Unveiling the Proton Conduction Mechanism in Phosphoric Acid

Phosphoric acid, a compound represented by the formula H₃PO₄, showcases a fascinating proton conduction mechanism that, while not as extensively studied as other materials, reveals significant insights into its properties and behavior. Above its melting point of 42 °C, this viscous liquid exhibits a rich network of intermolecular hydrogen bonding, which plays a critical role in its unique conductive capabilities. Unlike water, where the balance of donor and acceptor sites is relatively even, phosphoric acid demonstrates a pronounced amphoteric character, allowing it to act as both an acid and a base.

One of the standout features of phosphoric acid is its high degree of self-dissociation, or autoprotolysis, which occurs at approximately 7.4%. This process results in a variety of dissociation products, including H₂PO₄⁻, H₄PO₄⁺, and H₃O⁺. With such a high concentration of charge carriers, separating the overall conductivity into distinct terms of charge carrier concentration and mobility becomes a complex task. However, calculations based on total conductivities have revealed that proton mobility in phosphoric acid is nearly two orders of magnitude greater than the diffusion coefficients of its phosphate species, indicating an almost ideal proton conduction environment.

At its melting point, phosphoric acid boasts a conductivity of 7.7 × 10⁻² S/cm, with an estimated proton mobility of approximately 2 × 10⁻⁵ cm²/s. Notably, this proton mobility has been validated through various studies, including ¹H PFG NMR techniques, which suggest even higher values. The explanation for this remarkable mobility lies in the correlated motion of oppositely charged defects that arise shortly after the dissociation of H₃PO₄, highlighting the dynamic nature of proton transport within this medium.

While the molecular details that govern the diffusion mechanism—specifically the breaking and forming of hydrogen bonds and proton transfers among phosphate species—remain largely unexplored, the evidence of high self-dissociation suggests that these proton transfer events may be less correlated than in water. This characteristic implies that the transfer of protons in phosphoric acid is likely to be nearly barrierless, as indicated by minimal effects from isotopic substitutions.

Furthermore, the introduction of water into phosphoric acid enhances its conductivity, reaching values up to 0.25 S/cm under ambient conditions. Studies have shown that even an 85 wt% phosphoric acid system functions as an almost ideal proton conductor, with an impressive 98% of its conductivity attributed to the structural diffusion of protons. The combination of a high intrinsic charge carrier concentration and mobility in phosphoric acid-based systems renders them prime candidates for applications in various fields, from fuel cells to batteries, underscoring the importance of understanding their conductive mechanisms.