Water management system for fuel cells

A water management system reduces the problems with flooding and also enhances the flow of fuel gas to the anodes. Individual unit cells (20) in an array are separated by cell walls (24), the array is covered by a fuel manifold (36), and the manifold is arranged so that the individual unit cells have their own respective chambers. Each chamber is arranged so that the fuel gas flows from one chamber into another through an opening or vent (35) in the chamber wall. The opening contains a hydrophobic portion (38) that serves to urge liquid water that accumulates in the opening to migrate away.

TECHNICAL FIELD
 This invention relates in general to fuel cells, and in particular, to
 water management for fuel cells having a solid electrolyte.
 BACKGROUND
 Fuel cells are electrochemical cells in which a free energy change
 resulting from a fuel oxidation reaction is converted into electrical
 energy. A typical fuel cell consists of a fuel electrode (anode) and an
 oxidant electrode (cathode), separated by an ion-conducting polymer
 electrolyte. The electrodes are connected electrically to a load (such as
 an electronic circuit) by an external circuit conductor. In the circuit
 conductor, electric current is transported by the flow of electrons,
 whereas in the electrolyte it is transported by the flow of ions, such as
 the hydrogen ion (H.sup.+) in acid electrolytes, or the hydroxyl ion
 (OH.sup.-) in alkaline electrolytes. Gaseous hydrogen is the fuel of
 choice for most applications, because of its high reactivity in the
 presence of suitable catalysts and because of its high energy density.
 Similarly, the most common oxidant is gaseous oxygen, which is readily and
 economically available from the air. At the anode, incoming hydrogen gas
 ionizes to produce hydrogen ions and electrons. Since the electrolyte is a
 non-electronic conductor, the electrons flow away from the anode via the
 metallic external circuit. At the cathode, oxygen gas reacts with the
 hydrogen ions migrating through the electrolyte and the incoming electrons
 from the external circuit to produce water as a byproduct. The byproduct
 water is typically extracted as vapor. The overall reaction that takes
 place in the fuel cell is the sum of the anode and cathode reactions, with
 part of the free energy of reaction released directly as electrical
 energy. Some of the energy is produced as heat.
 In practice, a number of unit cells are normally stacked or `ganged`
 together to form a fuel cell assembly. The individual cells are typically
 electrically connected in series. Fuel and oxidant are introduced through
 manifolds into respective chambers. One style of fuel cell is a
 side-by-side air breathing configuration in which a number of individual
 cells are placed next to each other in a planar arrangement (see, for
 example, U.S. Pat. No. 5,783,324). Air breathing fuel cells do not rely on
 forced flow of oxidant or air past the cathodes, but instead utilize the
 ambient air and rely on natural convection in the surrounding environment.
 A classical problem with air breathing planar fuel cells is water
 management. Since the byproduct water is produced at the cathode (air
 side), it normally evaporates away during operation. However, under heavy
 loads, the evaporation rate lags the rate of formation and water tends to
 migrate back through the polymer electrolyte to the anode side. Some spots
 on the planar fuel cell are cooler than others, and the H.sub.2 O
 condenses at these locations into liquid water, flooding both the anode
 and the cathode, impeding the reactions at the catalyst sites and impeding
 the flow of hydrogen gas to the anode. Thus, although air breathing fuel
 cells continue to hold technological promise, they remain a dream that has
 so far proven to be elusive to the skilled artisan. An improved planar
 fuel cell that is less complex and less prone to failure would be a
 significant addition to the field.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
 A system of water management reduces the problems with flooding and also
 enhances the flow of fuel gas to the anodes. In a planar fuel cell there
 are a number of unit cells, and each unit cell has an anode side and a
 cathode side. The unit cells are typically arranged in a plane such that
 all the anode sides face in the same direction (i.e. on the same side of
 the planar fuel cell). A fuel manifold covers the anodes, and the manifold
 is arranged so that the individual unit cells have their own respective
 chambers, each of the chambers arranged so that the fuel gas flows from
 one chamber into another. This is accomplished by an opening or vent in
 the chamber wall. The opening contains a hydrophobic portion that serves
 to urge any liquid water that accumulates in the opening to migrate away.
 By keeping the opening free of liquid water drops, the fuel gas can freely
 move throughout the manifold, thus keeping the fuel cell running at
 maximum efficiency.
 While the specification concludes with claims defining the features of the
 invention that are regarded as novel, it is believed that the
 construction, method of operation and advantages of the invention will be
 better understood from a consideration of the following description in
 conjunction with the drawing figures, in which like reference numerals are
 carried forward. Referring now to FIG. 1, a fuel cell 10 has two sides, a
 cathode side 12 and an anode side 14. On each side 12 and 14, there are a
 plurality of individual unit cells 20. The reader will appreciate that
 each individual unit cell has an anode, cathode, current collector,
 catalyst, polymer electrolyte, etc. with various of these components
 disposed on the appropriate side(s). Each unit cell is separated from
 adjacent unit cells by one or more walls 24. These cell walls 24 create a
 manifold, serving to physically isolate the unit cells, and in some
 designs, to prevent the unit cell from electrically shorting to the
 adjacent cell. The walls 24 typically have a passage, opening or aperture
 35 in them so as to allow gas to pass from one unit cell to another. Note,
 that when assembled, the fuel cell 10 also has a manifold cover 36 with a
 10 fuel inlet 37, the manifold cover serving to trap the fuel gas
 substantially at the surface of the anodes. In practice, the hydrogen gas
 is admitted to the inlet 37 and the manifold is pressurized to a desired
 level. There is no outlet or vent for the hydrogen gas, so the fuel cell
 is said to be `dead-ended`. One can see that the apertures 35 allow the
 hydrogen gas to be equally distributed to all the unit cells 20, and that
 as the hydrogen is catalyzed to hydrogen ions that migrate through the
 polymer electrolyte membrane to produce electricity and water, the
 hydrogen in the manifold is consumed, thus lowering the pressure and
 allowing a pressure regulator (not shown) to admit additional hydrogen gas
 to maintain the desired pressurization.
 During operation of the fuel cell, the internal resistance of the system
 produces heat. This heat evaporates the byproduct water that is produced
 on or in the membrane electrolyte, creating a very humid environment, and
 the water vapor or gaseous H.sub.2 O condenses on the cooler portions of
 the fuel cell. In `dead-ended` systems the hydrogen gas is not constantly
 vented to atmosphere, that is, it does not continuously flow through the
 gas channels on the anode side, but is only replenished as the equilibrium
 is disturbed by consumption of hydrogen. As noted above, since air
 breathing fuel cells do not utilize forced air flow across the cathode, in
 some situations (e.g. heavy electrical load) the byproduct water is not
 removed fast enough and not only causes problems at the cathode catalyst
 sites, but it begins to migrate back through the polymer electrolyte
 membrane and to `flood` the anode side. When this excess water accumulates
 around or in the narrow passages 35, it can easily block the flow of
 hydrogen through the system and may cut off the access of new fuel to the
 downstream cells, adversely affecting the performance of the entire fuel
 cell system. As the size of fuel cells decreases, the accompanying
 dimensions of all components also decrease, and in fuel cells for small
 portable electronic applications, the apertures 35 are typically small
 enough that the smallest of water droplets can cause significant problems.
 One way to prevent the accumulation of water in the passages is to make the
 passages hydrophobic. In our invention, a hydrophobic layer 38 is added to
 the fuel manifold in the areas where the gas passages constrict or neck
 down, and also to other areas that are prone to water droplet formation.
 The hydrophobic treatment aids in water management of the fuel cell system
 by preventing the accumulation of water droplets at critical locations or
 by forcing the water drops to other areas where they can be evaporated or
 where they will not cause problems. The hydrophobic surfaces reduce the
 surface friction at the narrow regions. Polytetrafluoroethylene is the
 most common hydrophobic material, but other fluorinated polymers or
 molecules having a hydrophobic endgroup can also be used with
 effectiveness. For example, a soap-like chemical having a hydrophilic
 endgroup that attaches easily to the cell walls and a hydrophobic endgroup
 that mitigates the formation of water drops can be used. In addition to
 coatings, the apertures can be `plugged` with open cell fluorinated foams
 so as to permit the passage of gas and prevent the buildup of water drops.
 Although we believe our invention will find the most application in
 dead-ended planar fuel cells, we also realize that it can be used in
 conventional stacked fuel cells that have bipolar plates. Referring now to
 FIG. 2, the openings 36 in the bipolar plate 50 that allow the hydrogen
 gas to flow from one unit cell in the stack to the next unit cell are
 similarly coated with a hydrophobic material or plugged with hydrophobic
 foam. In addition, other strategic locations 52, 53 in the stack 60 can
 also be treated to prevent water buildup.
 In summary, the use of a hydrophobic coating material on critical surfaces
 in a fuel cell system aids in water management by preventing the buildup
 of water drops that can impede or stop the flow of fuel gas to the
 individual unit cells. While the preferred embodiments of the invention
 have been illustrated and described, it will be clear that the invention
 is not so limited. Numerous modifications, changes, variations,
 substitutions and equivalents will occur to those skilled in the art
 without departing from the spirit and scope of the present invention as
 defined by the appended claims. For example, channels on the cathode side
 could also be coated with a hydrophobic coating in systems that use and
 air manifold, forced air, or any other pressurized fuel cell where there
 is a flow of oxidant.