High-Flux Microporous Layers with Bimodal Pore Structures for Proton Exchange Membrane Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are critical for heavy-duty transportation due to their high efficiency and environmental benefits. However, achieving optimal water management and gas transport in the microporous layer (MPL) remains a challenge to improving its performance. In this paper, a high-flux MPL is presented, which was prepared using pure carbon nanotubes (CNT) by adding a nonionic surfactant to improve the dispersion of hydrophobic CNT. This aimed to improve the performance of the membrane electrode assembly (MEA) for PEMFCs at high current densities and enhance its stability at high voltages. The morphology, pore size distribution, porosity, gas/water permeability, and hydrophilicity of the MPL were characterized by transmission electron microscopy, scanning electron microscopy, an automatic mercury porosimeter, a comprehensive membrane pore size analyzer, and a contact angle meter, respectively. Electrochemical performance analyses were conducted by polarization curves and electrochemical AC impedance spectroscopy. The results indicated that the MPL prepared with pure CNT exhibited a bimodal pore structure, which significantly improved the water/air permeability and water management capability of the MPL. Compared to commercial XC-72-MPL, CNT-90-MPL reduced the charge transfer resistance (R ct) and mass transport resistance (R mt) of the MEA. The highest power density achieved by CNT-90-MPL was 1.813 W/cm2. Based on capillary pressure and bifunctional transport mechanism, the transport mechanism of water and air in the bimodal pore structure of the MLP was analyzed. Water primarily flows through the large pores in the MPL, while air diffuses oppositely via the small pores. This bifunctional transport mechanism is the primary factor contributing to the superior water management performance of CNT-90-MPL.