The effects of alloy chemistry on the electrochemical and hydriding properties of NI-substituted LaNi5

Citation

Witham, Charles Kincaid (2000) The effects of alloy chemistry on the electrochemical and hydriding properties of NI-substituted LaNi5. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/w3e0-5779. https://resolver.caltech.edu/CaltechETD:etd-12272004-145717

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The primary goal of this work was to verify the hypothesis that alloying LaNi5 with ternary elements that have a large heat of formation with La (and secondarily, with Ni) would slow the kinetics of metal (La) atom diffusion. This would have the effect of stabilizing the Haucke phase crystal structure of LaNi5 during electrochemical and gasphase hydrogen absorption/desorption cycling, and extending the material's useful lifetime. To test this hypothesis, I prepared a variety of single-phase alloys of composition [...]. Each alloy was annealed to insure equilibrium starting conditions. The lifetimes of these alloys were tested by charge-discharge cycling as the anode of an alkaline Ni-MH rechargeable cell. By characterizing the lifetimes of the alloys as an exponential capacity decay, I was able to determine a trend between the capacity decay and the heat of formation of an average 'B' element with La. The alloys were further characterized by obtaining gas-phase isotherms and, in the case of the [...] alloys, the thermodynamics of metal hydride formation and decomposition. X-ray diffraction was used to measure the effect of substitution on the alloy and its hydride phase. By examining the data obtained at Caltech as well as data published in the literature, several trends were noted. There is a fairly linear relationship between the solute's expansion of the LaNi5 unit cell and its radius. The total volume expansion an alloy experienced upon absorption of hydrogen was found to decrease with substituted composition. The discrete lattice expansion of [...] alloys was found to to decrease substantially for 0 < x < 0.2, but subsequent substitution had little effect on the volume expansion. The electrode electrochemical kinetice of charge transfer were investigated for each MH alloy. Measurements of the charge transfer exchange current by micropolarization and AC impedance were similar, while those measured by Tafel polarization did not have a clear relationship.

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