Modifications of membrane electrode assemblies to understand and improve water management and performance in PEM fuel cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) have been recognized as a promising zero-emission candidate for automobile applications. Through extensive research and development activities over the last 20 years, PEM fuel cells are now able to start broad commercialization. There is still a large potential to further improve efficiency and durability, where water management is of crucial importance to achieve these advancements. While membrane and ionomer in the catalyst layer should be fully humidified to enable proton conductivity, excessive water accumulation in the porous layers increases the mass transport losses and hence, reducing the cell performance. The present work investigates the influences of microstructural properties of the catalyst layer (CL) and its interactions with the cathode microporous layer (MPLC) on water balance and performance of PEMFCs operating under industrially relevant conditions. By establishing a lab type manufacturing process along with utilizing in situ high-resolution neutron imaging techniques, we studied for the first time the effects of systematic modifications of hydrophobicity and porosity of CLs in combination with the modified cathode MPL materials on water management of PEMFCs. This study helped to deepen the understanding of liquid water transport mechanisms in the porous materials which provides the necessary information for designing and optimizing the structural properties of the engineered materials to improve water management and performance in PEMFCs. Hydrophobicity of the materials was modified using hydrophobic polytetrafluorethylene (PTFE). We found that the operating conditions have a decisive role to define the optimum content of PTFE loading in the layers. While at dry conditions CLs with 5 wt.% PTFE loading enhanced the cell performance by retaining the water in the interface of membrane and CL and thus improving the membrane hydration, CLs with 10 wt.% PTFE improved the performance by decreasing the mass transport losses at humid conditions and high air flow rates. We further investigated the effects and interactions of hydrophobicity gradients within CLs and cathode MPLs on the cell performance and liquid water transport. It was found that the combination of CL with 5 wt.% PTFE and MPLC with 20 wt.% PTFE resulted in the best performance. The analyses of water content showed at humid conditions, higher PTFE loadings (>20%) of MPLC increased the water accumulation in adjacent CL and decreased the transfer of water into the cathode channels, which consequently increased the mass transport resistance at high current i densities. In addition, neutron radiographic studies revealed that water back-diffusion was escalated by increasing the hydrophobicity of the layers. To modify the porosity of CL and MPLC, polymeric pore formers were utilized to create macropores in the structure of materials. The perforated layers enhanced the performance of the cell at both dry and humid conditions, especially at higher current density regions. Neutron imaging analysis revealed different liquid water distributions under land and channel regions. The local water saturation beneath the land regions with the presence of perforated CL and MPLC was increased which is explained by increased water filling of the larger pores due to their lower capillary pressure and also elongated water transport pathways under the land regions. In contrast, under the channels, perforated layers decreased the liquid water content indicating that macropores provided preferential transport pathways to remove the liquid water to the channels. Therefore, the performance improvement is attributed to an enhanced parallel two-phase flow mechanism under channels region where liquid water is transported through the bigger pores, and oxygen is transported through the smaller pores. A maximum performance increase of 30 % at a cell voltage of 0.42 V was achieved for our investigated cell design with the optimized perforated materials.