Heterogeneous nanostructural ageing of fuel cell ionomer revealed by

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Heterogeneous nanostructural ageing of fuel cell ionomer revealed by operando SAXS Nicolas Martinez, Gérard Gebel, Nils Blanc, Nathalie Boudet, Jean-Sebastien MICHA, Sandrine Lyonnard, and Arnaud Morin ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b02004 • Publication Date (Web): 08 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Energy Materials

Heterogeneous Nanostructural Ageing of Fuel Cell Ionomer Revealed by Operando SAXS Nicolas Martinez,† Gerard Gebel,‡ Nils Blanc,¶ Nathalie Boudet,¶ Jean-Sebastien Micha,† Sandrine Lyonnard,∗,† and Arnaud Morin∗,‡ †Univ. Grenoble Alpes, CEA, CNRS, INAC, SyMMES, F-38000 Grenoble, France ‡Univ. Grenoble Alpes, CEA, LITEN, F-38000 Grenoble, France ¶Univ. Grenoble Alpes, CNRS, Grenoble INP, Institut Néel, 38000 Grenoble, France E-mail: [email protected]; [email protected]

Abstract Proton Exchange Membrane Fuel Cells (PEMFCs) represent one of the most interesting technologies for powering vehicles and small portable electronic devices in an eco-friendly way. Large scale implementation of PEMFCs requires to tailor novel resilient economically-viable materials with prolongated lifetimes. One key issue is the durablity of the ionomer membrane, which is responsible for the conduction of protons from anode to cathode. Here we report on the impact of ageing on the membrane structure by means of operando SAXS . By analyzing the most prominent features of the in-situ aged and pristine spectra at different relevant positions in the fuel cell, we could establish that structural ageing is highly heterogeneous and strongly dependent on the local conditions. The increase in current density produces a decrease in ionic nanodomain sizes in aged ionomer, due to continuous membrane drying, as in pristine materials. However, long term operation most dramatically affects the polymer organization into bundles, in particular at the air inlet and in the middle of the cell.

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Variations of the low-angle intensity of more than an order of magnitude are ascribed to significant increase in the area of interfacial regions, potentially impacting the diffusion within grain boundaries. With this study, we demonstrate that operando SAXS provide unique insights into ionomer structural ageing in dependence of the local hydration, and helps to identify the relevant scale for physical degradation. These informations are needed for optimizing fuel cells operating strategies and improving the durability of membranes.

Keywords PEMFC; ageing; ionomer nanostructure; degradation; in-situ SAXS; fuel cell water distribution

Introduction Reducing human related greenhouse effect is one of the challenges of our modern societies. In this context, efficient ways to convert energy from a non-carbon based fuel into electrical power is essential. Fuel cells represent one of the most promising technologies for automotive and stationary applications. Because of their low operation temperature, high efficiency and moderate cost, Proton Exchange Membrane Fuel cells (PEMFC) are one of the most advanced technologies. A central element of these devices is the ionomer membrane, which acts as a proton conductor while electrically insulating anode and cathode. This material should ideally have low resistance to protonic transfer 1 , as well as excellent resistance to chemical and mechanical stress 2,3 . In terms of efficiency and durability, two key aspects have to be taken into account: i) the distribution of water as a function of the operating conditions, and ii) the aging of the cell which results in components degradation, especially degradation of the ionomer material after repeated operation cycles leading to physical modifications induced by hygrothermomechanical stress or chemical degradation due to radical attacks.

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Aspect i) is linked to efficiency since proton transport property is dependent on hydration level 1 . The amount of water determines the nanoscale organisation of the proton-conducting network inside the polymer 4–11 , therefore directly impacting the mobility of confined protonic species at molecular 12–16 and mesoscopic scales 14,17 , and the available paths for structural diffusion 1,18 . Aspect ii) is related to fuel cell cost and maintenance 3,19 . Any defect of the membrane can dramatically reduce the lifespan of the fuel cell stack, therefore polymer degradation can result in cell failure, leading to a definitive stop of the system. The design of more efficient and durable devices requires to accumulate knowledge on both issues. In this context, it is of particular relevance to obtain the real-time monitoring of components and system behavior by realizing in-situ experiments on fuel cells operated in representative conditions.

In the last years, considerable efforts have been made in order to characterize the structurefunction relationship of PEMFC materials such as membranes 20 or catalysts 21 , which have ultimately led to the development of novel materials for catalyst layers 22–24 or proton conducting membranes 25–27 . In parallel, in-situ measurements of these materials have been carried out in order to understand the impact of operating conditions on these materials. Particular focus was put on the in-situ water distribution in PEMFCs. Using a variety of techniques such as NMR 28 , confocal Raman microscopy 29 , Small Angle Neutron Scattering (SANS) 30,31 , quasielastic neutron scattering 32 , neutron 33 and X-ray 34,35 imaging, it was concluded that water distribution in a functioning fuel cell is highly heterogeneous, both in-plane and through-plane. This intrinsic heterogeneity induces aging processes which in turn rule an heterogeneous degradation. 36–41 . Although aging potentially impacts all the materials composing the cell (current collecting plates 42,43 , gas diffusion layers 44,45 , catalyst layers 46,47 , ionomer membrane 48,49 ), catastrophic failure of the cell is often primarily due to membrane failure 50 . In-situ membrane degradation is a result of radical attack of the polymer 51–54 , introduction of cation pollutants in the ionic phase 54,55 , and physical constraints

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due to swelling-deswelling sequences 37,43 . These effects are expected to impact the structural integrity of the polymer on lengthscales spanning from nanometers (average size of ionic domains) to a few tens of nanometers (average size of the polymer bundles) 6,8,56,57 . The structural evolution of the ionomer can be probed at the relevant length-scales by Small Angle X-ray (SAXS) and Neutron (SANS) Scattering techniques. One main advantage of neutrons is their high sensitivity to hydrogen. The incoherent scattering cross section of H ( 80 barns) is much higher than that of any other atomic species (in the range of 1-2 barns usually). This makes SANS a very powerful tool to specifically evaluate the proton-containing hydrated ionomer membrane during operando fuel cell measurements, as the other components (mostly composed of C, O, Pt, Al, Au) produce moderate, H-free contributions to the total scattering 58–62 . However, SANS has several drawbacks, including limited time resolution, typically few tens of seconds, because of the intensity available at neutron sources 63 . Moreover, the dynamic Q-range accessible in a single measurement, which is determined by the neutron wavelengths and sample-detector distances achievable on modern spectrometers, is restricted to typically a decade. In situ SAXS appears to be highly complementary, in particular when performed using synchtrotron radiation. The high brilliance of the X-ray beam allows to resolve structural evolutions occurring within timescales in the range of ± 1 s in a single measurement over an extended Q-range (e.g. up to two decades). In this study, we report the first attempt at observing in-situ the effects of aging on the nano- and meso-scale organization of the polymer, and how this impacts the swelling properties of the membrane. By comparing operando SAXS measurements performed on pristine and aged fuel cells, we were able to clarify the interplay between local conditions, cell efficiency and microscopic structural effects, therefore providing clues for mitigating ionomer physical ageing in real functioning systems.

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Materials and Methods Materials and Fuel cell design The membrane-electrode-assembly (MEA) is composed of a low equivalent weight commerR membrane R79-02SX (20 µm thick) from Solvay Specialty polycially available Aquivion

mers sandwiched between home-made electrodes. The electrodes are composed of commercial Sigracet 24BC gas diffusion layer (GDL) from SGL coated with a catalyst layer (CL). PtCo alloy on Vulcan (TEC36V52 46,4% Pt from Tanaka Kikinzoku) was used as catalyst at the cathode. The anode is made of PtRu (TEC66E50_32,6% Pt et 16,9% Ru), supplied also by Tanaka Kikinzoku. The platinum loadings were about 0.4 and 0.6 mg.cm-2 for the anode and cathode, respectively. The MEA has been designed for micro combined heat and power (µCHP) application for operation in the medium temperature range, i.e. from 90◦ C to 120◦ C. As in our previous Small Angle Neutron Scattering experiments, we used a 25 cm2 single cell made of two gold-coated stainless steel plates serving as end-plate, current collector and monopolar plate, grooved with a single serpentine channel 59 . The channel depth is 1.4 mm. Both rib and channel have a width of 1.4 mm. Silicon gaskets were used for tightness, while mechanical bilges allowed to control the thickness of the compressed electrode corresponding to a stress of roughly 1 MPa on the active area. The homogeneity and the value of the R pressure sensitive films from FujiFilms. The cell stress were controlled using Prescale

temperature was controlled thanks to heaters stuck onto the end-plates. Six holes were drilled in the plates to define probed areas where the incoming X-ray beam was directed. The holes were closed by using X-ray transparent beryllium windows tightened with an O-ring to ensure cell sealing. The hole size (1 mm in diameter) was chosen to allow for illuminating the regions of interest with the microbeam (300 micron in size) and record scattered x-rays with an output angle sufficiently large to obtain data in the desired extended small angle range. The vertical positions of the windows along the flow field were defined to allow for probing

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the air outlet region (positions 1,2 at the top of the cell), the middle of the cell (positions 3,4), and the air inlet region (positions 5,6 at the bottom of the cell) (fig 1a). Because the water under current collecting ribs or gas flow fields was reported to be distinctly distributed 59,63 , the holes were performed in front of a rib (positions 2,3,5) and a channel (positions 1,4,6) at each vertical position. Adjacent holes designed in front of a rib and a channel are separated by one centimeter along the flow direction. This distance being small compared to the total length of the channel, rib and channel holes can be considered as very close one from the other. Horizontally, all holes are centered on the width of the electrode. Moreover, in order not to overwhelm the membrane signal by the strong scattered intensity of GDL and CL, the MEA was stripped from these components at the six probed locations, such that the beam only crosses the membrane (fig. 1a). Two identical single serpentine cells were assembled using MEAs coming from the same batch, and tested using the same test bench. They were operated with pure H2 at the anode and air at the cathode, with reactant stoechiometry of 1.2 and 2 respectively, in counter-flow configuration. In this configuration, the air inlet is located in front of the H2 outlet, yielding better performances as compared to the co-flow configuration due to an improved water management during operation, especially with partially humidified gases 64 at the cathode. A break-in procedure was conducted on both cells during 20 hours at constant current 0.8 A.cm-2 . The procedure was performed at 70◦ C and 1.2 bars, with a relative humidity (RH) of 80% and 100% at anode and cathode, respectively. One cell was further dried with pure nitrogen, closed, and labeled as "new cell". The second cell, further labeled as "aged cell", was operated during 600 hours at 90◦ C, using 68% and 46% RH at anode and cathode, respectively, and keeping the pressure and stoechiometry constant. The ageing was realized ˜ by applying a load cycle mimicking the on-field use of the µCHP system (fig.S1): 9 hours at 0.25 A.cm-2 and 15 hours at 0.5 A.cm-2 . In the middle of each of these periods, the current was decreased down to 0.16 A.cm-2 during 1 hour. The drop in performance during the ageing procedure was followed by measuring the drop in voltage in function of time (fig. S2).

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Polarization curves were recorded at the beginning and at the end of the test (fig. S3). From these data, it is clear that the function of the aged fuel cell has been deteriorated by the imposed cycle.

SAXS experiment SAXS data were collected on the D2AM/BM02 beamline at the ESRF (Grenoble, France). The X-ray beam was set to an energy of 13 keV, and the detector was positioned at 3 meters from the sample to cover the desired Q-range from 0.2-4 nm-1 .The size of the beam was reduced to 300 µm HWHM to fit with the size of the holes drilled in the fuel cell. All 2D SAXS spectra were obtained using exposure times of 5 s. After radial integration and empty cell subtraction, scattering angles were converted to wavevector transfer Q by using a calibrated scattering pattern (silver behenate standard) and the 1D scattering profiles were obtained. Absolute intensities were obtained by normalizing the spectra to a Lupolen standard. The new cell was mounted first to acquire reference data, then dismounted to allow for the aged cell to be installed. The SAXS measurements were performed on cells operated at 90◦ C, using 68% and 46% RH at anode and cathode, respectively, and keeping the pressure and stoichiometry constant. New and aged cell SAXS data were taken using the same protocols to allow for a direct comparison of the obtained structural evolutions in the fresh and degraded samples. The principle of the experiment is illustrated in fig. 1: the cell is scanned with the pinhole sized beam at all relevant positions (1 to 6) along the flow field (air inlet, air outlet and middle). The 6 position-sensitive SAXS spectra were first recorded at Open Circuit Voltage (OCV), e.g. without any production of current. Then, the current was varied stepwise in the range 0.1 to 0.8 A.cm-2 and data were continuously acquired at the 6 positions (kinetics measurements), until the selected current density was reached (equilibrium measurements). On fig. S4, a typical temporal sequence is shown, highlighting the evolution of remarkable structural features during the transient phases. Note that the data discussed hereafter were all recorded after equilibration, i.e. when the SAXS spectra were stable over 7



time, which occurred typically after 200 s 63 . During acquisition of the SAXS spectra, the ohmic resistance of the cell was measured by Electrochemical Impedance Spectroscopy (EIS). With this method, the ohmic resistance, also referred to as the high frequency resistance, is given by the value of the impedance reached when the imaginary part equals zero at high frequency. It can be reasonably assimilated to the membrane resistance.

In-situ water distribution In this section, we expose the results obtained on the new cell containing pristine ionomer. The 1D SAXS spectra were analyzed as a function of either the position (fig. 1b) or the current density (fig. 1c). As the PFSA ionomers are characterized by a complex structural organization spanning a range of lengthscales, the spectra were recorded over an extended Q-range, i.e. probing sizes and distances from 1 to 100 nm. The three typical features of PFSA ionomers are observed: a well-defined correlation peak located in the high-Q region (so-called ionomer peak), a broad bump visible in the intermediate Q-range (so-called matrix knee), and an intense low-Q up-turn. A number of structural models have been developed to account for the scattering profiles and their evolution with hydration, stretching, thermal annealing and/or process, density of charge, nature of side-chains, etc 65 . The attention was mostly devoted to describe the nanoscale phase separation, e.g. to unravel the size, shape, organization and connectivity of the proton-conducting hydrated nanodomains. Direct or reverse morphologies, as connected ionic clusters 66 , parallel ionic channels 5 , ribbon-like hydrophobic aggregates 7 , locally flat polymer particles 1 , were proposed. A general property, common to all models, is the existence of nanophase separation between the hydrophobic membrane moiety and the hydrophilic proton-containing domains, which is reflected by the ionomer peak located in the 1-2.5 nm-1 region of the spectra 4,6,8,66 . The peak position Q0 can be translated into a d-spacing by using the relation d=2π/Q0 . This quantity is defined as the average microscopic distance between the scattering objects. It corresponds 8


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Figure 1: Experimental principle: a) Photography of the SAXS fuel cell, with a picture of the single serpentine gas flow channel. The six probed positions are shown: air inlet (bottom), middle and air outlet (top), under ribs (closed circles) and channels (open circles). At each position, the GDL and CL were removed, and the cell is sealed using beryllium windows. A sketch of the cross-section of the cell showing the different components and the X-ray beam is represented underneath. b) 1D SAXS spectra taken at the three different representative flow-field positions at OCV. Filled circles represent measurements made in front of the rib, while open circles were made in front of the channel. c) 1D SAXS spectra recorded at the air inlet channel at different current densities. The curves are translated along the y-axis for illustrative purposes.

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to the correlation distance between ionic domains or, alternatively, to the mean separation between polymer aggregates/particles. When the ionomer hydration is increased, the continuous shift of the ionomer peak towards small Q reflects the microscopic swelling of the ionic network 56 . Recently, the swelling law of various PFSAs was established by combining sorption measurements and precise determination of d-spacings by ex-situ hydration-controlled SAXS experiments 13 . The variations of log(d) in function of the polymer volume fraction indicate the dilution of elongated objects having locally flat interfaces, therefore strongly supporting the ribbon-like and flat particles models. As a matter of fact, both Q0 and d are a direct indication of the membrane water content. As seen in fig 1c, variations in the position and intensity of the pristine ionomer peak are observed when current is gradually increased, highlighting the changes in water content with operating conditions. In contrast, the shape of the SAXS spectra at lower Q is not significantly altered when the new fuel cell is operated in the range 0 to 0.8 A.cm-2 . The matrix knee, which was shown to depend on the cristallinity of the polymer but whose origin is not elucidated, is hydration-dependent to a much lower extent than the ionomer peak. Its characteristics in terms of position, width and intensity, are difficult to establish as the large bump locates on the steep low-Q upturn. The excess scattered intensity at low-Q is completely insensitive to the fuel cell operation conditions, showing that the structure of pristine ionomer at scales larger than typically few tens of nanometers is mostly unaffected by the operating conditions. The origin of such low-Q signal has not been investigated in details in the literature. In fact, only Rubatat et al. 7 provided an interpretation by integrating the set of information measured over extended lengthscales by combining scattering techniques (e.g. USAXS, SAXS and WAXS) into a unique, self-consistent multi-scale structural model. They analysed the low-Q upturn using the Debye-Bueche formalism 67 , which describes large fluctuations in electronic densities. The domains associated to the Debye-Bueche behavior were found to extend over more than 50 nanometers in space. According to the authors, these so-called polymer bundles are formed by the preferential arrangement of neighboring ribbon-like particles, which are constituted

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by few polymer chains rejecting the side-chains in the outer part, e.g. in the continuous ionic phase containing sulfonic acid head groups, protons and water. Due to the aspect ratio of the aggregates and local flat topology of their surface, it was hypothesized that the ribbon aggregates are packed within a bundle in the same preferential direction. In this model, the material is therefore composed of the isotropic assembly of randomly oriented bundles. Such kind of structure is consistent with the presence of large scale heterogeneities in the PFSA, as evidenced by Atomic Force Microscopy measurements 7 . We adopt this bundle-ribbon-model in the following to discuss our SAXS results. Evidently, such scattering-based structure remains speculative and incomplete - for instance, the localization of polymer crystallites is unclear, but, at least, it provides an existing frame for cross-correlating nanoscale and larger scale behaviors. In the in-situ data obtained on the fresh membrane (Fig S5), we find that the low-Q upturn is well reproduced by a Q-4 power law. The Q-4 decay in intensity is typical of the existence of neat interfaces (so-called Porod’s law, see supporting information for more details). Here, we can not access the Debye-bueche behavior because our range in Q is limited, therefore the mean size of the bundles is not measurable. In the available Q-range corresponding to sizes smaller than the extension of the heterogeneities, the Q-4 scattering decay corresponds to the neat interfacial regions between bundle domains formed by large scale electron density fluctuations.

As the ionomer peak position and shape are highly sensitive to water activity we focus now on nanoscale effects by analyzing the variations in ionomer d-spacing as a function of both the position in the cell and the current density, and how they compare to previous experiments conducted by SANS 59 .

Fig. 2 shows the d-spacing distribution along the flow field at different current density conditions under ribs (closed symbols) and channels (open symbols). The relative position axis corresponds to a vertical scan starting from the bottom of the cell (air inlet cell positions

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Figure 2: d-spacing as a function of the relative position along the channel from the air inlet (0 being the air inlet, 1 the air outlet) and current density. Open (closed) symbols represent gas flow channels (current collecting ribs). 5,6 correspond to x-axis=0), going to the middle positions (positions 3,4; x-axis=0.5) towards the air outlet positions (positions 1,2, x-axis=1). At Open Circuit Voltage (0 A.cm-2 ), the d-spacing is roughly homogeneous, with a mean value of 3.09 nm. Note that a slightly lower hydration (d-spacing ∼ 3.05 nm) is recorded at the middle of the cell. Furthermore, no significant discrepancies between current collecting ribs and gas flow channels are observed, with an exception at the air outlet, where the channel is more hydrated. Since no current is drawn from the cell, no water can be formed from electrochemical reactions so one would expect a homogeneous hydration along the flow field. Previous experiments at similar conditions have shown that it was not the case 59 . It was hypothesized that heterogeneous water distribution at OCV could originate from an irreversible modification of the membrane swelling properties after the break-in procedure 68,69 .

Drawing current from the cell induces a significant heterogeneous water distribution

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across the flow field, reflecting the intrinsic heterogeneous behavior of the functioning fuel cell. The more (resp. less) hydrated regions are in the middle (resp. at the air inlet), as due to the counter-flow configuration using low relative humidity gases, especially at the cathode 46% RH, against 68% RH at the anode). On a much more local scale, e.g. between channels and ribs, the membrane appears to be slightly more hydrated in front of the ribs, except at the air outlet where the opposite behavior is observed. Because of the impediment of the rib, water molecules must follow a long path along the GDL plane to be evacuated in the channel, leading to larger transport limitations with consequently local accumulation. These trends have already been observed by SANS 59 , although for current densities larger than 0.6 A.cm-2 . The larger hydration in front of the channel could be explained by the accumulation of liquid water in this part of the flow field, as already observed by neutron radiography at the air outlet 70 .

When low current densities ( 50 nm (necessary to produce excess scattering in the observed Q-region) is a very unlikely situation. Moreover, no decrease in ohmic resistance, e.g. membrane conductivity, which is one of the footprint of pollution of the ionomer by cationic species, was measured on the aged MEA. Similar arguments stand for any impact of water content, because this can hardly lead to a ten-fold increase of contrast between hydrated and non-hydrated zones at this length scale. Regarding potential chemical modifications, the minimal changes observed in d-spacings after ageing indicate that a majority of the ionic groups remain intact and the nanoscale phase-separation between hydrophobic moieties and ionic domains is maintained, therefore excluding a prominent, massive chemical degradation affecting sulfonic acid groups. This is in contrast to hydrothermal ageing, where sulfonic acid moieties are modified by anhydride formation via cross-linking of SO3 groups, a process that hinders significantly the microscopic swelling ability of the membrane 69,73 . If the huge low-Q intensity variations do not originate from the presence of pollutants, large scale variations in water content or massive loss of ionic functions, we need to consider alterations in the nature (neat vs rough) and/or amount of interfaces. In general, any degradation of the quality of a neat interface between phase 1 and phase 2 would lead to a decreased slope (Q−(6−Df ) for a fractal of dimension Df 79 ), but all our data are satisfactorily represented by fitting the low-Q signal with an exponent -4, therefore giving no indication of rougher separation between large scale domains and their surrounding medium. Of course,

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the Q-range used for evaluating the low-Q data could be expanded towards small values, which would give access to a much more precise determination of these features, including the possibility to track the evolution of the Debye-Bueche behavior and characterize the observed correlation lengths. Accordingly, we are lacking quantitative information on the bundles, but, on the basis of our body of in-situ data, we can hypothesize an increase in specific surface of the objects probed at low angles, which could occur because their size and shape is modified. A possibility, in fact, is that the bundles could be decreased in size (reduction in correlation length) due to the reorganization of polymer chains, as a result, possibly, of enhanced disorder. Considering spherical bundle domains of size R, an increase of the specific surface S/V by a factor 10 would correspond to a diminution of radius by a factor 10. As a consequence, in this scenario, the ageing process would produce a significant amount of newly created inter-grain regions, with possibly specific grain boundaries properties in terms of mechanical and transport behavior. The ageing of the bundles structure could be related to a number of factors, as, for instance, chemical degradation of the backbone or evolution of the cristallinity. Hydroxyl radical attack on the fluorocarbon skeleton resulting in a stripping of CF groups from the membrane has been evidenced by theoretical calculations 80 and experimental data 51–54,81–83 . The presence of cristallites in the PFSA structure, as well as the global impact of the cristallinity on the overall membrane performance, is established, but information is lacking on the localization of the crystalline regions, their spatial correlations, and how they impact the multi-scale organization of the polymer. Cristallites may play a role in the bundles degradation. In the present state of understanding of membrane structure, it is impossible to unambiguously assess the origin of the physical polymer ageing at the mesoscale. More insights could be gained by performing operando scattering experiments using USAXS (bundle scale) and WAXS (cristallinity) to complement the body of experimental in situ observations we have collected. To assess the mechanical or chemical nature of degradation, additionnal mechanical, spectroscopic or dynamic techniques may need to be employed and correlated.

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However, despite the current limitations, our data clearly reveal the profound alteration of the ionomer mesostructure at the scale of polymer bundles 6 . This key finding highlights the importance of considering intermediate lentghscales in the attempt to rationalize the behavior of ionomer membranes, including critical processes as ageing in fuel cells.

Conclusion We have measured by operando synchrotron SAXS the structural evolution of pristine and aged ionomers in fuel cells operated in the usual range of current densities. By analyzing the spectra obtained in a single measurement over an extended Q-range, and using 6 different positions at the surface of the cell, we could identify the impact of ageing on the ionomer nanoand micro-structure and how it is modified by the operating conditions with respect to a pristine membrane. Increasing the current density results in membrane drying, in agreement with previous SANS measurement, although this effect appears to be less pronounced in the aged cell. We have observed that the nanoscale structure of the ionomer is not significantly altered by ageing. The ionic domains are intact after long term degradation. In contrast, huge cell position-dependent variations are observed in-situ at the mesoscale, typically beyond few tens of nanometers. Few hundreds of hours of operation therefore profoundly affect the nature and distribution of large-scale polymer bundles, which could potentially modify the efficiency of proton, water and gas transport by altering grain boundaries and interfaces. To evaluate the state of health of an aged ionomer membrane, observing the mesoscale structure evolution is highly indicative and more insightful than inspecting the commonly scrutinized nanoscale features, e.g. the typical sizes of the inter-connected ionic domains. These are not, in fact, the most manifest markers for diagnosing the membrane’s state in fuel cell operating in representative conditions. Further understanding of large scale bundle-like heterogeneities, their nature, organization and role, could be the clue to rationalize physical

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degradation of PFSAs and long-term performance limitations.

Acknowledgement The research exposed in this work received funding from the European Union’s Seventh Framework Program (FP7/2011), for the Fuel Cells and Hydrogen Joint Technology Initiative within the frame of PUMAMIND project under grant agreement n 303419, and for the Infrastructures within the frame of H2FC project under grant agreement n 284522. This work was also conducted in the frame of the internal CEA crosscutting action OPERANDO, which funded the post-doctoral position of N. Martinez. M. Ludovic Rouillon is thanked for his help in the cell design, as well as the ESRF for beamtime allocation.

Supporting Information Available SI include: Cyclic load profile and voltage evolution during the ageing procedure, polarization curves before and after ageing, time evolution of the nanoscopic swelling during operation, details and example of the SAXS data analysis, global correlation between d-spacings and ionomer peak integral, correlations between peak relative width and d-spacings. This material is available free of charge via the Internet at http://pubs.acs.org.

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Graphical TOC Entry

OPERANDO SAXS

PRISTINE 3 nm

AGED

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Heterogeneous nanostructural ageing of fuel cell ionomer revealed by

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