Dynamics and Thermodynamics of a Novel Phase of NaAlH4

We characterize a novel orthorhombic phase (gamma) of NaAlH4, discovered using first-principles molecular dynamics, and discuss its relevance to the dehydrogenation mechanism. This phase is close in energy to the known low-temperature structure and becomes the stabler phase above 320 K, thanks to a larger vibrational entropy associated with AlH4 rotational modes. The structural similarity of gamma-NaAlH4 to alpha-Na3AlH6 suggests it acts as a key intermediate during hydrogen release. Findings are consistent with recent experiments recording an unknown phase during dehydrogenation.Comment: 10 pages, 4 figures, 1 table + supplementary info; In press (Physical Review Letters

Assessing carbon-based anodes for lithium-ion batteries: A universal description of charge-transfer binding

Many key performance characteristics of carbon-based lithium-ion battery anodes are largely determined by the strength of binding between lithium (Li) and sp2 carbon (C), which can vary significantly with subtle changes in substrate structure, chemistry, and morphology. Here, we use density functional theory calculations to investigate the interactions of Li with a wide variety of sp2 C substrates, including pristine, defective, and strained graphene; planar C clusters; nanotubes; C edges; and multilayer stacks. In almost all cases, we find a universal linear relation between the Li-C binding energy and the work required to fill previously unoccupied electronic states within the substrate. This suggests that Li capacity is predominantly determined by two key factors -- namely, intrinsic quantum capacitance limitations and the absolute placement of the Fermi level. This simple descriptor allows for straightforward prediction of the Li-C binding energy and related battery characteristics in candidate C materials based solely on the substrate electronic structure. It further suggests specific guidelines for designing more effective C-based anodes. The method should be broadly applicable to charge-transfer adsorption on planar substrates, and provides a phenomenological connection to established principles in supercapacitor and catalyst design.Comment: accepted by Physical Review Letter

Exploring kinetics and thermodynamics in fast-ion conductors and hydrogen-storage materials using ab-initio molecular dynamics

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2007.Includes bibliographical references (p. 173-190).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.by Brandon C. Wood.Ph.D

An analysis method for conceptual design of complexity and autonomy in complex space system architectures

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Aeronautics and Astronautics; and, (S.M.)--Massachusetts Institute of Technology, Technology and Policy Program, 2001.Includes bibliographical references (p. 97-99).by Brandon C. Wood.S.M

Methane and carbon dioxide adsorption on edge-functionalized graphene: A comparative DFT study

With a view towards optimizing gas storage and separation in crystalline and disordered nanoporous carbon-based materials, we use ab initio density functional theory calculations to explore the effect of chemical functionalization on gas binding to exposed edges within model carbon nanostructures. We test the geometry, energetics, and charge distribution of in-plane and out-of-plane binding of CO2 and CH4 to model zigzag graphene nanoribbons edge-functionalized with COOH, OH, NH2, H2PO3, NO2, and CH3. Although different choices for the exchange-correlation functional lead to a spread of values for the binding energy, trends across the functional groups are largely preserved for each choice, as are the final orientations of the adsorbed gas molecules. We find binding of CO2 to exceed that of CH4 by roughly a factor of two. However, the two gases follow very similar trends with changes in the attached functional group, despite different molecular symmetries. Our results indicate that the presence of NH2, H2PO3, NO2, and COOH functional groups can significantly enhance gas binding with respect to a hydrogen-passivated edge, making the edges potentially viable binding sites in materials with high concentrations of edge carbons. To first order, in-plane binding strength correlates with the larger permanent and induced dipole moments on these groups. Implications for tailoring carbon structures for increased gas uptake and improved CO2/CH4 selectivity are discussed.Comment: 12 pages, 7 figure

Hydrogen Dynamics in Superprotonic CsHSO4

We present a detailed study of proton dynamics in the hydrogen-bonded superprotonic conductor CsHSO4 from first-principles molecular dynamics simulations, isolating the subtle interplay between the dynamics of the O--H chemical bonds, the O...H hydrogen bonds, and the SO4 tetrahedra in promoting proton diffusion. We find that the Grotthus mechanism of proton transport is primarily responsible for the dynamics of the chemical bonds, whereas the reorganization of the hydrogen-bond network is dominated by rapid angular hops in concert with small reorientations of the SO4 tetrahedra. Frequent proton jumping across the O--H...O complex is countered by a high rate of jump reversal, which we show is connected to the dynamics of the SO4 tetrahedra, resulting in a diminished CsHSO4/CsDSO4 isotope effect. We also find evidence of multiple timescales for SO4 reorientation events, leading to distinct diffusion mechanisms along the different crystal lattice directions. Finally, we employ graph-theoretic techniques to characterize the topology of the hydrogen-bond network and demonstrate a clear relationship between certain connectivity configurations and the likelihood for diffusive jump events.Comment: 12 pages, 10 figure

From Molecules to Materials: Pre-training Large Generalizable Models for Atomic Property Prediction

Foundation models have been transformational in machine learning fields such as natural language processing and computer vision. Similar success in atomic property prediction has been limited due to the challenges of training effective models across multiple chemical domains. To address this, we introduce Joint Multi-domain Pre-training (JMP), a supervised pre-training strategy that simultaneously trains on multiple datasets from different chemical domains, treating each dataset as a unique pre-training task within a multi-task framework. Our combined training dataset consists of $\sim$120M systems from OC20, OC22, ANI-1x, and Transition-1x. We evaluate performance and generalization by fine-tuning over a diverse set of downstream tasks and datasets including: QM9, rMD17, MatBench, QMOF, SPICE, and MD22. JMP demonstrates an average improvement of 59% over training from scratch, and matches or sets state-of-the-art on 34 out of 40 tasks. Our work highlights the potential of pre-training strategies that utilize diverse data to advance property prediction across chemical domains, especially for low-data tasks