The dehydrogenation reactions and kinetics of the 2LiBH4−Al composite were investigated by means of thermogravimetry allied with mass spectroscopy, differential scanning calorimetry, X-ray diffraction, Fourier transform infrared analysis, and isothermal dehydrogenation measurements. According to the analysis of the experimental results, the dehydrogenation of 2LiBH4−Al can be roughly divided into three stages that range from 553 to 648 K, 648 to 773 K, and 773 to 823 K. In the initial stage, it was observed that the decomposition of LiBH4 occurs simultaneously with the formation of AlB2. It was also found that the 3D diffusion mechanism in the form of the Jander equation dominates the kinetics of this stage. The second stage is the major dehydrogenation process. The well-fitted curves of the isothermal dehydrogenation by the Prout−Tompkins equation prove that this stage is mainly dominated by an autocatalysis reaction. The suggested reaction 2LiBH4 + AlB2 → 2LiH + Al + 4B + 3H2 takes place simultaneously with the reaction 2LiBH4 + Al → 2LiH + AlB2 + 3H2. AlB2 and Al serve not only as products but also as reagents for the decomposition of LiBH4. Within this stage, the activation energy is remarkably reduced to 94 ± 5 kJ/mol from that of bulk LiBH4. At the third stage, when the temperature is above 798 K, LiAl can be formed due to the decomposition of LiH. This stage was still characterized by the autocatalysis reaction.

The Mg(NH2)2–2LiH composite is a promising hydrogen storage material due to its relatively high reversible hydrogen capacity (∼5.6 wt%) and suitable thermodynamic properties that allow hydrogen sorption conducting at temperatures below 90 °C. However, the presence of a severe kinetic barrier inhibits its low-temperature operation. In the present work, Li3AlH6 was introduced to the Mg(NH2)2–2LiH system. Experimental results show that a 3.2% mol Li3AlH6-modified Mg(NH2)2–2LiH sample released hydrogen at a rate ca. 4.5 times as fast as that of the Li3AlH6-free sample at 140 °C. The enhancement of desorption kinetics was simultaneously demonstrated by activation energy (Ea) of ca. 96.3 ± 9 kJ mol−1 which was significantly decreased by 31 kJ mol−1 from that of the Li3AlH6-free sample. The interaction of Li3AlH6 and Mg(NH2)2 during ball milling results in the formation of LiAl(NH)2, LiNH2 and Mg3N2. LiAl(NH)2 was actually the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.

Stepwise reactions were observed in the ball milling and heating process of the LiBH4–NaNH2 system by means of X-ray diffraction (XRD) and Fourier transform infrared spectrometry (FT-IR). During the ball milling process, two concurrent reactions take place in the mixture: 3LiBH4 + 4NaNH2 → Li3Na(NH2)4 + 3NaBH4 and LiBH4 + NaNH2 → LiNH2 + NaBH4. The heating process from 50 °C to 400 °C is mainly the concurrent reactions of Li3Na(NH2)4 + 3LiBH4 → 2Li3BN2 + NaBH4 + 8H2 and 2LiNH2 + LiBH4 → Li3BN2H8 → Li3BN2 + 4H2, where the intermediate phases Li3Na(NH2)4 and LiNH2 serve as the reagents to decompose LiBH4. The merged equations for the mechanochemical and the heating reactions below 400 °C can be denoted as 3LiBH4 + 2NaNH2 → Li3BN2 + 2NaBH4 + 4H2. The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H2, which agrees well with the theoretical capacity (5.5 wt.% H2) of the merged equation and thus demonstrates the suggested pathway. The subsequent step consists of the decompositions of NaBH4 and Li3Na(NH2)4 within the temperature range of 400 °C–600 °C. The apparent activation energies of the two steps are 114.8 and 123.5 kJ/mol, respectively. They are all lower than that of our previously obtained bulk LiBH4.Highlights► Dehydrogenation in the LiBH4–NaNH2 starts from 50 °C. ► The intermediate phases Li3Na(NH2)4 and LiNH2 during heating process serve as the reagents to decompose LiBH4. ► The maximum dehydrogenation capacity in closed system below 400 °C is 5.1 wt.% H2. ► The apparent activation energies of the two steps (114.8 and 123.5 kJ/mol) are all lower than that of bulk LiBH4.

In this work we investigated the effect of Ti, TiH2, TiB2, TiCl3, and TiF3 additives on the hydrogen de/re-sorption kinetics and reaction pathways of LiH/MgB2 mixture. From high pressure differential scanning calorimeter (HP-DSC) measurements it was found that these additives all effectively decrease the onset temperature of hydrogenation. The isothermal hydrogenation/dehydrogenation measurements suggest that Ti, TiH2, and TiB2 can significantly improve the hydrogen sorption kinetics of LiH/MgB2 mixture. The absorption kinetics of TiF3 and TiCl3 doped samples are slower than the baseline (2LiH–MgB2 without additive), but their desorption kinetics are faster than the baseline and other additives doped systems. X-ray diffraction (XRD) analysis reveals that the additive Ti in LiH/MgB2 actively participates in both hydrogenation and dehydrogenation process, which can be regarded as an effective additive of this system.

Promotion of dehydrogenation based on the interaction of [BH4]− and [NH2]− sources has been demonstrated to be one of the most effective approaches in developing an advanced borohydride/amide hydrogen storage combined system. The Ca(BH4)2–2Mg(NH2)2 and Ca(BH4)2–2Ca(NH2)2 composites are thereby synthesized in the present work. It is found that the binary combined systems exhibit an onset dehydrogenation temperature of ∼220 °C, which is ∼100 °C lower than that of pristine Ca(BH4)2. The hydrogen release measurements for Ca(BH4)2–2Mg(NH2)2 and Ca(BH4)2–2Ca(NH2)2 samples below 480 °C show desorption amounts of 8.3 and 6.8 wt % hydrogen, respectively. The dehydrogenation of both samples is accompanied by an ammonia emission of <1.4 mol %. The characterizations such as X-ray diffraction and nuclear magnetic resonance on the postdehydrogenated samples indicate that the dehydrogenation reactions are in the pathways of Ca(BH4)2 + 2 Mg(NH2)2 → 1/3 [Ca3Mg6(BN2)6] + 8 H2 and Ca(BH4)2 + 2 Ca(NH2)2 → 1/3 Ca9(BN2)6 + 8 H2, respectively. Compared with pristine Ca(BH4)2 sample, both possess lower activation energy for dehydrogenation. Further investigation reveals that the interaction of B–H and N–H may be one of main driving forces for dehydrogenation of borohydride/amide combined system.

Mg-based hydrogen storage alloys are a type of promising cathode material of Nickel-Metal Hydride (Ni-MH) batteries. But inferior cycle life is their major shortcoming. Many methods, such as element substitution, have been attempted to enhance its life. However, these methods usually require time-consuming charge–discharge cycle experiments to obtain a result. In this work, we suggested a cycle life prediction method of Mg-based hydrogen storage alloys based on artificial neural network, which can be used to predict its cycle life rapidly with high precision. As a result, the network can accurately estimate the normalized discharge capacities vs. cycles (after the fifth cycle) for Mg0.8Ti0.1M0.1Ni (M = Ti, Al, Cr, etc.) and Mg0.9 − xTi0.1PdxNi (x = 0.04–0.1) alloys in the training and test process, respectively. The applicability of the model was further validated by estimating the cycle life of Mg0.9Al0.08Ce0.02Ni alloys and Nd5Mg41–Ni composites. The predicted results agreed well with experimental values, which verified the applicability of the network model in the estimation of discharge cycle life of Mg-based hydrogen storage alloys.

The synthesized LiBH4−MgH2−Al (4:1:1 mole ratio) composite exhibits reversible de/rehydrogenation properties. Thermogravimetry and differential scanning calorimetry indicate that the dehydrogenation onset temperature is reduced by 100 K from that of 2LiBH4−MgH2 and 2LiBH4−Al systems. The major dehydrogenation pathway for the 4LiBH4−MgH2−Al complex system can be identified as 4LiBH4 + MgH2 + Al → 4LiH + MgAlB4 + 7H2 by means of X-ray diffraction (XRD) measurements on the as-dehydrogenated samples. The isothermal dehydrogenation measurements exhibit that the maximum dehydrogenation amount (9.4 wt % H2, 673 K) approaches the theoretical value (9.9 wt % H2) of the reaction. Through pressure−composition isotherms (P−C−T) and the van’t Hoff equation, the dehydrogenation enthalpy and entropy of the 4LiBH4−MgH2−Al system can be determined as 57 kJ/mol-H2 and 75 J/K·mol-H2, respectively. The system is slightly destabilized from pristine LiBH4 (ΔHde° = 68 kJ/mol-H2) by coadditives of MgH2 and Al. The XRD measurements on the rehydrogenated samples suggest that the above reaction is partially reversible and the backward reaction takes place in two steps as 4LiH + MgAlB4 + 6H2 → Mg + 4LiBH4 + Al and Mg + H2 → MgH2. Because of the alloying of Mg with Al, MgH2 in the complex system cannot be fully recovered below the temperature of 673 K. The isothermal rehydrogenation measurements exhibit significantly enhanced kinetics for the LiH−MgAlB4 system compared with LiH−MgB2 and LiH−AlB2 systems.

LiBH4LiBH4–MgH2MgH2 composite exhibits excellent reversible hydrogen capacity, but it still presents high decomposition temperature over 350∘C and sluggish kinetics. For the purpose of optimizing its reaction performance, Nb2O5Nb2O5 was doped into this composite as a catalyst to form a more destabilized and reversible composite system. It possesses a maximum capacity of approximately 6–8 wt% hydrogen releasing below 400∘C and could be hydrogenated to 5–6 wt% hydrogen capacity at 400∘C under 1.9 MPa. XRD and SEM analysis revealed that NbH2NbH2, formed and highly dispersed in the composite, played a key role in changing the original path and resulted in the formation of an intermediate compound (MgB2)(MgB2) in the milling process. The hydrogen storage capacity of the LiBH4LiBH4–MgH2(massratio,1:2)+16wt%Nb2O5 composite decreased gradually during the dehydrogenation/hydrogenation cycles and still maintained 5.16 wt% in the third dehydrogenation process. The activation energies EAEA of LiBH4LiBH4–MgH2MgH2 (mass ratio, 1:2) with 16 wt% Nb2O5Nb2O5 and without Nb2O5Nb2O5 were estimated to be 139.96 and 156.75kJmol-1 by Kissinger method. It indicates that the additive Nb2O5Nb2O5 can decrease the activation energy of LiBH4LiBH4–MgH2MgH2 composite.

Cobalt-free AB3-type La0.7Mg0.3Ni3.5−x(MnAl2)x (x = 0–0.20) nonstoichiometric alloys were synthesized and investigated for their electrochemical hydrogen storage properties. It was found that the maximum discharge capacity of La0.7Mg0.3Ni3.5−x(MnAl2)x (x = 0–0.20) alloy electrodes decreases from 362.7 mAh/g (x = 0) to 293.9 mAh/g (x = 0.20) with increasing x. The cyclic capacity retention rate C100/Cmax of alloy electrodes initially increases to 43.7% (x = 0.10) due to the formation of Al-containing passive film, and subsequently decreases to 35.9% (x = 0.20) since the passive film is cracked. These phenomena are demonstrated by scanning electron microscopy (SEM) images of alloy electrodes after 70 charge–discharge cycles. It was also revealed that the electrochemical reaction kinetics is retarded when x < 0.10, which can be ascribed to the increase of charge-transfer resistance of the alloy electrodes due to the formation of Al-containing passive film. When x > 0.10, the electrochemical kinetics is enhanced because of the crack of the Al oxide film on the electrode surface resulting from the dissolution of Mn into the alkaline electrolyte. The optimal content of Mn and Al in La0.7Mg0.3Ni3.5−x(MnAl2)x alloys for negative electrodes in alkaline rechargeable secondary batteries is x = 0.10 in this study.

In the present study, a novel alloy composite has been synthesized by ball milling nonstoichiometric AB3-type La0.7Mg0.3Ni3.5 alloy with Ti0.17Zr0.08V0.35Cr0.1Ni0.3 alloy in order to improve the cyclic stability and other electrochemical properties of La0.7Mg0.3Ni3.5 alloy electrode. The phase structure, morphology and electrochemical performances of the composite have been investigated systematically. From X-ray diffraction (XRD) patterns, it can be found that the La0.7Mg0.3Ni3.5 and Ti0.17Zr0.08V0.35Cr0.1Ni0.3 alloys still retain their respective phase structures in the composite. Electrochemical studies show that the cyclic stability of the composite electrode is noticeably improved after 100 charge–discharge cycles in comparison with single La0.7Mg0.3Ni3.5 alloy electrode due to enhanced anti-corrosion performance in the alkaline electrolyte. The discharge capacity retention rate C100/Cmax of composite electrode is 62.3%, which is much higher than that of the La0.7Mg0.3Ni3.5 alloy electrode, although the maximum discharge capacity of the former decreases moderately. Both electrochemical impedance spectra (EIS) and linear polarization (LP) studies indicate that the electrochemical kinetics of the composite electrode is also improved. The charge-transfer resistance (Rct), the polarization resistance (Rp) and the exchange current density (I0) of the composite electrode are 160.2 mΩ, 129.5 mΩ and 201.6 mA/g, respectively, which are superior to those of the La0.7Mg0.3Ni3.5 alloy electrode.

LiBH4LiBH4 nanoparticles supported by disordered mesoporous carbon CMK-3, denoted as nano-LiBH4/CmesoporousLiBH4/Cmesoporous, were synthesized in the present work. Nano-LiBH4LiBH4 in this sample was destabilized significantly, resulting in more favorable latent heat of dehydrogenation (40kJ/molH2)H2), large amount of dehydrogenation capacity (14 wt%) below 600∘C and reversible capacity of 6.0 wt% H2H2 at 350∘C. It was revealed that such destabilization results from two kinds of mechanisms: nano-dispersion and reaction with mesoporous carbon. Both mechanisms exert synergistic effects on the dehydrogenation of nano-LiBH4LiBH4 and reverse hydrogenation.

We have reported that the cyclic stability of Mg0.9−xTi0.1PdxNi (x = 0.04, 0.06, 0.08, 0.1) amorphous electrode alloys prepared by mechanical alloying was enhanced to 100 cycles over 200 mAh/g in our previous work. The hydrogen desorption kinetics of the electrode alloys were studied in this work by potentiostatic discharge experiments. Experimental results showed that the three-dimensional hydrogen diffusion dominated the desorption process of electrode alloys. The activation energy of desorption was calculated according to the Arrenius equation. The values were 49.11 kJ mol−1, 45.99 kJ mol−1, 42.50 kJ mol−1 and 40.66 kJ mol−1 for x = 0.04, 0.06, 0.08 and 0.1 of Mg0.9−xTi0.1PdxNi electrode alloys, respectively. The limiting current densities were determined by anodic polarization experiments. The increasing Pd amount results in the reducing of activation energy and enhancement of the limiting current density of the quaternary Mg-based electrode alloys.

In the present work, new type hydrogen storage composites electrodes Ti0.9Zr0.2Mn1.5Cr0.3V0.3Ti0.9Zr0.2Mn1.5Cr0.3V0.3–x wt% La0.7Mg0.25Zr0.05Ni2.975La0.7Mg0.25Zr0.05Ni2.975Co0.525(x=0,5,10) were successively prepared by ball milling method. The structure and electrochemical properties of the composites were investigated by means of XRD, SEM, EDS and electrochemical measurements. It was found that the bulk of the composites still retained the hexagonal C14 Laves structure after short-term ball milling and that the maximum capacity of the composite electrodes was significantly improved to 292.4 and 314.0 mA h/g for x=5x=5 and 10, respectively, from maximum 48.6 mA h/g of Ti0.9Zr0.2Mn1.5Cr0.3V0.3Ti0.9Zr0.2Mn1.5Cr0.3V0.3 alloy electrode. Electrochemical impedance spectra and cyclic voltammograms (CV) measurements revealed that the charge-transfer resistance was reduced with increasing amount of La–Mg-based alloy. The increasing x amount also lifted the values of hydrogen diffusion coefficient D of the composite electrodes obtained by anodic polarization (AP) measurements. These indicated that the La–Mg-based alloy as a surface modifier not only increased the discharge capacity but also improved the charge–discharge kinetics of composite electrode greatly.

Hydrogen storage alloy composites Ti0.9Zr0.2Mn1.5Cr0.3V0.3-xwt%La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5(x=0,5,10) were prepared by ball milling method. Their structures, morphologies and the hydrogen storage characteristics were intensively studied in the present work. It was found that the bulk of composites maintained the hexagonal C14 Laves phase structure after ball milling with additional La–Mg-based alloy for two hours. Scanning electron microscopy (SEM) observations revealed that the average size of Ti0.9Zr0.2Mn1.5Cr0.3V0.3Ti0.9Zr0.2Mn1.5Cr0.3V0.3 and La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5 particles were reduced to several hundred nanometers after ball milling process. Energy dispersive X-ray spectrometer (EDS) patterns of the composites showed that the La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5 phase was uniformly distributed on the surface of Ti0.9Zr0.2Mn1.5Cr0.3V0.3Ti0.9Zr0.2Mn1.5Cr0.3V0.3. The hydrogen absorption–desorption characteristics of the composites were improved with increasing dispersivity of the La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5La0.7Mg0.25Zr0.05(Ni0.85Co0.15)3.5 alloy particles. Pressure–composition–temperature (PCT) measurements revealed that the pressure plateau and the hysteresis decreased with increasing x  . Among these composites, the sample of x=10x=10 exhibited more significant hydrogen storage performances. Its hydrogen absorption and desorption plateau pressure were 0.547 and 0.302 MPa, respectively, which were much lower than those of the Ti0.9Zr0.2Mn1.5Cr0.3V0.3Ti0.9Zr0.2Mn1.5Cr0.3V0.3 alloy. Moreover, it took only 200 and 2500 s to reach the constant pressure for the hydrogen absorption and desorption reaction. These improvements can be attributed to the preferable dispersivity of La–Mg-based alloy particles and also the formation of micro-crack after ball milling. Both factors synergetically facilitate the diffusion of hydrogen atoms in the composites and accelerate the dissociation of hydrogen molecules and recombination of hydrogen atoms on the surface of composites during the hydrogen absorption–desorption process.

The Mg(NH2)2–2LiH composite is a promising hydrogen storage material due to its relatively high reversible hydrogen capacity (∼5.6 wt%) and suitable thermodynamic properties that allow hydrogen sorption conducting at temperatures below 90 °C. However, the presence of a severe kinetic barrier inhibits its low-temperature operation. In the present work, Li3AlH6 was introduced to the Mg(NH2)2–2LiH system. Experimental results show that a 3.2% mol Li3AlH6-modified Mg(NH2)2–2LiH sample released hydrogen at a rate ca. 4.5 times as fast as that of the Li3AlH6-free sample at 140 °C. The enhancement of desorption kinetics was simultaneously demonstrated by activation energy (Ea) of ca. 96.3 ± 9 kJ mol−1 which was significantly decreased by 31 kJ mol−1 from that of the Li3AlH6-free sample. The interaction of Li3AlH6 and Mg(NH2)2 during ball milling results in the formation of LiAl(NH)2, LiNH2 and Mg3N2. LiAl(NH)2 was actually the active species for the enhancement of dehydrogenation/re-hydrogenation kinetics of the system.