Limiting Current Measurements of Oxygen Transport Resistance in Platinum Group Metal-Free Fuel Cell Cathodes

Abstract

Polymer electrolyte fuel cells (PEFCs) currently have a significant commercialization cost due to the price of the platinum or platinum-alloy catalysts they use on the cathode. Platinum group-metal (PGM) free catalysts have the potential to reduce this cost, but have lower volumetric activity, requiring the use of thicker catalyst layers. The increased catalyst layer thickness results in an increase in through-plane oxygen-transport resistance. In addition to that resistance, there are additional non-resistance due to diffusion through water filled pores and across ionomer films. Here, we elucidated the relative magnitude of the various oxygen-transport resistances from limiting current density measurements. To extract the various resistances, we measured the limiting current density for three PGM-free fuel cells with varied catalyst loading, testing each with varied total pressure and carrier gas (Helox, dry air, and 5% O2). Measuring limiting current density at various pressure separates pressure-dependent resistances from pressure-independent resistances. Pressure-dependent resistances include intermolecular gas diffusion while pressure-independent resistances include both Knudsen diffusion and diffusion through thin ionomer films that cover the catalyst. For increased loading and reduced local fluxes at the active site, local microstructure and ionomer thin-film diffusion resistances are less significant. Also, because of the lower volumetric activity and local oxygen flux, those resistances are generally lower within PGM-free cathodes. Varying the carrier gas influences the bulk gas diffusion. From this, and comparison to literature, the component of oxygen-transport-resistance due to the catalyst layer was determined apart from the channel and gas diffusion media of the cathode. Such information can be used to advance the morphology and materials of PGM-free cathodes for enhanced O2 transport and fuel cell power density. DOE ACKNOWLEDGEMENT This material is based upon work supported by the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE) under the Fuel Cell Technologies Office (FCTO) under Award Number DE-EE0008076. The authors gratefully acknowledge research support from the the Electrocatalysis Consortium (ElectroCat), established as part of the Energy Materials Network under theU.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, Fuel Cell Technologies Office, under Contract Number DE-EE0008076.