High-resolution Small-angle Neutron Scattering Research Articles

In Situ Measurement of Ionomer Water Content and Liquid Water Saturation in Fuel Cell Catalyst Layers by High-Resolution Small-Angle Neutron Scattering

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. However, 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 the 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 a low current density (150 mA/cm2), water was observed in the cathode CL as liquid and adsorbed in the ionomer. Improving the understanding of water management in the CL is of high interest in fuel cell community, and our results confirm that operando SANS will be indispensable for the development of catalysts and ionomer-based CLs.

Polymer hydration and microphase decomposition in poly(N-vinylcaprolactam)–water complex

Poly(N-vinylcaprolactam) (PVCL) is a synthetic analogue of biomolecules (enzymes, proteins). It demonstrates a specific hydration and undergoes a coil–globule transition. The PVCL–D2O system (PVCL massM= 106) has been investigated by small-angle neutron scattering (SANS) atT= 296–316 K to identify the structural features of the collapse at concentrationC= 0.5 wt% near the threshold of the coil overlap. (The collapse leads to the segregation of the phase enriched with polymer atT> 305 K). The SANS experiments atq= 0.1–5 nm−1(scales from monomer unit to globule gyration radiusRG≃ 16 nm) have revealed a stretched coil–globule transformation in the range 305–309 K. Using high-resolution SANS (q= 0.002–0.02 nm−1) the globule association to form fractal structures (sponge-like) of surface dimensionDF≃ 2.4–2.6 was examined. The coexistence of globules and disordered chains (regions ∼5–10 nm) was found. The growth of the content of globular phase was induced by the conformational transition in disordered molecular fragments from coiled (dimensionD≃ 1.8) to stretched chains (D≃ 1.2).

Vortex lattice transitions in YNi2B2C

We have performed extensive small-angle neutron scattering (SANS) diffraction studies of the vortex lattice in single crystal YNi2B2C for B‖c. High-resolution SANS, combined with a field-oscillation vortex lattice preparation technique, allows us to separate Bragg scattered intensities from two orthogonal domains and accurately determine the unit cell angle, β. The data suggest that upon increasing field there is a finite transition width where both low- and high-field distorted hexagonal vortex lattice phases, mutually rotated by 45°, coexist. The smooth variation of diffracted intensity from each phase through the transition corresponds to a redistribution of populations between the two types of domains.

Improvement of the double crystal diffractometer at the Geesthacht Neutron Facility (GeNF) by means of perfect channel-cut silicon crystals

High resolution small-angle neutron scattering (HR-SANS) investigations have been performed by means of the double crystal diffractometer (DCD) at the Geesthacht Neutron Facility (GeNF). The two single perfect silicon crystals of the instrument have recently been replaced by channel-cut ones in the non-dispersive (1, −1) setting to reduce the intensity of the rocking curve in its wings by three-fold reflections. Thereby a very strong decrease of this intensity has been achieved, whereby its former dependence on the scattering vector q of q −2 has been changed to q −6. This improvement of the rocking curve leading to a reduction of the inherent background is presented, and a new perspective for future HR-SANS investigations is pointed out.