This invention relates to a fuel cell with enhanced cross-flow serpentine flow fields.

A split serpentine flow field plate with enhanced cross flow which is proposed herein furnishes the quadruple advantage of (a) uniform reactant distribution over the entire active area of the cell achieved by using the serpentine flow design, (b) low pressure drop and hence low parasitic power losses (compared to a single serpentine flow field) achieved by splitting the single serpentine flow field into shorter segments of nearly equal length and flow rate (c) effective liquid water evacuation in the U- bends achieved by cross flow through the gas diffusion layer from the high pressure channel to the U-bends of the serpentine and (d) oxygen replenishment by enabling more cross flow into the oxygen-deficient portion of the serpentine channel achieved by an intelligent lay-out of the flow field.

In this invention the flow field plate has at least two independent single serpentine channels extending from the inlet manifold to the exit manifold.

The serpentine channels are arranged such that they are inverted alternately to each other.

The flow field plate can have split serpentine channels which are curved. The flow field plate can have sharp U-bends replaced by the rounded/ chamfered U-bends.

The flow field plate can have channel segments which are tapered.

The flow field plate can be used either on cathode side or on anode side or on both sides of the fuel cell.

The flow field plate can have the inlet/outlet arrangement at the centre, off- centre or at the extremes.

The flow field plates can have more than one of inlets or outlets per plate. The flow field plate can have the feed In either the gaseous form or in liquid form.

In the flow field plate the feed streams are either humidified or unhumidified or partially humidified.

The flow field plate can have the gas diffusion layer under the flow field made hydrophobic fully or partially.

The flow field plate can have one or more serpentine segments replaced by one or more interdigitated channels as illustrated in Figure 2.

The emerging fuel cell technology is one of the prominent solutions to the present and future energy and environmental crisis. Fuel cells are electrochemical energy devices that convert chemical energy of a fuel and oxidant directly into electrical energy. PEM fuel cells are gaining importance because of their low temperature operation (60-80°C) and high efficiency. These are being considered as good substitutes for traditional energy systems for both stationary and automotive applications, primarily due to their high efficiency and ultra low pollutant emission features. Though PEM fuel cell technology has seen significant progress over the past decade, its commercialization with high performance and at low cost is yet to be achieved. One of the critical issues that affect the cell performance is water management within the cell. In fact, the gas streams entering the fuel cell are sufficiently humidified to ensure proper membrane hydration and high membrane conductivity. If sufficient water is not available in the membrane, then membrane dehydration occurs, which causes an increase in the resistance to the proton flow, thereby damaging the cell performance. On the other hand, the presence of excess water either on the anode or on the cathode side results in flooding. It degrades the cell performance by preventing the reactants from reaching the catalyst sites and results in a rapid voltage drop at high loads. Both of these effects, which are extremes in improper water management, have an adverse effect on the overall efficiency of the fuel cell. Hence, an effective water management plays a crucial role in the operation of a fuel cell. In addition to this, reactant flow misdistribution in the flow channels of the flow field, which essentially determines the reactant distribution on the electrode of the fuel cell, is another important factor which has an adverse effect on the performance of the fuel cell.
The serpentine flow field is one of the widely used flow field designs in the operation of fuel cells. Since the serpentine flow channel gives the best performance and is durable compared to the other designs under the same operating and design conditions, it is often considered to be the 'industry standard'. Studies in the literature show that a significant pressure drop exists between the adjacent channels which leads to a considerable cross leakage between the adjacent channels in a serpentine flow field. This cross flow aids both in the feeding of the reactant to the catalyst layer and removal of the product water from the electrode, and hence serpentine flow field gives higher current compared to the conventional parallel flow fields. But these flow fields also suffer from many problems, which restrict their observed high performance to small area cells such as (I) relatively long flow paths leading to higher pressure drops and hence high parasitic power losses (ii) lower reactant concentration in the ending channels of the flow field (iii) localized flooding in the U-bend regions due to less cross flow and (iv) membrane dehydration near the channel inlet region, while liquid water flooding in the significant region near the channel exit due to excessive liquid water carried downstream by the reactant gas stream and collected along the flow channel.

Hence a suitable flow field with serpentine flow channel which overcomes these short-comings is desirable. Considering the advantages of the serpentine flow channel, the present invention provides a flow field based on the split up of the serpentine channels to address some of the above mentioned issues.

In the fuel cell with enhanced cross-flow serpentine flow fields, according to this invention, a serpentine channel is split into independent serpentine channels with individual inlets from a common inlet manifold, such that a high pressure differential is maintained between flow channel and the U- bends, causing a cross flow of the reactant through the porous diffusion layer from the flow channel to the U-bends, the lay-out of the flow field being such that the cross flow is higher in the oxygen-depleted portion of the adjacent serpentine flow field.

This invention will now be described with reference to the accompanying drawings which illustrate, by way of example, and not by way of limitation embodiments of the fuel cell proposed herein,

Fig. 1 illustrating a lay-out of the split serpentine flow field arrangement with three individual serpentine channels

Fig. 2 illustrating a lay-out of the split serpentine flow field arrangement with three serpentine channels and two pairs of interdigitated flow channels.

The flow field design according to the present invention draws upon the advantages of the serpentine channel and mitigates its disadvantages. Instead of having a single serpentine channel throughout the area of the flow field, which increases the fiow path for the reactants and hence causes higher pressure drop, it is split into different serpentine channels running individually from the inlet to the outlet on the flow field plate. Thus, a split serpentine gives lower pressure drop or helps in operating the fuel cell at higher stoichiometric ratio of the feed to combat flooding problem more effectively. Secondly, these split serpentine channels are arranged in such a way that the pressure drop between the inlet and outlet of each serpentine channel is almost same as the other, thereby ensuring uniform flow split among the channels. Thirdly, these serpentine channels are arranged in such a way that most of the U-bends in the serpentine channels get fed from the adjacent channel via cross-flow through the porous diffusion medium. This not only pushes out accumulated water under the rib portion, but also increases the effective stoichiometric ratio of the reactant leading to less possibility of localized flooding. Finally, the channel arrangement is such that higher cross flow occurs in the tail-end channels, which generally suffer from the lower reactant concentrations. This selective replenishment of the reactant to the more oxygen-deficient portions of the flow field is expected to increase the overall performance of the cell.

Fig. 1 shows the arrangement of the split serpentine flow field with three serpentine channels individually extending from the inlet to the outlet. The first serpentine channel is so arranged that one set of its U-bends will be fed by the fresh reactant coming from the inlet channel. As the flow goes from the left to the right, the pressure difference between the U-bend and the straight feed channel above the bends increases, causing more cross flow into the end channels on the right side of the serpentine. Since these channels are more deficient in the reactant concentration, the fresh entry of the reactant from the inlet channel causes replenishment of the reactant. The second serpentine channel is proposed to be inverted with respect to the first one and is so arranged that the other set of U-bends of the first serpentine channel and the one set of U-bends of the present serpentine channel are supplied with the fresh reactant via cross flow resulting from the pressure difference across the bends and the straight feed channel through the porous diffusion layer. This serpentine channel is also used in feeding one set of U-bends of the third serpentine channel. The third serpentine channel is arranged almost as a mirror to the first serpentine channel which will take care of feeding to another set of its U-bends through the cross flow mechanism.

Fig. 2 shows an alternative design of the split serpentine flow field coupled with interdigitated channels, which is also expected to improve the overall cell performance by the enhanced cross flow through the gas diffusion layer to the localized flooding prone areas of the serpentine channels. In this figure, the lay-out for the arrangement of three serpentine channels coupled with two-pairs of interdigitated channels is shown. The first serpentine channel is arranged in the same way as that of split serpentine flow field lay-out shown in Fig. 1. Between the first two serpentine channels, a two-channel interdigitated flow pattern is used. This is expected to feed the one set of U-bends of the first serpentine channel and one set of the second serpentine channel placed below the interdigitated channel and also allow for any liquid water accumulated in to the inlet channel to be flushed out. Similarly a second pair of interdigitated channel is placed between the second and third serpentine channels. This will feed the each set of U-bends of the second and third serpentine channels.

It will be appreciated that various other embodiments of this invention are possible without departing from the scope and ambit thereof.

We Claim:

1. A fuel cell with enhanced cross-flow serpentine flow fields,

wherein a serpentine channel is split into independent serpentine channels with individual inlets from a common inlet manifold, such that a high pressure differential is maintained between flow channel and the U-bends, causing a cross flow of the reactant through the porous diffusion layer from the flow channel to the U-bends, the lay-out of the flow field being such that the cross flow is higher in the oxygen-depleted portion of the adjacent serpentine flow field.

2. A fuel cell as claimed in Claim 1 wherein the split channels are of nearly equal length and flow rate.

3. A fuel cell as claimed in any one of the preceding Claims wherein the flow field plate consists of at least two independent single serpentine channels extending from the inlet manifold to the exit manifold.

4. A fuel cell as claimed in any one of the preceding Claims wherein the split serpentine channels are arranged such that they are inverted alternately to each other.

5. A fuel cell as claimed in any one of the preceding Claims wherein the split serpentine channels are curved.

6. A fuel cell as claimed in any one of the preceding Claims wherein the U-bends are rounded/ chamfered.

7. A fuel cell as claimed in any one of the preceding Claims wherein the channel segments are tapered

8. A fuel cell as claimed in any one of the preceding Claims wherein the flow field plate is provided on either or both sides of the cathode or anode and the flow field plate has the inlet/outlet arrangement at the centre, off- centre or at the extremes, the feed being either in the gaseous form or in liquid form, the feed streams being either humidified or unhumidified or partially humidified, the said flow field plate having a plurality of inlets or outlets per plate, the gas diffusion layer under the flow field being made hydrophobic fully or partially.

9. A fuel cell as claimed in any one of the preceding Claims wherein at least one serpentine segment is replaced by an interdigitated channel

10. A fuel cell with enhanced cross-flow serpentine flow fields substantially as herein described with reference to, and as illustrated by, the accompanying drawings.