Proton conduction mechanisms in the phosphoric acid-water system (H 4 P 2 O 7 -H 3 PO 4 ·2H 2 O): a 1 H, 31 P and 17 O PFG-NMR and conductivity study.

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 (H3 PO4 ) 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 H3 PO4 ·2H2 O, yet, the overall conductivity increases with increasing water content and reaches a maximum at a composition of H3 PO4 ·5H2 O. 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. H2 P2 O7 2- , H3 P3 O10 2- and H2 P3 O10 3- ). The results underline the singularity of structure diffusion in neat phosphoric acid (H3 PO4 ) and its sensitivity against any perturbation.