(220e) Exploring Ionomer-Related Transport Phenomena in Polymer-Electrolyte Fuel Cells

Authors: 
Weber, A., Lawrence Berkeley National Laboratory
Ion-conducting membranes serve critical functions within polymer-electrolyte fuel cells (PEFCs), both as a standalone separator as well as an ionic network within the catalyst layers. They are also critical components in enabling one to realize high power density and thus reduce stack count and cost. For proton-conducting PEFCs, the local ionomer within the cathode catalyst layer dominates the mass transport for such low loadings. For hydroxide-conducting PEFCs, such membranes can allow for less expensive membranes and catalysts, but have possible issues with water management and carbon dioxide interactions. In this talk, we will explore the limiting transport mechanisms and effects related to fuel-cell membranes through mathematical modeling and advanced diagnostics.

Thus, we will explore the gas transport limitations within PEFC catalyst layers, with a focus on the observed local transport resistance as measured using hydrogen and oxygen limiting current measurements. We will examine test cases for diagnosis of polarization curves in terms of different limiting phenomena. Different oxygen concentrations, catalyst-layer properties, and operating humidity and temperature will be introduced.

Water management is a more serious concern in hydroxide PEFCs, because OH- conductivity is more highly dependent on water content and the oxygen-reduction reaction consumes water. Compared to proton PEFCs, the lower performance of hydroxide PEFCs is mostly caused by a extremely nonuniform distribution of water in the ionomer phase between the anode and cathode as well as the increased overpotential for the hydrogen oxidation reaction. In this presentation, we will discuss the performance-limiting mechanisms specific to different operating conditions (e.g. varying inlet relative humidity (RH)) based on a cell-level mathematical model. In addition, the issue of carbonate formation with the membrane will be discussed.

Acknowledgements

We would like to thank helpful discussions and samples from Toyota Motor Company. This work was mainly funded under the Fuel Cell Performance and Durability Consortium (FC PAD) funded by the Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, of the U. S. Department of Energy, Program Development Manager Dimitrios Papageorgopoulos