Morphological Instability in Topologically Complex, Three-Dimensional Electrocatalytic Nanostructures

Meeting Program – March 2018

Yawei Li – Student Speaker

Advisor: Joshua Snyder
Department of Chemical and Biological Engineering
Drexel University, Philadelphia, Pennsylvania 19104

Abstract – Dealloying has shown increasing utility in the field of electrocatalysis as a tool for the synthesis and development of nanoporous materials possessing high surface-to-volume ratios with controlled morphology and compositional gradient (core-shell structure). After electrochemical dealloying, the open, bicontinuous, three-dimensional nanoporous nanoparticle electrocatalysts exhibit dramatically enhanced electrocatalytic properties.

In the development of efficient electrocatalysts for oxygen reduction reaction (ORR), durability is too often ignored in the pursuit of higher activities. For 3-dimensional, nanoporous materials, in addition to the standard mechanisms of electrocatalyst degradation including Pt dissolution/Ostwald ripening and coalescence/aggregation, new modes of morphological and compositional evolution must be considered. Here we use a combination of in-situ and ex-situ experimental techniques to develop insight into the structural and compositional evolution of nanoporous PtNi nanoparticles (np-NiPt) formed through the dealloying of Pt 20 Ni 80 precursor nanoparticles. We demonstrate that surface-diffusion facilitated coarsening, driven by the tendency to reduce the overall surface free energy of the system, is the dominant mechanism of electrochemical active surface area (ECSA) loss, consequently resulting in a decrease in activity.

With a better understanding of the interplay between nanoporous structure coarsening and transition metal loss, we have developed strategy to mitigate coarsening and improve operational catalyst stability by impeding step edge movement through the use of foreign adsorbates on the
surface. We show that partial monolayer decoration of np-NiPt with Ir, possessing a significantly lower rate of surface diffusion than Pt, acts to pin step edges and results in significant enhancement in catalyst durability as measured by ECSA and ORR activity retention. With this strategy we will show how more detailed insight into the atomic processes that govern electrocatalytic material instability can begin to break the inverse correlation between activity and durability.