An electrochemical model was developed to study the NH3-fed and H-2-fed solid oxide fuel cells based on proton conducting electrolyte (SCFC-H). The modeling results were consistent with experimental data in literature. It is found that there is little difference in working voltage and power density between the NH3-fed and the H-2-fed SCFC-H with an electrolyte-support configuration due to an extremely high ohmic overpotential in the SOFC-H. With an anode-supported configuration, especially when a thin film electrolyte is used, the H-2-fed SCFC-H shows significantly higher voltage and power density than the NH3-fed SCFC-H due to the significant difference in concentration overpotentials. The anode concentration overpotential of the NH3-fed SOFC-H is found much higher than the H-2-fed SCFC-H, as the presence of N-2 gas dilutes the H-2 concentration and slows down the transport of H-2. More importantly, the cathode concentration overpotential is found very significant despite of the thin cathode used in the anode-supported configuration. In the SOFC-H, H2O is produced in the cathode, which enables complete fuel utilization on one hand, but dilutes the concentration of O-2 and impedes the diffusion of O-2 to the reaction sites on the other hand. Thus, the cathode concentration overpotential is the limiting factor for the H-2-fed SOFC-H and an important voltage loss in the NH3-fed SCFC-H. How to reduce the concentration overpotentials at both electrodes is identified crucial to develop high performance SCFC-H. (C) 2008 Elsevier B.V. All rights reserved.