Neutron Metrology for Fuel Cells

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Hydrogen-Storage Systems • Metal Hydrides • Alkali-Metal Hydrides • Alkali Borohydrides • Nanoporous Materials 500 seconds Real time imaging of water dynamics in a fuel cell • location of H, OH, H2O • in materials • hydrogen vibrations • H bonding states • diffusion of H, H2O • in materials • nanostructure • e.g., H clustering • quantitative H analysis • in materials • H/H2O imaging • in storage vessels/fuel-cells • H in thin films • e.g. H density profile, • membrane structures • diffractionsensitivity> 2 % H (D) • vibrational spectroscopy • sensitivity: > 0.1% H (D) • quasielastic scattering • sensitivity: > 0.1% H (D), 10-8-10-12 s • small-angle scattering • sensitivity: > .01%, 10-10,000 Å • prompt- activation analysis • sensitivity: ~ 3g H • neutron imaging • sensitivity: ~100 m • reflectometry • sensitivity: > 2 %, ~ 5–1000 Å Average water distribution 1 mm water 2000 seconds 0 mm water Quantification of water content From the images the water content can be determined at the 1 mg level. Large areas can be summed to quantify the water mass during any frame. How it works Comparison of the relative size of the x-ray and thermal neutron scattering cross section s for various elements. x-ray cross section Sample t H D C O Al Si Fe neutron cross section E (meV) SrZr0.95Y0.05H0.02O2.985 1 Water Fraction 0.5 0 0 5 10 15 20 z (nm) Neutron Metrology for Fuel Cells David Jacobson, National Institute of Standards & Technology (NIST) Materials of Interest for Neutron Measurements and Theory Neutron Methods: Special Characteristics NIST Center for Neutron Research (NCNR) • Fuel-Cell Materials • High-Temperature Protonic Conductors • Inorganic Superprotonic Conductors • Polymeric Membranes • Very large H cross section: • - “see” H better than other atoms - H/D contrast, high sensitivity • Covers unique range: • - time (10-7-10-15s) • - distance (0.5-10,000 Å) • State-of-the art instrumentation available at NIST • Cover many phenomena at the atomic • and nanoscale • Especially powerful for H in materials Website: www.ncnr.nist.gov NR SANS QENS SANS NI NPD QENS Neutron Imaging Facility(NIF) NVS QENS PGAA SANS for more information, contact: David Jacobson (david.jacobson@nist.gov) Neutron Vibrational Spectroscopy (NVS) Quasielastic Neutron Scattering (QENS) Phenomena Probed in Hydrogenous Materials QENS simultaneously provides atomic-scale temporal and spatial information on the localized and diffusive motions of hydrogen in a host lattice. Diffusion mechanisms and pathways are keys to understanding performance of fuel-cell membranes and hydrogen-storage materials. Combining NVS with a first principles computational approach, we can pinpoint the hydrogen location in protonic conductors. M=Rb, CsX=S, Se, etc.Hydrogen bonds Measured and calculated hydrogen vibrational density of states for the rare-earth doped proton-conducting oxide SrZr1-xScxHyO3. Low-resolution QENS measurements of the inorganic solid acid CsH(SO4)0.76(SeO4)0.24 at 475 K suggest localized proton dynamics. A model described by two inequivalent, two-site reorientational jumps fits the data well. Very unusual softening of OH-tangential modes in RbHSO4suggests the presence of large-amplitude proton motions at temperatures >150 K. • N – numerical density of sample atoms per cm3 • I0 - incident neutrons per second per cm2 •  - neutron cross section in ~ 10-24 cm2 • t - sample thickness Small-Angle Neutron Scattering (SANS) Prompt- Activation Analysis (PGAA) Neutron Powder Diffraction (NPD) Neutron Time and Space Domain NPD is invaluable for determining the positions of light elements such as hydrogen in a crystal lattice. For example, it is essential for an accurate determination of the structures of the Brownmillerite-type oxides Ba2In2O5.xH2O. PGAA is a nondestructive technique for in situ quantitative analysis of hydrogen and many other elements based on the measured intensity of element-specific prompt gamma rays emitted upon nuclear capture of a neutron. In the present example, the small hydrogen concentration is accurately measured in a solid-oxide protonic conductor material. Using SANS measurements as a function of relative humidity level, it is possible to measure the water domain geometry and size within a fuel-cell membrane, a process directly analogous to the conditions present during the startup of a fuel-cell power source. Neutron methods at the NIST Center for Neutron Research (NCNR) encompass an enormous range of time and length scales. dope with H2O Membrane structure vs.water loading in a PEM. Neutron Reflectometry (NR) The reflection of neutrons from films or layered media deposited on flat surfaces directly probes the composition and distribution of the constituent materials, including water and/or hydrogen, on a sub-nanometer length scale both perpendicular and parallel to the film surface. In-situ NPD indicates at least two new phases with hydration. NVS measurements and first-principles calculations indicate that hydrogen ions are localized at the corners of distorted octahedra. Water profile in a biomembrane.