We have operated the single-cells in the stack structure using methane as a fuel under high fuel utilizations. Methane fuel was supplied directly to the cell-stack after addition of steam at a steam to carbon ratio of 2. A steam-reforming reaction was conducted directly on the anode (direct internal reforming: DIR). In the case of the single-cell with the anode-substrate having relatively high porosity, stack voltage was found to be 0.767 V at fuel utilization of 75%, at which d.c. energy conversion efficiency is 50.2%(HHV). A maximum energy conversion efficiency, 53.6%, was achieved at fuel utilization of 86% and cell voltage of 0.712 V. This methane-to-electricity conversion efficiency will be high enough for practical applications. For anode-supported SOFCs, concentration polarization loss in the anode can be a crucial problem because fresh fuel and electrochemical reaction products have to diffuse through a thick anode substrate in the opposite direction between the flow field (fuel distribution channel) and the anode/electrolyte interface. Limiting current density with a diluted CO at a fixed current density was measured and analyzed to investigate the diffusion characteristics of a fuel in the anode-substrates. Diffusion parameters obtained by the analysis were verified by measurement of the limiting current density with diluted H2 and voltage response for depolarization process after current-interruption.
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