1. Neutron Probes for the Hydrogen Economy David Jacobson, Terry Udovic, and Jack Rush, Muhammad Arif, National Institute of Standards & Technology (NIST) Phenomena Probed in Hydrogenous Materials Very large H cross section: - “see” H better than other atoms - H/D contrast, high sensitivity Covers unique range: - time (10 -7 -10 -15 s) - 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 Neutron Methods: Special Characteristics Neutron Powder Diffraction (NPD) Quasielastic Neutron Scattering (QENS) Materials of Interest for Neutron Measurements and Theory Fuel-Cell Materials High-Temperature Protonic Conductors Inorganic Superprotonic Conductors Polymeric Membranes 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 alkali alanates. Small-Angle Neutron Scattering (SANS) Using SANS, hydrogen distributions and internal strains that accompany hydriding of LaNi 5 are compared to those in the ternary alloy LaNi 4.75 Sn 0.25. Porod plots of the excess SANS intensity of LaNi 5 D x compared to LaNi 4.75 Sn 0.25 D x for partial D loadings (x=2,4) indicate a more homogeneous distribution of D in the latter alloy, at least on a scale of 4-15 nm. Increased homogeneity may suppress strain gradients that cause hydride decrepitation. 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 hydrogen-storage materials and fuel cells. A.V. Skripov, et al. B. Fultz, et al. For more information, contact: Jack Rush (jack.rush@nist.gov) Terry Udovic (udovic@nist.gov) David Jacobson (jacobson@nist.gov) Muhammad Arif (arif@nist.gov) Website: www.ncnr.nist.gov Neutron Time and Space Domain Neutron Vibrational Spectroscopy (NVS) Prompt-  Activation Analysis (PGAA) Alkali Alanates Hydrogen in Carbon Nanotubes Combining NVS with a first principles computational approach can yield detailed information about H-storage materials and their limits for the hydrogen kinetics and uptakes. (Neutron energy loss) J=0 J=1 J=2 J=4 J=3 J=5 Computation indicates 3 wt% at best. Neutron vibrational spectrum of NaAlH 4 compared with ab initio calculations that include one- and two-phonon processes Neutron methods at the NIST Center for Neutron Research (NCNR) encompass an enormous range of time and length scales. Neutron Imaging Facility(NIF) 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. T. Yildirim, et al. SrZr 0.95 Y 0.05 H 0.02 O 2.985 T. J. Udovic, et al. E. H. Majzoub, et al. E. Majzoub / C. Jensen, et al. NIST Center for Neutron Research (NCNR) diffraction sensitivity > 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, 1  g H reflectometry sensitivity: > 2 %, ~ 5–1000 Å location of H, OH, H 2 O in materials hydrogen vibrations H bonding states diffusion of H, H 2 O in materials nanostructure e.g., H clustering quantitative H analysis in materials H/H 2 O imaging in storage vessels/fuel-cells H in thin films e.g. H density profile, membrane structures The broad quasielastic component for the cubic Laves-phase ZrMo 2 H 0.92 below reflects fast localized H motion within the hexagons formed by interstitial g (Zr 2 Mo 2 ) sites. PGAA SANS QENS SANS NR NVS NPD NI Real time imaging of water dynamics in a fuel cell 500 seconds 2000 seconds Average water distribution 1 mm water 0 mm water N – numerical density of sample atoms per cm 3 I 0 - incident neutrons per second per cm 2  - neutron cross section in ~ 10 -24 cm 2 t - sample thickness How it works Comparison of the relative size of the x-ray and thermal neutron scattering cross section  for various elements. x-ray cross section HDCOAlSiFe neutron cross section Sample t Quantification of water content From the images the water content can be determined at the 1  g level. Large areas can be summed to quantify the water mass during any frame. Hydrogen-Storage Systems Metal Hydrides Alkali-Metal Hydrides Alkali Borohydrides Nanoporous Materials