INTRODUCTION
The rapid advancement of energy storage technologies has driven considerable interest in developing high-performance electrode materials for supercapacitors 1,2. Among various transition metal oxides, materials such as ruthenium dioxide (RuO2) 3, manganese dioxide (MnO2) 4,5, cobalt oxide (Co3O4) 6,7, and nickel oxide (NiO) 8,9 have been extensively investigated for supercapacitor applications owing to their high specific capacitance and pseudocapacitive behavior. However, issues such as high cost (e.g., RuO2), poor conductivity (e.g., MnO2), or limited cycle life (e.g., NiO and Co3O4) constrain their practical application. In this context, niobium pentoxide (Nb2O5) has emerged as a promising candidate due to its high theoretical capacitance, wide electrochemical window, and excellent structural stability 10. However, the practical application of Nb2O5 in supercapacitors remains limited by its inherently low electronic conductivity and relatively poor ionic diffusion kinetics, which result in suboptimal specific capacitance and rate performance 11.
To overcome these limitations, two principal strategies have been extensively explored: heteroatom doping and thermal treatment (calcination). Doping with transition metals such as molybdenum (Mo), tungsten (W), or vanadium (V) has been shown to effectively modulate the electronic structure of Nb2O5, introduce additional redox-active sites, and enhance electrical conductivity 12–14. In particular, Mo doping can introduce donor states near the conduction band, facilitating faster electron transport and enabling pseudocapacitive charge storage through reversible faradaic reactions 15,16. Furthermore, doping may also influence the material's morphology and crystallinity, promoting more accessible surface area and optimized ion diffusion pathways.
Calcination, on the other hand, plays a critical role in improving the crystallinity and structural integrity of the synthesized Nb2O5. Controlled thermal treatment can reduce structural defects, enhance grain connectivity, and tune porosity, all of which contribute to improved electrochemical performance 17–19. When combined with doping, calcination can synergistically reinforce the beneficial effects by stabilizing dopant distribution and promoting favorable microstructural evolution.
Despite previous efforts, the combined effect of Mo doping and calcination on the microstructure, elemental distribution, and electrochemical performance of Nb2O5 has not been fully elucidated. In this work, we synthesize Mo-doped Nb2O5 via a hydrothermal method followed by calcination and systematically investigate the impact of both modifications on the material's morphology, composition, and supercapacitor performance. The findings provide valuable insights into the structure–property relationships in doped Nb-based oxides and offer a viable strategy for designing advanced electrode materials with enhanced capacitive behavior.
MATERIALS AND METHODS
Materials
High-purity ammonium niobate(V) oxalate hydrate (ANO, C4H4NNbO9·xH2O, 99.99% trace metals basis) and ammonium molybdate tetrahydrate (AMT, (NH4)6Mo7O24·4H2O, 99.98% trace metals basis) were obtained from Sigma-Aldrich, Singapore. Deionized (DI) water was used as solvent during the hydrothermal process.
Preparation of Mo-Nb2O5 Nanoparticles
Mo-doped Nb2O5 nanostructures were synthesized via a hydrothermal method, adapted from previously reported studies 20. In a typical synthesis, 3.4 g of ammonium niobate(V) oxalate hydrate were dissolved in 70 mL of DI water under continuous magnetic stirring at 600 rpm to ensure complete dissolution. Once a clear and homogeneous solution was obtained, 0.3 g of ammonium molybdate tetrahydrate was introduced as the molybdenum source. The resulting solution was transferred into a 100-mL Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 175 °C for 72 hours. Upon completion of the reaction, the precipitate was collected by vacuum filtration, thoroughly washed with deionized water to eliminate residual impurities, and dried in an oven at 80 °C for 6 hours. To improve crystallinity and electrochemical properties, the dried product underwent calcination in ambient air at 500 °C for 2 hours. This final sample was designated as Mo-Nb2O5-500. For comparative analysis, two control samples were also prepared: undoped Nb2O5 subjected to identical calcination conditions (Nb2O5-500), and uncalcined, undoped Nb2O5 (Nb2O5).
Materials Characterizations
The morphological and microstructural features of the synthesized samples were examined using scanning electron microscopy (SEM) performed on a JEOL JSM-6510 microscope equipped with an energy-dispersive X-ray spectroscopy (EDS) detector for elemental analysis and mapping. High-resolution surface imaging was further conducted using field-emission scanning electron microscopy (FESEM) on a JEOL JSM-7610F to assess the finer details of particle morphology and surface texture. Phase composition and crystallographic structure were analyzed using X-ray diffraction (XRD) measurements carried out on a Rigaku SmartLab SE Basic diffractometer equipped with a Cu Kα radiation source (λ = 1.5406 Å). The XRD patterns were recorded over an appropriate 2θ range to identify phase purity and assess the effect of calcination and Mo doping on the crystal structure of the Nb2O5-based materials.
Electrochemical Investigation
Cyclic voltammetry (CV) measurements were conducted to evaluate the electrochemical performance of Nb2O5, Nb2O5-500, and Mo-Nb2O5-500 electrodes. The electrode slurry was prepared by mixing each active material (0.038 g) with PVDF binder (0.0127 g) and acetylene black (0.0127 g), followed by gradual addition of N-methyl-2-pyrrolidone (76 µL total, approximately 10 drops). The mixture was magnetically stirred for 1.5 h to obtain a homogeneous dispersion. Subsequently, 3 µL of slurry was drop-cast onto a glassy carbon electrode (GCE) and dried at 60 °C for 30 minutes to form the working electrode. CV tests were performed in 1 M Na2SO4 electrolyte using a three-electrode configuration: GCE as the working electrode, Ag/AgCl as the reference electrode, and Pt wire as the counter electrode. The measurements were carried out using a XPOW-AX-PST Potentiostat (Nicslab, Indonesia), and the obtained voltammograms were used to derive the effective specific capacitance of each sample 21,22.
RESULTS AND DISCUSSION
The crystalline structures of Nb2O5, Nb2O5-500, and Mo-Nb2O5-500 were analyzed using X-ray diffraction (XRD), with the results presented in Figure 1a–c. The uncalcined Nb2O5 sample (Figure 1a) displays broad and relatively low-intensity peaks that resemble a deformed orthorhombic structure, indicative of poor crystallinity and potential structural disorder typically associated with as-synthesized Nb2O5 from hydrothermal methods 23,24. Such features suggest partial amorphization or the presence of nanocrystalline domains with local lattice distortions. Upon calcination, the XRD pattern of the Nb2O5-500 sample (Figure 1b) reveals significantly sharpened and more intense peaks that match well with the pseudohexagonal Nb2O5 phase (PDF#07-0061) 25. This suggests a substantial improvement in crystallinity, driven by thermal energy promoting atomic rearrangement and long-range structural ordering. The emergence of well-resolved peaks confirms that the calcination process effectively transforms the initially disordered structure into a more ordered crystalline phase.
Further enhancement is observed in the Mo-doped and calcined sample (Figure 1c). Mo-Nb2O5-500 displays even sharper peaks, consistent with the pseudohexagonal Nb2O5 phase and no secondary phases such as MoO3, indicating successful Mo incorporation into the Nb2O5 lattice without inducing phase segregation. Additionally, minor features corresponding to the monoclinic phase (PDF#37-1468) are discernible 26, suggesting a possible coexistence or intermediate phase during lattice reorganization. The preserved pseudohexagonal structure and improved crystallinity are likely facilitated by both Mo doping and calcination. The close ionic radii of Mo6+ and Nb5+ support lattice substitution without significant distortion 27. Overall, the transition from a broad, amorphous-like pattern in Nb2O5 to well-defined crystalline features in Mo-Nb2O5-500 confirms the critical roles of both calcination and doping. The structural evolution toward a highly ordered pseudohexagonal phase is expected to enhance electron mobility, ion diffusion, and ultimately, electrochemical performance.
The Fourier-transform infrared (FTIR) spectra of Nb2O5, Nb2O5-500, and Mo-Nb2O5-500 (Figure 1d) are dominated by bands in the 900–500 cm-1 region, which are assigned to Nb–O stretching and Nb–O–Nb bridging vibrations of NbO6 octahedra, characteristic of Nb2O5-based oxides. The overall shape and positions of these Nb–O-related bands are very similar for all three samples, indicating that neither calcination nor Mo doping alters the primary Nb2O5 framework. This is consistent with previous Mo-doped Nb2O5 work, where doping did not introduce new vibrational features but preserved the Nb2O5 lattice 16. A broad absorption band between 3750 and 3000 cm-1 is observed for the as-prepared Nb2O5, associated with O–H stretching of surface hydroxyl groups and adsorbed water, accompanied by a weaker band near 1630 cm-1 from H–O–H bending. Both features are markedly reduced in Nb2O5-500 and Mo-Nb2O5-500, confirming that calcination at 500 °C effectively removes most physisorbed water and a significant fraction of surface –OH species 29. In the uncalcined Nb2O5, a weak band in the 1400–1500 cm-1 region can be ascribed to C=O vibrations from residual oxalate/organic precursor species, in agreement with previous Mo-Nb2O5 reports 16; this band disappears completely in Nb2O5-500 and Mo-Nb2O5-500, confirming almost complete elimination of these residues after heat treatment 30. Importantly, no new bands attributable to distinct Mo–O vibrations are observed for Mo-Nb2O5-500, and the positions of the Nb–O-related bands remain essentially unchanged, supporting that Mo is incorporated into the Nb2O5 lattice without forming separate MoOx phases and without disrupting the Nb–O framework.
The surface morphologies of Nb2O5, Nb2O5-500, and Mo-Nb2O5-500 samples were examined using scanning electron microscopy (SEM), as presented in Figure 2. All samples exhibit aggregated nanoparticulate structures, with no distinct long-range ordering, indicating that the hydrothermal synthesis route yields similar granular morphologies regardless of post-treatment or doping. Upon calcination at 500 °C, the Nb2O5-500 sample shows a slightly more compact and fused particle arrangement compared to the uncalcined Nb2O5, suggesting enhanced grain connectivity due to thermal sintering. Interestingly, the Mo-Nb2O5-500 sample displays a surface morphology that remains largely similar to the calcined undoped counterpart but exhibits a visibly rougher texture and more irregular particle boundaries 31,32.
The microstructure and elemental composition of the Mo-Nb2O5-500 sample were investigated using field-emission scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (FESEM-EDS), as presented in Figure 3. The SEM images (Figure 3a) reveal a rough, porous morphology composed of aggregated nanoparticles interspersed with thin, rod-like features 33. This hierarchical structure likely results from anisotropic crystal growth facilitated by Mo incorporation during hydrothermal synthesis, followed by morphological restructuring during calcination. Such a morphology is advantageous for supercapacitor applications, as it offers a high surface area and promotes efficient electrolyte penetration and ion diffusion. To examine the spatial distribution of the constituent elements, EDS elemental mapping was performed (Figure 3b). The uniform distribution of oxygen (O), niobium (Nb), and molybdenum (Mo) signals confirms the successful and homogeneous doping of Mo into the Nb2O5 framework, with no evidence of phase segregation or clustering. This uniformity is critical for ensuring consistent electronic properties throughout the material and maintaining structural integrity during electrochemical cycling.
The corresponding EDS spectra and quantitative analysis (Figures 3c–e) further validate the compositional differences among the samples. Both Nb2O5 and Nb2O5-500 show only Nb and O peaks, with stoichiometries close to the theoretical ratio, although slight shifts in O:Nb ratios are observed after calcination, likely due to structural densification and the removal of surface-bound species. In contrast, Mo-Nb2O5-500 exhibits a clear Mo peak, with a measured atomic concentration of 3.84%, confirming successful doping. The concurrent reduction in Nb atomic percentage suggests partial substitution or lattice incorporation of Mo, which can modulate the electronic structure and enhance redox activity.
The cyclic voltammetry (CV) curves of Nb2O5, Nb2O5-500, and Mo-Nb2O5-500 measured at various scan rates (5–50 mV s-1) are presented in Figures 4a–c. Each sample demonstrates markedly different capacitive behavior. The CV curves of pristine Nb2O5 (Figure 4a) exhibit a near-rectangular shape with relatively low current response, indicative of electric double-layer capacitance (EDLC)-dominated behavior with limited faradaic contribution, likely due to its poor electronic conductivity and lack of active redox sites 34,35. In contrast, the calcined Nb2O5-500 sample (Figure 4b) shows a slightly broader CV response, with mild redox features attributed to improved crystallinity and enhanced ion accessibility. The Mo-Nb2O5-500 sample (Figure 4c) demonstrates a markedly larger CV area and more pronounced redox features, highlighting dominant pseudocapacitive behavior. The combination of Mo incorporation and calcination synergistically contributes to the improved capacitive behavior, making Mo-Nb2O5-500 a promising candidate for high-performance supercapacitor electrodes.
The cycling stability of Nb2O5, Nb2O5-500, and Mo-Nb2O5-500 was further assessed over 10 consecutive CV cycles at 10 mV s-1, as depicted in Figures 4d, e, and f, respectively. All three samples demonstrate excellent electrochemical reversibility, with minimal distortion and consistent CV profiles across multiple cycles. The near overlap of the CV curves suggests good structural integrity and stable charge–discharge behavior, even for the uncalcined Nb2O5 sample. This indicates that the synthesized materials possess inherent robustness under repeated electrochemical operation.
The electrochemical performance of the three electrodes was directly compared through cyclic voltammetry (CV) and specific capacitance evaluation, as shown in Figure 5. The CV curves measured at 5 mV s-1 (Figure 5a) clearly show differences in capacitive behavior among the samples. Pristine Nb2O5 exhibits the lowest current response and the smallest CV area, indicating poor charge storage performance, which is consistent with its low crystallinity and absence of dopants. The calcined Nb2O5-500 electrode shows moderate improvement in current density and CV area, reflecting enhanced ion diffusion and conductivity resulting from improved crystallinity after thermal treatment. The Mo-Nb2O5-500 sample demonstrates a markedly larger CV area and more pronounced redox features, highlighting dominant pseudocapacitive behavior. This enhancement is attributed to the synergistic effect of Mo doping and calcination, which together improve surface reactivity, increase the number of redox-active sites, and enhance electron transport within the Nb2O5 framework.
To evaluate the cycling performance of each electrode, the specific capacitance over 10 consecutive CV cycles at 10 mV s-1 was monitored (Figure 5b). All samples exhibit stable cycling behavior with minimal capacitance loss, indicating good electrochemical reversibility. Notably, Mo-Nb2O5-500 retains the highest and most consistent capacitance across cycles, underscoring its superior structural and electrochemical stability 38. The specific capacitance values calculated from CV curves at 5 mV s-1 are summarized in Figure 5c. The pristine Nb2O5 electrode delivers a specific capacitance of 10.9 F g-1, which increases to 21.2 F g-1 after calcination. The Mo-doped and calcined sample achieves a significantly enhanced value of 55.3 F g-1, confirming the effectiveness of combined doping and thermal treatment in enhancing charge storage capacity. These findings reinforce that Mo-Nb2O5-500 is the most promising candidate among the tested materials for high-performance supercapacitor electrodes.
CONCLUSION
In this study, we successfully synthesized Mo-doped Nb2O5 nanostructures via a hydrothermal method followed by calcination and systematically investigated their structural and electrochemical properties for supercapacitor applications. Scanning electron microscopy revealed that all samples maintained a similar aggregated morphology, with the Mo-doped and calcined sample (Mo-Nb2O5-500) exhibiting a rougher and more porous surface, which is favorable for ion accessibility. Elemental mapping and EDS analysis confirmed the homogeneous incorporation of Mo into the Nb2O5 matrix without forming secondary phases. X-ray diffraction analysis showed that the pristine Nb2O5 exhibited features of a deformed orthorhombic structure, while the Mo-Nb2O5-500 sample displayed a pseudohexagonal phase with enhanced crystallinity due to calcination. This structural improvement is associated with better charge transport pathways. Electrochemical measurements demonstrated that both calcination and Mo doping significantly enhanced the capacitive performance of Nb2O5. The Mo-Nb2O5-500 electrode achieved a specific capacitance of 55.3 F g-1 at 5 mV s-1, substantially outperforming pristine Nb2O5 (10.9 F g-1) and calcined Nb2O5 (21.2 F g-1). All samples also exhibited stable cycling behavior, with Mo-Nb2O5-500 maintaining high capacitance and excellent reversibility over repeated cycles. Overall, the synergistic effect of Mo doping and calcination offers an effective strategy to enhance the electrochemical performance of Nb2O5-based electrodes, providing valuable insights for the design of advanced pseudocapacitive materials.