A quantitative evaluation method of gas diffusion resistance is proposed in which measurement of Ⅴ-Ⅰ characteristics under constant fuel utilization (U_f) and ac impedance were combined to separate diffusion and electromotive force (emf) change contributions from the low frequency semi-circle observed in complex impedance diagrams. Diffusion resistance was evaluated using various hydrogen content mixtures diluted with nitrogen, Uf, hydrogen flow rate and current as parameters. From the measurement under various U_f, it was found that the diffusion resistance at low U_f originates mainly from the diffusion of H_2O. Also, from the measurement under various hydrogen concentrations in inlet fuel, the diffusion resistance was found to become significantly larger with decreasing hydrogen content.
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