Morphology and adsorbate dependence of ionic transport in dye sensitized mesoporous TiO2 films

By modeling the steady-state mass transport in a thin-layer cell configuration involving an electrochemically inactive mesoporous colloidal film, the limiting currents of an iodine containing electrolyte can be predicted, as a function of the film porosity. The porosity of the layer is then determined by best fit of the porosity parameter in the model, given the experimentally determined limiting currents. Satisfactory agreement is found with the porosity values obtained by BET measurements. In photoelectrochemical cell applications such as dye sensitized nanocrystalline photoelectrodes for solar energy conversion devices, the porosities of the dye loaded films of e.g. anatase TiO2 play an essential role with respect to their efficiency. In particular, the findings of this work demonstrate that the presence of adsorbed sensitizer cis-(SCN-)(2) bis(2,2'-bipyridyl- 4,4'-dicarboxylate)ruthenium(II) in TiO2 colloidal films appears to decrease the original naked film porosity by up to 30%. This depends on the initial bare film porosity and the dye diameter, thus resulting in a considerably lower effective mass-transport limit within the coated films. The effective diffusion coefficient of triiodide in these porous electrodes was found to not deviate significantly with respect to the free-stream values, suggesting an influence of mesoporosity on the transport mechanisms involved.