The use of solid electrolytes in the determination of activities and the development of sensors
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9/11/03
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The Use of Solid Electrolytes in the Determination of Activities and the Development of Sensors DEREK J. FRAY Solid electrolytes have been known for decades, but it is only recently that the drive to develop new methods of electricity generation and storage has led to the development of new solid electrolytes. This article discusses the use of solid electrolytes in the measurement of activities of solutes, dissolved in molten metals, and the development of sensors that may find industrial application for on-line measurements in the metallurgical industry.
From
I. INTRODUCTION
SOLID electrolytes have been investigated for almost
a century, and ionic conductors were first used in 1908[1] to determine thermodynamic data. In the 1930s, Tubandt et al.[2] confirmed that many halides were ionic conductors, but applications were confined to simple cells operating over a limited temperature range, either due to the low melting points of the salts or the occurrence of nonstoichiometry at high temperatures. Following this active period, very little progress was made until 1957, when Kiukkola and Wagner,[3,4] Ure, [5] and Liddiard[6] restimulated interest in high-temperature galvanic cells incorporating solid electrolyte phases. These electrolytes only had one mobile species and, in the case of oxide systems based upon stabilized zirconia, have found wide application in the measurement of activities and as high-temperature fuel cells, high-temperature electrocatalytic reactors, and sensors. During the past three decades, environmental issues, especially the generation and storage of electricity, have lead to the discovery of other solid electrolytes and, although their initial application may be in batteries or fuel cells, these interesting materials can also be used for the determination of activities and development of sensors. II. FUNDAMENTALS OF ACTIVITY MEASUREMENTS USING SOLID ELECTROLYTES In the following solid electrolyte cell, A/A conductor/A(B) A is a pure element, the A conductor is an ionic conductor of A ions, and A (B) is A dissolved in B. If the cell is short-circuited, the following reactions will occur: A A e
[1]
A e A(B)
[2]
and
The net cell reaction is A A(B)
[3]
DEREK J. FRAY, Professor of Materials Chemistry and Head of Department, is with the Department of Materials Science and Metallurgy, University of Cambridge, Cambridge, CB2 3QZ, United Kingdom. Contact e-mail: [email protected] This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22-23, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS B
G G° RT ln Q
[4]
where G is the free-energy change for the reaction, G° is the standard free energy for the reaction, and Q aA/ aA° where a is the activity. G° 0, as A is on both sides of the equation, and G ZEF [5] where Z is the charge carried, E is the potential, and F is Farad
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