Fuel cell with passive water balance

1. A fuel cell ( 12 ) for generating electrical current from first and second reactant streams, comprising:

a. a membrane electrode assembly ( 46 );

b. a first reactant flow field ( 80 ) secured adjacent a surface selected from the group consisting of a first surface ( 48 ) and a second surface ( 50 ) of the membrane electrode assembly ( 46 ) for directing flow of a first reactant adjacent the selected surface ( 48 , 50 ) of the assembly ( 46 );

c. wherein the first reactant flow field ( 80 ) defines a plurality of two-pass circuits ( 82 , 84 , 86 , 88 ), each two-pass circuit being in fluid communication with a first reactant inlet ( 90 ) for directing the first reactant into the fuel cell ( 12 ), and in fluid communication with a first reactant outlet ( 92 ) for directing the first reactant out of the fuel cell ( 12 ); and,

d. wherein the reactant flow field ( 80 ) defines a common turn-around ( 96 ) in fluid communication with each two-pass circuit ( 82 , 84 , 86 , 88 ), and the common turn-around ( 96 ) being defined between the first reactant inlet ( 90 ) and the first reactant outlet ( 92 ) so that the first reactant flowing from the first reactant inlet ( 90 ) to the first reactant outlet ( 92 ) passes through the common turn-around ( 96 ) and raises within the common turn around ( 96 ) with the first reactant passing through the other of the plurality of two-pass circuits ( 84 , 86 , 88 ).

2. The fuel cell ( 12 ) of claim 1 , wherein each two-pass circuit ( 82 , 84 , 86 , 88 ) defines a width across the two-pass circuit ( 82 ), when divided by a cross-flow length ( 114 ) of the first reactant flow field, that is greater than 0.1 and less than 0.5, wherein the width across each two-pass circuit ( 82 ) is a shortest distance across the two-pass circuit ( 82 ) in a direction perpendicular to flow of the first reactant through the two-pass circuit ( 82 ), and the crossflow length ( 114 ) of the first reactant flow field ( 80 ) is a shortest distance across the first reactant flow field ( 80 ) in a direction perpendicular to flow of the first reactant through the flow field ( 80 ).

3. The fuel cell ( 12 ) of claim 1 , wherein the first reactant flow field ( 80 ) is a cathode flow field ( 20 ) for directing flow of an oxygen containing oxidant reactant adjacent the selected surface ( 48 ) of the membrane electrode assembly.

4. The fuel cell ( 12 ) of claim 1 , wherein the first reactant flow field ( 80 ) is an anode flow field ( 28 ) for directing flow of a hydrogen containing reducing fluid adjacent the selected surface ( 50 ) of the membrane electrode assembly.

5. The fuel cell ( 12 ) of claim 1 , further comprising a second reactant flow field ( 28 ) secured adjacent the first or second surface ( 48 , 50 ) for directing flow of a second reactant adjacent the first or second surface ( 48 , 50 ), wherein the second reactant flow field ( 28 ) defines a plurality of two-pass circuits ( 82 , 84 , 86 , 88 ), each two-pass circuit being in fluid communication with a reactant inlet ( 90 ) for directing the second reactant into the fuel cell ( 12 ), and in fluid communication with a reactant outlet ( 92 ) for directing the second reactant out of the fuel cell ( 12 ).

6. The fuel cell ( 12 ) of claim 1 , wherein the first reactant flow field ( 80 ) is a cathode flow field ( 20 ), the number of two-pass circuits ( 82 , 84 , 86 , 88 ) in each reactant flow field is greater than or equal to 2 and less than 10, a width across a two-pass circuit ( 82 ) divided by a parallel-flow length of the two-pass circuit ( 82 ) is greater than 0.3 and less than 1.0, wherein the parallelflow length of the two-pass circuit ( 82 ) is one-half of a shortest distance along the two-pass circuit ( 82 ) from a point of entry of the reactant stream into the circuit ( 82 ) to a point of exit of the reactant stream from the circuit ( 82 ) in a direction parallel to flow of the stream through the circuit ( 82 ).

7. A method of managing water within a fuel cell ( 12 ) comprising the steps of:

a. securing a first reactant flow field ( 80 ) within a fuel cell ( 12 ) adjacent a surface of a membrane electrode assembly ( 46 ) selected from the group consisting of a first surface ( 48 ) and a second surface ( 50 ) of the assembly ( 46 ); and,

b. directing a first reactant to flow through a plurality of two-pass circuits ( 82 , 84 , 86 , 88 ) defined within the first reactant flow field ( 80 ); and,

c. directing the first reactant to flow from reactant inlets ( 90 ) of the two-pass circuits ( 82 , 84 , 86 , 88 ) through a common turn-around ( 96 ) defined within the reactant flow field ( 80 ), then mixing the first reactants from the reactant inlets ( 90 ) within the common turnaround ( 96 ), and then directing the first reactant to flow from the common turn-around ( 96 ) through reactant outlets ( 92 ) of the two-pass circuits ( 82 , 84 , 86 , 88 .

8. A fuel cell ( 12 ) for generating electrical current from first and second reactant streams, comprising:

a. a membrane electrode assembly ( 46 );

b. a first reactant flow field ( 80 ) secured adjacent a surface selected from the group consisting of a first surface ( 48 ) and a second surface ( 50 ) of the membrane electrode assembly ( 46 ) for directing flow of a first reactant adjacent the selected surface ( 48 , 50 ) of the assembly ( 46 );

c. wherein the, first reactant flew field ( 80 ) defines a plurality of two-pass circuits ( 82 , 84 , 86 , 88 ) defined within porous layers of the flow field ( 80 ), each two-pass circuit being in fluid communication with a first reactant inlet ( 90 ) for directing the first reactant into the fuel cell ( 12 ), and in fluid communication with a first reactant outlet ( 92 ) for directing the first reactant out of the fuel cell ( 12 ); and,

d. wherein the first reactant flow field ( 80 ) defines a common turn-around ( 96 ) in fluid communication with each two-pass circuit ( 82 , 84 , 86 , 88 ), and the common turn-around ( 96 ) being defined between the first reactant inlet ( 90 ) and the first reactant outlet ( 92 ) so that the first reactant flowing from the first reactant inlet ( 90 ) to the first reactant outlet ( 92 ) passes through the common turnaround ( 96 ) and mixes within the common turn around ( 96 ) with the first reactant passing through the other of the plurality of two-pass circuits ( 84 , 86 , 88 ).