BIPOLAR SURFACE ELEMENT

Abstract

A bipolar flat element comprising a coating that contains expanded graphite and a binder, the coating being applied to at least one of the two primary surfaces of a flat, electrically conductive element.

Claims

1-15. (canceled)

16. A bipolar flat element comprising a coating that comprises expanded graphite and a binder, wherein the coating is applied to at least one of two primary surfaces of a flat, electrically conductive element.

17. The bipolar flat element according to claim 16, wherein the flat, electrically conductive element is a metallic flat element.

18. The bipolar flat element according to claim 16, wherein the flat, electrically conductive element is a graphite-comprising flat element.

19. The bipolar flat element according to claim 18, wherein the flat element comprises expanded graphite.

20. The bipolar flat element according to claim 18, wherein a mass fraction of binder comprised in the coating is higher than a mass fraction of binder of the flat element.

21. The bipolar flat element according to claim 16, wherein an area-specific volume resistivity of the bipolar flat element is at most 20 mΩ•cm.sup.2.

22. The bipolar flat element according to claim 16, wherein the binder comprises thermoplastics and/or thermosets.

23. The bipolar flat element according to claim 16, wherein the binder comprises a silicon compound comprising a moiety R, wherein R stands for —Si(OR.sup.1)(OR.sup.2)(OR.sup.3), —O—Si(OR.sup.1)(OR.sup.2)(R.sup.3), or —O—Si(OR.sup.1)(OR.sup.2)(OR.sup.3), wherein R.sup.1, R.sup.2 and R.sup.3 are moieties each bonded via a carbon atom.

24. The bipolar flat element according to claim 16, wherein the coating comprises a dispersing agent.

25. The bipolar flat element according to claim 16, wherein the thickness of the coating is in the range of from 5 to 500 .Math.m.

26. The bipolar flat element according to claim 16, wherein regions in the coating comprising the expanded graphite have an average length parallel to the surfaces of the coating that is at least twice as large as their average thickness.

27. The bipolar flat element according to claim 16, wherein, in the coating, the ratio Qs, which is calculated according to the following equation: ${\text{Q}}_{\text{S}}=\frac{{\text{m}}_{\text{SG}}}{{\text{m}}_{\text{SR}}}$ wherein m.sub.SG stands for the mass of the graphite comprised in the coating, and m.sub.SR stands for the mass of the non-volatile coating components comprised in the coating, is at least 0.25.

28. The bipolar flat element according to claim 16, obtainable by applying a coating agent to a flat, electrically conductive element, wherein the coating agent comprises expanded graphite and a binder.

29. A fuel cell or redox flow battery, having a bipolar flat element according to claim 16.

30. A method for producing a bipolar flat element, wherein a coating agent comprising expanded graphite and a binder is applied to a flat, electrically conductive element.

31. The bipolar flat element according to claim 19, wherein a mass fraction of binder comprised in the coating is higher than a mass fraction of binder of the flat element.

Description

[0075] FIGS. 1 and 2 show particle size distributions of expanded graphite present in the form of particles.

EXAMPLES

Production of a Water-Based Graphite Dispersion

[0076] To produce a water-based graphite dispersion, 1.5 g of the dispersing agent polyvinylpyrrolidone (PVP) and 0.75 g of benzoic acid were dissolved in 1.4 L of the diluent, water. 232.5 g of expanded graphite were added to the solution and dispersed therein by means of ultrasound. The total energy input was about 4.5 kWh.

[0077] The particle size distribution of the water-based graphite dispersion was measured. The distribution is shown in FIG. 1.

Production of a Premix

[0078] The water-based graphite dispersion was dried at 100° C. for 24 h. An easily (re)dispersible premix was obtained. This contained expanded graphite in the form of particles, and about 0.65 wt.% of the dispersing agent polyvinylpyrrolidone (PVP), and a small amount of benzoic acid.

Production of a Coating Agent

[0079] A solution of polyvinylidene fluoride/hexafluoropropylene copolymer (PVDF/HFP), as a binder was produced in a diluent (acetone) (9 wt.% PVDF/HFP in acetone). The solution was added to the premix and the premix was redispersed in the solution by ultrasonic treatment for 15 minutes.

Mass Fractions of the Coating Agent

[0080] PVDF/HFP: 7.8% [0081] Expanded graphite: 5.2% [0082] PVP: 0.09% [0083] small amount of benzoic acid

[0084] The particle size distribution of the coating agent was measured. This is shown in FIG. 2. The particle size distributions shown in FIGS. 1 and 2 were determined using a Shimadzu SALD-7500 measuring apparatus with batch cell by laser diffraction, in accordance with ISO 13320-2009.

[0085] Steel sheets and foils were coated with the coating agent.

[0086] It was also possible to produce free-standing, thin graphite coatings. For this purpose, a separating coating was first applied to a metal foil. The coating agent was then applied to the metal foil and the resulting coating was then carefully peeled off.

Production of a First Bipolar Flat Element According to the Invention

[0087] A coating agent containing 5.5 wt.% expanded graphite, 8 wt.% PVDF/HFP in the diluent acetone was prepared as described above. A metal foil having a thickness of 0.1 mm was coated on both sides with the coating agent, to a thickness of about 200 .Math.m. The coated metal foil was then embossed at 200° C. using an embossing tool. As a result, an embossed flow field could be created in the applied coating without deforming the metal foil. The depth of the channels was about 100 .Math.m.

Production of a Second Bipolar Flat Element According to the Invention

[0088] A metal foil having a thickness of 0.1 mm was coated on both sides with the coating agent, to a thickness of about 100 .Math.m. The coating agent used contained 5.5 wt.% of expanded graphite and 15 wt.% of PVDF/HFP in the diluent acetone. A second coating agent was then applied on both sides to a thickness of approx. 400 .Math.m. The coating agent used contained 15 wt.% of expanded graphite and 8 wt.% of PVDF/HFP in the diluent acetone. The metal foil coated in multiple coatings in this way was then embossed at 200° C. with an embossing tool. As a result, an embossed flow field could be created in the applied, multilayer coating without deforming the metal foil. The depth of the channels was about 350 .Math.m.

Production of a Third Bipolar Flat Element According to the Invention

[0089] A graphite foil having a density of 0.3 g/cm.sup.3 and a thickness of 2 mm was coated with a coating agent. The coating thickness was 100 .Math.m on both sides. The coating agent contained 5.5 wt.% expanded graphite, 8 wt.% PVDF/HFP, in the diluent acetone. It was made as described above. The graphite foil coated in this way was then embossed at 200° C. using an embossing tool. This made it possible to produce a sealed, embossed pattern.

[0090] Further tests showed that the coating agents can be calendered. A coating agent was applied to a metal foil with a doctor blade height of 300 .Math.m. The coating was then compressed to a thickness of just 25 .Math.m by calendering the composite coating. Metal and graphite foils can be coated on an industrial scale with the coating agents according to the invention in order to produce bipolar flat elements for fuel cells and redox flow batteries.