Neutron imaging of water distribution in full size automotive fuel cells

The high penetration of neutrons through large thicknesses of metals (e.g. > 10 cm of aluminum) allows imaging the water distribution in single fuel cells of full size design, even between the thick compression bodies requested to ensure enough flatness over a large area. For the single cell to have thermal boundary conditions similar to those of a cell integrated in a stack, the compression bodies are heated and a specific temperature gradient is applied to mimic the temperature gradient in the cell itself.

The beam diameter and detector size at the NEUTRA beam line of PSI allows imaging objects up to a size of 350 mm (horizontal) x 380 mm (vertical) in a single shot. Typical exposure times of 5 to 10 seconds allow observing the temporal evolution of water in the flow channels. With accumulated exposure time of 2-3 minutes, a low noise image is obtained, allowing identifying the presence of water in the GDLs in regions free of channel water.

Example: Imaging of the AutoStack-Core fuel cell (300 cm2 active area). The impact of cell design and the influence of nitrogen concentration in the anode loop on water distribution were studied in the frame of the AutoStack-Core project. The second evolution of the flow field design (EVO2) was shown to reduce significantly the amount of water accumulating at the end of the anode flow channels. The presence of up to 30% (in volume) of nitrogen in the anode further reduced the water accumulation. Avoiding these water accumulations are of high importance for long term stability, as they can induce hydrogen starvation as well as accelerate the coating degradation of bipolar plates.

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