EP1472390A2

EP1472390A2 - Electrochemical half-cell

Info

Publication number: EP1472390A2
Application number: EP03734598A
Authority: EP
Inventors: Fritz Gestermann; Andreas Bulan; Richard Malchow; Hans-Dieter Pinter
Original assignee: Bayer MaterialScience AG
Current assignee: Covestro Deutschland AG
Priority date: 2002-01-31
Filing date: 2003-01-20
Publication date: 2004-11-03
Also published as: WO2003064728A2; PL370278A1; JP2005516120A; WO2003064728A3; BR0307249A; KR20040089130A; US20030173211A1; DE10203689A1; CN1625610A

Abstract

The invention relates to an electrochemical half-cell, in particular for the electrolysis of aqueous solutions of hydrogen chloride. Said cell comprises at least one gas chamber, which has a gas supply and a gas discharge, in addition to a liquid outlet and comprises a gas diffusion electrode, which lies on an electrically conductive current distributor and makes contact with the latter in a conductive manner. According to the invention, the current distributor has an exposed surface ranging between 5 and 65 %, preferably between 10 and 60 % and more specifically between 15 and 50 %, in relation to the total surface of said current distributor and a thickness of between 0.3 mm and 5 mm and preferably between 0.35 and 0.6 mm.

Description

Electrochemical half cell

The 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.

A method for the electrolysis of hydrochloric acid using gas diffusion electrodes is known, for example, from US Pat. No. 5,770,035. An anode compartment with a suitable anode, consisting for example of a substrate made of a titanium-palladium alloy, which 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 to escape from the anode compartment and is an appropriate treatment conces- ^• leads. The anode compartment is separated from a cathode compartment by a commercially available cation exchange membrane. On the cathode side, there is a gas diffusion electrode on the cation exchange membrane. The gas diffusion electrode in turn rests on a power distributor. Gas diffusion electrodes are, for example, oxygen consumable cathodes (SVK). In the case of an SVK as a gas diffusion electrode, air, oxygen-enriched air or pure oxygen is usually introduced into the cathode compartment, which is converted at the SVK.

The known electrolysis of hydrochloric acid has the disadvantage that hydrogen evolution is observed on the cathode side at current densities that are greater than 4000 A / m ² . The hydrogen formed mixes with the excess supply of gas to the cathode half-cell, ie with the air, with the air enriched with oxygen or with the oxygen. Another disadvantage is that the high current densities also result in very high voltages. However, high current densities and low voltages are necessary for economic reasons when carrying out the process technically. A process for the electrolysis of hydrochloric acid by means of gas diffusion electrodes is also known from EP-A-785 294. A two-layer power distributor is described therein, the first layer of which consists of a mesh or an expanded metal with a large mesh size and a thickness which brings about sufficient mechanical stability. The second layer also consists of a mesh 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 lying thereon.

The object of the present invention is to operate the hydrochloric acid electrolysis with the highest possible current densities and the lowest possible voltages and to completely avoid the undesired evolution of hydrogen. Since the excess oxygen used is usually returned to the cathode half-cell, no hydrogen may be formed, as this would otherwise accumulate in the system.

The object is achieved by the features of claim 1.

The invention relates to 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 supply and a gas discharge and a liquid outlet, and a gas diffusion electrode which rests on an electrically conductive current distributor and the current distributor electrically contacted in a conductive manner, the current distributor preferably 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 0.3 mm to 5 mm from 0.35 to 2 mm.

The power distributor has several functions to perform. It is intended to make electrical contact with the gas diffusion electrode. At the same time, it must be ensured that the power distributor transports the gas in the gas space to the gas diffusion electrode and transports the reaction formed during the operation of the electrolysis. water and hydrochloric acid, which passes through the ion exchange membrane from the anode half-element into the cathode half-element.

In order that the current can be transported as evenly as possible over the surface of the gas diffusion electrode, uniform contacting of the gas diffusion electrode with the current distributor is necessary. The gas diffusion electrode therefore lies on the entire surface of the power distributor. The current distributor and the gas diffusion electrode form two planar layers lying one against the other. Furthermore, the current distributor must be connected to the cathode half element with the lowest possible contact resistance.

According to a preferred embodiment, the side of the gas diffusion electrode that rests on the current distributor (also referred to below as the rear side) is electrically conductive. The electrically conductive contact between the gas diffusion electrode and the current distributor can thus be achieved in that the gas diffusion electrode lies loosely on the current distributor. Due to the higher pressure in the anode half cell compared to 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 may also be attached to the power distributor. The attachment can be detachable, for example by means of a clamp connection, or fixed, for example by means of an adhesive connection or by sewing on. Alternatively, the gas diffusion electrode can also be electrically conductively connected to the current distributor. This is particularly necessary when the gas diffusion electrode does not have an electrically conductive back, but on its back with an additional, electrically non-conductive

Layer is provided.

If the current distributor is projected onto the gas diffusion electrode, a distinction must be made between areas which cover the gas diffusion electrode and those which do not. The sum of the areas not covering is subsequently referred to as

Open space of the power distributor. The sum of the areas of the electricity distribution Those who make contact with the gas diffusion electrode are also referred to below as the contact surface. If, for example, a perforated plate is used as a current distributor, the covering area coincides with the contact area. If the power distributor is an expanded metal, mesh, fabric or the like, then not the entire covering surface will contact the gas diffusion electrode, but only a smaller part, since the webs of the expanded metal or the like. do not lie on one level. If the expanded metal, mesh, fabric or the like. rolled flat, the contact area increases. In addition, the covering area of the power distributor increases.

The total area of the power distributor is understood here to mean the area resulting from the

Length and width of the power distributor is formed.

The contact surface can e.g. can be measured as follows: The power distributor is pressed like a stamp into a stamp pad and then pressed onto a sheet of paper, which rests on a gas diffusion electrode. The

Areas visible where the power distributor contacts the gas diffusion electrode. The contact area can be measured in this way. The covering area or the open area can be calculated from this.

In the special case of an expanded metal, the thickness of the current distributor is

Bridge thickness meant. The following parameters are used to identify the expanded metals: The web thickness corresponds to the thickness of the metal sheet used to produce the expanded metal. The web width results from the distance between two parallel but offset cuts. The mesh size characterizes the length of the cut, the mesh width of the maximum distance between two adjacent webs created by stretching deformation.

The current distributor preferably consists of at least one expanded metal, mesh, fabric, foam, fleece, slotted plate or perforated plate. It consists of an electrically conductive material, in particular metal. The current distributor preferably consists of titanium or a noble metal-stabilized titanium, for example one precious metal-doped titanium or a precious metal-titanium alloy. The power distributor is coated with a noble metal oxide. The noble metal stabilization of the titanium or the noble metal oxide coating takes place, for example, with an element of the platinum metal group, ie Ru, Rh, Pd, Os, Ir, Pt.

The current distributor is preferably an expanded metal with 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.

According to a preferred embodiment, the current distributor, if it is an expanded metal, is flat-rolled. The current distributor is particularly preferably completely flat-rolled. This creates a maximum contact area of the gas diffusion electrode on the power distributor. If the power distributor is rolled flat, the free area of the power distributor refers to the free area after rolling.

In order to achieve a higher mechanical stability, 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 is connected in an electrically conductive manner to the base support, the base support being made of at least one expanded metal,

Net, fabric, foam, fleece, slotted sheet or a perforated plate. Similar to the power distributor, the base support consists of titanium or a precious metal-stabilized titanium, the precious metal e.g. can be an element of the platinum metal group.

The base support is connected in particular to the power distributor with a low impedance. If the power distributor is connected to a base support, the base support is electrically conductively connected to the cathode half element in order to produce the power supply. Alternatively, the current distributor can also be connected in an electrically conductive manner to the cathode half element. The base support is connected to the electrode half-element in particular with a low resistance, ie with a slight excess contact resistance. A low-resistance connection is understood to mean, for example, a welded, sintered or soldered connection. It is essential for the base support as well as for the power distributor that they do not hinder the liquid transport through the gas diffusion electrode and the gas transport to the gas diffusion electrode.

The current distributor can be connected directly to the cathode half element with a low resistance. Likewise, if present, the base support can be directly connected to the cathode half element with a low resistance.

The median resistance connection of the power distributor or the base support to the cathode half element can be made, for example, with the aid of support elements. The support elements can e.g. Trapezoidal or Z profiles. The connection of the power distributor or the base support to the cathode half-cell must ensure full contact of the gas diffusion electrode with the power distributor.

Adequate stability can be achieved, for example, by the base support or by a sufficient number of support elements.

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

A network with a thickness of 1 to 4 mm and a mesh size of 7 to 25 mm is also preferably used as the base support.

Another preferred embodiment of the base support is a perforated plate or slotted plate with a free area of at most 70% and a thickness of 1 to 4 mm. Examples:

The examples were carried out in the electrolysis cell described below and shown schematically in FIG. 1 under the test conditions mentioned below:

The electrolytic cell has an anode half element 1 consisting of an electrolyte compartment 12 and an anode 3, for example a titanium electrode coated with noble metal oxide. The electrode area of the anode and cathode was 0.86 m ^{2 in} each case. The anode half element 1 is from the cathode half element 2 by a commercially available one

Cation exchange membrane 4, for example Nafion® type 324, separated. 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. The cation exchange membrane 4 usually lies on the gas diffusion electrode 5. If specified in the following examples, the current distributor 6 rests on a base support 14 and is connected to it in an electrically conductive manner. The gas diffusion electrode 5 requires good contact to the current distributor 6 and to the ion exchange membrane 4. This contact can e.g. can be produced in such a way that the pressure in the anode half element 1 is higher than the pressure in the cathode half element 2. In normal operation, the higher pressure in the anode half element presses the cation exchange membrane onto the gas diffusion cathode and this in turn onto the current distributor. This can e.g. by a liquid immersion 10 through which the chlorine gas formed during operation of the electrolytic cell is passed. The pressure difference between anode half cell and cathode half cell was 400 mbar, the pressure in the anode half element being higher.

During operation of the electrolysis cell, the hydrochloric acid was pumped through the anode half-element at a volume flow of approx. 450 l / h via a feed 7 and a discharge 15. The concentration of the pumped hydrochloric acid was 12-13

Wt .-%. At a current density of 5000 A / m ² , approx. 23 liters became 30% by weight Hydrochloric acid added. The used hydrochloric acid was replaced. The chlorine formed on the anode is also removed from the anode half element 1 via the discharge 15 and separated from the hydrochloric acid via the immersion 10. The chlorine is fed into a suitable treatment. Oxygen with a volume flow of 1750 l / h was introduced into the gas space 13 of the cathode half-element via a feed 8. The purity of the oxygen was 99.9%. Excess oxygen is removed from the cathode half element via the discharge 11. The water formed in the reduction of oxygen at the gas diffusion electrode is discharged from the gas space 13 via an outlet 9.

Example 1 (NergleichsbeispieD

In the electrolysis cell described above, an expanded metal with 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 open space was 68%. On this

Power distributor was on one side of the gas diffusion electrode. On the other side of the power distributor there was another coarser expanded metal with low resistance, which served as the base support. The medohohm connection of the power distributor to the base support was carried out by welding. The base support is also attached to the cathode half element with a low resistance. The base carrier 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 space of the base beam was 24%.

The voltage during operation of the electrolysis was 2.02 V at a current density of 5 kA / m ² . The concentration of hydrogen in oxygen which was removed from the cathode half-element was 2000 ppm. This was due to the comparatively high voltage. Example 2

In the electrolysis cell described above, an expanded metal with 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 open space was 68%. The expanded metal was rolled flat. The free space after rolling was 53%. The gas diffusion electrode was on one side of this current distributor. On the other side of the power distributor there was another coarser expanded metal with low resistance, which served as the base support. The medohohm connection of the power distributor to the base support was carried out by welding. The base support is also attached to the cathode half element with a low resistance. The base carrier 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 space of the base beam was 24%.

The voltage during the operation of the electrolysis was 1.57 V at a current density of

5 kA / m ² . The concentration of hydrogen in oxygen which was removed from the cathode half-element was less than 1 ppm.

Example 3

In the electrolysis cell described above, an expanded metal with 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 was rolled flat. The free space after rolling was 24%. The gas diffusion electrode was on one side of this current distributor. On the other side of the power distributor there was another coarser expanded metal with low resistance, which served as the base support. The medohohm connection of the power distributor to the base support was carried out by welding. The base support is also attached to the cathode half element with a low resistance. The base carrier 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 space of the base beam was 24%. The voltage during operation of the electrolysis was 1.44 V at a current density of 5 kA / m ² . The concentration of hydrogen in oxygen which was removed from the cathode half-element was less than 1 ppm.

Example 4

In the electrolysis cell described above, an expanded metal with 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 was rolled flat. The free area after rolling was 35%. The gas diffusion electrode was on one side of this current distributor. The power distributor was attached to the cathode half element in a low-resistance manner without a base support.

The voltage during operation of the electrolysis was 1.55 V at a current density of

Table 1: Overview of the results

Claims

claims

1. Electrochemical half-cell, in particular for the electrolysis of aqueous solutions of hydrogen chloride, comprising at least one gas space (13), the gas space (13) having a gas supply (8) and a gas discharge (11) and a liquid outlet (9), and a gas diffusion electrode (5), which rests on an electrically conductive current distributor (6) and contacts the current distributor (6) in an electrically conductive manner, characterized in that the current distributor (6) has an open 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 power distributor (6) and a thickness of 0.3 mm to 5 mm, preferably from 0.35 to 2 mm.

2. Electrochemical half-cell according to claim 1, characterized in that the current distributor (6) consists of at least one expanded metal, mesh, tissue,

Foam, fleece, slotted sheet or a perforated plate.

3. Electrochemical half-cell according to one of claims 1 or 2, characterized in that the current distributor (6) is an expanded metal with 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.

4. Electrochemical half-cell according to one of claims 1 to 3, characterized in that the current distributor (6) on an electrically conductive

Base support (14) rests and is connected to the base support (14) in an electrically conductive manner, the base support (14) consisting of at least one expanded metal, mesh, fabric, foam, fleece, slotted plate or perforated plate.

5. Electrochemical half cell according to claim 4, characterized in that

Base carrier (14) an expanded metal with a mesh length of 10 to 40 mm, a mesh width of 5 to 15 mm, a web width of 2 to 5 mm and a web thickness of 0.8 to 4 mm.

6. Electrochemical half cell according to claim 4, characterized in that the base support (14) is a network with a thickness of 1 to 4 mm and one

Mesh size from 7 to 25 mm.

7. Electrochemical half cell according to claim 4, characterized in that the base support (14) is a perforated plate or slotted plate with a free area of at most 70% and a thickness of 1 to 4 mm.

8. Electrochemical half cell according to one of claims 1 to 7, characterized in that the current distributor (6) consists of titanium or a noble metal-stabilized titanium and a noble metal oxide coating.

Electrochemical half-cell according to one of Claims 1 to 8, characterized in that the current distributor (6) is an expanded metal which is rolled flat.