The performance of a solid oxide fuel cell fueled with flames of combustible gases, liquids, and solids was studied. A cell structure in which cells were serially integrated within a disk was also examined. Open-circuit voltages indicated with a single cell and the integrated cell were ϳ0.8 and ϳ3.5 V, respectively. Maximum power density obtained with the flames of n-butane, kerosine, paraffin wax ͑candle͒, and wood were respectively 75, 65, 62, and 5 mW/cm 2 . The integrated cell gave a maximum power density of 318 mW/cm 2 with n-butane flame. Fuel to air ratio distinctly influenced the output.Commercialization of fuel cells has already begun. Most popular is a proton exchange membrane ͑PEM͒ type that can only use hydrogen as the sole fuel. Because the exhaust is only water, hydrogen is ideal to minimize environmental load. Currently, however, most hydrogen is produced from hydrocarbons, emitting carbon dioxide (CO 2 ), consuming an energy externally supplied. 1 Water electrolysis also imposes a heavy load after all, as the power used is derived from hydrocarbons, typically the fossil fuels. Hydrogen production using a nuclear reactor is proposed, 2 but other concerns arise ͑e.g., nuclear waste͒. Efficiency of the other alternative processes, photocatalysis 3 and biocatalysis, 4,5 is still insufficient. Moreover, it will take time to realize safe and efficient storage and transport of hydrogen. 6,1 At present, efficient use of the readily available hydrocarbons is accordingly important. Solid oxide fuel cells ͑SOFCs͒ can directly use hydrocarbons. In contrast to PEM, fuel reformer, and humidifier are unnecessary. Applicable fuels, however, are quite limited within few relatively low molecular gases and liquids. Furthermore, adequately reliable separator 7,8 and sealant 9 are hard to obtain so that their practical use is still restricted.A structure of fuel cell in which fuel and air ͑oxidant͒ are not separated had been suggested in early 1960s, 10 whereas confirmation of its performance in practical level with SOFC type was in the 1990s. 11-15 This is often called the single-chamber SOFC. This needs neither separator nor gas-seal structure and accordingly, can be remarkably simplified. To avoid an explosion of the mixed fuelair, however, a fuel concentration in the single gas chamber should be rigidly kept higher than its upper explosion limit, for example, 15 vol % for methane. In this safe range, oxygen is deficient for complete oxidation of the fuel. This remains a practical difficulty to manage. Here we report a fuel cell that generates electricity directly from combustion flame. Considerably short start-up time is realizable without extra energy. Because power generation can be done from anything combustible, fuel is allowed to be gases, liquids, or solids; consequently, it is advantageous in storage and delivery.
ExperimentalNo gas chamber exists in the SOFC system we report here. On a surface of 0.2 mm thick Sm 0.2 Ce 0.8 O 1.9 ͑SDC͒ disk, a cathode consisting of 50 wt % Sm 0.5 Sr 0.5 CoO 3 ͑SSC͒-50 wt % ...
