Investigation of the Structure and Electrochemical Performance of Perovskite Oxide La_1−xCa_xCrO₃ Utilized as Electrode Materials for Supercapacitors

Abstract

Lanthanide perovskite materials are promising candidates for supercapacitor applications. In this study, a series of La_1−xCa_xCrO₃ (x = 0–0.2) materials were prepared by sol-gel method, incorporating bivalent ions calcium at A-site. La_0.85Ca_0.15CrO₃ exhibited the lowest charge transfer resistance and highest specific surface area. At 1 A/g, La_0.85Ca_0.15CrO₃ achieved a maximum specific capacitance of 306 F/g, about 2.3 times higher than that of the LaCrO₃ (133 F/g). Based on the observed data, a mechanism involving oxygen anion charge storage during the charging-discharging process is proposed. After 5000 long cycle, the coulomb efficiency of the electrode remains above 94%. These results demonstrate that Ca-substituted compounds exhibit significant potential for A-site engineering in supercapacitor applications.

1. Introduction

In the contemporary era, the environmental energy crisis has emerged as a pivotal global concern [1,2,3]. Rapid population growth and industrialization have driven escalating global energy demands. This has led to a significant increase in the dependence on fossil fuels, triggering serious environmental degradation [4,5,6]. Nations pursue sustainable energy solutions like enhancing energy efficiency and developing renewable energy sources to balance economic growth with environmental protection [7,8]. Supercapacitors mitigate energy storage challenges with advantages like ultrafast charge and discharge, long cycle stability, and high-power density [9,10,11]. Unlike conventional batteries, supercapacitors are particularly well-suited for energy storage applications for intermittent energy sources, such as wind and solar energy [12]. Electrode material selection significantly impacts on supercapacitor performance. Consequently, a number of studies have investigated the potential of efficient electrode materials with excellent specific capacitance and long-term cycle stability. In recent years, lanthanide perovskite materials have attracted considerable attention in supercapacitors due to a number of advantageous properties, including high degrees of crystallinity, reversible redox capabilities, outstanding electrical characteristics, a profusion of oxygen vacancies, a highly stable architectural framework, straightforward synthesis procedures, and cost-efficiency [13,14].

ABO₃-type perovskites have been studied as supercapacitors electrode materials, including LaCoO₃ [15,16,17], LaMnO₃ and LaCrO₃ [18]. Among these materials, LaCrO₃ exhibits the highest specific surface area and electrochemical properties, making it a prime research focus for supercapacitor research [19]. Nonetheless, LaCrO₃’s ion diffusion and electron transfer speed are limited, restricting its practical use as an anode material [20]. LaCrO₃ represents a prototypical ABO₃ perovskite oxide structure. The basic building block of the structure is the CrO₆ octahedron. The physical characteristics of the crystal structure depend on the deformation and configuration of these CrO₆ octahedra [21]. Octahedron stability can be adjusted by doping the La site, thereby altering the performance of LaCrO₃. LaCrO₃ perovskite exhibits intrinsic ion vacancies that enhance their catalytic and electrical properties [22]. As a result, lanthanum chromite (LaCrO₃) is widely utilized in fuel cell electrodes, catalytic substances, and electromagnetic apparatuses. In addition, chromium-containing materials hold considerable promise for energy storage uses due to their protons/electrons conductivity, multiple oxidation states (specifically Cr³⁺/Cr⁶⁺), high redox capacity, outstanding thermal and structural stability, and natural abundance [23]. Perovskite-type oxides can be modified to achieve desired performance. This tunability facilitates the development of specific supercapacitor applications. Oxygen vacancies play a crucial role in optimizing the specific capacitance. Effective strategies for generating oxygen vacancies include doping the ABO₃ structure with A-site or B-site impurities to alter the oxidation state of the transition metal, or introducing oxygen sites for charge balance [24,25]. Transition metal dopants’ precise modulation of A/B-site cations significantly enhance the perovskite oxide electrodes’ capacitance, cycling stability, and the overall electrochemical characteristics. Doping with Ca, Sr, Co, and K at the A-site of LaMnO₃ [26,27,28,29], LaAlO₃ [30], LaFeO₃ [31], LaNiO₃ [32], and LaCoO₃ [33], as well as Zn, Fe, and Ni at the B-site, has demonstrated significant performance enhancements. Partial A-site substitution with lower-cost elements results in B-site transition metal oxidation state instability and generates more oxygen vacancies. This modification enhances both electrical conductivity and electrochemical properties. The specific capacitances of LaMnO₃, LaAlO₃, LaFeO₃ and LaNiO₃ are 72, 122, 17.15 and 48.85 F/g, respectively. It is noteworthy that LaCrO₃ exhibits a relatively high specific capacitance of 86.4 F/g [34]. To further enhance the specific capacitance of LaCrO₃, researchers investigated the doping of Ca at the A-site as a potential avenue. For example, following the testing of the electrochemical properties of LaCrO₃ and La_0.7Ca_0.3CrO₃, Jiang et al. [35] demonstrated that the La_0.7Ca_0.3CrO₃’s electrode polarization resistance decreased from 45.3 Ω cm² to 6.3 Ω cm² compared to undoped LaCrO₃. La_0.7Ca_0.3CrO₃ exhibits significantly lower electrode polarization resistance for oxygen reduction, enhancing the electrocatalytic activity of LaCrO₃ for fuel cell reaction. Rashtchi et al. [36] prepared an AISI 430 interconnect metal coating by means of the sol-gel method and investigated the effect of doped calcium on the conductivity and oxidation properties of the coating. Their results show that the oxidation rate and antioxidant properties of the La_0.8Ca_0.2CrO₃ coating are significantly reduced by about 2 times and 6 times than LaCrO₃ metal coating. Their findings indicated that strontium doping improves the electrochemical properties of LaCrO₃ materials. Consequently, it is crucial to examine the impact of Ca doping on the electrochemical characteristics of LaCrO₃ materials.

This study introduces several innovations. Firstly, it systematically explores the effects of different calcium doping ratios (x = 0–0.2) on the microstructure, morphology and electrochemical performance of La_1−xCa_xCrO₃. Previous studies have mostly focused on a single doping ratio. The present study combines XRD, SEM, TEM and XPS analysis, supplemented by electrochemical testing and EIS analysis, to provide new theoretical and experimental foundations for related applications. The results demonstrate that La_0.85Ca_0.15CrO₃ exhibits the optimal electrochemical performance, characterized by a specific capacitance of 306 F/g. Despite the enhancement of the electrochemical performance of LaCrO₃ material through the doping of Ca elements, the performance difference between Ca²⁺ and La³⁺ can readily result in uneven element distribution and composition segregation, thereby affecting the uniformity of performance. Future research endeavors could explore avenues for expanding the application prospects of the process by optimizing process parameters, developing new chelating agents to achieve uniform doping, and investigating green preparation routes to reduce costs.

2. Experimental Section

The requisite pharmaceuticals for the preparation of La_1−xCa_xCrO₃ (x = 0–0.2) series samples by sol-gel methodology are as follows: The requisite chemicals include citric acid monohydrate (≥99.5%), lanthanum nitrate hexahydrate (La(NO₃)₃·6H₂O), chromium nitrate hydrate (≥99.9%) and Cr(NO₃)₂·9H₂O (≥99%), as well as calcium nitrate Ca(NO₃)₂ (≥99.5%). The aforementioned reagents were all procured from Aladin Chemical Reagent Co., Ltd, Shanghai, China. These chemicals are of analytical grade and can be used directly without further purification.

Citric acid monohydrate, La(NO₃)₃·6H₂O and Cr(NO₃)₂·9H₂O were added in succession to a 30 ml beaker at a molar ratio of 1:1:1. A series of La_1−xCa_xCrO₃ (x = 0–0.2) were prepared by altering the molar ratio of Ca(NO₃)₂. The solution should then be stirred for 24 h on a magnetic stirrer in order to form a transparent solution, resulting in the formation of a transparent solution. Following this, the solution underwent a drying procedure in a forced-air drying oven at a temperature of 80 °C for a duration of 72 h. Thereafter, the dried sample was removed and placed in a crucible and calcined in a muffle furnace. The Muffle furnace(KSL-1200, HF-Kejing, Hefei, China) was heated to 700 °C at a rate of 10 °C/min, maintained in an air atmosphere for a period of 2 h, and subsequently ground to produce a black powder sample. The synthesized black powder specimens underwent X-ray powder diffraction (XRD) examination. X-ray powder diffractometer (Bruker D8, Karlsruhe, Germany) was employed to conduct a phase analysis of these specimens. The X-ray powder diffractometer has a scanning range of 20–70° and a scanning speed of 6°/min. The microstructure of the samples was characterized by field emission scanning electron microscopy (Merlin Compact, Carl Zeiss, Munich, Germany). The elemental composition and chemical valence changes of the samples were analyzed by means of X-ray photoelectron spectroscopy (XPS, Thermofisher, Waltham, MA, USA). A specific surface area test was conducted using a specific surface area tester (BET, V-Sorb 2800TP, Hefei, China).

In order to conduct electrochemical testing of the prepared electrodes, a three-electrode system was employed, utilizing 3 M KOH as the electrolyte. The three electrodes are the working electrode, the counter electrode (platinum electrode), and the reference electrode (Hg/HgO electrode, Yueci, Shanghai, China). The electrode is prepared in the following manner: Firstly, the sample, comprising polyvinylidene fluoride (PVDF, Aladin Chemical Reagent Co., Ltd., Shanghai, China) and acetylene black, is mixed at a ratio of 8:1:1 in a specified quantity of N-methylpyrrolidone (NMP, Aladin Chemical Reagent Co., Ltd., Shanghai, China) and stirred for a designated period of time. Subsequently, the resulting paste is uniformly distributed on the 20 mm × 10 mm nickel foam sheet, with a coating area of 100 mm². The mass of the active substance coated on the nickel foam is approximately 0.8 mg. After subjecting the electrode to a vacuum oven at 80 °C for 24 h, the nickel foam is ready for use. Prior to this, however, the foam nickel be deoiled with acetone and deoxidized with diluted hydrochloric acid. The electrochemical properties of the electrodes were investigated using an Electrochemistry Workstation (CHI660e, Chenhua, Shanghai, China), which allowed for the determination of cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and alternating-current impedance (EIS). The stability of the electrodes during cycling was assessed with a multi-channel battery testing apparatus (Land, CT2001A, Wuhan, China).

3. Results and Discussion

Figure 1a depicts the XRD patterns of the La_1−xCa_xCrO₃ (x = 0–0.2), which are compared to the standard card PDF#75–0441. It reveals the presence of peaks corresponding to pure LaCrO₃. No impurity peaks were detected, indicating that Ca²⁺ has been completely dissolved into the LaCrO₃ lattice. These results demonstrated the successful synthesis of the La_1−xCa_xCrO₃ sample. Diffraction peaks observed at 2θ = 22.90°, 32.61°, 40.22°, 46.78°, 52.70°, 58.19°, and 68.32°. The diffraction peaks point to the (100), (110), (111), (200), (210), (211) and (220) crystal faces, respectively. As illustrated in Figure 1b, the diffraction peak of the locally enlarged (110) crystal plane demonstrates a shift towards higher angles with Ca²⁺ doping. The angular shift is more pronounced than that observed in pure LaCrO₃. This shift originates from doped Ca²⁺ occupying the A-site to minimize lattice stress. As Ca doping level increases, smaller Ca ion (0.99 A) replaces La ion (1.06 Å), inducing some Cr³⁺ to Cr⁶⁺ to maintain electrical neutrality. The ionic radius of Cr⁶⁺ (0.52 Å) is smaller than that of Cr³⁺ (0.69 Å). According to Bragg’s formula, increasing Ca doping shifts the diffraction peak to a larger angle. Calcium introduction affects ion transport pathways primarily through the lattice parameter alterations, defect formation, and microstructure change. Lattice expansion alters the particle size and pore structure, which in turn affects the ion transport efficiency. Ca doping in LaCrO₃ enhances redox activity and reaction rate by changing electronic structure, increasing active sites, and reducing charge transfer resistance [37]. As illustrated in Figure 1c, the XPS full spectrum of La_1−xCa_xCrO₃ (x = 0–0.2) samples demonstrates the presence of La, Cr, O and Ca. The outcomes of XRD and XPS validate that Ca has been successfully integrated into the samples.

As illustrated in the SEM diagram of Figure 2, La_1−xCa_xCrO₃ (x = 0–0.2) sample exhibits a microstructure comprising the aggregation of nanoparticles. This structure possesses a high specific surface area and small grain size, which facilitate enhanced surface activity and reaction rate [38]. Homogeneous particle distribution ensures consistent material properties and structural stability. Grain size decreases with elevated calcium doping levels correlates with lattice distortion induced by Ca²⁺ incorporation, which inhibits crystal growth [39]. The increased lattice defects are the primary factor reducing grain size. These defects result in the incompleteness and distorted crystal structure, thereby influencing the crystal growth process. During crystal growth, lattice defects may impede grain growth, ultimately reducing the overall grain size [40,41]. Grain size reduction increases grain boundary density, hindering ion diffusion and stabilizing the electrolyte interface. The increased surface-area-to-volume ratio in smaller grains mitigates material degradation. Calcium doping relieves internal stresses and enhances mechanical stability. Reduced grains shorten ion diffusion paths, while Ca doping improves both ionic and electronic conductivity. With more active sites, rate capability was enhanced at high current densities [42].

To investigate the impact of Ca doping on the specific surface area of La_1−xCa_xCrO₃ (x = 0–0.2) series samples, the N₂ adsorption-desorption method was employed. The particular surface area and the distribution of pore sizes of LaCrO₃ and La_0.85Ca_0.15CrO₃ nanoparticles were determined. Figure 3a,c illustrate the N₂ adsorption-desorption isotherm curves of LaCrO₃ and La_0.85Ca_0.15CrO₃ samples. The results demonstrate that with an increase in relative pressure, the adsorption capacity continues to rise. Above a relative pressure of 0.8, adsorption capacity increases sharply, exhibiting an evident hysteresis ring [43]. The observed hysteresis confirms mesoporous architectures in both samples [44]. Figure 3b,d show both samples have pore sizes predominantly in the range of 1–10 nm, confirming their mesoporous structure. Mesoporous structures enhance ion diffusion kinetics and surface redox reactions in electrodes. Interconnected pores serve as rapid ion pathways, reducing ion transport distances. Additionally, the high specific surface area and porosity of mesoporous structures improve electrolyte infiltration and ion transmission efficiency, while exposing more active sites for redox reactions [45]. Enhanced accessibility and chemical activity of the mesoporous surfaces promote redox reactions [46]. Specific surface area measurements demonstrate that the specific surface area of the LaCrO₃ and La_0.85Ca_0.15CrO₃ samples are 38.01 and 48.21 m²g⁻¹, respectively. These results suggest that the Ca doping enhances the specific surface area of the LaCrO₃ system.

Figure 4a–d depict La 3d, Cr 2p, O 1s, and Ca 3d high-resolution XPS spectra of La_1−xCa_xCrO₃ (x = 0–0.2). The high-resolution La 3d XPS spectrum is presented in Figure 4a. Peaks at 856.0 and 851.0 eV attribute to the orbital peak of La 3d_3/2, while peaks at 839.0 and 834.0 eV correspond to the orbital peak of La 3d_5/2 [47]. The difference between the two La 3d peaks is approximately 17 eV, indicating that the La element exists in a +3 valence state in the La_1−xCa_xCrO₃ (x = 0–0.2) nanoparticles [48]. Figure 4b depicts the Cr 2p high-resolution XPS diagram. The spectrum exhibits two distinct peaks. The peak with the lower binding energy is identified as Cr 2p_1/2, while the other is Cr 2p_3/2. The Cr 2p_1/2 and Cr 2p_3/2 peaks are composed of two smaller peaks [49]. The Cr 2p_1/2 peak occurs at 586.6 eV, while the Cr 2p_3/2 peak occurs at 576.7 eV. Similarly, the Cr 2p_1/2 peak occurs at 589.8 eV, while the Cr 2p_3/2 peak occurs at 579.9 eV. In the Cr 2p_1/2 and Cr 2p_3/2 spectra, the peaks with lower binding energies are attributed to Cr³⁺, while those with higher binding energies are ascribed to Cr⁶⁺. This suggests that chromium ions in LaCrO₃ exhibit two distinct valence states [50]. XPS analysis shows that changes in oxygen vacancy concentration and Cr oxidation state in La_1−xCa_xCrO₃ are directly linked to its charge storage mechanism in supercapacitors [51]. Oxygen vacancies can alter the material’s electronic structure, enhancing conductivity and ion diffusion pathways, thereby improving charge storage efficiency. Meanwhile, changes in Cr oxidation state directly impact electrochemical activity by providing additional redox pairs, increasing specific capacitance and pseudocapacitive properties [52]. Figure 4c illustrates the sub-peak fitting outcomes for the O 1s characteristic peaks of La_1−xCa_xCrO₃ (x = 0–0.2) nanoparticles. Three distinctive O 1s peaks are observed at 532.9, 531.5 and 529.4 eV, respectively [53]. The peak observed at 532.9 eV is attributed to the presence of an oxygen-containing functional group, specifically a hydroxyl group (OH). The characteristic peaks observed at 531.5 eV are attributed to the presence of adsorbed oxygen, which may be in the forms of O⁻, O²⁻, or O₂²⁻. The peak at 529.4 eV is attributed to lattice oxygen (O²⁻) [53].

Table 1. Relative concentration of three oxygen in La_1−xCa_xCrO₃ (x = 0–0.2).

	LaCrO3	La0.9Ca0.1CrO3	La0.85Ca0.15CrO3	La0.8Ca0.2CrO3
O 1(%)	45.8	44.7	42.1	43.9
O 2(%)	47.6	49.6	55.6	49.2
O 3(%)	6.6	5.7	2.3	6.9
O 2/O 1	1.04	1.11	1.32	1.12

presents the relative concentrations of the three types of oxygen present in the La_1−xCa_xCrO₃ (x = 0–0.2) sample. The oxygen vacancy of the electrode material is intimately associated with its electrical conductivity. High adsorbed oxygen (O⁻, O²⁻, O₂²⁻) concentration enhances OH adsorption, surface redox reactions, and electrochemical performance [54]. The concentration of oxygen vacancies is determined by calculating the ratio of the amount of adsorbed oxygen to the amount of lattice oxygen present. Table 1 illustrates that the introduction of Ca into the La_1−xCa_xCrO₃ (x = 0–0.2) series increases oxygen vacancy concentration. Among these samples, La_0.85Ca_0.15CrO₃ exhibits the highest oxygen vacancy concentration. Compared with LaCrO₃, Ca doping reduce the oxygen vacancy formation energy in La_0.75Ca_0.25CrO₃, as the charge of Ca replacing La is opposite to that of oxygen vacancies. Oxygen sites are inequivalent, making O sites easier to form oxygen vacancies. Under tensile strain, oxygen vacancy formation energy decreases without obvious minimum value; Under compressive strain, the O 1/O 2 formation energy decreases and the O 3 formation energy increases [55]. As illustrated in Figure 4d, the high-resolution XPS diagram of the La_1−xCa_xCrO₃ (x = 0–0.2) nanoparticle sample Ca 2p demonstrates a characteristic peak at 350.5eV, corresponding to the orbital peak of Ca 2p_1/2. The presence of a characteristic peak at 347.1 eV is indicative of the Ca 2p_3/2 orbital peak. The results obtained thus far demonstrate that the Ca elements present in the La_1−xCa_xCrO₃ (x = 0–0.2) nanoparticle samples exist at + 2 valence [56].

Figure 5a–d illustrates the cyclic voltammetry curves of the La_1−xCa_xCrO₃ (x = 0–0.2) electrodes at a scanning rate of 10–100 mV/s. The electrodes exhibit excellent reversibility, which is evidenced by the unaltered shape of the cyclic voltammetry curve despite increasing current density. A distinct redox peak suggests that redox reactions contribute to the charge storage mechanism [57]. Figure 5e illustrates the CV curve of nickel foam. The active substance is carried on the nickel foam, which may affect electrochemical tests. As demonstrated in Figure 5e, the nickel foam CV curve area is negligible at 10–100 mV/s. This observation indicates that nickel foam exhibits no inherent capacity to facilitate charge or material transfer. Consequently, the earlier electrochemical performance enhancement is exclusively attributable to La_1−xCa_xCrO₃. As the Ca doping concentration increased, the enclosed area of the cyclic voltammetry curves for the La_1−xCa_xCrO₃ series electrodes (x = 0–0.2) exhibited a trend of initial increase followed by subsequent decrease. Ca doping has an impact on the electrochemical characteristics of electrode materials. La_0.85Ca_0.15CrO₃ electrode displays the most favorable electrochemical performance, as evidenced by the largest enclosed cyclic voltammetry curve area. La_0.85Ca_0.15CrO₃ electrode exhibits enhanced capacitance and superior cycle stability during capacitor discharge and charging [58]. This phenomenon can be accounted for by referring to the oxygen insertion process of perovskite oxide substances. The equations for the reaction are as follows [59]:

$$\mathrm{L}\mathrm{a}\left[{\mathrm{C}\mathrm{r}}_{2\delta }^{3+};{\mathrm{C}\mathrm{r}}_{1-2\delta }^{6+}\right]{\mathrm{O}}_{3-\delta }+2\delta {\mathrm{O}\mathrm{H}}^{-}\leftrightarrow \mathrm{L}\mathrm{a}{\mathrm{C}\mathrm{r}}^{3+}{\mathrm{O}}_3+\delta {\mathrm{H}}_2\mathrm{O}+2\delta {\mathrm{e}}^{-}$$

(1)

$$\mathrm{L}\mathrm{a}{\mathrm{C}\mathrm{r}}^{3+}{\mathrm{O}}_3+2\delta {\mathrm{O}\mathrm{H}}^{-}\leftrightarrow \mathrm{L}\mathrm{a}\left[{\mathrm{C}\mathrm{r}}_{2\delta }^{6+};{\mathrm{C}\mathrm{r}}_{1-2\delta }^{3+}\right]{\mathrm{O}}_{3+\delta }+\delta {\mathrm{H}}_2\mathrm{O}+2\delta {\mathrm{e}}^{-}$$

(2)

$${\mathrm{L}\mathrm{a}}_{0.85}{\mathrm{C}\mathrm{a}}_{0.15}\left[{\mathrm{C}\mathrm{r}}_{2\delta }^{3+};{\mathrm{C}\mathrm{r}}_{1-2\delta }^{3+}\right]{\mathrm{O}}_{2.925-\delta }+2\delta {\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {\mathrm{L}\mathrm{a}}_{0.85}{\mathrm{C}\mathrm{a}}_{0.15}{\mathrm{C}\mathrm{r}}^{3+}{\mathrm{O}}_{2.925}+\delta {\mathrm{H}}_2\mathrm{O}+2\delta {\mathrm{e}}^{-}$$

(3)

$${\mathrm{L}\mathrm{a}}_{0.85}{\mathrm{C}\mathrm{a}}_{0.15}{\mathrm{C}\mathrm{r}}^{3+}{\mathrm{O}}_{2.925}+2\delta {\mathrm{O}\mathrm{H}}^{-}\leftrightarrow {\mathrm{L}\mathrm{a}}_{0.85}{\mathrm{C}\mathrm{a}}_{0.15}\left[{\mathrm{C}\mathrm{r}}_{2\delta }^{6+};{\mathrm{C}\mathrm{r}}_{1-2\delta }^{3+}\right]{\mathrm{O}}_{2.925+\delta }+\delta {\mathrm{H}}_2\mathrm{O}+2\delta {\mathrm{e}}^{-}$$

(4)

$$\mathrm{E}\left(\mathrm{R}\mathrm{H}\mathrm{E}\right)=\mathrm{E}\left(\mathrm{H}\mathrm{g}/\mathrm{H}\mathrm{g}\mathrm{O}\right)+0.0592\mathrm{p}\mathrm{H}+0.098$$

(5)

According to Equation (5), the voltage window of the working electrode during measurement is determined based on the electrode of the reference electrode. The CV and XPS results show that the electrochemical storage mechanism of LaCrO₃ perovskite in alkaline electrolytes involves oxygen ion intercalation mediated by Cr oxidation. This can be expressed by Equations (1) and (2). Oxygen ions adsorbed on the surface of perovskite release a proton (H⁺) to insert into the oxygen vacancies. Subsequently, the proton (H⁺) combines with another oxygen ion (OH⁻) to form H₂O. The oxygen ion diffusion along the octahedron edge is responsible for the filling of oxygen vacancies. This process leads to the intercalation of each oxygen ion, thereby oxidizing Cr³⁺ to Cr⁶⁺. As illustrated by Equations (3) and (4), the excess oxygen ions diffuse to the surface via the cobalt centers, concurrently oxidizing the Cr³⁺ atoms to the Cr⁶⁺ state. These results confirm that the LaCrO₃ electrodes contains a greater number of oxygen vacancies, aligning with prior XPS findings. With increasing in Ca doping content, as illustrated in Table 1, oxygen vacancy concentration within the crystal structure rises correspondingly. Enhanced oxygen ion conductivity of the electrode material improves its electrochemical performance. Nevertheless, excessive Ca doping alters lattice parameter, leading to crystal structure distortion and defect site formation [60]. Such alterations degrade electrochemical performance, including a reduction in cycle stability and energy storage efficiency in capacitors [61,62,63].

Figure 6a–d illustrate the galvanostatic charge-discharge curve of the La_1−xCa_xCrO₃ (x = 0–0.2) electrodes sample at current densities of 0.5–5 A/g. As the current density rises, the duration of discharge diminishes. This trend is due to the increased internal resistance at higher current densities, leading to a decrease in the specific capacitance [64]. At a constant current density, the discharge duration the La_1−xCa_xCrO₃ (x = 0–0.2) electrodes first increases then decreases with increasing Ca doping concentration. The specific capacitance of the La_1−xCa_xCrO₃ (x = 0–0.2) electrodes sample at a current density of 0.5 A/g is 133, 211, 306 and 211 F/g, respectively. The specific capacitance shows a volcano-shaped dependence on Ca content, peaking at x = 0.15. The electrode composed of La_0.85Ca_0.15CrO₃ demonstrates the greatest specific capacitance, and this result is consistent with the outcomes obtained from the cyclic voltammetry curve test [65].

The electrode performance contingents upon the redox reactive sites, as evidenced by the charge-discharge curve.

Table 2. The columbic efficiency and available redox reaction active sites of La_1−xCa_xCrO₃ (x = 0–0.2) at a current density of 0.5 A/g.

	Coulombic Efficiency (%)	Active Site
LaCrO3	88.4	0.072
La0.9Ca0.1CrO3	87.7	0.083
La0.85Ca0.15CrO3	91.4	0.118
La0.8Ca0.2CrO3	91.1	0.094

summarizes the coulomb efficiency and the number of available redox reactive sites of the La_1−xCa_xCrO₃ (x = 0–0.2) electrode at a current density of 0.5 A/g. La_1−xCa_xCrO₃ (x = 0–0.2) electrodes participated in the redox reaction to a limited extent, with participation rates of 7.2%, 8.3%, 11.8% and 9.4%, respectively. La_0.85Ca_0.15CrO₃ electrode achieves optimal active site density (11.8%), correlating with its highest specific capacitance. Current density changes significantly affect the coulomb efficiency and active site utilization of La_1−xCa_xCrO₃ electrodes. At low current density, the electrochemical reaction is stable with high coulomb efficiency but lower active site utilization. Conversely, high current density increases reaction irreversibility and reduces coulomb efficiency, while significantly improving active site utilization. Optimizing the electrode structure, material modification, electrolyte, and battery design can reduce internal resistance at high current density and enhance electrochemical performance [66].

The AC impedance spectra of La_1−xCa_xCrO₃ (x = 0–0.2) series, as illustrated in Figure 7a, reveals two distinct regions: a semi-circular section at high frequencies and a linear region at low frequencies [67,68,69]. The semi-circular portion of the high-frequency spectrum represents the redox reaction process occurring at the electrode surface, while the linear portion represents the capacitive effect generated at the electrode surface [70,71]. The charge transfer resistance is determined by fitting a semicircular curve, with the magnitude of the resistance directly reflected in the diameter of the semicircle [72]. Figure 7b,c show the bode plots for La_1−xCa_xCrO₃. bode plots show that the low-frequency impedance mode value increases and the phase angle changes, providing evidence of the adsorption phenomenon on the electrode surface. Calculation concluded that LaCrO₃ exhibits the largest transfer resistance. This finding is consistent with the results of the previous GCD curve. Electrochemical impedance spectroscopy reveals impedance values of 0.66, 0.56, 0.52, and 0.58 Ω for x = 0, 0.1, 0.15, and 0.2, respectively. La_0.85Ca_0.15CrO₃ electrode exhibited the lowest equivalent resistance, confirming superior electrochemical performance. The stability of the La_0.85Ca_0.15CrO₃ electrode was evaluated over 5000 continuous charge and discharge cycles at a constant current density of 1 A·g⁻¹ in 0.1–0.55 V. Figure 7d confirms the exceptional cyclability of La_0.85Ca_0.15CrO₃ electrode. The capacitance remains at 94% demonstrates even after 5000 cycles. To verify the cycling performance of La_1−xCa_xCrO₃ in practical applications, a symmetrical supercapacitor was assembled using two La_0.85Ca_0.15CrO₃ electrodes, PET films and 1 M KOH solution. Long-cycle test was conducted at a current density of 1 A·g⁻¹, as shown in Figure 7e. After 5000 long-cycle tests, the electrode’s specific capacity gradually increased. This increase is due to initial electrolyte penetration and ion intercalation/deintercalation reopening pores. This impeded the effective entry of ions, consequently leading to an incomplete release of the designated capacity. As the cycle progresses, electrolyte penetration and ion intercalation/deintercalation progressively open closed pores, increasing specific surface area and active sites, thereby enhancing capacity. The coulombic efficiency remained above 94%, and the partial charge and discharge curves of the electrode also proved La_1−xCa_xCrO₃’s excellent stability. These findings suggest that La_0.85Ca_0.15CrO₃ nanoparticles are a promising negative electrode material for supercapacitors.

4. Conclusions

In this study, a series of La_1−xCa_xCrO₃ (x = 0–0.2) samples were successfully synthesized by the sol-gel method, and their microstructure, morphology and electrochemical properties were systematically studied. Results indicate that the incorporation of calcium has a notable impact on both the crystal structure and the electrochemical properties of LaCrO₃. Calcium doping leads to changes in the lattice parameters of LaCrO₃ and simultaneously increases the concentration of oxygen vacancies. These oxygen vacancies act as active sites, promoting the adsorption of OH⁻ and the diffusion of O₂⁻ ions in the electrolyte, thereby enhancing the specific capacitance of the material. In addition, calcium doping significantly increased the specific surface area of LaCrO₃ from 38.01 m²/g to 48.21 m²/g, providing more active sites for electrochemical reactions and further enhancing the electrochemical performance. In electrochemical tests, La_0.85Ca_0.15CrO₃ sample exhibited the highest specific capacitance of 306 F/g at 1 A/g, which was approximately 2.3 times higher than the undoped sample (133 F/g). Meanwhile, its charge transfer resistance is the lowest, only 0.52 Ω, indicating the fastest electrode reaction kinetics. After 5000 charging-discharge cycles, the specific capacitance retention rate of La_0.85Ca_0.15CrO₃ electrode was 94%, demonstrating excellent cycling stability. In conclusion, calcium doping significantly enhances the electrochemical performance of LaCrO₃ by altering the crystal structure, increasing oxygen vacancies, and improving the specific surface area, making it a promising candidate for supercapacitor electrode materials. Future research could explore other dopants or composite structures to further enhance lanthanide perovskite performance.