Exploring kinetics and thermodynamics in fast-ion conductors and hydrogen-storage materials using ab-initio molecular dynamics
Author(s)Wood, Brandon C. (Brandon Christopher)
Massachusetts Institute of Technology. Dept. of Materials Science and Engineering.
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We investigate the interplay between various kinetic processes and thermodynamic factors in three materials--silver iodide (AgI), cesium hydrogen sulfate (CsHSO4), and sodium alanate (NaAlH4)-using ab-initio molecular dynamics simulations. The time-averaged and instantaneous silver substructure in the fast-ion conductor AgI is analyzed, resulting in a set of ordering rules that govern the distribution of the mobile silvers in the first coordination shell surrounding an iodine. We find evidence of an independent phase transition of the silver ions which drives the structural transformation to the high-mobility phase. A thermodynamic motivation for the existence of fast-ion conduction is suggested in terms of an entropic stabilization associated with the decrease in silver mobility upon melting. We also find a unique chemical signature for the fourth nearest-neighbor silver to an iodine. This fourth silver is weakly bound and relatively unconstrained, and we isolate it as the predominant agent in the diffusion process. Next, a detailed statistical analysis is performed on simulations of the fuel-cell electrolyte CsHSO4 to isolate the interplay between the dynamics of the O-H chemical bonds, the ... H hydrogen bonds, and the SO4 tetrahedra in promoting proton conduction. A high reversal rate limits the apparent success rate of the otherwise rapid chemical-bond dynamics, which are dominated by the Grotthuss mechanism of proton transfer. Rapid angular hops in concert with small reorientations of the SO4 tetrahedra constitute a new dominant mechanism for hydrogen-bond network reorganization. The SO4 dynamics are found to control the attempt rate of chemical-bond dynamical events and the success rate of hydrogen-bond dynamical events; this enables a novel interpretation of the diminished CsHSO4/CsDSO4 isotope effect.(cont.) Two distinct timescales for SO4 reorientation events are linked to different diffusion mechanisms along different crystal directions. Finally, a graph-theoretic analysis of the hydrogen-bond network topology demonstrates an increased likelihood for diffusion in connectivity configurations favoring linear network chains over closed rings. We have discovered and characterized a new phase (-y) of the hydrogen-storage material NaAlH4 that is energetically close to the known ground state. The manifestation of this phase is kinetically inhibited in the bulk but is favored in a (001) surface slab above 225 K. The transition involves first activating the surface AlH4 rotational modes. This is followed by a lattice expansion perpendicular to the slab and a shear of successive lattice planes. A possible connection between 7-NaAlH4 and the dehydrogenation product Na3aAH6 is suggested. We also show that hydrogen transport in NaAlH4 can be treated independently from the observed phase transition, and that the presence of certain point defects can enable transport of hydrogen via a structural diffusion mechanism. A link between long-range hydrogen migration and the rotational mobility of A1Hz groups is demonstrated.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.Includes bibliographical references (p. 173-190).
DepartmentMassachusetts Institute of Technology. Dept. of Materials Science and Engineering.
Massachusetts Institute of Technology
Materials Science and Engineering.