Data processing: measuring – calibrating – or testing – Measurement system in a specific environment – Electrical signal parameter measurement system
Reexamination Certificate
2000-09-29
2003-02-11
Hoff, Marc S. (Department: 2857)
Data processing: measuring, calibrating, or testing
Measurement system in a specific environment
Electrical signal parameter measurement system
C702S064000
Reexamination Certificate
active
06519539
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to measurement of fuel cell impedance, and in particular, to a self-contained, portable apparatus for obtaining real-time measurements of a fuel cell's internal resistance.
BACKGROUND OF THE INVENTION
Fuel cells are becoming increasingly important as alternative energy sources as seen by the estimated 3 billion dollar market for fuel cells in 2000. This is due to their advantages over conventional power sources such as the battery and the internal-combustion engine. For instance, a fuel cell can supply electrical energy over a longer time period than a battery because it can be constantly supplied with air and fuel (i.e. hydrogen, reformed natural gas (hydrogen-rich gas) and methanol). Furthermore, a fuel cell does not run down or require recharging. Fuel cells are also high-efficiency devices, with efficiencies as high as 60 percent. This is much better than the internal-combustion engine which has an efficiency of up to 40 percent. Fuel cells also emit no noxious gases, since the fuel cell relies on a chemical reaction versus combustion, and generate very little noise when in operation. All of these features make the fuel cell highly desirable as power sources for automobiles, buses, municipal power generation stations, space missions and cellular phones.
To evaluate a fuel cell's electrical efficiency, its internal resistance is determined which is achieved through AC Impedance measurement. This measurement is important because it allows for the examination of various physical and chemical characteristics of the fuel cell. This impedance measurement may also be used in a feedback mechanism to improve the fuel cell's performance.
The literature indicates that complex impedance measurements on fuel cells can only be performed using expensive bench-top laboratory equipment, consisting of many sub-systems interfaced with one another. For example: T. E. Springer, T. A. Zawodzinski, M. S. Wilson and S. Gottesfield, “
Characterization of polymer electrolyte fuel cells using AC Impedance spectroscopy
”, Journal of the Electrochemical Society of America, 143(2), p. 587-599, 1996; J. R. Selman and Y. P. Lin, “
Application of AC impedance in fuel cell research and development
”, Electrochemica Acta, 38(14), p. 2063-2073, 1993; B. Elsener and H. Bolmi, “
Computer
-
assisted DC and AC techniques in electrochemical investigations of the active
-
passive transition
”, Corrosion Science, 23(4), p. 341-352, 1983. Such known equipment is manually controlled, with no automation in place. No single known approach allows the use of a portable, integrated measurement system. In addition, no measurement equipment is integrated into these systems which permits modification of fuel cell operating parameters.
Furthermore, the patent literature shows that the measurement of complex impedance is primarily known for use on batteries. In addition, these patents only claimed to measure a single quantity, namely “impedance” (U.S. Pat Nos. 4,697,134 and 5,773,978) or “resistance” (U.S. Pat Nos. 3,753,094, 3,676,770 and 5,047,722). The previous patent relating to measuring impedance on an electrochemical cell (U.S. Pat No. 6,002,238), not necessarily a fuel cell, used an entirely different, yet complicated approach. Furthermore, this approach could not be directly applied to fuel cells due to the high currents associated with the latter.
Thus the issues which still need to be addressed and improved in fuel cell impedance measurement are portability, fuel cell applicability, measurement variety and resolution, automation and cost.
Generally, a fuel cell is a device which converts the energy of a chemical reaction into electricity. It differs from a battery in that the fuel cell can generate power as long as the fuel and oxidant are supplied.
A fuel cell produces an electromotive force by bringing the fuel and oxidant into contact with two suitable electrodes and an electrolyte. A fuel, such as hydrogen gas, for example, is introduced at a first electrode where it reacts electrochemically in the presence of the electrolyte and catalyst to produce electrons and cations in the first electrode. The electrons are circulated from the first electrode to a second electrode through an electrical circuit connected between the electrodes. Cations pass through the electrolyte to the second electrode. Simultaneously, an oxidant, typically air, oxygen enriched air or oxygen, is introduced to the second electrode where the oxidant reacts electrochemically in presence of the electrolyte and catalyst, producing anions and consuming the electrons circulated through the electrical circuit; the cations are consumed at the second electrode. The anions formed at the second electrode or cathode react with the cations to form a reaction product such as water. The first electrode or anode may alternatively be referred to as a fuel or oxidizing electrode, and the second electrode may alternatively be referred to as an oxidant or reducing electrode. The half-cell reactions at the two electrodes are, respectively, as follows:
First Electrode: H
2
→2H
+
+2
e
−
Second Electrode: 1/2O
2
+2H
+
+2
e
−
→H
2
O
The external electrical circuit withdraws electrical current and thus receives electrical power from the cell. The overall fuel cell reaction produces electrical energy which is the sum of the separate half-cell reactions written above. Water and heat are typical by-products of the reaction.
In practice, fuel cells are not operated as single units. Rather, fuel cells are connected in series, stacked one on top of the other, or placed side by side. A series of fuel cells, referred to as fuel cell stack, is normally enclosed in a housing. The fuel and oxidant are directed through manifolds to the electrodes, while cooling is provided either by the reactants or by a cooling medium. Also within the stack are current collectors, cell-to-cell seals and insulation, with required piping and instrumentation provided externally of the fuel cell stack. The stack, housing, and associated hardware make up the fuel cell module.
Fuel cells may be classified by the type of electrolyte, which is either liquid or solid. The present invention is primarily concerned with fuel cells using a solid electrolyte, such as a proton exchange membrane (PEM). The PEM has to be kept moist with water because the membranes that are currently available will not operate efficiently when dry. Consequently, the membrane requires constant humidification during the operation of the fuel cell, normally by adding water to the reactant gases, usually hydrogen and air.
The proton exchange membrane used in a solid polymer fuel cell acts as the electrolyte as well as a barrier for preventing the mixing of the reactant gases. An example of a suitable membrane is a copolymeric perfluorocarbon material containing basic units of a fluorinated carbon chain and sulphonic acid groups. There may be variations in the molecular configurations of this membrane. Excellent performances are obtained using these membranes if the fuel cells are operated under fully hydrated, essentially water-saturated conditions. As such, the membrane must be continuously humidified, but at the same time the membrane must not be over humidified or flooded as this degrades performances. Furthermore, the temperature of the fuel cell stack must be kept above freezing in order to prevent freezing of the stack.
Cooling, humidification and pressurization requirements increase the cost and complexity of the fuel cell, reducing its commercial appeal as an alternative energy supply in many applications. Accordingly, advances in fuel cell research are enabling fuel cells to operate without reactant conditioning, and under air-breathing, atmospheric conditions while maintaining usable power output.
Where a solid polymer proton exchange membrane (PEM) is employed, this is generally disposed between two electrodes formed of porous, electrically conductive material. The electrodes are generally impregnated or coated wi
Freeman Norman A.
Gopal Ravi B.
Masse Stephane
Rivard Pierre
Bereskin & Parr
Hoff Marc S.
Hydrogenics Corporation
Kim Paul
LandOfFree
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