Flow cell

Abstract

The present application relates to a method of reconditioning a flow cell The flow cell comprises a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber. The method comprises delivering a flow cell reconditioning liquid into the gas anode chamber.

Description

[0001] FLOW CELL

[0002] TECHNICAL FIELD

[0003] The present disclosure relates to redox flow cells. The disclosure relates more particularly, but not necessarily exclusively, to methods of reconditioning flow cells using a flow cell reconditioning liquid.

[0004] BACKGROUND

[0005] Redox flow cells, such as redox flow batteries (RFBs), are electrochemical apparatus for power delivery by means of a chemical redox reaction. In the context of an RFB, a chemical redox reaction typically proceeds in one direction in a power delivery mode (e.g. with a redox active species becoming reduced and another redox active species becoming oxidised) and in the opposite direction during an energy storage mode.

[0006] In the power delivery mode, redox active species are supplied to electrodes where they react electrochemically to produce electrochemical power. However, a problem associated with such flow cells is that they degrade over their lifetime (i.e. their energy efficiency decreases).

[0007] It is desirable to provide an improved or alternative flow cell and / or method of operating or using same (or components thereof) and / or to obviate or mitigate issues with existing flow cells and / or methods of operating a flow cell, whether identified herein or otherwise.

[0008] SUMMARY

[0009] According to a first aspect of the present disclosure, there is provided a method of reconditioning a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; and wherein the method comprises delivering a flow cell reconditioning liquid into the gas anode chamber. According to a second aspect of the present disclosure, there is provided a method of operating a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; wherein the method comprises: operating the flow cell to generate electrical power; and delivering a flow cell reconditioning liquid into the gas anode chamber, optionally wherein the method further comprises withdrawing the flow cell reconditioning liquid from the gas anode chamber.

[0010] According to a third aspect of the present disclosure, there is provided a use of a flow cell reconditioning liquid to recondition a flow cell which comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; and the use comprises delivering the flow cell reconditioning liquid into the gas anode chamber.

[0011] According to a fourth aspect of the present disclosure, there is provided a flow cell comprising: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface; a liquid cathode chamber having a flow cell liquid cathode; a flow cell membrane between the gas anode chamber and the liquid cathode chamber, wherein the flow cell membrane is capable of selectively passing protons; conduits configured to supply electrochemically active species to the anode and to the cathode in a power delivery mode, and to carry generated electrochemically active species away from the anode and away from the cathode in an energy storage mode; and a flow cell reconditioning liquid reservoir and a pump, wherein the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber. According to a fifth aspect of the present disclosure, there is provided a retrofit kit adapted to be fitted to a flow cell having a gas anode chamber, wherein: the retrofit kit comprises a flow cell reconditioning liquid reservoir and a pump; and the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber (e.g. over a flow cell gas anode active surface of a flow cell gas anode therein and / or a gas anode chamber face of a flow cell membrane therein).

[0012] DEFINITIONS

[0013] Unless otherwise defined herein, terms have their usual meaning, e.g. as defined in the IUPAC Compendium of Chemical Terminology (2019), version 3.0.1 , informally known as the "Gold Book".

[0014] In accordance with standard terminology in the field of flow cells, the terms “anode” and “cathode” are defined by the functions of the electrodes in the power delivery mode. To avoid confusion, the same terms are maintained herein to denote the same electrodes whether in a power delivery mode of operation or an energy storage mode of operation.

[0015] The terms “anolyte” and “catholyte” are used to denote the electrolyte in contact with the “anode” and “cathode”.

[0016] A flow cell is a reversible electrochemical cell for the conversion of chemical energy into electricity. A flow cell comprises a gas anode chamber comprising a reversible flow cell gas anode and an anolyte fluid (i.e. a gas, in the context of the present application) and a liquid cathode chamber comprising a reversible flow cell liquid cathode and a catholyte fluid (i.e. a liquid, in the context of the present application). A flow cell membrane is provided between the two chambers and is configured to exchange ions (e.g. protons, particularly in the context of a hydrogen anolyte) between the two chambers.

[0017] Here, it will be understood that a “flow cell gas anode” is an anode configured to be used with a gaseous anolyte. This term is not intended to imply necessarily that the anode is composed of gas. Similarly, the term “flow cell liquid cathode” refers to a cathode configured to be used with a liquid catholyte and is not intended to imply necessarily that the anode is composed of liquid.

[0018] The flow cell membrane may have a “gas anode chamber face” and a “liquid cathode chamber face”. These terms refer to the faces of the membrane that are on the gas anode chamber side and the liquid cathode chamber side of the flow cell, respectively. Thus, the gas anode chamber is partially walled by the gas anode chamber face of the flow cell membrane, and the liquid cathode chamber is partially walled by the liquid cathode chamber face of the flow cell membrane.

[0019] The chambers of electrolyte (catholyte and anolyte) fluid may be charged separately with different power delivery / energy storage species that are each able to undergo reversible reduction-oxidation (redox) reactions. This allows the power delivery / energy storage species in one chamber to undergo, for example, an oxidation reaction while the power delivery / energy storage species in the other chamber undergoes a reduction reaction. The redox reactions cause a net flow of electrons between the chambers, thus generating an electrical current.

[0020] A flow cell may be kept in idle mode, wherein no load is applied and so no net redox reaction is occurring.

[0021] The term “reconditioning”, as used herein in the context of a flow cell, refers to a process of returning the flow cell to a desired state for use. In the present disclosure, reconditioning a flow cell may involve the use of a flow cell reconditioning liquid to (A) unblock a blocked flow cell gas anode active surface and / or (B) rehydrate a dehydrated flow cell membrane.

[0022] In process (A), it has been found that washing a blocked flow cell gas anode active surface with the flow cell reconditioning liquid removes blockages (e.g. blocking metal species) from the active surface. Without wishing to be bound by theory, it is believed that a combination of mechanical removal and solvation by the reconditioning liquid is responsible for the removal of the blockages. After washing, these blockages may be suspended and / or dissolved in the reconditioning liquid, which can then be removed from the flow cell gas anode and away from the anode. This process increases the useful lifetime of the flow cell since it restores / preserves (reconditions) the active ability of the active surface (e.g. in comparison to a flow cell which is not reconditioned in this way).

[0023] The term “flow cell gas anode active surface”, as used herein, refers to a surface in a flow cell gas anode upon which electrochemical reactions (e.g. H++ e- %H2, in the context of a hydrogen gas anode) occur. Such a surface may be a catalytic surface or a non-catalytic surface. The term “blocked” (and / or “blockages”, and the like), as used herein in the context of a flow cell gas anode active surface, means that sites on the active surface upon which chemical / electrochemical reactions would usually occur are obstructed / clogged, e.g. by a deposited / adsorbed chemical species, such as a metal species. Blocking may comprise (i) physical obstruction (e.g. of a pore or opening in the surface) which sterically prevents attachment of an electrochemical reactant, and / or (ii) chemical deposition which reduces a reactive functionality of a site on the surface through chemical effects. Such obstruction may prevent the active catalytic sites from being able to catalyse / facilitate redox reactions and so a blocked catalytic surface may have a reduced catalytic performance as compared with a pristine / unblocked catalytic surface. Blocking may occur primarily in pores on the catalytic surface (e.g. those pores may become obstructed / clogged).

[0024] Process (B) has been found to be useful in flow cells having a flow cell gas anode, since the flow cell membrane in such flow cells may become dehydrated. This dehydration is particularly apparent on the gas anode chamber face of the flow cell membrane, as this face is generally in contact with a gaseous (rather than aqueous) anolyte. It will be appreciated that exposure of a wet membrane to a gaseous medium (particularly a flowing gaseous medium such as the anolyte) will cause dehydration of the membrane over time. As discussed above, flow cell membranes which are dehydrated in this way tend to have lower ionic conductivity and therefore poorer permeability to protons, which reduces the energy efficiency of the flow cell. It has been found that rehydrating the membrane by applying a reconditioning liquid to the gas anode chamber restores the energy efficiency of the flow cell and therefore increases its useful lifetime.

[0025] The term “dehydrated”, as used herein in the context of a flow cell membrane, means that the water content in the membrane has reached a lower-than-desired level (e.g. as defined elsewhere herein) which negatively affects the performance of the membrane. A dehydrated flow cell membrane may still comprise an amount of water (but still be at an unfavourably low level, i.e. it may be partially dehydrated). For example, the water content may have decreased to a level that reduces the ionic conductivity of the membrane, which therefore reduces the permeability of the membrane to protons and thus reduces the energy efficiency of the flow cell.

[0026] In contrast, a “hydrated” flow cell membrane comprises a desired water content which facilitates performance of the membrane (e.g. ionic conductivity, e.g. substantially optimal performance / ionic conductivity). The terms “rehydrate”, “rehydrated”, “rehydrating” (and the like), as used herein in the context of a flow cell membrane, means that the flow cell membrane was initially hydrated before becoming dehydrated, and has subsequently been brought back into a hydrated state (e.g. by an active step of rehydrating the membrane by passing the flow cell reconditioning liquid over the flow cell membrane). In other words, the membrane has been used previously in a flow cell and has been dehydrated over time (e.g. by operation of the flow cell as described herein).

[0027] The term “flow cell reconditioning liquid”, as used herein, refers to a liquid-state substance that is suitable for washing / rehydrating components of a flow cell and thereby reconditioning the flow cell. This term is not intended to encompass a humidified gas. For example, the term is not intended to encompass a gaseous anolyte comprising vapour (e.g. water vapour) therein.

[0028] The term “supporting electrolyte” is well-known in the art. For the avoidance of any doubt, as used herein, this term refers to a solution containing ions which are not electroactive in the range of potentials applied to the flow cell. In other words, the ions of the supporting electrolyte are not involved in an electron-transfer process, but instead support the overall operation of the flow cell by providing additional charged species.

[0029] The term “strong acid” is well-known in the art. For the avoidance of any doubt, this term refers to a compound which, in aqueous solution, substantially and / or fully dissociates into proton(s) and associated conjugate base(s), e.g. wherein at least around 95% of the acid dissociates, preferably at least around 96%, preferably at least around 97%, preferably at least around 98%, preferably at least around 99%, preferably around 100%, based on the molar amount of acid in the aqueous solution.

[0030] The term “redox-active metal species”, as used herein, refers to a metal species which can be reversibly oxidised or reduced under flow cell conditions. The direction of oxidation / reduction depends on the mode of operation of the flow cell, i.e. whether the flow cell is in power delivery mode or energy storage mode.

[0031] The term “redox-inert species”, as used herein, refers to a species which cannot be oxidised or reduced under flow cell conditions.

[0032] When a flow cell reconditioning liquid is said to “consist essentially of” redox-inert species, this means that the flow cell reconditioning liquid comprises the redox-inert species at an amount that one skilled in the art would consider equivalent to a flow cell reconditioning liquid consisting only of redox-inert species (e.g. having the same function or result, and / or achieved that function or result substantially in the same way). Suitably, a flow cell reconditioning liquid which consists essentially of redox-inert species comprises greater than or equal to about 90 vol% of redox-inert species, more suitably greater than or equal to about 95 vol%, more suitably greater than or equal to about 98 vol%, more suitably greater than or equal to about 99 vol%; based on the total volume of the flow cell reconditioning liquid.

[0033] The term “ionomer” is well-known in the art and refers to a polymer / oligomer which provides a pathway for H+to move from inside a gaseous electrode to the flow cell membrane during discharge (and vice versa in charge). This term refers to a molecule comprising two or more (such as three or more, four or more, five or more, six or more, seven or more, eight or more, nine or more, or ten or more) monomer units, a portion of which (e.g. up to around 15%) are ionic and / or readily ionisable. For example, around 10% of monomer units in an ionomer may be ionic and / or readily ionisable. An ionomer may comprise many monomer units, such as 100 or more or 1000 or more monomer units. The chemical building blocks of the membrane and ionomers are very similar, so they suffer from similar de-hydration / blocking problems. If the H+transport functionality (of ionomers or of the active sites in the membrane) is compromised, the H+mobility decreases. In other words, the reduction of H+mobility is the root cause for both the failure of the gaseous electrode and the decrease in conductivity of the membrane.

[0034] The ionic conductivity of a proton exchange membrane (PEM) is primarily determined by its water content (i.e. , hydration). There is extensive literature on the topic (see for instance: Kusoglu et al., “New Insights into Perfluorinated Sulfonic-Acid Ionomers”, Chem. Rev. 2017, 117, 3, 987-1104, January 23, 2017, https: / / doi.org / 10.1021 / acs.chemrev.6b00159, https: / / doi.Org / 10.1021 / acs.chemrev.6b00159 (accessible December 2023), the entire contents of which are incorporated herein by reference). Any process or event that affects the hydration of the membrane will therefore compromise or alter its ionic conductivity.

[0035] The term “proton selective”, as used herein in the context of a flow cell membrane, means the membrane will preferentially allow protons to pass through it in comparison to other species (e.g. other ions such as manganese and titanium ions). In other words, a proton selective membrane is predominantly permeable to protons only. BRIEF DESCRIPTION OF THE FIGURES

[0036] Fig. 1 Lifetime test (full charge / discharge cycles at 150 mA-cm-2at 50°C) showing the efficiency recovery of a flow cell after multiple reconditioning steps carried out every -100 h.

[0037] Fig. 2 Lifetime test (full charge / discharge cycles at 150 mA-cm-2) showing the recovery in performance of a flow cell after two reconditioning steps carried out with DI water. In blue (dots) is the cell voltage efficiency; in red (crosses) is the cell Ohmic resistance, measured as High Frequency Resistance (HFR) via Electrochemical Impedance Spectroscopy.

[0038] Fig. 3 Constant current (150 mA-cnv2) charging test (made of 2 h sub-charging blocks) performed in the absence of supporting electrolyte in the H2 compartment of a flow cell (red trace) and with a supporting electrolyte made of 7.5 M H2SO4 in DI water (blue trace).

[0039] Fig. 4 Constant current (150 mA-cnv2) charging test (made of 2 h sub-charging blocks) performed in the absence of supporting electrolyte in the H2 compartment of a flow cell (green trace) and with a supporting electrolyte made of Mg-based supporting electrolyte (black) and Mg / Ti- based supporting electrolyte (blue).

[0040] Fig. 5 Schematic of a flow cell that may require reconditioning in accordance with the present disclosure.

[0041] DETAILED DESCRIPTION

[0042] According to a first aspect of the present disclosure, there is provided a method of reconditioning a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; and wherein the method comprises delivering a flow cell reconditioning liquid into the gas anode chamber. The flow cell gas anode active surface may be or have a portion which is blocked, and reconditioning may comprise washing the blocked flow cell gas anode active surface with a flow cell reconditioning liquid to unblock the flow cell gas anode active surface during said delivering the flow cell reconditioning liquid into the gas anode chamber.

[0043] The flow cell membrane may have a gas anode chamber face and a liquid cathode chamber face, the flow cell membrane, or a portion thereof, may be dehydrated, and reconditioning may comprise rehydrating the dehydrated flow cell membrane, from the gas anode chamber face thereof, with the flow cell reconditioning liquid during said delivering the flow cell reconditioning liquid into the gas anode chamber.

[0044] During standard operation, a flow cell can degrade over time (i.e. its energy efficiency decreases). It is believed that chemical / physical processes occurring in and around the gas anode chamber during standard operation contribute to degradation.

[0045] As a first example, the flow cell gas anode active surface can become blocked, e.g. by a blocking metal species that has crossed over from a catholyte to the gas anode chamber (through the flow cell membrane). Without wishing to be bound by theory, it is believed that such blocking leads to degradation of the flow cell by obstructing sites on the active surface that would otherwise facilitate electrochemical reactions (e.g. H++ e- %H2, in the context of a hydrogen gas anode). This therefore reduces the energy efficiency of the flow cell over time.

[0046] A second issue is that the flow cell membrane can become dehydrated, since a face of the membrane (which faces the gas anode chamber) is typically only in contact with gases (rather than liquids). Over time, the gases carry away moisture, leading to said dehydration. Membranes that have become dehydrated tend to have a lower ionic conductivity and therefore poorer permeability to protons (e.g. due to suppression and thus inefficiency of the transport mechanism of ionomers in the membrane), which reduces the energy efficiency of the flow cell through Ohmic losses.

[0047] It has been found that reconditioning the flow cell by delivering a reconditioning liquid into the gas anode chamber mitigates both of these issues.

[0048] Firstly, it is found that said delivering has the effect of washing the blocked flow cell gas anode active surface with the reconditioning liquid, thereby removing blockages and freeing up sites on the surface for facilitation (e.g. catalysis) of electrochemical reactions. Indeed, washing can be achieved merely by delivering said flow cell reconditioning liquid into the gas anode chamber, e.g. by contact of said flow cell reconditioning liquid with the flow cell gas anode active surface during said delivering. Washing may comprise additional activity, such as agitation, pumping, etc. as described more fully below. Usefully, after said washing, the blockages may then be removed from the flow cell as a suspension or solution in the reconditioning liquid.

[0049] Secondly, it is found that reconditioning the flow cell with the reconditioning liquid also rehydrates the flow cell membrane, thus improving the ionic conductivity and / or overall performance of the membrane. For example, the proton conductivity at an interface between the membrane and a catalytic layer can be improved. Indeed, rehydrating can be achieved merely by delivering said flow cell reconditioning liquid into the gas anode chamber (which is partially walled by the flow cell membrane), e.g. by contact of said flow cell reconditioning liquid with the flow cell membrane during said delivering.

[0050] Reconditioning in this way has been found to reduce drastically the degradation rate of the flow cell and thus increase its useful lifetime significantly. Surprisingly, in some cases reconditioning has been found to lead to a full recovery of the original performance of the flow cell.

[0051] The flow cell gas anode may be a flow cell hydrogen gas anode.

[0052] The flow cell reconditioning liquid may comprise an acid, optionally a strong acid, optionally wherein the flow cell reconditioning liquid has a pH of at most around 7, optionally at most around 3 and / or optionally around 0 or lower, optionally wherein the pH is around 0 or lower to around 3, optionally wherein the flow cell reconditioning liquid comprises HCI, H3PO4, HCIO4 and / or H2SO4 (optionally H2SO4).

[0053] The acid may be present in the flow cell reconditioning liquid at a concentration of around 1 M to 10 M , optionally 1 to 8 M (e.g. in the context of H2SO4).

[0054] The flow cell reconditioning liquid may comprise a salt, optionally a metal salt, optionally a sodium, magnesium, potassium, aluminium, manganese or titanium salt, optionally a manganese or titanium salt. The salt may be present in the flow cell reconditioning liquid at a concentration of around 0.1 to 7 M, optionally around 0.1 to 5 M, optionally around 0.5 to 3 M, optionally around 1 to 2 M (e.g. in the context of a manganese or titanium salt).

[0055] Although reconditioning is a useful process for the reasons given above, it has been found that flowing a reconditioning liquid into / through a gas anode chamber can (in some instances) lead to undesired water permeation into the catholyte, through the flow cell membrane. Without wishing to be bound by theory, it is believed that this water permeation is due to an osmotic pull towards the catholyte in instances where the solvent concentration in the catholyte is lower than that of reconditioning liquid. In some instances, this is undesirable because it can lead to dilution of the catholyte.

[0056] It has been found that including acids and / or salts in the flow cell reconditioning liquid, particularly at the concentrations above, reduces water permeation through the membrane. Without wishing to be bound by theory, it is believed that this is because inclusion of these ionic species in the reconditioning liquid helps to match the ionic strength and osmotic pressure of the catholyte and the reconditioning liquid, reducing the thermodynamic pull towards the catholyte.

[0057] Usefully, the acid and / or salt can be chosen to be the same as acids / salts in the catholyte. This would mean that any crossing over of acids / salts through the membrane does not lead to the introduction of new species in either the catholyte or the reconditioning liquid.

[0058] It is also preferred that acids / salts are chosen which do not lead to poisoning of the flow cell membrane active surface (e.g. a catalytic surface).

[0059] The flow cell reconditioning liquid may be aqueous. The flow cell reconditioning liquid may be water, optionally deionised water.

[0060] The flow cell reconditioning liquid may be a supporting electrolyte. A supporting electrolyte is a useful form of reconditioning liquid because its constituent ionic species are not electrochemically active in the range of potentials applied to the anode and can therefore be pumped into / through the gas anode chamber during charge of the flow cell without interfering with normal operation. This is a particularly useful embodiment because there is no need for a discrete reconditioning stage separate from the charge and discharge stages; the charge and reconditioning stages can be combined. The flow cell gas anode active surface may:

[0061] (a) be a catalytic surface; and / or

[0062] (b) comprise a metal, optionally a catalyst suitable for catalysing redox reactions with a hydrogen anolyte, optionally platinum; and / or

[0063] (c) comprise an ionomer, optionally a perfluorinated sulfonic acid (PFSA) ionomer (e.g. a long-chain or short-chain PFSA ionomer) or a hydrocarbon ionomer (e.g. an aromatic, nonaromatic or polyimide ionomer).

[0064] The flow cell membrane may also comprise said ionomer (c).

[0065] The flow cell gas anode active surface may be blocked with a blocking metal species, optionally a catholyte crossover metal species, optionally a redox-active metal species, optionally a redox-active transition metal species, optionally a redox-active transition metal salt, optionally a sodium, magnesium, potassium, aluminium, manganese or titanium salt, optionally a manganese or titanium salt. The salt may comprise a sulphate.

[0066] The blocking metal species may be soluble in the flow cell reconditioning liquid. It has been found that the use of a flow cell reconditioning liquid in which a blocking metal species is soluble is particularly effective at removing said blocking metal species, since solvation of the blocking metal species provides an additional thermodynamic driving force for unblocking the active surface.

[0067] The gas anode chamber may be held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar.

[0068] Said delivering may comprise delivering the flow cell reconditioning liquid from a flow cell reconditioning liquid reservoir separated from and in fluid communication with the gas anode chamber. The method may comprise pumping the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir into the gas anode chamber. The method may further comprise pumping the flow cell reconditioning liquid from the gas anode chamber back to the flow cell reconditioning liquid reservoir. It has been found that such an embodiment is particularly efficient, because the reconditioning liquid can be continuously circulated through the flow cell and reused in several reconditioning steps.

[0069] Reconditioning may comprise:

[0070] (a) circulating the flow cell reconditioning liquid within the gas anode chamber; and / or (b) inserting the flow cell reconditioning liquid into the gas anode chamber and subsequently removing the flow cell reconditioning liquid from the gas anode chamber; or

[0071] (c) flowing the flow cell reconditioning liquid through the gas anode chamber in a continuous stream.

[0072] Reconditioning may be carried out when the flow cell is in:

[0073] (a) an idle mode;

[0074] (b) an energy storage mode; and / or

[0075] (c) a power delivery mode.

[0076] Said reconditioning may be carried out when the state-of-charge of the flow cell is at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%.

[0077] Said reconditioning may be carried out every 10 hours to 100 days, optionally every 1 to 100 days, optionally every 2 to 100 days, optionally every 3 to 100 days, optionally every 4 to 100 days, optionally every 5 to 100 days, optionally every 10 to 100 days, optionally every 15 to 100 days. It has been found that reconditioning a flow cell at this range of frequencies is a particularly favourable balance between the expense of time / resources needed to perform the reconditioning and the maintenance of an adequate performance of the flow cell.

[0078] Said reconditioning may be carried out: for at least around 10 seconds, optionally at least around 2 minutes; for at most around 60 minutes, optionally at most around 30 minutes; optionally at most around 5 minutes; and / or for around 10 seconds to 60 minutes, optionally around 2 to 5 minutes.

[0079] It has been found that reconditioning for this duration is a particularly favourable balance between the expense of time / resources needed to perform the reconditioning and the restoration of an adequate performance of the flow cell.

[0080] Said reconditioning may be carried out: at a flow rate of around 0.5 to 4 mL-min'1-cnr2, optionally around

[0081] 0.5 to 3 mL-min'1-cm'2; and / or at a temperature of at least around 0°C, optionally at least around 20°C, optionally at most around 80°C, optionally at most around 50°C, optionally around 20°C to 50°C. It has been found that reconditioning under these conditions is particularly effective at unblocking the flow cell gas anode active surface and / or rehydrating the dehydrated flow cell membrane. For example, the range of temperatures above has been found to maximise the unblocking ability of the reconditioning liquid (e.g. by providing sufficient thermal energy for solvation or other unblocking means) whilst avoiding issues with the stability of the flow cell membrane (which may occur at higher temperatures).

[0082] The liquid cathode chamber may comprise a liquid catholyte.

[0083] The liquid catholyte may comprise a primary metal species, optionally a transition metal species, optionally a sodium, magnesium, potassium, aluminium, manganese species, optionally manganese, titanium or aluminium (e.g. Al3+), optionally Mn2+and / or Mn3+; or optionally Ti3+and / or Ti4+. The liquid catholyte may comprise a sulphate and / or hydrogensulphate.

[0084] The liquid catholyte may further comprise:

[0085] (a) a secondary metal species, optionally a transition metal species, optionally a titanium species, optionally a Ti3+ion complex and / or a Ti4+ion complex; and / or

[0086] (b) an acid, optionally a strong acid. The liquid catholyte may have a pH of at most around 7, optionally at most around 3 and / or optionally around 0 or lower. The pH may be around 0 or lower to around 3. The liquid catholyte may comprise HCI, H3PO4, CH3SO3H, HCIO4 and / or H2SO4 (optionally H2SO4).

[0087] As mentioned above, ions in the catholyte (e.g. metal species and / or acids) can usefully be chosen to match those of the reconditioning liquid, so that any crossing over of acids / salts through the membrane does not lead to the introduction of new species in either the catholyte or the reconditioning liquid.

[0088] The flow cell reconditioning liquid and the liquid catholyte may each have molar solvent concentrations which differ by at most around 30%, optionally at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%, optionally at most around 8%, optionally at most around 6%, optionally at most around 4%, optionally at most around 2%.

[0089] As mentioned above, it has been found that flowing a reconditioning liquid into / through a gas anode chamber can lead to undesired water permeation into the catholyte, through the flow cell membrane. Usefully, it has been found that minimising the difference in solvent concentrations between the reconditioning liquid and the catholyte mitigates this undesired water permeation.

[0090] Without wishing to be bound by theory, it is believed that a smaller difference in solvent concentration leads to a smaller difference in osmotic pressure between the two substances, which in turn means that there is less of a thermodynamic pull of solvent across the flow cell membrane.

[0091] The liquid catholyte may be kept stagnant in the liquid cathode chamber during reconditioning. Here, the liquid cathode chamber may be isolated from the rest of the flow cell.

[0092] The liquid catholyte may alternatively be drained from the liquid cathode chamber prior to reconditioning. Here, the method may further comprise reintroducing the liquid catholyte after reconditioning.

[0093] The flow cell membrane may be a proton selective membrane. The use of a flow cell membrane which is proton selective helps to minimise crossing over of non-proton species through the membrane during reconditioning, such as water, acids and salts.

[0094] Prior to rehydrating the dehydrated flow cell membrane, the dehydrated flow cell membrane may have a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%. The water content of a flow cell membrane can be measured, for example, by weighing the membrane before and after drying the membrane in a vacuum oven (e.g. in accordance with the method set out in ASTM D570-22). If other substances (e.g. H2SO4) are present in the membrane, then an intermediate washing step may be needed to remove said other substances.

[0095] The method may comprise increasing the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%. It has been found that increasing the water content of a dehydrated flow cell membrane by the amounts above substantially, if not entirely, restores the original ionic conductivity of the membrane.

[0096] The flow cell gas anode may be configured to facilitate an anolyte redox reaction, and the flow cell reconditioning liquid may: (a) optionally not be an electrolyte in the flow cell (e.g. in the anode chamber thereof); and / or

[0097] (b) optionally not be involved in said anolyte redox reaction; and / or

[0098] (c) consist essentially of redox-inert species.

[0099] In other words, the anolyte and the flow cell reconditioning liquid may be two distinct entities, the anolyte being responsible for undergoing a redox reaction during operation of the flow cell, and the reconditioning liquid being responsible for reconditioning the flow cell.

[0100] The flow cell gas anode active surface may be in physical contact with the flow cell membrane. When the flow cell gas anode is a hydrogen gas anode (for instance), such an arrangement allows direct passage of protons from the flow cell gas anode active surface (having been adsorbed thereto from oxidation of hydrogen) to the flow cell membrane, and vice versa. Similar considerations apply to other gas anodes. This means that it is not necessary for a liquid to be present in the gas anode chamber (e.g. to carry said protons between the active surface and the membrane) other than during reconditioning. This minimises crossing over of the catholyte into the anode chamber and vice versa.

[0101] According to a second aspect of the present disclosure, there is provided a method of operating a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; wherein the method comprises: operating the flow cell to generate electrical power; and delivering a flow cell reconditioning liquid into the gas anode chamber, optionally wherein the method further comprises withdrawing the flow cell reconditioning liquid from the gas anode chamber.

[0102] As mentioned above in the context of the first aspect, standard operation of a flow cell can lead to degradation of the flow cell over time (e.g. due to blocking of the flow cell gas anode active surface and / or dehydration of the flow cell membrane), which reduces the energy efficiency of the flow cell.

[0103] It has been found that reconditioning the flow cell by delivering a flow cell reconditioning liquid into the gas anode chamber (e.g. to wash the blocked flow cell gas anode active surface and / or rehydrate the dehydrated flow cell membrane) can mitigate these issues and reduce the degradation rate of the flow cell.

[0104] Said operating the flow cell to generate electrical power may comprise:

[0105] (a) applying a high current density, optionally a current density of at least around 100 mA-crm2;

[0106] (b) operating at a temperature of at least around 50°C, optionally at least around 60°C, optionally at least around 70°C, optionally at least around 80°C;

[0107] (c) cycling the flow cell for a duration of at least around 5 hours, optionally at least around 6 hours, optionally at least around 7 hours, optionally at least around 8 hours, optionally at least around 9 hours, optionally at least around 10 hours; and / or

[0108] (d) charging the flow cell for a duration of at least around 5 hours, optionally at least around 6 hours, optionally at least around 7 hours, optionally at least around 8 hours, optionally at least around 9 hours, optionally at least around 10 hours.

[0109] The typical operating conditions above particularly lead to fast degradation of the flow cell (e.g. due to blocking of the flow cell gas anode active surface and / or dehydration of the flow cell membrane). Reconditioning the flow cell by delivering a flow cell reconditioning liquid into the gas anode chamber has therefore been found to be particularly effective in such methods of operating a flow cell, where said use can significantly increase the lifetime of the flow cell.

[0110] The flow cell gas anode may be a flow cell hydrogen gas anode.

[0111] The flow cell reconditioning liquid may be as defined above in the context of the first aspect.

[0112] The flow cell membrane may be a proton selective membrane.

[0113] The gas anode chamber may be held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar.

[0114] Reconditioning may be as defined above in the context of the first aspect.

[0115] The liquid cathode chamber may comprise a liquid catholyte as defined above in the context of the first aspect. Said operating the flow cell to generate electrical power may cause the flow cell gas anode active surface, or a portion thereof, to become blocked; and reconditioning may comprise washing the blocked flow cell gas anode active surface with the flow cell reconditioning liquid to unblock the flow cell gas anode active surface during said delivering the flow cell reconditioning liquid into the gas anode chamber.

[0116] The flow cell gas anode active surface may be as defined above in the context of the first aspect.

[0117] The flow cell membrane may have a gas anode chamber face and a liquid cathode chamber face, said operating the flow cell to generate electrical power may cause the flow cell membrane, or a portion thereof, to become dehydrated, and reconditioning may comprise rehydrating the dehydrated flow cell membrane from the gas anode chamber face with the flow cell reconditioning liquid during said delivering the flow cell reconditioning liquid into the gas anode chamber.

[0118] Prior to rehydrating the dehydrated flow cell membrane, the dehydrated flow cell membrane may have a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%.

[0119] The method may comprise increasing the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%.

[0120] The flow cell gas anode active surface may be in physical contact with the flow cell membrane.

[0121] During said operating the flow cell to generate electrical power, the gas anode chamber may be substantially free of liquid.

[0122] According to a third aspect of the present disclosure, there is provided a use of a flow cell reconditioning liquid to recondition a flow cell which comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; and the use comprises delivering the flow cell reconditioning liquid into the gas anode chamber.

[0123] The flow cell reconditioning liquid may be as defined above in the context of the first aspect.

[0124] The flow cell membrane may be a proton selective membrane.

[0125] The gas anode chamber may be held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar.

[0126] The use may be to recondition as defined above in the context of the first aspect.

[0127] The liquid cathode chamber may comprise a liquid catholyte as defined above in the context of the first aspect.

[0128] The flow cell gas anode may be a flow cell hydrogen gas anode.

[0129] The use may be to recondition:

[0130] (a) when the state-of-charge of the flow cell is at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%;

[0131] (b) every 10 hours to 100 days, optionally every 1 to 100 days, optionally every 2 to 100 days, optionally every 3 to 100 days, optionally every 4 to 100 days, optionally every 5 to 100 days, optionally every 10 to 100 days, optionally every 15 to 100 days; and / or

[0132] (c) for: at least around 10 seconds, optionally at least around 2 minutes, at most around 60 minutes, optionally at most around 30 minutes, optionally at most around 5 minutes, and / or around 10 seconds to 60 minutes, optionally around 2 to 5 minutes.

[0133] The flow cell gas anode active surface may be or have a portion which is blocked; and the use may be to wash the blocked flow cell gas anode active surface with the flow cell reconditioning liquid and thereby unblock the flow cell gas anode active surface by said delivering the flow cell reconditioning liquid into the gas anode chamber. The flow cell gas anode active surface may be, or be blocked, as is defined above in the context of the first aspect.

[0134] The flow cell membrane may have a gas anode chamber face and a liquid cathode chamber face; the flow cell membrane, or a portion thereof, may be dehydrated; and the use may be to rehydrate from the gas anode chamber face by said delivering the flow cell reconditioning liquid into the gas anode chamber.

[0135] As explained above, flow cells comprising a gas anode are prone to dehydration of the flow cell membrane. This is particularly apparent on the gas anode chamber face of the flow cell membrane, as this face is generally in contact with a gaseous (rather than aqueous) anolyte. It has been found that rehydrating the flow cell membrane from the gas anode chamber face is therefore a particularly effective form of rehydration which rapidly improves the ionic conductivity of a dehydrated flow cell membrane.

[0136] Prior to rehydrating the dehydrated flow cell membrane, the dehydrated flow cell membrane may have a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%.

[0137] The use may be to increase the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%.

[0138] The use may be to increase the ionic conductivity of the flow cell membrane by: at least around 50%; and / or at most around 400%, optionally at most around 300%, optionally at most around 200%, optionally at most around 100%.

[0139] An increase in the ionic conductivity of a flow cell membrane can be calculated, for example, by measuring the Ohmic specific resistance of the membrane before and after reconditioning and comparing the results. The Ohmic specific resistance is inversely proportional to the ionic conductivity, such that a decrease in Ohmic specific resistance corresponds to an increase in ionic conductivity.

[0140] The use may be to reduce the degradation rate of the flow cell. As explained above, it has been found that the use of a reconditioning liquid mitigates various issues that can lead to rapid degradation of a flow cell. These include blocking of a flow cell gas anode active surface and dehydration of a flow cell membrane. It is thought, without wishing to be bound by theory, that blocking of a flow cell gas anode active surface causes degradation because the number of sites on the active surface which can facilitate (e.g. catalyse) electrochemical reactions decreases, since these become blocked. Dehydration of a flow cell membrane is thought, again without wishing to be bound by theory, to cause degradation because dehydration of the flow cell membrane leads to a decrease in ionic conductivity and therefore a decrease in the efficiency of ion (e.g. proton) transport across the membrane during charge / discharge of the flow cell.

[0141] The use may be to increase the energy efficiency of the flow cell, optionally by reducing Ohmic loss. It has been found that the energy efficiency of the flow cell is increased by the use of a reconditioning liquid. Without wishing to be bound by theory, it is thought for example that the above-described effects on the availability of sites on the active surface and on the ionic conductivity of the membrane are responsible for this increase. A flow cell membrane which has an increase ionic conductivity (e.g. by virtue of becoming dehydrated) reduces Ohmic loss in the flow cell.

[0142] The use may be to increase the cyclability of the flow cell. Here, the use may be to restore the energy efficiency of the flow cell to at least around 90% of the energy efficiency of a pristine flow cell, optionally to at least around 92%, optionally to at least around 94%, optionally to at least around 96%, optionally to at least around 98%. Since the use of a reconditioning liquid has been found to reduce the degradation rate and increase the energy efficiency of a flow cell, the cyclability of the flow cell (i.e. the number of times the flow cell can undergo a charge / discharge cycle whilst still generating an adequate level of electrical power) is increased by this use.

[0143] The flow cell gas anode may be configured to facilitate an anolyte redox reaction, and the flow cell reconditioning liquid may:

[0144] (a) optionally not be an electrolyte in the flow cell (e.g. in the anode chamber thereof); and / or

[0145] (b) optionally not be involved in said anolyte redox reaction; and / or

[0146] (c) consist essentially of redox-inert species. The flow cell gas anode active surface may be in physical contact with the flow cell membrane.

[0147] According to a fourth aspect of the present disclosure, there is provided a flow cell comprising: a gas anode chamber having a flow cell gas anode and a flow cell gas anode active surface; a liquid cathode chamber having a flow cell liquid cathode; a flow cell membrane between the gas anode chamber and the liquid cathode chamber, wherein the flow cell membrane is capable of selectively passing protons; conduits configured to supply electrochemically active species to the anode and to the cathode in a power delivery mode, and to carry generated electrochemically active species away from the anode and away from the cathode in an energy storage mode; and a flow cell reconditioning liquid reservoir and a pump, wherein the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber.

[0148] A flow cell which comprises a flow cell reconditioning liquid reservoir and a pump is optimised for performing the method of reconditioning described above. For example, such an arrangement would be suitable for washing and thus unblocking a blocked flow cell gas anode active surface, and also rehydrating a dehydrated flow cell membrane.

[0149] The flow cell may further comprise the flow cell reconditioning liquid (e.g. in the liquid reservoir).

[0150] The flow cell may further comprise conduits configured to deliver the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir to the gas anode chamber, and to carry the flow cell reconditioning liquid away from the gas anode chamber, as required. The same conduit may be configured to deliver both flow cell reconditioning liquid and electrochemically active species to the gas anode chamber, and / or the same conduit is configured to carry both flow cell reconditioning liquid and electrochemically active species away from the gas anode chamber.

[0151] The flow cell reconditioning liquid may be as defined above in the context of the first aspect.

[0152] The flow cell gas anode may be a hydrogen flow cell gas anode. The gas anode chamber may be held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar

[0153] The liquid cathode chamber may comprise a liquid catholyte as defined above in the context of the first aspect.

[0154] Reconditioning may be as defined above in the context of the first aspect.

[0155] The pump may be configured to deliver the flow cell reconditioning liquid over the flow cell gas anode active surface, optionally wherein the flow cell gas anode is configured to distribute the flow cell reconditioning liquid over the flow cell gas anode active surface (e.g. during a washing step to effect reconditioning thereof), optionally wherein the gas anode substantially does not comprise a fluoropolymer (e.g. PTFE). Here, to “distribute” means to spread the flow cell reconditioning liquid over a significant portion of the flow cell gas anode active surface.

[0156] The flow cell gas anode active surface may be as defined above in the context of the first aspect.

[0157] The flow cell membrane may have a gas anode chamber face and a liquid cathode chamber face; and the pump may be configured to deliver the flow cell reconditioning liquid over the gas anode chamber face.

[0158] The flow cell membrane may be dehydrated, optionally having a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%.

[0159] The flow cell gas anode active surface may be in physical contact with the flow cell membrane.

[0160] According to a fifth aspect of the present disclosure, there is provided a retrofit kit adapted to be fitted to a flow cell having a gas anode chamber, wherein: the retrofit kit comprises a flow cell reconditioning liquid reservoir and a pump; and the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber (e.g. over a flow cell gas anode active surface of a flow cell gas anode therein and / or a gas anode chamber face of a flow cell membrane therein). The retrofit kit may further comprise the flow cell reconditioning liquid (optionally wherein the flow cell reconditioning liquid is as defined above in the context of the first aspect).

[0161] The retrofit kit may further comprise conduits configured to supply the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir to the gas anode chamber, and to carry the flow cell reconditioning liquid away from the gas anode chamber. The same conduit may be configured to supply both flow cell reconditioning liquid and electrochemically active species to the gas anode chamber, and / or the same conduit may be configured to carry both flow cell reconditioning liquid and electrochemically active species away from the gas anode chamber.

[0162] EXAMPLES

[0163] Example 1: Reconditioning of a flow cell in idle mode

[0164] Methods

[0165] Reconditioning liquid formulations and operating procedure

[0166] H2O reconditioning:

[0167] The H2 compartment of the test cell is filled with Ch-free (i.e. purged with H2) DI water (18 MQ-cm) at a flow rate of 3 mL-min'1-cm'2and then left stagnant for 2 minutes. Other variations of the reconditioning involve continuously flowing the DI Water (1-10 min) and then leaving the H2 compartment filled for different amounts of time (2-30 min). The liquid is drained from the H2 compartment of the test cell using a flow of H2 at 1.25 mL-min'1-cm'2.

[0168] H2SO4 - H2O reconditioning:

[0169] 1 M sulphuric acid solution is circulated around the test cell hydrogen compartment for 30 minutes at a flow rate of 3 mL-min'1-cm'2. Subsequently, DI water is used to rinse the hydrogen compartment. Initially DI water is flushed (i.e. a single pass of water that is not collected and recirculated) through the H2 compartment, then fresh DI water is circulated for 10 minutes. Between each step the hydrogen compartment is drained using a H2 flow of 1.25 mL-min'1-cm'2. A variation of the H2SO4 - H2O reconditioning involves heating the acid to 50°C during the initial 30 minutes circulation step.

[0170] Cell configuration (communal to all the electrochemical tests shown below) H2 electrode: Pt / C on carbon

[0171] Membrane: Nation N212 or Gore PEM membrane Mn electrode: 2 layers (stacked) of woven carbon cloth

[0172] Electrolyte and cell temperature: 20-50°C

[0173] Charging / discharging current density: 100-300 mA-crm2

[0174] Electrolyte used:

[0175] Mn (Mn2+ / 3+), at a concentration between 0.25 and 3.5 M;

[0176] H2SO4, at a concentration between 1 and 8 M; and

[0177] Ti, at a concentration of up to 1 M.

[0178] Mn electrolyte flow rate: 2 - 6 mL-min'1-cm'2

[0179] H2 flow rate (in discharge): 1 - 3 mL-min'1-cm'2

[0180] Other comments

[0181] Small-scale testing so far has been conducted with the test cell in “open loop” (i.e., the H2 gas that was generated in charge was not stored and the H2 necessary for the discharge phase was provided by an electrolyser), and the H2 compartment in the cell is at atmospheric pressure. For closed loop systems the hydrogen compartment will operate at variable H2 pressure, depending on the state-of-charge (SoC) of the battery. In general, during reconditioning the Mn catholyte electrolyte is kept stagnant (no flow). Other variations involve draining the electrolyte from the liquid compartment of the test cell.

[0182] Typically, the reconditioning is carried out at <20% SoC, but we do expect the same effectiveness if done at a different SoC. It is preferred to perform the reconditioning before a charge phase, primarily to avoid having a “flooded” compartment right before the discharge. At any rate, the effectiveness of the reconditioning is maintained regardless of the mode of the ensuing phase.

[0183] Testing & validation

[0184] Fig. 1 shows an example of reconditioning with H2O applied to a cell during a lifetime test where the cell was operated at 150 mA-cnv2(2.5 h charge / discharge cycles at full capacity). The reconditioning procedure is carried out at every -100 h and the performance is repeatedly recovered.

[0185] Fig. 2 and accompanying Table 1 show some additional performance metrics recorded before / after the reconditioning step in another set of experiments. Similarly to what shown earlier, the reconditioning was carried out with water. Both the energy efficiency and the Ohmic cell resistance regain > 97% of the initial (fresh) performance. Table 1: Voltage efficiency and Ohmic cell resistance recorded at beginning of life, in a degraded state and after reconditioning. sum of contact resistance and membrane ionic resistance

[0186] Example 2: Reconditioning of a flow cell during charging phases, using a supporting electrolyte

[0187] Methods

[0188] Reconditioning liquid formulations and operating procedure

[0189] The supporting electrolyte is a water-H2SO4 mixture that may contain specific salts (Mn / Ti / Mg, etc.). This liquid is continuously circulated in the H2 electrode compartment during the charging phase. In discharge, the H2 compartment is drained and H2 gas is fed to the electrode. This invention has been validated in a small test cell (20 cm2of active area). To simplify the R&D tasks, the cell was operated in “open mode” (i.e. , the H2 gas that was generated in charge was not stored and the H2 necessary for the discharge phase was provided by an electrolyser). The H2 compartment was kept at atmospheric pressure (-1 bar). Two cells were “paired” and connected to the same electrolyte tank; they were operated synchronously applying the same current density, but in opposite modes (i.e., one is charging, the other discharging). Adopting this approach, the state of charge of the electrolyte was maintained, and long (e.g., > 100 h) charge / discharge cycles were possible. It is important to note that while the SoC of the electrolyte does not vary during the test, the H2 electrode experiences a realistic charge / discharge cycle and therefore the experiment is representative of the H2-Mn technology in operation.

[0190] Supporting electrolyte formulations H2O-H2SO4: 7.5 M H2SO4 in H2O (deionized) Mg-based: 2 M MgSC>4 + 3 M H2SO4 in H2O (deionized) Mn-Ti-based: 0.75 M MnCCh + 0.396 M TiOSC>4 + 6 M H2SO4 in H2O (deionized)

[0191] Cell configuration (communal to all the tests)

[0192] H2 electrode: Pt / C on carbon

[0193] Membrane: Nation N212 or Gore PEM membrane

[0194] Mn electrode: 2 layers (stacked) of woven carbon cloth (unless otherwise specified)

[0195] Electrolyte and cell temperature: 20-50°C

[0196] Charging / discharging current density: 100-300 mA-cnv2

[0197] Electrolyte used:

[0198] Mn (Mn2+ / 3+), at a concentration between 0.25 and 3.5 M;

[0199] H2SO4, at a concentration between 1 and 8 M; and

[0200] Ti, at a concentration of up to 1 M.

[0201] Electrolyte Flow rate: 2 - 6 mL-min'1-cm'2

[0202] H2 flow rate (in discharge): 1 - 3 mL-min'1-cm'2

[0203] Supporting electrolyte flow rate: 0.5 - 2 mL-min'1-cm'2

[0204] Testing and validation

[0205] Fig. 3 shows a prolonged charging test (made of 2 h sub-charging blocks) performed in the absence of supporting electrolyte in the H2 compartment (red trace) and with a supporting electrolyte (SE) made of 7.5 M H2SO4 in DI water. The cut-off voltage (i.e. , maximum positive voltage allowed due to safety concerns) was set to 1.7 V. Without SE this limit is reached already within ~12 h. The degradation is caused by the concomitant decrease in H+mobility in the membrane (dehydration) and by the blocking of the H2 electrode ionomers. In contrast, a continuous charging phase of more than 60 h was successfully achieved with minimal decay in efficiency when the supporting electrolyte is used.

[0206] Similar results are obtained with different supporting electrolyte formulations. Fig. 4 shows the same charging tests carried out with a Mn-Ti-H2SO4 and with a Mg-FhSOt mixtures (see methods description for exact formulations).

[0207] Example 3: Schematic example of a flow cell suitable for reconditioning

[0208] Fig. 5 shows a schematic of a flow cell that may require reconditioning in accordance with the present disclosure. In this particular example, the following electrochemically active species are used to generate power: (a) hydrogen gas (supplied to the anode) and (b) a generic metal species Mn+1(supplied to the cathode).

[0209] In the power delivery mode, a liquid catholyte containing Mn+1is pumped by a pump (11) from a compartment of fresh catholyte storage container (12A), through a conduit (12B) and into a liquid cathode chamber (9), where it is reduced at a cathode (2) according to the half reaction:

[0210] Mn+1+ e" Mn

[0211] The catholyte containing the spent electrolyte species Mnis then carried away from the catholyte compartment through a second conduit (1) to the catholyte storage container (12A), where it is stored in a compartment separate from the fresh catholyte compartment.

[0212] The anode and at least part of a gas anode chamber (8) are formed by a porous gas flow electrode (4) and hydrogen is supplied from a pressurised gas source vessel (7) through a conduit (13), to the anode I gas anode chamber (8), where the hydrogen is oxidised to protons (H+) according to the half reaction: and the current is collected by a current collector (also labelled 4). A flow cell membrane (3) separates the gas anode chamber and liquid cathode chamber (8 & 9) and selectively passes the protons from the anolyte to the catholyte side of the membrane (3) to balance the charge, thereby completing the electrical circuit. Any unreacted hydrogen is carried away from the gas anode chamber (8) by a second conduit (5) and returned to the pressurised gas source vessel (7) via compressor (6).

[0213] In the energy storage mode, the system is reversed so that the redox active species Mnis pumped from the catholyte storage container (12A), through the conduit (1) to the liquid cathode chamber (9), where the spent electrolyte species Mnis oxidised at the cathode (2) to form the redox active species Mn+1. The resulting regenerated electrolyte is transferred away from the liquid cathode chamber (9) by the pump (11), through the second conduit (12B) to the catholyte storage container (12A). Meanwhile, protons at the anolyte side of the proton exchange membrane (3) are catalytically reduced at the porous gas anode (4) to hydrogen gas; the hydrogen is transferred away from the porous anode (4) through the conduit (5) and compressed by the compressor (6) before being stored in the pressurised gas source vessel (7).

[0214] The disclosure also comprises the following clauses, which may be claimed:

[0215] 1. A method of reconditioning a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; wherein the method comprises delivering a flow cell reconditioning liquid into the gas anode chamber.

[0216] 2. The method according to clause 1 , wherein the flow cell gas anode is a flow cell hydrogen gas anode.

[0217] 3. The method according to clause 1 or 2, wherein the flow cell reconditioning liquid comprises an acid, optionally a strong acid, optionally wherein the flow cell reconditioning liquid has a pH of at most around 7, optionally at most around 3 and / or optionally around 0 or lower, optionally wherein the pH is around 0 or lower to around 3, optionally wherein the flow cell reconditioning liquid comprises HCI, H3PO4, HCIO4 and / or H2SO4 (optionally H2SO4).

[0218] 4. The method according to clause 3, wherein the acid is present in the flow cell reconditioning liquid at a concentration of around 1 M to 10 M, optionally 1 to 8 M (e.g. in the context of H2SO4).

[0219] 5. The method according to any preceding clause, wherein the flow cell reconditioning liquid comprises a salt, optionally a metal salt, optionally a sodium, magnesium, potassium, aluminium, manganese or titanium salt, optionally a manganese or titanium salt.

[0220] 6. The method according to clause 5, wherein the salt is present in the flow cell reconditioning liquid at a concentration of around 0.1 to 7 M, optionally around 0.5 to 5 M, optionally around 0.5 to 3 M, optionally around 1 to 2 M (e.g. in the context of a manganese or titanium salt). The method according to any preceding clause, wherein the flow cell reconditioning liquid is aqueous. The method according to any preceding clause, wherein the flow cell reconditioning liquid is a supporting electrolyte. The method according to clause 1 or 2, wherein the flow cell reconditioning liquid is water, optionally deionised water. The method according to any preceding clause, wherein: the flow cell gas anode active surface is or has a portion which is blocked, and reconditioning comprises washing the blocked flow cell gas anode active surface with a flow cell reconditioning liquid to unblock the flow cell gas anode active surface during said delivering the flow cell reconditioning liquid into the gas anode chamber. The method according to any preceding clause, wherein the flow cell gas anode active surface:

[0221] (a) is a catalytic surface; and / or

[0222] (b) comprises a metal, optionally a catalyst suitable for catalysing redox reactions with a hydrogen anolyte, optionally platinum; and / or

[0223] (c) comprises an ionomer, optionally a perfluorinated sulfonic acid (PFSA) ionomer (e.g. a long-chain or short-chain PFSA ionomer) or a hydrocarbon ionomer (e.g. an aromatic, non-aromatic or polyimide ionomer); optionally wherein the flow cell membrane also comprises said ionomer (c). The method according to any preceding clause, wherein the flow cell gas anode active surface is blocked with a blocking metal species, optionally wherein the blocking metal species is a catholyte crossover metal species, optionally a redoxactive metal species, optionally a redox-active transition metal species, optionally a redox-active transition metal salt, optionally a sodium, magnesium, potassium, aluminium, manganese or titanium salt, optionally a manganese or titanium salt; and / or optionally wherein the salt comprises a sulphate. The method according to clause 12, wherein the blocking metal species is soluble in the flow cell reconditioning liquid. The method according to any preceding clause, wherein the gas anode chamber is held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar. The method according to any preceding clause, wherein said delivering comprises delivering the flow cell reconditioning liquid from a flow cell reconditioning liquid reservoir separated from and in fluid communication with the gas anode chamber; optionally wherein the method comprises pumping the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir into the gas anode chamber, optionally wherein the method further comprises pumping the flow cell reconditioning liquid from the gas anode chamber back to the flow cell reconditioning liquid reservoir. The method according to any preceding clause, wherein reconditioning comprises:

[0224] (a) circulating the flow cell reconditioning liquid within the gas anode chamber; and / or

[0225] (b) inserting the flow cell reconditioning liquid into the gas anode chamber and subsequently removing the flow cell reconditioning liquid from the gas anode chamber; or

[0226] (c) flowing the flow cell reconditioning liquid through the gas anode chamber in a continuous stream. The method according to any preceding clause, wherein reconditioning is carried out when the flow cell is in:

[0227] (a) an idle mode;

[0228] (b) an energy storage mode; and / or

[0229] (c) a power delivery mode. The method according to any preceding clause, wherein said reconditioning is carried out every 10 hours to 100 days, optionally every 1 to 100 days, optionally every 2 to 100 days, optionally every 3 to 100 days, optionally every 4 to 100 days, optionally every 5 to 100 days, optionally every 10 to 100 days, optionally every 15 to 100 days. The method according to any preceding clause, wherein said reconditioning is carried out:

[0230] (I) for at least around 10 seconds, optionally at least around 2 minutes; for at most around 60 minutes, optionally at most around 30 minutes; optionally at most around 5 minutes; and / or for around 10 seconds to 60 minutes, optionally around 2 to 5 minutes; and / or

[0231] (II) at a flow rate of around 0.5 to 4 mL-min'1-cnr2, optionally around 0.5 to 3 mL-min'1-cm'2; and / or

[0232] (III) at a temperature of at least around 0°C, optionally at least around 20°C, optionally at most around 80°C, optionally at most around 50°C, optionally around 20°C to 50°C. The method according to any preceding clause, wherein the liquid cathode chamber comprises a liquid catholyte. The method according to clause 20, wherein the liquid catholyte comprises a primary metal species, optionally a transition metal species, optionally a sodium, magnesium, potassium, aluminium, manganese species, optionally manganese, titanium or aluminium (e.g. Al3+), optionally Mn2+and / or Mn3+; or optionally Ti3+and / or Ti4+; and / or optionally wherein the liquid catholyte comprises a sulphate and / or hydrogensulphate. The method according to clause 21 , wherein the liquid catholyte further comprises:

[0233] (a) a secondary metal species, optionally a transition metal species, optionally a titanium species, optionally a Ti3+ion complex and / or a Ti4+ion complex; and / or

[0234] (b) an acid, optionally a strong acid, optionally wherein the liquid catholyte has a pH of at most around 7, optionally at most around 3 and / or optionally around 0 or lower, optionally wherein the pH is around 0 or lower to around 3, optionally wherein the liquid catholyte comprises HCI, H3PO4, CH3SO3H, HCIO4 and / or H2SO4 (optionally H2SO4). The method according to any one of clauses 20-22, wherein the flow cell reconditioning liquid and the liquid catholyte each have molar solvent concentrations which differ by at most around 30%, optionally at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%, optionally at most around 8%, optionally at most around 6%, optionally at most around 4%, optionally at most around 2%. The method according to any one of clauses 20-23, wherein the liquid catholyte is:

[0235] (a) kept stagnant in the liquid cathode chamber during reconditioning, optionally wherein the liquid cathode chamber is isolated from the rest of the flow cell; or

[0236] (b) drained from the liquid cathode chamber prior to reconditioning (optionally wherein the method further comprises reintroducing the liquid catholyte after reconditioning). The method according to any preceding clause, wherein the flow cell membrane is a proton selective membrane. The method according to any preceding clause, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face, the flow cell membrane, or a portion thereof, is dehydrated, and reconditioning comprises rehydrating the dehydrated flow cell membrane, from the gas anode chamber face thereof, with the flow cell reconditioning liquid during said delivering the flow cell reconditioning liquid into the gas anode chamber. The method according to clause 26, wherein, prior to rehydrating the dehydrated flow cell membrane, the dehydrated flow cell membrane has a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%. The method according to clause 26 or 27, wherein the method comprises increasing the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%. The method according to any preceding clause, wherein said reconditioning is carried out when the state-of-charge of the flow cell is at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%. A method of operating a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; wherein the method comprises: operating the flow cell to generate electrical power; and delivering a flow cell reconditioning liquid into the gas anode chamber, optionally wherein the method further comprises withdrawing the flow cell reconditioning liquid from the gas anode chamber. The method according to clause 30, wherein said operating the flow cell to generate electrical power comprises:

[0237] (a) applying a high current density, optionally a current density of at least around 100 mA-crm2;

[0238] (b) operating at a temperature of at least around 50°C, optionally at least around 60°C, optionally at least around 70°C, optionally at least around 80°C;

[0239] (c) cycling the flow cell for a duration of at least around 5 hours, optionally at least around 6 hours, optionally at least around 7 hours, optionally at least around 8 hours, optionally at least around 9 hours, optionally at least around 10 hours; and / or

[0240] (d) charging the flow cell for a duration of at least around 5 hours, optionally at least around 6 hours, optionally at least around 7 hours, optionally at least around 8 hours, optionally at least around 9 hours, optionally at least around 10 hours. The method according to clause 30 or 31 , wherein:

[0241] (a) the flow cell gas anode is a flow cell hydrogen gas anode;

[0242] (b) the flow cell reconditioning liquid is as defined in any one of clauses 3-9;

[0243] (c) the flow cell membrane is a proton selective membrane;

[0244] (d) the gas anode chamber is held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar;

[0245] (e) reconditioning is as defined in any one of clauses 15-19; and / or

[0246] (f) the liquid cathode chamber comprises a liquid catholyte as defined in any one of clauses 20-24. The method according to any one of clauses 30-32, wherein: said operating the flow cell to generate electrical power causes the flow cell gas anode active surface, or a portion thereof, to become blocked; and reconditioning comprises washing the blocked flow cell gas anode active surface with the flow cell reconditioning liquid to unblock the flow cell gas anode active surface during said delivering the flow cell reconditioning liquid into the gas anode chamber. The method of any one of clauses 30-33, wherein the flow cell gas anode active surface is as defined in any one of clauses 11-13. The method according to any one of clauses 30-34, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face, said operating the flow cell to generate electrical power causes the flow cell membrane, or a portion thereof, to become dehydrated, and reconditioning comprises rehydrating the dehydrated flow cell membrane from the gas anode chamber face with the flow cell reconditioning liquid during said delivering the flow cell reconditioning liquid into the gas anode chamber. The method according to clause 35, wherein, prior to rehydrating the dehydrated flow cell membrane, the dehydrated flow cell membrane has a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%. The method according to clause 35 or 36, wherein the method comprises increasing the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%. Use of a flow cell reconditioning liquid to recondition a flow cell which comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; and the use comprises delivering the flow cell reconditioning liquid into the gas anode chamber. The use according to clause 38, wherein:

[0247] (a) the flow cell reconditioning liquid is as defined in any one of clauses 3-9;

[0248] (b) the flow cell membrane is a proton selective membrane;

[0249] (c) the gas anode chamber is held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar;

[0250] (d) the use is to recondition as defined in any one of clauses 15-19;

[0251] (e) the liquid cathode chamber may comprise a liquid catholyte as defined in any one of clauses 20-24; and / or

[0252] (f) the flow cell gas anode is a flow cell hydrogen gas anode. The use according to clause 38 or 39, wherein the use is to recondition:

[0253] (a) when the state-of-charge of the flow cell is at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%;

[0254] (b) every 10 hours to 100 days, optionally every 1 to 100 days, optionally every 2 to 100 days, optionally every 3 to 100 days, optionally every 4 to 100 days, optionally every 5 to 100 days, optionally every 10 to 100 days, optionally every 15 to 100 days; and / or

[0255] (c) for: at least around 10 seconds, optionally at least around 2 minutes, at most around 60 minutes, optionally at most around 30 minutes, optionally at most around 5 minutes, and / or around 10 seconds to 60 minutes, optionally around 2 to 5 minutes. The use according to any one of clauses 38-40, wherein: the flow cell gas anode active surface is or has a portion which is blocked; and the use is to wash the blocked flow cell gas anode active surface with the flow cell reconditioning liquid and thereby unblock the flow cell gas anode active surface by said delivering the flow cell reconditioning liquid into the gas anode chamber. The use according to any one of clauses 38-41 , wherein the flow cell gas anode active surface is, or is blocked, as is defined in any one of clauses 11-13. The use according to any one of clauses 38-42, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face; the flow cell membrane, or a portion thereof, is dehydrated; and the use is to rehydrate the dehydrated flow cell membrane from the gas anode chamber face with the flow cell reconditioning liquid by said delivering the flow cell reconditioning liquid into the gas anode chamber. The use according to clause 43, wherein, prior to rehydrating the dehydrated flow cell membrane, the dehydrated flow cell membrane has a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around 2 wt%. The use according to clause 43 or 44, wherein the use is to increase the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%. The use according to any one of clauses 43-45, wherein the use is to increase the ionic conductivity of the flow cell membrane by: at least around 50%; and / or at most around 400%, optionally at most around 300%, optionally at most around 200%, optionally at most around 100%. The use according to any one of clauses 38-46, wherein the use is to reduce the degradation rate of the flow cell. The use according to any one of clauses 38-47, wherein the use is to increase the energy efficiency of the flow cell, optionally by reducing Ohmic loss. The use according to any one of clauses 38-48, wherein the use is to increase the cyclability of the flow cell, optionally wherein the use is to restore the energy efficiency of the flow cell to at least around 90% of the energy efficiency of a pristine flow cell, optionally to at least around 92%, optionally to at least around 94%, optionally to at least around 96%, optionally to at least around 98%. A flow cell comprising: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber, wherein the flow cell membrane is capable of selectively passing protons; conduits configured to supply electrochemically active species to the anode and to the cathode in a power delivery mode, and to carry generated electrochemically active species away from the anode and away from the cathode in an energy storage mode; and a flow cell reconditioning liquid reservoir and a pump, wherein the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber. The flow cell according to clause 50, further comprising the flow cell reconditioning liquid (e.g. in the liquid reservoir). The flow cell according to clause 50 or 51 , further comprising conduits configured to deliver the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir to the gas anode chamber, and to carry the flow cell reconditioning liquid away from the gas anode chamber, as required; optionally wherein the same conduit is configured to deliver both flow cell reconditioning liquid and electrochemically active species to the gas anode chamber, and / or the same conduit is configured to carry both flow cell reconditioning liquid and electrochemically active species away from the gas anode chamber. The flow cell according to any one of clauses 50-52, wherein:

[0256] (a) the flow cell reconditioning liquid is as defined in any one of clauses 3-9;

[0257] (b) the flow cell gas anode is a flow cell hydrogen gas anode;

[0258] (c) the gas anode chamber is held at a pressure (e.g. a hydrogen pressure) of around 1 to 10 bar, optionally around 1 to 9 bar, optionally around 1 to 8 bar, optionally around 1 to 7 bar;

[0259] (d) reconditioning is as defined in any one of clauses 15-19; and / or

[0260] (e) the liquid cathode chamber comprises a liquid catholyte as defined in any one of clauses 20-24. The flow cell according to any one of clauses 50-53, wherein the pump is configured to deliver the flow cell reconditioning liquid over the flow cell gas anode active surface, optionally wherein the flow cell gas anode is configured to distribute the flow cell reconditioning liquid over the flow cell gas anode active surface (e.g. during a washing step to effect reconditioning thereof), optionally wherein the gas anode substantially does not comprise a fluoropolymer (e.g. PTFE). The flow cell according to any one of clauses 50-54, wherein the flow cell gas anode active surface is as defined in any one of clauses 11-13. The flow cell according to any one of clauses 50-55, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face; and the pump is configured to deliver the flow cell reconditioning liquid over the gas anode chamber face. The flow cell according to any one of clauses 50-56, wherein the flow cell membrane is dehydrated, optionally having a water content of at most around 5 wt%, optionally at most around 4 wt%, optionally at most around 3 wt%, optionally at most around

[0261] 2 wt%. A retrofit kit adapted to be fitted to a flow cell having a gas anode chamber, wherein: the retrofit kit comprises a flow cell reconditioning liquid reservoir and a pump; and the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber (e.g. over a flow cell gas anode active surface of a flow cell gas anode therein and / or a gas anode chamber face of a flow cell membrane therein). The retrofit kit according to clause 58, further comprising the flow cell reconditioning liquid (optionally wherein the flow cell reconditioning liquid is as defined in any one of clauses 3 to 9). The retrofit kit according to clause 58 or 59, further comprising conduits configured to supply the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir to the gas anode chamber, and to carry the flow cell reconditioning liquid away from the gas anode chamber; optionally wherein the same conduit is configured to supply both flow cell reconditioning liquid and electrochemically active species to the gas anode chamber, and / or the same conduit is configured to carry both flow cell reconditioning liquid and electrochemically active species away from the gas anode chamber.

[0262] 61. A method, use, flow cell or retrofit kit substantially as described herein with reference to the accompanying description and / or drawings.

[0263] Any listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or common general knowledge. All references disclosed herein are to be considered to be incorporated herein by reference.

[0264] All features discussed herein in respect of any of the methods, uses or products relate to all other methods, uses or products mutatis mutandis. For example, features described in relation to the method of reconditioning a flow cell according to the first aspect apply equally, mutatis mutandis to the method of operating a flow cell according to the second aspect; and also to the use of a flow cell reconditioning liquid according to the third aspect, the flow cell according to the fourth aspect or the retrofit kit according to the fifth aspect, mutatis mutandis.

[0265] Those skilled in the art will recognise or be able to ascertain using no more than routine experimentation many equivalents to the specific embodiments described herein. The scope of the present disclosure herein is not intended to be limited to the above description, but rather is as set forth in the appended claims. Those of ordinary skill in the art will appreciate that various changes and modifications to this description may be made without departing from the spirit or scope of the present disclosure.

Claims

CLAIMS1. A method of reconditioning a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; wherein the method comprises delivering a flow cell reconditioning liquid into the gas anode chamber.

2. The method according to claim 1, wherein the flow cell gas anode is a flow cell hydrogen gas anode.

3. The method according to claim 1 or 2, wherein the flow cell reconditioning liquid comprises an acid.

4. The method according to claim 3, wherein the flow cell reconditioning liquid has a pH of at most around 7.

5. The method according to claim 4, wherein the flow cell reconditioning liquid has a pH of at most around 3.

6. The method according to claim 4 or 5, wherein the flow cell reconditioning liquid comprises HCI, H3PO4, HCIO4 and / or H2SO4.

7. The method according to claim 6, wherein the flow cell reconditioning liquid comprises H2SO4.

8. The method according to claim 7, wherein the H2SO4 is at a concentration of around 1 to 8 M.

9. The method according to any preceding claim, wherein the flow cell reconditioning liquid comprises a salt.

10. The method according to claim 9, wherein the salt is a metal salt.

11. The method according to claim 10, wherein the salt is a sodium, magnesium, potassium, aluminium, manganese or titanium salt.

12. The method according to claim 11, wherein the salt is a manganese or titanium salt.

13. The method according to any one of claims 9 to 12, wherein the salt comprises a sulfate.

14. The method according to any preceding claim, wherein the flow cell reconditioning liquid is a supporting electrolyte.

15. The method according to claim 1 or 2, wherein the flow cell reconditioning liquid is water, optionally deionised water.

16. The method according to any preceding claim, wherein: the flow cell gas anode active surface is or has a portion which is blocked, and reconditioning comprises washing the blocked flow cell gas anode active surface with a flow cell reconditioning liquid to unblock the flow cell gas anode active surface during said delivering the flow cell reconditioning liquid into the gas anode chamber; optionally wherein the flow cell gas anode active surface comprises platinum.

17. The method according to any preceding claim, wherein the flow cell gas anode active surface is blocked with a blocking metal species.

18. The method according to claim 17, wherein the blocking metal species is a catholyte crossover metal species.

19. The method according to any preceding claim, wherein the flow cell gas anode active surface further comprises an ionomer, optionally a perfluorinated sulfonic acid (PFSA) ionomer (e.g. a long-chain or short-chain PFSA ionomer) or a hydrocarbon ionomer (e.g. an aromatic, non-aromatic or polyimide ionomer).

20. The method according to any preceding claim, wherein reconditioning comprises circulating the flow cell reconditioning liquid within the gas anode chamber.

21. The method according to any preceding claim, wherein reconditioning comprises:(a) inserting the flow cell reconditioning liquid into the gas anode chamber and subsequently removing the flow cell reconditioning liquid from the gas anode chamber; or(b) flowing the flow cell reconditioning liquid through the gas anode chamber in a continuous stream.

22. The method according to any preceding claim, wherein: the liquid cathode chamber comprises a liquid catholyte; the flow cell membrane separates the liquid cathode chamber from the gas anode chamber; and the flow cell reconditioning liquid and the liquid catholyte each have molar solvent concentrations which differ by at most around 30%, optionally at most around 20%, optionally at most around 18%, optionally at most around 16%, optionally at most around 14%, optionally at most around 12%, optionally at most around 10%, optionally at most around 8%, optionally at most around 6%, optionally at most around 4%, optionally at most around 2%.

23. The method according to any preceding claim, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face; the flow cell membrane, or a portion thereof, is dehydrated, and reconditioning comprises rehydrating the dehydrated flow cell membrane, from the gas anode chamber face thereof, with the flow cell reconditioning liquid during said delivering the flow cell reconditioning liquid into the gas anode chamber; wherein rehydrating comprises increasing the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%.

24. The method according to any preceding claim, wherein said reconditioning is carried out: for at least around 10 seconds, optionally at least around 2 minutes; for at most around 60 minutes, optionally at most around 30 minutes; optionally at most around 5 minutes; and / or for around 10 seconds to 60 minutes, optionally around 2 to 5 minutes.

25. The method according to any preceding claim, wherein said reconditioning is carried out at a flow rate of around 0.5 to 4 mL-min'1-cm'2, optionally around 0.5 to3 mL-min'1-cnr2.

26. The method according to any preceding claim, wherein said reconditioning is carried out at a temperature of at least around 0°C, optionally at least around 20°C, optionally at most around 80°C, optionally at most around 50°C, optionally around 20°C to 50°C.

27. The method according to any preceding claim, wherein: the liquid cathode chamber comprises a liquid catholyte; the liquid catholyte comprises a primary metal species; and the primary metal species is a manganese species, optionally Mn2+and / or Mn3+.

28. The method according to claim 27, wherein the liquid catholyte further comprises a secondary metal species.

29. The method according to claim 28, wherein the secondary metal species is a titanium species.

30. The method according to claim 1 , wherein:(a) the flow cell gas anode is a flow cell hydrogen gas anode; and(b) the flow cell reconditioning liquid comprises an acid.

31. The method according to claim 30, wherein the flow cell reconditioning liquid has a pH of at most around 3.

32. The method according to claim 1 , wherein:(a) the flow cell gas anode is a flow cell hydrogen gas anode; and(b) the flow cell reconditioning liquid comprises a salt.

33. The method according to claim 32, wherein the salt is a manganese or titanium salt.

34. The method according to claim 1 , wherein:(a) the flow cell gas anode is a flow cell hydrogen gas anode; and(b) the flow cell reconditioning liquid is a supporting electrolyte.

35. The method according to claim 1 , wherein:(a) the flow cell gas anode is a flow cell hydrogen gas anode; and(b) the flow cell reconditioning liquid is water.

36. The method according to any preceding claim, wherein: the flow cell gas anode is configured to facilitate an anolyte redox reaction; and the flow cell reconditioning liquid:(a) is not an electrolyte in the flow cell (e.g. in the anode chamber thereof); and / or(b) is not involved in said anolyte redox reaction; and / or(c) consists essentially of redox-inert species.

37. The method according to any preceding claim, wherein the flow cell gas anode active surface is in physical contact with the flow cell membrane.

38. A method of operating a flow cell, wherein the flow cell comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; wherein the method comprises: operating the flow cell to generate electrical power; and delivering a flow cell reconditioning liquid into the gas anode chamber, optionally wherein the method further comprises withdrawing the flow cell reconditioning liquid from the gas anode chamber.

39. The method according to claim 38, wherein the flow cell gas anode active surface is in physical contact with the flow cell membrane.

40. The method according to claim 39, wherein, during said operating the flow cell to generate electrical power, the gas anode chamber is substantially free of liquid.

41. Use of a flow cell reconditioning liquid to recondition a flow cell which comprises: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber; and the use comprises delivering the flow cell reconditioning liquid into the gas anode chamber.

42. The use according to claim 41, wherein the flow cell gas anode is a flow cell hydrogen gas anode.

43. The use according to claim 41 or 42, wherein the flow cell reconditioning liquid comprises an acid.

44. The use according to claim 43, wherein the flow cell reconditioning liquid has a pH of at most around 7.

45. The use according to claim 44, wherein the flow cell reconditioning liquid has a pH of at most around 3.

46. The use according to claim 44 or 45, wherein the flow cell reconditioning liquid comprises HCI, H3PO4, HCIO4 and / or H2SO4.

47. The use according to claim 46, wherein the flow cell reconditioning liquid comprises H2SO4.

48. The use according to claim 47, wherein the H2SO4 is at a concentration of around 1 to 8 M.

49. The use according to any one of claims 41 to 48, wherein the flow cell reconditioning liquid comprises a salt.

50. The use according to claim 49, wherein the salt is a metal salt.

51. The use according to claim 50, wherein the salt is a sodium, magnesium, potassium, aluminium, manganese or titanium salt.

52. The use according to claim 51, wherein the salt is a manganese or titanium salt.

53. The use according to any one of claims 50 to 52, wherein the salt comprises a sulfate.

54. The use according to any one of claims 41 to 53, wherein the flow cell reconditioning liquid is a supporting electrolyte.

55. The use according to claim 41 or 42, wherein the flow cell reconditioning liquid is water, optionally deionised water.

56. The use according to any one of claims 41 to 55, wherein: the flow cell gas anode active surface is or has a portion which is blocked; and the use is to wash the blocked flow cell gas anode active surface with the flow cell reconditioning liquid and thereby unblock the flow cell gas anode active surface by said delivering the flow cell reconditioning liquid into the gas anode chamber.

57. The use according to any one of claims 41 to 56, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face; the flow cell membrane, or a portion thereof, is dehydrated; and the use is to rehydrate the dehydrated flow cell membrane from the gas anode chamber face with the flow cell reconditioning liquid by said delivering the flow cell reconditioning liquid into the gas anode chamber; wherein the use is to increase the water content of the flow cell membrane by at least around 25 wt%, optionally at least around 67 wt%, optionally at least around 150 wt%, optionally at least around 400 wt%.

58. The use according to any one of claims 41 to 57, wherein the use is to increase the ionic conductivity of the flow cell membrane by at least around 50%.

59. The use according to any one of claims 41 to 58, wherein the use is to restore the energy efficiency of the flow cell to at least around 90% of the energy efficiency of a pristine flow cell, optionally to at least around 92%, optionally to at least around 94%, optionally to at least around 96%, optionally to at least around 98%.

60. The use according to any one of claims 41 to 59, wherein: the flow cell gas anode is configured to facilitate an anolyte redox reaction; and the flow cell reconditioning liquid:(a) is not an electrolyte in the flow cell (e.g. in the anode chamber thereof); and / or(b) is not involved in said anolyte redox reaction; and / or(c) consists essentially of redox-inert species.

61. The use according to any one of claims 41 to 60, wherein the flow cell gas anode active surface is in physical contact with the flow cell membrane.

62. A flow cell comprising: a gas anode chamber having a flow cell gas anode with a flow cell gas anode active surface, a liquid cathode chamber having a flow cell liquid cathode, and a flow cell membrane between the gas anode chamber and liquid cathode chamber, wherein the flow cell membrane is capable of selectively passing protons; conduits configured to supply electrochemically active species to the anode and to the cathode in a power delivery mode, and to carry generated electrochemically active species away from the anode and away from the cathode in an energy storage mode; and a flow cell reconditioning liquid reservoir and a pump, wherein the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber.

63. The flow cell according to claim 62, wherein the flow cell gas anode is a flow cell hydrogen gas anode.

64. The flow cell according to claim 62 or 63, further comprising conduits configured to deliver the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir to the gas anode chamber, and to carry the flow cell reconditioning liquid away from the gas anode chamber, as required.

65. The flow cell according to any one of claims 62 to 64, wherein the pump is configured to deliver the flow cell reconditioning liquid over the flow cell gas anode active surface.

66. The flow cell according to any one of claims 62 to 65, wherein: the flow cell membrane has a gas anode chamber face and a liquid cathode chamber face; and the pump is configured to deliver the flow cell reconditioning liquid over the gas anode chamber face.

67. The flow cell according to any one of claims 62 to 66, wherein the flow cell gas anode active surface is in physical contact with the flow cell membrane.

68. A retrofit kit adapted to be fitted to a flow cell having a gas anode chamber, wherein: the retrofit kit comprises a flow cell reconditioning liquid reservoir and a pump; and the pump is configured to deliver a flow cell reconditioning liquid into the gas anode chamber (e.g. over a flow cell gas anode active surface of a flow cell gas anode therein and / or a gas anode chamber face of a flow cell membrane therein).

69. The retrofit kit according to claim 68, further comprising the flow cell reconditioning liquid (optionally wherein the flow cell reconditioning liquid is as defined in any one of claims 3 to 15).

70. The retrofit kit according to claim 68 or 69, further comprising conduits configured to supply the flow cell reconditioning liquid from the flow cell reconditioning liquid reservoir to the gas anode chamber, and to carry the flow cell reconditioning liquid away from the gas anode chamber; optionally wherein the same conduit is configured to supply both flow cell reconditioning liquid and electrochemically active species to the gas anode chamber, and / or the same conduit is configured to carry both flow cell reconditioning liquid and electrochemically active species away from the gas anode chamber.

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