Sensor cell for an electrochemical fuel cell stack

Details Sensor cell for an electrochemical fuel cell stack Sensor cell for an electrochemical fuel cell stack

2001-02-23

2004-01-06

Gulakowski, Randy (Department: 1746)

Chemistry: electrical current producing apparatus, product, and

Having magnetic field feature

C429S006000, C429S006000

Reexamination Certificate

active

06673480

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to electrochemical fuel cells, and more particularly to the incorporation into a fuel cell stack of one or more specialized sensor fuel cells for detecting problematic conditions before the other cells in the stack are adversely affected by those conditions.
BACKGROUND OF THE INVENTION
Electrochemical fuel cells convert fuel and oxidant to electricity and reaction product. Solid polymer electrochemical fuel cells generally employ a membrane electrode assembly (“MEA”) which comprises an ion exchange membrane or solid polymer electrolyte disposed between two electrodes typically comprising a layer of porous, electrically conductive sheet material, such as carbon fiber paper or carbon cloth. The MEA contains a layer of catalyst, typically in the form of finely comminuted platinum, at each membrane/electrode interface to induce the desired electrochemical reaction. In operation the electrodes are electrically coupled to provide a circuit for conducting electrons between the electrodes through an external circuit.
At the anode, the fuel stream moves through the porous anode substrate and is oxidized at the anode electrocatalyst layer. At the cathode, the oxidant stream moves through the porous cathode substrate and is reduced at the cathode electrocatalyst layer to form a reaction product.
In fuel cells employing hydrogen as the fuel and oxygen-containing air (or substantially pure oxygen) as the oxidant, the catalyzed reaction at the anode produces hydrogen cations (protons) from the fuel supply. The ion exchange membrane facilitates the migration of protons from the anode to the cathode. In addition to conducting protons, the membrane isolates the hydrogen-containing fuel stream from the oxygen-containing oxidant stream. At the cathode electrocatalyst layer, oxygen reacts with the protons that have crossed the membrane to form water as the reaction product. The anode and cathode reactions in hydrogen/oxygen fuel cells are shown in the following equations:
Anode reaction: H
2
→2H
+
+2e
−
Cathode reaction: 1/2O
2
+2H
+
+2e
−
→H
2
O
In typical fuel cells, the MEA is disposed between two electrically conductive fluid flow field plates or separator plates. Fluid flow field plates have at least one flow passage formed in at least one of the major planar surfaces thereof. The flow passages direct the fuel and oxidant to or across the respective electrodes, namely, the anode on the fuel side and the cathode on the oxidant side. The fluid flow field plates act as current collectors, provide support for the electrodes, provide access channels for the fuel and oxidant to the respective anode and cathode surfaces, and provide channels for the removal of reaction products, such as water, formed during operation of the cell. Separator plates typically do not have flow passages formed in the surfaces thereof, but are used in combination with an adjacent layer of material which provides access passages for the fuel and oxidant to the respective anode and cathode electrocatalyst, and provides passages for the removal of reaction products.
Two or more fuel cells can be electrically connected together in series to increase the overall power output of the assembly. In series arrangements, one side of a given fluid flow field or separator plate can serve as an anode plate for one cell and the other side of the fluid flow field or separator plate can serve as the cathode plate for the adjacent cell. Such a multiple fuel cell arrangement is referred to as a fuel cell stack, and is usually held together in its assembled state by tie rods and end plates, with the series of fuel cell assemblies interposed between the pair of end plates. The stack typically includes inlet ports and manifolds for directing the fluid fuel stream (such as substantially pure hydrogen, methanol reformate or natural gas reformate, or a methanol-containing stream in a direct methanol fuel cell) and the fluid oxidant stream (such as substantially pure oxygen, oxygen-containing air or oxygen in a carrier gas such as nitrogen) to the individual fuel cell reactant flow passages. The stack also commonly includes an inlet port and manifold for directing a coolant fluid stream, typically water, to interior passages within the stack to absorb heat generated by the fuel cell during operation. The stack also generally includes exhaust manifolds and outlet ports for expelling the depleted reactant streams, and the reaction products such as water, as well as an exhaust manifold and outlet port for the coolant stream exiting the stack. In a power generation system various fuel, oxidant and coolant conduits carry these fluid streams to and from the fuel cell stack.
Typically, fuel cell stack performance is monitored by detecting the voltage of individual cells or groups of cells in the stack. A typical stack generally comprises 30 to 200 individual cells. Voltage detection of individual cells or groups of cells is expensive and requires a complex data acquisition system and control algorithm to detect and identify a voltage condition outside a preset voltage range and to take corrective action or shut down the stack until normal operating conditions (i.e. conditions within a desired or preferable range) can be restored. A typical approach to monitoring fuel cell performance using voltage detection is described in U.S. Pat. No. 5,170,124. The '124 patent describes an apparatus and method for measuring and comparing the voltages of groups of cells in a fuel cell stack to a reference voltage. If the measured and reference voltages differ by more than a predetermined amount, an alarm signal or process control procedures can be initiated to implement a shut-down sequence or commence remedial action. While this voltage detection approach identifies the existence of an out-of-bounds condition, the approach is imprecise as to the source and/or nature of the problem which triggered the out-of-bounds condition.
SUMMARY OF THE INVENTION
In an electrochemical fuel cell stack comprising a plurality of fuel cells, each of the fuel cells comprises an anode comprising an anode electrocatalyst, a cathode comprising a cathode electrocatalyst, and an ion exchange membrane interposed between the anode and the cathode. At least one of the plurality of fuel cells is a sensor cell which has at least one structural dissimilarity with respect to the remaining fuel cells of the plurality. During operation of the stack, the structural dissimilarity induces an electrical and/or thermal response in the sensor cell which is not simultaneously induced in the remaining fuel cells under substantially the same operating conditions.
Thus, the sensor cell preferably operates under substantially the same conditions as the remaining non-sensor cells. However, in response to a change in a particular conditions an electrical and/or thermal response (preferably a voltage change) is induced in the sensor cell which is not simultaneously induced in the remaining fuel cells. The sensor cell reacts differently to, and can therefore be used to detect, undesirable conditions before they adversely affect other cells in the stack. The different electrical and/or thermal response of the sensor cell to the particular operating condition may provide diagnostic information, or a warning signal, or may be used to initiate a specific correction sequence to restore the stack to normal operating conditions or to shut down the stack if normal conditions cannot be restored. More than one type of sensor cell, specific to different types of conditions, may be employed in a fuel cell stack.
During operation of a fuel cell stack the anodes have a fuel stream directed thereto, typically via anode flow fields, the cathodes have an oxidant stream directed thereto typically via cathode flow fields, and each of the fuel stream and the oxidant stream directed to the fuel cells of the stack have an inlet pressure and a stoichiometry associated therewith. Each of the plurality of fuel cells generate an electr

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