Proton Conductivity Studies of Zirconium Silicate/Ionic Liquids Membranes for PEM Fuel Cells Applications

Abstract

This work reports the synthesis of a novel, high-temperature composite membrane for proton exchange membrane (PEM) fuel cells. High-temperature operation (> 100oC) is preferred in PEM fuel cells as it results in better water management and an enhancement to the overall kinetics of fuel cells’ electrodes. The current state of the art Nafion membrane is not suitable for high-temperature operation as their proton conductivities decline drastically. Hence, a novel Nafion-free membrane, based on zirconium silicates (ZrSi) and ionic liquids (ILs) is described in this thesis. Proton-conducting ZrSi/IL composite membranes were synthesized via precipitation of ZrSi/IL within the pores of polytetrafluoroethylene (PTFE) at room temperature. Seven different ILs were investigated to produce ZrSi/IL modified materials with ILs content ranging from 0.56% to 32% by mass. Significant enhancements in proton conductivities were observed upon the addition of the IL to ZrSi. Electrochemical impedance spectroscopy measurements showed proton conductivity values in the range of 0.1-0.2 S/cm, exceeding that of the Nafion membrane. The modified ZrSi/IL samples were further investigated by thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), X-ray diffraction (XRD), and energy-dispersive X-ray spectroscopy (EDX). TGA analysis of the modified ZrSi/IL materials demonstrated an approximately 20% weight loss at around 500°C . Water uptake analysis indicated that the new composite membrane could hold more than 50% by weight of water. SEM analysis showed the changes in particles' morphology of the modified ZrSi/IL. The interactions of ILs with ZrSi were studied using FTIR & XRD. Changes in the ZrSi/IL crystalline structures in comparison with pure ZrSi were observed. The samples processed at a high-temperature (200°C ) and exhibited a high proton conductivity in the order of 0.001 S/cm. Thus, the ZrSi/IL membranes reported in this work are very promising as proton conductors for high-temperature fuel cell applications.