186
4 – Electrode reactions
sites per unit surface area, by θ the fractional surface coverage, and
by k ads and k des the respective adsorption and desorption constants,
assuming that the adsorption follows the Langmuir equation with
θ % 1.
2 the equation relating the coverage rate at equilibrium θ eq to the oxygen partial pressure.
2 the charge-transfer reaction at the triple phase boundary between the
gas, the electrode material, and the electrolyte and the corresponding
rate. Denote by E the potential applied to the electrode and by α the
anodic transfer coefficient. These reactions are generally accepted for
modeling the reduction of oxygen at the (La 0.8 Sr 0.2 MnO 3−δ -YSZ) /
YSZ interface.
b. Deduce the formula for the current density i flowing through the electrode
as a function of potential E.
3. Derive the formula for the exchange-current density i 0 .
4. a. Express the formula for the current density under polarization as a function of the exchange-current density and the electrode overpotential η.
b. What happens to this formula when the fractional surface coverage is
negligible and the electrode is only weakly polarized?
c. Deduce the formula for the polarization η as a function of the current I
and of the contact surface area S.
5. Compare the theoretical formula obtained in question 4(a) to that observed
experimentally in question 1 and deduce the exchange current I 0 of the
electrode under study.
Exercise 4.5 – Reduction of water vapor at the M / YSZ interface
with M = Pt, Ni
Table 41 lists the results of potentiostatic measurements of the current I as a
function of the cathodic overpotential η corresponding to the reduction of water
vapor at 860 °C at the Pt /YSZ and Ni / YSZ interfaces.
4 – Electrode reactions
sites per unit surface area, by θ the fractional surface coverage, and
by k ads and k des the respective adsorption and desorption constants,
assuming that the adsorption follows the Langmuir equation with
θ % 1.
2 the equation relating the coverage rate at equilibrium θ eq to the oxygen partial pressure.
2 the charge-transfer reaction at the triple phase boundary between the
gas, the electrode material, and the electrolyte and the corresponding
rate. Denote by E the potential applied to the electrode and by α the
anodic transfer coefficient. These reactions are generally accepted for
modeling the reduction of oxygen at the (La 0.8 Sr 0.2 MnO 3−δ -YSZ) /
YSZ interface.
b. Deduce the formula for the current density i flowing through the electrode
as a function of potential E.
3. Derive the formula for the exchange-current density i 0 .
4. a. Express the formula for the current density under polarization as a function of the exchange-current density and the electrode overpotential η.
b. What happens to this formula when the fractional surface coverage is
negligible and the electrode is only weakly polarized?
c. Deduce the formula for the polarization η as a function of the current I
and of the contact surface area S.
5. Compare the theoretical formula obtained in question 4(a) to that observed
experimentally in question 1 and deduce the exchange current I 0 of the
electrode under study.
Exercise 4.5 – Reduction of water vapor at the M / YSZ interface
with M = Pt, Ni
Table 41 lists the results of potentiostatic measurements of the current I as a
function of the cathodic overpotential η corresponding to the reduction of water
vapor at 860 °C at the Pt /YSZ and Ni / YSZ interfaces.