Contributed by Héctor D. Abruña, December 14, 2018 (sent for review September 10, 2018; reviewed by Plamen Atanassov, Marc Koper, and Eugene S. Smotkin)
We present a comprehensive quantitative study of the dynamic order–disorder phase transition of Pt3Co nanoparticles, binary intermetallic oxygen-reduction reaction fuel cell electrocatalysts, during postsynthesis annealing. We employed in situ synchrotron-based X-ray diffraction (XRD) and in situ scanning transmission EM (STEM) to study the phase transition and morphological and structural changes during real-time annealing. In situ XRD revealed the impact of annealing/cooling conditions on the degree of ordering, particle size and lattice strain. In situ heating STEM enabled visualization of nanoparticle migration and growth. We find that a higher degree of ordering leads to more active and durable electrocatalysts. Our findings represent a groundbreaking advance in electrocatalyst development/design with a broad impact on energy materials, in general, and fuel cells, in particular.
Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). The ordered phase is generally identified based on the existence of superlattice ordering peaks in powder X-ray diffraction (PXRD). However, after employing a widely used postsynthesis annealing treatment, we have found that claims of “ordered” catalysts were possibly/likely mixed phases of ordered intermetallics and disordered solid solutions. Here, we employed in situ heating, synchrotron-based, X-ray diffraction to quantitatively investigate the impact of a variety of annealing conditions on the degree of ordering of large ensembles of Pt3Co nanoparticles. Monte Carlo simulations suggest that Pt3Co nanoparticles have a lower order–disorder phase transition (ODPT) temperature relative to the bulk counterpart. Furthermore, we employed microscopic-level in situ heating electron microscopy to directly visualize the morphological changes and the formation of both fully and partially ordered nanoparticles at the atomic scale. In general, a higher degree of ordering leads to more active and durable electrocatalysts. The annealed Pt3Co/C with an optimal degree of ordering exhibited significantly enhanced durability, relative to the disordered counterpart, in practical membrane electrode assembly (MEA) measurements. The results highlight the importance of understanding the annealing process to maximize the degree of ordering in intermetallics to optimize electrocatalytic activity.
↵1Y.X., Y.Y., and H.J. contributed equally to this work.
Author contributions: Y.X., Y.Y., A.K., F.A.E., J.D.B., F.J.D., D.A.M., and H.D.A. designed research; H.J. designed the in situ heating setup; Y.X., Y.Y., H.J., E.P., U.G., V.Y., D.N.A.-B., X.H., T.E.M., and R.Z. performed research; Y.X., Y.Y., and H.J. performed the in situ heating XRD experiment; Y.Y. and E.P. performed the in situ heating TEM experiment; U.G. performed the Monte Carlo simulation; Y.X., Y.Y., H.J., and E.P. analyzed data; and Y.X., Y.Y., H.J., and E.P. wrote the paper.
Reviewers: P.A., University of California, Irvine; M.K., Leiden University; and E.S.S., Northeastern University.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1815643116/-/DCSupplemental.
Published under the PNAS license.
