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.