Growth and degradation of advanced octahedral Pt‐alloy nanoparticle catalysts for fuel cells

Abstract

Octahedral Pt‐Ni nanoparticles are highly attractive as fuel‐cell catalysts due to their extraordinarily high activity for the oxygen‐reduction‐reaction (ORR). A deep understanding of their atomic‐scale structure, degradation and formation is a prerequisite for their use as rationally designed nanoparticle catalysts with high activity and long‐term stability. Here we present an extensive microstructural study of the growth and degradation behavior of various octahedral Pt‐alloy nanoparticles using in situ transmission electron microscopy (TEM) and Cs‐corrected high‐angle annular dark‐field scanning transmission electron microscopy (HAADF‐STEM) combined with electron energy‐loss spectroscopy (EELS) and energy‐dispersive X‐ray spectroscopy (EDX). We show that octahedral nanoparticles often show compositional anisotropy with Ni‐rich {111} facets leading to complex structural degradation during ORR electrocatalysis. The Ni‐rich {111} facets are preferentially etched, resulting in the formation of first concave octahedra and then Pt‐rich skeletons that have less active facets (Figure 1)[1]. Furthermore, we reveal element‐specific anisotropic growth as the reason for the compositional anisotropy and the limited stability. During the solvothermal synthesis, a Pt‐rich nucleus evolves into precursor nanohexapods, followed by the slower step‐induced deposition of Ni on the concave hexapod surface, to form octahedral facets (Figure 2)[2]. While the growth of Pt‐rich hexapod is a ligand‐controlled kinetic process, the step‐induced deposition of the Ni‐rich phase at the concave surface resembles a thermodynamically controlled process accomplished in much longer time. In order to tune the atomic‐scale microstructure of the octahedra for long‐term stability, we illustrate the effect of varying the growth conditions on morphology and compositional segregation by producing trimetallic PtNiCo nanooctahedra and comparing “one‐step” and newly‐developed “two‐step” synthesis routes [3]. Furthermore we demonstrate how Pt atom surface diffusion may produce a protective Pt surface layer on top of the Ni‐rich facets, resulting in advanced and more stable octahedral catalysts. Figure 3 shows a sequence of structural changes taking place on an octahedral nanoparticle during in situ heating up to 800°C using a MEMS chip heating holder (DENSsolutions, Delft, NL). It can be observed that Pt‐rich corner atoms diffuse and subsequently fill the concave Ni‐rich {111} facets, forming perfectly octahedral nanoparticles with flat Pt‐rich {111} surfaces (Figure 3) [4].