The present invention relates generally to fuel cells, and in particular to improvements in performance of polymer fuel cells.

Background of the Invention Polymer fuel cells are on the fringe of commercialization. The progress made in catalyst and membrane research in the last few years has enabled very high power densities (>lW/cm2) with moderate efficiencies for the fuel cell (40%). The catalyst loading of electrodes has been reduced to 0. 1 mg Pt/cm2 while maintaining a high performance. The price of the commonly used PFSI (perfluorosulfonic ionomer) membranes is expected to decrease, with increasing production, whilst other proton conducting membrane candidates have been discovered.

However, serious problems are encountered when polymer fuel cell technology is scaled up to larger cells and stacks. One of the main problems, in the stacks themselves, is the water management, since the proton conducting membrane must be kept well humidified under operating conditions.

The dominating component, at the iR loss in the stack, is due to the limiting proton conductivity of the membrane. Membranes tend to dry out, especially on the anode side, at high current densities, since proton migration"drags"water molecules away from the anode.

Drying out of anodes does not only affect resistance but also the kinetics of hydrogen reduction reaction (HRR) at the anode.

Therefore, in attempts to remedy this problem the anode side is often humidified more intensively than the cathode side. The cathode side of the cell can also be pressurized to use the pressure gradient to press the water back to the anode.

One solution for this problem is to use thinner membranes, but this approach has limitations since mechanical rigidity of the membrane must be sufficient. Furthermore, the crossover of

the gases over the membrane increases and this, in turn, lowers the faradic efficiency of the cell.

Another solution is to have a direct water contact with the membrane at the anode side, since the water content and conductivity of the membrane are much higher when the membrane is in equilibrium with water. Also, when liquid is vaporated inside the fuel cell, a considerable amount (40-50%) of the heat can be removed from the cell with the produced water vapor, thus bringing about a significant cooling.

US-5, 958, 613 (Hamada et al) relates to such direct water humidification of fuel cell membranes. Therein is disclosed a polymer fuel cell system with a capability to moisten the solid-polymer film without providing a special humidifier which humidifies the fuel gas or the oxidizer gas, and that cools down the main cell body without providing cooling channels.

In US-5, 935, 726 (Chow et al) there is disclosed a method and apparatus for improved humidification of membranes in polymer fuel cells, by periodically reversing the flow direction of the oxidant stream through a fuel cell flow field. However, this patent is not concerned with cooling of the fuel cell.

US-5, 952, 119 (Wilson) discloses a fuel cell humidification technique based on"spot-wise" wetting of a membrane by using a wick, contacting the membrane at discrete spots with a certain spacing therebetween. Water is introduced in the gas flow, and not separate therefrom.

Summary of the Invention Despite the numerous attempts to improve the water management in polymer fuel cells, there is still room for improvements.

Thus, the object of the present invention is to provide a method and a system for achieving better humidification, at low cost and low cell complexity. The trade off between performance and cost should be acceptable.

In the method of the present invention, an aqueous phase, preferably water, is used for the direct humidification of the membranes. The term"aquous phase"shall be taken to encompass, all of the following but not limited thereto, namely liquid water, solutions where water is the solvent, moist air, steam. The method comprises periodically pulsing the flow of the aqueous phase, of individual cells in a cell stack, in sequence, such that only a fraction of the active membrane area is flooded at any one time. The method is defined in claim 1, and comprises periodically exposing membranes of one cell and/or a block of cells at one time, at least partially over the surface of said membranes, to pulses of an aqueous phase, in order to humidify said membranes (4a-c) ; said pulses having a duration that is shorter than the period between pulses, and by repeating the step of exposing the membranes on a cell and/or block of cells that was not previously exposed to a pulse.

By introducing the pulses of aqueous phase, the supply of fuel will be interrupted, or at least reduced, and the efficiency of the cell in question will drop. If the entire cell is flooded, the output power will be practically zero. It is also possible to control the flow of fuel by actively interrupting the supply of fuel, e. g. by closing a valve in the fuel supply line.

Thereby, the overall efficiency of a cell stack having a plurality of cells is increased compared to the overall efficiency attainable without pulsing. Also, the cooling of the stack is improved.

In a second aspect of the invention there is provided a system for the controlled humidification of the membranes in a polymer electrolyte membrane fuel cell assembly, which is defined in claim 12. The system comprises a control unit, suitably programmed according to a desired algorithm for the pulsing sequence.

Brief Description of the Drawings The invention will now be described in detail with reference to the drawings, in which Fig. 1 illustrates a prior art fuel cell assembly suitable for the method according to the present invention ;

Fig. 2 is a schematic illustration of a system for performing the method according to the present invention ; Fig. 3 is a graph showing the behavior of one cell subjected to pulsing according to the invention ; Fig. 4 is a graph showing the behavior of an assembly of three cells subjected to pulsing according to the invention ; and Fig. 5 is a graph showing average current density for the cell assembly of Fig. 4.

Detailed Description of the Invention The pulsing technique according to the present invention is applicable to fuel cells in general under certain conditions, but the inventive method will be described with reference to a specific cell design, namely the one disclosed in US-5, 952, 119.

In Fig. 1 a fuel cell in exploded view according to US-5, 952, 119 is shown in cross-section. A unit fuel cell 1 is formed with anode side 2 and cathode side 4 separated by catalyzed membrane 4. Cathode side 3 comprises flow field plate 5 with oxidant feed channels 6 and hydrophobic gas backing 7 adjacent one surface of catalyzed membrane 4. Anode side 2 comprises flow field plate 8 with fuel distribution channels 9 and hydrophobic gas diffusion backing 10 adjacent a second side of catalyzed membrane 4. A two-part hydrophobic/hydrophilic structure is formed by wicking thread 11 that has been sewn through conventional hydrophobic gas diffusion backing 10 to supply the hydrophilic component for the anode side 2.

However, as will be understood, this is only one possible structure that is suitable for carrying out the method according to the present invention. Any other structure which permits water and gas to alternately access the membrane, and preferably at different regions, will do. E. g. any structure wherein hydrophilic and hydrophobic portions of the distribution member are separated is suitable, as long as the water is given access to the membrane surface at defined areas or spots only. This is the case in the example according to US-5, 952, 119, wherein the

hydrophobic gas backing 7 has channels or holes wherein the hydrophilic wick is located. The water does not penetrate the hydrophobic regions, but will access the membrane through the wicking action inside the channels or holes. Thus, a requirement is that the water shall only be given access to the membrane at certain areas or"spots", and these areas or spots must be close enough that the region between them will be sufficiently wetted or humidified by diffusion of water within the membrane itself.

When a polymer electrolyte membrane in a fuel cell unit is flooded, the reactant gases will not be able to reach the catalytic surface of the membrane, and the reaction rate will drop dramatically. The power output will of course also drop together with the drop in reaction rate, and there will be a strong fluctuation. Thus, it is not possible to flood the entire surface of a membrane, or of a cell assembly, at the same time.

In accordance with the present invention in a preferred embodiment thereof, therefore the electrode surface on the membrane of one single cell at a time in a stack or assembly of a plurality of cells will be partially flooded in a pulsing fashion, in sequence.

A pulsing sequence controlled by the control system, can be implemented such that different cells have different phase in pulsing. For example, it is within the inventive concept to pulse individual cells in sequence, or blocks of cells, at a time.

By the expression"in sequence"it is meant that one cell and/or block of cells is pulsed before another cell or block of cells, however not neccessarily adjacent cells or blocks of cells, i. e. the sequence of cells/blocks of cells can be arbitrary in the stack.

In another embodiment, only a fraction of the membrane surface of a single cell is flooded at a time. However, with the latter approach a more complex stack structure would be required.

Both sides of a cell can be humidified with the pulsing technique according to the invention, although the anode side is more important, because of proton migration that takes place. This proton migration causes a"drag"of water molecules from the anode surface, which thus tends to dry out.

An embodiment of the invention will now be described with reference to Fig. 2, which schematically shows an example of a system for performing a pulsing algorithm. The example shows pulsing on one side only of a cell (anode side) for clarity. Pulsing on the anode side may be preferable in some cases. However, also only the cathode side can be subjected to puling. The membrane would then have to be so permeable that water migrates through it in order that the anode side be sufficiently humidified. Also, both sides of the membrane can be subjected to pulsing of aqueous phase, simultaneously or alternatingly. In the figure only three unit cells are assembled to a cell stack, but of course any number of cells could in principle be assembled. For optimum performance 3-6 cells should be used. If a larger number of cells are used, each group of 3-6 cells should be considered as a subunit, on each of which the pulsing algorithm is performed.

Thus, as shown in Fig. 2, fuel gas 12 (H2) is supplied to the anode compartments 24a-c of the various cells 13a-c in fuel cell assembly 14 through a meter 15 via lines 16a-c. The lines 16a- c are provided with control valves 17a-c selectively operable to pass gas to a selected cell in the cell assembly.

In a similar manner an aqueous phase, preferably water, is supplied from a storage tank 18 via pump 19 and conduits 20, 20a-c to feed the individual cells 13a-c. There are provided valves 21a-c operable to selectively pass water into the desired cell of the assembly, so as to humidify one selected cell at a time.

Any unreacted gas and accumulated water will be output through the conduit 22. It is also conceivable to have separate outlets 23, 23a-c for water.

In alternative embodiment it is not necessary to actively interrupt or stop the supply of fuel by closing the valves 17a-c. Instead the actual pulse of water (or other aqueous phase) will effectively hinder the fuel from accessing the active surface on the membrane, and thereby essentially the same effect will be achieved.

The pulsing can be carried out as follows.

Cells are pre-humidified (or kept moist when inactive) to moisten the membranes to a sufficient degree that they are operable. Then reactant gases are introduced into the cell anode compartments 24a-c, and the reaction will start. The cathode compartment is indicated at 25a- c, and in the described embodiment no humidification is performed on this side, although it is of course within the inventive concept to perform an analogous humidification by pulsing also on the cathode side. After a short start-up period, valve 17a is closed and valve 21 a is opened to inject a"pulse"of water into the anode compartment 24a. Thereby the anode compartment of the first cell 13 a will become flooded with water. The duration of the pulse may be 0, 1-5 seconds, and is preferably 1-2 seconds. Then the water valve 21a is closed and the gas valve 17a is opened again. It will remain open for about 1-30 seconds, preferably 5-10 seconds, during which time reactions will take place and the cell will deliver power.

Let us assume as an example that the pulse has a duration of 1 second and the active time, i. e. the time during which the cell produces energy, is 9 seconds. Then the scheme for the entire three-cell assembly would be as follows : No.

Cell 1 Cell 2 Cell 3 In this scheme the short arrows represent the water pulsing, and the long arrows represent the "active"time during which the unit cells produce energy, and as can be seen in the scheme, the pulses are shorter than the period between pulses.

The behavior of the cells during pulsing will now be discussed in some detail, with reference to Figs. 3-5, which show diagrams of current density vs time for fuel cells when run using the pulsing technique according to the method of the invention.

Referring first to Fig. 3, which shows the behavior of a single cell when pulsed in accordance with the invention. The duration of the pulse is about 0. 5 seconds, and the active electrode time is about 3 seconds. Thus, the total cycle time is about 3. 5 seconds. As can be seen there is a rapid drop in output the instant when the electrode is flooded at the spots or areas of water

access, which is due to the water preventing the reactant gas from accessing the catalytic surface of the membrane, and therefore no reaction can occur. When the pulse is over, the water will diffuse into the membrane and some of the water will also evaporate, and the reaction rate will increase again. Unexpectedly, however, the power output is seen to increase to a level exceeding the output level existing before the pulse. This peak power will thereafter slowly decay to the normal level.

At all times the duration of the pulses of the aqueous phase have a longer duration than the duration of the active time, i. e. the time during which the cell produces energy with full effect.

In other words, the fuel supply period is always longer than the pulses of aqueous phase.

In a cell assembly, such as the one shown in Fig. 2, where a plurality (three in the example) of unit cells are stacked, the above described behavior can be used to advantage, to improve the overall cell efficiency. Namely, by pulsing the cell assembly according to a scheme as that shown above, a power output profile as shown in Fig. 4 (wherein the outputs for each single cell unit is shown as separate curves), will be obtained.

Fig. 5 illustrates the average current density for the three cells, the performance of which is shown in fig. 4, whereby it is clear that the overall average current density (approx. 550 mA/cm2) exceeds the nominal current density of a cell (500 mA/cm2), when not run according to the invention.

The specific cell structure shown in Fig. 1, representing a known fuel cell assembly, can be used as the fuel cell 13 in the scheme of Fig. 2. The function when the cell according to Fig. 1 is used, will then be as follows.

In order to avoid humidification of the reactant gases, preferably there are provided separate inlets for gas and water respectively, as schematically indicated in Fig. 2. However, the gas channels 9 will be used for the humidification. Thus, assuming that the cell is ready for operation by being suitably humidified, reactant gases are introduced such that the reactions will be initiated. Following the scheme shown above Cell No. 1 will be subjected to a switch- off of reactant gas supply, and a water pulse of a duration of say 1 second will be initiated.

After 1 second water supply is terminated and reactant gases will be introduced for a period of

9 seconds. 2 seconds after the termination of the water pulse in Cell No. 1, Cell No. 2 will be subjected to a first water pulse during 1 second, and finally Cell No. 3 will be subjected to a pulse 2 seconds later. This cycle will then be repeated during cell operation. During water pulsing, the channels 9 will be flooded, and the wick 11 will become soaked with water. By virtue of its hydrophilicity, the wick will function as a water distribution element, and it will relatively rapidly transport the water through the hydrophobic gas distribution element through the holes. Water will not adhere to the gas distribution member to any significant degree because of its hydrophobic properties. Should nevertheless any droplets adhere to the surface thereof, they will relatively rapidly evaporate because of the reaction heat generated.

In the process, the system will be cooled, which is a further benefit of the invention.

As can be understood from the above disclosure, an important aspect of the inventive method is that the gas distribution elements and the water distribution members be at least partially physically separated, in order to avoid that the water blocks the access of the gas to the membranes as far as possible.

It is of course possible to employ any pulsing algorithm that achieves the goal of humidifying the membrane surfaces sequentially. Thus, the expression sequentially shall be taken to encompass pulsing in any order, with respect to the cell order in a cell stack or assembly.

It is also conceivable, as already indicated, to inject water only over a fraction of each cell, but to do so for all cells in the stack at the same time.

Although of course the pulsing method according to the invention has general applicability, it is more suitable for relatively large, stationary fuel cell assemblies, since the mechanisms for the valves tend to be more and more complicated when dimensions are reduced.

Suitably a system for the implementation of the inventive method would comprise a fuel cell assembly as the one shown in Fig. 2, wherein a control unit 26, e. g. a microprocessor, would be programmed according to the desired pulsing algorithm for controlling the settings of the valves. Namely, the system comprises a plurality of polymer electrolyte membrane fuel cells coupled to a source of aqueous phase for the humidification of said membranes. There are inlets for reactant gases to the reaction compartments of the cells. The cells are provided with

a plurality of valves controlling the inflow of aqueous phase and reactants, respectively, into the cells. There is provided a control unit 26, programmed to control said valves to open and close according to a predetermined algorithm, such that the supply of aqueous phase will be in the form of pulses.

The skilled man would appreciate that the invention could be modified without departing from the scope and spirit of the invention, as defined in the claims.