Electrochemical cell having reduced cell pressure

Details Electrochemical cell having reduced cell pressure Electrochemical cell having reduced cell pressure

1999-04-14

2002-12-10

Langel, Wayne A. (Department: 1754)

Chemistry: electrical current producing apparatus, product, and

Fluid active material or two-fluid electrolyte combination...

Active material in solution

C429S201000, C429S206000, C429S207000, C429S347000

Reexamination Certificate

active

06492057

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to rechargeable electrochemical cells. In particular this invention relates to a metal hydride rechargeable electrochemical cell.
BACKGROUND OF THE INVENTION
Rechargeable electrochemical cells using a hydrogen absorbing alloy as the active material for the negative electrode are known in the art. The negative electrode is capable of the reversible electrochemical storage of hydrogen. The positive electrode typically comprises a nickel hydroxide active material. The negative and positive electrodes are spaced apart in an alkaline electrolyte, and a suitable separator (i.e., a membrane) may be positioned between the electrodes.
As shown by reaction (1), upon application of an electrical current to the negative electrode, the hydrogen absorbing alloy (M) is charged by the absorption of hydrogen to form a hydride (M—H).
M+H
2
O+e
−
→M—H+OH
−
(Charging)  (1)
During discharge, the stored hydrogen is released by the hydride to provide an electric current and participates in electrochemical reaction, as shown by reaction (2).
 M—H+OH
−
→M+H
2
O+e
−
(Discharging)  (2)
Examples of hydrogen absorbing alloys are disclosed in U.S. Pat. Nos. 4,551,400, 4,728,586, 5,096,667, 5,135,589, 5,277,999, 5,238,756, 5,407,761, and 5,536,591, the contents of which are incorporated herein by reference.
The reactions at a conventional nickel hydroxide positive electrode as utilized in a nickel-metal hydride electrochemical cell are as follows:
Ni(OH)
2
+OH
−
→NiOOH+H
2
O+e
−
(Charging)  (3)
NIOOH+H
2
O+e
−
→Ni(OH)
2
+OH
−
(Discharging)  (4)
Hence, as shown by reactions (1) and (2) above, the charging and discharging of the electrochemical cell involves the hydriding and dehydriding of the hydrogen storage alloys. Generally, the hydriding and dehydriding reactions in the electrochemical cell are accompanied by electrochemical charge transfer. These reactions also involve the transport of hydrogen atoms into and out of the hydrogen absorbing alloy. During the operation of the cells, particularly during high rate charge and discharge, significant hydrogen pressures can develop (especially if the hydrogen transport lags behind in one direction or the other).
Specifically, during cell charging, atomic hydrogen is formed at the surface of the negative electrode. Preferably, the atomic hydrogen reacts with the hydrogen absorbing alloy as shown by reaction (1) to form a hydride. However, depending on charge conditions and surface properties of the hydrogen absorbing alloy, some of the atomic hydrogen may instead recombine with another atomic hydrogen to form molecular hydrogen gas.
While cells typically operate at pressures greater than atmospheric pressure, excessive hydrogen pressure is undesirable since it can result in a loss of aqueous-based electrolyte material, thereby limiting cell life. Also, if excess hydrogen pressure is not vented, the cell can burst, deform, or otherwise be destroyed.
Clearly, it is desirable to limit excessive hydrogen overpressure in electrochemical cells. Reduction of the internal cell pressure increases the cycle life of the electrochemical cell. The present invention relates to additive materials which may be added to cell components for reducing cell pressure by decreasing hydrogen gas evolution within the electrochemical cell.
SUMMARY OF THE INVENTION
An objective of the present invention is an electrochemical cell having decreased cell pressure. Another objective of the present invention is an electrochemical cell with improved cycle life.
These and other objectives of the invention are satisfied by an alkaline electrochemical cell, comprising: a positive electrode; a negative electrode comprising a hydrogen absorbing alloy active electrode material; and an alkaline electrolyte comprising an additive material reducing cell pressure by decreasing hydrogen gas evolution.
These and other objectives of the invention are also satisfied by an electrolyte for an alkaline electrochemical cell having a negative electrode including a hydrogen absorbing alloy, the electrolyte comprising: an alkaline ion-conducting medium; and an additive material reducing cell pressure by decreasing hydrogen gas evolution.
These and other objectives of the invention are also satisfied by an alkaline electrochemical cell, comprising: at least one positive electrode; at least one negative electrode; and an alkaline electrolyte, the negative electrode comprising: a conductive substrate; and an active composition affixed to the substrate, the active composition comprising: a hydrogen absorbing alloy, and an additive material reducing cell pressure by decreasing hydrogen gas evolution.
These and other objectives of the invention are also satisfied by an electrode of an alkaline electrochemical cell, comprising: a conductive substrate; and a active composition comprising: a hydrogen absorbing alloy active electrode material, and a additive material reducing cell pressure by decreasing hydrogen gas evolution.
DETAILED DESCRIPTION OF THE INVENTION
Disclosed herein is an alkaline, metal hydride electrochemical cell. The electrochemical cell comprises one or more positive electrodes, one or more negative electrode, and an alkaline electrolyte. Separators are typically positioned between the positive and negative electrodes to electrically isolate the positive electrodes from negative electrodes.
Each of the positive electrodes comprises a positive electrode active material that is affixed to a conductive substrate. Preferably, the positive electrode active material comprises a nickel hydroxide active electrode material. Each of the negative electrodes comprises a negative electrode active material that is affixed to a conductive substrate. In the present invention, the negative electrode active material is a hydrogen absorbing alloy. It is within the spirit and scope of the present invention that any hydrogen absorbing alloy can be used.
Generally, the conductive substrates used for the positive and negative electrodes may be any electrically conductive support structure. Examples of conductive substrates include expanded metal, mesh, foam, grid, and plate. The substrate may be made from any electrically conductive material. The materials are preferably chosen so as to be immune from corrosion at the pH and potential at which the electrodes operate. The conductive substrate for the positive electrode is preferably a nickel foam or a nickel alloy foam. The conductive substrate for the negative electrode preferably has the form of an expanded metal. The substrate may comprise nickel, a nickel alloy, copper, a copper alloy, nickel-plated copper, or copper-plated nickel.
The alkaline electrolyte is an ion-conducting medium. The electrolyte is preferably an aqueous solution of an alkali metal hydroxide. Examples of alkali metal hydroxides include potassium hydroxide, sodium hydroxide, lithium hydroxide, and mixtures thereof. Preferably, the alkali metal hydroxide comprises potassium hydroxide. More preferably, the alkali metal hydroxide comprises about a 30 weight percent aqueous solution of potassium hydroxide.
In one embodiment of the present invention, the electrolyte further comprises an additive material which decreases the pressure within the electrochemical cell by reducing the evolution of hydrogen gas. Preferably, the additive material is selected from the group consisting of thiourea, urea, thiophene, thioalcohol, thiocyanate, pyrrole, cysteine, cynanide, sulfide, bisulfide, and mixtures thereof. More preferably, the additive comprises thiourea.
Specifically, the additive material may be added to the electrolyte by being mixed into the aqueous solution with the alkali metal hydroxide. Preferably, about 0.01 weight percent to about 5 weight percent of the additive material is added to the electrolyte. More preferably, about 0.05 weight percent to about 1 weight percent of the additive materi

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