Bipolar separators with improved fluid distribution

1. A fuel cell separator comprising a first plate, the first plate comprising:

a first active surface comprising a first reactant flow field comprising a number of flow channels and a first header fluidly connected to the flow channels;

an opposing first non-active surface; and

a first header channel fluidly connected to the first header;

wherein the first header further comprises a first recess directly fluidly connected to one end of the first header channel, wherein the first recess comprises a first top perimeter and a first side wall, and wherein said recess is dimensioned to reduce pressure gradients within the first header, to decrease fluid velocity, to provide a more uniform supply of fluid to the flow channels, thereby creating a more uniform channel-to-channel flow distribution than in the absence of the recess.

2. The fuel cell separator of claim 1 wherein the first side wall has a draft angle of less than about 90 degrees with respect to the first active surface.

3. The fuel cell separator of claim 1 further comprising at least one protrusion extending from the first active surface.

4. The fuel cell separator of claim 1 wherein the first plate is comprised of a material selected from the group consisting of a metallic, carbonaceous, graphitic, and polymeric materials.

5. The fuel cell separator of claim 1 wherein the first header is formed on the first active surface.

6. The fuel cell separator of claim 1 wherein the first header channel is formed in the first non-active surface, and the first side wall of the first header further comprises at least one fluid port fluidly connected to the end of the first header channel.

7. The fuel cell separator of claim 6 wherein at least a portion of the first side wall is curved.

8. The fuel cell separator of claim 1 wherein the first plate further comprises a manifold opening fluidly connected to the other end of the first header channel that cooperate to form an internal manifold.

9. The fuel cell separator of claim 1 further comprising a second plate comprising a second active surface and an opposing second non-active surface adjoining the first non-active surface.

10. The fuel cell separator of claim 9 wherein the first and second non-active surfaces are bonded together.

11. The fuel cell separator of claim 9 wherein the first and second non-active surfaces cooperate to form a coolant flow field therebetween.

12. The fuel cell separator of claim 9 wherein the first plate is an anode plate and the second plate is a cathode plate.

13. The fuel cell separator of claim 9 wherein the first header is a supply header.

14. The fuel cell separator of claim 9 wherein the first header is an exhaust header.

15. The fuel cell separator of claim 9 wherein the second plate further comprises: a second flow field comprising a number of flow channels and a second header fluidly connected to the flow channels on the second active surface; and

a second header channel fluidly connected to the second header,

wherein the second header further comprises a second recess directly fluidly connected to one end of the second header channel, wherein the second recess comprises a second top perimeter and a second side wall, and wherein said second recess is dimensioned to reduce pressure gradients within the second header, to decrease fluid velocity, to provide a more uniform supply of fluid to the flow channels, thereby creating a more uniform channel-to-channel flow distribution than in the absence of the recess.

16. An electrochemical fuel cell stack comprising:

a plurality of electrode assemblies each comprising a first electrode, a second electrode, and an electrolyte interposed between the first and second electrodes;

a separator interposed between adjacent pairs of the plurality of electrode assemblies, the separator comprising a first plate, the first plate comprising:

a first active surface comprising a first reactant flow field comprising a number of flow channels and a first header fluidly connected to the flow channels;

an opposing first non-active surface; and

a first header channel fluidly connected to the first header;

wherein the first header further comprises a first recess directly fluidly connected to one end of the first header channel, wherein the first recess comprises a first top perimeter and a first side wall, and wherein said recess is dimensioned to reduce pressure gradients within the first header, to decrease fluid velocity, to provide a more uniform supply of fluid to the flow channels, thereby creating a more uniform channel-to-channel flow distribution than in the absence of the recess.

17. The electrochemical fuel cell stack of claim 16 wherein the first header is formed on the first active surface.

18. The electrochemical fuel cell stack of claim 16 wherein the first header channel is formed in the first non-active surface, and the first side wall of the first header further comprises at least one fluid port fluidly connected to the end of the first header channel.

19. The electrochemical fuel cell stack of claim 16 further comprising a second plate comprising a second active surface and an opposing second non-active surface adjoining the first non-active surface.

20. The electrochemical fuel cell stack of claim 19 wherein the second plate further comprises:

a second flow field and a second header fluidly connected thereto on the second active surface; and

a second header channel fluidly connected to the second header;

wherein the second header further comprises a second recess directly fluidly connected to one end of the second header channel, wherein the second recess comprises a second top perimeter and a second side wall.