Non-aqueous redox flow batteries

Abstract

Non-aqueous redox flow battery (RFB) comprising: a positive compartment in which a positive electrode is positioned and in which a positive non-aqueous liquid electrolyte is caused to flow; a negative compartment in which a negative electrode is positioned and in which a negative non-aqueous liquid electrolyte is caused to flow; an ion-exchange membrane positioned between the positive compartment and the negative compartment in which: said positive non-aqueous liquid electrolyte comprises a solution of copper triflate or tetrafluoroborate complexes [Cu(I) or Cu(II)] in at least one organic solvent; said negative non-aqueous liquid electrolyte comprises a solution of at least one benzothiadiazole or a derivative thereof in at least one organic solvent.

Claims

1. Non-aqueous redox flow battery (RFB) comprising: a positive compartment in which a positive electrode is positioned and in which a positive non-aqueous liquid electrolyte is caused to flow; a negative compartment in which a negative electrode is positioned and in which a negative non-aqueous liquid electrolyte is caused to flow; an ion-exchange membrane positioned between the positive compartment and the negative compartment in which: said positive non-aqueous liquid electrolyte comprises a solution of copper triflate or tetrafluoroborate complexes [Cu(I) or Cu(II)] in at least one organic solvent; said negative non-aqueous liquid electrolyte comprises a solution of at least one benzothiadiazole in at least one organic solvent.

2. Non-aqueous redox flow battery (RFB) according to claim 1, in which said copper triflate or tetrafluoroborate complexes [Cu(I) or Cu(II)] are selected from: tetrakisacetonitrile copper(I) triflate [Cu(NCCH.sub.3).sub.4.CF.sub.3SO.sub.3], copper(II) trifluoromethanesulfonate [Cu(CF.sub.3SO.sub.3).sub.2], tetrakisacetonitrile copper(I) tetrafluoroborate [Cu(NCCH.sub.3).sub.4.BF.sub.4], or mixtures thereof.

3. Non-aqueous redox flow battery (RFB) according to claim 1, in which said benzothiadiazole is selected from benzothiadiazoles having general formula (I): ##STR00005## in which R.sub.1, R.sub.2, R.sub.3 and mutually identical or different, represent a hydrogen atom, or a halogen atom; or represent one of the following groups: CN, NO.sub.2, COOH, SO.sub.3H, SH; or are selected from: linear or branched, saturated or unsaturated C.sub.1-C.sub.10 alkyl groups, linear or branched, saturated or unsaturated C.sub.1-C.sub.10 alkoxy groups, carboxylic: esters having general formula RCOOR in which R and R, mutually identical or different, are selected from linear or branched, saturated or unsaturated C.sub.1-C.sub.10 alkyl groups, sulfonic esters having general formula ROSO.sub.2R in which R and R have the same meanings described above, thioesters having general formula RSOR in which R and R have the same meanings described above, (OCH.sub.2CH.sub.2).sub.nOH groups in which n is an integer ranging from 1 to 4, (OCH(CH.sub.3)CH.sub.2).sub.nOH groups in which n is an integer ranging from 1 to 4, optionally substituted aryl groups, or optionally substituted heteroaryl groups.

4. Non-aqueous redox flow battery (RFB) according to claim 3, in which in said general formula (I), R.sub.1, R.sub.2, R.sub.3 and R.sub.4, mutually identical, represent a hydrogen atom.

5. Non-aqueous redox flow battery (RFB) according to claim 1, in which said electrolytes comprise at least one supporting electrolyte selected from lithium tetrafluoroborate (LiBF.sub.4), lithium hexafluorophosphate (LiPF.sub.6), lithium perchlorate (LiClO.sub.4), lithium methyltrifluoromethanesulfonate (LiCF.sub.3SO.sub.3), lithium bis(trifluoromethylsulfonyl)imide [Li(CF.sub.3SO.sub.2).sub.2N], tetraethylammonium tetrafluorborate (TBABF.sub.4), tetrabutylammonium tetrafluorborate (TBABF.sub.4), or mixtures thereof.

6. Non-aqueous redox flow battery (RFB) according to claim 1, in which said organic solvent is selected from acetonitrile, dimethylacetamide, diethyl carbonate, dimethyl carbonate, -butyrolactone (GBL), propylene carbonate (PC), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), fluoroethylene carbonate, N,N-dimethylacetamide, or mixtures thereof.

7. Non-aqueous redox flow battery (RFB) according to claim 1, in which said ion-exchange membrane is selected from the following polymeric membranes: anion-exchange membranes selected from: membranes based on a styrene-divinylbenzene copolymer or on a chloromethylstyrene-divinylbenzene copolymer containing amino groups, membranes based on poly(ether ether ketones), membranes based on a divinylbenzene-vinylpyridine copolymer containing a quaternary pyridine group; membranes based on an aromatic polysulfonic copolymer containing a chloromethyl group and amino groups, or membranes based on polytetrafluoroethylene (PTFE); cation-exchange membranes selected from: membranes based on a fluoropolymer-copolymer based on tetrafluoroethylene sulfonate, membranes based on poly(ether ether ketones), membranes based on polysulfones, membranes based on polyethylene, membranes based on polypropylene, membranes based on ethylene-propylene copolymers, membranes based on polyimides, or membranes based on polyvinyl fluorides.

Description

BRIEF DESCRIPTION OF THE DRAWINGS:

(1) FIG. 1 is a schematice representation of an embodiments of a non-aqueous redox flow battery (RFB) according to the present disclosure.

(2) FIG. 2 shows a cyclic voltammogram obtained from solutions referred to in Example I below, BTD and Cu triflate in acetonitrileI at a scanning speed of 200 mV/s.

(3) FIG. 3 shows a cyclic voltammogram obtained from solutions referred to in Example I below, BTD and Cu(I) tetrafluoroborate in acetonitrile, at a scanning speed of 200 mV/s.

(4) FIG. 4 shows a cyclic voltammogram obtained from solutions referred to in Example I below BTD and CU (I) in ro lene carbonate at a scanning speed of 200 mV/s.

(5) FIG. 5 shows 150 consecutive cycles carried out for the solution referred to in Example 2 below BTD in acetonitrile.

(6) FIG. 6 shows 150 consecutive cycles carried out for the solution referred to in Example 2 below, Cu triflate.

(7) FIG. 7 shows a charge/discharge curve carried out for the solutions referred to in Example 3 below, BTD and copper(ll) trifluoromethanesulfonate Cu(CF.sub.3SO.sub.3).sub.2] in acetonitrile.

(8) FIG. 8 shows a charge/discharge curve carried out for the solutions referred to in Example 4 below BTD and Cu(I) in propylene carbonate.

(9) The present invention will now be illustrated in more detail by an embodiment with reference to FIG. 1 shown below.

(10) In particular, FIG. 1 is a schematic representation of an embodiment of a non-aqueous redox flow battery (RFB) according to the present invention. In this connection, the non-aqueous redox flow battery (RFB) (1) comprises a positive compartment (6a) in which a positive electrode (6) is positioned and in which a positive non-aqueous liquid electrolyte (not shown in FIG. 1) is caused to flow, a negative compartment (8a) in which a negative electrode (8) is positioned and in which a negative non-aqueous liquid electrolyte (not shown in FIG. 1) is caused to flow and an ion-exchange membrane (7) positioned between the positive compartment (6a) and the negative compartment (8a).

(11) The positive compartment (6a) is connected to a tank (2) containing the positive non-aqueous liquid electrolyte comprising a solution of copper triflate or tetrafluoroborate complexes [Cu(I) or Cu(II)] in at least one organic solvent, by an inlet tube (3) and a pump (4a) (for example, a peristaltic pump) and an outlet tube (5) so as to allow the feeding and discharge of said positive non-aqueous liquid electrolyte during the operating cycle (i.e. during the charge-discharge phase).

(12) The negative compartment (8a) is connected to a tank (12) containing the negative non-aqueous liquid electrolyte comprising a solution of at least one benzothiadiazole or a derivative thereof in at least one organic solvent, by an inlet tube (11) and a pump (4b) (for example, a peristaltic pump) and an outlet tube (10) so as to allow the feeding and discharge of said negative non-aqueous liquid electrolyte during the operating cycle (i.e. during the charge-discharge phase).

(13) A voltmeter (9) is connected to the positive electrode (6) and to the negative electrode (8). During the charge phase of the non-aqueous redox flow battery (RFB) (1), a potential difference is applied between the positive electrode and the negative electrode by the voltmeter (9) while, simultaneously, the positive non-aqueous liquid electrolyte is fed by the pump (4a) from the positive electrolyte tank (2) to the positive compartment (6a) and the negative non-aqueous liquid electrolyte is fed by the pump (4b) from the negative electrolyte tank (12) to the negative compartment (8a). Said positive non-aqueous liquid electrolyte present in the positive compartment (6a) undergoes an oxidation reaction on the positive electrode (6) and said negative non-aqueous liquid electrolyte present in the negative compartment (8a) undergoes a reduction reaction on the negative electrode (8): the ions involved in the above-stated oxidation and reduction reactions flow through the ion-exchange membrane (7) in the opposite direction to balance the charges. The reverse reactions occur during the discharge phase of the non-aqueous redox flow battery (RFB) (1). The above-stated charge phase and discharge phase may be schematically represented as follows:

(14) negative electrode:

(15) ##STR00002##
positive electrode:

(16) ##STR00003##
cell:

(17) ##STR00004##
in which: BTD=benzothiadiazole; Cu=copper; e.sup.=electrons.

(18) During the operating cycle (i.e. during the charge-discharge phase) both the positive non-aqueous liquid electrolyte and the negative non-aqueous liquid electrolyte, are continuously pumped within the positive and negative compartments, respectively, in order to feed said positive and negative compartments continuously.

(19) The energy stored in the non-aqueous redox flow battery (RFB) (1), may be directly used for operating the apparatus in which it is installed, or may be transferred into an electrical network during peak consumption periods to integrate the power supply. An alternating current/direct current (AC/DC) converter (not shown in FIG. 1) may optionally be used to facilitate transfer of energy to and from an alternating current (AC) power supply network. The present invention will be further illustrated below by means of the following examples which are stated for purely indicative purposes and without limiting the present invention in any way.

EXAMPLE 1

(20) Cyclic Voltammetry Measurements

(21) The cyclic voltammetry measurements were carried out in a half-cell with a three electrode configuration, a glassy carbon working electrode, a platinum counter-electrode and a silver/silver chloride (Ag/AgCl) reference electrode. The redox potentials E.sup.o.sub.Ox/Red were obtained from the position of the forward peak (E.sub.pf) and the return peak (E.sub.pr):
E.sup.o.sub.Ox/Red=(E.sub.pf+E.sub.pr)
and the values were normalized with reference to the intersolvent ferrocene/ferrocenium (Fc/Fc.sup.+) couple.

(22) The evaluations were carried out on an Autolab PGSTAT 128N analytical instrument at a scanning speed of 10, 20, 50, 70, 100, and 200 mV/s. All the evaluations were carried out in triplicate at room temperature (25 C.). The solutions used for this purpose contained: benzothiadiazole (Aldrich) (510.sup.4M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (negative non-aqueous liquid electrolyte of the negative compartment) (BTD); benzothiadiazole (Aldrich) (510.sup.4M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in propylene carbonate (Aldrich) (negative non-aqueous liquid electrolyte of the negative compartment) (BTD); copper(II) trifluoromethanesulfonate [Cu(CF.sub.3SO.sub.3).sub.2] (Aldrich) (510.sup.4M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (positive non-aqueous liquid electrolyte of the positive compartment) (Cu triflate); tetrakisacetonitrile copper(I) tetrafluoroborate [Cu(NCCH.sub.3).sub.4.BF.sub.4] (Aldrich) (510.sup.4M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (positive non-aqueous liquid electrolyte of the positive compartment) [Cu(I) tetrafluoroborate]; tetrakisacetonitrile copper(I) triflate [Cu(NCCH.sub.3).sub.4.CF.sub.3SO.sub.3] (Aldrich) (510.sup.4M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in propylene carbonate (Aldrich) (positive non-aqueous liquid electrolyte of the positive compartment) [Cu(I)].

(23) FIG. 2 [potential (E) measured in volts (V) is reported on the x axis and current density (J) measured in amperes/cm.sup.2 (A cm.sup.2) is reported on the y axis] shows the cyclic voltammogram obtained from the above-stated solutions (BTD) and (Cu triflate) in acetonitrile, at a scanning speed of 200 mV/s. A high open-circuit potential difference (E.sup.o) of 2.52 V calculated according to the following formula may be observed to be obtained:
E.sup.o=(E.sup.o.sub.1)(E.sup.o.sub.2)
in which: (E.sup.o.sub.1) is the redox potential for (Cu triflate) calculated as described above and is 0.62 V against (Fc/Fc.sup.+); (E.sup.o.sub.2) is the redox potential for (BTD) calculated as described above and is 1.90 V against (Fc/Fc.sup.+).

(24) FIG. 3 [potential (E) measured in volts (V) is reported on the x axis and current density (J) measured in amperes/cm.sup.2 (A cm.sup.2) is reported on the y axis] shows the cyclic voltammogram obtained from the above-stated solutions (BTD) and [Cu(I) tetrafluoroborate] in acetonitrile at a scanning speed of 200 mV/s.

(25) A high open-circuit potential difference (E.sup.o) of 2.52 V calculated according to the following formula may be observed to be obtained:
E.sup.o=(E.sup.o.sub.1)(E.sup.o.sub.2)
in which: (E.sup.o.sub.1) is the redox potential for [Cu(I) tetrafluoroborate] calculated as described above and is 0.62 V against (Fc/Fc.sup.+); (E.sup.o.sub.2) is the redox potential for BTD calculated as described above and is 1.90 V against (Fc/Fc+).

(26) FIG. 4 [potential (E) measured in volts (V) is reported on the x axis and current density (J) measured in amperes/cm.sup.2 (A cm.sup.2) is reported on the y axis] shows the cyclic voltammogram obtained from the above-stated solutions (BTD) and [Cu(I)] in propylene carbonate at a scanning speed of 200 mV/s.

(27) A high open-circuit potential difference (E.sup.o) of 2.29 V calculated according to the following formula may be observed to be obtained:
E.sup.o=(E.sup.o.sub.1)(E.sup.o.sub.2)
in which: E.sup.o.sub.1 is the redox potential for [Cu(I)] calculated as described above and is 0.43 V against (Fc/Fc.sup.+); E.sup.o.sub.2 is the redox potential for (BTD) calculated as described above and is 1.86 V against (Fc/Fc.sup.+).

EXAMPLE 2

(28) Cyclic Voltammetry Stability Test

(29) The stability tests were carried out using the same electrochemical cell as in Example 1. The solutions used for this purpose contained: benzothiadiazole (Aldrich) (210.sup.3 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) 0.1 M in acetonitrile (Aldrich) (negative non-aqueous liquid electrolyte of the negative compartment) (BTD); copper(II) trifluoromethanesulfonate [Cu(CF.sub.3SO.sub.3).sub.2] (Aldrich) (210.sup.3 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (non-aqueous liquid electrolyte of the positive compartment) (Cu triflate).

(30) FIG. 5 [potential (E) measured in volts (V) is reported on the x axis and current density (i) measured in amperes (A) is reported on the y axis] shows the 150 consecutive cycles carried out for the above-stated solution of BTD: it may be noted how the cycles can be superimposed, which means that no deposition of material occurs on the electrode due to parasitic reactions or polymerisation reactions and that the radical which is formed is stable.

(31) FIG. 6 [potential (E) measured in volts (V) is reported on the x axis and current density (i) measured in amperes (A) is reported on the y axis] shows the 150 consecutive cycles carried out for the above-stated solution of (Cu triflate): it may noted how the cycles can be superimposed, which indicates good stability of the redox couple.

EXAMPLE 3

(32) Charge/Discharge Tests of the Non-Aqueous Redox Flow Battery (RFB) [Electrolytes: Benzothiadiazole (BTD) and Copper(II) Trifluoromethanesulfonate [Cu(CF.sub.3SO.sub.3).sub.2] in Acetonitrile]

(33) The charge-discharge tests were carried out using an electrochemical cell with Teflon membrane (DuPont), having a surface area of about 0.8 cm.sup.2, placed between two platinum electrodes (Metrohm) having a surface area of about 0.7 cm.sup.2. The electrochemical cell was then assembled and sealed in a container containing argon (Ar). The solutions used for this purpose contained: benzothiadiazole (Aldrich) (1 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (negative non-aqueous liquid electrolyte of the negative compartment) (BTD), degassed with argon (Ar) and subjected to electrolysis in order to obtain benzothiadiazole in reduced form (BTD..sup.); copper(II) trifluoromethanesulfonate [Cu(CF.sub.3SO.sub.3).sub.2] (1 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (positive non-aqueous liquid electrolyte of the positive compartment) (Cu triflate), degassed with argon (Ar).

(34) 6 ml of the above-stated solutions were introduced into the respective compartments. The test was carried out using an Autolab PGSTAT 128N potentiostat/galvanostat (Metrohm) at room temperature (25 C.).

(35) Charge and discharge curves were carried out to evaluate the performance of the electrolytes in the cell. The tests were carried out in potentiostatic mode by applying a charge potential of 1.1 V and a discharge potential of 0.5 V. Each potential was applied for a period of 120 seconds.

(36) FIG. 7 [time measured in seconds (t/s) is reported on the x axis; current density (J) measured in milliamperes/cm.sup.2 (mA cm.sup.2) is reported on the y axis] shows the obtained charge/discharge curve. During discharge, the current has a negative sign due to the flow of electrons from the negative pole (BTD) to the positive pole (Cu). Conversely, during charging, the current has a positive sign. The current density values are stable and consequently both species are characterised by good stability during the oxidation-reduction (or redox) cycles.

EXAMPLE 4

(37) Charge/Discharge Tests of the Non-Aqueous Redox Flow Battery (RFB) [Electrolytes: Benzothiadiazole (BTD) and Tetrakisacetonitrile Copper(I) Triflate [Cu(I)] in Propylene Carbonate]

(38) The charge-discharge tests were carried out using the same electrochemical cell as in Example 3.

(39) The solutions used for this purpose contained: benzothiadiazole (Aldrich) (1 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in propylene carbonate (Aldrich) (negative non-aqueous liquid electrolyte of the negative compartment) (BTD), degassed with argon (Ar); tetrakisacetonitrile copper(I) triflate [Cu(NCCH.sub.3).sub.4.CF.sub.3SO.sub.3] (1 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in propylene carbonate (positive non-aqueous liquid electrolyte of the positive compartment) [Cu(I)].

(40) 6 ml of the above-stated solutions were introduced into the respective compartments. The test was carried out using an Autolab PGSTAT 128N potentiostat/galvanostat (Metrohm) at room temperature (25 C.).

(41) Charge and discharge curves were carried out to evaluate the performance of the electrolytes in the cell. The tests were carried out in potentiostatic mode by applying a charge potential of 1.1 V and a discharge potential of 0.5 V. Each potential was applied for a period of 120 seconds.

(42) FIG. 8 [time measured in seconds (t/s) is reported on the x axis; current density (J) measured in milliamperes/cm.sup.2 (mA cm.sup.2) is reported on the y axis] shows the resultant charge/discharge curve. During discharge, a negative current is obtained due to the flow of electrons from the negative pole (BTD) to the positive pole (Cu). Conversely, during charging, the current has a positive sign. The current density values are stable and consequently both species are characterised by good stability during the oxidation-reduction (or redox) cycles.

EXAMPLE 5

(43) Calculation of Energy Density

(44) The energy density (.sub.e) of a non-aqueous redox flow battery (RFB) is defined as the chemical energy contained in both the compartments (i.e. positive compartment and negative compartment) of the charged battery per unit volume.

(45) For each individual compartment (i.e. positive compartment and negative compartment) of the non-aqueous redox flow battery (RFB) it is possible to define the specific capacity (.sub.c) [expressed in amp hours/litre (Ah/l)] of the solution according to the following equation:
.sub.c=26.8.Math.conc.Math.n
in which conc is the concentration of the active species and n is the number of electrons involved in the reaction.

(46) The energy density (.sub.e) [expressed in watt hours/litre (Wh/l)] of the non-aqueous redox flow battery (RFB) is defined according to the following equation:

(47) ${}_e=\frac{\min \left({}_{c+}.Math.V_{+};{}_{c-}.Math.V_{-}\right)}{V_{+}+V_{-}}.Math.E_0$
in which: min is the minimum value between the two products placed in the numerator; c.sub.+ is the specific capacity measured at the positive pole [expressed in amp hours/litre (Ah/l)]; V.sub.+ is the volume of the positive non-aqueous liquid electrolyte solution [expressed in litres (I)]; c.sub. is the specific capacity measured at the negative pole [expressed in amp hours/litre (Ah/l)]; V.sub. is the volume of the negative non-aqueous liquid electrolyte solution [expressed in litres (I)]; E.sub.0 is the thermodynamic reaction potential on discharge [expressed in volts (V)].

(48) The following equation must be satisfied in order to have a balanced non-aqueous redox flow battery (RFB) with an equal charge both at the negative pole and at the positive pole:
.sub.c+.Math.V.sub.+=.sub.c.Math.V.sub.
in which c.sub.+, V.sub.+, c.sub. and V.sub. have the same meanings described above.

(49) In order to obtain high energy densities, it is thus important to maximise the following parameters: the concentration in solution of the reacting species; the number of electrons transferred into the positive compartment and into the negative compartment; the electrochemical cell potential.

(50) In the case of a non-aqueous redox flow battery (RFB) containing the following solutions: benzothiadiazole (Aldrich) (510.sup.4 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (negative non-aqueous liquid electrolyte of the negative compartment) (BTD), degassed with argon (Ar); tetrakisacetonitrile copper(I) triflate [Cu(NCCH.sub.3).sub.4.CF.sub.3SO.sub.3] (Aldrich) (510.sup.4 M) and tetrabutylammonium tetrafluoroborate (TBABF.sub.4) (Aldrich) (0.1 M) in acetonitrile (Aldrich) (positive non-aqueous liquid electrolyte of the positive compartment) [Cu(I)];
the theoretical energy density (.sub.e) is 51 Wh/l, said theoretical energy density (.sub.e) having been calculated assuming: the process is monoelectronic; the maximum concentration of the species is 1.5 M; the open-circuit potential difference value (E.sup.o) is 2.52 V.