Nanocomposite membranes of Nafion and Fe3O4-anchored and Nafion-functionalized multiwalled carbon nanotubes exhibiting high proton conductivity and low methanol permeability for direct methanol fuel cells

Abstract

Fe3O4 magnetic nanoparticle-anchored and Nafion-functionalized multiwalled carbon nanotubes (MWCNT–MNP–Nafion) have been prepared and used as a nano-additive for Nafion. The addition of 0.1 wt% of MWCNT–MNP–Nafion to Nafion results in the membranes showing a reduction of methanol permeability from 30.1 × 10−7 cm2 s−1 to 6.8 × 10−7 cm2 s−1 and an increase of proton conductivity from 31 mS cm−1 to 78 mS cm−1. The presence of MWCNT–MNP–Nafion reduces the ionic cluster sizes and decreases the free volume element concentration of the modified Nafion membrane. Both have been verified with small angle X-ray scattering spectroscopy and positron annihilation lifetime spectroscopy and contribute to the reduction of the methanol permeability of the membrane. The increase in the proton conductivity originates from the formation of proton-conducting pathways on the MWCNT bundles. For the membrane fabricated under a magnetic field, its proton conductivity further increases to 85 mS cm−1 due to the magnetically-driven alignment of MWCNT–MNP–Nafion in the Nafion matrix (A-Nafion-NM-0.1). Compared to the plain recast Nafion membrane, the A-Nafion-NM-0.1 membrane has a 15-fold membrane selectivity, an 8.7-fold maximum power density, and a 6.6-fold current density at an operating voltage of 0.4 V in the tests for direct methanol fuel cells (DMFC). The high performance of the membrane in the DMFC tests demonstrates that the MWCNT–MNP–Nafion is a highly efficient additive for the preparation of Nafion-based membranes for DMFCs.