Experimental and Theoretical Strategies for a Rational Design of Polyamine-based Anion Exchange Membranes

The use of anion exchange membranes (AEMs) in electrochemical applications, including fuel cells and water electrolyzers, has gained significant attention in recent years. However, their lower ionic conductivity and chemical stability compared to proton exchange membranes (PEMs) remain critical challenges. Numerous experimental and theoretical studies have explored different approaches to improve AEMs, including optimizing degree of functionalization, designing comb-shaped, crosslinked, or interpenetrating structures, incorporating reinforcements or nanoparticles, and chemically modifying polymer structures to achieve the desired properties. Additionally, computational studies at different length- and time-scales enabled the efficient prediction of novel or existing materials behavior, facilitating the rational design of AEMs. This thesis explores different strategies to efficiently tune the properties of polyamine-based AEMs. Polyamine (PA) has been synthesized from polyketone (PK) via the Paal-Knorr reaction with 1,2-diaminopropane. Controlling the polymer’s degree of functionalization by adjusting the reaction time enhances water uptake and conductivity. However, an excessive functionalization can negatively impact AEMs’ mechanical properties. The best compromise was achieved with PA synthesized over 14 days (31% conversion), yielding a tensile strength of 14 MPa and elongation at break of 15.5% at 30 °C and a maximum ionic conductivity of 3.81 mS/cm at 85 °C. Despite these improvements, the ionic conductivity remains insufficient. Therefore, additional strategies were explored to further enhance conductivity. Nanocomposite PA-based AEMs were developed by incorporating functionalized TiO2 nanoparticles into the membrane matrix. The nanoparticles were synthesized using the sol-gel method, grafting long-chain N1-(3-trimethoxysilylpropyl) diethylenetriamine (3-silane). The incorporation of modified nanofillers enhanced the overall properties of the AEM. The membrane with 10 wt% nanoparticles achieved a maximum conductivity of 24.3 mS/cm at 85 °C, approximately 6 times higher than the AEM without fillers. The modified nanoparticles were also incorporated into novel PET-based AEMs, demonstrating promising results. To conclude, new PA-based polymers were synthesized via chemical grafting of heterocyclic (pyrrolidinium and piperidinium) quaternary ammonium groups through long side chains containing hydroxyl groups. While these hydroxyl groups were expected to improve the water uptake and alkaline stability, excessive swelling compromised the mechanical integrity under OH- conditions despite a slight increase in ionic conductivity. Among all strategies, the incorporation of functionalized nanoparticles yielded the most effective results, enhancing all AEMs properties. Theoretical studies were conducted using both density functional theory (DFT) and molecular dynamics (MD) simulations. PA polymer and PA-based membranes were initially studied to investigate molecule’s reactivity and membrane’s static and dynamic properties as a function of the degree of functionalization, hydration level, and temperature. Subsequently, newly proposed structures were thoroughly analyzed using the same computational techniques. This study provides valuable insights into emerging polyamine materials for anion exchange membranes applications. Moreover, these findings may serve as a useful reference for the improvement of various AEMs used in electrochemical applications.