Numerical investigation of spatial variation of hydrophobicity in diffusion media along the through-plane direction in direct methanol fuel cells

Abstract

The wetting characteristics of the diffusion media (DM) of direct methanol fuel cells (DMFCs) are studied using a one-dimensional (1-D) DMFC model. The model, which is based on the capillary transport theory for porous media, facilitates investigation of the impact of spatial variation of hydrophobicity in the DM on methanol transport/crossover and the resultant cell performance. The 1-D DMFC simulation results were compared with the empirical data acquired at a methanol feed concentration of 8 M. The theoretical simulations agree well with the experimental data and also indicate that methanol crossover from the anode to the cathode can be decreased by employing an anode DM design in which the hydrophobicity increases toward the anode catalyst layer. Under the anode DM design, a maximum power density of 73.15 mW cm−2 was predicted by the model at a methanol feed concentration of 8 M, i.e. sufficiently close to the experimental measurement (71.17 mW cm−2). Detailed simulation results clearly show that the reduced methanol crossover is mainly derived from liquid transport characteristics; the rate of liquid transport through the DM is reduced as the liquid flows from the relatively hydrophilic region to the relatively hydrophobic one. This numerical study demonstrates that the present DMFC model is a valuable tool for the design and optimization of DM with spatial wettability variation to effectively control the methanol and water transport in DMFCs.