Structurally modulated and functionalized carbon nanotubes as potential filler for Nafion matrix toward improved power output and durability in proton exchange membrane fuel cells operating at reduced relative humidity

Development of a high-performance and durable proton exchange membrane (PEM) for low relative humidity (RH) operation is imperative for further commercialization of hydrogen-air fuel cell (HAFC). In this work, we reported the synthesize of carbon nanotubes-unzipped carbon nanotubes (CNTs-UCNTs) hybrid via calcination process with ammonium fluoride. Further, the 4-benzenediazonium sulfonate was used to sulfonate both CNTs and UCNTs to form sulfonated CNTs-sulfonated UCNTs (SCNTs-SUCNTs) hybrid. The unzipping enlarged the surface area of CNTs-UCNTs hybrid and facilitated the accommodation of a large density of sulfonic acid (-SO₃H) groups during sulfonation. Thus, SCNTs-SUCNTs hybrid incorporation with the host Nafion matrix results a uniform composite membrane with high density of –SO₃H groups per unit volume. It positively impacts thermal, mechanical and oxidative stabilities, and physicochemical performances of composite membrane. Besides, Nafion/SCNTs-SUCNTs composite membrane afforded credible proton conductivity, current output, power output, and durability, which are 0.015 S cm⁻¹, 1.000 A cm⁻², 0.343 W cm⁻², and 0.15 mV h⁻¹, respectively when operating the HAFC at 60 °C under 20% RH. In contrast, bare Nafion, Nafion/CNTs, and Nafion-212 membranes exhibited inferior proton conductivity, current output, power output, and durability under identical HAFC operating conditions.