Yakıt Hücresi Uygulamaları İçin Stiren-(Etilen-Bütilen)-Stiren (Sebs) Esaslı Kompozit Polielektrolit

Abstract

SEBS (styrene-(ethylene-buthylene)-styrene) is a triblock copolymer exhibiting both thermoplastic and elastomeric properties. In case of functionalization via sulfonic acid groups (SO3H) with a suitable method, proton conductive polyelectrolyte membrane (PEM) for use in fuel cell applications could be prepared by SEBS triblock copolymer. In the first stage of this study, cross-linked SEBS films were prepared by using different peroxides and accelerators. In order to prepare cross-linked SEBS, firstly uncross-linked SEBS films were prepared by solvent casting, then converted to cross-linked structure by hot-press method. Possibility of use in fuel cell applications of these membranes was investigated after sulfonation by immersing in a solution of chlorosulfonic acid in 1,2-dichloroethane with different time intervals. However, it was shown that although sulfonated cross-linked SEBS films eliminated high swelling disadvantage of sulfonated pristine SEBS films and had high ion exchange values, rough surface and excessive thickness reduced the chance of practical uses in fuel cell applications. iv For this reason, at the second stage of this study , composite films were prepared by blending SEBS and polypropylene (PP) which is a polymer belongs to polyolefine group. These composite films were prepared by extrusion and hot-press method. Wax was used as a process aid. Three different blends were prepared by changing the ratios of SEBS, PP and WAX. These blends prepared by using single screw extruder had mixing ratios respectively 35% SEBS, 24% PP, 40% WAX and 1% stabiliser as a first blend, 35% SEBS, 10% PP, 54% WAX and 1% stabiliser as 35S coded second blend and 67,5% SEBS, 5% PP, 27% WAX and 0,5% stabiliser as 68S coded third blend which was prepared by mixing equal amounts of SEBS and 35S coded second blend. First blend was not utilized in sulfonation because of its highly shrinking tendency in the film making phase. Other blends were extruded first to 300 μm thickness via twin screw extruder and then hot pressed to 60 μm and 125 μm thick films for 35S and 68S coded blends respectively. Again these composite films were sulfonated and proton conductivity was determined with the same method mentioned before. They were characterized by means of spectroscopic, mechanical and thermal analysis methods and then usability as a polyelectrolyte membrane was assessed by comparing their properties with Nafion 112 and Nafion 117 which are one of the most successfull membranes of the present-time. Proton conductivity values at 25ºC of 35S coded membranes prepared from SEBS/PP/WAX blend were found to be much lower than that of 68S coded ones, additionally these low conductivity values disappeared when fuel cell operating temperatures were increased to 75-80ºC. Such a reduction in proton conductivity was a result of membrane drying at these temperatures. As to 68S coded membranes, only two of them which were sulfonated at 45 and 60 minutes durations gave comparable proton conductivity values with Nafion 117 at 75-80ºC. Futhermore methanol permeability of the membrane which were sulfonated for 5 minutes was also measured and found to have a chance for using in methanol fuel cells.