AU2017358721B2 - Termination assembly of a cell stack of a redox flow battery - Google Patents

Abstract

The aim of the invention is to specify a termination assembly of a cell stack (2) of a redox flow battery (1), which termination assembly enables sufficiently high efficiency of the redox flow battery (1) and does not contaminate an electrolyte liquid when in contact with said electrolyte liquid. This aim is achieved in that the termination assembly is designed having a current collector (3) and an electrode end plate (70), wherein in a contact region (5) the electrode end plate (70) and the current collector (3) lie against each other and are in electrical contact, wherein in at least 20 % of the contact region (5) the current collector (3) is coated with an electrically conductive current collector coating (30), the electrode end plate (70) is coated with an electrically conductive electrode coating (71) in said at least 20 % of the contact region (5), the combination of the current collector (3), the current collector coating (30) and the electrode coating (71) consists of at most 5 wt% of Cu and Ag in total and at most 0.05 % of elements of the platinum group in total and the electrical resistance of the combination of the current collector (3), the current collector coating (30) and the electrode coating (71) does not exceed 1 ohm · cm

Description

Termination arrangement of a cell stack of a redox flow battery

The subject disclosure relates to a terminal arrangement of a cell stack of a redox flow battery with a current collector and an electrode end plate, wherein the electrode end plate and the current collector are arranged adjacent to each other in a contact area and in electrical contact, wherein in at least 20% of the contact region of the current collector is coated with an electrically conductive current collector coating. In addition, the subject invention relates to a cell stack of a redox flow battery consisting of a plurality of adjacent cells arranged between two end cells, wherein the cells and end cells each consist of two half-cells separated by a semipermeable membrane, electrodes are located between the cells, and a termination arrangement according to the invention is located on at least one end cell on an axially outer side with respect to the cell stack, and a redox flow battery comprising at least one such cell stack.

A redox flow battery is an electrochemical-based power generation and storage system typically consisting of tanks for storing positive and negative electrolyte fluids, and pumps and conduits for circulating the electrolyte fluids through one or more cell stacks, each of which is comprised of a number of cells. The cells of the cell stack are each formed by a positive half-cell and a negative half-cell, with the positive and negative half-cells of a cell separated by a semipermeable membrane, typically an ion exchange membrane. The positive half-cell contains a frame-mounted positive electrode through which the positive .0 electrolyte fluid flows. The negative half-cell contains a frame-mounted negative electrode through which the negative electrolyte fluid flows. For example, in the case of a vanadium redox flow battery, the positive electrolyte fluid may contain, in the charged state, tetravalent and pentavalent vanadium, sulfuric acid, and optionally other additives. The negative electrolyte fluid may, for example in the charged state, contain di- and trivalent vanadium, sulfuric acid and optionally further additives (and is thus more "negative" than the positive electrolyte fluid). The positive and negative electrodes are usually made as porous mats made of graphite which can be made to flow through by the electrolyte fluid.

In redox flow batteries, two types of frames are used, namely frames of resilient plastics (elastomers) and non-elastic plastics such as PVC, PP, PE, PTFE, epoxy, etc. Non-elastic plastics, typically thermoplastics, are stiff and moldable only within a certain temperature range. Elastomers are dimensionally stable, but elastically deformable plastics, i.e. such that an elastomer returns to its original shape after deformation.

Bipolar plates are arranged between individual adjacent cells of the cell stack, which is usually made of a composite material made of carbon and plastic, since the electrode plates - just as the frame and the electrodes - must be resistant to the electrolyte fluid. The higher the carbon content in the electrode plate, the higher the electrical conductivity of the electrode plate, which is why electrode plates usually consist of at least 50% carbon by weight, but usually more than 80% carbon by weight. It is known that carbon is far less electrically conductive than metals such as copper or aluminum. Metals would thus suggest themselves as material for the electrode plate, but metallic electrode plates are out of the question, since these materials are known not to be resistant to the electrolyte fluid. Metallic electrode plates would dissolve from contact with the electrolyte fluid after a short time.

End cells are arranged at the axial ends of the juxtaposed cells of the cell stack, which are structurally identical to the cells therebetween. But there are electrode end plates, instead of electrode plates, on each axial outer side of the end cells for double-sided completion of the cell stack, and these electrode end plates can be basically identical to the electrode plates. However, the electrode end plates can also be made of a different material or have a different thickness than an electrode plate. However, the essential difference between electrode end plates and electrode plates is that current collectors rest on the surfaces of the electrode end plates, which are opposite the last cell of the cell stack, also called the end cell. In contrast to this - as mentioned - with electrode plates, electrodes lie on both surfaces. An electrical contact can be led to the outside via the current collectors in order to be able to pick up electrical voltage (discharging the redox flow battery) or to create an electrical voltage (charging the redox flow battery).

The cell stack is completed in each case by an end plate on the axial outside of the end .0 frame. Thus, the following are located at the axial ends of the cell stack, each listed from the inside to the outside: End cells, end electrodes, end frames with current collectors and end plates. The cell stack is held together by the end plates arranged on both sides.

The current collectors are connected to an outgoing electrical connection of the redox flow battery and are often made of copper, since copper has a high electrical conductivity and is also inexpensive. In addition, a current collector made of copper is often coated with silver to provide even better contact with the electrode endplate. A production of the current collector made of carbon or graphite would not be effective, since the current collector must have a very high electrical conductivity in order not to adversely affect the efficiency of the redox flow battery.

A contact between the electrolyte fluid of a redox flow battery and an existing current collector is fundamentally (in particular structurally) to be prevented, since the acidic (e.g. in the case of a vanadium redox battery) or even basic electrolyte fluid inevitably causes corrosion of the usually metallic current collector. However, such a contact between the electrolyte fluid and the current collector could occur if a leak occurs in the region of the current collector, for example due to a defective electrode end plate or due to a faulty seal between the end cell and the current collector. If the leak goes unnoticed, the current collector will be completely dissolved by the electrolyte fluid, usually within 10-20 hours. This effect is not noticeable in particular during idling of the redox flow battery, but even in normal operation, the dissolution of a current collector is often not noticed until it is already completely in solution in the electrolyte, since the efficiency of the redox flow battery during dissolution of the current collector does not sink abruptly, but rather slowly. Especially when using multiple cell stacks and thus several current collectors in a redox flow battery, the decrease in efficiency is not serious enough to conclude that a dissolution of a current collector has taken place. But even if the dissolution of a current collector is noticed, the leak would first have to be located, which in turn requires time.

Upon contact with the electrolyte fluid, the material of the current collector, such as copper in the form of Cu 2 ' ions and silver in the form of Ag' ions, thus passes into the electrolyte fluid, which still flows through the cells of existing, often even of several cell stacks. Through the ion exchange membrane, the dissolved material of the current collector can also pass from the negative to the positive electrolyte fluid and vice-versa. The copper dissolved in the electrolyte fluid, and optionally other materials such as silver, would be deposited on the negative electrodes of the existing cells of all connected cell stacks due to electrochemical processes. This leads to an impairment of the flow behavior of the electrolyte fluid through the negative electrodes, since the pores of the negative electrodes are clogged with copper particles, and possibly other materials.

.0 In addition, the copper particles, and possibly other particles, due to their property as hydrogen catalysts on the negative electrodes, lead to increased hydrogen evolution in the redox flow battery.

Both the coating or blockage of the electrodes with copper or other materials, as well as the resulting hydrogen formation, are extremely harmful to the redox flow battery and therefore are to be avoided. The harmful effect of hydrogen in redox flow batteries is well known, so it will not be further discussed here. Even if the dissolution of the current collector is noticed, the redox flow battery is already damaged at this time such that at least complex maintenance is necessary.

A leak in the area of the current collector can thus cause a contamination of the entire electrolyte fluid and subsequently a total failure of all cell stacks and thus a total failure of the entire redox flow battery after a certain time. In the worst case, upon contact of the electrolyte fluid with the current collector, not only the electrolyte fluid must be exchanged or cleaned and the current collector replaced, but every cell stack present in the redox flow battery must be exchanged or at least the electrodes located therein must be freed from the copper deposits or deposits of other materials.

It is known that aluminum is also dissolved in contact with the electrolyte fluid of a redox flow battery in this, but, in contrast to copper, does not cause the subsequent negative effects described above (such as hydrogen formation). Patent AT 513 834 B2, for example, discloses a current collector made of aluminum. In addition, the current collector is coated with an electrically conductive material such as Zn, Sn, Ni, Pb, Sb, Cd, Cr, C, In, Al, V, Fe, or alloys thereof. Also mentioned as materials for current collector coating are inorganic compounds such as oxides, hydroxides, carbides, phosphides, sulfides, borides, etc., or electrically conductive polymers. Likewise, interlayers, e.g. of Ni, between the coating and the base material of the current collector are also disclosed. Specifically, the following is cited as an example of a current collector: 200 pm of aluminum with an interlayer of 5 pm of Ni and an outer coating of 10-100 pm of Sn. But no justification is given in the patent specification for this embodiment of the current collector.

JP 11329474 A2 proposes the production of current collectors made of aluminum, but in order to achieve a lower total weight of the redox flow battery compared to a production using copper.

Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the append .0 ed claims.

It is an aim of the current disclosure to provide a termination arrangement with a current collector of a redox flow battery which may allow a sufficiently high efficiency of the redox flow battery and preferably does not contaminate the electrolyte fluid on contact with an electrolyte fluid.

According to a first aspect, the present invention provides a termination arrangement of a cell stack of a redox flow battery with a current collector and an electrode end plate, wherein the electrode end plate and the current collector are arranged adjacent to each other in a contact region and are in electrical contact, wherein in at least 20% of the contact region the current collector is coated with an electrically conductive current collector coating, wherein the electrode end plate in this at least 20% of the contact region is coated with an electrically conductive electrode coating, wherein the combination of the current collector, the current collector coating and the electrode coating consists of a maximum of 5% by weight of Cu and Ag and a maximum of 0.05% of elements of the platinum group, wherein the electrical resistance of the combination of the current collector, the current collector coating and the electrode coating does not exceed 1 ohm.cm 2 over a period of at least 2000 hours.

. It should be noted that the mentioned combination does not include the electrode end plate, which, as is known, is usually non-metallic anyway. The determination of the time course of this resistance can take place on a termination arrangement located in an assembled cell stack (which may be in operation or even out of operation). Alternatively, the resistance profile can also be measured in a single termination arrangement not yet installed in a cell stack. The 2000 hours can thus be operating hours, but need not necessarily be operating hours, with a mixture of operating hours and non-operating hours also being conceivable. The resistance deteriorates primarily due to corrosion effects, as will be described below.

In order to achieve an even better electrical contact between the current collector and the electrode end plate, the current collector may be coated in at least 50%, preferably at least 90%, of the contact region with an electrically conductive current collector coating, and the electrode end plate in these at least 50%, preferably at least 90%, of the contact area may be coated with an electrically conductive electrode coating. Of course, the best electrical contact is made when the current collector is coated with an electrically conductive current collector coating in 100% of the contact region and the electrode end plate is coated with an electrically conductive electrode coating in 100% of the contact region. Due to the process, the current collector coating and/or the electrode coating can have pinholes (i.e. small defects in the coating) which are not used for the definition of the percentage coverage of the contact area. This means that, in spite of pinholes, a current collector coating or electrode .0 coating can, for example, coat 100% of the contact area.

In order to prevent hydrogen evolution in the case of dissolution of the material of the current collector in the electrolyte fluid, the production of the current collector from pure aluminum would theoretically be possible. Aluminum dissolved in the electrolyte fluid would not deposit on the negative electrodes and therefore would not generate hydrogen. In addition, aluminum has a lower density than copper, whereby the use of aluminum current collectors causes weight to be saved, as is known in the prior art. However, it has been found that current collectors made of pure aluminum have a high surface resistance due to the formation of a high-resistance oxide film on the surface and thus are immediately unusable due to the low electrical conductivity between the electrode end plate and current collector or become unusable over time (after one week at most), since the efficiency of the redox flow battery decreases rapidly. A (further) oxidation of the aluminum current collector takes place even at a conventional contact pressure of 10 kPa between current collector and electrode end plate.

A coating of the current collector, for example made of aluminum in at least 20% of the contact region, i.e. on the side facing the electrode end plate, with a thin current collector coating of conductive material would at least partially prevent the formation of this oxide layer. The material of the current collector coating would dissolve on contact with the electrolyte fluid as well as the base material of the current collector, which is why the choice of the material of the current collector coating is critical. Of course, current collector coatings dissolved in the electrolyte fluid should cause as few negative effects in the redox flow battery as possible, as they occur for example in copper, silver or elements of the platinum group.

However, coating the current collector with a current collector coating results in contact corrosion especially in the conventional use of high carbon-content electrode endplates (typically>80% by weight). This contact corrosion in turn leads to an undesired increased electrical contact resistance and thus again to a poor electrical contact between the electrode end plate and current collector, with this effect increasing over time.

However, it has been shown that there is a low contact resistance between the electrode end plate and current collector when in the at least 20% of the contact region in which the current collector is already provided with a current collector coating, an additional electrode coating of the electrode end plates is provided. With this, no further significant contact corrosion is detectable. It also applies to the choice of the material of the electrode coating that the electrode coating must not be contaminated when dissolved in the electrolyte fluid.

In a redox flow battery with a typical current density of 100 mA/cm 2 , an electrical resistance of the combination of current collector, current collector coating and electrode coating of 1 ohm.cm2 would cause 100 mV voltage losses. A cell stack of a redox flow battery has an .0 open circuit voltage of about 25-40 V, depending on how many cells the cell stack contains. In the presence of two termination arrangements, the total voltage loss due to the respective current collectors, current collector coatings and electrode coatings would be a maximum of 200 mV. Therefore, this corresponds to 0.5%-0.8% of the no-load voltage (depending on the level of the open-circuit voltage) in each case one negative and one positive electrode, resulting in total round-trip voltage losses of 1%-1.4% of the no-load voltage. This represents a limit that is just barely tolerable. A round-trip voltage loss is known to be twice the voltage loss, since it is assumed to be once for a charge, once for a discharge of a battery. In principle, the electrical resistance of the current collector, including the current collector coating and the electrode coating, must of course be kept as low as possible in order to keep the voltage losses low.

It should be noted that the resistance of this combination of the current collector, the current collector coating and the electrode coating results not only from the electrical resistances of the individual materials, but also from contact resistance, corrosion resistance, etc. of the combination of materials. As is known, electrically conductive metals have a very low electrical resistance, but the electrical resistance of a combination of layers of electrically conductive metals may be much higher than the sum of the electrical resistances of the individual materials due to contact resistance, corrosion resistance, etc. Experiments have shown that the electrical resistance of the combination of the current collector, the current collector coating and the electrode coating no longer significantly changes due to contact corrosion after 2000 hours.

Typically, current collector and end electrode plate are made as separate components, but are considered here as a whole (together with current collector coating and electrode end plate coating) as a termination arrangement.

Of course, electrode end plate and current collector could be provided outside the contact area with an electrode coating, or current collector coating, for example, so they can be completely surface-coated. The electrode coating as well as the current collector coating can be applied to the electrode end plates or current collectors by any known method, e.g. chemical deposition, (plasma-assisted) chemical vapor deposition, electrochemical coating, sputtering, calendering, etc., with a complete and gapless coating of the electrode end plate with the electrode end plate coating and the current collector with the current collector coating in the contact region not necessarily being required, but being preferred.

If the current collector coating and the electrode end plate coating are not present in the entire contact region (but only in 20% of the contact region, for example), then this would of course mean that contact corrosion and increased contact resistance can occur in the remaining contact region, resulting in increased electrical resistance. The total electrical .0 resistance of the combination of current collector, current collector coating and electrode end plate coating must therefore not exceed 1 ohm.cm 2 . Experiments have shown that this value occurs when the current collector coating and the electrode end plate coating were selected in 20% of the contact area. Of course, a higher coverage of the contact area with the current collector coating and the electrode end plate coating will reduce the electrical resistance to below 1 ohm.cm 2 .

According to the disclosure, the combination of the current collector, the current collector coating and the electrode coating comprises a maximum of 5% by weight of copper and silver and at most 0.05% by weight of elements of the platinum group (Rhuthenium Ru, Rhodium Rh, Palladium Pd, Osmium Os, Iridium Ir and Platinum Pt). These limits ensure that upon dissolution of the parts which are electrolytic fluid-soluble, that is, the current collector, the current collector coating and the electrode coating, the electrolyte fluid is not contaminated in such a way that the function of the redox flow battery is critically affected. It is known that the elements of the platinum group are very good catalysts for hydrogen, about a factor of 100 better than silver or copper, which results in the different limit values. These limits for the content of copper, silver and elements of the platinum group ensure that current collectors, including the current collector coating and electrode coating, dissolve on contact with an electrolyte fluid (which cannot be prevented on contact), but do not critically contaminate the electrolyte fluid. By selecting the materials of the current collector, the current collector coating and the electrode end plate coating, a low electrical resistance and thus good electrical contact between the current collector, the current collector coating, the electrode end plate coating and ultimately the electrode end plate is - as mentioned above guaranteed.

Preferably, the current collector (that is, the "core" of the current collector, without current collector coating) consists of at least 90% by weight of aluminum Al, nickel Ni, tin Sn or an alloy thereof, with a high aluminum content being preferred because of its low weight and high conductivity. A high nickel content would lead to a high weight and a slightly poorer electrical conductivity, but is still conceivable. For example, a current collector made of pure tin would be too soft.

Advantageously, the material of the electrode coating and/or the material of the current collector coating consists of 90% by weight of one of the following elements or alloys thereof: Zn, Sn, Ni, Pb, Sb, Cd, Cr, C, In, Al, V, Fe, Cu, Ag. These materials have high conductivity and are easy to process. It is even conceivable that the electrode coating and/or the current collector coating contain copper, silver, or even elements of the platinum group, as long as the total of the current collector, current collector coating and electrode coating contains less than 5% by weight of copper, silver and less than 0.05% by weight of elements of the .0 platinum group.

The electrode coating and/or the current collector coating can advantageously have a thickness of at least 5 pm if they are made of at least 90% by weight of one of the following elements or alloys thereof: Zn, Sn, Ni, Pb, Sb, Cd, Cr, C, In, Al, V, Fe, Cu, Ag. This ensures that no pinholes or very few pinholes occur in the electrode coating or current collector coating. If the electrode coating and/or the current collector coating consists of nickel, layer thicknesses of 50 pm should preferably not be exceeded, since otherwise the electrode coating or the current collector coating could become brittle.

However, the material of the electrode coating and/or the material of the current collector coating may also consist of 90% by weight of inorganic compounds, such as oxides, hydroxides, carbides, phosphides, sulfides, borides, etc., or electrically conductive polymers. These materials do usually have a lower electrical conductivity than metals, but they are more stable against corrosion and reactions in contact with the electrolyte fluid. Particularly thin current collector coatings of this type can theoretically consist 100% of said inorganic compounds. However, such an electrode coating or current collector coating should advantageously have a thickness of at least 0.1 pm in order to avoid pinholes. Layers which are too thick, however, could adversely affect the electrical conductivity, as mentioned.

Overall, of course, the above-mentioned limits for the electrical resistance and the proportion of copper, silver and elements of the platinum group must, of course, be maintained.

Advantageously, the material of the electrode coating and the material of the current collector coating are identical. In any case, it is essential that the electrical resistance of the combination of electrode coating, current collector and current collector coating does not exceed 1 ohm.cm 2 over a period of at least 2000 hours. The measurement of the electrical resistance takes place at 45 °C and a humidity of 60%.

The electrode coating may have an electrode interlayer on the side of the electrode end plate, which preferably has a layer thickness of at least 5 pm, which serves to avoid pinholes.

The current collector coating can have a current collector interlayer on the side of the current collector, preferably with a layer thickness of at least 4 pm.

Electrode interlayer and current collector interlayer serve primarily as a bonding agent and further to avoid diffusion between the materials of the end electrode and the electrode coating, and between the current collector and the current collector coating. The material of the electrode interlayer and/or the current collector interlayer advantageously consists of 90% by weight of nickel, chromium, titanium, cobalt or an alloy thereof.

The electrode interlayer is considered to be part of the electrode coating and the current collector interlayer to be part of the current collector coating. This means that also the electrode interlayer, as well as the current collector interlayer, are considered to be part of .0 the combination of the current collector, the current collector coating and the electrode coating, which comprises a maximum of 5% by weight of Cu, Ag and at most 0.05% of elements of the platinum group. Of course, this limit value must be taken into account when choosing the material of the electrode interlayer and current collector interlayer. This consistently follows from avoiding the contamination of the electrolyte fluid.

A cell stack of a redox flow battery may consist of a plurality of adjacent cells arranged between two end cells, wherein the cells and end cells each consist of two half-cells separated by a semipermeable membrane, electrodes are located between the cells and a termination arrangement according to the invention is located on at least one end cell on an axially outer side with respect to the cell stack..

A redox flow battery may further comprise at least one such cell stack. The termination arrangement according to the invention or the cell stack according to the invention can in principle be used for all redox flow batteries with aqueous and acid electrolyte fluids, in particular vanadium, zinc bromine and iron-chromium redox flow batteries. However, it is also conceivable to use the termination arrangement or the cell stack for alkaline redox flow batteries or for metal-gas flow batteries, such as aluminum-air, vanadium-air flow batteries.

In a redox flow battery, a termination arrangement may be such that the combination of the current collector, the current collector coating, and the termination electrode coating in relation to the electrolyte fluid present in the redox flow battery, is less than 5 ppm by weight of Cu, Ag and contains less than 0.05 ppm by weight of elements of the platinum group.

By taking into account the amount of Cu, Ag, and elements of the platinum group in the termination arrangement in relation to the potentially contaminable electrolyte fluid (as mentioned, the contaminating materials may also pass through the semipermeable membrane from the negative to the positive electrolyte fluid and vice-versa) proper operation can be ensured despite the dissolution of the current collector, the current collector coating and the electrode end plate coating. It is clear that the cell stack with a dissolved current collector is no longer fully functional, but the other existing cell stacks, in particular their negative electrodes, are not affected.

If 0 ppm of copper and silver are present in the electrolyte fluid, current redox flow batteries have maintenance intervals of 5-10 years. With 1 ppm copper and silver in the electrolyte fluid, the maintenance intervals shorten to 3-6 months. Such short maintenance intervals represent the still reasonable limit, which is why the limits of 1 ppm for copper and silver and 0.01 ppm for elements of the platinum group have been established. This means that acceptable maintenance intervals are still possible in the case of a partial dissolution of the termination arrangement in the electrolyte fluid.

.0 Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

The present invention is explained in more detail below referring to Figures 1 to 5, which show exemplary, schematic and non-limiting advantageous embodiments of the invention. In the drawings,

Fig. 1 shows a redox flow battery with a cell stack,

Fig. 2 shows a section of the cell stack,

Fig. 3 shows a termination arrangement with a current collector, a current collector coating, an intermediate current collector layer, an electrode coating and an electrode interlayer,

Fig. 4 shows the contact resistance of different combinations of current collector and electrode end plate,

Fig. 5 shows a termination arrangement according to the invention consisting of a current collector and an electrode end plate.

With reference to Figures 1 and 2, the known construction of a redox flow battery 1 according to the prior art will be explained. A cell stack 2 of a redox flow battery 1 comprises a plurality of cells 4, which in turn are each formed from two half-cells 40. A semipermeable membrane 6, typically an ion exchange membrane (cation and/or anion exchange membrane, e.g. Nafion@) is arranged in each case between two half-cells 40 of a cell 4. An electrode plate 7, for example a bipolar plate, is arranged between two adjacent cells 4. A positive or negative electrode 402, for example mats made of carbon or graphite fibers, are arranged in each case in the frames 401 of the positive and negative half-cells 40 (which are alternately arranged). Via recesses 80 in the frame 401 of the half-cells 40, or cells 4, electrolytically differentially charged electrolyte fluids are pumped through the cells 4 by means of the pumps 92, 93, wherein the positive electrode 402 in a cell 2 or the respective positive half cell 40 is perfused by the positive electrolyte fluid, and the negative electrode 402 of the negative half-cell 40 is perfused by the negative electrolyte fluid. This leads, as is well known for electrochemical processes, to the generation of electric current or to a charging of the redox flow battery 1, or more precisely of the electrolyte fluids. In some types of redox flow batteries 1, such as a vanadium redox flow battery or a vanadium polyhalide battery, the two electrolyte fluids are chemically similar or have only a different oxidation state (e.g. V 2 . and V3 +, VO 2 ' and VO 2 ).

Fig. 1 also shows the tanks 90, 91 of a redox flow battery 1, in which the electrolyte fluids are .0 stored. In the course of power generation or storage, the electrolyte fluids are circulated between the half-cells 40 and the tanks 90, 91 using the pumps 92, 93. The tanks 90, 91 may be spatially separate containers, but may also be formed, for example, as two compartments separated by a partition, in a common container.

The cell stack 2 is completed at the two axial ends by an end plate 60, made, for example, of plastic. The end plates 60 are clamped by means of clamping means 4, such as, for example, passing bolts, which are clamped by means of nuts 42, washers 43 and springs 44, and thus compress the frames 401 of the half-cells 40 of the cell stack 2. Furthermore, an electrical connection 11 can be provided on the end plates 60, via which the current collectors 3 in the interior of the redox flow battery 1 can be connected to an external circuit on both sides of the redox flow battery 1. For reasons of clarity, the electrical connection is shown only in Fig. 1, and the connection between the current collector 3 and the electrical connection 11 cannot be seen in the figures. Furthermore, the electrolyte fluid connections 9, 10 are provided for the supply and discharge of the electrolyte fluids in the illustrated embodiment at the end plates 60.

In order to prevent a possible setting of the, for example elastic, frames of the cells 4 by the contact pressure, spacers 8 may be provided between the end plates 60 in order to ensure a constant distance 8' between the end plates 60.

Fig. 2 shows a section of a cell stack 2, wherein some half-cells 40 are shown at the axially lower end of the cell stack 2 as a detail. In this case, a cell 4 is formed by two half-cells 40, each half-cell 40 including an electrode 402 surrounded by a frame 401. At the axial end of the cell stack 2, an end cell 41 is formed by two half-cells 40 separated from each other by a semipermeable membrane 6 and terminated by an end frame 61 and an end plate 60. The uppermost half-cell 40 shown in Fig. 2 is part of a cell 4 which is not shown completely. An electrode end plate 70 is arranged between the end cell 41 (or the axially outer half-cell 40 of the end cell 41) and the adjoining end frame 61, as shown in Fig. 2, instead of an electrode plate 7. In addition, the end frame 61 holds a current collector 3, which abuts at least partially in a contact region 5 to the electrode end plate 70 and is in electrical contact with the electrode end plate 70, as indicated in Fig. 2. Of course, the arrangement sketched in Fig. 2 and described below can also apply to the opposite end cell 41 at the other axial end of the cell stack 2.

According to the invention, the current collector 3 comprises a maximum of 5% by weight of copper and silver and a maximum of 0.05% by weight of elements of the platinum group (ruthenium Ru, rhodium Rh, palladium Pd, osmium Os, iridium Ir and platinum Pt), and preferably at least 90% by weight of aluminum, nickel, tin or an alloy thereof. If, however, the current collector 3 is manufactured, for example, from pure aluminum and this current collector 3 is without a current collector coating 30 directly at the electrode end plate 70, then .0 despite the good conductivity of aluminum due to the surface oxidation, a very high contact resistance of more than 1 ohm.cm 2 can be detected for the current collector. Therefore, the current collector 3 is provided with a current collector coating 30 made of an electrically conductive material at least in 20% of the contact region 5. Fig. 3 shows a schematic, enlarged section of an advantageously designed termination arrangement. This termination arrangement consists of a current collector 3 made of aluminum, for example, having a thickness of 200 pm, a current collector coating 30 applied to the current collector 3, for example of 13.84 pm tin, wherein a current collector intermediate layer 32, for example of 5.18 pm nickel is located between current collector 3 and current collector coating 30.

The current collector interlayer 32 is also electrically conductive and can advantageously consist of nickel, chromium, titanium, cobalt, or an alloy thereof, and can serve as a bonding agent between current collector 3 and current collector coating 30, on the one hand, and can act, on the other hand, as a barrier to possible material diffusion between the current collector 3 and the current collector coating 30.

In measurements, a contact resistance of <0.01 ohm.cm 2 was determined in the combination in Fig. 3, since the oxide layer on the aluminum current collector 3 was at least partially prevented by the at least partial current collector coating 30 or the current collector interlayer 30. In a cell stack 2, however, this current collector 30 is brought into contact with the current collector coating 30 and the current collector interlayer 32 with an electrode end plate 70. Copper has a resistivity of 1.7 10-8 ohm.m, aluminum 2.7 10-8 ohm.m, nickel 7.10-8 ohm.m, and tin 11.10-8 ohm.m. With the above-mentioned layer thicknesses and the associated specific resistances, arithmetically, an electrical resistance of well below 0.1 ohm.cm 2 would result in each case for the current collector 3, the current collector coating 30 and the current collector interlayer 32. Due to these very low resistance values, one could expect a low total resistance of the termination arrangement.

However, as can be seen from curve B in the graph in Fig. 4, for the layer sequence, current collectors 3 made of 200 pm aluminum, current collector interlayer 32 made of 5 pm nickel, current collector coating 30 made of 15 pm tin (and an electrode end plate 70), a resistance value of just under 10 ohm.cm 2 initially results, which is due to an increased contact resistance between the current collector 3, current collector coating 30, current collector intermediate layer 32 and the end electrode 7 and is already unacceptably high. The electrical resistance of the electrode end plate 70, which, however, is only 0.02-0.04 ohm.cm2 in each case, is also "measured along with this". However, due to contact corrosion, the resistance increases over a period of 2000 hours to over 100 ohm.cm 2

In order to reduce this contact resistance and corrosion resistance between current collector 3 and electrode end plate 70, the electrode end plate 70 is additionally provided in the contact region 5 with an electrode coating 71 made of an electrically conductive material - as .0 sketched in Fig. 3. Of course, the electrode coating 71 must be applied to at least 20% of the contact region 5 already provided with the current collector coating 30, so that the electrode coating 71 actually abuts the current collector coating 30. Also shown in Fig. 3 is an electrode interlayer 72 between electrode end plate 70 and electrode coating 71, which may be the same as the current collector interlayer 32, advantageously made of Ni, Cr, titanium, cobalt, or an alloy thereof. The electrode interlayer 72 can thus serve both as a bonding agent between the electrode end plate 70 and electrode coating 71, and as a barrier to a possible material diffusion between the electrode end plate 70 and the electrode coating 71.

Fig. 5 shows a cross section through an inventive termination arrangement consisting of a current collector 3 and an electrode end plate 70, wherein the electrode end plate 70 and the current collector 3 are arranged adjacent to each other in a contact region 5 and are in electrical contact.

The resistance values therefore do not arise primarily from the electrical resistance of the materials used for the individual layers, i.e. the current collector 3, the current collector coating 30, the electrode coating 71 (and possibly the current collector interlayer 32 or the electrode intermediate layer 72), but from the contact resistance of the combination of the current collector 3, the current collector coating 30 and the electrode coating 71 (and possibly the current collector interlayer 32 and the electrode interlayer 72), as well as from the corrosion resistance of this combination. Although both the electrode coating 71 and the current collector coating 30 are electrically conductive, they need not be made of the same material. The current collector coating 30 as well as the endplate coating may be made of Zn, Sn, Ni, Pb, Sb, Cd, Cr, C, In, Al, Vd, Fe, or an alloy thereof. Likewise, inorganic compounds, such as oxides, hydroxides, carbides, phosphides, sulfides, borides, etc., or electrically conductive polymers are conceivable as a coating. The thicknesses and materials of the current collector 3, the current collector coating 30 and the electrode coating 71 are selected such that the electrical resistance of the combination of the current collector 3, the current collector coating 30 and the electrode coating 71 does not exceed 1 ohm.cm 2 over a period of at least 2000 hours. This period can, for example, during normal operation of a redox flow battery 1 in which the termination arrangement is installed, be measured, and a measurement while out of service or not installed in a redox flow battery 1 is also possible.

In Fig. 4, a termination arrangement is shown as the curve C consisting of the layer sequence of the current collector 3 made of 200 pm aluminum, the current collector interlayer 32 made of 5 pm tin, the current collector coating 30 made of 10 pm nickel, the end electrode coating 71 made of 10 pm nickel, the end electrode interlayer 72 made of 5 pm tin (and the 2 electrode end plate 70). As can be seen, resistance values of less than 0.1 ohm.cm were measured over a period of 2000 hours for this layer sequence. If the current collector 3, .0 together with the electrode end plate 70, came into contact with an electrolyte fluid of the redox flow battery, the current collector 3, the current collector coating 30, the electrode coating 71, the current collector interlayer 32 and the electrode interlayer 72 would dissolve but the electrolyte fluid would not be contaminated, since the materials here are, for example, aluminum, nickel and tin and cause no negative effects in the redox flow battery.

If the electrode interlayer 72 is dispensed with, the resistance value does not rise above 1 ohm.cm2 over a period of 2000 hours, as also illustrated in Fig. 4 as curve D. This slightly increased resistance compared to the termination arrangement with existing electrode interlayer 72 is presumably due to a slight but actual oxidation of one or more of the layers present (i.e., current collector coating 30, electrode coating 71, current collector interlayer 32), wherein the oxidation is able to proceed particularly from the edge or also possibly from existing pinholes or cracks in the relevant layer.

Likewise, in Fig. 4. in the form of the curve A, the time course of the resistance of a current collector 3 according to the prior art is shown. The current collector 3 consists of 200 pm copper and has a current collector coating 30 of 3 pm silver. Although resistance values of less than 1 ohm.cm 2 are possible, the current collector 3 with current collector coating 30 would contaminate it when in contact with an electrolyte fluid.

A typical redox flow battery by Gildemeister energy solutions of the type CellCube FB30-70 includes tanks 90, 91 with 3500 liters of electrolyte fluid and twenty cell stacks 2 each with two current collectors 3, wherein a current collector has 600 cm 2 area. Type FB-10-130 redox flow batteries have 90.91 tanks of 6500 liters of electrolyte fluid, with six cell stacks 2 in these systems. There are 20-27 cells per cell stack. Advantageously, in at least one, advantageously in each termination arrangement of the redox flow battery in the combination of current collector 3, current collector coating 30 and electrode coating 71, less than 1 ppm by weight of Cu, Ag, and less than 0.01 ppm by weight of elements of the platinum group is present in relation to the electrolyte fluid present in the redox flow battery. Thus, in a redox o flow battery of the type FB-10-130, 6500 liters of electrolyte fluid would be present, and the electrolyte fluid has a density of 1.35 g/cm 2. Thus, 1 ppm by weight of the redox flow battery would correspond to a mass of 8.8 g. This means with 20 cell stacks, i.e. 40 current collectors, each with an area of 600 cm 3 each and a thickness of 200 pm, that a current collector may consist of a maximum of 0.2% copper and silver in order to dissolve 1 ppm by weight in 6500 liters of electrolyte fluid. This corresponds, for example, to an area of 600 cm of a pure copper or silver layer with a thickness of 0.4 pm.

Claims (15)

Claims

1. A termination arrangement of a cell stack of a redox flow battery with a current collector and an electrode end plate, wherein the electrode end plate and the current collector are arranged adjacent to each other in a contact region and are in electrical contact, wherein in at least 20% of the contact region the current collector is coated with an electrically conductive current collector coating, wherein the electrode end plate in this at least 20% of the contact region is coated with an electrically conductive electrode coating, wherein the combination of the current collector, the current collector coating and the electrode coating consists of a maximum of 5% by weight of Cu and Ag and a maximum of 0.05% of elements of the platinum group, wherein the electrical resistance of the combination of the current 2 collector, the current collector coating and the electrode coating does not exceed 1 ohm.cm over a period of at least 2000 hours.

2. The termination arrangement according to claim 1, wherein the current collector is coated in at least 50%, preferably at least 90%, of the contact region with an electrically conductive current collector coating, and wherein the electrode end plate in these at least %, preferably at least 90%, of the contact region is coated with an electrically conductive electrode coating.

3. The termination arrangement according to claim 1 or 2, wherein the current collector consists of at least 90% by weight of Al, Sn, Ni or alloys thereof.

4. The termination arrangement according to any of the claims 1 to 3, wherein the electrode coating and/or the current collector coating consist of at least 90% by weight of one of the following elements or alloys thereof: Zn, Sn, Ni, Pb, Sb, Cd, Cr, C, In, Al, V, Fe, Cu, Ag.

5. The termination arrangement according to claim 4, wherein the electrode coating and/or the current collector coating have a layer thickness of at least 5 pm.

6. The termination arrangement according to any one of claims 1 to 3, wherein the electrode coating and/or the current collector coating consists 90% by weight of electrically conductive inorganic compounds, and/or electrically conductive polymers.

7. The termination arrangement according to claim 6, wherein the electrode coating and/or the current collector coating have a layer thickness of at least 0.1 pm.

8. The termination arrangement according to any one of claims 1 to 8, wherein the material of the electrode coating and the material of the current collector coating are identical.

9. The termination arrangement according to any one of claims 1 to 8, wherein an electrically conductive electrode interlayer, preferably with a layer thickness of at least 5 pm, is located between the electrode coating and the electrode end plate.

10. The termination arrangement according to any one of claims 1 to 9, wherein there is a current collector interlayer, preferably with a layer thickness of at least 5 pm, between the current collector coating and the current collector.

11. The termination arrangement according to claim 9 or 10, wherein the material of the electrode interlayer and/or the current collector interlayer consists of at least 90% by weight of Ni, Cr, Ti, Co, or alloys thereof.

12. A cell stack of a redox flow battery consisting of a plurality of adjacent cells arranged between two end cells, wherein the cells and end cells each consist of two half-cells separated by a semipermeable membrane, electrodes are located between the cells, and a termination arrangement according to any one of claims 1 to 11 is located on at least one end cell on a axially outer side with respect to the cell stack.

13. A redox flow battery comprising at least one cell stack according to claim 12.

14. The redox flow battery according to claim 13, wherein the combination of the current collector, the current collector coating and the electrode coating of at least one termination arrangement contains less than 1 ppm by weight of Cu and Ag in relation to the electrolyte fluid present in the redox flow battery.

15. The redox flow battery according to claim 13 or 14, wherein the combination of the current collector, the current collector coating and the electrode coating of at least one termination arrangement contains less than 0.01 ppm by weight of elements of the platinum group in relation to the electrolyte fluid present in the redox flow battery.