Water management system for fuel cell

Details Water management system for fuel cell Water management system for fuel cell

2002-02-08

2004-04-20

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S047000

Reexamination Certificate

active

06723461

ABSTRACT:

TECHNICAL FIELD
This invention is directed to fuel cells, and more particularly, to a comprehensive water management system for use with a fuel cell.
BACKGROUND ART
Various fuel cell types exist in the prior art. Each fuel cell type, as defined by its electrolyte, has particular design requirements. In a proton exchange membrane (PEM) fuel cell one requirement is to provide an effective water management system. A PEM fuel cell includes a membrane confined between respective porous cathode and anode electrodes. These electrodes comprise a relatively thin catalyst layer on support plate members where the catalyst layer may be deposited either directly on respective major surfaces of the proton exchange membrane or on a porous support plate, wherein the porous support plate, also known as a substrate is in contact with the major surfaces of the proton exchange membrane. These respective cathode and anode catalyst layers reside at the interfaces between the respective electrode plates and the proton exchange membrane. In general, fuel cells function by supplying a gaseous fuel and an oxidant, through supply means, to the anode electrode and cathode electrode, respectively. These supply means distribute the fuel and oxidant gas as uniformly as possible over the surfaces of catalyzed layers of the respective electrodes. When a PEM fuel cell operates, the electrochemical reaction occurring at the catalyzed anode results in electrons and hydrogen ions being formed at the anode. The electrons flow through an external load circuit and the hydrogen ions flow through the membrane to the catalyzed cathode where they react with oxygen to form water and also release thermal energy.
Typically, fuel cell devices include more than one fuel cell, as described above, arranged in electrical series in a stack. Separator plates separate the individual fuel cells from adjacent cells. Usually, these plates have been nonporous, electrically conductive, impermeable separators. However, this invention uses separator plates that have a fine porous construction enabling water transfer therethrough.
In PEM fuel cell devices, water forms at the cathode catalyst layer. As hydrogen ions travel through the proton exchange membrane, the ions drag water at the anode side and carry the same to the cathode side. This activity causes several problematic events. Water accumulates at the cathode catalyst layer, requiring removal to avoid denying the gaseous oxidant access to the reaction surface thereof. Secondly, the anode side dries out due to the water depletion, thereby requiring measures for water replenishment.
It is known that the porosity of the separator plate along with a system created pressure differential unsatisfactorily achieves water removal from the cathode side and water replenishment to the anode side. U.S. Pat. No. 4,729,932 to McElroy, U.S. Pat. No. 5,503,944 to Meyer and U.S. Pat. No. 4,824,741 to Kunz disclose fuel cell designs that inadequately achieve these principles. Additional problems with PEM fuel cell devices can include excessive water loss at the cathode electrode due to dry inlet oxidant gas as well as evaporation of water into the oxidant stream, particularly at the oxidant inlet. That is, these events without corrective measures can lead to membrane dry out and water insufficiency thereby requiring water replenishment. Some prior art fuel cell stack configurations attempt to circumvent the dry out problem by using a condenser external to the stack. The condenser condenses water from the exiting air stream by heat exchange with a cooling medium such as ambient air and returns the water to the cell stack by way of an external loop. Such an approach adds complexity to the fuel cell power plant system and results in increased power plant weight and volume. Similar arrangements humidifying the oxidant gas entering the cathode electrode area to prevent dry out of the cells are known in U.S. Pat. No. 5,382,478 to Chow et al which uses humidifiers external to the cell stack or in the front segment of the stack not used for electrical reaction. Such humidification techniques using external saturators can only saturate the oxidant gas to an average temperature, but cannot account for the temperature variations that occur within the individual fuel cells of the fuel cell stacks.
Various patents disclose devices addressing these water management problems in fuel cells. For example, U.S. Pat. No. 4,345,008 to Breault discloses an apparatus for reducing electrolyte loss from an electrochemical cell using phosphoric acid as an electrolyte to facilitate the electrochemical reaction. The cell includes a condensation zone at the outlet of the oxidant gas. The condensation zone is an electrochemically inactive portion of the cell because it lacks a catalyst. Accordingly, the condensation zone operates at a cooler temperature than the active-catalyst-containing portion of the cell. To further facilitate a temperature reduction, coolant tube density in the area of the condensation zone is increased. Accordingly, the oxidant gas passes through the condensation zone thereby cooling the oxidant gas and condensing the electrolyte out of the gas. The electrode substrate absorbs the condensed electrolyte and returns it to the active portion of the cell. This fuel cell design does not include a similar humidification region for the prevention of cell dry out. Since the fuel cell does not use a PEM membrane, therefore the requirement for hydrating the inlet reactant gas does not exist. It also requires a more complex manufacturing process for forming the coolant tubes used at the condensation zone.
Further, U.S. Pat. No. 4,769,297 to Reiser et al discloses a water management system for a solid polymer electrolyte fuel cell stack. In U.S. Pat. No. 4,769,297 water feeds into the fuel cell stack in the hydrogen reactant stream where some water evaporates to provide cooling while other water migrates through the stack from cell to cell. Water migrates as a result of being dragged from the anode to the cathode through the electrolyte membrane and via the porous separator plates interposed between two adjacent cells in the stack. The reactant pressure differential pressure maintained between the cathode and anode forces the water through the porous separator plates. The anode support plates provide a large enough surface area from which water is evaporated to perform the cell cooling function. This system uses a condenser for removing water from hydrogen exhaust during fuel cell operation, but does not indicate the supply or withdrawal of water to the oxygen reactant gas.
There exists a need for a water management system for use with a PEM fuel cell stack, which system humidifies the oxidant gas and fuel gas at their inlets and condenses water out of the oxidant gas at its outlet, without the use of exterior humidifiers, for preventing membrane dry out and maintaining water self sufficiency of the fuel cell.
DISCLOSURE OF THE INVENTION
The present invention provides a water management system for the cell anode and cathode and integrates this function with the fuel cell coolant system. This integration allows system operation and control that can handle high current density operation at ambient pressure, and start up and shut down conditions because the system efficiently transfers water throughout the cells, and to and from the coolant system, on an as needed basis and at a rate which may be different for each cell.
The anode and cathode components of the invention consists of a tri-element assembly: 1) a water transport plate (WTP), 2) a bilayer plate, and 3) a catalyst layer/membrane surface. At the anode, water and fuel are required at the catalyst/membrane interface in sufficient quantity to replace water being lost through the membrane by proton drag, or from evaporation into the gas stream, and to supply the fuel necessary for the anode reaction. In the present invention, the WTP provides a full planar surface to the bilayer plate and the WTP acts as a water source that may be augmented by inlet stream water s

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