Boosted electrochemical performance of magnetic caterpillar-like Mg0.5Ni0.5Fe2O4 nanospinels as a novel pseudocapacitive electrode material

Ni-incorporated MgFe2O4 (Mg0.5Ni0.5Fe2O4) porous nanofibers were synthesized using the sol–gel electrospinning method. The optical bandgap, magnetic parameters, and electrochemical capacitive behaviors of the prepared sample were compared with pristine electrospun MgFe2O4 and NiFe2O4 based on structural and morphological properties. XRD analysis affirmed the cubic spinel structure of samples and their crystallite size is evaluated to be less than 25 nm using the Williamson–Hall equation. FESEM images demonstrated interesting nanobelts, nanotubes, and caterpillar-like fibers for electrospun MgFe2O4, NiFe2O4, and Mg0.5Ni0.5Fe2O4, respectively. Diffuse reflectance spectroscopy revealed that Mg0.5Ni0.5Fe2O4 porous nanofibers possess the band gap (1.85 eV) between the calculated value for MgFe2O4 nanobelts and NiFe2O4 nanotubes due to alloying effects. The VSM analysis revealed that the saturation magnetization and coercivity of MgFe2O4 nanobelts were enhanced by Ni2+ incorporation. The electrochemical properties of samples coated on nickel foam (NF) were tested by CV, GCD, and EIS analysis in a 3 M KOH electrolyte. The Mg0.5Ni0.5Fe2O4@Ni electrode disclosed the highest specific capacitance of 647 F g−1 at 1 A g−1 owing to the synergistic effects of multiple valence states, exceptional porous morphology, and lowest charge transfer resistance. The Mg0.5Ni0.5Fe2O4 porous fibers showed superior capacitance retention of 91% after 3000 cycles at 10 A g−1 and notable Coulombic efficiency of 97%. Moreover, the Mg0.5Ni0.5Fe2O4//Activated carbon asymmetric supercapacitor divulged a good energy density of 83 W h Kg−1 at a power density of 700 W Kg−1.


Scientific Reports
| (2023) 13:7822 | https://doi.org/10.1038/s41598-023-35014-w www.nature.com/scientificreports/ Material characterizations. Structural properties of prepared nanomaterials were investigated through X-ray diffraction analysis using the X'Pert Pro Philips device. The chemical bonds of nanofibers were studied by Fourier transform infrared analysis using the Alpha-Bruker device. The MIRA3TESCAN-XMU instrument was used to perform field emission scanning electron microscopy and energy dispersive spectroscopy to investigate nanofibers' morphological characteristics and elemental composition, respectively. N 2 adsorption/ desorption isotherms at 77 K were used to evaluate the textural properties of the samples using a BELSORPmini II instrument. Barrett-Joyner-Halenda (BJH) and Brunauer-Emmett-Teller (BET) techniques were used to measure the pore size distribution and surface area of the samples, respectively. The optical features of samples were explored using the Scinco-S4100 device. Magnetic measurements were done using MDK (Magnetic Daghigh Kavir Co., Iran) device. A three-electrode system, consisting of a prepared electrode, Ag/AgCl, and platinum wire as working, reference, and counter electrodes in a 3 M KOH electrolyte, was used to test the electrochemical efficiency of the synthesized specimens. Using a ZAHNER-ZENNIUM device, cyclic Voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 0.01-10 5 Hz at an AC amplitude of 10 mV. The Zview software was used to fit the EIS plot.

Results and discussion
Structural characterization. The 22,31 . No additional peaks related to any secondary phase were detected in the XRD pattern of synthesized nanofibers, indicating the high-purity phase in samples. On the XRD patterns, Rietveld fitting is conducted using MAUD software to further check the phase purity of the samples. The fitted XRD patterns are shown in Fig. 2d-f. The goodness of fit, Sig, evaluated the fitting quality of the experimental data. The Sig-goodness of those fittings was close to 1 which confirmed that the XRD patterns are compatible with a cubic spinel structure with space group symmetry Fd3m. Furthermore, the Ni 2+ incorporation in magnesium ferrite not only did not disturb the spinel structure but also the sharp diffraction peaks of the Mg 0. 5 21 . The broadening of the diffraction peak can be assigned to the small crystallite size of the sample, strain, and instrumental factors 32 . The average crystallite size (D) and lattice strain (ε) of prepared samples are obtained from the intercept and slope of the linear fit of the Williamson-Hall (W-H) plot (Fig. 3), which βcosθ was plotted    where λ is the x-ray wavelength, β is the width of the peak in radians, θ is the Bragg angle, and K is a constant which is considered 0.94. The W-H plot revealed a compressive strain for the MFO sample due to lattice shrinkage.
FTIR study. Figure 4 represents the Fourier transform infrared spectra of prepared nanofibers, revealing valuable information about the different functional groups. In general, the spinel ferrite structure consists of two sub-lattices in which divalent ions (Ni 2+ and Mg 2+ ) occupy octahedral B-sites and trivalent ions (Fe 3+ ) are equally distributed among tetrahedral A-sites and octahedral B-sites 34  . In the MNFO sample, the metal-oxygen absorption bands slightly shifted with the Ni 2+ incorporation. This may be due to Mg, Ni, and Fe cations redistribution on both sites. The broad band around 3419 and the less intense band around 1637 cm −1 indicated the characteristic vibrational modes of O-H groups and vibrations of the absorbed water molecules, respectively 36 . The bands observed at 2859 and 2937 cm −1 disclosed the asymmetric and symmetric stretching vibrations of methylene (-CH 2 ) groups, respectively 12 . The band at 1380 cm −1 was ascribed to the carboxylate group 37 . The bands in the range of 1000 to 1250 cm −1 correspond to nitrate ion traces as well as C-O bonding 22,38 . After calcination, they become weaker. In other terms, the collapse of the as-spun polymeric precursor fibers during the calcination led to nanobelts morphology 40 . The NiFe 2 O 4 depicts short and broken hollow-interior uniform nanotubes ( Fig. 5d-f). It has been reported that fast solvent evaporation rate and phase separation during electrospinning tend to the production of hollow nanofibers after calcination 41 . As clearly observed in Fig. 5g-i, the Ni incorporation in MgFe 2 O 4 causes the formation of roughly porous caterpillar-like nanofibers with numerous grain boundaries. The formation of (1) β hkl cos θ hkl = K D + 4ε sin θ hkl www.nature.com/scientificreports/ the rough surface is attributed to the properties of precursor solution, which will be valuable for electrochemical applications where the surface area has a huge impact on determining their performance 41,42 . Figure 6 showed the EDS spectra of prepared samples, which revealed the presence of desired elements such as magnesium (Mg), nickel (Ni), iron (Fe), and oxygen (O) and confirmed the chemical purity of samples 43 . Au peak was normally detected at ~ 2 keV due to the coating of Au thin layer over prepared samples to reduce charging influence 44 .   Fig. 7. The isotherm exhibited a typical type-IV behavior with an H3-type hysteresis loop, indicating the presence of mesoporous structure 45 . The formation of mesoporous structure is due to the removal of PVP polymeric matrix after calcination at temperature of 600 °C. The BET specific surface area and average pore diameter of the samples are listed in Optical analysis. In order to investigate the optical properties of prepared nanofibers, the UV-vis-DRS spectra were recorded in the region of 300-900 nm (Fig. 8a). Kubelka-Munk function ( F(R) = (1−R) 2 2R ) of each sample was utilized to calculate the optical band gap using Tauc equation (Eq. 2) 47 where α, h, ν , and A represent the material's absorption coefficient proportional to F(R), Planck's constant, the light frequency, and the constant parameters-containing characteristics of the bands, respectively. Also, n = 1/2 is considered to determine the direct optical band gap (E g ) of MgFe 2 O 4 and NiFe 2 O 4 , and the plot of (αhν) 2 versus hν is shown in Fig. 8b. It is known that the structural parameters, crystallite size, and impurities are potential factors that affect the band gap value 44 . The band gap of MFO nanobelts, NFO nanotubes, and MNFO caterpillar-like nanofibers was found to be 1.90, 1.80, and 1.85 eV, respectively. It is seen that the band gap of MNFO nanofibers is narrower than the MFO nanobelts sample because adding nickel in the MNFO preparation process induces inner states in the band gap, providing additional levels between the conduction and the valence bands 17,44 . The calculated band gap value of MNFO nanocomposite lies between the values obtained for the band gap of magnesium ferrite and nickel ferrite due to alloying effect and indicated suitable substitution of Ni ions on Mg sites in MgFe 2 O 4 . The band gap narrowing of magnesium ferrite nanoparticles by Ni substitution was reported by other researchers 11,48 .    49,50 .
The reduction of M s relatively to bulk can be attributed to the decreased particle size (enhanced surface/volume ratio) and spin canting at the surface of nanoparticles 49,50 . It is known that the crystallinity, surface imperfection, chemical composition, and cation distribution variation on octahedral and tetrahedral sites have a huge impact on the saturation magnetization of spinel ferrite nanostructures 9,13 Table 2). Moreover, the coercive field, H c , was increased from 0.40 for the MgFe 2 O 4 nanotubes to 3.26 Oe for the MNFO sample. The increase of the surface anisotropy of small crystallites contributed to the enhancement of coercivity 51,53 . This sort of increase in saturation magnetization for nickel-substituted magnesium ferrite has also been reported earlier 54,55 . Electrochemical measurements. The electrochemical performance of MgFe 2 O 4 , NiFe 2 O 4 , and Mg 0.5 Ni 0.5 Fe 2 O 4 electrodes was tested using a three-electrode system in 3 M KOH. The Cyclic Voltammetry (CV) of prepared samples at various scan rates of 10, 30, 50, and 80 mV s −1 with a potential window of 0-0.5 V is shown in Fig. 10a-c. The CV profiles with distinct anodic and cathodic redox peaks demonstrate the supercapacitive nature of prepared nanomaterials. As it is known, the enhancement of ion-electrode interaction (diffusionreaction at the electrolyte and electrode interface) leads to excellent capacitive behavior 56 . The energy storage mechanism of prepared samples is suggested by the following reactions 57,58 :   www.nature.com/scientificreports/ The shape of the cyclic voltammogram of samples remains unchanged as the sweep rate increases from 10 to 80 mV s −1 , revealing excellent electrochemical reversibility and prominent high-rate performance. However, the shifts of redox peaks towards lower/higher potentials may be attributed to the polarization effect. The specific capacitance from the CV profile was calculated according to the following equation: where C sp , ∫ IdV , m, ν , and ∆V denote the specific capacitance (F g −1 ), the integrated area under the CV plot, the mass of active material (g), the scan rate (V s −1 ), and the potential window (V), respectively 59 . The specific capacitance values of MgFe 2 O 4 , NiFe 2 O 4 , and Mg 0.5 Ni 0.5 Fe 2 O 4 at the scan rate of 10 mV s −1 were 98, 385, and 965 F g −1 , respectively, as shown in Fig. 10d. The specific capacitance is significantly influenced by the scan rate enhancement. The calculated data of C sp exhibited a higher specific capacitance at lower scan rates. This implies that the electrolyte ions had sufficient time to penetrate and access all the inner microstructures of the electrode material for charge storage 60 .
The galvanostatic charge-discharge (GCD) profiles of prepared nanomaterials were recorded at a current density of 1, 3, 5, 7, and 10 A g −1 demonstrated in Fig. 11a-c. To prevent an oxygen evolution reaction (OER) at a higher potential during the charging process in an aqueous electrolyte, the GCD test voltage was set in the range of 0-0.4 V. The specific capacitance was calculated from the GCD profile using the following equation: where C sp , I, ∆t, m, and ∆V denote the current (A), the time of a full discharge (s), the mass of the active material (g), and the potential window (V) 59 . The specific capacitance of prepared samples at various current densities of 1, 3, 5, 7, and 10 A g −1 is exhibited in Fig. 11d. The specific capacitance values of MgFe 2 O 4 , NiFe 2 O 4 , and Mg 0.5 Ni 0.5 Fe 2 O 4 at the current density of 1 A g −1 were obtained 97, 240, and 647 F g −1 , respectively, which declined to 75, 150, and 325 F g −1 at the current density of 10 A g −1 due to reduced accessibility of active sites in high diffusion rate.  www.nature.com/scientificreports/ ferrite has also been reported in the literature and some of them are listed in Table 3, indicating that obtained specific capacitance values are higher than those reported earlier by some authors. Long-term cycling stability as a criterion is also studied (see Fig. 12a). The prepared electrodes of MFO, NFO, and MNFO samples exhibited capacitance retention of about 81, 87, and 91% after 3000 cycles at 10 A g −1 , respectively. The Coulombic efficiency is also estimated according to the below equation: where η, t D , and t C represent the Coulombic efficiency, charge time (s), and discharge time (s), respectively 73 . The Coulombic efficiency of prepared samples at various current densities of 1, 3, 5, 7, and 10 A g −1 is exhibited in Fig. 12b. The MgFe 2 O 4 nanobelts, NiFe 2 O 4 nanotubes, and Mg 0.5 Ni 0.5 Fe 2 O 4 nanofibers demonstrated high Coulombic efficiency of 92, 95, and 97%, respectively, at the current density of 10 A g −1 . Herein, the superior Coulombic efficiency of MNFO may be attributed to the appropriate formation of 1D surface area and designed architecture with high surface area, which cause the unique reversibility of the charge-discharge process of the prepared sample 46,74 .
The electrochemical impedance spectroscopy (EIS) measurements of prepared nanomaterials were carried out in the frequency range of 0.01 Hz to 100 kHz.
The Nyquist plot and the equivalent circuit diagram consist of charge transfer resistance (R ct ), solution resistance (sum of electrolyte ionic resistance, electrode-to-current collector contact resistance, and electrode material intrinsic resistance; R s ), Warburg resistance (Z w ), and the constant phase element (CPE) are displayed in Fig. 12c. The intercept and semicircle diameter on the real axis in the Nyquist plot denotes the solution resistance and the Faradaic charge transfer resistance, respectively 46,61 . The R S value of electrodes is very low, allowing the electrolyte to access the electrodes surface efficiently 75 . The R ct of MFO, NFO, and MNFO was calculated to be 6.64, 4.79, and 3.16 Ω, respectively. The lowest R ct value of nickel-substituted magnesium ferrite revealed that Ni incorporation in magnesium ferrite structure facilitates the charge transfer efficiency at the interface of electrode and electrolyte, affirming the higher specific capacitance of this electrode 46,75 . Also, steeper slopes of the plot in the low-frequency regions imply that the Mg 0.5 Ni 0.5 Fe 2 O 4 sample has lower Warburg resistance (greater ionic conductivity) than other samples. Therefore, nickel incorporation in MgFe 2 O 4 provided better ionic and electronic conductivity in the Mg 0.5 Ni 0.5 Fe 2 O 4 sample.
Further exploration is done to study the accurate potential of MNFO sample in real applications, assembling two-electrode cell utilizing MNFO and Active Carbon (AC) as positive and negative electrodes, respectively, in a 3 M KOH electrolyte. At first, the electrochemical properties of AC electrode were investigated by the standard three-electrode system. Figure 13a demonstrates the CV curves of individual AC and MNFO electrodes at a scan rate of 30 mV s −1 and complementary potentials within − 1 to 0 V and 0-0.5 V, respectively. As shown in Fig. 13b, the stable potential windows of MNFO//AC asymmetric supercapacitor (ASC) are capable of being extended to 1.5 V, displaying cyclic voltammograms along with weak redox peaks with no polarization. As the scanning rate increases, the enlargement of the CV curves occurs, manifesting the suitable rate performance of the cell. The specific capacitance of MNFO//AC is 306, 206, 150, 126, 100, and 86 F g −1 at 1, 2, 3, 4, 5, and 7 A g −1 obtained from GCD results (Fig. 13c). The energy density (E) and power density (P) of MNFO//AC were calculated using Eqs. (9) and (10) 17 , as the Ragone plot is demonstrated in Fig. 13d.

Conclusion
In this work, an attempt has been made to achieve the superior electrocapacitive performance from the novel and well-designed ternary Mg 0.5 Ni 0.5 Fe 2 O 4 spinel ferrite nanofibers compared to pure MgFe 2 O 4 nanobelts and NiFe 2 O 4 nanotubes prepared by electrospinning technique. The XRD, FTIR, FESEM, EDS, DRS, and VSM studies are also done to show the maximum functionality of samples. XRD and FTIR results showed the well-crystallized cubic spinel phase and metal-oxygen bonds of the samples on the octahedral and tetrahedral sites, respectively. The optical band gap of Mg 0.5 Ni 0.5 Fe 2 O 4 was narrower than MgFe 2 O 4 nanobelts. The enhancement of saturation magnetization and coercivity of MgFe 2 O 4 nanobelts via Ni 2+ ions substation was confirmed using the VSM test. The electrochemical study revealed that although the specific capacitance obtained for the pristine magnesium ferrite nanobelts was small, the incorporation of nickel into its structure caused the formation of a novel ternary ferrite with a significant capacitance. The highest specific capacitance of 647 F g −1 for Mg 0.5 Ni 0.5 Fe 2 O 4 with outstanding cycling stability of 91% after 3000 cycles at 10 A g −1 was achieved which is far greater than pristine MgFe 2 O 4 and NiFe 2 O 4 . Furthermore, the Mg 0.5 Ni 0.5 Fe 2 O 4 //Activated carbon asymmetric supercapacitor cell could be cycled reversibly in the high-voltage range of 0 to 1.5 V and divulged intriguing performances with an energy density of 83 W h Kg −1 at a power density of 700 W Kg −1 . Mg 0.5 Ni 0.5 Fe 2 O 4 electrode with safe and suitable electrochemical performance is promising for practical application in energy storage devices and might play an important role in renewable energy, potentially reducing pollution and decreasing the consumption of hydrocarbon fuels. We hope that this work can open up new possibilities for exploring novel ternary ferrite spinels as electrode materials for application in the energy storage field.

Data availability
All data generated or analyzed during this study are included in this published article, and the datasets used and analyzed during the current study are available from the corresponding author upon reasonable request.