1. Neutron Imaging at NIST: An in situ method for visualizing and quantifying water dynamics in low temperature PEM fuel cells.
National Institute of Standards and Technology
Technology Administration
U.S. Department of Commerce
Neutron Imaging
Fuel Cells
David Jacobson
Daniel Hussey (NIST)
Muhammad Arif (NIST)
Jon Owejan (RIT)
Satish Kandlikar (RIT)
Thomas Trabold (GM – FCA)

2. Support DOE – Energy Efficiency and Renewable Energy DOC – NIST
Interagency agreement # DE\_AI0101EE50660
Nancy Garland Program Coordinator
DOC – NIST
NIST Directors office competence funding
NIST Intramural Advanced Technology Program
Gerald Caesar
NIST Physics Laboratory (
NIST Center for Neutron Research (
Patrick Gallagher (director), and many others who provide tremendous technical assitance.

3. Some Neutron Radiography Facilities
Paul Scherrer Institute - NEUTRA
Pennsylvania State University - Breazeale Nuclear Reactor Facility
Institute Laue Langevin (Grenoble, France)
FRM-II (Munich, Germany)
JRR-3M (JAERI) (Japan)
HANARO (KAERI) (Taejon, Korea)
Many other smaller reactors

4. Neutron Imaging Facility (NIF)
LN Cooled Bismuth Filter
Neutron Imaging Facility (NIF)
6 meter flight path
Beam Stop
Drum shutter and collimator
New facility 14.6 m2 (157 ft2) floor space
Accessible 2 meters to 6 meters
Variable L/d ratio
At 2 m L/d = 100 → ∞
At 6 m L/d = 300 → ∞
Maximum Intensity without filters
At 2 m = 1 x 109 n cm-2 sec-1 (L/d =100), 8 cm diameter beam size
At 6 m = 1 x 108 n cm-2 sec-1 (L/d =300), 25 cm diameter beam size
Maximum Intensity with 15 cm LN cooled Bismuth Filter
At 2 m = 2 x 108 n cm-2 sec-1 (L/d =100), 8 cm diameter beam size
At 6 m = 2 x 107 n cm-2 sec-1 (L/d =300), 25 cm diameter beam size
Support for fuel cell experiments
Hydrogen flow rates 18.8 lpm
50 cm2 fuel cell controller with 5 lpm flow rates.
Nitrogen, Air, Coolant and Hydrogen Venting
Detection capabilities
Real-Time Varian Paxscan, mm pitch or mm pitch
Second Varian detector will upgrade to mm pitch
2048 x 2048 Cooled (50° C) Andor CCD based box with 30 cm maximum field of view.
2 more 1024 x 1024 Cooled (30° C) Apogee CCD based
Sample Manipulation
Motor controlled
5 axis tomography capability
Open for business January 2006
2.13 m
Cable Ports
Cable Ports
Steel pellet and wax filled shield walls

5. Neutron sensitive screen
Point Source
Fuel cell

6. Why Neutrons
Neutrons are an excellent probe for hydrogen in metal since metals can have a much smaller cross section to thermal neutrons than hydrogen does.
Sample
N – numerical density of sample atoms per cm3
I0 - incident neutrons per second per cm2
 - neutron cross section in ~ cm2
t - sample thickness t
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

7. Water Sensitivity  = -ln 1 s exposure time 50 micron water thickness
Wet cuvet
Dry cuvet
water only
-ln
1 s exposure time
50 micron water thickness
Steps machined with 50 micron.
CCD camera exposure of 1 s yields a sensitivity of g cm-2 s-1
After 100 s a factor of 10 improvement gives g cm-2 s-1
New amorphous silicon detector should have a least a factor of 7 improvement in temporal sensitivity

8. Sensitivity required for fuel cells (assumes maximum water content)
Flow fields g cm-2
Gas diffusion media g cm-2
Electrode g cm-2
Membrane g cm-2

9. Neutron scintillator CCD Converts neutrons to light 6LiF/ZnS:Cu,Al,Au
Neutron absorption cross section for 6Li is huge (940 barns)
6Li + n0  4He + 3H MeV
Light is emitted in the green part of the spectrum
Neutrons in
Green light out
Neutron to light conversion efficiency is 20%
Scintillator

10. Real-Time Detector Technology
Amorphous silicon
Radiation hard
High frame rate (30 fps)
127 micron spatial resolution
Picture is of water with He bubbling through it
No optics – scintillator directly couples to the sensor to optimize light input efficiency
Helium through water at 30 fps
Front view
Scintillator aSi sensor
Readout electronics
Side view
Neutron beam
scintillator
aSi sensor

11. New technology
Currently the spatial resolution is of order 100 microns
Not a fundamental limitation, but is due to light blooming out in the ZnS, which is 0.1 mm – 0.3 mm thick
Currently have tested detectors with 30 micron resolution (potentially 15 microns).
Major innovation in detection technology
Resolution has been measured to be 30 microns
Final testing and development expected to be completed in 2006

12. Borated MCP Neutron Detection Mechanism
10B  7Li + 4He + Q (2.79 MeV)
Typical MCP structure
Neutron conversion
to electron pulse
Neutron
7Li
10B
4He
Secondary
Electrons e-
~5-10 µm channels
~25 mm
Secondary e- emitting channel wall

13. Orientation of Cell in all Images
The flow field geometry was selected to have DP and land width that model full scale hardware
channel width = 1.37mm; channel depth = 0.48 mm; land width = 1.45 mm
Inlet
Anode
Inlet
Cathode

14. Orientation of Cell in all Images
The flow field geometry was selected to have DP and land width that model full scale hardware
channel width = 1.37mm; channel depth = 0.48 mm; land width = 1.45 mm
Inlet
Anode
Inlet
Cathode

15. Amount of Water Possible
Volume of one channel = cm3
Volume of one port = cm3
Volume of one flow field = cm3
Volume of anode DM + cathode DM (70% porosity) + electrode (50% porosity) + membrane (20% uptake) = cm3
Max water volume possible = 3.12 cm3

16. Part I: Diffusion media study

17. Gas Diffusion Media Study
Three DM Used
Toray 060/090 Teflon Ground
SGL 21 BC
SGL 20 BC
Test Parameters
Gore 25mm 0.4/0.4 mg Pt/cm2
Rectangular channels with no PTFE Coating
80°C
2/2 Stoich H2/Air
100 kPag
100% Humidified
Exit RH approx. 150%
1 hr 0.6V Start Up
Permeability
Substrate
Thickness
Densometer
Pereometer
In-Plane (DP) (kPa)
Porosity
GDM
MPL
PTFE (%)
(mm)
(sec/100cc)
(ft3/min/ft2)
( psi)
(% void)
Toray 090, grd No 7
190 - 56
4.2 70
Toray 060 57
2.8
SGL 20BC
Yes 5
260 63 34
SGL 21BC
259 19 27

18. Comparison of Permeability
Current Density A/cm2

19. Excluding the Water in Channels

20. Excluding the Water in Channels
Permeability
Current Density A/cm2

22. Part II: Channel Geometries

23. Channel Geometries explored
Rectangular channels
Water flow is laminar tending to constrict and plug the channels
Water plugs form as large slugs and can be difficult to remove.
Triangular channels
Water stays at the corner interface with the diffusion media leaving the apex of the channel more clear.
Water tends to come out in smaller droplets instead of large slugs, which require a high pressure differential to remove

24. Flow Field Properties Contact Resistance Values Graphite Uncoated Gold
Gold Coated w/PTFE
Contact Angle = 93°
Gold Uncoated
Contact Angle = 50°
1.37 mm
1.45 mm
0.38 mm
Rectangular X-sect
Xsect Area = 0.52 mm2
1.37 mm
1.45 mm
94°
0.76 mm
Triangular X-sect
Contact Resistance Values
Graphite
Uncoated Gold
Coated Gold w/PTFE
ohm/cm2
ohm/cm2
ohm/cm2

25. Cathode Channel Cross Section Geometry and Surface Energy Study
Cathode Flow Field Variation
(Anode constant rect. x-sect no coating)
2 Channel Geometries
Rectangular
Triangular
2 Surface Energies
Gold
Gold coated ionic PTFE
4 Cathode FFs Total
Rect and Tri (gold only)
Rect and Tri (gold coated w/ ionic PTFE)
Test Parameters
100% Humidified
80°C
100kPag
Approx. 150% exit RH
1 Hr 0.6V Start Up
Gore 25mm 0.4/0.4
Toray 060/090 Teflon ground

26. Rectangular Comparison 0.5 A/cm2
Uncoated
PTFE Coated

27. Triangular Comparison 0.5 A/cm2
Uncoated
PTFE Coated

28. Geometry Comparison 0.5 A/cm2
Uncoated Rectangular
Uncoated Triangular

29. Total Water Mass Tends

30. Key Observations and Conclusions
For all cells tested, water accumulation in the channels decreased with load, while accumulation in the diffusion media/MEA increased with load.
There was a significant difference in channel water retention for Toray and SGL materials due to material surface energy.

31. Key Observations and Conclusions (cont’)
Lower cell performance at 1.0 A/cm2 using Toray is associated with only 0.05 g more water accumulation in the channels and non-channel regions.
Channel surface energy has a consistent effect on water slug shape and size. Higher contact angle increases average water mass retained, but distribution of smaller slugs more evenly in the channel area increases performance.
Triangular cross-sectional geometry accumulates water in the corners adjacent to diffusion media. The center of the channel does not become obstructed by stagnant slugs.

32. Thank You Questions?