Advances in alkaline stable guanidinium based anion exchange membranes

Abstract

Anion exchange membranes (AEMs) are solid polymer electrolytes that contain cationic groups covalently bound to or embedded in a polymer backbone. They act as key role of hydroxide conduction and separator between two electrodes in anion exchange membrane fuel cells (AEMFCs). To achieve high performance in AEMFCs, ideal AEMs should own high ion conductivity and good alkaline stability. Considering to the alkaline stability of AEMs, ionic groups play very important role in determining the overall stability for the AEMs apart from polymer backbones. To date, various ion conductive moieties have been investigated as new tethered ionic groups to replace conventional quaternary ammoniums (QAs) in AEMs because of the multiple degradation pathways under alkaline conditions including: (1) nucleophilic substitution (SN2), (2) Hoffmann elimination (E2), or (3) nitrogen ylide formation. Guanidiniums are found to be a kind of novel ionic groups for the application of AEMs. In the structure of guanidinium, the positive charge is uniformly distributed over the central carbon atom and three nitrogen atoms, leading to stabilized charge delocalization and good thermal and chemical stability. There are six substitutions distributed over three N atoms which can be easily tuned by a mature and feasible route. Additionally, the kinetics of hydrogen evolution/oxidation reaction (HER/HOR) and ORR catalyzed by Pt and carbon-based non precious metal could be greatly enhanced through using guanidiniums. Guanidinium cations can be introduced into the polymer backbone easily by the synthetic methods as follows: (1) halomethylation of a phenyl group and subsequent amination with pentalkylguanidine; (2) nucleophilic substitution of a Vilsmeier salt and a secondary amine; (3) activated fluoroamine reaction; (4) nucleophilic substitution of alkyl halid and guanidine to prepare guanidinium functional monomer; (5) polycondensation between guanidine hydrochloride and diamine. Recently, several types of guanidinium-functionalized AEMs have been explored including benzylic guanidinium AEMs, alkylic guanidinium AEMs and phenylic guanidinium AEMs. Some of the AEMs have been proven to possess excellent ion conductivity, and some have been used in AEMFCs and show good electrochemical performance. Considering to degradation mechanism of guanidinium, it can be concluded as a nucleophilic addition-elimination reaction: firstly the center C atom is attacked by OH- and an intermediate is obtained (addition reaction); then one of N atoms combines with the H atom and eliminates from the intermediate (elimination reaction); finally the degradation products urea and amine are formed. It is worth noting that the degradation process of guanidinium is evidently different from quaternary ammoniums. Their degradation reaction sites only occur at the central carbon atom rather than the benzyl carbon which indicate the promising stability for guanidinium. Though the stabilities of guanidiniums are under debate by now, the usability of guanidiniums as ionic group is still a topic worthy of study. On the one hand, almost all of the studies are confined to guanidiniums with methyl groups substituted at the N1 and N3 positions, hence comprehensive and systematic analysis of the structure-stability relationship of guanidinium cations in alkaline media is still rarely developed. On the other hand, the degradation process of guanidiniums is only speculated by a few studies in theoretical way, more specific experimental studies of this process are badly needed to prove it. Therefore, revealing the structure-stability relationship and the detailed degradation mechanism are vital to design alkaline stable guanidinums which can be applied to AEMs preparation in the future. In addition, the polymer backbone is also very important to improve the stability of guanidiniums based AEMs. More guanidiniums based AEMs with various structures should be designed and prepared. The ion conductivity, alkaline stability and cell performace should be assessed comprehensively to figure out the relationship between the structures and properties of AEMs, and finally the preparation of high performance anion exchange membranes can be achieved.