Influence of alkalizers–capping agents on hydrothermal synthesis, phase content, magnetization and pseudocapacitive behavior of iron oxides

A conceptually new approach is developed for hydrothermal synthesis of nanostructured iron oxides for energy storage in anodes of supercapacitors. The use of biocompatible organic additives facilitates the fabrication of electrodes with enhanced capacitance. The use of amino acids, such as L-histidine (HS), L-lysine (LS) and L-arginine (AR), as capping agents results in the crystallization of antiferromagnetic α-Fe2O3 phases, which show higher capacitance than that of α-Fe2O3 formed without additives using NaOH for pH modification. Surprisingly, the use of meglumine (ML) as an alkalizer–capping agent instead of NaOH results in the fabrication of a ferrimagnetic γ-Fe2O3 phase. The γ-Fe2O3 electrodes prepared using ML show a capacitance of 2.97 F cm−2 from cyclic voltammetry data at 2 mV s−1 and 3.22 F cm−2 from galvanostatic charge–discharge data at 3 mA cm−2 in a potential window of −0.8–0 V in 0.5 M Na2SO4 electrolyte. The high active mass γ-Fe2O3 electrodes, prepared using ML show higher capacitance and lower resistance, compared to the α-Fe2O3 electrodes, prepared using HS, LS and AR. Moreover, the γ-Fe2O3 electrodes prepared using ML exhibit higher capacitance, compared to the literature data for γ-Fe2O3 electrodes, prepared by other methods. The particle size, magnetization and capacitance of the γ-Fe2O3 electrodes can be varied by the variation in synthesis temperature. The obtained γ-Fe2O3 is a promising magnetically ordered pseudocapacitive material for energy storage and other applications based on magnetocapacitive properties. The synthesis method developed in this investigation can be used for the fabrication of other functional nanomaterials.Graphical abstract