Multifunctionality of metal hydrides for energy storage – developments and perspectives

Resume : Metal borohydride systems, such as eutectic LiBH4-Mg(BH4)2, LiBH4-Ca(BH4)2, LiBH4-NaBH4 and LiBH4-KBH4 are of interest as H2 storage materials due to their high H2 capacity but are usually hindered by poor kinetics and or limited reversibility. Real-world applications require metal hydrides with stable long-term cyclical H2 capacity but few the borohydride systems that have undergone this testing generally reveal progressively worse capacity with cycling. The 0.68LiBH4 + 0.32Ca(BH4)2 system forms a eutectic with a melting point at ~200oC, compared to ~280oC for LiBH4 and ~370oC for Ca(BH4)2 [1]. It has been studied for H2 storage and the experimental conditions have a large impact on the decomposition pathway [2, 3] but its reversibility has not been studied beyond 3 cycles [4]. Here we show that, under the right experimental conditions, full reversibility of LiBH4 over 5 cycles can be facilitated by initial decomposition of the LiBH4-Ca(BH4)2 eutectic. The characteristics of this system will be discussed including its potential use in high performance niche applications such as high temperature H2 storage and high temperature thermal energy storage. References [1] M. Paskevicius et al., Phys.Chem. Chem. Phys., 2013, 15, 19774. [2] J. Y. Lee et al., J. Phys. Chem. C, 2009, 113, 15080. [3] Y. Yan et al., J. Phys. Chem. C, 2013, 117, 8878. [4] P. Javadian et al., Nano Energy, 2015, 11, 96.

Resume : Mg2FeH6 has gravimetric and volumetric capacity of 5.5 wt.% and 150 kg/m3, respectively and excellent reversibility over 500 cycles above 723K and below 100 bar. However, the reaction kinetics becomes too slow below 673K which is the practical upper limit temperature of the storage system combined with a thermal management unit based on commercially viable heat transfer fluid. We have been studying three different scalable mixing processes, tumbling ball milling, vertical roll milling, and thermochemical mixing, to produce fine and uniform mixtures which can be used as hydrogen storage materials below 673K for stationary applications. The starting materials are commercially available low cost magnesium and iron powders with or without processing control agents. The effects of particle and crystallite size, microstructure, and level of unavoidable oxidation during different mixing processes on hydrogenation reaction kinetics and storage capacity of Mg/Fe composite powders have been compared. The influences of temperature and hydrogen pressure as well as PCAs on long term cycle performance have also been investigated in details. It has been found that the amount and distribution of oxygen inside Mg/Fe composite powders is one of the most important factors which strongly controls both the yield of Mg2FeH6 formation as well as the hydrogenation reaction kinetics. The particle size and uniform distribution of Fe also contributes significantly to the hydrogen storage capacity in the early stage of cycling.

Resume : In order to meet the demanding criteria for light vehicle applications, the development of hydrides with higher mass capacity than conventional metal hydrides (1-2 wt%) is required. Complex hydrides (based on LiBH4, LiNH2) have hydrogen storage capacities higher than 8 wt%, but at high temperatures (over 300 °C) and with very slow loading / unloading kinetics. In order to make optimal use of all the advantages of MgH2 (low pressure, low cost, good capacity and reversibility) whilst addressing the main impediment (i.e. the high temperature) to commercialization at larger scale as hydrogen storage material, we study the development of innovative composite materials based on MgH2 and complex hydrides (e.g., LiNH2, LiBH4) integrated into a functionalized polymer matrix (for stable composite materials with increased thermal conductivity and cycling durability). The mixed hydrides studied within this work were prepared by ball-milling of commercial raw materials (MgH2, LiNH2) with different ratios, 2:1 or 1:1. In order to improve the poor kinetics of this mixtures, we used some catalysts and additives such as TiCl3, LiBH4, ZrCo (as already described in literature). The hydrogen storage performances have been investigated by PCT coupled with DSC-TG apparatus and the results will be presented. Finally, the incorporation of these mixtures into a functionalized thermoplastic polymer matrix has been studied and the impact on the thermodynamic and kinetics properties will also be described.

Resume : Magnesium is an attractive hydrogen store candidate because of its high capacity (7.6 wt% H for MgH2), high abundance and low cost. However, the use of MgH2 in practical applications is limited due to its sluggish hydrogen kinetics and thermodynamical stability, which dictate operation temperatures higher than 300 °C. To solve the kinetic issue, both nanostructuration and dopant addition —such as TiH2— are commonly investigated. The present work aims to determine the optimal TiH2-amount to get the best reversible hydrogen capacity and cycling properties in the (1 y)MgH2+yTiH2 system. Nanocomposites with different TiH2-contents (0 < y < 0.3) were synthetized by mechanochemistry of Mg and Ti under hydrogen gas. Kinetic and cycling sorption properties were measured at T = 300 °C for 20 cycles by switching the hydrogen pressure between P = 0.8 and 0.03 MPa for absorption and desorption, respectively. For each sorption sweep, reaction time was constrained to 15 min. Under these conditions, the reversible capacity (RC) of non-doped MgH2 (y = 0) is almost negligible, whereas it increases to 1.3 H/f.u. (hydrogen atoms per formula unit) for y = 0.05. Thus, a small amount of TiH2 is enough to drastically fasten reaction kinetics. Moreover, RC keeps constant when the TiH2 increases from y = 0.05 to = 0.3. Indeed, by increasing the TiH2 amount the Mg/MgH2 sorption kinetics are enhanced but this beneficial effect is counterbalanced by the irreversible hydrogen storage capacity of TiH2.

Resume : The present study is aimed to extend the study of mixtures of complex hydrides up to quinary systems. The approach is to design combinations with multiple cations and anions in equimolar ratio, following the concept of high entropy alloys. In fact, by entropy effect, the presence of multiple cations and anions in the structure is expected to promote the mutual solubility, leading to highly substituted complex hydrides. For a cation substitutions, the BH4- anion has been fixed and various ternary, quaternary and quinary equimolar combinations in the LiBH4-NaBH4-KBH4-Mg(BH4)2-Ca(BH4)2 system were synthetized by ball milling. On the other hand, for anion substitution, Li+ cation has been taken fixed and equimolar combinations in the LiBH4-LiCl-LiBr-LiI-LiNH2 system were studied. The obtained structures were analysed by X-ray diffraction, in order to establish the amount of cations and anions incorporated in the obtained crystal phases. The thermal behaviour of the mixtures were analysed by DSC and TGA. An effect of the presence of solid solutions and multi-cation compounds on the hydrogen desorption reactions has been observed, depending on the interaction among the components. The role of multi-anion substitution in Li-ion conductivity has been investigated. The structure and properties of highly substituted complex hydrides will be discussed considering volume and electronic effects.

Resume : An integrated experimental-theoretical approach, X-Ray diffraction, solid-state NMR, and periodic plane wave DFT calculations are employed to investigate a series of hydrogen storage materials. X-Ray single crystal diffraction data provides the set of atomic coordinates, solid state NMR is a powerful spectroscopic technique for investigating the atomic chemical local environment, and plane-wave periodic DFT calculations helps in understanding the properties of the solid material, providing several set of physical (crystal structure) and spectroscopic (i.e. NMR chemical shifts and chemical anisotropies) data to be compared with the experimental ones. The combination of the three techniques will be shown is some examples. For example, the Li5(BH4)3NH complex hydride, obtained by ball milling LiBH4 and Li2NH in various molar ratios, has been investigated. The crystal structure of Li5(BH4)3NH has been initially solved by single crystal X-ray diffraction, but DFT calculations suggested alternative structure. Solid-state nuclear magnetic resonance measurements confirmed the chemical shifts calculated by DFT from the computed structure. Finally, synchrotron radiation X-ray powder diffraction data have been obtained for a 3LiBH4:2Li2NH ball-milled and annealed sample. The DFT calculations confirmed the ionic character of this lithium-rich compound. Each Li+ cation is coordinated by three BH4− and one NH2− anion in a tetrahedral configuration. The room temperature ionic conductivity of the new orthorhombic compound is close to10−6 S/cm at room temperature, with activation energy of 0.73 eV.

Resume : Metal hydrides (MH) offer a safe, compact and reliable solution for hydrogen storage applications in the private sector. Designing and planning a system based on the three main elements for an electro-thermal energy storage with (a) electrolysis, (b) metal hydride tank and (c) fuel cell with all important interfaces reveals complexity in many different areas. The latter starts with sizing the system components for the application of use, in the present case, a river-connected 2 - 8 kW turbine supplying an off-grid residential home. Further questions arise when discussing the media used for pressure tests of MH filled tanks, the planning of the transportation of activated MH tanks, or the regulations defining the storage size of MH tanks compared to pure pressure vessels etc. The present analysis discusses these open points and shows the overall concept of the system.

Resume : Sodium borohydride (NaBH4, SB) can generate hydrogen by thermolysis at high temperature (600K). Alternatively, its hydrolysis reaction (NaBH4 + 4 H2O = 2H2 + NaBO2) is exothermic and produces hydrogen at appreciable rates if proper catalysts are added with a maximum hydrogen storage capacity of 5 wt%. In this sense, stabilized SB solutions can be considered as liquid hydrogen carriers which can generate hydrogen on-demand. Cobalt based catalysts are the most used to accelerate SB hydrolysis because of their good cost-efficiency ratio [1-2]. The hydrolysis of SB was studied in our laboratory in two directions: i) Regarding hydrogen generation for portable applications, a continuous reactor was designed to generate 1Liter/min pure hydrogen to directly feed a 60W PEMFC. The hydrogen generation rate was varied on demand just by tuning the fuel (stabilized SB solutions) addition rate. For this application, supported Co-B catalysts were prepared by wet chemistry on different supports, including a homemade stainless steel monolith. Deactivation processes occurring in real high conversion operation conditions were studied. ii) The tailor-made fabrication of supported catalysts, with controlled microstructure and composition, was investigated by using magnetron sputtering (MS) deposition on different substrates (porous, metallic or polymeric). Novel Co-C catalysts were investigated and compared to Co-B and pure Co what permitted to get new insights into the structure-composition and activation/deactivation effects for the catalyzed NaBH4 hydrolysis reaction. [1] G.M. Arzac, T.C. Rojas, A. Fernandez. ChemCatChem, 3 (2011) 1305–1313. [2] M. Paladini, V. Godinho, G. M. Arzac, M. C. Jimenez de Haro, A. M. Beltran and A. Fernandez. RSC Advances, 6 (2016) 108611-108620

Resume : The developments both in passenger and commercial hydrogen vehicles necessitate a rapid expansion in the centrally developed hydrogen distribution infrastructure. Easy on-site generation of hydrogen will make it attractive for domestic hydrogen generation and distribution. The required high hydrogen pressure (>350 bar) for refueling the hydrogen vehicles can be achieved by a reliable Metal Hydride thermal sorption compression (MH compressor). However, design and the alloy selection of the MH compressor has an immediate impact on performance and efficiency of the system. In particular, the performance of a multi-stage MH compressor is governed by the alloys thermodynamic and kinetic properties. In addition, other requirements, such as acceptable hydrogen capacity, plateau slope, hysteresis and the alloy stability during cycling. This study focuses on the selection and development of high pressure Ti-Mn based alloy for a domestic two-stage MH compressor capable of compressing 600 g hydrogen within 10 h to over 350 bar. The plateau pressure of the commercially available Ti-Mn alloy was shown to be dependent on the unit cell volume of C14 laves phase. Hence, its plateau pressure and hydrogen capacity were tuned by modifying the alloy’s composition to achieve the MH compressor operation temperature of RT-130 °C. A significant reduction in the plateau slope of the modified alloy was achieved by minimizing the compositional fluctuations and encouraging the formation of C14 laves phase when compared to the commercial alloy. The room temperature cyclic stability of the modified alloy was investigated under hydrogen and it was noted that the structure and hydrogen capacity of the alloy remains almost constant even after 1000 cycles. Effective improvement in the hydrogen sorption kinetics of the Ti- Mn alloy was achieved after modification. Whilst, a full hydrogen cycle (based on 80 % of hydrogen capacity) in the as-received Ti- Mn takes more than 45 min, it takes less than 20 min for the modified sample. This will result in a considerable reduction in the required amount of alloy.

Resume : Metal borohydrides are multifunctional class of materials that may also be used as fast ion conductors in batteries or for gas adsorption.[1] LiRE(BH4)3Cl, RE = La, Ce, Gd is an interesting class of bimetallic borohydrides carrying significant amounts of hydrogen and simultaneously having high Li-ion conductivities.[2] In this work LiLa(BH4)3X, X = Cl, Br, I are synthesized with high purity by a new type of addition reaction between La(BH4)3 and LiX. These new compounds are isostructural to LiLa(BH4)3Cl and the highest Li ion conductivity is observed for LiLa(BH4)3Br, 1.8×10-3 S/cm2 at 140 °C with an activation energy of 0.272 eV. Topological analysis suggests a new lithium ion conduction pathway with two new different types of bottleneck windows. The sizes of these windows reveal an opposite size change with increasing lattice parameter, i.e. increasing size of the halide ion in the structure. Thus, we conclude that the sizes of both windows are important for the lithium ion conduction in LiLa(BH4)3X compounds. Moreover, 11B MAS NMR is used to verify the contents of the samples whereas thermogravimetric analysis shows 4.8 and 3.6 wt% of hydrogen release for LiLa(BH4)3Cl and LiLa(BH4)3Br in the temperature range RT to 400 °C. [3] [1] M. Paskevicius. Chem. Soc. Rev, 2017, 46, 1565-1634. [2] M. Ley. Chem. Mater, 2012, 24, 1654–1663. [3] SHP. GharibDoust, J. Phys. Chem. C, 2017, Submitted.

Resume : Owing to its proper thermodynamic properties (ΔH≈40kJ/molH2) and relatively low desorption temperatures (T<200°C),1 the Mg(NH2)2 2LiH composite is one of the most promising hydrogen storage candidate systems for mobile applications. Recent studies showed that Rb-based additives could further reduce the onset temperature of the desorption process by 50-100°C as a combination of kinetic and thermodynamic improvement.2 The reduced reaction enthalpy of the doped system was ascribed to the formation of a new amide-imide product of formula RbMgNH2NH.2 However, the crystal structure of this compound was still unknown. Here we present its structural characterization using synchrotron radiation powder X-ray diffraction (SR-PXD) and powder neutron diffraction (PND) experiments. The structure was solved in the orthorhombic system (a=9.5413Å, b=3.7028Å, c=10.0713Å) with symmetry Pnma. Moreover, new intermediates were identified in the Rb-N-H system and their structural properties studied by means of in situ SR-PXD and solid-state NMR (nuclear magnetic resonance). The differences and analogies with the K-Mg-N-H system3 and the K-N-H system4 will be discussed. [1] Z. T. Xiong et al., Journal of Alloys and Compounds 2005, 398, 235-239. [2] C. Li et al., Chemistry – An Asian Journal 2013, 8, 2136-2143. [3] A. Santoru et al., Physical Chemistry Chemical Physics 2016, 18, 3910-3920. [4] A. Santoru et al., Chemical communications 2016, 52, 11760-11763.

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 for efficient and large scale applications. Eutectic melting and nanoconfinement can affect and improve kinetics and hydrogen reversibility. The eutectic mixture of 0.725 LiBH4 − 0.275 KBH4 melts at 105°C and can be easily infiltrated in carbon scaffolds. 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 in small mesopores makes the system reversible, unlike the bulk (or scaffolds with larger pores). In an attempt to investigate the thermodynamic and kinetic behavior of infiltrated LiBH4/KBH4 and make a comparison with the bulk borohydrides, the study included compounds with porous and non porous carbons. All composites were systematically studied with several analysis techniques, namely N2 adsorption/desorption at 77K, X-ray diffraction, TPD-MS and H2 absorption/desorption cycles. The support of the FP7 Marie Curie ITN project ECOSTORE (Grant Agreement n°607040 is gratefully acknowledged.

Resume : Magnesium borohydride (Mg(BH4)2) is a promising material for solid state hydrogen storage, albeit the predicted properties cannot be reached in real life applications due to sluggish kinetics. HD isotopic substitution has been used to investigate the kinetics of the gas-solid state equilibrium in a range of samples: pure γ-Mg(BH4)2, ball milled γ-Mg(BH4)2 and composites ball milled with the additives nickel triboride (Ni3B) and diniobium pentaoxide (Nb2O5). Ball milling always led to partial or almost complete amorphization of the samples. Infrared (IR) and thermogravimetric (TG) analyses showed that as received, ball milled and γ-Mg(BH4)2 Nb2O5 have comparable HD exchange during long exposures (23 h) to 3 bar deuterium atmosphere at 120 °C, while the Ni3B additive reduces the isotopic exchange between the gas and solid state under the same conditions. Synchrotron radiation powder X-ray diffraction (SR-PXD) after HD exchange reveals α- and ε-Mg-borohydride in addition to the major phase of γ-Mg-borohydride in all ball milled samples. Furthermore, IR data showed that in all samples the exchange reaction even occurred at room temperature after HD exchange for six days under 3 bar D2. It is for the first time, to our knowledge, that the H-D isotopic exchange in alkali- or alkaline-earth boron-based complex metal hydrides has been observed at room temperature. However, in situ Raman analysis at a heating rate of 2 K min-1 under 3 bar D2 demonstrated that in pure γ-Mg(BH4)2 the isotopic exchange reaction started close to 80 °C. The lower specific surface area (SSA) in the ball milled sample resulted in no D substitution during short time frames of in situ Raman measurements at 2 K min-1 under 3 bar D2, nevertheless, HD exchange was observed under 7 bar. The effect of the lower SSA will be investigated further with the premise to discover the point when diffusion is changing from a surface to a bulk process.The funding from the Marie Curie Initial Training Network ECOSTORE is thankfully acknowledged.

Resume : The utilization of eutectic melting in metal borohydrides is considered as one of the potential routes for achieving high-capacity hydrogen storage materials. One of the interesting systems received attention is 0.62LiBH4 - 0.38NaBH4, due to its low cost among known eutectic systems and relatively high theoretical H2 capacity by weight (14.5 wt%). The dehydrogenation of this mixture started above 287 °C (in Ar) and released 9.8 wt.% of H2 upon heating to 600 °C. This process was partially reversible. However, the reversible H2 content reduced from 5.5 wt.% (when kept for 8 h at 500 °C in 1 bar H2 for desorption; and for 10 h at 400 °C in 130 bar H2 for absorption) to 1.1 wt. % at the 2nd cycle and to 0.8 wt. % at the 3rd cycle. No LiBH4 was reformed. The addition of nano-sized Ni reduced the dehydrogenation temperatures by 20-40 °C by altering reaction pathways, where Ni4B3 and Li1.2Ni2.5B2 were formed. A total of 7.3 wt. % of H2 release was gained (lower than the Ni-free sample). The reversible H2 content reduced from 5.1 wt.% to 1.1 wt.% to 0.6 wt.% (similar to the Ni-free sample). After rehydrogenation, some LiBH4-like signals were observed using FTIR. In addition, Ni3B, Ni2B were observed together with the disappearances of NaH and Ni4B3, suggesting NaBH4 was possibly reformed. This study suggests the addition of nano-sized Ni decreases the dehydrogenation temperatures of 0.62LiBH4 - 0.38NaBH4 and affects the rehydrogenation by changing reaction pathways.

Resume : Storage in metal hydrides is presented as a proposed solution to solve the hydrogen storage problem. MgH2 is one of the ideal materials, studied intensively for the hydrogen fuel based economy. In this work, Mg thin films (50-800nm) enable to monitor the growth process of the hydride and study the mechanism of formation. Pd is used as a catalyst as well as a protective layer preventing oxidation. The hydride formation is followed by XRD and by TEM imaging the co-existence of MgH2 and Mg. The microstructural change is clearly visible from columnar to an equi-axed grainy structure, despite the fact that electron microscopy damages the hydride phase. For kinetic measurements, the samples are fully hydrogenated at different conditions and the time of full hydrogenation is evaluated. These combined techniques are suitable to follow the kinetics of hydride formation within the layer, and to study quantitatively the diffusion coefficients and mechanism of hydrogenation. An interface limited growth is observed at low temperatures (T<250°C), whereas at higher temperatures hydride growth becomes diffusion limited. The interfacial barrier coefficient and the diffusion coefficient are quantified. In parallel, Ti/TiH2 system is investigated because of its catalytic effects. We show however, how the oxide passivating layer formed when Ti is exposed in air controls hydrogenation. This is demonstrated clearly by comparison with samples, protected by Pd to exclude any oxide formation. Hydrogenation Kinetics is measured at different conditions by means of XRD, while microstructural changes are characterized by TEM. The hydride growth is shown to be controlled by the atom transfer across the oxide layer. Furthermore, the dependence on hydrogen pressure is determined. Acceleration of hydrogenation with increasing pressure reaches a saturation at about 1 bar H2. As a consequence of this observation, the sticking coefficient of hydrogen at the TiO2 surface can be estimated.

Resume : A systematic investigation on the effect of the addition of Ti-Al-Cl dopant (3TiCl3.AlCl3) on the kinetic and cycling behavior of the lithium reactive hydride composite system (Li-RHC): 2LiBH4+MgH2 / 2LiH+MgB2 is herein carried out. Upon dehydrogenation, in the doped materials the incubation period necessary for the nucleation of MgB2 is sensibly reduced compared to the pristine material. The material doped with 3TiCl3.AlCl3 exhibits better hydrogen storage stability over cycling as well as faster dehydrogenation/hydrogenation kinetics than the material doped with TiCl3. The best performance in terms of reversible hydrogen capacity and fast reaction kinetics was observed for the system 2LiH+MgB2 doped with 5.7 wt% of 3TiCl3.AlCl3.

Resume : Complex hydrides have recently shown large interest as fast Li+ and Na+ solid conductors for all-solid-state batteries.1,2 The fast cation motion firstly discovered in LiBH4 was later studied in higher boranes systems showing low activation energy for diffusion process and ionic conductivity approaching the liquid electrolyte regime already at room temperature.3,4 This class of compounds is represented by closo-boranes and carba-closo-boranes. Here we report the sodium ionic conductor Na3(CB11H12)(B12H12) exhibiting a high Na-conductivity of 1 mS cm-1 at 20°C. The conductivity increased to 10 mS cm-1 at 100 °C with an activation energy of 136 meV. Cyclic voltammetry measurements revealed electrochemical stability in the window voltage 0-5 V (vs Na/Na+). In-situ X-ray diffraction showed a thermal stability up to 300°C. Half and complete Na-cells were assembled using this new electrolyte. We present the electrochemical performance of several all solid state cells using sulphates and sulphides as positive materials and graphite as negative electrode. 1DQMP - Université de Genève, Geneva, Switzerland 2Saft, 111 bd Alfred Daney, 33074 Bordeaux Cedex, France (1) Matsuo et al. Appl. Phys. Lett. 2007, 91 (2) Matsuo et al. Appl. Phys. Lett. 2012, 100 (3) Tang et al. Adv. Energy Mater. 2016, 6 (4) Tang et al. ACS Energy Letters 2016, 659

Resume : In recent years there has been increasing interest about use of metal hydrides as conversion type electrodes for rechargeable lithium batteries.[1] The complex transition metal hydride Mg2FeH6 has high specific capacities of 1455 mAh/g and 3995 mAh/cm3. Unfortunately, like other hydride electrodes, Mg2FeH6 suffers from poor reversibility in conventional Li-ions batteries using organic liquid electrolyte.[1,2] We show that in solid state batteries using LiBH4 as electrolyte, an initial Coulombic efficiency over 65 % can be obtained for Mg2FeH6, while that in liquid electrolyte system is ~ 24 %. The polarization of electrode is also reduced in solid state batteries. After ten repeated cycles, the retained capacity of Mg2FeH6 can be higher than 410 mAh/g. The performance of Mg2FeH6 is further investigated using several techniques: galvanostatic intermittent titration technique, cyclic voltammetry, electrochemical impedance spectroscopy and powder X-ray diffraction. It seems that higher operation temperature alters the reaction mechanism of the conversion of Mg2FeH6. In addition to the direct decomposition of Mg2FeH6 to amorphous Mg and Fe, conversion from Mg2FeH6 to MgH2 has also been observed. References: [1] S. Sartori, F. Cuevas, M. Latroche, Appl. Phys. A 2016, 122, 135. [2] W. Zaïdi, J.-P. Bonnet, J. Zhang, F. Cuevas, M. Latroche, S. Couillaud, J.-L. Bobet, M. T. Sougrati, J.-C. Jumas, L. Aymard, Int. J. Hydrogen Energy 2013, 38, 4798.

Resume : The present study reports on the effect of Mg alloying with lithium in the electrochemical performance of metal hydride MH nanocomposites xMgH2(1-x)TiH2. These hydride materials are used as active material of all-solid-state batteries using LiBH4 as solid electrolyte and Li as counter-electrode. Nanocomposites with molar composition x = 0.2 and 0.7 were prepared by mechanochemistry of the elemental metal powders under hydrogen pressure. Their electrochemical properties were investigated by galvanostatic cycling at 120 °C under two potential ranges: from 0.12 to 1.0 V and from 0.05 to 1.0 V. Within the first range, the conversion reaction of MgH2 and TiH2 occurs, whereas in the second one additional Mg alloying reaction with lithium takes place. The cells undergoing Mg alloying exhibit very high initial discharge capacities at C/50: 1650 and 1900 mAh g-1 for x = 0.2 and 0.7, respectively. Without Mg alloying, this capacities decrease by ca. 20%. After 10 cycles, the capacity retention is about 60 % for the two potential ranges at C/50 . However, when the C-rate is increased to 1C, the capacity retention improves for the cells undergoing Mg alloying: 30% compared to 10% for x = 0.2. Impedance spectroscopy studies also prove that, after 35 cycles, the cells undergoing Mg alloying exhibit lower resistance. The experiments show that Mg alloying plays role in improving conductivity of the all-solid-state cell then resulting in better C-rate and cycling performance.

Resume : In a hydrogen economy, hydrogen sensors will play a critical role due to the safety-concerns related to the high flammability of hydrogen gas mixed with air. Thus, to prevent the formation of flammable gas mixtures, fast, cheap, accurate and selective sensors are required for the timely detection of leaks. Moreover, the application of hydrogen sensors is not limited to the context of hydrogen as energy carrier but there is a need for accurate hydrogen detection also in the chemical and food industries, as well as in life science[1], [2]. For any new hydrogen detection technology to be viable in a real application, it has to fulfill a number of criteria. To this end, nanoplasmonic hydrogen sensors show great promise in terms of both accuracy and response time, as well as miniaturization potential [3], [4]. However, their resistance to poisoning and deactivation due to chemical species such as CO, NOx or SOx has so far not been explored. As a first effort in this direction, we have developed a polymer-coated nanoplasmonic hydrogen sensor, in which the polymer acts as selective and protective membrane and the plasmonic and hydride-forming metal nanoparticles as optical signal transducers. For the latter, a PdAu alloy is the material of choice due to its hysteresis-free nature combined with fast response time [5]. For the polymer coating, we investigated a range of materials, including PMMA and PTFE, to assess the hydrogen sensing performance in the presence of different background gases, such as O2, CO2, CH4, NO2, and CO. As the key results, we find dramatically improved poisoning resistance for the polymer-coated sensors even in CO background, as well as generic polymer-mediated drastic improvements of sensor response times and absolute sensitivity.

Resume : In this study, the hydrogen storage properties of the single substituted CaNi4.8Mn0.2, wchih is prepared by the partial substitution of Ni by Mn compared with the CaNi5 parent compound using mechanical alloying, were tested systematically at different grinding time (20, 30, 40, 50 and 60 hours). The structural transformation of the working electrode was characterized by XRD and SEM. It is found that, at 20 hours of grinding, the structural study showed that this compound is poly-phasic and crystallizes in the hexagonal structure of CaCu5 type (P6 / mmm spacegroup) as the base alloy CaCu5. At the 30 hours of grinding, the Ca-Ni peak becomes even larger, while the intensity of the peaks decreases, indicating an additional amorphization process. For alloys milled for 50, only the Ni peaks are found. The hydrogen storage capacity was measured electro- chemically in alkaline solution (6M), and the best discharge capacity (96mAh/g) is observed when the sample milled at 40h.

Resume : Lithium-ion batteries (LiBs) are today largely used as mobile energy storage systems. However, insertion compounds that currently operate in this technology reach intrinsic limitations. For instance, the reversible capacity of the usual graphite anode is limited to 370 mAh/g. Enhanced battery performances are expected from novel reaction schemes such as conversion reactions. In the last ten years, several families of metal and complex hydrides that react with lithium through a conversion reaction, have been proposed as efficient anodes of LiBs. Binary metal hydrides (e.g. MgH2 and TiH2), related nanocomposites, Mg- and Al-based complex hydrides (e.g. Mg2FeH6, NaAlH4) all have been shown to react with lithium to deliver electrochemical capacities over 1000 mAh/g. Moreover, the equilibrium potentials of the conversion reaction are typically below 0.7 V, which is satisfactory for their use as negative LiBs electrodes. However, reaction reversibility, kinetics and cycle-life need to be improved for practical applications, which requires a better understanding of the limiting reaction mechanisms in these systems. In this talk, we will review the state-of-the art and last advances on this research field. A particular attention will be paid to the recent progress on MgH2-TiH2 nanocomposites, initially tested as anodes in half-cells using liquid organic electrolytes and recently used to build-up a complete all-solid state LiB: P. López-Aranguren et al. J. Power Sources 357 (2017) 56.

Resume : Electrochemical devices with high energy and power densities are currently powered by lithium batteries with organic liquid electrolytes. However, such batteries require relatively stringent safety precautions, making large-scale systems very complicated and expensive. Lithium solid electrolytes promise the potential to replace organic liquid electrolytes and thereby improve the safety of next-generation batteries [1]. Compared with lithium batteries with organic liquid electrolytes, all solid-state batteries offer an attractive option owing to their potential in improving the safety and achieving both high energy and power densities. Despite extensive research efforts, the development of all solid-state batteries still falls short of expectation largely because of the lack of suitable candidate materials for the electrolyte required for practical applications. Lithium-conducting complex hydrides are considered possible candidates as solid electrolytes in all solid-state rechargeable batteries [2]. The advantages of complex hydrides as lithium solid electrolytes are as follows: 1) high ionic conductivity, 2) wide electrochemical window, 3) high compatibility with lithium metal electrode. Since the discovery of fast lithium ionic conduction in the high temperature phase of LiBH4, we have developed numerous solid electrolytes based on the complex hydrides that exhibit fast lithium ionic conductions. It this talk, we first review conduction properties of various complex hydrides including LiBH4-based and closo-borate materials [3]. In addition, several battery system and their electrochemical properties using related solid electrolytes are reported. Moreover, we deliver our recent results on the enhancement of lithium ionic conductivity by simple experimental treatments such as mechanical milling and heat treatment [2], along with a conduction mechanism. Based on these results, the future prospects of the all solid-state batteries using complex hydride electrolytes will be discussed. [1] N. Kamaya et al., Nat. Mater. 10, 682 (2011) [2] R. Mohtadi, S. Orimo, Nat. Rev. Mater., 2, 16091, (2016) [3] K. Yoshida et al., Appl. Phys. Lett. 110, 103901 (2017)

Resume : Metal borohydrides and closo-boranes attract since few years the attention of researchers working on all solid batteries. Since the discovery of the superionic high temperature phase of LiBH4, many other borohydrides and closo-boranes have been reported as close to room temperature fast ionic conductors, and tested as solid electrolytes. The chemical composition space to be investigated in the search for novel compounds becomes large as double and triple cation compounds appear, and anion mixing became powerful tool in the search for crystal structures with high cation mobility. The crystal chemistry knowledge accumulated on oxides and halides, superionics with packing of mono-atomic anions, can be extended to borohydrides and closo-boranes with poly-atomic bulky anions [1]. Examples of the crystal chemistry design will be discussed on mixed anion Na3BH4B12H12 [2], double cation A’A’’B12H12 (A’= Li, Na; A’’= Na, K, Cs) and anion modified Na2B12H12-yXy (X=Cl, I) [3]. [1] Cerny R. and Schouwink P. The crystal chemistry of inorganic metal borohydrides and their relation to metal oxides. Acta Cryst. B. (2015), B71, 619-640. [2] Sadikin Y., Brighi M., Schouwink P., and Cerny R. Superionic Conduction of Sodium and Lithium in Anion-Mixed Hydroborates Na3BH4B12H12 and (Li0.7Na0.3)3BH4B12H12. Adv. Energy Mater. (2015), 1501016. [3] Sadikin Y., Schouwink P., Brighi M., Lodziana Z. and Cerny R. Modified anion packing of Na2B12H12 in close to room temperature superionic conductors. Inorg. Chem. (2017), 56, 5006-5016.

Resume : La2MgNi9-related alloys are superior metal hydride battery anodes as compared to the commercial AB5 alloys. Nd-substituted La2-yNdyMgNi9 intermetallics are of particular interest because of increased diffusion rate of hydrogen and thus improved performance at high discharge currents. The present work presents in operando characterization of the LaNdMgNi9 intermetallic as anode for the nickel metal hydride (Ni-MH) battery. We have studied the structural evolution of LaNdMgNi9 during its charge and discharge using in situ neutron powder diffraction. The work included experiments using deuterium gas and electrochemical charge-discharge measurements. The alloy exhibited a high electrochemical discharge capacity (373 mAh/g) which is 25% higher than the AB5 type alloys. A saturated β-deuteride synthesized by solid-gas reaction at PD2 = 1.6 MPa contained 12.9 deuterium atoms per formula unit (D/f.u.) which resulted in a volume expansion of 26.1%. During the electrochemical charging, the volume expansion (23.4%) and D-contents were found to be slightly reduced. The reversible electrochemical cycling is performed through the formation of a two-phase mixture of the alpha-solid solution and beta-hydride phases. Nd substitution contributes to the high-rate dischargeability, while maintaining a good cyclic stability. Electrochemical Impedance Spectroscopy (EIS) experiments showed a decreased hydrogen transport rate during long-term cycling.

Resume : Metal closo-dodecaboranes can be functionalised by exchanging hydrogen atoms with different functional groups including halides. The modified closo-boranes exhibit dramatically different structural and thermal properties that follow a trend with anion size. Here, a focus will be made towards sodium-based halogenated closo-boranes directed towards their crystal structures, polymorphic phase transitions, thermal properties, solid-state ion conductivity, and anionic electrostatic potential. Comparisons to other known metal boranes will be made in regards to their ion conductivity and a discussion will be drawn around why there are differences in properties based solely on anion type. It is hoped that this information can lead to the further design and testing of other high performing solid-state ion conductors in the future.

Resume : Nowadays the rechargeable batteries operate usually with liquid organic electrolytes, which have disadventages as a narrow range of operation temperature, liquid leakage, deformation and flammability. Therefore has been a push to develop alternative solid-electrolyte technologies to address these concerns. The alkali salts of closo-borane clusters such as [B12H12]2− and [B10H10]2− investigated as thermal and kinetic stable byproducts in hydrogen storage chemistries could find suitable a use just as electrolytes in batteries. Recently one broad class of polyborane salts based on [B12H12]2−, [B10H10]2−, [CB11H12]−, and [CB9H10]− has appeared as highly promising solid electrolyte due to superionic conductivity that makes them attractive as electrolytes in solid-state batteries. The pseudoaromatic character of mentioned borane anions, where the anionic charge is part of the cage bonding and is fully delocalized over the cage, have a high thermal and electochemical stability, as well as are non-corrosive and air/H2O stableOur company belongs to the one of most important producer in the field of mentioned polyborane salts. Herein we present the comparison of laboratory syntheses of mentioned borane and carborane clusters and the influence of their structures to the properties of polyborane salts.

Resume : Solid-state electrolytes have the potential to improve the safety of Li-ion batteries. Many different materials have been proposed over the years. However, especially at lower temperatures, most solid-state electrolytes display lithium ion conductivities lower than the one of conventional organic liquid electrolytes (about 3-5 mS.cm-1 at room temperature). Several sulfides are among the compounds reported which display Li-ion conductivities in or above this range. However, issues regarding costs and/or electrochemical stability of sulfide electrolytes still remain. The addition of lithium halides to the glass electrolyte Li2S-P2S5 has been shown to improve the ionic conductivities and form favorable interface contacts. Recently, the LiBH4-Li2S-P2S5 system has attracted attention owing to its interesting ionic properties. LiBH4 is a good Li-ion conductor only above its phase transition temperature (110°C). However, the high-T phase can be stabilized by partly substituting BH4 - with halides, Li(BH4)0.75I0.25, thus preserving high ionic conductivity on cooling down to room temperature. The present work deals with the investigation of the Li-ion conduction properties of the Li(BH4)0.75I0.25 phase embedded in a sulfide-based amorphous matrix. Significant enhancement of the ionic conductivities were achieved for some privileged compositions. The study is supplemented by electrochemical stability (I-E) measurements and battery tests, first using a standard conversion type TiS2 electrode and then with high-capacity MgH2 based anodes (2037 mAh.g-1), paving the way for high-power lithium solid-state batteries at moderate temperatures.

Resume : The battery safety is a serious concern especially for large-scale applications owing to the utilization of flammable organic liquid electrolytes. Replacing liquid electrolytes by a solid electrolyte will overcome the safety issue.[1] Therefore, identification of solid electrolyte with ionic conductivity comparable to organic liquids is the major scientific challenge. Recently, lithium amide-borohydrides, i.e. Li(BH4)1-x(NH2)x, were reported as a new class of Li-ion conductors [2]. When 2/3 ≤ x ≤ 3/4, Li(BH4)1-x(NH2)x are composed of a major cubic α-phase (space group: I213 ) and a minor side phase. A first-order transition at 40 °C and the high Li-ion conductivities of Li(BH4)1-x(NH2)x were found to be strongly related to the side phase. The ionic conductivities achieve above 6.0×10-3 cm-1 at 40 °C, which is comparable to the value of liquid organic electrolytes. An Li4Ti5O12 half-cell based on such an Li(BH4)1-x(NH2)x electrolyte displays >60% capacity retention at 3.5 mA/cm2 (5C) and stable cycling for 400 cycles. In this presentation, we will report the synthesis, crystal structure and electro-chemical properties of the side phase and pure cubic α-phases. We further nanostructured the side phase, which stabilized the highly conductive state to below room temperature. [1] Jürgen Janek and Wolfgang G. Zeier, Nature Energy, 2016, 16141. [2] Y. Yan, R.-S. Kühnel, A. Remhof, et al., Adv. Energy Mater., accepted.

Resume : Light metal hydrides are relevant for a range of applications including reversible hydrogen storage, thermal energy storage and batteries. Nanosizing often improves the functionality, leading for instance to enhanced Li+ ionic conductivity and largely enhanced reversibility and sorption kinetics in hydrogen storage.[1] The reversible interaction of these materials with hydrogen might also lead to new applications in (de)hydrogenation catalysis, relevant for indirect hydrogen storage,[2] in which they might replace expensive transition metals such as Pd, Pt, and Ru. I will highlight recent progress in our group regarding the use of carbon supported light metal hydride nanoparticles as catalysts. A first example are carbon supported LiH and LiNH2 nanoparticles, which where investigated for ammonia sorption and decomposition. Another interesting line of research is the use of carbon-supported NaAlH4 as a hydrogenation catalyst for organic compounds in the liquid phase, which we investigate using amongst others norborylene as a probe molecule.[3] [1] de Jongh and Adelhelm, ChemSusChem, 3 (2010), 1332; Blanchard, de Jongh et al., Adv. Funct. Mater. 25 (2015), 182. [2] Streukens and Schüth, J. Alloys Comp. 474 (2009), 57; Makepeace, David et al, Chem Sc. 6 (2015), 3805; Chen et al, Nature Chem. 9 (2017), 64 [3] Bramwell, de Jongh et al. J. Phys. Chem C 120 (2016), 27212; Int. J. Hydr. En. 42 (2017), 5188; J. Catal. 344 (2016), 129;

Resume : Solid state compounds with ionic conductivity larger than 1 mS/cm at room temperature may compete with liquid electrolytes in rechargeable batteries. New sulfide materials recently discovered extend family of solid state ionic conductors toward this direction. Another new class of new fast ionic conductors was found as a by-product of hydrogen storage research in borate materials. Especially, very high conductivity of sodium reported in Na2B12H12 can compete with the best alumina based solid state conductors known for this metal. Further developments reveal borate based Na superionic conductors that are by orders of magnitude better than those based on beta-alumina. On the contrary conductivity of lithium cations is lower than sodium in these materials. The origin of cation mobility in this class of materials still remains puzzled. We will present examples of theoretical studies, based on DFT calculations, covering problems related to structure, dynamics, and the ionic conductivity mechanism in compounds with B12H122- anions. The relation between the superionic conductivity and the crystal structure will be presented.

Resume : Alkali metals hydrides are characterized by high hydrogen gravimetric densities, although their bulk stability complicates the use in reversible systems. However, when intercalated in C60 crystals, Na and Li metals have the ability to activate a reversible hydrogen storage process, which involve the hydrogen chemisorption on the fullerene molecule and the fast decomposition of LiH and NaH at temperatures and enthalpies well below their bulk counterparts (i.e. 250-350 C, 40-60 kJ/mol). Alkali cluster-intercalated fullerides consist in crystalline nanostructures in which positively charged Li (Na) metal clusters are ionically bond to negatively charged C60 molecules, forming charge-transfer salts. Former studies of the hydrogen storage properties of these compounds highlighted the synergic interplay between the carbon nanostructure and the alkali metal, that allows to store up to 6 wt% H2 in LixC60 (x=6, 12) and 3.5 wt% H2 in NaxC60 (x=6, 10) at temperatures slightly above 300 C.[1–4] The H2 desorption occurs below 400 C, with desorption enthalpies in the order of 56-66 kJ/mol H2. In this work, we will discuss the optimization of the hydrogen storage capabilities of these compounds after modification of the metal composition. In particular, we will show how the substitution of small fractions of Li in Li6C60 and Li12C60 by other alkali and alkali-earth metals (i.e. Na, K, Rb, Mg, and Ca) can affect the structure, charge, dynamics, and the final performance of such materials. The synthesis of ACIF compounds is carried out by solid state reaction after mechanical milling of metals and C60 in controlled atmosphere. Kinetic hydrogen absorption has been carried out at 100 bar H2. Thermodynamic features were investigated by DSC analysis during hydrogen discharge under 1 bar H2.[5] NMR was adopted to study the dynamics.[6,7] In situ neutron diffraction (under D2 pressure) has been carried out to study the absorption processes.[8] Muon spin relaxation experiments were carried out to determine the hydrogen interaction mechanisms.[9,10] We found that the destabilizing effect of Na in the co-intercalated NaxLi6-xC60 compounds leads to an improvement of the hydrogen-sorption kinetics by about 70%, accompanied by a decrease in desorption enthalpy from 62 to 44 kJ/mol H2. Overall, our results suggest that these materials are of potential interest for mobile applications. [1] J.A. Teprovich, M.S. Wellons, et al. Nano Lett. 12 (2012) 582–9. [2] P. Mauron, M. Gaboardi, et al. Int. J. Hydrogen Energy. 37 (2012) 14307–14314. [3] P. Mauron, M. Gaboardi, et al. J. Phys. Chem. C. 117 (2013) 22598–22602. [4] D. a Knight, J. a Teprovich, et al. Nanotechnology. 24 (2013) 455601. [5] M. Gaboardi, C. Milanese, et al. Cond. Mat. arxiv.org/abs/1611.06512. (2016). [6] N. Sarzi Amadè, M. Gaboardi, et al. J. Phys. Chem. C. 121 (2017) [7] N. Sarzi Amadè, M. Gaboardi, et al. Int. J. Hydrogen Energy. (2017) 1–7. [8] M. Gaboardi, S. Duyker, et al. J. Phys. Chem. C. 119 (2015) 19715–19721. [9] M. Gaboardi, C. Cavallari, et al. Carbon N. Y. 90 (2015) 130–137. [10] M. Gaboardi, N. Sarzi-Amadé, et al. Carbon N. Y. (2017).

Resume : In recent years, borane compounds have been studied as materials for various energy applications. Especially, recent discovery of super-ionic conductivity in closo-borane salts gives hope to apply them as solid state electrolytes in rechargeable batteries. Increasing interest makes the correct computational representation of these materials imperative for further studies. In this work, we focus on structures containing the closo-borane anion B12H122- with alkali metals and magnesium. The thermodynamic and structural properties of these compounds are investigated for various approximations of the Density Functional Theory and compared with available experimental results. We show that the incorporation of van der Waals forces is essential for the proper description of their static and dynamical properties. Furthermore, the thermodynamic profile of alkali metal and Mg closo-boranes is presented. All compounds are thermodynamically stable below 100 oC, and stability window covers conditions required for the solid state electrolyte. Particularly for MgB12H12 a new crystal structure is discovered[1]. The structure has higher density than the one previously presented by Ozolins et al. [2]. The properties of MgB12H12 raise questions about the stability of this pure compound in the solid state form. [1] A. E. Maniadaki et al. submitted [2] J. Am. Chem. Soc., 131, 230-237, (2009)