Direct Observation of Yolk–Shell Transforming to Gold Single Atoms and Clusters with Superior Oxygen Evolution Reaction Efficiency

Noble metal particles and atoms tend not to react with other media even at elevated temperatures due to their inert nature. Herein, we report surprising findings of Au diffusion and dissolution in a Ni2P matrix, forming single Au atoms and tiny clusters. The dynamic atomic diffusion process was directly probed by in situ heating scanning transmission electron microscopy (STEM). The Au yolks can diffuse and dissolve completely into the Ni2P shell at temperatures of 350 °C or higher, resulting in an inward volume expansion of the Ni2P shell and formation of single Au atoms and tiny Au clusters in Ni2P. The resulting structure exhibited outstanding oxygen evolution reaction (OER) efficiency that is 16-fold that of commercial IrO2. The in situ STEM finding explains its drastically improved catalytic performance after annealing, confirming Au single atoms and clusters on a Ni2P surface as active OER sites. Our finding emphasizes the role of dynamic structural changes during simple heat treatment to enhance catalysis efficiency.