Mitigation of Structural and Compositional Instability in 3-Dimensional, Nanoporous Electrocatalysts

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) [1]. After electrochemical dealloying, the open, bicontinuous, three-dimensional nanoporous nanoparticle electrocatalysts exhibit dramatically enhanced electrocatalytic properties [2,3]. 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 Pt20Ni80 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 strategies to mitigate coarsening and improve operational catalyst stability. The first approach uses ionic liquids (IL) strategically placed at the metal/electrolyte interface to limit electrochemically enhanced surface diffusion. Taking advantage of the free volume within the nanoporous nanoparticles and creating a composite catalyst architecture through the incorporation of [MTBD][beti] IL, in addition to the improved oxygen reduction reaction kinetics [3,4], significant improvements in retention of ECSA during accelerated durability testing is observed, see Figure 1. The interfacial IL acts to limit the charge dependent formation of a surface metal/electrolyte anion complex which is responsible for the potential dependent enhancement in surface diffusion. A second approach involves the minimization of morphology evolution 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, see Figure 1. With these strategies 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. References [1] Snyder, J.; McCue, I.; Livi, K. & Erlebacher, J., J. Am. Chem. Soc. 134, 8633-8645 (2012). [2] Snyder, J.; Fujita, T.; Chen, M. W. & Erlebacher, J., Nature Mater. 9, 904-907 (2010). [3] Snyder, J.; Livi, K. & Erlebacher, J., Adv. Funct. Mater. 23, 5494-5501 (2013). Figure 1