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 (10-7-10-15s)- distance (0.5-10,000 Å)

• State-of-the art instrumentation available

at NIST

• Cover many phenomena at the atomicand 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

-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

0

2

4

6

8

10

ZrMo2H

0.92

T = 325 K

Q = 1.90 A-1

experiment fit quasielastic elastic

Inte

nsity

(a.

u.)

Energy transfer (meV)

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 LaNi5 are compared to those in the ternary alloy LaNi4.75Sn0.25. Porod plots of the excess SANS intensity of LaNi5Dx compared to LaNi4.75Sn0.25Dx 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 NaAlH4 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.

SrZr0.95Y0.05H0.02O2.985

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 scatteringsensitivity: > 0.1% H (D), 10-8-10-12 s

small-angle scatteringsensitivity: > .01%, 10-10,000 Å

prompt- activation analysissensitivity: ~ 3g H

neutron imagingsensitivity: ~100 m, 1 g H

reflectometrysensitivity: > 2 %, ~ 5–1000 Å

• 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 The broad quasielastic component for the cubic Laves-phase ZrMo2H0.92 below reflects fast localized H motion within the hexagons formed by interstitial g (Zr2Mo2) sites.

PGAA

SANS

SANS

QENS

QENS

QENSSANS

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 cm3

I0 - incident neutrons per second per cm2

- neutron cross section in ~ 10-24 cm2

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

H D C O Al Si Fe

neutron cross section

0I tNeII 0

Sample

t

Quantification of water content

Fuel Cell Water Content vs. Time

-20

0

20

40

60

80

100

120

0 200 400 600 800 1000

Time (seconds)

Wate

r C

on

ten

t (m

illig

ram

s)

Total Water Content

Channel Water Content

Diffusion Layer/Membrane WaterContent

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