Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Gas – vapor – or critical fluid
Reexamination Certificate
2000-12-14
2003-05-20
Phasge, Arun S. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Gas, vapor, or critical fluid
C205S781000, C204S252000, C204S421000, C204S424000, C204S425000, C204S426000, C204S431000
Reexamination Certificate
active
06565737
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an electrochemical cell and the use of the electrochemical cell. More specifically, the present invention relates to an improved electrochemical cell that is useful in selectively removing oxygen from a gas stream without decomposing NO
x
and particularly useful when employed in a NO
x
sensor.
BACKGROUND OF THE INVENTION
Depending on how they are run, internal combustion engines that use air as an oxidant are going to produce some amount of NO
x
. Large amounts of NO
x
emitted from internal combustion engines have proven to be a significant hazard to human health and to the quality of the environment. At the same time, the near ubiquitous presence of internal combustion engines throughout the industrial world insures that they will be a significant feature of every modern industrial society for at least the immediate future. Thus, if human health and environmental quality are to be preserved, internal combustion engines must be engineered to minimize the amount of NO
x
emissions.
To compel manufacturers to engineer their products to protect human health and environmental quality, regulatory agencies have imposed standards for NO
x
emissions that set forth the maximum levels that are permissible from a particular source. As more and more internal combustion engines are operated in ever smaller geographical boundaries, these standards are destined to grow more and more stringent if the overall goal of pollution reduction is to be achieved. To meet these standards, gasoline and diesel engine manufactures must build engines with the ability to adjust their own operation, to optimize the exhaust gas mix. Key to this ability is real time knowledge of the amount NO
x
being generated by the engine. Thus, the development of improved NO
x
sensors is critical to developing engines which can meet evolving, tough emissions standards that are in turn key to a healthy environment.
Due to their sensitivity, thermal stability, the ease with which they may be tested and manufactured, and the developed base technology, compact solid electrolyte NO
x
sensors are of particular interest to designers. Compared to nitrate-based metal salts or binary systems as an auxiliary phase, solid oxide electrolytes bring in high chemical and thermal stability.
A typical sensor utilizing a solid electrolyte consists of a two serial chambered system. The exhaust gas from an internal combustion engine (NO, NO
2
, CO, CO
2
, SO
2
, O
2
, hydrocarbons, etc.) enters the first chamber configured as an oxygen pump and the coexisting oxygen is removed via the YSZ solid electrolyte according to equation (1):
O
2
+4e
−
=2O
2−
(1)
The remainder of the gas diffuses into a second chamber where the NO
x
is decomposed electrochemically to N
2
and O
2−
on another electrode according to equations (2) and (3):
NO
2
+2e
−
=NO+O
2−
(2)
2NO+4e
−
=N
2
+2O
2−
(3)
The current drawn in the second chamber is proportional to the amount of NO
x
since the only oxygen, in theory, passing through the electrolyte is formed during NO
x
decomposition. Therefore, there is a great need for a first-chamber electrode, which is capable of catalyzing oxygen reduction (reaction 1) without catalyzing NO
x
decomposition (reactions 2 and 3). The desire to improve the operation of such systems creates a need for an electrochemical cell that is capable of catalyzing oxygen reduction (reaction 1) without catalyzing NO
x
decomposition (reactions 2 and 3). As will be apparent to those having skill in the art, such a material is generally desirable in any system wherein it is desirable to remove oxygen from a gas stream containing oxygen and NO
x
and is particularly desirable as an electrochemical cell utilized in the first chamber of a two chamber NO
x
sensor that utilized solid electrolyte electrochemical cells.
SUMMARY OF THE INVENTION
Accordingly, the present invention is broadly drawn to an electrochemical cell and the use of the electrochemical cell to selectively remove oxygen from a gas stream without decomposing NOx. This aspect of the present invention provides particular utility when the present invention is employed in a NO
x
sensor. More particularly, an especially useful aspect of the present invention is found when the present invention is employed in a two chambered NO
x
sensor utilizing solid electrolyte electrochemical cells, wherein the present invention is employed as forming at least a part of the surface of the first chamber, thereby providing an electrochemical cell on the surface of the first chamber capable of catalyzing oxygen reduction without catalyzing NOx decomposition.
The electrochemical cell consists of a solid oxide electrolyte sandwiched between a working, or positive, electrode and a counter, or negative, electrode. The working electrode consists of La
1-X
M
X
FeO
3
, where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5, but preferably is about 0.2. The counter electrode may be any conductive material, but is preferably is a metal stable in air at the operating temperatures of the electrochemical cell. The electrolyte is an oxide with sufficient oxygen ion conductivity, preferably an MO
X
-stabilized zirconia, where M is Y, Sc, Yb or Ca. By applying a current to the working electrode, and placing it in contact with a gas containing oxygen and NO
x
, oxygen is selectively reduced (equation 1) and thereby removed from the gas stream through the electrolyte, while NO
x
concentration is unchanged.
OBJECTS
Accordingly, it is an object of the present invention to provide a method of selectively removing oxygen from a gas stream containing NO
x
and oxygen by contacting the gas stream with an electrochemical cell made from an electrode consisting of La
1-X
M
X
FeO
3
, (where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5), wherein the electrode is on one side of a solid oxide electrolyte, and a counter electrode is on the opposite side of the solid oxide electrolyte, and applying a voltage to the electrochemical cell.
It is a further object of the present invention to provide the counter electrode as a metal.
It is a further object of the present invention to provide the counter electrode as a metal stable in air at the operating temperatures of the electrochemical cell.
It is a further object of the present invention to provide an electrochemical cell for selectively removing oxygen from a gas stream containing NO
x
and oxygen as an electrode consisting of La
1-X
M
X
FeO
3
, (where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5), and the electrode is affixed to one side of a solid oxide electrolyte, and a counter electrode affixed to the opposite side of the solid oxide electrolyte.
It is a further object of the present invention to provide an improved two chambered NO
x
sensor utilizing solid oxide electrolyte electrochemical cells, wherein an electrochemical cell capable of catalyzing oxygen reduction without catalyzing NO
x
decomposition is integral to the first chamber.
It is a further object of the present invention to provide an improved NO
x
sensor having an electrochemical cell fashioned of a porous electrode consisting of La
1-X
M
X
FeO
3
, (where M is selected from the group consisting of Sr, Ba, Ca, and combinations thereof, and X is between 0.05 and 0.5,) the electrode being affixed to one side of a solid oxide electrolyte, and a counter electrode affixed to the opposite side of the solid oxide electrolyte.
These and other objects of the present invention are particularly pointed out and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following des
Habeger Craig F.
Marina Olga A.
Battelle (Memorial Institute)
May Stephen R.
McKinley, Jr. Douglas E.
Phasge Arun S,.
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