Phosphoric Acid-Functionalized Mesoporous Silica/Nafion Composite Membrane for High Temperature PEMFCs

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The proton exchange membrane fuel cell (PEMFC) has received a great attention as a clean power generator to convert the chemical energy of hydrogen to electric energy electrochemically. It has major advantages for the portable electronic devices and transportation/stationary power systems such as the high power density and high energy efficiency, rapid start-up, etc. One of the most widely used proton exchange membrane is perfluorosulfonic acid (Nafion, DuPont Co.). It has excellent thermal and chemical stability and high proton conductivity below 80 C. However, PEMFC with Nafion membrane shows the poor performance at higher temperature due to the dehydration and low proton conductivity of the membrane. Recently, many studies have been performed to develop proton conducting membrane for operation above 100 C because the high temperature operation of PEMFC has the advantages of low CO poisoning of Pt catalyst and simple thermal and water management. Even if the Nafion composite membranes with incorporation of water retentive and inherent proton conductive fillers have been studied extensively under elevated temperature and low humidity conditions, the proton conductivity of the composite membrane is occasionally lower than the pristine Nafion because the wellconnected ionic domain of the Nafion is deformed by the inorganic fillers. In this paper, we report the physico-chemical and electrochemical properties of the phosphoric acid-functionalized mesoporous silica/Nafion composite membrane. The mesopore of silica increases the water retention capacity more effectively, and the phosphor-silicate has thermally and chemically stable silica networks and the surface-terminated phosphate interacts strongly with the water molecules. Therefore, the phosphoric acid-functionalized mesoporous silica/Nafion composite membrane shows the higher proton conductivities and cell performance than the pristine Nafion at high temperature and low humidity. The morphology and particle size of the prepared inorganic materials were investigated by TEM, and the data are shown in Figure 1. The shape of mesoporous SiO2 is spherical with a diameter of ~45 nm and their pore size is ~5 nm as shown in Figure 1(a). The specific surface area and the mean pore size of the mesoporous SiO2 are 560 m ·g and 4.82 nm, respectively. However, the morphology of the phosphoric acid-functionalized mesoporous SiO2 was quite different from that of pristine one. As shown in Figure 1(b)-(d), the porous structure is destructed and particle size increases as the annealing temperature increases after the mixing of mesoporous SiO2 with phosphoric acid. Figure 1(b) shows the sample heated at 80 C in the vacuum, and the result shows that the porosity of the sample is slightly reduced. With an annealing at 200 C, the morphology change is significant in that the mesopore disappears and large pore is formed. Interestingly, the further annealing above 200 C, the pore completely disappears and their particle size also increases, because of the formation of phosphor silicate through the reaction of silica and phosphoric acid, and the bulk-like phosphor silicate phase is confirmed by XRD (not shown for brevity). In order to elucidate the chemical structure of the prepared