Browsing by Subject "Gel electrolyte"

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    ItemOpen Access
    Highly proton conductive phosphoric acid-nonionic surfactant lyotropic liquid crystalline mesophases and application in graphene optical modulators
    (American Chemical Society, 2014) Tunkara, E.; Albayrak, C.; Polat, E. O.; Kocabas, C.; Dag, Ö.
    Proton conducting gel electrolytes are very important components of clean energy devices. Phosphoric acid (PA, H3PO4 3 H2O) is one of the best proton conductors, but needs to be incorporated into some matrix for real device applications, such as into lyotropic liquid crystalline mesophases (LLCMs). Herein, we show that PA and nonionic surfactant (NS, C12H25(OCH2CH2)10OH, C12E10) molecules self-assemble into PANS LLCMs and display high proton conductivity. The content of the PANS LLCM can be as high 75% H3PO4 3 H2O and 25% 10-lauryl ether (C12H25(OCH2CH2)10OH, C12E10), and the mesophase follows the usual LLC trend, bicontinuous cubic (V1) normal hexagonal (H1) micelle cubic (I1), by increasing the PA concentration in the media. The PANS LLCMs are stable under ambient conditions, as well as at high (up to 130 C) and low ( 100 C) temperatures with a high proton conductivity, in the range of 10 2 to 10 6 S/cm. The mesophase becomes a mesostructured solid with decent proton conductivity below 100 C. The mesophase can be used in many applications as a proton-conducting media as well as a phosphate source for the synthesis of various metal phosphates. As an application, we demonstrate a graphene-based optical modulator using supercapacitor structure formed by graphene electrodes and a PANS electrolyte. A PANS LLC electrolyte-based supercapacitor enables efficient optical modulation of graphene electrodes over a range of wavelengths, from 500 nm to 2 μm, under ambient conditions.
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    ItemOpen Access
    Investigation of lithium salts-nonionic surfactant lyotropic liquid crystalline mesophases in a dye sensitized solar cell as gel electrolyte
    (2015-09) Yılmaz, Ezgi
    Liquid crystals are one of the most widely studied and used materials in chemistry. The properties of liquid crystals make them interesting to research also for various electrochemical applications. In this context, lithium salts such as LiI, LiCl, LiBr, and LiNO3 can be assembled using non-ionic surfactants into lyotropic liquid crystalline (LLC) mesophases[1], [2] and used as gel electrolytes in various applications. In this work, the LLC mesophases of LiI with and without other lithium salts (such as LiCl, LiBr, and LiNO3) were prepared using 10-lauryl ether (C12H25(CH2CH2O)10OH, denoted as C12EO10) and characterized using the FT-IR (Fourier Transform Infrared Spectroscopy), Raman spectroscopy, XRD (X-Ray Diffraction), POM (Polarized Optical Microscopy), and AC conductivity measurements. Beside from single salt-surfactant mesophases, we also prepared LiI/I2 redox couple in an LLC phase with the help of a non-ionic surfactant and they were also characterized using the same techniques. We found out that the mesophases can be prepared as gels by directly mixing salt and surfactant with certain amounts of water or as solutions using excess solvent (such as water, ethanol, or acetonitrile) that can be evaporated to form the LLC mesophases. The water content of both sets of samples is the same upon exposing to the atmosphere for a certain time and it only depends on the salt amount and humidity under the ambient conditions (around room temperature and 20-25 % RH). The required water/salt ratio for a stable mesophase is around 3.0 which, it is much lower than the water needed to dissolve those salts in an aqueous media. The water/salt mole ratio closely follows the Hofmeister series of anions, where the water amount order is as follows; LiCl>LiBr>LiI>LiNO3, however, the AC ionic conductivity follows a different order; LiNO3>LiCl>LiBr>LiI. Adding I2 by 1/10 mole ratio of the LiI into the media does not change the properties of the mesophases. The AC conductivity increases with increasing salt and water content of the mesophases with a typical conductivity of around 0.1 to 1.0 mS/cm-1. The mesophases are also stable in a very broad temperature (below 0 °C to 60-130 °C) and salt concentration (2-10 salt/surfactant mole ratio) ranges. Finally, the LiI/I2 mesophases were used as gel-electrolytes in dye sensitized solar cells (DSSCs) as gel-electrolytes and redox couples. A set of samples were prepared with different ratios of the LiI:I2 redox couple (such as, 1:0.1, 1:0.2, 2:0.2, 2:0.3, 3:0.2, 3:0.3, 4:0.4, and 5:0.5) and the solar performances were tested in a DSSC, which contains N719 dye sensitized TiO2 anode and Pt cathode using a solar simulator. However, the LLC phases have gel like structure and it is hard to infiltrate the gel into the pores of dye modified nano-TiO2 films. To overcome the diffusion problem, the gel-electrolytes were also prepared as a solution in excess water, ethanol or acetonitrile that evaporates upon infiltration over time. In addition to this, by changing the procedure of preparing the TiO2 paste, improvement on results was also obtained. The DSSC provides 0.2 % efficiencies with 0.50 fill factors when gel-electrolytes are used. Since water is used for preparing the LLC phases, we had always lower Voc values. However, when it is prepared as a solution with excess ethanol, it provides up to 3.33 % efficiencies with 9.58 mA/cm2 short circuit current and 0.6 V open circuit voltage. Also, new procedure for preparing the TiO2 paste provides us even higher Voc values such as 0.76 V, which is unusual for the water based LLC electrolytes in this area.
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    ItemOpen Access
    Investigation on lithium salt - surfactant lyotropic liquid crystalline mesophases: characterization, role of water, and electrochemical behaviors
    (2021-09) Topuzlu, Ezgi Yılmaz
    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.
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    ItemOpen Access
    Lithium salt-nonionic surfactant lyotropic liquid crystalline gel-electrolytes with redox couple for dye sensitized solar cells
    (Royal Society of Chemistry, 2016) Yılmaz, E.; Olutaş, E. B.; Barım, G.; Bandara, J.; Dag, Ö.
    Lithium salt (LiCl, LiBr, LiI, or LiNO3) and a non-ionic surfactant (such as 10-lauryl ether, C12E10) form lyotropic liquid crystalline (LLC) mesophases in the presence of a small amount of water. The mesophases can be prepared as gels by mixing all the ingredients in one pot or in the solution phase that they can be prepared by coating over any substrate where the LLC phase is formed by evaporating excess solvent. The second method is easier and produces the same mesophase as the first method. A typical composition of the LLC phases consists of 2-3 water per salt species depending on the counter anion. The LiI-C12E10 mesophases can also be prepared by adding I2 to the media to introduce an I-/I3 - redox couple that may be used as a gel-electrolyte in a dye-sensitized solar cell. Even though the mesophases contain a large amount of water in the media, this does not affect the cell performance. The water molecules in the mesophase are in the hydration sphere of the ions and do not act like bulk water, which is harmful to the anode of the dye-sensitized solar cells (DSSC). There are two major drawbacks of the salt-surfactant LLC mesophases in the DSSCs; one is the diffusion of the gels into the pores of the anode electrode and the other is the low ionic conductivity. The first issue was partially overcome by introducing the gel content as a solution and the gelation was carried in/over the pores of the dye modified titania films. To increase the ionic conductivity of the gels, other salts (such as LiCl, LiBr, and LiNO3) with better ionic conductivity were added to the media, however, those gels behave less effectively than pure LiI/I2 systems. Overall, the DSSCs constructed using the LLC electrolyte display high short circuit current (Isc of around 10 mA), high open circuit voltage (Voc of 0.81 V) and good fill factor (0.69) and good efficiency (3.3%). There is still room for improvement in addressing the above issues in order to enhance the cell efficiency by developing new methods of introducing the gel-electrolytes into the mesopores of the anode electrode.
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    ItemEmbargo
    Understanding the role of water in the lyotropic liquid crystalline mesophase of high performance flexible supercapacitor electrolytes using a rheological approach
    (Elsevier, 2024-01-15) Özkaynak, M. U.; Kocaağa, B.; Dönmez, K. B.; Dağlar, S.; Tuerker, Y.; Karatepe, N.; Güner, F. S.; Dağ, Ömer
    The effect of water on the structure, properties, and flexibility of lyotropic liquid crystalline (LLC) C12E23-LiCl-H2O gel electrolytes was explored. Structural techniques, such as X-ray Diffraction (XRD), Polarized Optical Microscopy (POM), and five dynamic measurements, were employed to examine the rheological properties of the LLC mesophase across various water contents. These analyses provided quantitative insights into the influence of water content and LiCl concentration on gel strength, gelation point, and structural recovery. The three-dimensional network of the gel encapsulates Li+ and Cl− ions within hydrophilic domains, showing significant performance in supercapacitor applications. The observed increase in storage modules with decreasing water content is attributed to variations in the quantity and average size of junction points owing to system entanglement. These research findings highlight that excess water molecules, which break down micellar connections, are responsible for the weakening of the gel. Conversely, at low water concentrations, the micellar domains entangle, displaying viscoelastic behavior akin to that of a transitory polymer network.