Electrode assembly and method for its preparation

Abstract

The invention provides electrodes suitable for use as air electrodes, processes for their preparation and metal/air cells utilizing such electrodes as air cathodes. The invention relates to an electrode comprising a catalytically active layer applied on one face of a hydrophobic porous film and a conductive current collector pressed onto said catalytically active face, wherein at least a portion of the marginal area of said face is free from catalyst, and wherein a sealant is provided around at least part of the perimeter of said catalytically active layer, said sealant forming a coating onto the catalyst-free marginal area of said hydrophobic film.

Claims

1. An electrode assembly comprising an electronically conductive metal frame surrounding the perimeter of a perforated, essentially planar member having an electrode material applied within pores thereof, thereby providing a central electrochemically active region, said assembly further comprises a hydrophobic film attached to one face thereof, wherein said frame consists of a flat, non-folded border that is co-planar with the perforated member, and wherein a gap which contains a sealant separates the metal frame and the central electrochemically active region, such that said electrochemically active region is not in contact with said frame.

2. An electrode assembly according to claim 1, wherein the frame is welded or soldered to said perforated member.

3. An electrode assembly according to claim 1, wherein the electrode material comprises a catalyst for promoting oxygen reduction and a hydrophobic binder, wherein the weight ratio between the catalyst and the binder is not less than 6:1.

4. A process for preparing a curved electrode assembly, comprising providing an essentially planar current collector consisting of a perforated member having the shape of a either a rectangle or a square and a conductive metal frame surrounding three sides of said member, introducing a catalyst composition into pores of said perforated member, applying an aqueous electrolyte-impermeable film or layer onto one face of said current collector either before, after or concurrently with the introduction of said catalyst, sintering the resultant electrode assembly, and turning the essentially planar electrode assembly into a curved spatial body, with the outer lateral curved surface of said body being said aqueous electrolyte-impermeable film or layer.

5. A cylindrical electrode assembly defined by a lateral surface and two open bases, wherein the outer face of said lateral surface is provided by an aqueous electrolyte-impermeable film or layer, and the inner face of said lateral surface is provided by a perforated metallic member having an electrode material applied within pores thereof, said electrode assembly further comprises electronically conductive metal frames encircling the two open bases of said cylinder and an electronically conductive metal segment extending along said lateral surface in parallel to the axis of said cylinder.

6. An electrode assembly suitable for use as an air electrode, comprising a catalytically active layer and a conductive current collector pressed onto a catalytically active face of a hydrophobic porous PTFE film, wherein at least a portion of the marginal area of said face is free from catalyst, and wherein a sealant is provided around at least part of the perimeter of said catalytically active layer, said sealant forming a coating onto the catalyst-free marginal area of said hydrophobic PTFE film, said sealant penetrating into the porous hydrophobic PTFE film, such that the porosity of the film is partially filled.

7. An electrode assembly according to claim 6, wherein the sealant comprises an epoxy sealant.

8. An electrode assembly according to claim 7, wherein the sealant is present in at least some of the pores of the hydrophobic porous film, as determined by both scanning electron microscopy and energy dispersive x-ray analysis of the lateral side of electrode assembly, revealing variation in fluorine concentration across said lateral side.

9. A process for preparing an electrode assembly, comprising: assembling a hydrophobic film and a catalytically active layer together with a current collector, wherein said catalytically active layer and said current collector are placed on one face of said hydrophobic film and wherein at least part a marginal area of said face is free of catalyst; heat treating the so formed structure; and applying a sealant onto the catalyst-free marginal area of the catalytically active face of said structure to form a sealant coating on the marginal area of the hydrophobic film.

10. A process according to claim 9, wherein the sealant is an epoxy sealant.

11. A process according to claim 10, wherein the epoxy sealant is a two-component system comprising an epoxy base and a hardener, wherein the epoxy base, hardener or both, comprise one or more organic solvents.

12. A process according to claim 10, comprising the step of adding an organic diluant to the epoxy sealant prior to its application.

13. A method for minimizing an electrolyte seepage in metal/air cells, comprising utilizing in said cells an air cathode which comprises a catalytically active layer applied on a central area of an internal face of air permeable hydrophobic PTFE film, such that a marginal area of said internal face is free of catalyst, with current collector metal grid being pressed onto said internal face, wherein a sealant coating is applied on the catalyst-free marginal area of said internal face of said film, said sealant penetrating into the porous hydrophobic PTFE film, such that the porosity of the film is partially filled.

Description

(1) In the drawings:

(2) FIG. 1 illustrates a metal/air cell.

(3) FIG. 2 illustrates the preparation of a current collector.

(4) FIG. 3 illustrates the printing of a catalyst composition.

(5) FIG. 4 illustrates the coupling of a hydrophobic porous film.

(6) FIG. 5 illustrates the preparation of an electrode assembly of the invention, with a sealant coating applied on the catalyst-free marginal area of the hydrophobic film.

(7) FIG. 6 is a SEM image of the lateral side an electrode assembly produced by the process shown in FIG. 5.

(8) FIG. 7 is voltage vs. time plot demonstrating the performance of the electrode assembly produced by the process shown in FIG. 5.

EXAMPLES

Preparation 1

Printable Catalyst Formulation

(9) 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

(10) 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

(11) Preparation of the current collector: Nickel mesh (commercially available from Gerard Daniel Worldwide (0.007 thickness nickel wire, plain weave 200 mesh) is cut with a guillotine knife to form a square shape with 16.5 cm*16.5 cm dimensions. The resultant mesh is thoroughly cleaned with ethanol and metallic particles are blown off from the mesh using air pressure.

(12) A 500 m thick copper sheet is cut using a stamp to remove its central area, producing a square frame with an open area which is slightly smaller than the area of the nickel mesh. The outer and inner dimensions of the frame are (16.5 cm16.5 cm) and (14.5 cm14.5 cm), respectively. An electrical conducing tab of rectangular shape (5*3 cm.sup.2) is provided on one side of the frame. The copper frame is then plated with nickel by means of electroless nickel plating, thereby forming a nickel coating which is about 20 microns thick.

(13) The nickel mesh is then welded to the nickel-coated copper frame by means of point welding.

(14) Preparation of the electrode: The catalyst formulation of Preparation 1 is applied on the current collector as follows. A 200-300 m thick polypropylene sheet, the current collector and a 500 m stainless steel stencil 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 of Preparation 1 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 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 gently from the polypropylene sheet.

(15) Electrode assembly: the electrode and the hydrophobic film are combined as follows. 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.

Example 2

Air Electrode Assembly with a Sealant Layer on the Marginal Area of the Hydrophobic Film

(16) The procedure of Example 1 was repeated. However, at the electrode printing stage, the catalyst-containing formulation was applied onto the current collector to form a centrally placed electrode, with a narrow gap of about 1-7 mm between the inner boundary of the frame and the perimeter of said centrally printed electrode. The following example illustrates the application of a sealant into said narrow gap.

(17) The so formed electrode assembly is placed on a screen printing table, with the side having the PTFE film provided thereon facing the printing table and the opposite side, namely, the electrode side, facing upwardly. A polyester screen of 10-30 mesh, having a suitable open area which essentially coincides in shape and size with the gap located between the catalytic region and the conductive metal frame (the open area of the screen may overlap with the catalytic region by 1-3 mm and may also overlap with the conductive metal frame) is used in order to transfer the sealant into the gap.

(18) A suitable quantity of an epoxy such as DP270 produced by 3M is applied onto the polyester screen. The screen is lowered such that it is situated above the electrode by a distance of 200-400 microns. A 50 durometer squeegee is passed over the screen at a rate of 3-5 cm/sec. The screen is raised and the epoxy-containing electrode assembly is removed from underneath the screen. The epoxy is allowed to gel at room temperature for half an hour to one hour, and then the electrode assembly is placed gently in oven at a temperature of 60 C. for a period of 1 hour, thereby curing the epoxy.

Example 3

Air Electrode Assembly

(19) The procedure of Example 1 was repeated. However, the final stage of assembling the electrode was accomplished through the in-situ formation of a hydrophobic coating onto the catalyst layer (instead of attaching a commercially available hydrophobic film onto the catalyst).

(20) FEP particles (Ultraflon FP-15 produced by Laurel) are added to ethanol at a weight ratio of 1:10. The mixture is vigorously stirred until a homogenous blend is formed. The mixture is then loaded into a spray gun. The opening of the gun is held about 20 centimeters above the surface of the catalyst layer of the electrode, and the coating composition is uniformly sprayed onto the catalytic region.

(21) The electrode assembly is then allowed to dry at room temperature for 30 minutes for solvent removal, followed by oven-sintering for curing the coating at 275 C. for 20 minutes.

Example 4

A Tubular Air Electrode Assembly

(22) A flat rectangular electrode is produced according to the procedures set forth in previous examples, with dimensions of 7 centimeters by 12 centimeters. A frame is attached to three sides of the mesh via point welding, but one of the short sides of the rectangular mesh is without a side frame. The thickness of the nickel coated copper frame is 0.35 mm and its width is frame is 4 mm.

(23) The electrode obtained after the sintering step is rolled to form a cylinder, such that the outer lateral surface of the cylinder is the hydrophobic film. The height and diameter of the tubular structure thus formed are 7 centimeters and roughly 4 centimeters, respectively. The two short sides of the original rectangular electrode, which following the rolling extend in parallel to the cylinder axis, are connected to one another (one side is provided with a nickel coated copper frame while the other side is not). The two sides are welded, e.g., by means of point welding, such that a minimum of four points are welded between the mesh metal and the nickel coated copper frame.

Example 5

Aluminum-Air Battery

(24) An exemplary aluminum-air cell utilizing the air electrode assembly of the invention as a cathode, which cell is suitable for use in electric vehicle, is fabricated as follows:

(25) A flat square block of aluminum anode having area of about 160160 mm and thickness of 10-15 mm, is symmetrically positioned in the space between a pair of air cathodes of the invention that are placed parallel to each other at a distance of about 20 mm from one another, such that the catalyst side of each air cathode is facing the aluminum anode. The electrodes arrangement is mounted within a plastic housing, such that the sides of the air cathodes having the PTFE porous film provided thereon face the air.

(26) The electrolyte used is an aqueous solution of potassium hydroxide (350-500 g/L), which may further comprise efficiency-improving additives, such as stannate salts, glucose, poly-acrylic acid or polyacrylates, etc. The electrolyte is stored in a suitable tank. Typical electrolyte volume is determined by the desired working resource of the system, e.g., approximately 1 L for 500-600 Ah. The Electrolyte is forced to flow in the space between the air cathodes and the aluminum anode at a flow rate 0.05-0.1 L/min under pressure generated by a diaphragm pump.

(27) Typical working temperature lies in the range from 40 to 80 C. The current drawn from the cell is in the range of 100-200 mA/cm2, at voltage 1.0-1.2V.

Example 6

Air Electrode Assembly with a Sealant Layer on the Marginal Area of the Hydrophobic Film

(28) Preparation of the current collector: Nickel mesh (commercially available from Haver & Bocker (nickel 99.2 Nickel wire, 34 mesh, wire thickness 250 M, calendered to 0.23 mm thickness) is cut with a guillotine knife to form a square shape with 16.5 cm16.5 cm dimensions. The resultant mesh is thoroughly cleaned with ethanol and metallic particles are blown off from the mesh using air pressure.

(29) A 500 m thick copper sheet is cut to form a rectangular piece (16.5 cm0.5 cm). An electrical conducing tab (2.5 cm3 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.

(30) The rectangular nickel-coated copper piece is then welded to the edge of the nickel mesh by means of point welding.

(31) Preparation of the electrode: The catalyst formulation of Preparation 1 is applied on the current collector as follows. A 200-300 m thick polypropylene sheet, the current collector and a 500 m stainless steel stencil with a cavity of 15 cm15 cm 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 of Preparation 1 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.

(32) 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.

(33) The electrode is allowed to cool to room temperature prior to the printing of an epoxy sealant. An epoxy mixture prepared from 100 grams of Z-65 base epoxy and 25 grams of HM-Z/H hardener, which is further diluted with 6 g of VD 60 diluent, (the products are available from Coates Screen Ink GmbH) is screen printed through a 10 mesh polyester with a square shape rim of 14.5 cm and 0.7 cm width. The epoxy is printed with a 45 shore polyurethane squeegee onto the catalyst and the overlapping rim of the nickel mesh and the underlying hydrophobic PTFE film. The epoxy is left to gel for one hour and then sintered at 70 C. for a period of one hour.

(34) SEM images were obtained by FEI Inspect SEM (USA) instrument equipped with an Energy-Dispersive X-ray (EDX) spectroscopy. FIG. 6 presents a SEM image of the lateral side of the electrode. As shown in the SEM image obtained, the lateral side of the marginal area of the electrode consists of three distinct layers. The lowermost, highly uniform, cohesive layer is the natural PTFE film. The uppermost cohesive layer is the sealant coating; the small cavities are presumably due to diluent evaporation. The intermediate layer interposed between the PTFE film and the epoxy coating exhibits hybrid character indicative of the penetration of sealant into the porous PTFE film. EDX analysis shows that the highest fluorine content (indicative of the PTFE film) is in the lowermost section of the film.

Example 7

Testing the Performance of the Electrode

(35) The air cathode of Example 6 was utilized in a half cell 3-electrode setup described below.

(36) The air cathode and nickel electrode are spaced 2 cm apart and connected to the positive and negative poles of a power supply with a suitable internal load. The two electrodes are of approximately the same geometrical form and size. The nickel electrode is of 99.5% purity and is 400 m thick. The reference electrode consists of a luggin capillary with a zinc wire. An aqueous potassium hydroxide solution (30% weight concentration) is held in a storage tank at 60 C.

(37) The experimental conditions were as follows. The current density applied was 175 mA/cm.sup.2 and the electrolyte was circulated through the cell. The spent electrolyte was replaced every day with a fresh electrolyte.

(38) A discharge curve was used to assess the performance of the electrode of the invention, as shown in FIG. 7, where the curve is plotted as voltage versus time, demonstrating a stable electrochemical performance over a long service period.