Patent Publication Number: US-2013252041-A1

Title: Electrode for High Performance Metal Halogen Flow Battery

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
     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. 
    
    
     FIELD 
     The present invention is generally directed to flow batteries and more specifically to electrodes for flow batteries. 
     BACKGROUND 
     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. 
     SUMMARY 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A ,  1 B,  1 C,  1 D and  2  are schematic side cross sectional view illustrations of multi-porous electrodes according to various embodiments. 
         FIG. 3A  is a perspective view of a multi-porous electrode with junction ribs and a sealing rim according to an embodiment.  FIG. 3B  is a close up of the multi-porous electrode of  FIG. 3A .  FIG. 3C  is a close up of  FIG. 3B .  FIG. 3D  is a micrograph illustrating the microstructure of a cross section of the multi-porous electrode of  FIG. 3A . 
         FIGS. 4A ,  4 B,  4 C and  4 D are alternative schematic side cross sectional view illustrations of a cell with a restriction layer according to various embodiments. 
         FIG. 4E  is a top view of a cell frame. 
         FIG. 4F  is a schematic side cross sectional view of a flow battery system of the embodiments of the invention. 
         FIG. 5  is a schematic side cross sectional view illustration of six electrode configurations used in a computational fluid dynamics simulation. 
         FIG. 6  is a plot illustrating results comparing electrolyte velocity in electrodes with and without an additional restriction layer. 
         FIG. 7  is a plot illustrating results comparing electrolyte velocity in electrodes having baffle configurations with a configuration lacking a gap. 
         FIG. 8  is a plot illustrating average pore size of multi-layer electrodes according to embodiments. 
         FIG. 9  is a plot illustrating surface area of multi-layer electrodes according to embodiments. 
         FIG. 10A  is a micrograph illustrating the microstructure of porous electrodes with a mesh-325 monolayer.  FIG. 10B  is a micrograph illustrating the microstructure of porous electrodes with a mesh-325 and mesh-100 bilayer. 
         FIG. 11  is a simulation showing flow velocity with and without restriction layer through an electrode according to an embodiment. 
         FIG. 12A  is a plot showing the effect of a restriction layer on flow velocity.  FIG. 12B  is a plot showing the effect of a restriction layer and a gap on flow velocity. 
         FIG. 13  is a plot illustrating electrochemical polarization curves at different charge and discharge current densities obtained with the porous electrodes made from different mesh sizes material. 
     
    
    
     DETAILED DESCRIPTION 
     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 ,  106 A 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  106 A, 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. 
       FIGS. 1A-1D  and  2  illustrate embodiments of an electrochemical cell  100 , such as a flow cell of a flow battery, having a multi-porous positive electrode  102  and a non-porous (i.e., fluid impermeable) negative electrode  104  separated by a reaction zone  103 . Preferably, the battery comprises a hybrid flow battery having a single flow loop and no separator in the reaction zone  103  between the electrodes  102 ,  104 . In the hybrid metal halide flow battery, at least a part of the electrolyte flows through the porous positive electrode  102  and the metal (e.g., zinc) is plated in charge mode onto the surface of the negative electrode  104  facing the reaction zone  103 . 
     In the embodiment illustrated in  FIG. 1A , the multi-porous electrode  102  is a bi-layer electrode. The bilayer multi-porous electrode  102  includes two layers  106 ,  108  that are made from powders of different mesh size and/or different distribution. For example, a powder that is designated as “325 mesh” passed though a 325 mesh screen and the powder&#39;s particle size (i.e., the maximum particle diameter) is less than 44 microns. That is, the bilayer multi-porous electrode  102  includes a first, coarse and/or looser distributed sintered powder layer  106  that is made from coarser (e.g., larger mesh size) and/or looser distributed particles, and a second, finer and/or tighter distributed sintered powder  108  made from finer particles (e.g., with a smaller mesh size) and/or tighter distributed particles than that of the layer  106 . 
     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. 
       FIG. 1B  illustrates an alternative embodiment of the multi-porous electrode  102 . In this embodiment, the layer  108  includes a layer of metal powder that is sprayed on one side (e.g., the reaction zone  103  facing side) of the layer  106 . Any spraying, such as plasma-arc spray, shrouded plasma-arc spray, high velocity, oxy-fuel (HVOF), electric arc-spray, flame spray, or cold spray, may be used. Layer  108  may be sprayed onto a sintered layer  106  followed by a second sintering step or layer  108  may be sprayed onto layer  106  which comprises a green precursor or packed powder followed by a single common sintering step. Layer  108  may be formed by spraying either a sintered or unsintered powder or a green precursor onto layer  106 . A post spraying sintering step is optional. The mixed metal oxide catalytic coating may then be applied to layer  108 . 
     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. 
       FIG. 1C  illustrates an alternative embodiment in which the position of layers  106  and  108  is reversed with respect to  FIGS. 1A and 1B , such that layer  106  faces the reaction zone  103  and the negative electrode  104  of the same cell while layer  108  faces away from the reaction zone  103 . The particle size of the layer  106  facing the negative electrode  104  is optimized to facilitate the application of a catalytic coating (e.g. the mixed metal oxide coating), while the particle size of the layer  108  facing away from the negative electrode  104  is optimized to achieve desirable flow control properties, such as permeability. Thus, the coarser sintered powder layer  106  faces the negative electrode  104  and the finer sintered powder layer faces away from the negative electrode  104 . The catalytic coating is applied to the surface of layer  106  facing the reaction zone  103  and the negative electrode  104 . 
       FIG. 1D  illustrates an alternative embodiment in which the positive porous electrode  102  comprises a single porous substrate layer  107 , such as a sintered refractory metal powder layer (e.g., sintered titanium powder layer) designed to achieve the desired conductivity, stiffness, and surface area. To achieve the desired electrochemical activity, the catalytic coating  109  is applied to the surface  107 A of this substrate layer  107  facing the reaction zone  103  and the negative electrode  104 . However, since the catalytic coating  109  material is expensive, it is applied so that it does not penetrate and coat the entire substrate layer  107  thickness, but rather only penetrates to a predetermined depth  107 B from surface  107 A achieve the desired catalytic activity. 
     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  107 A 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  107 B into the substrate layer  107 . 
     The catalytic coating  109  and the substrate layer  107  portion from surface  107 A to depth  107 B forms mixed porous sintered metal powder structure having a thin coating  109  of the mixed metal oxide on surface  107 A 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  107 A and depth  107 B slightly smaller than in the rest of the porous sintered metal powder layer  107  beyond depth  107 B. Thus, in this embodiment, the flow resistance through the portion of layer  107  between surface  107 A and depth  107 B is substantially the same (e.g., slightly smaller) than in the rest of the porous sintered metal powder layer  107  beyond depth  107 B. In contrast, the flow resistance through layer  108  facing the negative electrode  104  in  FIG. 1A  is lower than through layer  106  facing away from the negative electrode, while the flow through layer  106  facing the negative electrode  104  in  FIG. 1C  is higher than through layer  108  facing away from the negative electrode. 
     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 m 2 /g, such as 0.02 to 0.05 m 2 /g. 
     The penetration depth  107 B 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  107 A 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  FIG. 1A ,  1 B or  1 C. 
     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  FIG. 2 . In this embodiment, one or more of the porous electrode layers, such as the structural backbone layer  106 A is made from the metal foam. Examples of metal foams include pure titanium, tantalum, and tungsten metal foams, or carbon or aluminum foams coated with a thin layer of refractory metal, such as titanium, tantalum, or tungsten deposited by techniques such as electroplating, or physical vapor deposition or chemical vapor deposition. The metal foam creates a lower cost stiff and conductive backbone for a thinner layer  108  which may comprise a catalytic metal oxide or which may be coated with a mixed metal oxide catalytic coating  109  described above. Layer  108  may be welded, sintered on or otherwise attached to the foam layer  106 , or layer  108  may be sprayed onto layer  106  as described above. 
       FIGS. 3A-3D  illustrate a multi-porous electrode  102  with junction ribs  110  and a sealing rim  112  according to a second embodiment. This design illustrates an economical method of integrating the multi-porous electrode  102  with similarly fabricated junction ribs  110  and/or a sealing rim  112 . The performance of the multi-porous electrode  102  is sensitive to the size of mesh used during the sintering process. Using smaller meshes with a tight tolerance on the particle size is more costly than using larger meshes with looser tolerances. In this embodiment, particles larger mesh sizes are used to produce the sealing rim  112  and the junction ribs  110 . The ribs  110  connect the positive/porous electrode  102  of one cell with a negative electrode  104  of an adjacent cell in the stack of cells to form an electrode assembly. Electrolyte flow channels are located between adjacent ribs on the side of the positive electrode  102  facing away from the reaction zone  103 . 
     In an embodiment of a method of making the multi-porous electrode  102  illustrated in  FIG. 3A , an electrode portion  100  is produced in a “green” pre-sintered state from powder of desired mesh(es) (e.g., the fine and coarse layers described above). Separately, the sealing rim  112  and junction ribs  110  are formed in a green state from a coarse mesh powder. Then, the green sealing rim  112  and junction ribs  110  are provided in contact with the multi-porous electrode  102 . The entire green assembly is then sintered. Optionally, the coarsness and final density tolerance of the sealing rim  112  and junction ribs  110  may be selected to meet stiffness and weldability requirements. 
     In a third embodiment illustrated in  FIGS. 4A and 4B , the electrochemical cell  100  may include a non-conductive porous flow restriction layer  114 . The non-conductive porous layer  114  acts as a flow restrictor, improving the uniformity of fluid flow within the porous electrode  102 . The cell  100  may be oriented with the porous electrode  102  above the reaction zone  103  as shown in  FIG. 4A  or with the non-porous electrode  104  located above the reaction zone as shown in  FIG. 4B . 
       FIG. 4B  shows a side cross sectional view of a stack  200  of cells  100  supported by a frame  201 . Each cell  100  includes the porous  102  and non-porous  104  electrodes separated by a reaction zone  103 . A porous electrode  102  of one cell is connected to the non-porous electrode  104  of an adjacent cell by the ribs  110  to form an electrode assembly  202 . Electrolyte flow channels  204  are located between the ribs  110  in each electrode assembly  202 . Each flow channel  204  is bounded by two ribs (or a rib and a frame  201 ) on the sides, the flow restriction layer  114  on top (or on the bottom if the stack  200  is flipped upside down as shown in  FIG. 4A ), and surface of the non-porous electrode  104  facing away from the reaction zone  103  on the bottom (or on top in  FIG. 4A  configuration). The stack  200  may be located in a flow battery system containing a housing, an electrolyte (e.g., zinc-bromide or zinc-chloride) reservoir and an electrolyte pump. 
     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  FIG. 4B . Alternatively, the restriction layer  114  may be spaced apart from the porous electrode  102  and be held in place by the frame  200  and/or the ribs  110 , as shown in  FIG. 4A . 
     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  FIG. 4B . Alternatively, layer  114  may comprise plural discrete strips which are placed or wedged between the ribs  110  in the flow channels  204 . The non-conductive porous restriction layer  114  may be a porous sintered plastic, a plastic felt, a porous ceramic, or a variety of other electrically insulating materials. Advantageously, the non-conductive porous layer  114  may be made of a less expensive material than the porous electrode  102 . Optionally, a baffle type structure (electrically insulating baffles located between spaced apart layer  114  and electrode  102 ) may also be used to improve the fluid flow distribution. 
       FIG. 4C  illustrates an exemplary embodiment of a flow battery cell with the porous restriction layer. The cell shown in  FIG. 4C  is similar to the cell shown in  FIG. 1A , except that the cell in  FIG. 4C  contains an additional porous restriction layer  114  on the opposite side of layer  106  from layer  108 . 
     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 m 2 /g, such as between 0.02 to 0.05 m 2 /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 cm 2 , such as between 1×10 −8  and 5×10 −7  cm 2 . 
       FIG. 4D  illustrates another exemplary embodiment of a flow battery cell with the porous restriction layer. The cell shown in  FIG. 4D  is similar to the cell shown in  FIG. 1D , except that the cell in  FIG. 4D  contains an additional porous restriction layer  114  on the opposite side of the substrate layer  107  from the surface  107 A coated with the catalytic coating  109 . Since the permeability and thickness of the positive electrode  102  (composed of layer  107  and coating  109 ) may not yield the desired flow resistance, the low cost flow control layer  114  is mated against the opposite surface of the porous electrode  102  from surface  107 A. Since this layer  114  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. 
     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 ,  106 A 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  FIGS. 3A and 3B ) has a significant impact on the resistivity/thickness characteristic of these layers  106 ,  106 A or  107 . In other words, the number of junction ribs  110  varies inversely with thickness and conductance of layers  106 ,  106 A or  107  (i.e., more ribs allows the use of a thinner layer or a more resistive layer). 
     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  FIG. 4E , then the electrode thickness and modulus may be decreased. In other words, the electrode  102  thickness and flexural modulus are inversely proportional to the area between the spacer ribs. 
       FIG. 4E  illustrates a top view of the cell frame  201  for holding the horizontally positioned flow battery cells, which are illustrated is side cross sectional view in  FIG. 4B . The cell frame  201  is described in more detail in U.S. patent application Ser. No. 13/630,572, filed on Sep. 28, 2012 and incorporated herein by reference in its entirety. The frame  201  includes an inlet manifold  205  and the outlet manifolds  207 ,  209 . The manifolds are respective openings through the frame  201  which align with similar openings in other stacked frames  201  to form the manifolds. 
     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 cm 2 , such as between 1×10 −8  and 5×10 −7  cm 2 , 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  FIG. 4A , electrolyte enters the electrochemical cell  100  via the inlet  120  between adjacent cells in the flow battery stack and spreads through flow channels  204  (shown in  FIG. 4B ) across the non-conductive porous layer  114  before passing to the porous electrode  102 , thereby evening the flow to the porous electrode  102  below. After passing through the porous electrode  102 , the electrolyte flows through the reaction zone  103  and exits the electrochemical cell  100  via an outlet  122 . 
     Alternatively, as shown in  FIG. 4F , the electrolyte enters the cell  100  through the reaction zone  103 , and then at least a portion of the electrolyte flows through the porous electrode  102  and out of the cell through the flow channels  204 . The electrolyte inlet flow is provided by pump  123  from reservoir  119  via inlet conduit  115  into the inlet manifold  205  and then into the reaction zone  103 . The electrolyte then flows through the reaction zone  103  and through the porous electrodes  102  into the flow channels  204  and out through respective manifolds  209 ,  207  and respective return conduits  120 A and  120 B back into the reservoir  119 . The respective return conduits  120 A and  120 B may be configured with calibrated pipe restrictions  602   a ,  602   b  and on/off valves  604   a ,  604   b , in order to control the flow ratios of the exit flow streams. Valve  604   a  is closed and valve  604   b  is open in charge mode. In contrast, valve  604   a  is open and valve  604   b  is closed in discharge mode. 
     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.  FIG. 5  shows several configurations were modeled in a 2-D CFD study. Configurations modeled include: (1) no non-conductive porous layer  114 , (2) a non-conductive porous layer  114  located immediately adjacent the porous electrode  102  (no gap), (3) a non-conductive porous layer  114  located with a 1 mm gap between the non-conductive porous layer  114  and the porous electrode  102 , (4) a non-conductive porous layer  114  located with a 1 mm gap between the non-conductive porous layer  114  and the porous electrode  102  and a baffle structure of 1 mm wide junction ribs  110  (or additional insulating baffles) separated 9 mm apart, (5) a non-conductive porous layer  114  located with a 1 mm gap between the non-conductive porous layer  114  and the porous electrode  102  and a baffle structure of 0.5 mm wide junction ribs  110  (or additional insulating baffles) separated 4.5 mm apart and (6) a non-conductive porous layer  114  located with a 1 mm gap between the non-conductive porous layer  114  and the porous electrode  102  and a baffle structure of junction ribs  110  (or additional insulating baffles) with varying separation distance (e.g., decreasing in electrolyte flow direction) between adjacent junction ribs. 
     The results of the CFD simulations are illustrated in  FIGS. 6 and 7 .  FIG. 6  compares the electrolyte velocity in the porous electrode  102  with and without an additional non-conductive porous restriction layer  114  (with and without a gap between electrode  102  and layer  114 ).  FIG. 7  compares the electrolyte velocity in the device having the restriction layer  114  separated from the porous electrode  102  with baffle structures (e.g., junction ribs  110 ) to the configuration with no gap between layer  114  and electrode  102 ). The results of the CFD studies suggest that the “no gap” configuration (the porous restriction layer bonded to the porous electrode) appear effective at evening the fluid flow, as shown in  FIG. 6 . A configuration including a gap between the non-conductive porous restriction layer  114  and the porous electrode  102  showed an improvement in electrolyte velocity distribution at the inlet  120 , but showed little impact at the outlet  122 . Each of the “baffle” configurations in  FIG. 7  provided a similar improvement as the configuration in which layer  114  contacts the electrode  104 . 
     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. 
     Test Results 
     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.  FIG. 8  illustrates the average pore sizes of these electrodes. As can be seen in  FIG. 8 , the average pore size of the multi-layer electrodes ( 3 ), ( 4 ) is as small as the monolayer ( 5 ) of fine mesh particles.  FIG. 9  illustrates the average surface area of the electrodes. The average surface area of multilayer electrodes ( 3 ), ( 4 ) is higher than any of the monolayer electrodes ( 1 ), ( 2 ), ( 5 ). 
       FIGS. 10A and 10B  are micrographs illustrating the microstructure of a comparative monolayer electrode and a bilayer multi-porous electrode, respectively.  FIG. 10A  illustrates the microstructure of a single layer porous electrode made from a mesh-325, while  FIG. 10B  illustrates the microstructure of a bilayer multi-porous electrode made from a mesh-325 layer and a mesh-100 powder. In  FIG. 10B , the coarse mesh-100 microstructure is on the right side and the fine mesh-325 microstructure is on the left side. 
       FIG. 11  is a simulation illustrating the fluid velocity through a porous electrode with and without a porous restriction layer  114  of the third embodiment. As can be seen in the lower simulation, the restriction layer provides much uniform flow velocity.  FIG. 12A  is a plot of the velocity through the electrode as a function of normalized distance (0-100%) along the electrode.  FIG. 12A  clearly shows that the addition of a porous restriction layer provides a more uniform flow velocity.  FIG. 12B  illustrates effect of using a porous restriction layer in combination with a gap on the flow velocity distribution. As can been seen in  FIG. 12B , the addition of the gap decreases the uniformity of the velocity distribution relative to the use of a restriction layer without a gap. However, the use of a restriction layer and a gap provides a more uniform velocity distribution than not using a restriction layer. 
       FIG. 13  below presents the electrochemical polarization curves at different charge and discharge current densities obtained with the porous electrodes made from different mesh sizes material. As can be seen  FIG. 13 , the electrode with the finer size shows lower overpotential for charge and much lower overpotentials for discharge current. The finer mesh size porous electrode has a much higher surface area. Thus, the finer mesh size porous electrode has a significantly higher electrochemical activity and superior voltaic efficiency for a given current density. 
     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.