Proton conducting membranes based on PEG/SiO2 nanocomposites for direct methanol fuel cells

A series of organic/inorganic composite materials based on polyethylene glycol (PEG)/SiO2 to be used as electrolytic membrane in direct methanol fuel cell (DMFC) have been synthesized through sol-gel processes. Acidic moieties of 4-dodecylbenzene sulfonic acid (DBSA) were doped into the network structure at different levels to provide the hybrid membrane with proton conducting behavior. Continuous, flat, flexible membranes were obtained with all of the DBSA doping levels. An increasing trend of proton conductivity with DBSA doping was observed and it indicates that the mobile protonic-carriers in the hybrid membrane originate from DBSA. At higher concentrations of DBSA, a part of the DBSA molecules could not be accommodated into network structure as it was formed and was segregated from the cast films. However, the fact that there is no significant difference in both ion-exchange capacity (IEC) values and ionic conductivities measured before and after it was soaked in water indicates that the doped DBSA is well within the network structure of hybrid membranes. For water equilibrated membranes, the conductivity varies from 2.79 x 10(-3) to 2.39 x 10(-2) S/cm. Due to the presence of SiO2 framework in the nanocomposite, thermal stability of the hybrid membrane enhanced enormously. The relationship between the concerned properties and the loading levels of DBSA and monophenyl trimethoxysilane (MPh) are discussed. Some of the hybrid membranes exhibited low methanol permeability without sacrificing their conductivities significantly and were thus felt to be potentially useful in DMFC. (C) 2003 Elsevier Science B.V. All rights reserved.