Thermodynamics and kinetics of hydrogen storage in magnesium hydride: a theoretical study of catalyst-dopant, defect, and size effects

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Chemical Physics

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2014

Author dc:creator

• Reich, Jason

Contributors dc:contributor

• Johnson, Duane D.
• Makri, Nancy
• Nuzzo, Ralph G.
• Ceperley, David M.

• hydrogen storage
• thermodynamics
• kinetics
• magnesium hydride (MgH2)
• size effect
• hydrogen
• H2
• desorption
• catalysis
• magnesium hydride defects
• rutile semi-infinite surface
• magnesium hydride nanocluster
• Mg31H62
• 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

Statement dc:rights

• 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)

Language dc:language

en

Handle dc:identifier

http://hdl.handle.net/2142/46805

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/46805