EP3231031A1

EP3231031A1 - Method for regenerating the electrolyte solution of a rechargeable redox flow battery

Info

Publication number: EP3231031A1
Application number: EP15820453.7A
Authority: EP
Inventors: Jens Noack; Peter Fischer; Jens TÜBKE; Karsten Pinkwart
Original assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Current assignee: Fraunhofer Gesellschaft zur Forderung der Angewandten Forschung eV
Priority date: 2014-12-11
Filing date: 2015-12-10
Publication date: 2017-10-18
Also published as: WO2016092004A1; DE102014225612A1; KR20170094343A; JP2018503222A; CA2970178A1

Abstract

The present invention relates to a method for releasing oxygen from an aqueous electrolyte solution of a rechargeable redox flow battery, wherein at least two electrodes (E) are in electrically conductive contact with the electrolyte solution, at least one of the electrodes (E) is connected as an anode and at least one of the said electrodes is connected as a cathode and oxygen (O2) is formed on the anode, wherein no hydrogen or not more than 1.5 mol of hydrogen (H2) per 1.0 mol of oxygen (O2) formed is/are formed on the cathode. The invention further relates to an electrochemical cell comprising at least two electrodes (E), of which at least one is connected as a cathode and at least one is connected as an anode, wherein the electrodes (E) are located in at least one half-cell of a rechargeable redox flow battery and/or there is a fluid connection between the electrochemical cell and at least one half-cell of a rechargeable redox flow battery. The invention further relates to the use of electric current for releasing oxygen from an electrolyte solution of a rechargeable redox flow battery.

Description

Process for the regeneration of the electrolyte solution of a redox flow accumulator

Electrical energy can be stored by various processes. One possibility is the conversion of electrical energy into chemical energy by chemical reactions on electrode surfaces by electric current. This type of energy storage is used technically in secondary batteries (accumulators) on a large scale.

A secondary battery is an electrochemical cell that consists of two half-cells. The two half-cells are usually separated by an ion-conducting separator. The separator ensures a charge balance, but prevents the mass transfer between the half-cells. In the negative half cell takes place during the storage process, a reduction of the active material, in the positive half-cell oxidation. In the storage process, electrons thus flow from the positive half-cell into the negative half-cell, in the discharge process in the opposite direction. In order to allow a compensation of the charge and a movement of the ions, in both half-cells as an inner conductor is a liquid substance or

Mixture of substances, referred to as electrolyte, necessary. The electrode is the phase boundary between the electrical conductor and the ionic conductor. The active material may be the electrode itself, a substance dissolved in the electrolyte or substances stored in the electrode material.

If the active material consists of substances dissolved in the electrolyte, then the result is that with this type of battery the amount of energy and the power can be scaled independently of each other, since the electrolyte flows from reservoirs to the

Electrodes can be passed. This type of electrochemical

Energy storage is called redox flow accumulator and includes a positively charged electrolyte component (catholyte) and a negatively charged

Electrolyte component (anolyte). The general battery equations for redox flow accumulators are as follows:

The full capacity of the redox flow accumulator is only available as long as equivalent amounts of anolyte and catholyte are provided.

During transport, storage or operation of the accumulator, a can

Full exclusion of air often can not be guaranteed and oxidation processes can occur, causing the Elyte to become higher

Oxidation levels are shifted, resulting in a loss of capacity. This problem is known in the prior art and it has hitherto been attempted to prevent the oxidation processes by inerting the electrolyte solution. For example, nitrogen was used as the inert gas or inert organic liquids having a lower density than the electrolytic solution in the reservoirs so as to prevent contact of the electrolytic solution with atmospheric oxygen. However, due to Umwälzvorgängen or diffusion processes such a contact of the electrolyte solution with atmospheric oxygen can not be completely prevented. There is therefore a need for methods in which the described loss of capacity as a result of the oxidation of the Elyte can be reversed. These and other objects are achieved by the method according to the invention and the electrochemical cell according to the invention.

Therefore, the present invention provides a method for oxygen release from an aqueous electrolyte solution of a redox flow accumulator, wherein at least two electrodes (E) with the electrolyte solution in electrically conductive

Contact, of the electrodes (E) is at least one as an anode and at least a switched as a cathode and at the anode, oxygen (O ₂₎ is formed, wherein formed at the cathode is not hydrogen or per 1.0 mole of formed oxygen (O ₂₎ not more than 1.5 moles of hydrogen (H _2).

Surprisingly, it has been found that the electrolyte solution of a redox flow accumulator can be regenerated by means of electrolysis. For example, the regeneration can take place during the operation of the redox flow accumulator, for example during the charging process, without interruptions in operation being necessary. Regeneration in the batch process, for example by separation of a part of the electrolyte solution, release of oxygen according to the method of the present invention, and subsequent return to the redox flow accumulator is also possible. Finally, one can

Electrolytic solution prior to addition to the redox flow accumulator the method according to the present invention are subjected.

Instead of a complete electrolysis of water, instead of H ⁺ ions, the constituents of the electrolyte are reduced.

In the context of the present invention, the term "redox flow accumulator" is used in its usual meaning The basic structure of a redox flow accumulator, for example a vanadium redox flow accumulator, is known to the person skilled in the art The invention uses the term "electrochemical cell" for an electrolytic cell and refers to an array of electrodes which are conductively connected by an electrolyte. The passage of electricity through the electrolyte causes a chemical change resulting in a direct conversion of electrical energy into chemical energy

Expressing electrode reactions and ion migration.

In the context of the present invention, the term "electrolyte" is used in its usual meaning and refers to ion-conducting media electrical conductivity is caused by electrolytic dissociation in ions.

The positive half cell of the redox flow accumulator is the part of the galvanic cell of the redox flow accumulator which represents the plus pole of the redox flow accumulator, based on the current draw.

When the term "half-cells" is used in the present application, this refers to the half-cells of the galvanic cell of the redox-flow accumulator, unless explicitly stated to the contrary.

In the context of the present invention, the term "anolyte" denotes the material of a redox flow accumulator which, during discharging, is in direct influence of the anode The corresponding half cell is the negative half cell.

In the context of the present invention, the term "catholyte" refers to the material of a redox flow accumulator which, during discharging, is in direct influence of the cathode The corresponding half cell is the positive half cell. "Circulation or" circulation of the redox flow accumulator " etc. refers to one of the two separate circuits of the redox flow accumulator. These circuits are separated in the galvanic cell of the redox flow accumulator by a membrane.

Accordingly, there is the circulation of the positive half cell and the cycle of the negative half cell.

As described above, the capacity of a redox flow accumulator is determined by the amount of active material dissolved in the electrolyte and can only be fully utilized if an equivalent mixture of anolyte and catholyte is present. Due to unwanted

Oxidation processes, the equimolar mixture of anolyte and catholyte in Directed higher oxidation levels shifted which has a capacity loss of the accumulator result.

In the case of a vanadium redox flow accumulator, this means, for example, that in the fully charged state in the catholyte exclusively V ⁵⁺ is present while in the anolyte a mixture of V ²⁺ and V ³⁺ is present.

In the context of this invention it has been found that it is possible to

To reverse oxidation processes on the electrolyte solution of a redox flow accumulator by electrolysis.

As stated above, at least two electrodes (E) are in electrically conductive contact with the electrolyte solution, wherein at least one of the electrodes (E) is connected as anode and at least one as cathode and oxygen (O ₂ ) is formed at the anode.

The electrodes (E) may be part of an electrolysis cell. The process may be batch or continuous, preferably continuous.

The electrodes (E) may also contain one or more, usually one, reference electrode (s) and / or one or more, usually one, electrode (s) for measuring the redox potential of the electrolyte solution.

The electrodes (E) can only consist of anode (s) and cathode (s). However, the potential of the cathode should then not be too negative by one

Prevent hydrogen evolution. Should hydrogen nevertheless be produced, it can be converted back to water with oxygen, as explained in detail below.

By means of a reference electrode, the potential of the cathode can be regulated. Preferably, therefore, a reference electrode is present, more preferably the cathodic potential is chosen so that no hydrogen is formed. As a reference electrode, for example, a Hg / Hg ₂ SO ₄ - reference electrode Ag / AgCl reference electrode or a suitable

Normal hydrogen electrode. The reference electrode is also referred to as a reference electrode. Both terms are used synonymously in this application. By means of the electrode for measuring the redox potential of the electrolyte solution, the course of the reaction of the oxygen release can be followed as explained in detail below.

Any electrode suitable for determining the redox potential of the electrolyte solution can be used as the electrode for measuring the redox potential of the electrolyte solution and is selected accordingly by a person skilled in the art. Usually, inert electrodes such as, for example, carbon-based electrodes, platinum or gold electrodes, preferably one

Glassy carbon electrode used.

Preferably, the electrodes (E) consist of - one or more anode (s), one or more cathodes (n), optionally one or more reference electrodes optionally one or more, usually 1 or 2 electrodes for measuring the redox potential , To determine the redox potential, for example, a combination electrode can be used which normally includes two electrodes. Alternatively, two electrodes may be used which are not connected to a single-rod measuring chain, for example two separate electrodes.

The redox potential can also be determined relative to the reference electrode, if present. Usually, the electrodes (E) then contain, preferably consist of, only one electrode for measuring the redox potential. Usually, a combination electrode is used or the redox potential is determined relative to the reference electrode.

If the reference electrode is used to measure the redox potential, it will not be counted among the electrodes for redox potential measurements. Usually, an anode and a cathode are used. Alternatively, multiple cathodes and / or anodes may be used. The use of multiple cathodes and multiple anodes may be advantageous, for example, if higher powers are desired. If several anodes and / or cathodes are used, the above-mentioned quantities of oxygen and hydrogen formed refer to the respective total amount of oxygen formed and hydrogen formed.

It is also possible that the oxygen release at several points in the redox flow accumulator is performed simultaneously or sequentially. In this case, the method of the present invention is carried out several times simultaneously or sequentially.

If present, usually only one reference electrode is present.

If present, usually only one or two electrodes are present for measuring the redox potential. As stated above, the determination of the redox potential by means of a combination electrode, usually containing two electrodes, two separate electrodes or by means of an electrode over the

Reference electrode can be determined.

Particularly preferably, the electrodes (E) are made

an anode, a cathode, optionally a reference electrode - Optionally one or two electrodes (s) for measuring the redox potential, for example, two electrodes in the form of a combination electrode.

Preferably, the electrodes (E) are not the electrodes used for current drain from the redox flow accumulator. In this preferred embodiment, the electrodes (E) include all electrodes other than the electrodes

Electrodes are used to draw current from the redox flow accumulator. In other words, in this preferred embodiment, the redox flow accumulator contains only the electrodes that are used for current drain from the redox flow accumulator and the electrodes (E). The efficiency of the oxygen release can be increased if the anode (s) of the electrodes (E) has a catalyst which facilitates the oxidation of H ₂ O to O ₂ . In one embodiment, the anode (s) of the electrodes (E) comprise a catalyst selected from (i) Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (ii) alloys of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (iii) oxides of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, preferably selected from (i) Fe , Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (ii) alloys of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (iii) oxides of Fe , Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt. In a particularly preferred embodiment, the anode (s) consist of Ir.

It is also possible to use a dimensionally stable electrode (DSA) as the anode (s) of the electrodes (E). Dimensionally stable electrodes are with regard to their

Microstructure, their phase composition and also their surface is not homogeneous. Dimensionally stable electrodes are generally titanium electrodes which are provided with a mixed oxide coating of titanium oxide with one or more rutile-type noble metal oxides (eg Ru _x Ti ₁ - _x O ₂ ). In one embodiment, the anode (s) of the electrodes (E) are constructed of a coating of a mixed oxide of the noble metals Pt, Ir, Rh, Pd, Ru and Os with the elements Mn, Pb, Cr, Co, Fe, Ti , Ta, Zr and Si are on a carrier Anode of Ti, Ta, Zr, Nb or their alloys is applied. Particularly preferred dimensionally stable electrode (s) (DSA) as the anode (s) of the electrodes (E) comprises / comprise a titanium support and a coating of TiO ₂ -RuO ₂ or TiO ₂ -RuO ₂ .

At the cathode of the electrodes (E) takes place during the release of oxygen, the reduction of the electrolyte instead. For example, in the case of a vanadium redox flow accumulator, the reduction of VO ₂ ⁺ to lower oxidation states takes place.

The potential of the cathode of the electrodes (E) should be as low as possible to ensure efficient reduction, with the potential of the cathode being limited by the potential at which hydrogen is formed by the electrolytic decomposition of the aqueous solution.

The electrochemical potential window in an aqueous solution, i. H. of the

Voltage range of the required for the desired reaction potential and the decomposition of water is dependent on the material of the electrode. Metallic electrodes usually have a relatively small potential window or form passivating layers. A large electrochemical potential window can be achieved by a high hydrogen overvoltage. In aqueous systems, carbon has diamond (doped), graphite, as well as modifications in its modifications

Glassy carbon, lead, zinc, cadmium and mercury a high hydrogen overvoltage.

Preferably, the cathode (s) of the electrodes (E) comprise a material selected from the group consisting of glassy carbon, graphite and diamond. Preferably, the cathode (s) of the electrodes (E) consist of one

Material selected from the group consisting of glassy carbon, graphite and diamond (doped). Particularly preferred is a graphite electrode. No hydrogen or per 1.0 mol formed oxygen (O ₂₎ not more than 1.5 moles of hydrogen (H ₂₎ is performed as described above formed at the cathode. If hydrogen is formed, it is preferably formed per 1.0 mol

Oxygen (O ₂ ) formed not more than 1.0 mol of hydrogen (H ₂ ), more preferably not more than 0.5 mol of hydrogen (H ₂ ). In a preferred

Embodiment is formed not more than 0.1 mol of hydrogen (H ₂ ).

Should hydrogen be formed, the hydrogen formed is reacted with oxygen, preferably with the oxygen formed at the anode, to form one or more catalyst (s) to form water. The water thus formed is preferably returned to the electrolyte solution.

Catalyst (s) for the conversion of hydrogen with oxygen to water are catalysts which can also be used for the formation of oxygen. For example, the catalysts mentioned above are for

Oxygen generation suitable. Preferably, the catalyst (s) for the reaction of hydrogen with

Oxygen to water selected from the group consisting of (i) Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (ii) alloys of Fe, Co, Ni, Ru, Rh, Pd, Os , Ir and / or Pt, (iii) oxides of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, preferably selected from (i) Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (ii) alloys of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt, (iii) oxides of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir and / or Pt. In a particularly preferred embodiment

the catalysts consist of Ir or platinum.

A dimensionally stable catalyst can also be used. When

Dimensionally stable catalysts are generally referred to as titanium catalysts provided with a mixed oxide coating of titanium oxide with one or more rutile type noble metal oxides (eg Ru _x Ti ₁ - _x O ₂ ). In a

Embodiment is / are the catalyst (s) composed of a coating from a mixed oxide of the noble metals Pt, Ir, Rh, Pd, Ru and Os with the elements Mn, Pb, Cr, Co, Fe, Ti, Ta, Zr and Si on a support of Ti, Ta, Zr, Nb or their Alloys is applied. Particularly preferred dimensionally stable

Catalyst (s) comprises a titanium support and a coating of TiO ₂ - RuO ₂ or TiO ₂ - RuO ₂ .

Usually, the reaction of the hydrogen formed with oxygen, preferably with the oxygen formed at the anode, takes place in one

Recombinant unit normally containing a catalyst, preferably the catalyst mentioned above, for the conversion of hydrogen and oxygen to water, which is so connected to the electrochemical cell that both the hydrogen formed during the electrolysis and the oxygen formed during the electrolysis in the Recombination unit can be initiated.

Most preferably, no hydrogen is formed at the cathode. The capacity loss in ampere hours (Ah) is proportional to the amount of oxygen absorbed by the redox flow battery. For example, a capacity loss of one ampere hour corresponds to the uptake of 9.33 mmol

Oxygen (O ₂ ). The calculation is based on the first Faraday law: Q = n * z * F with F = 96485 As / mol or 26.8 Ah / mol: n = 1 Ah 1 (4 * 26.8) since this is a four-electron transition is.

Thus, based on the amount of oxygen released and, optionally, hydrogen, it can be determined how far the regeneration of the electrolyte solution has progressed.

Preferably, the process of the invention is terminated when the net amount of oxygen released equals 90% of the theoretically releasable oxygen, preferably corresponds to 95% of the theoretically releasable oxygen, and most preferably corresponds to 99% of the theoretically releasable oxygen. "Net amount of oxygen released corresponds to the total amount of oxygen released minus the amount of oxygen which, if released, can be recombined with water with the released hydrogen. The "net amount of oxygen released thus corresponds to: total released oxygen (in moles) minus 0.5 times the total amount of released hydrogen in (moles).

The "theoretically releasable oxygen" is the amount of oxygen calculated from the capacity loss as described above using the following equation.

KV is the capacity loss in [As] O ₂ (theoretically) is the theoretically releasable oxygen in mol

Alternatively, the oxygen release can be carried out as follows.

For this purpose, the following equations are given first.

Reactions of the battery in the direction of charging:

Definition of the loading degree α (SOC):

Concentration of the reaction product divided by the total concentrations of educt and product

where α _{A is} the state of charge of the anolyte and a _{K is} the state of charge of the catholyte,

In the case where the redox flow accumulator has no capacity loss, ie the capacity is highest, α _A = α _κ ·

As a result of oxidation of the electrolyte by, for example, atmospheric oxygen, the concentration of oxidized species, in particular A ^m , is increased without producing a corresponding reduced species. As a result, α _{A is} smaller than α _κ , since in a state of charge of α _κ = 1 in the corresponding anolyte not exclusively reduced species A ^mz but still unreduced species A ^m is present.

The potentials as a function of the charge level are as follows: Anolyte:

catholyte

where z can be taken from equation (1) and (2)

R Universal Gas Constant, R = 8.31447 J. mol ^-1 . KT ^-1

F Faraday constant, F = 96485.34 C. mol ^-1

T is the temperature

(α _A and α _κ can be taken from equations (4) and (5)

Φ _κ and φ _Α are the respective standard potential

Φ _κ , φ _Α and φ _z are the current potentials of the catholyte, anolyte or the redox flow accumulator.

Normally, in a redox-flow accumulator, different proton concentrations are present in the catholyte and the anolyte. These usually lead to a membrane potential

wherein R, T, F are as defined above; the proton concentration in the catholyte; and the proton concentration in the anolyte;

is.

This membrane potential is considered in equation (8) as follows.

If two of the three values Φ _κ , (pA and φ _z are present, the third value can be calculated according to equation (10). Usually φ _κ and φ _{z are} measured and cpA calculated.

If φ is measured _κ can using equation (7) can be calculated from Φ _{_κ} α _κ. α _A then results from Equation (10) and cpA thus from Equation (6). The capacity loss can then be calculated from a _A and α _κ . This results in a relative capacity loss. The absolute value can be calculated by the volume of the respective solutions. α _A then results from Equation (9) and cpA thus from Equation (6).

In a first embodiment, the electrodes (E) are in the positive half-cell of the redox flow accumulator and / or there is a Fluidverbmdung between the electrodes (E) and the positive half-cell of the redox flow accumulator during the oxygen release.

Hereinafter, preferred features of the first embodiment of the present invention will be described. Fluid connection in this context means that an exchange of fluid including ions contained therein is possible. In particular, in this embodiment there is no membrane between the electrodes (E) and the electrodes of the positive half cell of the redox flow accumulator. Preferably, there is fluid communication between the electrodes (E) and the positive half cell of the redox flow accumulator during the

S auerstofffreisetzung.

Usually, the electrolyte is circulated in the process according to the first embodiment during the oxygen release. In other words, usually a continuous release of oxygen is performed.

The release of oxygen is preferably carried out in an electrochemical cell which is in the circulation of the electrolyte. The electrochemical cell may be part of the circulation of the electrolyte while none

Oxygen release takes place and the oxygen release can take place if necessary. Alternatively, for oxygen release at least a portion of the

Electrolytes are diverted from the circuit, passed through the electrochemical cell, and then returned to the circuit. Valves required for the branch are known from the prior art and are therefore not explained in detail.

In this first embodiment, the oxygen release may occur during operation of the redox flow accumulator, i. The redox flow accumulator can be charged or discharged during the oxygen release, or it can not be charged or discharged. Preferably, the redox flow accumulator is not discharged during the oxygen release.

Capacity changes of the redox flow accumulator by the

Oxygen release is not referred to as charging or discharging the redox flow battery. α _κ before starting the oxygen release is preferably 0 to 100, more preferably 50 to 100, particularly preferred are 70 to 90. In a preferred embodiment, α _κ is 80 to 90 before the start of the oxygen release. In a variant of this embodiment, the redox flow accumulator is neither charged nor discharged during the oxygen release and the

Oxygen release is usually carried out in the catholyte. By the release of oxygen, the charging degree α _{κ is} lowered because oxygen ions O ² are oxidized to elemental oxygen and accordingly the electrolyte itself is reduced. The charge level of the anolyte α _A changes during the

Oxygen release not.

If the charge level α _{κ is} again equal to α _A , the redox-flow accumulator has again reached its full capacity and the oxygen release can be ended. In this variant, the redox flow accumulator during the

Oxygen release is not charged and the oxygen release is performed until α _κ = α _A ± 5%, preferably to α _κ = α _A ± 2%, more preferably to α _κ = α _A ± 1%, most preferably to α _κ = α _A is. In this variant, α _κ before starting the oxygen release is preferably between 50 and 100, more preferably 70 to 90, particularly preferably 80 to 90. α _κ can be determined during the oxygen release by measuring φ _κ and equation (7). Measuring φ _Α and φ _z is then not necessary.

As stated above, overcompensation can be carried out, ie, after completion of the release of oxygen, α _κ <α _A. Although not the full capacity is initially available, however, during the oxidation of the electrolyte solutions, the redox flow accumulator passes through the region of the maximum capacity until it drops again.

In a further variant of this embodiment, the redox flow accumulator is charged during the oxygen release and the

Oxygen release is usually carried out in the catholyte.

In this variant, α _{A increases} due to the charging process. With respect to α _κ , the following opposite effects result. - by the loading process as such, α _{κ is} increased

- By the release of oxygen as such, α _{κ is} lowered.

Depending on which effect predominates, α _{κ is} increased, decreased or remains constant.

Even when α is increased rises _κ α _κ slower than, and hence there is an approximation of α _κ and

In this further variant, the redox flow accumulator during the

Oxygen release is charged and the oxygen release is performed to α _κ ± 5%, preferably to α _κ = ± 2%, more preferably to α _κ = α _A ± 1%, most preferably to α _κ = α _A. In this variant, α _κ before starting the release of oxygen is preferably between 50 and 100, more preferably 70 to 90, particularly preferably 80 to 90.

As stated above, overcompensation can be carried out, ie, after the end of the release of oxygen, α _κ <. Although not the full capacity is initially available, however, during the oxidation of the electrolyte solutions, the redox flow accumulator passes through the region of the maximum capacity until it drops again.

The oxidation of oxygen and the constituents of the electrolyte can represent competing reactions, depending on the respective redox potential of the constituents of the electrolyte. That is, in addition to an oxidation of 2 O ² to O ₂ , it may incidentally lead to oxidation of the electrolyte components. Therefore, it would be advantageous in terms of oxygen release alone if there are no ions in the electrolyte solution that can be oxidized in a competing reaction. However, the accumulator would usually have to be fully charged first and then the oxygen release be performed, resulting in a

Interruption in accumulator operation leads, which is not preferred for economic reasons. In another variant of the first embodiment, the electrodes (E) are in the positive half cell of the redox flow accumulator and / or there is a fluid connection between the electrodes (E) and the positive half cell of the redox flow accumulator during the oxygen release and the

Electrolyte solution is circulated during the oxygen release. In this variant, the voltage of the redox flow accumulator, usually

Called clamping voltage, measured before the start of the oxygen release and kept constant during the oxygen release. This can be done for example by an external power source, usually a potentiostat. Such a method is also called potentiostatic loading.

By the release of oxygen, the charging degree α _{κ is} lowered because oxygen ions O ² are oxidized to elemental oxygen and accordingly the electrolyte itself is reduced. The charge level of the anolyte α _A does not change initially.

By lowering α _κ and consequently φ _κ , the clamping voltage of the redox flow accumulator decreases. Since the clamping voltage is kept constant, the

Redox flow accumulator loaded, ie α _A increases. As a result, α _A and α _κ approach the same value, namely, the average of α _A and α _κ before the beginning of

S auerstofffreisetzung.

In this variant, the clamping voltage of the redox flow accumulator is measured before the oxygen release and kept constant during the oxygen release, also preferably the oxygen release is carried out in this variant to α _κ = α _A ± 5%, preferably to α _κ = α _A. ± 2%, more preferably to α _κ = α _A ± 1%, most preferably to α _κ = α _A. In this variant, α _κ before starting the oxygen release is preferably between 50 and 100, more preferably 70 to 90, particularly preferably 80 to 90. α _κ during the oxygen release can be determined by measuring φ _κ and equation (7). Measuring φ _Α and φ _z is then not necessary. The course of the current density of the redox flow accumulator in this variant is shown in FIG. ii is the diffusion current density, which is normally unavoidable for technical reasons;

At the beginning of the oxygen release, the current density increases as the rate and efficiency of the initial electrolysis is highest. The stronger the _{convergence of} α _κ and α _A, the lower the current density becomes.

The current density is based on the membrane area of the redox flow accumulator.

In an alternative of the abovementioned variant, the terminal voltage of the redox flow accumulator is measured before the oxygen release begins and kept constant during the release of oxygen; moreover, the oxygen release in this variant is preferably carried out until the following inequality is fulfilled

preferably until

and most preferably to

where di / dt is the change in current density relative to the membrane area of the redox-flow accumulator over time. In this alternative, α _κ before starting the oxygen release is preferably between 50 and 100, more preferably 70 to 90, particularly preferably 80 to 90.

In this alternative, the determination of redox potentials during the

Oxygen release not required. Under ideal conditions, no current flow occurs when the

Oxygen release is complete. Technically, however, a low current flow (diffusion current) can not normally be avoided. Therefore, the oxygen release is usually terminated when the current flow no longer or only slightly changes.

In a second embodiment, during the oxygen release, there is no fluid communication between the electrodes (E) and the half cells of the redox flow accumulator, and after the oxygen release, fluid communication is established or the electrolyte solution becomes the redox flow accumulator, preferably the positive one, after the oxygen release Half cell, fed.

In the method according to the second embodiment, the electrolytic solution is preferably taken out of the redox flow accumulator, the oxygen release is performed by the electrodes (E), and the electrolytic solution is returned to the redox flow accumulator after the oxygen release. For example, the electrolyte solution can be removed from the circulation of the redox flow

Diverted accumulator, are passed into a separate container in which the electrodes (E) are and the oxygen release are performed. After the release of oxygen, the electrolyte solution is returned to the circulation of the redox flow accumulator. During the oxygen release there is no fluid connection between the electrodes (E) and the half cells of the redox flow accumulator.

Preferably, the electrolyte solution is removed from the circulation of the positive half-cell of the redox flow accumulator.

Alternatively, the release of oxygen can also be carried out in the tank (s) of the circuit of the redox flow accumulator, after the fluid connection to the half cell (s) of the redox flow accumulator has been interrupted. Preferably, the tank or tanks are part of the circulation of the positive half-cell of the redox flow accumulator (catholyte).

In the second embodiment, α _κ before starting the oxygen release is preferably 0 to 100, more preferably 50 to 100, even more preferably 70 to 90, particularly preferably 80 to 90.

In this embodiment, the oxygen release is usually in the

Catholics performed. As a result, the loading degree α _{κ is} lowered because

Oxygen ions O ² are oxidized to elemental oxygen and

Accordingly, the electrolyte itself is reduced. The charge level of the anolyte α _A does not change during the oxygen release.

If the charge level α _{κ is} again equal to α _A , the redox-flow accumulator has again reached its full capacity and the oxygen release can be ended.

The oxygen release is preferably carried out until α _κ = α _A ± 5%, preferably to α _κ = α _A ± 2%, more preferably to α _κ = α _A ± 1%, most preferably to α _κ = α _A. α _κ during the oxygen release can be determined by measuring φ _κ and equation (7). Measuring φ _Α and φ _z is then not necessary.

In the following, preferred features of all embodiments of the present invention will be described, unless explicitly stated otherwise. Preferably, a reference electrode is present and the potential of the anode at the beginning of the oxygen release is set to at least 1.230 V vs

Normal hydrogen electrode (NHE) compared to the reference electrode set, preferably at least 1.500 V vs NHE, more preferably at least 2,000 V vs NHE, even more preferably at least 2,500 V vs NHE, based on the normal hydrogen electrode. Usually, the potential of the anode at the beginning of the oxygen release is 10 V vs NHE or less.

Preferably, a reference electrode is present and the potential of the cathode at the beginning of the oxygen release is set in a range of -0.800 to + 0.300 V vs NHE with respect to the reference electrode, preferably in a range of -0.500 to 0.000 V, more preferably in a range of 0,500 to -0,200 V vs NHE.

When the potential of the cathode is adjusted, the potential of the anode is automatically and vice versa. When a potential is set, either the potential of the cathode or the potential of the anode is adjusted.

To adjust the potential of the anode or cathode, a reference electrode is used as explained above.

The method of the present invention is suitable for all types of redox flow accumulators, including hybrid flow batteries. Suitable battery systems are in particular the following.

The redox flow accumulator is particularly preferably a vanadium redox flow accumulator. In the following, preferred features of the vanadium redox flow accumulator are described unless explicitly stated to the contrary.

In a vanadium redox flow accumulator, vanadium is in the

oxidation levels (+11) to (+ V). The corresponding oxidation part equations are as follows.

The oxidation part equation for oxygen generation is as follows.

The anode and cathode equations are as follows.

Due to the potentials it is believed that in a vanadium-containing

Electrolytic solution, a V (+ II), V (+ III) and V (+ IV) compound, if included, is first oxidized to V (+ V) before the oxygen release commences, i. the reactions according to equations (I) to (III) first occur.

During the release of oxygen, the following reactions take place

(i) oxidation of H ₂ O to O ₂ at the anode, (ii) reduction of VO ₂ ⁺ to lower oxidation states at the cathode,

Theoretically, a reduction to V ²⁺ occurs at the cathode ^. The resulting V ^{2+ is} symproportionated with V (+ IV) and V (+ V) compounds present in the solution. When V (+ V) compounds are present, V ²⁺ with V (+ V) usually symproportionates to V (+ IV) compounds. Only when no V (V +) - compounds are no longer present symproportioniert V ²⁺ V (+ IV) - connections to V (+ III) - compounds.

In the course of the oxygen release, therefore, the average oxidation state of the electrolyte is lowered. As a result, the redox potential of the electrolyte solution decreases.

That there are proportionally fewer V (+ V) in the solution, i. the number of vanadium compounds to be oxidized initially increases and the efficiency of the vanadium compounds increases

Oxygen release decreases.

The efficiency of electrolysis is directly related to the redox potential of

Electrolyte solution dependent. The kinetics of the oxidation of vanadium occurring simultaneously with the oxidation of oxygen at the anode decrease with increasing redox potential. The higher the proportion of vanadium ions with high Oxidation level, the higher the redox potential of the electrolyte solution, the more efficient the electrolysis. It is therefore advantageous if the proportion of

oxidation-capable vanadium ions at the beginning of the electrolysis is as low as possible. The higher the proportion of VO ₂ ⁺ , the lower the proportion of oxidation-capable

Vanadium ions, as these can not be further oxidized. It therefore expedient not directly an electrolyte lytlösung containing a mixture of V ³⁺ and V0 ²⁺ to subject the electrolysis, but an electrolyte solution with the highest possible proportion of VO ₂ ⁺ . Thus, the proportion of V0 ^{2+ increases} only during the course of the electrolysis.

Preferably, prior to initiation of oxygen release, the electrolyte solution has no V (+ II) compounds, more preferably no V (+ II) and V (+ III) compounds, even more preferably the molar ratio of V (+ V) - Compounds of V (+ IV) compounds in each case based on vanadium ions between 50 and 100, even more preferably between 70 and 90 and particularly preferably between 80 and 90.

Due to the progressive release of oxygen and the concomitant reduction of the average oxidation state of the electrolyte, the efficiency of the oxygen release is reduced. Preferably, therefore, the oxygen release is stopped before the efficiency falls below a certain value. This endpoint can be determined as generally described above

Another aspect of the present invention relates to an electrochemical cell comprising at least two electrodes (E) of which at least one is connected as a cathode and at least one as an anode, wherein the electrochemical cell are located in at least one half-cell of a redox flow accumulator and / or there is a fluid connection between the electrochemical cell and at least one half cell of a redox flow accumulator.

Preferred features of the electrochemical cell of the present invention are also preferred features of the method of the present invention, including all preferred embodiments, and vice versa. The electrochemical cell is preferably located in the positive half cell of the redox flow accumulator and / or there is a fluid connection between the electrodes (E) and the positive half cell of the redox flow accumulator

The present invention further relates to the use of electrical current for the release of oxygen from an electrolyte solution of a redox flow accumulator.

Typically (O ₂₎ not more than 1.5 moles of hydrogen (H ₂₎ is formed per 1.0 mol formed oxygen.

Preferred features of the electrochemical cell of the present invention and preferred features of the method of the present invention including all preferred embodiments are also preferred features of the use of the present invention and vice versa.

characters

FIG. 1 shows the electrochemical cell of the example.

List of Reference Numerals: A: Beaker

B: electrolyte solution C: reference electrode (Hg / Hg ₂ SO ₄ electrode)

D: measuring electrode (glassy carbon electrode)

E: magnetic stirrer

F: counterelectrode (iridium electrode), cathode

G: working electrode (graphite felt electrode), anode

φz: cell voltage

φc: cathode potential

ΦΑ: anode potential

φR: redox potential

FIG. 2 shows the potentials and cell voltage before the electrolysis of a 1.6 M VO ₂ ⁺ in 2 MH ₂ SO ₄ .

FIG. 3 shows the potentials, electric current and cell voltage during the electrolysis of a 1.6 M VO ₂ ⁺ solution in 2 MH ₂ SO ₄ .

FIG. 4 shows the redox potential and electric current during the electrolysis of a 1.6 M VO ₂ ⁺ solution in 2 MH ₂ SO ₄ .

FIG. 5 shows the potentials and cell voltage after the electrolysis of a 1.6 M VO ₂ ⁺ in 2 MH ₂ SO ₄ .

FIG. 6 shows the profile of the current density of the redox flow accumulator at a constant clamping voltage (potentiostatic charging) ii is the diffusion current density, which is normally unavoidable for technical reasons;

The invention is further described in the following points: 1. Method for the release of oxygen from an aqueous electrolyte solution of a redox flow accumulator, wherein at least two electrodes (E) are in electrically conductive contact with the electrolyte solution, of the electrodes (E) at least one as the anode and at least one connected as a cathode and at the anode oxygen (O ₂ ) is formed, wherein at the cathode no hydrogen or per 1.0 mol of oxygen (O ₂ ) formed not more than 1.5 mol of hydrogen (H ₂ ) are formed.

2. The method according to item 1, wherein the electrodes (E) are not the electrodes which are used for current drain from the redox flow accumulator. 3. The method according to any one of the preceding points, wherein the hydrogen formed with oxygen, preferably with that formed at the anode

Oxygen, is converted to water on a catalyst.

4. The method according to item 3, wherein the water is returned to the electrolyte solution. 5. The method according to item 1 wherein no hydrogen is formed at the cathode.

6. The method according to one of the preceding points, wherein the

Electrodes (E) are in the positive half-cell of the redox flow accumulator and / or there is a fluid connection between the electrodes (E) and the positive half-cell of the redox flow accumulator during the oxygen release. 7. The method according to item 6, wherein the clamping voltage of the redox flow accumulator measured before the start of the oxygen release and is kept constant during the oxygen release.

8. The method according to item 6, wherein the redox-flow accumulator is charged during the oxygen release.

9. The method according to any one of the preceding points 1 to 5, wherein during the oxygen release no fluid connection between the electrodes (E) and the half-cells of the redox flow accumulator and after

Oxygen release a fluid connection is made or the

Electrolyte solution after the oxygen release the redox flow accumulator, preferably the positive half-cell, is supplied.

10. The method according to item 9, wherein the electro lytlösung the redox flow accumulator is removed, the oxygen release by means of the electrodes (E) is performed and the electrolyte solution after the oxygen release the redox flow accumulator is fed again.

11. The method according to any one of the preceding points, wherein the redox flow accumulator is a vanadium redox flow accumulator.

12. An electrochemical cell comprising at least two electrodes (E) of which at least one is connected as a cathode and at least one as an anode, wherein the electrodes (E) are located in at least one half cell of a redox flow accumulator and / or a fluid connection between the electrochemical cell and at least one half-cell of a redox flow accumulator.

13. Electrochemical cell according to item 12, wherein the electrodes (E) are in the positive half-cell of the redox flow accumulator and / or a

Fluid connection between the electrodes (E) and the positive half-cell of the redox flow accumulator consists. 14. Use of electric current for oxygen release

Electrolyte solution of a redox flow accumulator.

example

400 mL of a 1.6 molar VO ₂ solution in 2 molar H ₂ SO ₄ is placed in a beaker. The first electrode used is an iridium sheet with a surface area of 16 cm ² (Heraeus, Germany). The second electrode used is a thermally activated (lh, 400 ° C.) graphite felt having a surface area of 40 cm ² (GFA5 from SGL-Carbon, Germany). The third electrode used is an Hg / Hg ₂ SO ₄ electrode. The fourth electrode used is a glassy carbon electrode. A potentiostat (Modulab + 20A Booster, Solatron Analytical, USA) controls the potential of the second electrode and measures the current between the first and second electrodes. About three additional

Voltage measurement inputs, the cell voltage (φζ), the anode potential (φΑ) and the redox potential (cpR) of the electrolyte solution is determined. Immediately after addition of the electrolyte solution, all voltages and potentials are measured for a period of 60 s (Figure 2). Subsequently, the cathode potential is adjusted for a period of 120 minutes to a value of -0.90 V against the Hg / Hg ₂ SO ₄ electrode and thus the electrolysis is started (Figure 3). Thereafter, the individual potentials are again measured without current for 300 s (Figure 4).

The electrolytic solution is sampled before the beginning of the electrolysis and after the end of the electrolysis, and the vanadium concentration and oxidation state fractions are determined by potentiometric titration. In the diagrams all potentials against the normal hydrogen electrode were given. As can be seen from FIG. 2, the redox potential of the VO ₂ solution before the start of the electrolysis is 1.23 V. The cathode potential has the same value as the anode potential, so that a cell voltage of 0 V results.

Figure 3 shows the potentials, electric current and cell voltage during the electrolysis of the 1.6 M VO ₂ ⁺ solution in 2 MH ₂ SO ₄ . The

Cathode potential was set to -0.90 V vs. Hg / Hg ₂ SO ₄ (-0.25 V vs. NHE) and did not change during the electrolysis period. The

Cathode potential corresponded to a value of about 200-250 mV below the potential of significant hydrogen generation on carbon electrodes. The choice of potential was a compromise between unwanted hydrogen generation and driving force for the oxygenation reaction. Between the working electrode (anode) and the counter electrode, an electric current with a current density of about 0.25-0.20 A / cm ^{2 was established} , which decreased continuously during the reaction time. With the beginning of the electrolysis, a strong one appeared at the anode

Gas evolution, which lasted until the end of the experiment. The implemented

Amperage was the integral of the current density curve and was 7.3 Ah.

The anode potential increased with the onset of electrolysis by polarization to a value of 2.46 V and dropped continuously to about 2.2 V after 120 min electrolysis time. The value was well above the standard electrode potential of the oxygenation reaction and thereby allowed the evolution of oxygen at the anode. The difference between anode and cathode potential was the measured cell voltage of 2.7-2.5 V, which had the same tendency during the electrolysis as the anode potential, since the cathode potential was kept at a constant potential. The fall of the anode potential and thus of the cell voltage can be associated with an increase of tetravalent vanadyl cations (V0 ²⁺ ) in the solution with progressive release of oxygen and thus a

Mixed potential formation (parallel oxidation reaction of V ⁴⁺ to V ⁵ ) can be explained. As the reaction progresses, the redox potential of the electrolyte solution should decrease by increasing the amount of tetravalent V0 ²⁺ . Figure 4 shows the redox potential of the electrolyte solution and the electrical current during the electrolysis. At the beginning of the electrolysis, the redox potential corresponds to the value of the de-energized state and decreases in the further course. The value of the redox potential was equal to the value in the de-energized state with a value of 1.23 V and decreased during the further course of the measurement with a typical behavior according to the Nernst equation with logarithmic

Concentration influence. FIG. 5 shows the course of the potentials and the cell voltage after the electrolysis of a 1.6 M VO ₂ solution in 2 MH ₂ SO ₄ within 5 minutes.

Due to polarization effects of the previously current-carrying electrodes, anode and cathode potentials differed from those of the redox potential. Only in the further course did the potentials equal those of the redox potential with a value of 1.09V. Thus, the redox potential of the solution was 114 mV lower than before the electrolysis, indicating an increase in V0 ²⁺ ions

was due.

Claims

Method for the release of oxygen from an aqueous electrolyte solution of a redox flow accumulator, wherein at least two electrodes (E) are in electrically conductive contact with the electrolyte solution, of the electrodes (E) is at least one connected as an anode and at least one as a cathode and at the Anode oxygen (O ₂ ) is formed, wherein at the cathode no hydrogen or per 1.0 mol of oxygen (O ₂ ) formed not more than 1.5 mol of hydrogen (H ₂ ) are formed, wherein the electrodes (E) in the positive half-cell of the redox flow accumulator and / or there is a fluid connection between the electrodes (E) and the positive half-cell of the redox flow accumulator during the oxygen release or wherein during the oxygen release no fluid communication between the electrodes (E) and the Half-cells of the redox flow accumulator is made and after the oxygen release, a fluid connection is made.

A method according to claim 1, wherein the electrodes (E) are not the electrodes used for current drain from the redox flow accumulator.

Method according to one of the preceding claims, wherein the hydrogen formed with oxygen, preferably with the oxygen formed at the anode, is converted to water on a catalyst. A method according to claim 3, wherein the water is again supplied to the electrolyte solution.

The method of claim 1 wherein no hydrogen is formed at the cathode.

Method according to one of the preceding claims, wherein the electrodes (E) are in the positive half-cell of the redox flow accumulator and / or a Fluidverbmdung between the electrodes (E) and the positive half-cell of the redox flow accumulator during the

Oxygen release exists and wherein the clamping voltage of the redox flow accumulator measured before the start of the oxygen release and kept constant during the oxygen release.

Oxygen release is and wherein the redox flow accumulator is charged during the oxygen release.

Method according to one of the preceding claims 1 to 5, wherein during the oxygen release there is no fluid connection between the electrodes (E) and the half cells of the redox flow accumulator and after the release of oxygen a Fluidverbmdung is prepared and wherein the Elektro lytlösung the redox flow Accumulator is removed, the oxygen release by means of the electrodes (E) is performed and the electrolyte solution after the oxygen release the redox flow accumulator is fed again.

Method according to one of the preceding claims, wherein the redox flow accumulator is a vanadium redox flow accumulator.

10. An electrochemical cell comprising at least two electrodes (E) of which at least one is connected as a cathode and at least one as an anode, wherein the electrodes (E) are located in at least one half cell of a redox flow accumulator and / or a fluid connection between the electrochemical cell and at least one half-cell of a redox flow accumulator.

11. An electrochemical cell according to claim 10, wherein the electrodes (E) are in the positive half-cell of the redox flow accumulator and / or a fluid connection between the electrodes (E) and the positive

Half cell of the redox flow accumulator is.

12. Use of electric current for oxygen release from an electrolyte solution of a redox flow accumulator.