The microporous layer (MPL), which is part of the gas diffusion layer, is an essential component for high-performance polymer electrolyte fuel cells (PEFCs). The mechanisms that improve cell performance are still under debate, but there has been increasing evidence that performance is promoted through superior water management induced by the MPL, especially at high current density and humid operating conditions. Unfortunately, due to its small thickness (typically 20 – 70 μm) and nanoscale pore size distribution, it has been difficult to characterize this layer under operando conditions that reflect the actual water progression during PEFC operation. Here, we use fast X-ray tomographic microscopy (XTM) to image an operando PEFC, and obtain three-dimensional (3D) quantitative liquid water distribution with high time resolution.

The operando imaging is performed using a synchrotron radiation source (TOMCAT beamline, Swiss Light Source), whose monoenergetic (16 keV, 2% bandwidth) high photon flux enables sub-second fast tomography at the acquisition rate of 0.8 s/scan, allowing time-resolved imaging of PEFC operation. X-ray absorption contrast imaging was used to restore the linear attenuation coefficient of liquid water in all PEFC porous layers. The liquid water volume fraction (LVF) distribution is extracted from the tomograms by subtraction of the operando scans and the dry reference scans.

The results show that that imaging error taking all artifacts and noise into account is generally within ±5% liquid volume fraction (LVF), allowing any water LVF over 5% in the CL and MPL to be identified. At 45°C, with oversaturated H₂/air feed gas (47°C dew point), liquid water is discovered to mainly accumulate at the CL/MPL interface both under the rib and under the channels. Furthermore, little liquid water is observed in the MPL and CL for current densities below 1.5 A cm^-2. Overall, the MPL LVF remains low at all humid conditions tested, suggesting dominant vapor transport through the MPL. The transient liquid distribution characterization in the porous diffusion media and in the CL provides insight into the two-phase flow from CL to channels, and allows direct visualization of the distribution of liquid water a few seconds after it is generated from the catalyst layer.