Neutron Imaging: The key to understanding water management in hydrogen fuel cells

1. Neutron Imaging: The key to understanding water management in hydrogen fuel cells D.S. Hussey, D.L. Jacobson, E. Baltic, M. Arif National Institute of Standards and Technology

2. Muhammad Arif David Jacobson Daniel Hussey Elias Baltic Neutron Imaging Team

3. Overview • Water management in Fuel Cells • Neutron Imaging • Fuel Cell anatomy • Exploring Water Management Strategies: • Channel Geometry • Freeze operation • Inlet gas conditions • Membrane Hydration

4. AIR FUEL CELL Electricity HYDROGEN Water Hydrogen Economy “Hydrogen Economy” is an energy system based upon hydrogen for energy storage, distribution, and utilization. The term was first coined at General Motors in 1970.

5. Managing Water • Water is GOOD: • Product water is only 7% more than currently produced by internal combustion engines • Water is needed by the fuel cell membrane • Water is BAD: • Ice creates frost heaves which are as bad for fuel cells as they roads • Water slugs impede performance • Trapped water promotes degradation

6. Measuring the Water distribution • Managing water requires measuring the water distribution • Fuel cells are metal boxes • Metal is opaque • Metal strongly absorbs x-rays (as Roentgen demonstrated) • Metal shields any MRI signal that could be obtained

7. Neutron Imaging & PEMFCs • Neutrons see material differently than x-rays • The fine details of the water in this Asiatic Lily are clear to neutrons even in a lead cask • Subtle changes in the water distribution inside a fuel cell impact performance and durability • Neutron Imaging measures these small changes at video frame rate

8. Neutron Radiography Sample t T = I/I0= transmission and what we measure directly with neutrons  = cross sectional area due to absorption and scattering that the atom presents to the neutron N = atom density t = Sample or Water thickness Simply measure water from the transmission: N t = -ln(T)

9. Fuel cell Point Source Neutron Imaging Fuel Cells

10. Neutron Detectors I: 6Li-doped ZnS • Old technology – Rutherford’s lab used ZnS in the backscattering of 4He particles from gold nuclei that determined the nuclear radius • Nuclear reaction: 6Li + n → 4He + 3H + 4.8 MeV • Detection efficiency improves with thicker scintillator • 4πemission (“blooming”) of light limits spatial resolution to about the scintillator thickness • We use 0.3 mm thick scintillators • Scintillation light is imaged by a digital camera (CCD or amorphous silicon) • Our scintillator system has a spatial resolution of ~0.25 mm 0.3 mm thick Neutrons in Green light out Scintillator

11. Neutron Detectors II: MicroChannel Plates • Similar in operation to a photo-multiplier tube • 10B or natGd in wall glass absorbs neutron • Reaction particles initiate electron avalanche • Charge cloud detected with position sensitive anode • Spatial resolution limited by channel separation and range of charged particle • Current detector has 0.025 mm resolution • Ultimate resolution of 0.01 mm expected by Fall 08

12. The NIST Neutron Imaging Facility

13. How a fuel cell works “Through-plane” direction is from anode to cathode – study membrane and GDL hydration “In-plane” direction is along the length of the gas flow channels

14. Anatomy of a PEM Fuel Cell Flow field - 1mm cross sectional area - and soft goods Assembled 50 cm2 Cell Porous gas diffusion layer GDL - 0.25 mm thick Membrane Electrode Assembly MEA - 0.05 to 0.2 mm thick

15. Optimum Channel Geometry • Water slugs trapped in channels impede reactant flow • Active water purges require additional energy • A passive water removal mechanism (surface coating or geometry) is preferred

16. Gold Coated w/PTFE Contact Angle = 93° Gold Uncoated Contact Angle = 50° Channel Geometries and Surface Treatments • 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 J. P. Owejan, T. A. Trabold, D. L. Jacobson, M. Arif and S.G. Kandlikar, "Effects of Flow Field and Diffusion Layer Properties on Water Accumulation in a PEM Fuel Cell", (submitted JHE).

17. Rectangular Comparison at 0.5 A/cm2 Uncoated PTFE Coated

18. Triangular Comparison 0.5 A/cm2 Uncoated PTFE Coated

19. Geometry Comparison 0.5 A/cm2 UncoatedRectangular Uncoated Triangular

20. First Freeze Data • Neutron imaging of ice formation in a 50 cm2 fuel cell operated at 0.5V at -10 oC. • Average water/ice density over the first 100s of the experiment • Average water/ice density over the last 100 sec (800 – 900 s) of the experiment • Calculated and measured water/ice accumulation in the fuel cell Neutron imaging quantitatively monitors ice formation in single fuel cells operated at sub-freezing temperatures. Rangachary Mukundan, Yu Seung Kim, Tommy Rockward , John R. Davey, Bryan S. Pivovar, Rodney L. Borup - Los Alamos National Laboratory

21. GDL Water content and Anode flow rate • High anode flow rates reduced the channel flooding • Both flow rates produce similar GDL and MEA water profiles • Cell conditions: 60 C, 1 A cm-2

22. Through-plane Water Content for Different RHs Varying Inlet RH

23. Cathode Flow Channel Anode Flow Channel Cathode GDL Membrane Anode GDL Membrane Conductivity vs. Hydration • Membrane must be wet to be a proton conductor • Neutron imaging measures membrane water content • Impendence spectrometry measures conductivity • Use models of hydration vs conductivity to calculate conductivity from neutron derived water content • Initial results show discrepancy possibly due to: • Membrane preparation, compression, gas flow, etc. In collaboration with D.J. Ludlow, M. Silva, M.K. Jensen, G.A. Eisman

24. Conclusions • Proton Exchange Membrane Fuel Cell vehicles are nearing production • Neutron Imaging has played a crucial role in optimizing water management strategies • We continue to improve our spatial resolution to study the fundamental properties of membranes • Thank you for your attention • This work was supported by the U.S. Department of Commerce, the NIST Ionizing Radiation Division, the Director’s office of NIST, the NIST Center for Neutron Research, and the Department of Energy through interagency agreement no. DE-AI01-01EE50660.