SC-SOFC Systems

In this case, compared with the direct electrochemical oxidation of the fuel [Eq. (2.2)], the Faraday efficiency is only 75% since the production of syngas (H, + CO) is not an electrochemical process, and electrons (six instead of eight) are generated only by the electrochemical oxidation of the syngas. In addition to this intrinsically reduced efficiency, current SC-SOFC systems show very low fuel utilization (1-8%) and thus efficiencies [4,18, 19]. While gas intermixing, small-scale electrodes, and high flow rates contribute to the low efficiency, non-ideally selective electrode materials and parasitic reactions are the primary reason. The further development of SC-SOFCs therefore requires active and selective materials for optimized performance. 

Recently, Savoie etal. [24] reported an extensive investigation on catalysis in SC-SOFCs with Ni-YSZ (yttria-stabilized zirconia) anodes. Their investigations were performed on three different half-cells exposed at various temperatures to a methane-air gas mixture. The detected outlet gases contained CO and H2, but also CO2, suggesting that the complete oxidation reaction also occurred. Furthermore, at a fixed gas flow rate, the outlet gas composition was found to depend on both Rmix and the thickness of the anode. At 600 °C and Rmix = 1-2, 33% of methane was catalytically converted on a thin anode (0.05 mm), and more than double on a thick anode (1.52 mm). An increase in temperature led to an increase in methane conversion. At 800 °C and R ix = 1-2, the yield of H2 was 14 and 38% for thin and thick anodes, respectively. The production of syngas was significantly reduced at lower temperatures. From this study, it is obvious that the optimization of SC-SOFC systems requires extensive catalytic studies on the electrode materials. 

A fuel-to-oxygen ratio corresponding to the partial oxidation of the fuel (i.e., = 2 in the CH4-O2 system) was found appropriate for SC-SOFC operation [13, 15, 20, 21]. This optimized R ix value also depends on the electrode structures and thickness. Fuel-rich gas mixtures can cause carbon deposition whereas oxygen-rich gas mixtures can favor complete fuel oxidation and increase the risk of explosions. 

The proof of concept that SC-SOFCs can be considered for portable power generation was provided by Shao etal. [19], who successfully powered a 1.5 V MP3 player with a miniature two-ceU SC-SOFC stack. The heat generated from the exothermic reactions allowed rapid start-up from cold start to stable power in less than 1 min and thermally self-sustained the system. However, the low cell efliciency of current SC-SOFCs questions their practical implementation, and energy-harvesting appHcations where energy is generated from waste gases and efficiency plays a secondary role seem more adequate. Hibino and co-workers... 

Therefore, their SC-SOFCs inherently show the low efficiency of the overall fuel cell system. 

Suzuki M, Iwata S, Higaki K, Inoue S, Shigehisa T, Miyoshi I, Nakabayashi H, Shimazu K (2009) Development and field test results of residential SOFC CHP system. In Singhal SC, Yokokawa H (eds) Solid oxide fuel cells XI -Part 1. ECS Trans 25(2) 143-148... 

Mizutani Y, TamuraM, Kawai M, Nomura K, Nakamura Y, Yamamoto O (1995) Characterization of the Sc203-Zr02 system and its application as the electrolyte in Planar SOFC. In Dokiya M, Yamamoto O, Tagawa T, Singhai SC (eds) Solid oxide fuel cells IV, PV 95-1. The Electrochem. Soc. Inc., Pennington, pp 301-317... 

Suzuki M, Sogi T, Higaki K et al (2007) Development of SOFC residentieil cogeneration system at Osaka Gas and Kyocera. In Eguchi K, Singhal SC, Yokokawa H, Mizusaki J (eds) Sohd Oxide Fuel Cells-X, The Electrochemical Society, 27—30... 

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