A high-performance supercapacitor electrode based on Zn/V co-doped NiMoO₄: a cation–cation doping strategy

Abstract

Rational design of electrode materials with tailored structural and electronic properties is crucial for the development of high-performance supercapacitors. Here, we report a Zn/V co-doping strategy for NiMoO₄ (ZV-NM), which simultaneously induces abundant oxygen vacancies and a phase transformation from the α to the β phase. The synergistic effects of lattice distortion, enhanced redox activity, and improved electrical conductivity collectively endow ZV-NM3 with extraordinary charge storage capabilities, achieving a remarkable specific capacitance of 1423.1 F g⁻¹ (316.24 mA h g⁻¹) at a current density of 0.4 A g⁻¹. When integrated into an asymmetric supercapacitor device (ZV-NM3/NF//GS), this optimized electrode exhibits a specific capacitance of 101 F g⁻¹ at a current density of 0.4 A g⁻¹. It offers a high energy density of 45.4 Wh kg⁻¹ and a power density of 1.31 kW kg⁻¹, while retaining 90.45% of its capacitance after 10 000 charge/discharge cycles. These results highlight the effectiveness of binary doping in engineering vacancy-rich, phase-optimized transition metal oxides for next-generation energy storage applications.

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1. Introduction

The increasing global energy crisis, coupled with the continued depletion of fossil fuels and associated environmental degradation, is one of the major global issues. In this context, the development of cost-effective and environmentally friendly energy technologies is crucial for mitigating the greenhouse effect and environmental pollution, thereby ensuring eco-balance and long-term sustainability for future generations.^1–3 This highlights the urgent need for the development of innovative and adaptable renewable energy conversion and storage devices. Among various energy storage systems including metal–air batteries,³ fuel cells,⁴ and lithium-ion batteries,⁵ supercapacitors (SCs) are considered highly promising for future technologies due to their fast charge/discharge rates, high power density, extended cycling stability, and environmental friendliness.^6–10 Based on their distinct energy storage mechanisms, electrochemical capacitors are generally classified into electric double-layer capacitors (EDLCs) and pseudocapacitors. The latter typically exhibit higher specific capacitance (SC) owing to fast and reversible faradaic redox reactions at the electrode–electrolyte interface.⁸ However, supercapacitors' inherent limitation in the working voltage window restricts their energy density, posing a challenge for widespread practical applications.^9–11 To overcome this, asymmetric supercapacitors (ASCs) have emerged, merging the advantages of both electrostatic double-layer capacitors and pseudocapacitors to enhance power density, energy density, working voltage window, and cycle stability.^12–15 The development of advanced electrode materials is crucial for improving sophisticated energy storage devices.¹³ To date, a wide range of transition metal sulfides, hydroxides and oxides exhibiting pseudocapacitive or battery-type behavior have been extensively explored.^14–19 In particular, transition metal oxides (TMOs) have garnered significant interest due to their excellent chemical stability and considerable specific capacitance performance.^20,21 Among these, nickel molybdate (NiMoO₄) has attracted extensive research attention as a promising candidate for supercapacitor applications.^18–20 NiMoO₄ offers a high theoretical SC (exceeding 3000 F g⁻¹), is easy to synthesize, environmentally friendly, low cost, and possesses abundant active sites.²¹ Its appeal stems from its tunable morphology, rich d-orbital electronic structure, multivalent electroactive cations (Ni²⁺/Ni³⁺), and the synergistic effects between Ni and Mo species.²² Specifically, nickel (Ni) acts as an excellent redox material, while molybdenum (Mo) significantly contributes to electrical conductivity during reversible faradaic processes in alkaline environments. Despite these advantages, pristine NiMoO₄ electrodes suffer from drawbacks such as marginal electronic conductivity, slow reaction kinetics, and significant volume expansion, leading to suboptimal rate capability and inadequate cycle stability.^22–24 To address these limitations and further enhance the electrochemical performance of NiMoO₄, incorporating suitable metal cations through doping has proven to be an effective strategy.^25–28 For example, Prabhu et al.²⁷ reported Ni_(1−α)Co_(α)MoO₄ nanorods to explore the impact of Co²⁺ ions in boosting the electrochemical performance and cycling stability. Qin et al.²⁸ developed V doped Ni microspheres coated with NiMoO₄/Ni₁₂P₅ with a microspherical structure, which show quick electron transfer and excellent catalytic performance. Unlike conventional single-element doping strategies, this study introduces a dual cation co-doping approach using Zn²⁺ and V⁵⁺ ions to simultaneously tailor the electronic structure and crystallographic features of NiMoO₄ nanorods. The co-doping not only promotes a controlled phase transition from α- to β-NiMoO₄ but also induces synergistic lattice distortions and oxygen vacancies that enhance redox activity and electrical conductivity. Vanadium(V), characterized by multiple valence states and unique 3d orbital electronic configurations,²⁹ and Zn, known for its ability to modify the electronic structure and induce lattice distortion,³⁰ are expected to boost the redox activity and electrical conductivity of the NiMoO₄ matrix, thereby improving energy density and addressing the material's inherently low cycling stability. This work highlights the effectiveness of Zn/V co-doping in potentially enhancing the specific capacitance of NiMoO₄-based electrodes in alkaline media, alongside superior long-term cycling stability and operational voltage. The integration of these dopants also enables the practical fabrication of a high-voltage asymmetric supercapacitor device with remarkable energy and power density performance. These findings provide a new pathway for engineering multifunctional electrode materials through rational cation–cation doping strategies.

2. Experimental section

2.1. Preparation of dual-doped (Zn,V):NiMoO₄ 1D nanorods

Fig. S1 in the SI schematically illustrates the synthesis strategy used in this study. Co-doped Zn–V NiMoO₄ (Zn_x/V_y–N_(1−(x+y))MO) was synthesized via a single-step green hydrothermal method. In this procedure, (1−(x + y)) mmol of Ni(NO₃)₂·H₂O, x mmol of Zn(NO₃)₂·6H₂O, and y mmol of NH₄VO₃, together with 1 mmol of Na₂MoO₄·2H₂O, were dissolved in 30 mL of DI water under vigorous stirring at room temperature to form a homogeneous green transparent solution. The Zn/V co-doped NiMoO₄ samples are denoted as ZV-NM1, ZV-NM2, and ZV-NM3, corresponding to increasing Zn/V doping concentrations (see Table S4 for detailed composition). The resulting precursor solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave, sealed tightly, and subjected to hydrothermal treatment at 130 °C for 6 h. Upon completion, the obtained product was thoroughly washed with ethanol and DI water to remove residual ions and then dried at 60 °C overnight. The final calcination step was carried out at 450 °C for 2 h, yielding the Zn_x/V_y–N_(1−(x+y))MO compound.

3. Results and discussion

3.1. Structural characterization

To investigate the crystal and phase structures of the synthesized materials, X-ray diffraction (XRD) analysis was performed on pristine NM, Z-NM, V-NM, and ZV-NM3 nanorods. As shown in Fig. 1(a), the XRD pattern of the NM sample confirmed the formation of the α-NiMoO₄ phase, consistent with JCPDS card no. 031-0902. The prominent diffraction peaks located at 2θ = 14.32°, 25.35°, 28.85°, 32.36°, 32.85°, and 43.87° correspond to the (110), (−112), (220), (310), (112), and (−330) planes, respectively. The doping strategy serves as an effective approach to induce phase transformation in NiMoO₄ by altering the MoO_x coordination environment, facilitating the transition from the α- to the β-phase.³¹ Upon doping, noticeable changes in the phase structure were observed. In the Z-NM sample, the XRD pattern remained largely similar to that of pristine NM; however, slight shifts in the main peaks were evident. These shifts are attributed to the difference in ionic radii between Zn²⁺ (0.74 Å) and Ni²⁺ (0.69 Å) ions, confirming the successful incorporation of Zn into the NiMoO₄ crystal lattice. Moreover, the appearance of a weak additional peak at 2θ = 26.6°, corresponding to the (220) plane of β-NiMoO₄ (JCPDS card no. 045-0142), indicates the onset of a partial phase transition from the α- to the β-phase.³² For the V-NM sample, doping with vanadium led to subtle modifications in the XRD pattern. Although the overall intensity of the peaks remained comparable to that of NM, a slight sharpening of specific reflections was observed, suggesting improved local ordering rather than an increase in crystallite size. The enhanced intensity of diffraction peaks at 2θ = 26.6°, 27.3°, and 33.8° aligns with the characteristic reflections of the β-NiMoO₄ phase, confirming a more pronounced phase transition. The integrated XRD peak-area analysis of V-NM reveals approximately 49.8 ± 7.4% α-phase and 50.2 ± 7.9% β-phase, demonstrating that vanadium doping significantly promotes the α → β phase transformation. The XRD pattern of ZV-NM3 displayed features consistent with both Zn and V doping. The pattern was highly similar to that of the V-NM sample, with diffraction peaks attributable to both α- and β-NiMoO₄ phases. It is estimated that the ZV-NM3 sample contains approximately 67.0 ± 1.1% α-phase and 33.0 ± 0.92% β-phase within the total crystalline fraction. This indicates the coexistence of the two polymorphs within the co-doped material, as also seen in the V-NM sample. Additionally, the investigation of the XRD pattern of two ZV-NM1 and ZV-NM2 samples indicates consistency with both JCPDS card no. 031-0902 and 045-0142, where the main diffraction plane is (220), as shown in Fig. S2(a).

Fig. 1 (a) XRD and (b and c) Raman shift patterns of pure NM, Z-NM, V-NM and ZV-NM3 electrodes.

Beyond the observed phase transformation, the microstructural properties of the synthesized materials were notably influenced by the doping strategy. The crystallite size (D) and the lattice strain (ε) were determined through the Williamson–Hall (W–H) method.³³ As summarized in Table S2, pristine NiMoO₄ exhibited the largest crystallite size of 103 nm, along with the lowest lattice strain, indicating a well-ordered crystalline structure. Upon individual doping, both Zn- and V-doped samples showed significant reductions in crystallite size to 62 nm and 45 nm, respectively. Zn doping induced a slight increase in strain, likely due to lattice mismatch caused by the substitution of Ni²⁺ with larger Zn²⁺ ions, resulting in moderate lattice distortion. Conversely, V doping led to a further rise in strain, potentially due to enhanced defect formation and oxygen vacancy generation associated with the substitution of Mo⁶⁺ by V⁵⁺. The most significant structural evolution was observed in the ZV-NM3 sample, which exhibited the smallest crystallite size (28 nm) and the highest lattice strain. These observations suggest substantial microstructural disorder arising from the combined effects of dual doping. The significant suppression of crystal growth and the increased lattice distortion are likely attributed to enhanced structural mismatch and the synergistic introduction of defects, which increase the surface area and grain boundaries, thereby exposing more redox-active sites, facilitating faster ion diffusion.^34,35

Raman spectroscopy was employed to investigate the impact of individual doping of Zn and V and simultaneous Zn/V dual doping on the chemical composition and molecular structure modifications in NiMoO₄. As depicted in Fig. 1(b and c), all spectra showed a dominant α-NiMoO₄ phase, particularly symmetric Mo=O at ∼961 cm⁻¹ and asymmetric Mo–O peak stretching at ∼910 cm⁻¹, confirming retention of the MoO₆ octahedral structure across all four samples.³⁶ The Z-NM Raman profile preserved sharp Mo–O bands with an evident red shift, reflecting that Zn²⁺ incorporation mainly causes local lattice strain without significantly disrupting the phase development. Zn doping predominantly affects Ni–O–Mo connectivity without compromising Mo–O structural integrity.³⁵ The presence of vanadium induced significant broadening of the Mo=O peak and enhanced mid-frequency bands (∼493–708 cm⁻¹), suggesting lattice distortion from V⁵⁺ substitution at Mo⁶⁺ sites.³⁷ Weak β-phase features (e.g. peaks at ∼354, ∼367, and ∼827 cm⁻¹) were more pronounced in V-NM, indicating that V promotes partial formation of β-NiMoO₄ through structural perturbation.^38,39 The co-doped sample (ZV-NM3) displayed broadened Mo–O peaks and mid-frequency features combining both V- and Zn-related effects. Notably, β-phase bands were more evident than in Z-NM, implying that V⁵⁺ plays a dominant role in driving β-phase formation, while Zn²⁺ modulates local distortions at Ni²⁺ sites. Moreover, as with V-NM and ZV-NM3, the lack of bands near ∼816 cm⁻¹ affirms that vanadium is integrated into the host lattice rather than forming segregated V–O phases. Notably, the main Raman peaks associated with the α and β phases of dual-doped NM (e.g. ZV-NM1 and ZV-NM2 samples) are observed clearly in Fig. S2(b). These Raman results further confirm the successful dopant incorporation in the NiMoO₄ configuration, demonstrating their impact on the phase development.

The morphologies, EDX, and mapping analysis of the synthesized samples are identified by FESEM analysis. FE-SEM images of NM and ZV-NM3 (Fig. 2(a–d)) show the 1D nanorods of NM. The 1D nanorods are uniform and the morphology remains unchanged after Zn and V doping. Such a type of morphology creates a free space between the rods, facilitating good contact with electrolyte ions and promoting rapid diffusion, resulting in high electrochemical performance.^40,41 As depicted in Fig. 2(e), the mapping analysis of ZV-NM3 shows the uniform distribution of nickel (Ni), molybdenum (Mo), oxygen (O), zinc (Zn) and vanadium (V).

Fig. 2 (a and b) FE-SEM of NM and (c and d) ZV-NM3 at different magnifications, and (e) EDX elemental mapping of ZV-NM3.

Fig. 3 presents the nitrogen adsorption–desorption isotherms (a) and the corresponding pore size distribution (b) for the synthesized materials. The ZV-NM3 composite exhibits a characteristic Type IV isotherm with an H4 hysteresis loop, indicative of a highly porous structure dominated by mesopores. Crucially, ZV-NM3 displays a massive enhancement in adsorption capacity (518.7 m² g⁻¹), while the other samples (V-NM, Z-NM, and NM) show minimal porosity. The BJH pore size distribution (3(b)) confirms this observation, showing a sharp, dominant peak for ZV-NM3 centered in the small mesopore range (estimated at 3-10 nm), suggesting a highly tailored porous network critical for efficient surface-based performance. This exceptionally high surface area and ideal mesopore size distribution provide abundant, accessible active sites and facilitate rapid ion transport, which are essential prerequisites for achieving superior charge storage capacity and rate capability in supercapacitors.^41,42

Fig. 3 (a) Absorption–desorption isotherm of samples and (b) pore size distribution plot.

To investigate the surface electronic structures and chemical states of the synthesized materials, high-resolution X-ray Photoelectron Spectroscopy (XPS) was conducted for NM and ZV-NM3, focusing on Ni 2p, Mo 3d, Zn 2p, V 2p, and O 1s core levels (Fig. 4). The schematic presentation of the simultaneous incorporation of Zn and V, which induces lattice distortion and oxygen vacancies in the NiMoO₄ host material, is provided in Fig. 4(a). Quantitative atomic percentages were determined by normalizing the peak areas with relative sensitivity factors according to eqn (S4). The dopant concentrations are relatively low (1–2 at%), consistent with their role as minor cationic substitutions. Detailed atomic percentages for all elements in each sample are provided in Table S3 of the SI. The relatively low concentrations of Zn and V are consistent with their role as dopants rather than major lattice constituents, supporting the effective cationic substitution without altering the primary NiMoO₄ framework. The broad XPS survey spectrum (Fig. 4(b)) confirms successful incorporation of Zn and V dopants, as evidenced by clear Zn 2p and V 2p signatures alongside preserved Mo, Ni, and O peaks. The formation of a quaternary composite is thus validated. As shown in Fig. 4(c), both samples exhibit two prominent spin–orbit doublets Ni 2p_3/2 and Ni 2p_1/2 alongside associated shake-up satellite peaks. For NM, characteristic peaks at ∼855.7 eV and ∼858.0 eV (Ni 2p_3/2) are assigned to Ni²⁺ and Ni³⁺, respectively. The corresponding Ni 2p_1/2 peaks emerge at ∼873.3 eV (Ni²⁺) and ∼875.5 eV (Ni³⁺), with satellite features at ∼862.0 eV and ∼880.6 eV. The observed spin–orbit separation of 17.6 eV confirms the coexistence of Ni²⁺/Ni³⁺ species, essential for faradaic charge storage.^43,44 Additionally, the enhanced satellite peak intensity and elevated Ni³⁺ content in ZV-NM3 suggest a higher density of redox-active sites, contributing to improved charge transfer dynamics and enhanced pseudocapacitive behavior.⁴⁵Fig. 4(d) presents Mo 3d spectra deconvoluted into Mo⁶⁺ and Mo⁴⁺ doublets, separated by ∼3.1 eV. In pristine NM, the peaks at ∼232.3 eV and ∼235.6 eV are attributed to Mo⁶⁺, while those at ∼233.2 eV and ∼235.3 eV correspond to Mo⁴⁺.⁴⁶ The presence of a faint satellite near the Mo 3d_3/2 feature suggests a shake-up process consistent with ionic Mo–O bonding in the NM pattern. ZV-NM3 exhibits an intensified Mo⁴⁺ signature and reduced satellite peak intensity, pointing to greater electron delocalization and enhanced covalency.⁴⁷ This shift implies increased oxygen deficiency and a more conductive framework that favors charge mobility and redox interactions. Fig. 4(e) details the deconvoluted O 1s spectra, consisting of lattice oxygen (O_L), oxygen vacancies (O_V), and chemisorbed oxygen (O_C). In NM, the dominant component at ∼530.1 eV reflects an ordered crystal structure with few defects. Subordinate peaks at ∼531.3 eV (O_V) and ∼533.1 eV (O_C) suggest limited vacancies and surface adsorption. Conversely, ZV-NM3 exhibits a notable increase in the oxygen vacancy signal (∼531.0 eV) and a corresponding decrease in lattice oxygen intensity. This trend denotes a higher defect concentration and improved conductivity, facilitating enhanced ion diffusion and redox activity.⁴⁸ A slight reduction in chemisorbed oxygen suggests a cleaner, more stable surface. Fig. 4(f and g) illustrate the chemical states of Zn and V in ZV-NM3. The Zn 2p spectrum shows well-resolved peaks at ∼1022.3 eV (Zn 2p_3/2) and ∼1045.4 eV (Zn 2p_1/2), with a spin–orbit splitting of ∼23.1 eV, consistent with Zn²⁺ in a stable oxide environment.³⁰ The V 2p region is deconvoluted into four peaks: V⁴⁺ (at ∼516.8 eV and ∼524.7 eV) and V⁵⁺ (at ∼517.7 eV and ∼525.7 eV). Quantitative analysis reveals a dominant V⁴⁺ contribution, suggesting partial reduction likely driven by substitutional doping and lattice oxygen deficiencies.^49,50 Moreover, XPS analysis was conducted to further evaluate the compositional characteristics of Z-NM and V-NM nanorods, as evidenced in Fig. S3(a–i). The result demonstrates an increased presence of Ni³⁺, Mo⁴⁺, and oxygen deficiency upon vanadium incorporation into the NM configuration. These findings reveal the importance of the induced dopants (Zn and V) in enhancing redox-active sites, promising for the increase of electrochemical performance.

Fig. 4 (a) Schematic illustration of the NiMoO₄ crystal structure before (left) and after (right) Zn/V co-doping, (b) comparative survey of the two samples, (c–e) high-resolution Ni 2p, Mo 3d, and O 1s XPS spectra of NM and ZV-NM3, and (f and g) Zn 2p and V 2p spectra of ZV-NM3 from the ZV-NM3 powder.

Fig. S4(a) illustrates the optical absorption spectra of the four prepared samples. Moreover, the direct optical bandgaps derived from Tauc plots (Fig. S4(b)) reveal a systematic reduction upon single and co-metal doping. The pristine NM shows a bandgap of 2.46 eV, which decreases to 2.28 eV for Z-NM, 2.32 eV for V-NM, and further to 2.26 eV for Zn and V co-doped ZV-NM3, consistent with values reported in the literature^51,52. This bandgap narrowing reflects the introduction of defect states, primarily oxygen vacancies, as confirmed by XPS and Raman analyses. Notably, Zn doping significantly enhances electronic conductivity by facilitating charge carrier mobility through these defect states, while V incorporation further modifies the electronic structure⁵³. The combined Zn–V co-doping thus synergistically improves the electronic conductivity and is expected to enhance the charge storage performance of NiMoO₄ electrodes. Fig. S4(c) presents the Mott–Schottky curve of the pristine NM and the ZV-NM3 electrodes measured in 3 M KOH. Both samples exhibit a negative slope, confirming p-type semiconductor behavior, which is consistent with hole-dominated charge transport in NiMoO₄-based materials^54–56. However, the ZV-NM3 electrode shows a distinctly different slope and intercept compared to the pristine NM, indicating that Zn/V co-doping significantly alters the semiconductor characteristics. The flat-band potential (E_fb), extracted from the x-intercept of the linear region, shifts from 0.167 V (NM) to 0.252 V (ZV-NM3) vs. Ag/AgCl. This positive shift suggests an increased hole concentration and modified band alignment due to the introduction of Zn²⁺ and V⁵⁺/V⁴⁺ species, which is consistent with the formation of additional oxygen vacancies induced by Zn and V co-doping⁵⁷. Additionally, the reduced slope of the ZV-NM3 curve implies a higher acceptor density, which facilitates improved charge transport and enhances electrical conductivity.

3.2. Electrochemical properties and performance evaluation

The electrochemical performance of pristine, individually doped, and co-doped nanomaterials was investigated using a three-electrode aqueous system. Pure and heteroatom-doped NM samples on nickel foam served as the working electrodes, while a platinum wire and an Ag/AgCl electrode functioned as the counter and reference electrodes, respectively. The cell utilized a 3 M KOH aqueous electrolyte, chosen for its high ionic concentration (K⁺ and OH⁻), rapid ion mobility, and excellent ionic conductivity. This electrolyte minimizes electrode/electrolyte interfacial resistance and supports a broad operating voltage range. Additionally, its simple preparation, low cost, and chemical and electrochemical stability make it a practical choice for energy storage applications. The use of 3 M KOH is critical because its abundant OH⁻ ions directly participate in the Ni²⁺/Ni³⁺ and Mo⁶⁺/Mo^x redox reactions, enabling the doped NiMoO₄ electrode to express its full pseudocapacitive behavior. In contrast to neutral electrolytes, KOH provides superior ionic conductivity and stronger surface interactions, ensuring maximum capacitance and allowing the Zn/V co-doped structure to achieve its enhanced electrochemical performance.^58,59 All electrochemical measurements were carried out at room temperature (25 °C), with the data presented in Fig. 5.The cross-sectional FESEM image shows the 3D porous architecture of the nickel foam uniformly coated with the active material with a thickness of 7.5 µm (Fig. S5). The interconnected ligaments and open macropores remain clearly visible, indicating that the coating forms a conformal layer without blocking the foam's intrinsic channels. Fig. 5(a) shows the CV profiles of pure NM, Z-NM, V-NM, ZV-NM1, ZV-NM2, and ZV-NM3 electrodes. These analyses were performed at a scan rate of 4 mV s⁻¹ within a potential window of −0.2 to 0.6 V (vs. Ag/AgCl). Redox peaks observed in the CV curves for all electrodes indicate faradaic reactions driven by OH⁻ ions.⁶⁰ These peaks reflect valence state transitions, suggesting battery-type energy storage behavior. Possible reversible redox reactions involve Ni²⁺/Ni³⁺ and V⁴⁺/V⁵⁺ couples, accompanied by OH⁻ ion insertion/extraction processes. Although molybdenum atoms do not engage in ion exchange, their presence enhances electrical conductivity significantly.^61,62 The redox activity observed in the alkaline electrolyte can be attributed to the following reactions within the NM structure:

NiMoO₄(s) + OH⁻ → NiOOH(s) + MoO₄²⁻(aq) + e⁻ (1)

NiOOH(s) + MoO₄²⁻(aq) + e⁻ → NiMoO₄(s) + OH⁻ (2)

Fig. 5 (a) Comparative CV profiles and (b) specific capacities of NM, Z-NM, V-NM, and ZV-NM3 electrodes at a sweep rate of 4 mV s⁻¹. (c–f) CV performed at different scan rates ranging from 1 to 50 mV s⁻¹ on NM, Z-NM, V-NM, and ZV-NM3.

Analysis of the CV curves reveals that NF, pristine NM, and individually doped NM electrodes exhibit relatively small enclosed CV areas, signifying a limited contribution to overall electrochemical performance. In contrast, the ZV-NM3 electrode displays the largest integrated CV area among all tested samples, clearly indicating its superior specific capacitance. The specific capacitance values, calculated using eqn (S6) and presented in Fig. 5(b), are as follows: NM: 529 F g⁻¹, Z-NM: 467 F g⁻¹, V-NM: 561 F g⁻¹, ZV-NM1: 769 F g⁻¹, ZV-NM2: 825 F g⁻¹, and ZV-NM3: 1175 F g⁻¹.

ZV-NM3 notably outperforms the others, suggesting a greater number of electrochemically active sites. This enhancement aligns with structural and compositional analyses, pointing to a synergistic effect from the optimized incorporation of Ni, Zn, and V dopants. The phase development and distinct oxygen vacancies created by Zn–V co-doping at optimal concentrations provide abundant, accessible active sites for electrolyte ion interactions, as evidenced by Raman and XPS analysis, reinforcing the importance of such cation–cation doping strategies in boosting charge storage capabilities within the NM matrix.⁶³Fig. 5(c–f) further illustrate CV profiles of NM, Z-NM, V-NM, and ZV-NM3 electrodes recorded at varying scan rates (1–50 mV s⁻¹) across a potential window of −0.2 to 0.6 V (vs. Ag/AgCl). The consistent shape of these profiles across different scan rates highlights rapid redox processes and efficient electron/ion transfer kinetics.^22,64 With increasing scan rates, both the integrated area and current density rise significantly while maintaining peak symmetry and reversibility, signs of robust electrochemical activity. Prominent redox peaks are attributed to fast and reversible interactions involving Ni²⁺/Ni³⁺ and V⁵⁺/V⁴⁺ species. At low scan rates, diffusion dominates, allowing more ions to reach the electrode interface, although fewer participate in actual charge transfer. Conversely, at higher scan rates, ion diffusion is limited by time, and redox reactions are mainly governed by quick adsorption/desorption dynamics at the electrode/electrolyte interface.^65,66 Nonetheless, even under rapid cycling conditions, the system maintains high current densities and integral areas, underscoring its excellent rate capability. An essential parameter linking electroactive site density to accessible surface area is the voltammetric charge (Q*), calculated using eqn (S11).^67,68 As expected, Q* increases with decreasing scan rate, reflecting enhanced ion diffusion and more effective interaction between electrolyte ions and electrode surfaces. Among all tested samples, the ZV-NM3 electrode demonstrates a remarkable Q* of 6.27 C cm⁻² at a scan rate of 4 mV s⁻¹, surpassing those of NM (2.82 C cm⁻²), Z-NM (2.49 C cm⁻²), and V-NM (2.98 C cm⁻²). This elevated specific charge confirms that ZV-NM3 hosts a higher concentration of electrochemically active sites than the other materials. The result complements earlier findings and underscores the impact of optimized dual-doping strategies on improving charge storage efficiency in NM-based electrodes.

The relationship between peak current (i) and scan rate (ν) can be described by the equation

where k₁ν and k₂ν^1/2 represent the contributions from capacitive current and diffusion-controlled current, respectively. The parameters k₁ and k₂ represent the slope and intercept, respectively, of the linear fit derived from the plot of i(ν)/ν^1/2versus ν^1/2. Fig. 6(a and b) illustrate the log i versus log ν plots recorded at a potential of 0.4 V for various electrodes. All tested samples show near-linear behavior, and the calculated b values for pristine and single doped electrodes fall between 0.5 and 1.0, suggesting dominantly capacitive mechanisms with partial faradaic effects.⁶⁹ In contrast, the ZV-NM3 electrode displays a distinct trend (pink line in Fig. 6(a and b)), indicating a diffusion-controlled process driven by redox reactions, highlighting its potential for high specific capacitance and energy density. Specifically, the b values for anodic and cathodic peaks of the pure NM electrode are 0.98 and 0.99, respectively, consistent with capacitive behavior. For ZV-NM3, these values are significantly lower, 0.44 (oxidation) and 0.71 (reduction), underscoring the impact of dopants in promoting ionic diffusion through the NM matrix. These findings suggest enhanced energy storage capability facilitated by improved charge transport pathways. By employing the above equation, this analysis provides a robust method for differentiating between surface-controlled and diffusion-controlled processes, offering valuable insights into electrode kinetics. Fig. 6(c–f) illustrate how the total charge storage behavior evolves with scan rate, differentiating between capacitive and diffusion mechanisms. In these figures, the purple regions correspond to capacitive current, while the blue zones indicate diffusion-driven contributions. The data reveal that the dominant charge storage mechanism at lower scan rates is diffusion-controlled, whereas capacitive contributions grow more prominent at higher scan rates. This trend is consistent with the previously discussed b-values derived from CV curves. At elevated scan rates, ions have insufficient time to engage in deep redox interactions within the NM matrix. Instead, they undergo rapid adsorption/desorption alongside superficial redox reactions near the electrode/electrolyte interface. At lower scan rates, however, diffusion processes dominate due to extended interaction time, allowing ions to penetrate deeper into the electrode structure, supporting pseudocapacitive behavior and contributing to high specific capacitance.^70,71 In addition, Fig. S6(a–f) indicate the CV profiles and charge storage mechanism of ZV-NM1 and ZV-NM2 electrodes, which indicates that their anodic b-values are 1 and 0.71 at a potential of 0.4 V, respectively. Therefore, this dual nature suggests that charge storage in the doped electrodes stems primarily from pseudocapacitance, complemented by electric double-layer capacitance (EDLC).⁷² The enhanced performance can be attributed to the synergistic effect of the quaternary metal combination: Ni, Zn, V, and Mo. Ni, Zn, and V improve electrical conductivity and activate redox reactions, while Zn and V promote efficient electron transport and increase the number of electrochemically active sites.

Fig. 6 (a and b) Variation of log(i) versus log(v) for Power's law for anodic and cathodic peaks, and variation of capacitive and diffusion specific capacitance as a function of scan rate of (c) NM, (d) Z-NM, (e) V-NM, and (f) ZV-NM3, respectively.

Galvanostatic charge/discharge (GCD) analysis was employed to evaluate the charge storage capabilities of the synthesized electrode materials in a 3 M KOH electrolyte. Measurements were conducted at a current density of 0.4 A g⁻¹ across a potential window of 0–0.8 V (Fig. 7(a)). Notably, the ZV-NM3 electrode exhibited the longest discharge time, underscoring its superior energy storage capacity, a result consistent with CV profiles. This exceptional performance stems from strategic dual-cation doping of Zn and V into the NiMoO₄ framework, coupled with its 1D nanorod morphology. These compositional and structural modifications significantly enhance the specific capacitance. Fig. 7(b–e) present the GCD curves of NM, Z-NM, V-NM, and ZV-NM3 electrodes at various current densities. Specific capacitance values, calculated from the GCD profiles (using eqn (S10)), are as follows: NM: 13.7 F g⁻¹, Z-NM: 63.3 F g⁻¹, V-NM: 140.4 F g⁻¹, ZV-NM1: 351 F g⁻¹, ZV-NM2: 807.2 F g⁻¹, and ZV-NM3: 1423.1 F g⁻¹ (316.24 mAh g⁻¹).

Fig. 7 (a) Comparison of GCD signals, galvanostatic charge–discharge profiles recorded at different current densities from 0.1 to 1 A g⁻¹ in the potential range of 0.0–0.8 V versus Ag/AgCl of pristine NM (b), Z-NM (c), V-NM (d) and ZV-NM3 (e) electrodes, (f) variation of specific capacitance as a function of current density of the ZV-NM3 electrode, and (g) cycling stability at a current density of 8 A g⁻¹ for NM, Z-NM, V-NM and ZV-NM3 electrodes. (h) Electron transportation and ion diffusion mechanism of the ZV-NM3 electrode.

GCD was also used to select the potential window for all four electrodes. The absence of distortion in these curves indicates that the operating voltage range of 0.8 V is suitable for the prepared electrodes, as shown in Fig. S7(a–d). Fig. 7(f) further explores the influence of current density on Cs for ZV-NM3. At low current densities, electrolyte ions have sufficient time to penetrate the electrode matrix, accessing abundant electrochemically active sites throughout the bulk and surface. This deep ion diffusion facilitates extensive redox activity, thereby maximizing charge storage. Conversely, at higher current densities, ion transport is confined to the outer surface due to time constraints, reducing C_s accordingly.⁷³ Despite these changes, all four electrodes retained the shape of their GCD curves across varying current densities, demonstrating excellent reversibility and highlighting their pseudocapacitive characteristics in alignment with CV analyses. Additionally, the IR drop was minimal (0.005 V), even at elevated current densities, indicating low internal resistance and efficient energy retention.⁷⁴ The optimal Zn/V doping concentration contributes to enhanced electrochemical performance through modified electronic conductivity and accelerated electron transit, which is well in line with structural and chemical characterization. Fig. 7(g) showcases cycle stability data following 10 000 charge/discharge cycles at 8 A g⁻¹. Capacitance retention rates for NM, Z-NM, V-NM, and ZV-NM3 were 74%, 93.91%, 93%, and an impressive 100%, respectively. The decrease in NM performance is attributed to material dissolution and the inherent instability of NiMoO₄, which suffers from mechanical stress and poor conductivity during repeated cycling. In contrast, ZV-NM3's robust stability is credited to its reduced band gap, enhanced ion mobility, and the dynamic redox behavior of Ni and V. Structurally, pristine NM adopts a monoclinic α-phase, where Ni and Mo occupy octahedral sites. Upon dual doping, Ni participates via its octahedral sites, while V contributes through both tetrahedral and octahedral sites, expanding redox possibilities and significantly boosting Cs. The higher density of active sites and optimal doping ratios further elevate the performance of ZV-NM3 above that of pristine and single-doped counterparts. The structural and chemical stability of the ZV-NM3/NF electrode before and after 10 000 GCD cycles was confirmed by FESEM, EDX, and XRD analyses. As depicted in Fig. S8(a and b), FESEM images show that the porous nanorod architecture is largely preserved after cycling, with only slight thickening and partial fusion of the rods, likely due to mild surface reconstruction during repeated redox processes. EDX mapping further verifies that the homogeneous distribution of Ni, Mo, O, Zn, and V remains unchanged, indicating the absence of elemental leaching or segregation. Consistently, the XRD patterns before and after cycling display the same α/β-NiMoO₄ phase signatures without peak shifting, demonstrating that the crystal structure remains intact; only modest peak broadening is observed, reflecting minor surface relaxation rather than bulk degradation. In addition, EIS results support this stability, showing only a small increase in R_s (2.21 to 2.78 Ω); the non-appearance of the semicircle indicates a low interfacial R_ct and a steeper low-frequency slope after cycling, suggesting improved ion transport due to electrode activation. These combined analyses confirm the excellent structural, chemical, and electrochemical durability of the Zn/V-doped NiMoO₄ electrode during long-term operation. Fig. 7(h) provides a schematic of ion/electron transport in ZV-NM3 nanorods. The crystal structure facilitates strong ligand–metal interaction between O 2p orbitals and metal 2p and 3d orbitals, promoting reversible electronic transitions during charging and discharging. This dynamic bonding creates abundant redox-active sites, enhancing both pseudocapacitive behavior and electrochemical reaction rates (Table 1).

Table 1 Comparison of the electrochemical performance with recently reported NiMoO₄-based electrodes

Material	Electrolyte	Current density (A g^−1)	C s (F g^−1)	C s retention (%) with cycles	Ref.
NiMoO4/WO3/NF	2 M KOH	1	429.4	89.9% (10 000)	75
δ-MnO2/NiMoO4/NF	6 M KOH	1	1866.2	96.6% (5000)	76
NiMoO4@CNTs	1 M KOH	2	915.32 C g^−1	—	77
NiMoO4@Co(OH)2	2 M KOH	1	1357	80.66% (5000)	78
NiMoO4 nanoflower	1 M KOH	1	947	94% (6000)	79
WO3/NiMoO4–120	2 M KOH	1	875	90.48% (5000)	80
NiMoO4@Ni0.5Co-MOF/CC	2 M KOH	1	1210	—	20
C@Co9S8/NiMoO4	0.6 M Na2SO4	1	986.3	74.9% (10 000)	81
MNM-160	2 M KOH	0.5	1160.5	93.2% (5000)	82
ZV-NM3//NF	3 M KOH	0.4	1433	100% (10 000)	This work

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To explore the practical applicability of the ZV-NM3/NF electrode, hybrid asymmetric supercapacitor (ASC) devices were fabricated and systematically evaluated. These pouch-type devices employed ZV-NM3 as the positive electrode, a graphite sheet (GS) as the negative electrode, and a PVA/KOH gel electrolyte as the ion-conducting medium. A Whatman filter paper was used as a separator to prevent direct electrode contact. The overall architecture is illustrated in Fig. 8(a). In addition, FESEM analysis (Fig. S10) shows that the graphite sheet consists of overlapping graphite platelets with visible micro-voids and interlayer separations, indicating a porous and non-compact structure that facilitates ion access during charge/discharge processes. Moreover, the electrochemical performance of the negative electrode is also conducted for comparison in a three-electrode system, and the results are shown in Fig. S11.

Fig. 8 (a) Schematic illustration of the fabricated ZV-NM3/NF//GS device, (b) comparative three-electrode CV profiles of the GS and the ZV-NM3/NF electrode, (c) optimization of the CV potential window, (d) diverse sweep rates in the potential window: 0.0 to 1.8 V, (e) GCD curves at different current densities, (f) the stability of the device with inset GCD curves after the 1st and 10000th cycles, (g) LSV plot of KOH/gel electrolyte, and (h) LSV profile of the ZV-NM3/NF//GS solid state device.

Fig. 8(b) compares the CV profiles of ZV-NM3 and GS electrodes within their respective stable voltage windows of −0.2 to 0.6 V (ZV-NM3) and −1.0 to 0 V (GS) vs. Ag/AgCl, at a scan rate of 20 mV s⁻¹. The assembled ZV-NM3/NF//GS device was optimized to operate within a broad potential window of 0.0 to 1.8 V (Fig. 8(c)), exhibiting minimal polarization. However, further expansion to 1.9 V triggers the onset of the oxygen evolution reaction (OER), characterized by gas bubble formation and a sharp rise in current density, which compromises device durability and ideal cell performance. Fig. 8(d) presents CV curves of the ZV-NM3/NF//GS ASC at sweep rates ranging from 3 to 50 mV s⁻¹ within the 0.0–1.8 V window. Increasing sweep rates result in an expansion of the enclosed area, indicating excellent reversibility and predominant capacitive behavior. This confirms that ZV-NM3 paired with GS and PVA/KOH gel electrolyte offers robust charge/discharge dynamics. GCD measurements for the ZV-NM3/NF//GS ASC device (Fig. 8(e)) were performed at current densities from 0.3 to 1.4 A g⁻¹, showing stable operation with a low IR drop across a wide voltage window (0.0–1.8 V). The distorted plateau observed in the GCD profile arises from the dual contribution of electrochemical double-layer capacitance (EDLC) from GS and faradaic processes from ZV-NM3. The highest stored SC of 101 F g⁻¹ is achieved at a current density of 0.4 A g⁻¹. In addition, when evaluating the performance of the device, it is crucial to consider the energy and power densities obtained using eqn (S8) and (S9). At a specific power of 1.31 kW kg⁻¹, the as-designed flexible asymmetric supercapacitor device exhibits a maximum specific energy of 45.4 Wh kg⁻¹. Long-term cycling stability was assessed over 10 000 cycles at 4 A g⁻¹ (Fig. 8(f)), revealing a capacitance retention of 90.45% and a coulombic efficiency maintained at nearly 99%. This impressive durability is attributed to ZV-NM3's intrinsic low resistance and high density of active surface sites, which minimize inter-electrode resistance and preserve electrochemical integrity during prolonged cycling.⁸³ Moreover, the electrochemical stability of the PVA/KOH gel electrolyte was evaluated by linear sweep voltammetry (LSV) in a three-electrode configuration versus Ag/AgCl (Fig. 8(g)). The current remained negligible up to ≈1.7 V, after which a sharp rise associated with water decomposition was observed, confirming an electrochemical stability window of about 1.7 V. The LSV curve of the solid-state supercapacitor (Fig. 8(h)) shows a nearly linear current response up to 1.8 V without any abrupt current increase, indicating the absence of electrolyte decomposition or side reactions. This confirms that the device can safely operate within the 0–1.8 V potential window. Electrochemical impedance spectroscopy (EIS) data before and after cycling in the frequency range of 0.01 Hz to 100 kHz, along with the equivalent circuit model, are presented in the inset of Fig. S11(a). As shown in Table 2, the series resistance (R_s) showed a slight increase from 18.54 to 20.45 Ω, indicating that the intrinsic conductivity was largely retained. The charge-transfer resistance (R_ct) increased moderately by 12.5%, reflecting minor interfacial resistance growth during cycling. The double-layer capacitance (Q₁) decreased by 18.1%, while the pseudocapacitive component (Q₂) declined significantly, likely due to surface reconfiguration or partial loss of active sites. The Warburg impedance, typically observed as a sloped feature in the low-frequency region, reflects the diffusive resistance of OH⁻ ions within the electrode system. The gradual change in slope can be attributed to reversible faradaic redox processes and the progressive insertion of OH⁻ ions into the electrode material.⁸⁴ After cycling, the Warburg resistance (W_0R) increased, indicating higher diffusion impedance and more restricted ion transport. Meanwhile, the W_0P exponent decreased slightly from 0.473 to 0.451, suggesting stronger finite-length diffusion effects, consistent with structural and ionic-transport changes induced during long-term cycling.⁸⁵ These results confirm the electrode's strong structural integrity and stable electrochemical behavior under long-term operation.^35,37

Table 2 Quantitative parameters extracted from fitting of EIS data before and after cycling of the ZV-NM3/NF//GS device

Parameter	Initial value	Value after 10 000 cycles	Unit
R s	18.54	20.45	Ω
R ct	19.67	24.73	Ω
Q 1	0.595	0.425	mF cm^−2
Q 2	3.15 × 10^−4	3.35 × 10^−4	mF cm^−2
W 0R	108.42	281.5	Ω s^−1/2
W 0P	0.473	0.451	—

---

Additionally, flexibility is a crucial factor for portable and compact devices.^86,87 To demonstrate the flexible properties of the prepared device, we tested its CV and GCD curves at various bending angles (0°, 90°, and 180°). The curves remained well-aligned across these angles, showing no significant changes in shape, which indicates the device's excellent flexibility (Fig. S12(b and c)). Fig. S12(d) shows the remarkable electrochemical and mechanical stability of the full supercapacitor device, maintaining 88.97% of its initial capacitance after 10 000 charge/discharge cycles while being subjected to an aggressive 180° bending angle. Collectively, these findings demonstrate that the ZV-NM3/NF//GS hybrid flexible device delivers robust pseudocapacitive behavior, excellent structural stability, prominent flexibility, and promising potential for advanced energy storage applications.

In addition, leakage current plays a crucial role in predicting energy loss in supercapacitors. To comprehensively evaluate both leakage current and self-discharge behaviors, solid-state asymmetric supercapacitors based on Zn, V co-doped NiMoO₄ (ZV-NM3) were fabricated. The leakage current and self-discharge characteristics of the asymmetric devices were investigated under open-circuit conditions at room temperature. As shown in Fig. 9(a), the ZV-NM3/NF//GS device exhibited a low leakage current. The leakage current initially dropped sharply to 0.50 mA within the first 3 h, followed by a gradual decrease to 0.42 mA over the subsequent 21 h, indicating stable and low leakage behavior that favors voltage retention. This suggests that the device exhibits lower leakage current, which is beneficial for maintaining the stability of voltage holding.⁸⁸ After charging to 1.8 V, the self-discharge process was monitored as a function of time for 24 h. The ZV-NM3/NF//GS device exhibited a potential decay from 1.8 V to 0.91 V within 24 h (Fig. 9(b)), indicating that it retained nearly half of its initial voltage over this period. The noticeable voltage drop is associated with the intrinsic self-discharge characteristics of high-voltage supercapacitors. It has been reported that an increase in charging voltage intensifies the self-discharge process due to a larger potential difference between the pore mouth and bottom, which enhances the ion concentration gradient within the electrode structure.⁸⁹ This gradient promotes ion redistribution or desorption from the electrode surface, leading to a gradual potential decay over time. Therefore, the observed self-discharge behavior at 1.8 V is in good agreement with previously reported results and confirms the voltage-dependent nature of this phenomenon.⁹⁰ These results demonstrate that the binary Zn, V co-doping strategy effectively minimizes energy loss in NiMoO₄-based supercapacitors and broadens their potential for practical applications in next-generation energy storage systems.

Fig. 9 (a) Leakage current behavior and (b) the self-discharge curve of the devices over 24 h.

4. Conclusion

In summary, this study presents a successful cation–cation co-doping strategy for enhancing the electrochemical performance of NiMoO₄-based supercapacitor electrodes. Zn and V were co-doped into NiMoO₄ nanorods via a facile hydrothermal method, resulting in significant improvements in structural, electronic, and electrochemical properties. Comprehensive structural analyses confirmed a partial phase transformation from α- to β-NiMoO4, a reduction in crystallite size, increased lattice strain, enhanced electrical conductivity, and the creation of oxygen vacancies, all of which contribute to improved electrochemical activity. Electrochemical testing revealed that the optimally co-doped ZV-NM3 electrode exhibited a remarkable specific capacitance of 1423.1 F g⁻¹ at 0.4 A g⁻¹ current density, with outstanding cycling stability (100% retention after 10 000 cycles). These enhancements are attributed to synergistic effects between Zn and V dopants, which modulate the local electronic environment, increases the number of redox-active sites, and facilitate rapid ion diffusion and charge transfer. Furthermore, an asymmetric supercapacitor (ZV-NM3/NF//GS) demonstrated a high working voltage of 1.8 V, delivering an impressive energy density of 45.4 Wh kg⁻¹ and a power density of 1.31 kW kg⁻¹, along with excellent rate capability and 90.45% capacitance retention after prolonged cycling. These results establish ZV-NM3 as a promising electrode material for next-generation energy storage devices and provide valuable insights into the design of advanced transition metal oxide-based pseudocapacitors through strategic co-doping approaches.

Author contributions

Fatemeh Kazemi Kerdabadi: writing – original draft, validation, investigation, and formal analysis. Parviz Kameli: writing – review & editing, supervision, resources, methodology, investigation, funding acquisition, and conceptualization. Mohamad Mohsen Momeni: writing – review & editing, supervision, resources, methodology, investigation, funding acquisition, and conceptualization. Tapati Sarkar: writing – review & editing, visualization, and conceptualization. Bagher Aslibeiki: formal analysis, writing – review & editing, visualization, and conceptualization.

Conflicts of interest

The authors declare no competing financial interests or personal relationships that could have influenced the work reported in this paper.

Data availability

The data used to support the findings of this study are included in the article and supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d5ta07465g.

Acknowledgements

This work is based upon research funded by the Iran National Science Foundation (INSF) under project no. 40403086. TS and BA gratefully acknowledge Stiftelsen Olle Engkvist Byggmästare (grant no. 214-0346) and the Swedish Research Council (grant no. 2021-03675) for financial support.

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