On the evolution of proton conductivity of Aquivion membranes loaded with CeO2 based nanofillers: Effect of temperature and relative humidity

Journal of Membrane Science 574 (2019) 17–23

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On the evolution of proton conductivity of Aquivion membranes loaded with CeO2 based nanofillers: Effect of temperature and relative humidity

T

⁎

Anna Donnadioa,b, , Roberto D’Amatoc, Fabio Marmottinic, Gianmarco Panzettac, Monica Picaa,b, ⁎ Chiara Battocchiod, Donatella Capitanie, Fabio Ziarellif, Mario Casciolab,c, a

Department of Pharmaceutical Sciences, University of Perugia, Via del Liceo 1, 06123 Perugia, Italy Centro di Eccellenza su Materiali Innovativi Nanostrutturati per Applicazioni Chimiche Fisiche e Biomediche – CEMIN, via Elce di Sotto 8, 06123 Perugia, Italy c Department of Chemistry, Biology and Biotechnologies, University of Perugia, via Elce di Sotto 8, 06123 Perugia, Italy d Department of Sciences, University of Roma Tre, Via della Vasca Navale 79, 00146 Rome, Italy e CNR-IMC, Laboratorio NMR ‘‘Annalaura Segre”, Via Salaria km 29,300, 00015 Monterotondo, Rome, Italy f Aix-Marseille Université - Centrale Marseille - CNRS, Fédération des Sciences Chimiques FR1739, Campus Scientifique de Saint Jérôme, Marseille Cedex 20 13397, France b

A R T I C LE I N FO

A B S T R A C T

Keywords: Aquivion Proton conductivity Fluorophosphonic acid modified cerium oxide Fluoride emission rate Radical scavenger

Surface modified CeO2 nanoparticles were prepared by reacting nanometric CeO2 with a fluoroalkyl (C8F17C2H4-PO(OH)2) or a fluorobenzyl phosphonic acid (C6F5-CH2-PO(OH)2). Both non-modified and fluorophosphonic acid modified CeO2 were used as fillers of composite Aquivion membranes, with loadings up to 5 wt%. Ex situ accelerated ageing using the Fenton reaction shows that non-modified CeO2 is a more efficient radical scavenger than the two fluorophosphonic acid modified fillers. Fluoride emission rate data for pristine Aquivion and for the composite membranes are consistent with the corresponding changes in the values of ion exchange capacity (IEC) and elastic modulus. The conductivity of membranes containing non-modified CeO2, measured as a function of relative humidity at 80 and 110 °C, decreases with increasing filler loading. For loadings > 2 wt%, an increase in temperature from 80 to 110 °C results in further conductivity drop and in the concomitant IEC decrease, due to partial filler solubilization, thus indicating that the composite membranes are unstable at temperature > 80 °C. However such conductivity drops are strongly reduced, especially at 80 °C, for composites with fluorophosphonic acid modified CeO2.

1. Introduction Polymer electrolyte membrane (PEM) fuel cells have been extensively studied as a clean and efficient power source for transportation and stationary power generation. Among different PEM materials, perfluorosulfonic acid (PFSA) ionomers, such as Nafion, Aquivion, and 3 M membranes, are considered as the state-of-art PEM materials because of their excellent chemical, mechanical, and thermal stability, as well as their relatively high proton conductivity when fully hydrated. However, an adoption on large scale of the PEM fuel cell based technology is currently limited by poor component durability under working conditions. Specifically, because of the PEM multiple functions both as an electrolyte and a separator of the reactant gases, the integrity of the PEM is one of the most crucial factors affecting the cell longevity and the PEM degradation remains a critical issue in fuel cell systems to meet the required lifetime. PEMs can suffer from both mechanical

degradation, determined by membrane hydration – dehydration, and chemical degradation due to radical species, generated by electrochemical or chemical processes during fuel cell operation, that attack weak links in the polymer electrolyte backbone or side-chains and lead to chain scission, as well as to the release of fluoride ions [1–9]. Chemical degradation can be mitigated by incorporation of radical scavenger species such as terephthalic acid [10], metal particles [11–14] or cations either in the PEM [15] or in the catalyst layer [16,17]. However, cations such as Ce3+ or Mn2+ give rise to a decrease in membrane conductivity and tend to be eluted from the membraneelectrode assembly especially when current is drawn from the cell [18]. A different approach consists in the incorporation of cerium and manganese oxide nanoparticles [19–25]. The radical scavenging properties of cerium dioxide have been explored by many researchers that have established its role in reducing FER (Fluorine Emission Rate) during fuel cell operation. Cerium has two stable oxidation states, + 4 and

⁎ Corresponding authors at: Centro di Eccellenza su Materiali Innovativi Nanostrutturati per Applicazioni Chimiche Fisiche e Biomediche – CEMIN, via Elce di Sotto 8, 06123 Perugia, Italy. E-mail addresses: [email protected] (A. Donnadio), [email protected] (M. Casciola).

https://doi.org/10.1016/j.memsci.2018.12.045 Received 14 October 2018; Received in revised form 14 December 2018; Accepted 16 December 2018 Available online 17 December 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

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2.4. Membrane preparation

+ 3, and the relatively ease to switch between these two states is the fundamental factor for its scavenging activity. Nevertheless, the proton transport properties of such composite PEMs as a function of temperature and relative humidity have not been adequately investigated up to date. A preliminary investigation carried out in our laboratory showed that the conductivity of composite membranes made of a short-sidechain perfluorosulfonic acid (Aquivion, hereafter AQ) and CeO2 decreased with increasing temperature. This behaviour suggested that the ionomer protons could react with the oxide surface. This reaction was expected to be hindered to some extent by forming, on the oxide surface, a protective shell made of phosphonates bonded to Ce ions through the –PO3 function. To this aim, in the present work, CeO2 nanoparticles were reacted with a fluoroalkylphosphonic acid (C8F17C2H4-PO(OH)2, hereafter C8) or with a fluorobenzylphosphonic acid (C6F5-CH2-PO(OH)2, hereafter Bz) and used, together with unmodified CeO2, as fillers of AQ based composite membranes. After the synthesis and characterization of the fillers, the paper reports the proton conductivity evolution as a function of temperature (80–110 °C) and relative humidity (RH, 50–90%) for composite membranes with filler loadings up to 5 wt%. The radical scavenger efficiency of the fillers was also investigated through accelerated membrane ageing by the Fenton test.

The Aquivion dispersion was homogenized, then cast on a Petri dish and dried at 80 °C for 2 h in an oven. The obtained membrane (1 g) was redissolved in 20 mL of propanol at 80 °C in a closed vessel. The dispersion thus obtained was recast on a glass support by an Elcometer Doctor Blade Film Applicator 4340 with transverse speed of 2 mm min-1 and knight height of 0.7 mm. After heating in an oven at 50 °C for 2 h, the temperature was raised to 90 °C to remove the residual solvent. The membrane was then detached from the support and treated according the following procedure: 2 h in HCl 1 M, 1 h in H2O at room temperature, 2 h at 90 °C and 1 h at 160 °C. The obtained membrane was thick ca. 20 µm. 2.5. Preparation of composite membranes containing CeO2 based nanoparticles A weighed amount of pristine or functionalized CeO2 nanoparticles was added to the dispersion of Aquivion in propanol. The mixture was sonicated for 10 min, stirred for 2 h and cast on a Petri dish. Solvent was evaporated in an oven at 80 °C. The membrane thus obtained was redissolved in 20 mL of propanol and the resulting dispersion was treated as described in the previous section. Membranes loaded with 1%, 2%, 3%, 4%, 5% w/w of either pristine or functionalised CeO2, as well as one membrane loaded with 10% w/w of Bz functionalized CeO2, were prepared.

2. Experimental 2.1. Materials

3. Methods

Cerium (III) nitrate hexahydrate (Ce(NO3)3·6H2O) was supplied by Carlo Erba, fluorophosphonic acid (C8F17-C2H4-PO(OH)2) was supplied by Specific Polymers. A 20 wt% Aquivion dispersion in water (D83-6A, ionomer equivalent weight = 830 g equiv-1) was kindly supplied by Solvay Specialty Polymers, Italy. Citric acid (C6H8O7·H2O), 2,3,4,5,6pentafluorobenzylphosphonic acid (C6F5-CH2-PO(OH)2) and all other reagents were from Sigma-Aldrich and used without purification.

3.1. Ex situ accelerated membrane ageing using Fenton's reagent To prepare the Fenton's reagent, 20 ppm iron sulfate (FeSO4·7H2O) was added to the hydrogen peroxide solution (30 wt% in H2O). The accelerated ageing test was carried out at 75 °C by contacting a membranes sample (50–60 mg) with the Fenton's reagent (20 mL). After 4 h ageing, the membrane sample was removed from solution, washed with DI water and dried at room temperature. The fluoride ion concentration in the Fenton's reagent was measured by using an ion selective electrode Mettler Toledo. The electrode was calibrated using standard solutions before making a series of measurements. A total ionic strength adjustment buffer (TISAB) was added to the electrolyte solution in order to keep the pH in the range of 5–7.

2.2. Synthetic procedures Nanopolyhedral shaped cerium dioxide was synthesized by thermal decomposition of a cerium cerium citrate precursor. Specifically, cerium nitrate hexahydrate 0.2 M and citric acid 0.5 M were solubilized in water separately. The two solutions were mixed, so that the molar ratio Ce3+/citric acid was 1:4, and the mixture was stirred at 70 °C in an oil bath for 24 h; then, the temperature was raised at 120 °C and kept at this value for 3 h. The obtained powders were ground in a mortar for few minutes and then heated in air at 500 °C for 2 h in order to convert cerium citrate into CeO2.

3.2. Conductivity measurements The in-plane conductivity of the membranes was determined on 5 cm ± 0.5 cm membrane strips in the frequency range 10 Hz to 100 kHz with 100 mV signal amplitude by four-probe impedance measurements by using an Autolab, PGSTAT30 potentiostat/galvanostat equipped with a frequency response analysis (FRA) module as described in Ref. [27].

2.3. Functionalization of cerium dioxide with phosphonic acids Surface functionalized CeO2 was obtained by adding the CeO2 powder to a solution of 10-2 M phosphonic acid in a 1:1 mixture of water and n-propanol. The amount of phosphonic acid was three times the amount needed for a full surface coverage. In particular, the moles (n) of phosphonic acids were calculated by the following relation:

n=3

3.3.

31

P solid-state NMR experiments

Solid state 31P MAS NMR spectra were performed at 161.97 MHz on a Bruker Advance 400 spectrometer. The powders were packed into 4 mm zirconia rotors and sealed with Kel-F caps. The spin rate was 8 kHz. The π/2 pulse width was 3.5 μs, and the recycle delay was 140 s; 1200 scans were collected for each spectrum. Spectra of powdered samples were acquired using 2048 data points. All spectra were zero filled and Fourier transformed. The Gaussian/ Lorentzian model was selected. Each resonance was characterized by the amplitude, the resonance frequency in parts per million (ppm), and the width at halfheight [28].

SCeO2 N ·APA

where SCeO2 is the surface area of cerium dioxide (calculated by BET), APA is the area (24 Å2) covered by the -PO3 group [26] and N is the Avogadro number. Cerium dioxide and the phosphonic acid solution were kept under stirring for 2 h at room temperature and then heated for 24 h at a constant temperature of 25 °C, or 80 °C or 130 °C. The functionalized solid was recovered by centrifugation, washed twice with n-propanol and finally dried at 100 °C. 18

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3.4.

19

temperature of boiling liquid nitrogen by a Micromeritics ASAP 2010 apparatus on samples previously outgassed overnight at 100 °C. The specific surface area and the pore size distribution of CeO2 powders were evaluated by the Brunauer–Emmett–Teller (BET) method and the B.J.H. analysis [32], respectively.

F solid-state NMR experiments

The solid-state 19F MAS quantitative NMR spectra were obtained on a Bruker Advance-400 MHz NMR spectrometer (magnetic field 9.4 T) operating at a 19F resonance frequency of 376.8 MHz and using a commercial Bruker 4 mm probe. About 30 mg of samples were placed in the quantitative part of a zirconium dioxide rotor of 4-mm outer diameter and spun at a Magic Angle Spinning rate of 10 kHz. The 19F Single Pulse Experiments (SPE) were performed at ambient temperature, 256 scans were accumulated using a delay of 7.5 s, equal to 5 × the longest spin-lattice relaxation time T1 (T1 = 1.5 s). The absolute amount of fluorine of the fluorophosphonic acid modified CeO2 nanoparticles was obtained by integration of 19F NMR signal of the sample and compared to the 19F NMR spectra of the pure substance. The 19F chemical shifts were referenced to 0.1 M aqueous NaF solution.

3.10. IEC determination The ion-exchange capacity (IEC) of the membranes was determined by acid-base titrations with a Radiometer automatic titrimeter (TIM900 TitraLab) operating at the equilibrium point method. Before titration, the membrane was heated at 120 °C for 3 h in order to determine the anhydrous weight of the sample. The dry membrane (∼ 150 mg) was equilibrated overnight, at room temperature, with 20 mL of a 0.1 M NaCl aqueous solution in order to exchange Na+ ions for the membrane protons [33]. The error on the determination of IEC was evaluated by titrating five AQ membranes. The average IEC value turned out to be 1.206 meq g-1, with a standard deviation of 0.0032 meq g-1.

3.5. TEM Transmission electron microscopy (TEM) analysis was carried out by a Philips 208 transmission electron microscope, operating at an accelerating voltage of 100 kV. Sample powders were diluted in ethanol and sonicated for 2 min, then supported on copper grids (200 mesh) precoated with Formvar carbon films and dried in air overnight.

4. Results and discussion 4.1. CeO2 based fillers The X-ray diffraction pattern of cerium dioxide (CeO2) nanoparticles prepared by thermal decomposition of a cerium citrate precursor is shown in Fig. SI1. Diffraction peaks are observed at 2θ values of 28.69°, 33.24°, 47.49°, 56.58°, and 59.17° associated with reflections from (111), (200), (220), (311) and (222) planes, respectively, of the face centred cubic fluorite type unit cell (space group Fm3m). The crystallite size of CeO2 was estimated in the range 9–10 nm from the width at half height of the first reflection by using the Scherrer equation, and turned to be in excellent agreement with the TEM micrographs, showing a quite narrow size distribution of the ceria particles the range 7–12 nm, with an average particle size of 9 nm (Fig. 1). The CeO2 powder samples synthesized in the present work had a specific surface area in the range 30–37 m2/g. The analysis of the nitrogen adsorption/desorption isotherms (Fig. SI2) indicates that the powders are mesoporous with a cumulative pore volume around 0.12 cm3/g. In order to modify the surface of the CeO2 nanoparticles with organic groups, CeO2 was treated with solutions of two phosphonic acids as described in the experimental section. The presence of phosphonic acids bonded to the CeO2 nanoparticles was investigated by solid state NMR. The 31P MAS spectra of the nanoparticles treated with the phosphonic acids (Fig. 2) show a main broad resonance, around 18 ppm for Bz and 24 ppm for C8, with a shoulder at 24 ppm for Bz and 31 ppm for C8. The fact that, in both cases, the main 31P resonance is shifted by -10/-12 ppm with respect to that of the free phosphonic acids (27.6 ppm

3.6. X-ray diffraction X-ray diffraction (XRD) patterns of powders were collected with a Philips X’Pert PRO MPD diffractometer as described in Ref. [29]. 3.7. X-ray photoelectron spectroscopy The equipment used for X ray photoelectron spectroscopy (XPS) studies consists of a preparation and an analysis chambers separated by a gate valve [30]. The analysis chamber is equipped with a six degrees of freedom manipulator and a 150 mm mean radius hemispherical electron analyzer with five-lens output system combined with a 16channel detector. Measurements were performed at normal take-off (θ = 90°). Samples were introduced in the preparation chamber and left outgassing overnight at a base pressure of about 10–8 Torr, before introduction in the analysis chamber. Typical vacuum pressure in the analysis chamber during measurements was in the 10-9–10-10 Torr range. Mg Kα non-monochromatized X-radiation (hν = 1253.6 eV) was used for recording C1s, Ce3d, O1s, F1s and P2p core level spectra, with a pass energy of 25 eV. Measurements were performed on at least two different specimens for each sample in order to check data reproducibility; average values are reported. The measured Binding Energies ( ± 0.1 eV) were calibrated to the C1s signal of aliphatic C-C carbons, arising by impurities on the sample surface, located at a Binding Energy (BE) of 285.0 eV. Experimental spectra were analyzed by curve fitting using Gaussian curves as fitting functions. 3.8. Mechanical tests Stress–strain mechanical tests were performed by using a Zwick Roell Z1.0 testing machine with a 200 N static load cell as described in Ref. [31]. The elastic modulus was determined on rectangular shaped film strips with length and width of 100 and 5 mm, respectively. Before the tests, the samples were equilibrated for seven days in vacuum desiccators at 53% RH and room temperature (20–23 °C). At least five replicate film stripes were analyzed. The data were elaborated by the TestXpert V11.0 Master software. 3.9. Surface area determination

Fig. 1. TEM image of CeO2 nanoparticles.

Nitrogen adsorption/desorption isotherms were determined at the 19

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Fig. 2. Solid-state temperature.

31

Fig. 4. Comparison of X-ray powder diffraction spectra of C8 and Bz, synthesized at different temperatures, with the pattern of non-modified CeO2.

P CP-MAS NMR spectra of Bz and C8 prepared at room

Table 1 P/Cerium atomic ratio (P/Ce) for Bz and C8 prepared at different temperatures.

for C6F5-CH2-PO(OH)2 and 36 ppm for C8F17-C2H4-PO(OH)2) suggests that the phosphonic groups are bonded to the nanoparticles [34]. Moreover, the broadness of the main resonance and the presence of the shoulder are the signature of a non-uniform binding configuration. Although the assignment to specific binding configurations is difficult on the basis of the present data, it can be observed that when a –PO3 group is bonded to two or three Zr(IV) atoms the 31P resonance is shifted by − 9 or − 18 ppm, respectively, relative to the resonance of the free –PO3H2 group. Such shifts are expected to be reduced for –PO3 groups bonded to a larger and less electronegative atom, such as Ce(IV). Therefore, the observed shift of -10/-12 ppm appears to be consistent with the presence of tri-dentate –PO3 groups, while the shoulder of the main resonance could be assigned to bi-dentate groups. The surface functionalization of CeO2 does not alter the morphology of the nanoparticles, whose size ranges from 5 to 8 nm (Fig. 3). To ascertain that the reaction of CeO2 with the phosphonic acids is limited to the surface and does not involve the bulk of the nanoparticles, X-ray powder patterns were collected on CeO2 samples treated with the phosphonic acids at different temperature in the range 25–130 °C and compared with the pattern of pristine CeO2. Fig. 4 shows that in all cases the reaction with the two phosphonic acids does not alter the X-ray pattern of pristine CeO2, thus proving that the reaction is limited to the surface of the nanoparticles. The phosphonate content of surface modified CeO2, determined by 19 F-MAS NMR analysis, is shown in Table 1. Within the experimental error, the phosphonate/Ce ratio (averaged over the results of two series of experiments) is the same for both Bz and C8 (0.032 ± 0.002) independent of reaction conditions. This seems to

Sample

Temperature/°C

P/Ce

Bz

25 80 130

0,034 0,033 0,035

C8

25 80 130

0,034 0,030 0,034

indicate that the affinity of the two phosphonates is high enough to allow complete surface coverage independent of temperature and phosphonate type. In agreement with these considerations a surface coverage percentage of 83% can be estimated on the basis of the BET surface area of pristine CeO2 by assuming a surface coverage of 24 Å2 per molecule of phosphonic acids [26]. For determining the Ce oxidation state, the sample surface was investigated by X-ray photoelectron spectroscopy (XPS). The measurement of the Ce 3d core level allows to differentiate the Ce3+ and Ce4+ species due to the characteristic shape of their respective 3d signals. As shown in Fig. SI3 the Ce 3d XPS spectra are complex and the detailed quantification of the two oxidation states on the particle surface is arduous. As expected, Ce3d signal arising from Ce(III) and Ce(IV) mixed oxides is always observed. In the Table S1, XPS data (BE, FWHM, relative percents of atomic species values) collected on all samples at C1s and Ce3d core levels are reported, as well as Ce(III) and Ce(IV) percentages. Specifically, a Ce3+ and Ce4+ concentration of 40% and 60%,

Fig. 3. TEM images of C8 (left) and Bz (right) nanoparticles. 20

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respectively, was determined for CeO2 and Bz, while the Ce3+ concentration was slightly lower (30%) in the case of C8, coherently with literature findings for CeO2 nanoparticles. Actually, a Ce3+ concentration between 20% and 30% is reported by several authors for nanoparticles with mean sizes below 50 nm and different shapes [35–37]. Furthermore, the concentration of Ce3+ relative to Ce4+ is expected to increase as particles size decreases justifying the slightly higher amount of Ce(III) percentage (30–40%) observed for our samples (mean size 7–9 nm) [37]. 4.2. Composite membranes CeO2, C8 and Bz nanoparticles were used to prepare composite membranes based on Aquivion ionomer, with filler loading in the range 1–5 wt%. Considering the different hydrophilic / hydrophobic character of CeO2, Bz and C8, as well as the nanophase separated morphology of the ionomer, it is expected that CeO2 may interact mainly with the hydrophilic moieties of the ionomer, while Bz and C8 have preferred interaction with the perfluorinated ionomer backbone. This kind of selective interaction was recently suggested [31] to explain the mechanical properties of composite membranes made of Aquivion 700 loaded with hydrophilic and hydrophobic zirconium phosphate. The IEC values of the composite membranes loaded with CeO2, C8 and Bz nanoparticles are plotted in Fig. 5 as a function of filler loading. Since the IEC value of the filler (< 0.1 meq g-1) is small in comparison with that of the ionomer, the IEC of the composite membranes decreases with increasing filler loading and,is lower than that calculated on the basis of the ionomer weight percentage in the composite (solid line in Fig. 5). Moreover, for each filler loading, the following IEC sequence is observed:

Fig. 6. Conductivity as a function of relative humidity for AQ and the indicated composite membranes at 80 and 110 °C.

(from 0.028 S cm-1 to 0.048 S cm -1 at 50% RH, and from 0.123 S cm-1 to 0.157 S cm-1 at 90% RH) and a decrease in the conductivity of the three composite membranes. To highlight the conductivity dependence on temperature and filler loading, the membrane conductivity measured at 80 °C and 110 °C (for RH = 50% and 90% at each temperature) is plotted in Fig. 7 as a function of filler loading. As a general trend, it can be observed that: – the conductivity of all composites is nearly independent of temperature for filler loadings up to 2 wt% (and up to 3 wt% for AQ/ C8), but decreases with increasing temperature for higher loadings; – the conductivity sequence: AQ/Bz > AQ/C8 > AQ/CeO2 (which reflects the IEC sequence) is observed for all membranes with filler loadings > 3 wt%.

AQ/Bz > AQ/C8 > AQ/CeO2. These findings suggest that a small fraction of the ionomer protons (3.5% at most) react with the oxide surface and that this reaction is hindered to some extent when the surface of CeO2 is functionalized with the phosphonic acids. The in-plane membrane conductivity (σ) of the composite membranes was measured at 80 and 110 °C as a function of filler loading (from 1 to 5 wt%) and RH (from 50% to 90%). In all cases, log σ depends linearly on RH as shown, for example, in Fig. 6 where the conductivity of membranes containing 5 wt% filler (AQ/CeO2-5, AQ/C8-5 and AQ/Bz-5) is compared with the conductivity of a pristine Aquivion membrane. It can be observed that, in the investigated RH range, the temperature increase from 80 °C to 110 °C gives rise to an opposite effect: an increase in the conductivity of the pristine Aquivion membrane

Interestingly, at 80 °C the AQ/Bz-x membranes are as conductive as Aquivion regardless of filler content (x): this is not surprising since it was reported that the conductivity of PFSA composite membranes can even be higher than the conductivity of the PFSA in the presence of hydrophobic fillers [38,39]. All other composite membranes are less conductive than Aquivion, especially at 50% RH. To get further insight into the dependence of membrane conductivity on temperature, the conductivity of the AQ/CeO2-5 composite was measured as a function of temperature from 60 to 110 °C, by keeping RH constant at 50 or 90%, and compared with the conductivity of pure Aquivion under the same conditions. All measurements were performed after membrane equilibration for one day at a certain T and RH value. At 90% RH, the conductivity of the composite shows the same temperature dependence as the Aquivion membrane up to 80 °C, but starts decreasing at higher temperature (Fig. 8). Differently, at 50% RH, the conductivity of the composite decreases continuously over the entire temperature range. It can be observed that the conductivity decrease with increasing temperature can hardly be attributed to changes in membrane composition due to elution of cerium and/or phophonic acids because all measurements were carried out at RH < 100% (and in the absence of any humidified gas flow). Accordingly, the composition of an AQ/CeO2 and an AQ/Bz membrane turned out to be unchanged after one-day treatment in the conductivity cell at 110 °C and 90% RH (see SI for details). The conductivity decrease with increasing temperature is likely due to the reaction between the protons of the ionomer sulfonic groups and the oxygen anionic species of the surface of cerium dioxide, which is expected to decrease the ion exchange capacity of the ionomer. To check this suggestion, membrane samples were heated from 60 to

Fig. 5. IEC values for AQ/CeO2, AQ/C8 and AQ/Bz composite membranes as a function of filler loading. The solid line represents the IEC values calculated on the basis of the ionomer weight percentage. 21

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Fig. 8. Conductivity as a function of temperature at 50% and 90% relative humidity for AQ and AQ/CeO2-5 membranes.

Fig. 9. Fluoride emission rate (FER) as a function of cerium content (Ce/SO3H) for AQ and for AQ/CeO2, AQ/Bz and AQ/C8 composite membranes.

with the Fenton solution. The fluoride emission rate (FER, defined as the ratio between the mass of released fluoride ions and the initial mass of the membrane) decreases with increasing the number of cerium atoms per sulfonic group, Ce/SO3H (Fig. 9). The fluorophosphonic acid modified fillers show similar FER values for Ce/SO3H ratios between 0.14 and 0.25 (filler loadings in the range 3–5 wt%), while Bz is more efficient than C8 for lower ratios. In all cases, CeO2 is the most efficient radical scavenger: specifically, a FER of 4·10-3 is obtained with CeO2 and Bz for Ce/SO3H = 0.15 and 0.25, respectively. After exposure to the Fenton reagent, the integrity of the membranes containing 5 wt% filler was evaluated by determining the IEC and by testing the mechanical properties. The results of these tests are reported in Table 2 in terms of percentage change in IEC and elastic modulus (E). On the whole, both IEC and E values reflect qualitatively

Fig. 7. Conductivity as a function of filler loading for AQ/CeO2, AQ/C8 and AQ/Bz composite membranes.

110 °C in the conductivity cell under the same conditions of RH and equilibration time used for the conductivity measurements. The membrane IEC was determined after the first day of heating at 60 °C and at the end of each heating run. At both RHs, the IEC at 110 °C turned out to be 80% of the IEC at 60 °C. It can be observed that, if all the Ce atoms of the oxide were in the oxidation state +IV, then the IEC decrease would imply the solubilization of about 19% of the oxide mass. All these findings indicate that the AQ/CeO2-5 composite membrane is intrinsically unstable over the whole investigated temperature range at 50% RH and above 80 °C at 90% RH. To test the ability of the different fillers to mitigate the membrane chemical degradation, ex-situ accelerated degradation tests were carried out by reacting pristine Aquivion and the composite membranes

Table 2 Percentage changes in ion exchange capacity (ΔIEC) and elastic modulus (ΔE) for AQ and for AQ/CeO2-5, AQ/C8-5 and AQ/Bz-5 composite membranes after the Fenton test.

22

Membrane

ΔIEC (%)

ΔE (%)

AQUIVION AQ/CeO2-5 AQ/C8-5 AQ/Bz-5

− 35 − 22 − 29 − 27

− 35 − 17 − 34 − 13

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the FER sequence, although the mechanical deterioration of AQ/Bz-5 (-13%) is surprisingly lower in comparison with AQ/CeO2-5 (-17%).

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5. Conclusions Cerium dioxide based nanoparticles, including non-modified CeO2 and fluorophosphonic acid modified CeO2, were synthesized and used as fillers of Aquivion membranes. Composite membranes with different filler loadings were characterized by conductivity measurements, under different conditions of temperature and relative humidity, and subjected to ex situ chemical degradation to test the activity of the fillers as radical scavengers. The membrane conductivity decreases with increasing filler loading according to the sequence: AQ/Bz > AQ/C8 > AQ/CeO2. A further conductivity decay is observed especially at low relative humidity when temperature is increased from 80 to 110 °C. The concomitant decrease in the IEC indicates that a partial filler solubilization occurs at 110 °C. Ex situ chemical degradation tests were performed with the Fenton reagent. The data of fluoride emission rate for both pristine Aquivion and the composite membranes are consistent with the corresponding changes in IEC and elastic modulus induced by the degradation process and show that non-modified CeO2 is a more efficient radical scavenger than the two fluorophosphonic acid modified fillers. Therefore, the advantages arising from the use of fluorophosphonic acid modified CeO2 in terms of a reduced conductivity drop are offset to some extent by their lower efficiency as radical scavengers, although a reasonable compromise between stable conductivity and improved membrane stability is reached with the AQ/Bz composite. Further investigation is however necessary to mitigate as much as possible the conductivity drop, while keeping high radical scavenger efficiency. Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at doi:10.1016/j.memsci.2018.12.045. References [1] R. Borup, J. Meyers, B. Pivovar, Y.S. Kim, R. Mukundan, N. Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J.E. McGrath, M. Inaba, K. Miyatake, M. Hori, Kenichiro Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K. Kimijima, N. Iwashita, Scientific aspects of polymer electrolyte fuel cell durability and degradation, Chem. Rev. 107 (2007) 3904–3951. [2] A. Sadeghi Alavijeh, M.A. Goulet, R.M.H. Khorasany, J. Ghataurah, C. Lim, M. Lauritzen, E. Kjeang, G.G. Wang, R.K.N.D. Rajapakse, Decay in mechanical properties of catalyst coated membranes subjected to combined chemical and mechanical membrane degradation, Fuel Cells 15 (2015) 204–213. [3] F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Review: durability and degradation issues of PEM fuel cell components, Fuel Cells 8 (2008) 3–22. [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. Caqu´e, A review of PEM fuel cell durability: materials degradation, local heterogeneities of aging and possible mitigation strategies, Wiley Interdiscip. Annu. Rev. Energy Environ. 3 (2014) 540–560. [5] M.P. Rodgers, L.J. Bonville, H.R. Kunz, D.K. Slattery, J.M. Fenton, Fuel cell perfluorinated sulfonic acid membrane degradation correlating accelerated stress testing and lifetime, Chem. Rev. 112 (2012) 6075–6103. [6] M. Zaton, B. Prélot, N. Donzel, J. Rozière, D.J. Jones, Migration of Ce and Mn ions in PEMFC and its impact on PFSA membrane degradation, J. Electrochem. Soc. 165 (2018) F3281–F3289. [7] J. Wu, X.Z. Yuan, J.J. Martin, H. Wang, J. Zhang, J. Shen, S. Wu, W. Merida, Degradation mechanisms and mitigation strategies, J. Power Sources 184 (2008) 104–119. [8] M. Zaton, J. Roziere, D.J. Jones, Current understanding of chemical degradation mechanisms of perfluorosulfonic acid membranes and their mitigation strategies: a review, Sustain. Energy Fuels 1 (2017) 409–438. [9] L. Ghassemzadeh, T.J. Peckham, T. Weissbach, X. Luo, S. Holdcroft, J. Am. Chem. Soc. 135 (2013) 15923. [10] Y. Zhu, S. Pei, J. Tang, H. Li, L. Wang, W.Z. Yuan, Y. Zhang, Enhanced chemical durability of perfluorosulfonic acid membranes through incorporation of terephthalic acid as radical scavenger, J. Membr. Sci. 432 (2013) 66–72. [11] M. Baker, R. Mukundan, D. Spernjak, S.G. Advani, A.K. Prasad, R.L. Borup, Cerium migration in polymer electrolyte, Membr. ECS Trans. 75 (2016) 707–714. [12] P. Trogadas, J. Parrondo, F. Mijangos, V. Ramani, Degradation mitigation in PEM fuel cells using metal nanoparticle additives, J. Mater. Chem. 21 (2011)

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