Influence of membrane-type and flow field design on methanol crossover on a single-cell DMFC: An experimental and multi-physics modeling study

The performance of a 5 cm2 single-cell direct methanol fuel cell (DMFC) was evaluated experimentally by using two different electrolyte membranes (Fumapem® F-1850 and Nafion® N-117) for assembling the electrodes and three different types of flow field design (a unique serpentine, four parallel serpentines, four inlet serpentines). A 3D multi-physics, multi-component, two-phase, and not-isothermal model was computed with Comsol® Multiphysics v4.4 platform, to analyze and understand the behavior of the various configuration tested. The model consists of Maxwell-Stefan, Stokes-Brinckman, extended two-phase Darcy Law, modified Butler-Volmer and Tafel equations to simulate the performance of the DMFC. Pulse Field Gradient (PFG) NMR spectroscopy was used to get a direct measurement of the diffusion coefficients of water and methanol through the membranes. These values were then implemented in the multi-physics model. The model well reproduces the cell performance of all the MEA tested regarding polarization curves obtained under various experimental conditions (varying the inlet mass flows, the methanol concentration, the type of oxidant, the temperature). Thus, the model was used as a tool to investigate anodic overpotentials, water and methanol crossover flow rates, current density distribution at the catalyst layer/membrane interface, understanding the relationship between flow fields and cell performance. At similar specific power density, and similar anodic overpotentials, the methanol crossover flow rate is one order of magnitude lower for Fumapem® F-1850 than for Nafion® N-117, notwithstanding the much lower thickness of the F-1850 membrane.