Mechanism of the Electrocatalytic Reduction of Oxygen in a Tubular Solid Oxide Electrolyte Flow Reactor

The nature of the electrode polarization process involving O 2 gas mixtures has been investigated in a 10 mole percent Y 2 O 3 ‐ stabilized ZrO 2 tubular solid electrolyte cell with porous Au electrodes in the temperature range 650°–850°C. The polarization phenomena are dominated by cathodic processes occurring on the inside wall of the tube. In the intermediate oxygen pressure range ( P O 2 = 10 − 2 − 0.21 atm ) , the reactor operated in a differential mode. The shapes of the experimental current‐overpotential curves are in accordance with the form of a modified Butler‐Volmer relationship with the forward and backward cathodic transfer coefficients being equal to 1 and with a model being employed in which charge transfer takes place in the two‐phase region between the Au‐electrode and ZrO 2 electrolyte. The most plausible rate‐determining step is found to be the surface diffusion of adsorbed O 2 species on the electrolyte surface inside this two‐dimensional ZrO 2 ‐ Au interface. The apparent exchange current I 0 app is found experimentally to be proportional to P O 2 to the 0.53 ± 0.06 power. This behavior of I 0 app eliminates many possible mechanisms and corroborates the one involving surface diffusion of O 2 in addition to charge transfer steps involving species O 2 − and/or O − . For very low values of P O 2 (<10 −3 atm), the reactor operated in an integral mode with large conversions. In this case, significant changes that occur in the oxygen concentration owing to axial gas‐phase diffusion must be accounted for in employing the above model. In the case of high pressures, both the processes of charge transfer and surface diffusion play a rate‐determining role in the overall electrode dynamics. The activation enthalpy for the overall cathodic reaction is estimated to be 64 ± 8 kcal / mol .