Field of the invention

The present invention relates to an electrolyser, a device in which a liquid electrolyte is decomposed by an electric current flowing through the electrolyte, based on ionization. The electrolyte can be in molten state, for example molten NaCl, or the electrolyte can be an aqueous solution, for example an aqueous solution of NaCl, or it can be pure water.

Background of the invention An electrolyser comprises a housing containing at least two electrodes facing each other and an electrolyte located between the electrodes. During normal operation, a direct current source having a positive pole and a negative pole is electrically connected to the electrodes, such that one electrode, the anode, is connected to the positive pole and the other, the cathode, is connected to the negative pole.

A direct current flows through the electrolyte from the cathode to the anode. In such an electrolyser the electrodes are solid and impermeable. For example, the electrolysis of molten NaCl is forced by a sufficiently high direct current. Bubbles of chlorine gas form at the anode, and globules of metal Na form at the cathode. The chlorine gas is removed through an outlet.

In electrolysis of an aqueous solution of NaCl, bubbles of hydrogen form and globules of metal Na form at the cathode. What happens at the anode depends on the concentration of NaCl in water: in dilute solutions bubbles of oxygen form at the anode, whereas in concentrated solutions bubbles of chlorine gas form at the anode. The hydrogen and, if formed, the chlorine gas are removed separately through separate outlets.

In electrolysis of pure water, at the cathode bubbles of hydrogen form and the anode bubbles of oxygen. Hydrogen and oxygen are removed separately through separate outlets.

USA patent specification No. 4086 155 discloses an electrolyser in which gas is released. The electrolyser comprises a pair of electrodes disposed in a liquid electrolyte. The electrodes consist of an insulating layer covered by a conducting layer, wherein the electrodes are so arranged that their insulating layers face each other. Both layers are permeable, so that electrolyte can pass through the insulating layer to the conducting layer, where, during normal operation, gas bubbles are formed. The diameters of the pores in the conducting layer are so selected that gas bubbles can escape from the conducting layer. Because the insulating layers of the electrodes face each other, the escaping gas bubbles do not interfere with the direct current between the electrodes.

The present invention aims to improve electrolysers that have a electrodes comprising an impermeable conducting layer.

A problem with electrolyser having an impermeable conducting layer is that gas bubbles are formed at the electrodes in an uneven pattern: at some places of the surface of the electrode a larger number of bubbles is formed than at other places. The uneven distribution of bubbles causes an uneven distribution of the direct current through the electrolyte, which adversely affects the hydrolysis efficiency. Summary of the invention

It is an object of the present invention to provide an improved electrolyser wherein the uneven distribution of gas bubbles at the electrodes is avoided.

To this end the electrolyser according to the invention comprises a housing containing at least two electrodes facing each other and an electrolyte located between the electrodes, wherein each electrode comprises an impermeable conducting layer covered at the surface facing the electrolyte by an insulating layer provided with a plurality of cavities, wherein each cavity comprises a central hole extending through the insulating layer to the impermeable conducting layer and at least one lateral channel extending at least partly through the insulating layer, of which one end is in fluid communication with the central hole.

In the specification and the claims, the term 'conducting' is used to refer to electrically conducting, and 'insulating' refers to electrically insulating.

Detailed description of the invention

The invention will now be described by way of example in more detail with reference to the accompanying drawings, wherein

Figure 1 is a 3/4 view, drawn not to scale, showing schematically an electrolyser with three electrodes, wherein the walls of the electrolyser have been omitted;

Figure 2 is a front view, drawn not to scale, of part of an electrode for use in the electrolyser according to the invention;

Figure 3A is a sectional view of the electrode of Figure 2 along the line III-III; and

Figure 3B shows an alternative to the embodiment shown in Figure 3A. Reference is made to Figure 1 showing an electrolyser wherein the container has been omitted. The electrolyser is provided with three electrodes, referred to with reference numerals 1, 3 and 5, respectively. Impermeable conducting layers of the electrodes 1 and 5 are electrically connected to a negative pole 10 and 12 of a direct current source (not shown) by means of wires 10a and 12a. The impermeable conducting layer of the electrode 3 is connected to a positive pole 13 of the direct current source (not shown) by means of wire 13a. Since, during normal operation electrodes 1 and 5 are negatively charged, electrodes 1 and 5 are the cathodes. Since, during normal operation, electrode 3 is positively charged, electrode 3 is the anode. The electrodes 1, 3 and 5 are submerged in a liquid electrolyte, schematically shown by horizontal bars, referred to by reference numeral 15.

During normal operation, the direct current source (not shown) is switched on, and a direct current the flows through the electrolyte 15 from the cathodes 1 and 5 to the anode 3. In case the electrolyte 15 is pure water, we have electrolysis of pure water, wherein bubbles of hydrogen 16 form at the sides of the cathodes 1 and 5 facing the anode 3, and bubbles of oxygen 17 at the sides of the anode 3 facing the cathodes 1 and 5. The bubbles 16 and 17 move upwards through the electrolyte and the gases are removed separately from the electrolyser through separate outlets (not shown).

The bubbles form at the surface of each electrode 1, 3 or 5 in an uneven pattern.

Reference is now made to Figures 2 and 3A, showing the electrode for the electrolyser according to the invention. Each of the electrodes 1 and 5 comprises an impermeable conducting layer 20 covered at the surface facing the electrolyte by an insulating layer 22 provided with a plurality of cavities 25. For the sake of clarity, in Figure 2, only one cavity 25 of the plurality of cavities is shown. Each cavity 25 comprises a central hole 27 extending through the insulating layer 22 to the impermeable conducting layer 20 and at least one lateral channel 29, of which one end is in fluid communication with the central hole 27. The lateral channels 29 extend partly through the insulating layer 29.

It will be understood that electrode 3, Figure 1, comprises an impermeable conducting layer covered at both surfaces with an insulating layer that is provided with a plurality of cavities.

Reference is now made to Figure 1, wherein the electrodes 1,3 and 5 are replaced by the electrodes described with reference to Figures 2 and 3A. During normal operation, gas bubbles will form at the spots where the electrolyte is in contact with the impermeable conducting layer 20 of the electrode 1, 5, that is to say at the bottom of the central hole 27 of each of the cavities 25. Gas bubbles are ejected from the central holes 27 and rise through the electrolyte 15, and gas is removed from the electrolyser through separate outlets (not shown). The formation and ejection of a gas bubble out of the central hole 27 causes a pressure reduction behind the gas bubble, which causes electrolyte to flow through the lateral channels 29 into the central hole 27. Thus, there is a regular supply of electrolyte to the central hole 27.

Because the bubbles are only formed at the cavities 25, a regular pattern of gas bubbles is obtained when the cavities have the same dimensions and are regularly distributed over the surface of the electrode. The number of lateral channels is suitably between 1 and 8.

Suitably, the lateral channels 29 extend through the insulating layer 22 to the impermeable conducting layer 20 as shown in Figure 3B.

Suitably, the walls of the cavity 25 are covered with an electrically conductive layer (not shown) that is in electric contact with the impermeable conducting layer 20. In this case, bubbles are formed in the entire cavity 25.

Suitably, the insulating layer is a metal oxide that is deposited on the electrode, and the cavities are etched in the insulating layer.

Suitably, the thickness of the insulating layer 21 is in the range of from 0.1 to 2 000 micron, and more suitably in the range of from 2 to 2 000 micron. Suitably, the diameter of the central hole 27 is in the range of from 0.1 to 2000 micron and more suitably in the range of from 2 to 2 000 micron. Suitably the length of a lateral channel 29 is in the range of from 0.2 to 4000 micron and more suitable in the range of from 4 to 4000 micron, and its width in the range of from 0.1 to 2000 micron and more suitably in the range of from 1 to 2000 micron.

Suitably, the electrode 3 is omitted, and the electrolyser comprises two electrodes, electrodes 1 and 5, facing each other. During normal operation, electrodes 1 and 5 are in this case differently charged, one electrode is positively charged and the other is negatively charged.

More suitably, the electrolyser comprises a set of more than three electrodes facing each other. The first and the last electrode of the set are comprising an impermeable conductive layer provided at the surface facing the electrodes with an insulating layer with cavities. The other electrodes comprise an impermeable conductive layer provided with an insulating layer with cavities covering both surfaces of the impermeable conductive layer. During normal operation, odd numbered electrodes are positively charged and even numbered electrodes are positively charged, or the reverse.