AB2 hydride electrodes

PCT diagrams of ABj (electrxxle alloys) /H systems reflect multiphase or nonideal behaviour [54]. This is illustrated in Fig. 19, in which both the equilibrium pressure and the open—circuit equilibrium voltage, are plotted for Zr 5Tio5 

The cycle lives of several AB2 electrodes are illustrated in Fig. 20 [56]. In some cases alloys require many charge-discharge cycles to become fully activated preactivation via direct reaction with Hj gas is helpful in this regard. Some pertinent properties and results are given in table 9. 

It is of interest to note that V in the hydride phase is significantly less than in AB5 hydrides. Consequently, lattice expansion is also significantly reduced. However, the corrosion rate of the electrodes in Table 9 is still appreciable. Indeed, for the electrode wither = 0.25 the 

X Qma (mAhg ) n (H atoms/ unit cell) AVIV (%) Corrosion (wt.% / cycle) 

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There are few systematic guidelines which can be used to predict the properties of AB2 metal hydride electrodes. Alloy formulation is primarily an empirical process where the composition is designed to provide a bulk hydride-forming phase (or phases) which form, in situ, a corrosion— resistance surface of semipassivating oxide (hydroxide) layers. Lattice expansion is usually reduced relative to the ABS hydrides because of a lower VH. Pressure-composition isotherms of complex AB2 electrode materials indicate nonideal behaviour. 

Two types of metal hydrides electrodes, comprising the AB, and AB2 classes of intermetallic compounds, are currently of interest. The AB, alloys have the hexagonal CaCu, structure where the A component comprises one or more rare earth elements and B consists of Ni, or another transition metal, or a transition metal combined with other metals, The paradigm compound of this class is LaNi, which has been well investigated because of its utility in conventional hydrogen-storage applications. Unfortunately LaNi, is too costly, too unstable, and too corrosion— sensitive for use as a battery electrode. Thus commercial AB, electrodes use mischmetal, a low-cost combination of rare earth elements, as a substitute for La. The B, component remains primarily Ni but is substituted in part with Co, Mn, Al, etc. The partial substitution of Ni increases the thermodynamic stability of the hydride phase fl2] and corrosion resistance. Such an alloy is commonly written as MmB, where Mm represents the mischmetal component. The compositions of normal and cerium-free mischmetal are given in Table 2. 

The other electrode type is usuaUy referred to as the AB2 or Laves phase type electrode and is discussed in Section 9.3. These electrodes are complicated, multiphase alloys with as many as nine metal components. AUoy formulation is primarily an empirical process where the compxjsition is adjusted to provide one or more hydride-forming phases in the particle bulk but which has a surface that is presumed to be corrosion resistant because of the formation of semi-passivating oxide layers. Unlike the AB5 aUoys there are few systematic guidelines which can be used to predict aUoy properties. Eventually AB2 alloy electrodes may be more attractive than AB5 electrodes in terms of cost and energy density, but that potential is not yet reahzed. 

Laboratories around the world have exhausted nearly all the elements in the periodic table to synthesise various AB2 and AB5 alloys to improve the cycle life and capacity of metal hydride cells. The major problem with metal hydride electrodes is that the function of the alloying elements either pristine or in combination with other alloying elements cannot be predicted with certainty. Conventional AB5 and AB2 alloys based on LaNis and (Ti, Zr)Ni2 have relatively low coulombic capacity values between 300 and 450 Ah/kg. Present research has focused on alloys such as TiZrNi2 and Mg2Ni as low-cost, lightweight and safer electrode alternatives, thereby increasing the energy density of the Ni-MH battery. 

This survey presents an overview of the chemistry of metal-hydrogen systems which form hydride phases by the reversible reaction with hydrogen. The discussion then focuses on the AB5 class and, to a lesser extent, the AB2 class of MHs, both of which are of interest for battery applications. A new section has been introduced on super-stoichiometric La(Ni, Sn)5+ e electrodes, which have a higher storage capacity and cycle life than commercial-type electrodes containing Co. 

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