Session D26: Focus Session: Physics of Energy Storage Materials - Catalysis and H2 Storage

Design Principles for Oxygen Reduction and Evolution on Oxide Catalysts

Driven by growing concerns about global warming and the depletion of petroleum resources, developing renewable energy production and storage technologies represent one of the major scientific challenges of the 21$^{st}$ century. A critical element in pursuit of this quest is the discovery of efficient and cost-effective catalysts used in solar fuel production via electrochemical energy conversion processes such as oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), both of which are central to the efficiencies of direct{\-}solar and electrolytic water-splitting devices, fuel cells, and metal-air batteries. Although the Sabatier's principle provides a qualitative argument in tuning catalytic activity by varying the bond strength between catalyst surface and reactant/product (neither too strong nor too weak leading to the maximum activity at moderate bond strength), it has no predictive power to find catalysts with enhanced activity. Identifying a ``design principle'' that links catalyst properties to the catalytic activity is critical to accelerate the search for highly active catalysts based on abundant elements, and minimize the use of precious metals. Here we establish a molecular principle that governs the activities of oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) for oxide catalysts, where the activities primarily correlate to the $\sigma $* orbital (``e$_{g}$'') occupation of surface transition{\-}metal cations established by systematic examination of more than ten to fifteen transition{\-}metal oxides. The intrinsic ORR and OER activities exhibit a volcano-shaped dependence on the e$_{g}$ occupancy and the activities peak at an e$_{g}$ occupancy close to unity. Our findings reflect the critical influence of the $\sigma $* orbital on the energetics of surface reaction intermediates on surface transition metal ions such as the O$_{2}^{2-}$/OH$^{-}$ displacement and the OH$^{-}$ regeneration, and thus highlight the importance of surface oxide electronic structure in controlling catalytic activities. Using the established molecular principle, we further demonstrate that an alkaline earth cobalt oxide with a chemical formula of Ba$_{0.5}$Sr$_{0.5}$Co$_{0.8}$Fe$_{0.2}$O$_{3{\-}\delta }$ (BSCF), catalyzes the OER with intrinsic activity that is at least an order of magnitude higher than the state-of-the-art iridium oxide catalyst in basic solutions. \\[4pt] [1] J. Suntivich, H.A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B. Goodenough and Y. Shao-Horn, Design Principles for Oxygen Reduction Activity on Perovskite Oxide Catalysts for Fuel Cells and Metal-Air Batteries, Nature Chemistry, \underline {3}, 546--550 (2011).\\[0pt] [2] Jin Suntivich, Kevin J. May, Hubert A. Gasteiger, John B. Goodenough and Yang Shao-Horn, A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles, ScienceExpress, Science DOI: 10.1126/science.1212858, (2011).   

Modeling of hydrogen evolution reaction on the surface of GaInP$_{2}$

GaInP$_{2}$ is promising candidate material for hydrogen production using sunlight. It reduces solvated proton into hydrogen molecule using light-induced excited electrons in the photoelectrochemical cell. However, it is challenging to model hydrogen evolution reaction (HER) using first-principles molecular dynamics. Instead, we use Anderson-Newns model and generalized solvent coordinate in Marcus-Hush theory to describe adiabatic free energy surface of HER. Model parameters are fitted from the DFT calculations. We model Volmer-Heyrovsky reaction path on the surfaces of CuPt phase of GaInP$_{2}$. We also discuss effects of surface oxide and catalyst atoms that exist on top of bare surfaces in experimental circumstances.   

In situ Raman Evidence for Reversible Room-Temperature Hydrogenation in Pt-doped Carbons

Atomic hydrogen spillover was investigated by in situ Raman spectroscopy and density functional theory. In the presence of Pt nanoparticles, modes related to Basal plain hydrogenation were observed for activated carbon and graphene, respectively, during Raman measurements in 100 bar H2. The modes were absent when Pt, carbon, or H2 were omitted from the experiment. Substituting H2 with D2 led to the expected isotopic shift for a hydrogen-dominated vibrational mode. The mode disappeared and reappeared over several cycles of exposure to H2 or He at room temperature, consistent with room-temperature reversibility of H chemisorbed to the activated carbon in the presence of Pt nanoparticles. Reversibility apparently arises from a facilitated transition of H from a chemisorbed state to a more mobile physisorbed state, followed by recombination and release. Reversibility for Pt/graphene was much less pronounced, suggesting that structural heterogeneities in the sample carbon support and/or catalyst-carbon contact are important factors.   

Hydrogen storage on calcium coated boron (hetero-)fullerenes: A DFT study

Using density functional theory (DFT), we investigate hydrogen storage properties of calcium-coated molecular systems of B$_{80}$ boron fullerene, C$_{48}$B$_{12}$ boron-doped heterofullerenes, and well-known C$_{60}$ fullerene. Here, we consider the most common and low-lying isomers of B$_{80}$ and C$_{48}$B$_{12}$. We find that the Ca-coated molecules have the following properties: (\emph{1}) The binding of metal atoms to B$_{80}$ or to C$_{48}$B$_{12}$ molecules is much stronger than their binding to a C$_{60}$ molecule. (\emph{2}) B$_{80}$ and C$_{48}$B$_{12}$ have larger electron affinities than their carbon only counterpart, and accordingly discharge the surface Ca atoms more efficiently. (\emph{3}) B$_{80}$ molecule, however, shows structural deformations upon reacting with Ca atoms. (\emph{4}) C$_{48}$B$_{12}$Ca$_{6}$, however, is stable at elevated temperatures. C$_{48}$B$_{12}$ has well-exposed, positively charged Ca atoms on its surface, and binds up to six hydrogen molecules per metal center with hydrogen binding energies of 0.17-0.14 eV/H$_{2}$, that are suitable for ambient temperature hydrogen storage.   

Role of nano in catalysis: Pd catalyzed H desorption from MgH$_{2}$

Magnesium hydride (MgH$_{2})$ is promising for on-board hydrogen (H) storage with the major hurdle being the slow desorption kinetics. H desorption from ball-milled MgH$_{2}$ peaks at two slightly different temperatures, which further split in the presence of palladium catalyst. It has been experimentally demonstrated that nanostructuring can eliminate the high temperature peak. However, the effect of nanostructuring cannot be explained by thermodynamic destabilization due to quantum size effect. Our first-principles calculation reveals that there exist two reaction pathways for H desorption from MgH$_{2}$. One involves H vacancy (SV) diffusion at surface, while the other one involves H atom diffusion in bulk. The SV pathway self-terminates as dehydrogenation eventually eliminates the exposed MgH$_{2}$ region. Therefore, it is size-sensitive and fully functions only when the surface-to-bulk ratio is large, which is available only in nanostructures. Our calculation further shows that the SV pathway significantly lowers the desorption barrier, because it decouples the H transport process with the surface liftoff process and benefits from a fact that diffusion of vacancies at surface can have significantly lower barrier than that in bulk.   

Particle-size dependence of the activation energy for decomposition of lithium amide

Lithium amide (LiNH$_{2})$ is a promising material for reversible hydrogen storage, yet atomistic mechanisms behind the dehydrogenation process are unknown. The activation energy for LiNH$_{2}$ decomposition has been observed to strongly vary with ball milling, suggesting a dependence of the thermodynamics and kinetics of the decomposition on the particle size. We have examined these mechanisms based on first-principles calculations for native point defects and defect complexes in LiNH$_{2}$. We propose that the decomposition of LiNH$_{2}$ into lithium imide (Li$_{2}$NH) and ammonia (NH$_{3})$ occurs through two competing mechanisms, one involving the formation of native defects in the interior of the material and the other at the surface. As a result, the prevailing mechanism and hence the activation energy depend on the surface-to-volume ratio, or the specific surface area, which changes with the particle size. We explain the observed variations of activation energy, and address the role played by LiH in the dehydrogenation of (LiNH$_{2}$+LiH) mixtures. The relationship between the structure of hydrogen-related defects and the end products in the decomposition reaction can be extended to other complex hydrides.   

Deliquescence of NaBH$_4$ computed from density functional theory

Complex hydrides are promising hydrogen storage materials and have received significant attention due to their high hydrogen-capacity. The hydrolysis reaction of NaBH$_4$ releases hydrogen with both fast kinetics and high extent of reaction under technical conditions by using steam deliquescence of NaBH4. This catalyst-free reaction has many advantages over traditional catalytic aqueous phase hydrolysis. The first step in the reaction is deliquescence, i.e. adsorption of water onto NaBH$_4$ surface and then formation of a liquid layer of a concentrated NaBH$_4$ solution, which is quickly followed by hydrogen generation. We have used periodic plane wave density functional theory to compute the energetics and dynamics of the initial stages of deliquescence on the (001) surface of NaBH$_4$. Comparison of results from standard generalized gradient approximation functionals with a dispersion-corrected density functional show that dispersion forces are important for adsorption. We used DFT molecular dynamics to assess the elementary steps in the deliquescence process.   

Electronic Structure and Molecular Dynamics Calculations for KBH$_{4}$

In the search for hydrogen storage materials, alkali borohydrides MBH$_{4}$ (M=Li, Na, K) are especially interesting because of their light weight and the high number of hydrogen atoms per metal atom. Electronic structure calculations can give insights into the properties of these complex hydrides and provide understanding of the structural properties and of the bonding of hydrogen. We have performed first-principles density-functional theory (DFT) and tight-binding (TB) calculations for KBH$_{4}$ in both the high temperature (HT) and low temperature (LT) phases to understand its electronic and structural properties. Our DFT calculations were carried out using the VASP code. The results were then used as a database to develop a tight-binding Hamiltonian using the NRL-TB method. This approach allowed for computationally efficient calculations of phonon frequencies and elastic constants using the static module of the NRL-TB, and also using the molecular dynamics module to calculate mean-square displacements and formation energies of hydrogen vacancies.   

Dehydrogenation of LiBH$_4$ nanoclusters: A first-principles study

Recent experimental studies\footnote{A.F. Gross et al. J. Phys. Chem. C 112, 5651, 2008}$^,$\footnote{X. Liu et al. Chem. Mater. 23, 1331, 2011} show faster desorption kinetics, improved reversibility and more favorable thermodynamics for confined LiBH$_4$ nanoparticles than the bulk. Using density functional theory calculations, we first discuss the geometries and energetics of LiBH$_4$, LiH, LiB, Li and B clusters. Secondly, we study the effects of particle size on the decomposition pathway of LiBH$_4$ clusters. Our calculations show that only very small clusters of LiBH$_4$ (up to 12 formula units) are significantly destabilized relative to the bulk. High stability of small clusters of LiBH$_4$ originates from the fact that surface energies are very low for bulk LiBH$_4$. (100), (010), (101) and (011) surfaces are almost degenerate with surface energies of 0.113, 0.102, 0.115 and 0.097~J/m$^2$, respectively. Clusters of LiH, LiB, Li and B are more strongly destabilized than the LiBH$_4$ clusters upon decreasing the cluster size. We show that, in contrast to the bulk, destabilized clusters of LiBH$_4$ decompose to (LiB)$_n$ clusters. Finally, we present some of our preliminary NMR chemical shift results for different LiBH$_4$ surface terminations.   

First-principles Modeling of Diffusion during Hydrogenation of LiBH$_4$

LiBH$_4$ has been studied extensively because of its high volumetric and gravimetric hydrogen content. However, experiments show that hydrogen release is very slow at temperatures up to 300 C, which severely limits applications in mobile storage. Using density-functional theory calculations, we systematically study bulk diffusion of defects during solid-state hydrogenation reactions. The defect concentration and concentration gradients are calculated for a variety of defects, including charged vacancies and interstitials. We find that low concentration gradients limit the rate of hydrogen desorption.   

Bulk diffusion of defects in LiAlH4/Li3AlH6

From various experimental studies on decomposition of hydrogen storage materials it has been proposed that bulk diffusion of metal species may be the rate limiting step in hydrogen storage reactions. A recently developed theoretical model uses density functional theory to study the underlying processes involved in bulk diffusion. To date this model has been applied to study dehydrogenation of NaAlH4. However further study of alkaline and alkali earth metals merits attention, particularly LiAlH4 with a high gravimetric hydrogen density, accessible at moderate temperatures. This study uses density functional theory to obtain concentration gradients and diffusivities of native charged and neutral defects in Li3AlH6 and LiAlH4. The flux of each defect is obtained and thus the activation energy for each defect. Our results show that diffusion of metal species is a possible rate limiting process in the system.   

First-Principles Study of the Li-Na-Ca-N-H System: Compound Structures and Hydrogen-Storage Properties

Mixed-metal amides and imides are being widely investigated as potential hydrogen storage materials. Using a combination of first-principle DFT calculations, grand-canonical linear programming, and prototype electrostatic ground state (PEGS) approaches, we predict hydrogen storage reactions in the Li-Na-Ca-N-H system. The enthalpies, entropies, static, zero-point, and T\textgreater{}0K vibrational energies of known compounds together with our predictions of some incomplete experimental crystal structures are presented.