Nicolas Martinez

The proliferation of hydrogen fuel cell systems is hindered by a degradation of the platinum catalyst. Here, we provide a device-level assessment of the catalyst degradation phenomena and its coupling to nanoscale hydration gradients, using advanced operando X-ray scattering tomography tailored for device-scale imaging. Gradients formed inside the fuel cell produce a heterogeneous degradation of the catalyst nanostructure, which can be linked to the flow field design and water distribution in the cell. Striking differences in catalyst degradation are observed between operating fuel cell devices and the liquid cell routinely used for catalyst stability studies, highlighting the crucial impact of the complex operating environment on the catalyst degradation phenomena. This degradation knowledge gap accentuates the necessity of multimodal, in situ characterization of real devices when assessing the performance and durability of electrocatalysts and, more generally, electrochemically active phases used in energy conversion and storage technologies.

In fuel cell catalyst layers (CLs), the ionomer plays roles as a binder and proton conductor. The ionomer requires hydration for enabling proton transport; however, the accumulation of excess liquid water in CL pores hinders oxygen diffusion. Therefore, a key to the improvement of CLs is understanding the distribution and the quantity of water in an operando fuel cell. Yet, there is a lack of information on the water content in conventional Pt-based CLs during operation, especially within the ionomer phase. Here, we present the application of high-resolution operando small angle neutron scattering (SANS) on custom fuel cells to probe CL water content. Using this technique, we quantified liquid water saturation and ionomer nanostructure (swelling) in in situ CLs, unique information not accessible via conventional diagnostic tools such as imaging. Even at the low current density (150 mA/cm²), water was observed in the cathode catalyst layer as liquid and adsorbed in the ionomer. Improving the understanding of water management in the catalyst layer is the high interest of fuel cell community, and our results confirm that operando SANS will be indispensable for the development of catalyst and ionomer-based CLs.

Bacterial spores are among the most resistant forms of life on Earth. Their exceptional resistance properties rely on various strategies, among them the core singular structure, organization and hydration. By using elastic incoherent neutron scattering, we probed the dynamics of Bacillus subtilis spores to determine whether core macromolecular motions at the sub-nanosecond timescale could also contribute to their resistance to physical stresses. In addition, in order to better specify the role of the various spore components, we used different mutants lacking essential structure such as the coat (PS4150 mutant), or the calcium dipicolinic acid complex (CaDPA) located in the core (FB122 mutant). PS4150 allows to better probe the core’s dynamics, as proteins of the coat represent an important part of spore proteins, and FB122 gives information about the role of the large CaDPA depot for the mobility of core’s components. We show that core’s macromolecular mobility is not particularly constrained at the sub-nanosecond timescale in spite of its low water content as some dynamical characteristics as force constants are very close to those of vegetative bacteria such as Escherichia coli or to those of fully hydrated proteins. Although the force constants of the coatless mutant are similar to the wild-type’s ones, it has lower mean square displacements (MSDs) at high Q showing that core macromolecules are somewhat more constrained than the rest of spore components. However, no behavior reflecting the glassy state regularly evoked in the literature could be drawn from our data. As hydration and macromolecules’ mobility are highly correlated, the previous assumption, that core low water content might explain spores’ exceptional resistance properties seems unlikely. Thus, we confirm recent theories, suggesting that core water is mostly as free as bulk water and proteins/macromolecules are fully hydrated. The germination of spores leads to a much less stable system with a force constant of 0.1 N/m and MSDs ~2.5 times higher at low Q than in the dormant state. DPA has also an influence on core mobility with a slightly lower force constant for the DPA-less mutant than for the wild-type, and MSDs that are ~ 1.8 times higher on average than for the wild-type at low Q. At high Q, germinated and DPA-less spores were very similar to the wild-type ones, showing that DPA and core compact structure might influence large amplitude motions rather than local dynamics of macromolecules.

The dynamics of chymotrypsin increases when bound to two different covalent inhibitors. These effects were analyzed by univariate and multivariate methods.

Human low-density lipoprotein (LDL) is a highly complex nano-particle built up of various lipid classes and a single large protein moiety (apoB-100) owning essential physiological functions...

We report on a quantitative method to diagnose water distribution at various relevant length scales inside an operating proton membrane exchange fuel cell (PEMFC). By means of Small Angle Neutron Scattering experiments designed to achieve unprecedented spatial and temporal resolutions, we provide the first mapping of local water quantity, simultaneously over the entire cell as well as inside the polymer membrane, and we analyze the observed heterogeneities in light of various functional parameters. We evidence non-homogeneous water distribution at the scale of the flow field pattern, with significant differences in water content in front of the channel and the ribs, both inside and outside the membrane, depending on the positions close to gas inlet or gas outlet. In particular, it is shown that the amount of liquid water outside the membrane and the hydration state of the membrane are not directly cross-correlated. The operando water mapping at the level of the unit fuel cell allows us to identify the major phenomena controlling water formation and/or accumulation, therefore providing unique insights to improve the complex operando behavior of a fuel cell.

Water and protein dynamics on a nanometer scale were measured by quasi-elastic neutron scattering in the piezophile archaeon Thermococcus barophilus and the closely related pressure-sensitive Thermococcus kodakarensis, at 0.1 and 40 MPa. We show that cells of the pressure sensitive organism exhibit higher intrinsic stability. Both the hydration water dynamics and the fast protein and lipid dynamics are reduced under pressure. In contrast, the proteome of T. barophilus is more pressure sensitive than that of T. kodakarensis. The diffusion coefficient of hydration water is reduced, while the fast protein and lipid dynamics are slightly enhanced with increasing pressure. These findings show that the coupling between hydration water and cellular constituents might not be simply a master-slave relationship. We propose that the high flexibility of the T. barophilus proteome associated with reduced hydration water may be the keys to the molecular adaptation of the cells to high hydrostatic pressure.