Flow Battery State of Health Indicator

Abstract

1. A state of charge indicator arrangement for a redox flow battery system having a reference cell arrangement for measuring a potential difference between a positive electrolyte and a negative electrolyte, an auxiliary reference electrolyte arrangement comprising a separate auxiliary electrolyte reservoir for housing a redox electrode corresponding to the reference electrolyte, a means for measuring a potential difference between the auxiliary reference electrolyte and an electrolyte of the reference cell arrangement, and an ion pathway conduit configured for low flow diffusion rates connecting the auxiliary reference electrolyte reservoir to an electrolyte of the reference cell arrangement, the conduit comprising a wicking means for absorbing electrolyte and maintaining an ionic connection between the auxiliary reference electrolyte reservoir and each electrolyte of the reference cell arrangement.

Description

[Technical field]

[0001] The present invention relates to the field of redox flow batteries, more specifically to a flow battery's electrolyte state of health or state of charge, a device or reference cell for detecting the electrolyte state of health or state of charge, a method of manufacturing such a device, a method of detecting, monitoring or correcting the electrolyte state of health or state of charge in a flow battery, and a redox flow battery having a state of health indicator therein. [Background technology]

[0002] Redox flow batteries, such as vanadium redox flow batteries, can become imbalanced with respect to the state of charge of their positive and negative electrolytes over time or through use.

[0003] When a vanadium redox flow battery becomes unbalanced in terms of state of charge, the result is a reduction in the energy storage capacity and performance of the flow battery.

[0004] In order to identify an imbalance in the state of charge of a flow battery, it is necessary to obtain a measurement or indication of the state of charge of each of the positive and negative electrolytes. In the absence of reliable information about the balance of charge between the two electrolytes, the flow battery may operate based on inaccurate information, which may lead to hazards posed by attempts to overcharge or overdischarge the electrolyte, or at least limit the available depth of discharge and battery efficiency. The lack of reliable information about the balance of charge between the two electrolytes also has the effect of not prompting corrective or corrective action (whether manual or automatic) in a timely manner.

[0005] Several methods have been proposed to measure the state of charge of the electrolyte in a flow battery.

[0006] US Patent No. 5,399,633 describes how the state of charge of both electrolytes can be determined indirectly by utilizing optical absorption, density and viscosity measurements, however, optical measurements are subject to instrument drift (due to changes in light source and detector over time and temperature) while in-line density and viscosity measurements require expensive equipment (especially in the relatively harsh chemical conditions and for high levels of resolution required for VRFB).

[0007] In Patent Document 2, an in-line potentiometric titration technique is proposed, however, this method is limited by the extremely high requirements imposed on controlling the titration volume of the electrolyte and is therefore impractical for commercial systems.

[0008] Measurement of electrolyte potential at inert redox electrodes has also been proposed previously.

[0009] In US Pat. No. 5,999,136, a conventional reference electrode is proposed, however, the reference electrode is subject to contamination and subsequent voltage drift after long-term immersion in the test electrolyte and is therefore not practical for commercial systems with service intervals of months or years.

[0010] In Patent Document 3, a dynamic hydrogen electrode is proposed. The dynamic hydrogen electrode uses platinum group metals to catalyze hydrogen generation. Unfortunately, these are prone to dissolution or contamination in the electrolyte, thus resulting in unstable results after a long period of time. Furthermore, the dissolved catalyst deposits on the negative electrode of the flow battery, resulting in the acceleration of the disproportion reaction (hydrogen generation).

[0011] In US Patent No. 5,399,436, a reference cell is proposed in which a reference electrolyte of similar composition and known state of charge is contained in one half-cell, and a test electrolyte is flowed through the other half-cell, however, membrane-separated cells are subject to mass transfer through the membrane, resulting in relatively rapid changes in the composition of the reference electrolyte.

[0012] The inventors have devised an easy to implement, cost effective and robust device and arrangement, by which the state of charge of an electrolyte in a redox flow battery can be detected or verified. [Prior art documents] [Patent documents]

[0013] [Patent Document 1] International Publication No. 90 / 03666 [Patent Document 2] Japanese Patent Application Publication No. 09-101286 [Patent Document 3] International Publication No. 2014 / 184617 [Patent Document 4] US Patent Application Publication No. 2018 / 0375132

[0014] There is a need for a robust and inexpensive method and apparatus to independently measure or detect the state of charge of each electrolyte in a redox flow battery.

[0015] It is an object of the present invention to provide a method and apparatus or system that can measure or detect the state of charge of one or both electrolytes and / or determine the state of health of a flow battery.

[0016] It is a further object of the present invention to provide a method and apparatus or system for identifying when action should be taken to address imbalances in the state of charge between electrolytes in a flow battery. Summary of the Invention

[0017] According to a first aspect of the invention, there is provided a state of charge or state of health indicator arrangement for a redox flow battery system comprising a redox flow battery cell stack, a positive electrolyte tank and plumbing for circulating a positive electrolyte through the cell stack, and a negative electrolyte tank and plumbing for circulating a negative electrolyte through the flow battery cell stack, the indicator arrangement comprising: a reference cell arrangement comprising means for measuring a potential difference between a positive electrolyte in or from the positive electrolyte tank of the flow battery and a negative electrolyte in or from the negative electrolyte tank of the flow battery; at least one auxiliary reference electrolyte configuration, a separate auxiliary electrolyte reservoir containing a redox electrode corresponding to a reference electrolyte of known composition equivalent to the desired or initial composition of the electrolyte of the reference providing flow battery and having a known state of charge; means for measuring the potential difference between the or each auxiliary reference electrolyte and each electrolyte of the reference cell arrangement; and an ion pathway conduit connecting the or each auxiliary reference electrolyte reservoir to a respective electrolyte of the reference cell configuration, the conduit being configured for low flow diffusion capacity or rate, preferably the conduit comprising a wicking means for absorbing electrolyte and maintaining an ionic connection between the or each auxiliary reference electrolyte reservoir and the respective electrolyte of the reference cell configuration; and a supplemental reference electrolyte configuration comprising:

[0018] In a second aspect of the present invention, there is provided a state of charge or state of health indicator arrangement (or device or system) for a redox flow battery system comprising a redox flow battery cell stack, a positive electrolyte tank and plumbing for circulating a positive electrolyte through the cell stack, and a negative electrolyte tank and plumbing for circulating a negative electrolyte through the flow battery cell stack, the indicator arrangement comprising: a positive half-cell having a positive electrolyte reservoir configured for fluid circulation communication with a positive electrolyte tank of the flow battery, a negative half-cell having a negative electrolyte reservoir configured for fluid circulation communication with the negative electrolyte tank, and a reference cell comprising a means for measuring a potential difference across the reference cell; At least one auxiliary reference electrolyte configuration (or device, system, or subsystem), a separate auxiliary electrolyte reservoir containing a redox electrode corresponding to a reference electrolyte of known composition equivalent to the desired or initial composition of the electrolyte of the reference providing flow battery and having a known state of charge; means for measuring the potential difference between the or each auxiliary reference electrolyte and each half-cell of the reference cell; and an ion pathway conduit connecting the or each auxiliary reference electrolyte reservoir to electrolyte in a respective half-cell of the reference cell, the conduit being configured for low flow diffusion capacity or rate, preferably the conduit comprising a wicking means for absorbing electrolyte and maintaining an ionic connection between the or each auxiliary reference electrolyte reservoir and the electrolyte in the respective half-cell of the reference cell; and a supplemental reference electrolyte configuration comprising:

[0019] In a third aspect of the present invention, there is provided a state of charge or state of health indicator arrangement for a redox flow battery system comprising a redox flow battery cell stack, a positive electrolyte tank and plumbing for circulating a positive electrolyte through the cell stack, and a negative electrolyte tank and plumbing for circulating a negative electrolyte through the flow battery cell stack, the indicator arrangement comprising: at least one auxiliary reference electrolyte configuration, a separate auxiliary electrolyte reservoir containing a redox electrode corresponding to a reference electrolyte of known composition equivalent to the desired or initial composition of the electrolyte of the reference providing flow battery and having a known state of charge; means for measuring the potential difference between the or each auxiliary reference electrolyte and the respective electrolyte of the flow battery (or reference cell configuration); and an ion pathway conduit connecting the or each auxiliary reference electrolyte reservoir to a respective electrolyte of the flow battery (or reference cell configuration thereof), the conduit being configured for low flow diffusion capacity or rate, preferably the conduit comprising a wicking means, preferably for absorbing electrolyte and maintaining an ionic connection between the auxiliary reference electrolyte reservoir and the respective electrolyte of the flow battery (or reference cell configuration thereof); an auxiliary reference electrolyte configuration comprising: and means for determining a state of charge, or a surrogate for the state of charge, of the other electrolyte of the flow battery (usually the negative electrolyte), such as by providing a reference cell comprising a positive half-cell having a positive electrolyte reservoir configured for fluid circulation communication with the positive electrolyte tank of the flow battery, a negative half-cell having a negative electrolyte reservoir configured for fluid circulation communication with the negative electrolyte tank, and means for measuring a potential difference across the reference cell.

[0020] In a fourth aspect of the present invention there is provided a state of health indicator system for a redox flow battery system comprising a state of charge indicator arrangement as defined above and configured to determine a state of health of the redox flow battery system from the state of charge indicator arrangement, preferably by obtaining (optionally continuously, periodically or intermittently) measurements of the state of charge of at least one electrolyte and the other electrolyte or surrogate thereof of the flow battery, preferably determining a relative oxidation state of the respective electrolytes, and preferably causing an alarm, indication or corrective action in response to the determined relative oxidation state being outside predetermined limits.

[0021] In a fifth aspect of the present invention there is provided an auxiliary reference electrolyte configuration for a state of charge or state of health indicator as defined above, the auxiliary reference electrolyte configuration comprising: a separate auxiliary electrolyte reservoir containing a redox electrode corresponding to a reference electrolyte of known composition equivalent to the desired or initial composition of the electrolyte of the reference providing flow battery, and having a known state of charge; means for measuring a potential difference between the or each auxiliary reference electrolyte and an associated electrolyte of the reference cell arrangement or an associated half-cell of the reference cell; an ion pathway conduit for connecting the or each auxiliary reference electrolyte reservoir to an electrolyte in the respective electrolyte in the respective half-cell of the reference cell configuration or of the reference cell, the conduit being configured for low flow diffusion capacity or rate, preferably the conduit comprising a wicking means for preferably absorbing electrolyte and maintaining an ionic connection between the or each auxiliary reference electrolyte reservoir and the respective electrolyte in the respective half-cell of the reference cell configuration or of the reference cell.

[0022] In a sixth aspect of the present invention there is provided a redox flow battery comprising a redox flow battery cell stack, a positive electrolyte tank and plumbing for circulating positive electrolyte through the cell stack, a negative electrolyte tank and plumbing for circulating negative electrolyte through the flow battery cell stack, and a state of charge or state of health indicator as defined above.

[0023] In a seventh aspect of the present invention, there is provided a method of monitoring a state of charge or state of health in a redox flow battery comprising the steps of providing a state of charge or state of health indicator as defined above and causing the state of charge or state of health indicator to perform periodic measurements of the charge on a reference cell and the charge between an auxiliary reference electrolyte and each half-cell of the reference cell to identify a state of charge of the system, and optionally issuing an alert of an electrolyte charge imbalance of the flow battery in response to states of charge on the reference cells of the flow battery differing by a predetermined threshold.

[0024] In an eighth aspect of the present invention, there is provided a method of maintaining a balanced state of charge in a redox flow battery, the method comprising: monitoring the state of charge of the flow battery by providing a state of charge or state of health indicator as defined above and having the state of charge or state of health indicator make periodic or operation- or event-dependent measurements of the charge on the reference cell and the charge between the auxiliary reference electrolyte and each half-cell of the reference cell, thereby determining the state of charge of the system; and causing one or more maintenance actions to be applied to the flow battery in response to a variance in the state of charge between the positive and negative electrolytes exceeding one or more predetermined thresholds or meeting one or more predetermined criteria.

[0025] The state of charge or state of health indicator of the present invention offers the advantageous benefits of the robustness of a standard reference cell, but with consistency in measurement over the life of the flow battery by taking into account voltage drift in the reference cell resulting from contamination of the battery electrolyte, thereby providing a more accurate measurement of the battery's state of health over its life and a more complete and safer use of the battery's capacity. [Brief description of the drawings]

[0026] [Figure 1] 1 illustrates a perspective view of a charge status indicator arrangement according to one embodiment of the present invention; [Figure 2a] FIG. 1 illustrates a perspective view of an ion pathway conduit tube for use in a charge state indicator configuration according to an embodiment of the present invention. [Figure 2b] FIG. 1 illustrates a perspective view of an ion pathway conduit tube for use in a charge state indicator configuration according to an embodiment of the present invention. [Figure 2c] FIG. 1 illustrates a perspective view of an ion pathway conduit tube for use in a charge state indicator configuration according to an embodiment of the present invention. [Diagram 3] FIG. 1 is an illustration of a state of charge or state of health indicator according to an embodiment of the present invention. [Figure 4] FIG. 1 is a simplified plumbing and instrumentation diagram of a flow battery according to an embodiment of an aspect of the present invention incorporating a state of charge or state of health indicator according to an embodiment of another aspect of the present invention. [Figure 5a] 1 is a graph of voltage versus absolute current for a flow battery reference cell in a flow battery incorporating a state of charge indicator according to an embodiment of the present invention. [Figure 5b]1 is a graph of voltage versus absolute current between the positive electrode and auxiliary electrolyte of a flow battery reference cell in a flow battery incorporating a state of charge indicator according to an embodiment of the present invention; [Figure 6] 1 shows an overview of an experimental setup for determining diffusion rates through ion pathway conduits in an auxiliary reference electrolyte configuration for use in embodiments of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] A state of charge indicator or state of health indicator for a redox flow battery system according to the present invention is for a redox flow battery having a redox flow battery cell stack, a positive electrolyte tank, a negative electrolyte tank, and plumbing for circulating electrolyte from the respective tanks through respective portions of the cell stack.

[0028] By "cell stack" as used herein is meant a flow battery cell along with any membranes, electrodes or current collectors and cell frame, and may comprise one or more cells (usually arranged in parallel and receiving a single combined feed or multiple parallel feeds of electrolyte from each electrolyte tank).

[0029] A flow battery cell typically comprises two half-cells for containing electrolytes, each supplied from a respective electrolyte tank and separated by an ion-selective membrane. Each half-cell has an electrode or current collector connected to an electrical circuit that provides a power source or load for use in charging or discharging the battery.

[0030] The electrolyte is typically circulated from each electrolyte tank through the cells or cell stack using a pump.

[0031] The state of charge or state of health indicator arrangement comprises at least one auxiliary reference electrolyte (interchangeably referred to herein as auxiliary electrolyte) arrangement and is configured to determine the state of charge of one electrolyte (usually the positive electrode electrolyte) of the flow battery. Optionally, a second auxiliary electrolyte arrangement is provided to determine the state of charge of a second (usually the negative electrode) electrolyte of the flow battery.

[0032] The state of charge or state of health indicator comprises one of a reference cell, a reference cell configuration or a means for identifying a state of charge of a second / other electrolyte of the flow battery, where the second / other electrolyte may typically be the negative electrode electrolyte (the first electrolyte being that identified by the auxiliary electrolyte configuration), or the state of charge or state of health indicator comprises a proxy means for identifying information as a proxy for the state of charge of the second (usually negative) electrolyte.

[0033] The means for determining the state of charge of the second / other electrolyte of the flow battery may be any suitable means, for example a second auxiliary electrolyte configuration provided to determine the state of charge of the second (usually negative) electrolyte of the flow battery. The proxy means for determining information as a surrogate for the state of charge of the second (usually negative) electrolyte may be any suitable means that can be approximated to or inferred from measurements of the proxy means. In a particularly preferred embodiment, the proxy means is a reference cell configuration or reference cell for the flow battery.

[0034] The reference cell arrangement comprises means for measuring a potential difference between a positive electrolyte in or from a positive electrolyte tank of the flow battery and a negative electrolyte in or from a negative electrolyte tank of the flow battery. Preferably, the state of charge or state of health indicator arrangement comprises at least one reference cell for the flow battery (the reference cell arrangement is a reference cell for at least one flow battery).

[0035] The means for measuring the potential difference between the positive electrolyte in or from the positive electrolyte tank of the flow battery and the negative electrolyte in or from the negative electrolyte tank of the flow battery in a reference cell configuration may comprise an electrode disposed in association with each electrolyte, such as in association with each electrolyte tank, and a voltmeter or similar instrumentation connected to both electrodes (to measure the potential difference).

[0036] The at least one reference cell (included in certain aspects and preferred embodiments of the invention) comprises a positive half-cell having a positive electrolyte reservoir for fluid circulation communication with the positive electrolyte tank of the flow battery and a negative half-cell having a negative electrolyte reservoir configured for fluid circulation communication with the negative electrolyte tank. The at least one reference cell comprises a means for measuring a potential difference across the reference cell. This may comprise, for example, electrodes disposed in association with each half-cell and a voltmeter or similar instrumentation connected to each electrode. This is preferably configured to determine an open circuit voltage across the reference cell, i.e., the open circuit voltage between the positive and negative electrolytes in the flow battery.

[0037] The positive and negative half-cells of the reference cell may be in fluid communication with the respective electrolyte tanks of the flow battery by any suitable means, such as pipes connecting the positive and negative half-cells of the reference cell to the respective electrolyte tanks, more preferably to piping that circulates electrolyte from the tanks to and / or from the cell stack of the flow battery. Optionally, the half-cells of the reference cell are connected to the electrolyte tank via piping to and from a return arm of the piping for circulating electrolyte from the tank through the cell stack of the flow battery, but preferably from a pipe that delivers electrolyte from the tank to the cell stack (e.g., just before the electrolyte enters the cell stack).

[0038] Preferably, the electrolyte flows through the reference cell in parallel with its flow through the cell stack. The positive and negative electrolytes are taken at a point near the inlet of the cell stack (using a piping branch) and can be returned to any point downstream of the stack (between the stack outlet and the tank) or directly into the tank.

[0039] In a reference cell there is usually one inlet and one outlet for the positive electrolyte (through one half-cell of the reference cell) and one inlet and one outlet for the negative electrolyte (through the opposing half-cell of the reference cell). Preferably, the electrolytes are electronically separated but ionically connected through an ion-exchange (or porous) membrane in the reference cell.

[0040] The height of the reference cell is not critical with respect to the tank and stack, in fact it is preferred that the reference cell be placed at approximately the same height as the cell stack (to prevent electrolyte from being sucked out of the tank if the tank is placed too low and leaks).

[0041] Preferably, the reference cell is provided with a temperature sensor or thermometer so that any measurement of the potential difference relative to the reference cell can be adjusted for temperature. Optionally, the temperature sensor may be configured to measure or determine the temperature of the reference cell, at the electrolyte in one or both half-cells of the reference cell and / or at other components of the reference cell (e.g. the housing).

[0042] The auxiliary reference electrolyte configuration, which is a feature of the state of charge or state of health indicator and a further aspect of the invention, comprises a separate auxiliary electrolyte reservoir, means for measuring a potential difference between the or each auxiliary reference electrolyte and each half-cell of the reference cell (or the respective electrolyte of the reference cell configuration or flow battery) and an ion pathway conduit connecting the or each auxiliary reference electrolyte reservoir to each half-cell of the reference cell (or the respective electrolyte of the reference cell configuration or flow battery). Preferably, the ion pathway conduit comprises a wicking means, preferably for absorbing electrolyte and maintaining an ionic connection between the auxiliary reference electrolyte reservoir and each half-cell of the reference cell (or the respective electrolyte of the reference cell configuration or flow battery).

[0043] In some applications, temperature gradients may occur that cause outgassing of gas dissolved in the electrolyte located in the ion pathway conduit. Outgassing may also result in the formation of gas bubbles in the ion pathway conduit, which may disrupt ionic connectivity through the ion pathway conduit. An advantageous effect of the wicking means is that in the event of bubble formation, the electrolyte absorbed by the wicking means maintains ionic connectivity through the ion pathway conduit.

[0044] Optionally, the wicking means may extend through only a portion of the ion path conduit, for example at one high point or at or through two or more high points of the ion path conduit where there is a risk of bubble formation or retention.

[0045] Preferably the wicking means extends continuously along the entire length of the ion path conduit.

[0046] Preferably, the wicking means extends from the ion pathway conduit into the auxiliary reference electrolyte reservoir and / or into each half-cell of the reference cell (or the reference cell configuration or each electrolyte of the flow battery). Preferably, the wicking means extends from the ion pathway conduit by up to 10 mm, more preferably by up to 8 mm, even more preferably by up to 6 mm, for example by 5 mm. Preferably, the wicking means extends from the ion pathway conduit by at least 0.5 mm, more preferably by at least 1 mm, even more preferably by at least 2 mm, for example by at least 3 mm.

[0047] Preferably, the wicking means is hydrophilic and / or electrically non-conductive (typically σ<10 -3 Preferably, the wicking means comprises (or consists of) silica, ceramic and / or hydrophilic polymer, such as silica, ceramic and / or hydrophilic polymer fibres.

[0048] Preferably the wicking means is a wick or cord, preferably made of bundled (e.g. woven, twisted or braided) fibres. Preferably the wicking means is a wick or cord having a diameter of 0.4mm to 8mm, preferably 1mm to 6mm, more preferably 2mm to 4mm, for example about 3mm.

[0049] In one embodiment, the ion path conduit is defined by a wicking means extending through an optionally flexible sleeve or tube.

[0050] The separate auxiliary electrolyte reservoir is for housing the redox electrode and the reference electrolyte. The reference electrolyte is preferably selected to be of a known composition similar to the desired composition of each electrolyte of the flow battery at a predetermined and known state of charge. The desired composition of each electrolyte of the flow battery is usually the initial composition (before degradation or contamination occurs). The separate auxiliary electrolyte reservoir may be of any suitable size. For example, the separate auxiliary electrolyte reservoir may have an electrolyte capacity of preferably at least 10 ml and preferably not more than 10 L. More preferably, the electrolyte capacity is in the range of 30 ml to 1000 ml, more preferably 50 ml to 750 ml, and even more preferably at least 100 ml. In one embodiment, the electrolyte capacity is at least 400 ml, such as 400 ml to 600 ml. In other embodiments, particularly where the ion pathway conduit is relatively narrow, long or curved / looped (as described below), the separate auxiliary electrolyte reservoir has a capacity of up to 500ml, for example from 100ml to 350ml, more preferably up to 250ml, and even more preferably up to 200ml.

[0051] The separate auxiliary electrolyte reservoir is preferably a container that includes a space or volume for receiving a quantity of electrolyte, as described above, and for receiving or housing a redox electrode for use in measuring or detecting a potential difference between the auxiliary electrolyte in the reservoir and an electrolyte elsewhere.

[0052] The redox electrode can take virtually any form. It can be a simple plate, rod or space-filling porous 3D shape (felt, foam, etc.). It must be positioned so that it is (at least partially) immersed in the electrolyte in the auxiliary reservoir. The redox electrode should be chemically stable with respect to the electrolyte in this reservoir. Therefore, carbon or carbon composite materials (e.g., carbon and polypropylene) are preferred.

[0053] Preferably, the auxiliary reference electrolyte configuration further comprises a temperature sensor, preferably associated with a separate auxiliary electrolyte reservoir or containment or a fixture in thermal communication therewith, to correct any potential difference measurements for temperature. The temperature sensor may take any suitable form. Since the temperature sensor is intended to measure the temperature of the electrolyte in the separate auxiliary electrolyte reservoir, it should be in good thermal contact with the electrolyte. For example, it may be immersed in the electrolyte (possibly in a thermowell or with a suitable protective coating) or in intimate contact with the electrode (carbon generally has a high thermal conductivity and may therefore be used to transfer heat to the thermal sensor). The second option usually requires some insulation around the air side of the thermal sensor to obtain accurate measurements.

[0054] The means for measuring the potential difference between the or each auxiliary reference electrolyte and the respective half-cell of the reference cell (or the respective electrolyte of the reference cell configuration or flow battery) may be any suitable device or instrument, such as a redox electrode in a separate auxiliary electrolyte reservoir and a voltmeter connected to an appropriate electrode in the or each respective half-cell of the reference cell (or corresponding to the or each electrolyte of the reference cell configuration or flow battery, such as an electrolyte tank).

[0055] The or each auxiliary reference electrolyte reservoir to the respective half-cell of the reference cell is configured for low flow diffusion capacity or rate. The ion path conduit may be provided by any suitable means, but typically comprises a tubular member providing a fluid connection between the auxiliary reference electrolyte reservoir and the half-cell of the reference cell (or the electrolyte therein). Preferably, when the ion path conduit comprises a tubular member, a wicking means (which may be, for example, a wick or cord as defined above) is disposed inside the tubular member. Preferably, the ion path conduit has a resistivity of 1 MOhm or less along its length (e.g., between the auxiliary reference electrolyte reservoir and the half-cell of the reference cell). Preferably, the ion path conduit does not have a membrane or barrier that may prevent (or inhibit) ionic and fluid communication between the auxiliary reference electrolyte and the respective half-cell of the reference cell, but rather is preferably openly fluidly connected by the ion path conduit.

[0056] The ion pathway conduit may be of any suitable length (depending on the geometry of bore diameter, curvature, loops, etc.) suitable to inhibit rapid fluid mixing of the auxiliary electrolyte and the flow battery electrolyte in fluid communication by the ion pathway conduit. Preferably, the ion pathway conduit has a length of at least 5 cm, more preferably up to 10 m. More preferably, the ion pathway conduit has a length of 5 m or less, preferably about 2 to 2.5 m or less. Preferably, the ion pathway conduit has a length in the range of 10 cm to 1.5 m, more preferably 15 cm to 1.2 m, such as 20 cm to 1 m or optionally up to 75 cm, and preferably about 30 to about 50 cm. In one embodiment, for example, when a larger bore diameter tube is utilized as the ion pathway conduit, the length may be longer, for example, 50 cm to 5 m, such as 1.5 m to 2.5 m.

[0057] The ion pathway conduit may have any suitable bore diameter, which may be selected depending on the length and other geometric characteristics of the conduit, to inhibit mixing of the auxiliary electrolyte and the flow battery electrolyte through the conduit. Preferably, the ion pathway conduit has a bore diameter of at least 0.5 mm. The bore diameter may be up to 10 mm, preferably 7.5 mm or less. Preferably, the ion pathway conduit is a small bore conduit, preferably of sufficiently small bore to inhibit laminar flow of electrolyte through the conduit. More preferably, the ion pathway conduit has an internal bore diameter in the range of 2.5-4 mm, such as 1-5 mm, preferably 3-3.5 mm.

[0058] Preferably, where the ion path conduit comprises a tubular member, the wicking means (e.g. a wick or cord as defined above) has a diameter smaller than the inner / bore diameter of the tubular member. Thus, preferably, the wicking means fits loosely within the bore of the ion path conduit tubular member. An advantageous effect of this feature is that the wicking means may be easily inserted through the ion path conduit tubular member during assembly. A further advantageous effect of this feature is that electrolyte may be disposed in the space between the wicking means and the inner / bore surface of the conduit tubular member along the length of the ion path conduit.

[0059] The inner diameter of the ion pathway conduit should be small enough to prevent laminar flow (e.g., to facilitate mixing of the auxiliary electrolyte with the flow battery electrolyte, but large enough so that there is not a high risk of blockage by particulates that may be present in the flow battery electrolyte).

[0060] In one option, for example when the ion path conduit does not include a wicking means, it is advantageous to have a slightly larger conduit bore diameter. This is because air bubbles resulting from degassing of the electrolyte in the ion path conduit are less likely to then close off the complete inner / bore diameter of the conduit. Thus, in this option, it is preferred that the ion path conduit has a bore diameter of at least 3mm, more preferably at least 4mm, even more preferably at least 5mm. The bore diameter may be up to 10mm, preferably 8mm or less. Preferably, the ion path conduit has an internal bore diameter in the range of 4mm to 7mm, such as 3.5mm to 7.5mm, more preferably 4.5mm to 6mm, for example an internal bore diameter of 5mm.

[0061] In one embodiment, the ion path conduit has a length between 20 cm and 2 m and an inner diameter between 2.5 and 4 mm.

[0062] Preferably, the ion path conduit has one or more curved portions or bends along its length. Preferably, the ion path conduit has one or more vertical components associated with the one or more curved portions or bends. The curved portions or bends may result in a change in tangential direction along the length of the ion path of at least 30°, preferably at least 60°, more preferably at least 90°, more preferably at least 120°, such as at least 180°, more preferably at least 270°, even more preferably greater than 360°.

[0063] Preferably, at least one curved portion or bend in the ion path conduit defines at least one U-shaped bend or loop. The orientation of the U-shaped bend or loop preferably has a vertical component. Preferably, the ion path conduit defines at least one loop along its length, more preferably two or more loops, such as three loops. Having at least two loops in the ion path conduit between the auxiliary electrolyte reservoir and the reference cell is believed to be particularly advantageous for slowing down the mixing of the flow battery electrolyte with the auxiliary electrolyte (e.g., compared to one loop or no loops). As a result, including one loop or preferably two loops (preferably with a vertical component) reduces the mixing rate of the auxiliary electrolyte with the electrolyte of the flow battery (or reference cell), and therefore a smaller volume of auxiliary electrolyte may be used for a similar useful service life of the state of charge indicator, or the service life of the state of charge indicator may be increased for a similar volume of auxiliary electrolyte.

[0064] A state of charge or state of health indicator according to the present invention may be configured such that the auxiliary electrolyte corresponds to the positive electrode electrolyte of the flow battery (e.g., has a composition corresponding to a desired or initial composition of the positive electrode electrolyte of the flow battery) or corresponds to the negative electrode electrolyte of the flow battery (e.g., has a composition corresponding to a desired or initial composition of the positive electrode electrolyte of the flow battery).

[0065] Optionally, there are two auxiliary reference electrolyte configurations. In one such embodiment, one auxiliary reference electrolyte configuration corresponds to the positive electrolyte of the flow battery (and preferably the first half-cell of the reference cell), while the other auxiliary reference electrolyte configuration corresponds to the negative electrolyte of the flow battery (and preferably the second half-cell of the reference cell). In another such embodiment, both auxiliary reference electrolyte configurations are selected to comprise an electrolyte corresponding to the positive electrolyte of the flow battery, one connected to the positive electrolyte (e.g., the positive half-cell of the reference cell) via an ion pathway conduit, and the other connected to the negative electrolyte (e.g., the negative half-cell of the reference cell) via a pathway conduit, and the state of charge of each electrolyte of the flow battery is determined from a measurement of the potential difference between the respective electrolyte of the flow battery (or half-cell of the reference cell) and the auxiliary reference configuration to which the respective ion pathway conduit is connected.

[0066] Preferably, the at least one auxiliary electrolyte corresponds to the positive electrode electrolyte, and the auxiliary reference electrolyte arrangement (or pseudo reference cell) is configured such that the means for measuring the potential difference is between the auxiliary reference electrolyte and the positive electrode half-cell of the reference cell, and an ion path conduit connects the auxiliary reference electrolyte reservoir to the positive electrode half-cell of the reference cell. Optionally, the state of charge or state of health indicator also comprises a second auxiliary reference electrolyte arrangement, the second auxiliary electrolyte corresponds to the negative electrode electrolyte of the flow battery (or may also correspond to the positive electrode electrolyte), and the second auxiliary reference electrolyte arrangement (or pseudo reference cell) is configured such that the means for measuring the potential difference is between the second auxiliary reference electrolyte and the negative electrode half-cell of the reference cell, and an ion path conduit connects the auxiliary reference electrolyte reservoir to the negative electrode half-cell of the reference cell.

[0067] Preferably, the state of charge or state of health indicator comprises a temperature sensor for measuring the temperature of the or each flow battery electrolyte.

[0068] The state of charge or state of health indicator may further comprise a processor for controlling measurements of obtaining and recording potential differences and temperatures for each or each half-cell of the or each auxiliary reference electrode configuration and reference cell, and optionally configured to communicate said measurements to a controller or data logger of the flow battery. Preferably, the processor is or is part of a processor for controlling or managing the operation of the flow battery to which the state of charge or state of health indicator is connected.

[0069] Preferably, the state of charge indicator is configured to measure the state of charge at a predetermined time period or in response to a predetermined system operation. For example, the state of charge indicator arrangement may be configured to determine the state of charge every 24 hours or every 7 days, preferably every 24 hours to every 3 months, more preferably every 2 days to every 2 months, e.g., once a week to once a month. Additionally or alternatively, the state of charge indicator arrangement may be configured to determine the state of charge after every charge / discharge cycle or after every thousand charge / discharge cycles, such as 10 to 500 charge / discharge cycles, e.g., 50 to 250 charge / discharge cycles.

[0070] The state of charge indicator may be configured to take measurements (e.g., potential difference / temperature) from which a state of charge can be determined, at any time during a charge or discharge cycle, whether the flow battery is cycling or at rest, but preferably when the flow battery is cycling, and such measurements may be taken at any anticipated state of charge, but preferably at intermediate anticipated states of charge, e.g., between 20-80% state of charge, e.g., between 40-60% state of charge, and preferably about 50% anticipated state of charge.

[0071] The state of charge indicators described herein are typically and preferably incorporated within or fitted to a redox flow battery.

[0072] Thus, in a further aspect of the present invention there is provided a redox flow battery comprising a redox flow battery cell stack, a positive electrolyte tank and plumbing for circulating the positive electrolyte through the cell stack, a negative electrolyte tank and plumbing for circulating the negative electrolyte through the flow battery cell stack and a state of charge or state of health indicator as described above.

[0073] The redox flow battery may be of any suitable type, particularly where a state of charge imbalance may occur (eg through hydrogen generation), but in any case is preferably a vanadium redox flow battery.

[0074] In another aspect of the invention as described above, there is provided a method of monitoring a state of charge or state of health in a redox flow battery, the method comprising the steps of providing a state of charge or state of health indicator as described above, causing the state of charge indicator to perform periodic or irregular measurements of the charge on the reference cell and the charge between the auxiliary reference electrolyte and each half-cell of the reference cell to identify a state of charge of the system, and optionally issuing an alert of an electrolyte charge imbalance of the flow battery in response to a state of charge on a reference cell of the flow battery differing by a predetermined threshold.

[0075] Such an alert may be, for example, an alarm, a warning light, a notification (eg, via email or SMS to a technician or contact), or any other suitable alerting means.

[0076] By state of health (i.e., SOH), we include state of charge (i.e., SOC). When the term "state of health indicator" is used herein, it may also mean state of charge indicator, and vice versa, if the context permits. By state of charge of an electrolyte, we mean the charge level of that electrolyte. As used herein, the state of charge of a flow battery is preferably the state of charge of each (or both) electrolytes. In a preferred embodiment of a vanadium redox flow battery, by state of charge (SOC) of the positive electrolyte we mean the concentration ratio of V(V) to the total vanadium in the positive electrolyte, while the state of charge of the negative electrolyte is the concentration ratio of V(II) to the total vanadium in the negative electrolyte. In a perfectly balanced (and healthy) system, the SOC of the positive and negative electrolytes will be equal.

[0077] State of health (SOH) can be defined in a number of different ways for different battery chemistries. Preferably, in the context of a vanadium redox flow battery system, state of health refers to the degree to which the average oxidation state in the entire electrolyte (both positive and negative electrolytes) of the system deviates from its original value (which is about 3.50 for a vanadium redox flow battery system).

[0078] If the electrolyte oxidizes (e.g., through parasitic side reactions such as hydrogen evolution or oxygen ingress into the tank), the average oxidation state will increase. This will also be manifested as a difference in the SOC of the positive and negative electrolytes. In situations where the average oxidation state reaches at least 3.65, the vanadium flow battery may be considered to be in a "dangerous" state of health where the positive half-cell may be accidentally overcharged, causing irreversible damage to the stack. The increase in the average oxidation state will also be manifested as a decrease in discharge energy.

[0079] In a preferred embodiment of the present invention, if the average oxidation state deviates from a balanced or healthy state (i.e., normally the original state) by 0.10 (e.g., having an average oxidation state of 3.6 or greater), the system is determined to have poor health, and if the average oxidation state is 3.55 or greater, the system may be considered to have diminished health.

[0080] The average oxidation state may be determined from measurements of the state of charge (e.g., across the positive electrolyte and across a reference cell) using the present system / configuration on a one-off basis, but is preferably calculated over an extended period of time, such as over hours or days or a week or more, preferably relying on multiple measurements.

[0081] In one embodiment, the arrangement is configured to determine the state of charge of the positive electrolyte by determining the voltage difference between the positive half-cell of the reference cell and the auxiliary reference electrolyte arrangement defined above, which is connected to the positive half-cell of the reference cell via an ion path conduit. The state of charge of the negative electrolyte may be determined by determining the voltage difference between the negative half-cell of the reference cell and a second auxiliary reference cell defined above, which is connected to the negative half-cell of the reference cell via an ion path conduit, or (or additionally) by determining the difference between the measured open circuit voltage across the reference cell and the determined state of charge of the positive electrolyte (determined using the auxiliary reference electrolyte arrangement described above), preferably with compensation for temperature changes. The state of charge of the negative electrolyte may be estimated as the "average" state of charge, which is rather a value obtained from the reference cell, and is generally understood to be between the states of charge of the positive and negative electrolytes.

[0082] When making these measurements, the positive electrode presents a potential that depends on the state of charge of the positive electrolyte. The negative electrode presents a potential that depends on the state of charge of the negative electrolyte. The reference cell measures the potential difference between the positive and negative electrodes, thus giving a value for the "total battery state of charge" that is actually between the values ​​for the positive and negative electrodes, assuming both electrolytes are well balanced. This follows a rather complex relationship and is not a simple average value.

[0083] In this preferred embodiment, the auxiliary reference electrode (after temperature compensation) provides a fixed voltage to compare to the positive electrode. This allows the state of charge of the positive electrolyte to be determined. This value can then be compared to a "total battery state of charge" value (determined by measuring the voltage across a reference cell). If they are close, the negative and positive state of charge values ​​should be similar and the battery is considered "healthy". If the values ​​differ, there is a discrepancy between the positive electrolyte state of charge value and the negative electrolyte state of charge value and the battery is "unhealthy".

[0084] The state of charge may be determined by the inventive configurations by measuring the potential difference between each electrolyte and the auxiliary electrolyte configurations described above, and then measuring and / or determining a surrogate for the state of charge of the other electrolyte (e.g., by measuring the potential difference across the electrolyte of the flow battery or, more preferably, a reference cell). The state of charge of each electrolyte (or surrogate for the state of charge) may then be determined by any suitable method, such as by a predefined look-up table for the particular system or by using a suitable empirical formula.

[0085] According to a preferred embodiment, the arrangement or system comprises a potential difference (E ref a sensor for determining the potential difference (E ref-aux a sensor for determining the reference cell temperature (T 1 The temperature (T 2 The reference cell is equipped with a sensor for determining the state of charge α and the positive electrolyte charge α pos can be determined from a suitable look-up table or by application of an empirical formula.

[0086] In one embodiment, α is determined by iterating an empirical equation, for example of the form shown in Equation 1 below for an electrolyte containing 1.6M total vanadium and 4.0M total sulfate, until convergence. This equation has a form similar to the Nernst equation and cannot be implemented directly because the activity of the electroactive species is unknown.

number

[0087] State of charge of the positive electrolyte, α pos can be determined, for example, by repeating an empirical equation of the form shown in Equation 2 below (where both the reference and active electrolytes contain 1.6M total vanadium and 4.0M total sulfate):

number

[0088] In a preferred embodiment, measurements are taken at about 50% charge, but measurements may be taken at any suitable charge level, preferably taken relatively consistently at that charge level. Intermediate charge portions of the flow battery's state of charge (e.g., 20%-80% charged, more preferably 25%-75% charged, even more preferably 30%-70% charged, even more preferably 40%-60% charged, or even 45%-55% charged) are preferred, particularly since the flow battery is more likely to be in those portions than in other portions, and measurements and characterizations made in those portions are associated with smaller errors.

[0089] For example, as determined from the empirical formula above (or otherwise by a look-up table or the like), α and α pos By comparing the values ​​of , a determination can be made as to the balance of the state of charge of the positive and negative electrolytes. α posIf >α, the electrolyte is oxidized and rebalancing is necessary. α pos = α, the electrolytes are in balance and no rebalancing is required; α pos If <α, the electrolyte is reduced and no re-equilibration is necessary.

[0090] In a preferred embodiment, α and α pos The values ​​of α and α can be integrated over a long period of time (or an increased number of charge-discharge cycles). For example, pos The values ​​of may be integrated over periods of up to 30 days, more preferably over periods of 1-10 days. This is a useful period because systems under typical operating conditions oxidize at a rather slow rate (typically the average oxidation state can change by about 0.001-0.02 per month).

[0091] Preferably, the measurements (of the potential difference across the reference cell and between the reference cell and the auxiliary reference cell, and preferably temperature) are taken during a discharge or charge cycle or during operation while electrolyte is flowing (and through the reference cell).

[0092] Furthermore, for cells using membranes that induce significant concentration dispersion in the electroactive material, it is preferred that any definitive measurements be made immediately following complete remixing of the electrolyte. For membranes that do not induce significant changes in the electroactive species, concentration measurements may be made at any time.

[0093] In a further aspect of the invention, there is a method of maintaining a balanced state of charge or state of health, such as a balanced oxidation state, in a redox flow battery, comprising the steps of monitoring the state of charge in the flow battery by providing a state of charge or state of health indicator as described above and having the state of charge or state of health indicator (or a controller configured accordingly) make periodic or operation- or event-dependent measurements of the charge on the reference cell and the charge between the auxiliary reference electrolyte and each half-cell of the reference cell, thereby determining the state of charge and / or state of health of the system, and causing one or more maintenance actions to be applied to the flow battery in response to a variance in the state of charge or oxidation state between the positive and negative electrolytes exceeding one or more predefined thresholds or meeting one or more predefined criteria.

[0094] In one embodiment, a method comprises monitoring the state of charge of the flow battery by providing a state of charge or state of health indicator as described above and having the state of charge or state of health indicator make periodic or operation- or event-dependent measurements of the charge on the reference cell and the charge between the auxiliary reference electrolyte and each half-cell of the reference cell to thereby identify a state of charge of the system, and causing one or more maintenance actions to be applied to the flow battery in response to the identified state of charge variance between the positive and negative electrolytes exceeding one or more predetermined thresholds or meeting one or more predetermined evaluation criteria.

[0095] In the event that the flow battery's state of health decays and reaches, at least for example, a critical state (defined above), optionally the system may be configured to introduce performance limits to the flow battery, such as limiting the maximum state of charge of the battery (e.g., determined from a reference cell that gives a value between the positive and negative electrolytes), which reduces the risk of damage to the system, but also reduces the discharge energy of the battery.

[0096] In one embodiment, the method and system (e.g., its control system) are configured to cause (or recommend) a corrective action. Preferably, the system is configured to automate the corrective action in response to a state-of-health determination that the oxidation state is greater than a predetermined value (e.g., greater than 0.05 above the original level). The corrective action may be selected from the addition of a reducing agent to the electrolyte tank (e.g., automated dosing of a reducing agent into the electrolyte tank in response to a predetermined state-of-charge distribution) to convert a portion of V(V) to V(IV) [e.g., as described in WO 2018 / 047079] and the use of an electrochemical rebalancing cell to generate oxygen for introduction into the positive electrolyte tank to electrochemically reduce the average oxidation state of vanadium in the electrolyte [e.g., as described in U.S. Pat. No. 3,315,508].

[0097] The rebalancing action (e.g., reductant addition rate or rebalancing cell current) may, for example, be proportional to the difference between the average oxidation state and the target oxidation state, or may have on / off action if it deviates by more than a preset amount.

[0098] In either case, any measured or determined state of charge values ​​are preferably corrected for temperature to generate state of charge or state of health data.

[0099] The invention will now be explained in more detail, without limitation, with reference to the drawings, in which:

[0100] In FIG. 1, a state-of-charge or state-of-health indicator configuration 1 is shown having a reference cell 3 and a single auxiliary reference electrolyte configuration 5 associated therewith. Configuration 1 is configured for use with a vanadium redox flow battery. Reference cell 3 comprises an anode half-cell 7 and a cathode half-cell 13, each of which comprises a respective electrolyte reservoir (not shown). The anode half-cell 7 is configured via an anode electrolyte inlet 9 and outlet 11 for anode electrolyte circulation with an anode electrolyte tank (not shown) or circuit (not shown) of a flow battery to which it may be connected. The cathode half-cell 13 is configured via a cathode electrolyte inlet (not shown) and outlet 17 for electrolyte circulation with a cathode electrolyte tank (not shown) or circuit (not shown) of a flow battery to which it may be connected.

[0101] As with a standard reference cell, a potential difference can be measured across the reference cell 3 between the positive and negative half-cells 7, 13. This provides an overview of the measured state of charge of the flow battery, since the positive and negative half-cells 7, 13 are in fluid circulation with the positive and negative electrolyte tanks of the flow battery.

[0102] The auxiliary reference electrolyte configuration 5 has a cylindrical auxiliary electrolyte reservoir 19 for containing a reference electrolyte, which may be the original composition of the flow battery's positive electrolyte or a comparison electrolyte composition at a given state of charge, typically around 50% state of charge. The reference electrolyte in the auxiliary electrolyte reservoir 19 is in ionic communication with the positive electrolyte reservoir of the positive half-cell 13 of the reference cell 3 by tubing 21, which provides an ionic pathway conduit between the auxiliary electrolyte reservoir 19 and the positive half-cell 13, via a tubing connector 23 in the lower portion of the auxiliary electrolyte reservoir 19 and a reference cell connection (not shown) at the bottom of the positive half-cell 13.

[0103] The conduit pathway tube 21 provides an open and continuous fluid connection between the auxiliary electrolyte reservoir 19 and the electrolyte in the positive half-cell 13 without blockages or obstructions such as membranes or valves. The conduit pathway tube 21 provides an uninterrupted ionic connection between the auxiliary electrolyte reservoir 19 and the positive half-cell 13 of the reference cell 3 that allows accurate voltage difference measurements to be made between the reference electrolyte in the auxiliary electrolyte reservoir 19 and the positive electrolyte of the flow battery in the positive half-cell 13.

[0104] Tube 21 has a length of 60 cm (but could be up to 1.5 m) and an internal bore diameter of 3.2 mm, which is sufficiently inhibitory to mixing of the reference electrolyte (500 ml volume, but preferably less, which could be 100 ml, for example) with the positive electrolyte in the positive half-cell 13, so as to essentially maintain the composition of the reference electrolyte over an extended period of time, while providing an ionic connection between the reference electrolyte in auxiliary electrolyte reservoir 19 and the positive electrolyte in the positive half-cell 13, allowing continuous, reliable and consistent reference measurements.

[0105] To further inhibit fluid mixing between the reference electrolyte and the positive electrolyte in the positive half-cell 13, the tube 21 is provided with multiple bends 25, each of which changes the angle of the tube by approximately 90°. The two vertical portions 29 and horizontal portion 31 of the tube and the separate bends 25 together form a U-shaped bend. In the resulting U-shaped bend configuration, the lowest point (horizontal portion 31) is lower than both the auxiliary electrolyte reservoir 19 and the positive electrode 13.

[0106] The absence of obstructions or interruptions in the tube 21 between the auxiliary electrolyte reservoir 19 and the positive electrode 13 helps to maintain a low resistivity throughout the tube 21, ideally less than 1 MOhm, so that the voltage difference between the reference electrolyte and the positive electrolyte of the flow battery in the positive electrode half-cell 13 can be accurately measured with little interference from electronic "noise."

[0107] In use, the auxiliary electrolyte reservoir 19 should be filled with enough reference electrolyte to substantially fill the reservoir 19 and the tube 21, and should be essentially free of gas.

[0108] The auxiliary reference electrolyte arrangement 5 is physically attached to the reference cell 3 by a bracket 27 attached to the front or side of the reference cell 3 relative to the top of the auxiliary electrolyte reservoir 19. The auxiliary electrolyte reservoir 19 is thus positioned lower than the reference cell in use, further reducing the risk of reference electrolyte mixing and backflow and inflow into the positive half-cell 13.

[0109] A reference cell electrode (not shown) is disposed in each of the positive and negative half-cells 7, 13 of the reference cell 3 and associated means (not shown) for measuring the potential difference across the half-cells are provided along with means for storing and / or communicating the resulting data. An auxiliary electrode (not shown) is disposed in the auxiliary electrolyte reservoir 19 and in the measuring means.

[0110] The positive half-cell 13 and the auxiliary electrolyte reservoir 19 are each provided with a thermal sensor (not shown) for measuring the respective electrolyte temperatures.

[0111] In Figures 2a, 2b and 2c three versions of an ion pathway conduit or tube 21 for use in the state of charge indicator configuration 1 are shown. The tube 21 in Figure 2a is for use in the auxiliary reservoir configuration 5 of Figure 1. According to Figure 2a, the tube 21 has an auxiliary reservoir end 33 for connection to the auxiliary electrolyte reservoir 19 (Figure 1) via an ion pathway tube connector 23 and a reference cell end for connection to the positive electrode half-cell 13 (or any half-cell), the two ends 33, 35 being separated by a length of the tube 21 of about 60 cm. The tube in Figure 2a has an internal bore diameter of 3.2 mm and is preferably characterized by multiple bends 25 along its length to form U-shaped bends.

[0112] The tubing material can be any suitable material that is stable to the electrolyte. It can typically include one or a mixture of polymers such as polyethylene, polypropylene, polyvinyl chloride, polytetrafluoroethylene, and polyvinylidene fluoride, and optionally flexible polymers (e.g., Tygon® tubing). Preferably, the tubing is translucent or transparent (to observe any gas locks or particulate blockages).

[0113] Preferably the tubing is provided with wicking means (as described above) extending through the tubing (wicking means not shown).

[0114] Figures 2b and 2c show modified tubes 21 that may be used in place of the tube 21 of Figure 2a in any particular system. The tubes of Figures 2b and 2c differ in that they are provided with additional bends 25 to form one or more loops 37. Although the illustrated loops 37 are formed by a series of straight tube sections interspersed with 90° bends, the loops 37 may have any geometric shape, such as elliptical or circular / spiral. The provision of one loop 37 as in Figure 2b reduces the diffusion and therefore exchange of electrolyte (and in particular vanadium) between the auxiliary electrolyte reservoir 19 and the reference cell 3, thereby extending the period during which a stable reading from the auxiliary electrolyte reservoir can be achieved. The provision of a second loop 37 as in Figure 2c significantly reduces the diffusion of electrolyte between the auxiliary electrolyte reservoir 19 and the reference cell 3 with respect to the U-shaped bend of Figure 2a and the single loop version of Figure 2b. Thus, by providing one or more loops 37 (or further U-shaped bends) in tube 21 for use in configuration 1 of Figure 1, tube 21 will act to extend the period before electrolyte from reference cell 3 mixes with the electrolyte in auxiliary electrolyte reservoir 19 to an unacceptable extent, or tube 21 can be adapted by shortening it or increasing its internal bore diameter to achieve the same performance (in terms of diffusion inhibition) as that of Figure 2a.

[0115] In Figure 3, a schematic diagram of a state of charge indicator arrangement 1 according to an embodiment of the present invention comprises a reference cell 3 having a positive side 7, a negative side 13 and a voltmeter 5 for measuring potential difference disposed across the cell. The indicator arrangement further comprises an auxiliary reference electrolyte arrangement 5 comprising a reservoir 19 and a narrow bore tube 21 connected (as an ionically conductive path) to the positive side 7 of the reference cell 3. The positive side 7 and negative side 13 have respective inlets 15, 9 for positive and negative electrolyte into the reference cell 3 from pipes that supply the positive and negative electrolytes respectively from the fuel cell's electrolyte tank into the cell stack (ideally just before the cell stack) and outlets 17, 11 for returning the positive and negative electrolytes from the reference cell 3 to pipes returning from the cell stack to the electrolyte tank (or directly to the electrolyte tank).

[0116] The potential difference across the reference cell 3 may be measured by a voltmeter 5 , while the potential difference between the positive side 7 of the reference cell 3 and the auxiliary reservoir 19 can be measured via a voltmeter 39 .

[0117] 4, the location of the state of charge indicator arrangement is shown in terms of a vanadium redox flow battery 41 with a positive electrolyte tank 43 containing a positive electrolyte 45 and a negative electrolyte tank 47 containing a negative electrolyte 49. The positive and negative electrolytes 45 and 49 are circulated by a pump 51 through a cell stack 53 by a positive supply line 55 and a positive return 57 and a negative supply line 59 and return line 61. A reference cell 3 is placed in parallel with the cell stack 53 and is fed by a positive inlet 15 and a negative inlet 9 from a positive supply line 55 and a negative supply line 59 and returned to a positive return 57 and a negative return 61 via a positive outlet 17 and a negative outlet 11. The positive side 7 of the reference cell 3 is connected to an auxiliary electrolyte reservoir 19 by a curved narrow bore pipe 21.

[0118] When the pump 51 is running, electrolyte may be circulated through the cell stack 53 and through the reference cell 3. A measurement of the potential difference is best taken when the pump is running.

[0119] If the state of charge of the positive electrolyte 45 is measured to have a different state of charge than the state of charge of the negative electrolyte 49 (as estimated by measuring the potential difference across the reference cell 3), the flow battery 41 may be configured to enable corrective action, such as dosing a reducing agent to the electrolyte. EXAMPLES

[0120] In the context of vanadium oxide redox flow batteries, a state-of-charge or state-of-health indicator was set up by connecting a reference cell in parallel with the stack in the redox flow battery (5 kW stack in a 40 kWh battery) in the configuration shown in Figure 1. The auxiliary reference electrolyte was the positive electrolyte at 50% SOC.

[0121] The cells initially contained discharged electrolyte at near 0% state of charge, and the pump was run continuously to charge the cells.

[0122] 5a and 5b are plots of the potential difference across the reference cell (FIG. 5a) and the potential difference between the positive electrode of the reference cell and the auxiliary electrolyte of the auxiliary reference configuration (FIG. 5b), both plotted against the absolute charge passed through the stack.

[0123] As can be seen from Figure 5a, continued charging increases the voltage of the reference cell (as expected), and a decrease in the voltage of the reference cell is observed as the battery is discharged (followed by a very short final charging period when the voltage of the reference cell increases again). The measurable voltage range across the reference cell was 0-1.6V, with an active range of 1.25V-1.45V.

[0124] The potential difference between the positive electrode of the reference cell and the auxiliary electrode (in the auxiliary reference configuration) was also measured by a voltmeter and is shown in Figure 5b. This measurement has a low value if the positive electrolyte has a lower state of charge than the auxiliary electrolyte (at 50% state of charge) and a higher value if it exceeds the charge level of the auxiliary electrolyte. To obtain an accurate differential, temperature compensation is required for the reference cell and the auxiliary reservoir (assuming they are isothermal, then when the potential difference is zero, the positive electrolyte will be at 50% state of charge). The active voltage range between the positive electrode of the reference cell and the auxiliary electrolyte in the auxiliary reference configuration was -0.01 to 0.04 V.

[0125] Measurements of the potential difference at any particular time during the charge / discharge of the battery (or averaged over a portion of a charge cycle, a charge cycle, or multiple charge cycles) can be used to assess the state of health of the flow battery. pos and may be inserted into the above empirical formula (or used against a look-up table) to specify a value for α. EXAMPLES

[0126] A series of diffusion tests were performed to compare conduit dimensions and geometries in the context of fluid diffusion / mixing for use in the auxiliary reference electrolyte configuration of the present invention.

[0127] In this experiment, a tank of electrolyte was connected to a test tube of sulfuric acid (acting as the reference and pseudo-reference cells, respectively) and the progress of the electrolyte through the tubing was monitored. The tank and test tube were connected using various lengths of tubing in different geometric configurations to understand the relationship each of these factors has to the diffusion of the electrolyte. Because sulfuric acid is colorless, they were able to measure the concentration of the electrolyte (blue color) in the test tube using UV-vis. The concentration of electrolyte could be compared to the elapsed time to quantify the rate of diffusion.

[0128] Six comparative experiments were set up using 4.8 mm inner diameter Tygon® tubing in the following lengths and geometries: A 30cm long, double loop B Length 30cm, straight (no loop) C 30cm long, single loop D Length 50cm, straight (no loop) E 50cm long, single loop F Length 50cm, double loop

[0129] The experiment was set up as follows, using the configuration shown in FIG. 6 (showing experiments with tubes E and F above).

[0130] Two sealed branched test tubes 73 were placed in a test tube rack 75. The branched test tubes 73 were connected to an outlet 67 via 50 cm lengths of 4.8 mm inner diameter Tygon® tubing 69, 71, which extended horizontally. One length of tubing 71 had one vertically oriented loop 79 by wrapping the tubing around a rod (cork) 81 having a diameter of approximately 150-200 mm (Experiment E above). A second length of tubing 69 had two vertically oriented loops 77 by wrapping the tubing twice around the rod 81 (Experiment F above).

[0131] Before connecting to outlet 67, test tube 73 was filled with 4.2 M sulfuric acid (15.2 ml) until test tube 73 and connected tubes 69, 71 were filled, and the tube ends were clamped near their free ends. Test tube 73 was then sealed with a cap. Tubes 69, 71 were then connected to electrolyte storage container 65 via two outlets 67 located near the bottom of the container and at approximately the same height as the branches of test tube 73.

[0132] A quantity of 1.6M TMS 2 Vanadium electrolyte 63 was added to the electrolyte reservoir 65 up to a height about the height of the two outlets 67. The clamp was then removed.

[0133] To test for diffusion, 1 ml samples were taken from the tubes (at irregular intervals starting approximately 2 months after the start of the experiment) and replaced with 1 ml of sulfuric acid. The extracted samples were measured by UV / vis against a sulfuric acid standard (4.2 M).

[0134] The average permeation rates of vanadium were calculated from the measured UV / vis data and are shown below in Table 1 as diffusion rates in mol / day. [Table 1] Note: No results were recorded for Sample B, which was thoroughly mixed at the end of the experiment.

[0135] From the above testing, it was determined that with the criteria that the state of charge in the auxiliary reservoir is within 2% of the initial value after 12 months and that there are two loops in the tubing, the auxiliary reservoir can be less than 100 ml in volume, which provides an advantage in terms of cost and integration into the system while still maintaining the effectiveness of the auxiliary reservoir.

[0136] This was calculated using the following approach / assumptions: The initial state of charge of the auxiliary electrolyte reference configuration = 0.50, and the total vanadium concentration [V] = 1.8 mol dm -3 is · Concentration of V(IV) in the auxiliary electrolyte reference composition = [V(IV)] D V , there is a constant migration of V(IV) into the reference cell (and V(V) does not diffuse into the reference cell, which is obviously a "worst case" approximation). Vanadium substitution (at the charge state in the auxiliary electrolyte reference configuration) occurs at equal rates The capacity of the auxiliary electrolyte reference composition is = V

[0137] The V(IV) concentration in the auxiliary electrolyte reference configuration [V(IV)] is calculated using the following formula:

number

number

[0138] If the maximum allowable deviation from the starting SOC is 0.02 and the minimum time to this deviation is 1 year,

number

[0139] For the above tubing connections, the supplemental electrolyte standard make-up volumes shown in Table 2 below will meet the indicated evaluation criteria. [Table 2] EXAMPLES

[0140] Tests were conducted to investigate the ability of a 3 mm amorphous silica wick to maintain ionic connectivity through an ion pathway tube (having an inner diameter of 3.18 mm) when the tube contained trapped gas bubbles.

[0141] A tube material was selected that was known to react with the electrolyte and generate gas during testing, such that gas bubbles were generated. A 3 mm amorphous silica wick was inserted along the entire length of the tube. The tube was placed in fluid communication between two electrolyte reservoirs. The reservoirs and tube were filled with electrolyte. Large gas bubbles formed along nearly the entire length of the tube during testing.

[0142] Testing continued under these conditions to monitor the performance of the silica wick. The resistance between the carbon fiber rods placed in the electrolyte reservoir increased over time as large bubbles formed, from approximately 1.6k ohms to a peak of approximately 40k ohms at day 21. The peak is estimated to have occurred at the maximum extent of bubble formation. As testing continued, the resistance steadily decreased to approximately 15k ohms at day 43 and remained stable until the time of reporting (i.e., day 70). Over the test period, the wick changed color from dark blue to light green and appeared to remain stable with no signs of degradation.

[0143] As can be seen, despite complete occlusion of the tube by the large air bubbles, the resistance between the carbon fiber rods in each reservoir remained at a relatively low level and stable over time. Thus, the tests of Example 3 show that even when a large air bubble completely occludes the tube acting as a conduit for the ionic pathway between two electrolyte reservoirs, the wicking means disposed in the tube is still capable of maintaining ionic conductivity through the length of the tube and between the electrolyte reservoirs.

[0144] The present invention has been described with reference to preferred embodiments, however, it will be understood that variations and modifications can be effected by those skilled in the art without departing from the spirit of the invention.

Claims

1. A charge state indicator configuration for a redox flow battery system comprising a cell stack of redox flow batteries, a positive electrode electrolyte tank and piping for circulating the positive electrode electrolyte through the cell stack, and a negative electrode electrolyte tank and piping for circulating the negative electrode electrolyte through the cell stack of the flow battery, A reference cell configuration comprising means for measuring the potential difference between the positive electrode electrolyte in or from the positive electrode electrolyte tank of a flow battery and the negative electrode electrolyte in or from the negative electrode electrolyte tank of a flow battery, At least one auxiliary reference electrolyte configuration, A separate auxiliary reference electrolyte reservoir having a known charge state accommodates a redox electrode corresponding to a reference electrolyte of a known composition equivalent to the desired or initial composition of the electrolyte of the flow battery that provides the reference, Means for measuring the potential difference between the aforementioned or each auxiliary reference electrolyte and each of the electrolytes in the reference cell configuration, and Ion pathway conduits connecting the above or each auxiliary reference electrolyte reservoir to the respective electrolytes of the reference cell configuration, wherein the conduits are configured for low flow diffusion capacity or velocity, and the conduits include wicking means for absorbing electrolytes and maintaining ionic connections between the auxiliary reference electrolyte reservoirs and the respective electrolytes of the reference cell configuration. A supplementary reference electrolyte configuration comprising, A charging status indicator equipped with a charging status indicator.

2. The charge state indicator according to claim 1, wherein the wicking means extends continuously along the entire length of the ion pathway conduit.

3. The charge state indicator according to claim 1, wherein the wicking means extends from the ion pathway conduit to the auxiliary reference electrolyte reservoir and / or into the respective electrolytes of the reference cell configuration.

4. The charging state indicator according to claim 1, wherein the wicking means is hydrophilic and / or electrically nonconductive.

5. The charge state indicator according to claim 1, wherein the wicking means comprises silica (e.g., silica fibers), ceramic (e.g., ceramic fibers), and / or hydrophilic polymer (e.g., hydrophilic polymer fibers).

6. The charging state indicator according to claim 1, wherein the wicking means is preferably a wick or cord made of bundled (e.g., woven, twisted, or braided) fibers.

7. The charge state indicator according to claim 1, wherein the ion pathway conduit comprises a tubular member that provides a fluid connection between the auxiliary reference electrolyte reservoir and the respective electrolytes of the reference cell configuration.

8. The charging status indicator according to claim 7, wherein the wicking means is disposed inside the tubular member, and preferably the wicking means is a wick or cord disposed inside the tubular member.

9. The charge state indicator according to claim 1, wherein the ion pathway conduit has a resistivity of 1 M ohm or less.

10. The charge state indicator according to claim 1, wherein the ion pathway conduit is openly fluid-connected to the auxiliary reference electrolyte and the respective electrolytes of the reference cell configuration without having a membrane or barrier between them.

11. The charge state indicator according to claim 1, wherein the ion pathway conduit and the separate auxiliary reference electrolyte reservoir are immersed in water and substantially free of gas.

12. The charge state indicator according to claim 1, wherein the ion pathway conduit has an inner diameter of 0.5 mm to 10 mm, preferably 1 to 5 mm, more preferably 2.5 to 4 mm, for example 3 to 3.5 mm.

13. The charge state indicator according to claim 1, wherein the wicking means is a wick or cord having a diameter smaller than the inner diameter of the ion path conduit (for example, such that the wick or cord is loosely fitted into the conduit).

14. The charge state indicator according to claim 1, wherein the ion pathway conduit has a length of 5 cm to 10 m, preferably up to about 5 m, more preferably up to about 2 m, even more preferably 10 cm to 1.5 m, more preferably 15 cm to 1.2 m, for example, 20 cm to 1 m or 75 cm and preferably up to about 30 to 50 cm.

15. The charge state indicator according to claim 12, wherein the ion pathway conduit is a tubular member having an inner diameter of 2.5 to 4 mm and a length of 20 cm to 2 m.

16. The charge state indicator according to claim 1, wherein the ion pathway conduit preferably has a vertical component along its length and has one or more curved or bent portions, for example, a U-shaped bend or a loop.

17. The charge state indicator according to claim 16, wherein the ion pathway conduit comprises one or more loops along its length.

18. The charge state indicator according to claim 1, wherein the auxiliary reference electrolyte reservoir is preferably configured to hold at least 100 ml of reference electrolyte, in a volume of 200 ml to 1000 ml, such as 400 to 600 ml, which is 10 L or less.

19. The charge state indicator according to claim 1, wherein the auxiliary reference electrolyte configuration comprises a temperature sensor configured to measure the temperature of the electrolyte in the auxiliary reference electrolyte reservoir.

20. The charge state indicator according to claim 19, wherein a temperature sensor is provided to measure the temperature of the respective electrolytes in the reference cell configuration of the flow battery and / or in the respective electrolyte tanks or associated circulation systems.

21. The charge state indicator according to claim 1, wherein the auxiliary reference electrolyte corresponds to the positive electrode electrolyte of the flow battery, the auxiliary reference electrolyte configuration is configured for ion path conduit connection, and is configured for measuring the potential difference between the auxiliary reference electrolyte reservoir and the positive electrode electrolyte of the reference cell configuration.

22. The charge state indicator according to claim 1, comprising an auxiliary reference electrolyte configuration, wherein the auxiliary reference electrolyte corresponds to the negative electrode electrolyte of the flow battery, the auxiliary reference electrolyte configuration is configured for ion path conduit connection, and is configured for measuring the potential difference between the auxiliary reference electrolyte reservoir and the negative electrode electrolyte of the reference cell configuration.

23. The charge state indicator according to claim 1, wherein the reference cell configuration comprises a reference cell having a positive electrode half cell configured for fluid circulation communication with the positive electrode electrolyte tank of a flow battery, and a negative electrode half cell having a negative electrode electrolyte reservoir configured for fluid circulation communication with the negative electrode electrolyte tank, the means for measuring the potential difference is configured to measure the potential difference across the reference cell, and the means for measuring the potential difference between the or each auxiliary reference electrolyte and the respective electrolytes of the reference cell configuration is configured to measure the potential difference between the or each auxiliary reference electrolyte and the respective half cells of the reference cell.

24. A charge state indicator according to claim 1, configured to measure the charge state over a predetermined period of time in accordance with a predetermined system operation.

25. The charge state indicator according to claim 1, further comprising a processor for controlling temperature and / or voltage measurement of the charge state indicator, and / or configured to communicate the measured values ​​to a control device or data logger of the flow battery.

26. The charge state indicator according to claim 1, wherein the redox flow battery is a vanadium redox flow battery.

27. A healthy state indicator system for a redox flow battery system, comprising a charge state indicator configuration, the charge state indicator configuration comprising the charge state indicator configuration defined in claim 1, and configured to determine the healthy state of the redox flow battery system from the charge state indicator configuration, preferably (optionally, continuously, periodically or intermittently) taking the steps of: obtaining measured values ​​of the charge state of at least one electrolyte and the other electrolyte or substitute thereof of the flow battery; preferably identifying the relative oxidation state of each of the electrolytes; and preferably generating an alarm, display or corrective action in response to the identified relative oxidation state which is outside a predetermined limit value.

28. A supplementary reference electrolyte configuration for a healthy state indicator as defined in claim 1, A separate auxiliary reference electrolyte reservoir having a known charge state and housing a redox electrode corresponding to a reference electrolyte of a known composition equivalent to the desired or initial composition of the electrolyte of the flow battery that provides the reference, Means for measuring the potential difference between the aforementioned or each auxiliary reference electrolyte and the associated electrolyte of the reference cell configuration or the associated half cell of the reference cell, An ion pathway conduit for connecting the above or each auxiliary reference electrolyte reservoir to the electrolyte in each electrolyte in the reference cell configuration or in each half cell of the reference cell, wherein the conduit is configured for low flow diffusion capacity or velocity, and the conduit includes wicking means for absorbing electrolyte and maintaining the ionic connection between the auxiliary reference electrolyte reservoir and the respective electrolyte in each half cell of the reference cell configuration or the reference cell, A supplementary reference electrolyte configuration comprising the above.

29. A redox flow battery comprising: a cell stack of a redox flow battery; a positive electrode electrolyte tank and piping for circulating the positive electrode electrolyte through the cell stack; a negative electrode electrolyte tank and piping for circulating the negative electrode electrolyte through the cell stack of the flow battery; and a charge state indicator as defined in claim 1.

30. A method for monitoring the charge state and / or health state of a redox flow battery, comprising the steps of: providing a charge state indicator as described in claim 1, causing the charge state indicator to periodically measure the charge on the reference cell and the charge between the auxiliary reference electrolyte and each half cell of the reference cell to determine the charge state of the system; and optionally, issuing an alert for an imbalance in the electrolyte charge of the flow battery in accordance with the charge state which differs by a predetermined threshold on the reference cell of the flow battery.

31. A method for maintaining a balanced charge state or oxidation state in a redox flow battery, A step of monitoring the charge state in the flow battery by providing a charge state indicator as described in claim 1, and having the charge state indicator perform periodic, operation-dependent, or event-dependent measurements of the charge on the reference cell and the charge between the auxiliary reference electrolyte and each half cell of the reference cell, thereby identifying the charge state and / or healthy state of the system; A step of generating one or more maintenance operations applied to the flow battery depending on the dispersion of the charge state or oxidation state between the positive electrode electrolyte and the negative electrode electrolyte, which exceeds one or more predetermined thresholds or satisfies one or more predetermined evaluation criteria, A method for providing this.

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