CLIAMS:We Claim:

1. A bipolar plate for a fuel cell comprising:
a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path; and the flow channel of each sub group forms a combination of serpentine and parallel flow paths; and the number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same.

2. A bipolar plate as claimed in claim 1, wherein each flow channel extends between a fuel inlet and a fuel outlet.

3. A bipolar plate as claimed in claim 1, wherein each flow channel extends between an oxidant inlet and an oxidant outlet.

4. A bipolar plate as claimed in claim 1, wherein the length of flow channel of a group and the total length of flow channel of all sub groups of a group is substantially the same.

5. A bipolar plate as claimed in claim 1, wherein the length of flow channel of each group is the same.

6. A bipolar plate as claimed in claim 1, wherein each flow channel has a straight line path followed by an artery that converts the flow channel into a sub group.
7. A polymer electrolyte membrane fuel cell comprising a bipolar plate, the bipolar plate comprising:
a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path; and the flow channel of each sub group forms a combination of serpentine and parallel flow paths; and the number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same.

8. A polymer electrolyte membrane fuel cell comprising a bipolar plate as claimed in claim 7, wherein each flow channel extends between a fuel inlet and a fuel outlet.

9. A polymer electrolyte membrane fuel cell comprising a bipolar plate as claimed in claim 7, wherein each flow channel extends between an oxidant inlet and an oxidant outlet.

10. A polymer electrolyte membrane fuel cell comprising a bipolar plate as claimed in claim 7, wherein the length of flow channel of a group and the total length of flow channel of all sub groups of a group is substantially the same.

11. A polymer electrolyte membrane fuel cell comprising a bipolar plate as claimed in claim 7, wherein the length of flow channel of each group is the same.

12. A polymer electrolyte membrane fuel cell comprising a bipolar plate as claimed in claim 7, wherein each flow channel has a straight line path followed by an artery that converts the flow channel into a sub group.
,TagSPECI:FIELD OF INVENTION
The present disclosure provides a bipolar plate and a polymer electrolyte membrane fuel cell comprising said bipolar plate.

BACKGROUND
Polymer electrolyte membrane (PEM) fuel cell is an electrochemical energy conversion device that converts chemical energy of a fuel into electrical energy and heat as long as the fuel is supplied. A fuel cell stack is formed by stacking a plurality of single cells that are electrically connected in series. Bipolar plates are important and effective elements as far as efficiency and power density gains of fuel cells are concerned. The bipolar plates supply fuel and oxidant to the reactive zones, remove reaction products, collect produced current and provide mechanical support to the cells in the stack. Typically, more than 60% of the weight and 30% of the total cost of fuel cells stacks is due to bipolar plates.
Efficiency of PEM fuel cells can be improved by improving the performance, reliability and durability of bipolar plates. Conventional bipolar plates are not efficient as regards gas pressure drop in the flow channels, concentration gradient between the flow channels, gas flow distribution over the plate and electrode surface, fuel utilization etc. FIG. 1 is a plan view of a surface of a conventional bipolar plate having serpentine flow path. In the conventional bipolar plate 11, a plurality of fuel/oxidant flow channels 13 are formed in a serpentine pattern. Manifolds 15a, 15b, 25a and 25b coupled with inlet and outlet of the fuel/oxidant flow channels 13 through which liquid fuel or oxidant is supplied or discharged, are formed through the bipolar plate 11. The fuel/oxidant path holes 15a, 15b, 25a and 25b form an inlet 15a and an outlet 15b of the liquid fuel and an inlet 25a and an outlet 25b of the oxidant. Active area of the bipolar plate is indicated as 37 and flow channel depth (channel depth) is indicated as 35.
In the flow channel with the serpentine shape shown in FIG. 1, the fuel/oxidant concentration gradient between the fuel/oxidant manifold into which fuel/oxidant flows and the fuel/oxidant manifold through which fuel/oxidant and a reaction product are discharged, is large. In addition, when the fuel/oxidant manifolds and are formed at the same side, a plurality of flow paths may vary in length between the fuel/oxidant path holes and, and thus, flow velocities at the flow paths can be different. In addition, since the length of a flow path is large, a pressure loss is large.
Therefore, there is a need for an improved bipolar plate which increases the efficiency of fuel cells in particular PEM fuel cells.

SUMMARY
The present disclosure provides a bipolar plate for a fuel cell. The bipolar plate comprising a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path; and the flow channel of each sub group forms a combination of serpentine and parallel flow paths; and the number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same.

Further the present disclosure also provides a polymer electrolyte membrane fuel cell comprising a bipolar plate, the bipolar plate comprising a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path; and the flow channel of each sub group forms a combination of serpentine and parallel flow paths; and the number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same.

BRIEF DESCRIPTION OF DRAWINGS
Figure 1 is a plan view of a conventional bipolar plate.
Figure 2 is a plan view of a bipolar plate in accordance with the present invention.
Figure 3 is an enlarged view of a section (one group) of a bipolar plate in accordance with the present invention.

DETAILED DESCRIPTION
For the purpose of promoting an understanding of the principles of the invention, reference will now be made to embodiments and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the disclosed process and system, and such further applications of the principles of the invention therein being contemplated as would normally occur to one skilled in the art to which the invention relates.
It will be understood by those skilled in the art that the foregoing general description and the following detailed description are exemplary and explanatory of the invention and are not intended to be restrictive thereof.
Reference throughout this specification to “one embodiment” “an embodiment” or similar language means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrase “in one embodiment”, “in an embodiment” and similar language throughout this specification may, but do not necessarily, all refer to the same embodiment.
The present disclosure relates to a bipolar plate for a fuel cell. The bipolar plate comprises a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path and the flow channel of each sub group forms a combination of serpentine and parallel flow paths. The number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same.
Figure 2 is a plan view of a surface of a bipolar plate in accordance with an embodiment of the present invention. Referring to figure 2, flow channels 13 through which fuel/ oxidant flows is formed on a top 39 and bottom surface 41 of the bipolar plate 11. The flow channels 13 communicate with fuel/ oxidant path holes 15a, 15b, 25a and 25b through which fuel or oxidant is supplied or discharged. The fuel/ oxidant path holes include an inlet 15a and outlet 15b for fuel and an inlet 25a and an outlet 25b for oxidant, which are formed through the bipolar plate 11.
In accordance with an embodiment, the fuel is hydrogen and methanol and the oxidant is air/ oxygen.
In accordance with an embodiment, the flow channels extend between a fuel inlet and a fuel outlet in the bipolar plate. The flow channels extend between an oxidant inlet and an oxidant outlet in the bipolar plate. Referring to figure 2, flow channels 13 formed on top surface 39 of the bipolar plate 11 are divided in to plurality of groups 17, such that each flow channel has the same length and forms one group. Each group forms a plurality of subgroups 19. Length of the flow channels of the groups and the sub groups is substantially the same. Figure 3 illustrates an enlarged view of a section (one group) of the bipolar plate 11 indicating that the flow channel of each group forms a serpentine flow path and the flow channel of each sub group forms a combination of serpentine and parallel flow paths. Presence of same length flow channels and the combination of parallel and serpentine flow paths facilitates enhanced reduction in gas pressure drop throughout the bipolar plate. Further, the combination of parallel and serpentine flow paths provide high residence time for improved mass diffusion rate. The flow channels 13 are separated by the ribs 31 having rib edge 33.
In accordance with an embodiment, the length of the flow channel of each group is the same. Further, each flow channel has a straight line path followed by an artery that converts the flow channel into a sub group. Referring to figure 2, each flow channel 13 has a straight line path followed by an artery 29 that converts the flow channel into a sub group 19. The length of a flow channel path in a sub group is short which facilitates enhanced reduction in gas pressure drop throughout the bipolar plate.
In accordance with an embodiment, in the bipolar plate the length of flow channel of a group and the total length of flow channel of all sub groups of a group is substantially the same. The length of flow channel of each group is the same.
The disclosed bipolar plate for a fuel cell provides enhanced reduction in gas pressure drop in the flow channels and concentration gradient between the flow channels. Further, the fuel cell having such bipolar plate has improved gas flow distribution over the plate and electrode surface; and mean residence time resulting in enhanced mass diffusion rate. Such fuel cell has improved fuel utilization rate i.e. up to 95% to 98%.
The present disclosure also relates to a polymer electrolyte membrane fuel cell comprising a bipolar plate disclosed herein above.

Experiments:
Use of a conventional bipolar plate (having serpentine flow paths) for a polymer electrolyte membrane fuel cell has indicated the current density as 150 – 200 mA/cm2 and power density as 100 – 130 mW/cm2.
The following experiments are provided to illustrate the most important and effective elements on efficiency and power density gains of fuel cells.

Experiment 1:
The following experiment is for design of a bipolar plate, where the number of groups in the bipolar plate is changed from 2 to 10. The design parameters of bipolar plate and fuel cell stack are as follows:

PARAMETERS VALUES
Channel width 0.3 to 3 mm
Channel depth 0.3 to 3 mm
Rib width 0.3 to 3 mm
Channel edge radius 0.1 to 1mm
Arteries edge radius 0.1 to 1 mm
No of sub groups 2 to 10
No of parallel channels in sub group 2 to 7
No of serpentine channels in sub group 1 to 7
Electrode Active area 130 to 200cm2
No of cells 5 to 50
Anode catalyst Pt-MASA
Cathode catalyst Pt-AASA
Membrane Nafion 212
Catalyst loading 0.2 mg/cm2 on both side

The power density obtained by changing the number of groups from 2 to 10 is 195-260 mW/cm2 at a constant voltage of 0.65V, and the same clearly indicates improved efficiency and power density gains of fuel cell when compared to the fuel cell using conventional bipolar plate.

Experiment 2:
The following experiment is for design of a bipolar plate, where the number of sub groups in the bipolar plate is changed from 2 to 10. The design parameters of bipolar plate and a fuel cell stack are as follows:

PARAMETERS VALUES
Channel width
Channel depth 0.3 to 3 mm
0.3 to 3 mm
Rib width 0.3 to 3 mm
Channel edge radius 0.1 to 1mm
Arteries edge radius 0.1 to 1 mm
No of groups 2 to 10
No of parallel channels in sub group 2 to 7
No of serpentine channels in sub group 1 to 7
Electrode Active area 130 to 200cm2
No of cells 5 to 50
Anode catalyst Pt-MASA
Cathode catalyst Pt-AASA
Membrane Nafion 212
Catalyst loading 0.2 mg/cm2 on both side

The power density obtained by changing the number of sub groups from 2 to 10 is 163-260mW/cm2 at a constant voltage of 0.65V, and the same clearly indicates improved efficiency and power density gains of fuel cell when compared to the fuel cell using conventional bipolar plate.

Experiment 3:
The following experiment is for design of bipolar plate, where the channel depth of the sub group and group in the bipolar plate is changed from 0.3 to 1.8 mm. The design parameters of bipolar plate and fuel cell stack are as follows:

PARAMETERS VALUES
Channel width 0.3 to 3 mm
Rib width 0.3 to 3 mm
Channel edge radius 0.1 to 1mm
Arteries edge radius 0.1 to 1 mm
No of groups 2 to 10
No of sub groups 2 to 10
No of parallel channels in sub group 2 to 7
No of serpentine channels in sub group 1 to 7
Electrode Active area 130 to 200cm2
No of cells 5 to 50
Anode catalyst Pt-MASA
Cathode catalyst Pt-AASA
Membrane Nafion 212
Catalyst loading 0.2 mg/cm2 on both side

The power density obtained by changing the channel depth of the sub group and group from 0.3 to 1.8 mm is 195-260mW/cm2 at a constant voltage of 0.65V, and the same clearly indicates improved efficiency and power density gains of fuel cell when compared to the fuel cell using conventional bipolar plate.

Experiment 4:
The following experiment is for design of bipolar plate, where the channel width in the sub group and group in the bipolar plate is changed from 0.3 to 2.5mm. The design parameters of bipolar plate and a fuel cell stack are as follows:

PARAMETERS VALUES
Channel depth 0.3 to 3 mm
Rib width 0.3 to 3 mm
Channel edge radius 0.1 to 1mm
Arteries edge radius 0.1 to 1 mm
No of groups 2 to 10
No of sub groups 2 to 10
No of parallel channels in sub group 2 to 7
No of serpentine channels in sub group 1 to 7
Electrode Active area 130 to 200cm2
No of cells 5 to 50
Anode catalyst Pt-MASA
Cathode catalyst Pt-AASA
Membrane Nafion 212
Catalyst loading 0.2 mg/cm2 on both side

The power density obtained by changing the channel width of the sub group and group from 0.3 to 2.5 mm is 130-260 mW/cm2 at a constant voltage of 0.65V, and the same clearly indicates improved efficiency and power density gains of fuel cell when compared to the fuel cell using conventional bipolar plate.

Experiment 5:
The following experiment is for design of bipolar plate, where the rib width in the sub group and group in the bipolar plate is changed from 0.3 to 2.5mm. The design parameters of bipolar plate and a fuel cell stack are as follows:

PARAMETERS VALUES
Channel width 0.3 to 3 mm
Channel depth 0.3 to 3 mm
Channel edge radius 0.1 to 1mm
Arteries edge radius 0.1 to 1 mm
No of groups 2 to 10
No of sub groups 2 to 10
No of parallel channels in sub group 2 to 7
No of serpentine channels in sub group 1 to 7
Electrode Active area 130 to 200cm2
Anode catalyst = Pt-MASA 5 to 50
Cathode catalyst Pt-AASA
Membrane Nafion 212
Catalyst loading 0.2 mg/cm2 on both side

The power density obtained by changing the rib width of the sub group and group from 0.3 to 2.5 mm is 195-260mW/cm2 at a constant voltage of 0.65V, and the same clearly indicates improved efficiency and power density gains of fuel cell when compared to the fuel cell using conventional bipolar plate.

Experiment 6:
The following experiment is for design of bipolar plate, where the channel edge radius in the sub group and group in the bipolar plate is changed from 0.1 to 1mm. The design parameters of bipolar plate and fuel cell stack are as follows:

PARAMETERS VALUES
Channel width 0.3 to 3 mm
Channel depth 0.3 to 3 mm
Rib width 0.3 to 3 mm
Arteries edge radius 0.1 to 1 mm
No of groups 2 to 10
No of sub groups 2 to 10
No of parallel channels in sub group 2 to 7
No of serpentine channels in sub group 1 to 7
Electrode Active area 130 to 200cm2
No of cells 5 to 50
Anode catalyst Pt-MASA
Cathode catalyst Pt-AASA
Membrane Nafion 212
Catalyst loading 0.2 mg/cm2 on both side

The power density obtained by changing the rib width of the sub group and group from 0.1 to 1 mm is 195-260mW/cm2 at a constant voltage of 0.65V, and the same clearly indicates improved efficiency and power density gains of fuel cell when compared to the fuel cell using conventional bipolar plate.
Following Table provides comparative results of a fuel cell using conventional bipolar plate and bipolar plate (experiment no. 1 to 6) as per present disclosure:

SNo, Bipolar plate and experiments Current density @ 0.65 V Power density @ 0.65 V
1 Conventional bipolar plate 150 - 200 mA/cm2 100 - 130 mW/cm2
2 Experiments = 1 300 - 400 mA/cm2 195 - 260 mW/cm2
3 Experiments = 2 250 - 400 mA/cm2 163 -260 mW/cm2
4 Experiments = 3 300 - 400 mA/cm2 195 -260 mW/cm2
5 Experiments = 4 200 - 400 mA/cm2 130 - 260 mW/cm2
6 Experiments = 5 250 - 400 mA/cm2 195 -260 mW/cm2
7 Experiments = 6 300 - 400 mA/cm2 195 -260 mW/cm2

SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
A bipolar plate for a fuel cell comprising a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path; and the flow channel of each sub group forms a combination of serpentine and parallel flow paths; and the number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same.
Such bipolar plate(s), wherein each flow channel extends between a fuel inlet and a fuel outlet.
Such bipolar plate(s), wherein each flow channel extends between an oxidant inlet and an oxidant outlet.
Such bipolar plate(s), wherein the length of flow channel of a group and the total length of flow channel of all sub groups of a group is substantially the same.
Such bipolar plate(s), wherein the length of flow channel of each group is the same.
Such bipolar plate(s), wherein each flow channel has a straight line path followed by an artery that converts the flow channel into a sub group.
FURTHER SPECIFIC EMBODIMENTS ARE DESCRIBED BELOW
A polymer electrolyte membrane fuel cell comprising a bipolar plate, the bipolar plate comprising a plurality of flow channels extending between an inlet and an outlet, each flow channel having the same length and forming one group, each group forming a plurality of sub groups; wherein the flow channel of each group forms a serpentine flow path; and the flow channel of each sub group forms a combination of serpentine and parallel flow paths; and the number of sub groups is equal to the number of flow channels and the length of the flow channel of each sub group is the same. Such polymer electrolyte membrane fuel cell(s), wherein each flow channel extends between a fuel inlet and a fuel outlet.
Such polymer electrolyte membrane fuel cell(s), wherein each flow channel extends between an oxidant inlet and an oxidant outlet.
Such polymer electrolyte membrane fuel cell(s), wherein the length of flow channel of a group and the total length of flow channel of all sub groups of a group is substantially the same.
Such polymer electrolyte membrane fuel cell(s), wherein the length of flow channel of each group is the same.
Such polymer electrolyte membrane fuel cell(s), wherein each flow channel has a straight line path followed by an artery that converts the flow channel into a sub group.

INDUSTRIAL APPLICABILITY
The disclosed bipolar plate comprising plurality of flow channels such that each flow channel forms one group and each group forms a plurality of sub groups. Such bipolar plate is advantageous in terms of providing enhanced reduction in gas pressure drop, enhanced reduction in concentration gradient between the flow channels and increased mean residence tine facilitating improved mass diffusion rate, as compared to conventional bipolar plates. The disclosed bipolar plate for PEM fuel cell facilitates improved uniform gas flow distribution over the plate and electrode surface; and also utilizes fuel at a rate up to 95% to 98%.