•Zr substituted compositions show better hydrogen absorption kinetics.•Zr substitution improves the cyclic durability of (VFe)60(TiCrCo)40 alloy.•Micro-strain accumulation during the cycles is reduced by Zr substitution.•Alloys with less micro-strain accumulation show better cyclic durabilities.Hydrogen storage and cyclic properties of (VFe)60(TiCrCo)40-xZrx (0 ≤ x ≤ 2) alloys had been investigated systematically. The results showed that the alloys when x = 0 and 0.5 consist of BCC main phase, C14 Laves phase and CeO2 phase, while Zr-based FCC phase appears in the alloys when x = 1 and 2, and the contents of the Laves phase and the Zr-based FCC phase increase with the increment of Zr substitution for Ti. Zr substituted compositions were found to have better hydrogen absorption kinetics but the hydrogen storage capacities decrease as the substitution amounts increase. Owing to that Zr element can reduce the micro-strain accumulation in the crystal lattice during the hydrogen absorption-desorption cycles, the cyclic properties of the (VFe)60(TiCrCo)40-xZrx alloys are improved. As Zr content increases, the desorption capacity fading rate of (VFe)60(TiCrCo)40-xZrx decreases and the best cyclic property was obtained when x = 2 with only 4.5% capacity decay after 10 cycles in this work.

•Hydrogen is generated by hydrolysis of NaBH4-based composite NaBH4NH3BH3 accelerated by AlCl3 salt.•The maximum hydrogen release of 2314 ml/g at 50 °C is achieved by optimized composition.•The direct addition of AlCl3 by hand mixing leads to better properties than that by ball-milling.•The detailed catalytic factors and hydrolysis mechanism are discussed.In this work, AlCl3 is used as a catalyst for hydrolysis of NaBH4-based composite xNaBH4-NH3BH3 (xSB-AB, x = 4, 6, 8). Four factors affecting the hydrogen generation performances are studied in this paper, including different addition method of AlCl3, molar ratio of SB/AB by varying the value of x, AlCl3 amount and temperature. The experimental results demonstrate that direct addition of AlCl3 by hand mixing leads to better hydrogen generation properties than that by ball-milling owing to concentrated heat caused by uneven distribution of AlCl3. The optimized composite achieves complete dehydrogenation within 1 h at 50 °C, whose maximum hydrogen release reaches 2314 ml/g. The improved hydrogen generation properties are attributed to thermal stimulation induced by dissolving AlCl3 in water, decline of pH value caused by the formation of Al(OH)3 precipitation, and the synergy between SB and AB. The kinetic simulation results of the xSB-AB/yAlCl3 system show that the hydrolysis kinetics is influenced by composition design. The activation energies of 15.12–29.5 kJ/mol are obtained from 4SB-AB/yAlCl3 (y = 5, 10, or 20 in wt%), and 12.59–15.12 kJ/mol from xSB-AB/20AlCl3 (x = 4, 6, or 8). The main solid byproducts of this hydrolysis system are Na2B4O7·10H2O, NaCl, Na2ClB(OH)4 and Al(OH)3.

•After hydrogenation, Mg–Ca alloys change into mixtures of MgH2 and Ca4Mg3H14.•The hydrolysis of Ca4Mg3H14 and solubility of Ca(OH)2 improve the hydrogen release.•Ca content is responsible for the specific surface areas of Mg–Ca alloy hydrides.•The increment of Ca content lowers the activation energies of Mg–Ca alloy hydrides.The microstructure characteristics and hydrolysis mechanism of Mg–Ca alloy hydrides were investigated in this paper. It was found that x wt.% Ca–Mg alloys (x = 10, 20 and 30) hydrogenated after ball-milling are mainly composed of MgH2 and Ca4Mg3H14 phases, and they show much better hydrolysis properties than pure MgH2 at the temperature range of 25–70 °C. The superior performances of Mg–Ca alloy hydrides may be attributed to an easy hydrolysis nature of Ca4Mg3H14 and a high solubility of the by-product Ca(OH)2, which provides gates to make water penetrate deeply inside the particles. Moreover, increasing the content of Ca enhances the hydrolysis performances of Mg–Ca alloy hydrides further due to the formation of more Ca4Mg3H14 and larger specific surface areas as well. The best comprehensive hydrolysis performances are obtained by 30 wt.% Ca–Mg alloy hydride, which releases 1419.8 mL g−1 hydrogen within 1 h at 70 °C and its conversion yield is about 95%. The hydrogen generation kinetics of Mg–Ca alloy hydrides was also investigated and it showed that the activation energies decrease with the increase of the Ca content, which is in accordance with their hydrolysis properties.

•Hydrogen is generated via self-hydrolysis of NaBH4-based composite (xNaBH4–yNH3BH3) without any catalyst.•More than 10 wt% hydrogen yield (taking reacted water into account) is achieved by optimized composition.•The hydrolysis behaviors are affected by pH value, microstructure and the content of NH3BH3 in the composite.•The detailed hydrolysis reaction processes were discussed.In this work, NH3BH3 (AB) is used to induce hydrogen generation during NaBH4 (SB) hydrolysis in order to reduce the use of catalysts, simplify the preparation process, reduce the cost and improve desorption kinetics and hydrogen capacity as well. xNaBH4–yNH3BH3 composites are prepared by ball-milling in different proportions (from x:y = 1:1 to 8:1). The experimental results demonstrate that all composites can release more than 90% of hydrogen at 70 °C within 1 h, and their hydrogen yields can reach 9 wt% (taking reacted water into account). Among them, the composites in the proportion of 4:1 and 5:1, whose hydrogen yields reach no less than 10 wt%, show the best hydrogen generation properties. This is due to the impact of the following aspects: AB additive improves the dispersibility of SB particles, makes the composite more porous, hampers the generated metaborate from adhering to the surface of SB, and decreases the pH value of the composite during hydrolysis. The main solid byproduct of this hydrolysis system is NaBO2·2H2O. By hydrolytic kinetic simulation of the composites, the fitted activation energies of the complexes are between 37.2 and 45.6 kJ mol−1, which are comparable to the catalytic system with some precious metals and alloys.

The bcc alloy V40(TiCr)51Fe8Mn shows a high reversible hydrogen capacity of 2.22 mass% at room temperature and is therefore expected to be applied as a hydrogen storage material. During the first 10 hydrogenation/dehydrogenation cycles, the capacity decreases markedly from 2.22 to 1.86 mass%. Possible reasons are both internal (including lattice variation, strain, phase transformation, phase disproportion, etc.) and external factors (poisoning by impurities in the gas after decades of cycles mainly).In this work, the surface properties of the V40(TiCr)51Fe8Mn alloy during 20 hydrogenation/dehydrogenation cycles were investigated by XPS analysis. During hydrogenation/dehydrogenation cycles, the surface becomes oxidized. Particular oxides were known to block hydrogen uptake and might thus be responsible for the capacity decay. It was found that Cr is enriched at the surface. Dense and passive chromia forms thereafter around alloy particles and hinder further oxidation and therefore stops further degradation of the hydrogen capacity of the alloy.Graphical abstractHighlights► Cheaper FeV80 master alloy was used in new V40(TiCr)51Fe8Mn alloy. ► Desorption capacity decreases dramatically during the first 10 cycles. ► Formation of main element oxides are responsible for the decay. ► Dense and passive chromia forms around alloy particles after 15 cycles. ►Chromia hinders further oxidation and explains the stabilization of the alloy.

NaBH4–NH3BH3 composites were prepared by high-energy ball-milling processes for hydrogen generation through hydrolysis. After ball milling, there were no new phases found in the XRD patterns of NaBH4–NH3BH3 composites. The experimental results demonstrate that when the molar ratios of NaBH4–NH3BH3 composites range from 1:4 to 2:1, these composites can release above 90% hydrogen in 30 min at 70 °C. Comparing with neat NaBH4 or NH3BH3, the hydrolysis properties of these composites are greatly enhanced. And the hydrolysis reaction mechanism is turned out to be more explicit as Na2B4O5(OH)4·8H2O appears in the hydrolysis products. Since the preparation processes of these composites are simple and cost-effective and the hydrolysis of these composites achieves efficient hydrogen release, it is promising that this kind of composites can be applied in hydrogen generation.Highlights► The hydrolysis of NaBH4 -NH3BH3 composites is catalyst-free. ► The hydrolysis properties of both NH3BH3 and NaBH4 have been greatly improved. ► The complex reaction mechanism was discussed.

•Low-cost (FeV80)48Ti26Cr26 alloy was firstly synthesized by hydride powder sintering method.•Ti compensated sintered alloy exhibits better hydrogen storage capacities.•The mechanism for the enhanced hydrogen storage performance of Ti compensated V-Ti-based alloy is proposed.The V-Ti-based hydrogen storage alloy (FeV80)48Ti26Cr26 was synthesized by the hydride powder sintering method. X-ray diffraction (XRD) analysis results show that the alloy consists of a main BCC phase, a La2O3 phase and a Ti-rich oxide phase. The existence of the Ti-rich oxide phase leads to a loss of Ti element content in the main BCC phase and further results in a lattice contraction. Thus, the hydrogen absorption capacity of the alloy synthesized by the hydride powder sintering method is only 2.8 wt%, with a hydrogen desorption capacity of 1.5 wt%. To address this problem, the effects of Ti element compensation on the microstructure and the hydrogen storage capacities of the alloy were investigated. It was found that the lattice constant of the BCC phase increases and the amount of Ti-rich oxide phase is almost unchanged with increased Ti element compensation, leading to an improved hydrogen storage capacity. When Ti element compensation reaches 4 at%, the hydrogen absorption capacity of the alloy reaches 3.3 wt% with a desorption capacity of 2.0 wt% at room temperature and at a pressure of 10−4MPa. The hydrogen desorption plateau slope factor of the (FeV80)48Ti26+xCr26 (x = 0–4) alloy prepared by the hydride powder sintering method is as small as that of the alloy prepared by the arc-melting method. Thus the sintering method is a suitable option for the manufacture of V-Ti-based hydrogen storage alloys.