Hydrogen storage in solids: materials, systems and application trends

Resume : Storing hydrogen in materials is based on the observation that metals can reversibly absorb hydrogen, however practical application of such a finding is found to be rather challenging especially for vehicular applications. The ideal material should reversibly store a significant amount of hydrogen under moderate conditions of pressures and temperatures. To date, such a material does not exist, and the high expectations of achieving the scientific discovery of a suitable material simultaneously with engineering innovations are out of reach. Of course, major breakthroughs have been achieved in the field, but the most promising materials still bind hydrogen too strongly and often suffer from poor hydrogen kinetics and/or lack of reversibility. Herein, progress made toward the practical use of hydrides as a hydrogen store and the barriers still remaining are reviewed. In this context, the new approach of tailoring the properties of hydrides through size restriction at the nanoscale is discussed. Such an approach already shows great promise in leading to further breakthroughs because both thermodynamics and kinetics can be effectively controlled at molecular levels. The effects of size restriction on the storage properties of magnesium and other complex hydrides such as LiBH4 would be discussed as well as potential core-shell strategies to design practical store based on these nanosized hydrides.

Resume : LiBH4 is a so-called ?complex hydride?, a solid consisting of a lattice of Li cations and BH4- anions. It is potentially interesting for reversible solid state hydrogen storage (containing 18.5 wt% hydrogen) and as a solid state Li-ion battery electrolyte. However, in macrocrystallline LiBH4, high hydrogen and lithium ion mobilities are only found at relatively high temperatures. For instance hydrogen can only be released at an appreciable rate above the melting point (280 oC), while appreciable Li ion mobility is only found in the high temperature hexagonal phase (above 110 oC). Nanoconfinement of LiBH4 in mesoporous scaffolds (2-20 nm pores) greatly changes its properties. [1] Confinement in turbostratic carbon led to an altered hydrogen release pathway of LiBH4 to B and LiCx, releasing the full content hydrogen at 375 ?C under Ar. Under 1 bar H2, decomposition started at 150 ?C lower than the equilibrium decomposition temperature. Confinement into SiO2 nanoscaffolds led to high hydrogen and Li local mobilities at room temperature, dominated by LiBH4 within 1-2 nm from the SiO2 pore walls [2]. Conductivity measurements show that the Li-ion conductivity of LiBH4 at room temperature is increased by more than three orders of magnitude upon nanoconfinement, making it a promising electrolyte for all solid-state batteries [3]. References: [1] Ngene, Chem. Comm. 46 (2010), 8201; [2] Verkuijlen, JPCC 116 (2012); [3] Blanchard, Adv. Funct. Mater. 25 (2015), 182

Resume : Efficient storage of hydrogen is widely recognized as a key challenge in the transition towards a hydrogen-based energy economy. We here explore the mechanochemistry under hydrogen gas of the light-weight systems Li-N-H and Li-Mg-N-H. Starting reactants were Li3N and 2Li3N+Mg, respectively, and hydrogen gas at 80 atm. In-situ hydrogen absorption curves were monitored during mechanochemical synthesis. The reaction paths were elucidated by means of ex-situ X-ray (XRD) and neutron diffraction. Starting from Li3N, hydrogen absorption of 9.8 wt.% H occurs in 2 h following two steps of equal hydrogen uptake: Li3N + 2H2 -> Li2NH + LiH +H2 -> LiNH2 + 2LiH. Interestingly, the second step entails the formation of non-stoichiometric imide phases. Starting from 2Li3N+Mg powder mixture, two reactions steps of 3.9 and 4.7 wt.% were noticed during 4 h of milling. The reaction path also entails two steps: 2Li3N + Mg + 5H2 -> Li3MgN2H + 3LiH +3H2 -> Mg(NH2)2 + 6LiH. The metastable mixed-cation imide Li3MgN2H is formed as intermediate and magnesium amide is obtained in amorphous state. The structural and hydrogenation properties of the metastable Li3MgN2H and amorphous Mg(NH2)2 phases where further studied by in-situ neutron diffraction using deuterated samples. Results will be unveiled during the conference. Mechanochemistry under hydrogen gas is an efficient method for fast synthesis of light-weight hydrides leading to the formation of novel metastable phases with unexplored properties.

Resume : Ammine metal borohydrides, M(BH4)m∙nNH3, have recently received significant attention owing to their promising hydrogen storage properties [1] We present more than 30 new halide-free and solvent-free ammine metal borohydrides based on M = Mg, Ca, Sr, Mn, Y, La, Ce, Gd and Dy. The compounds are synthesized by combining solvent-based techniques, mechanochemistry, solid-gas reactions and thermal treatment. This allows to efficiently control the NH3/BH4 (n/m) ratio, e.g. the first long series of Y(BH4)3∙nNH3 (n = 1, 2, 4, 5, 6 and 7) is presented,2 where an increased hydrogen purity is obtained for lower n/m ratios [2,3]. The structures of all new compounds are solved from powder X-ray diffraction and subsequently optimized by DFT calculations. Interestingly, destabilization is observed for most metal borohydrides with low electronegativity, while metal borohydrides with high electronegativity are stabilized by NH3 [1,4]. We propose a new mechanism for gas release, which depends on the ammonia release temperature and the stability of the metal borohydride, which will be discussed in more detail. References [1] L. H. Jepsen. Mater. Today 2014, 17, 129–135. [2] L. H. Jepsen. Submitted 2015. [3] L. H. Jepsen. ChemSusChem 2015. [4] L. H. Jepsen. Submitted 2015.

Resume : Lithium-magnesium alloys are the lightest metallic alloy with a density of 1.35-1.65 g/cm3. Hydrogenation of lithium-magnesium alloy appears difficult and is rarely reported in the literature. An attempt to synthesize LiMgH3 was made by substitution of Na by Li in LixNa1-xMgH3 but there was no evidence of the perovskite phase LiMgH3 [1]. The authors suggested that LiMgH3 is impossible to form under solid state conditions due to geometric restrictions. The synthesis of a series of novel Lithium-Magnesium hydride compounds is reported using a unique ultra-high vacuum high throughput physical vapour deposition, HT-PVD [2], [3]. The behaviour of the alkali-earth binary hydride has been characterized by Temperature Programmed Desorption (TPD) analysis and an optimum composition with a high gravimetric capacity (> 10 wt.%) was identified. This system shows a reversibility under very mild condition (10 bar H2, room temperature). A high-energy reactive milling of a Li-Mg alloy was subsequently attempted and the result of the trial will be discussed. References [1] K. Ikeda, Y. Nakamori, S. Orimo, Acta Mater. 53 (2005) 3453 [2] Patent WO 2009/101046 [3] B.E. Hayden, J-Ph. Soulié et al., Faraday Discuss. 151 (2011) 369

Resume : The interaction of H2 with carbon-based materials has been studied intensively for the adsorption-based characterisation of nanoporous adsorbents but also as a result of the interest in using such substrates for hydrogen storage. Porous carbon materials can in general achieve adequate gravimetric storage but only at cryogenic conditions (e.g. 77 K) due to the weak interactions involved. Physisorption forces may be enhanced in narrow micropores, with a size close to the kinetic diameter of H2 molecules. H2 molecules, confined in very narrow spaces at low temperatures, cannot be treated as classical Lennard-Jones particles, as quantum effects become significant. Indeed, the quantum nature of H2 (and its isotopes) gives rise to the so-called quantum molecular sieving, which can be exploited for e.g. the separation of H2/D2 mixtures. We herein aim at providing additional insight into the crucial effect of pore size and pressure on the adsorption of H2 (and D2) in porous carbons by Grand Canonical Monte Carlo simulations in model slit micropores at 77 K. GCMC results are also coupled with experimental high pressure H2 (and D2) adsorption data at 77K for different nanoporous carbons.

Resume : There is a considerable interest in carbon coating of metal hydrides for energy storage purposes. This might result in improved thermal conductivity of metal hydrides, an aspect which is of considerable interest in Mg based hydrogen storage tanks. In batteries, such coatings might improve the electrical conductivity, thus obviating the need for the special additives used for such purposes in the electrode make-up. In the current work, following a successful synthesis of Mg2Ni and Mg nanoparticles using thermal plasma [1], we investigate whether in-situ encapsulation of such particles with carbonaceous material would be possible with the same method. [1] B Aktekin, G Çakmak and T Öztürk, Induction thermal plasma synthesis of Mg2Ni nanoparticles. Int. J of Hydrogen Energy, 39, 2014,9859.

Resume : The Mg-Si-H system could be used for a range of practical applications including mobile transport since thermodynamic calculations indicate that it has equilibrium conditions of 1 bar of hydrogen pressure at room temperature. Experimentally, these conditions have never been met, for either absorption or desorption, indicating that reaction kinetics play a dominant role in the reaction. Presented here is a kinetic hydrogen desorption study involving MgH2 with Si prepared using different methods to obtain different crystallite sizes (or grain size in the case of amorphous Si nanoparticles). An empirical understanding of the relationship between crystallite size and reaction kinetics for the dehydrogenation of MgH2 in the presence of Si was determined. It was found that there is a strong correlation between crystallite size and activation energy for the growth of the Mg2Si phase and the three dimensional Carter-Valensi (or contracting volume) diffusion model could be used to describe the rate limiting step of the reactions. A reaction mechanism has been proposed showing that nucleation occurs at the surfaces/interfaces at a fast rate followed by slow diffusion of H out of the Mg matrix whilst Mg bonds with the Si as it moves through the Si matrix to form Mg2Si.

Resume : Magnesium hydride (MgH?2) is noteworthy because of its dual use as both a hydrogen storage material and as a potential battery electrode material. This work focuses specifically on characterizing the hydrogenation of Mg thin films (≈200nm) deposited by ion beam sputtering. Hydrogenation of the Mg layer was studied in the temperature range of 100 to 300?C in a H2 atmosphere up to 100bar. Monitoring the lattice structure by XRD allowed us to gain insight into the sorption of hydrogen and the subsequent hydride formation. TEM measurements before and after hydrogenation demonstrated the changes in microstructure. It is known that the creation of MgH2 creates a ?blocking effect? that slows down hydrogenation kinetics. In the presented experiment, complete hydrogenation of a 200nm Mg layer was achieved after 3 hours at 300?C under a 10bar H2 atmosphere. Performing the hydrogenation in smaller steps we determined the hydrogenation kinetics. In order to accelerate the absorption/desorption process, small amounts of Pd catalyst (5-20nm of Pd) were also deposited on top of the Mg layer. The observed kinetics were compared with volume diffusion and interface diffusion models.

Resume : Mixes of carbon nanotubes (CNT) are generally recognized as perspective materials for adsorption of gas molecules. However, application of such materials for efficient hydrogen storage is under question since it was found that materials based on undoped CNTs possess low hydrogen sorption capacity [1]. An intensive search for modifications of the CNT structures that can improve hydrogen uptake by materials, in particular by doping the CNTs with non-isovalent ions, is currently in progress. In this search, using the first-principles computational studies of molecular adsorption on the CNT surfaces is a very advantageous approach since the calculations can independently predict sorption capabilities of the CNT-based materials. In this work, computational studies are applied to estimate the hydrogen storage capabilities of two types of doped CNT-based materials, namely the boron-doped and the nitrogen-doped ones. Adsorption of hydrogen molecules on the surfaces of undoped and B(N)-doped various chiralities CNTs as well as on graphene surface is studied by the DFT-based electronic structures quantum-chemical calculations. The relaxed geometries, binding energies of the molecules to the CNTs, density contours of electronic vawefunctions, dependencies of binding energies on tube- molecule distance are obtained and analyzed in view of studied materials potential application for hydrogen storage. [1] S.H. Barghi, T.T. Tsotsis, M. Sahimi, Int. J. of Hyd

Resume : The main aim of new metal-organic material investigation was to set alternative adsorbent for hydrogen storage. It is why, mono anionic mono dentate oratato complex of Co(II) was synthesized. The final molecular formula of the compound is Co(H3Or)2.nH2O which was synthesized according to room temperature method. The characterization process carried out Uv-vis, FT-IR, elemental and thermal analysis. Also the crystal structure was refined by using single crystal XRD data. Hydrogen storage property of the compound was measured by using HPVA at 77 K and up to 100 bars pressure. In the other hand, final crystal structure was used to simulate hydrogen storage property theoretically. GCMC ensemble with LJ potentials was used to simulate hydrogen storage property of the material. In addition, HOMO and LUMOs were determined for repeated molecular structure for clarification of adsorption process. It is found that the compound could uptake 2.2 wt. % hydrogen at 77 K and 85 bars pressure experimentally. It is also found, the hydrogen molecules placed close to LUMOs and the most important factor for storage process was accessible surfaces according to simulations.

Resume : Magnesium and titanium hydrides (MgH¬2, TiH2) are good candidate materials for hydrogen storage. Furthermore, MgH2 shows promise as a potential battery electrode material. In thin film geometry, both materials may also be used as optical switches. In our work, we used thin films with a defined thickness in order to quantitatively determine diffusion coefficients and measure kinetic barriers at the interfaces. Mg and Ti thin films (≈200 nm) were deposited using ion beam sputtering. Hydrogenation of these layers was studied at temperatures up to 300°C under 10bars of H2 atmosphere for different durations. Microstructural changes were studied by TEM and hydrogen sorption was quantified by XRD, with emphasis on quantitatively comparing the behavior of both materials.

Resume : Among various nanostructures used as potential material for hydrogen storage, MgH2 thin films attract attention for several reasons, easy way of synthesis and low reactivity not being least important. Moreover, thin films offer the possibility to study influence of microstructure and additives on hydrides sorption properties in controlled way. The mechanism of desorption from thin films the is described by nucleation and growth process, but in capped films an interface mechanism is proposed as well. To clarify the mechanism of reaction, in situ desorption from MgH2-TiO2 films coupled with optical microscopy were used, followed by numeric simulations. On the other hand, titania draws a lot of interest because of its non-toxicity, safe usage and low cost. Further, it has the same (rutile) structure as MgH2, with similar lattice parameters. The interaction of hydrogen with catalyst (TiO2- (110) – (1x1) surface were investigated using PAW method as implemented in Abinit code. The hydrogen diffusion behavior and the thermodynamic properties were calculated by means of the full relaxation of the structure in every step of bulk diffusion, followed by activation energies calculations using NEB method. The results show the existence of potential barriers close to every atomic layer and the trends of barriers and overall system energy lowering away from surface. In-situ optical study shows that nucleation process depends on sample thickness.