Operando X-ray Tomographic Microscopy of Polymer Electrolyte Fuel Cell Freeze Start

Understanding the water management in polymer electrolyte fuel cells (PEFCs) during sub-zero operation is crucial for designing effective freeze start strategies. In collaboration with Toyota Motor Europe sub-second X-ray tomographic microscopy was used to study the water distributions in the gas diffusion layer (GDL) of PEFCs during dynamic freeze starts from −30 °C that mimic automotive freeze start conditions at different pre-drying levels and  varying the feed gas humidity.

Caption: Gap between the cathode GDL & MPL (CGDL) and the membrane (white) in a polymer electrolyte fuel cell imaged by X-ray tomographic microscopy during transient freeze start at -3°C visualized in two perpendicular cross-sections. Reproduced from 10.1016/j.jpowsour.2020.229447, Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/).

Proper drying of PEFCs before shutdown and freeze has significant influence on freeze start performance. The operando tomographic imaging at the TOMCAT beamline of the Swiss Light Source (SLS) at PSI revealed that no additional water formed in the GDL between -30°C and 0°C independent of the of the pre-drying level. Mechanical machining of the microporous layer (MPL) enabled to verified the existence of super-cooled liquid water at the interface between the catalyst layer and the MPL/GDL at intermediate humidity conditions, which cannot drain into the GDL due to the high capillary pressure barrier of the hydrophobic MPL with sub-micrometer pores. The formation of temporary gaps between the catalyst layer and the MPL was observed between −20 to 18 ◦C due to the hygro-mechanical stresses caused by water uptake in the cell’s membrane during the sub-zero operation and helps to explain fuel cell degradation during freeze-thaw-cycles (see Figure 1). Overall, the insights will guide material development to reduce water saturation in the CL for improved freeze-start performance and to increase lifetime of PEFCs.