Metal bipolar plate

1. A fuel cell comprising:

a metal bipolar plate; and

a layer coating the plate, the layer having a contacting external surface and having a plurality of phases at least in a region of the contacting external surface, a first phase being a metallic phase and/or an associated α-phase of the metallic phase, and second phase being a compound phase including one or more metals of the metallic phase and one of the elements N, O, C and B, wherein the layer has a surface resistance lower than about 20 mΩ/cm 2 .

2. The fuel cell of claim 1 wherein the metallic phase is composed of Cr, Ti, Mo, W, V, Fe, or an alloy thereof.

3. The fuel cell of claim 1 wherein the metallic phase is composed of Cr or of the α-phase of Cr.

4. The fuel cell of claim 3 wherein the compound phase is composed of Cr 2 N and/or Cr 7 C 3 .

5. The fuel cell of claim 1 wherein the metallic phase is composed of an alloy of Cr—Ti or Cr—Mo.

6. The fuel cell of claim 5 wherein the compound phase is composed of a ternary compound (Cr,Mo) 2 N or (Cr,Ti) 2 N.

7. The fuel cell of claim 1 wherein a composition of a heterogeneous mixed phase including the first and second phases has a content of nitrogen, carbon, oxygen or boron between x=4 atom % and x=45 atom %.

8. The fuel cell of claim 1 wherein a concentration of the compound phase decreases toward the external surface.

9. The fuel cell of claim 1 wherein in a region of the layer on a side of the plate, only the metallic phase is present.

10. The fuel cell of claim 1 further comprising a Pt film on the external surface of the layer.

11. The fuel cell of claim 10 wherein the Pt film has a thickness of less than 100 nm.

12. The fuel cell of claim 11 wherein the thickness of the Pt film is less than or equal to 50 nm.