Ion Transport in Hydrated Polymer Electrolytes - Role of Segmental Dynamics, Water Concentration, and Interface

Solid polymer electrolytes (SPEs) have gained prominence for their potential in fuel cell performance. However, the acidic environment in which PEMs operate limits the choice of catalysts to precious metals such as platinum. As an alternative, anion exchange membrane fuel cells (AEMFCs) operate in an alkaline environment, facilitating oxygen reduction and fuel oxidation reactions and permitting the use of low-cost, non-noble metal catalysts. Despite this, AEMs typically exhibit lower ionic conductivity than PEMs due to the intrinsically lower mobility of OH-.In this thesis, we studied ion transport in polymer electrolytes in thin film format, with which extrinsic factors can be avoided and insights into the molecular level ion transport mechanism can be obtained. In chapter 3, we used crosslinking to tune the segmental dynamics of the polymer chain to study the effect on ion transport property. The reduced chain dynamics has no influence on ion transport, instead, the water concentration in the polymer was found to be the key parameters that determined the ion transport efficiency. In chapter 4, we investigated how the interface in block copolymer electrolytes affect ion transport through the confinement of water. BCEs with different molecular weights were chosen to change the volume fraction of interface. The higher volume fraction of interface in lower molecular weight polymer can more effectively confine water in the interface, leading to higher conductivity due to higher water concentration. MD simulation further explains the reason for the confinement of water in the interface. More free volume presents at the interface compared to the bulk, which enables accommodation of water without experiencing excessive swelling. These studies can shed light on the design and optimization of polymer electrolytes to solve the inferior conductivity problem for the AEMFCs.