Spatially Resolved Analysis of Freezing during Isothermal PEFC Cold Starts with Time-of-Flight Neutron Imaging

Abstract

We demonstrate that the use of high duty cycle time-of-flight (HDC-TOF) neutron transmission imaging allows a spatially resolved analysis of the aggregate state of water during isothermal cold starts of polymer electrolyte fuel cells (PEFCs). Neutron attenuation at long wavelengths depends on the mobility of protons and is therefore lower for ice compared to liquid water. With the experimental setup used in our experiment (chopper disk duty cycle of 30%) attenuation at long wavelengths normalized to attenuation at short wavelengths leads to a contrast between ice and super-cooled water higher than 6% and a measurement time of a few minutes is sufficient to detect reliably phase changes of thick water layers (≈0.5 mm), i.e. when water is present in the flow field channels. Using this method, local freezing events were identified during an isothermal cold start, which was corroborated by locally resolved measurements of the latent heat released during freezing. The impact of systematic biases such as the scattered background was evaluated, showing that a precise correction is of utmost importance if the aggregate state is to be determined in an absolute way. If such a highly precise correction is not possible, the detection of freezing events is still possible by observing relative changes during time series.