Three-phase boundaries sensors

Figure. For a modeling of the coupled ion and electron transfer at a three-phase boundary see [v-vii]. Three-phase boundaries play a crucial rule in fuel cells, and many electrochemical sensors.

Fig. 5.6.2 Zr02-based oxygen sensor principle and three-phase boundary between zirco-nia, platinum and exhaust gas (triple points)...

A very important aspect of gas sensors in automotive exhaust-gas environments is aftertreatment of the electrodes to control a specific sensor behavior. For example, to measure nonequilibrium raw emissions, the sensor needs excellent catalytic ability. Various methods are known to improve electrodes in Zr02-based sensors. One well known method is to increase the active platinum surface area and the three-phase boundary area by partial reduction of zirconia close to the electrode. This occurs when the ceramic is exposed to a reducing atmosphere at high temperatures or when an electrical cathodic current is applied through the electrode and electrolyte. A similar effect can be achieved by chemical etching of the elec-... 

Due to this process, a concavo-convex surface is formed on the sensor element and it is possible that robust adhesion and effective surface area of Pt electrode increases. Pt electrode has several small pores because of heat treatment. A large amount of three-phase boundary, which is called triple point of Pt electrode material, zirconia solid electrolyte, and atmosphere gases, is created, which allows easy electrode reaction (see Figure 3.1.6). 

The steady-state potential of this sensing electrode (mixed potential) and the corresponding EMF of the sensor are established when the rates of the two electrochemical reactions are equal [13]. In order to estimate the mixed potential, one should consider the absolute values of the cathodic and anodic currents, expressed by the Butler-Volmer equation, taking also into account the rate from the catalytic reaction which determines the amounts of adsorbed species at the three-phase boundaries. A detailed analysis [8] concluded either a logarithmic or linear dependence for the concentration of reacting gas on mixed potential. The former... 

In many cases, the three-phase boundary system is formed with the layer of oxygen adsorbed on the solid electrolyte surface. Then, the triple contact diagram is that shown in Fig. 4b. It is assumed that oxygen adsorbs and dissociates at a rate proportional to its pressure in the gas and desorbs proportionally to the concentration of atomic species in the adsorbed layer. The response curves of the Pt/YSZTPt sensor are shown in Fig. 5. 

The obvious interest of this diagram is that it expands the list of materials available to develop many kinds of gas sensing devices based on the potentiometric oxygen sensor, since adsorbed layer can be also controlled by the modification of the surface of the three-phase boundary with the additional materials having molecular sieve effects and catalytic activities [12-14]. 

As shown in Table 6.1, at present three types of potentiometric gas sensors can be designed based on solid electrolytes (Yamazoe and Miura 1998). The well-known oxygen YSZ-based probe with an oxygen ion conductor is a sensor of Type I. This means that a gas sensor of Type I is constructed from a solid electrolyte for which the mobile ion is the same as that electrochemically derived from the gas phase. In this case, the interface potential can be obtained from the local equilibrium reaction, occurring at the electrode/electrolyte/gas three-phase boundary involving the mobile 0 ions in the electrolyte, electrons... 

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