Nernst equation oxygen

Air is normally the reference gas used in the exhaust gas sensor. If the oxygen partial pressure in the engine exhaust gas is known as a function of the engine air/fuel ratio, the theoretical galvanic potential of the sensor is easily determined by the Nernst equation. 

This standard potential is for an OH concentration of 1 mol-L 1, which corresponds to pH = 14, a strongly basic solution. However, from the Nernst equation, we can calculate that, at pH = 7, this couple has E = —0.42 V. Any metal with a standard potential more negative than —0.42 V can therefore reduce water at pH = 7 that is, at this pH, any such metal can be oxidized by water. Because E° = — 0.44 V for Fe2+(aq) 4- 2 e Fe(s), iron has only a very slight tendency to be oxidized by water at pH = 7. For this reason, iron can be used for pipes in water supply systems and can be stored in oxygen-free water without rusting (Fig. 12.17). 

Since the electrolyte membrane only allows the conduction of ions, the electrons are forced through an exterior circuit, creating an electromotive force. The voltage generated by such a cell is given by the Nernst equation. For the hydrogen-oxygen reaction we can write ... 

The oxygen content in the exhaust is measured against a suitable reference, in this case atmospheric air. The response is given by the Nernst equation ... 

When a zirconia electrolyte is exposed on different sides to gases with different oxygen partial pressures a relationship such as shown in Figure 1 is obtained. The voltage, E, developed with this type of galvanic cell is given by the Nernst equation as shown below ... 

Figure 4 shows the theoretical output of a sensor calculated using the Nernst equation and the oxygen partial pressures of Figure 3. Again note the step change at the stoichiometric air-fuel ratio. Many commercially available sensors have outputs that closely approach this theoretical relationship.

At operating temperature (100-400°C), the oxygen anions have sufficient mobility in the solid electrolyte. The cell voltage ECeii is then related to the partial pressure of oxygen by the Nernst equation written for the concentration cell. 

The complete current-voltage characteristics of the sensor can be derived from the similar consideration that was used for derivation of the i-E curve for liquid electrolytes. Because the potentials at each electrode are reversible, their difference can be expressed by the Nernst equation for the concentration of oxygen at the anode Co(0) and at the cathode Co (A). The current flowing through the layer generates a voltage drop iRb, where Rb is the bulk resistance of the ZrC>2 layer. 

The standard states to which this E° value refers are 1 atm for oxygen gas and 1 mol/L for H+. We can calculate E for the above half-cell for neutral solutions, in which [H+] = 10-7, by using the Nernst equation. Assuming the oxygen remains at its standard state, P(O2) = 1 atm,... 

Platinum catalyses the dissociation and recombination of the oxygen molecules so that O2 ions can be formed at one electrode and converted to 02 molecules at the other. The e.m.f. developed by such a cell is given by the Nernst equation (see Eq. (4.33)). In the present case z = 4 because an oxygen molecule consists of two atoms each acquiring two electrons on being ionized. Hence, the equation becomes... 

A final example of electrochemical kinetics will consider a return of the Pt WE from before but now exposed to a neutral (pH 7.2) solution into which oxygen is bubbled. The kinetics of the oxygen reduction reaction (ORR) will be studied. The data generated might appear as shown in Fig. 24. The reversible potential for the ORR in pH 7 solution, according to the Nernst equation, is... 

The cell reaction is the transfer of oxygen from one side to the other. (See also Lambda probe). In the case of an - electrochemical equilibrium (subentry of -> equilibrium) the measured -> open circuit potential (subentry of - potential) or -> equilibrium potential (subentry of -> potential) Ueq or E (emf) can be calculated by the -> Nernst equation ... 

Therefore such sensors are called Nernstian sensors. As a reference air with defined humidity is used. In reducing gases that are in chemical equilibrium (e.g., H2, H2Oj CO, C02 water gas) the oxygen partial pressure is determined by the mass law constant Kv and this in turn depends on the temperature. In the case of H2,H20-mixtures the cell voltage is obtained by insertion of a temperature function of log Kp into the Nernst equation... 

As discussed in table 10.1, the mobile species within a fuel cell are ions, which necessitate the electrolyte being an ionic conductor and electronic insulator. If the oxygen ions are the only charge carriers, the electron motive force (EMF) of the cell is determined from the chemical potential of oxygen (i.e., oxygen activity), which is expressed by the Nernst equation as... 

Fig. 3.1. A, The respiratory chain. Q and c stand for ubiquinone and cytochrome c, respectively. Auxiliary enzymes that reduce ubiquinone include succinate dehydrogenase (Complex II), a-glycerophosphate dehydrogenase and the electron-transferring flavoprotein (ETF) of fatty acid oxidation. Auxiliary enzymes that reduce cytochrome c include sulphite oxidase. B, Thermodynamic view of the respiratory chain in the resting state (State 4). Approximate values are calculated according to the Nernst equation using oxidoreduction states from work by Muraoka and Slater, (NAD, Q, cytochromes c c, and a oxidation of succinate [6]), and Wilson and Erecinska (b-562 and b-566 [7]). The NAD, Q, cytochrome b-562 and oxygen/water couples are assumed to equilibrate protonically with the M phase at pH 8 [7,8]. E j (A ,/ApH) for NAD, Q, 6-562, and oxygen/water are taken as —320 mV ( — 30 mV/pH), 66 mV (- 60 mV/pH), 40 mV (- 60 mV/pH), and 800 mV (- 60 mV/pH) [7-10]. FMN and the FeS centres of Complex I (except N-2) are assumed to be in redox equilibrium with the NAD/NADH couple, FeS(N-2) with ubiquinone [11], and cytochrome c, and the Rieske FeS centre with cytochrome c [10]. The position of cytochrome a in the figure stems from its redox state [6] and its apparent effective E -, 285 mV in...

The value of for a hydrogen/air fuel cell, 1.23 V at 25 °C and unity activities of hydrogen, oxygen, and water would be further corrected by the Nernst equation for specific values of pu, Po, h>o>... 

A higher polarization curve when cell operating pressure is increased can be expected on the base of the Nernst equation (3.15), but the concomitant increase of / o, due to the higher concentration of reactant gases on electrodes, with the consequent improvement of the hydrogen/oxygen reaction rate, has to be also considered [34]. 

In this process, air and fuel are fed to the reactor at opposite sides of a dense oxygen ion-conducting solid electrolyte membrane. The theoretical electromotive force (EMF), Eth or f ocv> is calculated from the Nernst equation (1.01 V at 800 °C with pure hydrogen at the anode and air at the cathode). The voltage output (U) under load conditions obeys the following equation ... 

Nernst equation is not straightforward however whit thermodynamics data one can demonstrate that CeOj can act as an efficient oxidation catalyst [33]. According to its ability to "deliver" oxygen, Ce02 is consider as a n-type semiconductor. This type of oxide is generally assumed to have good oxidation properties [7,20]. 

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