Experiments have shown that improved hydride-forming negative electrodes for rechargeable nickel/metal hydride (Ni/MH) electrochemical cells can be made by substituting Ge for some of the Ni in LaNi5. A similar discovery regarding the substitution of Sn for some of the Ni was reported in the preceding article.

Figure 1. The Equilibrium Plateau Pressure for LaNi5 - xGex (x > 0) is less than 1 atmosphere - essential for preventing self-discharge in cells that must operate at atmospheric pressure.

Hydride-forming electrodes made of LaNi5 rapidly lose reversible hydrogen-storage capacities during charge/discharge cycling and thus have short cycle lives. It has been known for some years that the loss of reversible hydrogen-storage capacity can be slowed by substituting small amounts of other elements for both La and Ni. However, early attempts to prolong cycle lives through substitutions of elements other than Sn or Ge resulted in undesired side effects in the form of large decreases in hydrogen-absorption capacities, prolongation of activation cycles (cycles of initial charge/discharge cycling needed to achieve full capacities), or slow kinetics.

Specimens of LaNi5 -xGex (0.1 ≤x ≤ 0.5) for use in the experiments were made by induction melting in an argon atmosphere and annealed in vacuum at a temperature of 950 °C for 72 hours. In one of several experiments, gas-phase hydrogen-absorption capacities of specimens with x = 0, 0.3, and 0.4 were measured at a temperature of 23 °C. The data from these measurements (see Figure 1) show that the gas-phase hydrogen-absorption capacities of the Ge-substituted alloys are marginally lower (a small disadvantage) than those of the binary alloy LaNi5. However, the data also show a significant advantage for Ge substitution in that the equilibrium plateau pressure of the Ge-substituted alloys is less than 1 atm (< 0.1 MPa) - less than half the equilibrium plateau pressure of LaNi5.

Figure 2. The Specific Charge Capacity retained after many charge/discharge cycles is greater for at least one of the Ge-substituted alloys than it is for the Sn-substituted alloy. The performance of a misch-metal-based alloy is also shown for comparison.

In another experiment, electrochemical capacities of prismatic LaNi4.6Ge0.4and LaNi4.7Ge0.3 electrodes were measured in charge/discharge cycles in a negative-limited glass cell with NiOOH counter electrodes and an Hg/HgO reference electrode. For comparison, some measurements were performed on an Sn-substituted electrode with optimal composition of LaNi4.75Sn0.25, and on an electrode of composition Mm(NiCoMnAl)5 (where "Mm" denotes misch metal). The data from this experiment (see Figure 2) show that all the alloys exhibit rapid loss of capacity during the first 10 to 20 cycles, but thereafter, the loss of capacity slows. The data also suggest that in the long term, the Ge-substituted alloys retain more capacity than does the optimal Sn-substituted alloy.

Another experiment focused on electrochemical kinetics for absorption and desorption of hydrogen. As quantified in terms of exchange-current densities, the kinetics of LaNi4.6Ge0.4 and LaNi4.7Ge0.3were found to be improved over those of LaNi5 - comparable to the kinetics of LaNi4.8Sn0.2.

In conclusion, the substitution of appropriate amounts of Ge for Ni in LaNi5results in alloys that are better suited for use in negative electrodes in rechargeable electrochemical cells. When developed further, these cells can be expected to exhibit high specific energy and power densities, low internal pressures and self discharge, and long cycle lives.

This work was done by Ratnakumar Bugga, Charles K. Witham, Brent T. Fultz, Subbarao Surampudi, Robert C. Bowman, and Adrian Hightower of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Materials category.

In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to

Technology Reporting Office
JPL
Mail Stop 122-116
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(818) 354-2240

Refer to NPO-19962, volume and number of this NASA Tech Briefs issue, and the page number.


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LaNi5-xGex electrodes for Ni/MH electrochemical cells

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This article first appeared in the August, 1998 issue of NASA Tech Briefs Magazine.

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