Post-Synthesis Heat Treatment of Doped PtNi-Alloy Fuel-Cell Catalyst Nanoparticles Studied by In-Situ Electron Microscopy

Octahedral-shaped PtNi-alloy nanoparticles are highly active oxygen reduction reaction catalysts for the cathode in proton exchange membrane fuel cells. However, one major drawback in their application is their limited long-term morphological and compositional stability. Here, we present a detailed in situ electron microscopy characterization of thermal annealing on octahedral-shaped PtNi catalysts as well as on doped octahedral PtNi(Mo) and PtNi(MoRh) catalysts. The evolution of their morphology and composition was quantified during both ex situ and in situ experiments using energy dispersive X-ray spectroscopy in a scanning transmission electron microscope under a hydrogen atmosphere and in vacuum. Morphological changes upon heating, i.e., a gradual loss of the octahedral shape and a continuous rounding of the particles, were observed, as well as evidence for increased alloying. Furthermore, the evolution of the shape of the PtNi(Mo) nanoparticles was quantified using in situ experiments under hydrogen atmosphere in a transmission electron microscope. The shape change of the particles was quantified using segmentation maps created by a neural network. It has been demonstrated that morphological changes crucially depend on the composition and surface doping: doping with Mo or Mo/Rh significantly stabilizes the structure, allowing for persistence of a truncated octahedral shape during heat treatments.