Experimental characterization of water transport in polymer electrolyte membrane fuel cells

Water management is among the key factors in maximizing the performance and durability of a polymer electrolyte membrane (PEM) fuel cell, since the cell operation is strongly influenced by water transport and distribution. Reactant starvation caused by flooding on either side of the cell may lead to substantial reduction in performance, and in the case of anode flooding, permanent damage to the catalyst layer. However, correlating the water dynamics to cell performance has been a challenge, mainly due to limited ability of current experimental tools to obtain spatially resolved information about the water content.;In the present work, liquid water formation, transport and removal were studied in situ at the catalyst layer, gas diffusion layer (GDL), and the flow field channels. Two factors that greatly influence water management, the gas diffusion layer and the flow field configuration, were investigated in operating fuel cells. The results show that the limiting current and maximum power can be more than doubled through efficient liquid water removal: by changing the GDL or by altering the flow field configuration.;The level of cathode flow field flooding was visualized in situ and recognized as a criterion for evaluating the water removal capacity of the GDL materials. When compared at same current (i.e. water production rate), higher amount of liquid water in the cathode channel indicated that the water has been efficiently removed from the catalyst layer.;Visualization of the anode channel was used to investigate the influence of the microporous layer (MPL) on water transport. No liquid water was observed in the anode flow field unless cathode GDLs had an MPL. MPL on the cathode side creates a pressure barrier for water produced at the catalyst layer. Water is pushed across the membrane to the anode side, resulting in the anode flow field flooding. Throughout this work, water dynamics on both cathode and anode sides were investigated.;A novel experimental approach for water metrology in PEM fuel cells is presented, where neutron and optical images were obtained simultaneously in situ. Liquid water dynamics in the flow field (channels and manifolds) were recorded on a digital camera through the optical window, while the integrated water content across the cell thickness was measured by neutron imaging. The concurrent images provide complementary data that allow one to separate the water distribution in the flow field from the remainder of the cell thickness, which has been a great challenge in water management studies. It was shown that such an approach enables improved interpretation of the water metrology data obtained independently from neutron and optical imaging. Utility of the combined technique was demonstrated through case studies typical of fuel cell operation.;Water content and dynamics were characterized and compared in an operating cell by simultaneous neutron and optical imaging for three basic flow field configurations in bipolar plates of PEM fuel cells: parallel, serpentine, and interdigitated. The transient water content within the cell measured using neutron imaging is correlated with optical data as well as with temporal variations in the cell output and pressure differentials across the flow fields. The flow field layout had a major impact on cell performance and its dynamic response as it resulted in substantially different water content and distribution. Among the three configurations explored, serpentine flow field showed stable output across the current range and resulted in the highest limiting current, which came at the expense of the largest pressure differential across the flow field. Flooding in the anode channels occurred in all flow fields, however it was in the parallel and interdigitated flow field that it caused substantial reduction in cell performance at high currents.;The collective contribution of this work should prove useful in (i) improving the understanding of water transport in PEM fuel cells, (ii) advancing experimental techniques for water management studies, and (iii) validating and developing fuel cell models with multiphase flow.