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

Tuesday, October 13, 2015: 14:40
211-B (Phoenix Convention Center)
I. Mayrhuber (Electrochemistry Laboratory, Paul Scherrer Institut), F. Marone (Paul Scherrer Institut), M. Stampanoni (Institut of Biomedical Engineering, ETH Zürich, Swiss Light Source, Paul Scherrer Institut), T. J. Schmidt (Laboratory of Physical Chemistry, ETH Zürich, Electrochemistry Laboratory, Paul Scherrer Institute), and F. N. Büchi (Electrochemistry Laboratory, Paul Scherrer Institut)
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.