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Open Access Publications from the University of California

Scholarly Works (5 results)

  • Thesis
  • Peer Reviewed
UC Berkeley Electronic Theses and Dissertations (2013)

The van der Waals force is ubiquitous in nature, however, first principles calculations of this interaction for large systems, i.e., around 1000 atoms, have been performed only recently. In the following are presented results on the application of the van der Waals density functional (vdW-DF) to gas adsorption and transport in zeolitic imidazolate frameworks (ZIFs).

Carbon dioxide and methane binding energies and positions are calculated with the vdW-DF in three distinct binding sites in a series of five rho topology ZIFs. The isostructural set of ZIFs was selected in order to isolate the effect of framework functionalization. Gas molecules are found to bind in locations with high coordination to framework atoms at distances of around 3 A. Contributions to the binding energy from induced polarization and dispersion are quantified in order to elucidate the origins of strong CO2 adsorption and selectivity over CH4. The dispersion energy is found to dominate the interactions, however, CO2 adsorption is also enhanced by electrostatic interactions with asymmetrically functionalized linkers. Steric constraints for methane molecules, that do not impede carbon dioxide binding, further contribute to selectivity.

Binding energy landscapes for CO2 and CH4 are calculated using classical force fields for the same set of rho ZIFs and several other ZIFs that differ in functionalization and topology. Quantities extracted from these landscapes are used to explain the effect of framework topology on gas adsorption at low and high pressure as well as how the positions of adsorbed gas molecules evolve as a function of pressure. Materials with large surface areas have greater gas uptake at high pressure, while smaller pores, which are associated with stronger binding, adsorb more gas at low pressure.

Finally, the effect of framework flexibility on CO2 transport through the double 8-ring channel of ZIF-97 is investigated with computationally intensive climbing-nudged elastic band calculations utilizing two versions of the vdW-DF. The results are largely consistent between the two versions and show a small decrease (12-33 meV) in the transport barrier with flexibility.

In addition, several versions of the vdW-DF are applied to the Kr dimer, graphite, and H2, Al, and Li on graphene. For these systems experimental binding energies and separations are available, such that they provide useful benchmarks for the accuracy of the vdW-DF type methods. The vdW-DF2 and vdW-optB88 methods are found to both produce accurate results for dispersion dominated binding. Analyses of mixed ionic-dispersion binding highlight the importance of further study of these functionals at short-range.

    Cover page: van der Waals Corrected Density Functional Theory Calculations on Zeolitic Imidazolate Frameworks
    • Article
    • Peer Reviewed
    LBL Publications (2017)
    Mg(BH4)2 is a promising solid-state hydrogen storage material, releasing 14.9 wt% hydrogen upon conversion to MgB2. Although several dehydrogenation pathways have been proposed, the hydrogenation process is less well understood. Here, we present a joint experimental-theoretical study that elucidates the key atomistic mechanisms associated with the initial stages of hydrogen uptake within MgB2. Fourier transform infrared, X-ray absorption, and X-ray emission spectroscopies are integrated with spectroscopic simulations to show that hydrogenation can initially proceed via direct conversion of MgB2 to Mg(BH4)2 complexes. The associated energy landscape is mapped by combining ab initio calculations with barriers extracted from the experimental uptake curves, from which a kinetic model is constructed. The results from the kinetic model suggest that initial hydrogenation takes place via a multi-step process: molecular H2 dissociation, likely at Mg-terminated MgB2 surfaces, is followed by migration of atomic hydrogen to defective boron sites, where the formation of stable B-H bonds ultimately leads to the direct creation of Mg(BH4)2 complexes without persistent BxHy intermediates. Implications for understanding the chemical, structural, and electronic changes upon hydrogenation of MgB2 are discussed.
      Cover page: Elucidating the mechanism of MgB 2 initial hydrogenation via a combined experimental–theoretical study
      • Article
      • Peer Reviewed
      UC Santa Cruz Previously Published Works (2023)
      Metal boride nanostructures have shown significant promise for hydrogen storage applications. However, the synthesis of nanoscale metal boride particles is challenging because of their high surface energy, strong inter- and intraplanar bonding, and difficult-to-control surface termination. Here, it is demonstrated that mechanochemical exfoliation of magnesium diboride in zirconia produces 3-4 nm ultrathin MgB2 nanosheets (multilayers) in high yield. High-pressure hydrogenation of these multilayers at 70 MPa and 330 °C followed by dehydrogenation at 390 °C reveals a hydrogen capacity of 5.1 wt%, which is ≈50 times larger than the capacity of bulk MgB2 under the same conditions. This enhancement is attributed to the creation of defective sites by ball-milling and incomplete Mg surface coverage in MgB2 multilayers, which disrupts the stable boron-boron ring structure. The density functional theory calculations indicate that the balance of Mg on the MgB2 nanosheet surface changes as the material hydrogenates, as it is energetically favorable to trade a small number of Mg vacancies in Mg(BH4 )2 for greater Mg coverage on the MgB2 surface. The exfoliation and creation of ultrathin layers is a promising new direction for 2D metal boride/borohydride research with the potential to achieve high-capacity reversible hydrogen storage at more moderate pressures and temperatures.
        Cover page: Hydrogen Storage in Partially Exfoliated Magnesium Diboride MultilayersCreative Commons 'BY-NC' version 4.0 license
        • Article
        • Peer Reviewed
        LBL Publications (2021)
        Layered boron compounds have attracted significant interest in applications from energy storage to electronic materials to device applications, owing in part to a diversity of surface properties tied to specific arrangements of boron atoms. Here we report the energy landscape for surface atomic configurations of MgB2 by combining first-principles calculations, global optimization, material synthesis and characterization. We demonstrate that contrary to previous assumptions, multiple disordered reconstructions are thermodynamically preferred and kinetically accessible within exposed B surfaces in MgB2 and other layered metal diborides at low boron chemical potentials. Such a dynamic environment and intrinsic disordering of the B surface atoms present new opportunities to realize a diverse set of 2D boron structures. We validated the predicted surface disorder by characterizing exfoliated boron-terminated MgB2 nanosheets. We further discuss application-relevant implications, with a particular view towards understanding the impact of boron surface heterogeneity on hydrogen storage performance.
          Cover page: Spontaneous dynamical disordering of borophenes in MgB2 and related metal borides
          • Article
          • Peer Reviewed
          UC Berkeley Previously Published Works (2020)
          Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has received tremendous attention as a promising onboard hydrogen storage medium due to its excellent gravimetric and volumetric hydrogen storage capacities. While the polymorphs of MBH-alpha (α), beta (β), and gamma (γ)-have distinct properties, their synthetic homogeneity can be difficult to control, mainly due to their structural complexity and similar thermodynamic properties. Here, we describe an effective approach for obtaining pure polymorphic phases of MBH nanomaterials within a reduced graphene oxide support (abbreviated MBHg) under mild conditions (60-190 °C under mild vacuum, 2 Torr), starting from two distinct samples initially dried under Ar and vacuum. Specifically, we selectively synthesize the thermodynamically stable α phase and metastable β phase from the γ-phase within the temperature range of 150-180 °C. The relevant underlying phase evolution mechanism is elucidated by theoretical thermodynamics and kinetic nucleation modeling. The resulting MBHg composites exhibit structural stability, resistance to oxidation, and partially reversible formation of diverse [BH4]- species during de- and rehydrogenation processes, rendering them intriguing candidates for further optimization toward hydrogen storage applications.
            Cover page: A Mechanistic Analysis of Phase Evolution and Hydrogen Storage Behavior in Nanocrystalline Mg(BH4)2 within Reduced Graphene Oxide
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