Functional hydride materials

Resume : Solar electromagnetic radiation is a highly underutilized sustainable energy source capable of providing large amounts of energy if appropriately exploited. Concentrated solar thermal power plants aim at utilizing the majority of the solar irradiance spectrum by concentrating the solar energy to produce heat. In order to maintain electricity generation overnight, an appropriate thermal energy storage system is required. Practical thermal energy storage systems require low cost which in turn demands high efficiencies, low cost materials, and long term stability of the thermal energy storage system. In order to achieve high efficiencies of the thermal energy storage system, high operating temperatures are desired (≥ 600 °C). Thermal energy storage systems for concentrated solar power fall into three different categories including sensible heat, latent heat, and thermochemical heat. Thermochemical heat storage systems have the advantage of much higher energy densities than systems based on sensible or latent heat materials. Metal hydrides have recently gained interest as thermochemical energy storage materials due to the high enthalpy associated with hydrogenation of many hydrides. Herein, we describe a metal hydride based thermal energy storage system based on Calcium Aluminum alloys. Calcium Aluminum alloys provide a solution to many of the challenges associated with the high temperature metal hydride required for metal hydride base thermal energy storage systems. Calcium aluminum can be operated at 600 °C to reversibly produce CaH2/Al and the CaAlx alloy by manipulation of the hydrogen overpressure. The use of calcium hydride for this application is impractical due to corrosion of the molten hydride, very low equilibrium pressures, and volatility of calcium at the desorption temperatures. The addition of aluminum allows for the tuning of the thermodynamic properties to give appropriate equilibrium pressures for pairing with a suitable low temperature metal hydride and also prevents the corrosion associated with molten CaH2. The thermodynamic properties of CaAlx alloys and appropriate low temperature metal hydride pairs are presented.

Resume : Metal hydrides have long been explored for their potential application in a variety of technological applications including hydrogen storage materials for energy applications, fast-ion conductors and sensors. For thermal energy storage, such as concentrating solar thermal energy storage, metal hydrides are required to operate at temperatures in excess of 500 °C [1]. This temperature is too high for even the more stable, reversible hydrogen storage materials, and as such, work has been undertaken to synthesise metal hydrides that are stable and reversible at these high temperatures. One proven strategic method to stabilise these materials is to substitute fluorine atoms for hydrogen [2]. Fluorides are more stable than their hydride equivalents and this can be exploited to increase the stability of various metal hydrides by partially substituting hydrogen for fluorine. One recent example is the NaMgH3-xFx system, where pure NaMgH3 releases H2 at ~400 °C, whereas NaMgH2F decomposes at ~478 °C [3]. More recently, NaH1−xFx solid solutions have been synthesised from stoichiometric mixtures of NaH and NaF and their physical properties determined by in situ powder XRD, pressure-composition-isotherm (PCI) and temperature-programmed-desorption (TPD) measurements [4]. Decomposition, under reduced pressure, is observed at 443 °C for NaH0.5F0.5 compared to 408 °C for pure NaH, while a ΔHdes of 106 ± 5 kJ/mol.H2 and ΔSdes of 143 ± 5 J/mol.H2/K was established by PCI analysis (compared to ΔHdes of 117 kJ/mol.H2 and ΔSdes of 167 J/mol.H2/K for pure NaH [5]). This indicates that the reduced operating pressure/temperature of NaH1-xFx is primarily driven by changes in entropy induced by fluorine substitution). Theoretically, these materials may operate between 470 − 775 °C and as such, these low-cost materials show great potential as thermal energy storage materials for concentrating solar thermal power applications. To demonstrate their viability, cycling studies prove that NaH0.5F0.5 is stable over at least seven hydrogen sorption cycles, with only a slight decrease in capacity while operating between 470 and 520 °C. References 1. D. A. Sheppard, M. Paskevicius, T. D. Humphries, M. Felderhoff, G. Capurso, J. Bellosta von Colbe, M. Dornheim, T. Klassen, P. A. Ward, J. A. Teprovich, C. Corgnale, R. Zidan, D. M. Grant and C. E. Buckley, Appl. Phys. A, 2016, 122, 395. 2. D. A. Sheppard, T. D. Humphries and C. E. Buckley, Mater. Today, 2015, 18, 414. 3. D. A. Sheppard, C. Corgnale, B. Hardy, T. Motyka, R. Zidan, M. Paskevicius and C. E. Buckley, RSC Adv., 2014, 4, 26552. 4. T. D. Humphries, D. A. Sheppard, M. R. Rowles, M. V. Sofianos and C. E. Buckley, 2016, Unpublished results. 5. F. D. Manchester and A. San-Martin, Phase Diagrams of Binary Hydrogen Alloys, ASM International, Ohio, 2000.

Resume : Alkali-cluster intercalated fullerides 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. The structural properties were clarified by means of in-situ neutron diffraction and the analysis of the Pair Distribution Function (PDF) obtained from high-energy synchrotron diffraction. The mechanism of hydrogenation was unveiled by Muon Spin Relaxation spectroscopy (μSR). By coupled manometric - calorimetric analyses and thermogravimetric measurements, we proved that C60 covalently binds up to 5 wt% H2 at moderate temperature and pressure, thanks to the catalytic effect of the intercalated alkali clusters. Recently, we also identified some strategies to further improve the absorption performance in this class of materials. For example, we succeeded to catalyze Li-fullerides with Pt and Pd nanoparticles, whose known activity towards hydrogen dissociation allows increasing the H2 absorption up to 5.9 wt% H2 and the absorption rate of about 35 % with respect to the pure compound.

Resume : Hydrogen is a promising energy carrier, compatible with the sustainable energy concept. Generally, hydrogen is stored either in high pressure tanks or in liquid form in cryogenic tanks. These forms of storage are not suitable for widespread commercial application. For example, a hydrogen fuel cell car needs to store at least 4 kg hydrogen to match the range of a gasoline-powered car! The promising alternative is to use solid materials for hydrogen storage. Indeed, it has been known for more than a century that hydrogen can be stored reversibly in metals such as Pd. Knowledge about the ground-state crystal structure is a prerequisite for the rational understanding of solid-state properties of hydrogen storage materials. To act as an efficient energy carrier, hydrogen should be absorbed and desorbed in materials easily and in high quantities. Owing to the complexity in structural arrangements and difficulties involved in establishing hydrogen positions by x-ray diffraction methods, the structural information of hydrides are very limited compared to other classes of materials (like oxides, etc.). This can be overcome by conducting computational simulations combined with selected experimental study which can save environment, money, and man power. In this work, the results of evolutionary algorithm calculations, within the first-principles framework of density functional theory (DFT), performed on palladium-arsenic system and their hydrides are presented. The obtained results show excellent correlation with the experimental results. Moreover, new hypothetic stable hydride structure, experimentally unknown, was predicted.

Resume : The attractiveness of metal-organic frameworks (MOFs) for hydrogen storage is derived from the high surface area and tunability of their pore structure. Carbon materials such as templated carbons and graphene have also been identified as having attractive properties for hydrogen storage. For instance, graphene is considered promising for hydrogen storage due to its many interesting features such as high mechanical strength, lightweight, and high thermal and electrical conductivity. Synthesis of MOF/carbon composite materials has the potential of tapping into the attractive properties of the individual carbon and MOF materials. In this study, hybrid composites of zeolite templated carbon (ZTC) and Cr-MOF (MIL-101), as well as reduced graphene oxide (rGO) and Zr-MOF (UiO-66) were synthesised and analysed using various techniques such as SEM, XRD, BET, and also tested for hydrogen uptake. Rather than follow the conventional method of physical mixing, our synthesis strategy involved an in situ method of incorporating the carbon material into the synthesis mixture of the MOF. The results showed that the surface areas and the hydrogen uptake capacities of individual MIL-101, ZTC, and UiO-66 could be enhanced when the hybrids MIL-101/ZTC composite and UiO-66/rGO composite were synthesized. The incorporation of ZTC and rGO was found not to interfere with the crystallization of the MOF materials. While some MIL-101 crystals were observed to have grown on the surface of the ZTC, in the other hybrid material, Zr-MOF crystals were able to form between the graphene sheets. This intergrowth characteristic is expected to induce graphene’s intrinsic properties to the MOF materials such as enhanced thermal conductivity which is a favorable property during the design of a hydrogen storage system.

Resume : Many energy and information storage processes rely on phase changes of nanostructured materials in reactive environments. In ensemble studies of these materials, it is however often difficult to discriminate between intrinsic size-dependent properties and effects due to sample size and shape dispersity. Here, we use in-situ transmission electron microscopy to show the first direct measurement of hydrogen-induced phase transitions in colloidally-synthesized palladium nanocrystals, both at the single particle level [1] and within individual nanosized particles [2]. The combination of nanoscale spectroscopy and imaging with single-particle diffraction in an environmental TEM offers unprecedented insight into the phase transition of nanomaterials and can be extended to the study of a variety of processes, from ion intercalation in battery materials to nanoparticle degradation in heterogeneous catalysis. [1] A. Baldi, T. C. Narayan, A. L. Koh, and J. A. Dionne, Nature Materials 13, 1143-1148 (2014) [2] T. C. Narayan, A. Baldi, A. L. Koh, R. Sinclair, and J. A. Dionne, Nature Materials AOP (2016)

Resume : The use of hydrogen absorption to tune the magnetic properties of alloys and compounds has a long and prosperous history. From the use of hydrogen processing of high performance permanent magnets [1] to tuning the Curie temperature of magnetocaloric materials [2], hydrogen absorption has been important in the development of the modern magnetic material. The magnetic properties of a material are given by the electronic structure. The interaction between unpaired electrons in different ways can yield different magnetic behaviour from the simple ferromagnets to more complex interaction like magnetic frustration and spin ice. Hydrogen incorporation in magnetic materials provides a unique possibility to probe the electronic structure and provide insight into the interplay between chemical bonding and magnetic properties. In particular, itinerant electrons may be localized as H- through the formation of interstitial hydrides. This generally leads to a change of the strength and possibly also the sign of the magnetic interaction. During this lecture, I will give a general introduction to magnetism in general as well as the field of metal hydrides in magnetism, followed by a discussion of some recent results from my group [3] and others. The talk will finish with some perspectives, challenges and the future prospects of this research field. References 1. Harris, I.R. and P.J. McGuiness, Hydrogen: its use in the processing of neodymium-iron-boron-type magnets. J Less-Common Met, 1991. 174(1-2): p. 1273-1284. 2. Fukamichi, K., A. Fujita, and S. Fujieda, Large magnetocaloric effects and thermal transport properties of La(FeSi)13 and their hydrides. Journal of Alloys and Compounds, 2006. 408–412: p. 307-312. 3. Angström, J., et al., Hydrogenation-Induced Structure and Property Changes in the Rare-Earth Metal Gallide NdGa: Evolution of a [GaH]2– Polyanion Containing Peierls-like Ga–H Chains. Inorganic Chemistry, 2016. 55(1): p. 345-352.

Resume : Hydrogen is recognized as a potential and extremely interesting energy carrier [1], which can facilitate efficient utilization of unevenly distributed renewable energy. Furthermore, hydrogen has an also an extremely interesting chemistry and form compounds with most elements in the periodic table and with a variety of different types of bonds. Here we report selected recent results within new hydrogen containing materials: (i) synthesis of novel metal borohydrides and studies of their properties for hydrogen storage or as ion conductors, (ii) tailoring materials properties by formation of eutectic melting systems, and (iii) in situ powder X-ray diffraction studies of hydrogen release and uptake reactions. We also demonstrate that structural dynamics in the solid state, i.e. entropy effects, are of extreme importance for detailed material property analysis. We present a ‘paddle wheel’ mechanism, which may be responsible for fast ionic conductivity [2]. Entropy effects may also be responsible for anion substitution, which mainly occur in some polymorphs. We conclude that the chemistry of hydrides is very divers, towards rational design of multi-functional materials [3], including new ion-conductors for batteries, hydrogen storage materials, and possibly materials with new types of optical properties. [1] Ley, et al, Mater. Today 17(3), 122 (2014) [2] Skripov, et al., J. Phys. Chem. C 117, 14965 (2013) [3] Schouwink, et al, Nature Comm., 5, 5706 (2014)

Resume : Magnesium borohydride is a very high-capacity hydrogen complex hydrides (14.84 wt% of H2) although its H2 sorption reversibility remains a challenge. Thermodynamics of Mg(BH4)2 dehydrogenation is calculated to be near ideal for effective hydrogen storage, but experiments reveal competing decomposition pathways with the formation of very stable intermediates limiting the lifecycle. In practice, 11 wt% H2 reversibility was demonstrated for very high pressures and temperatures, while only 2.5 wt% H2 at reasonable conditions. In this work, the composites made of commercial γ-Mg(BH4)2 and synthesized silica aerogel microparticles were prepared by thermal treatment in hydrogen under 120 bar H2 and 200 ºC for 3 h. As a result, calorimetric measurements showed a decrease in decomposition temperature by 60 ºC with respect to the pure hydride, and a single step decomposition in range of 220-400 ºC in the prepared composite. The kinetic of the first dehydrogenation at 300 ºC for Mg(BH4)2–SiO2 composites was two times faster compared to that of the bulk γ-Mg(BH4)2. Experimental results suggested that silica aerogel acting as scaffold for γ-Mg(BH4)2 nanoconfinement resulted in an inherent mechanism of reversible hydrogen sorption: for bulk γ-Mg(BH4)2 and the prepared Mg(BH4)2–SiO2 composite 42 % of reversible hydrogen sorption was experimentally observed for the first time at 390 ºC and 110 bar H2 during the 2-nd and 3-rd cycles.

Resume : Bimetallic borohydrides synthesized from alkali, alkali earth and/or transition metal borohydrides by mechanochemical treatment have received significant attention, because of their structural diversity and very high volumetric density of hydrogen [1-3]. Recently, new bimetallic compounds were found in the KBH4-M(BH4)2 (M = Mg or Mn) system. Particularly, the perovskite structure of KMn(BH4)3 is interesting, as it initiated a thorough study of novel perovskite-type metal borohydrides [4]. Recently, strontium borohydride, Sr(BH4)2, and halide free Sm(BH4)2 has been reported [5,6]. Thus, we here study the formation and properties of new MM’(BH4)3 from MBH4-M’(BH4)2, M = Na, K, Rb and Cs; M’ = Sr, Sm mixtures using mechanochemical treatment i.e. ball milling. We discovered new bimetallic compounds, KM’(BH4)3, RbM’(BH4)3 and CsM’(BH4)3, M’ = Sr, Sm, which have an orthorhombic perovskite crystal structure, similar to KMn(BH4)3. These new bimetallic compounds have been investigated by in situ synchrotron radiation X-ray powder diffraction (SR-PXD), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC) and Sieverts’ method [7]. The new bimetallic compounds, MSr(BH4)3, behaves similar to Sr(BH4)2 as the decomposition is very complex, thus decomposition products have been determined ex situ. Additionally, reversiblity has been tested using a Sieverts apparatus by conducting a desorption-absorption-desorption cycle. However, thermal stability of solid borohydride compounds owing to strong covalent and ionic bonding often provides high decomposition temperatures alongside slow kinetics and poor reversibility [8]. Indeed, high thermal stability might be an advantage in respect to other applications e.g. ion conductors or magnetic compounds, exsemplified by gadolinium borohydrides, AnGd(BH4)n+3 (A = K, Cs) with interesting magnetic refrigeration properties [9]. References [1] L. H. Rude et al., Phys. Status Solidi A, 208 (2011) 1754 – 1773. [2] P. Schouwink et al, J. Phys. Chem. C 116 (2012) 10829–10840. [3] R. Černý and P. Schouwink, Acta Cryst. B71 (2015), 619-640. [5] D.B. Ravnsb?k et al. Inorg. Chem. 52 (2013), 10877–10885. [6] T. Humphries et al., J. Mater. Chem. A 3 (2014), 691–698. [7] K. T. M?ller, Dalton Trans. 45 (2015), 831-840. [8] L. George, Int. J. Hydrog. Energy 35 (2010), 5454–5470. [9] P. Schouwink, J. Alloys Compd. 664 (2016), 378-384.

Resume : Hydrogen technology can be incorporated into intermittent renewable energy systems to realise a complete clean, green energy harvesting and storage cycle. An efficient hydrogen storage system based on solid state hydrogen requires optimised material properties for each particular application. There are several approaches to tailoring material properties towards particular applications including the alloying additions to change the thermodynamic properties, additives to improve reaction kinetics and combining hydrides together to form so call ‘’reactive hydride composite’’ (RHC) systems. The mixture of two or more phases has vastly improved properties in terms of reaction kinetics and thermodynamics compared to their individual counterparts, however, the reaction mechanisms are complex and particular to each individual system and some of these issues will be presented here. In all of these approaches aiming to tailor thermodynamics, high capacity at moderate temperatures as a critical application requirement has not been met by any of the investigated systems so far. Complex hydrides are very promising hydrogen-storage materials, due to their high gravimetric and volumetric capacities. At moderate temperatures and pressures both borohydride and alanate systems have increased functionality when used in multi-component systems. The most state-of-the-art multicomponent systems will be presented with respect to several complex hydride systems and their potential towards application. This technology can provide compact and efficient energy solutions to reduce climate change and in turn, improve the global environment.

Resume : Over the last decade dozens of new hydride structures have been synthesised based on complex anions including [BH4]−, [B12H12]2−, [AlH4]−, [AlH6]3−, [NH2]−, [NH]2− and [FeH6]4−. Many of these new structures are comprised of combinations of cations, complex hydride anions and halide anions. In addition, a large number of complex hydride metal ammines have also been synthesised. The prediction of the decomposition pathway of these complex hydrides requires knowledge of their thermodynamic properties. This can be achieved using Density Functional Theory (DFT), but DFT has a number of limitations: functionals that are suitable for hydrides must first be chosen, the calculations are performed at 0 K and approximations need to be made to obtain estimates of the enthalpies and entropies at room temperature and above. Lastly, it cannot easily be applied if the crystal structures of reaction products are unknown or amorphous. The thermodynamics for complex hydrides can be estimated using the Thermodynamics by Difference Rules (TDR) and Volume Based Thermodynamics (VBT) method. The TDR and VBT methods achieve this by relating the unknown thermodynamic properties of a new complex hydride to the experimental thermodynamic properties of a related phase. The TDR and VBT methods are most accurate if the crystal structure or density of the new complex hydride is known, but they can also be applied to amorphous compounds and compounds where only the chemical formula might be known. Thermodynamic quantities such as enthalpy of formation, entropy and heat capacity can be readily estimated and used to predict the reaction pathways of new complex hydrides. Application of the TDR method and an improved VBT method to complex hydrides of the alkali metals shows that the calculated enthalpy of formation (per mole of formula unit) can be estimated to within 15 kJ.mol-1 of the experimental value. Similarly, the entropy can typically be estimated to within 15 J.mol-1.K-1. The extension of this technique to multi-cation and multi-anion complex hydrides will be discussed.

Resume : Rare earth (RE) borohydrides have recently received attention as possible hydrogen storage materials and solid-state Li-ion conductors. They have mostly been synthesized by metathesis reactions between LiBH4 and RE halides, thus yielding Li halides as byproducts. The current syntheses for RE borohydrides with RE = La, Gd and Er were performed by a combination of a metathesis reactions and wet chemistry with removal of the LiCl byproduct. This is the first time halide free Er(BH4)3 has been synthesized. Rehydrogenation of pure decomposed Er(BH4)3 was not successful. However, a composite of decomposed Er(BH4)3 + 50 wt% LiH was rehydrogenated and resulted in formation of ErH3 and LiBH4. Furthermore, the decomposition and reversibility of composite mixtures of RE(BH4)3 with 3LiBH4 and 3LiH with hydrogen capacities up to 10 wt% were investigated during in situ synchrotron radiation powder X-ray diffraction (SR-PXD) and during ex situ measurements in a Sieverts-type apparatus. Three desorption-absorption cycles of the 3LiBH4 + Er(BH4)3 + 3LiH composite at 400/ 340 °C and 5/ 100 bar H2, respectively, showed a reversible hydrogen capacity of 3.7 wt%. This amount corresponds to 87 % of the initially released 4.3 wt% hydrogen. The research leading to these results has received funding from the People Program (Marie Curie Actions) of the European Union's Seventh Framework Program FP7/2007-2013/ under REA grant agreement n° 607040 (Marie Curie ITN ECOSTORE) and is thankfully acknowledged.

Resume : A central goal in current battery research is to increase the safety and energy density of Li-ion batteries. Electrolytes nowadays typically consist of lithium salts dissolved in organic solvents. Solid electrolytes could facilitate safer batteries with higher capacities, as they are compatible with Li metal anodes, prevent Li dendrite formation and sulphur shuttling, and eliminate risks associated with flammable organic solvents. Less than 10 years ago, LiBH4 was proposed as a solid state electrolyte. It showed a high ionic conductivity, but only at elevated temperatures. Since then strategies have been developed to extend the high ionic conductivity of LiBH4 down to room temperature, and other light metal hydrides have been explored as solid electrolytes [1]. Using LiBH4 as an example we will discuss how the properties of solid electrolytes can be modified by forming nanocomposites with metal oxides, leading to an enhancement of the room temperature ionic conductivity of more than three orders of magnitude[2]. DSC measurements combined with solid state NMR allow to identify how the nanoconfinement and presence of interfaces modify the phase stability and the Li mobility [3,4]. Systematic studies show how the ionic conductivity can be optimized by tuning the nanostructure and interfaces in these nanocomposites. Finally first results have been obtained in using these materials as solid-state electrolytes in next generation all-solid state lithium-sulphur batteries. [5] [1] de Jongh et al. J. Appl. Phys. A (2016), 122:251. [2] Blanchard et al., Adv. Funct. Mater. 25 (2015), 182. [3] Verkuijlen et al., J. Phys. Chem. C 116 (2012) 22169. [4] Suwarno et al, submitted

Resume : All-solid-state batteries promise to overcome the safety issues related to flammable organic liquid electrolytes used in state-of-the-art lithium ion batteries. So far, no solid-state electrolyte satisfies all the requirements for a competitive solid-state battery for the mass market. We report the discovery of a new lithium borohydride based superionic phase near room temperature , enabling ionic conductivities of up to 7x10-3 S cm-1, comparable to values of common organic liquid electrolytes. The enhanced lithium ion motion results from an anion supported vacancy mechanism. A model device, using Li4Ti5O12 electrodes and the a new lithium borohydride based superionic phase as solid electrolyte exhibits good rate performance up to 5C and stable cycling over hundreds of cycles at 1C at 40 °C, indicating high bulk and interfacial stability. Our results show the potential of lithium borohydride based solid-state electrolytes for high-power lithium ion batteries.

Resume : With the steady depletion of fossil fuels, concerns over climate change and the necessity for secure sources of fuel supply, the need to explore alternatives to a carbon-based economy is becoming more urgent. One could consider storing sustainably generated electrical energy directly (for example in batteries) or indirectly using an energy vector, such as hydrogen. In fact, some materials with similar origins can serve both these purposes and the release of hydrogen from a number of complex hydrides, such as lithium borohydride, is linked to a transition to a fast ion conducting state. Partial anion replacement with appropriate halides can stabilise the high temperature structures of these hydrides, engendering fast ion conductivity at room temperature and enabling the design of potential new solid state electrolyte materials for secondary batteries. This talk will consider how one might design and produce new fast ion conducting complex hydride materials, taking lithium borohydride, LiBH4, as a basis. In contrast to their use in hydrogen storage applications, the hydrides should be thermodynamically robust to allow operation over a range of working temperatures. “Soft chemistry” synthesis and the ionic conductivity of these materials will be described. Stabilisation of the high temperature and high pressure phases of LiBH4 via anion and cation substitution will be discussed and the relationships between conduction mechanism and structure elucidated. Anion and cation disorder are crucial factors in determining the fast ion conduction properties of the materials. Finally, families of complex hydrides-“chemical” hydrides, incorporating ammonia borane and derivatives will be introduced. The open structures formed by these hybrid materials create new pathways for Li+ ion conduction, which can be tailored by anion substitution.