Energetics of Base–Acid Pairs for the Design of High-Temperature Fuel Cell Polymer Electrolytes
Ivana Matanović, Albert S. Lee, Yu Seung Kim
Abstract
The interaction energy of base–acid plays a key role in acid retention of phosphoric acid (PA)-doped polymer electrolytes under fuel cell operating conditions. Here, we investigate the energetics of proton-accepting and hydroxide-donating organic bases using density functional theory calculations. Because of their weak basicity, proton-accepting organic bases such as benzimidazole have relatively low interaction energy with the acid in the absence of water (15.3–28.0 kcal mol–1). Energetics of the proton-accepting base–PA complex increases by adding water, indicating that the interactions in the base–acid complex strengthen in the presence of water. On the other hand, hydroxide-donating organic bases, such as tetramethylammonium hydroxide, have high interaction energy with PA (∼110 kcal mol–1), which remains high in the presence of water. The chemical shifts of 31P NMR support the energetics of the base–acid complexes. This study further discusses the benefit of incorporating hydroxide-donating organic bases into the polymeric structure over proton-accepting bases as a way to increase acid retention.