Effect of SiO2 on the Properties of Sulfonated Polyimide and Poly(Arylene Ether) Block Copolymer Membranes

Perfluorosulfonic acid (PFSA) ionomers have been extensively studied as electrolyte membranes of polymer electrolyte fuel cells (PEMFCs). However, they suffer from problems relating to stability at high temperature (over 100C) and production cost. We reported that sulfonated polyimide (SPI-8) and sulfonated poly (arylene ether sulfone ketone) block copolymer (SPE-bl1) were highly proton conductive at high humidity and durable in operating fuel cells. However, it remains as a subject that they have lower proton conductivity compared to PFSA membranes at low relative humidity. On the other hand, we achieved a high cell performance under low humidity conditions by using PFSA membrane (Nafion) with highly dispersed oxide nanoparticles such as SiO2 or TiO2. In this study, we report composite membranes of SPI-8 and SPE-bl-1 with SiO2 nanoparticles and compare the effects of the amounts and distribution of SiO2 on the membrane properties. SiO2 particles (AEROSIL380) were added to SPI-8 or SPE-bl-1 polymer solution, followed by ultrasonic dispersion. All PEMs were prepared by casting from the polymer solution. Besides, SiO2/polymer bilayer membranes were prepared by stacking two electrolyte membranes with different SiO2 contents (the layer with higher SiO2 content was arranged on the anode side of the cell). The SiO2/polymer composite membranes were analyzed by SEM equipped with EDX and STEM. A catalyst paste was prepared by mixing Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo K.K.) and 5 wt% Nafion solution (Aldrich Co.) in a ball-mill. The resulting catalyst paste was directly sprayed onto the membrane by pulseswirl-spray technique to prepare the catalyst-coated membrane. The geometric electrode area was 3.8 cm. The anode and cathode contained 0.5±0.1 mg-Pt/cm of Pt electrocatalysts. Cells were operated at 80°C and the flow rates of reactant gases (anode: H2, cathode: O2 or air) were constant at 200 ml/min. Fig. 1 shows the performance of cells using the SiO2/ SPI-8 composite membranes. It was found that each cell using the SiO2/SPI-8 composite membrane exhibited improved performance under low humidity (53% RH), compared to that using a normal SPI-8 membrane. This is due to a decrease in the ohmic resistance of the cell by modification with the SiO2. This result suggests that SiO2 particles are effective in increasing water content in the SPI-8 membrane under the present PEFC operating conditions. Moreover, it is indicated that that utilization of SiO2/SPI-8 bilayer membrane is more effective in improving the cell performance. In contrast, as can be seen from Fig. 2, modification of SPE-bl-1 with SiO2 did not lead to enhanced performance compared to that using a normal SPE-bl-1 under low humidity (53% RH). The ohmic resistance of each cell using SPE-bl-1 membrane with SiO2 was slightly higher than that for the normal SPE-bl-1 membrane. For SiO2/SPI-8 composite membrane (10%) and SiO2/SPI-8 bilayer membrane (3-10%), the O2 gain was found to be clearly reduced, compared to the normal SPI8 membrane. This result indicates that the diffusivity of reactant gas in the cathode was improved by promoting of the back diffusion of generated water from the cathode to the anode. In contrast, addition of SiO2 to SPE-bl-1 increased O2 gain, indicating inhibition of diffusion of reactant gas in the cathode. It can be considered that the difference in tendency between the two electrolyte membranes is due to their morphology. Their STEM images showed that SPI-8 had highly-distributed sulfonic acid groups, while SPE-bl-1 showed large hydrophilic clusters of ca. 5-10 nm diameter as reported previously. However, it was found that their connection was partially broken by the addition of SiO2. This may cause suppression of the back diffusion of generated water from cathode. This work was supported by funds for the “Research on Nanotechnology for High Performance Fuel Cells (HiPer-FC)” project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.