WO2008058577A1

WO2008058577A1 - Electrode and its use

Info

Publication number: WO2008058577A1
Application number: PCT/EP2007/002376
Authority: WO
Inventors: Klaus Nohl
Original assignee: Hydrodivide Ag
Priority date: 2006-11-16
Filing date: 2007-03-17
Publication date: 2008-05-22
Also published as: DE102006054442A1

Abstract

Electrode for the electrolysis of water-containing electrolytes, which comprises a material suitable for the electrolysis of water, with the surface of the electrode being configured so that it has at least two internal regions having different charge densities when a voltage is applied.

Description

Electrode and its use

The present invention relates to electrodes for the electrolysis of aqueous electrolytes and more particularly to electrodes for the production of hydrogen and oxygen.

Hydrogen has long been an important, universally applicable raw material for the synthesis of chemical compounds and reduction reactions in metallurgy. Furthermore, hydrogen is used in the processing of crude oil to fuels and high-quality chemical products.

The production of hydrogen currently takes place almost exclusively from hydrogen-containing fossil raw materials and as a reaction product in the chloralkali electrolysis. However, for environmental reasons increasingly desired manufacturing process, in which instead of exhaustible primary energy, such. As natural gas, uranium ore, coal, crude oil, in principle inexhaustible primary energies, such as. As renewable resources, sun, water, wind, can be used.

For this purpose, the water electrolysis offers itself as a proven in practice and working in its advanced version with good efficiency method. Regenerative, non-biogenic primary energies, which can be harnessed in the form of electricity, can thus in principle also be converted into hydrogen.

The water splitting by electrolysis is a relatively simple and at the same time effective method of hydrogen production. In this case, water is decomposed by means of electric current. In a single alkaline electrolysis cell is at a DC voltage of at least 1.5 volts (equivalent to 3.54 kWh per standard cubic meter (Nm ³ ) hydrogen) at the cathode hydrogen and formed at the anode oxygen. The electrolyte used is 20 to 40% potassium hydroxide solution. A gas-tight membrane, the so-called diaphragm, leaves the necessary Transport of OH ^' ions, but at the same time prevents the mixing of the product gases. Technical electrolysers consist of many individual cells connected one after the other - the cell block - as well as auxiliary equipment for the electrolyte circuit, the power supply, the deionization of the feed water, the gas separation and drying and the cooling. Such a cell block can usually produce between 200 m ³ and 500 m ^{3 of} hydrogen per hour, for which an electric charge between 860 kW and 2150 kW is required at a typical current density of 300 mA / m ² electrode surface (corresponding to 4.3 kW per Nm ³⁾ Hydrogen). The modular cell blocks can be combined to any desired performance. The largest electrolysis plants currently have connection capacities of 150 MW. Electrolysers operate at ambient pressure as well as under overpressure up to 30 bar.

The object of the present invention is to further improve the efficiency of water electrolysis and, in particular, to identify possibilities for reducing the required energy requirement.

These and other objects which can be directly deduced from the foregoing considerations are achieved by providing an electrode having all of the features of independent claim 1 herein. The dependent claims on this claim describe particularly expedient embodiments of the electrode. Furthermore, an electrolysis cell, comprising at least one electrode according to the invention, and the use of the electrolysis cell for the electrolysis of a water-containing electrolyte are protected.

The electrodes of the present invention permit easier detachment of the hydrogen gas bubbles from the electrode surface and result in a significant reduction in the energy normally required in the electrolysis of aqueous electrolytes. They therefore allow a highly efficient production of hydrogen with very high efficiency and very high purity with relatively low energy consumption. The at the Electrolysis optionally as by-product resulting oxygen also has a very high purity and can be used again for the oxidation of hydrogen, if necessary, for. B. in a fuel cell.

The present invention thus relates to an electrode for the electrolysis of aqueous electrolytes.

Electrodes have long been known and refer to electron-conducting materials that can be used for the electrolysis of electrolytes. In electrolysis, by applying an external voltage, a chemical reaction is forced in the electrolyte comprising two partial reactions, oxidation at the cathode and reduction at the anode. So go z. As in the electrolysis of water at the cathode, the hydrogen ions (H ⁺ ) by receiving electrons (from the circuit) in neutral atoms, which combine in pairs to form H ₂ molecules and escape as gas. Conversely, at the same time the hydroxide ions (OH ^" ) at the anode lose their negative charge .The emitted electrons are conducted through the current source to the cathode, while the formed oxygen atoms combine to form O ₂ molecules and also escape as gas ⁺ Ions are immediately neutralized by hydroxide ions to water molecules, but this redox reaction occurs only when the voltage applied to the electrodes is greater than the decomposition voltage dependent on the individual redox potentials.

In the context of the present invention, the electrode has a material suitable for the electrolysis of water, preferably a metal and / or graphite, in particular copper, silver, lead, mercury, platinum, palladium, gold, nickel, graphite and / or iron.

Furthermore, the surface of the electrode is designed such that it has at least two internal regions with different charge density with applied voltage. The charge density is the ratio of the charge to the size of the charged surface. Consequently, the Charge distribution on the surface uneven (inhomogeneous), the edges, ie the limitations of the electrode are not taken into account in this context. In the context of the present invention, at least one plane can be applied to each point of the edge or edges of the electrode, without the electrode cutting the plane at another point. However, touching the electrode with the plane is possible.

The charge distribution on the surface is expediently determined under conditions customary for the electrolysis of aqueous electrolytes. Conveniently, two electrodes are immersed in an electrolyte, connected together and a DC voltage of 1, 7 V and 0.6 A applied. The electrolyte used is preferably an aqueous KOH solution having a pH of> 10. The electrolysis is particularly preferably carried out at 20 ⁰ C and 1033 mbar.

The inhomogeneity of the charge distribution on the surface is preferably such that the charge density of the region of greater charge density is at least 10%, preferably at least 20%, more preferably at least 50%, especially at least 100%, greater than the charge density of the region of lower charge density.

The concrete shape of the electrode is basically of minor importance. Expediently, its surface comprises at least one inner curved partial surface, in which the first main radius of curvature is smaller than 8 mm at at least one point and the second main radius of curvature of the surface at this point is preferably smaller than 1 mm.

In this case, a Cartesian coordinate system is used in such a way that the point considered lies in the origin and the x, y plane coincides with the tangent plane through this point. In this system, the area in the vicinity of the considered point Po has the representation z = z (x, y) with z (0,0) = Oz (O ₁ O) / Sx = S (O ₁ O) / Sy = 0. The associated accompanying tripod in point P ₀ consists of the three unit vectors ei , β ₂ , N = ei x β ₂ pointing in the direction of the coordinate axes. The Taylor expansion around Po is z = V z O ² Z (O ₁ O) / Sx ² + O ² Z (O ₁ O) / dxdy + Vz ^O2 Z (O ₁ O) / ² + Sy ... by rotating the coordinate system about the z-axis can be z = ^_1/2 (k ₁ x ² + k ₂ y ²⁾ + ... achieve. One defines: ki, k _{2 main} curvatures

Ri = 1 / k-ι first major radius of curvature

R ₂ = 1 / k ₂ second major radius of curvature

K = kik ₂ Gaussian curvature

H = Vz (ki + k ₂ ) mean curvature at point P ₀ .

For further details is on the standard literature, z. See, for example, I.N. Bronstein, K.A. Semendjajew Taschenbuch der Mathematik 24th edition, Teubner, 1989, p. 603 ff.

In the context of a particularly preferred embodiment of the invention, the surface of the electrode comprises at least two, particularly preferably at least three, mutually different sub-areas, which intersect in at least one inner point. Preferably, the faces have a different spatial orientation and / or are curved differently. The line of intersection of two preferably non-curved partial surfaces is usually referred to as edge and the intersection of at least 3 preferably non-curved partial surfaces generally as corner.

Expediently, the electrode comprises at least one, preferably at least two, more preferably, at least 5, internal recess (s), which preferably has (have) at least one non-curved edge and is particularly preferably designed as a rectangular recess of the electrode. In the context of a particularly favorable Embodiment of the present invention, the recess is an opening through the electrode.

In the electrolysis cell, the electrode is then expediently arranged such that the non-curved edge of the recess, with several non-curved edges, preferably the longest edge of the recess, is not perpendicular to the gravity vector (Lot) of the earth's gravity field, so that the gas bubbles are easier from the electrode edge solve, because they are carried away by other rising gas bubbles. The usually necessary electrical energy for desorption of the gas bubbles is drastically reduced by the utilization of the earth's gravity field. The angle between the edge and the gravity vector is favorably as small as possible and is preferably in the range of 0 ° to 30 °, preferably in the range of 0 ° to 15 °, more preferably in the range of 0 ° to 5 °, most preferably between 0 ° and 0.5 °, and is 0 in the most preferred embodiment.

At least one recess is favorably at least 3 mm, preferably at least 5 mm, particularly preferably at least 15 mm long and at least 1 mm, preferably at least 2 mm, particularly preferably at least 3 mm wide, wherein in principle is to be placed on the maximum length and width of the recess. The depth of the recess, which in the case of a hole corresponds to the thickness of the electrode at this point, is preferably at least 0.1 mm, preferably at least 0.2 mm, particularly preferably at least 0.3 mm. Again, the maximum value should be turned off.

In the context of a further particularly preferred variant of the present invention, the electrode comprises at least one prong, ie at least one protruding tip, which extends, for example, conically or pyramidally outwards. The use of a plurality of prongs, preferably of at least 5, in particular of at least 16 prongs has proven particularly useful in the context of the present invention. The length of the tines should preferably be at least 0.1 mm, preferably at least 0.5 mm, in particular at least 1 mm. Furthermore, the ratio of length of the tine to the maximum diameter of the tine should preferably be greater than 5: 1, preferably greater than 10: 1, in particular greater than 20: 1. The maximum diameter of the tine should preferably be less than 1.0 mm, preferably less than 0.5 mm, in particular less than 0.1 mm.

Moreover, for the purposes of the present invention, it is particularly advantageous if, when using a plurality of tines, the tips of the tines are not in contact with each other and are preferably at least 0.1 mm apart.

The surface density of the tines can in principle be freely selected depending on the particular electrolysis conditions. However, according to the invention, electrodes having at least 16 tines per cm ^{2 of} surface area of the main body of the electrode have proven especially useful.

The concrete form of the tines is basically of secondary importance. However, within the scope of the present invention, electrodes having non-curved surfaces and outwardly tapering teeth are preferably used.

In the context of a very particularly preferred embodiment of the present invention, the electrode has the shape of a cylinder, the surface of which is designed so that it has at least two internal regions with different charge density when the voltage is applied. The length and the diameter of the cylinder can be chosen arbitrarily. However, thread-like cylinders with a diameter of less than 5 mm, advantageously less than 2 mm, in particular less than 1 mm, are particularly preferred. The minimum diameter is preferably at least 0.1 mm, more preferably at least 0.3 mm, in particular at least 0.5 mm. The ratio of length The diameter of the cylinder is furthermore preferably in the range from 10000: 1 to 1: 1, in particular in the range from 2500: 1 to 10: 1.

The electrode according to the invention is preferably used for the electrolysis of aqueous electrolytes, such as. As aqueous NaOH used. An electrolysis cell which is preferred for this purpose comprises at least two electrodes according to the invention, one being connected as cathode. The cathode surface preferably contains a material from the following series,

Pd - Au - Pt - Ag - Ni - C (graphite) - Fe - Cu - Pb - Hg with the materials arranged according to their suitability (left best).

The anode surface preferably contains a material from the following series,

Ni - Fe - Cu - Pb - Ag - Pd - Pt - Au with the materials arranged according to their suitability (left best).

The cathode space and the anode space are advantageously separated by a gas-tight membrane, the so-called diaphragm from one another, while permitting the necessary transport of OH ^"ions, but also prevents the mixing of the product gases. For these purposes, suitable membranes are known from the prior art well known.

In this context, it has proved to be particularly advantageous to use a membrane electrode assembly comprising at least two electrodes according to the invention, which are separated by the membrane. In this case, the electrodes expediently each have at least one opening which communicates with the electrolyte on one side and with the membrane on the other side, thus permitting the transport of the ions through the openings of the electrode and the membrane. The position of the openings in the electrodes should furthermore preferably be such that, when the membrane is (mentally) removed, the openings of the two electrodes are in direct contact with each other. The electrolyte used is preferably an aqueous solution having a pH greater than 10, preferably greater than 11, particularly preferably greater than 12, in particular greater than 13, measured at the electrolysis temperature, and very particularly preferably up to 40% potassium hydroxide solution. Within the scope of a further particularly preferred embodiment of the invention, an aqueous solution having a pH in the range from 11 to 13, measured at the electrolysis temperature, is used.

In the electrolysis, a DC voltage of at least 1.23 volts, more preferably at least 1.5 volts, is desirably applied to induce the decomposition of the water.

To further increase the efficiency of several such electrolysis cells may optionally be interconnected.

Moreover, for the purposes of the present invention, it has proven particularly useful to apply a pulsed DC voltage during the electrolysis in order to additionally facilitate the desorption of the hydrogen. The pulsed DC voltage, also called pulse current, is preferably generated by pulse rectifiers, potentiostats or galvanostats and consists of individual pulse sequences, which may include both anodic and cathodic currents and pulse pauses. In principle, a wide variety of pulse shapes can be used, but square-wave pulses are very particularly favorable for the purposes of the present invention. Particularly preferred pulse frequencies are in the range of 0.2 Hz to 800 Hz. The pause-pulse ratio is preferably selected in parallel operated generators in the range of 1: 1000 to 1000: 1, more preferably in the range of 1: 100 to 100: 1. The pulse current intensity and the pulse voltage are set as in the DC voltage arrangement. To keep the loss in the control as low as possible, preferably two generators are operated in parallel, between which is switched back and forth.

Furthermore, it is advantageous to superimpose the DC voltage pulses with a high frequency in order to further excite the ions and to further promote the discharge. Radio frequency refers in this context to an alternating voltage of high frequency, which is preferably between 2,000 Hz (2 kHz) and 5,000,000 Hz (5 MHz).

In principle, the electrolysis can be carried out at normal, underpressure or overpressure. Furthermore, the temperature can in principle be chosen arbitrarily, with temperatures in the range of 5 ° C to 95 ° C have proven particularly useful.

Claims

claims:

1. Electrode for the electrolysis of aqueous electrolytes, comprising a suitable material for the electrolysis of water, characterized in that the surface of the electrode is formed such that it has at least two internal areas with different charge density at applied voltage.

An electrode according to claim 1, characterized in that the charge density of the region of greater charge density is at least 10% greater than the charge density of the region of lower charge density.

3. Electrode according to claim 1 or 2, characterized in that the electrode comprises at least one inner curved partial surface, in which the first main radius of curvature is at least one point less than 8 mm.

4. An electrode according to claim 3, characterized in that the second main radius of curvature of the surface at this point is less than 1 mm.

5. An electrode according to at least one of the preceding claims, characterized in that the surface comprises at least two mutually different sub-areas, which intersect in at least one inner point.

6. An electrode according to claim 5, characterized in that the surface comprises at least three mutually different sub-areas, which intersect in at least one inner point.

7. An electrode according to claim 5 or 6, characterized in that the partial surfaces have a different spatial orientation.

8. An electrode according to claim 5, 6 or 7, characterized in that the partial surfaces are curved differently.

9. An electrode according to at least one of the preceding claims, characterized in that the electrode comprises at least one internal recess.

10. An electrode according to claim 9, characterized in that the recess has at least one non-curved edge.

11. An electrode according to claim 9 or 10, characterized in that the recess is formed as a rectangular opening through the electrode.

12. An electrode according to claim 11, characterized in that the recess is at least 3 mm long and at least 0.5 mm wide.

13. An electrode according to at least one of the preceding claims, characterized in that the electrode comprises at least one prong.

14. An electrode according to claim 13, characterized in that the tine is at least 0.5 mm long, wherein the ratio of length of the tine to the maximum diameter of the tine is greater than 5: 1.

15. An electrode according to claim 13 or 14, characterized in that the tips of the tines are at least 0.1 mm apart.

16. An electrode according to claim 13, 14 or 15, characterized in that the electrode comprises at least 16 tines per cm ² surface of the electrode body.

17. An electrode according to claim 13, 14, 15 or 16, characterized in that the electrode comprises at least one diameter tapering towards the outside tines.

18. An electrode according to claim 13, 14, 15, 16 or 17, characterized in that the tine is at least 1, 0 mm long.

19. An electrode according to claim 13, 14, 15, 16, 17 or 18, characterized in that the maximum thickness of the tine is less than 1, 0 mm.

20. Electrode according to at least one of the preceding claims, characterized in that the electrode comprises a metal and / or graphite.

21. An electrode according to claim 20, characterized in that the electrode comprises copper, silver, lead, mercury, platinum, palladium, gold, nickel, graphite and / or iron.

22. An electrolytic cell comprising at least one electrode according to at least one of the preceding claims.

23. An electrolytic cell according to claim 22, comprising a nickel-containing anode and a palladium-containing cathode.

24. An electrolytic cell according to claim 22 or 23, comprising at least two electrodes according to claim 11, which are separated by a membrane, wherein the holes are in contact with the electrolyte and the membrane surface, respectively.

25. Use of an electrolytic cell according to claim 22, 23 or 24 for the electrolysis of a water-containing electrolyte.

26. Use according to claim 25 for the production of hydrogen.

27. Use according to claim 25 or 26, characterized in that one applies a pulsed DC voltage.

28. Use according to claim 27, characterized in that two electrolysis cells connected in parallel and switches between them back and forth.

29. Use according to claim 27 or 28, characterized in that one superimposes the DC voltage pulses with a high frequency.