Electrodes oxygen partial pressure

A signihcant problem in tire combination of solid electrolytes with oxide electrodes arises from the difference in thermal expansion coefficients of the materials, leading to rupture of tire electrode/electrolyte interface when the fuel cell is, inevitably, subject to temperature cycles. Insufficient experimental data are available for most of tire elecuolytes and the perovskites as a function of temperature and oxygen partial pressure, which determines the stoichiometty of the perovskites, to make a quantitative assessment at the present time, and mostly decisions must be made from direct experiment. However, Steele (loc. cit.) observes that tire electrode Lao.eSro.rCoo.aFeo.sOs-j functions well in combination widr a ceria-gadolinia electrolyte since botlr have closely similar thermal expansion coefficients. 

Description of the cell composition is based - as far as possible - on the Stockholm convention (1953), i.e. the left-hand electrode constitutes the negative terminal of the cell. Cells are listed according to the metallic constituent of the electrode mentioned first which is involved in the electrode reaction establishing the respective electrode potential. Contact materials and conductive additives may be mentioned first before the actual element of interest only for the sake of correct materials sequence. The sequence of electrode components is stated as reported in the original publications. When an oxygen electrode is used as reference electrode an oxygen partial pressure of 0.21 atm is assumed. 

At mercury and graphite electrodes the kinetics of reactions (15.21) and (15.22) can be studied separately (in different regions of potential). It follows from the experimental data (Fig. 15.6) that in acidic solutions the slope b 0.12 V. The reaction rate is proportional to the oxygen partial pressure (its solution concentration). At a given current density the electrode potential is independent of solution pH because of the shift of equilibrium potential, the electrode s polarization decreases by 0.06 V when the pH is raised by a unit. These data indicate that the rate-determining step is addition of the first electron to the oxygen molecule ... 

Oxides play many roles in modem electronic technology from insulators which can be used as capacitors, such as the perovskite BaTiOs, to the superconductors, of which the prototype was also a perovskite, Lao.sSro CutT A, where the value of x is a function of the temperature cycle and oxygen pressure which were used in the preparation of the material. Clearly the chemical difference between these two materials is that the capacitor production does not require oxygen partial pressure control as is the case in the superconductor. Intermediate between these extremes of electrical conduction are many semiconducting materials which are used as magnetic ferrites or fuel cell electrodes. The electrical properties of the semiconductors depend on the presence of transition metal ions which can be in two valence states, and the conduction mechanism involves the transfer of electrons or positive holes from one ion to another of the same species. The production problem associated with this behaviour arises from the fact that the relative concentration of each valence state depends on both the temperature and the oxygen partial pressure of the atmosphere. 

Such an electrochemical arrangement can also be used to transport oxygen from one electrode to the other by the imposition of an externally applied potential. This technique, known as coulometric titration , has been used to prepare flowing gas mixtures of oxygen/argon with a controlled oxygen partial pressure, to vary the non-stoichiometry of oxides, to study the thermodynamics of dilute oxygen solutions in metals, and to measure the kinetics of metal oxidation, as examples. 

FIGURE 3.2 Cyclic voltammograms of an LSM electrode as a function of reverse potential at 900°C and oxygen partial pressure of 100 Pa. The scan rate is 200 mVs-1. (From Chen, X, J. et al., Electrochem. Solid-State Lett., 7 A144-A147, 2004. With permission.)... 

The next two chapters discuss two supporting components of the fuel cell stack —specifically, interconnects and sealants. The interconnect conducts the electrical current between the two electrodes through the external circuit and is thus simultaneously exposed to both high oxygen partial pressure (air) and low oxygen partial pressure (fuel), which places stringent requirements on the materials stability. Ceramic interconnects have been used, but metallic interconnects offer promise... 

While a number of designs have been used, most oxygen sensors for automotive applications consist of a hollow, closed end tube, a schematic of which is shown in Figure 2. As shown, the interior of the closed end tube is open to the atmosphere which serves as a constant or reference oxygen partial pressure while the exterior is exposed to the exhaust gas. The voltage signal produced by the electrolyte is sensed by electrodes on the inner and outer surface of the sensor. These, in turn, are connected to the electronics package of the closed loop system. 

Consider a chamber which is partitioned into two rooms (I) and (II) by a wall made of solid electrolyte, as shown in Fig. 1.47(a). and P Sl represent the oxygen partial pressure of each room, where is higher than P Q. This can be regarded as a kind of reversible cell. When the cell operates the following chemical reactions take place at each surface of the electrolyte (denoted by electrode (I) and (II)) ... 

Sensor voltage characteristics of the crucible type oxygen sensor. According to the oxygen sensor model used in this analysis, the oxygen partial pressure Po2(0) at the measurement electrode can be expressed by Equations 28- 32, 3, and 35. Calculated results for the sensor voltage are shown in Figure 9-... 

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