Recent Progress and New Perspectives on Metal Amide and Imide Systems for Solid-State Hydrogen Storage

Unveiling the Structure-Property Relationship of MgO-Supported Ni Ammonia Decomposition Catalysts from Bulk to Atomic Structure by In Situ/Operando Studies

Ammonia is currently being studied intensively as a hydrogen carrier in the context of the energy transition. The endothermic decomposition reaction requires the use of suitable catalysts. In this study, transition metal Ni on MgO as a support is investigated with respect to its catalytic properties. The synthesis method and the type of activation process contribute significantly to the catalytic properties. Both methods, coprecipitation (CP) and wet impregnation (WI), lead to the formation of Mg1-xNixO solid solutions as catalyst precursors. X-ray absorption studies reveal that CP leads to a more homogeneous distribution of Ni2+ cations in the solid solution, which is advantageous for a homogeneous distribution of active Ni catalysts on the MgO support. Activation in hydrogen at 900 °C reduces nickel, which migrates to the support surface and forms metal nanoparticles between 6 nm (CP) and 9 nm (WI), as shown by ex situ STEM. Due to the homogeneously distributed Ni2+ cations in the solid solution structure, CP samples are more difficult to activate and require harsher conditions to reduce the Ni. The combination of in situ X-ray diffraction (XRD) and operando total scattering experiments allows a structure-property investigation of the bulk down to the atomic level during the catalytic reaction. Activation in H2 at 900 °C for 2 h leads to the formation of large Ni particles (20-30 nm) for the samples synthesized by the WI method, whereas Ni stays significantly smaller for the CP samples (10-20 nm). Sintering has a negative influence on the catalytic conversion of the WI samples, which is significantly lower compared to the conversion observed for the CP samples. Interestingly, metallic Ni redisperses during cooling and becomes invisible for conventional XRD but can still be detected by total scattering methods. The conditions of activation in NH3 at 650 °C are not suitable to form enough reduced Ni nanoparticles from the solid solution and are, therefore, not a suitable activation procedure. The activity steadily increases in the samples activated at 650 °C in NH3 (Group 1) compared to the samples activated at 650 °C in H2 and then reaches the best activity in the samples activated at 900 °C in H2. Only the combination of complementary in situ and ex situ characterization methods provides enough information to identify important structure-property relationships among these promising ammonia decomposition catalysts.

Pseudo-Binary Phase diagram of LiNH2-MH (M = Na, K) Eutectic Mixture

The hunt for a cleaner energy carrier leads us to consider a source that produces no toxic byproducts. One of the targeted alternatives in this approach is hydrogen energy, which, unfortunately, suffers from a lack of efficient storage media. Solid-state hydrogen absorption systems, such as lithium amide (LiNH₂) systems, may store up to 6.5 weight percent hydrogen. However, the temperature of hydrogenation and dehydrogenation is too high for practical use. Various molar ratios of LiNH₂ with sodium hydride (NaH) and potassium hydride (KH) have been explored in this paper. The temperature of hydrogenation for LiNH₂ combined with KH and NaH was found to be substantially lower than the temperature of individual LiNH₂. This lower temperature operation of both LiNH₂-NaH and LiNH₂-KH systems was investigated in depth, and the eutectic melting phenomenon was observed. Systematic thermal studies of this amide-hydride system in different compositions were carried out, which enabled the plotting of a pseudo-binary phase diagram. The occurrence of eutectic interaction increased atomic mobility, which resulted in the kinetic modification followed by an increase in the reactivity of two materials. For these eutectic compositions, i.e., 0.15LiNH₂-0.85NaH and 0.25LiNH₂-0.75KH, the lowest melting temperature was found to be 307 °C and 235 °C, respectively. Morphological studies were used to investigate and present the detailed mechanism linked with this phenomenon.

De-hydrogenation/Rehydrogenation Properties and Reaction Mechanism of AmZn(NH2)n-2nLiH Systems (A = Li, K, Na, and Rb)

With the aim to find suitable hydrogen storage materials for stationary and mobile applications, multi-cation amide-based systems have attracted considerable attention, due to their unique hydrogenation kinetics. In this work, AmZn(NH2)n (with A = Li, K, Na, and Rb) were synthesized via an ammonothermal method. The synthesized phases were mixed via ball milling with LiH to form the systems AmZn(NH2)n-2nLiH (with m = 2, 4 and n = 4, 6), as well as Na2Zn(NH2)4∙0.5NH3-8LiH. The hydrogen storage properties of the obtained materials were investigated via a combination of calorimetric, spectroscopic, and diffraction methods. As a result of the performed analyses, Rb2Zn(NH2)4-8LiH appears as the most appealing system. This composite, after de-hydrogenation, can be fully rehydrogenated within 30 s at a temperature between 190 °C and 200 °C under a pressure of 50 bar of hydrogen.

Solid-State Hydrogen Storage Systems and the Relevance of a Gender Perspective

This paper aims at addressing the exploitation of solid-state carriers for hydrogen storage, with attention paid both to the technical aspects, through a wide review of the available integrated systems, and to the social aspects, through a preliminary overview of the connected impacts from a gender perspective. As for the technical perspective, carriers to be used for solid-state hydrogen storage for various applications can be classified into two classes: metal and complex hydrides. Related crystal structures and corresponding hydrogen sorption properties are reviewed and discussed. Fundamentals of thermodynamics of hydrogen sorption evidence the key role of the enthalpy of reaction, which determines the operating conditions (i.e., temperatures and pressures). In addition, it rules the heat to be removed from the tank during hydrogen absorption and to be delivered to the tank during hydrogen desorption. Suitable values for the enthalpy of hydrogen sorption reaction for operating conditions close to ambient (i.e., room temperature and 1–10 bar of hydrogen) are close to 30 kJ·molH2−1. The kinetics of the hydrogen sorption reaction is strongly related to the microstructure and to the morphology (i.e., loose powder or pellets) of the carriers. Usually, the kinetics of the hydrogen sorption reaction is rather fast, and the thermal management of the tank is the rate-determining step of the processes. As for the social perspective, the paper arguments that, as it occurs with the exploitation of other renewable innovative technologies, a wide consideration of the social factors connected to these processes is needed to reach a twofold objective: To assess the extent to which a specific innovation might produce positive or negative impacts in the recipient socioeconomic system and, from a sociotechnical perspective, to explore the potential role of the social components and dynamics in fostering the diffusion of the innovation itself. Within the social domain, attention has been paid to address the underexplored relationship between the gender perspective and the enhancement of hydrogen-related energy storage systems. This relationship is taken into account both in terms of the role of women in triggering the exploitation of hydrogen-based storage playing as experimenter and promoter, and in terms of the intertwined impact of this innovation in their current conditions, at work, and in daily life.

Recent Advances in Applications of Co-B Catalysts in NaBH4-Based Portable Hydrogen Generators

This review highlights the opportunities of catalytic hydrolysis of NaBH4 with the use of inexpensive and active Co-B catalysts among the other systems of hydrogen storage and generation based on water reactive materials. This process is important for the creation of H2 generators required for the operation of portable compact power devices based on low-temperature proton exchange membrane fuel cells (LT PEM FC). Special attention is paid to the influence of the reaction medium on the formation of active state of Co-B catalysts and the problem of their deactivation in NaBH4 solution stabilized by alkali. The novelty of this review consists in the discussion of basic designs of hydrogen generators based on NaBH4 hydrolysis using cobalt catalysts and the challenges of their integration with LT PEM FC. The potential of using batch reactors in which there is no need to use aggressive alkaline NaBH4 solutions is discussed. Solid-phase compositions or pellets based on NaBH4 and cobalt-containing catalytic additives are proposed, the hydrogen generation from which starts immediately after the addition of water. The review made it possible to formulate the most acute problems, which require new sci-tech solutions.

Designing an AB2-Type Alloy (TiZr-CrMnMo) for the Hybrid Hydrogen Storage Concept

The hybrid hydrogen storage method consists of the combination of both solid-state metal hydrides and gas hydrogen storage. This method is regarded as a promising trade-off solution between the already developed high-pressure storage reservoir, utilized in the automobile industry, and solid-state storage through the formation of metal hydrides. Therefore, it is possible to lower the hydrogen pressure and to increase the hydrogen volumetric density. In this work, we design a non-stoichiometric AB2 C14-Laves alloy composed of (Ti0.9Zr0.1)1.25Cr0.85Mn1.1Mo0.05. This alloy is synthesized by arc-melting, and the thermodynamic and kinetic behaviors are evaluated in a high-pressure Sieverts apparatus. Proper thermodynamic parameters are obtained in the range of temperature and pressure from 3 to 85 °C and from 15 to 500 bar: ΔHabs. = 22 ± 1 kJ/mol H2, ΔSabs. = 107 ± 2 J/K mol H2, and ΔHdes. = 24 ± 1 kJ/mol H2, ΔSdes. = 110 ± 3 J/K mol H2. The addition of 10 wt.% of expanded natural graphite (ENG) allows the improvement of the heat transfer properties, showing a reversible capacity of about 1.5 wt.%, cycling stability and hydrogenation/dehydrogenation times between 25 to 70 s. The feasibility for the utilization of the designed material in a high-pressure tank is also evaluated, considering practical design parameters.

Metals in Hydrogen Technology

The world transition to a sustainable and reliable carbon-free economy is the greatest challenge of the 21st century [...]

A Review of the MSCA ITN ECOSTORE—Novel Complex Metal Hydrides for Efficient and Compact Storage of Renewable Energy as Hydrogen and Electricity

Hydrogen as an energy carrier is very versatile in energy storage applications. Developments in novel, sustainable technologies towards a CO2-free society are needed and the exploration of all-solid-state batteries (ASSBs) as well as solid-state hydrogen storage applications based on metal hydrides can provide solutions for such technologies. However, there are still many technical challenges for both hydrogen storage material and ASSBs related to designing low-cost materials with low-environmental impact. The current materials considered for all-solid-state batteries should have high conductivities for Na+, Mg2+ and Ca2+, while Al3+-based compounds are often marginalised due to the lack of suitable electrode and electrolyte materials. In hydrogen storage materials, the sluggish kinetic behaviour of solid-state hydride materials is one of the key constraints that limit their practical uses. Therefore, it is necessary to overcome the kinetic issues of hydride materials before discussing and considering them on the system level. This review summarizes the achievements of the Marie Skłodowska-Curie Actions (MSCA) innovative training network (ITN) ECOSTORE, the aim of which was the investigation of different aspects of (complex) metal hydride materials. Advances in battery and hydrogen storage materials for the efficient and compact storage of renewable energy production are discussed.

A dual borohydride (Li and Na borohydride) catalyst/additive together with intermetallic FeTi for the optimization of the hydrogen sorption characteristics of Mg(NH2)2/2LiH

The present study deals with the material tailoring of Mg(NH₂)₂-2LiH through dual borohydrides: the reactive LiBH₄ and the non-reactive NaBH₄. Furthermore, a pulverizer, as well as a catalyst FeTi, has been added in order to facilitate hydrogen sorption. Addition of LiBH₄ to LiNH₂ in a 1 : 3 molar ratio leads to the formation of Li₄(BH₄)(NH₂)₃ which also acts as a catalyst. However, the addition of NaBH₄ doesn't lead to any compound formation but shows a catalytic effect. The onset dehydrogenation temperature of thermally treated Mg(NH₂)₂-2LiH/(Li₄(BH₄)(NH₂)₃-NaBH₄) is 142 °C as against 196 °C for the basic material Mg(NH₂)₂-2LiH. However, with the FeTi catalyzed Mg(NH₂)₂-2LiH/(Li₄(BH₄)(NH₂)₃-NaBH₄, it has been reduced to 120 °C. This is better than other similar amide/hydride composites where it is 149 °C (when the basic material is catalyzed with LiBH₄). The FeTi catalyzed Mg(NH₂)₂-2LiH/(Li₄(BH₄)(NH₂)₃-NaBH₄ sample shows better de/re-hydrogenation kinetics as it desorbs 3.9 ± 0.04 wt% and absorbs nearly 4.1 ± 0.04 wt% both within 30 min at 170 °C (with the H₂ pressure being 0.1 MPa for desorption and 7 MPa for absorption). The eventual hydrogen storage capacity of Mg(NH₂)₂-2LiH/(Li₄(BH₄)(NH₂)₃-NaBH₄ together with FeTi has been found to be ∼5.0 wt%. To make the effect of catalysts intelligible, we have put forward in a schematic way the role of Li and Na borohydrides with FeTi for improving the hydrogen sorption properties of Mg(NH₂)₂-2LiH.

Enhancement Effect of Bimetallic Amide K2Mn(NH2)4 and In-Situ Formed KH and Mn4N on the Dehydrogenation/Hydrogenation Properties of Li–Mg–N–H System

In this work, we investigated the influence of the K2Mn(NH2)4 additive on the hydrogen sorption properties of the Mg(NH2)2 + 2LiH (Li–Mg–N–H) system. The addition of 5 mol% of K2Mn(NH2)4 to the Li–Mg–N–H system leads to a decrease of the dehydrogenation peak temperature from 200 °C to 172 °C compared to the pristine sample. This sample exhibits a constant hydrogen storage capacity of 4.2 wt.% over 25 dehydrogenation/rehydrogenation cycles. Besides that, the in-situ synchrotron powder X-ray diffraction analysis performed on the as prepared Mg(NH2)2 + 2LiH containing K2Mn(NH2)4 indicates the presence of Mn4N. However, no crystalline K-containing phases were detected. Upon dehydrogenation, the formation of KH is observed. The presence of KH and Mn4N positively influences the hydrogen sorption properties of this system, especially at the later stage of rehydrogenation. Under the applied conditions, hydrogenation of the last 1 wt.% takes place in only 2 min. This feature is preserved in the following three cycles.

Decomposition pathway of KAlH4 altered by the addition of Al2S3

Altering the decomposition pathway of potassium alanate, KAlH4, with aluminium sulfide, Al2S3, presents a new opportunity to release all of the hydrogen, increase the volumetric hydrogen capacity and avoid complications associated with the formation of KH and molten K. Decomposition of 6KAlH4-Al2S3 during heating under dynamic vacuum began at 185 °C, 65 °C lower than for pure KAlH4, and released 71% of the theoretical hydrogen content below 300 °C via several unknown compounds. The major hydrogen release event, centred at 276 °C, was associated with two new compounds indexed with monoclinic (a = 10.505, b = 7.492, c = 11.772 Å, β = 122.88°) and hexagonal (a = 10.079, c = 7.429 Å) unit cells, respectively. Unlike the 6NaAlH4-Al2S3 system, the 6KAlH4-Al2S3 system did not have M3AlH6 (M = alkali metal) as one of the intermediate decomposition products nor were the final products M2S and Al observed. Decomposition performed under hydrogen pressure initially followed a similar reaction pathway to that observed during heating under vacuum but resulted in partial melting of the sample between 300 and 350 °C. The measured enthalpy of hydrogen absorption (ΔHabs) was in the range -44.5 to -51.1 kJ mol-1 H2, which is favourable for moderate temperature hydrogen applications. Although, the hydrogen capacity decreases during consecutive H2 release and uptake cycles, the presence of excess amounts of aluminium allow for further optimisation of hydrogen storage properties.

Effect of the Process Parameters on the Energy Transfer during the Synthesis of the 2LiBH4-MgH2 Reactive Hydride Composite for Hydrogen Storage

Several different milling parameters (additive content, rotation velocity, ball-to-powder ratio, degree of filling, and time) affect the hydrogen absorption and desorption properties of a reactive hydride composite (RHC). In this paper, these effects were thoroughly tested and analyzed. The milling process investigated in such detail was performed on the 2LiH-MgB2 system doped with TiCl3. Applying an upgraded empirical model, the transfer of energy to the material during the milling process was determined. In this way, it is possible to compare the obtained experimental results with those from processes at different scales. In addition, the different milling parameters were evaluated independently according to their individual effect on the transferred energy. Their influence on the reaction kinetics and hydrogen capacity was discussed and the results were correlated to characteristics like particle and crystallite size, specific surface area, presence of nucleation sites and contaminants. Finally, an optimal value for the transferred energy was determined, above which the powder characteristics do not change and therefore the RHC system properties do not further improve.

Improvements in the hydrogen storage properties of the Mg(NH2)2–LiH composite by KOH addition

KH thermodynamically destabilizes the Mg(NH₂)₂–LiH composite as it is actively involved in the dehydrogenation process.