Toward alkaline‑stable anion exchange membranes in fuel cells:cycloaliphatic quaternary ammonium‑based anion conductors

Anion exchange membrane(AEM)stability has been a long-standing challenge that limited the widespread development and adoption of AEM fuel cells(AEMFCs).The past five years have been a period of exceptional progress in the development of several alkaline-stable AEMs with remarkable both ex situ and in situ AEMFC stability.Certain cycloaliphatic quaternary ammonium(cQA)(mainly five-and six-membered)based AEMs appear to be among those having the most promising overall performance.In this review,we categorize cQAs as cage-like(such as quaternized 1,4-diazabicyclo[2.2.2]octane,(QDABCO)and quinuclidinium),non-cage-like(such as pyrrolidinium and piperidinium)and N-spirocyclic(such as 6-azonia-spiro[5.5]undecane(ASU)).The degradation mechanisms of categorized cQAs are first elucidated.Through an understanding of how the cations are attacked by strongly nucleophilic OH–,improved structural design of incorporating alkaline-stable cations into AEMs is facilitated.Before a detailed description and comparison of the alkaline stability of cQAs and their respective AEMs,current protocols for the assessment of alkaline stability are discussed in detail.Furthermore,the initial AEMFC performance and fuel cell performance stability based on cQA AEMs are also examined.The main focus and highlight of this review are recent advances(2015–2020)of cQA-based AEMs,which exhibit both excellent cation and membrane alka-line stability.We aim to shed light on the development of alkaline-stable cQA-type AEMs,which are trending in the AEM community,and to provide insights into possible solutions for designing long-lived AEM materials.

Coordination-driven structure reconstruction in polymer of intrinsic microporosity membranes for efficient propylene/propane separation

Polymers of intrinsic microporosity(PIMs),integrating unique microporous structure and solution-processability,are one class of the most promising membrane materials for energy-efficient gas separations.However,the micropores generated from inefficient chain packing often exhibit wide pore size distribution,making it very challenging to achieve efficient olefin/paraffin separations.Here,we propose a coordination-driven reconstruction(CDR)strategy,where metal ions are incorporated into amidoxime-functionalized PIM-1(AO-PIM)to in situ generate coordination crosslinking networks.By varying the type and content of metal ions,the resulting crosslinking structures can be optimized,and the molecular sieving capability of PIM membranes can be dramatically enhanced.Particularly,the introduction of alkali or alkaline earth metals renders more precise micropores contributing to superior C3H6/C3H8 separation performance.K+incorporated AO-PIM membranes exhibit a high ideal C3H6/C3H8 selectivity of 50,surpassing almost all the reported polymer membranes.Moreover,the coordination crosslinking structure significantly improves the membrane stability under higher pressure as well as the plasticization resistant performance.We envision that this straightforward and generic CDR strategy could potentially unlock the potentials of PIMs for olefin/paraffin separations and many other challenging gas separations.