SECONDARY CELL, ACCUMULATOR COMPRISING ONE OR MORE SECONDARY CELLS, AND METHOD FOR CHARGING AND DISCHARGING

Abstract

The present invention relates to a secondary cell in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte. The anode contains zinc (Zn) and/or zinc oxide (ZnO2), and the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag2O/AgO) with a catalyst for the electrochemical oxygen evolution, wherein the catalyst is selected from cobalt oxide Co3O4), manganese oxide (Mn3O4 or MnO2), cobalt-nickel oxide (CoNiO2), lanthanum-calcium-cobalt oxide (LaxCa1-xCoO3), ruthenium oxide (RuO2), iridium oxide (IrO2), platinum (Pt), palladium (Pd), and mixtures thereof.

The invention further relates to an accumulator which comprises one or a plurality of secondary cells, as well as a method for charging and a method for discharging a secondary cell or an accumulator.

Claims

1. A secondary cell in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte, wherein: the anode contains zinc (Zn) and/or zinc oxide (ZnO), the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag.sub.2O/AgO) with a catalyst for the electrochemical oxygen evolution, the catalyst is selected from cobalt oxide (Co.sub.3O.sub.4), manganese oxides (Mn.sub.3O.sub.4 or MnO.sub.2), cobalt-nickel oxide (CoNiO.sub.2), lanthanum-calcium-cobalt oxide (La.sub.xCa.sub.1-xCoO.sub.3), ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), platinum (Pt), palladium (Pd), and mixtures thereof.

2. The secondary cell in accordance with claim 1, wherein the catalyst contains Co.sub.3O.sub.4, in particular as the sole component.

3. The secondary cell in accordance with claim 1, wherein the cathode contains a proportion of 5 to 20% by weight of the catalyst.

4. The secondary cell in accordance with claim 1, wherein the cathode further contains a binder which is selected from polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF), and/or polyethylene (PE).

5. The secondary cell in accordance with claim 4, wherein the cathode contains 5 to 15% by weight of the binder.

6. The secondary cell in accordance with claim 1, wherein the cathode is produced using silver particles with a particle diameter in the range of 5 to 30 m.

7. The secondary cell in accordance with claim 1, wherein the cathode is produced using particles of the catalyst, the particle diameter of which is smaller than 100 nm.

8. The secondary cell in accordance with claim 1, wherein the cathode has a porosity in a range of 40% to 80%.

9. The secondary cell in accordance with claim 1, wherein the cathode contains no carbon.

10. The secondary cell in accordance with claim 1, wherein the cathode has a thickness in a range of 350 to 700 m.

11. The secondary cell in accordance with claim 1, wherein the entire amount of substance of silver in the cathode in the form of Ag, Ag.sup.+ and Ag.sup.3+ is less than the entire amount of substance of zinc in the anode in the form of Zn and Zn.sup.2+.

12. The secondary cell in accordance with claim 1, wherein the electrolyte is an alkaline aqueous solution.

13. An accumulator, comprising one or a plurality of secondary cells, wherein: each of the one or a plurality of secondary cells being in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte, the anode contains zinc (Zn) and/or zinc oxide (ZnO), the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag.sub.2O/AgO) with a catalyst for the electrochemical oxygen evolution, the catalyst is selected from cobalt oxide (Co.sub.3O.sub.4), manganese oxides (Mn.sub.3O.sub.4 or MnO.sub.2), cobalt-nickel oxide (CoNiO.sub.2), lanthanum-calcium-cobalt oxide (La.sub.xCa.sub.1-xCoO.sub.3), ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), platinum (Pt), palladium (Pd), and mixtures thereof.

14. A method for charging a secondary cell, the secondary cell being in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte, wherein: the anode contains zinc (Zn) and/or zinc oxide (ZnO), the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag.sub.2O/AgO) with a catalyst for the electrochemical oxygen evolution, the catalyst is selected from cobalt oxide (Co.sub.3O.sub.4), manganese oxides (Mn.sub.3O.sub.4 or MnO.sub.2), cobalt-nickel oxide (CoNiO.sub.2), lanthanum-calcium-cobalt oxide (La.sub.xCa.sub.1-xCoO.sub.3), ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), platinum (Pt), palladium (Pd), and mixtures thereof, wherein the method comprises in a first phase of the charging process, substantially only the oxidation of the present silver to silver oxide occurs in the cathode, and in a second phase additionally or exclusively the electrochemical evolution of oxygen.

15. A method for discharging a secondary cell, the secondary cell being in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte, wherein: the anode contains zinc (Zn) and/or zinc oxide (ZnO), the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag.sub.2O/AgO) with a catalyst for the electrochemical oxygen evolution, the catalyst is selected from cobalt oxide (Co.sub.3O.sub.4), manganese oxides (Mn.sub.3O.sub.4 or MnO.sub.2), cobalt-nickel oxide (CoNiO.sub.2), lanthanum-calcium-cobalt oxide (La.sub.xCa.sub.1-xCoO.sub.3), ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), platinum (Pt), palladium (Pd), and mixtures thereof, wherein the method comprises in a first phase of the discharging process, substantially only the reduction of the present silver oxide to silver occurs in the cathode, and in a second phase additionally or exclusively the electrochemical reduction of oxygen.

16. A method for charging an accumulator, the accumulator comprising one or a plurality of secondary cells, wherein: each of the one or a plurality of secondary cells being in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte, the anode contains zinc (Zn) and/or zinc oxide (ZnO), the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag.sub.2O/AgO) with a catalyst for the electrochemical oxygen evolution, the catalyst is selected from cobalt oxide (Co.sub.3O.sub.4), manganese oxides (Mn.sub.3O.sub.4 or MnO.sub.2), cobalt-nickel oxide (CoNiO.sub.2), lanthanum-calcium-cobalt oxide (La.sub.xCa.sub.1-xCoO.sub.3), ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), platinum (Pt), palladium (Pd), and mixtures thereof, wherein the method comprises in a first phase of the charging process, substantially only the oxidation of the present silver to silver oxide occurs in the cathode, and in a second phase additionally or exclusively the electrochemical evolution of oxygen.

17. A method for discharging an accumulator, the accumulator comprising one or a plurality of secondary cells, wherein: each of the one or a plurality of secondary cells being in the form of a hybrid system of a zinc-air battery and a silver oxide-zinc battery, comprising an anode, a cathode, and an electrolyte, the anode contains zinc (Zn) and/or zinc oxide (ZnO), the cathode is configured as a gas diffusion electrode which contains a mixture of silver (Ag) and/or silver oxide (Ag.sub.2O/AgO) with a catalyst for the electrochemical oxygen evolution, the catalyst is selected from cobalt oxide (Co.sub.3O.sub.4), manganese oxides (Mn.sub.3O.sub.4 or MnO.sub.2), cobalt-nickel oxide (CoNiO.sub.2), lanthanum-calcium-cobalt oxide (La.sub.xCa.sub.1-xCoO.sub.3), ruthenium oxide (RuO.sub.2), iridium oxide (IrO.sub.2), platinum (Pt), palladium (Pd), and mixtures thereof, wherein the method comprises in a first phase of the discharging process, substantially only the reduction of the present silver oxide to silver occurs in the cathode, and in a second phase additionally or exclusively the electrochemical reduction of oxygen.

Description

[0040] These and further advantages of the invention are explained in more detail using the following exemplary embodiments. In the drawings:

[0041] FIG. 1: shows a schematic depiction of a secondary cell in accordance with the invention; and

[0042] FIG. 2: shows cyclic voltammograms of a gas diffusion electrode with a mixture of Ag and Co.sub.3O.sub.4 and a pure nickel electrode.

[0043] Schematically depicted in FIG. 1 is the structure of a secondary cell in accordance with the present invention, referred to as a whole with the reference numeral 10. The secondary cell 10 is configured in the form of a hybrid system 12 of a zinc-air battery and a silver oxide-zinc battery. It comprises an anode 14 which is in contact with an electrolyte 16. Further, a cathode 18 of the secondary cell 10 is also in contact with the electrolyte 16.

[0044] The anode 14 contains zinc and/or zinc oxide 20, depending on the charge state of the secondary cell 10, wherein in the fully charged state only zinc and in the fully discharged state only zinc oxide is present.

[0045] The electrolyte 16 may in particular be an aqueous alkaline electrolyte. For example, a 7 molar KOH solution may be used.

[0046] The cathode 18 is configured as a gas diffusion electrode 22 with a porous structure in order to enable the electrochemical reactions involving oxygen (O.sub.2) at a three-phase boundary of the solid/liquid/gas phases. The cathode 18 contains in accordance with the invention a mixture 24 of silver and/or silver oxide with a catalyst for the electrochemical oxygen evolution, like for example Co.sub.3O.sub.4. Optionally, the cathode 18 may be arranged on a cathode support 26, which allows the passage of oxygen. The support 26 may be configured, e.g., as metal foam, as metal mesh, or as expanded metal, in particular of stainless steel, nickel, or silver.

[0047] Optionally, the secondary cell 10 (or an accumulator comprising a plurality of secondary cells) may comprise a housing 28, out of which an anode contact 30 electrically conductively connected to the anode 14 protrudes. Analogously, a cathode contact 32 may be electrically conductively connected to the cathode 18, which contact projects out of the housing 28.

[0048] The anode contact 30 may be connected to a consumer 36 by way of a connecting line 34 and to the cathode contact 32 by way of a further connecting line 38 in order to bring about a current flow from the cathode 18 to the anode 14. Electrons hereby flow from the anode 14 to the cathode 18. The secondary cell 10 is hereby discharged. When charging the secondary cell 10, the current and the electrons flow in the respective other direction.

[0049] For producing a cathode 18 for a secondary cell 10 in accordance with the invention, in one embodiment, 70% by weight silver powder with a particle size of 10 to 30 m, 20% by weight cobalt oxide powder with a particle size of under 50 nm and 10% by weight PTFE particles with a particle size of about 4 m are mixed and preferably milled in a blade mill in order to obtain a homogeneous mixture. Strands form when mixing, which hold the mixture together, similarly to a spider web. A milling duration of the constituents is about 2 seconds.

[0050] After milling, the mixture 24 is filled into the flexible frame, covered with a stainless steel mesh, and compressed to a solid composite, preferably with a hydraulic press at a pressure of about 2.5 bar. The temperature when pressing is about 25 C. The mesh of stainless steel serves as a cathode support 26 on the one hand for improving the mechanical stability of the cathode 18 and as a current collector on the other hand.

[0051] Gas diffusion electrodes 22 are produced in the described manner as cathodes 18 for a secondary cell 10 with a thickness in the range of 450 m.

[0052] For the electrochemical characterization of these gas diffusion electrodes, a cyclic voltammogram was measured, wherein a reversible hydrogen electrode (RHE) was used as a reference electrode and a platinum electrode and a counter electrode. A 7 molar KOH solution was used as an electrolyte and pure oxygen as a test gas.

[0053] Depicted in FIG. 2 is the cyclic voltammogram of the gas diffusion electrode with Ag/Co.sub.3O.sub.4 described above (continuous line), and for comparison the cyclic voltammogram of a pure nickel electrode measured under the same conditions (dotted line). The current density is plotted in mA/cm.sup.2 against the potential voltage with respect to the RHE in V.

[0054] The progression in the lower part of the diagram for the discharging process of the Ag/Co.sub.3O.sub.4 electrode clearly shows a first peak at about 1.3 V for the reduction of AgO to Ag.sub.2O and a second peak at about 0.8 V for the reduction of Ag.sub.2O to Ag. Below this voltage, the reduction of oxygen occurs (ORR). Corresponding peaks are also visible in the upper part of the diagram for the charging process, at about 1.4 V for the oxidation of Ag to Ag.sub.2O and at about 1.7 V for the oxidation of Ag.sub.2O to AgO, wherein the latter is only weakly present. The evolution of oxygen (OER) occurs above this voltage.

REFERENCE NUMERAL LIST

[0055] 10 secondary cell [0056] 12 hybrid system [0057] 14 anode [0058] 16 electrolyte [0059] 18 cathode [0060] 20 zinc/zinc oxide [0061] 22 gas diffusion electrode [0062] 24 mixture of Ag/Ag.sub.2O/AgO and catalyst [0063] 26 cathode support [0064] 28 housing [0065] 30 anode contact [0066] 32 cathode contact [0067] 34 connecting line [0068] 36 consumer [0069] 38 connecting line