Beyond hydrogen storage – Metal hydrides as multifunctional materials for energy storage and conversion

Resume : Solar driven plants can produce electric power efficiently and at low cost only if equipped with suitable thermal energy storage systems. Thermal energy storage systems based on metal hydrides have several advantages over other systems comprised of molten salt or phase change materials, namely low costs, high exergetic efficiencies and high volumetric energy densities [1]. A recent techno-economic analysis demonstrated that some of the high temperature metal hydrides have the potential to closely approach the US Department Of Energy (DOE) SunShot program targets for thermal energy storage [1]. For solar driven steam power plants, one of the most promising and appealing metal hydrides, working at temperatures of about 400-500 °C, is Mg2FeH6. A system comprised of Mg-Fe material, coupled with a suitable inexpensive low temperature hydride, represents a very promising economic energy storage concept. Results of modeling analysis of the proposed concept will be shown and discussed, demonstrating the technical feasibility of the proposed concept under several operating conditions and scenarios. A new project within the DOE SunShot program is aiming to demonstrate a prototype integrated solar receiver-thermal energy storage system for a high temperature s-CO2 solar driven power plant. The overall system is comprised of a novel Ca-based metal hydride material, coupled with a novel solar receiver concept. The initial outcomes of the novel prototype tests will be shown and discussed. Results of a detailed techno-economic performance analysis of the system, operating at large scale, will also be presented and discussed. [1] Corgnale C, et al. Renewable and Sustainable Energy Reviews 38(2014)821-833

Resume : Solid state hydrogen storage in metal hydrides is a promising form of energy storage for stationary energy storage. TiFe alloy is a promising candidate because of its low price, reasonable storage capacity, low operation temperature and good reversibility. However, the first hydrogenation (activation) of TiFe alloy is usually difficult. In order to overcome this problem, different approaches have been investigated, among them the element substitution for Fe or Ti with transition metals (TMs), including Mn, Cr, Ni and Zr. In this talk we will present the effect of mechanochemistry (ball milling and cryomilling) on TiFeZr-type alloys with respect to their hydrogenation properties, their microstructure and element distribution. Ball milling and cryomilling showed a completely different behavior in respect to hydrogenation. Ball milling improved the initial kinetics of processed powders compared with the as-cast sample but reduced the H-storage capacity. The faster kinetics is likely due to the reduction of the crystallite sizes and formation of new grain boundaries. The reduced hydrogen storage capacity may be explained by the formation of grain boundaries that enhance the hydrogen diffusion but do not store hydrogen in their structures. Following measurements at ESCA microscopy beamline (Elettra), we found phase separations which are more pronounced during hydrogenation. This indicates that the variation of composition at the interface matrix/secondary phase may play an important role in the transfer of hydrogen from the secondary phase to the matrix.

Resume : Metal hydrides are perfectly suitable for high thermal power devices as they show two important properties: high rates of reactions even at temperatures well below 0 °C and high enthalpies of reaction with values of 20-30 kJ/molH2. However, due to the low intrinsic thermal conductivity of the powder material, it remains a challenge to transfer the produced heat (or cold) of reaction to the required component or heat transfer fluid. Thus, as for many applications the sensible mass of the final reactor is also an important characteristic, the reactor design is always a compromise between heat transfer enhancement measurements and system weight. In the presentation, the focus will be on two main thermal applications for metal hydrides: air-conditioning systems as well as pre-heating devices. For the corresponding boundary conditions, the performance of conventional commercial reactor designs based on plate heat exchanger and tubular heat exchanger will be presented and discussed. Furthermore, a complete new reactor design based on additive manufacturing will be presented and discussed in the context of new opportunities for reactor design in the future!

Resume : Hydrazine borane N2H4BH3 (HB) and its derivatives are boron and nitrogen-based compounds that have been studied as potential hydrogen storage materials. Hydrazine borane stores 15.4 wt % of hydrogen but it presents some issues during its dehydrogenation like a high dehydrogenation temperature and the formation of hazardous by-products. Derivatives of hydrazine borane, named hydrazinidoboranes, overcome some of the issues present in HB thermolysis. To date, some alkaline hydrazinidoboranes have been synthesized (lithium, sodium, potassium), but the others are still missing. Obtaining new derivatives could shed some light on the chemistry of these B-N-based compounds. Rubidium hydrazinidoborane RbN2H3BH3 (RbHB) and cesium hydrazinidoborane CsN2H3BH3 (CsHB) have been synthesized for the first time. The obtention of these compounds was achieved by the reaction of metallic rubidium/cesium with HB in THF. The characterization of the materials was done by FTIR, 11B MAS-NMR, PXRD and molecular modeling. RbHB was found as a new crystalline compound and its structure was solved, the evaluation of its dehydrogenation properties and analyses of the solids residues were carried out and compared to those of other alkali hydrazinidoboranes. On the other hand, CsHB has shown an unexpected behavior compared to the other derivatives. The analyses of these compounds open up prospects on the chemistry of B-N-based materials for chemical hydrogen storage.

Resume : Understanding the transport of hydrogen within metallic nanomaterials is crucial for the advancement of energy storage and the mitigation of hydrogen embrittlement. Using nanosized palladium particles as a model, recent experimental studies have revealed several highly nonlinear phenomena that occur over long time periods. The time scale of these phenomena is beyond the capability of established atomistic models such as molecular dynamics. In this work, we present a new approach, referred to as diffusive molecular dynamics (DMD), to the simulation of long-term diffusive mass transport at the atomic scale. The basic assumption underlying DMD is that the time scale of diffusion is much larger than that of microscopic state transitions. In terms of numerical implementation, our approach involves the numerical integration of the master equation, and the numerical solution of a highly nonlinear optimization problem at every time-step. By working with atomic fractions, the characteristic time-step size of our DMD simulations can be much larger than those based on either AMD and KMC methods. Thus, the time-step size in our calculations is only limited by the diffusive time scale, e. g., by the speed of the propagation of a phase boundary, which can be as slow as 1 nm/s. The ability of DMD to predict the propagation of atomically-sharp phase boundaries over a time window of more than 30 s with full atomistic realism is particularly noteworthy. We also note that the scope of DMD is not limited to metal hydrides and a broad range of multi-species systems of practical interest suggest themselves as worthwhile foci for future studies.

Resume : Hydrogen, as an efficient energy carrier, can be used for storing and managing the energy produced from intermittent renewable sources, facilitating the transition towards CO2-free energy. Metal hydrides are safe materials for hydrogen storage under mild conditions of pressure and temperature. Among them, TiFe can efficiently store up to 1.86 wt.% H2 through consecutive formation of a monohydride β-FeTiH and dihydride, γ-FeTiH2. 1 However, FeTi exhibits difficult activation towards first hydrogen absorption and the presence of two plateau pressures.1 Elemental substitutions in intermetallic FeTi can solve activation issues and tune its hydrogen storage properties. In this work, characteristics of substituted FeTi alloys will be discussed, featuring their homogeneity domain, structures and hydrogen storage properties, i.e. both thermodynamics and kinetics, activation issues and cycling properties. In fact, FeTi substituted alloys, as in the case of Mn and Cu substitutions, can be tailored to tune plateau pressures to the specifications required by the final application.2–4 Their activation conditions, kinetic and thermodynamic properties, in relation to phase formation and stabilities will be discussed. This study enables remarkable understanding on hydrogen storage, structural knowledge and support to the industrial application of FeTi-type alloys for the development of large-scale hydrogen storage. References (1) Cuevas, F.; Burzo, E. Materials for Hydrogen Storage: AB Compounds. In Hydrogen Storage Materials, vol. 8; Springer-Verlag Berlin Heidelberg, 2018; pp 45–78. (2) Guéguen, A.; Latroche, M. Influence of the Addition of Vanadium on the Hydrogenation Properties of the Compounds TiFe0.9Vx and TiFe0.8Mn0.1Vx (X=0, 0.05 and 0.1). J. Alloys Compd. 2011, 509 (18), 5562–5566. https://doi.org/10.1016/j.jallcom.2011.02.036. (3) Challet, S.; Latroche, M.; Heurtaux, F. Hydrogen Storage in TiFe(0.70+x)Mn(0.20-x) (0 <= x <= 0.15) and TiFe(0.70)Mn(0.20-y)Ni(y) (0 <= y <= 0.08) Metallic Alloys. Mater. Sci. Technol. 2005, 13–21. (4) Ali, W.; Hao, Z.; Li, Z.; Chen, G.; Wu, Z.; Lu, X.; Li, C. Effects of Cu and Y Substitution on Hydrogen Storage Performance of TiFe0.86Mn0.1Y0.1−xCux. Int. J. Hydrogen Energy 2017, 42 (26), 16620–16631. https://doi.org/10.1016/j.ijhydene.2017.04.247.

Resume : Alkali cluster-intercalated fullerides (ACIF) consist in crystalline nanostructures in which positively charged metal clusters are ionically bond to negatively charged C60 molecules, forming charge-transfer salts. These compounds have been recently investigated with renewed interest, appearing as a novel class of materials for hydrogen storage, thanks to their proved capability to uptake reversibly high amounts of hydrogen via a complex chemisorption mechanism. In this presentation, the synthesis, the structural investigation and the hydrogen storage properties of Li, Na and mixed Li-Na clusters intercalated fullerides belonging to the families NaxLi12-xC60 (0 ≤ x ≤ 12) and NaxLi6-xC60 (0 ≤ x ≤ 6) will be presented. By manometric and thermal analyses it has been proved that C60 covalently binds up to 5.5 wt% H2 at moderate temperature and pressure, thanks to the catalytic effect of the intercalated alkali clusters. Moreover, the destabilizing effect of Na in the co-intercalated NaxLi(6-x)C60 compounds leads to an improvement of the hydrogen-sorption kinetics by about 70%, linked to a decrease in the desorption enthalpy from 62 to 44 kJ/mol H2. The addition of Pt and Pd nanoparticles to Li fullerides increases up to 5.9 wt% H2 the absorption performances and of about 35 % the absorption rate. The ammonia storage properties of Li6C60 have also been investigated, resulting quite appealing. Being the price of C60 quite high for large scale practical applications, new cheaper C based materials are under examination. In particular, porous biochar from agricultural waste are giving interesting results as electrode materials for high-performance supercapacitors.

Resume : The holy grail for efficient seasonal and onboard hydrogen storage is a system that makes use of light and cheap materials and exchanges H2 at temperature and pressure close to the ambient ones. Mg-based H2 storage systems partly fulfill these requirements showing high volumetric and gravimetric H2-storage capability. The main drawbacks of MgH2 are the high stability and the sluggish sorption kinetics. Nanostructuring and mixing with catalytic additives (such as Nb2O5, Pd, TiH2) has been successfully demonstrated to accelerate H2 exchange kinetics, but still in the 200-350°C range. In this work, we present a detailed study of the thermodynamics and kinetics of H2 sorption by Mg/MgH2 in the 100-150°C range. We are able to investigate this unexplored range thanks to a novel concept of nanocomposite (Mg-Ti-H) in which TiH2 and MgH2 coexist at the single nanoparticle level, determining remarkably fast H2 sorption kinetics. Mg-Ti-H nanoparticles can complete the H2 absorption in minutes (up to 4.8 wt%) and fully release it in 1-2 hours in the <150°C range without the addition of expensive noble metal catalysts [2]. We critically evaluate the influence of nanostructure and phase mixing at the single nanoparticle level on the equilibrium and kinetic properties of H2 sorption. The proposed nanocomposite opens an unexpected window for Mg-based reversible H2 storage close to ambient temperature, even if the equilibrium pressures are just slightly above those of bulk Mg/MgH2.

Resume : There is an increasing interest in using the favourable thermodynamic properties of metal hydrides for thermal applications, either as a thermal energy store, to provide cooling, or for compression of hydrogen. The Mg/MgH2 system operating between 350 and 400oC offers opportunities as a thermal store for parabolic trough concentrated solar thermal power plants and for process heat in industrial plants. The use of the endothermic dehydrogenation can be used to provide cooling for refrigeration and air conditioning, in a solar thermal cycle. The last application, compression, capitalises on the van't Hoff relationship between pressure and temperature, in order to provide an effective heat powered solid state compressor. This presentation will explore some of the technical challenges in the design of vessels that can rapidly transfer heat in out of a powdered bed and the advantages of using a metal hydride slurry instead of a packed powdered bed. The presentation will also investigate the impact of non-idealised thermodynamic properties on the performance of the metal hydride and the selection of high temperature and low temperature metal hydride couples that most of these applications require.

Resume : Metallic hydrides (MH) are widely studied and used as materials for energy storage due to their high reversible gas sorption capacity, and their potential application as negative electrode in Ni-MH or Li-ion batteries. LaNix (2 ≤ x < 5) compounds exhibit attractive hydrogen sorption properties by partial Mg substitution of lanthanum [1]. They adopt either a rhombohedral 3R (space group R-3m) or hexagonal 2H (space group P63/mmc) structure and can be described by the stacking along the c axis of two different subunits [La2Ni4] and [LaNi5]. The La-Mg-Ni system has been largely studied, but other ANix (A = rare earths) systems have been barely reported [2]. In this study, binary A2Ni7 alloys were synthesized, for A = La, Sm, Gd or Y. Their structural and hydrogen sorption properties have been studied. Furthermore, optimized alloy compositions by A-site substitution have been achieved. The influence of the A composition on the structural and H2 sorption properties will be discussed. References [1] Kohno, T.; Yoshida, H.; Kawashima, F.; Inaba, T.; Sakai, I.; Yamamoto, M.; Kanda, M. Journal of Alloys and Compounds 2000, 311, L5-L7. [2] Zhang, J.; Latroche, M, Magén, C.; Serin, V.; Hÿtch, M. J.; Knosp, B.; Bernard, P. J. Phys. Chem. C, 2014, 118, 27808

Resume : Intermetallic alloys such as AB, AB2, and AB5 type have been studied due to their capability to reversibly store hydrogen. These alloys exhibit varying hydrogen storage properties depending on the crystal structure and composition. Compositional modification is commonly known as an effective method to modify the alloys thermodynamic and kinetics for various applications such as metal hydride batteries, metal hydrides hydrogen storage and compression. However, the effects of the compositional modification on the cyclic stability of these alloys are not usually well studied. Here, the hydrogen cycling stabilities of Ti-Mn based alloys with C14 type structure are studied. Hyper-stoichiometry, stoichiometry and hypo-stoichiometry alloys were prepared accordingly: Ti30.6V16.4Mn48.7 (Zr0.7Cr0.8Fe2.8) (B/A=2.19), Ti32.8V15.1Mn47.1 (Zr0.9Cr1.2Fe2.9) (B/A=1.97) and Ti34.5V15.4Mn44.7 (Zr0.9Cr1.3Fe3.2) (B/A=1.87). Whilst the hyper-stoichiometry alloy showed almost a stable (about 9 % capacity reduction) hydrogen capacity after 1000 cycles of hydrogenation and dehydrogenation, the stoichiometry and hypo-stoichiometry alloys failed to hydrogenate after about 950 and 500 cycles respectively. A limited reduction in the calculated crystalline size of the alloys was observed before and after the hydrogen cycling, denoting that pulverisation plays a less significant role on the observed hydrogen capacity loss. In addition, a reduction in the B/A ratio from 2.19 to 1.82 (hyper to hypo-stoichiometry) encouraged the formation of more stable hydride and a higher level of heterogeneous lattice strain. Whilst a small loss of hydrogen capacity (9%) in the hyper-stoichiometry alloy was attributed to the trapped hydrogen, the complete loss of hydrogen capacity in the stoichiometry and hypo-stoichiometry alloys seemed to originate from the formation of stable hydride and the lattice distortion.

Resume : Ti is the lightest and richest among metals that form reversible metal hydrides under ambient temperature and hydrogen pressure. Ti based hydrogen absorbing alloys are the most suitable candidates for stationary energy storage in the form of hydrogen. TiFe that was reported in 1974 has the hydrogen capacity of ~1.8 wt%. TiFe is the most cost-effective hydrogen absorbing alloy but activation and control of equilibrium hydrogen pressure are issues. Ti-based BCC solid solution alloys are developed by one of the authors in 1996. The hydrogen capacity reaches to ~4 wt% but absorbed hydrogen is reversible up to 75% of their content. To solve the roadblocks of TiFe, the third elements (M=V, Cr, Mn, Co, Ni, Zr, Nb and Sn) were added and TiFe0.8M0.2 were successfully synthesized. Every TiFe-based alloy could be activated under moderate conditions. In addition, hydrogen equilibrium pressure was controlled to some extent with substitution of the third elements. Ti-based BCC solid solution alloys contain hydrogen up to 4 wt% but reversible hydrogen is less than 3 wt%. The residual hydrogen in the hydride is very stable for desorption at room temperature. In addition, equilibrium pressure is more sensitive to lattice parameter than conventional intermetallic compounds. To reduce the amount of residual hydrogen and control equilibrium pressure, metal elements were added to Ti-based BCC alloys. The results of addition of the fourth (fifth) elements may be introduced at the conference.

Resume : Firstly, I will show recent attempts to design a molecular system capable to store hydrogen reversibly. We have developed systems based on Al with complex and chemical hydrides coordinated to it, such as ammonia borane NH3BH3 (AB), amidoborane NH2BH3– and borohydrides BH4– [1]. We showed [2] that the ability of the strong Lewis acid Al3+ to coordinate both the initial hydrogenated species as well as their dehydrogenation products makes it a good template for chemical transformations of hydrides. In particular, AB coordinated to Al3+ in Al(BH4)3∙NH3BH3 endothermically dehydrogenates to a single product identified as Al(BH4)3∙NHBH, with a potential for a direct rehydrogenation of AB. More recent work explores the possibility to substitute the anions coordinated to Al, as well as modify AB by a substitution on the N-side [3]. Secondly, porous materials can be also made of hydrides [4]. We investigated gas sorption in the porous γ-Mg(BH4)2 using neutron powder diffraction to accurately localize the guests and synchrotron X-ray powder diffraction to collect data along the adsorption isobars. We are trying to extend the family of hydridic porous solids by combining imidazolates (ligands known to build stable frameworks) and complex hydrides (should be responsible for the unusual functionality). The approach and recent results will be presented. [1] Dovgaliuk, I. et al. (2017). ChemSusChem 10, 4725-4734. [2] Dovgaliuk, I. & Filinchuk, Y. (2016). Int. J. Hydr. Energy 41, 15489-15504. [3] Dovgaliuk, I., Møller, K.T., Robeyns, K., Louppe, V., Jensen, T.R. & Filinchuk, Y. (2019) Inorg. Chem. DOI: 10.1021/ acs.inorgchem.8b02630 [4] Filinchuk, Y. et al. (2011). Angew. Chem. Int. Ed. 50, 11162-11166. [5] Burazer, S., Morelle, F., Filinchuk, Y., Černý, R. & Popović J. (2019) Inorg. Chem. DOI: 10.1021/ acs.inorgchem.9b00446

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Resume : This study of shows a flexible system that offers promising candidates for Li-based solid state electrolyte. In this work, the effect of the halogenation on the electrochemical properties in the LiBH4-LiBr-LiCl system has been investigated. The LiBH4-LiBr-LiCl ternary phase diagram has been determined at RT by X-ray Powder Diffraction, coupled with Rietveld refinement. For the first time, a ternary hexagonal solid solution containing chloride anion was stabilized at RT, decreasing the weight of the electrolyte and hence increasing the energy density. The effect of the halogenation on the Li-ion conductivity and electrochemical stability has been investigated by Electrochemical Impedance Spectroscopy and Cyclic Voltammetry. The h-Li(BH4)0.7(Br)0.2(Cl)0.1 sample showed the highest value of Li ion conductivity at RT (1.3 × 10^-5 S/cm), suggesting possible applications of these fast ion conductors as solid-state electrolyte in Li-ion batteries. The values of Li-ion conductivity at room temperature depend only on the BH4- content in the solid solution, suggesting that the Br/Cl ratio does not affect the defect formation energy in the structure. The chloride anion substitution in the hexagonal structure increases the activation energy, moving from about 0.45 eV for sample without Cl-, up to about 0.63 eV for h-Li(BH4)0.6(Br)0.2(Cl)0.2. In addition, a wide electrochemical window of 4.04 V vs. Li+/Li is reached in the Li-Br system, while the further addition of Cl is a trade-off between oxidative stability and weight reduction.

Resume : Alkali metal and alkali earth borohydrides, due to their high hydrogen content and availability, are interesting materials for hydrogen storage in stationary or mobile systems. However, the high thermal stability and the limited cyclability are obstacles to an efficient and large-scale application. Eutectic melting and nanoconfinement can affect and improve kinetics and hydrogen reversibility. The eutectic mixture of 0.71 LiBH4 − 0.29 NaBH4 melts at 219°C and can be easily infiltrated in carbon scaffolds.1,2 The presence of carbon has a catalytic effect on the release of hydrogen, lowering decomposition temperatures and improving the desorption rate. On the other hand, the nanoconfinement of the eutectic mixture in small mesopores makes the system reversible, unlike the bulk material. In an attempt to investigate the thermodynamic and kinetic behaviour of nanoconfined LiBH4/NaBH4 and make a comparison with the bulk borohydrides, the study included mesoporous and non-porous carbons. All composites were systematically studied with several techniques such as N2 adsorption/desorption at 77K, X-ray diffraction, TPD-MS and H2 absorption/desorption cycles. (1) Dematteis, E. M.; Roedern, E.; Pinatel, E. R.; Corno, M.; Jensen, T. R.; Baricco, M. A Thermodynamic Investigation of the LiBH4-NaBH4 System. RSC Adv. 2016, 6, 60101–60108. (2) Javadian, P.; Sheppard, D. A.; Buckley, C. E.; Jensen, T. R. Hydrogen Storage Properties of Nanoconfined LiBH4-NaBH4. Int. J. Hydrogen Energy 2015, 40 (43), 14916–14924.

Resume : Hydrogen storage is one of the key obstacles to the commercialization as well as market acceptance of hydrogen fueled vehicle. Besides the efficiency of power system, it is an extremely challenging technology to store sufficient hydrogen on the vehicle without compromising consumer requirement such as safety, space, driving range, and fuel cost. There are three main hydrogen storage methods including compression, liquefaction and hydrogen storage materials. Among the technologies currently under development, the hydrogen storage as a highly pressurized gas is the most prominent candidate for the hydrogen powered vehicle now. The advanced automobile industries have already demonstrated the highly pressurized hydrogen system on fuel cell vehicles for past several years. The hydrogen storage materials in solid state have some advantages such as high volumetric storage capacity, little energy loss, longer storage time and highest safety. Various carbonaceous and non-carbonaceous hydrogen storage materials have been studied over the past few years in Korea. To improve the adsorption/desorption performance of hydrogen storage alloys in hydrogen storage device, it is advantageous to use a hydrogen storage alloy having a highly effective thermal conductivity. Sodium aluminum hydride NaAlH4 (SAH) is one type of promising hydrogen storage alloys, and used in powder or compacted pellet forms. In order to increase the volumetric storage density in the hydrogen storage device, a compacted pellet shape having a high effective density of the hydrogen storage alloy is used. The enthalpy change of SAH due to adsorption and desorption of hydrogen is approximately 40 kJ/mole of H2. The hydrogen storage alloy should be heated and cooled rapidly to adsorb and desorb hydrogen within a short time. In this case, it is highly advantageous to increase the effective thermal conductivity of the hydrogen storage alloy. In this presentation, we would like to introduce our new developing hydrogen storage system including a internal heat exchanger for FCV based on a SAH.

Resume : The development of the ErwiN – Energy research with Neutrons – neutron powder diffraction (NPD) beamline is presented with three possible advancements: Firstly, the primary beam optics will be replaced to bring this diffractometer to the same level as the high flux and high resolution instrument D20 at the ILL. The ErwiN instrument will be used for the investigation of energy storage materials, also integrated in complete components and under real operating conditions. Thus, it is possible to scan a large parameter space (e.g. temperature, state of charge, charge rate, fatigue degree) for the investigation of modern functional materials in kinetic and time-resolved experiments. Diffraction data will be obtained from the entire sample volume or in a spatially resolved mode from individual parts of the sample. ErwiN is designed for different scenarios: for very fast measurements at medium resolution, for medium fast measurements at higher resolution and for very high resolution still at a reasonable time frame. The final commissioning and integration of ErwiN is the second important objective which will be addressed, while thirdly, the integration of newly developed and strongly needed sample environment, e.g. H2-gas system, will enhance the attractiveness for a wider community in energy research as well as materials science while furthermore developing novel methods for the neutron science community.

Resume : Metal closo-hydridoborates, e.g. M2B12H12, have received increased attention as ion conductors due to extraordinary ionic conductivity in their high-temperature structural polymorphs. Recently, nanoconfinement of Li2B12H12 was achieved via a solid-gas reaction inside a nanoporous SiO2 (SBA-15) scaffold between LiBH4 and gaseous B2H6. However, the reaction yields moderate purity (94 mol%) Li2B12H12 with 6 mol% Li2B10H10. The ionic conductivity of the nanoconfined sample was 1.0 x 10-7 S·cm-1 at room temperature, which is similar to bulk Li2B12H12. In this work, solvated lithium closo-dodecaborate, Li2B12H12·Solv. (Solv. = tetrahydrofuran, acetonitrile), show unexpected melting at low temperature (T < 145 °C) and subsequent recrystallisation of the parent compound at higher temperature. This feature has been explored to melt infiltrate Li2B12H12 into a nanoporous scaffold (SBA-15), which ensures a 100% purity. Small-angle X-ray scattering confirms that melt infiltration occurred, while the ionic conductivity of the nanoconfined and molten Li2B12H12 was measured by EIS.

Resume : Magnesium-based materials are promising as hydrogen storage media due to their high hydrogen absorption capacity and low price. The subject of this study are the hydrogen sorption characteristics of the composites 80wt.% MgH2-15 wt. % Ni- 5wt.% activated carbon (synthesized from polyolefin wax, a waste product of polyethylene production at low pressure- POW) and 90 wt% MgH2-5 wt. % Ni- 5wt.% POW prepared by ball milling under argon atmosphere. The polyolefin wax was submitted to pyrolysis and steam activation, in order to prepare nanoporous carbon POW and used as additive in the preparation of MgH2-Ni composites. Structure, phase and surface composition of the starting compounds and the sample before and after hydriding are determined by XRD and TEM. The maximum achieved absorption capacity of the composites at 573 K and after 60 min of hydriding are 5.3 wt. % H2 for the material with higher Ni content and 5.5 wt % H2 for the other sample. The presence of both additives - nickel and activated carbon derived from POW has a positive impact on hydrogenation kinetics and the capacity achieved. The results from TEM characterization e. g. the polycrystalline SAED show the presence of graphite, Mg and monoclinic Mg2NiH4. Keywords: Mg- based composites, sorption kinetics, hydrogen storage, activated carbon Acknowledgments: Financial support from Bulgarian Academy of Sciences and Bulgarian Ministry of Education (Project DFNI-KП-06-H27/9, 08/12/2018) is gratefully acknowledged.

Resume : In recent years, hybrid nanocrystals (HNs) have emerged as an important class of materials to tune the optical, electrical, magnetic and catalytic properties of nanocrystals. In HNs, two disparate functional material systems (i.e., metal/magnet, metal/semiconductor and magnet/semiconductor) are combined through their crystal facets, which results in the nontrivial synergetic effects including extinction enhancement due to the coupling of surface plasmon resonance and electronic doping by the intraparticle charge transfer. Among different types of HNs, metal-semiconductor HNs are of particular interest in photocatalysis because it can provide a very good light absorbing semiconductor properties and catalytically active metal nanostructure properties. In this paper, we investigated a new and simple strategy to construct Pt-CsPbBr3 HNs, and shows that light-driven CO2 reduction reaction could be enhanced by synergetic effects between CsPbBr3 and Pt as they promote enhanced light absorption and facilitate the charge separation of the photogenerated carriers.

Resume : The aim of this work is to realize a metal hydride (MH) compressor to provide compressed H2 for a small scale hydrogen refuelling station (HRS). In the refuelling station, H2 is produced by an electrolyser, compressed through a MH-compressor (MHC) and stored in high pressure bottles. The latter are used to supply a demonstrative PEM-FC drone, planned for a flight time of 2 hours. The use of MHs to compress H2 is an innovative and efficient way to convert energy from a heat source into compressed gas, avoiding the use of electricity. Compression is linked to the thermodynamics of the reversible reaction between H2 and a metal (M) to form MH and producing heat. Compression occurs since M can absorb H2 at low pressures and low temperatures, forming MH. Afterwards, MH is heated up and H2 is released at a significantly higher pressure. In this work, compression occurs between 20 and 150 °C, from 20 to 200 bar, using two commercial alloys (La0.9Ce0.1Ni5 and Hydralloy®) in series. Lumped simulations were performed based on experimental kinetic and thermodynamic data of hydrogen sorption reactions. Moreover, a lab-scale compressor was also realized to validate the model. The work involves also the study of a TiCr2-based alloy to reach a pressure up to 400 bar. Results of the lab-scale compressor were used for the scale-up. The design of compressor for the HRS is based on the amount of H2 necessary to supply the PEM-FC of the drone (i.e. about 83 g H2 for 2 hours of flight). For the MHC, about 1 kg of M is used in each stage and H2 is compressed and stored up to 200 bar. The integration of the compressor in the HRS, including a proper electrolyser and photovoltaic panels for its electrical supply, will be outlined.

Resume : Hydrogen appears to be the most attractive source of energy alternative to carbon-based fuels, it can be produced from a variety of renewable resources, e.g. biomass and water electrolysis. Hydrogen is environmentally friendly, it releases water vapour into the environment during combustion which offers near-zero emissions of pollutants and greenhouse gases. The economy of hydrogen is still under development. The major challenge is finding an effective route of storing hydrogen for vehicles applications. The conventional storage methods such as liquefaction or compression resulted in energy efficiency and safety concerns. E.g. liquid storage in term of a low boiling point -252.87 ⁰C, high-cost material is needed to maintain a cryogenic state. The focus is to develop sustainable solid-state storage materials. Magnesium hydride is considered as the potential storage medium. Mg has the highest hydrogen storage capacity of 7.6 wt. %. However, Mg has undesirable thermodynamics and kinetics. The MgH2 sorption resulted in a high temperature approximately 250⁰ C which needs to be reduced to 100⁰ C and the hydrogen sorption rate needs to be accelerated. The sorption kinetics can be improved by catalyzing the surface and nanosizing to provide a pathway for hydrogen diffusion. The thermodynamics by alloying Mg to form a new compound or phase. The MgH2 – catalyst mixture material was prepared by ball milling of pure MgH2 with nano-sizer ZrNi, where the catalyst was prepared using an innovative method. The resulted nanostructure powder was characterized by using both the EDS-SEM and X-ray Diffraction. The results determined the characteristics of the synthesized powder. More characteristic studies need to be done such as the TGA (Thermogravimetric analysis), and the XRD crystallographic and TEM to determine the actual size of the particle. Further investigations are required on the PCT isotherm of hydrogen absorption/desorption.

Resume : Hydroborates are a promising alternative class of solid electrolytes that combine liquid-like ionic conductivity with high electrochemical stability, high thermal stability and favorable mechanical properties [1-3]. Using the mixed-anion compound Na4(B12H12)(B10H10) as electrolyte, we real-ized stable cycling of 3 V all-solid-state battery using sodium metal as anode and NaCrO2 as cath-ode. We achieved a capacity of 85 mAh/g at C/20 and 80 mAh/g at C/5 with more than 90% capacity retention after 20 cycles at C/20 and 85% after 250 cycles at C/5 [1,2]. Here, we discuss the implementation of this solid electrolyte in all-solid-state batteries using solution-based techniques and we elucidate the conduction mechanism of this electrolyte. We show that the fast sodium ion diffusion is correlated to the rotational and librational motions of the anions, resulting in a complex temperature dependence of the conductivity [4,5]. References [1] L. Duchêne et al., Chem. Commun., 53, 4195 (2017). [2] L. Duchêne et al., Energy Environ. Sci., 10, 2609 (2017). [3] R. Moury et al., Acta Cryst. B, in press [4] L. Duchêne et al., Chem. Mat., 31, 3449, (2019). [5] T. Burankova et al., J. Phys. Chem. Lett. 9, 6450 (2018). --------------------------------------------------------------

Resume : There is a pressing need for material possessing decent hydrogen storage properties. Magnesium is considered as one of the most attractive hydrogen storage materials, mainly because of its high storage capacity (7.6 wt.%), lightweight and low cost. Unfortunately the high desorption temperature of MgH2 and the slow kinetics of sorption reactions prevent its use in a wide range of applications. However there are multiple ways to improve the hydrogen sorption properties of Mg. Ball-milling and different severe plastic deformation (SPD) methods are shown to be effective techniques to enhance the kinetics of absorption and desorption. Different catalysts (like TiO2) and alloying elements are also often used to improve the hydrogen storage properties of Mg. In the present work titanate nanotubes were prepared by microwave assisted hydrothermal method. High energy ball-milling of Mg was carried out with the addition of titanate nanotubes, using different milling times. The milled powder was then subjected to high-pressure torsion for further deformation. For comparison, a sample with TiO2 addition was also prepared. In order to investigate the effect of the additives, absorption-desorption measurements were carried out. The microstructure of the material was studied by X-ray diffraction, for local analysis of the morphology of the nanotubes, transmission electron microscopy was employed.

Resume : Photocatalytic water splitting with colloidal suspensions has attracted considerable attention recently. The approach that involves linking a particle light absorber with a molecular catalyst combines the advantages of homogeneous and heterogeneous catalysis. Recently, layered double hydroxides (LDHs) have been widely used in photocatalysis, due to their abundant active metal centers. Inspired by the previous studies on colloidal systems, in this work, our efforts on coupling Prussian blue analogues as catalyst with other light-harvesting semiconductors such as layered-double hydroxides (LDHs) will be presented.

Resume : An efficient storage system based on solid state hydrogen requires a fundamental understanding of the material properties. Intermetallics (Ti-based) and complex hydrides (magnesium amide based) are very promising hydrogen-storage materials, due to their volumetric energy storage capacities and for their operation under moderate temperatures and pressures. These materials also have increased functionality once introduced into high pressure conditions either for compressor systems or high-pressure tanks up to 700 bar and above. Storing hydrogen in such a system would yield a significant increase in volumetric capacities and potentially reducing maintenance costs thereby allowing hydrogen powered vehicles to be more widespread. Currently, there is no single solution that meets the range of requirements, neither metal hydride nor compressed gas storage tanks. Novel high content hydrogen materials have been characterised under high pressure and will be presented here. These use of these materials will lead to breakthroughs regarding overall efficiency of either high pressure tank or non-mechanical compressor systems.

Resume : Transparent conducting oxides (TCOs), which combine high electrical conductivity and high optical transmission in the visible spectral range, are needed in many modern optoelectronic devices, such as solar cells, flat-panel displays, touch-screen sensors, light emitting diodes and transparent thin film transistors. In this talk, I will highlight our study on the band structure and doping control of TCOs. In particular, I will discuss (i) the fundamental band structures and defect properties for the TCOs; (ii) how to achieve simultaneously high transparency and conductivity in n-type TCOs; (iii) why p-type TCOs are difficult to achieve; (iv) how to modify the band structure or design new materials to achieve p-type TCOs or even bipolarly dopable TCOs. Based on the understanding above, we hope to provide useful guidelines for the rational design of novel TCOs that are critical for the development of the next-generation optoelectronic materials.

Resume : Photocatalytic H2 evolution on virtually any pristine semiconductor surface is characterized by low efficiencies due to trapping and recombination of charge carriers, and due to a sluggish kinetics of charge transfer (1). Thus, co-catalyst metal nanoparticles are typically decorated at the semiconductor surface to reach reasonable photocatalytic yields. For this, most common strategies involve wet chemical methods, e.g. impregnation, chemical reduction or photo-deposition. An intriguing, alternative pathway is ?solid state dewetting?: as-deposited thin (nm-thick) metal films on a given surface are usually metastable and when heated to a certain temperature tend to ?dewet?, i.e. they break up and aggregate into defined metal particles (2). Factors such as the initial metal film thickness and composition, treatment temperature, atmosphere and time, can influence morphological and physicochemical features of the dewetted particles (2,3) (e.g. shape, size, dispersion, composition and crystallinity, among others) that are key for their co-catalytic ability in photocatalytic reactions. This talk discusses the use of dewetting as a precise nanostructuring tool to form functional metal nanoparticles at highly-defined TiO2 nanotube surfaces (4), namely to engineer catalyst/semiconductor platforms for light-driven reactions. In particular, we discuss how metal dewetting, steered in size, order, and composition, and synergistically combined with additional self-ordering principles (e.g. alloying, dealloying), can enable various types of functionalization of semiconductor surfaces for photocatalytic applications (5?8). (1) Takanabe, K. ACS Catal. 2017, 7 (11), 8006?8022. (2) Thompson, C. V. Annu. Rev. Mater. Res. 2012, 42 (1), 399?434. (3) Leroy, F.; Borowik, ?.; Cheynis, F.; Almadori, Y.; Curiotto, S.; Trautmann, M.; Barbé, J. C.; Müller, P. Surf. Sci. Rep. 2016, 71 (2), 391?409. (4) Yoo, J. E.; Lee, K.; Altomare, M.; Selli, E.; Schmuki, P. Angew. Chemie Int. Ed. 2013, 52 (29), 7514?7517. (5) Nguyen, N. T.; Altomare, M.; Yoo, J.; Schmuki, P. Adv. Mater. 2015, 27 (20), 3208?3215. (6) Altomare, M.; Nguyen, N. T.; Hejazi, S.; Schmuki, P. Adv. Funct. Mater. 2018, 28 (2), 1704259. (7) Spanu, D.; Recchia, S.; Mohajernia, S.; Schmuki, P.; Altomare, M. Appl. Catal. B Environ. 2018, 237 (May), 198?205. (8) Spanu, D.; Recchia, S.; Mohajernia, S.; Tomanec, O.; Kment, ?.; Zboril, R.; Schmuki, P.; Altomare, M. ACS Catal. 2018, 8 (6), 5298?5305.

Resume : The growing interest for environmental protection and the pursuit for renewable and clean energy sources have made the TiO2-assisted photoelectrochemical water splitting a very active research area. For such application, TiO2 nanoparticles are commonly used due to their superior performances, with anatase being the most commonly observed polymorph in small particles. The study and characterization of native point defects is of paramount importance for understanding the electronic properties of the material and the impact on the photocatalytic activity. Moreover, native defects play a fundamental role also in doped systems, where they can compensate for the defect-introduced carriers. Although several first-principles studies of native point defects in TiO2 anatase exist, they are mostly consider a few of them at a time. The different levels of theory employed in the literature do not allow for a direct calculation of the equilibrium concentrations as different formation energies are often reported with different functionals and corrections schemes.In this study, we comprehensively investigate the stability of native point defects in TiO2 anatase using the GGA+U approach. All native point defects and their ionized states are considered. We systematically compare state-of-the-art methods for obtaining reliable formation energies and correcting for finite-size-errors. Equilibrium defect and carriers concentrations are then calculated with respect to different growth conditions.

Resume : Semiconductor oxides nanowires (NWs) are an excellent benchmark to produce complex nanostructures, which could lead to novel architectures in a NW-heterostructure morphology with enhanced physical properties. Recently, wide band gap oxides, such as Zn2GeO4, are emerging as potential candidates for applications in high power devices or in ultraviolet-solar blind photodetectors [1]. In this work, we report the synthesis and characterization of complex nanostructures of oxide NWs in which Zn2GeO4 NWs combine with SnO2 particles, in a self-organized manner. The synthesis method is the thermal evaporation of the suitable chemical precursors on a catalyst-free basis via a vapour-solid mechanism. Electron microscopy measurements have been carried out to assess the morphology, the microstructure and determine the orientation of the NWs. The results support a model growth in which the Plateau-Rayleigh mechanism [2] would produce a pattern of germanium oxide amorphous particles along the Zn2GeO4 NW. In addition, under suitable kinetic conditions, these particles act as nucleation sites for well-faceted SnO2 crystals. The luminescence properties have been assessed by means of cathodoluminescence and photoluminescence techniques. In addition, optical confinement effects have been observed, what could be further exploited in the design of optical microcavities. References [1] H. Mizoguchi, T. Kamiya, S. Matsuishi, H. Hosono, ?A germanate transparent conductive oxide?, Nature Communications, 2, 470 (2011). [2] R. W. Day, M. N. Mankin, R. Gao, Y. No, S. Kim, D. C. Bell, H. Park and C. M. Lieber, ?Plateau?Rayleigh crystal growth of periodic shells on one-dimensional substrates? Nature Nanotechnology, 10, 345 (2015).

Resume : Many materials for energy storage undergo phase transformations to store and release the energy in form of hydrogen, ions or electrons involving mass transport. To understand and finally overcome the rate limiting processes detailed understanding of the systems is necessary. This includes the structure of crystalline systems, the reaction pathway with possible metastable phases as well as the mesoscale to identify inhomogeneities and long term effects. Here X-rays, especially synchrotron radiation, can provide a wide range of methods spanning from XRD to characterize the crystalline phases, spectroscopy to characterize chemical species to spatially resolved techniques (X-ray microscopy) or combinations thereof. This presentation will show some examples for different techniques and results from metal hydrides for hydrogen storage, intercalation and conversion materials for Li-ion batteries and catalyst particles. With the advance of X-ray free electron lasers the door has opened to directly study ultrafast phenomena such as i.e. nucleation also in the time domain.

Resume : Many materials for energy storage undergo phase transformations to store and release the energy in form of hydrogen, ions or electrons involving mass transport. To understand and finally overcome the rate limiting processes detailed understanding of the systems is necessary. This includes the structure of crystalline systems, the reaction pathway with possible metastable phases as well as the mesoscale to identify inhomogeneities and long term effects. Here X-rays, especially synchrotron radiation, can provide a wide range of methods spanning from XRD to characterize the crystalline phases, spectroscopy to characterize chemical species to spatially resolved techniques (X-ray microscopy) or combinations thereof. This presentation will show some examples for different techniques and results from metal hydrides for hydrogen storage, intercalation and conversion materials for Li-ion batteries and catalyst particles. With the advance of X-ray free electron lasers the door has opened to directly study ultrafast phenomena such as i.e. nucleation also in the time domain.

Resume : Multi-Principal-Element Alloys (MPEAs) belong to a new metallurgical paradigm based on the alloying of four or more elements with equal concentrations. Most of reports concerning these alloys describe their structure, microstructure and mechanical properties, whereas functional properties such as, hydrogen sorption, are only scarcely investigated. We present here the study of hydrogen absorption properties of MPEAs based on refractory metals. The TiVZrNbX (X = Mg, Al and Ta) alloys have been synthesized by classical high temperature methods or mechanosynthesis under inert atmosphere. To directly produce hydrides we have employed the reactive ball milling under hydrogen gas starting from the pure metal powders. The properties of materials have been studied by several experimental techniques: X-ray diffraction, electron microscopy, neutron diffraction, pressure-composition-isotherm, thermal desorption spectroscopy. All the alloys are bcc single-phased and undergo one-step reaction with hydrogen. We suggest that the lattice distortion, δ, might play an important role: larger δ would favours a single-step reaction, whereas small δ would favour a two-steps phase, as encountered for conventional bcc alloys. The hydrogen absorption/desorption is completely reversible and the capacities varies between 2 and 3 wt%. In summary, this is an original research topic in the field of solid-state hydrogen storage that might open new routes for the design of multifunctional materials.

Resume : MgH2 is one of the most investigated solid-state hydrides as promising hydrogen storage materials due to its large hydrogen capacity of 7.6 mass% and the high abundance of Mg in Earth's crust. The high thermodynamics (-75 kJ/mol H2) of MgH2 originated from the strong Mg-H binding energy results in the serious issue of high dehydrogenation temperature, i.e. above 300°C for pure MgH2. Forming an alloy with a hydride non-forming transition metal has been approved to be a feasible approach to destabilize MgH2. For example, when alloying with Ni to form a binary Mg2Ni, the hydride transforms from MgH2 to Mg2NiH4, and the enthalpy is reduced from -75 to -64 kJ/mol H2. Ternary alloys like RMg2Ni9 and (Mg,Ca)Ni2 can further destabilize MgH2 and can absorb and desorb hydrogen at room temperature, whereas the gravimetric hydrogen capacity is limited due to the Ni-rich composition. It is hard to produce an Mg-based ternary alloy with a homogeneous elemental distribution by melting process because of the thermodynamic immiscibility of Mg in many systems. This technical issue may be solved by the application of severe plastic deformation using high-pressure torsion (HPT). In this study, a new Mg-rich ternary alloy Mg4NiPd was designed and prepared successfully by HPT. Preliminary results indicate hydrogen absorption and desorption proceed at room temperature, suggesting a new approach to design and prepare Mg-based alloys exceed the scope of known equilibrium phase diagrams for room temperature hydrogen storage.

Resume : Hydrogen-rich hydrides attract great attention due to recent theoretical [1] and then experimental discovery of record high-temperature superconductivity in H3S (TC = 203 K at 155 GPa [2]). Here we perform a systematic evolutionary search for new phases in the Fe-H [3], Th-H [4], U-H [5] and other numerous systems under pressure [6] in order to predict new materials which are unique high-temperature superconductors. We predict new hydride phases at various pressures using the variable-composition search as implemented in evolutionary algorithm USPEX [7-9]. Among the Fe-H system two potentially high-TC FeH5 and FeH6 phases in the pressure range from 150 to 300 GPa were predicted and were found to be superconducting within Bardeen-Cooper-Schrieffer theory, with TC values of up to 46 K. Several new thorium hydrides were predicted to be stable under pressure using evolutionary algorithm USPEX, including ThH3, Th3H10, ThH4, ThH6, ThH7 and ThH10. Fm3 ̅mThH10 was found to be the highest-temperature superconductor with TC in the range 221-305 K at 100 GPa. Actinide hydrides show, i.e. AcH16 was predicted to be stable at 110 GPa with TC of 241 K. To continue this theoretical study, we performed an experimental synthesis of Th-H phases at high-pressures including ThH10. Obteined results can be found in Ref. [10]. Acknowledgments: Authors thank RFBR foundation № 19-03-00100. This work was supported by RFBR foundation № 19-03-00100 and facie foundation, grant UMNIK №13408GU/2018. References [1] D. Duan et al., Sci. Rep. 2018, 4, 6968. [2] A.P. Drozdov et al. Nature. 2015, 525, 73–76. [3] A.G. Kvashnin at al. J. Phys. Chem. C 2018, 122 4731-4736. [4] A.G. Kvashnin et al. ACS Applied Materials & Interfaces 2018, 10, 43809–43816. [5] I.A. Kruglov et al. Sci. Adv. 2018, 4, eaat9776. [6] D.V. Semenok et al. J. Phys. Chem. Lett. 2018, 8, 1920-1926. [7] A.O. Lyakhov et al. Comp. Phys. Comm. 2013, 184, 1172-1182. [8] A.R. Oganov et al. J. Chem. Phys. 2006, 124, 244704. [9] A.R. Oganov et al. Acc. Chem. Res. 2011, 44 227-237. [10] D.V. Semenok et al. 2019, arXiv:1902.10206.