Cell components, with emphasis on hydride electrodes for nickel/metal hydride batteries

The objectives of this work are to tailor-make and evaluate promising alloys for hydride electrodes; the investigations include the elucidation of the role of individual elements on the thermodynamics and kinetics of hydriding/dehydriding and the optimization of the composition and structure of the metal hydride electrodes. A number of LaNi{dollar}sb5{dollar} based alloys were prepared and tested as electrodes in Ni/metal hydride batteries. Ce and Co substitution result in an improvement in the cycle life of the alloys. Sn substituted compounds exhibit high hydriding capacities. It is also necessary to gain insight on the electrode kinetics of hydriding/dehydriding reaction. The proposed equivalent circuit for the MH{dollar}sb{lcub}rm x{rcub}{dollar} electrode reaction fitted the EIS data and yielded the electrode kinetic parameters. The EIS and steady state experiments revealed that the rate determining step in the electrode hydriding reaction is charge transfer processes. For the gas phase hydriding reaction, the shrinking core model assumes that the diffusion of hydrogen in the {dollar}beta{dollar} hydride phase is the rate-controlling step. The jump mass balance and differential mass balance theories were used to derive the equations for the core shrinking rate. The transient analysis could more precisely describe the hydriding processes. The microcalorimetric technique was used to determine the thermodynamic and kinetic parameters of the gas-phase hydriding/dehydriding reactions. The rate determining step for the gas phase hydriding/dehydriding reaction is that of diffusion of atomic hydrogen into and out of the alloy. Electrochemical cycling results show that Ce lowers the capacity loss in the alloys and that this effect is not a simple function of the extent of lattice expansion during hydriding as was previously suggested. Correlation of the electrochemical and XAS results show that capacity loss is directly related to the extent of Ni corrosion. The corrosion behavior of AB{dollar}sb5{dollar} metal hydride electrodes has been studied using the scanning vibrating electrode techniques (SVET). The positive and negative currents indicate the local anodic and cathodic reactions, presumably caused by corrosion of the alloy. The reduced corrosion current in the Ce containing electrode confirmed that Ce substitution for La in the AB{dollar}sb5{dollar} alloy has a beneficial effect on electrode lifetime.