SOLID-STATE POTENTIOMETRIC SENSORS FOR NITROGEN MONOXIDE, NITROGEN DIOXIDE, AND CARBON MONOXIDE
By
BRIGGS MCKENNEY WHITE
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
2007
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© 2007 Briggs Mckenney White
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To My Parents
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ACKNOWLEDGMENTS
A researcher develops only if those around him and those that have passed before him are of ample quality and agreeable predisposition. I thank Vincenzo Esposito, Suman Chatterjee, and Keith Duncan, for their help, advice, and inspiration. I thank Bryan Blackburn, Martin Van
Assche, and Laure Chevallier for their enthusiasm and support in the laboratory. I acknowledge the efforts of the many students in the research groups at the University of Florida and The
University of Rome “Tor Vergata” for making this Ph.D. enjoyable and productive.
I thank my advisor, Eric Wachsman, for the resources, motivation, recognition, advice, and responsibility that helped me to develop as a scientist and thinker. I also thank my Italian co- advisor, Enrico Traversa, for his friendship, scientific philosophy, and technical instruction. Both
Eric Wachsman and Enrico Traversa afforded me with the possibility to do many great things in the laboratory and also to go learn and speak in Lindau (Germany), Quebec City (Canada),
Genoa (Italy), Villa Adrianna (Italy), Los Angeles, and Cancun (Mexico). I thank my committee of Mark Orazem, Anthony Brennen, Amelia Dempere, and Daryll Butt for their scientific/technical discussions.
I acknowledge the Major Analytical Instrumentation Center, the Engineering Research
Center, and the Italian Center of National Research at Tor Vergata for use of the facilities.
Several researchers directly assisted me by either preparing materials or executing experiments on my behalf. Sean Bishop, Eric Armstrong and Alberto Reiner pressed the
Zirconia pellets used in the three electrode cells (Chapter 6). Martin Van Assche and Jiho Yoo completed all temperature programmed desorption (TPD) and temperature programmed reaction
(TPR) experiments (Chapter 4). Suman Chatterjee participated heavily in all phases of the
La2CuO4 synthesis, La2CuO4 sensor fabrication and sensor response (OCP) characterization
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(Chapter 3). Eric Macam fabricated and characterized the sensors that were prepared with the auto-ignition fuel-lean La2CuO4 and the co-precipitated La2CuO4 (Chapter 3).
A researcher, like any professional, needs a clear view of reality and a strong sense of self- confidence so that he/she can rationally select ambitious goals from the multitude available without fear of failure. To my parents, I owe a debt of gratitude for their persistant love, guidance, and support. They assisted me more than anyone else in the development of my personal skill set which ultimately enabled me to achieve this doctoral degree. I thank Tony,
Kerry, Dan, and John for opening their hearts and home to me. They made my last year in
Gainesville my most productive and enjoyable.
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TABLE OF CONTENTS
Page
ACKNOWLEDGMENTS ...... 4
LIST OF TABLES...... 9
LIST OF FIGURES ...... 10
ABSTRACT...... 16
CHAPTER
1 INTRODUCTION ...... 18
1.1 Why Do We Need Sensors for NO or NO2?...... 18 1.2 Why Use Solid-State Potentiometric NOX Sensors? ...... 19 1.3 Why Aren’t We Already Using Solid-State Potentiometric Sensors to Monitor NOX? ...... 20 1.4 Summary...... 20
2 BACKGROUND ...... 22
2.1 Nernstian and Non-Nernstian Sensors...... 22 2.2 Sensor Materials ...... 24 2.3 Background Information on La2CuO4 ...... 28 2.4 Homogeneous Gas Phase NOX and CO Chemistry...... 30 2.5 Sensing Mechanisms ...... 32 2.5.1 Introduction ...... 32 2.5.2 Mixed Potential Theory Discussion ...... 33 2.6 Summary...... 36
3 EFFECT OF ELECTRODE MICROSTRUCTURE ON SENSITIVITY AND RESPONSE TIME...... 44
3.1 Introduction...... 44 3.2 Experimental...... 47 3.2.1 Powder Synthesis...... 47 3.2.2 Powder Characterization ...... 48 3.2.3 Sensor Fabrication...... 48 3.2.4 Electrical Characterization ...... 50 3.3 Results and Discussion ...... 50 3.3.1 Powder Synthesis...... 50 3.3.2 Effect of Synthesis Technique on Sensor Electrode Microstructure...... 51 3.3.3 Effect of Sintering Temperature and Time on Sensor Electrode Microstructure...51 3.3.4 Open-Circuit Potential Responses to Changes in NO Concentration ...... 52 3.4 Summary...... 56
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4 EVIDENCE THAT NOX AND CO SHIFT THE FERMI LEVEL OF La2CuO4 ...... 66
4.1 Introduction...... 66 4.2 Background...... 66 4.3 Experimental...... 68 4.3.1 Powder Preparation and Characterization ...... 68 4.3.2 Potentiometric Sensor Fabrication and Voltage Monitoring...... 69 4.3.3 Resistance-type Sensor Fabrication and Electrical Characterization ...... 69 4.3.4 Mass Spectrometry ...... 70 4.4 Results...... 70 4.4.1 DTA-TGA, X-Ray Diffraction and FE-SEM Results of Calcined La2CuO4 Powder ...... 70 4.4.2 Potentiometric Sensor...... 72 4.4.3 Resistance-type Sensor...... 73 4.5 Summary...... 75
5 INVESTIGATION OF SENSORS WITH ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY...... 87
5.1 Introduction...... 87 5.2 Experimental...... 89 5.3 Results and Discussion ...... 90 5.3.1 High Frequency ...... 90 5.3.1.1 La2CuO4 conduction mechanism ...... 92 5.3.1.2 Effect of NOX on La2CuO4 surface conductivity ...... 93 5.3.2 Low Frequency...... 96 5.4 Summary...... 97
6 EFFECT OF NO AND NO2 ON THE POLARIZATION RESISTANCE AND POTENTIOMETRIC RESPONSES OF La2CuO4 AND Pt ELECTRODES ...... 106
6.1 Introduction...... 106 6.2 Luggin’s Probe Configuration ...... 110 6.3 Experimental...... 111 6.3.1 Sample Preparation...... 111 6.3.2 Electrical Characterization ...... 112 6.4 Results...... 112 6.4.1 EIS Measurements Part 1 ...... 112 6.4.2 Potentiometric Characterization ...... 114 6.4.3 D.C. Polarization Measurements...... 116 6.4.4 EIS Measurements Part 2 ...... 121 6.4 Summary...... 123
7 CONCLUSIONS ...... 134
APPENDIX OPTIMIZATION OF SCREEN-PRINTING INK CONTAINING La2CuO4 ...... 137
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LIST OF REFERENCES...... 139
BIOGRAPHICAL SKETCH ...... 158
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LIST OF TABLES
Table Page
2-1 Publications relevant to several materials commonly used as gas sensing elements. Common applications for the materials other than as sensing elements for a resistance-type or potentiometric gas sensors are also listed...... 38
2-2 Factors determining steady-state current resulting from the electrochemical reaction of a pollutant gas on a sensor electrode...... 42
3-1 Characteristics of La2CuO4 powders prepared with different synthesis techniques...... 58
3-2 La2CuO4 synthesis techniques with physical and electrical characterization results...... 65
6-1 D.C. and A.C. results for La2CuO4 and Pt in 3% O2 at 500°C...... 129
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LIST OF FIGURES
Figure Page
2-1 La2O3-CuO binary phase diagram...... 39
2-2 Four-point resistance measurement of La2CuO4 in air as a function of temperature...... 39
2-3 Equilibrium gas composition calculated using thermodynamics software (FACTSAGE) for combustion exhaust containing 3% H2O, 15% CO2, 3% O2 and 650 ppm NO...... 40
2-4 Equilibrium constants for NO+1/2O2=NO2 and NO=N2O+1/2O2...... 41
2-5 Mixed-potential schematics of current-overpotential characteristics. A) Theoretical current-voltage curves for Pt and Au in air and air + NO with a logarithmic current dependence on electrode potential. B) Theoretical current-voltage curves for an electrode in air and in air + NO at two different concentrations assuming a linear current dependence on electrode potential and a mass transfer limited current at high overpotentials...... 43
3-1 Potentiometric NOX sensor configuration: front view (left) and side view (right)...... 57
3-2 B.E.T. surface area and X-ray diffraction results for La2CuO4 powders synthesized with different synthesis routes. The powders synthesized with the co-precipitation and Pechini techniques were calcined at 650°C for 10 h and the powders synthesized with the two auto-ignition techniques were calcined at 600°C for 10 h. A) Specific surface area for each powder. B) X-ray diffraction pattern for each powder...... 57
3-3 FE-SEM images of La2CuO4 powder prepared by calcining precursors which were synthesized with different techniques. A) Powders prepared by calcining (600 °C / 10 h) precursors synthesized with the auto-ignition, fuel-rich process. B) Powders prepared by calcining (600 °C / 10 h) precursors synthesized with the auto-ignition, fuel-lean process. C) Powders prepared by calcining (650 °C / 10 h) precursors synthesized with the co-precipitation techniques. D) Powders prepared by calcining (650 °C / 10 h) precursors synthesized with the Pechini method...... 58
3-4 FE-SEM images of electrode microstructures that were fabricated by screen-printing La2CuO4 powders synthesized with four different techniques. All samples were sintered at 800°C for 10 h. A) Synthesized with the auto-ignition, fuel-rich process. B) Synthesized with the auto-ignition, fuel-lean process. C) Synthesized via co- precipitation. D) Synthesized with the Pechini method...... 59
3-5 Electrode microstructures consisting of co-precipitated La2CuO4 powders which were screen-printed and then sintered for 10 h at various temperatures...... 60
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3-6 FE-SEM images of electrodes fabricated by screen-printing La2CuO4 powders synthesized with four different techniques and sintered at different temperatures. A) Powders synthesized with the fuel-lean, auto-ignition technique, electrode sintered at 700°C for 10 h. B) Powders synthesized with the fuel-lean, auto-ignition technique, electrode sintered at 900°C for 10 h. C) Powders synthesized with the fuel-rich, auto-ignition technique, electrodes sintered at 700°C for 10 h. D) Powders synthesized with the fuel-rich, auto-ignition technique, electrode sintered at 800°C for 18 h. E) Powders synthesized with the Pechini method, electrode sintered at 700°C for 10 h. F) Powders synthesized with the Pechini method, electrode sintered at 800°C for 18 h...... 61
3-7 Sensor responses to step changes in NO concentration in a background of simulated combustion exhaust at A) 450°C and B) 500 ºC. The sensors were fabricated with La2CuO4 powders synthesized with four different techniques and all sintered at 800°C for 10 h...... 62
3-8 Sensor responses to step changes in NO concentration in a background of simulated combustion exhaust at various temperatures. The sensors were fabricated with La2CuO4 powders synthesized with A) fuel-lean and B) fuel-rich variations of the auto-ignition techniques and the C) co-precipitation and D) Pechini methods...... 63
3-9 Voltage change and response time at 450°C (Figures A and C) and 500°C (Figures B and D) of sensors fabricated from powders prepared through different techniques and sintered at 800°C for 10 h...... 64
3-10 NO response times for potentiometric sensors operating at 500°C that were sintered with different isothermal dwell temperatures (indicated in graph) and exposed to step changes in NO concentration in simulated combustion exhaust. The sensors were fabricated with La2CuO4 powders synthesized via co-precipitation...... 65
4-1 Simultaneous DTA-TGA measurement of dried XST precursor in air...... 77
4-2 X-Ray diffraction pattern of calcined La2CuO4 powder between 25°C and 700°C...... 78
4-3 Detail X-ray diffraction pattern of calcined La2CuO4 showing tetragonal- orthorhombic transformation between 25°C and 250°C...... 79
4-4 La2CuO4 lattice parameters calculated from X-ray diffraction and neutron diffraction data as a function of temperature. A) Lengths of the A-axis and the B-axis as a function of temperature for La2CuO4 in air. B) Length of the C-axis as a function of 201 temperature for La2CuO4 in air. Low temperature data from Campi et. al...... 79
4-5 Tetragonal La2CuO4 unit cell schematic. Local CuO6 structure is represented by green polygons, Oxygen atoms are in red and Lanthanum atoms are in blue...... 80
4-6 Unit cell volume of La2CuO4 as a function of temperature calculated from diffraction data collected in air. Low temperature data from Campi et. al.201...... 80
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4-7 FE-SEM image of a La2CuO4 electrode sintered at 800°C for 10 h...... 81
4-8 FE-SEM image of the cross-section of a typical prototype potentiometric NOX/CO sensor based on La2CuO4. The sensor was screen-printed at sintered at 800°C for 10 h. A) The Pt|YSZ interface. B) The YSZ|La2CuO4 interface...... 81
4-9 Potentiometric response of a La2CuO4|YSZ|Pt sensor in the presence of NO and NO2....82
4-10 Conductance of porous La2CuO4 as a function of temperature in air...... 82
4-11 Nyquist plot of the impedance measured for porous La2CuO4 in various oxygen partial pressures at 500°C...... 83
4-12 Bode plot of the real impedance (Z’, left Y-axis) and imaginary impedance (Z”, right Y-axis) as a function of measurement frequency of porous La2CuO4 in air at 500°C...... 83
4-13 Conductance of porous La2CuO4 in various oxygen partial pressures between 483°C and 665°C...... 84
4-14 Normalized resistance of porous La2CuO4 upon exposure to step changes in NO concentration with a background of 3% O2 (balance N2) at A) 450-500°C and B) 550-650°C...... 84
4-15 NO adsorption/desorption and heterogeneous catalysis results for La2CuO4 powder. A) Results from a NO + O2 Temperature Programmed Reaction. B) Results from a 1% NO Temperature Programmed Desorption...... 85
4-16 Normalized resistance of porous La2CuO4 upon exposure to step changes in NO2 concentration with a background of 3% O2 (balance N2) at A) 450-500°C and B) 550-650°C...... 85
4-17 NO2 adsorption/desorption and heterogeneous catalysis results for La2CuO4 powder. A) Results from a 400 ppm NO2 Temperature Programmed Reaction. B) Results from a 1000 ppm NO2 Temperature Programmed Desorption...... 86
4-18 Steady-state responses of La2CuO4-based potentiometric and resistance-type sensors plotted as a function of pollutant concentration. A) Steady-state potentiometric sensor responses to NO, NO2, CO, CO2, and O2 at 450°C. B) steady-state normalized resistance changes in NO, NO2, CO and O2 at 600°C...... 86
5-1 Nyquist plot of a potentiometric sensor and two symmetrical cells measured at 600°C in 3% O2 (balance N2)...... 99
5-2 Arrhenius plot of the high frequency real-axis intercepts for each of the three samples measured in 3% O2 (balance N2)...... 99
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5-3 DC surface emphasized La2CuO4 conductance plotted together with the AC La2CuO4 component calculated from La2CuO4 and Pt symmetrical cells and multiplied by 5 for clarity...... 100
5-4 Nyquist plot showing the effect of 200 ppm of either NO or NO2 on the high frequency impedance response for each of the three cells at 500°C...... 100
5-5 Impedance of a single La2CuO4 electrode in 200 ppm NO or NO2 normalized to the impedance in the absence of NOX calculated from impedance data measured with a La2CuO4|YSZ|La2CuO4 sample in a background of 3 % O2 (balance N2). Sensor (La2CuO4|YSZ|Pt) OCP response data to 200 ppm of NO and NO2 is included...... 101
5-6 The real component of complex impedance measured at 100 kHz for a La2CuO4|YSZ| La2CuO4 cell. NO (closed symbols) and NO2 (open symbols) concentrations were individually stepped every 5 minutes at 450°C, 500°C and 550°C...... 102
5-7 Real component of complex impedance associated with surface conduction on La2CuO4 when exposed to various NO and NO2 concentrations at 450°C, 500°C, and 550°C normalized to its initial value when unexposed to NOX. Impedance measured at 100 kHz in a background of 3% O2 (N2 bal.)...... 103
5-8 Nyquist representation of impedance data obtained for a potentiometric NOX sensor (La2CuO4|YSZ|Pt)and two symmetrical cells (Pt|YSZ|Pt and La2CuO4|YSZ|La2CuO4) measured at 500°C in 3% O2 (balance N2)...... 104
5-9 Nyquist representation of the impedance data obtained for a potentiometric NOX sensor (La2CuO4|YSZ|Pt) at 500°C in different NO gas concentrations and a background of 3% O2 (balance N2)...... 105
6-1 Idealized current-potential schematics illustrating method for constructing quantitative mixed potential diagrams. A) Representation of process used to calculate the current resulting from the electrochemical oxidation of a pollutant gas. The two lines with data points represent D. C. polarization measurements made in a (1) base gas containing O2 and a (2) base gas plus some pollutant gas. B) The x calculated current-potential characteristics for the Red/Ox and the O2/OO redox couples...... 124
6-2 Luggin’s Probe configuration for solid-state electrochemical measurements...... 124
6-3 Impedance of a sample with all electrodes made of porous Pt measured at 800°C in stagnant air in two-point and three-point configurations. The two-point spectrum was calculated by summing the two three-point spectra...... 125
6-4 Impedance of a sample with a Pt electrode and a La2CuO4 electrode measured at 700°C in stagnant air in two-point and three-point configurations. The two-point spectrum was calculated by summing the two three-point spectra...... 125
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6-5 Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. In separate experiments, the sensor was exposed to step changes in A) CO2 concentration and B) O2 concentration, both at 500°C with a background 3% O2 concentration (N2 balance)...... 126
6-6 Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. The sensor was exposed to step changes in NO concentration at 500°C with a background of 3% O2 (N2 balance). A) Potential differences as a function of time. B) Steady-state potential differences as a function on NO concentration...... 126
6-7 Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. The sensor was exposed to step changes in CO concentration at 500°C with a background of 3% O2 (N2 balance). A.) Potential differences as a function of time. B) Steady-state potential differences as a function on NO concentration...... 127
6-8 Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. The sensor was exposed to step changes in NO2 concentration at 500°C with a background of 3% O2 (N2 balance). A.) Potential differences as a function of time. B) Steady-state potential differences as a function on NO2 concentration. Data collected with a sensor (La2CuO4|YSZ|Pt) fabricated with a 0.1 mm thick YSZ sheet is included for comparison...... 127
6-9 Current-potential characteristics measured with a La2CuO4 WE in 0-800 ppm of NO2 and a background of 3% O2 (N2 balance) at 500°C. A) Actual measured data with interpolated line fits. Potentials were IRYSZ subtracted using RYSZ from impedance measurements at 0 V bias. B) Current-potential diagram constructed by analysis of the polarization data (part A)...... 128
6-10 Current-potential diagram constructed by analysis of polarization data measured on A) a La2CuO4 electrode and B) a Pt electrode at 500°C in 0-800 ppm of NO2 and a background of 3% O2 (N2 balance)...... 128
6-11 Current-overpotential diagrams for a Pt electrode. A) Low overpotential results. B) High overpotential results...... 129
6-12 Current-overpotential diagrams for a La2CuO4 electrode. A) Low overpotential results. B) High overpotential results...... 130
6-13 Nyquist plot for a Pt electrode at 0V DC bias exposed to various concentrations of NO...... 130
6-14 Nyquist plot for a La2CuO4 electrode at 0V DC bias exposed to various concentrations of NO...... 131
6-15 Nyquist plot for a Pt electrode at 0V DC bias exposed to various concentrations of NO2...... 131
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6-16 Nyquist plot for a La2CuO4 electrode at 0V DC bias exposed to various concentrations of NO2...... 132
6-17 Plot of log |Zj| vs. log frequency for a Pt electrode at 0V DC bias exposed to various concentrations of NO...... 132
6-18 Normalized polarization resistance of La2CuO4 and Pt in various NO and NO2 concentrations and a background of 3% O2 (N2 balance) at 500°C...... 133
A-1 SEM image of electrode prepared by screen-printing 16 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X...... 137
A-2 SEM image of electrode prepared by screen-printing 17 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X...... 137
A-3 SEM image of electrode prepared by screen-printing 18 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X...... 137
A-4 SEM image of electrode prepared by screen-printing 19 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X...... 138
A-5 SEM image of electrode prepared by screen-printing 20 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X...... 138
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Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy
SOLID-STATE POTENTIOMETRIC SENSORS FOR NITROGEN MONOXIDE, NITROGEN DIOXIDE, AND CARBON MONOXIDE
By
Briggs Mckenney White
May 2007
Chair: Eric D. Wachsman Major: Materials Science and Engineering
There is currently a large need for the development of solid-state potentiometric sensors for monitoring the NOX emanating from combustion processes. In an effort to better understand the phenomena behind the sensor response, sensors of the form: La2CuO4|YSZ|Pt were investigated. These sensors showed promising results.
The effect of electrode microstructure on sensor response time and sensitivity was investigated. Electrodes that have high surface/bulk ratios respond faster and are more sensitive than electrodes with low surface/bulk ratios. They are also stable at lower operating temperatures where signals are inherently larger.
The resistance of La2CuO4, a p-type semiconductor, was measured as a function of various pollutants to verify that the possibility that Fermi level changes are responsible for the potentiometric response of the sensor. The resistance of La2CuO4 and the potential of
La2CuO4|YSZ|Pt sensors have similar dependencies on temperature and NOX/CO concentration.
This correspondence along with temperature-programmed desorption results suggest that the adsorption/desorption of NOX/CO on/from La2CuO4 shifts the Fermi level and is measured by the electrometer as the electrode potential.
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The electrochemistry at Pt and La2CuO4 electrodes in NOX environments was investigated
with D.C. polarization and electrochemical impedance spectroscopy. The polarization resistance
values for both electrodes decreased with increasing concentrations of NO and NO2. Theoretical
considerations and experimental approaches were refined concerning three-electrode polarization
measurements in the solid-state.
Using an independent reference electrode, the sensor response was shown to be a
composite of the individual electrode responses. In the case of O2, the potentials of both
electrodes change significantly; however, they do so in identical fashion and the net sensor signal
is insignificant to O2. The sensor responds to NO, NO2, and CO because the Fermi level of the
La2CuO4 shifts when they adsorb on it. This contribution to the sensor response is often
overlooked; however, in the case of La2CuO4-based sensors it is not only a contributor to the response, it dominants the response. In other words, the sensor response is the net difference between the sums of the electrode potential contributions (heterogeneous catalysis,
adsorption/desorption, and electrochemical reactions) or in other words, the “Differential
Electrode Equilibria.”
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CHAPTER 1 INTRODUCTION
1.1 Why Do We Need Sensors for NO or NO2?
Devices capable of monitoring the concentrations of NO or NO2, collectively known as
NOX, are greatly needed to maintain and improve air quality, especially in urban areas. The
currently-available NOX sensors do not meet the requirements of the transportation industry, the
largest producer of NOX. NOX reacts with volatile organic compounds in the presence of
sunlight to form tropospheric, low-level, Ozone (O3). NOX is a key component in photochemical smog. O3 and smog, which emanate from dense vehicular traffic and stationary combustion
sources, cause respiratory ailments such as bronchitis and asthma. NOX also reacts with
atmospheric moisture to form acids which are easily transported by weather systems from urban
1 areas to rural locations where they are deposited as acid rain . If left alone, NOX levels are only
likely to increase.
The largest source of NOX is the transportation industry (53%), followed by electric
utilities (25%)2. The number of highway miles driven annually is increasing and in an effort to
increase fuel-efficiency, combustion is being progressively shifted toward fuel-lean (excess O2)
conditions. More air is used for fuel-lean combustion than for stoichiometric combustion;
therefore, more N2 and O2 are present, which ultimately react, resulting in higher levels of
emitted NOX. High fuel prices and global warming will shift combustion even leaner and
considering the increasing number of drivers on the road, NOX concentrations will persistently
rise. In fact of all the EPA’s criteria pollutants, NOX is the only one which has increased despite
regulation efforts beginning with the passing of the clear air act in 19701.
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NOX sensors could enable technologies which can reduce NOX such as selective catalytic
reduction. They could also indicate to the end-user exactly how much NOX he/she is producing,
thereby increasing awareness and accountability.
1.2 Why Use Solid-State Potentiometric NOX Sensors?
Solid-state potentiometric sensors, in particular, are the most promising candidates for real-
time monitoring of NOX in lean exhaust (1-15% O2) at high temperatures (300°C to 800°C).
They have suitable NOX sensitivities (mV/ppm NOX) and low enough detection limits (below 1
ppm) for immediate application. By tailoring the sensor fabrication route and optimizing the
sensor’s operating temperature, small response times (<1 min) were observed. The sensors can
also respond reproducibly to changes in NOX concentration changes over large periods of time.
Finally, they are simple to fabricate and cheap making them good candidates for
commercialization.
Potentiometric sensors do not need a power source or complicated electronics like
amperometric sensors for NOX sensing; however, they do need local heating, which requires
electrical power not unlike the well-established O2 sensor. In fact, solid-state potentiometric
NOX sensors are based on the same materials as the O2 sensor. They can also withstand the high
temperatures, vibrations and corrosive agents found in vehicular exhaust. Solid-state
potentiometric sensors are insensitive to the vacillations of either O2 or CO2 found in transportation-related exhaust.
They can be fabricated in planar and tubular designs with or without both electrodes on the
same surface facing the exhaust. Planar designs are conducive to screen-printing and tape-
casting, which are both cheap and scalable. Photoresist/etching techniques for miniaturization
are also more easily realized in a planar design as opposed to tubular designs currently employed
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for the traditional O2-sensor. Finally, planar designs lend themselves to multi-layer technology
enabling the integration of heating elements, temperature sensors and electrical conductors.
1.3 Why Aren’t We Already Using Solid-State Potentiometric Sensors to Monitor NOX?
There are no commercially-available NOX sensors which can selectively sense NO or NO2.
There are some resistive sensors which can sense NOX; however, they are also sensitive to O2,
CO, humidity and just about everything else in the exhaust. In fact, all NOX sensors for high
temperature exhaust application lack the required selectivity. Many researchers reported high
NOX sensitivities for particular sensors and then found in later experiments that the sensors were
also strongly sensitive to at least one of the other gases in exhaust. In response, many materials
were used as sensing electrodes for potentiometric, resistance-type, and amperometric sensors
and several theories were subsequently postulated. Unfortunately, the gas-material-response
relationship is not adequately understood for the practical selection of sensor parameters (e.g.,
materials, microstructure and operating temperature) to improve selectivity.
1.4 Summary
There is currently a large and growing need for NOX sensors. Of the candidate
technologies available, solid-state potentiometric sensors show the most promise for fuel-lean combustion exhaust monitoring at high temperatures. However, solid-state potentiometric sensors lack the required selectivity; and no alternative is clearly envisioned at present due to the current inadequate understanding of the sensing mechanism(s). In this work, this lack of understanding was addressed by quantifying the fundamental properties of the electrode materials and their interactions with NOX, CO, CO2, O2 and humidity. To this same end,
Differential Electrode Equilibria, a theory detailing the generation of electrode potentials by pollutant species, was better defined. In clarifying several key points, this research has enabled progress toward the commercialization of a multifunctional exhaust sensor, which can measure
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the concentration of O2, CO, NO and NO2 simultaneously. This device provides information to on-board diagnostic systems enabling new NOX reduction technologies and improving the performance of catalytic converters.
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CHAPTER 2 BACKGROUND
2.1 Nernstian and Non-Nernstian Sensors
Sensors based on solid-electrolytes have been used to measure the concentration of O2 in
3-10 gaseous mixtures since about 1950 . They consist of two metal electrodes separated by an O2- ion conductor, typically Yittria-stabilized-Zirconia (YSZ). One of the electrodes is exposed to an invariant atmosphere with a known concentration of O2, usually air, and the other electrode is
exposed to the gas mixture containing an unknown concentration of O2. This electrode exposed to the unknown gas mixture is often called a “sensing electrode” or an “exhaust electrode.” The potential difference or voltage measured between the two electrodes is related to the concentration of O2 at each electrode by Equation 2-1, the Nernst Equation.
RT [O (Electrode1)] ΔV = ln 2 (2-1) 4F [O2 (Electrode2)]
The Nernst Equation coupled with an invariant reference electrode and a voltage
measurement between the two electrodes are all that is needed to calculate the concentration of
O2 in the unknown gas mixture.
These sensors are also known as λ-sensors, “lambda sensors,” because they are most
commonly used to measure air/fuel ratios, usually denoted by a “λ.” When the air/fuel ratio is
14.5 by mass at the stoichiometric point, the fuel is burnt stoichiometrically with the oxygen in the air. If the air/fuel ratio is less than 14.5, the combustion is called “fuel-rich” and the sensor registers a voltage around 100-200 mV. Conversely, if the air/fuel ratio is greater than 14.5, the combustion is called “fuel-lean” and the sensor registers a voltage greater than 1 V. The change in voltage that occurs at the stoichiometric point ideally is a step function, making it possible to quickly gage if combustion is fuel-rich or fuel-lean. However, an oxygen sensor only registers
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the correct estimate of the O2 concentration in a gas mixture, such as automotive exhaust, if the
exhaust has come to equilibrium at the electrode surface. As a result, the majority of efforts to
improve λ-sensors have centered on the replacement of the exhaust electrode with more catalytic
metals.
Non-catalytic electrodes exposed to non-equilibrium gas mixtures register potentials which
are shifted from their Nernstian values8,11-28. Fleming16 first reported evidence of this “non-
Nernstian” phenomenon in 1977. He reported that a few thousand ppm of CO could shift the potential of an electrode down 100-200 mV relative to a reference electrode16,29. This is rather
3 remarkable considering that the concentration of O2 in fuel-lean exhaust is between 1 and 21% .
A change between 1 and 15% O2 on a Pt electrode shifts the potential ~30 mV. This can be
compared to a change from 100 to 500 ppm CO which also shifts the electrode potential by ~30
mV30.
Sensitivity (S) is defined as the ratio of the sensor output to the change in the concentration
of the measurant, in this case O2 and CO. The O2 and CO sensitivities for the above sensor are
calculated in Eqs. 2-2 and 2-3.
ΔV (O2 ) 30mV mV S(O2 ) = = = 0.000214 (2-2) Δ[O2 ] 150000 ppm −10000 ppm ppmO2
ΔV (CO) 30mV mV S(CO) = = = 0.075 (2-3) Δ[CO] 500 ppm −100 ppm ppmCO
In the case where only two detectable species are present, the selectivity is equal to the ratios of
the individual gas sensitivities (Equation 2-4),
S(CO) 0.075 = = 350 (2-4) S(O2 ) 0.000214
23
From this calculation, it is evident that Pt electrodes are 350 times more sensitive to CO than they are to O2 or in other words they are very selective to CO vs. O2. Naturally, this large CO selectivity is very attractive from a scientific point-of-view as well as for developing CO sensors with all the practically-beneficial attributes of λ-sensors, like durability, size, and low cost.
This same phenomenon occurs when an electrode is exposed to NOX; the potential shifts negatively in NO and positively in NO2. Sensors that exhibit large sensitivities to pollutants are called “non-Nernstian” sensors and they are the most intensively-researched NOX sensors of the last decade8,11,17,23,25,27,30-44,44-59,59-74.
2.2 Sensor Materials
Since 1977, non-Nernstian sensor electrodes were fabricated from metals and metal oxides having a wide variety of compositions in attempts to amplify this selectivity effect and/or extend
28,42,75-77 78 19,20,70,71,79-84 85 46 it to other gases like H2 , H2S , hydrocarbons , SOX and N2O .
Initial efforts to produce NOX sensors began with Nernstian sensors. Researchers used binary salts with an anion in common with the measurant and a cation in common with the solid electrolyte, which they termed “auxiliary-phase” electrodes. Such electrode|electrolyte couples are Nernstian sensors because a proper thermodynamic can be written which relates the activity of the mobile species in the electrolyte to the activity of the measurant gas (Equation 2-5).
+ − Na (NASICON) + NO2 (g) + e ↔ NaNO3 (s) (2-5)
- + Since NO2 ions are mobile in neither NASICON (Na conductor) nor YSZ, an auxiliary phase is required to complete the thermodynamic chain. For example, NaNO3 can be used to communicate the concentration of NOX to NASICON (Equation 2-5). This chemical equation is then used to derive an expression relating the dependence of the electrode potential on NO2 concentration.
24
Additional examples of Nernstian sensors include Na2CO3|NASICON for CO2 sensing.
However, NASICON is unstable in moist gases at high temperatures and as a result the scope of potentiometric gas sensor researched focused on YSZ. Unlike NASICON, YSZ has proven record of stability in high temperature combustion exhaust environments. Subsequently, metal nitrates were interfaced with YSZ instead of the previously used NASICON; however, no thermodynamic chain can be written for such a sensor. These were classified as non-Nernstian sensors. A complete classification system is given here41.
Several nitrates were investigated for NOx sensing. Examples include Na, Ba, Sr and Ca-
24,36,86-88 based nitrates for sensing NOX . Unfortunately, these auxiliary phases have low melting points (Ba(NO3)2 is the highest at 590°C) and like resistance-type sensors they are also sensitive to O2. These electrodes may also exhibit poor selectivities; however, in most cases, cross- sensitivity studies were not completed before researchers moved onto metal oxides with higher- melting points.
Metal oxides have been used in gas sensing applications as resistance-type sensing elements for a while89. Table 2-1 shows some of the most commonly researched materials for both resistance-type sensors and potentiometric sensors. The list shows that many of the most important semiconductors used for resistance-type sensors are also the same ones most commonly used for potentiometric sensors. Additionally, semiconductors utilized as sensing elements for resistance-type sensors rather than metals, which do however find frequent application in potentiometric sensing. All of these materials have interesting electronic or catalytic properties making them suitable for a variety of important applications. The semiconductors have relatively low conductivities in air and their conductivities change
89 considerably (1-10x) when they are exposed to NOX .
25
NOX can physisorb or chemisorb on semiconducting metal oxides. Physisorption entails a weak attraction between the adsorbed gas and the surface, not necessarily requiring the transfer of charge between the solid and the adsorbate. However, physisorption can significantly impact sensor performance because physisorbed gas molecules can block the approach of other potential adsorbates. Conversely, during chemisorption, the adsorbates ionizes by extracting charge from the surface of the semiconductor or injecting charge into the semiconductor. As a result, a layer forms near the surface of the metal oxide which depleted or enriched in charge carriers. This depletion or enrichment of charge carriers can be large in semiconducting metal oxides whereas it is non-existent in metals. Hence, semiconducting metal oxides make much better resistance- type sensing elements than metals because their conductivities are strongly affected by the presence of NOX, whereas metals are unaffected. Metals are full of charge carriers, compared to semiconductors, so losing or gaining charge carriers due to NOX adsorption or desorption is negligible. Furthermore, if charge carriers are lost or gained at the surface of metal, charges within the metal are mobile enough to redistribute such that an electronically homogeneous solid is maintained.
As stated before, the chemisorption of a gas on a semiconductor changes its resistance.
During chemisorption, a bond is made, and some charge transfers between the gas and the material. Since the charge transfer occurs at the surface, the material’s charge carrier concentration is either depleted or enhanced near the surface. This layer near the surface of the semiconductor is called a space-charge layer and its depth is scaled by the Debye length (LD) of the semiconductor. The Debye length is given by Equation 2-6.
1 ⎛ εε kT ⎞ 2 L = ⎜ o ⎟ (2-6) D ⎜ 2 ⎟ ⎝ e nb ⎠
26
where ε and nb are the dielectric constant and bulk concentration of charge carriers in the
semiconductor, respectively. If the semiconductor is relatively full of charge carriers (large nb )
then its Debye-length ( LD ) will be small and the extent of the space-charging will be limited.
However, if the material is nearly an insulator (small nb ), the Debye length will be large and the space charge region will extend deep into the material.
Resistance-type gas sensors are based on porous networks of semiconducting grains
prepared by screen-printing and sintering. If the grain diameters are large compared to LD , then a depletion region (space-charge layer) exists at each grain’s extremity, where it interfaces the gaseous environment, and also at each grain-to-grain contact. If the grains are small compared
to LD , the depletion region encompasses the entirety of the grains. Furthermore, if the grains are
small compared to LD , the bulk concentration of charge carriers is directly effected by each charge transferred during adsorption/desorption.
For resistance-type sensors, charge must traverse the depletion regions and grain core regions (if they are not depleted themselves). This is widely observed and translates to the frequently observed relationship that decreasing grain size enhances gas sensitivity. This relationship, initially proven for SnO2 CO sensors, was observed for many semiconducting materials as well as different gases90,113,120,125,145.
The Fermi level is directly related to the bulk charge carrier concentration; therefore, the
Fermi level also shifts as gas adsorbs/desorbs on/from semiconductors. Many of the semiconductors used as sensing elements for resistance-type gas sensors are also used as gas sensing electrodes for potentiometric sensors (Table 2-1). The potential of an electrode as measured with an electrometer (in open-circuit conditions, i.e. passing no current) is determined
27
by its Fermi level, which can be measured indirectly by measuring its resistance or its workfunction with a Kelvin probe104,117,118.
The interrelationship of charge carrier concentration and workfunction was recently
104 173 demonstrated clearly with a SnO2 sensor , which was understood for some time in aqueous and solid-state electrochemistry174. In other words, gas adsorbing or desorbing on a semiconductor electrode changes the bulk charge carrier concentration, which in turn changes the potential of the electrode.
2.3 Background Information on La2CuO4
La2CuO4 is the parent compound of the doped high-temperature (Tc ≤ 40 K) superconductors La2-X(Sr,Ba)XCuO4+y. As such, its structure and electronic properties have been investigated extensively, mostly below room temperature. Various proprieties of La2CuO4 were quantified with four-point resistivity measurements, thermoelectric power measurements, and
D.C. magnetic susceptibility measurements, all of which require dense samples (bars or pellets)175. The solid-state synthesis route was found acceptable for producing powder which could be pressed and sintered to sufficiently high densities for experimentation.
Solid-state synthesized La2CuO4 powders were reported to be phase-pure with relatively
2 low specific surface areas, typically less than 1 m /g. La2CuO4 was also synthesized by the
Pechini method176, co-precipitation177, reaction of mixed-oxalates178,179, reaction of metal-nitrates in supercritical water180, the sol-gel method181, inverse microemulsions182 and freeze-drying44,56.
These methods also yielded phase-pure powder; however, the powders were generally finer (25 nm-5000 nm) with larger specific surface areas (1-15 m2/g).
La2CuO4 is a line-compound (Figure 2-1) and as such the tolerance of its structure to excess Cu and La are extremely small; therefore, the massing of reagents is critical. The typical reagents used are nitrates, acetates and oxalates of La and Cu. La and Cu nitrates are cheap,
28
widely-available, very hydroscopic (especially Cu-nitrate) and yield nitrate groups which provide oxidants during subsequent thermal treatments. Acetates and oxalates are more expensive and based on carbon-containing groups; therefore, they are preferred when the researcher wishes to avoid auto-combustion during calcination of the subsequently formed precursor.
La2CuO4 is a superconductor below 30 K, an intrinsic semiconductor between 30 K and 50
175 K and a heavily acceptor-doped (P-type) semiconductor above 50 K . O2 acts as an acceptor
2- 179,183 dopant forming Oxygen-interstitials (Oi ) and holes in La2CuO4 . The concentration of the
2- Oi is determined by the concentration of O2 around the La2CuO4 and the degree to which the
La2CuO4 equilibrates with it. Above about 30 K, intrinsic charge carrier activation is evidenced by an exponential drop in resistivity as temperature increases (see Figure 2-2). Above around 50
K, resistivity increases with increasing temperature. Seebeck Effect measurements showed that the mobility of the extrinsically-formed holes decreases as temperature is increased which results in the rising resisitivity seen above 50 K175.
La2CuO4 has been investigated as a candidate material for several environmental sensor applications. The resistance of dense bars of La2CuO4 measured in a four-point configuration
158 increases in NO and decreases in NO2 (consistent as a p-type semiconductor) . It was also
159 sensitive to NOX, CO, O2 when screen-printed on interdigitated electrodes for a resistance- type sensor application. La2CuO4 was interfaced with ZnO forming a varistor which was sensitive to humidity184-186.
La2CuO4 was selected for trial as a potentiometric sensor electrode based not only on its semiconductor sensor response but also on its inability to reduce NO (to N2 and O2) in the
187 187 presence of excess O2 or to oxidize NO to NO2 . Experiments showed that it was 2000 times
29
more sensitive to NO than to O2. In fact, La2CuO4-based sensors are totally insensitive to
30,44,56,74 O2 . Furthermore, the sensor’s response was reproducible over the course of several
56 hundred hours in both 15% O2 (N2 balance) and a simulated combustion exhaust . Minor interference from CO was observed56.
2.4 Homogeneous Gas Phase NOX and CO Chemistry
NOX chemistry is complex and kinetically limited. NO and NO2 form by the reaction of
N2 and O2 at high temperatures where the equilibrium constant (Keq) for the reaction becomes sufficiently large. Choosing 1 to 10 ppm of NO as a point of reference and recognizing that NO is always formed in larger quantities than NO2 (9:1), high temperatures are those above 600°C.
At lower temperatures, there is a strong thermodynamic driving force (large Keq) for NO and
NO2 to decompose into N2 and O2. Unfortunately, this doesn’t happen in any substantial quantity due to kinetic restrictions. Instead, NO and NO2 remain unchanged or react to form other nitrogen- and oxygen-bearing compounds (e.g., N2O and NO3).
While NOX chemistry is dominated by kinetics, thermodynamic predictions are useful because they indicate the direction of spontaneous change for the system. Calculating the equilibrium concentrations of all possible components requires: (1) the identification of all possible species, (2) the Keqs (ΔGformation) for all independent reactions and (3) numerical solving software (e.g., MathCad and MATLAB). There are several software packages available for equilibrium thermodynamic calculations that combine a Keq database with a numerical solver.
One such program, FACTSAGE, was used to calculate the equilibrium concentrations of all possible species for a simulated combustion exhaust (650 ppm NO, 3% O2, 15% CO2, 3%
H2O, balance N2) (Figure 2-3). At low temperatures, thermodynamics predicts that all of the NO would decompose to N2 and O2, which shows that there is a strong thermodynamic driving force
30
for NO to decompose; however, in practice, it does not due to kinetic constraints. Between
200°C and 350°C, NO2 is more thermodynamically favorable than NO. This suggests that between 200°C and 350°C, any NO should decompose to form N2 and O2 or oxidize to form
NO2. In practice, NO is stable in this temperature range because the processes of NO oxidation and NO decomposition are too slow. Above 350°C, NO is more thermodynamically favorable than NO2; however, the majority of the NO is thermodynamically predicted to decompose to N2 and O2. In addition, above 800°C, the equilibrium concentration of N2O rises above the ppb
(parts-per-billion) level. The equilibrium concentration of NO2 never rises above 1 ppm; therefore, NO should never spontaneously oxidize to NO2, rather the vast majority of it should decompose to N2 and O2. Other nitrogen oxide species (NO3, N2O5) are possible; however, their equilibrium concentrations are below a ppb over the entire temperature range. Again, these are thermodynamic predictions, so the real situation can differ substantially; however, in the presence of sufficient catalyst the gas composition will approach this value.
NO may transform into N2O, NO2 or N2 and O2 (Figure 2-3) and the equilibrium constants for the reaction of NO to NO2 and NO to N2O dependent exponentially on temperature (Figure 2-
4). The equilibrium constant for the decomposition of NO to N2 and O2 is so large over the entire temperature range plotted that it would require replotting the data on a logarithmic scale.
The equilibrium constant for NO2 decomposition to N2 and O2 is also very large at all temperatures practically important for sensors. Figure 2-4 shows that NO2 is more thermodynamically favorable compared to NO below 500°C and N2O is more thermodynamically favorable than NO below 700°C. Reports of NO converting to N2O in catalysis studies are rather common; however, NO reacting to form NO2 less commonly observed150.
31
In summary, thermodynamics predicts that NO will decompose to N2 and O2. If NO cannot go to N2 and O2, the next most favorable products are N2O and NO2, which become favorable only below 700°C and 500°C, respectively. Furthermore, NO2 and N2O are expected to decompose forming NO and O2. The decomposition of NO2 to NO and O2 is commonly observed150.
Like the decomposition of NO to N2 and O2, there is a strong thermodynamic driving force for the oxidation of CO and hydrocarbons. In oxidizing environments, CO oxidizes to equilibrate with CO2 and hydrocarbons oxidize to form H2O and possibly some OH groups
(Figure 2-3). Solid C is thermodynamically unfavorable in oxidizing environments
2.5 Sensing Mechanisms
2.5.1 Introduction
Discussion of the sensing mechanism of solid-state potentiometric sensors is still very much open and it dates to Fleming’s paper in 197716. Since the Nernst Equation is used with λ- sensors, some researchers tried to explain the non-Nernstian potentials by local Po2 (O2 partial pressure) arguments. For example, it was suggested that CO reacts very near the electrode surface consuming large quantities of O2 hence depleting the local Po2 in the immediate vicinity of the electrode. This stemmed somewhat from the concurrent research at the time on resistance- type sensors where it was seen that decorating a semiconductor sensing electrode with catalyst powders increased the response time and sensitivity, which was explained by enhanced reaction of CO near the electrode surface.
In 1982, it was suggested that Mixed Potential theory, a theory developed in the field of aqueous corrosion, might be applied to solid-state potentiometric gas sensors189. Mixed potentials are generated when multiple electrochemical reactions, each with its own half-cell
32
potential, occur on the same conductive surface. The net potential of the surface is intermediate of the half-cell potentials for the reactions occurring.
Shortly afterwards, it became fashionable to call nearly every non-Nernstian sensor a mixed potential sensor. Mixed Potential theory can explain some experimental results (non-
Nernstian potentials) but not all of them. Furthermore, the existences of the said electrochemical reactions were never confirmed with gas compositional analysis.
Differential Electrode Equilibria was proposed by Wachsman44 in 2000. Differential
Electrode Equilibria takes into account the changes in Fermi level which can occur in semiconductor electrodes. A shift Fermi level is equivalent to a change in electrode potential.
This contribution is often overlooked; however, it is well established by previous research on resistance-type semiconductor gas sensors. This theory is the current state-of-the-art. In all cases, gas composition analysis is prerequisite to ascribing any particular theory or mechanism to the generation of a potentiometric response.
2.5.2 Mixed Potential Theory Discussion
Mixed Potential theory is complex and its implications are somewhat unapparent so a few words of clarification are warranted here. Mixed Potential theory is the most commonly invoked mechanism for explaining the observance of non-Nernstian potentials. Several reviews can be found here40,41,79,190. The word “mixed” in Mixed Potential indicates that more than one electrochemical reaction occurs on the same conducting surface, be it at the triple phase boundary (TPB, between the electrode, electrolyte, and gas) or on the electrode surface.
In Mixed Potential theory, more than one electrochemical reaction occurs on the same conductive surface. Since the electrolyte, YSZ, in this case is an O2-ion conductor some exchange with the external environment will occur as given by reaction 1 (Equation 2-7) until a steady-state is achieved.
33
1 Reaction 1 O (g) +V •• (YSZ) + 2e − (electrode) ↔ O x (YSZ) (2-7) 2 2 O O
Eventually, the system reaches a steady-state and the flux of O2 leaving the YSZ equals that entering from the gas. The electrochemical reaction of a pollutant gas (e.g., NO) may be generically represented by reaction 2 (Equation 2-8) where Re(g) and Ox(g) represent a pollutant gas in its reduced and oxidized forms, respectively.
X − •• Reaction 2 Re(g) + OO (YSZ) ↔ ne (electrode) +VO (YSZ) + Ox(g ) (2-8)
The currents resulting from reactions 1 and 2 are given by their individual rates (k) and the number of electrons (n) transferred in each reaction (Equation 2-9).
i = nFk (2-9)
The rate of each reaction depends on several factors which are listed in Table 2-2. The sum of the anodic currents must be equal to the sum of cathodic currents to maintain local charge balance. At equilibrium, reactions 1 and 2 have formal electrode potentials (E°) as defined by the Nernst Equation. E2°
The rate of electrochemical reactions can be determined with compositional analysis; however, in most cases it is easier and more accurate to use electrical means. Two possible mixed-potential schematics are shown in Figure 2-5. The Pt (blue) electrode in Figure 2-5A has two X-axis intercepts which correspond to the formal potentials (E°) for reaction 1(Equation 2-8) and reaction 2 (Equation 2-8). The difference between these two values is the maximum mixed-
40 potential (Emix(Pt) and Emix(Au)) possible . They are determined by the half-cell potential for the reaction plus a concentration term which has log arithmetic dependence and therefore they
34
are the same regardless of the electrode material. However, the slopes of the current-electrode potential lines are determined by both the kinetics for the individual reactions and each electrode will have its own reaction rate as well.
The O2-reduction current-potential line can be determined by biasing the electrode and measuring the current; however, the current-potential line for the oxidation/reduction of pollutant gases is difficult to obtain without some major assumptions because oxygen is involved in this reaction as well. Some authors have taken the difference between current-potential lines in the absence and presence of pollutants and ascribed this current-potential line to the oxidation/reduction of pollutant gas; however, this is not so straight-forward, so further development will wait until Chapter 6 of this dissertation.
Other authors have measured the current-potential characteristics pollutants in the absence of O2. These current-potential characteristics are only equal to those directly resulting from the reduction or oxidation of pollutant species if the major assumption that the kinetics of an
84 electrode in the absence of O2 are the same as those in O2 .
Despite these difficulties, some researchers have speculated about the form of the current- potential characteristics for the electrochemical reaction of various pollutants. These theoretical approaches require the simplification of many system parameters; therefore, even qualitative predictions about the interrelationship between temperature, material, and gas concentration on electrode potential are difficult to ascertain. They do however advance the state-of-the-art understanding behind sensor technology and provide a means for initiating discussion.
The electrode potential usually varies as a function of pollutant concentration. Some researchers suggest this is evidence of the mixed-potential mechanism (Figure 2-5A, note the log current scale on Y-axis)41,45,59,169. However, in other cases, a linear dependence on gas
35
concentration was observed42,191 and also in this case the mixed-potential mechanism was cited.
In this case, it was suggested that this is a result of mass-transfer limited kinetics for the reduction/oxidation of pollutants (Figure 2-5B, note the linear current scale). Finally, the potential of some electrodes exhibited both a linear dependence on gas concentration for some gases and a logarithmic dependence on concentration for other gases80.
These concentration dependencies were initially derived with various assumptions and schematics were drawn to illustrate how the electrode materials, gas concentration, and electrode potential are tied together by an electrode’s current-overpotential characteristic and its dependence on gas concentration (Figures 2-5A and 2-5B). Clearly, the mixed-potential mechanism is complicated, difficult to verify, and difficult to use for making predictions; never- the-less it is the most often cited mechanism to explain non-Nernstian behavior.
2.6 Summary
Solid-state potentiometric sensors are best candidates for NOX and CO sensing in combustion exhaust. Non-Nernstian sensors showed impressive sensitivities and response times.
Their selectivity for NOX and CO over O2 gives them a key performance advantage over resistance-type gas sensors. However, the fundamental mechanisms behind non-Nernstian potentiometric responses to NOX and CO are currently poorly understood. As a result, the remaining hurdle to the commercialization of potentiometric sensors, that of selectivity between the individual pollutant species themselves (e.g., NO vs. NO2), still remains.
Many materials were tested as sensing electrodes for potentiometric gas sensors. Many of those same materials were also used as sensing electrodes for resistance-type gas sensors. This illustrates the link between the two types of sensors and how their sensing mechanisms are interrelated.
36
La2CuO4, a p-type semiconducting metal oxide, showed some promising results as a sensing electrode for a potentiometric NOX sensor. Powders and dense samples of La2CuO4 were extensively characterized in previous work. Exhaust chemistry especially that of NOX is complex and is not predictable from thermodynamic calculations alone. La2CuO4 is a poor catalyst for NO reduction, which simplifies the heterogeneous catalysis component of the NOX chemistry in its vicinity. Ultimately, its low catalytic activity and p-type semiconductivity make it possible to identify and separate competing sensing mechanisms, whose origins are in the semiconductivity and catalytic activity of the electrode.
37
Table 2-1. Publications relevant to several materials commonly used as gas sensing elements. Common applications for the materials other than as sensing elements for a resistance-type or potentiometric gas sensors are also listed. Material Publications as a Publications as a Common resistance-type potentiometric sensor applications other sensing element electrode than gas sensors 64,90-117 46,118,119 SnO2 (pure or doped) 29 3 ITO layer in solarcells 94,100,112,120-133 62,71 TiO2 (pure or doped) 17 2 Conductometric O2 sensors 94,95,112,134-140 39,45,47,49,53,57,58 LaFeO3 10 7 100,141-148 49,50,53,54,57,58,149,150 WO3 9 8 Electrochromic applications 94,100,106,112,115,151-154 65,83,84,155 In2O3 9 4 115,126,128,156 76,77,157 ZnO (pure or doped) 4 3 FET H2 sensors, Barrier-layer varistors 126,128,158,159 30,44,56,62,71,74,159 La2CuO4 4 7 Superconductor 134,160,161 39 SmFeO3 3 1 162 82,163,164 Nb2O5 1 3 Pt 4 93,103,131,144 (as a 38 15- Catalyst (aqueous dopant or modifier) 17,25,28,30,37,39,42,44,48,50,54,56 and gaseous ,57,65-67,70,74,78- applications) 80,93,103,119,131,144,150,159,165- 172 Au 3 131,148,153 (as a 10 19,20,28,42,48,54,79-81,172 Low temperature dopant or modifier) applications
38
Figure 2-1. La2O3-CuO binary phase diagram.
Figure 2-2. Four-point resistance measurement of La2CuO4 in air as a function of temperature.
39
0 H2
N2
-2 O2 O
OH
-4 H2O OOH CO -6 CO2 log (Volume Fraction) NO N2O -8 NO2
0 200 400 600 800 1000 Temperature (C)
Figure 2-3. Equilibrium gas composition calculated using thermodynamics software (FACTSAGE) for combustion exhaust containing 3% H2O, 15% CO2, 3% O2 and 650 ppm NO.
40
100
10 NO= N O+1/2O 2 2
1 Keq
NO+1/2O = NO 2 2
0.1
0.01 0 200 400 600 800 1000 Temperature (C)
Figure 2-4. Equilibrium constants for NO+1/2O2=NO2 and NO=N2O+1/2O2.
41
Table 2-2. Factors determining steady-state current resulting from the electrochemical reaction of a pollutant gas on a sensor electrode. Factors Relationships Driving force for reaction Directly proportional to reaction quotient, Q, which is determined by the activities of the reactants and products Directly proportional to the magnitude of the difference in Gibbs free energy of formation between the products and reactants. Activity of reactants Decreased by steady-state reaction rate (k) adjacent to reaction sites Increased by gaseous bulk reactant concentration and diffusivity of reactant. Decreased by heterogeneous catalysis enroot to reaction sites. Activity of products Increased by steady-state reaction rate (k) adjacent to reaction sites Increased by gaseous bulk reactant concentration and diffusivity of reactant. Increased or decreased by heterogeneous catalysis depending on what reaction was heterogeneously catalyzed Reactant and product Dependent on reaction mechanism concentration profiles Determined by concentrations of reactants and products in gaseous bulk Rate of surface exchange between electrode surface and gaseous bulk Diffusivity of reactant/product through the electrode or on electrode surface important Total number of reaction Large for high surface area electrodes sites Large for small grained electrodes Occupancy of reaction Dependent on gas history (small gas is strongly adsorbed on sites electrode surface)
Activation energy of Dependent on the bond strength of intermediate complexes. intermediate complexes Especially important in NOX chemistry
42
Log |i| (A) |i| (A) A) B) Pt Pt
Au [NO] Au E(V) E(V) E°1 Emix (Au) E°2 E°1 Emix (2) E°2
Emix (Pt) Emix (1)
Figure 2-5. Mixed-potential schematics of current-overpotential characteristics. A) Theoretical current-voltage curves for Pt and Au in air and air + NO with a logarithmic current dependence on electrode potential. B) Theoretical current-voltage curves for an electrode in air and in air + NO at two different concentrations assuming a linear current dependence on electrode potential and a mass transfer limited current at high overpotentials.
43
CHAPTER 3 EFFECT OF ELECTRODE MICROSTRUCTURE ON SENSITIVITY AND RESPONSE TIME
3.1 Introduction
The microstructure of a potentiometric sensor’s electrodes plays a pivotal role in determining the sensitivity and response time of the sensor. The sensitivities of both resistance- type semiconductor gas sensors and potentiometric gas sensors are strongly dependent on the microstructure of their sensing electrodes. The response times of both types of sensors are also strongly dependent on the grain-size of the sensing electrode microstructures. As an example,
134-136,140 45,47,49,53,57,58 both resistance-type and potentiometric sensors based on LaFeO3, a p-type semiconductor, respond well to NO2. Decreasing the grain-size of the LaFeO3 used for both types of sensors improved gas response/sensitivity and response time.
There is some discrepancy in the literature as to the affect of sensing electrode grain-size on potentiometric sensor performance. In some cases, increasing the grain-size of the electrode improved sensitivity59,60,192 and in other cases it worsened it72. These are limiting cases where the potentiometric response is dominated by a particular mechanism. For example, the potentiometric response of metallic electrodes is commonly ascribed to electrochemical reaction of NOX at the TPB, whereas the potentiometric response of semiconductor electrodes is usually due to gas adsorption/desorption.
There are also cases where the potentiometric response is determined by more than one sensing mechanism. Different mechanisms are dominant at different temperatures for a given electrode material and microstructure. Electrochemical or catalytic mechanisms dominate the potentiometric response at high temperatures (>500°C) and sorption-type mechanisms dominate at lower temperatures (300°C-500°C). For example, a sorption-type mechanism dominated the
44
potentiometric response of a TiO2-based sensor at lower temperatures and an electrocatalytic mechanism dominated it’s response at higher temperatures62.
Given the array of microstructures, catalytic activities, bulk charge-carrier concentrations, and operating temperatures investigated in the literature, it is not surprising that the potentiometric response often has a mixed character, in other words, the electrode potential is determined commonly by a combination of several mechanisms simultaneously rather than a single mechanism82. These mechanisms determine each of the individual electrode responses and the difference between the individual responses is what is registered by multimeters. As a result, the difference in the equilibria attained at each electrode directly determines the potentiometric response of the sensor to changes in NOX concentration. This is the heart of the theory Differential Electrode Equilibria, which takes into account all the contributions to the variations in electrode potential possible.
Heterogeneous catalysis plays an important role in potentiometric NOX sensing. NO and
NO2 are non-equilibrium gases and in the presence of a catalyst with sufficient activity and surface area, may react to form other NOX species. The surface area of an electrode is determined by its grain-size and the catalytic activity of the electrode is determined by what material it is. Catalytic activity increases when the grain-size of a catalyst decreases because decreasing the grain-size of a catalyst also increases its surface area. NOX, being a non- equilibrium mixture, may react as it diffuses through the electrode enroot to the TPB coming closer to thermodynamic equilibrium in the process72. Therefore, decreasing the grain-size of an electrode (increasing the number of surface sites) should increase the extent to which NOX reactions are catalyzed as it diffuses through the electrode.
45
For the cases in which decreasing the electrode grain-size deteriorated sensitivity, it was suggested that the decreased grain-size increased the number of reaction sites allowing the NOX to equilibrate before reaching the TPB. If the NOX is at equilibrium when it reaches the TPB, it does not electrochemically react and the electrode potential remains constant72. NO rarely converts to N2 and evidence of such has yet to be observed. In fact, reduction of NO to N2O is more commonly observed although usually over very catalytic powders. Conversely, NO2 does decompose to NO + O2. So the above explanation of poor NOX sensitivity for electrodes of small grain-size makes sense for NO2 but not NO.
For the cases in which decreasing the electrode grain-size improved sensitivity, it was suggested that the increased number of surface sites increased the quantity of gas adsorbed on the electrode and the degree to which it affected the bulk charge-carrier concentration of the electrode material. This was evidenced by FT-IR and XPS which showed the formation of
193 chemical bonds between NOX and these materials . Additionally, temperature-programmed desorption experiments also showed that NOX desorbs from these materials at the same temperatures where the maximum potentiometric response sensitivities were observed159,193.
To better understand the relationship between the grain-size of potentiometric sensor electrodes and their potentiometric responses, La2CuO4 powders were synthesized using several wet-chemical methods for ultimately fabricating sensing electrodes with various surface/bulk ratios. The potential of these electrodes were referenced to porous Pt electrodes, deposited and sintered in identical fashion, on the opposite side of thin, dense, YSZ sheets. Variations in NO concentration change the potential of Pt electrodes30,74; therefore, the response performance characteristics (sensitivity and response time) were directly related to the microstructures of the
46
various La2CuO4 electrodes under the assumption that the response of each of the Pt electrodes was equivalent.
3.2 Experimental
3.2.1 Powder Synthesis
194 176 La2CuO4 was synthesized with three techniques: auto-ignition , Pechini and co- precipitation177 leading to sensor electrodes with grains of the same bulk composition but with different microstructures and surface/bulk ratios.
For the auto-ignition process, metal nitrates were dissolved in distilled H2O. Citric acid was then stirred into the solution. Next, the water was evaporated by heating on a hot plate to form a gel, which upon further heating began to smolder, char and finally combusted. The temperature and homogeneity of this combustion reaction are determined by the ratio of citric acid/metal nitrates in the gel195. After gel combustion, the product was calcined, burning any remaining organics and forming crystalline particles of La2CuO4. The citrate/nitrate ratio (γ) is a major factor that determines the powder morphology after calcination. A calculation of the coefficients for the stoichiometric reaction of citrate ions and nitrate ions yields a citrate/nitrate molar ratio of γ=0.277. Two different citrate/nitrate ratios were used to synthesize powders, which were named auto-ignition fuel-lean (γ=0.098) and auto-ignition fuel-rich (γ=0.50).
In the co-precipitation process, metal-nitrates were dissolved in distilled H2O. A polymer
(H3DPTA) was added to this solution in order to coordinate the cations in a stoichiometric ratio.
The metals were precipitated as hydroxides by slowly adding 10% excess of a basic solution of tetra-methyl ammonium hydroxide in 25% methanol. The precursor metal-hydroxide powders were recovered, rinsed and air-dried at 110 ºC overnight.
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For the Pechini process, metal-nitrates were dissolved in distilled H2O. Then citric acid and ethylene glycol were added to the solution. The entire solution was heated nearly to boiling, and then cooled to room temperature forming a solid mass of gel. This gel was calcined yielding crystalline La2CuO4 particles.
3.2.2 Powder Characterization
X-ray Diffraction patterns were collected in the 2θ range of 10 to 80° for the calcined powders using a powder diffractometer (APD 3720, Phillips) with a step size of 0.05° and a count time of 1 s. Specific surface areas were measured using the B.E.T. N2-adsorption method
(Nova, Quantachrome). Calcined powders were observed using a Field Emission Scanning
Electron Microscope (FE-SEM, 6335F). Sintered electrode surfaces were also viewed using the
FE-SEM.
3.2.3 Sensor Fabrication
There are no commercially-available inks containing La2CuO4 particles; therefore, an ink suitable for screen-printing was prepared. There are three main carrier systems: (1) alpha- terpineol-based, (2) water-ethyl cellulose-based and (3) PEG-based. These three carrier systems
196 197 were directly compared using TiO2 and NASICON . The PEG-based inks formed the most porous layers. The PEG-based inks, which consist of only PEG and powder, are relatively straight-forward to optimize because there is only one parameter (solids loading) which needs to be optimized.
The optimal La2CuO4 solids-loading was determined by preparing inks containing 16 to 20 volume % co-precipitated La2CuO4, screen-printing them onto YSZ rectangles, firing them at
700°C, 800°C and 900°C and observing them with a SEM. There are many parameters in screen-printing like squeegee pressure, squeegee speed, mesh thickness, mesh count, etc and all of these were initially established through trial and error. These parameters, especially squeegee
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speed, dictate the rheological characteristics of the ink which enable the realization of optimal printing performance198. The relatively high solids-loading inks (19 and 20 volume %) adhered best to the YSZ rectangles and had the best print definition. Conversely, if was difficult to print complete squares of the lower solids-loading inks. Furthermore, the lower solids-loading (15 and 16 volume %) inks were more fluid so they ran at the edges. After screen-printing, all of the samples were sintered. The high solids-loading samples were denser than the low solids-loading samples and they also tended to form islands and crack (Appendix A). The islands are a result of the rest viscosity (no shear, after printing) being too high for the layer to settle (Appendix A)198.
A compromise between electrode porosity, print definition and screen-print-ability was found at
18 volume % for co-precipitated La2CuO4. A similar procedure was followed to determine the optimal solids-loading for each of the other types of La2CuO4 (Pechini-synthesized, auto- ignition, fuel-rich, etc.). Temperature had a large effect on the electrode porosity as will be discussed in section 3.3.3 and the samples fired at 700°C were the most indicative of print quality (islanding) since at high temperatures the microstructure homogenizes to some extent during sintering (Appendix A).
The sensors were fabricated as follows. An ink containing Pt particles (Haereus) specifically prepared for screen-printing was used to screen-print a square on one side of a 20 mm × 10 mm × 100 μm tape-cast YSZ rectangles (Marketech International Inc.). Next, a square of the La2CuO4 ink was printed on the YSZ rectangle directly opposite the Pt square (Figure 3-
1). Thin Pt wires were connected to both electrodes. Finally, the sensor was sintered at temperatures ranging from 700 ºC to 1050 ºC for 2, 10 or 18 h to see the effect of electrode microstructure on sensing performance.
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3.2.4 Electrical Characterization
The fabricated sensors were systematically evaluated in simulated combustion exhaust containing 3% O2, 16% CO2, 100 ppm NO2, 100 ppm CO, and balance N2, bubbled through water. Sensing experiments were carried out with a computer controlled gas flow system (647c,
MKS) connected to a quartz tube inside of a furnace. The sensors were exposed to 50, 100, 200,
400 and 650 ppm NO step changes in the simulated combustion environment at a constant total flow rate of 300 sccm between 400°C and 700°C. The voltage was monitored between the two sensor electrodes using a digital electrometer (2000, Keithley). The La2CuO4 and Pt electrodes were connected to the positive and negative terminals of the electrometer, respectively, and both electrodes were exposed to the same gas environment.
3.3 Results and Discussion
3.3.1 Powder Synthesis
X-ray diffraction showed that a heat treatment at 600°C for 10 h was sufficient for phase formation in the case of the auto-ignition technique whereas the precursors formed using the co- precipitation and Pechini techniques required calcination at 650°C for 10 h to form La2CuO4.
This could be a result of the high local temperatures reached during auto-ignition. The auto- ignition reaction initiated at 180±10°C, a temperature similar to what other researchers have
195 observed . B.E.T. N2 adsorption results (Figure 3-2) showed that the highest surface area was obtained for the auto-ignition fuel-rich powders. Pechini powders were the least phase-pure of any of the methods and also had the least specific surface area.
FE-SEM pictures of the different powders are presented in Figures 3-3. The particle sizes
(Table 3-1) were estimated from the micrographs (Figure 3-3) and typically ranged between 50-
200 nm. Micrographs of these agglomerated structures are inset within each of the pictures in
Figure 3-3.
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The morphologies of the agglomerated calcined powders were dependent on the method of powder synthesis. The powder agglomerates synthesized with both variations of the auto- ignition technique were net-like webs made up of small primary particles. The primary particle size for the fuel-rich case was broad with a large portion of fine particles. The fuel-lean case was more mono-sized and slightly larger. The co-precipitation process also resulted in net-like agglomerates but the primary particles were slightly larger than in the auto-ignition, fuel-lean case. The Pechini process produced solid lumps of particles that appear well-sintered together
(Figure 3-3 D). Typical agglomerate sizes are summarized in Table 3-1. It was verified with
SEM that the agglomerated structures were broken during the subsequent ball-milling procedure; therefore, the size of the primary particles directly influenced the size-scale of the sintered electrode microstructure.
3.3.2 Effect of Synthesis Technique on Sensor Electrode Microstructure
The electrode microstructures obtained from powders synthesized with these techniques were studied to identify the morphologies of the sensing surfaces. FE-SEM micrographs of the sensor surfaces were taken for samples all fired at 800°C for 10 h (Figures 3-4). The microstructure of electrodes prepared with powders synthesized with the auto-ignition, fuel-rich process were fine (Figure 3-4A) with average particle sizes of 100-300 nm and flakey microstructures, while the auto-ignition fuel-lean technique produced comparatively coarser microstructures (Figure 3-4B). The co-precipitation technique produced larger particles in the range of 0.5-0.7 μm and the grains were more spherical (Figure 3-5C). Finally, the Pechini process gave an intermediate range of grain sizes (Figure 3-5D).
3.3.3 Effect of Sintering Temperature and Time on Sensor Electrode Microstructure
The effect of sintering temperature was studied with the co-precipitated powders (Figure 3-
5 and Appendix Figure A-1). A large increase in grain size occurred at temperatures above
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800°C. By 1050°C, the grains were much larger and sintered together. Similar large variations in microstructure with sintering temperature were observed for powders produced through the three other synthesis routes (Figures 3-6, 3-7 and 3-8). Powders prepared through the other techniques also exhibited limited grain growth up to 800°C and relatively large grain growth at
900°C and above.
The effect of sintering time was less dramatic. Some difference in grain growth can be observed between samples sintered for 10 h and 18 h at 800ºC as presented in Figure 3-4A
(800ºC/10 h) and Figure 3-6D (800ºC/18 h) for auto-ignition fuel-rich powders and Figure 3-4D
(800ºC/10 h) and Figure 3-6F (800ºC/18 h) for Pechini powders. Based on these observations, the sintering condition of 800°C for 10 h was selected for further study because it yielded a uniform distribution of nanometric grains and pores.
3.3.4 Open-Circuit Potential Responses to Changes in NO Concentration
The potentiometric responses at 450°C and 500°C for the samples prepared with La2CuO4 powders synthesized with the various techniques (Figure 3-7). All the sensors showed a sharp step response at 450°C, although most overshot or monotonically came to a steady-state voltage.
The notable exception was the auto-ignition fuel-rich sensors which reached a steady-state quicker than the rest. Sensors prepared from La2CuO4 synthesized via the Pechini method showed the largest response (20 mV at 650 ppm of NO), whereas co-precipitated powders showed the lowest (13 mV at 650 ppm of NO). At 500°C, sensors fabricated with the powders synthesized via the auto-ignition fuel-lean route showed the largest response (14 mV at 650 ppm
NO) and the Pechini-derived samples had one of the lowest. At 500°C, the voltage of all the sensors reached a steady-state in less than a minute. These results were both reproducible and reversible.
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The sensor responses for the samples prepared from the different powders were temperature dependent (Figure 3-8). The sensitivity was inversely proportional to the operating temperature. While decreasing the operating temperature did improve sensitivity, it also destabilized it. This instability was manifested as a baseline shift from zero to non-zero values
(not shown for clarity) and as an initial oscillation in the step response before arriving at a steady-state value.
Sensors prepared with the La2CuO4 synthesized with the Auto-ignition, fuel-rich process were the only ones that were reasonably stable at 400°C. They were also sensitive to changes in
NO concentration below 50 ppm (Figure 3-8). The sensors prepared with the powders synthesized using the Pechini process were more sensitive at higher NO concentrations than the rest of the sensors. This result indicates that coarse grained electrodes are more sensitive at higher NO concentrations while fine-grained electrodes are mores sensitive at lower NO concentrations. In other words, sensitivity scales with the dimensions of the microstructural features.
The mean values of voltage change for four measurements (up, down, up, down) at each
NO gas concentration were calculated from Figure 3-8 and tabulated in Table 3-2 and plotted in
Figure 3-9. At 450°C, all of the samples were sensitive to NO. Sensitivity (mV/dec) is defined as the change in signal (mV) per change in gas concentration (ppm). The sensor responses plotted versus the logarithm of the NO concentration were the straightest for the sensors prepared with the auto-ignition, fuel rich and Pechini techniques. The auto-ignition, fuel-lean and the co- precipitation samples were non-linear at higher NO concentrations indicating saturation. When a sample saturates, the calculated sensitivity averaged over the entire gas range goes down. The co-precipitation samples had the lowest sensitivity as calculated in mV/dec (Table 3-2);
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however, the auto-ignition, fuel-rich samples were the most sensitive at low NO concentrations.
Therefore, both the range (for example, 50–200 ppm NO) and the sensitivity, in mV/dec, are important performance characteristics when selecting candidate sensors.
The dependencies of all responses on the log of the NO concentration were all straighter at
500°C. The magnitude of all the responses went down and the sensors prepared with the auto- ignition, fuel-rich powders actually had the least sensitivity. The auto-ignition, fuel-lean and co- precipitation had nearly identically grain-sizes and their sensitivities were the largest at 500°C.
The grain-size of their electrodes were larger than the auto-ignition, fuel-rich samples but not as large as the Pechini samples which could be the reason for their enhanced sensitivity at 500°C compared to that of the sensors prepared with the auto-ignition fuel-rich powders.
The response times were calculated from the transient potentiometric measurements by taking the times required for the voltages to reach 90% of their final stable values. At 450°C, response times ranged from 10-80 s and were faster at higher NO concentrations (Figure 3-9C).
Response time was relatively independent of NO concentration for sensors fabricated with auto- ignition fuel-rich powders. In contrast, the response time for the other samples decreased with increasing NO concentration. At 500°C, the response times decreased to less than 40 s and became relatively independent of NO concentration for all the sensors (Figure 3-9D). Again, the sensor prepared with the auto-ignition, fuel-rich method had the fastest response of the samples tested.
A comparison of the response time and sensitivity, as a function of powder surface area and electrode grain size, is presented in Table 3-2. In the table, the powders are presented in order of increasing particle size. At 450°C, Pechini powders had the highest sensitivity and co-
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precipitation powders had the least. At 500°C, auto-ignition fuel-lean powders had the highest sensitivity and auto-ignition fuel-rich powders had the least.
These results show that microstructure plays a role in sensitivity; however, no direct correlation can be made between microstructure and sensitivity at this time. The use of reference electrodes show that the potential of each of the sensor electrodes changes when exposed to pollutant gas30,80. In this study, the voltage of the sensor, ΔV(sensor), was monitored, which is equal to the sum of individual potentiometric responses for each electrode (Equation 3-1).
ΔV (sensor) = ΔV (La2CuO4 ) − ΔV (Pt) (3-1)
Separation of the La2CuO4 electrode response, ΔV(La2CuO4), with a reference electrode and correlating its characteristics (sensitivity, range, stability, and response time) to its microstructure
(grain size, surface/bulk ratio, and thickness) should clarify the relationship between electrode microstructure and potentiometric response sensitivity and will be investigated in the future.
However, there was a relationship between electrode microstructure and response time.
The highest surface area powders, from auto-ignition, fuel-rich processing, resulted in the finest grained electrode microstructures and when used for fabricating sensors, those sensors had the fastest response times at both 450°C and 500°C. All three of the other techniques resulted in significantly lower surface area powders and similarly-sized, larger-grained (0.5 μm) electrode microstructures. Furthermore, all three had essentially the same response time (within the statistical significance of the data). Hence, it may be concluded that electrode grain size is directly related to response time.
In order to investigate the relationship between electrode grain-size and sensor response time further, four sensors were fabricated with the same co-precipitated La2CuO4 powders, sintered at four different temperatures, and measured at 500°C. The responses were similar to
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those before (Figure 3-9C) and the response times were calculated by averaging the time required to reach 90% of the steady-state response voltage for all steps in gas concentrations.
The response time increases from approximately 25 s for 100 nm grained electrodes to more than
5 min for 5 μm grained electrodes (Figure 3-10). This result was not surprising since it has been repeatedly observed in other studies of potentiometric and resistance pollutant sensors117 but never before in such systemic fashion for potentiometric sensors.
3.4 Summary
Higher sensitivity at low NO concentrations and faster response times were obtained with sintered electrodes having finer grain sizes. Response stability and speed improved at elevated operating temperatures at the expense of response sensitivity. Coarse-grained electrodes exhibited slow responses and poor sensitivities in low NO concentrations; however, it was also observed that they exhibit large sensitivities at high gas concentrations. In general, it was observed that sensitivity decreases significantly at temperatures above 550 ºC. La2CuO4|YSZ|Pt sensors, when operated between 450-500 ºC in simulated combustion exhaust, respond quickly
(<1 min) and reproducibly to step changes in NO concentration with adequate sensitivity to resolve 50 ppm of NO.
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Pt electrode
8 mm La2CuO4 electrode
20 mm current collectors
Figure 3-1. Potentiometric NOX sensor configuration: front view (left) and side view (right).
20 B) A) 16.7 /g) 2 15 Pechini
Co-precipitation
10 9.15 7.57 Auto-Ign. Rich Relative Intensity Relative
Auto-Ign. Lean 5 3.7 Specific SurfaceArea (m
0 10 20 30 40 50 60 70 80 Degree (2θ) Pechini Auto-ign. Lean Co-precipitation Auto-ign. Rich
Figure 3-2. B.E.T. surface area and X-ray diffraction results for La2CuO4 powders synthesized with different synthesis routes. The powders synthesized with the co-precipitation and Pechini techniques were calcined at 650°C for 10 h and the powders synthesized with the two auto-ignition techniques were calcined at 600°C for 10 h. A) Specific surface area for each powder. B) X-ray diffraction pattern for each powder.
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A) B)
C) D)
Figure 3-3. FE-SEM images of La2CuO4 powder prepared by calcining precursors which were synthesized with different techniques. A) Powders prepared by calcining (600 °C / 10 h) precursors synthesized with the auto-ignition, fuel-rich process. B) Powders prepared by calcining (600 °C / 10 h) precursors synthesized with the auto-ignition, fuel-lean process. C) Powders prepared by calcining (650 °C / 10 h) precursors synthesized with the co-precipitation techniques. D) Powders prepared by calcining (650 °C / 10 h) precursors synthesized with the Pechini method.
Table 3-1. Characteristics of La2CuO4 powders prepared with different synthesis techniques. Synthesis Technique Agglomerate Agglomerate Grain Size Specific Surface Area Size (μm) Morphology (nm) (m2/g) Auto-ignition, fuel-rich 24±8 flakes 75±25 16.7 Auto-ignition, fuel-lean 19±5 nearly 85±50 7.57 spherical Co-precipitation 35±5 nets 140±50 9.15 Pechini 17±2 spherical 62±12 3.7
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2 μm A) 2 μm B)
2 μm C) 2 μm D)
Figure 3-4. FE-SEM images of electrode microstructures that were fabricated by screen-printing La2CuO4 powders synthesized with four different techniques. All samples were sintered at 800°C for 10 h. A) Synthesized with the auto-ignition, fuel-rich process. B) Synthesized with the auto-ignition, fuel-lean process. C) Synthesized via co- precipitation. D) SynthesizedA) with the Pechini method. B)
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2 μm 2 μm 700°C x 10 h 800°C x 10 h
A) B)
2 μm 2 μm 900°C x 10 h 1050°C x 10 h
C) D)
Figure 3-5. Electrode microstructures consisting of co-precipitated La2CuO4 powders which were screen-printed and then sintered for 10 h at various temperatures.
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2 μm A) 2 μm B)
2 μm C) 2 μm D)
2 μm E) 2 μm F)
Figure 3-6. FE-SEM images of electrodes fabricated by screen-printing La2CuO4 powders synthesized with four different techniques and sintered at different temperatures. A) Powders synthesized with the fuel-lean, auto-ignition technique, electrode sintered at 700°C for 10 h. B) Powders synthesized with the fuel-lean, auto-ignition technique, electrode sintered at 900°C for 10 h. C) Powders synthesized with the fuel-rich, auto- ignition technique, electrodes sintered at 700°C for 10 h. D) Powders synthesized with the fuel-rich, auto-ignition technique, electrode sintered at 800°C for 18 h. E) Powders synthesized with the Pechini method, electrode sintered at 700°C for 10 h. F) Powders synthesized with the Pechini method, electrode sintered at 800°C for 18 h.
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20 A) Pechini 650 ppm Auto-ignition 400 ppm fuel-lean 15 200 ppm
100 ppm
10 50 ppm
Co-precipitation 5 Voltage Change (mV) Voltage
Auto-ignition fuel-rich 0 0 20 40 60 80 100 120 Time (min) 15 B) Auto-ignition 650 ppm fuel-lean 400 ppm Co-precipitation
10 200 ppm
Pechini Auto-ignition 100 ppm fuel-rich 5
Voltage Change (mV) Change Voltage 50 ppm
0 0 20 40 60 80 100 120 Time (min)
Figure 3-7. Sensor responses to step changes in NO concentration in a background of simulated combustion exhaust at A) 450°C and B) 500 ºC. The sensors were fabricated with La2CuO4 powders synthesized with four different techniques and all sintered at 800°C for 10 h.
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20 30 A) B) 450 °C 650 ppm 400 °C 650 ppm 400 25 400 15 200 200 20 100 100 450 °C 10 15 50 50 500 °C 10 550 °C 5 5 500 °C Voltage Change (mV)
600 °C Voltage Change (mV) 0 0 020406080100120 020406080100120 Time (min) Time (min) 14 20 C) 450 °C 650 ppm D) 650 ppm 12 400 450 °C 400 200 15 10 200 100 8 500 °C 10 100 6 50 550 °C 500 °C 50 4 5 2 Voltage Change (mV) Change Voltage Voltage Change(mV) 550 °C 0 0 0 20406080100120 0 20406080100120 600 °C Time (min) Time (min)
Figure 3-8. Sensor responses to step changes in NO concentration in a background of simulated combustion exhaust at various temperatures. The sensors were fabricated with La2CuO4 powders synthesized with A) fuel-lean and B) fuel-rich variations of the auto-ignition techniques and the C) co-precipitation and D) Pechini methods.
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20 14 A) Pechini B) Auto-ignition fuel-lean 18 12 Co-precipitation 16 10 14 Auto-ignition Auto-ignition fuel-lean 8 fuel-rich 12 Pechini 6 10 Co-precipi tation 8 4
Voltage Change (mV) 6 Voltage Change (mV) 2 Auto-ignition fuel-rich 4 0 100 1000 100 1000 NO Concentration (ppm) NO Concentration (ppm) 100 40 C) D) Pechini 35 Co-precipitation 80 Co-precipitation 30 Auto-ignition 60 fuel-lean 25 20 40 15 Pechini Auto-ignition Auto-ignition fuel-rich 10 fuel-lean
Response Time Response (s) 20 Response Time (s) 5 Auto-igntion fuel-rich 0 0 10 100 1000 10 100 1000 NO Concentration (ppm) NO Concentration (ppm)
Figure 3-9. Voltage change and response time at 450°C (Figures A and C) and 500°C (Figures B and D) of sensors fabricated from powders prepared through different techniques and sintered at 800°C for 10 h.
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Table 3-2. La2CuO4 synthesis techniques with physical and electrical characterization results. Synthesis Surface Grain Sensitivity Response Time Technique Areaa Sizeb (mV/decade) (s)c (m2/g) (μm) 450 °C 500 °C 450 °C 500 °C Auto-ignition 16.7 0.2 9.69±4.09 2.63±1.17 21.29±3.74 15.65±3.68 fuel-rich Auto-ignition 0.25- 7.6 7.22±3.47 9.72±3.25 33.74±16.18 31.01±5.53 fuel-lean 0.40 Pechini 3.7 0.40 11.11±4.69 4.66±2.01 49.81±17.36 32.67±4.63 Co-precipitation 9.2 0.45 5.89±2.85 8.26±1.90 40.32±19.36 35.67±7.20 Notes. aB.E.T. surface areas measured on calcined powders. bGrain sizes estimated from micrographs of sintered electrodes. cAverage values of response times for five different ppm ranges based on 90% criteria.
3 R=0.977
slope=0.778 o intercept=-0.229 1050 C x 10 h 2.5
900 oC x 10 h 2
1.5 o log(response time)log(response (s) 800 C x 10 h
700 oC x 10 h
1 1.522.533.54 log(grain size) (nm)
Figure 3-10. NO response times for potentiometric sensors operating at 500°C that were sintered with different isothermal dwell temperatures (indicated in graph) and exposed to step changes in NO concentration in simulated combustion exhaust. The sensors were fabricated with La2CuO4 powders synthesized via co-precipitation.
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CHAPTER 4 EVIDENCE THAT NOX AND CO SHIFT THE FERMI LEVEL OF La2CuO4
4.1 Introduction
Potentiometric sensors with p-type semiconducting La2CuO4 electrodes exhibited ppm- level NO sensitivity, a fast response time, and O2-insensitivity in simulated combustion exhausts44. However, like most potentiometric sensors with metal oxide electrodes, the sensor responds not only to NO but also to NO2 and CO gases coexisting in the typical exhaust stream.
Furthermore, the sensing mechanism has not been well understood in the case of p-type metal oxide electrodes where the directionality of the signal (+mV for NO) is contrary to Mixed-
Potential theory predictions47,59. These sensors are better described by a more general theory called Differential Electrode Equilibria44. This theory additionally includes a contribution due to
Fermi level changes in semiconducting metal oxide electrodes resulting from gas adsorption.
4.2 Background
Mixed Potential theory has been successfully employed for explaining the non-Nernstian responses measured on oxygen sensors of the configuration (air, ref.)Pt|YSZ|Pt(exhaust)15,40,41.
According to Mixed-Potential theory, NO electrochemically converts to NO2 at the three-phase interface between gas, metal, and O2-ion conductor. The anodic reaction of NO produces electrons that are consumed at an equal rate by the cathodic reaction of O2 to Oxygen ions also at the three-phase interface41. The rates of these reactions are dependent on the TPB length and the electrocatalytic activity of the interface. The roles of other kinetic parameters such as diffusion and surface exchange coefficients have not been well established. The electrochemical reaction of NO to NO2 has a lower standard potential than the O2/Oxygen ion reaction; therefore, the net mixed-potential will be negative relative to an electrode in the absence of NO with a similar oxygen activity40. The sign of the mixed-potential is only dependent on the standard potential of
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the electrochemical reaction in question. Therefore, for electrochemical oxidation of CO for example, a negative mixed-potential is produced and for the electrochemical reduction of NO2 to
NO a positive change in potential is seen169. Naturally, as with all voltage measurements, the measurement of a potential requires a comparison of the potential of the electrode under study with the fixed potential of a reference electrode. Sensors with noble metal electrodes have been investigated extensively in a tubular geometry which employs a Pt electrode exposed to ambient or synthetic air149,169. The potential of this electrode is inherently fixed because temperature and oxygen activity are fixed. In this way, mixed potentials measured on noble metal electrodes showed signal directionalities consistent with the above discussion of standard potentials. One often overlooked caveat of Mixed Potential theory is that the sign of the mixed-potential is totally independent of the electrode material only the electrochemical reactions at the electrode/electrolyte interface. However, several studies showed that this is not always the case when using semiconducting metal oxides as electrodes47,59.
It is well established that when redox gases adsorb onto semiconductor electrodes, charge is transferred across the gas/semiconductor interface89,98,117,199. This alteration in charge carrier density can be measured as a change in resistance. Semiconductors that exhibit large changes in resistance, when exposed to small changes in gas concentration are said to be sensitive and have been used as commercial devices. These changes in electron concentration can be described as
Fermi level changes and the corresponding changes in work function have been measured104,117,118. In this way, gas adsorption can alter electrode potentials in a way altogether separate from the electrochemical reduction/oxidation reactions described by Mixed Potential theory.
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To illustrate this concept, a potentiometric sensor (Pt|YSZ|La2CuO4) and a resistance-type sensor based on a La2CuO4 sensing electrode were fabricated and electrically characterized.
These sensors were tested in various concentrations of NO, NO2, CO, O2 and CO2. Temperature
Programmed Reaction (TPR) and Temperature Programmed Desorption (TPD) were used to measure the heterogeneous catalytic activity of La2CuO4 and the kinetics of NO and NO2 adsorption/desorption.
4.3 Experimental
4.3.1 Powder Preparation and Characterization
La2CuO4 powder was synthesized using a new, patent-pending, wet-chemical combustion technique called the “Xerogel supported technique” or “XST.” La and Cu nitrates were dissolved in distilled water, a gelling agent (high purity Agarose) was mixed in carefully and the solution was left to cure and transform into a light-blue, soft, free-standing gel. The gel was diced into 1 cm cubes and dried at 80°C for 24 h in a drying oven. During the drying, the gel expands as it foams turning a florescent light-green. This foam was calcined at 650°C for 10 h in air.
The dried XST precursor was measured by simultaneous differential-thermal analysis and thermal-gravimetric analysis (DTA-TGA). The temperature of the sample was increased from room temperature to 1000°C at 3°C/min in air flowing at 30 sccm.
Diffraction patterns were collected for the calcined La2CuO4 powder using an X-ray diffractometer (X-pert, Phillips) in the Bragg-Brentano configuration. A pattern was collected from 20° to 70° 2θ using a step size of 0.02° 2θ with a count time at each step of 3 s. The sample was placed on a hot stage within the diffractometer and initially heated in air to 700°C and held for 3 h. Diffraction patterns were collected in flowing air on cooling at 1°C/min between each measurement temperature and after an equilibrating the powder sample for 1 h at each
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measurement temperature. The patterns were Rietveld refined using GSAS software available via NIST.
4.3.2 Potentiometric Sensor Fabrication and Voltage Monitoring
A paste for screen-printing sensor electrodes was prepared by ball-milling the XST- synthesized La2CuO4 powders with poly ethylene glycol (Avocado, PEG 400) and excess ethyl alcohol for 2 h. After evaporating the ethyl alcohol, the paste was screen-printed onto commercial dense YSZ rectangles (Marketech) of dimension (20 mm × 10 mm × 100 μm). On the opposite side, Pt (Heraeus) paste was screen-printed and Pt wires were embedded into each electrode. The device was sintered at 800°C for 10 h and subsequently viewed using a FE-SEM
(LEO, Supra 35).
The open circuit potential (OCP) of the sensor was monitored by attaching the Pt electrode to the negative terminal of a digital multimeter (Keithley 2000) and the La2CuO4 electrode to the positive terminal. Gas mixing was carried out using MKS mass flow controllers. The total flow was kept constant at 300 sccm with the typical background gas containing 3% O2 and a balance of N2. The concentration of either NO or NO2 was stepped up and down using steps of 0 ppm,
50 ppm, 100 ppm, 200 ppm, 400 ppm and 650 ppm for sensor OCP measurements.
4.3.3 Resistance-type Sensor Fabrication and Electrical Characterization
A thin square-shaped (2 mm x 2 mm) layer of the same La2CuO4 paste was deposited on a laser-cut Al2O3 substrate equipped with Au interdigitized electrodes. The Au electrodes were
200 μm thick, 200 μm wide and separated from one another by 200 μm. Gold paste was used to attach Au lead wires to the Au electrodes and the device was fired at 800°C for 10 h.
The resistance was measured with a digital multimeter (Keithley 2000).
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Electrochemical impedance spectroscopy (EIS) measurements were also made at 500°C in varied oxygen partial pressures. A 50 mV amplitude was used for the measurements which were done over a frequency range of 0.1 Hz to 10 MHz.
4.3.4 Mass Spectrometry
Temperature programmed reaction (TPR) and temperature programmed desorption (TPD) were used to characterize the catalytic activity and adsorptive properties of La2CuO4 concerning
NO and NO2. The basic setup consists of gas cylinders and mass flow controllers that regulate a gas stream. This gas stream is passed through a ~50 mg La2CuO4 powder sample, which is suspended by a quartz frit in a quartz tube. After passing through the powder sample, the effluent gas goes directly into a mass spectrometer (Extrel) for compositional analysis. The experimental procedure for both TPR and TPD begins by holding the powder sample at 500°C for several hours in 30 sccm of He to desorb any unwanted surface species.
For a TPR, the sample is then cooled to room temperature under He at which point the reaction gases to be characterized begin flowing and the temperature is ramped up to 800°C at
30°C/min.
For a TPD, the sample is cooled from 500°C to 300°C under He and exposed to the gas to be characterized for several hours at 300°C. The sample is then cooled to 50°C under a flow of the same gas. The gas is stopped and 100% He gas flows to purge the sample and then the temperature is ramped at 30°C/min until reaching 800°C.
4.4 Results
4.4.1 DTA-TGA, X-Ray Diffraction and FE-SEM Results of Calcined La2CuO4 Powder
DTA-TGA measurements of the precursor in flowing air showed that the precursor is transformed into La2CuO4 by 630°C during calcination (Figure 4-1). During heating, the precursor initially dehydrates around 100 °C accompanied by a large loss in mass. Upon further
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heating two more large mass losses are seen at 200 °C and 400 °C, the first of which is endothermic and the second is exothermic probably associated with the pyrolization of hydrocarbon groups.
The DTA-TGA results correlated with the results of the calcining trials. It was possible to synthesize 100 % phase-pure La2CuO4 only at and above 650°C. Below 650°C, there were always at least a few percent of secondary phases present in the powders.
Figure 4-2 shows the X-ray diffraction patterns collected for the calcined powder. At room temperature, the peak positions corresponded well with the peak positions list on powder diffraction file card 30-0487. The reflections were indexed with the space group Fmmm
(tetragonal) above 200°C and with F4/mmm (orthorhombic) below 200°C. As shown in the previous Chapter, La2CuO4-based potentiometric sensors are best suited to an operating temperature between 400°C and 600°C. In this temperature range, La2CuO4 is stable in the tetragonal form. Furthermore, in this dissertation, many electrochemical characterization and catalysis measurements were made on La2CuO4 as an electrode or as a powder at temperatures between 300°C and 700°C. The X-ray diffraction results tell us that, at least in air, no phase change occurred during any of these measurements. Near 200°C, the phase is mixed: partially tetragonal/partially orthorhombic as evidenced by the transition from two peaks at low temperature to one at high temperature in Figure 4-3. These two low temperature peaks are the
200 and 020 peaks and their coincidence above 200°C illustrates how the structure transitions from orthorhombic to tetragonal.
The lattice parameters were extracted from a reitveld refinement of a model following the atomic positions of Radaelli200. The lattice parameters are plotted in Figures 4-4A and 4-4B as a function of temperature along with the low temperature data of Campi et. al201. The linear
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thermal expansion coefficients of the tetragonal phase were calculated as: 15.2±0.7 x 10-6 K-1 for the A-axis, 15.2±0.7 x 10-6 K-1 for the B-axis and 16.0±1.4 x 10-6 K-1 for the C-axis. The symmetric expansion of the A and B axis makes sense in the tetragonal structure (Figure 4-5) the atoms and their relative 3-D positioning are equivalent in the X-axis and the Y-axis of the unit cell. The volume of the unit cell was calculated at each temperature and plotted in Figure 4-6.
The volumetric expansion coefficients were 1.96±0.63 x 10-5 K-1 for the orthorhombic phase and
4.65±0.24 x 10-5 K-1 for the tetragonal phase.
4.4.2 Potentiometric Sensor
The FE-SEM image in Figure 4-7 shows the highly asymmetrical grains of La2CuO4 illustrative of the room-temperature orthorhombic structure. The grain size was ca. 50 nm x 100 nm x 200 nm indicative of a relatively large surface to bulk ratio for La2CuO4. The sintered electrode was highly porous (Figure 4-7) and both of the sensor electrodes (Pt and La2CuO4) were about 10 μm thick and adhered well to the YSZ (Figure 4-8).
The sensor OCP was positive for NO and negative for NO2 (Figure 4-9). The response was faster at high temperature and larger in magnitude at lower temperature. These results can be explained based on the discussion of semiconductor properties given earlier in the text. NO (a reducing gas) adsorbs on the La2CuO4 surface forming a resistive space charge layer at the grain surface. The concentration of holes is reduced and the Fermi level rises proportionally. The Pt counter electrode Fermi-level is constant because Pt is a metal and the electron concentration is comparatively undisturbed by NO adsorption. Therefore, when the sensor is exposed to NO, the
Fermi level rises on the La2CuO4 electrode and the sensor response increases positively.
Recognizing that NO2 is an oxidizing gas, the NO2 response can be explained using the same rationale. Williams and Mosley stated that the “response of semiconducting oxides could be
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described using a simple effective medium model, in which the porous oxide was treated as a homogeneous semiconductor, but with a concentration of one kind of acceptor state which varied as a function of gas concentration”199. The diminishing response sensitivity with increasing temperature is indicative of the eventual extrinsic-intrinsic transition. As the temperature rises, so does the intrinsic semiconducting contribution to the total conductivity. Therefore, as the temperature is increased and nears the extrinsic-intrinsic transition temperature, a reduction in sensitivity is expected.
4.4.3 Resistance-type Sensor
The conductance of the porous La2CuO4 was measured in air and was seen to increase with increasing temperature (Figure 4-10). The conductance increased with increasing temperature.
This is contrary to the conductivity trend of bulk La2CuO4; which decreases with increasing temperature much like a metal. This confirms that high surface/bulk ratios, only achievable with nanometric powders and low temperature sintering conditions, emphasize the surface character of semiconducting electrodes.
Impedance measurements at 500°C showed a single semi-circle in the complex impedance plane suggesting a single process related to charge transport through the porous La2CuO4 (Figure
4-11). This was further verified by analyzing plots of log |Zj| vs log f (Figure 4-12). This process had a capacitance of 29.7 pF typical of a grain conduction process. Double layer effects at metal oxide/conductor interfaces are typically in the μF range ruling out the possibility of attributing a contact resistance to the semicircle202.
The oxygen partial pressure dependence was further investigated in the temperature range of 483°C-665°C using the two-pt. D.C. resistance method described in Section 4.4.3 (Figure 4-
13). The logarithm of the conductance had a 1/6 log PO2 dependence between 483°C and 665°C
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183 in accordance with literature . In general, researchers agree that every ½ O2 incorporated into
200,201 the La2CuO4 lattice forms an O2 interstitial and two holes .
The resistance-type sensor was exposed to steps in NO gas concentrations from 450°C to
650°C. Figure 4-14 shows the gas response at low temperature and high temperature, respectively. Like the potentiometric sensor, the conductometric sensor showed a slower response at low temperature making longer step times unavoidable. The ordinate axis for each plot shows the resistance value at any time normalized to the initial resistance value in the absence of NO. The resistance increases with each additional NO concentration increase as expected for a p-type semiconductor. The gas responses were also larger at lower measurement temperatures as in the case of the potentiometric sensor indicating an enhanced sensitivity.
Additionally, NO is stable under all the conditions tested and does not transform in the presence of O2 (Figure 4-15A). Therefore, NO adsorption is directly responsible for the resistance changes seen. TPD (Figure 4-15B) shows that NO is strongly adsorbed to La2CuO4 at temperatures less than 450°C. Above 450°C, NO can come to a dynamic equilibrium at the surface and the response time is improved.
Figure 4-16 shows the gas response of the semiconducting sensor upon exposure to NO2.
Focusing on the initial step from 0 ppm NO2 to 50 ppm NO2, there is a large decrease in
158 resistance in accordance with our earlier work on p-type LaFeO3 and SmFeO3 . The step change in resistance is larger for the lower temperatures as before and slower. However, NO2 is not stable like NO and heterogeneously decomposes over La2CuO4 above 450°C to form NO and
O2 (Figure 4-17). TPD showed that NO2 adsorption/desorption on La2CuO4 is much more complex than NO desorption (Figure 4-17). Unlike for NO, a direct relationship between the conductometric sensor and the potentiometric sensor was less obvious. Fermi level changes due
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to NO2 adsorption could be responsible for the potentiometric response; however, this claim becomes weaker at lower temperatures where the disparities in sensor responses become exacerbated. These disparities between the two sensors at low temperature may be because the potentiometric sensor had a ~50 μm thick La2CuO4 layer and the semiconducting sensor had a
~200 μm thick La2CuO4 layer.
A final comparison can be drawn between the general sensitivities and directionalities for the two sensors in the presence of typical pollutant gases. In Figure 4-18, the stable OCP values from each gas concentration step are plotted against the gas concentration on a log scale. In
Figure 4-18, the stable normalized resistance values are also plotted versus gas concentration on a logarithmic scale. The most obvious feature is that the directionalities and the relative magnitudes are rather similar for both sensors with the noted exception of O2. This is quite reasonable because both electrodes are exposed to the same environment in the case of the potentiometric sensor; therefore, the Nernst potential is zero. Of course, the resistance-type sensor is quite sensitive to oxygen, as was explained earlier, illustrating one of the key advantages of potentiometric sensors for combustion monitoring applications.
4.5 Summary
The NO potentiometric response can be entirely explained by semiconducting changes in
La2CuO4 due to gas adsorption. NO does not decompose or heterogeneously react over
La2CuO4. The NO2 potentiometric response can be explained in terms of conductivity changes in the semiconducting electrode at high temperatures. However, at low temperatures, where the sensor is especially sensitive to NO2, a direct relationship between La2CuO4 resistance changes and the potentiometric response is still difficult to draw. It was found that NO2 reacts over
La2CuO4 to form NO and O2 and that the NO2 desorption process is more complex than that of
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NO. Electrode thickness could be the cause of this non-uniformity and needs to be looked into further. The potentiometric sensor responded to reducing gases (NO, CO) with a positive OCP and oxidizing gases (NO2) with a negative OCP, all of which are contrary to the directions predicted by Mixed-Potential Theory. Differential Electrode Equilibria Theory can account for this disparity because it takes into account adsorption induced Fermi level changes and their effect on the measured potential. Therefore, Differential Electrode Equilibria is more appropriate than Mixed-Potential Theory for describing the sensing behavior of potentiometric sensors with La2CuO4 sensing electrodes.
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100 10 90 Heat Flow (C/ Flow Heat 80 0 70 -10 60 50 -20 Mass (%) Mass 40 μ g) 30 -30 20 -40 0 200 400 600 800 1000 Temperature (C)
Figure 4-1. Simultaneous DTA-TGA measurement of dried XST precursor in air.
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700 oC
600 oC
500 oC
450 oC
400 oC
350 oC
300 oC Relative Intensity(%)
250 oC
Tetragonal 200 oC
100 oC
Orthorhombic 25 oC
20 30 40 50 60 70 Diffraction Angle, 2Θ (o)
Figure 4-2. X-Ray diffraction pattern of calcined La2CuO4 powder between 25°C and 700°C.
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Counts
250°C 200°C 100°C 25°C
500
0 33 34 º2Theta
Figure 4-3. Detail X-ray diffraction pattern of calcined La2CuO4 showing tetragonal- orthorhombic transformation between 25°C and 250°C.
5.44 13.35 B Axis A & B Axis C Axis 5.42 13.30
5.40 13.25
5.38 13.20
5.36 13.15
5.34 13.10 Lattice Parameter (A) Parameter Lattice Lattice Parameter (A) B) A Axis A) 5.32 13.05 0 200 400 600 800 1000 0 200 400 600 800 1000 Temperature (K) Temperature (K)
Figure 4-4. La2CuO4 lattice parameters calculated from X-ray diffraction and neutron diffraction data as a function of temperature. A) Lengths of the A-axis and the B-axis as a function of temperature for La2CuO4 in air. B) Length of the C-axis as a function of 201 temperature for La2CuO4 in air. Low temperature data from Campi et. al. .
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Figure 4-5. Tetragonal La2CuO4 unit cell schematic. Local CuO6 structure is represented by green polygons, oxygen atoms are in red and lanthanum atoms are in blue.
392
390
) 388 3 386
384
Volume (A 382
380
378 0 200 400 600 800 1000 Temperature (K)
Figure 4-6. Unit cell volume of La2CuO4 as a function of temperature calculated from diffraction data collected in air. Low temperature data from Campi et. al.201.
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Figure 4-7. FE-SEM image of a La2CuO4 electrode sintered at 800°C for 10 h.
Platinum YSZ YSZ La2CuO4
A) B)
Figure 4-8. FE-SEM image of the cross-section of a typical prototype potentiometric NOX/CO sensor based on La2CuO4. The sensor was screen-printed at sintered at 800°C for 10 h. A) The Pt|YSZ interface. B) The YSZ|La2CuO4 interface.
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20 20 o o 450 C NO NO 450 C 10 10 o 0 ppm 500 oC 500 C 0 0 550 oC 50 -10 -10 100
o -20 o -20 550 C 500 C o 200 500 C
-30 450 oC -30 o 400 450 C Sensor Response (mV) Response Sensor Sensor Response (mV) Response Sensor -40 -40 650 ppm NO NO 2 2 -50 -50 0 102030405060 1.6 1.8 2 2.2 2.4 2.6 2.8 3 log [NO ] (ppm) Time (min) X
Figure 4-9. Potentiometric response of a La2CuO4|YSZ|Pt sensor in the presence of NO and NO2 in 3% O2 (balance N2) 0.005
0.004
) 0.003 -1 Ω
G ( 0.002
0.001
0 0 100 200 300 400 500 600 Temperature (C)
Figure 4-10. Conductance of porous La2CuO4 as a function of temperature in air.
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-6000 104.0 Pa -5000 Air -4000 ) 1 MHz
Ω 103.5 Pa ( -3000 imag
-Z -2000 104.5 Pa -1000 105.0 Pa 0 0 2000 4000 6000 8000 10000 12000 Z (Ω) real
Figure 4-11. Nyquist plot of the impedance measured for porous La2CuO4 in various oxygen partial pressures at 500°C.
10000 -5000
8000 -4000 -Z" (Ohms) -Z" 6000 -3000
4000 -2000 Z' (Ohms) Z'
2000 -1000
0 0 10-1 100 101 102 103 104 105 106 107 Frequency (Htz)
Figure 4-12. Bode plot of the real impedance (Z’, left Y-axis) and imaginary impedance (Z”, right Y-axis) as a function of measurement frequency of porous La2CuO4 in air at 500°C.
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-2.2 574oC 1/4 o 665 C o 1/6 483 C { o -2.3 606 C ) 1 - 507oC
-2.4
log G (ohm log G -2.5
3%O 21%O 2 2 -2.6 -2 -1.5 -1 -0.5 0 log P (Atm) O2
Figure 4-13. Conductance of porous La2CuO4 in various oxygen partial pressures between 483°C and 665°C.
1.8 1.4
A) B) o 450 oC 550 C 650 ppm 1.3 1.6 400
o 200 i 1.2 600 C 1.4 650 ppm 400 i 100 R/R 200 50 R/R 1.1 1.2 100
o 50 500 C 1 0 ppm 1 650 oC 0 ppm 0.9 0 40 80 120 160 200 240 0 40 80 120 160 200 240 Time (min) Time (min)
Figure 4-14. Normalized resistance of porous La2CuO4 upon exposure to step changes in NO concentration with a background of 3% O2 (balance N2) at A) 450-500°C and B) 550- 650°C.
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1100 14 A) B) 1000 O 12 2 900 10
800 8
700 6
600 4 NO 500 2 Concentration (ppm)Concentration
400 NO Concentration (ppm) 0 0 200 400 600 800 0 100 200 300 400 500 600 700 Temperature (C) Temperature (C)
Figure 4-15. NO adsorption/desorption and heterogeneous catalysis results for La2CuO4 powder. A) Results from a NO + O2 Temperature Programmed Reaction. B) Results from a 1% NO Temperature Programmed Desorption.
1.3 1.2 A) B) 0 ppm 1.2 1.1 0 ppm 1.1 1
o 50 ppm i i 450 C 1 0 ppm 0.9 o R/R R/R 0.9 650 C 0.8 600 oC o 0.8 500 C 50 ppm 0.7 100 0.7 50 200 100 650 0.6 o 200400 550 C 400 0.6 650 0 40 80 120 160 200 240 0 20 40 60 80 100 120 Time (min) Time (min)
Figure 4-16. Normalized resistance of porous La2CuO4 upon exposure to step changes in NO2 concentration with a background of 3% O2 (balance N2) at A) 450-500°C and B) 550- 650°C.
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500 40 B) A) 35 400 NO 30 2 NO NO 2 NO 25 300 20 O CO 2 O 2 200 2 15 N O 10 2 100
Concentration (ppm) Concentration CO Concentration (ppm) 5 0 0 0 200 400 600 800 200 400 600 Temperature (C) Temperature (C)
Figure 4-17. NO2 adsorption/desorption and heterogeneous catalysis results for La2CuO4 powder. A) Results from a 400 ppm NO2 Temperature Programmed Reaction. B) Results from a 1000 ppm NO2 Temperature Programmed Desorption.
80 A) 1.25 B) CO 60 CO 40 NO 1.00 NO 20 i O 0 R/R 2 0.75 -20 CO O 2 2 NO NO 2 -40 2 Sensor Response (mV) -60 0.50 2 3 4 5 6 102 103 104 105 106 10 10 10 10 10 Concentration (ppm) Concentration (ppm)
Figure 4-18. Steady-state responses of La2CuO4-based potentiometric and resistance-type sensors plotted as a function of pollutant concentration. A) Steady-state potentiometric sensor responses to NO, NO2, CO, CO2, and O2 at 450°C. B) steady- state normalized resistance changes in NO, NO2, CO and O2 at 600°C.
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CHAPTER 5 INVESTIGATION OF SENSORS WITH ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY
5.1 Introduction
Electrochemical impedance spectroscopy (EIS) is widely used in the fields of aqueous corrosion science and solid state ionics for probing the nature of electrochemical mechanisms173,203. Impedance, Z(ω), is the frequency-dependent, A.C., analog of resistance, R.
As such, an additional variable, the time domain, is available which can be manipulated by the experimenter to separate time-dependent phenomena. The advent of high quality electronics permits the experimentalist to access a wealth of information about their system of study by sweeping the frequency over at least eight orders of magnitude. In fact, great care must be taken to avoid artifacts in the data as well as in the selection of the proper model for extracting meaningful information.
The accurate measurement of impedance and the modeling/analysis of the data which follow are quite involved yet the accessibility of impedance measurements has made it one of the ~ most useful techniques in electrochemistry. A small sinusoidal voltage, ΔV , is applied between ~ two electrodes, the resulting current, I , is measured, and the impedance, Z(ω) , is calculated.
~ ΔV ΔV sin(ωt) Z(ω) = ~ = (5-1) I I sin(ωt + φ)
Since Z(w) is a time-dependent variable, it has real, Zreal, and imaginary, Zimag, components. It is possible to sweep the frequency of the sinusoidal voltage between 1 mHz and 1 MHz enabling the exploration of systems with time constants ranging more than nine orders of magnitude. The transport of O2-ions within various solid electrolytes including YSZ was studied many times with EIS. Electrodics were also investigated with EIS and in some cases it was possible to
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identify and quantify processes such as diffusion, adsorption/desorption, and charge- transfer204,205.
These processes are identifiable if they have time constants at least one to two orders of magnitude from one another. Reactant diffusion and adsorption/desorption are both slow processes so they limit the current in the steady-state, D.C. case, and at low frequencies, A.C.,
EIS case. The transport of O2-ions through solid electrolytes is generally the fastest process so it is observed at high frequencies. Charge-transfer is a fast electrode process but not as fast at O2- ion transport in electrolytes so its contribution to the sample impedance is observed at intermediate frequencies.
The time-constants of these processes can be envisioned as the time-scales for the processes and each has its own dependency on temperature, reactant concentration, materials, and microstructure. As a result, it is sometimes possible to extract a very detailed understanding of electrochemical systems by varying dependent variables such as temperature or gas concentration, measuring a process impedance, and developing a model which fits the collected data. If the model is valid then the researcher has an accurate understanding of the system.
EIS was used in the past to study potentiometric sensors, mostly focusing on the low frequency processes and their dependencies on NO, NO2, CO, C3H6, and
22,42,45,49,50,166,171,172,206-209 CH4 . Generally, but not always, the impedance at low frequencies decreased quite significantly as pollutant gases were exposed to the sample.
This result has been interpreted in several different ways. Firstly, it could be a result of the pollutants electrochemically reacting at the electrode|electrolyte interface thus altering the current-voltage curve for the electrode of study. Another interpretation is that the pollutants simply alter the electrolyte|gas interface, assisting or deterring the O2 exchange reaction. Finally,
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where heterogeneous catalysis occurs, it could be possible that pollutants react with O2 or decompose, producing O2 (e.g., NO2 decomposition). In either case, the O2 activity in the vicinity of the electrode would be changed. This would have an effect analogous to changing the
204 bulk gas stream O2 concentration by a very small amount since the pollutants were investigated usually at the ppm level. In any case, some theoretical analysis would be helpful for discerning the mechanism behind these results.
Some authors suggested that impedametric sensors could be used as pollutant sensing devices operating at a fixed frequency207-209. This may work in principle; however, a much easier application of the given transduction mechanism would be to measured the DC resistance of the device unless there was some other advantage (e.g., response time).
A potentiometric sensor based on La2CuO4 (LCO|YSZ|Pt) was prepared and characterized by EIS. Of course, a sensor of this form is an asymmetric cell. It is difficult to define the contributions to the impedance of an asymmetric electrochemical cell; therefore, symmetric cells
(LCO|YSZ|LCO and Pt|YSZ|Pt), which are easier to define theoretically, were also fabricated and measured to separate the competing electrode effects.
5.2 Experimental
La2CuO4 powder was synthesized as detailed in Chapter 4 using the XST method and a paste for screen-printing electrodes was prepared by ball-milling the La2CuO4 powders with poly ethylene glycol (Avocado, PEG 400) and excess ethyl alcohol for 2 h. After evaporating the ethyl alcohol, the paste was screen-printed onto dense, thin YSZ rectangles (Marketech) of dimension (20 mm × 10 mm × 100 μm). On the opposite side, Pt (Heraeus) paste was screen- printed and Pt wires were embedded into each electrode. The device was sintered at 800°C for
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10 h. These same pastes were also used to fabricate the symmetrical cells that were also fired at
800°C for 10 h.
Impedance spectra were measured with a frequency response analyzer (Solartron, 1260) in the frequency range of 10 mHz to 10 MHz using a signal of 30 mV. The impedance at each frequency was registered after a delay of four cycles, using a maximum of 3 cycles for auto- integration analysis. A quartz tube with a thermocouple very close to the sample was used for mounting the samples inside a 40 mm diameter tube furnace. Gas was mixed with mass flow controllers (MKS, 647C) and the total flow was maintained at a constant value of 300 sccm. The typical background gas contained 3% O2 and a balance of N2. Three static gas conditions were used: 3% O2 (N2 bal.), 200 ppm NO + 3% O2 (N2 bal.), and 200 ppm NO2 + 3% O2 (N2 bal.).
The samples were equilibrated overnight at 700°C in one of three gas compositions and EIS measurements were performed every 50°C from 700°C to 300°C on cooling. The concentration of either NO or NO2 was also stepped up and down using steps of 0 ppm, 50 ppm, 100 ppm, 200 ppm, 400 ppm, and 650 ppm at 450°C, 500°C, and 550°C.
5.3 Results and Discussion
5.3.1 High Frequency
Figure 5-1 shows a typical Nyquist plot of the impedance data for the sensor and the two symmetrical cells in 3 % O2 at 600 °C. The high-frequency intercept, usually associated with O2 ion transport in the YSZ, showed a significant contribution from each of the La2CuO4 electrodes.
175,179 This is because the bulk resistivity of La2CuO4 is at least five orders of magnitude greater than Pt and at 600 °C, they both conduct species whose transport processes have time constants smaller than those accessible by EIS at 1 MHz. Pt electrodes should have a negligible effect on the impedance associated with O2 ion transport in YSZ; therefore, the Pt|YSZ|Pt high frequency real-axis intercept was plotted in Arrhenius fashion to identify the YSZ contribution by itself
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(Figure 5-2). The activation energy of 1.073 eV/atom matched well with the typical values reported for O2 ion conduction in the YSZ lattice (1.1±0.1 eV/atom). Figure 5-2 also makes apparent the additional impedance of the La2CuO4 electrodes at high frequency.
A subtraction procedure was completed to extract the impedance of the La2CuO4 electrode from the total impedance measured at high frequencies. The high frequency real-impedance-axis intercept is equal to the sum of the contributions from the YSZ and each of the electrodes as shown in Equation 5-2.
Z Z Z (total) = Z (YSZ) + (#(LCO layers))( R,∞f ,LCO ) + (#(Pt layers))( R,∞f ,Pt ) (5-2) R,∞f R,∞f LCO layer Pt layer
Since the conductivity of Pt is so much higher than that of La2CuO4 and the YSZ, the
Z contribution of R,∞f ,Pt to the total impedance can be neglected. The Arrhenius plot in Figure Pt layer
5-2 shows that Z R,∞f (Pt YSZ Pt) is a reasonable substitution for Z R,∞f (YSZ) , thus Equation 5-2 simplifies to Equation 5-3.
Z Z (total) = Z (Pt YSZ Pt) + (#of LCO layers)( R,∞f ,LCO ) (5-3) R,∞f R,∞f LCO layer
The effect of each La2CuO4 electrode can be seen in Figure 5-1, where the sensor (having one
La2CuO4 electrode) has a real-axis intercept intermediate between that of the symmetrical Pt cell
(no La2CuO4 electrodes) and that of the symmetrical La2CuO4 cell (two La2CuO4 electrodes).
Since, the symmetrical La2CuO4 sample has two La2CuO4 electrodes, the impedance of each
La2CuO4 electrode on a symmetrical La2CuO4 cell is given by Equation 5-4.
Z (LCOYSZ LCO) − Z (Pt YSZ Pt) Z = R,∞f R,∞f (5-4) R,∞f ,LCO 2
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The dependence of Z R,∞f ,LCO on temperature and NOX was further investigated to identify its role in the potentiometric sensing mechanism.
5.3.1.1 La2CuO4 conduction mechanism
As stated above, the La2CuO4 electrode contributes to the high frequency impedance of the potentiometric sensor. Many EIS studies of resistance-type semiconductor sensors showed that the key conduction processes in porous semiconductor electrodes occur at electrode|contact interfaces, through the semiconductor bulk, and at intergranualar contacts. Discerning between these processes can be difficult if they have similar time constants. In this case, it was possible to do so by comparing the temperature dependence of the impedance data to the temperature dependence of the DC resistance of dense and porous La2CuO4 samples.
179 Dense La2CuO4 shows metallic conductivity above approximately 100 K . Conversely, porous layers of La2CuO4 show a temperature activated conductivity typical of a
159 semiconductor . Therefore, if the La2CuO4 conductivity decreases or stays constant as temperature increases, then bulk transport processes dominate. However, if the conductivity is temperature activated, then conduction through the surface of the La2CuO4 or intergranular contacts dominates the conduction mechanism. In Figure 5-3, the temperature dependence of the
DC conductance of a porous La2CuO4 layer is plotted together with the high frequency, real
impedance of a single La2CuO4 electrode ( Z R,∞f ,LCO ) measured with EIS and calculated with
Equation 5-4. The impedance of the La2CuO4 clearly decreases with increasing temperature just like the DC resistance of the porous sample (Figure 5-3). This indicates that surface conduction on La2CuO4 contributes to the sensor impedance at high frequency.
The low conductivity of the porous La2CuO4 is responsible for the impedance contribution and this effect depends on the electrode materials and the geometrical parameters of the sample.
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We selected the term “ohmic” contribution, in the sense of an “ohmic drop” in an electrolyte, to describe the additional impedance at high frequency from a resistive electrode because it has no imaginary component (like the impedance response of a solid electrolyte at intermediate/high temperatures).
The ohmic electrode contribution to the total impedance arises from the fact that charge needs to be transferred between the electrical lead, which goes to the impedance analyzer, and the electrode|electrolyte interface. The ohmic contribution of an electrode will scale with the resistivity of the electrode material, increasing for less conductive electrodes (e.g., WO3) and decreasing for more conductive electrodes like Pt or Au.
Furthermore, this resistance will scale with the area of the electrode because the average distance between the lead wire and the electrode|electrolyte interface is longer. However, this effect can be suppressed by the use of a current collector mesh since the total path length within the semiconductor is minimized. Finally, this ohmic electrode effect may have been overlooked in other work because the electrolytes were thick (mm-scale vs. this study 0.1 mm) so the impedances of the electrolytes were much larger than the relatively small ohmic electrode impedances.
5.3.1.2 Effect of NOX on La2CuO4 surface conductivity
The high frequency real axis intercept of the samples with La2CuO4 electrodes changed in
NO and NO2 but that of the symmetrical Pt sample did not (Figure 5-4). This is because the
159 conductivity of Pt is relatively unaffected by NO or NO2 but the conductivity of La2CuO4 is .
Upon exposure to NO or NO2, the sensor and the symmetrical La2CuO4 cell both shifted to larger impedances for NO exposures and smaller impedances for NO2 exposures, consistent with the p-
158,159 type semiconducting behavior of La2CuO4 . Although not shown, the impedance of the
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sensor and symmetrical La2CuO4 sample also decreased in O2 consistent with p-type semiconductivity.
The relative effect of NO and NO2 on the ohmic La2CuO4 contribution was dependent on temperature. Impedance spectra were measured every 50 °C between 300 °C and 650 °C. The effects of NO and NO2 on the ohmic La2CuO4 impedance were calculated from impedance data for symmetrical La2CuO4 cells using Equation 5-4 and normalizing the impedance of a single
La2CuO4 electrode (ZR,LCO) in NOX to that in the absence of NOX with Equation 5-5.
Z Z (t) Z (t) R = R = R,∞f ,LCO (5-5) Z R,o Z R ([NOX ] = 0 ppm) Z R,∞f ,LCO ([NOX ] = 0)
Z R There is a maximum in at 450 °C for NO2 and between 350 °C and 400 °C for NO (Figure Z R,o
5-5). The temperatures at which these maximums in resistance change take place are the same as the temperatures where the sensor device is most sensitive (ΔmV/dec) to NOX, which demonstrates that the two are likely linked.
Temperature programmed desorption showed that NO strongly adsorbs onto La2CuO4 at temperatures at and below 350 °C159,193 in good correspondence with the normalized impedance changes in NO (Figure 5-5). Temperature programmed desorption experiments showed that NO2 and its decomposition products fully desorb between 400°C and 500 °C74,159,193. The steady-state
159 potentiometric responses of the sensor to 200 ppm of NO and NO2, from previous work , were also included in Figure 5-5 for comparison. At 550 °C the NO response is basically zero and the conductivity of the La2CuO4 was also unaffected by NO at 550 °C.
Meanwhile, the sensor OCP decreased in NO2 at 550 °C and the impedance of the
La2CuO4 correspondingly decreased as well (Figure 5-5). The relative impedance change for the
La2CuO4 increased with decreasing temperature for both NO and NO2. The magnitude of the
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sensor response (ΔOCP) to 200 ppm of NO and NO2 also increased with decreasing temperature.
In our previous work, we observed similar temperature dependence of the relative change in DC resistance of porous La2CuO4 when exposed to NOX.
By isolating the impedance contribution of the La2CuO4 at high frequency, it was possible also to track the conductivity of the La2CuO4 layers during transient exposures to NOX. This was accomplished by measuring the impedance of symmetrical La2CuO4 cells at a fixed frequency rather than sweeping it. 100 kHz was selected because the impedance results remained near to the real impedance axis between 450 °C and 550 °C and for NOX exposures between 0 and 650 ppm. The real impedance at 100 kHz for an entire La2CuO4|YSZ|La2CuO4 cell is shown in Figure 5-6 as NO and NO2 concentrations are stepped up and down every 5 min.
The impedance in the absence of NOX decreases as temperature is increased because conduction in both the YSZ and La2CuO4 are temperature activated
The impedance increased in NO and decreased in NO2, again consistent with the p-type semiconductivity of La2CuO4. The impedance reached a steady state after five min at 500 °C and 550 °C and was somewhat slower at 450 °C (Figure 5-6). This corresponds almost exactly with the transient measurements we reported for La2CuO4-based potentiometric sensors30,44,56,74,159. Furthermore, the impedance increased much more because of the NO at 450
°C than it did at 550 °C as shown earlier in this study while sweeping the frequency at fixed gas concentrations (Figure 5-6).
The impedance of a single La2CuO4 electrode was calculated with Equation 5-3 from the
100 kHz impedance data. This value, which depended on the NOX concentration at any point during the experiment, was normalized with Equation 5-4 to focus on the sensitivity of the resistance changes to gas concentration. This ratio is not influenced by the temperature
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Z R dependent conductivity of La2CuO4 or YSZ. This normalized resistance, , tabulated at the Z R,o end of each five min exposure to NO or NO2, was plotted in Figure 5-7 versus the log of the NO or NO2 concentration at each step. The concentration and temperature dependencies of the sensor OCP responses (measured in our previous work) and the La2CuO4 resistance changes
(Figure 5-7) were very similar. The resistance changes of La2CuO4-based resistance-type
159 sensors resulted from adsorption/desorption of NO and NO2 . The similarities between the trending of the ohmic La2CuO4 impedance and the potentiometric sensor response with NOX and temperature suggest that adsorption/desorption of NOX on La2CuO4 electrode determines the potentiometric sensor response.
5.3.2 Low Frequency
The low frequency part of the impedance spectra for all three samples consisted of a single semi-circle (Figure 5-8). The Pt symmetrical cell had a much larger semicircle than the La2CuO4 symmetrical cell and the semicircle for the sensor was intermediate in size of the two symmetrical cells. The centers of the semicircles were depressed below the real impedance axis; therefore, the semicircles were fit with an equivalent circuit consisting of a resistor and a constant phase element (CPE) in parallel. Both NO and NO2 decreased the impedance for all three samples at low frequency. Typically, NO2 decreased the impedance more than NO. NO2 also changed the OCP of the sensor (Figure 4-9) more than NO. This is described further in
Chapter 6.
Impedance spectra were taken at 500°C every five minutes until a steady-state was reached. Upon NO introduction the impedance decreased and reached a steady-state after 20 minutes. The impedance of the sensor decreased continuously as the NO concentration was
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increased (Figure 5-9). Although not shown here, NO2 and O2 both changed the size of the semicircle for the sensor.
In summary, NO, NO2, and O2 all decrease the impedance of all three samples at low frequency which corresponds to the steady-state exchange between the electrodes, electrolyte, and the gases. NO2 changed the impedance more than NO similar to how the potentiometric sensor responded more to NO2 than NO. This is a third contribution to the overall Differential
Electrode Equilibria, where the potential of the La2CuO4 electrode can be altered by: (1) the heterogeneous catalysis of NO2, but not NO; (2) the adsorption of NO and NO2 on its surface; and (3) the rate of exchange between the electrode|electrolyte and the external environment. The potential of the Pt electrode is affected by heterogeneous catalysis and external exchange but not adsorption of NOX because of its metallic character. The sensor response equals the difference between the potentials of each electrode which are determined by the various contributions to the equilibria pertinent to each electrode44,56.
5.4 Summary
The impedance spectra for the sensor consisted of three features that were subsequently identified. O2-ion conduction in the YSZ and the transport of charged species on the surface of the La2CuO4 electrode manifest as the high-frequency real-impedance-axis intercept. In the case of a Pt symmetrical cell, no changes in impedance with NO or NO2 exposure were seen at high frequencies. In contrast the high frequency intercept of cells containing La2CuO4 did. This additional impedance was attributed to surface conduction on the La2CuO4 electrodes.
The temperature, gas and gas concentration trends of the La2CuO4 surface conductivity were all very similar to that of the sensor OCP. Conductivity changes in resistance-type sensors are most frequently explained by adsorption and other non-electrochemical mechanisms.
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Therefore, non-electrochemical processes fundamentally contribute to the differential electrode equilibria of sensors containing semiconductor electrodes.
A semicircle was observed at low frequencies related to the capacitance and resistance associated with charge-transfer during the oxygen-reduction reaction at the YSZ/electrode/gas interface. The size of this semicircle decreased when either the sensor or the symmetrical cells were exposed to NO and NO2. This is an additional source of differential electrode equilibria.
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-400 Pt / YSZ / Pt 102 Hz -300
Z (Ω) imag -200
103 Hz Sensor 104 Hz -100 103 Hz LCO/YSZ/LCO 0 0 100 200 300 400 Z (Ω) real
Figure 5-1. Nyquist plot of a potentiometric sensor and two symmetrical cells measured at 600°C in 3% O2 (balance N2).
6 Pt|YSZ|Pt 4 Sensor 2
0 LCO|YSZ|LCO -2 ln G (S) -4
-6
-8 1 1.2 1.4 1.6 1.8 2 2.2 1000/T (K -1)
Figure 5-2. Arrhenius plot of the high frequency real-axis intercepts for each of the three samples measured in 3% O2 (balance N2).
99
0.003 La CuO Surface Conductance 2 4 0.0025 AC La CuO 2 4 0.002 Component
0.0015
0.001 Conductance (S) Conductance
0.0005
0 0 100 200 300 400 500 600 700 800 o Temperature ( C)
Figure 5-3. DC surface emphasized La2CuO4 conductance plotted together with the AC La2CuO4 component calculated from La2CuO4 and Pt symmetrical cells and multiplied by 5 for clarity.
-100
-80 LCO|YSZ|LCO Pt|YSZ|Pt
-60 Z (Ω) 100 kHz imag -40 Sensor
-20
0 0 50 100 150 200 w/ 200 ppm NO 2 Z (Ω) w/ 200 ppm NO real
Figure 5-4. Nyquist plot showing the effect of 200 ppm of either NO or NO2 on the high frequency impedance response for each of the three cells at 500°C.
100
1.3 30 NO 1.2 20 Sensor(mV) OCP
1.1 10 R,o
/Z 1 0 R Z
0.9 -10
0.8 NO -20 2 -30 0.7 350 400 450 500 550 600 650 Temperature ( oC)
Figure 5-5. Impedance of a single La2CuO4 electrode in 200 ppm NO or NO2 normalized to the impedance in the absence of NOX calculated from impedance data measured with a La2CuO4|YSZ|La2CuO4 sample in a background of 3 % O2 (balance N2). Sensor (La2CuO4|YSZ|Pt) OCP response data to 200 ppm of NO and NO2 is included.
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450 oC NO 250 ) o
Ω 450 C NO 2 ( 200
Real o Z 500 C NO
550 oC NO
150 500 oC NO 2 550 oC NO 2 0 1020304050 Time (min)
Figure 5-6. The real component of complex impedance measured at 100 kHz for a La2CuO4|YSZ| La2CuO4 cell. NO (closed symbols) and NO2 (open symbols) concentrations were individually stepped every 5 minutes at 450°C, 500°C and 550°C.
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1.3
1.2 NO 450 oC
1.1 NO 500 oC ]=0) X 1 NO 550 oC ([NO
R o 0.9 NO 550 C 2 / Z R Z 0.8 NO 500 oC 2 0.7 NO 450 oC 2 0.6 1.6 1.8 2 2.2 2.4 2.6 2.8 3 log [NO ] (ppm) X
Figure 5-7. Real component of complex impedance associated with surface conduction on La2CuO4 when exposed to various NO and NO2 concentrations at 450°C, 500°C, and 550°C normalized to its initial value when unexposed to NOX. Impedance measured at 100 kHz in a background of 3% O2 (N2 bal.).
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-80000 Pt|YSZ|Pt
-70000
-60000
-50000 Z (Ω) imag Sensor -40000 Pt|YSZ|Pt NO
-30000 1 Hz Pt|YSZ|Pt NO Sensor 2 -20000 NO Sensor NO -10000 2 LCO|YSZ|LCO
0 0 10000 20000 30000 40000 50000 60000 70000 80000 LCO|YSZ|LCO LCO|YSZ|LCO Z (Ω) NO NO real 2
Figure 5-8. Nyquist representation of impedance data obtained for a potentiometric NOX sensor (La2CuO4|YSZ|Pt)and two symmetrical cells (Pt|YSZ|Pt and La2CuO4|YSZ|La2CuO4) measured at 500°C in 3% O2 (balance N2).
104
30
0.1Hz 0 ppm NO 20 100 ppm NO -Z (kΩ) imag 1Hz 200 ppm NO
10 400 ppm NO 650 ppm NO
0 0 10203040 Z (kΩ) real
Figure 5-9. Nyquist representation of the impedance data obtained for a potentiometric NOX sensor (La2CuO4|YSZ|Pt) at 500°C in different NO gas concentrations and a background of 3% O2 (balance N2).
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CHAPTER 6 EFFECT OF NO AND NO2 ON THE POLARIZATION RESISTANCE AND POTENTIOMETRIC RESPONSES OF La2CuO4 AND Pt ELECTRODES
6.1 Introduction
One intriguing method for discriminating between sensing mechanisms is the analysis of
D. C. polarization data. In many cases, researchers have seemingly confirmed that the mixed- potential mechanism is the origin of observed non-Nernstian potentials through analysis of D. C. polarization data41,46,169,210. They do so by constructing “quantitative mixed potential diagrams” by plotting “modified polarization curves” in order to “verify the sensing mechanism.” While these diagrams do illustrate how non-Nernstian potentials could result from multiple electrochemical reactions, the exclusivity of this approach has yet to be determined.
In fact, it is also possible to construct an identical diagram even if the electrode potential has been shifted to lower or higher values without the occurrence of multiple electrochemical reactions. This approach, by itself, yields results that may falsely confirm the sensing mechanism and therefore need reconsideration.
As discussed in detail in Chapter 2, in environments containing O2, a steady state is achieved when the flux of O2 ions entering an O2-ion conductor is equal to that exiting (Equation
6-1). The electrochemical reaction of NOX may be generically represented by Equation 6-2.
Re(g) and Ox(g) in Equation 6-2 represent NOX in its reduced and oxidized forms, respectively and the magnitude of the current resulting from it is related to the reaction rate.
1 Reaction 1 O (g) +V •• (YSZ) + 2e − (electrode) ↔ O x (YSZ) (6-1) 2 2 O O
X − •• Reaction 2 Re(g) + OO (YSZ) ↔ ne (electrode) +VO (YSZ) + Ox(g ) (6-2)
As is typically done in the literature, a modified polarization curve is constructed by polarizing a sensing electrode in a base gas (typically air) and the resulting net current (I1) is
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measured (data points in Figure 6-1A (base)). This polarization curve represents the current- potential characteristic for the reaction given by Equation 6-1. The electrode is then polarized a second time after introducing the pollutant gas under study and the net current (I2) is measured
(data points in Figure 6-1B, base + redox gas). It is assumed that any difference in current (IΔ) between the two measurements (I1 and I2) is equal to the current contribution of a reaction represented by Equation 6-2. The net current is equal to the sum of each of the current contributions (Equation 6-3).
Inet =+II1 Δ or IIInet==2Re1Re2 action + I action (6-3)
A comparison of the currents (I2 - I1) at any specific potential (E) will be different by an amount
(IΔ) and a list of (E, IΔ) coordinates can be tabulated. I1(E) and IΔ(E) then represent the half-cell
I(E) characteristics for the reactions given by Equation 6-1 and Equation 6-2, respectively.
The slopes (m1 and mΔ) for each of the half-cell I(E) characteristics are inversely proportional to their polarization resistances (R1 and RΔ). Each of the half-cell I(E) characteristics intersect the current axis at I°1 and I°Δ. If I1(E) and I2(E) are straight lines, they can be described by Equation 6-4 and Equation 6-5, respectively.
o I1(E) = m1E + I1 (6-4)
o I 2 (E) = m2 E + I 2 (6-5)
o o o IΔ will also be a straight line given by Equation 6-6, where mΔ = m2 − m1 and I Δ = I 2 − I1 .
o I Δ (E) = mΔ E + I Δ = I 2 (E) − I1(E) (6-6)
The open circuit potentials are then given by Equation 6-7A and Equation 6-7B.
o o − I1 − I 2 EOCP (base) = , EOCP (base + redox gas) = (6-7A and 6-7B) m1 m2
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According to Mixed Potential theory, I1 and IΔ are opposite in sign because they are the sums of the anodic currents and the sum of the cathodic currents. The net current is zero at open-circuit is given by Equation 6-8.
I net = I1 + I Δ = 0 (6-8)
Having already solved for IΔ(E), the mixed potential can be solved for after rearranging Equation
6-8 is given by Equation 6-9.
I1 = −I Δ (6-9)
By substituting Equations 6-4, 6-5, and 6-6 into Equation 6-9 yields Equation 6-10.
o o o m1E + I1 = −1[(m2 − m1 )E + (I 2 − I1 )] (6-10)
This can be represented graphically by multiplying one of the currents, either I1(E) or IΔ(Ε), by -
1 or by taking their absolute values such that after plotting I1(E) and IΔ(Ε) intersect (Figure 6-
1B)41,46,169. Solving for E gives the mixed potential (Equation 6-11).
o I 2 Emix = − (6-11) m2
The potential shift due to the NOX is equal to Equation 6-11 minus Equation 6-7A (Figure 6-1B) and is given by Equation 6-12.
o o I1 I 2 ΔE = Emix − EOCP (base) = − (6-12) m1 m2
This result is identical to the potential difference (ΔOCP) given by subtracting Equations 6-8A from 6-8B (Figure 6-1A) giving Equation 6-13.
o o I1 I2 ΔOCP = EOCP (base + redox gas) − EOCP (base) = − (6-13) m1 m2
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The mixed-potential calculated by analyzing polarization curves in this manner will always yield a potential difference equal to that measured at open-circuit. This identity is true irrespective of the I-V slopes or the formal electrode potentials of the redox couples.
It has become customary in the literature to attribute significance to the close agreement between the OCPs measured during step changes in pollutant gas concentration and the mixed- potentials determined by analyzing polarization curves in the aforesaid manner41,169,210.
However, in the total absence of supporting evidence, the sensing mechanism cannot be determined based solely on the similarity between ΔOCP and Emix-EOCP(base gas). Furthermore, it is possible to predict the same OCP shift considering an entirely different mechanism.
Consider a semiconductor electrode, interfaced with an ionic conductor, situated in the absence of pollutant gas. Suppose a D. C. polarization measurement results in I1(E) (Figure 6-
1A (base)). A pollutant gas is introduced and it subsequently adsorbs on the electrode changing its work function, Fermi-level and potential45,89,112,117,118. If the electrode is polarized again, this time in the presence of pollutant gas, the OCP has shifted to some new value (Figure 6-1A, base
+ redox gas). IΔ(E) is again given by Equation 7 and presented in Figures 6-1A and 6-1B.
Equations 6-12 and 6-13 still hold; therefore, ΔOCP equals Emix-EOCP(base gas) as before.
In summary, the coincidence of OCP changes as measured directly with an electrometer and those revealed by constructing quantitative I(E) diagrams from D. C. polarization data is not standalone confirmation that the Mixed-Potential mechanism occurs. Additionally, it is well known that parallel redox reactions can contribute significant errors to the half-cell currents estimated from polarization data; therefore, reaction schemes should be confirmed211. Additional experimental evidence (e.g. gas composition analysis) is prerequisite to attributing D. C.
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polarization results to a single sensing mechanism. In fact, several authors have commented on the complexity of the situation40,42,44,59 .
In order to investigate the sensing mechanism(s), solid-state electrochemical cells were constructed using a solid-state adaptation of the Luggin’s-probe configuration. This configuration is relatively new to the field of solid-state electrochemistry and has several advantages over traditional configurations including, reduced frequency dispersion during EIS measurements, D. C. overpotential measurements with a bias-independent error structure and a novel benefit in our case: the ability to separate individual electrode responses (ΔOCP) from each other without the use of a reference environment.
6.2 Luggin’s Probe Configuration
The Luggin’s Probe configuration has been discussed by several authors in the field of solid-state electrochemistry as a tool for accurately measuring electrode overpotentials using both A. C. and D. C. techniques205,212,213. The configuration consists of two current carrying electrodes: a working electrode (WE) and a counter electrode (CE); and a reference electrode
(RE), which is a local voltage probe. Luggins developed REs for aqueous electrochemistry by embedding REs in capillaries such that a RE could be placed as close as possible to a WE thereby reducing the ohmic drop between the two. Ideally, the RE is of negligible size compared to the WE (small rRE/rWE in the solid-state analog) and requires very little current to obtain an accurate reading of the local potential. The solid-state analog of the Luggin’s Probe configuration used in this study consists of an RE inserted inside of a cylindrical YSZ pellet
(Figure 6-2). Discussion of the various cell configurations including: consideration of electrical circuit analogs, finite elemental analysis, and laboratory measurements can be found here 212-214.
This is most likely the first application of this measurement configuration within the field of
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solid-state gas sensors and while it bears some resemblance to the embedded electrode work done by Garzon et al., the work is totally different in purpose17.
6.3 Experimental
6.3.1 Sample Preparation
Electrochemical cells, in the Luggin’s Probe configuration, were fabricated as follows.
Nanometric YSZ powder (8YSZ, TOSOH, Japan) was mixed with a solution of poly vinyl buytral (PVB, Alfa Aesar, Ward Hill, MA) dissolved in Acetone such that after evaporating the
Acetone the PVB was 2 weight percent of the total mixture. This mixture was isostatically pressed into cylinders at 200 MPa for 3 min. The cylinders were cut to lengths of approximately
1.5 cm and a hole was drilled into one end to a depth of ~0.5 cm with a drill bit of diameter 1.04 mm yielding a l2/rRE ratio of ~20. The ends of the pellets were sanded and the samples were heated at 5°C/min until reaching 1500°C where they were sintered for 6 hrs and then cooled at
1°C/min to room temperature. The pellets were inspected with an electron microscope (6335F,
JEOL, Japan) and found to be dense.
A thin Pt wire was then dipped into Pt paste (Heraeus, West Conshohocken, PA) and inserted into each pellet followed by a loose backfilling of YSZ powder. The samples were then sintered at 1200°C for 30 min with a heating and cooling rate of 5°C/min. The pellet ends were then sanded and electrode pastes consisting of either La2CuO4 or Pt were applied. Details of the
La2CuO4 synthesis and subsequent paste preparation procedure were given in Chapter 2. Finally,
Pt lead wires were applied to each electrode and the devices were fired at 800°C for 10 h with heating and cooling rates of 5°C/min.
The final pellet diameter was 7.35 mm and this number was used for calculating current densities. In this study, the CE was always porous Pt and the WE was either porous Pt or porous
La2CuO4. The sample with a Pt WE had an l1/LT=0.325 and an l2/LT=0.675, while the sample
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with a La2CuO4 WE had an l1/LT=0.301 and an l2/LT=0.699 with an estimated error maximum of
±0.065.
A second set of Luggin’s probe cells were made in a similar manner; however, Al2O3 powder was substituted for the backfilling over the Pt reference electrode. The cross-sectional area of the cell was also increased substantially and the thickness reduced while retaining an l2/rRE ratio greater than four.
6.3.2 Electrical Characterization
Each sample was tested in the same quartz tube (40mm I.D., 350 mm length) equipped with gold lead wires, a K-type thermocouple and ports for gas inlet and exhaust. The temperature was recorded within +/-0.2 K of the set point during all measurements. Six gas cylinders: 1000 ppm NO (balance N2), 1000 ppm NO2 (balance N2), 1000 ppm CO (balance N2),
100% O2, 100% N2 and 100% CO2 were manifolded together and their individual flow rates were monitored and controlled with mass flow controllers (M100B, MKS, Wilmington, MA) maintaining a constant total flow rate of 100 sccm. The potential differences between the WE,
CE and RE were monitored with a digital electrometer (2000, Keithley, Cleveland, OH) equipped with a scanning card. A dual potentiostat/frequency response analyzer (PARSTAT
2273, AMETEK, Paoli, PA) was used to make D. C. and A. C. measurements. The potential of the WE was linearly swept at 0.1 mV/s typically from OCP to ±1 V. For A.C. measurements, the potential of the WE was sinusoidally varied by 30 mV r.m.s. with a frequency that was varied from 1 MHz down to 10 mHz.
6.4 Results
6.4.1 EIS Measurements Part 1
The integrity of the cell configuration was verified with EIS. The impedance of a sample with a Pt WE and a Pt CE was measured in a conventional two-point configuration and also in
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two different three-point configurations to assess the degree with which the RE measured a localized potential. An indication of the electrolyte resistance (RYSZ) can be seen in Figure 6-3 as the high frequency, Zreal-axis intercept for each of the data sets. RYSZ,WE/RYSZ,2pt=0.707 and
RYSZ,CE/RYSZ,2pt=0.280 in agreement with the l1/LT and l2/LT ratios for the sample. The concurrence of the electrolyte resistance partitioning and the length ratios confirms that the RE measured a localized potential. The electrode processes can be visualized at lower frequencies.
Both three-point configurations yielded nearly identical spectra for two nominally identical Pt electrodes. The Zreal and Zimag were tabulated as a function of frequency for each of the data sets acquired in three-point configurations and they were summed to yield the “calculated” locus of points (Figure 6-3). A 45° angle is seen at intermediate frequencies which has been ascribed to
215 the perturbation of the O2 concentration profile adjacent to the TPB .
A similar set of measurements was completed with a La2CuO4 WE instead of Pt WE
(Figure 6-4). As seen before, the electrolyte resistance was partitioned according to the geometrical parameters of the cell (Figure 6-4). At low frequencies, a single semicircle is apparent in the complex plane plots with a leading 45° angle suggesting a diffusional process is also important in the reduction/oxidation of O2 on La2CuO4. The La2CuO4 electrode clearly exhibits a much lower resistance to the reaction given by Equation 6-1 compared to the Pt electrode. Since R1(Pt) is greater than R1(La2CuO4), the slopes (m) of I-V plots measured in the absence of pollutant gases should be larger for the La2CuO4 electrode than the Pt one, i.e. m1(La2CuO4)>m1(Pt). Pt has been termed an equilibrium electrode due to its ability to bring redox species to their equilibrium concentrations quickly. At least in the case of O2, La2CuO4 shows greater kinetic facility than Pt. In the mixed-potential scenario, electrodes with poor O2 kinetics should exhibit larger potentiometric responses191.
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6.4.2 Potentiometric Characterization
Potentiometric sensors with La2CuO4 and Pt electrodes were sensitive to NO, NO2 and CO in the temperature range of 400°C to 600°C. 500°C was selected for initial investigation because at 500 °C, the time required to reach a steady-state potential on each electrode relative to the reference electrode was experimentally attainable and the potentials were large enough to be separated easily from the baseline noise level. The second set of samples with Al2O3 backfills were used for these experiments.
A sample with a La2CuO4 and Pt electrodes was exposed to varying concentrations of
NOX, COX and O2 at 500°C. The potential of the La2CuO4 electrode relative to the RE,
E(La2CuO4 vs. RE), and the Pt electrode relative to the RE, E(La2CuO4 vs. RE), were both monitored as well as the potential difference between the two exposed electrodes, E(La2CuO4 vs.
Pt).
The sample was exposed to CO2 concentrations between 0 and 30 vol. % in the baseline of
3% O2 (N2 bal.). The potentials of the exposed electrodes relative to the RE, E(La2CuO4 vs. RE) and E(Pt vs. RE), changed less than 3 mV at all CO2 concentrations tested (Figure 6-5A). The potential difference between the two exposed electrodes La2CuO4 and Pt electrodes was less than
1 mV.
Unlike CO2, the potentials of the La2CuO4 and Pt electrodes, referenced to the RE, were strongly dependent on O2 concentration (Figure 6-5B). The potentials of the exposed electrodes, referenced to the RE, changed from -20mV in 3% O2 to + 35 mV in 100% O2 (Figure 6-5B).
The potentials of the exposed electrodes, E(La2CuO4 vs. RE) and E(Pt vs. RE), changed at similar rates upon step changes in O2 concentration which showed up in the La2CuO4 vs. Pt signal as small (<1mV) transient voltage discontinuities up or down that relaxed back to zero relatively quickly (~1 min). EIS measurements (Figures 6-3 and 6-4) showed that La2CuO4 had
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superior O2 electroreduction kinetics than Pt; however, this was not evident in the electrodes’ transient and steady-state potentiometric responses to changes in O2 concentration.
Changes in the concentration of both O2 and CO2 resulted in changes in the potential of the
La2CuO4 electrode that were very similar to those of the Pt electrode. As a result, the steady- state potentiometric response of sensors based on La2CuO4 and Pt electrodes that are both exposed to the same environement are insensitive to changes in CO2 and O2 concentration because the potential difference between the electrodes, ΔE(La2CuO4 vs. Pt), is zero (Equation 6-
15).
The potential differences between the individual electrodes are mathematically related by
Equation 6-15. This was confirmed by comparing the measured ΔE(La2CuO4 vs. Pt) values with those calculated using Equation 6-15 and the experimental data for ΔE(La2CuO4 vs. RE) and
ΔE(Pt vs. RE). The experimental data and the calculated data overlapped exactly in every case.
ΔE(La2CuO4 vs. Pt) = ΔE(La2CuO4 vs. RE) – ΔE(Pt vs. RE) (6-15)
Exposures to NO (Figure 6-6) and CO (Figure 6-7) similarly resulted in negative potential changes for both of the exposed electrodes. The response times were also similar for both gases and both electrodes. However, |ΔE(Pt vs. RE)| was larger (higher sensitivity) than |ΔE(La2CuO4 vs. RE)| in both CO and NO. This may be a result of the superior electroreduction O2 kinetics of the La2CuO4 electrode compared to the Pt electrode. It also may be a result of the additional potential contribution on the La2CuO4 due to adsorption-induced Fermi-level. The difference in potential between the two exposed electrodes, ΔE(La2CuO4 vs. Pt), was positive in NO (+5 mV for 800 ppm NO) and CO (+20 mV for 800 ppm CO).
In NO2, the potentials of both the exposed electrodes increased positively relative to the
RE (Figure 6-8). Again, the potential of the Pt electrode was increased more than that of the
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La2CuO4 electrode. The difference in potential between the two exposed electrodes,
ΔE(La2CuO4 vs. Pt), was negative (-30 mV for 800 ppm NO2).
The potentials of both of the exposed electrodes responded (ΔV) in the same potential direction, positive for O2 and NO2, negative for NO, CO, and CO2 for all of the gasses tested. It is because of the difference in magnitude of the individual electrode potentials that the sensor results in a voltage ΔE(La2CuO4 vs. Pt), and because of the sensor electrode convention,
(+)La2CuO4|YSZ|Pt(-), the sensor voltage, ΔE(La2CuO4 vs. Pt), is opposite in sign to the response of the two electrodes. This results in ΔE(La2CuO4 vs. Pt) being always positive when the sample was exposed to NO and CO and negative when exposed to NO2.
The ΔE(La2CuO4 vs. Pt) sensor OCP trends are similar to those of the sensors based on 0.1 mm YSZ sheets (included in Figure 6-8 for comparison) illustrating the comparability of this study to real, prototype sensors. Similar shifts in electrode potential are also recognizable in the potential-axis intercept of Figure 6-9A, which shows data collected during polarization measurements made on a La2CuO4 electrode as a function of NO2 concentration.
6.4.3 D.C. Polarization Measurements
The La2CuO4 electrode was polarized in the positive potential direction at 500°C in various concentrations of NO2 (Figure 6-9A). The data were subsequently fit with polynomial functions giving a R2>0.9984 in all cases (Figure 6-9A). These equations were then used to recalculate the currents as a function of potential. The current (measured) in the absence of NO2, I1(E), was subtracted from the net current, I2(E), in the presence of NO2 yielding the current contribution,
IΔ,NO2/NO(E), from the NO2/NO reaction by itself. I1(E) and IΔ,NO2/NO(E) are plotted as a function of potential in Figure 6-9B.
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The absolute values of the net currents were taken and plotted as a quantitative “mixed- potential” diagram for exposures of NO2 to the La2CuO4 electrode (Figure 6-10A) and the Pt electrode (Figure 6-10B). The currents calculated from the difference between the currents when the sample was exposed to NO or NO2 compared to in their absence was plotted on the same plot as the currents from only the O2 reactions. As the concentration of NO2 increases the potentials where the curves intersect is progressively shift in the positive direction. The NO2 increases the slope of the i(E) characteristics more on the Pt electrode than on the La2CuO4 electrode. It seems that the high electrochemical activity of the La2CuO4 electrode makes it less sensitive to changes in polarization resistance due to changes in NO2 concentration.
As illustrated earlier, a treatment of this sort requires the confirmation that only two reactions occur on the electrode surface. NO2 begins decomposing at temperatures above 300°C
159,193 on La2CuO4 and totally decomposes above 500°C . Additionally, NO2 decomposes to NO
150 and O2 over Pt electrodes at temperatures above 450-500°C . No additional reactions such as
NO2 reduction to N2O or N2 were observed. Therefore, in the case of NO2 reacting on Pt and
La2CuO4 electrodes, it is confirmed that NO2 nearly completely converts to NO and O2 at and above 500°C. This has two effects that need consideration: (1) change in local or effective Po2 and (2) change in Q, the proper quotient of activities for the reaction, at the TPB.
The local Po2 pertinent to the discussion is dependent on the extent to which NO2 is heterogeneously catalyzed prior to reaching its ultimate electrochemical reaction site. In the case of Pt, it is assumed that the reaction occurs at the TPB whereas in the case of La2CuO4 it is only possible to speculate that, in principle, it is possible for NO2 to react anywhere on the electrode or at the TPB because La2CuO4 has both O2-ions and holes. If a large quantity of NO2 decomposes adjacent to the reaction sites, as in the case of Pt and La2CuO4, it is expected that the
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local Po2 will be augmented. This increases the potential of the electrode relative to some fixed reference potential as given by the Nernst Equation (Equation 6-16).
RT Po() local E = ln(2 ) (6-16) 4()FPoref2
The exchange current density (io) for Equation 6-1 is dependent on Po2 and is typically
n 215 described by an expression like io ∝ Po2 , where n is between 1/8 and 1/2 . The slope I1(E) is directly proportional to io; therefore, if the local Po2 increases, the slope of an I(E) curve will increase as well for a given electrode. If electrochemical or heterogeneous reactions occur in the vicinity of an electrode, the slope (m1) and current intercept (I1°) of I1(E) will shift to some new slope (m1′) and some new current intercept (I1°′). As a result, the subtraction of I1(E) from I2(E) to find half-cell characteristics, IΔ(E), may be misleading because I1(E) has a new slope (m1′) and current intercept (I1°′) due to the decomposition of NO2.
Assuming that the local Po2 is in equilibrium with the NO2/NO mixture adjacent to the
TPB, the proper quotient of activities at a reaction site determines the electrode potential
(Equations 6-17 and 6-18). E′ is the standard electrode potential which is calculated from the change in free energy between reactants and products. Its value is +3.6 mV relative to the
X 188 E´(O2/OO ) at 500°C for the reduction of NO2 to NO and O2 .
1/ 2 RT aNO aO EEo =−′ ln Q and Q = 2 (6-17A and 6-17B) nF a NO2
For the most part, E°(NO2/NO) increased with NO2 concentration (Figures 6-8 and 6-9) as suggested by Equations 6-17A and 6-17B. However, at 800 ppm of NO2 the potential of the
La2CuO4 electrode actually decreased suggesting that the local NO2/NO ratio was less than in
500 ppm of NO2. Coincidentally, this sort of reversal was seen in D. C. resistance measurements
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159 of porous La2CuO4 layers for NO2 concentrations greater than 400-650 ppm at 500°C .
Therefore, adsorption-induced Fermi level shifts may also explain the potential shifts seen at open-circuit.
In the absence of NO2, the I(E) lines of the La2CuO4 electrode are much straighter than those of the Pt electrode (Figure 6-10). The magnitude of the slope is also larger for the
La2CuO4 electrode than the Pt electrode. As a result, nearly three times as much current density is obtained for the same 30 mV overpotential with the La2CuO4 electrode as compared to the Pt electrode in the same gas condition at 500°C.
The slopes of all the NO2/NO half-cell I(E) curves increased linearly with NO2 concentration, which could be interpreted as NO2 electrochemically reacting or a result of augmented local Po2. For example, reviewing the raw data (Figure 6-9A), the OCP shifted to higher potentials with increased NO2 concentration. This could indicate that the local Po2 is higher as given by Equation 6-16. Of course, if NO2 decomposes far from the reaction sites, then this effect will not happen because the Po2 (0.03 in 3% O2) will be hardly affected by the addition of several tens of ppms of O2.
Current-overpotential characteristics of the individual electrodes were collected to better understanding the electrochemical activity f both electrodes. The Pt electrode was polarized over a broader range (-1V to +1V) and the current was measured (Figure 6-11). The same was done for the La2CuO4 electrode (Figure 6-12). As before, the i(E) characteristic of both electrodes were linear near 0 V (Figures 6-11A and 6-12B). The extent of the linearity was greater for the
Pt electrode than for the La2CuO4 electrode.
Close to the OCP (η<10 mV), diffusion contributions can be neglected and a charge transfer resistance (RCT) can be calculated with Equation 6-18.
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η − iR R = YSZ (6-18) CT i
The exchange current density (iexch) is given by Equation 6-19.
RT 1 iexch = ( )( ) (6-19) nF RCT
RCT and iexch were calculated for both electrodes in 3% O2 at 500°C (Table 6-2). These RCT values are equivalent to the electrode polarization resistance values obtained with EIS measurements using small amplitude (<10 mV) signals. The iexch values (Table 6-2) calculated with Equation 6-19 are determined solely by the kinetics of O2 electroreduction reaction pathway steps, assuming that diffusion contributions are negligible.
Regions under kinetic and diffusion control at larger overpotentials were identified by calculating the transfer coefficients (α) and iexch again, (Table 6-2) by fitting the I(E) data at large
173 positive bias and large negative biases (Figures 6-11B and 6-12B) . The iexch values calculated from the fits at positive and negative biases were not the same. In some cases they were similar to the iexch calculated at OCP and other cases they were not. The coincidence of iexch values calculated at OCP and those calculated from fitting a branch of I(E) data indicated that diffusion processes are not rate-limiting under that bias condition. This was true for both the Pt and the
La2CuO4 electrodes under oxidizing potentials (Figures 6-11B and 6-12B). Furthermore, the transfer coefficient values calculated from fitting the large positive bias data were near to one- half, within the typical range of 0.4 and 0.6. Conversely, the low α values (<0.1) on both electrodes and dissidence between iexch values at OCP and at reducing potentials indicates that an additional process is limiting the current (Figures 6-11B and 6-12B). Performing a similar analysis for an electrode in the presence of NOX or CO could make it possible to determine if the reaction of NOX and CO are diffusion-limited or kinetically-limited. Determining the rate-
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limiting process for the reaction of NOX and CO would be useful in constructing predictive sensor models.
6.4.4 EIS Measurements Part 2
The impedance of the Pt electrode and the La2CuO4 electrode were individually measured in 3% O2 with and without NO or NO2. A single semicircle dominated the impedance spectra for the Pt electrode wheras the spectra for the La2CuO4 electrode had two semicircles. The number of features in the spectra for both electrodes was constant regardless of the NO or NO2 concentration (0 to 800 ppm).
The impedance data were fit with a resistor and a constant-phase element (CPE) in parallel.
The impedance of a CPE has the generic formula given by Equation 6-20, where ω is the angular frequency, Q is the effective capacitance, and α quantifies the dispersion in the capacitance. The leading resistor was included in the equivalent circuit, in series with the R-CPE element, to extract the electrolyte resistance (RYSZ). The equivalent circuit fit well to the impedance data collected with the Pt electrode but not the La2CuO4 electrode. Regardless of the poor fit, reistance values were extracted from the La2CuO4 electrode impedance data.
The resistance values (Rp, polarization resistance) for the electrode polarization resistance of each electrode were extracted from the impedance data for both electrodes in 0-800 ppm of either NO or NO2. In the absence of NO or NO2, the Rp values were quite similar to the RCT values measured at OCP with low overpotentials (Table 6-2). The Rp values of both electrodes decreased when they were exposed to both NO (Figure 6-13 and Figure 6-14) and NO2 (Figure
6-15 and Figure 6-16). No features in the impedance spectra were obscured nor were any additional features generated in the impedance spectra by the presence of NO or NO2.
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The frequency dependence of the imaginary impedance (Zimag) for the Pt electrode was plotted because the formulism is useful for deconvoluting processes with similar time constants and extracting other parameters (Figures 6-17). For example, the slope of such a plot is indicative of the dispersion (α) of the sample capacitance. For a perfectly capactive electrode process α-values approach 1 and the slopes of data on log|Zj| vs. log f plots do as well. The absolute values of the slopes at low frequencies (0.8) were inconsistent with those at high frequencies (-0.56) for the Pt electrode (Figure 6-17). This indicates that the impedance spectra have contributions from at least two processes because single processes, even if they are dispersed, have slopes which are symmetrical about their peaks in plots of log|Zj| vs. log f.
The slopes at intermediate frequency were unchanged in both NO (0.56±0.01) and NO2
(0.55 ±0.01) for the Pt electrode. The impedance spectra for the La2CuO4 electrode were too convoluted to permit extraction of useful data about the dispersion of the capacitance; however, the slopes were near ½ at intermediate frequencies. Dispersion parameters so near to ½ suggest that diffusion plays a key role in the electroreduction kinetics of both Pt and La2CuO4 electrodes.
1 Z = (6-20) CPE [ j(ωQ)α ]
The polarization resistance values (Rp) for both electrodes in NO and NO2 were normalized to the Rp values in the absence of NO or NO2 (Ro) giving the ratio Rp/Ro. Rp/Ro decreased in both NO and NO2 on both electrodes. The effect of NO2 on the polarization resistances of both electrodes was very strong, reducing Rp values by 50% with only 50 ppm
(Figure 6-18). Rp values for both electrodes continued to decrease for increasing concentrations of NO2. NO also decreased the polarization resistance of both electrodes but did so less than the
NO2 (Figure 6-18).
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Recalling that the potential of both electrodes increased in NO2 and decreased in NO
(Figure 6-6), the polarization resistance values do not follow the same trend. This was contrary to the OCP trends. Currently, there is no link between electrode polarization resistance and the open-circuit potential of an electrode. When NOX or CO interacts with the electrode they change
Rp as shown here. Mixed-Potential theory suggests that the net current-potential characteristic between multiple electrochemical should be the basis for extracting an electrode polarization resistance. The form of this net current-potential curve is not simply dependent on the gas in question but also the electrode kinetics.
6.4 Summary
An embedded reference electrode was employed in making overpotential measurements on porous Pt and La2CuO4 electrodes between 450°C-800°C in NOX and CO containing atmospheres. The embedded reference electrode provided a convenient way to separate the individual potentiometric responses of the two electrodes when they were both exposed to the same pollutant-containing atmospheres. It was shown that sensors of the form La2CuO4|YSZ|Pt respond (ΔOCP) to NO and CO positively and NO2 negatively because the Pt electrode response is larger than the La2CuO4 response. It was shown that analysis of D.C. polarization data is not straightforward, as many authors have suggested. It was possible to explain the sensing responses to NO2 in terms of three different mechanisms including: Mixed-Potential theory, changes in localized O2 concentration at the electrode surface and adsorption-induced Fermi level shifts in the case of semiconducting La2CuO4.
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A) I (A) I (A) I 2 ( E ) I 1 ( E ) B) m 2 (Emix , I corr ) o m1 I2 I Δ ( E )
I Δ ( E ) o IΔ o m mΔ I Δ 1 E (V) E (V)
EOCP (base) − I 1 ( E ) o X E (O2 / OO ) − m1 EOCP (base + redox gas) − ΔE +
Figure 6-1. Idealized current-potential schematics illustrating method for constructing quantitative mixed potential diagrams. A) Representation of process used to calculate the current resulting from the electrochemical oxidation of a pollutant gas. The two lines with data points represent D. C. polarization measurements made in a (1) base gas containing O2 and a (2) base gas plus some pollutant gas. B) The calculated x current-potential characteristics for the Red/Ox and the O2/OO redox couples.
RE CE
l1
LT r l2
WE
Figure 6-2. Luggin’s Probe configuration for solid-state electrochemical measurements.
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-20
) 2-point config. (CE + WE)
Ω 3-point config. 3-point config. Calculated ( Pt CE Pt WE imag
Z Measured 0 20 40 60 80 100 120 Z (Ω) real
Figure 6-3. Impedance of a sample with all electrodes made of porous Pt measured at 800°C in stagnant air in two-point and three-point configurations. The two-point spectrum was calculated by summing the two three-point spectra.
3-point Config. 3-point Config. 2-point Config. (CE+WE) ) -100 Pt CE La CuO WE Ω 2 4 ( 100 Hz measured imag -Z 100 Hz 100 Hz calculated 0 100 200 300 400 500 Z (Ω) real
Figure 6-4. Impedance of a sample with a Pt electrode and a La2CuO4 electrode measured at 700°C in stagnant air in two-point and three-point configurations. The two-point spectrum was calculated by summing the two three-point spectra.
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1 35 La CuO vs. Pt A) 32 La CuO vs. RE B) 100 0 2 4 30 2 4 24 Pt vs. RE 80 -1 25 [CO
16 [O
-2 20 2 60 2 Pt vs. RE ] (%) 8 ] (%) -3 15 E (mV) E (mV) 0 40 Δ -4 10 Δ -8 La CuO vs.Pt 2 4 20 -5 5 La CuO vs. RE -16 2 4 -6 0 0 0 5 10 15 20 25 30 35 0 10203040 Time (min) Time (min)
Figure 6-5. Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. In separate experiments, the sensor was exposed to step changes in A) CO2 concentration and B) O2 concentration, both at 500°C with a background 3% O2 concentration (N2 balance).
1000 La CuO vs. Pt La CuO vs. RE 2 4 2 4 La CuO vs. Pt 5 5 2 4 800 [NO] (ppm) 0 600 0 La CuO vs. RE 2 4 E (mV) 400 E (mV) -5 Δ -5 Δ
200 -10 -10 Pt vs. RE A) B) 0 1600 1650 1700 1750 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Time (min) Pt vs. RE log [NO] (ppm)
Figure 6-6. Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. The sensor was exposed to step changes in NO concentration at 500°C with a background of 3% O2 (N2 balance). A) Potential differences as a function of time. B) Steady-state potential differences as a function on NO concentration.
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1000 La CuO vs. Pt A) 2 4 B) La CuO vs. Pt 20 20 2 4 800
10 (ppm) [CO] 10 600 La CuO vs. RE 2 4 0 0 E (mV) E (mV) La CuO 400 Δ
Δ 2 4 Pt vs. vs. RE RE -10 200 -10 Pt vs. RE -20 0 -20 0 20 40 60 80 100 120 1.6 1.8 2 2.2 2.4 2.6 2.8 3 Time (min) log [CO] (ppm)
Figure 6-7. Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. The sensor was exposed to step changes in CO concentration at 500°C with a background of 3% O2 (N2 balance). A.) Potential differences as a function of time. B) Steady-state potential differences as a function on NO concentration.
1000 Pt vs. RE 60 Pt vs. RE La CuO A) B) 60 2 4 vs. RE 800 40 40 [NO La CuO vs. RE 2 4 600 2 20 20 ] (ppm) E (mV) E (mV) 400 Δ 0 Δ 0 Sensor -20 La CuO 200 -20 2 4 La CuO vs. Pt vs. Pt 2 4 -40 -40 0 1.6 1.8 2 2.2 2.4 2.6 2.8 3 0 20 40 60 80 100 120 log [NO ] (ppm) Time (min) 2
Figure 6-8. Potential differences measured between a La2CuO4 electrode, a Pt electrode and an embedded RE. The sensor was exposed to step changes in NO2 concentration at 500°C with a background of 3% O2 (N2 balance). A.) Potential differences as a function of time. B) Steady-state potential differences as a function on NO2 concentration. Data collected with a sensor (La2CuO4|YSZ|Pt) fabricated with a 0.1 mm thick YSZ sheet is included for comparison.
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x 0 E°(O /o ) E°(100 ppm NO ) 0 2 o 2 100 ppm NO 500 ppm 2 -2 NO -2 2 0 ppm NO 0 ppm NO 2 2 -4 500 ppm NO 50 ppm NO 2 2
A) -4 μ 50 ppm NO A) 100 ppm NO 2 2 μ i (
-6 i ( -6 E (800 ppm NO ), i mi x 2 corr
-8 -8 800 ppm NO 800 ppm NO A) 2 B) 2 -10 -10 0 20406080 0 20406080 E - IR (mV) E-IR (mV) YSZ YSZ
Figure 6-9. Current-potential characteristics measured with a La2CuO4 WE in 0-800 ppm of NO2 and a background of 3% O2 (N2 balance) at 500°C. A) Actual measured data with interpolated line fits. Potentials were IRYSZ subtracted using RYSZ from impedance measurements at 0 V bias. B) Current-potential diagram constructed by analysis of the polarization data (part A).
800 ppm NO 800 ppm NO 1 10-5 2 5 10-6 2 A) O only 2 B) -6 500 -6 500 ppm NO 8 10 4 10 2 ppm ) ) NO 2 2 -6 2 -6 50 ppm NO 6 10 100 ppm 3 10 2 100 ppm NO NO 2 4 10-6 2 2 10-6 |i|(A/cm 50 ppm |I| (A/cm -6 -6 2 10 NO 1 10 O only 2 2
0 0 0 1020304050607080 0 1020304050607080 E - IR (mV) E - IR (mV) YSZ YSZ Figure 6-10. Current-potential diagram constructed by analysis of polarization data measured on A) a La2CuO4 electrode and B) a Pt electrode at 500°C in 0-800 ppm of NO2 and a background of 3% O2 (N2 balance).
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Table 6-1. D.C. and A.C. results for La2CuO4 and Pt in 3% O2 at 500°C. La2CuO4 Pt RCT Iexch α 1- α RCT Iexch α 1- α (Ω*cm2) (A/cm2) (Ω*cm2) (A/cm2) At OCP 1723 1.93x10-5 NA NA 9609 3.47x10-6 NA NA Reducing 5.50x10-5 0.119 0.881 4.12x10-5 0.0797 0.920 Bias (-) Oxidizing 1.30x10-5 0.367 0.633 4.70x10-6 0.671 0.329 Bias (+)
15 600 10 400 5 200 ) tafel region 2 0 0 (mV) -200 A/cm YSZ μ -5 -400 i ( -iR
η -600 -10 -800 transport A) B) limited -15 -1,000 -100 -50 0 50 100 -16 -14 -12 -10 -8 -6 η-iR (mV) 2 YSZ ln |i| (A/cm )
Figure 6-11. Current-overpotential diagrams for a Pt electrode. A) Low overpotential results. B) High overpotential results.
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30 200 20 tafel region 10 0 ) 2
0 (mV) -200 A/cm YSZ μ -10 -400 -iR i ( η -20 A) -600 B) -30 -50 0 50 -14 -12 -10 -8 -6 η-iR (mV) 2 YSZ ln |i| (A/cm )
Figure 6-12. Current-overpotential diagrams for a La2CuO4 electrode. A) Low overpotential results. B) High overpotential results.
-4,000 900 ppm NO R =266.3 Ω∗cm2 650 ppm NO YSZ
) 400 ppm NO 0.1 Hz 2 200 ppm NO 100 ppm NO *cm -2,000 0 ppm NO Ω 0 ppm NO ( imag Z 900 ppm NO 0 0 2,000 4,000 6,000 8,000 10,000 Z' (Ω*cm2)
Figure 6-13. Nyquist plot for a Pt electrode at 0V DC bias exposed to various concentrations of NO.
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-1,000 0 ppm NO 100 ppm NO 200 ppm NO 650 ppm NO ) 2
*cm -500 100 Hz
Ω 1 Hz ( imag Z
0 1,000 1,500 2,000 2,500 3,000 Z (Ω*cm2) real
Figure 6-14. Nyquist plot for a La2CuO4 electrode at 0V DC bias exposed to various concentrations of NO.
-6,000 100 ppm NO 400 ppm NO 900 ppm NO 2 2 2 2 R =267.7 Ω∗cm YSZ ) 0 ppm NO 200 ppm NO 650 ppm NO 2 2 2 2 -4,000 *cm Ω ( 0.1 Hz
imag -2,000 Z 0 ppm NO 2 0 0 2,000 4,000 6,000 8,000 10,000 12,000 Z' (Ω*cm2)
Figure 6-15. Nyquist plot for a Pt electrode at 0V DC bias exposed to various concentrations of NO2.
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-1,000 0 ppm NO 100 ppm NO 200 ppm NO 650 ppm NO 2 2 2 2 ) 2
*cm 100 Hz Ω -500 ( 1 Hz imag Z
0 1,000 1,500 2,000 2,500 Z (Ω*cm2) real
Figure 6-16. Nyquist plot for a La2CuO4 electrode at 0V DC bias exposed to various concentrations of NO2.
4 900 ppm NO 0 ppm NO 650 ppm NO 400 ppm NO ) 2 3 200 ppm NO 100 ppm NO *cm 0 ppm NO
Ω slope = 0.80 | ( j 2 slope = -0.56 log |Z
1 -2-101234 log f (Hz)
Figure 6-17. Plot of log |Zj| vs. log frequency for a Pt electrode at 0V DC bias exposed to various concentrations of NO.
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1
Pt NO 0.8 La CuO NO 2 4 0.6
R/Ro La CuO NO 0.4 2 4 2
0.2 Pt NO 2 0 1.8 2 2.2 2.4 2.6 2.8 3 log [NO ] (ppm) X
Figure 6-18. Normalized polarization resistance of La2CuO4 and Pt in various NO and NO2 concentrations and a background of 3% O2 (N2 balance) at 500°C.
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CHAPTER 7 CONCLUSIONS
Electrode microstructure has a large effect on the response characteristics of potentiometric sensors. Response stability and speed improved at elevated operating temperatures at the expense of response sensitivity. Coarse-grained electrodes exhibited slow response and poor sensitivities in low NO concentrations; however, it was also observed that they exhibit larger sensitivities at high gas concentrations. Decreasing the electrode grain size exponentially decreases the response time. In general, it was observed that sensitivity decreases significantly at temperatures above 550 ºC. La2CuO4|YSZ|Pt sensors, when operated between 450-500 ºC in simulated combustion exhaust, respond quickly (<1 min) and reproducibly to step changes in NO concentration with adequate sensitivity to resolve 50 ppm of NO.
The NO potentiometric response can be explained entirely by semiconducting changes in
La2CuO4 due to gas adsorption. NO does not decompose or heterogeneously react over
La2CuO4. The NO2 potentiometric response can be explained in terms of conductivity changes in the semiconducting electrode at high temperatures. However, at low temperatures, where the sensor is especially sensitive to NO2, a direct relationship between La2CuO4 resistance changes and the potentiometric response is still difficult to draw. It was found that NO2 reacts over
La2CuO4 to form NO and O2 and that the NO2 desorption process is more complex than that of
NO. Electrode thickness could be the cause of this non-uniformity and needs to be looked into further. The potentiometric sensor responded to reducing gases (NO, CO) with a positive OCP and oxidizing gases (NO2) with a negative OCP, all of which are contrary to the directions predicted by Mixed-Potential Theory. Differential Electrode Equilibria Theory can account for this disparity because it takes into account adsorption induced Fermi level changes and their effect on the measured potential.
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The impedance spectra for La2CuO4-based potentiometric sensors consisted of three features that were subsequently identified. At high frequencies, there were the contributions by the O2-ion conduction in the YSZ and the transport of charged species on the surface of the
La2CuO4 electrode. In the case of the Pt symmetrical cell, no changes in impedance with NO or
NO2 exposure were seen at high frequencies; however, the high frequency intercept of cells containing La2CuO4 did. This was determined to be the p-type, semiconducting, surface conduction on the La2CuO4 electrodes.
The temperature, gas, and gas concentration trends of the La2CuO4 surface conductivity were all very similar to that of the sensor OCP. Conductivity changes in semiconductor sensors are most frequently explained by adsorption and other non-electrochemical mechanisms.
Therefore, non-electrochemical processes fundamentally contribute to the Differential Electrode
Equilibria of sensors containing semiconductor electrodes.
A semicircle was observed at low frequencies related to the capacitance and resistance associated with charge-transfer during the oxygen-reduction reaction at the YSZ/electrode/gas interface. The size of this semicircle decreased when either the sensor or the symmetrical cells were exposed to NO and NO2. It can be concluded that O2 exchange between the electrolyte and the external environment is affected by NOX and this is another source of each electrode potential.
An embedded reference electrode was employed in making overpotential measurements on porous Pt and La2CuO4 electrodes between 450°C-800°C in NOX and CO containing atmospheres. The embedded reference electrode provided a convenient way to separate the individual potentiometric responses of the two electrodes when they were both exposed to the same pollutant-containing atmospheres. It was shown that sensors of the form La2CuO4|YSZ|Pt
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respond (ΔOCP) to NO and CO positively and NO2 negatively because the Pt electrode response is larger than the La2CuO4 response, but not O2 because the responses of both electrodes are equal.
Analysis of D.C. polarization data is not straightforward, as many authors have suggested.
Temperature Programmed Reaction measurements were used to qualitatively compare the heterogeneous catalysis of NO2 on La2CuO4 and Pt electrodes. It was possible to explain the sensing responses to NO2 in terms of three different mechanisms including: Mixed Potential
Theory, changes in localized O2 concentration at the electrode surface and adsorption-induced
Fermi level shifts in the case of semiconducting La2CuO4. These three combined are
“Differential Electrode Equilibria.”
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APPENDIX A OPTIMIZATION OF SCREEN-PRINTING INK CONTAINING La2CuO4
20 μm 20 μm 20 μm 700°C 800°C 900°C
Figure A-1. SEM image of electrode prepared by screen-printing 16 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X
20 μm 20 μm 20 μm 700°C 800°C 900°C
Figure A-2. SEM image of electrode prepared by screen-printing 17 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X
20 μm 20 μm 20 μm 700°C 800°C 900°C
Figure A-3. SEM image of electrode prepared by screen-printing 18 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X
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20 μm 20 μm 20 μm 700°C 800°C 900°C
Figure A-4. SEM image of electrode prepared by screen-printing 19 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X
20 μm 20 μm 20 μm 700°C 800°C 900°C
Figure A-5. SEM image of electrode prepared by screen-printing 20 volume % co-precipitated La2CuO4 mixed with PEG400 and sintered at 700°C, 800°C, and 900°C for 10 h. Magnification = 2,500 X
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BIOGRAPHICAL SKETCH
Briggs was born in Cheverly, Maryland in July of 1980. After two years in Alexandria,
Virginia he moved with his Mother, Betsy and Father, Ron to the countryside in western
Maryland. His formative years were spent in his Father’s pottery studio and herb gardens, while his Mother commuted to her job with the U.S. Fish and Wildlife Service in Washington, D.C.
His parents divorced when Briggs was ten and his Mother relocated to Illinois. Thereafter,
Briggs lived with his Father near Sharpsburg, MD.
Briggs attended the New York State College of Ceramics at Alfred University and received a Bachelor of Science degree in Materials Science and Engineering in May of 2002. While at
Alfred, he worked with Dr. William Carty of the Whitewares Research Center as a research assistant and spent one semester studying at the University of Sheffield, in Sheffield, England.
During this period, Briggs gained an appreciation for the extent to which scientific knowledge can affect engineering progress. He did his senior thesis on developing new Bismuth oxide- based superionic conductors for solid oxide fuel cells. He did an additional project on characterizing plasticity via high-pressure shear rheology.
After graduating from Alfred, Briggs was accepted into the graduate school at University of Florida to work with Dr. Eric Wachsman on solid oxide fuel cells. Within his first month at
UF, Briggs’ Mother passed away very suddenly and quite unexpectedly and his ability to carry out his graduate assignments was deeply affected. Before long, Dr. Wachsman suggested that
Briggs switch from working on fuel cells to working on sensors where he would work with Dr.
Suman Chatterjee. Briggs was given the task of improving the rheology of screen-printing pastes for sensor electrodes and he subsequently enjoyed some success working closely with Dr.
Chatterjee. Suman injected a lot of enthusiasm into their collaborative work and very quickly,
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Briggs and Suman made some large gains in the lab. Their work together ultimately led to a presentation at the Cocoa Beach meeting of the American ceramic Society in January of 2004.
In October of 2003 Briggs accepted an offer from Dr. Enrico Traversa to study in Rome,
Italy. Briggs worked for Dr. Traversa at the University of Rome, Tor Vergata for one year beginning in September of 2004. During that year Briggs made a number of lasting friendships and he became fluent in Italian. He also taught a group of European students, gave a presentation in Quebec City, Canada, and worked on several projects including a collaboration with an Italian national laboratory to develop large-area solid oxide fuel cells.
During the final year of his Ph.D., Briggs gave presentations in Los Angeles, CA and
Cancun, Mexico as well as leading the sensor group of graduate students. Briggs has accepted a job offer to manage fuel cell-related fuel reformation projects with the National Energy
Technology Laboratory in Morgantown, WV upon completion of his Ph.D. degree.
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