Anion Exchange Membranes’ Evolution toward High Hydroxide Ion Conductivity and Alkaline Resiliency

Abstract

The development of alkaline fuel cells over the past decade has led to exciting developments in low resistant and alkaline stable anion exchange membranes (AEMs). This Review highlights new material chemistries and macromolecular designs that have fueled AEMs with ionic conductivities greater than 100 mS cm–1, while demonstrating stability for extended periods in base bath solutions of 1 M potassium (or sodium) hydroxide solutions at temperature of 80 °C or greater. The new AEMs have led to AEM fuel cells (AEMFCs) with power density values that exceed 1 W cm–2 with hydrogen and oxygen. AEM research activities are motivated in large part by their prospect to realize fuel cells free of platinum group metals, which is paramount for cost reduction of fuel cell technology. In addition to highlighting the remarkable achievements of AEMs in the past 4 years, this Review discusses future priorities for the scientific community to address in AEM development. These priorities include stability and conductivity under low humidity or dry conditions, resisting carbonation and oxidation, and AEMFC device stability studies.

Density functional theory study on the degradation of fuel cell anion exchange membranes via removal of vinylbenzyl quaternary ammonium head group

“…Under the usual environment of fuel cell and electrolyser operation, the degradation of QA groups can occur through four mechanisms, namely, nucleophilic substitution reaction (S N 2), Hofmann elimination, Stevens rearrangement, and Sommelet-Hauser rearrangement. [1,2,11,56] TMA is one of the most studied QA functionality for AEMs due to the high ionic conductivity that it imparts. [41] The degradation scheme via S N 2 for TMA-based head group is shown in Figure 1.…”

“…Even though DABCO and ABCO produce β‐carbons (consequently β‐hydrogens) upon reaction with benzylic chloride, Hofmann elimination will not proceed due to the impossibility of attaining the required stereoelectronics . NMP‐functionalised AEMs, on the other hand, can undergo degradation via any or combination of three possible mechanisms, namely, OH − ion attack through nucleophilic substitution, ring‐opening elimination, and ring‐opening substitution …”

“…Quaternary ammonium (QA)-based anion exchange membranes (AEMs) have been studied extensively for fuel cell and electrolyser application due to their ease of functionalisation and high hydroxide ion conductivity. [1][2][3] Commonly employed cationic head groups based on QA are benzyl-trialkylammonium such as functionalised with trimethyl amine (TMA), QA with bridged bicyclic amines such as 1,4-diazabicyclo[2.2.2] octane (DABCO) and 1-azabicylo[2.2.2]octane (ABCO), and cycloaliphatic QA such as N-methylpiperidine (NMP). However, QA-functionalised AEMs have poor stability in high pH conditions due to base-induced degradation mechanisms, namely, Hofmann elimination, nucleophilic substitution, and ylide formation pathway.…”

“…However, QA-functionalised AEMs have poor stability in high pH conditions due to base-induced degradation mechanisms, namely, Hofmann elimination, nucleophilic substitution, and ylide formation pathway. [2][3][4][5] Richard Espiritu and John Lester Tan contributed equally to this work.…”

“…[1,9,10] NMP-functionalised AEMs, on the other hand, can undergo degradation via any or combination of three possible mechanisms, namely, OH − ion attack through nucleophilic substitution, ring-opening elimination, and ring-opening substitution. [2,11,12] The nucleophilic substitution reaction in a TMAfunctionalised AEM is an S N 2 attack of the OHions on the methyl group resulting to the formation of benzyl dimethylamine and methanol byproduct or attack of the OHions on the benzylic carbon causing the release of TMA as the preferred leaving group. [6][7][8]13] In the case of the mono-quaternised ABCO and DABCO AEMs, nucleophilic substitution reaction causes the detachment of the head group from the polymer membrane.…”