Electrochemical Properties of Nb-Substituted Zr-Ti-Ni Hydrogen Storage Alloy Negative Electrodes for Nickel-Metal Hydride Batteries

Abstract

Abstract:Crystal structure, pressure-composition isotherms and electrochemical properties of the Zr 0.6−x Ti 0.4 Nb x Ni (x = 0.01, 0.02, and 0.05) alloys were investigated. Their X-ray diffraction profiles demonstrated that all the Zr 0.6−x Ti 0.4 Nb x Ni alloys consisted of the primary phase with the B33-type orthorhombic structure and the secondary phase with the B2-type Ti 0.6 Zr 0.4 Ni cubic structure. Rietveld refinement demonstrated that the atomic fraction of the secondary phase increased with the Nb content. The Zr 0.6−x Ti 0.4 Nb x Ni alloys were lower in hydrogen storage capacity than the Nb-free Zr 0.6 Ti 0.4 Ni alloy due to an increase in the abundance of the secondary phase. In the charge-discharge tests with the Zr 0.6−x Ti 0.4 Nb x Ni alloy negative electrodes, all the initial discharge curves had two potential plateaus due to the electrochemical hydrogen desorption of trihydride to monohydride and monohydride to alloy of the primary phase. The total discharge capacities at 333 and 303 K for the Zr 0.58 Ti 0.4 Nb 0.02 Ni alloy negative electrode were 384 and 335 mAh g −1 , respectively, which were higher than those of the other Zr 0.6−x Ti 0.4 Nb x Ni and Zr 0.6 Ti 0.4 Ni alloy negative electrodes.

Effect on Electrochemical Properties of Phases in AB-Type Zr–Ti–Nb–Ni Alloys as Nickel-Metal Hydride Batteries

“…The mother alloy was composed of two phases, the B33-type orthorhombic primary phase and the B2-type cubic secondary phase, which was consistent with the other ZrTiNbNi alloys. 15) Table 1 shows lattice parameters and abundance of the primary-and secondary-phases estimated by the Rietveld refinement for the Zr 0.49 Ti 0.5 Nb 0.01 Ni mother alloy powders. The residue of the weighted pattern (R wp ) and goodness-of-fit indicator (S) are used to ascertain the quality of the fit between the experimental and calculated patterns.…”

“…The Zr 0.49 Ti 0.5 Nb 0.01 Ni mother alloy or its primary-or secondary-phase alloys were prepared along the previous method. 15) All reagents used in this study were purchased from the Koujundo Chemical Laboratory. The stoichiometric amounts of sponge Zr (98.0%), Ti powder (99.0%), Nb powder (99.9%) and Ni powder (99.9%) were mixed and alloyed by arc-melting method on a water-cooled copper mold in an Ar atmosphere.…”

“…Before the PCT measurement, the initial activation treatment was performed according to our previous study. 15) Each sample was placed into a stainless steel reactor tube in an Ar-filled glove box, heated in vacuum at 423 K for 1 h and gradually introduced at 0.9 MPa H 2 pressure. After the reactor tube was heated to 623 K, it was evacuated at the same temperature for 2 h. After the initial activation treatment, the PCT curves were measured in the hydrogen pressure between 0.005 and 0.92 MPa at given temperatures.…”

“…The Zr 0.49 Ti 0.5 Nb 0.01 Ni mother alloy and its primary-and secondary-phase alloy negative electrodes were prepared according to our previous study. 15) Briefly, the alloy powder, Cu powder and 10 mass% polyvinyl alcohol aqueous solution were mixed to make a paste, and the resultant paste was cast on a Ni mesh as a current collector. For the activation treatment, the resultant ZrTiNbNi negative electrodes were immersed in a boiling 6 M KOH alkaline solution for 4 h. 12) The positive and reference electrodes were NiOOH/ Ni(OH) 2 and Hg/HgO electrodes, respectively.…”

“…AB-type ZrNi hydrogen storage alloy has the theoretical capacity of 536 mAh g ¹1 , 9) which is higher than that of commercial AB 5 -or AB 3 -type rare earth-based alloys, 10,11) so the ZrNi alloy is a promising negative electrode active material. 1214) Recently, we have developed new AB-type ZrTiNbNi quaternary hydrogen storage alloys, and the electrochemical capacities of the Zr 0.58 Ti 0.4 Nb 0.02 Ni negative electrode at 333 and 303 K were 384 and 335 mAh g ¹1 , 15) respectively. The ZrTiNbNi alloys consist of two phases, the primary-phase with B33-type ZrNi orthorhombic structure and the secondary-phase with B2-type Ti 0.6 Zr 0.4 Ni cubic structure.…”