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

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Dağ, Ömer
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Bilkent University
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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)42, Co(H2O)62, Ni(H2O)62) 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.

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