Coating and layer system, and bipolar plate, fuel cell and electrolyser

Abstract

A coating for a bipolar plate of a fuel cell or an electrolyzer contains a homogeneous or heterogeneous solid metal solution. The coating contains at least 15% Iridium and up to 84% Ruthenium with a total combined concentration of Iridium and Ruthenium of at least 99% (atomic). The coating also contains at least one of Nitrogen, Carbon, and Flourine. The coating may contain traces of Oxygen or Hydrogen. The coating may be used as part of a layer system that includes one or more undercoat layers and the coating as a covering layer.

Claims

1. A coating for an energy converter, wherein the coating comprises a homogeneous or heterogeneous solid metallic solution which either contains a first chemical element of iridium in a concentration of at least 99 at. % or contains a first chemical element of iridium and a second chemical element of ruthenium, with the first chemical element and the second chemical element being present in a total concentration of at least 99 at. %, and also contains at least one further nonmetallic chemical element selected from the group consisting of nitrogen, carbon, and fluorine present in a concentration in the range from 0.1 to 1 at. %, with oxygen and hydrogen being additionally present only in traces.

2. The coating as claimed in claim 1, wherein the coating comprises at least 99 at. % iridium and additionally carbon; or comprises at least 99 at. % iridium and additionally carbon and traces of oxygen or hydrogen; or comprises at least 99 at. % iridium and additionally carbon and fluorine; or comprises 15 to 98.9 at. % iridium and 0.1 to 84 at. % ruthenium and additionally carbon; or comprises 15 to 98.9 at. % iridium and 0.1 to 84 at. % of ruthenium and additionally carbon and traces of oxygen or hydrogen; or comprises 15 to 98.9 at. % iridium and 0.1 to 84 at. % of ruthenium and additionally carbon and fluorine.

3. The coating as claimed in claim 2, wherein the coating comprises iridium in a concentration of at least 99 at. % and carbon in a concentration in the range from 0.1 to 1 at. %, or the coating comprises iridium in a concentration of at least 99 at. % and nitrogen in a concentration in the range from 0.1 to 1 at. %, or the coating comprises iridium in a concentration in the range from 15 to 98.9 at. %, ruthenium in a concentration in the range from 0.1 to 84 at. % and carbon in a concentration in the range from 0.1 to 1 at. %, or the coating comprises iridium in a concentration in the range from 15 to 98.9 at. %, ruthenium in a concentration in the range from 0.1 to 84 at. % and nitrogen in a concentration in the range from 0.1 to 1 at. %.

4. The coating as claimed in claim 1, further comprising fluorine in a concentration not more than 0.5 at. %.

5. The coating as claimed claim 1, further comprising at least one chemical element selected from the group consisting of the base metals.

6. The coating as claimed in claim 5, wherein the at least one chemical element selected from the group consisting of the base metals is aluminum, iron, nickel, cobalt, zinc, cerium or tin.

7. The coating as claimed in claim 5, wherein the at least one chemical element selected from the group consisting of the base metals is present in a concentration range from 0.005 to 0.01 at. %.

8. The coating as claimed in claim 1 further comprising at least one refractory metal element selected from the group consisting of titanium, zirconium, hafnium, niobium, and tantalum.

9. The coating as claimed in claim 8, wherein the at least one refractory metal element is present in a concentration range from 0.005 to 0.01 at. %.

10. The coating as claimed in claim 1, further comprising at least one additional chemical element selected from the group consisting of the noble metals in a concentration range from 0.005 to 0.9 at. %.

11. The coating as claimed in claim 10, wherein the at least one chemical element selected from the group consisting of the noble metals is platinum, gold, silver, rhodium, or palladium.

12. The coating as claimed in claim 1, wherein the coating has a layer thickness between 1 nm and 50 nm.

13. A layer system for a bipolar plate of a fuel cell or an electrolyser, comprising a covering layer as claimed in claim 1 and an undercoat layer system.

14. The layer system as claimed in claim 13, wherein the undercoat layer system comprises at least one undercoat layer comprising at least one chemical element selected from the group consisting of titanium, niobium, hafnium, zirconium, and tantalum.

15. The layer system as claimed in claim 14, wherein the undercoat layer system comprises at least one first undercoat layer in the form of a metallic alloy layer comprising the chemical elements titanium and niobium.

16. The layer system as claimed in claim 15, wherein the undercoat layer system comprises a second undercoat layer comprising at least one chemical element selected from the group consisting of titanium, niobium, hafnium, zirconium, and tantalum and at least one nonmetallic element selected from the group consisting of nitrogen, carbon, boron, and fluorine.

17. The layer system as claimed in claim 16, wherein the second undercoat layer is arranged between the first undercoat layer and the covering layer.

18. The layer system as claimed in claim 16, wherein the second undercoat layer contains up to 5 at. % of oxygen.

19. A bipolar plate comprising a metallic substrate and a layer system as claimed in claim 13 applied to at least parts of a surface of the metallic substrate.

Description

DETAILED DESCRIPTION

(1) The FIGURE shows a bipolar plate 1 comprising a substrate 2 composed of stainless steel and a layer system 3 applied over the full area on one side of the substrate 2. The layer system 2 comprises a covering layer 3a and an undercoat layer system 4 comprising a first undercoat layer 4a and a second undercoat layer 4b.

(2) In a first working example, a metallic substrate 2 in the form of a conductor, here for a bipolar plate 1 of a polymer electrolyte fuel cell for reaction of (reformed) hydrogen, composed of a stainless steel, in particular an austenitic steel meeting very demanding known requirements in respect of corrosion resistance, e.g. having the DIN ISO material number 1.4404, is produced.

(3) A layer system 3 is formed on the substrate 2 of the bipolar plate 1 by means of a coating process, for example a vacuum-based coating process (PVD), with the substrate 2 firstly being coated with a first undercoat layer 4a in the form of a 0.5 μm thick titanium layer in one process pass, subsequently with a 1 μm thick second undercoat layer 4b in the form of a titanium nitride layer and subsequently with a covering layer 3a in the form of a 10 nm thick iridium-carbon layer. The covering layer 3a corresponds to a layer which is open on one side since only a covering layer area of a further layer, here the second undercoat layer 4b, contacts it. The free surface 30 of the covering layer 3a in a fuel cell thus directly adjoins and is exposed to an electrolyte, in particular a polymer electrolyte.

(4) In a second working example, the metallic substrate 2 for the bipolar plate 1 is firstly coated with a first undercoat layer 4a in the form of a metallic alloy layer having a thickness of 100 nm, with the metallic alloy layer having the composition Ti.sub.0.67 Nb.sub.0.33. A further application of a second undercoat layer 4b having a thickness of 400 nm and the composition (Ti.sub.0.67 Nb.sub.0.33).sub.1-x N.sub.x where x=0.40-0.55 is subsequently carried out. A covering layer 3a having a thickness of 10 nm and the composition iridium-carbon is then applied thereto.

(5) The advantage is an extraordinarily high stability to oxidation of the bipolar plate 1. Even under a long-term electric potential of +3000 mV relative to a standard hydrogen electrode, no increase in resistance is observed in sulfuric acid solution having a pH of 3. On the outside, the free surface 30 of the covering layer 3a, i.e. the surface of the covering layer 3a facing away from the substrate 2, remains silvery and shining even after long-term application of +2000 mV relative to a standard hydrogen electrode for 50 hours. Even in a scanning electron microscopic examination, no traces of corrosion extending through the thickness of the covering layer 3a to the substrate 2 or reaching the substrate 2 can be discerned.

(6) The covering layer 3a of the second working example can be applied either by means of the vacuum-based PVD sputtering technique or by means of a cathodic ARC coating process, also known as vacuum electric arc vaporization. Despite a higher number of droplets, in other words an increased number of metal droplets compared to the sputtering technology, the covering layer 3a produced in the cathodic ARC process also has the advantageous properties of high corrosion resistance combined with time-stable surface conductivity of the covering layer 3a produced by means of the sputtering technique.

(7) In a third working example, the layer system 3 is formed on a substrate 2 in the form of a structured perforated stainless steel plate. The substrate 2 has been electrolytically polished in an H.sub.2SO.sub.4/H.sub.3PO.sub.4 bath before application of a layer system 3. After application of a single undercoat layer in the form of a tantalum carbide layer having a thickness of several 1000 nm, a covering layer 3a in the form of an iridium-carbon layer having a thickness of several 100 nm is applied.

(8) The advantage of the undercoat layer composed of tantalum carbide is not only its extraordinary corrosion resistance but also that it does not absorb hydrogen and thus serves as a hydrogen barrier for the substrate 2. This is particularly advantageous when titanium is used as substrate material.

(9) The layer system 3 of the third working example is suitable for use of an electrolysis cell for producing hydrogen at current densities i which are greater than 500 mA cm.sup.−2.

(10) The advantage of the metalloid layer which is located in an intermediate position and/or is closed on both sides in the layer system or the second undercoat layer, which in the simplest case is composed of, for example, titanium nitride, is its low electrical resistance of 10-12 mΩ cm.sup.−2. Likewise, the coating or covering layer can also be formed without a second undercoat layer or metalloid layer, possibly with an increase in resistance.

(11) Some layer systems together with their characteristic values are shown by way of example in table 1.

(12) TABLE-US-00001 TABLE 1 Layers and selected characteristic values Corrosion current at Oxidation 2000 mV stability at vs. standard 2000 mV hydrogen measured as Specific electrode in change in surface μA cm.sup.−2 in the surface resistance aqueous resistance in in mΩ sulfuric acid mΩ cm.sup.−2 Layer system/layer cm.sup.−2 at solution (pH = 3) Value: <20 thickness T = 20° C. at T = 80° C. mΩ cm.sup.−2 1 Gold/3 μm 9 >100 pitting  9-10 (as reference) current 2 Ti/0.5 μm 8 0.001 12 TiN/1 μm Ir.sub.0.99-C.sub.0.01/10 nm 3 Ti.sub.0.67Nb.sub.0.33/0.1 μm 7-8 0.01 1-2 (Ti.sub.0.67Nb.sub.0.33).sub.1-x N.sub.x where x = 0.40-0.55/0.4 μm Ir.sub.0.99-C.sub.0.01/10 nm 4 Zr/0.5 μm 11 0.001 11-12 ZrN/1 μm Ir.sub.0.99-C.sub.0.01/10 nm 5 Ta/0.05 μm 10 0.001 17-18 TaC/0.5 μm Ir.sub.0.991-C.sub.0.009/5 nm 6 ZrB.sub.2/0.3 μm 7 Pitting reaction Ir.sub.0.7-B.sub.0.3/5 nm after stressing for 4 h

(13) Only some illustrative layer systems are presented in table 1. The layer systems advantageously display no increase in resistance over a number of weeks at an anodic voltage of +2000 mV relative to a standard hydrogen electrode in sulfuric acid solution at a temperature of 80° C. The layer systems applied in a high vacuum by means of a sputtering or ARC process or in a fine vacuum by means of PECVD processes (plasma-enhanced chemical vapor deposition processes) or in an ALD process (atomic layer deposition) in some cases had a dark discoloration after this stressing time. However, no visible corrosion phenomena or significant changes in the surface resistance occurred.

LIST OF REFERENCE NUMERALS

(14) 1 Bipolar plate 2 Substrate 3 Layer system 3a Covering layer 4 Undercoat layer system 4a First undercoat layer 4b Second undercoat layer 5 Free surface