Progress in the morphology control and stability of shaped Pt alloy nanocrystal catalysts for fuel cells

Abstract

With the increasing demand in new clean energy conversion and storage technologies in recent years, metal alloy nanocrystals have drawn considerable interests as high performance electrocatalysts for proton-exchange-membrane fuel cells (PEFMCs), lithium air batteries and (photo)electrochemical electrolyzers. In particular, PEMFCs are considered to be the ideal clean energy techniques for automobiles due to their high energy density and environment-friendly. Through continuous efforts from both academy and industry for decades, spring in the commercialization of PEMFCs is now coming marked by the first mass-produced hydrogen fuel cell car released by Toyota in 2014. Still, one of the most challenging tasks towards the commercialization of PEMFCs is to develop highly active, stable and low cost electrocatalysts for the sluggish oxygen reduction reaction (ORR) at the cathode, for which Pt is the best pure metal catalyst yet expensive for large scale applications. Alloying of Pt with non-noble 3D transition metal M (M=Fe, Co, Ni, etc.) are known to improve the ORR catalytic activity, while recent breakthrough results further demonstrat that certain Pt alloy facets particularly Pt3Ni {111} surface processes the potential to greatly increase the ORR activity by 90-fold compared to the state-of-the-art commercial Pt catalyst. Inspired by the exceptional ORR activity reported on the extended Pt-skin {111} surface, numerus studies have been devoted to the synthesis of shaped Pt alloy nanocrystals with {111}-oriented surfaces, e.g., nanooctahedra enclosed by six {111} surfaces. These particles are perceived to be the realistic “dream-ORR-catalyst” and are expected to bring in the 90-fold enhancement over the commercial Pt catalysts. Significant progress in the synthesis of shaped-controlled platinum alloy nanocrystals has been made in recent years, resulting in several families of highly active shaped Pt alloy electrocatalysts including nanooctahedra, icosahedra, nanoframe and nanocage catalysts. Meanwhile, their ORR activity improvements over commercial Pt catalyst have also increased greatly from 3–4 fold reported in 2010 to more than 30-fold reported recently. Despite these progresses, the long-term stability of the shaped Pt alloy nanocrystals particularly under realistic fuel cell condition is still the most challenging issue. In this review, we summarize recent progresses in the morphology control, catalytic activity and durability of shaped Pt-alloy nanocrystals for application in ORR electrocatlaysts in PEMFCs. We first introduce the syntheses of shaped Pt alloy nanocrystals such as octahedra, icosahedra, nanoframes and nanocages and their ORR activity improvement over commercial Pt catalyst. We then review recent efforts on a deeper understanding of the relationship among the element-specific growth, structure and catalytic stability of shaped Pt alloy catalysts. Emphasis will be placed on atomic insights into the structural evolutions of shaped Pt nanocrystals across their full life cycle (from growth to application in electrocatalysis) gained by advanced high-resolution scanning transmission electron microscopy combined with X-ray energy dispersive spectroscopy and electron energy loss spectroscopy. Finally, we summarize several approaches reported recently to improve the catalytic stability of the shaped Pt alloy nanocrystals, for instance, by rational design of tri-metallic shaped alloy nanocrystals and new nanostructures. These results are helpful for designing next-generation shaped Pt alloy nanocatalysts with both high activity and high durability in realistic PEFMCs and may also provide useful insights into metal alloy nanocatalysts for other catalytic reactions.