Oxidation potential solid electrolyte sensors

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). 

Zirconia solid electrolyte and zinc oxide sensing electrodes were used as a high-temperature NOx sensor [470, 471]. The response of the electrode potential was linear for the logarithm of NOx (NO) concentration from 40 to 450 ppm. 

The nonequilibrium potentials measured in solid-electrolyte cells are established by electrochemical reactions, just as for equilibrium-based sensors. For example, in an environment containing CO and O2, the CO could be oxidized chemically according to the following reaction ... 

Taking into account the results obtained by the different research groups, it can be concluded that the sensing mechanism of the solid electrolyte NO sensor with the oxide-SE is based on the mixed-potential model under the coexistence of NO,j and O2. This mechanism is rather complex, and the NO2 sensitivity can be indirectly determined by the following factors [50] ... 

The situations depicted in Figs. 7.18a and 7.19c are very similar in that in both cases a driving force exists for mass transport. The origin of this force is the chemical potential gradient dji/dx that exists across the growing oxide layer in one case and the solid electrolyte or sensor in the other. 

LT is the standard cell potential difference, which is determined only by the reactants in definited standard states. This quantity results as the difference of standard electrode potentials. The power term Ila contains the corrected composition quantities a, (fugacities and activities) with the stoichiometric coefficients v, of the gases and condensed substances taking part in the cell reaction [10,12]. If a sensor at equilibrium delivers signals in agreement with Equation (25-7) then we have a reaction celt. In this case at solid electrolytes with oxide ion vacancies Vo> two reactions can be found besides... 

The common element in all the electrochemical solid-state CO2 sensors is the solid electrolyte as an ionic conductor. According to the way the measurement is performed, they can be classified into two groups amperimetric and potentiometric. Electrochemical sensors for CO2 detection are usually mixed potential sensors, a subcategory of potentiometric sensors to detect other gases than oxygen. The mixed potential sensors generally involve oxide semiconductors as electrode materials. 

The partial pressures of oxygen, po, on both sides of the membrane are used instead of activities as in electrolyte solutions. When air is used as the reference gas on one side of the membrane, the potential of the sensor will be determined by the partial pressure of oxygen in the sample gas. In some of the latest modifications air as the reference gas has been replaced by solid nickelous oxide in contact with the zirconium dioxide providing a constant concentration of ions at the platinum contact electrode. 

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