CFD simulation of the effect of membrane thickness and reactants flow rate on water management in PEM fuel cells

Polymeric Electrolyte Membrane Fuel Cells (PEMFCs) are receiving a higher-than-ever interest to maximize their specific performance and reach the industrial maturity for large-scale application. One of the most promising development directions consists in using ultra-thin electrolytes, which are known to lower the ohmic overpotential. However, thin membranes effects extend largely beyond the mere internal resistance reduction, encompassing the often-overlooked full spectrum of water-related processes and of species crossover. In this study a three-dimensional multi-phase computational fluid dynamics (CFD) simulation model is presented and used to characterize the coupled current/water transport for two membrane thicknesses (30 and 6 µm), using experimental data from literature at high stoichiometry for model validation and extending the simulations to low flow rates corresponding to realistic stoichiometry. The simulation results highlight the complexity of the transport processes involved, resulting in a promoted self-humidification for thin membranes and under low stoichiometry. Two original figures of merit are introduced to (i) quantify the dominant water transport mode, and (ii) to attribute a self-humidification quality to the produced electric power, innovatively identifying which transport mode prevails and how a given power density is produced in terms of external water need, thus proposing a new method to design highly-efficient and self-humidified PEM fuel cells.