Effect of Ammonium Cations on the Diffusivity and Structure of Hydroxide Ions in Low Hydration Media
I Zadok and DR Dekel and S Srebnik, JOURNAL OF PHYSICAL CHEMISTRY C, 123, 27355-27362 (2019).
Anion exchange membrane (AEM) fuel cells are an attractive alternative technology to the acidic proton exchange membrane-based fuel cells. Conduction of hydroxide ions in AEMs creates an alkaline operating environment that allows using platinum-free catalysts, while still maintaining the performance needed for commercial application (e.g., the automotive industry). However, this technology is very sensitive to the behavior of hydroxide ions under low hydration conditions because of the consumption of water near the cathode. We use molecular dynamics simulation to investigate the behavior of two model quaternary ammonium cations used in AEM technologies at low hydration states in the presence of hydroxide anions. Both systems show the existence of an interesting ion complex-water-bridged hydroxide pair- that surprisingly involves two hydroxide anions in close proximity that is found to be highly stable. We use these new insights to explain the observed change in diffusivity of hydroxide and water from high hydration to low hydration regimes. The prevalence of these structures at different levels of hydration also explains the difference in diffusivity observed between the two studied cations. We believe that this hydroxide pair complex is key to understanding and controlling performance and stability in AEMs and in similar electrolyte systems.
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