Bipolar plate channel structure with knobs for the improvement of water management in particular on the cathode side of a fuel cell

A fluid distribution assembly for use in a fuel cell includes a separator plate having a major face. A boundary element is disposed over the major face. A flow field communicates reactant in a flow direction across the separator plate. The flow field is defined by a plurality of knobs formed on the separator plate extending from the major face toward the boundary element.

FIELD OF THE INVENTION

The present invention relates to a fuel cell. More particularly, the present invention relates to a flow field on a bipolar plate for a fuel cell.

BACKGROUND OF THE INVENTION

Fuel cells have been proposed as a power source for electric vehicles and other applications. One such fuel cell is a PEM (i.e. Proton Exchange Membrane) fuel cell that includes a so-called “membrane-electrode-assembly” (MEA) having a thin, solid polymer membrane-electrolyte. The MEA is sandwiched between a pair of electrically conductive fluid distribution elements (i.e., bipolar plates) which serve as current collectors for the electrodes, and contain a so-called “flow field” which is an array of lands and grooves formed in the surface of the plate opposing the MEA.

The lands conduct current from the electrodes, while the grooves between the lands serve to distribute the fuel cell's gaseous reactants evenly over the faces of the electrodes. Gas diffusion media are positioned between each of the electrically conductive fluid distribution elements and the electrode faces of the MEA, to support the MEA where it confronts grooves in the flow field, and to conduct current therefrom to the adjacent lands.

A drawback of fuel cells, however, is the phenomenon of water being impeded from flowing outward from the MEA, often referred to as “flooding”. Flooding can hinder a fuel cell's operation at low current density when the air flow through the cathode flow field is not sufficient to drive the water removal process. Excess liquid water also tends to plug the pores in gas diffusion media, and thereby isolate the catalytic sites from the reactant oxygen flow.

Typically, conventional flow fields employ discrete channels that induce strong non-uniform flow under the lands. The non-uniform flow under the lands tends to lead to a non-equilibrated water management. In some regions high flows may lead to a dry out of the MEA. Moreover, in some regions negligible flows tend to promote a conglomeration of liquid water which may lead to flooding and ultimately a reduction of the efficiency of the fuel cell stack as a whole. Therefore, there is a need for an improved fuel cell design to minimize the aforesaid drawbacks.

SUMMARY OF THE INVENTION

A fluid distribution assembly for use in a fuel cell includes a separator plate having a major face. A boundary element is disposed over the major face. A flow field communicates reactant in a flow direction across the separator plate. The flow field is defined by a plurality of knobs formed on the separator plate extending from the major face toward the boundary element.

According to other features, the plurality of knobs includes a first series of knobs arranged in a repeating manner across the separator plate in a direction generally transverse to the flow direction. A second series of knobs are arranged in a repeating manner across the separator plate in a direction generally transverse to the flow direction. A first series of gaps is defined between adjacent knobs of the first series of knobs and a second series of gaps is defined between adjacent knobs of the second series of knobs. A knob of the first series of knobs and a knob of the second series of knobs define a first footprint and a second footprint, respectively, for impeding flow of the reactant in the flow direction. The first footprint is offset from the second footprint in a direction transverse to the flow direction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1depicts a two cell, bipolar fuel cell stack2having a pair of membrane-electrode-assemblies (MEAs)4and6separated from each other by an electrically conductive fluid distribution element8, hereinafter bipolar plate8. The MEAs4and6and bipolar plate8, are stacked together between clamping plates, or end plates10and12, and end contact elements14and16. The end contact elements14and16, as well as both working faces of the bipolar plate8, contain a plurality of grooves or channels18,20,22and24, respectively, for distributing fuel and oxidant gases (i.e. H2and O2) to the MEAs4and6.

Nonconductive gaskets26,28,30and32provide seals and electrical insulation between the several components of the fuel cell stack. Gas permeable conductive materials or diffusion media34,36,38and40press up against the electrode faces of the MEAs4and6. The diffusion media34-40may be referred to herein as boundary elements. The end contact elements14and16press up against the diffusion media34and40respectively, while the bipolar plate8presses up against the diffusion media36on the anode face of the MEA4, and against the diffusion media38on the cathode face of the MEA6.

Oxygen is supplied to the cathode side of the fuel cell stack from a storage tank46via appropriate supply plumbing42, while hydrogen is supplied to the anode side of the fuel cell from a storage tank48, via appropriate supply plumbing44. Alternatively, ambient air may be supplied to the cathode side as an oxygen source and hydrogen to the anode side from a methanol or gasoline reformer, or the like. Exhaust plumbing (not shown) for both the H2and O2sides of the MEAs4and6will also be provided. Additional plumbing50,52and54is provided for supplying liquid coolant to the bipolar plate8and the end plates14and16. Appropriate plumbing for exhausting coolant from the bipolar plate8and the end plates14and16is also provided, but not shown.

FIG. 2is an enlarged, exploded partial view of various components of a fuel cell according to the teachings of the present invention. As can be seen inFIG. 2, the fuel cell comprises a membrane electrode assembly (MEA)56that includes an ionically conductive membrane58sandwiched by an anode electrode60and a cathode electrode62. The MEA56is further sandwiched by an anodic surface bipolar plate68and a cathodic surface bipolar plate70. It is appreciated that gas diffusion media are preferably disposed between the bipolar plates68and70and the MEA56, but are not necessary to the present invention and, therefore, have been omitted from this Figure for simplicity.

When a fuel stream of pure H2or hydrogen reformate is dispersed over the anode60, electrons that are produced by the hydrogen oxidation reaction are conducted a short distance to the adjacently disposed electrically conductive fluid distribution element, or bipolar plate68. Since the lands72of the bipolar plate68directly contact the anode electrode60(or diffusion media if used), electrical conductivity is facilitated and enhanced. Protons (H+) produced from the anodic reaction, combined with water from the humid fuel stream pass through the anode60to the ionically conductive membrane58and through to the cathode62. At the cathode side of the MEA56, a stream of O2or ambient air that contains oxygen is dispersed over the cathode62. The oxygen undergoes a reduction and the electrons that are produced are also conducted a short distance to another adjacently disposed bipolar plate70. The reduced oxygen then reacts with the protons from the anode60and liquid water is produced.

With continued reference toFIG. 2and further reference toFIGS. 3 and 4, the bipolar plate70will be described in greater detail. In accordance to the present teachings and in order to further facilitate the electrochemical reaction and improve the convective removal or water from the fuel cell, the bipolar plate70includes a major face78defining a plurality of knobs80extending therefrom. As used herein, the term “knobs” defines individual extension portions each having an outer boundary defining a perimeter. The plurality of knobs80cooperate to define a flow field84for directing reactant across the major face78of the bipolar plate70in a flow direction (F). As will be described herein, the flow field84employing knobs80leads to a more homogenous flow over the cathode62(or diffusion media) in the channels74and under the lands72. In this regard, water management is improved, especially the extraction of liquid water. While the flow direction (F) is generally depicted in the drawings as a straight line, it is appreciated that the reactant flows between respective knobs80in a generally serpentine manner from an upstream side to a downstream side of the bipolar plate. In addition, a significant and quite homogeneous flow passes under the knobs80and enters the diffusion media, improving water management.

The plurality of knobs80generally include a first series of knobs86and a second series of knobs88arranged in a repeating manner along the bipolar plate70in a direction generally transverse to the flow direction (F). The first and second series of knobs86and88, respectively, repeat in an alternating manner across the bipolar plate70. A first series of gaps90(FIG. 4) are defined between adjacent knobs of the first series of knobs86. Likewise a second series of gaps92is defined between adjacent knobs of said second series of knobs88.

As best illustrated inFIG. 4, the first series of knobs86are staggered relative to the second series of knobs88in a direction perpendicular to the flow direction (F). The plurality of knobs80each define an upstream surface94and a downstream surface96, respectively (FIG. 3). The upstream and the downstream surfaces94and96, respectfully, define a convex contour. The staggered relationship between the first and second series of knobs86,88, along with the convex contour of each knob80reduces dead water areas in the flow field84and promotes extraction of liquid water. More specifically, the varying cross-sections of the spaces between adjacent knobs (referred generally as a passage area97,FIG. 4) discourage slugs of liquid water from becoming stuck between adjacent knobs. In the smallest cross-sections, higher flow velocities lead to a transport of possible slugs in the downstream direction.

With specific reference now toFIGS. 3 and 4, dimensional aspects of the knobs80will be explained. It is appreciated however, that the dimensions associated with the knobs80, the bipolar plate70and the flow field84as a whole are merely exemplary, and other dimensions may be similarly employed. Each knob80extends from the major face78a distance (H1,FIG. 3) defining a channel height. Preferably the channel height is approximately 0.3 mm. Each knob80defines a footprint having a width (W1) extending transverse to the flow direction (F) and a length (L) extending lateral to the flow direction (F). Preferably the width (W1) and the length (L) are approximately 5 mm and 1 mm, respectively, providing a W1:L ratio of 5:1. A distance between respective centerlines of the first series of knobs86and the second series of knobs88defines an offset (O). Preferably the offset (O) is approximately 1.25 mm. The series of gaps90,92define a distance (G), preferably 0.5 mm. As a result, a given space between adjacent knobs (at the gaps90,92and at the passage area97) varies between 0.5 mm to 1 mm.

Of particular note, an axis (A1) defined along respective centerpoints of repeating first series of knobs86is aligned with an outer edge of repeating second series of knobs88. Such a relationship presents an irregular pattern whereby a peak98of the convex downstream surface96is offset from an axis (A2) defined at a centerpoint of a proximate downstream gap90,92. Moreover, edges that may tend to hinder water movement, are noticeably absent. As a result, transportation of water is facilitated around the arcuate surfaces of respective knobs80and ultimately across the flow field84of the bipolar plate70in the flow direction (F).

With reference now toFIGS. 5 and 6, a bipolar plate110according to other features will be described. The bipolar plate110includes a major face112defining a plurality of knobs120extending therefrom. The knobs120each generally define a distorted diamond shape. The plurality of knobs120cooperate to define a flow field126for directing reactant across the major face112of the bipolar plate110in a flow direction (F).

The plurality of knobs120generally include a first series of knobs130and a second series of knobs132arranged in a repeating manner along the bipolar plate110in a direction generally transverse to the flow direction (F). The plurality of knobs120each define an upstream surface133and a downstream surface134, respectively. The first and second series of knobs130and132, respectively, repeat in an alternating manner across the bipolar plate110.

With specific reference now toFIG. 6, dimensional aspects of the knobs120will be described. Again, the dimensions associated with the bipolar plate110are merely exemplary. A channel height H2is preferably 0.3 mm. Each knob120generally defines an upstream triangle136and a downstream triangle138. The upstream triangle and the downstream triangle136and138define a height D1and D2respectively. Preferably the upstream triangle height D1is 0.25 mm and the downstream triangle height D2is 0.75 mm, providing an appropriate 1:3, D1:D2ratio. A span (S) between adjacent knobs120is preferably 2 mm and the width (W2) of a knob120is preferably 3 mm, providing an approximate S:W2ratio of 2:3.

The knobs120present varying cross-sections (referred generally as a passage area142) between adjacent upstream surfaces and downstream surfaces133and134respectively. The passage area discourages slugs of liquid water from becoming stuck between adjacent knobs by promoting higher flow velocities at the smallest cross-sections.

Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the present invention can be implemented in a variety of forms. For example which the knobs80and120illustrated herein are shown as having a distorted elliptical and diamond shape respectively, other shapes may be employed such as pure ellipses having a width to height ratio of 5:1. Therefore, while this invention has been described in connection with particular examples thereof, the true scope of the invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification and the following claims.