Atomic scale imaging of platinum based catalysts for polymer electrolyte membrane fuel cell cathodes

Aberration-corrected scanning transmission electron microscopy (STEM) is being used to characterize the sub-Å-scale atomic structure of Pt-based electrocatalysts, which are used in the cathodes of polymer electrolyte membrane (PEM) fuel cells. Structural characteristics of the catalysts nanoparticles that are of particular interest include the surface structure (as related to composition, atomic ordering, and particle faceting), surface area (as related to particle size, dispersion, and overall particle morphology), and relationship of individual particles with the carbon support material (i.e., bonding). The contributions of each of these catalyst structural parameters will ultimately determine the activity (for the oxygen reduction reaction) and long-term stability of a particular catalyst in the cathode. In addition to initial characterization of as-prepared catalysts, in-situ, high-resolution microscopy techniques are being used to evaluate the stability of potential cathode catalysts under relevant PEM fuel cell exposures and to identify the mechanisms of catalyst degradation. These studies utilize specially designed holders for the aberration-corrected STEM to expose fuel cell catalysts to temperature, water-vapor, and potential cycling and track changes to individual catalyst particles. The behavior of several catalyst compositions during in-situ microscopy exposures will be discussed.