Description of Nernstian Behavior

If the SE and RE of the zirconia-based sensor are exposed to different oxygen partial pressures, P02 (gas) and 7 02 (reference), this induces different chemical potentials of oxygen ions in zirconia at the interfaces with gas phases. In order that the electrochemical potential remains constant, the electrical potential has to be different. Therefore, the output emf of the electrochemical cell (3.2), represented as a difference between potentials on the RE and SE, obeys the well-known Nemst s law  

This equation contains only thermodynamic quantities and does not require any information about the microstructure of the system. Hence, aging effects on the microstmeture of typical YSZ/Pt electrodes do not influence the sensor signals, and current X.-sensors have lifetimes of more than 1(X),(XX) miles (10 years) [51]. 

It is noteworthy that the surface chemistry of zirconia sensors under practical operating conditions is much more complicated than expected from the oxygen partial pressures in a simple Equation (3.3). This equation imphes that only oxygen is involved in the potential forming electrode reaction, whereas in reality, a series of electrode reactions occurs together with Equation (3.1) on the SE [8]  

These reactions determine the apparent potential of the zirconia-based sensor. Since the raw exhaust gas at automotive and combustion applications constitutes a nonequilibrium gas mixture, thermodynamic equilibrium has to be achieved at the SE surface of the zirconia-based sensor before monitoring the potential. Consequently, such sensors contain catalytically active materials and are operated at temperatures above 600°C, when the average ionic transference number % 0.99. For less active materials or temperatures below 600°C the apparent em/starts to deviate significantly from the value under equilibrium conditions due to insufficient catalytic activity. Earlier research of the YSZ-based sensors was focused on the electrode materials with high exchange currents and high catalytic activity for the desired electrode reactions. Pt electrodes were found to be the most suitable for this type of application. 

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