(Invited) Understanding Oxygen Transport in Polymer Electrolyte Membrane Water Electrolyzers Using X-Ray Computed Tomography

Abstract

This study aims to understand the fundamental processes that affect catalyst utilization and gas transport through the porous media in polymer electrolyte water electrolyzers (PEWEs) to optimize porous transport layers (PTLs) and flow-fields. We designed a novel operando x-ray computed tomography (CT) cell that can resolve oxygen transport along a 10 cm x 1 cm membrane electrode assembly (MEA) active area. The flow field with two straight channels was designed for this operando cell. X-ray CT with the field of view (FOV) of about 3 mm x 2 mm and resolution of 0.7 um allowed visualizing MEA and flow channels of operando PEWEs. The cell features three observation windows that are transparent to x-rays. These are located near the inlet, in the middle of the flow-channels and at the outlet. In addition, thermocouples are embedded in various locations in the cell to measure temperature profiles of the cell. Oxygen content in the flow-channels was observed for various current densities using 2D radiography technique, with 5 ms time resolution step size. The cell morphology and steady-state oxygen distribution in the PTL were observed using stained water (stained with NaI). The results show significant oxygen enrichment in the channels near the outlet, as well as higher oxygen presence in the PTLs near the outlet. This new cell design can enable simultaneous heat and mass transport measurements along the flow channels. Inlet-outlet variations in oxygen content at scale of 10 cm is of relevant to single cell testing in the laboratory.