This study investigates the CO2 behavior and its effect on the performance of a passive direct methanol fuel cell (DMFC) by use of visualization method. Different flow field designs are introduced to the anode of the DMFC. The parallel-fence structure and the new composite structure with a sintered porous metal fiber felt (SPMFF) are experimentally compared in terms of the cell performance and mechanisms of reactant and product managements. Results show that the cell based on the composite structure yields the best performance during high-concentration operation. The vertical parallel-fence structure is more suitable at a lower methanol concentration. The visualization tests indicate that the use of a composite structure can help enhance convection due to CO2 self-promoting behavior and also enhance gas storage in the channel to control the methanol concentration. The presence of CO2 helps to restrain methanol crossover, which should be actively controlled to enhance internal convection and thereby improve the cell performance. The use of a composite structure enables the cell to perform stably for continuous operation at a higher methanol concentration. The open-circuit and dynamic characteristics of this composite-structure-based DMFC are also evaluated in this study. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.