Diffusion-dominated redox performance of hydrated copper molybdate for high-performance energy storage

Abstract

The development of cost-effective metal molybdates with enhanced energy storage capabilities has garnered significant attention as promising redox-active electrodes for asymmetric supercapacitors (ASCs). In this work, we synthesized binder-free copper molybdate (CMO) nanostructures on nickel foam using a simple hydrothermal process and thoroughly investigated their structural and electrochemical properties. The resulting CMO nanostructures exhibited a hybrid nanosheet–nanoplate morphology with a layered structure, providing an increased electroactive surface area. The structural integrity and elemental composition were confirmed using X-ray diffraction, X-ray photoelectron and X-ray (EDX) spectroscopy, showing a homogeneous distribution of copper, molybdenum, and oxygen elements. Electrochemical analysis showed that the hydrated CMO (CMO_BH) electrode provides higher specific capacitance and redox behavior than the thermally treated CMO (CMO_AH) electrode. The higher performance is attributed to the superior conductivity of CMO_BH and the presence of hydroxyl groups, which enhance redox-type charge storage. Moreover, the ASC device fabricated using the hydrated CMO_BH and activated carbon electrodes achieved a high operating voltage of 1.6 V with a maximum specific capacitance of 142.1 F g⁻¹ at 1 A g⁻¹, an energy density of 48.6 W h kg⁻¹ and a power density of 12.5 kW kg⁻¹, respectively. Additionally, the device demonstrated excellent cycling stability, retaining 89.1% of its capacitance after 10 000 cycles. The ASCs also successfully powered light-emitting diodes, emphasizing their potential for practical energy storage applications.

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1. Introduction

With the ever-increasing demand for efficient energy storage systems in portable devices, such as smartphones, wearables, MP3 players, and other consumer electronics, supercapacitors (SCs) have emerged as promising energy storage technology.^1–3 Unlike traditional batteries, SCs have desirable merits including reliable safety, power density and rapid charge/discharge cycles.⁴ Moreover, SCs generally use materials that are more environmentally friendly, contributing to sustainable and eco-friendly designs for portable electronics.⁵ However, SCs suffer from low energy density owing to their limited cell voltage and energy storage performance.^6,7 Consequently, the advancement of energy storage technologies has driven the exploration of asymmetric SCs (ASCs) comprising novel materials capable of delivering superior performance.^8,9 The ASCs consist of two electrodes made of distinct materials, such as capacitive-type and redox-type materials.^10–12 The combination of these distinct electrodes allows ASCs to offer higher energy density compared to symmetric SCs while retaining high power density and long cycle life.¹³ Specifically, charge storage mechanisms in ASCs involve both electrostatic and faradaic processes.¹⁴ The electrostatic charge storage occurs on the negative electrode side with capacitive-like behavior (carbon-based materials are extensively used as electrodes, such as activated carbon or graphene). The capacitive-type charge storage is also termed a non-faradaic process and involves no charge transfer between the electrode and the electrolyte, leading to a high-power density and long cycle life.¹⁵ On the positive electrode side, transition metal oxides and metal sulfides/selenides have been explored as electrode materials and they undergo redox reactions (faradaic reactions) that involve charge transfer between the electrode and the electrolyte.^16,17 The redox-type charge allows for higher energy storage compared to pure electrostatic mechanisms but at the expense of slightly slower charge/discharge rates.^18,19 By combining these two materials, ASCs can achieve a higher energy density than traditional SCs while maintaining high power density and rapid response times.²⁰ Therefore, to improve the energy density of ASCs, it is crucial to develop efficient electrode materials with versatile nanostructures, modified crystal structures, superior electrochemical stability, and redox charge storage performance.²¹

Recently, the number of redox-type energy storage materials for ASCs has expanded to include a wide variety of metal molybdates (NiMoO₄,²² CoMoO₄,²³ MnMoO₄,²⁴ CuMoO₄,²⁵ and Cu₃Mo₂O₉²⁶) each offering unique properties and electrochemical benefits.²⁷ In particular, copper molybdate-based materials (CMOs) have been considered as promising positive electrodes owing to their favorable electrochemical characteristics, such as high theoretical specific capacitance, multiple redox states, and superior conductivity.²⁵ Specifically, CMOs offer a compelling balance between cost, environmental friendliness, and electrochemical performance compared to other metal molybdates.²⁸ Recently, Harichandran et al. designed SCs using the Cu₃(MoO₄)₂(OH)₂ nanostructure as a redox-type electrode, which exhibited a specific capacitance of 489 F g⁻¹ at 1 A g⁻¹ with an energy density of 13 W h kg⁻¹ at a power density of 224 W kg⁻¹ due to the large surface area and unique nanostructure of the composite structure, contributing to improved electrochemical performance.²⁹ In another work, Gajraj et al. explored the use of porous Cu₃Mo₂O₉ petals combined with La₂Mo₃O₁₂ nanoparticles as the negative electrode in ASCs.³⁰ The designed ASCs showed enhanced conductivity and electrochemical activity, resulting in a device with a high energy density of 21.84 W h kg⁻¹ and a power density of 10.86 kW kg⁻¹. Alagarsamy et al. reported a high-performance ASC using a redox-type chrysanthemum flower-like CuMoO₄-graphene composite in combination with activated carbon and the corresponding device exhibited the maximum energy density of 41.7 W h kg⁻¹ and a power density of 1058 W kg⁻¹, respectively.³¹ Furthermore, Liu et al. explored the use of microwave-assisted synthesis of Cu₃Mo₂O₉ for Li-ion battery and SC applications, which shows a high specific capacitance of 136.3 F g⁻¹ with good cyclability (1000 cycles, 95%).³² Although these works showed decent electrochemical performance, most of the works used high temperature, conventional binders to prepare the electrodes, which impact the electrochemical performance.

Furthermore, binder-free synthesis of CMO electrodes is highly desirable for high-energy-density ASCs.³³ In conventional electrode fabrication, binders such as polyvinylidene fluoride (PVDF) are mixed with active materials to form a composite electrode.³⁴ However, binders can reduce the electrochemical performance by introducing non-conductive phases that hinder electron flow and block active sites, thus lowering the overall energy storage capability.^35–37 In binder-free synthesis, CMO is directly grown on a conductive substrate (such as nickel foam and metal foil), without the need for insulating binders.²⁵ The direct growth of active materials maximizes electrical conductivity by forming a strong, uninterrupted connection between the active material and the current collector.^37,38 By eliminating binders, the overall structure and electroactive area of the CMO can be exposed to the electrolyte, resulting in more active sites for ion exchange and faster ion diffusion, which leads to a significant improvement in redox properties, specific capacitance, energy density, and charge/discharge rates.³⁹ Additionally, because the active material is directly anchored to the substrate, it remains mechanically stable during the repeated expansion and contraction that occurs during charge/discharge cycles. The enhanced mechanical integrity improves the cycling stability of the device, preventing the detachment of the active material and extending the longevity of ASCs.⁴⁰ Several growth methods such as hydrothermal, chemical bath deposition and electrochemical deposition methods are often employed in the development of binder-free electrodes in ASCs.⁴¹ Among the growth methods explored, a low-temperature-based hydrothermal method enables precise control over the size, shape, and arrangement of the CMO nanostructures, which could be expected to further enhance the electrochemical properties.

In this work, we synthesized hydrated CMO nanostructures on nickel foam (Ni foam) using a facile hydrothermal method. The CMO nanostructures exhibit a nanosheet–nanoplatelet morphology which provides a high electroactive area and enhanced ion diffusion properties. The hydrated CMO (before heating) was annealed in air to obtain a more crystalline phase of CMO (after heating) and their physical and electrochemical properties were compared. When explored as electrode materials, the binder-free hydrated CMO showed superior redox activity with high specific capacitance compared to the annealed electrode, which could be due to the improved conductivity and hydrated species in between the layers of CMO nanostructures. Furthermore, the coin-cell based ASC was assembled using hydrated CMO as the positive electrode and activated carbon as the negative electrode to examine the energy and power densities. The fabricated CMO//activated carbon-based ASC demonstrates high energy and power densities with excellent cycling stability, which are beneficial for practical electronic applications. This work provides insights into the development of facile large-scale synthesis of binder-free redox-type electrodes for energy storage applications.

2. Results and discussion

The hydrothermal synthesis and formation process of CMO nanostructures (before and after the heating process) are schematically presented in Fig. 1a. During the hydrothermal process, copper nitrate and sodium molybdate dissolve in water and release Cu²⁺ and MoO₄²⁻ ions. In an aqueous solution, hexamine also slowly hydrolyzes to form ammonia (NH₃) and formaldehyde (CH₂O). Ammonia then reacts with water to produce hydroxide ions (OH^–), which will participate in the precipitation of Cu²⁺ ions to form copper hydroxide. The hydroxide form eventually reacts with MoO₄²⁻ ions, resulting in the formation of hydrated CMO. After annealing in air, CMO_AH is formed by the release of water molecules, according to the chemical equations below:

3Cu(OH)₂ + 2MoO₄²⁻ → Cu₃(MoO₄)₂(OH)₂ + 4OH⁻

Fig. 1 (a) Hydrothermal synthesis scheme for the preparation of CMO nanostructures on Ni foam. (b) SEM images of the Ni foam and (c–e) low- and magnified SEM images of CMO nanosheet–nanoplate hybrid nanostructures. (f) Crystal structure of CMO and (g) XPS survey scan spectrum and high-resolution XPS spectra of (h) Cu 2p, (i) O 1s and (j) Mo 3d of CMO.

The morphological characteristics of the synthesized CMO nanostructures on the Ni foam were investigated using scanning electron microscopy (SEM), which provides detailed insights into the surface morphology and structural integrity of the material at different magnifications. Fig. 2b shows the pristine Ni foam before the deposition of CMO. The Ni foam exhibits a highly porous and interconnected conductive frame structure, which is advantageous for charge storage applications. Moreover, the pores in the Ni foam substrate increase the surface area available for high mass loading of active materials. The inset in Fig. 1b provides a magnified view of the pore walls and structure, highlighting the rough texture of the Ni foam surface. This texture is beneficial for the good adhesion of deposited materials. Fig. 1c shows the SEM images of Ni foam after the deposition of CMO_BH nanostructures. The SEM image clearly shows a uniform and dense coverage of the Ni foam surface by the CMO nanosheets, indicating successful deposition. The magnified SEM images included in Fig. 1d further provide a more detailed view of CMO_BH nanosheets, showcasing their high-density arrangement and intricate morphology. The nanosheets appear to have a layered structure, expected to offer a high electroactive area for energy storage. Further increasing the magnification (Fig. 1e), the SEM image reveals that several nano-platelets are decorated on nanosheets, which are thin, flat structures that maximize the exposed surface area, enhancing the material reactivity and efficiency. The morphology of the CMO_AH electrode appears the same even after heating, indicating the good thermal stability of CMO nanostructures Fig. S1.†Fig. 1f includes the crystal structure of CMO_AH, illustrating the atomic arrangement and structural characteristics. In the crystal structure of CMO, the Cu atoms are coordinated in a distorted octahedral geometry, which means that each Cu atom is surrounded by four to six oxygen atoms, forming a polyhedral structure. The specific coordination environment of Cu atoms affects their electronic properties and reactivity. In CMO, Cu atoms form CuO₄ or CuO₆ units, which can influence the material's electronic and charge transportation behavior. In addition, Mo atoms are coordinated in a tetrahedral or octahedral geometry, surrounded by oxygen atoms. In CMO, Mo forms a MoO₄ tetrahedral coordination, which leads to the formation of MoO₄ units. The O atoms bridge between Cu and Mo atoms (Cu–O and Mo–O bonds), creating a network of interconnected polyhedral units. The bonding between these units creates a three-dimensional network that provides structural integrity and influences the electrochemical properties. The valency state and elemental information of the CMO/Ni foam electrodes were investigated using X-ray photoelectron spectroscopy (XPS) analysis (Fig. 1g–j). As displayed in the survey scan spectrum of CMO/Ni foam (Fig. 1g), the peaks related to Cu 2p, Mo 3d, Ni 2p and O 1s were well presented. The presence of Ni 2p in the XPS survey scan spectrum was due to the presence of Ni foam substrate. The high-resolution Cu 2p spectrum in Fig. 1h shows two peaks related to Cu 2p^3/2 and Cu 2p^1/2 along with the corresponding satellite peaks. These peaks appeared at binding energies of ∼933.0 eV for Cu 2p^3/2 and ∼953.0 eV for Cu 2p^1/2 indicating the +2-oxidation state of Cu species.⁴² Shake-up satellite peaks appeared in the Cu 2p spectra due to the d9 configuration in Cu²⁺, providing further evidence for the +2 oxidation state. The O 1s spectrum in Fig. 1i shows a single peak with binding energies for lattice oxygen (O²⁻) in metal oxides typically in a range of around 529–531 eV.⁴³Fig. 1j shows the high-resolution XPS Mo 3d spectrum, which further exhibits a doublet due to spin–orbit coupling, resulting in Mo 3d^5/2 and Mo 3d^3/2 peaks. The binding energies for Mo 3d^5/2 and Mo 3d^3/2 peaks in the CMO sample appeared around 232.5 eV for Mo 3d^5/2 (Mo⁶⁺ in MoO₃) and 235.5 eV for Mo 3d^3/2, respectively.⁴⁴ The multivalent Mo in the CMO could be expected to increase the electronic conductivity and stability of the material during the electrochemical measurements.

Fig. 2 (a) EDX spectrum and (b–d) elemental mapping images of the CMO nanostructure deposited Ni foam electrode. (e) Photographic images of the CMO_BH and CMO_AH electrodes before and after the annealing process. (f–h) XRD and FT-IR spectra of corresponding samples.

The elemental composition and distribution of the synthesized CMO nanosheets were analyzed using Energy Dispersive X-ray (EDX) spectroscopy and elemental mapping. The EDX spectrum in Fig. 2a confirms the presence of copper (Cu), molybdenum (Mo), and oxygen (O) in the synthesized CMO. The elemental mapping images in Fig. 2b–d show the distribution of Mo, Cu, and O, respectively, across the nanostructures synthesized. The elemental mapping images reveal a homogeneous distribution of corresponding elements, indicating a uniform composition of the CMO nanostructures on Ni foam. Fig. 2e shows digital pictures of the CMO electrodes before and after the annealing process, which indicate a brownish color for the hydrated CMO and a dark color for the CMO_AH electrode. The XRD spectra of the CMO electrode are shown in Fig. 2f. The XRD spectrum shows the dominant peaks related to the Ni foam, while the CMO peaks appear with low intensities, which match well with the Cu₃(MoO₄)₂O JCPDS card (240055).⁴⁵Fig. 2g and h shows the FT-IR spectra of the CMO_BH and CMO_AH samples. As shown in the magnified FT-IR plot of the CMO_BH electrode, the vibrational peak observed at 3428 cm⁻¹ (before heating) is associated with the O–H stretching vibrations, which indicates the presence of water molecules or hydroxyl groups in the sample. Such a broad peak at higher wavelengths suggests that the hydrothermally synthesized CMO comprises a hydrated phase. Moreover, the same sample shows a peak at 1601 cm⁻¹, which is mainly attributed to the bending vibrations of water molecules (H–O–H bending).²⁹ The presence of this peak further confirms that the sample before heating contained water, likely in the form of adsorbed or crystalline water. Meanwhile, these peaks are absent in the after-heating sample (CMO_AH), indicating that the heating process effectively removed the water content (both adsorbed and crystalline), as the peaks related to O–H and H–O–H vibrations are absent in the CMO_AH spectrum. Meanwhile, vibrational peaks present at 779, 650, 562, and 526 cm⁻¹ in both CMO_BH and CMO_AH samples are evident, which correspond to the characteristic stretching vibrations of the Mo–O and Cu–O bonds in the CMO structure. These peaks are present in both the before and after heating samples, indicating that the fundamental structure of CMO is retained regardless of the heating process.

The electrochemical properties of the prepared CMO electrodes were initially investigated in a three-electrode system with aqueous 1 M KOH as an electrolyte at room temperature. A comparison of the CMO electrode performance before and after heating is shown in Fig. 3a and b. From the cyclic voltammetry (CV) curves measured at a constant scan rate of 20 mV s⁻¹ (Fig. 3a), two strong and intense oxidation–reduction (redox) peaks are visible in the hydrated CMO_BH (0.53 V and 0.16 V) and CMO_AH (0.47 V and 0.23 V) electrodes, which indicate a faradaic-type redox behavior. The faradaic behavior from the electrode material usually provides higher charge storage than the conventional capacitive-type carbon materials. Moreover, the comparative CV curves also show a larger integral area for the hydrated CMO_BH electrode compared to the CMO_AH electrode, reflecting superior electrochemical performance. The higher redox peak current from the hydrated CMO_BH electrode could be due to the associated water molecules or hydroxyl groups (OH⁻) related to the CMO nanostructures, which provide higher electrochemical activity and conductivity, thereby enhancing the electrochemical performance. Meanwhile, the CMO_AH electrode undergoes dehydration after heating, where water molecules are removed from the structure, which could reduce the number of available active sites for electrochemical reactions, leading to a lower capacity. The galvanostatic charge–discharge (GCD) curves of the CMO electrodes were also measured at the potential range of 0–0.5 V at a constant current density of 1 A g⁻¹. From the GCD curves, the symmetrical charge–discharge times and non-linear behavior of CMO_BH and CMO_AH electrodes further suggest good reversibility and capacitive behavior, respectively. The CMO_BH displays higher charge–discharge times compared to the CMO_AH electrode indicating higher specific capacitance of the corresponding material. The ideal non-linear behavior of the CMO electrodes during the faradaic redox process can be explained as follows. During the reduction half-reaction or discharging process (i.e., when the potential sweeps in the lower potential regions in the GCD or CV curves) Cu²⁺ in the CMO is reduced to Cu⁺ and Cu⁰ and Mo⁶⁺ may be reduced to lower oxidation states of Mo⁴⁺/Mo⁵⁺. Meanwhile, in the oxidation charging process (the potential sweeps in the higher potential direction), Cu⁺ and Cu⁰ are oxidized back to Cu²⁺ and Mo⁴⁺ or Mo⁵⁺ can be oxidized to Mo⁶⁺. The hydroxide ions from the electrolyte participate in balancing the charges during the redox process, typically forming water, or combining with cations. The higher CV current densities and GCD times of the hydrated CMO_BH electrode were further investigated to analyze the charge storage kinetics. Fig. 3c shows the CV curves of the hydrated CMO_BH electrode recorded at various scan rates ranging from 2 mV s⁻¹ to 20 mV s⁻¹. The CV curves retain a redox-type shape even at higher scan rates, indicating good faradaic behavior and fast ion transport within the electrode material. Specifically, at lower scan rates, the redox peaks are better defined, indicating the controlled and reversible redox process. At a higher scan rate of 20 mV s⁻¹, the redox process becomes broader, which suggests faster charge–discharge dynamics but possibly less reversible behavior due to the diffusion limitations/kinetics. The constant redox behavior of the CMO_BH electrode across different scan rates demonstrates the suitability of hydrated CMO_BH for high-performance electrodes in asymmetric supercapacitors. To differentiate between capacitive and diffusion-controlled processes in the CMO electrode, a power law and a modified power law were used. A linear relationship on the log–log plot suggests a power-law dependence, with the slope indicating the dominant charge storage mechanism. As per the power law (i = aν^b, where a and b denote the variable factors), if the b value (slope) is 0.5, it indicates a diffusion-controlled process and if the b value is 1.0, it indicates a capacitive process. The b-values (0.74 and 0.84) of the CMO_BH electrode during the oxidation and reduction process imply a transition from diffusion-controlled to capacitive behavior as the scan rate increases (Fig. 3d). The modified power law was used to quantify the contributions of capacitive and diffusion-controlled processes, providing more insights into the charge storage mechanisms of the CMO electrode.⁴⁶

I(V) = s₁ν + s₂ν^1/2 (1)

Fig. 3 (a) and (b) Comparative CV curves of CMO_BH and CMO_AH electrodes measured at 20 mV s⁻¹, demonstrating enhanced performance. (c) CV curves of the CMO_BH electrode analyzed at various scan rates of 2–20 mV s⁻¹ highlighting redox behavior with consistency of redox peaks. (d) Relationship between peak current and scan rate for CMO, validating diffusion-limited kinetics. (e) Bar chart percentage comparison of diffusion-limited and capacitive-type contribution of the CMO_BH electrode under various scan rates. (f) Comparison of diffusion-limited behavior in the CV curve of the CMO_BH electrode. (g) Charge–discharge curves of the CMO_BH electrode at different current densities and (h) rate capability of CMO_BH and CMO_AH, with specific capacitance and specific capacity plots as a function of increasing current density. (i) Capacitance distribution of CMO across different charge–discharge cycles, emphasizing high retention. The inset of (i) shows the SEM image of CMO, showcasing its nanoscale structure and morphology after the cycling process.

and it can be written as

I(V)/ν^1/2 = s₁ν + s₂ (2)

Here, ‘I(V)’ is the redox peak current at the corresponding potential of V. ‘ν’ is the scan rate and ‘s₁’ and ‘s₂’ are constants that represent the contributions of different charge storage mechanisms. Fig. 3e shows the percentage of capacitance derived from capacitive processes and diffusion-controlled processes. Predominantly, at low scan rates, diffusion-controlled processes dominate (85.1% at 2 mV s⁻¹). As the scan rate increases, capacitive contributions become more dominant (53.4% at 20 mV s⁻¹). The CV curves of the diffusion-dominated CV curves are presented in Fig. 3f. To evaluate the charge storage capability and rate performance of the CMO electrodes, GCD analysis was tested at various current densities of 0.5–10 A g⁻¹ (Fig. 3g). From the GCD curves, stable charge–discharge times and a non-linear potential plateau indicate good coulombic efficiency and redox-type energy storage behavior. At higher current densities, the charge–discharge times are shorter, which could be due to the surface controlled electrochemical reactions, and this further reflects the material's ability to handle high rates. Furthermore, the CMO_BH electrode shows a smaller drop in potential (IR drop), indicating low internal resistance and efficient charge transfer. Meanwhile, the electrochemical properties of the CMO_AH electrode measured at different scan rates and current densities are included in Fig. S2.† The calculated specific capacitance of the CMO_BH electrode in comparison with the CMO_AH electrode is shown in Fig. 3h. Apparently, specific capacitance decreases (731.1 F g⁻¹ to 307.5 F g⁻¹ for the CMO_BH electrode) with increasing current density, which is a common behavior due to the surface reactions or limited diffusion of electrolyte ions on the electrode, specifically at higher rates. Owing to the high electrochemical activity and conductivity, the CMO_BH electrode consistently exhibits higher specific capacitance compared to CMO_AH at all current densities. The specific capacities of the CMO_BH electrode are also included in the lower panel of Fig. 3h, which are 306.5 C g⁻¹ and 153.8 C g⁻¹ at current densities of 0.5 A g⁻¹ and 10 A g⁻¹, which are higher than the values for the CMO_AH electrode (295.1 C g⁻¹ and 127.3 C g⁻¹), respectively. We also analyzed electrochemical impedance spectroscopy (EIS) analysis (100 kHz to 0.01 Hz at 5 mV sinusoidal voltage) to understand the charge transportation properties of the CMO_BH electrode compared to the CMO_AH electrode. The Nyquist curves in Fig. S3† show that CMO_BH has a reduced charge transfer resistance (1.9 Ω) compared to the sample after annealing (6.7 Ω; CMO_AH), as evident from the smaller semicircle diameter, which suggests enhanced conductivity and better electrochemical performance for the hydrated sample. The long-term cycling stability of the electrode material is also crucial for practical applications, ensuring that the material can maintain its performance over prolonged charge–discharge cycles. To evaluate the long-term cycling stability and durability of the CMO_BH electrode, GCD analysis was conducted at a constant current density of 5 A g⁻¹ (Fig. 3i). The CMO_BH electrode shows excellent cycling stability, retaining a high percentage (87.3%) of its initial capacitance after 5000 cycles. After the cycling process, the morphology and surface characteristics of CMO_BH were similar (inset of Fig. 3i), demonstrating the good structural integrity of the prepared electrode. The slightly decreased capacity, despite there being no visible structural collapse in CMO_BH during increased cycling, may result from surface passivation, where a passivation layer forms, blocking active sites. Additionally, minor side reactions between the electrode and electrolyte could consume active ions, reducing capacitance. Moreover, degradation at the electrode–electrolyte interface might also hinder ion transport, causing a performance decline without affecting the material's overall structure. The structural merits of the CMO nanosheet–nanoplate hybrid structure for enhanced electrochemical performance and its advantages in electrochemical performance are schematically presented in Fig. S4.† Firstly, the CMO nanosheet–nanoplate hybrid structure provides a large surface area, which increases the contact area between the electrolyte and the electrode material. This leads to more active sites being available for the redox reactions, thus improving the charge storage capacity. Secondly, the hybrid structure further benefits by providing shorter ion diffusion paths for ions/charges. This means electrolyte ions can more easily and quickly penetrate the material, which enhances the rate capability and overall conductivity. Thirdly, the binder-free growth of CMO hybrid morphology may also provide better mechanical stability as the nanostructures hold better adhesion with the 3D foam current collector and can accommodate the strain from ion insertion and extraction during charge–discharge cycles, which leads to improved cycling stability. Lastly, the hydrated phase of CMO provides better conductivity and facilitates better electron transport throughout the material. The interconnected structure allows for efficient pathways for electron movement, reducing resistance and enhancing the overall redox-type electrochemical performance.

To evaluate the hydrated CMOBH electrode potential for practical applications, we assembled a coin-cell based ASC using the hydrated CMO electrode as a positive electrode and activated carbon as a negative electrode with a piece of glassy fiber paper as a separator and a required amount of 1 M KOH as an electrolyte (Fig. 4a). The use of a separator prevents physical contact between the positive and negative electrodes, which allows the free movement of electrolyte ions through its porous structure. The charge storage mechanism in the ASC is a hybrid process, with the CMO electrode utilizing faradaic redox reactions to achieve high specific capacitance and the activated carbon electrode employing a capacitive-type mechanism through ion adsorption, and it provides a higher voltage window. It is known that a high operating voltage is a desirable feature for energy storage devices as it increases the energy density (which is proportional to the square of the voltage). Combining the CMO and activated carbon electrode in an asymmetric device (hydrated CMO//activated carbon), the operating voltage could be expected to be 1.6 V, as shown in Fig. 4b. Preceding the device assembly, it is essential to optimize the charge balancing between the positive and negative electrodes, which ensures that the device works efficiently. The charge stored (Q) in each electrode is Q+ = Q− (where Q+ is the charge stored in the positive electrode and Q− is the charge stored in the negative electrode). The above equation can be further written as to adjust the mass ratio of each electrode (where m+ and m− are masses (g) of the active materials in the positive and negative electrodes, Cm+ and Cm−are their specific capacitances, and V+ and V− are the voltages), respectively.28 The corresponding mass ratio between the positive and negative electrode was optimized as 0.84% in the fabricated device. Furthermore, the optimization of the working voltage is essential to ensure the stable operation of the fabricated ASC device. Fig. 4c illustrates the CV curves of the ASC recorded at varying applied voltages, which are crucial for determining the optimal working voltage range of the device. By incrementally increasing the voltages (from 0–0.5 V to 0–1.7 V), the CV profiles reveal how the ASC charge storage behavior evolves with higher potentials. The gradual broadening of the CV curves as the voltage increases indicates that the ASC can store more charges until it reaches an optimal point (0–1.6 V). In this voltage range, the device performs efficiently without significant degradation. However, when the device was operated at excessive voltages (>1.7 V), the shape of the CV curve was slightly altered, which could be due to electrolyte decomposition or other breakdown issues (gas evaluation reactions) that can occur at the electrodes. Fig. 4d provides a GCD analysis of the ASC device measured at different working voltages (0–0.5 V to 0–1.7 V), which further assesses the device performance at different working voltages. The corresponding GCD curves show the non-linear and mixed electrochemical behavior (ascribed by the faradaic CMO and capacitive carbon) of the charging and discharging processes of the ASCs across different voltages. Symmetric charge–discharge times over a certain voltage range confirm the ASCs’ ability to operate effectively within that voltage window, while deviations at the higher voltage (1.7) could indicate degradation issues such as high internal resistance or electrolyte decomposition. The GCD curves therefore offer a more practical estimation of the maximum operating voltage while ensuring that the device remains safe and efficient during repeated cycling. As such, we optimized the operating voltage of ASC in the range of 0–1.6 V for the remaining electrochemical properties. Fig. 4e provides CV curves of ASC behavior under its optimal working voltage, which were recorded at different scan rates of 5–100 mV s−1, reflecting how the ASC charge storage behavior changes with an increase in the speed of the voltage sweep. At lower scan rates, the CV curves exhibit a redox behavior as well as ideal capacitive behavior, as the slower voltage sweep allows the ions in the electrolyte sufficient time to fully interact with the electrode surfaces. With further scan rate increases, the area under the CV curves also increases, which indicates higher current flow and confirms that the device can maintain efficient charge storage at higher operational rates. The mixed charge storage behavior without changing the CV shapes at higher scan rates reflects excellent electrochemical kinetics and reversibility of the fabricated ASCs.

Fig. 4 (a) Schematic representation of the assembly process and working principle of the ASC, depicting the configuration of the device and the flow of ions during operation. (b) Cyclic voltammetry (CV) comparison between CMO (red) and activated carbon (black) within an operating voltage range of 0–1.6 V, illustrating differences in electrochemical performance. (c) CV curves and (d) GCD plots of ASC at different operating voltages. (e) CV curves of ASC with a fixed operating voltage of 0–1.6 V at increasing scan rates (5–100 mV s⁻¹).

To further dissect the capacitive and diffusion contributions in the device, a modified power-law or Dunn's method is applied (using eqn (1) and (2)), allowing the separation of the total current response into capacitive and diffusion-controlled components. Specifically, we have extracted the slope and intercept values from the CV curves of the device, which were recorded at various scan rates (5–100 mV s⁻¹), and they reveal important insights into the charge storage mechanisms within the device. As explained in Fig. 4, charge storage occurs in ASCs via two primary mechanisms: electrostatic (capacitive) and faradaic (diffusion-controlled). The capacitive contribution is generally associated with the electrostatic adsorption of ions at the electrode–electrolyte interface or near-surface redox reactions, which can occur rapidly and are independent of ion diffusion in the bulk material. In contrast, the diffusion-controlled processes involve redox reactions that occur deeper within the electrode material, where ion diffusion is the rate-limiting step. As illustrated in the CV curves of Fig. 5(a–h) and the accompanying comparative bar graph (Fig. 5i), at low scan rates (e.g., 5 mV s⁻¹), the diffusion-controlled processes dominate in the fabricated device, as indicated by the larger portion of the current that scales with ν^1/2. This is because at slower scan rates, ions have sufficient time to diffuse into the bulk of the electrode material, facilitating deeper redox reactions. However, as the scan rate increases (e.g., up to 100 mV s⁻¹), the capacitive processes become more dominant. This shift is because at higher scan rates, the ions are more likely to be confined to surface or near-surface reactions, where diffusion limitations are less significant, thus favouring faster capacitive storage mechanisms. The comparative bar graph (in Fig. 5i) quantifies the transition of the capacitive and diffusion contributions, showing a clear increase in the capacitive contribution with increasing scan rate. The transition from a diffusion-dominated mechanism at low scan rates to a capacitive-dominated mechanism at high scan rates is a common behaviour for ASC under increased scan rates. The comparative charge storage mechanisms suggest that the fabricated ASC exhibits a desirable high-rate performance, with a significant portion of its charge storage coming from fast, surface-associated processes as the scan rate increases.

Fig. 5 (a–h) CV curves of the ASC measured at different scan rates of 5–100 mV s⁻¹. The outer colored regions illustrate the total contributions, and inner curves demonstrate diffusion-controlled contributions as derived from the modified power law/Dunn method. (i) Bar graph summarizing the quantitative contributions of capacitive (blue) and diffusion-controlled (yellow) processes to the total charge storage at each scan rate, illustrating the transition from diffusion-controlled to capacitive-dominated mechanisms as the scan rate increases.

To estimate the charge storage performance of ASCs, including specific capacitance and energy density, GCD analysis was conducted at different current densities of 1–10 A g⁻¹ (Fig. 6a). The GCD curves of the hydrated CMO_BH//activated carbon device show distinct charging and discharging behaviors for the different current densities investigated. Specifically, as the current density increases (from 1 A g⁻¹ to 10 A g⁻¹), the charge–discharge time decreases significantly, which is expected because, at higher current densities, the device is charged and discharged more quickly due to the larger amount of current being applied. At current densities of 1 A g⁻¹ and 10 A g⁻¹, the device shows relatively low internal resistance drops of 0.03 V and 0.25 V from the overall working voltage of 1.6 V, which indicates that the device has a lower voltage drop. From the GCD curves, long charge–discharge times at lower current densities (1 A g⁻¹) suggest that our ASC has good energy storage capability when operated at low current densities. The gradual discharge profile at these lower current densities is indicative of efficient energy storage and release. Conversely at higher current densities, the ASC device operates more quickly, but with a trade-off in energy efficiency, as the shorter discharge times indicate lower energy retention. The calculated specific capacitance values of the device at different current densities are plotted in Fig. 6b and it provides a comprehensive view of the ASC's ability to retain capacitance across different rates. At a current density of 1 A g⁻¹, the hydrated CMO_BH//activated carbon device shows a specific capacitance of 142.1 F g⁻¹ (852.9 mF cm⁻²) with a good rate capability of 40.5% (at a high current density of 10 A g⁻¹). The reduced specific capacitance could be due to the limited access to the interior of the electrode's porous structures at higher discharge current densities, which is a common behavior of energy storage devices. Furthermore, we also quantified the energy and power densities of the hydrated CMO_BH//activated carbon device, making it a promising option for applications requiring both fast charge/discharge rates and high energy storage. As shown in Fig. 6c, the device demonstrated a maximum energy density of 48.6 W h kg⁻¹ at a power density of 785 W kg⁻¹. When the power density reached 6250 W kg⁻¹, the device still retained an energy density of 12.5 W h kg⁻¹, confirming the suitability of the ASCs for practical electrical applications. As compared in the Ragone plot, the energy density of our CMO_BH//activated carbon is higher than or comparable to the previously reported metal molybdate-based asymmetric/hybrid SCs, such as Co₃O₄@NiMoO4//AC,⁴⁷ ND-CoMoO₄/NF//rGO/NF,⁴⁸ CoMoO₄-NiMoO₄//AC,⁴⁹ CoMoO₄-graphene,⁵⁰ Cu₃(MoO₄)₂(OH)₂,²⁹ Cu₃Mo₂O₉-La₂Mo₃O₁₂,³⁰ and CuMoO₄-graphene,³¹ respectively. Determination of the cycling durability of ASCs is also crucial to exploring long-term usage of the device. Therefore, charge–discharge analysis was conducted on the ASC at a constant current density of 5 A g⁻¹ for 10 000 cycles (Fig. 6d). Noticeably, the device retained a specific capacitance of 89.9% after 10 000 cycles compared to its initial cycle, reflecting the excellent cycling durability of the CMO_BH//activated carbon, which is a feasible advantage for employing useage of the device in practical portable electronics. The EIS spectra of the ASC before and after cycling are displayed in Fig. 6e. The EIS plots indicate that the ASC has undergone a slight shift in internal resistance after cycling (1.45 vs. 2.03 Ω), but the charge transfer resistances are quite similar. However, the device still maintains good capacitive behavior, with a relatively linear Warburg region at lower frequencies, suggesting that ion diffusion processes are still efficient. The increase in internal resistance post-cycling is possibly due to corrosion or other aging effects within the ASC components. To demonstrate the device for practical applications, two ASCs are connected in series to add up the voltage, which is beneficial for powering devices requiring higher voltages. The ASCs connected in series provide sufficient voltage (∼3.1 V, Fig. S5†) and current to light up green, blue, and red-light emitting diodes (LEDs), showcasing the ASCs’ capability in efficient energy storage and discharge (Fig. 6f–j). These results suggest that the ASCs have good energy density and are effective in delivering power for small electronic devices.

Fig. 6 (a) GCD curves of the ASC at different current densities of 1–10 A g⁻¹, (b) specific capacitance and areal capacitance of the ASC as a function of the current density and (c) Ragone plot showing the relationship between energy density and power density for the ASC. (d) Cycling stability and (e) EIS plots of the corresponding device before and after the cycling process. (f) Schematic image of series connected coin-cell type ASCs to maximize the voltage, (g) photograph of ASCs before connecting to LEDs and (h–j) sequential demonstration of the ASCs, powering different colored LEDs (green, blue, red), indicating the practical application of the device in powering low-power electronic devices.

3. Conclusions

The binder-free synthesis of copper molybdate (CMO) nanostructures on Ni foam has been successfully demonstrated, resulting in a material with excellent structural and electrochemical properties suitable for asymmetric supercapacitor (ASC applications. The hydrothermal process followed by thermal treatment yields CMO nanosheets with a highly porous, layered structure that maximizes the electroactive surface area. Electrochemical analysis reveals that the hydrated CMO (CMO_BH) electrode exhibits superior charge storage capabilities due to its enhanced redox behavior, facilitated by the presence of hydroxyl groups. The asymmetric supercapacitor (ASC) device assembled using the CMO_BH electrode achieves a high operating voltage of 1.6 V, a specific capacitance of 142.1 F g⁻¹, and an energy density of 48.6 W h kg⁻¹, with excellent cycling stability over 10 000 cycles. The successful powering of LEDs by coin-cell based ASCs underlines their potential in real-world energy storage applications. These findings highlight the potential of CMO nanostructures as high-performance electrode materials in next-generation SCs, particularly in applications requiring both high energy and power densities.

4. Experimental details

The synthesis of CMO nanostructures was carried out using a one-pot hydrothermal method. Initially, copper nitrate (Cu(NO₃)₂, Sigma Aldrich) and sodium molybdate (NaMoO₄, Sigma Aldrich) with a molar ratio of 1 : 1 were dissolved in 50 ml of water. Then hexamethylenetetramine (3 mmol, C₆H₁₂N₄, Sigma Aldrich) was also added to the above solution and magnetically stirred for 2 h at room temperature. The as formed precursor solution was used for the hydrothermal growth of CMO nanostructures on nickel (Ni) foam. Prior to the growth process, two pieces of Ni foam were prepared and acid treated with diluted hydrochloric acid (HCl, Sigma Aldrich) to remove the native oxide layer and cleaned thoroughly with deionized water to remove any further impurities. The cleaned Ni foam was then immersed in the prepared precursor solution and transferred to a sealed autoclave (a high-pressure vessel that facilitates the hydrothermal reaction). The autoclave was then heated to 150 °C and maintained for 10 h. During the hydrothermal process, CMO nanostructures spontaneously crystallized on the surface of the Ni foam substrate through chemical reactions between the precursors. After the hydrothermal process, the autoclave was cooled to room temperature, and the sample was rinsed with deionized water to eliminate any residual reactants and dried in an oven. The CMO synthesized before heating was marked as hydrated CMO or CMO_BH. The mass loading of active materials grown on Ni foam was measured as ∼1.5–2 mg cm⁻². The as-deposited hydrated CMO was then annealed in a box furnace at 350 °C per 3 h under air and the corresponding sample was labeled as CMO_AH. All the characterization details and device fabrication procedures are included in the ESI.†

Data availability

The authors confirm that the data will be made available upon request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the funding of the Deanship of Graduate Studies and Scientific Research, Jazan University, Saudi Arabia, through project number: RG24-S0127.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02229g

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