Visualization of Thawing and Desaturation in Frozen Gas Diffusion Layers of Proton Exchange Membrane Fuel Cells via Synchrotron X-ray Computed Tomography

Abstract

Proton exchange membrane fuel cells (PEM fuel cells) are one of the promising clean energy solutions to replace fossil fuel in applications such as automobiles and stationary power systems. Though significant research progress has been made, there are still some key technical challenges to be solved including the water management and cold-start problems, hindering large scale commercialization of this technology. A successful water management requires the amount of water content in a PEM fuel cell system to be kept at an optimal level. A poor water management would lead to membrane dehydration or liquid water flooding, which would cause temporary or permanent losses in performance and durability. The water flooding problem would become more serious in the subzero temperature during the cold-start process, which could lead to irreversible damages on cell components, or even cell failure in some extreme cases. Due to opaque nature of PEM fuel cell components, visualization and understanding of water transport behavior remains a challenge. Therefore, thawing and desaturation processes of gas diffusion layers (GDLs) under cold-start operating conditions were studied in this research via synchrotron X-ray computed tomography (CT) imaging techniques. The high speed and high resolution CT scan made it possible to capture the dynamic water behavior during the thawing and desaturation process for both qualitative and quantitative analyses. The experiments were performed on a half cell (cathode side) with a 40 mm serpentine channel, where Sigracet® 35AA and 35BA graphite GDLs were selected in different trials, with the superficial gas velocity of the purging air set to 2.88 m/s, 4.26 m/s, 5.98 m/s and 9.02 m/s. A similar desaturation pattern was observed in both global and local GDL regions; however, heterogeneity in water transfer was found over the entire GDL domains, both in-plane and through-plane. It was also found that the air purging rate, purging distance, and flow field geometry would affect the desaturation pattern, while the GDL hydrophobicity would mainly affect the initial saturation level. These data provide valuable information for future experimental and modeling studies that involve the thawing process in the GDL, and could be used to optimize the cell design and develop the cold-start protocols.