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

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.