Asymmetric Hysteresis in Nanoscopic Single-Metal Hydrides: Palladium Nanorings

Using direct nanoplasmonic sensing (DNPS), we have studied hydrogen sorption in Pd nanorings with a constant average inner diameter of 240 nm and varying wall thickness. As the main finding we observe an unusual feature above a critical wall thickness in the appearance of a double plateau in the hydrogenation coexistence curve, whereas a single plateau is seen during dehydrogenation for all wall thicknesses. This is the first experimental observation of asymmetric hysteresis in a single metal system. We explain this effect by structural defect-related sample heterogeneity and the interplay of strain-dependent hydrogenation thermodynamics and kinetics.