Effect of Silicone-Containing Charge-Transfer Polymer Blend Membrane for PEFC Application

Abstract

Introduction Silicone polymers generally have characteristics such as thermal/chemical stability compared to hydrocarbon polymer [1,2]. As such, we have focused on silicone polymers as new alternative polymer electrolyte membranes (PEMs) in this study. However, silicone polymers have some drawback such as low glass transition temperature (Tg) and relatively high gas permeability. We attempt to develop PEMs consisting of a blend of silicone polymer and a hydrocarbon polymer, to enhance the merits of silicone polymers, while suppressing the demerits (Fig. 1). Our group has previously developed the polymer blend method applied to supramolecular interactions to prepare PEMs [3-5], and charge-transfer (CT) interaction has been used as supramolecular interaction. Polymer blends using the CT interaction are composed of two different polymers. One polymer is an electron-donating unit, and the other polymer is an electron-accepting unit [3-5]. In CT blend membranes, the donor polymer and the acceptor polymer can align alternatively due to the CT interaction, and miscible polymer blend at the molecular level can be prepared. In addition, the CT polymer blend method can be used to control several properties such as mechanical strength and the Tg [3-5]. In this study, we developed silicone-containing polymer, Si-Epoxy, and evaluated their mechanical, thermal stability, and performance.Experimental Silicon-containing polymer, Si-Epoxy, was synthesized by epoxy ring open reaction from 2,6-dihydroxynaphthalene and 1,3-Bis (3-glycidyloxypropyl) tetramethyldisiloxane using phosphine catalyst. Si-Epoxy was used as the electron-donating polymer while sulfonated polyimide homopolymer (SPI) was used as the electron-accepting polymer. CT blend membranes (SPI/Si-Epoxy) were prepared by solvent casting method. Tg of Si-Epoxy, SPI and SPI/Si-Epoxy were measured by differential scanning calorimetry (DSC). To confirm CT complex formation, visible spectroscopy was used. Thermal stability and mechanical strength of the SPI/Si-Epoxy membranes were evaluated by TGA, tensile test. IEC, proton conductivity and fuel cell performance were also evaluated.Results and discussion From the DSC results, Tg of Si-Epoxy was observed at 12.5°C. The reason for the relatively high Tg is that Si-Epoxy have naphthalene moiety into main chain. The Tg of the SPI was not observed within the measured temperature range. But Tg of the CT blend membrane (SPI/Si-Epoxy-0.5) also was not observed. This phenomenon is a result of blending which eliminates the Tg of Si-Epoxy. The TGA result of Si-Epoxy has no weight loss until 350 ˚C. This result indicates that Si-Epoxy is very thermally stable and will not thermally decompose under PEFC operation conditions. For the SPI/Si-Epoxy, The TGA tendency was similar with SPI, not Si-Epoxy. It means that thermal property of SPI/Si-Epoxy membrane depends on property of SPI. Mechanical properties of SPI/Si-Epoxy membranes with different ratio, Nafion 211 and SPI were measured. The stress at break of SPI/Si-Epoxy was similar with Nafion 211. Elongation of SPI/Si-Epoxy was less than 20 %, and there was almost no deformation compared to Nafion 211. This is a great advantage for mechanical durability, such as buckling deformation. The proton conductivity of SPI/Si-Epoxy was measured at 80 ˚C, 90 % RH (Fig. 2). The proton conductivity of SPI/Si-Epoxy 0.5 (SPI : Si-Epoxy = 1 : 1 (mol eq.)), SPI, Nafion was 23.1, 49.1, 56.8 mS/cm, respectively. The proton conductivity of SPI/Si-Epoxy depends on the amount of sulfonic acid. In fuel cell test, OCV was decreased with increase of Si-Epoxy in SPI/Si-Epoxy membranes. For CT membranes with low Si-Epoxy ratio (< 30 %), OCV and mechanical strength will have a positive effect on improvement. In addition, SPI/Si-Epoxy can be used as an ionomer by using the advantage of high gas permeability.