NIST neutron imaging facility for fuel cell imaging.

David JacobsonDaniel Hussey (NIST)

Muhammad Arif (NIST)

Jon Owejan (GM – FCA)Thomas Trabold (GM – FCA)

Daniel Baker(GM – FCA)Satish Kandlikar (RIT)

Fuel CellsNeutron Imaging

National Institute of Standards and Technology Technology AdministrationU.S. Department of Commerce

Support

• DOE – Energy Efficiency and Renewable Energy– Nancy Garland Program Coordinator

• DOC – NIST– NIST Directors office competence funding

• NIST Intramural Advanced Technology Program– Gerald Caesar

• NIST Physics Laboratory (www.physics.nist.gov)• NIST Center for Neutron Research (www.ncnr.nist.gov)

– Patrick Gallagher (director), Charlie Glinka and many others who provide tremendous technical assistance.

Variable beam diameter Variable resolution

Intense neutron beam Single or multi-stack cell

OLD NIST Neutron Imaging Facility

Beam Stop

Cable Ports

Drum shutter and collimator

6 meter flight

path

LN Cooled Bismuth Filter

2.13 mCable Ports

Steel pellet and wax filled shield walls

1. New facility 14.6 m2 (157 ft2) floor space2. Accessible 2 meters to 6 meters3. Variable L/d ratio

1. At 2 m L/d = 100 → ∞2. At 6 m L/d = 300 → ∞

4. Maximum Intensity without filters1. At 2 m = 1 x 109 n cm-2 sec-1 (L/d =100), 8 cm diameter beam size2. At 6 m = 1 x 108 n cm-2 sec-1 (L/d =300), 25 cm diameter beam size

5. Maximum Intensity with 15 cm LN cooled Bismuth Filter1. At 2 m = 2 x 108 n cm-2 sec-1 (L/d =100), 8 cm diameter beam size2. At 6 m = 2 x 107 n cm-2 sec-1 (L/d =300), 25 cm diameter beam size

6. Support for fuel cell experiments1. Hydrogen flow rates 18.8 lpm2. 50 cm2 fuel cell controller with 5 lpm flow rates.3. Nitrogen, Air, Coolant and Hydrogen Venting

7. Detection capabilities1. Real-Time Varian Paxscan, 30 fps @ 0.254 mm pitch or 7.5 fps @ 0.127 mm pitch2. Second Varian detector will upgrade to 30 fps @ 0.127 mm pitch3. 2048 x 2048 Cooled (50° C) Andor CCD based box with 30 cm maximum field of view.4. 2 more 1024 x 1024 Cooled (30° C) Apogee CCD based

8. Sample Manipulation1. Motor controlled2. 5 axis tomography capability

9. Phase imaging capable10. Open for business January-March 2006

NEW Neutron Imaging Facility (NIF)

1. New facility 14.6 m2 (157 ft2) floor space2. Accessible 2 meters to 6 meters3. Variable L/d ratio

1. At 2 m L/d = 100 → ∞2. At 6 m L/d = 300 → ∞

4. Maximum Intensity without filters1. At 2 m = 1 x 109 n cm-2 sec-1 (L/d =100), 8 cm diameter beam size2. At 6 m = 1 x 108 n cm-2 sec-1 (L/d =300), 25 cm diameter beam size

5. Maximum Intensity with 15 cm LN cooled Bismuth Filter1. At 2 m = 2 x 108 n cm-2 sec-1 (L/d =100), 8 cm diameter beam size2. At 6 m = 2 x 107 n cm-2 sec-1 (L/d =300), 25 cm diameter beam size

6. Support for fuel cell experiments1. Hydrogen flow rates 18.8 lpm2. 50 cm2 fuel cell controller with 5 lpm flow rates.3. Nitrogen, Air, Coolant and Hydrogen Venting

7. Detection capabilities1. Real-Time Varian Paxscan, 30 fps @ 0.254 mm pitch or 7.5 fps @ 0.127 mm pitch2. Second Varian detector will upgrade to 30 fps @ 0.127 mm pitch3. 2048 x 2048 Cooled (50° C) Andor CCD based box with 30 cm maximum field of view.4. 2 more 1024 x 1024 Cooled (30° C) Apogee CCD based

8. Sample Manipulation1. Motor controlled2. 5 axis tomography capability

9. Phase imaging capable10. Open for business January-March 2006

NEW Neutron Imaging Facility (NIF)

Hydrogen Safety

• Computational Fluid Dynamics modeling

– Free software

– NIST Fire Dynamics Simulator (FDS)

– http://www.fire.nist.gov/fds/

• Release point in reactor confinement building

• Extremely high buoyancy turbulently mixes hydrogen resulting in low concentrations throughout room

Diffusion coefficient at STP in air = 0.61 cm2 s-1

Diffusion velocity at STP in air 2 cm/s

Buoyant velocity at STP in air = (1.9 m/s to 9 m/s)

Explosive equivalent at 28% H2 in air 1 g H2

= 24 g TNT

Lower explosive limit by volume fraction 4%

Upper explosive limit by volume fraction 77%

Hydrogen Plume 22 lpm

Modeling Building ReleaseM

ass (kg/kg) x 10-4

Modeling the Release Point

Below 1 meter a maximum of 68 mg of hydrogen is expected to be within the range of 77% to 4% and so an unlikely detonation of such a mixture is expected to have an explosive yield similar to a few firecrackers.

Lower flammability limit

Upper flammability limit

Neutron scintillator• Converts neutrons to light 6LiF/ZnS:Cu,Al,Au

6Li + n0 4He + 3H + 4.8 MeV

• Light is emitted in the green part of the spectrum

• Neutron absorption cross section for 6Li is huge (940 barns)

CCD

Scintillator

Neutrons inGreen light out

• Neutron to light conversion efficiency is 20%

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

Neutron beam

scintillator

aSi sensor

Side view

Readout electronics

Scintillator aSi sensor

Front view

Helium through water at 30 fps

Hydrogen Fuel Cells

Water Sensitivity

=

Wet cuvet Dry cuvet water only

=-ln

• Steps machined with 50 micron.• CCD camera exposure of 1 s yields a

sensitivity of 0.005 g cm-2 s-1

• After 100 s a factor of 10 improvement gives 0.0005 g cm-2 s-1

• New amorphous silicon detector should have a least a factor of 7 improvement in temporal sensitivity

1 s exposure time

50 micron water thickness

Sensitivity required for fuel cells (assumes maximum water content)

• Flow fields 0.020 g cm-2

• Gas diffusion media 0.012 g cm-2

• Electrode 0.0005 g cm-2

• Membrane 0.0005 g cm-2

Single Cell Assembly

Flow Fields

Current Collectors

Compression Plates

Locating Pins

Gaskets - GDM - MEA

PEM Fuel Cell Operation

200 m 200 m25 m

15 m 15 m

Flow rates of reactants

• Definitions

– J : Current density Amp cm-2

– A : Active area of cell

– n : (mols electrons)/(mol reactant)

RatioStoich1

RateFlowVol.reactant

ideal

tot

p

pA

nF

J

– F : Farday constant 96484 Coulomb mol-1

ideal : ideal molar gas density (1/22400) mol cm-3

– Stoich. Ratio

Fuel Cell Performance

1.2H2 + ½ O2 H2O

Neutron sensitive screen

Point Source

Fuel cell

Orientation of Cell in all Images

Cathode

Anode

Inlet

Inlet

channel width = 1.4 mm; channel depth = 0.5 mm; land width = 1.5 mm

Orientation of Cell in all Images

Cathode

Anode

Inlet

Inlet

channel width = 1.4 mm; channel depth = 0.5 mm; land width = 1.5 mm

Amount of Water Possible

Volume of one channel = 0.176 cm3

Volume of one flow field = 0.980 cm3Volume of anode DM + cathode DM (70% porosity) + electrode (50% porosity) + membrane (20% uptake) = 1.160 cm3

Max water volume possible = 3.12 cm3

Volume of one port = 0.050 cm3

Channel Geometries

Channel Geometries explored

Triangular X-sect

Rectangular X-sect

94°

1.37 mm 1.45 mm

0.76

mm

1.37 mm 1.45 mm

0.38

mm

X-sect Area = 0.52 mm2

Triangular X-sect

Rectangular X-sect

94°

1.37 mm 1.45 mm

0.76

mm

1.37 mm 1.45 mm

0.38

mm

X-sect Area = 0.52 mm2Rectangular X-sect

94°

1.37 mm 1.45 mm

0.76

mm

1.37 mm 1.45 mm

0.38

mm

X-sect Area = 0.52 mm2

• 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

Flow Field Properties

Rectangular X-sect

Triangular X-sect

Gold Coated w/PTFEContact Angle = 93°

Graphite Uncoated Gold Coated Gold w/PTFE

0.01170 ohm/cm2 0.00044 ohm/cm2 0.00052 ohm/cm2

Gold Uncoated Contact Angle = 50°

Contact Resistance Values

94°

1.37 mm 1.45 mm

0.7

6 m

m

1.37 mm 1.45 mm

0.3

8 m

m

Xsect Area = 0.52 mm2

Cathode Channel Cross Section Geometry and Surface Energy Study

Test Parameters• 100% Humidified• 80°C• 100kPag • Approx. 150% exit RH• 1 Hr 0.6V Start Up• Gore 25m 0.4/0.4 • Toray 060/090 Teflon

ground

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)

Rectangular Comparison 0.5 A/cm2

Uncoated PTFE Coated

Triangular Comparison 0.5 A/cm2

Uncoated PTFE Coated

Geometry Comparison 0.5 A/cm2

Uncoated Triangular

Uncoated Rectangular

Total Water Mass TendsWater Mass Comparison

0

0.10.2

0.3

0.40.5

0.6

0.7

0.80.9

1

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6Current Density (A/cm2)

Wate

r M

ass (g)

Rect No PTFERect PTFETri No PTFETri PTFE

Flow Field Study Performance Data

0.35

0.45

0.55

0.65

0.75

0.85

0.95

0 0.2 0.4 0.6 0.8 1

I (A/cm2)

Vo

lta

ge

(V

)

Toray Rect No PTFE

Toray Rect PTFE

Toray Tri No PTFE

Toray Tri PTFE

Conclusions• Neutron imaging is an important and effective tool to study fuel cells in situ.

• Computational fluid dynamics can be an extremely useful tool in analyzing safety of hydrogen gas released in a reactor hall.

• 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.

Key Observations and Conclusions (cont’)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.