Performance of Stratified Fuel Cell Catalyst Layer

Abstract

The development of stratified catalyst layers promises an increase in catalyst layer performance compared to conventional flat catalyst layers.1,2 Irregular catalyst layer thickness and porosity can lead to enhanced gas and water transport in and out of the catalyst layer, respectively. The stratified structure is expected to have the same performance in the kinetic region where the performance is controlled by the overall Pt loading. However at high current densities, the thinner sections of the stratified catalyst layers should allow for better mass transport properties. Electrode fabrication is done in house with a custom designed spray coating procedure and catalyst ink recipe. Various approaches are being explored to achieve the desired electrode structure. One approach is to densify the catalyst layer in localized regions, and is based on Ion Power proprietary manufacturing techniques. This can involve the use of glass epoxy masks during the spray coating process to directly create a patterned electrode on the membrane that has thicker and thinner regions. Figure 1 illustrates the effect of stratification on the fuel cell performance of a MEA. The second approach is to pattern thicker and thinner sections of an electrode using masks. During spray coating, one mask exposes the lands and the other mask exposes the channels. This way the two extreme cases can be investigated and the resulting performance compared. The concept of having more catalyst material in the channels may be beneficial due to rapid reaction times during influx of reactant gases through the channels. However, the presence of a thicker catalyst layer under the channel may later become an issue for product water removal and oxygen diffusion. On the other hand, spray coating more of the electrode in the land areas may support better product water removal and oxygen diffusion by leaving a thinner catalyst layer under the channels. The best performance is expected from a combination of ordered thin and thick regions in the catalyst layer. Results from various MEAs tested in a single cell on a fully automated test station to evaluate catalyst performance will be presented. The evaluation of new catalyst ink recipes with reduced ionomer to carbon ratios is included in this study to further increase mass transport by reducing ionomer swelling in the catalyst layer at high relative humidity. Acknowledgments This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.