ELECTRODE ASSEMBLY AND METHOD FOR ITS PREPARATION

Abstract

The invention relates to a metal-air electrochemical cell comprising a frame (100) defining an electrolyte chamber having an anode side and a cathode side, wherein an air cathode assembly is provided in the cathode side, said air cathode assembly (20) comprising hydrophobic porous film having a first face and a second face, with current collector (21) and catalyst-containing active layer (26) provided on said first face, with the planar dimensions of the catalyst-containing active layer on said first face being smaller than that of said hydrophobic film and said current collector, such that the catalyst-containing active layer does not reach the edges of said hydrophobic film and said current collector, thereby creating a catalyst-free margin (27) on the hydrophobic film (31) and current collector which surrounds the catalyst-containing active layer, and wherein said first face of the hydrophobic film and said frame of the cell arm joined together by thermoplastic (101) applied onto the catalyst-free margin of the hydrophobic film. A method of assembling the metal/air cell is also described.

Claims

1. A method for assembling a metal/air cell, comprising the steps of: (i) providing an air cathode assembly that comprises hydrophobic porous film having a first face and a second face, with a current collector and catalyst-containing active layer provided on said first face, with the planar dimensions of the catalyst-containing active layer being smaller than that of said film and the current collector, such that the catalyst-containing active layer does not reach the edges of said film and current collector, thereby creating a catalyst-free margin on the film and current collector, which surrounds the catalyst-containing active layer; (ii) placing said cathode assembly atop of a cell frame defining an electrolyte chamber, with a thermoplastic body that is capable of softening to a mobile, flowable state upon heating to an elevated temperature being positioned between said cell frame and said cathode assembly, wherein the thermoplastic body coincides in geometrical shape and position with said catalyst-free margin area and essentially coincides also in width with said catalyst free-margin; and (iii) applying heat and pressure to soften the thermoplastic body and forcing a thermoplastic melt to flow in the direction of the pores in the catalyst-free margin of the hydrophobic porous film, thereby welding the air cathode assembly and the frame of the cell upon solidification of the melt.

2. A method according to claim 1, wherein the thermoplastic body comprises a raised region of the cell frame protruding from the surface of the frame, said raised region forming an integral part of the frame.

3. A method according to claim 2, which comprises a step of applying hot melt polyolefin onto the surface of the cell frame adjacent to, and in outward direction from, the raised region of the cell frame.

4. A method according to claim 1, wherein the thermoplastic body is polypropylene body.

5. A method according to claim 1, wherein the thermoplastic body softens at a temperature in the range from 120 to 280° C.

6. A method according to claim 1, wherein the air cathode and the frame of the cell are welded by coating of the current collector with the molten thermoplastic and/or by penetration of the molten thermoplastic into the pores of the hydrophobic film in the catalyst-free margin.

7. A method according to claim 1, wherein the hydrophobic film is PTFE film.

8. A metal-air electrochemical cell comprising a frame defining an electrolyte chamber having an anode side and a cathode side, wherein an air cathode assembly is provided in the cathode side, said air cathode assembly comprising hydrophobic porous film having a first face and a second face, with current collector and catalyst-containing active layer provided on said first face, with the planar dimensions of the catalyst-containing active layer on said first face being smaller than that of said hydrophobic film and said current collector, such that the catalyst-containing active layer does not reach the edges of said hydrophobic film and said current collector, thereby creating a catalyst-free margin on the hydrophobic film and current collector which surrounds the catalyst-containing active layer, and wherein said first face of the hydrophobic film and said frame of the cell are joined together by thermoplastic applied onto the catalyst-tree margin of the hydrophobic film.

9. A metal-air electrochemical cell according to claim 8, wherein the thermoplastic coats the current collector and/or penetrates to the pores of the hydrophobic film in said catalyst-free margin.

10. A metal-air electrochemical cell according to claim 8, wherein the thermoplastic comprises polypropylene.

11. A metal-air electrochemical cell according to claim 10, wherein the thermoplastic comprises polypropylene and a second polyolefin.

12. A metal-air electrochemical cell according to claim 8, wherein the hydrophobic porous film is PTFE fil.

13. A metal-air electrochemical cell according to claim 12, characterized in that EDX spectroscopy indicates variation in the relative proportion between fluorine and carbon across the thickness of the PTFE film in said catalyst-free margin area, identifying segments in the film where carbon and fluorine are the major and minor chemical elements, respectively.

14. An electrode assembly comprising PTFE film having a first face and a second face, with current collector and catalyst-containing active layer provided on said first face, with the planar dimensions of the catalyst-containing active layer being smaller than that of said PTFE film and said current collector, such that the catalyst-containing active layer does not reach the edges of the PTFE film and current collector, thereby creating a catalyst-free margin on the PTFE film and current collector which surrounds the catalyst-containing active layer, characterized in that a thermoplastic is applied onto said first face in the catalyst-free margin area, and further characterized in that EDX spectroscopy indicates variation in the relative proportion between fluorine and carbon across the thickness of the PTFE film in said catalyst-free margin area, identifying segments in the film where carbon and fluorine are the major and minor chemical elements, respectively.

Description

EXAMPLES

Preparation 1

Printable Catalyst Formulation

[0069] Silver catalyst (70 grams; prepared as described in U.S. Pat. No. 8,142,938) is mixed with 10 grams of FEP (available from Dupont in the form of aqueous dispersion, TE-9568). The mixture is placed in a rotating shaker for 1 hour. Water (20 grams) and isopropanol (20 grams) are then added to the mixture and the shaker is allowed to rotate for an additional period of 25 minutes. The resultant composition is allowed to stand for 1 hr and is then remixed for 25 minutes. The resultant composition exhibits good flowability and thixotropicity and is suitable for use as a printable material.

Preparation 2

Printable Catalyst Formulation

[0070] Silver catalyst (70 grams; prepared as described in U.S. Pat. No. 8,142,938) and 70 grams of nickel-coated carbon particles (60% w/w, such as E-2701 or E-2702 commercially available from Sulzer) are mixed with 14 grams of FEP (available from Dupont in the form of aqueous dispersion, TE-9568). The mixture is placed in a rotating shaker for 1 hr. Water (40 grams) and ethanol (40 grams) are then added to the mixture and the shaker is allowed to rotate for additional 25 minutes. The resultant composition is allowed to stand for 1 hour and is then remixed for 25 minutes. The composition thus formed exhibits good flowability and thixotropicity and is suitable for use as a printable material.

Example 1

Air Electrode Assembly with a Thermoplastic Applied onto the Margin Area of the PTFE Film

[0071] Preparation of the current collector: Nickel mesh (commercially available from Haver & Bocker (nickel 99.2 Nickel wire, 34 mesh, wire thickness 250 μM, calendered to 0.23 mm thickness) is cut with a guillotine knife to form a pentagon-shaped mesh (the shape is shown in FIG. 5; a=22.5 cm, b=18.5 cm, c=13.5 cm, d=10.0 cm and e=5.0 cm). The resultant mesh is thoroughly cleaned with ethanol and metallic particles are blown off from the mesh using air pressure.

[0072] A 500 μm thick copper sheet is cut to form a pentagon shaped frame (element 22 in FIG. 5). An electrical conducting tab 24 (2.5 cm×3 cm) is attached to one side of the copper piece. The copper piece is then plated with nickel by means of electroless nickel plating, thereby forming a nickel coating which is about 20 microns thick.

[0073] The nickel-coated copper pentagon-shaped frame 22 is then welded to the edge of the nickel mesh by means of point (resistance) welding.

[0074] Preparation of the electrode: The catalyst formulation (see Preparations) is applied on the current collector as follows. A 200-300 μm thick polypropylene sheet, the current collector and a ˜500 μm stainless steel stencil with a suitably sized cavity (to enable the formation of a catalyst-free margin area) are placed on a printing machine (Ami Presco model MSP-9155) one on top of the other, such that the polypropylene sheet and the stainless steel stencil constitute the lowermost and uppermost layers, respectively. The catalyst formulation is then applied using a blade or squeegee which is passed above the stencil such that the silver catalyst formulation penetrates through the stencil into the pores of the current collector mesh. The stainless steel stencil is then removed, and 10 sheets of standard A4 paper are placed above the current collector and the stack is transferred to a press in which a 10 ton pressure is applied. The papers are carefully peeled off the electrode and the electrode is then detached from the polypropylene sheet.

[0075] Electrode assembly: the electrode and the hydrophobic film are combined as follows. The outer perimeter of the Haber & Bocker mesh are coated with an aqueous form of FEP such as TE9568 or FEPD121 produced by DuPont via a thin paint brush and the emulsion is allowed to dry for 10 minutes. A porous hydrophobic PTFE film (manufactured by Saint Gobain or Gore) which is slightly larger than the electrode is placed above the electrode and a pressure of 10 tons is applied using a press. The electrode assembly is then oven-sintered at about 280° C. for a period of about 20 minutes. In order to prevent the membrane from shrinking or detaching from the mesh a heavy external metallic frame that coincides with the external area of the current collector is placed on the mesh and membrane thus reducing membrane detachment during the 280° C. sintering process.

[0076] The electrode is allowed to cool to room temperature prior to welding to the cell frame.

[0077] Welding the Air Cathode to the Cell Frame

[0078] The set-up is shown in FIG. 5. An electrode assembly (20) with an exposed area of PTFE is placed on a thermoplastic frame of polypropylene (100). The face of the electrode assembly with the catalyst layer applied thereon faces the thermoplastic frame and hence not shown in FIG. 5; towards the outer side is the PTFE film. The frame may be filled with a plastic or metal insert such that the plastic does not buckle in when exposed to high pressure and temperature.

[0079] Above the electrode is placed a Teflon based sheet [available from AMI-TUF® (TGL) PTFE Coated Cloth of 0.012 inch thickness, to prevent adhesion of the hot shoe to the PTFE film of electrode assembly (20)]. The heat press (40) with the hot shoe (50) is heated to 260° C. and the shoe is applied with a pressure of 10-15 kg per cm.sup.2. The pressure is held for 30 seconds. The heat head is lifted and a brass slab of 4 mm thickness with the exact dimensions of the electrode is placed on the electrode for a period of 1-3 minutes until electrode temperature has reached below 80° C.

Example 2

Air Electrode Assembly with a Thermoplastic Applied onto the Margin Area of the PTFE Film

[0080] The procedure of Example 1 was repeated, with the exception of the last step, which was carried out as described below.

[0081] Welding the Air Cathode to the Cell Frame

[0082] The set-up is shown in FIG. 5. An electrode assembly (20) with an exposed area of PTFE is placed on a thermoplastic frame of polypropylene (100). The face of the electrode assembly with the catalyst layer applied thereon faces the thermoplastic frame and hence not shown in FIG. 5; towards the outer side is the PTFE film. The frame may be filled with a plastic or metal insert such that the plastic does not buckle in when exposed to high pressures and temperatures.

[0083] A polyolefin hot melt bead is applied onto the external circumference of the frame so that the polypropylene elevated area (101) is inwards and more outwards is a hot melt ring, as shown in FIG. 6 (the rib indicates the polypropylene wall protruding from the surface of the frame). This structure provides a combination of a stronger mechanical welding along with an additional leak proof barrier.

[0084] Above the electrode is placed a Teflon based sheet [available from AMI-TUF® (TGL) PTFE Coated Cloth of 0.012 inch thickness, to prevent adhesion of the hot shoe to the PTFE film of electrode assembly (20)]. The heat press (40) with the hot shoe (50) is heated to 260° C. and the shoe is applied with a pressure of 10-15 kg per cm.sup.2. The pressure is held for 30 seconds. The heat head is lifted and a brass slab of 4 mm thickness with the exact dimensions of the electrode is placed on the electrode for a period of 1-3 minutes until electrode temperature has reached below 80° C.