Fuel Cell Flow Channels and Flow Fields

Abstract

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.

Claims

1. A fuel cell reactant flow field plate comprising: (a) a reactant inlet; (b) a reactant outlet; and (c) an at least one channel formed in a major surface of said fuel cell reactant flow field plate, said at least one channel having a floor and a pair of side walls extending between said floor and said major surface, said at least one channel having a channel length that fluidly interconnects said reactant inlet and said reactant outlet, said side walls separated by a channel width, said floor and said major surface separated by a channel depth, wherein said at least one channel has a cross-sectional area that is substantially constant along a first portion of said channel length and that decreases along a second portion of said channel length.

2. The fuel cell reactant flow field plate of claim 1, wherein said channel depth is substantially constant and said channel width decreases along said second portion of said channel length.

3. The fuel cell reactant flow field plate of claim 1, wherein said channel depth is substantially constant and said channel width decreases monotonically along said second portion of said channel length.

4. The fuel cell reactant flow field plate of claim 1, wherein said channel depth is substantially constant and said channel width decreases continuously along said second portion of said channel length.

5. The fuel cell reactant flow field plate of claim 1, wherein said channel portion having a decreasing cross sectional area is proximal said reactant inlet and said channel portion having a substantially constant cross-sectional area is proximal said reactant outlet.

6. The fuel cell reactant flow field plate of claim 1, wherein said channel portion having a substantially constant cross-sectional area is proximal said reactant inlet and said channel portion having a decreasing cross-sectional area is proximal said reactant outlet.

7. The fuel cell reactant flow field plate of claim 1, wherein said cross-sectional area decreases exponentially along said second portion of said channel length.

8. The fuel cell reactant flow field plate of claim 7, wherein said channel depth is substantially constant and said channel width decreases exponentially along said second portion of said channel length.

9. The fuel cell reactant flow field plate of claim 7, wherein said channel portion having an exponentially decreasing cross-sectional area is proximal said reactant inlet and said channel portion having a substantially constant cross-sectional area is proximal said reactant outlet.

10. The reactant flow field plate of claim 7, wherein said channel portion having a substantially constant cross-sectional area is proximal said reactant inlet and said channel portion having an exponentially decreasing cross-sectional area is proximal said reactant outlet.

11. The reactant flow field plate of claim 10, wherein said plate has a substantially circular major planar surface, said reactant outlet is centrally disposed on said plate, and said reactant inlet is disposed at the circumferential edge of said plate.

12. The fuel cell reactant flow field plate of claim 10, wherein said fuel cell reactant flow field plate has a substantially trapezoidal major planar surface.

13. The fuel cell reactant flow field plate of claim 7, wherein said at least one channel is a plurality of channels.

14. The fuel cell reactant flow field plate of claim 1, wherein said at least one channel is a plurality of channels.

15. The fuel cell reactant flow field plate of claim 14, wherein said plurality of channels defines a flow field area.

16. The fuel cell reactant flow field plate of claim 14, wherein said plurality of channels is arranged in a nested configuration.

17. The fuel cell reactant flow field plate of claim 1, wherein said at least one channel extends in a meandering path between said reactant inlet and said reactant outlet.

18. The fuel cell reactant flow field plate of claim 16, wherein said at least one channel extends in a serpentine path along at least a portion of said channel length.

19. The fuel cell reactant flow field plate of claim 16, wherein said at least one channel extends in a sinusoidal path along at least a portion of said channel length.

20. The fuel cell reactant flow field plate of claim 17, wherein said at least one channel is a plurality of channels.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0036] FIG. 1A is a simplified representation of an anode flow field plate comprising a flow channel that decreases in depth, with constant width, along its length.

[0037] FIG. 1B is a simplified representation of an anode flow field plate comprising a flow channel that decreases exponentially in width, with constant depth, along its length.

[0038] FIG. 2 shows a trapezoidal anode flow field plate comprising multiple flow channels that decrease exponentially in width along their length.

[0039] FIG. 3 is a simplified representation showing an example of how a serpentine flow channel, in which the channel width varies, can be applied to a rectangular flow field plate.

[0040] FIG. 4A is a simplified representation showing an example of how a wavy flow channel, in which the channel width varies, can be applied to a rectangular flow field plate.

[0041] FIG. 4B is a simplified representation showing an example of how multiple wavy flow channels can be nested on a rectangular flow field plate.

[0042] FIG. 5A (prior art) shows a square flow field plate comprising a conventional serpentine flow field with 3 flow channels extending between a supply manifold opening and a discharge manifold opening.

[0043] FIG. 5B shows a similar serpentine flow field to FIG. 5A, but where the width of each serpentine flow channel decreases exponentially along its length.

[0044] FIG. 6A is a simplified representation of a flow field plate comprising a flow channel that decreases exponentially in width for a first portion of the channel length and is then constant for a second portion of the channel

[0045] FIG. 6B is a simplified representation of a flow field plate comprising a flow channel that is constant in width a first portion of the channel length and decreases exponentially for a second portion of the channel length.

[0046] FIG. 7A is a simplified representation showing how 3 flow channels of the type shown in FIG. 6B can be arranged radially on an annular flow field plate.

[0047] FIG. 7B is a simplified representation showing how multiple flow channels of the type shown in FIG. 6B can be arranged in a radial array on an annular flow field plate.

[0048] FIG. 8 is a simplified representation of a flow field plate comprising two flow channels that are initially serpentine with constant width, and then the channel width decreases exponentially for a second portion of the channel length.

[0049] FIG. 9 is a simplified representation of a flow field plate comprising a flow channel in which the channel depth is constant along a first portion of the channel length and then decreases along second a portion of the channel length.

[0050] FIG. 10A (prior art) shows a rectangular flow field plate comprising a multi-channel serpentine flow field extending between a supply and a discharge manifold opening.

[0051] FIG. 10B shows a modification to the flow field plate of FIG. 10A, in which the width of each channel decreases exponentially along a middle portion of the length of each channel.

[0052] FIG. 11 is a simplified representation of a flow field plate comprising a substantially rectangular flow channel having a central rib with exponentially curved side walls.

[0053] FIG. 12A shows, in a simplified representation a flow field plate comprising a flow channel that has a conventional rectangular cross-section at one end and is gradually filleted to reduce its cross-section towards the other end, in the flow direction. FIG. 12B shows another view of the flow field plate of FIG. 12A.

[0054] FIG. 13 is a simplified representation of a flow field plate comprising a rectangular flow channel incorporating rib dots, where density of the rib dots increases in the flow direction.

[0055] FIG. 14 is a simplified representation of a flow field plate comprising a wavy flow channel incorporating rib dots, where density of the rib dots increases in the flow direction.

[0056] FIG. 15 is a simplified representation illustrating an example where the flow channel width decreases in a stepwise, non-linear fashion in the flow direction.

[0057] FIG. 16 is a simplified representation illustrating another example where the flow channel width decreases in a stepwise, non-linear fashion in the flow direction.

[0058] FIG. 17 is a graphical representation illustrating how stepwise or discrete changes in channel width can be used to approximate e-flow.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

Anode Flow Channels—for Fuel Cell Operation on Pure or Dilute Fuel

[0059] According to one embodiment, an anode flow field channel for a PEM fuel cell is designed for maintaining substantially constant fuel availability for the fuel cell electrochemical reaction along at least a portion of the length of the channel, for a given channel length and fuel stoichiometry, when the fuel cell is operating on either a pure or a dilute fuel stream.

[0060] We theorize that fuel availability is proportionate to fuel cell performance, and that uniform fuel availability promotes uniform current density, which is desirable for efficient fuel cell operation and improved performance. In the equations and description below, the fuel referred to is hydrogen, although the description would be applicable to other fuels such as methanol (although the value of the constant would change).

[0061] Hydrogen availability is a function of hydrogen mass flow and velocity, and is defined as follows:

[00003]$\begin{array}{cc}{\mathrm{AVAIL}}_{H2}\left(x\right)={\dot{m}}_{H_2}\left(x\right)v\left(x\right)& \left(1a\right)\end{array}$$\begin{array}{cc}{\mathrm{AVAIL}}_{H2}\left(x\right)=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I.Math.\mathrm{Stoich}\right)}{A\left(x\right)}\left[\mathrm{kg}.Math.m/s^2\right]& \left(1b\right)\end{array}$

wherein, [0062] AVAIL.sub.H2(x)=Hydrogen availability at position x [kg.Math.m/s.sup.2] [0063] {dot over (m)}.sub.H.sub.2(x)=Mass flow rate of hydrogen at position x [kg/s] [0064] v(x)=Velocity of flow at position x [m/s] [0065] ρ.sub.fuel=Fuel stream density [kg/m.sup.3] [0066] I.sub.d=Current density (constant) [A/m.sup.2] [0067] =Entire channel current load [A] [0068] Stoich=Fuel stoichiometry [0069] W(x)=Width of channel at position x [m] [0070] A(x)=Cross-sectional area of channel at position x [m.sup.2] [0071] C=Initial concentration of hydrogen (%)

[0072] Assumptions. To derive equation (1b), the following assumptions were made: [0073] 1. Uniform current density: an objective is to provide an anode flow channel that can achieve or approach uniform current density; [0074] 2. Single phase state (gas form): to reduce thermodynamic complexity, liquid water produced by the fuel cell reaction is considered to be the only fluid in liquid form; all other masses are considered to be in gas form; [0075] 3. Evenly distributed hydrogen concentration, velocity, and mass flow across flow section: complex flow patterns are not considered in the interest of reducing mass flow complexity; [0076] 4. Reaction is considered to be local to the flow channel only: the model does not consider above-rib activity (namely, in the region between channels where the MEA is in contact with plate, and is not directly exposed to fuel flowing in the flow channel); [0077] 5. Steady state system: the reaction and flows are considered to be steady state, or unchanging.

[0078] H2 Availability Equation Derivation. Derivation of equation (1b) from equation (1a) is described as follows: [0079] Definition of additional variables used in the derivation: [0080] x=Position along channel length [m] [0081] {dot over (m)}.sub.H.sub.2 consumer(x)=Mass flow rate of hydrogen consumed up to position x [kg/s] [0082] {dot over (v)}.sub.fuel(x)=Volumetric flow rate of fuel stream at position x [SLPM] [0083] I.sub.acc(x)=Accumulated current up to position x [A]
As noted above, hydrogen availability is defined as the hydrogen mass flow rate by velocity [kg.Math.m/s.sup.2]:

AVAIL.sub.H2(x)={dot over (m)}.sub.H.sub.2(x)v(x)   (1a)

[0084] In the anode flow channel, hydrogen is consumed as the fuel stream moves along the flow channel. The mass flow rate of hydrogen at a given position x along the channel is: [0085] H.sub.2 mass flow at x=H.sub.2 mass flow at inlet−H.sub.2 mass flow consumed to x:

{dot over (m)}.sub.H.sub.2(x)={dot over (m)}.sub.H.sub.2(0)−{dot over (m)}.sub.H.sub.2 Consumed(x)

Where {dot over (m)}.sub.H.sub.2(0)=(1.16×10.sup.−7)(C) ρ.sub.fuel.Math.I.Math.Stoich [kg/s]

Where {dot over (m)}.sub.H.sub.2 Consumed(x)=(1.16×10.sup.−7)(C) ρ.sub.fuel.Math.I.sub.acc(x) [kg/s]

{dot over (m)}.sub.H.sub.2(x)=(1.16×10.sup.−7)(C) ρ.sub.fuel((I.Math.Stoich)−I.sub.acc(x)) [kg/s]  (2)

[0086] These equations are based on a well-known empirically derived fuel cell reaction fundamental principle: Hydrogen flow [SLPM]=0.006965×fuel stoichiometry (Stoich)×current load (I). The value 1.16×10.sup.−7 (units m.sup.3/A.s) is obtained by converting 0.006965 [SLPM] to [m.sup.3/s].

[0087] The velocity of the fuel stream at a given position x along the channel is: [0088] Velocity at x=Fuel stream volumetric flow rate at x/Flow area at x

[00004]$$\begin{gathered} \begin{array}{cc}v\left(x\right)=\frac{{\dot{V}}_{\mathrm{fuel}}\left(x\right)}{A\left(x\right)} \\ v\left(x\right)=\frac{\left(1\text{.16}⨯{10}^{-7}\right)I.Math.\mathrm{Stoich}}{A\left(x\right)}\left[m/s\right]& \left(2b\right)\end{array} \end{gathered}$$

Combining equations (2a) and (2b) then gives:

[00005]$\begin{array}{cc}{\mathrm{AVAIL}}_{H2}\left(x\right)=\left(1.35\times {10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_{acc}\left(x\right)\right)\left(I.Math.\mathrm{Stoich}\right)}{A\left(x\right)}\left[\mathrm{kg}.Math.m/s^2\right]& \left(1b\right)\end{array}$$\mathrm{Where}{I}_{\mathrm{acc}}\left(x\right)=I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\left[A\right]$${\mathrm{AVAIL}}_{H2}\left(x\right)=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I.Math.\mathrm{Stoich}\right)}{A\left(x\right)}\left[\mathrm{kg}.Math.m/s^2\right]$

[0089] Equation (1b) shows that increasing the hydrogen availability can be achieved by: [0090] Increasing current density (I.sub.d) [0091] Increasing fuel stoichiometry (Stoich) [0092] Increasing in channel length (L) [0093] Increasing average channel width (W) [0094] Increasing fuel stream density (ρ.sub.fuel) [0095] Decreasing channel depth (D)

[0096] As previously discussed, it is desirable to manufacture a fuel cell that can achieve substantially uniform current density in operation. Assuming that uniform current density can be achieved by maintaining uniform fuel and oxygen availability along the length (x) of the anode and cathode flow channels respectively, equation (1b) shows that holding hydrogen availability constant along x requires changes in flow area. The flow area A(x) for each position along the channel length can be determined by solving equation (1b) for A(x) as shown in equation (7) below. For a rectangular flow channel cross-section (namely, channel with a straight floor and side walls), the channel width and depth can be determined at any given lengthwise position x in the channel by defining area A(x) as the product of width W(x) and depth D(x), then changing the channel width or depth (W or D) along channel length x:

[00006]$\begin{array}{cc}{\mathrm{AVAIL}}_{H2}\left(x\right)=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I.Math.\mathrm{Stoich}\right)}{D\left(x\right)W\left(x\right)}\left[\mathrm{kg}.Math.m/s^2\right]& \left(3\right)\end{array}$

Anode Flow Channel Having Varied Depth Profile

[0097] An anode flow channel can be manufactured with a constant width and a varying depth profile to achieve constant hydrogen availability. Such a channel profile is calculated as follows:

Using the hydrogen availability equation as previously derived in equation (3):

[00007]$\begin{array}{cc}{\mathrm{AVAIL}}_{H2}\left(x\right)=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I.Math.\mathrm{Stoich}\right)}{D\left(x\right)W\left(x\right)}\left[\mathrm{kg}.Math.m/s^2\right]& \left(3\right)\end{array}$

and solving for channel depth D(x):

[00008]$D\left(x\right)=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I.Math.\mathrm{Stoich}\right)}{{\mathrm{AVAIL}}_{H2}\left(x\right)W\left(x\right)}$

[0098] Assuming constant hydrogen availability Avail.sub.H2 and channel width W, the following equation 4 is obtained:

[00009]$\begin{array}{cc}\mathrm{where}\underset{0}{\overset{x}{\int }}W\left(x\right)\mathrm{dx}=W.Math.x& \left(4\right)\end{array}$$\mathrm{where}I=I_d.Math.W.Math.L$$D\left(x\right)=\left(1\text{.35}⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I_d.Math.W.Math.L.Math.\mathrm{Stoich}\right)-\left(I_d.Math.W.Math.x\right)\right)\left(I_d.Math.W.Math.L.Math.\mathrm{Stoich}\right)}{{\mathrm{AVAIL}}_{H2}.Math.W}$$D\left(x\right)=\frac{\left(1\text{.35}⨯{10}^{-14}\right)C.Math.{\rho }_{\mathrm{fuel}}.Math.\mathrm{Stoich}.Math.L.Math.I_d^2.Math.W}{{\mathrm{AVAIL}}_{H2}}\left(L.Math.\mathrm{Stoich}-x\right)\left[m\right]$$D\left(x\right)=D\left(0\right)\left(1-\frac{x}{L.Math.\mathrm{Stoich}}\right)\left[m\right]$

The result is the depth profile is linear to x.

[0099] FIG. 1A is a simplified representation of an anode flow field plate 100A comprising a flow channel 110A that decreases in depth, with constant width, along its length. A channel profile can be defined by solving for D(x) in equation 4 at each position (x) along the length of the channel, given a specified operating fuel stoichiometry STOICH and channel length L, and assuming a constant channel width. Referring to FIG. 1A, the resulting anode channel 110A extends between fuel supply manifold opening 120A and discharge manifold opening 130A, and has a linearly decreasing depth floor 112A from inlet 116A to outlet 118A, with straight (parallel) side walls 114A.

[0100] For the varied depth approach, to increase hydrogen availability along the channel requires, ordered in effectiveness, an: [0101] increase in current density (I.sub.d); [0102] increase in fuel stoichiometry (Stoich); [0103] increase in channel length (L); [0104] increase in channel width (W); [0105] increase in fuel stream density (ρ.sub.fuel); or, [0106] decrease in average depth (D)

Anode Flow Channel Having Varied Width Profile

[0107] Given the desire to reduce or minimize the thickness of the separator plates in a fuel cell stack, it is desirable to keep the depth of the channel shallow. Therefore, instead of varying the depth of the channel, which would require a sufficiently thick plate to accommodate the deepest part of the channel, it can be preferred to keep the channel depth constant and to vary just the width of the channel to achieve constant hydrogen availability along the length of the channel.

[0108] Again, the H.sub.2 availability equation (3) is:

[00010]$\begin{array}{cc}{\mathrm{AVAIL}}_{H2}\left(x\right)=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\stackrel{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I.Math.\mathrm{Stoich}\right)}{D\left(x\right)W\left(x\right)}\left[\mathrm{kg}.Math.m/s^2\right]& \left(3\right)\end{array}$

[0109] Applying constant hydrogen availability Avail.sub.H2 and channel depth

[00011]$\mathrm{where}I=I_d\underset{0}{\overset{L}{\int }}W\left(x\right)dx{\mathrm{AVAIL}}_{H2}=\left(1.35⨯{10}^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(Sto\mathrm{ich}.Math.I_d\underset{0}{\overset{L}{\int }}W\left(x\right)\mathrm{dx}-I_d\underset{0}{\overset{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I_d.Math.\underset{0}{\overset{L}{\int }}W\left(x\right)\mathrm{dx}.Math.\mathrm{Stoich}\right)}{D.Math.W\left(x\right)}$

Solving for W(x), equation (5) is as follows:

[00012]$\begin{array}{cc}\frac{W\left(x\right)}{\left(\mathrm{Stoich}.Math.I_d\underset{0}{\overset{L}{\int }}W\left(x\right)\mathrm{dx}-I_d\underset{0}{\overset{x}{\int }}W\left(x\right)\mathrm{dx}\right)\left(I_d.Math.\underset{0}{\overset{L}{\int }}W\left(x\right)\mathrm{dx}.Math.\mathrm{Stoich}\right)}=\frac{{\rho }_{\mathrm{fuel}}}{\left(1.35⨯{10}^{-14}\right)C.Math.{\mathrm{AVAIL}}_{H2}.Math.D}& \left(5\right)\end{array}$

[0110] Equation (5) can be simplified to obtain

[00013]$\begin{array}{cc}W\left(x\right)=W\left(0\right)e^{-\frac{x}{L}\ln \left(\frac{\mathrm{STOICH}}{\mathrm{STOICH}-1}\right)}\left[m\right]& \left(6\right)\end{array}$

[0111] FIG. 1B is a simplified representation of an anode flow field plate 100B comprising a flow channel 110B that decreases in width along its length. A channel profile can be defined by solving for W(x) in equation (6) at each position (x) along the length of the channel, given a specified operating fuel stoichiometry STOICH and channel length L, and assuming a flat channel floor (constant depth). Referring to FIG. 1B, the resulting anode channel 110B extends between fuel supply manifold opening 120B and discharge manifold opening 130B, and has a constant depth floor 112B with convexly curved side walls 114B that converge inwards from inlet to outlet. The walls 114B converge inwards towards an outlet end 118B with an inlet 116B having the largest width and the channel profile delineating at a diminishing rate. That is, the channel width decreases exponentially along the length of the channel from the inlet to the outlet according to the equation (6). It would be possible for one of the side walls to be straight and the other to be convexly curved.

[0112] Referring to FIG. 2, multiple channels 210 having the channel profile shown in FIG. 1B can be applied to a separator plate 200, to form an anode flow field 222 extending between fuel supply manifold opening 220 and discharge manifold opening 230. The anode flow field 222 is arrayed in a generally trapezoidal geometry to enable separating ribs 224 to have a relatively even width along their length; it can be seen that using a conventional rectangular flow field geometry with narrowing flow channels would require the ribs to also have a narrowing profile. Such a rib profile would result in significant amounts of MEA contacting the ribs, resulting in reduced membrane active area and less efficient usage of membrane material. Since MEA material is relatively expensive, it can be desirable to maximize the MEA active area using a generally even rib width. Using a generally trapezoidal flow field geometry also enables the flow field 222 to fit onto a trapezoidal separator plate 200, or to fit onto a traditional rectangular separator plate with room to spare on the separator plate for other components such as manifolding (not shown).

[0113] The separator plate 200 includes partial ribs 226 located at the inlet region of each channel 210. The partial ribs 226 serve to reduce the distance between channel side walls, and serve as a bridging or support structure for the adjacent MEA (not shown).

Anode Flow Channel Having Varied Cross-Sectional Area

[0114] If alternate techniques are used to generate a constant H.sub.2 availability profile without a rectangular channel cross-section (flat floor, vertical walls), then a new variable W.sub.R(x) is introduced into equation (1(b)). W.sub.R(x) is defined as the width of the hydrogen reaction area at a given lengthwise position x along the channel (for a rectangular channel cross-section, W.sub.R(x)=W(x) as the width of the channel that is exposed to the adjacent MEA or gas diffusion layer is the same as the channel width). A(x) is then calculated through iteration based on channel profile. The resulting equation (7) encompasses various channel cross-sectional flow shapes that maintain a constant H.sub.2 availability along the channel length. For example, alternative channel cross-sectional profiles can include, but are not limited to: U-shaped channel, polygonal channel, semi-circular channel, varying fillet channel corner, varying chamfer channel corner, varying side wall slope angle channel, or varying floor bevel.

[00014]$\begin{array}{cc}A\left(x\right)=\left(1.35\times 10^{-14}\right)C\frac{{\rho }_{\mathrm{fuel}}\left(\left(I.Math.\mathrm{Stoich}\right)-I_d\underset{0}{\overset{x}{\int }}W\left(x\right)dx\right)\left(I.Math.\mathrm{Stoich}\right)\left[m^2\right]}{{\mathrm{AVAIL}}_{H2}\left(x\right)}& \left(7\right)\end{array}$

[0115] The preferred anode flow channel dimensions or characteristics based on the equations set forth above are applicable to operation on pure or dilute fuels as the equations take into account concentration (C).

Improved Reactant Flow Field Designs

[0116] As used herein, the terms “e-flow”, “based on e-flow”, “based on the e-flow equations”, “in accordance with e-flow principles”, “e-flow channels”, and similar phrases are intended to refer to reactant flow channel dimensions, flow characteristics and/or flow field designs that are selected based on the equations and description set forth above with respect to the anode, and in U.S. Pat. No. 7,838,169 with respect to the cathode. Such channels or flow field designs can be incorporated into the anode and cathode flow field plates or other components of a fuel cell.

[0117] Flow fields based on e-flow principles are more likely to be adopted if they can be accommodated within conventional flow field plate geometries and into conventional fuel cell stack architectures (which typically have rectangular flow field plates). Flow channels where the depth profile changes along the length of the channel (such as shown in FIG. 1A) can be accommodated by using the existing flow field design (pattern) and merely altering the depth profile of the channels along their length (keeping the channel width and ribs the same as in the original flow field design). However, plates with channels where the depth profile changes are generally more challenging to fabricate. They also result in a need for thicker plates, in order to accommodate the deepest part of the channel, leading to decreased fuel cell stack power density and higher cost.

[0118] FIGS. 3-5 show some examples of ways in which flow fields based on e-flow where the flow channel width varies, can be applied to a rectangular reactant flow field plate. FIG. 3 shows a rectangular reactant flow field plate 300 with a serpentine channel 310 where the channel width is decreasing exponentially as it zig-zags across the plate between supply manifold opening 320 and discharge manifold opening 330. FIG. 4A shows a rectangular reactant flow field plate 400A with a wavy channel 410A extending between supply manifold opening 420A and discharge manifold opening 430A, where the channel width is decreasing exponentially along its length. In FIG. 4A the amplitude of the path of the center-line of the flow channel 410A increases as the width of the channel decreases, so that the channel still occupies most of the width of the plate 400A. Making the variable width e-flow channel serpentine or wavy, rather than straight, allows the channel to occupy a more rectangular shape making more efficient use of the surface area of the plate. FIGS. 3 and 4A show a single flow channel, however, it is apparent that such channels can be repeated or arrayed across a rectangular plate so that a large portion of the plate area can be “active area” (for example, so that a large portion of the plate surface is covered in channels, with a large open channel area exposed to the adjacent electrode or MEA). FIG. 4B shows a rectangular reactant flow field plate 400B with multiple flow channels 410B (like flow channel 410A of FIG. 4A repeated) extending between supply manifold opening 420B and discharge manifold opening 430B, arranged so that the channels nest together. This example describes a single direction array (x-axis shown) resulting in an overall approximately rectangular active area, and maintains substantially uniform rib width (channel spacing). This differs from a radial or arced array pattern resulting from maintaining substantially uniform rib width in the straight e-flow channel profile of FIG. 2. FIG. 5A shows a square flow field plate 500A comprising a conventional (prior art) serpentine flow field with three flow channels 510A extending between supply manifold opening 520A and discharge manifold opening 530A. FIG. 5B shows a similar serpentine flow field plate 500B, but where the width of each serpentine channel 510B decreases exponentially along its length as it extends from supply manifold opening 520B to discharge manifold opening 530B.

[0119] Another approach is to take a radial array of channels of decreasing width, such as is shown in FIG. 2, and incorporate a 90° “fan” of such channels on a square plate with discharge manifold opening in one corner (where narrower ends of the channels converge) and supply manifold opening in the opposite corner, with a feed header extending along two sides of the square plate. Similarly a 180° fan of channels could be incorporated on a rectangular plate. Rectangular active area shapes are generally preferred for flow field plates (and other fuel cell components), as they generally make more efficient use of bulk sheet materials with less waste during manufacturing. It is possible to efficiently “nest” circular or trapezoidal plates onto a rectangular sheet, however there is generally more unused area than with rectangular cut shapes.

[0120] Improvements in fuel cell performance can be obtained by incorporating e-flow along only a portion of the length of the reactant flow channel. The performance improvements are not necessarily as great as if e-flow is employed along the entire channel length, but such flow field designs can in some cases provide most of the benefit, and can allow more efficient use of the plate area. For example, current density maps of conventional flow fields, such as those having serpentine or straight flow channels, generally show reasonable uniformity of current density near the supply manifold followed by a reduction in current density further downstream. This indicates that e-flow can provide the most benefit if used in the latter portion of the channel length, where the current density is more sensitive to reactant availability. However, it is possible to incorporate e-flow into the flow field near the beginning of the channel length followed by a downstream section that has conventional flow channels. This embodiment can be used to hold the reactant availability substantially constant over an initial portion of the fuel cell active area, and allow the downstream “conventional” section to operate as though there was no upstream active area. In this way the e-flow region could be regarded as a power generating “manifold” for the subsequent conventional flow field.

[0121] FIGS. 6-8 show some examples where the flow channel width varies along just a portion of the length of the channel. FIG. 6A shows a rectangular reactant flow field plate 600A with a flow channel 610A extending between supply manifold opening 620A and discharge manifold opening 630A. Similarly, FIG. 6B shows a rectangular reactant flow field plate 600B with a flow channel 610B extending between supply manifold opening 620B and discharge manifold opening 630B. In FIG. 6A the flow channel width decreases exponentially for a first portion 625A of the channel length (near the supply manifold), and is then constant for a second portion 635A of the channel length (towards the discharge manifold). Conversely, in FIG. 6B the flow channel width is constant for a first portion 625B and decreases exponentially for a second portion 635B of the channel length.

[0122] FIGS. 7A & 7B show how flow channels 710 of the type shown in FIG. 6B can be arranged radially on an annular reactant flow field plate 700. FIG. 7A is a partial view showing just a few channels 710, and FIG. 7B shows a complete flow field.

[0123] FIG. 8 shows a reactant flow field plate 800 comprising two flow channels 810. The channels are initially serpentine with constant width in portion 825 near the supply manifold opening 820, and then, after abruptly increasing, the channel width decreases exponentially for a second portion 835 of the channel length (towards discharge manifold opening 830). FIG. 9 shows an example of a reactant flow field plate 900 comprising a flow channel 910 extending between a supply manifold opening 920 and a discharge manifold opening 930. The flow channel depth is constant along a first portion 925 of the channel length and then decreases along a second portion 935 of the length of the channel 910.

[0124] In some embodiments, the flow channels can incorporate an e-flow based variation in both width and depth along their entire length, or a portion of their length.

[0125] FIGS. 10A & 10B illustrate how an existing flow field design can be readily modified to incorporate e-flow along a portion of the length of the flow channels. FIG. 10A (prior art) shows a rectangular flow field plate 1000A comprising a fairly complex serpentine flow field with multiple serpentine channels 1010A extending between a supply and a discharge manifold opening. FIG. 10B shows a modification in which the width of each channel 1010B decreases exponentially along a middle portion 1025B of the length of each channel.

[0126] It is also possible to take a “conventional” flow channel (for example, a channel with a rectangular and constant cross-sectional shape and area along its length) and incorporate a shaped rib, fillet or other features within the volume of the original channel to reduce the channel cross-sectional area in a way that provides at least some of the benefits of e-flow. FIG. 11 shows an example of a reactant flow field plate 1100 with a single flow channel 1110 extending between a supply manifold opening 1120 and a discharge manifold opening 1130. The channel 1110 comprises a central rib 1140 with exponentially curved side walls. The rib splits the flow channel 1110 in two and effectively reduces its width gradually along most of its length in accordance with e-flow principles. FIGS. 12A and 12B show two different views of another example of a flow field plate 1200 with a single flow channel 1210 extending between a supply manifold opening 1220 and a discharge manifold opening 1230. The channel 1210 is of a conventional rectangular cross-section at one end 1225, and is gradually filleted to reduce its cross-section towards the other end 1235, in accordance with e-flow principles.

[0127] In the examples described above, the flow channel dimensions vary along at least a portion of the channel length in a smooth and continuous fashion in accordance with e-flow principles. However, performance benefits can also be obtained by using reactant flow channels that incorporate a “discrete approximation” of e-flow. In other words, the characteristics of the channel can be varied as a function of distance along the channel in a stepwise or discontinuous fashion, but where the overall variation trends the smooth e-flow profile, either in fluctuations about the calculated profile, or in discrete approximations of the e-flow profile (so that it is in accordance with e-flow at a “macro” level). This approach can be used to achieve at least some of the performance benefits of e-flow, and can provide some options for improved flow fields that are easier to fabricate or to incorporate into existing fuel cell plate geometries. In all cases the outlet, or region near the outlet is smaller or more constricted that the inlet or inlet region. In some embodiments, the channels can contain discrete features that obstruct reactant flow, where the density and/or size of those features increases in the flow direction. An example of a flow field plate 1300 where the flow channels incorporate rib dots or raised columns 1350 is shown in FIG. 13. The density of the rib dots 1350 can increase in the reactant flow direction (indicated by the arrow) in accordance with the e-flow equations. Such features can be as high as the channel is deep (so that they touch the adjacent electrode) or can obstruct only part of the channel depth. In the example illustrated in FIG. 13, the “channel” is the entire active area and the rib dots (or other such features that obstruct reactant flow) are distributed across the active area in a varied density array approximating e-flow. In other examples, the rib dots or other features can be incorporated into one or more separate channels. FIG. 14 is a simplified representation of an anode flow field plate 1400 comprising a wavy flow channel 1410 incorporating rib dots 1450, where the density of the rib dots increases in the flow direction (indicated by the arrow).

[0128] In other examples, the flow channel dimensions (for example, width or depth) can decrease in the flow direction in a stepwise fashion. The increments by which the dimensions change and the distance between the step-changes are selected so that the changes in channel dimensions in the flow direction are consistent with the e-flow equations. In some embodiments the increments by which the channel dimensions change can be the same along the channel length, and in other embodiments it can vary along the channel length. Similarly, in some embodiments the distance between (or frequency of) the step-changes in channel dimensions can be the same along the channel length, and in other embodiments it can vary along the channel length.

[0129] FIGS. 15 and 16 illustrate examples where the channel width decreases in a stepwise, non-linear fashion in the flow direction in accordance with the e-flow equations. FIG. 15 is a simplified representation illustrating an example flow field plate 1500 where the width of flow channel 1510 decreases in a stepwise, non-linear fashion in the flow direction between a supply manifold opening 1520 and a discharge manifold opening 1530. FIG. 16 is a simplified representation illustrating another example flow field plate 1600 where the width of flow channel 1610 decreases in a stepwise, non-linear fashion in the flow direction between a supply manifold opening 1620 and a discharge manifold opening 1630.

[0130] FIG. 17 is a graphical representation 1700 illustrating how stepwise or discrete changes in channel width can be used to approximate e-flow. The solid line 1710 represents changes in channel width and the dashed line 1720 shows a smooth exponential variation in channel width.

[0131] In other examples the porosity of the flow channel could vary in a continuous or stepwise fashion in accordance with e-flow principles.

[0132] FIGS. 1A, 1B, 3, 4A, 4B, 6A, 6B, 7A, 7B, 8, 9, 11, 12A, 12B, 13, 14, 15 and 16 are simplified drawings, in which the size of the flow channel and the manifold openings, and e-flow based variations in channel characteristics are exaggerated for the purposes of clear illustration.

[0133] In all of the above-described embodiments, the flow characteristics of the flow channel vary along at least a portion of the channel length substantially in accordance with the e-flow equations. The variations can be continuous or discrete. In the latter case, a continuous curve fitted to the discrete changes would be substantially consistent with the e-flow equations.

[0134] Flow channels with characteristics as described herein can be used at the anode or the cathode or both. Also they can be used for some or all of the fuel cells in a particular fuel cell stack.

[0135] The open channel area versus the rib or landing area on a reactant flow field plate is generally selected to give sufficient electrical contact between the plates and the adjacent MEAs for efficient current collection, while providing sufficient reactant access to the electrodes of the MEA to support the electrochemical reactions. Using a wider rib area (between flow channels) improves electrical connectivity and current collection in the fuel cell.

[0136] As used herein the “inlet” refers to either the start of the flow channel where reactant enters the channel, or the start of a region where the channel characteristics vary as a function of channel length as described herein; and “outlet” refers to either the downstream end of the channel, or the end of a region over which channel characteristics vary as a function of channel length as described herein.

[0137] Fuel cell flow field plates can utilize the reactant flow channels or flow field designs described above. Such plates can be made from suitable materials or combination of materials, and can be fabricated by suitable methods.

[0138] Other fuel cell components can also incorporate the flow channels or passageways as described herein. For example, such channels could be incorporated into the gas diffusion layer, or other components of the unit cell.

[0139] Fuel cells and fuel cell stacks can also incorporate the flow field plates and/or other components. The reactant flow channels and flow field designs described herein have been found to be particularly advantageous in PEM fuel cells, however they can be applied in other types of fuel cells or other electrochemical devices, such as electrolyzers.

[0140] Where a component is referred to above, unless otherwise indicated, reference to that component should be interpreted as including the equivalents of that component and any component which performs the function of the described component (namely, that is functionally equivalent), including components which are not structurally equivalent to the disclosed structure but which perform the function in the illustrated exemplary embodiments.

[0141] While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, that the invention is not limited thereto since modifications can be made without departing from the scope of the present disclosure, particularly in light of the foregoing teachings.