Oxygen Evolution on La1 − x Sr x Fe1 − y Co y O 3 Series Oxides

Abstract

This is the volume change that results when it is taken into account that the oxygen fugacity increases with pressure. All other factors being constant, the current should increase when the oxygen fugacity is increased. When this is not so, it must be because the other processes going on have a total pressure dependence that more than compensates for the effect of the fugacity increase. The result from the studies of the membrane permeability as a function of pressure is shown in Fig. 6. The ordinate is the logarithm of a relative measure of the amount of oxygen that has permeated the membrane during 1 hr. The initial slope of the curve corresponds to a volume change which can be interpreted as the activation volume for the diffusion process in the membrane. The oxygen flux j through the membrane has a pressure dependence given by 01nj~ = Df/8 In fo2) T OP OP associated with the volume changes AVj=~ : ~Vk+-V(02) The average value of the slope was found to be hV/e _: 26 cm 3 mole-1 and this leads to the value ~Vk4 = 58 cm ~ mole-1. This is an appreciable pressure dependence , and it is clear that the membrane diffusion is rate determining. The experimental uncertainty here is about-+2 cm 3 mole-~, so this value is in good agreement with the value 57.2 cm 3 mole-L Then, as could be expected, the whole electrode is operating under total mass transfer control also at high pressure. The pressure effects on the Au and the Ag/AgC1 electrodes are screened by the effect on the membrane and cannot be observed.