Seed-Mediated Synthesis and Catalytic ORR Reactivity of Facet-Stable, Monodisperse Platinum Nano-Octahedra

Abstract

Shape-selective, sub-10 nm-sized metal nanoparticles are of high fundamental and practical interest in catalysis and electrocatalysis, where the surface structure dictates the kinetic properties of the nanomaterials. Unlike their bimetallic analogues, the synthesis of size-controlled, pure Pt octahedral nanocatalysts has remained a formidable chemical challenge. In bimetallic shaped systems, however, the benefit of shape is often convoluted with surface composition in complex ways. In the present work, a seed-templated approach is presented for the preparation of ultrasmall octahedral platinum nanoparticles (Pt NPs), harnessing the effect of monocrystalline anisotropic seeds and strict control of the reduction rate and other physicochemical parameters while avoiding polymers, surfactants, and organic solvents. The procedure yields previously elusive 6.7 nm, strictly single-crystal, Pt NPs with partially truncated octahedral shape and prevalent extended {111} surface facets. Electrochemical measurements using rotating disk electrodes in an acid electrolyte revealed a much higher electrochemical active surface area (ECSA) over the state-of-the-art octahedral Pt NPs, which is ascribed to small-sized, poison-free, and preferentially {111} orientated facets. The dramatic kinetic benefit for the oxygen reduction reaction (ORR) of the octahedral shape over spherical particle shapes of same size is convincingly demonstrated. More important for practical applications is the fact that the intrinsic specific ORR activity is about 2.4-fold higher than commercial optimized spherical Pt NPs deployed in fuel cell cathodes at comparable ORR stability. In doing this analysis, we validate the voltammetric correspondence between Pt single crystals and Pt nanoparticulate materials and highlight the kinetic benefits of limiting the proportion of {100} facets. Prolonged suppression of {100} facet growth in octahedral Pt catalysts is the reason for the unusually high specific activity and fair stability and calls for their integration and testing as cathode catalysts in fuel cell membrane electrode assemblies.