Electrochemical half-cell

Abstract

The invention describes an electrochemical half-cell, in particular for the electrolysis of aqueous solutions of hydrogen chloride, at least comprising a gas space, the gas space having a gas feed and a gas discharge as well as a liquid outlet, and a gas diffusion electrode which rests on an electrically conductive current distributor and makes an electrically conductive contact with the current distributor, the current distributor having a free area in the range from 5 to 65%, preferably from 10 to 60%, particularly preferably from 15 to 50%, based on the total area of the current distributor, and a thickness of from 0.3 mm to 5 mm, preferably from 0.35 to 0.6 mm.

Description

CLAIM FOR PRIORITY

[0001] The present application claims priority under 35 U.S.C. § 119 from German Patent Application No. 102 03 689.6 filed Jan. 31, 2002, the content of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an electrochemical half-cell, in particular for the electrolysis of an aqueous solution of hydrogen chloride (hydrochloric acid) by means of gas diffusion electrodes.

[0004] 2. Description of Related Art

[0005] A method for the electrolysis of hydrochloric acid by means of gas diffusion electrodes is disclosed, for example, in U.S. Pat. No. 5,770,035. An anode space having a suitable anode, consisting of, for example, a substrate which comprises a titanium-palladium alloy and is coated with a mixed oxide of ruthenium, iridium and titanium, is filled with the aqueous solution of hydrogen chloride. The chlorine formed at the anode escapes from the anode space and is fed to a suitable treatment. The anode space is separated from a cathode space by a commercial cation exchange membrane. On the cathode side, a gas diffusion electrode rests on the cationic exchange membrane. The gas diffusion electrode also rests on a current distributor. Gas diffusion electrodes are, for example, oxygen-consuming cathodes (OCC). In the case of an OCC as a gas diffusion electrode, air, air enriched with oxygen or pure oxygen is usually passed into the cathode space and is reacted at the OCC.

[0006] The hydrochloric acid electrolysis known per se has the disadvantage that, at current densities which are greater than 4 000 A/m2, hydrogen evolution is observed on the cathode side. The hydrogen formed mixes with the gas fed in excess to the cathode half-cell, i.e. with the air, with the air enriched with oxygen or with the oxygen. A further disadvantage is that, at high current densities, very high voltages also occur. However, high current densities and low voltages are necessary for economic reasons when carrying out the method industrially.

[0007] EP-A-785 294 likewise discloses a method for the electrolysis of hydrochloric acid by means of gas diffusion electrodes. A two-layer current distributor whose first layer consists of a net or an expanded metal having a large mesh size and a thickness which results in sufficient mechanical stability is described therein. The second layer likewise consists of a net or an expanded metal but has a smaller mesh size than the first layer and thus offers a large number of contact points with the gas diffusion electrode resting thereon.

[0008] It is an object of the present invention to operate the hydrochloric acid electrolysis at as high current densities as possible and as low voltages as possible and completely to avoid the undesired hydrogen evolution. Since, as a rule, the oxygen used in excess is recycled into the cathode half-cell, hydrogen must not be formed since otherwise this would accumulate in the system.

[0009] In accordance with these and other objects, the present invention is directed to an electrochemical half-cell, in particular for electrolysis of aqueous solutions of hydrogen chloride, at least comprising a gas space, the gas space having a gas feed and a gas discharge as well as a liquid outlet, and a gas diffusion electrode which rests on an electrically conductive current distributor and makes electrically conductive contact with the current distributor, the current distributor having a free area in the range from 5 to 65%, preferably from 10 to 60%, particularly preferably from 15 to 50%, based on the total area of the current distributor, and a thickness of from 0.3 mm to 5 mm, preferably from 0.35 to 2 mm.

[0010] Additional objects, features and advantages of the invention will be set forth in the description which follows, and in part, will be obvious from the description, or may be learned by practice of the invention. The objects, features and advantages of the invention may be realized and obtained by means of the instrumentalities and combination particularly pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] FIG. 1 is a schematic of an electrolysis cell according to an embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0012] A current distributor of the present invention generally is capable of performing several functions. It should typically produce the electrical contact with a gas diffusion electrode. At the same time, it should generally ensure that the current distributor does not hinder the transport of gas in the gas space to the gas diffusion electrode and the transport of water of reaction formed during operation of the electrolysis and of hydrochloric acid, which passes through an ion exchange membrane from the anode half-element into the cathode half-element.

[0013] So that the current transport can take place as uniformly as possible over the surface of the gas diffusion electrode, uniform contact between the gas diffusion electrode and the current distributor is highly desirable. The gas diffusion electrode therefore preferably rests on the current distributor over substantially its whole surface. The current distributor and the gas diffusion electrode generally are capable of forming two planar layers lying one on top of the other. Furthermore, the current distributor should generally be connected to the cathode half-element with as low a contact resistance as possible.

[0014] According to a preferred embodiment, at least one side of the gas diffusion electrode, generally the side which rests on the current distributor (also referred to below as the “back”) is electrically conductive. Thus, the electrically conductive contact between the gas diffusion electrode and the current conductor can be achieved by virtue of the fact that the gas diffusion electrode rests loosely on the current distributor. Owing to the higher pressure in the anode half-cell in comparison with the cathode half-cell, the ion exchange membrane is pressed onto the gas diffusion electrode, which in turn is pressed onto the current distributor. The gas diffusion electrode is optionally additionally fastened to the current distributor. The fastening may be effected if desired, in a detachable manner, for example, by means of a clamp connection, or in a fixed manner, for example by means of an adhesive bond or by sewing on, or in any suitable manner. Alternatively, if desired for any reason, the gas diffusion electrode may also be connected to the current distributor in an electrically conductive manner. This is desirable in particular when the gas diffusion electrode does not have an electrically conductive back, per se, but is instead provided on its back with an additional, electrically nonconductive layer.

[0015] If the current distributor is projected onto the gas diffusion electrode, it is generally possible to distinguish regions which cover the gas diffusion electrode and regions which do not cover it. The total uncovered area is referred to below as a “free area” of the current distributor. The total area of the current distributor which makes contact with the gas diffusion electrode is referred to as “contact area.” If, for example, a perforated metal sheet is used as the current distributor, the covered area corresponds almost exactly to the contact area. If the current distributor is an expanded metal, net, woven fabric or the like, the total covered area may not make contact with the gas diffusion electrode but only a relatively small part thereof, since the webs of the expanded metal or the like do not lie in a plane. If the expanded metal, net, woven fabric or the like is rolled flat, the contact area increases. Moreover, the covered area of the current distributor would also increase accordingly.

[0016] The total area of the current distributor is understood here as meaning the total surface area which is calculated by measuring the length and width of the current distributor.

[0017] The contact area can be measured, for example, as follows: the current distributor is pressed into an inkpad like a stamp, and is then pressed onto a sheet of paper which rests on a gas diffusion electrode. This visualizes the regions in which the current distributor makes contact with the gas diffusion electrode. The contact area can be measured in this manner. The covered area or the free area can then be calculated based on the contact area.

[0018] In the special case when the current distributor comprises an expanded metal, the thickness of the current distributor means the web thickness. The following parameters are used for characterizing the expanded metals: The web thickness corresponds to the thickness of the metal sheet used for producing the expanded metal. The web width results from the distance between two cuts which are parallel to one another but offset. The mesh size characterizes the length of the cut and the mesh width the maximum distance between two adjacent webs which is formed by stretching deformation.

[0019] The current distributor preferably comprises at least one of expanded metal, net, woven fabric, foam, nonwoven, slotted metal sheet and/or perforated plate. The current distributor preferably comprises an electrically conductive material, in particular of metal. The current distributor preferably includes titanium or a noble metal-stabilized titanium, for example a noble metal-doped titanium or a noble metal-titanium alloy. The current distributor can be coated in some embodiments with a noble metal oxide. The noble metal stabilization of the titanium or the noble metal oxide coating can be produced, for example, using a metal of the platinum metal group, i.e. Ru, Rh, Pd, Os, Ir, Pt.

[0020] The current distributor is preferably an expanded metal having a mesh length in the range from 4 to 8 mm, a mesh width in the range from 3 to 5 mm, a web width in the range from 0.4 to 1.8 mm and a web thickness in the range from 0.4 to 2 mm.

[0021] According to a preferred embodiment, the current distributor, if it comprises an expanded metal, is rolled flat. The current distributor is particularly preferably completely rolled flat. This produces a maximum contact area of the gas diffusion electrode on the current distributor. If the current distributor is rolled flat, the free area of the current distributor relates to the free area after rolling.

[0022] According to a preferred embodiment of the electrochemical half-cell according to the invention, the current distributor rests on an electrically conductive base support and has an electrically conductive connection to the base support, and, in order to achieve higher mechanical stability, the base support preferably comprises at least one of an expanded metal, net, woven fabric, foam, nonwoven, slotted metal sheet and/or perforated plate. Similarly to the current distributor, the base support preferably comprises titanium or a noble metal-stabilized titanium, it being possible for the noble metal to be, for example, an element of the platinum metal group.

[0023] The base support can be, in particular embodiments, connected with low resistance to the current distributor. If the current distributor is connected to a base support, the base support advantageously can have an electrically conductive connection to the cathode half-element in order to establish the current supply. Alternatively, however, the current distributor may also have an electrically conductive connection to the cathode half-element. The connection of the base support to the electrode half-element has in particular low resistance, i.e. a low contact resistance. A low-resistance connection is understood as meaning, for example, a weld joint, sinter connection or solder connection. What is important for the base support as well as for the current distributor is that they generally do not substantially hinder transport of liquid through the gas diffusion electrode and the transport of gas to the gas diffusion electrode.

[0024] A current distributor of the present invention can be connected directly to the cathode half-element with low resistance if desired. If present, the base support may also be connected directly to the cathode half-element with low resistance.

[0025] A low-resistance connection of the current distributor or of the base support to the cathode half-element can be effected, for example, with the aid of support elements. The support elements may be, for example, trapezoidal profiles or Z-profiles as well known in the art. The connection of the current distributor or of the base support to the cathode half-cell should generally ensure contact of the gas diffusion electrode over substantially the entire area thereof with the current distributor. Sufficient stability can be achieved, for example, by the base support or by a sufficient number of support elements.

[0026] The base support is preferably an expanded metal having a mesh length of from 10 to 40 mm, a mesh width of from 5 to 15 mm, a web width of from 2 to 5 mm and a web thickness of from 0.8 to 4 mm.

[0027] Moreover, a net having a thickness of from 1 to 4 mm and a mesh size of from 7 to 25 mm is preferably used as the base support.

[0028] A further preferred embodiment of the base support is a perforated metal sheet or slotted metal sheet having a free area of not more than 70% and a thickness from 1 to 4 mm.

EXAMPLES

[0029] The examples were carried out in the electrolysis cell described below and shown schematically in FIG. 1, under the experimental conditions mentioned below:

[0030] The electrolysis cell has an anode half-element 1 consisting of an electrolyte space 12 and an anode 3, for example a noble metal oxide-coated titanium electrode. The electrode area of the anode and cathode is in each case 0.86 m2. The anode half-element 1 is separated from the cathode half-element 2 by a commercial cation exchange membrane 4, for example Nafion®0 type 324. The cathode half-element 2 consists of a gas space 13 and a cathode which is formed from a current distributor 6 and a gas diffusion electrode 5. Usually, the cation exchange membrane 4 rests on the gas diffusion electrode 5. Where specified in the following examples, the current distributor 6 rests on a base support 14 and has an electrically conductive connection to it. The gas diffusion electrode 5 requires good contact with the current distributor 6 and with the ion exchange membrane 4. This contact can be produced, for example, by ensuring that the pressure in the anode half-element 1 is higher than the pressure in the cathode half-element 2. In normal operation, the cation exchange membrane is pressed onto the gas diffusion cathode by higher pressure in the anode half-element and said gas diffusion cathode in turn is pressed onto the current distributor. This can be effected, for example, by a liquid immersion seal 10 through which the chlorine gas form during operation of the electrolysis cell is passed. The pressure difference between anode half-cell and cathode half-cell was 400 mbar, the pressure in the anode half-element having been higher.

[0031] During operation of the electrolysis cell, the hydrochloric acid was pumped through the anode half-element at a volume flow rate of about 450 l/h via a feed 7 and a discharge 15. The concentration of the hydrochloric acid circulated by pumping was 12-13% by weight. At a current density of 5 000 A/m2, about 23 litres of 30% strength by weight hydrochloric acid were added. The hydrochloric acid consumed was replaced. The chlorine formed at the anode was likewise removed from the anode half-element 1 via the discharge 15 and separated from the hydrochloric acid via the immersion seal 10. The chlorine is fed to a suitable treatment. Oxygen was passed into the gas space 13 of the cathode half-element via a feed 8 at a volume flow rate of 1 750 l/h. The purity of the oxygen was 99.9%. Excess oxygen was removed from the cathode half-element via the discharge 11. The water formed during the reduction of the oxygen at the gas diffusion electrode is removed from the gas space 13 via a discharge 9.

Example 1 (Comparative Example)

[0032] In the electrolysis cell described above, an expanded metal having a mesh length of 4.2 mm, a mesh width of 3.1 mm, a web width of 0.5 mm and a web thickness of 0.4 mm was used as the current distributor. The free area was 68%. The gas diffusion electrode rested on one side of this current distributor. A further coarser expanded metal which served as a base support had been mounted with a low resistance on the other side of the current distributor. The low-resistance connection of the current distributor to the base support was effected by welding. The base support is moreover mounted with a low resistance on the cathode half-element. The base support had the following dimensions: mesh length 13.2 mm, mesh width 6.3 mm, web width 2.4 mm and web thickness 1.5 mm. The free area of the base support was 24%.

[0033] The voltage during operation of the electrolysis was 2.02 V at a current density of 5 kA/m2. The concentration of hydrogen in the oxygen which was removed from the cathode half-element was 2 000 ppm. This was due to the comparatively high voltage.

Example 2

[0034] In the electrolysis cell described above, an expanded metal having a mesh length of 6 mm, a mesh width of 3.3 mm, a web width of 0.5 mm and a web thickness of 0.5 mm was used as the current distributor. The free area was 68%. The expanded metal had been rolled flat. The free area after rolling was 53%. The gas diffusion electrode rested on one side of this current distributor. A further coarser expanded metal which served as a base support had been mounted with a low resistance on the other side of the current distributor. The low-resistance connection of the current distributor to the base support was effected by welding. The base support is moreover mounted with low resistance on the cathode half-element. The base support had the following dimensions: mesh length 13.2 mm, mesh width 6.3 mm, web width 2.4 mm and web thickness 1.5 mm. The free area of the base support was 24%.

[0035] The voltage during operation of the electrolysis was 1.57 V at a current density of 5 kA/m2. The concentration of hydrogen in the oxygen which was removed from the cathode half-element was less than 1 ppm.

Example 3

[0036] In the electrolysis cell described above, an expanded metal having a mesh length of 6 mm, a mesh width of 3.4 mm, a web width of 1.3 mm and a web thickness of 1 mm was used as the current distributor. The expanded metal had been rolled flat. The free area after rolling was 24%. The gas diffusion electrode rested on one side of this current distributor. A further coarser expanded metal which served as a base support had been mounted with a low resistance on the other side of the current distributor. The low-resistance connection of the current distributor to the base support was effected by welding. The base support is moreover mounted with a low resistance on the cathode half-element. The base support had the following dimensions: mesh length 13.2 mm, mesh width 6.3 mm, web width 2.4 mm and web thickness 1.5 mm. The free area of the base support was 24%.

[0037] The voltage during operation of the electrolysis was 1.44 V at a current density of 5 kA/m2. The concentration of hydrogen in the oxygen which was removed from the cathode half-element was less than 1 ppm.

Example 4

[0038] In the electrolysis cell described above, an expanded metal having a mesh length of 6.2 mm, a mesh width of 3.4 mm, a web width of 1.1 mm and a web thickness of 1 mm was used as the current distributor. The expanded metal had been rolled flat. The free area after rolling was 35%. The gas diffusion electrode rested on one side of this current distributor. The current distributor was mounted with a low resistance on the cathode half-element without a base support by welding.

[0039] The voltage during operation of the electrolysis was 1.55 V at a current density of 5 kA/m2. The concentration of hydrogen in the oxygen which was removed from the cathode half-element was less than 1 ppm. 1 TABLE 1 Overview of the results Free area of the Operating current distributor in voltage in % according to Thickness V at 5 Example manufacturer's data in mm kA/m2 1 (Current distributor 68 0.4 2.02 with base support- comparative example) 2 (Current distributor 53 0.5 1.57 with base support) 3 (Current distributor 24 1.0 1.44 with base support) 4 (Current distributor 35 1.0 1.55 without base support)

[0040] Additional advantages, features and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details, and representative devices, shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

[0041] All documents referred to herein are specifically incorporated herein by reference in their entireties.

[0042] As used herein and in the following claims, articles such as “the”, “a” and “an” can connote the singular or plural.

Claims

1. An electrochemical half-cell, suitable for the electrolysis of aqueous solutions of hydrogen chloride, said electrochemical half-cell comprising:

at least one gas space, the gas space having a gas feed and a gas discharge as well as a liquid outlet, and

a gas diffusion electrode which rests on an electrically conductive current distributor and makes electrically conductive contact with the current distributor, wherein the current distributor has a free area in the range from 5 to 65%, based on the total area of the current distributor, and a thickness of from 0.3 mm to 5 mm.

2. An electrochemical half-cell according to claim 1 wherein the current distributor has a free area from 10 to 60% based on the total area of the current distributor.

3. An electrochemical half-cell according to claim 1, wherein the current distributor has a free area from 15 to 50%, based on the total area of the current distributor.

4. An electrochemical half-cell according to claim 1, wherein the current distributor has a thickness from 0.35 to 2 mm.

5. An electrochemical half-cell according to claim 1, wherein the current distributor at least comprises an expanded metal, net, woven fabric, foam, nonwoven, slotted metal sheet or perforated plate.

6. An electrochemical half-cell according to claim 1, wherein the current distributor comprises an expanded metal having a mesh length in the range from 4 to 8 mm, a mesh width in the range from 3 to 5 mm, a web width in the range from 0.4 to 1.8 mm and a web thickness in the range from 0.35 to 2 mm.

7. An electrochemical half-cell according to claim 1, wherein the current distributor rests on an electrically conductive base support and has an electrically conductive connection to the base support, the base support at least comprising an expanded metal, net, woven fabric, foam, nonwoven, slotted metal sheet or perforated plate.

8. An electrochemical half-cell according to claim 7, wherein the base support comprises an expanded metal having a mesh length of from 10 to 40 mm, a mesh width of from 5 to 15 mm, a web width of from 2 to 5 mm and a web thickness of from 0.8 to 4 mm.

9. An electrochemical half-cell according to claim 7, wherein the base support comprises a net having a thickness of from 1 to 4 mm and a mesh width of from 7 to 25 mm.

10. An electrochemical half-cell according to claim 7, wherein the base support comprises a perforated metal sheet or slotted metal sheet having a free area of not more than 70% and a thickness of from 1 to 4 mm.

11. An electrochemical half-cell according to claim 1, wherein the current distributor comprises at least one of (i) titanium or (ii) a noble metal-stabilized titanium, and said current distributor is further optionally provided with a noble metal oxide coating.

12. An electrochemical half-cell according to claim 1, wherein the current distributor comprises expanded metal which has been rolled flat.