Investigation on lithium salt - surfactant lyotropic liquid crystalline mesophases: characterization, role of water, and electrochemical behaviors

Abstract

In this thesis, lyotropic liquid crystalline (LLC) mesophases of lithium salts (LiCl, LiBr, LiNO3, LiSCN, LiI, LiI/I2, and LiH2PO4) and 10-lauryl ether (C12H25(OCH2CH2)10OH, C12EO10) have been investigated and the LiI/I2 LLC mesophases was used as gel electrolyte in a dye sensitized solar cell (DSSC). The LLC mesophases of LiCl, LiBr, LiNO3, LiSCN, and LiH2PO4 salts were prepared in a broad range of salt concentrations and found that the mesophases of LiCl, LiBr, LiNO3, and LiSCN salts are stable between 2 and 10 salt/surfactant mole ratios. The LiI-C12EO10 samples undergo meso-crystallization over a 3 mole ratio and not investigated at high salt mole ratios. The LiH2PO4-C12EO10 LLC samples are semi stable and leach out salt crystals upon aging because they cannot hold sufficient water; the water holding capacity and stability of the LiH2PO4 mesophases can be improved by adding various amount of H3PO4 that prevents salt crystallization by increasing the water content of the mesophase. Therefore, this thesis was divided into three main chapters, the first chapter is on the role of water in the LLC mesophases of LiCl, LiBr, LiNO3, and LiSCN. The second chapter is on the LiH2PO4 LLC mesophases, and addition of H3PO4 to prevent crystal formation. And the last one is about the use of the LiI/I2 LLC mesophase as gel electrolyte in DSSCs. The mesophases were obtained by evaporating clear solutions of all ingredients. First, they were fully characterized by using Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR), Raman, and UV-Vis absorption spectroscopy, X-ray diffraction (XRD), AC conductivity, gravimetric measurements, and polarized optical microscope (POM) imaging to identify structural and electrical properties and water up takes of the mesophases. The water evaporation has been monitored by gravimetric, spectroscopic, and conductivity measurements. The water evaporation rate follows a 2/3 power of the evaporated water weight. After complete water evaporation, the LiCl and LiBr systems keep a similar amount of water in their mesophases (typically 3-8 water/Li+, depending on the humidity condition). However, the LiSCN and LiNO3 systems keep a significantly lower amount of water inside their mesophase (1.5-5 water/Li+ depending on the humidity condition). Water has a critical role in forming the lithium salt-surfactant mesophases and their structural properties. To investigate the role of water, humidity-dependent measurements have been carried by using gravimetry, spectroscopy, AC conductivity, and POM techniques. The gravimetric and spectroscopic data show that the H2O/Li+ mole ratio increases linearly by 0.08-0.09 per humidity. Increasing water amount in the mesophases improves their ionic conductivities with at least a factor of two when the humidity was changed from 20 % to 40 %. The water/Li+ mole ratios, water-ion, water-water, water-surfactant, and ion-surfactant interactions strongly depend on the counter anions of the lithium salts and closely follows the Hofmeister series of anions, such that the water molecules are strongly held in the LiCl mesophase but weakly held in the LiNO3 and LiSCN. Controlling the humidity over the lithium salt-surfactant mesophases is a promising way of tuning the properties of the LLC mesophases as needed. Furthermore, mixing two different lithium salts may also improve/adjust the water amount in the gel-phases. For instance, adding LiCl to LiNO3 mesophase increases the final water content in the mesophase and, therefore, the conductivity. Controlling the water amount in the mesophase can be beneficial for the use of the lithium salt-surfactant mesophases in various electrochemical applications. The LiH2PO4 mesophase has also been extensively investigated by changing the salt/surfactant mole ratio and found out that it forms at high humidity but is unstable at lower humidity. The LiH2PO4-mesophase undergoes a phase change by leaching out first surfactant and then the salt crystals. Increasing the salt amount in the mesophase can postpone the salt crystallization but it cannot stop completely. However, the LiH2PO4 mesophases can also be stabilized by adding H3PO4 that holds an extensive amount of water. The LiH2PO4-H3PO4 mesophase forms a buffer LLC mesophase that can be used in electrochemical devices as gel-electrolytes where constant pH is needed. The LiI-I2 redox couple was also prepared in LLC mesophase, characterized by the above approaches, and used in a DSSC as gel-electrolyte. To investigate the importance of the water content in the mesophase different humidity levels were tested for gelation. It was shown that 40 % humidity is the best since it increases the efficiency of DSSC from 3.67 % to 5.36 %. Further increase in the humidity level causes dye detachment and free iodine formation; therefore, the efficiency of DSSC start to decline. Altering how to introduce dye and electrolyte over the working electrode, such as introducing dye and electrolyte simultaneously and carrying gelation at 40 % humidity, results a further increase in the cell efficiency to 7.32 %, which is the highest efficiency achieved for an LLC gel electrolyte in DSSC.