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Water and proton dynamics in an functioning proton exchange membrane fuel cell (PEMFC) were probed with quasi-elastic neutron scattering (QENS). By operating the specially designed neutron-transparent fuel cell in a “proton pump” mode, we were able to perfectly control the membrane hydration, which allowed us to discriminate the impact of electric field on water molecules diffusivity from pure hydration effects. By cross-analyzing the fuel cell QENS spectra taken over extended range of time scales, we demonstrate that protons exhibit the same dynamical behavior as in an ex-situ membrane at the same level of hydration, e.g., slow hopping motions of adsorbed protons (mostly H3O+) and subdiffusive confined motions of hydration protons (H2O). These mechanisms are maintained on applying a current up to 0.1 A/cm2. With this work, we establish the applicability of QENS to monitor in situ the molecular dynamics in PEMFC, and prove that knowledge extracted from ex situ proton conducting electrolytes can be transposed to the full operando cells.

Trifluoromethanesulfonate of triethylammmonium (TFTEA) doped Nafion-TEA membranes have been evaluated to understand the impact of different elaboration methods, i.e., swelling and casting, on their nanoscale morphology using SANS and resulting functional properties such as electrochemical, thermomechanical, and transport properties. The effect of transformation of acidic side chains of Nafion into ammonium side chains (Nafion-TEA) was first compared for the Nafion-TEA membrane elaborated by casting with the one elaborated by modification of extruded commercial Nafion117 membrane. Concerning TFTEA doped membranes, their nanostructuration is very similar whatever the processing mode despite some key differences. The casting based membranes exhibit higher and/or more heterogeneous swelling of ionic domains of Nafion-TEA by TFTEA and lower impact of TFTEA on the long-range crystalline order in Nafion-TEA. Casting based membranes present better thermomechanical properties compared to swelling based membranes above 150 °C and sustain a storage modulus of 1 MPa in the temperature range of 150–180 °C. With increasing TFTEA content as well as temperature, both types of membranes show an increase in the ionic conductivity of a similar order but no significant changes in hydrogen and oxygen-permeability coefficients. Both kinds of doped membranes exhibit similar sorption behavior (e.g., an increase in water uptake with increasing TFTEA content), though the results from GAB modeling suggest that morphology developed by casting is more favorable for the sorption of water molecules in middle range activity. Current density of 0.85 A/cm2at 0.6 V was obtained with a membrane containing 20% TFTEA.

The structure of polymer electrolyte membranes, e.g., Nafion, inside fuel cell catalyst layers has significant impact on the electrochemical activity and transport phenomena that determine cell performance. In those regions, Nafion can be found as an ultrathin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic/hydrophobic character of these surfaces on the structural formation of the films and, in turn, on transport properties has not been sufficiently explored yet. Here, we report classical molecular dynamics simulations of hydrated Nafion thin films in contact with unstructured supports, characterized by their global wetting properties only. We have investigated structure and transport in different regions of the film and found evidence of strongly heterogeneous behavior. We speculate about the implications of our work on experimental and technological activity.

Structure of Nafion inside Polymer Electrolyte Membrane Fuel Cells (PEMFC) catalyst layers is suspected to have a significant impact on the electrochemical activity and transport phenomena that determine the overall cell performances. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic character of these surfaces on the structural formation of the films has not been explored in details yet. Here, we report about our Molecular Dynamic computer simulations studies of the formation of Nafion thin films in contact with unstructured continuous surfaces, characterized by their global wetting properties only. We investigate changes of morphology and nanostructure of the perfluorosulfonic acid polymer as a result of water and polymer-surface affinities. Charge and water distributions are also analyzed. Possible impact of Nafion thin film self-assembly on the actual activity properties of realistic catalysts layers is shortly discussed.

The impact of triethylammmonium trifluorosulfonate (TFTEA) concentration on the nanostructuration and electrochemical, thermo mechanical, transport properties of doped Nafion membranes has been studied. The Nafion membranes neutralized with triethylamine (Nafion–TEA) have been doped with various amounts of TFTEA using the swelling method. The effect of the TEA neutralization was first studied. The results suggest a specific arrangement of TEA cations and a quite low water uptake. Concerning TFTEA doped membranes, the evolution trend of nanostructure of TFTEA doped Nafion is very similar to that of acidic Nafion swollen by water, with a slope equal to 1.33. However, at high TFTEA concentration (29 wt %) in this composite membrane, the average hydrophobic–hydrophilic phase separation distance appears to be 59 Å while it is 41 Å in hydrated acidic Nafion at the same volume fraction of polymer, which could be related to a much more heterogeneous distribution of TFTEA in Nafion–TEA than water molecules in acidic Nafion. Increasing TFTEA concentration in Nafion–TEA membranes results in significant increase of anhydrous ionic conductivity but no significant change in gas-permeability coefficients (hydrogen, oxygen). Water sorption experiments at 25 °C show that water uptake increases with increasing water activity and percentage of TFTEA in the membrane. However, the water sorption capacity of the TFTEA molecules within the membrane is limited due to the hindering effect of the polymer matrix.

Investigation of membrane/electrode assembly (MEA) microstructure has become an essential step to optimize the MEA components and manufacturing processes or to study the MEA degradation. For these investigations, transmission electron microscopy (TEM) is a tool of choice as it provides direct imaging of the different components. TEM is then widely used for analyzing the catalyst nanoparticles and their carbon support. However, the ionomer inside the electrode is more difficult to be imaged. The difficulties come from the fact that the ionomer forms an ultrathin layer surrounding the carbon particles and in addition, these two components, having similar density, present no difference in contrast. In this work, we show how the recent progresses in TEM techniques as spherical aberration (Cs) corrected HRTEM, electron tomography and X-EDS elemental mapping provide new possibilities for imaging this ionomer network and consequently to study its degradation.

Structure of Nafion inside Polymer Electrolyte Membrane Fuel Cells (PEMFC) catalyst layers is suspected to have a significant impact on the electrochemical activity and transport phenomena that determine the overall cell performances. In those regions, Nafion can be found as an ultra-thin film, coating the catalyst and the catalyst support surfaces. The impact of the hydrophilic character of these surfaces on the structural formation of the films has not been explored in details yet. Here, we report about our Molecular Dynamic computer simulations studies of the formation of Nafion thin films in contact with unstructured continuous surfaces, characterized by their global wetting properties only. We investigate changes of morphology and nanostructure of the perfluorosulfonic acid polymer as a result of water and polymer-surface affinities. Charge and water distributions are also analyzed. Possible impact of Nafion thin film self-assembly on the actual activity properties of realistic catalysts layers is shortly discussed.