Enhanced dehydrogenation/hydrogenation kinetics of the Mg(NH2)2-2LiH system with NaOH additive

The effects of NaOH addition on hydrogen absorption/desorption properties of the Mg(NH₂)₂-2LiH system were investigated systematically by means of dehydrogenation/hydrogenation measurements and structural analyses. It is found that the NaOH-added Mg(NH₂)₂-2LiH samples exhibit an enhanced dehydrogenation/hydrogenation kinetics. In particular, a ∼36 °C reduction in the peak temperature for dehydrogenation is achieved for the Mg(NH₂)₂-2LiH-0.5NaOH sample with respect to the pristine sample. Structural examinations reveal that NaOH reacts with Mg(NH₂)₂ and LiH to convert to NaH, LiNH₂ and MgO during ball milling. Then, their co-catalytic effects result in a significant improvement in the dehydrogenation/hydrogenation kinetics of the Mg(NH₂)₂-2LiH system. This finding will help in designing and optimizing the novel high-performance catalysts to further improve hydrogen storage in the amide-hydride combined systems.     

The enhanced hydrogen storage performance of (Mg–B–N–H)-doped Mg(NH2)–2LiH system

Doping Mg(NH₂)₂–2LiH by Mg₂(BH₄)₂(NH₂)₂ compound exhibits enhanced hydrogen de/re-hydrogenation performance. The peak width in temperature-programmed desorption (TPD) profile for the Mg(NH₂)₂–2LiH–0.1Mg₂(BH₄)₂(NH₂)₂ was remarkably shrunk by 60 °C from that of pristine Mg(NH₂)₂–2LiH, and the peak temperature was lowered by 12 °C from the latter. Its isothermal dehydrogenation rate was greatly improved by five times from the latter at 200 °C. XRD, FTIR and NMR analyses revealed that a series of reactions occurred in the dehydrogenation of the composite. The prior interaction between LiH and Mg–B–N–H yielded intermediate LiBH₄, which subsequently reacted with Mg(NH₂)₂ and LiH in molar ratio of 1:6:9 to form Li₂Mg₂(NH)₃ and Li₄BN₃H₁₀ phases. The observed 6Mg(NH₂)₂–9LiH–LiBH₄ combination dominated the hydrogen release and soak in the composite system, and enhanced the kinetics of the system.     

Thermal dehydrogenation behaviors and mechanisms of Mg(BH4)2∙6NH3-xLiH combination systems

A reactive composite of Mg(BH₄)₂⋅6NH₃-xLiH is prepared, and the effects of the LiH content on the dehydrogenation/hydrogenation properties of the material are investigated. The results show that the presence of LiH with x = 3 reduces the onset dehydrogenation temperature of Mg(BH₄)₂⋅6NH₃ from 130 °C to 80 °C in TPD mode. Approximately 14.3 wt% hydrogen is released from the Mg(BH₄)₂⋅6NH₃-6LiH composite with distinctly reduced ammonia evolution while heating to 340 °C. Upon heating, Mg(BH₄)₂⋅6NH₃ first reacts with LiH to form Mg(NH₂)₂, Li₃BN₂H₈ and LiBH₄ with the release of H₂ and the evolution of a minor amount of NH₃. The newly formed Mg(NH₂)₂ then reacts with LiH to produce H₂ and Li₂Mg(NH)₂. Further elevating the operating temperature induces chemical reactions between Li₂Mg(NH)₂, LiBH₄ and Li₃BN₂H₈, causing the release of additional H₂ and production of Li₃BN₂, LiMgBN₂ and LiH. The dehydrogenated sample at 210 °C absorbs 2.2 wt% of hydrogen, exhibiting partial reversibility for hydrogen storage.     

Effect of CO on hydrogen storage performance of 2LiNH2 + MgH2 system

(2LiNH₂ + MgH₂) system is one of the most promising hydrogen storage materials due to its suitable operation temperature and high reversible hydrogen storage capacity. In studies and applications, impurities such as CO, CO₂, O₂, N₂ and CH₄ are potential factors which may influence its performance. In the present work, hydrogen containing 1 mol% CO is employed as the hydrogenation gas source, and directly participates in the reaction to investigate the effect of CO on the hydrogen sorption properties of (2LiNH₂ + MgH₂) system. The results indicate that the hydrogen capacity of the (Mg(NH₂)₂ + 2LiH) system declines from 5 wt.% to 3.45 wt.% after 6 cycles of hydrogenation and dehydrogenation, and can not restore to its initial level when use purified hydrogen again. The hydrogen desorption kinetics decreases obviously and the dehydrogenation activation energy increases from 133.35 kJ/mol to 153.35 kJ/mol. The main reason for these is that two new products Li₂CN₂ and MgO appear after (2LiNH₂ + MgH₂) react with CO. They are formed on the surface of materials particles, which may not only cause a permanent loss of NH²⁻, but also prevent the substance transmission during the reaction process. After re-mechanically milling, both kinetics and dehydrogenation activation energy can be recovered to the initial level.     

Achievement of excellent hydrogen sorption through swift hydrogen transport in 1:2 Mg(NH2)2–LiH catalyzed by Li4BH4(NH2)3and carbon nanostructures

The present studies deal with the catalytic character of carbon nanostructure (Graphene (Gr) and single-wall carbon nanotubes (SWNTs), and their composite versions) on the hydrogen sorption behavior of 1:2 Mg(NH₂)₂–LiH/Li₄BH₄(NH₂)₃. The inclusion of an optimal quantity of 2 wt% SWNTs in Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ resulted in superior hydrogen sorption over 2 wt% Gr and 2 wt% of (Gr and SWNT) composite. The onset desorption temperature for SWNTs catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ is 108 °C which is 32 °C, 44 °C lower compared to Gr catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ and uncatalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ respectively. The de/re-hydrogenation kinetics of the SWNT catalyzed sample has been found to be 4.02 wt% and 4.63 wt% within 15min at 170 °C and 7 MPa H₂ pressure, correspondingly. The activation energy for SWNT catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ has been found to be 69.75 kJ/mol. The SWNT catalyzed Mg(NH₂)₂–2LiH/Li₄BH₄(NH₂)₃ shows good cyclic stability (almost no degradation) up to 10 cycles. The better hydrogen sorption for SWNTs is attributed to the ballistic transport of hydrogen atoms within and across the amide/hydride matrix. In contrast, Gr sheets agglomerate, which adversely affects hydrogen sorption from Gr and Gr+SWNT composites. A hydrogen sorption mechanism has been proposed based on structural, microstructural, Fourier-transform infrared spectroscopy, and Raman characterization results.     

Improved hydrogen storage performance of Mg(NH2)2/LiH mixture by addition of carbon nanostructured materials

The effect of different carbon nanostructures specifically carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the improvement of the de/re-hydrogenation characteristics of a Mg(NH₂)₂/LiH mixture have been studied. Amongst CNTs and CNFs, the improvement in the hydrogenation properties for the Mg(NH₂)₂/LiH mixture is higher when CNFs are used as a catalyst. Investigations are also focused on the deployment of two different types of CNF (a) CNF1 (synthesized using a ZrFe₂ catalyst) and (b) CNF2 (synthesized using a LaNi₅ catalyst). The results show that CNF2 is better. The maximum decomposition temperature for the pristine Mg(NH₂)₂/LiH mixture is found to be ∼250 °C, which is reduced to ∼180 and ∼150 °C for the sample mixed with 4 wt% of multi-walled carbon nanotubes (MWCNTs) and CNF2 respectively. The activation energy for the dehydrogenation reaction is found to be 74 and 68 kJ mol⁻¹ for the samples mixed with MWCNT and CNF2 respectively, whereas the activation energy for the dehydrogenation reaction of the pristine Mg(NH₂)₂/LiH mixture is 97 kJ mol⁻¹. The catalytic activity and the de/re-hydrogenation characteristics of the Mg(NH₂)₂/LiH mixture mixed with different carbon nanostructures are described and discussed.     

Synthesis and thermal decomposition properties of a novel dual-cation/anion complex hydride Li2Mg(BH4)2(NH2)2

A novel dual-cation/anion complex hydride (Li₂Mg(BH₄)₂(NH₂)₂), which contains a theoretical hydrogen capacity of 12.1 wt%, is successfully synthesized for the first time by ball milling a mixture consisting of MgBH₄NH₂ and Li₂BH₄NH₂. The prepared Li₂Mg(BH₄)₂(NH₂)₂ crystallizes in a triclinic structure, and the [NH₂] and [BH₄] groups remain intact within the structure. Upon heating, the prepared Li₂Mg(BH₄)₂(NH₂)₂ decomposes to release approximately 8.7 wt% hydrogen in a three-step reaction at 100–450 °C. In addition, a small amount of ammonia is evolved during the first and second thermal decomposition steps as a side product. This ammonia is responsible for the lower experimental dehydrogenation amount compared to the theoretical hydrogen capacity. The XRD and FTIR results reveal that Li₂Mg(BH₄)₂(NH₂)₂ first decomposes to LiMgBN₂, LiBH₄, BN, LiH and MgBNH₈ at 100–250 °C, and then, the newly formed MgBNH₈ reacts with LiH to form Mg, LiBH₄ and BN at 250–340 °C. Finally, the decomposition of LiBH₄ releases hydrogen and generates LiH and B at 340–450 °C.     

Studies on the de/re-hydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with carbon nanofibres

The effect of carbon nanofibres (CNFs) on the de/re-hydrogenation characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture is investigated. It is found that the desorption as well as absorption characteristic of the 1:2 Mg(NH₂)₂/LiH mixture is improved with admixing of different shaped (planar and helical) CNFs separately. The different shaped CNFs were synthesized through catalytic decomposition of acetylene gas over LaNi₅ alloy. The synthesized CNFs contain Ni-metal nano particles. Among two different types of nanofibres namely planar carbon nanofibres (PCNFs) and helical carbon nanofibres (HCNFs), the later was found to act as a better catalyst. The decomposition temperature of the pristine Mg(NH₂)₂/LiH mixture is ∼250 °C, reduced to 150 and 140 °C for the PCNF and HCNF admixed Mg(NH₂)₂/LiH mixture respectively. The activation energy for dehydrogenation reaction was found to ∼97.2 kJ/mol, which is further reduced to ∼67 and ∼65 kJ/mol for the PCNF and HCNF admixed Mg(NH₂)₂/LiH mixture respectively. The lowering of decomposition temperature and enhancement in desorption kinetics, with admixing of different shaped CNFs are described and discussed.     

Releasing 9.6 wt% of H2 from Mg(NH2)2–3LiH–NH3BH3 through mechanochemical reaction

Ball milling the mixture of Mg(NH₂)₂, LiH and NH₃BH₃ in a molar ratio of 1:3:1 results in the direct liberation of 9.6 wt% H₂ (11 equiv. H), which is superior to binary systems such as LiH–AB (6 equiv. H), AB–Mg(NH₂)₂ (No H₂ release) and LiH–Mg(NH₂)₂ (4 equiv. H), respectively. The overall dehydrogenation is a three-step process in which LiH firstly reacts with AB to yield LiNH₂BH₃ and LiNH₂BH₃ further reacts with Mg(NH₂)₂ to form LiMgBN₃H₃. LiMgBN₃H₃ subsequently interacts with additional 2 equivalents of LiH to form Li₃BN₂ and MgNH as well as hydrogen.     

Studies on dehydrogenation characteristic of Mg(NH2)2/LiH mixture admixed with vanadium and vanadium based catalysts (V,V2O5 and VCl3)

In the present study, we have investigated the effect of vanadium and its compounds (V, V₂O₅ and VCl₃) on desorption characteristics of 1:2 magnesium amide (Mg(NH₂)₂) and lithium hydride (LiH) mixture. The hydrogen storage characteristics of 1:2 Mg(NH₂)₂/LiH mixture gets enhanced with admixing of V, V₂O₅ and VCl₃ separately. The VCl₃ has been found to be the most effective followed by V and V₂O₅. The activation energy for dehydrogenation process of 1:2 Mg(NH₂)₂/LiH mixture with and without catalyst has been evaluated using a method suggested by Ozawa et al. [25]. Based on the experimental results, schematic reaction scheme for decomposition of Mg(NH₂)₂ in the presence of VCl₃ has also been proposed.     

Effect of Al on the hydrogen storage properties of Mg(BH4)2

The various Mg–B–Al–H systems composed of Mg(BH₄)₂ and different Al-sources (metallic Al, LiAlH₄ and its decomposition products) have been investigated as potential hydrogen storage materials. The role of Al was studied on the dehydrogenation and the rehydrogenation of the systems. The results indicate that the different Al-sources exhibit a similar improving effect on the dehydrogenation properties of Mg(BH₄)₂. Taking the Mg(BH₄)₂ + LiAlH₄ system as an example, at first LiAlH₄ rapidly decomposes into LiH and Al, then Mg(BH₄)₂ decomposes into MgH₂ and B, finally the MgH₂ reacts with Al, LiH and B, which forms Mg₃Al₂ and MgAlB₄. The system starts to desorb H₂ at 140 °C and desorbs 3.6 wt.% H₂ below 300 °C, while individual Mg(BH₄)₂ starts to desorb H₂ at 250 °C and desorbs only 1.3 wt.% H₂ below 300 °C. The isothermal desorption kinetics of the Mg–B–Al–H systems is about 40% faster than that of Mg(BH₄)₂ at the hydrogen desorption ratio of 90%. In addition, the Mg–B–Al–H systems show partial reversibility at moderate temperature and pressure. For Al-added system, the product of rehydrogenation is MgH₂, while for LiAlH₄-added system the product is composed of LiBH₄ and MgH₂.     

Mechanistic investigations on significantly improved hydrogen storage performance of the Ca(BH4)2-added 2LiNH2/MgH2 system

The hydrogen storage properties and mechanisms of the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system were systematically investigated. The results showed that the addition of Ca(BH₄)₂ pronouncedly improved hydrogen storage properties of the 2LiNH₂–MgH₂ system. The onset temperature for dehydrogenation of the 2LiNH₂–MgH₂–0.3Ca(BH₄)₂ sample is only 80 °C, a ca. 40 °C decline with respect to the pristine sample. Further hydrogenation examination indicated that the dehydrogenated 2LiNH₂–MgH₂–0.1Ca(BH₄)₂ sample could absorb ca. 4.7 wt% of hydrogen at 160 °C and 100 atm while only 0.8 wt% of hydrogen was recharged into the dehydrogenated pristine sample under the same conditions. Structural analyses revealed that during ball milling, a metathesis reaction between Ca(BH₄)₂ and LiNH₂ firstly occurred to convert to Ca(NH₂)₂ and LiBH₄, and then, the newly developed LiBH₄ reacted with LiNH₂ to form Li₄(BH₄)(NH₂)₃. Upon heating, the in situ formed Ca(NH₂)₂ and Li₄(BH₄)(NH₂)₃ work together to significantly decrease the operating temperatures for hydrogen storage in the Ca(BH₄)₂-added 2LiNH₂–MgH₂ system.     

Investigation of the processes for reversible hydrogen storage in the Li–Mg–N–H system

The hydrogen storage performances of the Li–Mg–N–H system are investigated starting either from 1:2 Mg(NH₂)₂–LiH or 1:2 MgH₂–LiNH₂ ball-milled mixtures. It is shown that, for 1:2 MgH₂–LiNH₂, an ammonia release occurs if the first heating is conducted under a dynamic vacuum, leading to a fast degradation of the material. The positive role of LiH, if initially present in the mixture, is therefore emphasized as LiH rapidly reacts with ammonia and avoids the contamination of the hydrogen desorbing flow. The desorption kinetics of the ball-milled 1:2 Mg(NH₂)₂–LiH mixture are fast: a total amount of 5.0 wt.% of hydrogen is desorbed in 25 min at 220 °C. This material exhibits a nice reversibility at 200 °C with an experimental capacity around 4.8 wt.%. Preliminary results are given on the structure of Li₂Mg(NH)₂, formed upon desorption: this phase crystallizes in a cubic unit cell with a lattice parameter of 10.06(1) Å. In addition, by plotting an absorption isotherm of the Li₂Mg(NH)₂ phase at 200 °C, two pressure plateaus are observed revealing the existence of an intermediary phase between Li₂Mg(NH)₂ and the rehydrided material, which is the 1:2 Mg(NH₂)₂–LiH mixture.     

Hydrogen absorption–desorption mechanisms for the ball-milled Li3N–MgH2 (1:1) mixture

The Li–Mg–N–H system is a very promising hydrogen storage material due to its high capacity, reversibility and moderate operating conditions. In this work, the LiMgN/2LiH was directly synthesized by ball-milling the mixture of Li₃N–MgH₂ at 1:1 molar ratio by a reaction of Li₃N + MgH₂ → LiMgN + 2LiH. The hydrogenation/dehydrogenation properties of the as-prepared LiMgN/2LiH were investigated by a Sieverts'-type apparatus. The mixture of LiMgN/2LiH started to absorb hydrogen at 130 °C, and 2.2 wt%, 3.2 wt% hydrogen were absorbed under a pressure of 5 MPa and 10 MPa, respectively. Powder X-ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) spectrometer measurements were used to identify the phase characterizations of the products during the hydrogen absorption–desorption process. The reaction mechanism during the hydrogenation/dehydrogenation process for the Li₃N–MgH₂ system is discussed.     

Raman spectroscopic observation of dehydrogenation in ball-milled LiNH2–LiBH4–MgH2 nanoparticles

In situ Raman spectroscopy was used to monitor the dehydrogenation of ball-milled mixtures of LiNH₂–LiBH₄–MgH₂ nanoparticles. The as-milled powders were found to contain a mixture of Li₄BN₃H₁₀ and Mg(NH₂)₂, with no evidence of residual LiNH₂ or LiBH₄. It was observed that the dehydrogenation of both of Li₄BN₃H₁₀ and Mg(NH₂)₂ begins at 353 K. The Mg(NH₂)₂ was completely consumed by 415 K, while Li₄BN₃H₁₀ persisted and continued to release hydrogen up to 453 K. At higher temperatures Li₄BN₃H₁₀ melts and reacts with MgH₂ to form Li₂Mg(NH)₂ and hydrogen gas. Cycling studies of the ball-milled mixture at 423 K and 8 MPa (80 bar) found that during rehydrogenation of Li₄BN₃H₁₀ Raman spectral modes reappear, indicating partial reversal of the Li₄BN₃H₁₀ to Li₂Mg(NH)₂ transformation.     

Structural and reaction pathway analyses of Mg(BH4)2·2NH3 for hydrogen storage : A first-principles study

Mg(BH₄)₂·2NH₃ is a relatively new compound considered for hydrogen storage. The fundamental properties of the compound were comprehensively studied using first-principles calculations, such as crystal structure and electronic structure, reaction Gibbs free energy and possible reaction pathway. The calculated crystal structure is in good agreement with the experimental and other theoretical results. Results from electronic density of states (DOS) and electron localization function (ELF) show the covalent characteristics of the N–H and the B–H bonds, and the weak ionic interactions between the Mg atom and the NH₃ ligands or the (BH₄)⁻ ligands. The reaction Gibbs free energies of several possible decomposition reactions were calculated between 0 and 700 K. All the reactions are exothermic. The most likely reaction pathway of the dehydrogenation reaction was clarified to show five distinct steps.     

Combined effects of molar ratio and ball milling energy on the phase transformations and mechanical dehydrogenation in the lithium amide-magnesium hydride (LiNH2 + nMgH2)(n = 0.5–2.0) nanocomposites

The phase transformations occurring as a function of the ball milling energy injected into the hydride system (LiNH₂ + nMgH₂), having molar ratios n = 0.5–2.0, have been thoroughly studied. The milling energy in a magneto-mill is estimated by a semi-empirical method. X-ray diffraction (XRD) and Fourier Transform Infrared (FT-IR) measurements show that for the molar ratios n < 1.0 three new phases such as LiH, amorphous Mg(NH₂)₂ (a-Mg(NH₂)₂) and Li₂Mg(NH)₂ are formed during ball milling depending on the injected quantity of milling energy. Hydrogen is not released during milling when the LiH and a-Mg(NH₂)₂ hydrides are being formed whereas the formation of the Li₂Mg(NH)₂ hydride phase is always accompanied by a profound release of hydrogen. For the molar ratios n ≥ 1.0, at a low level of injected milling energy, the hydride phases formed are LiH and a-Mg(NH₂)₂. The latter reacts with MgH₂ during further milling to form the new phase MgNH whose formation is also accompanied by a profound release of hydrogen. Based on the experimental data we established an approximate hydride phase-injected milling energy diagram for various levels of injected milling energy and the molar ratios.     

Tuning the reaction mechanism and hydrogenation/dehydrogenation properties of 6Mg(NH2)29LiH system by adding LiBH4

The hydrogen storage properties of 6Mg(NH₂)₂9LiH-x(LiBH₄) (x = 0, 0.5, 1, 2) system and the role of LiBH₄ on the kinetic behaviour and the dehydrogenation/hydrogenation reaction mechanism were herein systematically investigated. Among the studied compositions, 6Mg(NH₂)₂9LiH2LiBH₄ showed the best hydrogen storage properties. The presence of 2 mol of LiBH₄ improved the thermal behaviour of the 6Mg(NH₂)₂9LiH by lowering the dehydrogenation peak temperature nearly 25 °C and by reducing the apparent dehydrogenation activation energy of about 40 kJ/mol. Furthermore, this material exhibited fast dehydrogenation (10 min) and hydrogenation kinetics (3 min) and excellent cycling stability with a reversible hydrogen capacity of 3.5 wt % at isothermal 180 °C. Investigations on the reaction pathway indicated that the observed superior kinetic behaviour likely related to the formation of Li₄(BH₄)(NH₂)₃. Studies on the rate-limiting steps hinted that the sluggish kinetic behaviour of the 6Mg(NH₂)₂9LiH pristine material are attributed to an interface-controlled mechanism. On the contrary, LiBH₄-containing samples show a diffusion-controlled mechanism. During the first dehydrogenation reaction, the possible formation of Li₄(BH₄)(NH₂)₃ accelerates the reaction rates at the interface. Upon hydrogenation, this ‘liquid like’ of Li₄(BH₄)(NH₂)₃ phase assists the diffusion of small ions into the interfaces of the amide-hydride matrix.     

Improved hydrogen storage reversibility of LiBH4 destabilized by Y(BH4)3 and YH3

In the present study, the synthesis of two different LiBH₄–Y(BH₄)₃ and LiBH₄–YH₃ composites was performed by mechanochemical processing of the 4LiBH₄–YCl₃ mixture and as-milled 4LiBH₄–YCl₃ plus 3LiH. It was found that Y(BH₄)₃ and YH₃ formed in situ during milling are effective to promote LiBH₄ destabilization but differ substantially from each other in terms of the dehydrogenation pathway. During LiBH₄–Y(BH₄)₃ dehydriding, Y(BH₄)₃ decomposes first generating in situ freshly YH₃ and subsequently, it destabilizes LiBH₄ with the formation of minor amounts of YB₄. About 20% of the theoretical hydrogen storage was obtained via the rehydriding of YB₄–4LiH–3LiCl at 400 °C and 6.5 MPa. As a novel result, a compound containing (B₁₂H₁₂)²⁻ group was identified during dehydriding of Y(BH₄)₃. In the case of 4LiBH₄–YH₃ dehydrogenation, the increase of the hydrogen back pressure favors the formation of crystalline YB_4, whereas a reduction to ≤0.1 MPa induces the formation of minor amounts of Li₂B₁₂H₁₂. Although for hydrogen pressures ≤0.1 MPa direct LiBH₄ decomposition can occur, the main dehydriding pathway of 4LiBH₄–YH₃ composite yields YB₄ and LiH. The nanostructured composite obtained by mechanochemical processing gives good hydrogen storage reversibility (about 80%) regardless of the hydrogen back pressure.     

The reaction pathway and rate-limiting step of dehydrogenation of the LiHN2 + LiH mixture

The reaction pathway and rate-limiting step of dehydrogenation of the LiNH₂ + LiH mixture have been investigated. The study reveals that dehydrogenation of the LiNH₂ + LiH mixture is diffusion-controlled and the rate-limiting step is NH₃ diffusion through the Li₂NH product layer outside the LiNH₂ shrinking core. This phenomenon is explained based on a model describing the major steps of the dehydriding reaction of the mixture, and related to the evidence obtained from X-ray diffraction and specific surface area measurements of the mixture before and after isothermal hydrogen uptake/release cycles at high homologous temperatures.