Roles of Photon Scattering in Synchrotron X-ray Radiography of In Operando Visualizations of the Polymer Electrolyte Membrane Fuel Cell
Authors: Ge Nan , Chevalier Stéphane , Hinebaugh James , George Michael Gerald , Lee Jongmin , Banerjee Rupak , Liu Hang , Muirhead Daniel , Shrestha Pranay , Bazylak Aimy 
Published: February 2020

Synchrotron X-ray radiography has been proven to be a powerful tool for visualizing and quantifying the in operando liquid water distributions in polymer electrolyte membrane (PEM) fuel cells. Water thickness, the accumulated water content at an image pixel, was calculated by normalizing the images collected to isolate the presence of liquid water. Images containing liquid water are called wet images, and are evaluated with respect to a reference image in the absence of liquid water, called the dry image. This normalization process is based on the Beer-Lambert law, and the attenuation coefficient of liquid water (at the applied photon energy level) is vital for this calculation. The attenuation coefficient for monoenergetic X-ray photons, provided by the National Institute of Standards and Technology (NIST), describes the probability that a photon will interact with the particles in a material as it travels a unit distance in the direction of the beam. Hence, the liquid water thickness calculation is highly dependent on the correct use of the attenuation coefficient.

The incorrect use of a monoenergetic attenuation coefficient in synchrotron X-ray image analysis may lead to significant inaccuracies attributed to the neglect of scattering effects and higher harmonics contamination, which may be present at a synchrotron beamline. In our previous work (1), a calibration experiment was developed to experimentally obtain the water attenuation coefficient based from six known thicknesses of liquid water. The calibrated attenuation coefficient was 18.9% lower than the value for the monoenergetic photons at 20 keV. This deficit implied that at least one of the two prescribed phenomena (scattering and higher harmonics) contributed to the measured intensity in the obtained images. The purpose of this study is to determine the ratios of produced X-ray intensities that arise due to the scattering effect and higher harmonics, respectively. From this work, a correction method for calculating the water thickness can be proposed based on the scattering ratio. This correction method is vital for increasing the accuracy of water quantification.

The calibration experiments were performed with a range of photon energy levels and filters at the Canadian Light Source.  It was observed that the scattered signal increased as a function of water thickness, and this trend was more significant for lower experimental energy levels. The results suggested that the calibration experiment should be conducted with customized devices that contain liquid water with comparable quantities to that which one would find in the PEM fuel cell, in order to accurately simulate the in operando scattering effect.

The existence of the higher harmonics was verified at the experimental energy level of 20 keV, and the water attenuation coefficient was experimentally measured to be 0.258 cm-1, which is in agreement with the value reported by NIST at 40 keV (0.268 cm-1). From the results obtained at 20 keV with the high-transmission filter, the ratio of higher harmonics was determined to be 0.9%–1.4% of the incident beam intensity. From this work, the authors urge that caution should be exercised when using selecting energy levels that could contain higher harmonic photons.

References

1. N. Ge, S. Chevalier, J. Hinebaugh, R. Yip, J. Lee, P. Antonacci, T. Kotaka, Y. Tabuchi and A. Bazylak, Journal of Synchrotron Radiation.,
23, 2 (2016).