Decreasing the dimensions of heterogeneous mixtures of ionic conductors towards the nanoscale results in ionic conduction enhancements, caused by the increased infl uence 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 10 21 cm − 3 in TiO 2 are measured, which means that about 10% of the available sites for H + ions are fi lled on average. Such high concentrations require a specifi c 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 fi rst-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 profi les, are in good agreement with the experimental observations.

Space charge induced 2H+ densities up to 2H+ (0.17)TiO2 are observed directly using neutron diffraction on two different nanoscale particle sizes of anatase TiO2 immersed in sulfuric acid, and consistent with experimental evidence modelling shows that these ions show rapid self diffusion.

The field of nanoionics is of great importance for the development of superior materials for devices that rely on the transport of charged ions, like fuel cells, batteries, and sensors. Often nanostructuring leads to enhanced ionic mobilities due to the induced space‐charge effects. Here these large space‐charge effects occurring in composites of the proton‐donating solid acid CsHSO4 and the proton‐accepting TiO2 or SiO2 are studied. CsHSO4 is chosen for this study because it can operate effectively as a fuel‐cell electrolyte at elevated temperature while its low‐temperature conductivity is increased upon nano­structuring. The composites have a negative enthalpy of formation for defects involving the transfer of protons from the acid to the acceptor. Very high defect densities of up to 10% of the available sites are observed by neutron diffraction. The effect on the mobility of the protons is observed directly using quasielastic neutron scattering and nuclear magnetic resonance spectroscopy. Surprisingly large fractions of up to 25% of the hydrogen ions show orders‐of‐magnitude enhanced mobility in the nanostructured composites of TiO2 or SiO2, both in crystalline CsHSO4 and an amorphous fraction.

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 TiO2 anatase and solid acid CsHSO4, 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 1021 cm−3 in TiO2 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 CsHSO4 into the TiO2 particles is preferred compared to neutral hydrogen atom insertion and indeed that the formation enthalpy is negative. Moreover, the average proton fractions in TiO2, estimated by the theoretical ionic density profiles, are in good agreement with the experimental observations.