Mass transport and charge transfer rates for Co(III)/Co(II) redox couple in a thin-layer cell

Recently, the complex Co(dtb)3n+ (dtb = 4,4 di tert-butyl-2,2′ bipyridine) in methoxypropionitrile (MPN) solvent has been proposed as an alternative redox mediator in the thin-layer dye sensitized solar cells. The electrochemical properties of this new mediator as a function of temperature were investigated by mean of symmetric golden electrodes thin-layer cell, using three electro-analytical techniques: electrochemical impedance spectroscopy (EIS), slow scan cyclic voltammetry (SCCV) and chronoamperometry (CA). Our study pointed out that, at room temperature, both the electron transfer rate k° = 1.24 10−4 cm s−1 as well as the diffusion coefficient D = 5.85 × 10−7 cm s−1 are rather low. Raising the temperature has a beneficial effect, increasing more than 6 times the standard rate constant of electron transfer and more than 3 times the ionic diffusion coefficient at 80 °C. However, for all the studied temperatures, the slow mass transport of Co(III)/Co(II) species still remains the rate determining step. Viscosity measurements have demonstrated that the ionic mass transport in MPN follows the Stokes’ law and the Walden product is constant, in the temperature range investigated.