Mechanism of hydrogen-induced phase transformations in sub-20 nm Pd nanocrystals

Determine whether hydrogen-induced phase transformations in palladium nanocrystals smaller than 20 nm follow the diffusion-limited shrinking-core mechanism or the insertion-limited intercalation wave mechanism during hydrogen absorption and desorption, in order to resolve the size-dependent pathway of phase separation in PdHx.

Background

Hydrogen intercalation in palladium (Pd) produces phase separation between dilute α-PdHx and hydrogen-rich β-PdHx. Prior ensemble studies for sub-10 nm Pd nanoparticles suggested diffusion-limited shrinking-core behavior, while in situ single-particle imaging for particles ≥20 nm indicated insertion-limited intercalation waves propagating across the particle.

These divergent observations implied a gap for smaller nanocrystals (<20 nm), leaving uncertainty about which mechanism governs phase transformations in this size regime. Clarifying the operative mechanism is important for understanding nanoscale kinetics, strain evolution, and performance in hydrogen storage and related energy devices.