Probing Strain in Individual Palladium Nanocrystals during Electrochemically-Induced Phase Transitions

The palladium-hydrogen system is essential for applications in water electrolysis, catalysis, hydrogen storage, and hydrogen sensing technologies. Understanding the transition of Pd nanocrystals from the hydrogen-poor (α) phase to the hydrogen-rich (β) phase is crucial for elucidating hydrogen absorption/desorption mechanisms, as well as related phenomena such as hydrogen trapping. In this study, we employ in situ Bragg Coherent Diffraction Imaging (BCDI) to investigate individual palladium nanocrystals,mapping their strain and lattice parameter distributions across electrochemical potentials typical of hydrogen absorption, while avoiding undesired ‘beam effects’. We analyse lattice parameter changes in both α and β phases and observe reversible strain inversion during the α-to-β phase transition. Through strain and reciprocal space analysis, and molecular simulations, we propose a model for the α-to-β phase transition which includes a hydrogen-saturated subsurface shell, hydrogen depletion from the α phase during β nucleation, and propagation of the β phase in a spherical-cap fashion.