Understanding ion and solvent transport in anion exchange membranes under humidified conditions

Abstract

Anion exchange membranes (AEM) have been studied for more than a decade for potential applications in low temperature fuel cells and other electrochemical devices. They offer the advantage of faster reaction kinetics under alkaline conditions and ability to perform without costly platinum catalyst. Inherently slow diffusion of hydroxide ions compared to protons is a primary reason for synthesizing and studying the ion transport properties in AEMs. The aim of this thesis is to understand ion transport in novel AEMs using Pulse Gradient stimulated Spin Echo Nuclear Magnetic Resonance technique (PGSE NMR), water uptake, ionic conductivity, Small Angle X-ray Scattering (SAXS) etc. All experiments were performed under humidified conditions (80-95% relative humidity) and fuel cell operating temperatures of 30-90°C. In this work, the NMR tube design was modified for humidifying the entire NMR tube evenly from our previous design. We have developed a new protocol for replacing caustic hydroxide with harmless fluoride or bicarbonate ions for 19F and 13C NMR diffusion experiments. After performing these NMR experiments, we have obtained in-depth understanding of the morphology linked ion transport in AEMs. We have obtained the highest fluoride self-diffusion coefficient of > 1 x 10-5 cm2/sec (@ 55°C) for ETFE-g-PVBTMA membrane which is a result of low tortuosity of 1 obtained for the membrane. This faster fluoride transport combined with low tortuosity of the membrane resulted in > 100mS/cm hydroxide conductivity for the membrane. Polycyclooctene (PCOE) based triblock copolymers are also studied for in-depth understanding of molecular weight, IEC, mechanical and transport properties. Effect of melting temperature of PCOE has favorable effect on increasing ion conductivity and lowering activation energy. Mechanical properties of these types of membranes were studied showing detrimental effect of water plasticization which results in unsuitable mechanical properties. Hydroxide conductivity was studied to measure the effectiveness of AEMs for practical applications. PPO-b-PVBTMA membrane showed more than 100mS/cm conductivity and PCOE based membranes showed ~ 70mS/cm conductivity which is a combined effect of Grotthuss hopping and vehicular mode of ion transport, which lowers the activation energy to < 14 kJ/mol. Overall this thesis sheds light on one of the most important aspect of AEMs: ion/solvent transport, we have studied effect of membrane chemistry, IEC, morphology, polymer molecular weight on self-diffusion, ionic conductivity to have a better understanding for development of a good AEM for practical applications.