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. 2026 Oct 5. Online ahead of print. doi: 10.1039/d6ra07046a

Design and optimization of CdS–CeO2 composite electrodes for enhanced electrochemical performance for asymmetric supercapacitors

Junaid Riaz a, Muhammad Arif a,✉, Samira Elaissi b, Amina Bibi c,✉
PMCID: PMC13637679  PMID: 42836151

Abstract

The advance of high-performance energy storage devices (ESD) is critical for modern semiconductor technology, rechargeable vehicles, and renewable energy integration. In this work, a CdS–CeO2 composite was successfully synthesized via a hydrothermal method and investigated as an electrode material for an asymmetric supercapacitor (ASCs). The CdS–CeO2 composite leverages the high redox activity and structural stability of CeO2 along with the enhanced electrical conductivity and charge transport of CdS. Structural and morphological characterization using XRD, BET surface analysis, SEM, and EDX mapping confirmed the formation of a well-dispersed, hierarchical composite with abundant electro active sites. Electrochemical measurements revealed that the CdS–CeO2 electrode delivers a specific capacitance of 476 F g−1 at 1 A g−1, along with good rate performance and outstanding long-term stability, retaining 95.83% of its initial capacitance after 15 000 charge–discharge cycles. The asymmetric supercapacitor ASC (CdS–CeO2//AC) exhibited a highest energy density of 57.74 Wh kg−1, and power density of 6298 W kg−1, indicating the effectiveness of the CdS incorporation in enhancing the electrochemical performance. These results highlight the potential of oxide-sulfide composite engineering in designing next-generation electrodes for high-performance ASCs.

The advance of high-performance energy storage devices (ESD) is critical for modern semiconductor technology, rechargeable vehicles, and renewable energy integration.graphic file with name d6ra07046a-ga.webp

1. Introduction

The constantly growing need for eco-friendly and efficient energy storage systems has motivated much research in advanced electrochemical devices. Supercapacitors (SCs) are appealing for use in portable electronics, electric vehicles, and grid stabilization because they have high power density, fast charge–discharge performance, and long cycle life.1–3 However, traditional supercapacitors have low energy density compared to batteries, which limits their use in devices that need high energy density and high power density. To address this shortcoming, asymmetric supercapacitors (ASCs) have been invented with a pseudocapacitive electrode paired with an electrical double layer capacitance (EDLC) electrode, which allows operating voltages to be increased and energy densities to be enhanced with good power performance and cyclability.4–6 Electrode architectures and materials are important factors that define the overall performance of ASCs since the electrochemical properties are dependent on the capacities to store charges through faradaic and surface adsorption processes. Studies on the electrode materials have been on transition metal oxides (TMOs), metal sulfides, carbonaceous materials, and composite materials. TMOs like RuO2, MnO2, NiO, Co3O4, and V2O5 have been shown to be highly pseudocapacitive because they have rich redox activity, but have weaknesses including low electrical conductivity and slow ion diffusion, which limits their rate behavior and practical energy storage density.7–9 Research has shown that hybrid materials that use TMOs and conductive supports can improve conductivity and electrochemical performance by providing more active sites and speeding up the transfer of charge.10–12 There is also interest in composite electrode materials that incorporate transition metal sulfides due to their high redox activity and controllable conductivity. NiCo2S4, MoS2 and other chalcogenides are metal sulfides with promising pseudocapacitive properties and better electrical conductivity than plain metal oxides, which help to increase specific capacitance and cycling stability.13–15 Other intriguing electrode materials have also been developed, including oxides of rare earth metals, like CeO2, whose redox chemistry (Ce3+/Ce4+) is reversible, which has high oxygen vacancies, and is stable in an alkaline electrolyte, enabling the reaction kinetics, and charge storage behavior to be improved.16,17 Nevertheless, pure CeO2 has poor electrical conductivity and agglomeration of nanoparticles, which may hinder ion access and restrict electrochemical activity. To overcome such challenges, conductive carbon derivatives, polymers, and other active materials have been incorporated with CeO2 to create nanocomposites that are more conductive, possess a larger electrochemically active surface area, and have enhanced redox kinetics.18–20 Cadmium sulfide (CdS) is a metal sulfide semiconductor with a relatively small bandgap (∼2.4 eV), high intrinsic conductivity and favourable charge carrier mobility, so it is a possible pseudocapacitive electrode material. Electrochemical CdS based electrodes have been shown to have good electrochemical properties such as relatively high specific capacitance, and stable cycling performance when incorporated into supercapacitor architecture.21–23 CdS may be used to speed up electron transfer and increase the overall electrochemical activity when used in combination with conductive carbon materials, and other transition metal oxides. Recent reports indicate that CdS-based composites are very effective in improving the electrochemical activity than the individual elements. Arif et al. reported that the integration of CdS with MnO significantly improved its electrochemical performance, providing 1020 F g−1 at 1 A g−1 and retaining 90% of its capacitance after 5000 charge discharge cycles.24 The composite also exhibited better charge-transfer efficiency and ion-transport kinetics compared to the individual CdS and MnO electrodes. A different work, NiCo2O4/CdS composites on Ni foam showed much higher specific capacity 331 mA h g−1 compared to pristine NiCo2O4 260 mA h g−1 as a result of the increased conductivity and pseudocapacitive effect of CdS.25 CdS–Co3O4 composites also displayed the best specific capacitance 390 F g−1 and a better charge transfer kinetics over pure CdS, and Co3O4, and how this synergistic behavior of the oxide/sulfide interface and better ion transport.26

This work develops a CdS–CeO2 composite as the positive electrode of an asymmetric supercapacitor coupled with activated carbon. The morphological and structural studies verified formation of a well-dispersed and hierarchical composite with a rich electroactive sites. Electrochemical analysis of the CdS–CeO2//AC ASC demonstrated that the CdS–CeO2 electrode delivered a high specific capacitance of 476 F g−1 at 1 A g−1, excellent rate capability, and a high energy density, demonstrating its potential for high-performance asymmetric supercapacitor applications. The device also reached a top energy density of 57.74 Wh kg−1 and power density of 6298 W kg−1 indicating that the incorporation of CdS gave a substantial boost to charge storage and transfer. These findings affirm that engineering oxide/sulfide composite is a viable approach to enhancing electrochemical performance, and it offers a solid platform to develop the next generation, high-performance supercapacitor electrodes.

2. Experimental section

2.1. Materials

All reagents employed in the experiments were commercially sourced at analytical purity and directly used as received. The precursor materials were: cerium nitrate hexahydrate Ce(NO3)3·6H2O, 99%, sodium hydroxide (NaOH, 98%), cadmium acetate (Cd(CH3COO)2, 99%), sodium sulfide hydrate (Na2S·H2O, 98%). The deionized (DI) water was used as the main solvent, with absolute ethanol (C2H5OH, 99.9%) serving as the washing reagent and a co-solvent of the synthesis.

2.2. Synthesis of CeO2

The amount of 2.6 g Ce(NO3)3·6H2O remained liquefied in 10 mL of deionized water under magnetic continue magnetic stirring (100 rpm). Meanwhile, 8 g of NaOH was dissolved in separate 15 mL of deionized water to get a clear solution. Then under vigorously stirring, the translucent solution was slowly injected into Ce(NO3)3·6H2O solution for half hour to acquire the solution. The solution was sealed in a 50 mL Teflon-lined autoclave and heated at 180 °C for 12 h. The precipitate was gathered and washed with using centrifuge machine with deionized water and absolute ethanol for three times, respectively. CeO2 powder was prepared following 12 h drying at 80 °C trailed by oppressive.

2.3. Synthesis of CdS

The cadmium sulphide (CdS) nanoparticle was prepared through a simple method of precipitation. A 0.2 M Cd(CH3COO)2 was dissolved in the mixture of 99.8 mL ethanol and 100 mL deionized water. Self-reliantly, 0.2 M Na2S·H2O dissolved in deionized water and ethanol. The Cd(CH3COO)2 solution was stirred at 600 rpm and the Na2S·H2O solution was added dropwise. The resulting mixture was continuously stirred with the heat up to 70 °C temperature for 80 min. After which the product was filtered and washed with a lot of ethanol and deionized water a few times to remove contaminants. A 30 min drying of material in an oven at a temperature of 80 °C was used to clean the sample. CdS dried nanoparticles underwent an annealing process of 500 °C in 2 h to attain purification and crystallization.

2.4. Preparation of CdS–CeO2 composite

Preparation of CdS–CeO2 composite was done through a coordinated hydrothermal-assisted precipitation method. The CdS–CeO2 composite was prepared by dispersing the as-prepared CeO2 nanoparticles in a mixed solvent, deionized water and ethanol to obtain a homogeneous suspension. A 0.2 M solution of cadmium acetate was then put into CeO2 suspension and stirred a mixture to entail the due interaction between the Cd2+ ions and the CeO2 particles. A 0.2 M Sodium sulfide hydrate (Na2S·H2O) solution was prepared separately and drop-by-drop solutions added to the previously prepared solution of CeO2 under constant stirring. The reaction mixture was thoroughly mixed and allowed to heat at 70 °C over a period of 80 min to enable the in situ growth of CeO2 on the surface of CdS nanoparticles. After completion of the reaction, the resultant composite was filtered and washed with DI water and ethanol to remove the impurities. The product obtained was dried at 80 °C and then annealed at the temperature of 500 °C (2 h) to enhance the crystallinity and intensify the bond between CdS and CeO2. The resulting CdS–CeO2 structure was then refined to a fine powder to undertake further characterization and electrochemical tests (Scheme 1).

Scheme 1. Schematic representation of synthesis steps of CeO2, CdS, and CdS–CeO2 composite.

Scheme 1

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2.5. Preparation of electrodes

The supercapacitors electrodes were prepared using nickel foam of size 1 × 1 cm2 as the conductive substrate. At the first stage the nickel foam was thoroughly cleaned. For eliminating surface contaminants, the foam was subjected to sequential ultra-sonication in 3 M HCl, ethanol, and distilled water for 10 min each. The foam was dried at 60 °C for 1 h afterwards. The selected active material (CdS, CeO2, or CdS–CeO2) was dispersed in a NMP-based mixture that included 80 wt% of the active material and 10 wt% of AC and 10 wt% of PVDF for electrode fabrication. The mixture was homogenized and evenly spread on the pre-cleaned Ni foam with a spatula. These coated substrates were then placed in an evaporator at 80 °C for 12 h to remove the solvent. The electrodes were then compressed under 5 MPa to enhance the interfacial contact and mechanical adhesion. About 2 mg of active material was used in each electrode.

2.6. Fabrication of asymmetric device

For the fabrication of ASC device was accumulated using the prepared CdS–CeO2 on Ni foam substrate the positive side and activated carbon functioning as the negative side. The device was assembled using a porous cellulose membrane as a separator impregnated with 3 M KOH solution and the electrodes placed on both sides of this separator. The quantity of the active materials of the electrodes was fine tuned to achieve charge equivalence (q+ = q−), using the charge-storage equation (q = C × ΔV × m) as the basis. The electrodes were separated by a porous cellulose-based membrane soaked in 3 M KOH aqueous electrolyte. The mass balance between the positive and negative electrodes was adjusted according to the charge balance equation q+ = q−, where q = C × ΔV × m. This assured optimum performance and prohibited overcharging of every prepared electrode. The electrodes, along with the separator, were stacked and assembled in a coin-cell configuration for laboratory testing. Electrical contacts were secured, and the device was allowed to stabilize for 2 h at room temperature before electrochemical characterization. The assembled device was denoted as CdS–CeO2//AC ASC and was used to evaluate CV, GCD, and EIS in a two-electrode configuration.

2.7. Characterization

The crystal structures of the prepared CdS, CeO2 and CdS–CeO2 samples were studied by XRD (Cu Kα radiation). Specific surface area and pore distribution were used to evaluate their textural properties by using BET analysis. The surface morphology and particle dimensions were analyzed with SEM, and the elemental composition and spatial distribution information was achieved using EDX. CV, GCD, and EIS methods were employed and measurements were carried out in a three-electrode set-up for each of the electrodes, and in a two-electrode set-up for the constructed ASC. Electrochemical impedance spectroscopy (EIS) measurements were performed over the frequency range of 0.01 Hz to 100 kHz. The experiments were directed at room temperature in 3 M KOH electrolyte solution to assess the capacitance, rate performance, and charge-transport properties. The performance of electrodes and assembled asymmetric supercapacitor (CdS–CeO2//AC) device was evaluated using conventional equations for calculating capacitance, energy and power density.27–29 The specific capacitance (Cs) of electrodes configuration was calculated from galvanostatic charge–discharge measurements using:

2.7. 1

The device's energy density and power density were determined from the following equations:

2.7. 2
2.7. 3

where the factor 1/3.6 converts energy from J g−1 to Wh kg−1, and Δt is the discharge time (s). The coulombic efficiency η (%) was evaluated as:

2.7. 4

The equivalent series resistance (ESR) was estimated from the initial voltage drop Vdrop in the GCD curves:

2.7. 5

The current density was calculated by dividing the applied current by the mass of active material (J = I/m), where I is the applied current and m is the active material mass.

3. Results & discussion

3.1. Microstructure analysis

Fig. 1 presents the structural and morphological characteristics of the synthesized composite, with its XRD patterns shown in Fig. 1a. For pure CeO2, the main diffraction peaks appear at planes indexed as (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (3 3 1), and (4 2 0), which are characteristic of the cubic fluorite structure and are in good agreement with JCPDS card no. 34-0394. In the case of pure CdS, the diffraction peaks can be assigned to the (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (2 0 1), (2 0 3), (2 1 2), and (2 0 4) planes, matching well with the hexagonal CdS phase and JCPDS card no. 010-0454. For the CdS–CeO2 composite, the diffraction peaks of CeO2 and CdS are evident, indicating the successful synthesis of the composite. This suggests that the phases maintained their crystallinity in the composite. The lack of additional diffraction peaks further confirms the high phase purity of the synthesized material. This confirms the successful incorporation of CdS with CeO2, which is likely to enhance the potential of the electrode for electrochemical applications.

Fig. 1. (a) XRD pattern and (b) nitrogen adsorption–desorption isotherm of CdS–CeO2 composite demonstrating crystalline structure and mesoporous nature.

Fig. 1

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Fig. 1b on the right side represent the adsorption–desorption isotherm of nitrogen which demonstrates the porosity and surface area of the material synthesized. The isotherm is of the usual type-IV type having a clear hysteresis loop, which shows that the structure is mesoporous. This meso porosity is important in improving the electrochemical performance due to faster ion diffusion and greater electro active surface area. The progressive augmentation of the adsorption at elevated relative pressure (P/P0) is an additional indicator that a well-established pore channels are present in the composite structure. Both structural and surface analysis indicates that the CdS–CeO2 composite has a high crystallinity and porous structure. These characteristics prove to be extremely advantageous in the context of supercapacitor uses, because they facilitate effective charge exchange, better accessibility of electrolytes, and electrochemical activity. Consequently, the material is likely to have increased capacitance and rate capability when used in asymmetric supercapacitor devices.

The surface structure and elemental mapping of the samples CeO2, CdS and CdS–CeO2 composite-shown in Fig. 2, which are acquired from scanning electron microscopy (SEM) and elemental mapping. The SEM image of CeO2 (Fig. 2a) shows densely packed, cauliflower-like morphology composed of interconnected nano sized particles with rough and porous surfaces. Such a hierarchical architecture provides abundant electrochemically active sites and facilitates electrolyte penetration. In contrast, CdS (Fig. 2b) presents irregularly shaped polygonal and block-like particles with relatively smooth surfaces. These larger particles are closely stacked, resulting in a comparatively lower surface roughness and reduced accessible surface area than CeO2. After composite formation, the CdS–CeO2 sample Fig. 2c displays a homogeneous distribution of fine CeO2 nanoparticles anchored onto the CdS particles. The incorporation of CeO2 effectively suppresses excessive particle growth and produces a rougher and more porous surface. The intimate interfacial contact between CdS and CeO2 establishes a well-integrated structure, which is expected to enhance electron transport, provide more accessible electroactive sites, and promote efficient ion transport, during electrochemical processes. Fig. 2d shows the region from where we observed the elemental mapping of the composite. The elemental mapping images (Fig. 2e–h) also provide more insight on the composite. These demonstrate the presence and uniform distribution of Cd, S, Ce and O elements in the composite. The even distribution of elements indicates the successful formation of the CdS–CeO2 composite without phase segregation. This uniform dispersion leads to an optimal interaction between CdS and CeO2 that is required for enhancing the electrical conductivity and redox activity. XPS was used to explore the chemical state and elemental composition of CdS–CeO2 composite. The survey spectrum in Fig. S1(a) also confirms the presence of Cd, S, Ce, O and C. The high resolution Cd 3d spectrum in Fig. S1(b) shows two main peaks: one at 404.5 eV, assigned to Cd 3d5/2, and another at 411.2 eV (assigned to Cd 3d3/2), both of which are characteristic of Cd2+. The S2− state of CdS is evident in the S 2p spectrum of Fig. S1(c). In addition, the Ce 3d spectrum in Fig. S1(d) shows typical Ce3+ and Ce4+ features, thus demonstrating the presence of both of these oxidation states in the CeO2 phase. The Ce3+/Ce4+ redox states indicate that CeO2 is reversibly redox-active and contributes to pseudocapacitive charge storage. The O 1s signal of Fig. S1(e) also confirms the oxygen species that are associated with the surface of CeO2. The overall XPS results indicate the chemical states of both phases, and the proposed interaction at the CdS–CeO2 interface is confirmed.30–33

Fig. 2. (a–h) SEM images and elemental mapping of CeO2, CdS, and CdS–CeO2 composite showing morphology and uniform elemental distribution.

Fig. 2

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3.2. Electrochemical analysis

Fig. 3 presents the cyclic voltammetry (CV) behavior of CeO2, CdS, and the CdS–CeO2 composite electrodes at various scan rates, providing insight into their electrochemical charge storage mechanisms and performance.

Fig. 3. (a) CV curves of CdS–CeO2, (b) CdS, and (c) CeO2 electrodes at 10–70 mV s−1 scan rates and (d) comparative electrochemical performance of all electrodes at 40 mV s−1.

Fig. 3

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As shown in Fig. 3a, the composite electrode exhibits quasi-rectangular CV profiles with discernible redox characteristics over scan rates of 10–70 mV s−1, indicating a combined EDLC and faradaic pseudocapacitive charge-storage mechanism. With increasing scan rate, the charge/discharge currents increase at a linear rate and the CV curves retain their shape. This behavior suggests a good rate capability and efficient charge-transfer kinetics. The CV curves of the composite have a relatively larger enclosed area, indicating better charge storage capacity than that of the individual components. The CV curves of the CdS electrode are shown in Fig. 3b. The curves have a smaller enclosed area and weaker redox peaks than the composite. While CdS can contribute to charge storage via surface redox reactions, its contribution is hindered by lower activity and potential structural degradation during cycling. The distortion at high scan rates also suggests relatively slow ion diffusion. The CV curves of CeO2 are shown in Fig. 3c. The two pairs of redox peaks are related to the reversible redox pair, Ce3+/Ce4+. But the current response and the area under the curve are lower than those for the composite, suggesting low capacitance. This can be attributed to the low intrinsic conductivity of CeO2, thereby limiting electron transport. Besides, the slight shift of peaks at high scan rate indicates polarization and slow reaction rate. Fig. 3d presents the CV curves of CeO2, CdS and CdS– CeO2 at a scan rate of 40 mV s−1. Clearly, the CdS– CeO2 composite displays the largest integrated area and the highest current response of all the samples, demonstrating its excellent performance. This improvement is due to the complementary effect of CdS and CeO2, where CdS enhances electrical conductivity and charge transport, and CeO2 offers a large number of redox sites. This results in increased capacitance, improved reaction kinetics, and reversibility. The redox reactions that occur during the charge–discharge process in alkaline KOH electrolyte are as follows:

The pseudocapacitive behavior arises from the reversible redox transition between Ce3+ and Ce4+

CeO2 + e− + OH− ↔ CeOOH 6
Ce4+ + e− ↔ Ce3+ 7

CdS contributes through surface redox reactions involving Cd2+ species:

CdS + OH− ↔ CdSOH + e− 8

The overall enhanced performance is due to combined reactions:

Ce4+/Ce3+ + Cd2+/Cdx+ 9

Charge storage via ion adsorption at electrode/electrolyte interface.

Fig. 4 provides a detailed electrochemical study of the CdS–CeO2 composite and its pure components CeO2, and CdS, focusing on the charge storage mechanism via capacitive and diffusive processes. Fig. 4a displays the CV curve at 70 mV s−1 and the shaded area corresponds to the capacitive charge storage. The small redox peaks in the near rectangular CV curves indicate a hybrid charge storage mechanism of EDLC and faradaic pseudocapacitance. The large green shaded area indicates that a large fraction of the current arises from surface-capacitive processes that are generally related to the fast-charge storage and excellent rate performance. To separate the capacitive and diffusion-controlled contributions, the current at a given potential (i) can be written as:

i (V) = K1ν + K2ν1/2 10

where i (V) is the current, ν is the scan rate, K1ν is the capacitive (surface-controlled) component and K2ν1/2 is the diffusion-controlled (battery type) component. This equation helps to separate the total current into these two parts, thereby providing a better insight into charge storage mechanism. The contribution of capacitive and diffusion-controlled processes for the CdS–CeO2 composite is shown in Fig. 4b at various scan rates from 10–70 mV s−1. The capacitive contribution of the CdS–CeO2 electrode increases from 19% at 10 mV s−1 to 62% at 70 mV s−1, indicating an increasing contribution of surface-controlled charge-storage processes at higher scan rates. This suggests that with increasing scan rate, the contribution of ion diffusion becomes negligible, and the capacitive contribution becomes predominant. The increased capacitive contribution of the composite indicates better electrochemical kinetics, which is presumably because of the interaction between CdS and CeO2 in the composite, offering more active sites and fast electron/ion transfer. However, Fig. 4c and d shows this analysis for the reference CdS and CeO2 electrodes, respectively. The capacitive contribution rises with the scan rate but is relatively lower than the composite at entire scan rates. This proposes that the CdS electrode tends to have a higher contribution from diffusion-controlled processes, implying slower ion diffusion and lower surface activity. Similarly, CeO2Fig. 4d also exhibits a rising capacitive contribution with increasing scan rate; however, the capacitive contribution remains smaller than that of the CdS–CeO2 composite. This is due to the inherent lower conductivity and surface activity of CeO2. The results clearly show that the CdS–CeO2 composite has a greater capacitive contribution than the single materials, particularly at high scan rates. This demonstrates that the composite structure helps to enhance surface charge-controlled behavior, increases the accessibility of ions and facilitates the transport of electrons. Thus, the interaction between CdS and CeO2 is essential for attaining the excellent electrochemical properties, and the composite can be used as a potential high-rate energy storage device.

Fig. 4. Separation of capacitive and diffusion-controlled contributions: (a) CV curve with capacitive region at 70 mV s−1, and contribution ratios at various scan rates for (b) CdS–CeO2, (c) CdS, and (d) CeO2 electrodes.

Fig. 4

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Fig. 5 presents the galvanostatic charge–discharge (GCD), specific capacitance, and electrochemical impedance spectroscopy (EIS) of CeO2, CdS and CdS–CeO2 electrodes to understand the mechanism of charge storage, specific capacitance and kinetic behaviors of the electrodes. Fig. 5a presents the GCD results of the CeO2, CdS and CdS–CeO2 electrodes at 3 A g−1 current density. The longer discharge time of CdS–CeO2 compared to CdS and CeO2 electrodes indicates a greater specific capacitance of CdS–CeO2. The almost symmetric charge and discharge with slightly different charge and discharge curve suggests the presence of both electric double-layer and faradaic reactions (pseudocapacitive). The GCD curves of CeO2, CdS and CdS–CeO2 electrodes at different current densities (1–5 A g−1) are shown in Fig. 5b–d. As usual, the discharge time of all electrodes gradually decreases with increasing current density due to the slower ion diffusion process. However, the composite CdS–CeO2 electrode shows a much longer discharge time than the other electrodes, indicating enhanced rate capability. Also, the lower IR drop of the composite electrode suggests a lower resistance and higher electrical conductivity, possibly owing to the synergy between the CdS and CeO2. The specific capacitances at different current densities are shown in Fig. 5e. At 1 A g−1, CeO2, CdS, and CdS–CeO2 exhibit capacitances of 142, 284, and 476 F g−1, respectively. The composite displays a specific capacitance of 306 F g−1, which is much higher than CdS (138 F g−1) and CeO2 (112 F g−1) at a higher current density of 5 A g−1. The composite's capacitance retention (∼64% from 1–5 A g−1), suggests it has a better rate performance, while single CdS and CeO2 exhibit higher capacitance decay. The enhanced electrochemically active surface, quick ion diffusion and electron transport in the composite result in a high performance. Fig. 5f shows the Nyquist plots of the electrochemical impedance spectroscopy (EIS). The x-axis (Z′) intercept of the high-frequency part is the equivalent series resistance (Rs) and the diameter of the semicircle is the charge transfer resistance (Rct). The diameter of the semicircle is smaller for the CdS–CeO2 composite than that for CdS, implying that the charge transfer resistance is smaller and charge transfer is faster. Moreover, the noticeable low-frequency slope specifies enhanced ion diffusion and mainly capacitive charge-storage behavior. The larger resistive component compared to CeO2 and CdS suggests a slow charge transfer process. Based on these results, it can be concluded that the CdS–CeO2 composite possesses better capacitance, rate capability and charge transfer ability than the individual components. The combined effect of the CdS and CeO2 particles improve the electrical conductivity, surface area and ion/electron diffusion of the composite and thus make it an excellent supercapacitor electrode (Table 1).

Fig. 5. Electrochemical performance analysis: (a) GCD comparison at 3 A g−1, (b–d) GCD curves for CeO2, CdS, and CdS–CeO2, (e) specific capacitance comparison, and (f) Nyquist plots showing charge transfer characteristics.

Fig. 5

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Table 1. 3E specific capacitance calculated from GCD results of CeO2, CdS, and CdS–CeO2 composite.

Current density (A g−1) CeO2 (F g−1) CdS (F g−1) CdS–CeO2 (F g−1)
1 142 284 476
2 135 265 447
3 130 218 394
4 120 190 355
5 112 138 306
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4. Asymmetric supercapacitor (CdS–CeO2//AC)

The electrochemical properties of the CdS–CeO2//AC ASC device that was made are shown in Fig. 6. This shows its range of operating voltages, rate capability, energy storage properties, and stability. Fig. 6a shows CV results of the device at a constant scan rate of 50 mV s−1 in the various potential windows between 1.0 and 1.8 V. The CV area enclosed by the voltage window grows exponentially with the expansion of the voltage window, which represents a large charge storage capacity. Interestingly, the CV curves still have a quasi-rectangular shape with minor redox characteristics even at elevated voltages, which confirm the sum of electric double-layer capacitance of activated carbon (AC) and pseudocapacitance by the CdS CeO2 electrode. The fact that it can be operated at 1.8 V or more indicates that it has good electrochemical stability and can be used to effectively suppress the decomposition of electrolytes, which is vital to higher energy density. Fig. 6b indicates the CV results (10–70 mV s−1) of the CdS–CeO2//AC device within the optimized voltage range. As the scan rate is increased, the current response also increases but with a similar shape of the curve signifying great rate capability and rapid charge propagation. This small deviation at a high scan rate is explained by the influence of internal resistance and reduced ion diffusion; but the fact that the curve shape is retained in general proves efficient ion movement and high reversibility of the electrochemical reactions. Fig. 6c displays the GCD curves of various voltages windows at 4 A g−1. The potential window directly correlates with the higher the energy storage and the higher the discharge time. The almost symmetric triangular shapes suggest high coulombic efficiency and reversibility and the low voltage drop (IR drop) implies low internal resistance of the device. The GCD curves of different current densities (1–7 A g−1) are shown in Fig. 6d. As anticipated, the discharge time reduces with the density of current because there is limited ion diffusion at higher currents. Nevertheless, the device has a relatively long discharge time at higher current densities, indicating good rate performance. The high reversibility and stability of the device in the case of the rapid charge–discharge processes are further supported by the good symmetry of the charge–discharge curves. The calculated values of specific capacitance using the GCD curves are summarized in Fig. 6e. This equipment provides a high capacitance of 128.3 F g−1 at 1 A g−1, which reduces slowly to 65.3 F g−1 at 7 A g−1. The capacitance decrease at higher-current densities is because electrolyte ions do not have sufficient time to reach the active sites. But capacitance retention is still important, which means that the ion diffusion pathways work well and the electrode kinetics are good in the asymmetric setup. Finally, Fig. 6f shows the Nyquist plots prior to and after the stability test. The slope of the post-cycling curve is a little smaller and the resistance is a little higher than in the initial condition, indicating that there is a slight deterioration in the charge transfer kinetics. Nevertheless, the general appearance is still the same, which proves that the device is still in good electrochemical state following extended cycling. The equivalent series resistance is also low and the semicircle diameter is relatively small which also indicates efficient transport of electrons and good interaction between the electrode and the electrolyte. The CdS–CeO2//AC asymmetric device has a large operating voltage range, high specific capacitance, high rate capability, and electrochemical stability. These findings substantiate the fact that the synergistic integration of CdS–CeO2 and activated carbon is an effective way to improve energy and power properties, providing the device with an excellent prospect of finding application in the world of advanced energy storage options (Table 2).

Fig. 6. Electrochemical performance of CdS–CeO2//AC asymmetric supercapacitor: (a) CV at different voltage window at 50 mV s−1, (b) CV at 10–70 mV s−1, (c) GCD window adjustment at 4 A g−1, (d) GCD at 1–7 A g−1, (e) specific capacitance, and (f) Nyquist plots before and after stability test.

Fig. 6

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Table 2. Electrochemical results of ASC CdS–CeO2//AC.

Current density (A g−1) Specific capacitance (F g−1) Energy density (Wh kg−1) Power density (W kg−1) Coulombic efficiency (%)
1 128.3 57.74 899.7 41.00
2 118.8 53.46 1799 60.33
3 107.0 48.45 2700 73.12
4 98.00 44.10 3600 81.45
5 86.00 38.70 4500 85.90
6 76.00 34.20 5400 86.37
7 65.30 29.30 6298 89.78
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Fig. 7 presents the practical electrochemical performance and long-term stability of the assembled CdS–CeO2//AC ASC. The Ragone plot in Fig. 7a shows the relationship between the energy density and the power density of the CdS–CeO2//AC ASC device. The assembled ASC achieves a peak energy density of ∼57.74 Wh kg−1 at a power density of ∼899.7 W kg−1. With the continuous enhancement of the power density, the energy density also steadily falls to ∼29.30 Wh kg−1 at a highest power density of 6298 W kg−1. This inverse relation is common to supercapacitor systems, since the higher the power that is requested the shorter the ion diffusion time and thus the less the electroactive materials are used. However, it exhibits a high energy density at high power density, with excellent rate capability and fast charge transport kinetics. The coulombic efficiency of the CdS–CeO2//AC device in long duration cycling is shown in Fig. 7b. Coulombic efficiency of the device is almost 95.33%, suggesting high charge–discharge reversibility and very little loss of energy on repeated electrochemical cycling. The coulombic efficiency of these cells remains stable, indicating that the electron transport is efficient and the faradaic reactions are reversible, thus the materials are reversible. The long term stability of their cycling is additionally presented in Fig. 7c. The device has shown excellent electrochemical durability with capacitance retention of ∼95.83% after 15 000 cycles. A slight decrease in capacitance is seen during cycling, which shows that the electrode material retains its electrochemical activity and integrity over extended cycling. The inset GCD profiles recorded as a function of cycling show nearly symmetric charge–discharge behaviour, very little distorted, further confirming excellent reversibility and stable capacitive performance (Table 3).

Fig. 7. Electrochemical performance of the assembled CdS–CeO2//AC ASC: (a) energy density vs. power density, (b) coulombic efficiency, and (c) cycling retention over 15 000 charge–discharge cycles.

Fig. 7

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Table 3. Comparative analysis of CeO2 with different composites used as electrodes materials.

Material/device Specific capacitance (F g−1) Potential window (V) Energy density (Wh kg−1) Power density (W kg−1) Retention @ cycles Ref.
CeO2/AC 162 1.0 4.86 3500 86% 20
CeO2@γ-Fe2O3//rGO 1169.3 1.0 36.8 700 89.7% @ 1000 34
CeO2/MWCNTs composite 486.5 1.2 85.7 5300 92.3% @ 10k 35
CeO2/SiO2/CMK-3 403 1.0 26.39 2500 91.98% @ 3000 36
CeO2/CNTs hybrid 818 1.0 NR — ∼95.3% @ 2000 37
CeO2/NiS 710 NR NR — 96.56% @ 3000 38
CeO2/PANI 242.7 1.0 ∼85.7 ∼566 — @1000 39
CdS//CeO2–TNT ∼1165 1.0 ∼14.58 1900 100% @ 2000 40
CeO2–Sm2O3 397.5 C g−1 — 32.85 4223.91 86.92% @ 3000 41
CdS–CeO2//AC 476 1.8 57.74 6298 95.83% @ 15  000 This study
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5. Conclusion

In summary, a CdS–CeO2 composite was successfully synthesized and employed as the positive electrode in an asymmetric supercapacitor, with activated carbon serving as the negative electrode. The composite is able to incorporate high redox activity and structural stability of CeO2 and high electrical conductivity and charge transport of CdS to form a hierarchical structure of the material with high electro active sites. The electrochemical measurement showed that the specific capacitance of the CdS–CeO2 electrode is 476 F g−1 at 1 A g−1 and has excellent rate capability and long-term durability. The electrode maintained 95.83% of its capacitance after 15 000 charge–discharge cycles. Moreover, the fabricated asymmetric supercapacitor of CdS–CeO2 showed an energy density of 57.74 Wh kg−1 and maximum power density of 6298 W kg−1, which revealed the positive effect of the incorporation of CdS on the energy-storage performance. This demonstrates the efficiency of oxide-sulfide composites engineering to enhance charge transfer and electrochemical kinetics making CdS–CeO2 composites as prospective electrodes in the next generation, high-performance ASCs.

Author contributions

Junaid Riaz: wrote the original manuscript draft, data creation, characterization, and electrochemical analysis. Samira Elaissi: software analysis, funding, characterization, and review and editing. Muhammad Arif & Amina Bibi: supervision, review and editing, electrochemical analysis, and formal analysis.

Conflicts of interest

The authors declare that they have no conflict of interest.

Supplementary Material

RA-OLF-D6RA07046A-s001

Acknowledgments

The authors extend their appreciation to Princess Nourah Bint Abdulrahman University Researchers Supporting Project Number (PNURSP2026R738), Princess Nourah Bint Abdulrahman University, Riyadh, Saudi Arabia.

Data availability

The data supporting this article have been included as part of the Supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra07046a.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-OLF-D6RA07046A-s001
RA-OLF-D6RA07046A-s001.pdf (245.5KB, pdf)

Data Availability Statement

The data supporting this article have been included as part of the Supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6ra07046a.

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