Abstract
The paper presents a method for obtaining electrochemically active ultrafine composites of iron oxides, superparamagnetic ‘core/shell’ γ-Fe2O3/defective α-Fe2O3, which involved modifying sol-gel citrate synthesis, hydrothermal treatment of the formed sol, and subsequent annealing of materials in the air. The synthesized materials’ phase composition, magnetic microstructure, and structural, morphological characteristics have been determined via X-ray analysis, Mossbauer spectroscopy, scanning electron microscopy (SEM), and adsorption porometry. The mechanisms of phase stability were analyzed, and the model was suggested as FeOOH → γ-Fe2O3 → α-Fe2O3. It was found that the presence of chelating agents in hydrothermal synthesis encapsulated the nucleus of the new phase in the reactor and interfered with the direct processes of recrystallization of the structure with the subsequent formation of the α-Fe2O3 crystalline phase. Additionally, the conductive properties of the synthesized materials were determined by impedance spectroscopy. The electrochemical activity of the synthesized materials was evaluated by the method of cyclic voltammetry using a three-electrode cell in a 3.5 M aqueous solution of KOH. For the ultrafine superparamagnetic ‘core/shell’ γ-Fe2O3/defective α-Fe2O composite with defective hematite structure and the presence of ultra-dispersed maghemite with particles in the superparamagnetic state was fixed increased electrochemical activity, and specific discharge capacity of the material is 177 F/g with a Coulomb efficiency of 85%. The prototypes of hybrid supercapacitor with work electrodes based on ultrafine composites superparamagnetic ‘core/shell’ γ-Fe2O3/defective α-Fe2O3 have a specific discharge capacity of 124 F/g with a Coulomb efficiency of 93% for current 10 mA.
Highlights
Iron oxide compounds continue to be one of the most important transition metal oxides of technological importance
It is known that the hydrothermal treatment of solutions of iron nitrate in the temperature range 130–250 ◦C initiates the processes of high-temperature hydrolysis, which leads to the formation of crystalline hematite [17]
Weakly crystalline particles of γ-Fe2O3 in the magnetically ordered state are transformed into the structure of hematite
Summary
Iron oxide compounds continue to be one of the most important transition metal oxides of technological importance. The effective use of the material, especially of ultrafine iron compounds, is determined by an adapted set of structural and morphological characteristics, phase composition, and physicochemical properties, the control of which should be carried out at the synthesis stage [10,11,12,13]. There are such parameters as temperature and pH of the reaction medium, concentration of ions in solution, nature of salt anion and cation of an alkaline agent, presence of uncontrolled or specially introduced impurities in solution, and reagent feed rate that are typical of synthesized by all known ‘wet soft chemical methods’. The question of the formation of metastable structures is open, especially in post-processing
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