Proton conductivity and methanol transport behavior of cross-linked PVA/PAA/silica hybrid membranes

Cross-linked poly(vinyl alcohol) (PVA)/poly(acrylic acid) (PAA)/silica hybrid membranes were prepared to evaluate the possibility of use as a proton exchange membrane for direct methanol fuel cell (DMFC). A chemical cross-linking agent having sulfonic acid group (-SO3H) was used to increase proton conductivity, and simultaneously to prevent methanol transport through the cross-linked membranes. In addition, silica particles were dispersed into polymer matrices via sol-gel reaction under acidic conditions, expecting the barrier to the methanol transport. The proton and the methanol transport were investigated in terms of PVA/PAA compositions and cross-linker (sulfosuccinic acid, SSA) concentration. It was found that the compositions of PVA/PAA and the cross-linker concentration affected the transport properties of the membranes. Particularly, the concentration of cross-linker markedly affected the proton and the methanol transport because SSA was used not only as a chemical cross-linker but also as a donor of fixed anionic group (-SO3−H+). The proton conductivities of the hybrid membrane were in the range of 10−3-10−2 S/cm, and the methanol permeabilities ranged between 10−8 and 10−7 cm2/s. Noticeably, the methanol permeabilities were reduced by cross-linking between PVA, PAA and SSA chains without a large sacrifice of proton conductivity. Moreover, the silica particles embedded in the cross-linked polymer membranes acted as a reducing material for fraction of free water as well as a methanol barrier to hinder pathway from penetrating methanol molecules.