Browsing by Subject "Lyotropic liquid crystalline mesophase"

Now showing 1 - 3 of 3

Open Access
Fabricaiton, characterization, and electrolysis of mesoporous CaFe2O4 thin film electrodes
(2023-06) Raza, Hamid Ali
Transition metal ferrites have attracted the attention of many scientists because of their low cost, high earth abundance, low band gap, and biocompatibility. They can be prepared in different morphologies, and because of this, they may have a high surface area and excellent electrochemical and photoelectrochemical properties. In this thesis, we have prepared mesoporous calcium iron oxide (CFO) thin films using the molten-salt assisted self assembly (MASA) method and analyzed its electrochemical applications for oxygen evolution reaction (OER). The clear and homogeneous aqueous solution of metal salts (Calcium nitrate tetrahydrate, iron (III) nitrate nonahydrate) and surfactants (cethyltrimethyl ammonium bromid, CTAB, and C12H25(OCH2CH2)10OH, C12EO10) were coated on microscope glass slides by various coating techniques to obtain mesophases. Later on, the mesophases and their aging process were analyzed by the small-angle XRD measurements, ATR-FTIR and POM. Diffraction lines between 1 and 5°, 2θ, indicate the formation of ordered lyotropic liquid crystalline mesophases. These mesophases were subjected to calcination at various temperatures, and the powders obtained were further characterized by wide-angle XRD measurements, SEM, EDX, TEM, XPS, ATR-FTIR, and N2-adsorption and desorption techniques. The calcium iron oxide in highly crystalline form are prepared at 800 °C, having thin film morphology. Interestingly, we are able to retain the porous structure even at such a high temperature. The amorphous phase contains calcium carbonate as a side product that was confirmed by ATR-FTIR, XRD and XPS data. The maximum surface area of mesoporous material is 145 m2/g, while water being used as a solvent. Similarly, we prepared the same materials using different precursors (chlorides) and solvent (ethanol) to see the effect of counter anion and solvent on the porosity, self-assembly, morphology, and electrocatalytic performance of the material in the OER. We observed that while using chloride precursors, the material was quite crystalline even at low calcination temperature, i.e., 300 °C. Iron oxide forms at low temperatures and with the increase in temperature, it finally transforms to calcium iron oxide. But in this case, the materials are not as porous and display a surface area of only 5 m2/g at 300 °C. Similarly, we also characterized these materials using the above-mentioned techniques. While using ethanol as a solvent, keeping nitrate precursors the same, and using two different mole ratios of calcium and iron (2:4, 3:6), we also tried to elucidate the effect of solvent on morphology and catalytic properties of materials. In this case, we observed that the surface area did not drop immediately (as in the case of water) but gradually. The maximum surface area, obtained are almost similar to the material prepared by water as a solvent. All the solutions mentioned above (prepared by using different precursors, solvent, and mole ratios) are coated (by dip-coating) on the graphite rod to determine the catalytic activities by various electrochemical experiments (cyclic Voltammetry (CV), chronopotentiometry (CP), and chronoamperometry (CA)). Electrodes are quite stable in all cases, even in harsh conditions (CP at 100 mA for 2 h). Also, enhanced activity may be because of reduced resistance and increased conductivity with the usage. In all cases, the minimum Tafel slopes are almost similar, and vary between 47 and 83 mV/dec. The overpotentials at various current densities are 260 mV for 1 mA/cm2, about 450 mV for 10 mA/cm2, and about 700 mV for 100 mA/cm2. Additionally, effect of the coated material's thickness on the electrocatalyst's activity is also investigated. It has been found that by decreasing the amount of coated material (by diluting up to 100 times), there is no change in the activity of the material. Finally, our results indicate that these types of energy material's (CFO) performance depends on the surface's characteristics rather than the coating material's thickness or the pores' size. Also, we found it unnecessary to waste a large amount of metal salts to fabricate these materials; OER performance is similar regardless of coating thickness. Therefore, the surface reaction is the primary factor in electrode activity, with pore shape being the critical characteristic.
Embargo
Synthesis, characterization, and electrochemical properties of mesoporous spinel LiMn2-xMxO4 (M = Mn, Fe, Co, Ni, AND Cu) thin films
(2024-05) Durukan, Irmak Karakaya
Mesoporous LiMn2-xMxO4 electrodes are promising candidates for efficient oxygen evolution (OER) electrocatalysis. In this study, mesoporous LiMn2-xMxO4 (where M is Mn, Fe, Co, Ni, and Cu) thin films have been fabricated by employing molten-salt assisted self-assembly (MASA) method on fluorine doped tin oxide (FTO) surface. The electrodes are characterized according to their structure, morphology, and thicknesses using various characterization techniques. The electrochemical properties of the films are comprehensively investigated under acidic, alkaline, neutral, and non-aqueous solutions. The manganese oxide-based electrodes undergo Mn(III) and Mn(VI) disproportionation reactions. Here, we have extensively investigated these disproportionation reactions by post-characterization techniques after electrochemical experiments using LiMn2-xMxO4 (M is Fe, Co, Ni, and Cu and x is 0, 0.1, 0.3, 0.4, and 0.67) and Mn3O4 electrodes. The LiMn2O4 thin films are found to be more stable in OER compared to Mn3O4. Lithium de-intercalation of the LiMn2O4 films produces a λ-MnO2 phase robust against Mn(VI) disproportionation. The electrochemical degradation rates are investigated using the LiMn2O4 electrodes, fabricated at various spin rates (from 2000 and 10000 rpm). The film thicknesses are between 150 and 500 nm. The LiMn2O4 electrode at 5000 rpm is more resistant to physical degradation during electrochemical tests. Charge capacity values of the thin films are determined by electrochemical experiments in LiNO3 electrolyte and found to be between 136 and 273 mC/cm2 for the films, then these values are used to calculate their approximate weights (between 30 and 60 μg/cm2). The annealing temperature for the LiMn2O4 thin films is also optimized for a stable OER. The LiMn2O4 film, fabricated at 5000 rpm spin rate and annealed at 300 oC, is found to be a more robust and efficient electrode with a 60 mV/dec Tafel slope and 812 mV overpotential at 10 mA/cm2 current density. The same fabrication parameters are used for the other mesoporous LiMn2-xMxO4 thin films. The LiMn2-xMxO4 thin films are used to collect their N2-adsorption-desorption isotherms. The isotherms display type IV hysteresis, characteristic of mesoporous materials. BET surface areas are estimated as 98, 99, 116, 112, and 75 m2/g for the LiMn2O4, LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 and LiMn1.7Cu0.3O4, films, respectively. Moreover, the LiMn2-xMxO4 electrodes (fabricated at 5000 rpm spin rate and 300 oC annealing temperature) are investigated for lithium de-intercalation/intercalation behavior in 1 M LiNO3 solution. Then, the same electrodes are used to collect 300 CVs, CAs, and CPs in 1 M KOH solution to evaluate electrochemical behaviors. From these measurements, the origin of phase separation and bearing lower oxidation states of the nickel and copper at higher x values are identified in the spinel structure. The Mn(VI) disproportionation reaction on the LiMn2-xMxO4 electrodes is investigated by CV cycling experiments in 1 M KOH. The LiMn2O4, LiMn2-xFexO4, and LiMn2-xCuxO4 electrodes undergo fast degradation compared to LiMn2-xCoxO4 and LiMn2-xNixO4 through the dispersion of [MnO4]- and [FeO4]2- ions and dissolution of the CuO phase formed in the electrodes during OER. The LiMn1.7M0.3O4 thin films on FTO are used in OER electrocatalysis and the overpotential values at 10 mA/cm2 are evaluated as 645, 686, and 657 mV for the LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 electrodes, respectively. The exact compositions are also coated on graphite substrates and their overpotential values are also evaluated as 629, 462, 440, and 532 mV at 10 mA/cm2 for the LiMn2O4, LiMn1.7Fe0.3O4, LiMn1.7Co0.3O4, LiMn1.7Ni0.3O4 electrodes, respectively. The LiMn1.7Co0.3O4 on graphite and LiMn1.7Ni0.3O4 on FTO electrodes are found to be the most robust and efficient electrodes at a 50 mA/cm2 current density.
Embargo
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.