Approaches to Understanding AEM Degradation Using Model Compounds

The development of anion exchange membranes (AEMs) with sufficient conductivity and durability for use in alkaline fuel cells has become an increasingly active area of research.  And while a number of polymer scaffolds bearing a variety of different cationic groups have been examined as AEMs, no clear solution has been found.  The degradation of cationic groups, the polymer backbones, or both components of candidate AEMs upon prolonged exposure to alkaline media presents the greatest challenge and hence greatest opportunity in the field of AEM discovery. 

            Several studies have examined the degradation of intact AEMs using techniques suited to the characterization of polymeric materials including infrared spectroscopy, Raman spectroscopy, and NMR spectroscopy.^1,2  This approach to unraveling AEM degradation allows simultaneous, parallel investigation of conductivity, water uptake, ion exchange capacities, as well as experiments in fuel cells.  Recent work in this area has even implicated cation degradation as a trigger for further decomposition of polymer backbones leading to membrane failure.³

            Some groups have begun to approach AEM degradation using small model compounds that mimic the structure of the cationic component.  These small organic models and their degradation products are amenable to study by techniques not readily suited to polymeric systems.  For example, NMR experiments on model quaternary ammonium and azolium species have provided baseline data on the mechanisms of decomposition operative under alkaline conditions. 

            Our work seeks to expand the use of model compounds for understanding the mechanism(s) of AEM degradation.  Our approach includes small molecule mimics for different cationic groups, polymer backbones, and hybrid models wherein cationic groups are attached to polymer resins like chloromethylated polystyrene.  The aim of our approach is to increase the throughput of techniques that can be used to evaluate the stability of different cation-polymer combinations while providing chemical insights into their mechanisms of degradation.  By attaching cationic models to robust resins like cross-linked chloromethylpolystyrene, the decomposition products can be isolated by filtration and analyzed by mass spectrometry (MS).  The sensitivity and amenability of MS to high throughput formats compliments NMR-based experiments on model compunds or dissolved membranes. 

References

1.  C. G. Arges and V. Ramani, JECS, 160, F1006 (2013).

2.  S. A. Nunez and M. A. Hickner, ACS Macro Lett., 2, 49 (2013).

3.  C. G. Arges and V. Ramani, PNAS, 110, 2490 (2013).