Enhanced proton conduction of chitosan membrane enabled by halloysite nanotubes bearing sulfonate polyelectrolyte brushes

Currently, enhancing the proton conductivity is one challenge for chitosan (CS, an industrial waste around the world) membrane to work as proton exchange membrane for direct methanol fuel cell. In this study, halloysite nanotubes bearing sulfonate polyelectrolyte brushes (SHNTs) are synthesized via distillation–precipitation polymerization and then incorporated into CS matrix to fabricate nanohybrid membranes. The membranes are characterized using field emission scanning electron microscope, fourier transform infrared, thermogravimetric analysis, differential scanning calorimetry, and mechanical tester. It is found that SHNTs generate strong electrostatic attractions to CS chains, which inhibit the chain mobility and thus enhance the thermal and mechanical stabilities of nanohybrid membranes. The results of water uptake, area swelling, proton conductivity, and activation energy reveal that the high aspect nanotube and long polyelectrolyte brush allow SHNTs to construct continuous and wide pathways, along which sulfonic acid–amide acid–base pairs are formed and work as low-barrier proton-hoping sites, imparting an enhanced proton transfer via Grotthuss mechanism. In such a way, the proton conductivity of CS membrane is obviously enhanced, and 15% SHNTs can afford a 60% enhancement in conductivity to the nanohybrid membrane, particularly. Moreover, the methanol permeability and selectivity of the as-prepared membranes are investigated in detail.