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

Octahedral-shaped PtNi-alloy nanoparticlesare highlyactive oxygenreduction reaction catalysts for the cathode in proton exchange membranefuel cells. However, one major drawback in their application is theirlimited long-term morphological and compositional stability. Here,we present a detailed in situ electron microscopy characterizationof thermal annealing on octahedral-shaped PtNi catalysts as well ason doped octahedral PtNi-(Mo) and PtNi-(MoRh) catalysts. The evolutionof their morphology and composition was quantified during both exsitu and in situ experiments using energy dispersive X-ray spectroscopyin a scanning transmission electron microscope under a hydrogen atmosphereand in vacuum. Morphological changes upon heating, i.e., a gradualloss 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 quantifiedusing in situ experiments under hydrogen atmosphere in a transmissionelectron microscope. The shape change of the particles was quantifiedusing segmentation maps created by a neural network. It has been demonstratedthat morphological changes crucially depend on the composition andsurface doping: doping with Mo or Mo/Rh significantly stabilizes thestructure, allowing for persistence of a truncated octahedral shapeduring heat treatments.