Nafion based organic/inorganic composite membrane for air-breathing direct methanol fuel cells

Membrane electrode assemblies (MEA) with organic/inorganic composite membranes were fabricated and tested in air-breathing direct methanol fuel cells (DMFCs). Various composite membranes based on Nafion® ionomer with functionalized montmorillonite (MMT) were employed to reduce methanol cross-over without sacrificing proton conductivity by introducing sulfonic acid group at the MMT surface followed by blending with Nafion ionomer. Through a systematic investigation, the composite membrane containing 5 wt.% of MMT modified with 1,3-propane sultone was found to exhibit the best membrane selectivity among other membranes. Membrane–electrode interface of the composite membrane was improved by additional ionomer layer incorporated between membrane and catalyst layers. As a result, an improved DMFC performance was achieved compared to that with Nafion 112 or 115. Employing a self-humidifying diffusion layer, semi-passive DMFC tests were performed for various membranes, and it was confirmed that the composite membranes exhibit higher power density than the Nafion membranes. In particular, MEA with a thin composite membrane showed the highest power density and fuel efficiency among the membranes tested in semi-passive DMFCs.