title: Improved Oxygen Reduction Activity and Durability of Dealloyed PtCoxCatalysts for Proton Exchange Membrane Fuel Cells: Strain, Ligand, and Particle Size Effects.
abstract: The development of active and durable catalysts with reduced platinum content is essential for fuel cell commercialization. Herein we report that the dealloyed PtCo/HSC and PtCo3/HSC nanoparticle (NP) catalysts exhibit the same levels of enhancement in oxygen reduction activity (~4-fold) and durability over pure Pt/C NPs. Surprisingly, ex situ high-angle annular dark field scanning transmission electron microscopy (HAADF STEM) shows that the bulk morphologies of the two catalysts are distinctly different: D-PtCo/HSC catalyst is dominated by NPs with solid Pt shells surrounding a single ordered PtCo core; however, the D-PtCo3/HSC catalyst is dominated by NPs with porous Pt shells surrounding multiple disordered PtCo cores with local concentration of Co. In situ X-ray absorption spectroscopy (XAS) reveals that these two catalysts possess similar Pt-Pt and Pt-Co bond distances and Pt coordination numbers (CNs), despite their dissimilar morphologies. The similar activity of the two catalysts is thus ascribed to their comparable strain, ligand, and particle size effects. Ex situ XAS performed on D-PtCo3/HSC under different voltage cycling stage shows that the continuous dissolution of Co leaves behind the NPs with a Pt-like structure after 30k cycles. The attenuated strain and/or ligand effects caused by Co dissolution are presumably counterbalanced by the particle size effects with particle growth, which likely accounts for the constant specific activity of the catalysts along with voltage cycling.
authors.affiliation: Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States.
authors.forename: Qingying
authors.initials: Q
authors.lastname: Jia
authors.affiliation: Department of Chemistry, George Washington University, Washington, DC 20052, United States.
authors.forename: Keegan
authors.initials: K
authors.lastname: Caldwell
authors.affiliation: Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States.
authors.forename: Kara
authors.initials: K
authors.lastname: Strickland
authors.affiliation: Electrochemical Energy Research Lab, General Motors Central Research and Development, Warren, Michigan 48090, United States.
authors.forename: Joseph M
authors.initials: JM
authors.lastname: Ziegelbauer
authors.affiliation: Electrochemical Energy Research Lab, General Motors Central Research and Development, Warren, Michigan 48090, United States.
authors.forename: Zhongyi
authors.initials: Z
authors.lastname: Liu
authors.affiliation: Electrochemical Energy Research Lab, General Motors Central Research and Development, Warren, Michigan 48090, United States.
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Yu
authors.affiliation: Department of Chemistry, George Washington University, Washington, DC 20052, United States.
authors.forename: David E
authors.initials: DE
authors.lastname: Ramaker
authors.affiliation: Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States.
authors.forename: Sanjeev
authors.initials: S
authors.lastname: Mukerjee
keywords: ORR; Pt-based nanocatalysts; in situ XAS; ligand effects; particle size effects; strain effects