Development of a Novel Sulfated Poly (ether sulfone) with Aliphatic Chain for High Temperature Fuel Cell

Abstract

Polymer electrolyte membrane fuel cells (PEFCs) as one of fuel cell systems, offer a board range of benefits, including: (1) high efficiency, (2) clean process (no CO2 emission), (3) compact design1, and (4) quiet operation. As one of solutions to current technical problems, PEFC operation at high temperature (low humidity) has been considered as a promising system to reduce cost of PEFCs. High temperature PEFCs have some additional merits such as fast reaction kinetic, low activation energy for power generation and high CO catalyst poisoning tolerance2. These advantages make high temperature PEFCs very attractive. However, there are also some technical barriers to develop this attractive system. Especially, harsh environment such as high temperature and low humid condition are very severe for polymer electrolyte membranes (PEMs) as components of PEFCs. Sulfonated poly(ether sulfone)s (SPESs) have been commonly developed as hydrocarbon-type PEMs3,4for high temperature PEFCs, because of the high durability at high temperature condition. Several kinds of SPESs with different structures were investigated previously. However, most of them used only aromatic chain as backbone, which lead the membranes to be tough. In this research, we have investigated a series of copolymerized sulfated poly (ether sulfone) (SPES) with aliphatic chain for high temperature PEFCs. Sulfuric acid was attached to the hydroxyl group connected to the aliphatic main chain. The attached sulfuric acid would be able to move easily and work as a proton conductor because aliphatic unit is more flexible than aromatic unit. We evaluated the basic property of the aliphatic SPES as high temperature PEMs although chemical stability of the aliphatic unit is not higher than that of the aromatic unit. SPES shown in Figure 1 was prepared by sulfation of the hydroxyl group of aliphatic PES. Purification and protonation of the obtained SPES were carried out thoroughly by dialysis from SPES HCl aq. The structure of SPES was confirmed by 1H NMR. SPES films were prepared by cast method. SPES DMSO solution was cast on a petri dish and dried at 60 ˚C in vacuo overnight. The membrane showed hydrophobicity. This property indicated that it showed strong durability against water, which is necessary in fuel cell system. Thermal stability of PEMs is significantly required for high temperature PEFC operation. The obtained SPES was investigated by TGA/DSC experiment as shown in Figure 2. From the result of TGA, SPES had three stages of weight loss. The weight loss was assigned to the removing of water from 66 ˚C to 100 ˚C, thermal decomposition of sulfuric acid2from 176 ˚C to 343 ˚C and the thermal decomposition of the main chain from 343 ˚C to 600 ˚C. And DSC result indicated that no glass transition temperature was observed from SPES at operation temperature region for high temperature PEFC (<120 ˚C). In addition, the proton conductivity of SPES film was observed. The obtained proton conductivity was 0.3 mS/cm at 120 ˚C, 100%RH, which may be caused the high hydrophobic and long distance between sulfonic acid group. We introduce more sulfuric acid group in SPES, and the effect of the sulfuric acid amount and introduction position of sulfonic acid on the proton conductivity is discussed and an optimum condition for preparing MEA for high temperature PEFC is investigated. Reference T. Husaboe, J. A. Rittenhouse, M. D. Polanka, P. J. Litke, J. Hoke, AIAA Paper. DOI, 10, (2013).S. Bose, T. Kuila, T.X. Nguyen, N. H. Kim, K. T. Lau, J. H. Lee, Prog. Polym. Sci., 36, 813 (2011).Y. S. Ye, J. Rick, B. J. Hwang, Polymers, 4, 913 (2012).M. Ulbricht, Polymer, 47, 2217 (2006). Figure 1