Fast X-Ray Tomographic Imaging: Investigation of Freezing Mechanisms in PEFC during Sub-Zero Start-Ups

Starting from subfreezing temperatures is a challenge for polymer electrolyte fuel cells. The product water removal is a problem since ice may form, limiting the gas transport in gas diffusion and catalyst layers.

X-ray tomographic imaging is used to investigate the liquid/solid water phase in the GDL and to deduce information on the mechanism of freezing of the water, which induces a performance drop during freeze starts of PEFC. As freeze starts are transient processes tomographic imaging is tuned for fast acquisition while conserving acceptable image quality. Tomographic scans of 4.9 s are recorded.

As freezing of the super-cooled water is a temperature dependent nucleation process [1,2], the time to freezing of the super-cooled water depends on the cell temperature. The figure shows an isothermal freezing experiment with X-ray imaging and a slice through the tomographic scan of the cathode GDL 30 s before performance drop.

[1]        Y. Ishikawa, et al., J. Power Sources, 179 (2008), pp. 547-552

[2]        T.J. Dursch, et al., Langmuir 28, (2012), pp 1222-1234

Different freezing mechanisms are observed. At low temperatures (i.e. -20 °C) and low current density no water is observed in the GDL. The product water seems to freeze instantly in the catalyst layer (CL) which leads to a slow performance drop with filling of the CL with ice. At the same low temperatures, but higher current densities super-cooled liquid water emerges from the CL and fills part of the macro-porous gas diffusion layer and even penetrates into the channel. In this case the freezing dynamics are different: as long as the water remains liquid, operation continues. Then all water freezes instantly at the same time and operation stops abruptly.