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)

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 (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

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, 30 fps @ 0.254 mm pitch or 7.5 fps @ 0.127 mm pitch Second Varian detector will upgrade to 30 fps @ 0.127 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

Neutron sensitive screen Point Source Fuel cell

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 ~ 10-24 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

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 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

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 CCD Converts neutrons to light 6LiF/ZnS:Cu,Al,Au Neutron absorption cross section for 6Li is huge (940 barns) 6Li + n0  4He + 3H + 4.8 MeV Light is emitted in the green part of the spectrum Neutrons in Green light out Neutron to light conversion efficiency is 20% Scintillator

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

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 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

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

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

Amount of Water Possible Volume of one channel = 0.176 cm3 Volume of one port = 0.050 cm3 Volume of one flow field = 0.980 cm3 Volume of anode DM + cathode DM (70% porosity) + electrode (50% porosity) + membrane (20% uptake) = 1.160 cm3 Max water volume possible = 3.12 cm3

Part I: Diffusion media study

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) (44 sccm @ 458 psi) (% void) Toray 090, grd No 7 190 - 56 4.2 70 Toray 060 57 2.8 SGL 20BC Yes 5 260 63 34 70 - 80 SGL 21BC 259 19 27

Comparison of Permeability Current Density A/cm2

Excluding the Water in Channels

Excluding the Water in Channels Permeability Current Density A/cm2

Part II: Channel Geometries

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

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

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

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 Rectangular Uncoated Triangular

Total Water Mass Tends

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?