NiFe₂O₄ nanospheres with size-tunable magnetic and electrochemical properties for superior supercapacitor electrode performance

Highlights

• Ferric ions induced size reduction of NiFe₂O₄ nanospheres.

• Size depended variation in saturation magnetization from 45 to 29 emu/g.

• Fabrication and performance analysis of asymmetric solid-state cell.

• Electrode exhibited a higher energy density of 22.5 Wh/ Kg.

• Cyclic stability of the cell showed capacity retention of 126 %.

Abstract

Hereby, we present our efforts to understand the influence of ferric ions in the preparation of NiFe₂O₄ magnetic nanoparticles (MNPs) by facile chemical oxidation method. The variation in the Ferric ions content provoked size tunability of the MNPs (particle size reduced with increasing ferric ions content). The effect of ferric ions content on the prepared MNPs were analyzed for phase, morphological and magnetic characteristics by using X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Vibrating Sample Magnetometer (VSM) and Thermomagnetic Analysis (TMA) measurements. The saturation magnetization and coercivity values were observed to be 45 emu/g and 90 Oe for the nanoparticles prepared without ferric ions. The size depended reduction in magnetization and coercivity values were evident for increased ferric ion concentration. The curie transition temperature was observed to be 584 °C from TMA analysis. The cyclic voltammogram, galvanostatic charge/discharge and electrochemical impedance measurements were carried out to evaluate the electrochemical characteristics of prepared NiFe₂O₄ nanoparticles for supercapacitor application. The highest specific capacitance of 277 F/g was observed for the NiFe₂O₄ nanoparticles that prepared with 50 % of ferric ions (NF50). From the cyclic stability study, 101 % of capacitance retention was demonstrated up to 5000 cycles. An asymmetric two-electrode cell that fabricated with NF50 exhibited a maximum specific capacitance of 56 F/g (with an applied current density of 1 A/g) and higher energy density of 22.5 Wh/ Kg (with a power density of 0.85 kW/Kg). The cyclic stability study of the cell revealed the increased capacity retention (126 % up to 5000 cycles) due to the particle size reduction. The observed enhancements in the magnetic and electrochemical properties with the size reduction are discussed in-detail. The facile strategy presented by this work would give insights for high-performance supercapacitor electrode material.

Introduction

Magnetic field assisted capacitance enhancement results in superior specific capacitance by virtue of compositional and morphological improvements in the electrode materials [1,2]. Another important factor to be considered for the magnetic field assisted applications is the particle size tunability to achieve favorable magnetic characteristics of the nanoparticles (NPs) [3,4]. Among the various nanostructures, spinel ferrites have attracted much attention for the supercapacitor (SC) applications due to their various oxidation states within the structure, greater electrochemical stability and higher theoretical capacitance [5]. Within the different spinel ferrites, NiFe₂O₄ with inverse spinel structure has not been explored much for electrochemical applications due to the difficulties in obtaining the NPs without agglomeration [6,7]. Previous reports suggest the better electronic conductivity in bi-metallic or multi-metallic oxides with mixed valence states than the mono-metallic oxides [8]. In the recent past, the global research fraternity actively engaged in noticeable research activity to overcome the electronic conductivity and other issues in hybrid materials of carbon along with NiFe₂O₄ [9], [10], [11], [12], [13], [14]. Various attempts have been done to improve the storage capacity and the cyclic stability [15,16]. Perhaps, the viable improvement of electrochemical characteristics of NiFe₂O₄ alone is still an open research theme. Earlier reports on NiFe₂O₄ NPs that obtained from the different experimental methods suggested the morphological dependence of the electrochemical capacitance behavior [17].

Gao et al., insisted the enhanced electrochemical characteristics and observed higher specific capacitance of 240.9 F/g at 1 A/g of current density for NiFe₂O₄ nanosheets when compared to the spherical NiFe₂O₄ nanoparticles [18]. As the morphological parameters are vital for a supercapacitor electrode material, exploration of various methodologies to obtain definite morphology with enhanced characteristics is inevitable. Singh and Kant, discussed the theoretical aspects of morphological dependence and the impact of pores for the charge storage applications [19]. The size dependent capacitance behavior of NiO NPs were demonstrated by preparing different sized NiO nanoparticles by cathodic contact glow discharge electrolysis process, in which the 70 nm NiO NPs showed the higher capacitance of 218 F/g and the 107 nm NiO NP showed the capacitance of 63 F/g, which indicates the particle sizes influence on the capacitance [20]. Hematite NPs have been developed by hydrothermal synthesis and their size depended electrochemical characteristics were studied by cyclic voltammetry and galvanostatic charge/discharge measurements. For the same current density, the bigger hematite particles exhibited lower specific capacitance of 170 F/g with less discharge time, whereas the smaller hematite particles demonstrated higher specific capacitance of 340.5 F/g [21]. Tiwari et al., [22] reported about the Cd and Sr ions substituted nickel nano ferrites and proposed nano ferrites were found to be sensitive to the ionic radii of substituent ions. Shan et al., discussed about the pH depended preferential growth of NiFe₂O₄ nanostructures (spheres, rods and octahedrons) using hydrothermal method at 160 °C [23]. The earlier reports emphasized the influence of the particles size on the electrochemical characteristics that suited for high-performance SC applications. The selection synthesis methods for the development of size and morphology controllable MNPs are important. The experimental parameters and fraction of precursors could be adjusted in chemical oxidation method which is modified chemical oxidation process. Modified chemical oxidation process has been utilized here for the tuning of MNPs size by adjusting the ferric ion concentration. Though plenty of studies reported the size dependent properties of the various MNPs [24,25], the size dependent enhanced electrochemical characteristics of NiFe₂O₄ needs to be addressed further.

By this work, we report the importance of the utilization of cost-effective ferric ions content for improved crystallinity in the NiFe₂O₄ NPs that obtained by facile chemical oxidation method. We discussed the size dependent micro-structural, magnetic, and electrochemical characteristics of NiFe₂O₄ nanospheres. Based on the superior properties, NiFe₂O₄ NPs prepared with 50% of ferric ions were endorsed with galvanostatic charge/discharge experiments suggested the suitability as superior electrode material for high-performance SC application.