Valutazione della fase di Laves Zr(Ni_(0.55)Mn_(0.30)V_(0.10)Co_(0.05))_(2.1) come materiale attivo per elettrodi a idruro metallico

摘要

The use of hydrogen storage alloys as negative electrodes for secondary batteries has been intensively studied in recent years. The Nickel-Metal Hydride (Ni-MH) batteries consist of a positive electrode of Ni(OH)_2 and a negative electrode, made of an alloy with a strong tendency to charge and discharge hydrogen. Intermetallic AB_5 compounds, La-Ni5-type, are able to charge a huge amount of hydrogen, but they tend to form stable hydrides, so that the discharge is difficult. More recently, AB_2 Laves-phases have been proposed as negative electrodes. They are characterised by a very compacted structure, which can be obtained when a suitable ratio between the radii of components is reached. These compounds are known to form cubic (C15) and hexagonal (C14 and C36) crystalline lattices. Hydrogen is absorbed in tetragonal interstices and induces a lattice expansion. Zr-Ni_2-based intermetallic compounds have shown high absorption/desoprtion capacity and have been already used for commercial products. The aim of this paper is to study the electrochemical properties of the Zr(Ni_(0.55)Mn_(0.30)V_(0.10)Co_(0.05))_(2.1) Laves-phase. The master alloy was prepared by arc melting pure components and then it was crushed by milling. The electrodes have been prepared pressing the Laves-phase with Ni powder. In order to improve the mechanical properties, a composite electrode was prepared pressing the powders inside a Ni sponge. The structure of the electrodes was analysed by X-ray diffraction (XRD). The microstructure was observed by Scanning Electron Microscopy (SEM), equipped with EDS for compositional analysis. The electrochemical properties of the alloy were determined in 6M KOH solution. During the charge-discharge cycles, the hydrogen absorption and desorption was observed following the volume expansion of the material, through the measurement of the force necessary to maintain the electrode in a fixed position (fig. 1). The XRD patterns of the Laves phase and of the electrodes are shown in fig. 2. The Laves phase is cubic (C15), with a lattice constant of 7.05 A. An average crystallite size of 80 nm has been estimated from a Scherrer analysis of the peak broadening. From the SEM images (figs. 3 and 4) and from the EDS analysis (table Ⅰ), it appears clear that, in the composite electrodes, the Laves-phase is surrounded by the nickel powder. The first cycles of absorption and desorption of hydrogen have shown a very low capacity. The electrode need about 40 cycles to become active, as shown in fig. 5, where the ratio between the absorption and desorption capacity is shown as a function of the number of cycles. It can be observed that, after activation, the capacity of the electrode remains quite stable. As an example, the potential (dashed curve) referred to Hg/HgO electrode and the force (solid curve) are shown in fig. 6 as a function of time, during charge/discharge cycle n 80. The charging current was 30 mA, corresponding to 75 mA per gram of active material. The maximum capacity of the composite electrode turns out around 300 mAh/g, a value higher than that of commercial material. The electrode remains mechanically stable after several working cycles, as shown in fig. 7. In conclusion, it was observed that the Zr(Ni_(0.05)Mn_(0.30)V_(0.10)Co_(0.05))_(2.1) Laves-phase is an active material for the absorption and desorption of hydrogen and it is a good candidate as a material for negative electrodes for secondary batteries.