Fuel cell flow channels and flow fields
A fuel cell anode flow field includes at least one flow channel with a cross-sectional area that varies along at least a portion of its length. In some embodiments, the channel width decreases along at least a portion of the channel length according to a natural exponential function. This type of anode flow field can improve performance, reduce fuel consumption and/or reduce detrimental effects such as carbon corrosion and catalyst degradation, thereby improving fuel cell longevity and durability. When operating the fuel cell on either a substantially pure or a dilute fuel stream, this type of anode flow field can provide more uniform current density. These flow channels can be incorporated into reactant flow field plates, fuel cells and fuel cell stacks.
1. A method of operating a fuel cell on substantially pure hydrogen and air to produce electric power, the method comprising:
(a) connecting said fuel cell to an electrical load, wherein said fuel cell comprises a membrane electrode assembly interposed between a first separator plate and a second separator plate, said membrane electrode assembly comprising an anode, a cathode, and a proton exchange membrane interposed between said anode and said cathode;
(b) supplying substantially pure hydrogen to said anode via an anode flow channel formed in a major surface of said first separator plate, wherein said anode flow channel has an anode flow channel profile with a cross-sectional area that decreases continuously in the fuel flow direction;
(c) supplying air to said cathode via a cathode flow channel formed in a major surface of said second separator plate,
(d) selecting, based at least in part on said anode flow channel profile, a fuel stoichiometry for said supplying said substantially pure hydrogen to said anode flow channel that causes said fuel cell to produce said electric power
with a current density distribution across said fuel cell that approaches or achieves uniform current density distribution.
2. The method according to claim 1 , wherein said anode flow channel has a width that decreases exponentially in the fuel flow direction.
3. The method according to claim 1 , wherein said anode flow channel has a cross-sectional area that decreases continuously along its entire length.
4. The method according to claim 1 , wherein said cathode flow channel has a cathode channel profile with a cross-sectional area that decreases continuously in the air flow direction, and air is supplied to said cathode flow channel at a stoichiometry selected in combination with said cathode channel profile, such that oxidant availability at progressively downstream lengthwise positions along said cathode flow channel approaches or achieves constant oxidant availability.
5. The method according to claim 1 , wherein said anode flow channel has a width that decreases substantially continuously in the fuel flow direction.
6. The method according to claim 1 , wherein said anode flow channel has a channel depth that is substantially constant and a channel width that decreases exponentially in the fuel flow direction.
7. The method according to claim 1 , wherein said anode flow channel has a channel width at a selected lengthwise position along said anode flow channel that is proportional to a natural exponential function of said selected lengthwise position, said natural exponential function comprising a constant derived from said fuel stoichiometry.
8. The method according to claim 1 , wherein said anode flow channel has a channel width that decreases continuously as a function of distance along its length, and wherein the relationship between said anode channel profile and said fuel stoichiometry is such that:
W
(
x
)
=
W
(
0
)
e
-
x
L
ln
(
STOICH
STOICH
-
1
)
where W(x) is the channel width at lengthwise position x, x is a selected position along the channel length [m], W(0) is the channel width at the fuel inlet, L is the channel length, and STOICH is said fuel stoichiometry.
9. The method according to claim 1 , said method comprising selecting, based at least in part on said anode flow channel profile, a fuel stoichiometry for supplying said substantially pure hydrogen to said anode flow channel that causes said fuel cell to produce said electric power with uniform current density distribution across said fuel cell.