Ni-based Hydrogen Storage Alloys Research Articles

The hydriding kinetics of Mg–Ni based hydrogen storage alloys: A comparative study on Chou model and Jander model

Two kinds of kinetic models, which are Jander model and Chou model, were applied to investigate the hydriding kinetic behavior of Mg–Ni based alloys. By comparing the calculated values with experimental data, it can be seen that both models were successfully used in the diffusion-controlled hydrogen absorption process of Mg–Ni system. However, Chou model was not only convenient for use but also gave a set of physical meaningful explicit analytic expressions. Chou model should be preferentially recommended to deal with the calculation at multi-temperatures and multi-pressures without multistep calculation. The application of Chou model to Mg 20Ni 8Cu 2 and Mg 20Ni 8Co 2 alloys shows that the calculated results agreed well with the experimental data and it is reasonable to expect that this model will also suitable for other Mg–Ni based alloys if the mechanism is similar.

The correlative effects of Al and Co on the structure and electrochemical properties of a La–Mg–Ni-based hydrogen storage electrode alloy

Replacing Co in the La–Mg–Ni-based alloys by other cheap materials is an expected goal to reduce the cost of the hydrogen storage alloys. The present study aims to partially substitute Co by Al for the La–Mg–Ni-based hydrogen storage alloys and to get the correlative effects of Al and Co on the structure and electrochemical properties of the alloys. Alloys with composition of La 0.7Mg 0.3Ni 2.75Co 0.75− x Al x ( x = 0–0.4) were investigated. The results from X-ray diffraction analyses show that all of the alloys mainly consist of a LaNi 5 phase and a (La, Mg)Ni 3 phase. While LaNi 5 phase increases, the (La, Mg)Ni 3 phase decreases and the cell volumes of both the phases are enlarged with the increasing substitution of Al for Co. Results of the electrochemical studies indicate that the substitution of Al for Co can enhance the cycle stability of the alloy electrode effectively. The capacity retention after 100 cycles is improved from 45.9% ( x = 0) to 82.1% ( x = 0.4). However, the maximum capacity and the electrochemical kinetics decrease. The correlative actions of Al and Co on the corrosion and pulverization resistance of the alloys and hence on the cycle stability and electrochemical kinetics of the alloy electrodes were discussed.

Electrochemical performance studies on cobalt and nickel electroplated La–Mg–Ni-based hydrogen storage alloys

A novel electroplating treatment was applied onto La–Mg–Ni-based La 0.88Mg 0.12Ni 2.95Mn 0.10Co 0.55Al 0.10 alloy powders. The effect of cobalt or nickel metallic coating on morphological and electrochemical properties was studied. FESEM results showed that a dense layer of spherical cobalt particles with uniform radius and an undulate layer of lamellar nickel formed on the surface of the Co- and Ni-coated alloys, respectively. These coatings enhanced the conductivity and the catalytic activity, besides acting as a protective layer, thereby improving the electrochemical properties. The maximum discharge capacity increased from original 316 mAh/g to 335 mAh/g on Co-coated alloys and 336 mAh/g on Ni-coated ones, the cycling stability was enhanced and the self-discharge was suppressed. The high rate dischargeability (HRD) was ameliorated remarkably, and the HRD value at 1500 mA/g rose by 10% and 17%, for cobalt- and nickel-coated alloy electrodes respectively, which is believed to be ascribed to the improved kinetics from the metallic coatings on the surface.

Effect of rare earth elements on electrochemical properties of La–Mg–Ni-based hydrogen storage alloys

La 0.60R 0.20Mg 0.20(NiCoMnAl) 3.5 (R = La, Ce, Pr, Nd) alloys were prepared by inductive melting. Variations in phase structure and electrochemical properties due to partial replacement of La by Ce, Pr and Nd, were investigated. The alloys consist mainly of LaNi 5 phase, La 2Ni 7 phase and LaNi 3 phase as explored by XRD and SEM. The maximum discharge capacity decreases with Ce, Pr and Nd substitution for La. However, the cycling stability is improved by substituting Pr and Nd at La sites, capacity retention rate at the 100th cycle increases by 13.4% for the Nd-substituted alloy. The electrochemical kinetics measurements show that Ce and Pr substitution improves kinetics and thus ameliorates the high rate dischargeability (HRD) and low temperature dischargeability. The HRD at 1200 mA g −1 increases from 22.1% to 61.3% and the capacity at 233 K mounts up from 90 mAh g −1 to 220 mAh g −1 for the Ce-substituted alloy.

Effects of substituting Ni with Al on the microstructure and electrochemical performances of the as-cast and quenched La–Mg–Ni-based (PuNi 3-type) hydrogen storage alloys

In order to improve the cycle stability of La–Mg–Ni system (PuNi 3-type) hydrogen storage alloy, Ni in the alloy was partially substituted by Al, and La 0.7Mg 0.3Ni 2.55− x Co 0.45Al x ( x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of substituting Ni with Al on the microstructures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni 3, the LaNi 5 and the LaNi 2 phases. The substitution of Al for Ni increases the amount of the LaNi 2 phase in the as-cast alloys, but it has an inappreciable influence on the abundance of the LaNi 2 phase in the as-quenched alloy. The results obtained by SEM and TEM indicate that the substitution of Al for Ni leads to the grains of the as-quenched alloys coarsen and restrain an amorphous phase formed in the as-quenched alloys. The results of the electrochemical measurement indicate that the capacities of the as-cast and quenched alloys monotonously decrease and their cycle stabilities significantly increase with the increase of Al content. When the amount of substitution Al for Ni increased from 0 to 0.4, the cycle life of the as-cast alloys increased from 72 to 132 cycles, and that of the as-quenched (20 m s −1) alloys from 86 to 134 cycles.

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La 0.80− x Nd x Mg 0.20Ni 3.20Co 0.20Al 0.20 ( x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi 5 phase, La 2Ni 7 phase and minor LaNi 3 phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g ( x = 0.20) to 374 mAh/g ( x = 0.30), and then decreases to 338 mAh/g ( x = 0.60). The electrochemical kinetics test shows exchange current density I 0 increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.

Hydrogen storage alloys as the anode materials of the direct borohydride fuel cell

The direct borohydride fuel cell (DBFC) employs an alkaline borohydride solution as the anodic fuel. In this study, several Ni-based hydrogen storage alloys were studied as the anode materials for the DBFC. It was proved that hydrogen storage materials can absorb hydrogen during immersing in the borohydride solution. When used as the anode catalysts, these alloys usually showed less negative potentials than the pure nickel electrode. It was observed that hydrogen evolution was partially depressed through the electrochemical oxidation of the absorbed hydrogen at low borohydride concentrations. However, at high borohydride concentrations, the electrochemical reaction starting from borohydride species seems to be kinetically more favorable than the oxidation of the absorbed hydrogen. It was also found that the increases in specific surface area, surface activity and temperature improved the polarization properties of the alloy electrodes but hydrogen evolution was also increased.

Cycling durability and degradation behavior of La–Mg–Ni–Co-type metal hydride electrodes

The cycling durability and degradation behavior of the La–Mg–Ni-based hydrogen storage alloys La 0.7Mg 0.3Ni 3.4− x Co x Mn 0.1 ( x = 0, 0.75, 1.3) during charge/discharge cycling has been systematically studied by XRD, SEM, EIS, XPS and AES measurements. The reasons for the improvement of the cycling stability of the alloy electrodes with increasing Co content have also been analyzed and discussed. The results show that the pulverization of the alloy particles and the oxidation/corrosion of the active components of the alloys during charge/discharge cycling in the alkaline electrolyte are the two main factors responsible for the fast capacity degradation of the La–Mg–Ni-based alloy electrodes, and the capacity degradation mechanism can be decomposed into three consequent stages, i.e., the pulverization and Mg oxidation stage, the Mg and La oxidation stage and the oxidation and passivation stage. With the increase in Co content, the cell volume expansion ratio Δ V/ V of the two main phases during hydrogenation/dehydrogenation was obviously decreased, which results in a reduction of the pulverization of the alloy particles and, consequently, in an increase in the charge and discharge efficiency and a decrease in the rate of contact of the fresh alloy surface with alkaline electrolyte and a subsequent lower rate of oxidation/corrosion. It is believed to be the most important reason responsible for the improvement of the cycling stability of the alloy electrodes with increasing Co content.

The analysis of the two discharge plateaus for Ti–Ni-based metal hydride electrode alloys

In some conditions, the multi-phase composition of the hydrogen storage electrode alloy can be reflected on its discharge curve. In this paper, the discharge curves of the Ti–Ni-based hydrogen storage alloys have be analyzed using electrochemical techniques and XRD. We concluded that the appearance of the two discharge plateaus on the discharge curves of the Ti 0.5Al 0.25Ni 0.25 alloy and Ti 3Ni 1.7M 0.3 (M=Ni, Al, Co, Cr) alloys is related to their multi-phase composition and multi-phase hydride formation. The appearance of the first fastigiated plateau is the result of together releasing hydrogen of TiNi and Ti 2Ni phases. The secondary flat plateau represents releasing hydrogen process of only the Ti 2Ni phase.

The hydrogen electrode reaction characteristics of thin film electrodes of Ni-based hydrogen storage alloys

Film-type electrodes of hydrogen absorbing intermetallic compound and alloys, LaNi 5, Ni 0.11 Ti 0.89, Ni 0.50 Ti 0.50 and Ni 0.76 Ti 0.24 were prepared by a flash evaporation method. The hydrogen electrode reaction characteristics of the LaNi 5 and NiTi alloy films in 1 M NaOH are very similar to each other. The reaction proceeds via the Volmer-Tafel reaction route with mixed rate-determining characteristics. The exchange current densities of the constituent steps, as well as the overall reaction, are in the range of 10 −6 A cm −2 (true). Surface analysis by an XPS technique has shown that La or Ti on the electrode surface exists as an electrocatalytically innert oxide of La 2O 3 or TiO 2. Close similarities of these electrodes with pure Ni electrodes indicate that Ni is responsible for the electrocatalytic activity. No synergistic effect is thus noticeable.