The invention relates to a proton selective membrane and a system and method comprising a proton selective membrane.

Membranes for transferring cations are known from practice in the form of cation- exchange membranes. Such membranes are for example used in (bio-) electrochemical cells and (bio-based) fuel cells. Regardless of the system in which cation-exchange membranes are used, the membranes should be designed to have a high selectivity towards protons, especially in situations in which pure proton transport is desired. In other words, the efficiency of the membrane, and consequently the system in which that membrane is used, depends on the membrane being able to effectively block other cations from passing, especially in situations in which pure proton transport is wanted.

A disadvantage of the known cation-exchange however is the presence of co-ion transport, which is (unwanted) transport of anions across the membrane. This is a direct consequence of the fact that most cation-exchange membranes have an insufficiently high level of selectivity for protons. A consequence of the co-ion transport is that the efficiency of the system in which the membrane is used, is reduced and the maintenance costs of such a system increase.

The membrane according to the invention is aimed at obviating or at least reducing the abovementioned disadvantage.

To that end, the invention provides a membrane for proton conduction, the membrane comprising a layer of water, wherein the layer of water is substantially entirely in a solid state.

It should be noted that, for the purpose of the invention, water in a solid state can also be read as 'ice' , frozen water and/or similar known synonyms for water in a solid state. Furthermore, the solid-phase water as referred to in the application may be formed of any one of the known crystalline phases of solid state water, such as the crystalline phases ice-I to ice-XV, a combination thereof or any other additional crystalline phase of solid-state water. The invention is not limited to any one or any combination of such crystalline phases. Furthermore, it should be noted that the term 'membrane' should in view of the invention also be read as 'proton conduction layer', which amongst other can be used in an (electro)chemical system. The invention therefore may alternatively be formulated as a device for substantially pure proton conduction comprising a layer of water, wherein the layer of water is substantially entirely in a solid state. The advantages and embodiments as described below are also applicable to the device for pure proton conduction.

The membrane according to the invention provides several advantages over the prior art. Contrary to conventional cation-exchange membranes, the membrane according to the invention has the surprising advantage that it is a substantially pure proton conduction membrane in which substantially no co-ion transport is present. Several tests that were conducted using solid state water membranes have proven that the membrane according to the invention is up to 100% selective with regard to proton conduction in an electrochemical cell. Up to 100% should be read as being at least 95% selective, preferably at least 99% selective and more preferably at least 99,5% selective. The use of the membrane as substantially pure proton conduction layer therewith provides a significant improvement over conventional cation-exchange membranes, which are less selective. The increased selectivity of the membrane according to the invention eliminates or at least significantly reduces the co-ion transport across the membrane and significantly improves the lifetime and stability of a system in which in the membrane is used. This is for example useful in Vanadium-redox-flow batteries, which are known per se and which have the disadvantage that unwanted co-ion transport takes place through the currently used anion-exchange membranes. The membrane according to the invention has the advantage that is substantially inhibits any transport save pure proton transport, therewith significantly improving the existing Vanadium-redox-flow batteries.

Another advantage of the membrane according to the invention is that the membrane is substantially impervious to water transport caused by the proton transport. As a result, the efficiency of the membrane is increased compared to known cation-exchange membranes, in which water transport is also an issue.

Another advantage of the membrane according to the invention is that the membrane is manufactured from low-cost materials, most notably water, that is transferred from a gas state or a fluid state to a solid state. Furthermore, the manufacturing process to manufacture the membrane according to the invention is also relatively low-cost, therewith allowing the membrane according to the invention to be produced at relatively low cost. As a result, the cost of the membrane according to the invention is relatively low compared to the known cation-exchange membranes.

Furthermore, the membrane according to the invention also drives out any 'foreign' substances, such as salts during the formation or growing of the membrane.

Yet another advantage is that the membrane according to the invention can be used in existing systems, such as redox-recirculation devices or systems. The membrane according to the invention may also be used in novel applications (which may also include redox-recirculation systems or devices.

Yet another advantage is that the membrane according to the invention can easily be replaced and/or recycled after use. The membrane may, for example when used in a (bio-) electrochemical cell, be melted down to liquid phase after use and subsequently recycled into a new membrane. Alternatively, the water may be mixed with the fluids in the system in which the membrane is used, such as an electrochemical cell, and subsequently disposed with those fluids.

Yet another advantage of the membrane according to the invention is that the membrane is exceptionally environmentally friendly, since it does not contain any rare metals and/or transition metals and/or other rare or scarce raw materials and/or toxic materials. Instead, it contains a readily available source of material.

The membrane according to the invention is useable in a great variety of different applications, such as energy storage and/or energy back-up for example for server parks, hospitals and banks, and may also be used in (large-scale) energy storage solutions.

In an embodiment according to the invention, the membrane may be manufactured from demineralised water, destilled water and/or sonicated water.

The advantage of manufacturing the membrane from deminerisalised or demi-water, destilled water and/or sonicated water is that the membrane has a reduced amount of minerals and/or gases and/or contaminations contained in the crystalline structure of the solid state water. As a result, the amount of impurities in the membrane is reduced and the performance of the membrane is increased. Alternatively, milli-Q water may also be used.

In an embodiment according to the invention, the membrane may be manufactured from boiled water that is transferred from a fluid-state to the solid state.

The advantage of using boiled water, preferably boiled demi-water, is that boiled water is substantially devoid of oxygen, which reduces the formation of 'bubbles' in the solid-state water of the membrane during formation. This provides a more stable crystalline structure and a more efficient membrane.

In an embodiment according to the invention, the water is doped with a strong acid.

It has been found that doping the water with a strong acid to manufacture the membrane with provides a membrane which is more efficient terms of conductivity and potential.

In an embodiment according to the invention, the strong acid is chosen from the group of HF, HC1, HI, HBr, CH ₃ S0 ₃ H, HCIO ₄ , HN0 ₃ , H ₂ S0 ₄ , H ₃ P0 ₄ , or a mixture thereof.

The advantage of using the strong acid is the ability to get incorporated in the crystal structure of the membrane and to provide the ability for proton hopping. The skilled person would understand that proton hopping, also known as the Grotfhuss mechanism, is the proton defect diffuses through the hydrogen bond network of water molecules through the formation and concomitant cleavage of covalent bonds involving neighbouring molecules.

Membrane according to one of the presently preferred embodiments the concentration of the strong acid is in the range of 0.1M to 6M, preferably in the range of 1M to 4M, more preferably in the range of 1.5M to 3M, most preferably in the range of 2M to 3M.

It has been found that the membrane is more efficient in terms of conductivity and potential when the strong acid is provided in a concentration of 2M to 3M.

In an embodiment according to the invention, the membrane may be transparent.

It has been found that the membrane is more efficient in terms of conductivity and potential when it is transparent. Manufacturing a layer or block of transparent solid state water can be performed using various techniques. Preferably, the manufacturing includes the use of demi- water and/or boiled water and/or strong acid as raw material for growing the membrane layer to provide a transparent layer containing a relatively low amount of impurities. Preferably the membrane is formed in situ in the water. Furthermore, the layer of solid-state water is preferably formed under controlled conditions, which may include providing a closed space having low oxygen-conditions or even a vacuum.

In an embodiment according to the invention, the membrane may have a thickness, wherein the thickness is in the range of 1 μιη to 20 cm, preferably in the range of 1 μιη to 10 cm, and more preferably in the range of 1 μιη to 1 cm.

An advantage of the membrane according to the invention is that the thickness of the membrane may be adapted to the specific use that is required. In most applications, the thickness of the membrane will be limited to a thickness in the range of 1 micrometre - 1 centimetre. Having a thickness in this particular range allows for a membrane having sufficient strength, while simultaneously requiring relatively low amount of space and allowing a high performance.

Alternatively, the thickness may also be chosen to be larger in order to provide an increased resistance of the membrane due to osmotic pressure.

In an embodiment, the thickness of the membrane may vary across the surface of the membrane. Such variation may for example be used to provide an outer edge that is configured to cooperate with a system in which the membrane is to be placed to provide a substantially tight fit of the membrane in the system. In an example, the membrane may be formed as a square, rectangular or round membrane having relatively high thickness in a middle section and having a decreased thickness near the edges. Alternatively, a reduced thickness in a middle section and an increased thickness near an outer edge is also possible for a membrane according to the invention. It is however preferred to have a substantially constant thickness over the entire surface area of the membrane.

In an embodiment of the invention the thickness of the membrane is measured using at least one sensor, wherein the sensor is an optical sensor, a laser, a resistance sensor, conductivity sensor.

The advantage of the at least one sensor is that the membrane thickness can be measured on demand and even during operation to maintain the desired thickness. This allows a reduced energy consumption and simultaneously prevents the membrane from reaching an undesired thickness. In an elaboration of the invention the sensor may be combined with a controller to actively influence the thickness of the membrane during operation for example by switching on and/or off a cooling device. It is preferred that the controller comprises a feedback control.

In an embodiment according to the invention, the membrane may comprise a support structure that is configured for supporting the solid state layer of water. The advantage of a support structure is that it provides additional strength to the membrane, which reduces the risk of fracturing and/or breaking.

Additionally, the support structure may be used during manufacturing for providing a basis on which the solid-state layer of water is grown to form the membrane. Preferably, the support structure is reusable or at least recyclable. Preferably, the support structure and the membrane cooperate to provide a fluid-tight seal between the first and second compartment. In a preferred embodiment, the solid-state layer of water is melted during or after removal from the system in which the membrane is used, after which the support structure is re-used for growing a new layer of solid state water on the support structure. This allows a cost-effective and environmentally friendly manufacturing of the membrane. Additionally or alternatively, the support structure may also be manufactured from a recyclable material.

In an embodiment according to the invention, the support structure may be provided by a number of cooling pipes or tubes that extend parallel to each other in a horizontal or vertical direction. The cooling pipes may for example be metal pipes, such as copper or stainless steel pipes, or for example plastic pipes or tubes, having a predetermined inner and outer diameter and which are configured for guiding a cooling medium. In an elaboration of the embodiment, the cooling pipes may have an inner diameter in the range of 1 - 3 mm. In an elaboration of the embodiment, the cooling pipes may have an outer diameter in the range of 1 - 6 mm, and may preferably have an outer diameter in the range of 2 - 4 mm. Furthermore, the cooling pipes may extend horizontally or vertically and parallel to each other with a predetermined center-to-center distance. The center-to-center distance may for example be in the range of 2 - 8 mm, and preferably may be in the range of 3 - 5 mm. A suitable cooling fluid, such as glycol, may be chosen for forming the membrane. Preferably, the pipes are connected to an external source for cooling (i.e. discharging heat), which often results in the coolant being at substantially the same temperature.

In an embodiment according to the invention, the support structure may be and/or may be manufactured from a porous material, wherein the porous structure is preferably configured for holding and/or incorporating solid state water.

A porous support structure may, in addition to having openings for the easy transfer of protons, also be advantageously used during the manufacturing of the membrane, in that it can be provided as a starting point for forming the layer of solid state water.

In an embodiment according to the invention, the support structure may be a mesh structure, preferably a mesh structure having a plurality of relatively large openings.

A mesh structure provides the advantage that is requires a relatively small amount of raw material and provides a relatively high degree of flexibility compared to a solid support structure. Furthermore, the mesh structure reduces or obviates the possibility of interference with regard to the transport of protons. The mesh structure is preferably made of an inert material, such as mentioned above. Preferably, the mesh structure is chosen to provide support during the formation and/or operation of the membrane, wherein it may be coupled with an active cooling function of the membrane.

In an embodiment, the support structure is manufactured from an electrically nonconducting and/or chemically inert material. An inert material for the support structure is a material that is chemically inactive and/or electrically non-conductive, such that the support structure does not interfere with the function of the membrane and does not promote and/or support any co-ion transport and/or leak-currents.

In an embodiment according to the invention, the support structure may be connectable to a cooling device that is configured for cooling the membrane and/or forming the membrane from fluid-state phase water.

The support structure may be used to provide cooling to the membrane during operation, which reduces melting and increases the life -time of the membrane. The support structure is preferably manufactured from a material capable of transporting and/or extracting heat from the layer of solid state water of the membrane in order to maintain the layer at cryogenic temperatures, preferably a temperature of a few degrees below 0 °C, although other temperatures are possible as well. Preferably, the cooling device is provided with a control system or is connectable thereto for regulating the amount and/or intensity of cooling.

More preferably, the material of the support structure is chosen to be capable of heat transport and extraction, while simultaneously being chemically inactive and electrically non- conductive.

The support structure may be provided in any form that is suitable for use in or on the membrane and/or may be connectable to (part of) a device or system in which the membrane is useable.

In an embodiment according to the invention, the support structure may comprise a cooling device configured for forming a layer of solid state water to manufacture the membrane.

This embodiment provides a highly integrated membrane in that the support structure is used for supporting the layer of solid state water during operation as well as forming the layer of solid state water during manufacturing of the membrane. This for example allows the membrane to be formed in-situ rather than manufacturing the membrane off-site and therewith obviates transportation of the membrane to the location of use.

In an embodiment according to the invention, the membrane is substantially pure in that the water in the membrane contains less than 1 volume-% of impurities, and preferably less than 0.1 volume-% of impurities. By reducing the amount of impurities in the layer of solid state water of the membrane, a high performance membrane can be formed. Especially the presence of minerals, oxygen and/or other gases reduces the purity of the membrane, therewith resulting in a reduced performance of the membrane. It is found that if the level of impurities, measures as a volume-percentage of the layer of water in the membrane, is kept within the range as mentioned before, the performance of the membrane is not adversely affected.

In an embodiment according to the invention, the membrane may be configured for operation under cryogenic conditions, and may preferably be configured for operation in a temperature range between -15 °C and 4 °C, more preferably in a temperature range of - 10 °C and 2 °C and most preferably in a temperature range of -6 °C to -2° C.

The temperature range of the membrane is preferably kept at or even more preferably slightly below 0 °C, which prevents melting of the membrane during use in a system, for example an (bio-) electrochemical cell. Simultaneously, a temperature at or even slightly below 0 °C prevents any fluids in the system, for example an (bio-) electrochemical cell, from completely freezing up and reducing or obviating operation of the system.

The invention also relates to a device for storing and/or producing energy, the device comprising:

a housing that is tillable with fluid;

a first compartment in the housing in which an electrode is positioned; - a second compartment in the housing in which an electrode is positioned;

a membrane according to one of the preceding claims, the membrane being configured for placement in the housing for forming the first and second compartments; and wherein the electrodes are connectable to an external power source and/or a load for forming an electrical circuit.

The device according to the invention provides the same effects and advantages as the membrane according to the invention. The device according to the invention is usable in a wide variety of different devices, all of which fall within the scope of the application. In a preferred embodiment, the device according to the invention is configured for producing and/or storing energy. This includes capacitive devices in which only a proton gradient between the first and the second compartment is provided to store and/or generate energy, and also includes devices having electrochemical devices having reversible electrodes and/or electrochemical couples. It also includes fuel cells for generating energy.

It has been found in various tests that the device according to the invention advantageously provides a cost-effective, efficient and environmentally friendly device for storing and/or generating energy. The device according to the invention may be provided in various different forms and shapes, depending on the specific requirements of the device, such as storage capacity and size. Preferably, the housing is chosen from a material that is suitable for holding acids and/or bases in order to provide a reusable housing for the device. The housing may be provided with means, such as connectors, for holding the electrodes, wherein the means are preferably electrically insulated from the housing. The device may be configured for coupling the device with a second and/or further device, wherein the coupling may be provided between devices, or wherein the device may be coupled using bipolar stacking, in which different devices are connected using a bipolar plate for connecting a first compartment of a second device to a second compartment of a first device.

The device according to the invention preferably is a reversible device in that it can be used as a rechargeable battery. This is achievable by choosing a correct combination of electrolytes and/or redox-reactants. The electrolytes and/or redox-reactants are in such a situation configured to facilitate proton transport across the membrane in both directions, depending on the configuration of an electrical circuit attached to the device.

Preferably, the device is isolated in that it is provided in an encapsulating layer of insulation that prevents heating of the device (and therewith melting of the membrane). The isolation is preferably configured to retain the device, or at least the membrane therein, at a temperature below 0 C°.

Furthermore, the device, and specifically the walls thereof, may be configured to be actively cooled in order to retain the device, or at least the membrane therein, at a temperature below 0 C°. A combination of active cooling and providing an encapsulating layer of isolation also falls within the scope of the invention as described in the application.

In a non-limiting example of the device that is used for illustration purposes only, the housing is a square or rectangular box having four side walls which define an inner space, wherein the housing is preferably formed of a suitable plastic. The inner space is divided in the first and the second compartment, which are at least partially separated from each other by the membrane. The membrane may be formed externally from the housing and be provided into the housing prior to filling the first and second compartment with fluid. In such a situation, it is preferred that the side walls of the housing, at the location in which the membrane is inserted, are provided with a slit or vertical opening through which the membrane is slidable insertable in the housing. The membrane may however also be formed in the housing before filling the compartments with fluid. This may for example be performed by providing a support structure which is connected to and/or slidable into the housing. Preferably, the support structure is connectable to a cooling device and/or is part of a cooling device, which allows the membrane to be formed directly onto the support structure. In this situation, the membrane is grown from an inside to an outside of the membrane. Such a support structure may be a cooling element, a pipe-structure or a wire-structure in which the pipes or wires extend substantially parallel to each other, a mesh structure or any other suitable structure. Alternatively, two vertical plates may be provided that provide a space formed between the plates and the side walls of the housing. The space is fillable with water and can be provided with a freezing or cooling element, or one or both of the plates may be cooled, which allows the water to transfer from a fluid state to a solid state for forming the membrane. The plates are subsequently removed from the housing, which makes the device ready to use. In yet another example, the device may even be integrally formed from a block of ice, in which the membrane is nherently' available. The block of ice may be formed into the device by creating two compartments having a layer of ice between them. The compartments may for example be provided by drilling, milling, grinding, sawing or even laser cutting, although other suitable means can be used as well.

It is found that any pollution in the fluid used to form the membrane is transported to the outer sides of the membrane during formation thereof. In other words, when a membrane is grown, any pollution present in the base material will not be enclosed in the crystal structure, thus leading to a substantially pure membrane.

In an embodiment of the device according to the invention, the fluid may be a compatible couple of an acid and base.

The device according to the invention may be provided using a couple of an acid and a base, wherein the acid is provided in one of the first and second compartment, whereas the base is provided in the other of the first and second compartment.

In an embodiment of the device according to the invention, the electrodes may be capacitive electrodes.

By using capacitive electrodes in the device, the device may advantageously provide a relatively low-cost device for storing (electrical) energy. In this embodiment, both the membrane and the electrodes are low-cost materials that are abundantly available. The device according to this embodiment may therefore be manufactured relatively easy and the use of (expensive) electrodes is obviated.

In an embodiment of the device according to the invention, the electrodes may be reversible electrodes.

The device according to the invention may also be provided with reversible electrodes, which is for example advantageous when refitting existing devices to the device according to the invention. Existing devices for energy storage are often provided with a cation-exchange membrane and reversible electrodes. Such devices may easily be converted into devices according to the invention by exchanging the existing membranes with a membrane according to the invention as provided above. This reduces the cost of the devices and obviates having to replace the entire device.

Furthermore, the device having reversible electrodes may in some cases provide additional advantages over the use of capacitive electrodes. Reversible electrodes have the advantage that they can be chosen to match other substances that may be used in the device, which may for example include acids, bases and/or reversible redox couples. This allows the voltage across the cell to be increased. In addition, it allows a rechargeable cell to be manufactured.

In an embodiment of the device according to the invention, the first compartment may be filled with an acid and the second compartment may be filled with a base.

The device may be operated with an acid and a base as primary fluids for the storage of electrical energy, which electrical energy may than be discharged from the device to a load.

In a currently preferred embodiment of the device according to the invention, the first compartment may be filled with an acid, wherein the second compartment may be filled with an acid comprising, and wherein preferably the pH of the acid in the first compartment preferably may differ from the pH of the acid in the second compartment.

An advantage of the embodiment according to the invention is that it allows a rechargeable device to be manufactured. The application of two different acids allows the proton transport to take place in either direction across the membrane. By providing a difference in the pH- value between the different acids, the proton exchange is stimulated.

In an embodiment of the device according to the invention, at least one of the

compartments may additionally be provided with a reversible redox couple, wherein, when each compartment is provided with a reversible redox couple, the reversible redox couples may be different reversible redox couples, wherein the reversible redox couples are preferably chosen to be compatible.

The device may additionally comprise redox couples which are added to the

compartments, which advantageously increases the voltage over the electrodes. The redox couples are chosen to be compatible, which means that they (at least) provide a cell voltage. Preferably, the redox couples are chosen to complement each other to provide the highest effect, i.e. the largest voltage and/or the most stable and cost-effective solution. Furthermore, the redox couples preferably are stable redox couples in at least one of an acid and a base. In the embodiment in which both compartments are provided with an acid, each of the redox couples is configured to be stable in an acid.

In the known devices, the number of redox couples that is useable is relatively limited due to the fact that cation-exchange membranes are unable to provide sufficient selectivity to prevent co-ion transport in the device and due to the fact that the cation-exchange membrane transports other cations than protons as well. The device according to the invention has the advantage that a wide range of redox couples can be used, because the membrane is exclusively configured for proton transport as well as because the membrane is not reactive and/or affected by the redox couples in the compartments. In practice, several options for manufacturing a device are available. An example of such a device is provided below, wherein it is noted that this example is in no manner limiting other options. In the example, a redox couple of Fe ³⁺ /Fe ²⁺ in a solution of 1 M HC1 and a redox couple of Zn/Zn ²⁺ in a solution of 1 M HC1 is used. The voltage over the electrodes is provided by the difference in standard potential of both redox couples. The total voltage over the electrodes amounts to 1.57 V.

In an embodiment of the device according to the invention, the electrodes may be chosen from a group of activated carbon cloth, graphite, silver (Ag), anodised silver, lead (Pb), Zinc (Zn), Cadmium (Cd), Nickel (Ni), Platinum (Pt), Titanium (Ti), Iridium (Ir), Ruthenium (Ru), Tantalum (Ta), Indium (In) and mixed metal oxides, including Platinum-lridium (Pt-Ir), Ruthenium-Iridium (Ru-Ir), Tantalum-Iridium (Ta-Ir), Titanium-Ruthenium (Ti-Ru), Titanium-Tantalum (Ti-Ta), wherein the mixed metal oxides are preferably mixed metal oxides used in dimensionally stable anodes (DSAs).

The device according to the invention may be provided with a great variety of different electrodes, wherein both reversible and capacitive electrodes can be used. Preferably, capacitive electrodes are formed by activated carbon cloth, which is a cost-effective material. The electrode may however also be a capacitive slurry or capacitive solution provided in the compartment.

In an embodiment of the device according to the invention, each of the redox couples may chosen from a group of: Fe ²⁺ /Fe ³⁺ , Ce ⁴ 7Ce ³⁺ , Cr ²⁺ /Cr ³⁺ , V ³⁺ /V ²⁺ , Cu ²⁺ /Cu ³⁺ and V ⁴⁺ V ⁵⁺ .

The device according to the invention can be used in conjunction with a large number of different redox couples, which includes redox couples that may not be usable or prove difficult to use in known devices.

This may for example include the use of V ³ 7V ⁺ as redox couple, preferably in combination with 0 ₂ /H ₂ 0 as bifunctional gas diffusion electrode. The use of this particular redox couple in the device according to the invention provides the advantage that both transport of V ²⁺ and V ' ⁺ as well as the cross-over of 0 ₂ through the membrane is substantially obviated.

In an embodiment of the device according to the invention, the reversible redox couple may be a solid/solid-couple, a solid/liquid-couple, a liquid/liquid-couple or a gas/liquid-couple.

An advantage of the device according to the invention is that it can be used in conjunction with substantially all types of reversible redox couples. It is preferred to use redox couples that are stable in an acidic medium. This increases the flexibility and effeclivity of the device, since the type of redox couple than is chosen may be adapted to the specific availability of substances in particular area, such as a geographic region.

In an embodiment of the device according to the invention, the fluid may be a solution of one or more salts. In an embodiment of the device according to the invention, the fluid may be a solution of acid, wherein the acid may be chosen from a group of HF, HC1, HBr, HI, CH ₃ SO ₃ H, HCIO ₄ , HNO ₃ , H ₂ SO ₄ , H ₃ PO ₄ , HCOOH or CH ₃ COOH.

Several acids may be used in the device, although preferably the acid is chosen to be compatible with the redox couples which are provided to the acid in the housing.

In an embodiment of the device according to the invention, the first compartment may comprise a solution of acid and the second compartment may comprise a solution of acid, wherein the acid in the first and second compartments may be different acids, wherein preferably the acids may be chosen from a group of HF, HC1, HBr, HI, CH ₃ SO ₃ H, HCIO ₄ , HNO ₃ , H ₂ S0 ₄ or H ₃ PO ₄ .

Several acids may be used in the device, although preferably the acid is chosen to be compatible with the redox couples which are provided to the acid in the housing. More preferably, each compartment in the housing is provided with a different acid, which may increase the voltage. In case a device having more than two compartments is provided, the adjacent compartments are preferred to contain different acids or a similar acid having different concentrations. Furthermore, also in a device having two compartments, the concentration of the acids may be different, which also allows a single type of acid in different concentrations to be used in the different

compartments. The use of the device according to the invention, especially when using acid and/or a combination of acid with redox couples, provides a further additional advantage that a relatively high concentration difference of acid and/or redox couples can be used between the two compartments. In other words, the concentration in one of the first and second compartments may be significantly higher than the concentration in the other of the first and second compartment. In existing membranes the concentration difference must be kept limited due to the lack of resistance against the increasing osmotic pressure between the compartments. The device according to the invention, by virtue of the membrane, can be used with relatively high concentration differences due to the high resistance to the ensuing osmotic pressure. Furthermore, the membrane may even be provided at a relatively high thickness to increase the resistance to the osmotic pressure and provide higher concentration differences. Therewith, the device according to the invention provides a more efficient and compact storage for electrical energy than existing devices.

Yet another advantage of the embodiment according to the invention is that the concentrations in the first and second compartment can be significantly higher than with existing conventional membranes. In conventional cation-exchange membranes the permeation selectivity decreases with increasing concentrations of the acid and/or redox couple. This does not occur in the device according to the invention, which allows the use of higher concentrations of acid and/or redox couples, leading to a more efficient and compact storage cell for (electrical) energy.

In an embodiment of the device according to the invention, each of the electrodes may be a reversible electrode and wherein each of the first and second compartments is provided with an acid, wherein, in a charging state involving a charging operation, the reversible electrode in the first compartment is configured to be subjected to a reduction reaction and the reversible electrode in the second compartment is subjected to an oxidation reaction and the reduction reaction and the oxidation reaction provide a conversion of electrical energy into chemical energy. Furthermore, in a discharging state involving a discharging operation, the reversible electrode in the first compartment is subjected to an oxidation reaction and the reversible electrode in the second compartment is subjected to a reduction reaction, and the oxidation reaction and the reduction reaction provide a conversion reaction for converting chemical energy into electrical energy.

The invention also relates to a system for storing and/or generating energy, comprising a number of devices according to any one of the preceding claims, wherein the devices are connected to each other.

The system according to the invention provides the same effects and advantages as the membrane according to the invention and the device according to the invention. A system for energy storage may be formed by a number of devices according to the invention. Depending on the specific need that is to be fulfilled, a number of devices may be chosen and connected with each other for increasing the energy storage capacity. As such, the number may comprise one, i.e. a single device, yet may also be two, three, four, five or more. Even ten or a hundred or more devices can be used to form a single system according to the invention. In essence, the system according to the invention can be scaled by using any number of devices to obtain the desired storage.

An advantage of the system, especially when provided with at least one sensor and preferably a controller, may be used to control the thickness of the membrane. To this end the temperature variation of the system can be controlled to the within specific margins of for example 0.1 °C. furthermore, the system provides the advantage that the temperature of the membrane can be controlled with a very small margin of for example 0.01 °C. it is preferred that the sensor input is used by the controller to maintain the temperature with the above mentioned small margins of respectively 0.1 °C and 0.01 °C.The invention also relates to a method for storing and/or discharging energy, the method comprising the steps of:

providing a device or a system according to the invention;

filling the housing with a solution comprising a salt and/or an acid;

- providing the membrane to the housing for forming compartments in the housing; and applying a potential over to the electrodes for charging the device or the system.

The method according to the invention provides the same effects and advantages as the membrane, the device and the system according to the invention.

It should be noted that providing the device according to the invention also comprises manufacturing the membrane, which may be performed using various production techniques, including the boiled water-method (regular freezing at -18 to -20 °C), the top-down freezing method (freezing at -3 to -8 °C), the high-temperature freezing method (freezing at -1 °C) and/or the bottom freezing method (using very cold saltwater inside a freezer).

In an embodiment of the method according to the invention, the step of filling the housing with a fluid may comprise filling the first compartment with a fluid, preferably an acid, and filling the second compartment with a fluid, preferably an acid, wherein filling the first and second compartment is performed after the membrane is provided to the housing.

In a preferred embodiment according to the method, the housing is provided with a membrane, after which the first and second compartments are filled with acid. This may constitute the same type of acid having different pH-values or may constitute different acids for each compartment.

In an embodiment of the method according to the invention, the method additionally may comprise the steps of:

providing a redox couple to the first compartment; and

providing a redox couple to the second compartment.

Further advantages, features and details of the invention are elucidated on the basis of preferred embodiments thereof, wherein reference is made to the accompanying drawings, in which:

Figure 1 shows a schematic example of a device according to the invention;

Figure 2 shows a second schematic example of a device according to the invention;

Figure 3 shows a third schematic example of a device according to the invention;

Figure 4 shows a fourth schematic example of a device according to the invention;

Figure 5 shows a fifth schematic example of a device according to the invention; and Figure 6 shows experimental results with the device according to the invention.

The membrane according to the invention can advantageously be used in a device for energy storage, such as an electrochemical or bio-electrochemical cell. A schematic example of device 2 is an electrochemical cell having housing 4 and electrodes 6 (see figure 1 ). Electrodes 6 are connectable to an external load or power source 8 (not shown) for closing the electrical circuit and charging or discharging electrochemical cell 2. Furthermore, housing 4 is provided with first compartment 10, which is fillable with a fluid, and second compartment 12, which is fillable with a different fluid than first compartment 10. First compartment 10 and second compartment 12 are separated by membrane 14. Membrane 14 is provided with cooling device 16, which is used to cool membrane 14 during operation to a temperature that is preferably below 0 °C.

In use, first compartment 10 of housing 4 is filled with a fluid, preferably an acid, and is additionally preferably provided with a redox couple. Second compartment 12 of housing 4 is filled with a fluid, preferably an acid, which is a different fluid than the fluid in first compartment 10 or is a similar fluid having a different pH. Preferably, a redox couple is provided to second compartment 12 as well. Furthermore, the electrodes are connected by external power source or load 8 to form an electrical circuit.

During operation, an oxidation reaction takes place at one electrodes, for example electrode 6 in first compartment 10, which produces electrons, whereas a reduction reaction takes place at electrode 6 in compartment 12 in which electrons are used. This process may be reversible, which allows electrochemical cell 2 to function as a rechargeable battery.

In a second example (see figure 2), device 102 is an electrochemical cell having housing 104 and reversible electrodes 106. Electrodes 106 are connected to an external load or power source 108 (not shown) for closing the electrical circuit. Housing 104 is provided with first compartment 1 10 and second compartment 1 12. First compartment 1 10 in this example is filled with a HCl-solution having a redox couple of Zn/Zn ²⁺ and second compartment 1 12 is filled with a HCl/FeCL ₃ -solution having a redox couple of Fe ²⁺ /Fe ' ⁺ . First compartment 1 10 and second compartment 1 12 are separated by membrane 114, which is formed out of crystalline ice and forms a fluid-tight seal between first compartment 110 and second compartment 1 12. Membrane 1 14 is provided with cooling device 1 16, which is used to cool membrane 1 14 during operation to a temperature that is preferably below 0 °C.

During operation, when a load is provided to device 102, an oxidation reaction takes place at electrode 106 in first compartment 1 10, which oxidizes zinc (Zn) to zinc-ions (Zn ²⁺ ), according to the formula Zn→ Zn ²⁺ + 2 e ^~ , while at the same time protons (H ⁺ ) are transported. The protons migrate through membrane 1 14 to second compartment 1 12 in which a reduction reaction takes place, which reduces the iron ions according to the formula 2Fe ³⁺ + 2e ^~ → Fe ²⁺ . Conversely, when external power source 108 is provided to close the electrical circuit with electrodes 106, the reverse process, which is a charging process, takes place, in which Fe ²⁺ ions are oxidized and zinc-ions are reduced to metallic Zinc. This occurs according to the following formulas:

2Fe ²⁺ → 2(Fe ⁺ + el

Zn ²⁺ + 2 e ^" → Zn

Zn ²⁺ + 2Fe ²⁺ → 2Fe ³⁺ + Zn

In a third example (see figure 3), device 202 is an electrochemical cell having housing 204 and reversible electrodes 206. Electrodes 206 are connected to a (not shown) external load or power source 208 for closing the electrical circuit. Housing 204 is provided with first compartment 210 and second compartment 212. First compartment 210 in this example is filled with an acid provided with a redox couple of Fe ²⁺ /Fe ⁺ and second compartment 212 is filled with an acid with a redox couple of Cr ²⁺ /Cr ³⁺ . First compartment 210 and second compartment 212 are separated by membrane 214, which is formed out of crystalline ice and forms a fluid-tight seal between first compartment 210 and second compartment 212. Membrane 214 is provided with cooling device 216, which is used to cool membrane 214 during operation to a temperature that is preferably below 0 °C.

During operation, when a load is provided to device 202, an oxidation reaction takes place at electrode 206 in first compartment 210, which oxidizes Fe ²⁺ -ions to Fe ³⁺ -ions, while at the same time protons (H ⁺ ) are transported through membrane 214. The protons migrate through membrane 214 to second compartment 212 in which a reduction reaction takes place, which reduces the Cr ⁺ - ions to Cr ²⁺ -ions. Conversely, when external power source 208 is provided to close the electrical circuit with electrodes 206, the reverse process takes place.

In a fourth example (see figure 4), device 302 is a known Vanadium/Vanadium electrochemical cell provided with ice membrane 314 according to the invention. Device 302 is provided with housing 304 and reversible electrodes 306. Electrodes 306 are connected to an external load or power source 308 (not shown) for closing the electrical circuit. Housing 304 is provided with first compartment 310 and second compartment 312. First compartment 310 in this example is an acid having a redox couple of V ³⁺ /V ²⁺ and second compartment 312 is filled with an acid having a redox couple of V0 ²⁺ V 0 ₂ ⁺ . First compartment 310 and second compartment 312 are separated by membrane 314, which is formed out of crystalline ice and forms a fluid-tight seal between first compartment 310 and second compartment 312. Membrane 314 is provided with cooling device 316, which is used to cool membrane 314 during operation to a temperature that is preferably below 0 °C.

Membrane 314 is provided instead of the commonly used anion-exchange membranes in

Vanadium-redox-flow batteries and have the advantage that they block substantially all unwanted anion and cation transport (other than so-called 'proton hopping'). In this example, and in general for the invention, it is preferable to provide circulation for the redoxfluid (in this example acid having the redox -reactants) in a closed system. In an example of a device according to the invention (see figure 5), device 402 is formed by Plexiglas container 404 having side walls 404a and bottom wall 404b. Upper side 404c is closeable with container lid 404d (not shown). Housing 404 of device 402 has inner length L, inner width W and inner height H. The outer side of housing 404 is provided with vacuum insulating layer 405 that envelops device 402 to maintain cryogenic circumstances within device 402.

In this example container 404 is provided with separation wall 411 to form compartments

410, 412. In this example, separation wall 41 1 comprises a Plexiglas wall having height H, outer width W, inner width Wi and thickness D. Furthermore, separation wall 41 1 is, near a lower end thereof, provided with opening 413 having width W ₀ and height H ₀ . In this example, opening 413 has a height and width of approximately 10 - 15 cm. Opening 413 is closed off by membrane 414 that seals opening 413 and prevents fluids from compartments 410 and 412 from mixing. A number of horizontally placed copper tubes 416 is provided in opening 413 for forming and subsequently cooling membrane 414 in the opening. In this example, tubes 416 are copper tubes having an inner diameter of about 2 mm and an outer diameter of about 3 mm. Tubes 416 are placed at a center-to-center distance of about 3 - 4 mm and are, in use, provided with a refrigerant having a temperature well below 0 °C, for example -6 °C. The refrigerant is circulated through lubes 416 for cooling membrane 414, which in this example has a thickness of about 1 cm.

Furthermore, the amount and intensity of the cooling process may be regulated using control system 415.

In use, device 402 is first of all provided with water, preferably boiled and/or demi-water, for forming membrane 414 by circulating refrigerant through lubes 416. During a predetermined period of time, membrane 414 is grown inside Plexiglas container 404. When the growing of membrane 414 is complete, the water is removed from compartments 410, 412 and replaced with acid that has a temperature of at most 0 °C, for example -4 °C. The acid provided to compartment 410 is a different acid or an acid with a different pH than the acid in compartment 412.

Furthermore, device 402 may be provided with a compatible redox-couple, wherein each of the compartments 410, 412 is provided with a reagent forming the compatible redox-couple.

In a subsequent step the respective electrodes 406, which in this example are provided in container lid 404d (not shown), are placed respectively in compartment 410, 412 and connected to an electrical circuit for discharging and/or charging device 402.

An experiment was performed using the setup of figure 5, in which membrane 414 was a transparent ice membrane made of demi-water having height H ₀ and width W ₀ . Membrane 414 has thickness D ₀ of 1 cm and a surface of approximately 25 cm ² . Membrane 414 was internally cooled having a set point of -6 °C. Tubes 416 are copper tubes having an inner diameter of 2 mm and an outer diameter of 3 mm. First compartment 410 was filled with a 0.5 M HC1 solution at 0 °C having a Zn/Zn ²⁺ redox-couple and second compartment 412 was filled with a 0.1 M FeCWO.1 M Fed-, solution at 0 °C, thus providing a redox-couple Fe ²⁺ /Fe ³⁺ . Electrodes 406 were formed by a Zinc -plate in first compartment 410 and a Titanium mesh provided with a Ru/Ir-coating in second compartment 412. After measurement, it was established that the open circuit voltage was 1 ,46 V. During the experiment, a load was applied to device 402, which lead to the following reactions: oxidation reaction first compartment 410: Zn→ Zn ²⁺ + 2 e ^~ ;

reduction reaction second compartment 412: 2Fe ³⁺ + 2e ^~ → Fe ²⁺ ;

total reaction device 402: Zn + 2Fe ⁺ → Zn ²⁺ + 2Fe ²⁺

The ion transfer consisted of pure proton (H ⁺ ) transfer by means of proton hopping in membrane 414.

In a fifth example (see figure 6) the polarisation curve of an iron redox electrochemical cell separated by ice doped with 1M HC1 is shown. The internal resistance of the cell was 0.03 Ωηι ² and the specific resistance of the membrane was estimated at 86.32 cm the result of the resistance determination shows an inverse relationship between the concentration of the HCl in the ice and the specific resistance of the membrane. The specific resistance of 0.5M HCl doped ice membrane was reduced by a vector 2.2 compared to a 0.1M HCl ice membrane in the cell. The specific resistance of a 1M HCl doped ice membrane was reduced by a vector of 5.4 compared to the 0.5M HCl doped membrane. The current power density in this example reaches apex value of about 6 Wm ^'2 . This value may be increased even further by optimising the system for example by using conduits of different material, such as copper, stainless steel, plastic, preferably thin walled plastics. The fit using a 2 ^nd order polynomial resulted in the equation y = -0.0034x ² + 0.2643x and R ² = 0.9976.

Further tests were performed. The viability of the concept was proven in laboratory test 1 and test 2, of which a brief summary is provided below.

Test 1

The test comprised preparing a layer of solid state water (i.e. ice) which was sufficiently thick to prepare two holes on opposite sides of the layer. The holes were positioned a few centimetres apart. In a first hole a solution of 1 M HCl was provided, whereas the second hole was provided with a solution of 1 M NaOH. Both holes were provided with electrode material, which in this test was activated carbon cloth. The activated carbon cloth was rolled like a wick in a conical tube, and subsequently prepared for insertion. The preparation involved dipping the wicks in respectively the HCl-solution (first hole electrode) and the NaOH-solution (second hole electrode).

After insertion, the voltage difference measured between the electrodes at 273 was 0.392

Volt. The test was performed in a cooled chamber of 6 °C. The calculated potential difference was 0.379 V if Nernst Law was used with T = 273K and a perm selectivity equal to 1 of the ice for the protons and a concentration of 1 M of protons of the acid-base-salt used. If a temperature of T = 279 K is used (actual temperature of the climate room) a voltage difference of 0.387 V was calculated, which is substantially equal to the voltage difference measured.

Test 2

In a second test, the first compartment was provided with a solution of 0.1 M HCl and the second compartment was provided with a solution of 1 M NaOH, which were separated by a membrane according to the invention. Furthermore, each compartment was provided with a reversible Ag/AgCl-electrode. The voltage measured over the electrodes was 0.35 Volts, which was the expected value.

Test 3

Test 3 included the increasing the acidity of the solutions to form the membrane to 2M and 3M solutions respectively. It was found that doing so increased the power density from 6 Wm ^'2 to approximately 100 Win ² . In other words the increase of the acidity leads to a significant increase of the power density of the system. The present invention is by no means limited to the above described preferred embodiments thereof. The rights sought are defined by the following claims, within the scope of which many modifications can be envisaged.