Time of flight

The successful start-up of polymer electrolyte fuel cell stacks (PEFCs) under sub-zero conditions (cold-start) with a minimal input of auxiliary power is an important requirement for the broad market introduction of fuel cell cars. Typically, cold start failures occur when the water produced by the electrochemical reaction freezes and blocks the access of oxygen to the catalyst. However, water produced by the reaction in sub-zero conditions can remain in liquid (super-cooled) state [1]. Methods that allow visualizing the location of freezing events during cold-starts help to understand which parameters influence the phase transitions.


To this purpose our group applies time-of-flight neutron imaging (TOF-NI). We exploit the fact that the attenuation differs for ice and super-cooled water at low neutron energies (< 5.11 meV) while it is identical for high energies [1]. TOF-NI allows for discriminating the neutron wavelengths according to their travel time from the disk to the detector, as the speed of neutrons is energy and thus wavelength dependent (Figure 1). Our proposed concept uses broad neutron pulses, which strongly increases the flux in comparison to conventional TOF imaging.


When water freezes in the pores of the GDL its volume expands in all directions and the density decreases. To account for the density change, we normalize the image recorded based on low energy neutron attenuation with the attenuation image for high energy neutrons and obtain a relative attenuation image, which allows for determining the state of aggregation. With this method, we can clearly distinguish between super-cooled water and ice (Figure 2) and we could significantly improve the contrast compared to other energy selective methods [2] (reaching approximately 6%) while keeping a high neutron flux.


In summary, TOF-NI allows for the distinction between super-cooled water and ice during fuel cell cold-starts with reasonable high…
•    … special resolution (xx µm)
•    … time resolution (few minutes)
•    … contrast-to-noise ratio (> xx with contrast > 6 %)

 

Neutron images of six GDLs. Partially filled with water/ice. (a) ice (b) partially ice, partially liquid water, (c) liquid (super-cooled) water. Left: High energy attenuation. Right: Ratio between low and high energy attenuation.