WO2013105956A1

WO2013105956A1 - Fuel cell reactant flow field having impediments to flow

Info

Publication number: WO2013105956A1
Application number: PCT/US2012/021012
Authority: WO
Inventors: Robert M. Darling
Original assignee: Utc Power Corporation
Priority date: 2012-01-12
Filing date: 2012-01-12
Publication date: 2013-07-18

Abstract

Rows (50-55, 60-65, 70-73, 80-83, 90-93) of projections (27-28) disposed in reactant gas channels (21) are disposed at intervals along the length of reactant flow fields (20), the rows being more sparse near the inlet end (47) of the flow field and more dense near the outlet end (48) of the flow field or being increasingly more obstructive in shape or size, so that the obstruction to reactant gas flow is approximately proportional to the degree of reactant depletion in the flow. Alternatively, projections (29) may be more shallow near the inlet end of a reactant flow field but larger and therefore more occlusive at the downstream end of the flow field, to provide obstruction to the flow which is approximately proportional to the degree of reactant depletion in the flow. Projections may be in staggered rows (80-87, 90-97) or in random patterns.

Description

FUEL CELL REACTANT FLOW FIELD HAVING IMPEDIMENTS TO FLOW

Technical Field

[0001] Fuel cell reactant flow field channels are provided with a series of flow obstructions at intervals, characterized as projections extending outwardly into the flow path from the bottom wall of the flow path. In flow fields made of thin, stamped metal, the projections are characterized as dents in the floor of the flow field channels. If the structure is formed of molded carbon or other conductive material, the projections are molded into the bottom walls of each flow path. To optionally improve reactant utilization with a minimum impact on flow field pressure requirements, the projections are sparse (or small) in the upstream end of the flow field, and become increasingly dense (or larger) in successive downstream positions, to provide occlusion generally proportional to reactant dilution.

Background Art

[0002] The electrochemical reactions in fuel cells are well known. The reactants in alkaline, acid, or solid polymer electrolyte fuel cells are hydrogen or a hydrogen rich fuel at the anode, and oxygen or air at the cathode. When the fuel cell is operating on dilute reactants, such as air or reformed hydrocarbon fuel, the extraction of the desired reactant gas component becomes more difficult.

[0003] Interdigitated reactant flow fields, in which entrance gas flow channels do not directly connect to exit gas flow channels, force the reactant gas to flow into an adjacent layer of the fuel cell. This results in forced convection of the reactant toward the electro catalyst so that a greater proportion of the reactant flowing through the reactant flow fields is utilized more efficiently. However, the forced convection cannot be achieved effectively without an increase in the pressure drop across the flow field. Fuel pressure is usually not a problem. The increased pressure required of an air pump consumes a greater proportion of the electricity which is generated by the fuel cell, which is called parasitic power. The suitability of fuel cells for any particular utilization is at least partly

dependent upon its overall efficiency, including not only the efficiency of generating the electricity, but the cost (in power) of generating that electricity. The the overall efficiency of the fuel cell is of paramount importance, particularly in mobile equipment, such as vehicles, which not only must transport a load, and the fuel cell, but also the fuel which is to be utilized, in one form or another.

[0004] US patent 6,472,095 is predicated on the fact that the reactant utilization advantage of interdigitated flow fields is not necessary when the reactant concentration has been depleted only slightly, as in upstream portions of the fuel cell. The advantage of interdigitated flow fields becomes operative part way through the reactant flow field, when the reactant concentration has been significantly depleted.

[0005] One problem with the use of interdigitated flow fields is that it is very difficult to get the character of the flow field to change incrementally at successive positions along the path of the reactant gas. As illustrated in said patent, the flow fields are typically partially of flow-through type and then partially of the interdigitated type, so that the flow field is provided in two sections. Other, more incremental utilizations of interdigitated flow fields might be achieved with difficulty.

Summary

[0006] The apparatus herein includes projections at positions along the reactant flow fields, to redirect flow toward the membrane-electrode assembly, and increase reactant transport rates, thereby increasing reactant utilization. Either the spacing of the projections is sparse near the inlet to the flow fields, and decreases progressively toward the exit of the flow fields, or the obstruction provided by projections near the inlet to the flow fields is low, and increases progressively toward the exit of the flow fields. These increases in partial obstruction along the reactant flow path provide obstruction to the flow approximately in proportion to the reduction of reactant concentration, as the flow progresses through the reactant flow fields. As used herein, the phrase "approximately in proportion to the reduction of reactant concentration" includes: flow fields with no

obstruction for a significant fraction of the flow field length near the entrance, such as in fuel flow fields when industrial grade hydrogen is the fuel; obstruction in clumps; obstruction which increases in steps; and other arrangements which do provide increased obstruction along the reactant flow from inlet to exit in a manner generally related to the concentration of reactant along the reactant flow.

[0007] The projections may be stamped in flow fields formed of thin metal stampings, or the projections may be molded into composites of carbon or other conductive materials.

[0008] The projections may have various shapes and sizes, such as simply being quasi conical and not impeding the full width of the flow field channel, or the projections may extend from one side wall of each flow channel to the opposite side wall thereof. Generally speaking, a suitable form of a projection is one which is approximately the same length as the width of the flow field channel, and extends from one side wall to the other.

[0009] In the examples herein, the projections are on the order of half of the depth of the flow field channels, but that is only exemplary, and the extent of occlusion of the flow field channels, by selecting the width, number and/or the height of projections, is a factor which can be selected to suit other design considerations of the fuel cells involved.

[0010] Other variations will become more apparent in the light of the following detailed description of exemplary embodiments, as illustrated in the accompanying drawings.

Brief Description of the Drawings

[0011] Fig. 1 is a fragmentary perspective view of reactant gas flow field channels having projections therein in the form of inverse cones.

[0012] Fig. 2 is a partial perspective view illustrating projections in reactant gas flow field channels which are wider and longer than those of Fig. 1.

[0013] Fig. 3 is a fragmentary perspective view of reactant gas flow field channels having projections extending across the entire width of the channels.

[0014] Fig. 4 is a partial front elevation section of a fuel cell employing reactant gas flow field channels according to the subject matter hereof. [0015] Fig. 5 is a stylized top plan view, not to scale, of an air reactant gas flow field plate in accordance with the subject matter hereof,

illustrating the increasing density of projections as a function of distance downstream from the air inlet.

[0016] Fig. 6 is a section taken on the line 6-6 of Fig. 5

[0017] Fig. 7 is a section taken on the line 7-7 of Fig. 5.

[0018] Fig. 8 is a stylized top plan view, not to scale, of a fuel reactant gas flow field plate in accordance with the subject matter hereof,

illustrating the increasing density of projections as a function of distance downstream from the fuel inlet.

[0019] Fig. 9 is a fragmentary side elevation view of the fuel flow field plate of Fig. 8.

[0020] Fig. 10 is a fragmentary side elevation view of a portion of a reactant gas flow field plate having projections which provide increasing obstruction to reactant gas flow as a function of distance downstream from the reactant gas inlet.

[0021] Fig. 11 is a partial perspective view of fuel flow field plates having reactant gas channels of different width.

[0022] Fig. 12 is a stylized top plan view, not to scale, of a fuel reactant gas flow field plate illustrating staggered projections 27 of increasing density as a function of distance downstream from the fuel inlet.

Mode(s) of Implementation

[0023] An exemplary fuel cell reactant flow field plate 20 is shown in Fig. 1. A plurality of flow field channels 21 are formed, in this example, in thin metal 23, stamped to provide the channels interspersed with what will be referred to herein as ribs 24. In Fig. 1 , a projection extending into the flow field channel from the floor of the channel is an inverse crater 27 having a general cone-like shape. In much of the illustration of various

embodiments herein, this form of impediment to reactant gas flow is shown for simplicity of understanding.

[0024] Other shapes of projections may be used. In Fig. 2, a projection 28 is more extensive than the projection 27 of Fig. 1 , being wider and extending a greater length along the flow channel 21. [0025] In Fig. 3, the projection 29 is still more extensive, extending completely from one wall 32 of the flow channel 21 to another wall 33 thereof. If such a projection were formed by stamping in sheet metal, there would be elements of crumpling not shown in Fig. 3. However, the projection could be more nearly as shown in Fig. 3 if the reactant flow field plate 20 were molded of suitably conductive and structurally robust material.

[0026] Fig. 4 is a fragmentary sectional view of a fuel cell stack taken at a point along the flow fields where there are no projections. In Fig. 4, a fragment of a fuel cell 36 is illustrated with some adjoining apparatus. Fig. 4 illustrates front elevation views of projections of the type shown in Fig. 1 or Fig. 2, for clarity of illustration. In Fig. 4, although the thin stamped metal 23 has thickness as illustrated in Figs. 1-3, it is illustrated in Fig. 4 with single solid lines for clarity of illustration. All of the various channels are labeled, the flow field channels 21 of Figs. 1-3 are utilized in fuel cells either for air or for fuel. The fuel channels 38 are open toward and adjacent to the anode GDLs (gas diffusion layers) 39, and the air channels 42 are open toward and adjacent to the cathode GDLs 44. Between the two GDLs 39, 44 of each fuel cell 36 is an MEA (membrane electrode assembly) 45 which includes, in the example herein, a proton exchange membrane with electrode catalysts on either side thereof.

[0027] When arranged in a stack of fuel cells, as depicted in Fig. 4, with the fuel and air channels 21 being of the same size, they are aligned with each other. The space 43 in-between the ribs 24 provides coolant passageways, which in this example is assumed to be water, although any suitable coolant can be utilized since there is no fluid communication through the thin metal 23. However, as described briefly hereinafter, other physical arrangements may be utilized within the purview of the subject matter herein.

[0028] Fig. 5 is a fragmentary top plan view of an air flow field of the type described with respect to Fig. 1. The air flow is from left to right as indicated by an arrow. The projections 27 are arranged in rows 50-55 transverse to the channels which have an increasing density in the direction of flow from an inlet end 47 to an outlet end 48. The rows 54, 55 are closer together than the rows 50, 51 ; similarly with respect to the rows 51-53 therebetween. The rows are therefore spaced in a manner which is approximately proportional to the dilution of air at any row 50-55 along the flow through the flow field 20. Figs. 6 and 7 are further illustrations of the air flow field. Fig. 6 is taken on the line 6-6 in Fig. 5 which is between the rows 53, 54 of projections 27. Fig. 7 is taken on the line 7-7 in Fig. 5 which is through the row 55 of projections 27.

[0029] Referring to Fig. 8, fuel enters from the right as shown by an arrow 59. The projections 27 are arranged in rows 60-65 which become progressively closer together in successive downstream positions. This provides spacing of the projections which is approximately proportional to depletion of fuel (particularly if the fuel is dilute, as in reformate fuel).

[0030] Fig. 9 is a fragmentary section taken on the line 9-9 in Fig. 8, which is down the middle of the fuel flow channel 21. Small, dashed arrows 68, 69 illustrate the flow of fuel along the channel 21. The arrows

68 indicate that some fuel will continue to flow in the fuel flow channel 21 , whereas the arrows 69 indicate that a portion of the fuel will be forced into convective flow within the gas diffusion layer 39 adjacent to the anode of the MEA 45. The extent to which more or less fuel will follow the arrows

69 into convective flow within the GDL is a function of the degree of occlusion which the projections 27 (or other projections such as 28 and 29 in Figs. 2 and 3) occlude the flow of fuel within the channel 21.

[0031] Although illustrated in Figs. 8 and 9 with respect to fuel, the utilization of the subject matter herein is far more significant in the air flow fields, as illustrated in Fig. 5. When the fuel is high grade industrial hydrogen or the like, it is quite possible that little benefit would be obtained by utilizing any sort of occlusion to the fuel flow. However, the air starts out with only about 20% oxygen, and as oxygen is utilized, it becomes successively more rarified as the flow progresses through the air flow field. Although the fuel and air flow fields of Figs. 5 and 8 appear to be approximately the same, in the usual case there will be more obstruction in the downstream area of the air flow field than would be necessary in the fuel flow field. [0032] A plurality of variations may be made in the manner in which the subject matter herein is implemented in any given utilization thereof. For example, the method of increasing convective flow into the GDL by virtue of increasing the occlusion of reactant gas as a function of the dilution of that gas in successive portions of the fuel cells can be effected in ways other than the positioning of rows of projections. For example, Fig. 10 illustrates projections 30 which are small in the upstream end and do not occlude very much, while at the downstream end, they are large and provide a more significant occlusion to the flow of reactant gas. This will provide convective flow into the GDL approximately proportional to the depletion of the reactant gas at the various positions along the flow field where the projections are provided.

[0033] In Figs. 1-3, the projections are shown as extending upwardly about one-half the height of the flow field channels 21. However, the height may be selected to suit characteristics of any particular fuel cell design in which they are used. Furthermore, the height of the projections may be varied so as to adjust the impediment to reactant gas flow, such as, for instance, as shown in Fig. 10.

[0034] In Fig. 1 1 , the flow field channels are shown without any projections for clarity of illustration. Fig. 1 1 illustrates that adjacent flow field plates may have different channel widths, if appropriate to improve electrical conduction, or to accommodate different flow dynamics in the respective reactants. The plates may have different channel depths, as well or otherwise.

[0035] Fig. 12 illustrates projections 27 which are arranged at each of the positions in a first set of rows 80-87 which are downstream from a second set of rows 90-97. In Fig. 12, the rows 80 comprise a first group of projections 27 which are at different distances from either end of the flow field than the group of projections 27 in the rows 90. The staggering of rows as illustrated in Fig. 12 may be beneficial to a more even distribution of reactant gas in the adjacent layer. The projections 27 could be staggered in groups of three or four or more, rather than the groups of two as illustrated in Fig. 12. Such groups could form patterns found useful for deploying reactant gas into the adjacent layer of the fuel cell. [0036] The rows need not be in perfect alignment; if desired, the pattern of projections at each interval could be random (not shown).

However, the pattern of projections throughout the flow field plate needs to be more dense near the outlet end and generally progressively less dense toward the inlet end, as in other embodiments.

[0037] It is within the purview of the subject matter that total occlusion might occur at or near the downstream end of a flow field. It is also within the purview of the subject matter that less than all channels of a flow field may be obstructed. Some fraction of a flow field, such as ¼ or ½, near the inlet end may be devoid of any projections. Although some of the figures herein depict only air flow fields or only fuel flow fields, it is to be understood that anything depicted herein may be utilized for either fuel reactant gas flow fields or oxidant reactant gas flow fields.

[0038] As used herein, the term "partially" excludes "completely", and excludes "substantially".

[0039] Since changes and variations of the disclosed embodiments may be made without departing from the concept's intent, it is not intended to limit the disclosure other than as required by the appended claims.

Claims

1. A method of improving reactant gas utilization in a fuel cell apparatus, comprising:

providing a flow of reactant gas in reactant gas flow channels from a flow inlet end of a reactant gas flow field plate to a flow exit end of said reactant gas flow field plate;

providing obstruction to flow of reactant gas in said reactant gas flow channels;

characterized by:

partially obstructing at least selected ones of the reactant gas flow channels at successive positions along the reactant gas flow channels, thereby to force a portion of the reactant gas in each selected flow channel to flow by convection into an adjacent layer of the reactant gas flow field plate.

2. A method according to claim 1 further characterized in that: said step of partially obstructing comprises providing a degree of obstruction to flow at each of said successive positions in general proportion to the degree of depletion of reactant in the flow at each of said successive positions.

3. A method according to claim 1 further characterized in that: intervals between the successive positions are progressively shorter from the inlet end of the reactant gas flow field plate to the exit end of the reactant gas flow field plate.

4. A method according to claim 1 further characterized in that: the degree of partial obstruction provided at said successive positions is increased from the inlet end of the reactant gas flow field plate to the exit end of the reactant gas flow field plate.

5. A method according to claim 1 further characterized by: in addition to partially obstructing the flow of said reactant gas flow channels, totally obstructing the flow of at least some of said reactant gas flow channels near said exit end.

6. A fuel cell apparatus, comprising:

a fuel cell reactant gas flow field plate having reactant gas flow channels extending from a flow inlet end of said reactant gas flow field plate to a flow exit end of said reactant gas flow field plate;

characterized by:

a plurality of projections, at successive positions in at least selected ones of the reactant gas flow channels, each projection configured to partially obstruct the flow of reactant gas in the corresponding reactant gas flow channel, thereby to force a portion of the reactant gas in each selected reactant gas flow channel to flow by convection into an adjacent layer of the reactant gas flow field plate.

7. A fuel cell apparatus according to claim 6 further characterized in that:

intervals between the successive positions are progressively shorter from the flow inlet end of the reactant gas flow field plate to the flow exit end of the reactant gas flow field plate.

8. A fuel cell apparatus according to claim 6 further characterized in that:

the degree of partial obstruction provided by each of the projections is increased in successive positions of said reactant gas flow channels from the flow inlet end of the reactant gas flow field plate to the flow exit end of the reactant gas flow field plate.

9. A fuel cell apparatus according to claim 6 further characterized in that:

said projections are configured to provide a degree of obstruction to flow which is approximately proportional to the degree of depletion of reactant in the flow at each of said successive positions.

10. A fuel cell apparatus according to claim 9 further characterized in that:

the projections at each position are in rows transverse to the channels.

11. A fuel cell apparatus according to claim 10 further characterized in that:

the rows of projections in at least a first group of said selected channels are at different distances from said flow inlet end of the reactant gas flow field plates than the rows of projections in at least a second group of said selected channels.

12. A fuel cell apparatus according to claim 9 further characterized in that:

the arrangement of said projections is random.

13. A fuel cell apparatus according to claim 9 further characterized in that:

the arrangement of said projections is to provide projections in each channel which are longitudinally offset from projections in adjacent channels.