Statistical and Block Copolymers for Anion-Exchange Membranes with High Local Concentrations of Quaternary Ammonium Groups

Cationic polymers in the form of anion-exchange membranes (AEMs) may enable a route towards low platinum or even platinum-free alkaline membrane fuel cells with the potential to oxidize more complex fuels than hydrogen and methanol [1-4]. The main challenges for the AEMs are currently to enhance the alkaline stability of the cationic group and to improve the ion conductivity. In this context, there have been many attempts to enhance the properties by varying the backbone polymer structure and the nature and content of the cations [1-4]. However, clearly promising strategies to prepare high-performance AEMs have not yet been described. While several lessons can be learnt from the development of proton-exchange membranes (PEMs), the cationic polymers in AEMs have different characteristics. For example, the organic cations of AEMs, such as quaternary ammonium (QA) groups, are larger and have a lower propensity to cluster and form a distinct ionic phase in the polymer matrix in comparison with the sulfonic groups of PEMs. This characteristic may seriously limit the transport properties of AEMs.

We have recently developed synthetic strategies where single phenylene rings in aromatic polymers can be directly functionalized with precisely two, three, or four QA groups, allowing an excellent and systematic control over the local density and distribution of ionic groups along the polymer backbone. We have used this approach to prepare several series of statistical and multiblock copolysulfones with very high local QA concentrations to promote membrane properties. Small angle X-ray scattering of AEMs based statistical copolymers containing phenylene rings functionalized with two, three and four QA groups, respectively, showed clear ionomer peaks to indicate efficient ionic clustering. The clustering was promoted by an increasing density of QA on the single phenylene rings. Corresponding randomly functionalized copolymer membranes showed no ionomer peaks. At moderate ion-exchange capacities, the Br^- conductivity of the densely functionalized copolymers was higher than that of randomly functionalized copolymers, despite a significantly higher water uptake of the latter. In the current presentation, we will discuss selected results on the synthesis and structure-property relationships of the copolymers and AEMs.

References

[1] M. A. Hickner, A. M. Herring, E. B. Coughlin, J. Polym. Sci., Part B: Polym. Phys., 2013, 51, 1727.

[2] J. R. Varcoe, R. C. T. Slade, Fuel Cells, 2005, 5, 187.

[3] N. Li, M. D. Guiver, Macromolecules, 2014, online DOI: 10.1021/ma402254h.

[4] K. D. Kreuer, Chem. Mater., 2014, 26, 361.