Energetics of Base–Acid Pairs for the Design of High-Temperature Fuel Cell Polymer Electrolytes

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 31 P 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.