Nafion® nanocomposite membranes: Effect of fluorosurfactants on hydrophobic silica nanoparticle dispersion and direct methanol fuel cell performance

Nafion®–silica nanocomposite membranes are successfully prepared by adding hydrophobic silica nanoparticles to a Nafion® solution. To distribute these nanoparticles evenly in the Nafion® matrix, various fluorosurfactants of different ionic character are employed. Fluorosurfactants with acid groups such as phosphonic acid and sulfonic acid play an important role in simultaneously increasing the homogeneous dispersion of silica nanoparticles, enhancing proton conductivity, and reducing the methanol permeability of the nanocomposite membranes. Therefore, the dispersion properties of inorganic fillers such as silica can significantly affect nanocomposite performance in direct methanol fuel cell (DMFC) applications, whereas surfactants, if used properly, can improve the nanocomposite membrane properties. In particular, a commercial fluorosurfactant containing a sulfonic acid group (Zonyl® TBS) at the end of the surfactant chain exhibits better miscibility with the Nafion® ionomer. This feature results in a reduction in the dimensional change of the nanocomposite membrane due to relatively lower water swelling and significantly reduced methanol permeability through the membrane. A membrane–electrode assembly (MEA) prepared from a Nafion®–silica nanocomposite membrane with TBS shows the highest DMFC performance in terms of voltage vs. current density (V–I) and power density vs. current density (P–I). The current densities at 0.4 V and 90 °C are 342, 508, and 538 mA cm−2 with 1, 3 and 5 M methanol being fed at the anode side, respectively.