Solid cage fuel cell stack

Details Solid cage fuel cell stack Solid cage fuel cell stack

2001-06-08

2004-04-13

Bell, Bruce F. (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C429S006000, C429S006000

Reexamination Certificate

active

06720101

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to electrochemical fuel cell stacks and, more particularly, to proton-exchange-membrane (“PEM”) fuel cell stacks.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and an oxidant to electricity and a reaction product. In electrochemical fuel cells employing hydrogen as the fuel and oxygen as the oxidant, the reaction product is water. Solid polymer fuel cells generally include a membrane electrode assembly (“MEA”) layer comprising a solid polymer electrolyte or ion exchange membrane disposed between two electrode layers. The electrode layers typically comprise porous, electrically conductive sheet material and an electrocatalyst at each membrane-electrode interface to promote the desired electrochemical reaction.
At the anode, the fuel (typically hydrogen) moves through the porous electrode material and is oxidized at the anode electrocatalyst to form cations, which migrate through the membrane to the cathode. At the cathode, the oxidizing gas (typically oxygen contained in air) moves through the porous electrode material and is reduced by reaction with the cations at the cathode electrocatalyst to form the reaction product (water).
In conventional fuel cells, the MEA layer is interposed between two substantially fluid-impermeable, electrically-conductive plates, commonly referred to as separator plates. The separator plates serve as current conductors; provide structural support for the electrode layers; typically provide means for directing the fuel and oxidant to the anode and cathode layers, respectively; and typically provide means for exhausting products, such as water, formed during operation of the fuel cell. Separator plates having reactant channels, that is, channels for the fuel or oxidizing gas, are sometimes referred to as fluid flow field plates.
Typically, the reaction product water migrates through the MEA layer to the adjoining reactant channels and is carried from the fuel cell by the fuel gas and oxidizing gas. Typical MEA layers work most effectively if they are humidified with water. Despite the production of the reaction product water and its migration through the MEA layer, providing dry fuel gas and oxidizing gas to the fuel cell tends to dehydrate portions of the MEA layer, reducing their efficiency. The portions of the MEA layer that are susceptible to dehydration are those with which the fuel gas and oxidizing gas come into contact before the fuel gas and oxidizing gas have picked up appreciable reactant water, that is, at the upstream ends of the fuel-gas and oxidizing-gas channels. To overcome the problem of dehydration of the MEA layer, the fuel gas and oxidizing gas are typically humidified, that is, water vapour is added to the fuel gas and oxidizing gas.
Fuel cell stacks are well known, comprising an aligned assembly of fuel cells connected together electrically in series to obtain desired voltage and power output. An early example of a fuel cell stack is illustrated in Maru U.S. Pat. No. 4,444,851 granted Apr. 24, 1984; a later example is illustrated in Washington U.S. Pat. No. 5,514,487 granted May 7, 1996.
The fuel cell reaction is exothermic and fuel cell stacks typically have cooling means associated with them, such as plates interposed between the fuel cells having cooling passages for circulating a coolant fluid so as to absorb and carry away heat.
Typically, in any such fuel cell stack, one side of a given fluid flow field plate (separator plate) provides means (typically fuel-gas channels) for directing the fuel to the anode layer of one cell, and the other side of the plate provides means (typically oxidizing-gas channels) for directing the oxidizing gas to the cathode layer of the adjacent cell, and so on seriatim. As well, separator plates may also have channels for a coolant fluid on one side and channels for either fuel gas or oxidizing gas on the other side. Such plates, that is, plates having channels on each side, are sometimes referred to as bipolar plates.
With current MEA technology, the greatest power density is achieved with fuel gas and oxidant gas both compressed to roughly 3 bar, roughly 44 pounds per square inch, and this is the sort of pressures at which most known PEM fuel cell stacks are designed to function.
Typically, for a fuel cell stack to operate properly the fuel cell stack assembly must, in general terms, include: means for containing the fuel cell stack; means for providing fuel gas and oxidizing gas to the reactant channels and for conducting reaction product, fuel gas and oxidizing gas from the reactant channels; and means for cooling the fuel cell stack.
The means for containing a fuel cell stack typically must both compress and align the fuel cell stack. The fuel cell stacks must be compressed so as to press the individual fuel cell components and the fuel cells together in order to prevent leaks from between the plates and to provide good electrical contact between the components. Proper alignment of the plates aids in preventing leaks from between the plates, particularly in the case of fuel cell stacks having internal manifolds. In use, the vertical dimension of the fuel cell stack tends to fluctuate due to: temperature changes (the fuel cell stack generates heat and expands during operation, and cools and contracts after use); and absorption of water and resulting expansion of the proton exchange membrane of the MEA layer. Typically the means for containing fuel cell stacks include means for compensating for this size fluctuation, such as compression springs.
Various means for containing fuel cell stacks are known. For example, many known fuel cell stack assemblies use conventional rod and plate configurations that comprise two end plates between which the fuel cells are sandwiched. The end plates are larger than the fuel cells and have holes spaced about their peripheries. Rods having threaded ends are inserted in the holes so as to run through a hole in each end plate. Nuts, threaded onto each end of the rods and tightened down, provide the compressive force. Coil springs and washers, placed between the nuts and one of the end plates, permit the end plate to move relative to the rods to compensate for expansion and contraction of the fuel cells. With typical PEM fuel cells operating with fuel gas and oxidant gas at pressures of roughly 44 pounds per square inch, the components of the containment must be very heavy and robust. Such conventional rod and plate containment means typically rely on the compression of the fuel cell stack and the resulting friction between fuel cell components to maintain the required rigidity of the fuel cell stack assembly and the required alignment of the fuel cell components.
In all previous PEM fuel cell stack designs, the means for providing fuel gas and oxidizing gas to the reactant channels and for conducting reaction product, fuel gas and oxidizing gas from the reactant channels can be generally divided into two categories: those with internal manifolds and those with external manifolds/plena. Internal manifolds have been almost universally used in preference to external plena or manifolds for proton exchange membrane (PEM) fuel cell stacks. In part this is because hydrogen, which has the smallest atomic structure of all elements and thus is difficult to contain, and which is used at relatively high pressures, can generally be more effectively contained by internal manifolds than by external plena. In internal-manifold fuel cell stacks, the hydrogen is typically provided to the stack via conventional tubing attached to internal manifold ports in the top or bottom end plate. In external-plena PEM fuel cell stacks, the joints between the plena and the fuel cell stack containment means, and the joint between the containment means and the fuel cell stack itself, must be somehow sealed so as to contain the hydrogen, which can be difficult.
In internal-manifold fuel cell stacks, each plate and MEA layer of the fuel cell stack has holes that align with corresponding holes in the other components

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