Crystallographic Characteristics of Nanostructured Thin-Film Fuel Cell Electrocatalysts: A HRTEM Study

This paper reports a study of thin films of Pt and Pt-alloys deposited on oriented crystalline organic whiskers using high resolution transmission electron microscopy imaging. It is our opinion that these nanostructured thin film (NSTF) electrocatalysts are the leading electrode technology for polymer electrolyte fuel cells because of their significant advantages in durability and specific activity. The sputter-deposited thin catalyst films on the whiskers grow as polycrystalline layers that expose highly oriented fcc crystallites. Within the films, truncated pyramid-shaped crystallites grow into acicular whiskerettes on the whisker sides, and the whiskerette shapes display a growth mechanism governed by a strong support−metal interaction and surface energy minimization. The surface structure of the crystalline whiskers facilitates metal nucleation sites that are spaced evenly along the whisker length by ∼6–8 nm. The whiskerettes are the dominant structural form of the NSTF catalyst coatings at the loading levels investigated. Along just a few nanometers of a whisker’s long axis, the whiskerettes can exhibit changes in aspect ratios up to a factor of 5 and thus may increase the surface roughness by a factor of 6. Diffractogram indexing suggests that the truncated pyramids expose four (111) and one (100) planes, which grow further into pillars by elongating their base along two of the (111) facets. When the atomic scale roughness is disregarded, the supported NSTF Pt-based electrocatalysts expose predominantly (111) facets.