DE102015119159A1 - Light-Rechargeable Redox Flow Battery - Google Patents

Abstract

A light-rechargeable redox flow battery has a first and a second half-cell (1, 2), wherein the first half-cell (1) has a chamber (5) filled with an electrolyte, to which a flat with the electrolyte Electrode (13) is adjacent, wherein the chamber (5) to a from an electrolyte tank (9) and an electrolyte pump (11) existing electrolyte circuit (7) is connected. The electrode (13) consists of a photo semiconductor which is located in front of a window (19) in the housing (3) of the battery. By the substitution of at least one of the two conventional electrodes (13, 15) of a redox flow battery by a photoelectrode (20, 21), it can be charged directly with sunlight. An external connection to an electrical energy source is no longer necessary.

Description

The invention relates to a light-rechargeable redox flow battery having a housing which is divided by a membrane into a first and a second half-cell, wherein at least the first half-cell has a filled with an electrolyte chamber, to which a surface with the electrolyte-related electrode is adjacent and this chamber is connected to a consisting of an electrolyte tank and an electrolyte pump electrolyte circuit.

Redox flow batteries (RFBs) are chemical energy storage systems in which the electroactive substances are dissolved in electrolytes. These electrolyte solutions are stored in large tanks and continuously pumped through the electrode unit, in which the energy conversion is realized. Due to the decoupling of capacity (electrolyte tanks) and power (electrode unit) more flexible designs than with conventional batteries are possible and also the costs are predicted significantly lower. Therefore, RFB are particularly suitable for stationary energy storage to compensate for volatile regenerative energy sources.

As electroactive substances usually metal ions are used, but also the use of organic redox mediators has already been described. This can increase the energy density and further reduce the cost of the electrolyte.

The positive half-cell can be replaced by a gas diffusion electrode operated with oxygen or air. In such a redox flow air battery, the redox pair O2 / H2O replaces the electrolyte of the positive half cell. Since the oxygen can be taken from the ambient air, it does not have to be cached separately. Accordingly, the electrolyte pump and the electric tank for the positive half-cell are no longer needed, which reduces the cost, weight and volume of the system.

Like other electrochemical energy stores, RFBs can only be charged with electrical energy. If solar energy is to be stored, it must first be converted by a separate photovoltaic system into electrical energy, which is then stored in the battery.

In the past, attempts have been made to combine solar energy conversion and storage in solar batteries. However, these concepts were based on conventional battery systems such as nickel metal hydride, ion intercalation and metal / sulfide / polysulfide. Consequently, they also have the corresponding limitations and disadvantages of these battery technologies, such as. As low life, fast self-discharge, coupled capacity / performance and high costs and have therefore not prevailed.

A 1984 publication by Sharon and Sinha describes a solar battery based on an n-type semiconductor in which the chemical energy is stored in metal ions (Fe2 + / 3 + and Ce3 + / 4 +) dissolved in the electrolyte.

Due to the small volume of the cell, however, only a small amount of energy can be stored. A storage of the electrolyte in large external tanks with recycling by the electrode unit, as is usual in a redox flow battery, does not take place.

Two publications by Yan, Li and Gao in 2013 and 2014 describe a solar redox flow battery. The solar energy conversion is realized in the described system, however, not by the semiconductor of a photoelectrode, but via photosensitizers deposited on the semiconductor electrode, which then inject the electrons into the semiconductor electrode.

The publication EP 0 553 023 A1 describes an air-solar battery based on an n-type semiconductor and metal electrodes. However, the battery does not correspond to the redox flow design, also explicitly an n-type semiconductor photoanode is provided, not a photocathode. The disadvantages are therefore a shorter life and limited storage capacity.

The patent US 4,215,182 describes an air solar battery in which the electrolyte is pumped through the electrode unit as in a redox flow battery. However, the solar energy conversion is not realized in the described system via a photoelectrode, but via a photosensitizer applied to the membrane (which in turn may be an n-type semiconductor), which redirects the electrons to a conventional electrode via a redox shuttle system. A disadvantage is the additional potential drop through the redox shuttle system, which reduces the usable potential difference and thus the energy conversion efficiency.

The invention is based on the task of creating an efficient, light-rechargeable and compact redox flow battery.

To solve the issue, the invention provides that the said electrode consists of a photo-semiconductor, that the housing has at least one optically transparent window, and that said electrode is located in front of the window.

By substituting at least one of the two electrodes in a redox flow battery by a photoelectrode, it can therefore be charged with sunlight instead of electrical energy. Since the photoelectrode converts light energy into electrical energy, part of the energy required to charge the battery can be provided in the form of light and the required electrical energy is correspondingly reduced. With a suitable choice of the photoelectrode and electrolytes, it is even possible to charge the battery exclusively with light, without an additional voltage source is necessary.

The conversion of light energy into stored chemical energy can be done directly in the battery and no separate solar cell is needed anymore. The stored energy can be released later as needed as electrical energy. The structure is similar to that of a photoelectrochemical solar cell, except that the electrical energy is not obtained immediately by regeneration of the electrolyte, but is cached in the form of the charged electrolyte in large tanks. In contrast to conventional solar batteries, the use of the solar-chargeable redox flow technology enables a much higher capacity, more flexible design, lower costs and a long service life.

For the discharge of the battery, a conventional auxiliary electrode is still provided in the half-cell, which has the electrode with the photo-semiconductor, the z. B. is metallic.

In a first embodiment of the battery, both half cells are equipped with electrodes of photo semiconductors. Thus, the second half-cell also has a chamber filled with an electrolyte, to which a counterelectrode, which is in contact with the electrolyte, adjoins and which is connected to a further electrolyte circuit consisting of an electrolyte tank and an electrolyte pump.

This arrangement makes it possible to take advantage of the light transmitted through the battery to the counter electrode for the charging process.

Furthermore, in particular, the positive half-cell can be replaced by a gas diffusion electrode which is operated with oxygen or air. In such a light-rechargeable redox flow-air battery, the redox pair O2 / H2O replaces the electrolyte of the positive half-cell. Since the oxygen can be taken from the ambient air, it does not have to be cached separately. The electrolyte pump and electric tank for the positive half-cell are no longer needed, which reduces the cost, weight and volume of the system.

As semiconductor material both n-type semiconductor and p-type semiconductor can be used. Thus, the electrode functions either as a photoanode consisting of an n-type semiconductor or as a photocathode consisting of a p-type semiconductor.

Preferably, the substrate for the photo-semiconductors is transparent and serves as a translucent disk in the housing of the battery. It is also conductive so that the external circuit can be connected to the substrate.

Furthermore, it can be provided that the battery has a heat exchanger, which is in a thermally conductive contact with the electrolyte in at least one of the half-cells, preferably with the electrolyte in the half-cell with the transparent window, or the electrolyte is supplied via a circuit.

In addition to the recovery and storage of electrical energy, the invention can thus also be used for the recovery of usable heat. In fact, irradiation with sunlight converts a significant portion of the solar energy into heat, which heats the battery. The heat thus generated can be dissipated by means of the liquid electrolyte and transferred via a heat exchanger to another medium, such as a hot water supply, and thus made usable. As a result of the additional use of heat, a significantly higher overall efficiency (electrical and thermal) of the system can be achieved.

Further embodiments of the invention with regard to the material to be used will become apparent from the dependent claims.

Accordingly, the photoanode consists of one or more of the following materials: TiO 2, ZnO, BiVO 4, InVO 4, Fe 2 O 3, MgFe 2 O 4, Si, WO 3, and the photocathode of one or more of Si, SiC, CuO, Cu 2 O, CoO, Bi 2 O 3 , V2O5, MoS2, ZnMn2O4, CaFe2O4, FeS2

The photocathode is additionally coated with a protective layer of ZnO, TiO2 or WO3 for corrosion protection.

The material of the electrodes may additionally be doped with foreign elements.

In the following, the invention will be explained in more detail with reference to an embodiment. To show:

1 a schematic representation of a first embodiment of the invention, in which both half-cells of a redox flow battery each having a filled with an electrolyte chamber and in which either only one or both chambers each have an existing of a photo-semiconductor electrode, and

2 a schematic representation of a second embodiment of the invention, in which a half-cell of a redox flow battery has a filled with an electrolyte chamber and wherein the other cell is a gas diffusion electrode.

Basically, three variants are possible for the first embodiment, whose operating principle based on 1 should be explained:
A light-chargeable redox flow battery consists of a first half-cell 1 and a second half cell 2 in a housing 3 through a membrane 4 are separated. Every half cell 1 . 2 consists of a chamber filled with an electrolyte 5 . 6 those with an electrolyte circuit 7 . 8th connected is. Each of these electrolyte circuits 7 . 8th has an electrolyte tank 9 . 10 and an electrolyte pump 11 . 12 ,

The first half cell 1 has an electrode 13 with an external front contact 14 and the second half cell 2 a counter electrode 15 with a back contact 16 that have a circuit 18 which is an electrical consumer 17 may be interconnected.

In a first variant, the electrode 13 as photoanode 20 executed, which consists of an n-doped semiconductor material. The front contact 14 serves as a substrate for the semiconductor material. Furthermore, the front contact 14 a transparent window 19 in the case 3 , so it is passable for photons. The first half cell 1 is therefore the positive half-cell of the battery.

In a second variant, the electrode 13 as a photocathode 21 executed. The front contact 14 also serves as a substrate for the semiconductor material and is also a transparent window 19 executed. The first half cell 1 is therefore the negative half cell of the battery.

In a third variant is the electrode 13 as photoanode 20 and the counter electrode 15 as a photocathode 21 executed.

In addition, in each chamber, which is provided with a semiconductor material equipped electrode, still an auxiliary electrode 22 provided by means of a switch 23 can be switched parallel to the respective electrode.

The mode of action will be explained below with reference to the first variant. The conversion of sunlight into electrical energy is essentially in the photoanode 20 realized. This preferably consists of an n-type semiconductor material which is applied to a layer of a transparent conductive material (eg FTO, ITO). Sunlight can now through the front contact 14 (Window 19 ) on the photoanode 20 fall. There photons whose energy corresponds to at least the band gap of the semiconductor material used, absorbed. Due to the absorption of the photons, electrons are excited from the valence band into the higher energy conduction band of the semiconductor, leaving behind positively charged holes (electron defects) in the valence band. By the contact of the n-type semiconductor with the electrolyte of the positive half-cell 1 An electric field (space charge zone) forms in the semiconductor, which brings the electrons to the front contact 14 and drives the holes to the contact surface with the electrolyte. There, the valence band holes oxidize the electroactive species present in the electrolyte. The excited conduction band electrons reach over the front contact 14 , the external electrical circuit 18 and the back contact 16 to the counter electrode 15 where they match the electroactive species in the electrolyte of the negative second half cell 2 to reduce.

The two half cells 1 . 2 are inside the case 1 through a membrane 4 separated, which allows the exchange of protons to maintain charge neutrality.

The electrolytes are in large electrolyte tanks 9 and 10 stored and stored and circulate continuously by means of the electrolyte pumps 11 . 12 through the half cells 1 . 2 ,

This charging process, oxidation in the positive half-cell 1 and reduction in the negative half-cell 2 , according to the invention is effected within the battery by light-excited electrons and not by an external influx of electrons. However, such an external inflow of electrons is still possible in addition by an external voltage is connected to the electrodes.

After the described charging process, the stored in the electrolyte chemical energy is later retrieved as needed and returned to electrical energy. Thereby, the electroactive species of the positive electrolyte become the first half cell 1 again reduced and that of the negative electrolyte of the second half-cell 2 again oxidized. It can be one in the circuit 18 connected consumer 17 by the tension between the two half-cells 1 . 2 are driven.

Due to the rectifying effect of the electrolyte-semiconductor contact between the photoanode 20 and the electrolyte in the first half-cell 1 the back reaction can be difficult via the photoanode 2 will be realized. For this reason, in addition, the auxiliary electrode 22 provided, which acts as a positive electrode during the discharging of the battery. The conventional counterelectrode 15 will be used during discharge (as a negative electrode).

The second variant provides that, instead of a photoanode, a photocathode 21 is used in the first half cell. In this case, the structure of the battery is identical to that described above, only with the difference that the electrode 13 a photocathode 21 is. The photons are now absorbed at the photocathode, the resulting electrons migrate to the negative electrolyte in the first half-cell 1 and reduce this. The resulting holes get over the front contact 14 , the external circuit 18 and the back contact 16 to the counter electrode 15 where she gets the electrolyte of the second half-cell 2 oxidize.

A third variant is the combination of photoanode and photocathode in a tandem cell. In this embodiment, both the electrode 13 as well as the counter electrode 15 photoactive. This does not necessarily have to be on a transparent back contact 16 be applied as only one side of the battery, namely the one with the transparent front contact 14 , can be aligned with the sun. However, the photoanode becomes 20 the electrode 13 still transmit a portion of the received sunlight through the battery, which then on the photocathode 21 meets, who can still use this. However, this requires the band gap of the photoanode 20 larger than that of the photocathode 21 ,

By using two photons, a larger potential difference than in the first two variants can be achieved. However, since both electrodes have rectifying character, but also an additional auxiliary electrode 22 - not shown here - in the second half cell 2 needed for the unloading process.

The 2 shows a second embodiment of the invention. the entire positive second half-cell will be doing this 2 through a gas diffusion electrode 24 replaced, which is operated with atmospheric oxygen. Accordingly, only a liquid electrolyte for the first half cell 1 needed, one as a photocathode 21 executed electrode 13 having. This results in the following simplified structure:
As described above, in the photocathode 21 the first half cell 1 Photons absorbed and electron / hole pairs generated. The electrons pass through the electric field to the electrolyte of the first half-cell 1 where they reduce the electroactive species contained therein. The holes get over the front contact 14 and the external circuit 18 to the gas diffusion electrode 24 where they oxidize water to oxygen, which is released into the ambient air. When discharging the battery, oxygen contained in the air at the gas diffusion electrode 24 reduced to water and at the same time at the auxiliary electrode 22 the electroactive species of the first half-cell 1 again oxidized.

Preferred versions:

• Photoanode: TiO 2, ZnO, BiVO 4, InVO 4, Fe 2 O 3, MgFe 2 O 4, Si, WO 3
• Front contact: ITO (tin oxide doped indium oxide), FTO (fluorine doped tin oxide), AZO (aluminum-doped zinc oxide), antimony-doped tin oxide, Nb-doped TiO2.
• Electrolytes: Water or aqueous solutions of acids such as H2SO4, HCl, HBr, HClO4, HNO3 or H3PO4, in which salts of Fe, Cr, Zn, Cu or V and / or organic compounds of the family of quinones, naphthaquinones, anthraquinones, quinolines , Quinoxalines, viologens, phenothiazines, piperidinyloxyls, anthocyanins and indophenols or polymers dissolved from these compounds.
• Auxiliary electrode / counter electrode: glassy carbon, graphite, platinum, metals.
• Membrane: Sulfonated perfluorinated polymers such as Nafion.
• Back contact: like front contact or metal.
• Photocathode: Si, SiC, CuO, Cu2O, CoO, Bi2O3, V2O5, MoS2, ZnMn2O4, CaFe2O4, FeS2, MoS2, and in addition to the corrosion protection may be coated with a protective layer of ZnO, TiO2 or WO3; In addition, all of these materials can also be doped with foreign elements.

LIST OF REFERENCE NUMBERS

1

half-cell

2

half-cell

3

casing

4

membrane

5

chamber

6

chamber

7

Electrolyte circuit

8th

Electrolyte circuit

9

electrolyte tank

10

electrolyte tank

11

electrolyte pump

12

electrolyte pump

13

electrode

14

front contact

15

counter electrode

16

back contact

17

consumer

18

circuit

19

window

20

photoanode

21

photocathode

22

auxiliary electrode

23

switch

24

Gas diffusion electrode

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• EP 0553023 A1 [0010]
• US 4215182 [0011]

Claims (13)

A light-chargeable redox flow battery with a housing ( 3 ), by a membrane ( 4 ) into a first and a second half cell ( 1 . 2 ), wherein at least the first half-cell ( 1 ) a chamber filled with an electrolyte ( 5 ), to which a surface in contact with the electrolyte electrode ( 13 ) and this chamber ( 5 ) to one of an electrolyte tank ( 9 ) and an electrolyte pump ( 11 ) existing electrolyte circuit ( 7 ), characterized in that said electrode ( 13 ) consists of a photo semiconductor that the housing ( 3 ) at least one optically transparent window ( 19 ), and that said electrode ( 13 ) in front of the window ( 19 ) is located. Redox flow battery according to claim 1, characterized in that in the first half cell ( 1 ) a conventional auxiliary electrode ( 22 ) is provided. Redox flow battery according to claim 1 or 2, characterized in that the second half cell ( 2 ) a chamber filled with an electrolyte ( 6 ), to which a counterelectrode ( 15 ) and the chamber ( 6 ) to one of an electrolyte tank ( 10 ) and an electrolyte pump ( 12 ) existing further electrolyte circuit ( 8th ) connected. Redox flow battery according to claim 1 or 2, characterized in that the second half cell ( 2 ) a gas diffusion electrode ( 24 ). Redox flow battery according to one of the preceding claims, characterized in that the electrode is used as photoanode ( 20 ), which consists of an n-type semiconductor. Redox flow battery according to one of Claims 1 to 5, characterized in that the electrode is in the form of a photocathode ( 21 ), which consists of a p-type semiconductor. Redox flow battery according to Claim 5 or 6, characterized in that the semiconductor material which contains the electrode ( 13 ) is applied to an electrically conductive substrate and that the substrate is the optically transparent window ( 19 ). Redox flow battery according to claim 5, characterized in that the photoanode ( 20 ) consists of one or more of the following materials: TiO 2, ZnO, BiVO 4, InVO 4, Fe 2 O 3, MgFe 2 O 4, Si, WO 3. Redox flow battery according to claim 6, characterized in that the photocathode ( 21 ) consists of one or more of the following materials: Si, SiC, CuO, Cu2O, CoO, Bi2O3, V2O5, MoS2, ZnMn2O4, CaFe2O4, FeS2 Redox flow battery according to claim 9, characterized in that the photocathode ( 21 ) for corrosion protection is additionally coated with a protective layer of ZnO, TiO2 or WO3. Redox flow battery according to claim 8, 9 or 10, characterized in that the material of the electrodes ( 13 ) is doped with foreign elements. Redox flow battery according to one of the preceding claims, characterized in that the electrolytes consist of water or aqueous solutions of acids such as H2SO4, HCl, HBr, HClO4, HNO3 or H3PO4 in which salts of Fe, Cr, Zn, Cu or V and / or organic compounds from the family of quinones, naphthaquinones, anthraquinones, quinolines, quinoxalines, viologens, phenothiazines, piperidinyloxy, anthocyanins and indophenols or polymers of these compounds are dissolved. Redox flow battery according to one of the preceding claims, characterized in that it comprises a heat exchanger which is in heat-conductive contact with the electrolyte in at least one of the half-cells ( 1 . 2 ), preferably with the electrolyte in the half-cell ( 1 ) with the transparent window ( 19 ) or to which the electrolyte is supplied via a circuit.

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