Understanding the Effect of Feed Gas Humidity on the Freeze Start Behavior of Polymer Electrolyte Fuel Cells

Abstract

Understanding the water dynamics during the sub-zero operation of polymer electrolyte fuel cells can help to optimize the materials and operating strategies to achieve successful freeze starts. This study employs sub-second X-ray tomographic microscopy to study the effect of downstream flow conditions on the water distributions during dynamic freeze starts from −30 °C by varying the feed gas humidity. An increase in the feed gas humidity resulted in a decrease in electrochemical performance. To probe the catalyst layer (CL)-micro-porous layer (MPL) interface, a MPL with grooves across it’s entire thickness was used. Imaging results showed that during the initial phase of performance drop due to ice formation, all the water produced was limited to the CL and membrane. The total water observed during the sub-zero operation in the MPL grooves was 1–2 orders of magnitude lower than the theoretical water produced and increased with an increase in the gas RH from 0% to 50% but reduced with further increase to 100% RH due to a lower cell performance. The overall saturation in the GDL was zero during the sub-zero operation and increased to less than 1% above 0 °C indicating minimal effect of the GDL mass transport on the freeze start performance.