Fuel cell rejuvenation of hygrothermally aged Nafion

International audienceNafion ® membranes stored for long periods at 80 °C under elevated relative humidity up to 95%RH exhibit large modifications of their properties attributed to the sulfonic acid end-group condensation into sulfonic anhydrides. The present study is devoted to the membrane property rejuvenation, namely the hydrolysis of the sulfonic anhydrides under different experimental conditions. Aged membranes were exposed to pure water and to acid solutions or vapors in order to check the reversibility of the condensation reaction. Indeed, the hydrolysis process is slow in pure water and limited while it is fast and complete in the presence of acid or base. The native polymer chemical structure and the main membrane properties (mechanical properties, hydrophilicity, etc.) are completely restored. No evidence of hygrothermal aging was observed after fuel cell operation and it is shown that a membrane previously aged under ex situ conditions can be completely rejuvenated when operated in fuel cell

Fuel cell rejuvenation of hygrothermally aged Nafion

Nafion ® membranes stored for long periods at 80 °C under elevated relative humidity up to 95%RH exhibit large modifications of their properties attributed to the sulfonic acid end-group condensation into sulfonic anhydrides. The present study is devoted to the membrane property rejuvenation, namely the hydrolysis of the sulfonic anhydrides under different experimental conditions. Aged membranes were exposed to pure water and to acid solutions or vapors in order to check the reversibility of the condensation reaction. Indeed, the hydrolysis process is slow in pure water and limited while it is fast and complete in the presence of acid or base. The native polymer chemical structure and the main membrane properties (mechanical properties, hydrophilicity, etc.) are completely restored. No evidence of hygrothermal aging was observed after fuel cell operation and it is shown that a membrane previously aged under ex situ conditions can be completely rejuvenated when operated in fuel cell

Disentangling water, ion and polymer dynamics in an anion exchange membrane

Semipermeable polymeric anion exchange membranes are essential for separation, filtration and energy conversion technologies including reverse electrodialysis systems that produce energy from salinity gradients, fuel cells to generate electrical power from the electrochemical reaction between hydrogen and oxygen, and water electrolyser systems that provide H2 fuel. Anion exchange membrane fuel cells and anion exchange membrane water electrolysers rely on the membrane to transport OH− ions between the cathode and anode in a process that involves cooperative interactions with H2O molecules and polymer dynamics. Understanding and controlling the interactions between the relaxation and diffusional processes pose a main scientific and critical membrane design challenge. Here quasi-elastic neutron scattering is applied over a wide range of timescales (100–103 ps) to disentangle the water, polymer relaxation and OH− diffusional dynamics in commercially available anion exchange membranes (Fumatech FAD-55) designed for selective anion transport across different technology platforms, using the concept of serial decoupling of relaxation and diffusional processes to analyse the data. Preliminary data are also reported for a laboratory-prepared anion exchange membrane especially designed for fuel cell applications

Etude des interactions moléculaires polymère-eau lors de l'hydratation de la membrane Nafion, électrolyte de référence de la pile à combustible

Le polymère Nafion est l'électrolyte de référence de la pile à combustible. Lorsqu'il est hydraté, il présente un conductivité élevée (10-2 S.cm-1). Néanmoins cette conductivité chute à faible taux d'hydratation. L'ajout d'un compos hygroscopique dans la membrane, telle phosphate de zirconium (ZrP), a été proposé dans la littérature pour répondre à c problème. La conductivité est le fait de la structure du matériau, des mécanismes de diffusion du proton, et des interactions eaq polymère au sein de la membrane. Nous nous sommes intéressés à cette dernière partie du problème. Nous avons étudi' les mécanismes d'hydratation à l'échelle moléculaire pour les membranes Nafion puis Nafion-ZrP par technique d spectrométrie infrarouge sur toute la gamme d'hydratation.. Cette technique peut être couplée à une étude par dynamiqu moléculaire que nous avons initié sur le polymère Nafion. Nos résultats font état de 5 mécanismes d'hydratation successifs pour la membrane Nafion. L'ionisation des group sulfoniques S03H est très rapide en début d'hydratation. Elle est suivie d'un éloignement des protons H+ par rapport aux groupes sulfonates S03- et d'une réorganisation du réseau de liaisons H autour de ces groupes ioniques. Enfin une eau d type bulk apparaît vers 40% d'hydratation. Nous avons ainsi une photographie de la membrane à chaque tal d'hydratation. L'ajout d'un composé inorganique ZrP n'influe pas sur les mécanismes d'hydratation. D'après la comparaison entre nos mécanismes et la courbe de conductivité, il est nécessaire de dissocier tous les groupes sulfoniaues DOur atteindre une diffusion oDtimale du Droton, probablement assurée par le mécanisme de Grotthuss.The Nafion is a polymer. Thanks to its high conductivity (up to ] 0-'< S.cm-') at high relative humidity (RH), it is a reference electrolyt for a fuel cell. However its conductivity falls during low hydration conditions. To solve this problem, we can add a hygroscopic compound, like ziconium phosphate (ZrP), into the membrane. The conductivity is linked to the structure of the membrane, the proton diffusion mechanisms and the interactions between water molecules and the polymer; we are interested by this last field of research. Infrared spectroscopy are used to establish the hydration mechanisms at a molecular scale for a Nafion and a Nafion-ZrP membrane. This technique can be coupled with a molecular dynamic study, which we have begun for the Nafion. The inftared spectra ofNafion and Nafion-ZrP have been measured on the whole range of RH. We found 5 hydration mechanisms for the Nafion membrane. The ionisation of sulfonic groups S03H is very fast at the beginning ofhydration. Then the protons H+ move away from the sulfonate groups S03- and the net ofhydrogen bonds around these ionic groups changes. For a RH of 40%, bulk water appears inside the membrane. We have thus a "photograph" of the inner membrane at each stage of RH. The adding of an inoganic compound ZrP has no influence on the hydration mechanisms. According to the comparison between our mechanisms and the curve of conductivity, all the sulfonic groups have to be dissociated to reach optimal diffusion ofthe Droton, probablv assured bv the Grotthuss mechanism.GRENOBLE1-BU Sciences (384212103) / SudocSudocFranceF

Small-Angle Scattering Study of Short Pendant Chain Perfuorosulfonated Ionomer Membranes

A structural study using both small-angle X-ray and neutron scattering (SAXS and SANS) of dry and water-swollen short pendant side chain perfluorosulfonated ionomer (SPC PFSI) membranes is presented. The SAXS and SANS profiles are shown to be equivalent, confirming the phase separation between the water and the perfluorinated matrix. The study of the asymptotic behavior at large angles of the scattering curves confirms that the structure is dominated by the polymer-solvent interfacial energy. The local order model is shown to satisfactorily reproduce the scattering profiles obtained for SPC PFSI membranes depending on the equivalent weight (EW) with a low number of free parameters. However, the values of the parameters for large EWs are not in agreement with those determined from the Pored analysis due to the presence of a well-defined maximum at low q values. This maximum, attributed to a crystalline component, slightly modifies both the position and the intensity of the ionomer peak. For the low EW membrane, which generates a very large water uptake, we propose to describe the swollen membrane as a connected network of rods

Advanced Fuel Cell Catalyst based on Graphene

International audienceCarbon blacks supported Pt, currently widely used as electrocatalysts in Polymer Electrolyte Membrane Fuel Cells (PEMFC) are thermochemically unstable in PEMFC operating conditions. This is especially true at the cathode side where, on top of relatively elevated temperature (80°C) and acidic conditions, both the potential and the relative humidity may be high. The resulting carbon oxidation is partially responsible for the PEMFC performance decrease observed over time. Hence, long term durability still needs to be improved in order to consider PEMFC as credible alternatives to conventional power sources for automotive, stationary or portable applications.Much effort have been directed to identify and synthesize alternative carbon materials as catalyst supports for PEMFCs. One strategy to decrease carbon support corrosion is to use carbon with high extent of graphitization, which is supposed to decrease defect sites on the carbon structure, where carbon oxidation starts [1], [2]. However high graphitic content of carbon can be a brake for particle nucleation and dispersion. Among the different forms of carbon, graphene, a monolayer of graphite, has attracted increasing attention because of its unique two-dimensional (2D) single sheet of sp2 carbon in a hexagonal arrangement. Graphene possesses unique properties, such as high charge-carrier mobility (up to 105 cm2 V-1 s-1), super conductivity, ambipolar electric field effect, high mechanical strength (130 GPa), quantum Hall effects at room temperature, and high surface area (2,600 m2 g-1). These properties make graphene ideal for a wide potential applications such as in nanoelectronics, electrode support for catalysts in electrochemical energy systems, chemical and biological sensors, composite materials and biotechnology.Although graphene exhibits an attractive range of properties as catalyst supports for fuel cells (FC), they suffer from a serious issue. Integration of such catalyst in catalyst layer (CL) is complex due to the high cohesive van der Walls attractions between graphene sheets, named also pi-stacking of graphene layers, leading to serious blocking of the active surface area. To overcome this phenomena, one solution consists to add spacers or additives in graphene sheets such as carbon black, carbon nanotube and urea have been reported to be intercalated into Pt/graphene or with several applications in FCs [3]. In this work, platinum catalysts were synthetized on Graphene support, nanocharacterised, processed in ink and incorporated in fuel cell. Several techniques of MEA fabrication were used: CCB (catalyst coated backing), CCM (catalyst coated membrane) or CCM by decal (transfert on inert substrate). We investigated these new active layers at the cathode side in term of electrocatalytic performance and compared the electrochemical properties of these hybrid materials with a commercial Pt/C catalyst using carbon blacks as carbon support. Moreover, the use of additive was also studied to enhance performance by aerate active layer and avoid pi-stacking of graphene layer.The goal of the project is to demonstrate that the use of GRM based carbon supports can be promising in effectively reducing the carbon corrosion and then increase lifetime of the cell. Accelerated stress test has been done in 25cm² fuel cell setup on both AST for carbon support and fuel cell Dynamic Load Cycle, FC-DLC cycles.The GrapheneCore3 European program funds this work.References[1] Lu, Y; Applied Catalysis B: Environmental 199 (2016) 292-314.[2] Yadav, R.; Industrial & Engineering Chemistry Research 57 (2018) 9333-9350.[3] Suter, T. A. M. et al. Nanomaterials 11, 2530 (2021)