Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-09-26
2003-03-11
Kalafut, Stephen (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C264S042000
Reexamination Certificate
active
06531238
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates in general to the field of proton exchange membrane (“PEM”) fuel cell systems, and more particularly, to an improved PEM fuel cell system having improved discrete fuel cell modules with improved mass transport for ternary reaction optimization and a method for manufacturing same.
A fuel cell is an electrochemical device that converts fuel and oxidant into electricity and a reaction by-product through an electrolytic reaction that strips hydrogen molecules of their electrons and protons. Ultimately, the stripped electrons are collected into some form of usable electric current, by resistance or by some other suitable means. The protons react with oxygen to form water as a reaction by-product.
Natural gas is the primary fuel used as the source of hydrogen for a fuel cell. If natural gas is used, however, it must be reformed prior to entering the fuel cell. Pure hydrogen may also be used if stored correctly. The products of the electrochemical exchange in the fuel cell are DC electricity, liquid water, and heat. The overall PEM fuel cell reaction produces electrical energy equal to the sum of the separate half-cell reactions occurring in the fuel cell, less its internal and parasitic losses. Parasitic losses are those losses of energy that are attributable to any energy required to facilitate the ternary reactions in the fuel cell.
Although fuel cells have been used in a few applications, engineering solutions to successfully adapt fuel cell technology for use in electric utility systems have been elusive. The challenge is for the generation of power in the range of 1 to 100 kW that is affordable, reliable, and requires little maintenance. Fuel cells would be desirable in this application because they convert fuel directly to electricity at much higher efficiencies than internal combustion engines, thereby extracting more power from the same amount of fuel. This need has not been satisfied, however, because of the prohibitive expense associated with such fuel cell systems. For example, the initial selling price of the 200 kW PEM fuel cell was about $3500/kW to about $4500/kW. For a fuel cell to be useful in utility applications, the life of the fuel cell stack must
be a minimum of five years and operations must be reliable and maintenance-free. Heretofore known fuel cell assemblies have not shown sufficient reliability and have disadvantageous maintenance issues. Despite the expense, reliability, and maintenance problems associated with heretofore known fuel cell systems, because of their environmental friendliness and operating efficiency, there remains a clear and present need for economical and efficient fuel cell technology for use in residential and light-commercial applications.
Fuel cells are usually classified according to the type of electrolyte used in the cell. There are four primary classes of fuel cells: (1) proton exchange membrane (“PEM”) fuel cells, (2) phosphoric acid fuel cells, and (3) molten carbonate fuel cells. Another more recently developed type of fuel cell is a solid oxide fuel cell. PEM fuel cells, such as those in the present invention, are low temperature low pressure systems, and are, therefore, well-suited for residential and light-commercial applications. PEM fuel cells are also advantageous in these applications because there is no corrosive liquid in the fuel cell and, consequently, there are minimal corrosion problems.
Characteristically, a single PEM fuel cell consists of three major components-an anode gas dispersion field (“anode”); a membrane electrode assembly (“MEA”); and a cathode gas and liquid dispersion field (“cathode”). As shown in
FIG. 1
, the anode typically comprises an anode gas dispersion layer
502
and an anode gas flow field
504
; the cathode typically comprises a cathode gas and liquid dispersion layer
506
and a cathode gas and liquid flow field
508
. In a single cell, the anode and the cathode are electrically coupled to provide a path for conducting electrons between the electrodes through an external load. MEA
500
facilitates the flow of electrons and protons produced in the anode, and substantially isolates the fuel stream on the anode side of the membrane from the oxidant stream on the cathode side of the membrane. The ultimate purpose of these base components, namely the anode, the cathode, and MEA
500
, is to maintain proper ternary phase distribution in the fuel cell. Ternary phase distribution as used herein refers to the three simultaneous reactants in the fuel cell, namely hydrogen gas, water vapor and air. Heretofore known PEM fuel cells, however, have not been able to efficiently maintain proper ternary phase distribution. Catalytic active layers
501
and
503
are located between the anode, the cathode and the electrolyte. The catalytic active layers
501
and
503
induce the desired electrochemical reactions in the fuel cell. Specifically, the catalytic active layer
501
, the anode catalytic active layer, rejects the electrons produced in the anode in the form of electric current. The oxidant from the air that moves through the cathode is reduced at the catalytic active layer
503
, referred to as the cathode catalytic active layer, so that it can oxidate the protons flowing from anode catalytic active layer
501
to form water as the reaction by-product. The protons produced by the anode are transported by the anode catalytic active layer
501
to the cathode through the electrolyte polymeric membrane.
The anode gas flow field and cathode gas and liquid flow field are typically comprised of pressed, polished carbon sheets machined with serpentine grooves or channels to provide a means of access for the fuel and oxidant streams to the anode and cathode catalytic active layers. The costs of manufacturing these plates and the associated materials costs are very expensive and have placed constraints on the use of fuel cells in residential and light-commercial applications. Further, the use of these planar serpentine arrangements to facilitate the flow of the fuel and oxidant through the anode and cathode has presented additional operational drawbacks in that they unduly limit mass transport through the electrodes, and therefore, limit the maximum power achievable by the fuel cell.
One of the most problematic drawbacks of the planar serpentine arrangement in the anode and cathode relates to efficiency. In conventional electrodes, the reactants move through the serpentine pattern of the electrodes and are activated at the respective catalytic layers located at the interface of the electrode and the electrolyte. The actual chemical reaction that occurs at the anode catalyst layer is: H
2
⇄2H
+
+2e
−
. The chemical reaction at the cathode catalyst layer is: 2H
+
+2e
−
+½O
2
⇄H
2
O. The overall reaction is: H
2
+½O
2
⇄H
2
O. The anode disburses the anode gas onto the surface of the active catalyst layer comprised of a platinum catalyst electrolyte, and the cathode disburses the cathode gas onto the surface of the catalytic active layer of the electrolyte. However, when utilizing a conventional serpentine construction, the anode gas and the cathode gas are not uniformly disbursed onto the electrolyte. Nonuniformn distribution of the anode and cathode gas at the membrane surface results in an imbalance in the water content of the electrolyte. This results in a significant decrease in efficiency in the fuel cell.
The second most problematic drawback associated with serpentine arrangements in the electrodes relates to the ternary reactions that take place in the fuel cell itself. Serpentine arrangements provide no pressure differential within the electrodes. This prohibits the necessary ternary reactions from taking place simultaneously. This is particularly problematic in the cathode as both a liquid and a gas are transported simultaneously through the electrode's serpentine pattern.
Another shortcoming of the conventional serpentine arrangement in the anode in particular is
Baker & Botts L.L.P.
Kalafut Stephen
Reliant Energy Power Systems, Inc.
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