Application of SrFeO3 perovskite as electrode material for supercapacitor and investigation of Co-doping effect on the B-site

Energy storage by supercapacitors with short charging time and high power density is one of the types of electrochemical storage systems. Perovskite oxides have been significantly investigated as promising materials for energy storage in electrochemical systems. In this study, three perovskites, SrFeO3, SrCoO3, and SrCo0.5Fe0.5O3, were prepared using the sol-gel method and used as supercapacitor electrode materials. In fact, in this research, two consecutive elements (Fe, Co) from the periodic table that differ by one unit in atomic number are placed in the perovskite structure to study their electrochemical properties for use in supercapacitors. From the obtained results, it was found that Co doping with a ratio of 1/1 (Co/Fe) at B site of SrFeO3 reduced the specific capacitance from 101.687 F g−1 to 60.912 F g−1 at a scan rate of 10 mV s−1. Also, the specific capacitance of SrCoO3 decreased from 68.639 F g−1 to 60.912 F g−1 at the same substitution rate at B site.

with 6 oxygen atoms to form an octagonal (BO 6 ) and each A site cation bonds with 12 oxygen atoms to obtain a stabilized perovskite structure [22,23]. In fact, the main controller of electrochemical properties of perovskite can be considered related to BO 6 in the structure of perovskite. In addition, the physicochemical properties of perovskites can be improved by substitution at the A and B sites with another cation with different radii and valence [24][25][26][27][28]. One of the properties of perovskites that lead us to study them as supercapacitor electrode is their ionic and electronic conductivity [29][30][31][32][33][34][35]. In 2014, Mefford et al. [36] investigated the storage of anionic charge in the LaMnO3 perovskite as a supercapacitor electrode. For the first time, they used oxygen intercalation for rapid energy storage. Zhu et al. [37] synthesized SrCo 0.9 Nb 0.1 O 3-δ for use in supercapacitor as electrode material. Their results showed that in this type of perovskite oxide, when the discharge rate increases from 0.1 to 10 A g -1 , the storage capacitance will be more than 92%.
In this study, the electrochemical properties of SrCoO 3 , SrFeO 3 , SrFe 0.5 Co 0.5 O 3 perovskites as supercapacitor electrodes were investigated to evaluate the effect of partial substitution at the B-site of SrFeO 3 and SrCoO 3 on the performance and specific capacitance of the supercapacitor electrode. As far as we know, it is may be the first time that Co and Fe elements, which differ by one unit in atomic number, are placed in the B site of the perovskite structure, and their electrochemical properties are studied as supercapacitor electrodes. The electrochemical activity of SrFeO 3 , SrCoO 3 and SrFe 0.5 Co 0.5 O 3 was determined by cyclic voltametric (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) analyses.  3 .9H 2 O were mixed in stoichiometric ratio (to produce 1 g of SrFeO 3 , each raw material was used: 1.105, 0, and 2.11 g, respectively. Also for SrCoO 3 : 1.11, 1.32, and 0 g and for SrFe 0.5 Co 0.5 O 3 : 1.108, 0.657, and 1.057 g of the mentioned raw materials were used, respectively.) at 60 °C in 50 mL deionized water, then glycine with a 5:1 molar ratio (glycine/perovskite; about 1.96 g of glycine is needed to produce 1 g of each of the three mentioned perovskite samples) added to the solution at 80 °C. By continued heating and stirring, the gel was formed, and by auto-combustion, a black-brown perovskite powder was obtained.

Synthesis and characterization
Crystalline phases were identified by X-ray diffraction (Tongda TD-3700, China) and Cu Kα radiation (λ = 1.5406 Å). Diffractograms were recorded with a step of 0.02 degree per 0.5 s for 2θ between 10 and 80. The synthesized materials were morphologically investigated by scanning electron microscopy (FE-SEM, model MIRA3-TESCAN, Czech) analysis. Further analysis of the structure was carried out by energy dispersive X-ray spectroscopy (EDX, MIRA3 FEG-SEM, Tescan, Czech Republic) analyzes and elemental mapping. The pore size distribution of samples was determined by Brunauer-Emmett-Teller (BET) through nitrogen adsorption-desorption analyzer (Micromeritics, TriStar II 3020).

Electrochemical measurement
The electrochemical evaluations were conducted using an Autolab, PGSTAT30 Potentiostat-Galvanostat. All electrochemical analysis and measurements were done in a symmetric two-electrode system. In which copper plates were used as current collectors and paper impregnated with 1 mol/L KOH (as electrolyte material) was used as separator. Cyclic voltammetry (CV) curves were obtained at various potential scan rates between −0.8 and (+1.0) V. Electrochemical impedance spectrometry (EIS) measurements were carried out at open circuit potential (OCP) in a frequency range of 100 kHz -10 mHz. Galvanostatic charge-discharge (GCD) tests were performed in a voltage range of -0.8 to 1.0 V with different current densities. Using CV curves and GCD curves, the specific capacitance (Cs, F g -1 ) of perovskite compounds can be obtained from Equations (1) and (2), respectively [4]: where (A) is the current , (g) is the mass of electrode material, (s) is the discharge time, (mV s -1 ) is the scan rate, and (V) is the potential sweep window. The energy density (E, Wh kg -1 ) and power density (P, W kg -1 ) of the supercapacitor were also obtained from the GCD curves according to Equations (3) and (4) [4,38]: where (F g -1 ) is the specific capacitance, (V) is the potential drop. Figure 1 shows the XRD pattern of SrCoO 3 , SrFeO 3 , and SrFe 0.5 Co 0.5 O 3 powders. From the obtained diffraction patterns, the peak sharpness of SrCoO 3 , SrFeO 3 , and SrFe 0.5 Co 0.5 O 3 occurred at angles (2ɵ) of 32.655°, 32.816°, and 32.803°, respectively. This showed the sharp peak of the SF shifted slightly to the left relative to the sharp peak of the SrFe 0.5 Co 0.5 O 3 while the sharp peak of the SrCoO 3 shifted to the right, indicating that the SrCoO 3 is distorted in structure of perovskite [18]. The reason for the observed shift in the XRD patterns of the synthesized samples is the difference in the elements in B site of the perovskite structure; because the oxidation states of each of the elements are different and these oxidation states directly affect the chemical bonds in the perovskite structure and cause structural differences among them. The oxygen vacancy of the perovskite structure, which is related to the oxidation states of the B site element, can be determined to some extent by the same structural differences observed from the XRD analysis of the samples. According to the obtained XRD results and comparing with the related standard XRD cards, the presence of oxygen vacancy in the structure of synthesized perovskites was confirmed. According to the obtained XRD diffraction for SrCoO 3 structure of this perovskite has a rhombohedral structure and its diffraction pattern according to Figure 1 is very close to SrCoO x (JCPDS card 49-0692), and after doping Fe along with Co (SrFe 0.5 Co 0.5 O 3 ), the structure has changed to cubic (JCPDS card 46-0335). While the structure of SrFeO is cubic, its diffraction pattern is close to SrFeO 2.97 (JCPDS card 40-0905).

Crystal structure and morphology
The morphological structure of the synthesized perovskite compounds was obtained by SEM analysis, as shown in Figures 2a-2c. All of the synthesized perovskites have shown rough and highly porous structures that may result in an increased active surface area for the electrodes. More homogenous distribution of the pores is obvious for SrFe 0.5 Co 0.5 O 3 compared to SrFeO 3 . Both SrCoO 3 and SrFe 0.5 Co 0.5 O 3 samples have also indicated small average pore sizes. These characteristics are expected to be critical factors for their supercapacitive activities. Also, analysis of nitrogen adsorption and desorption measurements was performed to obtain detailed information about the porosity nature of the synthesized samples. As shown in Figure 3 most of the pores were in the size range of 5 to 17 nm. In addition, the elemental mapping of the SrFeO 3 composition in Figures 2d-2g shows that the distribution of Sr, Fe and O elements on the SrFeO 3 surface is uniform, which confirms the success of SF synthesis. SrFeO 3 was also analyzed by energy dispersive X-ray (EDX). According to the EDX results as shown in Figure 2h

Electrochemical performance
The oxygen vacancy sites in the structure of perovskites can provide diffusion pathways for electrolyte ions. Because the compounds are synthesized in porous form, electrolyte transport is more efficient for redox reactions during the Faraday charge storage process. What is important in charge storage in this way is the mobility of oxygen vacancy space as a charge carrier. The concentration of oxygen vacancy also depends on the structure of the perovskite. In addition, the oxidation states of the B site and the ease of access to these oxidation states have a great effect on the specific capacitance (and oxygen vacancy) [39,40].
CV and GCD analysis were performed to clarify perovskite oxides' charge storage capacitance and stability, and to evaluate their specific capacitance. Figure 4 shows the CV curves of SrCoO 3 , SrFe 0.5 Co 0.5 O 3 , SrFeO 3 with distinct redox peaks showing typical pseudo-capacitive properties. SrCoO 3 and SrFeO 3 capacitance can be attributed to Co 3+ /Co 2+ and Fe 3+ /Fe 2+ redox reactions, respectively [41,22]. As the scan rate increases, it is observed that the area related to the CV curves increases and as a result, the specific capacitance decreases. The decrease in specific capacitance is visible by increasing the scan rate. At lower scan rates, more electrolyte ions can react electrochemically with the active material on the electrode surface, as a result, the specific capacity increases. The specific capacitance of the synthesized perovskite compounds was calculated using the CV curve by Equation (1), the results of which are given in Table 1.
The GCD curves of the SrCoO 3 , SrFeO 3 and SrFe 0.5 Co 0.5 O 3 electrodes at different current densities are shown in Figure 5. Table 2 contains information on the specific capacitance of perovskite compounds calculated from Equation 2. In addition, EIS analysis was performed in the frequency range from 100 kHz to 10 mHz. The results are shown in Figure 6. R ct shows the resistance of the transfer of ions at the interface of the electrode/electrolyte materials. The straight line also shows the diffusion resistance of electrolyte ions in the pores of the electrode material [42,43]. At high frequencies, the presence of a small semicircle indicates the low charge transfer resistance (R ct ) at the electrode/electrolyte interface and the rapid charge propagation between the electrolyte and the perovskite electrode material.  The results obtained for the specific capacitance of SrFeO 3 , SrCoO 3 and SrFe 0.5 Co 0.5 O 3 from CV and GCD curves show that in low scanning rates and high current densities, the specific capacitances of SrFeO 3 are higher than those of other compounds, which could indicate the existence of the proper oxygen vacancy in the structure of this perovskite as well as the proper stability of these active points in the high current density. The partial substitution made has a negative effect on the specific capacitance of SrFeO 3 . This means that the oxygen vacancy concentration in SrFeO 3 decreased after the partial substitution of Co at B site of SrFeO 3 .   Table 3 shows the specific capacitance obtained for different compounds that have been studied as electrode material for supercapacitor. As can be seen, perovskite compounds have high specific capacitance. The main advantage of these compounds is its proper stability during redox cycles, which causes a better preservation of the specific capacitance of the perovskite supercapacitor. The compounds synthesized in this study, which have been studied as two electrodes, in other words in the symmetric cell of the supercapacitor, have specific capacitance suitable for the supercapacitor. Nevertheless, it is still possible to increase their specific capacitance with a substitution and finding the right percentage combination of A and B site elements.

Conclusion
In this paper, three electrode materials including SrCoO 3 , SrFeO 3 and SrCo 0.5 Fe 0.5 O 3 , were prepared to be used as supercapacitor electrode candidates; and the effect of partial substitution on electrochemical properties was investigated. The specific capacitance obtained for the SrCoO 3 , SrFeO 3 and SrCo 0.5 Fe 0.5 O 3 samples at the scan rate of 10 mV s -1 was 68.639, 101.687, and 60.912 F g -1 , respectively, also similar to these results with a slight difference in the high current density. It was observed that accordingly SrFeO 3 showed higher specific capacitance at low scan rates and higher current densities than other samples. Therefore, the partial substitution of Co at the B site of SrFeO 3 in low scanning rates and high current densities has a negative effect on the specific capacitance and reduces it. This means that partial substitution reduces oxygen vacancy concentration as a charge carrier in the SrFeO 3 structure. These observations were in argument with the results of EIS, where the SrFeO 3 had the highest R ct indicating slower charge transfer inside the electrode material, which decreases the contribution of faradaic currents in overall current density. The CV plats also confirmed this observation with a more rectangular shape of CVs for SrFeO 3 . CNTs/MnO 2 1229 F g -1 at 1 A g -1 1 mol L -1 Na 2 SO 4 [44] ZnO/MnO x 556 F g -1 at 1 A g -1 1 mol L -1 Na 2 SO 4 [45] CoO/NiO-Cu@CuO 2035 mF cm -2 at 2 mA cm -2 Solid-state PVA/KOH hydrogel [46] CNT-Fe 3 O 4 373 F g -1 at 10 mV s -1 1 mol L -1 Na 2 SO 4 [47] LaCrO 3 1268 F g -1 at 2 A g -1 Neutral LiCl aqueous [48] SrCo