Neutron Imaging at NIST: An in situ method for visualizing and quantifying water dynamics in low temperature PEM fuel cells.

David JacobsonDaniel Hussey (NIST)

Muhammad Arif (NIST)

Jon Owejan (RIT)Satish Kandlikar (RIT)

Thomas Trabold (GM – FCA)

Fuel CellsNeutron Imaging

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

Support

• DOE – Energy Efficiency and Renewable Energy– 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 (www.physics.nist.gov)• NIST Center for Neutron Research (www.ncnr.nist.gov)

– Patrick Gallagher (director), and many others who provide tremendous technical assitance.

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

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. Open for business January 2006

Neutron Imaging Facility (NIF)

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

Neutron sensitive screen

Point Source

Fuel cell

Neutrons are an excellent probe for hydrogen in metal since metals can have a much smaller cross section to thermal neutrons than hydrogen does.

Comparison of the relative size of the x-ray and thermal neutron scattering cross section for various elements.

x-ray cross sectionH D C O Al Si Fe

neutron cross section

0I tNeII 0

Sample

t

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

Why Neutrons

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

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

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

Borated MCP Neutron Detection Mechanism10B 7Li + 4He + Q (2.79 MeV)

Typical MCP structure

Neutron conversion to electron pulse

Neutron

7Li

10B

4He

SecondaryElectrons

e- e-e-

e-e-

~5-10 µm channels

~25 mm

Secondary e- emitting

channel wall

Orientation of Cell in all Images

Cathode

Anode

Inlet

Inlet

The flow field geometry was selected to have P and land width that model full scale hardware

channel width = 1.37mm; channel depth = 0.48 mm; land width = 1.45 mm

Orientation of Cell in all Images

Cathode

Anode

Inlet

Inlet

The flow field geometry was selected to have P and land width that model full scale hardware

channel width = 1.37mm; channel depth = 0.48 mm; land width = 1.45 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

Part I: Diffusion media study

Gas Diffusion Media StudyThree DM Used• Toray 060/090

Teflon Ground• SGL 21 BC• SGL 20 BC

Test Parameters• Gore 25m 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 (P) (kPa) Porosity

GDM MPL PTFE (%) (m) (sec/100cc) (ft3/min/ft2) (44 sccm @ 458 psi) (% void)

Toray 090, grd No 7 190 - 56 4.2 70

Toray 060 No 7 190 - 57 2.8 70

SGL 20BC Yes 5 260 63 - 34 70 - 80

SGL 21BC Yes 5 259 19 - 27 70 - 80

Comparison of Permeability

Average Water Mass Comparison

0.38

0.4

0.42

0.44

0.460.48

0.5

0.52

0.54

0.56

0.58

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

Wat

er M

ass

(g)

ToraySGL 20BCSGL 21BC

Current Density A/cm2

Per

mea

bil

ity

Excluding the Water in Channels

Excluding the Water in Channels

Current Density A/cm2

Per

mea

bil

ity

Average Water Mass in DM Only Comparison

0.28

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

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

Wat

er M

ass

(g)

ToraySGL 20BCSGL 21BC

Performance Comparison

0.35

0.45

0.55

0.65

0.75

0.85

0.95

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

Vol

tage

(V)

ToraySGL 20BCSGL 21BC

Average Water Mass in DM Only

0.28

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

I (A/cm2)

Wat

er M

ass

(g)

ToraySGL 20BCSGL 21BC

Average Water Mass in Channels Only

0

0.05

0.1

0.15

0.2

0.25

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6I (A/cm 2)

Wat

er M

ass

(g)

Toray

SGL 20 BC

SGL 21 BC

Part II: 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

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.

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.

Thank You

Questions?