The present invention is generally directed to flow batteries and more specifically to electrodes for flow batteries.
The development of renewable energy sources has revitalized the need for large-scale batteries for off-peak energy storage. The requirements for such an application differ from those of other types of rechargeable batteries such as lead-acid batteries. Batteries for off-peak energy storage in the power grid generally are required to be of low capital cost, long cycle life, high efficiency, and low maintenance.
One type of electrochemical energy system suitable for such an energy storage is a so-called “flow battery” which uses a halogen component for reduction at a normally positive electrode, and an oxidizable metal adapted to become oxidized at a normally negative electrode during the normal operation of the electrochemical system. An aqueous metal halide electrolyte is used to replenish the supply of halogen component as it becomes reduced at the positive electrode. The electrolyte is circulated between the electrode area and a reservoir area. One example of such a system uses zinc as the metal and chlorine as the halogen.
Such electrochemical energy systems are described in, for example, U.S. Pat. Nos. 3,713,888, 3,993,502, 4,001,036, 4,072,540, 4,146,680, and 4,414,292, and in EPRI Report EM-I051 (Parts 1-3) dated April 1979, published by the Electric Power Research Institute, the disclosures of which are hereby incorporated by reference in their entirety.
An embodiment relates to a porous electrode for a flow battery which includes a first layer and a second layer, wherein the first layer has at least one of a different catalytic property or a different permeability than the second layer.
Another embodiment relates to a method of making a porous electrode for a flow battery, comprising providing a first substrate layer comprising a sintered metal or metal oxide powder substrate layer, and coating a portion of the first substrate layer with a mixed metal oxide catalytic coating.
Embodiments include a multilayer positive electrode structure for a metal halogen flow cell. The multilayer electrode structure provides one or more of the following advantages over conventional positive electrodes: a more uniform fluid flow and pressure distribution, high electrochemical reaction kinetics, high mechanical integrity, excellent manufacturing tolerance as well as lower cost.
In some embodiments, the porous electrode includes a first layer and a second layer, where the first layer has a different catalytic property and permeability than the second layer. Specifically, in some embodiments, the first layer (e.g., layer 106, 106A or 107 described below) has a lower catalytic property and a higher flow resistance than the second layer (e.g., layer 108 and/or 109 described below). The first layer may comprise at least one of a porous metal or metal oxide foam layer 106A, or a porous sintered metal or metal oxide powder layer 106 or 107. The second layer may comprise at least one of sintered metal oxide powder layer 108 which catalyzes oxidation of a metal-halide electrolyte to form halogen ions, or a sintered metal or metal oxide powder layer 107 portion which is coated with a mixed metal oxide catalytic coating 109 which catalyzes oxidation of the metal-halide electrolyte to form the halogen ions. The first layer is preferably thicker than the second layer.
An embodiment is drawn to an electrode that is permeable to the electrolyte and fabricated by sintering metal oxide powder and/or by sintering a metal powder and then coating it with a metal oxide (i.e., catalytic) coating. The metal oxide powder can be, but is not limited to, titanium oxide, tantalum oxide, tungsten oxide and oxides of other refractory metals, the metal powder can be, but is not limited to, titanium, tantalum, tungsten, or other refractory metals and their alloys, and the metal oxide coating (e.g., catalytic coating) may be a mixed metal oxide comprising a mixed refractory and noble metal oxide, such as a mixed titanium oxide and ruthenium oxide (i.e., ruthenized titania), or mixtures of other refractory and noble metal oxides. The catalytic coating catalyzes conversion of a metal halide electrolyte (e.g., a zinc-bromine or zinc-chloride aqueous electrolyte) to metal and halogen ions (e.g., zinc ions and bromine or chlorine ions). In other words, the catalytic coating catalyzes oxidation of the metal-halide electrolyte to form the halogen ions.
In a first embodiment, the positive electrode is produced by sintering metal powders or metal oxide powders such as titanium, tantalum, tungsten, titanium oxide, tantalum oxide, tungsten oxide or combinations thereof. The sintered powder becomes a porous structure with high surface area, uniform thickness and desired pore size and permeability. In one embodiment, the porous structure acts as a positive electrode substrate which is at least partially coated with a mixed metal oxide catalytic coating to complete the positive electrode. In another embodiment, the mixed metal oxide catalytic coating is omitted when at least a part of the porous structure comprises a sintered metal oxide powder which itself acts as the catalyst.
Typically, finer and/or tighter distributed particles are more expensive to make than coarser and/or looser distributed particles. As used herein, particle distribution refers to the half maximum width of a peak in a plot of particle size (e.g., diameter) versus number of particles of that size in the powder. A tighter distribution has a smaller half maximum peak width in this plot than a looser distribution.
Thus, to save cost, electrodes made by powder metallurgy are typically fabricated with coarser and/or looser distributed particles. However, fabrication with smaller and/or tighter distributed particles yields an electrode with increased surface area which produces superior electrochemical performance in the battery. By fabricating a multi-layer electrode having a layer made of coarser and/or looser distributed particles and a layer of finer and/or tighter distributed particles, an electrode with the superior electrochemical performance of the finer particles can be achieved at less cost than an electrode made entirely from finer and/or tighter distributed particles.
In the embodiment illustrated in
Preferably, when using the bilayer multi-porous electrode 102 in a flow battery, the finer and/or tighter distributed particle side 108 of the (positive) electrode 102 is placed facing the reaction zone 103 and the negative electrode 104 of the electrochemical cell 100 to take advantage of the higher surface area and/or an increased functional surface area of the layer 108 during the electrochemical reaction in the flow battery. An increased functional surface area has a more uniform roughness and/or pore size as a function of area of the electrode 102 facing the reaction zone due to the tighter sintered particle distribution of layer 108 in the electrode 102. In contrast, layer 106 provides a less expensive, electrically conductive structural backbone for the electrode 102.
The electrode 102 may be made by separately sintering powders to form layers 106, 108 and then joining the layers 106, 108 to form the electrode. Alternatively, green layers 106, 108 or packed powder layer 106, 108 may be placed in contact with each other followed by a single common sintering step to form electrode 102. Alternatively, one layer (e.g., layer 106) is formed and sintered first, followed by forming the other green layer (e.g., layer 106) on the sintered layer (e.g., 108), followed by a second sintering step. If desired, the mixed metal oxide catalytic coating may be applied to layer 108, especially if the layer 108 is made from metal rather than metal oxide sintered particles.
The layer 108 may include, but is not limited to, particles, of finer and/or tighter distributed titanium powder. The layer 106 may also include, but is not limited to sintered titanium powder having a coarser and/or looser distributed powder particles. As with the first embodiment, the layer 108 of the electrode 102 is preferably placed facing the negative electrode 104 in a flow battery cell to take advantage of the higher and/or improved functional surface area during the electrochemical reaction in the flow battery. In this embodiment, the mixed metal oxide coated sintered powder titanium layer 108 provides a region for high electrochemical activity, while layer 106 provides structural integrity at a lower cost.
For example, the substrate layer 107 may comprise a sintered refractory metal (e.g., titanium) powder layer having a relatively loose distribution of powder particles. Layer 107 is then coated from the side facing the reaction zone using a mix of a solid catalyst phase (e.g., mixed metal oxide, such as ruthenized titania) and a liquid carrier phase (e.g., organic liquid, such as an alcohol) to form the catalytic coating 109. The mix may comprise a colloid or suspension, e.g., slurry or another mixture, that is formed by wet spraying, brushing on, dip coating, spin coating, etc. on surface 107A of substrate layer 107. Preferably, the organic liquid carrier is selected such that it evaporates before penetrating the entire thickness of the substrate layer. This allows the catalytic coating to achieve the desired penetration depth 107B into the substrate layer 107.
The catalytic coating 109 and the substrate layer 107 portion from surface 107A to depth 107B forms mixed porous sintered metal powder structure having a thin coating 109 of the mixed metal oxide on surface 107A and on the surface of the pores in layer 107. Thus, the coating 109 makes the porosity in the portion of layer 107 between surface 107A and depth 107B slightly smaller than in the rest of the porous sintered metal powder layer 107 beyond depth 107B. Thus, in this embodiment, the flow resistance through the portion of layer 107 between surface 107A and depth 107B is substantially the same (e.g., slightly smaller) than in the rest of the porous sintered metal powder layer 107 beyond depth 107B. In contrast, the flow resistance through layer 108 facing the negative electrode 104 in
The portion of layer 107 coated with the catalytic coating 109 may have a BET surface area that is greater than that of a flat, non-porous titanium layer. For example, it may have a BET surface area that is greater than 1 and less than 20 times, such as between 5 and 10 times that of the flat, non-porous titanium layer. The portion of layer 107 coated with the catalytic coating 109 may have a BET surface area between 0.001 and 0.5 m2/g, such as 0.02 to 0.05 m2/g.
The penetration depth 107B may be between 0.1 and 1 mm, such as 0.25 to 0.5 mm. The thickness of the coating 109 on the surface 107A and on the surface of the pores in the substrate layer 107 may be between 100 and 500 nm, such as 200 to 400 nm. It should be noted that coating 109 may also be applied to the bi-layer structure shown in
In another alternative embodiment, the multi-porous electrodes 102 are made with multilayer wire meshes (e.g., stacked or joined fine and coarse wire meshes). A wire mesh provides more surface area than a solid plate. Fine or coarse meshes in this embodiment could be, but are not limited to be, manufactured from titanium, tantalum or tungsten wire, or an aluminum wire coated with a thin layer of titanium, tantalum or tungsten deposited by techniques such as electroplating, physical vapor deposition or chemical vapor deposition.
In an alternative embodiment, the multilayer porous electrode contains one or more layers made from a metal foam, as shown in
In an embodiment of a method of making the multi-porous electrode 102 illustrated in
In a third embodiment illustrated in
In an embodiment, the non-conductive porous restriction layer 114 may be affixed to the porous electrode 102 (e.g., the multi-porous electrode of the above embodiments or another porous electrode having a single porosity and/or made by other suitable methods that those described above), as shown in
Alternatively, the stack may include an optional alignment part (e.g. molded plastic) that presses the restriction layer 114 against the porous electrode 102. The restriction layer 114 may be co-molded, welded, or otherwise integrated with this alignment part. The restriction layer(s) and corresponding alignment part(s) may be installed during the fabrication of the bipolar electrode assembly 202, such that they are captive, or installed after the bipolar electrode assembly 202 is fabricated such that they are removable.
Layer 114 may comprise layer having slit shaped openings (e.g., cut-outs) such that the ribs 110 protrude through the openings, as shown in
As described above, the layer 108 facing the negative electrode 104 and the reaction zone 103 is designed to maximize catalytic activity by achieving a high surface area and a structure that facilitates the uniform application of the catalytic coating 109. An example construction for this layer 108 is a relatively tightly controlled distribution of titanium particle sizes sintered together. Layer 108 may have a BET surface area that is greater than that of a flat, non-porous titanium layer. For example, layer 108 may have a BET surface area that is greater than 1 and less than 20 times, such as between 5 and 10 times that of the flat, non-porous titanium layer. Layer 108 may have a BET surface area between 0.001 and 0.5 m2/g, such as between 0.02 to 0.05 m2/g.
Since the coating and substrate material in this layer 108 may be fairly expensive, the layer 108 (i.e., layer 108 comprised of sintered metal oxide catalyst particles, or layer 108 comprised of sintered metal or metal oxide particles coated with a thin mixed metal oxide catalytic coating 109) is only as thick as necessary to provide sufficient catalytic activity. As a result, this layer 108 may not be thick enough to provide sufficient conductivity or stiffness.
The next layer 106 takes care of this issue by providing a lower cost electrically conductive and structural backbone. An example construction for this layer 106 is a loosely distributed range of titanium particle sizes sintered together and subsequently sintered or welded to the layer 108. The conductive material in this layer 106 may still be relatively expensive, so it is only thick enough to achieve the required conductivity and stiffness. Layer 106 may be thicker than layer 108. For example, the total thickness of layers 106 and 108 (i.e., of electrode 102) should be sufficient to provide an area specific resistance that is equal to that of a 0.25 mm to 1 mm thick, non-porous titanium plate, to provide a sufficient conductivity for the positive electrode 102. For example, the in-plane resistance of a planar electrode sheet 102 (e.g., combination of layers 106 and 108 or layers 107 and 109) per centimeter width and centimeter depth is between 2×10−4 and 5×10−1 ohms, such as 2×10−3 and 5×10−2.
However, this thickness of layers 106 and 108 may not be thick enough, given their permeability characteristics, to provide the desired flow resistance for the flow of the electrolyte. The non-conductive porous restriction layer 114 provides an even lower cost flow control layer. Since this layer does not need to be conductive, it can be made from a much lower cost material, such as a plastic. An example construction for this layer 114 is a relatively tightly controlled range of HDPE particle sizes sintered together. Layer 114 may be thicker than layers 106 and 108 to provide sufficient flow resistance (i.e., a desired permeability). For example, the gas permeability of the porous restriction layer 114 may be between 1×10−10 and 5×10 cm2, such as between 1×10−8 and 5×10−7 cm2.
The positive electrode 102 (i.e., layers 107/109 or layers 106/108) preferably has a sufficient stiffness to be suspended across the reaction zone. Preferably, the flexural modulus times thickness cubed parameter of the positive electrode is between 0.1 and 1200 Newton-meters, such as between 10 and 100 Nm. This parameter correlates to the bending stiffness per cm width and cm length of the electrode 102. If the electrode is connected to the porous restriction layer 114, then the flexural modulus times thickness cubed parameter of combination of the positive electrode 102 and layer 114 is between 0.1 and 1200 Newton-meters, such as between 10 and 100 Nm.
The values of the gas permeability, in-plane resistance, and bending stiffness are a function of the porous electrode geometry. For example, the porous restriction layer 114 (i.e., a flow control layer) increases a fluidic resistance of the electrolyte flowing through the porous positive electrode 102 and thereby distributes the flow more uniformly. For a shorter electrode 102, the desired flow uniformity can be achieved with a smaller fluidic resistance in the flow control layer (i.e. thinner and/or more permeable layer) because the flow resistance along the reaction zone is a smaller fraction of the total.
For layers 106, 106A or 107 (i.e., the conductivity enhancing layer), the lateral distance that current must flow through the electrode 102 before it can flow into an adjacent cell (e.g., through a junction rib 110 shown in
Similarly, the bending stiffness depends on the length of the unsupported span of the electrode 102 (or the combination of the electrode 102 and layer 114). If the porous electrode is supported in the reaction zone 103 by one or more plastic spacer ribs 211 shown in
The plastic cell frame 201 contains a plurality of plastic spacer ribs 211 which support the porous electrode 102 over the reaction zone 103. The active area 213 (e.g., opening in middle of frame 201 containing the electrodes 102, 104) is separated into flow areas 215.
The flow areas 215 may be between 200 mm and 1000 mm long (i.e., in the direction between manifolds 205 and 207/209), such as 300 to 500 mm long, and between 50 to 150 mm wide (i.e., in the direction perpendicular to the length direction), such as 75 to 100 mm wide. The above described values of gas permeability, in-plane resistance, and bending stiffness are suitable for the flow area 215 dimensions described above. In other words, for the flow areas 215 described above, the gas permeability of the porous restriction layer 114 may be between 1×10−10 and 5×10 cm2, such as between 1×10−8 and 5×10−7 cm2, the in-plane resistance of a planar electrode sheet 102 per centimeter width and centimeter depth may be between 2×10−4 and 5×10−1 ohms, such as between 2×10−3 and 5×10−2 and the flexural modulus times thickness cubed parameter of combination of the positive electrode 102 (or the combination of electrode 102 and layer 114) may be between 0.1 and 1200 Newton-meters, such as between 10 and 100 Nm. Other values may be used for different flow area dimensions.
As shown in
Alternatively, as shown in
Using computational fluid dynamics (CFD), the potential impact of a separate porous restriction layer 114, the effect of any gap between the restriction layer 114 and the porous electrode 102 and the effect of an additional baffle structure in the gap were analyzed.
The results of the CFD simulations are illustrated in
Another embodiment is drawn to an electrode assembly which includes a porous electrode 102 affixed to an impermeable electrode 102. The electrodes may be affixed by any suitable method, such as welding or brazing. Example electrode assemblies are described in U.S. patent application Ser. No. 12/877,884, filed Sep. 8, 2010, hereby incorporated in its entirety.
The average pore size and surface area data for five porous electrodes 102 made of various powder sizes and assemblies are presented for comparison: (1) mono-layer made from mesh-100 powder, (2) mono-layer made from mesh-100 and -325 mixed powders, (3) bilayer made from mesh-100 and mesh-325 powders, (4) bilayer made from coarse layer made from mesh-100 powder and sprayed on mesh-325 fine layer and (5) monolayer made from mesh-325. The pore size and surface area were measured by the capillary flow porosimetry technique.
Although the foregoing refers to particular preferred embodiments, it will be understood that the invention is not so limited. It will occur to those of ordinary skill in the art that various modifications may be made to the disclosed embodiments and that such modifications are intended to be within the scope of the invention. All of the publications, patent applications and patents cited herein are incorporated herein by reference in their entirety.
The present application claims benefit of priority of U.S. Provisional Patent Application No. 61/615,544, filed on Mar. 26, 2012 which is incorporated herein by reference in its entirety.
This invention was made with Government support under contract DE-AR0000143 awarded by Department of Energy. The Government has certain rights in the invention.
Number | Date | Country | |
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61615544 | Mar 2012 | US |