Electrochemical Energy Storage Properties of Sm2FeCuO6 Double Perovskite Synthesized via Sol-Gel Route

The rare Earth-based double perovskites have been widely studied due to their exceptional physical properties and wide range of technological applications. Despite the extensive investigation of copper-based rare Earth double perovskites and a limited study of samarium-based double perovskites, no reports on the synthesis and characterization of Sm2FeCuO6 have been found in the literature. This work presents the experimental investigation on the synthesis of Sm2FeCuO6 via a wet chemical sol-gel route and the characterization of its structural and electrochemical properties using various techniques. The results showed that Sm2FeCuO6 have good electrochemical properties, making it a promising candidate for use in electrochemical energy storage applications.

The rare Earth based double perovskites with formula Re 2 TT ′ O 6 (Re-rare Earth cations, T/T ′ -transition metal cations) have been widely studied due to their exceptional configuration {Xe} 4 f n−1 5d 0-1 6s 2 (where n = 1-15). 1 The experimental and theoretical investigations have revealed interesting physical properties such as antiferromagnetism, magnetoresistance, and giant dielectric permittivity. 2 The double perovskites have been used for various technological applications such as light absorption in perovskite solar cells, catalysis, solid oxide fuel cells, spintronics, and more. 1,[3][4][5][6][7] Limited experimental investigation of copper-based rare Earth double perovskites such as La 2 NiCuO 6 , La 2 CuSnO 6 , La 2 CuCoO 6 , and La 2 CuMnO 6 has been performed for applications such as catalysis and electrochemical performance. [8][9][10][11][12][13][14] Most of the experimentally investigated rare Earth double perovskites are based on lanthanum, with a number of combinations studied experimentally for different applications. The investigation of samarium-based double perovskites is limited, with Sm 2 NiMnO 6 , Sm 2 CoMnO 6 and Sm 2 CuRuO 6 having been studied. [15][16][17] Sm 2 NiMnO 6 has been widely studied for applications such as lead-free solar cells, electrochemical supercapacitors, and spintronic devices, while Sm 2 CoMnO 6 has been investigated for its dielectric and magnetic behavior. 15,16,[18][19][20][21][22] For electrochemical performance, various compositions of perovskites and metal oxides including those based on carbon, conducting polymers, MXenes, hydroxides and Metal organic frameworks etc. have been used as electrodes. [23][24][25][26][27][28] The transition metal-based compounds have gained popularity due to oxygen vacancies that lead to increased specific capacitance and high energy density of the materials. 29 Lanthanide and copper-based double perovskites such as La 2 CuCoO 6 and La 2 CuMnO 6 , as well as Ln 2 CuRuO 6 , have been studied for use as electrochemical supercapacitor electrodes. [12][13][14][15]30 The presence of different cations in the double perovskite structures provides several advantages such as enhanced electrochemical activity and increased active area for faradaic redox processes. 31 Cations such as La, Sm, Nd, etc. have fixed oxidation states, but they can affect the oxidation states of the cations at T/T ′ -sites, the active area for electrochemical reactions, porosity, and oxygen vacancies. 1,16 The rare Earth-based double perovskites have been synthesized using a variety of conventional techniques such as solid-state reactions, wet chemical sol-gel methods, and co-precipitation techniques. [32][33][34] To date, no experimental reports on the synthesis and characterization of Sm 2 FeCuO 6 have been found in the literature. The work presents, the first experimental report on the synthesis of Sm 2 FeCuO 6 via the wet chemical sol-gel route, along with investigations of its structural and electrochemical properties using X-ray diffraction, Raman spectroscopy, and cyclic voltammetry and chronopotentiometry techniques.

Experimental
Material synthesis and characterization.-The double perovskite Sm 2 FeCuO 6 was synthesized using a wet chemical sol-gel method with precursors samarium trinitrate hexahydrate Sm(NO 3 ) 3  O], in a stoichiometric ratio of 2:1:1. The precursors were dissolved in 50 ml of DI water, with constant stirring, and citric acid (as a chelating agent) and ethylene glycol (as surfactant) were added in the metal equivalent ratios of 1:2 and 1:4, respectively. The solution was stirred continuously and heated to 180°C to form a gel, which was then dried at 140°C for 24 h and ground into a fine powder. The powder was decomposed at 1050°C for 10 h to obtain Sm 2 FeCuO 6 . The material was characterized using X-ray diffraction (M/s Bruker AXS, Germany), and Raman spectroscopy (Bruker Alpha Platinum-ATR and WITEC model with an Ar laser, λ = 532 nm), X-ray photoelectron spectroscopy (PHI 5000 Versa Probe III), and highresolution transmission electron microscopy (TECNAI G 2 20 S-TWIN, FEI Netherlands).
Electrode fabrication and electrochemical analysis.-To analyze the electrochemical properties of the material, a three-electrode configuration was utilized. The working electrodes were prepared using the double perovskite Sm 2 FeCuO 6 . To make the slurry for the working electrodes, Sm 2 FeCuO 6 powder, activated carbon, and the binder polyvinylidene fluoride (PVDF) were first ground separately and then mixed together in a mortar in a weight ratio of 80 wt%: 10 wt%: 10 wt%. Afterwards, the solvent N-methyl-2-pyrrolidone (NMP) was added and the mixture was stirred for 12 h to create a uniform slurry. The slurry was then coated evenly onto a cleaned nickel mesh (1 cm 2 ) and dried first at room temperature and then for 8 h at 80°C in an oven. The electrodes were weighed before and after coating and the net weight was approximately 1 mg. The z E-mail: ashokku@nitttrchd.ac.in electrochemical measurements were performed using the Biologic SP-150 potentiostat instrument and EC Lab software. The working electrode consisted of the double perovskite material, while the counter electrode was Platinum (Pt) and the reference electrode was Ag/AgCl. An aqueous olution of 6 M KOH was used as the electrolyte. Figure 1a shows the powder X-ray diffraction (PXRD) pattern of the pristine Sm 2 FeCuO 6 sample obtained by the decomposition of the sol-gel product at 1050°C for 10 h. The pattern exhibits an orthorhombic structure with Pbnm space group symmetry, and the high-intensity peaks at Bragg ′ s positions (2θ) (110) Figure 2 shows the Raman spectra of Sm 2 FeCuO 6 at room temperature. As demonstrated by the PXRD pattern, the sample exhibits an orthorhombic structure with Pbnm space group symmetry. The Raman spectra have been analyzed by comparing it to RFeO 3 compounds with the same Pbnm space group. 35,[37][38][39] The observed Raman modes for Sm 2 FeCuO 6 match those of SmFeO 3 compounds, but with some shift in the modes due to differences in the bond lengths of Fe-O and Cu-O. 34 According to S. Gupta et al., there are 24 Raman active modes for SmFeO 3 , which has Pbnm space group symmetry. 35 These modes can be characterized as Г Raman = 7A g + 7B 1g + 5B 2g + 5B 3g . The modes appearing at 120, 167, 242, 323 120, 167, 242, 323 cm −1 correspond to the Ag symmetry, while the modes appearing at 52, 386, 469 cm X-ray photoelectron spectroscopy (XPS) was used to study the oxidation states of samarium, iron, copper, and oxygen as shown in Fig. 3. The XPS data was fitted using XPSPEAK 4.1 software with a Gaussian peak fit and Shirley background correction. Figure 3a shows the XPS spectrum for Sm-3d, with peaks for Sm3d 5/2 and Sm3d 3/2 appearing at binding energies of 1110.0 eV and 1082.8 eV, respectively, indicating the +3 oxidation state of Sm. 16 Figure 3b shows the XPS spectrum for Fe-2p in the binding energy range of 705-730 eV, with peaks at binding energies of 710.7 eV and 724.0 eV corresponding to Fe2p 3/2 and Fe2p 1/2 , respectively, indicating the +3 oxidation state of Fe, and a satellite peak at 718.8 eV. 40 Figure 3c shows the XPS spectrum of Cu-2p, with peaks for Cu2p 3/2 and Cu2p 1/2 at binding energies of 933.5 eV and 936.7 eV, respectively, and strong satellite peaks at 942.2 eV and 964.4 eV, indicating the presence of copper in the Cu 2+ oxidation state. 13,16 Figure 3d shows the high-resolution XPS spectrum for O1s, with a high-intensity peak at 529.2 eV due to lattice oxygen and

Results and Discussion
The retention of the shape of CV curves demonstrates the material's slow reaction rate and good rate behavior. The specific capacitance (Csp) from the CV has been calculated using the following formula: 46 where ∫ IdV is the area under the CV curve at each scan rate, "υ" (mV/s) is the scan rate, "m" (mg) is the mass of the active material, and "ΔV" (V) represents the potential window. The specific capacity (Cs) was also calculated from the specific capacitance by multiplying it by ΔV. The calculated Csp and Cs were found to be 60.2 F g −1 , 30 .1 C g −1 at a scan rate of 10 mV s −1 and decreased to 46.2 F g −1 and 23.1 C g −1 with a ten-fold increase in scan rate. In a CV experiment, the current (i) response at a fixed potential when changing the scan rate (υ) is a characteristic of the charge storage kinetics. The power law relationship can be used to determine the current dependence on scan rate: i = av b , where "a" is a constant and "b" value determines the charge storage mechanisms. The value of "b" can be determined from the slope of the graph of log (i) versus log (υ). If "b" = 0.5, the charge storage is by semiinfinite diffusion process; for spherical diffusion process "b" value is 0.75, and for non-faradaic (surface-controlled) mechanism "b" = 1. However, if "b" value lies between 0.85 and 1, surface-controlled reactions dominate over diffusion-controlled reactions. Figure 4b shows the variation of the anodic peak current density with scan rate at 0.27 V and 0.35 V. The oxidation peaks at 0.27 V and 0.35 V had "b" values of 0.712 and 0.803, respectively, which suggest that the charge storage process is a mixture of diffusioncontrolled and capacitive response of the anodic current. Figure 4c represents the charging-discharging profiles of the Sm 2 FeCuO 6 electrode recorded at current densities of 1, 2, 3, and 6 A g −1 , respectively. The specific capacitance was directly calculated from the discharge curves using the GCD method with the following formula, 40 where, "C sp " (F/g) is the specific capacitance, "I" (mA) is current, "ΔV" (volt) is the potential window and "m" (mg) is mass of the active material of electrode and "Δt" (s) is the discharging time.  can be improved by controlling its morphology and decreasing particle size through changes to the synthesis method's surfactants. Figure 4d shows the plot of the specific capacitance variation with current density. Other important parameters to study the electrochemical behavior are energy density and power density. Since the GCD curve is non-linear, Eq. 6 was used to accurately measure the energy density, and power density was calculated from Eq. 7: 39 where, various terms are stated as; energy density as "E d " (Wh/kg), power density as "P d " (W/kg), current density as "I" (A/g), area under the discharging curve as "∫V(t)dt" and discharging time as "t" (s). Figure 4e depicts the Ragone plot, which compares the energy density to the power density at different current densities. The energy density was found to be 1.2 Wh kg −1 and the power density was 239.1 W kg −1 at a current density of 1 A g −1 . The stability of the electrode is crucial to study its electrochemical performance for use in supercapacitors. Figure 4f shows the cyclic stability of the Sm 2 FeuO 6 electrode after 2,000 cycles at a current density of 6 A g −1 , demonstrating a retention of ∼72% specific capacitance over 2,000 cycles. 39,47,48 Figure 4g represents the Nyquist plot of the Sm 2 FeCuO 6 electrode in the frequency range of 200 kHz to 100 mHz at the voltage of 10 mV before cycling. In this figure the intercept of imaginary part on the real part axis of impedance in the high frequency region shows the equivalent charge series resistance. The resistance is combination of electrolyte solution resistance and electrode internal resistance. In low frequency region, beyond charge series resistance, the resistance is called the charge transfer which depends on the area and conductivity of electrode. In the low frequency region, the straight line in the EIS plot represents the Warburg line, and the resistance because of diffusion of ions from electrolyte to electrode surface is called Warburg resistance.

Conclusions
The synthesis of Sm 2 FeCuO 6 and its characterization showed that it has good electrochemical properties, making it a potential candidate for use in electrochemical applications. The results of this study contribute to the advancement of rare Earth-based double perovskites as materials for various technological applications. Further research can focus on optimizing the synthesis conditions and investigating the properties of Sm 2 FeCuO 6 for different applications.