Abstract
Product water accumulations in polymer electrolyte fuel cells can cause performance losses and reactant starvation leading to cell degradation. Liquid water removal in the form of droplets, fed by percolation networks in the gas diffusion layer (GDL), is one of the main transport mechanisms by which the water is evacuated from the GDL. In this study, the effect of droplet detachment in the gas channel on the water cluster inside the GDL has been investigated using X-ray tomographic microscopy and X-ray radiography. The droplet growth is captured in varying stages over a sequence of consecutive droplet releases, during which an inflation and deflation of the gas-liquid interface menisci of the percolating water structure in the GDL has been observed and correlated to changes in pressure fluctuations in the water phase via gas-liquid curvature analysis.
Highlights
Studying the water percolation networks is not trivial, as many methods of investigation are inapplicable in the confines of a fuel cell
While many projections have to be recorded during X-ray tomographic microscopy (XTM), inherently lowering the temporal resolution compared to radiography, developments in fast XTM of polymer electrolyte fuel cell (PEFC) with scan times of about 1 s24,25 make it possible for dynamic effects to be investigated in 3D with tomography
The 3D information of the water cluster that can be deduced from XTM in the range of seconds linked to information about the dynamics of droplet detachment in the gas channel recorded at a higher temporal resolution via X-ray radiography, providing an unprecedented level of insight
Summary
Studying the water percolation networks is not trivial, as many methods of investigation are inapplicable in the confines of a fuel cell. An ex situ liquid injection cell was used to investigate the behavior of the percolating water in the GDL, droplet evolution and detachment in the gas channel and their interaction (Fig. 1a) It consisted of a GDL sandwiched between a flow field plate, emulating a channel section of a fuel cell cathode bipolar plate, and a water injector. It was determined to have an average porosity of 0.75 (based on XTM segmenation) which can not consider binder porosity compared to 0.74.26 Advancing and receding contact angle values of 159° and 119° respectively, were measured using the sessile droplet method on a GDL sample of the same type It was positioned between the injector and flow channel and compressed from its original thickness of 190 μm to 180 μm in the gas channel and 150 μm under the ribs.
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