1. Neutron Metrology for Fuel Cells David Jacobson, 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) 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 Ba 2 In 2 O 5.xH 2 O. Small-Angle Neutron Scattering (SANS) 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. 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. for more information, contact: David Jacobson (david.jacobson@nist.gov) Website: www.ncnr.nist.gov Neutron Time and Space Domain Neutron Vibrational Spectroscopy (NVS) Prompt-  Activation Analysis (PGAA) Combining NVS with a first principles computational approach, we can pinpoint the hydrogen location in protonic conductors. Neutron methods at the NIST Center for Neutron Research (NCNR) encompass an enormous range of time and length scales. 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. SrZr 0.95 Y 0.05 H 0.02 O 2.985 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 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 PGAA SANS QENS SANS NR NVS NPD NI Very unusual softening of OH-tangential modes in RbHSO 4 suggests the presence of large-amplitude proton motions at temperatures >150 K. Measured and calculated hydrogen vibrational density of states for the rare-earth doped proton-conducting oxide SrZr 1-x Sc x H y O 3. E (meV) M=Rb, Cs X=S, Se, etc. Hydrogen bonds Low-resolution QENS measurements of the inorganic solid acid CsH(SO 4 ) 0.76 (SeO 4 ) 0.24 at 475 K suggest localized proton dynamics. A model described by two inequivalent, two- site reorientational jumps fits the data well. 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. dope with H 2 O Water profile in a biomembrane. 0 5 10 15 20 z (nm) 0 0.5 1 Water Fraction 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. Materials of Interest for Neutron Measurements and Theory Fuel-Cell Materials High-Temperature Protonic Conductors Inorganic Superprotonic Conductors Polymeric Membranes Neutron Imaging Facility(NIF) 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