Computationally Predicted High-Throughput Free-Energy Phase Diagrams for the Discovery of Solid-State Hydrogen Storage Reactions

The design of multinary solid-state material systems that undergo reversible phase changes via changes in temperature and pressure provides a potential means of safely storing hydrogen. However, fully mapping the stabilities of known or newly targeted compounds relative to competing phases at reaction conditions has previously required many stringent experiments or computationally demanding calculations of each compound's change in Gibbs energy with respect to temperature, G(T). In this work, we have extended the approach of constructing chemical potential phase diagrams based on ΔG_f(T) to enable the analysis of phase stability at non-zero temperatures. We first performed density functional theory calculations to compute the formation enthalpies of binary, ternary, and quaternary compounds within several compositional spaces of current interest for solid-state hydrogen storage. Temperature effects on solid compound stability were then accounted for using our recently introduced machine learned descriptor for the temperature-dependent contribution G^δ(T) to the Gibbs energy G(T). From these Gibbs energies, we evaluated each compound's stability relative to competing compounds over a wide range of conditions and show using chemical potential and composition phase diagrams that the predicted stable phases and H₂ release reactions are consistent with experimental observations. This demonstrates that our approach rapidly computes the thermochemistry of hydrogen release reactions for compounds at sufficiently high accuracy relative to experiment to provide a powerful framework for analyzing hydrogen storage materials. This framework based on G(T) enables the accelerated discovery of active materials for a variety of technologies that rely on solid-state reactions involving these materials.

Hydrogen Pressure-Dependent Dehydrogenation Performance of the Mg(NH2)2-2LiH-0.07KOH System

The Mg(NH₂)₂-2LiH system with KOH additive is a promising high-capacity hydrogen storage material in terms of low dehydrogenation temperatures, good reversibility, and excellent cycling stability. Various mechanisms have been reported to elucidate the reasons for the K-containing additive improving the hydrogen storage performance. Herein, the dehydrogenation performance of Mg(NH₂)₂-2LiH-0.07KOH is found to be strongly associated with hydrogen pressures. The Li₂K(NH₂)₃ and KH produced from the reaction between KOH, LiH, and Mg(NH₂)₂ in the ball milling process are converted into Li₃K(NH₂)₄, MgNH, and LiNH₂ in the heating dehydrogenation process under Ar carrier gas or very low hydrogen pressure, exhibiting a two-peak dehydrogenation process. For the sample under high hydrogen pressure, Li₂K(NH₂)₃ can react with LiH to convert into Li₃K(NH₂)₄ and further to form KH and LiNH₂ in the heating process, showing a one-peak dehydrogenation process under 5 bar hydrogen. The hydrogen pressure-dependent reactions of K-containing additives in the Mg(NH₂)₂-2LiH system lead to a different hydrogen storage performance under different dehydrogenation conditions.

Kinetic alteration of the 6Mg(NH2)2-9LiH-LiBH4 system by co-adding YCl3 and Li3N

The 6Mg(NH₂)₂-9LiH-LiBH₄ composite system has a maximum reversible hydrogen content of 4.2 wt% and a predicted dehydrogenation temperature of about 64 °C at 1 bar of H₂. However, the existence of severe kinetic barriers precludes the occurrence of de/re-hydrogenation processes at such a low temperature (H. Cao, G. Wu, Y. Zhang, Z. Xiong, J. Qiu and P. Chen, J. Mater. Chem. A, 2014, 2, 15816-15822). In this work, Li₃N and YCl₃ have been chosen as co-additives for this system. These additives increase the hydrogen storage capacity and hasten the de/re-hydrogenation kinetics: a hydrogen uptake of 4.2 wt% of H₂ was achieved in only 8 min under isothermal conditions at 180 °C and 85 bar of H₂ pressure. The re-hydrogenation temperature, necessary for a complete absorption process, can be lowered below 90 °C by increasing the H₂ pressure above 185 bar. Moreover, the results indicate that the hydrogenation capacity and absorption kinetics can be maintained roughly constant over several cycles. Low operating temperatures, together with fast absorption kinetics and good reversibility, make this system a promising on-board hydrogen storage material. The reasons for the improved de/re-hydrogenation properties are thoroughly investigated and discussed.

The effect of Sr(OH)2 on the hydrogen storage properties of the Mg(NH2)2–2LiH system

Sr(OH)₂ influences both the thermodynamics and kinetics of the Mg(NH₂)₂–2LiH system, lowering the dehydrogenation onset and peak temperatures by ca. 70 °C and 13 °C.

New amide-chloride phases in the Li-Al-N-H-Cl system: formation and hydrogen storage behaviour

New amide-chloride phases were successfully synthesized by mechanical milling of the LiNH2-AlCl3 mixture at a molar ratio of 1 : 0.11 and further heating at 150 °C under argon (0.1 MPa) or under hydrogen pressure (0.7 MPa). Powder X-ray diffraction measurements as a function of milling time increase revealed that the milling of the LiNH2-0.11AlCl3 mixture results in the formation of a FCC solid solution with an excess of LiNH2. Subsequent heating of the LiNH2-0.11AlCl3 sample ball milled for 5 hours at 150 °C under argon or under hydrogen induces the appearance of an amide-chloride phase isostructural with cubic Li4(NH2)3Cl. This Li-Al-N-H-Cl phase transforms progressively into the trigonal phase after prolonged heating at 300 °C under hydrogen pressure. The thermal behaviour of the amide-chloride without and with LiH addition displays dissimilar decomposition pathways. The decomposition of amide-chloride alone involves the formation of ammonia and hydrogen from 120 to 300 °C. Conversely, the amide-chloride material in the presence of LiH only releases hydrogen avoiding the emission of ammonia. The resultant material is able to be rehydrogenated under moderate conditions (300 °C, 0.7 MPa H2), providing a new reversible hydrogen storage system.

Li4(NH2)3Cl amide-chloride: a new synthesis route, and hydrogen storage kinetic and thermodynamic properties

A Li₄(NH₂)₃Cl–3LiH composite was synthesized by the milling of LiNH₂–LiCl–3LiH and heating at 200 °C under 6.0 MPa of hydrogen pressure.

Effect of LiCl presence on the hydrogen storage performance of the Mg(NH2)2–2LiH composite

The decrease in the dehydrogenation kinetics and hydrogen storage capacity during Mg(NH₂)₂–2LiH cycling is related with the Li₄(NH₂)₃Cl formation and the incomplete Li₂Mg₂(NH₂)₃ rehydrogenation.

CNT addition to the LiBH4–MgH2 composite: the effect of milling sequence on the hydrogen cycling properties

The effect of the addition of CNT to the 2LiBH₄ : MgH₂ system was studied. The enhanced kinetic behaviour disappeared after several absorption/desorption cycles.

Crystal feature and electronic structure of novel mixed alanate LiCa(AlH4)3: a density functional theory investigation

The H coordinates are predicted for LiCa(AlH₄)₃ in which Li–H bonding and Ca–H anti-bonding interactions are illustrated.