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. 2026 Aug 13;16(40):47246–47257. doi: 10.1039/d6ra06581c

Synergistic enhancement of supercapacitor performance in Ce–Bi co-doped CdO: from structural engineering to electrochemical insights

Muhammad Umar Farooq a, Ikram Ahmad a,✉, Muzammal Aslam a,✉, A F A Kadir b, Sajjad Gull a, Ali Mujtaba b,c, Tahira Ramzan a, Ahmed Sheraz a
PMCID: PMC13470749  PMID: 42597938

Abstract

Along with the growing interest in sustainable energy storage solutions, research into developing advanced electrode materials with enhanced electrochemical properties has intensified. In the present work, Ce–Bi co-doped CdO nanostructures were successfully synthesized by an eco-friendly hydrothermal route. The novelty of this work is the synergistic Ce–Bi co-doping approach to simultaneously improve the structural stability and electrochemical performance of CdO electrodes. X-ray diffraction revealed the successful incorporation of Ce and Bi in the phase-pure cubic CdO with decreased crystallite size (16.3 to 13.2 nm) and increased dislocation density. The results of EDX confirmed the homogeneous incorporation of Ce and Bi, whereas TEM and HR-TEM showed uniformly distributed nanoparticles with high crystallinity and reduced lattice spacing which were in agreement with the XRD results. SEM showed the conversion of CdO from a densely packed to a porous interconnected structure that is desirable for electrolyte diffusion. The values of b obtained from cyclic voltammetry are 0.48 and 0.52, which suggests that the charge-storage mechanism is predominantly diffusion-controlled. The Ce–Bi co-doped CdO electrode had a maximum specific capacitance of 252.7 F g−1 at a current density of 0.8 A g−1, with a decrease in charge transfer resistance (Rct) from 0.41 to 0.26 Ω, and an 87.3% capacitance retention after 5000 cycles, which indicated the excellent long-term electrochemical stability for supercapacitor applications. These results show that the co-doping of Ce–Bi is an effective approach to enhance the microstructure, charge transfer properties and durability of CdO-based supercapacitor electrodes.

The Ce–Bi co-doped CdO electrode had a maximum specific capacitance of 252.7 F g−1 at a current density of 0.8 A g−1, with a decrease in charge transfer resistance (Rct) from 0.41 to 0.26 Ω.graphic file with name d6ra06581c-ga.webp

1. Introduction

The modern world is forced to cope with several challenging issues including energy storage and environmental degradation caused by the increased human population and industrialization.1 This alarming situation makes us depend upon renewable energy sources such as solar, geothermal, biomass, hydropower, wind, and so on.2 The ever-increasing energy demands in the world have forced material scientists to develop highly efficient, environmentally friendly and sustainable energy storage technologies.3 An electrochemical capacitor called a supercapacitor has emerged as a potentially useful energy storage technology due to its greater power density, fast charge and discharge, and longer cycle life, as compared to commercial batteries.4 To acquire specific properties for energy and environmental applications, there have so far been great efforts to construct efficient and functional nanomaterials.5,6

Electrodes made of metal oxides have been of interest in supercapacitors because of their high capacitance and multi-oxidation states.7,8 Doping and composite techniques have been used to improve the electrochemical properties of CdO based electrodes. Cu/Zn doped CdO demonstrated by M. Iniya Pratheepa et al. improved conductivity with decreased band gap (2.48–2.28 eV) but charge transfer kinetics remain unclear.9 Kenesi et al. achieved 647 F g−1 at 1 A g−1 using a PANI/CdO/GO nanocomposite, while Sajjad et al. reported 1195 F g−1 for an optimized CdO–25% rGO electrode. Similarly, Sandhu et al. demonstrated that 3% Ce-doped CdO delivered 1027 F g−1 at 1 A g−1 with 90% retention after 3000 cycles, highlighting the effectiveness of compositional modification in improving CdO electrochemical behavior.10–12 Tauseef Munawar et al. found CrCo co-doped CdO to have high performance (500 F g−1, 72 Wh kg−1) but through structural complexity limits scalability.13 K. Dhamodharan et al. have reported Nd-doped CdO of high capacitance (652 F g−1) with 94% retention but diffusion-limited behavior predominates.14 A. Robert Xavier et al. announced Sr-doped CdO with high capacitance (383 F g−1)and low resistance, but the long-term stability is an issue.15,16 CdO is a promising material due to its excellent electrical conductivity and stability; but with low capacity to store charge and sluggish ion diffusion, its performance is hampered. In this respect, Ce–Bi co-doped CdO is likely to achieve further conductivity and defect active sites, as well as better kinetics of charge transfer, and improved electrochemical performance than those reported in CdO-based systems in the past.17

Hydrothermal synthesis has been extensively used in the production of nanostructured materials because this method offers a means to achieve control of morphology, uniform particle size distribution and increased crystallinity.18 Crystina Simanjuntak et al. used hydrothermal technique to prepare nano-Si/graphite composites with high specific capacity (3219 mAh g−1) and high retention 98.36% because their bonding between layers is good and uniform growth.19 Equally, Muhammad Rafeeq et al. have reported In2O3/Fe2O3 nanocomposites with a high capacitance (1868 F g−1) and they emphasized that hydrothermal synthesis can increase the surface area and ion transport.20 Shama Sadaf et al. reported enhanced functional performance of MXene-based nanocomposites because of the formation of well-structured layered structures.21 Hongtao Yang et al. reported that incorporation of NiS in hydrothermal reaction with carbon frameworks produced enhanced capacitance (1720 F g−1) because of increased porosity and active sites.22,23 Moreover, Mohammad W. Marashdeh et al. verified that hydrothermal processes can be used to successfully dope and defect engineer with enhanced structural stability.24 Here, the hydrothermal synthesis of Ce : Bi doped CdO is predicted to yield uniform dopant incorporation, and improved surface defects that, in combination, will enhance electrical conductivity, and electrochemical performance in supercapacitor applications.17

In this Study, Ce–Bi co-doped CdO nanostructures were successfully prepared using an eco-friendly hydrothermal method for improved energy storage applications. Characterization studies confirmed lattice shrinkage and a smaller crystallite size, with morphological investigations revealing the formation of an interconnected porous structure, allowing for easy ion transport. Electrochemical performance showed a high specific capacitance (252.7 F g−1), low charge transfer resistance and enhanced ion diffusion compared to undoped CdO. The morphology and electrochemical performance of our synthesized material are superior, according to Table 1, which compares various studies on CdO with our current work. This work demonstrates the promise of Ce–Bi co-doped CdO as an eco-friendly, high-performance electrode material for future supercapacitor devices.

Table 1. Comparison of electrochemical performance of CdO-based supercapacitor electrodes.

Electrode material Method Specific capacitance Energy density Powerdensity Cycling stability Ref.
MoO3/CdO symmetric device Hydrothermal 671 F g−1 124 Wh kg−1 1067 W kg−1 88.9% after 5000 cycles 51
Cu-doped CdO symmetric device Hydrothermal 69.26 F g−1 4.56 Wh kg−1 410 W kg−1 70% after 4000 cycles 52
CdO//AC@CdO device Facile chemical route 320 F g−1 48.6 Wh kg−1 3499.2 W kg−1 99.2–99.5% after cycling 53
NiSe2@CdO//AC device Hydrothermal 255 F g−1 48 Wh kg−1 2000 W kg−1 96.7% after 5000 cycles 54
Ce–Bi doped CdO Hydrothermal 252.7 F g −1 — — 87.3% after 5000 cycles This work
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2. Experimental section

2.1. Material required

Cadmium chloride (CdCl2, ≥99%), cerium nitrate hexahydrate (Ce(NO3)3·6H2O, ≥99%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O, ≥98%), sodium hydroxide (NaOH, ≥98%), N-methyl-2-pyrrolidone (NMP), polyvinylidene fluoride (PVDF), and carbon black were all purchased from Sigma-Aldrich (St. Louis, MO, USA). Ethanol (≧99.9%) was purchased from Merck KGaA (Darmstadt, Germany) and deionized water was used during the synthesis and washing steps. All the chemicals are purchased from local supplier and were analytical grade and were used as received.

2.2. Synthesis of Ce–Bi doped CdO

Ce–Bi doped CdO nanoparticles were prepared through hydrothermal process as shown in Fig. 1. The aqueous solutions of cadmium chloride, cerium nitrate, and bismuth nitrate were made with a ratio of 97% Cd and 3% Ce–Bi dopants. Cadmium chloride stock solution (0.01 M), Cerium nitrate stock solution (0.01 M) and Bismuth nitrate stock solution (0.01 M) were made in deionized water. These solutions were mixed to obtain the desired final concentrations of 9.7 mM CdCl2, 0.15 mM Ce(NO3)3, and 0.15 mM Bi(NO3)3, which is a Cd : Ce : Bi molar ratio of 97 : 1.5 : 1.5. This mixed solution was continuously stirred and the pH value of it was adjusted by dropwise addition of NaOH. The Cd solution was stirred and then the dopant solutions were added, and the pH was regulated by adding drop by drop of NaOH. The mixture was placed in a Teflon lined autoclave and warmed at 180 °C, 6 hours. The precipitate was cooled, and then washed with deionized water and ethanol, dried at 80–100 °C, and subjected to 650 °C calcination to get CeBi doped CdO.25

Fig. 1. Stepwise hydrothermal synthesis of Ce–Bi doped CdO.

Fig. 1

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2.3. Electrode fabrication

The pure CdO and Ce–Bi doped CdO were used as the active material in preparing two working electrodes.26 The carbon black and PVDF binder were added to each material in the right proportion and dispersed in NMP to a homogeneous mixture. The active material, carbon black and polyvinylidene fluoride (PVDF) binder were thoroughly mixed in a weight ratio of 80 : 10 : 10 in N-methyl-2-pyrrolidone (NMP) to form a homogeneous paste as the working electrode slurry. The slurry was evenly coated on Ni foam and dried at 80 °C for 12 h followed by pressing prior to electrochemical measurement. The electrochemical measurements were then made using the prepared electrodes in KOH electrolyte (Fig. 2).27

Fig. 2. Electrode fabrication of Ce–Bi doped CdO.

Fig. 2

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2.4. Characterizations

The crystal structure of the synthesized samples was identified with the help of an X-ray diffractometer (Bruker D2 PHASER, Bruker AXS GmbH Germany) at Lahore University of Management Sciences, Pakistan. The surface morphology and elemental composition were analyzed by field-emission scanning electron microscope (FEI Nova NanoSEM 450, Thermo Fisher Scientific, USA) with an attached energy-dispersive X-ray spectroscopy (EDX) detector at LUMS, Pakistan. To investigate microstructure and lattice fringes of the sample, transmission electron microscopy (TEM) was performed. The Brunauer–Emmett–Teller (BET) analysis was used to determine the specific surface area and the pore characteristics. The electrochemical measurements such as cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), electrochemical impedance spectroscopy (EIS), and cycling stability tests were conducted with a three-electrode electrochemical workstation in a 3 M KOH electrolyte at The University of Lahore, Lahore, Pakistan.

3. Results and discussion

3.1. X-ray diffraction analysis

XRD is a powerful technique that can be used to study the crystalline phase, crystal structure, preferred orientation, lattice variation and microstructural parameters of synthesized materials. The XRD patterns of pristine CdO and Ce–Bi co-doped CdO are shown in Fig. 3(a). The diffraction peaks observed at approximately 2θ = 33°, 38°, 55°, 65°, and 69° are indexed to the (111), (200), (220), (311), and (222) planes of cubic CdO, respectively. These peaks can be matched with JCPDS card No.65-2908, which indicates the formation of cubic CdO phase with Fm–3m symmetry. No other diffraction peaks corresponding to CeO2, Bi2O3 or any other impurity phase were found indicating that Ce and Bi dopants were successfully incorporated in CdO matrix without any secondary phase formation.28 The enlarged XRD profile of the major (111) peak is shown in Fig. 3(b) to clearly evaluate the variation in the structure as a function of doping. The diffraction peak (111) of Ce–Bi co-doped sample is moved towards higher angle than CdO, suggesting the lattice contraction and lowering of interplanar spacing. This change is in line with the partial replacement of Cd2+ ions with smaller dopant ions, such as Ce4+ ions, which can reduce the distance between atoms and cause local lattice strains. Bragg's law was used to calculate the d-spacing:29

3.1. 1

Fig. 3. (a)The XRD patterns of pristine CdO, Ce–Bi doped CdO, (b) enlarged view of major (111) diffraction peak showing 2θ shifts due to doping, and (c) calculated values of crystallite size, DLD and d-spacing values.

Fig. 3

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The diffraction order, n, the X-ray wavelength, λ, the interplanar spacing, d and the Bragg angle, θ, relate in the equation It was calculated that the d spacing reduced from 2.71 Å (CdO) to 2.68 Å (Ce–Bi doped CdO) which indicated that the lattice of CdO was contracted after doping. The crystallite size was calculated using the Debye–Scherrer equation:30

3.1. 2

where D is the crystallite size, K is the shape factor, λ is the X-ray wavelength, β is the full width at half maximum in radians, and θ is the diffraction angle. The density of dislocation lines was computed using:31

3.1. 3

where δ is the dislocation line density and D is the crystallite size. The crystallite size of undoped CdO was 16.3 nm and that of Ce–Bi doped CdO was 13.2 nm as shown in Fig. 3(c) while the dislocation line density was 3.78 × 1015 lines m−2 for undoped CdO and it was increased to 5.76 × 1015 lines m−2 for Ce–Bi doped CdO. This suggests that Ce–Bi co-doping retards the crystal growth and creates more lattice defects. These structural defects can create more electrochemically active sites and charge transfer pathways that will benefit the performance of the supercapacitor.32

3.2. Scanning electron microscopy

SEM is a high-resolution surface characterization instrument that offers a detailed understanding of the morphology, size, and microstructural development of a particle, which is vital to electrochemical performance. Fig. 4 (a–c) shows pristine CdO has highly agglomerated and densely packed microstructures with particles forming large clusters owing to the high surface energy and uncontrollable crystal growth.33 At increased magnifications, the surface is compact with few pores and insufficient inter-particle connectivity, limiting accessibility of electrolytes, and decreasing the availability of electrochemically active sites. This high-density morphology results in slow ion diffusion and increased internal resistance, thus reducing its supercapacitive functionality.

Fig. 4. Morphology analysis (a–c) SEM images of CdO (d–f) SEM images of Ce–Bi co-doped CdO composite (g and h) particle size distribution histograms along with the Gaussian fitting.

Fig. 4

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By contrast, Fig. 4(d–f) shows that a strong morphological change is caused by CeBi doping. The Ce–Bi–CdO doped exhibits less agglomeration and a more homogeneous distribution of particles and a porous loosely packed architecture. As the magnification is magnified, the structure transforms itself into a fluffy, interconnected network of smaller nanostructured structures, which greatly enhances surface roughness and pore volume. This porosity structure is hierarchical, which improves the penetration of electrolytes and reduces ion diffusion distances. Besides, the addition of Ce and Bi brings about distortion of the lattice and defect sites that do not only inhibit grain growth but also enhance electrical conductivity and redox-active centers. Subsequently, Ce–Bi co-doped CdO offers a greater effective surface area, greater number of available active sites and a faster charge transfer rate than pristine CdO. The synergistic effects result in a much more efficient doped material in terms of supercapacitor usage, resulting in increased capacitance, rate capability, and better cycling stability.34 The histograms of the particle size distribution were calculated from the SEM images, as shown in Fig. 4(g and h). The average particle size of the pristine CdO is 75.45 nm, which reduces to 69.12 nm with Ce–Bi co-doping. The reduction demonstrates that the hydrothermal synthesized particles are effectively suppressed in the grain growth process during Ce and Bi co-doping, and leads to finer particles with more electrochemically accessible active sites, which is beneficial to improving diffusion of the electrolyte and charge-storage properties.

3.3. Energy dispersive X-ray spectroscopy and BET surface area analysis

EDX analysis is a technique used to identify elements within a sample. The EDX spectra of the pristine CdO and Ce–Bi co-doped CdO is shown in Fig. 5. The CdO sample is very clean, showing only Cd and O peaks, which means that the CdO is formed in very high purity without significant impurity elements. The characteristic peaks of Ce and Bi are seen and clearly distinguished after co-doping which shows that both elements are successfully incorporated in the CdO matrix. The elemental compositions of the co-doped sample were found to be 25.32 wt% O, 71.68 wt% Cd, 1.52 wt% Bi and 1.48 wt% Ce which were consistent with the designed composition. These results are a confirmation of successful synthesis of the Ce–Bi co-doped CdO material and thus validate the structural analyses that they got from XRD.35 As can be seen from the N2 adsorption–desorption isotherms displayed in Fig. 5(c), both pristine CdO and doped CdO are mesoporous in nature with a typical isotherm type-IV having hysteresis loop at high relative pressure (P/P0 > 0.8). Nitrogen uptake in the low- and intermediate-P/P0 range shows a gradual increase, which is consistent with the phenomenon of monolayer–multilayer adsorption, followed by a dramatic rise near P/P0 = 1.0 that is indicative of capillary condensation in the mesopores. The doped CdO has higher adsorption of nitrogen gas throughout the range of pressure when compared to pristine CdO, which is about 97 cm3 g−1 STP at P/P0 ≈ 1.0 while the pristine CdO is 71 cm3 g−1 STP at the same pressure.33 Thus, specific surface area of pure CdO is 36.8 m2 g−1 but it is 52.4 m2 g−1 for doped CdO and average pore radius decreases from 8.9 nm to 6.7 nm after doping indicating that finer mesopores are formed after doping. The increased surface area and optimized pore structure leads to the increase in the number of electrochemically active sites and ease the diffusion of electrolyte ions, which results in better electrochemical performance of the doped CdO electrode.36

Fig. 5. (a) EDX spectra and elemental composition of pristine CdO (b) Ce–Bi co-doped CdO, which shows correct incorporation of Ce and Bi (c) BET N2 adsorption–desorption isotherms of CdO and Ce–Bi doped CdO.

Fig. 5

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3.4. Transmission electron microscopy analysis

The morphology and microstructure of the pristine CdO and Ce–Bi doped CdO was further investigated by transmission electron microscopy (TEM) as displayed in Fig. 6. TEM images of pristine CdO (Fig. 6(a and b)) indicate relatively larger particle size of the agglomerated nanoparticles. However, the Ce–Bi doped CdO sample (Fig. 6(e and f)), shows the presence of fine and uniformly distributed nanoparticles with porous interconnected morphology. The reduced particle size and enhanced dispersion by co-doping also leads to an increased accessible surface area and the number of electrochemically active sites for improved ion diffusion and charge storage.37 Well-defined lattice fringes are observed in the corresponding HR-TEM images as shown in Fig. 6(c, d, g and h) which indicates high crystallinity of both the samples. After co-doping, the interplanar spacing is reduced from 2.704 Å of pristine CdO to 2.675 Å of the Ce–Bi doped CdO. This is in good agreement with the XRD and it indicates that the Ce and Bi have been successfully incorporated into the CdO crystal lattice without altering the crystalline structure.38 Particle size distribution histograms that were obtained from the TEM images also show the size reduction after co-doping. The average particle size of pristine CdO is 61.95 nm whereas after Ce–Bi co-doping, it is reduced to 47.35 nm, which indicates the formation of uniformly distributed nanostructures with lesser agglomeration. The reduction in particle size reduces the pathways for diffusion of ions across the electrode and also increases number of electrochemically active sites which are exposed to the diffusion medium, thus accounting for the improvement in the electrochemical performance of the co-doped electrode.

Fig. 6. TEM and HR-TEM images of (a–d) pristine CdO and (e–h) Ce–Bi doped CdO, revealing the morphology of nanoparticles and lattice fringes with decreased interplanar spacing after co-doping with Ce–Bi, (i and j) particle size distribution histograms along with the Gaussian fitting.

Fig. 6

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3.5. Galvanostatic charge–discharge

GCD is an electrochemical method where the electrode is charged and discharged with a constant current while measuring the potential vs. time. The charge–storage performance of the pristine CdO electrode and the Ce–Bi doped CdO electrode was checked by the galvanostatic charge–discharge (GCD) measurements carried out at various current density in 3 M KOH electrolyte. The GCD curves of pure CdO at different current densities (0.8–2 A g−1) shown in Fig. 7(a) are quasi-linear and slightly asymmetrical, with pronounced IR drop, revealing faradaic battery-like behavior with low conductivity. Shorter discharge times at higher currents are caused by decreased ion diffusion. Fig. 7(b) shows the specific capacitance values, ranging from 157.4 to 87.5 F g−1 with increasing current density, indicating fair rate capability.39

Fig. 7. (a) GCD curves in a 3 M KOH electrolyte at different scan rates (b) variation of capacitance w.r.t to current density.

Fig. 7

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In Contrast, the GCD curves of Ce–Bi doped CdO shown in Fig. 8(a) show an increase in discharge time, with more symmetrical shape and higher reversibility, reflecting lower resistance. The increased utilization of the potential window and the low IR drop suggest enhanced electrical conductivity and fast charge transfer in the presence of Ce–Bi. Despite the moderate energy density reached, the enhancement of the same over pristine CdO proves the effectiveness of Ce–Bi co-doping and serves as a good starting point for compositional and morphological engineering to further improve. This high rate performance of the doped electrode is a direct result of the fine microstructure and enhanced charge transfer kinetics provided by the presence of Ce4+ and Bi3+, providing high specific capacitance across the entire current density range and with competitive energy density at high power density – firmly establishing Ce/Bi co-doped CdO as an electrochemically superior pseudocapacitive electrode material in high performance supercapacitor applications.40,41 The morphology and electrochemical performance of our synthesized material are superior, according to Table 1, which compares various studies on CdO with our current work.

Fig. 8. (a) GCD curves in a 3 M KOH electrolyte at different scan rates (b) variation of capacitance w.r.t to current density. (c) Cycling stability and capacitance retention over 5000 GCD cycles at 0.8 A g−1.

Fig. 8

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In addition, the long-term electrochemical stability of the electrodes was also investigated by repeated GCD cycling for 5000 cycles at a current density of 0.8 A g−1 (Fig. 8(c)). The CdO electrode showed 79.2% capacitance retention after 5000 cycles while Ce–Bi co-doped CdO electrode showed a high capacitance retention of 87.3%. The overall improved cycling stability has been accredited to the synergistic contribution of the Ce and Bi co-doped material, which helps in maintaining the structural integrity of the electrode, promotes charge transport and minimizes the degradation of the electrode structure after cycling. The obtained results also show that the supercapacitor application of the Ce–Bi@CdO electrode is extremely durable in the long term.

3.6. Cyclic voltammetry

CV is an electrochemical method to characterize the charge storage mechanism, redox behavior, and kinetics of electrode materials by calculating current response as a function of applied potential at variable scan rates.42 The electrochemical properties of the pure CdO and the Ce–Bi doped CdO at different scan rates (5–50 mV s−1) are indicated in the CV curves of pure CdO and the Ce–Bi doped CdO in Fig. 9(a–d). The CV curves of pristine CdO (Fig. 9(a)) show well-defined anodic and cathodic redox peaks, which confirm that the charge-storage process is mainly dominated by the faradaic pseudocapacitive reactions. The CV enclosed area is relatively small and the current response is low, which means that less electrochemically active surface area and slower electrochemical reaction rate. In contrast, the CV area of Ce–Bi co-doped CdO electrode is larger with higher current response and the redox peaks are clearly defined (Fig. 9(b)), which can be explained as the improved electrochemical activity, reaction kinetics and charge-storage capability after Ce–Bi co-doping. Nevertheless, the enclosed area is relatively small indicating that there is minimal utilization of the electroactive surface and charge-storing capacity. Conversely, Fig. 9(b) indicates that Ce–Bi doped CdO has a much greater CV area and current response, which supports the enhanced electrochemical behavior following doping. This enhancement can largely be credited to higher electrical conductivity, active sites generated by defects, and better ion diffusion pathways by incorporation of Ce and Bi.43 Moreover, the CV curves do not change their overall shape as the scan rate increases, which implies good reversibility and structural stability of both electrodes. The redox increment observed at the higher scan rates is likely due to polarization, but the doped sample shows less distortion, which indicates improved charge transfer kinetics than pure CdO.26

Fig. 9. (a and b) CV curves of CdO and Ce–Bi co-doped CdO at different scan rates (c and d) linear fit of log peak current versus log scan rate of CdO and Ce–Bi co-doped CdO.

Fig. 9

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The kinetic analysis obtained in Fig. 10(c–f) gives a better understanding of the charge storage mechanism. Pristine CdO has a b-value of 0.48, which is indicative of the diffusion-controlled faradaic charge storage processes dominating the charge storage mechanism. The b-value increases slightly to 0.52 after Ce–Bi co-doping, which denotes a slight increase of the surface-controlled contribution, but the overall charge-storage mechanism is still mainly diffusion controlled. This slight increase indicates that co-doping promotes faster surface redox reactions, diffusion-controlled ion transport still playing a dominant part in the electrochemical behavior.44 Also, Fig. 10(a and b) show the doped CdO (5.4) has a higher fitting parameter (n) value than pure CdO (4.3), which means that it has better redox kinetics and higher electron transport pathways. The appearance of a linear relationship between the scan rate and the diffusion-controlled nature of the process is further proved by the diffusion-controlled dependence on the scan rate, that was seen in Fig. 10(c and d). Further, in Fig. 10(e), pure CdO has a major contribution in diffusion (72.7% at 5 mV s−1) and in Fig. 10(f) Ce–Bi doped CdO has a larger contribution in capacitance which increases with scan rate 35.9% to 63.9%. This is a clear indication that doping enhances quicker surface reactions and accessibility of electrolytes resulting to the enhancement of electrochemical kinetics and enhancement of overall functionality.45

Fig. 10. (a and b) Tafel slope of CdO and Ce–Bi co-doped CdO (c and d) reaction kinetics (e and f) capacitance vs. diffusion contribution.

Fig. 10

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3.6.1. Reaction kinetics in KOH electrolyte

The electrochemical reaction kinetics of Ce–Bi–CdO with KOH as the electrolyte involve reversible faradaic redox reactions with hydroxide ions (OH−). Oxidation of CdO occurs during the anodic sweep during which the CdO becomes oxidized by interacting with OH− ions to form oxyhydroxide species as follows:CdO + OH− ↔ CdOOH + e−

During the cathodic sweep, the reverse reduction occurs:CdOOH + H2O + e− ↔ Cd(OH)2 + OH−

The overall kinetics can be described using the scan-rate-dependent power law:i = avbwhere i is the peak current, b is the charge-storage mechanism (0.5 for diffusion-controlled and 1 for capacitive behavior). The b-value of Ce–Bi at CdO is a little higher, which suggests better contribution to the surface. Also, it is possible to isolate the current response into capacitive and diffusion-controlled responses according to the Dunn model:46(V) = k1v + k2v1/2where k1v is the surface-controlled capacitive process and k2v1/2 is the diffusion limited intercalation. The improved contribution to capacitance in Ce–Bi co-doped CdO is due to more active sites and better electronic conductivity caused by the doping of Ce and Bi. Furthermore, Ce ions are able to create oxygen vacancies and allow electrons to hop, whereas Bi enhances charge transport routes, which jointly speeds up redox reactions. As such, the Ce–Bi doped CdO electrode has a higher ion diffusion rate, lower charge-transfer resistance and electrochemical performance than pure CdO.

3.7. Electrochemical impedance spectroscopy

The kinetics of charge transport and interfacial performance of pure CdO and Ce/Bi co-doped CdO electrodes were assessed using electrochemical impedance spectroscopy (EIS). In order to visualise the high-frequency region of the Nyquist plot, an enlarged inset of it has been added in Fig. 11(a) where the semicircular part for the determination of the solution resistance (Rs) and charge-transfer resistance (Rct) is clearly visible. Both samples shown in the Nyquist plots (Fig. 11(a)), exhibit an almost vertical line in the low-frequency range, which is a sign of dominating capacitive behavior and good ion diffusion essential properties of supercapacitor electrodes.47 It is noteworthy that the slope of the Ce/Bi co-doped CdO is steeper towards the imaginary axis than that of pure CdO and thus indicates a more ideal capacitive response and low diffusion resistance, which is attributed to the doping.48 The parameters obtained Fig. 11(b) reveal that the solution resistance (Rs) is reduced from 1.56 Ω to 1.41 Ω, and the charge transfer resistance (Rct) is reduced significantly from 0.41 Ω to 0.26 Ω (≈37% reduction) upon Ce/Bi co-doping). Such an improvement suggests that there is improved electrical conductivity and higher interfacial charge transfer which is probably caused by more oxygen vacancies and charge carriers. Ohmic losses are reduced by the concomitant decrease in both Rs and Rct, and faster redox reactions ensue, resulting in increased energy efficiency, increased capacitance, and increased rate capability. All in all, EIS findings validate that Ce/Bi co-doped CdO has low resistance, fast charge transfer, and quick ion diffusion and is therefore very suitable in high-performance supercapacitor.49,50

Fig. 11. (a) Nyquist plot of CdO and Ce–Bi co-doped CdO (b) parameters drived from EIS including solution resistance (Rs), charge-transfer resistance (Rct).

Fig. 11

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The present Ce–Bi doped CdO electrode has been compared with reported CdO based supercapacitor systems in Table 1. The present electrode shows higher specific capacitance and improved cycling retention as compared to the CdO symmetric device with the Cu-doping. While CdO-based composite devices like CdO//AC@CdO, MoO3/CdO and NiSe2@CdO exhibit improved energy and power densities on the device level, these systems require extra composite/device engineering. The present work on the other hand is based on direct co-doping of CdO with Ce and Bi yielding a balanced improvement in capacitance, low charge transfer resistance and stable 5000 cycle durability. This comparison also demonstrates the effect of Ce–Bi co-doping as a promising structural engineering approach to enhance the performance of CdO-based electrodes.

4. Conclusion

In summary, Cerium and bismuth co-doped CdO nanostructures were successfully synthesized by hydrothermal method and confirmed as high performance pseudocapacitive electrodes for next-generation supercapacitors. X-ray diffraction reveals phase-pure substitutional doping with resultant lattice shrinkage (from 2.71 Å to 2.68 Å), crystallite size shrinkage (from 16.3 nm to 13.2 nm) and increased dislocation density, while scanning electron microscopy confirms significant morphological transformation from highly agglomerated pristine CdO to a highly porous, interconnected nanostructured morphology that facilitates electrolyte wetting and amplifies electroactive sites. The results of the cyclic voltammetry showed a mostly diffusion-controlled charge storage mechanism with the b value increasing from 0.48 to 0.52 following Ce–Bi co-doping, which suggests a slight improvement in surface-controlled charge storage, and galvanostatic charge–discharge testing confirmed a maximum specific capacitance of 252.7 F g−1 at 0.8 A g−1 (68% greater than pristine CdO). Electrochemical impedance spectroscopy also confirmed a decrease in charge transfer resistance from 0.41 Ω to 0.26 Ω and solution resistance from 1.56 Ω to 1.41 Ω. The energy density and overall electrochemical performance of the Ce–Bi co-doped CdO electrode has been greatly improved compared to the pristine CdO electrode, but further optimization of the synthesis parameters, dopant concentration and electrode architecture is expected to improve its electrochemical performance. The future research will be carried out in these aspects for high performance supercapacitor electrodes.

Conflicts of interest

The authors declare no known competing financial or personal relationships that could have been viewed as influencing the work reported in this paper.

Data availability

Data will be made available from the corresponding authors upon reasonable request.

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