Silicon Oxide Nafion Composite Membranes for Proton-Exchange Membrane Fuel Cell Operation at 80-140°C

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Abstract
Silicon oxide/Nafion composite membranes were studied for operation in hydrogen/oxygen proton-exchange membrane fuel cells (PEMFCs) from 80 to 140°C. 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 SiO2SiO2 and siloxane polymer with product hydroxide and ethoxide groups. This material is referred to as SiOs/­OH/­OEt.SiOs/­OH/­OEt. When Nafion is used as the acidic medium, the SiO2SiO2 /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 SiO2/­OH/­OEtSiO2/­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 H2/O2H2/O2 PEMFC at 130°C 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. © 2002 The Electrochemical Society. All rights reserved.