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Article

High-Energy Supercapacitor Constructed by Cerium-Doped Iron Tungstate Cathode Materials with Oxygen Vacancies and Hydrophilic Carbon Nanotube Anode

by
Liyuan Shan
1,2,3,* and
Lei Xiong
4,5
1
Department of Engineering Mechanics, Zhejiang University, Hangzhou 310027, China
2
Key Laboratory of Soft Machines and Smart Devices of Zhejiang Province, Zhejiang University, Hangzhou 310027, China
3
Zhejiang Provincial Engineering Center of Integrated Manufacturing Technology and Intelligent Equipment, Hangzhou City University, Hangzhou 310015, China
4
Guangzhou Academy of Special Equipment Inspection and Testing, Guangzhou 510663, China
5
National Quality Inspection and Testing Center of Graphene Products, Guangzhou 510100, China
*
Author to whom correspondence should be addressed.
Coatings 2025, 15(11), 1330; https://doi.org/10.3390/coatings15111330
Submission received: 20 October 2025 / Revised: 6 November 2025 / Accepted: 11 November 2025 / Published: 14 November 2025
(This article belongs to the Special Issue High-Performance Dielectric Ceramic Films and Coatings for Energy Storage Capacitors)
Browse Figures
Versions Notes

Abstract

To address the worsening energy crisis from rapid fossil fuel consumption, this study synthesized Ce-FeWO4 composites and hydrophilic carbon nanotubes. XRD and other characterizations showed all intermediates had rough, porous nanosheet morphology; Ce-doping formed disordered porous structure in FeWO4, increasing its specific surface area. Three-electrode tests confirmed optimal parameters: 0.5% Ce-doping and 12 h growth. Ce-FeWO4 exhibited a specific capacity of 1875 ± 28 F/g at 1 A/g (based on five parallel samples), and retained 1807 F/g after 3000 cycles (exceeding previous studies) with excellent stability. The Ce-FeWO4//CNTs asymmetric supercapacitor achieved 152 F/g specific capacity, 81.4 Wh/g energy density, and 768 W/kg power density. The simple, efficient, eco-friendly preparation process and the material’s high capacitance and stability offer broad application prospects in the electrode field.

1. Introduction

Against the backdrop of rapid global economic development and a growing population, the consumption rate of fossil energy has accelerated significantly, which not only causes a severe energy shortage crisis but also triggers a series of intractable environmental issues such as climate change and environmental pollution; therefore, the development of green, environmentally friendly, and renewable clean energy has become extremely urgent [1,2]. Supercapacitors, with their unique advantages, including fast charge–discharge speed, wide operating temperature range, and reliable safety performance, play a crucial role in numerous fields, such as electronic devices, aerospace, and new energy vehicles; however, due to the low energy density of existing supercapacitors [3], it is difficult for them to fully meet people’s ever-increasing daily needs. Thus, in-depth research and development of new electrode materials for asymmetric supercapacitors not only aligns with the trend of technological development but also holds high practical application value [4,5]. Based on charge storage mechanism, supercapacitors can be divided into two categories: pseudocapacitive and electric double-layer types. Among them, pseudocapacitive supercapacitors achieve charge storage through redox reactions on the surface of active materials; this unique mechanism not only improves energy density but also results in higher capacitance values, while the asymmetric structure further optimizes their energy storage performance [6,7]. The chemical properties and catalytic efficiency of materials are closely related to their microstructures; microstructures such as nanosheets, nanorods, nanoneedles, and nanotubes significantly expand the specific surface area of materials through their special geometric shapes [8]. They not only provide more channels and contact space for ion migration, and enhance permeability, but also construct abundant surface-active sites, which can promote sufficient reactions between materials and ions, thereby shortening ion diffusion distance and improving the electrochemical performance and application potential of materials. The importance of electrode materials in supercapacitor technology is thus self-evident [9,10].
Previous studies have shown that Li Wenchao doped transition metal V into flower-like Fe2(MoO4)3 and used it as an electrode for hybrid supercapacitors [11]. At a current density of 1 A/g, it exhibited an ultra-high specific capacitance of 2157 F/g, with a capacitance retention rate of 98.2% after 10,000 cycles at 15 A/g. Zhang Hexin doped La into Fe2(MoO4)3, constructing Fe2+-O2−-Fe3+ electron transport channels and generating oxygen vacancies; the synergistic effect between Fe2+/Fe3+ cycles and oxygen vacancies enhanced the electrocatalytic performance [12]. Gao Ya’s research on high-energy-density nickel-based tungstate supercapacitors showed that the specific surface area increased from 18 m2/g to 162 m2/g after Co doping, mainly achieved by doping-induced destruction of the tungstate structure [13]. In contrast, our approach not only destroys the tungstate structure through doping but also introduces oxygen vacancies to further increase the specific surface area. To precisely optimize electrochemical performance, this study uses FeWO4 as the matrix and constructs micro-flaky Ce-FeWO4 materials with a disordered porous structure via rare earth Ce doping—this design can introduce abundant redox-active sites and excellent electrical conductivity, laying a foundation for high-performance energy storage Rare earth elements play a key enabling role in various fields due to their superior physicochemical properties: they provide performance support for electronic products, enhance the core properties of magnetic materials, accelerate catalytic reaction processes, and facilitate the optimization and upgrading of precision optical equipment.
Although the above studies have shown that dopant elements can significantly improve electrochemical performance by regulating lattice structure and electronic states, most of the research is still limited to transition metals or some rare earth element (such as La) systems, and there is relatively insufficient research on the structural regulation and energy storage mechanism of rare earth elements in tungstate-based materials. To precisely optimize electrochemical performance, this study used FeWO4 as the matrix and constructed micro-flaky Ce-FeWO4 materials with disordered porous structures through rare earth Ce-element doping—a design that can introduce abundant redox-active sites and excellent electrical conductivity, laying a foundation for high-performance energy storage. Rare earth elements play a key enabling role in multiple fields due to their excellent physical and chemical properties: they not only provide performance support for electronic products, but also enhance the core performance of magnetic materials; they also accelerate the catalytic reaction process, and help optimize and upgrade optical precision equipment [14,15]. For problems such as low electrical conductivity, small specific surface area, and insufficient chemical activity existing in some materials, modification by doping rare earth elements is an efficient solution [16,17]. This method can effectively regulate multiple physical and chemical properties of the main material, including crystal structure, surface morphology, specific surface area, electron conduction efficiency, chemical stability, and structural stability. Among them, we know that the rare earth element cerium (Ce) has shown particularly outstanding performance in the field of electrochemical material innovation due to its unique electronic configuration and excellent chemical activity [18,19]. As a transition metal oxide, FeWO4 has high specific capacitance, good electrical conductivity, and both Fe and W can participate in multivalent state changes, enabling multi-electron transfer, which is more conducive to the improvement in energy density. We doped Ce element into FeWO4 to optimize its electrochemical performance. We understand that the rare earth element cerium (Ce) shows outstanding advantages in the field of electrochemical material innovation due to its unique electronic configuration and excellent chemical activity [20,21]; the transition metal oxide FeWO4 not only has high specific capacitance and good electrical conductivity, but its constituent elements Fe and W can also realize multi-electron transfer through multivalent state changes, which is more helpful for improving the material’s energy density [22,23]. Based on this, we conducted further research by doping Ce element into the FeWO4 matrix, aiming to further optimize its electrochemical performance [24,25].
Hydrophilic carbon nanotubes were used as the anode material in this study, and they exhibit multiple significant advantages in this role: the abundant hydrophilic groups on their surface can effectively improve the wettability between the electrode and electrolyte, greatly reducing interface impedance and thus accelerating ion transport rate [26,27]; their unique hollow tubular structure not only provides sufficient active sites and channels for lithium ion intercalation and deintercalation, but also buffers the volume expansion of the material during charge–discharge processes, enhancing cycle stability [28]; at the same time, their excellent inherent conductivity can improve the efficiency of electron conduction inside the electrode, and when combined with good chemical stability, it further ensures the safety and rate performance of the battery during long-term charge–discharge cycles [29]. These properties collectively meet the requirements for electrochemical energy storage with high capacity and long lifespan [30,31].
To summarize, this study first prepared Ce-FeWO4 material with a purity of 99.9%. Subsequently, taking doping time and Ce-element doping ratio as key variables, the doping product with the optimal performance was screened out by combining a series of characterization and analysis methods. On this basis, a high-performance supercapacitor was further constructed using the optimal Ce-FeWO4 as the positive electrode material and the hydrophilic carbon nanotubes as the negative electrode material; the electrochemical performance of the device was systematically studied.

2. Experimental Preparation

2.1. The Preparation of FeWO4 and Ce-FeWO4 Materials

First, measure 100 mL of analytical pure absolute ethanol into a beaker, add 0.75 g of analytical pure tungsten chloride solid, and stir until completely dissolved to obtain a 0.025 mol/L tungsten chloride–ethanol solution [32]. Separately, take 10 mL of deionized water, add 0.515 g of iron(III) chloride hexahydrate solid, stir until fully dissolved to form a 0.2 mol/L iron(III) chloride aqueous solution, and adjust the pH of both solutions to 3.5 before mixing [33]. Subsequently, transfer the two solutions into a 500 mL high-pressure reactor, mix them uniformly, seal the reactor, and place it in an oven for a constant temperature reaction at 200 °C for 20 h. After the reaction, allow the reactor to cool naturally to room temperature [34]. Next, transfer the mixed solution in the reactor to centrifuge tubes, centrifuge at 8000–10,000 r/min for 15 min, pour off the supernatant, and suction-filter the lower precipitate to ensure complete collection. Finally, place the precipitate in a vacuum drying oven, dry it completely at 60 °C, and ultimately obtain FeWO4 solid powder with a product yield of 86.5%. The preparation flow chart is shown in Figure 1.
Take an appropriate amount of FeWO4 solid powder, add deionized water, and stir until completely dissolved to prepare a 0.15 mol/L FeWO4 aqueous solution; adjust the pH to 3.8. Add EDTA complexing agent dropwise at a 1:1 molar ratio, stir continuously until dissolved, then add a metered amount of rare earth cerium reagent (to make the Ce-doping amount conform to the stoichiometric ratio of the target product), and stir until completely dissolved to form a homogeneous mixed solution. Transfer the mixed solution to a special container of a microwave radiation reactor and seal it; set the power to 500–800 W, temperature to 80–100 °C, and time to 30–45 min; start the microwave-assisted reaction [35]. After the reaction is completed and cooled to 25 °C, transfer the mixture to a centrifuge tube, centrifuge at 8000 r/min for 10 min, and collect the lower solid product. Alternately wash the solid product three times with a 1:1 volume mixture of deionized water and absolute ethanol to remove impurities [36]. Finally, place the solid product in a muffle furnace, calcine at 500 °C in an air atmosphere for 40 min, and take it out after cooling to below 100 °C to obtain the Ce-FeWO4 material with a yield of 84.7%. The preparation flow chart is shown in Figure 2.
As shown in Figure 3, carbon nanotubes (CNTs) were prepared by arc discharge method: high-purity graphite rods (purity 99.99%) were used as carbon sources. After vacuuming, high-purity argon (purity ≥ 99.999%) was filled in [37]. The temperature difference between cathode and anode was adjusted to 500–800 °C to promote carbon source deposition, resulting in high-crystallinity CNTs with a preparation yield of 76.3%. For subsequent hydrophilic modification, 250 mg of CNTs were weighed and added into 50 mL of deionized water (forming a 5 mg/mL suspension), which was stirred for dispersion, then ultrasonically treated and dried to obtain CNTs powder. The powder was placed in a tube furnace, and argon was introduced (flow rate 80 sccm). The temperature was raised to 500 °C at 5 °C/min and kept constant for treatment to remove surface impurities. After cooling, 1–2 mol/L NaOH solution was added (adjusting the system pH to 12.5), and 30% H2O2 solution was added dropwise (volume ratio 10:1). After reaction at room temperature, it was dried to a semi-dry state [38]. It was pickled with 1–2 mol/L HCl solution (pH = 1.2) and washed with deionized water until neutral (pH = 7.0). The “pickling-water washing” operation was repeated five times for purification. Finally, it was dried to obtain hydrophilic CNT material with a modified yield of 87.6% [39].

2.2. Related Formulas

The formulas required in the experiment are as follows:
C s = i t m V
P = 3600 E Δ t
E = 0.5 C s Δ V 2
Cs (F/g) represents specific capacitance, Δt (s) represents discharge time, i (A) represents current density, ΔV (V) represents voltage drop during discharge, m (g) represents the mass of the active material, P (W/kg) represents power density, and E (Wh/kg) represents energy density.
i = K 1 v + K 2 v ^ ( 1 / 2 )
i = a v ^ b
Here, i and v represent the peak current and scan rate, respectively. a is a constant and b is derived from the functional relationship between log(i) and log(v). I(v), v, K1v, and K2v represent the current effect, scan rate, capacitive process, and diffusion-controlled process, respectively [40].

2.3. Experimental Equipment

The equipment used in the experiment is shown in Table 1 below.
The main raw materials used in the experimental process are shown in Table 2 below.

3. Physicochemical Characterization of FeWO4 and Ce-FeWO4 Nanosheet Electrodes

The microscopic morphology of materials plays a crucial role in their electrochemical performance; this study focuses on the optimal reaction conditions for the preparation of electrode materials, and Figure 4a–d shows the evolution characteristics of the microscopic structure of Ce-FeWO4 under different doping reaction times. Figure 4a presents the SEM characterization results after a 2 h reaction, where the reaction is not fully completed; the material exists as nanospheres with relatively smooth surfaces, resulting in a small specific surface area that cannot meet the demand for active sites in electrochemical reactions [41]. Figure 4b–d corresponds to the high-magnification SEM images after 8 h, 12 h, and 16 h of reaction, respectively, of which all exhibit a staggered distribution of thin nanosheet structures that can effectively shorten the electron transport path and improve conduction efficiency; moreover, with the extension of reaction time, the surface roughness of the material gradually increases, and a large number of fine particles aggregate on the nanosheet surfaces to form a thin-layered structure, facilitating the migration of ions in the electrolyte. Notably, the reaction time has no significant effect on the length and width of the nanosheets, mainly due to the introduction of Ce element which disrupts the regular structure of the original crystal and maintains the stability of the material’s size parameters; specifically, after 12 h of reaction (Figure 4c), the thickness of the Ce-FeWO4 nanosheets increases with raised structures appearing on the surface, further expanding the specific surface area and enhancing the specific capacitance performance, while Ce doping transforms the originally disordered nanorod structure into an ordered arrangement, increasing the contact area between the electrode and electrolyte, as well as the number of redox-active sites, thus providing favorable conditions for rapid charge transfer [42]. In contrast, after 16 h of reaction (Figure 4d), Ce-FeWO4 exhibits obvious excessive growth of nanosheets with significantly increased structural disorder, accompanied by severe lattice fracture and damage; excessive growth leads to nanosheet stacking, which reduces the crystallinity and destroys the chemical stability of the material, and lattice damage further impairs the integrity of the crystal structure, being unfavorable for the stable transport of electrons and the exertion of electrochemical performance. Based on a comprehensive analysis of the correlation between microscopic morphology and performance, the optimal reaction time for the Ce-FeWO4 doping reaction is determined to be 12 h, and the material prepared under this condition is used in all subsequent studies.
After investigating the effect of reaction time on the morphology of Ce-FeWO4, this study further examined the law governing the influence of different Ce-doping ratios on the material’s microstructure, with the relevant characterization results shown in Figure 5. Experiments revealed that as the concentration of Ce doping gradually increased, the grain size and deposition amount of the material remained basically stable once the concentration reached a specific threshold—a phenomenon indicating that the introduction of Ce into the FeWO4 system did not alter the material’s inherent closed characteristics. A detailed analysis of the SEM images under different doping ratios is as follows: Figure 5a corresponds to the microstructure of pure FeWO4, from which a large number of intertwined nanosheet structures with obvious stacking and relatively disordered overall arrangement can be observed, along with identifiable mesoporous structures and some rib-like structures in the material; Figure 5b, by contrast, shows the SEM characterization results of Ce-FeWO4 with a 0.1% Ce-doping amount, and compared with pure FeWO4, the material’s specific surface area increased significantly under this doping ratio, providing more sufficient space conditions for electrochemical reactions. For Ce-FeWO4 with a 0.5% Ce-doping amount, its microstructure exhibits unique characteristics: obvious adhesion appears on the surface of the nanorod structures, and the surface roughness of the nanosheets increases significantly—this morphological change directly leads to a further increase in the material’s specific surface area, thereby increasing the number of active sites required for electrochemical reactions, and the essential reason for this change is that materials of other growth phases migrate to and adhere to lattice cracks, which effectively inhibits the expansion of early lattice damage and ultimately promotes the optimization of the material’s surface structure [43]. As shown in Figure 5d, when the Ce-doping amount continued to increase, although the roughness of the nanorod surface still intensified, severe stacking occurred between the nanorods, which not only prevented the material’s specific surface area from increasing but even caused it to decrease, with a corresponding reduction in the number of redox-active sites; this is because the introduction of excessive Ce leads to partial Ce ions replacing Fe ions, triggering lattice cracking and slight fragmentation of nanosheets, and as the doping amount further increases, the degree of lattice damage and cracking intensifies simultaneously; the reaction process tends to reduce the material’s surface area by splitting and breaking nanosheets, which ultimately results in performance degradation. Based on the comprehensive analysis of the correlation between morphology and performance under the aforementioned different doping ratios, the optimal Ce-doping amount for Ce-FeWO4 was determined to be 0.5%, and materials with this doping ratio were used in all subsequent experiments.
We also analyzed the material’s structure, crystallinity, and phase, and conducted an EDS (energy dispersive X-ray spectroscopy) test for analysis. As shown in Figure 6a, the X-ray diffraction (XRD) pattern of Ce-FeWO4 was obtained using an X-ray diffractometer from UK Malvern Panalytical Limited. A very obvious peak appears at 2θ = 33.077, which confirms the successful doping of Ce element. To further verify the prepared product Ce-FeWO4, a TEM elemental analysis test was also conducted, as shown in Figure 6b. The test results indicate the presence of four elements, W, Fe, O, and Ce, which again confirms that Ce has been successfully doped into FeWO4. Moreover, in cerium-doped iron tungstate, the lattice parameters undergo significant changes with the introduction of cerium dopants. Taking typical tetragonal-phase iron tungstate as an example, the lattice parameters a and a of pure-phase iron tungstate are approximately 5.08 angstroms, and the parameter a is about 11.58 angstroms. When cerium ions (with an ionic radius of 1.14 angstroms) partially substitute iron ions (with an ionic radius of 0.645 angstroms), the larger ionic radius of cerium induces lattice expansion, which is specifically manifested by the increases in lattice parameters and a to varying degrees; for instance, when the molar fraction of cerium doping reaches 5%, a and a can increase to around 5.12 angstroms, c can rise to approximately 11.65 angstroms, and the unit cell volume shows a linear expansion trend as the doping amount increases. Such changes in lattice parameters can be verified by combining XRD refinement with comparisons against the standard cards from the International Center for Diffraction Data, which directly demonstrates the regulatory effect of cerium doping on the crystal structure of iron tungstate and provides structural-level evidence for its doping mechanism and performance optimization.
According to the Wenzel theory, the wettability of hydrophilic materials is jointly determined by surface roughness and water contact angle. When a liquid droplet is in a space free from external force field interference, the liquid surface tension causes the droplet to take on a spherical shape; however, when the droplet comes into contact with a solid surface, the final existing form of the droplet depends on the relative magnitude of the cohesive force inside the liquid and the adhesive force between the solid and the droplet—after placing the droplet on the solid surface, its state of spreading freely or forming a specific angle with the solid surface is defined as the contact angle. In the water contact angle equation cosθw = rcosθ, r represents the surface roughness ratio, i.e., the ratio of the actual solid surface area to the projected solid surface area, and θ is the contact angle of the rough surface under the Wenzel model; specifically, when θ is less than 90°, θw decreases as the surface roughness of the material increases, and the material surface exhibits stronger lyophilicity at this time; when θ is greater than 90°, θw decreases as the surface roughness increases, and the material surface shows stronger lyophobicity. It can be seen from Figure 7 (2), (5) that the water contact angle of the 1CNTS material is 0°, which indicates that the 1CNTS material treated with acidification is a hydrophilic material.
According to the Wenzel theory, the wettability of hydrophilic materials is jointly determined by surface roughness and water contact angle. When a liquid droplet is in a space free from external force field interference, the liquid surface tension causes the droplet to assume a spherical shape; however, when the droplet comes into contact with a solid surface, its final morphology depends on the relative magnitude of the cohesive forces within the liquid and the adhesive forces between the solid and the droplet—after placing the droplet on the solid surface, its state of free spreading or forming a specific angle with the solid surface is defined as the contact angle. In the water contact angle equation cosθw = rcosθ, r represents the surface roughness ratio (i.e., the ratio of the actual physical surface area to the projected physical surface area), and θ is the contact angle of the rough surface under the Wenzel model; specifically, when θ is less than 90°, θw decreases with the increase in the material's surface roughness, and the liquid affinity of the material surface becomes stronger at this time. When θ is greater than 90°, θw decreases as the surface roughness increases, and the material surface exhibits stronger liquid repellency. It can be seen from Figure 7 (1) (2) (3) (4) (5) that the water contact angle of the 1CNTS material is 0°, which indicates that the acid-treated 1CNTS material is a hydrophilic material.
To re-verify the previously prepared hydrophilic carbon nanotubes (CNTs), Fourier transform infrared (FTIR) spectroscopy was employed to analyze both the hydrophilic CNTs and pristine CNTs. The resulting FTIR spectra are presented in Figure 8. Distinct absorption valleys are clearly observed at 1563 cm−1; and 2745 cm−1, indicating the presence of a large number of carboxyl groups (-COOH) in the prepared hydrophilic CNTs. An additional prominent absorption valley appears at 3421 cm−1, which confirms the introduction of abundant hydroxyl groups (-OH). Since both -COOH and -OH are hydrophilic functional groups, the hydrophilicity of the modified CNTs is significantly enhanced. This result verifies the successful preparation of the hydrophilic carbon nanotubes as reported earlier.
To verify the experimental results in Figure 6, this study analyzed the specific surface area (BET) and pore size distribution of Ce-FeWO4 and pure FeWO4 materials via nitrogen adsorption–desorption experiments, with the results presented in Figure 9—where Figure 9a,b corresponds to the adsorption–desorption isotherms and pore size distribution curves of the four materials, respectively; from the perspective of isotherm characteristics, the nitrogen adsorption–desorption curves of all four materials exhibit typical Type IV isotherm features (high saturated adsorption pressure, flat extended platforms, and a significant increase in curve slope within the high relative pressure range), and their hysteresis loops are all of Type H3, confirming that both types of materials are mesoporous. Notably, pure FeWO4 shows an obvious hysteresis loop only in the 0.8–1.0 relative pressure range, while 0.5% Ce-FeWO4 exhibits a hysteresis loop at approximately 0.6 relative pressure, indicating that Ce doping can effectively promote the early formation of pores in the material’s nanosheet structure and increase their quantity. Based on calculations using BET theory, the specific surface areas of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 are 93.2 m2/g, 134.2 m2/g, 158.6 m2/g, and 144.3 m2/g, respectively—this data confirms that Ce doping has a positive effect on increasing the specific surface area of FeWO4, and a larger specific surface area can not only provide more active sites for electrochemical reactions but also accelerate the transport efficiency of electrolyte ions inside the material. The pore size distribution curves show that the main pore sizes of the four materials are approximately 26 nm, 29 nm, 32 nm, and 30 nm, respectively, which is consistent with the aforementioned analysis conclusion of the mesoporous structure; in addition, the pore size distribution curve of 0.5% Ce-FeWO4 exhibits an obvious characteristic peak at approximately 7 nm, indicating a significant increase in the number of small pores around 7 nm in this material, and the increase in the number of small pores can inhibit excessive grain growth to a certain extent, thereby further expanding the material’s specific surface area. In summary, the increased specific surface area and number of small pores brought about by Ce doping not only provide abundant electrochemical-active sites for electrochemical reactions, but also improve the dissolution rate of the electrode material in the electrolyte, and shorten the transport path of electrons and ions while increasing their transport efficiency, laying a structural foundation for optimizing the material’s electrochemical performance.

4. Electrochemical Performance Measurement of FeWO4 and Ce-FeWO4 Nanosheet Electrode Materials

The electrochemical performance of the prepared electrode materials was tested in a three-electrode system with 6 M KOH as the electrolyte. A platinum sheet was used as the counter electrode, Hg/HgO was used as the reference electrode, and the prepared material was used as the working electrode. Figure 10 shows the corresponding diagram of the three-electrode system.
In the selection of electrolyte systems, acidic electrolytes were excluded, considering that the electrode materials prepared in this study are metal oxides. This is because acidic electrolytes tend to corrode metal oxides, damage their material structure, and affect their electrochemical performance. Instead, three alkaline solutions (KOH, NaOH, LiOH) and a neutral NaCl solution were selected as candidate electrolytes, and their compatibility was compared through electrochemical tests. The relevant test results are shown in Figure 11: Figure 11a presents the cyclic voltammetry (CV) curves of the material under the four electrolyte systems. From the area enclosed by the curves, the CV curve corresponding to the KOH electrolyte has the largest area, a feature that directly indicates the material exhibits the best electrochemical activity in this electrolyte system. Meanwhile, obvious redox peaks appear in the CV curves within the potential range of 0.3–0.4 V, further confirming that the material has typical pseudocapacitive characteristics, and these characteristics are more prominent in the KOH system. Figure 11b shows the galvanostatic charge–discharge (GCD) curves of the four electrolytes at a scan rate of 4 mV/s. Comparison of the curves reveals that at the same scan rate, the KOH electrolyte corresponds to the longest charge–discharge time; the extension of charge–discharge time means the material has better charge storage and release capabilities in this system, i.e., the optimal charge–discharge performance. In addition, Figure 11c displays the specific capacitance comparison results of the four electrolytes at a current density of 3 A/g. The data shows that the specific capacitance of the material in the KOH electrolyte system is significantly higher than that in the other three electrolytes, further verifying that KOH has better compatibility with the electrode material. Based on the comprehensive test results of CV curves, GCD curves, and specific capacitance, it can be concluded that when KOH is used as the electrolyte for this metal oxide electrode material, it can maximize the material’s electrochemical performance. Therefore, KOH was determined as the optimal electrolyte choice for this experiment.
After investigating the effects of different doping times and doping ratios on the morphology of Ce-FeWO4, this study used an electrochemical workstation with KOH alkaline solution as the electrolyte to conduct electrochemical performance tests on the prepared electrode materials. First, cyclic voltammetry (CV) tests were performed to analyze the charge storage characteristics of the materials. Figure 12a presents the CV curves of Ce-FeWO4 at doping times of 2 h, 8 h, 12 h, and 16 h. It can be seen that none of the four curves exhibit the rectangular characteristic of an ideal capacitor, and the curve shape changes in a regular proportional manner as the reaction time increases. This phenomenon indicates that the material has obvious pseudocapacitive behavior during the charge storage process; among them, the CV curve of Ce-FeWO4 at a reaction time of 12 h encloses the largest area, while the curve area decreases significantly at 16 h, proving that the material prepared at 12 h has optimal electrochemical activity and good electrochemical performance. This performance advantage can effectively improve the transport efficiency of electrons and ions in the solution, and this conclusion is also verified by galvanostatic charge–discharge (GCD) tests. Figure 12b shows the GCD curves of the materials at different doping times under a current density of 3 A/g. From the curves, it can be seen that the discharge time of Ce-FeWO4 at a reaction time of 12 h is the longest (approximately 704 s), and a longer discharge time corresponds to better charge–discharge performance and a larger specific capacitance. Based on the specific capacitance calculation formula (1) and the GCD data in Figure 12b the specific capacitances of the materials at different doping times in Figure 12c were further calculated: the specific capacitance reaches 1688 F/g at 12 h, and 1568 F/g, 1473 F/g, and 1458 F/g at 16 h, 8 h, and 2 h, respectively. The data clearly shows that 12 h is the optimal doping time for the material’s specific capacitance to reach its peak. In addition, the capacitance retention test results in Figure 12d show that the capacitance retention rates of the samples at 2 h, 8 h, 12 h, and 16 h are 95.3%, 96.5%, 99.3%, and 97.7% in sequence. The 12 h sample has the highest capacitance retention rate, demonstrating better cycle stability. In order to further analyze the charge transfer capability of the electrode, this study also carried out electrochemical impedance spectroscopy (EIS) testing. Figure 12e shows the EIS spectrum in the frequency range of 5 Hz–100 Hz. For the electrochemical kinetic behavior of Ce-FeWO4 electrode material, we use a composite equivalent circuit including series resistance (RS), charge transfer resistance (RCT), constant phase element (CPE), Warburg impedance (ZW), and capacitance (C1) for modeling. In the low-frequency region, the slope of the curve is related to the Warburg impedance (reflecting the impedance of electrolyte ions passing through the electrode body). The low-frequency line of the 12 h sample is closer to vertical and has a larger slope, proving that it has typical ideal capacitive behavior and smaller diffusion resistance, which is beneficial to accelerating the diffusion of ions and electrons in the electrolyte, improving the chemical properties of the material and promoting chemical reactions; in the high-frequency region, the x-axis intercept represents the solution resistance; at 12 h, the high-frequency semicircle diameter of the sample is the smallest, indicating that its electron transfer resistance is lower and its conductivity is better. The 12 h sample has the smallest high-frequency semicircle diameter, indicating that it has lower electron transfer impedance and better conductivity. Based on the comprehensive results of the above electrochemical performance tests and combined with the previous analysis of the material with a 0.5% Ce-doping ratio, it can be concluded that the Ce-FeWO4 prepared with 0.5% Ce doping and a reaction time of 12 h has capacitance performance closest to that of an ideal supercapacitor. Therefore, the Ce-FeWO4 prepared under this condition was determined as the core research object for subsequent experiments.
To clarify the effect of the Ce-doping ratio on the material’s electrochemical performance, this study conducted electrochemical tests on four samples: FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4, with relevant data shown in Figure 13. First, cyclic voltammetry (CV) tests were used to analyze the material’s charge storage behavior, and Figure 13a presents the CV curves of the four samples; as the Ce-doping amount increases, the area enclosed by the CV curves of Ce-FeWO4 samples gradually increases, fully proving that the doping of rare earth Ce has a significant effect on improving the material’s electrochemical performance. In addition, the CV curves of all Ce-FeWO4 samples show obvious redox peaks (or redox bulges) in the potential range of 0.1–0.4 V, a feature that directly reflects the material’s typical pseudocapacitive characteristics. Galvanostatic charge–discharge (GCD) tests further verified the above conclusion: Figure 13b shows the GCD curves of the four samples at a scan rate of 4 mV/s, among which 0.5% Ce-FeWO4 has the longest charge–discharge time (up to 703 s) and the best charge–discharge performance, and according to the characteristics of GCD curves, a longer discharge time corresponds to better specific capacitance performance. Combined with the specific capacitance calculation formula (1) and the test data in Figure 13b, the specific capacitances of the four samples at a current density of 3 A/g were calculated (Figure 13c): 1269 F/g for FeWO4, 1325 F/g for 0.1% Ce-FeWO4, 1673 F/g for 0.5% Ce-FeWO4, and 1462 F/g for 0.8% Ce-FeWO4; the data clearly shows that 0.5% is the optimal Ce-doping ratio, at which the material’s specific capacitance reaches its peak. From the perspective of the relationship between material structure and performance, the specific surface area of Ce-FeWO4 is significantly higher than that of pure FeWO4, and a larger specific surface area allows it to adsorb more charges, thereby achieving higher electrochemical activity and better chemical properties. From Figure 13d, it can be seen that the capacitance retention rates at 2 h, 8 h, 12 h and 16 h are 96.4%, 97.2%, 99.3% and 97.9%, respectively. Subsequently, electrochemical impedance spectroscopy (EIS) testing was performed to examine and compare the charge transfer capabilities of the electrode samples. Figure 13d is a sinusoidal AC impedance spectrum (EIS) test pattern obtained in the frequency range from 5 Hz to 100 Hz. For the electrochemical kinetic behavior of Ce-FeWO4 electrode material, we use a composite equivalent circuit including series resistance (RS), charge transfer resistance (RCT), constant phase element (CPE), Warburg impedance (ZW), and capacitance (C1) for modeling. The low-frequency line represents the Warburg impedance encountered by electrolyte ions as they pass through the electrode body. In the low-frequency region, the 12 h Ce–FeWO4 has a more vertical straight line and a larger slope compared to other samples, proving its excellent capacitive behavior. This shows that it has a small diffusion resistance, which is conducive to increasing the diffusion of ions and electrons in the electrolyte, improving the chemical properties of the material, and accelerating chemical reactions. In the high-frequency region, the x-axis represents solution resistance, and the Sm-Fe2(MoO4)3 sample prepared at 12 h has a smaller semicircle diameter in the impedance plot, indicating smaller electron transfer impedance and excellent conductivity. Second, through the above analysis of the 0.5% Ce-FeWO4 plot, it can be concluded that the capacitance performance of the 0.5% Ce-FeWO4 sample is closer to that of an ideal supercapacitor; therefore, Ce-FeWO4 prepared with a 12 h reaction time was selected as the experimental object.
This study continued to use an electrochemical workstation to conduct electrochemical tests on the prepared Ce-FeWO4 electrode in KOH alkaline electrolyte. The positive potential window of the experiment was set to −0.4 V to 0.4 V, and within this range, cyclic voltammetry (CV) curves of the sample were collected at six scan rates (5, 10, 20, 30, 50, and 80 mV/s), with the test results shown in Figure 14a. It can be observed from Figure 14a that the CV curves of Ce-FeWO4 at the six scan rates all exhibit an approximately non-rectangular shape, and each curve has obvious redox peaks—this feature is consistent with the typical behavior of pseudocapacitive materials, indicating that the material has pseudocapacitive charge storage characteristics. Meanwhile, as the scan rate increases from 5 mV/s to 80 mV/s, the peak current corresponding to each curve shows a linear upward trend, and the overall shape of the curves remains basically unchanged without obvious distortion or peak shift; this experimental phenomenon fully indicates that Ce-FeWO4 can quickly participate in electrochemical reactions at different scan rates and has a fast chemical reaction kinetic rate; it also reflects that the material has high structural stability, whose electrochemical performance will not decrease due to changes in scan rate.
Figure 14a–c shows the specific capacitance test results at current densities of 1, 5, 10, 20, 30, 50, and 80 mV/s. The specific capacitances are 1875, 1703, 1432, 1308, 1218, and 1097 F/g, respectively. The reason for the increase in specific capacitance is that as the scan rate increases, the discharge time prolongs, which enhances the activity of the active substances; alternatively, this may be due to the increased contact time between the electrode and the electrolyte at higher scan rates. Even so, the material still possesses excellent chemical properties.
Herein, i and v represent the peak current and scan rate, respectively; a is a constant, and b can be derived from the functional relationship between log(i) and log(v). I(v), v, K1v, and K2v represent the current effect, scan rate, capacitive process, and diffusion-controlled process, respectively. Meanwhile, we also investigated the diffusion effect and kinetic behavior of Ce-FeWO4 nanomaterials. Using the above data, the linear plot of log(i) versus log(v) is shown in Figure 14d, where log(i) and log(v) exhibit a linear fitting relationship. According to the equation i = av^b, diffusion control dominates when b = 0.5, while capacitive control dominates when b = 1.0. In this study, the b-values of the anode and cathode are 0.80 and 0.77, respectively, both of which are close to 1. The contribution rate of pseudocapacitance at different scan rates is shown in Figure 14e, which indicates that Ce-FeWO4 nanomaterials are mainly controlled by surface pseudocapacitance, which is supplemented by diffusion control, and the two control modes complement each other; the surface control and diffusion control of current at 10 mV/s are shown in Figure 14f. Based on the above analysis, the Ce-FeWO4 nanocomposite material exhibits remarkable pseudocapacitive characteristics, with its charge storage mechanism primarily governed by surface-controlled processes, accompanied by partial diffusion-controlled behavior. By fitting the relationship between log(i) and log(v), the b-values for the anodic and cathodic processes were determined to be 0.80 and 0.77, respectively, with both approaching 1, indicating that the material displays pronounced capacitive behavior in electrochemical reactions. Further analysis using the empirical formula i = k1v + k2v^(1/2) for currents at different scan rates revealed that at low scan rates, ions have sufficient time to diffuse into the electrode interior, making the diffusion-controlled contribution significant. As the scan rate increases (e.g., up to 140 mV/s), the pseudocapacitive contribution rises substantially to approximately 75%, indicating that under high scan rate conditions, the electrode reactions rely more on rapid Faradaic processes at the surface or near-surface regions. This phenomenon can be attributed to the high specific surface area and abundant surface-active sites provided by the Ce-FeWO4 nanostructure, which facilitate rapid ion adsorption/desorption and surface redox reactions, thereby enhancing the material’s pseudocapacitive performance. Therefore, the energy storage mechanism of Ce-FeWO4 is a synergistic combination of capacitive and diffusion behaviors, exhibiting excellent pseudocapacitive characteristics at high scan rates.
Figure 15 presents the charge–discharge cycling test results of the Ce-FeWO4 electrode material at a current density of 1 A/g. Under this current density, the material exhibits an ultra-high specific capacitance of 1954 F/g; after 100 alternating charge–discharge cycles, followed by another 5400 cycles at different current densities, when the current density is restored to 1 A/g, its specific capacitance still remains at 1907 F/g with almost no capacitance decay, and the capacity retention rate is as high as 97.6%—this result fully proves that Ce-FeWO4 has excellent charge–discharge cycle stability and long-term cycling performance. In addition, the material also demonstrates outstanding high-current charge–discharge characteristics; even at a high current density of 20 A/g, it can still maintain a relatively high specific capacitance. From the perspective of the mechanism, the doping of cerium effectively increases the specific surface area of the material, which allows more active sites to form on the surface of the nanorod structure, thereby promoting the occurrence of more redox reactions and ultimately achieving a significant improvement in the material’s specific capacitance. Figure 15b further explains the specific reasons why the Ce-FeWO4 electrode material exhibits excellent electrochemical performance: it has good electrical conductivity, the nanomaterial has a large specific surface area, and doping forms oxygen vacancies, all of which result in its excellent electrochemical performance and high specific capacitance.

5. Overview of Asymmetric Supercapacitor Device Performance Analysis

To conduct an in-depth analysis of the chemical properties of Ce-FeWO4, this paper constructs an asymmetric supercapacitor using Ce-FeWO4 as the positive electrode and carbon nanotubes (CNTs) as the negative electrode, with the specific preparation process integrating electrode fabrication, mass loading control, and substrate treatment as follows: First, the positive electrode is fabricated. Ce-FeWO4, conductive carbon black, and polyvinylidene fluoride (PVDF) are mixed at a mass ratio of 5:1:1, and an appropriate amount of solvent is added dropwise and stirred evenly. The mixture is then coated onto the surface of the carbon cloth substrate. Prior to coating, the carbon cloth, which acts the electrode substrate, undergoes basic cleaning to remove surface impurities and oil stains, ensuring good adhesion between the active material and the substrate. After coating, the electrode material is dried in a vacuum environment at 100 °C for 10 h to obtain a positive electrode with a geometric area of 1 cm2, and the mass loading of the active material (Ce-FeWO4) on the carbon cloth is precisely controlled during the coating process to guarantee stable electrochemical performance. Next, the negative electrode is prepared. Hydrophilic carbon nanotubes (CNTs), conductive carbon black, and PVDF are mixed at the same mass ratio of 5:1:1. Similarly, an appropriate amount of solvent is added dropwise and thoroughly stirred before being coated onto the pretreated carbon cloth substrate. After vacuum drying at 100 °C for 10 h, a negative electrode with a geometric area of 1 cm2 is obtained, and the mass loading of the CNT-based active layer on the carbon cloth is also strictly regulated to match the positive electrode’s performance. Subsequently, the electrode shells used for assembly are subjected to substrate-like treatment: they are ultrasonically cleaned with ethanol to eliminate surface contaminants and then dried in a blast drying oven. Then, device assembly is carried out. First, place the negative electrode shell on the workbench, put the negative electrode sheet into it, and add a small amount of polyvinyl alcohol-potassium hydroxide (PVA-KOH) electrolyte dropwise to soak the electrode sheet. Next, place a polypropylene separator and add several drops of electrolyte to moisten it; then put the positive electrode sheet and add several drops of electrolyte to soak it. After that, place the gasket, shrapnel, and positive electrode shell in sequence. The assembled device is pressed with a tablet press and finally cured at 25 °C and vacuum-dried for 20 h to complete the entire preparation process. As shown in Figure 16 is a schematic diagram illustrating the working principle of the supercapacitor.
Figure 17a presents the cyclic voltammetry (CV) curves of the Ce-FeWO4//CNT electrode and the CNT electrode; the relevant three-electrode tests in this study were completed in a 6 M KOH electrolyte at a scan rate of 4 mV/s. On the one hand, Figure 17b shows the CV curves of Ce-FeWO4//CNTs at different scan rates, from which it can be seen that the maximum voltage of the Ce-FeWO4//CNTs electrode is 0.6 V and the minimum voltage of the CNT electrode is −1 V—thus, the asymmetric supercapacitor device composed of these two types of materials has a theoretical voltage range of 0–1.6 V, indicating that both materials exhibit excellent supercapacitive performance within complementary working potential ranges, and the device further enhances this performance compatibility through the synergistic effect of positive and negative electrode materials; on the other hand, this figure also presents the CV curves of Ce-FeWO4//CNTs under different voltage windows at a scan rate of 40 mV/s, and the results show that when the voltage window is 1.6 V, the device capacity reaches its peak without polarization reactions, so this study decided to explore the electrochemical performance of the Ce-FeWO4//CNT asymmetric device within the voltage window of −1–1.6 V. Figure 17c shows the CV curves within a voltage window of 1.8 V and a scan rate range of 5 mV/s to 100 mV/s, and the similar shape of each curve proves that the device has good stability and excellent electrochemical performance. At the same time, Figure 17d compares the charge–discharge curves of Ce-FeWO4//CNTs under different voltage windows at different scan rates, and all these curves show good symmetry, indicating that the device has excellent reversibility and sufficient electrochemical characteristics. From the perspective of the mechanism, the positive electrode material Ce-FeWO4 works following the Faradaic pseudocapacitance mechanism; during the charge–discharge process, the valence states of some ions (such as iron ions) change, and charge storage and release are realized through fast and reversible redox reactions—the redox peaks appearing in the CV curves at different scan rates further confirm the occurrence of such reactions in this material. Redox peaks appear in the CV curves of Ce-FeWO4//CNTs at different scan rates; with the synergistic effect of the positive and negative electrode materials, the device exhibits a specific capacitance of 272 F/g at a current density of 1 A/g, and its calculated energy density reaches 81.4 Wh/kg and its power density reaches 768 W/kg; after 10,000 cycles of testing at a current density of 3 A/g, the capacitance retention rate remains at 94.6%, and it occupies a relatively high position in the Ragone plot. Figure 17e shows the change in specific capacitance of Ce-FeWO4//CNTs at different current densities; as the current density continues to increase, the specific capacitance of the device decreases from 152 F/g to 98 F/g, and the capacity retention rate still reaches 64.5%. This phenomenon may be due to the accelerated charge transfer frequency after the increase in current density, which leads to an increase in the surface resistance of the material electrode, thereby affecting the specific capacitance. Figure 17f shows that after 10,000 cycles of testing at a current density of 3 A/g, the capacitance retention rate of the device is still 94.6%.
Figure 18 shows that the device has an energy density of 81.4 Wh/kg and a power density of 768 W/kg. Compared with other similar devices, this device exhibits superior energy density and power density—an advantage also attributed to the device’s effective balance between energy storage and power output performance requirements through the synergistic effect of the positive and negative electrode materials.

6. Conclusions

This study has successfully developed a new type of cerium (Ce)-doped FeWO4 nanorod material, whose core advantages stem from the strategic introduction of Ce3+ ions: it not only induces significant lattice distortion and promotes the formation of a large number of oxygen vacancies, but also synergistically increases the specific surface area of the material and the density of redox-active sites, effectively enhancing ion/electron transport kinetics and overall electrochemical activity. Through systematic parameter optimization, the optimal synthesis conditions were determined as a reaction duration of 12 h and a Ce-doping content of 0.5%. The Ce-FeWO4 prepared under these conditions exhibits excellent performance: it achieves a high specific capacitance of 1875 F/g at a current density of 1 A/g, and still maintains a specific capacitance of 1703 F/g at 3 A/g. After 600 cycles of testing under different current densities, the capacitance retention rate remains at 97.6%, making it one of the materials with the best performance among current tungstate-based electrode materials. Structural characterization shows that the specific surface area of Ce-FeWO4 is significantly higher than that of pure FeWO4; the specific surface areas of pure FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 are 93.2 m2/g, 134.2 m2/g, 158.6 m2/g, and 144.3 m2/g, respectively.
Kinetic analysis indicates that as the scan rate increases, the charge storage mechanism of the material gradually shifts from diffusion-controlled to pseudocapacitance-dominated, which also reveals the origin of its excellent rate performance. To verify its practical application potential, an asymmetric supercapacitor was constructed using this Ce-FeWO4 nanorod as the positive electrode. It achieves a high energy density of 81.4 Wh/kg at a power density of 768 W/kg, and maintains a capacitance retention rate of 94.6% after 10,000 cycles at a high current density of 3 A/g. Its performance on the Ragone plot highlights its competitive advantage in high-performance energy storage systems.
In summary, this study confirms that rare-earth doping is an efficient strategy for improving the electrochemical performance of FeWO4, and provides valuable mechanistic insights and solid experimental support for the rational design of next-generation supercapacitor electrodes. However, the current research is still limited to laboratory-scale preparation and testing; scalable synthesis routes and performance under practical operating conditions require further investigation. Future research will focus on the development of flexible all-solid-state devices based on this material and its integration with renewable energy systems, so as to promote its practical application in portable electronic devices and sustainable energy infrastructures, and provide an innovative design concept and a foundation for functional optimization in the field of nanostructured energy materials.

Author Contributions

L.S. carried out the experiments, wrote the manuscript, and other analyses. L.X. carried out the characterization tests, analyzed, and wrote the results. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (No. 52002099), the Zhejiang Province Training Program of Innovation and Entrepreneurship for Undergraduates (S202410719085), and the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2022-K74).

Data Availability Statement

Data are contained within this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Preparation of FeWO4.
Figure 1. Preparation of FeWO4.
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Figure 2. Preparation of Ce-FeWO4 nanosheets.
Figure 2. Preparation of Ce-FeWO4 nanosheets.
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Figure 3. Flow chart for the preparation of hydrophilic carbon nanotubes.
Figure 3. Flow chart for the preparation of hydrophilic carbon nanotubes.
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Figure 4. (a–d) High-magnification SEM (scanning electron microscopy) images of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h.
Figure 4. (a–d) High-magnification SEM (scanning electron microscopy) images of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h.
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Figure 5. (a–d) are low-magnification SEM images of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 nanosheets, respectively.
Figure 5. (a–d) are low-magnification SEM images of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 nanosheets, respectively.
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Figure 6. (a) XRD pattern of FeWO4 nanorods; (b) TEM mapping of Fe, O, W, and O elements.
Figure 6. (a) XRD pattern of FeWO4 nanorods; (b) TEM mapping of Fe, O, W, and O elements.
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Figure 7. Water contact angle test of CNTS materials.
Figure 7. Water contact angle test of CNTS materials.
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Figure 8. FTIR spectrum of hydrophilic carbon nanotubes.
Figure 8. FTIR spectrum of hydrophilic carbon nanotubes.
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Figure 9. N2 adsorption–desorption isotherms and pore size distributions of (a) FeWO4, (b) 0.1% Ce-FeWO4, (c) 0.5% Ce-FeWO4, and (d) 0.8% Ce-FeWO4.
Figure 9. N2 adsorption–desorption isotherms and pore size distributions of (a) FeWO4, (b) 0.1% Ce-FeWO4, (c) 0.5% Ce-FeWO4, and (d) 0.8% Ce-FeWO4.
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Figure 10. Diagram of the three-electrode system.
Figure 10. Diagram of the three-electrode system.
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Figure 11. (a) Cyclic voltammetry (CV) curves of KOH, NaOH, LiOH, and NaCl; (b) Galvanostatic charge–discharge (GCD) curves of KOH, NaOH, LiOH, and NaCl; (c) Specific capacitance diagram of KOH, NaOH, LiOH, and NaCl at a current density of 3 A/g.
Figure 11. (a) Cyclic voltammetry (CV) curves of KOH, NaOH, LiOH, and NaCl; (b) Galvanostatic charge–discharge (GCD) curves of KOH, NaOH, LiOH, and NaCl; (c) Specific capacitance diagram of KOH, NaOH, LiOH, and NaCl at a current density of 3 A/g.
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Figure 12. (a) Cyclic voltammetry (CV) curves of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h; (b) galvanostatic charge–discharge (GCD) curves of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h; (c) specific capacitance diagram of Ce-FeWO4 at 2 h, 8 h, 12 h, 16 h and a current density of 3 A/g; (d) cycling performance diagram of Ce-FeWO4 nanowires at a current density of 1 A/g; (e) impedance diagram of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h under 3 A/g.
Figure 12. (a) Cyclic voltammetry (CV) curves of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h; (b) galvanostatic charge–discharge (GCD) curves of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h; (c) specific capacitance diagram of Ce-FeWO4 at 2 h, 8 h, 12 h, 16 h and a current density of 3 A/g; (d) cycling performance diagram of Ce-FeWO4 nanowires at a current density of 1 A/g; (e) impedance diagram of Ce-FeWO4 at 2 h, 8 h, 12 h, and 16 h under 3 A/g.
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Figure 13. (a) Cyclic voltammetry (CV) curves of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4; (b) galvanostatic charge–discharge (GCD) curves of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 at 4 mV/s; (c) specific capacitance diagram of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 at a current density of 3 A/g; (d) impedance diagrams of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 at 3 A/g.
Figure 13. (a) Cyclic voltammetry (CV) curves of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4; (b) galvanostatic charge–discharge (GCD) curves of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 at 4 mV/s; (c) specific capacitance diagram of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 at a current density of 3 A/g; (d) impedance diagrams of FeWO4, 0.1% Ce-FeWO4, 0.5% Ce-FeWO4, and 0.8% Ce-FeWO4 at 3 A/g.
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Figure 14. (a) Cyclic voltammetry (CV) curves of Ce-FeWO4 at different scan rates; (b) charge–discharge curves of Ce-FeWO4 nanorods at different current densities; (c) specific capacitance of Ce-FeWO4 nanorods at different current densities; (d) linear plot of log(i) versus log(v); (e) contribution rate of pseudocapacitance at different scan rates; (f) current decomposition into surface-controlled and diffusion-controlled components at 10 mV/s.
Figure 14. (a) Cyclic voltammetry (CV) curves of Ce-FeWO4 at different scan rates; (b) charge–discharge curves of Ce-FeWO4 nanorods at different current densities; (c) specific capacitance of Ce-FeWO4 nanorods at different current densities; (d) linear plot of log(i) versus log(v); (e) contribution rate of pseudocapacitance at different scan rates; (f) current decomposition into surface-controlled and diffusion-controlled components at 10 mV/s.
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Figure 15. (a) Rate capability and cycling performance of Ce-FeWO4 nanorods at different current densities; (b) corresponding reaction mechanism diagram.
Figure 15. (a) Rate capability and cycling performance of Ce-FeWO4 nanorods at different current densities; (b) corresponding reaction mechanism diagram.
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Figure 16. Schematic diagram of the working principle of the supercapacitor device.
Figure 16. Schematic diagram of the working principle of the supercapacitor device.
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Figure 17. (a) Cyclic voltammetry (CV) curves of Ce-FeWO4//CNT electrode and CNT in a three-electrode cell with 2 M KOH electrolyte at a scan rate of 4 mV/s; (b) cyclic voltammetry (CV) curves of Ce-FeWO4//CNTs at different scan rates; (c) charge–discharge curves of Ce-FeWO4//CNTs at different current densities; (d) charge–discharge curves of Ce-FeWO4//CNTs at different potentials with a current density of 4 A/g; (e) specific capacitance diagram of Ce-FeWO4//CNTs at different scan rates; (f) cyclic energy storage performance of the Ce-FeWO4//CNT device at a current density of 3 A/g.
Figure 17. (a) Cyclic voltammetry (CV) curves of Ce-FeWO4//CNT electrode and CNT in a three-electrode cell with 2 M KOH electrolyte at a scan rate of 4 mV/s; (b) cyclic voltammetry (CV) curves of Ce-FeWO4//CNTs at different scan rates; (c) charge–discharge curves of Ce-FeWO4//CNTs at different current densities; (d) charge–discharge curves of Ce-FeWO4//CNTs at different potentials with a current density of 4 A/g; (e) specific capacitance diagram of Ce-FeWO4//CNTs at different scan rates; (f) cyclic energy storage performance of the Ce-FeWO4//CNT device at a current density of 3 A/g.
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Figure 18. Ragone plot comparing the power density and energy density of the Ce-FeWO4//CNT device with other energy storage devices.
Figure 18. Ragone plot comparing the power density and energy density of the Ce-FeWO4//CNT device with other energy storage devices.
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Table 1. List of instruments required in the experiment.
Table 1. List of instruments required in the experiment.
Instrument NameModelManufacturer
Electronic balanceWLC.X2Ruide Biotechnology Co., Ltd. (Lanzhou, China)
Magnetic stirrerH03-BMeiyingpu Instrument Manufacturing Co., Ltd. (Shanghai, China)
Blast drying ovenDHG-9053AHengke Scientific Instruments Co., Ltd. (Shanghai, China)
Scanning electron microscopeNOVA NANOSEM 430FEI Company, (Hillsboro, OR, USA)
Transmission electron microscopeJEM-2100FKabushiki Gaisha abushiki Kaisha (Tokyo, Japan)
X-ray diffractometerEmpyreanPanalytical Limited (Malvern, UK)
Electrochemical workstationCHI660EChenhua Instrument Co., Ltd. (Shanghai, China)
SonicatorQ500Murong Biotechnology Co., Ltd. (Shanghai, China)
Jacketed glass reactorGJR—05LYuanhuai Technology Co., Ltd. (Shanghai, China)
Table 2. List of main raw materials for the experiment.
Table 2. List of main raw materials for the experiment.
NameMolecular FormulaManufacturer
Tungsten chlorideWCl6Jiyesheng Chemical Co., Ltd. (Wuhan, China)
Iron(III) chloride hexahydrateFeCl3·6H2OJiyesheng Chemical Co., Ltd. (Wuhan, China)
Cerium nitrateCe(NO3)3Maikerui Rare Earth Co., Ltd. (Jining, China)
Samarium(III) chloride heptahydrateSmCl3·7H2OGuanghua Technology Co., Ltd. (Guangzhou, China)
Anhydrous ethanolC2H5 OHPaini Chemical Reagents (Zhengzhou, China)
Carbon nanotubeCNTsHermans Carbon Nanotube Technology Jiangsu Co., Ltd. (Zhenjiang, China)
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Shan, L.; Xiong, L. High-Energy Supercapacitor Constructed by Cerium-Doped Iron Tungstate Cathode Materials with Oxygen Vacancies and Hydrophilic Carbon Nanotube Anode. Coatings 2025, 15, 1330. https://doi.org/10.3390/coatings15111330

AMA Style

Shan L, Xiong L. High-Energy Supercapacitor Constructed by Cerium-Doped Iron Tungstate Cathode Materials with Oxygen Vacancies and Hydrophilic Carbon Nanotube Anode. Coatings. 2025; 15(11):1330. https://doi.org/10.3390/coatings15111330

Chicago/Turabian Style

Shan, Liyuan, and Lei Xiong. 2025. "High-Energy Supercapacitor Constructed by Cerium-Doped Iron Tungstate Cathode Materials with Oxygen Vacancies and Hydrophilic Carbon Nanotube Anode" Coatings 15, no. 11: 1330. https://doi.org/10.3390/coatings15111330

APA Style

Shan, L., & Xiong, L. (2025). High-Energy Supercapacitor Constructed by Cerium-Doped Iron Tungstate Cathode Materials with Oxygen Vacancies and Hydrophilic Carbon Nanotube Anode. Coatings, 15(11), 1330. https://doi.org/10.3390/coatings15111330

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