Metallic nanoparticles have been intensively studied as catalytic materials,¹ significant understanding of their physio-chemical properties has been achieved to date, paving the way for their usage in wide range of industrial applications, from pharmaceuticals to sensing and energy. Over time, the importance of nanoparticle shape in defining its performance has been recognised; the rate of catalytic activity also depends on the shape and size of nanoparticles and therefore the synthesis of well-controlled shapes and sizes of colloidal nanoparticles can significantly improve catalytic performance.² Given the increasing demand for durable platinum catalysts for applications such as fuel cells and electrolysers, shaped nanoparticles offer an effective solution with minimal or no cost differences.

Sub-5 nm shaped noble metal nanoparticles with high fraction of {111} surface domains have been shown to demonstrate significantly superior resistance to surface rearrangement and dissolution.^{3 ­}Hence, are of fundamental and practical interest as electrocatalysts, especially in fuel cells. The nanoparticles surface structure dictates its catalytic properties, including kinetics and stability.⁴ However, unlike their bimetallic analogues that typically deliver poor durability, the synthesis of size-controlled, pure Pt shaped nano-catalysts has remained a formidable chemical challenge. Therefore, there is great need for an industrially scalable and greener synthetic method for the production of ultra-small, size-controlled nano-catalysts. We report a green innovative one-step approach used for the preparation of ultra-small pyramidal nano-catalysts with a high fraction of {111} surface domains. This is achieved by harnessing the shape-directing effect of citrate molecules, together with a strict control of oxidative etching whilst avoiding polymers, surfactants, and organic solvents. The low costs and green synthetic method can also be easily scaled up, as it is simple, low temperature, ‘one-pot’ and fast. The procedure yields single-crystal 3.4 nm Pt NPs with pyramidal shape and prevalent extended {111} facets, as proved by HR-TEM and electrochemical characterization. This was followed by Pt pyramidal NPs deposition on Vulcan carbon, ink formation and spray deposition to create gas diffusion electrodes. In a preliminary study, these pyramidal Pt catalysts are shown to offer significantly enhanced stability, using the standardised US Department of Energy (DoE) accelerated stress tests (ASTs) metrics, as cathode catalysts in full polymer electrolyte fuel cells, both compared to non-faceted equivalents and highly optimised commercial Pt/C catalysts, while providing equivalent current and power densities. Post mortem HR-TEM images of the catalyst show reduced agglomeration for the pyramidal Pt NPs compared to the commercial Pt NPs. Demonstrating that the {111} surface domains in pyramidal Pt NPs (as opposed to spherical Pt NPs) can improve aggregation/corrosion resistance, leading to a significant improvement in membrane-electrode assembly (MEA) stability and lifetime.

1. Iben Ayad, A.; Belda Marín, et all. “Water Soluble” Palladium Nanoparticle Engineering for C-C Coupling, Reduction and Cyclization Catalysis. Green Chem. 2019, 21 (24), 6646–6657.
2. Stepanov, A. L.; Golubev, et all. A Review on the Fabrication and Properties of Platinum Nanoparticles. Adv. Mater. Sci. 2014, 38 (2), 160–175.
3. Fuchs, T.; Drnec, J.; Calle-Vallejo, et all. Structure Dependency of the Atomic-Scale Mechanisms of Platinum Electro-Oxidation and Dissolution. Catal. 2020, 3 (9).
4. Lopes, P. P.; Li, D.; et all. Eliminating Dissolution of Platinum-Based Electrocatalysts at the Atomic Scale. Mater. 2020, 19 (11), 1207–1214.