Investigation of PEFC Freeze Start by X-ray Tomographic Microscopy

Starting from subfreezing temperatures presents a challenge for PEFC since ice may form in the gas flow channel, gas diffusion layer and catalyst layer and restrict the gas transport. Under non-isothermal conditions, a start-up is successful if the cell temperature rises above 0°C before freezing occurs. Under isothermal conditions below 0°C cells operate for a certain time followed by a sudden drop in power. The distribution of liquid/solid water during an isothermal start at subfreezing temperatures is quantitatively mapped by means of X-ray tomographic microscopy to improve understanding the circumstances at the phase transition and its effects on gas diffusion. Evidence was found that the produced water is in supercooled state at -10 °C initially and the drop of power is associated with freezing of the water in the cell at a GDL saturation of 20 to 30%.