Browsing by Subject "Mesoporous thin film"

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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.
Open Access
Synthesis & characterization of mesoporous zinc cobaltite thin films and its electrochemical application for OER
(2021-07) Kalaycı, Nesibe Akmanşen
Transition metal cobaltite materials were widely used as electrode material due to their excellent electrochemical performance, flexibility to be prepared with different morphologies and, high surface area. In this thesis, mesoporous zinc cobaltite thin films were synthesized in cubic spinel structure via molten-salt assisted surfactant assembly (MASA) method with a high surface area and its electro-catalytic performance in oxygen evolution reaction (OER) was analyzed. Clear and homogenous aqueous solution of surfactants (P123 and CTAB), zinc nitrate hexahydrate and cobalt(II) nitrate hexahydrate (as precursors) are coated on glass substrate to obtain mesophases, thereafter mesophases are calcined to synthesize mesoporous zinc cobaltite (denoted as m-ZnCo) as powder. m-ZnCo-60 (with a total salt/P123 ratio of 60) samples were synthesized with a smooth film morphology and maximum surface area of 102 m2/g. The mesophases with different compositions were analyzed using X-Ray Diffraction (XRD) technique. The line(s) between 1.5 and 2°, 2θ, in the XRD pattern is an indication for the formation of ordered lyotropic liquid crystalline mesophase. Aging of the mesophase was monitored via XRD and POM techniques to establish its stability. The stable mesophases were used to synthesize m-ZnCo film and powder samples. The powder samples were collected after calcination process and characterized by XRD, N2 adsorption-desorption, SEM, HR-TEM techniques. The precursor solutions were spin coated on half of 1cm x 2cm size FTO glasses, then calcined and used in three-electrode system as working electrodes. The electrocatalytic performance of the materials was analyzed by cyclic voltammetry (CV), chronopotentiometry (CP), and chronoamperometry (CA) experiments for oxygen evolution reaction (OER). All electrodes were stable up to 100 mA/cm2 current density and displayed minimum Tafel slope value of 41 mV/dec. Mesoporous zinc cobaltite materials were also synthesized through precursor solutions without CTAB. Removing CTAB from the synthesis results films with rougher surface and reduced crystallinity. Same techniques were also employed for characterization. The prepared electrodes of non-CTAB samples exhibited a lower Tafel slope of 40 mV/dec and overpotential of 256 mV at 1mA/cm2 current density. In addition, silica templated mesoporous zinc cobaltite was synthesized by adding TMOS to the precursor solution of ZnCo-60 to increase the surface area, the calcined samples were denoted m-ZnCo-60-S20-300 (S20 is represents 20 TMOS/P123 mole ratio). The m-ZnCo-60-S20-300 sample has the highest specific surface area of 215 m2/g. However, despite having higher surface area due to high resistance of silica material, silicated samples exhibited higher overpotential values.