Session P20: Focus Session: Physics of Energy Storage Materials IV -- Complex Hydrides and Methane

NaAlH$_{4}$ -- Carbon Aerogel: Kinetic Enhancement of a Complex Hydride by Nanoporous Carbon

Complex hydrides promise high gravimetric and volumetric hydrogen storage densities, but considerable modification of their thermodynamic and kinetic properties will be required in order to make them feasible for on-vehicle applications. Catalyst additions to achieve fast hydrogen cycling kinetics have been studied for more than a decade. More recently, the concept of nanoconfinement has been explored as a means to improve kinetics, using melt infusion or solvent infusion to embed the hydride into nanoscale pores within a solid. We have achieved enhanced kinetic performance and reversibility of NaAlH$_{4}$ incorporated into nanoporous carbon aerogel by melt infusion, even in the absence of a catalyst. In fact, hydrogen cycling of uncatalyzed NaAlH$_{4}$ in aerogel is almost as good as unconfined NaAlH$_{4}$ catalyzed by addition of TiCl$_{3}$. It remains challenging, however, to obtain NaAlH$_{4}$-carbon aerogel infusions with high hydride loading and/or co-incorporated catalyst. We have therefore investigated combining NaAlH$_{4}$ with carbon aerogel and nanoporous activated carbon by ball milling. The kinetic performance is similar to that of melt-infused NaAlH$_{4}$ at the same loading, and importantly, higher NaAlH$_{4}$ loading can be easily achieved with only modest loss of kinetics. Furthermore, TiCl$_{3}$ catalyst can be easily co-incorporated. In the latter case, a small but significant improvement over TiCl$_{3}$-catalyzed NaAlH$_{4}$ without carbon is observed.   

Hydrogen release reactions in the {\{}H,Li,B,Na,Al{\}} system

A thermodynamic investigation of the {\{}H,Li,B,Na,Al{\}} system for new solid state hydrogen storage reactions is performed using first-principles DFT calculations and the the grand-canonical linear programming approach (Akbarzadeh, et al. Adv. Mater. 2007, 19, 3233). We report the static, zero-point, and T $>$ 0 K vibrational energies of all known compounds in this system. Enthalpies, entropies, and hydrogen release temperatures are calculated for all thermodynamically reversible dehydrogenation reactions occurring from 0-1000K. Several novel mixtures of reactants with high gravimetric hydrogen storage densities are found using the calculated {\{}H,Li,B,Na,Al{\}} phase diagrams.   

First-principles investigations of the quaternary Li-Zn-B-H hydrogen storage system

Mixed metal borohydride hydrogen storage materials are a new class of materials which may possess better thermodynamic and kinetic properties than their separate phases. $LiBH_4$ has an undesirably high $H_2$ desorption temperature, while $Zn(BH_4)_2$ has a lower desorption temperature, but releases $B_2H_6$ upon desorption. We have used density functional theory, as well as Monte Carlo-based crystal structure prediction tools and phase diagram computational methods to explore the stability and decomposition reactions of mixed Li and Zn borohydrides to ascertain whether they possesses an intermediate decomposition temperature. Based on a combination of classical potentials, Monte Carlo optimization, and DFT calculations, we search for low-energy quarternary borohydrides as a function of the Li/Zn context. We find that this system has compounds that are lower in energy than the isolated borohydrides. In agreement with prior work, we confirm the existence of a $LiZn(BH_4)_3$ compound, which as yet has been unobserved. We find that this new mixed compound $LiZn(BH_4)_3$ decomposes via an initial decomposition of $Zn(BH_4)_2$ , and a subsequent decomposition of $LiBH_4$. This sequential decomposition is favored due to the lack of stable intermediate products which involve both Li and Zn. Using this framework, we are searching for stable mixed metal borohydrides in a wide variety of other systems.   

First-principles studies of intermediate products in the decomposition of metal amidoboranes

Metal amidoboranes [MAB, M=metal cation] form an interesting class of recently-discovered hydrogen storage compounds. However, the decomposition products remain largely unknown. Armed with the combination of the prototype electrostatic ground state search and density-functional theory methodology (PEGS+DFT), we have searched for crystal structures of possible reaction products with [NHBH$_2$]$^-$, [NBH]$^-$, [NBH$_5$]$^-$, polymer-[NHBH$_2$] anion groups in the decomposition of LiAB and CaAB. All these reaction pathways are significantly endothermic, which is in disagreement with the experimentally measured enthalpies in these systems, which are found to be nearly thermoneutral [$-3\sim-5$ kJ/(mol H$_2$) in LiAB and 3.5 kJ/(mol H$_2$) in CaAB]. Using newly developed dianion group [NHBHNHBH$_3$]$^{2-}$, our PEGS+DFT methodology predicts structures and energies of Li/Ca-dianion compounds. Including vibrational thermodynamics and zero-point effects, we successfully obtain a nearly thermoneutral enthalpy of decomposition into these dianion compounds. This agreement lends strong support to the dianion phases as energetically preferred products in the decomposition of metal amidoboranes.   

Theoretical study of the vibrational properties of ${\rm NaAlH_4}$ with ${\rm AlH_3}$ vacancies

We investigate from first-principles calculations the vibrational properties in the presence of the ${\rm AlH_3}$ vacancy in both $\alpha$ and $\gamma$ phases of ${\rm NaAlH_4}$. When ${\rm AlH_3}$ is removed from an ${\rm AlH_4^-}$ anion, the remaining H recombines with another neighboring ${\rm AlH_4^-}$ anion and forms an ${\rm AlH_5^{2-}}$ unit with slightly deformed $D_{3h}$ symmetry. For both $\alpha$- and $\gamma$-${\rm NaAlH_4}$, the ${\rm AlH_3}$ vacancy induces several isolated phonon modes that are highly localized on the ${\rm AlH_5^{2-}}$ unit with frequencies within the band gap separating the Al-H stretching modes and Al-H bending modes in pure ${\rm NaAlH_4}$. Similar localized phonon modes also exist in the gap separating the Al-H bending modes and the modes involving the rotation of ${\rm AlH_4^-}$ anions for the $\gamma$ phase. On the other hand, for both $\alpha$ and $\gamma$ phases of ${\rm NaAlH_4}$ with charged ${\rm AlH_4^-}$ vacancies, no isolated phonon modes were found to be localized in the vacancy region with frequencies within the band gap of the pure crystal. These theoretical findings suggest further experimental studies to identify the defects that are involved in the decomposition of ${\rm NaAlH_4}$.   

Hexagonal Antiprismatic Metallacarborane Clusters for Hydrogen Storage

We investigated the adsorption properties of molecular hydrogen attached to hexagonal antiprismatic metallacarborane clusters, RuNiC$_2$B$_{10}$H$_{12}$ and Ru$_2$C$_2$B$_{10}$H$_{12}$, using density functional theory. These clusters have been recently synthesized using the reduction-metallation (RedMet) approach [1] and their structures have been resolved. The hydrogen molecules are sequentially attached to these clusters until the H$_2$ binding energies fall below 0.2 eV, which is the minimum value of ideal H$_2$ binding energy in the range of 0.2-0.4 eV/H$_2$ for the practical vehicle applications [2]. We included the van der Waals interactions between metallacarborane clusters and molecular hydrogens. We also evaluated the contribution of zero point vibrational energies to the H$_2$ binding energy. The kinetic stability of these clusters before and after hydrogen adsorption is discussed by analyzing the energy gap. The results show that RuNiC$_2$B$_{10}$H$_{12}$ and Ru$_2$C$_2$B$_{10}$H$_{12}$ clusters can bind up to 8.5 wt \% and 9.8 wt \% molecular hydrogen, respectively. These results suggest that these metallacarborane clusters are potential hydrogen storage materials to meet the targets of DOE for 2015. \\[4pt] [1] D. Ellis et al., Chem. Commu. {\bf 14}, 1917 (2005).\\[0pt] [2] http://www.sc.doe.gov/bes/hydrogen.pdf.   

Diffusion-limited Kinetic Pathway for Hydrogen Release from LiNH$_{2}$/LiH

From experimental work on decomposition of hydrogen storage materials it has been suggested that bulk diffusion of metal species is the bottleneck for hydrogen release. In this work we study the underlying mechanism for diffusion reactions in the dehydrogenation of LiNH$_{2}$. Using first-principle, density functional theory methods we have calculated concentration gradients and diffusivities of neutral and charged defects in LiNH$_{2}$ and Li$_{2}$NH phases. The overall activation energy is obtained from these calculations. The calculated activation energies are found to agree well with experimental work on the kinetics of LiNH$_{2}$ decomposition, suggesting that diffusion of metal species is a possible method for dehydrogenation of Lithium Amide.   

Reaction Pathways in the Reactive Composite Mg(NH$_{2})_{2}$ + LiH

Chen \textit{et al} [1] reported reversible hydrogen storage in a mixture of LiH + LiNH$_{2}$ with a storage capacity of 6.5 wt {\%}. However, this system requires an operating temperature in excess of 250 C to achieve a hydrogen pressure of 1 bar. Several efforts including cation substitution have been considered in order to improve the operating conditions, which is necessary for onboard applications. For instance, replacing LiH with MgH$_{2}$ markedly reduces the operating temperature through the reaction MgH$_{2 }$+ 2LiNH$_{2} \quad \to $Li$_{2}$Mg(NH)$_{2 }$+ 2H$_{2} \quad \leftrightarrow $Mg(NH$_{2})_{2}$ + 2LiH. Recent experimental results however indicate that the latter is not a simple one-step reaction and full hydrogenation of Li$_{2}$Mg(NH)$_{2}$ occurs in a two-step sequence via an intermediate Li$_{2}$Mg$_{2}$(NH)$_{3}$ [2]. In this work we examine the stability and structure of possible intermediates compounds, namely Li$_{2-2x}$Mg$_{x}$NH, Li$_{1-2x}$Mg$_{x}$NH$_{2}$, and Li$_{2-x}$Mg(NH)$_{2-x}$(NH$_{2})_{x}$, by means of first-principles DFT calculations. All intermediate compounds are thermodynamically stable with respect to the elements. The hydrogenation reaction of Li$_{2}$Mg(NH)$_{2}$ via the intermediate imides Li$_{2-2x}$Mg$_{x}$NH is energetically favorable compared to other intermediates.\\[0pt] Ref~: [1] Nature \textbf{420}, 302 (2002). [2] J. Phys. Chem. C \textbf{113}, 15772 (2009).   

First-Principles Study of Native Defects in Li$_4$BN$_3$H$_{10}$ Under Varied Chemical Conditions

Hydrogen desorption from many complex hydrides, such as Li$_4$BN$_3$H$_{10}$, is known to be kinetically limited. At temperatures below melting, the motion of point defects is one possible factor affecting chemical reactions. Therefore, an understanding of their formation and migration will yield insight into the kinetic limitation of hydrogen desorption. To explore this, we have determined the 0~K formation energy for a number of neutral and charged point defects in Li$_4$BN$_3$H$_{10}$ under a variety of chemical conditions via density functional theory calculations. We determined chemical potentials based on thermodynamically predicted hydrogen desorption reactions and provide a physical interpretation of the resulting equilibrium conditions. Our results indicate that: 1) The lowest energy defect varies with chemical conditions. 2) neutral defects are always lower energy than analogous pairs of oppositely charged defects. 3) Hydrogen defects are rarely the lowest energy defect.   

First-principles Modeling of Diffusion Reactions in the Hydrogenation of NaAlH$_{4}$

The hydrogenation of NaAlH$_{4}$ has been studied extensively since it was discovered that doping with Ti greatly increases its reversible hydrogen storage capacity. Experimental studies have suggested that diffusion of metal-containing defects may be the rate-limiting step in this reaction. We present a model to study the diffusion of defects during a solid-state reaction and apply it to this hydrogen storage reaction. The flux of defects in simple, model systems is calculated and from these values the activation energy for these processes is determined. We find that the activation energy for the diffusion of metal defects matches well to the experimental activation energy for the reaction when doped with Ti. The model that is presented can easily be applied to other systems in which a reaction takes place in the solid state.   

Catalytic effect of carbon nanomaterials on light metal hydride systems

Carbon nanomaterials are becoming recognized for their use in catalyzing hydrogen desorption from light metal hydride systems, in particular complex borohydrides and alanates. For example, it was shown by us that graphene, carbon nanotubes, and especially fullerenes can improve the hydrogen sorption properties of sodium alanate [Nano Lett. 9, 1501 (2009)]. In parallel to ongoing experimental investigations, we have carried out further theoretical studies in order to better understand the underlying catalyzing mechanism. Our most recent work is concentrated on the interaction of lithium borohydride with fullerene where a complete dehydrogenation process was simulated using the cluster approach. Furthermore, the catalytic effect of graphene nanofibres on sodium alanate has been experimentally demonstrated by our collaborators, and we have studied this system from first principles as well, to better understand the origin of its catalytic effect.   

Adsorbed Natural Gas Storage in Optimized High Surface Area Microporous Carbon

Adsorbed natural gas (ANG) is an attractive alternative technology to compressed natural gas (CNG) or liquefied natural gas (LNG) for the efficient storage of natural gas, in particular for vehicular applications. In adsorbants engineered to have pores of a few molecular diameters, a strong van der Walls force allows reversible physisorption of methane at low pressures and room temperature. Activated carbons were optimized for storage by varying KOH:C ratio and activation temperature. We also consider the effect of mechanical compression of powders to further enhance the volumetric storage capacity. We will present standard porous material characterization (BET surface area and pore-size distribution from subcritical N$_{2}$ adsorption) and methane isotherms up to 250 bar at 293K. At sufficiently high pressure, specific surface area, methane binding energy and film density can be extracted from supercritical methane adsorption isotherms. Research supported by the California Energy Commission (500-08-022).   

On the reversibility of the adsorption of methane-mercaptan for natural gas storage

Methane is the main constituent of natural gas (NG). As fuel for vehicular applications NG requires sorbents that allow efficient, reversible and safe storage at room temperature and moderate pressure. To enable easy human detection of gas leaks the fuel gas should be added with compounds having low odor threshold, such as thiols (mercaptans). Thus a full understanding of the behavior of methane-mercaptan mixtures is necessary for the development of safe storage systems. In this talk we present results of molecular dynamics simulations in the temperature range 150--350 K and for a large range of methane partial pressures, up to the saturation pressure of methane. We observe the presence of 2D (and to a lesser degree 3D) diffusion of the thiols indicating that though thiols adsorb preferentially relative to methane, the adsorption is still reversible. We estimate that only a small increase in mercaptan concentration is necessary for the desorbed phase to be above the threshold for human detection.