Nanoparticle catalysis plays a crucial role in the development of renewable energy cycles, such as splitting water into hydrogen and oxygen and storing the hydrogen for on-demand power generation in a proton exchange membrane fuel cell (PEMFC). Though PEMFCs are entering commercialization, primarily in passenger vehicles, major challenges face this technology in cost and durability of the fuel cell stack. In particular, the oxygen reduction catalyst, typically platinum, contributes significantly to these cost and durability issues, with Pt being one of the most expensive metals and Pt dissolution occurring at the oxidizing potentials of the oxygen reduction reaction. Therefore, advanced designs for oxygen reduction nanocatalysts must be developed to obtain higher levels of activity and stability required for cost-effective commercialization of PEMFCs. Common strategies include composition and shape control and have demonstrated improvement over state-of-the-art Pt electrocatalysts, but these approaches do not obtain the level of atomic precision necessary to make significant improvement in catalyst activity and durability. We present nanoscopic and atomic segregation of the elements in Pt-Ni nanoparticles as next-level variables that can be controlled to create a three-dimensional Pt3Ni nanoframe composed of highly active interconnected one-dimensional struts. Starting from solid Pt-Ni rhombic dodecahedra with a Pt-rich phase segregated to their edges, selectively removing the interior Ni-rich phase results in hollow nanoframes. The synthesis of the rhombic dodecahedra is revealed as a complex concurrent evolution of their composition, element spatial distribution, and morphology. It can be summarized as a series of fundamental chemistries including unmatched reduction potentials of Pt and Ni precursors, anisotropic overgrowth on preformed seeds, step-induced metal deposition, and site-dependent phase segregation and migration. Once evolved to Pt3Ni nanoframes, in situ EXAFS analysis illustrates that these catalysts exhibit excellent oxygen reduction activity due to atomic-level segregation of Pt from Ni, forming a Pt-skin on the nanoframe surface. Ultimately, we demonstrate how nanoscopic and atomic level segregation of Pt and Ni can be used to innovate new catalyst morphologies with unprecedented levels of activity and stability for the oxygen reduction reaction.

Controlling nanoscopic and atomic segregation of Pt within Pt-Ni rhombic dodecahedra and nanoframes for fuel cell catalysis

253rd National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.