Nafion membranes with vertically-aligned CNTs for mixed proton and electron conduction

The interest in clean energy is of increasing importance nowadays. In this regard, hydrogen production from sun-assisted water splitting is a field of intense research in the scientific community. In the design of artificial innovative devices devoted to this task, the simultaneous transport of protons and electrons from an anode (where they are generated by water oxidation) to a cathode (where they have to recombine in order to form molecular hydrogen) is essential. Nafion, a sulfonated fluoropolymer, is of great interest because of its proven ability as a proton-conducting membrane in fuel cells, and also for its good thermal and mechanical stability. Carbon nanotubes (CNTs), rolls of graphene sheets, possess a high thermal and mechanical stability together with excellent electron conductivity. Nafion/CNTs nanocomposites with CNT amounts above the percolation threshold have been recently obtained which are capable of simultaneously conducting protons and electrons. Here we report on the fabrication of nafion/CNTs membranes in an alternative configuration, namely vertically-aligned carbon nanotubes columns embedded in a nafion matrix. Nanocomposite films of nafion embedded with an array of CNT columns were prepared by solvent casting. Films of thickness around 100 μm were obtained. They were characterized by scanning electron microscopy, electrochemical impedance spectroscopy, electronic conductivity tests, water uptake and water contact angle. Electronic and protonic conductivities were measured in ambient and wet conditions and turned out to be higher than those previously reported for nafion/CNTs nanocomposites in the conditions where such devices are expected to operate.