LiNH2 LiH Research Articles

Hydrogen sorption properties of Li–N–F–H pellets in laboratory and small tank scales

We propose the improvement of de/rehydrogenation kinetics and reversibility of LiNH2–LiH system via F substitution for H in LiNH2. Slight content of LiF–TiH2 composite is milled with LiNH2–3LiH and the obtained mixture is compacted into the pellet. By compositing LiNH2 with excess LiH and compaction, NH3 emission poisoning fuel cell catalysts and degrading hydrogen capacity can be prevented. The formation of LiNH(2-x)Fx achieved from F substitution in LiNH2 significantly enhances kinetic properties, for example, the 1st dehydrogenation at 280 °C completes within 5 min with the capacity of 3.6 wt % H2. Although LiNH(2-x)Fx cannot be recovered, good kinetics and reversibility upon five de/absorption cycles of LiNH2–LiH (up to 3.0 wt % H2 within 20 min) are preserved due to catalytic effects of LiF. Phase compositions and hydrogen capacities in laboratory and tank scales are comparable. This suggests the maintained de/rehydrogenation performance of Li–N–F–H pellets even after upscaling.

Improved hydrogen sorption kinetics of compacted LiNH2–LiH based small hydrogen storage tank by doping with TiF4 and MWCNTs

Abstract Kinetic properties of compacted LiNH2–LiH developed by doping with TiF4 and multi-walled nanotubes (MWCNTs) as well as upscaling to small hydrogen storage tank are proposed. During de/rehydrogenation, transition metal-based catalyst (TiF4) provides the catalytic effects on hydrogen dissociation/recombination, while MWCNTs benefit thermal conductivity and hydrogen permeability. Enhanced dehydrogenation kinetics is observed from single-step reaction at narrower and lower temperature range of 150–350 °C (100 °C lower than the compacted LiNH2–LiH without additives) together with long plateau temperature and constant hydrogen flow rate (50 SCCM) up to 30 min during desorption of the small tank. Hydrogen contents de/absorbed during 5–6 cycles increase from 1.90-2.40 to 3.10-4.70 wt % H2 (from 29 to up to 80% of theoretical capacity). Li5TiN3 detected upon cycling absorbs NH3 to form Li5TiN3(NH3)x, favoring hydrogen sorption properties of LiNH2–LiH system. Moreover, comparable reaction mechanisms and performances are found at different positions inside the tank of compacted LiNH2–LiH doped with TiF4 and MWCNTs.

Cyclic reaction-induced enhancement in the dehydrogenation performances of the KNH2-doped LiNH2 and LiH system

Ammonia is a vital intermediate in the hydrogen desorption process of Metal-N-H system. KH has strong reactivity with NH3 to form KNH2. We speculate that KNH2 is also an intermediate formed during hydrogen desorption of the potassium-doped M-N-H systems. In this research, the dehydrogenation performance of the KNH2-doped LiNH2 and LiH composition was first studied. Compared with the broad dehydrogenation curve of the composite material of LiNH2 and LiH without the catalyst, the dehydrogenation curve of 0.05 mol KNH2-doped composite material was significantly narrowed. The initial and peak dehydrogenation temperature of the composite to which 0.05 mol of KNH2 was added was lowered remarkably. Besides, the cyclic dehydrogenation properties of the LiNH2 and LiH system was also significantly enhanced by the introduction of KNH2. The cyclic conversion of KNH2 to KH is the main reason for the enhancement of the hydrogen evolution performance of the LiNH2–LiH system doped with KNH2. We found the KNH2-doped Li–N–H system exhibits similar dehydrogenation property with that of the KH-doped Li–N–H system. This work proves that KNH2 plays a key role in improving the hydrogen desorption performances of the potassium-doped M-N-H systems.

The effect of KH on enhancing the dehydrogenation properties of the Li-N-H system and its catalytic mechanism.

Although recent works demonstrated that some potassium compounds that can be converted to KH during ball-milling or heat-treatment have obvious effects on enhancing the dehydrogenation properties of the Li-N-H system, the effect of KH on enhancing the dehydrogenation properties of the Li-N-H system and its catalytic mechanism remain unclear. In this study, the hydrogen desorption properties of the LiNH2-LiH system with alkali metal hydrides (LiH, NaH, or KH) were investigated and discussed. We find that the three types of hydrides are effective for enhancing the hydrogen desorption properties of the LiNH2-LiH system, among which, KH shows the best effect. In comparison with the broad shaped hydrogen desorption curve of the LiNH2-LiH composite without additive, the hydrogen desorption curve of the LiNH2-LiH-0.05KH composite becomes narrow. The dehydrogenation onset temperature of the LiNH2-LiH-0.05KH composite is decreased by approximately 20 °C, and the dehydrogenation peak temperature is lowered by approximately 30 °C. Moreover, the reversibility of the LiNH2-LiH system is enhanced drastically by the addition of KH. On the basis of previous reports and present experimental results, the mechanism for the enhancement of the dehydrogenation properties in the KH-added Li-N-H system is proposed. The reason for the improvement of the hydrogen desorption kinetics is that KH has superior reactivity with NH3 and plays the role of a catalyst to accelerate hydrogen release by cyclic reactions.

Remarkably improved hydrogen storage properties of LiNH2-LiH composite via the addition of CeF4

Amide-hydride composite materials, such as LiNH2–LiH and Mg(NH2)2–2LiHcomposites, are promising candidates for hydrogen storage due to the high hydrogen storage capacity and moderate hydrogen storage enthalpies and entropies. However, the hydrogen absorption/desorption kinetics of the amide-based system is too sluggish, and emission of ammonia (NH3) during dehydrogenation also fatally deteriorates the hydrogen storage cyclic ability. In this study, we report that CeF4 shows a remarkable catalytic effect in improving hydrogen storage kinetic and cyclic properties, reducing dehydrogenation temperature, and suppressing emission of NH3 of LiNH2-LiH composite. Mechanism study reveals that the catalysis should be due to the in situ formed F-containing CeFx species during ball milling process.

Ammonia suppression during decomposition of sodium amide by the addition of metal hydride

The decomposition of NaNH2 has been reported, mainly decomposing into NaH, N2 and H2. Ammonia is also produced in addition to N2 and H2. To the best of our knowledge, very few scattered reports on the effect of alkali hydrides on NaNH2 exist in literature. Thus, we choose NaNH2–MH (M = Li, Na, K, Mg, Ca) system to be investigated in detail. Since NaNH2–NaH is the simplest combination due to same cation, it was tested for the establishment of reaction mechanism using transmission electron microscopy (TEM). It is observed that the entire process follows NH3 mediated reaction similar to LiNH2–LiH system. Sodium amide first decompose into Na metal and NH3, then generated NH3 reacted with added NaH to form NaNH2 and release H2. This process continues until the consumption of NaH, thus suppresses NH3 evolution to a great extent. The investigation has been extended further to the other metal hydrides and it is found that the addition of other metal hydride i.e. LiH, KH, MgH2, and CaH2 have also effectively suppressed the NH3 evolution. The detailed reaction mechanism has been elucidated for all the amide hydride systems. It is observed that the decomposition takes place through an intermediate step of double-cation amide formation.

Hydrogen desorption improvement of the LiNH2–LiH–KF composite

Interestingly, an obvious enhancement of the hydrogen storage property is achieved by introducing KF into the LiNH2–LiH system, while KCl, KBr or KI does not show obvious effect on improving the hydrogen storage properties of the LiNH2–LiH system. In comparison with that of the LiNH2–LiH composite, the hydrogen desorption curve of the LiNH2–LiH–0.05 KF composite becomes sharp and the dehydrogenation onset and peak temperatures are decreased by about 36 °C and 38 °C, respectively. The cyclic capacity and desorption kinetic of the LiNH2–LiH–0.05 KF composite in the tenth cycle are still much better than that of the LiNH2–LiH composite in the second cycle. Cyclic performance can be improved at least 4 times by addition of 5 mol% KF. Detailed structural investigations reveal that during the heat-treatment, the doped potassium fluoride can react with LiH and convert to potassium hydride, which induces the improvement in hydrogen storage properties of the LiNH2–LiH system.

Improvement of hydrogen storage property of three-component Mg(NH2)2-LiNH2-LiH composites by additives.

The three-component Mg(NH2)2-LiNH2-4LiH composite reversibly stores hydrogen exceeding 5 wt% at a temperature as low as 150 °C. In this work, a number of additives such as CeF4, CeO2, TiCl3, TiH2, NaH, KBH4 and KH are added to the Mg(NH2)2-LiNH2-4LiH composite in order to improve its kinetics, thermodynamics and cycling properties. Addition of 3 wt% of KH reduces the dehydrogenation onset temperature of the Mg(NH2)2-LiNH2-4LiH composite to below 90 °C without emission of NH3 during the whole dehydrogenation process up to 450 °C. Moreover, the dehydrogenation kinetics and cycling ability are remarkably enhanced upon KH-addition. The reaction model of the Mg(NH2)2-LiNH2-4LiH composite is altered upon KH-addition with the active molecule density improved by about 200 times. In addition, by optimization of the ratio of Mg2+ to Li+ in the Mg(NH2)2-LiNH2-LiH system, several novel composites, e.g., Mg(NH2)2-2LiNH2-5.9LiH-0.1KH and Mg(NH2)2-LiNH2-5.9LiH-0.1KH, with the hydrogen storage capacity exceeding 6 wt% without emission of NH3 below 250 °C are developed. Our study demonstrates that there are various undiscovered candidates with promising hydrogen storage properties in the three-component Li-Mg-N-H system.

Role of aluminum chloride on the reversible hydrogen storage properties of the Li–N–H system

In order to understand the role of AlCl3 addition on the Li–N–H system, we have systematically investigated the hydrogen sorption kinetics and the reactions between LiNH2–LiH and AlCl3 additive with a multitechnique approach involving differential scanning calorimetry (DSC), hydrogen volumetric measurements, X-ray powder diffraction (XRPD), Fourier transform infrared analysis (FTIR) and solid-state nuclear magnetic resonance (NMR). Different interactions were identified as a function of the amount of added AlCl3. For low AlCl3 addition (0.03 mol), the Al3+ is incorporated into the interstitial sites by the LiNH2 structure. When AlCl3 amount increased (0.08 and 0.13 mol), the formation of new amide-chloride phases were detected by XRPD and indexed with cubic and hexagonal Li–Al–N–H–Cl geometries. Occurrence of such new phases was also confirmed by FTIR and NMR. The formation of these new Li–Al–N–H–Cl phases modifies the kinetics as well as the thermodynamic behavior of the original Li–N–H system. Interesting, in all AlCl3-doped composites, hydrogen was stored reversibly with faster sorption kinetics than un-doped Li–N–H system and with a significant reduction of NH3 emission. This improvement can be associated with the Al3+ incorporation into LiNH2 that promotes the migration of Li+, while for high AlCl3 doping, the formation of new phases Li–Al–N–H–Cl also weakens the N–H bond.

Hydrogen storage properties of LiNH2–LiH system with MgH2, CaH2 and TiH2 added

The addition of different metal hydrides to Li–N–H system offers a possible way to modify its thermodynamic properties and/or the dehydrogenation/hydrogenation kinetics. In this paper we report the hydrogen storage properties of LiNH2–LiH system with MH2 added (M = Mg, Ca, Ti) and clarify the chemical interactions occurring during hydrogen cycling. Detailed structural investigations reveal that during heating under hydrogen, MH2 (M = Mg, Ca) reacts with LiNH2 to form Li2Mg(NH)2 and 2CaNH–Ca(NH2)2 solid solution, with simultaneous hydrogen release. Formation of the Li2NH–CaNH mixture after dehydrogenation of the LiNH2–LiH with CaH2 added is proved by XRPD and FTIR, providing a new reversible pathway for hydrogen storage in the Li–Ca–N–H system. Notable improvement in the dehydrogenation temperature and kinetics was observed for LiNH2–LiH system with CaH2 and MgH2 added, without clear effects in the case of TiH2. The kinetic analysis reveals that dehydrogenation process is diffusion-controlled and the beneficial effect of MgH2 and CaH2 is due to the enhancement of Li+ mobility.

Improved dehydrogenation properties of the LiNH2–LiH system by doping with alkali metal hydroxide

The hydrogen desorption properties of the LiNH2–LiH system were dramatically improved by the addition of 5 mol% KOH.

Destabilization of the LiNH2–LiH hydrogen storage system by aluminum incorporation

The lithium amide–lithium hydride system (LiNH2–LiH) is one of the most attractive light-weight materials for hydrogen storage. In an effort to improve its hydrogen sorption kinetics, the effect of 1 mol% AlCl3 addition to LiNH2–LiH system was systematically investigated by differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis and hydrogen volumetric measurements. It is shown that Al3+ is incorporated into the LiNH2 structure by partial substitution of Li+ forming a new amide in the Li–Al–N–H system, which is reversible under hydriding/dehydriding cycles. This new substituted amide displays improved hydrogen storage properties with respect to LiNH2–LiH. In fact, a stable hydrogen storage capacity of about 4.5–5.0 wt% is observed under cycling and is completely desorbed in 30 min at 275 °C for the Li–Al–N–H system. Moreover, the concurrent incorporation of Al3+ and the presence of LiH are effective for mitigating the ammonia release. The results reveal a common reaction pathway for LiNH2–LiH and LiNH2–LiH plus 1 mol% AlCl3 systems, but the thermodynamic properties are changed by the inclusion of Al3+ in the LiNH2 structure. These findings have important implications for tailoring the properties of the Li–N–H system.

In situ hybridization of LiNH2–LiH–Mg(BH4)2 nano-composites: intermediate and optimized hydrogenation properties

Nano-composites of LiNH(2)-LiH-xMg(BH(4))(2) (0 ≤ x ≤ 2) were prepared by plasma metal reaction followed by a nucleation growth method. Highly reactive LiNH(2)-LiH hollow nanoparticles offered a favorable nucleus during a precipitation process of liquid Mg(BH(4))(2)·OEt(2). The electron microscopy results suggested that more than 90% of the obtained nano-composites were in the range 200-400 nm. Because of the short diffusion distance and ternary mixture self-catalyzing effect, this material possesses enhanced hydrogen (de)sorption attributes, including facile low-temperature kinetics, impure gases attenuation and partial reversibility. The optimal hydrogen storage properties were found at the composition of LiNH(2)-LiH-0.5Mg(BH(4))(2), which was tentatively attributed to a Li(4)(NH(2))(2)(BH(4))(2) intermediate. 5.3 wt% hydrogen desorption could be recorded at 150 °C, with the first 2.2 wt% release being reversible. This work suggests that controlled in situ hybridization combined with formula optimization can improve hydrogen storage properties.