Gaseous Phase Hydrogen Storage Capacities Research Articles

Effects of rare-earth element additions to Laves phase-related body-centered-cubic solid solution metal hydride alloys: Thermodynamic and electrochemical properties

The influences of rare-earth element additions on the structural, gaseous phase hydrogen storage, and electrochemical properties of Laves phase-related body centered cubic (BCC) metal hydride alloys were studied using a series of alloys with the design composition of Ti14.6Zr2.1V44.1Cr12.2Mn6.9Fe2.7Co1.4Ni14.7Al0.3RE1.0, where RE (rare earth) = Y, La, Ce, Nd. Samples were prepared by induction melting and then annealed, and only 0.2 to 0.3 at% of the rare-earth element was included in the final samples in the elemental form, which varies from the RENi phase formed in the C14-predominated metal hydride alloy. The Al-content in the final alloy is about 1 at% because of a small reaction to the crucible material. The beneficial effects of rare-earth element addition on the low-temperature performance of the C14-predominated alloy was not observed in this group of BCC/C14 alloys. The gaseous phase and electrochemical hydrogen characteristics were influenced by the rare-earth elements addition mainly through changes in both compositions and abundances of the constituent phase. Y increased reversibility of gaseous phase hydrogen storage. La raised plateau pressure, decreased discharge capacity, and hindered activation and surface electrochemical reaction. Ce increased BCC phase abundance and discharge capacity with reduced room temperature surface charge-transfer resistance. Nd increased C14 phase abundance, decreased the plateau pressure, increased gaseous phase hydrogen storage capacities, and increased discharge capacity and bulk hydrogen diffusion.

Gaseous Phase and Electrochemical Hydrogen Storage Properties of Ti50Zr1Ni44X5 (X = Ni, Cr, Mn, Fe, Co, or Cu) for Nickel Metal Hydride Battery Applications

Structural, gaseous phase hydrogen storage, and electrochemical properties of a series of the Ti50Zr1Ni44X5 (X = Ni, Cr, Mn, Fe, Co, or Cu) metal hydride alloys were studied. X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed the multi-phase nature of all alloys, which were composed of a stoichiometric TiNi matrix, a hyperstoichiometric TiNi minor phase, and a Ti2Ni secondary phase. Improvement in synergetic effects between the main TiNi and secondary Ti2Ni phases, determined by the amount of distorted lattice region in TiNi near Ti2Ni, was accomplished by the substitution of an element with a higher work function, which consequently causes a dramatic increase in gaseous phase hydrogen storage capacity compared to the Ti50Zr1Ni49 base alloy. Capacity performance is further enhanced in the electrochemical environment, especially in the cases of the Ti50Zr1Ni49 base alloy and Ti50Zr1Ni44Co5 alloy. Although the TiNi-based alloys in the current study show poorer high-rate performances compared to the commonly used AB5, AB2, and A2B7 alloys, they have adequate capacity performances and also excel in terms of cost and cycle stability. Among the alloys investigated, the Ti50Zr1Ni44Fe5 alloy demonstrated the best balance among capacity (394 mAh·g−1), high-rate performance, activation, and cycle stability and is recommended for follow-up full-cell testing and as the base composition for future formula optimization. A review of previous research works regarding the TiNi metal hydride alloys is also included.

The effects of Al substitution on the phase abundance, structure and electrochemical performance of La0.7Mg0.3Ni2.8Co0.5−xAlx (x = 0, 0.1, 0.2) alloys

A series of (La, Mg)Ni2.8Co0.5 metal hydride alloys was studied in order to understand the effects of partial Al-substitution for Co on the alloy's structure, gaseous phase hydrogen storage properties, and electrochemical performance. The alloys were characterized by X-ray diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy, pressure-concentration-temperature characterization, half-cell electrochemical tests, and sealed full-cell tests. After an annealing process at 960 °C for 5 h, the main matrix of the Al-free base alloy shifted from AB3 phase into an A2B7 phase. Annealed alloy samples incorporating Al promoted the AB5 phase over the A2B7 phase. The annealing treatment (AB3 to A2B7) increased the gaseous phase hydrogen storage capacity and lowered the plateau pressure and hysteresis in the absorption–desorption isotherms. The addition of Al in the annealed samples (A2B7 to AB5) decreased gaseous phase hydrogen storage and electrochemical capacities, decreased high rate dischargeability, but increased cycle stability. Continuous pulverization of the metal hydride alloy at the negative electrode appeared to be the main failure mode of this series of alloys despite the lower starting rate of pulverization when compared to commercially available AB5 alloys.

Studies on the hydrogen storage characteristic of La1−xCex(NiCoMnAlCuSiZr)5.7 with a B2 secondary phase

Abstract Six alloys with Ce partially substituting for La, having nominal alloy compositions of La15−xCexNi68.7Co4.7Mn4.3Al5.6Cu1.2Zr0.2Si0.3 (x = 5, 6, 7, 8, 9, and 10), were prepared by induction melting. The structure, chemical composition, gaseous phase hydrogen storage, and electrochemical properties of these alloys in the as-cast and annealed (960 °C for 10 h in vacuum) states were investigated. It was found that both the hydrogen reactions in the gaseous phase and the electrochemistry of these Pr, Nd-free hyper-stoichiometric alloys were dominated by the “synergetic effect” between the AB5 main phase and a B2 (close to AlMnNi2) secondary phase (3–9 vol.%). As the abundance of the B2 secondary phase decreased with the annealing process, although the gaseous phase storage capacities improved, all the electrochemical properties (capacity, high-rate dischargeability, surface exchange current, and hydrogen diffusion in the bulk) deteriorated. Besides, with the increase in Ce-content (less La), the unit cell volume of the AB5 main phase decreased, the c/a ratio in the same phase increased, the abundance of the B2 secondary phase increased, the hydrogen equilibrium pressure and the hydride entropy and enthalpy increased, and the maximum and reversible gaseous phase hydrogen storage capacities and the electrochemical discharge capacity increased first and then decreased. The surface exchange currents of the as-cast alloys were higher than those from the annealed alloys. Both the as-cast and annealed alloys showed higher hydrogen bulk diffusion coefficients (up to four times) than that from an AB5 sample.

Gaseous phase hydrogen storage and electrochemical properties of Zr8Ni21, Zr7Ni10, Zr9Ni11, and ZrNi metal hydride alloys

A systematic study of four important non-Laves phase alloys, Zr8Ni21, Zr7Ni10, Zr9Ni11, and ZrNi, commonly seen in the Zr-based AB2 metal hydride alloys was presented. In order to investigate the synergetic effect between the major and secondary phases, an annealing treatment was used to change the abundances of various phases in the alloys. The structure, gaseous phase hydrogen storage, and electrochemical properties were obtained for each of the four alloy compositions before and after annealing, and the correlations among these properties were explored. Annealing generally suppressed secondary phases except for the case of Zr9Ni11, where its secondary ZrNi phase abundance increased. As the Zr/Ni ratio in the average composition increased, the maximum gaseous phase hydrogen storage capacity increased but maximized at Zr:Ni = 9:11. Comparing the properties before and after annealing, it was established that the change in phase distribution influenced the gaseous phase storage. Through the electrochemical measurements, it was found that the highest full discharge capacity was obtained at Zr:Ni = 7:10, which represents a compromise between the hydrogen desorption/discharge rate and the theoretical maximum gaseous phase hydrogen storage. As the Zr/Ni ratio increased, the high-rate dischargeability decreased, which followed the trend of the amount of metallic Ni in the surface oxide determined by magnetic susceptibility measurement. The synergetic effect was observed in the electrochemical environment by comparing the results before and after annealing. In general, annealing deteriorated and improved the electrochemical discharge capacity and high-rate dischargeability, respectively, due to the reduction in secondary phase abundance and consequent synergetic effect. Among all alloys investigated, the unannealed Zr7Ni10 demonstrated the best overall gaseous phase hydrogen storage and electrochemical capacity and could be considered as a candidate to replace the AB5 and AB2 metal hydride alloys in Ni/MH battery applications. Furthermore, the unannealed Zr8Ni21 showed a good balance between high-rate dischargeability and ease of formation.

Improvement in high-rate dischargeability, activation, and low-temperature performance in multi-phase AB2 alloys by partial substitution of Zr with Y

The changes in structure, gaseous phase hydrogen storage, and electrochemical properties due to the incorporation of Y in the alloy formula have been studied on two series of AB2 alloys, with an originally C14- (relatively higher Cr-content) and a C15- (relatively higher Co-content) predominated microstructure, respectively. In each series, 1–4 at.% of Y was added to the designed formula and partially replaced Zr. A majority of the added Y went into the secondary phases, such as YNi, (Y, Sn)-rich, and Y2O3, and only a small amount being found in the main AB2 phase. The lattice constants and the unit cell volume decreased due to the reduction in Zr-content. The C15 phase abundances of the Y-containing alloys were higher than those of the Y-free alloys in both series. As the Y-content increased, the gaseous phase hydrogen storage capacity measured at 30 °C decreased in the high-Cr series and increased in the high-Co series. Both the hydrogen mid-point pressures and the electrochemical discharge capacities of the Y-containing alloys were lower than those in the Y-free alloys in both series. Both the activation and high-rate dischargeability (HRD) improved with the increase in Y-content. The −40 °C resistance was substantially reduced with the incorporation of Y in both series.

Effects of annealing on Zr8Ni19X2 (X = Ni, Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf): Hydrogen storage and electrochemical properties

The effects of 8-h annealing at 960 °C on the gaseous phase hydrogen storage and electrochemical properties of partial-Ni substituted Zr8Ni21 alloys were studied. The substituting elements included Mg, Al, Sc, V, Mn, Co, Sn, La, and Hf. Only the main phase of the annealed Sn-substitution remained Zr8Ni21-structured while those of other substitutions turned into Zr7Ni10 or Zr2Ni7. The observed trend in maximum gaseous phase hydrogen storage capacity followed the increasing order of B/A ratio of the main phase as orthogonal Zr7Ni10 > tetragonal Zr7Ni10 > Zr8Ni21 > Zr2Ni7. After annealing, due to the increase in abundance of the main phase, the maximum gaseous phase hydrogen storage capacities of alloys with higher capacities before annealing increased while others decreased. The full discharge capacity also improved in the same increasing order of B/A ratio in the main phase. Hf-substitution showed the highest electrochemical discharge capacity at 200 mAh g−1. After annealing, all alloys with the same main phase as the as-cast alloys showed degradation in full electrochemical capacity due to the reduction in both number and abundance of the catalytic secondary phases. All supplements assisted in improving surface exchange current from the base binary Zr8Ni21 alloy. Except for La- and Hf-substitutions, annealing reduced the surface exchange current density. The bulk hydrogen diffusion coefficient decreased with most of the supplements except for V- and Sn-substitutions. All supplements, except for Sc, showed improvement in the bulk diffusion after annealing. Furthermore, the maximum gaseous phase hydrogen storage capacity showed a strong correlation to the full electrochemical discharge capacity. Among all alloys in this study, the as-cast Hf-substituted Zr8Ni21 alloy demonstrated the best overall gaseous hydrogen storage and electrochemical properties.