Sulfonated poly(arylene ether sulfone)/ORMOSIL (organically modified silicate) composite membranes for direct methanol fuel cell

Abstract

In order to retain the proton conductivity and reduce the methanol crossover in the direct methanol fuel cell (DMFC), a new approach to improve the DMFC membrane transport properties of sulfonated poly(arylene ether sulfone) copolymers (sPAES-60) has been tried, which is based on controlling the water-channel structure and the state of water by in-situ hybridization of ORMOSIL (organically modified silicates). Diethoxydimethylsilane (DEDMS) was employed as a functional ORMOSIL precursor. Its hydrophobic nature made it possible to adjust the hydrophilicity of the resulting hybrid membranes. The morphology of the hybrid membranes was investigated by FE-SEM and TM-AFM. The transport properties such as proton conductivity and methanol permeability were measured by the AC impedance spectroscopy and the liquid permeability measuring instrument, respectively. An increase of DEDMS uptake into sPAES-60 membranes allowed the water uptake to decrease and the water channels to become narrow and tortuous. The influence of DEDMS uptake on the state of water was investigated by measuring the characteristic melting peak of water at $0\degC$. Based on this investigation, the dependence of water uptake on the state of water was quantitatively identified. A noteworthy observation is that the incorporation of DEDMS into sPAES-60 was effective in suppressing the methanol crossover with slightly sacrificing the proton conductivity, resulting in the higher selectivity. This improvement in DFMC membrane transport properties was discussed in terms of the water-channel structure and the state of water.