An Y2O3@rGO nanocomposite was synthesized via an impregnation method and the catalytic effect of the nanocomposite on the hydrogen-storage properties of a Mg–Al alloy was investigated. The pressure composition isotherm measurement results revealed that the Mg–Al–Y2O3@rGO composite underwent a reversible hydrogenation/dehydrogenation process at 250 °C. Furthermore, the onset temperatures of hydrogenation and dehydrogenation were significantly (i.e., 102 °C and 122 °C, respectively) lower than the respective values corresponding to the Mg–Al alloy. The Y2O3@rGO nanocomposite enhanced the hydriding kinetic properties of the alloy. The hydrogenation kinetic parameter of the Mg–Al alloy increased from 0.008 to 0.195 at 300 °C with 5 wt% of the Y2O3@rGO composite. The values of 162.6 kJ mol−1 H2 and 145.9 kJ mol−1 H2 were obtained for the dehydrogenation energy barrier (evaluated by means of a Kissinger plot) of the Mg–Al alloy and the Mg–Al–Y2O3@rGO hydride, respectively. The reaction enthalpy of hydrogenation/dehydrogenation (determined from a van't Hoff plot) of the alloy decreased with the addition of the Y2O3@rGO nanocomposite. For example, the values of 70.7 kJ mol−1 H2 and 54.3 kJ mol−1 H2 were obtained for the reaction enthalpy of hydrogenation associated with the Mg–Al alloy and the Mg–Al–Y2O3@rGO composite, respectively. Therefore, the addition of the Y2O3@rGO nanocomposite is conducive for improving the thermodynamic and kinetic properties of hydrogenation/dehydrogenation of the Mg–Al alloy.

•Influence of the Ni-rGO nanocomposite on the electrochemical property of the AB3.5 alloy is investigated.•The addition of the Ni-rGO nanocomposite improves the reversibility hydrogen storage property of the AB3.5 alloy.•The Ni-rGO framework facilitates the hydrogen atom diffusion in the AB3.5 alloy bulk.•AB3.5-Ni-rGO electrode exhibits an excellent electrochemical kinetic property.A Ni-rGO nanocomposite was synthesized by a hydrothermal process and La0.7Mg0.3(Ni0.85Co0.15)3.5, an AB3.5-type hydrogen storage alloy, was prepared by magnetic levitation melting under argon atmosphere. The influences of the Ni-rGO nanocomposite on the hydrogen storage and electrochemical performance of the La0.7Mg0.3(Ni0.85Co0.15)3.5 alloy were investigated via pressure composition isotherms (PCT) and electrochemical measurements. The PCT curves revealed that the addition of the Ni-rGO nanocomposite improved the reversibility of hydrogen absorption and desorption for the La0.7Mg0.3(Ni0.85Co0.15)3.5 alloy. The electrochemical measurements showed that the electrochemical impedance of the La0.7Mg0.3(Ni0.85Co0.15)3.5 alloy electrode was significantly reduced, the high rate dischargeability, HRD1200, increased from 60% to 86%, the limiting current density, IL, increased from 1216.7 mA·g−1 to 2287.6 mA·g−1, and the hydrogen diffusion coefficient, D, increased with the added Ni-rGO nanocomposite. These improvements to the electrochemical performance are mainly attributed to the Ni-rGO framework, with the large specific surface area of the graphene, and to the high conductivity of metal nickel.

•Porous flower-like NiCoO2/rGO/NFnanoarrays were grown directly on Ni foam without any binders by ahydrothermal method.•NiCoO2/rGO/NFshowshigh specific capacity (1286 C g-1)and outstandingrate capability (1071.5 C g-1 at 12 A g-1).•The excellentperformance is attributed to the introduction of graphene into the ultrathin NiCoO2 nanoflakes.•It demonstrates the potential application of NiCoO2/rGO/NF compositeas an electrode material for supercapacitors.Porous flower-like NiCoO2 and graphene (NiCoO2/rGO) composite nanoarrays were grown directly on Ni foam (NF) without any binders by a simple hydrothermal synthesis method. Due to the porous flowers-like architecture of ultrathin NiCoO2 nanoflakes and the introduction of graphene with the high electrical conductivity, NiCoO2/rGO composite nanoarrays showed superior electrochemical performance including high specific capacity (1286C g−1 at 0.5 A g−1), and good rate capability (1071.5C g−1, 83.3% capacity retention at 12 A g−1). The asymmetric supercapacitor assembled with NiCoO2/rGO and rGO/NF also showed good performance with the energy density of 38.5 Wh kg−1 at 288 W kg−1 power density and good cycle stability of 86.9% after 2000 cycles. These results demonstrate the potential application of NiCoO2/rGO composite nanoarrays as a promising electrode material for supercapacitors.Download high-res image (128KB)Download full-size image

•γ-CuCl is an intrinsic p-type semiconductor owing to the low formation energy of Cu vacancy.•The low diffusion barrier of Cu vacancy make the long-standing n-type conductivity cannot be realized in γ-CuCl.•The microscopic origin of doping asymmetry in γ-CuCl is discussed.Doping asymmetry is a pervasive issue in wide band gap semiconductors. We demonstrated that γ-CuCl is one of them with an intrinsic p-type semiconductor by first-principles calculations. The valence band maximum of γ-CuCl is dominated by the antibonding state of Cu-3d and Cl-3p, resulting in a high energy position. We further find that Cu vacancy has a relatively low diffusion barrier in addition to its low formation energy, implying that the long-standing n-type conductivity is hard to realize in γ-CuCl even with non-equilibrium approaches.

•A non-equilibrium process is used to prepare Mg17Al12 alloy.•The hydrogenation process of the Mg17Al12 alloy can be regulated by different processing.•The hydrogenation performance of the Mg17Al12 alloy is improved by quick quenching.•Ball milling significantly improves hydrogen sorption kinetics of the Mg17Al12 alloy.•The hydrogen storage capacity of the Mg17Al12 alloy treated by the non-equilibrium process reaches 4.0 wt%.In this work, the Mg17Al12 alloy was prepared by sintering, annealing, and mechanical ball milling, while the tuning of its hydrogen storage performance in relation to the preparation process was investigated. For the Mg17Al12 alloy, the hydrogenation process can be regulated by using different processing. Mechanical milling not only improves the hydrogen storage capacity, but also reduces the dehydrogenation temperature and increases the dehydrogenation rate. The non-equilibrium process of quick quenching at liquid nitrogen temperature can effectively enhance the hydrogenation reaction of the Mg17Al12 alloy and improve the kinetic performance of the hydrogenation. The maximum hydrogen storage capacity of the alloy treated by quick quenching and mechanical milling is 4.0 wt%, which is close to the theoretical hydrogen storage capacity of 4.4 wt%.

•Quenching can enhance hydrogen desorption kinetics of the Mg-Al-Li composite.•Milling can improve hydrogen storage properties of the Mg-Al-Li composite.•The hydrogen storage capacity of Mg17Al12 (Li) solid solution reaches 3.7 wt.%.•The Mg17Al12 (Li) solid solution has good thermodynamic property.In this work, a Mg-Al-Li solid solution was prepared and the influence of the preparation process on its hydrogen storage properties was investigated. The Mg17Al12 (Li) solid solution was obtained by sintering, annealing at liquid nitrogen temperature and mechanical ball milling using a molar ratio of Mg:Al:LiH = 6.5:1.5:2.0. The maximum hydrogen storage capacity of the Mg17Al12 (Li) solid solution reached 3.7 wt.% at 300 °C in 80 min. The enthalpy of the solid solution hydride decomposition was 53 kJ mol−1 H2, much lower than that of bulk Mg hydride decomposition (75 kJ mol−1 H2). The solid solution had higher hydrogen storage capacity and better hydrogen absorption kinetics than Mg17Al12 alloy.

•The electronic structures of AgAlTe2 and CuAlTe2 have been studied.•The band edge positions of AgAlTe2 straddle the water redox potentials.•Ga-doping can optimize the band structure of AgAlTe2 for water splitting.I–III–VI2 chalcopyrite compounds (I = Cu or Ag; III = Al, Ga, or In; VI = S, Se, or Te), especially sulfides and selelides, were studied as the promising photocathodes for water splitting. In this work, tellurides with a proper band gap [i.e. CuAlTe2 (2.06 eV) and AgAlTe2 (2.27 eV)] had been investigated as the photocathode candidates by our first-principles calculations. Our results show that the band edge positions of AgAlTe2 straddle the water redox potentials. In combination with the suitable band gap energy and light carrier effective masses along [001]-direction, AgAlTe2 has the capacity of being a good candidate for water splitting. Moreover, to increase the absorbing ability on visible light, Ga-doping can adjust the band gap of AgAlTe2 to an appropriate value and remain the reasonable band edge positions of AgAlTe2, which is suggested as an effective approach to optimize its electronic structure for water splitting.

•The structures and enthalpies of compounds in the Li–N–H system are investigated.•The chemical potential phase diagrams of compounds are computed.•Changing the hydrogen chemical potential, six reversible reactions are discussed.•Evaluating the competition between each reaction in the Li–N–H system.The electronic structures and formation enthalpies of compounds in the Li–N–H system have been studied by using the density functional theory. In order to evaluate the competition between each reaction in the system, the chemical potential phase diagrams of compounds in the Li–N–H system have been computed and discussed. Our calculations show that for LiNH2, Li+ combines with [NH2]- by an ionic bond. For Li2NH, the N–H bond displays covalent characteristics. The calculated formation enthalpy of compounds in the Li–N–H system is in agreement with previous results, the LiNH2 is −212.27 kJ mol−1, LiH is −91.66 kJ mol−1, Li2NH is −243.14 kJ mol−1, Li4NH is −309.72 kJ mol−1, Li3N is −189.11 kJ mol−1, and NH3 is −102.27 kJ mol−1, respectively. Using the chemical potential phase diagrams, six reversible reactions are discussed. It is found that Li4NH takes part in the three reversible reactions and some NH3 formed in the system react with other compounds in the Li–N–H system. These reversible reactions are confirmed by the proposed mechanism from experiments.

La1.8Ti0.2MgNi8.9Al0.1 alloys have been prepared by magnetic levitation melting followed by annealing treatment. The effects of annealing on the structure and electrochemical properties of the alloys are investigated by XRD, SEM–EDS and electrochemical measurement. For as-cast and annealed La1.8Ti0.2MgNi8.9Al0.1 alloys, La(Ni,Al)5 and LaMg2Ni9 are the main phases. Annealing not only causes LaNi2 phase to disappear, but also brings higher compositional homogeneity, which contributes to the improvement of cycling stability. At 60% discharge capacity retention, the maximum charge–discharge cycles, 160 cycles (at 900 °C), is more than three times the minimum, 51 cycles (as-cast). The positive impact of annealing (at 800 °C and 900 °C) on the charge transfer rate at the surface and the hydrogen diffusion rate in the bulk enhances the HRD. The optimum annealing temperature is found to be 900 °C.Graphical abstractDischarge cycling performance of as-cast and annealed La1.8Ti0.2MgNi8.9Al0.1 alloy electrodes.Highlights► As-cast and annealed La1.8Ti0.2MgNi8.9Al0.1 alloys are prepared. ► After annealing, the composition becomes homogenous. ► Suitable annealing markedly improves cycle life and high rate dischargeability. ► The most appropriate annealing temperature for the alloy studied is 900 °C.

First-principle density functional theory calculations were used to investigate the electronic structure and mechanism of the LiH + NH3 → LiNH2 + H2 reaction. Along the reaction pathway, intermediate complexes HLi…NH3 and LiNH2…H2 and a transition state can be found. The N-2p electron in the highest occupied molecular orbital (HOMO) of NH3 transfers to the Li-2s orbital in lowest unoccupied molecular orbital (LUMO) of LiH and forms the initial state HLi…NH3. In the transition state, H1 of LiH and H2 of NH3 turn toward each other, resulting in the formation of a H2 bond. From the transition state to the final state, the geometric configuration changes from Cs to C2v, and the improvement of geometric configuration symmetry results in a decrease in the energy gap between HOMO and LUMO. The LiH + NH3 → LiNH2 + H2 reaction is exothermic.Highlights► The mechanism of the LiH + NH3 → LiNH2 + H2 reaction was investigated. ► The intermediate complexes HLi…NH3 and LiNH2…H2 and a transition state were found. ► The process for formation of H…H bond was shown.

ReNi2.6−xMnxCo0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys were prepared by induction melting. The effects of partially substituting Mn for Ni on the phase structure and electrochemical properties of the alloys were investigated systematically. In the alloys, (La, Ce)2Ni7 phase with a Ce2Ni7-type structure, (Pr, Ce)Co3 phase with a PuNi3-type structure, and (La, Pr)Ni5 phase with a CaCu5-type structure were the main phases. The (La,Pr)Ni phase appeared when x increased to 0.45, and the (La, Pr)Ni5 phase disappeared with further increasing x (x > 0.45). The hydrogen-storage capacity of the ReNi2.6−xMnxCo0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloys initially increased and reached a maximum when Mn content was x = 0.45, and then decreased with further increasing Mn content. The ReNi2.6−xMnxCo0.9 (x = 0.0, 0.225, 0.45, 0.675, 0.90) alloy exhibited a hydrogen-storage capacity of 0.81, 0.98, 1.04, 0.83 and 0.53 wt.%, respectively. Electrochemical studies showed that the maximum discharge capacity of the alloy electrodes initially increased from 205 mAh/g (x = 0.0) to 352 mAh/g (x = 0.45) and then decreased to 307 mAh/g (x = 90). The hydrogen absorption rate first increased and then decreased with addition of Mn element. The ReNi2.15Mn0.45Co0.9 alloy showed faster hydrogen absorption kinetics than that of the other alloys. The presence of Mn element slowed hydrogen desorption kinetics.

La2−xTixMgNi9 (x = 0.2, 0.3) alloys have been prepared by magnetic levitation melting under an Argon atmosphere, and the as-cast alloys were annealed at 800 °C, 900 °C for 10 h under vacuum. The effects of annealing on the hydrogen storage properties of the alloys were investigated systematically by XRD, PCT and electrochemical measurements. For the La2−xTixMgNi9 (x = 0.2, 0.3) alloys, LaNi5, LaMg2Ni9 and LaNi3 are the main phases and a Ti2Ni phase appears at 900 °C. The effective hydrogen storage capacity increases from 1.10, 1.10 wt.% (as-cast) to 1.22, 1.16 wt.% (annealed 800 °C) and 1.31, 1.27 wt.% (annealed 900 °C), respectively. The annealing not only improves the hydrogen absorption/desorption kinetics but also increases the maximum discharge capacity and enhances the cycling stability. The La1.8Ti0.2MgNi9 alloy annealed at 900 °C exhibits good electrochemical properties, and the discharge capacities decrease from 366.1 mA h/g to 219.6 mA h/g after 177 charge-discharge cycles.

La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys were prepared by induction melting, and the effects of partially substituting Al for Co on the structure and hydrogen-storage properties of the alloys were investigated systematically. It is found that La(Ni, Co, Al)5 phase with hexagonal CaCu5-type structure, LaNi3 phase with PuNi3 structure and MgNi2 phase exist as the main phases in La0.7Mg0.3Ni2.6AlxCo0.5−x (x = 0.0–0.3) alloys, and the cell volume of the La(Ni, Co, Al)5 phase increases with the amount of Al added. The results show that the substitution of Al for Co can reduce the plateau pressure and the hysteresis between hydrogen absorption and desorption, and improve the hydrogen-absorption capacity and thermal stability of the hydride. Moreover, the addition of Al can delay the oxidation of the surface layer of the alloy electrodes in electrolyte, slow down the capacity degradation and prolong the cycling lifetime, and enhance the electrocatalytic activity of the hydrogen-storage electrodes for hydrogen oxidation.

The La2−xTixMgNi9 (x = 0.1, 0.2, 0.3, 0.4) alloys were prepared by magnetic levitation melting under Ar atmosphere. The effects of partial substitution Ti for La on the phase structures, hydrogen-storage properties and electrochemical characteristics of the alloys were investigated systematically. For La2−xTixMgNi9 (x = 0.1, 0.2, 0.3, 0.4) alloys, LaNi5, LaNi3 and LaMg2Ni9 are the main phases, the maximum hydrogen-storage capacity is 1.51, 1.36, 1.35 and 1.22 wt%, respectively. The absorption–desorption plateau pressure of the alloys first decreases and then increases with increase of Ti content, and the La1.8MgTi0.2Ni9 alloy has the lowest absorption–desorption plateau pressure. The discharge voltage of the alloy electrodes rises with increasing the amount of Ti content. The La1.8Ti0.2MgNi9 alloy electrode presents good electrochemical performance.

Mg76Ti12Fe12-xNixMg76Ti12Fe12-xNix(x=0,4,8,12)(x=0,4,8,12) alloys were prepared by mechanical alloying and the hydrogen storage properties were investigated systematically. In Mg76Ti12Fe12Mg76Ti12Fe12 and Mg76Ti12Ni12Ti12Mg76Ti12Ni12Ti12 alloys, the main binary alloy phase is Fe2TiFe2Ti and Mg2NiMg2Ni, respectively. There are same binary alloy phase structures included Fe2TiFe2Ti, Mg2NiMg2Ni and NiTi in Mg76Ti12Fe8Ni4Mg76Ti12Fe8Ni4 and Mg76Ti12Fe4Ni8Mg76Ti12Fe4Ni8 alloys. For Mg76Ti12Fe12-xNixMg76Ti12Fe12-xNix(x=0,4,8,12)(x=0,4,8,12) alloys, the hydrogen storage capacity is 2.88, 3.31, 3.12 and 2.24 wt%, respectively. The hysteresis between hydrogen absorption and desorption decreases gradually with increasing the amount of substitution Ni for Fe. Mg76Ti12Fe8Ni4Mg76Ti12Fe8Ni4 shows the highest hydrogen absorption and desorption rate among Mg76Ti12Fe12-xNixMg76Ti12Fe12-xNix(x=0,4,8,12)(x=0,4,8,12) alloys. Fe and Ni coexistence is favorable to improve hydrogen storage properties. For Mg76Ti12Fe8Ni4Mg76Ti12Fe8Ni4 alloy, the amorphous degree increase with the milling time, and the amorphous degree increase is unfavorable to improve hydrogen storage capacity.

La0.7Mg0.3Ni2.75−xAlxCo0.75 (x = 0.0–0.4) hydrogen storage alloys were prepared and the effects of partial substitution Al for Ni on the structure, electrochemical properties of the alloys were investigated systematically. In the alloys, Mg2Ni phase appears in all alloys and La (Ni, Co, Al)5 phase with the hexagonal CaCu5-type structure replaces La(Ni,Co)5 phase with Al substituting for Ni. The plateau pressure declines gradually and the hysteresis between the hydrogen absorption and desorption decreases with the amount of substitution Al for Ni increasing. The substitution Al for Ni can improve thermal stability for La0.7Mg0.3Ni2.75−xAlxCo0.75 (x = 0.0–0.4) alloy hydrides, slow down the capacity degradation and prolong cycle life for La0.7Mg0.3Ni2.75−xAlxCo0.75 (x = 0.0–0.4) alloy electrodes.

The effects of La and Mg relative content on the electrochemical performances of La1+xMg2−xNi9(x = 0, 0.5, 1.0, 1.5) hydrogen storage alloys were investigated. The crystal structure, hydrogen storage capacities and electrochemical performances of the alloys were evaluated. It was found that increase of La content was favorable to decrease hysteresis of hydrogen absorption/desorption, and La2MgNi9 alloy exhibited higher hydrogen storage capacity and better electrochemical properties than the other three alloys.

The electronic structure of LaNi4M (M=Ni, Cu, Mn, Al) and their hydrides were investigated by the self-consistent-charge discrete variational Xα method. The results show that the stabilities of LaNi4M hydrides are related to charge transferred to hydrogen atom, which is found to be strengthen by decrease of the transferred charge; the cycle lifetime of LaNi4M is effected by strength of bond between 4f orbit in La atom and 4p orbit in Ni(1) atom, the stronger the bond is, the longer the cycle lifetime is; there is also a orbital action between 3d orbit in Ni(3) atom and 4f orbit in La atom, the orbital action, however, is weaken when hydride is formed.

The electronic structures of RENi5 (RE=La, Ce, Pr, Nd) and their hydrides were investigated by the SCC-DV-Xα (Self-Consistent-Charge Discrete Variational Xα) method. The results show that the stabilities of RENi5 hydrides are related to the charge transferred to the hydrogen atom, and decrease with increase of the transferred charge. There is a strong orbital interaction between the 3d orbit in the Ni atom and the 4f orbit in the RE atom, which is weakened when the hydride is formed. The equilibrium plateau pressure of RENi5 rises with lowering of the Fermi energy of the hydride.

RE3–xMgx(Ni0.7Co0.2Mn0.1)9 (x=0.5–1.25) alloys were prepared by induction melting and the influence of the partial substitution of RE (where RE stands for La-rich mischmetal) by Mg on the hydrogen storage and electrochemical properties of the alloys were investigated systematically. These alloys mainly consisted of three phases, La(Ni,Mn,Co)5 phase, La2Ni7 phase and Mg2Ni phase. The P-C-T isotherms showed that with Mg content increasing in the alloys, the hydrogen storage capacity first increased and reached the maximum capacity of 1.36 wt.% when x=1.0, and then decreased with x increasing further. Electrochemical studies revealed that the discharge capacity reached the maximum value of 380 mAh/g and the alloy electrode presented better cyclic stability when RE/Mg=2. The high rate discharge ability of the alloy electrodes was also improved by the substitution of Mg for RE. The RE2Mg(Ni0.7Co0.2Mn0.1)9 alloy exhibited better hydrogen absorption kinetics (x=1.0).

•H adsorption and diffusion in Mg2Ni were studied using first-principles calculations.•Adsorption energies for H on Mg2Ni (100) surface are much higher than that in bulk.•Hydrogen diffusion path from NiNi site to nearby MgNi site is most likely.First-principles calculations have been used to study the hydrogen adsorption and diffusion in both hexagonal Mg2Ni surface and bulk Mg2Ni, and the hydrogen diffusion rate in bulk Mg2Ni is evaluated. The calculation shows that the adsorption energies on Mg2Ni (100) surface are much higher than in bulk Mg2Ni. For hydrogen diffusing from Mg2Ni (100) surface to subsurface, three diffusion pathways have been studied, and the lowest energy barrier is found to be 0.64 eV. To study the hydrogen atom diffusion in bulk Mg2Ni, nine possible diffusion paths have been considered. The results show hydrogen diffusion from NiNi bridge sites to the most nearby MgNi bridges site is energetically more favorable than the other paths. The energy barrier of this process is 0.34eV, which is close to experimental data of 0.28 ± 0.04 eV. The temperature dependence of the jump rates has been investigated in some details using harmonic transition state theory and semi-classically corrected harmonic transition state theory. The calculated diffusion constant is in good agreement with the experimental data.

An Y2O3@rGO nanocomposite was synthesized via an impregnation method and the catalytic effect of the nanocomposite on the hydrogen-storage properties of a Mg–Al alloy was investigated. The pressure composition isotherm measurement results revealed that the Mg–Al–Y2O3@rGO composite underwent a reversible hydrogenation/dehydrogenation process at 250 °C. Furthermore, the onset temperatures of hydrogenation and dehydrogenation were significantly (i.e., 102 °C and 122 °C, respectively) lower than the respective values corresponding to the Mg–Al alloy. The Y2O3@rGO nanocomposite enhanced the hydriding kinetic properties of the alloy. The hydrogenation kinetic parameter of the Mg–Al alloy increased from 0.008 to 0.195 at 300 °C with 5 wt% of the Y2O3@rGO composite. The values of 162.6 kJ mol−1 H2 and 145.9 kJ mol−1 H2 were obtained for the dehydrogenation energy barrier (evaluated by means of a Kissinger plot) of the Mg–Al alloy and the Mg–Al–Y2O3@rGO hydride, respectively. The reaction enthalpy of hydrogenation/dehydrogenation (determined from a van't Hoff plot) of the alloy decreased with the addition of the Y2O3@rGO nanocomposite. For example, the values of 70.7 kJ mol−1 H2 and 54.3 kJ mol−1 H2 were obtained for the reaction enthalpy of hydrogenation associated with the Mg–Al alloy and the Mg–Al–Y2O3@rGO composite, respectively. Therefore, the addition of the Y2O3@rGO nanocomposite is conducive for improving the thermodynamic and kinetic properties of hydrogenation/dehydrogenation of the Mg–Al alloy.