TOPICAL DAY - A multiscale study of proton dynamics and ionic transport in phosphoric acid the substance with the highest intrinsic proton conductivity

Orateur : Jan-Patrick MELCHIOR (Max-Planck-Institut für Festkörperforschung Heisenbergstraße 1, 70569 Stuttgart, Germany)

The proton conduction mechanism of phosphoric acid is important for the functions of complex phosphate containing biological and technological systems (e.g. phospholipid membranes and polybenzimidazole phosphoric acid membranes for high-temperature PEM fuel cells). In neat phosphoric acid structural proton diffusion, i.e., proton hopping between phosphoric acid molecules, is superimposed onto hydrodynamic diffusion of the molecules in the viscous liquid. Though structural diffusion of protons is also known to occur in other hydrogen bonded liquids, e.g., in water, phosphoric acid constitutes a special case : 97% of the acid’s conductivity is through structural proton diffusion and its structural diffusion mechanism is, in fact, unique. Its a consequences of the acid’s frustrated hydrogen bond network (there is a severe imbalance of potential proton donors and acceptors) and the strength of its highly polarizable hydrogen bonds.[1-3] In this study we separate hydrodynamic and structural contributions to proton diffusion and conductivity on the millisecond and nanosecond timescale. Phosphoric acid—water and phosphoric acid—benzimidazole mixtures are investigated as model systems for hygroscopic polybenzimidazole phosphoric acid fuel cell membranes with a focus on the general influence of additional proton acceptors on structural proton diffusion. Millisecond dynamics is investigated through a combination of different electrochemical, NMR and pulsed field gradient NMR experiments (1H, 31P and 17O).[4,5] Nanosecond dynamics is investigated through 1H and 17O nuclear magnetic relaxation measurements and high resolution backscattering quasi-elastic neutron scattering (IN16B).[6] Here we describe the peculiar nature of proton transport in phosphoric acid and the different underlying physics of the experiments that allow for a separation of structural and hydrodynamic diffusion on the different timescales.

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