Enhanced electrochemical performance and long-term durability of composite membranes through a binary interface with sulfonated unzipped graphite nanofibers for polymer electrolyte fuel cells operating under low relative humidity

The phenylsulfonic-acid functionalized, and unzipped graphite nanofiber (SO₃H-UGNF) was studied as a potential proton-conductive material to embed in a sulfonated poly(ether ether ketone) (SPEEK) to create proton exchange membrane (PEM) for polymer electrolyte fuel cell (PEFC) working under low relative humidity (RH). First, we used ammonium fluoride as an unzipping agent to adapt GNF to UGNF, and then functionalized UGNF with 4-benzene diazonium sulfonate to obtain SO₃H-UGNF. Subsequently, we prepared a nanohybrid membrane by integrating SO₃H-UGNF in SPEEK at 0.1, 0.5, 1, and 1.5 wt%. Compared to SPEEK alone, SPEEK/SO₃H-UGNF demonstrated higher water uptake, ion exchange capacity, water sorption, wettability, thermal stability, and tensile strength. The optimized SPEEK/SO₃H-UGNF (1 wt%) showed the improved physicochemical, thermal, and tensile properties. This nanohybrid membrane also showed excellent proton conductivity under 20% RH at 90 °C, which is 2.1 times better than SPEEK membrane. When applied as a PEM in a PEFC operating under 20% RH at 60 °C, the maximum power density and durability of SPEEK/SO₃H-UGNF (1 wt%) membrane was higher than SPEEK membrane. After durability operation, it also demonstrated better morphological integrity. This study affords valuable insights on SPEEK-based nanohybrid membrane fabrication and optimization for PEFC operating under low RH condition.