Ph.D Thesis

Ph.D StudentZadok Israel
SubjectWater Transport in Anion Exchange Membranes for Fuel Cell
DepartmentDepartment of Chemical Engineering
Supervisors PROF. Dario Dekel
ASSOCIATE PROF. Simcha Srebnik
Full Thesis textFull thesis text - English Version


Anion exchange membrane (AEM) fuel cells are an attractive alternative technology to the acidic proton exchange membrane-based fuel cells. Conduction of hydroxide ions in AEMs creates an alkaline operating environment that allows using platinum-free catalysts, while still maintaining the performance needed for commercial application (e.g. the automotive industry). However, this technology is very sensitive to the behavior of hydroxide ions in low hydration states. Under such low hydration conditions in the membrane, the hydroxide is more reactive and less diffusive leading to loss of performance and stability.

Understanding the behavior of hydroxide ions in aqueous and non-aqueous media is also fundamental to many chemical, biological, and electrochemical processes. Research has primarily focused on a single fully solvated hydroxide ion, either as an isolated cluster or in bulk. This work presents the first computational study to consider hydroxide under low hydration levels, where the anion may not be fully solvated.

Using the power of molecular simulation, both atomistic and ab-initio, we simulated several model systems under various temperature and hydration conditions. We found the anions are predominantly present as unique water-bridged hydroxide pairs complexes, distinct from previously reported structures under fully hydrated conditions. Although some double-hydroxide layers were reported in the crystalline state, this is the first time to observe them in the disordered liquid state. These anion complexes have a higher ionic strength, which may explain the unusual diffusion behavior as well as the higher reactivity of hydroxide anions observed under low hydration conditions.

We later used a simple schematic model to connect the hydration structure with the diffusivity results. We believe this advances the goal of understanding performance and stability in low hydration states of anion exchange membrane-based fuel cells.