Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-04-05
2002-04-02
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000
Reexamination Certificate
active
06365291
ABSTRACT:
TECHNICAL FIELD
The present invention relates to fuel cells that are suited for usage in transportation vehicles, portable power plants, or as stationary power plants, and the invention especially relates to a fuel cell power plant that utilizes an antifreeze solution passing through components of the power plant.
BACKGROUND OF THE INVENTION
Fuel cell power plants are well-known and are commonly used to produce electrical energy from reducing and oxidizing fluids to power electrical apparatus such as apparatus on-board space vehicles. In such power plants, a plurality of planar fuel cells are typically arranged in a stack surrounded by an electrically insulating frame structure that defines manifolds for directing flow of reducing, oxidant, coolant and product fluids. Each individual cell generally includes an anode electrode and a cathode electrode separated by an electrolyte. A reactant or reducing fluid such as hydrogen is supplied to the anode electrode, and an oxidant such as oxygen or air is supplied to the cathode electrode. In a cell utilizing a proton exchange membrane (“PEM”) as the electrolyte, the hydrogen electrochemically reacts at a surface of the anode electrode to produce hydrogen ions and electrons. The electrons are conducted to an external load circuit and then returned to the cathode electrode, while the hydrogen ions transfer through the electrolyte to the cathode electrode, where they react with the oxidant and electrons to produce water and release thermal energy.
The anode and cathode electrodes of such fuel cells are separated by different types of electrolytes depending on operating requirements and limitations of the working environment of the fuel cell. One such electrolyte is the aforesaid proton exchange membrane (“PEM”) electrolyte, which consists of a solid polymer well-known in the art. Other common electrolytes used in fuel cells include phosphoric acid or potassium hydroxide held within a porous, non-conductive matrix between the anode and cathode electrodes. It has been found that PEM cells have substantial advantages over cells with liquid acid or alkaline electrolytes in satisfying specific operating parameters because the membrane of the PEM provides a barrier between the reducing fluid and oxidant that is more tolerant to pressure differentials than a liquid electrolyte held by capillary forces within a porous matrix. Additionally, the PEM electrolyte is fixed, and cannot be leached from the cell, and the membrane has a relatively stable capacity for water retention.
Manufacture of fuel cells utilizing PEM electrolytes typically involves securing an appropriate first catalyst layer, such as a platinum alloy, between a first surface of the PEM and a first or anode porous substrate or support layer to form an anode electrode adjacent the first surface of the PEM, and securing a second catalyst layer between a second surface of the PEM opposed to the first surface and a second or cathode porous substrate or support layer to form a cathode electrode on the opposed second surface of the PEM. The anode catalyst, PEM, and cathode catalyst secured in such a manner are well-known in the art, and are frequently referred to as a “membrane electrode assembly”, or “M.E.A.”, and will be referred to herein as a membrane electrode assembly. In operation of PEM fuel cells, the membrane is saturated with water, and the anode electrode adjacent the membrane must remain wet. As hydrogen ions produced at the anode electrode transfer through the electrolyte, they drag water molecules in the form of hydronium ions with them from the anode to the cathode electrode or catalyst. Water also transfers back to the anode from the cathode by osmosis. Product water formed at the cathode electrode is removed from the cell as a liquid through a porous water transport plate, or by evaporation or entrainment into a gaseous stream of either the process oxidant or reducing fluid.
While having important advantages, PEM cells are also known to have significant limitations especially related to liquid water transport to, through and away from the PEM, and related to simultaneous transport of gaseous reducing fluids and process oxidant fluids to and from the electrodes adjacent opposed surfaces of the PEM. The prior art includes many efforts to minimize the effect of those limitations. Use of such fuel cells to power a transportation vehicle gives rise to additional problems associated with water management, such as preventing the product water from freezing, and rapidly melting any frozen water during start up whenever the fuel-cell powered vehicle is operated in sub-freezing conditions.
Known fuel cells typically utilize an open or closed thermal management or coolant system supplying a flow of cooling fluid through a porous or sealed cooler plate within the fuel cell to maintain the cell within an optimal temperature range. Where the cooling fluid is a solution including water it also must be kept from freezing. It is known to utilize a conventional antifreeze solution such as ethylene glycol and water or propylene glycol and water as a cooling fluid in such a closed coolant system having a sealed cooler plate. However, such antifreeze solutions are not acceptable in an open coolant system because they are known to be adsorbed by and poison the catalysts that form electrodes. Furthermore, those antifreeze solutions have low surface tensions which result in the solutions wetting any porous, wetproofed support layers adjacent cell catalysts, thereby impeding diffusion of reactant fluids through the support layers to the catalysts, which further decreases performance of the electrodes. Also, the vapor pressure of such conventional antifreezes is high, resulting in excessive loss rates of the antifreeze solutions through fuel cell exhaust streams.
A fuel cell power plant includes a fuel cell or fuel cell stack to generate electricity and a variety of systems to support the fuel cell stack. For example, if the plant is to be utilized to power a transportation vehicle, it is necessary that the power plant be self-sufficient in water to be viable. Self-sufficiency in water means that enough water must be retained within the plant to offset losses from reactant fluids exiting the plant in order to efficiently operate the plant. Any water exiting the plant through a plant process exhaust stream consisting of a cathode exhaust stream of gaseous oxidant and/or an anode exhaust stream of fluid exiting the anode side of the fuel cell or a burner exhaust stream must be balanced by water produced electrochemically at the cathode electrode and water retained within the plant. To maintain water self-sufficiency, it is common that the plant include a water recovery device, controls, and piping to recover and direct water into the fuel cell stack to maintain proper wetting of the PEM electrolytes, and humidity of the reactant streams, etc.
Additionally, it is known that some fuel cell power plants operate on pure hydrogen gas, while others utilize a reformate wherein a hydrogen enriched reducing fluid is formed from any of a variety of hydrocarbon fuels by fuel processing components including for example use of steam mixed with the fuel at high temperatures within a reformer, as is well known in the art. If a fuel cell power plant included a steam generator for such fuel processing components, as with the water recovery device, the water in such components would have to be protected against freezing, such as by use of an antifreeze solution.
Water retention by the recovery devices of such fuel cell power plants will vary depending upon power output of the plant. For example, the water recovery device may be a condenser positioned to pass a process exhaust stream exiting the fuel cell in heat exchange relationship with a cooling fluid to condense and recover water within the process exhaust stream. When the power output of the plant is low, more water will be recovered by the condenser than when the power output is high. If the water recover ed by the condenser is mixed with an antifreeze to p
Chisholm, Jr. Malcolm J
Kalafut Stephen
UTC Fuel Cells LLC
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