Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For organic compound
Reexamination Certificate
1998-04-06
2001-10-23
Warden, Sr., Robert J. (Department: 1744)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For organic compound
C205S783500, C204S421000, C204S422000
Reexamination Certificate
active
06306285
ABSTRACT:
FIELD
This disclosure generally relates to analyte concentration sensors and in particular fuel concentration sensors for use with a liquid direct-feed fuel cell.
BACKGROUND
The liquid direct-feed fuel cell is a device that generates electrical energy from the oxidation of organic fuels. Jet Propulsion Laboratory “JPL” developed a liquid direct-feed fuel cell using a solid-state electrolyte, preferably a solid polymer cation exchange electrolyte membrane. The subject matter of this implementation is described in U.S. Pat. No. 5,599,638, U.S. patent application Ser. No. 08/569,452 (Patent Pending), and U.S. patent application Ser. No. 08/827,319 (Patent Pending) the disclosures of which are incorporated by reference to the extent necessary for proper understanding.
FIG. 1
illustrates a typical structure of a JPL fuel cell with an anode
110
, a solid electrolyte membrane
120
, and a cathode
130
. An anode
110
is formed on a first surface
140
of the solid electrolyte membrane
120
with a first catalyst for electro-oxidation. A cathode
130
is formed on a second surface
145
thereof opposing the first surface
140
with a second catalyst for electro-reduction. The anode
110
, the solid electrolyte membrane
120
, and the cathode
130
are hot press bonded to form a single multi-layer composite structure, referred to herein as a membrane electrode assembly “MEA”
150
. An electrical load
160
is connected to the anode
110
and the cathode
130
for electrical power output.
A fuel pump
170
is provided for pumping an organic fuel and water solution into an anode chamber
180
. The organic fuel and water mixture is withdrawn through an outlet port
190
and is re-circulated. Carbon dioxide formed in the anode chamber
180
is vented through a port
1100
within tank
1120
. An oxygen or air compressor
1130
is provided to feed oxygen or air into a cathode chamber
1140
.
Prior to use, the anode chamber
180
is filled with the organic fuel and water mixture. The cathode chamber
1140
is filled with air or oxygen either at ambient pressure or in a pressurized state. During operation, the organic fuel in the anode chamber
180
is circulated past the anode
110
. Oxygen or air is pumped into the cathode chamber
1140
and circulated past the cathode
130
. When an electrical load
160
is connected between the anode
110
and the cathode
130
, electro-oxidation of the organic fuel occurs at the anode
110
and electro-reduction of oxygen occurs at the cathode
130
. Electrons generated by electro-oxidation at the anode
110
are conducted through the external load
160
and are captured at the cathode
130
. Hydrogen ions or protons generated at the anode
110
are transported directly across the solid electrolyte membrane
120
to the cathode
130
. Thus, a flow of current is sustained by a flow of ions through the cell and electrons through the external load
160
.
During operation, a fuel and water mixture in the concentration range of 0.5-3.0 mole/liter is circulated past the anode
110
within anode chamber
180
. Preferably, flow rates in the range of 10-500 ml/min are used. As the fuel and water mixture circulates past the anode
110
, the following electro-chemical reaction, for an exemplary methanol cell, occurs releasing electrons:
Anode: CH
3
OH+H
2
O →CO
2
+6H
+
+6e
−
(1)
Carbon dioxide produced by the above reaction is withdrawn along with the fuel and water solution through outlet
190
and separated from the solution in a gas-liquid separator. The fuel and water solution is then re-circulated into the cell by pump
170
.
Simultaneous with the electrochemical reaction described in equation (1) above, another electrochemical reaction involving the electro-reduction of oxygen, which captures electrons, occurs at the cathode
130
and is given by:
Cathode: O
2
+4H
+
+4e
−
→2H
2
O (2)
The individual electrode reactions described by equations (1) and (2) result in an overall reaction for the exemplary methanol fuel cell given by:
Cell: CH
3
OH+1.5O
2
→CO
2
+2H
2
O (3)
During operation of the fuel cell, methanol is consumed at the anode
110
. In order to maintain steady operation of the fuel cell system, the methanol concentration in the anode compartment
180
should be maintained. The concentration of methanol in the compartment can be sensed so that an appropriate amount of methanol is metered. The rate in which methanol is added to the system should be related to the rate of depletion of methanol in the system. Therefore, an accurate measure of fuel concentration is desirable for a fuel cell system.
SUMMARY
The inventors disclose an analyte concentration sensor that is capable of fast and reliable sensing of analyte concentration in aqueous environments with high concentrations of the analyte. Preferably, the present invention is a fuel concentration sensor device coupled to a fuel metering control system for use in a liquid direct-feed fuel cell. The present invention performs reliably in aqueous environments in the analyte concentration range 0.01 M-5 M and the temperature range 0-100 degrees Celsius.
The concentration sensor device includes a sensor element connected to a sensor response circuit. A reference element can be incorporated in several ways. One mode features a reference element coupled to both the sensor element and to the fuel metering control system. A preferred reference element for this mode is a thermocouple placed within the analyte fuel bath of the sensor element.
Another mode features a second sensor element as a reference element. This second sensor element is connected to the first sensor element; both the first and second sensor elements are connected to the sensor response circuit. In this mode, the concentration sensor device has a sensor element, a reference element, and a sensor response circuit.
The sensor element has a membrane electrode assembly mounted on supports. This sensor element resembles the fuel cell structurally. Like the fuel cell, the sensor element is fabricated by forming two catalyzed electrodes sandwiching a solid electrolyte membrane. The anode is preferably coated with platinum-ruthenium catalyst; the cathode is coated with platinum catalyst. Other catalyst formulations are possible.
However, the sensor element is operated quite differently from the standard fuel cell. The sensor element is connected to a sensor response circuit. This sensor response circuit provides a means to detect a concentration-dependent response from the sensor element.
In the preferred embodiment, both the anode and the cathode of the sensor element are immersed in a fuel bath. The anode is connected to the positive terminal of a constant voltage power supply or “potentiostat” and the cathode is connected to the negative terminal. The current flows through the sensor element causing electrochemical reactions to occur. This current is measured by an ammeter or a current measuring circuit connected in series in the path of the sensor response circuit, e.g. in series with the sensor element and the power supply.
At a certain high anode potential threshold, the current passing through the sensor cell becomes sensitive to the concentration of the fuel. This is because the mass transport of the fuel to the surface of the electrode becomes the current-limiting mechanism. This means that higher concentrations of methanol can sustain higher current densities. Therefore, the concentration sensor operates on the principle of the electrochemical oxidation of the fuel under mass transport limited conditions.
As discussed above, the current measured by the ammeter or the current measuring circuit represents the sensor response. The ammeter is connected to a fuel metering control system. The control system translates the ammeter output into a fuel concentration, e.g. using a pre-stored mathematical relation, or using a look up table. The fuel concentration verses measured current relationship may be highly temperature dependent. A reference
Chun William
Narayanan Sekharipuram R.
Valdez Thomas I.
California Institute of Technology
Fish & Richardson P.C.
Olsen Kaj K.
Warden, Sr. Robert J.
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