Design of an effective methanol-blocking membrane with purple membrane for direct methanol fuel cells

In this study, purple membrane (PM) was applied as a methanol-blocking agent on Nafion^® membranes. A series of well-organized poly(diallyldimethylammonium chloride)/PM multilayer films were obtained on a Nafion^® 212 surface (PDDA/PM/Nafion^®) to form composite membranes by the electrostatic layer-by-layer (LbL) self-assembly method. The effect of the PDDA/PM/Nafion^® heterogenic interface on proton conductivity and methanol permeability was studied by alternating the deposited surface. With five PDDA/PM bilayers, double-sided modification (PDDA/PM)_DF-5 and single-side modification (PDDA/PM)_SF-5 resulted in excellent methanol blocking with a 73.4% and 64.7% reduction in methanol permeability in comparison with unmodified Nafion^® 212, respectively. Indeed, the involvement of PM suppressed the methanol crossover. With regard to the selectivity factor (PDDA/PM)_DF-1 composite membranes showed approximately 2-fold improvement as compared with unmodified Nafion^® membranes. The cell performance of (PDDA/PM)_SF-1 composite membranes achieved a power density of 27.0mWcm^-2 which is 48.4% increase compared to cells using unmodified Nafion^® 212 membranes. Moreover, with (PDDA/PM)_DF-1, we achieved a power density of 34.4mWcm^-2. This study highlights the potential application of PM as multifunctional protein membrane in direct methanol fuel cells.