Especially, the relative humidity w

Especially, the relative humidity was the most dominant parameter affecting the power output. The power output increased at the elevated temperature due to the higher electrochemical reaction rate, ionic conductivity of the membrane, and mass diffusion coefficient. The increased relative humidity caused higher ionic conductivity and the increased stoichiometric ratio reduced the concentration loss. The ionic conductivity of the membrane was strongly influenced by the relative humidity, showing the most critical effect on the cell performance. For example, the ionic conductivity was respectively 2.03 S/m and 12.93 S/m at the relative humidities of 20.0% and 100.0%. As shown in Fig.6, the system efficiency dramatically decreased with increasing operating temperature because the saturation vapor pressure and thermal energy for preheating/humidification exponentially increased with the operating temperature. The compression work also rapidly increased at the elevated operating temperature due to the higher water vapor partial pressure and inlet pressure. The operating pressure was determined by summing the partial pressures of air and water vapor [21]. As shown in Fig. 7, the power output, thermal energy, and compression work increased with the increase in the relative humidity. However, the effect of the relative humidity on the system efficiency was almost negligible because the increased power output corresponded to the increased thermal energy and compression work. In addition, as shown in Fig. 8, the power output slightly increased with the increase in the stoichiometric ratio due to the reduced concentration loss. The thermal energy and compression work almost linearly increased with the stoichiometric ratio because of the elevated flow rate. Consequently, the system efficiency decreased with the stoichiometric ratio because the increased thermal energy and compression work was more substantial than the increasedpower output. At high current density conditions, the cell performance increment might be more pronounced because the concentration loss becomes more significant.