Correlating Structure and Chemistry of PEM Fuel Cell Materials with Performance & Durability Using Advanced Microscopy Methods

Abstract: Polymer electrolyte membrane (PEM) fuel cell performance and materials degradation, particularly associated with the cathode catalyst layer (CCL), can be directly attributed to the structure and chemistry of individual material components, as well as their uniformity/homogeneity within a CCL. The individual material constituents used to form the CCL within the membrane electrode assemblies (MEAs), which include the electrocatalyst, catalyst support, and ionomer films, and especially the critical interfaces that are formed between these various constituents, are critically important in controlling fuel cell performance. Understanding the specific microstructural characteristics of the individual materials within the CCL, and how the materials interact to “form” the CCL, is important for identifying materials optimization parameters that can significantly enhance performance and durability. Materials in several states/conditions, e.g., prior to incorporation in the CCL (as-synthesized), after MEA preparation (CCL), and after fuel cell testing, are evaluated and quantified using a combination of advanced electron microscopy methods, which are used to interrogate the materials constituents and interfacial structures and chemistries from the m- to the Å-level. The as-processed (prior to and following incorporation into a CCL) microstructural evidence is directly correlated with observations of materials-specific degradation mechanisms that contribute to fuel cell (CCL) performance loss, and are then used to identify potential processing variables (materials-based mitigation strategies) to improve the microstructure and compositional homogeneity within the electrode structure, and enhance MEA durability and stability during fuel cell operation.

Research efforts at Oak Ridge National Laboratory are focused on the high-resolution microstructural and microchemical characterization of MEAs fabricated using different electocatalysts (Pt-based) and catalyst loadings, carbon-based support materials, and ionomer solutions, as well as the same MEAs subjected to accelerated stress tests (ASTs) designed to degrade specific MEA components. Recently, high-resolution analytical microscopy methods have been used to directly image/map the distribution and chemistry of the ionomer films/layers within CCLs, results of which are being combined with high-resolution imaging and 3-D tomography data to provide unprecedented insight into the structure and interfaces (ionomer/support, ionomer/catalyst, catalyst/support, ionomer/pore) in “real” MEAs. This presentation will focus on understanding materials distributions within the CCL as a function of processing variables, e.g., initial ionomer and/or ink chemistry, electrocatalyst (type, content, and dispersion), and the type of carbon support used.

Bio: Karren More received her Ph.D. in 1992 from the Department of Materials Science and Engineering at North Carolina State University, Raleigh, NC under the direction of Professor Robert F. Davis (now at Carnegie Mellon University). She joined Oak Ridge National Laboratory (ORNL) as a full-time research staff member in 1988 and performed most of her PhD research on understanding structure-property relationships in silicon nitride ceramics with a focus on high-resolution electron microscopy.

Karren has led ORNL’s efforts in the area of characterization of fuel cell (FC) materials since 2005. She is currently the PI on multiple FC projects that are primarily focused on developing new microscopy methods to characterize FC materials (catalysts, supports, ionomer, membranes, bipolar plates) and to understand the critical structural and chemical characteristics that contribute to enhanced FC performance and durability. Two of these projects are national laboratory consortiums (ElectroCat and FC-PAD) that are part of the DOE’s Energy Materials Network (EMN), which aims to dramatically decrease the time-to- market for advanced materials innovations critical to many clean energy technologies. Karren’s work in fuel cell materials characterization has been recognized as a “core” component of fuel cell research by DOE, industry, and other national laboratory partners. She has been awarded the DOE Hydrogen and Fuel Cells R&D Award two times, once in 2006 and again in 2013. She has developed an extensive network of industrial and academic collaborators.