Current-voltage ͑I-V͒ characteristics and electrode impedance of a tubular-type solid oxide fuel cell ͑SOFC͒ were analyzed for mixed fuel gases, consisting mainly of CO, H 2 , H 2 O, and a carrier gas, as simulated reformed gas of hydrocarbons or coal gas. I-V characteristics of a single cell were measured as a function of various operational parameters including the H 2 -to-CO ratio, the type of carrier gas such as He, N 2 , and Ar, the temperature, the fuel-to-carrier gas ratio, and the water vapor concentration. It has been experimentally confirmed that the use of CO-rich gases results in comparable performance to that of H 2 -rich gases and thus mixed gas such as coal gas is useful as a SOFC fuel. We have found, for the first time, that the I-V characteristics depend on the carrier gas, indicating the importance of gas transport in porous anodes for anodic polarization. The change in cell voltage by varying fuel compositions was mainly caused by the change in anode impedance associated with a low frequency semicircle in a Cole-Cole impedance plot at 1000°C. The fuel gas compositions in thermodynamic equilibrium were calculated and compared with the initial gas compositions, suggesting the importance of water vapor concentration to control the equilibrium H 2 -to-CO ratio for CO-rich fuel gases.One of the most advantageous features of solid oxide fuel cells ͑SOFCs͒ compared with other types of fuel cells, is the flexibility for types of fuel that can be used, in addition to their excellent electrical generation efficiency, system simplicity, and long-term stability for possible stationary and mobile applications. In principle, besides hydrogen, we can indirectly ͑using an external reformer͒ or even directly use various kinds of fuels including carbon monoxide, hydrocarbons, and alcohols from fossil fuels as well as natural energy resources such as biomass.Hydrocarbon-based fuel gases, such as natural gas, petroleum gas, and even gasoline, may be reformed 1 in a prereformer ͑or at the electrodes͒ to mixed gases consisting mainly of H 2 , CO, and H 2 O, the ratio of which varies considerably depending on their reforming conditions. Biogas and alcohols, produced from, e.g., biomass and other biological or industrial wastes, can be useful energy resources, 2 which may also be reformed to such mixed gases before the electrochemical oxidation at the fuel electrodes. As one of the most abundant and inexpensive resources, coal can be used to produce coal gas, also a mixed gas based on CO and H 2 . Coal gas could be applied as a fuel for a combined-cycle power plant with coal gasifier, SOFCs, and gas ͑and steam͒ turbine͑s͒. 3-7 The concentration of water vapor and carrier gas in such a gas depends on the gasification procedures for the partial oxidation of coal by air, oxygen, or water. 3 It is therefore of technological relevance to analyze power generation characteristics of SOFCs operated with mixed H 2 and CO fuel gases. The ratio of H 2 and CO varies depending on the kinds of fuel gases supplied to the fuel cells:...
