Diffusion-limited transport of I3− through nanoporous TiO2-polymer gel networks

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Abstract
Tri-iodide transport in a polymer gel electrolyte embedded in nanoporous TiO2 networks and its diffusion limits are investigated by means of current-voltage (I–V) characteristics of simple Pt-gel-Pt sandwich devices with a thin porous TiO2 layer sintered directly onto one of the Pt electrodes. At voltages between 0.2 and 0.7 V, the I–V curves of such devices show the typical plateau of diffusion-limited redox reactions, in this case I−/I3−, at the platinum electrodes. From the dependence of the limiting current density on layer thickness, the diffusion constants Dbulk and Dp,eff of tri-iodide in the bulk polymer gel and through a polymer gel penetrated TiO2 network, respectively, have been found to be Dbulk=3.2(±0.2)×10−6 cm2/s and Dp,eff=1.5(±0.1)×10−6 cm2/s. Temperature-dependent measurements show diffusion in the gel to be activated by about 0.16 eV. The results are discussed in comparison to diffusion in liquid electrolytes as well as with respect to the implications for dye-sensitized solar cell devices.