Technical Field

The present invention generally relates to the field of devices which are capable of storing and delivering electricity.

Background

Renewable energy sources (e.g. solar, wind, geothermal and others) are essential to combat oil addiction, alleviate anthropogenic factors affecting global climate change and help address increasing energy demands. However, due to their intermittency, the energy produced by these sources needs to be stored when available and utilised when demanded. In addition, grid level electricity generated via conventional combustion still has to circumvent periods of excess and insufficient production. These surpluses and lags are costly to the industry and can cause instabilities on the electricity network. There are many existing technologies at different stages of maturity that can store energy mechanically, magnetically, electrochemically, chemically, thermally and in other forms. The ability of redox flow batteries (RFBs) or regenerative fuel cells (RFCs) to store energy electrochemically is particularly attractive as these systems can decouple power and energy, are site-independent, have very fast response times, require very low

maintenance and have extensively long cycle lives.

RFBs, and the methods by which they are able to store and deliver electricity, have been known for many years. They are electrochemical apparatus for energy storage and power delivery. In the power delivery phase, active species are supplied to electrodes, where they react electrochemically to produce electrical power. In a storage phase, electrical power is used to regenerate the electrochemically active species, which are stored.

Because the electrochemically active species can be stored separately from the electrode compartments and supplied when required, the generating/storage capacity of this equipment can be relatively large.

The electrochemical reactions take place on either side of an ion transport system separating the two half cells (such as a membrane) with selective charge carriers either being transported or exchanged by the membrane.

The fundamental process in these redox flow battery (RFB) systems is characterised by a chemical equation where the reaction proceeds in one direction in the energy storage mode of the system and in the opposite direction during the power delivery mode by the system. This chemical process can be exemplified by the following redox chemical equation, wherein the term "redox" defines reactions in which a reduction and a complementary oxidation occur together.

V ^lv _(sol) + V'" _(sol) V" _(sol) + V ^v _(SO i ₎ Equation 1

Two examples of redox flow technologies which have been commercially implemented are all-vanadium and zinc-bromine batteries. All vanadium redox flow batteries use aqueous vanadium redox couples at 4 different oxidation states (see Equation 1 ), and the vanadium serves as the electrochemically active species in both the anolyte and the catholyte. Zinc-bromine batteries utilise electrochemical deposition and dissolution of zinc in aqueous bromide solutions. Both technologies have reached the level of fully commercial products. Though existing redox flow batteries (RFB) are ideal candidates for storing energy, there are some general drawbacks impeding widespread use of these systems such as electrolyte cross-contamination, uncontrollable hydrogen or oxygen co- generation, metal dendrite formation, danger of formation of toxic gases, large space requirements for electrolyte reservoirs and high cost.

The increasing interest towards grid connected energy storage devices helps in generating a strong motivation in the scientific community to develop new sources for large scale energy storage to store electrical power when demand is low and to release it when demand is high, therefore making intermittent renewable energy sources an increasingly commercially viable option as well as mitigating grid level fluctuations in present demand/usage.

It is therefore an objective of the invention to provide a device which is capable of storing and delivering electricity, and which overcomes one or more of the issues associated with commercially available energy storage and delivery devices. For example, the device may reduce or prevent metal dendrite formation, or reduce or eliminate cross contamination. Furthermore, the inventive devices may be cheaper and may have higher power density and/or energy density than the commercially available methods and devices for generating and storing electricity.

Rechargeable batteries and battery materials are well known in many forms and for many uses. While existing interest has been centred upon non-aqueous lithium-ion batteries, aqueous systems utilising lithium insertion solid state electrodes have been gaining significant attention for their potential use in grid storage applications. One of the most striking characteristics of aqueous lithium-ion batteries is their exceptionally high charge and discharge rate (ca.l OO'O rates) due to significantly higher electrolyte conductivity, which is only matched through specialised electrochemical devices such as

supercapacitors.

It is important to realise that RFBs are distinct from standard fuel cells. Standard fuel cells consume fuel and can normally only be run in a power delivery mode; they either cannot be run in an energy storage mode (in which power is stored) or, if they can, they can only do so in a highly inefficient way. Furthermore, reversing the electrochemical reaction in a fuel cell can cause permanent damage to the catalyst. Standard fuel cells are optimised for operating in the energy generating mode only while RFBs are optimised for the combined power delivery mode and the energy storage mode. Thus only electrochemical reactions that are readily reversible can be used in a RFB, while in standard fuel cells (such as direct alcohol, or direct borohydride fuel cells, or hydrogen/oxygen fuel cells) the reactions need not be reversible and indeed they are usually not.

Disclosure of the Invention

The present invention is defined in the accompanying claims. The present invention relates to a hybrid redox flow battery (HyRFB), that is to say an electrochemical apparatus configured for both energy storage and power delivery wherein one of the electrodes comprises a material that is capable of reversibly taking up and releasing alkali metal ions during the power delivery and energy storage modes of operation, respectively.

In accordance with standard terminology in the field of redox flow batteries, 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 to denote the same electrodes throughout the two modes of operation (power deliver and energy storage) of the HyRFB. The terms "anolyte" and "catholyte" will be used to denote the electrolyte in contact with the "anode" and "cathode".

In a power delivery mode, an electrochemically active species is oxidised at the anode and an electrochemically active species is reduced at the cathode to form reacted (or "spent") species. In the energy storage mode, the electrochemical system is reversed and the "spent" anode species is electrochemically reduced at the anode and the "spent" cathode species is electrochemically oxidised at the cathode to regenerate the corresponding electrochemically active species. Ions are either selectively passed by, or exchanged across, the membrane to maintain charge neutrality in order to complete the electrical circuit in both the energy storage mode and the power delivery mode.

Therefore, in accordance with the present invention there is provided a hybrid redox flow battery (HyRFB) capable of operating in a power delivery mode in which it generates electrical power by the reaction of electrochemically active species at an anode and at a cathode and in an energy storage mode in which it consumes electrical power to generate said electrochemically active species. The HyRFB comprises:

• a reversible first electrode in a first electrode compartment containing a first aqueous electrolyte,

• a reversible second electrode in a second electrode compartment

containing a second aqueous electrolyte; and

• a conduit arrangement configured, in said power delivery mode, for

carrying electrochemically active species to the first electrode and, in an energy storage mode, for carrying generated electrochemically active species away from the first electrode;

wherein the second electrode comprises a solid state insertion material that is capable of reversibly taking up and releasing alkali metal ions (e.g. lithium, sodium, etc.) during the said modes of operation, and wherein the second aqueous electrolyte comprises the alkali metal ions.

Accordingly, it is apparent that a HyRFB comprises an electrochemical cell and can therefore be referred to herein as the cell.

The hybrid redox flow battery of the invention comprises a conduit arrangement configured to carry electrochemically active species to the first electrode in the power delivery mode and to carry generated electrochemically active species away from the first electrode in the energy storage mode.

The electrochemically active species may be present in the liquid electrolyte supplied to the first electrode compartment. However, the electrochemically active species may be a gas (especially hydrogen or air (oxygen)), in which case the first electrode must be a gas- permeable electrode.

Preferably the electrochemically active species is supplied in liquid electrolyte to the first electrode compartment. When the electrochemically active species is to be supplied to the first electrode compartment in liquid electrolyte, the liquid electrolyte comprising the electrochemically active species may be stored in a first vessel ready for passing to the first electrode compartment in the power delivery mode. However, where the electrochemically active species is gaseous, it will generally be in the form of a pressurised gas source, except when it is readily obtainable at source, notably air (oxygen), in which case there may be no need to store it in a vessel. Hydrogen storage can also be in the form of a solid state powder, for example as one or more metal hydride compounds, or in a metal organic framework.

During the power delivery mode, the electrolyte comprising the spent species may be collected in a second vessel or, if it can be safely discharged and further supplies are readily available, e.g. if it is water, there may be no need to collect it. If the

electrochemically active species is gaseous and readily available (e.g. oxygen), any non- reacted gas (such as oxygen-depleted air) can be discharged to atmosphere, if permissible, or collected if not or if it can be re-used. If the spent species is gaseous, it can be collected for storage, e.g. in a pressurised vessel or a solid state (i.e. metal hydride) compound(s).

During the energy storage mode, the electrolyte comprising the spent species may be supplied from the second vessel to the first electrode compartment, where the

electrochemically active species is regenerated. The first and second vessels may be different compartments of a single container, or they may be the same compartment.

When the electrochemically active species is included in the electrolyte, the HyRFB of the present invention may additionally include a pump which allows the liquid electrolyte to be circulated through the conduit arrangement between the storage vessels and the first electrode compartment. When the electrochemically active species is a gas, a compressor may be provided that allows the gas to be stored at pressure in a vessel exterior to the electrochemical cell. The HyRFB of the present invention may also include an additional vessel for holding an anode supporting electrolyte, wherein the additional vessel is connected to a further conduit arrangement and wherein the further conduit arrangement is configured during the power delivery mode to carry the anode supporting electrolyte from the anode

compartment to the additional vessel and during the energy storage mode to supply anode supporting electrolyte from the additional vessel to the anode compartment. An anode supporting electrolyte may be required when the second electrode is the anode.

It will be appreciated that discussion in relation to an anode supporting electrolyte applies mutatis mutandis to a cathode supporting electrolyte when the second electrode is the cathode. Thus, the HyRFB of the present invention may include an additional vessel for holding an cathode supporting electrolyte, wherein the additional vessel is connected to a further conduit arrangement and wherein the further conduit arrangement is configured during the power delivery mode to carry the cathode supporting electrolyte from the cathode compartment to the additional vessel and during the energy storage mode to supply cathode supporting electrolyte from the additional vessel to the cathode compartment.

It will be appreciated that the anode supporting electrolyte may be used when the ion exchange membrane is an anionic membrane capable of selectively passing anions, that is to say the membrane can transfer negatively charged ions, e.g. by being selectively permeable to anions as opposed to cations, such as alkali metal ions or alkaline earth metal ions. Therefore, in this instance, it is desirable to circulate an electrolyte which provides the alkali metal ions or alkaline earth metal ions to the anode in the energy storage mode, and removes the alkali metal ions or alkaline earth metal ions together with crossed (or exchanged) anions from the cathode compartment away from the anode in power delivery mode. Thus, an anode supporting electrolyte container may be used when the active cation, such as Li, Na or K, is not passed across the ion exchange membrane. It will be appreciated that the anode supporting electrolyte is not undergoing redox reaction and is therefore referred to as a supporting electrolyte. A skilled person will appreciate that when, the second electrode is the cathode, the above discussion in relation to the anode supporting electrolyte applies to the cathode supporting electrolyte.

An exemplary HyRFB comprising an additional vessel configured to supply cathode supporting electrolye to the cathode compartment include a HyRFB wherein the second electrode is the cathode and comprises LiMn0 ₂ and the first electrolyte is the anolyte and comprises an acidic solution of a V(2)/V(3) redox couple.

Use of an anode or cathode supporting electrolyte may be required when it is desired to prevent mixing of either cations or anions. For example, when there is a risk that the second electrode comprising the solid state insertion material may be damaged by concentrated acidic conditions. Therefore, it may be desirable to prevent crossing of protons across the membrane using an anionic exchange. A skilled person will appreciate that where the electrolyte is an alkaline system, a cation exchange membrane may be required to prevent crossing of hydroxide ions that may damage the second electrode comprising the solid state insertion material. In such situations it may be desired to provide an anode or cathode supporting electrolyte depending on whether the second electrode is the anode or the cathode. In an exemplary HyRFB according to the invention, the anode is the second electrode and comprises LiTi(P0 ₄ ) _x , the first electrolyte is the catholyte and comprises Fe(CN) _x . Where the first electrolyte is alkaline, it is desired to prevent the crossover of hydroxide ions to prevent the second electrode comprising the solid state insertion material being damaged. In such case the use of cation exchange membrane is desired. It will be appreciated that the cathode may be the second electrode comprising the solid state insertion material and first electrolyte may be the anolyte comprising the redox electrolyte.

The electrochemical reactions may take place at a discrete electrodes (i.e. at the anode and cathode) or they may take place at least partly in the electrolyte or the membrane. Therefore, it may not always be easy to identify a discrete anode and cathode and the main manifestations of the anode and the cathode may simply be the anodic and cathodic current collectors, which facilitate the supply of electrons to an electrode from an external circuit and the removal of electrons from an electrode to the external circuit (in the energy storage mode, the cathodic current collector will transfer electrons away from the cathode, and the anodic current collector will supply electrons to the anode. In the power delivery mode, this will be reversed.)

When the electrochemically active species is gaseous, the first electrode will be porous, and when the electrolyte is a liquid, it may be porous or non-porous. Examples of suitable electrodes are well known in the art, however catalysed porous carbon electrodes are particularly preferred in the present invention, for example catalysed carbon paper, cloth, felt or composite. The carbon may be graphitic, amorphous, glassy, or nano structured, such as graphene, nanotubes etc. When the first electrode is an anode, it electrochemically catalyses a redox reaction, that is, the first electrode facilitates the conversion of the electrochemically active species to the "spent" species in the power delivery mode, and the conversion of the "spent" species to the electrochemically active species in the energy storage mode. When the first electrode is a cathode, this process is reversed.

The second electrode comprises a solid state insertion material that is capable of taking up and releasing alkali metal ions during the operation of the inventive HyRFB. The second electrode can be the anode or the cathode, and is a solid state electrode. It will be appreciated that whether the material of the second electrode takes up or releases the alkali metal ions during the power delivery and energy storage modes will depend on whether the second electrode is the anode or the cathode. When the second electrode is the anode, the material that is capable of reversibly taking up and releasing alkali metal ions will take up the alkali metal ions during the energy storage mode and release the alkali metal ions during the power delivery mode. When the second electrode is the cathode, the material that is capable of reversibly taking up and releasing alkali metal ions will release the alkali metal ions during the energy storage mode and take up the alkali metal ions during the power delivery mode. Preferably, the alkali metal ions comprise Li ⁺ , K ⁺ , Na ⁺ or a mixture thereof, even more preferably the alkali metal ions comprise Li ⁺ . The second electrode is preferably a composite material comprising a mixture of conductive, binding and electrochemically active phases, supported by a current collector. Such composite electrodes are well known in the art, and are commonly used in battery manufacturing. Examples of such electrodes are set out in US 8,148,015, the entire contents of which is hereby incorporated by reference.

The material that is capable of taking up and releasing alkali metal ions is preferably an alkali metal insertion compound, such as a lithium insertion compound. An insertion compound is a compound which allows alkali metals to be inserted into and de-inserted from its structure. Alkali metal insertion compounds which are useful in the present invention are well known to the skilled person, for example, from Islam S. M. and Fisher C. A. J, Chemical Society Reviews, 2014, 43(1 ), 185-204, the entire contents of which is hereby incorporated by reference. Examples of suitable alkali metal insertion compounds include LiCo0 ₂ , LiMn ₂ 0 ₄ , LiFeP0 ₄ , Li[Ni _1/3 Co ₁ / ₃ Mn _{1 /} 3]02, LiNi0 ₂ , LiNiP0 ₄ , LiTi ₂ (P0 ₄ ) ₃ , LiV ₂ 0 ₅ , LiV ₃ 0 ₈ and Li ₂ Mn ₄ 0 ₉ . It is also contemplated that metal hexacyanoferrate compounds, such as NiFe(CN) ₆ , can be used as the material of the second electrode.

Preferably, the material of the second electrode has the general formula:

M _A [Mi]nAi or M _A [Mi]nA ₂ A3

wherein: M _A is one or more alkali metal ions, each of which is independently selected from Li, Na, K or a mixture thereof;

n is 1 , 2 or 3;

each Mi is independently a transition metal or a mixture thereof, the transition metal is preferably selected from Co, Ni, Mn, Fe, Al, Y, Ti, V or W;

Ai is a one or more chalcogenides which is preferably selected from O, S, Se, or a mixture thereof;

A ₂ is an oxygen-containing inorganic anion which is preferably selected from P0 ₄ ^3~ , Si0 ₄ ^4" , S0 ₄ ^2" ;

A ₃ may be absent or may be F.

The materials defined by the above formulae have high electrochemical potential and are therefore particularly useful when the HyRFB comprises a solid cathode and an anode where liquid or gaseous electrochemically active species can be reduced or oxidised. Therefore, when the second electrode is the cathode, the material of the second electrode preferably has the general formula (M _A )m[M ₁ ] _n (A ₁ ) _r or M _A [M ₁ ]nA ₂ A ₃ .

The material which is capable of taking up and releasing alkali metal ions does not necessarily comprise alkali metal ions in its general formula, particularly when the second electrode is the anode. For example, the material of the second electrode may have the general formula:

[M ₂ ] _r H _x A ₄

wherein:

each M ₂ is independently selected from W, V, Cr and Mo;

r is 1 , 2 or 3

H is hydrogen

x is an integer of from 0-3, preferably 0, 1 or 2

A ₄ is one or more chalcogenides which is preferably selected from O, S, Se, or a mixture thereof, even more preferably A ₄ is one or more oxygen atoms (i.e. A ₄ may be 0 ₂ , 0 ₃ , 0 ₅ , 0 ₈ etc, depending on the oxidation state of M ₂ and the number of H atoms).

Examples of materials which are capable of taking up and releasing alkali metal ions, but which do not comprise an alkali metal in their general formula include V0 ₂ , V ₂ 0 ₅ , H ₂ V ₃ 0 ₈ , Cr ₃ 0 ₈ , W0 ₃ , Mo0 ₃ and mixtures thereof.

The skilled person will also appreciate that the second electrode may be a supercapacitor electrode. Electrodes which are used in supercapacitors are capable of adsorbing and desorbing alkali metal ions during energy storage and power delivery cycles. Suitable electrodes are well known to the skilled person in the field of supercapacitors. Examples of suitable electrodes are set out in US 5,993,996, the entire contents of which is hereby incorporated by reference. Thus, the second electrode may be a supercapacitor electrode, that is, it may comprise a porous stable inert material which is capable of adsorbing and desorbing alkali metal ions. For example, the material of the second electrode may be porous activated carbon.

Preferably, the material of the second electrode is selected from LiCo0 ₂ , LiMn ₂ 0 ₄ , LiFeP0 ₄ , Li[Ni _1/3 Co _1/3 Mn _{1 /} 3]0 ₂ , LiNi0 ₂ , LiNiP0 ₄ , V0 ₂ , V ₂ 0 ₅ , LiV ₂ 0 ₅ , H ₂ V ₃ 0 ₈ , LiV ₃ 0 _8, Cr ₃ 0 ₈ , W0 ₃ , Mo0 ₃ , LiTi ₂ (P0 ₄ ) ₃ , Li ₂ Mn ₄ 0 ₉ and combinations thereof. For example, the material of the second electrode may be selected from LiCo0 ₂ , LiMn ₂ 0 ₄ , LiFeP0 ₄ ,

Li[Ni _1/3 Co _1/3 Mn _1/3 ]0 ₂ , LiNi0 ₂ , LiNiP0 ₄ , V0 ₂ , V ₂ 0 ₅ , H ₂ V ₃ 0 ₈ , Cr ₃ 0 ₈ , W0 ₃ , Mo0 ₃ , LiTi ₂ (P0 ₄ ) ₃ , Li ₂ Mn ₄ 0 ₉ and combinations thereof. Preferably, the material of the second electrode, wherein the second electrode is a cathode, is LiCo0 ₂ , LiV ₂ 0 ₅ , LiMn ₂ 0 ₄ , LiFeP0 ₄ , Li[Ni _1/3 Co _1/3 Mn _1/3 ]0 ₂ , LiNi0 ₂ , LiNiP0 ₄ , and combinations thereof. Preferably, the material of the second electrode, wherein the second electrode is the anode, is selected from V0 ₂ , H ₂ V ₃ 0 ₈ , Mo0 ₃ , LiTi ₂ (P0 ₄ ) ₃ , Li ₂ Mn ₄ 0 ₉ , LiV ₃ 0 ₈ and combinations thereof.

The solid state insertion material may comprise one or more electronically conducting polymers. Thus the second electrode may comprise one or more electronically conducting polymers. Such electrodes may comprise a single electronically conducting polymer or a mixture of electronically conducting polymers. Electronically conducting polymers may be selected from a Polyacytelene, Polyaniline, Polypyrrole, Polythiophene, Polyphenylene, Polytriphenylene, Polyazulene, Polyacene, Polyoxyphenazine, Carbazole-Substituted Polyethylene, Ferrocene-Substituted Polyethylene, Tetrathiafulvalene-Substituted

Polystyrene and those set out in Novak P. et al., Chem. Rev., 1997, 97(1), 207-281 , the entire contents of which is hereby incorporated by reference, or a mixture thereof.

The second electrolyte comprises the alkali metal ions. The second electrolyte is aqueous and may be acidic, neutral or alkaline. The second electrolyte may be prepared by dissolving a salt of the alkali metal in water. Examples of suitable salts include nitrate (N0 ₃ ), chloride (CI ), perchlorate (CI0 ₄ ^~ ), methanesulfonate (CH ₃ S0 ₃ ^~ ) and sulphate (S0 ₄ ^2~ ) salts. Aqueous electrolytes have the advantage of being cheap and readily available, as well as having higher conductivity than non-aqueous electrolytes.

The first and the second electrode can function as either the anode or the cathode. For example, the first electrode may be the anode and the second electrode may be the cathode. Alternatively, first electrode may be the cathode and the second electrode may be the anode.

The first electrode catalyses a redox reaction between the electrochemically active species and the spent species (i.e. the redox couple) and may be called the redox electrode. Accordingly, the invention provides a HyRFB comprising a first electrode and a second electrode, wherein the first electrode catalyses the redox reactions of the active species in the first electrolyte (anolyte or catholyte) and the second electrode comprises a solid state insertion material that is capable of reversibly taking up and releasing alkali metal ions.

When the second electrode is the cathode, the material of the second electrode is preferably selected from LiCo0 ₂ , LiMn ₂ 0 ₄ , LiFeP0 ₄ , Li[Ni _{1 3} Co _{1 3} Mn ₁ 3]0 ₂ , LiNi0 ₂ , LiNiP0 ₄ , LiV ₂ 0 ₅ , and combinations thereof. For example, the material of the second electrode is preferably selected from LiCo0 ₂ , LiMn ₂ 0 ₄ , LiFeP0 ₄ , Li[Ni _{1 3} Co _{1 3} Mn ₁ 3]0 ₂ , LiNi0 ₂ , LiNiP0 ₄ , and combinations thereof. When the first electrode is the anode, the redox reaction at the anode is preferably selected from:

Te(aq) + 2e ·.-·: Te ^2" ,

Se(aq) + 2e :. - Se ² .

S(aq) + 2e S ² .

T ^' + e - Ti ² \

HPb0 ₂ ^" + H ₂ 0 + 2e -- Pb + 30H .

Te0 ₂ ^2" + 3H ₂ 0 + 4e Te + 60H .

Cr ³⁺ + e ·*···* Cr ²⁺ ,

V ³ + e , V ² · . and

Sn " ^' + 2e Sn ^2' .

When the first electrode is the anode, the redox reaction at the anode may also preferably be selected from:

Mn ² - + 2e Mn.

V ² · + 2e , - V.

Cr ²⁺ + 2e ^" Cr.

Zn ²⁺ + 2e ^" ::· - Zn.

Cr ^3' + 3e Cr, Fe ^2' + 2e Fe,

Cd ²⁺ + 2e , - Cd.

Co ²⁺ + 2e ^ Co,

Ni ²⁺ + 2e ^" ::·- Ni.

Sn ²⁺ + 2e ^" ^ Sn, and

Pb ²⁺ + 2e Pb.

It will also be appreciated that the first electrode may be the cathode and the second electrode may be the anode.

When the second electrode is the anode, the material of the second electrode is preferably selected from V0 ₂ , V ₂ 0 ₅ , H ₂ V ₃ 0 ₈ , Mo0 ₃ , LiTi ₂ (P0 ₄ )3, Li ₂ Mn ₄ 0 ₉ , LiV ₃ 0 ₈ and combinations thereof. When the first electrode is the cathode, the redox reaction at the cathode is preferably selected from:

Fe(CN) ₆ ³ + e ·*···.* Fe(CN) ₆ "

l ₂ (aq) + 2e 21

Br ₂ (aq) + 2e ·:.··-· 2Br

Cl ₂ (aq) + 2e -- 2CI

Fe ^3' + e - ^ Fe ^2'

V0 ₂ ⁺ + 2 FT + e ^" V0 ²⁺ + H ₂ 0

Mn ³⁺ + e ^" Mn ²⁺ , and

Ce"- + e — Ce ³ - .

Even more preferably, the redox reaction at the cathode is preferably selected from:

Fe ^3' + e - Fe ^2' .

V0 ₂ ⁺ + 2FT + e ^" -..·:·: V0 ²⁺ + H ₂ 0,

Mn ³⁺ + e ^" :;··- Mn ²⁺ , and

Ce ⁴⁺ + e - Ce ^3' . The skilled person will appreciate that when the first electrode is the anode, the first electrolyte will be referred to as the anolyte, and when the first electrode is the cathode, the first electrolyte will be referred to as the catholyte. Similarly, when the second electrode is the cathode, the second electrolyte will be referred to as the catholyte and when the second electrode is the anode, the second electrolyte will be referred to as the anolyte. When the first electrode is the anode and the second electrode is the cathode, it will be understood by the person skilled in the art that when the HyRFB is charged, the predominant species in the anolyte will be the reduced species of the above equations, while at the end of the power delivery phase, when the HyRFB is fully discharged, the predominant species in the anolyte will be the oxidised species. For the second electrode (the cathode), when the HyRFB is fully discharged, the material of the second electrode will have taken up, or "inserted" alkali metal ions, while at the end of the charging or energy storage phase, the inserted alkali metal ions will have been released by the material of the second electrode. The position is reversed for when the first electrode is the cathode and the second electrode is the anode. Namely, when the HyRFB is charged, the predominant species in the catholyte will be the oxidised species of the above equations, while at the end of the power delivery phase, when the HyRFB is fully discharged, the predominant species in the catholyte will be the reduced species. For the second electrode (the anode), when the HyRFB is charged, the material of the second electrode will have taken up, or "inserted" alkali metal ions, while at the end of the power delivery phase, the inserted alkali metal ions will have been released by the material of the second electrode.

The concentration of a non-gaseous electrochemically active species in the electrolytes (the anolyte and the catholyte) determines the power and energy density of the regenerative fuel cell (HyRFB). Therefore, the concentration of electrochemically active species in the electrolytes is preferably at least 0.5M, and more preferably greater than 1 M. However, the skilled person will appreciate that electrolyte concentration can fluctuate during operation of the HyRFB. The maximum practical concentration of the

electrochemically active species will generally be governed by its solubility in the electrolyte as precipitation from the electrolyte becomes an increasing problem at higher concentrations, and the presence of precipitated materials in the cell is preferably avoided since it interferes with the flow of the electrolyte and the functioning of the regenerative fuel cell in question.

The skilled person will appreciate that if the first electrode and the second electrode are physically and electrically separated, the HyRFB may not require a membrane separating the first electrode compartment and the second electrode compartment. Membranes can be expensive and therefore a HyRFB which avoids the use of a membrane will have the advantage of being cheaper than systems which require the use of such membranes. When the HyRFB of the invention comprises a membrane separating the first electrode compartment from the second electrode compartment, the membrane preferably comprises a porous material that allows charge carriers to pass between the first electrode compartment and the second electrode compartment. The membrane may be a cation exchange membrane, an anion exchange membrane or a combined anion and cation exchange membrane (e.g. a bipolar membrane). For example, the membrane may be capable of selectively passing cations, i.e. protons or alkali metal ions. The membrane may be an anionic membrane capable of selectively passing anions, that is to say the membrane can transfer negatively charged ions, e.g. by being selectively permeable to anions or is an anion exchange material. Preferably the anion exchange membrane is impermeable to cations.

The temperature at which the HyRFB of the present invention is operated at impacts the stability of the species present in the electrolyte. Therefore an operating temperature between about 0 ^e C and about 80 ^e C, for example between about Ι Ο 'Ό and about 70 °C, is preferred in the present invention.

In some embodiments, individual HyRFBs of the present invention can be connected in series or in parallel to one another, or as part of a larger circuit comprising other balance- of-plant systems.

It will be appreciated that the various preferred features described above for the HyRFB may be present in combination mutatis mutandis.

Skilled person will appreciate that the invention can also be applied to a solid state insertion material that is capable of reversibly taking up and releasing alkaline earth metal ions (e.g. Magnesium, Calcium etc). Therefore, the second electrode may comprise a solid state insertion material that is capable of reversibly taking up and releasing alkaline earth metal ions. Accordingly, the invention also provides a hybrid redox flow battery (HyRFB) capable of operating in a power delivery mode in which it generates electrical power by the reaction of electrochemically active species at a first and second electrode and in an energy storage mode in which it consumes electrical power to generate at least one

electrochemically active species. The HyRFB comprises:

• a reversible first electrode in a first electrode compartment containing a first aqueous electrolyte,

· a reversible second electrode in a second electrode compartment

containing a second aqueous electrolyte; and

• a conduit arrangement configured, in said power delivery mode, for

carrying electrochemically active species to the first electrode and, in an energy storage mode, for carrying generated electrochemically active species away from the first electrode;

wherein the second electrode comprises a material that is capable of reversibly taking up and releasing alkaline earth metal ions during the said modes of operation of the cell, and wherein the second electrolyte comprises the alkaline earth metal ions.

The material of the second electrode may be a divalent cation solid state insertion material, preferably having the general formula:

AM _x [Fe ²⁺ (CN) ₆ ] _2x [Fe ³⁺ (CN ₆ )] ₁ . _2x , where:

A is one or more divalent alkaline metal ions selected from Ba, Sr, Ca or Mg, or a mixture thereof; and M _x is a transition metal or a mixture thereof (see Wang R.Y. et al., Nano Lett, 2013, 13(1 1 ), 5748-5752, the entire contents of which is hereby incorporated by reference). Preferably M _x may be Cu or Ni or a mixture thereof. The thermodynamic potential of these compounds is around 0.6 V vs SHE, which places them into category of solid state cathodes.

A skilled person will appreciate that the various features described above in relation to the HyRFB comprising an alkali metal insertion compound may be present in combination with the HyRFB comprising an alkaline earth metal insertion compound mutatis mutandis. The present invention also provides a method for operating a hybrid redox flow battery (HyRFB) as defined above. Each of the preferred embodiments of the HyRFB apply to the method for operating the HyRFB mutatis mutandis.

Brief Description of the Figures Figures 1 , 2 and 3 are schematic sectional views of exemplary hybrid redox flow batteries of the invention.

Detailed Description of the Figures

Figure 1 shows a schematic of a HyRFB in which the first electrode is the cathode and the second electrode is the anode. The electrochemically active species supplied to the cathode is dissolved in liquid (aqueous) electrolyte.

In the power delivery mode, the material of the second electrode (3) is oxidised according to the following half reaction, and releases lithium ions into the second electrolyte (the catholyte):

LiTi ₂ (P0 ₄ ) ₃ Li ₁ . _x Ti ₂ (P0 ₄ ) ₃ + xLi ⁺ + xe ^"

The liquid catholyte containing the electrochemically active species Fe ³⁺ is pumped by a pump (7) from a compartment of a catholyte storage container (9), through a conduit (8) and into the catholyte compartment (6), where it is reduced at the cathode (10) according to the half reaction:

Fe ^3' + e ,...·:· Fe ²⁺

The catholyte containing the spent species Fe ²⁺ is then carried away from the catholyte compartment through a second conduit (1 ) to the catholyte storage container (9), where it is stored in a compartment separate from the fresh catholyte compartment.

Lithium ions pass from the anolyte compartment (4) to the catholyte compartment (6) through the membrane (5), thereby balancing the charge and completing the electrical circuit.

In the energy storage mode, the system is reversed so that the spent Fe ²⁺ is pumped from the catholyte storage container (9) through a conduit (1 ) to the catholyte compartment (6) where it is oxidised at the cathode (10) to the regenerated electrochemically active species Fe ³⁺ . The resulting regenerated electrolyte is transferred away from the catholyte compartment (10) by a pump (7) through a conduit (8) to the catholyte storage container. Meanwhile, the material of the second electrode (3) is reduced, and lithium ions present in the anolyte are taken up, or "inserted" into the material of the second electrode (3).

Lithium ions pass from the catholyte compartment (6) to the anolyte compartment (4) through the membrane (5), thereby balancing the charge and completing the electrical circuit. It will be appreciated that any of the following redox couples may be used instead of

Fe ³ 7Fe ²⁺ :

Fe(CN) ₆ ³ - + e ^" <→ Fe(CN) ₆ ⁴ -,

l ₂ (aq) + 2e ^" ^ 21 ^" ,

Br ₂ (aq) + 2e ^" ^ 2Br ^" ,

Cl ₂ (aq) + 2e ^" <→ 2CI ^" ,

V0 ₂ ⁺ + 2 FT + e ^" <→ V0 ²⁺ + H ₂ 0,

Mn ³⁺ + e Mn ²⁺ , or

Ce ⁴⁺ + e - → Ce ³⁺ .

Figure 2 shows a schematic of a hybrid redox flow battery in which the first electrode is the anode and the second electrode is the cathode. The electrochemically active species supplied to the anode is dissolved in liquid (aqueous) electrolyte and the anode is a metal deposition electrode.

In the energy storage mode, the material of the second electrode (107) is oxidised according to the following redox reaction, and releases lithium ions into the second electrolyte (the catholyte):

LiCo0 ₂ S ^ Li _1-x Co0 ₂ + xLi ⁺ + xe ^"

The liquid anolyte containing the electrochemically active species Zn is pumped by a pump (103) from a compartment of an anolyte storage container (101 ), through a conduit (102) and into the anolyte compartment (104), where it is reduced at the anode (108) according to the half reaction:

Zn ²⁺ + 2e :;··- Zn

The solid metal (Zn) produced by the above reaction is deposited on the anode (108). The spent anolyte is then carried away from the anolyte compartment through a second conduit (109) to the anolyte storage container (101 ), where it may be stored in a compartment separate from the fresh anolyte compartment. Lithium ions pass from the catholyte compartment (106) to the anolyte compartment (104) through the membrane (105), thereby balancing the charge and completing the electrical circuit. In the power delivery mode, the system is reversed so that the metal (Zn) deposited on the anode is oxidised to the species Zn ²⁺ at the anode (108). The anolyte may be pumped from the anolyte storage container (101 ) through a conduit (109) to the anolyte

compartment (104) where it is replenished by the electrochemically active species Zn ²⁺ . The resulting regenerated electrolyte is transferred away from the anolyte compartment (104) by a pump (103) through a conduit (102) to the anolyte storage container.

Meanwhile, the material of the second electrode (107) is reduced, and lithium ions present in the catholyte are taken up, or "inserted" into the material of the second electrode (107). Lithium ions pass from the anolyte compartment (104) to the catholyte compartment (106) through the membrane (105), thereby balancing the charge and completing the electrical circuit.

It will be appreciated that any of the following redox couples may be used instead of Zn ² 7Zn:

Mn ²⁺ + 2e ^~ ·:··- Mn,

V ²⁺ + 2e ^" v,

Cr ²⁺ + 2e ^ Cr,

Cr ³⁺ + 3e ^" ==- Cr,

Fe ²⁺ + 2e ^" Fe,

Cd ²⁺ + 2e ^" ^ Cd,

Co ²⁺ + 2e Co,

Ni ²⁺ + 2e ^" ~* Ni,

Sn ²⁺ + 2e ·*· Sn, or

Pb ²⁺ + 2e ^" -... ^· :· Pb.

Figure 3 shows a schematic of a hybrid redox flow battery in which the first electrode is the cathode and the second electrode is the anode.

In the power delivery mode, the liquid catholyte containing the electrochemically active species V0 ₂ ⁺ is pumped by a pump (207) from a compartment of a catholyte storage container (209), through a conduit (208) and into the catholyte compartment (206), where it is reduced at the cathode (210) according to the half reaction:

V0 ₂ ⁺ + 2H ⁺ + e ^" V0 ²⁺ + H ₂ 0

The catholyte containing the spent species V0 ²⁺ is then carried away from the catholyte compartment through a second conduit (201 ) to the catholyte storage container (209), where it is stored in a compartment separate from the fresh catholyte compartment. The counter anions present in the catholyte (i.e. S0 ₄ ^2~ ) pass from the catholyte compartment (206) to the anolyte compartment (204) through the anion permeable membrane (205), thereby balancing the charge and completing the electrical circuit.

The material of the second electrode (203) is oxidised according to the following half reaction, and releases lithium ions into the second electrolyte (the anolyte):

LiV ₃ 0 ₈ Lii _-x V ₃ O _e + xLi ⁺ + xe ^~

The lithium ions are counter-balanced by the counter anions (i.e. S0 ₄ ^2~ ) coming from the catholyte. The lithium ion electrolyte (i.e. Li ₂ S0 ₄ ) is pumped away from the reaction zone into the anode supporting electrolyte container (21 1 ) thus preventing Li ₂ S0 ₄ precipitation there.

In the energy storage mode, the system is reversed so that the spent V0 ²⁺ is pumped from the catholyte storage container (209) through a conduit (201 ) to the catholyte compartment (206) where it is oxidised at the cathode (210) to the regenerated electrochemically active species V0 ₂ ⁺ . The resulting regenerated electrolyte is transferred away from the catholyte compartment (206) by a pump (207) through a conduit (208) to the catholyte storage container. Meanwhile, the material of the second electrode (203) is reduced, and lithium ions present in the anolyte are taken up, or "inserted" into the material of the second electrode (203). Counter anions (i.e. S0 ₄ ^2" ) pass from the anolyte compartment (204) to the catholyte compartment (206) through the membrane (205), thereby balancing the charge and completing the electrical circuit. It will be appreciated that an anode supporting electrolyte may be used when the ion exchange membrane is a membrane permeable to anions as opposed to cations, such as alkali metal ions or alkaline earth metal ions. As result of this there is no cation exchange in the system thus preventing diffusion of protons from catholyte to the anode

compartment (204). Only counter anions (in this case S0 ₄ ^2" ) are exchanged. Therefore, in this instance, it is desirable to circulate an electrolyte which provides the alkali metal ions or alkaline earth metal ions to the anode in the energy storage mode, and removes the alkali metal ions or alkaline earth metal ions away from the anode in power delivery mode. Thus, an anode supporting electrolyte container (21 1 ) may be used when the active cation, such as Li, Na or K, is not passed across the ion exchange membrane. Such a system may be utilised when it is desired to prevent mixing of either cations or anions. For example, in the present case it is desired to prevent protons crossing into the anode compartment and lithium ions crossing into cathode compartment. In this case, if protons cross to the anolyte they may deteriorate the performance of the anode solid state insertion electrode. Therefore, an anion exchange membrane may be used to prevent this.