Facile synthesis of nanorod-like NiCo₂O₄ particles via PVP-assisted hydrothermal processing for high-performance supercapacitors

Summary

Nanorod-like NiCo₂O₄ powders were synthesized for supercapacitor applications. Hydrothermal treatment of oxalate precipitates with different amounts of polyvinylpyrrolidone (PVP) surfactant (0–8 mmol) led to the formation of NiCo₂O₄ particles with rectangular-like to nanorod-like morphologies. Variations in electrochemical properties with changes in PVP amount were evaluated. Due to the nanorod-like morphology of the NiCo₂O₄ powders, the effective surface area increased from 36.3 to 67.7 m² g⁻¹. At an appropriate PVP content, the specific capacitance increased from 981 to 2,717 F g⁻¹ (at a current density of 1 A g⁻¹) with a rate capability of 71.2% by increasing the current density up to 10 A g⁻¹. The contribution of capacitive-controlled process was initially decreased with the addition of low PVP content, but subsequently increased at higher PVP dosage. The supercapacitor fabricated from NiCo₂O₄ and activated carbon, at the current density of 1 A g⁻¹, exhibited 25.4 Wh·kg⁻¹ energy density at 1,428 W kg⁻¹ power density.

Graphical abstract

Highlights

• •Nanorod-like NiCo₂O₄ (NCO) powders were synthesized by proper amount of PVP addition
• •At proper PVP amount the specific capacitance reached 2,717 F g⁻¹ (current density 1 A g⁻¹)
• •Energy & power density of hybrid NCO//activated carbon are 25.4 Wh·kg⁻¹ at 1,428 W kg⁻¹

Chemistry; Engineering; Materials science; Physics

Introduction

Portable electronic devices rely on various electrochemical energy storage systems, such as supercapacitors, batteries, and fuel cells. These storage systems provide rapid charging and discharging capabilities, making them suitable for high-power applications. Consequently, they meet diverse demands ranging from long-term energy supply to short-term power bursts, establishing them as indispensable components in modern portable electronics.^1,2 Batteries are known for their high energy storage capacity, whereas supercapacitors stand out due to their superior power density and significantly longer lifespan in terms of charge-discharge cycles.^3,4 Among these, electrochemical capacitors (ECs) have emerged as one of the most promising energy storage technologies, owing to their unique characteristics, including fast charge-discharge processes, high power density, and excellent cycling stability.^2,5,6

Based on charge storage mechanisms, supercapacitors are generally classified into two main categories: electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. The performance of EDLCs relies on a non-Faradaic process that involves the electrostatic adsorption of electrolyte ions (e.g., OH⁻) at the electrode-electrolyte interface, without electron transfer between the electrode material and the adsorbed ions. In this case, energy storage arises from the formation of an electrical double layer, and the process is purely physical.^7,8,9 The specific surface area of porous electrodes plays a critical role in determining the efficiency of EDLCs. To maximize ion adsorption and enhance the capacitance of electrode materials, high-surface-area materials such as carbon nanotubes, reduced graphene oxide (rGO), graphene oxide (GO), and activated carbon (AC) are widely employed as electrode candidates.^10,11

A larger specific surface area enables greater ion adsorption from the electrolyte, which in turn results in higher specific capacitance.^12,13 The high power density of EDLCs, enabling rapid energy release, originates from the weak physical adsorption of ions on the electrode surface. Another storage technique of electrochemical energy is the reversible faradaic redox reaction of the electroactive materials and the electrolyte by electron subscription. The critical characteristics of pseudo-capacitive materials include hierarchical microstructure, chemical stability, and the ability to exhibit multiple oxidation states.^14,15

The impressive specific capacitance and energy density of pseudocapacitive materials, which exhibit battery-like behavior (including transition metal oxides, hydroxides, nitrides, sulfides, etc.), are attributed to their charge storage mechanisms via redox reactions.^16,17 The electrochemical performance of these redox-active materials is critically dependent on their chemical composition and nanostructural features. Specifically, factors such as elemental constituents, crystal structure, particle size, shape, and surface characteristics play pivotal roles in determining the material’s efficiency for charge storage and transfer processes.^18,19,20

Recently, research focus has shifted toward binary and ternary transition metal oxides as promising electrode materials for supercapacitor applications. These compounds have garnered significant interest due to their advantageous properties, such as superior electronic conductivity and enhanced electrochemical reactivity. These characteristics make them particularly well-suited for high-performance energy storage devices.^21,22,23

Among these, nickel cobalt oxide (NiCo₂O₄), a binary metal oxide with several oxidation states (Ni²⁺/Nⁱ³⁺ and Co²⁺/Co³⁺, Co³⁺/Co⁴⁺), has superior chemical and electrochemical properties, making it suitable for applications in supercapacitors, batteries, sensors, and more.^24,25,26,27 NiCo₂O₄ is advantageous due to its abundance, versatile morphology, excellent chemical durability, and cost-effectiveness.^28,29,30 The electrochemical performance of NiCo₂O₄ is inherently influenced by its electroactive sites, which are determined by key factors including crystallinity, particle morphology (size and shape), and the materials structural porosity.^31,32 Therefore, enhancing the electro-catalytic activity of NiCo₂O₄ by developing a hierarchical morphology with a large active surface area through a cost-effective, simple, and chemical synthesis route is essential.³³

Among synthesis methods, the hydrothermal approach is a popular choice due to its comparatively mild operating conditions, environmental friendliness, and capability to modify shape.^33,34 Furthermore, surfactants can influence morphology by regulating nucleation and growth during hydrothermal treatment.³⁵ They also prevent particle aggregation and limit particle development through steric hindrance.³⁶ Polyvinylpyrrolidone (PVP), a neutral surfactant, can modify the morphology by reducing the surface energy of crystal faces.

In most studies, the molar ratio of nickel to cobalt is maintained at 1:2 to correspond with the stoichiometry of NiCo₂O₄, and oxalic acid is applied in either stoichiometric quantities or slightly in excess to ensure complete precipitation.^37,38

This research, presents an optimized and straightforward synthetic approach for the preparation of NiCo₂O₄ materials, utilizing PVP as a structure-directing agent. The investigation systematically explores the impact of varying PVP concentrations on the crystalline structure, morphological characteristics and electrochemical performance of the resultant NiCo₂O₄ particulates. This comprehensive analysis aims to elucidate the structure-property relationships and optimize the material’s functionality for potential energy storage applications.

Results and discussion

Structural characterization

The obtained phase identifications of the intermediate products in P0, P2, P4, and P8 samples after hydrothermal treatment are presented in Figure 1A. The precipitates display an orthorhombic β-(Ni,Co)C₂O₄·2H₂O phase (PDF2#00-025-0581), coexisting with some organic impurities. Increasing PVP content correlates with enhanced peak intensity and reduced peak width, indicating improved crystallinity without changes in the phase constituents.

Figure 1.

XRD results

Diffraction patterns of (A) the intermediate products obtained after hydrothermal treatment (♥: (Ni, Co)C₂O₄·2H₂O) and (B) the as-calcined NiCo₂O₄ powders.

Previous research indicates that PVP plays a role in impacting the nucleation and crystal growth during hydrothermal synthesis.^39,40 Consequently, the observed enhancements in peak intensity and the reduction in peak width in the samples may be attributed to greater crystallinity, rather than modifications in phase composition.

Figure 1B presents the XRD patterns of the calcined powders at 450°C. The reflections corresponding to the cubic spinel structure of the NiCo2O4 phase (PDF2#00-073-1702) are noted at positions (111), (220), (311), (222), (400), (422), (511), and (440), with a space group of Fd-3m. According to the JCPDS reference card (PDF2#00-073-1702), the (422) diffraction peak should be present, but in many published works, this peak is much less intense compared to the stronger reflections such as (111), (220), (311), (400), and (511). For instance, in sample P2, the (422) peak has been observed but is typically very faint. As a result, the lack or the extremely weak appearance of this reflection in the current samples is likely due to the small particle size and the intrinsically low intensity of the (422) peak, making it difficult to distinguish from background noise and obscuring weaker signals. Additionally, the (111) peak was missing in sample P2 but became apparent in samples P4 and P8. This behavior may be explained by the effect of PVP on preferential crystallographic growth: with low PVP amounts, growth in certain directions may be inhibited, reducing some peak intensities, while larger amounts can enhance the visibility of those reflections again.

There are no additional peaks in the diffraction patterns, indicating the absence of impurity phases in the sample. Formation of the NiCo₂O₄ phase at low calcination temperatures is likely due to the homogeneous cation distribution associated with the crystallization of the mixed oxalate (NiCo₂(C₂O₄)₃.6H₂O) precipitates.⁴¹

Upon oxidizing the oxalate precipitates during calcination, the NiCo₂O₄ phase is formed according to the reaction as follows⁴²:

$$NiC{o}_2{\left(C_2{O}_4\right)}_3.6H_{2}O+2O_2\to NiC{o}_2{O}_4+6C{O}_2+6H_{2}O$$ (Equation 1)

The PVP molecules can modify the oxalate precipitation by affecting the surface energy of the precursor solution.⁴³ The increase in peak intensity and the narrowing of peak width observed with higher PVP content could be related to enhanced crystallinity. PVP may function as a structure-directing or capping agent, influencing the nucleation and growth processes during hydrothermal synthesis to improve crystal quality.⁴⁴

Morphological studies

Scanning electron microscopy (SEM) micrographs of NiCo₂O₄ powders prepared with varying amounts of PVP are shown in Figure 2. In sample P0 (which contains no PVP), the NiCo₂O₄ powders consist of large, rectangular particles. However, with the addition of PVP, these rectangular particles transform into rod-like particles that are randomly arranged. By adding surfactant, the P0 sample, which had diameters in the range of 150–650 nm and length of about 1 μm, was transformed into rods with diameters in the range of 200–350 nm and a length of 2.5–3 μm in P4. In the oxalate precipitates, two C₂O₄²⁻ ions bond with Co²⁺ and Ni²⁺ cations in a square planar arrangement. As a result, the oxalate precipitates exhibit a rectangular morphology, which persists even after the calcination treatment.⁴⁵ The non-ionic PVP surfactant acts as a hard barrier, causing the precipitates to develop preferentially around the compact PVP molecules. Therefore, the PVP molecules are responsible for the preferential growth of the precipitates, leading to the rod-like morphology due to spatial confinement (Figure 3). A comparison of the oxalate precipitates and the calcined powders (Figure S1) shows that the calcination treatment preserves the original morphology. However, the calcined NiCo₂O₄ powder exhibits porosity due to the release of large amounts of gases produced by combustion of residual organic compounds during calcination.⁴⁶ The excessive PVP content in sample P8 likely altered the nucleation and growth dynamics during the hydrothermal process, resulting in denser and more aggregated structures. This morphological change reduces the number of accessible electroactive sites, which can negatively impact ion transport and the material’s charge storage capabilities.

Figure 2.

SEM micrographs

SEM micrographs of (a1 [5 μm], a2 [1 μm]) P0, (b1 [5 μm], b2 [1 μm]) P2, (c1 [5 μm], c2 [1 μm]) P4, and (d1 [10 μm], d2 [2 μm]) P8 samples.

Figure 3.

The effect of PVP on the morphological evolution of the NiCo₂O₄ powders

Figure 4 presents nitrogen gas (N₂) physisorption data for the (P0) and (P4) nickel cobalt oxide materials: (a) adsorption and desorption isotherms and (b) pore diameter distribution analyses. The observed isotherms exhibit characteristics of type IV behavior with an H3 hysteresis loop. This pattern suggests the presence of mesoporous structures formed by the loose agglomeration of NiCo₂O₄ nanoparticles.^47,48 With the addition of PVP, the effective surface increases from 36.3 m² g⁻¹ to 67.7 m² g⁻¹. The pore volumes for the P0 and P4 samples are 0.14 cm³ g⁻¹ and 0.31 cm³ g⁻¹, respectively. The increased pore volume in the P4 sample demonstrates the beneficial role of the gaseous products released during calcination. Both the P0 and P4 samples are mesoporous, by pores with the mean sizes of 16.0 nm and 18.4 nm, respectively. The highly porous architecture of the NiCo₂O₄ particles confers two key advantages. It creates an abundance of redox-active surface sites, promoting efficient electron transfer reactions; and the second one, it enhances electrolyte penetration and ion diffusion throughout the material’s structure. These synergistic effects culminate in superior electrochemical performance of the electrode material.

Figure 4.

BET analysis

BET analysis (A) nitrogen gas (N₂) physisorption isotherms depicting adsorption and desorption behavior; (B) pore diameter distribution analyses for both P0 and P4 electrode materials.

Electrochemical performance in three-electrode configuration

Figure 5A is the cyclic voltammetry (CV) tests at the scan rate of 5 mVs^–1 for the samples in the potential range of 0–0.5 V. When the NiCo₂O₄ nanoparticles are subjected to an alkaline environment, they undergo the following redox reactions^49,50: NiCo₂O₄ undergoes reversible electrochemical oxidation in the presence of hydroxide ions (OH⁻) and water (H₂O), forming nickel oxyhydroxide (NiOOH) and two equivalents of cobalt oxyhydroxide (CoOOH), accompanied by electron release:

$$NiC{o}_2{O}_4+O{H}^{-}+H_{2}O\rightleftharpoons NiOOH+2CoOOH+e^{-}$$ (Equation 2)

Figure 5.

Electrochemical performance of the electrodes

(A and B) Cyclic voltammetry curves for various electrodes, (B) illustrates the relationship between specific capacitance and scan rate.

(C and D) Cyclic voltammetry curves for the P4 electrode at several scan rates; (D) presents the plot of ln(i_p) versus ln(ν).

(E–G) A comparison of the current response between capacitive and diffusion-controlled processes for the P4 electrode; (F) highlights the contribution ratio as a function of PVP content and (G) schematic illustration of ion transport and charge storage mechanisms in the nanorod electrode. Blue arrows on the rod surfaces represent surface adsorption of ions (capacitive-controlled), while longer black arrows penetrating into the nanorod interiors indicate diffusion-controlled Faradaic processes.

The CoOOH further participates in a subsequent oxidation step with hydroxide ions (OH⁻), producing cobalt dioxide (CoO₂) and H₂O, with an additional released electron:

$$CoOOH+O{H}^{-}\rightleftharpoons Co{O}_2+H_{2}O+e^{-}$$ (Equation 3)

The oxidation peak observed in the anodic scan represents the transformation of nickel from its divalent (Ni²⁺) to trivalent (Ni³⁺) state and cobalt from its divalent (Co²⁺) to trivalent (Co³⁺) and tetravalent (Co⁴⁺) states. Conversely, the reduction peak in the cathodic scan signifies the reverse process, where tetravalent cobalt (Co⁴⁺) is reduced to its trivalent (Co³⁺) and divalent (Co²⁺) forms, and trivalent nickel (Ni³⁺) reverts to its divalent (Ni²⁺) state.⁵¹ The redox transitions of divalent (Ni²⁺) to trivalent (Ni³⁺) and trivalent (Co³⁺) to tetravalent (Co⁴⁺) states occur at adjacent potentials; hence, only one redox peaks pair is detectable. The presence of redox peaks indicates that NiCo₂O₄ behaves as a battery-type material.⁵² The P4 sample exhibits the largest area, corresponding to the highest specific capacitance (1,711 F g⁻¹) at a scan rate of 5 mV s⁻¹. Figure 5B compares the specific capacitance at various scan rates, revealing that the P4 sample has the maximum specific capacitance across all rates. Figure 5C shows the CV graphs of the P4 sample at different scan rates of 1–5 mV s⁻¹. It is observed that at a scan rate of 1 mV s⁻¹, the specific capacitance reaches up to 1,978 F g⁻¹.

The charge storage mechanism is characterized using the power law relationship, expressed as i = aυ^b, where i represents the measured current response; υ denotes the applied potential sweep rate; and (a and b) are empirical parameters. These parameters (a and b) are derived from the linear regression of the natural logarithm of peak current (ln(i_p)) versus the natural logarithm of sweep rate (ln(υ)). The b value ranges from 0.5 to 1: diffusion-controlled behavior during electron sharing for faradaic redox reactions corresponds to b-value of 0.5, while b value of 1 indicates capacitive behavior due to the physical adsorption of electrolyte ions (OH⁻).⁵³ As illustrated in Figure 5D, the calculated b values (0.64–0.74) suggest a mixed mechanism involving both diffusion-controlled and capacitive-controlled charge storage processes. The relative contributions of these mechanisms—capacitive (denoted by K₁υ) and diffusion-governed (K₂υ⁰·⁵)—can be determined quantitatively using the following relationships⁵⁴:

$$\mathrm{i}={\mathrm{K}}_1\mathrm{\upsilon }+{\mathrm{K}}_2\mathrm{\upsilon }{}^{0.5}$$ (Equation 4)

The determination of coefficients K₁ and K₂ involves plotting the normalized current (i/υ⁰·⁵) against the parameter (K₁υ⁰·⁵ + K₂), followed by linear regression analysis to quantify their respective contributions. Figure 5E illustrates the distinctive current responses associated with diffusion-governed and capacitive charge storage mechanisms for the P4 electrode composition. The summary of the energy storage mechanisms at a scan rate of 5 mV s⁻¹ for the synthesized electrodes (Figure 5F) shows that the P0 electrode is mainly governed by capacitive-controlled storage. For the P2 electrode, the contribution from diffusion-controlled mechanisms increases to 27%, but it declines sharply to 9% in the P8 electrode. As the PVP content rises, capacitive-controlled processes dominate due to increased specific surface area. However, for P8, the decrease in diffusion-controlled contribution and dominant capacitive behavior likely result from the denser, more aggregated morphology seen in SEM images, which restricts ion diffusion. Thus, excessive PVP alters the microstructure, causing a shift in the electrode’s charge storage mechanism.

Figure 5F shows that the capacitive contribution is greatest for P0, decreases for P2, and rises again for P4, implying that P4 achieves a more balanced combination of capacitive- and diffusion-controlled mechanisms. This balance is advantageous as it allows for both rapid charge/discharge kinetics and improved energy storage capacity. Similar findings have been reported in NiCo₂O₄ nanostructures, where controlling morphology creates structures that facilitate efficient ion diffusion alongside abundant surface redox reactions, thereby optimizing electrochemical performance.^49,55

A schematic inset (Figure 5G) has been incorporated to more effectively illustrate the fundamental electrochemical processes. Surface-limited adsorption of electrolyte ions takes place directly on the outside surfaces of the rods in the nanorod architecture (capacitive-controlled process). On the other hand, diffusion-controlled Faradaic reactions are made possible by ion penetration into the porous interiors and inter-rod gaps. The morphology and porosity of nanorods have a significant impact on how these two mechanisms balance out.

Figure 6A depicts the Galvanostatic charge-discharge (GCD) plots of the samples P0, P2, P4, and P8, measured over a potential range of 0–0.4 V against an Ag/AgCl reference electrode at a constant current density of 1 A g⁻¹. Two distinct voltage plateaus are visible between 0.22 and 0.35 V, which correspond to the returnable redox reactions that occur during the charge and discharge cycles, thereby confirming their battery-type behavior. The specific capacitance values for the P0, P2, P4, and P8 samples are determined as 981, 1,836, 2,717, and 960 F g⁻¹, respectively, at a current density of 1 A g⁻¹. Figure 6B displays the GCD curves for the P4 electrode at different current densities. The P4 sample-specific capacitance decrement from 2,717 to 1,936 F g⁻¹, with current density increment to 10 A g⁻¹ results in rate capability of 71.2%. This specific capacitance drop at higher current densities arises from insufficient time for diffusion-controlled processes governing Faradaic redox reactions to fully occur. The GCD curves remain stable without distortion, which demonstrates the highly reversible redox reactions facilitated due to the high effective surface provided by the porous microstructure. Figure 6C illustrates the specific capacitance and current density relationship across the samples, revealing that sample P8 has the lowest rate capability at 57.6% (Figure 6D), where the specific capacitance drops from 960 to 553 F g⁻¹ with the current density increment to 10 A g⁻¹. The inferior performance of P8 can be attributed to its denser morphology, likely due to reduced ion diffusion pathways, according to SEM images. This is in consistent with the reduced diffusion-controlled contribution observed in Figure 5F.

Figure 6.

Diagrams of electrochemical performance consisting

(A) GCD curves of P0 to P8 electrodes (current density of 1 A g⁻¹); (B) GCD curves for the P4 electrode across a range of current densities (1–10 A g⁻¹); (C) plots of specific capacitance against current density; (D) a comparison of the rate capabilities of various electrodes; and (E) comparison of the rate capability of the P4 electrode with previously reported materials.

Figure 6E provides a comparison of the rate capability of the P4 electrode with that of previously reported NiCo₂O₄-based electrodes (NiCo₂O₄,⁴⁹ NiCo₂O₄/RGO,⁵⁶ NiCo₂O₄,⁵⁷ NiCo₂O₄/vanadium,⁵⁸ NiCo₂O₄/graphene,⁵⁹ and NiCo₂O₄/NiO/Co₃O₄⁶⁰).

Figure 7A features Nyquist plots along with corresponding fitting circuits. At elevated frequencies, the Nyquist plot exhibits a characteristic semicircular arc. In the low-frequency part, a diagonal line is visible. At high frequencies, the x axis intercept of the plots corresponds to Rs, which encompasses the combined resistances of the electrolyte, the intrinsic materials, and the interface between the electroactive materials and the current collector. The Rs value remains relatively consistent for all electrodes. The charge-transfer resistance (Rct) is represented by the diameter of the semicircle, reflecting the impedance associated with the Faradic redox reaction.⁶¹ The inclined line is due to Warburg impedance (W₀), which arises from the diffusion of electrolyte ions, specifically OH^-.⁶² An electrode with a smaller Rct value indicates a faster charge/discharge response and a higher rate capability. The PVP-assisted NiCo₂O₄ samples exhibit lower Rct values (as shown in Figure 7B) which is due to the rod-like morphology, inducing the shorter diffusion path and the higher electroactive sites.

Figure 7.

Electrochemical impedance spectroscopy analysis

(A and B) Nyquist plots of samples along with its corresponding equivalent circuit model, (B) quantitative resistance parameters derived from circuit fitting.

Device-level evaluation (two-electrode configuration)

Using the P4 material as cathode and AC as anode, a hybrid capacitor was fabricated to evaluate real-world performance. Figure 8A compares the CV profiles of both electrodes measured in a three-electrode configuration. AC electrode stores charge through electrostatic adsorption on its surface, a mechanism known as the electrochemical double-layer effect, resulting in a rectangular CV curve. Conversely, the P4 electrode exhibits two asymmetric oxidation-reduction peaks, indicating its battery-type behavior. Figure 8B demonstrates CV analysis across progressively expanded voltage ranges (1.0–1.6 V) at 20 mV s⁻¹, with 1.5 V identified as the optimal operational limit to prevent water decomposition observed at elevated potentials. Figure 8C corroborates this threshold through GCD testing at 1 A g⁻¹, verifying complete discharge functionality up to 1.5 V while confirming instability at 1.6 V. Thus, the maximum potential window is established at 1.5 V, as discharge does not occur at 1.6 V. Figures 8D and 8E present rate capability analyses through CV and GCD techniques. CV profiles (Figure 8D) maintain quasi-rectangular shapes across increasing scan rates, indicating robust redox reversibility at the electrode interface. Peak position evolution is observed, with anodic (oxidation) peaks shifting positively and cathodic (reduction) peaks shifting negatively at elevated scan rates. GCD curves (Figure 8E) demonstrate consistent profiles across various current densities. The GCD curves exhibit symmetric triangular profiles, characteristic of capacitive energy storage. This shape strongly indicates the dominance of electrochemical double-layer capacitance. The curves display minimal IR drop, suggesting low internal resistance in the hybrid capacitor system. The specific capacitance of NiCo₂O₄//AC device at current densities of 1 A g⁻¹ and 10 A g⁻¹ reaches to 40.6 F g⁻¹ and 28.9 F g⁻¹ respectively. Figure 8F presents the Ragone plot of the fabricated NiCo₂O₄//AC device (energy density versus power density) in comparison with previously reported NiCo₂O₄-based electrodes. The NiCo₂O₄//AC device has an electrochemical energy storage capacity of 25.4 Wh·kg⁻¹ and a power density of 1,428 W kg⁻¹ at the low current density of 1 A g⁻¹. However, with the current density increment to 10 A g⁻¹; a lower energy storage of 18 Wh·kg⁻¹ can be achieved at a greater power density of 7,200 W kg⁻¹.

Figure 8.

Electrochemical performance analysis

Electrochemical performance analysis consisting (A) the cyclic voltammetry of AC and P4 electrodes at 2 mV s⁻¹, (B) CV stability assessment across 1–1.6 V potential windows, (C) Galvanostatic charge-discharge (GCD) profile evaluation, (D) scan-rate-dependent of CV curves (5–100 mV s^–1), (E) current-density-dependent of GCD response (1–5 A g⁻¹), (F) Ragone plot performance comparison (energy vs. power density), and (G) cycling performance of the NiCo₂O₄//AC capacitor at 10 A g⁻¹ current density.

This representation not only facilitates comparison of energy and power densities but also emphasizes the curvature of the Ragone plot, thereby highlighting the operational range of our system. While absolute values differ among reports due to variations in synthesis routes, morphologies, and composite structures, the present plot situates our results within the general performance window rather than aiming for one-to-one comparisons. The performance of the NiCo₂O₄//AC capacitor is compared with previously reported devices (NiCo₂O₄,⁴⁹ NiCo₂O₄/RGO,⁶³ NiCo₂O₄/NiO/Co₃O₄,⁶⁰ NiCo₂O₄/vanadium,⁵⁸ NiCo₂O₄/NiCo₂S₄/rGO,⁶⁴ NiCo₂O₄,⁶⁵ and NiCo₂O₄⁶⁶). One crucial factor contributing to the satisfactory performance of PVP-assisted NiCo₂O₄ powders is their porous morphology combined with an elevated effective surface. The cycle performance of the NiCo₂O₄//AC asymmetric capacitor at a current density of 10 A g⁻¹ demonstrates gradual capacity retention over extended cycling (Figure 8G). The device retains approximately 92% of its primary specific capacitance after 650 cycles of charge and discharge. This stability reflects efficient ion diffusion and structural integrity during redox reactions. However, capacitance retention declines further with prolonged cycling, reaching ∼70% of the initial value by the 2000th cycle.

Finally, Nickel and cobalt nitrate compounds were combined with oxalic acid to form crystalline oxalate salts, which were then subjected to high-pressure water vapor treatment to create NiCo₂O₄ powder material, followed by calcination. The inclusion of PVP surfactant significantly altered the powder morphology, transforming large rectangular particles into narrow rod-like structures averaging 200–350 nm in diameter and 2.5–3 μm in length by adding 4 mmol of PVP. This morphological modification substantially enhanced material characteristics, increasing effective surface from 36.3 m² g⁻¹ to 67.7 m² g⁻¹ and expanding total pore volume from 0.14 cm³ g⁻¹ to 0.31 cm³ g⁻¹. These changes were attributed to gas evolution during calcination. CV analysis revealed a hybrid charge storage mechanism in the rod-like NiCo₂O₄ powder, combining capacitive and diffusion-controlled processes. The material demonstrated exceptional electrochemical performance with a peak specific capacitance of 2,717 F g⁻¹ at 1 A g⁻¹ and maintained 71.2% capacitance retention. When paired with activated carbon in a capacitor configuration, the system achieved an energy density of 25.4 Wh·kg⁻¹ at 1,428 W kg⁻¹ power density, showcasing its potential for high-performance energy storage applications.

Limitations of the study

Due to limitations in the equipment available to us at this time, we were unable to conduct additional materials characterization, such as X-ray photoelectron spectroscopy and transmission electron microscopy analyses.

Resource availability

Lead contact

Further information and requests for resources should be directed to the lead contact, Somaye Alamolhoda (alamolhoda@iust.ac.ir).

Materials availability

The only material generated in this study was NiCo₂O₄ which was prepared in our laboratory according to the experimental section.

Data and code availability

• •This article does not report any original code.
• •Any additional information required to reanalyze the data reported in this article is available from the lead contact upon request.
• •No additional resources are reported.

Acknowledgments

Authors are gratefully appreciating the Iran University of Science and Technology for supporting this work.

Author contributions

S.P.G.: data curation, formal analysis. and writing – original draft. S.A.: supervision, conceptualization, and writing – review & editing. S.M.M.: supervision, conceptualization, and writing – review & editing.

Declaration of interests

The authors declare no competing interests.

STAR★Methods

Key resources table

REAGENT or RESOURCE	SOURCE	IDENTIFIER
Chemicals, peptides, and recombinant proteins
Nickel(II) nitrate hexahydrate	Merck	13478-00-7
Cobalt(II) nitrate hexahydrate	Merck	10026-22-9
Oxalic acid	Merck	144-62-7
Polyvinylpyrrolidone (PVP)	Merck	9003-39-8
Hydrochloric acid	Merck	7647-01-0
Polyvinylidene fluoride (PVDF)	Alfa Aesar	24937-79-9
N-methyl-2-pyrrolidone (NMP)	Merck	872-50-4
Potassium hydroxide	Merck	1310-58-3

For the synthesis of NiCo₂O₄, nickel (II) and cobalt (II) nitrates [Ni(NO₃)₂·6H₂O and Co(NO₃)₂·6H₂O], along with oxalic acid [H₂C₂O₄·2H₂O] and polyvinylpyrrolidone [(C₆H₉NO)_n, MW = 3000], were obtained from Merck.

Method details

Synthesis route

The synthesis process began with the preparation of an aqueous precursor solution of nickel (II) nitrate (2.0 mmol) and cobalt (II) nitrate (4.0 mmol) as metal salt components. 30 mL of deionized water was used as solvent.

Polyvinylpyrrolidone (PVP) was used as the structural guiding agent at concentrations of (0 (for control), 2, 4, and 8 mmol). Additionally, 15.0 mmol of H₂C₂O₄·2H₂O was dissolved in 30 mL of water. The aqueous precursor solution of metal nitrates was then added dropwise to the oxalic acid solution over a period of 15 minutes. For the hydrothermal treatment, the resulting oxalate precipitates were placed in an autoclave at 120°C for 3 hours. After drying, calcination of the precipitates was performed in air at 450°C for 1 hour. The samples were labeled P0 to P8 based on the amount of PVP used (0 to 8 mmol).

The nickel (II) nitrate to cobalt (II) nitrate molar ratio was established at 1:2, aligning with the composition of NiCo₂O₄. Oxalic acid (H₂C₂O₄·2H₂O) was introduced in an amount sufficient to fully precipitate the combined nickel and cobalt ions as oxalates. Comparable precursor proportions and precipitation techniques have been routinely utilized in earlier research focused on NiCo₂O₄ nanostructure synthesis.^49,67,68 Thus, in this study, the main variable examined was the PVP concentration, while the precursor ratios were kept unchanged according to common methodologies reported in the literature.

Materials characterization

Phase identification of the samples was carried out using X-ray diffraction (XRD) on a D8 ADVANCE diffractometer (Bruker, Japan) equipped with Cu Kα radiation (λ = 1.54060 Å), with a step size of 0.04° and a dwell time of 0.5 s per step. Particle morphology and size were examined using a Vega II TESCAN scanning electron microscope (TESCAN, Czech Republic). Nitrogen adsorption–desorption isotherms of the degassed samples were measured at 250°C for 5 h using a Tristar 3000 analyzer (Micromeritics, USA).

Electrochemical measurements

Electrochemical characterization was conducted using a Radstat 10A system (manufactured by Kianshardanesh, Iran). This versatile instrument facilitated multiple analytical techniques, including: Cyclic voltammetry (CV), Galvanostatic charge-discharge (GCD) measurements, Electrochemical impedance spectroscopy (EIS). These complementary methods provided comprehensive insights into the electrode materials’ electrochemical behavior and performance characteristics.

The experimental configuration utilized a standard three-electrode setup, comprising:

• •Reference Electrode: Silver/silver chloride (Ag/AgCl)
• •Counter Electrode: Platinum wire (Pt)
• •Working Electrode: The material under investigation

This setup enabled precise control and measurement of electrochemical reactions during the experiments.

An aqueous solution of KOH (2 M) served as the electrolyte. The slurry for preparation of working electrode consists of 75 wt% active material along with 15 wt% carbon black, and 10 wt% polyvinylidene fluoride (PVDF) that were dissolved in approximately 1 mL of N-methyl-2-pyrrolidinone (NMP). After 4 hours of ultrasonication, the slurry was coated onto cleaned nickel foam (1×1 cm²) and dried at 80°C for 24 hours, resulting in a loaded mass of approximately 1-2 mg·cm^-2.

An asymmetric capacitor was also assembled to evaluate the energy and power densities of the P4 powders. The negative and positive electrodes consist of Activated carbon (AC) and P4 powders respectively. A filter paper (separator) was placed between the two electrodes. A few drops of aqueous solution of 2 M KOH used as the electrolyte.

Analytical formulae

Cyclic voltammograms were recorded within a potential window of 0–0.5 V (vs. Ag/AgCl reference electrode) at various potential sweep rates. The specific capacitance (C_s) of the electrode materials was quantified by the integral equation as follows:

$$C_s=\frac{\int idV}{2m\vartheta \Delta V}$$ (Equation 5)

Where, i is the measured current response (A), ϑ denotes the applied potential sweep rate (V.s^-1), ΔV defines the operational potential window (V) and m signifies the active material mass loading (g).

Galvanostatic charge-discharge (GCD) profiles were recorded within an operational voltage window of 0–0.4 V (vs. Ag/AgCl reference electrode) under applied current densities spanning 1–10 A g^-1. Quantitative analysis of charge storage capacity was performed using the fundamental relationship derived from these GCD measurements:

$$C_s=\frac{I\Delta t}{m\Delta V}$$ (Equation 6)

The variables in the equation represent

I: The applied constant current during both charging and discharging processes (measured in amperes, A).

Δt: The time interval required for complete discharge of the electrode (measured in seconds, s).

Energy and power densities were calculated based on the potential window (ΔV), specific capacity (Cs), and discharge time (Δt), using the following formulas:

$$Energydensity\left(E\_D\right)=\frac{C_s{\Delta V}^2}{2}$$ (Equation 7)

$$Powerdensity\left(P\_D\right)=\frac{ED}{\Delta t}$$ (Equation 8)

Quantification and statistical analysis

There are no quantification or statistical analyses to include in this study.

Published: December 19, 2025