Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For organic compound
Reexamination Certificate
1999-11-08
2003-03-04
Warden, Sr., Robert J. (Department: 1743)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For organic compound
C204S422000, C204S431000, C429S006000
Reexamination Certificate
active
06527943
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to fuel cell based concentration sensors and extending the measuring range thereof. Further, it relates to methanol concentration sensors suitable for use in direct methanol fuel cell systems.
BACKGROUND OF THE INVENTION
Fuel cells have been used as sensors for measuring the concentration of various oxidizable species in gas or liquid mixtures. An important application has been in the determination of methanol content in breath samples or in the headspace above beer and/or wine during fermentation processes. Generally, the operation of fuel cell based sensors involves supplying the fluid mixture to be analyzed to the fuel cell anode and then measuring the electrical output of the fuel cell. No external potential or current is supplied to the sensor and the measured electrical output is typically proportional to the concentration of the oxidizable species present in the mixture.
Fuel cell systems have also been used historically as power supplies in certain specialized applications but are receiving increased attention of late for use in more general applications, including power supplies for various portable, motive, and stationary applications. In some fuel cell systems, it is necessary to determine the concentration of oxidizable species in certain fluid mixtures and thus it may be useful to employ a fuel cell sensor as a component in the fuel cell based power supply.
In general, electrochemical fuel cells convert reactants, namely fuel and oxidants, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A solid polymer fuel cell is a specific type of fuel cell that employs a membrane electrode assembly (“MEA”) which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrode layers. An electrocatalyst is needed to induce the desired electrochemical reactions at the electrodes. The electrocatalyst used may be a metal black, an alloy or a supported metal catalyst, for example, platinum on carbon. The electrocatalyst is typically incorporated at the electrode/electrolyte interfaces. This can be accomplished, for example, by depositing it on a porous electrically conductive sheet material, or “electrode substrate”, or on the membrane electrolyte. Flow field plates for directing the reactants across one surface of each electrode substrate are generally disposed on each side of the MEA. Solid polymer fuel cells typically operate in a range from about 40° C. to about 150° C.
A broad range of reactants has been contemplated for use in solid polymer fuel cells and such reactants may be delivered in gaseous or liquid streams. The oxidant may, for example, be substantially pure oxygen or a dilute oxygen stream such as air. The fuel stream may, for example, be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream derived from a suitable feedstock, or a suitable gaseous or liquid organic fuel mixture. Liquid feedstocks and/or fuels, such as methanol, are preferred, particularly in non-stationary applications, since they are relatively easy to store and handle. Where possible, it is advantageous to react a fuel mixture directly in the fuel cell (that is, to supply the fuel unreformed to the fuel cell anodes) in order to avoid using a reformer in the fuel cell system. Inside the fuel cell, the fuel mixture may be reacted electrochemically (directly oxidized) to generate electricity or instead it may be reformed in-situ (internally reformed), as in certain high temperature fuel cells (for example, solid oxide fuel cells).
A direct methanol fuel cell (DMFC) is a type of solid polymer fuel cell that operates directly on a methanol fuel stream typically supplied as a methanol/water vapor or as an aqueous methanol solution in liquid feed DMFCs. The methanol in the fuel stream is directly oxidized at the anode therein. There is often a problem in DMFCs with crossover of methanol fuel from the anode to the cathode side through the membrane electrolyte. The methanol that crosses over typically then reacts with oxidant at the cathode and cannot be recovered, resulting in significant fuel inefficiency and deterioration in fuel cell performance. To reduce crossover, very dilute solutions of methanol (for example, about 5% methanol in water) are typically used as fuel streams in liquid feed DMFCs. The fuel streams in DMFCs are usually recirculated in order to remove carbon dioxide (a by-product of the reaction at the anode) and to re-use the diluent and any unreacted fuel in the depleted fuel stream exiting the DMFC). Methanol is added to the circulating fuel stream before it re-enters the fuel cell in order to compensate for the amount consumed, thereby providing a fresh mixture at the desired methanol concentration. Since the amount of methanol consumed is variable (depending on the load, crossover, and other operating parameters), the methanol concentration in the circulating fuel stream is usually measured continuously with a suitable sensor, and fresh methanol is admitted in accordance with the signal from the sensor.
Various types of sensors may be considered for purposes of measuring the concentration of methanol in aqueous solution and thus for use in a recirculating fuel stream in a liquid feed DMFC. For instance, a fuel cell based sensor may be considered. In Japanese Published Patent Application No. 56-118273, a small capacity, temperature-compensated fuel cell is suggested for use as a concentration sensor for a liquid electrolyte air-methanol fuel cell. A signal from the sensor is obtained by measuring the voltage across a large resistance connected across the small capacity fuel cell. In “Fuel Cell Sensors”, Selective Electrode Rev., 1992, Vol. 14, pp. 125-223, W. J. Criddle et al. discuss the principles and applications of fuel cell sensors generally. It was noted that diffusion from the air is usually sufficient for the oxygen supply in fuel cell sensors. Thus in principle, as long as the species concentration to be measured is in an appropriate range, a conventional direct methanol fuel cell may be employed as the concentration sensor. Saturation of such a sensor (where the electrical output of the sensor levels off and the sensor is no longer responsive to an increase in concentration) may occur however at higher methanol concentrations.
A fuel cell based concentration sensor for use in DMFCs is disclosed in U.S. Pat. No. 5,624,538. Therein, an additional, diffusion limiting, membrane is employed against the side of the anode opposite the ion conducting membrane electrolyte in the sensor fuel cell. The diffusion limiting membrane is used to limit the transport of methanol. It was suggested that the measuring range of this sensor could be expanded by varying the thickness of the diffusion limiting membrane.
Other types of sensors include capacitance devices or amperometric devices. The former measure the change in dielectric constant of the fuel stream with methanol concentration. The latter measure the current output from electrochemical cells in which an external potential is applied across the electrochemical cells and current is generated in accordance with the species concentration. The devices described in PCT/International Publication No. WO 98/45694 (Application No. PCT/US98/07244) and J. Electrochem. Soc., Vol. 145, No. 11, Nov. 1998, p. 3783 are examples of the latter. Along with an electrical meter for measuring the output of these devices, an additional apparatus is required to apply an external potential.
A preferred sensor however would measure methanol concentration over an extended range, without saturating at the higher methanol concentrations of interest, and without requiring an external power supply.
SUMMARY OF THE INVENTION
The present invention provides for extending the measuring range of a fuel cell based sensor that measures the concentration of an oxidizable species in a gaseous or liquid fluid mixture. This is accomplished by decreasin
Colbow Kevin M.
Müller Jens
Wilkinson David P.
Zhang Jiujun
Ballard Power Systems Inc.
McAndrews Held & Malloy Ltd.
Olsen Kaj K.
Warden, Sr. Robert J.
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