Understanding the Impedance of Mesoporous Oxides: Reliable Determination of the Material-Specific Conductivity

The unique architecture of ordered mesoporous oxidesmakes thema promising class of materials for various electrochemical applications,such as gas sensing or energy storage and conversion. The high accessibilityof the internal surface allows tailoring of their electrochemicalproperties, e.g., by adjusting the pore size or surface functionalization,resulting in superior device performance compared to nanoparticlesor disordered mesoporous counterparts. However, optimization of themesoporous architecture requires reliable electrochemical characterizationof the system. Unfortunately, the interplay between nanocrystallinegrains, grain boundaries, and the open pore framework hinders a simpleestimation of material-specific transport quantities by using impedancespectroscopy. Here, we use a 3D electric network model to elucidatethe impact of the pore structure on the electrical transport propertiesof mesoporous thin films. It is demonstrated that the impedance responseis dominated only by the geometric current constriction effect arisingfrom the regular pore network. Estimating the effective conductivityfrom the total resistance and the electrode geometry, thus, differsby more than 1 order of magnitude from the material-specific conductivityof the solid mesoporous framework. A detailed analysis of computedimpedances for varying pore size allows for the correlation of theeffective conductivity with the material-specific conductivity. Wederive an empirical expression that accounts for the porous structureof the thin films and allows a reliable determination of the material-specificconductivity with an error of less than 8%.