Integrating bimetallic MOF-derived sulfides with MnO₂: synergistic Cu–Co–S@MnO₂ heterojunctions for flexible hybrid supercapacitors†

Jiale Hou ^a, Ziheng Huang ^a, Haofeng Lu ^a, Cheng Chen *^a, Xinfeng Wu ^a, Yonghou Xiao ^a, Wanghui Wei ^b, Minjie Xue ^b, Yanyun Ma ^c, Xinzhou Ma ^d, Shigang Sun ^e and Donghai Lin *^a
aSchool of Energy and Materials, Shanghai Thermophysical Properties Big Data Professional Technical Service Platform, Shanghai Engineering Research Center of Advanced Thermal Functional Materials, Shanghai Key Laboratory of Engineering Materials Application and Evaluation, Shanghai Polytechnic University, Shanghai 201209, China. E-mail: chencheng@sspu.edu.cn; dhlin@sspu.edu.cn
^bShanghai Institute of Measurement and Testing Technology, Shanghai 201203, China
^cInstitute of Functional Nano & Soft Materials (FUNSOM), Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Soochow University, Suzhou, Jiangsu 215123, China
^dSchool of Materials Science and Hydrogen Energy, Foshan University, Foshan 528000, China
^eState Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, China

First published on 25th June 2025

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Introduction

The rapid development of renewable energy systems and portable electronics has intensified the demand for advanced energy storage technologies that combine high power/energy density, mechanical flexibility, and long-term durability. Supercapacitors, bridging the gap between conventional capacitors and batteries, have emerged as promising candidates due to their ultrafast charge–discharge kinetics, exceptional cycling stability, and robust operational safety.^1–3 However, the widespread adoption of supercapacitors in emerging applications—such as wearable electronics and grid-scale energy storage—remains hindered by their inherently low energy density, which stems from the limited charge-storage mechanisms of traditional carbon-based electrodes.^4,5

Metal–organic frameworks (MOFs), with their ultrahigh surface areas, tunable pore architectures, and diverse metal–ligand combinations, have recently garnered significant attention as electrode materials.^6–8 Shim et al.⁹ found that nickel-based MOFs exhibit higher conductivity than Cu–Co-MOFs owing to the superior electronic properties of Ni. However, Cu–Co-MOFs show better pseudocapacitance, which is beneficial for high-energy-density supercapacitors. Despite these advantages, practical implementation of pristine MOFs in energy storage is hampered by intrinsic limitations, including poor electrical conductivity, structural fragility during cycling, and insufficient exposure of redox-active sites.^10,11 To address these challenges, post-synthetic modifications such as nitridation,¹² sulfidation¹³ have proven effective in enhancing conductivity by optimizing electronic structures while preserving MOF-derived hierarchical morphologies.¹⁴ For instance, Wang et al.¹⁵ demonstrated that partial sulfidation of MOFs significantly enhances ion adsorption capacity through surface electron density modulation. Nevertheless, sulfidized MOFs often suffer from structural degradation in alkaline electrolytes, particularly under high-current-density operations.¹⁶

Recent advances in heterojunction engineering provide a compelling strategy to overcome these limitations. The built-in electric field formed at heterointerfaces between dissimilar materials (e.g., sulfides and oxides) can dramatically accelerate charge transfer kinetics while suppressing volume expansion.^17,18 Notably, MnO₂ has emerged as an ideal partner for heterostructure construction due to its high theoretical capacitance, natural abundance, and pseudocapacitive-dominated charge storage mechanism. Guo et al.¹⁹ developed a CuCo₂S₄@MnO₂ composite on a foam nickel substrate to create a hollow porous heterostructure for supercapacitors. However, conventional MnO₂ synthesis methods often yield aggregated particles with poor interfacial contact, undermining the synergistic advantages of heterojunctions.²⁰ Electrodeposition, as a controllable and scalable technique, enables the growth of conformal MnO₂ nanofilms on conductive substrates, offering opportunities to engineer intimate interfacial coupling and hierarchical architectures.^21,22 The transition metal oxide MnO₂ has been reported to accelerate the redox reaction rate and as a rational pseudocapacitive material,^21,22 which can be used as a thin film to form a “protective film” structure for the internal material and at the same time form a heterojunction with the transition metal sulfide, resulting in a better performance of the material.

In this work, we present a multi-step interfacial engineering strategy to construct flexible Cu–Co–S@MnO₂ heterojunction electrodes derived from bimetallic MOF precursors. The design rationale encompasses three synergistic aspects: (1) bimetallic Cu–Co-MOF nanoflowers provide a high-surface-area template with abundant coordinatively unsaturated metal sites, leveraging the synergistic redox activity of Cu²⁺/Cu⁺ and Co³⁺/Co²⁺ couples; (2) controlled sulfidation transforms the MOF into conductive Cu–Co–S while preserving its nanoflower morphology, addressing the conductivity-stability trade-off; (3) electrodeposited MnO₂ nanofilms serve dual roles as protective layers and heterojunction partners, where the vertical alignment of MnO₂ nanosheets creates 3D ion diffusion channels and stabilizes the sulfide-electrolyte interface. Crucially, the Cu–Co–S/MnO₂ heterointerface induces charge redistribution via Fermi-level alignment, generating a built-in electric field that facilitates interfacial electron transfer and enhances ion adsorption energetics.

Results and discussion

Building on the synthetic strategy outlined in Fig. 1, we systematically characterize the structural evolution and corresponding electrochemical enhancements through three key modifications: bimetallic MOF construction, sulfidation treatment, and heterojunction engineering. Three-dimensional structured bimetallic nanoflower structures were prepared by a simple pyrolysis method according to the previous literature.^23,24 Bimetallic MOFs have significant advantages in terms of electrochemical properties compared to monometallic coordination structures. By introducing bimetallic nodes into MOFs, unique defect structures can be created and synergistic effects between the metals can be triggered. This synergistic effect not only increases the number and variety of active sites, but also leads to a significant enhancement of the electrochemical performance. In situ coordination-driven growth of Cu–Co bimetallic MOFs mediated by 2-methylimidazole involves a dynamic interplay of competitive metal–ligand interactions and synergistic structural assembly. Initially, Cu²⁺ and Co²⁺ ions coordinate preferentially with 2-methylimidazole via its nitrogen donors, where the stronger affinity of Co²⁺ establishes a nucleation framework, while Cu²⁺ integrates into mixed-metal nodes via bridging oxygen or shared ligands.^25,26 The alkaline nature of 2-methylimidazole simultaneously facilitates ligand deprotonation and modulates crystallization kinetics, guiding hierarchical porosity through solvent-dependent anisotropic growth.²⁷ Dual-metal synergy enhances electronic redistribution at Cu–Co–O interfaces, optimizing charge transfer and active site accessibility. Coupled with epitaxial ZIF-67 shell formation, this mechanism yields a thermally stable composite with multiscale pore networks, demonstrating exceptional electrocatalytic activity in oxygen reduction and CO₂ conversion, thus offering a paradigm for designing multifunctional MOF heterostructures. Cu–Co-MOF serves as a good template for the precursor, which allows Cu–Co–S to maintain a unique nanoflower structure (Fig. 1). Compared with the single ZIF-67, the potential difference between Cu²⁺ and Co²⁺ can improve the electron transfer rate and further enhance the electrochemical activity.^28,29 In addition, the agglomeration phenomenon occurred during the synthesis of Cu–Co-MOF, which may be attributed to the special nanoflower structure of the material, which gives it a high surface energy, and the material reduces the surface energy by spontaneous agglomeration.

To solve the problem of poor electrical conductivity and stability of MOF materials, the method of sulfidation was used to modify and reconstruct MOF materials. Cu–Co-MOF was sulfidized by hydrothermal method, and the metal sites of MOF materials reacted with sulfur source (Na₂S·9H₂O) to form bimetallic sulfides at high temperature. Temperature is the core parameter in the sulfidation process, which directly affects the reaction kinetics, material structure and final properties.³⁰ The structural collapse of Cu–Co-MOF due to high temperature was avoided at 140 °C, and the nanoflower structure was better preserved to ensure the abundance of active sites while improving the electrical conductivity. The multivalent states of Cu–Co–S (e.g., Cu²⁺/Cu⁺, Co³⁺/Co²⁺) optimizes the electronic states on the sulfide surface, due to the electronic coupling between Cu and Co. Currently, MnO₂ is a highly promising electrode material for supercapacitors due to its high reserves and low cost. It is worth noting that the conventional water/solution thermal method for synthesizing MnO₂ thin films has some problems, such as poor electrical conductivity as well as the susceptibility of MnO₂ particles to buildup caused by intermolecular forces, which will reduce the number of active sites, slow down the electron transport rate, and affect the electrochemical performance.³¹ Using electrodeposition combined with surfactants, a layered MnO₂ film can be formed on the surface of the sample, solving the buildup problem caused by the traditional aqueous/solution thermal method, and at the same time, the MnO₂ combines with the well-conducting Cu–Co–S substrate to build a layered nanocomposite structure with controllable morphology and size. The MnO₂ film anchors metal sulfides within the electrolyte, thereby inhibiting their expansion and shedding during charge–discharge cycles. This stabilization mechanism effectively reduces active material loss, particularly in alkaline environments, which significantly enhances the structural durability of the composite system.

A series of characterization methods were used to correlate Cu–Co–S@MnO₂ and its precursors. Under scanning electron microscopy observation, it is very obvious that Cu–Co-MOF is anchored on NF (Fig. 2a), Combined with the EDS elemental mappings (Fig. S1 and Table S1†), the growth of Cu–Co-MOF on the NF substrate is clearly observed. The homogeneous distribution of Cu and Co elements across the NF surface, as evidenced by the EDS analysis, confirms the successful deposition of Cu–Co-MOF, indicating uniform coverage and effective integration with the substrate. And the high magnification SEM images show that the MOF nanoflower consists of ultra-thin 2D nanosheets self-assembled, and its hierarchical structure exhibits the typical three-dimensional radiating morphology, with a single the diameter of the “nanoflower” is about 1–2 μm, and the surface is rich in folds and pores (Fig. 2b). This multilevel structure not only significantly increases the specific surface area of the material, but also provides high-quality channels for ions from the electrolyte.³² Typical XRD peak positions of ZIF-67 include 7.4° (011), 12.7° (112), 14.7° (022), 16.4° (013), 18.0° (222), etc.³³. The XRD peak positions of Cu–Co-MOF at 11.5°, 14.7°, 16.4°, and 18.7° (Fig. S5†) are close to the standard peaks but with slight shifts, which may be due to the fact that the partial substitution of Cu²⁺ (ionic radius 0.73 Å) for Co²⁺ (0.745 Å) leads to lattice contraction, which shifts the peaks to higher angles (e.g., 16.0° vs. 16.7°).³⁴ The almost intact preservation of the nanoflower-like structure of Cu–Co–S display in Fig. 2c and d demonstrates that the low-temperature solvent-thermal method can effectively avoid the collapse of the structure of the MOF material, and the “etch-recrystallization” equilibrium in the topological transformation process was achieved by optimizing the sulfur source concentration (3 mmol) and the reaction time to achieve the ‘etch-recrystallization’ equilibrium during the transformation of the components in the macroscopic morphology was maintained at the same time, and the combination of mapping analysis (Fig. S2 and Table S2†) showed the uniform distribution of Cu, Co, and S elements on the nanosheets, indicating that the sulfidation process did not trigger metal segregation, further confirming the structural homogeneity. Unlike the conventional high-temperature calcination vulcanization process,^35,36 the present study achieves dual structure-component regulation through a ligand-assisted strategy, which simplifies the reaction process and reduces the production of NH₃ and H₂S during the vulcanization process. This “soft template” transformation mechanism (MOF to sulfide) achieves synergy between morphological inheritance and component upgrading through localized ion exchange rather than overall structural destruction.

Based on previous reports,³⁷ the MnO₂ layer, fabricated via electrochemical deposition, exhibits a unique hierarchical and highly textured morphology characterized by interconnected wrinkled nanosheets, forming a porous network structure akin to tripe. As revealed by high-resolution SEM imaging (Fig. 2e), the MnO₂ nanosheets are uniformly aligned on the Cu–Co–S substrate, constructing a three-dimensional labyrinthine architecture with undulating surfaces and nanoscale ridges. Individual nanosheets display a crumpled, leaf-like morphology with thicknesses of 15–30 nm and lateral dimensions of 350–400 nm, creating interwoven macro-/mesopores (∼5 nm in diameter), which significantly enhance the accessible specific surface area. This tripe-like topological structure originates from anisotropic growth kinetics during electrodeposition (Fig. 2f). The distinct wrinkled configuration not only provides mechanical flexibility to mitigate volume fluctuations during electrochemical cycling but also facilitates rapid ion diffusion through shortened transport pathways. The MnO₂ layer conformally coats the Cu–Co–S scaffold, with intimate interfacial contact suggesting robust chemical bonding between components. EDS elemental mappings (Fig. 2k) further confirm the homogeneous distribution of Mn and O across the tripe-like architecture, while localized enrichment of Cu and Co at the interface implies electron transfer interactions critical for synergistic effects. These structural and compositional features collectively underpin the enhanced electrochemical performance of the composite. The dense packing of nanosheets highlights the uniformity of MnO₂ deposition, which effectively prevents structural collapse during electrochemical processes. This strong interfacial coupling endows the composite with exceptional mechanical stability and interfacial charge transfer capabilities.

Additionally, the SEM image of the standalone MnO₂ film (Fig. S4†) provides a comparative perspective. Unlike the tightly stacked nanosheets in Cu–Co–S@MnO₂ (Fig. S3†), the pure MnO₂ film exhibits a more disordered arrangement with visible interlayer gaps, underscoring the critical role of the Cu–Co–S substrate in guiding the vertical growth and densification of MnO₂ nanosheets during electrodeposition. From an electrochemical kinetics perspective, this three-dimensional interconnected nanosheet network not only provides abundant active sites but also establishes a high-speed electron transport network, facilitating efficient electron migration during charge/discharge processes.^38,39

Fig. 2f highlights the presence of numerous irregular cavities on the surface of the MnO₂ ultrathin nanosheets. These cavities arise from the ion exchange behavior between K⁺ and CTA⁺, as well as the regulation of MnO₂'s layered structure, which reflects critical mechanisms in intercalation chemistry and interlayer engineering.⁴⁰ By introducing CTAB (hexadecyl trimethyl ammonium bromide) as a surfactant, the spatial effects induced by CTA⁺ weaken interlayer van der Waals forces, preventing aggregation of MnO₂ particles⁴¹ and enabling their uniform distribution on the Cu–Co–S surface. This design mitigates structural damage or collapse of Cu–Co–S during electrochemical cycling. Furthermore, the persistence of S elements in Fig. 2k confirms the successful synthesis of Cu–Co–S@MnO₂, which shortens ion diffusion distances and enhances electrochemical performance. This unique structural design effectively addresses the issues of poor conductivity and severe volume expansion in traditional MnO₂ electrode materials, offering novel insights for developing high-performance energy storage devices.

Regarding Fig. 2g, the Cu–Co-MOF, directly hydrothermally grown on nickel foam, exhibits a thin thickness. During XRD testing, its diffraction peaks may be overshadowed by those of the NF substrate. Additionally, Fourier-transform infrared spectroscopy (FTIR) analysis of the sample reveals characteristic peaks at 1250 cm⁻¹, corresponding to C–O–C stretching vibrations (Fig. 2h). The peak at 2400 cm⁻¹ is unusual in common IR spectra and may indicate the presence of a specific chemical bond or functional group in Cu–Co-MOF. The peak at 3173 cm⁻¹ is attributed to hydroxyl (–OH) or other hydrogen-containing functional groups, likely originating from the stretching vibrations of C–H bonds in imidazole rings and methyl groups. Through nitrogen adsorption–desorption isotherms and pore size distribution analysis, the porosity of Cu–Co-MOF was systematically studied. BET results show that the specific surface area of Cu–Co-MOF is 50.81 m² g⁻¹. The adsorption isotherm exhibits a typical Type IV profile (Fig. 2i), with a distinct hysteresis loop at a relative pressure of 0.4 to 0.8, confirming the highly ordered mesoporous structure of the material.^42,43 The steep adsorption at P/P₀ < 0.1 indicates the presence of some micropores, while the gradual adsorption in the medium and high-pressure regions is attributed to nitrogen capillary condensation in mesopores.^44–46 Further analysis of the BJH pore size distribution (Fig. 2j) reveals that the mesopores of Cu–Co-MOF are mainly concentrated in the range of 2 to 5 nm, with a cumulative pore volume of 0.16 cm³ g⁻¹. This narrow mesoporous distribution is closely related to the steric hindrance effect of 2-methylimidazole ligands and the dynamic assembly process of bimetallic nodes.⁴⁷ The mesopore size of less than 5 nm can provide a high density of active sites, and the moderate pore volume is conducive to the diffusion and mass transfer of reactant molecules, thereby enhancing the electrochemical performance. Additionally, the micropore-mesopore hierarchical structure may stabilize intermediates through confinement effects, further optimizing the reaction pathway.

As shown in Fig. 3a, under a resolution of 50 nm in the TEM spectrum, Cu–Co-MOF presents a flower-like shape, which is consistent with the SEM observations. HRTEM and selected area electron diffraction (SAED) analysis confirm that Cu–Co-MOF has a face-centered cubic (FCC) lattice structure. The lattice spacings of its crystal planes are 0.251 nm for (111), 0.24 nm for (200), and 0.18 nm for (222). Refined unit cell parameters (a = 0.416 nm) a indicate significant structural contraction compared to conventional MOFs. This is attributed to the dense atomic packing induced by the synergistic coordination of Cu–Co bimetallic centers, which enhances electronic interactions. Fig. 3b shows, in the TEM image of Cu–Co–S, HRTEM reveals clear lattice fringes with a spacing of 0.29 nm, corresponding to the (311) plane of the spinel phase CuCo₂S₄ (JCPDS#42-1450). Fourier transform analysis further identifies a periodic feature of 0.191 nm along the (110) zone axis, matching the (116) plane. These results confirm the single-crystalline nature of the sulfide, where the synergistic coordination of Cu–Co stabilizes the cubic close-packed structure, significantly improving structural stability. Under a resolution of 1 μm, the conformal coating of δ-MnO₂ nanosheets (with a layer spacing of 3–5 nm) on CuCo₂S₄ polyhedrons is clearly visible (Fig. 3c), confirming a uniform core–shell structure. High-resolution imaging at 500 nm resolution reveals stacking faults in the layered structure of MnO₂, implying a kinetically controlled mechanism for electrodeposition growth. Atomic-level HRTEM analysis identifies a dual-lattice period at the heterojunction: a lattice spacing of 0.238 nm corresponds to the (101) plane of MnO₂ (JCPDS#80-1098), and 0.2133 nm matches the (220) direction of CuCo₂S₄ (JCPDS#42-1450). The deviation in the angle between the crystal planes is less than 1°, confirming epitaxial growth. The Fourier transform spot pattern shows that the (101) plane of MnO₂ is aligned with the [220] zone axis of CuCo₂S₄, further confirming atomically ordered arrangements. This structure enhances the d-electron coupling between Co³⁺ and Mn⁴⁺ centers. SAED rings confirm the coexistence of two phases, showing the superposition of polycrystalline MnO₂ and single-crystalline CuCo₂S₄, which supports the integrity of the interface. In the EDS elemental analysis, Mn is uniformly distributed in the shell, indicating the successful synthesis of MnO₂. This electrodeposition structure maximizes the utilization of active sites and suppresses the dissolution of MnO₂.

Comprehensive XPS analysis of the Cu–Co–S@MnO₂/NF composite delves into the intricate distribution of chemical states and electronic structures shaped by multi-element synergy. The survey spectrum comparison serves as confirmation for the successful incorporation of all elements as shown in Fig. 3d. The Mn 2p_3/2 main peak at 644.5 eV indicates Mn⁴⁺ dominance, attributed to the MnO₂ lattice oxygen environment and signifying successful MnO₂ synthesis.⁴⁸ The shoulder peak at 640.8 eV points to Mn³⁺, possibly from oxygen vacancies or interfacial charge transfer. The satellite peak at 655.8 eV strengthens the case for Mn³⁺ presence. This suggests electronic interactions between Cu/Co/S components and MnO₂, causing partial Mn⁴⁺ reduction. Turning to the Cu 2p spectrum (Fig. 3d), the Cu 2p_3/2 main peak at 936.2 eV aligns with Cu²⁺ characteristics. The weak peak at 934 eV may correspond to Cu⁺ in Cu₂S, hinting at interfacial sulfide formation. The strong satellite peak at 944.7 eV on the high-binding-energy side confirms Cu²⁺ prevalence. However, the binding energy exceeds that of pure CuO (952.7 eV), likely due to interfacial charge redistribution involving Co/MnO₂. As for the Co 2p spectrum, the 2p_3/2 peak at 782.4 eV implies Co²⁺ dominance, along with a small amount of Co³⁺ at 778.5 eV. The satellite peak at 775.3 eV arises from ligand-to-metal charge transfer, shedding light on Co²⁺'s chemical environment.⁴⁹

As exhibition in Fig. 3d, the S 2p spectrum reveals two chemical states. The main peak at 165.3 eV is assigned to sulfate (SO₄²⁻)⁵⁰ or sulfonic groups (–SO₃H), indicating highly oxidized sulfur. The weak peak at 160.5 eV possibly represents metal sulfides like Cu–S or Co–S, suggesting residual under-oxidized sulfides. In the O 1s spectrum, lattice oxygen from MnO₂'s O²⁻ is at 530.1 eV. The peak at 531.5 eV corresponds to hydroxyl oxygen due to surface hydroxylation, indicating abundant active oxygen sites on the material's surface. Fig. 3d is dominated by a peak at 284.8 eV from adventitious carbon. The small peak at 288.5 eV represents oxidized carbon species, likely from synthetic residues or environmental adsorption, which may aid electron transport due to its conductivity. The coexistence of mixed valence states (Mn³⁺/Mn⁴⁺, Cu²⁺/Cu⁺, Co²⁺/Co³⁺) and interfacial synergy is vital for optimizing the material's properties. Electron transfer from Cu/Co to MnO₂ partially reduces Mn⁴⁺. The co-existence of sulfides and oxidized sulfur modulates surface hydrophilicity/hydrophobicity and active-site exposure. Oxygen vacancies and hydroxyl oxygen enhance proton transport and redox kinetics. This multi-scale electronic structure modulation endows the composite with potential in the electrocatalysis and energy storage.

Electrochemical tests were performed on Cu–Co-MOF/NF (Fig. S6†), Cu–Co–S/NF (Fig. S7†), and Cu–Co–S@MnO₂/NF (Fig. S8†) in a 3 M KOH solution. The transition from bimetallic MOFs to bimetallic sulfides significantly enhances conductivity, which is attributed to the improvement in material conductivity due to the sulfidation process, addressing the poor conductivity issue inherent to MOF materials.^51,52 As illustrated in Fig. 4a, at a scan rate of 10 mV s⁻¹, the CV curve of Cu–Co–S@MnO₂/NF encloses a larger area than the others. This is due to the heterostructure between MnO₂ and Cu–Co–S, which synergizes the strong oxidizing nature of transition metal sulfides with the capacitive compensation of metal oxides. This unique core–shell structure accelerates ion diffusion pathways and enhances structural stability during charge–discharge processes.⁵³ Comparing the GCD curves of the three materials (Fig. 4b), it is evident that Cu–Co–S@MnO₂/NF has a longer discharge time, demonstrating its superior charge storage capability. Relative to the pure NF material (Fig. S9†) under identical current density conditions, the charge–discharge time is markedly prolonged, achieving nearly 1700 seconds, with a calculated specific capacitance surpassing 10-fold that of pure NF. Furthermore, at the same current density, Cu–Co–S@MnO₂/NF consistently outperforms Cu–Co-MOF/NF and Cu–Co–S/NF (Fig. 4c). For example, at a current density of 1 A g⁻¹, the specific capacity of Cu–Co–S@MnO₂/NF is 1483.3 F g⁻¹, which is higher than that of Cu–Co–S/NF (1051.77 F g⁻¹) and Cu–Co-MOF/NF (260 F g⁻¹). This clearly proves the synergistic effects of hierarchical structure engineering and multicomponent integration.

The initial transformation from Cu–Co-MOF to Cu–Co–S significantly enhances electrochemical activity by replacing organic ligands with conductive sulfide species, which not only improves charge transfer kinetics but also retains a nanoflower framework rich in redox active sites.⁵⁴ Subsequently, the introduction of MnO₂ nanosheets onto Cu–Co–S further amplifies the capacitance through two key mechanisms: (1) MnO₂ provides additional capacitive contribution through surface redox reactions involving Mn²⁺/Mn³⁺ pairs,⁵⁵ and (2) the heterointerface formed between Cu–Co–S and MnO₂ optimizes charge redistribution, achieving efficient electron transfer across the composite material. The specific capacity increases threefold from Cu–Co-MOF to Cu–Co–S@MnO₂, highlighting the crucial role of phase evolution and interface engineering in activating and stabilizing redox active centers. The specific capacity improvement of 41% from Cu–Co–S to Cu–Co–S@MnO₂ emphasizes the effectiveness of hybridizing transition metal sulfides with metal oxides for synergistic charge storage.⁵⁶ This performance hierarchy validates the design principle of combining conductive sulfide matrices with electroactive oxide coatings to maximize ion-accessible surface area and faradaic reaction kinetics in alkaline energy storage systems.

Fig. 4d presents the electrochemical impedance spectroscopy (EIS) analysis for evaluating charge transfer efficiency, revealing key electrochemical properties.^57–59 The slope in the low-frequency region reflects the ionic diffusion characteristics of the material, which are closely related to the capacitive behavior of the electrode and the migration efficiency of electrolyte ions.^60,61 Significantly, the Nyquist plot of Cu–Co-MOF/NF exhibits a steeper slope approaching the Y-axis, indicating more pronounced capacitive behavior and lower internal ion diffusion resistance within the electrode. In the high-frequency region, Cu–Co–S@MnO₂/NF demonstrates the smallest intercept on the X-axis (ohmic resistance) and the smallest semicircle radius (charge transfer resistance), highlighting its superior conductivity and interfacial charge transfer kinetics. This enhancement is attributed to the electron coupling effects at the heterostructured interface, where strong electronic interactions between Cu–Co–S and MnO₂ (e.g., S–O–Mn bonds) optimize interfacial charge distribution and reduce the activation energy for redox reactions.

The synthesis strategy of Cu–Co–S@MnO₂/NF—starting from the hydrothermal growth of bimetallic MOF (Cu–Co-MOF) on a conductive substrate (Cu–Co–S), followed by surface functionalization (Cu–Co–S@MnO₂)—enables precise interfacial electronic structure modulation. This design effectively addresses the common challenges of poor conductivity and sluggish reaction kinetics in transition metal compounds, offering a novel approach for developing high-power-density energy storage devices. Combined with Bode plot (Fig. S10†), the three electrode materials show inconsistent electrochemical impedance characteristics. In the low-frequency 1 Hz region, Cu–Co-MOF/NF exhibits the lowest impedance, close to 1011 Ω, indicating its smaller R_ct and optimal ion diffusion kinetics.⁶² This may be attributed to the MOF's nanoflower structure promoting electrolyte penetration and ion transport. In contrast, Cu–Co–S@MnO₂/NF shows significantly increased low-frequency impedance. It's assumed that introducing MnO₂ nanocoatings enhances active site density but increases interfacial resistance. Cu–Co–S/NF's impedance lies in between (approximately 1011–1022 Ω), suggesting its charge transport is restricted by the sulfide substrate's compatibility with the electrolyte. Interestingly, despite low-frequency kinetic limitations, the high-frequency (>1 kHz) impedance (∼1000 Ω) is similar to Cu–Co–S/NF and Cu–Co-MOF/NF, indicating MnO₂ modification doesn't notably worsen the material's bulk conductivity or current collector interface contact. Given its high impedance and MnO₂'s inherent redox activity, Cu–Co–S@MnO₂/NF is predicted to deliver high-energy-density storage via synergistic multi-level heterogeneous interfaces. This makes it suitable for battery-type electrode systems requiring high specific capacity and stable redox cycles. Moreover, MnO₂ coating's structural protective role may enhance the electrode's stability during long-term charge–discharge, which presents a possible benefit for hybrid supercapacitor applications. Furthermore, as shown in Fig. 4e, the anodic peak current of Cu–Co–S@MnO₂/NF increases linearly with the square root of the scan rate. From an electrochemical kinetics perspective, this linear relationship confirms that the oxidation process in Cu–Co–S@MnO₂/NF is predominantly diffusion-controlled. The anodic and cathodic peaks indicate a mixed mechanism with enhanced diffusion contributions. Both b values, ranging between 0.5–1, align with hybrid charge storage systems. At higher scan rates, capacitive effects dominate, which is consistent with hierarchical structures that enhance interfacial kinetics. These findings confirm the material's balanced synergy between rapid capacitive response and sustained diffusion-based capacity, which is crucial for high rate energy storage applications. At higher scan rates, the concentration gradient of reactants at the electrode surface intensifies, accelerating diffusion rates and thereby increasing the current. This behavior underscores the critical role of reactant diffusion transport rates in governing the electrode reaction dynamics of the material.

Fig. 4f summarizes the specific capacitance performance of various Co-based materials (Table S4†), among which the Cu–Co–S@MnO₂/NF material shows a remarkable superiority. The specific capacitance of Cu–Co–S@MnO₂/NF reaches 1483.3 F g⁻¹, and this value is much higher than that of the other compared materials. This result indicates that Cu–Co–S@MnO₂/NF has a high efficiency and potential for electrochemical energy storage, and its excellent specific capacitance performance makes it stand out among many Co-based materials, which provides an important reference for the research and development of electrode materials for high-performance supercapacitors. A constant-current full electric cycling process of 10000 cycles at a current density of 10 A g⁻¹ was performed on Cu–Co–S@MnO₂/NF electrode sheets, and the capacity retention rate was 99.47% and the coulombic efficiency was 99.85% after 10000 cycles (Fig. 4g), and this excellent cycling stability and highly efficient coulombic efficiency indicate that the Cu–Co–S@MnO₂/NF material is not only highly active and stable during electrochemical energy storage, but also capable of maintaining very low active material loss and excellent ion/electron transfer efficiency during long charging and discharging processes. This outstanding performance may be ascribed to the composite structure design of the material, in which the synergistic effect of Cu and Co and the introduction of S optimize the electronic conductivity,⁶³ while MnO₂ as the outer shell material provides a stable structural framework, which enables the material to maintain high performance at both high current density and long cycling process.

To further explore the applicability of Cu–Co–S@MnO₂ in supercapacitors, an asymmetric flexible supercapacitor (Cu–Co–S@MnO₂//AC) was constructed using a Cu–Co–S@MnO₂ composite electrode as the positive electrode, activated carbon (AC) as the negative electrode, and PVA-KOH gel as the solid-state electrolyte (Fig. 5a). Fig. 5b presents the CV curves of the Cu–Co–S@MnO₂//AC at various scan rates. As the scan rate increases, the enclosed area of the CV curves gradually expands, indicating that the charge storage capacity of the capacitor enhances with the increase in scan rate. Even at a high scan rate of 50 mV s⁻¹, the CV curves maintain stable. This is attributed to the complementary effect of the double-layer capacitance of the negative electrode AC⁶⁴ and the pseudocapacitance of the positive electrode, jointly supporting the stable increase in the overall capacitance of the device with the increase in scan rate.

Fig. 5c displays the GCD curves of the asymmetric flexible supercapacitor device at different current densities, yielding an excellent specific capacity of 164.6 F g⁻¹. Notably, the device possesses a wide voltage window of 1.55 V, demonstrating the integration of the wide voltage characteristics of the MnO₂ film and excellent capacitor performance.⁶⁵ Based on the CV curves, a linear analysis was conducted between the logarithm of the scan rate and the logarithm of the peak current (Fig. 5d). The b-values for peak 1 and peak 2 are 0.8746 and 0.6867, respectively, confirming the hybrid capacitive mechanism of the device, i.e., the pseudocapacitive behavior of Cu–Co–S@MnO₂ and the double-layer capacitive behavior of AC.^66,67 An R² value of 0.9310 indicates good linear correlation, suggesting that the reaction kinetics corresponding to this peak highly conform to the theoretical model within the tested scan rate range, with excellent data consistency. However, it is slightly lower than peak 2, possibly due to minor polarization effects at high scan rates leading to data dispersion. It is evident that as the scan rate increases, the capacitive contribution ratio increases from 35% to 68% (Fig. 5e, f and S12†). The heterointerface in Cu–Co–S@MnO₂ can reduce charge transfer resistance (R_ct), accelerate electron transfer, and simultaneously optimize ion adsorption kinetics through lattice strain or defect engineering, thus maintaining a high pseudocapacitive contribution at high scan rates.

In addition, the device has an energy density of 185.25 Wh kg⁻¹ at a high power density of 0.75 kW kg⁻¹, which is much higher than the relevant supercapacitor performances in recent years (Fig. 5g and Table S2†). This only indicates that Cu–Co–S@MnO₂ can be directly used and fully integrated into a FASC device without any further modifications, which would greatly reduce the time cost and the material costs. In order to evaluate the stability of the device, 5000 GCD tests were performed on the device at a current density of 5 A g⁻¹, in which the coulomb efficiency was stable and remained at 99.53% after 5000 cycles, which reflects good stability, the capacity retention started to show a decreasing trend after 3000 cycles, and there was still 88.1% capacity retention after 5000 cycles, which was attributed to the fact that the PVA-KOH begins to age, reducing the ion mobility,^68,69 nevertheless, the capacity retention of the device also reflects a good data performance. Fig. S11† shows the device's GCD curves in bent and unbent states. The similar charge/discharge times and voltages indicate good stability. In short, the excellent performance of the Cu–Co–S@MnO₂ asymmetric flexible supercapacitor is attributed to the multifunctionalization of the electrode materials. First, the built-in electric field formed through charge redistribution between Cu–Co–S and MnO₂ greatly enhances the ion transport rate between the active material and the electrolyte, and at the same time reduces the interfacial R_ct, which significantly strengthens the reaction kinetics; second, the nanoflower-like Cu–Co-MOF is successfully converted into Cu–Co–S, and this template inheritance strategy retains the excellent structure with abundant active sites, while exploiting the sulfidation process to expose the active sites in the asymmetric flexible supercapacitor. Structure, and at the same time, the sulfur vacancies as well as bimetallic active sites were exposed by the sulfidation process, which provided a dense network of active sites for efficient pseudocapacitive behaviors; finally, under the architecture of MnO₂ film, the wide voltage performance of 1.55 V obtained by the FASC, and its interlayer gap (∼0.7 nm) constructed a high-speed OH⁻ ion transport in the PVA-KOH channel, providing an ideal electrode solution for wearable energy storage devices.

Further systematic electronic structure analysis of the Cu–Co–S@MnO₂ heterostructure (Fig. 6a) was carried out using density functional theory (DFT), with a focus on exploring the regulatory mechanism of the interfacial coupling effect between the bimetallic sulfide and layered MnO₂ on the material's electronic properties. In the model construction phase, based on the body-centered tetragonal crystal structure of layered MnO₂ (space group I4/m, unit cell parameters a = 9.72 Å), the spinel structure of Co₂CuS₄ (space group Fdm) was designed following the symmetry-principle, with its lattice parameter (a = 9.42 Å) achieving a good lattice compatibility with the MnO₂ substrate (lattice mismatch rate <5%).⁷⁰ This atomic-scale structural compatibility provides the foundation for constructing a stable hetero-interface. Band structure calculations indicated that the pure-phase Cu–Co–S shown in Fig. 6b exhibited an indirect bandgap of 1.31 eV along the W–L high-symmetry path. Combined with the projected density of states (PDOS) of Cu–Co–S as Fig. 6c, the valence band top was mainly composed of the hybridization of Co-3d and S-3p orbitals (Fig. S13†), while the conduction band bottom originated from the contribution of the Cu-3d orbitals. Such orbital distribution characteristics endow the material with typical semiconductor properties. In contrast, the band structure of layered MnO₂ (Fig. 6d) showed continuous band overlap near the Fermi level, with the valence and conduction bands directly crossing the Fermi level along the X–P path, forming a typical metallic electronic structure. Fig. 6e also clearly showed that the density of states was concentrated near the Fermi level. This unique conductivity can be attributed to the Mn–O octahedral edge-sharing network in the layered MnO₂ structure, which provides effective delocalized channels for electron transport.⁷¹

After constructing the Cu–Co–S@MnO₂ heterostructure (Fig. 6f), the interfacial coupling effect led to significant electronic structure reconstruction. The heterojunction exhibited dense band crossing points near the Fermi level, and PDOS analysis revealed obvious electron cloud overlap at the interface. Specifically, as shown in Fig. 6g, the PDOS of Cu–Co–S@MnO₂ was analyzed, with the O-2d orbitals of MnO₂ (Fig. S14†) forming a strong hybridization with the S-3p orbitals of the sulfide, and the Co-3d and Cu-3d orbitals producing synergistic coupling with the Mn-3d orbitals through interfacial charge transfer. This multi-orbital synergistic effect resulted in electron delocalization in the interfacial region of the heterojunction, causing the overall system to exhibit quasi-metallic conductivity. DFT simulations further confirmed that the charge redistribution at the heterointerface significantly reduced the electron migration barrier, providing a theoretical basis for the experimentally observed enhanced electrochemical performance. These results validate our initial hypothesis that the synergistic combination of conductive Cu–Co–S framework and MnO₂ protective layer could overcome the traditional limitations of MOF-based supercapacitors.

Conclusions

The bimetallic nanoflower MOF was used as a precursor, and a template inheritance strategy was employed to vulcanize it and still have a good nanoflower structure. In order to optimize the material structure, a layered MnO₂ was constructed on the Cu–Co–S surface, and the synergistic coupling between the two was utilized to establish an efficient heterojunction interface, which accelerated the ion/electron transport by lowering the interfacial charge transfer resistance and optimizing the redox kinetics. Cu–Co–S@MnO₂ exhibits a high degree of 1483.3 F g⁻¹ at 10 Ag⁻¹ with 99.47% capacity retention after 10000 GCD cycles. PVA-KOH is used as the gel electrolyte to construct an asymmetric flexible supercapacitor, and the device achieves a high power density of 185.25 Wh kg ⁻¹ at 0.75 kW kg ⁻¹ with a high energy, an excellent rate capability (88.4% capacity retention after 5000 GCD cycles), and the structure inherited from the Cu–Co-MOF precursor is transformed into a porous Cu–Co–S nanoflower framework, which provides a rich variety of accessible active sites (e.g., Co³⁺/Co²⁺, Cu³⁺/Cu²⁺ and sulfur vacancies), realizing a high pseudocapacitance contribution (b ≈ 0.87 for the main surface-controlled reaction). At the same time, vertically aligned MnO₂ nanosheets improve the electrolyte ion intercalation/decalcification efficiency, while the conducting Cu–Co–S core ensures fast electron transport. These remarkable properties stem from the dual optimization of surface-dominated pseudocapacitance and bulk ion diffusion pathways, and are validated by scan-rate-dependent capacitance contribution analysis. In addition, combined with DFT calculations, the superiority of the built-in electric field in the heterojunction was fully demonstrated, which promoted the electron migration rate in the electrochemical reaction process, enabling Cu–Co–S@MnO₂ to achieve better conductivity.

Data availability

Data will be made available on request.

Author contributions

Jiale Hou: data curation, formal analysis, investigation, writing – original draft. Ziheng Huang: formal analysis, investigation. Cheng Chen: data curation, writing – review & editing. Xinfeng Wu: data curation, investigation. Yonghou Xiao: investigation, formal analysis. Wanghui Wei: data curation, formal analysis. Yanyun Ma: methodology. Xinzhou Ma: funding acquisition, data curation, formal analysis. Shigang Sun: funding acquisition, writing – review & editing. Donghai Lin: conceptualization, project administration, funding acquisition, supervision, writing – review & editing.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

This work is supported by the Program for Professor of Special Appointment (Eastern Scholar) at SIHL, Gaoyuan Discipline of Shanghai-Materials Science and Engineering, and Shanghai Polytechnic University-Drexel University Joint Research Center for Optoelectronics and Sensing. This work is also supported by Science Fund for Distinguished Young Scholars of Fujian Province (2019J06027), open project of Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices (Soochow University) (KS2022), Collaborative Innovation Center of Suzhou Nano Science & Technology, the 111 Project, Joint International Research Laboratory of Carbon-Based Functional Materials and Devices.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta03226a

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