Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2026 Sep 3;11(37):55475–55485. doi: 10.1021/acsomega.6c04292

Tailoring Phase-Dependent Energy Storage in Nonstoichiometric Tungsten Oxide Nanostructures for Enhanced Supercapacitor Performance

Krishna KC †,‡, Santiago J Dopico †,‡, Janak Paudel †,‡, Assem Basurrah †,‡,, Noureen Siraj †, Shawn E Bourdo §, Fumiya Watanabe §, John Nichols †,*
PMCID: PMC13613467  PMID: 42798913

Abstract

The multivalent states of tungsten oxide offer multiple stoichiometric (WO3 and WO2) and nonstoichiometric WO x (2.72 ≤ x < 3) crystal structures. As x decreases in WO x , the reduced oxygen stoichiometry coincides with enhanced electronic conductivity and the emergence of structural channels within the unit cell. Despite these promising properties, most research has focused on WO3 due to the significant challenges in precisely synthesizing the various WO x phases. This study overcomes this limitation by employing a versatile resistive hot-wire oxidation (RHWO) method, enabling the precise engineering of tungsten-to-oxygen ratios and morphologies in WO x phases. Electrochemical characterization of these compounds indicates that energy storage performance is strongly influenced by crystal phase, morphology, and electrolyte cation size. In particular, clear performance distinction emerged between WO2.90 which shows enhanced capacitance (electric double-layer capacitance (EDLC)) due to fine nanostructures, whereas WO2.72 exhibits both capacitive and diffusion-controlled energy storage behavior, attributed to its larger open channels, higher carrier concentration, and larger concentration of W5+ and W4+ oxidation states. These findings highlight the correlation between crystal structure, morphology, redox behavior, and ionic transport in WO x , providing a foundation for advanced energy storage systems and an eco-friendly pathway for practical supercapacitor (SCs) applications.

graphic file with name ao6c04292_0013.webp

graphic file with name ao6c04292_0011.webp

Introduction

Societal demand to utilize portable/miniature electronics, electric vehicles, and artificial intelligence is leading to a perpetually increasing demand for new energy technologies. Despite much recent research effort committed to society evolving to exclusively utilize sustainable energy practices, viable solutions to the climate and energy crises require significant advancement in energy storage devices. − Although metal ion batteries are currently a popular solution to problems requiring large energy densities, the bulk diffusion-controlled chemical redox reaction intrinsic to their function result in relatively long charge/discharge cycles when compared to supercapacitors (SCs) along with relatively short lifetimes. In addition, current practices of the mining industry for extracting metals required for metal ion batteries commonly have negative environmental impacts, further compounding the sustainability of this technology. , Although SCs have a modest energy density compared to ion batteries, their rapid charge/discharge rates and extended lifetimes make them ideal solutions for applications requiring high power density that are inaccessible to traditional dielectric capacitors that have significantly lower energy density than their SC counterparts. − Potential applications for SCs include regenerative braking in vehicles with an electric power train, power supplies for miniature consumer electronics, and grid stabilization for wind and solar energy generation. −

While SCs hold significant promises for next-generation energy storage, their commercial viability remains constrained by limitations in electrode performance and scalability. , A common example for a SC electrode is activated carbon. However, despite being low cost and offering a large surface area, it exhibits low specific capacitance because its capacitive properties arise solely from electric double-layer capacitance (EDLC) , For this charge storage mechanism, an electric dipole layer forms at the electrode/electrolyte interface similar to the arrangement in a conventional capacitor, where only a limited amount of electrostatic charge accumulates at the electrode/electrolyte interface, limiting the specific capacitance. Another approach to overcome this fundamental limitation is by utilizing pseudocapacitive materials, such as conductive polymers. An advantage of these materials is their utilization of fast and reversible Faradaic (redox) reactions occurring at or near the electrode surface alongside the EDLC. Although conductive polymers commonly yield greater electrical performance than activated carbon electrodes, they commonly have poor mechanical and electrochemical stability. Thus, there is demand to develop novel robust electrode materials that promote efficient redox reactions for next-generation SCs. Tungsten oxides have emerged as highly promising SC electrode materials to overcome these limitations. This is due to their open crystalline frameworks and abundant redox active sites, which enables high theoretical capacitance through combined EDLC with pseudocapacitance charge storage, excellent electrochemical stability, dual-functional electrochromic energy storage systems, and scalable synthesis. − Furthermore, since conventional SC devices which utilize binders such as Nafion or PVDF commonly suffer from reduced performance due to dead mass and resistive loss, we aim to overcome these challenges by focusing on binder-free SCs. ,−

We have identified nonstoichiometric tungsten oxide phases, WO x (2.72 ≤ x < 3) as a viable alternative to WO3, for high-performance SC applications. ,, These compounds offer unique crystal structures, enhanced electronic conductivity, and superior ion-transport characteristics. ,− For instance, increasing the degree of nonstoichiometry (decreasing x) within these intrinsic n-type semiconductors not only boosts carrier concentration but also introduces multivalent tungsten cations (W5+ and W4+). This facilitates structural rearrangements, that create larger open channels that promote ion-intercalation and redox reaction, thereby promoting significant enhancement in pseudocapacitance performance. − Despite this promise, realizing the full potential of these WO x phases is hindered by significant synthesis challenges. The stabilization of specific, narrow-phase WO x compositions requires meticulous control over synthesis parameters due to them only occupies a small region of phase space which primarily limited them to fundamental research. , Furthermore, most research on tungsten oxide SCs has largely concentrated on acidic electrolytes, where the narrow potential window results in a hydrogen evolution reaction (HER) that compromises efficiency. , We propose that neutral electrolytes can mitigate HER overpotentials, expand the operational potential window, and enhance both stability and capacitance.

In this study, we have systematically investigated SC properties of different WO x phases synthesized through versatile resistive hot-wire oxidation (RHWO). Here, we report on the carefully curated compounds, WO2.98, WO2.90 (W20O58), and WO2.72 (W18O49) (see Figure ), which represent the diverse crystal structures and electronic properties within the tungsten oxide system, with WO2.90 being that to the best of our knowledge has yet to be investigated for potential SC applications. This research serves as an ideal platform for exploring the correlation among crystal structure, morphology, stoichiometry, and electrochemical performance in both acidic and neutral electrolyte environments. By conducting a comprehensive electrochemical evaluation of these WO x phases, we aim to offer critical insights into the charge storage mechanism of tungsten oxide electrodes and elucidate the fundamental relationship between material properties and electrochemical behavior.

1.

1

Open in a new tab

Schematic illustration of the resistive hot-wire oxidation (RHWO) method for synthesizing three distinct nonstoichiometric phasesWO2.98 (yellow), WO2.90 (green), and WO2.72 (blue) on a W (silver) substrate. The flowchart outlines the synthesis process for each phase transformation. Bottom view illustrating the corresponding crystal structures of W (space group Im3̅m, a = 0.320 nm) (COD 9006488), WO2.98 (space group no. 10 P21/c with a = 0.730 nm, b = 0.753 nm, c = 0.768 nm, and β = 90.9°) (COD 1521533), WO2.90 (space group no. 10 P2/m with a = 1.210 nm, b = 0.378 nm, c = 2.340 nm, and β = 95.0°) (COD 1538317), and WO2.72 (space group no. 10 P2/m with a = 1.830 nm, b = 0.378 nm, c = 1.404 nm, and β = 115.2°) (COD 1528166), viewed along the ac plane. Tungsten and oxygen atoms are represented by dark green and brown spheres, respectively. Open channels, characteristic of the WO2.90 and WO2.72 structures, are highlighted with yellow X’s symbol. Top enlarged views highlighting the distinct morphological variations associated with each phase: a general microstructure for WO2.98, bundles of small nanorods for WO2.90, and dense nanorods for WO2.72, all synthesized on the W core–shell via RHWO method.

Experiments

A low-cost resistive hot-wire oxidation (RHWO) technique was employed to synthesize three distinct nonstoichiometric tungsten oxide phases WO2.98, WO2.90, and WO2.72 each exhibiting unique morphology. This method has been described in detail elsewhere. The synthesis process is schematically illustrated in Figure . To synthesize WO2.98 on W substrate, the growth was carried out at a filament temperature T f = 750 °C under standard temperature and pressure (STP) for 75 s. Both WO2.90 and WO2.72 were synthesized by reducing WO2.98 in an annealing process. For WO2.90 the annealing conditions are a filament temperature of T f = 750 °C and a rough vacuum of air P air = 65 mTorr for 10 min while the conditions for WO2.72 are a filament temperature of T f = 850 °C and a nitrogen atmosphere of P N2 = 600 Torr for 40 min.

Material Characterization

The surface morphology was observed using a JEOL JSM-7000F scanning electron microscope (SEM). An X-ray diffractometer Rigaku MiniFlex 600 (Cu Kα radiation at λ = 0.154 nm) and a Horiba LabRam 800 Raman spectrometer (512 nm wavelength at 10 mW) were employed to identify its crystal structure and chemical composition. X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo Scientific Kα, reference to the binding energy of carbon 1s (C–C peak at 284.80 eV), and XPS data is fitted with Thermo Scientific Avantage software; transmission electron microscopy (TEM) images were captured with a JEOL2100F TEM at 200k accelerating voltage. The TEM samples were mounted in 2 separate ways, WO2.98 in the form of a powder and the other a direct mount (WO2.90//W and WO2.72//W). The powder form of WO2.98 was created on several wire samples and then sonicated in ethanol to remove the oxide layer from the wire. This slurry was then drop cast onto a ultrathin lacey carbon grid.

Electrochemical Characterization

The electrochemical performance of the synthesized nonstoichiometric tungsten oxide samples was evaluated using a standard three-electrode setup. In this setup, a 3 mm diameter polished glassy carbon electrode loaded with tungsten oxide served as the working electrode, platinum (Pt) as the counter electrode, and Ag/AgCl as the reference electrode. We used two different electrolytes: 1 M H2SO4 and 1 M Na2SO4. Prior to each experiment, approximately 20 mL of the electrolyte was purged with nitrogen (N2) gas for 30 min to eliminate dissolved oxygen and ensure an inert chemical environment. To prepare the electrode material, 20 mg of WO x was dispersed in 8 mL of ethanol, without adding any binders, then sonicated for 1 h. A 3 μL aliquot of this binder-free slurry was then drop cast onto a glassy carbon electrode, achieving an active mass loading of 7.5 μg. Note that a single synthesis would yield roughly 1 mg of material with a mass/area ratio of roughly 80 g/mm2 which required roughly 20 samples to provide enough material for each SC test. Samples were adhered firmly to the electrode surface, ensuring stable contact during electrochemical measurements. Data was analyzed using custom Python 3.9 scripts that utilized the packages SciPy (1.13.1) for integration and nonlinear curve fitting along with NumPy (3.0.0) for basic mathematical operations.

We performed cyclic voltammetry (CV) using a Keithley 2450 source meter to assess the material’s redox behavior and capacitive properties. Electrochemical impedance spectroscopy (EIS) was performed using a custom-built potentiostat in conjunction with a Stanford Research Systems SR860 lock-in amplifier.

Results and Discussion

Scanning electron microscopy (SEM) was utilized to investigate the sample morphology on all of the samples. Representative images under various magnifications are shown in Figure . The low-magnification insets highlight the large-scale uniformity of all three samples. We observed that WO2.98 (Figure a) displays a globular morphology with an average particle diameter of 300 ± 40 nm. In contrast, WO2.90 (Figure b) and WO2.72 (Figure c) exhibit nanorods with characteristic diameters of 21 ± 2 and 100 ± 4 nm, respectively. Although the diameter for WO2.90 nanorods is significantly smaller than those of WO2.72 nanorods, they form in large bundles while WO2.72 forms isolated nanorods.

2.

2

Open in a new tab

Representative SEM images display the distinct morphological features across different nonstoichiometric tungsten oxide compositions: WO2.98 (a, b), WO2.90 (c, d), and WO2.72 (e, f). Low-magnifications images (10k and inset 1k) demonstrate the large coverage and notable uniformity. Corresponding high-magnification images reveal structural differences.

X-ray diffraction (XRD) was conducted for all samples to investigate the crystal structures, and the results are presented in Figure a. The XRD of WO2.98 (brown) shows diffraction peaks at 23.30, 23.60, and 24.47°, corresponding to the (002), (020), and (200) reflections consistent with the monoclinic WO3 structure, where the (002) reflection is dominant. Distinguishing WO2.98 from WO3 with small volume samples is quite formidable utilizing XRD alone because WO3 can accommodate loss of roughly 1% of its oxygen without undergoing significant changes in its crystal structure. Turning to XRD spectrum of WO2.90 (green) as shown in Figure a, it exhibits distinct reflections at approximately 22.80° (302̅), 40.60° (413), and 53.80° (615̅), suggesting a monoclinic crystal structure. , Similarly, XRD pattern of WO2.72 exhibits distinct reflections at 26.10° (104̅) and 30.50° (501) as shown in Figure a, suggesting the monoclinic WO2.72 phase. , Furthermore, both WO2.90 and WO2.72 exhibit a pronounced preferential orientation along the (0h0) plane, as evident from the spectral shift to (010) reflections as shown in Figure a. These results, corroborated by our SEM observations (discussed above), provide strong evidence for nanostructure growth along the b-axis.

3.

3

Open in a new tab

(a) XRD θ–2θ scans of WO2.98 (brown), WO2.90 (green), and WO2.72 (blue) sample on a W substrate. The prominent diffraction peaks are indexed to their respective Miller indices, referenced to appropriate Crystallographic Open Database (COD) entries. (b) Corresponding Raman spectra with normalized peak intensities (I max = 53,471, 3334, 2988 for WO2.98, WO2.90, and WO2.72 respectively). Note that each data presented represent an average of 9 spectra obtained uniformly from across 10 × 10 μm2 spaced region.

Raman spectroscopy in conjunction with XRD is a powerful tool to investigate these crystal systems. As illustrated in Figure , in order to accommodate the significant reduction in oxygen both WO2.90 and WO2.72 undergo significant structural rearrangements, which are intrinsically linked to variations in the local bonding environments. Raman spectroscopy is uniquely suited to probe the local bonding environment intrinsic to WO x , providing a powerful avenue to differentiate between WO x phases with varying oxygen-to-tungsten (O:W) ratios. The RS are presented in Figure b where features in the spectral range of 200–400 cm–1 are due to bending modes while those in the range 500–900 cm–1 are due to stretching modes. , The spectra were normalized to its absolute maximum; however, the signal strength of WO2.98 was roughly an order of magnitude larger than that for the other samples, which is consistent with its high degree of crystal symmetry. The RS of WO2.98 closely resembles that of monoclinic WO3, with intense, distinct peaks indicating uniform bond lengths and angles. However, upon closer inspection, a broad, weak feature appears around 600 cm–1, which is not typically observed in WO3. The presence of this feature suggests the existence of weaker W–O bonds consistent with the presence of oxygen vacancies in WO3. In contrast, both WO2.90 and WO2.72 show remarkable changes in RS due to a distribution of bond lengths and angles, which is supported by a change in their crystal structure (see Figure ). The RS of WO2.90 (green, Figure b) consists of two stretching mode features at approximately 810 cm–1, and addition 880 cm–1 along with a bending mode feature at approximately 378 cm–1. We interpret the shift to Raman features to higher energy as an indication of shorter or more rigid W–O bonds. − These bonds are likely associated with the edge-sharing octahedral structure of WO2.90, which occur through the rearrangement of WO6 octahedral to accommodate a lower O:W ratio compared to WO3. Further reduction in oxygen content like WO2.72 phase results in a broad Raman feature in both W–O stretching and bending mode as shown in Figure b. This result and its interpretation are consistent with that of WO2.76. Furthermore, these results are characteristic of increased structural disorder and a broader distribution of W–O bond lengths, which is consistent with our above discussion.

The chemical states of the sample surfaces were further investigated using X-ray photoelectron spectroscopy (XPS) which provides direct insight into elemental composition and the oxidation states of metal cations. Figure a,b shows the narrow-scan XPS spectra near the W 4f and O 1s edges, respectively (see Supporting Information for details). In these figures, the thick dark blue lines represent the raw data, and the thin light blue dashed lines represent the overall fit to the data. In panel (a), the fit includes contributions from the W 5p (orange) and 4f orbitals. For the latter, we allowed for individual contributions from W6+ (brown), W5+ (green), and W4+ (magenta) oxidation states. Focusing on the WO2.98 spectrum, it is predominantly W6+ in character, with a small presence of W5+. Quantitative analysis reveals an average oxidation state of +5.96, which is consistent with the results discussed above.

4.

4

Open in a new tab

XPS spectra of WO2.98 (top), WO2.90 (middle), WO2.72 (bottom) samples. (a) Narrow-scan W 4f core-level spectra are deconvoluted into distinct binding energy states for W6+ (brown), W5+ (green), and W4+ (magenta), each represented by their respective 7/2 (dark filled) and 5/2 (light filled) doublets. The weak features at slightly higher binding energies correspond to the W6+ and W5+ 5p3/2 orbitals. (b) Narrow-scan O 1s peak deconvolution shows three primary features: lattice oxygen (OL, dark brown), oxygen vacancy (OV, green) and absorbed hydroxide from water and absorbed hydrocarbon (−OH/–HC, orange). In both panels, the thick dark line represents the collected experimental data, and thin light blue dotted lines indicate the overall fit to the data.

Turning to other samples, we observe a large (small) spectral weight shift from W6+ to W5+ and W4+ states. , Quantitative analysis of the fitting parameters, presented in Table S1, shows an average oxidation state of +5.80 for WO2.90 and +5.44 for WO2.72, which are consistent with our assignment of each sample’s stoichiometry. The O 1s spectra shown in Figure b were analyzed to further investigate the chemical states and electronic properties of the samples (see Supporting Information Table S2 for detail). The XPS spectrum for WO2.98 is dominated by the lattice oxygen signal (brown), with a small presence of oxygen vacancies (green). A notable change is observed as the O:W ratio decreases: the O 1s spectra broaden, particularly for WO2.90 and WO2.72, reflecting increases in the oxygen reduction. XPS analysis revealed higher binding energies for oxygen species in samples with the reduced oxygen content consisting of (O–), indicating a stronger interaction between tungsten and oxygen (W–O bonds). , This observation is consistent with discussed above. Upon quantitative analysis of our XPS spectra, the O:W ratios for WO2.98, WO2.90, and WO2.72 are found to be 2.98, 2.89, and 2.73 respectively, which are consistent with results obtained from W 4f analysis. To further explore the electronic properties of WO x , XPS valence band spectra of WO2.98, WO2.90, and WO2.72 were analyzed as presented in Figure . A distinct feature was consistently observed near the Fermi level (0 eV) across all samples. This feature is attributed to in-gap states arising from inherent oxygen nonstoichiometry, specifically the formation of multivalent tungsten sites (W5+ and W4+). − These features are negligible or absent in both the stoichiometric WO3 and the metallic W substrate, confirming its characteristic nature for the nonstoichiometric phases. The intensity of this in-gap feature was most pronounced in WO2.72 followed by WO2.90 and WO2.98 (Figure ), indicating an enhanced carrier concentration. This finding suggests that the intrinsic conductivity is inversely proportional to the carrier concentration with subsequently scales with the W:O ratio. Note that this result is in good agreement with previous theoretical studies.

5.

5

Open in a new tab

(a) Valence band spectra for WO2.98 (brown), WO2.90 (green), and WO2.72 (blue) show distinct electronic features relative to the Fermi energy level (dotted vertical line at about 0 eV). (b) Magnified view near E f clearly illustrates the phase-dependent variation in electronic states linking stoichiometry to conductivity. Peak intensities are normalized to I max = I(7 eV).

Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) analyses were conducted to elucidate the atomic structures. Low- and high-magnification TEM images are presented in Figure , alongside SAED patterns. The SAED patterns for WO2.98 (Figure , top panel) reflections along the [100] and [010] zone axis. The measured a and b lattice constants were approximately 0.7 nm, consistent with the reported structural parameters of WO2.98. Critically, the observation of both (100) and (010) reflections support the WO2.98 phase assignment. This is distinct from the WO3 structure, which forbids odd-integer reflections. The middle row of Figure shows data for WO2.90 nanorods. SAED analysis confirmed a [100] zone axis, revealing orientations along the [010] and [001] directions with a preferred growth direction along the b-axis consistent with XRD and SEM result discussed above. An inset SAED pattern, taken with a large camera length, was necessary to resolve the (001) reflections due to the exceptionally large c lattice parameter (2.359 nm). The bottom row presents the SAED pattern for the monoclinic WO2.72 nanorods. We determine that d 001 = c sin­(β) = 1.27 nm where c = 14.04 nm and β = 115.03° which is consistent with the monoclinic symmetry. The [100] zone axis was again identified, confirming the dominant growth direction along the b-axis further supporting XRD and SEM results discussed above.

6.

6

Open in a new tab

Panels in left are real space TEM micrographs of WO2.98 (top), WO2.90 (middle), and WO2.72 (bottom) with low mag inset. Panel right presents corresponding selected area electron diffraction (SAED) patterns with respective plane labeling. 50 cm SAED pattern of WO2.98 indicate [010] and [100] directions, 200 cm SAED WO2.90 with 50 cm inset represents (001) reflections, and 50 cm SAED pattern of WO2.72 indicate [010] and [100] directions.

Electrochemistry

To evaluate the supercapacitive performance of WO x electrodes, cyclic voltammetry (CV) measurements were performed in a 1 M H2SO4, and representative CV profiles acquired at a scan rate of 100 mV/s are presented in Figure a. The CV curve for WO2.98 (brown) is similar to that of WO3, , while the CV results for WO2.72 (blue) and WO2.90 (green) are distinct. Both WO2.90 and WO2.72 shows an enhanced capacitive response [integral area under the current density (j)-potential (V) curve] as compared to WO2.98 with maximum values of 553 and 384 F/g (at 1 mV/s), respectively. Despite the simple synthesis process utilized here, our results are comparable to those on similar systems despite their utilization of more sophisticated fabrication techniques and material design. ,− We observe distinct reduction (oxidation) features when scanning to negative (positive) potential. More specifically, the oxygen reduction features in WO2.90 and WO2.72 near −0.19 V, are attributed to reduction from W5+ to W4+, , while the corresponding oxidation features in WO2.90 and WO2.72, observed at −0.10 and −0.13 V, respectively, are attributed to oxidation from W5+ to W6+. , This result shows that WO2.90 features a smaller redox peak separation which is an indication of higher cyclic reversibility and faster redox kinetics. Notably, WO2.90 exhibits the highest response indicated by the largest CV curve area, followed by WO2.72, when compared to WO2.98. Since WO2.72 has the largest carrier concentration and crystalline channel, we found this surprising, however when considering the specific surface area (SSA) which is estimated utilizing morphological data obtained from SEM results discussed above (Figure ) to be 5 and 27 m2/g for WO2.72 and WO2.90, respectively, we conclude that the supercapacitive response of tungsten oxide is highly sensitive to sample morphology. Quantitative analysis was performed by estimating specific capacitance by integrating the area under the CV curves at various scan rates as shown in Figure c (see Supporting Information for detail). Here, WO2.90 exhibited the highest specific capacitance across all scan rates, followed by WO2.72, while WO2.98 showed the lowest capacitance values compared to WO2.90 and WO2.72, yet still comparable to stoichiometric WO3 from other studies. , Although consistent with our interpretation discussed above, this result is at odds with our initial expectation that WO2.72 would exhibit the highest capacitive response due to larger open channels and higher carrier concentration. , However, this is clearly not the case. This discrepancy suggests each material has different charge storage mechanisms, as evident by the distinct shape of their CV curves. The quasi-rectangular shape of WO2.90 suggests a dominant EDLC capacitive behavior, while WO2.72 exhibits mixed contributions from EDLC and pseudocapacitive behavior, and WO2.98 is likely dominated with diffusion-limited without significant redox features. ,

7.

7

Open in a new tab

Cyclic Voltammograms (CVs) of WO2.98 (brown), WO2.90 (green), WO2.72 (blue) at a scan rate of 100 mV/s in (a) 1 M H2SO4 and (b) 1 M Na2SO4. The arrow indicates the scan direction. Specific capacitance of WO2.98 (brown), WO2.90 (green), WO2.72 (blue) calculated from integrated area of CV at different scan rates in (c) H2SO4 and (d) Na2SO4 electrolytes. Dunn analysis using a b-value threshold of 0.5 ≤ b ≤ 1.0 results evaluating the specific capacitance contribution from capacitive (dotted line) and diffusion (dash line) mechanisms relative to the total capacitance (dash dot line) for WO2.98, WO2.90, and WO2.72 at 1 mV/s in (e) H2SO4 and (f) Na2SO4 electrolytes.

To further support these findings, Dunn’s method was used to quantify the surface (capacitive) and diffusion (intercalation) controlled contributions for all three samples (see Supporting Information for detail). , The results for the scan rate of 1 mV/s are summarized in Figure e. The analysis reveals that the kinetics of WO2.98 are diffusion-limited, with surface-controlled processes contributing less than 10%. In contrast, WO2.90 and WO2.72 exhibit significant hybrid behavior, with surface-controlled kinetics accounting for approximately 65 and 55% of the total capacitance, respectively. This aligns with the CV curve analysis, which indicates substantial contributions from both surface and diffusion process for WO2.90 and WO2.72. Notably, among the three samples, WO2.90 possesses the highest surface-controlled contribution (a trend that becomes more pronounced at faster scan rates see Supporting Information for detail), which is likely due to substantial EDLC contributions. This large EDLC performance is attributed to its fine nanostructure morphology, which provides an exceptionally high surface-area-to-volume ratio compared to the larger crystallite sizes of the other samples. While the surface behavior differs significantly between WO2.90 and WO2.72, the diffusion contributions for WO2.90 and WO2.72 converge at lower scan rates, implying efficient cation intercalation kinetics within the lattice. This bulk diffusion of cations remains the intrinsic rate-limiting process in WO2.72 nonsurface charge storage. For, WO2.72 the unique structural phenomena, specifically large open channels, appear to mitigate diffusion limitations by facilitating faster ion transport through the bulk material. , These results are further supported by scan rate-dependent current density variation among different samples, and a log­(ν) versus log­(j) analysis (see Supporting Information for details). , These findings suggest that in 1 M H2SO4, WO2.90 is the best electrode material for SC applications, exhibiting enhanced EDLC contributions, while WO2.72 benefits from a balanced hybrid storage mechanism.

To isolate and evaluate the intrinsic pseudocapacitive behavior, all samples were characterized in a neutral 1 M Na2SO4 electrolyte, where the EDLC effect is significantly suppressed compared to an H2SO4 medium. , A key distinction emerged among the samples. While the energy storage response of WO2.98 remains consistent across both electrolytes, both WO2.90 and WO2.72 displayed a roughly 2-fold reduction in specific capacitance within the neutral medium (Figure b), underscoring a strong dependence of the pseudocapacitive charge storage mechanism on the electrolyte environment and the contributions of pseudocapacitive charge storage. As presented in Figure f, Dunn’s analysis was utilized to provide further insights into the charge storage mechanism showing that the pseudocapacitive response for all three materials are dominated by diffusion-controlled charge storage. While WO2.98 displayed negligible contributions for surface-controlled storage mechanisms, that for WO2.90 and WO2.72 was found to be roughly 15 and 6%, respectively. Furthermore, WO2.72 exhibited a uniquely enhanced diffusion-controlled energy storage contribution, particularly at the slowest scan rates, attributed to a facilitated bulk intercalation process. These insights were further supported by detailed scan rate-dependent current density and a log­(ν) versus log­(j) analysis (see Supporting Information for details). , The enhanced energy storage properties of WO2.72 highlight its positions for next-generation energy storage devices that demands both high energy and power density. This performance is directly linked to the material’s larger intrinsic channels, which facilitate superior Na+ intercalation kinetics compared to WO2.90. To further evaluate the robustness of the electrode materials for SC applications, cyclic stability tests were conducted on representative WO2.90 and WO2.72 samples in both H2SO4 and Na2SO4 electrolytes.

Cycling stability tests spanning 1000 cycles at 50 mV/s were performed in both H2SO4 (Figure ) and Na2SO4 (Figure ) electrolytes. The virgin (final) CV curve is represented by a solid (dotted) line. In the acidic medium, WO2.72 (blue) exhibited substantial degradation attributed to a prominent HER, driven by its high carrier concentration and structural openness. Conversely, WO2.90 demonstrated superior stability, retaining over 80% of its initial capacitance after 1000 cycles without any substantial changes. The modest reduction in WO2.90 performance correlates with the observed suppression of surface-controlled redox features in subsequent CV cycles (Figure ). Contour plot analysis provides further insights into electrode robustness. While WO2.90 maintained a consistent current response throughout cycling, WO2.72 showed a sharp change in current response (dark blue) after 800 cycles, confirming performance instability likely due to the HER onset. Since the HER instability does not emerge until after 800 cycles suggest that it is due to gradual proton insertion into the lattice rather than surface chemistry. In the neutral Na2SO4 electrolyte, both materials demonstrated excellent long-term stability with nearly overlapping CV curves. WO2.72 showed a steady ∼20% increase in specific capacitance over 1000 cycles, consistent with electroactivation via the gradual wetting of internal channels intrinsic to its crystal structure, resulting in enhanced charge intercalation. Furthermore, the capacitive performance of WO2.90 nanostructures, which possess smaller internal channels, remained largely constant after an initial stabilization period, which is consistent with this interpretation.

8.

8

Open in a new tab

Superior Long-term cyclic stability and enhanced capacitance retention of WO x phases in 1 M H2SO4 electrolyte. Left panel: WO2.72; Right panel: WO2.90. (a) CVs comparing the 1st cycle (solid line) and 1000th cycle (dotted line) for WO2.72 (left panel) and WO2.90 (right panel) in 1 M H2SO4 electrolyte at scan rate of 50 mV/s. The current for WO2.90 is scaled by a factor of 2 for visual clarity. (b) Change in cyclic performance across 1000 cycles, highlighting initial specific capacitance of C(N = 1)= 384 F/g for WO2.72 and C(N = 1)= 553 F/g for WO2.90. (c) Contour plot illustrating the potential-dependent current density profile for the endurance test across all 1000 cycles (discharging 0.1 to −0.5 V and charging −0.5 to 0.1 V).

9.

9

Open in a new tab

Superior long-term cyclic stability and enhanced capacitance retention of WO x phases in 1 M Na2SO4 electrolyte. Left panel: WO2.72; Right panel: WO2.90. (a) CVs comparing the 1st cycle (solid line) and 1000th cycle (dotted line) for WO2.72 (left panel) and WO2.90 (right panel) at scan rate of 50 mV/s. For clarity comparison of corresponding CV results within H2SO4 electrolyte, current density range in Na2SO4 are scaled by a factor of 15. (b) Change in cyclic performance across 1000 cycles, highlighting initial specific capacitance of C(N = 1)= 63 F/g for both WO2.72 and WO2.90. (c) Contour plot illustrating the potential-dependent current density profile for the endurance test across all 1000 cycles (discharging 0.1 to −0.7 V and charging −0.7 to 0.1 V).

Electrochemical impedance spectroscopy (EIS) was employed to further investigate the charge storage kinetics of the synthesized WO x materials (Figure ). Since our other electrochemical results discussed above clearly indicate contributions from surface-controlled and redox/diffusive processes, this necessitates the careful selection of a two-time-constant model that adequately accommodates this fact. Thus, we identified the circuit model illustrated in the right panel of Figure . Within this model, R 0 represents the electrolyte impedance, R 1 and C 1 account for high-frequency surface-controlled processes, while R 2, C 2, and the Warburg element (A W) model the low-frequency diffusion and redox dynamics. Here, C 1 and C 2 were modeled as constant phase elements due to the intrinsic roughness of our nanostructured samples. More specifically, we interpret C 1 to represent surface contributions to be in great agreement with CV discussed above. Furthermore, since R 2 is tied to redox reactions, we expect it to scale inversely with carrier concentration, which is consistent with our XPS analysis.

10.

10

Open in a new tab

Nyquist plot of WO2.98, WO2.90, and WO2.72 in 1 M H2SO4 (left panel) and Na2SO4 (right panel) electrolyte solutions. The symbol represent the experimental data points, and the dotted line shows the best fit obtained from the equivalent circuit model. The inset in the right panel illustrates the specific equivalent circuit model used for fitting the data, which provide the best representation of the electrochemical processes observed.

The spectra were fitted with a two-time constant equivalent circuit (Figure ), including a series resistance (R 0). The high-frequency response corresponds to surface-controlled processes (R 1 and CPE1), while the low-frequency region accounts for diffusion and redox dynamics (R 2, CPE2, and a Warburg element, W). ,− Fitting results (Table S3) reveal distinct behavior between electrolytes and electrode materials. In H2SO4, WO2.90 exhibits the highest surface-controlled capacitance (CPE1), whereas WO2.72 shows an enhanced diffusion-controlled capacitance (CPE2). In Na2SO4, mass transport limitations related to the larger Na+ ionic radius increase diffusion resistance, which results in larger Warburg coefficients. Among all samples, WO2.90 exhibits the lowest overall Warburg coefficient, indicating the fastest ion transport, attributed to its finely nanostructure morphology. Concurrently, WO2.72 demonstrates the lowest charge-transfer resistance (R 2), suggesting higher carrier concentration and efficient redox activity, consistent with the potent pseudocapacitive contributions identified via cyclic voltammetry. These results confirm that H2SO4 facilitates superior overall performance due to accelerated H+ ionic kinetics and enhanced redox activity. ,

Conclusions

This study systematically elucidates the structural, oxidation state, and electrochemical evolution of nonstoichiometric tungsten oxides (WO x ) synthesized by RHWO to investigate their viability for SC applications. The chemical composition of nonstoichiometric tungsten oxides can be finely tuned, accompanied by distinct morphological transformations from globular particles to nanorods. These results demonstrate how powerful RHWO synthesis is for new materials investigations and functional applications. This arises from the scalability, time efficiency, and environmental friendliness of the sample synthesis technique. Furthermore, electrochemical measurements demonstrate that WO2.90 exhibits the highest specific capacitance of 583 F/g, attributed to its morphology of finely bundled nanostructures. In contrast, WO2.72 shows a hybride energy storage mechanism and enhanced performance at slow scan rates, particularly in neutral electrolytes, owing to its open-channel structure and multivalent redox activity. Analysis of the EIS data further supports these findings, revealing low charge-transfer resistance and favorable ion diffusion in reduced phases. Overall, the results highlight the critical influence of oxygen stoichiometry, nanostructure, and electrolyte environment on the charge storage mechanisms of WO x , offering valuable insights into the rational design of advanced tungsten oxide-based energy storage systems. Moreover, these results show that the intrinsic properties of tungsten oxides make them highly promising electrode materials for energy storage across a wide pH range.

Supplementary Material

ao6c04292_si_001.pdf (1.4MB, pdf)

Acknowledgments

The authors would like to thank The Center for Integrative Nanotechnology Science (CINS) for the accessibility to SEM, TEM, XPS, and Raman instrumentation. K. KC. and J.N. acknowledge Anindya Ghosh and Md Mohsin Patwary for electrochemistry resources and valuable discussion.

Data used is available throughout the manuscript text.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c04292.

  • Additional experimental results, methods, and analysis for all samples (PDF)

Conceptualization: J.N. and K. KC. Instrumentation: J.N. and K. KC. Data collection, analysis, and validation: SEM: K. KC., F.W., and J.N.; XRD: K. KC. and J.N.; Raman spectroscopy: K. KC., S.E.B., and J.N.; TEM: S.J.D., K. KC., F.W., and J.N.; XPS: K. KC., F.W., and J.N.; electrochemistry: K. KC., J.P., A.B., N.S., and J.N. Manuscript writing, review, and editing: J.N. and K. KC. with contributions from all authors. Supervision: J.N., S.E.B., and F.W.

The authors declare no competing financial interest.

References

  1. Navarro G., Torres J., Blanco M., Nájera J., Santos-Herran M., Lafoz M.. Present and future of supercapacitor technology applied to powertrains, renewable generation and grid connection applications. Energies. 2021;14:3060. doi: 10.3390/en14113060. [DOI] [Google Scholar]
  2. Patwary M. M., Haque S., Szwedo P., Hasan G., Kondrapolu R. S., Watanabe F., KC K., Wang D., Ghosh A.. Exploring Enhanced Oxygen Reduction Reactions: A Study on Nanocellulose, Dopamine, and Cobalt Complex-Derived Non-Precious Electrocatalyst. Catalysts. 2024;14:613. doi: 10.3390/catal14090613. [DOI] [Google Scholar]
  3. Sinha P., Sharma A.. The prospect of supercapacitors in integrated energy harvesting and storage systems. Nanotechnology. 2024;35:382001. doi: 10.1088/1361-6528/ad5a7b. [DOI] [PubMed] [Google Scholar]
  4. Palacín M. R., de Guibert A.. Why do batteries fail? Science. 2016;351:1253292. doi: 10.1126/science.1253292. [DOI] [PubMed] [Google Scholar]
  5. Ramasubramanian B., Ling J., Jose R., Ramakrishna S.. Ten major challenges for sustainable lithium-ion batteries. Cell Rep. Phys. Sci. 2024;5:102032. doi: 10.1016/j.xcrp.2024.102032. [DOI] [Google Scholar]
  6. Li M., Lu J., Chen Z., Amine K.. 30 years of lithium-ion batteries. Adv. Mater. 2018;30:1800561. doi: 10.1002/adma.201800561. [DOI] [PubMed] [Google Scholar]
  7. Chen S., Xing W., Duan J., Hu X., Qiao S. Z.. Nanostructured morphology control for efficient supercapacitor electrodes. J. Mater. Chem. A. 2013;1:2941–2954. doi: 10.1039/C2TA00627H. [DOI] [Google Scholar]
  8. Sadavar S. V., Lee S.-Y., Park S.-J.. Advancements in asymmetric supercapacitors: from historical milestones to challenges and future directions. Adv. Sci. 2024;11:2403172. doi: 10.1002/advs.202403172. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Kumar Y. A., Roy N., Ramachandran T., Hussien M., Moniruzzaman M., Joo S. W.. Shaping the future of energy: The rise of supercapacitors progress in the last five years. J. Energy Storage. 2024;98:113040. doi: 10.1016/j.est.2024.113040. [DOI] [Google Scholar]
  10. Simon P., Gogotsi Y.. Materials for electrochemical capacitors. Nat. Mater. 2008;7:845–854. doi: 10.1038/nmat2297. [DOI] [PubMed] [Google Scholar]
  11. Guo L., Hu P., Wei H.. Development of supercapacitor hybrid electric vehicle. J. Energy Storage. 2023;65:107269. doi: 10.1016/j.est.2023.107269. [DOI] [Google Scholar]
  12. Gopi C. V. M., Ramesh R.. Review of battery-supercapacitor hybrid energy storage systems for electric vehicles. Results Eng. 2024;24:103598. doi: 10.1016/j.rineng.2024.103598. [DOI] [Google Scholar]
  13. He A., He J., Cao L., Chen J., Cheng B., Ma R., Ding Y., Yang G., Yi F.. Flexible supercapacitor integrated systems. Adv. Mater. Technol. 2024;9:2301931. doi: 10.1002/admt.202301931. [DOI] [Google Scholar]
  14. Ahlawat N., Bansal L., Sahu B., Rath D. K., Tiwari S., Chaudhary A., Kaladi Chondath S., Kumar R.. Complementary Oxides Based Electrochromic Supercapacitor: Multiwavelength Switchable Solid State Device. ACS Appl. Energy Mater. 2025;8:4288–4298. doi: 10.1021/acsaem.4c03222. [DOI] [Google Scholar]
  15. Wang Y., Song Y., Xia Y.. Electrochemical capacitors: mechanism, materials, systems, characterization and applications. Chem. Soc. Rev. 2016;45:5925–5950. doi: 10.1039/C5CS00580A. [DOI] [PubMed] [Google Scholar]
  16. Haque S., Basurrah A., Hammer B. I., Bourdo S., Karabacak T.. Novel core-shell CuO nanostructures grown on Cu powder particles by hot water treatment for binder-free supercapacitor electrodes. J. Mater. Sci.: Mater. Eng. 2025;20:28. doi: 10.1186/s40712-025-00239-2. [DOI] [Google Scholar]
  17. Parnell C. M., Chhetri B. P., Mitchell T. B., Watanabe F., Kannarpady G., RanguMagar A. B., Zhou H., Alghazali K. M., Biris A. S., Ghosh A.. Simultaneous electrochemical deposition of cobalt complex and poly (pyrrole) thin films for supercapacitor electrodes. Sci. Rep. 2019;9:5650. doi: 10.1038/s41598-019-41969-6. [DOI] [PMC free article] [PubMed] [Google Scholar]
  18. Veerakumar P., Sangili A., Manavalan S., Thanasekaran P., Lin K.-C.. Research progress on porous carbon supported metal/metal oxide nanomaterials for supercapacitor electrode applications. Ind. Eng. Chem. Res. 2020;59:6347–6374. doi: 10.1021/acs.iecr.9b06010. [DOI] [Google Scholar]
  19. Siraj N., Macchi S., Berry B., Viswanathan T.. Metal-free carbon-based supercapacitorsa comprehensive review. Electrochem. 2020;1:410–438. doi: 10.3390/electrochem1040028. [DOI] [Google Scholar]
  20. Society, A. C. Eco-Friendly Supercapacitors: Design and Future Perspectives in Sustainable and Green Energy Storage Devices; ACS Publications, 2024. [Google Scholar]
  21. Song Y., Liu T.-Y., Xu X.-X., Feng D.-Y., Li Y., Liu X.-X.. Pushing the cycling stability limit of polypyrrole for supercapacitors. Adv. Funct. Mater. 2015;25:4626–4632. doi: 10.1002/adfm.201501709. [DOI] [Google Scholar]
  22. Liu X., Chen S., Xiong Z., Li K., Zhang Y.. Tungsten oxide-based nanomaterials for supercapacitors: Mechanism, fabrication, characterization, multifunctionality, and electrochemical performance. Prog. Mater. Sci. 2022;130:100978. doi: 10.1016/j.pmatsci.2022.100978. [DOI] [Google Scholar]
  23. Shinde P. A., Jun S. C.. Review on recent progress in the development of tungsten oxide based electrodes for electrochemical energy storage. ChemSusChem. 2020;13:11–38. doi: 10.1002/cssc.201902071. [DOI] [PubMed] [Google Scholar]
  24. Huang Z.-H., Li H., Li W.-H., Henkelman G., Jia B., Ma T.. Electrical and structural dual function of oxygen vacancies for promoting electrochemical capacitance in tungsten oxide. Small. 2020;16:2004709. doi: 10.1002/smll.202004709. [DOI] [PubMed] [Google Scholar]
  25. Roy R., Greeshma R., Dutta P., Mondal I., Banerjee R., Singh A. K.. Electrochromic and Energy Storage Performance Enhancement by Introducing Jahn–Teller Distortion: Experimental and Theoretical Study. ACS Appl. Mater. Interfaces. 2024;16:39539–39550. doi: 10.1021/acsami.4c04445. [DOI] [PubMed] [Google Scholar]
  26. Zhang N., Wang M., Quan Y., Li X., Hu X., Yan J., Wang Y., Sun M., Li S.. A review of binder-free electrodes for advanced supercapacitors. J. Ind. Eng. Chem. 2025;141:1–31. doi: 10.1016/j.jiec.2024.06.025. [DOI] [Google Scholar]
  27. Dhakal G., Sahoo S., Kidie A. E., Sharma K. P., Abir M. R. A. R., Zhao G.-L., Shim J.-J.. Morphology-controlled vertically grown manganese-substituted nickel-sulfide nanosheets for binder-free asymmetric supercapacitors. Appl. Surf. Sci. 2025;711:164015. doi: 10.1016/j.apsusc.2025.164015. [DOI] [Google Scholar]
  28. Hou J.-B., Zhang K., Xiao J.-H., Xu Z.-Q., Gao W.-J., Gao X.-Y., Zhou S.-K., Jiao Z.-Z., Yi M.-R., Yin Y.-H., Wu Z. P.. Modified tungsten oxide as a binder-free anode in lithium-ion battery for improving electrochemical stability. Tungsten. 2022;4:356–369. doi: 10.1007/s42864-022-00162-5. [DOI] [Google Scholar]
  29. Gurav S. R., Chodankar G. R., Sawant S. A., Shembade U. V., Moholkar A. V., Sonkawade R. G.. Exploring the potential of simultaneous nanoarchitectonics and utilization of Co-MOFs electrode as well as powder for aqueous supercapacitors. J. Energy Storage. 2023;73:109254. doi: 10.1016/j.est.2023.109254. [DOI] [Google Scholar]
  30. Tian Y., Cong S., Su W., Chen H., Li Q., Geng F., Zhao Z.. Synergy of W18O49 and polyaniline for smart supercapacitor electrode integrated with energy level indicating functionality. Nano Lett. 2014;14:2150–2156. doi: 10.1021/nl5004448. [DOI] [PubMed] [Google Scholar]
  31. Kim H.-S., Cook J. B., Lin H., Ko J. S., Tolbert S. H., Ozolins V., Dunn B.. Oxygen vacancies enhance pseudocapacitive charge storage properties of MoO3–x . Nat. Mater. 2017;16:454–460. doi: 10.1038/nmat4810. [DOI] [PubMed] [Google Scholar]
  32. Roy R., R G., Basith A., Banerjee R., Singh A. K.. Self-rechargeable aqueous Zn2+/K+ electrochromic energy storage device via scalable spray-coating integrated with marangoni flow. Energy Storage Mater. 2024;71:103680. doi: 10.1016/j.ensm.2024.103680. [DOI] [Google Scholar]
  33. Dutta P., Palliyal K. V., Prabhu R., Singh A. K.. Oxygen-deficient bimetallic oxide M0.11W0.89O3-x for flexible energy storage and electrochromic applications. J. Mater. Chem. A. 2026;14:13504–13519. doi: 10.1039/D6TA01049K. [DOI] [Google Scholar]
  34. Park S., Shim H.-W., Lee C. W., Song H. J., Kim J.-C., Kim D.-W.. High-power and long-life supercapacitive performance of hierarchical, 3-D urchin-like W18O49 nanostructure electrodes. Nano Res. 2016;9:633–643. doi: 10.1007/s12274-015-0943-3. [DOI] [Google Scholar]
  35. Jo C., Hwang I., Lee J., Lee C. W., Yoon S.. Investigation of pseudocapacitive charge-storage behavior in highly conductive ordered mesoporous tungsten oxide electrodes. J. Phys. Chem. C. 2011;115:11880–11886. doi: 10.1021/jp2036982. [DOI] [Google Scholar]
  36. Xu J., Li C., Chen L., Li Z., Bing P.. Anchoring carbon layers and oxygen vacancies endow WO3–x /C electrode with high specific capacity and rate performance for supercapacitors. RSC Adv. 2019;9:28793–28798. doi: 10.1039/C9RA03886H. [DOI] [PMC free article] [PubMed] [Google Scholar]
  37. Migas D. B., Shaposhnikov V., Rodin V., Borisenko V.. Tungsten oxides. I. Effects of oxygen vacancies and doping on electronic and optical properties of different phases of WO3. J. Appl. Phys. 2010;108:093713. doi: 10.1063/1.3505688. [DOI] [Google Scholar]
  38. KC K., Dopico S. J., Paudel J., Bonney M. M., Ibrahim G. J., Herbold M., Kingston A. J., Bourdo S. E., Watanabe F., Nichols J.. A single-step low-cost synthesis of tungsten oxide nanostructures by resistive hot wire oxidation. CrystEngComm. 2025;27:516–522. doi: 10.1039/D4CE00977K. [DOI] [Google Scholar]
  39. Roy R., Ganesha M. K., Dutta P., Pal D., Singh A. K.. High-performance aqueous electrochromic battery for smart window application: mechanistic insights of Al-ion (de) intercalation kinetics in thickness-optimized WO3. ACS Appl. Energy Mater. 2023;6:11683–11693. doi: 10.1021/acsaem.3c02237. [DOI] [Google Scholar]
  40. Musicó V. J., Holzapfel N. P., Wang M., Tsai W.-Y., Proksch T., Kozinsky B., Balke N., Augustyn V.. Ion-dependent electrochemical behavior in shear-structured tungsten oxides. Solid State Ionics. 2026;434:117095. doi: 10.1016/j.ssi.2025.117095. [DOI] [Google Scholar]
  41. Lee Y.-J., Lee T., Soon A.. Phase stability diagrams of group 6 Magneli oxides and their implications for photon-assisted applications. Chem. Mater. 2019;31:4282–4290. doi: 10.1021/acs.chemmater.9b01430. [DOI] [Google Scholar]
  42. Gulbransen E. A., Andrew K.. Kinetics of the Oxidation of Pure Tungsten from 500 to 1300 C. J. Electrochem. Soc. 1960;107:619. doi: 10.1149/1.2427787. [DOI] [Google Scholar]
  43. Periasamy P., Krishnakumar T., Devarajan V., Sandhiya M., Sathish M., Chavali M.. Investigation of electrochemical supercapacitor performance of WO3-CdS nanocomposites in 1 M H2SO4 electrolyte prepared by microwave-assisted method. Mater. Lett. 2020;274:127998. doi: 10.1016/j.matlet.2020.127998. [DOI] [Google Scholar]
  44. Li C.-P., Lai G. Y.. Synthesis and capacitive properties of mesoporous tungsten oxide films prepared by ultrasonic spray deposition. ACS Omega. 2023;8:40878–40889. doi: 10.1021/acsomega.3c05677. [DOI] [PMC free article] [PubMed] [Google Scholar]
  45. Virtanen P., Gommers R., Oliphant T. E., Haberland M., Reddy T., Cournapeau D., Burovski E., Peterson P., Weckesser W., Bright J., van der Walt S. J., Brett M., Wilson J., Millman K. J., Mayorov N., Nelson A. R. J., Jones E., Kern R., Larson E.. et al. SciPy 1.0: fundamental algorithms for scientific computing in Python. Nat. Methods. 2020;17:261–272. doi: 10.1038/s41592-019-0686-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  46. Harris C. R., Millman K. J., van der Walt S. J., Gommers R., Virtanen P., Cournapeau D., Wieser E., Taylor J., Berg S., Smith N. J., Kern R., Picus M., Hoyer S., van Kerkwijk M. H., Brett M., Haldane A., del Río J. F., Wiebe M., Peterson P.. et al. Array programming with NumPy. Nature. 2020;585:357–362. doi: 10.1038/s41586-020-2649-2. [DOI] [PMC free article] [PubMed] [Google Scholar]
  47. Locherer K. R., Swainson I., Salje E.. Transition to a new tetragonal phase of WO3: crystalstructure and distortion parameters. J. Phys.: Condens. Matter. 1999;11:4143. doi: 10.1088/0953-8984/11/21/303. [DOI] [Google Scholar]
  48. Gebert E., Ackermann R.. Substoichiometry of tungsten trioxide; the crystal systems of WO3.00, WO2.98, and WO2.96. Inorg. Chem. 1966;5:136–142. doi: 10.1021/ic50035a033. [DOI] [Google Scholar]
  49. Magnéli A.. Crystal structure studies on beta-tungsten oxide. Ark. Kemi. 1949;1:223–230. [Google Scholar]
  50. Tran N. Q. M., Pham A. T. T., Nguyen C. C., Tran T. T. N., Lai H. T., Le T. B. N., Jayasankar C. K., Pecharapa W., Phan T. B.. A homogeneous composite approach for improved thermoelectric performance: Repressing lattice thermal conductivity in substoichiometric WO2.90 and WO2.72. Ceram. Int. 2023;49:40804–40813. doi: 10.1016/j.ceramint.2023.10.065. [DOI] [Google Scholar]
  51. Viswanathan K., Brandt K., Salje E.. Crystal structure and charge carrier concentration of W18O49. J. Solid State Chem. 1981;36:45–51. doi: 10.1016/0022-4596(81)90190-0. [DOI] [Google Scholar]
  52. Hardcastle F. D., Wachs I. E.. Determination of the molecular structures of tungstates by Raman spectroscopy. J. Raman Spectrosc. 1995;26:397–405. doi: 10.1002/jrs.1250260603. [DOI] [Google Scholar]
  53. Ross-Medgaarden E. I., Wachs I. E.. Structural determination of bulk and surface tungsten oxides with UV–vis diffuse reflectance spectroscopy and Raman spectroscopy. J. Phys. Chem. C. 2007;111:15089–15099. doi: 10.1021/jp074219c. [DOI] [Google Scholar]
  54. Bets V., Veispals A., Lusis A., Purans J., Ramans G., Sheromov M., Kochubei D., Fedorov V.. Studies of tungsten oxide electrochromic thin films and polycrystals by the EXAFS method. Nucl. Instrum. Methods Phys. Res., Sect. A. 1987;261:175–177. doi: 10.1016/0168-9002(87)90593-6. [DOI] [Google Scholar]
  55. Zhang X., Tang F., Wang M., Zhan W., Hu H., Li Y., Friend R. H., Song X.. Femtosecond visualization of oxygen vacancies in metal oxides. Sci. Adv. 2020;6:eaax9427. doi: 10.1126/sciadv.aax9427. [DOI] [PMC free article] [PubMed] [Google Scholar]
  56. Paudel J., Bonney M. M., KC K., Dopico S. J., Kingston A. J., Ojo O. P., Lackey T., Sharma A. M., Watanabe F., Nichols J.. Z-Scheme Tungsten Copper Oxide for Photocatalytic Water Splitting. J. Phys. Chem. C. 2025;129:10001–10008. doi: 10.1021/acs.jpcc.5c01730. [DOI] [Google Scholar]
  57. Chen J., Lu D., Zhang W., Xie F., Zhou J., Gong L., Liu X., Deng S., Xu N.. Synthesis and Raman spectroscopic study of W20O58 nanowires. J. Phys. D: Appl. Phys. 2008;41:115305. doi: 10.1088/0022-3727/41/11/115305. [DOI] [Google Scholar]
  58. Frey G., Rothschild A., Sloan J., Rosentsveig R., Popovitz-Biro R., Tenne R.. Investigations of nonstoichiometric tungsten oxide nanoparticles. J. Solid State Chem. 2001;162:300–314. doi: 10.1006/jssc.2001.9319. [DOI] [Google Scholar]
  59. Shengelaya A., Conder K., Müller K.. Signatures of filamentary superconductivity up to 94 K in tungsten oxide WO2.90. J. Supercond. Novel Magn. 2020;33:301–306. doi: 10.1007/s10948-019-05329-9. [DOI] [Google Scholar]
  60. Bandi S., Srivastav A. K.. Oxygen-deficient tungsten oxides. J. Mater. Sci. 2021;56:6615–6644. doi: 10.1007/s10853-020-05757-2. [DOI] [Google Scholar]
  61. Himpsel F. J., Morar J., McFeely F., Pollak R., Hollinger G.. Core-level shifts and oxidation states of Ta and W: Electron spectroscopy for chemical analysis applied to surfaces. Phys. Rev. B. 1984;30:7236. doi: 10.1103/PhysRevB.30.7236. [DOI] [Google Scholar]
  62. Suzuki N., Yabuki K., Oshimura N., Yoshio S., Adachi K.. X-ray photoelectron spectroscopic study of reduced alkali tungsten oxides with localized and delocalized electrons. J. Phys. Chem. C. 2022;126:15436–15445. doi: 10.1021/acs.jpcc.2c03499. [DOI] [Google Scholar]
  63. Kong H., Yang H., Park J.-S., Chae W.-S., Kim H. Y., Park J., Lee J. H., Choi S. Y., Park M., Kim H.. et al. Spatial control of oxygen vacancy concentration in monoclinic WO3 photoanodes for enhanced solar water splitting. Adv. Funct. Mater. 2022;32:2204106. doi: 10.1002/adfm.202204106. [DOI] [Google Scholar]
  64. Khyzhun O. Y.. XPS, XES and XAS studies of the electronic structure of tungsten oxides. J. Alloys Compd. 2000;305:1–6. doi: 10.1016/S0925-8388(00)00697-6. [DOI] [Google Scholar]
  65. Gehlig R., Salje E., Carley A., Roberts M.. XPS studies on WO2.90 and WO2.72 and the influence of metallic impurities. J. Solid State Chem. 1983;49:318–324. doi: 10.1016/S0022-4596(83)80009-7. [DOI] [Google Scholar]
  66. Bouvard O., Krammer A., Schüler A.. In situ core-level and valence-band photoelectron spectroscopy of reactively sputtered tungsten oxide films. Surf. Interface Anal. 2016;48:660–663. doi: 10.1002/sia.5927. [DOI] [Google Scholar]
  67. Xie F., Gong L., Liu X., Tao Y., Zhang W., Chen S., Meng H., Chen J.. XPS studies on surface reduction of tungsten oxide nanowire film by Ar+ bombardment. J. Electron Spectrosc. Relat. Phenom. 2012;185:112–118. doi: 10.1016/j.elspec.2012.01.004. [DOI] [Google Scholar]
  68. AbdEl-Salam A. H., Ewais H. A., Rabia M., Liu M., Al Angari Y. M.. High-performance supercapacitor based on tungsten oxide iodide/polymer nanocomposite for advanced energy storage. Sci. Rep. 2026;16:410. doi: 10.1038/s41598-025-29780-y. [DOI] [PMC free article] [PubMed] [Google Scholar]
  69. Subrahmanian S. K., Yaakob Z., Pullithadathil B., Narayanan B. N.. Tailoring Holey Graphene Architecture with Sulfur-Doped Oxygen-Deficient MoO3 for Synergistic Capacitance Enhancement. Small. 2025;21:e06976. doi: 10.1002/smll.202506976. [DOI] [PubMed] [Google Scholar]
  70. Mashkoor F., Shoeb M., Zhu S., Ahmed J., Noh S. M., Jeong C.. Synergistic effect of heterointerface engineering coupled with oxygen vacancies enriched Ag2O-WO2.8-SnO2 anchored carbon nanotubes nanocomposite for high-performance supercapacitor devices and their charge storage mechanism. Surf. Interfaces. 2025;62:106198. doi: 10.1016/j.surfin.2025.106198. [DOI] [Google Scholar]
  71. Liu S., Zeng T., Zhang Y., Wan Q., Yang N.. Coupling W18O49/Ti3C2Tx MXene pseudocapacitive electrodes with redox electrolytes to construct high-performance asymmetric supercapacitors. Small. 2022;18:2204829. doi: 10.1002/smll.202204829. [DOI] [PubMed] [Google Scholar]
  72. Yamada H., Yoshii K., Asahi M., Chiku M., Kitazumi Y.. Cyclic voltammetry part 1: fundamentals. Electrochemistry. 2022;90:102005. doi: 10.5796/electrochemistry.22-66082. [DOI] [Google Scholar]
  73. Pickering R., Tilley R.. An electron microscope study of tungsten oxides in the composition range. J. Solid State Chem. 1976;16:247–255. doi: 10.1016/0022-4596(76)90039-6. [DOI] [Google Scholar]
  74. Gogotsi Y., Penner R. M.. Energy storage in nanomaterials–capacitive, pseudocapacitive, or battery-like. ACS Nano. 2018;12(3):2081–2083. doi: 10.1021/acsnano.8b01914. [DOI] [PubMed] [Google Scholar]
  75. Wang J., Polleux J., Lim J., Dunn B.. Pseudocapacitive contributions to electrochemical energy storage in TiO2 (Anatase) Nanoparticles. J. Phys. Chem. A. 2007;111(40):14925–14931. [Google Scholar]
  76. Wei Y.-M., Kumar D., Zhang L., Li J.-F.. Pseudocapacitive materials for energy storage: properties, mechanisms, and applications in supercapacitors and batteries. Front. Chem. 2025;13:1636683. doi: 10.3389/fchem.2025.1636683. [DOI] [PMC free article] [PubMed] [Google Scholar]
  77. Nagy G., Schiller R.. Hydrogen in tungsten bronzes: mechanism of hydrogen intercalation. Int. J. Hydrogen Energy. 1989;14:567–572. doi: 10.1016/0360-3199(89)90115-8. [DOI] [Google Scholar]
  78. Bi S., Knijff L., Lian X., van Hees A., Zhang C., Salanne M.. Modeling of nanomaterials for supercapacitors: beyond carbon electrodes. ACS Nano. 2024;18:19931–19949. doi: 10.1021/acsnano.4c01787. [DOI] [PMC free article] [PubMed] [Google Scholar]
  79. Jiang H., Hong J. J., Wu X., Surta T. W., Qi Y., Dong S., Li Z., Leonard D. P., Holoubek J. J., Wong J. C.. et al. Insights on the proton insertion mechanism in the electrode of hexagonal tungsten oxide hydrate. J. Am. Chem. Soc. 2018;140:11556–11559. doi: 10.1021/jacs.8b03959. [DOI] [PubMed] [Google Scholar]
  80. M A., Paul A.. Importance of electrode preparation methodologies in supercapacitor applications. ACS Omega. 2017;2:8039. doi: 10.1021/acsomega.7b01275. [DOI] [PMC free article] [PubMed] [Google Scholar]
  81. Saini S., Chand P.. Effect of aqueous electrolytes on h-WO3 nanorods as an electrode material for supercapacitor application. Chem. Phys. Lett. 2022;802:139760. doi: 10.1016/j.cplett.2022.139760. [DOI] [Google Scholar]
  82. Sequeira C. A. C., Rodrigues L., Santos D.. Cation diffusivity in nonstoichiometric tungsten trioxide films. ECS J. Solid State Sci. Technol. 2012;1:R136. doi: 10.1149/2.010205jss. [DOI] [Google Scholar]
  83. Ding M., Qin Y., Ji W., Zhang Y., Zhao G.. A Review of Strategies to Improve the Electrocatalytic Performance of Tungsten Oxide Nanostructures for the Hydrogen Evolution Reaction. Nanomaterials. 2025;15:1163. doi: 10.3390/nano15151163. [DOI] [PMC free article] [PubMed] [Google Scholar]
  84. Wang W., Guo S., Lee I., Ahmed K., Zhong J., Favors Z., Zaera F., Ozkan M., Ozkan C. S.. Hydrous ruthenium oxide nanoparticles anchored to graphene and carbon nanotube hybrid foam for supercapacitors. Sci. Rep. 2014;4:4452. doi: 10.1038/srep04452. [DOI] [PMC free article] [PubMed] [Google Scholar]
  85. Han J., Chan H. L., Wartenberg M. G., Heinrich H. H., Scully J. R.. Distinguishing interfacial double layer and oxide-based capacitance on gold and pre-oxidized Fe-Cr in 1-ethyl-3-methylimidazolium methanesulfonate room temperature ionic liquid aqueous mixture. Electrochem. Commun. 2021;122:106900. doi: 10.1016/j.elecom.2020.106900. [DOI] [Google Scholar]
  86. Lavillunière H., Pham-Truong T.-N., Nguyen T.-K.-L., Vancaeyzeele C., Aubert P.-H.. Controlled microwave-assisted synthesis of covalent organic frameworks opens the way toward more suitable porous supercapacitor electrodes. ACS Appl. Energy Mater. 2024;7:1723–1734. doi: 10.1021/acsaem.3c02484. [DOI] [Google Scholar]
  87. Mendhe A., Panda H.. A review on electrolytes for supercapacitor device. Discover Mater. 2023;3:29. doi: 10.1007/s43939-023-00065-3. [DOI] [Google Scholar]
  88. Yunita A., Farma R., Awitdrus A., Apriyani I.. The effect of various electrolyte solutions on the electrochemical properties of the carbon electrodes of supercapacitor cells based on biomass waste. Mater. Today: Proc. 2023;87:246–252. doi: 10.1016/j.matpr.2023.03.102. [DOI] [Google Scholar]

Associated Data

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

Supplementary Materials

ao6c04292_si_001.pdf (1.4MB, pdf)

Data Availability Statement

Data used is available throughout the manuscript text.

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES