Hydrogen-bromine cell.

0001The invention relates to a hydrogen / bromine cell with a cell frame made of graphite surrounding the electrodes.

0002For a wide range of renewable energy sources, such as sun and wind, storage is required for most areas of application in order to be able to adapt the strongly fluctuating energy supply to demand. Furthermore, possibilities for energy storage in decentralized systems for load balancing are also being sought. The lead accumulator can be used in both cases, but it has the disadvantages that it is relatively expensive and heavy and in particular has a low energy density.

0003A new type of storage that can be used for the purposes mentioned is the hydrogen / bromine cell (see: "J. Electrochem. Soc.", Vol. 127 (1980), pages 549 to 555). In this cell, the electrolysis of hydrobromic acid (HBr) is used to store the electrical energy:<chemistry id="chem0001" num="0001"><img file="EP0108970A2_D0001.tif" /></chemistry>

0004HBr electrolysis uses hydrogen (H₂) and bromine (Br₂) which are stored separately and - if required - converted back to hydrobromic acid to generate electrical energy, in a fuel cell reaction (see for example: "Energy", Vol. 4 (1979), pages 61 to 66). The advantage of the hydrogen / bromine system in comparison with the corresponding hydrogen / oxygen system lies in the high reversibility of the bromine electrode.

0005In order to keep the power to weight ratio and the dimensions of the energy converter as low as possible, the two reactions mentioned above, ie the electrolysis and the fuel cell reaction, should take place in one and the same cell. Such hydrogen / bromine cells (H₂/ Br₂Cells) should be inexpensive on the one hand, but on the other hand they must be made of a material that is corrosion-resistant to the reactants.

0006As corrosive media occur in H₂/ Br₂-Cells in particular bromine and hydrobromic acid. Plastics, such as polytetrafluoroethylene, are therefore particularly suitable materials for cell construction, ie for the cell frame (see: "Hydrogen Energy System", Proceedings of the 2nd World Hydrogen Energy Conference, Pergamon Press, 1978, pages 709 to 730). However, these materials are not conductive, ie they have insulating properties, so that the current draw or the power supply to the electrodes is difficult.

0007Of the electrically conductive materials, only noble metals such as palladium and platinum as well as niobium and tantalum are resistant to the aggressive chemicals bromine and hydrobromic acid. However, these materials are expensive and, moreover, very heavy, so that they adversely affect the power-to-weight ratio of the cells.

0008Graphite has already been specified as the material for H2 / Br2 cells. This material is inexpensive and also electrically conductive. When using graphite as cell frame material in H₂/ Br2 cells has now been shown, however, that after a short period of operation - due to the porosity of the material - bromine diffuses through the graphite, ie bromine exits through the cell frame.

0009The object of the invention is an H₂/ Br₂- To design cells which have cell frames made of graphite in such a way that diffusion of bromine through the cell frames is prevented.

0010This is achieved according to the invention in that at least the surface of the cell frame facing the electrode is provided with a layer of pyrographite on the bromine electrode.

0011The H₂/ Br₂Cell is absolutely Br₂- And also HBr-tight, ie the diffusion of these substances through the cell frame is completely prevented.

0012The H₂/ Br₂Cell on the H₂Side have a corresponding cell frame as on the Br₂-Side, ie a cell frame in which at least the surface facing the hydrogen electrode is provided with a layer of pyrographite. Such a cell frame is namely absolutely H₂-tight and thus prevents the escape of hydrogen. In addition, the leakage of Br₂ or HBr are prevented, in the event that these substances through the membrane that the Br₂- from the H₂-Electrode separates, should pass through. If one accepts the use of different cell frames, then the H₂Side, for example, but also plastic-impregnated coal can be used as the frame material.

0013The layer of pyrographite located on the graphite cell frame preferably has a layer dioke of approximately 20 to 30 μm. “Pyrographite”, which is also referred to as pyrolytic graphite, is understood to mean a material which is obtained by pyrolytic decomposition of gaseous or vaporous hydrocarbons, such as methane, ethane, propane and acetylene, at temperatures above 1800 ° C. In contrast to the so-called pyrocarbon formed at low temperatures (&lt;1700 ° C), which is predominantly fine-crystalline and isotropic, pyrographite is characterized by a strongly anisotropic, graphite-like layer structure.

0014Using examples and a figure in which a preferred embodiment of the H₂/ Br₂-Cell is shown, the invention will be explained in more detail.

0015The H shown in the figure₂/ Br₂- Cell 10 has a cell frame 11 on the Br₂Side and a cell frame 12 on the H₂Page on. Both cell frames consist of graphite, with at least the surface of the frame facing the corresponding electrode being provided with a pyrographite layer; the graphite is preferably used in the form of electrographite. The cell frame 11 encloses the bromine electrode 13, the cell frame 12 the hydrogen electrode 14; For this purpose, both cell frames have a corresponding recess. The surface of the recess in the cell frame 11 and that of the recess in the cell frame 12 is then coated with pyrographite.

0016The Br₂Electrode 13 preferably consists of a graphite felt, which is also used for contacting.

0017The H₂Electrode 14 is preferably made of platinum-coated carbon. A graphite felt 15 ensures the electrical contact between the Hp electrode 14 and the cell frame 12. The two electrodes 13 and 14 are separated from one another by a membrane 16. A cation exchange membrane, in particular based on perfluorinated polytetrafluoroethylene, serves as membrane 16.

0018To feed and discharge the reactants or the reaction products on the Br₂-Side lines 17 and 18 and on the H₂-Side lines 19 and 20, which penetrate the cell frames 11 and 12 in the form of bores. Corresponding holes also have two metal plates 21 and 22 through which the two frames 11 and 12 are pressed together by means of insulated screws. The plates 21 and 22, which are made, for example, of stainless steel and function as the two poles of the cell 10, can also be omitted if the power supply or removal takes place in a different way. The electrolyte, ie the HBr-Br₂Mixing is carried out with the aid of a membrane pump in a circuit through the cell, ie its HBr-Br₂Part, pumped.

0019The cell frames are coated with pyrographite, for example, in such a way that a gaseous hydrocarbon is fed to the cell frames, the surface of which is heated to a high temperature. The hydrocarbon is thermally decomposed and the pyrographite is deposited on the substrate, ie the cell frame, as an anisotropic layer with a high density and laminar structure. The deposition is preferably carried out under the following reaction conditions: temperature: 1900 ° C., hydrocarbon: methane at a pressure of 5 mbar; the deposition rate is generally 0.5 to 0.8 pm / min.

0020A^-The laboratory design of the cell according to the invention has the following dimensions, for example: the round metal plates have a diameter of 8.5 μm and a thickness of 8 mm; the cell frames each have a diameter of 5.7 cm and a thickness of approximately 52 cm. The H₂Electrode contains approximately 2.5 to 5 mg Pt / cm² and has an active area of ​​12.5 cm²on.

0021Such a cell can be used in a simple manner to build batteries in which the individual cells are each electrically connected in series. Cell frames are used for this purpose, which are provided with cutouts on both sides. In this case, for example, a bromine electrode would then be arranged in the second cutout of the cell frame 12 and, with the plate 22 omitted, a further membrane on which a hydrogen electrode would then follow. On the br₂Side it would be the other way around. In this case, however, the supply and discharge lines must take a different route.

0022For electrolysis, ie for energy storage, 6 M HBr is assumed, for example, which is depleted up to 1 M HBr. 13.4 1 6 M HBr are then required to store 1 kWh (voltage: 560 mV). In the fuel cell reaction, the 1 M HBr is again concentrated to 6 M HBr. This results in an H₂- Requirement of approx. 500 1 / kWh. However, the cell according to the invention can also be operated, for example, in such a way that an electrolyte with an initial concentration of 3.65 M HBr and 0.1 M Br₂ and for the fuel cell reaction an electrolyte with an initial concentration of 1.65 M HBr and 1.3 M Br₂ is used (temperature: 50 ° C).