CaSr_xCu_3−xTi₄O₁₂ Ceramic Oxide Modified with Graphene Oxide and Reduced Graphene Oxide for Supercapacitor Applications

Abstract

This study investigates CaCu_3−xSr_xTi₄O₁₂ (CCSTO) systems synthesized using the solid-state method, with x compositions of 0.00, 0.15, and 3.00. The samples were modified using 6 wt% graphene oxide (GO) and reduced GO (rGO) prepared via Hummer’s method to evaluate their performance as electrodes in supercapacitors. The results indicate that the addition of 6wt% rGO to CCTO (CCTO-6rGO) led to an improvement in specific capacitance, reaching 237.76 mF·g⁻¹ at a scan rate of 10 mV/s, compared to 29.86 mF·g⁻¹ for pure CCTO and only 7.83 mF·g⁻¹ for CCTO-6GO, suggesting that rGO enhances charge storage. For the CCTO15Sr samples, CCTO15Sr-6rGO exhibited the highest specific capacitance, with 321.63 mF·g⁻¹ at 10 mV/s, surpassing both pure CCTO15Sr (80.19 mF·g⁻¹) and CCTO15Sr-6GO (25.73 mF·g⁻¹). These results stem from oxygen and metal vacancies, which aid charge accumulation and ion diffusion. In contrast, adding GO generally reduced specific capacitance in all samples. The findings highlight CCSTO’s potential—especially with rGO modification—as a supercapacitor electrode while also indicating areas for further optimization.

1. Introduction

The global electricity demand is rising, driven by high industrialization and population growth. This surge in demand needs technologies that support not only sustainable systems but also those that are autonomous. Efficient energy storage solutions are essential in this context [1]. These solutions enable better electricity supply and demand management, which is why they are undergoing extensive research and development. Their goal is to optimize the use of renewable energy sources and to provide the necessary flexibility and support for the future challenges of the energy distribution and consumption [2]. Moreover, current energy storage technologies can enhance the grid’s reliability and efficiency by improving the integration of renewable sources such as wind and solar [3,4].

The growing need for more efficient energy storage solutions, particularly in terms of capacity and charging time, has driven significant development in recent years [5]. Supercapacitors (SCs), sometimes called ultracapacitors, represent a promising field of research that falls into three main types that encompass electric double-layer capacitors (denoted as EDLCs), pseudocapacitors, and hybridcapacitors. The distinct properties of SCs, namely high power density, long lifespan, and fast charge/discharge rates, are emerging as an indispensable medium for various applications, including electric vehicles and portable electronic systems [6,7]. Additionally, SCs are being explored for their potential to address intermittency and energy quality issues in photovoltaic power generation systems [8].

As research in this area progresses, it is becoming increasingly evident that SCs could revolutionize a wide range of energy-related fields, making them an intriguing area of study for scientists and engineers [9]. The quest for materials boasting expansive surface areas is at the heart of developing efficient supercapacitors. This stems from the direct relationship between a supercapacitor’s energy storage capacity and the surface area of its electrodes exposed to the electrolyte [10,11]. Put simply, a larger surface area translates to greater charge storage potential. This principle holds particular significance in EDLCs, where energy storage primarily relies on charge accumulation at the electrode and electrolyte interface. Consequently, high-performance supercapacitors often utilize electrode materials known for their extensive specific surface areas, examples including activated carbon, carbon nanotubes, and graphene [12]. Owing to their capacity for faradaic redox reactions, metal oxides are usually employed as electrode materials, most notably in the design of pseudocapacitors [13]. These reactions contribute to an enhanced charge storage capacity compared to the electric double-layer mechanisms present in EDLC supercapacitors [10,14].

Recent progress in materials science has been focused on improving the properties and performance of supercapacitors. In this pursuit, perovskite-structured systems are showing significant promise, especially since some studies report that non-stoichiometric systems can increase charge transport when used as electrodes [15,16,17,18,19,20,21,22,23]. Among these perovskite-type materials, calcium copper titanate (CCTO) has captured considerable scientific and technological attention, owing to its remarkable dielectric properties [16], varistor behavior [17], photoluminescence [18], and, in addition, sensing abilities [19]. CCTO ceramics show giant permittivity (ε ≈ 10⁴), a phenomenon explained by the internal barrier layer capacitor (IBLC) model; however, their relatively high dielectric losses limit practical applications. To overcome this limitation, doping with elements such as Ni, Sr/Zr, Sr/Mg, or Li has been used as an effective strategy to balance high permittivity and low dielectric losses, making it more viable for many electronic applications [20]. More importantly, its performance can be significantly enhanced through strategic chemical modifications. Ji et al. explored the potential of the supercapacitor-based piezoelectric composite (Na,K)NbO₃–CaCu₃Ti₄O₁₂ (NKN–CCTO) for use in piezoelectric devices [21]. Their results showed that the dielectric and piezoelectric properties are highly dependent on composition and sintering temperature. Additionally, the highest permittivity (796) was achieved by 0.94NKN–0.06CCTO sintered at 975 °C, while a balanced permittivity (405) and piezoelectric constant (98 pC/N) were shown by 0.985NKN–0.015CCTO sintered at 1025 °C. For instance, Cortés et al. [22] found that incorporating low levels of Sr²⁺ leads to surprisingly higher dielectric constant values (ε ≈ 3.28 × 10⁵), even surpassing CCTO, making it interesting for conventional capacitors. However, for higher levels of Sr²⁺, the system presented higher resistance values, making it more suitable for non-ohmic devices. On the other hand, Sr substitution in CCTO markedly increases both grain (Rg) and grain-boundary (Rgb) resistances, thereby suppressing DC transport and lowering dielectric loss, Mechanistically, Sr lowers Ti³⁺ and oxygen-vacancy populations while raising Cu⁺, which limits Ti³⁺/Ti⁴⁺ hopping and strengthens internal barriers to charge migration [23]. Rhouma et al. [24] showed that increasing Sr content (with Mg co-doping) raises the dielectric constant while preserving near frequency-independence over 10²–10⁶ Hz. In the same study, Ca₀.₉Sr₀.₁Cu₂.₉Mg₀.₁Ti₄O₁₂ reached a low loss tangent of ~0.05 at 1 kHz, indicating cleaner capacitive behavior. Beyond these applications, modifying CCTO also shows promise for energy storage. Kumar et al. [25] substituted Cu²⁺ with Mn²⁺ and studied the electrochemical behavior, showing a decrease in the charge resistance for the sample with Mn compared to the pure sample. Modified CCTO or perovskite systems, sometimes with Sr, demonstrate improved specific capacity and lower resistance compared to pure CCTO.

Graphene oxide (GO) and reduced GO (rGO) are materials of significant interest in the supercapacitor field. Owing to their large specific surface areas promote excellent electrolyte interaction to boosting the storage capacity [26], and in parallel, recent research has intensively explored their fundamental impact on charge transport mechanisms within these devices [25,26].

Graphene and its derivatives acted as powerful interface-engineering agents in CCTO. In polymer composites, Qi et al. [27]. showed that co-aligning graphene sheets with CCTO nanowires in PVDF built sub-percolative “micro-capacitors” (graphene–CCTO–graphene) that raised the dielectric constant while keeping the loss tangent low across a broad frequency range; the layered architecture blocked continuous leakage paths and amplified Maxwell–Wagner polarization without sacrificing insulation.

In ceramic systems, Qu et al. [28], reported that graphene networks tuned transport from hopping to more Drude-like behavior and even enabled negative permittivity at radio frequencies—benefits that required tight control near the percolation threshold to avoid increased low-frequency losses and weakening of IBLC barriers. At the defect/interfacial level, Ahmadipour et al. [29] and Praxedes et al. [30] found that rGO promoted interfacial charge transfer and reconfigured defect chemistry (Cu/Ti valences, oxygen-vacancy population), strengthening interfacial polarization when used below percolation. Overall, rGO tended to deliver cleaner capacitive gains than GO (which can introduce extra dipolar loss if uncontrolled), and the most robust improvements came from pairing sub-percolative carbon architectures with microstructure-conscious processing.

In summary, although CCTO/CCSTO are promising, their evaluation as active supercapacitor electrodes remains underexplored, and the literature lacks head-to-head comparisons, under identical processing and testing conditions, between GO and rGO at controlled loadings. This work addresses the outstanding gap by systematically comparing CCSTO with a fixed 6 wt% of GO or rGO under a single processing–testing protocol, thereby isolating (i) the role of Sr-driven defect chemistry and (ii) the impact of graphene chemistry; we demonstrate a specific Sr–rGO synergy that reduces Rct and promotes vacancy-assisted charge storage, clarifying why rGO is beneficial whereas GO can be detrimental.

Therefore, this study investigates the supercapacitive properties of films based on CaCu_3−xSr_xTi₄O₁₂ (CCSTO) system, with specific compositions of x = 0.00, 0.15, and 3.00, in both pure and composite forms (with 6 wt% GO or rGO. Herein, we chose a 6 wt% GO/rGO concentration because preliminary studies indicated that different wt% loadings caused aggregation and compromised CCSTO film integrity). Our work encompasses the full scope of the process, from the material synthesis to an in-depth analysis of charge transport via ions or vacancy-type defects. The ultimate goal is to provide a more profound understanding of the electrochemical properties of pure and modified CCSTO films as electrodes in supercapacitor applications.

2. Materials and Methods

2.1. Synthesis of CaCu_3−xSr_xTi₄O₁₂ (0 ≤ x ≤ 3.0)

The powders were synthesized using the nominal formula CaCu_3−xSr_xTi₄O₁₂, with x ranging from 0.00 to 3.00 mol, to create pure phases of CCTO and CSTO and their composites in various ratios. The starting materials included TiO₂, CuO, CaCO₃, and SrCO₃ (all from Sigma-Aldrich, St. Louis, MO, USA, 99.9% purity). These were mixed and ground in a yttrium-stabilized zirconia medium within a polyethylene container with isopropyl alcohol for 24 h. Following a drying period at 100 °C for 12 h to eliminate the alcohol, the mixture underwent calcination. This calcination cycle was executed by heating the sample to 1050 °C at a rate of 5 °C/min, holding it at that temperature for 12 h, and finally cooling it to room temperature at 10 °C/min. Post-calcination, the powders were milled again for 2 h to break any particle coalescence that occurred during calcination. Finally, they were dried and sieved using a 200-mesh screen.

Our previous works extensively characterized the powders of the CaCu₃−_xSr_xTi₄O₁₂ system (0.00 ≤ x ≤ 3.00) in terms of their phase composition, particle size, and purity [22,31]. We observed that the Sr substitution progressively reduces the particle size from ~1.0 µm for x = 0.00 to ~0.05 µm for x = 3.00, while also modifying the CCTO/CSTO/CTO phase mixture. Although some compositions behaving as solid solutions in a crystallographic sense, we chose the term ‘ceramic composites’ to highlight the coexistence of distinct crystalline phases with different structural and functional roles.

2.2. Synthesis of Graphene Oxide

The synthesis of GO was carried out using a modified Hummers method, following an approach based on the work of Marcano et al. [32]. Initially, 3 g of graphite flakes (Sigma-Aldrich, St. Louis, MO, USA) were mixed with 18 g of potassium permanganate (KMnO₄, Vetec, Duque de Caxias, Brazil). This mixture was then gradually added to a solution of 360 mL sulfuric acid (H₂SO₄, SYNTH, Diadema, Brazil) and 40 mL phosphoric acid (H₃PO₄, SYNTH, Diadema, Brazil), stirring at 500 RPM. To achieve exfoliation, the mixture was subjected to continuous magnetic stirring at 50 °C for a 12-h period. Upon cooling to room temperature after the stirring, the mixture was transferred into 400 mL of distilled water ice, and 3 mL of 30% H₂O₂ (CAAL, São Paulo, Brazil) was subsequently added. Once the visible reaction ceased, and gas bubbles no longer appeared, the liquid was transferred to test tubes and centrifuged for 30 min at 4000 RPM. This step resulted in the sedimentation of the suspended solid, with the supernatant being removed and discarded. The sedimented material was then washed with distilled water, filling the tubes and mixing. After mixing, it was centrifuged again, and the supernatant discarded. This procedure was repeated three times with water, three times with 30% hydrochloric acid solution (SYNTH, Diadema, Brazil), and finally, three washes with ethanol (Sigma-Aldrich, St. Louis, MO, USA). After each stage, the material was mixed and centrifuged, with the supernatant discarded. When a pH greater than 5 was achieved, the material was then transferred to a Petri dish and subjected to a 5-h drying cycle in a vacuum oven at 60 °C.

2.3. Synthesis of Reduced Graphene Oxide

For the synthesis of the rGO was adapted from the procedure developed by Praxedes et al. [30]. For the oxidation step, 5 g of graphite was dispersed into 50 mL of H₂SO₄ under continuous stirring, with the temperature kept constant at below 10 °C via an ice bath. Subsequently, 6 g of KMnO₄ was slowly added to the graphite-acid suspension. The mixture was then subjected to a treatment cycle consisting of 20 min of constant stirring following by 10 min in an ultrasonic bath. This entire cycle of stirring and sonication was repeated a total of 12 times. Following the completion of these cycles, the reaction was terminated by adding 500 mL of deionized water, ensuring the mixture was kept under constant stirring and its temperature remained below 10 °C. The mixture was exfoliated in an ultrasonic bath for 2 h. Following exfoliation, the resultant solution-GO system underwent a rigorous washing procedure. This involved multiple washing cycles until the pH of the solution reached approximately 6, ensuring the removal of impurities. To reduce the GO, 500 mL of distilled water was added, and stirred for 1 h in an ultrasonic bath. Afterward, 500 mL of an ascorbic acid solution (C₆H₈O₆, SYNTH, Diadema, Brazil 20%) was added under constant stirring for 1 h. Finally, the system was heated to 90 °C ± 5 °C and stirred for 1 h. The resultant powder was then purified by filtration and washing, followed by drying at 85 °C for 12 h to obtain rGO.

2.4. Preparation of the Films and Electrodes

Initially, 60 mg of the CaCu_3−xSr_xTi₄O₁₂ with x = 0.00, 0.15, and 3.00 was weighed into a ceramic crucible and ground for 5 min. Three base combinations were prepared: the pure CCTO, CCTO15Sr, and CSTO. When incorporating 6wt% of GO and rGO, the samples were labeled with the respective additions, using suffixes -6GO or -6rGO. After grinding the powder, reagents were added, interspersed with 5-min grinding periods between each addition. The added reagents were: 60 μL of a PEG-1500 solution (0.3 mol L⁻¹) (Sigma-Aldrich^®) as a binding agent; 2 μL of acetylacetone (Metaquímica, Jaraguá do Sul, Brazil) as a complexing agent; 20 μL of PEG-1500 again; 2.4 μL of Triton X-100 (Sigma-Aldrich^®) as a dispersant; another 20 μL of PEG-1500; and 2 μL of absolute ethanol (Chemicals, São Paulo, Brazil) as a solvent. This process was repeated until the paste reached the appropriate consistency.

Electrodes were assembled using glass substrates coated with Indium Tin Oxide (ITO, In₂O₃·(SnO₂)_x), sized 2 × 1 cm². Initially, the ITO substrates underwent a successive ultrasonic cleaning process using a neutral Extran detergent in distilled water, followed by 95% ethyl alcohol, and then pure ethyl alcohol. After cleaning, they were dried with a hot air blower, weighed, and stored with the conductive side facing up. For fixing the electrodes, a 50 μm thick 3 M Scotch tape was used to isolate a central area of approximately 0.5 × 0.5 cm, leaving it exposed for material deposition. The material was deposited on the upper end of the 3 M tape and spread mechanically over the exposed ITO surface with a cylindrical glass rod. After material application, the 3 M tape was removed, and the electrodes underwent a calcination heat treatment in a JUNG muffle furnace (model LF00614) at 450 °C for 195 min.

The films then underwent a lateral insulation process using Araldite Hobby epoxy resin. Once dried, a layer of conductive silver paint (PC-145, Join Metal) was applied to the top of the electrodes. Following this, the electrodes were placed in an oven for 20 min at 60 °C to cure the conductive paste.

2.5. Characterization

The employed characterization techniques included X-ray Diffraction (XRD). XRD data were gathered using a PANalytical multipurpose diffractometer (Almelo, The Netherlands), model EMPYREAN. Measurements were conducted over an angular range of 5 to 90° (2θ) in a continuous scanning mode. The counting step was set at 0.0263° (2θ) with an average time of 36.47 s per count. The Cu Kα1 wavelength (Å) of 1.540598 was used. The operational settings were maintained at 40 kV × 20 mA. Scanning Electron Microscopy (SEM) was employed to examine the morphology of the prepared composites, focusing on cluster formation, particle distribution, and pore size. Prior to analysis, the films were coated with a gold and palladium layer to enhance image quality and surface conductivity. SEM scans were performed using the Zeiss EVO-MA10 microscope (Oberkochen, Germany), at magnifications ranging from 1000× to 25,000×. Cyclic Voltammetry (CV) was utilized for the electrochemical characterization of the films. Measurements were carried out using an Autolab Metrohm Potentiostat/Galvanostat (PGSTAT101) (Utrecht, The Netherlands). CV experiments were conducted within a potential window of −0.2 to 0.8 V in a three-electrode electrochemical cell comprising a reference electrode (Ag/AgCl in a 3 mol L⁻¹ KCl solution), a platinum counter electrode, and the working electrodes (prepared films). This specific potential range was selected based on prior studies utilizing CCTO for supercapacitors [33]. The choice of potential window is a crucial aspect of cyclic voltammetry, as it helps identify the range within which the material remains stable and operates efficiently [34]. Tests were carried out at various scan rates, namely 10, 25, 50, 75, and 100 mV/s. These electrodes were immersed in a 1 mol L⁻¹ H₂SO₄ electrolytic solution owing to its high ionic conductivity, proton availability, and frequent use in perovskite-type oxide studies, including CCTO systems [33]. The use of this specific electrolyte allows for a direct comparison with data from the existing literature. Considering the exploratory nature of this work, the electrolyte was intentionally kept fixed to systematically assess the effects of Sr-doping and graphene-based modifications while minimizing electrolyte-related variables. Although device-level studies (symmetric or asymmetric cells) are important for assessing practical performance, here we employed a three-electrode configuration to isolate the intrinsic behavior of each composition. Future work will explore both device assembly and alternative electrolytes, including alkaline ones such as KOH.

To assess the performance of the prepared electrodes, we calculated the specific capacitance (C_sp) from the cyclic voltammetry data. The specific capacitance was determined using the following formula:

$$C_{sp}={\displaystyle \frac{A}{2mv ∆E}}$$

(1)

where A is the area under the cyclic voltammetry curve, m is the mass of the material of interest on the electrode (measured after the calcination process), v is the scan rate, and ΔE is the potential window, which was predetermined as 1 V for this study [33].

Electrochemical impedance spectroscopy (EIS) measurements were carried out using a Metrohm Autolab potentiostat/galvanostat (PGSTAT101) (Utrecht, The Netherlands) with the FRA module. Spectra were recorded at the open circuit potential in the frequency range from 10⁻¹ to 10⁵ Hz, using 10 points per decade and an excitation amplitude of 10 mV (RMS).

3. Results and Discussion

3.1. Structural and Morphological Analysis

Figure 1a presents the XRD results for the synthesized GO and rGO. The GO is confirmed by its characteristic sharp peak at 2θ ≈ 10 degrees [35], which gives a calculated interplanar of 0.879 nm. This value aligns with those reported in the literature, indicating an interplanar distance for GO ranging from 0.74 to 0.89 nm, depending on the quantity of oxygen-containing functional groups incorporated into the graphite layers [35]. The successful conversion to rGO is evidenced by the disappearance of this peak and the appearance of a broad peak at 2θ ≈ 24.9° (and others at 42.68° and 78.00°) [29,30]. These two precursor materials were subsequently incorporated into the CCSTO system to form composite films, which were deposited using the doctor blade technique. Figure 1b shows the XRD for CaCu_3−xSr_xTi₄O₁₂ (0 ≤ x ≤ 3.0) films, which can be observed in all samples showing high crystallinity. Also, peaks associated with the FTO substrate can be observed. On the other hand, the composition with x = 0.00 exhibited only the CCTO phase (Inorganic Crystal Structure Database (ICSD) no. 259849), corresponding to a cubic perovskite structure with an Im3 space group. In contrast, the composition with x = 3.00 showed only the CSTO phase (ICSD no. 190332), with an orthorhombic structure and Ibmm space group. For intermediate composition, x = 0.15, the material was observed to be a phase mixture of CCTO and CSTO, which is in nice agreement with the literature [20].

Observations from the SEM images in Figure 2 reveal differences in the particle sizes among the three systems studied. Figure 2a, corresponding to CCTO, displays particles with a particle size of 1.943 µm as shown in the inset box plot graphs; besides that, the pure system shows limited porosity which can influence the capacitive response, as will be discussed shortly. Figure 2b shows the CCTO-6GO sample, which can be observed an increase in the particle sizes approximately 2.441 µm associated with the GO proportion added. Thus, we have a bigger particle distribution when compared with the CCTO sample being evidence in the porosity increase for this sample. Similar behavior can be observed in Figure 2c, corresponding to CCTO-6rGO, which shows particles with sizes less than 0.919 µm, which can be attributed to rGO particles adhering to the CCTO particles in a more homogeneous manner, even filling the existing pores. This behavior was noted when analyzing the second and third groups of SEM images in Figure 2; it can be observed that the samples showed smaller particles when compared with samples of the first group. In Figure 2d,e, corresponding to CCTO15Sr with and without GO, the particle sizes were approximately 0.851 µm and 0.910 µm, respectively, with greater homogeneity as observed in the inset box-plot, different when compared with the sample CCTO15Sr-6rGO (Figure 2f), which showed more heterogeneity with an average particle size of 0.919 µm.

Upon examining Figure 2g,h and i, presented particle sizes of 0.304 µm, 0.406 µm, and 0.403 µm, respectively. Although the particle distribution was similar, the distribution associated with CSTO-6rGO was larger when compared with CSTO. The CSTO and CSTO-6GO samples presented a more porous appearance.

3.2. Electrochemical Response

EIS measurements were performed to evaluate the charge transport characteristics of CCTO, CCTO15Sr, and CSTO with GO and rGO (Figure 3a–c). Neglecting ion transport resistances within the electrode and electrolyte allows the system to be modelled effectively using a simplified equivalent circuit where capacitors are substituted by constant phase elements [36,37]. This approach yields excellent results when compared with the experimental impedance response of many oxide-based thin film electrodes. The Nyquist plots reveal distinct differences based on the presence of GO and rGO. The CCTO film (Figure 3a), the pristine sample shows moderate charge transfer resistance (R_ct) values, while the CCTO–6rGO composite exhibits a significant reduction in R_ct, as evidenced by the smaller semicircle arc. In contrast, CCTO–6GO displays much higher impedance. A similar trend is observed in the CCTO15Sr films (Figure 3b). CSTO films and films containing GO and rGO have the highest load transfer resistance. This makes them unsuitable for load storage applications.

In this study, although further detailed investigation is warranted and is currently in progress, particular attention should be given to the influence on highly resistive systems. In such contexts, and as already observed in other classes of systems [38,39,40], the role of capacitance may also become a significant factor to consider (Figure 4). It should be emphasized that this work is still under development and the results presented here are preliminary. Nevertheless, our findings indicate that the capacitance of sample CCTO15Sr-6rGO is approximately 202 mF, whereas samples CCTO-6rGO and CSTO-6rGO exhibit capacitance values of 8.7 mF and 1.6 mF, respectively. The capacitance extracted from electrochemical impedance spectroscopy is consistent with that calculated from cyclic voltammetry. This finding supports the reliability of the electrochemical evaluation.

This trend correlates with the specific capacitance values obtained from CV measurements. Figure 5 illustrates the CV results for CCTO, CCTO15Sr and CSTO systems incorporated with 6 wt% GO and 6 wt% rGO electrodes. Figure 5a,d show the pseudocapacitive behavior of the systems [6], as pronounced by the characteristic charge and discharge curves representative of faradaic redox processes occurring on the electrode surfaces. Notably, the CSTO electrode, as shown in Figure 5, exhibited behavior similar to that of an electrode in EDLC-type supercapacitors [31]. As an extension of this analysis, Figure 5 also presents the voltametric responses of GO- and rGO-modified the variants. Notably, the curves for CCTO-6GO (Figure 5b) and CSTO-6GO (Figure 5h) show significant deviations from those for the unmodified system, shifting towards profiles more typical of EDLCs. This highlights the positive influence of GO on the capacitive properties of the materials. Conversely, the CCTO15Sr-6GO (Figure 5e) maintains shape similar to that for the unmodified system, which suggests that the pseudocapacitive characteristics were preserved despite GO addition.

Similarly, the responses of the rGO-modified variations are depicted in Figure 5c,f and i), and notable changes can be observed. CCTO-6rGO (Figure 5c) exhibited a response similar to that of the EDLC. CCTO15Sr-6rGO (Figure 5f) maintained its pseudocapacitive response, and CSTO-6rGO (Figure 5i) also maintained its original form of response. The CCTO-6rGO and CCTO15Sr-6rGO current scales showed significant increases, indicating that rGO addition altered the current conduction capacity of these materials. This phenomenon can be attributed to the characteristics of rGO, namely high surface area, high electrical conductivity, and good chemical stability, which collectively contributed to increased capacitance [32]. It is worth mentioning again that all films were calcined at 450 °C, and this type of thermal treatment invariably produces oxygen vacancies in these systems, which contribute to increases the material’s capacity to accumulate charges and enables its active participation in redox processes during the charging and discharging process [41]. This behavior is relevant because it can explain the results obtained in this study, which are subsequently.

We propose that the differences among the sample responses are directly associated with the number of oxygen vacancies. Specifically, the CCTO (Figure 5a) and CSTO (Figure 5g) systems exhibit different conductivities [22], these differences indicate that Sr²+ leads to the generation of a more resistive system because it possesses fewer free electrons and consequently fewer oxygen vacancies, resulting in a lower overall capacitive response than that of the CCTO system. Similar behaviors were observed in the GO-modified samples (Figure 5b,e), the conductivity of which was lower than that of the CCTO (Figure 5a) and CCTO15Sr (Figure 5d) systems. Such behaviors can be ascribed to the presence of functional groups (-C-O-C-, -OH, -COOH) [42], which interact with the oxygen vacancies and mitigate the generation of free electrons.

This interpretation also applies to the rGO-modified samples, where rGO promotes oxygen vacancy formation, as reported by Liu et al. [42], and this improves the free electron concentration as well as the charge accumulation, consistent with the reduced impedance observed in the EIS spectra of the CCTO–6rGO and CTO15Sr–6rGO systems (Figure 4).

Under these conditions, it is noteworthy that the CCTO15Sr sample was obtained via the substitution process defined by Cortés et al. [22]. The removal of Cu²⁺ promoted the formation of metal vacancies, which in turn warranted the generation of oxygen vacancies to maintain system stability. When Sr²⁺ is added, it occupies Ca²⁺ sites in the system, resulting in a neutral substitution. This is because the added ion possesses the same valence as that of the expelled cation, as presented in the following Kroger-Vink reaction:

$${\mathrm{CaCu}}_3{Ti}_4{O}_{12}\to {\mathrm{Sr}}_{\mathrm{Ca}}^{\mathrm{X}}+{3\mathrm{V}}_{\mathrm{Cu}}^{\mathrm{\prime }\mathrm{\prime }}+{4\mathrm{Ti}}_{\mathrm{Ti}}^{\mathrm{X}}+{12\mathrm{O}}_{\mathrm{O}}^{\mathrm{X}}+{3\mathrm{V}}_{\mathrm{O}}^{··}$$

(2)

The above equation enables us to associate the sample response with two characteristics: charge accumulation linked to oxygen vacancies (as previously discussed) and ion diffusion linked to with metal vacancies [43]. This explanation corroborates that for the behavior of the CCTO15Sr samples with 6% GO and rGO; specifically, as the possibility of two types of events in the system enhances the capacitive response, which is consistent with our results. The CCTO15Sr sample with 6 wt% GO (Figure 5e) showed specific capacitance values similar to those of pure CCTO despite the effect of GO, whereas the CCTO15Sr sample with 6 wt% rGO (Figure 5f) exhibited the highest specific capacitance.

Among the CSTO samples, the GO-modified sample demonstrated a higher conductivity than that of the rGO-modified sample. This is because the rGO-modified samples possess an inherently higher resistivity, as mentioned by Cortés et al. [22], and the GO-modified sample interacts more easily with the electrolyte owing to the presence of functional groups, which are absent in the rGO-modified sample. Furthermore, redox reactions involving the CSTO sample with 6 wt% GO (Figure 5h) are more noticeable in the range of 0.2–0.4 V as compared to those involving CCTO sample with 6 wt% GO (Figure 5b). Redox reactions involving the latter sample are noticeable in the range of 0.4–0.6 V owing to the presence of functional groups. The CCTO15Sr sample with 6 wt% GO (Figure 5e) did not undergo redox reactions because the functional groups interacted with the respective metal and oxygen vacancies.

The specific capacitance values calculated using Equation (1) for all the samples are listed in

Table 1. A comparison of the specific capacitance values (mF g⁻¹) for CCTO, CCTO15Sr, and CSTO samples, in both pure form and as composite containing 6 wt% GO and rGO, measured at various scan rates.

	Scan Rate (mV·s−1)
Samples	10	25	50	75	100
CCTO	29.86	22.32	17.43	14.24	14.21
CCTO-6GO	7.83	3.46	4.89	2.59	2.22
CCTO-6rGO	237.76	175.63	145.35	129.28	124.20
CCTO15Sr	80.19	45.79	31.33	26.33	22.48
CCTO15Sr-6GO	25.73	19.51	15.48	13.16	10.64
CCTO15Sr-6rGO	321.63	212.89	141.90	119.69	104.25
CSTO	3.51	1.63	1.15	0.60	0.85
CSTO-6GO	2.60	1.58	1.20	1.04	1.33
CSTO-6rGO	2.93	2.19	1.93	1.85	1.55

and presented graphically in Figure 6. Each entry in the table corresponds to one of the pure materials investigated and its two variations with the addition of 6 wt% GO and 6 wt% rGO, respectively. Notably, the addition of 6 wt% rGO (CCTO-6rGO) significantly improved the specific capacitance of the CCTO samples at all scan rates.

At a scan rate of 10 mV/s, C_sp for CCTO-6rGO reached 237.76 mF·g⁻¹, compared to 29.86 mF·g⁻¹ for pure CCTO and 7.83 mF·g⁻¹ for CCTO-6GO. This enhancement remained consistent even at higher scan rates. Similarly, among the CCTO15Sr samples, CCTO15Sr-6rGO demonstrated the highest specific capacitance values; specifically, a C_sp of 321.63 mF·g⁻¹ was obtained at 10 mV/s, which is better than those of both pure CCTO15Sr (80.19 mF·g⁻¹) and CCTO15Sr-6GO (25.73 mF·g⁻¹).

In the CSTO samples, specific capacitance values were generally low. The addition of 6 wt% GO or rGO did not result in substantial improvements, with the values remaining similar to or slightly higher than those of pure CSTO.

All the specific capacitance curves shown in Figure 6, except for CSTO, exhibit a declining trend. This indicates that as the scan rate increased, the specific capacitance decreased. This behavior aligns with observations typically made in previous studies on supercapacitors [44]. The decrease in specific capacitance with increasing scan rate can be attributed to the fact that at high scan rates, the electrochemical double layer forms slower within micropores and to a lesser extent as compared to the potential variation rate, resulting in a decrease in efficient utilization of the electrode material [45,46].

As shown in Figure 6, the addition of 6 wt% rGO to CCTO led to a significant increase in the specific capacitance at all scan rates compared to the case of CCTO and CCTO-6GO. This suggests that rGO addition had enhanced the specific capacitance of CCTO on account of the oxygen vacancies. However, the addition of GO resulted in a decrease in the specific capacitance as compared to the behavior in pure CCTO because of the presence of functional groups on GO, which limited the development of oxygen vacancies. Future studies involving analyses and computational simulations of these systems are being conducted, which will better elucidate the effect of defects, such as metal or oxygen vacancies, in hybrid supercapacitors.

When comparing our results with those reported in the literature, we observe notable differences in the specific capacitance based on the synthesis methods and material modifications adopted. Although CCTO, in either pure or compound form, has shown potential for use in various applications, few studies have reported its electrochemical response using CV. Most existing studies have focused on its electrical properties rather than its electrochemical performance in supercapacitors [36]. Maity et al. [47] reported an areal capacitance of 1185 mF cm⁻² for CCTO nanocubes in KOH at 10 mV s⁻¹, which reduced to 76 mF cm⁻² in a solid-state device. Similarly, Padmini et al. [33] found that adding 5 wt% CCTO to polyaniline increased its capacitance from 308 to 610 F g⁻¹ owing to enhanced morphology and improved interactions with the electrolyte. In contrast, the capacitance of our solid-state CCTO reached 29.86 mF g⁻¹ in 1 mol L⁻¹ H₂SO₄, additionally, Sr substitution (CCTO15Sr) raised the capacitance to 80.19 mF g⁻¹. Furthermore, the addition of 6 wt% rGO to CCTO15Sr further enhanced the capacitance to 321.63 mF g⁻¹, demonstrating the effectiveness of rGO in enhancing charge storage capabilities.

Our results indicate that the specific capacitance is influenced by several factors, including the electrolyte choice and electrode material, specifically in terms of the type of defect (oxygen or metal vacancies) when working with hybrid electrodes responsible for electronic conductivity and charge transfer. The surface area and pore structure are also critical factors to be considered.

4. Conclusions

In this study, CCSTO films with x = 0.00, 0.15, and 3.00, both with and without GO and rGO, were successfully fabricated and investigated for supercapacitor applications. SEM analysis revealed minimal changes in morphology and particle size across samples, regardless of the Cu/Sr ratio or the addition of GO or rGO. Electrochemical analysis through cyclic voltammetry demonstrated that the addition of 6 wt% rGO significantly enhances specific capacitance, with CCTO-6rGO achieving up to 237.76 mF·g⁻¹ at 10 mV/s compared to 29.86 mF·g⁻¹ for pure CCTO. This improvement is attributed to the presence of oxygen and metal vacancies, which facilitate charge accumulation and ion diffusion. In contrast, GO modification was found to reduce specific capacitance in CCTO samples, suggesting a limited enhancement effect on charge storage capacity.

For CSTO samples, the addition of GO or rGO resulted in specific capacitances that were similar to or slightly higher than those of pure CSTO, indicating that GO and rGO may have a limited impact on CSTO’s electrochemical properties. These observations suggest that while rGO enhances the capacitance in CCTO, its influence on CSTO is less pronounced.

Overall, this work highlights the potential of structural modifications, particularly rGO incorporation, to improve the charge storage properties of complex perovskite systems like CCTO. The results open new possibilities for utilizing modified CCTO in hybrid supercapacitors, underscoring the importance of oxygen and metal vacancies in optimizing the electrochemical performance of perovskite-based materials for energy storage applications.