Poly(oxyalkylene)diamine-functionalized carbon nanotube/perfluorosulfonated polymer composites: Synthesis, water state, and conductivity

A new class of organic-inorganic composite membranes has been constructed, as proton exchange electrolytes, in which the structures of the composites have been designed at the molecular scale to possess fast proton conduction. Polysiloxane-functionalized multiwall carbon nanotubes (CNTs) were synthesized by covalently grafting hydrophilic layers composed of poly(oxyalkylene)diamines and tetraethyl orthosilicate-reinforced polysiloxane in a layer-by-layer manner onto the tube walls. The modified carbon nanotubes were blended with Nafion to form a proton-conducting membrane. Scanning electron microscopy images showed that the functionalized nanotubes had a better dispersion than the unfunctionalized nanotubes on the composite fracture surfaces. The incorporation of the inorganic polysiloxane layer onto the CNT sidewalls forms a resistance, which effectively prohibits electron conduction between the nanotubes. The incorporation of the amino-containing polymer onto the CNTs promotes Nafion coalescence on the CNTs through ionic interactions between amines and sulfonate groups, as evidenced by the shift of the -SO₃^- symmetric stretching peak in the FTIR, the N 1s region in X-ray photoelectron spectroscopy, and the increase of the decomposition temperature of sulfonate groups in thermogravimetric analysis. This approach provides continuous paths suitable for fast proton conduction (σ = 2.8 × 10^-2 S/cm at 30 °C) and also maintains the proton conduction at high temperatures (σ = 6.3 × 10^-2 S/cm at 130 °C).