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 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 SiO₂ 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 SiO₂/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/-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/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.