Field of the invention

The present invention relates to an electrochemical cell, methods for using it and uses of said cell. It relates particularly to the field of sodium-seawater-batteries .

Related Art

The invention relates to the field of energy storage and, in particular, to electrochemical systems for stationary storage. Lithium-ion batteries (LIBs) are the technology of choice for portable electronics and the transportation sector, as they allow for the highest energy and power density at present, while the related costs are steadily decreasing. The tremendous success of this technology, however, raises concerns about the large-scale availability of elements like lithium and cobalt (the latter being comprised in the positive electrode of such batteries) and potentially critical materials as graphite (the state-of-the-art negative electrode in LIBs) . Vaalma et al show in Nature Review Materials, vol 3, (2018) Art. Nr. 18013 that sodium-based systems provide potentially lower costs and reduced dependency from critical raw materials. Sodium-seawater batteries are particularly appealing for stationary storage. These consist of: (i) a cathode, where a practically infinite supply of sodium is provided by seawater, (ii) a solid electrolyte sodium conductor and (iii) an anode compartment including a Na host electrode and a non-aqueous electrolyte.

Senthilkumar et al. disclose in Journal of Materials Chemistry

A, vol. 7 (2019) p. 22803 that choosing the right materials for the anode compartment is crucial, in particular the non- aqueous electrolyte (also called anolyte) , which is generally one of the main factors influencing the cycling performance granting interfacial stability to the anode and voltage window stability .

Kim et al. disclose in Advanced Functional Materials 2020, art. Nr. 2001249 such a seawater battery with Na-Biphenyl as anolyte. However Na-Biphenyl is an extremely reactive and thus dangerous compound for operation in open air even in a very dry atmosphere (e.g. dry room, < 1 ppm water) .

Problem to be solved

It is therefore an objective of the present invention to provide an electrochemical cell, which at least partially overcome the above-mentioned problems of the state of the art.

It is a particular objective of the present invention to provide an electrochemical cell, which allows to produce Na metal with higher energy efficiencies towards known methods.

In particular the objective of the present invention is to provide a multifunctional cell, which is easy to handle.

Summary of the invention

This problem is solved by an electrochemical cell for sodium- batteries, methods of using it and uses of said electrochemical cell. Preferred embodiments are listed in the dependent claims.

In a first aspect, the present invention relates to an electrochemical cell comprising:

- an anode compartment comprising at least an anolyte comprising at least an aromatic hydrocarbon preferably unbounded biphenyl (BP) and a first electrode in physical contact with the anolyte, - a cathode compartment comprising at least a catholyte comprising a source of Na ⁺ and a second electrode preferably coated with a catalyst in physical contact with the catholyte and

- a first separator placed between the anolyte and the catholyte, preferably a solid electrolyte particularly preferred a specific conductor for Na ⁺ ions and electron insulator .

The aromatic hydrocarbon is at least partially unsaturated for being able to react with Na. In a particular embodiment the aromatic hydrocarbon is a polycyclic aromatic hydrocarbon preferably including between two and six rings. More preferred the aromatic hydrocarbon is selected from the list: (Biphenyl, Naphthalene, Phenanthrene, Anthracene, Tetracene, Pyrene, and Perylene ) .

The solvents in which the aromatic hydrocarbon is preferably dissolved are in particular Dimethoxyethane (DME) , Diethylene glycol dimethyl ether (DEGDME) , Triethylene glycol dimethyl ether (Triglyme) , and Tetraethylene glycol dimethyl ether (TEGDME) .

In particular, the anolyte consists of unbounded biphenyl (BP) in diethylene glycol dimethyl ether (DEGDME) . As biphenyl is a solid at room temperature (25 °C in the SATP conditions) , it has to be dissolved in a solvent preferably DEGDME at a concentration in the range from 0,1M to 5M.

In a preferred embodiment the source of Na ⁺ is natural seawater or salty water composed of sodium cations and different anions (preferably within a concentration range from 0.01 M to 5.0

M) , wherein the sodium salt is NaCl, Nal, Na ₂ CO ₃ or Na ₂ SO ₄ or a mixture thereof. Preferably the source of Na ⁺ is seawater. In a preferred embodiment the anolyte compartment comprises a third electrode in physical contact with the anolyte.

In a further preferred embodiment the electrochemical comprises at least a first sodium storage compartment connected with the anode compartment via at least one fluid connection wherein the at least one sodium storage compartment comprises at least a fourth electrode and at least a second separator, wherein the second separator is placed between the at least third electrode and the at least fourth electrode.

The sodium storage compartments are spaces for harvesting pure sodium metal. In a further embodiment it is a detachable compartment, which would facilitate the transport and storage of Na on the long term.

In particular, the electrochemical cell comprises at least a second sodium storage compartment connected with the anode compartment via at least a second fluid connection and preferably with the first sodium storage compartment via at least a third fluid connection.

The sodium storage compartments especially the first sodium storage compartment are spaces for harvesting pure sodium metal. In a further embodiment it is a detachable compartment, which would facilitate the transport and storage of Na on the long term.

The sodium storage compartments especially the second sodium storage compartment are spaces that provide a sodium source (e.g. Sodium metal or biphenyl solution) for supplying energy, and it is easily designed to provide an additional sodium source when necessary. In a further preferred embodiment the electrochemical cell comprises at least one CI ₂ extraction compartment connected with the catholyte compartment via at least a fourth fluid connection. The CI ₂ has to be actively separated since it has a non-negligible solubility in water. During the charge process for sodium metal harvesting, sodium cations in seawater move to the anode compartment, and other anions undergo oxidation reactions in seawater. Chlorine gas is produced by this oxidation reaction. If the second electrode is coated with a suitable catalyst for example Ruthenium-based catalyst or nickel-iron hydroxide, chlorine evolution reaction occurs more actively. The evolved (produced) chorine gas is stored in a typical gas collection method. The separation of chlorine gas can be done for example via the Castner-Keller process or via suitable membranes.

In particular, the first, the third and the fourth electrode are metallic and comprises preferably Ni, Cu, Stainless steel, Al. These metals do not alloy with Na. The metals that have to been avoided are from the group IV and and V i.e. C, Si, Ge, Sn, Pb and P, As, Sb, Bi as they form easily alloys with Na. The electrode is formed preferably as a grid or a mesh or a foam. The first electrode is more preferred a foam. In a particular embodiment the first and the third electrodes act as anodes. The second electrode comprises preferably carbon. It is formed preferably as a felt. In a particular embodiment the second and the fourth electrodes act as cathodes.

In a preferred embodiment, the catalyst used is the same for all the configurations of the multifunctional electrochemical cell. Oxide-based catalysts that are stable in seawater are preferred. In a preferred embodiment the catalyst depends on the way the system operates. For the configuration for producing Na metal and as primary cell the catalyst is preferably a bifunctional catalyst for oxygen evolution reaction (OER) and chlorine evolution reaction (GER) . For the configuration as secondary cell or redox flow cell, the catalyst is preferably a tri-functional catalyst for OER, CER and oxygen reduction reaction (ORR) . As a single material will not be capable of fulfilling all requirements the catalyst are made of a mixture of noble metals (for example Pt, Ru, Ir) , oxides (for example IrO ₂ , RuO ₂ , Cobalt-Manganese oxides) and carbon doped with heteroatoms (for example S, N, 0, P, B) . In general a catalyst active for CER is also active for OER, so one catalyst can be used.

In a further embodiment the first and second separators are a solid-state electrolyte preferably a specific conductor for Na ⁺ ions and electron insulator and more preferred a sodium super ionic conductor (NASICON) .

This invention provides in particular a multifunctional electrochemical cell comprising an anode compartment comprising at least an anolyte comprising at least an at least partially unsaturated aromatic hydrocarbon preferably biphenyl (BP) and a first electrode in physical contact with the anolyte and comprising a third electrode in physical contact with the anolyte. Further it comprises a cathode compartment comprising at least a catholyte comprising a source of Na ⁺ and a second electrode preferably coated with a catalyst in physical contact with the catholyte and a first separator placed between the anolyte and the catholyte, preferably a solid electrolyte particularly preferred a specific conductor for Na ⁺ ions and electron insulator. Besides it comprises a sodium storage compartment, which is connected with the anode compartment via at least one fluid connection and/or a second sodium storage compartment connected with the anode compartment via at least a second fluid connection and with the first sodium storage compartment via at least a third fluid connection, wherein the at least one sodium storage compartment comprises at least a fourth electrode and at least a second separator, wherein the second separator is placed between the at least third electrode and the at least fourth electrode. Eventually it comprises a CI ₂ extraction compartment connected with the catholyte compartment via at least a fourth fluid connection.

In a further aspect, the present invention relates to a method for using the said electrochemical cell, wherein the anolyte is introduced in the anolyte compartment before the first charge of the cell. In the case where the anolyte comprises unbounded biphenyl, the reaction forming sodium biphenyl (Na- BP) occurs in the sealed electrochemical cell avoiding the handling of the extremely reactive and dangerous Na-BP. By a pre-charging step Na-BP is first generated in-situ in the anolyte followed by the deposition of Na metal on the first electrode. Afterwards Na metal is reversibly plated or stripped from the first electrode acting as rechargeable anode. Due to its low reactivity, BP can be easily handled in dry conditions, without major safety risks. This represents a potential cost reduction compared to state-of-the-art anolytes but enables also a radically new type of electrochemical cell technology capable of operating in different configurations.

In another particular embodiment, the electrochemical cell is used as primary battery cell by introducing sodium metal in the second sodium storage compartment before the first discharge. In contact with at least partially unsaturated aromatic hydrocarbon, Na metal spontaneously dissolve to Na ⁺ , which is then covalently bound to the at least partially unsaturated aromatic hydrocarbon, preferably Na-BP. The anolyte is pumped in the anode compartment for example with a water pump generating a flow, where the at least partially unsaturated aromatic hydrocarbon bounded to Na is oxidized at the at least first electrode to reform Na ⁺ and the at least unsaturated aromatic hydrocarbon. Na metal continuously dissolves to replenish the anolyte solution. When exhausted, the primary cell can be mechanically recharged by adding fresh sodium in the second sodium storage compartment.

A further aspect of the present invention relates to a method for using said electrochemical cell to produce Na metal. Therefore a positive potential V0 is applied to the second electrode, a negative potential V2 to the fourth electrode and a negative potential V1 to the first electrode and to the third electrode. In a particular embodiment the negative potential V2 of the fourth electrode is lower than the negative potential V1 of the first electrode and the third electrode and is preferably comprised between V2=V1-0,5 V and V2=V1-0.01 V, more preferred between V2=V1-0,2 V. In a further embodiment the charge profile is obtained by applying a current density between 0,1 and 5 mA.cm ^-2 , preferably of 0,5 mA.cm ^-2 .

For the Na metal production or harvesting, the at least partially unsaturated aromatic hydrocarbon (called hereafter PUAH) covalently bound with the sodium is generated in-situ via the migration of the Na ⁺ from the Na ⁺ source preferably the sea water through the first separator by applying a negative potential to the first electrode.

The chemical reaction occurring at the first electrode is: Na+ + PUAH + e- (electron) → Na-PUAH (soluble) The generated → Na-PUAH is circulated in the first sodium storage compartment, where the further reduction of Na-PUAH to Na metal occurs on the third electrode: Na ⁺ + e- → Na (solid) Both, the first separator and the first fluid connection can be used to move sodium ions. Sodium ions can move through the solution flowing by the first fluid connection, and sodium ions can move through the first separator by an electrochemical method (redox reaction) . Even if one of both is not used, sodium metal can be extracted by charging on the electrode. The second sodium storage compartment can be used for the purpose of adding sodium metal to supply additional energy and for the purpose of improving the amount of energy by increasing the amount of solution.

When the electrochemical cell is used as a redox flow cell on contrary to its use as a primary battery cell, the further reduction to Na metal does not occur. Rather, the Na-PAUH produced in the anode compartment is stored into a second sodium storage compartment, which size determines the storage capacity of the battery cell. It can also be stored in the anode compartment or in the first sodium storage compartment. The anolyte is then circulated and the electrochemical cell is operated as a redox flow cell using the Na-PAUH solution as the anolyte. The redox flow cells are used as secondary cell capable of charging and discharging, while sodium ions are stored and used. This occurs as a redox reaction in a circulating of flowing solution. The circulation occurs preferably via pumps located in the first fluid connection and/or the second fluid connection and/or the third fluid connection .

Finally a further aspect of the present electrochemical cell is the use of it as multifunctional device switching between use as primary battery cell and/or use as secondary battery cell and/or use for Na metal production and/or use as redox flow cell and/or for water desalinization and/or for chlorine (CI ₂ ) gas production, preferably as primary battery cell or as secondary primary cell or for the production of Na metal or as redox flow cell or for water desalinization and chlorine (CI ₂ ) gas production. The electrochemical cell can be used as a multifunctional device capable of operating in at least four different configurations :

- for sodium metal production

- as primary battery cell

- as secondary battery cell

- as rechargeable redox flow battery cell.

The advantage of the electrochemical cell is the multifunctionality i.e. the possibility of operating in at least four different configurations in a flexible way for many purposes going from energy storage for short term (primary, secondary and redox flow cell) and long term storage (sodium metal storage) . The size of the compartments (sodium storage unit and redox flow tank) can be adjusted to fit the user's needs .

Using the aromatic hydrocarbon in particular biphenyl only as initial anolyte makes the assembly process easier, safer and therefore cheaper. The assembly process is thus possible as mass production process in a space such as a dry room. Since sodium ions are directly extracted from the cathode, there are advantages in that sodium ions resources can be saved and process steps for preparation of aromatic hydrocarbon bounded to Na especially Na-biphenyl are reduced.

With regards to Na metal production, the presented invention allows extremely high energy efficiencies with respect to the state of the art industrial processes. Indeed, while the energy efficiency of such processes remains confined between 30% and 60%, with the present invention Na metal can be harvested from seawater with an efficiency well beyond 80%, almost approaching 100%. Working examples

The invention will be explained in more detail in the following working examples and figures, which each represent preferred embodiments of the invention, whereby:

Fig. la, b and c show schematic views of the electrochemical cell with 3 different configurations,

Fig. 2a and b shows schematic illustrations of the electrochemical cell setup for sodium metal harvesting, Fig. 2c shows the charge profile using the setup of Fig. 2a, Fig. 3a shows a schematic illustration of the electrochemical cell setup for primary cell employing the biphenyl solution as anolyte and seawater as catholyte,

Fig. 3b shows the discharge profile of the setup shown in Fig. 3a,

Fig. 4a shows a schematic illustration of the electrochemical cell setup for secondary cell,

Fig. 4b shows the pre-charge profile of the setup shown in Fig. 4a,

Fig. 4c shows charge and discharge profiles of the setup shown in Fig. 4a,

Fig. 5a shows a schematic illustration of the electrochemical cell setup for redox flow secondary cell,

Fig. 5b shows the precharge profile of the setup shown in Fig. 5a,

Fig. 5c shows charge and discharge profiles of the setup shown in Fig. 4a and

Fig. 6 shows the voltage vs capacity characteristic of the Na metal production with the present electrochemical cell towards the theoretical one.

Fig. 1a shows a configuration of the present multifunctional electrochemical cell comprising: - an anode compartment 8 comprising at least an anolyte 80 comprising at least an at least partially unsaturated aromatic hydrocarbon preferably biphenyl (BP) and a first electrode 1 in physical contact with the anolyte 80and comprising a third electrode 3 in physical contact with the anolyte 80

- a cathode compartment 7 comprising at least a catholyte 70 comprising a source of Na ⁺ and a second electrode 2 preferably coated with a catalyst 16 in physical contact with the catholyte 70

- a first separator 5 placed between the anolyte 8 and the catholyte 70, preferably a solid electrolyte particularly preferred a specific conductor for Na ⁺ ions and electron insulator.

- a sodium storage compartment 9, which is connected with the anode compartment 8 via at least one fluid connection 12, wherein the at least one sodium storage compartment 9 comprises at least a fourth electrode 4 and at least a second separator 6, wherein the second separator 6 is placed between the at least third electrode 3 and the at least fourth electrode 4.

- a CI ₂ extraction compartment 11 connected with the catholyte compartment 7 via at least a fourth fluid connection 15.

The catholyte 70 enters the catholyte compartment 7 via a channel indicated by an arrow.

VO represents the positive potential applied to the second electrode 2, V2 the negative potential applied to the fourth electrode 4 and V1 the negative potential applied to the first electrode 1 and to the third electrode 3.

The configuration shown in Fig.1b discloses an alternative to the configuration in Fig. la with a second sodium storage compartment 10 connected with the anode compartment 8 via at least a second fluid connection 13 and with the first sodium storage compartment via at least a third fluid connection 14.

Fig. 1c discloses both alternatives of the Fig. 1a and 1c in the same configuration.

The electrochemical cell setup used for sodium metal harvesting employing the biphenyl solution as anolyte and seawater as catholyte is shown in Fig. 2a with the reactions occurring in the cell. The electrodes are physically separated by a NASICON solid electrolyte layer. Nickel foam serves as current collector for the anode and carbon felt for the cathode .

A second setup has been used for sodium metal harvesting shown in Fig. 2b comprising the CI ₂ extraction unit.

Fig. 2c shows the charge profile using the setup of Fig. 2a applying a current density of 0,5 mAh.cm ^-2 up to charge capacity of 3,0 mAh.cm ^-2 . The voltage profile displays two distinct plateaus. The first plateau at lower voltage (<4,0 V, up to about 0.4 mAh.cm ^-2 ) is related to the sodiation of biphenyl into Na-biphenyl (BP + Na ⁺ + e- → Na-BP) The second plateau is related to the Na metal plating (Na ⁺ + e- → Na) on the current collector (Na-metal production) and is located at 4,0 V up to 3.0 mAh . cm ^-2 .

Fig. 3a shows a schematic illustration of the electrochemical cell setup for primary cell employing the biphenyl solution as anolyte and seawater as catholyte. The electrodes are physically separated by a NASICON solid electrolyte layer. Nickel foam serves as current collector for the anode and carbon felt for the cathode.

Fig. 3b shows the discharge profile of the setup shown in Fig. 3a applying a current density of 0,5 mAh.cm-. ² This result shows that the new concept of discharging by putting sodium metal into a biphenyl solution is possible and works. It shows also a performance of sufficiently high discharge voltage and a very high discharge areal capacity.

Fig. 4a shows a schematic illustration of the electrochemical cell setup for secondary cell employing the biphenyl solution as anolyte and seawater as catholyte. The electrodes are physically separated by a NASICON solid electrolyte layer. Nickel foam serves as current collector for the anode and carbon felt for the cathode.

Fig. 4b shows the pre-charge profile of the setup shown in Fig. 4a applying a current density of 0,1 mAh.cm ^-2 (up to charge capacity of 1,0 mAh.cm ^-2 ) ( In the initial state, since sodium ions do not exist in the anode, a peak in the voltage curve is observed due to polarization during the initial charge process. Afterwards, the two plateaus correspond to the sodiation of biphenyl (BP + Na ⁺ + e- → Na-BP) and Na metal plating on the current collector (Na ⁺ + e- → Na) , respectively.

Fig. 4c shows charge and discharge profiles of the setup shown in Fig. 4a of galvanostatic cycle, applying a current density of 0,5 mAh.cm ^-2 (up to charge capacity of 1,0 mAh.cm ^-2 ) . These results show that the new battery system that can store sodium ions as a sodium metal form through a charging process without sodium ions and a negative electrode in the initial state is working. In addition, it is experimentally proven that such a battery system can be used as a rechargeable secondary battery .

Fig. 5a shows a schematic illustration of the electrochemical cell setup for redox flow secondary cell employing the biphenyl solution as anolyte and seawater as catholyte. Fig. 5b shows the precharge profile of the setup shown in Fig. 5a applying a current density of 0,1 mAh.cm ^-2 (up to charge capacity of 1,0 mAh.cm ^-2 ) . The electrodes are physically separated by a NASICON solid electrolyte layer. Nickel foam serves as current collector for the anode and carbon felt for the cathode. In the initial state, since sodium ions do not exist in the anode, some overpotential is observed during the initial stage of the charge. Afterwards, sodium ions are stably reacted with biphenyl (the sodiation into the biphenyl; BP + Na+ + e- → Na-BP) .

Fig. 5c shows charge and discharge profiles of the setup shown in Fig. 5a of galvanostatic cycle, applying a current density of 0,5 mAh.cm ^-2 (up to charge capacity of 0,5 mAh.cm ^-2 ) . Sodium biphenyl (Na-BP) provides reversible redox behaviour over 20 cycles in the voltage range comprised between 2, 0 to 4, 0 volts. These results show that such a system can be used as a stable redox flow secondary cell.

Seetharaman et al. in Industrial Processes, Elsevier, Amsterdam, The Netherland 2014 and Ito et al. in Electrochemical Society, Pennington NJ 2000, Vol. 99 pp.1-11 show that the two most common industrial processes namely the Downs and Castner processes have around from 30% to 60% energy efficiency. Fig. 6 shows an energy efficiency reached with the present electrochemical cell of about 87%. List of numerical references

1 f irst electrode

2 second electrode

3 third electrode

4 fourth electrode

5 f irst separator

6 second separator

7 cathode compartment

8 anode compartment

9 f irst sodium storage compartment

10 second sodium storage compartment

11 CI ₂ extraction unit

12 f irst channel

13 second channel

14 third channel

15 fourth channel

16 catalyst

70 catholyte

80 anolyte