Trends in the development of solid state amperometric and potentiometric high temperature sensors
Introduction
Chemical sensors based on solid electrolytes have received great attention since several years. They became well-known with the introduction of the λ-sensor based on stabilized zirconia to detect the equilibrium oxygen partial pressure in automotive exhausts. In this context, the signal of the λ-sensor is used to regulate the air/fuel-ratio in a narrow range around the stoichiometric combustion, which is important for the successful operation of the 3-way-catalyst behind the λ-sensor. The main attraction of solid electrolyte-based λ-sensors arises from the thermodynamically controlled detection principle of the so-called potentiometric devices. The equilibrium oxygen partial pressure in the exhaust gas is monitored in relation to a known oxygen partial pressure of a reference system like air. The potential U of such a concentration cell is given by the well-known Nernst-equation:with R — gas constant, F — Faraday constant, and T — temperature.
This equation contains only thermodynamic quantities and does not require any information about the microstructure of the system. Hence, aging effects in the microstructure of typical Pt/ZrO2 electrodes do not influence the sensor signal a priori and current λ-sensors have lifetimes of more than 100 000 miles.
It must be noted that the surface chemistry of a λ-sensor under practical operation conditions is much more complicated than expected from the oxygen partial pressures in the simple Eq. (1). This equation implies that only oxygen is involved in the potential forming electrode reaction, whereas in reality a series of electrode reactions with different gases such asdetermine the apparent potential of the λ-sensor. Since the raw exhaust gas reaching the λ-sensor constitutes a non-equilibrium gas mixture, thermodynamic equilibrium has to be achieved at the active electrode surface of the λ-sensor before monitoring the potential. Such λ-sensors, therefore, contain catalytically active materials and are operated at temperatures above 600°C. For less active materials or temperatures below 600°C the apparent voltage starts to deviate significantly from the value under equilibrium conditions due to insufficient catalytic activity. Temperature-dependent deviations may hence be used to compare the quality of λ-sensors from different suppliers [1].
Earlier zirconia based sensor research and development focussed on electrode materials with high catalytic activity and high exchange currents for the desired electrode reactions. Platinum electrodes were found to be most suitable for this type of application.
In contrast to these potentiometric sensors, the amperometric sensors based on stabilized zirconia are operated under an externally applied voltage which drives certain electrode reactions electrochemically. Usually pronounced non-linear current–voltage relations are observed which are determined by the electrode kinetics and, in combination with a diffusion barrier, by the diffusion of the gas through the diffusion barrier. Common amperometric sensors operate in the diffusion-limited mode. Here each molecule passing the diffusion barrier reacts immediately at the electrode. The corresponding limiting current is a unique function of the geometric parameters of the diffusion barrier. Fig. 1 shows schematically such a sensor to monitor oxygen. For a diffusion channel with length L and cross section A the limiting current Ilimit is given by [2]with F — Faraday constant, R — gas constant, T — temperature, DO2 — oxygen diffusion coefficient, Ptot — total gas pressure, A — cross section of the diffusion channel, L — length of the diffusion channel, and xO2 — molar fraction of oxygen in the gas.
At relative oxygen concentrations below 10% a linear relation between oxygen partial pressure and limiting current holds to a good approximation withDue to their linear response amperometric oxygen sensors are suitable to also operate in systems with high oxygen excess. A typical application example concerns the wide range oxygen sensors used for lean burn automotive engines.
Besides these two examples of sensor principles based on stabilized zirconia recent progress can be seen in the development of new materials with oxygen ion conduction, proton conduction or alkali ion conduction operating in sensors with auxiliary phases which will not be stressed here. In this paper we will intentionally restrict ourselves to zirconia because it is currently the only solid electrolyte material which can meet the harsh requirements for real applications at high temperatures in chemically aggressive environments like in car exhaust gases or melts of steel and glasses.
This paper does, however, include recent developments of zirconia-based high temperature sensors which focus on the detection of oxygen containing gases like NO, NO2, H2O or oxidizable gases like CO, H2 and hydrocarbons. These components appear typically in non-equilibrium gas mixtures of exhausts but also as pollutants in air. In contrast to earlier work on zirconia-based sensors, high catalytic activity of electrode materials is often undesired for these new applications. Hence, various new electrode materials have been developed recently in this context. In the following we describe fundamentals of the different operation modes of zirconia-based sensors for these applications and report on recent progress.
Section snippets
Potentiometric non-equilibrium (mixed potential) sensors
If the gases are not in thermodynamic equilibrium at the electrode surface and at the interface to the solid electrolyte, so called ‘mixed potentials’ appear. The latter are determined by the kinetics of the different electrode reactions and by the catalytic reactions at the electrode surface. Corresponding sensors show significant deviations from the Nernst voltage expected from the equilibrium partial pressure [see Eq. (1)]. They are therefore called non-Nernstian or mixed potential sensors.
Single electrode amperometric sensors
To detect other gases than oxygen with amperometric cells, a separation of different electrode reactions can be achieved by an externally applied voltage. This separation is determined by the thermodynamics of the electrode reaction, i.e. the standard electrode potential (see Fig. 3) and by the kinetics of the electrode reaction at the considered electrode. The kinetics depends strongly on the electrode material and in particular on the microstructure of the interface to the solid electrolyte.
Fundamental studies for further improvements
These new applications of zirconia-based sensors require electrode materials with specific catalytic and electrochemical properties. A major part of recent activities therefore focussed on the development of new electrode materials, which were tested in simple setups to simulate the sensor application. Only very few fundamental (‘basic science’) studies have been performed to improve our principal understanding of the underlying elementary processes and to, thereby, improve the performance of
Summary
Recent trends in the development of solid state amperometric and potentiometric sensors based on stabilized zirconia aim at measuring oxygen-containing or combustible gases like NOx, CO, H2 and hydrocarbons. Monitoring exhaust gases and in particular automotive exhaust gases, and monitoring environmentally important pollutants are their main areas of application.
- •
In this context, non-Nernstian sensors offer several advantages. They allow a comparatively simple construction. Arrays with several
Acknowledgements
This work is supported by the Federal Ministry of Education and Research, BMBF.
References (35)
- et al.
Sens. Actuators B
(1998) - et al.
Sens. Actuators B
(1993) - et al.
Solid State Ionics
(1996) - et al.
Sens. Actuators B
(1996) - et al.
Sens. Actuators B
(1996) - et al.
Solid State Ionics
(1996) - et al.
Sens. Actuators B
(1996) - et al.
Sens. Actuators B
(1998) Solid State Ionics
(1990)Sens. Actuators B
(1994)
Solid State Ionics
Solid State Ionics
SAE Tech. Pap. Ser. No. 930352
Z. Phys. Chemie (Leipzig)
Diffus. Defect Data Pt. B: Solid State Phenom
Cited by (0)
- 1
Deceased.