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Perovskite oxides have garnered substantial attention in recent years due to their diverse and exceptional properties, making them compelling candidates for various applications, especially in the realm of energy storage technology. This class of materials exhibits a distinctive crystal structure characterized by the general formula ABX3, where A is typically an alkaline earth metal, B is a transition metal, and X is an anion4.
In the context of perovskite oxides, alkaline earth-based titanates, particularly those derived from barium (Ba) and strontium (Sr), have emerged as pivotal contributors to advancements in energy storage technologies. The unique combination of their crystal structure and electrochemical properties makes them promising candidates for applications such as supercapacitor electrodes4,5,6.
The MTiO3 series, which includes elements like Mg, Mn, Ni, and others, is of particular interest due to its exceptional dielectric constant and remarkably high-quality factor. These unique properties in perovskite titanates stem from the arrangement of TiO6 octahedra, which are isolated by MO6 octahedra and cation vacancies. Each layer of MO6 octahedra is situated between two layers of TiO6 octahedra, contributing to these distinctive characteristics3,7,8.
The utilization of MgTiO3 extends across various domains, contingent upon the specific modifiers employed. When MgTiO3 is modified with rare earth metals, its applications encompass a wide range of areas, including light-emitting and photovoltaic applications, plasma and flat panel devices, light-emitting and solid-state diodes, and optical devices, among others13,14. Meanwhile, MgTiO3 modified with transition metal ions can be used in microwave, satellite, and terrestrial communication, including radio software, GPS, and DBSTV for environmental monitoring15.
Perovskite materials at the nanoscale exhibit distinctive features, including extensive porous structures, a significant surface area, regulated transport and charge-carrier mobility, potent absorption, and photoluminescence. Additionally, their unique adaptability in terms of composition, morphology, and functionalities candidate perovskite nanocrystals as highly effective elements for energy applications such as photovoltaics, catalysis, thermoelectrics, batteries, supercapacitors, and hydrogen storage systems19,20,21,22.
The electrochemical performance of supercapacitors depends on electrode materials, electrolytes, and potential windows. Metal oxides are extensively employed in energy storage and conversion applications, mainly due to their cost-effectiveness, abundant availability, ease of preparation, multiple valence states, and environmental friendliness. They find applications in various fields, including sensors23,24,25,26, biosensors27,28, lithium batteries29, supercapacitors30,31,32, electrocatalysis, and fuel cells.
The current work aims to fabricate MgTiO3 modified with Li+ to extend their application in energy storage systems, including lithium-ion batteries and supercapacitors. The production of Li-MgTiO3 as a dielectric nanoceramic material for supercapacitors was achieved via the acetic acid sol–gel method, followed by 3-h calcination at 800 °C to promote crystalline development. This research explores into evaluating the electrical and optical attributes of the resultant Li-MgTiO3 perovskite nano-ceramics, encompassing properties such as impedance, Cole–Cole plot analysis, conductivity, absorbance, and energy band gap.
The electrochemical studies were produced by using impedance and cyclic voltammetry electrochemical techniques. The modified screen-printed electrode exhibited remarkably electrocatalytic properties, proving effective in direct electrochemical applications. Notably, this synthesis approach holds significance for advancing energy storage applications.
This study ensures a comprehensive exploration of the doping mechanisms, contributing valuable insights into the tailored design of titanate-based materials for enhanced energy storage applications.
Initially, the synthesis of MgTiO3 (MT) was carried out using the sol–gel reaction method. All the necessary chemicals were procured from Sigma Aldrich. The procedure commenced by dissolving precise amounts of highly pure magnesium acetate (Mg(CH3COO)2•4H2O) in 15 mL of water and acetic acid with continuous stirring. The required stoichiometric quantities of titanium isopropoxide were dissolved in acetylacetone (CH3COCH2COCH3) and introduced into the previously mentioned solution while maintaining a temperature of 50 °C.
To produce Mg(1-x)LixTiO3 (MTxLi), lithium acetate was dissolved in acetic acid and distilled water and subsequently combined with the MT solution, Fig. 1. This process led to the formation of the desired chemical structure, Mg(1-x)LixTiO3, with varying lithium content (x = 0, 0.01, 0.05, & 0.1 mol.%, Table 1). The combination was stirred by magnetic stirring for 3 h. Afterward, all gel systems were subjected to drying at 200 °C for 8 h. The resulting xerogels were exposed to calcination at 800 °C for 3 h in the air.
Schematic diagram of the synthesis process.
The structural phases of the samples were determined through Rigaku X-ray diffraction (D-max 2500), utilizing monochromatic (Cu Ka) radiation. The settings used were an acceleration voltage of 40 kV and an applied current of 100 mA.
Surface morphology was examined using a scanning electron microscope (SEM)—specifically, the Quanta FEG-250 from the Czech Republic.
UV–visible diffuse reflectance spectroscopy (DRS) assessments were conducted using a Jasco V570 UV–vis NIR spectrophotometer equipped with an integration sphere diffuse reflectance accessory, originating from the USA.
The ac conductivity and the impedance (Z'' and Z") of MT and MTxLi were determined by utilizing the Hioki LCR IM3536. The sample powders were compacted into tablets with a diameter of 13 mm and a thickness defined as d and sintered at 800 °C for 3 h. The measurements were conducted employing the parallel plate capacitor methodology. The measurements were carried out in the frequency range (ν = 4Hz to 8MHz) and temperature range (T = 25 to 120 °C).
The expression for the complex impedance is provided in Eq. (1):
The real (Z'') and imaginary (Z") impedance of the MT and MTxLi samples were determined using Eq. (2) and Eq. (3), respectively33.
Here A represents the electrode surface area, ε0 denotes the permittivity of the free space (8.854 × 10–12 F m−1), ε′(ν) and ε′′(ν) stand for the dielectric constant and dielectric loss of the samples, respectively7,34.
(C and tan δ are the measured capacitance loss tangent factor, respectively).
The ac conductivity is given by
Potassium chloride, potassium ferricyanide and potassium ferrocyanide were brought from sigma Aldrich.
Electrochemical studies were produced using electrochemical workstation CHI –potentiostat and screen-printed electrodes (SPEs). For modified SPEs preparation, 10.0 mg of MT or MTxLi was weight, dispersed (1 ml double distilled water) and sonicated for 30 min. Furthermore, 30 μl of sonicated solution were drop on the SPE surface and dry in air. For CV and EIS measurements a mixture solution of 5 mM of the ferri/ferrocyanide [Fe (CN)6]3−/4− and 0.1 M KCl are used. The following schematic shows the prepared materials SPEs modification method for electrochemical performance measurements (Fig. 2).
schematic diagram illustrates the modification steps of SPEs with the MTxLi and their electrochemical study using a potentiostat.
the XRD chart of MgTiO3 doped with different concentration of Li+ perovskite nano-ceramics.
The appearance of the impurity phase MgTi2O5 can be attributed to the decomposition of MgTiO3, which is likely caused by the volatilization of Mg and oxygen deficiency36,37.
The Rietveld refinement doesn''t show a secondary phase for Lithium oxide, and according to Hume-Rothery criteria, the difference between the ionic radii of Li+ (0.74 Å) and Mg2+ (0.72 Å) is less than 10% therefore, Li+ is best suited to replace Mg2+ in the perovskite structure38.
The XRD chart provides evidence that the incorporation of Li+ ions enhance the crystal structure of the samples. As the Li+ ion concentration increases, the peak intensity of the primary phase, MgTiO3, shows an upward trend. In contrast, the peak intensity of the secondary phase declines, and at elevated Li+ ion concentrations, certain peaks related to the secondary phase disappear entirely. Consequently, Li+ ions emerge as a suitable choice for altering the local crystal structure of MgTiO3 since they function as charge compensators39.
Figure 4a and c displays SEM micrographs of MT and MT10Li nanopowders that have undergone calcination at a temperature of 800 °C for 3 h, at magnification 25X. The micrographs of the Li+-MgTiO3 nanopowders exhibit significant regular formation due to the presence of an interconnected network structure and higher surface energy. Upon calcination at 800 °C, the particles grow to sizes ranging from approximately 38 to 62 nm, Fig. 4b and d, displaying a good-particles distribution. The SEM micrographs distinctly reveal the presence of many nanopores within the perovskite nanoceramics. These findings suggest that the process of calcination at 800 °C leads to increased particle sizes and reduced agglomeration tendencies among the Li+-MgTiO3 nano powders.
SEM micrographs and particle size distribution of MT (a and b) and MT10Li (c and d), respectively.
UV–Vis spectroscopy is dedicated to assessing the light absorption capabilities of a chemical system. Within UV–VIS spectroscopy, molecules absorb incident light, leading to the excitation of electrons from their ground state to a higher energy level40. The energy of the absorbed light matches the energy gap between these ground and higher energy states. The spectrophotometer is used to measure the diffuse reflectance (Rd) of the sample as a function of the wavelength. Using these data, the energy band gap of a semiconductor can be determined41.
Figure 5 presents the diffuse reflectance spectra of the prepared samples at room temperature. The samples demonstrate a high reflectance at the measuring start at about 190 nm and then drops to minimum values at about 275 nm. At wavelength 275 nm, the diffuse reflectance shows a sharp increase with increasing of wavelength and reaches a maximum value at about 430 nm for all samples. A slight decrease is observed with increasing of wavelength until the end of measuring range. Moreover, the presence of interference peaks becomes evident at higher wavelengths.
The diffuse reflectance spectra of the prepared samples at room temperature.
The decrease in diffuse reflectance and the absence of fringes at shorter wavelengths signify the fundamental absorption of the films. The addition of Li+ causes a blue shift in the absorption edge, indicating an increase in oxygen content and highlighting the influence of oxygen on the optical properties42.
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