Role of surface exchange kinetics in coated zirconia dual-phase membrane with high oxygen permeability

In order to comprehend the enhancement mechanism of oxygen permeability by surface coating in the zirconiabased dual-phase membrane, it is necessary to comprehend surface exchange reactions in coated-membrane. Surface exchange reactions significantly contribute to oxygen permeability in a thin membrane with a thickness lower than characteristic thickness; thus, it is extremely important to define the correlation of surface exchange kinetics between the membrane and coating material. The permeation fluxes of zirconia-rich composite membrane [70 vol% zr(0.79)Sc(0)(.)(2)Ce(0.01)O(2-delta) (ScSZ): 30 vol% La0.7Sr0.3MnO3 +/-delta (LSM), LSM30] in the thickness range of 40-340 mu m have been investigated as a function of oxygen partial pressure on the feed and permeate sides to elucidate the enhancement in the permeation flux by a Nd2NiO4+delta (NNO)-coating layer. The oxygen permeation resistances were estimated from oxygen fluxes as a function of P(O2 )through the oxygen permeation model. The surface exchange (k and k '') and diffusion (D) coefficients are obtained from the resistances. The diffusion coefficient of the coated membrane is similar to that of ScSZ, which corresponds to the main element of the membrane as the ionic conductor. Additionally, the surface exchange coefficient of the coated membrane is similar to that of NNO as the coating material. The surface exchange reaction on the feed side is faster than that on the permeate side under the operating condition. In order to ensure a more detailed elucidation, the k values obtained from the permeation model are compared with those obtained from the electrical conductivity relaxation method and characteristic length (L-C). Despite differences in the experimental conditions such as morphologies, the surface exchange coefficients exhibited similar values and trend in the temperature range of 750-900 degrees C, irrespective of the experimental method.