Investigation of lithium salt-nonionic surfactant mesophases and their applications in solar cells as gel electrolyte
Author(s)
Advisor
Dağ, ÖmerDate
2013Publisher
Bilkent University
Language
English
Type
ThesisItem Usage Stats
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Abstract
Some salts and some nonionic surfactants self-assemble together into
lyotropic liquid crystalline (LLC) mesophases. The salt can be either in aqueous
solution phase or in its molten phase in the self-assembly process. Concentrated
aqueous solutions of lithium salts (LiCl, LiBr, or LiI) and pluronics (triblock
copolymers, such as P65, P85, P103, or P123) or 10-lauryl ether
(C12H25(CH2CH2O)10OH, denoted as C12EO10) type nonionic surfactant mesophases
were investigated in this thesis work. The LLC mesophases are well ordered between
5.0 and 25.0 salt/pluronics and 2.0 and 10.0 salt/C12EO10 mole ratios, and remain
stable for months under the ambient conditions. The water molecules remain as the
hydrates under open atmosphere in the LLC mesophases of lithium salts-nonionic
surfactants. The lithium salt-pluronic LLC mesophases are birefringent and have a
hexagonal mesophase in a broad range of salt concentrations. The unit cell of the
mesophases increases and a transition from the hexagonal to a cubic mesophases
occurs upon increasing the salt content of the media. Moreover the LLC mesophases
are ordered and stable up to 25.0 salt to pluronic mole ratio. At higher salt content,
one can observe either a disordered phase or co-existence of salt crystals and
mesophase.
There is a big demand on the gel electrolytes for dye sensitized solar cells
(DSSC) in order to overcome solvent problems caused by liquid electrolytes. The
LLC mesophases of LiI, LiCl and LiBr salts with 10-lauryl ether (C12EO10) has been considered as gel-electrolyte for the DSSC. We demonstrate that the LiI/I2 couple can be
incorporated into above LLC mesophases of various lithium salt-nonionic surfactant
systems. Those LLC phases, with LiI/I2 couple have been characterized by means of
diffraction, microscopy, spectroscopy and conductivity measurements. The LLC
mesophases diffract at small angles and do not show any phase segregation upon
incorporating the LiI/I2 redox couple. The LLC mesophases of these systems are 2D
hexagonal, and they remain stable under ambient conditions for months. In the LLC
media, the iodide ion and iodine molecule react to produce triodide ion in the media.
The iodide/triodide (I-
/I3
-
) redox couples containing gel electrolytes were formed and
their solar performance was investigated by using a solar simulator and a cell
consisting of a dye sensitized anode (FTO-dye modified TiO2), gel-electrolyte, and a
cathode (FTO-Pt nanoparticles). The LLC mesophases of various lithium saltnonionic
surfactant systems with the I-
/I3
-
redox couple were characterized using
POM (Polarized Optical Microscope), XRD (X-ray Diffraction), FT-IR (Fourier
Transform Infrared Spectroscopy) and Raman techniques. These new LLC
mesophases can be used as gel electrolytes in solar cells after incorporation of redox
couple into the media and display responses as good as commonly used liquid
electrolytes.
Keywords
Lyotropic Liquid CrystalMesophases
Molten Salt
Concentrated Aqueous Solution
Self-assembly
Solar Cell
Gel Electrolyte
Redox Couple