was readjusted to 5.5 and the mixture was heated to 70 °C for 30 min. The solid was filtered, dried under vacuum at 100 °C overnight, and reduced under a 20% H_2_/He flow (40/160 mL min^−1^) at 70 °C for 1 h.

2.3. 30%-Pt/CeO2 Scavenger’s Physicochemical Characterization Techniques

A high-resolution X-ray diffractometer (Bruker D8 Advance diffractometer) with a monochromatic Cu Kα radiation source (λ = 1.5406 Å) at 40 kV and 40 mA was utilized to identify phases, determine crystal structures, and analyse crystallite sizes. The surface area from nitrogen adsorption–desorption isotherms was determined using the Brunauer–Emmett–Teller (BET) method on a Micromeritics ASAP 2020 (Micromeritics, Norcross, GA, USA) surface area and porosity analyser. A Transmission Electron Microscope (JEOL JEM-F200, JEOL, Tokyo, Japan) was used to analyse the morphology and uniform distribution of commercial CeO_2_ in a 30 wt.% Pt/CeO_2_ nanocomposite scavenger. To better characterize the surface chemistry and electronic properties of the newly prepared catalysts, X-ray photoelectron spectroscopy (XPS) was carried out on the commercial Pt/CeO_2_. The spectra were collected with a PHI 5800 ESCA instrument (Physical Electronics, Chanhassen, MN, USA) using a monochromatic Al Kα radiation source (hν = 1486.6 eV, 350 W).

2.4. Aquivion® Dry Residue and Membrane Preparation

Building on previous experience with Nafion membrane preparation [31,32], a method for casting Aquivion^®^ membranes was refined. The dry polymer residue was prepared starting from the commercial Aquivion^®^ D72-25BS polymer dispersion (25 wt.%, 99% water, free of ethers) by Syensqo, which has an EW of 720 g/meq. The data sheet for the D72-25BS commercial dispersion from Syensqo is provided in Table 1.

The preparation procedure, slightly modified from a previous study [33], involves first drying the Aquivion^®^ dispersion in an oven at 80 °C until a dry residue is obtained. The resulting material, appearing as flakes, corresponds to the theoretical dry content (25 wt.%). These flakes are then finely ground and dissolved at room temperature (rT) in dimethylacetamide (DMAc, Carlo Erba Reagents, Milan, Italy) to obtain an 8 wt.% solution under continuous stirring. After approximately 16 h of solubilization, the dispersion is cast onto a glass substrate at 40 °C using the doctor-blade technique and subsequently treated as reported below [32,33,34].

Post-casting thermal treatment:

• Drying on a heating plate at 60 °C for 2 h;
• Drying on a heating plate at 90 °C for 3 h;
• Annealing at 155 °C for 30 min.

Annealing at temperatures above the glass transition temperature (T_g_) [35] stabilizes the ionic morphology, improves durability, and ensures reproducible electrochemical properties under severe operating conditions. Moreover, thermal treatment above T_g_ promotes segmental relaxation and ionic domain reorganization in low-EW Aquivion^®^ membranes, resulting in a more stable phase-separated morphology, improved dimensional stability and reproducible proton conductivity (PC) under PEMFC operating conditions.

Post-casting acid treatment:

• Immersion in 1 M HCl solution at rT for 3 h;
• Repeated washings in distilled water;
• Drying between absorbent papers for 24 h.

The acid treatment is performed to remove impurities and enhance membrane wettability. Acids were supplied by Sigma-Aldrich (St. Louis, MO, USA). The standardised casting procedure yields polymeric membranes with a fairly constant thickness of approximately 20 µm (dry state). When required, 1.0 or 1.5 wt.% of filler (commercial CeO_2_ or synthesised Pt/CeO_2_), relative to the dry polymer weight, was first dispersed at rT under stirring in an aliquot of the solvent. The suspension was briefly ultrasonicated, and then added to the polymer/solvent system. After a slow stirring for approximately 30 min, the resulting polymer/solvent/filler mixture was further sonicated for 12 min. The prepared membranes and their compositions are reported in Table 2.

2.5. X-Ray Diffraction Analysis on Membranes

X-ray Diffraction (XRD) patterns of the membranes were acquired using a PANalytical EMPYREAN diffractometer (Malvern Panalytical, Almelo, The Netherlands) equipped with a copper anode (Cu Kα_1_ = 1.54060 Å, Kα_2_ = 1.54443 Å). The generator operated at 40 kV and 45 mA. Membrane samples were mounted independently on flat, zero-background-diffraction sample holders. Measurements were carried out in the 10° < 2° < 60° angular region, using divergence slits of 1/4° and 1/2° as the incident pathway. All data were collected in Bragg–Brentano geometry, under ambient conditions (rT, 55% relative humidity, atmospheric pressure).

2.6. Micro-Raman Measurements and Optical Images on Membranes

Micro-Raman spectroscopy was performed using a Renishaw inVia microscope equipped with a 785 nm laser source and a 2400 lines/mm edge filter. For each sample, the laser power and exposure time were carefully optimized to achieve the best signal-to-noise ratio while avoiding laser-induced damage (1sec/acquisition, overall, 300 acquisitions per point, 10% laser intensity). Optical images were acquired using the integrated microscope of the Micro-Raman system, a Leica model; specifically, a 100X objective lens was used.

2.7. SEM and EDX Characterisations on Membranes

The cross-sectional morphology of the samples was examined using an Ultra-High-Resolution Scanning Electron Microscope (UHR-SEM-FEG, Helios 5 UC DualBeam, Thermo Scientific, Waltham, MA, USA) equipped with a Field Emission Gun (FEG), which ensures high spatial resolution and enhanced image quality. SEM imaging was performed on untreated samples using the in-lens SE/BSE detector with an accelerating voltage between 5 and 20 kV and a beam current in the 13pA–0.4 nA range. Mapping analyses were performed by Energy Dispersive X-ray (EDX) spectroscopy.

3. Chemical–Physical Characterizations of Membranes

For the chemical–physical characterization, to statistically validate the results, the measurements were repeated three times on three different pieces of each membrane sample. Hence, the presented results are the average of the different obtained values.

3.1. Water Uptake Measurements and Lambda (λ) Determination

Water retention capacity (Wup, %) was calculated by the difference between wet (m_wet_) and dry (m_dry_) sample weight, as described elsewhere [32,34,36]. Wup values were calculated by the following equation:

$$[eqn]$$

The λ value (expressed as H_2_O moles/-SO_3_H moles) was calculated through the water uptake and IEC values ratio, both expressed in moles.

$$[eqn]$$

where MW_H2O_ is the water molecular weight, equal to 18.

3.2. Chemical Ex Situ Oxidative Degradation Tests (in a Fenton’s Reagent Solution)

As mentioned before, the chemical ex situ oxidative degradation of the membranes was evaluated by a Fenton’s test (solution method, here briefly named FT). At first, each sample was dried at 80 °C for 2 h. under vacuum in an oven to have the same grade of water content for all samples investigated. Successively, each membrane was immersed, under stirring, in an acidified 20 ppm Fe^2+^ solution (pH < 3) and H_2_O_2_ (30 vol. %) for a t = 200 h. After this time, the membranes were washed, dried, and analysed in terms of IEC and PC to evaluate the chemical oxidative degradation determined ex situ from Fenton’s reagent [34].

3.3. Ion Exchange Capacity (IEC) Measurements on Membranes

Ion Exchange Capacity (IEC) for each membrane was calculated using an acid–base titration. A 0.01 M NaOH solution was used to neutralize exchanged H^+^ ions with an automatic titrator (Metrohm model 751GPD Titrino, Herisau, Switzerland), as described elsewhere [31,32,34,37]. The volume of titrant at the equivalent point was used to calculate the IEC, as follows:

$$[eqn]$$

where V_tit._ is titrant volume (ml), [M] is titrant molarity, and m_dry_ is dry mass of the membranes. The same measurement on the developed membranes was carried out after the ex situ FT, and IEC results were compared in order to verify the degradation resistance attributable to the scavenger presence.

4. Electrochemical Characterization on Membranes

Proton Conductivity (PC) Measurements on Membranes

Proton conductivity (σ, S cm^−1^) measurements [32,34] were conducted using the four-probe method in DC current on a commercial conductivity cell (Bekktech, Aachen, Germany). The cell was fed with humidified hydrogen and connected to a potentiostat/galvanostat (AMEL, Milan, Italy). Proton conductivity was determined in the T range 80–120 °C under varying relative humidity levels (50%, 75%, 100% RH). Each condition was equilibrated for 30’ before measurement, to ensure the desired humidity level. The proton conductivity (PC) was calculated using Ohm’s law:

$$[eqn]$$

where L (0.425 cm) is the constant distance between the two Pt electrodes; R (Ω) is the measured resistance; W (cm) is the sample width; and T (cm) is the sample thickness. Each PC value reported in the conductivity plots inherently corresponds to an averaged value derived from multiple resistance determinations.

The same measurements on the developed membranes were carried out after ex situ FT, and the PC trends were compared in order to verify the variation in PC and, also, in this case, the degradation resistance.

5. Results and Discussion

5.1. Physicochemical Characterization Results on 30% Pt/CeO2 Scavenger

Figure 1a shows the XRD patterns of commercial ceria (CeO_2_) and the 30 wt.% Pt/CeO_2_ scavenger synthesised via the sulphite complex approach. The commercial CeO_2_ exhibits characteristic diffraction peaks at 2θ values of 28.5°, 33.0°, 47.5°, 56.3°, 59.1°, 69.4°, 76.7°, and 79.1°, corresponding to the (111), (200), (220), (311), (222), (400), (331), and (420) crystallographic planes, respectively. The crystallite size of CeO_2_, calculated using the Scherrer equation, is 18.2 nm. In the synthesised Pt/CeO_2_ scavenger, the crystalline structure of the CeO_2_ support remains well defined, with a similar crystallite size of 18.2 nm, indicating that Pt loading does not alter the ceria lattice. In addition, distinct diffraction peaks at 40.1° and 67.8° are observed, corresponding to the (111) and (220) planes of face-centred cubic (fcc) Pt, respectively. The Pt particle size is estimated to be 5.6 nm. These results are consistent with literature reports by Z. Tu et al. and Y. Turap et al., who observed comparable diffraction patterns for pure CeO_2_ and Pt-loaded CeO_2_ with a face-centred cubic crystal structure [38,39].

Figure 1b presents the nitrogen (N_2_) adsorption–desorption isotherms and Brunauer–Emmett–Teller (BET) analysis of commercial CeO_2_ and the Pt/CeO_2_ scavenger, both exhibiting a mesoporous structure. The BET specific surface area of commercial ceria (CeO_2_) was determined to be 42.5 m^2^ g^−1^. According to Barrett–Joyner–Halenda (BJH) analysis, the adsorption and desorption surface areas were 41.1 m^2^ g^−1^ and 49.1 m^2^ g^−1^, respectively, with corresponding pore volumes of 0.066 and 0.087 cm^3^ g^−1^ and average pore diameters of 6.42 and 7.12 nm. For the 30 wt.% Pt/CeO_2_ scavenger, the BET surface area decreased to 24.15 m^2^ g^−1^, likely due to partial pore blocking by Pt nanoparticles. BJH analysis revealed adsorption and desorption surface areas of 17.32 and 27.38 m^2^ g^−1^, respectively. The average pore diameters increased to 7.41 nm (adsorption) and 11.66 nm (desorption), while the corresponding pore volumes were 0.032 and 0.079 cm^3^ g^−1^. Comparable results were reported by S. Wang et al. for Pt/CeO_2_ scavengers synthesised via a hydrothermal leaching approach, which exhibited a mesoporous structure with an average crystallite size of approximately 10 nm and a BET surface area of 52.0 m^2^ g^−1^ [40].

Figure 1c presents HR-TEM images of the 30 wt.% Pt/CeO_2_ scavenger at different magnifications, along with the corresponding elemental mapping overlays. The micrographs reveal a uniform distribution of Pt nanoparticles on the CeO_2_ support, with particle sizes consistent with those estimated from XRD analysis. The scavenger nanoparticles exhibit a predominantly spherical morphology. In the HR-TEM images, the darker contrast regions correspond to CeO_2_ nanoparticles, while the lighter regions are attributed to Pt, with both phases evenly dispersed throughout the material. Wavy lattice fringes are observed at the Pt/CeO_2_ interface, suggesting possible lattice distortion at the junction between the two phases. Similar HR-TEM observations, including uniform Pt and Ce distribution and an average Pt particle size of approximately 3.55 nm, were reported by Wang et al. for Pt/CeO_2_ scavengers [41].

In Figure 2a, X-ray Photoelectron Spectroscopy (XPS) is presented. The Pt 4f region is characterized by a doublet arising from the 4f_7_/2 and 4f_5_/2 spin–orbit components, with the former appearing at lower binding energy. These two features are separated by roughly 3.3–3.4 eV, and their relative intensities follow the expected ~4:3 ratio determined by their different degeneracies. Metallic platinum (Pt^0^) typically exhibits the 4f_7_/2 contribution at approximately 71.3 eV and its 4f_5_/2 peak around 74.6 eV. When platinum is present as Pt^2+^ (for instance, in PtO or other Pt(II) species), both peaks shift to higher energies, appearing at 72.3 eV and 75.6 eV. An even more pronounced shift is observed for Pt^4+^ (such as in PtO_2_), where the doublet was found at 73.3 eV (4f_7_/2) and 77.2 eV (4f_5_/2). The relative percentages of each species, calculated by integrating the peak areas, correspond to 67.5, 25.4, and 7.1% for Pt^0^, Pt^2+,^ and Pt^4+^, respectively. The partial oxidation of Pt, indicated by the shifts to higher binding energies, compared with bulk metallic Pt, reflects electronic interactions between Pt and the ceria support, consistent with strong metal–support interactions (SMSI). These electronic modifications of Pt are expected to enhance radical scavenging activity, in agreement with the observed superior performance of Pt/CeO_2_, compared with bare CeO_2_. Figure 2b highlights the spectral features of cerium dioxide (CeO_2_), whose XPS response is dominated by the intricate multiplet structure of the Ce 3d core level, a region particularly sensitive to the Ce^4+^/Ce^3+^ redox state. Ce^4+^, characteristic of stoichiometric CeO_2_, produces six main peaks distributed between the 3d_5_/2 and 3d_3_/2 components. These are commonly denoted as v (≈882.7 eV), v″ (≈887.7 eV), v‴ (≈898.0 eV), and u (≈901.3 eV), u″ (≈907.2 eV), u‴ (≈917.1 eV). The highest-energy feature (u‴) acts as a distinct satellite that helps differentiate Ce^4+^ from Ce^3+^ environments. Reduced cerium species (Ce^3+^), as found in Ce_2_O_3_ or oxygen-defective CeO_2_, introduce additional signals at around 880.3, 885.4, 898.8, and 903.0 eV, typically labelled v_0_, v′, u_0_, and u′. These extra components serve as clear markers of the presence of Ce^3+^.

5.2. XRD Measurements on Membranes

Figure 3 reports the XRD pattern of the pristine Aquivion reference membrane AD72-28, compared to the Aquivion membranes incorporating the syn-scavenger, as well as to the membrane with CeO_2_. All samples exhibit the characteristic reflections at approximately 2θ = 17.30° and 39.36°, corresponding to the (100) and (101) crystallographic planes of PFSA, respectively, in agreement with the literature [42]. Negligible amorphous contribution was found, as confirmed by the symmetric shape of the PFSA (100) reflection observed in all patterns. Furthermore, scavenger-based membranes show diffraction peaks matching the cubic phase of CeO_2_, as expected. The experimental patterns match well with the reference data for CeO_2_ (ICDD 00-004-0593 [43]. A weak Pt signal—partially obscured by the broader PFSA (101) reflection—was also detected in samples AD72-34 and AD72-35 around 2θ = 40°, consistently with ICDD 00-001-1190 [44] included in Figure 3 (black bars). The average CeO_2_ crystallite size was estimated by applying a Gaussian fit to the (111) peak and calculating the full width at half maximum (FWHM). The derived crystallite sizes and relative ceria crystallinity for each sample are summarized in Table 3. Notably, both the crystallite size and crystallinity ratio remain consistent across all membranes, regardless of the scavenger content or its origin (synthesised or commercial). Additionally, the PFSA domain size [45] was obtained from (100) reflection and resulted in being unchanged, indicating that the inclusion of scavenger does not modify the XRD spectrum when compared to the equivalent membrane without scavenger, and thus suggesting that the addition of the scavenger does not significantly alter the observed membrane crystallinity.

5.3. Micro-Raman Spectroscopy Measurements on Membranes

Figure 4 presents the micro-Raman spectra acquired on the recast pristine Aquivion membrane (AD72-28) compared to those obtained on membranes with a synthesised or commercial scavenger. Raman profiles are consistent with literature data for PFSA-based membranes [46] exhibiting the characteristic vibrational modes of the polymer matrix. Such modes include CF_2_ twisting (~292 cm^−1^), scissoring (~323 cm^−1^), wagging (~382 cm^−1^), and symmetric stretching (~733 cm^−1^), as well as CS stretching (~723 cm^−1^), C–O–C symmetric stretching (~817 cm^−1^), SO_3_^−^ symmetric stretching (~1060 cm^−1^), SO_3_^−^ and CF_2_ degenerate stretching (~1130–1200 cm^−1^) and C–C symmetric and degenerate stretching (~1250–1320 cm^−1^).

All PFSA bands discussed above were also observed in the membranes containing the CeO_2_-based scavenger. Notably, in the samples AD72-34, AD72-35 and AD72-36, localized aggregates or particles were identified via optical microscopy at 100X magnification (see Figure 5). Raman spectra collected specifically on these aggregates revealed an additional broad band around 460 cm^−1^, which can be attributed to the F_2_g vibrational mode of cubic CeO_2_. This signal, consistent with previous observations for Pt/CeO_2_ systems [47], confirms the presence of ceria-rich domains within these membranes (see insets in Figure 4).

Figure 4 shows the micro-Raman spectra collected upon the recast pristine Aquivion membrane (AD72-28) compared to the membranes containing the differently synthetised or commercial scavengers. Raman spectra were specifically collected on the ceria-rich domains in the insets.

The optical micrographs shown in Figure 5 clearly reveal both the homogeneous regions of the membranes—clean and free from particulates—and the areas where distinct clusters and particles, associated with the scavengers, are present. Indeed, the membrane clean zones provided reference spectra of the membrane material, while the cluster zones enabled the investigation of the chemical composition of the scavenger-related particles, as shown in Figure 4.

5.4. SEM and EDX Characterisations on Membranes

SEM section analyses provide relevant information on the internal morphology and distribution of inorganic fillers within polymer membranes. In Figure 6a–g, SEM images and EDX mapping of cross-section membranes are reported. The AD72-28 sample section, Figure 6a, appears homogeneous and compact, with a dense microstructure, suggesting lower water-retention capacity and reduced formation of preferential pathways for proton transport. The AD72-36 sample, containing CeO_2_, shows a compact structure (Figure 6b) and a uniform distribution of particles along the section, analysed and confirmed by EDX mapping (Figure 6c). A significantly more favourable behaviour was observed in the AD72-34 sample, containing 1 wt.% of Pt supported on CeO_2_. The SEM image (Figure 6d) shows a more structured microstructure and a homogeneous distribution of the filler along the section, without the presence of agglomeration phenomena. The EDX mapping on the sample with a 1 wt.% of Pt/CeO_2_ (Figure 6e) confirms a fine and continuous dispersion of the scavenger, indicative of a good degree of integration with the polymer matrix. This morphological configuration could be particularly effective in promoting the formation of hydrophilic conduction channels, improving water retention, and facilitating the proton transport mechanism. These microstructural features are fully consistent with the superior proton conductivity observed for the membrane containing 1 wt.% of Pt/CeO_2_, which therefore represents an optimised balance between scavenger loading and dispersion quality. On the contrary, in the AD72-35 sample with 1.5 wt.% of Pt/CeO_2_, the SEM image (Figure 6f) shows greater microstructural heterogeneity with the appearance of denser domains attributable to Pt/CeO_2_ agglomerates. EDX mapping (Figure 6g) confirms a less-uniform distribution of the scavenger, suggesting a local aggregation phenomenon. This agglomeration can compromise the continuity of the polymer matrix and reduce the effectiveness of proton transport pathways, explaining its lower performance compared to the sample with 1.0 wt.% of scavenger.

5.5. Water Uptake Measurements and Lambda (λ) Determination

Figure 7 and Figure 8 show the trends of water uptake (Wup) and hydration number (λ), respectively, as a function of temperature, comparing the recast pristine Aquivion^®^ membrane with the developed scavenger-containing membranes.

The water uptake (Wup) and hydration number (λ) were calculated using Equations (1) and (2) to quantify, respectively, the amount of water retained at each temperature and the number of water molecules coordinated per sulfonic acid group (–SO_3_H). For all the membranes developed, Wup and λ exhibit very similar overall trends with temperature. This behaviour arises because the λ calculation includes the ion-exchange capacity (IEC), which, as reported in Table 4, lies within a narrow range (1.40 ± 0.04 meq g^−1^). Consequently, λ is mainly governed by the number of water molecules associated with each –SO_3_H group. Both Wup and λ increase with T up to 80 °C and decrease at higher T (95 °C) for most membranes. The notable exception is the membrane containing commercial CeO_2_ (AD72-36), which exhibits significantly higher values, particularly for water uptake (Wup ≈ 91%). This behaviour is attributed to the highly hydrophilic nature of CeO_2_, whose surface is rich in –OH groups and Ce^3+^/Ce^4+^ redox sites. As temperature increases, polymer-chain mobility and the accessibility of –SO_3_H groups in the presence of fillers are enhanced. This effect is particularly pronounced for low-EW ionomers, where the density of –SO_3_H groups is higher. These factors promote the formation of larger and more-interconnected hydrophilic domains capable of retaining greater amounts of water [48]. Unlike Pt/CeO_2_, pure CeO_2_ does not catalyse cross-linking reactions, nor does it reduce the acidity of –SO_3_H groups; therefore, Wup continues to increase with temperature compared to the membranes containing the scavengers. In fact, the membrane with the lowest loading of synthesised Pt/CeO_2_ (AD72-34) shows increasing Wup and λ up to 80 °C, while at 95 °C, both values are lower than those of AD72-36 (CeO_2_ only). This indicates that the introduction of Pt onto CeO_2_ substantially alters the hydration behaviour. Pt–SO_3_H interactions partially shield the acidic sites, as Pt nanoparticles can coordinate with –SO_3_^−^ groups, reducing the number of free ionic sites available to bind water molecules. As a result, osmotic swelling is reduced. In addition, Pt/CeO_2_ establishes stronger filler–polymer interactions than CeO_2_ alone. At elevated temperatures, this leads to a more structurally constrained system, rather than promoting membrane expansion. The reduced mobility of ionomer chains produces a “stiffening” effect associated with the presence of the scavenger. Compared with pure CeO_2_, Pt/CeO_2_ scavenger also exhibits a less hydrophilic surface and, therefore, retains less physically adsorbed water. This effect becomes more evident at higher temperatures, where weakly bound water is more easily desorbed. Moreover, Pt can catalyse condensation reactions between –SO_3_H groups, inducing local rearrangements of the ionic morphology, resulting in smaller hydrophilic domains and reduced free volume for water [19,49].

Finally, the membrane containing the highest loading of synthesised scavenger (AD72-35) exhibits increasing Wup and λ values up to 80 °C, but fails at 95 °C. This behaviour is likely due to excessive filler content, which increases the elastic modulus and induces mechanical stiffening, leading to reduced flexibility, increased brittleness and a diminished ability to accommodate hydration–dehydration cycles. Over time, this can raise the risk of cracking or pinhole formation. In summary, excessively high Pt/CeO_2_ loadings can result in a loss of functional efficacy due to scavenger aggregation, a reduction in specific active-surface area and the formation of morphological defects, such as poorly hydrated polymer–filler interfaces. These effects ultimately lead to a reduced radical scavenging efficiency and compromised membrane integrity [17,21,50,51].

5.6. Ion Exchange Capacity (IEC) Measurements

Table 4 reports the experimental ion-exchange capacity (IEC) values of the developed membranes, measured in the as-received state and after the chemical ex situ degradation test. Overall, the IEC values of all membranes are consistent with literature data and with those provided by the manufacturer for EW720 Aquivion^®^ [52], which has an equivalent weight (EW) in the range of 700–740 g eq^−1^ and a corresponding total acid capacity of 1.35–1.43 meq g^−1^.

The pristine membrane AD72-28 was used as the reference polymer. Among the developed samples, the AD72-34 membrane containing 1.0 wt.% of synthesised scavenger exhibits the highest IEC value compared with the pristine reference membrane, likely as a consequence of its higher proton conductivity (PC), as discussed in the following section on PC measurements [53]. Notably, this membrane also retains the highest IEC value after the FT, indicating superior resistance to chemical degradation. This behaviour can be attributed to an optimal scavenger loading, which promotes a favourable arrangement of the polymer matrix, uniform nanoparticle dispersion, increased pathway tortuosity, and well-connected hydrophilic phases for proton transport. Such features limit the loss of –SO_3_H groups and, thereby, preserve IEC values.

In contrast, the AD72-35 membrane containing 1.5 wt.% of syn-scavenger shows a lower IEC value, most likely due to excessive incorporation of the non-conductive filler, which reduces the connectivity of proton-conducting microphases and negatively affects both IEC and PC. Alternatively, a non-homogeneous distribution of the scavenger or temporary ion exchange between H^+^ and Ce^3+^/Ce^4+^ ions may also contribute to the observed IEC reduction [21,54]. After the FT, this membrane exhibits a slight decrease in IEC, suggesting lower resistance to chemical degradation. Both membranes containing the scavenger clearly demonstrate its protective role, likely enhanced by strong metal–support interactions (SMSIs). This occurs when noble metals such as Pt, Pd or Rh are supported on metal oxides, including CeO_2_, SnO_2_, TiO_2_, MnO_2_, WO_3_, Nb_2_O_5_, or V_2_O_5_ [24]. In contrast, the pristine membrane and the AD72-36 membrane containing 1.0 wt.% of commercial CeO_2_ experience a more pronounced reduction in IEC after the FT.

The AD72-36 sample also exhibits the lowest IEC values both before and after the degradation test, indicating more severe chemical deterioration. These results suggest that the presence of Pt enhances the scavenger’s protective function through SMSI, which promotes electron transfer from the noble metal to the oxide support, facilitating the migration of reactive oxygen species from the oxide surface to the metal [55]. Overall, considering the combined chemical–physical properties (Wup, λ, and IEC), the AD72-34 membrane containing 1.0 wt.% of syn-scavenger emerges as the most promising candidate for practical application.

5.7. Proton Conductivity (PC) Measurements

Figure 9a–c reports the PC measurements of the developed as-received membranes at different relative humidity levels: 50% RH (a); 75% RH (b); and 100% RH (c). Generally, PC increases with temperature (T) and relative-humidity (RH) levels. High PC values were obtained for all the membranes developed across the entire range of operative conditions.

Overall, all developed membranes exhibit satisfactory PC values. The pristine AD72-28 membrane shows PC values comparable to those of the membranes containing the scavenger; however, at lower RH levels (50–75%) and higher temperatures, it displays unstable behaviour, due to its high sensitivity to dehydration. The AD72-34 membrane containing 1.0 wt.% of synthesised scavenger delivers the best performance across all investigated T and RH values, maintaining a stable PC even under harsh conditions (low RH and high T). This behaviour suggests that both the scavenger loading and its dispersion within the polymer matrix are optimal. In this membrane, an effective balance is achieved between PC and stability at elevated temperatures, aided by a moderate reduction in water uptake at high T (which limits excessive swelling), the formation of more compact yet percolating ionic domains, and an improved dimensional stability.

In contrast, the AD72-35 membrane, containing the highest amount of syn-scavenger, exhibits the poorest performance among the scavenger-containing membranes under all tested conditions, with particularly unstable behaviour at 50% and 75% RH, whereas it fails to provide measurable data at 120 °C. Consistent with Wup, λ, and IEC results, this behaviour is attributed to excessive loading of a poorly conductive filler, which reduces the connectivity of proton-conducting microphases and leads to non-homogeneous scavenger distribution and non-linear proton-conduction pathways. Furthermore, the higher filler content induces mechanical stiffening, loss of flexibility, increased brittleness and a reduced ability to accommodate hydration–dehydration cycles. Overall, Pt/CeO_2_ overloadings result in loss of ion percolation, excessive shielding of –SO_3_H groups, filler aggregation, and marked stiffening. All are factors that negatively affect both chemical–physical properties and proton conductivity.

The AD72-36 membrane containing CeO_2_ alone shows a trend similar to that of the membranes containing Pt/CeO_2_ at 100% RH, although with lower PC values. At 75% and 50% RH, it exhibits the lowest PC values, particularly at higher temperatures.

In all cases, the presence of Pt and the strong interaction between the noble metal and the oxide support, together with the self-humidifying properties of Pt [56], significantly enhance the proton-conduction mechanism and proton mobility compared with both the pristine Aquivion^®^ membrane and the CeO_2_ membrane.

Figure 10a–c report the comparison in terms of PC measurements before and after the FT at different RH levels for each developed membrane: AD72-28 membrane (a); AD72-34 membrane (b); AD72-35 membrane (c); and AD72-36 membrane (d). Generally, the PC after an FT always decreases, but the amount of loss strongly depends on the scavenger, its amount, its distribution and its protecting ability.

Regarding the pristine Aquivion^®^ AD72-28 membrane (Figure 10a), a comparison between the as-received membrane and the membrane after FT reveals a significant decay in PC (at 110–120 °C, no measurable data are supplied). The dominant mechanisms likely include lack of radical protection, rapid loss of –SO_3_H groups, irreversible reduction of functional Wup, and disconnection of ionic domains. This results in a drastic reduction of PC across the entire RH range, with the post-FT PC values far lower than the initial ones and with the difference increasing as RH decreases. Furthermore, the PC-versus-temperature trend (80–120 °C) shows weak thermal activation. Above 100 °C, where PC typically drops rapidly, the membrane fails to provide a measurable PC value, likely due to dehydration.

The AD72-34 membrane containing 1.0 wt.% of syn-scavenger exhibits the best overall performance, representing an optimal case with the lowest loss of proton conductivity. This improvement is likely due to the combined effects of CeO_2_, which acts as a radical scavenger, and Pt, which provides two key benefits: i) catalytic decomposition of H_2_O_2_ into H_2_O and O_2_; and ii) reduction of •OH radical concentration. Its post-FT PC-values-versus-RH curve are the highest and most stable, closely resembling the values of the undegraded sample. Moreover, the curve increases with RH, indicating a lower dependence on humidity. At RH levels of 50–75%, its PC exceeds that of the pristine degraded polymer. Regarding the trend vs. the temperature, the post-FT PC increases up to 120 °C, although shifted toward lower values, showing smooth thermal activation and the smallest pre-/post-FT variation.

The AD72-35 membrane with 1.5 wt.% of syn-cavenger exhibits a somewhat ambiguous behaviour, due to competing mechanisms. On one hand, it offers high radical protection and minimal chemical degradation; on the other hand, excessive scavenger loading leads to filler aggregation, strong shielding of –SO_3_H groups, and poorly percolating ionic domains, even before the FT. Its post-FT PC values-versus-the-RH curve are similar to, or slightly worse than those of the fresh sample. At high RH, the post-FT PC is slightly lower than the fresh membrane, while at low RH it is severely penalised, more than AD72-34, but less than AD72-36. The post-FT PC versus temperature shows only a weak increase, less evident than in AD72-34, suggesting that the limitation is structural rather than chemical. After FT, the membrane does not degrade significantly, but its performance remains suboptimal under all conditions.

The AD72-36 membrane provides slightly better PC retention than the pristine membrane, but with some trade-offs. According to the literature, CeO_2_ acts as a radical scavenger (Ce^3+^/Ce^4+^), reducing the rate of chemical degradation; however, Ce migration may partially neutralise –SO_3_H groups after FT. As shown in Figure 10, the post-FT PC versus temperature is higher than that of the degraded pristine polymer (AD72-28, which does not supply any PC above 100 °C), particularly at higher RH. PC is maintained up to 120 °C with a trend similar to the as-received sample, although shifted towards lower values. The post-FT PC values versus RH indicate adequate retention at medium-to-high RH, but a noticeable penalty at low RH.

In conclusion, the membrane containing 1.0 wt.% of synthesised Pt/CeO_2_ (AD72-34) exhibits the best overall performance, with minimal IEC loss (≈0.7%), the highest PC, and the highest proton conductivity retention after the degradation test across the investigated ranges of temperature (80–120 °C) and relative humidity (50-75-100% RH). These results indicate that this membrane is highly suitable for PEFC applications. The use of Pt/CeO_2_ instead of bare commercial CeO_2_ is motivated by its superior scavenging efficiency at very low Pt loadings. In the proposed synthesis, the Pt content is below 1 wt.%, which limits the impact of the noble metal on the overall material cost. Moreover, the preparation method is based on aqueous impregnation/deposition steps carried out at mild temperatures and ambient pressure, without the need for complex equipment or expensive organic ligands, making the process readily scalable using conventional catalyst manufacturing routes. For large-scale PEMFC applications, the amount of scavenger required in the membrane electrode assembly is relatively small compared to the total catalyst loading at the electrodes; therefore, the incremental cost associated with the Pt/CeO_2_ additive is expected to be marginal. Future work will focus on evaluating its performance and stability benefits in a single-cell PEFC configuration.

6. Conclusions

Recast short-side-chain Aquivion^®^ membranes (EW = 720 g eq^−1^) incorporating CeO_2_-based radical scavengers were successfully prepared via a standardised doctor-blade casting process to mitigate chemical degradation in PEMFC environments. A synthesised Pt/CeO_2_ scavenger (30 wt.% Pt) was developed and incorporated at optimised loadings (1.0–1.5 wt.%) into the polymer matrix. Structural analyses (XRD, Raman) confirmed that scavenger incorporation does not alter the intrinsic PFSA crystallinity, while chemical–physical characterisation revealed a balance between hydration and mechanical stability governed by filler content. Chemical ex situ oxidative degradation tests (Fenton’s test) demonstrated the protective role of Ce-based scavengers, with Pt/CeO_2_ providing the most effective mitigation through combined radical scavenging and catalytic H_2_O_2_ decomposition. Among all formulations, the membrane with 1.0 wt.% of Pt/CeO_2_ (AD72-34) exhibited the best overall performance, showing minimal IEC loss (≈0.7%), the highest proton conductivity, and superior retention across 80-120 °C and 50-100% RH. In contrast, an excessive loading (1.5 wt.%) led to structural limitations and reduced proton conductivity, despite enhanced chemical stability. Overall, optimised Pt/CeO_2_ loading significantly enhances durability and proton transport in low-EW Aquivion^®^ membranes, making AD72-34 a promising candidate for PEMFC applications. Future work will focus on improving scavenger dispersion and validating performance under in situ single-cell operation and long-term durability tests.