Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-07-17
2002-12-31
Chaney, Carol (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C029S623100, C429S060000
Reexamination Certificate
active
06500583
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrochemical hydrogen storage alloys, rechargeable electrochemical cells, fuel cells using these alloys, and to methods of manufacturing the same.
More particularly, the invention relates to rechargeable cells, batteries, and fuel cells having at least one electrode formed of multicomponent, electrochemical hydrogen storage material or alloy. In one embodiment, such multicomponent alloys include discrete regions, small in size and widely distributed as will be hereinafter described, which differ compositionally from the bulk alloy and which contribute to the high rate capabilities of the alloys of the present invention. The present invention also includes methods of manufacturing the improved alloys to significantly further enhance such improved performance characteristics. The methods of the present invention assure that the size range and distribution of the aforementioned regions are optimized for enhancement of the rate capabilities of the alloys of the invention. Cells that incorporate these alloys have significantly improved performance characteristics, particularly with respect to exhibiting high discharge rates.
BACKGROUND OF THE INVENTION
Rechargeable cells that use a nickel hydroxide positive electrode and a metal hydride forming hydrogen storage negative electrode (“metal hydride cells”) are known in the art.
When an electrical potential is applied between the electrolyte and a metal hydride electrode in a metal hydride electrochemical cell, the negative electrode material (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxyl ion; upon discharge, the stored hydrogen is released to form a water molecule and evolve an electron:
M
-
H
+
e
-
⟷
charge
discharge
M
-
H
+
OH
-
The reactions that take place at the positive electrode of a nickel metal hydride cell are also reversible. Most metal hydride cells use a nickel hydroxide positive electrode. The following charge and discharge reactions take place at a nickel hydroxide positive electrode:
Ni
(
OH
)
2
-
OH
-
⟷
charge
discharge
NiOOH
+
H
2
O
+
e
-
In a metal hydride cell having a nickel hydroxide positive electrode and a hydrogen storage negative electrode, the electrodes are typically separated by a non-woven, felted, nylon or polypropylene separator. The electrolyte is usually an alkaline aqueous electrolyte, for example, 20 to 45 weight percent potassium hydroxide. The first hydrogen storage alloys to be investigated as battery electrode materials were TiNi and LaNi
5
. Many years were spent studying these simple binary intermetallics because they were known to have the proper hydrogen bond strength for use in electrochemical applications. Despite extensive efforts, however, researchers found these intermetallics to be extremely unstable and of marginal electrochemical value due to a variety of deleterious effects such as slow discharge, oxidation, corrosion, poor kinetics, and poor catalysis. These simple alloys for battery applications reflect the traditional bias of battery developers toward the use of single element couples of crystalline materials such as NiCd, NaS, LiMS, ZnBr, NiFe, NiZn, and Pb-acid. In order to improve the electrochemical properties of the binary intermetallics while maintaining the hydrogen storage efficiency, early workers began modifying TiNi and LaNi
5
systems.
The modification of TiNi and LaNi
5
was initiated by Stanford R. Ovshinsky at Energy Conversion Devices (ECD) of Troy, Mich. Ovshinsky and his team at ECD showed that reliance on simple, relatively pure compounds was a major shortcoming of the prior art. Prior work had determined that catalytic action depends on surface reactions at sites of irregularities in the crystal structure. Relatively pure compounds were found to have a relatively low density of hydrogen storage sites, and the type of sites available occurred accidently and were not designed into the bulk of the material. Thus, the efficiency of the storage of hydrogen and the subsequent release of hydrogen was determined to be substantially less than that which would be possible if a greater number and variety of active sites were available.
Ovshinsky had previously found that the number of surface sites could be increased significantly by making an amorphous film that resembled the surface of the desired relatively pure materials. One of the characteristics of hydrogen storage alloys, particularly electrochemically activated hydrogen storage alloys, which researchers have sought to improve is the hydriding-dehydriding kinetics, or the speed of hydrogen absorption and desorption. This affects the charge and discharge rates at which the battery can operate and determines the applications for which the battery is suitable. For example, such applications as hybrid electric vehicle propulsion require very high discharge rate capabilities in order to meet vehicle torque and acceleration requirements and also very high charge rates to accommodate regenerative braking requirements.
An approach to improving these kinetic characteristics has been to explore mixing alloys of the TiNi and LaNi
5
types. TiNi alloys are often referred to as AB
2
alloys and LaNi
5
alloys as AB
5
alloys.
Bououdina, et al. describe, in “Improved Kinetics by the Multiphase Alloys Prepared from Laves Phases and LaNi
5
” as published in
Journal of Alloys and Compounds
288 (1999) 22-237, a new method to prepare hydrogen absorbing alloys from single phase intermetallic ones. To improve speed of hydrogen absorption and desorbtion, these authors intended to improve the hydriding-dehydriding kinetics, or speed, of the single phase system found in chromium and nickel modified Zirconium/Titanium Laves alloy (an AB
2
type alloy). This was accomplished by melting of two single-phase intermetallic compounds. Of particular interest was addition of LaNi
5
(an AB
5
type alloy) to the described Laves AB
2
alloy. This combination was chosen in light of the difficulties of preparing homogeneous alloys using rare earth elements and a Laves phase alloy, which stem from the approximately 750° C. difference in melting temperatures. Another consideration was the ease of preparation of the LaNi
5
as a single-phase material and its higher hydrogen kinetics coupled with a flat plateau for charging and discharging.
Yang et al. describe, in “Contribution of Rare-Earths to Activation Property of Zr-based Hydride Electrodes” as published in the Journal of Alloys and Compounds, 293-295 (1999) pgs. 632-636, the effect of alloying cerium and either cerium-rich mischmetal or lanthanum-rich mischmetal on the crystalline characteristics and electrochemical performances of AB
2
type hydride electrodes. Alloys of compositions ZrMn
0.5
Cr
0.10
V
0.3
Ni
1.1
and Zr
0.9
T
0.1
Mn
0.5
Cr
0.10
V
0.3
Ni
1.1
(T=Ml, mischmetal, or cerium and Ml=lanthanum-rich mischmetal) were prepared by arc melting under argon atmosphere with as-cast alloy ingots being crushed mechanically in air. Hydride electrodes were prepared by cold pressing the mixtures of different alloy powder with powdered copper in a weight ratio of 1:2 to form porous pellets of 10 mm diameter. Electrochemical charge-discharge tests were carried out in a trielectrode electrolysis cell in which the counter-electrode was nickel oxyhydroxide with excess capacity, the reference electrode was Hg/HgO.6M KOH, and the electrolyte was 6 M KOH solution. Discharge capacities were determined by galvanostatic method.
It was determined that cerium or mischmetal alloying can “basically” solve the problem of activation of a ZrMn
0.5
Cr
0.10
V
0.3
Ni
1.1
alloy electrode and that after rare-earth alloying, the maximum capacities rise from 250 mAh/g of the mother alloy ZrMn
0.5
Cr
0.10
V
0.3
Ni
1.1
to 356 mAh/g of the cerium-containing one.
Yang et al. describe, in “Activation of AB
2
Type Zr-based Hydride Electrodes” as published in
ACTA METALLURGICA SINICA,
Vol 11, No. 2 (April 1998) pgs. 107
Chao Benjamin
Eisen William B.
Habel Ulrike
Huang Baoquan
Peretti Michael W.
Chaney Carol
Dove Tracy
Energy Conversion Devices Inc.
Schumaker David W.
Siskind Marvin S.
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