High-Temperature Ionic Sensors

A potentiometric electrochemical cell consisting of a reference electrode, solid-state electrolyte(s), and an indicator electrode can provide information about the partial pressure of gas in the same way as the cells utilizing ion-selective electrodes and liquid electrolytes can. The general mechanism is as follows. A sample gas, which is part of a redox couple, permeates into the solid-state structure usually through the porous metal electrode and sets up a reversible potential difference at that interface according to the reaction 

Three requirements must be met in order to obtain a valid measurement. First, we need a stable reference potential at one interface, which is not affected by the changes of the composition of the sample. This is the usual requirement of 

The symbol III indicates the porous metal. The electrochemical reactions taking place at the two electrodes are identical. 

Ru02 SrClg AgCI Alg03 Fig. 6.41 High-temperature chlorine sensor (adapted from Pelloux et al., 1985) 

At operating temperature (100-400°C), the oxygen anions have sufficient mobility in the solid electrolyte. The cell voltage ECeii is then related to the partial pressure of oxygen by the Nernst equation written for the concentration cell. 

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An entirely different selectivity principle known as phase equilibrium comes into play in high-temperature ionic conductors. Many important gases dissolve in ionic solids at elevated temperatures. However, the solubility is rather sharply defined for the gas and the solid by the lattice parameters and the size of the gas molecule. The best example is the solubility of oxygen in zirconium dioxide. When Z1O2 is doped with yttrium ions, it exhibits a high mobility for the O anion. The solubility and anion mobility then become the basis for several electrochemical gas sensors, using yttria-stabilized zirconia (YSZ). 

Yu L, Garcia D, Renb R, et al. Ionic liquid high temperature gas sensors. Chem. Commun. 2005. 2277-2279. 

Liu, Q. (1996) The development of high temperature electrochemical sensors for metallurgical processor. Solid State Ionics. 86-88. 1037 3. 

Zhuiykov, S. et al.. High-temperature NOx sensors using zirconia and zinc-famUy oxide sensing electrode. Solid State Ionics 152 (2002) 801-807. 

Zhuiykov, S. et al.. High-temperature NO sensors using zirconia solid electrolyte and zinc-family oxide sensing electrode. Solid State Ionics 152-153 (2002) 801-807. Miura, N., Nakatou, N., and Zhuiykov, S., Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NO at high temperature. Sens. Actuators B Chem. 93 (2003) 221-228. 

Introduction of room-temperature ionic liquids (RTIL) as electrochemical media promises to enhance the utility of fuel-cell-type sensors (Buzzeo et al., 2004). These highly versatile solvents have nearly ideal properties for the realization of fuelcell-type amperometric sensors. Their electrochemical window extends up to 5 V and they have near-zero vapor pressure. There are typically two cations used in RTIL V-dialkyl immidazolium and A-alkyl pyridinium cations. Their properties are controlled mostly by the anion (Table 7.4). The lower diffusion coefficient and lower solubility for some species is offset by the possibility of operation at higher temperatures. 

Solid-Electrolyte Hydrogen Sensor. Most of solid gas sensors so far developed need high temperature operation because of limited ionic conductivities when the electrolyte is near room temperature. If solid electrolytes with sufficiently large ionic conductivities are available, unique gas sensors operative near room temperature can be fabricated. An example is the following proton conductor hydrogen sensor proposed by our group (10, 11). 

Gopel, W., Reinhardt, G. and Rosch, M. (2000) Trends in the development of solid state amperometric and potentiometric high temperature sensors, Solid State Ionics, 136/7, 519-31. 

Electrolytes are used in electrochemistry to ensure the current passage in -> electrochemical cells. In many cases the electrolyte itself is -> electroactive, e.g., in copper refining, the copper(II) sulfate solution provides the ionic conductivity and the copper(II) ions are reduced at the - cathode simultaneous to a copper dissolution at the - anode. In other cases of -> electrosynthesis or - electroanalysis, or in case of - sensors, electrolytes have to be added or interfaces between the electrodes, as, e.g., in case of the -> Lambda probe, a high-temperature solid electrolyte. 

Kurosawa, H., Yan, Y., Miura, N. and Yamazoe, N. (1995) Stabilized zirconia-based NOx sensor operative at high temperature. Solid State Ionics, 79, 558 5. 

M. and Miura, N. (2006) Mixed-potential-type zircorria-based NO sensor using Rh-loaded NiO sensing electrode operating at high temperatures. Solid State Ionics, 177 (26-32), 2305-11. 

F. (2003) The role of heterogeneous catalysis in the gas-sensing selectivity of high-temperature mixed potential sensors. Proceedings of the Electrochemical Society, 2002—26 Solid-State Ionic Devices 111, The Electrochemical Society, Pennington, New Jersey, pp. 261—71. 

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