The effect of employing a porous carbon plate on the performance of a passive direct methanol fuel cell (DMFC) under closed circuit conditions was investigated. The porous carbon plate and a CO2 gas layer that formed between the anode and the porous plate stably controlled the mass transfer of the methanol and water from the reservoir to the anode, which made operation with very high concentrations of methanol, even neat methanol, possible. The i-V and i-t performances of the DMFC with and without the porous plate were measured at different methanol concentrations, and the performances were compared. The maximum power density, 24 mW cm(-2) at room temperature, obtained at 2 M without the porous plate was reproduced at 16 M with the porous plate. Also, the methanol crossover flux and water flux through the MEA was evaluated, and the Faraday efficiencies of the DMFC with and without the porous carbon plate were analyzed. When high concentrations of methanol were used with the porous plate, it was confirmed that the Faraday efficiency remained high, and the back diffusion of water from the cathode to the anode through the membrane occurred which resulted in no flooding at the cathode, contrary to the case without the porous plate. By increasing the distance between the anode and the porous plate, the power density decreased, suggesting that the distance of the CO2 layer played an important role in obstructing the mass transport. (c) 2006 Elsevier B.V. All rights reserved.