Incremental substitution of Ni with Mn in NiFe2O4 to largely enhance its supercapacitance properties

By using a facile hydrothermal method, we synthesized Ni1−xMnxFe2O4 nanoparticles as supercapacitor electrode materials and studied how the incremental substitution of Ni with Mn would affect their structural, electronic, and electrochemical properties. X-ray diffractometry confirmed the single-phase spinel structure of the nanoparticles. Raman spectroscopy showed the conversion of the inverse structure of NiFe2O4 to the almost normal structure of MnFe2O4. Field-emission scanning electron microscopy showed the spherical shape of the obtained nanoparticles with a size in the range of 20–30 nm. Optical bandgaps were found to decrease as the content of Mn increased. Electrochemical characterizations of the samples indicated the excellent performance and the desirable cycling stability of the prepared nanoparticles for supercapacitors. In particular, the specific capacitance of the prepared Ni1−xMnxFe2O4 nanoparticles was found to increase as the content of Mn increased, reaching the highest specific capacitance of 1,221 F/g for MnFe2O4 nanoparticles at the current density of 0.5 A/g with the corresponding power density of 473.96 W/kg and the energy density of 88.16 Wh/kg. We also demonstrated the real-world application of the prepared MnFe2O4 nanoparticles. We performed also a DFT study to verify the changes in the geometrical and electronic properties that could affect the electrochemical performance.

as supercapacitor electrodes. However, mixed ternary-transition-metal ferrites deserve more investigation to enter the commercial real-world applications.
Here, we carried out a systematic study to see how the incremental substitution of Ni with Mn in hydrothermally synthesized Ni 1−x Mn x Fe 2 O 4 nanoparticles will affect their structural, electronic, and electrochemical properties. Finally, we performed a density functional theory study on the same structures to confirm the changes in their geometrical, electronic, and electrochemical properties.

Experimental methods
Synthesis procedure. All chemicals, including Mn(NO 3 ) 2 .4H 2 O, Fe(NO 3 ) 3 .9H 2 O, Ni(NO 3 ) 2 .6H 2 O, and cetyltrimethylammonium bromide (CTAB), were purchased from Merck Co. (> 98%) and used without any further purification. The nanoparticle powders of Ni 1−x Mn x Fe 2 O 4 (x = 0.0, 0.2, 0.4, 0.6, 0.8, and 1) were prepared using a hydrothermal method, similar to our recent work on supercapacitance properties of Ni 1−x Co x Fe 2 O 4 nanoparticles 26 . First, stoichiometric amounts of the materials, listed in Table 1, were dissolved in 40 ml deionized (DI) water by stirring for about 1 h. Next, 1 ml of 25% ammonia solution was added into the prepared nitrate mixture under vigorous stirring until its pH reaches ~ 9. Next, we transferred the obtained mixture into a Teflon-lined stainless autoclave that subsequently heated in an oven at 180 °C for 15 h. Then, we allowed the autoclave to cool in ambient air to room temperature. Finally, we washed the obtained product with DI water and ethanol for several times and dried it in an oven at 70 °C for 3 h. The prepared powders were used for further characterizations. A schematic of the various steps followed in our synthesis procedure is shown in Fig. 1.
Characterization techniques. X-ray diffractometry (XRD) was used to investigate the crystal structures of the prepared materials by employing a PANalytical X'pert MPD (Philips, Cu-K α radiation source, λ = 1.54056 Å). Raman spectroscopy equipped with an Nd:YAG laser working at λ ex = 532 nm at room temperature was employed to characterize the structure of the materials. Field-emission scanning electron microscopy (FESEM) was employed to observe the morphologies of the prepared materials by employing a MIRA3TESCAN-XMU Electrochemical measurements. A three-electrode electrochemical setup was used to study the electrochemical properties of the materials by utilizing a VSP-300 Multichannel Potentiostat/Galvanostat/EIS instrument (Bio-Logic Science Instruments). The electrolyte was 3 M KOH solution, the reference was an Ag/AgCl electrode, the counter electrode was a square-shaped Platinum sheet (1 cm 2 , 99.99%), and the substrate for the working electrodes was a square-shaped nickel foam (1 × 1 cm 2 ) washed with ethanol, acetone, and DI water. The working electrode material was prepared from the active material, acetylene black, and polyvinylidene difluoride (PVDF), as a binder, with the weight ratio of 80:15:5 solved in N-Methyl-2-pyrrolidone (NMP). The prepared material was coated on a nickel foam substrate by a brush. Cyclic voltammetry (CV) tests were performed at different scan rates in the potential range of 0-0.4 V. The chronopotentiometry galvanostatic charge-discharge (GCD) tests were performed at different current densities. An asymmetric supercapacitor in a two-electrode setup was also assembled using the MnFe 2 O 4 nanoparticles as the positive electrode, activated carbon (AC) as the negative electrode, and a filter paper wetted with the electrolyte as the separator to demonstrate the realworld application of the prepared materials. To prepare the AC electrode, a mixture of AC powder and PVDF, as a binder, was mixed with the weight ratio of 90:10 and solved in NMP. The prepared ink was coated on a nickel foam substrate by a brush and dried at 60 °C for 10 h.
Computational methods. First-principles calculations were performed in the framework of density functional theory (DFT), as implemented in the Quantum Espresso package (version 6.2) 34 , using the plane-wave basis set and ultrasoft pseudopotentials 35 and the valence electrons included Ni 3d 4 s, Mn 3d 4 s, Fe 3d 4 s, and O 2 s 2p states. Spin polarization was included in both geometry optimizations and electronic structure calculations. The generalized gradient approximation (GGA) developed by Perdew, Burke, and Ernzerhof (PBE) 36 was applied for electron exchange-correlation functionals with the on-site Coulomb repulsion U terms 37 of U(Ni) = 6.2 eV, U(Mn) = 3.9 eV, and U(Fe) = 5.3 eV to reproduce experimental data. The kinetic energy cutoffs for wavefunctions and charge densities were set to 50 and 450 Ry, respectively, and the k-point grid of 6 × 6 × 5 was adopted for sampling the first Brillouin zone (BZ) for electronic structure calculations. All structures were fully relaxed until the convergence criteria of energy and force became less than 10 -6 Ry and 10 -3 Ry/Bohr, respectively. All crystal images and simulated XRD patterns were produced by VESTA 38 . where θ hkl and β khl are the diffraction angle and the full width at half maximum (FWHM) of the (hkl) diffraction peak, k is the shape factor (here, 0.94), λ is the wavelength of the X-ray radiation source (1.54056 Å), and D and ε correspond to the crystallite size and the lattice strain, respectively. Accordingly, D and ε were obtained  Table 2. The lattice parameter a, also reported in Table 2, was calculated using Eq. (2) for the most intense peak 41 :

Results and discussions
where d hkl is the inter-planar spacing for the most intense peak (311). It is seen that the lattice parameter a increases as the content of Mn +2 ions increases, which could be attributed to the larger ionic radius of Mn +2 (0.8 Å) than Ni +2 (0.69 Å). It is also seen that MnFe 2 O 4 has both a larger crystallite size and a higher compressive strain than NiFe 2 O 4 , which is because of the stronger bonds that Mn +2 ions can form, as confirmed by Raman spectra (Fig. 3). Raman spectra of the Ni 1−x Mn x Fe 2 O 4 nanoparticles are shown in Fig. 3. The A 1g band is due to the symmetric stretching of oxygen atoms along Fe-O (or M-O) tetrahedral bonds, the F 2g (1) band is due to the translatory movement of the whole tetrahedron (FeO 4 ), the F 2g (2) band is due to the asymmetric stretching of Fe/M-O bonds, and the E g band is due to the asymmetric and symmetric bending of O with respect to Fe 42 . The inverse spinel structure of the samples is confirmed by the Raman analysis 43 . As it is seen in Fig. 3 and Table 2, the A 1g peak shifts toward a lower frequency (from 687 to 630 1/cm) and the intensity of the F 2g (2) peak decreases as Ni +2 ions are substituted with Mn +2 ions, consistent with differences between the Raman spectra of inverse and normal spinel structures 44 . This is due to the smaller ionic radius of Ni +2 than Mn +2 . As a result, when Ni +2 ions are substituted with Mn +2 ions the lengths of bonds between the cations with the host atoms increase, leading to stronger interactions between atoms 45 Figure 4 shows the optical absorption spectra of the prepared Ni 1−x Mn x Fe 2 O 4 nanoparticles. It is seen that as the content of Mn +2 increases the absorption edge undergoes a redshift and the absorbance is enhanced in both visible and near-infrared regions. These absorption spectra are consistent with those of previous reports on crystalline Mn Fig. 7. The images exhibit highly agglomerated and spherical nanoparticles with a small grain size thanks to the use of CTAB 26 . It is seen that the average grain size increases when Ni +2 ions are substituted with Mn +2 ions, which is due to the larger ionic radius of Mn +2 ions than Ni +2 ions.
Cyclic voltammetry (CV) tests were carried out within the potential range of 0.0-0.5 V for three-electrode tests with scan rates varying from 5 to 100 mV/s, as shown in Fig. 8. The CV curves display faradic currents,  www.nature.com/scientificreports/ which are generated by the reduction or oxidation of some chemical active materials at the electrode. Accordingly, there are two peaks in the CV curves: (1) the oxidation peak in positive currents and (2) the reduction peak in negative currents. The oxidation and reduction peaks shift to higher and lower potentials as the scan rate increases. It is well known that the area within a CV curve is directly proportional to its specific capacitance. Thus, it is seen in all of the CV curves that the specific capacitance decreases as the scan rate increases 25 . Because at high scan rates, the electrolyte ions do not have enough time to diffuse entirely into the electrode nanopores wherever the faradaic reactions occur, making some part of the active surface areas inaccessible. Furthermore,  www.nature.com/scientificreports/ this could be attributed to the existence of a large ohmic resistance at high scan rates 25,26,31 . The comparative CV curves of the samples at the scan rate of 5 mV/s are also shown in Fig. 9. Based on the area within the CV curves, it is evident that the incremental substitution of Ni with Mn leads to the enhancement of the specific capacitance of the Ni 1−x Mn x Fe 2 O 4 -based electrodes, resulting from the decreased bandgap of Ni 1−x Mn x Fe 2 O 4 nanoparticles for a higher x, which itself enhances faradaic reactions at the electrode surface and enhances the specific capacitance. Galvanostatic charge-discharge (GCD) curves of the samples at different current densities for three-electrode tests are shown in Fig. 10. The nonlinear discharge curves show that the capacitive performance results from both the electric double-layer capacitance and the pseudocapacitance 31 . The potential drop observed at the beginning of the discharge curve indicates both the very low internal series resistance (R s ) of the prepared electrodes in the KOH electrolyte and the low contact resistance at the interface of the current collector and the electrolyte 25 . It is seen that the discharge time decreases as the current density increases, which is due to the lower accessibility of pores in the active material for electrolyte ions at higher currents 16 . The comparative GCD curves of the samples at the current density 0.5 A/g are also shown in Fig. 11. It is seen that the discharge time increases as Ni is incrementally substituted with Mn in Ni 1−x Mn x Fe 2 O 4 -based electrodes. The specific capacitance values were calculated from Eq. (4):  Table 3. As can be seen in Fig. 12, the specific capacitance decreases as the current density increases, which can be explained by considering the ion diffusion mechanism. In other words, at a lower current density, the electrolyte ions have enough time to access the highest number of active sites on the electrode material, leading to a higher specific capacitance 54 . According to Table 3, the specific capacitance of Ni 1−x Mn x Fe 2 O 4based electrodes increases considerably as the content of Mn increases, consistent with CV curves in Fig. 9, as discussed above. The Ragone plots of the sample are shown in Fig. 13, indicating that the MnFe 2 O 4 -based Table 3. Specific capacitances, energy densities, and power densities of the electrodes constructed from the prepared Ni 1−x Mn x Fe 2 O 4 nanoparticles at the current density of 0.5 A/g.   Cyclic stability tests were performed at the current density of 3 A/g for 1,500 GCD cycles, as shown in Fig. 14. It is seen that the substitution of Ni with Mn in Ni 1−x Mn x Fe 2 O 4 -based electrodes considerably improves their cycling stability. This is another strength of MnFe 2 O 4 , in addition to its higher specific capacitance, as compared to NiFe 2 O 4 for real-world commercial supercapacitor applications.
We observed that the electrode based on the prepared MnFe 2 O 4 nanoparticles exhibited the highest specific capacitance and a very good stability. To demonstrate the real-world application of the electrode material, an asymmetric supercapacitor was assembled using the MnFe 2 O 4 nanoparticles as the positive electrode, AC as the negative electrode (refer to the "Electrochemical measurements" section for the preparation method), and a filter paper wetted with the electrolyte as the separator. According to the specific capacitance of the AC electrode (150 F/g), and in order to achieve the maximum operating potential window and performance, the optimal mass ratio between the positive and negative electrodes ( m + m − ) was balanced according to Eq. (7) 4,56,57 : Accordingly, the weight of AC powder was calculated ~ 4 mg. The CV curves of the electrode in the potential windows of 0.5-1.5 V and at different scan rates are shown in Fig. 15a and b, respectively. We chose the potential window of 1.5 V for further tests because of its highest CV area. It is seen that the CV curves retain their rectangular shape without apparent distortions up to 100 mV/s, indicating the high rate capability of this asymmetric supercapacitor. Interestingly, the asymmetric cell presents a wide and stable operating potential window up to 1.5 V in the KOH electrolyte that should afford high energy densities. The GCD curves of the electrode at different current densities are also shown in Fig. 15c. Figure 15d shows a picture of the assembled asymmetric supercapacitor lighting up a red LED, indicating the real-world application of the electrode material.  Fig. 16). All the structures exhibited a ferrimagnetic character. To have a more clear understanding, we computed XRD patterns of the structure optimized by the DFT calculations, as shown in Fig. 17. It is seen that except inverse spinel MnFe 2 O 4 , the XRD patterns of all structures are largely similar to the experimentally obtained ones (Fig. 2). However, based on the shift in the position of the most-intense peak, it seems that the true XRD pattern of MnFe 2 O 4 is a combination of normal and inverse spinel XRD patterns. Furthermore, it is seen that if one assumes the normal spinel structure for MnFe 2 O 4 , the changes in the lattice www.nature.com/scientificreports/ constants of a and c are consistent with the experimental XRD results (Fig. 2), confirming that the incremental substitution of Ni ions with Mn ions increases the cell volume. Figure 18 shows the electronic band structures and atom-projected density of states of the considered structures. It is seen that except inverse spinel MnFe 2 O 4 , the other structures are an insulator with different gaps for spin-up and spin-down states. However, when Co is incorporated into the structure, the structure becomes a conductor. The phenomenon and the increase of states near the Fermi level could help the structure to store charges, increasing the specific capacitance. It should be noted that the inverse spinel structure could not provide a true representation of the crystal structure of MnFe 2 O 4 , which predicts the structure as a conductor, which is not true experimentally. Lattice constants, spin-up gap, and spin-down gap of the considered structures calculated by DFT are also listed in Table 4

Data availability
The datasets generated during and/or analyzed during the current study are available from the corresponding author on reasonable requests.