Achieving autothermal operation in internally reformed solid oxide fuel cells: Experimental studies

An integrated internal reforming solid oxide fuel cell (IIR-SOFC), in which the reformer is in good thermal contact with the SOFC, is an energy-efficient mode of operation. However, the rapid, highly endothermic methane steam reforming (SMR) reaction causes undesirable local cooling at one point or another within the SOFC system. Earlier simulation studies had indicated several system modifications that could eliminate this problem. Accordingly, experimental studies were carried out to find a suitable catalyst and reactor configuration that lowered the rate of the reforming reaction while retaining activity in the face of potential carbon deposition or sulfur poisoning. A 0.5 wt % Rh/CeZrO2 has been identified as a catalyst that could provide the features that are suitable for the application in an IIR-SOFC. The catalyst showed good activity and stability toward the SMR. Kinetic studies provided overall information that was incorporated into the SOFC model developed to investigate its feasibility as the reforming catalyst. A further set of experiments investigated the use of the Rh/CeZrO2 catalyst as the thin washcoat on the wall of the reformer. It was demonstrated that the coated catalyst had a suitably reduced but stable activity. It was shown that a barrier material (ZrO2) could also be used to lower the catalyst activity and provide a mass transfer limited operation. The barrier material could be used to create a catalyst activity profile along the length of the reactor. By the addition of small amounts of oxygen to the feed, methane oxidation could be used to reduce the overall endothermicity of the reforming process; this can serve to improve the temperature profile within the reformer. Simulations indicated that typical methane-to-oxygen ratios of 10: 1 to 20: 1 were most effective. From the results obtained, Rh/CeZrO2 has demonstrated its potential,as the reforming catalyst in a SOFC, especially in a coated-wall configuration. The experimental results demonstrated that the configurations investigated by the simulation were achievable in practice.