CURRENT DENSITY DISTRIBUTOR FOR USE IN AN ELECTRODE

Abstract

A mesh-shaped, porous electric current density distributor is for use with an electrode, and is adapted for providing electric current to an active layer of the electrode. The active layer contacts a face of the current density distributor, and the current density distributor includes a porous mesh having several electrically conductive paths. At least part of the electrically conductive paths extend along a direction of major current flow over the current density distributor. The porous mesh includes in a direction crosswise to the direction of major electric current flow, several paths of an electric insulator. The current carrying capacity of the current density distributor in crosswise direction to the major current flow over the current density distributor is smaller than the current carrying capacity in the direction along the major current flow over the current density distributor.

Claims

1. A current density distributor comprising: an electrically conductive frame and a porous mesh mounted into the electrically conductive frame, wherein the electrically conductive frame comprises a lug adapted for providing electric current to the current density distributor, wherein the electric current flows along a direction of major current flow along the current density distributor, wherein the current density distributor comprises a porous mesh having a plurality of electrically conductive paths, wherein at least part of the electrically conductive paths extend along the direction of major current flow over the current density distributor, wherein the porous mesh comprises, in a direction crosswise to the direction of major current flow, a plurality of first electrically insulating paths, and wherein the porous mesh comprises a plurality of open spaces between the electrically conductive paths and/or the first electrically insulating paths.

2. The current density distributor as claimed in claim 1, wherein a current carrying capacity of the current density distributor in the direction crosswise to the direction of major current flow over the current density distributor is smaller than a current carrying capacity in the direction along the direction of major current flow over the current density distributor,

3. The current density distributor as claimed in claim 1, wherein the porous mesh comprises in crosswise direction to the direction of major current flow a number of electrically conductive paths, wherein the number of electrically conductive paths in the direction crosswise to the direction of major current flow per length unit is smaller than the number of electrically conductive paths per length unit along the direction of major current flow.

4. The current density distributor as claimed in claim 1, wherein the electrically conductive paths comprise along the direction parallel to the current flow a plurality of first electrically conductive paths, and a plurality of second electrically conductive paths, wherein the first electrically conductive paths have a cross section which is larger in comparison to a cross section of the second electrically conductive paths.

5. The current density distributor according to claim 1, wherein the porous mesh comprises along the direction of major current flow over the current density distributor, a plurality of electrically insulating paths.

6. The current density distributor according to claim 5, wherein along the direction of major current flow the electrically conductive paths and the electrically insulating paths are alternatingly provided.

7. The current density distributor according to claim 1, further comprising a plurality of second electrically insulating paths, wherein the electrically conductive paths and the first and second electrically insulating paths are arranged according to a regular geometric pattern.

8. The current density distributor according to claim 1, wherein the electrically conductive paths comprise a plurality of electrically conductive wires, and wherein the first paths of an electric insulator comprise a plurality of threads of an electric insulator.

9. The current density distributor according to claim 1, wherein electrical conductivity of the electrically conductive paths at 20 C. is preferably at least 1.2510E3 S/m.

10. The current density distributor according to claim 1, wherein the resistivity of the electrically conductive paths is less than 810E(4) Ohm.Math.m.

11. The current density distributor according to claim 1, wherein a material of which the electrically conductive paths are produced offers a tensile strength of at least 120 MPa.

12. The current density distributor according to claim 1, wherein a material for the electrically insulating paths has an electrical resistance at 20 C. of at least 10E12 Ohm.Math.m.

13. The current density distributor according to claim 1, wherein a material for the electrically insulating paths has a tensile strength of at least 15 MPa.

14. The current density distributor according to claim 1, wherein a material for the electric insulator is selected from the group consisting of polyaramides, polyesters, polyolefins, polyetherketone, polyphenylene sulfide, polyether imide (PEI), aliphatic polyamides, fluoroplastics, and a composite material comprising a polymeric matrix of one or more of the polymers indicated above, or any combination or blend of two or more of the afore-mentioned polymers indicated above and wherein the material for the electric insulator further comprises a reinforcing component comprising glass fiber and/or ceramic fiber.

15. The current density distributor according to claim 14, wherein a material for the electric insulator is polyethylene terephthalate, UHMWPE, HDPE, polypropylene, PTFE, ETFE, FEP, PFA, EFEP, or PCTFE.

16. The current density distributor according to claim 1, wherein a material of which the electrically conductive paths are produced is selected from the group consisting of conductive polymers, metal alloys comprising one or more metals selected from the group consisting of gold, iridium, platinum, rhodium, palladium, silver, copper, nickel, zinc, tungsten, titanium, aluminum, tin, steel, stainless steel alloys, austenitic stainless steels, and duplex stainless steel.

17. The current density distributor according to claim 1, wherein the porous mesh comprises a material selected from the group consisting of a woven wire, a knitted wire, a braided wire, a welded wire, an expanded mesh, a plate having a plurality of holes, a screen having a plurality of holes, and a plate having a plurality of holes, which has been photo-chemically etched or electroformed to provide the electrically conductive paths.

18. The current density distributor according to claim 1, wherein the porous mesh has an open area of at least 50%, wherein the open area represents the ratio of the area occupied by the open spaces between the electrically conductive paths and/or the electrically insulating paths.

19. The current density distributor according to claim 1, wherein the electrically conductive paths have an average cross section of 50 m to 500 m.

20. The current density distributor according to claim 1, wherein the number of electrically conductive paths in the porous mesh is between 10 and 50 paths per cm.

21. The current density distributor according to claim 1, wherein the number of electrically insulating paths in the porous mesh is between 10 and 50 paths per cm.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] FIGS. 1A and 1B show a schematic example of a current density distributor respectively comprising a plain woven material and a leno woven material.

[0040] FIGS. 1C and 1D show a schematic example of a current distributor respectively comprising a plain woven material and a leno woven material, wherein part of the electric conductors in a direction parallel to the major direction of the current flow have a relatively larger average cross section than another part of the electric conductors.

[0041] FIG. 2A shows what is meant with open area in a mesh.

[0042] FIG. 2B shows a view to an example of a current distributor of the present invention, showing the y or thickness direction.

[0043] FIG. 3A shows a schematic example of a current distributor respectively comprising a plain woven material and a leno woven material, wherein part of paths in a direction parallel to the major direction of the current flow are made of an electric isolating material. In FIG. 3B non-conductive yarns have been braided/interlaced into a perforated conductive foil.

[0044] FIGS. 4A, 5A, 6A, and 7A show the current density distribution calculated using the above described computational model in x and z direction for a current collector having respectively 10 electrically conductive wires evenly distributed over a length of 10 cm in z direction along the direction of major current flow, and respectively 10, 15, 20 and 30 electrically conductive wires in x direction, perpendicular to the main flow of electric current, evenly distributed over a length of 10 cm. FIGS. 4B, 5B, 6B, and 7B show the electric current density distribution corresponding to the current collectors of respectively FIGS. 4A, 5A, 6A, and 7A.

[0045] FIGS. 8A, 9A, and 10A respectively show the electric current density distribution over a current density distributor having 10 threads of a non-conductive plastic material evenly distributed over a length of 10 cm in a direction perpendicular to the direction of main current flow (x-axis), and respectively 15, 20 and 30 electrically conductive wires evenly distributed over a length of 10 cm in the direction along the main current flow (z-axis). FIGS. 8B, 9B, and 10B show the corresponding current density profile along the z-axis.

[0046] FIGS. 11A, 12A, and 13A show the electric current density distribution for a current density distributor having in a direction perpendicular to the direction of main current flow (x-axis) a plurality of electrically conductive wires with a thickness of respectively 2 mm, 1 mm and 0.5 mm, the remaining electrically conductive wires having a thickness of 0.25 mm FIGS. 11B, 12B, and 13B show the corresponding current density profile along the z-axis.

[0047] FIGS. 14A, 15A, and 16A show the electric current density distribution for a current density distributor having in a direction perpendicular to the direction of main current flow (x-axis) a plurality of electrically conductive wires with a thickness of respectively 2 mm, and 0.25 mm, and in a direction along to the direction of main current flow (z-axis) a plurality of electrically conductive wires with a thickness of respectively 2 mm, 1 mm and 0.5 mm, the remaining electrically conductive wires having a thickness of 0.25 mm FIGS. 14B, 15B, and 16B show the corresponding current density profile along the z-axis.

[0048] FIG. 17 represents the current density distributor comprising a mixed metal and plastic mesh.

[0049] FIG. 18 shows a view to an electrode comprising the current density distributor of this invention, wherein the current density distributor comprises a lug along the top for feeding electric current.

DETAILED DESCRIPTION

[0050] In the drawings and examples it is assumed that the direction of the major electric current flow is the z-axis, the direction crosswise to the major current flow is the x-axis. The smallest dimension or thickness of the current density distributor and of an electrode containing the current density distributor is the direction of y-axis. Dimensions in the figures are indicative and not in scale.

[0051] The current density distributor shown in FIGS. 1A-1D comprises a circumferential electrically conductive frame 1 housing one single porous mesh 10, or an array of several porous meshes 10. Instead of a circumferential housing, other configurations may be used, such as for example shown in FIGS. 3A and 3B, where electric current is fed along a lug which extends only along part of a first side 2 of the current density distributor. It is understood that the dimensions of the lug, i.e. its height and length along the first side 2 may vary depending on the envisaged use of the current density distributor.

[0052] A first side 2 of the frame is connected to a source of electric energy using a current feeder 3, for supplying electrical energy to the current density distributor. The electric current is supposed to flow from the feeder 3 to the side 4 opposite the first side 2, in a direction indicated by the arrow, where current may be collected. The frame 1 may however be limited to a current feeder 3 and a current collector, mounted to opposite sides of the current density distributor.

[0053] The mesh 10 comprises a plurality of electrically conductive paths 5, 15, 25 which may be connected to each other or contact each other or not depending on the nature of the porous mesh. At least part of the electrically conductive paths 5 extend along the direction of major current flow over the current density distributor. The mesh 10 may comprise a plurality of electrically conductive paths 15 which extend in cross direction to a direction of major current flow over the current density distributor. The mesh 10 may further comprise in cross direction to the direction of major electric current flow, a plurality of paths 6 of an electric insulator. The mesh 10 may also comprise in a direction along the direction of major electric current flow, a plurality of paths 7 of an electric insulator. The mesh may extend mainly in x and z direction, but it may extend in y direction also as is shown by FIG. 2B. The mesh may be a square mesh, but other geometries may be used as well. The geometry of the mesh will in general vary with the nature of the material as described below.

[0054] Within the scope of the present invention with mesh is meant a woven, knitted, braided, welded, y expanded mesh of fibers, bars or threads. The wording mesh is meant to include a square meshes with a substantially rectangular shape and orientation of the conductive wires and insulating threads such as the one shown in FIG. 17, but the mesh may also be tubular, or a coil film, or a otherwise shaped three-dimensional materials. In a plate, sheet, foil, film or screen holes may for example be created using photo-chemical etching or electroforming. Where use is made of a perforated plate, sheet, foil, film, screen or woven material, the paths which extend along the direction of major current flow will generally run substantially parallel to the direction of major current flow, and the paths which extend in cross direction to the direction of major current flow will generally extend substantially perpendicular to the major direction of current flow. Also, the paths which extend along the direction of major current flow will generally run substantially perpendicular to the paths which extend in cross direction of the major current flow. Where use is made of a knitted, braided or expanded mesh, the holes may have a somewhat irregular shape and may be irregularly positioned and holes of two or more different shapes may be present. Also, the paths which extend along the direction of major current flow will generally extend under an angle >90 to the direction of major current flow, and the paths which extend in cross direction to the direction of major current flow will generally extend under an angle <90 to the major direction of current flow. Also, the paths which extend along the direction of major current flow may, but will often not run substantially perpendicular to the paths which extend in cross direction of the major current flow. Where use is made of a woven material, a screen or a plate with an open structure, the holes will often be arranged according to a geometric pattern. The holes may be evenly spaced apart in x direction of the mesh, but an irregular spacing is possible as well. According to another preferred embodiment the holes may be evenly spaced apart in z direction of the mesh, but an irregular spacing is possible as well. The porous mesh may be an isotropic material or an anisotropic material.

[0055] The woven material may for example be a plain weave (also called tabby weave, linen weave or taffeta weave), leno weave (also called cross weave or gauze weave), in other words the woven material may be made using the basic weaving process known to the skilled person. An example of a plain weave and leno weave are respectively shown in FIGS. 1A and 1B. Balanced plain weaves are fabrics in which the warp and weft are made of threads of the same size and the same number of ends per cm as picks per cm. The woven material may also be a leno weave, in which the warp yarns are arranged in pairs and are twisted around the weft yarns, keeping the weft firmly in place to provide a strong yet sheer fabric. Leno weave produces an open fabric with almost no yarn slippage or misplacement of threads.

[0056] Other types of meshes suitable for use with this invention include perforated screens sheets, plates, films or foils made of a conductive material, having a plurality of wires or threads of a non-conductive material interlaced in a direction perpendicular to the current flow, in order to ensure mechanical and dimensional stability. Still other types of meshes suitable for use with this invention include perforated sheets, plates or foils made of a non-conductive material, having a plurality of wires or threads of a conductive material interlaced in the direction parallel to the current flow. A further type of mesh suitable for use with the present invention includes lines/wires of a conductive material, which extend parallel to the current flow direction, printed on a perforated sheet, foil or plate.

[0057] The material used to produce the woven, braided, knitted mesh or screen may comprise a plurality of filaments, mono-filament or multi-filament, fibers, yarns, threads, tows, wires or cables of an electrically conductive and an electrically insulating material. Therefore, electrically conductive paths and electrically insulating paths is understood to comprise wires, filaments, mono-filament or multi- filament, fibers, yarns, threads, tows, ropes or cables of an electrically conductive material. A thread is understood to comprise a filament, a group of filaments twisted together, or a filamentous length formed by spinning and twisting short textile fibers into a continuous strand. A wire is understood to mean a single, thin, flexible thread, strand or rod of metal, which may be cylindrical, but may also have another shape. Yarn is understood to mean a continuous length of interlocked fibers. Filament, a group of filaments twisted together, or a filamentous length formed by spinning and twisting short textile fibers into a continuous strand. Strands are understood to comprise a plurality of fibers or filaments twisted, plaited, or laid parallel to form a unit for further twisting or plaiting into yarn, thread, rope, or cordage.

[0058] In the present invention the use of wires of electrically conductive material is preferred.

[0059] The nature and number of electrically conductive wires in the current density distributor of this invention is chosen such that the current carrying capacity of electrically conductive wires 15 in cross direction to the direction of major current flow over the current density distributor (x-direction in FIGS. 1A and 1B) is smaller than the current carrying capacity in the direction along the direction of major current flow over the current density distributor (z-direction in FIGS. 1A and 1B).

[0060] This may be achieved in that the number of electrically conductive wires 15 per length unit in the direction crosswise to the major current flow over the electrode is smaller than the number of electrically conductive wires 5 per length unit along the direction of the major current flow. In a practical example this means that the number of electrically conductive wires 15 per cm of mesh crosswise to the major current flow direction, is smaller than the number of electrically conductive wires 5 per cm of mesh along the direction of the major current flow. This may be achieved by the use of a woven mesh in which part of the electrically conductive wires in cross direction have been exchanged for threads of an electrical insulator.

[0061] According to a preferred embodiment, the number of electrically conductive wires 5, 25 per length unit in the direction along the major current flow over the electrode is increased relative to the number of electrically conductive wires per length unit in cross direction of the direction of major electric current flow, to ensure that the current carrying capacity in the direction along the major current flow may be increased and that the current flow towards parts of the current density distributor at a position remote from the current feed may be improved.

[0062] Alternatively or in addition to increasing the number of electrically conductive wires in the direction along the direction of major current flow, at least part of the electric conductive paths or wires 25 in the direction along the direction of major current flow (z-direction in FIG. 1D) have a relatively larger average cross section, and at least part of the electric conductive paths or wires 5 in the direction along the current flow have a relatively smaller average cross section. The electric conductive paths or wires 25 with a relatively larger average cross section ensure that the current carrying capacity in a direction along the major current flow may be increased and that the current flow towards parts of the current density distributor at a position remote from the current feed may be ensured.

[0063] The average diameter or cross section of the electric conductive paths or wires 5, 15, 25 may vary within wide ranges, but preferably varies from 50 m to 500 m, preferably 75-400 m, more preferably 100-300 m, most preferably 150-250 m but paths with a larger or smaller cross section may be used as well, for example 100 or 250 m. With a smaller cross section, the number of wires can be increased while keeping a high percentage of open area.

[0064] A further weight and cost reduction may be achieved in that at least part of the paths in the direction along the direction of major electric current flow (z-direction in FIG. 1D) are made of electrically insulating material. The negligible current flow over these threads may be compensated by increasing in other areas the number of electrically conductive wires per unit length, or by inserting electric conductive wires with a larger average cross section.

[0065] When selecting an arrangement, number, shape and dimensions of electrically conductive paths and paths of electric insulator, the person skilled in the art will take into account that a sufficient electric current flow may be guaranteed over the entire current density distributor so that a sufficiently homogeneous current density distribution may be obtained, without compromising too much on the mechanical and dimensional stability of the porous mesh. The skilled person will further take care that the percentage of open area provided by the mesh is sufficiently large, and that the dimensions of the open spaces are to achieve that the current extends to a desired portion of the surface of the pores of the mesh, preferably over virtually the entire portion of the surface of the pores. This way, it may be guaranteed that a desired portion of an active layer positioned adjacent the current density distributor, may be in contact with the electrolyte as well as being reached by the current flow. In practice, a mesh will often comprise between 10 and 50 electrically conductive wires per cm, preferably between 10 and 40, more preferably between 10 and 30, most preferably between 10 and 20. However larger or smaller numbers of conductive wires may be used as well.

[0066] The electrically conductive wires and the electrically insulating threads may be arranged in a random manner in the mesh which forms the current density distributor. Similarly, the pores of the porous mesh may be arranged in a random manner In some cases this may lead to a current density distribution and an open area available for reaction which is not as regular as would be desirable and a non uniform performance of the active layer of the electrode may be more difficult to achieve, but this will not always be the case. Therefore, preferably, the electrically conductive wires 5, 15, 25 and the non-conductive threads 6, 16 are arranged according to a regular geometric pattern as is shown in FIGS. 1A-1D and FIGS. 3A-3B. Thereby, the distance between adjacent threads of electrically insulating material in x direction may be the same as the distance in z direction, or it may be different. The distance between adjacent threads of electrically insulating material in respectively x or z direction may be constant over the entire mesh, or may vary according to a regular pattern. Similarly, the distance between adjacent threads of electrically conductive material in x direction may be the same as the distance in z direction, or it may be different. The distance between adjacent threads of electrically conductive material in respectively x or z direction may be constant over the entire mesh, or may vary according to a regular pattern. In case the use of the electrolytic cell so requires, the electrically conductive wires 5, 15, 25 and the non-conductive threads 6, 16 may also be are arranged in a random manner

[0067] Preferred porous meshes are those which present a sufficiently large percentage of open area, in order to ensure that the reactants can penetrate the porous mesh and contact the electrochemically active layer. Therefore, the current density distributor will usually have an open structure, in particular in the x-z plane. The % of open area of a square mesh may be represented by the formula I below (see FIG. 2A):

[00001]$\begin{array}{cc}\%.Math..Math.\mathrm{OPEN}.Math..Math.\mathrm{AREA}=\frac{\underset{i}{.Math.}.Math.x_{i.Math..Math.1}-y_{i.Math..Math.1}}{\underset{i}{.Math.}.Math.x_{i.Math..Math.2}-y_{i.Math..Math.2}}100& \mathrm{formula}.Math..Math.I\end{array}$

For a more general example of a non-square mesh, the % of open area is represented by the ratio of the surface of the open area to the sum of the open area and the closed area. Particularly preferred porous meshes are those with an open area of at least 48%, preferably in the range of 60-80%.

[0068] The contact positions where the active layer contacts the current density distributor, screen areas of the active layer from the reaction medium and thereby risk to locally reduce the reaction yield that may be achieved by the electrolytic cell. The person skilled in the art will however be capable of selecting the percentage of open area in such a way that an optimum compromise is achieved between providing a sufficient dimensional and mechanical stability on the one hand, and a sufficiently large available reactive surface of the active layer. In practice this means that the skilled person will be capable of selecting an appropriate number of electrically conductive and electrically insulating paths per unit length, and an appropriate average cross section to achieve an optimum compromise between a sufficient dimensional and mechanical stability on the one hand, and a sufficiently large available reactive surface of the active layer on the other hand.

[0069] To achieve an open area that is as large as possible while maintaining a good dimensional stability, preferably the electrically conductive wires and the threads of electric insulator have approximately the same average cross section, and a similar number of electrically conductive wires and threads of electric insulator is present in a direction along the major current flow and the direction crosswise of the major current flow. With similar is meant a ratio of electrically conductive wires to threads of electric insulator of 1:1.5, preferably 1:1.25, more preferably 1:1.1, most preferably 1:1.

[0070] The dimensions of the current density distributor according to the invention may vary within wide ranges, but preferably the conductive and insulating material is selected such that the average thickness of the current density distributor in y direction is at least 100 m, preferably at least 120 m in order to ensure a sufficient dimensional stability. The average thickness of the current density distributor will usually not be more than 750 m, with less than 600 m being preferred and less than 500 m or even less than 400 m being particularly preferred. With larger average thickness, the thickness of the electrode risks to become too high and with several electrodes mounted into an electrolytic cell the space occupied by the electrolytic cell may become to large. More preferably the thickness of the current density distributor varies from 150-400 m, in particular from 200-300 m. This range provides an optimum compromise between minimizing the electrical resistance and providing a sufficiently large fraction of open spaces.

[0071] Within the scope of this invention, the electrically conductive paths may comprise a wide variety of electrically conductive materials. Materials suitable for use as an electrical conductor with the present invention include solid state materials with an electrical conductivity of at least 1.2510E3 S/m at 20 C. and atmospheric pressure. The resistivity of the electrically conductive paths is preferably less than 810E(4) Ohm.Math.m, more preferably less than 10E(6) Ohm.Math.m. Therefore, within the scope of the present invention, electrical conductor is understood to mean materials with an electrical conductivity of at least 10E6 S.m.sup.1 at 20 C. and atmospheric pressure, and a resistivity of preferably less than 810E(4) Ohm.Math.m. However, when use is made of a material for electrically conductive paths of an electrical insulator thread coated with an electrically conductive material, the above-described material properties may be different.

[0072] Examples of electrically conductive material suitable for use with this invention include, without being restricted thereto, intrinsically conductive polymers (ICP'S) which may be doped or undoped with an electrically conductive material, alloys of metals for example alloys of gold, iridium, platinum, rhodium, palladium, silver, copper, nickel, zinc, tungsten, titanium, aluminum, tin, but the pure metals may be used as well. Preferred electrically conductive materials include stainless steel alloys, more preferably austenitic stainless steels and duplex stainless steel. Duplex stainless steels typically contain approximately 22-25 wt. % of chromium and 5 wt. % of nickel, some molybdenum and nitrogen. Duplex steels may be preferred for their high yield strength and stress corrosion cracking resistance to chloride, when compared to austenitic stainless steels. The latter may be preferred for their superior corrosion resistance over duplex stainless steel, and because of their low magnetic properties. Austenitic stainless steels and duplex stainless steel are preferred because of their good corrosion resistance, and minimal magnetic properties. Intrinsically conductive polymers conduct electricity due to the intrinsic nature of its chemical structure. Examples of intrinsically conductive polymers suitable for use with this invention include poly(3,4-ethylenedioxythiophene) (PEDOT), polypyrrole, polyaniline (PANI), polyacetylene, poly-p-phenylene, poly-p-phenylene ethylene, polythiophene, polyfluorene.

[0073] The skilled person will be capable of selecting the most appropriate electrically conductive material taking into account the desired chemical resistance in the reaction medium, and/or the desired electrochemical stability and corrosion resistance, the envisaged operational conditions, in particular the operational voltage range, current range, composition of the electrolyte in particular the pH and ions present in the reaction medium.

[0074] Suitable electrically conductive materials preferably offer a tensile strength that is sufficiently high, to offer the desired strength/hardness/stiffness/resistance to deformation and minimize the risk to deformation during use, also in case the temperature of the material of the current density distributor raises. Therefore, preferred electrically conductive materials preferably offer a tensile strength of at least 120 MPa, more preferably higher than 190 MPa, most preferably higher than 300 MPa, in particular at least 450 MPa. With ultimate tensile strength or tensile strength at break is meant, the force per unit area (MPa or psi) required to break a material under tensile stress measured according to ISO 527 or ASTM D638. The yield strength or yield point is defined as the stress at which a material begins to deform plastically.

[0075] Within the scope of this invention, electrically insulating materials are understood to comprise those material which have an electrical resistance at 20 C. which is higher than 10E12 Ohm.Math.m, preferably higher than 10E16 Ohm.Math.m.

[0076] A wide variety of electrical insulating materials are suitable for use with the present invention, for example polyaramides, polyesters, polyolefins, polyetherketone, polyphenylene sulfide (PPS), polyether imide (PEI) etc. Suitable examples of polyolefins include UHMWPE, HDPE, polypropylene or the like. A suitable example of a polyester includes polyethylene terephthalate (PET). Other suitable materials include fluoroplastics for example PTFE, ETFE, FEP, PFA, EFEP, PCTFE, aliphatic polyamides. The afore-mentioned materials may be used alone or in any combination. The electrically insulating path may in addition comprise fibers made of glass, ceramics, mineral fibers, carbon fibers etc. in an appropriate ratio to the above-mentioned plastic fibers. Therefore a suitable material for use as electrically insulating path is a monofilament or thread made of composite material comprising a polymeric matrix of one or more of the polymers indicated above and a reinforcing component, such as glass fiber, ceramic fiber, carbon fiber distributed therein.

[0077] Preferred electrically insulating materials are those which offer a sufficiently high tensile strength, in combination with a sufficient compression resistance, to minimize the risk to deformation of the current density distributor during use. Preferred materials are those which offer a tensile of at least 15 MPa, more preferably at least 40 MPa, most preferably at least 55 MPa, in particular at least 100 MPa. Preferred materials offer a compression strength which is higher than 20 MPa, more preferably higher than 55 MPa, most preferably higher than 80 MPa, in particular higher than 100 MPa. With tensile strength or tensile strength at break is meant, the force per unit area (MPa or psi) required to break a material under tensile stress measured according to ASTM D882 or ASTM D412, depending on the nature of the material. The paths of electrical insulator preferably have an average cross section of 50 m to 250 m, preferably 60-200 m, more preferably 60-150 m.

[0078] Thus, preferably the current density distributor is selected from the group consisting of a metal type, a metal-plastic type, a mixed metal type, a mixed metal-plastic type, a mixed metal-metal type, a mixed metal-metal-plastic type a mixture of metal and other non-metallic conductive (such as conductive polymers and conductive carbon e.g. graphite, grapheme and carbon nano-tubes), or non-conducting material (such as plastic), a porous electrically conductive web and any combination thereof.

[0079] According to another preferred embodiment of the present invention, the current density distributor may further comprise a porous electrically conductive web, for example a carbon electrode e.g. a pitch-bonded carbon electrode or a carbon foam, a metal-cast electrode or a metallic electrode e.g. a punched metal sheet, a woven metallic material, a metallic mesh, a metallic screen, a metal gauze, a metallic foam or an electrically conducting net or array. The selection of material for the porous electrically conductive web is dependent upon the requirements of the application. Suitable materials include stainless steel and nickel. The pore size in the porous electrically conductive web is preferably in the range of 149 m to 840 m. According to another preferred embodiment, the current density distributor further comprises a chemically or thermally treated porous electrically conductive web e.g. to avoid corrosion in strongly acidic electrolytes.

[0080] According to still another preferred embodiment of the invention, the current density distributor further comprises a porous electrically conductive web in direct contact with the current density distributor, which is an electrically conductive material with even higher electric conductivity that will take away or bring the electron from and to the porous electrically conductive web.

[0081] The present invention provides a current density distributor which provides an optimum compromise between maximizing the current flow over substantially the entire surface area of the current density distributor, optimizing the current distribution over the surface of the current density distributor taking into account the spaces of open area in the current density distributor on the one hand and keeping the material cost and weight of the current density distributor as low as possible on the other hand, while still guaranteeing a sufficient mechanical and dimensional stability. The present invention provides a current density distributor which is capable of providing an improved electrical current density distribution, since its configuration permits reducing internal current losses which would be due to ohmic polarization.

[0082] The present invention also relates to a gas diffusion electrode comprising at least one current density distributor as described above or a plurality of such current density distributors, wherein the electrode may either be an anode or a cathode. To one face of the current density distributor an active layer is arranged, a hydrophobic or water repellant layer is arranged to the opposite face of the current density distributor. According to the present invention either the entire face of the current density distributor may be covered by the active layer, respectively the hydrophobic layer, or only part of it. Similarly, the current density distributor of the present invention may be arranged to part of the surface of the active layer, but is preferably arranged to the entire surface of the active layer. Water Repellent Layer (WRL), is understood to comprise a layer having an external surface in contact with gas with porosity to the gas and the capability of preventing leakage of an aqueous electrolyte to the external surface of the gas diffusion layer. Electrochemically Active Layer is a layer in which the electrochemical reaction takes place having high electrical conductivity and porosity to gas and electrolyte and having an interface with electrolyte on one surface and a water repellant (hydrophobic gas diffusion) layer on the other.

[0083] The invention is further elucidated in the examples and comparative examples below.

[0084] The examples below make use of an electrode and a current density distributor with a square geometry as shown in FIGS. 3A-3B and an electrochemical half cell containing such an electrode. The electrochemical half cell contains a current density distributor. The current density distributor comprises a square porous web comprising a plurality of wires of an electrically conductive material which extend in z direction of the mesh, and a plurality of wires of an electrically conductive material which extend in x direction of the mesh. The mesh is made of a plurality of electrically conductive wires with an average cross section of 0.25 mm The dimensions of the current density distributor in x and z direction are respectively 10 cm10 cm, and 0.25 mm in y direction. The current density distributor is mounted into an electrically conductive frame. The current is assumed to flow in the direction indicated by the arrow, the z direction. A lug of an electrically conductive material is mounted to the side of the frame along which electrical current is fed to the current density distributor. The dimensions of the lug in respectively x and z direction are 11 cm1 cm, and 0.25 mm in y direction. The electrolyte is in full contact with the electrochemically active layer, thus having dimensions of 10 cm10 cm (x and z directions), with a thickness of 0.5 cm (y direction). The thickness of the electrochemically active layer was set at 0.25 mm, to match that of the current density distributor.

[0085] To on one face of the current density distributor, a layer of an electro catalytically active material has been arranged, in particular a matrix of a porous active carbon material. The active material extends partly in the pores of the porous current density distributor. The active layer is considered to be in direct contact with the electrolyte. In some embodiments, the catalytically active material is selected from the group of one or more micro-organisms, one or more enzymes, one or more organo-metallic compounds, one or more inorganic and/or organic compounds.

[0086] The current density distribution over the current density distributor is calculated using the COMSOL Multiphysics primary current distribution premises available from Comsol (The Netherlands). The following assumptions have been made: the passage of current through the current density distributor is controlled by the ohmic resistance of the current density distributor. The electrode and current density distributor which are adjacent to the electrolyte are considered to be equipotential surfaces, it is assumed that the concentration of ions within the electrolyte is uniform. Activation over-potentials due to electrode kinetics, and concentration over-potentials due to mass transfer have been neglected. The electrodes are considered as perfect electric conductors, the electrode potential (.sub.M) is assumed to be constant. The outer surface of the electrodes is considered to be insulating (.sub.s=0). The conductivity of the electrolyte () and the electrolyte potential over the electrodes (.sub.S) are assumed to be constant. The electrode is considered to be out of equilibrium, this is E>E.sub.eq. Both electrolyte and electrode obey Ohm's Law (j=.sub.s, where j refers to the current density).

[0087] The stationary model considers the solution of the primary current density distribution at the interfaces concerning the current density distributor, the electrochemically active layer and the electrolyte.

[0088] It is assumed that a discharge current of 20 mA is applied through the lug. The equilibrium potential .sub.S+.sub.M=0.7 V. This condition is effective at the boundary between the electrochemically active layer and the current density distributor. The electrical conductivity of the current density distributor is assumed to be 4.810.sup.6 S m.sup.1, this value is representative for a current density distributor made of good electrically conducting materials such as stainless steel, copper, austenitic steel, etc. The electrical conductivity of the electrochemically active layer is 9.510.sup.2 S m.sup.1 for, this value is representative for semiconducting materials such as graphite, activated carbon and other forms of amorphous carbon. The conductivity of the electrolyte is 10 mS cm.sup.1.

EXAMPLE 1

Impact of the Number of Electrical Conductors Per Length Unit in on the Current Distribution Density Over the Current Density Distributor of a Porous Electrode

[0089] Example 1 shows the impact of the number of electrical conductors per length unit in x direction, i.e. the direction perpendicular to the direction of the main current flow, on the current distribution density profile of a porous electrode, using computational modeling based on a three-dimensional model.

[0090] In z direction, 10 wires were evenly distributed over a length of 10 cm. The wires were positioned at a regular distance from each other. The number of electrically conductive wires in x direction perpendicular to the main current flow was varied as follows: respectively 10, 15, 20 and 30 wires were provided over a length of 10 cm. The wires were positioned at a regular distance from each other, so that a mesh with a regular rectangular pattern was obtained.

[0091] FIGS. 4A, 5A, 6A, and 7A respectively show the electric current density distribution corresponding to a current density distributor having 10 electrically conductive wires in z direction, and respectively 10, 15, 20 and 30 electrically conductive wires in x direction (perpendicular to the main flow of electric current), evenly distributed over a length of 10 cm. FIGS. 4B, 5B, 6B, and 7B show the current density profile along the z-axis of the electrochemically active layer, corresponding to the current density distributors of respectively FIGS. 4A, 5A, 6A, and 7A.

[0092] From FIGS. 4B, 5B, 6B, and 7B it can be appreciated that the number of electrically conductive wires present per length unit in a direction perpendicular to the main current flow, does not have a significant impact on current density or its distribution over the electrode.

EXAMPLE 2

Impact of the Number of Electrical Conductors Per Length Unit in on the Current Distribution Density Over the Current Density Distributor of a Porous Electrode

[0093] The number of wires per length unit in the current density distributor was varied in z direction, in particular a direction parallel to the main current flow as follows: respectively 15, 20 and 30 wires were evenly distributed over a length of 10 cm. In x direction, 10 wires were evenly distributed over a length of 10 cm.

[0094] FIGS. 8A, 9A, and 10A respectively show a current density distributor having 10 electrically conductive wires in x direction (perpendicular to the main flow of electric current), evenly distributed over a length of 10 cm, and respectively 15, 20 and 30 electrically conductive wires in x direction (perpendicular to the main flow of electric current), evenly distributed over a length of 10 cm.

[0095] FIGS. 8B, 9B, and 10B show the current density profile along the z-axis of an electrochemically active layer, corresponding to the current density distributors of respectively FIGS. 8A, 9A, and 10A.

[0096] From FIGS. 8B, 9B, and 10B it can be appreciated that varying the number of electrical conductors per length unit in a direction parallel to the main current flow (z direction), has a significant impact on current density distribution across the electrode. The comparison of FIG. 8B with FIG. 9B shows that increasing the number of electrically conductive wires in z direction, parallel to the main current flow, improves the homogeneity of the current density distribution over the current density distributor. Thus, a larger area of the electrochemically active layer located on other positions of the electrode will have a higher current density, and the voltage drop across the electrode may be reduced. The smaller voltage drop across the electrode presents the advantage that when used in a chemical reaction, the kinetic reaction rate and reaction selectivity may be improved over electrodes with a larger voltage drop across the electrode.

[0097] From FIGS. 8A and 8B it can be observed that with a smaller number of wires per length unit in a direction parallel to the main direction of the current flow, the current density magnitude in z direction is higher in some areas of the electrode (e.g. 50 A m.sup.2) than in other areas. In practice this means that a large area of the electrochemically active layer located on the opposite side of the electrode will have to operate at rather low current densities (e.g. at z=0). This effect is associated with a large voltage drop across the electrode. The large voltage drop across the electrode presents the problem that when used in a chemical reaction, the kinetic reaction rate and reaction selectivity will be inferior in comparison to an electrode with a smaller voltage drop across the electrode.

EXAMPLE 3

[0098] Example 2 was repeated, with a current density distributor having electrically conductive wires both in x and z direction. In both x and z direction two types of electrical conductive wires were used, i.e. wires having a smaller cross section and wires having a larger average cross section (see FIG. 11A). FIG. 11A shows the example in which the thicker wires in x and z direction both have an average cross section of 2 mm, and the smaller wires have an average cross section of 0.25 mm FIGS. 12A and 13A show the example in which the thicker wires in x direction perpendicular to the major current flow have an average cross section of 2 mm, while the thicker wires in z direction parallel to the major current flow have an average cross section of respectively 1 mm and 0 5 mm, and the smaller wires in x and z direction both have an average cross section of 0.25 mm The current density profile is shown in FIGS. 11B, 12B, and 13B.

[0099] From this model, it may be inferred that replacing of the electrically conductive wires in a direction perpendicular to the main current flow, by wires made of an electrical insulator has a similar effect on the current density distribution and the current density profile along the z-axis, than removing of the electrically conductive wires. The model herewith shows that the thicker wires in x direction perpendicular to the main current flow direction are not indispensable in the current flow over the electrode and that they can be removed if a current density distributor and electrode of reduced weight and material cost is envisaged, while maintaining a comparable electrochemical performance. However, replacing of the electrically conductive wires by wires made of an electrical insulator, will improve the mechanical and dimensional stability when compared to the situation where the electrical insulator threads would simply be removed.

EXAMPLE 4

[0100] Example 3 was repeated, with the thicker wires in x and z direction both having an average cross section of 2 mm, and the smaller wires have an average cross section of 0.25 mm The average cross section of the thicker electrically conductive wires in z direction was varied as follows: FIG. 14A 2mm, FIG. 15A 1 mm, FIG. 16A 0.5 mm

[0101] From the electric density profiles shown in FIGS. 14B, 15B, and 16B it can be inferred that increasing the average cross section of the thick wires in z direction along the direction of main current flow, significantly improves the performance of the current density distributor. Therefore, intercalation of plastic wires to replace the thick metal wires in z direction is only advisable in cases where the product selectivity or performance is not appreciated over electrode cost and weight.

[0102] As per the model results, as the thick wires in x direction are not indispensable to distribute the current through the electrode, they could be removed providing a current density distributor and therefore an electrode that is lighter in weight and less expensive while maintaining a comparable electrochemical performance, in analogous way to the results encountered in example 2.