By
Assel Amirzhanova Katırcı
Advisor: Ömer Dağ
Bilkent University, Department of Chemistry
Date: 17.01.2025
Time: 13:00
Place: Chemistry Meeting Room (SB Building)
All are cordially invited to attend.
Abstract:
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)42 and Ni(H2O)62 (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)62 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.