An electrochemical model was developed to study the NH 3 -fed and H 2 -fed solid oxide fuel cells based on proton conducting electrolyte (SOFC-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 NH 3 -fed and the H 2 -fed SOFC-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 SOFC-H shows significantly higher voltage and power density than the NH 3 -fed SOFC-H due to the significant difference in concentration overpotentials. The anode concentration overpotential of the NH 3 -fed SOFC-H is found much higher than the H 2 -fed SOFC-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, H 2 O 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 NH 3 -fed SOFC-H. How to reduce the concentration overpotentials at both electrodes is identified crucial to develop high performance SOFC-H.