Description of Invention The present invention relates to slurry electrodes. The present invention also relates to batteries comprising slurry electrodes.

Batteries store electrochemical energy by separating an ion source and an ion sink, the ion source and the ion sink having different ionic electrochemical potential. A difference in electrochemical potential produces a voltage difference between the positive and negative electrodes; this voltage difference produces an electric current if the electrodes are connected by a conductive element. In a battery, the negative electrode and the positive electrode are connected by two conductive elements in series. The external element conducts electrons only, and the internal element (electrolyte) conducts ions only.

Because a charge imbalance cannot be sustained between the negative electrode and positive electrode, ions and electrons are transported between positive and negative electrodes to maintain charge neutrality. In operation, the electric current can be used to power an external device. A rechargeable battery can be recharged by application of an opposing voltage difference that drives an external electric current and an internal ionic current in an opposite direction as that of a discharging battery in service. Therefore, the active materials of rechargeable batteries need to be able to accept electrons from an external power source and at the same time be able to provide electrons to an external load.

Increased electrochemical potential difference between anode and cathode produces larger voltage differences across a battery, and increased voltage difference increases the electrochemical power produced per unit mass of the device. Rechargeable batteries can be constructed using static negative

electrode/electrolyte and positive electrode/electrolyte materials. In

rechargeable batteries, non-energy storing elements of the device comprise a fixed volume or mass fraction of the device; thereby decreasing the device's energy and power density. The rate at which current can be extracted is also limited by the distance over which ions and electrons can be conducted.

Therefore, the power requirements of static cells constrain the total capacity by limiting device length scales.

Redox flow batteries (sometimes known as flow cells, redox batteries or reversible fuel cells) are energy storage devices in which the positive and negative electrode reactants are soluble metal ions in liquid solution that are oxidised or reduced during the operation of the cell. Using two reversible redox couples, liquid state redox reactions are carried out at the positive and negative electrodes. A redox flow cell typically has a power-generating assembly comprising at least: an ionically transporting membrane; positive and negative electrode reactants (also called catholyte and anolyte respectively) separated by the membrane; and, positive and negative carbon electrodes which facilitate the redox reactions. Positive and negative current collectors are found adjacent to the positive and negative electrodes, respectively, which allow for the transfer of electrons to the external circuit but do not participate in the redox reaction (i.e. the current collector materials themselves do not undergo Faradaic activity).

There are many types of flow battery which are categorised based on their redox chemistry (examples include all-vanadium redox flow batteries, zinc- bromine redox flow batteries and chromium-iron redox flow batteries). All these types of flow battery can store large amounts of electricity (depending on the size of the electrolyte tank and the electrolyte concentration) and can cycle for thousands of cycles with high efficiency. However, these types of flow battery require a high cost (typically greater than US $ 400 per kW.hr) and have a large footprint (their volumetric energy density is typically 20 to 30 W per litre).

There are differences in terminology for the components of a redox flow battery and those of conventional primary or secondary batteries (i.e. sealed batteries). The active solutions in a flow battery are typically referred to as electrolytes, and specifically as the catholyte and anolyte, in contrast to the practice in, for example, lithium ion batteries where the electrolyte is solely the ion transport medium and does not undergo Faradaic activity. In a flow battery, the electrochemically active components at which the redox reactions take place and electrons are transported to or from the external circuit are known as electrodes.

While redox flow batteries have many attractive features, including the fact that they can be built to almost any value of total charge capacity by increasing the size of the catholyte and anolyte reservoirs, one of their limitations is that their energy density, being in large part determined by the solubility of the metal ion redox couples in liquid solvents, is relatively low. Methods of increasing the energy density by increasing the solubility of the ions have been proposed. However, such measures which may be detrimental to other aspects of the cell operation, such as by increasing corrosion of cell components, storage vessels, and associated plumbing. Furthermore, the extent to which metal ion solubilities may be increased is limited. To improve the energy density and lower the cost of redox flow batteries, researchers have focussed on two components: the chemistry of the

electrolytes; and the design and engineering of the flow battery components (for example the electrodes) and the system. There is a need to further increase the energy density of catholytes and anolytes in redox flow batteries. According to a first aspect of the present invention, there is provided a slurry electrode for a redox flow battery comprising:

carbon; and,

a redox system;

wherein the carbon and the redox system are bonded together to form a carbon-redox system composite.

Preferably, wherein the carbon and the redox system are bonded together chemically or by adsorption.

Further preferably, wherein the redox system comprises a metal and/or a metal oxide.

Advantageously, wherein the redox system comprises A _x B _y wherein:

A is any one or more of: V ⁺² , V ⁺³ , VO ⁺² , Fe ⁺² , Zn ⁺² , Mg ⁺² , TiO ⁺² , Cr ⁺³ ,

Co ⁺² , Co(en) ₃ ⁺³ and/or Cu(en) ₂ ⁺² , where en = ethylenediamine;

B is any one or more of: CI ^" , Br ^" , Γ, Fe(CN) ₆ ^"3 , Fe(CN) ₄ ^"2 , Co(CN) ₆ ^"3 ,

Co(CO) ₄ ^"1 , Zn(CN) ₄ ^"2 , Cu(CN) ₂ ^"1 , Cr(CN) ₆ ^"3 , R-S0 ₃ ^" and/or R-P0 ₂ ^" , where R is phenyl or substituted phenyl;

x is 1 , 2, 3 or 4, or another integer; and,

y is 1 , 2, 3 or 4, or another integer.

Preferably, wherein the metal is vanadium and/or the metal oxide is a vanadium oxide.

Further preferably, wherein the vanadium is present as V ²⁺ and/or the vanadium oxide is present as V0 ²⁺ .

Advantageously, wherein the redox system comprises VO(RS0 ₃ ) ₂ and/or VO(RP0 ₂ ) ₂ , wherein R is phenyl or substituted phenyl.

Preferably, wherein the redox system comprises V ⁺² and [Fe(CN) ₆ ] ^"4 . Further preferably, wherein the redox system comprises a compound of Formula I:

Formula I

wherein:

X is P0 ₂ ^" , S0 ₃ ^" or N0 ₂ ^" ;

M is V ⁺² or VO ⁺² ; and

R is hydrogen, C-I -C-I O alkyl, C1 -C10 substituted alkyl, phenyl or substituted phenyl.

Advantageously, wherein the redox system comprises a quinone, a compound derived from a quinone, an anthraquinone or a compound derived from an anthraquinone.

Preferably, wherein the quinone or the compound derived from a quinone is, or is derived from, 1 ,2-benzoquinone, 1 ,4-benzoquinone, 1 ,4-naphthoquinone, 9, 10-anthraquinone, chloranil, lawsone, 2,3-dichloro-5,6-dicyano-1 ,4-

benzoquinone or Further preferably, wherein the compound derived from a quinone is a hydrogenated derivative of a quinone.

Advantageously, wherein the redox system comprises a compound of Formula II:

Formula II

wherein:

P0 ₂ ^" , S0 ₃ ^" or N0 ₂ ^" Preferably, wherein the redox system comprises a quantum dot.

Further preferably, wherein the quantum dot is a generally spherical semiconductor particle with a radius of 1 -10 nm (optionally a radius of 2-3 nm, or a radius of 5-6 nm).

Advantageously, wherein the quantum dot is formed of lead sulphide, lead selenide, cadmium selenide, cadmium sulphide, indium arsenide or indium phosphide, or any combination of 1 , 2, 3, 4, 5 or 6 of these compounds.

Preferably, wherein the carbon is in the form of graphite, carbon particles, carbon nanoparticles, carbon tubes, carbon nanotubes, carbon fibers or carbon nanofibers. Preferably, wherein the carbon is conductive.

Further preferably, wherein the carbon is in the form of carbon nanotubes, the carbon nanotubes being generally elongate hollow tubes of carbon with a maximum diameter of from 1 .0 nm to 100 nm, and a length to diameter ratio of from 2: 1 to 132,000,000: 1

Advantageously, wherein the carbon is in the form of carbon nanofibers, the carbon nanofibers being generally elongate non-hollow fibers of carbon (optionally formed of graphite) with a maximum diameter of from 1 nm to 100 nm, and a length to diameter ratio of from 2: 1 to 132,000,000: 1

In an aspect of the present invention, there is provided a redox flow battery comprising a slurry electrode according to any one of the above descriptions of a slurry electrode.

Preferably, wherein the redox flow battery further comprises an ion-exchange membrane between two slurry electrode-electrolyte systems.

Further preferably, wherein the redox flow battery further comprises two carbon plates built in bipolar configuration, each carbon plate in contact with the slurry electrode-electrolyte to serve as current collectors during charge and/or discharge of the slurry electrode.

Advantageously, wherein the redox flow battery further comprises one or more pumps to pump the slurry electrode-electrolyte on one or both of the sides of the ion-exchange membrane into or out of one or more catholyte tanks. In an aspect of the present invention, there is provided a method of forming a slurry electrode for a redox flow battery, the slurry electrode comprising:

carbon; and,

a redox system;

wherein the carbon and the redox system are bonded together to form a carbon-redox system composite;

the method comprising the steps of:

bonding the redox system to the carbon to form a carbon-redox composite. In an aspect of the present invention, there is provided a method of forming a slurry electrode according to according to any one of the above descriptions of a slurry electrode, the method comprising the steps of: bonding the redox system to the carbon to form a carbon-redox composite.

Preferably, wherein the step of bonding the redox system to the carbon to form a carbon-redox composite comprises:

reacting a primary amine within the redox system with the surface of the carbon by the diazonium method.

Further preferably, wherein during the step of reacting a primary amine within the redox system with the surface of the carbon by the diazonium method: the primary amine is reacted with NaN0 ₂ and HCI to form a diazonium cation; the diazonium cation subsequently reacts with the surface of the carbon, the surface of the carbon acting as a nucleophile. In an aspect of the present invention, there is provided a battery comprising a slurry electrode according to any one of the above descriptions of a slurry electrode.

Preferably, wherein the battery comprises:

a slurry anode and a slurry cathode;

a slurry anode and a gas cathode;

a gas anode and a slurry cathode;

an all-liquid anode and a slurry cathode;

a slurry anode and an all-liquid cathode;

a slurry anode and a solid cathode; or,

a solid anode and a slurry cathode.

The present inventors investigated ways of combining the advantage of a conventional battery (i.e. sealed batteries) and all-liquid flow batteries by dispersing the electroactive species (electrode active materials) on carbon surfaces to form a flowable slurry electrode. One of the advantages of a flowable electrode-electrolyte is to increase the concentration of redox systems in the electrolyte and hence to enhance the specific energy density (Wh/I). In addition, the slurry electrode-electrolyte has the potential to operate in different media, such as alkaline, acidic, and organic solutions, because the excess of active species are supported on a conducting carbon surface where their electrochemical reactions still take place. As such, cell voltage and materials compatibility can be improved, especially working with benign aqueous or non-aqueous solutions. Embodiments of the invention are described below with reference to the accompanying drawings, in which:

Figure 1 is a general reaction scheme showing the formation of redox-carbon composites (sometimes referred to as carbon-redox system composites in this specification).

Figure 2 is a general reaction scheme showing the functionalisation of carbon surfaces bearing grafted redox systems using a diazonium method. Figure 3 is an alternative general reaction scheme showing the

functionalisation of carbon surfaces using a diazonium method.

Figure 4 is a schematic diagram showing a redox flow battery including a slurry electrode.

Figure 5 is a schematic diagram showing a battery stack including a slurry electrode.

Figure 6 is a cyclic voltammogram of an anthraquinone adsorbed on Vulcan™ XC-72R carbon.

Some of the terms used to describe the present invention are set out below: "Redox flow battery" refers to an energy storage device in which the positive and negative electrode reactants are in liquid or dispersed solution and are oxidised or reduced during the operation of the device.

"Electrode" refers to, in a flow battery, the non-electrochemically active components at which the redox reactions take place and electrons are transported to or from the external circuit. "Slurry electrode" refers to a composite material where the electroactive species (electrode active materials) are grafted and/or adsorbed on to an electrically conductive solid surface (for example on to a carbon surface).

"Catholyte" refers to the positive electrode-active solution in a flow battery.

"Anolyte" refers to the negative electrode-active solution in a flow battery.

"Diazonium method" refers to the reaction of a primary amine to form a diazonium cation. Common reaction conditions for this reaction include reacting the primary amine with NaN0 ₂ and HCI. The diazonium cation can subsequently be functionalised by reaction with a nucleophile.

"Graphite" refers to an allotrope of carbon which is an electrical conductor. "Carbon particles" refers to particles of carbon, for example formed of graphite, with a maximum dimension of from 0.0001 mm to 1 mm. Optionally, carbon particles are generally spherical.

"Carbon nanoparticles" refers to particles of carbon of any shape with a maximum dimension of from 1 nm to 100 nm; optionally, 5 nm to 10nm. "Carbon tubes" refers to generally elongate hollow tubes of carbon, for example formed of graphite, with a maximum diameter of from 0.001 mm to 1 mm. "Carbon nanotubes" (or "CNTs") refers to generally elongate hollow tubes of carbon with a maximum diameter of 1 .0 nm to 100 nm, and a length to diameter ratio of from 2: 1 to 132,000,000: 1 .

"Carbon fibers" refers to generally elongate non-hollow fibers of carbon, for example formed of graphite, with a maximum diameter of from 0.0001 mm to 1 mm.

"Carbon nanofibers" (or "CNFs") refers to generally elongate non-hollow fibers of carbon, for example formed of graphite, with a maximum diameter of from 1 nm to 100 nm, and a length to diameter ratio of from 2: 1 to 132,000,000:1 .

"Quantum dots" refers to semiconductor particles, which are generally spherical, with a radius of 1 -10 nm (optionally a radius of 2-3 nm, or a radius of 5-6 nm). Examples of semiconductors which form quantum dots include lead sulphide, lead selenide, cadmium selenide, cadmium sulphide, indium arsenide and indium phosphide, or other chalcogenides.

"Composite" refers to a material formed of two or more constituent materials, each material having different physical or chemical properties. In the case of a carbon-redox system composite, the carbon is generally inert and acts as a support for the redox system, while the redox system includes moieties which act to lose and gain electrons in solution. Redox systems can be organic or inorganic, grafted or adsorbed on the surface of the support.

Figure 1 is a general reaction scheme showing the formation of redox-carbon composites. According to Figure 1 , a source of carbon is provided. The source of carbon can be graphite, carbon particles, carbon nanoparticles, carbon tubes, carbon nanotubes, carbon fibers or carbon nanofibers. In some examples, the carbon is conductive. The source of carbon acts as a support to a chemical species generally referred to as A _x B _y . This leads to the formation of A _x B _y particles bonded to the surface of the carbon. As shown in the second steps of Figure 1 , the A _x B _y particles bonded physically (adsorption) or chemically to the surface of the carbon can be further modified by reacting with Z (to form A _X Z _Z on the surface of carbon) or applying a different pH, either acidic or basic (to form AO _x on the surface of carbon). By way of this reaction, redox-carbon composites with the general formula A _x B _y /carbon are formed. Alternatively, quantum dots based on A _x B _y species can be grafted on to the carbon surface, by electrochemical or chemical methods.

The synthesis of a redox-carbon composite, with reference to Figure 1 , is based on chemical deposition of redox systems (denoted as A _x B _y ) onto the surface of carbon; optionally, the carbon being carbon nanoparticles having a size (at least one dimension) ranging from 5 to 10 nm, or from 10 to 100 nm, or from 5 to 20 nm. To form the redox-carbon composite (for example a matrix), methods such as hydrothermal and co-precipitation are used. Figure 1 shows general deposition pathways for the formation of redox-carbon composites. Ideally, the reaction between A and B species should occur on the surface of the carbon at the molecular level to obtain highly dispersed A _x B _y . Deposition can be achieved via various paths; through change of counter ions, change of solvent, or change in pH. For example, if A _x B _y is, sulfate of oxy-vanadate (VOS0 ₄ ) (where A = V0 ²⁺ and B = S0 ₄ ^2" ), which is soluble in aqueous solution, substitution of the counter anion, S0 ₄ ^2" , by another anion (referred to as Z in Figure 1 ) like R-SO3 ^" or R-PO ₂ ^" (R = organic moiety, such as phenyl or substituted phenyl) leads to final compounds, denoted as VO(RS0 ₃ ) ₂ and VO(RP0 ₂ ) ₂ (examples of A _X Z _Z in Figure 1 ), which can adsorb strongly onto the carbon surface due to the nature of the anions. Furthermore, the redox system A _x B _y can be designed in such a way that both species A and B are electrochemically active at two different potentials, which can serve as a single substrate for both anode and cathode reactions. For example, if species A is vanadium ions, V , species B can be an anion, such as [Fe(CN) ₆ ] ^"4 and as result, the redox system has a dual redox potential with a cell voltage near 1 V. Tuning the structure and composition of the redox system can have a direct impact on cell voltage.

Some examples of the system A _x B _y (with reference to Figure 1 ) are (where x is 1 , 2, 3 or 4, or another integer; and y is 1 , 2, 3 or 4, or another integer):

A is any one or more of: V ⁺² , V ⁺³ , VO ⁺² , Fe ⁺² , Zn ⁺² , Mg ⁺² , TiO ⁺² , Cr ⁺³ , Co ⁺² , Co(en) ₃ ⁺³ and/or Cu(en) ₂ ⁺² (where en = ethylenediamine);

B is any one or more of: CI ^" , Br ^" , Γ, Fe(CN) ₆ ^"3 , Fe(CN) ₄ ^"2 , Co(CN) ₆ ^"3 , Co(CO) ₄ ^"1 , Zn(CN) ₄ ^"2 , Cu(CN) ₂ ^"1 , Cr(CN) ₆ ^"3 , R-S0 ₃ ^" and/or R-P0 ₂ ^" .

In other examples, A and B are not limited to the above possibilities. A can be any cation and B can be any anion.

In the system A _X Z _Z , Z can, in some examples, have the same ions as B. However, the physical properties of A _x B _y and A _X Z _Z can be different as a result of a change in ion characteristics.

Some examples of the system A _X Z _Z (with reference to Figure 1 ) are (where x is 1 , 2, 3 or 4, or another integer; and z is 1 , 2, 3 or 4, or another integer):

A is any one or more of: V ⁺² , V ⁺³ , VO ⁺² , Fe ⁺² , Zn ⁺² , Mg ⁺² , TiO ⁺² , Cr ⁺³ , Co ⁺² , Co(en) ₃ ⁺³ and/or Cu(en) ₂ ⁺² (where en = ethylenediamine);

Z is any one or more of: CI ^" , Br ^" , I ^" , Fe(CN) ₆ ^"3 , Fe(CN) ₄ ^"2 , Co(CN) ₆ ^"3 , Co(CO) ₄ ^"1 , Zn(CN) ₄ ^"2 , Cu(CN) ₂ ^"1 , Cr(CN) ₆ ^"3 , R-S0 ₃ ^" and/or R-P0 ₂ ^" .

The design and synthesis of A _x B _y /Carbon composites, or A _x Z _z /Carbon composites, can be tailored according to the working conditions of the desired slurry. For example, the A _x B _y /Carbon composites, or A _x Z _z /Carbon composites, can be tailored according to the chemistry of the conducting electrolyte, which can be aqueous or non-aqueous, and, acidic or alkaline or neutral.

One example of forming a redox system on a carbon surface is by way of a diazonium method (sometimes referred to as grafting), examples of which are shown in Figures 2 and 3.

Figure 2 is a general reaction scheme showing the functionalisation of carbon surfaces using a diazonium method, where the redox system includes a metal species (M in Figure 2; examples of M include V ²⁺ and VO ²⁺ ). In the first step, the -XH position is functionalised to a metal salt. Examples of X include PO ₂ ^" and SO ₃ ^" . Examples of R include hydrogen, C-I-C-IO alkyl, C-I-C-IO substituted alkyl, phenyl or substituted phenyl, or any other groups that can donate or attract electrons (for example F, Br or NO2). In the second step, the amine group is transformed by the diazonium reaction (applying NaNO ₂ and HCI) to form an -N ₂ X group in its place. The -N ₂ X group is then reacted with a carbon surface, for example carbon particles, carbon nanoparticles, carbon nanotubes or carbon nanofibers, to form Y (with the structure shown in Figure 2) bonded to the carbon surface. Y can act as a redox system in an

electrolyte. Thus, the carbon-Y composites can act in a slurry electrode.

Attaching a redox system to a carbon surface can have dual effects; one effect is on the redox potential due to the change in the electronic environment induced by the linker; the other effect is to keep the redox system more attached to the carbon surface rather than the interaction between redox-redox systems in solution as is the case of conventional liquid redox. These two effects provide a high performance slurry electrode-electrolyte.

Figure 3 is a general reaction scheme showing the functionalisation of carbon surfaces using an alternative diazonium method. In Figure 3, in comparison to Figure 2, the redox system takes the form of an organic compound, namely a Quinone compound. The Quinone compound is attached to carbon surfaces in a similar way to that shown in Figure 2. The amine group is transformed by the diazonium reaction (applying NaN0 ₂ and HCI) to form an -N ₂ X group in its place. The -N ₂ X group is then reacted with a carbon surface, for example carbon particles, carbon nanoparticles, carbon nanotubes or carbon

nanofibers, to form Y (with the structure shown in Figure 3) bonded to the carbon surface. Y can act as a redox system in an electrolyte. Thus, the carbon-Y composites can act in a slurry electrode.

Figure 4 is a schematic diagram showing a redox flow cell 1 including a slurry electrode 2,3. A battery (not shown) can be composed of a number of single cells 1 electrically connected. Each redox flow cell 1 is composed of an ion- exchange membrane 4 sandwiched between two slurry electrode-electrolyte systems 5,6. Two carbon plates 7,8, built in bipolar configuration, are placed in contact with the slurry electrode-electrolyte 5,6 on each side of the cell 1 . These plates 7,8 serve as current collectors during charge and/or discharge of the slurry electrode 2,3. The slurry electrode-electrolyte 5 on the side of the anode is called the anolyte and the slurry electrode-electrolyte 6 on the side of the cathode is called the catholyte. The anolyte can be pumped into or out of the anolyte tank 9 by a pump (not shown). The catholyte can be pumped into or out of the catholyte tank 10 by a pump (not shown). The anolyte and catholyte can be continuously renewed and replaced from the anolyte tank and the catholyte tank (respectively), thus forming an energy storage system with a high energy capacity. With the bipolar configuration shown in Figure 4, a multicellular battery stack can be built to generate high power.

The present inventors discovered a slurry flow battery where the electrochemical active species (sometimes called a redox system) are grafted and/or adsorbed on carbon surfaces and dispersed in an electrolyte. In some aspects, the invention relates to a flow battery where electrode and electrolyte are flowable together and stored in tanks outside of the battery box. The electrode is based on carbon slurry bearing chemical redox, grafted or adsorbed on surface.

Some benefits of the flow battery shown in Figure 4, provided by way of the slurry electrode-electrolyte include:

- enhanced energy storage capacity in a slurry flow battery.

- low cost with a reduced footprint in a slurry flow battery.

- use of abundant chemical redox systems, i.e. removing the need for expensive and potentially hazardous metals.

- safe and environmentally friendly slurry electrode.

- system can operate on different electrolytes (aqueous and nonaqueous).

- improved cell coulombic efficiency due to lower crossover through the battery separator.

Figure 6 is a cyclic voltammogram of a slurry electrode formed of an

anthraquinone adsorbed on Vulcan™ XC-72R carbon. The top and bottom lines are the cyclic voltammogram readings at 100mV/s. The lines next in from the top and bottom lines are the cyclic voltammogram readings at

50mV/s. The lines next in from the readings at 50mV/s are the readings at 20mV/s. The lines next in from the readings at 20m V/s (the middle two lines closest together) are the readings at 10mV/s. In the example shown in Figure 6, the anthraquinone depicted in Figure 6 was mixed with Vulcan™ XC-72R carbon (a conductive carbon black with a surface area in the range of 230 to 250 m ² /g) and dispersed in slurry form using a mix of solvents, namely, acetonitrile, water and isopropanol (the volumetric ratio being 10% acetonitrile: 10% water: 80% isopropanol). In this example, the mass ratio of the anthraquinone to Vulcan™ XC-72R carbon was 0.2. In other examples, the Vulcan™ XC-72R carbon can be any form of carbon, optionally a conductive form of carbon. In other examples, the solvent can be any one, two or three of water, acetonitrile or isopropanol. In other examples, the volumetric ratio of the solvents can be: (20% acetonitrile: 20% water: 60% isopropanol); or, (30% acetonitrile: 30% water: 40% isopropanol); or, (40% acetonitrile: 40% water: 20% isopropanol); or, (45% acetonitrile: 45% water: 10% isopropanol); or, any set of ratios between these values. In other examples, the mass ratio of the anthraquinone to Vulcan™ XC-72R carbon can be from 0.1 to 0.99.

To test the electrochemical behaviour of the example slurry shown in Figure 6, a small amount of the slurry was deposited on a glassy carbon disc and dried in air. The response of this thin film electrode in sulfuric acid and at different scan rates is shown in Figure 6. From the electrode response (cyclic voltammetry), this slurry electrode works well as an anolyte for a flow battery cell. The working voltage of this half-cell battery is close to -0.15 V/Ag-AgCI.

The present inventors also disclose a hybrid flow battery consisting of gas diffusion anode/slurry cathode or vice versa; slurry anode/gas diffusion cathode (for example like H ₂ /redox-carbon slurry and redox-carbon slurry/air).

The present inventors disclose a slurry electrode-electrolyte based on redox- carbon composite, in which the chemical redox is supported or grafted on a carbon surface. At least one advantage of slurry-based system is to enable the use of concentrated electrolyte solution in the form of two-phase system (e.g., solid/liquid) without the concerns of precipitation of redox systems.

As mentioned above, the slurry battery can be built in different configurations, including:

Slurry (anode)/slurry (cathode) (as shown in Figure 4);

Slurry (anode)/Gas (cathode) (as shown in Figure 5); Gas (anodeySlurry (cathode) (not shown);

All-liquid (anode)/slurry (cathode) (not shown); and,

Slurry (anode)/AII-liquid (cathode) (not shown). As an example, Fig 5 shows the configuration where the slurry is used as fuel (anode side) and air as oxidant (cathode side).

Figure 5 is a schematic diagram showing a battery 20 including a slurry electrode 21 . In this example, the slurry electrode 21 is a slurry anode. The battery 20 includes a gas cathode 22. The gas cathode 22 utilises oxygen in air 23 as an oxidant. The air 23 can be provided to the gas cathode 22. A load 24 is applied across the anode terminal and cathode terminal to produce power. Mobilizing the electrode material in the form of slurry:

- eliminates the use of costly 3-dimensional porous electrode layers.

- relaxes many flow field design conditions (e.g. pressure drop) since the solutions are no longer need to flow through the 3D-porous electrode, as found in conventional flow batteries.

- when flow battery electrode material is degraded (e.g. due to corrosion), the carbon slurry can simply be replaced in the tanks without the need for costly replacement of the entire battery pack. When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components. The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. The following wording is incorporated from the priority application, GB1602201 .4, filed 8 February 2016 at the UK Intellectual Property Office:

A Slurry Electrode for High Energy Density Flow Battery System Background:

This disclosed concept of slurry-based flow battery can have potential applications in the field of energy and environment. The primary application will be for grid and off-grid. Also, development of high energy density slurry electrode-electrolyte could enable the application of flow batteries for transportation. Other applications are also envisaged in the area of water and waste treatment as well as electrocatalysis.

The key competitive advantages of the disclosed concept of slurry based flow battery over the current technology, existing in the market, reside in high energy density, reduced footprint, flexible design, and low cost.

Currently there are many competing technologies, such as all-vanadium redox flow battery, zinc bromine flow battery, chromium-Iron flow battery, and coordination chemistry flow battery. However these technologies suffer from low energy density and high cots. The concept of slurry electrode has the potential to circumvent these challenges.

Problem:

Among the different flow batteries, all-liquid flow batteries are suited for large- scale multi-hour applications. There are two types of flow batteries; the first generation type is called the redox flow battery (RFB), such as vanadium redox flow battery and the second generation is based on coordination compounds, known as "coordination chemistry flow battery (CCFB). Both systems can store and release huge amounts of electricity (depends on size of electrolyte tank) almost instantly and have a high cycle efficiency. Yet, these types of flow battery still suffer from high cost (total cost >$400/kw.h) and large foot print due to low volumetric energy density (20-30W/L). To improve the energy density and lower the cost, researchers are focusing on two

components, which are chemistry of redox/electrolyte and design and engineering of electrode/plates. Recent attempts have been made to combine the advantage of conventional battery (sealed battery) and all liquid flow battery by dispersing the electroactive species (electrode active materials) in carbon to form a flowable slurry electrode. One of the advantages of flowable electrode-electrolyte is to increase the concentration of redox systems in the electrolyte and hence to enhance the specific energy density. In addition, the slurry electrode-electrolyte has the potential to operate in alkaline, acidic, and organic media because the excess of active species are supported on conducting carbon surface. As such, cell voltage and materials compatibility will be significantly improved.

In the present invention we disclose a slurry flow battery concept where the electrochemical active species (called redox systems) are grafted and /or adsorbed on carbon surface and dispersed in an electrolyte. Also, we disclose hybrid flow battery systems consisting of gas diffusion anode /slurry cathode or vice versa; slurry anode /gas diffusion cathode (e.g. , like H2/redox-carbon slurry and redox-carbon slurry /air).

We disclose a slurry electrode-electrolyte based on redox-carbon composite, in which the chemical redox is supported or grafted on carbon surface. The major advantage of slurry-based system is to enable the use of concentrated electrolyte solution in the form of two-phase system (e.g., solid/liquid) without the concerns of precipitation of redox systems. In addition, mobilizing the electrode material in the form of slurry could potentially:

- eliminate the use of costly 3-dimensional porous electrode layers

- relaxes many flow field design conditions (e.g., pressure drop) since the solutions are no longer need to flow through the 3d- porous electrode, as found in conventional flow batteries

- when flow battery electrode is damaged (e.g., due to corrosion), the carbon slurry can simply be replaced without the need for costly replacement of the entire battery pack

Description

The invention relates to a flow battery where electrode and electrolyte are flowable together and stored in tanks outside of the battery box. The electrode is based on carbon slurry bearing chemical redox, grafted or adsorbed on surface. Details are as below: a) Redox-carbon composite: The synthesis of redox-carbon composite will be based on chemical deposition of redox systems (denoted as A _x B _y ) onto the surface of carbon particles, preferable with size ranging between 5 and 10 nm. To achieve this matrix, methods such as hydrothermal and co-precipitation will be used. Figure 1 below describes deposition pathways for the formation of redox-carbon composites. Ideally, reaction between A and B species should occur on the surface of the carbon at the molecular level to obtain highly dispersed A _x B _y . Deposition can be achieved via various paths; through change of counter ions, change of solvent, or change in pH. For example, if A _x B _y is, for instance, sulfate of oxy-vanadate (VOS0 ₄ ), which is soluble in aqueous solution, substitution of the counter anion, S0 ₄ ^"2 , by another anion like R-SO3 ^" or R- P02 ^" (R = organic substitute, such as phenyl, and substituted phenyl) leads to final compounds, denoted as VO(RS0 ₃ ) ₂ and VO(RP0 ₂ )2, which can adsorb strongly on carbon surface due to the nature of the anions. Furthermore, the redox system A _x B _y can be designed in such a way that both species A and B will be electrochemically active at two different potentials, which can serve as a single substrate for both anode and cathode reactions. For example, if specie A is vanadium ions, V ⁺² , specie B can be an anion, such as [Fe(CN) ₆ ] ^"4 and as result, the redox system is expected to have dual redox potential with cell voltage near 1 V. Tuning the structure and composition of the redox system has a direct impact on cell voltage. Figure 1 shows the formation of redox-carbon composites.

In general, the design and synthesis of A _x B _y /Carbon composite should be done according to the working condition of the slurry. Grafting redox system onto carbon surface in the form of quantum dots to make redox-carbon composites is also envisaged in embodiments. Grafting will be achieved by electrochemical or chemical methods. Diazonium method is one of the effective approaches that can be used for functionalization of carbon (particles, tubes, fibers) with organic or inorganic redox species as depicted in Figure 2.

Figure 2 shows the functionalization of carbon surface using Diazonium method. Attaching a redox system to carbon surface can have a dual effect; one effect is on redox potential due to the change in the electronic environment induced by the linker, and the other effect is to keep redox more attached to carbon surface rather than interaction between redox-redox in solution. These two effects are essential for high performance slurry electrode-electrolyte.

Furthermore, redox systems can be in the form of organic fragment instead of metal species, such as Quinone. Attaching Quinone compounds to carbon surface (especially well structured carbon) can be also done via diazonium approach according the reaction shown in Figure 3.

Figure 3 shows Quinones functionalization of carbon surface using Diazonium method. b) Concept design:

The battery is composed of a number of single cells electrically connected. Each cell is composed of an ion- exchange membrane sandwiched between two slurry electrode-electrolyte systems. Two carbon plates, built in bipolar configuration, are placed in contact with the slurry electrode-electrolyte for both sides of the battery cell. These plates are served as current collectors during charge and discharge of the slurry. The slurry electrode-electrolyte side of the anode called anolyte and the one side of the cathode is called catholyte (as shown in Figure 4). With benefit of bipolar configuration, a multicell battery stack can be built to generate high power.

Figure 4 shows a flow battery cell architecture.

Benefits:

- Enhanced energy storage capacity in slurry flow battery

- Low cost with reduced footprint flow battery system

- Use abundant chemical redox systems

- Safe and environmentally friendly slurry electrode

- System can operate on different electrolyte (aqueous and non-aqueous)