Electrochemical measurement of triiodide diffusion coefficients in blends of ionic liquids

Triiodide diffusion coefficients were determined in two ionic liquid based electrolyte systems for dye-sensitized solar cells. The electrolytes comprised iodine, 1-methyl-3-propylimidazolium iodide ([MPIM][I]), and either 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide or 1-ethyl-3-methylimidazolium trifluoromethanesulfonate. Determination of triiodide diffusion coefficients was performed by two independent methods, steady-state cyclic voltammetry at ultramicroelectrodes and polarization measurements at thin layer cells, both temperature-dependent and over a broad mixing range with varying ionic liquid molar ratios. The triiodide diffusion coefficients strongly increase with increasing temperature and therefore decreasing electrolyte viscosity. However, the triiodide diffusion coefficients stay almost constant with decreasing [MPIM][I] mole fraction or they even have a maximum at high [MPIM][I] mole fractions and therefore high viscosities. The Einstein-Stokes ratios for both systems strongly increase with increasing [MPIM][I] mole fraction. So as the Einstein-Stokes equation is not obeyed, a strong non-Stokesian charge transport can be assumed to occur in both electrolyte systems.