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Title:Thermodynamics and kinetics of hydrogen storage in magnesium hydride: a theoretical study of catalyst-dopant, defect, and size effects
Author(s):Reich, Jason
Director of Research:Johnson, Duane D.
Doctoral Committee Chair(s):Johnson, Duane D.
Doctoral Committee Member(s):Makri, Nancy; Nuzzo, Ralph G.; Ceperley, David M.
Department / Program:Chemistry
Discipline:Chemical Physics
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):hydrogen storage
magnesium hydride (MgH2)
size effect
magnesium hydride defects
rutile semi-infinite surface
magnesium hydride nanocluster
nudged elastic band
moment transition nudged elastic band
reversible H-storage
reversible H2 storage
reversible hydrogen storage
density functional theory (DFT)
semi-infinite stepped rutile (110) surface
semi-infinite stepped surface
semi-infinite stepped magnesium hydride (MgH2) surface
activation energy
transition state
metal hydride
hydrogen desorption mechanism
H2 desorption mechanism
hydrogen desorption
H2 desorption
Abstract:With their high capacity, light-metal hydrides – like MgH2 – remain under scrutiny as reversible H-storage materials. A key question persists: Is there a means to enhance the hydrogen desorption/adsorption properties of this “simple” hydride by decreasing size (e.g., creating nano-sized particles by ball-milling) and/or adding catalyst dopants? Thus, we need to determine accurately both the enthalpy and kinetic barriers controlling desorption, but for realistic, defected cases. Employing density functional theory (DFT) and simulated annealing, we studied initial H2 desorption from nanoclusters and semi-infinite stepped surfaces with and without transition-metal “catalyst” dopants (Ti or Fe). The large 450-atom supercell of the (110)x(1 ̅10) single stepped terrace permits the study of the effects of catalytic dopant with 10 unique dopant sites at step edges, kinks sites, and terrace sites. Extensive DFT-based simulated annealing studies were performed to find the dopants site preference and mechanism for catalyst-enhanced release of hydrogen, with additional detailed understanding from the spin-polarized electronic-structure (density of states) and charge densities. Different kink environments at the stable (110)x(1 ̅10) interface were explored to model the stability of diffusion of H to the dopant before desorption. For the most stable initial and final (possibly magnetic) states, extensive Nudged Elastic Band (NEB) calculations were performed to explore the potential energy surface (desorption enthalpies and kinetic barriers). A moment transition NEB calculation was created whereby each image was initialized to its most stable magnetic state and then images along the transition path were allowed to relax according to the NEB algorithm. This approach provided the lowest energy activation states. Together the DFT-based simulated annealing and NEB simulations determined the enthalpy change and transition-state (kinetic barrier) for desorption (H2 release to vacuum). Although small nanocluster (we focused on Mg31H62) structures are disordered (amorphous), the semi-infinite surfaces and nanoclusters have similar single, double, and triple H-to-metal bond configurations that yield similar H-desorption energies. Hence, we find that there is no size effect on desorption energetics with reduction in sample size, but dopants (as observed, e.g., Ti) do reduce the energy and kinetic barrier of H2 desorption. Overall, our results compare well with desorption experiments and elucidate the controlling chemistry for doped-MgH2 and its efficacy for use as a storage material. Notably, the same techniques used and developed here can be used for more complex hydrides or hydride reactions.
Issue Date:2014-01-16
Rights Information:Copyright 2013 Jason Reich. (Figure 1.1 and Figure 3.1 adapted by permissions that are on file with the Graduate College at the University of Illinois at Urbana Champaign)
Date Available in IDEALS:2014-01-16
Date Deposited:2013-12

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