MANUFACTURING IMPERVIOUS BIPOLAR MATERIALS FROM POROUS GRAPHITE

Abstract

The present invention includes bodies of flexible expanded graphite or of rigid body porous graphite impregnated with blended polymer-wax treatments to create composite bodies that exhibit properties critical in the function of electrochemical systems, and methods of manufacturing the same. High electrical conductivity is an inherent attribute of the untreated graphitic material that is retained through the impregnation process, while attributes of extremely low permeability and high mechanical strength are added to the composite via the polymer-wax blend. In one embodiment of the invention, the attributes of low ionic permeability, high flexural strength, and high electrical conductivity are achieved to create a component that could be useful in Redox Flow Battery (RFB) systems.

Claims

1. A composite material comprising: a porous graphitic material having opposed parallel planar outer surfaces, wherein said porous graphitic material is impregnated with a solidified mixture in an amount of about 0.1-50.0% by weight, wherein the solidified mixture comprises a wax and a thermoplastic polymer that is miscible with the wax of the solidified mixture, and wherein said material composite is configured to prevent the crossover and permeation of ions in an electrochemical system.

2. The composite in accordance with claim 1, wherein the solidified mixture remains a solid below about 65 degrees Celsius.

3. The composite in accordance with claim 1, wherein the solidified mixture remains a solid below about 150 degrees Celsius.

4. The composite in accordance with claim 1, wherein the solidified mixture is layered within the porous graphitic material with a first layer comprising the wax and a secondary layer comprising of the thermoplastic polymer.

5. The composite in accordance with claim 1, wherein the thermoplastic polymer is chosen from the family of thermoplastic polyolefins.

6. The composite in accordance with claim 1, wherein the electrical resistivity from a first planar outer surfaces to an opposed planar outer surface of the porous graphitic material is less than or about equal to 110.sup.3 -m.

7. The composite in accordance with claim 1, wherein the solidified mixture further comprises electrically conductive additives.

8. The composite in accordance with claim 1, wherein the solidified mixture further comprises a wax and a plurality of thermoplastic polymers miscible in the wax.

9. The composite in accordance with claim 1, wherein a bending strength of the composite is greater than 5 MPa.

10. The composite in accordance with claim 1, wherein the wax comprises a paraffin.

11. The composite in accordance with claim 1, wherein the thermoplastic polymer comprises an ethylene vinyl acetate.

12. The composite in accordance with claim 1 wherein the solidified mixture consists of about 70%-90% natural or synthetic wax by mass, about 10%-30% thermoplastic polymer by mass, and about 0.01-10% conductive additive by weight.

13. A method of manufacturing a composite suitable for use as a component in an electrochemical system, comprising the steps of: (a) providing a porous graphitic material having opposed parallel planar outer surfaces; (b) providing a homogeneous mixture of a wax and a thermoplastic polymer miscible in said wax; (c) exposing at least one of the planar outer surfaces of the porous graphitic material to the homogeneous mixture; (d) heating the homogeneous mixture and graphitic material above the melt temperature of the homogeneous mixture; (e) allowing the homogeneous mixture to impregnate the porous graphitic material; and (f) cooling the porous graphitic material below the melting point of the homogeneous mixture.

14. The method of claim 13 further comprising the step of removing excess homogeneous mixture from the porous graphitic material prior to cooling below the melting point of the homogeneous mixture.

15. The method of claim 13 further comprising the step of removing excess homogeneous mixture from the porous graphitic material prior to cooling below the melting point of the homogeneous mixture with an element selected from a flexible blade, a rigid blade, an air knife, a cloth, and a tissue.

16. The method of claim 13 further comprising the steps of reheating the composite to about the melting temperature of the homogeneous mixture after cooling the composite below the melting point of the homogeneous mixture, and buffing clean the composite with an absorbent cloth or a tissue.

17. The method of claim 13 further comprising the step of applying pressure to the homogeneous mixture while it is in contact with the porous graphitic material.

18. The method of claim 13 further comprising the step of applying vacuum to a first opposed planar surface of the porous graphitic material while a second planar surface of the porous graphitic material is in contact with the homogeneous mixture.

19. The method of claim 13 further comprising the step of applying vacuum to a first opposed planar surface of the porous graphitic material for less than ten minutes prior to the step of exposing at least one of the planar outer surfaces of the porous graphitic material to the homogeneous mixture.

20. The method of claim 13 wherein heat is applied first to the homogeneous mixture only, and wherein the porous graphitic material starts on a spool and is then fed into the homogeneous mixture in its liquid phase at a rate of about 0.001 meter/minute to about 10 meters/min, then the porous graphitic material is fed out of the liquid phase and past a blade to remove excess homogeneous mixture, and then followed by cooling the porous graphitic material.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic illustrating a semi-continuous process for producing an impervious bipolar material from flexible expanded graphite foil according to an embodiment of the present invention;

[0022] FIG. 2 is a schematic illustrating a batch process for impregnating porous graphite materials and removal of excess material process treatment according to an embodiment of the present invention;

[0023] FIG. 3 is a schematic cross section of graphite foil illustrating open porosity (left), surface porosity (middle), and internal porosity (right);

[0024] FIG. 4 is a schematic cross section of graphite foil illustrating the impregnation of two kinds of porosity, both rendering the graphite material impervious;

[0025] FIG. 5 is a conceptual illustration of expanded graphite pre- and post-compression showing the residual discontinuity in the graphite material; and

[0026] FIG. 6 is a flow chart showing a process for producing an impervious bipolar material from flexible expanded graphite foil according to an embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0027] The following explanations of terms and abbreviations are provided to better describe the present disclosure and to guide those of ordinary skill in the art in the practice of the present disclosure. As used herein, the singular forms a or an or the include plural references unless the context clearly dictates otherwise. The term or refers to a single element of stated alternative elements or a combination of two or more elements, unless the context clearly indicates otherwise. Unless explained otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present disclosure, suitable methods and materials are described below. The materials, methods, and examples are illustrative only and not intended to be limiting. Other features of the disclosure are apparent from the following detailed description and the claims. Unless otherwise indicated, all numbers expressing quantities of components, percentages, temperatures, times, and so forth, as used in the specification or claims are to be understood as being modified by the term about. Unless otherwise indicated, non-numerical properties such as continuous, homogeneous, and so forth as used in the specification or claims are to be understood as being modified by the term substantially, meaning to a great extent or degree. Accordingly, unless otherwise indicated, implicitly or explicitly, the numerical parameters set forth are approximations that may depend on the desired properties sought and/or limits of detection under standard test conditions/methods. When directly and explicitly distinguishing embodiments from discussed prior art, the embodiment numbers are not approximates unless the word about is recited.

[0028] Referring generally to FIGS. 1-6, in any electrochemical system, multiple cells typically need to be electrically stacked in series with one another in order to achieve a desired electrical potential across the entire system. In order to be cost effective, these electrochemical systems will typically have reaction cells that are physically stacked in such a way as to allow the anode on one cell to be the cathode on the next cell, such that the stack of cells are all electrically in series with one another. In this mode of operation in an electrochemical system, every reactant on an anode side of one cell that leaks into the cathode side of its adjacent cell will result in a loss in efficiency or, in some cases, render the electrochemical system completely inoperable. For example, the component that separates these two cells in a redox flow battery is called a bipolar plate, on account of its need to function as both the cathode and anode for two different cells. An apparent loss for the system is any internal resistance that this component adds to the electrical system, but as presently described, it is equally important for this component, or bipolar plate in the case of redox flow batteries, to be impermeable, or at least functionally impermeable. Achieving complete impermeability of any component is very difficult to impossible and thus currently impractical. The present invention focuses on satisfying these two needs of impermeability and low electrical resistance with regard to this component as used in most electrochemical systems via a composite formed of low cost graphitic material and a wax-polymer mixture impregnated therein. An additional benefit of the present invention is that the high degree of customization that the mixture affords can allow an ordinary practitioner in the art to develop composites with customized material strengths, flexibility, and even chemical resistance to degradation. The variety of products and electrochemical systems that can be supported with this composite are extensive, and as such, a plurality of embodiments are also shared which each describe alternative designs that may be used for satisfying the needs of a plurality of different electrochemical systems. As a form of demonstrating how the invention can be put to practice to satisfy the needs of a vanadium redox flow battery system's bipolar plates, a novel method by which this bipolar composite may be manufactured is also shared.

[0029] Referring to FIG. 1, one embodiment of a continuous process 7 includes unspooling a roll of expanded graphite foil 1 through a set of feed rollers 2 passed through a molten bath of a wax and polymer treatment 3 and is collected by a spool-up on the other end. As the impregnated expanded graphite foil composite 1 departs the molten bath of treatment 3 the composite can be fed through a set of post-treatment calendaring rolls 6 to further improve the impermeability and conductivity of the composite. Furthermore, the composite can then be fed through a blade system 5 to remove the excess treatment 4. The removed excess treatment 4 can be returned to the molten bath of treatment 3.

[0030] Referring now to FIG. 2, one embodiment of a batch process 11 includes immersing pieces of an expanded graphite foil 8 in a molten bath of a wax and polymer treatment 9. The impregnated expanded graphite foil composite 8 is then fed through feed rollers 6 and a blade system 5 to remove the excess treatment 10. The removed excess treatment 10 can then be returned to the molten bath of treatment 9. In some embodiments, the sheets, or pieces of an expanded graphite foil described above are placed on a rack and immersed into a molten bath, having first been evacuated and the void space filled with a reactive gas. In other embodiments, the void space is evacuated and filled with an inert gas phase. In other embodiments, the void space is evacuated only. In other embodiments, the blade system may be manually or autonomously operated to remove the excess treatment from the treated composite.

[0031] FIGS. 3-5 show how the wax and polymer treatments described above fill the porous regions of porous graphitic materials to help seal off the graphitic material to any ions that may try to permeate through the material due to either pressure or electrochemical potential. FIG. 3 specifically shows pores that go all the way through the graphitic material 12, partial pores 13 that are exposed to at least one of the two opposed planar surfaces of the graphitic material, and fully encapsulated pores 14 within the graphitic material that will not fill with treatment, and as such will not provide a leak path for permeation.

[0032] FIG. 4 shows the graphitic material after one of the above treatments is applied thereto. FIG. 4 shows specifically, in cross sectional view, filled through-hole pore with treatment 15, and filled partial pore with treatment 16.

[0033] Referring more specifically to FIG. 5, the vermiform shape of graphite 17 is compressed into expanded graphite foil 18. Expanded graphite foil 18 includes cracks/pores/openings 19 in the calendared graphite that are susceptible leak paths for the catholyte and analyte to leak through during operation, these openings 19 are filled by the present invention. Expanded graphite foil 18 is commonly used as a gasket material that is compressed between two hard surfaces to form a seal. In one embodiment of creating the composite, an expanded graphite foil 18 can have all the void spaces within the foil 19, 12, 13 filled by the wax-polymer treatment 15, 16.

[0034] Referring more particularly to FIGS. 2 and 6, methods of producing impervious bipolar material from flexible expanded graphite foil are described herein. First, the components of the wax-polymer treatment 24 are chosen as paraffin and EVA. In addition to these two components, a conductive additive of activated charcoal is included in the treatment in order to improve electrical conductivity/contact between the composite and the components of the electrochemical system that the composite may come in contact with, including but not limited to the electrodes, the catholyte, the analyte, or the current collectors. The paraffin, EVA, and charcoal are heated beyond the melt temperature of EVA and combined at a ratio of about 80-90%, 10-20%, and 0.1-2% by weight, respectively. The mixture, hereafter referred to as the treatment, is then continually mixed in order to ensure that the activated charcoal remains adequately dispersed throughout the treatment.

[0035] Second, a die-cut or pre-formed sheet of calendared expanded graphite of a density about 0.8-1.5 g/cc and a thickness of about 0.4 mm-0.8 mm 8, 20 is pre-treated by wiping off any detritus from both opposed, planar surfaces of the sheet 21. This ensures that any detritus remnants from the manufacturing process of the sheet are removed from the surface, allowing the treatment to access all the pores of the graphite sheet that have a continuous flow path or access to at least one of the two planar, opposed surfaces. For significantly thick sheets of graphitic material, it stands to reason that the wiping down process shall also encompass the other four sides.

[0036] Third, the said graphite sheet is fully submersed into the mixed liquid bath of the treatment 22, 9. The sheet is allowed to stay inside the heated, mixed treatment bath for at least ten minutes and for as long as two hours, thus allowing sufficient time for the treatment to be drawn into the porous structure of the graphite plate through capillary forces and diffusion.

[0037] Fourth, the newly made composite is withdrawn from the liquid bath and laid upon a heated flat surface elevated to approximately the same temperature as the liquid bath 25. While the composite is on this heated surface, the excess treatment on the surfaces of the graphite sheet can then be wiped off of each side of the sheet with a rubber blade.

[0038] Fifth, the resulting composite can then be removed from the hot plate and placed on a rack where it can be allowed to fully cool 26. The excess treatment that was removed in the fourth step can be returned to the treatment bath for reuse 23.

[0039] Sixth, once cooled, the composite can be moved to a different heated surface that is just at or below the melt temperature of the treatment 26. While on this heated surface, any remaining treatment that was not removed with the rubber blade can be wiped off of both sides of the composite, exposing the graphite structure that constitutes a portion of the composite.

[0040] Finally, the composite can be removed from this last heated plate to yield the final product 27, which can be configured as bipolar plates.

[0041] Although the invention has been described in detail for the purpose of illustration based on what is currently considered to be the most practical and preferred embodiments of the application, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the described embodiment. To the contrary, it is intended to cover modifications and equivalent arrangements that are within the spirit and scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.