Fighting the Noise: Towards the Limits of Subsecond X-ray Tomographic Microscopy of PEFC

Thursday, 5 October 2017: 15:00
National Harbor 3 (Gaylord National Resort and Convention Center)
H. Xu (Electrochemistry Laboratory, Paul Scherrer Institut), M. Bührer, F. Marone (Photon Science Department, Paul Scherrer Institut), T. J. Schmidt (Laboratory of Physical Chemistry, ETH Zürich, Paul Scherrer Institut), F. N. Büchi (Paul Scherrer Institut), and J. Eller (Electrochemistry Laboratory, Paul Scherrer Institut)
Management and removal of liquid water is essential to maintain and improve the overall performance of polymer electrolyte fuel cells (PEFC). X-ray tomographic microscopy (XTM) of PEFCs has been proven as a valuable tool in order to understand accumulation of liquid water in the gas diffusion layer (GDL), both with in-situ [1-3] and operando setups [4-8]. Progress in operando XTM of PEFCs has paved the way for 4D imaging studies of the water distribution in the GDL [9]. In order to capture the water dynamics at high current density operation a further decrease of the scan time towards 0.1 s is aspired and the consequences of the reduced signal to noise ratio in the XTM data on water detectability need to be quantified.

In this work different imaging parameters and their consequences on the contrast-to-noise ratio (CNR) of water versus void have been studied with an ex-situ XTM experiment at the TOMCAT beamline of the Swiss Light Source (SLS) at Paul Scherrer Institut (PSI). The cathode channels of a double channel XTM cell [8] have been filled with liquid water, while keeping the anode channels empty, as shown in Figure 1. The CNR(H2O/Void) in the gas channels serves as an indicator for contrast between liquid water and void in the GDL to derive optimized imaging parameters for operando conditions. Over a range of monochromatic beam energies (13.5 – 21 keV) and scan times (0.2 – 10 s) a beam energy of 13.5 keV was found to be mostly appropriate as it provides the highest CNR value. The presentation will focus on the consequences of sub-second XTM acquisition time on the image processing pipeline for the detectability of small water droplets and their connecting paths.


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Figure 1: Quantitative comparison of the contrast-to-noise ratio (CNR) between water and void for different beam energies and scan times (middle); qualitative through-plane slices are given for 0.5 s (left) and 10 s scan time (right).