How to make a bipolar plate

Abstract

Systems and methods are provided for a redox flow battery. In one embodiment, the redox flow battery includes a bipolar plate assembly including a bipolar plate formed from a thermoplastic composite material. The thermoplastic composite material of the bipolar plate allows the bipolar plate to be directly bonded to a dielectric frame of the bipolar plate assembly, thereby simplifying the fabrication process of the bipolar plate assembly.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 267,588, entitled "METHOD FOR BIPOLAR PLATE FABRICATION," filed February 4, 2022, and U.S. Provisional Application No. 63 / 269,515, entitled "METHOD FOR BIPOLAR PLATE FABRICATION," filed March 17, 2022. The entire contents of the above-identified applications are hereby incorporated by reference for all purposes.

[0002] The present description relates generally to redox flow battery systems. [Background technology]

[0003] Redox flow batteries are suitable for grid-scale energy storage applications due to their ability to scale power and capacity independently, and for charging and discharging for thousands of cycles with minimal loss of performance. A redox flow battery cell includes negative and positive electrodes, each connected to a bipolar plate. The bipolar plates may be made of highly conductive materials so that electrons can be transported to reaction sites on the negative and positive electrodes, and may further function as fluid separators for electrolyte flow and distribution.

[0004] In one example, a bipolar plate used in conjunction with a negative electrode in a plating electrode assembly may be formed from a graphite composite material. The graphite composite material may be formed from a thermosetting material via a compression molding process. Although the thermosetting material may be highly conductive, bonding of other components of the electrode assembly to the thermosetting material may suffer from various challenges. For example, the compression molding process may impose size and aspect ratio constraints on the bipolar plate, which may result in splitting the bipolar plate into two or more sections, each section being individually attached to the dielectric frame of the bipolar plate assembly. Furthermore, the thermosetting material may have a different coefficient of thermal expansion (CTE) than the material used to form the dielectric frame. As a result of the CTE difference, the bipolar plate may be attached to the dielectric frame via a rubber flange, adding cost and complexity to the fabrication process of the bipolar plate assembly. Furthermore, the thermosetting material may be brittle and prone to cracking under strain.

[0005] In one embodiment, the above-mentioned problems may be addressed by a bipolar plate assembly that includes a bipolar plate formed from a thermoplastic composite material. The thermoplastic composite material may include a thermoplastic polymer that is also used to form the dielectric frame. Thus, the thermoplastic composite bipolar plate may be bonded to the dielectric frame by a simple process such as welding or overmolding. Thus, the fabrication process may be simplified to produce a more robust bipolar plate assembly.

[0006] In this manner, the technical effect of increasing compatibility between the bipolar plate material and the dielectric frame material is achieved. The fabrication process of the electrode assembly is simplified due to the more compatible CTE with the dielectric frame material, the relatively low melting point, increased elasticity, and other material properties of the thermoplastic composite bipolar plate. The process may also eliminate the need to divide the bipolar plate into sections. Furthermore, the bipolar plate may be bonded directly to the dielectric frame, thereby eliminating the use of additional flange pieces to bond the bipolar plate to the dielectric frame. Furthermore, when assembled, the bipolar plate assembly incorporating the thermoplastic composite bipolar plate forms a robust unit that is resistant to cracking or breaking.

[0007] It should be appreciated that the foregoing summary is provided to introduce in a simplified form a selection of concepts that are further described in the Detailed Description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined solely by the claims that follow the Detailed Description. Moreover, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. [Brief description of the drawings]

[0008] [Figure 1] FIG. 1 shows a schematic diagram of an exemplary redox flow battery system including battery cells having bipolar plates. [Diagram 2] FIG. 2 illustrates a side view of an exemplary layout of the redox flow battery system of FIG. [Figure 3A] 1 shows an example image of a thermoplastic composite bipolar plate welded to a dielectric frame. [Figure 3B] FIG. 3B shows a detailed cross-sectional view of the thermoplastic composite bipolar plate of FIG. 3A. [Figure 4] 1 shows an example of a thermoplastic composite bipolar plate. [Figure 5A] 1 shows a first embodiment of bonding a thermoplastic bipolar plate to a dielectric frame. [Figure 5B] 1 shows a second embodiment of the bonding of a thermoplastic bipolar plate to a dielectric frame. [Figure 6] FIG. 2 shows an exploded view of a power module of the redox flow battery system. [Figure 7] 1 illustrates an example method for forming a thermoplastic bipolar plate assembly. [Figure 8] 1 shows a comparison of the in-plane resistivity measured for different types of bipolar plates. [Figure 9] 1 shows a comparison of through-plane resistivity for different types of bipolar plates. [Figure 10] 1 shows a graph depicting the variation in plating thickness at different locations on a thermoplastic bipolar plate. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] The following description relates to materials and methods for redox flow batteries. In one embodiment, the bipolar plate of the redox flow battery may be formed from a thermoplastic composite material. In another embodiment, a method for manufacturing the bipolar plate is provided. FIGS. 1-2 show schematics of a redox flow battery and a system including two or more redox flow batteries, respectively. FIGS. 3A-3B show a first example of a thermoplastic composite bipolar plate welded to a dielectric frame. A second example of a thermoplastic composite bipolar plate is depicted in FIG. 4, and bonding of the thermoplastic composite bipolar plate to a dielectric frame is illustrated in FIGS. 5A-5B. The thermoplastic composite bipolar plate and the dielectric frame fit into a larger electrode assembly. FIG. 6 shows an exploded view of the electrode assembly. FIG. 7 shows an example of a method for making and using the thermoplastic composite bipolar plate. FIGS. 8-10 show experimentally determined properties of the thermoplastic composite bipolar plate and an evaluation of its suitability as a bipolar plate.

[0010] 1, in the redox flow battery system 10, the negative electrode 26 may be referred to as the plating electrode and the positive electrode 28 may be referred to as the redox electrode. The negative electrolyte in the plating side (e.g., negative electrode compartment 20) of the redox flow battery cell 18 may be referred to as the plating electrolyte and the positive electrolyte on the redox side (e.g., positive electrode compartment 22) of the redox flow battery cell 18 may be referred to as the redox electrolyte.

[0011] "Anode" refers to an electrode at which an electroactive material loses electrons, and "cathode" refers to an electrode at which an electroactive material gains electrons. During battery charging, the negative electrolyte gains electrons at the negative electrode 26, which is the cathode of the electrochemical reaction. During battery discharging, the negative electrolyte loses electrons, and the negative electrode 26 is the anode of the electrochemical reaction. Alternatively, during battery discharging, the negative electrolyte and the negative electrode 26 may be referred to as the anolyte and anode, respectively, of the electrochemical reaction, while the positive electrolyte and the positive electrode 28 may be referred to as the catholyte and cathode, respectively, of the electrochemical reaction. During battery charging, the negative electrolyte and the negative electrode 26 may be referred to as the catholyte and cathode, respectively, of the electrochemical reaction, while the positive electrolyte and the positive electrode 28 may be referred to as the anolyte and anode, respectively, of the electrochemical reaction. For simplicity, the terms "positive" and "negative" are used herein to refer to the electrodes, electrolytes, and electrode compartments in a redox flow battery system.

[0012] One example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), in which the electrolyte includes iron ions in the form of iron salts (e.g., FeCl2, FeCl3, etc.) and the negative electrode 26 includes metallic iron. For example, ferrous iron (Fe 2+ ) gains two electrons during battery charging and becomes iron metal (Fe 0 ) on the negative electrode 26, and Fe 0 During battery discharge, it loses two electrons to become Fe 2+ At the positive electrode 28, Fe 2+ During battery charging, ferric iron (Fe 3+ ) and Fe 3+During battery discharge, it gains electrons to form Fe 2+ The electrochemical reactions are summarized in equations (1) and (2), where the forward reaction (left to right) represents the electrochemical reaction during battery charging, while the reverse reaction (right to left) represents the electrochemical reaction during battery discharging. Fe 2+ +2e - ←→Fe 0 -0.44V (negative electrode) (1) 2Fe 2+ ←→2Fe 3+ +2e - +0.77V (positive electrode) (2)

[0013] As mentioned above, the negative electrolyte used in the IFB is Fe 2+ accepts two electrons from the negative electrode 26 to form Fe 0 and plated onto the substrate. 2+ During battery discharge, the plated Fe 0 loses two electrons to become Fe 2+ and can dissolve back into the electrolyte. The equilibrium potential of the above reaction is -0.44 V, so this reaction provides the negative terminal for the desired system. On the positive side of the IFB, the electrolyte loses electrons and becomes Fe 3+ During battery charging, Fe is oxidized to 2+ During battery discharge, Fe provided by the electrolyte 3+ by absorbing electrons provided by the positive electrode 28. 2+ The equilibrium potential for this reaction is +0.77V, creating a positive terminal in the desired system.

[0014] The IFB may provide the ability to charge and recharge the electrolyte therein, in contrast to other battery types that utilize non-regenerative electrolytes. Charging may be accomplished by applying a current across electrodes 26 and 28 via terminals 40 and 42, respectively. The negative electrode 26 contains (e.g., Fe 2+ But Fe 3+The negative side of a voltage source may be electrically coupled via terminal 40 such that electrons may be delivered to the negative electrolyte via positive electrode 28 (as Fe is oxidized to Fe 2+ is reduced to form Fe on the (plating) substrate. 0 and Fe 2+ is plated onto the negative electrode 26.

[0015] Discharge is Fe 0 remains available in the negative electrolyte for oxidation, and Fe 3+ remains available in the positive electrolyte for reduction. 3+ Availability may be increased by adding additional Fe via an external source such as the external positive electrolyte chamber 52. 3+ This can be maintained by increasing the concentration or volume of positive electrolyte in the positive electrode compartment 22 of the redox flow battery cell 18 to provide ions. More generally, Fe 0 The availability of Fe can be an issue in IFB systems, where the Fe available for discharge 0 can be proportional to the surface area and volume of the negative electrode substrate and the plating efficiency. The charging capacity is 2+ As an example, Fe 2+ Availability is via an external source, such as the external negative electrolyte chamber 50, for additional Fe 2+ This can be maintained by providing ions to increase the concentration or volume of negative electrolyte to the negative electrode compartment 20 side of the redox flow battery cell 18.

[0016] In an IFB, the positive electrolyte may include ferrous, ferric, ferric complexes, or any combination thereof, while the negative electrolyte may include ferrous or ferrous complexes, depending on the state of charge (SOC) of the IFB system. As previously mentioned, the utilization of iron ions in both the negative and positive electrolytes may enable utilization of the same electrolyte species on both sides of the redox flow battery cell 18, which may reduce electrolyte cross-contamination and increase the efficiency of the IFB system, resulting in less electrolyte exchange compared to other redox flow battery systems.

[0017] Efficiency losses in the IFB can result from electrolyte crossover through the separator 24 (e.g., ion exchange membrane barrier, microporous membrane, etc.). For example, Fe in a positive electrolyte 3+ The ion is Fe 3+ The negative electrolyte can then be driven toward the negative electrolyte by the ion concentration gradient and the electrophoretic force across the separator 24. The Fe ions then penetrate the separator 24 and cross over into the negative electrode compartment 20. 3+ Fe ions crossing over from the low pH redox side (e.g., the more acidic positive electrode compartment 22) to the high pH plating side (e.g., the less acidic negative electrode compartment 20) can result in coulombic efficiency losses. 3+ The ions may result in the precipitation of Fe(OH)3. The precipitation of Fe(OH)3 may degrade the separator 24 and cause permanent battery performance and efficiency loss. For example, Fe(OH)3 precipitates may chemically foul the organic functional groups of the ion exchange membrane or physically clog the micropores of the ion exchange membrane. In either case, the Fe(OH)3 precipitates may cause the membrane's ohmic resistance to increase over time and reduce battery performance. The precipitates may be removed by washing the IFB with acid, but the constant maintenance and downtime may be disadvantageous for commercial battery applications. Furthermore, washing may depend on periodic preparation of the electrolyte, contributing to additional processing costs and complexity. Alternatively, the addition of certain organic acids to the positive and negative electrolytes in response to electrolyte pH changes may mitigate precipitate formation during battery charge and discharge cycling without driving up overall costs. Additionally, Fe3+ Implementing a membrane barrier that inhibits ionic crossover may also mitigate fouling.

[0018] Additional coulombic efficiency loss, H + (e.g., protons) and subsequent formation of H2 gas, and the reaction of protons in the negative electrode compartment 20 with electrons supplied to the plated iron metal of the negative electrode 26, may cause the formation of H2 gas.

[0019] IFB electrolytes (e.g., FeCl2, FeCl3, FeSO4, Fe2(SO4)3, etc.) may be readily available and may be produced at low cost. In one example, the IFB electrolyte may be formed from ferrous chloride (FeCl2), potassium chloride (KCl), manganese(II) chloride (MnCl2), and boric acid (H3BO3). The IFB electrolyte may provide a higher reuse value since the same electrolyte may be used for the negative and positive electrolytes, resulting in reduced cross-contamination issues compared to other systems. Furthermore, due to the electron configuration of iron, it may solidify into a generally uniform solid structure during plating onto the negative electrode substrate. In the case of zinc and other metals commonly used in hybrid redox batteries, solid dendritic structures may form during plating. The stable electrode morphology of the IFB system may increase the efficiency of the battery compared to other redox flow batteries. Still further, the iron redox flow battery may reduce the use of toxic raw materials and may operate at a relatively neutral pH compared to other redox flow battery electrolytes. Therefore, the IFB system may pose less environmental hazard than all other current advanced redox flow battery systems in production.

[0020] Continuing with FIG. 1, a schematic diagram of a redox flow battery system 10 is shown. The redox flow battery system 10 may include a redox flow battery cell 18 fluidly coupled to an integrated multi-chamber electrolyte storage tank 110. The redox flow battery cell 18 may include a negative electrode compartment 20, a separator 24, and a positive electrode compartment 22. The separator 24 may include an electrically insulating ionically conductive barrier that prevents bulk mixing of the positive and negative electrolytes while allowing conduction of certain ions. For example, as discussed above, the separator 24 may include an ion exchange membrane and / or a microporous membrane.

[0021] The negative electrode compartment 20 may include a negative electrode 26, and the negative electrolyte may include an electroactive material. The positive electrode compartment 22 may include a positive electrode 28, and the positive electrolyte may include an electroactive material. In some embodiments, multiple redox flow battery cells 18 may be combined in series or parallel to generate higher voltages or currents in the redox flow battery system 10.

[0022] 1 are negative and positive electrolyte pumps 30 and 32, both of which are used to pump electrolyte solution through the redox flow battery system 10. Electrolyte is stored in one or more tanks external to the cell and is pumped through the negative electrode compartment 20 and positive electrode compartment 22 sides of the redox flow battery cell 18 via the negative and positive electrolyte pumps 30 and 32, respectively.

[0023] The redox flow battery system 10 may also include a first bipolar plate 36 and a second bipolar plate 38, each positioned along a back side of the negative electrode 26 and the positive electrode 28, respectively, e.g., opposite the side facing the separator 24. The first bipolar plate 36 may be in contact with the negative electrode 26 and the second bipolar plate 38 may be in contact with the positive electrode 28. However, in other embodiments, the bipolar plates 36 and 38 may be arranged in close proximity to the electrodes 26 and 28, but spaced apart from the electrodes 26 and 28 and housed within the respective electrode compartments 20 and 22. In either case, the bipolar plates 36 and 38 may be electrically coupled to terminals 40 and 42, respectively, via direct contact with the negative and positive electrodes 26 and 28, respectively, or via the negative and positive electrodes 26 and 28, respectively. The IFB electrolyte may be transported to the reaction sites of the negative and positive electrodes 26 and 28 by the first and second bipolar plates 36 and 38 due to the conductive properties of the materials of the bipolar plates 36 and 38. The flow of electrolyte may also be assisted by the negative and positive electrolyte pumps 30 and 32 to facilitate forced convection through the redox flow battery cell 18. Reacted electrochemical species may also be directed away from the reaction sites by a combination of forced convection and the presence of the first and second bipolar plates 36 and 38.

[0024] As illustrated in FIG. 1 , the redox flow battery cell 18 may further include a negative battery terminal 40 and a positive battery terminal 42. When a charging current is applied to the battery terminals 40 and 42, the positive electrolyte may be oxidized (lose one or more electrons) at the positive electrode 28 and the negative electrolyte may be reduced (gain one or more electrons) at the negative electrode 26. During battery discharge, reverse redox reactions may occur on the electrodes 26 and 28. In other words, the positive electrolyte may be reduced (gain one or more electrons) at the positive electrode 28 and the negative electrolyte may be oxidized (lose one or more electrons) at the negative electrode 26. A potential difference across the battery may be maintained by electrochemical redox reactions in the positive electrode compartment 22 and the negative electrode compartment 20, which may induce a current through the current collectors while the reactions persist. The amount of energy stored by a redox battery may be limited by the amount of electroactive material available in the electrolyte for discharge, depending on the total amount of electrolyte and the solubility of the electroactive material.

[0025] The redox flow battery system 10 may further include an integrated multi-chamber electrolyte storage tank 110. The multi-chamber electrolyte storage tank 110 may be divided by a bulkhead 98. The bulkhead 98 may create multiple chambers within the multi-chamber electrolyte storage tank 110 such that both positive and negative electrolytes may be contained within a single tank. The negative electrolyte chamber 50 holds a negative electrolyte containing an electroactive material, and the positive electrolyte chamber 52 holds a positive electrolyte containing an electroactive material. The bulkhead 98 may be positioned within the multi-chamber electrolyte storage tank 110 to provide a desired volume ratio between the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In one embodiment, the bulkhead 98 may be positioned to set the volume ratio of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 according to a stoichiometric ratio between the negative redox reaction and the positive redox reaction. FIG. 1 further illustrates a fill height 112 of the multi-chamber electrolyte storage tank 110, which may indicate the liquid level within each tank compartment. 1 also shows a gas headspace 90 located above the fill height 112 of the negative electrolyte chamber 50, and a gas headspace 92 located above the fill height 112 of the positive electrolyte chamber 52. The gas headspace 92 may be utilized to store H2 gas that is generated during the operation of the redox flow battery (e.g., by the side reactions of proton reduction and iron corrosion) and is carried to the multi-chamber electrolyte storage tank 110 with electrolyte returning from the redox flow battery cell 18. The H2 gas may naturally separate at the gas-liquid interface (e.g., the fill height 112) in the multi-chamber electrolyte storage tank 110, thereby eliminating the need to have an additional gas-liquid separator as part of the redox flow battery system 10. Once separated from the electrolyte, the H2 gas may fill the gas headspaces 90 and 92. Thus, the stored H2 gas can help to purge other gases from the multi-chamber electrolyte storage tank 110, thereby acting as an inert gas blanket to reduce oxidation of electrolyte species, which can help reduce redox flow battery capacity loss.In this manner, utilizing an integrated multi-chamber electrolyte storage tank 110 may eliminate the need to have separate negative and positive electrolyte storage tanks, hydrogen storage tanks, and gas-liquid separators common to conventional redox flow battery systems, thereby simplifying the system design, reducing the physical footprint of the redox flow battery system 10, and reducing system costs.

[0026] FIG. 1 also shows a spillover hole 96, which may create an opening in a bulkhead 98 between the gas headspaces 90 and 92 and provide a means of equalizing gas pressure between the chambers 50 and 52. The spillover hole 96 may be located at a threshold height above the fill height 112. The spillover hole 96 may further enable the ability to self-balance the electrolyte in each of the negative and positive electrolyte chambers 50 and 52 in the event of a cell crossover. In the case of an all-iron redox flow battery system, the same electrolyte (Fe 2+ ) are used in both the negative and positive electrode compartments 20 and 22, so that electrolyte spillover between the negative electrolyte chamber 50 and the positive electrolyte chamber 52 may reduce overall system efficiency, but overall electrolyte composition, battery module performance, and battery module capacity may be maintained. Flange joints may be utilized for all plumbing connections between the inlet and outlet multi-chamber electrolyte storage tank 110 to maintain a leak-free, continuously pressurized condition. The multi-chamber electrolyte storage tank 110 may include at least one outlet from each of the negative and positive electrolyte chambers 50 and 52, and at least one inlet to each of the negative and positive electrolyte chambers 50 and 52. Additionally, one or more outlet connections may be provided from the gas headspaces 90 and 92 to direct H2 gas to the rebalancing reactors or cells 80 and 82.

[0027] Although not shown in FIG. 1 , the integrated multi-chamber electrolyte storage tank 110 may further include one or more heaters thermally coupled to each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In an alternative example, only one of the negative and positive electrolyte chambers 50 and 52 may include one or more heaters. In the case where only the positive electrolyte chamber 52 includes one or more heaters, the negative electrolyte may be heated by transferring heat generated in the redox flow battery cell 18 to the negative electrolyte. In this manner, the redox flow battery cell 18 may heat the negative electrolyte and facilitate temperature regulation of the negative electrolyte. The one or more heaters may be operated by the controller 88 to regulate the temperature of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 independently or together. For example, in response to the electrolyte temperature falling below a threshold temperature, the controller 88 may increase the power supplied to the one or more heaters such that the heat flux to the electrolyte may be increased. The electrolyte temperature may be indicated by one or more temperature sensors attached to the multi-chamber electrolyte storage tank 110, such as sensors 60 and 62. By way of example, the one or more heaters may include coil-type heaters or other immersion heaters immersed in the electrolyte fluid, or surface mantle-type heaters that conductively transfer heat through the walls of the negative and positive electrolyte chambers 50 and 52 to heat the fluid therein. Other known types of tank heaters may be used without departing from the scope of the present disclosure. Additionally, the controller 88 may deactivate one or more heaters in the negative and positive electrolyte chambers 50 and 52 in response to a liquid level below a solids fill threshold level. Stated differently, in some embodiments, the controller 88 may activate one or more heaters in the negative and positive electrolyte chambers 50 and 52 only in response to a liquid level above a solids fill threshold level. In this manner, activation of one or more heaters without sufficient liquid in the negative and / or positive electrolyte chambers 50, 52 may be avoided, thereby reducing the risk of overheating or burning out of the heater(s).

[0028] Still further, one or more inlet connections may be provided to each of the negative and positive electrolyte chambers 50 and 52 from a field hydration system (not shown). In this manner, the field hydration system may facilitate the start-up of the redox flow battery system 10, including installation, charging, and hydration of the redox flow battery system 10, at the end-use location. Furthermore, prior to the start-up of the redox flow battery system 10 at the end-use location, the redox flow battery system 10 may be dry assembled at a battery manufacturing facility different from the end-use location, without the need to charge and hydrate the redox flow battery system 10 before shipping it to the end-use location. In one embodiment, the end-use location may correspond to a location where the redox flow battery system 10 is installed and utilized for on-site energy storage. In other words, the redox flow battery system 10 may be designed such that once installed and hydrated at the end-use location, the location of the redox flow battery system 10 is fixed and the redox flow battery system 10 may no longer be considered a portable dry system. Thus, from an end user's perspective, the dry portable redox flow battery system 10 may be delivered to a site, after which the redox flow battery system 10 may be installed, hydrated, and commissioned. Prior to hydration, the redox flow battery system 10 may be referred to as a dry portable system, in which the redox flow battery system 10 does not include or is free of water and wet electrolyte. Once hydrated, the redox flow battery system 10 may be referred to as a wet non-portable system, in which the redox flow battery system 10 includes wet electrolyte.

[0029] 1, the electrolyte solution initially stored in the multi-chamber electrolyte storage tank 110 can be pumped throughout the redox flow battery system 10 via negative and positive electrolyte pumps 30 and 32. The electrolyte stored in the negative electrolyte chamber 50 can be pumped through the negative electrode compartment 20 side of the redox flow battery cell 18 via the negative electrolyte pump 30, and the electrolyte stored in the positive electrolyte chamber 52 can be pumped through the positive electrode compartment 22 side of the redox flow battery cell 18 via the positive electrolyte pump 32.

[0030] The electrolyte rebalancing reactors 80 and 82 may be connected in-line or in parallel with the electrolyte recirculation flow paths at the negative and positive sides of the redox flow battery cells 18 in the redox flow battery system 10, respectively. One or more rebalancing reactors may be connected in-line with the electrolyte recirculation flow paths at the negative and positive sides of the battery, and other rebalancing reactors may be connected in parallel for redundancy (e.g., the rebalancing reactors may be maintained without interrupting the battery and rebalancing operation) and for increased rebalancing capacity. In one embodiment, the electrolyte rebalancing reactors 80 and 82 may be disposed in the return flow paths from the negative and positive electrode compartments 20 and 22, respectively, to the negative and positive electrolyte chambers 50 and 52.

[0031] The electrolyte rebalancing reactors 80 and 82 can serve to rebalance electrolyte charge imbalances in the redox flow battery system 10 that occur due to side reactions, ionic crossover, etc., as described herein. In one embodiment, the rebalancing reactors 80 and 82 can include trickle bed reactors, where H2 gas and electrolyte can contact at a catalyst surface in a packed bed to perform the electrolyte rebalancing reaction. In another embodiment, the rebalancing reactors 80 and 82 can include flow-through reactors that can contact H2 gas and electrolyte liquid and perform the electrolyte rebalancing reaction without a packed catalyst bed.

[0032] During operation of the redox flow battery system 10, the sensors and probes may monitor and control electrolyte chemical properties such as electrolyte pH, concentration, SOC, etc. For example, as illustrated in FIG. 1, sensors 62 and 60 may be positioned to monitor the state of the positive and negative electrolytes in the positive and negative electrolyte chambers 52 and 50, respectively. In another example, the sensors 62 and 60 may each include one or more electrolyte level sensors to indicate the level of electrolyte in the positive and negative electrolyte chambers 52 and 50, respectively. As another example, the sensors 72 and 70, also illustrated in FIG. 1, may monitor the state of the positive and negative electrolytes in the positive and negative electrode compartments 22 and 20, respectively. The sensors 72 and 70 may be pH probes, optical probes, pressure sensors, voltage sensors, etc. It will be understood that sensors may be positioned elsewhere throughout the redox flow battery system 10 to monitor electrolyte chemical properties and other properties.

[0033] For example, sensors may be positioned in an external acid tank to monitor the amount of acid or pH of the external acid tank (not shown), and acid from the external acid tank may be supplied to the redox flow battery system 10 via an external pump (not shown) to reduce precipitate formation in the electrolyte. Additional external tanks and sensors may be installed to supply other additives to the redox flow battery system 10. For example, various sensors including temperature, conductivity, and level sensors of the field hydration system may send signals to the controller 88. Additionally, the controller 88 may send signals to actuators such as valves and pumps of the field hydration system during hydration of the redox flow battery system 10. The sensor information may be sent to the controller 88, which may then, by way of example, operate the pumps 30 and 32 to control electrolyte flow through the redox flow battery cells 18, or perform other control functions. In this manner, the controller 88 may respond to one or a combination of sensors and probes.

[0034] The redox flow battery system 10 may further include a source of H2 gas. In one embodiment, the source of H2 gas may include a separate dedicated hydrogen gas storage tank. In the embodiment of FIG. 1, H2 gas may be stored in and supplied from an integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supply additional H2 gas to the positive electrolyte chamber 52 and the negative electrolyte chamber 50. The integrated multi-chamber electrolyte storage tank 110 may alternately supply additional H2 gas to the inlets of the electrolyte rebalancing reactors 80 and 82. As an example, a mass flow meter or other flow control device (which may be controlled by the controller 88) may regulate the flow of H2 gas from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supplement the H2 gas generated within the redox flow battery system 10. For example, if a gas leak is detected in the redox flow battery system 10 or if the reduction reaction rate is too slow at low hydrogen partial pressure, H2 gas can be provided from the integrated multi-chamber electrolyte storage tank 110 to rebalance the SOC of the electroactive materials in the positive and negative electrolytes. As an example, the controller 88 can provide H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a measured change in pH or in response to a measured change in the SOC of the electrolyte or electroactive materials.

[0035] For example, an increase in pH in the negative electrolyte chamber 50 or the negative electrode compartment 20 may indicate that H2 is leaking from the redox flow battery system 10 and / or that the reaction rate is too slow at the available hydrogen partial pressure, and the controller 88 may increase the supply of H2 gas from the integrated multi-chamber electrolyte storage tank 110 to the redox flow battery system 10 in response to the increase in pH. As a further example, the controller 88 may supply H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a pH change that increases above a first threshold pH or decreases above a second threshold pH. In the case of an IFB, the controller 88 may supply additional H2 to increase the pH of the redox flow battery system 10.3+ This can increase the rate of reduction of ions and the rate of production of protons, thereby decreasing the pH of the positive electrolyte. Furthermore, the pH of the negative electrolyte can be increased by increasing the rate of Fe crossover from the positive electrolyte to the negative electrolyte. 3+ The pH of the negative electrolyte can be lowered by hydrogen reduction of the ion or by protons generated on the positive side crossing over into the negative electrolyte due to the proton concentration gradient and electrophoretic forces. In this way, the pH of the negative electrolyte is reduced by Fe(OH)3. 3+ The risk of precipitation of ions (crossing over from the positive electrode compartment 22) can be kept within a stable region.

[0036] Other control schemes may be implemented to control the rate of H2 gas supply from the integrated multi-chamber electrolyte storage tank 110 in response to changes in electrolyte pH or changes in electrolyte SOC detected by other sensors, such as an oxygen reduction potential (ORP) meter or optical sensor. Still further, the change in pH or SOC trigger action of the controller 88 may be based on a rate of change or a change measured over a period of time. The period of the rate of change may be predetermined or adjusted based on the time constant of the redox flow battery system 10. For example, if the recirculation rate is high, the period may be shortened and the time constant may be small so that local changes in concentration (e.g., due to side reactions or gas leaks) may be measured quickly.

[0037] It will be appreciated that all components except the sensors 60 and 62 and the integrated multi-chamber electrolyte storage tank 110 (and components included therein) may be considered to be included in the power module 120. Thus, the redox flow battery system 10 may be described as including the power module 120 fluidly coupled to the integrated multi-chamber electrolyte storage tank 110 and communicatively coupled to the sensors 60 and 62. In some embodiments, each of the power module 120 and the multi-chamber electrolyte storage tank 110 may be included in a single housing (not shown), such that the redox flow battery system 10 may be included as a single unit in a single location. It will be further appreciated that the positive electrolyte, the negative electrolyte, the sensors 60 and 62, the electrolyte rebalancing reactors 80 and 82, and the integrated multi-chamber electrolyte storage tank 110 (and components included therein) may be considered to be included in the electrolyte subsystem 130. Thus, the electrolyte subsystem 130 may provide one or more electrolytes to the redox flow battery cells 18 (and components included therein).

[0038] 2, a side view of an exemplary redox flow battery system layout 200 for the redox flow battery system 10 is shown. The redox flow battery system layout 200 may be housed within a housing 202 that facilitates long-distance transportation and delivery of the redox flow battery system 10. In some embodiments, the housing 202 may comprise a standard steel cargo container or cargo trailer that may be transported via rail, truck, or ship. The redox flow battery system layout 200 may include an integrated multi-chamber electrolyte storage tank 110, one or more rebalancing reactors (e.g., rebalancing reactor 80) positioned on a first side of the housing 202, a power module 210, and a power control system (PCS) 288 on a second side of the housing 202. Supports 206 and auxiliary components such as various tubing 204, pumps 230, valves (not shown in FIG. 2) may be included within the housing 202 (as further described above with reference to FIG. 1) to stabilize and fluidly connect the various components positioned therein. For example, one or more pumps 230 may be utilized to transport electrolyte from the integrated multi-chamber electrolyte storage tank 110 to one or more redox flow battery cell stacks 214 within the power module 210. Additionally, additional pumps 230 may be utilized to return electrolyte from the power module 210 to the negative electrolyte chamber 50 or the positive electrolyte chamber 52 of the integrated multi-chamber electrolyte storage tank 110.

[0039] The power module 210 may include one or more redox flow battery cell stacks 214 electrically connected in parallel and / or series. Each of the one or more redox flow battery cell stacks 214 may further include a plurality of redox flow battery cells, such as the redox flow battery cell 18 of FIG. 1, connected in parallel and / or series. In this manner, the power module 210 may be capable of supplying a range of currents and / or voltages to an external load. The PCS 288 includes the controller 88 of FIG. 1 and other electronics for controlling and monitoring the operation of the redox flow battery system 10. Furthermore, the PCS 288 may regulate and monitor the voltage supplied to the external load, as well as supply current and / or voltage from an external source for charging the power module 210. The PCS 288 may further regulate and control the operation of the redox flow battery system 10 during an idle state or mode. The redox flow battery system 10 in an idle state may include when the power module 210 is not in a charging mode or a discharging mode. As an example, the power module 210 may be in a charging mode when an external voltage or current is provided to one or more redox flow battery cells 18 of the power module 210, resulting in reduction of electrolyte at the bipolar plate 36 connected to the negative electrode(s) of the one or more redox flow battery cells 18 and plating of the reduced electrolyte. In the case of an IFB, ferrous ions may be reduced at the plating electrode(s) of the one or more redox flow battery cells 18, thereby plating iron therein during charging of the power module 210. As another example, the power module 210 may be in a discharging mode when a voltage or current is provided from one or more redox flow battery cells 18 of the power module 210, resulting in oxidation of the plated metal at the negative electrode, resulting in de-plating (e.g., loss of metal) and solubilization of the oxidized metal ions. In the case of an IFB, iron may be oxidized at the plating electrode(s) of the one or more redox flow battery cells 18, thereby solubilizing ferrous ions therein during discharging of the power module 210.

[0040] The redox flow battery system layout 200 includes many electrode assemblies, each having at least one bipolar plate assembly coupled to a plating electrode. In a conventional bipolar plate assembly, the bipolar plates of the bipolar plate assembly may be formed from a highly conductive thermosetting polymer composite material. The thermosetting polymer composite material may not be directly welded to the dielectric frame of the bipolar plate assembly, but instead may rely on a rubber flange for mounting. Furthermore, the thermosetting polymer composite material may be brittle and may crack during manufacturing of the bipolar plate assembly. In addition, the dielectric frame may be formed from a different material than the bipolar plates, such as a thermoplastic material. Alternatively, by incorporating bipolar plates made of the same or complementary thermoplastic material, the bipolar plates may be directly welded or overmolded to the dielectric frame, thereby simplifying the fabrication of the bipolar plate assembly. Additionally, thermoplastic composite bipolar plates may have improved flexibility and resilience leading to increased durability of the bipolar plates and reduced issues such as poor sealing and bonding, cracking, and size constraints during assembly.

[0041] In one embodiment, the bipolar plates of the bipolar plate assembly may be formed from a thermoplastic composite material. For example, a bipolar plate assembly 300 is depicted in FIG. 3A. The bipolar plate assembly 300 includes a plurality of bipolar plates 302, each of which is coupled to a bipolar plate frame 304. Each of the plurality of bipolar plates 302 may be formed from a thermoplastic composite material including a thermoplastic polymer mixed with a conductive material, such as a polypropylene / graphite composite material. As an example, the polypropylene / graphite may include a polypropylene (PP) base having 60% to 80% graphite by weight. As an alternative example, the polypropylene / graphite may include 40% to 95% graphite by weight. The bipolar plate frame 304, which may be a dielectric frame configured to store and polarize an electric charge, may be formed from a thermoplastic polymer, such as PP.

[0042] Incorporating at least a portion of the bipolar plate material as a thermoplastic polymer may broaden the methods for bonding the bipolar plate 302 to the bipolar plate frame 304, thereby allowing for simpler, more direct techniques and more robust attachment. For example, as opposed to bonding a bipolar plate formed from a thermoset material to a dielectric frame, the bipolar plate 302 may be directly welded and sealed to the bipolar plate frame 304, as shown in FIG. 3A, due to the common thermoplastic polymer used for both components. By using similar base resins for the components, the components may have similar melting points, allowing the components to physically adhere to one another.

[0043] In some examples, as illustrated in FIG. 3A, welding may result in waving of the surface of the bipolar plate 302. A more detailed view depicting the distortion, particularly at the inlet 306 of one of the bipolar plate 302 surfaces, is shown in FIG. 3B. The view shown in FIG. 3B corresponds to a portion of one of the bipolar plates 302 enclosed by box 305 in FIG. 3A. The wavy bipolar plate surface may result in uneven plating on the bipolar plate in some examples. However, other methods of bonding the bipolar plate 302 to the bipolar plate frame 304, such as injection molding, may reduce or mitigate the waviness. As another example, a plasma treatment may be applied to the bipolar plate 302 to enable the use of adhesives to bond the bipolar plate 302 to the bipolar plate frame 304. Despite the waviness in the bipolar plate 302, the bipolar plate assembly 300 can demonstrate the desired electrochemical performance of a redox battery cell, as shown in FIG. 10 and further described below.

[0044] Although the bipolar plate assembly 300 of FIG. 3A includes three bipolar plates 302, other embodiments may include different amounts of bipolar plates coupled to a bipolar plate frame. For example, as shown in FIG. 4, a single bipolar plate 400 may be used in the bipolar plate assembly, thereby allowing the bipolar plate 400 to have a larger width 402 and height 404 than, for example, each of the bipolar plates 302 of FIG. 3A. By incorporating bipolar plate assemblies into a redox flow battery cell with a reduced number of bipolar plates per assembly, manufacturing may be simplified (e.g., fewer component attachments leading to reduced manufacturing cycle times), less expensive welding equipment may be used, active bipolar plate surface area may be increased, and adverse effects of the attachment process, such as waviness due to welding, may be reduced.

[0045] The bipolar plate 400 may also be formed from a thermoplastic polymer (e.g., a thermoplastic composite), such as, but not limited to, PP, polyphenylene sulfide (PPS), polyphenylsulfone (PPSU), polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), or perfluoroalkoxyalkane (PFA), mixed with a conductive filler to form a thermoplastic composite. The thermoplastic composite bipolar plate 400 may be formed by methods known in the art for mass thermoplastic fabrication. Such methods may include, but are not limited to, injection molding, extrusion, thermoforming, and blow molding. In one embodiment, the thermoplastic composite bipolar plate 400 may be formed from 40% to 95% by weight of conductive filler and 5% to 60% by weight of thermoplastic polymer. As an alternative embodiment, the thermoplastic composite bipolar plate 400 may be formed from 60% to 80% by weight of conductive filler and 20% to 40% by weight of thermoplastic polymer. The conductive filler can be a conductive material compatible with redox flow battery electrolytes, such as carbon (including but not limited to carbon particles, carbon fibers, carbon black, graphene, and carbon nanotubes), titanium, titanium alloys, stainless steel, or any other conductive metal, element, or alloy. The conductive filler can have a number of different particle shapes, including but not limited to particulates, filaments, fibers, plates, tubes, and other crystalline structures. The inclusion of conductive fillers can result in bipolar plates having resistivities in the range of 0.01 to 15 Ω-mm. As a result of the incorporation of thermoplastic composite materials, the bipolar plates can be stronger and more resilient than bipolar plates formed from thermoset materials. For example, thermoplastic bipolar plates can have elastic moduli in the range of 0.8 GPa to 25 GPa, and yield strengths in the range of 15 MPa to 100 MPa.

[0046] Similar to the manufacture of thermosetting composite bipolar plates, the fabrication of thermoplastic composite bipolar plates can result in a thermoplastic resin-rich surface that, if left untreated, can lead to poor electrical conductivity at the surface. Thus, the surface conductivity of thermoplastic bipolar plates can be enhanced through known treatments to improve electrical conductivity in thermosetting composite bipolar plates. For example, surface treatments can be applied. The bipolar plate surface can be polished and textured by sanding, knurling, or milling. In addition, the electrical conductivity of thermoplastic composite bipolar plates can also be enhanced by embossing a texture on the surface of the bipolar plate, thereby interrupting the continuity of the surface. Furthermore, the properties due to the thermoplastic polymer of the bipolar plate can allow heated graphite particles to impinge on the surface, thereby forming conductive bridges across the resin-rich surface layer of the bipolar plate. Conductive bridges can also be formed by using longer lengths of graphite fibers and orienting the fibers outward along the bipolar plate.

[0047] Additionally, carbon particles may be impregnated into the surface of the thermoplastic composite bipolar plate. In one embodiment, the carbon particles may be formed from an ordered carbon material, such as graphite. Additionally or alternatively, the carbon particles may be formed from a glassy carbon material. The carbon particles may be roughly three-dimensional spherical particles having a diameter of 10 μm to 100 μm.

[0048] The carbon particles may be impregnated into the surface immediately after the thermoplastic molding process (e.g., extrusion) while the thermoplastic material is still hot and therefore sticky. The impregnation may include impacting or pushing the carbon particles onto the surface of the thermoplastic composite bipolar plate. As a result, the carbon particles may be located primarily at the surface of the thermoplastic composite bipolar plate. The carbon particles may be substantially evenly distributed across the surface. The surface density of the carbon particles (e.g., number of particles / m 2) may be such that each carbon particle may contact at least one other carbon particle and create a conductive bridge across the surface. The carbon particles may be located primarily at the surface, but a subset of the carbon particles may penetrate through the resin-rich surface of the bipolar plate during impregnation. Carbon particles that penetrate the resin-rich surface of the thermoplastic bipolar plate may create a conductive bridge from the resin-rich surface to the underlying, more conductive composite-rich layer.

[0049] Once fabricated and processed, the thermoplastic composite bipolar plate may be bonded to a dielectric frame. For example, FIG. 5A shows a bipolar plate assembly 500 including the bipolar plate 400 of FIG. 4 attached to a bipolar plate frame (e.g., dielectric frame) 502. The dielectric frame 502 may also be formed from a thermoplastic material or a thermoplastic composite material including PP, PPS, PPSU, PEEK, PTFE, and PFA. Unlike the bipolar plate 400, the thermoplastic composite material forming the dielectric frame 502 may include a blend of thermoplastic materials and may not include conductive fillers. Preferably, the dielectric frame 502 and the bipolar plate 400 are made from the same thermoplastic material or materials having similar CTEs. If the CTEs of the bipolar plate 400 and the dielectric frame 502 are different enough to cause distortion and reduced cell performance, an intermediate material may be used, as described below with reference to FIG. 5B.

[0050] The bipolar plate 400 may be formed such that the width 402 and height 404 correspond to the maximum allowable dimensions of the bipolar plate 400. In this manner, the bipolar plate 400 may be a single unit occupying the maximum allowable area within the dielectric frame 502. By forming the bipolar plate 400 from a thermoplastic material, the bipolar plate may be free of size and aspect ratio constraints imposed on thermoset-based bipolar plates that result from the manufacturing methods used to form the bipolar plate. Thus, the bipolar plate 400 may have larger dimensions and a wider range of aspect ratios than bipolar plates formed from thermoset materials. As shown in FIG. 5A, a seal 504 may be formed between the bipolar plate 400 and the dielectric frame 502. The seal 504 may be formed by welding the bipolar plate 400 to the dielectric frame 502, the thermoplastic materials of the components having similar melting points. In another embodiment, the bipolar plate 400 and the dielectric frame 502 may both be fabricated via injection molding, and the encapsulation 504 may be formed by an overmolding process. Both the encapsulation 504 and the overmolding may be examples of direct bonding between the bipolar plate 400 and the dielectric frame 502, both of which may be made possible by the matching material properties (e.g., CTE and / or melting point) of the bipolar plate 400 and the dielectric frame 502.

[0051] 5B, an alternative embodiment of a bipolar plate assembly 550 is shown, including a bipolar plate 400 coupled to a dielectric frame 502 via an intermediate bonding material. The bipolar plate assembly 550 may be used when the CTE of the bipolar plate 400 and the dielectric frame 502 are different. To correct for the difference in CTE, the bipolar plate 400 may be attached to a first edge of an intermediate flange 554 by a first seal 552. The intermediate flange 554 may be formed from a flexible thermoplastic material having a CTE between the bipolar plate 400 and the dielectric frame 502, such as thermoplastic rubber. The first seal 552 may be formed by thermal welding similar to the seal 504 of FIG. 5A. A second edge of the flange 554 opposite the first edge may be attached to the dielectric frame 502 by a second seal 556. The second seal 556 may also be formed by thermal welding. Alternatively, laser welding may be used instead of thermal welding. In this manner, the bipolar plate 400 may be attached to the dielectric frame 502 via the intermediate flange 554 .

[0052] Figure 6 shows an exploded view of one example of an electrode assembly 600. The components of the electrode assembly 600 may be included in a redox flow battery cell, such as the redox flow battery cell 18 of Figure 1. Multiple electrode assemblies, each configured similarly to the electrode assembly 600 of Figure 6, may form a redox flow battery cell stack, such as the redox flow battery cell stack 214 of Figure 2.

[0053] The electrode assembly 600 may include one or more bipolar plate assemblies 601, each formed from a dielectric frame 602 surrounding at least one thermoplastic composite bipolar plate 604. Additionally, a negative electrode spacer 606 may be bonded to the thermoplastic composite bipolar plate 604. In one embodiment, the negative electrode spacer 606 may also be a thermoplastic polymer. Thus, the thermoplastic bipolar plate and the negative electrode spacer may have similar melting points and may be joined by welding, simplifying the construction of the electrode assembly 600. A positive electrode 608 may be bonded to the composite bipolar plate 604 on the opposite side of the negative electrode spacer 606. In one embodiment, the positive electrode 608 may be formed from a felt, such as a carbon felt. In other embodiments, the positive electrode may also be any conductive material with a high surface area per unit volume that is also compatible with the electrolyte of the redox flow battery. Additionally, the positive electrode material may be selected based on electrochemical properties that are compatible with the electrochemistry of the redox flow battery. For example, the positive electrode can alternatively be a carbon-based paper, such as a gas diffusion layer (GDL) or reticulated carbon foam. A thermally driven welding process can also be used to attach the positive electrode 608 to the thermoplastic composite bipolar plate 604, resulting in high conduction between the two components and thus improving the performance of the redox flow battery. As another example, the positive electrode 608 and the thermoplastic composite bipolar plate 604 can be bonded using laser welding. Furthermore, as described above, bonding the negative electrode spacer, bipolar plate 604, and positive electrode 608 allows the electrode assembly 600 to be fabricated as a single unit, thereby simplifying the assembly process of the electrode assembly 600.

[0054] A membrane 610 surrounded by a membrane frame 616 may be positioned adjacent to the positive electrode 608, opposite the bipolar plate 604. A pressure plate 612 and a picture frame 614 may be disposed at either end of the electrode assembly 600. Together, the pressure plate 612 and the picture frame 614 may secure the components of the electrode assembly 600 to one another and may seal the fluid within the electrode assembly 600.

[0055] FIG. 7 illustrates a method 700 for manufacturing and incorporating a thermoplastic composite bipolar plate in a bipolar plate assembly. At 702, the method 700 includes forming a composite material by mixing a thermoplastic material with a conductive material. As described above, with reference to FIG. 4, the materials forming the thermoplastic composite material may be selected from several different thermoplastic polymers and carbon-based conductive materials. The weight ratio of the conductive material to the polymeric material may range from 2:3 to 1:4. Once synthesized, at 703, the method 700 includes forming the material into at least one bipolar plate having target dimensions according to the dimensions of an opening in a dielectric frame configured to receive the bipolar plate. Any known method for mass fabrication processes of thermoplastic materials, as described with reference to FIG. 4, may be used at 703. At 704, the method 700 includes treating a surface of the thermoplastic composite material of the bipolar plate to reduce resistivity (e.g., increase conductivity), as described above. For example, the surface treatment may include polishing, texturing, etc. Finally, the method 700 includes attaching the thermoplastic composite bipolar plate to a dielectric frame, at 705. As described above with reference to Figures 5A-5B, the thermoplastic composite bipolar plate may be welded or overmolded to the dielectric frame, or may be welded to an intermediate flange positioned between the bipolar plate and the dielectric flange and configured to couple the bipolar plate to the dielectric flange.

[0056] The resistivity of a bipolar plate may be measured as an indicator of the performance of the bipolar plate material. Low resistivity may be a desired property of a bipolar plate, since electrons are conducted from the electrodes to the surface of the bipolar plate to carry out the electrochemical reactions that drive the operation of the redox flow battery. In this way, the electrochemical reactions, including iron plating, may occur efficiently, thereby improving the efficiency of the redox flow battery. A graph 800 is depicted in FIG. 8, depicting the measured in-plane resistivity of bipolar plates made from different materials, including a thermoplastic polypropylene (PP) composite, and from a thermoset composite. The resistivity is plotted along the y-axis according to the type of bipolar plate and the surface treatment it has received. For example, first column 802 represents the resistivity of a PP bipolar plate without a surface treatment, measured along the direction in which the bipolar plate is extruded; second column 804 represents the resistivity of a machine sanded PP bipolar plate, similarly measured along the direction of extrusion; third column 806 represents the resistivity of an unsanded PP bipolar plate, measured along a direction transverse (e.g., perpendicular) to the extrusion direction; fourth column 808 represents the resistivity of a sanded PP bipolar plate, measured along the transverse direction; fifth column 810 represents the resistivity of a compression molded thermoset composite bipolar plate that suffers from orbital scuffing; and sixth column 812 represents the resistivity of the thermoset composite bipolar plate material based on manufacturer specifications.

[0057] The results shown in graph 800 show that even after undergoing a surface treatment, the in-plane resistivity of the PP bipolar plate is still higher (e.g., at least two times higher) than the in-plane resistivity of the PP bipolar plate without surface treatment (e.g., sixth column 812). The results shown in graph 800 also show that the extrusion direction has little effect on the in-plane resistivity of the extruded material, and the surface treatment has an equally small effect on reducing the resistivity in both directions. However, the resistivity of the PP bipolar plate is higher than that of the thermoset composite bipolar plate.

[0058] The inter-plane resistivity of the compression molded thermoset composite bipolar plate and the inter-plane resistivity of the extruded PP bipolar plate are shown in graph 900 of FIG. 9. Similar to graph 800 of FIG. 9, resistivity is plotted along the y-axis and sample types are presented as columns along the x-axis. Sample types include extruded PP bipolar plate (shown in first column 902), compression molded thermoset composite bipolar plate (shown in second column 904), and manufacturer's resistivity specification for thermoset composite bipolar plate (shown in third column 906). Similar to the in-plane resistivity, the inter-plane resistivity of the PP bipolar plate is higher than that of the thermoset composite bipolar plate.

[0059] Compared to thermosetting composite bipolar plates, the higher resistivity of the PP bipolar plate may indicate that the PP bipolar plate may accommodate a lower performance of the redox flow battery. However, the PP bipolar plate experimentally demonstrates a plating thickness and plating quality that meets the desired thickness and quality despite the higher resistivity. The performance of the bipolar plate may be measured by the yield, e.g., plating thickness, and uniformity, of the metal plating formed on the bipolar plate during the charging cycle of the redox flow battery system. For example, in an iron flow battery, when the redox flow battery operates in a charging mode, Fe 0 is plated on a bipolar plate coupled to the negative electrode.

[0060] The plating thickness and uniformity of the PP bipolar plate placed in the test redox flow battery cell after operation of the cell in a charging mode is shown in graph 1000 of FIG. 10. Graph 1000 shows the plating thickness measured on the PP bipolar plate at different locations of the PP bipolar plate after charging the test redox flow battery cell to 1.5 MC. The locations are located along the electrolyte inlet of the PP bipolar plate as shown by a first plot 1002, and along the electrolyte outlet of the PP bipolar plate as shown by a second plot 1004. The maximum difference between the thickness of the first plot 1002 and the thickness of the second plot 1004 is about 0.05 mm. The variation in plating thickness along each of the electrolyte inlet and outlet and across the entire width of the PP bipolar plate does not vary by more than 10%.

[0061] In this manner, fabrication of a bipolar plate assembly for a redox flow battery may be simplified without adversely affecting the performance of the redox flow battery. The bipolar plate assembly may include a bipolar plate formed from a thermoplastic composite material. The thermoplastic composite material may have properties, such as melting point and CTE, that are compatible with the properties of a bipolar plate frame, e.g., a dielectric frame, of the bipolar plate assembly. The bipolar plate may be directly welded to the bipolar plate frame, thereby eliminating the use of additional materials and mechanisms to bond and join the bipolar plate to the bipolar plate frame. Additionally, the thermoplastic composite bipolar plate may have desirable material properties, such as higher strength and flexibility than traditional thermoset materials for bipolar plates, resulting in a more robust bipolar plate assembly. Although the thermoplastic composite bipolar plate may demonstrate higher resistivity than bipolar plates formed from thermoset materials, implementations of the thermoplastic composite bipolar plate may exhibit at least comparable redox flow battery performance.

[0062] The present disclosure also provides a support for a redox flow battery comprising a bipolar plate assembly including a bipolar plate formed from a thermoplastic composite. In a first embodiment of the system, the thermoplastic composite is formed from a thermoplastic polymer and a conductive filler, the thermoplastic polymer comprising one or more of polypropylene (PP), polyphenylene sulfide (PPS), polyphenylsulfone (PPSU), polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), and perfluoroalkoxyalkane (PFA). In a second embodiment of the system, optionally including the first embodiment, the conductive filler is one or more of graphite, carbon fiber, carbon black, carbon nanotubes, graphene, titanium, titanium alloys, and conductive metals. In a third embodiment of the system, optionally including one or both of the first and second embodiments, the thermoplastic composite is formed from 5% to 60% of a thermoplastic polymer and 40% to 95% of a conductive filler. In a fourth embodiment of the system, optionally including one or more of the first to third embodiments, the bipolar plate is formed by one of injection molding, extrusion, thermoforming, and blow molding. In a fifth embodiment of the system, optionally including one or more of the first to fourth embodiments, the bipolar plate is coupled to the dielectric frame by one or more of thermal welding, laser welding, overmolding, adhesives, and intermediate flanges. In a sixth embodiment of the system, optionally including one or more of the first to fifth embodiments, the bipolar plate is divided into two or more sections, each section being coupled to the dielectric frame. In a seventh embodiment of the system, optionally including one or more of the first to sixth embodiments, the bipolar plate is coupled to the dielectric frame as a single unit having a maximum allowable dimension determined based on the dimensions of the dielectric frame.

[0063] The present disclosure also provides support for a method for forming a bipolar plate assembly, including forming a thermoplastic composite, forming a bipolar plate from the thermoplastic composite, treating a surface of the bipolar plate, and bonding the bipolar plate to a dielectric frame. In a first embodiment of the method, forming the thermoplastic composite includes forming a mixture of a thermoplastic polymer and a conductive filler. In a second embodiment of the method, optionally including the first embodiment, forming the bipolar plate from the thermoplastic composite includes forming the bipolar plate by a mass fabrication process. In a third embodiment of the method, optionally including one or both of the first and second embodiments, forming the bipolar plate from the thermoplastic composite includes forming conductive bridges from particles of the conductive filler in the thermoplastic composite, the conductive bridges being formed along a surface of the bipolar plate. In a fourth embodiment of the method, optionally including one or more of the first to third embodiments, treating a surface of the bipolar plate includes polishing and / or texturing to reduce resistivity of the thermoplastic composite and / or impregnating the surface with carbon particles having a diameter of 10 μm to 100 μm, and polishing and / or texturing includes one or more of sanding, knurling, milling, and embossing. In a fifth embodiment of the method, optionally including one or more of the first to fourth embodiments, bonding the bipolar plate includes one of directly welding the bipolar plate to the dielectric frame and welding the dielectric frame to a mid-flange, the mid-flange being welded to the dielectric frame, and the welding being one of thermal welding or laser welding.

[0064] The present disclosure also provides a support for a bipolar plate assembly for a redox flow battery, comprising a dielectric frame formed from a first thermoplastic composite and a bipolar plate coupled to the dielectric frame, the bipolar plate formed from a second thermoplastic composite, the first thermoplastic composite and the second thermoplastic composite having matching material properties that allow direct bonding of the bipolar plate to the dielectric frame. In a first embodiment of the system, the first thermoplastic composite and the second thermoplastic composite are formed from common thermoplastic polymers. In a second embodiment of the system, optionally including the first embodiment, the first thermoplastic composite and the second thermoplastic composite have similar melting points and thermal expansion coefficients. In a third embodiment of the system, optionally including one or both of the first and second embodiments, a negative electrode spacer is welded directly to the first side of the bipolar plate. In a fourth embodiment of the system, optionally including one or more or each of the first to third embodiments, the positive electrode is bonded to a second side of the bipolar plate by melting material of the positive electrode to the second side of the bipolar plate opposite the first side. In a fifth embodiment of the system, optionally including one or more or each of the first to fourth embodiments, the negative electrode spacer, the positive electrode and the bipolar plate assembly form an electrode assembly.

[0065] 4-6 show example configurations with relative positioning of various components. When elements are shown to be in direct contact or directly coupled to one another, then such elements may, at least in one embodiment, be referred to as being in direct contact or directly coupled, respectively. Similarly, elements shown contiguous or adjacent to one another may, at least in one embodiment, be contiguous or adjacent to one another, respectively. As one embodiment, components placed in surface-sharing contact with one another may be referred to as surface-sharing contact. As another embodiment, in at least one embodiment, elements positioned apart from one another have only space between them, and other components may not be so referred to. As yet another embodiment, elements shown above / below one another, opposite one another, or to the left / right of one another may be so referred to relative to one another. Additionally, as shown in the figures, the topmost element or point of an element may, at least in one embodiment, be referred to as the "top" of the component, and the bottommost element or point of an element may, at least in one embodiment, be referred to as the "bottom" of the component. As used herein, top / bottom, upper / lower, above / below may be relative to the vertical axis of the drawing and are used to describe the positioning of the elements of the drawing relative to one another. Thus, in one embodiment, elements shown above other elements are positioned vertically above the other elements. As yet another example, the shapes of elements depicted in the drawings may be referred to as having those shapes (e.g., circular, rectilinear, planar, curvilinear, rounded, chamfered, angled, etc.). Additionally, elements shown intersecting one another may, at least in one embodiment, be referred to as intersecting elements or elements intersecting one another. Still further, in one embodiment, elements shown within or outside of another element may be so referred to. Although Figures 4-6 are drawn approximately to scale, other dimensions or relative dimensions may be used.

[0066] The following claims particularly point out certain combinations and subcombinations that are deemed novel and unobvious. These claims may refer to "an" element or "first" element or equivalents. Such claims should be understood to include the incorporation of one or more such elements, and do not require or exclude two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed through amendment of the claims or through the presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, are also deemed to be included within the subject matter of this disclosure.

Claims

1. A redox flow battery, 1. A redox flow battery comprising a bipolar plate assembly including a bipolar plate formed from a thermoplastic composite material.

2. 2. The redox flow battery of claim 1, wherein the thermoplastic composite is formed from a thermoplastic polymer and a conductive filler, the thermoplastic polymer comprising one or more of polypropylene (PP), polyphenylene sulfide (PPS), polyphenylsulfone (PPSU), polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE), and perfluoroalkoxyalkane (PFA).

3. 3. The redox flow battery of claim 2, wherein the conductive filler is one or more of graphite, carbon fiber, carbon black, carbon nanotubes, graphene, titanium, titanium alloys, and conductive metals.

4. 4. The redox flow battery of claim 3, wherein the thermoplastic composite is formed from 5% to 60% of the thermoplastic polymer and 40% to 95% of the conductive filler.

5. 10. The redox flow battery of claim 1, wherein the bipolar plate is formed by one of injection molding, extrusion, thermoforming, and blow molding.

6. 10. The redox flow battery of claim 1, wherein the bipolar plate is coupled to a dielectric frame by one or more of thermal welding, laser welding, overmolding, adhesives, and mid-flanges.

7. 7. The redox flow battery of claim 6, wherein the bipolar plate is divided into two or more sections, each section being bonded to the dielectric frame.

8. 7. The redox flow battery of claim 6, wherein the bipolar plate is bonded to the dielectric frame as a single unit having maximum allowable dimensions determined based on the dimensions of the dielectric frame.

9. 1. A method for forming a bipolar plate assembly, comprising: forming a thermoplastic composite; forming a bipolar plate from the thermoplastic composite material; and treating a surface of the bipolar plate; and bonding the bipolar plate to a dielectric frame.

10. The method of claim 9 , wherein forming the thermoplastic composite material comprises forming a mixture of a thermoplastic polymer and a conductive filler.

11. The method of claim 9 , wherein forming the bipolar plate from the thermoplastic composite material comprises forming the bipolar plate by a mass fabrication process.

12. 10. The method of claim 9, wherein forming the bipolar plate from the thermoplastic composite material includes forming conductive bridges from conductive filler particles of the thermoplastic composite material, the conductive bridges being formed along a surface of the bipolar plate.

13. 10. The method of claim 9, wherein treating the surface of the bipolar plate comprises polishing and / or texturing to reduce resistivity of the thermoplastic composite and / or impregnating the surface with carbon particles having a diameter of 10 μm to 100 μm, and wherein the polishing and / or texturing comprises one or more of sanding, knurling, milling, and embossing.

14. 10. The method of claim 9, wherein coupling the bipolar plate includes one of: directly welding the bipolar plate to the dielectric frame; and welding the dielectric frame to an intermediate flange, the intermediate flange being welded to the dielectric frame, the welding being one of thermal welding or laser welding.

15. 1. A bipolar plate assembly for a redox flow battery, comprising: a dielectric frame formed from a first thermoplastic composite material; a bipolar plate coupled to the dielectric frame, the bipolar plate being formed from a second thermoplastic composite material, the first thermoplastic composite material and the second thermoplastic composite material having matching material properties that enable direct bonding of the bipolar plate to the dielectric frame.

16. The bipolar plate assembly of claim 15 , wherein the first thermoplastic composite material and the second thermoplastic composite material are formed from a common thermoplastic polymer.

17. The bipolar plate assembly of claim 15 , wherein the first thermoplastic composite material and the second thermoplastic composite material have similar melting points and coefficients of thermal expansion.

18. The bipolar plate assembly of claim 15 , wherein a negative electrode spacer is welded directly to a first side of the bipolar plate.

19. 20. The bipolar plate assembly of claim 18, wherein a positive electrode is bonded to the second side of the bipolar plate by melting material of the positive electrode to the second side of the bipolar plate opposite the first side.

20. 20. The bipolar plate assembly of claim 19, wherein the negative electrode spacer, the positive electrode and the bipolar plate assembly form an electrode assembly.

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