0 sensor electrodes porosity

Figure 9.4 shows a cross-sectional view of a laminated-type sensor for atmospheric nitrogen oxides based on YSZ, acting as an oxidation-catalyst electrode (Ono et al., 2004). The cell was fabricated by firing YSZ sheet on which the oxidation-catalyst platinum anode and the platinum counterelectrode were screen-printed. The porosity of the electrode is adjusted by adding resin powder into each electrode paste. 

Metal-organic frameworks (MOFs), as a new class of porous materials, are well-known for high specific surface areas and high porosity that have potential applications in gas storage and separation, catalysis, sensor, and drug delivery (Furukawa et al., 2010). In recent years, the exploration of MOFs as electrode materials for supercapacitors has also been... 

Pt of the electrode material is deposited on the ceramic coating layer so as not to lose the electric characteristics. The coating layer is deposited on the sensor element with plasma jet coating of the spinel powder. It needs to meet the following conditions as an important protection layer (i) it should not be peeled off easily and (ii) it should have the porosity such that it is not plugged easily by poisonous substances in the exhaust gas. Therefore, specification are such that it should not be too thick, or too thin, and must be less dense. 

The most important considerations with respect to sensor characteristics are surface properties (hydrophilic-hydrophobic), pore-size distribution, and electrical resistance. To ensure adequate sensitivity and response a sensor of this type should consist of a very thin film of porous ceramic with a porosity >30%. The contacting electrodes may be interdigitated or porous sandwich-type structures made from noble metals (e.g., Pd, Pt, Au) and so constructed that they do not obstruct the pores of the oxide film. Humidity affects not only the resistance of a porous ceramic but also its capacitance by extending the surface area in contact with the electrodes. The high dielectric constant of adsorbed water molecules also plays an important role. 

The majority of solid electrolyte sensors are based on proton conductors (Miura et al. 1989, Alberti and Casciola 2(X)1). Metal oxides that can potentially meet the requirements for application in solid electrolyte sensors are listed in Table 2.7. These proton condnctors typically do not have high porosity but rather can reach 96-99% of the theoretical density (Jacobs et al. 1993). Similar to oxygen sensors, solid-state electrochemical cells for hydrogen sensing are typically constructed by combining a membrane of solid electrolyte (proton conductor) with a pair of electrodes (electronic conductors) Most of the sensors that use solid electrolytes are operated potentiometrically. The voltage produced is from the concentration dependence of the chenucal potential, which at eqnihbrium is represented by the Nemst equation (Eq. 2.3). 

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). 

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