CS223860B2 - Method of preventing the formation of the passivating anode surface by discharging the galvanic primary current - Google Patents

Abstract

1471806 Primary cells S T POWERCELL AB 14 July 1975 [15 July 1974] 29454/75 Heading H1B In a primary metal-air cell having an anode active material of iron, and a cathode which is preferably an oxygen electrode, the formation of a passivating coating on the anode is prevented by providing a complexing agent for iron ions, e.g. e.d.t.a., n.t.a., cyanides, thiocyanates, citrates, ammonia, ethylene diamine tetrapropionic acid or diethylene triamine pentaacetic acid. In a modification a second complexing agent may be used for the cathode and oxygen may be supplied to the cathode in the liquid phase. The second complexing agent may be hydroxyquinolines or phenols. The oxygen electrode may be porous sintered Ni or Ag. Active iron powder may be held on a porous Ag or mixture of oxides of Fe and Ba plate electromagnetically. A battery of cells can be built up by providing bipolar electrodes.

Description

The invention relates to a method for preventing the formation of a passivating anode coating upon discharge of a primary cell comprising an alkaline electrolyte, an anode, and a cathode which are connected to each other by an external electrical energy withdrawal circuit, the active anode material being iron.

There are often difficulties associated with the operation of galvanic elements or cells containing a metal electrode, since reaction products formed during operation of the electrode tend to precipitate or adhere to the electrode. These are, for example, oxides or metal hydroxides, by means of which the electrode is passivated and cell efficiency is reduced.

This problem is particularly evident in metal / air or metal / oxygen cells in which the anode is of iron and the electrolyte is an alkali metal hydroxide solution, for example sodium or potassium hydroxide solution, since the iron ions formed on the anode precipitate or adhere to an anode in the form of a coating that greatly reduces or prevents further anode function.

Therefore, it is of great importance to prevent the formation of the aforementioned coating on the electrodes, thereby ensuring a stable and efficient function of the electrode.

Due to the mechanism of formation of the passivation coating on the electrode, it is known for some simple cases using, for example, zinc and cadmium anodes in an alkaline electrolyte, that the metal is dissolved in the reaction of the electrode before settling of the anti-reaction coating. Zn (OH) type anode intermediates⁺ and Cd (OH)⁺ they can subsequently react with OH according to equations

Zn (OH) ⁺ OH ^- - Zn (0H) 2 and

Cd (OH) ⁺ + OH - Cd (OH) 2, the reaction being braked only when there is an excess of the solubility product for the corresponding metal hydroxide. Intermediates can also provide passivating oxide according to equations

Zni (OH) ⁺ -> ZnO + H ⁺ a

Cd (OH) ⁺ - > CdO + H ⁺ .

If the solubility is slightly exceeded, i.e. at low supersaturation, the settling rate increases as the supersaturation increases. With considerable supersaturation, the formation rate of the crystallization seeds is high compared to the growth rate of the seeds, and settling is prevented. This is very common with hydroxides having a low solubility product. Any method carried out at low supersaturation results in a reduction in the concentration of such complex hydroxide intermediates, thus providing less risk of deposition on the electrode surface. However, at high supersaturation, a decrease in concentration causes an increase in settling rate. In the case of zinc or cadmium, the formation of an anti-reaction deposit on the anode is delayed by vigorous stirring, and with continuous supply of liquid fresh electrolyte, inhibition of the reaction is completely prevented.

For most other metals, including iron, this simple process is not sufficient to eliminate the reactions that cause the formation of such coatings. In addition, passivations typically occur prior to actuation of the galvanic cell. The difference may be due to the fact that the amount of the corresponding solubility products is much minor, or the fact that the mechanisms of the dissolution and passivation reactions of the metals involved are different from those of the zinc and cadmium reactions.

The passivation coatings themselves on the electrodes of iron or other metals which provide passivation coatings of the type corresponding to the passivation coating of iron, but cannot be eliminated in the same simple manner as described above for zinc and cadmium, and as is known, no simple so far has been found and an effective solution to this problem.

In addition to the above-mentioned problem of passivation coatings, metal / air or metal / oxygen cells are associated with a further problem which is the supply of air or oxygen to the cathode. In conventional electrodes of this type, air, usually under pressure, is fed to the cathode, where oxygen is reduced in the presence of a catalyst to form hydroxide ions in the electrolyte. The oxygen reduction efficiency at the cathode, i.e. the cathode reaction, at high current density is limited by the rate at which oxygen is able to diffuse into the active cathode area, i.e., by oxygen transport. Oxygen transport and cathode reactions are inhibited by the oxygen present in the gas phase, while the electrolyte is liquid and the electrode is solid. To improve the reaction of the cathode, it is desirable to make oxygen transport more efficient, by increasing the oxygen flow, for example by increasing the oxygen rate or by increasing the oxygen supply concentration.

Another particular problem inherent in the oxygen electrodes of metal / air cells is their complex and critical design, which is largely due to the fact that the oxygen reaction in the cathode reaction is a three-phase reaction (gaseous oxygen, liquid electrolyte and solid electrode), and by the fact that this three-phase interface must - always be maintained. Accordingly, conventional oxygen cathodes are made-porous, the pores having a precisely selected dimension, and furthermore, the electrode surfaces are hydrophobic at least on the gas-exposed side and / or a counter-pressure is applied on that side. Despite the difficulty and cost of manufacturing such oxygen cathodes, it would provide considerable simplification and improvement if oxygen could be supplied at a certain concentration, dissolved in a liquid, in which case it would not be necessary to maintain a three-phase interface and further eliminate other problems, such as the presence of nitrogen in the air, which tends to poison the catalytic material in conventional oxygen cathodes.

It is clear from the above that there are serious problems associated with both the passivation coatings on the iron electrodes and the oxygen supply to the oxygen electrodes. Both of these problems result from the phase interface.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to eliminate phase interfaces that lead to passivation or prevent reaction, and in short, the task is accomplished by complementing the process with complex reagents that dissolve the substance leading to passivation and inhibition phase interfaces.

The process according to the invention is characterized in that the anode of the cell is contacted with a complexing agent for iron ions added to the electrolyte. The complexing agent is, for example, ethylenediaminetetraacetic acid, cyanide, thiocyanate, citrate, nitrilotriacetate, ammonia, ethylenediaminetetrapropionic acid or diethylenetriaminepentaacetic acid.

Thus, according to the invention, an electrode on which the phase interfaces formed as a result of an electrode coating preventing its operation can be contacted with a first complexing agent which - during operation - is able, at least partially, to dissolve the electrode coating and further it may be contacted with a second complexing agent when the phase interfaces are formed by a heterogeneous supply of active material which is consumed at the electrode, the second complexing agent being capable of reversibly dissolving said active material.

For carrying out the method of the invention, an anode can be used which is formed by an electrically conductive porous electrode body with a front and a back side, the front side for contacting the electrolyte in the electrolyte chamber, while the back side is provided with active material which is it consumes in the anode process and adheres to the electrode body by magnetic forces. Notwithstanding the advantages associated with the magnetic-adhesion of the active anode material to the electrode body, it is particularly suitable to arrange the active-anode material on the back of the electrode body, since this eliminates the risk of short-circuiting to be taken into account. prádrn material on side ^d are particularly Poku so aa ^{to d} ate ^d and n ^{can be} arranged in the ^y Mato mutual distance.

· Since the active anode material is iron, have pores of the electrode body sized to prevent passage of the active matermte yes ^Dy Avşa ^to umožtovaty r ^{p h} o ^s c ^d electrolyte complex ions of the anode active material.

As mentioned above, in the practice of the invention, the first complexing agent is fed to the anode in order to supplement the reaction products generated during the operation of the electrode, thereby avoiding the formation of a passivation coating on the anode. Similarly, the cathode supplying a second complexing agent for the active material of the cathode (oxygen], for example in the article metal / air or metal / ^ky with ^{phorous. P} řesněj ^and ratchets, ^C o ^dl ev ^y nátezu is performed with respect to an oxygen cathode replenishment oxygen and its solubilization by means of a suitable complexing agent which selectively dissolves oxygen from the air, the oxygen being fed to the cathode in a liquid state, dissolved in the complexing agent, or in a liquid mixture containing the complexing agent. Since the dissolved oxygen-containing liquid flows around or preferably through the cathode, diffusion or transport of oxygen to the active cathode surface can be greatly increased compared to conventional oxygen cathodes where only the oxygen cathodes occur. transport of oxygen by diffusion and there is no fact The liquid flow in the process of the invention will also facilitate the transport of OH ions from the cathode.

The invention further permits the use of cathodes of simpler construction than hitherto, since there are no longer three different states, but only two states on the cathode (liquid state and solid state). The distance from the phase (air) showing the active material (oxygen) to the active surface of the electrode will be less critical.

Finally provision Herein ^y in ^y n and ^l · EZ ^EZ also the advantage that the poisonous substance Jato · ^ is nitrogen m ^the hou be eliminated ^yp dip ^s maiden tomptexmho agent ^p ouštějíc ^larger-p sH nly ^{ky k.}

^You will be n and ^l ez vn ^ leMjímm textiles described and exemplified in connection with iron and oxygen as the active materials. It can, however, also use other suitable active materiality, ^{and the} holding tomptexm agent must 'be adapted to the respective active material. What CIMdata complex ^here in ka m ^or special use and ^{a children's p} ro odborrnka in the art.

Iron is referred to as active material in the preceding text. The term active m aterial herein indicates that the iron is materlálem ^that with ^p otra OVaV ^{b y} electrodes during operation. Anode te ^dy may be têtes electro ^dy from jinteto in ^d iv ^eh about materiae Jato is different ^{for s,} which is a ^p atrane ^p ovlatom of active materiáto ^T Éles · electrodes ^Y are not ^at častn ^í vtestrn reaction eletarody but serve only for carrying the active material, i.e. iron, and for removing the electrons involved in the electrode reaction based on their electrical conductivity. In actual operation of an electrode varies with iron ions of iron, which according to the invention complemented tomptexrnm · · ^C n ^{i DLEM,} vtodným ro ^p e ^{r f Ezo,} preferably ethylenediaminetetraacetic acid (EDTA). Further examples of other suitable complex agents may be cyanide (CN ^_), thiocyanate (SCN), citrate, nitrilotriacetate, ammonia, acetic acid or diethylenetriaminepentaacetic etyléndiamintetrapropionová.

In a particularly preferred embodiment, the anode is ^no āsīt ^CLA ge ^the PU ^to OV ^/ conditi ^h Neto tov / / oxygen, which in addition to an anode comprising a chamber for electrolyte containing an electrolyte and a cathode and an oxygen electrode that is electrically conductive and porous, and is provided on its sulphates fežfc ^aNTI Tomori with EIE ^{kt rolytem,} fed with air or oxygen. Jato examples of suitable complexing agents for active material for the cathode include the mono- or polynuclear aromatic compounds having two or more atoms providing electrons in the rings or the functional skupnách ^s ited ^{to the} rings ^such as hydroxyquinoli · lanolin, polyhydric phenols, aminophenols or their oxidation products. Polyhydric phenols may be particularly hydroquinone, pyrogallol or oxteačrn ^p ro ^d u ^k t ^y. Equal to ^z · u ^l of ^Gd vat nonaromatic substances which form complexes reversibly with oxygen.

^P ro · pulse ^width will porozummn ^y n and ^l ez described for purposes of illustration rather than · OMe zujícím oNedem with ^Article Ane ^k t u ^yp tov ^/ / air. The attached drawings show:

^O br. 1 schematicto required displayed čtanku metal / air, FIG. 2 · voltage as a function of current density, and FIG. 3 as a function of current density fluids it con centration at r ^{y s} m ^{of the} Zn content of the complexing agent.

Zn ^and Zorn ^{currency Article} Ane ^k t ^y pu tov / ^d mod handles consists of anodes 1 and cathodes 2 which are ^of vz ^and gently connected to ^y 3 vedernm electric resistor 4 provided for collecting electrical energy generated in the cell. The electrodes are spaced apart so as to form an electrolyte chamber 5 therebetween.

When supplying the complexing agent only to the anode, the oxygen electrode was a conventional oxygen electrode consisting of a porous plate of sintered nickel and silver. The anode is formed by an electrode body in the form of a porous silver plate 6 which carries on its side facing away from the electrolyte chamber 5 a gelling powder 7 which forms the active material of the electrode. The iron powder is held magnetically to the electrode by an electromagnet located on the back of the oxygen electrode. Here, the pore size of the silver plate 6 is only critical to the extent that it cannot let the iron powder pass freely. A four-molar potassium hydroxide solution was used as the electrolyte.

With the described apparatus, an experiment was first carried out without the use of a complex reagent, i.e. not according to the present invention. The chamber 5 was filled with an electrolyte solution, and a fresh electrolyte solution in the form of a 4 molar potassium hydroxide solution was continuously fed into the chamber along the side of the anode facing away from the electrolyte chamber 5 as indicated by arrow 8 · o-br. 1. At the same time, an appropriate amount of electrolyte solution was withdrawn from chamber 5. The fresh electrolyte solution first passed through a layer of iron powder and then through a porous anode, and was then introduced into the electrolyte chamber. At the same time, air was supplied to the oxygen electrode associated with the anode as shown by arrow 9, and hydroxide ions were formed at the oxygen electrode. Due to the alkaline environment, the iron ions formed hydroxide on the anode and the cell's satisfactory operation was not achieved.

The experiment of the present invention was then carried out as described above, except that the fresh electrolyte solution fed to the electrolyte chamber was first mixed with 50 g EDTA per liter of solution. The electrolyte solution received iron ions resulting from the reaction of the electrode in the form of complexes with EDTA, so that they did not deposit on the metal anode. Instead, the filler iron ions passed through the void anode and were introduced into the electrolyte chamber, and only in this chamber was the iron precipitated in the form of hydrous iron oxide. Since this precipitate was freely present in the solution and was not deposited on the anode, it could easily be removed with electrolyte from the electrolyte chamber. It was also found that the precipitated hydrous iron oxide was ferromagnetic and could therefore be readily removed magnetically from the electrolyte solution. The results obtained in carrying out the experiment according to the invention are shown in Table 1.

TABLE 1

Temperature (° C)

Current density · (mA / cm ² )

Cell voltage (mVJ)

300

100 ALIGN!

500

The aforementioned anode may be replaced by another type of anode, in which case the aforementioned electrode body, i.e. a porous silver plate, a perforated or porous plate of another material that is electrically conductive and magnetic, for example a sintered plate of magnetic iron oxide. and barium oxide provided with an electrically conductive layer. This arrangement renders the aforementioned separate electromagnet unnecessary, wherein the iron powder is held to the electrode body by its own magnetism. In the embodiment described above, the active anode material, i.e. the iron powder, is disposed on the electrode side facing away from the electrolyte chamber. However, this is not necessary since the iron powder may also be located on the electrode side facing the electrolyte chamber. Furthermore, the apparatus described above consists of a single cell, but for practical purposes the apparatus is preferably equipped with double-acting electrodes, i.e. the anode shown has a corresponding electrode body on the other side of the iron powder, which electrode body is directed to another chamber. for electrolyte. Correspondingly, the oxygen electrode is equipped with another porous plate of sintered nickel and silver, which borders on the third electrolyte chamber. In this way, a battery of cells can be created in a simple manner. In such a battery, air is supplied to the oxygen electrodes of the space between the porous and sintered nickel and silver plates, while the electrolyte solution and the complexing agent are led to the iron powder in the space between the porous electrode bodies. Once the active material, i.e. the iron powder, is consumed, the fresh iron powder can be brought to the electrode bodies and magnetically. Attached to them. A specific system for this purpose is described in detail in U.S. Patent No. 3,811,952. There is also no need to continuously supply the cell with an electrolyte solution, a complexing agent and / or an active material, since the cell may also be present as a primary battery. the metal electrode was pre-filled with iron powder and a sufficient amount of a complex iron ion replenishing agent formed from the metal powder.

In another experiment, the effect of the liquid concentration and the concentration of the complexing agent in the metal / air cell of the above-mentioned type was contained. an anode consisting of a porous sintered silver plate with a perforated iron oxide and barium oxide magnet, the iron powder being held by magnetic forces to the side of the anode facing away from the electrolyte chamber. The cathode was a sintered plate of

1β nickel and silver. On the cathode side, facing away from knmory electrolyte, supplied with air by ^{d p} ttatam 0.01 to 0.05 MPa. Yes ^d ai cathode had ^yp jail 2 cm ^second Eletarotytem Potassium hydroxide with varying addition of EDTA ^to the I ^{to P} om LEXN rea of ^¹H-and electrolyte flowed through the cell at a rate 20 ml per minute. Cl ^{and was} not ^to umfetěn ohnvactoo tax in order to maintain a constant temperature of 50 ° C. The current versus anode voltage curve was measured against a reference electrode (calomel electrode) in the cell. Giant. 2 shows a current versus cell voltage curve at different concentrations of complex reagent (EDTA). From the figure is seen ^from a ^s e kftvka respects, rou ^{p d} in voltage after increase of the concentration of EDTA improved with a view to achieving the optimal value of 5 g of EDTA to 100 ml of electrolyte [5 Μ KOH). Upon further addition of EDTA at a certain voltage, the current density of the cell decreased, so that at 50 g EDTA per 100 ml electrolyte, the current density at a certain voltage was lower than when adding 2 g EDTA per 100 ml electrolyte. It is clear that under these conditions, there is ^one optimum with respect to the concentration of EDTA .obnášející 5-10 'g of EDTA to 100 ml of electrolyte.

Corresponding conditions are also shown in FIG. 3, which shows the current density ^to Jata ^f un ci tancentrace elekfrotyto at various concentrations of complexing agent (EDTA). The lowest curve represents the state without any addition of complexing agent. Then the curve shows that increasing concentrations of EDTA increases vyvotavá ^ JTO judgments with a density ^{of d} and a concentration of 5 to 10 ^grams of EDTA per 100 ml of the electrolyte. In another excit ^growth: at concentration ^E DTA proutavá density re ¹ decreases ^TA tancentraci that when ⁵⁰ g of EDTA to 100 ml of the electrolyte is achieved consume relatively low values of current density. However, these values were not higher than the densities obtained without any addition of the complexing agent.

It is further evident from FIG. 3 that the current density exhibits a relatively strict dependence on the electrolyte concentration. This dependency does not occur, as disclosed in the absence of a complexing agent, while the current density at the optimal addition of the complexing agent, i.e. at 5 to 10 g per 100 ml, changing Ö This 12 ^{was 5} mA / cm ² PN mtato the electrolyte concentration. ^{Z p} r ^{s ub} EH ^LR ive ^{to p atm,} the optimum value of the electrolyte concentration OBn ^as and MTII 5 KO ^H per liter.

^The onečně ^would l also made ^{of p okus, wherein} šstřující effect of supplying a complexing agent to the ^alkyl s ^phorous memory eletaodě In this experiment, using the above-mentioned apparatus and the electrolyte was 5 M KOH solution, to which ^was about ^{EXAMPLE id} Yes ^{5 g of EDTA} per l ⁰⁰ m ^{1 of} electrolyte. Instead of supplying air under pressure to the side of the oxygen electrode facing away from the electrolyte chamber, the above electrolyte was supplied to it. This electrolyte was mixed with oxygen-supplemented pyrogalol. Oxygen was introduced in the complex phase in the liquid phase and not in the phase! ^pl ynném. Hkom b ^y lo achieve satisfactory operation article. The best result was obtained using an oxygen-enriched electrolyte pore cathode.

B pyrocatechol ^yl tested straight ^from Jata there ^pl EXM ^alkyl of bile for ^the SH and in the use of graphite cathodes providing higher short circuit current than pyrogallol. Values obtained with hydroquinone and ascorbic acid, taken between the semi ^{hy d} uvetanými notes above.

In the above Uve ^d en ^eh of provetanú where it is used 'iron as the active material for the anodes were complex ions of iron, produced in the process, the precipitation in the of electrolyte in ^f orme magnetic aqueous' iron oxide, which can be separated from the electrolyte, and gives specific as described below.

As shown in FIG. 1, the electrolyte in the en ^d of ^NO to Tanta Tadrai M. Aqueous Electrolyte oljsahuje magnetic dapper ^ky ^ Leza ^to which was precipitated from complex iron. At the electrolyte inlet solution to the tank 10, the magnetic hydrous iron oxide is separated by magnets 11. If the precipitation of the hydrous iron oxide is imperfect, the solution in the tank will be grafted with crystals of magnetic iron oxide, preferably by stirring the solution. After the removal of all iron oxide from the solution, the purified solution is pumped from the tank and can be returned to the metal / air cell shown in FIG. 1 if desired. the hydrous iron oxides act as a filter contaminated electrolyte, which is fed by ^d noh ^^ rnmž the sweet wine of electrolyte becomes effective.

Claims (1)

SUBJECT OF THE INVENTION A method of preventing the formation of a passivating anode coating when discharging a galvanic primary cell comprising an alkaline electrolyte and an anode and a cathode which are connected to each other by an external circuit for drawing electrical energy, the active anode material being iron, The method of claim 1, wherein the anode is contacted with a complexing agent for iron ions added to the electrolyte, such as ethylenediaminetetraacetic acid, cyanide, thiocyanate, citrate, nitrotriacetate, ammonia, ethylenediaminetetrapropionic acid or diethylenetriaminepentaacetic acid.