First-Principles Study on Alloy Nanoparticles for Polymer Electrolyte Fuel Cell Catalyst
Authors: Nanba Yusuke , Rivera Rocabado David Samuel , Ishimoto Takayoshi , Koyama Michihisa 
Published: February 2020

Catalyst for polymer electrolyte fuel cell (PEFC) remains a central issue toward a large-scale penetration of technologies especially for automobile applications. Along with the development of synthesis as well as analytical methodologies, a few nano-meter scale nano-particle with manipulated structure or compositions, such as core-shell or skin-layer alloy catalysts, have been intensively developed (1). Because of the complexity of catalytic reactions as well as the porous microstructure of catalyst layer with multi-components, theoretical methods are also intensively applied (2). While majority of theoretical papers adopts slab models under periodic boundary condition for simpler computation, properties of nano-particles such as strain, stability, or electronic structures should be discussed by using a cluster model. In discussing electronic structure of nano-particles, it is reported that small cluster models show discrete electronic structure, which may lead to a molecular properties rather than metallic properties (3). Thus, it is necessary to adopt models with a realistic diameter. While we can find a few papers adopting models over 2 nm (4), we often find ca. 3 nm particles as PEFC electrocatalyst (5). In our preceding study, we have discussed the stability and electronic structures of Pd-Pt alloys with different atomic configurations by using ca. 3 nm model consisting of 711 atoms (6). In this study, we extend our efforts to Pt-based electrocatalyst with skin-layer configuration.
For density functional theory calculations, we used Vienna ab initio simulation package (VASP) (7) with the projector augmented wave method (8). The Perdew-Burke-Ernzerhof under generalized gradient approximations (9) was used as the exchange and correlation functional. The cutoff energy was set to be 400 eV. Pt-Co alloy nano-particle models with solid solution and Pt-skin configurations are prepared. To discuss the stability of nano-particles with different configuration, excess energy (10) are evaluated.
By comparing the excess energies of the Pt-Co models with solid solution and Pt-skin configurations, we found that the Pt-skin configuration is more stable than solid solution configuration at the same Pt-Co composition. To have insight on the activity toward oxygen reduction reaction, we have analyzed interatomic distances to find shorter distances for Pt-skin configuration. Details will be discussed in the presentation.

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