Preparation and Evaluation of Nafion/CeO2 Composite Electrolyte Membranes for Polymer Electrolyte Fuel Cells

Abstract

Polymer electrolyte fuel cells (PEFCs) are increasingly applied to the vehicles (FCVs) toward the suppression of carbon dioxide emission. The larger FCVs requires the high power density to exceed the presumed temperature limit of 80 ℃, causing some issues of material durability and system performances. In particular, a commercial electrolyte membrane of Nafion occurs the decrease of proton conductivity at the high temperatures under an atmospheric pressure. To overcome this problem, our research group has developed a composite electrolyte membrane consisting of Nafion and Ta-TiO2 nanoparticles with a chain-like network structure. The composite electrolyte membrane improves a proton conductivity in a wide temperature range over 100 ℃ by the effect of hydrophilic surface of the oxide. In this study, the other candidate materials of CeO2, nanoparticles with highly hydrophilic surface rather than that of Ta-TiO2 was selected to try to improve the proton conductivity of the composite electrolyte membranes.The hydrophilic CeO2 nanoparticles with a chain-like network structure were prepared using flame oxide-synthesis method. The hydrophilic CeO2 nanoparticles were dispersed in a dispersion solvent in desired ratio. The dispersion was then added to 20 wt.% Nafion dispersion solution (DE2020 CS, Chemours Co.) neutralized with potassium hydroxide. The resulting dispersion was coated onto a die coater and dried using a hot air. The CeO2/Nafion composite membrane was then treated in high purified water with a sulfuric acid solution. The transmission electron microscopy (TEM) and water adsorption, and four-probe alternating current method was conducted to detect the microstructure, water uptake and proton conductivity of the composite membrane.TEM images confirmed that the CeO2 nanoparticles were highly dispersed in the composite membrane. The proton conductivity of the 3 wt.% CeO2 containing composite membrane was 2.1 times higher than that of commercial Nafion membrane (NRE211, Chemours Co.) at 80 ℃, 20% RH, and was 1.5 times higher than that at 80 ℃, 80% RH (Fig. 1). The proton conductivity of the 3 wt.% CeO2 containing composite membrane was found to be higher than that of the Nafion monolayer in each water uptake (Fig. 2), indicating that the morphology of the membrane might be changed by the adding of CeO2 oxide. Further research will be needed to examine the proton conductivity mechanism of this composite membrane, and fuel cell evaluations using this composite membrane. Figure 1