System Hydrogen Storage Alloy Research Articles

Phase formation of Ce5Co19-type super-stacking structure and its effect on electrochemical and hydrogen storage properties of La0.60M0.20Mg0.20Ni3.80 (M = La, Pr, Nd, Gd) compounds

In order to investigate the formation mechanism of Ce5Co19-type super-stacking structure phase, La0.60M0.20Mg0.20Ni3.80 (M = La, Pr, Nd, Gd) compounds are synthesized by powder sintering method. Rietveld refinements of X-ray diffraction patterns find that La0.80Mg0.20Ni3.80 compound has a single Pr5Co19-type structure. The Ce5Co19-type phase appears and increases with the decrease of atomic radius of M, until the La0.60Gd0.20Mg0.20Ni3.80 compound shows a Ce5Co19-type single phase structure. The cycling stability and high rate dischargeability (HRD) of the alloy electrodes both improve with the increase of Ce5Co19-type phase. The capacity retention of La0.60Gd0.20Mg0.20Ni3.80 compound at the 100th cycle is high to 93.6% and the HRD reaches 66.9% at a discharge current density of 1500 mA g−1. Moreover after 50 charge/discharge cycles, the Ce5Co19-type particle retains an intact crystal structure while severe amorphization occurs to Pr5Co19-type particle as shown in graphical abstract. The cohesive energy obtained from the First-principle calculations is analyzed combined with the experimental results. It is found that the La0.60Gd0.20Mg0.20Ni3.80 compound with Ce5Co19-type single phase structure has the highest cohesive energy indicating a more stable structure. This work provides new insights into the superior composition-structure design of LaMgNi system hydrogen storage alloys that may improve the cycling stability.

Structure and Electrochemical Properties of Ball‐Milled La15Fe2Ni72Mn7B2Al2 Hydrogen Storage Alloys with Graphene

AbstractIn order to improve the electrochemical properties of the La‐Fe−B system hydrogen storage alloy, graphene was synthesized and added into the hydrogen storage alloy La15Fe2Ni72Mn7B2Al2. The La15Fe2Ni72Mn7B2Al2 alloy was prepared by arc‐melting and the La15Fe2Ni72Mn7B2Al2+ x wt. % graphene (x=1, 3, 5) composites were obtained by mechanical ball‐milling method. The effects of graphene addition on their electrochemical properties and dynamic performances have been discussed. Structure analyses showed that the La15Fe2Ni72Mn7B2Al2 alloy was composed of LaNi5 phase, La3Ni13B2 phase and (Fe, Ni) phase. The electrochemical experimental results demonstrate that the discharge capacity of ball‐milled La15Fe2Ni72Mn7B2Al2 hydrogen storage alloys with graphene was significantly improved. With 3 wt. % graphene addition, the maximum discharge capacity can reach 288.4 mAh⋅g−1, which is much higher than that of norm alloy (215.3 mAh⋅g−1). Moreover, the high‐rate dischargeability (HRD) and self‐discharge characteristics were also increased with graphene addition.

Investigations on AB3-, A2B7- and A5B19-type La[sbnd]Y[sbnd]Ni system hydrogen storage alloys

The structure and properties of new LaYNi system alloys with high hydrogen-storing capacity were investigated using X-ray diffraction (XRD), scanning electron microscope (SEM), solid-H2 reactions (P-C-I curves) and electrochemical measurements. The LaY2Ni8.2Mn0.5Al0.3 (AB3-type), LaY2Ni9.7Mn0.5Al0.3 (A2B7-type) and LaY2Ni10.6Mn0.5Al0.3 (A5B19-type) hydrogen storage alloys were prepared with the induction-melting rapid-quenching method and annealed at 1148 K for 16 h. The LaYNiMnAl alloys were also compared with commercial AB5-type hydrogen storage alloy with high capacity. Similarly to LaMgNi system hydrogen storage alloy, LaYNi system alloys are multiphase structures and Y element in the LaYNi alloys avoid or delay the hydrogen-induced amorphous (HIA) of the alloys in the hydrogenation/dehydrogenization process. The hydrogen storage capacities of the A2B7- and A5B19-type alloys at 313 K are 1.48 wt.% and 1.45 wt.%, respectively, which are larger than that of the AB5-type alloy (1.38 wt.%). The maximum discharge capacities of the A2B7- and A5B19-type alloy electrodes at 298 K are 385.7 mAh g−1 and 362.1 mAh g−1, respectively, which are larger than that of the AB5-type alloy (356.1 mAh g−1). The maximum discharge capacity of the A2B7-type alloy exceeds the theoretical capacity (372 mAh g−1) of the AB5-type alloy. The A2B7- and A5B19-type alloy electrodes have better cycling ability than the AB5-type alloy.

Effect of elemental substitution on the structure and hydrogen storage properties of LaMgNi4 alloy

Intermetallic compounds with the nominal formula LaMgNi3.6M0.4 (M=Ni, Co, Mn, Cu, Al) were prepared through induction melting, and the structure and hydrogen storage properties of the resultant alloys were extensively investigated. Results showed that the alloys exhibit sizable hydrogen absorption capacity and that elemental substitution significantly influences their microstructure and hydrogen storage properties. The discharge capacities of the alloy electrodes decrease in the order Co>Ni>Al>Cu>Mn. Moreover, the electrochemical kinetics of the alloys depend on their microstructures and phase compositions. Smaller grain size is helpful to improve the electrochemical kinetics. The gaseous hydrogen absorption capacities of the alloys are approximately 1.7wt.% in the first hydrogenation process. Cracking caused by hydrogenation and dehydrogenation also significantly improves the hydrogen absorption kinetics of the alloy particles. The hydrogen storage capacities of the alloys rapidly decrease with increasing cycle number. This result is attributed to amorphisation of the LaMgNi4 phase during hydrogen absorption–desorption cycling (H2-induced amorphisation). Our findings provide new insights into the capacity degradation mechanism of La–Mg–Ni system hydrogen storage alloys that may improve their cycling stability.

Investigation of the thermodynamic and kinetic properties of La–Fe–B system hydrogen-storage alloys

La–Fe–B hydrogen-storage alloys were prepared using a vacuum induction-quenching furnace with a rotating copper wheel. The thermodynamic and kinetic properties of the La–Fe–B hydrogen-storage alloys were investigated in this work. The P–C–I curves of the La–Fe–B alloys were measured over a H2 pressure range of 10−3 MPa to 2.0 MPa at temperatures of 313, 328, 343 and 353 K. The P–C–I curves revealed that the maximum hydrogen-storage capacity of the alloys exceeded 1.23 wt% at a pressure of approximately 1.0 MPa and temperature of 313 K. The standard enthalpy of formation ΔH and standard entropy of formation ΔS for the alloys' hydrides, obtained according to the van't Hoff equation, were consistent with their application as anode materials in alkaline media. The alloys also exhibited good absorption/desorption kinetics at room temperature.

New La–Fe–B ternary system hydrogen storage alloys

The new La 8Fe 28B 24-, La 15Fe 77B 8- and La 17Fe 76B 7-type alloys have multiphase structures including LaNi 5, La 3Ni 13B 2 and (Fe, Ni) phases. The amount of La 3Ni 13B 2 phase increased and that of (Fe, Ni) phase decreased with an increasing La/(Fe + B) atomic ratio. The measurement of P–C–I curves revealed that the maximum hydrogen capacity exceeded 1.12 wt% at 313 K in the pressure range of 10 −3 MPa–2.0 MPa. The alloys exhibited good absorption/desorption kinetics at room temperature, and electrochemical experiments showed that all of the alloy electrodes exhibited good activation characteristics, high-rate dischargeability (HRD) and low-temperature (233 K) dischargeability (LTD).

Effect of Heat-Treatment Process on Properties of Rare Earth Mg-Based System Hydrogen Storage Alloys with AB 3-Type

The effect of heat-treatment process on the properties of Mm 0.8Mg 0.2(NiCoAlMn) 3.5 hydrogen storage alloy was discussed. The electrochemical properties such as cycling stability, activation property, and the plateau voltage of the alloy which was heat-treated in various temperatures and times had different changes during the cycle process, the optimum heat-treatment conditions of this alloy were determined by this work.

Study on Nanocrystalline Rare Earth Mg-Based System Hydrogen Storage Alloys with AB 3-Type

A sort of rare earth Mg-based system hydrogen storage alloys with AB 3-type was prepared by double-roller rapid quenching method. The alloys were nanocrystalline multi-phase structures composed of LaNi 3 phase and LaNi 5 phase by X-ray diffraction and scanning electron microscopy analyses, and the suitable absorption/desorption plateau was revealed by the measurement of P-C-I curve. Electrochemical studies indicate that the alloys exhibit good electrochemical properties such as high capacity and stable cycle life, and the discharge capacity is 369 mAh·g -1 at 0.2 C (72 mA·g -1), after 460 cycles, the capacity decay was only 19.4% at 2 C (720 mA·g -1).

La4MgNi19水素吸蔵合金のMg蒸気圧制御におけるアニール条件と水素吸蔵特性

In La-Mg-Ni system hydrogen storage alloys, which have been attracting attention as a promising negative electrode material for Ni-MH secondary batteries, there are many phases with a variously layered stacking structure. When subjected to annealing at high temperature, the composition of these alloys becomes inhomogeneous due to the volatilization of Mg. It is therefore difficult to prepare a single phase by the normal annealing procedure. We carried out high-temperature annealing processing in which we controlled the Mg vapor pressure of La4MgNi19 alloy using a temperature-gradient furnace. By controlling the Mg vapor pressure under the sealing atmosphere, we successfully obtained homogeneous La4MgNi19. We prepared a PMg-T diagram of the phases formed for La4MgNi19 composition, and found the optimum annealing conditions for the La4MgNi19 phase. The optimum annealing conditions obtained were between T=1123 K, PMg=0.4 kPa and T=1223 K, PMg=2 kPa. However, we found the coexistence of 3R-type and 2H-type structures, which are polytypes of La4MgNi19. The lattice constants of the 3R type were a=0.5031 nm, c=4.826 nm, and those of the 2H type were a=0.5032 nm, c=3.216 nm. The hydrogen storage characteristics of the annealed alloy were obtained by measuring the PCT curves at 333 K. The PCT curves showed an almost flat plateau, PH2=0.1 MPa, H/M=1.1, 1.5 mass%H2, and good cycle characteristics. It was confirmed that the dehydrogenated alloy retained the same structure but an expansion of 1 to 2% was observed in the c-axis.

Effect of Co content on the structural and electrochemical properties of the La 0.7Mg 0.3Ni 3.4− xMn 0.1Co x hydride alloys : II. Electrochemical properties

Electrochemical studies were carried out on the La 0.7Mg 0.3Ni 3.4− x Mn 0.1Co x ( x=0.00, 0.30, 0.60, 0.75, 0.90, 1.05, 1.15, 1.30, 1.45, 1.60) hydrogen storage alloys. Firstly, it was found that partial substitution of Ni by Co could improve effectively the cyclic stability of the La–Mg–Ni–Mn–Co system hydrogen storage alloys with little loss of the maximum discharge capacity, which can be ascribed to a relative lower cell volume change during hydriding/dehydriding cycling and an increase in the surface passivation. Secondly, the high rate dischargeability (HRD) measurement shows that the electrochemical kinetics of the alloy electrodes first increases owing to the concentrations of Co and Ni on the surface of the alloy particles and the subsequent formation of the Raney Ni–Co film with higher electrocatalytic activity after moderate Co substitution for Ni, and then decreases with the increase in Co content because of the reduction of surface electrocatalytic activity due to the diminution of the effective surface area and the increase of the attractive interaction between absorbed hydrogen atoms. Thirdly, the electrochemical impedance spectroscopy (EIS), linear polarization, anodic polarization, potential step, and cyclic voltammetry measurements also confirm the change of the electrochemical kinetics of these type of alloy electrodes. Above all, the optimal content of Co in La 0.7Mg 0.3Ni 3.4− x Mn 0.1Co x alloys for negative electrodes in alkaline rechargeable secondary batteries lies in the range 0.75≤ x≤1.15.

The electrochemical performance of a La–Mg–Ni–Co–Mn metal hydride electrode alloy in the temperature range of −20 to 30 °C

The effect of temperature on the overall electrochemical properties of La 0.7Mg 0.3Ni 2.875Co 0.525Mn 0.1 hydrogen storage alloy has been studied systematically. The results show that temperature has a striking effect on the overall electrochemical properties, especially the electrochemical kinetic performance. The maximum discharge capacity and the high rate dischargeability (HRD) of La 0.7Mg 0.3Ni 2.875Co 0.525Mn 0.1 alloy electrode both decrease with decreasing test temperature, mainly due to the slower hydrogen transfer in the bulk of the alloy and the lower electrocatalytic activity at lower temperatures. Detailed studies on the temperature effect on the polarization resistance ( R D), the exchange current density ( I 0), the limiting current density ( I L) and the hydrogen diffusion coefficient ( D), indicate that the diffusion of hydrogen in the bulk for La–Mg–Ni–Co system hydrogen storage alloy electrodes is the rate-determining factor for the discharge process of the alloy electrode for the temperature over 10 °C and the charge-transfer reaction is rate-determining step at lower temperature.

Hydriding combustion synthesis of hydrogen storage alloys of Mg–Ni–Cu system

Hydriding combustion synthesis of Mg–Ni–Cu system hydrogen storage alloys from metal powder mixture was reported in this study. For studying this new process, the binary system of Mg2Cu was also synthesized at 753 and 798 K under hydrogen/argon atmosphere. The X-ray diffraction (XRD) patterns of the binary system alloy showed a good yield without any un-reacted Mg and Cu and lower synthesis temperature was preferred for avoiding evaporation loss of magnesium and for promoting the hydriding reaction. The XRD patterns of the ternary system alloys, Mg2Ni0.75Cu0.25 and Mg2Ni0.5Cu0.5, showed the catalytic effect of nickel or copper on hydriding reaction because no intermediate products of Mg2Ni and Mg2Cu existed in final product. The hydriding curves of Mg2Ni0.75Cu0.25 showed the hydrogen storage capacity of 1.7 mass% at 473 K after 120 min, although the hydriding rate was not very high. The images of scanning electron microscopy (SEM) showed very interesting character of particle surface like scales of fish, which gave an evidence of eutectic reaction and disproportionation reaction in synthesizing the ternary alloy of Mg-Ni-Cu system.