NO sensitivity of perovskite-type electrode materials La0.6Ca0.4B′1−xB″xO3±δ (B′ = Mn, Cr; B″ = Ni, Fe, Co; x = 0, 0.1, …, 0.6) in mixed potential sensors
Introduction
During the last decades, a growing demand for in situ NOx gas sensors based on solid electrolytes has been developed in the field of monitoring and control of automotive and industrial exhausts. In addition to the ability of being highly sensitive to NO at temperatures above 500 °C, the sensors should be resistant in harsh atmosphere, compact, inexpensive in production and of minor complexity. The development of these sensors is tied to the production of non-polluting automotive engines and industrial combustion processes in line with tightening emission standards.
Many metal oxides have been tested, regarding their sensing qualities in NO sensing devices. Miura et al. screened binary metal oxides such as MexOy with Me = W, Cd, Ti, Cr, Mn, Ni, Co, La and Ln. Mn2O3 and CeO2 were found to be the most sensitive electrode materials [1]. Miura also described experiments with spinel-type AB2O4 (A = Zn, Ni, Cd and B = Mn, Fe, Cr) [1], [2], [3], [4], [5] and perovskite-type ABO3 (A = Ln, Ni, Y; B = Cr, Mn, Fe, Co, Ni) [2]. The potentiometric measurements showed that most of the spinel-type electrode materials are sensitive to NO whereas the perovskites are not. Dutta as well showed that the partial substitution of the A and B ions in a perovskite does not increase the NO sensitivity of the electrode [6]. Nevertheless, the substitution of A and B ions with earth alkali and transition metal ions does vary the electronic structure, and thus, the electronic and catalytic character of the perovskite type oxide is changed as described elsewhere [7], [8], [9], [10], [11], [12], [13] and may also have an influence on the NO sensitivity of the electrode material. The oxidic materials mentioned above mostly show NO sensitivity up to 500 °C or 600 °C as well as poor selectivity.
In this work, the dependency of the NO sensitivity on the composition of the perovskites La0.6Ca0.4B′1−xB″xO3±δ (B′ = Mn, Cr; B″ = Ni, Fe, Co; x = 0, 0.1, …, 0.6) was investigated by means of potentiometric measurements and impedance spectroscopy at temperatures 500 °C, 550 °C and 650 °C. The cross-sensitivity to other possible gas components of exhaust gases (NO2, C3H6) was ascertained at the same conditions.
Section snippets
Materials and methods
The electrode materials La0.6Ca0.4B′1−xB″xO3±δ (with B′ = Mn, Cr; B″ = Ni, Fe, Co) were prepared via solid-state reaction. La2O3, CaCO3 and MeyOz (Me = Mn, Cr, Ni, Fe, Co) powders were mixed and ground in a planetary ball mill for 24 h and tempered in alumina crucibles for 17 h. After the reaction, the products were milled again for 8 h and used without further treatment.
The product powders were mixed with an organic binding agent and screen printed on YSZ disks (∅ 12 mm, height 1 mm) followed by
Crystal structure and specific surface area
XRD studies of the manganite and chromite powders given in Fig. 1 for selected materials show the typical reflections of the orthorhombic GdFeO3 crystal structure. Materials with low B″ content (x ≤ 0.2) are pure while a higher B″ content induces impurities of reactants and related perovskites up to 7 mass%. Detailed evaluation of the XRD data of the manganites is described elsewhere [14], [15], [16]. The XRD analysis of the chromites can be understood similarly.
Sorption measurements using the
Conclusions
XRD studies show that the main product of the solid-state reaction is a perovskite-type oxide with GdFeO3 structure. The materials with low B″ content emerge as pure phases. With increasing B″ metal content the perovskites contain impurities of reactants B″yOz in the range of 1–7%.
The electrode materials La0.6Ca0.4Mn0.8Ni0.2O3±δ, La0.6Ca0.4Mn0.7Ni0.3O3±δ, La0.6Ca0.4Mn0.9Fe0.1O3±δ, La0.6Ca0.4Mn0.8Fe0.2O3±δ and La0.75Ca0.25Mn0.5Fe0.5O3±δ turned out to be most suitable for the use in
Acknowledgements
The authors would like to thank K. Ahlborn and F. Gerlach (both Kurt Schwabe Institute for Measurement and Sensor Technology) for preparing the screen printed YSZ disks and K. Ahlborn for performing the sorption measurements.
Financial support by the German Federal Ministry of Economics and Technology in the program PRO INNO II (No. KF0137601DA5) is gratefully acknowledged.
Daniela Franke received her diploma in chemistry in 2006 and her Ph.D. in 2012 at Technische Universität Dresden (Germany). Until 2014, her scientific work included the development and synthesis of new electrode materials and their behavior in electrochemical solid state sensors for different gases as well as the electrochemical behavior of oxygen-deficient perovskite-type oxides. She is currently involved in the synthesis and characterization of responsive polymers for microelectronics at
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Daniela Franke received her diploma in chemistry in 2006 and her Ph.D. in 2012 at Technische Universität Dresden (Germany). Until 2014, her scientific work included the development and synthesis of new electrode materials and their behavior in electrochemical solid state sensors for different gases as well as the electrochemical behavior of oxygen-deficient perovskite-type oxides. She is currently involved in the synthesis and characterization of responsive polymers for microelectronics at Technische Universität Dresden (Germany).
Jens Zosel received his diploma in physics from the University of Greifswald (Germany) in 1990 and his Ph.D. from the University of Freiberg in 1997. Since 1992 he has been working at the Meinsberg Kurt-Schwabe Research Institute. His basic research interests are directed toward the behavior of electrochemical sensors in liquid and gaseous flows and the development of solid electrolyte sensors for different applications.
Ulrich Guth received his Ph.D. from the University of Greifswald (Germany) in 1975. In 1993, he became a professor for solid state chemistry at the University of Greifswald. From 1999 to 2010, he had been working as the director of the Kurt Schwabe Institute for Measurement and Sensor Technology in Meinsberg and as a professor for physical chemistry, especially sensor and measuring technology, at Technische Universität Dresden (both Germany). His principal research interests are directed toward solid electrolyte sensors and new materials for these high temperature sensors.