DOI 10.5162/IMCS2012/P2.6.5
Half-cell characterization of a novel NH3 gas sensor
1
1
2
1
Daniela Schönauer-Kamin , Maximilian Fleischer , Ralf Moos
Functional Materials, University of Bayreuth, 95440 Bayreuth, Germany
[email protected]
2
Siemens AG, Corporate Technology, 81739 Munich, Germany
Abstract:
The function of a novel electrochemical NH3 gas sensor for application in SCR-systems is
investigated. It provides a semi-logarithmic characteristic curve with a high NH3-sensitivity and
marginal NOx cross interference at 550 °C. The electrochemical cell of the sensor device can be
defined as Au | YSZ | Au, VWT. It is assumed that the sensing mechanism is based on nonequilibrium conditions (mixed potential theory) including electrochemical kinetics. This paper describes
the investigation of electrode potentials and polarization curves of the half-cells Au | YSZ and
Au, VWT | YSZ in dependence of NH3, which provides essential information about electrochemical
reactions at the three-phase boundary. All electrode potentials depend on reactive gas concentration,
whereby the electrode potential of the VWT-covered Au-electrode shows a stronger dependency on
the NH3 concentration. The formation of mixed potentials at both electrodes is confirmed. Additionally,
the influence of the VWT-catalyst coating on the sensing mechanism and the sensitivity is
demonstrated. The sensitivity increases with increasing coverage of the Au-electrode with VWT
catalyst. Voltage-current curves help to analyze the kinetics of electrochemical reactions at the TPB. A
clear shift in cathodic direction (to more negative potentials) can be observed with increasing NH3
concentration and the current increases at a fixed potential due to an enhanced electrochemical NH3
oxidation.
Key words: NH3 sensor, electrochemical cell, Au | YSZ, mixed potential, polarization curves, SCR
catalyst
Introduction
The introduction of the selective catalytic
reduction (SCR) system for the exhaust gas
aftertreatment of the NOx-emissions of Diesel
propelled vehicles requires novel sensors for
control and OBD purposes. NOx-sensors or
NH3-sensors would be appropriate candidates
to measure the NOx- or the NH3-concentration
downstream of the SCR-catalyst. In [1], the
control of the AdBlue dosing system by an
ammonia sensor is preferred. Different NH3
sensing principles are investigated for
application in harsh environments [2].
Promising seem solid electrolyte based sensors
with
varying
electrode
materials
and
configurations. Well-known examples for high
temperatures are lambda probes and the
amperometric NOx sensor [3]. Another
approach for detection of gas components like
CO and HC are mixed potential sensors [4].
A novel mixed potential type sensor following a
new concept for the functionalization of the
sensing electrode is introduced in [5]. The
planar sensor consists of an YSZ-electrolyte
and two Au-electrodes, whereby one Au-
electrode is covered by a porous SCR-catalyst
layer V2O5-WO3-TiO2 (VWT). Both electrodes
are exposed to the same gas atmosphere. The
potentiometric NH3-sensor provides a semilogarithmic characteristic curve. A high NH3sensitivity (88 mV / decade NH3) with a
marginal NOx-cross-sensitivity was shown at
550 °C. Initial investigations of the electrode
effects [6] confirmed the involvement of
electrochemical reactions, especially at the
catalyst-covered
electrode.
A
detailed
investigation of the electrode potential and
electrochemical kinetic with respect to mixed
potential formation is the focus of this work.
Half-cell setup
The half-cell probe, shown schematically in
Fig. 1, consists of an YSZ-substrate and two
screen-printed
electrodes.
As
reference
electrode, which is exposed to reference
atmosphere, Au or Pt is applied. The measuring
electrode consists of an Au layer covered by a
porous SCR-catalyst thick film.
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DOI 10.5162/IMCS2012/P2.6.5
The setup for half-cell measurements is
illustrated in Fig. 1. The reference electrode
(RE) is located in a constant reference
atmosphere whereas the sensing electrode
(SE) is exposed to the measuring gas. The
reference and the base measuring atmosphere
consist of 10 % O2, 6.5 % CO2 and 2.5 % H2O
in N2 balance. Varying NH3-concentrations (44 470 ppm) are added to the measuring gas. The
electrode potentials and polarization curves are
measured at 550 °C. The resulting changes in
the signal can be ascribed to effects on the
sensing electrode because only the gas
composition on the measuring side varies. As
sensing electrodes, Au and Au covered partially
or completely with VWT-catalyst are applied.
The electrode potentials are measured as a
potential difference to the reference electrode
by a digital multimeter. Polarization curves are
conducted by a potentiostat in a two electrode
setup. Therefore potential steps were applied to
the sensing electrode and the resulting current
was measured.
chamber furnace
steel cylinder
Messgas
Al2O3-tube
Pt
Al2O3-Rohr
YSZ
measuring
atmosphere
porous catalyst film
reference
atmosphere
Au- / Pt-electrode
sealing
Schematic setup for half-cell measurements with half-cell probe “measuring gas, sensing electrode (SE),
VWT, Au | YSZ | Au, reference electrode (RE), reference gas”.
Electrode potentials
Fig. 2 illustrates the potential differences
∆U = UNH3 - Ubase gas for the three sensing
electrode configurations Au, Au, VWTpartially and
Au, VWTcompletely in dependence of cNH3
measured in the half-cell setup with reference
to the platinum RE. The measurements are
conducted on the same half-cell probe, where
subsequentially VWT was added. The
measured voltage differences can be ascribed
to processes at the sensing electrode. Each
electrode configuration shows the typical semilogarithmic behavior whereby the NH3sensitivity m increases clearly with the VWTcoverage of the Au-electrode. The signals are
stable and reproducible.
Interestingly, even the pure Au-electrode
displays an electrode potential - depending on
the
NH3
concentration
with
m = 23.3 mV / decade. The electrochemical
activity of Au can not be neglected [7] and it can
be assumed that electrochemical reactions at
the TPB are responsible for the voltage shift.
The VWT-catalyst layer strongly enhances the
voltage difference. A higher electrode potential
is observed and increases with the VWTcoverage. The VWT-layer seems to be
responsible for the strong sensor effect and for
the shift of the electrode potential.
140
Au,VWTcompletely
120
100
∆U / mV
Fig. 1.
m = 86,7 mV/decade
m = 81,1
80
Au,VWTpartially
60
m = 52,1
40
20
0
10
m = 51,9
Au
m = 23,3
100
cNH / ppm
1000
3
Fig. 2. Electrode potentials of the
Au | YSZ and Au, VWT | YSZ vs. cNH3.
half-cells
It is assumed that two competing electrochemical reactions, the oxidation of NH3 with
oxygen ions and the reduction of O2 proceed at
the three-phase boundary (TPB) and establish
a mixed potential. Possible explanations for the
VWT-effect could be changing electrochemical
reaction rates due to the catalytic properties of
VWT or a changed gas composition at the TPB
due to heterogeneous catalysis at the VWT
layer (NH3 oxidation with gaseous O2).
The investigation of the electrode potentials of
Au and Au, VWTcompletely in dependence on O2
concentration indicates that both electrodes
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DOI 10.5162/IMCS2012/P2.6.5
Polarization curves
Voltage-current curves of the half cells Au | YSZ
and Au, VWT | YSZ are plotted in Fig. 4 and 5.
A voltage from -125 to 100 mV (25 mV steps à
300 sec) was applied to the SE and the
resulting current between SE and RE was
measured. Fig. 4 demonstrates the NH3dependency of the Au | YSZ half cell. The
electrode potential slightly shifts in cathodic
direction to more negative potentials whereas
the shapes of the curves remain unchanged. At
a fixed potential a slight increase of the current
with increasing NH3 concentration can be
measured. This suggests that more charge is
transferred due to generated electrons during
electrochemical NH3 oxidation.
40 m = 64,7
m = 33,9
20 m = 34,1
0
100
-50
300
200
I / nA
100
RE, Pt/YSZ/Au, SE
100 150
Fig. 4. Polarization curves for half-cell Au | YSZ in
dependence of cNH3.
In contrast, the effect of NH3 on the
Au, VWT | YSZ half cell characteristic (Fig. 5) is
more pronounced. The VWT-catalyst clearly
changes the characteristics and influences the
polarization behavior. A strong shift of the
cNH / ppm
Au
1000
470 ppm
230 ppm
180 ppm
90 ppm
44 ppm
lean gas
0
-100
0
50
U / mV
m= 0
electrode potential to negative values in
cathodic direction and a changed curve shape
is visible. This indicates the formation of a
mixed potential and differences in the
electrochemical kinetic. These effects and the
observed higher current can be attributed to the
electrochemical NH3-oxidation at the TPB
Au, VWT | YSZ, which is supported probably by
the catalytic active VWT-layer. Additionally, the
adsorbed species and their activity could be
affected by the VWT film and affects the
electrode potential, too [8].
-150 -100 -50
-100
-150 -100 -50
Au,VWTpartially
Fig. 3. Corresponding sensor signal of the half cell
electrode potentials corrected by the NH3dependency of UAu, giving a simulation of the
behavior of a full sensor
3
0
cNH increases
I / nA
50
60
3
230 ppm
44 ppm
lean gas
100
m = 69,2
80
3
The sensor signal decreases as a consequence
of the electrochemical NH3-activity of the half
cell Au | YSZ. The results confirm the
dependency of the electrode potentials on cNH3
and the higher electrochemical activity of the
VWT-covered electrode. The sensitivities of the
half-cell probes are in good accordance with
sensor results.
Au,VWTcompletely
100
cNH increases
The O2 effect is less pronounced than the NH3
signal and is not affected by the catalyst layer.
Concerning the planar sensor device, in which
both electrodes are exposed to sensing gas,
the sensor signal correlates to the difference
between the electrode potential of the Au (UAu)and the Au, VWT (UAu,VWT)-electrode (see
Fig. 3) The characteristic curves in Fig. 3 are
corrected with the NH3-dependency of UAu.
120
UAu,VWT - UAu / mV
show Nernstian behavior (not shown here). The
resulting slope (-39 mV / decade O2) agrees
with the calculated Nernst response at 550 °C.
RE, Pt/YSZ/Au,VWT, SE
0
50
U / mV
100 150
Fig. 5. U-I-curves for half-cell Au, VWT | YSZ in
dependence of cNH3. The different behaviour of the
half-cells can be ascribed to the VWT-catalyst
coating and differences in mixed potential formation.
The shift in cathodic direction can be explained
by the standard electrode potential of NH3
oxidation in equilibrium. In [8], a value of
-1.18 V is calculated from thermodynamic data
at 623 °C related to the electrode reaction with
O2 (0 V for 1 bar).
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DOI 10.5162/IMCS2012/P2.6.5
The reaction rates are characterized by the
exchange current and can be analyzed by
Butler-Volmer theory. The polarization curves
represent the kinetic behavior of the electrode
reactions and therefore a further investigation is
required. For a more detailed investigation it
would be necessary to clearly separate NH3Oxidation and O2-reduction reaction, which is
not realistic.
Conclusions
The half-cell measurements of Au | YSZ and
Au,VWT | YSZ demonstrate that both electrode
potentials depend on cNH3 and are shifted in
cathodic direction. The NH3-sensitivities are in
good accordance with sensor results in [5] and
allow deeper understanding of characteristics of
this sensor. UAu,VWT is strongly influenced by
NH3-exposure and dominates the sensor
characteristic. The potential shift confirms the
establishment of mixed potentials at both halfcells. Additionally, the electrochemical kinetic
seems to be affected by the VWT-catalyst layer.
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