Tailoring Flow Field Channel Aspect Ratio for Efficient Mass Transport and Compression in Fuel Cells

Abstract

The adverse effects of global warming have made it critical to transition our reliance on fossil fuels to more sustainable energy sources. Polymer electrolyte membrane fuel cells (PEMFCs) can facilitate this transition by providing on-demand power with zero local carbon emissions. However, the high cost and poor durability of fuel cells hinder their widespread adoption. Particularly, PEMFC performance is strongly dependent on the flow fields which should be designed to optimize the transport of reactants and byproducts while maintaining uniform compression with subsequent layers. An important flow field design parameter is the channel aspect ratio which directly influences compression and the transport of reactants and products. Although novel flow field configurations have been studied previously, a comprehensive investigation on the effects of channel aspect ratio on cell performance has yet to be performed. In this study, we compared the electrochemical performance across varying flow field channel aspect ratios (channel width by height) from 0.5-2.0 with a fixed active area. The ohmic and mass transport resistances were quantified using electrochemical impedance spectroscopy. Operando X-ray imaging was performed to spatially resolve water saturation under the land and channel regions of the flow fields. We observed that lower channel aspect ratios (or higher number of channels for the given active area) led to lower ohmic resistance but resulted in higher mass transport losses at higher current densities due to significant water saturation under the hydrophilic ribs of the flow field. Notably, from these results we elucidated that there exists an ideal channel to rib width ratio to facilitate efficient mass transport and effective contact between the porous microstructures.