Silicon oxide Nafion composite membranes for proton-exchange membrane fuel cell operation at 80-140° C

Silicon oxide/Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells (PEMFCs) from 80 to 140degreesC. The composite membranes were prepared either by an impregnation of Nafion 115 via sol-gel processing of tetraethoxysilane or by preparing a recast film, using solubilized Nafion 115 and a silicon oxide polymer/gel. Tetraethoxysilane, when reacted with water in an acidic medium, undergoes polymerization to form a mixture of SiO(2) and siloxane polymer with product hydroxide and ethoxide groups. This material is referred to as SiO(s)/-OH/-OEt. When Nafion is used as the acidic medium, the SiO(2)/siloxane polymer forms within the membrane. All composite membranes had a silicon oxide content of less than or equal to 10 wt %. The silicon oxide improved the water retention of the composite membranes, increasing proton conductivity at elevated temperatures. Attenuated total reflectance-Fourier transform infrared spectroscopy and scanning electron microscopy experiments indicated an evenly distributed siloxane polymer of SiO(2)/-OH/-OEt in the composite membranes. At a potential of 0.4 V, silicon oxide/Nafion 115 composite membranes delivered four times the current density obtained with unmodified Nafion 115 in a H(2)/O(2) PEMFC at 130degreesC and a pressure of 3 atm. Furthermore, silicon oxide-modified membranes were more robust than the control membranes (unmodified Nafion 115 and recast Nafion), which degraded after high operation temperature and thermal cycling. (C) 2002 The Electrochemical Society.