(513e) The Influence of Carrier Polymer on the Microstructure and Rheology of Catalyst Inks for Electrospun Proton Exchange Membrane Fuel Cell Electrodes

Authors: 
Khandavalli, S., National Renewable Energy Laboratory
Kabir, S., National Renewable Energy Laboratory
Sur, S., University of Massachusetts Amherst
Rothstein, J. P., University of Massachusetts Amherst
Neyerlin, K. C., National Renewable Energy Laboratory
Ulsh, M., National Renewable Energy Laboratory
Mauger, S. A., National Renewable Energy Laboratory
Fabrication of electrospun nanofiber electrodes for proton exchange membrane fuel cells has attracted interest recently. A nanofiber architecture of the electrode was found to result in improved electrode performance due to improvements in catalyst utilization, proton conductivity, gas transport and durability, compared to the conventional spray-coated electrodes. A catalyst ink formulation commonly consists of catalyst particles and ionomer dispersed in an alcohol-water mixture. The interactions between the catalyst ink components: catalyst-ionomer-solvent, and the ink microstructure plays a critical role in the evolution of the microstructure of the electrode, which has significant impact on performance. In the case of catalyst ink formulations for the electrospinning process, a high molecular weight (M.W.) carrier polymer is added to enable the creation of stable fibers. The concentration and M.W. of the carrier polymer in the ink formulation are empirically optimized for the electrospinning process. However, how the carrier polymer interacts with the ink components and influences ink microstructure lacks clear understanding. Furthermore, the filament stability and dynamics during the electrospinning process, which dictate the fiber characteristics, are strongly influenced by rheological properties, particularly the extensional rheology, of the inks. However, there have been no studies of how a carrier polymer interacts with the catalyst ink components, or the resulting impact on the rheological behavior of the ink formulation. In this talk, an investigation of carrier polymer interactions with catalyst ink components, and the influence on rheological properties of the inks, both in shear and extension, will be presented. The catalyst ink formulation in the study consists of platinum-decorated carbon particles and ionomer dispersed in a water-alcohol mixture. The carrier polymer studied was polyacrylic acid (PAA). Preliminary findings based on shear rheology suggest a strong influence of PAA concentration on the ink microstructure. PAA was found to induce flocculation of the catalyst particles. An optimal PAA concentration was found to exist to minimize the extent of flocculation of catalyst particles in the ink. The comparison of shear rheological behavior with extensional rheology as a function of PAA concentration and the implications on the processing behavior in electrospinning will be presented.