(Invited) Fundamental Studies of PEM Fuel Cell Catalyst Layer Architectures

Abstract

Although fuel cells are being deployed in cars in limited commercialization, they still fall short of the DOE targets for this technology, which are required for widespread consumer acceptance. The FC-PAD (Fuel Cell Performance and Durability) consortium was formed to advance the performance and durability of polymer electrolyte membrane fuel cells (PEMFCs) at a pre-competitive level to further enable their commercialization. The primary component that FC-PAD works to understand and improve is the catalyst layer. The primary catalyst layer architecture in use is a porous dispersed Pt-supported carbon catalyst with recast solid ionomer component forming a continuous (percolating) network through the electrode for proton transport; the carbon facilitating the electrical conduction; the active electrochemical reactions conducted on the metallic Pt sites. These thin-layer-electrode films were pioneered at Los Alamos National Labs in the late 1980’s under the direction of Dr. Shimshon Gottesfeld [1,2] As ubiquitous as the thin layer electrode structure has been used, it is still an active field of research to properly understand the structure and improve the reactant transport, water removal and catalyst utilization. The fuel cell catalyst layer is a complex structure that facilitates the electrochemical conversion of hydrogen and oxygen; it provides pathways for reactant transport, and provides both electrical and proton conducting pathways. This has been traditionally referred to as the 3-phase boundary of an electrolyte, an electrode, and a gaseous fuel. In addition to the gas-reactant transport to the active site, the product water must also be managed both in terms of product removal and optimal hydration of the membrane/ionomer providing the proton conductivity. Recent FC-PAD results have examined the structure of the electrode layer. Because performance is largely influenced by the ionomer/catalyst interface within the catalyst layers of a MEA, the “ideal” interface should contain platinum particles with 100% available proton and electrical pathways to maximize catalyst utilization. In addition, the thickness of ionomer film over the catalyst nanoparticles should be optimal to facilitate gas diffusion and water balance without sacrificing its protonic conductivity. LANL’s recent FC-PAD examinations have included using neutron reflectometry to examine the interaction of ionomer (with and without cations such as cerium present as a radical scavenger) with platinum and carbon simulating the electrode structure. Other fundamental properties of the electrode layer being examined includes, the structure and size of catalyst agglomerates, the porosity and relative hydrophobicity of electrode layer pores, and relative distribution of ionomer. A combination of AFM (atomic force microscopy) BET, MIP (mercury intrusion porosimetry), IGC (inverse gas chromatograph) have been employed to help better define these catalyst layer fundamental properties. These types of measurements suggest an inhomogeneous distribution in the electrode layer. Such an inhomogeneous distribution indicates that catalyst utilization is not fully optimized. This talk will discuss many of the factors affecting catalyst layer performance and provide strategies for catalyst layer optimization. Acknowledgments This work was funded through the DOE FC-PAD Consortium with thanks to DOE EERE FCTO, Fuel Cell Team Leader: Dimitrios Papageoropoulos and Technical Development Manager: Greg Kleen. References Wilson, M. S.; Gottesfeld, S., Thin- film catalyst layers for polymer electrolyte fuel cell electrodes, Journal of Applied Electrochemistry (1992), 22(1), 1-7. Wilson, M. S.; Gottesfeld, S, High Performance Catalyzed Membranes of Ultra‐low Pt Loadings for Polymer Electrolyte Fuel Cells, Journal of the Electrochemical Society, Electrochem. Soc. 1992,