Fracture Behavior of Solid Polymer Electrolyte Membranes with Different Ion Exchange Capacity for Fuel Cell

Abstract

INTRODUCTION: Fuel cells are known as one of power generation technologies with high efficiency, and are anticipated to be ecological and renewable energy. Polymer electrolyte membrane (PEM) fuel cells have excellent features including compact size and lightweight. Therefore, these are expected to be used in automotive and electric portable applications. PEMs are utilized as an ion conductive material between electrodes in the fuel cell. Perfluorosulfonic acid polymer is widely used as the electrolyte membrane because of its high proton conductivity. However, the major problem of PEMs is the mechanical degradation of the membrane. In this study, toughness properties were investigated to clarify the mechanical fracture behaviour of PEM. EXPERIMENTIAL: Flemion® CSH25 (Asahi Glass Co., Ltd) consisting of tetrafluoroethylene main chain and sulfonic acid group in the side chain was used. It exhibits good proton conductivity by dissociating protons from the side chain moiety. Three types of samples with different ion exchange capacity (IEC) were selected. The IEC is determined by the number of sulfonic acid groups which is one of the factors that greatly affect power generation efficiency of a fuel cell. These samples were prepared by casting method. Therefore, any residual strains were not observed in the optical strain gauge. Uniaxial tensile tests (Imoto Machinery Co., Ltd.) with dumbbell shape samples (JISK-6251-7) were performed at a stroke length of 15 mm, a temperature of 25.0 ± 1.0 °C, a test speed of 60 mm/ min, ambient conditions of 23 ± 1 °C, and 45 ± 5% RH. The essential work of fracture (EWF) was calculated from the load-displacement curve of the uniaxial tensile test result to evaluate fracture toughness. Double notches were applied to specimens at both ends of test pieces on parallel portions. A test speed of 5 mm/min and the ambient conditions were maintained the same as above tests. RESULTS AND DISCUSSION: Nominal stress-strain curves in uniaxial tensile tests were shown in Fig.1. With increasing the IEC, elastic modulus and yield stress of the samples were increased. Fig.2 show the results of EVF evaluation. The IEC 1.10 (the lowest IEC) showed the highest EWF (15.2 KJ / m2) compared with IEC 1.25 and IEC 1.43. In the uniaxial tensile test results without double notch, larger IEC showed higher mechanical properties, but it is opposite to the results of EWF. Figure 1