US3071638A - Dendritic zinc electrodes - Google Patents

Description

Jan. 1, 1963 M. B. CLARK ETAL DENDRITIC zmc ELECTRODES 3 Sheets-Sheet 1 Filed June 25, 1959 INVENTORS MILTON B. CLARK By GEORZIE R.DRENGLER ATTORNEY a 1963 M. B. CLARK ETA. 3,071,638

DENDRITIC ZINC ELECTROiDES Filed June 25, 1959 3 Sheets Sheet 2 mmvroxs MILTON B. CLARK By GEORGE DRENGLER A T TORNEY Jan. 1, 1963 M. B. CLARK ETAL 3,071,638

DENDRITIC ZINC ELECTRODES Filed June 25, 1959 5 Sheets-Sheet 3 INVENTORS MILTON B.CLARK By GEORGE R.DRENGLER jLQM A T TOR /V15) United States 3,071,638 Patented Jan. 1, 1963 This invention relates to the fabrication of high efficiency, high current density zinc electrodes from electrochemically deposited zinc powders.

To obtain satisfactory high current density performance from a zinc anode in an alkaline medium, the available area of the zinc must be as large as possible. This measure results in more efficient zinc utilization, and makes possible high current density operation for the zinc oxide film formed during discharge is spread over a larger area, and hence Will require a longer period of time to attain a suflicient thickness to result in passivation. Various methods of increasing the surface area of zinc electrodes have been suggested previously and include:

(1) Spraying molten zinc onto a suitable support.

(2) Using powdered zinc in a gelled electrolyte, either in bulk or pasted on a grid.

(3) Reducing electrolytically zinc oxide which has been pasted on a grid.

(4) Using pasted or pressed electrodes made from zinc powder or dust.

Commercial zinc powder used in making zinc anodes is obtained as a condensate in the distillation of zinc from roasted zinc sulfide ore blends, or by atomizing molten zinc with a blast of air. Either of these methods results in a zinc powder which contains as much as 10 percent of zinc oxide. Zinc powders can be made by electro-deposition, although there appears to be no commercial source for this type because of the pyrophoric property of the material.

The main object of the present invention is to provide a high efficiency zinc anode molded from a specially prepared electrolytic zinc powder.

An equally important object of the present invention is to provide means for preventing high surface zinc powders from becoming pyrophoric prior to their use in achieving the main object of the invention.

The present invention whereby the foregoing objects are attained comprises compressing dried amalgamated electrolytically deposited zinc sponge into an electrode. A screen or expanded metal grid may be molded-in for added strength but is not essential.

In the drawings:

FIG. 1 is a micrograph showing electrolytically deposited zinc particles under 333 fold magnification;

FIG. 2 is an electron micrograph showing electrolyti: cally deposited zinc particles under 11,500 fold magnification;

FIG. 3 is an electron micrograph magnified 11,500 and showing commercial reagent grade zinc particles;

FIG. 4 is a front elevational view of a support grid for the anode of the invention;

FIG. 5 is a front elevational view of an anode in accord with the invention; and

FIG. 6 is a perspective view of an assembly for making duplex electrodes.

During the course of experimentation leading to the present invention, very finely divided sponge zinc was prepared electrochemically, which probably would be an excellent anode material, except that after drying, it could not be exposed to air without spontaneous combustion. Unexpectedly it was found, however, that amalgamation of this sponge zinc before drying yielded a stable material which did not ignite after drying. The use of this amalgamation step allows one to make a very finely divided zinc powder, which can be dried safely and fabricated into an excellent electrode for high current density application by forming it into a plate under pressure.

Illustrative of the practice of this invention, sponge zinc was deposited from a solution having the following composition:

Grams/liter NaOH 200 Zn (added as ZnO) 10 A fiat zinc sheet was used as a cathode, and an amalgamated zinc plate as anode. The current density used was 1 amp/sq. in., and the process was carried out at room temperature. After one hour electro-deposition time, the cathode was removed. The deposited sponge zinc was scraped off and rinsed with water to remove excess alkali. It was then transferred to a fiashk containing distilled Water slightly acidified with HCl (5 drops/ cc.), in which was dissolved suflicient mercuric chloride to give 24 percent by weight of metallic mercury with respect to sponge zinc. The flask was shaken for a sufficient period of time to insure complete amalgamation, after which the amalgamated zinc sponge was filtered and dried in a vacuum desiccator at room temperature.

Pressed electrodes were prepared as follows:

Zinc sponge powder as prepared above was spread into a mold cavity and pressed. Various fill depths tried successfully ranged from 0.030 inch to 0.230 inch, with resulting pressed plates of about 0.015 to 0.100 inch thickness.

The powder was molded onto a copper screen at a pressure of 0.5 ton/m At the molding pressure the sponge zinc of the electrode has an apparent density of about 1.85 g./cc. equivalent to 74 percent porosity.

Duplex electrodes were also molded in which the anode was made from the above described zinc sponge. In this case the sponge was pressed through an expanded metal grid and against a thin metal shim.

Zinc sponge, when prepared in the manner of the invention, is dendritic in nature (FIGS. 1 and 2), and has an extremely large surface area. On the other hand, commercial zinc dust (flue dust) is comprised of minute spherical particles (FIG. 3). The latter material cannot be pressed into an electrode of satisfactory physical properties without application of excessive pressure which compresses the material and reduces its efiective surface area to such an extent that it becomes unusable as a high current density anode.

A comparison of the zinc utilization efliciency performance of electrodes prepared in accord with the invention and electrodes made otherwise appears in Table I below. The percent utilization efficiency is 100 times the ratio of the experimentally determined coulometer capacity of the electrode to the theoretical capacity calculated from the total zinc present. The electrode representative of the invention consisted of electrolytically deposited zinc sponge molded onto a copper screen under a pressure of 0.5 ton per inch square. In each instance, tests were run against a nickel cathode in 7.35 M potassium hydroxide. A current interrupting device was used which permitted measurement of the anode potential versus a reference electrode consisting of mercurymercuric oxide, without the inclusion of the voltage (IR) drop through the electrolyte. These efiiciency tests were terminated at a 1.0 v. cut-off. It will be seen that the percent utilization efiiciency of the present anode (sample I) exceeds considerably that of the other types (samples 2, 3, 4 and 5).

3 TABLE I Electrochemical Performance of Various Types of Zinc Anodes AMBIENT ROOM TEIWIERATURE (Nondendritic zinc) presumably formed electrolytically from ZnO pasted plate.

Zinc particles made in accord with the present invention also may be used in making electrodes of types other than those described. Such powders as herein described may be used with plastic binders to form self-supporting anodes in accord with the method of US. Patent 2,828,- 351 to H. R. Rade..

What is claimed is:

1. In an alkaline galvanic cell, the improvement which consists of an anode characterized by a high surface area, said anode comprising compressed, finely divided, electrochemically deposited, amalgamated, dendritic zinc particles.

2. In an alkaline galvanic cell the improvement which consists of an anode characterized by a high surface area, said anode comprising a metallic grid embedded in finely divided, electrochemically deposited, amalgamated dendritic zinc particles.

3. The improved alkaline galvanic cell of claim 2 wherein said grid is of copper.

4. A process for making high current density dendritic zinc electrodes comprising electro-depositing dendritic sponge zinc from an alkaline solution containing a hydroxide of an alkali metal and zinc, removing the electro-deposited zinc, removing excess alkali from said zinc, agitating said zinc with an acidified amalgamated solution, filtering the thus amalgamated zinc drying the thusobtained amalgamated dendritic zinc and forming the same to desired electrode shape.

5. The process of claim 4 wherein the dried amalgamated dendritic zinc is molded onto a metallic screen to form a reinforced electrode.

References Cited in the file of this patent UNITED STATES PATENTS Great Britain July 16, 1931 UNITED STATES PATENT OFFICE Certificate of Correction J a-nua/ry 1, 1963 Patent No. 3,071,638

Milton B. Clark et al.

the above numbered patent requiring hould read as corrected below.

S m n m we a hr 2% 6 mL m aw m mt a m m Figs. 4, 5, and 6 as shown below, should be contaming rted as part of the Letters Patent In the drawings, Sheet 4 inse M. B. CLARK ETAL DENISRITIC ZINC ELECTRODES Jan. 1, 1963 4 Sheets-Sheet 4 F iled June '25, 1959 VlsNTbRs GEORGE R. DRENGLER Arrok/wsr in the heading to the drawings, Sheets 1 t0 3, line 3, for 3 Sheets, each occurrence, read-4- Sheets; column 2, line 19, for flashk reedflask.

Signed and sealed this 23rd day of July 1963.

[SEAL] Attest:

ERNEST W. SWIDER, DAVID L. LADD,

Attesting Ofiicer. Gammz'ssz'ouer of Patents.

Claims (1)

1. IN AN ALKALINE GALVANIC CELL, THE IMPROVEMENT WHICH CONSISTS OF AN ANODE CHARACTERIZED BY A HIGH SURFACE AREA, SAID ANODE COMPRISING COMPRESSED, FINELY DIVIDED, ELECTROCHEMICALLY DEPOSITED, AMALGAMATED, DENDRITIC ZINC PARTICLES.

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