Electronically conducting fuel cell component with directly...

Details Electronically conducting fuel cell component with directly... Electronically conducting fuel cell component with directly...

2001-02-08

2004-12-07

Stinson, Frankie L. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000

Reexamination Certificate

active

06828054

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to the field of electrical power generation, and more particularly to the generation of electrical power using an electrochemical fuel cell.
BACKGROUND OF THE INVENTION
A fuel cell is an energy conversion device that efficiently converts stored chemical energy into electrical energy. Conventional fuel cells generally operate by combining hydrogen with oxygen to generate direct current electrical power. The overall chemical reaction for this process is described by Equation 1.
2H
2
+O
2
→2H
2
O  Equation 1.
In order to generate enough power to be usable for practical applications, multiple fuel cells are combined electrically in series. In this stacked configuration, the individual fuel cells are connected one after another, similar to the cells of a conventional voltaic battery.
At its fundamental level, the individual fuel cell contains an electrode at which oxygen is reduced (the cathode) and another electrode at which fuel is oxidized (the anode). Fuel cell electrodes are generally of the gas diffusion type and are made of an electrically-conducting support material, an active catalytic layer, and an electrolyte.
Chemical pore-formers have been used to control the porosity of these layers. These pore-formers, or porophoric agents, are added as powders or crystals to the compositions of the various layers. The pore-formers eventually decompose in the gas phase or are dissolved in solution in post-fabrication steps. Conventional pore formers include powdered ammonium bicarbonate, ammonium chloride, and urea, which are either lost in the gas phase or via dissolution. Such particles are generally coarse and operate like yeast in dough, forming a sponge-like structure with rather coarse porosity. These pore-formers are too large to be compatible with the thin active layers that can be prepared by methods such as paint- or ink-like application or rolling (calendering).
The membrane electrode assembly (MEA), which is composed of the anode, cathode and electrolyte membrane, is generally sandwiched between gas flow-fields. These flow fields allow the reactant gases (separate streams of H
2
and O
2
) to contact the MEA. Conventional flow-fields are formed by pore-free grooved graphite plates, wherein the reactant gases flow through a serpentine-shaped groove. A drawback to this type of flow field is that it requires heavy tie rods and end-plates to compress the flow-fields and MEAs together to maintain electrical contact.
In a fuel cell stack, the individual fuel cells are connected to each other by bipolar plates. The bipolar plates provide electrical contact between the cells and may also be involved in cooling the stack. Once again, the traditional approach is to maintain contact between the cells and the bipolar plates by applying pressure to the stack using end-plates squeezed together by heavy tie rods.
Porous metals are an attractive alternative to heavy graphite flow fields. Porous metals that have been used as flow field material include porous copper, porous nickel, porous aluminum, porous titanium, and porous aluminum-titanium alloys (U.S. Pat. Nos. 6,022,634 and 6,146,780). The fuel cells shown in U.S. Pat. No. 6,022,634 use porous metal flow fields and collapsed porous metal current collectors. These components are pressed together between endplates and still require tie rods or some means of applying external pressure to the cells to maintain electrical contact between the components. Flow-fields shown in U.S. Pat. No. 6,146,780 are made of metal foams that are spot welded to gas impermeable bipolar separator plates.
The polymer electrolyte membrane (PEM) material generally used in polymer electrolyte membrane fuel cells (PEMFCs) is generally composed of a linear, branched-chain perfluorinated polyether polymer with a non-crosslinked structure that has terminal sulfonic acid endgroups. An example of such material is Nafion® (Du Pont de Nemours and Company, Wilmington, Del.). Nafion® requires a substantial amount of water (typically 10-20 water molecules per sulfonic acid group) to give adequate proton conductivity. The high water requirements are due to the volume occupied by the hydrophobic fluorinated sulfonic acid polymer chain. Proton conduction only takes place down self-organizing hydrophilic channels or micelles which occupy only a small portion of the total superficial area of a PEM electrolyte film, reducing the corresponding specific conductivity compared with the local value in the channels. The hydrophobicity of the polymer chain also limits the local amount of water associated with the sulfonic acid. This amount of water increases rapidly as the equivalent weight or the ratio of the molecular weight of polymer chain to sulfonic acid becomes less, and it falls with increasing temperature. Conductivity is highest when liquid water is in contact with the membrane at any given temperature. For this reason, developers have generally supplied PEMFCs with the reactants (hydrogen and air) humidified to at least the cell operating temperature, so that product water is formed in the liquid state.
Since the system must generally operate below the local boiling point of water, excess water used for humidification, plus the product water from the reaction, collects in the cathode gas flow channels. Means must therefore be provided to continuously remove it. The fact that the PEMFC produces liquid water under normal circumstances is a major operational flaw and considerable ingenuity is required to deal with it, especially in larger cells.
In the General Electric Company fuel cell used in the Gemini space missions starting in 1965, water was required to be removed in a microgravity environment. Water management was accomplished by providing a wicking material in the cathode flow channels of each cell. The exit end of the wick in each individual cell communicated over the active width of the cell with a porous water separator plate arranged in parallel with the cells in the stack. A differential oxygen pressure drove the water through this separator plate to a product water accumulator for storage as drinking water and for recycle to the entry side of each wick to maintain conductivity (Appleby et al.,
Energy
11: 137 (1986); Liebhavsky et al.,
Fuel Cells and Fuel Batteries
, Academic Press, 587 (1964)).
In the Gemini cell, the membrane was not fluorinated for stability and operated at close to ambient temperature and at low current density for high efficiency. The non-fluorinated membrane was prone to hot-spots with gas-crossover, and was replaced by Nafion® as soon as it became available in the late 1960s. In the Ballard stack design (Prater et al.,
J. Power Sources,
61: 105 (1996), U.S. Pat. Nos. 5,521,018 5,527,363, and 5,547,776), the water is forced out of the cathode flow channels by applying a large pressure differential between the inlet and exit side. This requires the use of very long, serpentine flow channels, each with a length many times the cell width (U.S. Pat. Nos. 4,988,583 and 5,108,849). While Ballard has looked at other systems for removing water, a vapor phase feedback loop at the anode (after back-diffusion from the cathode channels) (U.S. Pat. No. 5,441,819), the serpentine channel design is still retained. This design means that the stack will only operate under pressurized conditions, with a minimum operating pressure between 2 and 3 atmospheres absolute (atma) of air. Pressurized operation requires either a stack with heavy filter-press components or a pressure vessel surrounding the stack. Both of these reduce the flexibility of stack design, and necessitate the use of liquid cooling system with an external radiator. The reactants are prehumidified to cell operating temperature by passing them through membrane-humidification cells which may or may not be arranged en bloc with the electrochemical stack. The water circulation in the humidification cells is deionized cooling water exiting the cooling plates (generally, one every 4 or 5 active cells) in the electroche

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