Large Space-Charge Effects in a Nanostructured Proton Conductor

摘要

Decreasing the dimensions of heterogeneous mixtures of ionic conductors towards the nanoscale results in ionic conduction enhancements, caused by the increased influence of the interfacial space-charge regions. For a composite of TiO_2 anatase and solid acid CsHSO_4, the strong enhancement of the ionic conductivity at the nanoscale also can be assigned to this space-charge effect. Surprisingly high hydrogen concentrations in the order of TO21 cm~(-3) in TiO_2 are measured, which means that about 10% of the available sites for H~+ ions are filled on average. Such high concentrations require a specific elaboration of the space-charge model that is explicitly performed here, by taking account of the large occupation numbers on the exhaustible sites. It is shown that ionic defects with negative formation enthalpy reach extremely high concentrations near the interfaces and throughout the material. By performing first-principles density functional theory calculations, it is found that proton insertion from CsHSO_4 into the TiO_2 particles is preferred compared to neutral hydrogen atom insertion and indeed that the formation enthalpy is negative. Moreover, the average proton fractions in TiO_2, estimated by the theoretical ionic density profiles, are in good agreement with the experimental observations.