EP0204126B1 - Electrode for membrane electrolysis - Google Patents

Abstract

An electrode for membrane electrolysis comprises an electrode body, whose surface is provided at least partially with an electro-catalytically active coating. The electrode body is constructed from a plurality of parallel, mutually spaced lamellas having a plurality of recesses on edge surfaces facing the membrane, edge surfaces of bridge portions, located between these recesses, not being coated for electro-catalytic activity.

Description

The invention relates to an electrode for membrane electrolysis in preferably vertical electrolysis cells, with an electrode body which is formed from a number of parallel, spaced-apart lamellae and whose surface is at least partially provided with an electrocatalytically active coating.

Coated electrodes of this type are used in particular as anodes in electrolysis devices which operate according to the membrane cell method. In membrane cell technology, an ion exchange membrane is placed between the cathode and the anode. This membrane is impermeable to liquids, but certain ions can diffuse through it. The membrane process for the production of chlorine, sodium or potassium hydroxide solution and hydrogen is becoming increasingly important. Favorable technical conditions thanks to a wider range of different ion exchange membranes and coating types optimized for different anode processes have opened up many other areas of application for the environmentally friendly membrane cell technology, such as the desalination of seawater, brackish water and waste water by dialysis, the recovery of valuable materials from contaminated industrial waste and waste water, the detoxification of waste water and pollutant solutions, the sodium sulfate electrolysis for the extraction of caustic soda and sulfuric acid, the sulfate electrolysis for the sulfur dioxide utilization from flue gases, electrochemical redox processes on various inorganic and organic substances, and dimerization of organic substances.

Compared to conventional mercury cells or electrolytic cells with asbestos diaphragms, the use of membranes in electrolytic cells offers great advantages, particularly with regard to environmental friendliness. Nevertheless, there are still a number of technical and economic problems to be solved. The membrane is a complicated and very sensitive structure. They are therefore expensive to manufacture and must be handled with special care. Membrane cell technology therefore requires comparatively high investment costs; these must be offset by correspondingly low operating costs in order to achieve a satisfactory cost-effectiveness of the process. In view of the sharp rise in electricity prices, the lowest possible power consumption of the individual electrolysis cells must be sought. The focus here is on increasing the current density prevailing in the electrolysis cell and reducing the voltage coefficient (so-called k-value) which characterizes the cell design.

Membrane cell systems in particular have a pronounced dependency of the specific energy consumption on the current density. If one can be satisfied with the relatively inexpensive diaphragm cells, given their high k-value of 0.37-0.50 V xm ² / kA, with relatively low current densities in the range of 2-3 kA / m2, the more expensive membrane cells, on the other hand, require much higher ones Current densities. Current densities of 3 to 6 kA / m ² and even up to 10 kAlm ^{2 are} aimed for in order to enable economical operation.

• -die vertikale Anordnung der gasentwickelnden Elektroden zu beiden Seiten der Membran, die den spannungskostenden Gasblaseneffekt erhöht
• -die durch die Membran stark beeinträchtigte Stromverteilung zwischen den Elektroden, die praktisch einer Verminderung des Leiterquerschnitts des Elektrodenzwischenraumes gleichkommt
• -die Adhäsion der Wasserstoffgasbisen an der Membranoberfläche, die den Spannungsabfall erhöht
• -die Bildung einer an Salz verarmten Grenzschicht anodenseitig der Membran, die in stromüberlasteten Zonen mit unzureichender Na+ -lonenzufuhr aus dem Elektrolyten zu höheren Spannungsabfällen führt. Diese lokale Polarisation bewirkt überdies eine Verminderung der Stromausbeute
• -die partiellen Verengungen und Erweiterungen des Elektrodenspalts, die durch die Planheitsabweichungen der Anode, Kathode, Membran und der Dichtung bedingt sind, bewirken Unregelmäßigkeiten im Elektrolyseverlauf und Energieverluste.

However, the membrane as a separator between the electrodes makes it difficult to use high operating current densities. But it is not just the loss of energy in the membrane that weighs heavily. In addition to the voltage drop in the membrane, other factors also contribute to the high voltage coefficients k of high-performance membrane cells, which are four to seven times higher than those of mercury cells at 0.35 to 0.55 V xm ² / kA. Which includes:

• -The vertical arrangement of the gas-generating electrodes on both sides of the membrane, which increases the gas bubble effect, which costs voltage
• -The current distribution between the electrodes, which is severely impaired by the membrane, is practically equivalent to a reduction in the conductor cross section of the electrode gap
• -The adhesion of the hydrogen gas bites to the membrane surface, which increases the voltage drop
• the formation of a salt-depleted boundary layer on the anode side of the membrane, which leads to higher voltage drops in the current-overloaded zones with insufficient Na + ion supply from the electrolyte. This local polarization also causes a reduction in the current efficiency
• -The partial narrowing and widening of the electrode gap, which are caused by the flatness deviations of the anode, cathode, membrane and the seal, cause irregularities in the course of the electrolysis and energy losses.

The problems listed, which primarily stand in the way of increasing the current density, have hitherto prevented the environmentally friendly membrane cell technology from advancing rapidly. However, since the shape, dimensions and design of the membrane are largely fixed, the design of the electrodes plays an outstanding role in the further development of the membrane cell process.

The basic structure of a membrane electrolysis cell is described for example in EP-A 0 121 608. There two flat planar electrodes are used as anode or cathode, between which the membrane is firmly clamped. With this arrangement, however, it is difficult to ensure a constant distance from the electrodes over the entire membrane surface. In order to compensate for tolerances, the minimum distance between the membrane and the electrode, especially the anode, must not be undercut. In order to achieve high current densities, the smallest possible distance is desirable.

DE-A 3 223 701 attempts to ensure a secure plane parallelism of the electrode surfaces and an energetically favorable minimum electrode spacing in that one of the two electrodes can be displaced by spring elements. The arrangement proposed there requires too additional constructive elements; a decrease in their spring properties or even jamming of the moving parts can easily lead to failure of the electrolytic cell.

In the electrolysis cell according to DE-A 3 132 947, the membrane is pressed against one of the flat electrodes by means of a special support structure. Although the distance between the membrane and the electrode becomes zero, one side of the membrane is completely covered by the electrode lying on it. The membrane is only in contact with the electrolyte on one side; the supply of ions from the electrolyte is therefore difficult. Furthermore, the resulting gas bubbles can only escape to one side. The additional support structure makes the electrolysis cell considerably more expensive. In addition, special precautions must be taken so that the sensitive membrane is not damaged by the resilient elements of the support structure.

The bipolar electrolysis cell described in DE-C 2 545 339 also has a flat electrode on which the membrane bears without a gap. The resulting poor gas discharge should be improved by gaps or openings in the electrode. In particular, the escape of gas bubbles upwards is made considerably more difficult by such a flat electrode with an overlying membrane. In addition, large parts of the membrane are also excluded from the electrolyte supply.

Finally, EP-A 0 095 039 proposes an electrode with a grid-like structure. The membrane is clamped between the bars of the electrodes assigned in pairs. The result of this is that the thin membrane comes to lie in a wave shape between the electrodes, which leads to a completely inhomogeneous current density distribution. As a result of the membrane resting on both the anode and the cathode, relatively large parts of the membrane do not come into contact with the electrolyte here either. Although gas bubbles can escape on both sides of the membrane, the essentially horizontal arrangement of the lattice elements prevents free gas removal from the cells. The voltage coefficient of such membrane electrolysis cells is unsatisfactory.

In EP-A 0 079 445, an electrode which is in principle planar is also proposed. Special elevations or depressions bent out of the surface are intended to reduce the current requirement of this electrode. This electrode is electrocatalytically coated on its entire surface. An adjacent ion exchanger membrane would therefore be damaged on the contact surfaces as a result of current peaks occurring there if one wanted to achieve the high current densities mentioned at the beginning with such an electrode. Again, a large proportion of the surface of the membrane on the electrode side is covered, which leads to an undersupply of electrolyte. Since the very thin, flat membrane rests on curved surfaces, there are also high mechanical loads locally, which can damage the sensitive and expensive membrane. In addition, gas bubbles easily settle in the round indentations, which can seriously disrupt the transport of electricity to the electrode. This electrode is therefore not very suitable for the construction of a membrane electrolysis cell with a good voltage coefficient, which can be operated at high current densities.

US-A 4 013 537 relates to a membrane electrolysis cell. It is equipped with vertically running electrode bodies, each of which has strips running vertically on the membrane side.

Finally, FR-A 2 244 836 also discloses a relevant membrane electrolysis cell. According to one exemplary embodiment, it comprises U-shaped electrode lamellae, the legs of which are connected with webs which, on the one hand, serve for welding or soldering to a plate and, on the other hand, connect and reinforce the lamellae.

The large number of previously known, very differently designed electrodes for membrane electrolysis make it clear how difficult it is to find an optimal electrode design.

The object of the invention is therefore to provide an electrode which, while avoiding the disadvantages described, is suitable for the construction of a membrane electrolysis cell which can be operated safely at high current densities and which has a good voltage coefficient and which, moreover, can be produced simply and therefore inexpensively.

This object is achieved in an electrode for membrane electrolysis, with an electrode body, the surface of which is at least partially provided with an electrocatalytically active coating, in that the electrode body is formed from a number of parallel, spaced-apart lamellae that the lamellae have a plurality of recesses on their end faces facing the membrane, and that the end faces of the webs located between these recesses are not actively electrocatalytically coated.

The electrode designed according to the invention is excellently suitable for the application of an ion exchange membrane. The membrane lies flat on the end faces of the webs located between the recesses, so that the effective distance between the membrane and the electrode is zero. This allows the construction of a so-called "zero gap cell". Since the end faces of the webs, on which the membrane rests, are uncoated, no current peaks can occur there. An overload of the membrane caused by this is therefore largely excluded. The entire surface of the membrane is leaned against the electrode. In contrast to rigid membrane clamping, this allows the separator to work freely, for example if the electrolyte level in the cell is too low.

Another significant advantage over conventional electrodes is that the membrane is largely free in the cell space and only to a very small extent from the bars of the electrical system the body is covered. It is therefore excellently supplied with electrolyte from all sides, which ensures the necessary replenishment of ions. Local polarizations that could damage the membrane are avoided. The loss of electrocatalytically active electrode area through the uncoated end faces of the webs is small, so that high current densities can nevertheless be achieved with the electrode according to the invention.

The proposed lamellar structure of the electrode in conjunction with the large number of recesses on the end faces facing the membrane further enables gas bubbles to escape rapidly.

The proposed electrode geometry thus allows the construction of high-quality membrane electrolysis cells with the desired low voltage coefficient.

A vertical arrangement of the fins in the vertical cells promotes the flow of electrolyte through the cell from bottom to top. A vertical cell structure is also advantageous with regard to the gas bubble effect which counteracts the high current densities.

The lamellae forming the electrode body are expediently designed as rectangular, flat plates. Such plates are easy to manufacture; in addition, the recesses according to the invention can be easily made.

In a preferred embodiment of the invention, the recesses of two adjacent slats are arranged offset from one another. This allows a particularly uniform support of the membrane on top.

The recesses of all lamellae expediently have the same dimensions and are arranged regularly. In this way, a particularly uniform current density distribution is achieved.

A particularly uniform, mechanical and electrical stress on the adjacent membrane results when the recesses of two adjacent lamellae are offset from one another by half the width of a recess.

A flat design of the end faces of the webs allows the membrane to lie flat. This can then shift slightly with respect to the electrode, for example in the event of changes in length due to the absorption of liquid or due to temperature fluctuations. Slats with flat end faces can also be produced particularly easily and inexpensively. The passivation of the web surfaces can namely be accomplished by simply grinding the electrocatalytically active coating using a surface grinder.

Rectangular recesses are particularly easy to work into the slats. In addition, the bottoms of such recesses are parallel to the membrane and thus also to the current direction. This leads to the largest possible, effective, electrocatalytically active surface of the electrode according to the invention. However, other, for example round, shapes of the recesses are also conceivable.

To avoid current peaks, the edges between the bottoms and the side surfaces of the recesses and the edges between the recesses and the end surfaces of the webs can be rounded. The edges between the end faces of the webs and the side faces of the slats can also be rounded.

In a preferred embodiment of the electrode according to the invention, the width of the recesses corresponds approximately to the width of the webs. This dimensioning represents a good compromise between the demands for the best possible support of the membrane and a simultaneous, as unimpeded supply of electrolyte as possible.

Dimensioning is considered advantageous in which the depth of the recesses is less than their width and the distance between two adjacent lamellae approximately corresponds to the width of the recesses. The width of the recesses and the width of the webs are each a few millimeters.

Particularly high current densities were achieved with a width of the recesses and the webs of between 3 and 10 mm, in particular with a width of 5 mm.

A depth of the recesses of a few millimeters is sufficient for an adequate supply of the membrane with electrolyte. Particularly good results were achieved with recesses whose depth was between 2 and 4 mm.

A sufficient electrolyte flow between the lamellae results with a lamella spacing of a few millimeters; in a particularly preferred embodiment, this distance is between 4 and 6 mm.

In an expedient embodiment of the invention, the lamellae are electrically conductively connected to one another with a current distributor. A largely unimpeded flow of electrolyte results from the arrangement of a rectangular power distributor on the back of the slats.

Electrolysis cells with electrode bodies made of valve metal, preferably made of titanium, are distinguished by a particularly high current efficiency.

• Fig. 1 einen Ausschnitt einer erfindungsgemäßen Elektrode mit als rechteckige, ebene Platten ausgebildeten, senkrecht angeordneten Lamellen, mit versetzt angeordneten Ausnehmungen von rechteckigem Querschnitt, in einer vereinfachten perspektivischen Ansicht,
• Fig. 2 einen Ausschnitt aus einer Membran-Elektrolysezelle, mit der Elektrode nach Fig. 1 als Anode, einer anliegenden lonen-Austauschermembran sowie einer Lamellen-Kathode als Gegenelektrode, in einer schematischen perspektivischen Ansicht,
• Fig. 3 einen Ausschnitt aus einer Membran-Elektrolysezelle gemäß Fig. 2, mit einer Vollblech-Kathode als Gegenelektrode,
• Fig. 4 einen Ausschnitt aus einer Membran-Elektrolysezelle gemäß Fig. 2, mit einer Lochblech-Kathode als Gegenelektrode und
• Fig. 5 einen Ausschnitt aus einer Membran-Elektrolysezelle gemäß Fig. 2, mit einer Streckgitter-Kathode als Gegenelektrode.

An embodiment of the invention is explained below with reference to the accompanying drawings. It shows:

• 1 shows a section of an electrode according to the invention with vertically arranged fins designed as rectangular, flat plates, with offset recesses of rectangular cross section, in a simplified perspective view,
• 2 shows a section of a membrane electrolysis cell, with the electrode according to FIG. 1 as an anode, an adjacent ion exchange membrane and a lamellar cathode as counter electrode, in a schematic perspective view,
• 3 shows a section of a membrane electrolysis cell according to FIG. 2, with a solid sheet cathode as the counter electrode,
• Fig. 4 shows a section of a membrane electrolysis cell according to FIG. 2, with a perforated plate cathode as a counter electrode and
• Fig. 5 shows a section of a membrane elec 2, with an expanded cathode as the counter electrode.

The electrode shown in FIG. 1 has an electrode body 10 with a number of vertical, parallel, spaced-apart lamellae 20. These lamellae 20 are designed as rectangular, flat plates. On their end faces 21 they have a large number of identical recesses 30 of rectangular cross section. Between the recesses 30 there are webs 40 with flat end faces 41. The lamellae 20 forming the electrode body 10 are made of titanium. With the exception of the end faces 41, the lamellae 20 are provided with an electrocatalytically active coating. The edges 50 between the bottom surfaces 31 and the side surfaces 32, 33 of the recesses 30 are rounded. Likewise, the edges 60 between the recesses 30 and the end faces 41 and the edges 70 between the end faces 41 and the side faces 23, 24 of the slats 20 are rounded. The width 34 of the recesses 30 corresponds to the width 42 of the webs 40. The depth 35 of the recesses 30 is less than their width 34; it is approximately 3 mm. The recesses 30 of all the slats 20 are arranged regularly. The recesses 30 of two adjacent slats 20 are offset from one another by exactly half the width 34.

All slats 20 are at the same distance 80 from one another. The distance 80 is approximately 5 mm. On their rear sides 22, the lamellae 20 are electrically conductively connected to one another with a current distributor 90 of rectangular cross section.

FIG. 2 schematically shows the structure of a membrane electrolysis cell using the electrode according to the invention shown in FIG. 1. The fins 20 are vertical in the cell and form the anode. A membrane 91 bears against the end faces 41 of the webs 40. The counter electrode 92 is designed as a lamellar cathode. The distance between the membrane 91 and the counter electrode 92 is a few millimeters.

FIG. 3 shows a similar arrangement in which the electrode according to the invention is opposed by a solid sheet cathode as counter electrode 92.

In the membrane electrolysis cell shown in FIG. 4, a perforated plate cathode is used as counter electrode 92. This design is characterized by a particularly favorable current density distribution and a good supply to the membrane 91. On the anode side, the liquid electrolyte can reach the membrane 91 unhindered through the spaces between the lamellae 20 and their recesses 30. On the cathode side, the electrolyte is supplied through the holes in the counter electrode 92.

Finally, FIG. 5 shows a membrane electrolysis cell with an electrode according to the invention as an anode and a counter electrode 92 designed as an expanded metal cathode. The membrane 91 is largely exposed in space. Only about 10% of the membrane 91 is covered by the end faces 41 of the webs 40. Together with the open structure of the counterelectrode 92, this provides an excellent possibility of replenishing Na + ions. The vertical structure of the electrolysis cell as a result of the vertical arrangement of the fins 20 allows gas bubbles which arise to escape unimpeded upwards.

Claims (23)

1. An electrode for membrane electrolysis, having an electrode body (10),

which is formed from a plurality of parallel mutually spaced lamellas (20), and the surface of which is provided at least partially with an electro-catalytically active coating, characterised in that the lamellas (20) have on their edges which face the membrane in the assembled position a plurality of alternately arranged recesses (20) and bridge portions (40), and surfaces the edge (41) of the bridge portions (40) facing the membrane in the assembled position are not coated for electro-catalytic activity.

2. An electrode according to claim 1 characterised in that the lamellas (20) are disposed vertically.

3. An electrode according to claim 1 or 2 characterised in that the lamellas (20) are formed as rectangular flat plates.

4. An electrode according to any of claims 1 to 3 characterised in that the recesses (30) of two neighbouring lamellas (20) are mutually offset.

5. An electrode according to any of claims 1 to 4 characterised in that the recesses (30) of all lamellas (20) have the same dimensions and are regularly arranged.

6. An electrode according to claim 5 characterised in that the recesses (30) of two neighbouring lamellas (20) are mutually offset by half the width (34) of a recess (30).

7. An electrode according to any of claims 1 to 6 characterised in that the edge surfaces (41) of the bridge portions (40) are flat.

8. An electrode according to any of claims 1 to 7 characterised in that the recesses (30) have rectangular cross-section.

9. A electrode according to claim 8 characterised in that the edges (50) between the base surfaces (31) and side surfaces (32, 33) of the recesses (30) are rounded.

10. An electrode according to any of claims 1 to 9 characterised in that the edges (60) between the recesses (30) and edge surfaces (41) of the bridge portions (40) are rounded.

11. An electrode according to any of claims 1 to 10 characterised in that the edges (70) between edge surfaces (41) of the bridge portions (40) and side surfaces (23, 24) of the lamellas (20) are rounded.

12. An electrode according to any of claims 1 to 10 characterised in that the width (34) of the recesses (30) is approximately equal to the width (42) of the bridge portions (40).

13. An electrode according to any of claims 1 to 12 characterised in that the depth (35) of the recesses (30) is smaller than the width (34) of the recesses (30).

14. An electrode according to any of claims 1 to 13 characterised in that the spacing (80) between two neighbouring lamellas (26) is approximately equal to the width (34) of the recesses (30).

15. An electrode according to claim 12 characterised in that the width (34) of the recesses (30) and the width (42) of the bridge portions (40) are each in the range of from 3 to 10 millimetres.

16. An electrode according to claim 15 characterised in that the width (34) of the recesses (30) and the width (42) of the bridge portions (40) are each equal to about 5 millimetres.

17. An electrode according to claim 13 characterised in that the depth (35) of the recesses (30) lies in the range of from 2 to 4 millimetres.

18. An electrode according to claim 14 characterised in that the spacing (80) between the lamellas (20) is approximately in the range of from 4 to 6 millimetres.

19. An electrode according to any of claims 1 to 18 characterised in that the lamellas (20) are electrically conductively interconnected by means of a current distributor (90).

20. An electrode according to claim 19 characterised in that the current distributor (90) is arranged on the rear surfaces (22) of the lamellas which face away from the membrane in the assembled position.

21. An electrode according to claim 19 to 20 characterised in that the current distributor (90) has rectangular cross-section.

22. An electrode according to any of claims 1 to 21 characterised in that the electrode body (10) consists of a valve metal.

23. An electrode according to claim 22 characterised in that the electrode body (10) is made of titanium.

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