Atomistic modeling and analysis of hydride phase transformation in palladium nanoparticles

Palladium-hydrogen (Pd-H) is a prototypical system for studying solute-induced phase transformation in various energy conversion and storage applications. While the behaviors of bulk Pd-H have been studied extensively, the detailed atomic picture of hydride phase transformation within individual Pd nanoparticles is still unclear. In this work, we employ a novel atomistic computational model, referred to as Diffusive Molecular Dynamics (DMD), to characterize the H absorption dynamics in Pd nanoparticles of spherical, octahedral and cubic shapes. The DMD model couples a non-equilibrium thermodynamic model with a discrete diffusion law, which allows it to reach diffusive time scales with atomic resolution. The model is capable of capturing the propagation of an atomistically sharp hydride phase boundary. A remarkable feature of the phase boundary structure that is predicted by the calculations is the emergence of misfit dislocations distributed over the interface. These dislocations relieve the elastic residual stresses induced by the change of volume that accompanies the phase transformation. Shape effects are also investigated in this work. More specifically, in both spherical and octahedral nanoparticles, we observe stacking faults during the H absorption process while the phase boundary in the cubic nanoparticle remains coherent. The spatial distribution of the stacking faults in the spherical sample is investigated in detail using an elastic core-shell model. We also identify the mechanisms that enable the movement of the stacking faults as they track the propagation of the phase boundary. Finally, we find that the rate of H absorption is reduced by the formation and movement of the stacking faults.