Premium
Large Space‐Charge Effects in a Nanostructured Proton Conductor
Author(s) -
Haverkate Lucas A.,
Chan Wing K.,
Mulder Fokko M.
Publication year - 2010
Publication title -
advanced functional materials
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 6.069
H-Index - 322
eISSN - 1616-3028
pISSN - 1616-301X
DOI - 10.1002/adfm.201000692
Subject(s) - materials science , ionic bonding , proton , ionic potential , anatase , enthalpy , ion , ionic conductivity , chemical physics , space charge , nanoscopic scale , hydrogen , charge density , density functional theory , thermodynamics , chemistry , nanotechnology , computational chemistry , electrolyte , electron , physics , electrode , organic chemistry , quantum mechanics , photocatalysis , catalysis
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 10 21 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.