Designing polymer electrolytes for alkaline anion exchange membrane fuel cells [electronic resource]
Keywords: Chemical Engineering.

Increasing global demand and dependence on fossil fuels, coupled with environmental concerns arising from their use, have sparked interest in alternative energy sources. Hydrogen-powered fuel cells are a promising solution, offering a clean, scalable method for energy production. The most prominent low-temperature fuel cell devices today operate under an acidic environment, using a semi-permeable proton exchange membrane (PEM) to separate the two electrodes. However, their caustic operating conditions present unique stability and activity issues for the metal catalysts and ultimately necessitates the use of platinum-group materials, severely limiting commercial viability. A potential solution is to operate the fuel cell device under an alkaline environment using an anion exchange membrane (AEM), transporting hydroxide ions in lieu of protons. The basic environment opens the door for cheaper catalysts based on nickel and molybdenum, eliminating the cost barrier associated with PEM fuel cells. Unfortunately, typical AEMs exhibit poorer ionic conductivity and stability compared to traditional acidic membranes (e.g. Nafion), offsetting any potential cost advantage they may afford. This dissertation discusses design rationales towards enhancing the macroscopic properties of AEMs. Specifically, I present two experimental design motifs for improving the device viability of AEMs. In the first case, I present a semi-interpenetrating network design where a linear AEM ionomer is stabilized by a crosslinked poly(styrene-co-divinylbenzene) matrix. The crosslinked network acts as a reinforcing scaffold, dramatically increasing dimensional stability while maintaining excellent anion conductivity. Prototypical single-stack fuel cells with enhanced performance and stability have been fabricated from these materials, validating the design choices. In the second approach, I demonstrate the ability to increase hydroxide conductivity by tuning the nanostructure of the polymer electrolyte. Specifically, I show that tethering hydrophilic poly(ethylene glycol) grafts onto a benzyltrimethylammonium polysulfone benchmark AEM results in phase-separated, water-rich domains on the order of 5 to 10 nm. These domains serve as an ion transport pathway, facilitating the diffusion of hydroxide anions and consequently enhancing the efficiency of hydroxide conduction. Finally, in order to better understand the phase behavior and structure-property relationships of typical AEM materials, we have developed coarse-grained simulations and fundamental polymer theory to elucidate the thermodynamic behavior of random copolymers. We find that both the stochastic distribution of monomers along the polymer backbone as well as the overall stiffness of the polymer chain heavily influences its phase behavior (i.e., morphology and critical point). The ultimate objective is to provide not only a theoretical basis for understanding and explaining structure-property relationships in existing AEM materials, but to provide a set of general design guidelines moving forward.