Electrodes for Solid Electrolyte-Based Gas Sensors

1 The Role of Electrode Configuration in Solid Electrolyte-Based 

As it was shown in Chaps. 2 and 6, solid electrolyte-based gas sensors include two electrodes, which should correspond to several requirements such as (1) electrodes should possess sufficiently high catalytic activity with respect to target gas (2) both electrodes must be stable at the operating temperature (3) they must have suitable porosity and pore size to improve the catalyst surface area and enhance the catalytic activity and (4) the catalyst should possess high electronic conductivity (Amar et al. 2011). 

Experiment has shown that reactions in a nonporous pure metallic electrode are relatively difficult as the TPB sites are located exclusively in the contour of the electrode in contact with the electrolyte. Although gas molecules may adsorb over the entire surface of the electrode, most of them cannot react, providing or extracting oxygen ions from the electrolyte as the contacts gas/catalyst/electrolyte are only in the contour of the electrode. This low density of TPB sites causes a current constriction as a high fraction of the contact area between metal and YSZ is inactive (Sridhar et al. 1997 Aaberg et al. 2000). Subsequently, the response time of sensors with this structure is slow. 

3 Pores Noble metal particles b Noble metal particles 

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Oxygen ion conductors are used in amperometric sensors for a variety of gas species. The selectivity is controlled by selecting electrode materials that catalyze particular reactions hence, multiple electrodes are required for some multicomponent gas mixtures. In such cases, the system is designed so that each electrode removes a particular gas from the gas stream. The particular gas removed by a particular electrode can also be controlled by the voltage at the electrode, just as in cyclic voltammetry, which has also been used in solid-electrolyte based sensors [34]. In addition to providing selectivity, control of the applied voltage can be used to improve the magnitude of the response [35]. 

Ceria affords a number of important applications, such as catalysts in redox reactions (Kaspar et al., 1999, 2000 Trovarelli, 2002), electrode and electrolyte materials in fuel cells, optical films, polishing materials, and gas sensors. In order to improve the performance and/or stability of ceria materials, the doped materials, solid solutions and composites based on ceria are fabricated. For example, the ceria-zirconia solid solution is used in the three way catalyst, rare earth (such as Sm, Gd, or Y) doped ceria is used in solid state fuel cells, and ceria-noble metal or ceria-metal oxide composite catalysts are used for water-gas-shift (WGS) reaction and selective CO oxidation. 

Dubbe, A., Wiemhdfer, H.-D., Sadaoka, Y. and Gdpel, W. (1995) Microstructure and response behaviour of electrodes for CO2 gas sensors based on solid electrolytes. Sens. Actuators B, 24-25, 600-2. 

FIGURE 2.1 Summarized structural scheme of measuring transformations in the solid electrolyte gas sensors. (Reprinted from Zhuiykov, S., Mathematical modelling of YSZ-based potentiometric gas sensors with oxide sensing electrodes part II Complete and numerical models for analysis of sensor characteristics, Sensors and Actuators B, Chem. 120 (2007) 645-656, with permission from Elsevier Science.)... 

FIGURE 3.1 Catalytic converter system equipped with NO sensors for the exhaust gas emitted from a new-type car engine. (Reprinted from Miura, N., Nakatou M., 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, with permission from Elsevier Science.)... 

Mizutania Y, Matsudaa H, Ishijia T, Furuya N, Takahashi K (2005) Improvement of electrochemical NOj sensor by use of carbon-fluorocaibon gas permeable electrode. Sens Actuators B 108 815-819 Mukundem E, Brosha E, Brown D, Garzon F (1999) Ceria-electrolyte-based mixed potential sensors for the detection of hydroceubons and ceubon monoxide. Electrochem Sohd State Lett 2 412-414 Neikayama S, Sadaoka Y (1994) Preparation of Na3ZrjSijPOjj-sodium aluminosilicate composite and its application as a solid-state electrochemiceil COj gas sensor. J Mater Chem 4(5) 663-668... 

A similar situation occurs with sensors based on several types of solid electrolytes (Fergus 2008). For example, carbonate and sulfate electrolytes could be used with CO and SO sensors. However, those electrolytes generally do not provide adequate stability (see Chap. 6 (Vol. 1)), and therefore the most promising sensors use common electrolytes, such as Nasicon, P-alumina, and yttria-stabilized zirconia (YSZ). These electrolytes require auxiliary electrodes to provide the desired response, but they provide good stability and long operating lives. Therefore, while optimizing the reactions responsible for a gas sensor s sensitivity, one should also aim to maximize the chemical, structural and time stability of the device. 

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