Increasing the Alkaline Stability of N,N-Diaryl Carbazolium Salts Using Substituent Electronic Effects.

Nansi Gjineci,Charles E Diesendruck,Sinai Aharonovich,Dario R Dekel

doi:10.1021/acsami.0c14132

Abstract

Anion-exchange membrane fuel cells (AEMFCs) have attracted the attention of the scientific community during the past years, mostly because of the potential for eliminating the need for using costly platinum catalysts in the cells. 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. Improving the stability of these groups is directly connected with the ability to recognize the different mechanisms of the OH– attack. In this work, we have synthesized eight different carbazolium cationic model molecules and investigated their alkaline stability as a function of their electronic substituent properties. Given that N,N-diaryl carbazolium salts decompose through a single-electron-transfer mechanism, the change in carbazolium electron density leads to a very significant impact on their chemical stability. Substituents with very negative Hammett parameters demonstrate unparalleled stability toward 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.

Highlights

Anion-exchange membrane fuel cells (AEMFCs) have emerged as attractive candidates for energy conversion and storage, especially for automotive and portable applications.[1−3] The ability to eliminate the need for platinum catalysts due to their alkaline media, and the subsequent cost reduction, is one of the driving forces for the research and development of AEMFCs.[4−8] there are significant scientific and technological obstacles that inhibit the broad utilization of AEMFCs
We have recently demonstrated that water-soluble N,N-diaryl carbazolium salts (DACs), in which only sp[2] carbons are directly connected to the nitrogen center,[32,33] react with dry OH− and degrade through a different mechanism: an inner sphere single-electron transfer (SET) in the ion pair, followed by radical coupling, similar to the first step of a Birch reduction (Scheme 2).[34]
The introduction of para-hydroxide groups enhanced the half-life by more than 3 orders of magnitude. This stable cationic molecule was further studied under harsher conditions where it was compared to leading quaternary ammonium (QA) in the literature, including benzyl trimethylammonium (BTMA), ASU, and a substituted imidazolium salt

Summary

Introduction

Anion-exchange membrane fuel cells (AEMFCs) have emerged as attractive candidates for energy conversion and storage, especially for automotive and portable applications.[1−3] The ability to eliminate the need for platinum catalysts due to their alkaline media, and the subsequent cost reduction, is one of the driving forces for the research and development of AEMFCs.[4−8] there are significant scientific and technological obstacles that inhibit the broad utilization of AEMFCs.

Methods

Results

Conclusion