Preparation of a Ga-doped MnMoO₄ porous flower-like structure and study on its supercapacitor performance

Abstract

In this study, Ga-doped MnMoO₄ self-supported flower-like structured electrode materials were successfully prepared by the sol–gel method. The research results show that the introduction of gallium not only enhances the conductivity and charge transfer rate of MnMoO₄, but also improves the electrolyte permeability and ionic transport capacity by introducing oxygen vacancies and lattice defects. In addition, the self-supported flower-like structure increases the specific surface area of the material, enhances the structural stability of the material, provides more transport channels for ions, and improves the electrochemical reaction rate and cycling stability of the material. In the three-electrode test system, the specific capacitance of Ga-doped flower-like MnMoO₄ decreased from 1376 F g⁻¹ to 1358 F g⁻¹ after 10 000 cycles at a high current density of 15 A g⁻¹, with a retention rate of 98.6%. This fully demonstrates that this material has excellent stability in terms of cycle life. It exhibits outstanding cycle life. Moreover, after the performance of the carbon nanotube (CNT) material is enhanced, its excellent conductivity and ionic diffusion properties provide strong support for efficient energy storage. The Ga-doped flower-like MnMoO₄//CNT device, after 10 000 cycles at 5 A g⁻¹, saw a decrease in specific capacitance from the initial 255 F g⁻¹ to 249 F g⁻¹, with a capacitance retention rate of 97.6%, providing an effective strategy for the design and development of high-performance supercapacitors.

---

Introduction

With the global energy structure transformation and the rapid development of renewable energy technologies, the development of efficient and stable electrochemical energy storage devices has become a common goal for both scientific research and the industry.¹ Supercapacitors, due to their high power density, rapid charging and discharging capabilities, and long cycle life, have shown broad application prospects in smart grids, new energy vehicles, and portable electronic devices.² However, the core bottleneck of their relatively low energy density still restricts large-scale application. The electrode materials, as the key factors determining the performance of supercapacitors, urgently need breakthroughs through material design and structural regulation.^3–5

Among various materials, bimetallic oxides⁶ possess the flexibility of composition elements, abundant Faraday redox reactions, multiple valence states of metal ions, and controllable morphological structures, which endow them with diverse phase structures, interlayer spacings, electronic structures, and chemical bonding properties, enabling numerous important applications in energy conversion and storage (such as electrochemical oxygen evolution, supercapacitors), biomedicine (such as drug loading and sustained release, tumor treatment), nanosensing, water treatment, etc.⁷ In particular, NiCo₂O₄, ZnCo₂O₄, and MnMoO₄ have been extensively studied in the fields of energy conversion, storage, and catalysis science.^8–10 Among them, MnMoO₄, as a material with excellent performance, has demonstrated high specific capacity and long cycle stability in electrochemical energy storage fields such as supercapacitors and lithium-ion batteries. MnMoO₄ has high electrochemical activity and can promote faradaic redox reactions, thus being regarded as a promising electrode material.¹¹ However, MnMoO₄ also has some drawbacks. Firstly, due to its poor conductivity, MnMoO₄ often exhibits relatively low energy conversion efficiency in practical applications. Moreover, the slow ion diffusion rate of MnMoO₄ also limits its application under high power density requirements. To address this issue, researchers have begun to explore strategies for material composites and structural optimization to significantly enhance the electrochemical performance of the composites. For instance, doping with rare earth elements serves as an effective modification method. By introducing defect states, regulating the energy band structure, enhancing lattice stability, and improving conductivity, it can significantly improve the electrochemical performance and cycle stability of the materials.¹² Among them, gallium (Ga), as an important transition metal, has multiple oxidation states (Ga¹⁺, Ga²⁺, Ga³⁺) and abundant redox activity. For instance, Zhibei Liu et al.¹³ demonstrated that gallium doping significantly enhanced the structural stability and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂. The 2% Ga-doped Li[Ni_0.6Co_0.2Mn_0.2]_0.98Ga_0.02O₂ electrode exhibited a significantly increased discharge capacity (from 169.3 mAh g⁻¹ to 177 mAh g⁻¹ at 0.5C), and improved thermal stability. Therefore, the modification based on bimetallic oxide materials, especially the doping of rare earth elements and transition metals, not only provides a new direction for improving electrochemical performance, but also offers important theoretical basis and technical support for the development of energy storage and conversion devices.^14,15

Based on this, this study proposes to combine Ga doping with the design of a flower-like structure to construct a Ga–MnMoO₄ composite cathode material, and assemble it with a high-conductivity carbon nanotube (CNT) anode to form an asymmetric supercapacitor. After 10 000 cycles, the specific capacitance decreased from the initial 255 F g⁻¹ to 249 F g⁻¹, and the capacitance retention rate was 97.6%, indicating that this ASC device has excellent cycling stability. After being tested at a current density of 3 A g⁻¹, the energy density of the Ga–MnMoO₄//CNT supercapacitor reached 110 Wh kg⁻¹, and the power density was 5745 W kg⁻¹. At a higher current density of 15 A g⁻¹, its power density increased to 9352 W kg⁻¹, while the energy density was 77.54 Wh kg⁻¹. Compared with other energy storage devices, this device demonstrated significant advantages, especially maintaining a high energy density even at high power densities. This indicates that the Ga–MnMoO₄//CNT supercapacitor has broad application potential in the energy storage field. Through systematic research on the influence of Ga doping on the crystal structure, electron transport, and energy storage mechanism of the material, the correlation between morphology and performance was revealed. Finally, an energy storage device with high energy density, excellent rate performance, and ultra-long cycling stability was achieved, providing theoretical basis and technical support for the development of next-generation high-performance supercapacitors.

Experimental section

Reagents

The reagents are manganese nitrate (Mn(NO₃)₂·4H₂O), ammonium molybdate ((NH₄)₆Mo₇O₂₄·4H₂O), sodium hydroxide (NaOH), nitric acid (HNO₃), a gallium source (Ga(NO₃)₃·xH₂O), ammonia water (NH₃·H₂O, 25%), anhydrous ethanol, ethylene glycol, hexamethylene tetramine (HMT), carbon nanotubes (CNTs), and deionized water. All are of analytical purity (AR) and do not require further purification.

Preparation of sheet-like MnMoO₄

Weigh 2.5 g of manganese nitrate tetrahydrate and 1.23 g of ammonium molybdate tetrahydrate, dissolve them in 40 mL of deionized water, and stir with a magnetic stirrer for 30 minutes (at a speed of 300 rpm). Gradually add ammonia water (NH₃·H₂O, 25%) to adjust the pH of the mixed solution to 3–4. Add 10 mL of ethylene glycol and stir in a water bath at 60 °C for 2 hours, forming a transparent sol. Transfer it to a sealed and clean container, heat up to 80 °C, and let it stand and age for 12 hours to completely convert it into a gel. Transfer the mixed solution to a high-pressure reactor with a polytetrafluoroethylene inner lining, react at 180 °C for 12 hours, and naturally cool down to room temperature. Wash 3 times alternately with deionized water and ethanol, removing the unreacted precursors. Dry it in a 60 °C oven for 12 hours to obtain sheet-like precursor powder. Finally, place it in a muffle furnace and calcine at 600 °C for 4 hours, and then naturally cool down to obtain sheet-like MnMoO₄.

Preparation of Ga-doped lamellar MnMoO₄

Weigh 2.5 g of manganese nitrate tetrahydrate, 1.23 g of ammonium molybdate tetrahydrate and 0.145 g of gallium nitrate and dissolve them in 45 mL of deionized water. Stir with magnetic stirring for 30 minutes (at 300 rpm). Gradually add ammonia water (NH₃·H₂O, 25%) to adjust the pH of the mixed solution to 3–4. Add 10 mL of ethylene glycol and stir in a water bath at 60 °C for 2 hours to form a transparent sol. Transfer it to a sealed and clean container, heat to 80 °C, and let it stand and age for 12 hours to completely convert it into a gel. Transfer the mixed solution to a high-pressure reactor with a polytetrafluoroethylene inner lining, react at 180 °C for 12 hours, and naturally cool to room temperature. Wash three times alternately with deionized water and ethanol to remove the unreacted precursors. Dry it in a 60 °C oven for 12 hours to obtain sheet-like precursor powder. Finally, place it in a muffle furnace and calcine at 600 °C for 4 hours, and then let it cool naturally to obtain sheet-like MnMoO₄.

Preparation of flower-like MnMoO₄

Weigh 2.5 g of manganese nitrate tetrahydrate and 1.23 g of ammonium molybdate tetrahydrate, dissolve them in 40 mL of deionized water, and stir with a magnetic stirrer for 30 minutes (at a speed of 300 rpm). Gradually add ammonia water (NH₃·H₂O, 25%) to adjust the pH of the mixed solution to 3–4. Add 10 mL of ethylene glycol and stir in a water bath at 60 °C for 2 hours, forming a transparent sol. Transfer it to a sealed and clean container, heat up to 80 °C, and let it stand and age for 12 hours to completely convert it into a gel. Disperse the gel in 30 mL of deionized water, add an appropriate amount of hexamethylene tetramine (HMT),¹⁶ and stir for 30 minutes. Transfer the mixed solution to a high-pressure reactor with a polytetrafluoroethylene inner lining, react at 180 °C for 12 hours, and naturally cool to room temperature. Wash the mixture alternately with deionized water and ethanol 3 times to remove the unreacted precursors. Dry it in a 60 °C oven for 12 hours to obtain flower-like precursor powder. Finally, place it in a muffle furnace and calcine at 600 °C for 4 hours, and naturally cool after that to obtain flower-like MnMoO₄.

Preparation of Ga-doped flower-shaped MnMoO₄

We weighed 2.5 g of manganese nitrate tetrahydrate, 1.23 g of ammonium molybdate tetrahydrate and 0.145 g of gallium nitrate and dissolved them in 45 mL of deionized water. The subsequent preparation method was the same as that for the flower-like MnMoO₄. Fig. 1 shows the complete preparation process diagram of the flower-shaped MnMoO₄ doped with Ga.

Fig. 1 Schematic diagram of the preparation process of the Ga–MnMoO₄ electrode material.

Assembly of electrodes and supercapacitors

In this experiment, a non-symmetric supercapacitor was fabricated by assembling Ga-doped flower-shaped MnMoO₄ as the positive electrode and CNTs as the negative electrode, using a PVA–KOH aqueous gel electrolyte (Fig. S1).¹⁷

Firstly, 1.5 g of KOH and 3 g of PVA were added to 30 mL of deionized water and stirred at 85 °C for 1 hour to prepare the gel electrolyte. The electrode materials for the positive and negative electrodes were coated on carbon cloth by mixing Ga–MnMoO₄ and CNTs with conductive carbon black and PVDF in a ratio of 8 : 1 : 1. The geometric area of each electrode was 1 cm². The coated electrode materials were then dried at 120 °C under vacuum for 12 hours to enhance their adhesion and conductivity. Next, the positive and negative electrodes and polypropylene separator were immersed in the PVA–KOH gel electrolyte for 10 minutes to ensure complete wetting. The assembled capacitor was cured at room temperature and dried in a vacuum environment for 24 hours to remove residual moisture and ensure adequate contact between the electrode materials and the electrolyte. The assembled three-electrode test element was tested electrochemically using a Shanghai Huaxun workstation CHI660E, with test methods including cyclic voltammetry (CV), constant current charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS), to evaluate its electrochemical performance and cycling stability.

Material characterization

The experiments employed a scanning electron microscope (SEM, JEOL JSM-7500F) and a transmission electron microscope (TEM, JEOL JEM-2100F) to observe the morphological characteristics of the Ga-doped flower-shaped manganese oxomolybdate (Ga–MnMoO₄) and CNT materials prepared in this study, in order to analyze their nanostructures and the microscopic morphology of the material surfaces. The crystal structure of the materials was analyzed using X-ray diffraction (XRD, Rigaku D/max-2600 PC, with the radiation source being Cu Kα, λ = 1.5406 Å), to confirm the crystal phase composition, the doping effect of gallium, and its influence on the lattice structure.

The electrochemical tests were conducted on the Shanghai Chenhua CHI660E electrochemical workstation. They mainly included cyclic voltammetry (CV), constant current charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) tests to evaluate the electrochemical performance and cycling stability of the Ga-doped molybdenum oxalate manganese positive electrode and CNT negative electrode. The CV curves showed the pseudocapacitive behavior of the materials at different scan rates, while the GCD curves were used to calculate specific capacitance, energy density, and power density, and to further examine the electrochemical stability of the materials at different current densities.

The formula for calculating specific capacitance (C_s) is as follows:

where C_s represents the specific capacitance (F g⁻¹), I is the discharge current (A), Δt is the discharge time (s), ΔV is the voltage difference (V), m is the mass of the active material (g), energy density (E, Wh kg⁻¹) and power density (P, W kg⁻¹).

Result analysis and discussion

Material characterization analysis on MnMoO₄ and Ga–MnMoO₄

Fig. S2(a) presents the scanning electron microscope (SEM) image of the MnMoO₄ electrode material. From Fig. S2(a), it is clearly observable that this material has a clear outline and a nano-scale sheet-like structure. The sheet layers are relatively uniform in size, the surface has slight undulations, some sheet layers exhibit stacking phenomena, and they are relatively loose. They interweave to form a flower-like porous structure. These porous flower-like structures interweave with each other to form many pores, which are densely accumulated and not conducive to the transfer of electrons. Fig. S2(b) shows the SEM image of the Ga–MnMoO₄ sheet-like structure nanomaterial. This indicates that after doping, Ga–MnMoO₄ has a larger specific surface area at the same size, which is beneficial for electron transport and ion diffusion. It can provide more active sites, improve catalytic performance, facilitate charge storage and transfer, and enhance the electrochemical performance of the electrode material. Fig. S2(c) presents the scanning electron microscope (SEM) image of the MnMoO₄ electrode material. In the SEM image of the undoped manganese oxomolybdate flower-like structure, the flower-like microstructure can be clearly observed. The flower-like structure is composed of many slender nanosheets as “petals”, which grow radially from the center to the periphery, and the overall shape is relatively regular and symmetrical. There are certain voids between the “petals”, forming a rich three-dimensional porous structure. The surfaces of these nanosheets or nanorods are relatively smooth, with only some minor atomic-level undulations. The size of each flower-like structure is relatively uniform, with sizes distributed within a certain range, and the flower-like structures are dispersed among each other without obvious agglomeration.¹⁸ This will introduce additional charge carriers (electrons or holes), thereby changing the electrical conductivity of MnMoO₄. It will generate vacancies or interstitial atoms in the lattice, thereby affecting the electron transmission path and increasing or decreasing the electrical conductivity. Fig. S2(d) presents the SEM image of the Ga–MnMoO₄ flower-shaped nano-materials. By comparing the SEM images, it can be seen that after doping MnMoO₄, the flower-like structure is largely retained, but some changes occur at the microscopic details. The SEM images show that in the doped flower-like manganese molybdate, the “petals” may undergo a certain degree of distortion or deformation, no longer being as regular as before doping. The surface of the “petals” becomes rougher, and more defects and irregular protrusions or depressions appear, which are caused by the doping atoms entering the crystal lattice structure. In addition, the size distribution of the flower-like structure becomes more widespread. The size of some flower-like structures is significantly different from that before doping, and there will be a certain degree of agglomeration between the flower-like structures, forming larger aggregates. Compared with MnMoO₄, the Ga–MnMoO₄ samples prepared by the sol–gel method exhibit a uniform porous flower-like structure. We also measured the scanning electron microscope images, and the morphologies of other doped samples also show similar porous flower-like structures, which can enhance the structural stability of MnMoO₄. Under high temperature or other harsh conditions, the doped MnMoO₄ can maintain the integrity of its flower-like structure and crystal structure, making it more durable in practical applications. Through in-depth analysis of the SEM images, we can conclude that Ga doping significantly alters the microstructure and electrochemical properties of MnMoO₄. The doped material has a larger specific surface area, more active sites, and better electron transport performance, which make it exhibit higher efficiency and stability in electrochemical applications. Additionally, the uniformity of the elemental distribution further confirms the success of the doping process and the high purity of the material. These analytical results provide important theoretical basis and experimental guidance for further optimizing material performance and applications. The results in Fig. S3(a) and S4(h) indicate that the prepared material contains Mn, O, Mo, and Ga elements. All are uniformly distributed within the MnMoO₄ nanostructure and match the overall structural outline of MnMoO₄, without any obvious enrichment or depletion areas. The above research results show that the prepared material does not contain any other impurities.

Transmission electron microscopy of cathode materials

To further observe the microstructure of MnMoO₄ and Ga–MnMoO₄ nanomaterials, we conducted transmission electron microscopy (TEM) analysis, and the results are shown in the Fig. 2. From the internal regions in Fig. 2(a) and (c), the presence of a porous structure can be clearly observed, further proving that this material has a porous structure with good porosity.¹⁹ This not only promotes the penetration of the electrolyte, but also significantly increases the electrochemical reaction active sites, thereby effectively improving the electrochemical performance of the material.²⁰ As can be seen in Fig. 2(a), the material presents a similar ball-like aggregation morphology, composed of many slender nanostructures, which radiate outward from the center and form a large specific surface area. This unique microscopic morphology enables the material to have abundant surface atoms and unsaturated bonds, providing more active sites for chemical reactions.²¹ Doping can introduce additional charge carriers, altering the electrical conductivity of the material. This nanostructure may also lead to the nanoscale effect, further influencing the behavior of electrons and potentially enabling their application value in electrochemical energy storage (such as supercapacitors) or electronic devices. The Ga–MnMoO₄ nano-material exhibits high activity in catalytic reactions. In some redox reactions, the atoms on the material's surface can adsorb reactant molecules, reducing the activation energy of the reaction and thereby accelerating the reaction. The doping of Ga may enhance the structural stability of the material, allowing it to maintain relatively stable performance under different environmental conditions (such as temperature and humidity changes), which is crucial for its long-term stability and reliability in practical applications. From the high-resolution transmission electron microscopy (HRTEM) images in Fig. 2(b) and (d), it can be determined that this material has a specific crystal structure. The crystal structure determines the arrangement of atoms within the material, thereby influencing its physical and chemical properties. The specific interplanar spacing (such as 0.34 nm corresponding to certain crystal planes corresponding to the (110) lattice planes) reflects the periodicity and orderliness of the crystal, and this ordered structure helps with the transmission of electrons and the diffusion of ions. The selected area electron diffraction (SAED) image inserted in the figure further provides information on the crystal structure. The diffraction spots of the (010), (110), and (111) crystal planes marked in the figure indicate that the crystal has a specific crystallographic orientation. The distribution and intensity of these diffraction spots are closely related to the symmetry and structure of the crystal, which is helpful for determining the crystal's space group and lattice parameters. Fig. 2(d) also shows clear lattice stripes, with a (110) crystal plane spacing of 0.34 nm, indicating that Ga doping has widened the crystal plane spacing of MnMoO₄. However, there might be some lattice distortions or defects caused by Ga doping in the image. These microscopic structural changes will affect the physical and chemical properties of the material. The (010), (110) and (111) crystal plane diffraction spots marked in the inserted SAED figure have some similarities with the SAED figure of MnMoO₄ in Fig. 2(b), but there are also some minor differences, which are due to the minor changes in the crystal structure caused by Ga doping. These changes will affect the electronic structure, optical properties and catalytic performance of the material. This indicates that the prepared porous Ga–MnMoO₄ is a polycrystalline material. Moreover, the results in Fig. 2(e–h) show that the prepared material contains Mn, O, Mo and Ga elements, which match the overall structural outline of MnMoO₄, without obvious enrichment or depletion areas. This proves that the prepared product does not contain other impurities.

Fig. 2 (a) TEM image of the petal-shaped MnMoO₄ nanomaterials; (b) HRTEM image of the petal-shaped MnMoO₄ (inset: the corresponding SAED image); (c) TEM image of the petal-shaped Ga–MnMoO₄ nanomaterials; (d) HRTEM image of the petal-shaped Ga–MnMoO₄ (inset: the corresponding SAED image); (e)–(h) TEM mapping images of Mo, O, Mn, and Ga elements.

To further analyze the structural characteristics, chemical state and valence state of the samples, we used an X-ray diffraction instrument (XRD) and an X-ray photoelectron spectrometer (XPS). Fig. 3(a) shows the XRD spectrum of the Ga–MnMoO₄ nanomaterials. According to the graph, the XRD diffraction peaks of all samples are consistent with the standard MnMoO₄ (JCPDF 27-1280), indicating that the crystal phase of the samples has not been significantly affected by the Ga doping. Although the main phase has not changed, after the Ga was incorporated, the intensity and shape of some diffraction peaks showed subtle changes. This phenomenon indicates that the doping of gallium has, to a certain extent, influenced the lattice parameters, grain size or crystallinity of the crystal.²² The introduction of Ga dopants introduces additional carriers, thereby altering the electrical conductivity of the material. Due to the presence of nanostructures, a quantum confinement effect is generated, further influencing the behavior of electrons. In electrochemical energy storage (such as supercapacitors), it can enhance the efficiency of ion storage and diffusion, improve charging and discharging performance and cycle stability; in electronic devices, it can optimize the conductive properties. The doping of Ga enhances the structural stability of the material to a certain extent. By optimizing the doping concentration and method, the material can maintain relatively stable performance under complex environmental conditions such as temperature fluctuations and humidity changes. This characteristic is extremely crucial for ensuring the long-term stability and reliability of the material in practical applications. Taking harsh environments as an example, the optimized-doped material can continuously maintain its catalytic activity or electrochemical performance, demonstrating excellent environmental adaptability and application potential. The abundant active sites and special crystal structure make the Ga–MnMoO₄ nanomaterial after doping more active in catalytic reactions. Surface atoms can adsorb reactant molecules, reducing the activation energy of the reaction and accelerating the reaction. In redox reactions, organic synthesis reactions, or catalytic decomposition of harmful gases, it exhibits excellent catalytic performance, such as an increasing reaction rate and selectivity. Fig. S3 shows the EDS maps of Ga–MnMoO₄ and MnMoO₄. From the figure, it can be seen that both materials contain elements Mn, Mo, and O, which is consistent with the chemical composition of manganese molybdate. At the same time, a clear peak of the Ga element was detected in the gallium-doped sample, indicating the successful incorporation of gallium into the manganese molybdate nanomaterials. It can be observed that with the increase of time, the diffraction peaks have significantly shifted to the right, which is caused by the increase of oxygen vacancies. Oxygen vacancies have an important influence on the chemical and energy storage properties of electrode materials. By adjusting the content and distribution of oxygen vacancies, the reaction performance, energy storage capacity, and cycle stability of the electrode materials can be improved, providing higher performance and efficiency for electrochemical energy storage and related applications.

Fig. 3 (a) XRD patterns of Ga–MnMoO₄ and MnMoO₄ nanomaterials; (b) XPS spectrum of Ga–MnMoO₄ and high resolution spectra of (c) Mn 2p; (d) O 1s; (e) Mo 3d; (f) Ga 2p.

We used XPS testing to analyze the surface elemental composition and chemical state of the Ga–MnMoO₄ sample. Fig. 3(b) shows the measured scanning XPS spectrum of the sample, which presents the overall measured spectrum of the sample, covering a wide range of binding energies. From the figure, multiple characteristic peaks of elements can be observed, including Mn 2p, Ga 2p, Mo 3d, C 1s, etc. Through this full spectrum, the types of elements present in the sample can be preliminarily determined. The presence of the C 1s peak is due to the organic pollutants adsorbed on the sample surface or the carbon elements introduced during sample preparation. The peaks of Mn, Ga, and Mo elements indicate that these elements are present in the sample, further verifying the composition of the material. Fig. 3(c), (d), (e), and (f) respectively show the XPS spectra of Mn, O, Ga, and Mo elements of Ga–MnMoO₄. These spectral lines were obtained by fitting with the Gaussian–Lorentz function. The XPS spectrum of the Mn 2p region is shown in Fig. 3(c). The oxidation state of Mn can be inferred from the position and relative intensity of the peaks. Generally, the characteristic peaks of the 2p orbitals of Mn elements in different oxidation states (such as Mn²⁺, Mn³⁺, and Mn⁴⁺) will have certain shifts and intensity changes. Specifically, the figure clearly shows two characteristic peaks of the Mn 2p orbital, located at approximately 642.8 eV and 653.3 eV, respectively, corresponding to the characteristic peaks of Mn 2p_3/2 and Mn 2p_1/2. Compared with MnMoO₄, the peak positions of Ga–MnMoO₄ have shifted significantly to the left, indicating that in the Ga–MnMoO₄ sample, the oxidation state of Mn has changed from Mn²⁺ to being dominated by Mn³⁺, accompanied by the presence of a small amount of Mn²⁺. When Ga³⁺ replaces Mn³⁺, it will exert stress on the original crystal structure, thereby causing lattice distortion. The peak fitting in Fig. 3(d) indicates the presence of multiple oxygen species with different chemical environments. The peak at 532.3 eV represents the metal–oxygen bond (O_I). The peak at 530.3 eV corresponds to the oxygen vacancy (O_II). The relative sizes of the metal–oxygen bonds and oxygen vacancies in the two samples indicate that Ga–MnMoO₄ has more oxygen vacancies than MnMoO₄. Fig. 3(e) shows that the Mo 3d_5/2 and 3d_3/2 peaks are located at 232.1 eV and 235.3 eV, respectively. The binding energies are close to the standard values of Mo⁶⁺ (232.5 eV and 235.6 eV), indicating that Mo exists in the +6 oxidation state and no low-valent Mo species (such as Mo⁴⁺ or Mo⁵⁺) were detected. This confirms the stability of the MoO₄²⁻ structure in the material. Fig. 3(f) shows the Ga 2p_3/2 and 2p_1/2 peaks at 1115.5 eV and 1143.2 eV, respectively. They are shifted approximately 2.3 eV to the negative side compared to the Ga³⁺ standard values (1117.8 eV and 1145.3 eV), which may be due to the lattice distortion caused by the substitution of Ga³⁺ for Mn²⁺. The successful doping of Ga was further confirmed by the EDS surface scan, showing a uniform distribution without forming a second phase. In summary, by analyzing the Ga–MnMoO₄ sample using XPS technology, it was found that the sample surface contains elements Mn, O, Mo, and Ga. The main oxidation state of Mn is Mn 2p, and the main oxidation state of Mo is Mo 3d. The oxygen element exists in the form of oxygen from the oxygen and hydrogen–oxygen ion groups bound to the metal ions in the lattice, as well as the oxygen from the physically adsorbed water molecules.

Material surface area

Fig. 4(a) and (b) present the nitrogen adsorption–desorption isotherms and pore size distributions of the lamellar and flower-like MnMO₄ phases, as well as those of their gallium-doped versions.

Fig. 4 (a) N₂ adsorption–desorption isotherms of platelet MnMO₄, platelet Ga–MnMO₄, platelet Ga–MnMO₄, and flower-like MnMO₄; (b) pore size distribution curves.

From Fig. 4(a), it can be seen that the adsorption capacity of the lamellar MnMO₄ is relatively low, reflecting its tightly stacked lamellar structure which results in limited porosity and a small specific surface area. The small specific surface area restricts the electrolyte's penetration path, while the low porosity limits the electrochemical reaction efficiency of the lamellar material and hinders the further improvement of its electrochemical performance.^23,24 In contrast, the gallium-doped lamellar material introduces oxygen vacancies, generating lattice defects, increasing active sites and conductivity, and to some extent, improving the material's performance.²⁵ In Fig. 4(b), the flower-shaped MnMO₄ exhibits significant adsorption advantages. Compared with the lamellar structure, the flower-shaped MnMO₄ has an adsorption volume of approximately 150 cm³ g⁻¹ at high relative pressure, indicating that it has a larger specific surface area and a more developed pore structure.²⁶ The three-dimensional porous self-supporting flower-shaped structure provides more contact points for the electrolyte and shortens the ion transmission path, accelerating the electrochemical reaction rate. When gallium is doped into the flower-shaped MnMO₄ material (Fig. 4b), the adsorption volume further increases to 190 cm³ g⁻¹. The doping of gallium not only promotes lattice distortion and the generation of oxygen vacancies, but also forms a more porous structure, increasing the material's specific surface area. This structural change not only improves the material's porosity but also enhances its conductivity and the number of redox active sites, thereby significantly improving the material's electrochemical performance. In terms of pore size distribution, the pore size of the sheet-like material is small and unevenly distributed, restricting the transmission of the electrolyte and affecting the reaction speed of the electrochemical reaction. In contrast, the pore size of the flower-shaped MnMO₄ mainly falls within the mesoporous range of 30–50 nm. This mesoporous structure provides a large specific surface area while also offering a smooth path for ion transmission, making it highly suitable for rapid charging and discharging processes.²⁷

Crystal structure analysis

Fig. 5 shows the crystal structure analysis of manganese molybdate before and after doping with gallium.

Fig. 5 Schematic diagram of the crystal structure of manganese molybdate (on the left is the undoped version, and on the right is the doped version).

By comparing the crystal structures before and after doping (Fig. 5), α-MnMoO₄ (on the left) belongs to the monoclinic crystal system (space group P2/c), with its unit cell parameters being a = 4.85 Å, b = 5.75 Å, c = 5.02 Å, and β = 91.5°. In this structure, Mn²⁺ (octahedral sites) and Mo⁶⁺ (tetrahedral sites) form a three-dimensional framework through shared edges/shared angles; the MnO₆ octahedral chains along the b-axis are bridged by MoO₄ tetrahedra, and this topological structure endows the material with inherent high mechanical stability (elastic modulus ∼120 GPa). For the Ga doping mechanism (on the right), Ga³⁺ occupies the Mn²⁺ sites in a substitutional doping form (Ga–O bond length: 1.98 A, shorter than 2.12 A of Mn–O), causing lattice distortion and generating ionic vacancies . Furthermore, the crystal structure of nano-sized MnMoO₄ (such as nanorods and nanosheets) often exhibits a higher specific surface area and ion diffusion rate compared to bulk materials due to the existence of size effects and surface defects (such as oxygen vacancies). This results in improved electrochemical performance of MnMoO₄ in energy storage devices (such as lithium-ion batteries and zinc-ion batteries). As shown in the figure on the right, the doped gallium element uniformly fills the oxygen vacancies in the manganese oxomolybdate crystals. Doping with Ga³⁺ can enhance the cycling stability of MnMoO₄, which can inhibit the dissolution of Mn²⁺ and enhance the structural rigidity.²⁸

Comparison of electrochemical characteristics

In order to deeply analyze the influence of gallium (Ga) doping on the electrochemical performance of MnMoO₄ electrode materials, we carried out a series of electrochemical experiments such as cyclic voltammetry (CV) test, constant current charge–discharge (GCD) test and electrochemical impedance spectroscopy (EIS) test, and analyzed the effects of different structures and gallium doping on improving electrochemical performance.

In the field of materials research, it is of great significance to explore the optimization of electrode material properties for the development of high-performance energy storage devices. Taking the MnMoO electrode material as an example, from the analysis results of the cyclic voltammetry (CV) curves (Fig. 6(a)), it can be seen that the current density of undoped sheet MnMoO is low, which directly reflects its poor electrochemical activity. However, after Ga doping, the current density of flaky MnMoO₄ shows an upward trend, which strongly proves that Ga doping can effectively improve the redox activity and charge transfer ability of the material. At the same time, the performance of materials with different morphologies is also different. The materials with a flower-like structure show a higher current density. Among many materials, the performance of Ga-doped flower-like MnMoO₄ is particularly prominent, and its current density reaches the highest value among all kinds of materials. This fully shows that the synergistic effect of morphology regulation and Ga doping makes the materials obtain better electrochemical activity. The reason for this improvement is that the flower-like structure has a large specific surface area and rich pore structure, which is not only conducive to the rapid infiltration of electrolyte, but also promotes the diffusion of ions, thus enhancing the energy storage performance of the material. The constant current charge–discharge (GCD) curves (Fig. 6(b)) further support the above conclusion. The charge and discharge time of undoped sheet MnMoO₄ is shorter, but it is obviously prolonged after doping with Ga, which directly reflects that the energy storage performance of the material has been improved. It is worth noting that the flower-like structure has more obvious advantages in this respect, especially the gallium-doped flower-like MnMoO₄, which has the longest charge and discharge time and a specific capacitance as high as 2414 F g⁻¹, far exceeding those of the undoped sheet material (612 F g⁻¹) and the Ga-doped sheet material (1241 F g⁻¹). On the one hand, this remarkable performance improvement benefits from the flower-like structure, increasing the specific surface area and porous structure, and improving the contact efficiency between electrolyte and electrode. On the other hand, Ga doping introduces more redox active sites, which enhances the charge storage capacity, thus effectively improving the electrochemical properties of materials. In addition, the self-supporting structure of the material also enhances the mechanical stability, reduces the structural damage caused by volume change, ensures the integrity and durability of the material in the process of charging and discharging, and enables the electrode to maintain a good shape after repeated use, thus improving the long-term stability and electrochemical efficiency.²⁹ From the electrochemical impedance spectroscopy (EIS) test results (Fig. 6(d)), there are significant differences in charge transfer resistance (R_ct) among different materials. Ga doping can significantly reduce the R_ct of flake and flower-shaped MnMoO₄, especially for Ga-doped flower-shaped MnMoO₄ materials, which shows that it has stronger electron conduction ability and faster charge transfer speed.³⁰ In addition, the slope of the low frequency band in the Nyquist diagram shows that the ion diffusion ability of Ga-doped flower-like MnMoO₄ materials is enhanced, which is very beneficial to the efficient electrochemical reaction. This fully shows that Ga doping not only improves the conductivity of the material, but also optimizes the diffusion path of ions, further improving the electrochemical properties of the material.³¹ To sum up, the synergistic effect of Ga doping and self-supporting flower-like structure can effectively improve the electrochemical properties of MnMoO₄ electrode materials, which opens up a new way for developing high-performance energy storage devices.

Fig. 6 Electrochemical characterization of MnMoO₄ materials with different morphologies and doping. (a) Cyclic voltammetric curves of the materials; (b) constant current charge–discharge curves under different current densities; (c) comparison of specific capacitance of each material; (d) electrochemical impedance spectroscopy results.

Electrochemical characteristics

In this paper, the optimal state of Ga-doped flower-like MnMoO₄ materials is deeply analyzed.

Fig. 7(a) shows the cyclic voltammetry (CV) curves of this material in different scanning rate intervals. When the scanning rate is increased from 5 mV s⁻¹ to 50 mV s⁻¹, the integral area surrounded by CV curves is gradually increasing, while the characteristic shapes of curves remain highly similar, which strongly proves that the materials have excellent rate performance. In-depth analysis shows that the doping of gallium significantly enhances the electrochemical activity of the material and effectively expands the active sites of the redox reaction; at the same time, the self-supporting flower-like microstructure gives the material a larger specific surface area, which greatly promotes the interfacial charge transfer process between the electrolyte and the electrode material, thus improving the electrochemical performance of the material in all directions. Fig. 7(b) shows the constant current charge–discharge (GCD) curves under different current densities. With the linear increase of current density from 1 A g⁻¹ to 5 A g⁻¹, the charging and discharging time is gradually shortened regularly, which clearly shows that there is a negative correlation between specific capacitance and current density. It is worth noting that even under the working condition of high current density, the Ga-doped flower-shaped MnMoO₄ material can still maintain a relatively long charge–discharge time, which fully demonstrates its excellent rate performance in a wide current density range. This excellent performance is mainly attributed to the improvement of intrinsic conductivity by doping gallium. At the same time, the self-supporting flower-like structure provides an efficient transport channel for the rapid migration of ions in the material, which ensures the structural stability and reliability of the electrochemical performance of the material under high current density. Fig. 7(c) shows the specific capacitance response characteristics of Ga-doped flower-shaped MnMoO₄ under different current densities. With the increasing current density, the specific capacitance presents a monotonic decreasing trend. However, under the extreme condition of high current density as high as 15 A g⁻¹ (Fig. 7(d)), after 10 000 charge and discharge cycles, the specific capacitance of the Ga-doped flower-shaped MnMoO₄ only slightly decays from the initial 1376 F g⁻¹ to 1358 F g⁻¹, and the material still maintains a high specific capacitance value, which fully proves that the material has good cycle life stability. It can be seen that the doping of gallium has many positive effects on the properties of materials. From the point of view of electronic conductivity, the introduction of gallium effectively enhances the electronic conductivity of the material; in terms of structural stability, gallium doping significantly improves the structural stability of the material and greatly inhibits the possible volume deformation of the material during long-term cycle. The above advantages enable the material to maintain high-efficiency energy storage capacity all the time during multiple cycles of charge and discharge. Fig. 7(e) shows the Nyquist impedance spectra of the material after the first and last cycles. Through the equivalent circuit model fitting analysis, the charge transfer resistance of Ga-doped flower-shaped MnMoO is significantly lower than that of undoped MnMoO, which clearly shows that Ga-doped flower-shaped MnMoO has better electronic conduction dynamics. At the same time, the slope of the impedance spectrum of gallium doped materials is steeper in the low frequency band, which intuitively shows that they have better ion diffusion kinetics, and this characteristic is closely related to the high specific surface area and good conductivity of the materials. Fig. 7(f) shows the specific capacitance recovery characteristics of materials during dynamic switching of different current densities.³² As can be seen from the data in the figure, when the current density is set to 8 A g⁻¹, the specific capacity of the material reaches 1256 F g⁻¹. After 700 times of dynamic switching of current density and returning to 8 A g⁻¹, the specific capacity is 1549 F g⁻¹, reaching 99.44% of the initial specific capacity of 1589 F g⁻¹. It can be clearly found that the attenuation amplitude of specific capacity is very small during the dynamic change of current density, which fully shows that the material has good rate performance adaptability and cyclic stability.

Fig. 7 Electrochemical characterization of Ga-doped flower-shaped MnMoO₄. (a) Cyclic voltammetric curves at different scanning rates; (b) constant current charge–discharge curves under different current densities; (c) specific capacitance at different current densities; (d) cyclic stability at 15 A g⁻¹; (e) electrochemical impedance spectroscopy (EIS) of the first and last cycle; (f) recovery of specific capacitance at different current densities.

To sum up, the synergistic coupling effect between gallium doping and self-supporting flower-like structure significantly improves the electrochemical comprehensive properties of MnMoO₄ materials. The multivalent characteristics of gallium effectively increase the active sites of the redox reaction and greatly improve the intrinsic conductivity of the material; the self-supporting flower-like structure provides a high specific surface area and efficient ion transport channel. Their synergistic effect makes Ga-doped flower-shaped MnMoO₄ a potential high-performance electrode material, which shows a broad application prospect in the field of advanced energy storage.

Fig. 8 shows the performance comparison between MnMoO₄ and different Ga doping contents, and further explores the influence of Ga doping content on the electrochemical performance of the material. Fig. 8(a) shows a comparison of CV curves of MnMoO₄, MnMoO₄(0.1% Ga), MnMoO₄(0.3% Ga), MnMoO₄(0.5% Ga) and MnMoO₄(0.8% Ga). It can be seen that the area enclosed by the curves of MnMoO₄(0.5% Ga) is the largest. Fig. 8(b) shows a comparison chart of the charge and discharge performance of the five samples. By comparison, it is found that MnMoO₄(0.5% Ga) has the longest discharge time, indicating that MnMoO₄(0.5% Ga) has excellent charge and discharge performance. According to the charge–discharge curves, the specific capacity of the five samples was calculated. As shown in Fig. 8(c), the specific capacity of the MnMoO₄(0.5% Ga) material was the highest at 2414 F g⁻¹, which was the best charge storage capacity compared with other electrode materials with doping contents. From the above tests, it is found that the electrochemical performance of the material reaches the best state when the content of Ga is 0.5%. The doping of Ga helps to improve the conductivity of the material, reduce the resistivity of the material, and improve the electron transport characteristics of the MnMoO₄ material. Too high doping content makes the material have more defects, and the material structure is too complicated, which leads to the decrease of conductivity, and at the same time, the material structure is too complicated, which reduces the electrochemical activity and further negatively affects the specific capacity of the material. Fig. 8(d) shows the CV curves of the MnMoO₄(0.5% Ga) material at different scanning speeds. With the increase of scanning speed, the shape of the curve has not changed obviously, indicating that the material has high redox activity, fast reaction rate and good stability during the reaction. Fig. 8(e) shows the GCD curves under different current densities, and the specific capacity under different current densities is calculated according to this curve. As shown in Fig. 8(f), the MnMoO₄ (0.5% Ga) material has a high specific capacity of 2414 F g⁻¹ at a current density of 1 A g⁻¹ and 1682 F g⁻¹ at a high current density of 20 A g⁻¹. After 10 000 cycles of charge and discharge tests on the MnMoO₄(0.5% Ga) material at a current density of 5 A g⁻¹, as shown in Fig. 12(g), after 10 000 cycles of charge and discharge, the high specific capacity of MnMoO₄(0.5% Ga) remained at 2152 F g⁻¹, and the capacitance retention rate is illustrated by the SEM image of the material before and after the test. It can be seen that the morphology of the material did not change significantly before and after the cycle, indicating that the structural stability of the MnMoO₄(0.5% Ga) material is relatively good.

Fig. 8 (a) CV curves of materials with different Ga doping contents; (b) GCD curves of materials with different Ga doping contents at a current density of 1 A g⁻¹; (c) specific capacity diagram of materials with different Ga doping content; (d) CV curves of the MnMoO₄(0.5% Ga) material at different scanning speeds; (e) GCD curves of the MnMoO₄(0.5% Ga) material at different current densities; (f) specific capacity of the MnMoO₄(0.5% Ga) material at different current densities; (g) cyclic test diagram of the MnMoO₄(0.5% Ga) material at a current density of 5 A g⁻¹.

Electrochemical characteristics

In Fig. 9, a detailed analysis was conducted on the capacitance behavior and charge transfer mechanism of this electrode material.

Fig. 9 (a) Contribution rates of pseudocapacitance at different scanning rates. (b) Capacitance and diffusion contribution at different scanning rates. (c) Current at 2 mV s⁻¹ is divided into surface control and diffusion control. (d) Relationship between the peak current and V^1/2.

Fig. 9(a) shows the linear fitting of anode and cathode peak currents with the logarithm of the scanning rate. According to the fitting formula I = av^b (where the peak current is represented by I, the scanning rate is represented by v, and a and b are constants), the b value of the anode is 0.82 and the b value of the cathode is 0.79, both of which are in the range of 0.5 to 1. This shows that the charge storage process of this material is affected by both diffusion control and capacitance control, and the degree of these two control mechanisms will change with the change of the scanning rate. Fig. 9(b) shows the contribution ratio of capacitance control and diffusion control to the current at different scanning rates. It can be found that with the rising scanning rate, the contribution of capacitance control to current gradually increases, while the contribution of diffusion control decreases. This is because at a high scanning rate, electrolyte ions do not have enough time to diffuse into the material, and charge storage mainly depends on the surface reaction.³³ This shows that the reaction of materials to charge transfer is different in the process of oxidation and reduction, which reflects that the cathode and anode processes have different contributions to diffusion control and capacitance control. Fig. 9(c) shows the current contribution of surface control and diffusion control separately under the condition of a scanning rate of 2 mV s⁻¹. The results show that the contribution ratio of the diffusion control process to current reaches 43.1%, which indicates that the diffusion process plays a major role at a low scanning rate, but some charges are still stored through the surface reaction. With the increase of the scanning rate, the influence of diffusion control gradually weakens, and the charge is mainly stored through a rapid surface reaction. Fig. 9(d) shows the proportional distribution of capacitance and diffusion contribution at different scanning rates. It can be observed that when the scanning rate is low, the contribution of diffusion control is relatively large; when the scanning rate is high, the capacitance control is dominant. This shows that the material has good fast response ability at a high scanning rate, and the charge is mainly stored on the surface of the material.

Based on the above analysis, it can be seen that the Ga-doped MnMoO₄ material shows excellent pseudo-capacitance characteristics under the condition of rapid charge and discharge, and can maintain good electrochemical performance under the condition of a high rate.³⁴ This is mainly due to the fact that gallium doping improves the surface reactivity of the material, increases the redox active sites, and reduces the resistance of ion diffusion, so that the material can quickly store and release charges at high power density. Therefore, using Ga-doped MnMoO₄ as a cathode material for supercapacitors has obvious advantages in improving specific capacitance and rate performance.

The electrochemical performance of the Ga–MnMoO₄ electrode material was compared with that in the literature (Table 1), and the results showed that the electrode we prepared was superior to some of those in the previously published studies.

Table 1 Performance comparison of Ga–MnMoO₄ electrode materials with references

Materials	Current density (A g^−1)	Capacitance (F g^−1)	Number of cycles	Retention (%)	Ref.
NiMoO4/rGO	1	1400	2000	91%	35
NiMoO4/carbon	1	805	1000	66.7%	36
NiCo2S4@NiMoO4	5	1447	10 000	88%	37
KCu7S4@NiMoO4	1	1194.6	10 000	92.3%	38
NiCo2S4/NiMoO4	5	2323	10 000	90%	39
ZnCo2O4@NiCo2O4	1	1728.1	10 000	91.3%	40
5% La–CoMoO4	1	1232.99	5000	97.87%	41
Ga–MnMoO4	1	2414	10 000	98.6%	This work

---

Negative electrode material CNTs

Carbon nanotubes (CNTs) are one-dimensional carbon materials with excellent electrochemical properties. Their high specific surface area and pore structure make them excellent electric double layer capacitor materials, but their poor hydrophilicity limits their application. Particularly in electrolyte solution, the specific capacity is poor. In order to enhance its hydrophilicity and improve its capacitance performance, it can be further applied in energy storage devices. In this paper, SFG (carbonyl group, carboxyl group and hydroxyl group) was formed on its surface by an acidification method to make it hydrophilic, and the electric double layer capacitance was formed between electrolyte and electrode materials, while its pseudo-capacitance performance was increased.⁴² The schematic diagram of the preparation process is shown in Fig. S4. The SFG group is introduced into the acidification treatment, which helps to improve the surface wettability of carbon nanotube materials and finally endows CNTs with super hydrophilicity. Acidification treatment is carried out at 80 degree Celsius, as shown in Fig. S4. The two SEM images are the SEM images of carbon nanotube materials before and after functionalization (1CNTs). By comparison, it is found that the surface of carbon nanotubes becomes uneven and loses its original smoothness after acidification, but the ratio of length to diameter is similar, which is caused by acidification.

Fig. 10(a) presents a comparison of the CV curves of CNTs material and 1CNTs material. In the figure, it is observed that 1CNTs has a distinct redox peak at approximately 0 V. In addition, there are some peaks caused by steric hindrance caused by the Faraday reaction of some groups. The original shape of CNTs is similar to a rectangle, which indicates that the charge storage of CNTs mainly comes from the electric double layer capacitance.^43,44 Based on the CV curve, it can be inferred that SFG enhances the material's charge storage capacity by generating pseudocapacitance through carbon nanotubes. Fig. 10(b) shows the GCD curves of the two materials. The charge–discharge curve of 1CNTs has a distinct plateau, which is due to the fact that in the charging and discharging process of the pseudocapacitive materials, the active substances in the electrode materials need to undergo highly reversible adsorption–desorption and oxidation–reduction reactions. These require a certain amount of time to complete. At the same time, internal factors such as the crystal structure of the material affect the reaction rate, causing the current to remain stable within a certain voltage range, thereby forming a plateau. The specific capacity of the materials calculated based on the GCD curve is shown in Fig. 10(c). It can be seen that the specific capacity of the functionalized 1CNT material has significantly increased. This is attributed to the redox reactions generated by SFG, providing additional contributions to the material. CV curve tests were conducted on 1CNTs at different scan rates, as shown in Fig. 10(d). As the scan rate increases, the area enclosed by the curve increases, indicating that the 1CNT material has good ion transfer ability and still maintains good electrochemical performance at high scan rates. Fig. 10(e) shows the charge–discharge curves at different current densities, maintaining good charge–discharge performance. Further, the specific capacity was calculated based on the GCD curve, and in Fig. 10(f), at a current density of 1 A g⁻¹, the specific capacity is 520 F g⁻¹, and at a high current density of 20 A g⁻¹, there is still a high specific capacity of 272 F g⁻¹.

Fig. 10 (a) CV curves of CNTs and 1CNTs at a scanning speed of 30 mV s⁻¹; (b) GCD curves of CNTs and 1CNTs at a current density of 1 A g⁻¹; (c) specific capacity diagram of CNTs and 1CNTs at a current density of 1 A g⁻¹; (d) CV curves of 1CNTs at different scanning speeds; (e) GCD curves of 1CNTs at different current densities; (f) specific capacity diagram of 1CNTs at different current densities.

The chemical properties of supercapacitors

Fig. S5 shows the electrochemical performance test results of an asymmetric supercapacitor assembled with a Ga-doped flower-shaped MnMoO₄ anode and a CNT cathode.

Fig. 11(a) shows the cyclic voltammetric curves of the Ga-doped flower-shaped MnMoO₄ electrode and CNT electrode. The potential window of the Ga-doped flower-shaped MnMoO₄ electrode is 0–0.6 V, and that of the CNT electrode is −1.0–0 V. The voltage range of the asymmetric supercapacitor assembled with these two electrodes is the potential difference between the positive electrode and the negative electrode. Therefore, the theoretical potential window of the asymmetric supercapacitor assembled with the Ga-doped flower-shaped MnMoO₄ electrode and CNT electrode can reach 1.6 V. Fig. 11(b) shows the CV curves of the Ga-doped flower-shaped MnMoO₄//CNT device at scanning rates of 5–100 mV s⁻¹. With the increase of the scanning rate, the area of the CV curve increases and the shape of the curve remains good, which shows that the device can maintain good electrochemical performance and reversibility even at a high scanning rate. This shows that the device has excellent ion transmission ability and fast charge response speed, and is suitable for fast charge and discharge application scenarios. Fig. 11(c) shows the CV curves of the asymmetric supercapacitor in different voltage windows (0–1.0 V, 0–1.2 V, 0–1.4 V, 0–1.6 V and 0–1.8 V). It can be seen that the shapes of the CV curves are similar in different voltage windows, indicating that the device has good reversibility. With the increase of the potential window, the area of the CV curve increases gradually, and the shape of the curve remains similar, indicating that the device has good electrochemical reversibility and stability. Under the potential window of 0–1.8 V, the capacity of the device reaches the maximum value, which reflects higher energy storage capacity. Fig. 11(d) shows the constant current charge–discharge curves of this asymmetric supercapacitor at different current densities (1–15 A g⁻¹). The curve shows an approximately symmetrical trend, which shows that the electrochemical performance and reversibility of the device are good. With the increase of current density, the discharge time is shortened, but the device can still maintain a high specific capacity, which is consistent with the results of the cyclic voltammetry test, further verifying the pseudo-capacitance characteristics of the material. Fig. 11(e) shows the change of the specific capacitance of the Ga-doped flower-shaped MnMoO₄//CNT device at different current densities (1–15 A g⁻¹). It can be seen that when the current density increases from 1 A g⁻¹ to 15 A g⁻¹, the specific capacity of the device decreases, but even at a higher current density, it can still maintain a higher specific capacity, which shows the excellent rate performance of the device. In addition, when the current density is restored to 1 A g⁻¹, the specific capacitance can be restored to 99.47% of the initial value (Fig. 11(f)), indicating that the material has good reversibility and stability. This shows that the performance of the device has little change under different current densities, and it can adapt to various power requirements, showing good rate performance and cycle stability.

Fig. 11 (a) Comparison of the cyclic voltammetry (CV) curves of 1CNTs and Ga–MnMoO₄; (b) CV curves at different scanning rates; (c) CV curves in different voltage ranges; (d) constant current charge–discharge curves under different current densities; (e) cyclic stability test at a current density of 1 A g⁻¹; (f) specific capacitance recovery curve under different current densities.

The charge–discharge cycle performance of the Ga–MnMoO₄//CNT asymmetric supercapacitor (ASC) at a current density of 5 A g⁻¹ and the energy density and power density of other devices are compared, as shown in Fig. 12.

Fig. 12 (a) Cyclic stability of the Ga–MnMoO₄//CNT device at a current density of 5 A g⁻¹. The illustration shows the charge–discharge curves of the first 10 cycles and the last 10 cycles. (b) Comparison of energy density and power density between the Ga–MnMoO₄//CNT device and other devices.

After 10 000 cycles (Fig. 12(a)), the specific capacitance decreased from the initial 255 F g⁻¹ to 249 F g⁻¹, and the capacitance retention rate was 96.2%, indicating that the ASC device has excellent cycle stability. The illustration shows the charge–discharge curves of the first 10 cycles and the last 10 cycles. It can be seen that the charge–discharge curves of the last 10 cycles are basically the same as those of the previous 10 cycles, and the voltage range is still controlled between 0 and 1.6 V. This further shows that the ASC device has small capacitance attenuation, good stability and long service life after a long charge–discharge cycle. Fig. 12(b) shows the Ragone diagram^45–48 of the Ga–MnMoO₄//CNT supercapacitor and other energy storage devices, and compares their energy density and power density. According to formulas (2) and (3), the energy density and power density of the Ga–MnMoO₄//CNT supercapacitor are 110 Wh kg⁻¹ and 5745 W kg⁻¹ at a current density of 3 A g⁻¹, and the energy density and power density are 9352 W kg⁻¹ and 77.54 Wh kg⁻¹ at a higher current density (17 A g⁻¹). Compared with other energy storage devices, this device shows remarkable advantages, especially at high power density, which shows that the Ga–MnMoO₄//CNT supercapacitor has wide application potential in the field of energy storage.

Conclusions

In this study, Ga-doped MnMoO₄ electrode materials with a self-supporting flower-like structure were successfully prepared by a sol–gel method, and a high-performance asymmetric supercapacitor was constructed by combining with the acidified carbon nanotube (CNT) anode. Experiments show that gallium doping significantly improves the intrinsic conductivity and ion diffusion kinetics of MnMoO₄ by introducing oxygen vacancies and lattice defects, and the unique three-dimensional porous flower-like structure not only greatly increases the specific surface area (nitrogen adsorption is confirmed to be 190 cm³ g⁻¹), but also provides an efficient channel for electrolyte penetration and charge transfer. This synergistic effect of morphology control and element doping makes the Ga–MnMoO₄ electrode exhibit a high specific capacitance of 1089 F g⁻¹ at a high current density of 15 A g⁻¹, and it still maintains a capacitance retention rate of 97.7% after 10 000 cycles, highlighting its excellent structural stability. The assembled Ga–MnMoO₄//CNTs asymmetric device realizes the cooperative optimization of energy density and power density in a wide voltage window of 1.6 V: the energy density reaches 110 Wh kg⁻¹ at 3 A g⁻¹ (the power density is 5745 W kg⁻¹), and the power density rises to 9352 W kg⁻¹ at 15 A g⁻¹, while the energy density still remains 77.54 Wh kg⁻¹. After 10 000 cycles, the capacitance retention rate of the device reaches 96.2%, and its performance advantages are due to the pseudo-capacitance characteristics of cathode materials and the enhanced hydrophilicity and pseudo-capacitance contribution of negative CNTs after acidification. This work provides an important theoretical and practical basis for developing energy storage devices with high energy density, excellent rate performance and long cycle life through innovative strategies of gallium doping control and microstructure design.

Author contributions

Xinjia Zhang and Jingmin Ma: writing – original draft and conceptualization. Jian Hao and Shimiao Peng: investigation. Jing Wang: writing – review & editing.

Conflicts of interest

The authors declare no conflict of interest.

Data availability

Supplementary information is available. See DOI: https://doi.org/10.1039/D5RE00343A.

The data presented in this study are available in this article.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 52002099), the Youth Scientific Research Item of Harbin Commercial University (18XN034), the Research Foundation of Education Bureau of Heilongjiang Province of China (Grant No. 145309113), the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2022-K74) and the College Student Innovation and Entrepreneurship Project Fund of Qiqihar University (Grant No. X202410232032).

Notes and references

1. M. De Rosa, O. Afanaseva and A. V. Fedyukhin, et al., Prospects and characteristics of thermal and electrochemical energy storage systems, J. Energy Storage, 2021, 44, 103443.
2. S. S. M. Raheela Akhter, MXenes: Acomprehensive review of synthesis, properties, and progress in supercapacitor applications, J. Materiomics, 2023, 9(6), 1196–1241.
3. A. G. Olabi, Q. Abbas and M. A. Abdelkareem, et al., Carbon-based materials for supercapacitors: recent progress, Challenges and Barriers, Batteries, 2023, 9(1), 19.
4. V. J. Vipu Vinayak, K. Deshmukh and V. R. K. Murthy, et al., Conducting polymer based nanocomposites for supercapacitor applications: A review of recent advances, challenges and future prospects, J. Energy Storage, 2024, 100, 113551.
5. Y. Zhang, J. Xu and S. Lu, et al., Engineering few-layer MoS2 and rGO heterostructure composites for high-performance supercapacitors, Adv. Compos. Hybrid Mater., 2025, 8(1), 108.
6. M. Wang, L. Li and Z. Liu, et al., Nanorod-like Bimetallic Oxide for Enhancing the Performance of Supercapacitor Electrodes, ACS Omega, 2024, 9(14), 16118–16127.
7. K. O. Oyedotun and B. B. Mamba, New trends in supercapacitors applications, Inorg. Chem. Commun., 2024, 170, 113154.
8. M. Ganesan, S. Alagar and V. Bagchi, et al., Surface oxygen engineered ZnCo₂O₄ planar hybrid supercapacitor electrode for high energy applications, J. Energy Storage, 2024, 98, 112954.
9. A. Mathivanan, M. Jothibas and N. Nesakumar, Synthesis and characterization of ZnCo₂O₄ nanocomposites with enhanced electrochemical features for supercapacitor applications, Surf. Interfaces, 2024, 49, 104443.
10. M. A. Yewale, R. A. Kadam and N. K. Kaushik, et al., Interconnected plate-like NiCo₂O₄ microstructures for supercapacitor application, Mater. Sci. Eng., B, 2023, 287, 116072.
11. K. Sureshkumar, K. Babu and S. Sakkaraiyan, et al., Synthesis of manganese molybdate (MnMoO₄) nanorods by co-precipitation method for supercapacitor applications, Semiconductors, 2024, 58(10), 825–833.
12. S. S. Nandi, V. Adimule and S. A. Kadapure, et al., Rare earth based nanocomposite materials for prominent performance supercapacitor: a review, Appl. Mech. Mater., 2022, 908, 3–18.
13. Z. Liu, J. Li and M. Zhu, et al., Enhanced structural stability and electrochemical performance of LiNi_0.6Co_0.2Mn_0.2O₂ cathode materials by Ga doping, Materials, 2021, 14(8), 1816.
14. Y. Zhang, C. Tang and S. Lu, et al., MnO₂ nanoflower intercalation on Ti₃C₂T_x MXene with expanded interlayer spacing for flexible asymmetric supercapacitors, Carbon Neutralization, 2025, 4(3), e70006.
15. C. Tang, Y. Zhang and S. Lu, et al., Vertical β-MnO₂@δ-MnO₂ Core–Shell Heterostructures with Superior Cycling Stability for All-in-One Flexible Supercapacitors, ACS Appl. Nano Mater., 2025, 8(3), 1568–1576.
16. J. Shi, H. Cui and J. Xu, et al., Fabrication of nitrogen doped and hierarchically porous carbon flowers for CO₂ adsorption, J. CO₂ Util., 2021, 51, 101617.
17. H. Ur-Rehman, A. Shuja and M. Ali, et al., Investigation of charge and current dynamics in PVA–KOH gel electrolyte-based supercapacitor, J. Mater. Sci.: Mater. Electron., 2022, 33(5), 2322–2335.
18. M. Selvamani, A. Kesavan and A. Arulraj, et al., Microwave-assisted synthesis of flower-like MnMoO₄ nanostructures and their photocatalytic performance, Materials, 2024, 17(7), 1451.
19. Y. Li, G. He and Z. Huang, et al., High-performance NF-loaded F-NiFe-LDH/rGO nanoflower/nanosheet composite electrode material achieving enhanced specific capacitance and energy density for supercapacitor, J. Energy Storage, 2024, 101, 113954.
20. F. N. I. Sari, H. Chen and A. K. Anbalagan, et al., V-doped, divacancy-containing β-FeOOH electrocatalyst for high performance oxygen evolution reaction, Chem. Eng. J., 2022, 438, 135515.
21. H. Sun, Y. Tan and M. Tang, et al., Phenolic resin-based nanoflower porous carbon synthesized by self-assembly of MgO hydrolysis for boosting supercapacitor performance, Appl. Surf. Sci., 2024, 673, 160874.
22. Y. Liao, R. He and W. Pan, et al., Lattice distortion induced Ce-doped NiFe-LDH for efficient oxygen evolution, Chem. Eng. J., 2023, 464, 142669.
23. Y. A. Kumar, N. Roy and T. Ramachandran, et al., Shaping the future of energy: The rise of supercapacitors progress in the last five years, J. Energy Storage, 2024, 98, 113040.
24. S. G. Sayyed, A. V. Shaikh and D. P. Dubal, et al., Paving the way towards Mn₃O₄ based energy storage systems, ES Energy Environ., 2021, 14(4), 3–21.
25. H. Fu, M. Wang and Q. Ma, et al., MnMoO₄-S nanosheets with rich oxygen vacancies for high-performance supercapacitors, Nanoscale Adv., 2022, 4(12), 2704–2712.
26. M. Selvamani, A. Kesavan and A. Arulraj, et al., Microwave-assisted synthesis of flower-like MnMoO₄ nanostructures and their photocatalytic performance, Materials, 2024, 17(7), 1451.
27. S. Banerjee, B. Mordina and P. Sinha, et al., Recent advancement of supercapacitors: A current era of supercapacitor devices through the development of electrical double layer, pseudo and their hybrid supercapacitor electrodes, J. Energy Storage, 2025, 108, 115075.
28. X. Pan, R. Chen and F. Wu, Development of electrochemical energy storage technology, Strategic Study of Chinese Academy of Engineering, 2024, 25(6), 225–236.
29. G. Qu, J. Wang and G. Liu, et al., Vanadium doping enhanced electrochemical performance of molybdenum oxide in lithium-ion batteries, Adv. Funct. Mater., 2019, 29(2), 1805227.
30. C. Zhang, R. Liu and R. Liu, et al., Ultrasonically assisted fabrication of electrochemical platform for tinidazole detection, Ultrason. Sonochem., 2024, 110, 107056.
31. A. F. Seliem, A. Y. A. Mohammed and A. Attia, et al., ZIF-67 MOF-derived Mn₃O₄@N-doped C as a supercapacitor electrode in different alkaline media, ACS Omega, 2024, 9(15), 17563–17576.
32. T. Yi, T. Wei and Y. Li, et al., Efforts on enhancing the Li-ion diffusion coefficient and electronic conductivity of titanate-based anode materials for advanced Li-ion batteries, Energy Storage Mater., 2020, 26, 165–197.
33. T. Zhai, S. Sun and X. Liu, et al., Achieving insertion-like capacity at ultrahigh rate via tunable surface pseudocapacitance, Adv. Mater., 2018, 30(12), 1706640.
34. H. Fu, M. Wang and Q. Ma, et al., MnMoO₄-S nanosheets with rich oxygen vacancies for high-performance supercapacitors, Nanoscale Adv., 2022, 4(12), 2704–2712.
35. E. Murugan, S. Govindaraju and S. Santhoshkumar, Hydrothermal synthesis, characterization and electrochemical behavior of NiMoO₄ nanoflower and NiMoO₄/rGO nanocomposite for high-performance supercapacitors, Electrochim. Acta, 2021, 392, 138973.
36. Y. Zhang, C. Chang and X. Jia, et al., Morphology-dependent NiMoO₄/carbon composites for high performance supercapacitors, Inorg. Chem. Commun., 2020, 111, 107631.
37. B. Ren, X. Wang and X. Zhang, et al., Designed formation of hierarchical core-shell NiCo₂S₄@ NiMoO₄ arrays on cornstalk biochar as battery-type electrodes for hybrid supercapacitors, J. Alloys Compd., 2023, 937, 168403.
38. Y. Sun, Z. Liu and X. Zheng, et al., Construction of KCu₇S₄@NiMoO₄ three-dimensional core-shell hollow structure with high hole mobility and fast ion transport for high-performance hybrid supercapacitors, Composites, Part B, 2023, 249, 110409.
39. B. Ren, X. Zhang and H. An, et al., Hollow cotton carbon based NiCo₂S₄/NiMoO₄ hybrid arrays for high performance supercapacitor, J. Energy Storage, 2023, 59, 106553.
40. Z. Wu, X. Yang and H. Gao, et al., Controllable synthesis of ZnCo₂O₄@NiCo₂O₄ heterostructures on Ni foam for hybrid supercapacitors with superior performance, J. Alloys Compd., 2022, 891, 162053.
41. A. Shameem, P. Devendran and A. Murugan, et al., Cost-effective synthesis of efficient La doped CoMoO₄ nanocomposite electrode for sustainable high-energy symmetric supercapacitors, J. Energy Storage, 2023, 73, 108856.
42. S. Arai, Fabrication of metal/carbon nanotube composites by electrochemical deposition, Electrochem, 2021, 2(4), 563–589.
43. R. K. Nare, S. Ramesh and V. Kakani, et al., CNTs supported NiCo₂O₄ nanostructures as advanced composite for high performance supercapacitors, Diamond Relat. Mater., 2024, 141, 110660.
44. V. S. Saji, Carbon nanostructure-based superhydrophobic surfaces and coatings, Nanotechnol. Rev., 2021, 10(1), 518–571.
45. J. M. Gonçalves, M. I. da Silva and M. N. T. Silva, et al., Recent progress in ZnCo₂O₄ and its composites for energy storage and conversion: a review, Energy Adv., 2022, 1(11), 793–841.
46. X. Sun and Z. Jian, Core–shell structured Co₃O₄@NiCo₂O₄ electrodes grown on Ni foam for high-performance supercapacitors, J. Solid State Electrochem., 2024, 28(8), 2659–2667.
47. K. Aruchamy, A. Balasankar and S. Ramasundaram, et al., Recent design and synthesis strategies for high-performance supercapacitors utilizing ZnCo₂O₄-based electrode materials, Energies, 2023, 16(15), 5604.
48. X. Lv, X. Min and X. Lin, et al., Battery-type NiMoO₄@NiMoS₄ composite electrodes for high-performance supercapacitors, Chem. Pap., 2023, 77(11), 6655–6667.

---