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Frontiers in Batteries and Electrochemistry ·

Volume 5 - 2026 | https://doi.org/10.3389/fbael.2026.1883507

Mg-doped hydrated tungsten oxide nanostructures as pseudocapacitive electrodes for high-performance aqueous asymmetric supercapacitors

  • Harishchandra S. Nishad1
  • Rajesh R. Jaiswar1
  • Manav Shirodkar1
  • Bhimarao M. Patil2
  • Atul C. Chaskar3
  • Pravin S. Walke1*corresponding author
  1. 1National Centre for Nanoscience and Nanotechnology, University of Mumbai, Mumbai, India
  2. 2Department of Chemistry, Institute of Science, Dr. Homi Bhabha State University, Mumbai, India
  3. 3Department of Chemistry, Institute of Chemical Technology, Mumbai, India
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Abstract

The doping of metallic impurities into hydrated oxide structures is a suitable strategy to enhance the pseudocapacitive storage and improve the strength of electrode materials. In this work, we have prepared Mg-doped WO 3 ·H 2 O nanoplates via a facile one-step wet-chemical synthesis route. The doping of Mg 2+ ions effectively improved the electrical conductivity, electroactive sites, and ion diffusion kinetics of WO 3 ·H 2 O while maintaining its layered hydrated structure. The rational doping of Mg in hydrated tungsten oxide (WOM5, ∼3% Mg doping) electrodes exhibited superior specific capacitance (158 F g -1 at 1 A g -1) behavior with higher b-values (0.82), enhanced redox activity, and improved charge storage kinetics than other compositions. Furthermore, the fabricated aqueous asymmetric device demonstrated excellent long-term cycling stability with improved coulombic efficiency up to 5,000 cycles. These results highlight Mg doping as an effective strategy for enhancing the electrochemical performance and stability of WO 3 ·H 2 O-based supercapacitors.

Keywords

1 Introduction

A major challenge of the 21st century is the development of rapid and efficient power supply systems for portable electronic devices, the electric vehicle sector, and Industry 4.0 technologies (Xie et al., 2015; Muralee Gopi et al., 2025; Vigneshwaran and Dhayalini, 2025). To meet these growing energy demands, electrochemical supercapacitors have emerged as the most promising energy storage technologies owing to their ability to deliver high power density, fast charge-discharge capability, and improved energy density than conventional capacitors (Govindhan and Sukumar, 2025). Among various types of supercapacitors, pseudocapacitors exhibit superior energy storage performance due to the harmonization of electric double-layer capacitance (EDLC) and fast surface redox reactions. However, the development of high-performance pseudocapacitive electrode materials remains a significant challenge (Sumathy et al., 2025; Zhang et al., 2025). To date, various transition metal oxides (TMOs) and transition metal hydroxides/hydrated oxides (TMHOs) have been extensively investigated for pseudocapacitor applications, including MoO3 (Prakash et al., 2018), V2O5 (Wang et al., 2022), NiO (Shah et al., 2023), Co3O4 (Pal et al., 2016), Fe3O4 (Payami et al., 2024), WO3·H2O (Gayathri et al., 2018), Ni(OH)2 (Patil et al., 2025), and Co (OH)2 (Keshavarz et al., 2023). Of these materials, TMHOs have attracted considerable attention due to their unique layered structures, tunable morphology and crystal structure, abundant electroactive sites, and high specific capacitance, making them promising candidates for advanced energy storage applications (Gayathri et al., 2018; Keshavarz et al., 2023; Patil et al., 2025). Hydrated tungsten oxide (WO3·H2O) is a promising pseudocapacitive material due to its layered structure and structural water molecules, which enable fast H+ intercalation/deintercalation. It also offers multiple oxidation states, high theoretical capacitance, good chemical stability, enhanced ion diffusion, and improved cycling stability. Despite its excellent pseudocapacitive properties, WO3·H2O faces several challenges, including low electrical conductivity, structural instability during long-term cycling, and agglomeration of nanostructures. In addition, volume changes during ion intercalation/deintercalation lead to capacitance fading and poor cycling stability. Therefore, improving its conductivity, structural durability, and ion transport kinetics remains essential for practical supercapacitor applications. Several strategies could improve the performance of WO3·H2O, including doping with other elements and composites such as graphene and MXene (Koo et al., 2021).

In this context, doping is considered an effective strategy due to its simultaneous modification of the electronic structure, electrical conductivity, defect concentration, and electrochemical activity of WO3·H2O without significantly altering the original crystal framework. The doping of Mg2+ ions modifies the electronic structure of WO3·H2O, increases electrical conductivity, and generates additional defects/oxygen vacancies that provide more electroactive sites for charge storage. Furthermore, the smaller ionic radius and stable valence state of Mg2+ facilitate faster electrolyte ion transport and enhance pseudocapacitive behavior, leading to improved specific capacitance and rate capability (Wang et al., 2017; Selvam et al., 2023).

Several studies have demonstrated that alkaline-earth metal doping and structural engineering significantly enhance the electrochemical performance of WO3/WO3·H2O-based supercapacitors. For instance, Mg-doped WO3 prepared through a hydrothermal synthesis route exhibited enhanced conductivity, rapid ion diffusion kinetics, and improved charge storage behavior due to the generation of additional electroactive sites and oxygen vacancies (Yao et al., 2017). Similarly, Lokhande et al. reported Nb-doped WO3 with a high specific capacitance of 782 F g-1 and excellent electrochemical stability in a Ca2+ electrolyte, demonstrating the importance of lattice engineering in WO3 systems (Lokhande et al., 2021). Furthermore, hydrated WO3·0.33H2O microrods synthesized via the hydrothermal method showed a high specific capacitance of nearly 798 F g-1 owing to their layered hydrated structure, which facilitated rapid proton diffusion and reversible redox reactions (Islam et al., 2025). In another study, hydrothermally synthesized WO3 nanotube/flexible WO3 electrodes exhibited improved cycling stability with ∼94% capacitance retention after prolonged cycling because of enhanced electrolyte accessibility and shortened ion transport pathways (Koo et al., 2021). Moreover, Ce-doped WO3 nanowire arrays demonstrated a specific capacitance of 493 F g-1 at 4 A g-1 with 90.4% capacitance retention after 5,000 cycles, highlighting that dopant-induced oxygen vacancies and widened lattice spacing can significantly improve electrochemical kinetics and cycling durability (Li et al., 2024). These studies confirm that alkaline-earth metal doping-induced defect engineering and hydrated nanostructure design are effective strategies for improving conductivity, ion transport, specific capacitance, and long-term cycling stability of electrodes in supercapacitors.

Here, different concentrations of Mg-doped WO3·H2O were successfully synthesized via a facile one-step wet-chemical synthesis route. The prepared samples retained their orthorhombic crystal structure and nanoplate-like morphology even after Mg doping into the WO3·H2O matrix. Furthermore, electrochemical investigations revealed enhanced specific capacitance and rate capability after Mg doping, demonstrating the positive effect of Mg doping on charge storage behavior. To evaluate the practical applicability of the Mg-doped hydrated tungsten oxide material, an aqueous asymmetric supercapacitor device was fabricated and its electrochemical properties were systematically studied. The obtained results support that Mg-doped WO3·H2O is a promising pseudocapacitive electrode material for high-performance supercapacitor applications.

2 Experimental

Oxalic acid (C2H2O4·2H2O, AR grade), ethanol (C2H5OH), hydrochloric acid (HCl, 35%–38%, AR grade), Sulphuric acid (H2SO4, 98%), and Magnesium chloride (MgCl2) were purchased from SD Fine-Chem Ltd., India. Whatman-42 filter paper was supplied by Moly Chem, India, and activated carbon (AC) was obtained from Global Nanotech, India.

2.1 Preparation of WO3.H2O and Mg-doped WO3.H2O

The WO3·H2O and Mg-doped WO3·H2O was prepared by the one-step wet-chemical method. To start, 0.5 g of Na2WO4.2H2O and 0.5 g of C2H2O4.2H2O were added to 100 mL of deionized (DI) water and stirred a till homogeneous solution was formed. Then, hydrochloric acid was added dropwise till pH 2 and the obtained solution was stirred for 4 h at 90 °C in the oil bath setup. The resulting product was washed with DDW and ethanol till the pH was neutral; the gained precipitate was dried at 80 °C for 12 h in a hot-air oven. Similarly, Mg-doped WO3·H2O was prepared by introducing 1.74, 3.46, and 6.93 mol% of MgCl2 into 100 mL of DI water, followed by the same synthesis procedure as that of pristine WO3·H2O. The resulting samples were labeled as WOM0 (0 mg), WOM2.5 (2.5 mg), WOM5 (5 mg), and WOM10 (10 mg), respectively (See Figure 1).

Schematic sketch for the synthesis of WO<sub>3</sub>.nH<sub>2</sub>O and Mg-doped WO<sub>3</sub>. **(a)** Synthesis of WO<sub>3</sub> nH<sub>2</sub>O. **(b)** Synthesis of Mg doped WO<sub>3</sub>.
Figure 1. Schematic sketch for the synthesis of WO3.nH2O and Mg-doped WO3. (a) Synthesis of WO3 nH2O. (b) Synthesis of Mg doped WO3.

2.2 Materials characterization

The morphological study of WOM0, WOM2.5, WOM5, and WOM10 was carried out using a field emission scanning electron microscope (FESEM) (FIB, Auriga compact, Carl Zeiss) and Nova Nano Sem. Furthermore, the detailed morphology analysis was done with high-resolution transmission electron microscopy (HR-TEM) with TECNAI G2-20. The crystal structure property was studied with a Shimadzu XRD-6100 diffractometer (Cu-Kα, λ = 1.5404 Å). Elemental analysis was performed by X-ray photoelectron spectroscopy (XPS) with the Versaprobe II system from ULVAC-PHI (Chigasaki, Kanagawa, Japan).

2.3 Electrochemical measurements

The electrochemical characterization of WOM0, WOM2.5, WOM5, and WOM10 was done with the Autolab (PGSTAT302N) system with two and three-electrode setup. The three electrode measurements were performed by using Ag/AgCl, Platinum wire and glassy carbon as reference, counter and working electrode respectively. The Cyclic Voltammetry (CV), Galvanic Charge-Discharge (GCD), and electrochemical impedance spectroscopy (EIS) were performed in 1M H2SO4 electrolyte. However, the specific capacitance, energy density, power density calculation, electrode preparation for three electrodes, and device fabrication were described in the Supplementary Material (SI).

3 Results and discussions

3.1 Crystal structure analysis

The crystal structure property of WOM0, WOM2.5, WOM5, and WOM10 were analyzed by powder XRD respectively. Figure 2a shows the XRD pattern of WOM0 (brown curve), WOM2.5 (green curve), WOM5 (magenta curve), and WOM10 (purple curve) in the 2θ range of 10° to 80°. The XRD patterns of WOM0, WOM2.5, WOM5, and WOM10 peaks matches with JCPDS No.00-43-0679 and reveals the orthorhombic crystal structure of WO3.H2O respectively. Figure 2b presents the magnified 2θ region (14°–28°) extracted from Figure 2a. The brown curve corresponds to the XRD pattern of undoped WO3·H2O, exhibiting prominent diffraction peaks at 16.68° and 25.95°, which are indexed to the (020) and (111) crystallographic planes, respectively. Upon Mg doping into the WO3·H2O lattice, the diffraction peaks shift toward higher 2θ values. The (020) reflection moves to 16.94°, 16.96°, and 16.95°, while the (111) reflection appears at 26.08°, 26.01°, and 26.08° for WOM2.5, WOM5, and WOM10, respectively. This systematic shift to a higher 2θ indicates lattice contraction, confirming the successful doping of Mg into the WO3·H2O framework. Additionally, Figures 2c–f shows the Williamson–Hall W-H plot of WOM0, WOM2.5, WOM5, and WOM10, respectively. The W-H analysis reveals that the macrostrain values for Mg doping levels of WOM0, WOM2.5, WOM5, and WOM10 are −0.000285085, 0.00156, 0.000673925, and −0.000127935, respectively. The change in micro-strain suggests that Mg doping initially induces lattice distortion and tensile strain, reaching an optimum at WOM2.5-WOM5. Beyond this level (WOM10), the reduction in strain suggests possible lattice relaxation or the onset of structural instability; hence, WOM5 is an effective doping limit for maintaining a stable WO3·H2O lattice. However, WOM0, WOM2.5, WOM5, and WOM10 exhibit crystallinity values of 92%, 86%, 78%, and 87%, respectively. This specifies that the Mg doping initially reduces the degree of crystallinity due to lattice distortion and defect generation, reaching a minimum at WOM5. With a higher Mg content (WOM10), a slight recovery in crystallinity is observed, which may be attributed to partial structural reorganization or phase stabilization within the WO3·H2O matrix.

**(a)** XRD patterns of WOM0, WOM2.5, WOM5, and WOM10 samples; **(b)** enlarged view of the selected 2θ range from 14° to 28° **(a)**; **(c–f)** Williamson–Hall (W–H) plots of WOM0, WOM2.5, WOM5, and WOM10, respectively.
Figure 2. (a) XRD patterns of WOM0, WOM2.5, WOM5, and WOM10 samples; (b) enlarged view of the selected 2θ range from 14° to 28° (a); (c–f) Williamson–Hall (W–H) plots of WOM0, WOM2.5, WOM5, and WOM10, respectively.

3.2 Morphology investigations

Figures 3a–c shows the FESEM images of WOM0 at 1,000 nm, 500 nm, and 250 nm, respectively. Figure 3c reveals the nanoplate-like morphology of WO3·H2O (WOM0). Figures 3d–f exhibits the FESEM images of WOM5 at 1,000 nm, 500 nm, and 250 nm respectively, emphasizing nanoplates morphology even after doping of Mg. Figure 3g shows the TEM image of WOM5 at 100 nm, exhibiting the transparent nanoplates. Figure 3h shows the HR-TEM image of WOM5 at 5 nm and Figure 3i shows the HR-TEM image of the selected region (yellow dotted rectangle) of Figure 3h, which display the ‘d’ spacing value of 0.168 nm, which belongs to the (160) plane of WO3·H2O.

FESEM images of WOM0 **(a–c)** and WOM5 **(d–f)**, **(g)** TEM image of WOM5, **(h)** HR-TEM image of WOM5, **(i)** Enlarged HR-TEM image of Figure 1h.
Figure 3. FESEM images of WOM0 (a–c) and WOM5 (d–f), (g) TEM image of WOM5, (h) HR-TEM image of WOM5, (i) Enlarged HR-TEM image of Figure 1h.

3.3 Elemental analysis

Figure 4a presents the XPS survey spectrum of WOM5, confirming the presence of tungsten (W), oxygen (O), and magnesium (Mg) elements in the sample. Figure 4b shows the W 4f XPS spectrum peaks at binding energies of 35.58 eV, 37.71 eV, and 41.38 eV. The peaks at 35.58 eV and 37.71 eV are attributed to the W 4f7/2 and W 4f5/2 spin–orbit components of W6+, confirming the dominant oxidation state of tungsten. The additional feature at 41.38 eV corresponds to the W 5p3/2 level, which commonly overlaps in this energy region (Wang et al., 2021; Shao et al., 2024). Figure 4c shows deconvoluted peaks at 530.41 eV, 531.20 eV, and 532.85 eV. The peak at 530.41 eV is attributed to lattice oxygen (O2-) bonded to tungsten (W-O) in the WO3 framework. The peak at 531.20 eV corresponds to oxygen associated with defects or oxygen vacancies, as well as hydroxyl groups (-OH) (Govindaraj et al., 2021). The higher binding energy peak at 532.85 eV is assigned to adsorbed water molecules or surface-adsorbed oxygen species, indicating the hydrated nature of the WO3·H2O structure. Figure 4d shows the XPS spectrum of Mg 1s and peak at 1,304.10 eV, which is characteristic of Mg2+ species (Wang et al., 2017). This confirms the successful doping of magnesium into the WO3·H2O matrix, where Mg exists in its oxidized state, likely coordinated with oxygen in the lattice or at defect sites. The amount of Mg doping for WOM5 (∼3%) is displayed in Supplementary Table S1.

**(a)** Survey scan of XPS spectra of WOM5, **(b–d)** Deconvoluted XPS spectra of W 4f, O 1s, and Mg 1s respectively of WOM5.
Figure 4. (a) Survey scan of XPS spectra of WOM5, (b–d) Deconvoluted XPS spectra of W 4f, O 1s, and Mg 1s respectively of WOM5.

3.4 Electrochemical analysis

The electrochemical analysis of WOM0, WOM2.5, WOM5, and WOM10 was done by the electrochemical three electrode setup in 1 M H2SO4 electrolyte respectively. Figure 5a shows the combined CV plot of WOM0, WOM2.5, WOM5, and WOM10 at 10 mV s-1 respectively. WOM0 exhibits a lower peak current along with a smaller area under the CV curve, indicating comparatively weaker electrochemical performance. In contrast, WOM5 demonstrates a higher peak current and a larger enclosed CV area, suggesting enhanced charge storage capability (Bhojane, 2022; Xu et al., 2022; Govindhan, 2025). Additionally, the CV curves of WOM0, WOM2.5, WOM5, and WOM10 at different scan rates are presented in Supplementary Figures S1a–d. Among all the samples, WOM5 exhibits the highest peak current compared to the others, which can be attributed to its reduced crystallinity (∼78%). This lower crystallinity provides a greater number of active sites that originated from defects, thereby enhancing the electrochemical reaction.

**(a,b)** combined CV and GCD curves, **(c,d)** Specific capacitance at various current densities and scan rates for WOM0, WOM2.5, WOM5, and WOM10, respectively, **(e)** Nyquist plot, and **(f)** Enlarged view of the selected region (red circle) from **(e)**, **(g,h)** Trasatti method analysis, and **(i)** Square root of scan rate vs. anodic peak current for WOM0, WOM2.5, WOM5, and WOM10, respectively.
Figure 5. (a,b) combined CV and GCD curves, (c,d) Specific capacitance at various current densities and scan rates for WOM0, WOM2.5, WOM5, and WOM10, respectively, (e) Nyquist plot, and (f) Enlarged view of the selected region (red circle) from (e), (g,h) Trasatti method analysis, and (i) Square root of scan rate vs. anodic peak current for WOM0, WOM2.5, WOM5, and WOM10, respectively.

The combined GCD curves of WOM0, WOM2.5, WOM5, and WOM10 are presented in Figure 5b. WOM0 exhibits a relatively short discharge time; however, with increasing doping concentration, the GCD duration increases, reaching a maximum for WOM5. At higher doping (10 mg), the GCD time decreases again. Notably, WOM5 shows the longest charge–discharge duration, indicating its superior electrochemical performance compared to the other samples. Additionally, the GCD curves of WOM0, WOM2.5, WOM5, and WOM10 at various current densities are presented in Supplementary Figures S2a–d. At a lower current density of 1 A g-1, distinct discharge plateaus are observed in the GCD profiles, which can be ascribed to Faradaic redox interactions between the WO3·H2O framework and H+ ions. With increasing current density, the GCD curves progressively evolve into more symmetric and quasi-triangular shapes, indicating enhanced charge-transfer kinetics. However, the specific capacitance of WOM0, WOM2.5, WOM5, and WOM10 was calculated from the GCD and CV curves using Equations 1, 2 of SI, respectively. Figure 5c presents the specific capacitance values of the samples at different current densities. The WOM0 electrode delivered specific capacitances of 115, 86, 68, and 53 F g-1 at current densities of 1, 2, 3, and 5 A g-1, respectively. In comparison, WOM2.5 exhibited enhanced capacitance values of 148, 124, 101, and 87 F g-1 at the corresponding current densities. The WOM5 electrode showed the highest electrochemical performance, achieving specific capacitances of 153, 140, 121, and 101 F g-1 at 1, 2, 3, and 5 A g-1, respectively. Upon further increasing the Mg content, WOM10 exhibited capacitance values of 132, 116, 98, and 82 F g-1 at the respective current densities. These results indicate that an optimal Mg doping level (WOM5) significantly improves the charge-storage performance, whereas excessive Mg doping (WOM10) leads to a slight decrease in capacitance. However, Figure 5d shows the specific capacitance of WOM0, WOM2.5, WOM5 and WOM10 at different scan rates respectively. The specific capacitance values of WOM0 were calculated to be 84, 69, 55, 49, and 40 F g-1 at scan rates of 5, 10, 20, 30, and 50 mV s-1, respectively. Upon Mg doping, the capacitance increased significantly, with WOM2.5 exhibiting specific capacitances of 136, 114, 103, 102, and 76 F g-1 at the corresponding scan rates. Among all samples, WOM5 delivered the highest electrochemical performance, achieving specific capacitances of 158, 139, 136, 132, and 105 F g-1 at 5, 10, 20, 30, and 50 mV s-1, respectively. Further increasing the Mg content to WOM10 resulted in a slight decline in capacitance, yielding values of 145, 120, 96, 84, and 59 F g-1 at the respective scan rates.

3.5 Electrochemical kinetics analysis

Moreover, the charge storage kinetics is investigated by electrochemical impedance spectroscopy using a three-electrode setup in 1 M H2SO4 electrolyte. Figures 5e,f shows the Nyquist plot for WOM0, WOM2.5, WOM5, and WOM10 respectively and inset is equivalence circuit diagram which represents the series resistance (Rs), charge transfer resistance (Rct), and Warburg resistance (Zw). Supplementary Table S2 displays the Rs, Rct, and Zw value of WOM0, WOM2.5, WOM5, and WOM10 respectively. The lower Rs, Rct, and Zw value of WOM5 suggests good conductivity, excellent interface between electrode and electrolyte, and faster H+ ion diffusion in to the WO3.H2O matrix (WOM5) (Mei et al., 2018; Santa-Cruz et al., 2024).

Trasatti methods were utilized for the analysis of total charge (QT), inner charge (Qi), and outer charge (Qo) of all prepared materials, as per Equation 1. Moreover, Figures 5g,h show the plot of inverse square root scan rates vs. outer charge and square root scan rates vs. 1/total charge. The QT of WOM0, WOM2.5, WOM5, and WOM10 are 100.50 C. g−1, 121.3 C. g−1 5, 119.76 C. g−1, and 314.46 C. g−1 respectively. The Qo values are 13.42 C. g−1, 37.20 C. g−1, 58.59 C. g−1, and 18.78 C g−1 respectively. The Qi values are 87.08 C. g−1, 84.1 C. g−1, 61.17 C. g−1, and 295.68 C. g−1 respectively (Nishad et al., 2022).

Q T = Qo + Qi (1)

Furthermore, the diffusion coefficient (D) of all prepared materials was estimated using the below Equation 2 (Feng et al., 2024).

i p = 2.69 x 10 5 n 3 / 2 A D 1 / 2 C o ν 1 / 2 (2)

Here, the diffusion coefficient (D) is expressed in cm2 s−1, the electrolyte concentration (Co) in mol cm−3, the scan rate (ν) in V s−1, the number of electrons involved (n) is 1, and the peak current density is given in mA cm−2. Figure 5i displays the square root of scan rate vs. anodic peak current of WOM0, WOM2.5, WOM5, and WOM10 respectively. The ‘D’ value of WOM0 exhibits a relatively low diffusion coefficient of 4.95 × 10−4 cm2 s−1, indicating slower ion diffusion within the electrode. Upon Mg doping, a significant enhancement in diffusion is observed in WOM2.5 and WOM5, indicating improved ‘D’ values of 1.162 × 10−3 cm2 s−1 and 1.175 × 10−3 cm2 s−1 respectively. This improvement suggests that Mg doping leads to defects and lattice strain, thereby facilitating faster ion diffusion and improved electrochemical kinetics. Notably, the WOM5 exhibits the highest diffusion coefficient, indicating an optimal doping concentration for maximizing ion transport. However, a slight decrease in D to 9.53 × 10−4 cm2 s−1 is observed for WOM10, which is attributed to excessive dopant leading to structural distortion, blockage of active sites, or reduced crystallinity. Nevertheless, the capacitive and diffusion-controlled contributions of WOM0, WOM2.5, WOM5, and WOM10 were quantitatively evaluated using Equation 3. The relationship between current and scan rate is expressed as (Jaiswar et al., 2026a; 2026c; 2026b; Nishad et al., 2026)where k1ν represents the capacitive-controlled contribution, while k2ν corresponds to the diffusion-controlled contribution. This equation enables the separation of charge storage mechanisms into surface-controlled (capacitive) and diffusion-limited processes, providing deeper insight into the electrochemical kinetics of the electrode materials. Furthermore, Figures 6a–d shows the CV plot of capacitive contribution and diffusive contribution percentages of WOM0, WOM2.5, WOM5, and WOM10 respectively. Of all the samples, WOM5 shows the highest capacitive contribution with 54% capacitive, while WOM0, WOM2.5, and WOM10 capacitive values are limited to 43%, 46%, and 10% respectively. Furthermore, Figures 6e–h shows the capacitive and diffusion contribution percentage at various scan rates of WOM0, WOM2.5, WOM5, and WOM10 respectively. and it shows WOM5 has higher capacitive contribution. However, the electrochemical charge storage kinetics are also explored with the correlation of current with respect to scan rate of cyclic voltammetry as per Equation 4.where i is current, ‘a’ and ‘b’ are adjustable parameters, and ν represents scan rates. Furthermore, when the ‘b' value is less than 0.5, it suggests a diffusion-controlled reaction, and when ‘b' value is 1 it suggests a surface dominant mechanism of the electrochemical cell. Figures 6i–l presents the relationship between log(scan rate) and log(peak current) for WOM0, WOM2.5, WOM5, and WOM10. The calculated anodic b-values are 0.78, 0.78, 0.82, and 0.58, whereas the cathodic b-values are 0.59, 0.77, 0.84, and 0.61 for WOM0, WOM2.5, WOM5, and WOM10, respectively. Of all the samples, WOM5 exhibits the highest anodic and cathodic b-values, indicating a greater contribution from capacitive charge storage kinetics with faster electrochemical reaction dynamics. In contrast, the lower b-values observed for WOM10 suggest comparatively higher diffusion-controlled behavior. Therefore, the enhanced b-values of WOM5 confirm its superior capacitive-dominated electrochemical performance compared to the other samples.

i n = k 1 ν + k 2 ν 1 / 2 (3)

i = a ν b (4)

**(a–d)** CV curve of capacitive and diffusive contribution, **(e–h)** Contribution percentage at various scan rates, **(i–l)** plot of Log(scan rates) vs. Log (peak current) of WOM0, WOM2.5, WOM5, and WOM10 respectively.
Figure 6. (a–d) CV curve of capacitive and diffusive contribution, (e–h) Contribution percentage at various scan rates, (i–l) plot of Log(scan rates) vs. Log (peak current) of WOM0, WOM2.5, WOM5, and WOM10 respectively.

3.6 Aqueous asymmetric supercapacitor (AAS)

The Aqueous Asymmetric Supercapacitor (AAS) device was fabricated using activated carbon (AC) as a positive and WOM5 as a negative electrode material. The CV curve of AC and WOM5 at 10 mVs-1 was shown in Figure 7a. Figure 7b illustrates the CV curves recorded at different scan rates. At lower scan rates, well-defined redox peaks are observed, which can be attributed to the reversible surface redox reactions associated with the insertion/extraction of H+ ions within the hydrated WO3 matrix. (Wang et al., 2017). As the scan rate increases, the intensity of the redox peaks gradually decreases due to the rapid movement of ions and insufficient time for effective ion diffusion and electrochemical interaction within the electrode material. This behavior indicates limited accessibility of active sites at higher scan rates, resulting in reduced faradaic contribution to the charge storage process. Similarly, the GCD curves shown in Figure 7c exhibit slight charge–discharge plateaus at lower current densities, indicating the presence of faradaic redox reactions during the electrochemical process. (Delbari et al., 2021). As the current density increases, the charge–discharge profiles gradually transform into a more symmetric triangular shape, suggesting enhanced capacitive behavior with reduced contribution from redox reactions. This observation is consistent with the CV results, where the redox peaks become less prominent at higher scan rates due to the limited time available for ion diffusion and electrochemical reactions. Figure 7d displays the specific capacitance vs. current density plot for the WOM5//AC device. Figure 7e presents the Ragone plot of the asymmetric supercapacitor device, demonstrating a maximum specific energy density of 10 W h kg-1 at a power density of 4,000 W kg−1, indicating its promising energy storage capability along with good power delivery performance.

**(a)** CV curve of AC and WOM5 at 10 mVs<sup>-1</sup>, **(b,c)** CV and GCD curves at different current densities, **(d)** Plot of specific capacitance at different current densities, **(e)** Ragone plot, **(f)** Nyquist plot, **(g)** Cyclic stability test for the WOM5//AC, **(h)** Schematic illustration of EDLC.
Figure 7. (a) CV curve of AC and WOM5 at 10 mVs-1, (b,c) CV and GCD curves at different current densities, (d) Plot of specific capacitance at different current densities, (e) Ragone plot, (f) Nyquist plot, (g) Cyclic stability test for the WOM5//AC, (h) Schematic illustration of EDLC.

Furthermore, Figure 7f shows the Nyquist plot of the AAS device and inset is the equivalence circuit diagram. The EIS analysis of the WOM5//AC device reveals Rs, Rct, and Zw values of 0.13 Ω, 0.64 Ω, and 0.77 Ω, respectively. The very low Rs value indicates excellent ionic conductivity and minimal internal resistance of the electrolyte and electrode interfaces. Meanwhile, the small Rct value suggests rapid charge transfer kinetics and efficient electron transport at the electrode/electrolyte interface. In addition, the relatively low Zw value confirms favorable ion diffusion behavior within the electrode material, which facilitates faster electrolyte ion transport during the charge–discharge process. (Mishra and Kant, 2020).

These low resistance parameters collectively demonstrate the superior electrochemical conductivity and enhanced charge storage performance of the WOM5//AC asymmetric supercapacitor device. Furthermore, Figure 7g illustrates the cycling stability and coulombic efficiency of the device at different charge–discharge cycles. The comparison of the electrochemical performance of various metals doped with WO3 and WO3.nH2O are shown in Supplementary Table S1. The device initially exhibits nearly 60% capacitance retention with a coulombic efficiency of approximately 85% and the capacity fading due to the occurrence of structural and interfacial degradation during prolonged cycling. Additionally, Figure 7h shows the schematic of two WOM5//AC devices connected in series and running LED light.

4 Conclusion

In conclusion, Mg-substituted WO3·H2O nanostructures were successfully developed as efficient pseudocapacitive electrode materials for aqueous asymmetric supercapacitor applications. Among all synthesized samples, the rationally doped Mg (WOM5) electrode exhibited superior specific capacitance (158 F g−1 at 1 A g−1) behavior with higher b-values (0.82), enhanced redox activity, and improved charge storage kinetics. Furthermore, the assembled WOM5//AC asymmetric supercapacitor device delivered promising energy density of 10 W h kg−1 and maximum power density of 4000 W kg−1 along with excellent long-term cycling stability of 60% capacitance retention and Coulombic efficiency of 85% up to 5,000. The improved electrochemical performance was attributed to the synergistic effect of Mg doping and the layered hydrated WO3 structure, which promoted efficient electrolyte ion transport and reversible redox reactions. Therefore, Mg doping is an effective and simple strategy to enhance the pseudocapacitive performance and cycling durability of WO3·H2O-based electrode materials.

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Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fbael.2026.1883507/full#supplementary-material

Supplementary material Supplementary material at the source

Supplementary material Supplementary material at the source

Footnotes

Edited by: Apurba Ray, German Aerospace Center (DLR), Germany

Reviewed by: Uma Shankar Veerasamy, Chiang Mai University, Thailand

Moganesh Govindhan, Saveetha Engineering College, India

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Funding

The author(s) declared that financial support was not received for this work and/or its publication.

Data availability

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding author.

Author contributions

HN: Data curation, Formal Analysis, Investigation, Methodology, Writing – original draft. RJ: Data curation, Formal Analysis, Investigation, Methodology, Writing – original draft. MS: Investigation, Methodology, Writing – original draft. BP: Formal Analysis, Writing – review and editing. AC: Writing – review and editing. PW: Formal Analysis, Supervision, Writing – original draft, Writing – review and editing.

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Citation

Nishad, H. S., et al. (2026). Mg-doped hydrated tungsten oxide nanostructures as pseudocapacitive electrodes for high-performance aqueous asymmetric supercapacitors. Frontiers in Batteries and Electrochemistry, Volume 5. https://doi.org/
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Volume
5 - 2026

Copyright

Copyright © 2026 Nishad, Jaiswar, Shirodkar, Patil, Chaskar and Walke.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Pravin S. Walke

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