Accelerating proton conduction in proton exchange membranes with sandwich structure based on carbon nanotubes oxide

Abstract

Carbon nanotubes (CNTs) with a high aspect ratio can theoretically realize the concept of channel-like proton transport clusters. In this research, carbon nanotubes oxide (OCNTs) with large amounts of oxygen-containing groups constituted successive proton conduction channels, deriving from the surface oxidation of CNTs. Proton exchange membranes (PEMs) with sandwich structure were constructed through a couple of polyvinyl chloride nanofbers (PNs) mats wrapping a thin OCNTs layer. In the prepared (PNs/OCNTs/PNs)es membrane, the outer polyvinyl chloride (PVC) nanofibers delayed the leakage of the inner OCNTs and enveloped phosphoric acid (PA) molecules. Furthermore, more PA molecules were combined by the oxygen-containing groups of hydroxyl, epoxy and carboxyl on the surface of OCNTs with intermolecular hydrogen bonds. For the (PNs/OCNTs/PNs)es/PA membrane, the good structure stability, high proton conductivity and reinforced mechanical property were derived from the compact structure, fast proton conduction and the formation of inorganic-organic composites. Specifically, the maximum proton conductivity was 4.46 × 10−2 S/cm at 160 °C, higher than 2.75 × 10−3 S/cm of the PVC/OCNTs/PA membrane. Notably, the tensile stress values reached 5.42 MPa of the (PNs/OCNTs/PNs)es membrane and 7.32 MPa of the (PNs/OCNTs/PNs)es/PA membrane. Even after a 350 h non-stop measurement at 120 °C, the (PNs/OCNTs/PNs)es/PA membrane could have the complete surface, albeit with tiny cracks on cross section. The proton conductivity could maintain the proton conductivity of 3.17 × 10−2 S/cm at 160 °C. The research revealed that OCNTs provided the channel-like ionic clusters for proton conduction and sandwich structure accelerated proton conduction in the OCNTs-based membranes.