Molecular Engineering of Ion-Conducting Polymers for Fuel Cell Membrane Applications

Ion-conducting polymer membranes (cation exchange and anion exchange) are a key component in clean energy conversion devices (e.g., fuel cells and water electrolysis).  For example, perfluorinated sulfonic acid polymers such as Nafion are state-of-the-art proton-conducting polymers, but they suffer from high cost, mediocre mechanical strength, and moderate operation temperature.  Thus, it is highly desirable to develop readily tunable, high-performance ion-conducting membranes based on low-cost hydrocarbon polymers.

Over the years we have developed a novel polymer functionalization method that allows incorporation of a variety of ionic groups to aromatic polymers.  Using the method we synthesized highly proton-conductive fuel cell membranes containing sulfonate groups with different acidity strengths (fluoroalkylsulfonic, arylsulfonic, and alkylsulfonic acid groups) [1-2].  The effect of these different sulfonate groups of polymers on proton transport properties and fuel cell performance will be discussed.

Recently, anion exchange membrane fuel cells (AEMFCs) have gained significant attention because they can efficiently and cleanly convert the chemical energy stored in fuels directly into electrical energy [3].  Since AEMFCs utilize a solid alkaline electrolyte [i.e. anion exchange membrane (AEM)] to transport hydroxide anions, they can avoid issues previously faced with the concentrated aqueous KOH electrolyte in alkaline fuel cells.  However, significant shortcomings still remain in AEMFCs such as poor long-term alkaline stability and low hydroxide conductivity.  In our continued effort to develop robust, high performance ion-conducting polymers, we prepared styrene-ethylene/butylene-styrene (SEBS)-based AEM materials bearing various ammonium-cation groups.  Utilizing our groups’ recently developed metal-catalyzed synthetic strategy, involving C-H borylation followed by Suzuki coupling [4], we successfully introduced chemically stable ammonium-cation species to the aromatic units of SEBS.  The synthesis, hydroxide transport properties, alkaline stability, and fuel cell performance of these SEBS-based AEMs will be presented.

References:

[1] Y. Chang, G. F. Brunello, J. Fuller, M. Hawley, Y. S. Kim, M. Disabb-Miller, M. A. Hickner, S. S. Jang, C. Bae, Macromolecules 2011, 44, 8458–8469.

[2] Y. Chang, G. F. Brunello, J. Fuller, M. Disabb-Miller, M. Hawley, Y. S. Kim, M. A. Hickner, S. S. Jang, C. Bae, Polym. Chem. 2013, 4, 272–281

[3] G. Couture, A. Alaaeddine, F. Boschet, and B. Ameduri, Prog. Polym. Sci., 2011, 36, 1521–1557.

[4] A. D. Mohanty, Y. –B. Lee, Liang, Zhu, M. A. Hickner, C. Bae, Macromolecules, 2014, 47, 1973–1980.