Systematic Analysis of Cation Stability in Alkaline Exchange Membrane Fuel Cells

Discovery and implementation of alternative energy resources have been a leading R&D task for several decades.  Of the clean energy technologies, fuel cells offer a viable option because they can efficiently convert the chemical energy stored in fuels directly into electrical energy without emission of harmful pollutants.  In recent years anion exchange membrane (AEM) fuel cells [in which a solid polymer electrolyte containing fixed cationic groups facilitates the transport of OH^-anions] have received increased attention owing to their unique advantages over proton exchange membrane fuel cells.  Some of the advantages include the use of low-cost non-precious metal catalysts, faster cathode reaction rates, and greater fuel flexibility with lower fuel crossover [1].

Currently the most challenging technical hurdle for the implementation of AEM fuel cells is the lack of membrane materials that are stable under high pH conditions.  Recently it has been discovered that polymers which include aromatic rings linked via ether bonds degrade in high alkaline environments [2],[3].  And among the various cationic species (such as sulfonium, phosphonium, imidazolium, and guanidinium), benzyltrimethylammonium is the most frequently adopted cation because of its simple preparation and reasonable thermal and chemical stability.  However, more robust cations are still needed for long-term uses in alkaline conditions.

Although AEM chemical degradation has been widely acknowledged, a systematic study on the stability of various cations has yet to be reported.  The majority of AEM stability reports study chemical degradation of the cationic species that are tethered to a polymer backbone and compare their stability based on changes in ion-exchange capacity and ionic conductivity as a function of time from exposure to various concentrations of hydroxide solution.  This approach creates inconsistencies in comparing cation degradation since different testing conditions are frequently used and the polymer backbones may affect the overall AEM stability.

Herein, we report a convenient way to systematically compare the chemical stability of various cation species.  We prepared a series of small molecule quaternary ammonium analogues in pure hydroxide form by conducting ion exchange reactions with silver oxide (Ag₂O) and quantitatively monitored their stability in D₂O via NMR spectroscopy.  By using small molecules for our degradation analysis, we were able to avoid inconsistencies previously faced in polymer-based studies.  With this approach we identified several quaternized ammonium cations that are more stable than benzyltrimethylammonium; of which some of them show minimal degradation even above 100 ^oC for an extended time period.  This simple test of small molecule analysis can provide the most optimal solution for predicting which cation structures may have high stability when later attached to the polymer backbone.

References:

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

[2] S. A. Nunez and M. A. Hickner, ACS Macro Letters, 2012, 2, 49-52.

[3] C. G. Arges and V. Ramani, Proc. Natl. Acad. Sci. USA 2013, 110, 2490-2495.