The electrode kinetics of the evolution and dissolution of oxygen at the urania-zirconia interfaces

In order to assess the potential of urania-yttria fluorite-type solid solutions as electrodes for high-temperature electrolysis of steam, oxygen evolution and dissolution reactions have been studied at the (Usub(0.7)Ysub(0.3))Osub (2+x)/YSZ interface. A current-interruption technique was used to separate overpotential and resistive potential drop. In oxygen and air the overpotential-current curves obey the Tafel law, suggesting that a charge-transfer process is rate determining. Activation energies of 120 kJ mole-1 and 165 kJ mole-1 were obtained for the cathodic reaction in oxygen and air respectively. The capacitance obtained from galvanostatic transients varied with potential, temperature, and oxygen partial pressure. The average value of n, the number of electrons involved in the overall charge-transfer reaction, was determined to be 4.01 from reversible potential measurements. The overpotential losses are small for porous electrodes at high psub(O2). A mechanism for the oxygen transfer reaction has been proposed and its limitations discussed. (author)