Membranes with Phosphonic Acid Moieties for High Temperature PEMFC

Abstract

Global warming expedites the development of zero emission vehicles and a lot of attention has been paid on polymer electrolyte membrane fuel cell (PEMFC). Recently special interest has been focused on PEMFC operating at high temperatures above 90 o C. The operation of PEMFCs at high temperatures and low relative humidity provides several advantages: 1) high system efficiency, 2) increased rates of reaction and diffusion, 3) better water management, and 4) low CO catalyst poisoning [1][2]. However, most commercialized membranes are based on sulfonic acid moieties and they rapidly lose conductivities at these conditions. This has been the main obstacle to the development of high temperature PEMFC [3][4].Phosphoric acid doped-polybenzimidazole (PBI) system has been well known because of high proton conductivity at high temperatures. Also they have almost zero electro-osmotic drag number compared with Nafion ® . However, phosphoric acid in PBI is washed out by water which was produced at the cathode side. It causes loss of conductivity with time in addition to such problems as corrosion of the systems and the refill of phosphoric acid [5][6].To solve these problems and to maintain the conductivities of membranes irrespective of operating conditions it is very important to keep the proton conductors inside proton channels of polymer electrolyte membranes. Therefore, we tried to chemically attach phosphoric acid at the side chains of polymers. This approach will ensure the high conductivities and long-term stability at elevated temperatures.We grafted phosphonic acid groups to Nafion ® by replacing the sulfonic acid groups with 3-aminopropyl triethoxy silane (APTES). The attached amino groups can be converted to phosphonic acid by reacting with phosphoric acid. APTES is a representative coupling agent and can also goes through sol-gel reactions under acidic conditions which would enhance the mechanical properties of the membranes at high temperatures. Nafion® was re-dissolved in dimethyl acetamide (DMAc) and mechanically mixed with desired amount of APTES. And the mixed solutions were cast on a teflon dish. To introduce the reaction between sulfonic acid and APTES, the cast membranes were boiled in aqueous formaldehyde solution, and then boiled in phosphoric acid solution. Figure 1 compares the proton conductivities of the grafted membranes from room temperature up to 110 o C. When the samples were fully humidified at temperatures less than 90 o C, all the grafted membranes showed surprisingly higher conductivities than Nafion ® . When the samples were not fully humidified above 90 o C, it was almost impossible to measure the conductivity of Nafion ® . However, the grafted samples showed very high conductivities, which were even higher than that of Nafion ® at RT. This result indicates that phosphonic acid groups could work as a proton conductors instead of sulfonic acid groups at elevated temperature.We also confirmed the chemical structures of the grafted membranes by FT-IR, 31 P-NMR, and 29Si-NMR. And we will also ...