Engineering multiscale hollow core–shell nanostructures via in situ surface functionalization for advanced electrochemical energy storage applications

Abstract

The development of sustainable energy storage technologies is critical in addressing the global challenges posed by climate change. Supercapacitors, while offering exceptional power density and cycling stability, suffer from relatively low energy density, limiting their widespread use in large-scale energy storage systems. To overcome this limitation, we designed a novel composite electrode material featuring a core–shell structure. The core derived from well-defined ZIF-67 nanocubes (NCs) was innovatively processed into a hollow structure, which enhanced ion diffusion and increased the overall energy storage capacity by reducing internal resistance. Meanwhile, the shell consisted of 3D hierarchical Ni–Co layered double hydroxides (NiCo-LDH) grown in situ employing an ambient-temperature method, offering high electrochemical activity and abundant active sites for efficient charge storage. The ultimately synthesized multi-scale hollow core–shell material, Co₃O₄-HNC@NiCo-LDH, integrated the respective merits of the shell and core materials, while simultaneously addressing issues that arise when these materials exist in isolation. It effectively mitigated problems such as volume expansion and agglomeration that materials might encounter during electrochemical reactions, thereby further enhancing the materials’ performance and service life. Notably, in situ Raman spectroscopy was utilized to trace the dynamic redox processes and structural changes occurring during electrochemical cycling, thereby validating the stability and effectiveness of the charge storage mechanism. The resulting material, Co₃O₄-HNC@NiCo-LDH, demonstrated impressive capacitance (1862.4 F g⁻¹ at 2 A g⁻¹), high energy density (76.8 Wh kg⁻¹ at 2 A g⁻¹), and excellent cycling stability (98.38% after 15 000 cycles at 15 A g⁻¹), offering a promising solution for next-generation supercapacitors.

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1. Introduction

The urgent global imperative to mitigate the impacts of climate change has precipitated a vast, multifaceted endeavor within the scientific community, catalyzing the development of pioneering, sustainable energy storage technologies that are promising to address the critical energy demands of the future.^1,2 Among these, supercapacitors have emerged as particularly promising candidates due to their unparalleled power density, swift charge/discharge cycles, and exceptional cycling stability, which make them ideal for applications that require rapid energy release and prolonged operational lifetimes.^3,4 However, despite these remarkable capabilities, the relatively suboptimal energy density of supercapacitors remains a significant impediment, limiting their widespread adoption in large-scale energy storage systems.^5,6 This performance gap has galvanized considerable research into novel electrode materials aimed at enhancing the energy density of supercapacitors, while simultaneously preserving their hallmark characteristics of high-power density and exceptional cycling stability, which are essential for their success in various technological applications.^7,8

Metal–organic frameworks (MOFs) and their derivatives, along with polymetallic hydroxides and transition metal oxides/sulfides, have emerged as frontrunners among pseudocapacitive materials, owing to their exceptional electrochemical properties.^9,10 These materials, prized for their high surface areas, tunable structures, and remarkable charge storage capabilities, are at the forefront of modern energy storage research.¹¹ Despite their promising electrochemical potential, they often face inherent limitations, including poor electrical conductivity, structural instability under prolonged cycling, and suboptimal ion diffusion rates, all of which degrade their overall performance in supercapacitor applications.^12,13 In addressing the aforementioned challenges, we meticulously engineered and refined both the core and shell components of these sophisticated architectures. Through a nuanced design approach, we enhanced the core material to optimize its structural stability and electrochemical activity, while simultaneously improving the shell material to elevate its conductive properties and surface area. This approach not only addressed the inherent challenges but also maximized the potential of core–shell architectures for advanced energy storage applications, offering unprecedented improvements in charge storage capacity, cycling stability, and overall material efficiency.¹⁴

We undertook a meticulous design and refinement process for both the core and shell components, aiming to address the challenges outlined above. By strategically enhancing the properties of each phase, we were able to optimize their individual contributions, ensuring that the core material provided structural integrity and the shell material maximized electrochemical activity. An especially effective strategy for enhancing the electrochemical performance of core materials was the creation of hollow structures, which reduced internal resistance and promoted more efficient ion diffusion. This approach maximized the available surface area for electrochemical reactions, while simultaneously increasing the overall ion storage capacity by providing ample space for ion intercalation. Hollow structures mitigated internal resistance by facilitating ion diffusion through interconnected cavities, which significantly improved charge/discharge efficiency and boosted the rate capability of materials.¹⁵ In the case of shell structures, the in situ growth of nanosheet architectures, particularly those based on layered double hydroxides (LDHs), has proven to be a highly effective strategy for significantly enhancing the performance of composite materials, offering clear advantages over traditional manufacturing techniques.^16–18 LDHs are renowned for their exceptional mechanical strength, multifunctional ion-exchange properties, and superior theoretical capacitance, which make them ideal candidates for in situ growth applications, facilitating the development of materials with superior electrochemical characteristics.¹⁹ In particular, bimetallic LDHs exhibit inherent synergistic effects, optimal active sites, and excellent redox reversibility, further enhancing their electrochemical performance.^20,21 Furthermore, the design and development of advanced synthetic methods for fabricating novel environmentally friendly, room-temperature-controllable nanostructures with tailored properties proved to be an effective strategy in materials science.²² NiCo-LDH, as a battery-type material, benefits from the synergistic coupling of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺/Co⁴⁺ redox reactions, which enhance both charge storage capacity and cycling stability.²³

In this work, we integrated the various advantages mentioned above by optimizing and designing a novel composite electrode material with a core–shell structure. The material was composed of NiCo-LDH nanosheets (NSs), which were grown in situ on Co₃O₄ hollow nanocubes (Co₃O₄-HNCs) derived from ZIF-67 to form a hierarchical structure. The synthesis followed a multi-step process, beginning with precise ratio control to produce uniformly structured and well-defined ZIF-67 nanocubes (ZIF-NCs). An etching process was then employed to create hollow ZIF-67 nanocubes (HZIF-NCs), followed by calcination to yield Co₃O₄-HNCs. This etching process generated a hollow and uniform mesoporous structure, significantly improving the overall electrochemical performance by facilitating more efficient reactions and ion diffusion. After carbonization, the fragile HZIF-NCs were transformed into robust Co₃O₄-HNCs with a nitrogen-doped carbon framework, which enhanced both the structural stability and electrochemical properties of the core material. In the final phase of the synthesis, NiCo-LDH NSs were grown in situ under room temperature conditions. This step was meticulously fine-tuned to regulate the quantity of the coating, ensuring a precise balance between the core and shell layers. Meanwhile, in situ Raman spectroscopy was utilized to investigate the redox dynamics of the Co₃O₄-HNC@NiCo-LDH core–shell structure, revealing reversible transition metal redox processes and stable framework retention, thereby confirming its excellent electrochemical resilience. The resulting robust structure, with abundant active sites, effectively promoted the intercalation of ions, exhibiting a remarkable increase in specific capacitance, achieving 1862.4 F g⁻¹ at 2 A g⁻¹. The assembled asymmetric supercapacitor (ASC) established an energy density of 76.8 Wh kg⁻¹ at a power density of 829.2 W kg⁻¹. Moreover, the ASC delivered exceptional stability, retaining 98.38% of its initial capacitance after 15 000 cycles, showcasing its potential for reliable performance in real-world applications. The floating test is a powerful tool for decoupled voltage tracking of the positive and negative electrodes of the ASC, facilitating precise analysis of potential windows and internal losses, which is crucial for optimizing electrode pairing and improving the electrochemical stability of ASCs. By employing environmentally friendly, room-temperature-controllable methods for LDH shell growth, this strategy elevated both sustainability and electrochemical performance, offering improved long-term stability under high current densities and positioning these materials as key candidates for next-generation energy storage technologies.

2 Experimental

2.1 Materials and instruments

The specific information is listed in the SI.

2.2 Preparation of the ZIF-67 nanocube (ZIF-67 NC) precursor

Initially, 4 mg of CTAB were dissolved in 10 mL of DI water. 1 mmol of Co(NO₃)₂·6H₂O was gradually added to this solution. Subsequently, the mixture was combined with 70 mL of DI water previously prepared with 55 mmol of 2-methylimidazole. The resulting solution was then subjected to further agitation for an additional 30 min. ZIF-67 NCs were precipitated by centrifugation, after which they were thoroughly washed with DI water and ethanol. Subsequently, the material was swiftly transferred to an oven and subjected to drying at 75 °C for 6 h.

2.3 Preparation of the hollow ZIF-67 nanocube (HZIF-NC) precursor

1.2 g of the as-prepared ZIF-67 NCs were first dispersed in 10 mL of water and 10 mL of ethanol and then transferred into 600 mL of tannic acid solution, prepared with a 1 : 1 volume ratio of water to ethanol. The resulting solution was stirred vigorously at room temperature for 10 min to ensure complete dispersion and interaction. Subsequently, the HZIF NCs were separated by centrifugation, thoroughly washed with ethanol and water multiple times, and then dried at 80 °C overnight. With the incorporation of 0.9 to 1.5 g of tannic acid, the resulting samples were labeled as HZIF-0.9, HZIF-1.2, and HZIF-1.5.

2.4 Preparation of Co₃O₄ hollow nanocubes (Co₃O₄-HNCs)

The porcelain boat containing the HZIF NCs was placed in a tube furnace and then heated to 350 °C at a rate of 2 °C min⁻¹ under a N₂ atmosphere. The resulting samples were named Co₃O₄-HNCs.

2.5 Synthesis of NiCo-LDH

4 mmol of Ni(NO₃)₂·6H₂O, 2 mmol of Co(NO₃)₂·6H₂O, 10 mmol of NH₄F, and 1 mmol of CTAB were dissolved in 100 mL of deionized water. Under stirring conditions, 5 mL of ammonia was added to the solution, and the mixture was allowed to react at room temperature for 4 h. Upon completion of the reaction, the mixture was centrifuged, and the precipitate was thoroughly washed with ethanol and deionized water. The resulting material was designated as NiCo-LDH.

2.6 Synthesis of Co₃O₄-HNC@NiCo-LDH

The synthesis of Co₃O₄-HNC@NiCo-LDH was conducted at room temperature. Initially, 100 mg of Co₃O₄-HNCs were uniformly dispersed in 10 mL of deionized water and sonicated for 5 min to ensure uniform dispersion. Then, 4 mmol of Ni(NO₃)₂·6H₂O, 2 mmol of Co(NO₃)₂·6H₂O, 10 mmol of NH₄F, and 1 mmol of CTAB were dissolved in 90 mL of deionized water. Afterward, the uniformly dispersed Co₃O₄-HNCs were added to the mixture. The reaction was initiated by the rapid addition of 5 mL of ammonia. The resulting solution was allowed to react at ambient temperature for 4 h. After the reaction, the mixture was centrifuged repeatedly and washed thoroughly with ethanol and deionized water. The product synthesized using the aforementioned proportions was designated as Co₃O₄-HNC@NiCo-LDH-1. To investigate the impact of the LDH shell thickness on material properties, the quantities of all reagents used for synthesizing NiCo-LDH were adjusted to 0.5 and 2, while the amount of Co₃O₄-HNC was kept constant. The resulting products were designated as Co₃O₄-HNC@NiCo-LDH-0.5 and Co₃O₄-HNC@NiCo-LDH-2, respectively.

2.7 Electrochemical characterization

In the fabrication of the working electrodes, a composite blend was meticulously prepared by combining the as-synthesized materials, acetylene carbon black, and PTFE in a precise weight ratio of 8.5 : 0.5 : 1. This composite mixture was then dispersed in absolute ethanol to form a homogeneous slurry. The resultant slurry was uniformly applied to nickel foam (1 × 2 cm²), followed by air-drying at 60 °C overnight to ensure optimal binding and a uniform distribution. The final electrodes exhibited a mass loading of the active material of approximately 1.6 mg cm⁻².

Electrochemical evaluation was performed in a 6.0 M KOH aqueous electrolyte. The electrochemical characterization included a suite of advanced techniques, namely, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD) testing, and electrochemical impedance spectroscopy (EIS). For all measurements, a standard three-electrode configuration was employed, wherein platinum foil (1 × 1 cm²) acted as the counter electrode and an Hg/HgO electrode was utilized as the reference electrode.

2.8 The assembly of a button-type asymmetric supercapacitor

The initial step involved positioning Co₃O₄-HNC@NiCo-LDH-1 at the base of a button-type cell casing. Subsequently, a separator was strategically placed atop the positive electrode. The AC was then placed above the separator, ensuring symmetrical alignment with the positive electrode. A spacer and a spring were introduced, affixed atop the negative electrode. The cell was then filled with 6 M KOH aqueous electrolyte. Finally, a compressive force of 7 MPa was applied for 20 s.

Based on the calculations using the formula (C₁V₁M₁ = C₂V₂M₂), the mass of the positive electrode was determined to be 1 mg, while the mass of the negative electrode was calculated to be 3 mg to match the capacity.

3 Results and discussion

3.1 Characterization of the electrode material

Scheme 1 reveals the detailed synthesis route of Co₃O₄-HNC@NiCo-LDH. Initially, ZIF-67 NCs were created by stirring at ambient temperature. Subsequently, hollow HZIF NCs were obtained through selective etching with tannic acid solutions of varying concentrations. By selectively etching the internal volume of ZIF-67 NCs, this strategy effectively eliminated inaccessible dead space and facilitated faster reaction kinetics in subsequent processes. The hollow HZIF NCs were subsequently calcined in a tube furnace. In addition to improving structural stability, this calcination phase provided more anchoring sites for NiCo-LDH in the following step. Finally, NiCo-LDH was produced in situ on the surface of Co₃O₄-HNCs, forming the core–shell structured Co₃O₄-HNC@NiCo-LDH composite. The tight interfacial contact between the NiCo-LDH shell and the Co₃O₄-HNC core facilitated strong interactions with electrolyte ions, which in turn promoted efficient electron transfer during the charging and discharging processes.

Scheme 1 The preparation procedures of Co₃O₄-HNC@NiCo-LDH.

The micromorphology of the samples was characterized using SEM and TEM. According to Fig. S1(a–f), the synthesized ZIF-67 NCs exhibited a highly regular cubic structure, while the etched HZIF-1.2 formed a cavity cube with reduced dead space, which facilitated ion penetration and transport within the material. Subsequently, NiCo-LDH was then grown in situ on Co₃O₄-HNCs that were produced by calcining HZIF-1.2 at ambient temperature. During this process, the original morphology of the Co₃O₄-HNCs was preserved, serving as the core structure for the surface growth of NiCo-LDH.

SEM images of Co₃O₄-HNC@NiCo-LDH-0.5, Co₃O₄-HNC@NiCo-LDH-1, and Co₃O₄-HNC@NiCo-LDH-2 are presented in Fig. 1(a–c). The low magnification SEM images corresponding to Co₃O₄-HNC@NiCo-LDH are shown in Fig. S1(g–i). The NiCo-LDH layer enveloping the Co₃O₄-HNCs expanded the material in all directions, thereby providing a larger specific surface area and more active sites. This unique core–shell structure offered rapid ion transport pathways while combining the benefits of both components to optimize the sample's electrochemical performance. Notably, Co₃O₄-HNC@NiCo-LDH-1 demonstrated the most favorable morphology with more uniform lamellar growth. In contrast, Co₃O₄-HNC@NiCo-LDH-0.5 exhibited sparse and less-loaded lamellae, resulting in a contraction of the specific surface area and scarcity of active sites. The surface of Co₃O₄-HNC@NiCo-LDH-2 was completely covered with NiCo-LDH, resulting in significant agglomeration and the lowest specific surface area, which was detrimental to electrolyte diffusion. The advantages of the Co₃O₄-HNC@NiCo-LDH-1 sample were further highlighted by the corresponding TEM images (Fig. 1(e)), and the TEM images for the other samples are shown in Fig. 1(d and f). The Co₃O₄-HNC width was approximately 500 nm, providing an ideal environment for the growth of the NiCo-LDH lamellar structure (width ∼140 nm). The shell thicknesses of all composite material products are listed in Table S1 according to TEM. Fig. 1(g) shows the HRTEM image of the optimally proportioned Co₃O₄-HNC@NiCo-LDH-1. The interplanar spacings of the three lattice fringes were 0.22 nm, 0.24 nm, and 0.28 nm, corresponding to the (015) and (012) crystal faces of NiCo-LDH and the (220) crystal face of Co₃O₄-HNCs, respectively, further confirming the successful synthesis of the Co₃O₄-HNC@NiCo-LDH-1 sample.^24,25 Additionally, the SAED pattern in Fig. 1(h) reveals diffraction spots and rings. To elucidate the elemental distribution within the sample, Fig. 1(i–n) presents the HAADF-STEM image and the corresponding EDS mapping, indicating that C, Co, Ni, N, and O elements are uniformly distributed throughout the sample. The elemental spectra and compositional ratios of the Co₃O₄-HNC@NiCo-LDH-1 sample are presented in Fig. S2 and Table S2, respectively.

Fig. 1 (a–c) The SEM images of Co₃O₄-HNC@NiCo-LDH-0.5, Co₃O₄-HNC@NiCo-LDH-1, and Co₃O₄-HNC@NiCo-LDH-2; (d–f) the TEM images of Co₃O₄-HNC@NiCo-LDH-0.5, Co₃O₄-HNC@NiCo-LDH-1, and Co₃O₄-HNC@NiCo-LDH-2; (g) the HRTEM image of Co₃O₄-HNC@NiCo-LDH-1; (h) the SAED of Co₃O₄-HNC@NiCo-LDH-1; (i) the HAADF-STEM image corresponding to EDS analysis; and (j–n) the EDS elemental mapping of Co₃O₄-HNC@NiCo-LDH-1.

XRD analysis was utilized to characterize the crystal structures and phase information. Fig. 2(a) and Fig. S3(a–e) show the XRD patterns of all materials. The characteristic peaks of the prepared ZIF-67 NCs, HZIF-1.2, Co₃O₄-HNCs, and NiCo-LDH aligned with those previously reported. For Co₃O₄-HNC@NiCo-LDH, the diffraction peaks at 19.1°, 31.3°, 36.9°, 38.5°, 44.8°, 59.4°, 65.2°, and 78.4° corresponded to the (111), (220), (331), (220), (222), (400), (511), and (440) crystal planes of Co₃O₄-HNCs (PDF#42-1467), respectively. The diffraction peaks at 11.7°, 22.8°, 34.2°, and 63.0° for NiCo-LDH are attributed to the (003), (006), (009), and (110) crystal planes.^26,27 The intensity of the characteristic peaks of Co₃O₄-HNCs in Co₃O₄-HNC@NiCo-LDH was lower than that of pure Co₃O₄-HNCs. This reduction in peak intensity was due to the coating of Co₃O₄-HNCs with NiCo-LDH, which attenuated the diffraction signals from the underlying Co₃O₄-HNCs. Fig. 2(b) presents the Raman spectra of the composites. The peak at 524 cm⁻¹ was characteristic of NiCo-LDH, while the peak at 662 cm⁻¹ corresponded to the vibration of M–O (Ni–O, Co–O).²⁸ The Raman peak observed at 1040 cm⁻¹ was ascribed to the vibrational mode associated with the CO₃²⁻ groups that were intercalated between the NiCo-LDH layers.²⁹ All composites exhibited a D band and a G band; the comparable intensities of the D and G peaks indicated a well-balanced structure featuring both graphitized domains and defect sites introduced during the calcination process. These results further confirmed the successful integration of NiCo-LDH onto the surface of Co₃O₄-HNCs. The FTIR spectra of Co₃O₄-HNCs, NiCo-LDH, and Co₃O₄-HNC@NiCo-LDH composites were recorded to confirm the successful hybridization. Fig. S4 reveals that Co₃O₄-HNCs exhibited a characteristic absorption band at ∼570 cm⁻¹, corresponding to the Co–O stretching vibration of the spinel phase.³⁰ For NiCo-LDH, distinct bands at ∼3440 cm⁻¹ and ∼1620 cm⁻¹ were attributed to the O–H stretching and bending vibrations of interlayer water molecules, while the peak at ∼1380 cm⁻¹ was attributed to intercalated NO₃⁻ anions.^31,32 In the spectrum of the Co₃O₄-HNC@NiCo-LDH hybrids, the coexistence of all major absorption bands confirmed the formation of a core–shell architecture without significant disruption of the original frameworks.

Fig. 2 . (a) XRD patterns of all materials; (b) Raman spectra of the composites Co₃O₄-HNC@NiCo-LDH; (c) N₂ adsorption–desorption isotherm loops of the composites Co₃O₄-HNC@NiCo-LDH; and (d) related pore size plots of the composites Co₃O₄-HNC@NiCo-LDH.

Fig. 2(c) shows the nitrogen adsorption isotherms of the synthesized samples. All composite samples exhibited type IV isotherms, indicating the presence of micropores, mesopores, and macropores.³³ This hierarchical porous structure provided effective pathways for electrolyte ion diffusion, facilitating the full utilization of the electrode material. It was found that the SSA initially increased and then decreased with the addition of NiCo-LDH. Co₃O₄-HNC@NiCo-LDH-1 had the highest SSA at 172.6 m² g⁻¹, while Co₃O₄-HNC@NiCo-LDH-0.5 and Co₃O₄-HNC@NiCo-LDH-2 had SSAs of 141.6 m² g⁻¹ and 80.7 m² g⁻¹, respectively. This trend was attributed to the initial increase in SSA due to the presence of NiCo-LDH, which enhanced the overall surface area. However, excessive NiCo-LDH led to agglomeration, reducing the SSA. Fig. 2(d) reveals that the mesopores in Co₃O₄-HNC@NiCo-LDH-2 are primarily concentrated at 4.3 nm, with macropores at 53.3 nm. In contrast, the macropores in Co₃O₄-HNC@NiCo-LDH-0.5 and Co₃O₄-HNC@NiCo-LDH-1 were mainly concentrated at 56.3 nm. The combined nitrogen adsorption and desorption curve and pore size distribution of a single composite sample are shown in Fig. S5(a–c).

To clarify their surface chemical characteristics and the elemental oxidation states, all samples underwent further XPS analysis. The total spectra of ZIF-67 NCs, HZIF-1.2, and Co₃O₄-HNCs, as well as the spectra of each element, are shown in Fig. S6–S9. The peak positions of C, N, O, and Co in ZIF-67 NCs and HZIF-1.2 were essentially identical. In contrast, partial oxidation of Co²⁺ to Co³⁺ was observed in Co₃O₄-HNCs, which enhanced electrochemical performance. The full-scan spectra of the composite Co₃O₄-HNC@NiCo-LDH are shown in Fig. 3(a). The electrode materials were primarily composed of Co, Ni, O, N, and C elements. In Fig. 3(b), the peaks at 780.4 eV and 795.7 eV correspond to Co²⁺, while those at 781.8 eV and 797.3 eV correspond to Co³⁺. Satellite peaks were located at 785.6 eV and 802.4 eV, respectively.³⁴Fig. 3(c) illustrates that for Ni 2p, the peaks at 855.5 eV and 873.2 eV are attributed to Ni²⁺, whereas those at 856.7 eV and 874.6 eV are attributed to Ni³⁺, and satellite peaks are observed at 861.6 eV and 879.5 eV.³⁵ These results confirmed the successful integration of properties from individual materials in Co₃O₄-HNC@NiCo-LDH. The increased intensity of the Co and Ni peaks with higher feed ratios indicated the formation of the NiCo-LDH layer on the surface. In Fig. 3(d), the C 1s XPS spectra are deconvoluted into three distinct peaks at binding energies of 284.8 eV (C–C/C=C), 286.1 eV (C–O/C–N), and 288.1 eV (O–C=O).³⁶ For N 1s (Fig. 3(e)), the peaks at 398.9 eV (C–N), 399.8 eV (pyrrolic N), 402.4 eV (N–O), and 406.6 eV (nitrate) were identified.³⁷ The O 1s spectra (Fig. 3(f)) showed three peaks at 530.4 eV (M–O), 531.2 eV (M–OH), and 532.3 eV (adsorbed H₂O).³⁸ Fig. S10–S12 specifically show the full spectra of Co₃O₄-HNC@NiCo-LDH and each element.

Fig. 3 (a) XPS survey spectra of the composites Co₃O₄-HNC@NiCo-LDH; (b–f) Co 2p, Ni 2p, C 1s, N 1s, and O 1s spectra of the composites Co₃O₄-HNC@NiCo-LDH.

3.2 Electrochemical measurements of the electrode material

Fig. S13(a–d) show the CV curves of ZIF-67 NCs, HZIF-0.9, HZIF-1.2, and HZIF-1.5 at various sweep rates. Fig. S14 indicates that the HZIF-1.2 sample contains the largest area of CV curve at 20 mV s⁻¹. The GCD curves of ZIF-67 NCs, HZIF-0.9, HZIF-1.2, and HZIF-1.5 at different current densities are presented in Fig. S15(a–d). Fig. S16 indicates that HZIF-1.2 exhibits the maximum discharge duration at 2 A g⁻¹. After etching with varying quantities of tannic acid, the CV and GCD curves revealed that the properties of all samples post etching surpassed those prior to etching. Notably, the optimal performance was achieved when the amount of tannic acid was 1.2 g.

After the carbonization of HZIF-1.2 with the optimal etching ratio, the fragile HZIF-1.2 was transformed into robust Co₃O₄-HNCs with a nitrogen-doped carbon framework, and varying proportions of NiCo-LDH were subsequently grown in situ on this foundation. To demonstrate that the enhanced performance of the composite material was not attributable to any single component, Fig. S17 shows the CV and GCD curves of Co₃O₄-HNCs, respectively, while Fig. S18 displays the corresponding CV and GCD curves of NiCo-LDH. The quantity of shell structures developed on the core structure was modulated by altering the ratio of the reaction solution. Fig. 4(a) shows the CV curves of NiCo-LDH and Co₃O₄-HNC@NiCo-LDH at 20 mV s⁻¹. Among the samples tested, the CV curve of pure NiCo-LDH enclosed the smallest area, while the CV curve of Co₃O₄-HNC@NiCo-LDH-1 enclosed the largest area. The CV curves of the samples with the optimal ratios, evaluated at various scanning speeds, are shown in Fig. 4(b). (The CV curves of the remaining samples are presented in Fig. S19.) As the sweep speed increased, the CV curves exhibited no significant distortion, demonstrating excellent rate performance. Additionally, the CV curves displayed pronounced redox peaks, which collectively confirmed the occurrence of redox reactions in all samples. These redox peaks corresponded to the electrochemical reactions of the electrode material in 6.0 M KOH electrolyte as shown in eqn (1)–(4):^29,39,40

Co(OH)₂ + OH⁻ ↔ CoOOH + H₂O + e⁻ (1)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (2)

Ni(OH)₂ + OH⁻ ↔ NiOOH + H₂O + e⁻ (3)

NiOOH + OH⁻ ↔ NiO₂ + H₂O + e⁻. (4)

Fig. 4 Electrochemical performance of the composites Co₃O₄-HNC@NiCo-LDH and NiCo-LDH. (a) Comparison of CV curves at 20 mV s⁻¹; (b) CV curves at different scan rates of the composite material Co₃O₄-HNC@NiCo-LDH-1; (c) comparison of GCD profiles at 2 A g⁻¹; (d) GCD curves at different current densities of the composite material Co₃O₄-HNC@NiCo-LDH-1; (e) specific capacitance of the composite material Co₃O₄-HNC@NiCo-LDH at different current densities; (f) log(i) vs. log(v) curves at specific peak currents for Co₃O₄-HNC@NiCo-LDH-1; (g) percentage of capacitive contribution and diffusion contribution at different scan rates; (h) the area proportion diagram of diffusion control in the CV curve at 5 mV s⁻¹; (i) cycling performance at 15 A g⁻¹ of the composites Co₃O₄-HNC@NiCo-LDH-1.

Fig. 4(c) shows the GCD curve of NiCo-LDH and Co₃O₄-HNC@NiCo-LDH at 2 A g⁻¹. At the identical current density, Co₃O₄-HNC@NiCo-LDH-1 showed the most extended discharge time. Specifically, at 2 A g⁻¹, Co₃O₄-HNC@NiCo-LDH-1 achieved a specific capacitance of 1862.4 F g⁻¹, which was notably superior to that of Co₃O₄-HNC@NiCo-LDH-0.5 (1740.0 F g⁻¹) and Co₃O₄-HNC@NiCo-LDH-2 (1789.6 F g⁻¹). Fig. 4(d) presents the GCD curves of the optimally proportioned sample Co₃O₄-HNC@NiCo-LDH-1 at various current densities, while the GCD curves of the other samples are shown in Fig. S20. The GCD curves of all Co₃O₄-HNC@NiCo-LDH samples exhibited nearly symmetrical charging and discharging plateaus. This symmetry in the charge–discharge profiles suggested that the electrochemical reactions occurring at the electrode surface were highly reversible.⁴¹

The CV and GCD curves jointly confirmed that Co₃O₄-HNC@NiCo-LDH exhibited the highest specific capacitance, attributed to the in situ growth of an optimal density of NiCo-LDH on the robust Co₃O₄-HNC framework. The distinctive architecture facilitated cooperative redox coupling between Ni²⁺ and Co²⁺ centers. The synergistic impact greatly increased the overall electrochemical performance by optimizing the ion transport routes and improving the electrode's electrical conductivity and structural stability.⁴²Fig. 4(e) presents the specific capacitance of all samples in this study across varying current densities. As evident from Fig. 4(e), the Co₃O₄-HNC@NiCo-LDH-1 composite demonstrated remarkable specific capacitance performance at both low and high current densities. These findings highlight the superior rate capability and electrochemical stability of the material, as reflected in its robust capacitance retention from low to high current densities.

To further elucidate the charge storage mechanism of the Co₃O₄-HNC@NiCo-LDH-1 electrode, eqn (5) and (6) were utilized to differentiate between capacitive and diffusion-limited charge storage mechanisms:^43,44

i = av^b (5)

log i = b log v + log a. (6)

The specific steps for calculating the b value could be found in Scheme S2. As shown in Fig. 4(f), the b values of the Co₃O₄-HNC@NiCo-LDH-1 electrode, calculated from the anodic and cathodic peak current densities, were 0.58 and 0.56, respectively. The calculated b value was close to 0.5, indicating that the charge storage mechanism of the material was primarily governed by diffusion-controlled behavior during the charge and discharge processes.⁴⁵ This finding was consistent with the results observed in the CV and GCD curves.

To further elucidate the proportion of capacitance control and diffusion control in the total capacitance of the sample, the following formula was employed for quantitative analysis (eqn (7)):⁴⁶

i(v) = k₁ + k₂v^1/2. (7)

Here, k₁ and k₂ represent the contribution coefficients of capacitance control and diffusion control, respectively. Fig. S21(a) depicts the CV curve of the Co₃O₄-HNC@NiCo-LDH-1 electrode from 1 mV s⁻¹ to 20 mV s⁻¹. By fitting the CV curves at different scan rates, the values of k₁ and k₂ were determined. The calculated variation trend of the capacitance-controlled ratio of the Co₃O₄-HNC@NiCo-LDH-1 electrode, increasing from 18% at 1 mV s⁻¹ to 49% at 20 mV s⁻¹, suggests that as the sweep rate increased, the contribution of the capacitance-controlled mechanism to the total capacitance became more significant. However, the diffusion-controlled process remained the dominant factor in determining the overall capacitance. Fig. 4(h) presents the area proportion diagram of diffusion control in the CV curve at 5 mV s⁻¹, while Fig. S21 illustrates the area proportion diagrams of diffusion control at other sweep rates. Concurrently, the area ratio diagrams of capacitive control in the CV curve at various sweep speeds are shown in Fig. S22.

The cycling durability of the Co₃O₄-HNC@NiCo-LDH-1 electrode was evaluated via prolonged galvanostatic cycling at 15 A g⁻¹, as shown in Fig. 4(i). The comparison chart illustrating the stability tests of other electrodes is shown in Fig. S23. The Co₃O₄-HNC@NiCo-LDH-1 electrode exhibited an outstanding capacitance retention rate of 98.15% after 10 000 consecutive GCD cycles. Based on the GCD profiles of the initial and final three cycles, the electrochemical performance of the electrode remained virtually unchanged, despite prolonged operation at high current densities. This remarkable cycling stability demonstrates that the electrode material could maintain its efficient charge storage capacity even under high current density conditions, while also exhibiting excellent structural integrity. This superior performance could be attributed to the unique core–shell structure design of the material. Specifically, the etching of the core removed inactive regions, providing a robust internal framework, while the outward growth of the surface NiCo-LDH layer significantly increased the specific surface area and created more active sites for electrochemical reactions. EIS analysis provided insights into the electron/ion transport mechanisms within the hybrid materials. As illustrated in Fig. S24(a–d) and S25(a), the R_s of the ZIF-67 NCs exhibited a slight decrease compared to their pre-etching state. This observation suggested that the overall resistance of the electrode system was partially reduced. Fig. S26(a) presents the EIS diagrams of NiCo-LDH, and the individual samples of Co₃O₄-HNC@NiCo-LDH are provided in Fig. S27. Fig. 5(a) reveals the comparison diagram of Co₃O₄-HNC@NiCo-LDH composite materials. Among these, Co₃O₄-HNC@NiCo-LDH-1 exhibited the lowest internal resistance (R_s) value of 0.62 Ω, while the R_s values of the other samples were relatively higher. This similarity in R_s values indicated that the internal resistance of the entire system was not significantly different among the samples.⁴⁷R_ct represents the charge transfer resistance, which reflects the resistance encountered during the charge transfer process in the electrode reaction.^48,49 The Co₃O₄-HNC@NiCo-LDH-1 composite exhibited the smallest R_ct value of 0.32 Ω, followed by Co₃O₄-HNC@NiCo-LDH-2 (0.34 Ω) and Co₃O₄-HNC@NiCo-LDH-0.5 (0.85 Ω). The smallest R_ct value of Co₃O₄-HNC@NiCo-LDH-1 indicated superior electrochemical activity compared to other samples. Additionally, the Nyquist plot of Co₃O₄-HNC@NiCo-LDH-1 showed the steepest linear slope in the low-frequency region, which suggested the least diffusion resistance at the electrode–electrolyte interface.^50,51 This further highlighted the excellent electrochemical performance and efficient ion transport capability of Co₃O₄-HNC@NiCo-LDH-1.

Fig. 5 Co₃O₄-HNC@NiCo-LDH: (a) Nyquist plots; (b) Bode plots of phase angle versus frequency; the normalized real (c) and imaginary (d) part capacitance versus frequency.

Furthermore, analysis of the Bode phase diagram (Fig. 5(b)) revealed that at a phase angle of −45°, the relaxation time (τ), determined from the reciprocal of frequency, could serve as an indicator of the electrode discharge rate.⁵² The relaxation times for ZIF-67 NCs, HZIF-0.9, HZIF-1.2, HZIF-1.5, and NiCo-LDH were found to be 10.00 s, 9.84 s, 8.24 s, 8.33 s, and 6.67 s, respectively (Fig. S25(b) and Fig. S26(b)). Fig. 5(b) demonstrates that Co₃O₄-HNC@NiCo-LDH-1 exhibits a value of 1.75 s, outperforming Co₃O₄-HNC@NiCo-LDH-0.5 (5.56 s) and Co₃O₄-HNC@NiCo-LDH-2 (2.63 s). This indicates that Co₃O₄-HNC@NiCo-LDH-1 achieved the most efficient ion transport and charge transfer kinetics during the electrochemical process.

To further analyze the electrochemical behavior, both the real and imaginary parts of capacitance were examined to reflect the complex impedance characteristics of the electrode system. Specifically, changes in the imaginary part of capacitance were indicative of diffusion-controlled processes within the electrode material, while variations in the real part of capacitance reflected the charge accumulation and release processes occurring at the electrode surface. After normalizing the values of the real and imaginary capacitance as functions of frequency, the performance of the electrode material could be assessed based on the position of the curve.^53,54 Specifically, the closer the curve shifted to the right as the real capacitance changed with frequency, the better the performance of the material (Fig. 5(c) and Fig. S26(c)). This shift indicated a longer time constant for charge accumulation and release processes, suggesting enhanced electrochemical stability and efficiency. When the imaginary capacitance reached its maximum value, the reciprocal of the corresponding frequency reflected the key time scale of the electrode material during the charge and discharge process. A shorter time scale indicated that the material could maintain high energy efficiency even during rapid charge and discharge cycles. Fig. 5(d) and Fig. S26(d) show the calculated times for various samples: Co₃O₄-HNC@NiCo-LDH-0.5 (10.00 s), Co₃O₄-HNC@NiCo-LDH-1 (3.15 s), Co₃O₄-HNC@NiCo-LDH-2 (3.16 s), and NiCo-LDH (14.68 s). Fig. S28 presents the linear fitting plots of Z′ versus the inverse square root of angular frequency (ω^−1/2) in the low-frequency region. The Warburg region was typically associated with ion diffusion resistance, showing well-defined linearity for all three nanomaterials, indicating that the electrochemical processes were governed by diffusive behavior. The corresponding Warburg slopes (K), extracted from the linear fits, exhibit distinct values. Notably, the Co₃O₄-HNC@NiMn-LDH-1 electrode exhibited the lowest Warburg slope, suggesting significantly reduced diffusion impedance and enhanced ion transport kinetics. In summary, the Co₃O₄-HNC@NiCo-LDH-1 electrode exhibited the most superior electrochemical performance, which could be attributed to the optimal etching ratio, the most suitable in situ growth ratio, and the synergistic effects between these two factors.

In situ Raman spectroscopy was conducted to elucidate the electrochemical reaction pathways of the Co₃O₄-HNC@NiCo-LDH-1 composite. Raman spectra, illustrated in Fig. 6(a), were captured throughout the electrode's oxidation within a potential range from 0 to 0.9 V. A detailed view in Fig. 6(b) highlights the characteristic Raman peak for M–O (M = Ni or Co) at 662 cm⁻¹. As the voltage increased, this peak intensity heightened and shifted to higher wavenumbers, a blue shift indicating enhanced activity of the corresponding vibration mode (M–O).⁵⁵ This observation was in accordance with the electrochemical reactions outlined in eqn (1)–(4), where Ni(OH)₂ and Co(OH)₂ were oxidized to their respective oxides. In contrast, as depicted in Fig. 6(c and d), the 662 cm⁻¹ peak intensity diminished during reduction as the voltage decreased. The Raman spectra displayed heightened fluctuations and instability at elevated voltages, stabilizing as the voltage reduced.⁵⁶ Throughout the redox cycle, the Raman curve preserved a nearly symmetrical configuration. Furthermore, following a single electrochemical cycle, the Raman spectra were substantially unaltered relative to its initial state, thereby validating the reversibility of the material's electrochemical reactions.

Fig. 6 (a) In situ Raman spectroscopy of Co₃O₄-HNC@NiCo-LDH-1 during the oxidation process and (b) local magnification in the range of 400–1200 cm⁻¹; (c) in situ Raman spectroscopy of Co₃O₄-HNC@NiCo-LDH-1 during the reduction process and (d) local magnification in the range of 400–1200 cm⁻¹.

3.3 Electrochemical measurements of the ASC device

In order to explore the application potential of the Co₃O₄-HNC@NiCo-LDH-1 electrode in practical energy storage, button-type Co₃O₄-HNC@NiCo-LDH-1//AC ASCs were assembled using Co₃O₄-HNC@NiCo-LDH-1 as the positive electrode and AC as the negative electrode, with 6 M KOH serving as the electrolyte.

Fig. 7(a) presents the CV curves of the positive and negative electrodes, and the Co₃O₄-HNC@NiCo-LDH-1//AC ASC at 20 mV s⁻¹. The potential window of AC was −1 to 0 V, while that of Co₃O₄-HNC@NiCo-LDH-1 was 0 to 0.6 V. Based on these ranges, it could be inferred that the potential window of the assembled ASC was approximately 0 to 1.60 V. Therefore, in the potential window exploration of the ASC, the maximum voltage was set at 1.5–1.65 V. During CV testing (Fig. 7(b)), it was observed that significant polarization and water decomposition reactions occurred at 1.65 V. Fig. 7(c) presents the CV curves of the device at various sweep speeds within the 0–1.6 V potential window. Notably, no significant polarization was observed even at low sweep speeds. Importantly, even at 100 mV s⁻¹, the CV curves maintained their shape without obvious distortion, indicating that the ASC maintained exceptional rate capability and stability. Meanwhile, these curves illustrated that the specific capacitance of the ASC was determined by the synergistic contribution of the double-layer capacitance of activated carbon and the pseudocapacitance of the Co₃O₄-HNC@NiCo-LDH-1 electrode.

Fig. 7 (a) CV curves at 20 mV s⁻¹; (b) CV curves at 50 mV s⁻¹ of the ASC under different voltage windows; (c) CV curves at different scan rates; (d) GCD curves at 1 A g⁻¹ across varying potential windows; (e) GCD curves at varied current densities; (f) Nyquist plots; (g) rate performance at 2–15 A g⁻¹; (h) cycling stability; (i) comparison of the key performance metrics of this work and other reports.

Furthermore, Fig. 7(d) reveals that the stable operating voltage under the GCD curve was 1.6 V. When the peak voltage reached 1.65 V, a charging plateau emerged in the GCD curve. Based on these combined results, the subsequent voltage test range for the Co₃O₄-HNC@NiCo-LDH-1//AC ASC device was determined to be 0–1.6 V. Fig. 7(e) presents the GCD curve of the Co₃O₄-HNC@NiCo-LDH-1//AC ASC device within a voltage range of 0 to 1.6 V (the comparison chart of GCD at high mass loading is shown in Fig. S29.) The nearly symmetrical GCD curves observed at various current densities demonstrated that the device exhibited remarkable reversibility and excellent charge–discharge efficiency. Upon calculation, the specific capacitance of the Co₃O₄-HNC@NiCo-LDH-1//AC ASC device was 98 F g⁻¹ at 2 A g⁻¹.

Fig. 7(f) displays the EIS diagram of the ASC. The device exhibited low R_s and R_ct in the high-frequency region, along with steeply inclined lines in the low-frequency region. These characteristics underscored the system's superior performance. The outstanding performance of the ASC could be primarily attributed to the superior characteristics of both the positive and negative electrodes. In particular, the unique core–shell structure of the Co₃O₄-HNC@NiCo-LDH-1 electrode played a pivotal role in enhancing the overall electrochemical performance of the device. This innovative architecture not only streamlined the engagement of active materials but also significantly amplified the efficiency of ion migration and charge transfer. The synergistic interaction between the core and shell components ensured that the electrode maintains stability even at elevated scan rates.

Remarkably, as illustrated in Fig. 7(g), the energy density reached 76.8 Wh kg⁻¹ at 2 A g⁻¹ and 68.2 Wh kg⁻¹ at 15 A g⁻¹. The retention rate of 88.80% under these conditions highlighted the system's superior rate capability. Fig. 7(h) further proves the stability of the ASC. The data revealed that even after 15 000 cycles at 15 A g⁻¹, the ASC maintained a retention rate of 98.38% with a coulombic efficiency of 98.94%. These nearly negligible attenuation figures underscored the exceptional stability of the ASC device. The inset SEM image depicts the morphology of the sample after cycling. It is evident that the structure remains largely intact, with negligible collapse, compared to its condition before cycling. The superior core–shell architecture endowed the electrode with remarkable stability. As demonstrated in Fig. 7(h), the assembled button-type supercapacitor successfully powered an LED lamp, showcasing its practical applicability. In Fig. 7(i) a pentagonal star diagram is used to compare the key performance metrics of the current work on supercapacitors, including specific capacitance, retention rate, rate performance, power density, and energy density, with those of similar studies.^{20,24,27,57–59} The results demonstrated that the current work outperforms most comparable efforts in these critical areas.

The floating test is a sophisticated method for real-time monitoring of the potential fluctuations of both positive and negative electrodes during cyclic charge–discharge testing.⁶⁰ This technique enables real-time monitoring of electrode responses under varying electrochemical conditions.^61,62Fig. 8(a and b) shows the real-time voltages of the positive and negative electrode materials, respectively, within the same system under different current densities. Fig. 8(c) reveals that as the current density increased, the voltage range of the positive material increased, while the voltage range of the negative material decreased. Additionally, the combined voltage of the positive and negative electrodes decreased with the rise in current density. As evidenced in Fig. S30, the sum of these values approached 1.6 V, indicating that the device could fully utilize the entire potential window to deliver its electrochemical performance, regardless of changes in current density. This further implied that the system exhibited negligible internal resistance. Fig. 8(d) illustrates the real-time voltage range of the Co₃O₄-HNC@NiCo-LDH-1 electrode over 20 cycles at 10 A g⁻¹, while also presenting the energy density profile of the Co₃O₄-HNC@NiCo-LDH-1//AC ASCs. The system demonstrated excellent stability, with minimal voltage fluctuations across the cycles.

Fig. 8 (a) The potential window provided by the positive electrode material from floating tests at different current densities; (b) potential window of the negative electrode from floating tests at varied current densities; (c) potential window of positive/negative electrodes obtained via floating tests at varied currents; (d) ASC cycle life was tested at 10 A g⁻¹ for 20 cycles, showing voltage evolution.

4. Conclusions

In this work, we employed a multi-scale hollow core–shell design strategy to meticulously control the dimensions of the core and shell, as well as the size of the hollow structure, across nanometer, micrometer, and even larger scales. This approach facilitated the comprehensive optimization of material performance from multiple perspectives. First, the hollow structure itself acted as a stable framework, while the shell design provided additional active sites. Second, the core–shell structure enabled a synergistic effect between different materials. Moreover, the multi-scale structures effectively mitigated issues such as volume expansion and agglomeration that materials might encounter during certain application processes (e.g., electrochemical and catalytic reactions), thereby further enhancing the materials’ performance and service life. As a result, the overall electrochemical performance of the composite was significantly enhanced. The synthesized composite demonstrated outstanding electrochemical performance. The Co₃O₄-HNC@NiCo-LDH-1 electrode had a significant specific capacitance of 1862.4 F g⁻¹ (2 A g⁻¹) and a remarkable energy density of 76.8 Wh kg⁻¹ (829.2 W kg⁻¹) when assembled with AC. Moreover, the material exhibited outstanding long-term stability, retaining 98.38% of its initial capacitance after 15 000 cycles. This work underscores the significant potential of the Co₃O₄-HNC@NiCo-LDH-1 composite for high-performance energy storage applications, highlighting its balanced combination of high specific capacitance, excellent rate capability, and superior cycling stability. The “multi-scale hollow core–shell” structure finds broad application across diverse battery systems, including lithium-ion batteries, sodium-ion batteries, and lithium–sulfur batteries. By augmenting the specific capacity, cycling stability, and rate performance of materials, it offers an effective solution to key challenges in energy storage.

Author contributions

Yan Wang: conceptualization, methodology, data curation, validation, formal analysis, and writing – original draft; Hanbo Wang: software, formal analysis, and data curation; Yahui Xu: visualization and investigation; Dongyu Zhu: writing – review & editing; Ziming Wang: supervision and visualization; Yiduo Li: validation; Tian Yumei: (co-corresponding author) and Lu Haiyan (co-corresponding author): project administration, funding acquisition, and resources.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors confirm that the data supporting the findings of this study are available within the article. Supplementary information (SI) is available. See DOI: https://doi.org/10.1039/d5qi01541c.

Acknowledgements

This work was supported by the Science and Technology Development Plan Project of Jilin Province, China (No. 20250102066JC), and the Major Science and Technology Projects for Independent Innovation of China FAW Group Co., Ltd (Grant No. 20220301018GX and 20220301019GX).

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