Browsing by Subject "Molten salt-assisted self-assembly"

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Open Access
Fabrication of mesoporous nickel oxide based thin film electrodes and their electrochemical properties
(2025-01) Katırcı, Assel Amirzhanova
In this thesis, robust electroactive mesoporous Ni1-xMnxO thin-film electrodes were synthesized on FTO and graphite rod substrates. Molten salt-assisted self-assembly (MASA) synthesis method was employed to produce uniform thin films. The synthesis started with preparing ethanol solutions, containing various molar ratios of [Mn(H2O)4](NO3)2 and [Ni(H2O)6](NO3)2 (between 1.0 to 0.1 Ni(II)/Mn(II) ratio ) and surfactants (C12H25(OCH2CH2)10OH, C12E10 and C16H33N(CH3)3Br, CTAB). Then, these solutions are coated over conductive substrates to obtain the salt-surfactant lyotropic liquid crystalline (LLC) mesophase. The thin mesophase is calcined in order to produce mesoporous Ni1-xMnxO thin-films on the FTO or graphite. The thin-films form solid solutions with the x value of up to 0.7. The Ni1-xMnxO thin-films transform to NiMnO3, Mn3O4, and Mn2O3 phases at increased Mn ratios and annealing temperatures. The films are mesoporous and were confirmed by N2 adsorption-desorption analysis and typical type IV isotherms characteristic for mesoporous materials. Pore sizes varied from 2.8 to 17.6 nm from Ni-rich to Mn-rich oxides. The surface area reaches to 211 m2/g in Ni0.9Mn1O, while the pure NiO has a BET surface area of 164 m2/g at 350 oC calcination temperature. The FTO and graphite-coated electrodes (FTO-Ni1-xMnxO and G-Ni1-xMnxO) display high charge capacities, but the FTO coated electrodes are unstable and undergo to degradation over extended time of usage. In the first few CV cycles of the FTO-based electrodes, they show an increased capacity, however, decline in further cycles. On the other hand, graphite-based electrodes show better stability and high charge capacity. Origin for increasing charge capacity with cycling is attributed to a transformation of the metal oxides to metal hydroxides. Thus, the electrochemical CV cycling of both pure NiO and Ni1-xMnxO electrodes results in a structural change into a NiO(core)/Ni(OH)2(shell) or Ni1-xMnxO(core)/Ni(OH)2(shell) configurations. The shell thickness is ranged from 2.0 nm (pure NiO) to 1.1 nm (Ni0.9Mn0.1O) at 350°C. Moreover, the shell thicknesses and charge capacities are affected by the pore-wall thicknesses, which increases with increasing annealing temperature. Despite these changes, the manganese addition improves the stability of the electrodes, but there is no improvements on the overpotential on oxygen evolution reaction (OER). Moreover, the annealing temperature reduces the charge capacity, whereas the OER performance remains the same. By using the same MASA method, m-NiO-SiO2 electrodes were synthesized using [Ni(H2O)6](NO3)2 and tetramethyl orthosilicate (TMOS) with CTAB and C12E10 surfactants at different Ni to TMOS ratios. Silica acts as hard template support for NiO, and the film is formed in good quality with bimodal pore size distribution. The sample pore size that was observed is 2.6 nm, which originates from the m-SiO2 domains. The second pore system had also mesopores; the average pore size is 15 nm, calcined at 350 oC. That property helps better infiltration of electrolytes, which is advantageous during electrochemistry. During electrochemical analysis, silica is etched out in basic electrolyte. These electrodes, prepared on graphite substrate have specific surface area of around 130 m2/g. The electrodes show an overpotential 381 mV in the CP experiment at 10 mA/cm2 current density.
Open Access
Mesoporous MnCo2O4 NiCo2O4 and ZnCo2O4 thin-film electrodes as electrocatalysts for the oxygen evolution reaction in alkaline solutions
(American Chemical Society, 2021-03-22) Amirzhanova, Assel; Akmanşen, Nesibe; Karakaya, Irmak; Dağ, Ömer
The oxygen evolution reaction (OER) is the bottleneck of the electrochemical water-splitting process, where the use of porous metal oxide electrodes is beneficial. In this work, we introduce a one-pot synthesis method to fabricate a series of mesoporous metal cobaltite (m-MCo2O4, M = Mn, Ni, and Zn) electrodes for the OER. The method involves preparation and coating of a homogeneous clear solution of all ingredients (metal salts and surfactants) over a fluorine-doped tin oxide surface as a thin lyotropic liquid crystalline film and calcination (as low as 250 °C) to obtain a 400 nm thick crystalline m-MCo2O4 electrode with a spinel structure. Mesophases and m-MCo2O4 films are characterized using structural and electrochemical techniques. All electrodes are stable during the electrochemical test in 1 M KOH aqueous solution and perform at as low as 204 mV overpotential at 1 mA/cm2 current density; the m-MnCo2O4 electrode works at current densities of 1, 10, and 100 mA/cm2 at 227, 300, and 383 mV overpotentials after compensating the IR drop, respectively. The Tafel slope is 60 mV/dec for the m-NiCo2O4 and m-ZnCo2O4 electrodes, but it gradually increases to 85 mV/dec in the m-MnCo2O4 electrode by thermal treatment, indicating a change in the OER mechanism.
Open Access
Role of silica in the self-assembly of salt-surfactant mesophases and synthesis of mesoporous metal oxides
(2023-07) Ullah, Najeeb
In recent years, mesoporous metal oxides have attracted great attraction due to their unique optical, electrochemical, and catalytic properties. Mesoporous nickel oxide (m-NiO) is a p-type semiconductor, versatile in its application due to its high surface area, and has been investigated towards electrochromic devices, electrodes, supercapacitors, and catalysts. The electrochemical properties of NiO depend on its morphology, surface area, and particle size. In this thesis, mesoporous nickel oxide has been synthesized by combining soft templating (molten salt-assisted self-assembly method) and hard templating methods to attain a high surface area. Homogeneous aqueous solutions of nickel(II) nitrate hexahydrate ([Ni(H2O)6](NO3)2), TMOS (as silica source), and two surfactants, CTAB (charged surfactant) and C12E10 (nonionic surfactant) are stable only if a concentrated nitric acid is added before the TMOS addition. In the absence of nitric acid, TMOS hydrolyzes and condenses quickly, resulting in silica precipitation. The silica precipitation also occurs by using other salts, such as nickel(II) chloride hexahydrate, nickel(II) sulfate hexahydrate, cobalt(II) nitrate hexahydrate, and manganese(II) nitrate tetrahydrate. The silica precipitate is characterized by ATR-FTIR, small-angle, and wide-angle XRD and N2 adsorption-desorption measurements. The diffraction lines at 1.7 and 23o, 2θ, indicate the formation of mesostructured amorphous silica, in which the surfactant species fill the pores. . The silica precipitate is calcined at 450 oC for two hours to remove the surfactant completely, and characterized by ATR-FTIR, small-angle and wide-angle XRD measurements, N2- adsorption-desorption analysis and SEM-EDX techniques. The maximum surface area (1395 m2/g) is obtained from the cobalt(II) nitrate hexahydrate salt, and the EDX analysis confirms that there is no element other than silicon and oxygen in its elemental detection limit. The homogeneous, stable aqueous solutions of the nickel(II) nitrate hexahydrate ([Ni(H2O)6](NO3)2), HNO3, TMOS (as silica source), and two surfactants, CTAB (charged surfactant) and C12E10 (nonionic surfactant) solution is drop-casted on a glass slide to form a mesophase and analyzed by small-angle XRD, ATR-FTIR and POM techniques. The diffraction lines at 1.5 and 1.6o, 2θ, show the formation of ordered lyotropic liquid crystalline mesophases. The mesophases are then calcined at different temperatures (from 250 to 500 °C), to obtain m-NiO/SiO2 powders and characterized by ATR-FTIR, XRD measurements, N2- adsorption-desorption analysis, and SEM-EDX techniques. The XRD patterns show broad lines at small- and wide-angles, indicating the formation of m-NiO/SiO2 at 300 °C, where the pore-walls are made up 2.6 nm crystalline NiO coated amorphous silica . The NiO particle size (on the pore wall) grows with increasing annealing temperature, and at 500 °C, the particle size reaches 7.9 nm. This is also supported by the BET surface area that decreases at higher temperatures. At 300 °C, the BET surface area is 305 m2/g, which drops to 174 m2/g at 500 °C. However, the pore size of m-NiO/SiO2 does not responds to annealing temperature. It means that the pore walls grow in 2D space rather than 3D due to the presence of silica as a hard template. Therefore, combining the hard- and soft-templating methods can efficiently synthesize the crystalline materials with a high surface area. The m-NiO/SiO2 films can be coated over the FTO glass and calcined at different temperatures to fabricate the electrodes for oxygen evolution reaction (OER). During CV measurement, the NiO pore-walls get oxidized to NiOOH and reduced to Ni(OH)2 in the back cycle. Moreover, overpotential that is determined for the OER improves with the usage of the electrode, independent of the electrode thickness.