Nanocrystals put new face on oxygen reduction

Polymer electrolyte fuel cells (PEMFCs) use hydrogen and oxygen to generate electricity cleanly. But the reduction of oxygen (or ORR) at the cathode needs a platinum (Pt) catalyst to drive the reaction. Now researchers have designed a catalyst that uses much less Pt but drives the ORR much more efficiently and is much more durable [Zhu et al., Materials Today (2019), https://doi.org/10.1016/j.mattod.2019.11.002]. Reducing the reliance of PEMFCs on expensive and scarce Pt could dramatically improve the cost-effectiveness and commercialization potential of PEMFCs. There are a number of ways to enhance the catalytic activity of Pt-based catalysts while reducing its use, such as increasing surface area, tuning the composition, or constructing hollow or core-shell nanostructures. The team from Georgia Tech, the University of Wisconsin-Madison, and Nanjing Tech University led by Manos Mavrikakis and Younan Xia has shown previously that the atomic structure – or crystal facets – of nanocrystal catalysts can also improve activity. “In an effort to rationally optimize their performance towards the oxygen reduction reaction (ORR), it remains an unmet challenge to precisely engineer the type of facet exposed on the surface of nanocrystals,” says Xia. The team used atomic layer-by-layer co-deposition to synthesize Pt-Ir alloy nanocrystals on differently shaped Pd seeds. Ir is one of the few elements that is thermodynamically stable and resistant to surface segregation and leaching in acidic conditions. By ultrathin Pt-Ir alloy shells of just 1.6 atomic layers in the form of cubes, octahedra, and icosohedra, the team were able to create catalysts with different predominant facets. Cubes and octahedral have mainly {1 0 0} and {1 1 1} facets respectively, while icosohedra have {1 1 1} facets and twin boundaries. “Alloying Pt with Ir, especially, Pt4Ir, can speed up the sluggish kinetics associated with the ORR relative to pure Pt,” explains Xia. “As a result, Pt-Ir alloy nanocrystals have been considered as one of the most promising catalysts towards ORR.” All the new core-shell Pd@Pt-Ir nanocrystal catalysts show a significant enhancement in reactivity compared with commercial Pt/C catalysts, with the icosohedra showing the biggest boost. The researchers’ observations match their theoretical calculations, which predict that {1 1 1} facets and twin boundaries rise to the best catalytic performance. While the current simple and easy synthesis route can produce milligram batches of nanocrystals, it cannot meet the demand of large-scale commercial applications. But the team are already working on a solution. “We will scale-up production of Pt-Ir nanocrystals by switching from batch synthesis to a continuous flow or droplet-based system [which has] the potential for automation,” says Xia. “And to make sustainable and cost-effective products, we will use cheaper materials as the core or even remove the core via wet etching.”