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.

Improved kinetic behaviour of Mg(NH2)2-2LiH doped with nanostructured K-modified-LixTiyOz for hydrogen storage

The system Mg(NH₂)₂ + 2LiH is considered as an interesting solid-state hydrogen storage material owing to its low thermodynamic stability of ca. 40 kJ/mol H₂ and high gravimetric hydrogen capacity of 5.6 wt.%. However, high kinetic barriers lead to slow absorption/desorption rates even at relatively high temperatures (>180 °C). In this work, we investigate the effects of the addition of K-modified Li_xTi_yO_z on the absorption/desorption behaviour of the Mg(NH₂)₂ + 2LiH system. In comparison with the pristine Mg(NH₂)₂ + 2LiH, the system containing a tiny amount of nanostructured K-modified Li_xTi_yO_z shows enhanced absorption/desorption behaviour. The doped material presents a sensibly reduced (∼30 °C) desorption onset temperature, notably shorter hydrogen absorption/desorption times and reversible hydrogen capacity of about 3 wt.% H₂ upon cycling. Studies on the absorption/desorption processes and micro/nanostructural characterizations of the Mg(NH₂)₂ + 2LiH + K-modified Li_xTi_yO_z system hint to the fact that the presence of in situ formed nanostructure K₂TiO₃ is the main responsible for the observed improved kinetic behaviour.

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.

Enhanced hydrogen sorption in a Li–Mg–N–H system by the synergistic role of Li4(NH2)3BH4 and ZrFe2

The present investigation describes the synergistic role of Li₄(BH₄)(NH₂)₃ and ZrFe₂ in the hydrogen storage behaviour of a Li–Mg–N–H hydride system.

Insights into the dehydrogenation reaction process of a K-containing Mg(NH2)2–2LiH system

KH and Li₂K(NH₂)₃, formed in situ during ball milling, participate as reactants in the dehydrogenation reaction of the Mg(NH₂)₂–2LiH system.

Solid-Solid heterogeneous catalysis: the role of potassium in promoting the dehydrogenation of the Mg(NH(2))(2)/2 LiH composite

Considerable efforts have been devoted to the catalytic modification of hydrogen storage materials. The K-modified Mg(NH2 )2 /2 LiH composite is a typical model for such studies. In this work, we analyze the origin of the kinetic barrier in the first step of the dehydrogenation and investigate how K catalyzes this heterogeneous solid-state reaction. Our results indicate that the interface reaction of Mg(NH2 )2 and LiH is the main source of the kinetic barrier at the early stage of the dehydrogenation for the intensively ball-milled Mg(NH2 )2 /2 LiH sample. K can effectively activate Mg(NH2 )2 as well as promote LiH to participate in the dehydrogenation. Three K species of KH, K2 Mg(NH2 )4 , and Li3 K(NH2 )4 likely transform circularly in the dehydrogenation (KH↔K2 Mg(NH2 )4 ↔KLi3 (NH2 )4 ), which creates a more energy-favorable pathway and thus leads to the overall kinetic enhancement. This catalytic role of K in the amide/hydride system is different from the conventional catalysis of transition metals in the alanate system.

Hydrogen storage materials discovery via high throughput ball milling and gas sorption

The lack of a high capacity hydrogen storage material is a major barrier to the implementation of the hydrogen economy. To accelerate discovery of such materials, we have developed a high-throughput workflow for screening of hydrogen storage materials in which candidate materials are synthesized and characterized via highly parallel ball mills and volumetric gas sorption instruments, respectively. The workflow was used to identify mixed imides with significantly enhanced absorption rates relative to Li2Mg(NH)2. The most promising material, 2LiNH2:MgH2 + 5 atom % LiBH4 + 0.5 atom % La, exhibits the best balance of absorption rate, capacity, and cycle-life, absorbing >4 wt % H2 in 1 h at 120 °C after 11 absorption-desorption cycles.