Synthesis and stability studies of N,N-diaryl-carbazolium salts for alkaline fuel cell applications

The chemical stability of the cationic functional groups in Alkaline Anion-Exchange Membranes Fuel Cells (AEMFCs) is, currently, the critical challenge towards the development of these devices. Degradation of the cationic functional groups in the anion-exchange membranes leads to severe decrease in anion conductivity and to a rapid reduction of cell performance. Therefore, novel chemistries and materials are necessary for the development of stable anion-exchange membranes with significantly improved lifetimes under the conditions of AEMFC operation.

Tetraaryl ammonium salts possess structural features which can make them significantly more stable compared to standard alkylammonium salts. Given all carbons connected to the nitrogen are sp², S_N2 reactions are inhibited. In addition, elimination reactions are also not favorable since the abstraction of a β-hydrogen (ortho) leads to high energy benzynes. Despite these features, the synthesis of tetraaryl ammonium-based compounds is not trivial. We will describe our work on the development of a simple synthesis procedure that, in only 3 steps, can lead to a variety of substituted N,N-diaryl carbazoliums salts. In addition, we will show that, even with these structural features, we found that these quaternary ammonium salts decompose quite rapidly under the severe dry and alkaline conditions that are typically found in operating AEMFCs⁴. Given these molecules can be further improved, we conducted a study using experimental measurements as well as DFT calculations to understand the mechanism of decomposition.

Based on these results, we have synthesized an additional series of carbazolium derivates with different electron donating substituents. We have compared their chemical stability in the presence of low hydration alkaline environment. Our results show a clear dependence between the strength of electron donating substituent and the chemical stability of the carbazolium salts. These new understandings bring a new light to the efforts to develop highly-stable AEMFCs for fuel cell and other electrochemical applications.