NL8600347A - METHOD FOR THE PRODUCTION OF AN ELECTROCHEMICAL CELL WITH REDUCED GASES AND ELECTROCHEMICAL CELL - Google Patents

Description

N.0. 33,675 *

Method for the production of an electrochemical cell with reduced gases, as well as an electrochemical cell.

The present invention relates to methods and materials used to reduce gases in electrochemical cells, as well as the amount of mercury required in anode amalgamation for such cells.

Metals such as zinc have usually been used as anodes in electrochemical cells, especially in cells with aqueous alkaline electrolytes. In such cells, the zinc is amalgamated with mercury in order to prevent or reduce the degree of reaction of the zinc with the aqueous electrolyte with the deleterious development of water gas. In the past, it has been necessary to use about 6 to 7 wt.% Mercury aration in the anode to reduce the amount of gas formation to acceptable levels. However, for environmental reasons, it has become desirable to use mercury used in such cells. eliminate or reduce the amount thereof to as little as possible, but without the conductive increase in the gas of the cell Various aids have been used to achieve such a reduction in mercury, such as special treatment of the zinc, the use of additives and very special amalgamation methods, however such methods have had economic disadvantages or limited success.

It is an object of the present invention to provide an economical means of reducing gas in electrochemical cells.

It is another object of the present invention to provide a relatively economizing means to enable the reduction of amounts of mercury used in the amalgamation of aqueous electrochemical anode metals without significant concomitant increase in cell gassing or reduction of the cell performance.

These and other objects, features and advantages of the present invention will become more apparent from the following discussion, as well as the drawings, in which: Fig. 1 is a cross-sectional micrograph of potycrystalline zinc particles, and Fig. 2 is a cross-sectional micrograph of polycrystalline zinc treated according to the present invention.

In general, the present invention includes a method of manufacturing an electrochemical cell with reduced gases. The invention also includes the cell containing the treated anode material. The method of the present invention generally involves reducing the number of grains in the polycrystalline anode material to one third or less of the original number of grains. Thereafter, the reduced grain anode metal is formed into an anode, for example, by compacting powder particles on a substrate or within a cavity. Also, the anode metal may be in the form of a plate, with the anode winding in a "queen bread" configuration along with the cell separator and cathode.

The sheet metal can also be used in a prismatic cell without winding. If desired, the anode metal "especially zinc" is amalgamated with mercury after the grain reduction and prior to placing the anode metal in the cell. In all of the aforementioned embodiments, with such a degree of grain reduction, there is an accompanying decrease in the degree of grain boundaries and a reduction in gases at such locations.

To further reduce the amount of the gases, a small amount of a surfactant heteropolar (surfactant) of a type that will act as an inhibitor of hydrogen development is added to the cell. Due to the heteropolar nature of the surfactant, it is generally at least slightly soluble in the cell electrolyte and has a polar affinity for the surface of the anode metal particles with a coating thereby formed. Such an affinity is particularly noticeable with respect to zinc particles commonly used in anodes of alkaline electrolyte cells. The surfactant can be efficiently incorporated into the cell in various ways. For example, it can be added to the anode, incorporated into the electrolyte, or incorporated into the separator by pre-wetting or impregnating the separator with the additive. The surfactant can even be added to the cathode. In all such cases, the surfactant migrates to the surface of the anode metal particles to form the required coating to counteract hydrogen gas. Addition of the surfactant to the anode material is by direct addition to the powdered metal (amalgamated or non-amalgamated) or formation of the surface coating for the anode metal. Also, the surfactant is added to the electrolyte, which is then mixed with the anode metal particles with resulting migration of the surfactant to the surface of the anode metal particles.

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Migration of the surfactant to the anode metal particles can also be accomplished by the addition of the surfactant to the separator or cathode. Also, or in addition, the anode material particles, such as zinc, are pre-alloyed, with a small amount of one or more of indium, cadmium, gallium, thallium, bismuth, tin and lead and then changed into particles with a reduced number of grains or into individual discrete single crystal particles, which are then amalgamated with mercury.

In order to effect reduction in the number of grains, polycrystalline anode materials, such as zinc, are heat treated to a temperature below their melting point for a sufficient time, the number of grains in the polycrystalline material becoming one third or less of the original material reduced.

Although the anode material remains polycrystalline after this heat treatment, the amount of grain boundaries is reduced with the reduction in grain count. As a result, the amount of gas formation in the cell with the treated particles is noticeably reduced, since it is the surface of the grain boundaries most conducive to high chemical activity and gas formation. In addition, mercury easily infiltrates the grain boundaries. With the reduction of grain boundaries, there is a reduction in the amount of mercury required for amalgamation with the anode material. With the anode material and reduced grain size, the amount of mercury required for amalgamation can be effectively reduced from about 6-7¾ to about 4¾.

Heat treatment of the anode material depends on the purity factors of the polycrystalline starting material, the temperature at which the heat treatment is carried out and the duration of such a heat treatment. It goes without saying that heat treatment of powder particles of different mass amount may differ in required time, since the interior of the aggregate is somewhat insulated by the outer material and does not "meet" the same amount of heat as the outer material in direct heat reception. In practice, a continuously tumbling calcining furnace will provide the most efficient heating, and as a result, with a suitably designed calciner at less than 10 minutes at a temperature above 370 ° C is sufficient to effect a sufficient grain reduction. Recrystallization and coarsening of the 40 grain depends on many factors such as temperature, time, voltage </ 1 φ Λ 4 energy within the material and purity. As a result, exact heat treatment parameters are determined according to the specific heat treatment equipment used. For clarity, the effective heat and temperature mentioned below refer to a direct application of heat to the material. In all cases, a reduction in the number of grains in the material to a third or less of the original material is the desired result.

The heat treatment of the polycrystalline anode material is effective with both powdered material commonly used in the construction of compressed anodes in coil-type cells, and such treatment is also effective with regard to the treatment of metal strips or plates, which used in prismatic or tortuous wound cell structures.

The purity of the original polycrystalline anode material partly determines the length of time required to provide the required grain reduction or, conversely, the temperature at which the material is to be heated for a given period of time; the lower the purity, the higher the required temperature 20 or longer the required time period. The most common anode material for electrochemical cells is zinc, the most common impurity being lead therein. Other less common anode materials include cadmium, nickel, magnesium, aluminum, manganese, calcium, copper, iron, lead, tin and mixtures thereof including mixtures with zinc.

The alkaline electrolyte solution, into which the anode material is placed and which is generally a factor in gas evolution (usually the anode reacts with the electrolyte with resulting gassing), is usually an aqueous solution of a hydroxide of alkali or alkaline earth metals such as NaOH and KOH. Common cathodes for the alkaline cells include manganese dioxide, cadmium oxide and hydroxide, mercury (II) oxide, lead oxide, nickel oxide and hydroxide, silver oxide and air. However, the reduced grain number anodes of the present invention are also useful in cells with other electrolytes, where anode gas is problematic, such as in acid type electrolytes.

Conventional alkaline type cells contain compressed polycrystalline zinc particles with an average particle size of about 100 microns. Each of these particles has about 16 or 40 more grains, and according to the present invention, the number of grain travel in each of the particles is reduced by the zinc particles at an effective temperature between about 50 and 419.5 ° C (the latter being the melting point of zinc) for a minimum period of time ranging from about 2 hours at 50 ° C to about 5 minutes at 419.5 ° C to increase the amount of granules to reduce an average of about 3 to 5 grains per particle. Zinc particles with lead impurities require a temperature of about 100 ° C for the minimum period of 2 hours to achieve a similar reduction in the number of grains.

Useful surfactants which can be added to the cell of the present invention to further reduce the amount of gases include ethylene oxide containing polymers, such as those having phosphate groups, saturated or unsaturated monocarboxylic acid with at least two ethanolamide groups, tridecyloxypoly (ethylene oxide-15) ethanol and most preferably organic phosphate esters. The preferred organic phosphate esters are generally monoesters or diesters of the following formula: [R0 (EtQ) n] x - P = 0 20 | (QM) y wherein x + y is 3, Μ Η, NH4, amino or an alkali or alkaline earth metal and 25 R is phenyl or alkyl or alkylaryl of 6-28 carbon atoms.

Specifically suitable surfactants of the organic phosphate ester type include products which can be identified by their trade name as GAFAC RA600 (an anionic organic phosphate ester supplied by GAF Corp. as the free acid, based on a linear primary alcohol and having a is not neutralized partial ester of phosphoric acid); GAFAC RA610 (an anionic complex organic phosphate ester supplied by GAF Corp. as the free acid with an aromatic hydrophobic, and which is a non-neutralized partial ester of phosphoric acid) and KLEARFAC AA-040 (an anionic mono-substituted orthophosphate ester supplied by BASF Wyandotte Corp.).

It has been found that the incorporation of a surfactant additive of the type mentioned herein into a cell in an amount of 0.001 to 5, preferably 0.005 to 1, and most preferably 0.01 to 0.3% by weight of the active anode component of the cell, precludes or at least significantly inhibits the development of hydrogen within the cell and "7 l **? J - /

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6 thereby increasing shelf life and useful useful life.

The addition of the surfactant to cells, which contains a reduced number of anode metal grains or single crystals of such anode metals, provides a synergistic further reduction in cell gassing.

Although the use of a single crystal anode material and the use of surfactants of the organic phosphate ester type (US Pat. Nos. 4,487,651 and 4,195,120) have been known separately to effectively reduce cell gassing or some reduction in mercury content in the anode without harmful increase in the gases, the effect of the combination has unexpectedly been found to be considerably more than additive. Thus, in cells with amalgamated single crystal zinc anodes, the amount of mercury in the amalgam can be effectively reduced from about 6-7% to about 4% or, in other words, the amount of gases of polycrystalline zinc amalgam containing 1.5% mercury, can be reduced by about two-fold by using single crystal zinc. Similarly, the use of an organic phosphate ester surfactant, such as GAFAC RA600, with polycrystalline zinc amalgam anodes results in an approximately fourfold reduction in gases, for example, 0.1% GAFAC RA600. However, according to the present invention, a combination of the two, ie a single crystal zinc amalgam with a surfactant, unexpectedly makes the effective reduction of the mercury to about 1.5% with an about 20-fold inhibition of the gases or about double what is to be expected. Obviously, combination of chemical gas reduction aids usually do not provide an additive effect, nor do excessive use of additives. The use of the surfactant material with the reduced grain number 30 zinc anode material provides an economic synergistic reduction of the cell's gases above that obtained with single crystal anode material, but well below that obtained with the polycrystalline zinc having high grain number.

The zinc single crystals are preferably prepared as described in U.S. Patent 4,487,651. One such method involves forming a thin-skinned crucible on each of the zinc particles by oxidation in air at a temperature just below the melting point (419 ° C) of the zinc, heating the skin-enclosed zinc particles in an inert atmosphere above the melting point of the zinc and slow cooling then with removal of the oxide JO 4/7 skins. Zinc particle sizes generally range between 80 and 600 micrometers for utility in electrochemical cells, and such a method provides an effective means of making single crystal particles of such small dimensions.

The amount of mercury in the anode amalgam can vary from 0 to 4% depending on the use of the cell and the amount of gas to be allowed.

The amalgamated single crystal metal particles with reduced grain number or with surfactant additives such as GAFAC RA600 are formed into anodes for electrochemical cells, especially alkaline electrochemical cells. Also, the anodes are formed from reduced grain number single crystal metal particles or the surfactant migrates thereto from other cell components such as the electrolyte, separator or cathode to which the surfactants are initially added. Other anode metals, which can be formed into reduced grain number or single crystal powders and are useful in electrochemical cells, include Al, Cd, Ca,

Cu, Pb, Mg, Ni and Sn.

Another additional or alternative means for reducing the gases is alloying polycrystalline anode metal particles with indium or other additives prior to the reduction of the anode metal grain or the formation of the single crystal metal particles, generally in amounts that vary between 25-5000 ppm and preferably between 100-1000 ppm. The amount of mercury going into the anode amal-25 can vary from 0-4¾ depending on the use of the cell and the amount of gas to be allowed.

The amalgamated single crystal metal particles with pre-alloyed inclusions of products, such as indium, are then formed into anodes for electrochemical cells, especially alkaline electrochemical cells. Other anode metals that can be formed into single crystal powders and are useful in electrochemical cells include Al, Cd, Ca, Cu, Pb, Mg, Ni and Sn. It goes without saying that with anodes of these metals the material of the previous alloy is not the same as the active anode material, but is less electrochemically active.

In order to clearly illustrate the effectiveness of the present invention in reducing cell gassing, the following comparative examples are provided. It goes without saying that such examples are for illustrative purposes only and 40 details contained therein should not be construed as meaning. 8 limitations of the present invention. Unless otherwise indicated in the present description, all parts relate to parts by weight.

Example I

Three batches of polycrystalline zinc with an average particle size of about 100 microns are heat treated for varying periods of time and temperatures and then amalgamated with about 4 wt% mercury. A fourth batch of 4¾ mercury amalgamated polycrystalline zinc is untreated and used as a control. Two grams of each charge are placed in 37¾s solutions of KOH (similar to the electrolyte of alkaline cells) at 90 ° C with heating parameters and rates of gas formation given in Table A.

Table A

Zinc treatment ml gas (24 hours) ml gas (93 hours) control, unheated 0.62 3.77 20 114 hours at 400 ° C 0.25 2.07 235 hours at 400 ° C 0.28 2.78 70 hours at 419 ° C 0.28 2.18

It is clear from the foregoing that the heat treatment of the present invention serves to more than halve the gasification rate of amalgamated zinc. It is further understood that continued prolonged heating does not substantially affect the rates of gas formation and is generally economically undesirable.

Example II

30 Polycrystalline Zinc powder (average particle size 100 micro meters) from New Jersey Zinc Co. (NJZ) is heated to 370 ° C by tumbling in a rotary calcination oven for 1 hour. The powder, as received from New Jersey Zinc, has the crystal structure shown in Figure 1. After the heat treatment, the powder has the crystalline structure, shown in Figure 2, in which the grain size has increased significantly, the number of grains has decreased, and the amount of grain boundaries has decreased simultaneously. The polycrystalline zinc as received and after heat treatment is amalgamated with 4% mercury and samples of 2 g each are gassed as in Example I. Another 2 g sample of 7% mercury amine amalgamated zinc containing 7% mercury from Royce Zinc Co., with a similar polycrystalline grain structure and average particle size, is also tested for gassing as an additional control (represents prior art amalgamated zinc) with the gassing results given in Table B.

Table B

Zinc type Gasification (ml) after 24 hours at 90 ° C

10 _____ as received from NJZ 4% Hg 0.85 heated to 370 ° C for 1 hour 4% Hg 0.4 7% Hg Royce 0.25 15 The heat treatment, as described, gives an anode material with noticeably better gas-forming properties compared to untreated polycrystalline zinc and slightly worse than prior art amalgamated zinc with significantly more mercury in the amalgam.

Example III

Two grams of each of the amalgamated zinc products of Example II are similarly screened for gassing at 71 ° after 7 and 14 day periods with the results given in Table 0 and D.

Table C

Zinc type 7 days 14 days __ (ml gas) (ml gas) _ as received from NJZ 4% Hg 0.98 1.95 30 heated to 370 ° C for 1 hour 4¾ Hg 0.50 1.19 7% Hg Royce 0 .46 0.95

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Table D

Zinc type degree of gassing (/ Ul / g day) 5 0-7 days 7-14 days 0-14 days as received from NJZ 4¾ Hg 70 69 70 heated to 370 ° C for 1 hour 4% Hg 36 49 43 7% Hg Royce 33 35 34 10

Both the total amount of gas evolved and the degree of gasification of heat-treated zinc powders, after extended periods of time, are comparable to that of zinc powders amalgamated with significantly more mercury.

It is clear from the micrographs of FIGS. 1 and 2 that the numerous polycrystalline fine grain boundaries have decreased in number with an accompanying decrease in the number of polycrystalline granules per particle without general change in the shape of the individual particles. The number of granules in the heat-treated particles is one-third or less of that of the original particles.

Example IV

Zinc powder amalgams containing 1.5% mercury are prepared using only polycrystalline line zinc with standard grain, polycrystalline line zinc with standard grain with 0.1% RA600 as an additive, single crystal zinc and 25 single crystal zinc with 0.1% RA600 as an additive . Equal amounts of the amalgam powders are then applied in equal amounts of 37% Κ0Η solution (a common electrolyte solution of alkaline cells) and examined for gases at a temperature of 71 ° C. The 0.1% GAFAC RA600 is added to the alkaline solution and stirring the zinc in such a solution results in the deposition of the surfactant on the zinc. The amount of gassing, measured in microliters / gram per day (µl / g day) and the rate reduction factors (with the polycrystalline zinc control as 1) are listed in Table E.

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TabeT Ε anode material degree of degree of 5 _-__._ gas formation reduction factor polycrystalline zinc, 1.5% Hg 295 1 polycrystalline zinc, 1.5% Hg 0.1% RA600 80 3.7 single crystal zinc, 1.5% Hg 140 2.1 10 single crystal zinc, 1.5% Hg 0.1% RA600 15.19.7

A reduction factor (optional) would be expected at most as about 7.8 (3.7 x 2.1) for combined use of single crystal zinc and RA600 with a degree of gas reduction to about 38 µl / g day . However, the combination synergistically reduces gassing to about double the expected reduction.

Example V

Zinc powder amalgams of polycrystalline and single crystal zinc with and without the 0.1% GAFAC RA600 additive are tested as in Example III, but with 0.5% mercury amalgam. The amount of gassing, measured in microliters / gram per day (/ ul / g day) and the degree of reduction factors (with the polycrystalline zinc control as 1) are listed in Table F.

Table F 'Anode Material Degree of Degree of Gas Formation Reduction Factor Polycrystalline Zinc, 0.5% Hg 720 1 Polycrystalline Zinc, 0.5% Hg 0.1% RA600 130 5.5 Single Crystal Zinc, 0.5% Hg 265 2.7 35 single crystal zinc, 0.5% Hg 0.1% RA600 26 28

A reduction factor (optional) would be expected to be at most about 14.9 (5.5 x 2.7) for a combined use of single crystal zinc and RA600 with a degree of gas formation reduction. - .v / * w »* '<t 12th to about 48 / Ul / g day. However, the combination synergistically reduces gas formation to almost double the expected reduction.

It is clear from the previous examples and tables that the single crystal zinc with one or more additives of the present invention is noticeably effective in allowing for large mercury reductions without an increase in the gassing of the cell.

Example VI

Polycrystalline zinc is pre-alloyed with 550 ppm gallium and 100 ppm indium. A first sample thereof is then amalgamated with 1.5% mercury. A second sample is made into separate single crystal alloy particles, as described above, prior to amalgamation with mercury. Two grams of each of the samples are placed in a 37% 1s KOH electrolyte solution, measuring gassing after 90 hours and 48 hours at 90 ° C as representative of corrosion. As a control, an amalgam is made with polycrystalline zinc with 7% mercury similar to that commonly used in cells of the alkaline type. Results of such tests are given in Table G.

Table G

sample gas volume urn (ml), 90 ° C

_24 hours_48 hours_ polycrystalline alloy 0.7 1.9 25 single crystal alloy 0.3 1.0 control (7% Hg) 0.2 0.5

Example VII

A first part of polycrystalline zinc powder, containing 0.04% lead, is amalgamated with 2% Hg and a second part is converted into separate single crystal particles prior to amalgamation. The amalgams are then tested for corrosion in 10 mol air KOH containing 2% ZnO. The degrees of gas formation at 71 ° C are 225 / Ul / g per day resp. 80 ml / g per day.

It is clear that the corrosion reduction of anode metals, such as zinc, for the pre-alloying with corrosion reducing additive material and is greatly enhanced by the formation of single crystal and from the anode metal additive alloy.

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Claims (15)

Process for the production of an electrochemical cell with reduced gas formation, which cell contains a polycrystalline metal anode 5 subject to gas formation, characterized in that a) the number of granules in the polycrystalline metal is up to a third of the original number of grains, b) the polycrystalline metal, with a reduced number of grains, forms into an anode for the cell and c) places the formed polycrystalline metal anode in the cell. 2. Process according to claim 1, characterized in that the polycrystalline metal is heated to a temperature, below the melting point of the metal, for a time sufficient for the number of grains in the polycrystalline metal to at most one third of reduce the original number of grains. Process according to claim 1 or 2, characterized in that the polycrystalline metal is selected from the group consisting of zinc, cadmium, nickel, magnesium, aluminum, manganese, calcium, copper, iron, lead, tin and mixtures thereof. Process according to claim 3, characterized in that zinc is used as the polycrystalline metal and the zinc is heated to a temperature between 50 ° C and 419.5 ° C for 5 minutes to 2 hours. 5. Process according to claims 1 to 4, characterized in that a step is further used in which a surfactant heteropolar additive with a polar affinity for the anode for the cell is added. Process according to claims 1 to 5, characterized in that the polycrystalline metal is converted into single crystals. 7. Process according to claims 5 and 6, characterized in that the surfactant heteropolar additive used is an additive comprising an organophosphate ester of the formula ER0 (Et0) n] x - P = 0 i 35 (0M) y x + y is 3, Μ Η, NH4., amino or an alkali or alkaline earth metal and R is phenyl, alkyl or alkylaryl of 6-28 carbon atoms, contains 40. '1 »<dc (CS" / Process according to claims 1 and 5, characterized in that the polycrystalline anode metal is alloyed with one or more members selected from the group consisting of indium, gallium, thallium, cadmium, bismuth, tin and lead before the number reduction granules 5 thereof. 9. Process according to claim 8, characterized in that the polycrystalline anode metal is reacted to form separate single crystal particles, one or more members forming part of the single crystal. 10. An electrochemical cell containing an anode, a cathode and an electrolyte in water, characterized in that the anode consists of one-crystal anode metal particles and the cell is a surfactant heteropolar additive with a polar affinity for the anode. contains. 11. Cell according to claim 10, characterized in that the surfactant heteropolar additive is an organophosphate ester of the formula [R0 (Et0) n] x - P = Ο Ι 20 (0M) y wherein x + y is 3, Μ Η, NH4, amino or an alkali or alkaline earth metal and R is phenyl, alkyl or alkylaryl having 6-28 carbon atoms. The cell of claim 11, wherein the organic phosphate ester is formed from the free acid of an anionic organic phosphate ester based on a linear primary alcohol and is a non-neutralized partial ester of phosphoric acid. 13. Electrochemical cell that forms less gas, which contains an alkaline electrolyte in water, a cathode and an anode, which anode consists of mercury-amalgamated single crystal zinc particles and an organic phosphate ester, the mercury constituting up to 4% by weight of the anode and the organic phosphate ester is formed from the free acid of an anionic organic phosphate ester based on a linear primary alcohol and an unneutralized partial ester of phosphoric acid, the organic phosphate ester being 0.01 to 0.3% by weight of the anode matters. Cell according to claim 13, characterized in that the mercury constitutes up to 1.5% by weight of the anode, the cathode consists of manganese dioxide and the aqueous electrolyte consists of a potassium hydroxide solution. -: Ü / 15. An electrochemical cell comprising an anode, a cathode and an electrolyte in water, characterized in that the anode is composed of particles of individual single crystals of anode metal and at least one member of the group consisting of indium, cadmium, gallium, Thallium, bismuth, tin and lead, said at least one member being present in the anode in a range of 25-5000 ppm and wherein the at least one member is alloyed with the anode metal prior to the formation of the individual single crystal particles, the at least one member being part of the single crystal. 10 ------ 7