Proton conduction mechanisms in the phosphoric acid–water system (H4P2O7–H3PO4·2H2O): a 1H, 31P and 17O PFG-NMR and conductivity study

Abstract

Ionic charge carrier formation and mobility, including the underlying conduction mechanisms, are investigated for phosphoric acid at water contents relevant for the acid's application as electrolyte in fuel cells. The high conductivity contribution from structural diffusion involving intermolecular proton transfer (∼97%) in neat phosphoric acid (H₃PO₄) passes through a maximum at this composition. Hydrogen bond network frustration (imbalance of the number of proton donors and acceptors), which is closely related to the appearance of structural diffusion, decreases with both elimination and addition of water. Structural diffusion is virtually disappearing for H₃PO₄·2H₂O, yet, the overall conductivity increases with increasing water content and reaches a maximum at a composition of H₃PO₄·5H₂O. The conductivity increase is a consequence of the progressive de-coupling of the diffusion of aqueous species from that of phosphate species and the strongly enhanced acidity of phosphoric acid at low water contents. High concentrations of protonated aqueous species with high diffusivity then lead to high conductivity contributions from vehicular transport. The increased water transport associated with the change in transport mechanism is suggested to have severe implications for fuel cell applications. At low water contents the conductivity contribution of structural diffusion is also reduced, but it is accompanied by conductivity contributions from a high concentration of multiply charged condensation products (e.g. H₂P₂O₇²⁻, H₃P₃O₁₀²⁻ and H₂P₃O₁₀³⁻). The results underline the singularity of structure diffusion in neat phosphoric acid (H₃PO₄) and its sensitivity against any perturbation.