|Ph.D Student||Gjineci Nansi|
|Subject||New Kinetically Stable Quaternary Ammonium Salts for|
Alkaline Fuel Cell Applications
|Department||Department of Chemistry||Supervisors||ASSOCIATE PROF. Charles E. Diesendruck|
|PROF. Dario Dekel|
Anion-Exchange Membrane Fuel Cells (AEMFCs) have attracted the attention of the scientific community, mostly due to the potential of eliminating the need for using costly platinum catalysts. However, the broad commercialization of AEMFCs is hampered by the low chemical stability of the cationic functional groups in the anion-conducting membranes required for the transportation of hydroxide ions in the cell. To tackle the stability issue, new cationic functional groups are developed, with quaternary ammonium groups (QAs) being the prevalent ones, with numerous studies assessing their stability and trying to improve their longevity.
In this thesis, I developed two molecular design approaches to increase the kinetic stability QAs. The first includes the synthesis of a new type of quaternary ammonium salts, in which only sp2 carbons are connected to the nitrogen center. This thesis begins describing a simple approach to prepare N,N-diaryl carbazolium salts whose solid-state structure is characterized for the first time. Their relative stability is tested and to further understand the relevance of these new QAs, the mechanism of the reaction between these tetraaryl ammonium salts and hydroxide is studied experimentally. Different N,N-diaryl carbazolium salts are designed, synthesized, characterized, and reacted with hydroxide under different reaction conditions. The products of the reactions are directly characterized and the possible reaction mechanisms are compared. An unexpected H/D exchange was observed in one of these salts, helping to discard some of the classical SNAr mechanisms, supporting instead an unexpected radical mechanism initiated by a single-electron transfer from the hydroxide. By understanding the preferred reaction pathways, better quaternary ammonium salts can be designed to withstand aggressive alkaline environments, critical for many practical applications such as AEMFCs.
In addition, I present the synthesis of eight different carbazolium cationic model molecules and investigate the electronic substituent effects on their alkaline stability. Substituents with very negative Hammett parameters demonstrate unparalleled stability towards dry hydroxide. This study provides guidelines for a different approach to develop stable quaternary ammonium salts for AEMFCs, making use of the unique parameters of this decomposition mechanism
The second approach presented in this thesis regards non-covalent steric shielding around the positive charge as a novel approach to affect the rate of reaction between hydroxide and QAs. For that purpose, four polyrotaxanes containing crown ethers of varying cavity size were prepared. The alkaline stability of these polyrotaxanes and an unthreaded control polymer were evaluated and compared. We have found that the addition of crown ether leads to an increase in stability in correlation to the size of the crown ether, indicating that the crown-ethers work as large steric shields, protecting the QAs from reaction with hydroxide. Furthermore, I used small molecule model compounds to investigate the mechanism of the reaction. The results from this study, in addition to supporting our hypothesis, provide a new approach to increase the chemical stability of different functional groups in polymers without tweaking their electronic properties, as the rotaxane shield is not-covalently bound to the functional group.