Electrochemical flow system, redox flow battery, and electrochemical reactor

Abstract

An electrochemical flow system is disclosed. In one arrangement, the electrochemical flow system comprises a plurality of flow cells. Each of the flow cells is configured to allow flow of an electrolyte through the flow cell and to contact the electrolyte in the flow cell with one or more electrodes. A flow management system is also provided, comprising a conduit arrangement comprising a first flow unit, the flow cells, and a second flow unit. The conduit arrangement is configured to guide a flow of the electrolyte from the first flow unit through the flow cells and into the second flow unit along an electrolyte flow path. A flow driving arrangement is also provided, configured to drive the flow of the electrolyte along the electrolyte flow path. The flow management system is configured to provide one or more insulating buffers, each insulating buffer spanning the electrolyte flow path upstream or downstream of the plurality of flow cells. The insulating buffer spans the flow path such that, during the flow of the electrolyte through the flow cells, electrolyte located within each flow cell is electrically separated from electrolyte in the other flow cells. This electrical separation is at least via paths through electrolyte respectively upstream or downstream of the plurality of flow cells, by the insulating buffer blocking the paths.

Description

[0001] ELECTROCHEMICAL FLOW SYSTEM, REDOX FLOW BATTERY, AND

[0002] ELECTROCHEMICAL REACTOR

[0003] The present disclosure relates to electrochemical flow systems, particularly flow systems that aim to reduce or remove parasitic shunt currents. The flow systems may be used in the context of redox flow batteries and / or electrochemical reactors.

[0004] Batteries used in static energy storage scenarios are typically lithium-ion or lead- acid batteries. Lead-acid batteries typically have short lifetimes in hot climates, and the lifetime of lithium-ion batteries is also shortened. Fully discharging both types further shortens their lifetime. Lithium-ion batteries for static applications require many (e.g., thousands of) individual cells and hence require complex control systems. Over-charging and over-discharging a lithium-ion battery is harmful and can cause thermal runaway and fire.

[0005] Redox flow batteries (RFBs) are designed for bulk energy storage with systems typically ranging from 100 kWh to 100 MWh capacity and 10 kW to 10 MW power. They differ from conventional batteries by storing their active materials in external electrolyte reservoirs. The active material is pumped through stacks of cells during operation. Engineering challenges associated with this include complex cell and fluid flow circuits that must balance pressure drop, cell sealing and mass transport. The soluble lead flow battery (SLFB) differs from other flow batteries as it uses the same active material, solvated Pb2+ions, for both electrode reactions. This means it only requires a single electrolyte. Without the requirement to maintain separation of positive and negative electrolyte there is the possibility of using an open cell / stack architecture, either incorporating the cells within the electrolyte reservoir or externally with a simplified flow circuit.

[0006] There are numerous bipolar stack designs for flow batteries; however, all of the current solutions use a fully enclosed cell chamber. Current designs require complex engineering flow channels and inter-cell sealing. They also result in significant pressure drop and associated pumping losses. The structure of a standard flow battery includes a number of flow cells. An electrolyte will flow into the cells and react with a positive and negative electrode located within the flow cell to either charge, or discharge. The flow cells can be positioned fluidically in parallel to each other, so the electrolyte flows into each cell simultaneously, and the flow cells are electrically in series. Alternatively, the flow cells can be positioned fluidically in series, so the electrolyte flows into each cell in series, and the flow cells are electrically in parallel.

[0007] A problem common to existing flow batteries, as well as many other forms of electrochemical systems is the occurrence of parasitic shunt currents that flow between series cells via the conductive electrolyte path. These lead to reduced coulombic efficiency, and in RFBs involving solid phase deposition, cause uneven deposits. An existing method of reducing shunt currents is to include long, narrow flow channels from the manifold to the stack to increase the resistance to ionic current. However, this increases the power required to circulate the electrolyte, due to friction.

[0008] It is an object of the present disclosure to improve the performance of electrochemical flow systems.

[0009] According to an aspect of the invention, there is provided an electrochemical flow system comprising: a plurality of flow cells, each flow cell configured to allow flow of an electrolyte through the flow cell and to contact the electrolyte in the flow cell with one or more electrodes; and a flow management system comprising: a conduit arrangement comprising a first flow unit, the flow cells, and a second flow unit, the conduit arrangement configured to guide a flow of the electrolyte from the first flow unit through the flow cells and into the second flow unit along an electrolyte flow path; and a flow driving arrangement configured to drive the flow of the electrolyte along the electrolyte flow path, wherein: the flow management system is configured to provide one or more insulating buffers, each insulating buffer spanning the electrolyte flow path upstream or downstream of the plurality of flow cells such that, during the flow of the electrolyte through the flow cells, electrolyte located within each flow cell is electrically separated from electrolyte in the other flow cells, at least via paths through electrolyte respectively upstream or downstream of the plurality of flow cells, by the insulating buffer blocking the paths.

[0010] Thus, a system is provided in which an insulating buffer spans the electrolyte flow path upstream or downstream of the flow cells and reduces or prevents parasitic shunt currents. Parasitic shunt currents are currents associated with electrical paths between different flow cells that bypass the electrodes, thereby reducing efficiency.

[0011] In an embodiment, each of one or more of the insulating buffers comprises a gaseous region. The inventors have demonstrated that using a gaseous region to provide the insulating buffer can be implemented in a mechanically simple and robust manner and / or without excessively inhibiting flow of the electrolyte.

[0012] In an embodiment, either or both of the first flow unit and the second flow unit comprises a flow constraining arrangement configured to convert the flow of the electrolyte into a flow of electrolyte droplets that allow the electrolyte to pass through the gaseous region without providing a continuous liquid connection across the gaseous region. The use of such a flow constraining arrangement, which may for example comprise a plurality of nozzles (which may be referred to as a showerhead outlet), can be implemented using mechanically simple and robust elements, thereby promoting low cost and high reliability. Such flow constraining arrangements can also be configured to be easily replaceable and / or removed for cleaning. The gaps between droplets provide electrical separations that prevent shunt currents without significantly increasing pumping requirements.

[0013] In an embodiment, the gaseous region is fluidically connected to the atmosphere. This allows the pressure within the cell to be the same pressure as the pressure in the surrounding environment, which promotes pressure stability and reduces or avoids pressure-related variations in flow. Having the gaseous region be fluidically connected to the atmosphere can also be described as having an open cell. The gaseous region being connected to the atmosphere also simplifies the design and removes significant engineering challenges and design complexity associated with cell sealing. It also allows access to the cells without dismantling the stack, which is useful if a blockage occurs.

[0014] In an embodiment, the conduit arrangement comprises a flow trap configured such that a flow of the electrolyte through the flow trap and thereby through the flow cells is allowed when a level of the electrolyte in the plurality of flow cells is above the electrodes and is prevented when the level of the electrolyte in the plurality of flow cells is below at least a portion of one of the electrodes. The flow trap provides a mechanically simple and reliable way of controlling a level of the electrolyte upstream of the flow trap, for example to ensure that the electrolyte level does not fall below a reference level. Arranging for the reference level to be above an electrode means that the electrode will remain fully submerged within the electrolyte, maximising the available active area of the electrodes. This prevents a reduction in flow immediately impacting the efficiency of the system. In an embodiment, the flow management system is configured to maintain the gaseous region of one or more of the insulating buffers at a pressure different from atmospheric pressure. Controlling the pressure of one or more of the gaseous regions may provide improved flexibility for controlling flow of electrolyte.

[0015] In an embodiment, the flow management system is configured to maintain the gaseous region of one of the insulating buffers upstream of the flow cells at a pressure above atmospheric pressure. Providing a higher pressure upstream may advantageously increase a flow rate of the electrolyte.

[0016] In an embodiment, each of the plurality of flow cells comprises an electrode region and an electrolyte holding region, wherein the electrode region is a region of the flow cell located between the electrodes and the electrolyte holding region is a region of the flow cell located outside of the region between the electrodes. In an embodiment, the electrolyte holding region is above the electrode region of the flow cell and the electrolyte holding region is electrically separated from the electrolyte holding regions of the plurality of other flow cells by the insulating buffer. This arrangement allows a reserve volume of electrolyte to be held above the electrodes. The reserve volume may ensure that the electrodes remain submerged even if the rate of flow of electrolyte is temporarily reduced or halted, thereby promoting operational robustness. The insulating buffer may be provided upstream of the electrolyte holding region.

[0017] In an embodiment, each electrolyte holding region of the plurality of flow cells further comprises an overflow channel configured to allow electrolyte to flow out of the flow cell without flowing through the electrode region when the level of electrolyte in the electrolyte holding region exceeds a reference level. The overflow channel prevents disruption of the insulating buffer by preventing the overflow causing a flow of electrolyte into a neighbouring cell.

[0018] In an embodiment, each of one or more of the insulating buffers comprises a solid barrier member actuatable between a blocking state and a passing state. The blocking state is configured to block the flow of the electrolyte past the barrier member and the passing state is configured to allow passage of the flow of the electrolyte past the barrier member. The system is configured to periodically switch the barrier member between the blocking state and the passing state to allow the flow of the electrolyte through the flow cells in batches. Using such a solid barrier member provides robust electrical separation and protection against shunt currents. The actuatable barrier allows flow of the electrolyte to be controlled with high flexibility and / or high precision.

[0019] In an embodiment, each of one or more of the insulating buffers comprises an insulating liquid immiscible with the electrolyte; and the flow management system is configured to drive the flow of electrolyte in a flow comprising batches of the electrolyte interleaved in series by batches of the insulating liquid. Using such interleaved batches of electrolyte and insulating liquid provides reliable electrical separation between the batches and can be controlled in a flexible manner (e.g., by varying volumes and / or separations of the batches).

[0020] In an embodiment, the flow management system comprises one or more flush valves, each flush valve configured to be selectively openable to remove products produced by the electrochemical reaction occurring in a respective one or plurality of flow cells, the flush valve being optionally located beneath the respective one or plurality of flow cells. Provision of such flush valves allows deposits that form within the cell and fall downwards to be removed easily and reliably, thereby preventing blockages and / or reduced system performance. Alternatively or additionally, the flush valves promote easy and / or efficient flushing of the system to remove electrolyte.

[0021] In an embodiment, the one or more electrodes comprise one or more bipolar electrodes, each bipolar electrode being configured to act as a positive electrode of a first flow cell of the plurality of flow cells and a negative electrode of a second flow cell of the plurality of flow cells, the second flow cell being adjacent to the first flow cell. The flow cells are provided in a stack configured to allow electrolyte to flow through the flow cells in parallel, and each bipolar electrode separates a respective pair of adjacent flow cells in the stack. Sharing electrodes between different flow cells may reduce size and / or cost. Sharing electrodes between different flow cells may also improve the distribution of current over the electrode. Flow cells can be positioned closer together to save space. Fewer electrodes are required overall.

[0022] In an embodiment, there is provided a redox flow battery or electrochemical reactor comprising an electrochemical flow system according to any embodiment of the present disclosure. The use of the electrochemical flow system provides improved performance of redox flow batteries and / or electrochemical reactors. Using the soluble lead flow battery as an example, techno-economic modelling has shown that a soluble lead flow battery with this configuration could eventually have capital costs of approximately half that of an equivalent lithium-ion battery.

[0023] Embodiments of the disclosure will now be further described, merely by way of example, with reference to the accompanying drawings.

[0024] Fig. l is a schematic side view of an example electrochemical flow system configured to operate with a single electrolyte.

[0025] Fig. 2 is a schematic end view of the system of Fig. 1 showing a gaseous region acting as an insulating buffer.

[0026] Fig. 3 is a schematic end view of a variation on the system of Fig. 1 and 2 configured to drive flow of electrolyte from an entry point to an exit point above the entry point.

[0027] Fig. 4 is a schematic end view of an example electrochemical flow system in which insulating buffers are implemented using actuatable solid barrier members.

[0028] Fig. 5 is a schematic side view of an example electrochemical flow system configured to operate with two electrolytes.

[0029] Fig. 6 is a schematic end view of the system of Fig. 5.

[0030] Fig. 7 is a schematic end view of an example electrochemical flow system in which electrolyte flows in series through a sequence of flow cells.

[0031] Various examples of an electrochemical flow system 1 are described below with reference to the figures.

[0032] As depicted in Fig. 1 and 2, the flow system comprises a plurality of flow cells 11. Each flow cell 11 is configured to allow flow of an electrolyte through the flow cell 11. The electrolyte is contacted with (i.e., is brought into contact with) one or more electrodes while the electrolyte is in the flow cell 11. The one or more electrodes 111 may thus be located within the flow cell 11 and / or may comprise at least one face (electrode surface) that protrudes and / or faces into the flow cell 11. When in contact with an electrode 111, the electrolyte may interact with the electrode 111 to generate a current. The one or more electrodes 111 may provide positive and negative electrode surfaces. In some arrangements, as exemplified in Fig. 1 and 2, the electrodes 111 comprise parallel planar plates. In other arrangements, the shapes and / or positions and / or alignments of the electrodes 111 may be different. For example, the electrodes 111 may comprise two or more plates that are non-parallel with respect to each other and / or non-planar. The electrodes 111 will typically be metallic and the electrolyte will typically be an electrically conductive liquid.

[0033] The flow system further comprises a flow management system having a conduit arrangement and a flow driving arrangement. The conduit arrangement comprises a first flow unit 1211, the plurality of flow cells 11 and a second flow unit 1212. The conduit arrangement is configured to guide a flow of the electrolyte. The flow of the electrolyte includes flow from the first flow unit 1211, through the flow cells 11 and into the second flow unit 1212 along an electrolyte flow path.

[0034] As shown in Fig. 1 and 2, the first flow unit 1211 may comprise an inlet manifold 1216. The inlet manifold 1216 may be configured to receive electrolyte for multiple flow cells 11 and to guide a flow of the electrolyte into the multiple flow cells 11.

[0035] In the example of Fig. 1 and 2, the conduit arrangement is configured to guide flow of electrolyte around a closed loop. The first flow unit 1211 and second flow unit 1212 are thus fluidically connected to each other both through the flow cells 11 and through a separate return path 16. In other arrangements, the conduit arrangement is configured to provide for the electrolyte to flow between the first and second flow units 1211, 1212 in a backwards and forwards manner through the flow cells 11 (i.e., without any separate return path). This may be achieved for example in a gravity fed system in which the conduit arrangement is mounted rotatably and can be driven to switch by rotation between a state in which the first flow unit 1211 is above the second flow unit 1212 to a state in which the second flow unit 1212 is above the first flow unit 1211. The flow management system may be arranged to cause the electrolyte to flow through the flow cells 11 at a range of different rates depending on the particular application. Typically, an average linear velocity may be between about 0 and 100 eras'1, preferably between about 1 and 10 eras'1.

[0036] The flow management system comprises a flow driving arrangement. The flow driving arrangement drives the flow of the electrolyte along the electrolyte flow path in the flow system. As exemplified in Fig. 1 and 2, in some arrangements the flow driving arrangement comprises a pump 123 configured to drive the flow. Alternatively or additionally, the flow driving arrangement may use gravity to drive the flow, at least for a portion of the electrolyte flow path. The flow may be driven exclusively using gravity, as in the gravity fed system mentioned above that uses a rotatable conduit arrangement.

[0037] Alternatively, as depicted in Fig. 1 and 2, the flow driving arrangement may use gravity to drive the flow only through a portion of the flow path, such as a portion of the flow path comprising the flow cells 11.

[0038] The flow management system is configured to provide one or more insulating buffers 122. The insulating buffers 122 span the electrolyte flow path upstream, downstream, or upstream and downstream of the plurality of flow cells 11. When electrolyte flows through the flow cells 11, the or each insulating buffer 122 electrically separates the electrolyte within each flow cell 11 from electrolyte located within other flow cells 11. In the example of Fig. 1 and 2, insulating buffers 122 are provided as gaseous regions upstream and downstream of the flow cells 11. When an insulating buffer 122 is located upstream of the flow cells 11, electrolyte located within each flow cell 11 is electrically separated from electrolyte in the other flow cells 11, at least via paths upstream of the plurality of flow cells 11, by the insulating buffer 122 blocking the paths upstream of the plurality of flow cells 11. Thus, the electrical separation may involve blocking of paths between flow cells 11 through electrolyte located upstream. When an insulating buffer 122 is located downstream of the flow cells 11, electrolyte located within each flow cell 11 is electrically separated from electrolyte in the plurality of other flow cells 11, at least via paths downstream of the plurality of flow cells 11, by the insulating buffer 122 blocking the paths downstream of the plurality of flow cells 11. Thus, the electrical separation may involve blocking of paths between flow cells 11 through electrolyte located downstream. The electrical separation reduces or prevents current flow between two separate flow cells 11 through the electrolyte, which are examples of the undesirable parasitic shunt currents mentioned in the introduction. The prevention of such currents increases the efficiency of the system, as less energy is lost to these currents.

[0039] As exemplified in Fig. 1 and 2, each of one or more of the insulating buffers 122 may comprise a gaseous region. Each such gaseous region may comprise an insulating gas and electrically separates the electrolyte located within each flow cell 11 from electrolyte in other flow cells 11. Either or both of the first flow unit 1211 and the second flow unit 1212 may comprise a flow constraining arrangement configured to convert the flow of the electrolyte into a flow of electrolyte droplets 131 that allow the electrolyte to pass through the gaseous region without providing a continuous liquid connection across the gaseous region. The flow constraining arrangement constrains flow (i.e., provides a localized increase in flow resistance) such that the electrolyte droplets 131 are formed when the electrolyte falls (or is driven) into the gaseous region from the flow constraining arrangement. The droplets 131 are separated from each other such that there is no continuous liquid connection across the gaseous region via the droplets. The lack of a continuous liquid connection means there is no path for current to flow efficiently across the gaseous region and / or between different flow cells 11 via the gaseous region. The formation of droplets 131 allows a continual or quasi-continual flow of electrolyte while maintaining the desired electrical separation provided by the gaseous region. The flow constraining arrangement may comprise one or a plurality of nozzles, for example in the form of a showerhead outlet. In the example of Fig. 1 and 2, a flow constraining arrangement in the first flow unit 1211 (upstream of the flow cells 11) comprises a plurality of nozzles. Droplets 131 can thus be formed simultaneously at different positions across a surface of the flow constraining arrangement facing the flow cells 11 and pass across the gaseous region in parallel. In the example shown, a flow constraining arrangement is also provided in the second flow unit 1212, which may also comprise a plurality of nozzles.

[0040] In some arrangements, the gaseous region of one or more insulating buffers 122 is fluidically connected to the atmosphere. Arranging for the gaseous region to be fluidically connected to the atmosphere ensures that the pressure on at least one side of the flow cells 11 is constant across all flow cells 11. This approach may promote a uniform rate of flow through the flow cells 11. Maintaining a uniform flow across the flow cells 11 may promote efficient operation, for example by avoiding under- or overfilling of individual flow cells 11.

[0041] In some arrangements, the gaseous region is upstream of the flow cells 11 and the first flow unit 1211 is spaced apart from the flow cells 11 to at least partially define an opening providing the fluidic connection to the atmosphere. The first flow unit 1211 may be offset from the flow cells 11. Alternatively or additionally, the first flow unit 1211 may be detachable to allow the first flow unit 1211 to be removed. Configuring the first flow unit 1211 to be offset and / or detachable may provide easier access to the interior of the flow cells 11, for example to facilitate maintenance operations.

[0042] In some arrangements, the conduit arrangement comprises a flow trap 24. In the example of Fig. 1 and 2, the flow trap 24 is provided as part of the second flow unit 1212 downstream of the flow cells 11. The flow trap 24 is configured such that a flow of the electrolyte through the flow trap 24 and thereby through the flow cells is allowed when a level of the electrolyte in the plurality of flow cells 11 is above the electrodes 111 and is prevented when the level of the electrolyte in the plurality of flow cells 11 is below at least a portion of one of the electrodes 111. Alternatively, flow of the electrolyte is prevented when the level of electrolyte in the plurality of flow cells 11 is below a reference level located above the entirety of the electrodes. In the example of Fig. 2, the flow trap 24 comprises an upflow portion 21 and a downflow portion 22 positioned adjacent to (i.e., extending over a range of heights overlapping with) the electrodes 111. The upflow portion 21 and the downflow portion 22 may be connected to each by an elbow portion 23. The elbow portion 23 may be a highest portion of the flow trap 24. Either or both of the upflow portion 21 and the downflow portion 22 may be substantially straight and / or vertical. A lower end of the upflow portion 21 is fluidically connected to the bottom of each of the flow cells 11 to allow electrolyte to flow from the flow cells 11 into the upflow portion. Electrolyte then flows up the upflow portion 21, around the elbow portion 23 and down the downflow portion 22, eventually exiting the flow trap 24 at a lower end of the downflow portion 22. Flow is prevented by the flow trap 24 when the level of electrolyte in the flow cells 11 is too low to allow the electrolyte to flow around the elbow portion 23. Preferably, the flow trap 24 is configured to keep electrolyte flows from different flow cells 11 separate until after the electrolyte has left the flow trap to reduce or prevent parasitic shunt currents.

[0043] The flow trap 24 ensures that a minimum amount of electrolyte remains in the flow cell 11 by preventing the flow of electrolyte when the level of electrolyte within the flow cell 11 falls too low. The electrodes 111 thus remain at least partially (ideally fully) submerged during use even when there are variations in the rate of flow of electrolyte into the flow cells 11.

[0044] In some arrangements, the flow management system is configured to maintain the gaseous region of one or more of the insulating buffers 122 at a pressure different from atmospheric pressure. Maintaining a gaseous region at a pressure different from atmospheric pressure may be used to control a flow rate. The flow management system may for example be configured to maintain the gaseous region of an insulating buffer 122 upstream of the flow cells 11 at a pressure above atmospheric pressure to increase a flow rate through the flow cells 11. In some arrangements, as depicted in Fig. 2, each of the plurality of flow cells 11 comprises an electrode region 113 and an electrolyte holding region 114. The electrode region 113 is a region of the flow cell 11 (i.e., a portion of the volume within the flow cell 11 through which electrolyte passes) located between the electrodes 111. The electrolyte holding region 114 is a region of the flow cell 11 located outside of the region between the electrodes 111 (i.e., a different portion of the volume within the flow cell 11 through which electrolyte passes). The electrolyte holding regions 114 may be above the electrode regions

[0045] 113. Electrolyte in each electrolyte holding region 114 is electrically separated from electrolyte in other electrolyte holding regions 114 by the insulating buffer 122. Electrolyte in the electrolyte holding regions 114 may, for example, face into a gaseous region forming the insulating buffer 122. When an electrolyte holding region 114 is not completely full with electrolyte, the electrolyte holding region 114 may define a portion of the volume delimiting the gaseous region of the insulating buffer 122. The electrolyte holding regions 114 provide a reserve volume for electrolyte that allows the electrodes 111 to stay appropriately (e.g., fully) submerged and / or avoid overflows when there are variations in the flow rate into and / or out of the flow cells 11. The electrolyte holding regions 114 can accommodate variations in flow in which the flow rate into the flow cells 11 is higher than a flow rate out of the flow cells 11 and / or in which the flow rate out of the flow cells 11 is higher than a flow rate into the flow cells 11.

[0046] In the example of Fig. 1 and 2, flow cells 11 having electrolyte holding regions 114 are provided in combination with a flow constraining arrangement configured to form droplets 131 upstream of the electrolyte holding regions 114. In this case, the droplets 131 fall into the electrolyte holding region 114.

[0047] In some arrangements, as depicted in Fig. 2, each of the electrolyte holding regions 114 comprises an overflow channel 117. The overflow channel 117 is configured to allow electrolyte to flow out of the flow cell 11, without flowing through the electrode region

[0048] 113, when the level of electrolyte in the electrolyte holding region 114 exceeds a reference level. As depicted in Fig. 2, the overflow channel 117 may for example comprise a conduit leading to an opening 118 in a lateral wall defining the electrolyte holding region

[0049] 114. The position of the opening 118 defines the reference level. Electrolyte will flow into the conduit of the overflow channel 117 when the level of electrolyte reaches the opening 118. The overflow channel 117 prevents electrolyte from spilling out above the electrolyte holding regions 114, which could compromise the integrity of the gaseous region and / or disrupt the electrical separation between different flow cells 11 provided by the gaseous region.

[0050] In some arrangements, as depicted in Fig. 2, the flow management system comprises one or more flush valves 1215. Each flush valve 1215 is configured to be selectively openable to remove products produced by electrochemical reactions in a respective one or plurality of flow cells 11. Each flush valve 1215 may be located beneath, optionally directly beneath, the respective one or plurality of flow cells 11. Opening the one or more flush valves 1215 causes the flow cell or flow cells 11 to be flushed, which may involve a relatively rapid flow of electrolyte out of the flow cell or flow cells that carries with it the products produced by the electrochemical reactions and / or other unwanted material (which may be referred to as a sludge or residue) that may have built up in the flow cells 11. The one or more flush valves 1215 can be periodically opened, for example every 10-100 charge / discharge cycles, to flush the flow cells 11 in this manner. The one or more flush valves 1215 also allow electrolyte to be drained from the flow cells 11 when this is desired.

[0051] Alternatively or additionally, the second flow unit 1212 may comprises a trapdoor outlet (not shown). The trapdoor outlet may be actuatable between a normal state and a maintenance state. During the normal state, the trapdoor outlet is configured to act as a small outlet (i.e., providing relatively high flow resistance), which may be used for example to regulate electrolyte flow out of the flow cells 11. During the maintenance step, the trapdoor outlet is configured to open wider (i.e., to provide a lower flow resistance); optionally the trapdoor outlet opens fully. Opening the trapdoor wider and / or fully causes the flow cells 11 to be flushed as described above. The flow system can be configured to cause the trapdoor outlet to periodically transition between the maintenance state and the normal state. Alternatively, the trapdoor outlet can be configured to open into the maintenance state on command and / or manually, for example to allow manual electrode cleaning (e.g., scraping). The trapdoor outlet facilitates access for such cleaning operations in comparison to alternative arrangements involving outlets that cannot be opened to the same extent.

[0052] The flow management system may further comprise one or more electrolyte reservoirs 1210 configured to store the electrolyte. The reservoir or reservoirs 1210 increase the amount of electrolyte that can be stored in the flow system. The electrolyte may flow between the reservoir 1210 and the first flow unit 1211 and / or second flow unit 1212. The amount of electrolyte stored within the flow system may be proportional to the total energy stored within the flow system. Increasing the amount of electrolyte using such reservoirs 1210 therefore increases the energy storage capacity of the flow system.

[0053] In the example of Fig. 1 and 2, a single reservoir 1210 is provided below the flow cells 11 and flow trap 24. This is an example of a class of arrangement in which the flow cells 11 are positioned above at least one reservoir 1210. Positioning the flow cells 11 above a reservoir 1210 facilitates provision of an insulating buffer 122 in the form of a gaseous region in a flow path downstream of the flow cells 11 between the flow cells 11 and the reservoir 1210. In such arrangements, the second flow unit 1212 may comprise a flow constraining arrangement (e.g., showerhead outlet etc.) configured to convert the flow of electrolyte into a flow of electrolyte droplets 131. The flow constraining arrangement of the second flow unit 1212 may take any of the forms described above for the flow constraining arrangement of the first flow unit 1211. In some arrangements, debris produced within the plurality of flow cells 11 may also fall into the reservoir 1210 where it can be processed (e.g., removed by filtering, re-dissolved into the electrolyte etc.).

[0054] The flow driving arrangement may be configured to drive the flow of the electrolyte such that an entry region where the flow enters each flow cell 11 is located above an exit region where the flow leaves the flow cell 11. The flow of electrolyte flows through the flow cell 11 from the entry region to the exit region, optionally substantially vertically. In some arrangements, as exemplified in Fig. 1 and 2, the flow of the electrolyte is driven from the entry region to the exit region predominantly (or exclusively) by gravity. Alternatively, the flow driving arrangement may be configured to drive the flow of the electrolyte such that an entry region where the flow enters each flow cell 11 is located below an exit region where the flow leaves the flow cell. In such arrangements a pump or gravity (see Fig. 3) may be used to drive the flow through the flow cells 11.

[0055] In some arrangements, including the example of Fig. 1 and 2, the one or more electrodes 111 may comprise one or more bipolar electrodes. Each of the one or more bipolar electrodes is configured to act as a positive electrode of a first flow cell 11 of the plurality of flow cells 11 and a negative electrode of a second flow cell 11 of the plurality of flow cells 11. The second flow cell 11 is positioned adjacent to the first flow cell 11. The plurality of flow cells 11 may be provided in a stack configured to allow electrolyte to flow through the flow cells 11 in parallel. An example of such a stack of flow cells 11 is shown in Fig. 1 and 2. The stack of flow cells 11 may be defined by substantially identical and / or planar electrodes 111 that are parallel to each other, spaced evenly apart along a direction perpendicular to the planes of the electrodes 111, and / or aligned with other along the direction perpendicular to the planes of the electrodes 111. In the example of Fig. 1 and 2, each of the electrodes 111 is a bipolar electrode and separates a respective pair of adjacent flow cells 11 in the stack. Each bipolar electrode thus provide a positive electrode surface to one flow cell 11 and a negative electrode surface to an adjacent flow cell 11. Each bipolar electrode may be configured to separate a respective pair of adjacent flow cells 11. Forming a stack of flow cells 11 separated by bipolar electrodes in this manner may reduce the size and complexity of the flow system. Fewer components are required for each flow cell 11 due to the sharing of bipolar electrodes. The sharing of bipolar electrodes also allows the flow cells 11 to be positioned closer together than if each flow cell 11 had two individual electrodes 111, thereby improving compactness. The sharing of bipolar electrodes equally improves the current distribution over the electrode.

[0056] Fig. 3 depicts a variation on the arrangement of Fig. 1 and 2 in which the electrolyte flows vertically upwards through the flow cells 11 rather than vertically downwards. A flow constraining arrangement in the inlet manifold 1216 forms droplets 131 of electrolyte that pass across an insulating buffer 122 in the form of a gaseous region in a similar way to the arrangement of Fig. 1 and 2 described above. However, unlike the arrangement of Fig. 1 and 2, the droplets 131 fall into a standpipe 50 fluidically connected to the flow cells 11 at the bottom of the flow cells 11. An outlet 115 configured to allow electrolyte to flow out of the flow cells 11 is provided above the electrodes 111 but below an upper limit of the standpipe 50. When the level of electrolyte in the standpipe 50 is higher than the outlet 115, gravity drives flow of the electrolyte vertically upwards through the flow cells 11 and out of the outlet 115, thereby providing the desired flow of electrolyte past the electrodes 111. The combination of the standpipe 50 providing a connection to the flow cells 11 at the bottom of the flow cells and the outlet 115 located above the electrodes 111 provides a flow trap having functionality corresponding to the flow trap 24 described above with reference to Fig. 1 and 2. Flow of the electrolyte through the flow trap (i.e., through the flow cells 11 in this case) is allowed when a level of the electrolyte in the plurality of flow cells 11 is above the electrodes 111 and is prevented when the level of the electrolyte in the plurality of flow cells 11 is below at least a portion of one of the electrodes 111.

[0057] Fig. 4 depicts a further variation on the arrangements of Fig. 1 and 2 in which the insulating buffer 122 is implemented upstream and downstream of the flow cells 11 by a respective solid barrier member (i.e., a solid body) instead of a gaseous region. Each barrier member is electrically insulating. Each barrier member is actuatable between a blocking state and a passing state. The blocking state is configured to block flow of the electrolyte past the barrier member, thereby electrically isolating electrolyte on one side of the barrier member from electrolyte on the other side of the barrier member. The passing state is configured to allow flow of the electrolyte past the barrier member. When in this state, electrolyte can enter the flow cell 11, exit the flow cell 11 or enter and exit the flow cell 11. The flow system may be configured to periodically switch the barrier member between the blocking state and the passing state to allow flow of the electrolyte through the flow cells 11 in batches. In the example of Fig. 4 where insulating buffers 122 comprising barrier members are provided upstream and downstream of the flow cells 11, the barrier members would be switched in unison such that both remain in the same state as each other (i.e., both in the passing state or both in the blocking state). The duration of the passing state can be selected according to user requirements. Longer passing states will allow larger batches of electrolyte to flow but losses due to parasitic shunt currents may be higher than shorter passing states because the barrier member may not fully prevent such currents while in the passing state. In some arrangements, the barrier member may be implemented as a valve. The passing state of the barrier member is provided when the valve is open and the blocking state of the barrier member is provided when the valve is closed. The valve can be periodically transitioned between the open and closed states to allow the electrolyte to flow through the flow cells 11 in batches.

[0058] The example of Fig. 4 differs further from the example of Fig. 1 and 2 in that the electrolyte is configured to be pumped upwards through the flow cells 11 by a pump 123 rather than the flow through the flow cells 11 being driven by gravity. An inlet manifold 1216 is provided below the flow cells 11. An outlet manifold 1217 is provided above the flow cells 11. Other arrangements using solid barrier members may, however, be configured to drive the flow downwards through the flow cells 11, driven predominantly or exclusively by gravity for example.

[0059] The example of Fig. 1 shows a stack of flow cells configured to allow the electrolyte to flow though the flow cells 11 in parallel. A variation of the example shown in Fig. 1 can include two or more stacks of flow cells, positioned either fluidically in parallel or in series with each other. For example, two stacks of flow cells positioned fluidically in series would connect the output of an upstream first stack of flow cells to the input of a downstream second stack of flow cells. The electrolyte flow paths between the stacks must carry more electrolyte than the electrolyte flow paths between individual flow cells 11 of the same stack. The connection between stacks of flow cells (such as a pipe) therefore tends to be larger and so have a lower resistance. Shunt currents can therefore have a greater impact. Current techniques attempt to mitigate shunt currents between stacks of flow cells through the use of very long pipe lengths between stacks. However, using long pipe length can become expensive, and increases the size of the system. Applying an insulating buffer 122 between the stacks of flow cells can therefore significantly reduce shunt currents even without use of an insulating buffer 112 between individual flow cells 11.

[0060] Fig. 5 and 6 depict a further variation on the arrangement of Fig. 1 and 2 in which the flow system is configured to handle two different electrolytes, referred to below as an electrolyte and a further electrolyte. In the example shown, each flow cell 11 comprises a first chamber 31 containing a first electrode 111, a second chamber 32 containing a second electrode 111, and a membrane 33 separating the first chamber 31 from the second chamber 32. For clarity of depiction, only the rightmost first and second chambers 31, 32 are labelled in Fig. 5. The membrane 33 prevents mixing of electrolyte in the first chamber 31 with electrolyte (e.g., a “further electrolyte”) in the second chamber 32. The membrane 33 allows for the movement of ions between the first chamber 31 and second chamber 32.

[0061] The flow management system comprises a conduit arrangement and a further conduit arrangement. The conduit arrangement may comprise first and second flow units 1211 and 1212 and take any of the forms described above with reference to Fig. 1-4. The further conduit arrangement comprises a third flow unit 1213 and a fourth flow unit 1214. For clarity of depiction, only portions of the first to fourth flow units 1211-1214 corresponding to the rightmost first and second chambers 31, 32 are labelled in Fig. 5. The flow management system is configured to drive a flow of an electrolyte from the first flow unit 1211 through the flow cells 11 and into the second flow unit 1212 along an electrolyte flow path. The flow management system is configured to drive a further electrolyte from the third flow unit 1213 through the flow cells 11 and into the fourth flow unit 1214 along a further electrolyte flow path. The third flow unit 1213 may comprise a further inlet manifold 1218 that is shared between each of the plurality of flow cells 11 and guides the flow of the further electrolyte into the individual flow cells 11 from the third flow unit 1213. The first flow unit 1211 and second flow unit 1212 are configured to guide and receive flow of electrolyte respectively to and from the first chambers 31 of the flow cells 11. The third flow unit 1213 and fourth flow unit 1214 are configured to guide and receive flow of the further electrolyte respectively to and from the second chambers 32 of the flow cells 11. The electrolyte flows through the first chambers 31 and the further electrolyte flows through the second chambers 32.

[0062] The flow management system may be configured to provide one or more insulating buffers 122 spanning the further flow path upstream or downstream of the plurality of flow cells 11. During the flow of the further electrolyte through the flow cells 11, further electrolyte located within each flow cell 11 is electrically separated from further electrolyte located in the other flow cells 11. When an insulating buffer 122 is located upstream of the flow cells 11, the further electrolyte is electrically separated from further electrolyte in other flow cells 11 at least via paths through further electrolyte upstream. The electrical separation may involve the insulating buffer 122 blocking any path through the further electrolyte located upstream of the plurality of flow cells 11. When an insulating buffer 122 is located downstream of the flow cells 11, the further electrolyte is electrically separated from further electrolyte in other flow cells 11 at least via paths through further electrolyte downstream of the plurality of flow cells 11. The electrical separation may involve the insulating buffer 122 blocking any path through the further electrolyte located downstream.

[0063] In the example of Fig. 6 a flow constraining arrangement is located upstream of the flow cells 11 to form droplets 131 of electrolyte and droplets 132 of further electrolyte to allow the electrolyte and further electrolyte to pass through an insulating buffer 122 in the form of a gaseous region before entering the respective chambers 31 and 32 of the flow cells 11. The example of Fig. 6 also includes a flow constraining arrangement located downstream of the flow cells 11 to form droplets 131 of electrolyte and droplets 132 of further electrolyte to allow the electrolyte and further electrolyte to pass through an insulating buffer 122 in the form of a gaseous region provided downstream of the flow cells 11.

[0064] As depicted in Fig. 6, the flow system in this example comprises two reservoirs 1210 respectively holding the two different electrolytes (the electrolyte and the further electrolyte). The leftmost reservoir 1210 contains the electrolyte and the rightmost reservoir contains the further electrolyte. The conduit arrangement and the further conduit arrangement may be fluidically separated from each to prevent intermixing between the different electrolytes. Separate pumps 123 are provided to drive flow of the electrolytes through the respective conduit arrangements. Each flow cell 11 comprises two fluidically separated electrode regions 113 for the two respective electrolytes and two fluidically separated electrolyte holding regions 114 for the two respective electrolytes. The electrode regions 113 and electrolyte holding regions 114 may take any of the forms described above with reference to Fig. 1 and 2.

[0065] Fig. 7 depicts a further variation on the flow system of Fig. 1 and 2 in which flow cells 11 are connected fluidically in series rather than in parallel. The flow management system may therefore be configured to guide flow of electrolyte through the flow cells 11 in sequence one after the other. In the example shown, the electrolyte flows through the uppermost flow cell 11 first, then through the middle flow cell 11, and finally through the lowermost flow cell 11. Three flow cells 11 in series are depicted in Fig. 7 as an example; fewer or more than three flow cells 11 may be provided in series in other arrangements. The electrolyte is driven by pump 123 to flow in a circuit comprising the sequence of flow cells 11 and a reservoir 1210. In arrangements of this type, the flow management system may be configured to provide insulating buffers 122 that span the electrolyte flow path upstream, downstream, or upstream and downstream of each of the plurality of flow cells 11. In the example shown, insulating buffers 122 comprising gaseous bodies are provided upstream and downstream of each of the flow cells 11 in the series. As described above with reference to Fig. 1 and 2, flow constraining arrangements may be configured to form droplets 131 of electrolyte that pass across each insulating buffer 122. In the example of Fig. 7, an insulating buffer 122 in the form of a gaseous region is provided upstream of each flow cell 11. In the example shown, the flow system comprises unipolar rather than bipolar electrodes 111, which provides more freedom for positioning the flow cells 11 in a convenient manner to achieve the series flow. In other arrangements, bipolar electrodes are used, for example where the flow cells 11 are provided in a stack arrangement such as that described above with reference to Fig. 1 and 2 (but configured to allow for a series rather than parallel flow through the flow cells 11).

[0066] In other arrangements, each of one or more of the insulating buffers 122 may comprise an electrically insulating liquid. The insulating liquid may be immiscible with the electrolyte. The insulating liquid may be configured to electrically separate the electrolyte within each of the flow cells 11 from electrolyte in the other flow cells. The flow management system may, for example, be configured to drive the flow of electrolyte in a flow comprising batches of the electrolyte interleaved in series by batches of the insulating liquid. The insulating liquid prevents any significant electrical current from flowing between different batches of the electrolyte. By spacing the electrolyte batches apart sufficiently it is possible to ensure that there is no continuous liquid path through the electrolyte between different flow cells 11 in use.

[0067] In some arrangements, the flow management system is configured to provide insulating buffers of different types configured to span the electrolyte flow path at different location. The different types of insulating buffer may comprise any of the examples given above, such as an insulating buffer comprising a gaseous region, an insulating buffer comprising a solid barrier member and / or an insulating buffer comprising an insulating liquid.

[0068] In some arrangements, the flow cell 11 may be submerged in electrolyte within an electrolyte reservoir 1210, such that at least portions of the electrodes 111 are below the level of electrolyte within the reservoir 1210. Submerging the flow cells 11 will prevent the level of electrolyte in the flow cells 11 from falling below the electrodes 111 as long as the level of electrolyte in the reservoir 1210 is maintained at a suitable level. Shunt currents may be reduced or avoided by arranging for the electrolyte to flow through the flow cells 11 in batches, for example as described above with reference to Fig. 4.

[0069] The flow system of the present disclosure can advantageously be used in any scenario where it is desired to reduce shunt currents. For example, a redox flow battery or electrochemical reactor may be provided that comprises the flow system. As depicted in Fig. 1 and 7, the flow system may be connected to a power supply / load 14 that produces a current through the flow system or has a current produced through it by the flow system. In the case of a redox flow battery, the battery may for example be charged using the flow system with a current density of about 1-100 mA cm'2. In one particular implementation, during such charging, solid lead and lead dioxide may form on the negative and positive electrodes. The stack of flow cells 11 may be discharged at a current density of about 1- 200 mA cm'2. As shown in Fig. 1, the flow system may be connected to a current collector 15 made from an electrically conductive material such as copper. The current collector 15 allows the electrodes to be connected to an external circuit. The current collector 15 may be located on the outermost electrodes.

Claims

CLAIMS1. An electrochemical flow system comprising: a plurality of flow cells, each flow cell configured to allow flow of an electrolyte through the flow cell and to contact the electrolyte in the flow cell with one or more electrodes; and a flow management system comprising: a conduit arrangement comprising a first flow unit, the flow cells, and a second flow unit, the conduit arrangement configured to guide a flow of the electrolyte from the first flow unit through the flow cells and into the second flow unit along an electrolyte flow path; and a flow driving arrangement configured to drive the flow of the electrolyte along the electrolyte flow path, wherein: the flow management system is configured to provide one or more insulating buffers, each insulating buffer spanning the electrolyte flow path upstream or downstream of the plurality of flow cells such that, during the flow of the electrolyte through the flow cells, electrolyte located within each flow cell is electrically separated from electrolyte in the other flow cells, at least via paths through electrolyte respectively upstream or downstream of the plurality of flow cells, by the insulating buffer blocking the paths.

2. The system of claim 1, wherein each of one or more of the insulating buffers comprises a gaseous region.

3. The system of claim 2, wherein either or both of the first flow unit and the second flow unit comprises a flow constraining arrangement configured to convert the flow of the electrolyte into a flow of electrolyte droplets that allow the electrolyte to pass through the gaseous region without providing a continuous liquid connection across the gaseous region.

4. The system of claim 2 or 3, wherein the gaseous region is fluidically connected to the atmosphere.

5. The system of claim 4, wherein the gaseous region is upstream of the flow cells and the first flow unit is spaced apart from the flow cells to at least partially define an opening providing the fluidic connection to the atmosphere.

6. The system of any of claims 2 to 5, wherein the conduit arrangement comprises a flow trap configured such that a flow of the electrolyte through the flow trap and thereby through the flow cells is allowed when a level of the electrolyte in the plurality of flow cells is above the electrodes and is prevented when the level of the electrolyte in the plurality of flow cells is below at least a portion of one of the electrodes.

7. The system of any of claims 2 to 6, wherein the flow management system is configured to maintain the gaseous region of one or more of the insulating buffers at a pressure different from atmospheric pressure.

8. The system of claim 7, wherein the flow management system is configured to maintain the gaseous region of one of the insulating buffers upstream of the flow cells at a pressure above atmospheric pressure.

9. The system of any of claims 2 to 8, wherein each of the plurality of flow cells comprises an electrode region and an electrolyte holding region, wherein the electrode region is a region of the flow cell located between the electrodes and the electrolyte holding region is a region of the flow cell located outside of the region between the electrodes.

10. The system of claim 9, wherein the electrolyte holding region of the flow cell is located above the electrode region of the flow cell and the electrolyte holding region is electrically separated from the electrolyte holding regions of the plurality of other flow cells by the insulating buffer.

11. The system of claim 10, wherein each electrolyte holding region of the plurality of flow cells further comprises an overflow channel configured to allow electrolyte toflow out of the flow cell without flowing through the electrode region when the level of electrolyte in the electrolyte holding region exceeds a reference level.

12. The system of any preceding claim, wherein: each of one or more of the insulating buffers comprises a solid barrier member actuatable between a blocking state and a passing state; the blocking state is configured to block flow of the electrolyte past the barrier member; the passing state is configured to allow flow of the electrolyte past the barrier member; and the system is configured to periodically switch the barrier member between the blocking state and the passing state to allow flow of the electrolyte through the flow cells in batches.

13. The system of any preceding claim, wherein: each of one or more of the insulating buffers comprises an insulating liquid immiscible with the electrolyte; and the flow management system is configured to drive the flow of electrolyte in a flow comprising batches of the electrolyte interleaved in series by batches of the insulating liquid.

14. The system of any preceding claim, wherein the flow management system comprises one or more flush valves, each flush valve configured to be selectively openable to remove products produced by the electrochemical reaction occurring in a respective one or plurality of flow cells, the flush valve being optionally located beneath the respective one or plurality of flow cells.

15. The system of any preceding claim, wherein the flow management system further comprises one or more reservoirs configured to store the electrolyte, optionally wherein the plurality of flow cells are positioned above the one or more reservoirs.

16. The system of any preceding claim, wherein the one or more electrodes are parallel planar plates.

17. The system of any preceding claim, wherein the flow driving arrangement is configured to drive the flow of the electrolyte such that an entry region where the flow enters each flow cell is located above an exit region where the flow leaves the flow cell.

18. The system of claim 17, wherein the flow management system is configured such that a flow of the electrolyte is driven from the entry region to the exit region predominantly by gravity.

19. The system of any of claims 1 to 16, wherein the flow driving arrangement is configured to drive the flow of the electrolyte such that an entry region where the flow enters each flow cell is located below an exit region where the flow leaves the flow cell.

20. The system of any preceding claim, wherein the one or more electrodes comprise one or more bipolar electrodes, each bipolar electrode being configured to act as a positive electrode of a first flow cell of the plurality of flow cells and a negative electrode of a second flow cell of the plurality of flow cells, the second flow cell being adjacent to the first flow cell.

21. The system of claim 20, wherein the flow cells are provided in a stack configured to allow electrolyte to flow through the flow cells in parallel, and each bipolar electrode separates a respective pair of adjacent flow cells in the stack.

22. The system of any preceding claim, wherein: each flow cell comprises a first chamber containing a first electrode, a second chamber containing a second electrode, and a membrane separating the first chamber from the second chamber; the flow management system comprises a further conduit arrangement, the further conduit arrangement comprising a third flow unit and a fourth flow unit,wherein the flow management system is configured to drive a flow of a further electrolyte from the third flow unit through the flow cells and into the fourth flow unit along a further electrolyte flow path; the first flow unit and second flow unit are configured to guide and receive flow of electrolyte respectively to and from the first chambers of the flow cells; the third flow unit and fourth flow unit are configured to guide and receive flow of further electrolyte respectively to and from the second chambers of the flow cells; and the flow management system is configured to provide one or more insulating buffers spanning the further flow path upstream or downstream of the plurality of flow cells such that, during the flow of the further electrolyte through the flow cells, further electrolyte located within each flow cell is electrically separated from further electrolyte in the other flow cells, at least via paths through further electrolyte respectively upstream or downstream of the plurality of flow cells, by the insulating buffer blocking the paths.

23. The system of any preceding claim, wherein the flow management system is configured to cause the electrolyte to flow through the flow cells with an average linear velocity between 0 and 100 cms'1and optionally between 1 and 10 eras'1.

24. The system of any preceding claim, wherein each of one or more of the insulating buffers is configured to prevent a portion of the electrolyte spanning across the insulating buffer and thereby allowing flow of ions across the insulating buffer through the portion of the electrolyte.

25. A redox flow battery or electrochemical reactor comprising the system of any preceding claim.

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