Cross-Aligned PTFE Nanofiber Reinforced Composite Membrane for PEM Fuel Cells

Polymer electrolyte membrane fuel cells (PEMFCs) have widely been considered environmental friendly alternative energy technologies due to their high energy conversion efficiency and zero emission. In the recent, the importance of performance and durability of electrolyte membrane, interfacial layer separating two electrodes, prohibiting fuel cross-over, while transporting only protons, has become much crucial for utilizing PEMFCs to the automobile applications such as fuel cell electric vehicles (FCEVs) and drones. Unfortunately, the current state membranes, mainly composed of perfluorosulfonic acid (PFSA) ionomers, have suffered from the high cost and low proton conductivity, and low dimensional stability during dynamic humidity changes during fuel cell operation. In order to deal with these issues, recently, a reinforced composite membranes (RCMs) have been developed in which a PFSA ionomer is impregnated into a porous support matrix consisted with nanofibrous polymer materials such as polytetrafluoroethylene (PTFE) due to their thermal/mechanical robustness. The PTFE based RCMs could provide enhanced mechanical stability and electrochemical performance with a reduced use of PFSA ionomer, but low impregnation of ionomers into nanosized hydrophobic PTFE networks, resulting in phase separation and irreversible creep deformation. Herein, we report a novel preparation of vertically alined PTFE aggregated fibers (PTFE AFs) as a reinforcement by using facile electrospinning hetero-polymer solutions containing PTFE nanoparticles and PEO (Polyethylene oxide) carrier polymer and subsequent thermal removal of PEO. Developed PTFE AF were effectively soaked by PFSA ionomers, and PTFE AF based RCMs delivered outstanding dimensional stability and proton conductivity compared to commercial RCMs. Furthermore, PTFE AF based RCMs used MEA exhibited exceptional single-cell performance and long-term stability. The enhanced performance was attributed to the facile proton conduction through the one dimensional channels between aligned fibers without fiber scattering, and effective suppression of volume expansion of hydrated PFSA ionomer by rectangular shaped PTFE fiber network. We believe the widespread use of novel PTFE fiber based membranes with an unique pattern in a variety of membrane applications where porous supports are used. Figure 1