Thermal activation of mass transport and charge transfer at Pt in the I3−/I− electrolyte of a dye-sensitized solar cell

Abstract

The electrochemical properties of the I₃⁻/I⁻ reaction mediator as a function of temperature in the range from 30 °C to 80 °C were investigated by means of symmetric Pt electrodes thin-layer cells (TLC), using three electro-analytical techniques: Electrochemical Impedance Spectroscopy (EIS), Slow Scan Cyclic Voltammetry (SSCV) and Chronoamperometry (CA). Our study pointed out that raising the cell temperature has a beneficial effect both on charge transfer and on mass transport, with an activation energy for the electron transfer process at equilibrium of 24 kJ mol⁻¹, and of 12 kJ mol⁻¹ for the mass transfer process at equilibrium. Viscosity and conductivity measurements have demonstrated that most of the ionic mass transport in the solvent (methoxypropionitrile) follows the Stokes' law and that the Walden product is constant, in the temperature range investigated. The diffusion of I₃⁻, however, was found to be partly “non-Stokesian” at lower temperature where the viscosity of the electrolyte is higher. We have shown that EIS and chronoamperometry are both valid methods to derive diffusion coefficients of redox ions in TLC, even if their exact concentration in the electrolyte is not known.