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 (10-7-10-15s)- 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)

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.

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.

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

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

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

PGAA

SANS

SANS

QENS

QENS

QENSSANS

NR

NVS

NPD

NI

Very unusual softening of OH-tangential modes in RbHSO4 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 SrZr1-xScxHyO3.

E (meV)

M=Rb, Cs X=S, Se, etc. Hydrogen bonds

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.

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 H2O

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 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