CA2970178A1 - Method for regenerating the electrolyte solution of a redox flow battery - Google Patents
Method for regenerating the electrolyte solution of a redox flow battery Download PDFInfo
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- CA2970178A1 CA2970178A1 CA2970178A CA2970178A CA2970178A1 CA 2970178 A1 CA2970178 A1 CA 2970178A1 CA 2970178 A CA2970178 A CA 2970178A CA 2970178 A CA2970178 A CA 2970178A CA 2970178 A1 CA2970178 A1 CA 2970178A1
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- flow battery
- redox flow
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- 239000008151 electrolyte solution Substances 0.000 title claims abstract description 60
- 238000000034 method Methods 0.000 title claims abstract description 49
- 230000001172 regenerating effect Effects 0.000 title description 2
- QVGXLLKOCUKJST-UHFFFAOYSA-N atomic oxygen Chemical compound [O] QVGXLLKOCUKJST-UHFFFAOYSA-N 0.000 claims abstract description 179
- 239000001301 oxygen Substances 0.000 claims abstract description 174
- 229910052760 oxygen Inorganic materials 0.000 claims abstract description 174
- 239000001257 hydrogen Substances 0.000 claims abstract description 45
- 229910052739 hydrogen Inorganic materials 0.000 claims abstract description 45
- UFHFLCQGNIYNRP-UHFFFAOYSA-N Hydrogen Chemical compound [H][H] UFHFLCQGNIYNRP-UHFFFAOYSA-N 0.000 claims abstract description 35
- 239000012530 fluid Substances 0.000 claims abstract description 26
- 150000002431 hydrogen Chemical class 0.000 claims abstract description 18
- 239000003054 catalyst Substances 0.000 claims description 17
- LEONUFNNVUYDNQ-UHFFFAOYSA-N vanadium atom Chemical group [V] LEONUFNNVUYDNQ-UHFFFAOYSA-N 0.000 claims description 13
- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 claims description 13
- 229910052720 vanadium Inorganic materials 0.000 claims description 11
- 238000004891 communication Methods 0.000 claims description 8
- 229940021013 electrolyte solution Drugs 0.000 description 39
- 238000005868 electrolysis reaction Methods 0.000 description 31
- 238000007254 oxidation reaction Methods 0.000 description 29
- 230000003647 oxidation Effects 0.000 description 28
- 239000003792 electrolyte Substances 0.000 description 26
- BASFCYQUMIYNBI-UHFFFAOYSA-N platinum Chemical compound [Pt] BASFCYQUMIYNBI-UHFFFAOYSA-N 0.000 description 18
- 230000008569 process Effects 0.000 description 18
- 229910052741 iridium Inorganic materials 0.000 description 15
- 238000006243 chemical reaction Methods 0.000 description 14
- 229910052742 iron Inorganic materials 0.000 description 14
- 229910052763 palladium Inorganic materials 0.000 description 14
- 229910052703 rhodium Inorganic materials 0.000 description 14
- 229910052707 ruthenium Inorganic materials 0.000 description 14
- 229910052762 osmium Inorganic materials 0.000 description 13
- 229910052759 nickel Inorganic materials 0.000 description 12
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 11
- 239000000243 solution Substances 0.000 description 11
- 239000010936 titanium Substances 0.000 description 9
- GWEVSGVZZGPLCZ-UHFFFAOYSA-N Titan oxide Chemical compound O=[Ti]=O GWEVSGVZZGPLCZ-UHFFFAOYSA-N 0.000 description 8
- 230000007423 decrease Effects 0.000 description 8
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- 229910052719 titanium Inorganic materials 0.000 description 8
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- 238000000576 coating method Methods 0.000 description 6
- 238000005755 formation reaction Methods 0.000 description 6
- 229910002804 graphite Inorganic materials 0.000 description 6
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- 239000000463 material Substances 0.000 description 6
- 239000012528 membrane Substances 0.000 description 6
- 239000000203 mixture Substances 0.000 description 6
- 239000011701 zinc Substances 0.000 description 6
- QAOWNCQODCNURD-UHFFFAOYSA-N Sulfuric acid Chemical compound OS(O)(=O)=O QAOWNCQODCNURD-UHFFFAOYSA-N 0.000 description 5
- 229910052799 carbon Inorganic materials 0.000 description 5
- 230000008859 change Effects 0.000 description 5
- 150000001875 compounds Chemical class 0.000 description 5
- 150000002500 ions Chemical class 0.000 description 5
- 238000005259 measurement Methods 0.000 description 5
- -1 Oxygen ions Chemical class 0.000 description 4
- RTAQQCXQSZGOHL-UHFFFAOYSA-N Titanium Chemical compound [Ti] RTAQQCXQSZGOHL-UHFFFAOYSA-N 0.000 description 4
- 239000011149 active material Substances 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 4
- 238000009792 diffusion process Methods 0.000 description 4
- 229910021397 glassy carbon Inorganic materials 0.000 description 4
- 229910000510 noble metal Inorganic materials 0.000 description 4
- 229910052697 platinum Inorganic materials 0.000 description 4
- WOCIAKWEIIZHES-UHFFFAOYSA-N ruthenium(IV) oxide Inorganic materials O=[Ru]=O WOCIAKWEIIZHES-UHFFFAOYSA-N 0.000 description 4
- 229910052715 tantalum Inorganic materials 0.000 description 4
- 229910001456 vanadium ion Inorganic materials 0.000 description 4
- 229910052726 zirconium Inorganic materials 0.000 description 4
- 230000003247 decreasing effect Effects 0.000 description 3
- 229910003460 diamond Inorganic materials 0.000 description 3
- 239000010432 diamond Substances 0.000 description 3
- 238000007599 discharging Methods 0.000 description 3
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- MINVSWONZWKMDC-UHFFFAOYSA-L mercuriooxysulfonyloxymercury Chemical compound [Hg+].[Hg+].[O-]S([O-])(=O)=O MINVSWONZWKMDC-UHFFFAOYSA-L 0.000 description 3
- 229910000371 mercury(I) sulfate Inorganic materials 0.000 description 3
- 230000008929 regeneration Effects 0.000 description 3
- 238000011069 regeneration method Methods 0.000 description 3
- KEQXNNJHMWSZHK-UHFFFAOYSA-L 1,3,2,4$l^{2}-dioxathiaplumbetane 2,2-dioxide Chemical compound [Pb+2].[O-]S([O-])(=O)=O KEQXNNJHMWSZHK-UHFFFAOYSA-L 0.000 description 2
- IJGRMHOSHXDMSA-UHFFFAOYSA-N Atomic nitrogen Chemical compound N#N IJGRMHOSHXDMSA-UHFFFAOYSA-N 0.000 description 2
- 229910052924 anglesite Inorganic materials 0.000 description 2
- 238000013459 approach Methods 0.000 description 2
- 239000007864 aqueous solution Substances 0.000 description 2
- 229910052804 chromium Inorganic materials 0.000 description 2
- 239000004020 conductor Substances 0.000 description 2
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- 238000000354 decomposition reaction Methods 0.000 description 2
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- 239000007789 gas Substances 0.000 description 2
- 239000011521 glass Substances 0.000 description 2
- GKOZUEZYRPOHIO-UHFFFAOYSA-N iridium atom Chemical compound [Ir] GKOZUEZYRPOHIO-UHFFFAOYSA-N 0.000 description 2
- 229910052745 lead Inorganic materials 0.000 description 2
- 239000007788 liquid Substances 0.000 description 2
- 229910052748 manganese Inorganic materials 0.000 description 2
- 150000004706 metal oxides Chemical class 0.000 description 2
- 229910052758 niobium Inorganic materials 0.000 description 2
- 230000010287 polarization Effects 0.000 description 2
- 230000006798 recombination Effects 0.000 description 2
- 238000005215 recombination Methods 0.000 description 2
- 229910052710 silicon Inorganic materials 0.000 description 2
- OGIDPMRJRNCKJF-UHFFFAOYSA-N titanium oxide Inorganic materials [Ti]=O OGIDPMRJRNCKJF-UHFFFAOYSA-N 0.000 description 2
- 101100165918 Caenorhabditis elegans cam-1 gene Proteins 0.000 description 1
- 229910002549 Fe–Cu Inorganic materials 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- 238000010521 absorption reaction Methods 0.000 description 1
- 238000010923 batch production Methods 0.000 description 1
- 229910052793 cadmium Inorganic materials 0.000 description 1
- BDOSMKKIYDKNTQ-UHFFFAOYSA-N cadmium atom Chemical compound [Cd] BDOSMKKIYDKNTQ-UHFFFAOYSA-N 0.000 description 1
- 239000007795 chemical reaction product Substances 0.000 description 1
- ARUVKPQLZAKDPS-UHFFFAOYSA-L copper(II) sulfate Chemical compound [Cu+2].[O-][S+2]([O-])([O-])[O-] ARUVKPQLZAKDPS-UHFFFAOYSA-L 0.000 description 1
- 229910000366 copper(II) sulfate Inorganic materials 0.000 description 1
- 230000001419 dependent effect Effects 0.000 description 1
- 238000013461 design Methods 0.000 description 1
- 238000010586 diagram Methods 0.000 description 1
- 230000005611 electricity Effects 0.000 description 1
- 238000012983 electrochemical energy storage Methods 0.000 description 1
- 238000009791 electrochemical migration reaction Methods 0.000 description 1
- 239000007772 electrode material Substances 0.000 description 1
- 238000003411 electrode reaction Methods 0.000 description 1
- 230000005592 electrolytic dissociation Effects 0.000 description 1
- 238000004146 energy storage Methods 0.000 description 1
- 230000007717 exclusion Effects 0.000 description 1
- 238000002474 experimental method Methods 0.000 description 1
- PCHJSUWPFVWCPO-UHFFFAOYSA-N gold Chemical compound [Au] PCHJSUWPFVWCPO-UHFFFAOYSA-N 0.000 description 1
- 229910052737 gold Inorganic materials 0.000 description 1
- 239000010931 gold Substances 0.000 description 1
- 239000011261 inert gas Substances 0.000 description 1
- 239000010416 ion conductor Substances 0.000 description 1
- QSHDDOUJBYECFT-UHFFFAOYSA-N mercury Chemical compound [Hg] QSHDDOUJBYECFT-UHFFFAOYSA-N 0.000 description 1
- 229910052753 mercury Inorganic materials 0.000 description 1
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 229910052757 nitrogen Inorganic materials 0.000 description 1
- 238000003918 potentiometric titration Methods 0.000 description 1
- 239000000047 product Substances 0.000 description 1
- 230000000750 progressive effect Effects 0.000 description 1
- 230000035484 reaction time Effects 0.000 description 1
- 238000012546 transfer Methods 0.000 description 1
- 230000007704 transition Effects 0.000 description 1
- 150000003682 vanadium compounds Chemical class 0.000 description 1
- 229910052725 zinc Inorganic materials 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M8/00—Fuel cells; Manufacture thereof
- H01M8/18—Regenerative fuel cells, e.g. redox flow batteries or secondary fuel cells
- H01M8/184—Regeneration by electrochemical means
- H01M8/188—Regeneration by electrochemical means by recharging of redox couples containing fluids; Redox flow type batteries
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/42—Methods or arrangements for servicing or maintenance of secondary cells or secondary half-cells
- H01M10/4242—Regeneration of electrolyte or reactants
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
-
- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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- Engineering & Computer Science (AREA)
- Manufacturing & Machinery (AREA)
- Chemical & Material Sciences (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Electrochemistry (AREA)
- General Chemical & Material Sciences (AREA)
- Life Sciences & Earth Sciences (AREA)
- Sustainable Development (AREA)
- Sustainable Energy (AREA)
- Fuel Cell (AREA)
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
Method for regenerating the electrolyte solution of a redox flow battery Electrical energy can be stored by different processes. One possibility is the conversion of electrical energy into chemical energy by chemical reactions at electrode surfaces by electrical current. This type of energy storage is used extensively in secondary batteries (accumulators) on a large scale.
A secondary battery is an electrochemical cell consisting of two half cells.
The two half-cells are usually separated by an ion-conducting separator. The separator ensures charge balancing, but prevents the material transfer between the half cells. In the negative half-cell, a reduction of the active material takes place during the storage process, and an oxidation takes place in the positive half-cell. Thus, electrons flow from the positive half-cell into the negative half-cell in the storage process, and in the opposite direction in the discharge process. In order to allow compensation of the charge and a movement of the ions, a liquid substance or a substance mixture is required as the inner conductor in both half cells, referred to as electrolyte. The electrode is the phase boundary between the electrical conductor and the ionic conductor. The active material can be the electrode itself, a substance dissolved in the electrolyte or substances embedded in the electrode material.
If the active material consists of substances dissolved in the electrolyte, then the case arises that in this type of battery energy and power can be scaled independently of each other, since the electrolyte can be guided from supply containers past the electrodes. This type of electrochemical energy storage is called redox flow battery and includes a positively charged electrolyte component (catholyte) and a negatively charged electrolyte component (anolyte).
The general battery equations for redox flow batteries are as follows:
, ,õ
A secondary battery is an electrochemical cell consisting of two half cells.
The two half-cells are usually separated by an ion-conducting separator. The separator ensures charge balancing, but prevents the material transfer between the half cells. In the negative half-cell, a reduction of the active material takes place during the storage process, and an oxidation takes place in the positive half-cell. Thus, electrons flow from the positive half-cell into the negative half-cell in the storage process, and in the opposite direction in the discharge process. In order to allow compensation of the charge and a movement of the ions, a liquid substance or a substance mixture is required as the inner conductor in both half cells, referred to as electrolyte. The electrode is the phase boundary between the electrical conductor and the ionic conductor. The active material can be the electrode itself, a substance dissolved in the electrolyte or substances embedded in the electrode material.
If the active material consists of substances dissolved in the electrolyte, then the case arises that in this type of battery energy and power can be scaled independently of each other, since the electrolyte can be guided from supply containers past the electrodes. This type of electrochemical energy storage is called redox flow battery and includes a positively charged electrolyte component (catholyte) and a negatively charged electrolyte component (anolyte).
The general battery equations for redox flow batteries are as follows:
, ,õ
2 Am + ze- __ ¨Am-z (Anolyte) Kn __________ z c (Catholyte) The full capacity of the redox flow battery is available only as long as equivalent amounts of anolyte and catholyte are present.
During transport, storage or operation of the battery, a complete exclusion of air can often not be ensured, and oxidation processes may occur, so that the elytes are shifted toward higher oxidation states, resulting in a loss of capacity.
This problem is known in the art and attempts have been made so far to prevent the oxidation processes by inertization of the electrolyte solution. For this purpose, nitrogen, for example, was used as inert gas or inert organic liquids with a lower density than the electrolyte solution in the storage containers were used in order to prevent contact of the electrolyte solution with oxygen. Owing to circulation processes or diffusion processes, however, a contact of the electrolyte solution with oxygen cannot be completely prevented. Therefore, there is a need for methods in which the described loss of capacity resulting from the oxidation of the electrolyte can be reversed.
These and other objects are achieved with the method according to the invention and with 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 battery, wherein at least two electrodes (E) are in electrically conductive contact with the electrolytic solution at least one of the electrodes (E) is an anode and at least one is connected as a cathode, and the anode forms oxygen (02), the cathode does not form hydrogen or forms not more than 1.5 moles of hydrogen (H2) per 1.0 mole of formed oxygen (02).
r 3 =
It has surprisingly been found that the electrolyte solution of a redox flow battery can be regenerated by means of electrolysis. For example, the regeneration can be carried out during the operation of the redox flow battery, for example during the charging process, without interruptions during operation. A regeneration in the batch process, for example by separating a part of the electrolyte solution, release of oxygen according to the method of the present invention, and subsequent reintroduction into the redox flow battery is also possible. Finally, an electrolyte solution can be subjected to the process according to the present invention before the addition to the redox flow battery.
Instead of a complete electrolysis of water, the components of the electrolyte are reduced instead of any H+ ions.
Within the scope of the present invention, the term "redox flow battery" is used in its usual meaning. The basic design of a redox flow battery, for example a vanadium-redox flow battery, is known to a person skilled in the art.
Within the scope of the present invention the term "electrochemical cell" is used for an electrolysis cell and relates to an arrangement of electrodes which are conductively connected by an electrolyte. During the circuit continuity through the electrolyte a chemical change is caused which results in a direct conversion of electrical energy into chemical energy by electrode reactions and ionic migration.
Within the scope of the present invention, the term "electrolyte" is used in its usual meaning and refers to ion-conducting media whose electrical conductivity is caused by electrolytic dissociation into ions.
The positive half-cell of the redox flow battery is the part of the galvanic cell of the redox flow battery, which represents the positive pole of the redox flow battery in relation to the current drain.
If the term "half-cells" is used in the present application, this refers to the half-cells of , 4 I
the galvanic cell of the redox flow battery, unless explicitly referred to the contrary.
Within the scope of the present invention, the term "anolyte" refers to the material of a redox flow battery, which is directly influenced by the anode during discharging. The corresponding half cell is the negative half cell.
For the purposes of the present invention, the term "catholyte" refers to the material of a redox flow battery which is directly influenced by the cathode during discharging.
The corresponding half cell is the positive half cell.
"Circulation" or "circulation of the redox flow battery", etc. denotes one of the two separate circuits of the redox flow battery. These circuits are separated by a membrane in the galvanic cell of the redox flow battery.
Accordingly, there is the circulation of the positive half cell and the circulation of the negative half cell.
As described above, the capacity of a redox flow battery is determined by the amount of active materials dissolved in the electrolyte, and can only be made fully utilizable when an equivalent mixture of anolyte and catholyte is present. Due to undesirable oxidation processes the equimolar mixture of anolyte and catholyte is shifted in the direction of higher oxidation states, resulting in a loss of capacity of the battery.
In the case of a vanadium redox flow battery, for example, this means that in the fully charged state, only V5+ is present in the catholyte, while a mixture of V2+
and V3+ is present in the anolyte.
Within the scope of this invention, it has been found that it is possible to revert the oxidation processes on the electrolyte solution of a redox flow battery by electrolysis.
As described above, at least two electrodes (E) are in electrically conductive contact with the electrolyte solution, with at least one of the electrodes (E) being connected as an anode and at least one as a cathode, and oxygen (02) being formed at the anode.
The electrodes (E) can be part of an electrolysis cell. The process can be carried out batchwise or continuously, preferably continuously.
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) may only consist of anode(s) and cathode(s). However, then the potential of the cathode should not be too negative in order to avoid hydrogen development. If hydrogen is nevertheless formed, this can be reacted with oxygen again to form water, as explained in detail below.
The potential of the cathode can be controlled by means of a reference electrode.
Preferably, therefore, a reference electrode is present, more preferably the cathodic potential is adjusted such that no hydrogen is produced.
As the reference electrode, for example, an Hg / H92SO4 reference electrode, Ag /
AgCI reference electrode or a normal hydrogen electrode may be used. The reference electrode is also designated as an indifferent electrode. Both terms are used synonymously in this application.
By means of the electrode for measuring the redox potential of the electrolyte solution, the reaction sequence of the oxygen release can be followed as explained in detail below.
As the electrode used for the determination of the redox potential of the electrolyte solution any electrode can be used for measuring the redox potential of the electrolyte solution, and the person skilled in the art usually selects inert electrodes such as, for example, carbon-based electrodes, platinum or gold electrodes, preferably a glassy , 6 carbon electrode.
Preferably, the electrodes (E) consist of - one or more anode(s), - one or more cathode(s), - optionally one or more reference electrode(s), - optionally one or more usually 1 or 2 electrode(s) for measuring the redox potential.
For the determination of the redox potential, for example, a single-rod measuring cell can be used, which normally comprises two electrodes. Alternatively, two electrodes which are not connected to a single-electrode measuring cell, for example two separate electrodes, can be used.
The redox potential may also be determined against the reference electrode, if any.
Usually, the electrodes (E) contain, preferably consist of, only one electrode for measuring the redox potential.
A single-rod measuring cell is usually used or the redox potential is determined against the reference electrode.
If the reference electrode is used to measure the redox potential, it does not belong to the electrodes for measuring the redox potential.
Conventionally, an anode and a cathode are used. Alternatively, a plurality of cathodes and / or anodes may be used. The use of a plurality of cathodes and a plurality of anodes can be advantageous, for example, if higher a power is desired. If a plurality of anodes and / or cathodes are used, the above-mentioned quantities of oxygen and hydrogen formed are related to the respective total quantity of oxygen produced and hydrogen formed.
It is also possible that the oxygen release is carried out simultaneously or w , 7 , successively at several points in the redox flow battery. In this case, the method of the present invention is performed several times simultaneously or successively.
If present, usually only one reference electrode is present.
If present, there is usually only one or two electrodes for measuring the redox potential. As stated above, the determination of the redox potential may be performed by means of a single-rod measuring cell, usually including two electrodes, two separate electrodes or by means of an electrode vis-à-vis a reference electrode.
In particular the electrodes (E) consist of - an anode, - a cathode, - optionally a reference electrode, - optionally one or two electrodes for measuring the redox potential, for example two electrodes in the form of a single-rod measuring cell.
Preferably, the electrodes (E) are not the electrodes used for the current drain from the redox flow battery. In this preferred embodiment, the electrodes (E) comprise all the electrodes which are not electrodes that are used for the current drain of the redox flow battery. In other words, in this preferred embodiment, the redox flow battery contains only the electrodes which are used for the current drain of the redox flow battery 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 H20 to 02. In one embodiment, the anode(s) of the electrodes (E) comprises 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, II.
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) is / are composed of Ir.
A dimensionally stable electrode (DSA) can also be used as anode(s) of the electrodes (E). Dimensionally stable electrodes are not homogeneous regarding their microstructure, their phase composition and also their surface. Titanium electrodes are generally referred to as dimensionally stable electrodes which are provided with a mixed oxide coating of titanium oxide with one or more noble metal oxides of the rutile type (e.g. RuxTi1_x02). In one embodiment, the anode(s) of the electrodes (E) is / are composed 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 applied to a carrier anode of Ti, Ta, Zr, Nb or their alloys. Particularly preferred dimensionally stable electrode(s) (DSA) as the anode(s) of the electrode(s) (E) comprise(s) a titanium support and a coating of Ti02-Ru02 or TiO2-Ru02.
At the cathode of the electrodes (E) the reduction of the electrolyte takes place during the oxygen release. For example, in the case of a vanadium redox flow battery, the reduction of V02+ takes place towards lower oxidation states.
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.
_________ LJ-The electrochemical potential window in an aqueous solution, i.e. the voltage range of the potential required for the desired reaction 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 in its modifications diamond (doped), graphite as well as glass(y) carbon, lead, zinc, cadmium and mercury have a high hydrogen overvoltage.
Preferably, the cathode(s) of the electrodes (E) comprise(s) a material selected from the group consisting of glassy carbon, graphite, and diamond. Preferably, the cathode(s) of the electrodes (E) is / are composed of one material selected from the group consisting of glassy carbon, graphite, and diamond (doped). Particular preference is given to a graphite electrode. As stated above, no hydrogen or not more than 1.5 mol of hydrogen (H2) is formed per 1.0 mol of oxygen (02) formed. If hydrogen is formed, preferably per 1.0 mol of formed oxygen (02) not more than 1.0 mol of hydrogen (H2), more preferably not more than 0.5 mol of hydrogen (H2) is formed. In a preferred embodiment not more than 0.1 mol of hydrogen (H2) is formed.
If hydrogen is formed, the hydrogen formed is reacted with oxygen, preferably with the oxygen formed at the anode, on one or more catalyst(s) to form water. The water thus formed is preferably returned to the electrolyte solution.
Catalyst(s) suitable for the conversion of hydrogen with oxygen to obtain water are catalysts which can also be used for the generation of oxygen. For example, the abovementioned catalysts are suitable for the oxygen generation.
The catalyst(s) for the reaction of hydrogen with oxygen to obtain water are 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. As dimensionally stable catalysts are generally referred to titanium catalysts which are provided with a mixed oxide coating of titanium oxide with one or more noble metal oxides of the rutile type (e.g.
RuJii,02). In an embodiment the catalyst(s) is / are composed of a coating of a 1 0 t 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 which are applied to a support of Ti, Ta, Zr, Nb or their alloys. Particularly preferred dimensionally stable catalyst(s) comprise(s) a titanium support and a coating of Ti02-Ru02 or Ti02-RuO2.
The reaction of the formed hydrogen with oxygen, preferably with the oxygen formed at the anode, is usually carried out in one recombination unit normally comprising a catalyst, preferably the above-mentioned catalyst, for converting hydrogen and oxygen to water, which unit is so connected to the electrochemical cell that both the hydrogen formed during the electrolysis and the oxygen formed during the electrolysis can be introduced into the recombination unit.
Most preferably, no hydrogen is formed at the cathode.
The loss of capacity in ampere hours (Ah) is proportional to the amount of oxygen absorbed by the redox flow battery. For example, a loss in capacity of one ampere hour corresponds to the absorption of 9.33 mmol oxygen (02). 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 / (4 * 26.8) since it is a four-electron transition.
Consequently, the extent to which the regeneration of the electrolyte solution has progressed can be determined by means of the amount of oxygen released and optionally by means of the amount of hydrogen.
The process according to the invention is preferably terminated when the net amount of liberated oxygen corresponds to 90% of the theoretically releasable oxygen, preferably 95% of the theoretically releasable oxygen, and most preferably 99%
of the theoretically releasable oxygen.
The "net quantity of released oxygen" corresponds to the total quantity of oxygen released minus the amount of oxygen which, if hydrogen is released, can be recombined with that hydrogen released to obtain water. Thus the "net quantity of released oxygen" corresponds to:
The total amount of oxygen (in mol) minus 0.5 times the total amount of hydrogen released in (mol).
The "theoretically releasable oxygen" is the amount of oxygen which, as described above, is calculated from the loss of capacity by the following equation.
6,24 1018 4 = As KV = 02(theoretica1) 6.02/ 1 023 ino1-1 wherein KV is the loss of capacity in [As]
02 (theoretical) is the theoretically releasable oxygen in mol Alternatively, the oxygen release can be carried out as follows.
For this purpose, at first the following equations are given.
Reactions of the battery towards charging:
Anode: Am + ze- Am -z (1) Cathode: K" Kn+z + ze- (2) Cell: Am + K" Am -z + K"z (3) Definition of the charge level a (SOC):
Concentration of the reaction product divided by the total concentrations of educt and product = 12 _ cAr-z C + CA,. (4) aK- __ C + C
K (5) wherein aA is the state of charge of the anolyte and al< is the state of charge of the catholyte, cAm--1 is the concentration of the species A' CA' is the concentration of the species Am Cmr-z is the concentration of the species Kn+z CKr1 is the concentration of the species Kr' wherein A", Am, Kn+z, and Kr' are different from each other.
In the case where the redox flow battery has no loss of capacity, i.e. the capacity is at the highest: aA =K.
As a result of oxidation of the electrolyte by, for example, oxygen, the concentration of oxidized species, in particular Am, is increased without producing a corresponding reduced species. As a result, aA is smaller than ak, since, given a state of charge of = 1 in the corresponding anolyte, not only reduced species A' but also unreduced species Am are present.
The potentials depending on the charge level are as follows:
Anolyte:
13' R- T a 0 in __________ A (6) (PA = PA Z = F aA
Catholyte:
(p,< :17 (1)K _____________________________________________________ - In (7) z= F 1-Cell:
=
0 0 __ T - 7 \ -In a + in{ \ aA (8) (Pz z= F aK 1- aA
wherein can be taken from equation (1) and (2) universal gas constant, R = 8.31447 J.mo1-1 K-1 Faraday constant, F = 96485.34 C mo1-1 is the temperature a A and aK can be taken from equations (4) and (5) (pK and (PA are the respective standard potentials (Pk, (PA and (Pz are the current potentials of the catholyte, the anolyte or the redox flow battery.
Normally, different proton concentrations are present in a redox flow battery in the catholyte and the anolyte. These usually lead to a membrane potential (pm R =T = In c;
(pm - _______________________________________________________________ (9) F
wherein R, T, F are as defined above;
c- is the proton concentration in the catholyte; and Ir is the proton concentration in the anolyte.
This membrane potential is taken into consideration in Equation (8) as follows.
\
(Pz = - T a, (PA + In K + In a A (pm ( I 0) Z = F 1-u11 If two of the three values (pK, (PA and (pz are present, the third value can be calculated according to equation (10). Usually (PK and (pz are measured and TA is calculated.
If (pK can be measured using equation (7), aK can be calculated from (PK. aA
is then obtained from equation (10) and (PA from equation (6).
The loss of capacity can then be calculated from aA and aK. This results in a relative loss of capacity. The absolute value can be calculated by the volume of the respective solutions.
aA is then obtained from equation (9) and (PA from equation (6).
In a first embodiment, the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid connection is present between the electrodes (E) and the positive half-cell of the redox flow battery during oxygen release.
Preferred features of the first embodiment of the present invention will be described below.
In this context, fluid connection 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 battery.
. I CA 02970178 2017-06-08 . 1.5 Preferably, a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release.
According to the process of the first embodiment the electrolyte is usually circulated during the oxygen release. In other words, a continuous oxygen release is usually carried out.
The oxygen release is preferably carried out in an electrochemical cell which is located in the circuit of the electrolyte. The electrochemical cell can be a part of the circuit of the electrolyte while none oxygen release takes place and the oxygen release can take place as required. Alternatively, for oxygen release, at least a part of the electrolytes are diverted or branched off 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 can take place during operation of the redox flow battery, i.e. the redox flow battery can be charged or discharged during the oxygen release or neither be charged nor discharged. Preferably, the redox flow battery is not discharged during the oxygen release.
Capacity changes of the redox flow battery by the oxygen release are not referred to as charging or discharging the redox flow battery.
aK before the start of the oxygen release is preferably 0 to 100, more preferably 50 to 100, more preferably 70 to 90. In a preferred embodiment, aK is 80 to 90 before the oxygen release begins.
In an option of this embodiment, the redox flow battery is not charged or discharged during the oxygen release and the oxygen release is usually carried out in the catholyte. Oxygen ions 02- are oxidized to elemental oxygen by the oxygen release and the electrolyte itself is reduced accordingly. The charge level of the anolyte aA
16, does not change during the oxygen release.
When the charge level aK is again equal to apk, the redox flow battery has reached its full capacity again and the oxygen release can be terminated.
In this option, the redox flow battery is not charged during the oxygen release and the oxygen release is carried out until aK = aA 5%, preferably to aK = A 2%, more preferably to aK = aA 1%, most preferably to aK = aA. In this option, aK is preferably between 50 and 100 before the start of the oxygen release, more preferably 70 to 90, particularly preferably 80 to 90. aK can be determined during the oxygen release by measuring (1)K and equation (7). In that case a measurement of (PA and cpz is not necessary.
As explained above, an overcompensation can be carried out, i.e., after the oxygen release is complete aK < aA. Although initially the full capacity is not available, the redox flow battery passes through the range of the maximum capacity during the oxidation of the electrolyte solutions, which decreases again.
In a further option of this embodiment, the redox flow battery is charged during the oxygen release and the oxygen release is usually carried out in the catholyte.
In this option aA increases by the charging process. Regarding aK, the following opposing effects are obtained.
- aK is increased by the charging process as such - aK is reduced by the oxygen release as such.
Depending on which effect is predominant, aK is increased, decreased or remains constant.
Even if aK is increased, a K increases slower than aA and thus an approximation of aK
, 17, and aA occurs.
In this further option, the redox flow battery is charged during the oxygen release and the oxygen release is carried out up to aK 5%, preferably up to aK = 2%, more preferably up to aK = aft, 1%, most preferably up to aK = aA. In this option, aK is preferably between 50 and 100 before the start of the oxygen release, more preferred 70 to 90, particularly preferred 80 to 90.
As described above, an overcompensation can be carried out, i.e., after the oxygen has been released, aK < aA. Although initially the full capacity is not available, the redox flow battery passes through the range of the maximum capacity during the oxidation of the electrolyte solutions, until it decreases again.
The oxidation of oxygen and the constituents of the electrolyte can be competition reactions depending on the respective redox potential of the components of the electrolyte. That is to say, in addition to an oxidation of 2 02- to 02, oxidation of the electrolyte constituents can also occur. Therefore, it would be advantageous for the oxygen release alone if no ions are present in the electrolyte solution which can be oxidized in a competition reaction. However, for this purpose, the battery would first have to be fully charged and subsequently the oxygen release should be carried out, resulting in an interruption in battery operation, which is not preferred for economic reasons.
In another option of the first embodiment, the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during oxygen release and the electrolyte solution is circulated during oxygen release. In this option, the voltage of the redox flow battery, usually terminal voltage, is measured before the start of oxygen release and kept constant during oxygen release. This can be effected, for example, by an external current source, usually a potentiostat.
Such a method is also referred to as potentiostatic loading.
18, By the oxygen release the charge level aK is reduced because oxygen ions 02-are oxidized to elemental oxygen and, accordingly, the electrolyte itself is reduced. The charge level of the anolyte aK is not initially changed.
By lowering of ak, and consequently (pk, the terminal voltage of the redox flow battery decreases. Since the terminal voltage is held constant, the redox flow battery is charged, i.e. aA increases. As a result, aA and aK approach the same value, namely the average of aA and aK before the start of oxygen release.
In this option, the terminal voltage of the redox flow battery is measured prior to the release of oxygen and kept constant during the release of oxygen, in addition, the oxygen release is preferably carried out in this option up to aK = aA 5%, preferably up to aK = sag 2%, more preferably up to aK =aA = 1%, most preferably up to aK =
A. In this option before the start of the oxygen release aK is preferably between 50 and 100, more preferably 70 to 90, most preferably 80 to 90. aK during oxygen release can be determined by measuring gm( and equation (7). In that case a measurement of (r)jek and (pz is not necessary.
The course of the current density of the redox flow battery in this option is shown in FIG. 6 is the diffusion current density, which cannot be avoided due to technical reasons;
At the beginning of the release of oxygen, the current density increases as speed and efficiency of the electrolysis are at the highest at the beginning. The more aK and aA
approach each other the lower the current density is.
The current density is related to the membrane surface of the redox flow battery.
In an alternative to the above-mentioned option the terminal voltage of the redox flow battery is measured prior to the release of oxygen and kept constant during the = 19, release of oxygen, in addition, the oxygen release is preferably carried out in this option until the following inequality is satisfied - di/dt < 0.010 A/(cm2s), preferably up - di/dt < 0.005 A/(cm2s), and most preferably up to - di/dt < 0.001 A/(cm2s), where di / dt is the change in current density with respect to the membrane surface of the redox flow battery over time. In this alternative, before the start of the oxygen release, aK is preferably between 50 and 100, more preferably 70 to 90, most preferably 80 to 90.
In this alternative, the determination of redox potentials is not required during the oxygen release.
Under ideal conditions, no current flows when the oxygen release is complete.
Technically, however, a small flow of current (diffusion current) usually cannot be avoided. Therefore, the release of oxygen is usually terminated when the current does not flow or changes only slightly.
In a second embodiment, no fluid communication exists between the electrodes (E) and the half-cells of the redox flow battery during oxygen release and after the oxygen release a fluid connection is established, or the electrolyte solution is supplied to the redox flow battery, preferably the positive half-cell, after the oxygen release.
In the process according to the second embodiment, the electrolyte solution is preferably removed from the redox flow battery, the oxygen release is performed by electrodes (E) and the electrolytic solution is supplied again to the redox flow battery after the oxygen release.
For example, the electrolyte solution can be diverted from the circulation of the redox flow battery, be directed into a separate container in which the electrodes (E) are located and the oxygen release is performed. After the release of oxygen, the electrolyte solution is returned to the circulation of the redox flow battery.
During the release of oxygen no fluid connection exists between the electrodes (E) and the half-cells of the redox flow battery.
Preferably, the electrolyte solution is taken from the circuit of the positive half-cell of the redox flow battery.
Alternatively, the oxygen release can also be carried out in the one or more tanks of the circulation of the redox flow battery after the fluid connection has been interrupted to the half-cell(s) of the redox flow battery.
Preferably, the or each tank(s) is/are part of the circuit of the positive half-cell of the redox flow battery (catholyte).
In the second embodiment before the oxygen release aK is preferably 0 to 100, more preferably 50 to 100, still more preferably 70 to 90, most preferably 80 to 90.
In this embodiment, the oxygen release is usually performed in the catholyte.
In this way, the charge level aK is reduced because oxygen ions 02- are oxidized to elemental oxygen and accordingly, the electrolyte itself is reduced. The charge level of the anolyte aA does not change during the oxygen release.
If the charge level aK is again aA the redox flow battery has reached again its full capacity of and the release of oxygen can be terminated.
. 21 The oxygen release is preferably performed up to aK = aA 5%, preferably up to aK =
saiek 2%, more preferably up to aK = aA = 1%, most preferably up to aK =
aA during oxygen release. aK can be determined by measuring TK and equation (7). In that case a measurement of (PA and (pz is not necessary.
As mentioned above, an overcompensation can be carried out, which means that after completion of oxygen release a K < aA. While initially the full capacity ist not available, the redox flow battery runs the range of the maximum capacity until the capacity drops again during the oxidation of the electrolyte solutions.
In the following preferred features of all embodiments of the present invention are described, unless it is explicitly referred to the contrary.
Preferably, a reference electrode is present and the potential of the anode at the beginning of the oxygen release is at least 1.230 V vs. the normal hydrogen electrode (NHE) compared with the reference electrode set preferably to at least 1,500 V
vs.
NHE, more preferably at least 2.000 V vs. NHE, even more preferably to at least 2,500 V vs. NHE, relative to the normal hydrogen electrode. Typically, the potential of the anode is at the beginning of the oxygen release 10 V vs. NHE or less.
Preferably, a reference electrode is present and the potential of the cathode at the start of the oxygen release is in a range from -0.800 to + 0.300 V vs. NHE set against the reference electrode, preferably in a range from -0.500 to 0.000 V, more preferably to a range of - 0.500 to -0.200 V vs. NHE.
When the potential of the cathode is set, then the potential of the anode is automatically obtained and vice versa. When a potential is adjusted either the cathode potential or the potential of the anode is set.
For setting 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 batteries, including hybrid flow batteries. Suitable battery systems are in particular the following.
Zn/Zn2 // V02'/V02 Anode: Zn2+ +2 e- 4 Zn (pA = -0.76 V
Cathode: V02- + H20 ¨} V02 + e + 2 H+ (PK = +1.00 V
Cell: Zn2+ + 2 V02+ + 2 H20 4 2 V02+ + 4 H+ + Zn ZniZn2 // Ce3 /Ce4 Anode: Zn2+ +2 c-3 Zn (pA = -0.76 V
Cathode: Ce3+ Ce4- + e- (pK = +1.28 bis 1.72 V
Cell: 2 Ce3 + Zn2 4 2 Ce41 + Zn Cr2+/Cr3- 11 Fe2-/Fe3-Anode: Cr3+ + e 4 Cr2+ (PA = -0.42 V
Cathode: Fc2- 4 Fe3- + (pi;o _ ¨ +0.77 Cell: Cr'+ + Fe" 4 Cr2- + Fe3-Cr2'/Cr3 I/ Cr3 /Cr042-Anode: Cr s+ + e 4 Cr2+ (PA = -0.42 V
Cathode: Crs' Cr042 + 3 c (pi(o = +1.35 Cell: 4 Cr3+ 4 3 Cr 2- + Cr042-V2 N31 7/ Fe2 '!Fe3 Anode: V3+ + e- 4 V2+ (pAo = -0.25 V
Cathode: Fe2 4 Fe3 + e- (1)K0 = +0.77 Cell: v3+ Fe2-+ 4 v2+ Fe3+
V2+/V3+ Ce37Ce4-Anode: V3+ + e- 4 V2+0 (PA = -0.25 V
Cathode: Ce3+4 Ce4- + e- (I)K = +1.28 bis 1.72 V
Cell: V3' + Ce3- 4 V2- + Cei Fe/Fe2 // Fe2 cd/cd24 Fe2- /Fe3 Pb,43b2+ Pb2+!Pb02 Anode: Pb2- + 2 e-4 Pb Cathode: Pb2- +2 H20 4 Pb02 +4 H- +2 e-Cell: 2 Pb2 + 2 H20 4 Pb + Pb02 + 4 H{
Polyoxometalates Anode: [SiV1v3Wv19040]1 - + 3 c 4 [SiV1v1Wv3WV16040]13-Cathode: [SiV1v3W\119040]1 -4 [SiVvilWv19040]7- + 3 e--Cell: 2 [SiVr\"3Wv19040]10- [Si7iv3Wv3Ww6040]13- + [SiVv3Wv19040]7-24 , Ti3+/Ti02- Fe27Fe3+
Anode: TiO2+ + 2 H+ + 2 e- 4 Ti + H20o ¨_ (pA + 0.04 V
Cathode: Fe2- 4 Fe3- + e- (PK + 0.77 V
Cell: TiO2+ + 2 H+ +2 Fe2-9 Ti3+ + H20 + 2 Fe-Cu/CuCI /1 CuCliCu2+
Anode: Cu- + e 4 Cu0 (PA = 0.153 V
Cathode: Cu 4 Cu2 + e- 0 (plc = + 0.521 V
Cell: 2 Cu- 4 Cu2- + Cu CulCu21 1/ Pb2 I/Pb02 Anode: Cu2+ + 2 e-9 Cu (PAO = +0.34 V
Cathode: PbSO4 + 2 H20 4 Pb01 + 4 H- +S042- +2 e- (pK = +1.69 V
Cell: CuSO4 +PbSO4 + 2 H20 4 Pb02 + Cu + 2 H2SO4 Particularly preferred the redox flow battery is a vanadium redox flow battery. In the following preferred features of the vanadium redox flow battery are described unless it is explicitly referred to the contrary.
In a vanadium redox flow battery vanadium is in the oxidation state (+II) to (+V) before. The corresponding oxidation part equations are as follows.
V02' + H20 - V02 + 2H +& E = +
0.998 V (I) V3' +H20 VO2' + 2H' + e- E = +
0.377 V (II) V2+ ___________ - V3- +e- e= - 0.255 V (HI) The partial oxidation equation for the oxygen formation is as follows.
2H20 - 02 + 4H+ + 4e- E = + 1,230 V
Thus, the equations for anodes and cathodes are as follows.
Anode: V3+ + e 4 V2+o = -0.25 V
Cathode: V02' + H20 4 V02' + e- + 2 HI_ o (pK. - +1.00 Cell: V02 + V3' + H70 4 V021 + V2 +2 H' Due to the potentials it is believed that in a vanadium-containing electrolyte solution a V (+II), V (+III) and V (+IV) compound, if contained, is firstly oxidized to V
(+V) before the oxygen release begins, that is, the reactions according to equations (I) to (III ) run first.
During the release of oxygen the following reactions take place (I) oxidation of H20 to 02 at the anode, (ii) reduction of V02+ to lower oxidation states at the cathode, Theoretically, a reduction to V2+ takes place at the cathode. The resulting V2+
symproportionates with V (+IV), and V (+V) - compounds which are present in the solution. If V (+V) compounds are present, V2+ symproportionates with V (+ V) usually to V (+IV) compounds. Only if V (+V) compounds are no longer present V2+
symproportionates with V (+ IV) - compounds to V (+III) - compounds.
Accordingly, in the course of the oxygen release, the average oxidation state of the electrolyte is lowered. As a consequence, the redox potential of the electrolyte solution decreases.
That is, there are proportionally fewer V (+V) in the solution, i.e. the number of = 26 =
vanadium compounds initially to be oxidized increases and the efficiency of release of oxygen decreases.
The efficiency of electrolysis depends directly on the redox potential of the electrolyte solution. The kinetics of the oxidation of vanadium, which concurrently takes place on the anode in addition to the oxygen oxidation, decreases with increasing redox potential. The higher the proportion of vanadium ions with high oxidation state, the higher the redox potential of the electrolyte solution, the more efficient electrolysis is.
It is therefore advantageous if the proportion of oxidizable vanadium ions is minimized at the beginning of electrolysis. The higher the percentage of V02+, the lower the proportion of oxidizable Vanadium ions is, since they cannot be oxidized further. Thus it is convenient if an electrolytic solution, containing a mixture of V3+ and V02+ is not immediately subjected electrolysis, but an electrolytic solution having a high proportion of V02+. So the proportion of V02+ increases during the course of the electrolysis.
Before the start of oxygen release, the electrolytic solution preferably does not has V
(+II) compounds, more preferably no V (+II) - and V (+III) - compounds, even more preferably, the molar ratio of V (+V) - compounds to V (+IV) - compounds each based on vanadium ions is between 50 and 100, even more preferably between 70 and 90 and most preferably between 80 and 90.
Due to the progressive release of oxygen and the concomitant reduction in the average oxidation state of the electrolyte, the efficiency of release of oxygen is reduced. Therefore, the release of oxygen is preferably terminated before the efficiency drops below a certain value. This endpoint may be determined as generally described above wherein = V2' Am =
Kn-4 = V02+
= v02+
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 is connected as an anode, wherein the electrochemical cell is located in at least a half-cell of a redox flow battery and / or a fluid connection exists between the electrochemical cell and at least a half-cell of a redox flow battery.
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. Preferably, the electrochemical cell is in the positive half-cell of the redox flow battery and / or a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery.
The present invention further relates to the use of electrical current for oxygen evolution from an electrolyte solution of a redox flow battery.
Usually, per 1.0 mol of formed oxygen (02) not more than 1.5 moles of hydrogen (H2) are formed.
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.
Figures In Figure 1, the electrochemical cell of the example is shown.
. .. CA 02970178 2017-06-08 , 28' LIST OF REFERENCE NUMBERS
A: Beaker B: Electrolyte solution C: Reference electrode (Hg / Hg2SO4 electrode) D: Measuring electrode (glassy carbon electrode) E: Magnetic stirrer F: Counter electrode (iridium electrode), cathode G: Working electrode (graphite electrode), the anode (Pz cell voltage (Pc: cathode potential (PA: anode potential (PR: redox potential Figure 2 shows the potentials and cell voltage before the electrolysis of a 1.6 M V02+
in 2M H2SO4.
Figure 3 shows the potentials, electric current and cell voltage during electrolysis of a 1.6 M V02+ solution in 2 M H2SO4.
Figure 4 shows the redox potential and electric current during the electrolysis of a 1.6 M V02+ - H2SO4 solution in 2 M.
29' Figure 5 shows the potential and cell voltage by the electrolysis of a 1.6 M
H2SO4 in 2M V02+.
Figure 6 shows the profile of the current density of the redox flow battery at a constant terminal voltage (potentiostatic charging) wherein is the diffusion current density, which normally cannot be avoided for technical reasons.
In the following items, the present invention will be further described:
1. A process for oxygen evolution from an aqueous electrolyte solution of a redox flow battery, wherein at least two electrodes (E) are in electrically conductive contact with the electrolytic solution at least one of the electrodes (E) is connected as an anode and at least one is connected as a cathode and at the anode oxygen (02) is formed, and wherein at the cathode no hydrogen is formed or per 1.0 mole of formed oxygen (02) not more than 1.5 moles of hydrogen (H2) are formed.
2. The method of item 1, wherein the electrodes (E) are not the electrodes being used in the current drain from the redox flow battery.
During transport, storage or operation of the battery, a complete exclusion of air can often not be ensured, and oxidation processes may occur, so that the elytes are shifted toward higher oxidation states, resulting in a loss of capacity.
This problem is known in the art and attempts have been made so far to prevent the oxidation processes by inertization of the electrolyte solution. For this purpose, nitrogen, for example, was used as inert gas or inert organic liquids with a lower density than the electrolyte solution in the storage containers were used in order to prevent contact of the electrolyte solution with oxygen. Owing to circulation processes or diffusion processes, however, a contact of the electrolyte solution with oxygen cannot be completely prevented. Therefore, there is a need for methods in which the described loss of capacity resulting from the oxidation of the electrolyte can be reversed.
These and other objects are achieved with the method according to the invention and with 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 battery, wherein at least two electrodes (E) are in electrically conductive contact with the electrolytic solution at least one of the electrodes (E) is an anode and at least one is connected as a cathode, and the anode forms oxygen (02), the cathode does not form hydrogen or forms not more than 1.5 moles of hydrogen (H2) per 1.0 mole of formed oxygen (02).
r 3 =
It has surprisingly been found that the electrolyte solution of a redox flow battery can be regenerated by means of electrolysis. For example, the regeneration can be carried out during the operation of the redox flow battery, for example during the charging process, without interruptions during operation. A regeneration in the batch process, for example by separating a part of the electrolyte solution, release of oxygen according to the method of the present invention, and subsequent reintroduction into the redox flow battery is also possible. Finally, an electrolyte solution can be subjected to the process according to the present invention before the addition to the redox flow battery.
Instead of a complete electrolysis of water, the components of the electrolyte are reduced instead of any H+ ions.
Within the scope of the present invention, the term "redox flow battery" is used in its usual meaning. The basic design of a redox flow battery, for example a vanadium-redox flow battery, is known to a person skilled in the art.
Within the scope of the present invention the term "electrochemical cell" is used for an electrolysis cell and relates to an arrangement of electrodes which are conductively connected by an electrolyte. During the circuit continuity through the electrolyte a chemical change is caused which results in a direct conversion of electrical energy into chemical energy by electrode reactions and ionic migration.
Within the scope of the present invention, the term "electrolyte" is used in its usual meaning and refers to ion-conducting media whose electrical conductivity is caused by electrolytic dissociation into ions.
The positive half-cell of the redox flow battery is the part of the galvanic cell of the redox flow battery, which represents the positive pole of the redox flow battery in relation to the current drain.
If the term "half-cells" is used in the present application, this refers to the half-cells of , 4 I
the galvanic cell of the redox flow battery, unless explicitly referred to the contrary.
Within the scope of the present invention, the term "anolyte" refers to the material of a redox flow battery, which is directly influenced by the anode during discharging. The corresponding half cell is the negative half cell.
For the purposes of the present invention, the term "catholyte" refers to the material of a redox flow battery which is directly influenced by the cathode during discharging.
The corresponding half cell is the positive half cell.
"Circulation" or "circulation of the redox flow battery", etc. denotes one of the two separate circuits of the redox flow battery. These circuits are separated by a membrane in the galvanic cell of the redox flow battery.
Accordingly, there is the circulation of the positive half cell and the circulation of the negative half cell.
As described above, the capacity of a redox flow battery is determined by the amount of active materials dissolved in the electrolyte, and can only be made fully utilizable when an equivalent mixture of anolyte and catholyte is present. Due to undesirable oxidation processes the equimolar mixture of anolyte and catholyte is shifted in the direction of higher oxidation states, resulting in a loss of capacity of the battery.
In the case of a vanadium redox flow battery, for example, this means that in the fully charged state, only V5+ is present in the catholyte, while a mixture of V2+
and V3+ is present in the anolyte.
Within the scope of this invention, it has been found that it is possible to revert the oxidation processes on the electrolyte solution of a redox flow battery by electrolysis.
As described above, at least two electrodes (E) are in electrically conductive contact with the electrolyte solution, with at least one of the electrodes (E) being connected as an anode and at least one as a cathode, and oxygen (02) being formed at the anode.
The electrodes (E) can be part of an electrolysis cell. The process can be carried out batchwise or continuously, preferably continuously.
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) may only consist of anode(s) and cathode(s). However, then the potential of the cathode should not be too negative in order to avoid hydrogen development. If hydrogen is nevertheless formed, this can be reacted with oxygen again to form water, as explained in detail below.
The potential of the cathode can be controlled by means of a reference electrode.
Preferably, therefore, a reference electrode is present, more preferably the cathodic potential is adjusted such that no hydrogen is produced.
As the reference electrode, for example, an Hg / H92SO4 reference electrode, Ag /
AgCI reference electrode or a normal hydrogen electrode may be used. The reference electrode is also designated as an indifferent electrode. Both terms are used synonymously in this application.
By means of the electrode for measuring the redox potential of the electrolyte solution, the reaction sequence of the oxygen release can be followed as explained in detail below.
As the electrode used for the determination of the redox potential of the electrolyte solution any electrode can be used for measuring the redox potential of the electrolyte solution, and the person skilled in the art usually selects inert electrodes such as, for example, carbon-based electrodes, platinum or gold electrodes, preferably a glassy , 6 carbon electrode.
Preferably, the electrodes (E) consist of - one or more anode(s), - one or more cathode(s), - optionally one or more reference electrode(s), - optionally one or more usually 1 or 2 electrode(s) for measuring the redox potential.
For the determination of the redox potential, for example, a single-rod measuring cell can be used, which normally comprises two electrodes. Alternatively, two electrodes which are not connected to a single-electrode measuring cell, for example two separate electrodes, can be used.
The redox potential may also be determined against the reference electrode, if any.
Usually, the electrodes (E) contain, preferably consist of, only one electrode for measuring the redox potential.
A single-rod measuring cell is usually used or the redox potential is determined against the reference electrode.
If the reference electrode is used to measure the redox potential, it does not belong to the electrodes for measuring the redox potential.
Conventionally, an anode and a cathode are used. Alternatively, a plurality of cathodes and / or anodes may be used. The use of a plurality of cathodes and a plurality of anodes can be advantageous, for example, if higher a power is desired. If a plurality of anodes and / or cathodes are used, the above-mentioned quantities of oxygen and hydrogen formed are related to the respective total quantity of oxygen produced and hydrogen formed.
It is also possible that the oxygen release is carried out simultaneously or w , 7 , successively at several points in the redox flow battery. In this case, the method of the present invention is performed several times simultaneously or successively.
If present, usually only one reference electrode is present.
If present, there is usually only one or two electrodes for measuring the redox potential. As stated above, the determination of the redox potential may be performed by means of a single-rod measuring cell, usually including two electrodes, two separate electrodes or by means of an electrode vis-à-vis a reference electrode.
In particular the electrodes (E) consist of - an anode, - a cathode, - optionally a reference electrode, - optionally one or two electrodes for measuring the redox potential, for example two electrodes in the form of a single-rod measuring cell.
Preferably, the electrodes (E) are not the electrodes used for the current drain from the redox flow battery. In this preferred embodiment, the electrodes (E) comprise all the electrodes which are not electrodes that are used for the current drain of the redox flow battery. In other words, in this preferred embodiment, the redox flow battery contains only the electrodes which are used for the current drain of the redox flow battery 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 H20 to 02. In one embodiment, the anode(s) of the electrodes (E) comprises 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, II.
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) is / are composed of Ir.
A dimensionally stable electrode (DSA) can also be used as anode(s) of the electrodes (E). Dimensionally stable electrodes are not homogeneous regarding their microstructure, their phase composition and also their surface. Titanium electrodes are generally referred to as dimensionally stable electrodes which are provided with a mixed oxide coating of titanium oxide with one or more noble metal oxides of the rutile type (e.g. RuxTi1_x02). In one embodiment, the anode(s) of the electrodes (E) is / are composed 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 applied to a carrier anode of Ti, Ta, Zr, Nb or their alloys. Particularly preferred dimensionally stable electrode(s) (DSA) as the anode(s) of the electrode(s) (E) comprise(s) a titanium support and a coating of Ti02-Ru02 or TiO2-Ru02.
At the cathode of the electrodes (E) the reduction of the electrolyte takes place during the oxygen release. For example, in the case of a vanadium redox flow battery, the reduction of V02+ takes place towards lower oxidation states.
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.
_________ LJ-The electrochemical potential window in an aqueous solution, i.e. the voltage range of the potential required for the desired reaction 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 in its modifications diamond (doped), graphite as well as glass(y) carbon, lead, zinc, cadmium and mercury have a high hydrogen overvoltage.
Preferably, the cathode(s) of the electrodes (E) comprise(s) a material selected from the group consisting of glassy carbon, graphite, and diamond. Preferably, the cathode(s) of the electrodes (E) is / are composed of one material selected from the group consisting of glassy carbon, graphite, and diamond (doped). Particular preference is given to a graphite electrode. As stated above, no hydrogen or not more than 1.5 mol of hydrogen (H2) is formed per 1.0 mol of oxygen (02) formed. If hydrogen is formed, preferably per 1.0 mol of formed oxygen (02) not more than 1.0 mol of hydrogen (H2), more preferably not more than 0.5 mol of hydrogen (H2) is formed. In a preferred embodiment not more than 0.1 mol of hydrogen (H2) is formed.
If hydrogen is formed, the hydrogen formed is reacted with oxygen, preferably with the oxygen formed at the anode, on one or more catalyst(s) to form water. The water thus formed is preferably returned to the electrolyte solution.
Catalyst(s) suitable for the conversion of hydrogen with oxygen to obtain water are catalysts which can also be used for the generation of oxygen. For example, the abovementioned catalysts are suitable for the oxygen generation.
The catalyst(s) for the reaction of hydrogen with oxygen to obtain water are 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. As dimensionally stable catalysts are generally referred to titanium catalysts which are provided with a mixed oxide coating of titanium oxide with one or more noble metal oxides of the rutile type (e.g.
RuJii,02). In an embodiment the catalyst(s) is / are composed of a coating of a 1 0 t 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 which are applied to a support of Ti, Ta, Zr, Nb or their alloys. Particularly preferred dimensionally stable catalyst(s) comprise(s) a titanium support and a coating of Ti02-Ru02 or Ti02-RuO2.
The reaction of the formed hydrogen with oxygen, preferably with the oxygen formed at the anode, is usually carried out in one recombination unit normally comprising a catalyst, preferably the above-mentioned catalyst, for converting hydrogen and oxygen to water, which unit is so connected to the electrochemical cell that both the hydrogen formed during the electrolysis and the oxygen formed during the electrolysis can be introduced into the recombination unit.
Most preferably, no hydrogen is formed at the cathode.
The loss of capacity in ampere hours (Ah) is proportional to the amount of oxygen absorbed by the redox flow battery. For example, a loss in capacity of one ampere hour corresponds to the absorption of 9.33 mmol oxygen (02). 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 / (4 * 26.8) since it is a four-electron transition.
Consequently, the extent to which the regeneration of the electrolyte solution has progressed can be determined by means of the amount of oxygen released and optionally by means of the amount of hydrogen.
The process according to the invention is preferably terminated when the net amount of liberated oxygen corresponds to 90% of the theoretically releasable oxygen, preferably 95% of the theoretically releasable oxygen, and most preferably 99%
of the theoretically releasable oxygen.
The "net quantity of released oxygen" corresponds to the total quantity of oxygen released minus the amount of oxygen which, if hydrogen is released, can be recombined with that hydrogen released to obtain water. Thus the "net quantity of released oxygen" corresponds to:
The total amount of oxygen (in mol) minus 0.5 times the total amount of hydrogen released in (mol).
The "theoretically releasable oxygen" is the amount of oxygen which, as described above, is calculated from the loss of capacity by the following equation.
6,24 1018 4 = As KV = 02(theoretica1) 6.02/ 1 023 ino1-1 wherein KV is the loss of capacity in [As]
02 (theoretical) is the theoretically releasable oxygen in mol Alternatively, the oxygen release can be carried out as follows.
For this purpose, at first the following equations are given.
Reactions of the battery towards charging:
Anode: Am + ze- Am -z (1) Cathode: K" Kn+z + ze- (2) Cell: Am + K" Am -z + K"z (3) Definition of the charge level a (SOC):
Concentration of the reaction product divided by the total concentrations of educt and product = 12 _ cAr-z C + CA,. (4) aK- __ C + C
K (5) wherein aA is the state of charge of the anolyte and al< is the state of charge of the catholyte, cAm--1 is the concentration of the species A' CA' is the concentration of the species Am Cmr-z is the concentration of the species Kn+z CKr1 is the concentration of the species Kr' wherein A", Am, Kn+z, and Kr' are different from each other.
In the case where the redox flow battery has no loss of capacity, i.e. the capacity is at the highest: aA =K.
As a result of oxidation of the electrolyte by, for example, oxygen, the concentration of oxidized species, in particular Am, is increased without producing a corresponding reduced species. As a result, aA is smaller than ak, since, given a state of charge of = 1 in the corresponding anolyte, not only reduced species A' but also unreduced species Am are present.
The potentials depending on the charge level are as follows:
Anolyte:
13' R- T a 0 in __________ A (6) (PA = PA Z = F aA
Catholyte:
(p,< :17 (1)K _____________________________________________________ - In (7) z= F 1-Cell:
=
0 0 __ T - 7 \ -In a + in{ \ aA (8) (Pz z= F aK 1- aA
wherein can be taken from equation (1) and (2) universal gas constant, R = 8.31447 J.mo1-1 K-1 Faraday constant, F = 96485.34 C mo1-1 is the temperature a A and aK can be taken from equations (4) and (5) (pK and (PA are the respective standard potentials (Pk, (PA and (Pz are the current potentials of the catholyte, the anolyte or the redox flow battery.
Normally, different proton concentrations are present in a redox flow battery in the catholyte and the anolyte. These usually lead to a membrane potential (pm R =T = In c;
(pm - _______________________________________________________________ (9) F
wherein R, T, F are as defined above;
c- is the proton concentration in the catholyte; and Ir is the proton concentration in the anolyte.
This membrane potential is taken into consideration in Equation (8) as follows.
\
(Pz = - T a, (PA + In K + In a A (pm ( I 0) Z = F 1-u11 If two of the three values (pK, (PA and (pz are present, the third value can be calculated according to equation (10). Usually (PK and (pz are measured and TA is calculated.
If (pK can be measured using equation (7), aK can be calculated from (PK. aA
is then obtained from equation (10) and (PA from equation (6).
The loss of capacity can then be calculated from aA and aK. This results in a relative loss of capacity. The absolute value can be calculated by the volume of the respective solutions.
aA is then obtained from equation (9) and (PA from equation (6).
In a first embodiment, the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid connection is present between the electrodes (E) and the positive half-cell of the redox flow battery during oxygen release.
Preferred features of the first embodiment of the present invention will be described below.
In this context, fluid connection 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 battery.
. I CA 02970178 2017-06-08 . 1.5 Preferably, a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release.
According to the process of the first embodiment the electrolyte is usually circulated during the oxygen release. In other words, a continuous oxygen release is usually carried out.
The oxygen release is preferably carried out in an electrochemical cell which is located in the circuit of the electrolyte. The electrochemical cell can be a part of the circuit of the electrolyte while none oxygen release takes place and the oxygen release can take place as required. Alternatively, for oxygen release, at least a part of the electrolytes are diverted or branched off 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 can take place during operation of the redox flow battery, i.e. the redox flow battery can be charged or discharged during the oxygen release or neither be charged nor discharged. Preferably, the redox flow battery is not discharged during the oxygen release.
Capacity changes of the redox flow battery by the oxygen release are not referred to as charging or discharging the redox flow battery.
aK before the start of the oxygen release is preferably 0 to 100, more preferably 50 to 100, more preferably 70 to 90. In a preferred embodiment, aK is 80 to 90 before the oxygen release begins.
In an option of this embodiment, the redox flow battery is not charged or discharged during the oxygen release and the oxygen release is usually carried out in the catholyte. Oxygen ions 02- are oxidized to elemental oxygen by the oxygen release and the electrolyte itself is reduced accordingly. The charge level of the anolyte aA
16, does not change during the oxygen release.
When the charge level aK is again equal to apk, the redox flow battery has reached its full capacity again and the oxygen release can be terminated.
In this option, the redox flow battery is not charged during the oxygen release and the oxygen release is carried out until aK = aA 5%, preferably to aK = A 2%, more preferably to aK = aA 1%, most preferably to aK = aA. In this option, aK is preferably between 50 and 100 before the start of the oxygen release, more preferably 70 to 90, particularly preferably 80 to 90. aK can be determined during the oxygen release by measuring (1)K and equation (7). In that case a measurement of (PA and cpz is not necessary.
As explained above, an overcompensation can be carried out, i.e., after the oxygen release is complete aK < aA. Although initially the full capacity is not available, the redox flow battery passes through the range of the maximum capacity during the oxidation of the electrolyte solutions, which decreases again.
In a further option of this embodiment, the redox flow battery is charged during the oxygen release and the oxygen release is usually carried out in the catholyte.
In this option aA increases by the charging process. Regarding aK, the following opposing effects are obtained.
- aK is increased by the charging process as such - aK is reduced by the oxygen release as such.
Depending on which effect is predominant, aK is increased, decreased or remains constant.
Even if aK is increased, a K increases slower than aA and thus an approximation of aK
, 17, and aA occurs.
In this further option, the redox flow battery is charged during the oxygen release and the oxygen release is carried out up to aK 5%, preferably up to aK = 2%, more preferably up to aK = aft, 1%, most preferably up to aK = aA. In this option, aK is preferably between 50 and 100 before the start of the oxygen release, more preferred 70 to 90, particularly preferred 80 to 90.
As described above, an overcompensation can be carried out, i.e., after the oxygen has been released, aK < aA. Although initially the full capacity is not available, the redox flow battery passes through the range of the maximum capacity during the oxidation of the electrolyte solutions, until it decreases again.
The oxidation of oxygen and the constituents of the electrolyte can be competition reactions depending on the respective redox potential of the components of the electrolyte. That is to say, in addition to an oxidation of 2 02- to 02, oxidation of the electrolyte constituents can also occur. Therefore, it would be advantageous for the oxygen release alone if no ions are present in the electrolyte solution which can be oxidized in a competition reaction. However, for this purpose, the battery would first have to be fully charged and subsequently the oxygen release should be carried out, resulting in an interruption in battery operation, which is not preferred for economic reasons.
In another option of the first embodiment, the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during oxygen release and the electrolyte solution is circulated during oxygen release. In this option, the voltage of the redox flow battery, usually terminal voltage, is measured before the start of oxygen release and kept constant during oxygen release. This can be effected, for example, by an external current source, usually a potentiostat.
Such a method is also referred to as potentiostatic loading.
18, By the oxygen release the charge level aK is reduced because oxygen ions 02-are oxidized to elemental oxygen and, accordingly, the electrolyte itself is reduced. The charge level of the anolyte aK is not initially changed.
By lowering of ak, and consequently (pk, the terminal voltage of the redox flow battery decreases. Since the terminal voltage is held constant, the redox flow battery is charged, i.e. aA increases. As a result, aA and aK approach the same value, namely the average of aA and aK before the start of oxygen release.
In this option, the terminal voltage of the redox flow battery is measured prior to the release of oxygen and kept constant during the release of oxygen, in addition, the oxygen release is preferably carried out in this option up to aK = aA 5%, preferably up to aK = sag 2%, more preferably up to aK =aA = 1%, most preferably up to aK =
A. In this option before the start of the oxygen release aK is preferably between 50 and 100, more preferably 70 to 90, most preferably 80 to 90. aK during oxygen release can be determined by measuring gm( and equation (7). In that case a measurement of (r)jek and (pz is not necessary.
The course of the current density of the redox flow battery in this option is shown in FIG. 6 is the diffusion current density, which cannot be avoided due to technical reasons;
At the beginning of the release of oxygen, the current density increases as speed and efficiency of the electrolysis are at the highest at the beginning. The more aK and aA
approach each other the lower the current density is.
The current density is related to the membrane surface of the redox flow battery.
In an alternative to the above-mentioned option the terminal voltage of the redox flow battery is measured prior to the release of oxygen and kept constant during the = 19, release of oxygen, in addition, the oxygen release is preferably carried out in this option until the following inequality is satisfied - di/dt < 0.010 A/(cm2s), preferably up - di/dt < 0.005 A/(cm2s), and most preferably up to - di/dt < 0.001 A/(cm2s), where di / dt is the change in current density with respect to the membrane surface of the redox flow battery over time. In this alternative, before the start of the oxygen release, aK is preferably between 50 and 100, more preferably 70 to 90, most preferably 80 to 90.
In this alternative, the determination of redox potentials is not required during the oxygen release.
Under ideal conditions, no current flows when the oxygen release is complete.
Technically, however, a small flow of current (diffusion current) usually cannot be avoided. Therefore, the release of oxygen is usually terminated when the current does not flow or changes only slightly.
In a second embodiment, no fluid communication exists between the electrodes (E) and the half-cells of the redox flow battery during oxygen release and after the oxygen release a fluid connection is established, or the electrolyte solution is supplied to the redox flow battery, preferably the positive half-cell, after the oxygen release.
In the process according to the second embodiment, the electrolyte solution is preferably removed from the redox flow battery, the oxygen release is performed by electrodes (E) and the electrolytic solution is supplied again to the redox flow battery after the oxygen release.
For example, the electrolyte solution can be diverted from the circulation of the redox flow battery, be directed into a separate container in which the electrodes (E) are located and the oxygen release is performed. After the release of oxygen, the electrolyte solution is returned to the circulation of the redox flow battery.
During the release of oxygen no fluid connection exists between the electrodes (E) and the half-cells of the redox flow battery.
Preferably, the electrolyte solution is taken from the circuit of the positive half-cell of the redox flow battery.
Alternatively, the oxygen release can also be carried out in the one or more tanks of the circulation of the redox flow battery after the fluid connection has been interrupted to the half-cell(s) of the redox flow battery.
Preferably, the or each tank(s) is/are part of the circuit of the positive half-cell of the redox flow battery (catholyte).
In the second embodiment before the oxygen release aK is preferably 0 to 100, more preferably 50 to 100, still more preferably 70 to 90, most preferably 80 to 90.
In this embodiment, the oxygen release is usually performed in the catholyte.
In this way, the charge level aK is reduced because oxygen ions 02- are oxidized to elemental oxygen and accordingly, the electrolyte itself is reduced. The charge level of the anolyte aA does not change during the oxygen release.
If the charge level aK is again aA the redox flow battery has reached again its full capacity of and the release of oxygen can be terminated.
. 21 The oxygen release is preferably performed up to aK = aA 5%, preferably up to aK =
saiek 2%, more preferably up to aK = aA = 1%, most preferably up to aK =
aA during oxygen release. aK can be determined by measuring TK and equation (7). In that case a measurement of (PA and (pz is not necessary.
As mentioned above, an overcompensation can be carried out, which means that after completion of oxygen release a K < aA. While initially the full capacity ist not available, the redox flow battery runs the range of the maximum capacity until the capacity drops again during the oxidation of the electrolyte solutions.
In the following preferred features of all embodiments of the present invention are described, unless it is explicitly referred to the contrary.
Preferably, a reference electrode is present and the potential of the anode at the beginning of the oxygen release is at least 1.230 V vs. the normal hydrogen electrode (NHE) compared with the reference electrode set preferably to at least 1,500 V
vs.
NHE, more preferably at least 2.000 V vs. NHE, even more preferably to at least 2,500 V vs. NHE, relative to the normal hydrogen electrode. Typically, the potential of the anode is at the beginning of the oxygen release 10 V vs. NHE or less.
Preferably, a reference electrode is present and the potential of the cathode at the start of the oxygen release is in a range from -0.800 to + 0.300 V vs. NHE set against the reference electrode, preferably in a range from -0.500 to 0.000 V, more preferably to a range of - 0.500 to -0.200 V vs. NHE.
When the potential of the cathode is set, then the potential of the anode is automatically obtained and vice versa. When a potential is adjusted either the cathode potential or the potential of the anode is set.
For setting 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 batteries, including hybrid flow batteries. Suitable battery systems are in particular the following.
Zn/Zn2 // V02'/V02 Anode: Zn2+ +2 e- 4 Zn (pA = -0.76 V
Cathode: V02- + H20 ¨} V02 + e + 2 H+ (PK = +1.00 V
Cell: Zn2+ + 2 V02+ + 2 H20 4 2 V02+ + 4 H+ + Zn ZniZn2 // Ce3 /Ce4 Anode: Zn2+ +2 c-3 Zn (pA = -0.76 V
Cathode: Ce3+ Ce4- + e- (pK = +1.28 bis 1.72 V
Cell: 2 Ce3 + Zn2 4 2 Ce41 + Zn Cr2+/Cr3- 11 Fe2-/Fe3-Anode: Cr3+ + e 4 Cr2+ (PA = -0.42 V
Cathode: Fc2- 4 Fe3- + (pi;o _ ¨ +0.77 Cell: Cr'+ + Fe" 4 Cr2- + Fe3-Cr2'/Cr3 I/ Cr3 /Cr042-Anode: Cr s+ + e 4 Cr2+ (PA = -0.42 V
Cathode: Crs' Cr042 + 3 c (pi(o = +1.35 Cell: 4 Cr3+ 4 3 Cr 2- + Cr042-V2 N31 7/ Fe2 '!Fe3 Anode: V3+ + e- 4 V2+ (pAo = -0.25 V
Cathode: Fe2 4 Fe3 + e- (1)K0 = +0.77 Cell: v3+ Fe2-+ 4 v2+ Fe3+
V2+/V3+ Ce37Ce4-Anode: V3+ + e- 4 V2+0 (PA = -0.25 V
Cathode: Ce3+4 Ce4- + e- (I)K = +1.28 bis 1.72 V
Cell: V3' + Ce3- 4 V2- + Cei Fe/Fe2 // Fe2 cd/cd24 Fe2- /Fe3 Pb,43b2+ Pb2+!Pb02 Anode: Pb2- + 2 e-4 Pb Cathode: Pb2- +2 H20 4 Pb02 +4 H- +2 e-Cell: 2 Pb2 + 2 H20 4 Pb + Pb02 + 4 H{
Polyoxometalates Anode: [SiV1v3Wv19040]1 - + 3 c 4 [SiV1v1Wv3WV16040]13-Cathode: [SiV1v3W\119040]1 -4 [SiVvilWv19040]7- + 3 e--Cell: 2 [SiVr\"3Wv19040]10- [Si7iv3Wv3Ww6040]13- + [SiVv3Wv19040]7-24 , Ti3+/Ti02- Fe27Fe3+
Anode: TiO2+ + 2 H+ + 2 e- 4 Ti + H20o ¨_ (pA + 0.04 V
Cathode: Fe2- 4 Fe3- + e- (PK + 0.77 V
Cell: TiO2+ + 2 H+ +2 Fe2-9 Ti3+ + H20 + 2 Fe-Cu/CuCI /1 CuCliCu2+
Anode: Cu- + e 4 Cu0 (PA = 0.153 V
Cathode: Cu 4 Cu2 + e- 0 (plc = + 0.521 V
Cell: 2 Cu- 4 Cu2- + Cu CulCu21 1/ Pb2 I/Pb02 Anode: Cu2+ + 2 e-9 Cu (PAO = +0.34 V
Cathode: PbSO4 + 2 H20 4 Pb01 + 4 H- +S042- +2 e- (pK = +1.69 V
Cell: CuSO4 +PbSO4 + 2 H20 4 Pb02 + Cu + 2 H2SO4 Particularly preferred the redox flow battery is a vanadium redox flow battery. In the following preferred features of the vanadium redox flow battery are described unless it is explicitly referred to the contrary.
In a vanadium redox flow battery vanadium is in the oxidation state (+II) to (+V) before. The corresponding oxidation part equations are as follows.
V02' + H20 - V02 + 2H +& E = +
0.998 V (I) V3' +H20 VO2' + 2H' + e- E = +
0.377 V (II) V2+ ___________ - V3- +e- e= - 0.255 V (HI) The partial oxidation equation for the oxygen formation is as follows.
2H20 - 02 + 4H+ + 4e- E = + 1,230 V
Thus, the equations for anodes and cathodes are as follows.
Anode: V3+ + e 4 V2+o = -0.25 V
Cathode: V02' + H20 4 V02' + e- + 2 HI_ o (pK. - +1.00 Cell: V02 + V3' + H70 4 V021 + V2 +2 H' Due to the potentials it is believed that in a vanadium-containing electrolyte solution a V (+II), V (+III) and V (+IV) compound, if contained, is firstly oxidized to V
(+V) before the oxygen release begins, that is, the reactions according to equations (I) to (III ) run first.
During the release of oxygen the following reactions take place (I) oxidation of H20 to 02 at the anode, (ii) reduction of V02+ to lower oxidation states at the cathode, Theoretically, a reduction to V2+ takes place at the cathode. The resulting V2+
symproportionates with V (+IV), and V (+V) - compounds which are present in the solution. If V (+V) compounds are present, V2+ symproportionates with V (+ V) usually to V (+IV) compounds. Only if V (+V) compounds are no longer present V2+
symproportionates with V (+ IV) - compounds to V (+III) - compounds.
Accordingly, in the course of the oxygen release, the average oxidation state of the electrolyte is lowered. As a consequence, the redox potential of the electrolyte solution decreases.
That is, there are proportionally fewer V (+V) in the solution, i.e. the number of = 26 =
vanadium compounds initially to be oxidized increases and the efficiency of release of oxygen decreases.
The efficiency of electrolysis depends directly on the redox potential of the electrolyte solution. The kinetics of the oxidation of vanadium, which concurrently takes place on the anode in addition to the oxygen oxidation, decreases with increasing redox potential. The higher the proportion of vanadium ions with high oxidation state, the higher the redox potential of the electrolyte solution, the more efficient electrolysis is.
It is therefore advantageous if the proportion of oxidizable vanadium ions is minimized at the beginning of electrolysis. The higher the percentage of V02+, the lower the proportion of oxidizable Vanadium ions is, since they cannot be oxidized further. Thus it is convenient if an electrolytic solution, containing a mixture of V3+ and V02+ is not immediately subjected electrolysis, but an electrolytic solution having a high proportion of V02+. So the proportion of V02+ increases during the course of the electrolysis.
Before the start of oxygen release, the electrolytic solution preferably does not has V
(+II) compounds, more preferably no V (+II) - and V (+III) - compounds, even more preferably, the molar ratio of V (+V) - compounds to V (+IV) - compounds each based on vanadium ions is between 50 and 100, even more preferably between 70 and 90 and most preferably between 80 and 90.
Due to the progressive release of oxygen and the concomitant reduction in the average oxidation state of the electrolyte, the efficiency of release of oxygen is reduced. Therefore, the release of oxygen is preferably terminated before the efficiency drops below a certain value. This endpoint may be determined as generally described above wherein = V2' Am =
Kn-4 = V02+
= v02+
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 is connected as an anode, wherein the electrochemical cell is located in at least a half-cell of a redox flow battery and / or a fluid connection exists between the electrochemical cell and at least a half-cell of a redox flow battery.
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. Preferably, the electrochemical cell is in the positive half-cell of the redox flow battery and / or a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery.
The present invention further relates to the use of electrical current for oxygen evolution from an electrolyte solution of a redox flow battery.
Usually, per 1.0 mol of formed oxygen (02) not more than 1.5 moles of hydrogen (H2) are formed.
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.
Figures In Figure 1, the electrochemical cell of the example is shown.
. .. CA 02970178 2017-06-08 , 28' LIST OF REFERENCE NUMBERS
A: Beaker B: Electrolyte solution C: Reference electrode (Hg / Hg2SO4 electrode) D: Measuring electrode (glassy carbon electrode) E: Magnetic stirrer F: Counter electrode (iridium electrode), cathode G: Working electrode (graphite electrode), the anode (Pz cell voltage (Pc: cathode potential (PA: anode potential (PR: redox potential Figure 2 shows the potentials and cell voltage before the electrolysis of a 1.6 M V02+
in 2M H2SO4.
Figure 3 shows the potentials, electric current and cell voltage during electrolysis of a 1.6 M V02+ solution in 2 M H2SO4.
Figure 4 shows the redox potential and electric current during the electrolysis of a 1.6 M V02+ - H2SO4 solution in 2 M.
29' Figure 5 shows the potential and cell voltage by the electrolysis of a 1.6 M
H2SO4 in 2M V02+.
Figure 6 shows the profile of the current density of the redox flow battery at a constant terminal voltage (potentiostatic charging) wherein is the diffusion current density, which normally cannot be avoided for technical reasons.
In the following items, the present invention will be further described:
1. A process for oxygen evolution from an aqueous electrolyte solution of a redox flow battery, wherein at least two electrodes (E) are in electrically conductive contact with the electrolytic solution at least one of the electrodes (E) is connected as an anode and at least one is connected as a cathode and at the anode oxygen (02) is formed, and wherein at the cathode no hydrogen is formed or per 1.0 mole of formed oxygen (02) not more than 1.5 moles of hydrogen (H2) are formed.
2. The method of item 1, wherein the electrodes (E) are not the electrodes being used in the current drain from the redox flow battery.
3. The method according to any one of the preceding items, wherein the formed hydrogen is reacted with oxygen, preferably with oxygen formed at the anode, on a catalyst to form water.
4. The method according to item 3, wherein the water is fed back into the electrolyte solution.
5. The method according to item 1, wherein no hydrogen is formed at the cathode.
6. The method according to any one of the preceding items, wherein the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release.
7. The method of item 6, wherein the terminal voltage of the redox flow battery measured before the start of the oxygen release and during the release of oxygen is kept constant.
8. The method of item 6, wherein the redox flow battery is charged during the oxygen release.
9. The method according to any one of the preceding items 1 to 5, wherein during the oxygen release no fluid communication exists between the electrodes (E) and the half-cells of the redox flow battery and after the oxygen release a fluid connection is established or the electrolytic solution is supplied to the redox flow battery, preferably to the positive half-cell, after the oxygen release.
10. The method of item 9, wherein the electrolyte solution is taken from the redox flow battery, the oxygen release is performed by means of the electrodes (E) and the electrolytic solution is fed again to the redox flow battery after the release of oxygen.
11. The method according to any one of the preceding items, wherein the redox flow battery is a vanadium redox flow battery.
12. Electrochemical cell having at least two electrodes (E) of which at least one is connected as cathode and at least one is connected as an anode, said electrodes (E) being located in at least a half-cell of a redox flow battery and / or a fluid communication exists between the electrochemical cell and at least a half-cell of a redox flow battery.
13. An electrochemical cell according to item 12, wherein the electrodes (E) are = 31 located in the positive half-cell of the redox flow battery and / or a fluid communication exists between the electrodes (E) and the positive half-cell of the redox flow battery.
14.
The use of electric current for oxygen release from an electrolytic solution of a redox flow battery.
Example 400 mL of a solution of 1.6 molar V02+ in 2 molar H2SO4 are filled in a glass beaker.
An iridium sheet having a surface area of 16 cm2 (Heraeus, Germany) is used as the first electrode. A thermally activated (1h, 400 C) graphite felt with a surface of 40 cm2 (GFA5 the SGL Carbon, Germany) is used as a second electrode. A Hg / Hg2SO4-electrode is used as a third electrode. A glassy carbon electrode is used as a fourth electrode. The potential of the second electrode is controlled by a potentiostat (Modulab + 20A booster Solatron Analytical, USA) and the current between the first and the second electrode is measured. The cell voltage (TR), the anode potential (TA) and the redox potential (CPR) of the electrolyte solution are determined via three additional voltage measurement ports. Immediately after the addition of the electrolyte solution all voltages and potentials are measured (Figure 2) for a time of 60 s.
Subsequently, the cathode potential is set to a value of -0.90 V versus the Hg /
Hg2SO4 electrode for a period of 120 min, and thus the electrolysis is started (Figure 3). Thereafter, the individual potentials are currentless measured again for 300 s (Figure 4).
Before the start of the electrolysis and after completion of electrolysis samples have been taken from the electrolytic solution and the vanadium concentration and the portions of oxidation state are determined by potentiometric titration.
In the diagrams, all potentials were given to the normal hydrogen electrode.
As seen in Figure 2 the redox potential of V02+ solution is about 1.23 V
before the electrolysis. The respective cathode potential has the same value as the anode 32 =
potential, so that a cell voltage of 0 V results.
In Figure 3, the potentials, electric current and the cell voltage during electrolysis of 1.6 M are V02+ solution in 2 M - H2SO4 are shown. The cathode potential has been controlled to a value of -0.90 V vs. Hg / Hg2SO4 (-0.25 V vs. NHE) and did not change during the electrolysis time. The cathode potential corresponds to a value of about 200 to 250 mV below the potential of a significant hydrogen formation at carbon electrodes. The choice of the potential formed a compromise between undesirable hydrogen formation and driving force for the oxygen formation reaction.
Between working electrode (anode) and counter electrode an electric current having a current density of about 0.25 - 0.20 a A / cm2 has been established, which decreased during the reaction time. With the start of the electrolysis a strong gas development has been observed at the anode, which continued until the end of the experiment.
The converted electricity was the integral of the current density curve and was 7.3 Ah.
The anode potential increased with the start of the electrolysis by polarization to a value of 2.46 V, and fell steadily to about 2.2 V after 120 min from electrolysis time.
The value was significantly higher than the standard electrode potential of the oxygen formation reaction and thereby enabled the evolution of oxygen at the anode.
The difference between anode and cathode potential was the measured cell voltage of 2.7 to 2.5 V, which had the same tendency during electrolysis as the anode potential, since the cathode potential was held at a constant potential. The drop of the anode potential and thus the drop of the cell voltage can be explained by an increase in tetravalent vanadyl cations (V02+) in the solution progress with increasing oxygen release and thus the formation of a mixed potential (parallel oxidation reaction of V4+
to V5+).
With the progress of reaction, the redox potential of the electrolyte solution should decrease by increasing the amount of tetravalent V02+. In Figure 4, the redox potential of the electrolytic solution and the electric current during electrolysis is shown. At the beginning of the electrolysis the redox potential corresponds to the value of the currentless state and decreases in the further course. The value of the redox potential of 1.23 was equal to the value V in the currentless state, and decreased in the further course of the measurement with a typical behavior according to the Nernst equation with a logarithmic influence of concentration.
Figure 5 shows the course of the potential and the cell voltage after the electrolysis of a 1.6 M V02+ solution in 2 M H2SO4within 5 minutes.
Because of the polarization effects of the anterior current-carrying electrodes anode and cathode potentials differed from the redox potential. Only in the further course the potentials approximated to the redox potential at a value of 1.09 V. Thus the redox potential of the solution was 114 mV lower than before the electrolysis, suggesting an increase in V02+ - ions.
The use of electric current for oxygen release from an electrolytic solution of a redox flow battery.
Example 400 mL of a solution of 1.6 molar V02+ in 2 molar H2SO4 are filled in a glass beaker.
An iridium sheet having a surface area of 16 cm2 (Heraeus, Germany) is used as the first electrode. A thermally activated (1h, 400 C) graphite felt with a surface of 40 cm2 (GFA5 the SGL Carbon, Germany) is used as a second electrode. A Hg / Hg2SO4-electrode is used as a third electrode. A glassy carbon electrode is used as a fourth electrode. The potential of the second electrode is controlled by a potentiostat (Modulab + 20A booster Solatron Analytical, USA) and the current between the first and the second electrode is measured. The cell voltage (TR), the anode potential (TA) and the redox potential (CPR) of the electrolyte solution are determined via three additional voltage measurement ports. Immediately after the addition of the electrolyte solution all voltages and potentials are measured (Figure 2) for a time of 60 s.
Subsequently, the cathode potential is set to a value of -0.90 V versus the Hg /
Hg2SO4 electrode for a period of 120 min, and thus the electrolysis is started (Figure 3). Thereafter, the individual potentials are currentless measured again for 300 s (Figure 4).
Before the start of the electrolysis and after completion of electrolysis samples have been taken from the electrolytic solution and the vanadium concentration and the portions of oxidation state are determined by potentiometric titration.
In the diagrams, all potentials were given to the normal hydrogen electrode.
As seen in Figure 2 the redox potential of V02+ solution is about 1.23 V
before the electrolysis. The respective cathode potential has the same value as the anode 32 =
potential, so that a cell voltage of 0 V results.
In Figure 3, the potentials, electric current and the cell voltage during electrolysis of 1.6 M are V02+ solution in 2 M - H2SO4 are shown. The cathode potential has been controlled to a value of -0.90 V vs. Hg / Hg2SO4 (-0.25 V vs. NHE) and did not change during the electrolysis time. The cathode potential corresponds to a value of about 200 to 250 mV below the potential of a significant hydrogen formation at carbon electrodes. The choice of the potential formed a compromise between undesirable hydrogen formation and driving force for the oxygen formation reaction.
Between working electrode (anode) and counter electrode an electric current having a current density of about 0.25 - 0.20 a A / cm2 has been established, which decreased during the reaction time. With the start of the electrolysis a strong gas development has been observed at the anode, which continued until the end of the experiment.
The converted electricity was the integral of the current density curve and was 7.3 Ah.
The anode potential increased with the start of the electrolysis by polarization to a value of 2.46 V, and fell steadily to about 2.2 V after 120 min from electrolysis time.
The value was significantly higher than the standard electrode potential of the oxygen formation reaction and thereby enabled the evolution of oxygen at the anode.
The difference between anode and cathode potential was the measured cell voltage of 2.7 to 2.5 V, which had the same tendency during electrolysis as the anode potential, since the cathode potential was held at a constant potential. The drop of the anode potential and thus the drop of the cell voltage can be explained by an increase in tetravalent vanadyl cations (V02+) in the solution progress with increasing oxygen release and thus the formation of a mixed potential (parallel oxidation reaction of V4+
to V5+).
With the progress of reaction, the redox potential of the electrolyte solution should decrease by increasing the amount of tetravalent V02+. In Figure 4, the redox potential of the electrolytic solution and the electric current during electrolysis is shown. At the beginning of the electrolysis the redox potential corresponds to the value of the currentless state and decreases in the further course. The value of the redox potential of 1.23 was equal to the value V in the currentless state, and decreased in the further course of the measurement with a typical behavior according to the Nernst equation with a logarithmic influence of concentration.
Figure 5 shows the course of the potential and the cell voltage after the electrolysis of a 1.6 M V02+ solution in 2 M H2SO4within 5 minutes.
Because of the polarization effects of the anterior current-carrying electrodes anode and cathode potentials differed from the redox potential. Only in the further course the potentials approximated to the redox potential at a value of 1.09 V. Thus the redox potential of the solution was 114 mV lower than before the electrolysis, suggesting an increase in V02+ - ions.
Claims (12)
1. A method for releasing oxygen from an aqueous electrolyte solution of a redox flow battery, wherein at least two electrodes (E) are in electrically conductive contact with the electrolytic solution at least one of the electrodes (E) is an anode and at least one is connected as a cathode, and the anode forms oxygen (O2), the cathode does not form hydrogen or forms not more than 1.5 moles of hydrogen (H2) per 1.0 mole of formed oxygen (O2), wherein the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid communication exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release or wherein no fluid communication exists between the electrodes (E) and the half-cells of the redox flow battery during the oxygen release and a fluid connection is established after the oxygen release.
2. The method of claim 1, wherein the electrodes (E) are not the electrodes being used in the current drain from the redox flow battery.
3. The method according to any one of the preceding claims, wherein the formed hydrogen is reacted with oxygen, preferably with oxygen formed at the anode, on a catalyst to form water.
4. The method according to claim 3, wherein said water is fed back into the electrolyte solution.
5. The method according to claim 1, wherein no hydrogen is formed at the cathode.
6. The method according to any one of the preceding claims, wherein the electrodes (E) are located in the positive half-cell of the redox flow battery and / or where a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release and wherein the terminal voltage of the redox flow battery is measured prior to the oxygen release and kept constant during the oxygen release.
7. The method according to any one of the preceding claims, wherein the electrodes (E) are in the positive half-cell of the redox flow battery and /
or where a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release, and wherein the redox flow battery is charged during the oxygen release.
or where a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery during the oxygen release, and wherein the redox flow battery is charged during the oxygen release.
8. The method according to any one of the preceding claims 1 to 5, wherein during the oxygen release no fluid communication exists between the electrodes (E) and the half-cells of the redox flow battery, and after the oxygen release a fluid connection is established and wherein the electrolyte solution is withdrawn from the redox flow battery, the oxygen release is carried out by the electrodes (E) and the electrolytic solution is fed to the redox flow battery again after the oxygen release.
9. The method according to any one of the preceding claims, wherein the redox flow battery is a vanadium redox flow battery.
10. 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, said electrodes (E) are located in at least one half-cell of a redox flow battery and / or a fluid communication exist between the electrochemical cell and at least one half-cell of a redox flow battery.
11. The electrochemical cell according to claim 10, wherein the electrodes (E) are located in the positive half-cell of the redox flow battery and / or a fluid connection exists between the electrodes (E) and the positive half-cell of the redox flow battery.
12. A
use of electrical current for an oxygen release from an electrolytic solution of a redox flow battery.
use of electrical current for an oxygen release from an electrolytic solution of a redox flow battery.
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DE102014225612.7 | 2014-12-11 | ||
| DE102014225612.7A DE102014225612A1 (en) | 2014-12-11 | 2014-12-11 | Process for the regeneration of the electrolyte solution of a redox flow accumulator |
| PCT/EP2015/079225 WO2016092004A1 (en) | 2014-12-11 | 2015-12-10 | Method for regenerating the electrolyte solution of a rechargeable redox flow battery |
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| CA2970178A1 true CA2970178A1 (en) | 2016-06-16 |
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| CA2970178A Abandoned CA2970178A1 (en) | 2014-12-11 | 2015-12-10 | Method for regenerating the electrolyte solution of a redox flow battery |
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| EP (1) | EP3231031A1 (en) |
| JP (1) | JP2018503222A (en) |
| KR (1) | KR20170094343A (en) |
| CA (1) | CA2970178A1 (en) |
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| CN109346756A (en) * | 2018-11-26 | 2019-02-15 | 上海电气集团股份有限公司 | A kind of flow battery system and its regeneration of electrolyte method of regeneration of electrolyte |
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JPS6290875A (en) * | 1985-10-16 | 1987-04-25 | Sumitomo Electric Ind Ltd | Electrolyte regeneration device of redox flow cell |
| JP2649700B2 (en) * | 1988-06-03 | 1997-09-03 | 関西電力株式会社 | Electrolyte regeneration device for redox flow battery |
| US5439757A (en) * | 1992-10-14 | 1995-08-08 | National Power Plc | Electrochemical energy storage and/or power delivery cell with pH control |
| JP3315508B2 (en) * | 1994-01-07 | 2002-08-19 | 住友電気工業株式会社 | Electrolyte flow battery with electrolyte readjustment device |
| JP3137548B2 (en) * | 1994-12-15 | 2001-02-26 | 三菱重工業株式会社 | Charged particle, X-ray detector |
| JP2002516755A (en) * | 1998-05-29 | 2002-06-11 | プロートン エネルギー システムズ．インク | Water electrolysis fluid management system |
| EP1310008A1 (en) * | 2000-08-16 | 2003-05-14 | Squirrel Holdings Ltd. | Vanadium electrolyte preparation using asymmetric vanadium reduction cells and use of an asymmetric vanadium reduction cell for rebalancing the state of charge of the electrolytes of an operating vanadium redox battery |
| JP2002175829A (en) * | 2000-12-07 | 2002-06-21 | Sumitomo Electric Ind Ltd | All-vanadium redox-flow battery and operating method of all vanadium redox-flow battery |
| DE102007011311A1 (en) * | 2006-12-22 | 2008-06-26 | Mtu Cfc Solutions Gmbh | Vanadium-redox-battery operating method, involves regenerating anolyte by contact with carbon monoxide, with metal such as iron, zinc and nickel, or with electrolytic cell in electro-chemical manner |
| JP2010018840A (en) * | 2008-07-10 | 2010-01-28 | Teijin Pharma Ltd | Method for removing water in electrolyte, device therefor and water content measurement apparatus |
| US8916281B2 (en) * | 2011-03-29 | 2014-12-23 | Enervault Corporation | Rebalancing electrolytes in redox flow battery systems |
| JP6204382B2 (en) * | 2012-03-05 | 2017-09-27 | ウオエス ホールディング ソシエテ アノニム | Redox flow battery for hydrogen generation |
| US8980454B2 (en) * | 2013-03-15 | 2015-03-17 | Enervault Corporation | Systems and methods for rebalancing redox flow battery electrolytes |
-
2014
- 2014-12-11 DE DE102014225612.7A patent/DE102014225612A1/en not_active Withdrawn
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- 2015-12-10 CA CA2970178A patent/CA2970178A1/en not_active Abandoned
- 2015-12-10 WO PCT/EP2015/079225 patent/WO2016092004A1/en active Application Filing
- 2015-12-10 KR KR1020177018989A patent/KR20170094343A/en not_active Application Discontinuation
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| KR20170094343A (en) | 2017-08-17 |
| JP2018503222A (en) | 2018-02-01 |
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