The invention relates to a perforated plate structure, such as a gas diffusion electrode, e.g., for the electrolysis of water to produce hydrogen and oxygen or for water purification or similar electrolysis processes, or fuel cells.

Electrodes for the electrolysis of water form an inter face supporting an electrochemical reaction between a liquid phase and a gaseous phase. The electrode must facilitate suffi cient mass flow, or mass transfer, as well as electric conduc tivity and mechanical stability. Mesh electrodes or perforated plate electrodes can be used to optimize mass flow and to in crease the surface area available for the electrochemical reac tion. The openings in the electrodes should be sufficiently large to prevent gas stagnation. Typically the electrodes com prise a catalyst material, e.g., as a coating, to catalyze the desired electrochemical reaction.

It is an object of the invention to provide a structure that can for example be used as a perforated plate electrode for electrolysis processes, combining a large surface area and in creased mass flow with sufficient mechanical stability.

The object of the invention is achieved with a metal plate structure comprising two outer layers and at least one in termediate layer, wherein both outer layers are provided with a pattern of recesses, the recesses on one outer layer being off set or staggered with respect to the recesses in the other outer layer. The intermediate layer comprises through-holes, each through-hole connecting a recess at one outer layer with a par tially overlapping recess at the opposite outer layer. The per forated plate structure combines a high mechanical stability and flatness with enlarged specific surface area and facilitates in creased mass flow. Due to its flatness and mechanical stability the perforated plate structure can make good conductive contact with an adjacent proton or anion exchange membrane. Therefore, the perforated plate structure is particularly suitable for use as an electrode, in particular for electrolytic hydrogen produc tion or for use in a fuel cell stack, e.g., at either side of a membrane or separator. The plate structure can be embodied with plane outer surfaces, e.g., only interrupted by the recesses, which makes it possible to realize a zero-gap configuration, e.g., for PEM and AEM electrodes. The perforated plate structure can also be used as an electrode for water purification or de salination, or similar gas forming electrolysis processes. The perforated plate structure can also be used for other purposes, such as a screen or sieve.

In this respect, plate structure means that the layers form an integral structure of the same material, such as a cor rosion resistant steel, nickel, titanium, niobium or alloys thereof, or plastic materials or other suitable materials. The plate does not, or at least not necessarily, show materially distinctive layers. The expressions "inner layer" and "outer layer" refer to the position of the through-holes and recesses rather than referring to distinctive material layers.

In a specific embodiment, the recesses at the outer layers are of equal size, shape and spacing, separated by parti tions of even thickness, wherein partitions at one outer layer join each other at junctions between three or four adjacent re cesses. The junctions of the partitions of one outer layer can for example be aligned with the centers of the recesses of the opposite outer layer. This results in a very regular and mechan ically stable structure. Alternatively, the recesses may have varying sizes, shapes and/or spacings.

In a specific embodiment, each recess at one outer layer partly overlaps three or more adjacent recesses of the op posite outer layer, and the through-holes are formed where a re cess of one outer layer overlaps a recess of the opposite outer layer. Hence, each recess encircles at least three through-holes leading to at least three different recesses at the opposite outer layer. The recesses can for example have a maximum width of at most 2 mm, e.g. at most 1 mm, e.g., at most 100 micron, e.g. at least 10 micron. The through-holes can have a diameter of at least 10 micrometers, e.g. at most 4 mm. Larger through-holes and/or recesses can also be used, if so desired. In a specific example the partitions may have a width of at most 2 mm, e.g., at most 1 mm, e.g., at least 10 micron. The plates structure may for example have a thickness of at most 2 mm, e.g. at most 1 mm, e.g., at most 300 micron, e.g., at most 100 micron, e.g., at least 10 micron.

Size, shape and spacing of the recesses and through holes can be varied depending on the intended use of the plate structure, e.g., as a cathode or as an anode. Also within a sin gle plate structure, the size, shape and spacing of the recesses and through holes can be varied, e.g., to optimize electric cur rent flow or the discharge of gas bubbles formed at the elec trode surface.

All layers can be made from the same starting metal plate or sheet, for example be made by etching, for example mi cro-etching and/or electrochemical etching. The recesses can be etched in the usual manner, using a regular photo-resist mate rial to mask the partitions. When the recesses are sufficiently deep, through-holes will be formed where the recesses overlap recesses at the opposite outer layer. The starting plate can be materially homogeneous over its thickness, but starting plates with a layered structure can also be used, if so desired.

Before etching, both sides of the starting plate are cleaned and coated with a light-sensitive photoresist. Parts of the photoresist layer are then selectively exposed to actinic radiation, in particular light, more in particular UV light. The photoresist can for example be a positive photoresist, in which the portion of the photoresist that is exposed to light becomes soluble to the photoresist developer, while the unexposed por- tion of the photoresist remains insoluble to the photoresist de veloper. Or the photoresist can be a negative photoresist. In that case, the portion of the photoresist that is exposed to light becomes insoluble to the photoresist developer, while the unexposed portion of the photoresist is dissolved by the photo resist developer.

Selective exposure of the photoresist can for example be achieved by using a mask or by using Laser Direct Imaging (LDI) technology, allowing projections of high resolution im ages, e.g., directly from a CAD file.

The actinic radiation cures the photoresist. In a next step, the parts of the photoresist layers that are soluble for the developer, are washed away. The remaining part of the photo resist reflects the desired pattern of partitions. The metal is bare where the recesses are to be etched.

In a next step, both sides of the plate are exposed to an etching medium, e.g., in a bath or as a spray. Examples of suitable etching fluids are for instance FeC13, FeN03, CuC12, and HF. Suitable etching techniques are for instance disclosed in the handbook Principles and Practice of Photochemical Machin ing and Photoetching, of D. Allen, published by Adam Hilger,

1986.

Both sides of the starting plate can be etched simulta neously. Optionally, different spraying pressures can be used at the two sides of the plate and/or a different number of etching units can be used at the two sides of the plate. This makes it possible to have different etching depths at the two sides. Op tionally, different spraying pressures can be used at different sections of the same side of the plate.

In a final step the remaining photoresist is removed, e.g., using a suitable solvent or cleaning medium. A burr free and stress free perforated plate structure remains.

Other machining techniques, such as mechanical machin ing, can also be used, if so desired. Optionally, a coating can be used enhancing the spe cific surface area, particularly if the plate is used as a per forated plate electrode for electrolysis processes. Such coat ings may for example be applied using sol-gel technology or dy namic hydrogen bubble template synthesis (DHBT).

Thin layers of catalysing materials can be applied on the electrode, for example by means of vapor deposition, sput tering or electrostatic spraying. Suitable catalysts include, but are not limited to platinum, palladium, yttrium, vanadium molybdenum, tellurium, Raney nickel or mixtures thereof.

Other methods to increase specific surface area include mechanical treatments to increase surface roughness.

The perforated plate structure will typically be flat, but it may also be shaped with a different geometry, e.g., as a cylinder.

The invention is further explained with reference to the accompanying drawings showing exemplary embodiment.

Figure 1: shows a section of a plate structure accord ing to the invention;

Figure 2: shows a cross section of the structure along line I-I in Figure 1.

Figures 3A-G: show consecutive steps of a micro-etching process for manufacturing the plate structure of Figure 1.

Figure 1 and 2 show a metal plate structure 1 compris ing two outer layers 2, 3 and an intermediate layer 4. In Figure

1, the pattern of the outer layer 2 facing the viewer is repre sented in drawn lines, while the pattern of the opposite outer layer 3 is drawn in dashed lines.

In the shown exemplary embodiment, both outer layers 2, 3 are provided with a honeycomb pattern of hexagonal recesses 5. In the shown embodiment, the recesses 5 at the two outer layers

2, 3 are of equal size, shape and spacing, and are separated by partitions 6 of even thickness. In alternative embodiments, the outer layers 2, 3 can have different thicknesses and/or the re cesses may have varying geometries. The partitions 6 join each other at junctions 7 between three adjacent recesses 5. This re sults in a very dense arrangement of recesses 5 and, conse quently, in a very high specific surface area. The recesses 5 on one outer layer 2 are offset with respect to the recesses 5 of the opposite outer layer 3, in such a way that junctions 7 of the partitions 6 of one outer layer 2 are aligned with the cen ters of the recesses 5 of the opposite outer layer 3.

The plate structure 1 has plane outer surfaces inter rupted only by the recesses 5.

The intermediate layer 4 comprises through-holes 8.

Each through-hole 8 connects a recess 5 at one outer layer 2 with a partially overlapping recess 5 at the opposite outer layer 3.

The outer layers 2, 3 and the intermediate layer 4 in tegrally form a single plate of a single metal or metal alloy material.

Figures 3A-F show consecutive steps of a micro-etching process for manufacturing the perforated metal plate structure 1. A starting plate or sheet 1 of nickel, a nickel alloy, a corrosion resistant steel or any other suitable etchable mate rial, is first cleaned, typically in a clean room, in order to optimize adhesion to a layer 10 of a light-sensitive photoresist material, applied in a next step on both sides of the plate 1 (Figure 3B). The photoresist material is exposed to actinic ra diation, in particular to UV light, and subsequently washed with a photoresist developer. The remaining part of the photoresist images the desired pattern of partitions 6.

Using Laser Direct Imaging (LDI) technology (reschemat- ically represented by arrows I in Figure 3C), laser sources di rectly image a desired pattern of cured photoresist material 11 on both sides of the plate 1'. The imaged pattern at one side of the plate is identical to the pattern at the other side, but offset. If a negative photoresist is used, then the photoresist cures where it is affected by the laser beam, but the other parts 12 of the photoresist remains removable by means of a pho toresist developer.

In a next step (Figure 3D) the plate 1 is washed to remove the uncured parts 12 of the photoresist. The cured parts 11 of the photoresist remain and reflect the honeycomb pattern of the partitions 6 of the perforated plate structure 1 to be made.

In a next step (Figure 3E), an etching fluid is sprayed over both sides of the plate 1. The cured photoresist 11 is re sistant to the etching fluid and shields the metal directly un derlying the cured photoresist parts. The etching fluid etches the hexagonal recesses 5, which gradually grow deeper. At a cer tain depth of the hexagonal recesses 5, through-holes 8 will oc cur connecting a hexagonal recess 5 at one side of the plate with an overlapping hexagonal recess 5 at the opposite side of the plate 1 (Figure 3F). In a final step (Figure 3G), the cured photoresist 11 is washed away, and the desired perforated plate structure 1 is ready. Optionally, it can be treated further, e.g., by applying a coating enhancing catalytic activity or en hancing specific surface area.