Lateral-aligned sulfonated carbon-nanotubes/Nafion composite membranes with high proton conductivity and improved mechanical properties

Proton exchange membranes, such as the widely used Nafion membranes, are essential components in industrial proton-exchange membrane fuel cells. In this study, multilayer sulfonated carbon-nanotubes/Nafion composite membranes with high performances have been prepared using layer-by-layer assembly. The membranes consist of a certain number of equally thick sulfonated carbon-nanotubes/Nafion layers. The thickness of the layers varies from 40 μm to 0.5 μm. This study indicates that the multilayered structure in composite membranes, especially in membranes with ultrathin layers ( 1 μm), encourages the alignment of the sulfonated carbon-nanotubes and the proton-conducting pathways in the lateral direction (parallel to the membrane interface). The multilayered structure also improves the dispersion of sulfonated carbon-nanotubes, which leads to a uniformly-distributed proton-conducting network. Therefore, the multilayer composite membranes are excellent proton conductors in the lateral direction at different temperatures. In addition, due to the formation of a hydrophilic interface between the layers, the composite membranes show a higher water-retention capacity. Furthermore, the lateral-aligned sulfonated carbon-nanotubes lead to significantly improved mechanical properties of the multilayer composite membranes, which results in a better electrochemical stability and a long-term performance stability of membranes.