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Long-term performance testes by CRIEPI (Central Research Institute for Electric Power Industry) on six industrial stacks have revealed an interesting correlation between cathode polarization loss and ohmic loss. To make clear the physicochemical meaning of this correlation, detailed analyses were made on the conductivity degradation of YSZ electrolyte in button cells and then on the ohmic losses in the industrial cells in terms of time constants which are determined from speed of the tetragonal transformation through the Y diffusion from the cubic phase to the tetragonal phase. In some cases, shorter time constants (faster degradations) were detected than those expected from the two-time-constant (with and without NiO reduction effects) model, suggesting that additional ohmic losses after subtracting the contribution from the tetragonal transformation must be caused from other sources such as cathode-degradation inducing effects. Main cathode degradations can be ascribed to sulfur poisoning due to contamination in air in the CRIEPI test site. An important feature was extracted as this cathode degradations became more severe when the gadolinium-doped ceria (GDC) interlayers were fabricated into dense film. Plausible mechanisms for cathode degradations were proposed based on the Sr/Co depletion on surface of lanthanum strontium cobalt ferrite (LSFC) in the active area. Peculiar cathode degradations found in stacks are interpreted in term of changes in surface concentration by reactions with sulfur oxide, electrochemical side reactions for water vapor emission or Sr volatilization, and diffusion of Sr/Co from inside LSCF.

To examine the effect of so-called "concentration polarization" on the performance of a membrane reactor with a highly hydrogen-permeable membrane, methane steam reforming was conducted, using a Pd/Ag membrane with a thickness of a few micrometers. First, the relation between the methane conversion and the hydrogen recovery was experimentally examined, and the relation was compared with that predicted by a rather simple simulation that assumes the instant achievement of equilibrium. When the hydrogen recovery was smallest, the experimental results agreed well with the simulation results. With increasing reaction pressure, the experimental methane conversion became lower than the simulated conversion. These results suggest that the reaction is limited by reduced hydrogen removal, because of the concentration polarization. The influence of concentration polarization was confirmed by the comparison of the experimental results of hydrogen permeation from a mixture of H 2 and N 2 with the simulation results based on the plug-flow model. It then was experimentally attempted to reduce the concentration polarization by changing the configuration of the reactor. The methane conversion was successfully improved using reactors that had narrower inner diameters and baffle plates, probably because of the reduction in concentration polarization. It was concluded that the reactor configuration was quite essential to make the best use of a membrane reactor with a highly permeable membrane.

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