Themodynamic and Kinetic Principles for Electrochemical Gas Sensors Based on Solid Ionic Conductors
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Themodynamic and Kinetic Principles for Electrochemical Gas Sensors Based on Solid Ionic Conductors Evangelos D. Tsagarakis 1 and Werner Weppner 2 1
Department of Materials Science and Engineering, Cornell University, Ithaca, N.Y. 14853, U.S.A. 2 Faculty of Engineering, Christian Albrechts University, Kaiserstr. 2, 24143 Kiel, Germany
ABSTRACT Several concepts can be applied for gas detection using solid ionic conductors. The fundamental characteristics and the essential variables for proper design of electrochemical cells are analyzed in terms of the sensing principle applied. The principal properties for materials as components of electrochemical devices are being illustrated. Employment of dynamic method allows implementation of the Fourier coefficients to generate a complex plane plot representation. This can be particularly useful for extracting information regarding the selectivity of an electrochemical cell to multiple gas species. Moreover, modeling the system response upon a periodical perturbation can be beneficial on understanding the individual processes and optimizing the overall performance.
INTRODUCTION Gas sensing using electrochemical sensors is a field of increasing interest with respect to scientific research and a variety of applications. Controlling air pollution, industrial safety, indoor air quality, medical and agriculture are areas where sensor usage benefits the society. In view of that, cells based on solid ionic conductors have been extensively investigated for use to monitorring gas partial pressures. All-solid-state cells show certain advantages since commonly only one type of species is mobile within a solid ionic conductor. Moreover, solids are rigid materials and show possibility of miniaturization both with regard to thickness and lateral dimension and intergration into mircoelectronic circuits. The general principle of electrochemical gas sensors is the direct conversion of chemical energy of the reaction involving species from the galvanic cell and the gaseous species under detection, into electrical energy. Or vice versa, by the application of electrical energy to a galvanic cell forcing a reaction to occur. In both cases chemical information is directly transduced into electrical information e.g. easily measurable electrical quantities, voltages or currents. Depending on the sensing principle applied they may be divided into smaller categories with common characteristics. CLASSIFICATION OF ELECTROCHEMICAL GAS SENSORS Electrochemical gas sensors employing solid ionic conductors may primary be divided into potentiometric devices operated under open circuit voltage conditions and amperometric ones where current is flowing through the cell. Potentiometric devices are furthermore categorized [1]
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into type I, II and III. Figure 1 illustrates the structure of the electrochemical cell employed in each case for the detection of gas A, B and C respectively. Main characteristic of type I sensors is that they make use of a solid ionic conductor of the same
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