Self-Consistent Field Modeling of Microstructure Formation In Fluorinated “Block” Ionic Liquids for Photovoltaic Cells

Ionic liquids are commonly used in electrolytes for dye-sensitized solar cells. To prevent leakage from devices, and to simplify device manufacture, it is desirable that the liquids have large viscosities. However, ionic conductivity is inversely proportional to viscosity [1]. To achieve high ionic conductivity at low fluidity, a new fluorinated imidazolium iodide ionic liquid with a block microstructure was synthesized. The chemical structure of this ionic liquid consists of linear blocks of fluoralkyl and poly(ethylene glycol) (PEG) segments with an imidazolium cation that is connected to the PEG block. The salt also contains free iodide anions. The conductivity and viscosity of electrolytes prepared using this ionic liquid were characterized using electrochemical impedance spectroscopy and cone and plate viscometry, respectively, at different temperatures. Microphase segregation of the immiscible fluoroalkyl and PEG segments resulted in an electrolyte with low fluidity (at temperatures below about 90 °C). The relative concentrations of different chemical groups in the electrolyte were found to have a strong effect on experimentally determined molar conductivity. The variations in conductivity could be explained based on the predictions of a self-consistent field model (SCMF) [2,3] on the formation and nature of microstructures in the electrolytes. This talk will discuss the results of the SCMF simulations in the context of experimental ionic conductivity data.

References

1. Ganapatibhotla, L. V. N. R.; Zheng, J.; Roy, D.; Krishnan, S. PEGylated Imidazolium Ionic Liquid Electrolytes: Thermophysical and Electrochemical Properties. Chem. Mater. 2010, 22, 6347-6360.
2. Scheutjens, J.M.H.M.; Fleer, G.J. Statistical Theory of the Adsorption of Interacting Chain Molecules. 1. Partition Function, Segment Density Distribution, and Adsorption Isotherms. J. Phys. Chem. 1979, 83, 1619-1635.
3. Scheutjens, J.M.H.M.; Fleer, G.J. Statistical Theory of the Adsorption of Interacting Chain Molecules. 2. Train, Loop, and Tail Size Distribution. J. Phys. Chem. 1980, 84, 178-190.