Mathematical modeling of liquid-feed direct methanol fuel cells

A two-phase, multicomponent model has been developed for liquid-feed direct methanol fuel cells (DMFC). In addition to the anode and cathode electrochemical reactions, the model considers diffusion and convection of both gas and liquid phases in the backing layers and flow channels. In particular, the model fully accounts for the mixed potential effect of methanol oxidation at the cathode as a result of methanol crossover caused by diffusion, convection, and electro-osmosis. This comprehensive model is solved numerically using computational fluid dynamics. The transport phenomena and electrochemical kinetics in a liquid-feed DMFC are delineated and the effects of the methanol feed concentration on cell performance are explored. The model is validated against DMFC experimental data with reasonable agreement. The void fraction at the anode outlet is found to be as high as 95% at a cell current density of 0.45 A/cm² for a 7 cm long channel. Increase in methanol feed concentration leads to a slight decrease in cell voltage and a proportional increase in the mass-transport limiting current density for a methanol concentration below 1 M. However, which the methanol feed concentration is larger than 2 M, the cell voltage is greatly reduced by excessive methanol crossover and the maximum current density begins to be limited by the oxygen supply at the cathode. The oxygen depletion results from excessive parasitic oxygen consumption by methanol crossing over.