Synchrotron X-ray Radiography as a Highly Precise and Accurate Method for Measuring the Spatial Distribution of Liquid Water in Operating Polymer Electrolyte Membrane Fuel Cells

For the first time, the precision and the accuracy of liquid water content measurements in operating polymer electrolyte membrane (PEM) fuel cells from synchrotron X-ray radiographic imaging are determined. We define the precision of X-ray radiographic-based water measurements by combining two types of uncertainties. In the uncertainty of type A, the standard deviation of the average through-plane water thickness distribution is determined as a function of time and space. In the uncertainty of type B, the effects of the instrumentation (camera readout noise and X-ray energy level) in the Beer-Lambert law are considered. The volume of liquid water in the gas diffusion layer (GDL) is quantified with the greatest precision (plus or minus 5%) when using a beam energy of 24 keV and a low camera readout noise. The accuracy of our synchrotron X-ray-based liquid water measurements is determined from the direct comparison to neutron radiography measurements reported by Kotaka et al. (Electrochim. Acta. 146, 2014). The GDL through-plane distributions and volumes of liquid water measured by two techniques are in excellent agreement. From this work, synchrotron X-ray radiography is shown to be a powerful technique that is both precise and accurate for measuring the spatial distribution of liquid water in miniature operating PEM fuel cells.