In the present study, a passive air-breathing direct methanol fuel cell (DMFC) with an active area of 4.41 cm2 and a built-in methanol solution reservoir of 8 ml was used to investigate the effects of relative humidity (RH) and temperature of ambient atmosphere on cell performance and cathodic flooding. The experimental results indicate that the effects of RH on cell performance are dependent upon temperature. When the temperature is below 10 °C, RH shows no effect on the cell performance. At the temperature of 20 °C and 35 °C, the cell performance is quite similar at low to medium current densities, and the cell performance increases with the decrease of RH at high current densities. In the case of 50 °C, it seems that RH has a greater effect on the cell performance than those at lower temperature: the cell performance is improved at low to medium current densities, whereas it is depraved at high current densities with the increase of RH. Furthermore, it is found that water flooding at the cathode was strongly dependent on the competition of water evaporation and production. High RH and low temperature led to more serious water flooding at the cathode. A better long-term discharge performance can be obtained under the condition of high temperature and low RH.

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