Gas diffusion electrode

Abstract

Gas diffusion electrode (10; 20; 30) for the reduction and formation of oxygen, in particular for a secondary metal-air battery, characterized in that the gas diffusion electrode (10; 20; 30) comprises: - a flat, electrically conductive substrate (12); - a first porous gas diffusion layer (14) for oxygen reduction arranged on the substrate and containing a first catalyst material and a hydrophobic binder; - a second porous gas diffusion layer (16) for oxygen formation, which is arranged above the first gas diffusion layer (14) with respect to the substrate (12) and which contains a second catalyst material; and - at least one current collector (18) which electrically connects the second gas diffusion layer (16) to the substrate (12), wherein the second gas diffusion layer (16) has a higher porosity than the first gas diffusion layer (14), and optionally a lower hydrophobicity.

Description

[0001] The present invention relates to a gas diffusion electrode for the reduction and formation of oxygen, in particular for a secondary metal-air battery.

[0002] Compared to most other battery types, metal-air batteries are characterized by a significantly higher gravimetric and volumetric energy density. This is primarily due to the fact that only one of the two reactants contributes to the initial weight of the cell, while oxygen is supplied from the ambient air during battery discharge. For this reason, metal-air batteries, especially lithium-air batteries, are of great practical importance, particularly for mobile applications, e.g., as energy storage devices in electric vehicles. In these applications, secondary metal-air batteries, which can be recharged with the formation of oxygen, are of particular interest.

[0003] In metal-air batteries, the cathode is designed as a gas diffusion electrode to enable electrochemical reactions involving oxygen at a three-phase boundary between the solid catalyst, the liquid electrolyte, and the gaseous air or oxygen. In the case of a secondary battery, the gas diffusion electrode must contain one or more catalysts that collectively exhibit sufficient activity for both oxygen reduction (ORR) during discharging and oxygen evolution (OER) during charging. In the cathodes known to date for secondary metal-air batteries, these two functionalities are combined within a single, essentially homogeneous gas diffusion layer.

[0004] A gas diffusion electrode suitable for secondary metal-air batteries is known, for example, from WO 2015 / 124713 A1. This contains, as catalysts, metallic silver, which has a high catalytic activity, particularly for oxygen reduction, and a metal oxide, such as Co. 3 O 4 , which has a high catalytic activity, particularly for oxygen formation.

[0005] A disadvantage of these and other state-of-the-art gas diffusion electrodes, however, remains that the composition and structure of the electrode cannot be optimized with regard to both reaction directions (ORR and OER). Oxygen reduction and oxygen formation place partly conflicting demands on the properties of the gas diffusion electrode, so a compromise must be found. This particularly applies to the porosity and hydrophobicity of the electrode: While a highly porous structure with low hydrophobicity, in which the pores can be flooded by the electrolyte, is advantageous for oxygen formation, such flooding would be extremely disadvantageous for oxygen reduction, as it would severely limit the diffusion of gaseous oxygen to the phase interface. The known gas diffusion electrodes therefore contain a sufficient proportion of a hydrophobic binder such as PTFE.

[0006] WO 2014 / 037828 A1 discloses a gas diffusion electrode for rechargeable electrochemical metal-oxygen cells, comprising at least one porous support and one or more layers applied to one side of the porous support, which layers contain at least one catalyst for a metal-oxygen cell. At least one functionally relevant parameter in the layer(s) containing the catalyst changes continuously or discontinuously with increasing distance from the porous support.

[0007] US 2015 / 0104714 A1 discloses a method for fabricating electrodes on a surface. The resulting electrodes have a three-dimensional current collector layer.

[0008] WO 2004 / 057692 A1 discloses a double-layer oxygen electrode impregnated with an active catalyst material. The oxygen electrode has layers of varying hydrophobicity, allowing chemical impregnation of the active catalyst material into the oxygen electrode where it is most needed.

[0009] Against this background, the invention is based on the object of proposing a gas diffusion electrode which has improved properties for both oxygen reduction and oxygen formation.

[0010] This object is achieved according to the invention in the gas diffusion electrode of the type mentioned at the outset in that the gas diffusion electrode comprises: - a flat, electrically conductive substrate; - a first porous gas diffusion layer for oxygen reduction arranged on the substrate and containing a first catalyst material and a hydrophobic binder; - a second porous gas diffusion layer for oxygen formation, which is arranged above the first gas diffusion layer with respect to the substrate and which contains a second catalyst material; and - at least one current collector which electrically connects the second gas diffusion layer to the substrate, wherein the second gas diffusion layer has a higher porosity and optionally a lower hydrophobicity than the first gas diffusion layer.

[0011] The gas diffusion electrode according to the invention is based on the idea of ​​implementing the different functionalities required for oxygen reduction and oxygen formation using two separate electrode layers, whereby the properties of these layers can be optimized largely independently of one another for the respective process. Thus, the second gas diffusion layer, which faces the electrolyte in a metal-air battery, is designed with a relatively high porosity and optionally with a relatively low hydrophobicity, so that the pores of this layer can be flooded by the electrolyte, thus providing a high concentration of the species to be reduced during battery charging (hydroxide ions in the case of an alkaline electrolyte) at the interface. The high porosity also promotes the escape of the oxygen formed.Since the oxygen reduction takes place at the first gas diffusion layer facing away from the electrolyte, which is less porous and possibly more hydrophobic, the flooding of the second gas diffusion layer is not critical even during battery discharge.

[0012] The at least one current conductor, which is additionally provided in the gas diffusion electrode according to the invention, represents a direct electrical connection between the second gas diffusion layer and the substrate, which serves as the electrical contact for the electrode as a whole. This results in a lower electrical resistance of the gas diffusion electrode during the charging process than if the current had to flow exclusively through the first gas diffusion layer. This applies in particular to the case where the electrical conductivity of the first gas diffusion layer is reduced due to corrosion processes after a large number of charging and discharging cycles.

[0013] The second gas diffusion layer has a higher porosity than the first gas diffusion layer, which, within the context of the present invention, is understood to mean that it has a higher volume fraction of pores and / or a larger average pore diameter. These parameters can be adjusted during the production of the gas diffusion layers by measures known to those skilled in the art, such as the particle size of the catalyst materials used and the pressure and temperature conditions during sintering of the particles. The gas diffusion layers can also be formed from metal sponges, which can be produced with a defined porosity.

[0014] In a preferred embodiment of the invention, the volume fraction of pores in the first gas diffusion layer is in the range of 40% to 90% and in the second gas diffusion layer is in the range of 40% to 70%.

[0015] In a further preferred embodiment, the average pore diameter of the first gas diffusion layer is in the range from 50 nm to 50 µm and of the second gas diffusion layer is in the range from 500 nm to 100 µm.

[0016] Since the second gas diffusion layer according to the invention should have at most a low hydrophobicity, it preferably contains no hydrophobic binder or a lower proportion of a hydrophobic binder compared to the first gas diffusion layer. Thus, in an advantageous embodiment of the invention, the second gas diffusion layer can be formed exclusively from the second catalyst material.

[0017] The first gas diffusion layer preferably contains a proportion of 5 to 15 wt.% of the hydrophobic binder, more preferably 8 to 12 wt.%. This essentially corresponds to the binder proportion in single-layer gas diffusion electrodes according to the prior art. If the second gas diffusion layer also contains a hydrophobic binder, the proportion there is preferably less than 8 wt.%, more preferably less than 5 wt.%.

[0018] The hydrophobic binder is preferably selected from polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF), and / or polyethylene (PE). The use of PTFE as the hydrophobic binder is particularly preferred.

[0019] The first catalyst material contained in the first gas diffusion layer preferably comprises a metal with high catalytic activity for oxygen reduction, particularly silver or platinum. The first catalyst material may also consist entirely of this metal.

[0020] In addition to a metal, which also ensures the electrical conductivity of the first gas diffusion layer, the first catalyst material may further comprise a metal oxide, which is in particular selected from cobalt oxide (Co 3 O 4 ), manganese oxide (Mn 3 O 4 ), iron oxide (Fe 3 O 4 ), cobalt nickel oxide (CoNio 2 ), lanthanum calcium cobalt oxide (La x Ca 1-x CoO 3 ), ruthenium oxide (RuO 2 ), iridium oxide (IrO 2 ) and mixtures thereof. Such metal oxides can further increase the catalytic activity for oxygen reduction.

[0021] The proportion of metal oxide in the first catalyst material is preferably in the range of 5 to 25 wt.%, more preferably in the range of 10 to 15 wt.%.

[0022] The second catalyst material of the second gas diffusion layer preferably comprises a metal with high catalytic activity for oxygen formation, in particular nickel. According to the invention, the second catalyst material can also consist entirely of this metal.

[0023] In addition to a metal, which also ensures the electrical conductivity of the second gas diffusion layer, the second catalyst material may further comprise a metal oxide, which is in particular selected from cobalt oxide (Co 3 O 4 ), manganese oxide (Mn 3 O 4 ), iron oxide (Fe 3 O 4 ), cobalt nickel oxide (CoNiO 2 ), lanthanum calcium cobalt oxide (La x Ca 1-x CoO 3), ruthenium oxide (RuO 2 ), iridium oxide (IrO 2 ) and mixtures thereof. The addition of such metal oxides can further increase the catalytic activity for oxygen formation.

[0024] The proportion of metal oxide in the second catalyst material is preferably in the range of 5 to 60 wt.%, more preferably in the range of 15 to 20 wt.%.

[0025] According to a preferred embodiment of the invention, the second gas diffusion layer contains no carbon. In the prior art, carbon is often added to ensure the electrical conductivity of the gas diffusion electrode when the latter contains exclusively oxides as catalyst material. However, the addition of carbon is disadvantageous because it corrodes, particularly under conditions of oxygen formation, and thus leads to structural failure of the gas diffusion electrode after a certain number of charge and discharge cycles. Preferably, both gas diffusion layers of the electrode according to the invention contain no carbon, especially since conductivity can be ensured in each case by a metal.

[0026] In the gas diffusion electrode according to the invention, the flat substrate not only serves for electrical contact but also contributes to the structural stability of the electrode. The substrate is electrically conductive and also has a gas-permeable structure to allow oxygen to enter the first gas diffusion layer. The substrate is preferably a metal mesh or expanded metal, in particular made of stainless steel, nickel, silver, gold, or platinum.

[0027] The first and second gas diffusion layers on the substrate can be produced using methods known from the prior art. For example, a gas diffusion layer in which the catalyst material consists entirely of a metal can be produced in the form of a metal sponge. In the case of the first diffusion layer, a hydrophobic binder can be infiltrated into the pores of such a metal sponge (e.g., a silver sponge).

[0028] According to a further advantageous manufacturing process for the gas diffusion electrode according to the invention, particles of one or more catalyst materials and, optionally, particles of a hydrophobic binder are mixed and applied to the substrate or to a preceding layer using dry or wet chemical methods. The gas diffusion layer is then compacted or sintered under the influence of elevated pressure and / or elevated temperature, optionally melting or melting the hydrophobic binder. Such a process for the production of a gas diffusion electrode is described, for example, in the publication WO 2015 / 124713 A1.

[0029] As already mentioned above, the different porosities of the first and second gas diffusion layers can be adjusted by the choice of starting materials and / or process parameters when carrying out these manufacturing processes.

[0030] The at least one current conductor, which is provided according to the invention for the electrically conductive connection of the second gas diffusion layer to the substrate, is preferably arranged in an edge region of the gas diffusion electrode. In particular, the at least one current conductor can extend along one or more side edges of the gas diffusion electrode, wherein, for example, a continuous current conductor can extend along all four side edges of a rectangular electrode, or two current conductors can extend along two opposite side edges.

[0031] The current collector is preferably made of a metal, whereby advantageously the same metal as the substrate can be used, or of carbon fibers. Depending on the arrangement, the current collector can also contribute to the mechanical stability of the gas diffusion electrode.

[0032] In an advantageous embodiment, the at least one current collector is covered by the second gas diffusion layer. This prevents direct contact between the current collector and the electrolyte.

[0033] According to one embodiment of the invention, the second gas diffusion layer can be arranged directly on the first gas diffusion layer. In this case, the gas diffusion electrode has a three-layer structure.

[0034] The first and second gas diffusion layers preferably each have a layer thickness in the range of 100 to 300 µm.

[0035] The layer thickness of the substrate is preferably in the range of 50 to 200 µm.

[0036] According to an alternative embodiment of the invention, an electrically conductive intermediate layer is arranged between the two gas diffusion layers, which is contacted by the at least one current collector. In this way, the contact area between the second gas diffusion layer and the current collector(s), whose extension is represented by the electrically conductive intermediate layer, can be increased many times over. The intermediate layer can, in particular, be formed from the same metal as the substrate and / or the at least one current collector.

[0037] The present invention further relates to a secondary metal-air battery, in particular a lithium-air battery, which comprises a gas diffusion electrode according to the invention as a cathode.

[0038] The electrolyte used in the metal-air battery according to the invention can be an alkaline or acidic aqueous solution or an organic electrolyte. In the case of a lithium-air battery, the electrolyte preferably comprises a lithium hydroxide solution.

[0039] These and other advantages of the invention are explained in more detail using the following exemplary embodiments. They show in detail: Fig. 1: schematic cross-sectional view of a first embodiment of a gas diffusion electrode according to the invention; Fig. 2: schematic cross-section of a second embodiment of a gas diffusion electrode according to the invention; and Fig. 3: schematic cross-section of a third embodiment of a gas diffusion electrode according to the invention.

[0040] The Fig. 1 shows a cross-section through a first embodiment of a gas diffusion electrode according to the invention, which is designated as a whole by 10. The illustration is purely schematic, and in particular the ratio of width to thickness of the gas diffusion electrode 10 does not correspond to the actual conditions.

[0041] The gas diffusion electrode 10 comprises a flat substrate 12, which enables electrical contact with the gas diffusion electrode 10 and simultaneously provides it with mechanical stability. The flat substrate 12 is, for example, a mesh made of stainless steel or nickel with a thickness of approximately 150 µm.

[0042] Directly on the substrate 12 is a first porous gas diffusion layer 14, which contains a first catalyst material with a high catalytic activity for oxygen reduction (ORR) and a hydrophobic binder. For example, the first gas diffusion layer 14 can be a PTFE-infiltrated silver sponge or a material composed of particles of silver, cobalt oxide (Co 3 O 4 ) and PTFE sintered material. The thickness of the first gas diffusion layer 14 can be, for example, 200 µm, with a volume fraction of pores in the range of 40% to 90% and an average pore diameter of 50 nm to 50 µm.

[0043] A second porous gas diffusion layer 16 containing a second catalyst material with a high catalytic activity for oxygen evolution (OER) is applied directly onto the first gas diffusion layer 14. The second gas diffusion layer 16 can be formed, for example, from a nickel sponge, or it can be produced by sintering nickel particles, optionally with the addition of up to 20 wt.% cobalt oxide particles (Co 3 O 4 ). The second gas diffusion layer 16 has, for example, a thickness of approximately 200 µm, with the volume fraction of pores in the range of 40% to 70% and the average pore diameter in the range of 500 nm to 100 µm.

[0044] The gas diffusion electrode 10 further comprises two current collectors 18 arranged in an edge region of the gas diffusion electrode 10, in this case along the left and right edges of the gas diffusion electrode 10 in the cross-section. The current collectors 18 are preferably made of the same metal as the substrate 12, for example, stainless steel or nickel, and represent a direct electrical connection between the second gas diffusion layer 16 and the substrate 12.

[0045] When operating a metal-air battery with the gas diffusion electrode 10 as the cathode, the second gas diffusion layer 16 is oriented toward the electrolyte. Due to the low or nonexistent hydrophobicity and the high porosity of the second gas diffusion layer 16, its pores can be flooded by the electrolyte, which is favorable for the oxygen evolution reaction (OER) catalyzed by the second catalyst material. The charging current between the substrate 12 and the second gas diffusion layer 16 flows via the current collectors 18, meaning that the first gas diffusion layer 14 is not limiting the current flow during charging.

[0046] During the discharge process, oxygen reduction reaction (ORR) occurs primarily at the first gas diffusion layer 14, where the pores are not flooded by the electrolyte due to its significantly higher hydrophobicity. This ensures sufficient transport of oxygen through the substrate 12 to the phase interface of the pore structure of the first gas diffusion layer 14.

[0047] The Fig. 2 shows a cross section through a second embodiment of a gas diffusion electrode 20 according to the invention. Identical or corresponding elements are each provided with the same reference numerals as in the first embodiment.

[0048] The gas diffusion electrode 20 differs from the gas diffusion electrode 10 only in that the current collectors 18 are covered by the second gas diffusion layer 16 at their upper end facing the electrolyte. In this way, direct contact between the current collectors 18 and the electrolyte can be avoided.

[0049] The Fig. 3 shows a cross section through a third embodiment of a gas diffusion electrode 30 according to the invention. In contrast to the first two embodiments, here the second gas diffusion layer 16 is not arranged directly on the first gas diffusion layer 14, but rather an electrically conductive intermediate layer 32 is located between the two gas diffusion layers. The intermediate layer 32 is contacted by the two current collectors 18, so that a significantly more effective current flow between the second gas diffusion layer 16 and the substrate 12 is enabled.

[0050] The intermediate layer 32 is advantageously formed from the same metal as the substrate 12. List of reference symbols 10 Gas diffusion electrode 12 Flat substrate 14 First gas diffusion layer 16 Second gas diffusion layer 18 current arresters 20 Gas diffusion electrode 30 Gas diffusion electrode 32 Intermediate layer

Claims

[1] Gas diffusion electrode (10; 20; 30) for the reduction and formation of oxygen, in particular for a secondary metal-air battery, characterized by that the gas diffusion electrode (10; 20; 30) comprises: - a flat, electrically conductive substrate (12); - a first porous gas diffusion layer (14) for oxygen reduction arranged on the substrate and containing a first catalyst material and a hydrophobic binder; - a second porous gas diffusion layer (16) for oxygen formation, which is arranged above the first gas diffusion layer (14) with respect to the substrate (12) and which contains a second catalyst material; and - at least one current collector (18) which electrically connects the second gas diffusion layer (16) to the substrate (12), wherein the second gas diffusion layer (16) has a higher porosity than the first gas diffusion layer (14), and optionally a lower hydrophobicity. [2] Gas diffusion electrode (10; 20; 30) according to claim 1, wherein the higher porosity of the second gas diffusion layer (16) is achieved by a higher volume fraction of pores and / or by a larger average pore diameter compared to the first gas diffusion layer (14). [3] Gas diffusion electrode (10; 20; 30) according to claim 2, wherein the average pore diameter of the first gas diffusion layer (14) is in the range of 50 nm to 50 µm and of the second gas diffusion layer (16) is in the range of 500 nm to 100 µm. [4] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the second gas diffusion layer (16) contains no hydrophobic binder or a smaller proportion of a hydrophobic binder compared to the first gas diffusion layer (14). [5] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the first gas diffusion layer (14) contains a proportion of 5 to 15 wt.% of the hydrophobic binder, preferably 8 to 12 wt.%. [6] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the hydrophobic binder is selected from polytetrafluoroethylene (PTFE), polypropylene (PP), polyvinylidene fluoride (PVDF) and / or polyethylene (PE), wherein the binder is preferably PTFE. [7] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the first catalyst material comprises or consists entirely of a metal with a high catalytic activity for oxygen reduction, in particular silver or platinum. [8] Gas diffusion electrode (10; 20; 30) according to claim 7, wherein the first catalyst material further comprises a metal oxide, which is in particular selected from cobalt oxide (Co3O4), manganese oxide (Mn3O4), iron oxide (Fe3O4), cobalt-nickel oxide (CoNiO2), lanthanum-calcium-cobalt oxide (La x Ca 1-x CoO3), ruthenium oxide (RuO2), iridium oxide (IrO2) and mixtures thereof. [9] Gas diffusion electrode (10; 20; 30) according to claim 8, wherein the first catalyst material comprises a proportion of metal oxide of 5 to 20 wt.%, preferably 10 to 15 wt.%. [10] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the second catalyst material comprises or consists entirely of a metal having a high catalytic activity for oxygen formation, in particular nickel. [11] Gas diffusion electrode (10; 20; 30) according to claim 10, wherein the second catalyst material further comprises a metal oxide, which is in particular selected from cobalt oxide (Co3O4), manganese oxide (Mn3O4), iron oxide (Fe3O4), cobalt-nickel oxide (CoNiO2), lanthanum-calcium-cobalt oxide (La x Ca 1-x CoO3), ruthenium oxide (RuO2), iridium oxide (IrO2) and mixtures thereof. [12] Gas diffusion electrode (10; 20; 30) according to claim 11, wherein the second catalyst material comprises a metal oxide content of 5 to 60 wt.%, preferably 15 to 20 wt.%. [13] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the second gas diffusion layer (16) does not contain carbon, and in particular both gas diffusion layers (14, 16) do not contain carbon. [14] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the substrate (12) is a metal mesh or an expanded metal, in particular made of stainless steel, nickel, silver, gold or platinum. [15] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the at least one current collector (18) is arranged in an edge region of the gas diffusion electrode (10; 20; 30) and extends in particular along one or more side edges of the gas diffusion electrode (10; 20; 30). [16] Gas diffusion electrode (10; 20; 30) according to one of the preceding claims, wherein the at least one current collector (18) is formed from a metal or from carbon fibers. [17] Gas diffusion electrode (20; 30) according to one of the preceding claims, wherein the at least one current collector (18) is covered by the second gas diffusion layer (16). [18] Gas diffusion electrode (10; 20) according to one of the preceding claims, wherein the second gas diffusion layer (16) is arranged directly on the first gas diffusion layer. [19] Gas diffusion electrode (30) according to one of claims 1 to 17, wherein an electrically conductive intermediate layer (32) is arranged between the two gas diffusion layers (14, 16), which is contacted by the at least one current collector (18).

Images