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Large-scale energy storage plants based on power-to-gas-to-power technologies incorporating high temperature electrolysis, catalytic methanation of H₂ and CO₂ and novel, highly efficient methane-fired Allam reconversion cycles allow for a confined and circular use of CO₂ and thus an emission-free storage of intermittent renewable energy. The Allam power cycle is considered as a beneficial power plant concept, which employs supercritical CO₂ as working fluid as well as an oxy-combustion process to reach high efficiencies of up to 66%. The combination of said process chain could reach a maximum roundtrip efficiency of 54.2 % assuming the presence of sufficient storage capacities for all relevant technical gases. In a technically feasible scenario, paired with a separate air separation unit instead of stationary O₂ storages, roundtrip efficiencies of 49.0 % were determined.. The implementation of said energy storage systems into existing national energy systems will pose a major challenge, since they will require far-reaching infrastructural changes to the respective systems itself, such as extensive installations of renewable generation and electrolysis capacities as well as sufficient subsurface storage capacities for both CO₂ and CH₂. Furthermore, an exemplary energy system forecast for Germany for the year of 2050 is presented to show the viability of the energy storage concept. In case of a fully circular use of CO₂, when electricity is solely generated by renewable energy sources (RES), 736 GW of RES, 234 GW of electrolysis and 62 GW of gas-to-power capacities are required. The total storage volume on the national scale of Germany for both CO₂ and CH₄ was determined to be 7.8 billion Nm³, respectively, leading to a CH₄ storage capacity of 54.5 TWh. The present investigation illustrates the feasibility of large-scale energy storage systems for renewable electricity based on high temperature electrolysis, catalytic methanation and Allam power cycles paired with lar

Germany 2050For the first time Germany reached a balance between its sources of anthropogenic CO2 to the atmosphere and newly created anthropogenic sinks. This backcasting study presents a fictional future in which this goal was achieved by avoiding (∼645 Mt CO2), reducing (∼50 Mt CO2) and removing (∼60 Mt CO2) carbon emissions. This meant substantial transformation of the energy system, increasing energy efficiency, sector coupling and electrification, energy storage solutions including synthetic energy carriers, sector-specific solutions for industry, transport and agriculture, as well as nature-based and technological carbon dioxide removal. All of the above was necessary to achieve a net-zero CO2 system for Germany by 2050.

The thermochemical energy storage properties of calcium hydride (CaH2) destabilised with either silicon (Si) or CaxSiy compounds at various molar ratios, were thoroughly studied by a combination of experimental and computer assisted thermodynamic calculations. Particularly, the destabilisation effect of Si on CaH2 at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH2 to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH2 and Si forming CaxSiy at varying temperatures, which was confirmed by in-situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and mass-spectroscopic measurements. The most suitable destabilisation reactions between CaH2 and Si or CaxSiy that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH2 and CaSi system (in a 2:3 molar ratio of CaH2 to CaSi) showed desirable operating conditions with a decomposition temperature of 747 ± 33 °C at a hydrogen pressure of 1 bar. Pressure composition isothermal measurements were conducted on this system to determine its practical enthalpy of decomposition to form Ca5Si3. The calculated value (107.3 kJ mol−1 H2) was lower compared to the experimentally determined value (154 ± 4 kJ mol−1 H2). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca5Si3 phase.

© 2020 The Authors Globally, the accelerating use of renewable energy sources, enabled by increased efficiencies and reduced costs, and driven by the need to mitigate the effects of climate change, has significantly increased research in the areas of renewable energy production, storage, distribution and end-use. Central to this discussion is the use of hydrogen, as a clean, efficient energy vector for energy storage. This review, by experts of Task 32, “Hydrogen-based Energy Storage” of the International Energy Agency, Hydrogen TCP, reports on the development over the last 6 years of hydrogen storage materials, methods and techniques, including electrochemical and thermal storage systems. An overview is given on the background to the various methods, the current state of development and the future prospects. The following areas are covered; porous materials, liquid hydrogen carriers, complex hydrides, intermetallic hydrides, electrochemical storage of energy, thermal energy storage, hydrogen energy systems and an outlook is presented for future prospects and research on hydrogen-based energy storage.

Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The “Magnesium group” of international experts contributing to IEA Task 32 “Hydrogen Based Energy Storage” recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures, kinetics and thermodynamics of the systems based on MgH 2 , nanostructuring, new Mg-based compounds and novel composites, and catalysis in the Mg based H storage systems. Finally, thermal energy storage and upscaled H storage systems accommodating MgH 2 are presented.

In the past decades, complex hydrides and complex hydrides-based materials have been thoroughly investigated as materials for energy storage, owing to their very high gravimetric and volumetric hydrogen capacities and interesting cation and hydrogen diffusion properties. Concerning hydrogen storage, the main limitations of this class of materials are the high working temperatures and pressures, the low hydrogen absorption and desorption rates and the poor cyclability. In the past years, research in this field has been focused on understanding the hydrogen release and uptake mechanism of the pristine and catalyzed materials and on the characterization of the thermodynamic aspects, in order to rationally choose the composition and the stoichiometry of the systems in terms of hydrogen active phases and catalysts/destabilizing agents. Moreover, new materials have been discovered and characterized in an attempt to find systems with properties suitable for practical on-board and stationary applications. A significant part of this rich and productive activity has been performed by the research groups led by the Experts of the International Energy Agreement Task 32, often in collaborative research projects. The most recent findings of these joint activities and other noteworthy recent results in the field are reported in this paper.

Metal hydrides are known as a potential efficient, low-risk option for high-density hydrogen storage since the late 1970s. In this paper, the present status and the future perspectives of the use of metal hydrides for hydrogen storage are discussed. Since the early 1990s, interstitial metal hydrides are known as base materials for Ni – metal hydride rechargeable batteries. For hydrogen storage, metal hydride systems have been developed in the 2010s [1] for use in emergency or backup power units, i. e. for stationary applications. With the development and completion of the first submarines of the U212 A series by HDW (now Thyssen Krupp Marine Systems) in 2003 and its export class U214 in 2004, the use of metal hydrides for hydrogen storage in mobile applications has been established, with new application fields coming into focus. In the last decades, a huge number of new intermetallic and partially covalent hydrogen absorbing compounds has been identified and partly more, partly less extensively characterized. In addition, based on the thermodynamic properties of metal hydrides, this class of materials gives the opportunity to develop a new hydrogen compression technology. They allow the direct conversion from thermal energy into the compression of hydrogen gas without the need of any moving parts. Such compressors have been developed and are nowadays commercially available for pressures up to 200 bar. Metal hydride based compressors for higher pressures are under development. Moreover, storage systems consisting of the combination of metal hydrides and high-pressure vessels have been proposed as a realistic solution for on-board hydrogen storage on fuel cell vehicles. In the frame of the “Hydrogen Storage Systems for Mobile and Stationary Applications” Group in the International Energy Agency (IEA) Hydrogen Task 32 “Hydrogen-based energy storage” different compounds have been and will be scaled-up in the near future and tested in the range of 500 g to several hundre

Magnesium hydride owns the largest share of publications on solid materials for hydrogen storage. The “Magnesium group” of international experts contributing to IEA Task 32 “Hydrogen Based Energy Storage” recently published two review papers presenting the activities of the group focused on magnesium hydride based materials and on Mg based compounds for hydrogen and energy storage. This review article not only overviews the latest activities on both fundamental aspects of Mg-based hydrides and their applications, but also presents a historic overview on the topic and outlines projected future developments. Particular attention is paid to the theoretical and experimental studies of Mg-H system at extreme pressures, kinetics and thermodynamics of the systems based on MgH2, nanostructuring, new Mg-based compounds and novel composites, and catalysis in the Mg based H storage systems. Finally, thermal energy storage and upscaled H storage systems accommodating MgH2 are presented.

Porous Al scaffolds were synthesised and melt-infiltrated with various eutectic metal borohydride mixtures (0.725LiBH4-0.275KBH4, 0.68NaBH4-0.32KBH4, 0.4NaBH4-0.6 Mg(BH4)2) to simultaneously act as both a confining framework and a reactive destabilising agent for H2 release. The scaffolds were synthesised by sintering a pellet of NaAlH4/2 mol%TiCl3 at 450 °C under dynamic vacuum. During the sintering process the sodium alanate (NaAlH4) decomposed to Al metal. The vacuum applied at elevated temperature promoted the Na metal to vaporise and be extruded from the pellet. The pores of the resulting Al scaffold were created during removal of the H2 and the Na from the body of the NaAlH4/2 mol%TiCl3 pellet. According to the morphological observations carried out by a Scanning Electron Microscope (SEM), melt-infiltrated eutectic mixtures of metal borohydrides were highly dispersed into the porous scaffolds. Temperature Programmed Desorption (TPD) experiments, revealed that the melt-infiltrated samples exhibited faster H2 desorption kinetics in comparison to bulk samples, with onset temperatures (Tdes) lower than the bulk by 150–250 °C. The as-synthesised porous Al scaffolds acted as a reactive containment vessel for these eutectic mixtures that simultaneously nanoconfined and destabilised the mixtures.

Metal hydrides (MHs) are promising candidates as thermal energy storage (TES) materials for concentrated solar thermal applications. A key requirement for this technology is a high temperature heat transfer fluid (HTF) that can deliver heat to the MHs for storage during the day, and remove heat at night time to produce electricity. In this study, supercritical water was used as a HTF to heat a prototype thermochemical heat storage reactor filled with MgH2 powder during H2 sorption, rather than electrical heating of the MH reactor. This is beneficial as the HTF flows through a coil of tubing embedded within the MH bed and is hence in better contact with the MgH2 powder. This internal heating mode produces a more uniform temperature distribution within the reactor by increasing the heat exchange surface area and reducing the characteristic heat exchange distances. Moreover, supercritical water can be implemented as a heat carrier for the entire thermal energy system within a concentrating solar thermal plant, from the receiver, through the heat storage system, and also within a conventional turbine-driven electric power generation system. Thus, the total system energy efficiency can be improved by minimising HTF heat exchangers.

© The Royal Society of Chemistry 2017.A wide variety of metal borohydrides, MBH4, have been discovered and characterized during the past decade, revealing an extremely rich chemistry including fascinating structural flexibility and a wide range of compositions and physical properties. Metal borohydrides receive increasing interest within the energy storage field due to their extremely high hydrogen density and possible uses in batteries as solid state ion conductors. Recently, new types of physical properties have been explored in lanthanide-bearing borohydrides related to solid state phosphors and magnetic refrigeration. Two major classes of metal borohydride derivatives have also been discovered: anion-substituted compounds where the complex borohydride anion, BH4-, is replaced by another anion, i.e. a halide or amide ion; and metal borohydrides modified with neutral molecules, such as NH3, NH3BH3, N2H4, etc. Here, we review new synthetic strategies along with structural, physical and chemical properties for metal borohydrides, revealing a number of new trends correlating composition, structure, bonding and thermal properties. These new trends provide general knowledge and may contribute to the design and discovery of new metal borohydrides with tailored properties towards the rational design of novel functional materials. This review also demonstrates that there is still room for discovering new combinations of light elements including boron and hydrogen, leading to complex hydrides with extreme flexibility in composition, structure and properties.

Copyright © 2015 Hydrogen Energy Publications, LLC.NaAlH4 was melt infiltrated within a CO2 activated carbon aerogel, which had been preloaded with TiCl3. Nanoconfinement was verified by Small Angle X-Ray Scattering (SAXS) and the nature of the Ti was investigated with Anomalous SAXS (ASAXS) and X-Ray Absorption Near Edge Structure (XANES) to determine its size and chemical state. The Ti is found to be in a similar state to that found in the bulk Ti-doped NaAlH4 system where it exists as Al1-xTix nanoalloys. Crystalline phases exist within the carbon aerogel pores, which are analysed by in-situ Powder X-Ray Diffraction (PXD) during hydrogen cycling. The in-situ data reveals that the hydrogen release from NaAlH4 and its hydrogen uptake occurs through the Na3AlH6 intermediate when confined at this size scale. The hydrogen capacity from the nanoconfined NaAlH4 is found to initially be much higher in this CO2 activated aerogel compared with previous studies into unactivated aerogels.

In the framework of the European Cooperation in Science and Technology (COST) Action MP1103 Nanostructured Materials for Solid-State Hydrogen Storage were synthesized, characterized and modeled. This Action dealt with the state of the art of energy storage and set up a competitive and coordinated network capable to define new and unexplored ways for Solid State Hydrogen Storage by innovative and interdisciplinary research within the European Research Area. An important number of new compounds have been synthesized: metal hydrides, complex hydrides, metal halide ammines and amidoboranes. Tuning the structure from bulk to thin film, nanoparticles and nanoconfined composites improved the hydrogen sorption properties and opened the perspective to new technological applications. Direct imaging of the hydrogenation reactions and in situ measurements under operando conditions have been carried out in these studies. Computational screening methods allowed the prediction of suitable compounds for hydrogen storage and the modeling of the hydrogen sorption reactions on mono-, bi-, and three-dimensional systems. This manuscript presents a review of the main achievements of this Action.

© 2016, Springer-Verlag Berlin Heidelberg.The research on complex hydrides for hydrogen storage was initiated by the discovery of Ti as a hydrogen sorption catalyst in NaAlH4 by Boris Bogdanovic in 1996. A large number of new complex hydride materials in various forms and combinations have been synthesized and characterized, and the knowledge regarding the properties of complex hydrides and the synthesis methods has grown enormously since then. A significant portion of the research groups active in the field of complex hydrides is collaborators in the International Energy Agreement Task 32. This paper reports about the important issues in the field of complex hydride research, i.e. the synthesis of borohydrides, the thermodynamics of complex hydrides, the effects of size and confinement, the hydrogen sorption mechanism and the complex hydride composites as well as the properties of liquid complex hydrides. This paper is the result of the collaboration of several groups and is an excellent summary of the recent achievements.

The development of alternative methods for thermal energy storage is important for improving the efficiency and decreasing the cost of concentrating solar thermal power. We focus on the underlying technology that allows metal hydrides to function as thermal energy storage (TES) systems and highlight the current state-of-the-art materials that can operate at temperatures as low as room temperature and as high as 1100 °C. The potential of metal hydrides for thermal storage is explored, while current knowledge gaps about hydride properties, such as hydride thermodynamics, intrinsic kinetics and cyclic stability, are identified. The engineering challenges associated with utilising metal hydrides for high-temperature TES are also addressed.

© 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. An empirical understanding of the relationship between crystallite size and reaction kinetics for the dehydrogenation of MgH2 in the presence of Si was determined. MgH2 was combined with Si under different conditions to obtain varying crystallite sizes of both reactants. Thermal analysis and isothermal desorption were undertaken to obtain reaction kinetic information and therefore determine activation energies as well as the rate limiting step for each of the different crystallite sizes. It was found that there is a strong correlation between crystallite size and activation energy for the growth of the Mg2Si phase, however, any correlation between the nucleation (of Mg2Si) activation energy was less evident. Direct measurements of kinetic behaviour from a manometric Sieverts apparatus showed that initial reaction kinetics were fastest when MgH2 was mixed with Si nanoparticles, however, this sample was not able to fully desorb. Data from the Sieverts measurements were then used with well-known theoretical models to determine the rate limiting step of the reaction. The three dimensional Carter-Valensi (or contracting volume) diffusion model could be used to describe the rate limiting step for most of the reactions. These results have led to a proposed mechanism for the formation of Mg2Si during the decomposition reaction.

© International Union of Crystallography 2015.New sample environments and techniques specifically designed for in situ powder X-ray diffraction studies up to 1000 bar (1 bar = 105 Pa) gas pressure are reported and discussed. The cells can be utilized for multiple purposes in a range of research fields. Specifically, investigations of gas-solid reactions and sample handling under inert conditions are undertaken here. Sample containers allowing the introduction of gas from one or both ends are considered, enabling the possibility of flow-through studies. Various containment materials are evaluated, e.g. capillaries of single-crystal sapphire (Al2O3), quartz glass (SiO2), stainless steel (S316) and glassy carbon (Sigradur K), and burst pressures are calculated and tested for the different tube materials. In these studies, high hydrogen pressure is generated with a metal hydride hydrogen compressor mounted in a closed system, which allows reuse of the hydrogen gas. The advantages and design considerations of the in situ cells are discussed and their usage is illustrated by a case study.

Designing safe, compact and high capacity hydrogen storage systems is the key step towards introducing a pollutant free hydrogen technology into a broad field of applications. Due to the chemical bonds of hydrogen–metal atoms, metal hydrides provide high energy density in safe hydrogen storage media. Reactive hydride composites (RHCs) are a promising class of high capacity solid state hydrogen storage systems. Ca(BH4)2 + MgH2 with a hydrogen content of 8.4 wt% is one of the most promising members of the RHCs. However, its relatively high desorption temperature of ∼350 °C is a major drawback to meeting the requirements for practical application. In this work, by using NbF5 as an additive, the dehydrogenation temperature of this RHC was significantly decreased. To elucidate the role of NbF5 in enhancing the desorption properties of the Ca(BH4)2 + MgH2 (Ca-RHC), a comprehensive investigation was carried out via manometric measurements, mass spectrometry, Differential Scanning Calorimetry (DSC), in situ Synchrotron Radiation-Powder X-ray Diffraction (SR-PXD), X-ray Absorption Spectroscopy (XAS), Anomalous Small-Angle X-ray Scattering (ASAXS), Scanning and Transmission Electron Microscopy (SEM, TEM) and Nuclear Magnetic Resonance (NMR) techniques.