Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2001-05-03
2004-01-27
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000
Reexamination Certificate
active
06682839
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to electrochemical fuel cells. In particular, the invention provides a method and apparatus for controlling the temperature within a fuel cell using a heat transfer liquid travelling in the same fluid passages as a reactant fluid.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluids, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise a quantity of electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area of the fuel cell.
Solid polymer fuel cells generally employ a membrane electrode assembly (“MEA”) which typically consists of a solid polymer electrolyte, or ion exchange membrane, disposed between two electrode layers comprising porous, electrically conductive sheet material. The membrane is typically proton conductive, and acts as a barrier for isolating the fuel and oxidant streams from each other on opposite sides of each MEA. The membrane also substantially electronically insulates the electrodes from each other.
In a fuel cell stack, the MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluids. The plates act as current collectors and provide support for the adjacent electrodes. To distribute the reactant fluids to the respective electrochemically active area of each electrode, the reactant fluid passages may comprise open-faced channels or grooves formed in the surfaces of the plates that face the MEA. Such channels or grooves define a flow field area that generally corresponds to the adjacent electrochemically active area. Such separator plates, which have reactant channels formed therein are commonly known as flow field plates. The flow field plates, together with the porous electrode layer define reactant fluid passages adjacent both sides of each membrane.
In a fuel cell stack a plurality of fuel cells is connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell. To improve the viability of fuel cells as a commercial power source, it is generally desirable to improve the power density of the stack, that is, to reduce the stack dimensions and weight for a given electrical power output capability.
The fuel fluid stream which is supplied to the anode may be a gas such as substantially pure hydrogen or a reformate stream comprising hydrogen. Alternatively, a liquid fuel stream such as, for example, aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air. In a fuel cell stack, the fuel and oxidant fluid streams are typically supplied to respective electrodes by respective supply manifolds and exhausted from the same electrodes by respective exhaust manifolds. Manifold ports fluidly connect the stack manifolds to the flow field area and the electrochemically active area of each fuel cell. (See U.S. Pat. Nos. 5,484,666 and 5,514,487, each of which is hereby incorporated by reference in its entirety, and which disclose examples of fuel cell stack manifold configurations.)
In conventional solid polymer fuel cells incorporating an ion exchange membrane, water is normally used to hydrate the membrane to improve its ionic conductivity. Hydration may also help to maintain the resilience of the membrane, reducing the potential for structural failure, which may occur if the membrane becomes too dry.
Conveniently, water is produced within the MEA as a product of the desired electrochemical reactions at the cathode. However, the quantity of water produced at the cathode is typically insufficient to keep the membrane suitably hydrated, so additional water is often introduced into one or both of the reactant streams, usually as water vapor. In conventional fuel cells, an objective is typically to keep water in the vapor phase in the vicinity of the MEA and to manage the cathode product water so that it evaporates into the cathode reactant stream.
The electrochemical reaction in a solid polymer fuel cell is typically exothermic. Accordingly, a cooling system is typically needed to control the temperature within a fuel cell to prevent overheating. Since a water supply is often present in conventional fuel cell systems for humidification, conventional designs have commonly used water as a coolant (see, for example, U.S. Pat. No. 5,200,278, hereby incorporated by reference herein in its entirety, which discloses a conventional fuel cell power generation system which employs water as a coolant).
However, because the solubility of the typical gaseous reactants in water is low, if water or aqueous liquids are introduced into a reactant fluid stream as a liquid coolant, the presence of the liquid coolant generally reduces the accessibility of the reactants to the electrocatalyst. Conventional fuel cells typically seek to avoid gas-liquid two phase flow within the reactant fluid passages by isolating liquid coolant streams from gaseous reactant streams by employing separate cooling fluid passages which are fluidly isolated from the reactant fluid passages (see for example, FIGS. 1, 2A and 2B in U.S. Pat. No. 5,230,966 and the accompanying description, which is hereby incorporated by reference herein in its entirety).
While conventional arrangements such as the one described in the preceding paragraph can control the temperature within a fuel cell stack, these arrangements generally require separate cooling layers (for example, coolant flow field plates) and manifolds for directing the coolant fluid through the stack. In a conventional fuel cell stack the cooling layers often occupy at least about one third of the plate volume. Therefore, the power density of a conventional fuel cell stack could be improved if the cooling layers could be eliminated. Furthermore, another disadvantage associated with separate cooling plates and a fluidly isolated cooling system is that additional seals and manifolds are required to contain the coolant and to keep the coolant fluidly isolated from the reactants.
Another problem associated with using water or other aqueous coolants in fuel cells is that such coolants may expand upon freezing and damage stack components. Also, frozen water in the active area can be difficult to remove since it must be melted and then purged to allow the reactant to access the catalyst. It is anticipated that fuel cells may be used in vehicles or installations where the fuel cell may be exposed to temperatures below the freezing temperature of water. For example, automobiles are typically designed for exposure to temperatures as low as −40° C. Therefore, when water or aqueous liquids are used as liquid coolants, conventional fuel cells intended for such applications need to incorporate features to prevent freeze expansion damage. This is another reason why conventional fuel cells avoid using water or aqueous coolants directly in the reactant stream passages. The porous electrodes and thin membrane are particularly susceptible to freeze expansion damage. As it is, precautions must be taken to deal with product water and reactant stream humidification water if the fuel cell is going to be exposed to freezing conditions.
Accordingly, there is a need for a method of operating a fuel cell, and an apparatus for implementing such a method, which reduces or eliminates some or all of the problems and disadvantages described above.
SUMMARY OF THE INVENTION
In the present approach, a method is provided for controlling temperature within an electrochemical fuel cell which comprises an ele
Chiem Bien Hung
Roberts Joy A.
St-Pierre Jean
Stumper Jürgen
Wilkinson David P.
Ballard Power Systems Inc.
Kalafut Stephen
McAndrews Held & Malloy Ltd.
Mercado Julian
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