Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B13/00—Diaphragms; Spacing elements
  • C25B13/04—Diaphragms; Spacing elements characterised by the material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B13/00—Diaphragms; Spacing elements
  • C25B13/04—Diaphragms; Spacing elements characterised by the material
  • C25B13/05—Diaphragms; Spacing elements characterised by the material based on inorganic materials
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
  • C25B9/60—Constructional parts of cells
  • C25B9/65—Means for supplying current; Electrode connections; Electric inter-cell connections
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0206—Metals or alloys
  • H01M8/0208—Alloys
  • H01M8/021—Alloys based on iron
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0223—Composites
  • H01M8/0228—Composites in the form of layered or coated products
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• the invention relates to a coated bipolar plate for electrochemical energy converters, as commonly used in fuel and / or electrolysis cells.
• the invention further relates to a method for producing the coated bipolar plate according to the invention.
• the electrolytes may be ion-conductive polymer membranes, dissolved alkalis or acids, alkali metal carbonate melts or ceramics.
• a porous electrically conductive gas diffusion layer of carbon nonwoven fabric or metal foam adjoins the electrode from both sides in each case.
• an electrochemical energy converter has on both sides a bipolar plate (also called flow distributor or current collector plates, engl., Current collector), which are usually made of electrically conductive carbon composite materials or metals.
• the focus is on fuel cells or electrolyzers, which are operated with a solid polymer electrolyte membrane.
• the electrolyte is a proton or hydroxide ion-conducting ionomer membrane. It is gas-tight and not electron-conducting. This is followed on both sides by a respective catalyst layer (electrode), a porous gas diffusion layer and a bipolar plate.
• the reactants hydrogen and oxygen in the case of fuel cells and water in the case of electrolysis cells
• the purpose of the bipolar plate is to mechanically stabilize the electrochemical cell, to supply and remove the reactants on both sides and to dissipate the generated electrical current.
• bipolar plates must be mechanically stable, since they have to withstand vibrations and vibrations as a mechanically supporting element within fuel cells and electrolyzers of thermal expansion, high contact pressures and in mobile applications. Bipolar plates must also have high electrical and thermal conductivity to efficiently dissipate the generated electrical current and deliver thermal energy to the cooling medium. To do this, they must have a dense surface texture so that neither reactants nor electrolytes are absorbed and spatially separated from the cooling medium. In addition, the water balance (water supply in electrolysers and water discharge in fuel cells) is controlled by them. In addition, a high stability against electrochemical corrosion at high temperature and external potential is needed. Bipolar plates u. a.
• graphite-based bipolar plates have a relatively high material thickness (> 2 mm).
• Coatings such as ceramic nitrides and carbides based on titanium, chromium, aluminum, silicon or zirconium, graphitic or gold-based coatings produced by means of physical or chemical vapor deposition (PVD / CVD) have hitherto been known.
• Electrochemical deposition processes of metal borides eg NiCoB, Ni 2 B or Ni 3 B
• gold or conductive organic polymers such as polyaniline or polypyrrole
• the difficulty is always to produce defect-free layers that provide high long-term stability. Even the smallest defects, such as cracks or holes (pinholes), cause the electrolyte to spread under the coating and lead to corrosion damage.
• Another option is to plate stainless steel as a bipolar plate material with a thin niobium layer that forms stable and electronically conductive oxide layers and thereby passivates the metal.
• Bipolar plates in polymer electrolyte fuel cells are subject to electrochemical corrosion due to the acidic environment as well as the influence of temperature and electrochemical potential.
• precious metals such as gold or very rare and expensive metals such as tantalum, which in the o. G.
• inexpensive stainless steels and nickel-based alloys have too high corrosion rates and / or a formation of nonconductive passive layers. This leads to a relatively rapid decrease in performance and adversely to the aging of the fuel cell.
• the object of the invention is to provide a further metallic bipolar plate for low-temperature or high-temperature polymer electrolyte fuel cells / electrolyzers which, owing to a suitable coating, is inexpensive to produce, sufficiently stable and resistant to corrosion and also has the necessary electronic and thermal conductivities having application in an electrochemical energy converter.
• the core idea of the invention is to provide a metallic bipolar plate with a coating comprising at least partially reduced graphene oxide layers (GO), which represents an effective corrosion protection of metallic materials and moreover enables a simple and favorable coating process.
• GO graphene oxide layers
• Suitable materials for such a bipolar plate include all metallic materials customary hitherto for bipolar plates, including iron-based steels, austenitic stainless steels and alloys having a high chromium, nickel and / or molybdenum content and additions of niobium, titanium and / or copper, manganese, Tungsten, tantalum and vanadium, copper alloys and precious metals, such as gold and platinum into consideration.
• a coating with a graphene-like material which consists of one or more at least partially reduced graphene oxide layers, is a promising option for the corrosion reduction of metallic materials.
• Such a coating advantageously has sufficient stability and the necessary electrical conductivity for use in an electrochemical cell.
• Graphene is usually understood to mean a monolayer of carbon which is present in a plane of 2-dimensional and hexagonal crosslinked. In contrast, graphite is present in a 3-dimensional structure of parallel, planar layers of graphene. Both graphene and graphite are electrically conductive.
• Graphite oxide is a non-stoichiometric compound of carbon, oxygen and hydrogen. If graphite oxide is dissolved in a polar solvent and treated by ultrasound, a homogeneous colloidal suspension of flakes of non-conductive monomolecular layers (graphene oxide) is formed.
• a chemical synthesis is carried out. Based on graphite powder, a graphite oxide powder is first prepared, which is subsequently converted by ultrasound dispersion into a stable graphene oxide (GO) suspension. Such a suspension is considered to be stable if it does not show any sedimentation even after more than 30 days. By depositing this suspension on a metallic carrier substrate, individual thin graphene oxide layers can then be applied and subsequently reduced to at least partially reduced graphene oxide (rGO), which in the following will be referred to as graphene-like material.
• rGO at least partially reduced graphene oxide
• the oxygen content in graphene oxide is in the order of 20-60 wt .-%, in particular between 25 wt .-% and 50 wt .-% (balance carbon) and can be significantly reduced depending on the reduction conditions after the reduction in the graphene-like material become. A reduction by more than 50%, in particular by about 75%, is advantageous.
• the at least partial reduction of graphene oxide to graphene in the coating is referred to in the context of this invention as at least partially reduced graphene oxide (rGO) and the material of the coating insofar as graphene-like.
• the layer thicknesses of the individual deposited graphene oxide layers vary depending on the application, deposition method and concentration of the GO suspension and are generally between 10 nm and 1 ⁇ .
• the spray, dip or spin coating process are suitable for the deposition.
• the coating and reduction steps take place alternately in several successive process steps until the desired layer thickness is reached.
• the GO concentration during the spraying process is typically 0.5-5 mg / ml and for dip and spin coating about 5-10 mg / ml.
• the required layer thickness can vary.
• the graphene oxide layers are advantageously sufficiently electrically conductive for the use of a bipolar plate.
• sufficient Conductive conductivity is understood to mean an electrical conductivity of at least 50 S / cm, preferably more than 100 S / cm.
• a decisive parameter for the coating according to the invention of a bipolar plate with a graphene-like material is thus the contact resistance at the interface bipolar plate and gas diffusion layer.
• Precious metals such as gold or platinum show very low contact resistance since they are not subject to surface passivation.
• precious metals are generally not suitable as bipolar plate material.
• contact resistance has a huge impact on the performance of fuel cells and electrolysers.
• the metallic bipolar plate is spatially shielded from the chemical electrolyte attack by the at least partially reduced graphene oxide coating.
• the reduced graphene oxide coating is free of defects, such as. As cracks or pinholes, is.
• low-temperature fuel cells and electrolysers typically operate temperatures of 80-90 ° C and there is a sulfuric acid aqueous environment (usually R. ⁇ 0.5 MH 2 S0 4 ) before.
• High-temperature fuel cells and electrolyzers have higher operating temperatures of 120-180 ° C and acidities of ⁇ 16 M H3PO4. It has been found that the "graphene-like" coating of the bipolar plate according to the invention is stable to the aforementioned temperatures and the acidic ambient conditions.
• the at least partially reduced graphene oxide coating In addition to the chemical corrosion stability compared to the corrosive electrolyte, the at least partially reduced graphene oxide coating also exhibits high electrochemical stability at external potentials of up to 1 V in fuel cells and 2.2 V in electrolyzers. Indicated potential values are measured against the reversible hydrogen electrode as reference standard.
• the reduction in the anodic corrosion current densities at the external potential due to the inventively reduced graphene oxide coating is a decisive improvement over hitherto conventional graphitic composite materials, uncoated metallic materials and coating concepts known from the literature.
• the at least partially reduced graphene oxide coating according to the invention has higher coefficients of expansion than graphite and ceramic coating concepts known from the literature.
• the metallic bipolar plate undergoes a measurable material expansion, which can disadvantageously lead to cracks and flaking in the case of rigid and inelastic coatings. This inevitably leads to corrosion phenomena, an increase in the contact resistance and consequently to a decrease in cell performance.
• the high modulus of elasticity of the inventive at least partially reduced graphene oxide layers regularly prevents the temperature-dependent degradation of the coating. Bending tests of coated metallic substrates have also shown that even with a bend of up to 45 °, no defects (cracks and flaking) of the at least partially reduced graphene oxide coating can be observed.
• Suitable materials for a suitable metallic bipolar plate include all hitherto customary metallic materials including iron-based steels, austenitic stainless steels and alloys with a high proportion of chromium, nickel and / or molybdenum and additions of niobium, titanium and / or copper, manganese, tungsten, Tantalum and vanadium, copper alloys and precious metals, such as gold and platinum into consideration.
• the graphene-like coating produced on the metallic bipolar plate according to the invention advantageously has a very good adhesion to the metallic material of the bipolar plate. Furthermore, it has been found that this coating is advantageously flexible.
• the corrosion resistance in acidic or basic media, at high temperatures and electrical / electrochemical potentials can be significantly increased compared to an uncoated metallic bipolar plate.
• the graphene-like coating according to the invention also has sufficiently good electrical conductivity, it can advantageously be used as a promising coating for metallic bipolar plates for use in fuel cells and electrolyzers, ie generally in electrochemical cells. This opens up completely new design possibilities, which are not transferable to graphitic materials. Since metallic bipolar plates generally have a material thickness of ⁇ 100 ⁇ m (cf., graphitic composite materials> 2 mm), significantly higher gravimetric and volumetric power densities of a fuel cell / electrolysis stack can be achieved with the graphene-like coating according to the invention.
• the graphene-like coating according to the invention is to be produced by means of the abovementioned coating process, which in comparison with known alternative coating technologies, eg.
• the graphene-like coating according to the invention is not limited to use in electrochemical cells. Further applications of the graphene-like coating according to the invention are generally the coating of corrosion-endangered metallic components, such as pipelines, ship hulls, vehicle bodies, metallic electrochemical and chemical reactors, etc. or also of steel components on buildings, in particular on bridges and similar constructions.
• FIG. 1a shows a metallic bipolar plate (material 1.4404) with an embossed channel structure (English, flow field) without a reduced graphene oxide coating according to the invention.
• FIG. 1 b shows a metallic bipolar plate (material 1.4404) with an embossed channel structure (English, flow field) with an inventive at least partially reduced graphene oxide coating.
• the application was carried out by spraying using an aqueous graphene oxide suspension at a concentration of 2 mg / ml.
• the reduction to the reduced graphene oxide layers was carried out thermally on a hot plate at temperatures up to 500 ° C. After each spraying process, the currently applied layer was thermally reduced before the next coating curtain followed.
• the total layer thickness is 250 nm in this bipolar plate.
• FIG. 2 shows a cross section of the metallic bipolar plate (material 1.4404) with the graphene-like coating according to the invention.
• Application and reduction methods de (thermal-reduced) analogous to FIG. 1.
• the total layer thickness is approximately 250 nm.
• Cross-sections were prepared by means of ion polishing technology.
• FIG. 3 shows a cross section of the metallic bipolar plate (material 1.4404) with a thermally reduced graphene oxide coating according to the invention. Application and reduction method comparable to Figure 1.
• the layer thickness is here about 250 nm.
• the cross section was prepared by means of a scalpel section. It can clearly be seen the layer structure of individual reduced graphene oxide layers.
• FIG. 4 shows the contact resistance between the uncoated and coated bipolar plate and the adjacent gas diffusion layer (carbon nonwoven) as a function of the contact pressure.
• the inventive thermally reduced graphene oxide coating on material 1.4404 (trGO / 1.4404) with a layer thickness of 200 nm advantageously shows a reduction of the contact resistance by more than one order of magnitude compared to a non-reduced graphene oxide coating on 1.4404 (GO / 1.4404).
• the contact resistance of an uncoated bipolar plate of material 1.4404 is shown with a surface passivation layer naturally formed on atmospheric oxygen and with a mechanically removed (polished) surface. Material sample 1.4404 was measured directly after mechanical polishing. However, since the surface passivation takes place within a few hours of atmospheric oxygen or in aqueous and oxygen-containing solutions, the Matenalpattern 1.4404 with passive layer shows the expected contact resistance in fuel cells or electrolyzers during operation.
• the contact resistance of thermally reduced graphene oxide layers on the material 1.4404 (trGO / 1.4404) with a layer thickness of ⁇ 100 nm increases after the thermal reduction of 1700 mQ cm 2 to 120 mQ cm 2 at a contact pressure of 140 N cm “2 and from 775 mQ cm 2 to 62 mQ cm 2 at a contact pressure of 300 N cm "2 by more than an order of magnitude compared to unreduced graphene oxide layers (GO / 1.4404).
• the contact resistance of trGO / 1.4404 is even lower than that of the uncoated material 1.4404, which is passivated by atmospheric oxygen. It has been shown that in fuel cell operation contact resistances ⁇ 100 mQ cm 2 are necessary. This specification can be achieved for the inventively produced thermally-reduced graphene oxide layers on a bipolar plate.
• FIG. 5 shows in the long-term test of 30 days the course of the free corrosion potential with a temperature rise to 130 ° C.
• the experiment was carried out in a three-electrode measuring cell in 175 ml of 85 wt .-% H 3 P0 4 .
• uncoated material 1.4404 is a rapid degradation of the passive layer to detect, which goes hand in hand with the drop in the free corrosion potential with increasing temperature.
• the temperature rise is shown in the diagram by diamonds with a temperature fluctuation of 5 ° C.
• the thermally at least partially reduced graphene oxide coatings on material 1.4404 with a thickness of 10 nm and 100 nm show only a slight improvement under these drastic conditions.
• thermally reduced graphene oxide layers with a thickness of approx. 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 under the coating is effectively protected from acid attack. Under real fuel cell / electrolysis conditions, significantly smaller quantities of electrolyte ( ⁇ 1 mg / cm 2 ) are usually in contact with the bipolar plate, so that in these cases even lower layer thicknesses are sufficient to protect the metal substrate from corrosion.
• the at least partial reduction of the graphene oxide to graphene takes place in a simple embodiment of the process in an oven or on a hotplate, preferably in a protective gas atmosphere (nitrogen, argon) or atmospheric oxygen in the temperature range from 200 to 500.degree. Due to the thermal energy supply functional groups are reduced (escape of CO / C0 2 ) and regression of the aromatic system. This was confirmed by thermogravimetic analyzes (TGA). By checking by means of XPS spectroscopy it is also possible to clearly distinguish whether the coating applied to a bipolar plate has graphene, graphite oxide or the graphene-like composition according to the invention.
• a protective gas atmosphere nitrogen, argon
• atmospheric oxygen in the temperature range from 200 to 500.degree. Due to the thermal energy supply functional groups are reduced (escape of CO / C0 2 ) and regression of the aromatic system. This was confirmed by thermogravimetic analyzes (TGA). By checking by means of XPS spectroscopy it is also possible to clearly distinguish whether the coating applied to
• the laser-induced reduction takes place by direct irradiation of the graphene oxide coating by means of a laser beam. For this, adjustments of intensity, energy, pulse duration, etc. are necessary in order to achieve an effective reduction to laser-reduced graphene oxide coating (LrGO) on the one hand and to avoid damaging the coating on the other hand.
• LrGO laser-reduced graphene oxide coating
• the thermal reduction is considered to be particularly simple and effective and insofar as particularly advantageous.

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• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Carbon And Carbon Compounds (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Other Surface Treatments For Metallic Materials (AREA)

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• 2014
  • 2014-11-03 DE DE102014016186.2A patent/DE102014016186A1/de not_active Withdrawn
• 2015
  • 2015-09-17 JP JP2017522673A patent/JP7256600B2/ja active Active
  • 2015-09-17 US US15/517,739 patent/US10418643B2/en active Active
  • 2015-09-17 WO PCT/DE2015/000466 patent/WO2016070862A1/de not_active Ceased
  • 2015-09-17 EP EP15778604.7A patent/EP3216075B1/de active Active
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  • 2015-09-17 CN CN201580059715.7A patent/CN107210456A/zh active Pending

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