Charles W.Tobias Young Investigator Award: Understanding Transport Phenomena in Polymer-Electrolyte Fuel Cells

One of the keys to optimizing polymer-electrolyte-fuel-cell performance is the understanding of transport phenomena. This is especially critical when liquid water exists in the cell as it can result in flooding and blockage of the electrocatalytic sites. In this award talk, transport phenomena in general will be discussed with a focus on efforts conducted at LBNL in terms of both modeling and advanced diagnostics. Thus, this talk will discuss the current macroscopic modeling approaches being undergone in terms of multiphase flow and fuel-cell behavior at low and subzero temperatures. In addition, diagnostic work on effective properties will be discussed. Topics to be explored in depth include transport in GDLs and catalyst-layer ionomer.

For GDLs, there has been a study on the effective properties including diffusivity, thermal conductivity, and adhesion force for GDLs that demonstrate different performance at low temperatures. Both modeling and experiments show that the key is water removal out of the anode to decrease the cathode flooding. Such removal is enhanced by decreasing adhesion force at the GDL/channel surface owing to structural hetergeities in fiber density.

For catalyst-layer ionomer, it will be shown how this could be an important issue for the observed increased mass-transport limitations for low Pt loadings. In this structure, confinement and substrate interactions dominate and result in increased transport resistance for the ionomer thin-film surrounding the catalyst sites. Ex-situ thin-film studies show how the morphology changes to more of a membrane interfacial-type behavior as the thickness decreases, thereby decreasing the water content in agreement with water-content measurements in catalyst layers. Mitigation of this issue is paramount to the future commercial viability of polymer-electrolyte fuel cells.   

Acknowledgements

This work was funded by the Assistant Secretary for Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy under contract number DE-AC02-05CH11231.