Coupling of Kinetic and Mass Transfer Processes in Direct Methanol Fuel Cells

Abstract

A model coupling momentum transport with reaction kinetics within the five-layer membrane electrode assembly has been developed for direct methanol fuel cells (DMFCs). The model accounts for the essential intermediate reaction steps on both anode and cathode catalyst layers, as well as the two-phase phenomena in the anode and cathode gas diffusion layers. The kinetics of the methanol reaction on the cathode catalyst layer that separately account for both chemical and electrochemical pathways are investigated. The model predictions agree with the DMFC experimental data. Simulation results indicate that the transport of methanol is essential in determining both the anode and cathode kinetics. Anode kinetics are not significantly improved for anode concentrations above 2 M. It is also revealed that the transport of methanol to the anode catalyst layer is significantly enhanced by the convection of bubbles toward the flow field. The influence of methanol crossover on the cathode potential is quantified by changing the anode feed from methanol to hydrogen. The cathode potential is seen to deteriorate at higher methanol feed concentrations mainly due to the depletion of oxygen by the crossed over methanol on the cathode catalyst. This model should prove useful in optimizing the methanol feed concentration in DMFCs.