Current oxygen pump

Yttrium stabilized zirconia (Zr02-Y203) as an electrolyte for reduction of molecular oxygen at elevated temperatures (400-800°C) has been already discussed in Section 6.23.4. In fact, both the reduction of oxygen and the oxidation of the oxide ion at the Pt/zirconia interface is reversible and the transport of both species in zirconia is so rapid it is possible to construct an electrochemical oxygen pump, which is the heart of the limiting current oxygen sensor described in this chapter (Saji, 1987). The overall electrochemical reaction that takes place at the porous Pt electrode is... 

In a blank test where nitrogen gas, instead of the reaction gas mixture, was passed over the Au anode at 723K, it was confirmed that the oxygen pumping was well controlled by the electric potential, i.e. the amount of oxygen gas evolved in the anode room (measured by the gas chromatography) coincided well with the value calculated from the electric current across the YSZ. 

Case 1 the DC current is lower than Jiiml- The mass balance within a layer ofthe solid electrolyte near the cathode indicates that no stoichiometry change is obtained rather, only an oxide ions flux from the cathode to the anode is observed. This flux obeys the Faraday law (i.e., J = I/2F, and the cell functions as an oxygen pump (Figure 12.2). 

In laboratory-prepared devices (such as oxygen pumps), porous platinum layers that function as both the electrode and current collector are generally deposited on both surfaces of the membrane. 

Dutta, A. and Ishihara, T. (2005) Amperometric NOx sensor based on oxygen pumping current by using LaCaOj-based solid electrolyte for monitoring exhaust gas. Sens. Actuators B, 108 (1-2), 309-13. 

Ishihara, T, Fukuyama, M., Dutta, A., Kabemura, K., Nishiguchi, H. and Takita, Y. (2003) Solid state amperometric hydrocarbon sensor for monitoring exhaust gas using oxygen pumping current. 7. Electrochem. Soc., 150 (10), H241-5. 

In other cases, the use of the oxygen pumps carries out in galvano-stalic mode, that is, at the constant current in circuit (4.43). Let us consider the main calculated characteristics of such pumps, which can be represented by the speed and depth of the liquid metal purification from oxygen as well as by the oxygen flow and by the time of the attainment of the set deepness of purification. 

Figure 5. Selected examples of e.m.f. vs. time dependencies of the YSZ cells, shown in Fig.2B, with fixed gas composition in the chamber and current through the oxygen pump.

Zirconia (Zr02) stabilized in its high temperature, cubic form by addition of 8 to 12 mol.% yttria or scandia is currently the materials of choice in devices utilizing a solid-state oxide ion conducting electrolyte, eg oxygen sensors, oxygen pumps and solid oxide fuel cells (SOFCs), because of its good oxide ion conductivity and redox resistance [1-4]. Improvements in conductivity, however, are necessary to enhance theirs performance and efficiency. 

Vs cell always monitors the atmosphere in the gas detection chamber. Oxygen pumping current is flowed into the Ip cell due to the constant atmosphere in the detection chamber. Ip signal output can detect the overall air/fuel ratio region, which corresponds to the air/fuel ratio [6]. The relationship between Ip signal output and air/fuel ratio is shown in Figure 3.1.14. 

However, the central portion of the element consists of a second diffusion chamber to induce the gas to the second detection chamber. The bottom sheet composes the second oxygen pumping cell (lp2 cell). An external voltage is applied to the pumping cell to dissociate NO, into N2 and O2, and finally transfers the generated O2 by the pump. At that time, Ip2 pumping current corresponds to the NO concentration in the exhaust gas. 

Membrane Technology, Fig. 1 Modifications of process applications of membranes with high oxide-ion transport (a) oxygen pump with an electrical current (b) transport membrane (c) heterogeneous catalysis (b) and (c) without an electrical field... 

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