Site-Specific Proton Dynamics in Indium-Doped Tin Pyrophosphate

Tin pyrophosphates are solid-state materials that have been proposed for use as proton conductors in intermediate-temperature fuel cells. However, protons must be added synthetically as these materials do not contain structural protons. Protonation via indium doping results in two NMR-resolvable proton signals that correspond to hydrogen-bonded interstitial protons at phosphate tetrahedral and metal octahedral sites. These sites were assigned by using 1H NMR spectroscopy, allowing for site assignment based on the assumption that protons occupying the octahedral sites are more mobile. The full width at half-maximum and longitudinal relaxation were measured to assess proton mobility. Exchange spectroscopy showed that intersite proton exchange (needed for long-range proton transport) is enhanced by increased indium doping up to 20%. Activation energies calculated from proton exchange and proton conductivity measurements suggest that indium doping does not affect the mechanism of proton conduction in these materials: hopping between octahedral and tetrahedral sites. The increased proton conductivity is instead attributable to increased proton concentration.