Low temperature synthesis of BiNi0.6Mn0.4O3 nanostructures via citric acid and ethylene glycol assisted hydrothermal process for energy storage applications

Present study reports the electrochemical behavior of BiNi0.6Mn0.4O3 nanostructures synthesized via citric acid and ethylene glycol assisted hydrothermal process at low temperature calcination of 400oC. Raman spectroscopy and Rietveld refinement have confirmed BiNi0.6Mn0.4O3 to crystallize in tetragonal phase with P4̅ 21c space group symmetry. X-ray photoelectron spectroscopic analysis showed the presence of ‘B’ cations, Ni and Mn in (+2) and (+4) oxidation states, respectively, which mainly contributed to faradaic reactions as observed in CV curves. The specific capacitance of BiNi0.6Mn0.4O3 electrodes has been found to be ∼243 F g−1 at the current density of 1 A g−1 in a 6 M KOH aqueous solution. The nanostructured electrodes showed a cyclic stability of ∼70% after 4000 charge–discharge cycles at the current density of 6 A g−1 .


Introduction
The quest for efficient energy storage is one of the biggest challenges facing the world today [1][2][3].As the world shifts from non-renewable energy sources to sustainable forms such as thermal, mechanical, physical, chemical, and electrochemical, the need for reliable and portable devices is on rise [4][5][6][7][8].Electrochemical storage systems thus have become a subject of intense study, and materials such as perovskites and double perovskites have drawn significant interest due to their multifunctional properties [9,10].
Perovskite oxides, with the general formula of ABO 3 and double perovskites with the formula AA'BB'O 6 , exhibit a wide range of physical and chemical properties due to the combination of rare earth metals/ lanthanides at A position and transition metals at B position [11,12].The structural stability of these materials depends on cationic size, charge, and the synthesis process [13].The development of efficient electrode materials for supercapacitors with high stability and electrochemical performance requires new combinations and environmentally benign materials [14][15][16][17][18][19].
Hydrothermal synthesis is an attractive process for the synthesis of perovskite nanomaterials due to being a controlled and safe process, which allows the precise tuning of morphology and composition [20,21].Bismuth is an economically and environmentally suitable candidate for an electrode material due to its magnetic and electrical properties, despite minor differences in ionic size with the commonly used lanthanum at A lattice site [22].
In this study, we synthesized BiNi 0.6 Mn 0.4 O 3 for the first time using the hydrothermal process and evaluated its electrochemical behavior.There are many reports in the literature regarding the electrochemical behavior of perovskites as an electrode material for supercapacitors [23,24].The perovskite compounds with transition metal oxides like Ni and Mn at B site can provide high charge storage due to fast redox reactions [25][26][27][28][29][30][31].
According to the availability of results of compounds with Ni and Mn on the B site, their morphological and electrochemical performance are shown in table 1. Bi based perovskite such as BiFeO 3 was synthesized for supercapacitor by S Yin et al [32].This enhanced our interest to synthesize the BiNi 0.6 Mn 0.4 O 3 compound and to explore its electrochemical performance.Our synthesis of BiNi 0.6 Mn 0.4 O 3 resulted in a single phase material at a low calcination temperature, as revealed by X-ray diffraction.Field emission scanning electron microscopy (FESEM) was performed to understand morphology, and X-ray photoelectron spectroscopy (XPS) analysis to investigate the surface oxidation states in BiNi 0.6 Mn 0.4 O 3 nanostructures.Electrochemical evaluations were conducted using a SP-150 Biologic potentiostat.

Experimental
2.1.Synthesis of BiNi 0.6 Mn 0.4 O 3 using hydrothermal process The process of BiNi 0.6 Mn 0.4 O 3 synthesis involved firstly, the mixing the precursors, Bi(NO 3 ) 3 .5H 2 O, Ni(NO 3 ) 2 .6H 2 O, and Mn (CH 3 CO 2 ) 2 •4H 2 O in the 2:1.2:0.8 stoichiometric ratio in 50 ml of deionized water under constant stirring to get a clear solution.Afterward, the citric acid anhydrous (C 6 H 8 O 7 ) and ethylene glycol (EG) were added as chelating agents in the ratio, 1(metal ions): 2(citric acid anhydrous): 4 (EG) in the former solution.Subsequently, after stirring of an hour, the solution was transferred into a 100 ml stainless steel autoclave and heated to 180 o C for 24 h.The sample was then cooled, washed, and dried (for 12 h at 70 °C) before being ground into a powder and calcined for 2 h each at 400 °C and 600 °C.The precursors used to synthesize BiNi 0.6 Mn 0.4 O 3 were of analytical grade and used without any further purification.The crystal structure of the precursors, Bi(NO 3 ) 3 .5H 2 O, Ni(NO 3 ) 2 .6H 2 O and Mn (CH 3 CO 2 ) 2 •4H 2 O have been known [33][34][35].Scheme 1 illustrates the synthesis process of BiNi 0.6 Mn 0.4 O 3 nanostructures.

Fabrication of working electrode
The Ni-mesh based working electrode was prepared by utilizing slurry composed of as-synthesized material, acetylene black, and polyvinylidene fluoride (PVDF) in the weight percentage ratios of 80:10:10 to investigate the electrochemical performance.As-synthesized material, acetylene black, and PVDF mixture was modified to slurry using organic solvent N-methyl-2-pyrrolidinone (NMP).For this, the slurry mixture was placed in an ultrasonicator bath for 30 min, followed by 10 h of vigorous stirring to achieve homogeneity.The nickel mesh was thoroughly cleaned with deionized water and ethanol (3 times) and dried in an oven at 70 °C for 1 h before drop-casting electrode on it (within an active area of 1cm 2 with mass loading ∼1 mg).The electrode was dried for 12 h at 80 °C.Electrochemical analysis was performed in a 6 M potassium hydroxide solution (KOH) at room temperature.

Characterization of BiNi 0.6 Mn 0.4 O 3 and its electrochemical performance
The X-ray diffraction pattern was obtained using the Rigaku MiniFlex X-ray diffractometer (CuKα 1 radiation, wavelength = 1.54056 nm) in the 2θ range of 10°to 90°.The morphology was studied with a Field Emission Electron Microscope (FE-SEM) (Nova Nano FE-SEM 450 (FEI)).The presence of different oxidation states of elements in the material was analyzed by X-ray Photoelectron Spectroscopy (XPS) (Kratos Analytical, AXIS Supra).The molecular structure was analyzed using Raman Spectroscopy, which utilized monochromatic light at 532 nm with an output power of 5 mW.Electrochemical observations, including Cyclic Voltammetry and Galvanostatic Charge/Discharge via Chronopotentiometry, were performed at room temperature in a 6 M KOH aqueous solution using the SP-150 Biologic instrument.[36,37].Meanwhile, the monoclinic phase which was formed at 600 °C, which is iso-structural to α-Bi 2 O 3 (JCPDS card No. 76-1730) [38].It is evident that the increase in temperature from 400 °C to 600 °C results in a complete transformation of tetragonal phase to monoclinic phase.Figure 1(b) presents the Rietveld refined X-ray diffraction pattern for material calcined at 400 °C.The refinement was performed using FullProf software, and the results show that the material has a tetragonal structure with space group symmetry of ̅ P 4 2 1 c.The refined lattice parameters were found to be a = b = 7.731 Å, c = 5.651 Å, and χ 2 = 2.24. Figure 1(c) presents the Rietveld refined X-ray diffraction pattern for material calcined at 600 °C has a monoclinic structure with space group of P12 1 /c 1 and refined lattice parameters were found to be a = 5.84860 Å b = 8.16610 Å c = 7.50970 Å β = 113.000Å and χ 2 <1.The average crystallite size was calculated by Scherer's formula (equation ( 1)) [39], where λ is the X-ray wavelength, β is the FWHM of the diffraction peak, and θ is the diffraction angle.It was observed that the average crystallite size increased from 19 nm at a temperature of 400 °C to 53 nm at a  A smaller particle size results in an increased surface area per unit volume, which an optimal condition for electrochemical energy storage.Therefore, the sample calcined at 400 °C was selected for further investigation.Figure 2(b) shows the Energy Dispersive X-ray (EDX) analysis of BiNi 0.6 Mn 0.4 O 3 , conducted to determine the elemental composition of the sample.It reveals peaks corresponding to each element present in the sample (Bi, Ni, Mn, O), and the elemental percentages were found to match the stoichiometry of the composition with minimal deviation.The atomic percentage for Bi, Ni, Mn and O estimated from EDS data is 15.47%, 15.54%, 8.84% and 60.15% respectively.A slightly higher amount of nickel and reduced amount of bismuth on the surface may be because of preferential occupation of the surface by nickel over bismuth [41].

X-ray photoelectron spectroscopic (XPS) analysis
The surface oxidation states of the elements in the sample BiNi 0.6 Mn 0.4 O 3 were analysed using XPS.The highresolution spectra of the Bi 4 f, Ni 2p, Mn 2p, and O 1 s are shown in figures 4(a)-(d).The measured binding energies were calibrated using C 1 s peak at 284.7 eV as a reference.The Bi 4 f core electron spectrum, depicted in figure 4(a), exhibits doublet peaks with higher binding energies of approximately 158 eV and 163.36 eV, corresponding to the Bi 3+ states of Bi 4f 7/2 and Bi 4f 5/2 , respectively [45,46].The additional smaller peaks of Bi could be due to a minimal surface charge effect caused by the crystal's polarization change [47].Ni in its +2 oxidation state.The satellite peaks at binding energies of 860.6 eV and 879.06 eV suggest the presence of oxidized nickel on the surface [26,30,48,49].The high-resolution (Mn 2p) spectrum is shown in figure 4(c).The broad peak appearing in the spectrum has been ascribed to Mn 2p 3/2 , which on deconvolution reveals the co-existence of a mixed valence states: Mn 4+ (646.2 eV) and Mn 3+ (641.6-642.3eV) [50][51][52][53].The characteristic peak at about 529.2 eV in the high-resolution O-1s spectrum, shown in figure 4(d), is attributed to a metal-oxygen bond, while the second peak at a binding energy of approximately 531 eV is ascribed to O 2 2species and/ or hydroxyl ions absorbed on the surface [54].

Electrochemical analysis
The cyclic voltammetry curves of BiNi 0.6 Mn 0.4 O 3 electrodes at varying scan rates of 5-100 mV s −1 in a potential range of 0.1-0.6V are presented in figure 5(a).The redox peaks observed in the CV of BiNi 0.6 Mn 0.4 O 3 demonstrate the faradaic redox processes [55].The redox reactions as represented by equations (2-3) result in a single oxidation peak and reduction peak.
The rate characteristics of the BiNi 0.6 Mn 0.4 O 3 electrode were confirmed by the CV shape, which remained undisturbed even at higher scanning rates (100 mV s −1 ), demonstrating its high rate capabilities and electrochemical reversibility [56].At high scan rates, the surface of the working electrode becomes more readily accessible to electrolyte ions diffusion, boosting current value.However, at lower scan, a diffusion layer is formed at the interface of the electrode and the electrolyte, which limits the movement of electrolyte ions across the direction of the electrode, leading to a long response time for redox reactions to take place [57].The charge storage in BiNi 0.6 Mn 0.4 O 3 electrode is a faradaic redox reaction controlled process, occurring due to the variable states Ni and Mn.The redox behavior is aligned well with the oxidation states confirmed through XPS analysis.The increase in scanning rate, shifts in the anodic peaks to positive (0.30 to 0.40 V) value while negative shifts is observed in the cathodic peaks (0.30 to 0.20 V), which is ascribed to the polarization effect limiting the faradaic type redox process [26,58].
The electrochemical performance of the BiNi 0.6 Mn 0.4 O 3 electrode was evaluated through galvanostatic charging/discharging (GCD) experiments in a 6 M KOH electrolyte solution, as recorded by chronopotentiometry (figure 5(b)).The GCD curves of the BiNi 0.6 Mn 0.4 O 3 electrode exhibit a non-linear characteristic attributed to the faradaic redox reaction.The discharge curves reveal a low internal resistance (IR) drop, which suggests that the BiNi 0.6 Mn 0.4 O 3 electrode is a promising candidate for supercapacitor applications.The specific capacitances (Csp) and capacities (Cs) of the BiNi 0.6 Mn 0.4 O 3 electrode were calculated using the discharge duration according to the following equation [59,60] ẃhere, I is the current, m is the effective mass of the material, ∆V is the potential window, and ∆t is discharge period.The specific capacitance was measured at current densities of 1, 2, 3, 4, 5, and 6 Ag −1 and was estimated to be 242.99,173.07, 139.19, 114.94, 98.20, and 84.66 F g −1 , respectively (figure 5(c)).The decrease in specific capacitance as current density increases was attributed to a lack of OH-ions available for intercalation at the electrode-electrolyte interface, as diffusion processes require fewer ions at lower current densities [61,62].The maximum specific capacity (C s ) was determined to be 30.374mAh g −1 at a current density of 1 A g −1 , as seen in the figure 5(c).The cycling performance of BiNi 0.6 Mn 0.4 O 3 was evaluated using chronopotentiometry for 4000 cycles at a current density of 6 A g −1 , as shown in figure 5(d).After 4000 cycles, the specific capacitance of BiNi 0.6 Mn 0.4 O 3 showed a gradual reduction, retaining approximately 70% of its initial value.The decline in specific capacitance over the charge/discharge cycles may be due to the degradation of the active material (assynthesized material) in the fabricated BiNi 0.6 Mn 0.4 O 3 electrode during the faradaic redox reactions [63].
The energy density (E d ) and power density (P d ) of the BiNi 0.6 Mn 0.4 O 3 electrode were assessed to evaluate its effectiveness as an energy storage system.The values of E d and P d were calculated using the specific capacitance and discharge time obtained from the GCD analysis, as per the equations provided below [64]: At 1 A g −1 current density, the maximum energy density of 6.83 Wh kg −1 and power density of 224.87 W kg −1 were achieved.
In brief, the BiNi 0.6 Mn 0.4 O 3 nanostructures were successfully synthesized involving hydrothermal method.The synthesized nanostructures showed a tetragonal structure at lower calcination temperature (400 °C).The XRD analysis suggests that the crystallite size increases at elevated temperatures as the atoms receive more energy to diffuse, though retain the least energy positions in the crystal lattice.The particle size also increased with an increase in temperature as observed during FESEM analysis.The EDS spectra confirmed the presence of Bi, Ni, Mn and O in the synthesized material.The BiNi 0.6 Mn 0.4 O 3 electrode stores charge through the faradaic redox reactions, occurring due to the variable states of B site (Ni, Mn).The redox behavior is in accordance with the oxidation states confirmed through XPS analysis.The synthesis approach employed for the BiNi 0.6 Mn 0.4 O 3 nanostructures demonstrated better transfer of charge at the nanoscale for a faradaic-type redox reaction leading to superior electrochemical performance.The electrochemical behavior of the as-synthesized electrodes revealed that they are favorable for energy storage applications.

Conclusions
The X-ray diffraction analysis of BiNi 0.6 Mn 0.4 O 3 synthesized at low temperature of 400 o C confirmed a tetragonal crystal phase with ̅ P 4 2 1 c symmetry in the sample.The field emission scanning electron microscopy revealed an uneven morphological pattern of BiNi 0.6 Mn 0.4 O 3 nanostructures.The highest recorded specific capacitance was approximately 243 F g −1 at a current density of 1 A g −1 .Moreover, after 4000 cycles, approximately 70% retention of specific capacitance was achieved at a current density of 6 Ag −1 using chronopotentiometry.These findings demonstrate the potential of BiNi 0.6 Mn 0.4 O 3 nanostructures' application in energy storage devices.

3 .
Results and discussion 3.1.Structural and morphological analysis Figures 1(a), (b) depicts the X-ray diffraction pattern of hydrothermally synthesized perovskite oxide BiNi 0.6 Mn 0.4 O 3 .Figure 1(a) illustrates the impact of the temperature on the structural symmetry of BiNi 0.6 Mn 0.4 O 3 .The tetragonal phase formed at 400 °C is iso-structural to β-Bi 2 O 3 (JCPDS card No.78-1793)

Figure 1 .Figure 2
Figure 1.(a) X-ray diffraction patterns showing the effect of temperature on the structure of BiNi 0.6 Mn 0.4 O 3 and (b) Rietveld refinement of X-ray pattern of BiNi 0.6 Mn 0.4 O 3 calcined at 400 °C (c) Rietveld refinement of X-ray pattern of BiNi 0.6 Mn 0.4 O 3 calcined at 600 °C.
The highresolution (Ni 2p) spectrum, shown in figure 4(b), contains two spin-orbit doublets and two satellite peaks.The 2p 1/2 and 2p 3/2 peaks at binding energies of 872.06 eV and 853.76 eV, respectively, demonstrate the presence of

Figure 5 .
Figure 5. (a) Cyclic voltammetry curves with varying scan rates, (b) Galvanostatic charge-discharge curves with varying current densities, (c) Specific capacitance/capacity with varying current densities and (d) Cyclic stability at 6 A/g for BiNi 0.6 Mn 0.4 O 3 .

Table 1 .
Comparison of the performance of perovskite oxides with Ni and Mn occupied BB' site.