Unveiling the synergistic potential of LaMnO₃–CeO₂ composites in supercapacitor applications

Abstract

This study investigates the synthesis of LaMnO₃–CeO₂ composites with varying CeO₂ contents ((100 − x)% LaMnO₃–x% CeO₂; x = 0, 10, 30, 50, 100 wt%) via an autocombustion method to elucidate their synergistic electrochemical properties. X-ray diffraction (XRD) confirmed the presence of both LaMnO₃ (LMO) and CeO₂ phases in the anticipated stoichiometric ratios. Nitrogen adsorption–desorption isotherms revealed a mesoporous structure, with the LMO–CeO₂ (70 : 30) composite exhibiting the highest specific surface area of 14.32 m² g⁻¹, as determined by the Brunauer–Emmett–Teller (BET) method. X-ray photoelectron spectroscopy (XPS) provided insights into the ion valences and chemical composition of the composites. Electrochemical performance was evaluated in a 1 M KOH aqueous electrolyte using a three-electrode configuration. The LMO–CeO₂ (70 : 30) composite demonstrated superior performance, achieving a specific capacitance of 830.3 F g⁻¹ at a scan rate of 1 mV s⁻¹ and 637.6 F g⁻¹ at a current density of 1 A g⁻¹, corresponding to an energy density of 31.9 Wh kg⁻¹ at a power density of 357.5 W kg⁻¹. These results underscore the synergistic enhancement of electrochemical properties through the integration of LaMnO₃ and CeO₂, offering significant potential for the development of high-performance materials for energy storage applications.

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Sustainability spotlight

The growing need for sustainable energy storage requires materials that are efficient, affordable, and environmentally friendly. In this work, we introduce LaMnO₃–CeO₂ composites as electrode materials for supercapacitors, synthesized by a simple and scalable autocombustion process. These oxides are composed of earth-abundant elements, avoiding the use of costly or scarce metals. By tuning the LaMnO₃–CeO₂ ratio, the composites show enhanced charge storage and long-term stability, offering a pathway to durable devices with reduced material waste. This research supports global efforts toward clean, sustainable energy storage solutions.

Introduction

The demand for power and energy resources is increasing due to the rapid population growth and industrialization's fast-paced development.¹ Supercapacitors offer several benefits, including high power density, remarkable cycling stability, and quick charging and discharging rates, in contrast to conventional batteries, which store energy through chemical reactions.^2–6 ABO₃ perovskite materials have recently garnered significant attention as potential replacements for traditional metal oxides in electrochemical applications. The key feature of ABO₃-type perovskites, such as LaMnO₃, is their ability to undergo redox reactions, which lead to pseudocapacitance. The Mn³⁺/Mn⁴⁺ redox couple governs the redox behavior in LaMnO₃. During charge/discharge cycles, Mn can switch between these oxidation states, contributing to the material's overall capacitance. This faradaic charge storage mechanism (pseudocapacitance) significantly enhances the energy density of the supercapacitor, which is one of the primary performance metrics of the capacitor. However, for redox reactions to occur efficiently, electrons must move between the Mn ions and the external circuit. Low conductivity hinders the movement of these electrons, which limits the efficiency of the pseudocapacitive charge storage and reduces the overall capacitance. Additionally, at higher current densities (faster charge/discharge), the high conductivity of a material is desirable to effectively engage redox couples during the cycle. This enhances charge transfer kinetics, minimizes internal resistance, and reduces voltage losses. One approach to improving the supercapacitor performance of LaMnO₃ is to combine it with conductive materials such as graphene,⁷ carbon nanotubes,⁸ or conducting polymers,⁹ thereby forming LaMnO₃ composites. In addition to conductivity mismatch, the structural mismatch between LaMnO₃ and graphene, for example, could cause the loss of contact between the two components, leading to poor charge/discharge performance.

The above issue is addressed by combining LaMnO₃ with similar oxides, forming a compatible interface with LaMnO₃. Along with reducing strain and facilitating better electron transport between the oxides, the oxide's pseudocapacitive behavior can lead to a synergistic effect, improving the total capacitance of the supercapacitor. For example, LaMnO₃@NiO composites were synthesized by A. Arya et al. via a hydrothermal method assisted by sol–gel processing. They claimed the C_SP of 170 F g⁻¹ when measured at 10 mV s⁻¹.¹⁰ P. M. Shafi et al. synthesized LaMnO₃/Mn₃O₄ composites using a one-pot synthesis method. The C_SP of 135 F g⁻¹ was achieved at 1 A g⁻¹ in 1 M Na₂SO₄ solution, corresponding to an energy density of 75 Wh kg⁻¹ and a power density of 1000 W kg⁻¹.¹¹ LaMnO₃–MnO nanoarrays supported by carbon cloth were synthesized through one-step electrodeposition by P. Ma et al. These nanoarrays demonstrated a C_SP of 260 F g⁻¹ in a 0.5 M Na₂SO₄ at 0.5 A g⁻¹.¹² LaMnO₃@NiCo₂O₄ nanoarchitecture in Ni foam was synthesized by H. Tian et al. using the hydrothermal route, which exhibited a C_SP of 811 F g⁻¹ in 6 M KOH and at 1 A g⁻¹.¹³

Considering its structural compatibility, CeO₂ is a promising choice for forming a composite structure with LaMnO₃. While LaMnO₃ has a perovskite structure, CeO₂ adopts a fluorite structure; however, both exhibit cubic symmetry, making them structurally compatible at the interface. While LaMnO₃ provides efficient electron transport via hopping through Mn–O–Mn bonds, CeO₂ provides oxygen vacancies for ionic transport. These two conductivity mechanisms combine at the interface, creating a simultaneous pathway for both electronic and ionic charge transport. Furthermore, Mn³⁺/Mn⁴⁺ redox transitions in LaMnO₃ could be coupled with Ce⁴⁺/Ce³⁺ redox transitions in CeO₂,¹⁴ promote electron exchange with multiple redox processes and thus enhance pseudocapacitance. Meanwhile, oxygen ions from LaMnO₃'s lattice can migrate into vacancies in CeO₂ and vice versa, and this movement could stabilize oxygen vacancies at the interface, increasing active sites for redox reactions. In fact, CeO₂ is known for its electrochemical stability in a wide range of electrolytes (both aqueous and organic), and it can withstand multiple charge/discharge cycles without significant degradation. This property enhances the cycling life of LaMnO₃–CeO₂ composites in supercapacitors.

This study presents a detailed electrochemical analysis of LaMnO₃–CeO₂ composites prepared via autocombustion. The process involves igniting a precursor mixture containing metal salts and fuels (such as urea or citric acid). When ignited, the heat generated by the combustion reaction leads to the rapid formation of highly porous and high surface area nanocrystalline particles.¹⁵ As discussed above, the choice of LaMnO₃ as a composite component is due to its excellent redox activity, structural stability, and high theoretical C_SP. At the same time, CeO₂ has a highly reversible Ce³⁺/Ce⁴⁺ redox transition, which is essential for its pseudocapacitive behavior. CeO₂ can easily switch between the +3 and +4 oxidation states in redox, allowing it to store and release charge through fast and reversible electron transfers. The choice further extends to CeO₂ as other rare-earth oxides, such as La₂O₃, Pr₂O₃, and Nd₂O₃, typically have more stable oxidation states (e.g., La³⁺, Pr³⁺, Nd³⁺) and do not exhibit the same level of redox versatility as CeO₂. Thus, enhanced electrochemical performance in LaMnO₃–CeO₂ composite is anticipated from the synergistic interactions across their interfaces, enabling simultaneous ionic and electronic transport. The nanoscale interface is expected to maximize the active area, shorten diffusion paths, and enhance charge separation, ensuring efficient charge storage with enhanced electrochemical performance.

Experimental section

Materials

The composites of LaMnO₂–CeO₂ were prepared using lanthanum(III) nitrate hexahydrate (La-nitrate, La(NO₃)₃·6H₂O), cerium(III) nitrate hexahydrate (Ce-nitrate, Ce(NO₃)₃·6H₂O), manganese(II) nitrate tetrahydrate (Mn-nitrate, Mn(NO₃)₂·4H₂O), and citric acid monohydrate (C₆H₈O₇·H₂O). All chemicals were purchased from Sigma-Aldrich, USA, and used without purification.

Preparation of LaMnO₃–CeO₂ composites

Composites of LaMnO₃–CeO₂ were synthesized via the autocombustion process.¹⁶ The weight% of LaMnO₃ and CeO₂ in the composite was adjusted according to the expression (100 − x)% LaMnO₃–x% CeO₂ (x wt% = 0, 10, 30, 50, and 100), and the resulting samples were designated as LMO, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), LMO–CeO₂ (50% : 50%), and CeO₂, respectively. Initially, the hydrous powders of La-nitrate, Mn-nitrate, and citric acid monohydrate were mixed in 20 mL of deionized (DI) water and stirred magnetically (solution-1). In a separate beaker, Ce-nitrate and citric acid monohydrate were dissolved in 20 mL of DI water and stirred for 20 minutes to ensure uniformity (solution-2). Subsequently, solution 2 was added to solution 1, and the mixture was heated at 120 °C with continued magnetic stirring until autoignition occurred. The resulting black powder was transferred to a crucible and calcined at 900 °C overnight in the furnace to yield LaMnO₃–CeO₂ composites. The chemical reaction in forming LaMnO₃–CeO₂ composites is presented as eqn (1), while Fig. 1 illustrates a schematic diagram of the synthesis process.

La(NO₃)₃·6H₂O + Ce(NO₃)₃·6H₂O + Mn(NO₃)₂·4H₂O + C₆H₈O₇·H₂O → LaMnO₃(powder) + CeO₂(powder) + 8N₂↑ + 6CO₂↑ + 21H₂O + 5O₂↑ (1)

Fig. 1 A schematic of the autocombustion synthesis process of LaMnO₃–CeO₂ composites. Created with https://BioRender.com.

Electrode fabrication

Electrode fabrication was carried out according to previous work.¹⁷ A 2 cm × 1 cm nickel foam (Ni-foam) was used for electrode preparation. It was first ultrasonicated for 10 minutes in 25 mL of 37% HCl solution and then rinsed with deionized (DI) water. Subsequently, it underwent another 10 minutes of ultrasonication in DI water and ethanol. Finally, after an additional 10 minutes of ultrasonication in ethanol, the samples were dried at 110 °C for 2 hours under vacuum conditions. To prepare the homogeneous electrode solution, 6 mg of the synthesized LaMnO₃–CeO₂ composite was stirred for 6 hours (magnetically) with 1 mg of polyvinylidene fluoride in 0.001 L of N-methylpyrrolidone to ensure uniform dispersion. After that, 0.6 mL of the resulting suspension was then deposited onto a 1 cm × 1 cm section of precleaned Ni-foam. The coated substrate was heated at 110 °C in a vacuum furnace for 20 hours. The loading of the active material was calculated by measuring the mass difference between the coated and uncoated Ni-foam.

Material characterization

The structural characteristics, including crystallographic phase and lattice parameters of the LaMnO₃–CeO₂ composites, were analyzed by using X-ray diffraction (XRD) using Cu Kα1 radiation of wavelength λ ∼ 1.5406 Å on a D8 Advance diffractometer (Bruker, Germany). Data were collected using a Vantec solid-state detector (Bruker) with a 2θ range of 20°–70°, a step interval of 0.0484°, and a collection time of 0.2 s per step. Morphological analysis was performed using a Hitachi S-470 field-emission scanning electron microscope (FESEM) operated at 10 μA emission current and 20 kV as an acceleration voltage. With the help of energy-dispersive X-ray spectroscopy (EDX) using the Bruker Esprit spectrum 2.3, the elemental composition was determined. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Scientific K-Alpha system at a 0.1 eV step size to determine the binding energies, elemental compositions, chemical states, and electron configuration of the LaMnO₃–CeO₂ composite. The Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution were determined using a NOVAtouch surface area analyzer (Quantachrome Instruments, USA) at 77 K, employing nitrogen adsorption. A Gamry instrument (Interface 1000, USA) with a three-electrode setup was used to perform the electrochemical measurements in 1 M KOH electrolyte. A stoichiometric quantity of KOH flakes was dissolved in 25 mL of deionized water to make a 1 M KOH. A platinum plate and Ag/AgCl were used as the counter and reference electrodes, respectively. A working electrode was fabricated by coating the sample onto a pure nickel foam substrate (area: 1 cm × 1 cm). The electrochemical performance was evaluated at room temperature using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and charge–discharge measurements.

Results and discussion

Structure and morphology study

The XRD spectra along with the corresponding Rietveld refinement profiles of LaMnO₃–CeO₂ composites are shown in Fig. 2. The diffraction peaks observed at 2θ = 28.7, 33.2, 47.7, 56.6, 59.3, and 69.7° match the PDF number 01-073-6318, confirming the presence of CeO₂ with a face-centered cubic structure (space group Fm3m). Additionally, the primary peaks at 2θ = 22.9, 32.5, 32.8, 40.2, 46.9, 52.6, 58.2, and 68.1° correspond to the PDF number 01-075-7697, indicating the presence of rhombohedral (space group R3̄c) for LaMnO₃. Sharp and intense diffraction peaks suggest that the LaMnO₃ and CeO₂ phases exhibit good crystallinity (Fig. 2(a)). Rietveld refinement was performed (Fig. 2(b)) to determine the composition percentages and lattice parameters, which are summarized in Table 1. Overall, the R_w% for all refinements was less than 2%. The calculated composition ratio matches our initial stoichiometry of the composite.

Fig. 2 (a) X-ray diffraction profiles and (b) corresponding Rietveld refinement results for the synthesized LaMnO₃–CeO₂ composites.

Table 1 Estimated composition of LaMnO₃–CeO₂ composites, lattice parameters, crystallite size, and microstrain derived from XRD data

Composites	Estimated composition	Lattice parameters					Crystallite size (nm)		Microstrain (10^−3)
		LMO			CeO2
		a (Å)	c (Å)	V (Å^3)	a (Å)	V (Å^3)	LMO	CeO2	LMO	CeO2
LMO	LMO	5.5027 (4)	13.3134 (6)	349.123 (23)	—	—	86.65 ± 0.03	—	1.6	—
CeO2	CeO2	—	—	—	5.40970 (8)	158.314 (7)	—	81.56 ± 0.05	—	1.3
LMO–CeO2 (90% : 10%)	91.0% : 9.0%	5.5078 (8)	13.3371 (12)	350.39 (5)	5.4256 (13)	159.72 (11)	60.28 ± 0.04	29.50 ± 0.48	1.8	1.6
LMO–CeO2 (70% : 30%)	68.0% : 32.0%	5.5054 (11)	13.3528 (24)	350.49 (8)	5.4231 (5)	159.49 (4)	33.01 ± 0.10	20.09 ± 1.38	1.2	0.1
LMO–CeO2 (50% : 50%)	50.5% : 49.5%	5.5063 (26)	13.367 (7)	350.98 (18)	5.4200 (4)	159.22 (4)	19.80 ± 0.44	33.013 ± 0.10	1.0	1.2

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The Williamson–Hall (W–H) equation β cos θ = kλ/D + 4ε sin θ,^18,19 is used to determine the crystallite size (D) from the XRD data. Here, β is the full width at half maximum (in radians) of the peaks, θ is the peak position, k is the Scherrer constant (0.9), λ is the wavelength of the X-ray used, and ε is the lattice strain. The crystallite size is obtained from the Y-intercept of the linear plot of β cos θ (y-axis) vs. 4 sin θ (x-axis). The W–H equation was plotted using the diffraction peaks at 2θ = 28.7, 47.7, and 56.6° for CeO₂ and 2θ = 22.9, 32.5, 32.8, 40.2, 46.9, 52.6, 58.2, and 68.1° for LaMnO₃. The crystallite size and lattice parameters of LMO and CeO₂ are listed in Table 1. The crystallite size of LMO decreased while CeO₂ increased with increasing CeO₂ content in the composite. The difference in grain growth between LMO and CeO₂ can be attributed to grain boundary migration, which typically occurs through long-range diffusion in a two-phase system with mutually limited solubility.²⁰ The impediment to grain growth of LMO is evident with the increase in CeO₂ content in the composite.²¹ The lattice parameters obtained from the Rietveld analysis of the respective components of the composite are listed in Table 1. The lattice parameters of LMO and CeO₂ remain unaltered across the compositions of the composites. Furthermore, CeO₂, with a cubic structure, and LaMnO₃, with a rhombohedral structure, display similar lattice parameters of approximately 5.503 Å. This similarity in lattice parameters reduces the lattice strain at the interface between the two components. With the close lattice matching, the energy cost associated with forming the grain boundary is minimized, thus making it easier for the grains to grow uniformly.

Fig. 3 displays the LaMnO₃–CeO₂ composite morphology, elemental map, and spectrum. Fig. 3(a and a′ to e and e′) shows 20 and 5 μm scale FESEM images of LMO, CeO₂, and composite particles. Fig. 3(b and b′) shows a compact and dense morphology of CeO₂ with flake-like or layered structure and granular clusters. The composite morphology becomes more granular with increasing CeO₂ content. The EDX mapping of La (green), Ce (cyan), Mn (blue), and O (red) of LaMnO₃–CeO₂ composites is shown in Fig. 3, and the EDX spectrum of synthesized composites is shown in Fig. 3(f–j). The EDX elemental mapping shows a homogeneous coverage of LMO particles with the CeO₂ phase. CeO₂ and LaMnO₃, having close lattice “a” parameter (Table 1), could facilitate the growth of CeO₂ over LaMnO₃ particles. The lattice match makes the growth of CeO₂ energetically favorable, aligning with the crystal structure of LMO. Consequently, this reduces the strain energy that would otherwise cause CeO₂ to agglomerate as separate particles.

Fig. 3 FESEM images at 20 μm and 5 μm magnifications for (a and a′) LMO, (b and b′) CeO₂, (c and c′) LMO–CeO₂ (90% : 10%), (d and d′) LMO–CeO₂ (70% : 30%), and (e and e′) LMO–CeO₂ (50% : 50%). Elemental mapping of O (red), Mn (blue), La (green), and Ce (cyan color) at 5 μm highlights the distribution of elements in different LaMnO₃–CeO₂ composites. (f–j) Corresponding EDX spectra of the LaMnO₃–CeO₂ composites. (NA: not applicable.)

The nitrogen adsorption and desorption isotherms of LaMnO₃–CeO₂ composites (Fig. 4) were measured at a temperature of 77 K using N₂ gas. The isotherm profile of LaMnO₃–CeO₂ composites corresponds to a type IV isotherm with mesoporous characteristics.²² The pore size distribution and specific surface area were calculated from the desorption data by using the BJH and BET models, respectively, and are listed in Table 2. The LMO–CeO₂ (70% : 30%) composite has the highest specific surface area of 14.32 m² g⁻¹, along with a total pore volume of 0.09 cm³ g⁻¹ and a pore radius of 11.91 nm. Fig. 4 inset shows the BJH pore size distribution. The differential pore volume (dV/dD) at each pore size indicates the volume available in pores of a given size range. The increase in the dV/dD value below 10 nm in all samples shows the presence of mesopores (2–50 nm) and micropores (<2 nm) in the material. The porous structure, larger surface area, and pore volume of the composite are beneficial to improving the electrochemical performance of the composite.²³

Fig. 4 (a–e) Nitrogen adsorption–desorption isotherms of LMO–CeO₂ composites, with corresponding BJH pore size distribution curves shown in the inset.

Table 2 Surface area and pore size derived from N₂ adsorption and desorption measurement of LaMnO₃–CeO₂ composites at 77 K

Composite (LMO–CeO2)	BET surface area (m^2 g^−1)	Average pore size (radius, nm)	Total pore volume (cc g^−1)
LMO	7.70	3.91	0.02
CeO2	8.86	4.22	0.02
LMO–CeO2 (90% : 10%)	10.35	5.00	0.03
LMO–CeO2 (70% : 30%)	14.32	11.91	0.09
LMO–CeO2 (50% : 50%)	13.54	10.60	0.07

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The composition and the oxidation states of LaMnO₃–CeO₂ composites were analyzed using XPS. The high-resolution spectra of La (Fig. 5(a)) exhibit binding energies at 834.4 and 838 eV for La 3d_5/2 and 850.8 and 855.4 eV for La 3d_3/2, respectively, corresponding to the +3-oxidation state. This implies that La ions are in the trivalent state (La³⁺), consistent with previously reported values.²⁴Fig. 5(b) presents the high-resolution spectra for Mn, displaying two peaks at 641.8 and 653.4 eV for Mn 2p_3/2 and Mn 2p_1/2, respectively. The deconvoluted Mn 2p_3/2 peaks reveal three oxidation states at 640.7, 641.6, and 642.9 eV, which correspond to Mn²⁺, Mn³⁺, and Mn⁴⁺. Similarly, the deconvoluted Mn 2p_3/2 peaks at 652.4, 653.2, and 654.4 eV corresponds to Mn²⁺, Mn³⁺, and Mn⁴⁺, respectively.²⁵ The 882.8, 888.8, 898.2, and 901.2 eV peaks correspond to the Ce⁴⁺ oxidation state.²⁶ Additionally, the peaks at 881.1, 885.5, 899.0, and 903.9 eV binding energy confirm the Ce³⁺ oxidation state (Fig. 5(c)).²⁷ The localization of the electrons left behind in the Ce 4f states due to oxygen defects could lead to the formation of Ce³⁺ ions.^14,28 The coexistence of Ce³⁺ and Ce⁴⁺ oxidation states verified the incorporation of CeO₂ in the composite. The high-resolution O 1s spectrum of the LaMnO₃–CeO₂ composites, illustrated in Fig. 5(d), was deconvoluted into three primary peaks. The first peak (O₁) at 529.4 eV corresponds to lattice oxygen (O_latt) present on the surface. The second peak (O₂), appearing at 531.1 eV, is linked to oxygen vacancies or under-coordinated sites (O_ads).¹³ The third component (O₃), located at 532.8 eV, is attributed to hydroxyl groups arising from adsorbed moisture or surface-bound water molecules.^13,29 The ratios of Mn²⁺/Mn³⁺ are 0.93, 0.81, 0.70, and 0.69, and Mn⁴⁺/Mn³⁺ are 0.98, 1.04, 1.12, and 0.71 for LMO, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%), respectively. For Ce ions, the ratio of Ce³⁺/Ce⁴⁺ is 0.79, 0.98, 1.03, and 0.57 for CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%), respectively. The presence of Mn²⁺, Mn⁴⁺, and Ce⁴⁺ ions indicates the presence of oxygen deficiency in the corresponding compounds in the composite. A large number of defects are observed in LMO and CeO₂ for the 70% : 30% composite. The presence of oxygen defects enhances the electrochemical properties of LaMnO₃–CeO₂ composites.³⁰ Oxygen vacancies act as active sites for faradaic charge storage in both LaMnO₃ and CeO₂, improving the C_SP by enhancing the ability of these materials to undergo redox reactions during supercapacitor operation.^23,31

Fig. 5 (a) High-resolution spectrum of La 3d_5/2, (b) high-resolution spectrum of Mn 2p_1/2, (c) high-resolution spectrum of Ce 3d_5/2, and (d) high-resolution spectrum of O 1s.

Electrochemical performance

Cyclic voltammetry (CV). The CV curves of LaMnO₃–CeO₂ composites at 10 mV s⁻¹ are shown in Fig. 6(a). The area enclosed by the LMO–CeO₂ (70% : 30%) composites is higher than that of others, indicating that the C_SP is expected to be higher for these composites. Fig. 6(b–f) shows the CV curves of LaMnO₃–CeO₂ composites at different scan rates of 1 to 300 mV s⁻¹. The redox peak can still be observed at 300 mV s⁻¹, indicating that the charge storage mechanism of the electrode is due to the reversible conversions of redox reactions between Ce⁴⁺ and Ce³⁺ in CeO₂ and Mn³⁺ and Mn⁴⁺ for LaMnO₃ electrodes. Moreover, the oxidation peaks shifted to higher potentials, and the reduction peaks shifted to lower potentials as the scan rates increased. The redox reaction is more kinetically controlled at higher scan rates than purely diffusion-controlled. At higher scan rates, the redox reaction does not occur instantly upon reaching its standard potential because the electrode and electrolyte need time for charge transfer and mass transport to catch up. The reaction becomes kinetically hindered, meaning that additional energy (a more negative reduction potential) is necessary to drive the process at the required rate. The reduction peaks further shifted to lower potentials in LMO–CeO₂ (50% : 50%) composites, which could be attributed to decreased reversibility in the redox processes. This behavior likely arises due to excess CeO₂ disrupting the conductive network within the composites, blocking an active redox site of LMO, obstructing ion transport pathways, and decreasing the intrinsic reversibility of the Ce³⁺ ↔ Ce⁴⁺ redox transition.³² Concomitantly, a voltage corresponding to the reduction drifts to a lower potential with an increase in CeO₂ content, primarily due to an increase in the overpotential required for reduction. The corresponding adsorption/desorption of OH⁻ ions from the CeO₂ and LaMnO₃ surface can be expressed as eqn (2)–(5):^33,34

Ce⁴⁺O₂ + H₂O + e⁻ → Ce³⁺O·OH + OH⁻ (2)

Ce³⁺OOH + OH⁻ → Ce⁴⁺O₂ + H₂O + e⁻ (3)

La[Mn_2δ²⁺;Mn_(1−2δ)³⁺]O_3−δ + 2δOH⁻ ↔ LaMn³⁺O₃ + 2δe⁻ + δH₂O (4)

LaMn³⁺O₃ + 2δOH⁻ ↔ La[Mn_2δ⁴⁺;Mn_(1−2δ)³⁺]O_3+δ + 2δe⁻ + δH₂O (5)

Fig. 6 Cyclic voltammetry measurements of LaMnO₃–CeO₂ composites: (a) at a scan rate of 10 mV s⁻¹ and (b–f) at different scan rates of 1 to 300 mV s⁻¹.

Pseudocapacitive materials such as LaMnO₃–CeO₂ composites are capable of storing charge in two different mechanisms: (1) through the faradaic electron transfer, which involves the reversible redox reactions of metal ions with their corresponding oxides (e.g. Mn(II), Mn(III), and Mn(IV), and Ce(II), and Ce(III)), and (2) via non-faradaic charge storage in the electrical double layer on the electrode's surface. Accordingly, the charge–discharge behavior was qualitatively assessed using eqn (6) or (7).³⁵

i = aν^b (6)

or

log(i) = log(a) + b log(ν) (7)

where a and b are the constants, while i denotes the current density and ν represents the scan rate of the CV measurement. In general, two limiting values exist for b: b = 0.5, where the electrochemical behavior is dominated by diffusion-controlled or battery-type processes, slowing the reaction kinetics. On the other hand, b = 1 is applied to surface-controlled or electric double-layer capacitance, and the reaction kinetics are fast. When the b values fall between 0.5 and 1, the energy storage kinetics are governed by a combination of both diffusion and surface-controlled reactions. In our work, as shown in Fig. 7(a), the calculated values of b range from 0.53 to 0.65 for the LaMnO₃–CeO₂ composites. The b-value of LMO, CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%) composites are calculated to be 0.54, 0.65, 0.62, 0.53, and 0.54, respectively. Furthermore, to evaluate the quantitative contributions to the overall energy storage of the LaMnO₃–CeO₂ composites, Dunn's method was applied to distinguish between surface-controlled capacitive effects and diffusion-limited processes, as described by eqn (8) or (9).^36–38

i(V) = k₁ν + k₂ν^1/2 (8)

orIn these equations, i(V) denotes the peak current as a function of scan rate (ν), with k₁ (slope) and k₂ (Y-intercept) are both constant terms.³⁸ Here, k₁ν and k₂ν^1/2 correspond to current contributions from surface-controlled capacitive processes and diffusion-limited mechanisms, respectively.³⁶ As illustrated in Fig. 7(b), the diffusion-controlled capacitance contributions at 300 mV s⁻¹ are approximately 71, 53, 62, 85, and 81% for LMO, CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%) composites, respectively. However, it is noticed that the diffusion-controlled process persists even at 300 mV s⁻¹, indicating a Faraday-controlled type of redox reaction.

Fig. 7 (a) Relationship between the logarithm of cathodic peak current and the logarithm of scan rates, (b) contribution of capacitive/diffusion-controlled processes at different scan rates, and (c) correlation between peak current from CV curves and the square root of the scan rate of LaMnO₃–CeO₂ composites.

The Randles–Sevcik equation describes the effect of scan rate (ν, mV s⁻¹) on peak current (i_p) for a reversible cyclic voltammogram, representing a diffusion-controlled process (eqn (10)):³⁹

i_p = 2.69 × 10⁵AC₀(n³Dν)^1/2 (10)

Here, i_p is directly influenced by several parameters, including the electrode area (A, cm²), the electroactive material's concentration (C₀, mol cm⁻³), the number of electrons transferred per molecule (n), the diffusion coefficient (D, cm² s⁻¹), and the scan rate (ν, mV s⁻¹). Fig. 7(c) shows the plot of peak current obtained from the CV curves versus the square root of the scan rate, which exhibits a linear trend. This linear dependence indicates that a diffusion-controlled mechanism dominates the charge storage behavior.

Fig. 8 presents the C_SP values of the LaMnO₃–CeO₂ composites, calculated from the integrated area under the CV curves using eqn (11):⁴⁰

where m denotes the mass of the active material in the electrode, I(V) is the response current, and ΔV is the potential window.³⁶ Here, the integrated part of eqn (11) gives the area enclosed by the current–potential curve or CV curve, which is directly proportional to the C_SP of the material. The C_SP values of LMO, CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%) are 304, 289, 453, 830, and 670 F g⁻¹ at 1 mV s⁻¹, respectively, and are shown in Table 3.

Fig. 8 C _SP of LaMnO₃–CeO₂ composites at different scan rates.

Table 3 C _SP, energy density, and power density of LaMnO₃–CeO₂ composites

Composites	C SP (F g^−1)		Energy density (Wh kg^−1)	Power density (W kg^−1)
	1 mV s^−1	1 A g^−1
LMO	303.8	290.7	14.5	354.0
CeO2	289.2	234.2	11.7	351.4
LMO–CeO2 (90% : 10%)	452.8	430.2	21.5	351.9
LMO–CeO2 (70% : 30%)	830.3	637.6	31.9	357.5
LMO–CeO2 (50% : 50%)	670.0	513.6	25.7	350.5

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It has been observed that an increase in scan rate leads to a decrease in the C_SP of LaMnO₃–CeO₂ composites. At low scan rates, the C_SP is high because ions with low kinetic energy and resistance have sufficient time to diffuse into the electrode surface. Conversely, at high scan rates, the increased kinetic energy of ions increases their resistance, reducing the time available for them to interact with the electrode surface and resulting in lower C_SP values. Furthermore, it demonstrates that the C_SP values increase with the increase in the CeO₂ ratio in LMO. This may be due to the presence of oxygen vacancies, defects, and enhanced interfacial and synergistic effects between the LMO and CeO₂ electrodes. Furthermore, the LMO–CeO₂ (70% : 30%) composites have a higher C_SP value than those of other composites, indicating this is the optimal composition of this LMO and CeO₂. However, with a higher ratio of CeO₂ in LMO, the C_SP of LMO–CeO₂ (50% : 50%) composites decrease. This may be due to particle agglomeration, which implies that the CeO₂ particles may block the ion flow path, thereby limiting ion diffusion and the availability of electroactive sites.

Chronopotentiometry studies. Fig. 9(a) shows the chronopotentiometry charge/discharge curves of LaMnO₃–CeO₂ composite electrodes at a current density of 1 A g⁻¹. In all cases, the nonlinear discharge curves are asymmetric to the corresponding charge curves, indicating supercapacitive behavior. Charge/discharge measurements are also carried out at different current densities, ranging from 1 to 15 A g⁻¹, for LaMnO₃–CeO₂ composites (Fig. 9(b–f)). At low current densities, the discharge time is observed to be longer for all composites with a small plateau, indicating the extrinsic pseudocapacitive behavior. Dunn et al. introduced the concept of the “extrinsic pseudocapacitor” for materials such as LiCoO₂. These materials behave as a battery in the bulk phase, but after size reduction, pseudocapacitive behavior emerges.^41,42

Fig. 9 Charge/discharge curves (a) at a scan rate of 1 A g⁻¹ and (b–f) at different current densities from 1 to 15 A g⁻¹ for LaMnO₃–CeO₂ composites.

Eqn (12) can be used to calculate the C_SP of working electrodes from the charge–discharge curves.⁴³

where I_m represents the current density, m denotes the mass of the active material, Δt refers to the discharge time, and ΔV indicates the potential window. The calculated C_SP is 291, 234, 430, 638, and 514 F g⁻¹ at a scan rate of 1 A g⁻¹ for LMO, CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%) composites, respectively. The LMO–CeO₂ (70 : 30%) composites have a higher C_SP, which is 2.19 times higher than that of LMO and 2.7 times higher than that of CeO₂. This result emphasizes the synergistic effects between the LMO and CeO₂ compounds. Fig. 10(a) shows the relationship between the C_SP and current densities from 1–15 A g⁻¹.

Fig. 10 (a) C_SP and (b) Ragone plot of LaMnO₃–CeO₂ composites at different current densities.

Fig. 10(b) represents the Ragone plot for the LaMnO₃–CeO₂ composites, demonstrating the correlation between the logarithmic values of energy density and power density. The energy density (E; Wh kg⁻¹) and power density (P; W kg⁻¹) were determined using eqn (13) and (14):^44,45

where C_SP represents specific capacitance (F g⁻¹), and ΔV is the voltage window (V), and Δt is the discharge time (s). The LMO–CeO₂ (70% : 30%) composites have the highest energy density of 31.9 Wh kg⁻¹ with a power density of 357.5 W kg⁻¹ (Table 3). Even at the high-power density of 5390.8 W kg⁻¹, the energy density is 20.0 Wh kg⁻¹, which is higher than the reported values of CeO₂ compounds.

The ability to maintain capacity over multiple cycles at higher current rates is a crucial requirement for a charge storage device, making it an essential feature of supercapacitors for real-world applications. The cyclic performance for the LaMnO₃–CeO₂ composites electrode is depicted in Fig. 11. This was carried out at a constant current density of 10 A g⁻¹ using chronopotentiometry charge–discharge cyclic measurement for 5000 continuous cycles. No perceptible degradation was observed in the LMO–CeO₂ (70% : 30%) composite. The capacity retention of LMO–CeO₂ (50% : 50%) composite increases after 2000 cycles, and no degradation is noticed until 4500 cycles. The retention percentages of electrodes prepared using LaMnO₃–CeO₂ composites were obtained to be 85.8, 90.7, 80.2, 98.3, and 93.1%, corresponding to a decrease in C_SP from 121.0, 122.8, 231.6, 394.2, and 182.3 F g⁻¹ to 103.8, 111.4, 185.7, 387.6, and 169.7 F g⁻¹ for LMO, CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%) composites, respectively. The higher stability of electrodes of LaMnO₃–CeO₂ composites can be attributed to several factors: (i) the structurally integrated composites provide interconnected porous channels that facilitate efficient diffusion of electrolyte ions toward the interior regions, (ii) the close interfacial contact between the LaMnO₃–CeO₂ mesoporous core and highly conductive intermediate shell can increase the electrical conductivity of the electrode material, resulting in faster electron transport in the electrode.⁴⁶ Additionally, it can effectively adapt to volume variations during faradaic redox reactions across the LaMnO₃–CeO₂ core surface, preventing the active electrode material from degradation, thereby ensuring high cycling stability.

Fig. 11 Cyclic performance and coulombic efficiency (inset) of LaMnO₃–CeO₂ composite electrode during 5000 cycles at a current density of 10 A g⁻¹.

The coulombic efficiency as a function of cycle number is shown in the inset of Fig. 11. It specifies the efficacy of converting electrical charge into a useful output during charging and discharging cycles. The coulombic efficiency of electrodes was evaluated using the formula η = (t_D/t_C) × 100%.⁴⁷ Where t_D and t_C represent discharging time and charging time, respectively. The result provides the coulombic efficiency of LMO–CeO₂ (70% : 30%) and LMO–CeO₂ (50% : 50%), which exhibit 100% efficiency throughout 5000 cycles at a current density of 10 A g⁻¹. According to these results, the optimal proportion of LMO and CeO₂ composites enhances the electrochemical stability and increases the viability of the redox process.

Table 4 compares the C_SP, energy density, and power density of LaMnO₃–CeO₂ composites to those of the published articles. It demonstrates that LMO–CeO₂ (70% : 30%) outperforms other composites in terms of C_SP, energy, and power density.

Table 4 Electrochemical performance metrics of LaMnO₃ and CeO₂ based electrode materials for energy storage devices

Material	Current density (A g^−1)	Electrolyte (KOH)	C SP (F g^−1)	Energy density, E (Wh kg^−1)	Power density, P (W kg^−1)	Ref.
LaMn1±xO3	1	1 M	202.1 mAh g^−1	—	—	25
LaMnO3–SiO2	5 mV s^−1	1 M	200	—	—	48
LaMnO3–Mn3O4 (70% : 30%)	1	6 M	478.8	23.9	355.7	38
CeO2–Fe2O3 nanospindles	5 mV s^−1	6 M	142.6	—	—	49
Porous NiO–CeO2	1	3 M	305	—	—	50
LaMnO3–Co3O4 (70% : 30%)	0.5	1 M	660.0	33.0	202.7	45
LaMnO3/rGO/PANI	1	PVA/KOH	802	—	—	51
CeO2/AC	2 mA cm^−2	1 M H2SO4	162	—	—	52
Co–CeO2	2	1 M	573 C g^−1	—	—	53
Polythiophene	0.5	Polymer gel	129.13	3	250	54
La0.75Sr0.25MnO3−δ/MnO2	2 mV s^−1	1 M Na2SO4	437.2	—	—	55
CeO2/CeS2	1	0.1 M	420	21.2	303.0	56
LMO–CeO 2 (70% : 30%)	1	1 M	637.6	31.9	357.5	Present work
MnO2–CeO2-1	0.5 A g^−1	3 M	274.3	—	—	33
LaMnO3/CeO2	1	1 M Na2SO4	262.0	—	—	57

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Electrochemical impedance spectroscopy (EIS). The impedance spectrum of a supercapacitor is another crucial component to understanding its performance. The primary goal of electrochemical impedance measurements is to analyze the electrode's capacitive and resistive characteristics. The impedance spectra of the LaMnO₃–CeO₂ composites are presented in Fig. 12. The Bode diagrams present the relationship between the impedance magnitude (|z|) (Fig. 12(a)) and the phase angle between the applied voltage signal and the resulting current (Φ, degrees) (Fig. 12(b)) as a function of AC frequency, while the corresponding Nyquist plots are shown in Fig. 12(c). As depicted in Fig. 12(a), the LMO–CeO₂ (70% : 30%) composite exhibits the lowest impedance values across the high-frequency region, indicating a reduced resistance to ion transport and enhanced electrochemical performance. In the low-frequency region, the phase angle of this composite approaches −80° (Fig. 12(b)), indicating near-ideal capacitive behavior. Although not perfectly capacitive (−90°), the high phase angle signifies rapid charge storage kinetics and efficient charge transport at the electrode–electrolyte interface. In the Nyquist plot of LaMnO₃–CeO₂ composites, where Z_re (X-axis) and Z_im (Y-axis) are the real and imaginary parts of the impedance, measured in the frequency range from 100 kHz to 0.01 Hz. The insets in Fig. 12(c) are the equivalent circuit, which consists of solution resistance (R₁), open Warburg impedance (W_o), interfacial resistance (R₂), and a constant phase element (CPE). The intersection of the EIS plots and the real axis represent the solution resistance of the electrode (R₁). This can be written as the combination of the contributions from uncompensated contact resistance and the interface.⁵⁸ The high-frequency loop corresponds to the charge transfer resistance (R₂) across the catalyst/electrolyte interface.⁵⁹ At low frequencies, the linear portion of the Nyquist plot typically represents Warburg impedance, associated with ion diffusion within the electrode and electrolyte. The R₂ value of the LMO–CeO₂ (70% : 30%) composite is the lowest among the composites, indicating the composite's ability for a rapid charge transfer rate, which increases its inherent electronic conductivity and electrochemical activity. The CPE arises from a distribution of time constants, which can be attributed to the variability in R₁ and/or the dispersion of interfacial capacitance.

Fig. 12 EIS Bode plots of (a) impedance magnitude and (b) phase change vs. AC frequency. Nyquist plot (c) of LaMnO₃–CeO₂ composites illustrated with the fitted equivalent circuit (inset figure).

The impedance of a CPE is expressed by where, Q and α are the parameters defining the CPE. In the above expression, Q is expressed in Ω⁻¹ s^a, and α is the dimensionless parameter that ranges between 0 and 1. When α = 1, 0, and 0.5, the system behaves as a pure capacitor, resistance, and Warburg impedance, respectively.⁶⁰ The EIS analysis revealed that the R₂ values are 21.21, 28.05, 8.83, 6.83, and 9.25 Ω from LMO, CeO₂, LMO–CeO₂ (90% : 10%), LMO–CeO₂ (70% : 30%), and LMO–CeO₂ (50% : 50%) composites, respectively, indicating that the composites can boost the conductivity of the individual compounds. R₂ is related to the electrode area where the electrolyte ions can access; that is, the power control factor for the supercapacitor.⁶¹ The lower R₂ values of the material further indicate that it had lower resistance and aided in maintaining stability during the electrochemical measurements. The impedance values of LaMnO₃–CeO₂ composites were determined by fitting in the “Gamry Echem Analyst” software and are presented in Table 5.

Table 5 Nyquist plot fitting values for LaMnO₃–CeO₂ composites

Composites	R 1 (Ω)	R 2 (Ω)	W o (Ω, ×10^−3)	CPE1 (α)	CPE1 (Q; Ω^−1 s^a)	CPE2 (α)	CPE2 (Q; Ω^−1 s^a)	Goodness of fit (×10^−3)
LMO	1.619	21.21	0.142	0.711	0.004	0.934	0.002	0.953
CeO2	3.721	28.05	0.195	0.750	0.004	0.852	0.001	0.646
LMO–CeO2 (90% : 10%)	2.573	8.830	0.200	0.713	0.008	0.891	0.002	0.176
LMO–CeO2 (70% : 30%)	1.618	6.831	0.106	0.689	0.008	0.944	0.003	0.703
LMO–CeO2 (50% : 50%)	1.493	9.253	0.186	0.711	0.006	0.946	0.002	1.364

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This study observed that increasing the CeO₂ concentrations on LMO by up to 30% enhances the C_SP, energy density, and power density. The LMO–CeO₂ (70% : 30%) composite exhibits the highest C_SP due to an optimal balance between structure, conductivity, and ion accessibility achieved at this ratio. This composition creates a highly interconnected conducting network that significantly enhances electron transport while reducing interfacial resistance, a crucial factor for the rapid redox reactions required for efficient charge storage. LMO provides excellent supercapacitive behavior in this setup, contributing to the primary capacitance. At the same time, CeO₂ introduces structural stability, oxygen vacancies, and a slight boost in conductivity, supporting both longevity and additional charge storage sites. The LMO–CeO₂ (70% : 30%) composites enable these materials to work synergistically, maximizing their electrochemical strengths. Moreover, the mesoporous structure observed in this composite, with a specific surface area of 14.32 m² g⁻¹, facilitates easy access to the electrolyte, allowing for the accommodation of more ions during charge–discharge cycles and thereby increasing the overall capacitance. Ratios with higher CeO₂ content, such as the LMO–CeO₂ (50% : 50%) composite, begin to block ion transport pathways and hinder access to active sites, thereby reducing capacitance. In contrast, a lower CeO₂ content, such as the LMO–CeO₂ (90% : 10%) composition, lacks structural enhancement and additional redox sites provided by CeO₂, resulting in a less efficient charge storage process. Therefore, the LMO–CeO₂ (70% : 30%) composition strikes an ideal balance, offering a high surface area, optimal pore structure, and efficient ion and electron transport, all of which contribute to its superior electrochemical performance.

Conclusion

Crystalline LaMnO₃–CeO₂ composites, synthesized through an autocombustion method, exhibit significantly enhanced electrochemical properties driven by synergistic interactions between LaMnO₃ (LMO) and CeO₂. The LMO–CeO₂ (70 : 30) composite, characterized by a BET specific surface area of 14.32 m² g⁻¹, achieved a superior specific capacitance of 637.6 F g⁻¹ at a current density of 1 A g⁻¹, with an energy density of 31.9 Wh kg⁻¹ and a power density of 357.5 W kg⁻¹. This enhanced performance is attributed to a robust, interconnected conductive network, reduced contact resistance, and improved electron transport at the electrode–electrolyte interface. However, increasing CeO₂ content beyond 30% led to diminished electrochemical performance, likely due to impeded ion diffusion pathways. These findings highlight the potential of optimized LaMnO₃–CeO₂ composites for advanced supercapacitor applications and provide critical insights into tailoring composite ratios to maximize electrochemical efficiency.

Conflicts of interest

The authors confirm that they have no known financial or personal conflicts of interest that could have influenced the work presented in this paper.

Data availability

The data are available in this link: https://zenodo.org/uploads/17058741.

Acknowledgements

The authors express their gratitude to the University of Memphis for its support in conducting this research work.

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