WO2014037851A2

WO2014037851A2 - Rechargeable electrochemical zinc-oxygen cells

Info

Publication number: WO2014037851A2
Application number: PCT/IB2013/058047
Authority: WO
Inventors: Tobias Urban; Thomas Giesenberg; Eva MUTORO; Sigmar BRÄUNINGER; Gunter Bechtloff
Original assignee: Basf Se; Basf (China) Company Limited; Basf Schweiz Ag
Priority date: 2012-09-07
Filing date: 2013-08-28
Publication date: 2014-03-13
Also published as: WO2014037851A3

Abstract

The present invention relates to rechargeable electrochemical zinc-oxygen cells comprising A) at least one anode comprising metallic zinc, B) at least one gas diffusion electrode comprising (B1) at least one cathode active material, and (B2) optionally at least one solid medium through which gas can diffuse, and C) an aqueous electrolyte comprising boric acid. The present invention further relates to uses of the inventive rechargeable electrochemical zinc- oxygen cells, to zinc-air batteries comprising the inventive rechargeable electrochemical zinc- oxygen cells, and to the use of an aqueous electrolyte comprising boric acid for production or for operation of rechargeable electrochemical zinc-oxygen cells.

Description

Rechargeable electrochemical zinc-oxygen cells

The present invention relates to rechargeable electrochemical zinc-oxygen cells containing

A) at least one anode containing metallic zinc,

B) at least one gas diffusion electrode comprising

(B1) at least one cathode active material, and

(B2) optionally at least one solid medium through which gas can diffuse, and

C) an aqueous electrolyte containing boric acid.

Furthermore, the present invention relates to uses of the rechargeable zinc-oxygen electrochemical cells according to the invention, to zinc-air batteries containing the rechargeable zinc-oxygen electrochemical cells according to the invention, and to the use or preparation of an aqueous electrolyte containing boric acid rechargeable electrochemical zinc-oxygen cells. Secondary batteries, accumulators, rechargeable batteries or "rechargeable batteries" are only a few embodiments for storing and using electrical energy after production because of the significantly better power density, it has recently been deviated from the water-based secondary batteries and developed especially for the However, alternative secondary water-based batteries that are more environmentally friendly and have higher power density and longer life than the long-term use of lead-acid batteries are being investigated Alternative to lead-acid batteries are the so-called metal-air batteries, in particular zinc-air batteries.

The known metal-air batteries contain as essential constituents a negative electrode, for example zinc, and a positive electrode, which preferably consists of an electronically conductive carrier material of finely divided carbon, on which a catalyst for oxygen reduction is applied. Here, the negative electrode and the positive electrode are separated by a separator which may be in the form of a membrane. In a common embodiment, metal, such as zinc, is oxidized with atmospheric oxygen in an alkaline electrolyte to form an oxide or hydroxide. The released energy is used electrochemically. Currently commercially sold zinc-air batteries are not rechargeable. Intensive, however, researched rechargeable zinc-oxygen electrochemical cells in which by applying an electric voltage zinc ions formed during the discharge are reduced again to the zinc and oxygen is released by oxidation of the oxides or hydroxides formed during the discharge. Rechargeable electrochemical zinc-oxygen cells can be operated both with aqueous acidic (WO2012 / 012558) and basic electrolytes (WO2007 / 065899). The known rechargeable zinc-oxygen cells, however, still have room for improvement, especially with regard to the following properties: cell performance, cell energy efficiency, and cycle stability. Furthermore, an optimization of the costs caused by material and production costs has to be taken into account in order to promote the spread of this new energy storage technology.

It was therefore the object to provide rechargeable zinc-oxygen cells, which constitute an advance over the prior art in terms of at least one of the aforementioned properties. A particularly important feature of the rechargeable zinc-oxygen cells is ultimately the cycle stability, which must be improved with otherwise comparable properties of the cells.

This object is achieved by an initially defined rechargeable zinc-oxygen electrochemical cell, which

A) at least one anode containing metallic zinc, at least one gas diffusion electrode comprising (B1) at least one cathode active material, and

(B2) optionally at least one solid medium through which gas can diffuse, and containing an aqueous electrolyte containing boric acid.

The anode of the rechargeable zinc-oxygen electrochemical cell according to the invention, also referred to as anode (A) in the context of the present invention, contains metallic zinc.

The metallic zinc can be present as a solid plate, as a layer on a Abieiter, for example made of sheet steel or copper, as a sintered, porous electrode, as a metal powder or granules, optionally sintered. In a preferred embodiment, the zinc is present in the form of a layer on a Abieiter in a charged inventive electrochemical cell according to the invention, wherein the layer can be produced for example during the first charging of the cell.

The gas diffusion electrode of the rechargeable electrochemical zinc-oxygen cell according to the invention, also referred to as gas diffusion electrode (B) or cathode (B) in the context of the present invention, comprises at least one cathode active material, hereinafter also briefly called cathode active material (B1), and optionally at least one solid medium, hereinafter also briefly called medium (B2), can diffuse through the gas.

The cathode active material (B1) usually comprises at least one catalyst, in the context of the present invention also called catalyst (b1a), where the catalyst (b1a) is used as catalytically active component for the reduction of oxygen during the discharge process and / or for the evolution of oxygen in the Charging the electrochemical zinc-oxygen cell is used. In the context of the present invention are suitable as catalyst (b1 a) in particular

Mixed oxides, for example cobalt oxides, nickel oxides, iron oxides, chromium oxides, tungsten oxides and also noble metals and noble metal alloys, in particular silver. In a preferred embodiment, a catalyst combination of a catalyst catalyzing the reduction of oxygen and a catalyst catalyzing the evolution of oxygen or a bifunctional catalyst according to WO 2007/065899 A1, page 7, line 14 to page 8, line 27 is used. A preferred catalyst that catalyzes both oxygen oxidation and reduction is La ₂ O ₃ . Preferred catalysts for reducing the oxygen are Mn0 ₂ , KMn0 ₄ , MnS0 ₄ , Sn0 ₂ , Fe ₂ 0 ₃ , Co ₃ 0 ₄ , Co, CoO, Fe, Pt, Pd, Ag ₂ O, Ag, spinels or perovskites. In a preferred embodiment of the present invention, the rechargeable zinc-oxygen electrochemical cell according to the invention is characterized in that the cathode active material (B1) comprises at least one catalyst (b1 a) selected from the group consisting of La ₂ O ₃ , WC, Ce (W0 ₄ ) ₃ , FeAgMo ₂ 0 ₈ , Fe ₂ (Wo ₄ ) ₃ , Mn ₃ 0 ₄ , Mn ₂ 0 ₃ , Mn0 ₂ , KMn0 ₄ , MnS0 ₄ , Sn0 ₂ , Fe ₂ 0 ₃ , Co ₃ 0 ₄ , CoO, Ir0 _2, Ag ₂ 0, Co, Ni, Fe, Pt, Pd, Ir, Pt _4, ₅ Ru ₄ Ir _0, _5, Ag, Pd-W, containing spinels and perovskites.

The cathode active material (B1) further comprises, in addition to the catalyst (b1 a), preferably at least one catalyst support material, in the context of the present invention also referred to as a short support material (b1 b). The support material (b1 b) is usually an electronically conductive material, which serves in particular for fixing the catalyst (b1 a). Preferably, the support material (b1 b) should be as stable as possible to oxygen and the oxygen compounds formed during the discharging and charging process. Suitable support materials are, for example, electrically conductive, carbonaceous materials, as described in WO 201 1/161598, page 5, line 1 to page 6, line 26. In addition, nitrogen-doped carbonaceous materials are suitable as support materials.

In addition to the cathode active material (B1), the gas diffusion electrode (B) optionally comprises at least one solid medium, also referred to in the context of the present invention as medium (B2), through which gas can diffuse. Furthermore, the medium (B2) also fulfills the function of serving as a carrier for the cathode active material. In principle, cathode active materials (B1) can also be used without a further porous medium, ie gas-permeable medium, which serves as a support for stabilizing and shaping the cathode active materials (B1) and furthermore the contact of the support material (b1b) and the catalyst (b1 a) guaranteed with oxygen. In this case, the support materials (b1 b) can be mixed directly with the catalyst (b1 a), or the support materials (b1 b) can be further processed into fibers or laminar structures and then coated with the catalyst (b1 a), wherein a porous, self-supporting structure is created. In a further embodiment, the support material (b1 b), optionally together with the catalyst (b1 a), is applied to a gas-permeable solid medium (B2). Such a gas-permeable solid medium (B2) may, for. B. a nonwoven, z. B. carbon fibers, or glass fibers. Further suitable gas-permeable solid media (B2) are, in particular, metal nets, metal foams, etc. The gas-permeable solid medium serves, as already mentioned, essentially the mechanical stability and shaping, but also improves the electrical contacting if it is itself electrically conductive. Further suitable solid media (B2) are mentioned and described in WO 201 1/161598, page 4, lines 4 to 40.

In a preferred embodiment of the present invention, the gas diffusion electrode of the cell according to the invention is characterized in that the gas diffusion electrode further comprises a gas-permeable solid medium (B2) on which the cathode active material (B1) is fixed.

In addition to the components of the cathode active material (B1), namely the catalyst (b1 a) and preferably at least one support material (b1 b), and the preferably present solid medium (B2), the gas diffusion electrode (B) of the electrochemical cell according to the invention preferably comprises at least one binder , which is usually an organic polymer, as described in more detail in WO 201 1/161598, page 6, line 28 to page 8, line 15, wherein the binder there called polymer (C) or binder (C) becomes. The binder serves primarily to mechanically stabilize the cathode active material (B1) by bonding together particles of the support material (b1b) and / or the catalyst (b1a) by the binder, and further causes the cathode active material to have sufficient adhesion to a solid Medium (B2) or a current conductor has. Preferably, the binder is chemically inert to the chemicals with which it comes into contact in the electrochemical cell.

Cathode (B) can be configured in various forms, for example rod-shaped, in the form of round, elliptical or square columns or cuboid, in particular also as a planar electrode. Thus, in the case of selecting the solid medium (B2) of metal mesh, it is possible that the shape of the cathode (B) is substantially predetermined by the shape of the metal mesh. In the context of the present invention, the term "sheet-like" means that the electrode, a three-dimensional body, in one of its three spatial dimensions (dimensions), namely the layer thickness, is smaller than in the other two dimensions, the length and the width is the layer thickness of the sheet-like electrode at least by a factor of 5, preferably at least by a factor of 10, more preferably at least by a factor of 20 smaller than the second largest extent.

In the rechargeable electrochemical zinc-oxygen cell according to the invention, oxygen is reduced during the discharge process of the same at the cathode (A), more precisely molecular oxygen (O 2). Molecular oxygen (O2) can be used in dilute form, such as in air, or in highly concentrated form.

Inventive rechargeable zinc-oxygen electrochemical cells furthermore contain an aqueous electrolyte, in the context of the present invention also referred to as electrolyte (C) for short, this electrolyte (C) containing boric acid. The aqueous electrolyte enables charge transport within the cell between the two electrodes by the migration of ions and serves as storage for the anode material which goes into solution during the discharge process, that is to say as storage for the zinc ions. The boric acid present in the electrolyte (C) serves to buffer a desired pH or pH range. In principle, it is possible to operate rechargeable electrochemical zinc-oxygen cells according to the invention in a wide pH range.

The amount of boric acid in the electrolyte (C) can be varied within a wide range and depends on both the desired buffering capacity and the solubility of boric acid in the electrolyte. Preferably, the boric acid (B (OH) 3) in the aqueous electrolyte in a concentration of 0.1 to 50 g / l, preferably from 1 to 35 g / l, more preferably 10 to 30 g / l, in particular 20 to 25 g / l used. The charge transport within the cell is effected by so-called conductive salts, which are readily soluble in water and whose ions do not cause any undesirable side reactions during operation of the rechargeable zinc-oxygen electrochemical cell according to the invention. In addition to the necessarily present water-soluble zinc salts, salts of alkali metals or ammonium salts, in particular salts of alkali metals, are preferably used as further conductive salts.

Examples of suitable zinc salts are zinc halides, such as zinc chloride, zinc bromide or zinc iodide, in particular zinc chloride, and zinc sulfate or zinc methanesulfonate. Examples of suitable salts of alkali metals are halides, preferably chlorides, bromides or iodides, in particular chlorides, sulfates or methanesulfonates of the alkali metals lithium, sodium, potassium, rubidium or cesium, preferably of sodium or potassium. In one embodiment of the present invention, the rechargeable electrochemical zinc-oxygen cell according to the invention is characterized in that the aqueous electrolyte contains at least one conductive salt selected from the group of salts consisting of alkali metal halides, alkali metal sulfates, alkali metal methanesulfonates, zinc halides, zinc sulfate and zinc methanesulfonate , Preferred conductive salts are selected from the group of salts consisting of sodium chloride, potassium chloride, sodium sulfate, potassium sulfate, sodium methanesulfonate, potassium methanesulfonate, zinc chloride, zinc sulfate and zinc methanesulfonate, more preferably selected from the group of salts consisting of potassium chloride, potassium sulfate, zinc chloride and zinc sulfate.

In the electrolyte (C), the concentration of the zinc salt used can be varied within a wide range. In general, the amount of Zn ²⁺ ions in the electrolyte (C) is in a range of at least 0.1 g / l to the saturation concentration of the respective zinc salts, preferably in a range of 1 g / l to 100 g / l. in particular in the range of 2.5 g / l to 25 g / l. While the amount of Zn ²⁺ ions in the electrolyte (C) decreases during charging of the rechargeable electrochemical zinc-oxygen cell, it is correspondingly increased during the discharge process.

The concentration of the other conductive salts used in addition to the zinc salt (s) used, in particular the abovementioned sodium or potassium salts, can likewise be varied within a wide range in the electrolyte (C). A limitation to the top is given in particular by avoiding the precipitation of a salt.

In the case of a chloride-containing electrolyte containing zinc chloride and sodium and / or potassium chloride, it is preferable for a molar ratio of Zn ²⁺ ions to CP ions to be in the range from 1: 2 to 1: 30, preferably in the range from 1: 5, 5 to 1: 14.8, in particular in the range of 1: 8.3 to 1: 9.2 set by the appropriate amounts of chloride, for example in the form of alkali metal chloride or hydrogen chloride are added. In a further embodiment of the present invention, the rechargeable zinc-oxygen electrochemical cell according to the invention is characterized in that the aqueous electrolyte has a pH in the range from 0 to 7, preferably in the range from 2 to 6.

The adjustment of the desired pH in the electrolyte to a pH in the range between 0 and 7 is preferably carried out by adding a Brönsted acid as proton donor to the electrolyte (C). Preferred Bronsted acids are the acids corresponding to the anions present in the electrolyte (C), that is to say aqueous solutions of hydrogen halides, in particular hydrochloric acid, sulfuric acid or methanesulfonic acid. Hydrochloric acid or sulfuric acid are particularly preferably used for adjusting the pH. In cases where the starting solvent used in the preparation of the electrolyte instead of water is already an acid such as hydrochloric acid or sulfuric acid, it is preferable to add the desired pH by adding the boric acid, the conductive salts and optionally other additives to the electrolyte a base, preferably an alkali metal hydroxide, in particular by addition of sodium hydroxide or potassium hydroxide, in solid form or as an aqueous solution.

The determination of the pH can be carried out by methods which are generally known to the person skilled in the art. Coarse pH determinations can already be made with universal indicator paper, while a more accurate adjustment of the pH value can be made potentiomet- rically with the aid of a pH electrode.

In the case of an electrolyte (C) containing chloride as anions, it is preferable to adjust the pH to a value in the range of 5.0 to 5.4.

In the case of an electrolyte (C) containing sulfate as anions, it is preferable to adjust the pH to a value in the range of 2.0 to 3.0.

The cycle stability and also the lifetime of a rechargeable, zinc-oxygen electrochemical cell are adversely affected, inter alia, by the formation of zinc dendrites at the anode during the charging process. It is known that the growth of dendrites can cause shorts within an electrochemical cell. A uniform, dendrite-free deposition of the zinc at the anode therefore has a positive effect on the cycle stability of a rechargeable zinc-oxygen electrochemical cell. From electroplating additives are known, which support a uniform, largely dendrite-free metal deposition on a surface. These additives are in particular surfactants and so-called brighteners, some of these additives may be referred to both as a surfactant and as a brightener. In a further embodiment of the present invention, the rechargeable zinc-oxygen electrochemical cell according to the invention is characterized in that the aqueous electrolyte (C) contains at least one surfactant.

The surfactant may in principle be a nonionic or ionic surfactant. Preference is given to nonionic or anionic surfactants, in particular nonionic surfactants. Examples of preferred nonionic surfactants are linear or branched alkyl ethoxylates. Examples of preferred anionic surfactants are alkyl ethoxylate sulfonates, alkyl ethoxylate sulfates, alkylphenol ethoxylate sulfonates or alkylphenol ethoxylate sulfates. The concentration of the surfactant in the electrolyte (C) can be varied within a wide range. Preferably, the surfactant is used in a concentration of 0.1 to 10 g / l. Commercially available suitable anionic surfactants are, for example, polyethylene glycol octyl (3-sulfopropyl) diether, potassium salt (CAS NUMBER 154906-10-2), polyethylene glycol alpha-alkyl omega-3-sulfopropyl diether, potassium salt (CAS NUMBER 1 19481-71 -9) or polyethylene / propylene glycol (beta-naphthyl) (3-sulfopropyl) diether, potassium salt (CAS NUMBER 120478-49-1).

Commercially available suitable nonionic surfactants are, for example Octaethyleneglycol octyl ether (CAS NUMBER 26468-86-0) or beta-Naphtholethoxylat (BNO Lugaivan ^® 12) In another embodiment of the present invention, the rechargeable electrochemical zinc-oxygen cell of the invention is characterized in that the wässri- ge electrolyte (C) at least one brightening agent, in particular a brightening agent selected from the group consisting of alkali metal salts of naphthalene sulfonic acid condensates (Tamol ^® NN 8906), contains Thiodiglycolethoxylate and benzalacetone.

Another object of the present invention is the use of an aqueous electrolyte containing boric acid for the production or operation of rechargeable zinc-oxygen electrochemical cells. The other constituents of the boric acid-containing electrolyte and preferred embodiments thereof have been described above.

The rechargeable zinc-oxygen electrochemical cell of the present invention may further include a separator for separating anode (A) and gas diffusion electrode (B), which prevents short-circuiting between anode (A) and gas diffusion electrode (B), but migration of ions between electrodes allowed.

Suitable separators are polymer films, in particular porous polymer films, which are unreactive with respect to the zinc of the anode, the reduction products formed at the cathode (B) during the discharge process and to the constituents of the electrolyte in the rechargeable zinc-oxygen electrochemical cells according to the invention. Particularly suitable materials for separators are polyolefins, in particular film-shaped porous polyethylene and film-shaped porous polypropylene.

Also suitable is glass fiber reinforced paper or inorganic nonwovens, such as glass fiber webs or ceramic nonwovens.

The separator used is preferably an acid-resistant, inert material in the zinc-oxygen cells according to the invention. In a preferred embodiment, polyolefins are used, in particular porous porous polyethylene and porous porous polypropylene. The separator preferably has a layer thickness of 10 to 200 μm. In addition, other known acid-resistant polymers or inorganic compounds are suitable as a separator. The separator may be, for example, a sulfonated polytetrafluoroethylene, a doped polybenzimidazole, a polyether ketone or polysulfone. In a preferred embodiment, the separator has a porosity of 30 to 80%, in particular 40 to 70%. By porosity is meant the ratio of void volume to total volume. Rechargeable, electrochemical zinc-oxygen cells according to the invention contain, as further components, electrical connections which connect cathode (B) and anode (A) to one another. These electrical connections are preferably produced by introducing, in a manner known per se, electrode layers of conductive and corrosion-resistant materials, preferably of carbon or nickel, which are connected to the corresponding electrodes. Further suitable compounds are known to those skilled Cu alloys, electrically conductive polymers, such as polyaniline 3,4-Polyethylenendioxithiophen- polystyrenesulfonate (PEDOT / PSS) or polyacetylene. In a particularly preferred embodiment, a composite of carbon and polymer is used. The rechargeable electrochemical zinc-oxygen cells of the present invention are incorporated into a suitable container for use. This container is preferably made of polymeric materials. It is provided with insulated terminals for the electrodes and has at least one opening through which air or oxygen can enter or escape to the operation of the cell.

Rechargeable electrochemical zinc-oxygen cells according to the invention show a small decrease in the theoretical cell voltage and are distinguished by increased energy efficiency and good stability. In particular, rechargeable electrochemical zinc-oxygen cells according to the invention are distinguished by improved cycle stability.

A further subject of the present invention is the use of rechargeable zinc-oxygen electrochemical cells according to the invention as described above in zinc-air batteries. A further subject matter of the present invention is zinc-air batteries containing at least one rechargeable zinc-oxygen electrochemical cell according to the invention as described above. Rechargeable, electrochemical zinc-oxygen cells according to the invention can be combined with one another in zinc-air batteries according to the invention, for example in series connection or in parallel connection. Series connection is preferred. Another object of the present invention is the use of rechargeable zinc-oxygen electrochemical cells according to the invention as described above in automobiles, electric motor-powered two-wheelers, aircraft, ships or in particular in stationary energy storage. The invention is illustrated by the following, but not limiting examples of the invention.

Percentages (%) are each by weight (% by weight), unless expressly stated otherwise.

I. Structure of a cell ZA according to the invention and a comparative cell VZ.B not according to the invention. The cell ZA according to the invention and the cell VZ.B not according to the invention were in each case rechargeable zinc-oxygen electrochemical cells, each of which comprises an anode of metallic Zn sheet (commercially available, thickness: 1.0 mm), a cathode consisting of a gas diffusion electrode (GDL, commercially available from SGL Carbon) and a cathode active material (catalyst: MnO2, commercially available from Alfa Aesar) and an aqueous electrolyte. The electrolyte E1 of the cell Z.A according to the invention contained 1.25% boric acid, while the electrolyte V-E2 of the comparison cell V-Z.B not according to the invention contained no boric acid. Both cells did not include a separator. Anode and cathode were arranged parallel to each other at a distance of 1, 5 cm. 1.1 Preparation of the cathode

On a gas diffusion layer (GDL) of thickness 300 μηη with microporous carbon layer (MPL) and a PTFE (polytetrafluoroethylene) content of 30%, which was not thermally treated prior to coating, the catalyst Mn02 by screen printing applied. For this purpose, a mixture of 30 parts by weight of Mn02, 50 parts by weight of conductive carbon was (Ketjenblack ^® EC300J the company Akzo Nobel) and 20 parts by weight of an aqueous dispersion of polytetrafluoroethylene prepared (60 wt .-% PTFE in the dispersion) under milling. 4.4 g of this mixture consisting of MnC carbon black / PTFE and water was added along with 6.0 g of isopropanol, 0.4 g of the dispersing agent EFKA ^® 4585 (an acrylic block copolymer having an active content of about 50%) and 40 g water mixed with a homogenizer (Kinematica Polytron) for 2-3 min at 10,000 r / min. A viscous (honey-like) mixture was prepared as an ink. The ink was applied by screen printing in several layers on the GDL, wherein after each printing the electrode was dried for about 4 min at 80 ° C and then laminated at 120 ° C. A total of about 0.45 mg MnO cm ^{2 was} applied to the GDL in this way.

I.2 electrolytes for the cell Z.A according to the invention and the comparative cell V-Z.B not according to the invention 1.2.1 E.1 for cell Z.A.

The boric acid-containing electrolyte E.1 for the cell ZA according to the invention contained: I .25% boric acid, 0.25 mol / l ZnC, 1, 75 mol / l KCl, and as additives 0.1% Tamol ^® NN 8906 and 0.1% Plurafac LF401 ^® (nonionic surfactant); the pH was adjusted to pH 4 with hydrochloric acid. The conductivity of the electrolyte E.1 was 178 mS / cm. I.2.2 VE.2 for cell VZ.B

The non-boric acid electrolyte VE.2 the cell VZ.B not according to the invention containing: 0.25 mol / l ZnCl _2, 1, 75 mol / l NH ₄ Cl, and as additives 0.1% Tamol ^® 8906 and 0.1 NN % Plurafac® LF401 (nonionic surfactant); the pH was adjusted to pH 4 with hydrochloric acid. The conductivity of the electrolyte VE.2 was 184 mS / cm.

Thus, both the electrolyte have the same concentrations of Zn ²⁺ ions and Ch ions, as well as identical pH values and similar conductivities.

II. Electrochemical Testing of Cells Z.A and V-Z.B

To determine the activity of the catalyst and its stability under electrochemical loading, the cells Z.A and V-Z.B were measured by cyclic voltammetry. 50 cycles were recorded at a feed rate of 100 mV / s. The resting potential was used as starting potential, the reversal potentials were +0.8 V and +2.0 V. With identical active area, the current value represents a parameter of the activity of the catalyst; a decrease in the current density from cycle to cycle shows a degradation of the catalyst and thus a lack of cycle stability of the cell.

III. Experimental results

The experimental data presented in Table 1 show that the cell VZ.B not according to the invention had a higher current intensity at the beginning of the measurement, ie the current increased at + 2 V during the first 10 cycles, but a strong overall increase with increasing number of cycles Decrease in the current is observed. This corresponds to a poor cycle stability of the rechargeable Zn-oxygen cell. The cell ZA according to the invention with boric acid in the electrolyte is more stable and shows almost no decrease in the current intensity at + 2 V with increasing number of cycles, as well as a significantly reduced decrease in the current density at 0.8 V in comparison with the cell VZ.B. After a few cycles (35 cycles at 2 V, 40 at 0.8 V), the current density of the boric acid-containing cell ZA according to the invention was greater than that of the comparative cell VZ.B. The experimental results show that the addition of boric acid has a positive effect on the cell cycle stability. Table 1: Test results of the above-described invention and not inventive electrochemical cells. The table contains current values (in mA) at the reversing potentials (+2.0 V and +0.8 V).

IV. Investigation of zinc deposition in different electrolytes:

In order to simulate the deposition behavior of the zinc during charging of the Zn / air battery, deposits were carried out in the so-called Hull cell. The morphology and optics of galvanic metal depositions can be mapped over a large current density range in an experiment.

For this purpose, steel plates were pickled with 15% hydrochloric acid, rinsed, degreased by electrolysis and dekapiert with 10% sulfuric acid. The Zn deposition was carried out according to DIN 50957 in a standard Hull cell (for example from McGean-Rohco) with 250 ml of electrolyte at 1 A cell stream for 10 min. As anodes (in terms of metal deposition), pure zinc electrodes (for example, AMPERE) were used.

The boric acid electrolyte E.3 contained:

1, 25% boric acid, 0.25 mol / l ZnC and 1.75 mol / l KCl; the pH was adjusted to pH 4 with hydrochloric acid.

The non-boric electrolyte V-E.4 contained:

0.25 mol / l ZnC and 1.75 mol / l NH ₄ Cl; the pH was adjusted to pH 4 with hydrochloric acid.

The two electrolytes E.3 and VE.4 thus had the same Zn content, chloride content and pH value. They differ only in the type of buffer substance. The boric acid electrolyte E.5 contained:

1, 25% boric acid, 0.25 mol / l ZnC, 1, 75 mol / l KCl, and as an additive 0.1% Tamol ^® NN 8906 (dispersants) and 0.2% Plurafac LF401 ^® (nonionic surfactant as a wetting agent) ; the pH was adjusted to pH 4 with hydrochloric acid.

The non-boric electrolyte V-E.6 contained:

0.25 mol / l ZnC, 1, 75 mol / l NH ₄ Cl, and as an additive 0.1% Tamol ^® NN 8906 (dispersants) and 0.2% Plurafac LF401 ^® (nonionic surfactant as a wetting agent); the pH was adjusted to pH 4 with hydrochloric acid.

Result:

In the high current density range, the deposition from the boric acid-containing electrolytes E.3 and E.5 (FIG. 1) has significantly lower amorphous structures than those from the ammonium-containing electrolytes V-E.4 and V-E-6 (FIG. 2); in particular no dendrites and a layer well adhering to the steel sheet. In addition, the deposition is uniform at lower current densities.

FIGS. 1 and 2 show photographs of the steel sheets after the zinc deposition in various electrolytes. The applicable current density range is indicated by the double arrow

4 »marked.

The current density range applicable to the operation of a Zn / air battery is greater in the boric acid electrolyte E.5 (FIG. 1) than in the ammonium-containing electrolyte V-E-6 (FIG. 2).

Electrolyte E.3: from 2.5 to 0.25 A / dm ²

Electrolyte VE.4: from 2.0 to 0.3 A / dm ²

By the addition of additives such as wetting agents e.g. 0.2% Plurafac LF 401 and dispersants e.g. 0.1% Tamol NN 8906 this effect is reinforced.

Electrolyte E.5: good deposition of 5 to 0.7A / dm ²

Electrolyte VE.6: good deposition of 4 to 1, 8 A / dm ²

Claims

claims

Rechargeable electrochemical zinc-oxygen cell containing

A) at least one anode containing metallic zinc,

B) at least one gas diffusion electrode comprising (B1) at least one cathode active material, and

(B2) optionally at least one solid medium through which gas can diffuse, and

C) an aqueous electrolyte containing boric acid.

Rechargeable electrochemical zinc-oxygen cell according to claim 1, characterized in that the cathode active material at least one catalyst selected from the group consisting of La ₂ 0 ₃ , WC, Ce (W0 ₄ ) 3, FeAgMo ₂ 0 ₈ , Fe ₂ (Wo ₄ ) ₃ , Mn ₃ 0 ₄ , Mn ₂ 0 ₃ , MnO ₂ , KMnO ₄ , MnSo ₄ , SnO ₂ , Fe ₂ O ₃ , Co ₃ O ₄ , CoO, IrO ₂ , Ag ₂ O, Co, Ni, Fe, Pt, Pd, Ir, Pt ₄ , ₅ Ru ₄ lro, 5, Ag, Pd-W, spinels and perovskites.

Rechargeable zinc-oxygen electrochemical cell according to claim 1 or 2, characterized in that the aqueous electrolyte contains at least one conductive salt selected from the group of salts consisting of alkali metal halides, alkali metal sulfates, alkali metal methanesulfonates, zinc halides, zinc sulfate and zinc methanesulfonate.

Rechargeable electrochemical zinc-oxygen cell according to one of claims 1 to 3, characterized in that the aqueous electrolyte has a pH in the range of 0 to 7.

Rechargeable electrochemical zinc-oxygen cell according to one of claims 1 to 4, characterized in that the aqueous electrolyte contains at least one surfactant.

Rechargeable electrochemical zinc-oxygen cell according to one of claims 1 to 5, characterized in that the aqueous electrolyte contains at least one brightener selected from the group consisting of alkali metal salts of naphthalenesulfonic acid condensates, thiodiglycol ethoxylates and benzalacetone.

7. Use of a rechargeable electrochemical zinc-oxygen cell according to one of claims 1 to 6 in zinc-air batteries.

A zinc-air battery containing at least one rechargeable electrochemical zinc-oxygen cell according to any one of claims 1 to 6.

9. Use of a rechargeable electrochemical zinc-oxygen cell according to one of claims 1 to 6 in automobiles, electric motor-powered two-wheelers, aircraft, ships or stationary energy storage.

10. Use of an aqueous electrolyte containing boric acid for the manufacture or operation of rechargeable zinc-oxygen electrochemical cells.