GB2570672A

GB2570672A - Reversible cell

Info

Publication number: GB2570672A
Application number: GB1801625.3A
Authority: GB
Inventors: Martin Creeth Andrew
Original assignee: Ac Chemical Systems Ltd
Current assignee: Ac Chemical Systems Ltd
Priority date: 2018-02-01
Filing date: 2018-02-01
Publication date: 2019-08-07
Also published as: GB201801625D0; WO2019149770A1

Abstract

A reversible cell 1 can be run in either a fuel cell or electrolyser mode, and comprises a fuel electrode 3 in a first region and an oxidant electrode 4 in a second region, separated by an ion selective PEM 5, and a chamber 7 in fluid communication with the second region to form an electrolyte flow path 11, wherein an electrolyte comprising two redox mediator couples having different redox potentials for use in each of the fuel cell and electrolyser modes circulates in the electrolyte flow path. The first redox mediator couple is at least partially reduced at the oxidant electrode and regenerated by reaction with the oxidant after such reduction, when the cell is run in the fuel cell mode, and a second redox mediator couple is at least partially oxidised at the oxidant electrode and regenerated by oxidation of water to produce oxygen after such oxidation, when the cell is run in the electrolyser mode. Fuel (H2) is introduced into, or removed from, the first region including the fuel electrode 3, oxidant is introduced into, or removed from, the second region including the oxidant electrode 4, and water is removed from or introduced into the electrolyte flow path, depending on the cell mode.

Description

The present invention concerns a reversible cell that can be run in either a fuel cell mode or an electrolyser mode.

Substantial progress has been made in reducing levels of carbon dioxide emissions from electricity generation, but heating is still primarily provided by burning natural gas or other fossil fuels. To meet future carbon dioxide emissions targets, heat must be provided in another way. Hydrogen can replace natural gas in the gas network, thereby providing a route to heating with minimal disruption. This provides an attractive alternative to employing heat pumps powered by electricity, which requires underfloor heating along with enhanced electricity supply infrastructure, implying substantial extra capital investment. Electrolysers can use renewable energy to generate hydrogen, but they are expensive and the hydrogen produced will have a high cost.

There is also a need to manage the variability of renewable energy to create a reliable electricity supply. Batteries can be used to smooth the output over short timescales but, due to the cost of the storage chemicals, are expensive at grid-scale for longer term storage.

Redox fuel cells, which use electrochemical species to generate electricity from fuels such as hydrogen, are well known in the art and are discussed, for example in EP1999811.

WO2013/104664 discloses a regenerative fuel cell, in which power can be used to regenerate the contained electrochemically active species, which act as an energy store when charged and can be used to generate electricity when discharged. The electrochemically active species comprises one redox couple at the cathode side of the fuel cell, which is reduced during the fuel cell mode and oxidised during the regeneration mode.

WO2013/131838 discloses a redox flow battery, which comprises a redox couple in each of the anolyte and catholyte solutions. These electrolyte solutions allow the battery to charged and can then either be discharged or used to generate hydrogen and oxygen, using a catalyst bed.

A reversible fuel cell/electrolyser can be used to generate both hydrogen and electricity. Such a cell has two modes, a fuel cell mode and an electrolyser mode. In the fuel cell mode, a fuel such as hydrogen is introduced to a fuel electrode (the anode in fuel cell operation), and an oxidant, such as oxygen or air, is introduced to the oxidant electrode (the cathode in fuel cell operation):

Fuel electrode: 2H₂ -> 4 H⁺ + 4 e^-

Oxidant electrode: O₂ + 4H⁺ + 4e^_ A 2H₂O

Hydrogen is consumed in the above reaction, thereby generating electricity through an external circuit linking the anode and cathode. The product (water) is often released to atmosphere.

When a potential is set up across the cell in the reverse direction of fuel cell operation and water is supplied to the oxidant side, the unit operates in electrolyser mode. In this case, hydrogen is generated at the fuel electrode (now the cathode), and oxygen at the oxidant electrode (now the anode):

Fuel electrode: 4 H⁺ + 4e' -> 2 H₂

Oxidant electrode: 2H₂O -> O₂ + 4H⁺ + 4e^_

Such a unit can be used to link the gas and electricity networks, providing gas for heating or storage when renewable energy is not needed for electricity. The unit can also be used to boost the electricity supply from stored hydrogen when the supply from variable renewable sources is low and the electricity price is high.

Reversible cells therefore provide a form of energy storage, which uses the components of water as the store and so are of low cost at grid scale. Thus, such a reversible fuel cell/electrolyser would be very valuable commercially.

US2014242479 discloses a reversible redox device, which can be used as either a fuel cell or for electrolysis. Hydrogen and oxygen are directly fed into or removed from the electrode regions. A recombination catalyst is included near to the electrodes, which catalyses the recombination of hydrogen and oxygen to form water, thereby purifying the gases that are released from the electrode regions.

A similar device is disclosed in US4048383, which relates to a combination electrolyser and fuel cell in which hydrogen and oxygen are directly fed into or removed from the electrode regions. US2015349368 discloses membrane electrode assembly which can perform both elecrolysis and electricity generation using a single bifunctional electrocatalyst for oxygen reduction and evolution.

A significant issue for reversible cells is that the conditions required for generating hydrogen and consuming it are very different. To produce oxygen, as part of the process to generate hydrogen in the electrolyser mode, the anode requires a highly oxidizing potential. This damages the materials used for reducing oxygen in the cathode when the unit in the fuel cell mode. In addition, the catalyst materials used for oxygen generation are comparatively poor at reducing oxygen.

Further, polymer electrolyte membrane (PEM)-type electrolysers often include thick membranes, generally of around 50 microns and above, to allow a pressure difference across the membrane to generate hydrogen at pressure, which is costly.

Thus, a cell that contains an oxidant electrode arrangement that can be used in both oxygen evolution and reduction is required. Further, it would be beneficial if a pressure difference across the membrane is not required to generate hydrogen at pressure.

According to the present invention there is provided a reversible cell that can be run in either a fuel cell mode or an electrolyser mode, wherein the cell comprises a fuel electrode in a first region, an oxidant electrode in a second region, wherein the fuel electrode and the oxidant electrode are electrically connected in a circuit and wherein the circuit is arranged such that a power supply can be connected to said circuit when the cell is run in the electrolyser mode and an electrical load can be connected to said circuit when the cell is run in the fuel cell mode, the first and second regions being separated by an ion selective polymer electrolyte membrane, at least one chamber in fluid communication with the second region, an electrolyte in the second region comprising two redox mediator couples, wherein the electrolyte can circulate between the second region and the at least one chamber around an electrolyte flow path, means for introducing a fuel into the first region and means for introducing an oxidant into the at least one chamber when the cell is run in the fuel cell mode, means for introducing water into the electrolyte flow path and means for removing hydrogen from the first region and means for removing oxygen from the at least one chamber when the cell is run in the electrolyser mode, wherein the redox mediator couples have different redox potentials such that a first redox mediator couple is at least partially reduced at the oxidant electrode and at least partially regenerated by reaction with the oxidant after such reduction when the cell is in the fuel cell mode and a second redox mediator couple is at least partially oxidised at the oxidant electrode and at least partially regenerated by oxidation of water to produce oxygen after such oxidation when the cell is in the electrolyser mode.

In one embodiment, a power supply is connected to the circuit in the electrolyser mode and an electrical load is connected to the circuit in the fuel cell mode.

The fuel electrode may be the same as a standard PEM-type reversible fuel cell.

The reversible cell of the present invention employs two different redox mediator couples in the electrolyte in contact with the oxidant electrode. The two different redox mediator couples have different redox potentials, such that a different redox mediator couple is either reduced or oxidised at the oxidant electrode depending on the mode of operation of the cell.

In the fuel cell mode, the fuel electrode is the anode and consumes the fuel (such as hydrogen), separating H2 into 2H⁺ and 2e^_. The oxidant electrode is the cathode and consumes the oxidant (such as oxygen).

In the fuel cell mode, a first redox mediator couple is reduced at the oxidant electrode. This reaction may be:

Electrode:

Med¹ ox + e^-Med¹ red

The first redox mediator couple is then oxidized in at least one chamber by an oxidant, such as oxygen. This reaction may be:

Regenerator: 4Med¹red + O2 + 4 H⁺-> 4Med¹ox + 2H2O

In the electrolyser mode, the direction of the current and thus the roles of the electrodes are reversed. The fuel electrode becomes the cathode and generates hydrogen. The oxidant electrode becomes the anode and generates oxygen.

Thus, in the electrolyser mode, a second redox mediator couple is oxidized at the oxidant electrode. This reaction may be:

Electrode: Med²red Med²0x + e^-

The second redox mediator couple is then reduced by reaction with water to produce oxygen. This reaction may be:

Oxygen forming step: 4Med²0x + 2H2O 4Med²red + O2 + 4H⁺

The first and the second redox mediator couples are compatible with one another when dissolved in the same electrolyte. Both are also durable at the necessary operating conditions.

The above reactions can occur in the electrolyte during the different modes of operation as the first and the second redox mediator couples have different redox potentials when present in the electrolyte. Preferably, the redox mediator couples have oxidation states that are stable across the range of redox potentials involved in both the fuel cell mode and the electrolyser mode.

The redox potential of the first redox mediator couple may be such that it can reduce oxygen. The redox potential of the second redox mediator couple may be such that it can oxidise water.

The present invention therefore provides an electrolyte that is stable across the range of redox potentials involved in both the fuel cell mode and the electrolyser mode. This ensures that the electrolyte is suitable for use at the oxidant electrode when the cell is in both the fuel cell and the electrolyser mode.

The redox properties of the first and second redox mediator couples are important for the performance of the cell.

The first redox mediator couple may have a redox potential of between 0.7 V and 1.2

V vs standard hydrogen electrode (SHE) in the electrolyte solution. More preferably, the redox potential of the first redox mediator couple is between 0.8 V and 1.1 V vs SHE in the electrolyte solution.

The second redox mediator couple may have a redox potential of between 1.4 and 1.8

V vs SHE in the electrolyte solution. More preferably, the redox potential of the second redox mediator couple is between 1.5 and 1.7 V vs SHE in the electrolyte solution.

Examples of suitable redox couples for use in the present invention are outlined in Table 1 below, in which the redox potentials vs SHE are as disclosed in the Handbook of Chemistry and Physics.

Table 1

Fuel Cell mode mediator couples	Redox Potential vs H₂	Electrolyser mode mediator couples	Redox Potential vs H₂
Polyoxometalates	Various	Cerium (4+/3+)	1.44 V
Vanadium (5+/4+)	1.00 V	Cobalt (3+/2+)	1.84
Manganese (3+/2+)	1.51 V	Manganese (3+/2+)	1.51 V
Iron (3+/2+)	0.77 V

Ferrocene and triphenylamine complexes may also be used as fuel cell mode mediator couples if they are stable at the high redox potentials involved in oxygen generation.

Polyoxometalates are known in the art for use as redox mediator couples in fuel cells. The polyoxometalate for use in the present invention may be represented by the formula below, as is disclosed in EP1999811:

Xa[ZbM_cOd] wherein:

X is selected from hydrogen, alkali metals, alkaline earth metals, ammonium and combinations of two or more thereof;

Z is selected from B, P, S, As, Si, Ge, Ni, Rh, Sn, Al, Cu, I, Br, F, Fe, Co, Cr, Zn, H₂, Te, Mn and Se and combinations of two or more thereof;

M is a metal selected from Mo, W, V, Nb, Ta, Mn, Fe, Co, Cr, Ni, Zn Rh, Ru, T I, Al, Ga, In and other metals selected from the 1^st, 2^nd and 3^rd transition metal series and the lanthanide series, and combinations of two or more thereof;

a is a number of X necessary to charge balance the [M_cOd] anion;

b is from 0 to 20, c is from 1 to 40; and d is from 1 to 180.

Preferred ranges for b are from 0 to 15, more preferably 0 to 10, still more preferably 0 to 5, even more preferably 0 to 3, and most preferably 0 to 2. Preferred ranges for c are from 5 to 20, more preferably from 10 to 18, most preferably 12. Preferred ranges for d are from 30 to 70, more preferably 34 to 62, most preferably 34 to 40.

Vanadium, molybdenum, tungsten and combinations thereof are particularly preferred for Μ. Z may preferably be B, P S, As, Si, Ge, Al, Co, Mn, or Se when the polyoxometalate comprises tungsten and vanadium as M, with P, S, Si, Al or Co being most preferred. Phosphorus is particularly preferred for Z.

A combination of hydrogen and an alkali metal and/or alkaline earth metal is preferred for X. One such preferred combination is hydrogen and sodium. In a preferred embodiment of the present invention, the polyoxometalate comprises vanadium, more preferably vanadium and molybdenum. Preferably, the polyoxometalate comprises from 2 to 4 vanadium centres.

Thus, suitable polyoxometalates for use in the present invention include H3Na2PMoioV204o, HsNasPMogVsCLo or H3Na4PMosV404o, and compounds of intermediate composition.

Other suitable polyoxometalates include the above examples where a portion or all of the molybdenum is replaced by tungsten, as disclosed in EP2823530. The content of Mo and W may be in equal proportion, for example in H3Na4PMo4W4V₄04o.

The polyoxometalate may be H6[AIWhVi04o]. The polyoxometalate may also be XfiSiWgVsCUo], where X can give rise to the general formula K2H5[SiWgV304o].

The polyoxometalates disclosed in EP2193568 are also suitable for use in the present invention, in which X is selected from hydrogen, alkali metals, alkaline earth metals, and combinations of two or more thereof with the proviso that at least one X is a divalent ion and/or the or each divalent ion is selected from Ca, Mg, Mn, Fe, Co, Ni, Cu, Zn, Sr, Ba, Be, Cr, Cd, Hg, Sn and other suitable ions from the second and third transition series or the lanthanides, or combinations of two or more thereof.

Another suitable polyoxometalate structure is disclosed in EP2867949, in which X is H, as represented by the formula:

HaPzVxMOyOb wherein:

a is a number of H necessary to charge balance the anion [P_zV_xMo_yOb];

z is from 1 to 3;

x is from 1 to 12;

y is from 7 to 20; and b is from 40 to 89.

Possible acid polyoxometalates according to the above formula for use in the present invention include H10P2V4M08O44, H10.75P2.25V4M045, H7PV4MO9O42.5,

H10P2V4M09O46.5, H7PV4M08O40, H12P3V7MO18O85, H10P2V4M08O44,

H10.75P2.25V4M08O45, or combinations of the above.

In another suitable embodiment disclosed in EP3014688, X includes an amount of non-hydrogen cation and the molar ratio of the non-hydrogen cation to vanadium is more than 0 and less than 1. Preferably in this embodiment, the polyoxometalate contains 2 to 10 vanadium centres. Specific examples of this embodiment include KH6PV4M08O40, NaHePV4Mo804o, K0.5H9.5P2V4M08O44.

One or more of the polyoxometalates discussed above may be included in the electrolyte.

The electrolyte may comprise more than two redox mediator couples. This can improve efficiency of the fuel cell mode and/or the electrolyser mode.

The electrolyte may further comprise additional counterions or complexes in order to adjust the redox potential of one or both of the redox mediator couples. For example, the use of nitrate increases the potential of cerium (4+/3+), while iron (3+/2+) bipyridyl and phenanthroline complexes have redox potentials as high as 1 V, whereas an iron (3+/2+) complex with EDTA has a potential as low as 0.1 V.

The adjusted redox potential of the first redox mediator couple may be such that it can reduce oxygen. The adjusted redox potential of the second redox mediator couple may be such that it can oxidise water.

The electrolyte may further comprise additional counterions to increase the solubility of the redox mediator couples, such as mixed sulphate/chloride for vanadium and methanesulphonate for cerium.

The fuel may be hydrogen. The fuel may be selected from the group including hydrogen, metal hydrides, ammonia, low molecular weight alcohols, aldehydes and carboxylic acids, sugars, biofuels, LPG, LNG or gasoline. The fuel may act as a fuel itself or may act to provide hydrogen.

The oxidant may be oxygen or air. The electrolyte solution may be aqueous.

The electrolyser mode of the present invention is disclosed in terms of electrolysing water. However, it is envisaged that other electrolysable components could be used instead of water. These electrolysable components may form products other than hydrogen and oxygen. Thus, references to water herein may also refer to an electrolysable component, while references to the production of hydrogen and oxygen may also refer to the production of a first and second product.

In these alternative embodiments, there is provided a means for introducing an electrolysable component into the electrolyte flow path, means for removing a first product from the first region and means for removing a second product from the at least one chamber when the cell is run in the electrolyser mode. The redox potential of the second redox mediator couple may be such that it can oxidise the electrolysable component.

The fuel electrode may be a standard membrane electrode assembly, which may comprise platinum or palladium or a combination thereof as a catalyst. The catalyst may be supported on a particulate carbon combined with a membrane ionomer and a gas diffusion layer.

Electrodes suitable for use in fuel cells and electrolysers are known in the art. The electrode on the oxidant side must be compatible with the redox conditions required in both the fuel cell and electrolyser modes. The oxidant electrode must also be durable for use in both the fuel cell and electrolyser modes of operation.

The electrode materials must therefore be compatible with the range of potentials involved in operation in both the fuel cell and electrolyser modes, whilst still providing porosity for liquid flow and conduction of the current.

Examples of electrode materials include titanium, platinised titanium, indium tin oxide and Magneli phase titanium oxide. The electrode may be a foam, mesh, sintered materials or layer adjacent to the membrane.

The cell may comprise more than one cell unit containing a fuel electrode in a first region, an oxidant electrode in a second region and an ion selective polymer electrolyte membrane. An arrangement of a plurality of cell units is known as a stack. The plurality of cell units may be connected in series within the stack. This allows each cell unit to be electrically connected in a single circuit. The stack may comprise one or more bipolar plates, which can be used to separate the plurality of cell units.

The at least one chamber with means for introducing an oxidant thereto and means for removing oxygen therefrom may provide electrolyte to each of the cell units. This may be done via a manifold flow channel. The means for introducing a fuel into the first region and means for removing hydrogen from the first region may do so for all of the first regions in the stack. This may be done via a manifold flow channel.

The one or more bipolar plates may comprise flow channels for allowing fuel (in the case of the fuel electrode side) to diffuse across the electrode surface in operation of the cell and a well to site the oxidant electrode and electrolyte (in the case of the oxidant electrode side). The bipolar plates may be compatible and durable with operation under both oxygen reduction and oxygen evolution conditions.

The fuel electrode and the oxidant electrode may each be connected to an end plate, which are attached to opposite ends of the circuit that connects the two electrodes. In a stack, one end plate may be located at the fuel electrode end of the stack and another may be located at the oxidant electrode end of the stack. This allows each cell unit to be electrically connected in a single circuit.

Ion selective polymer electrolyte membranes are well known in the art. The ion selective polymer electrolyte membrane of the present invention may be formed from any suitable material, but preferably comprises a polymeric substrate having ion exchange capability. Suitable examples include fluororesin-type ion exchange resins and non-fluororesin-type ion exchange resins. Fluororesin-type ion exchange resins include perfluorocarboxylic acid resins, perfluorosulphonic acid resins, and the like. Perfluorocarboxylic acid resins are preferred, for example “Nation” (Du Pont Inc.), “Flemion” (Asahi Gas Ltd), “Aciplex” (Asahi Kasei Inc) and the like. Non-fluororesintype ion exchange resins include polyvinyl alcohols, polyalkylene oxides, styrenedivinylbenzene ion exchange resins, and the like, and metal salts thereof. Preferred non-fluororesin-type ion exchange resins include polyalkylene oxide-alkali metal salt complexes. These are obtainable by polymerizing an ethylene oxide oligomer in the presence of lithium chlorate or another alkali metal salt, for example. Other examples include phenolsulphonic acid, polystyrene sulphonic, polytrifluorostyrene sulphonic, sulphonated trifluorostyrene, sulphonated copolymers based on α,β,β trifluorostyrene monomer, radiation-grafted membranes. Non-fluorinated membranes include sulphonated poly(phenylquinoxalines), poly (2,6 diphenyl-4-phenylene oxide), poly(arylether sulphone), poly(2,6-diphenylenol), acid-doped polybenzimidazole, sulphonated polyimides, styrene/ethylene-butadiene/styrene triblock copolymers, partially sulphonated polyarylene ether sulphone, partially sulphonated polyether ether ketone (PEEK) and polybenzyl suphonic acid siloxane (PBSS).

The reaction at the oxidant electrode may be catalysed by a catalyst. Catalysts may be employed for the reactions with oxygen in the fuel cell mode and/or with water to form oxygen in the electrolyser mode. Preferably, the electrolyte comprises two catalysts, namely a first that interacts with the first redox mediator couple in the fuel cell mode and a second that interacts with the second redox mediator couple in the electrolyser mode.

The catalyst/mediator system when the cell is in the fuel cell mode can be expressed in the scheme below:

Electrode

Mediator

Catalyst

Air/Oxygen Regenerator

Cat at cathode

Η,Ο

The catalyst/mediator system when the cell is in the electrolyser mode can be expressed in the scheme below:

Electrode

Mediator

Catalyst

Oxygen

Evolution

Chamber

The catalyst may be soluble and thus may be dissolved in the electrolyte. The catalyst may also be solid, in which case it may be dispersed or suspended in the electrolyte. Additionally or alternatively, the solid catalyst may be supported on a surface, optionally in solid particulate form, or present in another particulate-based form, such as in a packed bed through which the electrolyte flows during use of the cell.

The catalyst may be disposed at one or more fixed locations along the electrolyte flow path, for example in the form of a fixed bed type arrangement. The catalyst may be present in the one or more chamber. The catalyst may be present in the chamber in which oxygen is produced when the cell is run in the electrolyser mode.

The catalyst and the redox mediator couple may be the same species. The catalyst and the redox mediator couple may be different species.

Suitable catalysts for oxygen reduction in the fuel cell mode include polyoxometalates as discussed above, nitric acid, N-donor ligand metal complexes such as those disclosed in EP2041825, metal porphyrin complexes and metal phthalocyanine complexes.

Suitable catalysts for the oxidation of water in the electrolyser mode of the present invention include transition metals, especially those residing in Groups 6 to 9, such as manganese, osmium, rhodium, ruthenium, tungsten and/or iridium. Polyoxometalate catalysts containing these metals are particularly preferable. Especially preferred catalysts are lrO₂, lrRuO₂, iridium doped ruthenium oxide, [Ru^lll2Zn2(H2O)2(ZnW9O34)2]¹⁴-, Liio[Ru4(p-0)4(p-OH)₂(H₂0)4(Y-SiWio036)2],

Csio[Ru₄(p-0)4(p-OH)2(H₂0)4(Y-SiWio036)2] and/or Nai₄[Ru2Zn2(H₂O)2(ZnW9O34)2].

Other preferred catalysts include those described Yagi et al, Photochem. Photobiol. Sci., 2009, 8, 139-147; in particular di-μ-οχο dimanganese complexes such as [(terpy)(H₂0)Mn(p-0)Mn(terpy)(H₂0)]³⁺, Mn404 cubane complexes, Mn porphyrin dimers dinuclear ruthenium complexes such as Ru2(macroN6)(Rpy)4CI]³⁺, [(bpy)2(H2O)Ru(p-O)Ru(H₂O)(bpy)2]⁴⁺ [(terpy)2(H20)Ru(bpp)Ru(H20)(terpy)2]³⁺ and [(tBu2qui)(OH)(Ru(btpyan)Ru(OH)(tBu₂qui)]²⁺, mononuclear ruthenium complexes such as [Ru(tBudnpp)(Rpy)2OH2]²⁺ complexes, [Ru(Rterpy)(bpy)OH2]²⁺ complexes and iridium complexes, including cyclometalated iridium aquo complexes such as ([lr^lll(RiR2ppy)2(OH2)2]⁺) wherein R1 is hydrogen or alkyl, particularly methyl, and wherein R2 is hydrogen, phenyl or a halogen such as F or Cl.

Also suitable for use in the present invention are cobalt-containing polyoxometalate materials as disclosed in Cobalt Polyoxometalates as Heterogeneous Water Oxidation Catalysts, Joaquin Soriano-Lopez, Sara Goberna-Ferron, Laura Vigara, Jorge J. Carbo, Josep M. Poblet, and Jose Ramon Galan-Mascaros, Inorg. Chem., 2013, 52 (9), pp 4753-4755.

When nitric acid is used as the oxygen reduction catalyst, nitrogen oxide (NO) is produced at the oxidant electrode of the cell, as part of the following reaction:

HNOs + 3H⁺ + 3 e^- -> NO + 2H₂O

This requires a reaction with oxygen in the gas phase:

2NO + O₂ = 2 NO₂

2NO₂ = N2O4 before subsequent dissolution:

2NO₂ + H₂0 A NO2 + NO3· + 2H⁺

3NO₂ + H₂0 A 2HNOs + NO

Care must be taken not to release nitrogen oxides to the atmosphere, due to their harmful nature. There are various options for separating unreacted oxides of nitrogen, some of which are discussed in US9231266B1. Hydrogen peroxide may be used to produce nitric acid, or the waste nitrogen oxide-containing gas may be collected. The oxides of nitrogen may be reacted using a catalyst and hydrogen to form nitrogen, or the oxides may be reacted with an alkali to form a nitrate salt.

The reversible cell may comprise a chamber for the reaction with oxygen, optionally from air, and a gas/liquid contactor such as a spray tower or falling film tower to enable the dissolution either in electrolyte or water. Recirculation of the gas or successive gas/liquid contactors may be included to reduce the level of unreacted nitrogen oxide. Unreacted nitrogen oxide may be removed before release of the gaseous product to the atmosphere using methods such as a catalytic reaction to form nitrogen or absorption in an alkali solution.

The reversible cell of the present invention comprises one or more chambers in fluid communication with the second region. The reversible cell may comprise one chamber. The oxidant may be introduced into this chamber when the cell is run in the fuel cell mode via the means for introducing an oxidant into the at least one chamber. Water may be removed from this chamber when the cell is run in the fuel cell mode. Oxygen may be removed from this chamber when the cell is run in the electrolyser mode via the means for removing oxygen from the at least one chamber. Water may be introduced into this chamber via the means for introducing water into the electrolyte flow path when the cell is run in the electrolyser mode.

The reversible cell may comprise two chambers. The first chamber may be a regeneration chamber, into which the oxidant is introduced when the cell is run in the fuel cell mode via the means for introducing an oxidant into the at least one chamber. Water may be removed from this regeneration chamber when the cell is run in the fuel cell mode. The second chamber may be an oxygen evolution chamber, from which oxygen is removed when the cell is run in the electrolyser mode via the means for removing oxygen from the at least one chamber. Water may be introduced into this chamber via the means for introducing water into the electrolyte flow path when the cell is run in the electrolyser mode.

The means for introducing water into the electrolyte flow path may be the same as the means for introducing an oxidant into the at least one chamber. The means for introducing water into the electrolyte flow path may be different to the means for introducing an oxidant into the at least one chamber.

The electrolyte flow path may be variable such that it flows through the regeneration chamber in the fuel cell mode and through the oxygen evolution chamber in the electrolyser mode.

The regeneration chamber may comprise a means of improving the contact between the oxidant and the electrolyte. The regeneration chamber may be a gas/liquid twophase reactor and may take many forms. Suitable forms include a turbulent, baffled stirred vessel, a gas-liquid ejector, a plunging jet, a spinning cone or disc, a turbulent static mixer, a bubble column with sparge, a falling-film column, a packed column, a spray tower, a gas-liquid agitated vessel, a plate column, a rotating disc contactor and/or a Venturi tube.

The regeneration chamber may comprise the contactor disclosed in EP2543104, where the liquid flows past a porous member which allows air to bubble into the liquid and regeneration to occur within the flow of the liquid.

The regeneration chamber may comprise a porous member as disclosed in EP2542332, in which the holes have a specific shape in order to produce small bubbles.

The oxygen evolution chamber may have several forms, including a stirred tank for a soluble or dispersed catalyst. The dispersed catalyst may be present as a colloidal dispersion or as a slurry with a particle- or powder-based catalyst. The oxygen evolution chamber may comprise a means of producing flow of the electrolyte through a solid particulate-based catalyst, such as a packed bed reactor.

Both of the regeneration chamber and the oxygen evolution chamber may produce a dispersion of bubbles in a liquid. The reversible cell may comprise a means for removing excess gas and/or liquid from the system. Said means may extend from the electrolyte flow path, from the at least one chamber or from the first region. Said means may extend to the surrounding environment or to a storage means.

The excess gas may comprise air with a depleted level of oxygen in the fuel cell mode, which may be removed from the at least one chamber. The excess gas may comprise unreacted fuel, which may be removed from the first region. The excess gas may comprise water vapour from the water produced in the fuel cell mode and evaporated water in both modes, which may be removed from the at least one chamber. The excess liquid may comprise water produced during the fuel cell mode.

The hydrogen and/or the oxygen produced during operation of the cell in the electrolyser mode may be stored for future use. The hydrogen and/or oxygen may be stored under compression.

The reversible cell may comprise a separator for separating a mixture of a gas and a liquid that is removed from the cell. The mixture may be removed from the at least one chamber. This can help to retain the liquid within the electrolyte circuit. The liquid may comprise electrolyte and/or water. The gas may comprise oxygen and/or excess oxidant. Such means can include gravity separators, centrifugal separators, filter vane separators, mist eliminator pads and/or liquid/gas coalescers.

The gas/liquid separator may comprise a means of condensing evaporated water. The condensed water can be fed back into the reversible cell, for example back into the electrolyte. This can ensure that the water level remains constant during the fuel cell mode and helps to control water losses in the electrolyser mode.

The gas/liquid separator may comprise a centrifugal cyclone separator, as disclosed in EP2806959. The gas/liquid separator may comprise a coalescer comprising low surface energy materials, as disclosed in EP3138617.

The separator may be located downstream of the means for removing oxygen and/or the means to remove excess gas and/or liquid. The separator may be located in-line in the electrolyte flow path.

The means of introducing water into the electrolyte flow path when the cell is run in the electrolyser mode may also be used to supply water for cooling by evaporation in both modes of operation.

The cell may comprise a means for storing the water that is generated during the operation of the cell in the fuel cell mode. The water that is introduced into the electrolyte flow path during the electrolyser mode may comprise the water that was generated during the fuel cell mode.

One or more condensers may be located downstream of the means for removing oxygen and/or the means to remove excess gas. The condenser may capture water evaporated during the process of either regeneration of the redox mediator couple or oxygen evolution. The condenser may condense a controlled amount of water from the gas. The condenser may be liquid or air cooled.

One or more demisters may be located further downstream of the means for removing oxygen and/or the means to remove excess gas. The demister may remove any remaining droplets of liquid from the gas.

The condenser and demister may be arranged to return any captured water back to the electrolyte. The amount of water returned may be controlled in order to keep the water level constant in the fuel cell mode and to control cooling in both modes.

The cell may further comprise a separate heat exchanger to cool the electrolyte directly using a liquid/liquid or a liquid/air heat exchanger. The only method of cooling the electrolyte within the cell may be by circulation.

One or more pumps may drive circulation of the electrolyte and/or control the rate of flow of the electrolyte between the second region and the at least one chamber. The pumps may be at any convenient location in the electrolyte flow path.

The reversible cell may operate at atmospheric pressure. Alternatively, part or all of the cell may be in a region of increased pressure. The electrolyte may be cycled between atmospheric and high pressure by a pump, such as a positive displacement pump.

The at least one chamber may be located outside the region of increased pressure and may be at atmospheric pressure. The first and second region and the ion selective polymer electrolyte membrane may be in the region of increased pressure. The pressure may be increased to greater than 80 atm (8.1 MPa), preferably greater than 700 atm (70.9 MPa). The increased pressure may be different in the fuel cell mode and the electrolyser mode. The increased pressure in the electrolyser mode may be greater than the increased pressure in the fuel cell mode.

The increased pressure may be applied by compressing the gases and/or liquids that are introduced into the system. The compression of the fuel, the oxidant and the electrolyte will determine the pressure within the different regions of the cell. If the fuel is compressed, optionally as well as a portion of the electrolyte, the first and second region and the ion selective polymer electrolyte membrane may be in a region of increased pressure. Thus, the one or more cell units may be in a region of increased pressure. If the oxidant is also compressed, the entire cell may be in a region of increased pressure.

The oxidant used in the fuel cell mode and/or the oxygen created in the electrolyser mode may be compressed to below 3 atm (0.3 MPa).

The fuel gas used in the fuel cell mode and/or the hydrogen created in the electrolyser mode may be compressed to above 3 atm (0.3 MPa) and up to 1000 atm (101.3 MPa), preferably from 100 atm (10.1 MPa) to 1000 atm (101.3 MPa), more preferably from 500 atm (50.7 MPa) to 1000 atm (101.3 MPa) and most preferably from 700 atm (70.9 MPa) to 1000 atm (101.3 MPa). Pressures of above 1000 atm (101.3 MPa) may also be used.

The oxygen may be generated at increased pressure in the electrolyser mode and/or may be stored at increased pressure. This may be used, optionally in combination with the fuel stored at a similar pressure, to operate the cell at an increased pressure. This may result in a higher voltage output than if the system were operated at a lower pressure.

These pressure levels are higher than those generally used due to the compression of the gases and/or liquids that are introduced into the system. Generally, electrolyser cells of the prior art are run at up to around 80 atm (8.1 MPa). The level of increased pressure will depend on the intended use of the reversible cell and the efficiency requirements.

The pressure may be controlled using a pressure control valve in the first region. The pressure in the second region may be controlled using a pump and a pressure relief valve. The pressure may be varied depending on the intended use of the cell. The cell may further comprise sensors to monitor the pressures involved and to match the pressures in the first and second region as closely as possible.

Part or all of a non-reversible fuel cell or a non-reversible electrolyser cell could also be run at increased pressure, as discussed above. Non-reversible fuel cells and electrolyser cells have a similar structure to that of the reversible cell discussed above, but only include the features necessary for running in either the fuel cell mode or the electrolyser mode. Further, such cells generally only include redox mediator couples that can be operated at one redox potential, namely the potential required to either reduce oxygen or oxidise water respectively.

Regions of increased pressure as discussed above can be used in non-reversible fuel cells and non-reversible electrolyser cells. Said regions of increased pressure can be created in the same manner as discussed above.

This enables the pressurisation of hydrogen without the use of a separate compressor. As the high potential redox couple is oxidized in the liquid, the impact of the increased pressure on its redox potential is low, whereas if oxygen gas was produced under increased pressure in a standard reversible fuel cell, a higher potential would be required according to the Nernst equation.

The present invention allows for the use of thinner membranes compared to those in the prior art. The ion selective polymer electrolyte membrane may be around 25 microns thick, or less. Such thinner membranes may be used in the present invention at atmospheric pressure or at pressures up to around 80 atm (8.1 MPa).

As discussed above, pressure ranges higher than those used in the prior art may also be achieved with the present invention. When the pressure range is higher, for example between 700 and 1000 atm (70.9 MPa and 101.3 MPa), the thickness of the ion selective polymer electrolyte membrane may be increased. The cell may comprise a membrane of up to around 50 microns thick at higher pressure ranges.

According to a second aspect of the present invention, there is provided a method of producing electricity using a reversible cell as discussed above in the fuel cell mode, comprising connecting an electrical load to the circuit, providing a fuel to the first region and introducing an oxidant into the at least one chamber.

This method provides an environmentally friendly way of generating electricity from a fuel, such as hydrogen.

According to a third aspect of the present invention, there is provided a method of producing hydrogen and oxygen using a reversible cell as discussed above in the electrolyser mode, comprising connecting a power supply to the circuit, providing water into the electrolyte flow path, removing hydrogen from the first region and removing oxygen from the at least one chamber.

This method provides an efficient way of generating oxygen and hydrogen for storage or use in other applications.

Both methods can involve creating a region of increased pressure within the reversible cell. The fuel and/or the oxidant may be provided under compression when the cell is run in the fuel cell mode. The electrolyte may be maintained under compression when the cell is run in the fuel cell mode and/or the electrolyser mode.

According to a fourth aspect of the present invention, there is provided a method of operating a reversible cell as discussed above comprising switching between the method of producing electricity and the method of producing hydrogen and oxygen, as discussed above.

Thus, the reversible cell of the present invention provides a single device that can be used to produce either electricity or hydrogen and oxygen. This is due to the presence of two different redox mediator couples, one of which is reduced and oxidised when the cell is run in the fuel cell mode and the other of which is reduced and oxidised when the cell is run in the electrolyser mode.

The method of switching from the method of producing electricity to the method of producing hydrogen and oxygen may comprise switching the electrolyte flow path from flowing through a regeneration chamber to flowing through an oxygen evolution chamber.

The reversible cell of the present invention therefore can be used in combination with a means for generating electricity from a renewable energy source, such as solar, wind or wave power, all of which create highly variable electricity supplies. At times when the renewable energy source is generating levels of electricity above a first threshold, any excess electricity can be supplied to the reversible cell in the electrolyser mode, thereby creating hydrogen and oxygen. This hydrogen and/or oxygen can be stored and may be used as an energy source in the future.

At times when the renewable energy source is generating electricity levels below a second threshold, the reversible cell can be used as a fuel cell in order to generate additional electricity. The hydrogen produced during the electrolyser mode may be used as a fuel for the reversible cell when operating in the fuel cell mode. This therefore ensures a more reliable electricity supply.

The thresholds for the electricity level can be set depending on the requirements of the system. The first and the second threshold may be the same or different, depending on the intended use of the cell and the means for generating electricity from a renewable energy source.

The reversible cell according to the first aspect of the invention may therefore further comprise means of integration with a means for generating electricity from a renewable energy source. According to a fifth aspect of the present invention, there is provided a device comprising a reversible cell as described above and a means for generating electricity from a renewable energy source. The device may be operated as discussed above.

According to a sixth aspect of the present invention, there is provided an electrolyte comprising two redox mediator couples having different redox potentials, wherein a first redox mediator couple has a redox potential such that it is at least partially reduced at an electrode and at least partially regenerated by reaction with an oxidant after such reduction and wherein a second redox mediator couple has a redox potential such that it is at least partially oxidised at an electrode and at least partially regenerated by oxidation of water to produce oxygen after such oxidation.

The inclusion of these two compatible redox mediator couples in a single electrolyte allows said electrolyte to be used in both electricity generation and oxygen generation processes. The electrolyte may have any of the features outlined above in relation to the electrolyte in the reversible cell.

Thus, according to a seventh aspect of the present invention, there is provided a use of an electrolyte as described above in a reversible cell that can be run in either a fuel cell mode or an electrolyser mode. The reversible cell may include any of the features described above.

The invention will now be more particularly described with reference to the following figures, in which:

Figure 1 illustrates a reversible cell according to the present invention, without a region of increased pressure; and

Figure 2 illustrates a reversible cell according to the present invention, including a region of increased pressure.

Figure 1 illustrates a stack 1 comprising four cell units 2, each cell unit comprising a fuel electrode 3, an oxidant electrode 4 and an ion selective polymer electrolyte membrane 5 separating the two electrodes. Each of the cell units 2 are separated from adjacent cell units by bipolar plates 13.

The fuel electrode 3 end of the stack and the oxidant electrode 4 end of the stack are electrically connected in a single circuit 15. The electrically connected circuit 15 connects to the stack via stack end plates 14.

An inlet/outlet 6 allows the introduction of a hydrogen fuel to the first regions in which the fuel electrodes 3 are positioned when the stack 1 is run in the fuel cell mode. The inlet/outlet 6 also allows the removal of hydrogen from said regions when the stack 1 is run in the electrolyser mode.

A chamber 7 is in fluid communication with the second regions in which the oxidant electrodes 4 are positioned. An inlet 8 allows the introduction of an oxidant into the chamber 7 when the stack 1 is run in the fuel cell mode. The inlet 8 also allows the introduction of water into the chamber 7 when the stack 1 is run in the electrolyser mode.

The chamber 7 also comprises an outlet 9, which allows the removal of a gas from the chamber 7. Outlet 9 leads to a separator 10a, a condenser 10b and demister 10c. The separator 10a separates any liquid from the gas removed from the chamber 7. The condenser 10b condenses a controlled amount of water from the gas. The demister 10c removes any remaining droplets of water in the gas. This arrangement allows a controlled release of water vapour to the atmosphere and ensures that no droplets of liquid are released to the atmosphere.

An electrolyte 11 is located in the chamber 7 and circulates past the oxidant electrodes 4 and back to the chamber 7. The circulation of the electrolyte 11 is controlled by a pump 12. The electrolyte 11 contains two redox mediator couples, wherein the redox mediator couples have different redox potentials such that a first redox mediator couple is oxidised and reduced when the stack 1 is run in the fuel cell mode and a second redox mediator couple is oxidised and reduced when the stack 1 is run in the electrolyser mode.

In the fuel cell mode, an electrical load is connected to the circuit 15 between the fuel electrode 3 end of the stack 1 and the oxidant electrode 4 end of the stack 1. The fuel electrodes 3 are the anodes that consume the hydrogen fuel, which enters the first regions in which the fuel electrodes 3 are positioned via inlet/outlet 6. The hydrogen fuel is separated into 2H⁺ and 2e^_.

The oxidant electrodes 4 are the cathodes in the fuel cell mode and so a first redox mediator couple in the electrolyte 11 is reduced at the oxidant electrodes 4. The first redox mediator couple is then regenerated in the chamber 7 using oxygen, which is introduced into the chamber 7 via the inlet 8. This reaction produces water, which is removed along with any unreacted oxygen via outlet 9.

The unreacted oxygen and water then enter the separator 10a, condenser 10b and demister unit 10c and are separated. The water is passed back into the electrolyte 11 in the stack 1, while the gas is either released to the atmosphere or recycled back into chamber 7.

In the electrolyser mode, a power supply is connected to the circuit 15 between the fuel electrode 3 end of the stack 1 and the oxidant electrode 4 end of the stack 1. The direction of the current and thus the roles of the electrodes are reversed. The fuel electrodes 3 becomes the cathodes and generate hydrogen, which is removed from the stack 1 via inlet/outlet 6 and stored.

The oxidant electrodes 4 become the anodes and so a second redox mediator couple in the electrolyte 11 is oxidized at the oxidant electrodes 4. The second redox mediator couple is then reduced by reaction with water in the chamber 7, to produce oxygen.

Water is introduced into the chamber 7 via inlet 8 and the oxygen produced is removed from the chamber 7 via outlet 9.

The oxygen and electrolyte liquid mixture, as well as any evaporated water, then enter the separator 10a, condenser 10b and demister unit 10c and are separated as required. The separated electrolyte and water is passed back into the electrolyte 11 in the stack 1, while the oxygen is either released to the atmosphere or stored for future use.

Figure 2 illustrates a stack 21 comprising four cell units 22, each cell unit comprising a fuel electrode 23, an oxidant electrode 24 and an ion selective polymer electrolyte membrane 25 separating the two electrodes. Each of the cell units 22 are separated from adjacent cell units by bipolar plates 35.

The fuel electrode 23 end of the stack and the oxidant electrode 24 end of the stack are electrically connected in a circuit 37 from the ends of the stack. The electrically connected circuit 37 connects to the stack via stack end plates 36.

An inlet/outlet 26 allows the introduction of a hydrogen fuel to and the removal of hydrogen from the first regions in which the fuel electrodes 23 are positioned when the stack 21 is run in the fuel cell and the electrolyser modes respectively, as in Figure 1.

A chamber 27 is in fluid communication with the second regions in which the oxidant electrodes 24 are placed. An inlet 28 allows the introduction of an oxidant into the chamber 27 and the introduction of water into the chamber 27 when the stack 21 is run in the fuel cell and the electrolyser modes respectively, as in Figure 1.

The chamber 27 also comprises an outlet 29, which allows the removal of a gas from the chamber 27. Outlet 29 leads to a separator 30a, condenser 30b and demister unit 30c. The separator 30a separates any liquid from the gas removed from the chamber 27. The condenser 30b condenses a controlled amount of water from the gas. The demister 30c removes any remaining droplets of water in the gas. This arrangement allows a controlled release of water vapour to the atmosphere and ensures that no droplets of liquid released to the atmosphere.

An electrolyte 31 is located in the chamber 27 and circulates past the oxidant electrodes 24 and back to the chamber 27. The circulation of the electrolyte 31 is controlled by a pump 32. The electrolyte 31 contains two redox mediator couples, wherein the redox mediator couples have different redox potentials such that a first redox mediator couple is oxidised and reduced when the stack 21 is run in the fuel cell mode and a second redox mediator couple is oxidised and reduced when the stack 21 is run in the electrolyser mode.

In the fuel cell mode, an electrical load is connected to the circuit 37 between the fuel electrode 23 end of the stack 21 and the oxidant electrode 24 end of the stack 21. The fuel electrodes 23 are the anodes and consume the hydrogen fuel, which enters the first regions in which the fuel electrodes 23 are positioned via inlet/outlet 26. The hydrogen fuel is separated into 2H⁺ and 2e; as in Figure 1. However, in this embodiment, the hydrogen fuel is introduced into the first regions in which the fuel electrodes 23 are positioned under compression.

The oxidant electrodes 24 are the cathode in the fuel cell mode and so a first redox mediator couple in the electrolyte 31 is reduced at the oxidant electrodes 24. The first redox mediator couple is then regenerated in the chamber 27 using oxygen, which is introduced into the chamber 27 via the inlet 28, as in Figure 1. However, in this embodiment, the pump 32 compresses the electrolyte 31. This compression is controlled by a pressure release valve 34.

The pressure created by the compressed hydrogen fuel and electrolyte 31 creates a region of increased pressure 33 around the cell units 22. The chamber 27 is not within this region of increased pressure 33 and instead is under atmospheric pressure.

This reaction produces water, which is removed along with any unreacted oxygen via outlet 29. Gas and water then enter separator 30a, condenser 30b and demister 30c and are separated. The water is passed back into the electrolyte 31 in the stack 21, while the gas is either released to the atmosphere or recycled back into chamber 27.

In the electrolyser mode, a power supply is connected to the circuit 37 between the fuel electrode 23 end of the stack 21 and the oxidant electrode 24 end of the stack 21. The direction of the current and thus the roles of the electrodes are reversed. The fuel electrodes 23 become the cathodes and generate hydrogen, which is removed from the stack via inlet/outlet 26 and stored.

The oxidant electrodes 24 become the anodes and so a second redox mediator couple in the electrolyte 31 is oxidized at the oxidant electrodes 24. The second redox mediator couple is then reduced by reaction with water in the chamber 27, to produce oxygen. Water enters the chamber 27 via inlet 28 and the oxygen produced is removed from the chamber 27 via outlet 29.

The oxygen and any evaporated water then enter the separator 30a, condenser 30b and demister 30c and are separated. The water is passed back into the electrolyte 31 in the stack 21, while the oxygen is either released to the atmosphere or stored for future use.

Claims

1. A reversible cell that can be run in either a fuel cell mode or an electrolyser mode, wherein the cell comprises:

a fuel electrode in a first region;

an oxidant electrode in a second region, wherein the fuel electrode and the oxidant electrode are electrically connected in a circuit and wherein the circuit is arranged such that a power supply can be connected to said circuit when the cell is run in the electrolyser mode and an electrical load can be connected to said circuit when the cell is run in the fuel cell mode;

the first and second regions being separated by an ion selective polymer electrolyte membrane;

at least one chamber in fluid communication with the second region;

an electrolyte in the second region comprising two redox mediator couples, wherein the electrolyte can circulate between the second region and the at least one chamber, around an electrolyte flow path;

means for introducing a fuel into the first region and means for introducing an oxidant into the at least one chamber when the cell is run in the fuel cell mode; and means for introducing water into the electrolyte flow path, means for removing hydrogen from the first region and means for removing oxygen from the at least one chamber when the cell is run in the electrolyser mode;

wherein the redox mediator couples have different redox potentials such that a first redox mediator couple is at least partially reduced at the oxidant electrode and at least partially regenerated by reaction with the oxidant after such reduction when the cell is run in the fuel cell mode and a second redox mediator couple is at least partially oxidised at the oxidant electrode and at least partially regenerated by oxidation of water to produce oxygen after such oxidation when the cell is run in the electrolyser mode.

2. The reversible cell according to Claim 1 wherein the redox potential of the first redox mediator couple is such that it can reduce oxygen and/or the redox potential of the second redox mediator couple is such that it can oxidise water.

3. The reversible cell according to Claim 1 or Claim 2 wherein the first redox mediator couple has a redox potential of between 0.7 V and 1.2 V vs SHE in the electrolyte solution and/or wherein the second redox mediator couple has a redox potential of between 1.4 and 1.8 V vs SHE in the electrolyte solution.

4. The reversible cell according to any one of Claims 1 to 3 wherein the first redox mediator couple is selected from a polyoxometalate, vanadium (574+), manganese (372+), iron (372+) and/or wherein the second redox mediator couple is selected from cerium (473+), cobalt (372+) and manganese (372+).

5. The reversible cell according to any one of Claims 1 to 4 wherein the electrolyte includes additional counterions or complexes to adjust the redox potential and/or to increase the solubility of the first redox mediator couple and/or the second redox mediator couple.

6. The reversible cell according to any one of Claims 1 to 5 wherein one or more catalysts that catalyse the reduction of oxygen and/or the oxidation of water are dissolved or suspended in the electrolyte, or are in a fixed location along the electrolyte flow path.

7. The reversible cell according to any one of Claims 1 to 6 wherein the cell comprises one chamber in fluid communication with the second region for both oxidation of the redox couple in the fuel cell mode and oxygen evolution in the electrolyser mode.

8. The reversible cell according to any one of Claims 1 to 6 wherein the cell comprises a regeneration chamber in fluid communication with the second region for oxidation of the redox couple in the fuel cell mode and an oxygen evolution chamber in fluid communication with the second region for oxygen evolution in the electrolyser mode.

9. The reversible cell according to any one of Claims 1 to 8 further comprising a means for removing excess gas and/or liquid from the cell.

10. The reversible cell according to Claim 9 wherein the excess gas is air with depleted levels of oxygen or unreacted fuel and/or wherein the excess liquid is water produced during fuel cell mode.

11 .The reversible cell according to any one of Claims 1 to 10 further comprising a means for separating a mixture of gas and liquid that is removed from the cell.

12. The reversible cell according to Claim 11 wherein the means for separating a mixture of gas and liquid comprises a separator, condenser and/or a demister.

13. The reversible cell according to any one of Claims 1 to 12 further comprising a pump that controls the rate of flow of the electrolyte between the second region and the at least one chamber.

14. The reversible cell according to Claim 13 wherein the pump is arranged to maintain the electrolyte under compression, thereby creating a region of increased pressure within the cell.

15. The reversible cell according to any one of Claims 1 to 14 further comprising a means for introducing the fuel to the first region and/or the oxidant to the at least one chamber under compression, thereby creating a region of increased pressure within the cell.

16. The reversible cell according to any one of Claims 1 to 15 comprising a plurality of cell units, each cell unit comprising a fuel electrode in a first region, an oxidant electrode in a second region and an ion selective polymer electrolyte membrane, wherein the plurality of cell units are electrically connected in series in a single circuit.

17. A method of producing electricity using a reversible cell according to any preceding claim in the fuel cell mode, comprising:

connecting an electrical load to the circuit;

providing a fuel to the first region; and introducing an oxidant into the at least one chamber.

18. The method of producing electricity according to Claim 17, wherein the fuel and/or the oxidant is provided under compression and/or wherein the electrolyte is maintained under compression, thereby creating a region of increased pressure within the cell.

19. A method of producing hydrogen and oxygen using a reversible cell according to any of Claims 1 to 16 in the electrolyser mode, comprising:

connecting a power supply to the circuit;

providing water into the electrolyte flow path; removing hydrogen from the first region; and removing oxygen from the at least one chamber.

20. The method of producing hydrogen and oxygen according to Claim 19, wherein the electrolyte is maintained under compression, thereby creating a region of increased pressure within the cell.

21. A method of operating a reversible cell comprising switching between the method of Claim 17 or 18 to that of Claim 19 or 20.

22. The method of Claim 21 wherein the reversible cell is used in combination with a means for generating electricity from a renewable energy source, such that the reversible cell operates in the electrolyser mode when the renewable energy source is providing levels of electricity above a first threshold and operates in the fuel cell mode when the renewable energy source is providing levels of electricity below a second threshold.

23. A device comprising a reversible cell according to any one of Claims 1 to 16 and a means for generating electricity from a renewable energy source.

24. An electrolyte comprising two redox mediator couples having different redox potentials, wherein a first redox mediator couple has a redox potential such that it is at least partially reduced at an electrode and at least partially regenerated by reaction with an oxidant after such reduction; and wherein a second redox mediator couple has a redox potential such that it is at least partially oxidised at an electrode and at least partially regenerated by oxidation of water to produce oxygen after such oxidation.

25. A use of an electrolyte according to Claim 24 in a reversible cell that can be run in either a fuel cell mode or an electrolyser mode.