Nafion and carbon nanotube nanocomposites for mixed proton and electron conduction

Abstract

Carbon nanotubes (CNTs) exhibit extraordinary mechanical, electronic and thermal properties, because of which they have been used in several applications like mechanical reinforcement of polymers, electrocatalysis, sensors, electronics, batteries, etc. Nafion is a sulfonated fluoropolymer that has become standard material in fuel cell applications for its excellent proton conducting property, and for its thermal and mechanical stability. Recently, Nafion/CNTs composites with CNT amounts less than the percolation threshold have been used to increase the mechanical stability and decrease methanol permeation of nafion, with very little effect on proton conductivity and avoiding short circuit risks. Above the percolation threshold, such membranes show the potentialities to allow a separate proton/electron conduction paths within the membrane. This aspect opens new ways for applications in selective membranes for artificial innovative devices capable of using sunlight to produce hydrogen from water splitting. Nafion + multi-walled CNTs (MWCNTs) nanocomposite films with varying amounts of filler were fabricated and their proton/electron transport behaviors were characterized. Samples were prepared by recasting Nafion with MWCNTs and conditions were optimized to obtain freestanding films of thickness around 100 μm. After ultrasonication, we observed a uniform dispersion of the carbon fillers inside the matrix, with a good affinity between the two materials without the need of CNT surface functionalization. Membranes were characterized by Scanning Electron Microscopy, Electrochemical Impedance Spectroscopy and Electronic Conductivity tests.