Layer and layer system, as well as bipolar plate, fuel cell and electrolyser

Abstract

Layers for a bipolar plates are disclosed, as well as bipolar plates including the layers and fuel cells and/or electrolyzers including the bipolar plates. The layer may include a homogeneous or heterogeneous solid metallic solution or compound which either contains a first chemical element from the group of the noble metals in the form of iridium; or contains a first chemical element from the group of the noble metals in the form of iridium and a second chemical element from the group of the noble metals in the form of ruthenium. The layer may also include at least one further nonmetallic chemical element from the group consisting of nitrogen, carbon, boron, fluorine, and hydrogen.

Claims

1. A layer for a component of an electrochemical cell, wherein the layer comprises: a homogeneous or heterogeneous solid metallic solution which contains a first chemical element from the group of the noble metals in the form of iridium and a second chemical element from the group of the noble metals in the form of ruthenium; at least one further nonmetallic chemical element from the group consisting of nitrogen, carbon, boron, fluorine, and hydrogen embedded into a crystal lattice of the first chemical element.

2. The layer as claimed in claim 1, wherein the at least one nonmetallic chemical element is present in a concentration in the range from 0.1 at. % to 65 at.-% in the layer.

3. The layer as claimed in claim 1, wherein it a) comprises more than 35 at.-% of iridium and additionally carbon; or b) comprises more than 35 at.-% of iridium and additionally carbon and hydrogen; or c) comprises more than 35 at.-% of iridium and additionally carbon and fluorine, optionally additionally hydrogen d) comprises a total of more than 35 at.-% of iridium and ruthenium and additionally carbon; or e) comprises a total of more than 35 at.-% of iridium and ruthenium and additionally carbon and hydrogen; or f) comprises a total of more than 35 at.-% of iridium and ruthenium and additionally carbon and fluorine, optionally additionally hydrogen.

4. The layer as claimed in claim 1, including iridium and/or ruthenium, are present in a concentration range from 35 to 99 at.-% in the layer.

5. The layer as claimed in claim 1, wherein the at least one nonmetallic chemical element includes carbon and is present in a concentration range from 10 to 25 at.-% in the layer.

6. The layer as claimed in claim 1, wherein the layer has a layer thickness of from at least 1 nm to not more than 50 nm.

7. A layer system for an electrochemical cell, comprising: a base layer system; and a covering layer in the form of a solid metallic solution which contains a first chemical element from the group of the noble metals in the form of iridium; a second chemical element from the group of the noble metals in the form of ruthenium; and at least one further nonmetallic chemical element from the group consisting of nitrogen, carbon, boron, fluorine, and hydrogen embedded into a crystal lattice of the first chemical element.

8. The layer system as claimed in claim 7, wherein the base layer system has at least one base layer comprising at least one chemical element from the group consisting of titanium, niobium, hafnium, zirconium, and tantalum.

9. The layer system as claimed in claim 8, wherein the base layer system has at least one first base layer in the form of a metallic alloy layer comprising the chemical elements titanium and niobium.

10. The layer system as claimed in claim 8, wherein the base layer system has a second base layer comprising at least one chemical element from the group consisting of titanium, niobium, hafnium, zirconium, and tantalum and additionally at least one nonmetallic element from the group consisting of nitrogen, carbon, boron, and fluorine.

11. The layer system as claimed in claim 10, wherein the second base layer is arranged between the first base layer and the covering layer.

12. The layer system as claimed in claim 10, wherein the second base layer contains up to 5 at.-% of oxygen.

13. A bipolar plate comprising a metallic substrate and a layer system as claimed in claim 7 applied at least in partial areas of the surface of the metallic substrate.

14. An electrochemical cell comprising at least one bipolar plate as claimed in claim 13.

15. The layer of claim 1, wherein the solid metallic solution further contains at least one metal from transition groups IV or V of the periodic table.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Further advantages, features and details of the disclosure can be derived from the following description of preferred working examples and the FIGURE. The features and combinations of features mentioned above in the description can be employed not only in the combination indicated in each case but also in other combinations or on their own, without going outside the scope of the disclosure.

(2) The FIGURE shows an example cross-section of a bipolar plate having a layer system applied thereon.

DETAILED DESCRIPTION

(3) 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 of one side of the substrate 2. The layer system 2 comprises a covering layer 3a and a base layer system 4 comprising a first base layer 4a and a second base layer 4b.

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

(5) A layer system 3 according to the disclosure is formed on the substrate 2 of the bipolar plate 1 by a coating process, for example a vacuum-based coating process (PVD), with the substrate 2, in a pass through the process, being coated firstly with a first base layer 4a in the form of a 1.5 μm thick titanium layer, subsequently with an approximately equally thick second base layer 4b in the form of a titanium nitride layer and subsequently with a covering layer 3a in the form of a 25 nm thick titanium-iridium nitride layer. The covering layer 3a corresponds to a sublayer which is open on one side since only a covering layer surface of a further layer, here the second base layer 4b, is in contact with the covering layer. The free surface 30 of the covering layer 3a in a fuel cell is thus arranged directly adjacent to an electrolyte, in particular a polymer electrolyte, and is exposed thereto.

(6) In a second working example, the metallic substrate 2 for the bipolar plate 1 is firstly coated with a first base layer 4a in the form of a metallic alloy layer having a thickness of a number of 100 nm, with the metallic alloy layer having the composition Ti0.9 Nb0.1. A further application of a second base layer 4b having a thickness of again a number of 100 nm and the composition Ti0.9 Nb0.1 N1-x is subsequently carried out. A covering layer 3a is applied thereon in a thickness of a number of nm with the composition (Ti,Nb—Ir)N1-δ.

(7) The advantage is an extraordinarily high stability to oxidation of the bipolar plate 1 according to the disclosure. Even at long-term subjection to +3000 mV relative to a standard hydrogen electrode, no increase in resistance is found in sulfuric acid solution having a pH of 3. It appears to be particularly advantageous when a covering layer 3a having the composition (Ti0.9 Nb0.1 Iry)N1-φ Oφ, which has a comparatively high residual conductivity and reacts with iridium (Ir) under a high anodic load to form a stable quaternary mixed oxide, is formed during operation of a fuel cell. The free surface 30 of the covering layer 3a remains on the exterior, so that the surface of the covering layer 3a facing away from the substrate 2 has a shiny silvery appearance even after subjection to +2000 mV relative to a standard hydrogen electrode over a period of 50 hours. Even under a scanning electron microscope, it is not possible to discern any traces of corrosion extending through the thickness of the covering layer 3a to the substrate 2 or reaching the substrate 2.

(8) The covering layer 3a according to the disclosure of the second working example can be applied both by the sputtering technique and also by a cathodic arc coating process, also referred to as vacuum electric arc evaporation. Despite a higher droplet count, in other words a metal droplet count which is higher than in the case of sputtering technology, the covering layer 3a according to the disclosure produced in the cathodic arc process also has the advantageous properties of high corrosion resistance combined with temporally stable surface conductivity of the covering layer 3a according to the disclosure produced by the sputtering technique.

(9) In a third embodiment, the layer system 3 according to the disclosure is formed on a substrate 2 in the form of a structured perforated stainless steel sheet. The substrate 2 has been electrolytically polished in an H2SO4/H3PO4 bath before application of a layer system 3. After application of a single base layer in the form of a tantalum carbide layer having a thickness of a number of 1000 nm, a covering layer 3a in the form of an iridium carbide layer having a thickness of a number of 100 nm is applied.

(10) The advantage of the base layer formed by the tantalum carbide is not only its extraordinary corrosion resistance but also the fact that it does not absorb hydrogen and thus serves as hydrogen barrier in respect of the substrate 2. This is particularly advantageous if titanium is used as substrate material.

(11) The layer system 3 according to the disclosure 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-2.

(12) The advantage of the metalloid layer which has an intermediate position in the layer system and/or is closed on both sides or of the second base layer, which in the simplest case is formed, for example, by titanium nitride, is its low electrical resistance of 10-12 mΩ cm-2. Likewise, the layer or covering layer according to the disclosure can also be configured without a second base layer or metalloid layer, with a possible increase in resistance.

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

(14) TABLE-US-00001 TABLE 1 Layers and selected characteristic values Corrosion current at 2000 mV standard hydrogen column Oxidation stability at in μA cm.sup.−2 in 2000 mV measured as aqueous sulfuric change in the surface Layer system/layer Surface acid solution resistance in thickness resistance (pH 3) at T = 800° C. mΩ cm.sup.−2 1 Gold/3 μm 9 >100 pitting current  9-10 (as reference) 2 Ti/0.5 μm 8 0.001 12 TiN/1 μm (Nb.sub.0.1Ir.sub.0.9)C.sub.1−δ/10 nm 3 TiNb/0.5 μm 10-11 0.001 10-11 TiN/1 μm (Nb.sub.0.1Ir.sub.0.9)N.sub.1.05/10 nm 4 TiNb/0.1 μm 7-8 0.01 4-6 IrC/10 nm 5 Ta/0.05 μm 10  0.001 17-18 TaC/0.5 μm (Ta,Ir)C/5 nm 6 ZrB.sub.2/0.3 μm 7 Pitting reaction after (Zr.sub.0.3Ir.sub.0.7)B.sub.2−δ/10 nm exposure for 4 hours

(15) Table 1 shows only some illustrative layer systems. The layer systems according to the disclosure advantageously do not display any increase in resistance at an anodic potential of +2000 mV relative to a standard hydrogen column in sulfuric acid solution at a temperature of 80° C. over a number of weeks. Some of the layer systems applied in high vacuum by a sputtering or ARC process or in a fine vacuum by the PECVD process (plasma enhanced chemical vapor deposition process) had a dark discoloration after this period of exposure. However, there were no visible corrosion phenomena or significant changes in the surface resistances.

LIST OF REFERENCE NUMERALS

(16) 1 bipolar plate 2 substrate 3 layer system 3a covering layer 4 base layer system 4a first base layer 4b second base layer 30 free surface