Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2000-11-29
2004-09-21
Gulakowski, Randy (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000
Reexamination Certificate
active
06794067
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to fuel cells, and specifically, to using measurements of the dielectric constant to control and measure critical operations of the fuel cell.
BACKGROUND OF THE INVENTION
Fuel cells are becoming more widely used as an inexpensive, continuous source of energy for many different applications. Direct Methanol Fuel Cell Systems (DMFCs), in particular, have the potential to provide power for electronics at a significantly higher energy density than batteries, in a scalable, small system.
FIGS. 1 and 2
illustrate two variations of a basic fuel cell
1
, well known in the art, which in this illustration, could be either a reformer based fuel cell as in
FIG. 1
, or a direct oxidation fuel cell as in FIG.
2
. Whether reformer-based or direct oxidation, a fuel cell
1
comprises an anode chamber
11
, an anode electrode
12
, an electrolyte
13
, a cathode electrode
14
, and a cathode chamber
15
. The two electrodes, namely anode electrode
12
and cathode electrode
14
are sandwiched around electrolyte
13
as shown.
In reformer-based fuel cells, hydrogen
16
, extracted from a hydrocarbon source fuel such as, but not limited to, natural gas, methanol, ethanol, butane, propane, or even gasoline, is fed into anode chamber
11
, while oxygen, or a gas comprising oxygen
17
, such as air, is fed (or allowed to enter) to cathode chamber
15
. Anode electrode
12
contains a catalyst that promotes the chemical reaction that causes hydrogen atoms from a source fuel to split into protons and electrons. The protons pass through electrolyte
13
, but the electrons, which are unable to pass through electrolyte
13
, must instead take a different path around electrolyte
13
. This creates a current
19
which is utilized to provide power, before the electrons return to cathode electrode
14
. With the aid of cathode electrode
14
, the hydrogen protons and oxygen are reunited on the cathode side of the fuel cell to create the fuel cell waste product, namely, water
18
. Because fuel cell
1
relies on electrochemical oxidation and not combustion, emissions are not nearly as significant as emissions from even the cleanest fuel combustion processes.
In a reformer-based fuel cell system such as is illustrated in
FIG. 1
, hydrocarbons with multiple carbon atoms can be used as fuel. These fuels generally have higher energy density than fuels used in direct oxidation systems, but must be reformed by a separate fuel reformer
10
, to assist in extracting hydrogen from the source fuel. Reformation is a technically difficult process that consumes energy and increases complexity of the overall fuel cell system. Because direct oxidation fuel cell systems generally use fuels that are molecularly simple, no reformer is needed to promote the reaction that releases the protons and electrons to generate electricity. As such, direct oxidation systems are simpler and can be fabricated in a smaller volume.
FIG. 2
illustrates a Direct Oxidation Fuel Cell, as would be present in a DMFC. Other than the absence of a fuel reformer
10
, schematically, a Direct Oxidation Fuel Cell is identical to a Reformer Based Fuel such as was illustrated in
FIG. 1
, though it may be made from different materials to account for the different electrochemistry of the direct oxidation source fuels (including methanol), as opposed to a reformed source fuel. A Direct Oxidation Fuel Cell generates current in a fashion similar to reformer-based fuel cell systems, wherein anode electrode
12
promotes the desired oxidation of the fuel (methanol) and electrons flow, supporting a load. However, because the source fuel for a direct oxidation fuel cell is generally in a different phase than a reformed fuel, and because there are different by-products of the electricity-producing reaction, direct oxidation fuel cells have different ancillary support systems than reformer-based fuel cells.
In a DMFC, a liquid methanol (CH
3
OH) and water (H
2
O) mix
24
enters fuel cell
1
directly at anode chamber
11
, as opposed to a reformer-based fuel cell wherein a reformed fuel containing hydrogen
16
(H
2
) extracted with the aid of the reformer
10
enters anode chamber
11
. At anode electrode
12
, methanol is oxidized according to:
CH
3
OH+H
2
O→CO
2
+6H
+
+6e. (1)
The CO
2
23
, is discharged as a waste product from anode chamber
11
. The hydrogen ions (H
+
) pass through the membrane electrolyte
13
, which may comprise, for example, Nafion®, a commercially available material. Electrons (e) do not pass through the membrane electrolyte, and must take a different path through the load, creating a usable current
19
. At cathode electrode
14
, the oxygen
17
(O
2
) reunites with the electrons (e) from current
19
and the hydrogen ions (H
+
) according to:
O
2
+4e+4H
+
→2H
2
O, (2)
thus creating water
18
(H
2
O) as a waste product. The overall chemical reaction of the DMFC
1
is therefore given by:
CH
3
OH+3/2O
2
+2H
2
O. (3)
DMFC
1
also comprises an anode gas diffusion layer (GDL)
21
, and a cathode gas diffusion layer
22
, which are utilized to ensure that the fluids involved in these reactions are diffused in a substantially uniform manner over anode chamber
11
and cathode chamber
15
respectively. The gas diffusion layers
21
and
22
are also typically part of a reformer-based system such as illustrated by
FIG. 1
, but are omitted from
FIG. 1
for simplicity of illustration.
As noted in the earlier discussion and specified in eq. 1, pure methanol is not fed to anode chamber
11
. Rather, to operate fuel cell
1
at peak efficiency, it is preferred to feed to anode chamber
11
a dilute mixture of methanol and water. In particular, it is well known in the art that the membrane electrolyte
13
is, to varying degrees permeable to water, methanol, and protons. As such, if the methanol concentration is too high relative to the water on the anode side of the DMFC, some methanol will pass through electrolyte
13
and react with the source of oxygen or air
17
without contributing to current
19
. This reduces the efficiency of the DMFC, and wastes methanol. Alternatively, if not enough methanol is supplied, fuel cell
1
will not receive enough fuel to generate the desired current
19
.
A 3% methanol, 97% water mixture is typical using current technology and load requirements. However, it is anticipated that over the longer term, this concentration might be as low as 2% or even 1%, but may become substantially higher as advances in the fuel cell
1
, the electrolyte
13
, and the ancillary systems are realized. As such, variations in methanol concentration are to be considered within the scope of this disclosure and its associated claims, and these may run as high as 5%, 10%, 15%, 30%, 50%, 75%, 90%, and even 100% as the fuel cell
1
and fuel cell system technology progresses.
More generally, the exact fuel and water mixing proportion in any given fuel cell application is related to the particular technology of fuel cell
1
and the overall fuel cell system which comprises fuel cell
1
, and it is expected that these technologies will improve over time. Thus the desired mixing proportions will change as well. This anticipated change in optimum mixing proportions as these technologies progress is considered to be within the scope of this disclosure and its associated claims.
The water
18
produced as a by-product of DMFC operation is suitable as a water supply for mixing with the methanol source fuel, and indeed, is an attractive source for diluting water. In particular, provided that suitable methods for managing water are implemented, a DMFC system may be self-contained because the cathode-side fuel cell
1
reaction produces adequate water to operate the DMFC. It may, however, be necessary to remove some water from the DMFC to prevent saturation of the cathode electrode
14
of fuel cell
1
. To ensure a proper mix of methanol and water, it is necessary to measure the re
Acker William P.
Adler Michael S.
Crepeau Jonathan
Gulakowski Randy
MTI MicroFuel Cells Inc.
Yablon Jay R.
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