No.sub.x sensor using electrochemical reactions and differential pulse voltammetry (DPV)

A sensor system accurately measures nitrogen oxide (NO.sub.x) in a gas mixture via the use of at least one electrochemical sensing cell and differential pulse voltammetry (DPV). The sensor system has a sensor with an electrochemical sensing cell for producing an electrical signal (current, voltage, etc.) indicative of an amount of the nitrogen oxide within the gas mixture. The sensing cell has an electrolyte interposed between an anode electrocatalyst and a cathode electrocatalyst. Significantly, a DPV mechanism is connected to the sensing cell for enhancing the sensitivity and selectivity of the electrolyte associated with the sensing cell. The DPV mechanism has (1) a pulse superimposition mechanism for combining a pulse with a sensing cell bias imposed upon the sensing cell; (2) a measurement mechanism for measuring the electrical signal before and during superimposition of the pulse to derive first and second sample signals; and (3) a concentration derivation mechanism for mathematically combining (preferably, subtraction) the first and second sample signals to derive a DPV signal which is indicative of the NO.sub.x concentration. Optionally, based upon the material structure of the sensing cell, the sensor system may also be equipped with an electrochemical pumping cell for consuming oxygen (O.sub.2) within the gas mixture.

FIELD OF THE INVENTION 
The present invention generally relates to electrochemical analysis and 
measurement of specific gases within an environment containing a mixture 
of gases, and more particularly, to a sensor and method for accurately 
measuring nitrogen oxide (NO.sub.x) concentrations in gas mixtures, such 
as exhaust gases and emissions from combustion engines, furnaces, and 
facilities, which may contain oxygen (O.sub.2), via electrochemical 
reactions and differential pulse voltammetry (DPV). 
BACKGROUND OF THE INVENTION 
Nitrogen oxide (NO.sub.x, for example, N.sub.2 O, NO.sub.x, NO.sub.2, etc.) 
generated from combustion processes is a serious atmospheric pollutant. In 
fact, continuous on-line monitoring of NO.sub.x from combustion processes 
is often necessary to meet strict regulations of the U.S. Clean Air Act, 
which are expected to become more and more stringent in the future. 
Furthermore, because the amount of NO.sub.x in the exhaust of a combustion 
process is indicative of the air/fuel ratio, NO.sub.x concentration can 
also be used for feedback control of the air-to-fuel ratio of the 
combustion process in order to achieve optimal fuel efficiency. 
Various apparatus and techniques are known in the art for determining the 
concentration of NO.sub.x in a gas mixture, which may include, for 
instance, gaseous oxygen (O.sub.2), nitrogen (N.sub.2), and/or other 
gases. Typically, the electrochemical sensing of gaseous oxide compounds 
has been based on a well known "oxygen pumping principle," which is 
described briefly hereafter. The oxygen pumping principle has been widely 
publicized and is described in, for example, U.S. Pat. No. 4,005,001 to 
Pebler, U.S. Pat. No. 4,770,760 to Noda et al., U.S. Pat. No. 4,927,517 to 
Mizutani et al., U.S. Pat. No. 4,950,380 to Kurosawa et al., U.S. Pat. No. 
5,034,107 to Wang et al. and U.S. Pat. No. 5,034,112 to Murase et al. 
Generally, a solid electrolyte conductive to oxygen ions is utilized when 
employing the oxygen pumping principle. The electrolyte is commonly 
zirconia (ZrO.sub.2), bismuth oxide (Bi.sub.2 O.sub.3), ZrO.sub.2 and/or 
Bi.sub.2 O.sub.3 containing alkaline earth dopants, such as calcia (CaO), 
or containing rare earth dopants, such as yttria (Y.sub.2 O.sub.3), as a 
stabilizer, or some other suitable electrolyte having the properties more 
fully described hereafter. These electrolytes show a high permeability 
(conductance) to oxygen ions O.sup.2- when biased at a constant voltage 
and when maintained above a certain temperature, for instance, greater 
than 200.degree. C. in many applications. In other words, in an 
environment containing oxygen, these electrolytes can selectively permit 
oxygen to pass therethrough if certain biasing and temperature conditions 
are met. Said another way, these electrolytes exhibit high conductivity at 
elevated temperatures, and application of a voltage creates an O.sup.2- 
current or flux. 
In sensors utilizing these oxygen-ion-permeable electrolytes, 
electrocatalysts are usually disposed on opposing sides of the 
electrolyte, and a voltage is applied across the electrolyte via the 
electrocatalysts. The electrocatalysts typically comprise platinum (Pt), 
rhodium (Rh) and/or other noble metals. In this configuration, the 
combination of the electrocatalysts and the electrolyte disposed 
therebetween forms an electrochemical cell which is often referred to as a 
"pumping cell" because it pumps oxygen from the gas mixture exposed to the 
pumping cell. The pumping cell causes oxygen in the gas mixture to be 
reduced to oxygen ions O.sup.2- at the negative electrocatalyst 
(cathode), and then the oxygen ions O.sup.2- move through the electrolyte 
to the positive electrocatalyst (anode), where they are oxidized to oxygen 
again and discharged. 
Numerous techniques have been proposed in the art for determining the 
amount of oxygen and/or oxide compounds in the environment around 
electrochemical cells, particularly pumping cells, by monitoring the 
voltage and/or current generated across and/or through the electrolyte. A 
brief discussion of several exemplary types of prior art sensors is set 
forth hereafter, but it should be noted that this discussion is not 
exhaustive. 
One type of sensor is described in U.S. Pat. No. 5,217,588 to Wang. This 
sensor employs two electrochemical cells on a zirconian electrolyte. One 
cell senses only oxygen gas and the other cell senses all the gases which 
contain oxygen, including the oxygen gas. Both electrochemical cells are 
exposed to the same gas mixture, and the difference between the sensed 
signals is a measure of the concentration of NO.sub.x in the gas mixture. 
Another type of sensor is described in U.S. Pat. No. 5,034,112 to Murase et 
al. In this sensor, an electrocatalyst for reducing NO.sub.x is placed on 
an electrolyte adjacent to a pumping cell. A current is induced in the 
pumping cell so as to control the oxygen concentration in the environment 
around the pumping cell. When the oxygen concentration is depleted to a 
predetermined level, the electrocatalyst supposedly begins to deplete 
NO.sub.x, and the concentration of NO.sub.x is determined by measuring the 
current supplied to the pumping cell. 
Although the sensors of the prior art have some merit, they do not provide 
for highly accurate measurement of NO.sub.x or other oxide compounds in 
gas mixtures because the electrocatalysts utilized for the electrochemical 
cells do not provide for sufficient selectivity between oxygen and 
NO.sub.x. In other words, some amounts of oxygen and some amounts of these 
oxide compounds are undesirably consumed by the wrong electrocatalyst, and 
this phenomenon results in inaccurate measurements of oxygen as well as 
NO.sub.x concentrations. Moreover, if the gas mixture contains a 
relatively low NO.sub.x concentration as compared with that of oxygen, the 
signal-to-noise ratio is small, and an accurate determination of the 
NO.sub.x concentration is even more difficult. In exhaust gases or 
emissions produced by internal combustion engines or furnaces, the 
concentration of oxygen is typically several thousand times higher than 
the NO.sub.x concentration. Hence, measurements of NO.sub.x in exhaust 
gases using the prior art techniques are undesirably and unavoidably 
inaccurate. 
SUMMARY OF THE INVENTION 
An object of the present invention is to overcome the inadequacies and 
deficiencies of the prior art as noted above and as generally known in the 
industry. 
An object of the present invention is to provide a sensor and method for 
accurately measuring NO.sub.x in exhaust from gas combustion processes. 
Another object of the present invention is to provide a sensor with 
sufficient sensitivity and selectivity so that NO.sub.x can be accurately 
measured in exhaust from gas combustion processes. 
Another object of the present invention is to provide a sensor with 
sufficient sensitivity and selectivity so that NO.sub.x can be accurately 
measured in the presence of O.sub.2. 
Another object of the present invention is to provide an NO.sub.x sensor 
which is simple in design, reliable in operation, and exhibiting optimal 
sensitivity and selectivity to NO.sub.x so that NO.sub.x can be accurately 
measured in exhaust from gas combustion processes and particularly in 
exhaust having gaseous oxygen. 
Another object of the present invention is to provide a method for 
enhancing the sensitivity and selectivity of an electrolyte, such as one 
having zirconia (ZrO.sub.2), with respect to NO.sub.x. 
Another object of the present invention is to provide a highly effective 
NO.sub.x sensor which is inexpensive to manufacture. 
Briefly described, the present invention provides for a sensor system and 
method for accurately measuring NO.sub.x in a gas mixture via the use of 
differential pulse voltammetry (DPV). The sensor system has an NO.sub.x 
sensor. The NO.sub.x sensor has an electrochemical sensing cell for 
producing an electrical signal (current, voltage, etc.) indicative of an 
amount of the nitrogen oxide within the gas mixture. The sensing cell has 
an electrolyte interposed between a pair of electrocatalysts, or 
electrodes, one referred to as the anode and the other the cathode. The 
electrolyte is formed from yttria-stabilized-zirconia (YSZ), some other 
ZrOx compound, bismuth oxide (Bi.sub.2 O.sub.3), Bi.sub.2 O.sub.3 
containing alkaline earth dopants, such as calcia (CaO), or containing 
rare earth dopants, such as yttria (Y.sub.2 O.sub.3), as a stabilizer, 
some other suitable material, or combinations thereof. The 
electrocatalysts can be made from noble metals, (for example but not 
limited to, gold (Au), platinum (Pt), or rhodium (Rh)), metal oxides (for 
example but not limited to, a perovskite), other suitable materials, or 
combinations thereof. 
Optionally, depending upon the material structure of the sensing cell, the 
NO.sub.x sensor may be further equipped with an electrochemical pumping 
cell for consuming oxygen (O.sub.2) within the gas mixture. The O.sub.2 
pumping cell is constructed similar to the sensing cell, with a suitable 
electrolyte interposed between opposing electrocatalysts. 
In accordance with a significant feature of the present invention, the 
sensor system further comprises a DPV mechanism, which is connected to the 
sensing cell for enhancing the sensitivity and selectivity of the 
electrolyte associated with the sensing cell. The DPV mechanism has (1) a 
pulse superimposition mechanism for combining a pulse v.sub.pulse with a 
sensing cell bias V.sub.bs imposed upon the NO.sub.x sensing cell; (2) a 
measurement mechanism for measuring the electrical signal (preferably, 
current i.sub.s) before and during superimposition of the pulse 
v.sub.pulse to derive first and second sample signals; and (3) a 
concentration derivation mechanism for mathematically combining 
(preferably, subtraction) the first and second sample signals to derive a 
DPV signal which is indicative of the NO.sub.x concentration. In the DPV 
signal, the background noise current due to coexisting gases, such as 
NO.sub.x and O.sub.2, and the capacitive charging is substantially reduced 
or completely eliminated to provide inherent selectivity and better 
resolution (i.e., signal-to-background ratio). Moreover, the mathematical 
operation of combining the first and second sample signals eliminates the 
effect of drift on the NO.sub.x measurement. 
The sensitivity, selectivity, and resolution can be even further enhanced 
when using DPV if the reduction reactions of coexisting gases, such as 
NO.sub.x and O.sub.2, occur at different electrical potentials. This 
desirable operation can be accomplished by the use of electrocatalysts 
which are highly selective to NO.sub.x and O.sub.2, respectively. 
The physical structure of the NO.sub.x sensor can exhibit many possible 
configurations. As an example, the sensor could be designed with a 
single-hole housing having an internal cavity where the pumping cell's 
cathode and the sensing cell's cathode are disposed. In this 
configuration, the cavity has only a single hole for ingress and egress of 
the gas mixture. As another example, the sensor could have a porous layer 
enclosing one or both of the cathode electrocatalysts corresponding with 
the cells. In this configuration, the porous layer is permeable to the gas 
mixture for permitting passage of the gas mixture therethrough to the 
shielded cathode electrocatalyst. 
The electrical biasing of the sensing cell could take many possible forms. 
However, in the preferred embodiment, the electrical biasing is a periodic 
voltage signal in the form of a step function waveform. The DPV pulses are 
superimposed over the step function waveform commencing at a rising edge 
of the step function waveform and terminating during the step level after 
the rising edge. 
In addition to achieving all of the aforementioned objects, the present 
invention has many other advantages, a few of which are delineated 
hereafter. 
An advantage of the present invention is that a DPV NO.sub.x sensor system 
can be used to measure a very small amount of NO.sub.x in the presence of 
a very large amount of O.sub.2 (even as high as about 5% by volume). 
Another advantage of the present invention is that DPV can be utilized to 
make measurements based upon electrocatalysts that are originally 
nonselective to NO.sub.x, ultimately highly selective to NO.sub.x. 
Another advantage of the present invention is that a DPV NO.sub.x sensor 
system using either YSZ or some other ZrO.sub.2 compound as a sensing 
electrolyte can be inexpensively manufactured and easily afforded by 
individual gas consumers. 
Another advantage of the present invention is that a DPV NO.sub.x sensor 
system can provide a linear output over a wide range of NO.sub.x 
concentrations (100 percent (%) down to a few parts per million (ppm)). 
Another advantage of the present invention is that an NO.sub.x sensor in 
the DPV NO.sub.x sensor system can exhibit long-term operational stability 
by utilizing very stable electrocatalysts, which would be undesirably 
nonselective to NO.sub.x without the use of DPV. 
Another advantage of the present invention is that YSZ can be utilized in 
the NO.sub.x sensor of the DPV NO.sub.x sensor system, and YSZ is an 
extremely stable material in combustion exhaust. In fact, YSZ has been 
widely used for monitoring automotive exhaust gas and has a lifetime of 
between 5 to 10 years. 
Another advantage of the present invention is that the DPV NO.sub.x sensor 
system can accurately provide information for feedback control of the 
air-to-fuel ratio for a combustion process in order to achieve high fuel 
efficiency. 
Another advantage of the present invention is that the DPV NO.sub.x sensor 
system can be used as an exhaust diagnostic tool for NO.sub.x abating 
devices. 
Another advantage of the present invention is that DPV can be utilized to 
further enhance the sensitivity and selectivity of metal oxide perovskites 
relative to NO.sub.x and/or O.sub.2. Perovskites utilized as 
electrocatalysts in electrochemical cells are described in copending 
application entitled "Sensor And Method For Accurately Measuring 
Concentrations Of Oxide Compounds In Gas Mixtures", Ser. No. 08/208,449, 
filed Mar. 9, 1994, by inventor Eric Wachsman, which is now U.S. Pat. No. 
5,397,442. The foregoing disclosure is incorporated herein by reference. 
In the aforementioned document, it was determined that the perovskites 
La.sub.2 CuO.sub.4, LaNiO.sub.3, LaFeO.sub.3, LaCoO.sub.3, and 
LaSrCoO.sub.3 were highly selective to O.sub.2, and that the perovskites 
LaRuO.sub.3 and LaMnO.sub.3 were highly selective to NO.sub.x. These 
selectivities can be further optimized using DPV. 
Other objects, features, and advantages of the present invention will 
become apparent to one with skill in the art upon examination of the 
drawings and the following detailed description. All such additional 
objects, features and advantages are intended to be included herein within 
this disclosure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates the DPV NO.sub.x sensor system 20 in accordance with the 
present invention. The DPV NO.sub.x sensor system 20 can be used for 
accurately measuring NO.sub.x within a gas mixture 21 via the use of 
differential pulse voltammetry (DPV). The gas mixture 21 comprises 
NO.sub.x and possibly other gases, including oxygen (O.sub.2). Generally, 
the DPV NO.sub.x sensor system 20 has an NO.sub.x sensor 22 disposed 
within the gas mixture 21 and a data acquisition system 23 connected to 
the NO.sub.x sensor 22 for driving and monitoring the NO.sub.x sensor 22. 
The NO.sub.x sensor 22 has an NO.sub.x sensing cell 27 for consuming 
NO.sub.x within the gas mixture 21. Optionally, depending upon the 
material and selectivity of the sensing cell 27, as will further become 
apparent later in this document, an O.sub.2 pumping cell 26 may be 
disposed within the NO.sub.x sensor 22 for consuming O.sub.2 within the 
gas mixture 21. The electrochemical cells 26, 27 each have an electrolyte 
interposed between an anode electrocatalyst and a cathode electrocatalyst. 
The electrolyte is formed from yttria-stabilized-zirconia (YSZ), another 
ZrO.sub.2 compound, bismuth oxide (Bi.sub.2 O.sub.3), Bi.sub.2 O.sub.3 
containing alkaline earth dopants, such as calcia (CaO), or containing 
rare earth dopants, such as yttria (Y.sub.2 O.sub.3), as a stabilizer, or 
another suitable material. The electrocatalysts of the cells 26, 27 are 
made from gold (Au), platinum (Pt), rhodium (Rh), another nobel metal, or 
some other suitable material. In order to consume O.sub.2, the O.sub.2 
pumping cell 26 is provided with a voltage bias V.sub.bp. In order to 
consume and sense NO.sub.x, the NO.sub.x sensing cell 27 is provided with 
a voltage bias V.sub.bs. 
The data acquisition system 23 provides the voltage biases V.sub.bp, 
V.sub.bs to the cells 26, 27, respectively. These biases can be DC (steady 
state) or can vary over time, as is well known in the art. Further, 
circuits for generating these biases are well known in the art. In 
accordance with a significant feature of the present invention, the data 
acquisition system 23 comprises a DPV mechanism 28 for enhancing the 
sensitivity and selectivity of the NO.sub.x sensing cell 27, particularly 
the electrolyte corresponding thereto, and produces a DPV signal 31 which 
very accurately corresponds to the NO.sub.x concentration within the gas 
mixture 21. 
The DPV mechanism 28 may be implemented in hardware (via, e.g., logic 
gates), software (via executable instructions), or a combination thereof. 
As shown in FIG. 2, the DPV mechanism 28 comprises a pulse superimposition 
mechanism 41 for generating a voltage pulse v.sub.pulse and for combining 
the voltage pulse v.sub.pulse with the sensing cell bias v.sub.bp. A 
measurement mechanism 42 measures an electrical characteristic e (e.g., 
current i, voltage v, etc.) of the NO.sub.x sensing cell 27 both before 
and during superimposition of the voltage pulse v.sub.pulse to derive 
first and second sample signals. Moreover, an NO.sub.x concentration 
derivation mechanism 43 mathematically combines (e.g., subtraction, as in 
the preferred embodiment), the first and second sample signals in order to 
derive the DPV signal 31, which is indicative of the NO.sub.x 
concentration within the gas mixture 21. 
When the DPV signal 31 is derived in accordance with the aforementioned 
procedure, the noise caused by background current due to coexisting gases, 
such as O.sub.2, and capacitive charging is substantially reduced or 
completely eliminated. Hence, the inherent selectivity of the sensing cell 
27 is enhanced and better resolution (i.e., signal-to-background ratio) is 
provided. Furthermore, the mathematical operation of combining the first 
and second sample signals eliminates the effect of drift on the NO.sub.x 
measurement. 
The selectivity and resolution can be even further enhanced when using DPV 
if the reduction reactions of coexisting gases, such as NO.sub.x and 
O.sub.2, occur at different electrical potentials. This desirable 
operation can be accomplished by the use of electrocatalysts which are 
highly selective to NO.sub.x and O.sub.2, respectively. For instance, in 
copending application entitled "Sensor And Method For Accurately Measuring 
Concentrations Of Oxide Compounds In Gas Mixtures", Ser. No. 08/208,449, 
filed Mar. 9, 1994, by inventor Eric Wachsman, which is now U.S. Pat. No. 
5,397,442, metal oxide perovskites having high selectivity to NO.sub.x and 
O.sub.2 were described. In the aforementioned document, it was determined 
that the perovskites La.sub.2 CuO.sub.4, LaNiO.sub.3, LaFeO.sub.3, 
LaCoO.sub.3, and LaSrCoO.sub.3 were highly selective to O.sub.2, and that 
the perovskites LaRuO.sub.3 and LaMnO.sub.3 were highly selective to 
NO.sub.x. 
FIG. 3 shows a flow chart indicating the preferred functionality and 
architecture of the DPV mechanism 28 in FIGS. 1 and 2. In the preferred 
embodiment, the voltage bias V.sub.bs on the NO.sub.x sensing cell 27 is a 
step function waveform, as is well known in the art. With reference to 
FIG. 3, particularly block 51, the measurement mechanism 42 (FIG. 2) of 
the DPV mechanism 28 obtains a first sample signal from the NO.sub.x 
sensing cell 27 (FIG. 1). In the preferred embodiment, the sample signals 
are currents i.sub.s (FIG. 1), and the first sample signal is denoted by 
i.sub.s1 herein. Next, as indicated in block 52, the pulse superimposition 
mechanism 41 (FIG. 1) generates a substantially square voltage pulse 
v.sub.pulse and combines the voltage pulse v.sub.pulse with the sensing 
cell bias V.sub.bs starting with the rising edge of V.sub.bs and 
terminating during the steady state voltage step just after the rising 
edge. During the pulse, as indicated at block 53 in FIG. 3, the second 
sample signal i.sub.s2 is sensed by the measurement mechanism 42 (FIG. 1) 
from the NO.sub.x sensing cell 27 (FIG. 1). Moreover, the pulse 
v.sub.pulse is terminated so that the sensing cell bias V.sub.bs drops to 
its steady-state step level, as indicated in block 54 of FIG. 3. Finally, 
as shown in block 55, the currents i.sub.s2, i.sub.s1 are mathematically 
combined via subtraction in the preferred embodiment by the NO.sub.x 
concentration derivation mechanism 43 (FIG. 2) in order to derive the DPV 
signal 31, which is accurately indicative of the NO.sub.x concentration in 
the gas mixture 21. 
Reference numeral 61a in FIG. 4 graphically illustrates the resultant bias 
V.sub.bs imposed upon the NO.sub.x sensing cell 27 due to the 
superimposition of the voltage pulse v.sub.pulse. As shown in FIG. 4, the 
step function waveform is initially at the steady state voltage level 62 
and the first sample signal i.sub.s1 is obtained at time t.sub.1. The step 
function then begins to rise at time t.sub.2 , as indicated by the rising 
edge 63. During the rising edge 63, the voltage pulse V.sub.pulse is 
generated and additively superimposed on the step function waveform. 
Reference numeral 64 indicates the steady-state voltage level during the 
time when the pulse is superimposed on the step function waveform. At time 
t.sub.3, the second sample signal i.sub.s2 is obtained. Next, the pulse is 
terminated at time t.sub.4 and the voltage level of the step function 
waveform declines, as indicated by falling edge 65 to another steady-state 
voltage level 66, which represents the voltage level at which the step 
function waveform would have assumed after the rising edge 63 if the 
voltage pulse V.sub.pulse had not been superimposed. After the voltage 
level 66, the step function waveform again begins to rise as indicated by 
the rising edge 67. 
The physical structure of the NO.sub.x sensor 22 can exhibit many possible 
configurations. As an example, the NO.sub.x sensor 22 could be designed as 
shown in FIGS. 5A and 5B. FIGS. 5A and 5B illustrate a first embodiment, 
denoted as reference numeral 22', having a single hole for ingress and 
egress of the gas mixture 21. The NO.sub.x sensor 22' has (a) a planar, 
first YSZ electrolyte layer 71, (b) a C-shaped alumina insulating layer 72 
with an internal aperture 73 and a hole 74 connecting the aperture 73 to 
the outer edge of the alumina insulating layer 72, and (c) a planar, 
second YSZ electrolyte layer 76. Together, these layers form a single hole 
structure, as shown in FIG. 5b, having an internal chamber 78 with inlet 
74. 
The O.sub.2 pumping cell 26 (optional; not necessary) can be established at 
the second YSZ electrolyte layer 76, and the NO.sub.x sensing cell 27 is 
established at the first YSZ electrolyte layer 71. Further, each cell 26 
and 27 has a pair of electrocatalysts 26a, 26b and 27a, 27b, respectively. 
Electrocatalysts 26a, 27a, which are situated within the internal cavity 
78 (FIG. 5B), serve as cathodes for their respective cells, and the 
external electrocatalysts 26b, 27b, serve as anodes for their respective 
cells. 
In operation, the gas mixture 21 enters the hole 74. The O.sub.2 within the 
gas mixture 21 decomposes on the internal cathode electrocatalyst 26a, if 
present, and oxygen ions O.sub.2. pass through the second YSZ electrolyte 
layer 76 to the external anode electrocatalyst 26b. The decomposition of 
the O.sub.2 gas and the driving force of the ion transfer is caused by the 
pumping cell voltage bias V.sub.bp. 
The NO.sub.x sensing cell 27, which is situated farther in the cavity 78 
from the hole 74 than the optional O.sub.2 pumping cell 26, consumes 
NO.sub.x in the gas mixture 21. The NO.sub.x gas decomposes on the cathode 
electrocatalyst 27a, and the resultant ions O.sup.2- are transferred from 
the internal cathode electrocatalyst 27a to the external anode 
electrocatalyst 27b. The ion transfer results in the current i.sub.s which 
is measured by the DPV mechanism 28. 
As another example of a possible structure for the NO.sub.x sensor 22, 
FIGS. 6A and 6B show a second embodiment, denoted generally as reference 
22", having a single-cell for sensing NO.sub.x concentration. As shown, a 
porous YSZ layer 81 acts as both a gas diffusion barrier and an 
electrolyte for the sensing cell 27. Further, the sensing cell 27 has a 
cathode electrocatalyst 83a situated between the porous YSZ 
electrolyte/barrier layer 81 and the substrate 82, and an anode 
electrocatalyst 83b situated on the opposing side of the YSZ 
electrolyte/barrier layer 81. Further, an alumina insulating layer 84 
permits easier electrical connection to the cathode electrocatalyst 83a 
near the end of the structure. 
A third embodiment of the NO.sub.x sensor 22 is shown in FIGS. 7A and 7B, 
and generally delineated by reference numeral 22"'. The NO.sub.x sensor 
22"' has both an O.sub.2 pumping cell 26 and an NO.sub.x sensing cell 27. 
In the double-cell porous-type NO.sub.x sensor 22"' of the third 
embodiment, a porous YSZ layer 86 is used as both a gas diffusion barrier 
and an electrolyte for the O.sub.2 pumping cell 26. The NO.sub.x sensor 
22"' is situated about a substrate 87. The O.sub.2 pumping cell 26 is made 
up of the electrolyte YSZ layer 86, the cathode electrocatalyst 88a, and 
the anode electrocatalyst 88b. Moreover, the NO.sub.x sensing cell 27 is 
made up of the electrolyte substrate 87, the cathode electrocatalyst 89a, 
and the anode electrocatalyst 89b. An alumina insulating layer 91 is 
disposed between the YSZ layer 86 and the cathode electrocatalyst 88a of 
the O.sub.2 pumping cell 26 so that the O.sub.2 pumping cell 26 
effectively removes oxygen associated with the NO.sub.x sensing cell 27. 
Further, a porous layer 92, preferably alumina, is disposed between the 
YSZ layer 86 and the cathode electrocatalyst 89a of the NO.sub.x sensing 
cell 27 in order to permit passage of the gas mixture 21 to the cathode 
electrocatalyst 89a. Finally, an alumina insulating layer 93 is disposed 
between the cathode electrocatalyst 88a of the O.sub.2 pumping cell 26 and 
the electrolyte substrate 87 in order to isolate the cells 26, 27. 
EXPERIMENT 
1. Overview 
Operation parameters of DPV, two metal electrocatalysts (i.e., Au and Pt), 
and configurations of a gas-diffusion-limiting mode NO.sub.x sensor 22 
(i.e., single-hole-type and porous-type) were selected for determining the 
feasibility of the DPV NO.sub.x sensor system 20 (FIG. 1). Sensors 22 were 
fabricated having multilayers of ceramics (i.e., alumina and YSZ) and 
metals (i.e., Au and Pt), which are suitable for cyclic voltammetry (CV) 
and DPV experiments. Prior to the DPV experiment, a CV experiment was 
conducted to evaluate the metal electrocatalysts. It was determined that 
the Au electrocatalyst discriminated between the reductions of O.sub.2 and 
NO.sub.x : the onset of the reduction of O.sub.2 occurred at low voltage 
(0 to 0.3 V), and that of NO.sub.x occurred at high voltage (0.9 to 1.1 
V). The DPV experiment was conducted with the same NO.sub.x sensor 22 
having the Au electrocatalyst in the varying NO.sub.x concentrations in 
the presence of 0.5% O.sub.2 and 5% O.sub.2. The sensitivity of the 
NO.sub.x measurement greatly increased in DPV compared to CV as the effect 
of coexisting O.sub.2 was eliminated. 
To determine whether or not the sensitivity could be further improved, the 
DPV measurements were carried out on an NO.sub.x sensor 22, originally 
single-celled (FIG. 6), modified by addition of an O.sub.2 pumping cell 26 
(FIG. 7) in order to reduce the background O.sub.2 concentration by 
electrochemically pumping out O.sub.2. The results were as good as those 
of the case without the O.sub.2 pumping cell. 
2. Test Setup 
The data acquisition system 23 (FIG. 1) was implemented with a Apple 
Macintosh computer and its peripherals. The computer acquired and 
processed the data for CV and DPV. The voltage sweeping rate (i.e., from 0 
to 2 V) for the voltage pulse v.sub.pulse could vary from 2 mV/s to 0.8 
V/s, but the pulse times could be no shorter than 30 ms. The pulse time 
was limited by the Macintosh clocking speed. The Macintosh tick counter 
provided 60 ticks/s, resulting in a pulse width for the pulse v.sub.pulse 
of about 17 ms. Considering the overhead of both sending and receiving 
data from the system 23, a practical limit was 30-ms pulses v.sub.pulse. 
Smoothing, to remove noise, was carried out by the averaging of three to 
five values. Voltage and current waveforms of the DPV are given in FIG. 4. 
Materials for fabricating NO.sub.x sensors 22 were purchased, including 
sealing glass and Au paste. Three different NO.sub.x concentrations--2% 
(20,000 ppm), 2000 ppm, and 200 ppm balanced by NZ--and 1% O.sub.2 
balanced N.sub.2 were also purchased. (NO.sub.x, consists of NO.sub.x and 
NO.sub.2, but as NO.sub.x comprises most of the emitting NO.sub.x in the 
exhaust, the NO.sub.x detection was focused on.) 
2.1 Theoretical Evaluation of the Feasibility of an Amperometric NO.sub.x 
Sensor 
2.1.1 Electrocatalysts 
Au and Pt were selected and evaluated as electrocatalysts for the reduction 
of O.sub.2 and NO.sub.x because the two metals have different 
characteristics in reducing O.sub.2 at high temperatures. Pt is a very 
good electrocatalyst for O.sub.2 reduction. It is most frequently used as 
an electrocatalyst of an O.sub.2 sensor. However, Au is generally known to 
be an ineffective electrocatalyst for the reduction of O.sub.2 (at least 
in the Au/YSZ system). It is believed that either the adsorption, or 
transport kinetics, or both, of O.sub.2 are not very favorable on the Au 
surface. 
For the reduction of NO.sub.x on Pt, literature sources-although in 
considerable disagreement about the proposed mechanisms-generally agree 
that the reduction kinetics are slow. It has also been reported that the 
NO.sub.x decomposition rate is inversely dependent upon the O.sub.2 
partial pressure, which indicates the inhibiting role of O.sub.2. 
In the source-limiting mode of operation, the O.sub.2 pressure on the 
cathode electrocatalyst decreases with an increase of applied voltage bias 
V.sub.b. If the NO.sub.x sensor 22 operates in a gas mixture 21 of O.sub.2 
and NO.sub.x, the onset of NO.sub.x reduction should occur at a high 
voltage where the O.sub.2 pressure is low enough to diminish the 
inhibiting role of O.sub.2. Au, known to be a slow electrocatalyst, will 
further separate the potentials at which reduction occurs for O.sub.2 and 
NO.sub.x. Therefore, Au was selected as the electrocatalyst for the DPV 
experiment, but Pt was also used to compare the DPV results. 
Existing electrocatalysts were utilized without optimization. The two 
metals were prepared by RF sputtering. 
2.1.2 DPV for Sensitive and Selective Detection of NO.sub.x 
In DPV, as previously discussed, a small voltage pulse v.sub.pulse is 
periodically superimposed on the sensing cell bias V.sub.bs (reference 
numeral 61a in FIG. 4). The DPV signal (reference numeral 61d in FIG. 4) 
can be produced by the difference of the two currents, one current 
(reference numeral 6lb in FIG. 4 sampled just before the pulse and another 
current (reference numeral 61c in FIG. 4) sampled during the pulse. 
The observed current relaxation shown in FIG. 4 at reference numeral 61d 
primarily comes from two different physical origins: capacitive (reference 
numeral 6lb in FIG. 4) and mass transport (reference numeral 61c in FIG. 
4). The capacitive part is due to the rearrangement of the double layer at 
the electrocatalyst/electrolyte interface. In the solution electrochemical 
system, the relaxation time associated with the interface capacitance 
usually does not exceed 3 ms. The relaxation has an exponential relation 
with respect to time. 
The relaxation related to the mass transport is actually due to the 
depletion (or purging) of gases in the depletion layer on the 
electrocatalyst. The current decays inversely as t.sup.1/2. The depletion 
layer, formed during a pulse v.sub.pulse, is filled with the measuring gas 
(i.e., NO.sub.x) for the rest time following the pulse v.sub.pulse, so the 
relaxation of the Faradaic current during the pulse v.sub.pulse is 
repeated at every pulse application. The magnitude of the relaxed Faradaic 
current is related to the gas concentration. 
As the current subtraction in DPV yields a signal 61d that resembles the 
derivative of the conventional CV signals, the background current due to 
the coexisting gases can be eliminated in DPV operation. The DPV readout 
also eliminates most of the capacitive charging current and provides a 
significantly better signal-to-background ratio. The width and magnitude 
of the pulse v.sub.pulse should be adjusted to completely eliminate the 
capacitive current. The subtraction also eliminates the effect of drift on 
the measurement. 
DPV's selectivity and resolution would be increased if the reduction 
reactions of coexisting gases occur at different potentials. This can be 
accomplished by the use of a proper electrocatalyst, which, for example, 
discriminates between the reduction of NO.sub.x and O.sub.2. 
In summary, DPV has three inherent advantages over CV: (a) DPV sensitivity 
is several orders of magnitude better than CV; (b) DPV selectivity is at 
least 10 to 20 times better than selectivity obtained with conventional 
CV; and (c) DPV drift is negligible. 
2.1.3 Sensor Structure 
In a conventional solution electrochemical system, the depletion layer in 
front of the electrocatalyst naturally forms during the CV operation 
because of the slow diffusion of species in the liquid medium. In the gas 
phase, a rather rough, thick electrocatalyst can be sufficient. As was 
mentioned earlier, existing electrocatalysts (i.e., Au and Pt) were used 
in this experiment and each electrocatalyst was prepared by conventional 
RF sputtering. 
Several NO.sub.x sensors 22 having a gas-diffusion-limiting barrier were 
examined for our exploration of the feasibility of the NO.sub.x sensor 22. 
One species of design is the single-hole-type NO.sub.x sensor 22' (FIG. 5) 
and the other is the porous-type NO.sub.x sensors 22", 22"' (single-cell 
of FIGS. 6; double-cell of FIG. 7). FIG. 5 shows exploded and 
cross-sectional views of the single-hole-type NO.sub.x sensor 22'. FIGS. 
6A and 6B show top and cross-sectional views, respectively, of the 
porous-type NO.sub.x sensor 22" having a single NO.sub.x sensing cell. 
FIGS. 7A and 7B show top and cross-sectional views, respectively, of the 
porous-type NO.sub.x sensor 22"' having an NO.sub.x sensing cell and an 
O.sub.2 pumping cell. 
In the single-hole-type NO.sub.x sensor 22', the O.sub.2 pumping cell 26 
near the opening of the gas-diffusion-limiting hole 74 decreases the 
O.sub.2 pressure in the hole, and the NO.sub.x sensing cell 27 conducts 
the DPV experiment. The advantage of this configuration is the accurate 
design of the sensor geometry-that is, the hole diameter and length and 
electrocatalyst area. However, its fabrication may require the 
optimization of a number of processes. The bonding of the three ceramic 
layers (i.e., an alumina and two YSZ) without damaging the metal 
electrocatalyst is an especially challenging process. 
In the porous-type NO.sub.x sensors 22', 22"', the porous YSZ layers 81 
(FIGS. 6A, 6B), 86 (FIGS. 7A, 7B) act as both a gas-diffusion barrier and 
an electrolyte for a cell. The advantage of this design is easy 
fabrication of the structure. The porous layer 81, 86 can accommodate 
thermal strain to a certain level without mechanical failure. 
2.1.4 Material Compatibility 
The major parts of the NO.sub.x sensors 22 are YSZ, alumina as an 
insulator, and the metal electrocatalyst (i.e., Au and Pt). Chemical and 
thermal compatibilities among the layers were examined to confirm that the 
multilayered NO.sub.x sensor 22 could be tested at high temperatures 
without failure. 
Dense YSZ coupon was selected as a substrate for the porous-type NO.sub.x 
sensors 22', 22"'. Alumina was selected as an insulating layer between YSZ 
and Pt (or Au). Thermal expansion of alumina is approximately 10% less 
than that of YSZ, but the porous alumina (formed by the plasma spray 
coating) can accommodate the thermal strain produced by the thermal 
expansion mismatch between YSZ and alumina. 
Corning #1415 barium borosilicate glass was selected as a sealer for 
bonding the alumina and zirconia layers of the single-hole-type NO.sub.x 
sensor 22'. Its thermal expansion coefficient is 96.7 .times.10.sup.-7 
which is between alumina's and YSZ's thermal expansion coefficients. The 
softening temperature of the glass is 766.degree. C., which is sufficient 
for the amperometric-type sensor operation. 
Au paste was used to fill the gap between the two ceramic pieces and to 
provide electric lead continuity. 
2.2. Demonstration of the Feasibility of the DPV NO.sub.x. Sensor System 
2,2,1 Sensor Fabrication 
The NO.sub.x sensors 22', 22", 22"' were fabricated. The single-hole-type 
NO.sub.x sensor 22' was fabricated using YSZ and alumina green tapes. The 
green tapes were cut to form an appropriate shape, as shown in FIG. 5, and 
fired at 1550.degree. C. in air. The Au electrocatalysts 26a, 26b, 27a, 
27b were coated on the fired YSZ by RF sputtering, and then Au paste was 
coated on the thin-film Au for electric continuity. Electric leads from 
the electrocatalysts to the outer surface of the NO.sub.x sensor 22' were 
made by coating Au paste on the fired ceramic layers. The NO.sub.x sensors 
22' were completed by gluing the ceramic layers with the sealing glass. 
The O.sub.2 in the chamber 78 was electrochemically pumped out by the 
electrolyte pumping cell 26 near the opening of the gas-diffusion-limiting 
hole 74, while the O.sub.2 pressure in the chamber 78 was measured by the 
NO.sub.x sensing cell 27. The maximum EMF observed was approximately 40 
mV, which means that the O.sub.2 concentration on the measuring 
electrocatalyst 27a in the cavity is approximately one order of magnitude 
lower than in the test gas. The measured concentration is higher than 
expected. This is attributed to either ineffective pumping capacity, or 
gas leaking through cracks in the sealing glass, or both. Resolving the 
problem required gas-tight sealing of the ceramic layers, and decreasing 
the intake of the gas by reducing the cross-sectional area of the 
gas-diffusion-limiting hole 74. Because of the complexity of the sample 
fabrication, porous-type NO.sub.x sensors 22' were used for both the CV 
and DPV experiments. 
Porous-type NO.sub.x sensors 22", 22"' (FIGS. 6, 7) were made primarily by 
using plasma spray and RF sputtering techniques. Ceramic layers such as 
alumina and YSZ were coated by the plasma spray method, while the Au and 
Pt electrocatalysts 83a, 83b 88a, 88b, 89a, 89b were coated by RF 
sputtering. Masking techniques were used to selectively coat the foregoing 
layers. Layer thicknesses were as follows: YSZ electrolytes of about 150 
to 200 .mu.m, an alumina dielectric of about 10 to 20 .mu.m, and an Au or 
Pt electrocatalyst of about 0.4 to 0.7 .mu.m. 
The composite electrocatalyst Au/Pt was made by alternating the coating of 
very thin Au and Pt (each layer approximately 200 to 400 angstroms). 
The YSZ film actually acts as both a gas diffusion barrier and an O.sub.2 
ion-conducting electrolyte. Plasma-sprayed YSZ films are strongly bonded 
to the substrates and have exceptionally high integrity because YSZ powder 
is melted in a high-temperature gas plasma and propelled onto the 
substrate during deposition. However, its ionic conductivity is not as 
high as that of YSZ electrolytes sintered at high temperatures, as the 
films are not very dense. Because almost fully activated YSZ films can be 
obtained by the plasma spray method without excessive heating of the 
substrate already carrying the thin-film Pt electrocatalyst, the plasma 
spray method is very suitable for fabricating the multilayered NO.sub.x 
sensors 22 containing the ceramic materials and thin film metal 
electrocatalysts on various substrates. 
The area of the internal electrocatalyst 83b (FIG. 6) was approximately 5 
mm.times.5 mm, which produced the limiting current of 1 to 3 mA in 0.5% 
O.sub.2 at about 740.degree. C. Note that the limiting current is 
proportional to the area of the electrocatalyst and inversely proportional 
to the thickness of the gas-diffusion limiting layer 81. 
2.2.2 CV Experiment and Electrocatalyst Evaluation 
Experiments using conventional CV were conducted on the double-cell 
porous-type NO.sub.x sensor 22"' (FIG. 7) having different 
electrocatalysts (i.e., Pt, Au, and multilayered composite 
electrocatalysts Pt/Au) at about 740.degree. C. To vary the NO.sub.x and 
O.sub.2 concentrations, 2% NO.sub.x, 1% O.sub.2, and N.sub.2 were mixed. 
The total flow rate varied from 1000 to 1500 sccm. CV measurements were 
carried out using a potentiometer (BAR CV27). Scanning rate was 40 mV/s, 
but 80 and 400 mV/s were tried to analyze the effect of scanning rate on 
the measurement. Data was recorded on an X-Y plotter and sent to the data 
acquisition system 23 (FIG. 1) for further analysis. 
The test results of the NO.sub.x sensor 22"' having Pt, Au, and the Pt/Au 
composite are shown in FIGS. 8 through 10. Reductions of O.sub.2 and 
NO.sub.x occur on the Pt electrocatalyst from zero potential, as shown in 
FIG. 8. FIG. 8 shows current versus voltage characteristics of the 
porous-type NO.sub.x sensor 22"' having Pt electrocatalyst in 2% NO.sub.x, 
1% O.sub.2 at about 740.degree. C. Current flow at zero potential was 
observed on the Pt electrocatalyst. It is attributed to the chemical 
reaction of NO.sub.x on Pt electrocatalyst; NO.sub.x is decomposed to 
N.sub.2 and O.sub.2 to increase the O.sub.2 concentration, which causes 
the current flow at zero potential. Because the slopes of the NO.sub.x and 
O.sub.2 reduction curves on the Pt electrocatalyst are different, it is 
still possible, using DPV, to discriminate between the two gases. 
As shown in FIG. 9, the onset of reduction of NO.sub.x and O.sub.2 on the 
Au electrocatalyst occurs at different voltages. FIG. 9 shows current 
versus voltage characteristics of the porous-type NO.sub.x sensor 22"' 
having Au electrocatalyst in 2% NO.sub.x, 1% O.sub.2, and 0.5% O.sub.2 +1% 
NO.sub.x at about 740.degree. C. The reduction of NO.sub.x occurs between 
0.9 and 1.1 V with respect to the counter electrocatalyst, while the 
reduction of O.sub.2 occurs at approximately 0.3 V. When the NO.sub.x 
sensor 22"' is operated in the mixture 21 of NO.sub.x and O.sub.2, the 
measured current curve resembles the superimposition of the two curves 
shown for each gas (reference numeral 96c of FIG. 9). The slope of the 
curve increases at approximately 0.9 to 1.1 V because of the reduction of 
NO.sub.x. The observed behavior is primarily due to the different kinetics 
of O.sub.2 and NO.sub.x on the Au electrocatalysts; on the Pt 
electrocatalyst, the reduction of NO.sub.x is as fast as that of O.sub.2, 
but on the Au electrocatalyst, the reduction of NO.sub.x is more sluggish 
than that of O.sub.2. 
The Au/Pt composite was tested to explore the feasibility of tailoring the 
electrocatalyst material. FIG. 10 illustrates the test results. FIG. 10 
shows current versus voltage characteristics of the porous-type NO.sub.x 
sensor 22"' (FIG. 7) having Pt/Au composite electrocatalyst in about 0.5% 
O.sub.2, 1% NO.sub.x, and 0.5% O.sub.2 +1% NO.sub.x at about 740.degree. 
C. The reduction of O.sub.2 starts near the zero voltage, similar to the 
reaction on the Pt electrocatalyst, but the onset voltage of NO.sub.x 
reduction still requires a rather high potential, similar to the reaction 
on the Au electrocatalyst. A close look at the NO.sub.x reduction curve 
reveals a shift to a lower onset voltage compared to that of the Au 
electrocatalyst. The materials-tailoring concept is feasible, but further 
study is required to optimize the process. 
In the CV measurement with the present electrocatalyst and sensor 
configuration, the hysteresis loop in the current-voltage curve became 
wider with an increase in the scanning rate, but any characteristic peak 
associated with O.sub.2 or NO.sub.x did not appear. However, in the DPV 
measurement carried out on the same sample, the distinct peaks 
representing O.sub.2 and NO.sub.x were produced. 
2.2.3 DPV with Single-Cell Porous-Type Sensor 22" 
A conventional oscilloscope was used to analyze the current relaxation 
behavior of the single-cell porous-type NO.sub.x sensor 22" (FIG. 6) 
responding to the voltage pulse of DPV. The parameters of DPV were 
determined based on the results. An example of the current relaxation is 
given in FIG. 11. FIG. 11 shows current relaxation behavior of the 
porous-type NO.sub.x sensor 22" having Au electrocatalyst, responding to 
voltage pulse. As was discussed, the relaxing current consists of two 
components: capacitive and mass transport. Application of a pulse results 
in a current spike due to the charging of the double layer. The charging 
spike rapidly decays in a few milliseconds; this is followed by a slow 
decrease in current, indicating diffusion-controlled Faradaic current. The 
current-time behavior shows the t.sup.-1/2 relation. When the pulse 
application was stopped, a negative current spike appeared and decayed to 
zero. The relation time depends on the magnitude of voltage, sweeping 
speed, and gas concentration, but generally it was at least 200 to 300 ms 
until the relaxation was completed. 
As was described previously, the sampling interval of the present data 
acquisition system 23 is unfortunately limited to 30 ms. With the data 
acquisition system 23 in the specific embodiment described herein, it is 
difficult to measure the diffusion-controlled Faradaic current 61c (FIG. 
4), which immediately follows the charging spike appearing for a few 
milliseconds, in an accurate and consistent manner. For the consistent DPV 
measurement in this phase, a pulse width of approximately 400 ms and a 
rest time of 600 ms were used. A pulse height of about 150 mV was used as 
an optimum performance of DPV technique. The sweeping speed was about 0.05 
sec; it thus takes 40 s to sweep from 0 to 2 V. Under these conditions, 
the diffusion-controlled Faradaic current almost completely decays. This 
means that DPV sensitivity and resolution would not be fully maximized, 
but the present DPV still takes advantage of the subtraction process. 
DPV experiments were carried out on single-cell porous-type NO.sub.x sensor 
22" (FIG. 6) samples at about 740.degree. C. Three different NO.sub.x 
concentrations--(a) 2%, (b) 2000 ppm, and (c) 200 ppm--were mixed with 
about 1% O.sub.2 and N.sub.2 to vary the NO.sub.x concentration from a few 
ppm to about 1%. O.sub.2 concentration was maintained at about 0.5% and 
about 5%. 
2.2.4 Gold (Au) Electrocatalyst 
The results of DPV measurement on the NO.sub.x sensor 22" having the Au 
electrocatalyst are given in FIGS. 12A-12D. FIGS. 12A-12D show current 
versus voltage characteristics of DPV measurement in varying mixture 21s 
of NO.sub.x and O.sub.2 at about 740.degree. C. O.sub.2 concentration was 
maintained at approximately 0.5%, and NO.sub.x concentration varied from 
about 0% to 1%. Compared to the flat response of conventional CV (FIG. 9), 
the DPV measurements as shown in FIGS. 12A-12D give a peaked (bump-shaped) 
output. The peak at around 0.25 V in these figures represents the current 
increase due O.sub.2 reduction, and the peak near approximately 1.3 V (see 
FIG. 12C and 12D) represents the current increase due to NO.sub.x 
reduction. The current increase between about 1.6 V and about 1.8 V 
reflects the electronic leakage through the YSZ electrolyte 81 (FIG. 6), 
or the electrolyte decomposition. The background current appearing between 
about 0.6 V to about 1.4 V in FIG. 12A is caused by the current increase 
with voltage in the gas-diffusion-limited region. Ideally, the current 
should be zero if the current associated with the O.sub.2 reduction would 
be saturated in the gas-diffusion-limiting mode operation. The 
source-limiting process may be more easily controlled by using a thick 
electrocatalyst. 
The height of the peak near about 1.3 V associated with NO.sub.x is 
increased with NO.sub.x concentration. However, it is shown that the peak 
becomes broad with the increase of NO.sub.x concentration. 
Phenomenologically, this is due to the large cell resistance, as the slope 
of the current-voltage curve is determined by this resistance. If the 
resistance is reduced, the curve is steeper in the ohmic region, and the 
DPV peak should be sharper. It should be emphasized that the NO.sub.x 
sensor 22" can be optimized to decrease the resistance. This can be 
accomplished, for example, by improving the electrocatalyst 83b (FIG. 6). 
The peak heights in FIG. 12 are plotted as a function of NO.sub.x 
concentration in FIG. 13A. As the NO.sub.x concentration increases, the 
measured current i.sub.s appears to be lower than the expected linear 
trend. If the peak area were plotted, it would be close to the linear 
relation. The present NO.sub.x sensor 22" has approximately 3 mm.times.3 
mm electrocatalyst 83b (FIG. 6) and yields approximately 50 .mu.A in the 
1000-ppm NO.sub.x. This corresponds to the current of approximately 0.5 
.mu.A in 10-ppm NO.sub.x, which is large enough to electrochemically 
measure. If the electrocatalyst area is enlarged or if its coating is 
improved to yield a large current, the signal will be further increased. 
The results of the DPV measurement on the NO.sub.x sensor 22" in the 
presence of about 5% O.sub.2 are presented in FIG. 13B. The current 
increase with respect to NO.sub.x concentration is almost the same in the 
range from 0% to 0.1% as that in the presence of 0.5% O.sub.2. Even though 
the coexisting gas (i.e., O.sub.2) is increased tenfold in concentration, 
the background current is not much affected. This result is due to the 
subtraction process of DPV. 
2.2.5 Platinum (Pt) Electrocatalyst 
The reduction of both O.sub.2 and NO.sub.x occurs in the same voltage range 
(0 V to about 0.3 V), but the slopes of current-voltage curve are 
different (FIG. 8). Comparison of the DPV results indicated another peak 
associated with NO.sub.x reduction at approximately 0.25 V on the primary 
peak due to O.sub.2 reduction. As the two peaks are superimposed, the 
method was not pursued. 
2.2.6 DPV Experiment with Double-Cell Porous-Type Sensor 22"' in Reduced 
O.sub.2 Concentration 
DPV experiments were conducted on the double-cell porous-type NO.sub.x 
sensor 22"' (FIG. 7) at about 740.degree. C. Three different NO.sub.x 
concentrations-2%, 2000 ppm, and 200 ppm-were mixed with about 1% O.sub.2 
and N.sub.2 to vary the NO.sub.x concentration from a few ppm to about 1%. 
The O.sub.2 pumping cell 26 (88a, 86, 88b in FIG. 7B) electrochemically 
pumps out O.sub.2 at the cathode electrocatalyst 88a, and the reduced 
O.sub.2 concentration is monitored by the sensing cell 27 (89a, 87, 89b in 
FIG. 7B). As the O.sub.2 concentration in the test gas is known, the 
measured EMF can be converted to the O.sub.2 concentration between the 
pumping and sensing cells. By scanning the potential of the pumping cell 
from about 0 to 2 volts, a look-up table was made showing the O.sub.2 
concentration in the sensing cell associated with the pumping cell 
potential. The DPV experiment was conducted by the measuring the pumping 
cell 26, while the O.sub.2 concentration in the sensing cell was 
maintained at a predetermined value (i.e., approx. 1.5 V) by selectively 
pumping out O.sub.2. DPV measurements were carried out at the two 
different O.sub.2 pressures--about 0.5% and about 5%. 
The results of the regulation of O.sub.2 concentration are given in FIGS. 
14A and 14B. Specifically, FIG. 14A shows the current versus voltage 
behavior of the pumping cell 26 and particularly the reduction of O.sub.2 
pressure in the sensing cell 27 by electrochemically pumping out O.sub.2 
selectively with the pumping cell 26. The corresponding O.sub.2 pressure 
in the sensing cell 27 is presented in FIG. 14B. The O.sub.2 pressure 
noticeably decreases at the beginning of the gas-diffusion-limiting region 
(i.e., about 1.3 V). An electromotive force (EMF) of approximately 50 mV 
is equivalent to the tenfold O.sub.2 pressure difference at about 
740.degree. C. The O.sub.2 pressure difference is approximately two orders 
of magnitude at about 1.5 V, which reflects the O.sub.2 concentration of 
about 50 ppm in the sensing cell as the O.sub.2 concentration of the test 
gas is approximately 5000 ppm. 
The results of DPV experiment are shown in FIGS. 15A-15C. FIGS. 15A-15C 
show current versus NO.sub.x concentration of DPV measurement in the 
presence of 0.5% O.sub.2, with O.sub.2 concentration reduced by the 
O.sub.2 pumping cell 26. DPV currents measured at about 1.5 V (of sensing 
cell 27) were plotted with respect to NO.sub.x concentration. Deviations 
from the linear trend in FIGS. 15A-15C are mostly due to either the 
readout of peak height of current, or the gas-handling system purging the 
exhaust gas upward or both. The background current is comparable to that 
of the single-cell porous-type NO.sub.x sensor 22" shown in FIG. 13, which 
proves that the subtraction process of DPV alone actually eliminates the 
effect of coexisting gas on the signal-to-background ratio. According to 
the results, the double-cell porous-type NO.sub.x sensor 22"' (FIG. 7) 
does not have a significant advantage over the single-cell porous-type 
NO.sub.x sensor 22" (FIG. 6) as far as DPV is concerned. However, the 
double-cell porous-type NO.sub.x sensor 22"' is very useful when the 
electrocatalytic activity of the electrocatalyst is affected by the 
O.sub.2 pressure. Electrocatalytic activity in the reduced O.sub.2 
concentration is easily evaluated by the double-cell porous-type NO.sub.x 
sensor 22"'. 
3.0 Acheivements of the Present Invention 
3.1 DPV Experiment With Single-Cell Porous-Type NO.sub.x sensor 22" 
DPV can selectively monitor NO.sub.x in the presence of O.sub.2 with high 
precision. The concentration of background O.sub.2 does not significantly 
affect the NO.sub.x measurement because of the mathematical process 
associated with DPV of combining the first and second sample signals. With 
the specific setup in this experiment, the sensitivity of NO.sub.x 
detection down to a few hundred ppm could be obtained, but for this 
experiment, the sensitivity was limited by the data acquisition system 23 
(FIG. 1). A better electrocatalyst may also be necessary to improve 
sensitivity. The measurement of a few ppm of NO.sub.x is assuredly 
feasible with an improved electrocatalyst and data acquisition system 23. 
3.2 Electrocatalyst for Selective Detection of NO.sub.x in the Presence of 
O.sub.2 
Reduction of O.sub.2 and NO.sub.x on the Pt electrocatalyst occurs from 
zero potential. However, when the Au electrocatalyst is used, the 
reduction of NO.sub.x occurs at high potential (approximately 0.9 to 1.1 
V) with respect to the counter electrocatalyst, while the reduction of 
O.sub.2 occurs at low potential (approximately 0.3 V). When Au and Pt form 
a composite Au/Pt, the onset voltage of O.sub.2 reduction is lower than 
with Au alone, and the onset of NO.sub.x reduction appears to decrease. 
3.3 Experiment Results 
The combination of three components-DPV, Au electrocatalyst, and YSZ as the 
electrolyte material-has great promise in monitoring the low NO.sub.x 
concentration in the presence of O.sub.2. At least three important 
achievements were made during this experiment: (a) it was demonstrated 
that DPV can be used to measure a small amount of NO.sub.x in the presence 
of a much larger amount of O.sub.2 ; (b) it was found that the Au 
electrocatalyst can discriminate between the O.sub.2 and NO.sub.x 
reductions, and that the electrocatalyst can be, through materials 
engineering, further improved for the optimum operation of DPV; and (c) it 
was demonstrated that an NO.sub.x sensor 22 for use with DPV and having a 
ZrO.sub.2 -based electrolyte can be made using inexpensive fabrication 
processes. 
5. Conclusions 
DPV is a very useful technique for measuring a very low concentration of 
NO.sub.x. DPV reduces or completely eliminates the background current so 
as to enhance the resolution. With a proper electrocatalyst, DPV 
completely eliminates the effect of coexisting gases (e.g., O.sub.2). DPV 
with the Au electrocatalyst discriminates between the NO.sub.x and O.sub.2 
reductions, and the NO.sub.x measurement is not substantially affected by 
varying the O.sub.2 pressure from about 0.5% to about 5%. A few hundred 
ppm NO.sub.x were easily detected by the DPV measurement in this 
experiment, which can, if optimized, accurately measure a few ppm 
NO.sub.x. If the measured value is linearly extrapolated, it is possible 
to obtain a few tenths of a microampere (.mu.A) in a few ppm NO.sub.x 
concentration, which is easily measured by electronic instrumentation. 
The sensitivity can be further improved by (1) decreasing the resistance of 
electrocatalyst/electrolyte interface and (2) increasing the sampling 
speed of the data acquisition system 23. 
The major sensing components are YSZ, forming a stable material in exhaust 
gas. Its operational time is to about 5 to 10 years. An NO.sub.x sensor 22 
based on YSZ and for use with DPV can be miniaturized and manufactured at 
low cost. 
The present invention demonstrates the feasibility of an NO.sub.x sensor 22 
based on DPV and an O.sub.2 pumping cell, and demonstrates that the 
combination of the three components--DPV, an Au electrocatalyst, and a YSZ 
electrolyte--can be used to develop an NO.sub.x sensor 22 with optimized 
sensitivity and selectivity to NO.sub.x. 
It will be apparent to one of skill in the art that many variations and 
modifications may be made to the preferred embodiments as described above 
without substantially departing from the principles of the present 
invention. All such variations and modifications are intended to be 
included herein and within the scope of the present invention, as set 
forth in the following claims.