Reactant flow channels for electrolyzer applications
An electrolyzer or unitized regenerative fuel cell has a flow field with at least one channel, wherein the cross-sectional area of the channel varies along at least a portion of the channel length. In some embodiments the channel width decreases along at least a portion of the length of the channel according to a natural exponential function. The use of this type of improved flow field channel can improve performance and efficiency of operation of the electrolyzer device.
1. An electrolyzer assembly for generating hydrogen and oxygen from water, said electrolyzer assembly comprising:
a first unit cell wherein said first unit cell comprises:
(a) a membrane electrode assembly comprising a proton exchange membrane interposed between an anode and a cathode;
(b) a cathode flow field plate adjacent to said cathode; and
(c) an anode flow field plate adjacent to said anode, said anode flow field plate having an anode channel for directing water in contact with said anode, wherein said anode channel has a length, and a cross sectional area that varies along at least a portion of said length;
wherein the depth of said anode channel is substantially constant, and the width of said anode channel decreases along at least a portion of said channel length in a direction of reactant flow according to a natural exponential function.
2. The electrolyzer assembly of claim 1 , wherein said natural exponential function is a function of position along the length of said channel.
3. The electrolyzer assembly of claim 1 , wherein said channel width decreases along substantially the entire length of said anode channel.
4. The electrolyzer assembly of claim 1 , wherein the width of said anode channel is substantially constant for a portion of said channel length and said channel width varies along another portion of said channel length.
5. The electrolyzer assembly of claim 1 , wherein said natural exponential function is:
W
(
x
)
=
ST
H
2
O
k
H
2
O
i
t
Dv
ⅇ
l
n
(
ST
H
2
O
-
1
ST
H
2
O
)
x
L
where W(x) is the anode channel width at lengthwise position x; x is a selected position along the channel length; D is the channel depth; v is constant flow velocity; ST H 2 O is water stoichiometry; k H 2 O is flow rate coefficient for water; i d is the total channel current; and L is the channel length.
6. The electrolyzer assembly of claim 5 , further comprising a second unit cell wherein said first unit cell and said second unit cell are arranged in a stack.
7. The electrolyzer assembly of claim 6 , further comprising a water supply configured to deliver water to said anode channels.
8. The electrolyzer assembly of claim 7 , further comprising a power supply configured to deliver electrical power to said electrolyzer assembly.
9. The electrolyzer assembly of claim 8 , further comprising a hydrogen containment vessel configured to collect hydrogen generated by said electrolyzer assembly.
10. The electrolyzer assembly of claim 9 further comprising an oxygen containment vessel configured to collect oxygen generated by said electrolyzer assembly.
11. The electrolyzer assembly of claim 10 wherein said electrolyzer assembly is configured to operate as a fuel cell to generate electric power and water when oxygen and hydrogen are supplied to said anodes and cathodes.
12. The electrolyzer assembly of claim 1 , further comprising a second unit cell wherein said first unit cell and said second unit cell are arranged in a stack.
13. The electrolyzer assembly of claim 12 further comprising a water supply configured to deliver water to said anode channels.
14. The electrolyzer assembly of claim 13 further comprising a power supply configured to deliver electrical power to said electrolyzer assembly.
15. The electrolyzer assembly of claim 14 further comprising a hydrogen containment vessel configured to collect hydrogen generated by said electrolyzer assembly.
16. The electrolyzer assembly of claim 15 further comprising an oxygen containment vessel configured to collect oxygen generated by said electrolyzer assembly.
17. The electrolyzer assembly of claim 16 wherein said electrolyzer assembly is configured to operate as a fuel cell to generate electric power and water when oxygen and hydrogen are supplied to said anodes and cathodes.
18. An electrolyzer assembly for generating hydrogen and oxygen from water, said electrolyzer assembly comprising:
a first unit cell wherein said first unit cell comprises:
(a) a membrane electrode assembly comprising a proton exchange membrane interposed between an anode and a cathode;
(b) a cathode flow field plate adjacent to said cathode; and
(c) an anode flow field plate adjacent to said anode, said anode flow field plate having an anode channel for directing water in contact with said anode, wherein said anode channel has a length, and a cross sectional area that varies along at least a portion of said length;
wherein the width of said anode channel is substantially constant, and the depth of said anode channel varies as a function of distance along said portion of said channel length such that:
D
(
x
)
=
ST
H
2
O
k
H
2
O
i
d
L
v
(
1
-
x
ST
H
2
O
L
)
where D(x) is the anode channel depth at lengthwise position x; x is a selected position along the channel length; ST H 2 O is water stoichiometry; k H 2 O is flow rate coefficient for water; i d is current density; and L is the channel length.
19. A unitized regenerative fuel cell assembly configured to operate both as an electrolyzer to produce hydrogen and oxygen from water, and as a fuel cell to produce electric power from hydrogen and oxygen, said unitized regenerative fuel cell comprising a unit cell wherein said unit cell comprises:
(a) a membrane electrode assembly comprising a proton exchange membrane interposed between a first electrode and a second electrode;
(b) a first flow field plate adjacent to said first electrode, said first flow field plate comprising an oxygen-side channel for directing a first fluid stream in contact with said first electrode, said oxygen-side channel having a length and a cross-sectional area that varies along at least a portion of said channel length; and
(c) a second flow field plate adjacent to said second electrode, said flow field plate comprising a hydrogen-side channel for directing a second fluid stream in contact with said second electrode;
wherein for operation as an electrolyzer said one unit cell is connected to a source of electrical power and said one oxygen-side channel is fluidly connected to a water supply for flowing reactant water through said oxygen-side channel, and for operation as a fuel cell said oxygen-side channel is fluidly connected to receive an oxygen-containing reactant stream, said hydrogen-side channel is fluidly connected to receive a hydrogen-containing reactant stream, and said unit cell is connected to an electrical load; wherein the depth of said oxygen-side channel is substantially constant, and the width of said oxygen-side channel decreases along at least a portion of said channel length in a direction of water reactant flow during electrolyzer operation and in a direction of oxygen-containing reactant stream flow during fuel cell operation according to a natural exponential function.
20. The unitized regenerative fuel cell assembly of claim 19 , wherein the width of said oxygen-side channel decreases according to a natural exponential function along substantially the entire length of said oxygen-side channel.
21. The unitized regenerative fuel cell assembly of claim 19 further comprising a second unit cell wherein said unit cell and said second unit cell are arranged in a stack.