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Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity

Final Report Summary - ECOSTORE (Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity)

Borohydride- and nitride-based materials exhibit very high hydrogen storage capacities up to 18 wt%, and they also show excellent properties as novel solid room temperature ion conductors or negative electrode materials with improved capacity. The objectives of ECOSTORE have been to obtain a fundamental understanding of these metal hydride based energy storage materials, and to develop them towards industrial implementation. Detailed fundamental studies in close collaboration with those for hydrogen and those for electrochemical energy storage enabled ECOSTORE to go beyond the state-of-the-art towards its targets on energy storage materials:
• Development of (i) novel bi- and tri-metal borohydrides, complex nitrogen based hydrides, their combinations and Reactive Hydride Composites with high hydrogen densities, low decomposition temperatures, high ion conductivities at low temperatures, and high electrochemical capacities, respectively,
(ii) techniques for stabilisation of high temperature phases towards room temperature, exhibiting enhanced diffusion rates for ions and other mobile active species,
(iii) improved kinetics and thermodynamics of release and uptake of hydrogen or ions, respectively, and cycling stability using suitable additives and/or nanoconfinement for reduced phase separation and crystal growth during hydrogenation, (iv) improved safety of materials by nanoconfinement,
(vi) facilitated hydrogenation by functionalised scaffolds containing catalytically active nanoparticles.
• Theoretical modelling supported by advanced in situ and nanoscale characterisation.
• Cost reduction by utilisation of less pure raw materials, available on a larger industrial scale, while still preserving storage performance of the materials.
• Evaluation of the techno-economical potentials.

In the following a summary of the most important results achieved in the ECOSTORE project is given:
• New synthesis protocols for pure, high quality boron and nitrogen based materials were developed, leading to a series of rare earth metal borohydrides and their derivatives. New types of metal amide-imide-hydride composites as well new mesoporous carbon – metal hydride composites were discovered. For all, detailed structural investigations of several metal borohydrides and amide compounds were performed and their reaction pathways were elucidated.
• Identification of suitable materials for nanoconfinement in hydrogen and/or electrochemical energy storage systems:
o Hydrogen storage: For the 1:1 Mg(NH2)2/ LiNH2 mixture, cycling PCT measurements revealed an initial hydrogen capacity of around 8%wt and adequate reversibility. This low-melting mixture was for the first time successfully infiltrated in all the porous carbons mentioned above.
o Electrochemical energy storage: two hydride materials for infiltration have been selected based on prior studies and investigations within this project: MgH2 and NaAlH4.
• Identification of suitable materials for hydrogen storage with a focus on novel and cost effective materials
o The mechanism of function of a low cost additive in the dehydrogenation process of Li-B-Mg based RHC has been clarified, opening up a new approach towards the development of effective and cost-efficient additives to tailor the kinetic behaviour of promising hydrogen storage systems
o Routes for cost effective synthesis of Li-Mg-N based RHC have been identified and material for tank testing produced in the 0.5 kg range.
• Identification of suitable materials for electrochemical energy storage with a focus on novel and cost effective materials
o Capacities larger than 1000 Ah/kg have been achieved with the MgH2-TiH2 composite materials. Ionic conductivities of 6x10-7 and 3x10-7 S/cm have been obtained for garnet-borohydride as K3Ln2Li3(BH4)12 with Ln=La, Ce). Moreover, solid electrolytes for Na-ion batteries have also been developed like e.g. Na3B12H12CB11H12 and Na2B12H12-xIx. This research has allowed to develop several battery prototypes based on solid electrolytes with both Li and Na anions and reaching 900 Ah/kg and C/10 rate for the best one.
o New ionic conductors have been identified in the LiBH4-P2S5-LiI system; a material family has been patented by Saft and CNRS. The achieved conductivity is close to 1mS/cm at room temperature, which corresponds to the targeted value for the battery application.
o Solid state sulfur cathode electrode were also prepared electrochemically at discharged (lithiated) state in order to enable the assembly of (Mg-Ti)H2 anode and sulfur cathode; a very high capacity theoretical system has been demonstrated for the first time in the scientific community.
• Understanding of Dynamics of Reactions in Energy Storage Materials
o The thorough characterization of thermodynamics and reaction paths allowed for a full CalPhaD assessment of the LiBH4-LiNH2 quasi binary phase diagram. A detailed characterization of stabilities and reaction steps of hydrogen desorption in RE-BH4, partly with further additives, was achieved. The decomposition and dehydrogenation behaviour of eutectic mixtures of LiBH4, NaBH4 and KBH4 was studied comprehensively.
o The impact of Ni and SiO2 nanoparticles on the hydrogen (de-)sorption behaviour of boron based hydrides was demonstrated. Hydrogen/deuterium isotope exchange experiments demonstrated efficient hydrogen mobility already at 80°C in Mg(BH4)2. Additionally, a partial recovery of the open porosity in ball-milled Mg(BH4)2 by heat treatment was shown. The transition from interface to diffusion control in hydrogenation kinetics of MgH2 was identified.
o The diffusion and interfacial barrier transport coefficients of H in Mg, Ti, MgH2, TiH2 and TiO2 were quantified. The pressure dependence of sorption kinetics through a TiO2 surface coating was measured. A maximum technical useful pressure has been demonstrated and quantitatively determined.
o In ab initio based theoretical predictions, saddle points and activation energies of diffusion in mixed complex hydrides were determined by the nudged elastic band method as well as by topological analysis.
o It was discovered, that cubic Li2NH was identified as a promising solid state electrolyte, that rehydrogenated Er(BH4)3 + 6 LiH was identified as a promising high performance Li ion-conductor, that additions of inert nanoparticles can reduce hydrogen desorption temperature of borohydrides significantly, and that in presence of surface oxides acceleration of hydrogen loading by pressure increase becomes limited.

ECOSTORE could demonstrate routes for cost effective synthesis of hydrogen storage materials by high energy ball milling. Together with hydrogen release temperatures below 200°C, this opened up perspectives for cost effective low pressure, high capacity hydrogen storage for stationary and mobile hydrogen storage. This might lead to a boost in applications of hydrogen technologies in various markets, where the current inconvenient high pressure or liquid hydrogen storage technologies prevent commercialisation due to the high technical and cost efforts. ECOSTORE could demonstrate novel highly conductive solid state ion conductors as well as high capacity conversion-type electrode materials. The introduction of cost effective solid state ion conductors, together with novel electrode materials to Li-ion battery technology will allow for a significant increase of capacity and safety of rechargeable batteries. Furthermore, a cost decrease by introducing a straight-forward manufacturing technology, based on solid state materials only, is expected. The combination of “better” materials with cheaper manufacturing is expected to lead to a much broader application of electricity-based solutions e.g. in transport and many other applications.
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