WO2003052169A1

WO2003052169A1 - Method and apparatus for carrying out electrochemical reactions

Info

Publication number: WO2003052169A1
Application number: PCT/NL2002/000779
Authority: WO
Inventors: Machiel Saakes; Willem Rintcius Ter Veen
Original assignee: Nederlandse Organisatie Voor Toegepast- Natuurwetenschappelijk Onderzoek Tno
Priority date: 2001-11-30
Filing date: 2002-12-02
Publication date: 2003-06-26
Also published as: AU2002347663A1; NL1019454C2

Abstract

The invention relates to a method and an apparatus for carrying out electrochemical reactions, in particular electrochemical reactions with dissolved gaseous components, such as the electrochemical production of hydrogen peroxide. According to the invention, an electrochemical reaction is carried out by bringing an electrolyte, which comprises a liquid with a at least one gaseous component dissolved therein, into electrically conductive contact with an electrode, wherein said gaseous component is supplied or, conversely, discharged by using one or more hollow fibers, which hollow fibers have an at least partly microporous wall.

Description

Title: Method and apparatus for carrying out electrochemical reactions.

The invention relates to a method and an apparatus for carrying out electrochemical reactions, in particular electrochemical reactions with dissolved gaseous components, such as the electrochemical production of hydrogen peroxide from dissolved oxygen gas. In the state of the art, in the electrochemical synthesis of compounds starting from one or more gaseous components, use is made of conductive liquids in which the gaseous component has been dissolved. In the present application, such a liquid with one or more gaseous components dissolved therein is referred to as an electrolyte. The electrochemical reaction is carried out by bringing the electrolyte into contact with an electrode. If, for instance, oxidation of the gaseous component is desired, the electrolyte is brought into contact with an anode. For reduction of the gaseous component, the electrolyte is brought into contact with the cathode.

The surface of the electrolyte used can be co-determinative of the efficiency of the electrochemical cell in which the reaction is carried out. In order to enhance the efficiency, in electrical cells, such as batteries, use is sometimes made of so-called three-dimensional electrodes, i.e. an electrode based on a porous material, such as felt (as distinguished from a planar, or two-dimensional electrode). Due to the porous nature of a three-dimensional electrode, the electrolyte can penetrate into the electrode and a larger contact surface can be obtained, so that, in principle, a better efficiency of the cell is achieved.

However, it has appeared that enlarging the electrode surface of cells for carrying out electrochemical reactions with gaseous components leads to the electrolyte becoming exhausted sooner, because the gaseous component is converted faster. If the electrochemical cell is deployed for the production of gases, an analogous objection holds; the gases produced cannot be discharged from the solution fast enough, as a result of which, locally, around the electrode, supersaturation of the gaseous product occurs, resulting in an unfavorable position of the equilibrium. Consequently, also in this case, an unfavorable efficiency is obtained and the advantage of the larger electrode surface is undone, at least partially. For this reason, in practice, for carrying out electrochemical reactions with gaseous components, it is often not useful to use three- dimensional electrodes and for such applications, often, only conventional, planar electrodes are used. For carrying out electrochemical conversions, also because of the rapid exhaustion or, conversely, supersaturation and the large volume such cells occupy, the efficiency of such cells is usually too low to come to an economically interesting production.

DE-A-40 39 018 describes an electrolysis cell in which an electrode with a capillary structure is utilized, while the capillaries are at a specific angle to the longitudinal axis of the electrode. As a result, the direction of movement of gas bubbles can be influenced. The capillaries in these known electrodes are manufactured from conductive material. Hollow fibers whose wall is at least partly microporous are not mentioned or suggested at all in DE-A-40 39 018.

It has been found that by utilizing hollow microporous fibers, a highly efficient exchange of gas from or to the liquid can be achieved.

Therefore, the present invention relates to a method for carrying out an electrochemical reaction, wherein an electrolyte, comprising a liquid with at least one gaseous component dissolved therein, is brought into electrically conductive contact with an electrode, and wherein the gaseous component is supplied or, conversely, discharged, by using one or more hollow fibers which hollow fibers have an at least partly microporous wall.

The present invention is based on the insight that it has been found possible to introduce the gaseous component into the liquid so finely divided, that the exhaustion of the electrolyte as a result of a large electrode surface occurs considerably less rapidly. Surprisingly, it appears that the introduction of the gaseous component into the liquid can be carried out very well with the aid of hollow fibers. Conversely, it appears also possible to recover dissolved gas having formed in the electrolyte by electrochemical route, from the electrolyte with the aid of the hollow fibers. The present invention therefore relates to electrochemical conversions, wherein gaseous components dissolved in gas occur, while these components can be both reactants for and products of the electrochemical conversion.

Introducing or collecting gaseous components with the aid of hollow fibers proceeds very efficiently, which contributes to the compactness of the apparatus.

Owing to the gas transfer from or to the liquid being so efficient according to the invention, the electrochemical reaction can be carried out highly advantageously with the aid of three-dimensional electrodes. Suitable three-dimensional electrodes are electrodes which comprise graphite felt, carbon felt, metal foams and/or reticularly glazed carbon. The use of three- dimensional electrodes further contributes to a high efficiency, so that the apparatus according to the invention can be of even more compact design.

A suitable apparatus for carrying out an electrochemical reaction according to the invention comprises at least one pair of electrodes which can be brought into electrically conductive contact with an electrolyte, and means for supplying a gaseous component to a liquid for forming the electrolyte and/or means for discharging at least one gaseous component coming from the electrolyte which comprises a liquid with a gaseous product dissolved therein, which means comprise one or more hollow fibers with an at least partly microporous wall.

In principle, the dimensions of the hollow fibers can be selected freely and depend on the intended use. Preferably, hollow fibers with an inside diameter of 50 to 5000 μm are used, more preferably of 100 to 500 μm. The wall thickness is preferably 20 to 200 μm. The length can vary from several centimeters to tens of centimeters or more. The hollow fibers used according to the invention are wholly or partly microporous, with the micropores preferably having a size of 0.05 to 0.5 μm, more preferably of 0.1 to 0.2 μm.

The material of the hollow fibers is preferably substantially non- conductive, i.e. having a conductivity of less than 10 ¹⁰ ohm/cm, preferably less than 10 ¹¹ ohm/cm, more preferably less than 10 ¹² ohm/cm. Suitable materials for the hollow fibers are polymer plastics, in particular polyethylene, polypropylene, polytetrafluoroethylene.

The electrodes are placed in a space in which space the electrolyte can be introduced. Preferably, the space with the electrodes is placed directly behind the space containing the hollow fibers so that the path the electrolyte is to travel between the zone where gas exchange takes place and where the electrochemical conversion takes place is as short as possible. The method according to the invention is preferably carried out continuously.

The electrodes, which form the pair of electrodes, are introduced, in an otherwise conventional manner, in the space where the electrochemical conversion takes place. If desired, this space can be subdivided into compartments through the use of a membrane, so that the product formed is prevented from being converted on the counter electrode (anode or cathode). The membrane can be either cation-specific (for instance Navion™) or anion- specific.

An additional advantage of the method and apparatus according to the invention is that modules with hollow fibers and modules with electrodes can be easily connected. In this manner, an apparatus can be obtained with a capacity which can be geared to the desired size. The hollow fibers through which the gas is supplied or discharged can each be open at one or at both ends. For the supply of a gaseous component, a gas flow comprising the gaseous component is supplied to one end of the hollow fiber. In addition to the gaseous component, this gas flow can also comprise a carrier gas. If the hollow fiber is open at both ends, the gaseous component will diffuse through the wall of the fiber and dissolve in the Uquid which is present around the fiber on the outside. If one end of the fiber is closed, the gas flow through the fiber wall can be controlled by setting a suitable pressure. Preferably, no carrier gas is used in the variant where fibers with a closed end are used. For the production of gaseous components, also a carrier gas can be used for discharging the gaseous products from the fibers. In that case, fibers with two open ends are used. It is also possible to discharge the gaseous product through fibers with only one open end. In that case, no carrier gas needs to be used. According to a preferred embodiment of the present invention, the liquid is cross-flowed to the hollow fibers. Here, very suitably, use can be made of so-called cross flow modules (CFM). Such modules are described in US-A-6 103 118 and comprise several hollow fibers, which are clamped in at the top and the bottom side in a potting. In this manner, a module with two compartments is obtained, which are separated by the wall of the microporous hollow fibers. These modules can be designed such that they can be easily connected, for instance by alternately placing a CFM and an electrode compartment, if desired repeated a number of times. A schematic representation of such a combination of CFM and three-dimensional electrodes is represented in Figure 1 by way of example. The combination represented in this figure comprises the CFM-modules (1) and three-dimensional electrodes (2). The liquid flows into the combination at (3) and out of the combination at (4). A gas flow (5) is guided through the CFM-modules and flows perpendicularly to the hquid flow. Due to the liquid cross-flowing to the hollow fibers (i.e. in a direction perpendicular to the direction of the fibers), an exceptionally good mass transfer is obtained as a result of the very intensive gas/liquid contact.

The present invention can be deployed for various electrochemical conversions starting from reagents dissolved in hquid or for electrochemical conversions in which gaseous products dissolved in hquid are formed. The electrolyte can be an aqueous liquid, but it is also possible to use an organic hquid for this purpose. Examples of organic liquids are propylene carbonate, diethyl carbonate or dimethyl carbonate. If desired, the conductivity of the electrolyte can be enhanced by addition of salts. The invention can, for instance, be utilized for the production of hydrogen peroxide, wherein water is converted with oxygen on the cathode. According to the invention, very high oxygen concentrations can be achieved (up to 50 mg/liter or more). By using three-dimensional graphite felt electrodes, the dissolved oxygen can be converted very rapidly. Because the gas supply proceeds highly efficiently, the concentration of the oxygen can be kept at a sufficiently high level to guarantee a high production rate of hydrogen peroxide. In this manner, a hydrogen peroxide product can be obtained with a concentration which can be as high as 500 ppm. Such a concentration is adequate for many applications, such as cleaning, purification of waste water, etc.

According to the invention, a hydrogen peroxide solution can be produced in situ for various uses. Of particular interest is the use of the hydrogen peroxide produced according to the invention as disinfectant in the treatment of water, for instance swimming pool water. According to the invention, a compact apparatus suffices, to which, as reactants, exclusively water and oxygen need to be supplied, which products are generally amply available.

Another example is the production of oxygen through the oxidation of water on the anode, using, for instance, a DSA (dimensionally stable anode). These are coated titanium anodes, with the coating comprising a precious metal (Pt, Iridium oxide or Ruthenium oxide). The oxygen formed dissolves in the water and can be discharged through the hollow fibers.

In an analogous manner, by reduction on the cathode, hydrogen can be formed, which is then removed from the solution using the hollow fibers. Suitable materials for the cathode in the production of hydrogen are precious metals or metal foams of Ni of Cu.

The invention can also be used for the production of hydrogen, oxygen, chlorine gas, bromine gas or fluorine gas by oxidation on the anode. Suitable anode materials are the above-mentioned DSAs.

The invention will be elucidated on the basis of the following example which serves exclusively as illustration.

Example A cross-flow module was provided with 500 hollow fibers (material: polypropylene, inside diameter 280 μm, wall thickness 10 μm, pore size 0.2 μm). Three of such cross-flow modules were alternately placed between three- dimensional graphite felt cathodes of an electrochemical cell.

Pure oxygen was guided through the hollow fibers so that a concentration of minimally 30 ppm of oxygen in the liquid was obtained. Through reduction at the cathode of the oxygen present in the electrolyte (0.1 M KCl solution and, in a different example, tap water (0.25 mScπr¹) was used for this purpose), in both cases, a hydrogen peroxide concentration of 500 ppm was achieved.

Claims

1. A method for carrying out an electrochemical reaction, wherein an electrolyte, comprising a liquid with at least one gaseous component dissolved therein, is brought into electrically conductive contact with an electrode and wherein said gaseous component is supplied, or, conversely, discharged by using one or more hollow fibers, which hollow fibers have an at least partly microporous wall.

2. A method according to claim 1, wherein said liquid is caused to flow transversely towards said hollow fibers.

3. A method according to any one of the preceding claims, wherein said electrode comprises a porous material.

4. A method according to any one of the preceding claims, wherein hydrogen, oxygen, chlorine gas, bromine gas or fluorine gas is produced.

5. A method according to one of claims 1 - 4, wherein hydrogen peroxide is produced.

6. A method according to claim 5, wherein as electrode a graphite felt electrode is used.

7. A method according to any one of the preceding claims, wherein the material of the hollow fibers is substantially non-conductive, and preferably comprises a plastic selected from the group consisting of polyethylene, polypropylene, polytetrafluoroethylene, and combinations thereof.

8. An apparatus for carrying out an electrochemical reaction, comprising at least one pair of electrodes which can be brought into electrically conductive contact with an electrolyte, and means for supplying a gaseous component to a liquid in order to form said electrolyte and/or means for discharging at least one gaseous component coming from the electrolyte which comprises a liquid with a gaseous product dissolved therein, which means comprise one or more hollow fibers with an at least partly microporous wall.

9. An apparatus according to claim 8, wherein said hollow fibers are arranged such that said Hquid can flow transversely towards them.

10. Use of substantially electrically non-conductive hollow fibers with an at least partly microporous wall for carrying out electrochemical reactions with electrolytes which comprise a liquid with at least one gaseous component dissolved therein.

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