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

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Open Access
Acid-salt-surfactant lyotropic liquid crystalline mesophases: synthesis, characterization, and electrochemical properties of mesoporous M2P2O7 and M2-xM’xP2O7 (M and M’= MN(II), CO(II) and NI(II)) powders and films
(2023-10) Ulu, Işıl
The mesoporous metal pyrophosphates (M2P2O7) are considered to be important as energy storage materials. This thesis proposes that a surfactant-assisted approach for the synthesis of the mesoporous M2P2O7 would be a good solution since the high surface area is crucial for energy storage materials. A novel synthesis method for the synthesis of mesoporous metal pyrophosphates (Ni2P2O7, Co2P2O7, Mn2P2O7, and binary metal pyrophosphates) is investigated by using a modified MASA (Molten Salt Assisted Self-Assembly) method using related acid; phosphoric acid (PA, H3PO4) or pyrophosphoric acid (PPA, H4P2O7), salts ([Mn(H2O)4](NO3)2, [Co(H2O)6](NO3)2, [Ni(H2O)6](NO3)2) and surfactant (pluronic P123 (EO20PO70EO20, where EO is ethylene oxide and PO is propylene oxide)). Firstly, homogeneous solutions using a broad range of inorganic ingredients are prepared. These solutions are then coated (spin-coating, drop-cast coating, or dip-coating) over a substrate to form lyotropic liquid crystalline (LLC) mesophases that can be calcined at various temperatures to synthesize the mesoporous metal pyrophosphates. The mesophases are characterized using small-angle XRD, ATR-FTIR, POM, and gravimetric measurements during the water evaporation from the solution phases. Aging the mesophases at room temperature forms an ordered (display diffraction(s) at small angles) mesostructured semi-solid (exhibit some cracks under POM with particle-like morphology) M2HxP2O7(NO3)x∙nH2O materials as a result of a polymerization reaction (followed by ATR-FTIR) between transition metal species and PPA. This process initiates in the solution phase and continues within the mesophase by releasing water and nitrate species and becomes stable in 24 h under ambient conditions. The mesostructured semi-solid M2HxP2O7(NO3)x∙nH2O materials are calcined at 300 oC to produce mesoporous spherical M2P2O7 with surface areas of 60, 111, and 41 m2/g for Ni(II), Co(II), and Mn(II) pyrophosphates, respectively. These mesoporous M2P2O7 materials, calcined at 300 oC and higher temperatures, are further characterized using wide-angle XRD, ATR-FTIR, XPS, SEM-EDX, TEM, N2 adsorption-desorption, and electrochemical characterization techniques. Both Co2P2O7 and Mn2P2O7 are amorphous up to 600 oC, then crystallizing at around 600 oC to their alpha and beta phases, respectively. In contrast, the crystallization temperature of Ni2P2O7 is around 700 oC, and it has mainly alpha and minimal delta phases. Mesoporous NiCoP2O7 and MnCoP2O7 with surface areas of 68 and 70 m2/g, respectively, become crystalline at 600 oC to α-NiCoP2O7 and β-MnCoP2O7 phases, and they form solid-solutions when the mole ratio of the metal species is varied. The clear solutions are spin-coated onto an FTO surface and then calcined to produce FTO-coated electrodes; however, those electrodes are not stable during the electrochemical measurements. Therefore, the diluted solutions from the mother liquor are dip-coated over a pure graphite rod (GR) and subsequently calcined to fabricate electrodes of mesoporous metal pyrophosphates. The GR-electrodes, which remain stable during the measurements, are tested using cyclic voltammetry (CV) and galvanostatic charge-discharge measurements with a 3-electrode system in a 3M KOH electrolyte. It is important to note that the metal pyrophosphates transform to their corresponding hydroxides in an alkaline solution during the electrochemical measurements. As a result, the collected data from the electrochemical measurements originate from the M(OH)2 species rather than M2P2O7. The mesoporous spherical Ni2P2O7 material is converted into a very thin needle-like β-Ni(OH)2 (1.5 nm thick and 7 nm wide) in alkaline media, maintaining its spherical morphology. In contrast, the mesoporous spherical Co2P2O7 and Mn2P2O7 particles transform into much thicker plate-like β-Co(OH)2 and β-Mn(OH)2 particles. The transformation time differs depending on the type of metal; the Co2P2O7 and Mn2P2O7 materials transform rapidly (about 30 sec), whereas the complete transformation of Ni2P2O7 to its hydroxide takes around 1 hour. The transformation time determines the particle size and morphology, consequently influencing the capacitance values. The β-Ni(OH)2 exhibits a high charge capacity and specific capacitance (102 mA.s and 368 mF/cm2 at a current density of 1 mA/cm2). However, these values are nearly 10 times smaller in the β-Mn(OH)2 and β-Co(OH)2 electrodes. The addition of nickel ions to the cobalt system in the preparation of binary metal pyrophosphates enhances the capacity and specific capacitance values, with the sample having β-Ni0.67Co0.33(OH)2 composition displaying the highest capacity value in alkali media (170 mA.s at a current density of 1 mA/cm2). Nevertheless, other binary systems (Mn1-xCox(OH)2 and Ni1-xMnx(OH)2) display almost similar capacity behavior to pure cobalt and manganese systems.
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.
Open Access
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.