BIPOLAR PLATE FOR ELECTROCHEMICAL CELLS AND METHOD FOR THE PRODUCTION THEREOF

Abstract

The invention relates to a metallic bipolar plate for use in an electrochemical cell, wherein the bipolar plate comprises an electrically conductive graphene-like coating. The graphene-like coating has a layer thickness between 10 nm and 1 μm. Chemical synthesis is initially carried out to produce the graphene-like coating according to the invention comprising one or more at least partially reduced graphene oxide layers. Proceeding from graphite powder, a graphite oxide powder is initially produced, which is subsequently converted into a stable graphene oxide (GO) suspension by way of ultrasonic dispersion. By depositing this suspension on a metallic carrier substrate (bipolar plate), thin graphene oxide layers can then be applied and subsequently be reduced to obtain at least partially reduced graphene oxide (rGO), which is referred to as graphene-like. This coating advantageously has sufficient stability and the necessary electrical conductivity for use in an electrochemical cell.

Claims

1. A bipolar plate for use in an electrochemical cell, comprising a metal plate having an electrically conductive graphene-like coating.

2. The bipolar plate according to claim 1, wherein the graphene-like coating comprises one or more graphene oxide layers in at least partially reduced form.

3. The bipolar plate according to claim 1, wherein the graphene-like coating has a layer thickness between 10 nm and 1 μm.

4. The bipolar plate according to claim 1, wherein the graphene-like coating has an electrical conductivity of at least 50 S/cm.

5. A bipolar plate according to claim 1, wherein the metal is selected from the group consisting of: iron-based steels; austenitic stainless steels and alloys having a high content of chromium, nickel and/or molybdenum and additions of niobium, titanium and/or copper, manganese, tungsten, tantalum and vanadium; copper alloys; gold; and platinum.

6. A method for producing a bipolar plate according to claim 1, comprising applying a stable suspension comprising graphene oxide to a metal plate to form at least one graphene oxide layer on the metal plate; and subjecting the at least one applied graphene oxide layer to a reduction step so that an at least partially reduced electrically conductive graphene-like coating is generated on the metal plate.

7. The method according to claim 6, wherein a the stable suspension comprising graphene oxide is applied to the metal plate a plurality of times to form a plurality of graphene oxide layers and each applied graphene oxide layer is subjected to the reduction step before a next graphene oxide layer is applied.

8. The method according to claim 6, wherein the application of the stable suspension comprising graphene oxide takes place by way of a spray, dip or spin coating method.

9. The method according to claim 6, wherein the reduction of the at least one graphene oxide takes place chemically, electrochemically, induced by laser, or thermally.

10. The method according to claim 9, wherein the reduction takes place thermally and temperatures up to a maximum of 500° C. are used for the thermal reduction.

11. The method according to claim 6, wherein the metal is selected from the group consisting of; iron-based steels; austenitic stainless steels and alloys having a high content of chromium, nickel and/or molybdenum and additions of niobium, titanium and/or copper, manganese, tungsten, tantalum and vanadium; copper alloys; gold; and platinum.

12. An electrochemical cell, comprising the bipolar plate according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] FIG. 1a) shows a metallic bipolar plate not having a coating according to the invention;

[0043] FIG. 1b) shows a metallic bipolar plate having an at least partially reduced coating according to the invention;

[0044] FIG. 2 shows a cross-section of a metallic bipolar plate having a graphene-like coating according to the invention;

[0045] FIG. 3 shows a cross-section of a metallic bipolar plate having a thermally reduced graphing oxide coating according to the invention;

[0046] FIG. 4 shows the contact resistance between uncoated and coated bipolar plates and and abutting gas diffusion layer as a function of the pressing pressure; and

[0047] FIG. 5 shows the progression of the free corrosion potential during a temperature increase to 130° C. in a long-term experiment lasting 30 days.

DETAILED DESCRIPTION OF THE INVENTION

[0048] FIG. 1 a) shows a metallic bipolar plate (material 1.4404, which is an austentitic stainless steel) having an embossed flow field without the reduced graphene oxide coating according to the invention.

[0049] In contrast, FIG. 1 b) shows a metallic bipolar plate (material 1.4404) having an embossed flow field comprising an at least partially reduced graphene oxide coating according to the invention. This was applied by way of a spray method using an aqueous graphene oxide suspension having a concentration of 2 mg/ml. The reduction to obtain the reduced graphene oxide layers was carried out thermally, on a heating plate at temperatures up to 500° C. After every spraying operation, the currently applied layer was thermally reduced before the next coating process was carried out. The overall coating thickness of this bipolar plate is 250 nm.

[0050] FIG. 2 shows a cross-section of the metallic bipolar plate (material 1.4404) comprising the graphene-like coating according to the invention. The application and reduction methods (thermally reduced) were analogous to FIG. 1. The overall coating thickness is also 250 nm here. Cross-sections were prepared by way of an ion polishing technique.

[0051] FIG. 3 shows a cross-section of the metallic bipolar plate (material 1.4404) comprising a thermally reduced graphene oxide coating according to the invention. The application and reduction methods are comparable to those of FIG. 1. The layer thickness here is approximately 250 nm. The cross-section was prepared using a scalpel cut. The layer composition of individual reduced graphene oxide layers is clearly apparent.

[0052] FIG. 4 shows the contact resistance between uncoated and coated bipolar plates and the abutting gas diffusion layer (carbon non-woven) as a function of the pressing pressure. The thermally reduced graphene oxide coating according to the invention on material 1.4404 (trGO/1.4404) having a layer thickness of 200 nm advantageously shows a reduction in the contact resistance by more than one order of magnitude compared to a non-reduced graphene oxide coating on 1.4404 (GO/1.4404). For comparison, the contact resistance of an uncoated bipolar plate made of material 1.4404 comprising a surface passivation layer formed naturally when exposed to atmospheric oxygen and a mechanically ablated (polished) surface is shown. The material sample 1.4404 was measured directly after the mechanical polishing step. However, since the surface passivation takes place within a few hours when exposed to atmospheric oxygen or in aqueous oxygen-containing solutions, the material sample 1.4404 comprising the passivation layer shows the contact resistances in fuel cells or electrolyzers to be expected during operation.

[0053] The contact resistance of thermally reduced graphene oxide layers on material 1.4404 (trGO/1.4404) having a layer thickness of ˜100 nm decreases by more than one order of magnitude compared to unreduced graphene oxide layers (GO/1.4404) following the thermal reduction from 1700 mΩ cm.sup.2 to 120 mΩ cm.sup.2 at a pressing pressure of 140 N cm.sup.2, and from 775 mΩ cm.sup.2 to 62 mΩ cm.sup.2 at a pressing pressure of 300 N cm.sup.2. The contact resistance of trGO/1.4404 is even lower than that of the uncoated material 1.4404 passivated by exposure to atmospheric oxygen. It has been shown that contact resistances <100 mΩ cm.sup.2 are necessary during fuel cell operation. This specification can be achieved for the thermally reduced graphene oxide layers generated according to the invention on a bipolar plate.

[0054] FIG. 5 shows the progression of the free corrosion potential during a temperature increase to 130° C. in a long-term experiment lasting 30 days. The experiment was carried out in a three-electrode measuring cell in 175 ml 85 wt. % H.sub.3PO.sub.4. Rapid degradation of the passive layer is apparent on the uncoated material 1.4404, which is accompanied by a drop in the free corrosion potential as the temperature rises. The temperature increase is shown in the diagram by rhombi having a temperature fluctuation of 5° C. The at least partially thermally reduced graphene oxide coatings on material 1.4404 having a thickness of 10 nm and 100 nm show only minor improvement under these drastic conditions. In contrast, thermally reduced graphene oxide layers having a thickness of approximately 250 nm have a corrosion potential of 435 mV (vs. reversible hydrogen electrode) even after 30 days. This is an indication that the metal surface of the bipolar plate beneath the coating is effectively protected against acid attack. Under real fuel cell/electrolysis conditions, in general a considerably lower amount of electrolyte (˜1 mg/cm.sup.2) is in contact with the bipolar plate, so that lower layer thicknesses are sufficient in these instances to protect the metal substrate against corrosion.

[0055] As another experiment, a potentiodynamic corrosion test was carried out in 175 ml 1 M H.sub.3PO.sub.4 at room temperature in an electrochemical three-electrode measuring cell. After 100 cycles in the potential range of 0 to 1.3 V (vs. reversible hydrogen electrode) using a scan rate of 100 mV/s, an uncoated copper sample having a thickness of 100 μm (serving as a potential bipolar plate material) had completely dissolved, while a copper sample coated with at least partially thermally reduced graphene oxide (trGO) (as one embodiment of a bipolar plate according to the invention) only showed minor defects in the edge regions. This effect can be explained by the fact that, among other things, the sealing ring had damaged the thermally reduced graphene oxide coating during disassembly of the measuring cell.

[0056] To produce the graphene oxide (GO) suspension, the chemical synthesis described hereafter was carried out. Functionalization of graphite with hydrophilic groups (epoxy, hydroxy and carboxy groups, among other things) with subsequent ultrasonic dispersion ensures a stable graphene oxide (GO) suspension, which is reduced after the coating process, as was already described above. The aromatic system is restored as a result of the elimination of the hydrophilic groups.

Exemplary Synthesis (Synthesis According to Variation of Hummers' Method):

[0057] Charge graphite in a round-bottomed flask, mix it with 400 ml H.sub.2SO.sub.4/H.sub.3PO.sub.4 (360/40 ml) and stir. Then, slowly add KMnO.sub.4 in portions, so that the temperature remains relatively constant. Cool in an ice bath, since the reaction is highly exothermic. Thereafter, control the temperature to 50° C. and stir for 18 hours. Afterwards, allow to cool to room temperature and place 500 ml on ice. Then, add 7 ml 27% H.sub.2O.sub.2. Thereafter, centrifuge the graphite oxide powder thus obtained, wash several times with ethanol and water, and dry. This is followed by the direct ultrasonic dispersion of the graphite oxide powder by way of a sonotrode in a protic polar solvent (60 minutes per 100 ml suspension at 1 mg/ml concentration, using an intensity of approximately 100 W/cm.sup.2 sonotrode surface). Lastly, centrifuge again and dry (product is graphene oxide particles). It is possible to directly coat the substrate in the form of a suspension.

[0058] The at least partial reduction of the graphene oxide to obtain graphene takes place in a simple embodiment of the method in a furnace or on a heating plate, preferably in an inert gas atmosphere (nitrogen, argon) or by exposure to atmospheric oxygen in the temperature range of 200 to 500° C. As a result of the supply of thermal energy, functional groups are reduced (CO/CO.sub.2 escapes) and the aromatic system is restored. This was confirmed by way of thermogravimetric analyses (TGA). Checking with XPS spectroscopy, it is further possible to clearly distinguish whether the coating applied to a bipolar plate comprises graphene, graphite oxide or the graphene-like composition according to the invention.

[0059] Other methods for producing the graphene oxide (GO) suspension were also tested. The chemical synthesis is carried out with a strong reducing agent, such as hydrazine in solution or in the gas phase. Graphene oxide layers are reduced by way of hydrazine to obtain chemically reduced graphene oxide layers (crGO). The electrochemical reduction of graphene oxide layers to obtain electrochemically reduced graphene oxide layers (erGO) took place in an electrolyte (such as potassium dihydrogen phosphate) in the cathodic polarization range (up to −1 V vs. reversible hydrogen electrode). The disadvantage of the chemical and electrochemical reduction that should be mentioned is that a contamination of the coating with foreign ions of the reducing agent occurs. In fuel cells/electrolyzers, this may result in contamination of the polymer electrolyte membrane or of the catalyst. Laser-induced reduction takes place by the direct irradiation of the graphene oxide coating by way of a laser beam. This requires adjustments of the intensity, energy, pulse duration and the like, so as to achieve an effective reduction to obtain the laser-reduced graphene oxide coating (LrGO), while also avoiding damage to the coating.

[0060] Chemical and electrochemical reduction thus brings with it the disadvantages of impurities in the coating, and that partial defects are frequently caused in the case of laser reduction. Within the scope of the present invention, in particular thermal reduction is thus considered to be particularly simple and effective, and in this respect is regarded as particularly advantageous.