WO2016102373A1

WO2016102373A1 - Molten salt electrochemical flow cell

Info

Publication number: WO2016102373A1
Application number: PCT/EP2015/080515
Authority: WO
Inventors: Mihai Stefan VICIU; Cord-Henrich Dustmann; Akane Elisabeth HARTENBACH; Michael Harald BAYER
Original assignee: Battery Consult Gmbh
Priority date: 2014-12-23
Filing date: 2015-12-18
Publication date: 2016-06-30
Also published as: EP3238297A1

Abstract

A rechargeable molten salt electrochemical flow cell according to the present invention comprises a cathode compartment containing a cathode material comprising a liquid metal nitrite salt and nitrogen dioxide, an anode compartment containing an anode material comprising an liquid elemental metal, and a solid membrane separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing the exchange of metal ions between the cathode and anode material, whereby the underlying redox reaction is according to the equation: MeNO₂ (liq.) <-> Me (liq.) + NO₂ (gas. or liq.).

Description

TITLE

MOLTEN SALT ELECTROCHEMICAL FLOW CELL

TECHNICAL FIELD The present invention relates to a novel type of molten salt electrochemical flow cell as well as its use for storing electrical energy.

PRIOR ART

Electrochemical energy storage systems are mostly based on connected closed cells (e. g. lithium ion cells) with common energy and thermal management. While lithium ion batteries have a very good volumetric and/or mass energy ratio, the improvements in redox active components to the total mass ratio are rather limited and potentially stagnant for known closed cell geometries. Consequently, the price associated with purchasing, maintenance and replacement of lithium ion batteries is around 500 USD per kWh which represents a high economic burden especially affecting large electrochemical energy storage systems. Such systems would benefit from the decoupling of power and energy capacity, which are properties that can be independently scaled to a particular need in a special type of electrochemical energy storage systems known as flow battery systems. A flow battery system comprises separate storage compartments for liquid anode and cathode materials representing essentially the energy capacity of the flow cell. The liquids are pumped through a reaction area where the redox reaction takes place by selective mass transfer across a membrane which depending on its size will determine the available power of the flow cell. The "spent" solutions are then collected in storage tanks and are either replaced, or more desirably regenerated, in order to recharge the electrochemical flow cell.

At present, aqueous electrolytic solutions based on vanadium, zinc-cerium or zinc-bromine are explored for their use in electrochemical flow cells. The economic benefit of using aqueous electrolytes is counterbalanced by electrochemical limits of the water, namely: low output voltage, gas evolution outside of the operating limits of aqueous electrolytes and low to medium current due to inner resistivity.

In order -to overcome the aforementioned voltage limitation imposed by water, it is possible to turn to electrochemical cell systems using non-aqueous organic electrolytes or molten, salts.

Molten salts have a high degree of dissociation into their corresponding ions which then entails a high ionic conductivity. Molten salt electrochemical flow cells, such as sodium sulfur batteries which operate at temperatures of around 300 to 350 °C, have been used in industrial applications with success in Japan and USA for storing grid energy because they can be produced from cheap starting materials to form generally low melting point sodium poly- sulfides. Such sodium-sulfur cells are well known in the art and are disclosed in for example US 4,230,778 A. DE3319951 discloses an electrochemical cell including a molten alkali metal such as sodium as anode material and alkali metal nitrate salts as the cathode material separated by a solid barrier of alkali metal ion conducing material. The redox reaction underlying this cell is summarized by the equation NaN0₃ <-> Na₂0 + NaN0₂ .

US2012/0219832 Al discloses a flow electrochemical cell in which molten halogenide salts, in particular molten zinc or aluminum chlorides are used as the electrolyte.

Another approach is to devise an all-liquid cell using liquid metals of different electrochemical potential as cathode and anode materials separated by a layer of molten salt. Such batteries would however need to be operated at temperature as high as 650°C. An exemplary liquid metal cell is disclosed in US2008/0044725.

It is thus desirable to provide an electrochemical cell which can achieve higher voltages and lower internal resistivity than flo electrochemical cells based on aqueous electrolyte solutions while at the same time being as freely scalable with respect to their electrical capacity as said flow electrochemical cells, which can be manufactured from readily available low-cost commodity chemicals, and which can be operated at reasonable temperatures, preferably below 350 °C.

SUMMARY OF THE INVENTION The present invention solves the above-mentioned problem by providing a molten salt electrochemical flow cell according to claim 1. The underlying electrochemical principle of the molten salt electrochemical flow cell according to the present invention is the use of the liquid metal nitrite salt as both redox material and electrolyte. In molten salt electrochemical flow cell according to the present invention the redox material, i.e. the metal nitrite salt behaves according to the present equation, where Me is a metal: cathode MeN0₂ (liquid) cathode compartment compartment

membrane membrane

anode (fluid) + anode compartment (liquid) compartment

The theoretical capacity of such system is 388.4 Ah/kg and a theoretical energy at 3V discharge of 1.17kwh/kg. The energy is similar with lithium air systems without the complexity imposed by a separate electrolyte. In one embodiment the molten salt electrochemical flow cell according to the present, invention comprises a cathode compartment containing a cathode material comprising a liquid metal nitrite salt and nitrogen dioxide, an anode compartment containing an anode material comprising an liquid elemental metal, and a solid membrane separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing the exchange of metal ions between the cathode and anode material.

In the context of the present invention, the terms "liquid" and "solid" refer to the state . of a designated material at the operating temperature of the molten salt electrochemical flow cell. Thus, it is understood that the operating temperature of the electrochemical cell is such that the electrochemical cell can perform the electrochemical reaction inherent to its functioning. It is known in the art that molten salt electrochemical flow cells must be brought to a temperature at which the materials of the redox pair become liquid or gaseous. Thus, depending on the cathode and anode materials used, the temperature will be such that the cathode and anode materials are brought into a liquid or gaseous state.

In another embodiment the molten salt electrochemical flow cell according to the present invention further comprises a nitrogen dioxide compartment fluidly connected to the cathode compartment, said nitrogen dioxide compartment being adapted for storing nitrogen dioxide released in the cathode compartment from the cathode material during charging as well as returning released nitrogen dioxide to the cathode material in the cathode compartment during discharge. It is understood that depending on the conditions within the nitrogen dioxide compartment, the nitrogen dioxide can form a mixture of nitrogen dioxide and of its dimer, dinitrogen tetroxide . In yet another embodiment the molten salt electrochemical flow cell according to the present invention the nitrogen dioxide compartment is further adapted to be cooled below the boiling point of the nitrogen dioxide, preferably 0 °C or less, more preferably below -10 °C, such as to reduce the nitrogen dioxide pressure in the cathode compartment and/or heated above the melting point of nitrogen dioxide, preferably, above 0 °C, more preferably above the boiling point of the nitrogen dioxide such as to increase the nitrogen dioxide pressure in the cathode compartment. Controlling the temperature of the nitrogen dioxide has the effect of being able to control the pressure of nitrogen dioxide, by either using the compartment as a cold trap to remove nitrous dioxygen from the system or using the heated compartment as a nitrogen dioxide source by increasing the pressure of nitrogen dioxide. By controlling the availability of nitrogen dioxide through variation of the temperature in the nitrogen dioxide compartment, it is ultimately possible to control the output of electrical energy of the cell.

In yet another embodiment the molten salt electrochemical flow cell according to the present invention further comprises a liquid metal nitrite salt compartment fluidly connected to the cathode compartment, said liquid metal nitrite salt compartment being adapted for storing liquid metal nitrite salt and releasing liquid metal nitrite salt into the cathode compartment during charging as well as returning liquid metal nitrite salt to the liquid metal nitrite salt compartment during discharge.

In yet another embodiment the molten salt electrochemical flow cell according to the present invention further comprises a liquid elemental metal compartment fluidly connected to the anode compartment, said liquid > elemental metal compartment being adapted for storing liquid elemental metal and receiving liquid elemental metal from the anode compartment during charging as well as returning liquid elemental metal salt to the anode compartment during discharge.

In another embodiment the molten salt electrochemical flow cell according to the present invention the cathode and anode compartment are arranged adjacently to each other, i.e one compartment is arranged side-by-side to the other compartment.lt is understood that side-by-side arrangement can be either horizontal or vertical.

In another embodiment the molten salt electrochemical flow cell according to the present invention the cathode and anode compartment are arranged concentrically, i.e one compartment is surrounded by the other compartment.

In a further embodiment the molten salt electrochemical flow cell according to the present invention, the liquid metal salt is an alkali or earth alkali metal nitrite salt and the liquid elemental metal is an alkali or earth alkali metal, and more preferably the liquid metal salt is an alkali metal nitrite salt .

It is understood that both the anode and cathode compartments respectively may further comprise current collectors that electrically connect the anode compartment and the cathode compartment to the cathode and anode terminals of the electrochemical flow cell. In one embodiment, the anode current collector in contact with the anode material of the anode compartment comprises at least on its surface, preferably is made of, a metal chosen from nickel, copper, aluminium, steel or alloys thereof. In one embodiment, the cathode current collector in contact with the. cathode material of the cathode compartment comprises at least on its surface, preferably is made of, conductive carbon such as carbon fiber, vitreous carbon, carbon felt, and graphite, electronically conductive metal oxides (eg. - and/or β-Pb02, magnetite), electronic conductive perovskites, doped oxide glass (ITO, FTO, AZO) , noble metals (Pt, Ir, Pd) , niobium, steel, alloys such as Duriron®, Durimet-20, electronically conductive ferrites, (LaO ..8SrO .2)0.95FeO .6MnO .3CoO .1CH3 (LSFMC) , LaNiO .6FeO .403-δ (LNF) and LaNiO .6CoO .403-δ (LNC) , dimensionally stable anodes (eg. Ti-Ti02-Pb02 ) conductive doped diamond such as boron doped diamond (BDD) .

In a yet further embodiment the molten salt electrochemical flow cell according to the present invention, the liquid metal nitrite salt is sodium nitrite and the liquid elemental metal is sodium. In this case, the sodium nitrite and the elemental sodium, or the corresponding cathode and anode compartments, must be heated to a temperature where both are in the liquid state, i.e. preferably above 271 °C and more preferably of from 271°C to 320°C. In an even further embodiment the solid separator comprises beta-alumina solid electrolyte (BASE) or a sodium super ionic conductor (NASICON) , or a combination thereof.

In another embodiment the molten salt electrochemical flow cell according to the present invention, the electrochemical cell is used for storing electrical energy, in particular is used for storing grid energy. In another embodiment of the present invention, the molten salt electrochemical flow cell may be used in the production of elemental sodium. Further embodiments of the invention are laid down in the dependent claims .

BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

Fig. 1 shows a sectional drawing of a molten salt electrochemical flow cell (0) having a tubular cathode compartment (3^') containing a cathode material of liquid metal nitrite salt arranged in a larger tubular anode compartment (2) containing the anode material comprising a liquid elemental metal. The cathode and anode compartments are separated by a solid membrane (1) that is selectively permeable to metal ions and which is in contact on its outer side with the anode material and on its inner side the cathode material, said membrane being capable of selectively allowing the exchange of metal ions between the cathode and anode material. A nitrogen dioxide compartment (7) equipped with heating/cooling coils (9) is fluidly connected through a pipe (10) to the cathode compartment. Likewise, a liquid elemental metal compartment (4) and a liquid metal nitrite salt compartment (8) are fluidly connected through pipes (11, 5) to the cathode and anode compartment, respectively. The cell may be connected to a electrical circuit through the positive and negative terminals (12,13).

Fig.2 shows the charge discharge cycles for a NaN02 based electrochemical cell with glassy carbon working electrode and sodium counter electrode/reference. Both charge discharge -curves were performed at 1mA. Fig.3 shows the charge discharge cycles for a NaN02 based electrochemical cell with glassy carbon working electrode and sodium counter electrode/reference. Charging was performed at 5mA and discharge curves were performed at various currents.

Fig.4 shows the charge discharge cycles for a NaN02 based electrochemical cell with gas diffusion carbon working electrode and sodium counter electrode/reference. Charging and discharge curves were performed at various currents

DESCRIPTION OF PREFERRED EMBODIMENTS

In a preferred embodiment, the molten salt electrochemical flow cell comprises a cathode compartment containing a cathode material comprising a liquid metal nitrite salt and nitrogen dioxide, an anode compartment containing an anode material comprising an liquid elemental metal, a solid membrane separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing the exchange of metal ions between the cathode and anode material. The topology of the molten salt electrochemical flow cell according to the present invention may have different forms. , . For instance, the cathode compartment and the anode compartments which are separated by a solid membrane being in contact on one side with the cathode material and on the other side the anode material, may be arranged side by side, with the solid membrane being a planar solid membrane . separating the two compartments on each side of the membrane or may be a arranged concentrically with the solid membrane being a tubular solid membrane separating the inner compartment from the outer compartment .

In a preferred embodiment the molten salt electrochemical flow cell comprises a cathode compartment containing a cathode material of liquid sodium nitrite salt and nitrogen dioxide, an anode compartment containing an anode material of liquid elemental sodium, a tubular solid membrane of beta- alumina (BASE) separating the cathode and anode compartment and being in contact with the cathode material on its inner surface and in contact with the anode material on its outer surface, said membrane being capable of selectively allowing the exchange of sodium ions between the cathode and anion material. In a particular embodiment, the tubular solid membrane in the form of a closed end tube having preferably an essentially circular or polygonal cross-section. The tubular solid membrane may be fixed in the electrochemical cell through an appropriate fixture made of an ionic and electronic- non-conducting material, such as polymer, in particular fluoropolymer, or inorganic insulating material such as alpha-alumina.

The solid membrane capable of selectively allowing the exchange of metal ions between the cathode and anion material may be formed from several materials, such as the ceramic, glass ceramic, glass or a combination thereof. Examples of suitable materials include but not be limited to beta' or beta' ' phases of alumina, especially sodium-complexed alumina, NASICON, or sodium alumino-phosphates (garnet type) .

Beta-alumina (BASE) is an isomorphic form of aluminium oxide (AI₂O₃) , a _. hard polycrystalline ceramic, which, when prepared as an electrolyte, is complexed with a mobile ion, such as Na+, K+, Li+, Ag+, H+, Pb2+, Sr2+ or Ba2+ depending on the type of application. Beta-alumina is a good conductor of its mobile ion yet allows no non-ionic (i.e., electronic) conductivity. In a another preferred embodiment the molten salt electrochemical flow cell comprises a cathode compartment containing a cathode material of liquid sodium nitrite salt and nitrogen dioxide, an anode compartment containing an anode material of liquid elemental sodium, a solid membrane of beta-alumina (BASE) separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing the exchange of sodium ions between the cathode and anion material, and further comprises a nitrogen dioxide compartment fluidly connected to the cathode compartment, said nitrogen dioxide compartment being adapted for storing nitrogen dioxide released in the cathode compartment from the liquid sodium nitrite salt during charging as well as returning released nitrogen dioxide to the liquid sodium nitrite salt in the cathode compartment during discharge, a liquid sodium nitrite compartment fluidly connected to the cathode compartment, said liquid metal nitrite salt compartment being adapted for storing liquid .sodium nitrite and releasing liquid sodium nitrite into the cathode compartment during charging as well as returning liquid sodium nitrite to the liquid sodium nitrite compartment during discharge wherein optionally said compartment may be adapted to be heated to a temperature above the melting point of the sodium nitrite, and a liquid elemental sodium compartment 'fluidly connected to the anode compartment, said liquid elemental sodium compartment being adapted for storing liquid elemental sodium and receiving liquid elemental sodium from the anode compartment · during charging as well as returning liquid elemental sodium to the anode compartment during discharge, wherein optionally said compartment may be adapted to be heated to a temperature above the melting point of the elemental metal.

In a yet another preferred embodiment the molten salt electrochemical flow cell comprises a cathode compartment containing a cathode material of- liquid, mixture of sodium nitrite and lithium nitrite having a melting temperature of between 150 °C and 155 °C, more preferably of about 152 °C, an anode compartment containing an anode material of liquid elemental sodium, a solid membrane of beta-alumina (BASE) separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing the exchange of sodium ions between the cathode and anion material, and further comprises a nitrogen dioxide compartment fluidly connected to the cathode compartment .

In a yet another preferred embodiment the- molten salt electrochemical flo _. cell comprises a cathode compartment containing a cathode material of liquid mixture of sodium nitrite and sodium nitrate having a melting point of from 230 °C to 300 °C, an anode compartment containing an anode material of liquid elemental sodium, a solid membrane of beta-alumina (BASE) separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing the exchange of sodium ions between the cathode and anion material, and further comprises a nitrogen dioxide compartment fluidly connected to the cathode compartment.

EXPERIMENTS

NaN0₂ (45.7g, 99.99%) was dried under active vacuum for 16 hours at 150°C. A borosilicate glass tube was loaded with the above mentioned amount of NaN0₂ and heated up to temperature above the melting point of the salt (271°C) , at 300°C. A closed-end β''-alumina tube with an internal diameter of 6mm and external diameter of 8mm was loaded with metal sodium (approximatively 0.5g) and provided with an electron collector. The collector/sodium/^"-alumina acts as both reference electrode (Na/Na+) and counter electrode.

A working electrode made of pyrolytic graphite was connected to an external circuit by means of an electrical metallic wire, redox inactive under the electrochemical reaction conditions. The cell was provided with a path to remove nitrogen dioxide and prevent a self-discharge of the system.

The nitric oxide was removed from the system by pressure differential and stored as liquid in a refrigerated reservoir. The temperature of the refrigeration was -20 °C such that the N0₂ was stored as liquid. The thus obtained cell was charged under galvanostatic conditions at currents in the range of 0.5-130 mA for various amounts of time. The current density relative to the β"- alumina was in . the range of 5-1300 A/m2 for an estimated contact surface of 1 cm². The cell was discharged under diffusion conditions and galvanostatic regime from 0.1 to 25 mA or current densities of 1-250 A/m2.

Fig. 2. shows the charge discharge cycles for a NaN0₂ based electrochemical cell with glassy carbon working electrode and sodium counter electrode/reference. Both charge discharge curves were performed at 1 mA. The data presented in Fig.2 shows a high reversibility of the nitrite/NC>2 redox couple and the possibility to operate such battery. The open circuit voltage was 2.56V. The discharge voltage profile is strongly dependent on the pressure of NO2 as expected from Nernst equation with a strong voltage drop to roughly 2 V versus Na/Na+ reference when the sodium nitrite molten electrolyte was depleted of N0₂ due to limited diffusion of N0₂.

Fig. 3. shows the charge discharge cycles for a NaN0₂ based electrochemical cell with glassy carbon working electrode and sodium counter electrode/reference. Charging was performed at 5 mA and discharge curves were performed at various currents. When the discharge rate was smaller and the area of the working electrode was increased the system stabilized its discharge voltage to values around 2.5V. A maximum stable discharge of 2.61 V is very similar to the theoretical value calculated from the thermodynamic value of E0 (3V) and the reported solubility of N02 in molten sodium nitrite at 300 °C (3.2mmol/L) under diffusion conditions at ambient pressure.

Fig. 4 shows the charge discharge cycles for a NaN0₂ based lectrochemical cell with a gas diffusion carbon working lectrode and sodium counter electrode/reference. Charging nd discharge curves were performed at various currents.

LIST OF REFERENCE SIGNS solid membrane salt storage liquid elemental metal compartment liquid metal nitrite 9 heating apparatus salt 10 tube connection liquid metal storage 11 tube connection compartment 12 (+)cap

tube connection 13 (-) case

electrical insulator

N0₂ storage compartment

liquid metal nitrite

Claims

1. An molten salt electrochemical flow cell comprising a

a . cathode compartment containing a cathode material comprising a liquid metal nitrite salt and nitrogen dioxide,

b. an anode compartment containing an anode material comprising an liquid elemental metal,

c. a solid membrane separating the cathode and anode compartment and being in contact on one side with the cathode material and on the other side the anode material, said membrane being capable of selectively allowing · the exchange of metal ions between the cathode and anion material .

2. The molten salt electrochemical flow cell according to claim 1, further comprising

a. a nitrogen dioxide compartment fluidly connected to the cathode compartment, said nitrogen dioxide compartment being adapted for storing nitrogen dioxide released in the cathode compartment from the cathode material during charging as well as returning released nitrogen dioxide to the cathode material in the cathode compartment during discharge .

3. The molten salt electrochemical flow cell according to claim 2, wherein the nitrogen dioxide compartment is further adapted to be

a. cooled below the boiling point of the nitrogen dioxide, preferably below 0 °C, more preferably below -10 °C, such as to reduce the nitrogen dioxide pressure in the cathode compartment and/or b. heated above the melting point of nitrogen dioxide, preferably above 0 °C, more preferably above the boiling point of the nitrogen dioxide such^' as to increase the nitrogen dioxide pressure in the cathode compartment .

The molten salt electrochemical flow cell according to any preceding claim , further comprising

a. a liquid metal nitrite salt compartment fluidly connected to the cathode compartment, said liquid metal nitrite salt compartment being adapted for storing liquid metal nitrite salt and releasing liquid metal nitrite salt into the cathode compartment during charging as well as returning liquid metal nitrite salt to the liquid metal nitrite salt compartment during discharge, wherein optionally said compartment may be adapted to be heated to a temperature above the melting point of the metal nitrite salt.

a. a liquid elemental metal compartment fluidly connected to the anode compartment, said liquid elemental metal compartment being adapted for storing liquid elemental metal and receiving liquid elemental metal from the anode compartment during charging as well as returning liquid elemental metal to the anode compartment during discharge, wherein optionally said compartment may be adapted to be heated to a temperature above the melting point of the elemental metal.

The molten salt electrochemical flow cell according to any preceding claim, wherein the cathode and anode compartment are arranged concentrically, i.e. within each other, and preferably wherein the cathode material contained in the cathode compartment is in contact with the inner side of the solid membrane and the anode material contained in the anode compartment is in contact with the outer side of the solid membrane.

The molten salt electrochemical flow cell according to any preceding claim wherein it further comprises, a cathode current collector in contact with the cathode material of the cathode compartment comprising at least on its surface, and preferably is made of, conductive carbon such as carbon fiber, vitreous carbon, carbon felt, and graphite., electronically conductive metal oxides (eg. a- and/or f¾-Pb02, magnetite) , electronic conductive perovskites, doped oxide glass (ITO, FTO, AZO) , noble metals (Pt, Ir, Pd) , niobium, steel, alloys such as Duriron®, Durimet-20, electronically conductive ferrites, (LaO .8Sr0.2) 0.95FeO .6MnO .3CoO .10-.3 (LSFMC) , LaNi0.6Fe0.4O3-5 (LNF) and LaNiO .6CoO .403-δ (LNC) , dimensionally stable electrodes (eg. Ti-Ti02-Pb02 ) conductive doped diamond such as boron doped diamond (BDD) .

The molten salt electrochemical flow cell according to any preceding claim, wherein the liquid metal salt is an alkali or earth alkali metal nitrite salt and the liquid elemental metal is an alkali or earth alkali metal.

The molten salt electrochemical flow cell according to any preceding claim, wherein the liquid metal salt is a mixture of two or more alkali metal nitrite salts and the liquid elemental metal is an alkali metal, preferably wherein the liquid metal salt is a mixture of lithium nitrite and sodium nitrite.

I . The molten salt electrochemical flow cell according to any preceding claim, wherein the liquid metal salt is sodium nitrite and/or the liquid elemental . metal is sodium.

11. The molten salt electrochemical flow cell according to any preceding claim wherein - the solid separator comprises beta-alumina solid electrolyte (BASE) or a sodium super ionic conductor (NASICON) .

12. An electrical energy storage device, comprising at least two or more molten salt electrochemical flow cells according to the preceding claims electronically configured in a series, parallel or a mixture of both.

13. Use of a molten salt electrochemical flow cell or an electrical energy storage device according to any preceding claim for storing electrical energy, in particular for storing grid energy.

14. Use of a molten salt electrochemical flow cell or an electrical energy storage device according to any preceding claim for storing electrical energy in an automotive vehicle.

15. Use of a molten salt electrochemical flow cell or an electrical energy storage device according to any preceding claim, for the production of elemental sodium.