Direct sulfonation and photocrosslinking of unsaturated poly(styrene-b-butadiene-b-styrene) for proton exchange membrane of direct methanol fuel cell

In this report, we demonstrate a simple preparation of the partially sulfonated poly(styrene-b-butadiene-b-styrene) (sSBS), casting and photocrosslinking of tough films which can be used as a proton exchange membrane (PEM) for direct methanol fuel cell (DMFC). Upon reaction with acetylsulfate in a homogeneous solution phase, SBS was converted to sSBS with a degree of sulfonation (DS) in a controlled manner. Subsequent washing and redissolution of the product in the mixed solvent enabled the solution casting of the mechanically stable membranes, which could be further toughened by crosslinking the butadiene groups with the premixed divinylbenzene (DVB) upon exposure to UV light. The sSBS membranes showed excellent proton conductivity as well as low methanol permeability presumably due to the efficient formation of hydrophilic channels aided by microphase separation. The nanostructures of the sSBS PEMs were rigorously characterized by the small angle X-ray scattering (SAXS) and electron microscopic analyses. Finally the membrane electrode assembly (MEA) was fabricated using the sSBS as PEM, and an active mode DMFC test was performed to show a power density of a single cell containing noncrosslinked sSBS to be ∼80 mW/cm2 at 60 °C which is 15% higher than that of Nafion115-containing single cell under the same test conditions.

► We synthesize block copolymer ionomers of sulfonated SBS by using acetyl sulfate.
► By using a mixed casting solvent of 1-butanol and toluene, stable PEM of sSBS is obtained.
► SAXS measurements reveal nanostructures of sSBS PEMs.
► sSBS PEM shows 15% improved power performance than Nafion-115 in active DMFC mode.