Novel bio-chelating agent-assisted eco-friendly synthesis of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes for supercapacitor applications

Abstract

The current work pioneers the synthesis of a novel composition of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrode materials for supercapacitors through an innovative semi-green route, utilizing lemon powder as the bio-chelating agent. The synergy between the biomolecules and organic citric acid in lemon powder resulted in minimal impurities and enhanced the crystallinity of the desired perovskite electrodes. XRD analysis confirmed the cubic perovskite structure of all Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes synthesized via both semi-green and chemical approaches. Notably, samples synthesized through the semi-green approach exhibited better crystallinity with no secondary phases. Microstructural analysis revealed a dense and agglomerated morphology for all samples, while EDX analysis confirmed the elemental composition with no prominent impurities. FTIR analysis confirmed the presence of identical functional groups in samples synthesized through both routes. Electrochemical studies demonstrated the highest specific capacitance of 1176.36 F g⁻¹ and excellent electrochemical stability, with 88.2% capacity retention after 5000 galvanostatic charge–discharge cycles for Sr_0.9X_0.1CoO_3−δ (X = Ce) synthesized through the semi-green route. Meanwhile, Sr_0.9X_0.1CoO_3−δ (X = Ba) also exhibited a reasonable specific capacitance. These findings confirm that the novel perovskite composition Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) can be successfully utilized for supercapacitor applications and that the innovative semi-green route can also be employed for the efficient synthesis of perovskite materials, ensuring minimal ecological impact and reduced impurities compared to conventional chemical and green synthesis routes.

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1. Introduction

In the modern era, there is a dire need to explore energy devices that are sustainable, environmentally friendly, and capable of providing quick responsiveness with high energy and power densities.¹ Supercapacitors (SCs) have emerged in recent years as a promising energy storage technology due to their high cyclic stability, high power density, and fast charging/discharging rates.^2–4 Many applications require high power rather than high energy, such as phone chargers, regenerative braking systems, and portable vacuum cleaners.^5,6 Therefore, supercapacitors can serve as an alternative to conventional lithium-ion batteries.⁷ Supercapacitors are intermediate between conventional capacitors and rechargeable batteries, but they have a similar configuration with two electrodes—an anode and cathode—immersed in an electrolyte, separated by a membrane.⁸

There are two types of supercapacitors based on the charge storage mechanism: electrical double-layer capacitors (EDLCs) and pseudocapacitors.^9,10 In EDLCs, charge is stored electrostatically at the interface between the electrolyte and the surface of the electrodes without any faradaic reaction (referred to as non-faradaic). During charging, the ions in the electrolyte move toward their respective electrodes, and an inner Helmholtz layer is formed, known as the double layer, which enhances the specific capacitance.^11–13 In pseudocapacitors, the charge storage mechanism is achieved through redox reactions (oxidation–reduction), electrosorption, or ion intercalation mechanisms.¹⁴ Thus, pseudocapacitors store charge both electrostatically and electrochemically, leading to higher specific capacitance than EDLCs. Carbon-based materials are commonly used as electrode materials in EDLCs due to their facile processing, chemical stability, wide operational temperature range, ability to function without binder agents, and well-established activation techniques.¹⁵ EDLCs store the charge at their electrochemical surfaces; however, they have some drawback, such as a low energy density and specific capacitance, along with excessive cost.^13,16 Pseudocapacitive-type materials, on the other hand, exhibit higher energy densities but possess shorter life cycles. The shorter life cycles of pseudocapacitive materials can be improved by employing conductive, sustainable methods to synthesize transition metal oxides.¹⁷ Chi-Chang Hu et al. first reported that RuO, a transition metal oxide, showed pseudocapacitive behavior due to its various corrosive states that made the faradaic process possible. However, while much work followed on transition metal oxides in this field, they are generally too costly.^18,19 Meanwhile, perovskite oxides have attracted increasing attraction for different applications and now as potential electrode materials in supercapacitors due to their structural flexibility, low cost, high charge-carrier mobility, compositional and stoichiometric flexibility, high tapping density, and appropriate oxygen vacancies.^9,20 The presence of oxygen vacancies in perovskite-based materials makes the intercalation process faster than in transition metal oxides. Various perovskite oxides have been investigated as active materials for SCs due to their high number of oxygen vacancies and high tapping density.²¹ Recently, SrCoO has been explored as a potential electrode material for supercapacitors. It has been reported that SrCoO_3−δ perovskite with a cubic structure exhibits higher conductivity than other structures, but it faces issues with achieving structural stability at room temperature.²² However, it has been reported that doping different dopants at the A or B site can help SrCoO_3−δ attain structural stability. Furthermore, it was found that the substitution of a mere 5% can introduce significant changes in the conductivity, structure, and oxygen diffusion of perovskite structures. It has also been reported that dopants such as Ce⁴⁺, Y⁴⁺, Zr⁴⁺, Al³⁺, and Sc³⁺ can increase the oxygen permeability and conductivity of perovskites by 1–2 orders of magnitude due to the high concentrations of oxygen vacancies present.^23–26 Considering the aforementioned findings, the doping of SrCoO with Ba and Ce could be expected to have a significant impact on the structural stability and conductivity of SrCoO_3−δ. It was previously reported that the larger ionic radii of Ba ions than Sr ions would cause lattice distortions and lead to the formation of more oxygen vacancies to maintain charge neutrality.²⁷ Similarly, Ce doping can also lead to the formation of oxygen vacancies. This is because Ce can exist in multiple oxidation states, primarily Ce³⁺ and Ce⁴⁺, and the transition between these states can lead to the formation or annihilation of oxygen vacancies to maintain charge neutrality.²⁸ Generally, the charge-storage capacity of the perovskites is dependent on the availability of oxygen vacancies, and therefore, the doping of Ba and Ce in SrCoO_3−δ would not only lead to enhanced electrochemical performance for supercapacitor electrodes but at the same time would also improve their structural stability.^29,30

In addition to the investigation of the novel composition of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) as perovskite electrodes for supercapacitors, the current project also developed an innovative synthesis approach in which organic citric acid (lemon) and biomolecules played a synergistic role as reducing and capping agents to facilitate the synthesis of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) for the first time. Although chemical approaches are commonly used to synthesize perovskites, they involve the use of hazardous solvents and reagents, posing substantial ecological and health concerns.³¹ In contrast, the reported green approach can mitigate these concerns, but the end product obtained through green approach lacks phase purity and the desired crystallinity, and suffers from a reduced reproducibility, and lower yield.³² Therefore, balancing the efficiency and environmental impact of both synthesis routes remains a critical challenge for the advancement of perovskite technology. The novel approach we propose can attain this balance by simultaneously utilizing the reducing and capping role of both biomolecules and organic citric acid present in lemon to synthesize Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes.

Herein, the Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite composition was synthesized for the first time for use as electrodes for pseudocapacitive applications. In addition, an innovative semi-green approach was also employed to synthesize this novel composition along with a conventional chemical approach for comparison. Our novel semi-green approach utilized the synergy of biomolecules and the organic citric acid present in lemon as reducing and capping agents, which enabled more control over the synthesis process, together with minimized toxicity compared to the conventional approach and impurities compared to the green approach. The synthesized electrodes were investigated through various characterizations.

2. Experimental section

Semi-green and chemical combustion methods were employed to synthesize Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) as anode materials for supercapacitors. The starting precursors Sr (NO₃)₂, Co (NO₃)₂·6H₂O, Ba (NO₃)₂·6H₂O, and Ce (NO₃)₂·6H₂O in an exact stoichiometric ratio were dissolved in 200 mL deionized water under constant stirring and heating to prepare a transparent solution. Lemon powder (LP) and citric acid (CA) were used as chelating agents in 20–21 wt% of the total precursor solution. Next, the clear precursor solution was put in a ceramic beaker, and placed on a magnetic hot plate. The solution was continuously stirred and heated at 80 °C on the hot plate. The viscosity of the homogeneous solution was increased by boiling off the water and it changed into a gel at 110 °C. Further heating evaporated all the water from the pores of the gel and this then self-ignited to produce a fine powder of perovskite materials. The obtained powder was then calcinated at 200 °C for 1 h and then sintered at 1150 °C for 5 h to obtain a further fine powder. The complete synthesis process is illustrated schematically in Fig. 1. Pellets of the sintered powder were formed using a hydraulic press under a pressure of 300 MPa and the samples were then characterized by different techniques, including XRD, FESEM, EDX, FTIR, CV and GCD.

Fig. 1 Schematic of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite anode materials synthesized through semi-green (LP) and chemical (CA) approaches.

All the synthesized samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Fourier-transform infrared spectroscopy (FTIR) to study their structural properties, morphology, elemental composition, and functional groups. The electrochemical performances of the synthesized electrodes in a 1 M KOH electrolyte solution were assessed by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) analyses.

3. Results and discussions

Fig. 2 represents the XRD spectra of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through semi-green (LP) and chemical (CA) approaches. The diffraction planes (100), (101), (110), (111), (200), (211), (220) and (221) confirmed their cubic structure (JCPDS = 00-038-1148) with the space group Pm3m for all the Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) samples synthesized with either the semi-green (LP) or chemical (CA) approach.

Fig. 2 XRD spectra of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through semi-green and chemical approaches, showing the cubic perovskite structure for all samples with no extra phases. However, a small BaO phase appeared for Sr_0.9Ba_0.1O_3−δ synthesized through the chemical approach.

It was clear that Sr_0.9X_0.1CoO_3−δ synthesized by the semi-green (LP) synthesis exhibited no secondary phases. Meanwhile, it was interesting to observe that Sr_0.9X_0.1CoO_3−δ (X = Ba) synthesized with the chemical approach exhibited a secondary phase of BaO, which was unexpected for the chemical approach. The presence of this secondary phase could be attributed to the low sintering temperature, as previously reported by researchers, wherein the BaO phase appeared for samples sintered at temperatures lower than 1400 °C.^33–38 The absence of BaO in Sr_0.9X_0.1CoO_3−δ (X = Ba) synthesized with the semi-green (LP) approach could be credited to the presence of metallic impurities, which may have acted as a sintering aid.

Table 1 presents the variations in the structural parameters, such as crystallite size, dislocation density, microstrain, and lattice constant, for Sr_0.9X_0.1Co O_3−δ synthesized through the semi-green (LP) and chemical (CA) approaches.

Table 1 Structural parameters of Sr_0.9X_0.1CoO_3−δ perovskite anodes, showing larger crystallite size for the semi-green (LP)-synthesized anodes compared to the chemically (CA)-synthesized anodes

Sample	Crystallite size (nm)	Lattice constant (nm)	Dislocation density (m^−2)	Microstrain (nm)
Sr0.9Ce0.1CoO (semi-green method)	49	0.072731	0.401214	0.002452
Sr0.9Ce0.1CoO (chemical method)	47	0.072745	0.446863	0.002586
Sr0.9Ba0.1CoO (semi-green method)	56	0.072764	0.317514	0.00218
Sr0.9Ba0.1CoO (chemical method)	52	0.072804	0.367166	0.002343

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Fig. 3(a) and (b) present the crystallite sizes and lattice constants of Sr_0.9X_0.1Co O_3−δ (X = Ba, Ce) perovskite anodes synthesized through the semi-green (LP) and chemical (CA) routes. It is clear from Fig. 3(a) that the crystallite sizes of Sr_0.9X_0.1Co O_3−δ (X = Ba, Ce) synthesized by the semi-green (LP) approach were larger than those obtained from the chemical (CA) approach, which could be ascribed to the fact that LP contained metallic impurities, such as Mg and Ca, which may act as a sintering aid to promote crystal diffusion, and hence the crystallite size increased.^39–41

Fig. 3 Variation of (a) crystallite size and (b) lattice constant for the (110) plane of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through semi-green (LP) and chemical (CA) approaches.

It can be further inferred from Table 1 and Fig. 3(a) that comparing the semi-green (LP) and chemically (CA) synthesized Sr_0.9X_0.1CoO_3−δ and Sr_0.9Ba_0.1CoO_3−δ, Sr_0.9Ba_0.1CoO_3−δ exhibited a larger crystallite size than Sr_0.9Ce_0.1CoO_3−δ, which implies that Ba ions have larger ionic radii and with their incorporation in the lattice they occupy the interstitial sites, leading to lattice expansion and larger grain boundaries.⁴² Another reason for the small crystallite of Sr_0.9Ce_0.1CoO_3−δ compared to Sr_0.9Ba_0.1CoO_3−δ is the small ionic radii of Ce ions, resulting in the effective incorporation of Ce into the lattice, which may enhance the grain boundary formation and inhibit the grain growth, leading to finer crystallites.⁴³

Similarly, Fig. 3(b) presents the variation of the lattice constant of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through the semi-green (LP) and chemical (CA) approaches, showing the smaller lattice constant for the semi-green synthesis, which may be ascribed to the fact that the sustainable approach (LP) improved the crystallinity leading to smaller defect densities in the lattice formation.⁴⁴ It can be further inferred from Table 1 and Fig. 3(b) that Sr_0.9Ce_0.1CoO_3−δ had a smaller lattice constant compared to Sr_0.9Ba_0.1CoO_3−δ, which was ascribed to the small difference in the ionic radii of Ce ions compared to Ba ions.⁴⁵

Fig. 4(a) and (b) present the variation of the dislocation density and microstrain of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through the two approaches (semi-green and chemical). It is clear from Fig. 4(a) that the dislocation density was higher for the semi-green (LP) method due to the presence of metallic impurities, which influenced the lattice symmetry and led to larger dislocations.⁴⁶ It can be observed from Fig. 4(b) that the samples synthesized by the semi-green (LP) route had larger microstrain compared to those synthesized by the chemical (CA) route due to the presence of metallic impurities, such as Mg and Ca; whereby the semi-green method led to higher lattice distortions. Similarly, Sr_0.9Ce_0.1CoO_3−δ showed higher microstrain than Sr_0.9Ba_0.1CoO_3−δ, which may be due to the higher valency of Ce³⁺ than Ba²⁺, thus more oxygen vacancies were produced and resulting in higher lattice distortion in the unit cell,^42,47 as shown in Fig. 4(b).

Fig. 4 Variation of (a) dislocation density and (b) microstrain for the (110) plane of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through the semi-green (LP) and chemical (CA) approaches.

4. Scanning electron microscopy (SEM) analysis

Fig. 5(a)–(d) present the SEM images of Sr_0.9X_0.1CoO_3−δ synthesized through the semi-green (LP) and chemical (CA) routes, while Fig. 5(e)–(h) show their corresponding histograms. It is clear from the SEM image (Fig. 5(a)) of the Sr_0.9Ba_0.1CoO_3−δ anode synthesized through the semi-green (LP) approach that it exhibited a regular and porous structure with a more defined shape and less agglomeration compared to the Sr_0.9Ba_0.1CoO_3−δ anode (Fig. 5(b)) synthesized with the chemical (CA) approach, which could be credited to the presence of biomolecules and organic citric acid present in the lemon powder. It has been reported that biomolecules present in organic compounds can act as both reducing and capping agents during the semi-green synthesis; therefore, the combined effect of organic citric acid and biomolecules present in the lemon powder can provide more control over the growth and agglomeration of the nanoparticles.^48–50 The microstructure of the Sr_0.9Ba_0.1CoO_3−δ anode synthesized through the chemical method showed irregular particle shapes with multiple crevices and protrusions, indicating the heterogeneous nature of the particles. Fig. 5(e) and (f) show the range of the Sr_0.9Ba_0.1CoO_3−δ anodes’ particle sizes synthesized by the two approaches. It is evident from the statistical histogram (Fig. 5(e)) that the particle size of Sr_0.9Ba_0.1CoO_3−δ synthesized through the semi-green (LP) route ranged from 10–55 nm, with most particles existing in the 20–25 nm range, while the particles for the Sr_0.9Ba_0.1CoO_3−δ anode synthesized through the chemical (CA) route lay in a wider range from 10–110 nm, with most particles existing in the 80–90 nm range. This variation in the particle sizes of the Sr_0.9Ba_0.1CoO_3−δ synthesized through the two routes clearly showed that the synergistic role of natural citric acid and biomolecules in the semi-green (LP) route effectively reduced and capped the Sr_0.9Ba_0.1CoO_3−δ compared to the chemical synthesis route, where only synthetic citric acid acted as the reducing and capping agent.^51,52

Fig. 5 SEM images and corresponding histograms (a, e) Sr_0.9Ba_0.1CoO_3−δ anode synthesized through the semi-green (LP) approach, exhibiting a regular and porous structure; and (b, f) Sr_0.9Ba_0.1CoO_3−δ anode synthesized through the chemical (CA) approach, showing agglomeration; (c, g) Sr_0.9Ce_0.1CoO_3−δ anode synthesized through the semi-green (LP) approach; and (d, h) Sr_0.9Ce_0.1CoO_3−δ anode synthesized through the chemical (CA) approach, showing agglomeration.

The microstructures of Sr_0.9Ce_0.1CoO_3−δ synthesized through both approaches are shown in Fig. 5(c) and (d), while the corresponding histograms for the particle sizes are shown in Fig. 5(g) and (h). The Sr_0.9Ce_0.1CoO_3−δ anode exhibited a similar pattern to that of Sr_0.9Ba_0.1CoO_3−δ, whereby the Sr_0.9Ce_0.1CoO_3−δ anode particles synthesized through the semi-green (LP) route exhibited small and round particles with less agglomeration, while the chemically synthesized Sr_0.9Ce_0.1CoO_3−δ anode had larger and more agglomerated particles. The particle sizes of the semi-green-synthesized Sr_0.9Ce_0.1CoO_3−δ lay in the 4–24 nm range, with most particles existing in the 8–12 nm range, whereas the sizes of the chemically synthesized particles were in the 10–120 nm range, with most particles having a size of 40–50 nm.

It can be further inferred from Fig. 5(a) and (d) that between the semi-green (LP) and chemically (CA) synthesized Sr_0.9Ce_0.1CoO_3−δ and Sr_0.9Ba_0.1CoO_3−δ, Sr_0.9Ba_0.1CoO_3−δ exhibited larger crystallite sizes compared to Sr_0.9Ce_0.1CoO_3−δ, which implies that the Ce ions in Sr_0.9Ce_0.1CoO_3−δ can act as nucleation sites, facilitating the production of nuclei during the synthesis process. Ce can accelerate crystal growth by occupying these favorable sites, providing stable anchoring points for the precursor molecules to attach and arrange themselves, thus facilitating nucleation, resulting in the formation of smaller particles with a more uniform size distribution.

5. EDX analysis

Fig. 6(a)–(d) present the EDX spectra of the Sr_0.9X_0.1CoO_3−δ anode materials synthesized through the semi-green (LP) and chemical (CA) routes. The surface areas were carefully selected for the EDX analysis to avoid surface contaminants. The qualitative spectra shown in Fig. 6(a)–(d) confirmed the presence of strontium, cobalt, and oxygen in all the samples, while Ba and Ce were confirmed in Sr_0.9Ba_0.1CoO_3−δ and Sr_0.9Ce_0.1CoO_3−δ synthesized by both routes, respectively. The quantitative data shown in the inset provide the percentage elemental compositions of the Sr_0.9X_0.1CoO_3−δ anode materials. It can be observed from the spectra that no impurity-related peaks appeared, depicting the high purity of the Sr_0.9X_0.1CoO_3−δ anodes. The absence of impurity-related peaks was unexpected for Sr_0.9X_0.1CoO_3−δ synthesized through the semi-green (LP) approach because lemon powder contains several minerals, but their absence from the spectra can be ascribed to their concentration being lower than the detection limit of EDX.^53,54 The compositional analysis confirmed that the semi-green route can be employed for the successful synthesis of Sr_0.9X_0.1CoO_3−δ anodes without significant impurities.

Fig. 6 EDX spectra of Sr_0.9X_0.1CoO_3−δ (a) doped with (X = Ba) using citric acid as a chelating agent, (b) doped with (X = Ba) using lemon powder as a chelating agent, (c) doped with (X = Ce) using citric acid as a chelating agent, and (d) doped with (X = Ce) using lemon powder as a chelating agent.

6. Fourier-transform infrared (FTIR) analysis

Fig. 7 depicts the FTIR spectra of Sr_0.9X_0.1CoO_3−δ (a) doped with Ba, and (b) doped with Ce, synthesized using citric acid and lemon powder as chelating agents. The small peaks around 3735, 2980, 2361, 1461, and 994–911 cm⁻¹ were related to O–H stretching, C–H stretching, O=C=O stretching, C–H bending and M–O stretching, respectively, in all the samples. Oxygen vacancies in perovskite oxides may cause hydroxyl groups (OH⁻) to develop on the surface. These hydroxyl groups may have originated from the interaction of surface oxygen species with ambient moisture, and may be responsible for the peaks for O–H stretching vibrations in the FTIR spectra.⁵⁵ The C–H stretching and bending vibrations may arise due to the presence of organic residue within the synthesized material derived from the chelating agents used, i.e., citric acid and lemon powder. The peaks associated with O=C=O stretching vibrations imply the presence of carbonate species in the produced materials. The existence of carbonate species could be due to ambient CO₂ absorption or residual carbonate from the precursor materials. Also, there was an indication of M–O stretching, which implied the formation of metal oxides, showing the perovskite nature of the samples, as previously reported in the literature.^56–62

Fig. 7 FTIR spectra of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce), synthesized through semi-green (LP) and chemical (CA) approaches.

7. Electrochemical analysis

a. Cyclic voltammetry (CV)

The electrochemical performances of the Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes synthesized through the semi-green (LP) and chemical (CA) approaches were evaluated through cyclic voltammetry and are shown in Fig. 8. Fig. 8(a) displays the CV curves of the Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes synthesized through the semi-green (LP) and chemical (CA) approaches in 1 M KOH in a potential window range of 0.1–0.8 V at a scan rate of 20 mV s⁻¹. It can be observed from CV curves that all the materials exhibited a pseudocapacitive behavior because of the faradaic redox reactions. Also, these curves displayed rectangular and steep slopes, suggestive of higher power densities.⁶³ It is clear from Fig. 8(a) that the Sr_0.9X_0.1CoO_3−δ electrodes synthesized through the semi-green (LP) route exhibited better capacitance compared to the same electrodes synthesized through the chemical (CA) route. The specific capacitance (C_s) of the working electrodes was calculated using the following equation:⁶⁴

Fig. 8 (a) Cyclic voltammetry curves of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) within the potential window range of −0.1 to 0.7 V at a scan rate of 20 mV s⁻¹. (b) Specific capacitance of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce). (c) Anodic and cathodic peak currents obtained from the CV curves of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through the semi-green (LP) and chemical (CA) approaches.

The specific capacitances of Sr_0.9Ce_0.1CoO_3−δ synthesized by the semi-green (LP) and chemical (CA) approaches were 1176 and 871 F g⁻¹, respectively, while Sr_0.9Ba_0.1CoO_3−δ synthesized through the semi-green (LP) and chemical (CA) routes exhibited specific capacitances of 475 and 284 F g⁻¹, respectively, and are depicted in Fig. 8(b). Fig. 8(c) shows the anodic and cathodic current peaks of Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) synthesized through both routes. It can be observed from Fig. 8(a)–(c) that the semi-green-synthesized Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) electrodes exhibited better capacitance compared to the corresponding chemically synthesized Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) electrodes, which could be ascribed to the presence of metallic impurities in the lemon powder, which can facilitate the redox process due to the availability of higher charge carriers. In addition, the large crystallite size of the semi-green-synthesized electrodes compared to the chemically synthesized electrodes may also contribute to the better electrochemical performance.⁶⁵ Generally, a larger crystallite size leads to the reduction in the grain boundary resistance, which reduces the number of scattering sites. This permits the electrons to move freely through the material, enhancing the electrical conductivity, which facilitates the charge carriers moving faster, leading to rapid charging and discharging.^66–68

It was interesting to observe that among the semi-green and chemically synthesized Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) electrodes, the Sr_0.9Ce_0.1CoO_3−δ electrode exhibited better performance compared to Sr_0.9Ba_0.1CoO_3−δ, which could be ascribed to the higher valence state of Ce³⁺ than Ba²⁺, which can provide more charges, leading to higher oxygen vacancies and resulting in a better electrochemical performance.^69–71

b. Galvanostatic charge–discharge (GCD)

GCD tests were performed on Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes synthesized through the semi-green (LP) and chemical (CA) approaches. These tests, conducted in 1 M KOH electrolyte within a 0–0.45 V potential window (vs. Ag/AgCl), compared the electrochemical behavior between the 1st and 5000th cycles. Fig. 9 depicts the GCD curves of Sr_0.9Ce_0.1CoO_3−δ for both the 1st and 5000th cycles, highlighting the material's impressive performance over extended cycling. Notably, the GCD tests revealed exceptional capacity retention after 5000 cycles, as evidenced by the comparative curves in Fig. 9, showing that the semi-green-synthesized Sr_0.9Ce_0.1CoO_3−δ yielded a specific capacitance of 1176 F g⁻¹ with 88.2% capacitance retention after 5000 cycles, demonstrating performance comparable to, and in some instances surpassing, previously reported perovskite materials. For example, MnO₂@SrCo_0.875Nb_0.125O₃@CC exhibited a specific capacitance of 2066.0 mF cm⁻²,⁷² while SrFe_0.85Zr_0.15O_3−δ achieved a specific capacitance of 163.92 F g⁻¹.⁷³ Similarly, Ti-substituted SrCo_0.9Ti_0.1O_3−δ delivered a specific capacitance of 625.0 F g⁻¹, attributed to its stable cubic structure and abundant oxygen vacancies.²² Cr-substituted SrCo_0.95Cr_0.05O_3−δ@CC, reported by Jiahao et al., maintained 95.8% capacitance after 10 000 cycles in an asymmetric configuration with PPy@CC.⁷⁴ Other perovskites, such as SrCoO_2.5 (475 F g⁻¹, 85% retention after 2000 cycles) and LaMnO₃-based electrodes (720 F g⁻¹, 80% retention after 3000 cycles), have also been documented.^27,29 Table 2 provides a further comprehensive comparison among different supercapacitor electrodes.

Fig. 9 Galvanostatic charge–discharge (GCD) plots of Sr_0.9X_0.1CoO_3−δ (X = Ce) perovskite electrodes synthesized through the semi-green (LP) approach, comparing the electrochemical behavior between the 1st and 5000th cycles.

Table 2 Comprehensive comparison of different electrodes

Electrode material	Cycling stability (%)	Charge–discharge rate (A g^−1)	Operating voltage window (V)	Specific capacitance (F g^−1)	Ref.
Ni3Se4/Co3Se4	83.4	1	1.5–1.8	1120.4	75
Reduced graphene oxide	97.14 (5000 cycles)	0.2	0.2–1.0	585.44	76
SbTe/SnSe	96.08 (5000 cycles)	1–3	0.1–0.7	1276	77
RGO/LaAlO3	55.47 (5000 cycles)	0.5	0–1.2	283	78
La0.75Sr0.25Cr0.5Mn0.5O3 LSCM	92 (5000 cycles)	10	0.2–1.0	630	79

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The enhanced stability obtained for the electrodes could be ascribed to the semi-green synthesis approach, which leveraged lemon powder-derived biomolecules and citric acid to minimize agglomeration, enhance the crystallinity (as confirmed by XRD and SEM), reduce defects, and improve the ionic conductivity. In contrast, the chemically synthesized counterparts, such as Sr_0.9Ba_0.1CoO_3−δ, suffer from accelerated capacity fading due to their irregular morphologies and secondary phases (e.g., BaO). Furthermore, the findings also reflect the extraordinary structural integrity of our perovskite framework, even under repeated ion insertion and extraction. The selective substitution of strontium with barium and cerium enhanced the lattice stability and electrochemical reversibility, while the semi-green (LP) and chemical (CA) synthesis methods yielded optimal particle shapes and crystallinity. These factors collectively minimize the degradation mechanisms, such as phase transitions or resistive layer formation, ensuring consistent performance over 5000 cycles, as visually demonstrated in Fig. 9. It is worth noting that 5000 cycles is a widely accepted benchmark in energy-storage research, providing a reliable indicator of material stability for preliminary evaluations. The superior performance of the semi-green-synthesized electrodes, as reflected in both the CV data and the GCD curves, could also be attributed to metallic impurities (e.g., Mg and Ca) from lemon powder, which can boost redox processes, and the larger crystallite sizes, which can reduce the grain boundary resistance, thereby enhancing the electrical conductivity. In addition, the semi-green route followed for the synthesis of perovskite electrodes is more efficient compared to the already existing green routes used to synthesize perovskite electrodes for the supercapacitors. A comparative analysis of some reported green routes is shown in Table 3.

Table 3 Performance metrics of some reported green-synthesized perovskite-based supercapacitors

Electrode material	Synthesis approach	Energy density (W h kg^−1)	Power density (W kg^−1)	Specific capacitance (F g^−1)	Ref.
LaMnO3	Green synthesis (Lemon juice)	52.5	1000	375	80
RGO/LaAlO3	Green synthesis (Green tea)	57	569	721	78
Ag-Zirconia	Green synthesis (Sauropus androgynus)	31.94	500.86	256	81
KMnCl3/C60	Antisolvent (KCl and MnCl2)	150.58	3.1	936	82
SnO2/g-C3N4	Green synthesis (Ananas comosus)	11.5	1705	302.7	83

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The remarkable stability exhibited by Sr_0.9X_0.1CoO_3−δ (X = Ce) synthesized through the semi-green route over 5000 cycles, paired with its high capacitance, can position these electrodes and the semi-green routes as promising candidates for next-generation energy-storage devices, where long-term operational reliability and sustainability are paramount.

8. Conclusions

In summary, Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) materials were synthesized for the first time as perovskite electrode materials for supercapacitors using an innovative semi-green approach leveraging lemon powder as a bio-chelating agent. This pioneering eco-friendly method achieved high-purity perovskite electrodes with minimal impurities, highlighting its potential for sustainable supercapacitor development. The synthesized electrodes were investigated and compared with chemically synthesized electrodes through XRD, FESEM, EDX, FTIR, and CV analyses. The XRD analysis validated the successful synthesis of the Ba- and Ce-doped Sr_0.9X_0.1CoO_3−δ perovskite structure through a semi-green approach with no secondary phase, as evidenced by the characteristic diffraction peaks. The FESEM micrographs revealed a homogenous microstructure with well-defined grains for the semi-green-synthesized Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce), which was attributed to the synergistic role of biomolecules and bio-chelating agents present in the lemon powder, while the chemically synthesized perovskite anodes exhibited irregular-shaped particles. The EDX mapping confirmed the presence of all the expected elements in all the samples synthesized by both approaches, while FTIR provided insights into the vibrational modes and confirmed the presence of the identical functional groups in all samples, further verifying the structural integrity of the materials synthesized by both approaches. Electrochemical performance evaluations by CV indicated that the semi-green-synthesized Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) anodes exhibited promising redox behavior, with distinct oxidation and reduction peaks, compared to the chemically synthesized anodes. It was also observed that Ce-doped Sr_0.9X_0.1CoO_3−δ exhibited better electrochemical performance than Ba-doped Sr_0.9X_0.1CoO_3−δ, suggesting its good potential for application in energy-storage and -conversion devices. GCD analysis of the Sr_0.9X_0.1CoO_3−δ (X = Ce) perovskite electrode demonstrated its remarkable electrochemical stability with 88.2% capacity retention after 5000 galvanostatic charge–discharge cycles, demonstrating that strategic A-site substitution could effectively preserve the electrode's performance over extended cycling. In conclusion, it can be deduced that not only did the Sr_0.9X_0.1CoO_3−δ (X = Ba, Ce) perovskite electrodes demonstrate significant potential as advanced materials for electrochemical storage applications, but the semi-green (LP) route can be effectively used as a synthesis approach for the synthesis of perovskite structures with minimal impurities or ecological impact.

Data availability

The data will be made available upon request.

Conflicts of interest

The authors declare no competing interests or conflicts of interest.

Acknowledgements

The authors extend their appreciation to the Deanship of Research and Graduate Studies at King Khalid University for supporting this work through the Large Research Project under RGP2/125/46.

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