Case 5 and Case 12 were considered

Case 5 and Case 12 were considered as the references for the maximum system efficiency and power output, respectively, to explain their variations at a given current density. At all current conditions, Case 12 showed higher cell voltage and power density than the optimal condition, but it showed lower system efficiency. The power density increased with the increase in the operating temperature, but the system efficiency decreased due to the elevated thermal energy for preheating and humidification. Case 5 represented higher system efficiency than the optimal condition, but it showed lower power density due to the loweroperating temperature. Therefore, the optimal condition was determined to obtain relatively high system efficiency with allowable cell voltage and power density at a given current density. In addition, the system efficiency decreased with increasing current density because the fuel energy and thermal energy for preheating and humidification increased linearly even though the increasing slope of the power density decreased gradually (Fig. 4(b)). Case 5 showed rapid drops in the cell voltage and system efficiency at the high current density because the concentration loss became significant with the lower stoichiometric ratio.