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
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
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
Neutron Imaging Fuel Cells
10
Neutron Detectors I 6Li-doped ZnS

• Old technology Rutherfords 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
• 4p emission (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
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
Uncoated Triangular
Uncoated Rectangular
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
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 Directors office of NIST, the NIST Center
  for Neutron Research, and the Department of
  Energy through interagency agreement no.
  DE-AI01-01EE50660.