Novel core–shell nanoclusters composed of multiple nickel–cobalt-oxyselenide nanowires wrapped with NiCo-LDH nanosheets for high energy density supercapacitors

Abstract

Transition metal selenides are widely used in supercapacitors because of their high theoretical capacity but their poor electron or ion accessibility in electrochemical tests leads to low conductivity and weak energy density. Herein, a novel nanocluster made up of a nickel–cobalt-oxyselenide (NiCoO₄Se₃, denoted as NCOSe) nanowire core and a Ni_xCo_y-LDH nanosheet shell on carbon fiber (CF@NCOSe/Ni_xCo_y-LDH) is synthesized for the first time. Nanosheets encapsulated with multiple active nanowires have a multitude of active sites and high electrical conductivity simultaneously. This core–shell structure can enhance material stability and also further stimulate the redox reaction activity of the internal core nanowires. Due to the synergistic effect of NCOSe and Ni₂Co₁-LDH, the nanocluster CF@NCOSe/Ni₂Co₁-LDH exhibits an outstanding capacity of 3270 F g⁻¹ (454.17 mA h g⁻¹) at 1 A g⁻¹. Significantly, the prepared hybrid supercapacitor (HSC) with CF@NCOSe/Ni₂Co₁-LDH//AC shows a particularly high energy density of 89.7 W h kg⁻¹ at a power density of 800 W kg⁻¹. In addition, the HSC exhibits an outstanding cycling performance of 95.6% capacitance retention after 10 000 charge/discharge cycles. Therefore, constructing core–shell nanoclusters with an effective electrolyte buffer space can obtain high capacitance electrode materials, which is an effective strategy to build high energy density HSCs.

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1. Introduction

In recent years, transition metal selenides (TMSes) have been widely used in supercapacitors due to their excellent redox activity and cycling stability.^1,2 However, in electrochemical performance tests, the active Se utilization of TMSe is low and it eventually turns into the corresponding hydroxide, leading to a decrease in electrode conductivity.³ Conductivity is one of the important parameters determining electrochemical performance, and a lower conductivity leads to the rapid decay of performance at high current densities, i.e., poor multiplicative performance. In addition, unfavorable ionic or electronic conductivity can limit the energy density and electrochemical performance.^4–6 Therefore, how to increase the electrical conductivity to improve the electrochemical performance is currently the bottleneck facing TMSe in supercapacitor applications.

Many effective strategies, such as introducing defect engineering,⁷ designing composite structures,⁸ and non-metal doping,⁹ can enhance the conductivity of TMSes. Based on the synergistic effect of the composite structures, the redox reactions of multiple components can promote effective charge transfer and conductivity enhancement.^10,11 For example, Ameri et al. synthesized NiCoSe₂@NiMn-LDH layered core–shell materials by using synergistic effects.¹² Both components undergo redox reactions with enhanced conductivity, and more importantly, the composite 2D-LDH nanosheets with highly interconnected morphology and high specific surface area can effectively contact the electrolyte, shorten the ion transport path, and improve conductivity.¹³ The core–shell material exhibits outstanding multiplicative performance and excellent energy density. Furthermore, the use of quasi-metallic doping can generate more active sites and improve intrinsic conductivity.^14–16 For example, Zong et al. constructed P-(Ni, Co)Se₂ nanomaterials using a nonmetallic doping strategy.¹⁷ P doping transformed the Prussian blue analog (PBA) from a nanocube into a hollow cube, and the PBA hollow cube, nanowires, and nanosheets formed a continuous conductive network with significantly enhanced conductivity, and the nanomaterials showed excellent area-specific capacity. Especially, methods have been developed for polyanion-doped bimetallic compounds (e.g. NiCoOS) with higher conductivity and theoretical capacity than single anion bimetallic compounds (NiCoS) because the polyanion strategy can adjust the composition of the metal compound and thus the electronic structure to improve the conductivity and electrochemical properties.^18,19 Given this, two means of rational design of composite structures and the introduction of nonmetallic doping are feasible for improving the electrical conductivity of electrode materials. However, no systematic studies have been reported on the advantages of the simultaneous application of both approaches, i.e., composite 2D-LDH nanosheets and polyanion doping, to improve the electrical conductivity and to obtain excellent electrochemical properties of bimetallic compound electrode materials.

Herein, we first synthesized NCOSe nanowire cores and then wrapped Ni_xCo_y-LDH nanosheets around them to form nanocluster composites, in which NCOSe active nanowires can promote electron transport along the axial direction and improve energy storage kinetics, while the modulation of Ni_xCo_y-LDH nano component helps to form a more stable three-phase system. Finally, the nanocluster structure can provide sufficient space to effectively mitigate the volume changes generated during the continuous charge/discharge process and enhance cycling stability. Based on the synergistic effect of NCOSe and Ni_xCo_y-LDH, both nanowire cores and nanosheets can have great advantages in redox, resulting in a core–shell electrode with ultra-high energy density and cycling stability. The NCOSe/Ni₂Co₁-LDH electrode outperforms other electrodes in terms of capacity (an electrochemical capacity of 454.17 mA h g⁻¹ at 1 A g⁻¹). In addition, the hybrid supercapacitor constructed using the core–shell NCOSe/Ni₂Co₁-LDH electrode exhibits an excellent energy density of 89.7 W h kg⁻¹ at a power density of 800 W kg⁻¹ and a capacity retention of 95.6% after 10 000 charge/discharge cycles. Therefore, the construction of core–shell electrodes has a promising future in applications.

2. Experimental section

For details of physical characterization and the corresponding electrochemical tests, please refer to the ESI.†

2.1 Chemicals

NiCl₂·6H₂O, CoCl₂·6H₂O, NiNO₃·6H₂O, CoNO₃·6H₂O, urea, Se powder, carbon black (10%), and polyethylene-difluoride (10%), and N-methylpyrrolidone (NMP) were obtained by Zhenjiang Dewei Chemical Co. SCNs were bought from China's XFNAN. Carbon fiber fabrics (CF, 1 × 2 cm²) were washed for 30 min each with acetone (CH₃COCH₃), ethanol (C₂H₆O) and DI water.

2.2 Preparation of NiCo-CH

First, 95 mg of NiCl₂·6H₂O and CoCl₂·6H₂O and 200 mg of urea were weighed and added to 30 mL of distilled water, stirred evenly, and subjected to ultrasound for 10 min. Then, a piece of CF (1 × 2 cm²) was placed into the solution prepared for the mixture and reacted at 120 °C for 10 hours. After 10 hours of reaction, the product is obtained by exchanging it several times with DI water and ethanol and baking in an oven at 60 °C for 12 hours.

2.3 Preparation of NCOSe

NCOSe was obtained by selenization based on NiCo-CH nanowires. 0.114 g of Se powder and 0.285 g of NaOH were weighed, dissolved in 25 mL of deionized water, sonicated for 1 h, put into the oven at 140 °C, sealed for 4 h, 6 h, 8 h, and 10 h, and then removed, rinsed with a few exchanges of deionized water and ethanol, and dried in the oven at 60 °C for 12 h. According to the selenization time, it is named NCOSe-4, NCOSe-6, NCOSe-8(NCOSe), and NCOSe-10. For comparison, (Ni,Co)Se₂ formed by heating at 180 °C for 8 h under the same reaction conditions was also prepared.

2.4 Preparation of NCOSe/Ni_xCo_y-LDH

NCOSe/Ni_xCo_y-LDH was prepared by electrodeposition. First, 2.475 mmol of NiNO₃·6H₂O and CoNO₃·6H₂O were dissolved in 50 mL of deionized water containing and sonicated for 10 min to obtain the electrolyte solution. The prepared NCOSe, Pt plate (area of 4 cm²), and saturated AgCl₂ electrode were put into the electrolyte solution as the working, counting, and comparison electrodes and the electrodeposition potential was controlled at −1.0 V for 300 s to obtain NCOSe/Ni₁Co₁-LDH. Then, NCOSe/Ni₂Co₁-LDH and NCOSe/Ni₁Co₂-LDH, NCOSe/Ni₃Co₁-LDH, and CF@Ni₂Co₁-LDH were obtained by varying the ratio of Ni/Co concentration in the electrolyte solution and designing Ni₂Co₁, Ni₁Co₂, Ni₃Co₁, and Ni₂Co₁-LDH by electrodeposition directly on carbon cloth at constant potential and time.

2.5 Preparation of negative electrode materials

First, the nickel foam was cleaned with hydrochloric acid and the powder from the mixed active electrode material (80%), carbon black (10%), and polyethylene diene difluoride (PVDF 10%) was ground into a fine powder. The fine powder was placed in the center of the mortar, 1–2 drops of NMP were added, continued to be ground, and then the slurry was evenly coated on the nickel foam. The electrode was dried in an oven at 60 °C for 12 h and the mass loading of the electrode was about 4 mg cm⁻².

3. Results and discussion

To obtain high purity core–shell nanocluster structures of the NCOSe/Ni_xCo_y-LDH electrode, the synthesis strategy shown in Fig. 1 was followed. First, dense and smooth nanowires NiCo-CH [Fig. S1, ESI†] were synthesized on carbon fabrics by a solvothermal method, and these nanowires could be used as scaffolds or supports. Then, after a high-temperature selenization process, the NiCo-CH precursors were further converted into NiCo-oxyselenium (i.e., NCOSe) via an anion-exchange reaction and this active selenized nanowire slowed down the volume expansion in electrochemical cycles and provided an efficient transport pathway for ion and electron mobilization.²⁰ Finally, to expose more active sites of NCOSe, we wrapped 2D-Ni_xCo_y-LDH nanosheets on the surface of multiple selenide nanowires by electrodeposition to obtain a core–shell nanocluster structure of NCOSe/Ni_xCo_y-LDH, which has a large specific surface area and can effectively promote charge transfer efficiency and also promote further core NCOSe redox reactions to improve the conductivity and electrochemical performance of the electrode.

Fig. 1 Schematic strategy for the heterogeneous nanocluster NCOSe/Ni_xCo_y-LDH on CFs.

Regarding the X-ray diffraction of NiCo-CH, NCOSe-6, NCOSe-8(NCOSe), and NCOSe-10 (Fig. 2a), the XRD spectra show that although the selenization times are different, all of them show diffraction peaks typical of both mixed phases of CoSeO₄ and NiSe₂, indicating that the oxygen atoms in NiCo-CH are partially replaced by selenium atoms. The two diffraction peaks are shifted, probably due to the nature of the selenization. The 2θ values of the 19°, 27.3°, 32.9°, and 61.4° diffraction peaks are from the (110), (002), (200), and (151) planar CoSeO₄ (JCPDS card no. 17-0844), while the four diffraction peaks at 36.8°, 52.2°, 58.8°, 74.3° correspond well to the (200), (031), (310), and (240) planes of NiSe₂ (JCPDS card no. 18-0886). When selenized for 10 h, two different crystalline surfaces appeared, one containing (111) NiSe₂ and one containing CoSeO₄, respectively. The amount of selenium doped increased with the increase in selenization time, and when the selenization time was 10 h, it led to nanostructure agglomeration due to excessive selenization, which was verified by SEM. Fig. 2b shows the XRD data of NCOSe-/Ni_xCo_y-LDH formed by the electrodeposition of Ni_xCo_y-LDH. The crystallinity of Ni_xCo_y-LDH is relatively high and a series of diffraction planes can be indexed to the (003), (006), (100), (015), (102), and (110) of Ni_xCo_y-LDH planes.^21,22 All diffraction peaks are broad and weak, indicating low crystallinity and the generation of lattice defects during the reaction process, which is attributed to interphase multiphase diffusion.^23–25 This is in line with the results of the TEM and SEM.

Fig. 2 (a) XRD patterns of NiCo-CH, NCOSe-6, NCOSe-8, NCOSe-10. (b) XRD patterns of NCOSe-/Ni_xCo_y-LDH and NCOSe-8. XPS survey spectra of (c) Ni 2p; (d) Co 2p; (e) O 1s and (f) Se 3d of NCOSe-8 nanowire and NCOSe-/Ni₂Co₁-LDH nanoclusters.

To further determine the atomic valence and elemental composition of the composite, X-ray photoelectron spectroscopy (XPS) was carried out for NCOSe-8 and NCOSe/Ni₂Co₁-LDH. The XPS spectra of NCOSe/Ni₂Co₁-LDH are shown in Fig. S2.†Fig. 2(c–f) present the characteristic peaks of the Ni, Co, O, and Se elements. For NCOSe-8, the Ni 2p_3/2 spectra of 856.8, 855.6, and 853.5 eV correspond to Ni³⁺, Ni-O-Se, and Ni²⁺, respectively,^26–28 and similarly, the Co 2p spectra can be fitted to spectra with Co²⁺, Co-O-Se, Co³⁺, and associated recombination satellites (Fig. 2(c and d)). The peaks of each element in NCOSe/Ni₂Co₁-LDH spectra are shifted towards the high binding energy indicating that the molecular transfer to Ni, Co and O atoms in LDH increases the density of Ni, Co and O. Ni has two spin orbitals with peaks located at 876.1 eV and 856.6 eV attributed to the presence of Ni²⁺; the other two peaks at 881.2 eV and 858.1 eV belong to Ni³⁺. In the spectrum of Co 2p in Fig. 2d, the energy of binding 2p_1/2 and 2p_3/2 are 796.8 eV, 793.6 eV, 781.1 eV, and 778.6 eV indexed to the +2 and +3 states of the Co element, respectively.²⁹ The XPS pattern of the seized O shows many oxygen defects and little surface adsorbed oxygen, while the electrodeposited O does not show oxygen vacancies, probably because the LDH nanosheets attached to the nanowire surface are too thick and the maximum depth that XPS can penetrate is 10 nm. Therefore, XPS did not penetrate the whole core–shell structure and only the O in LDH can be measured (Fig. 2e). In Fig. 2f, the fit peak at 59.1 eV is attributed to selenium oxide.³⁰

Fig. 3(a–c) show the scanning electron microscopy (SEM) spectra after selenization for 6 h, 8 h, and 10 h. The homogeneous and monodisperse active nanowires can be seen after selenization until 8 h. However, after selenization until 10 h, the selenium oxide starts to show agglomeration, indicating that its shuttle effect in the redox reaction is severe and the nanowires are not enough to provide enough space to accommodate the volume expansion, leading to a decrease in performance.³Fig. 3(d–f) show the NCOSe/Ni_xCo_y-LDH morphology of the core–shell nanoclusters. It can be seen that the active nanowires can provide support for the deposition of nanosheets when the optimal Ni/Co ion ratio is 2 : 1 (Fig. 3d). When the nanocluster structure is almost complete, it provides the largest specific surface area, which facilitates electron transport and ion diffusion in the inner layer, and the structure exhibits stable kinetic performance properties (Fig. S3(a and b)†). However, when Ni/Co ions are deposited in other ratios, the nanosheets thickly cover the active nanowires, hindering the electrolyte from reaching the nanowire cores and thus reducing the utilization of active sites (Fig. 3(d, f)). In addition, the effect of different ratios of Ni and Co on the growth of selenide was investigated. Among different Ni_xCo_y-CH arrangements, the most regular arrangement of nanowires was found to be NiCo-CH. Upon hydrothermal selenization of Ni_xCo_y-CH, it was found that different ratios of Ni and Co affected not only the growth of selenides but also the electrochemical properties (Fig. S4, S8†). The different morphologies of NCOSe-x were also found to affect the growth of LDH by plating LDH on the basis of different selenization times (Fig. S5†).

Fig. 3 SEM images of (a) NCOSe-6 NAs; (b) NCOSe-8 NAs; (c) NCOSe-10 NAs; (d) NCOSe/Ni₂Co₁-LDH; (e) NCOSe/Ni₁Co₁-LDH; and (f) NCOSe/Ni₁Co₂-LDH.

Transmission electron microscopy (TEM) further confirmed the structural features of the active nanowire NCOSe-8 and nanoclusters NCOSe/Ni₂Co₁-LDH in Fig. 4(a–c). Fig. 4a shows small nanosheets grown at the edges of the nanowires after high-temperature selenization, constituting the active nanowires, which have a larger specific surface area than normal nanowires and provide more active sites, as can be seen in the SEM. Fig. 4(b and c) show the Ni₂Co₁-LDH nanosheets anchor wrapped around the outside of multiple active selenium nanowires, forming a nanocluster structure. From Fig. S6,† it can be confirmed that the size of NCOSe/Ni₂Co₁-LDH nanoclusters is about 245 nm. Furthermore, high magnification transmission electron microscopy (HRTEM) shows the NCOSe/Ni₂Co₁-LDH visible lattice stripes with a spacing of 3.37, 2.44, and 2.31 nm pointing to the CoSeO₄ (002) plane, the (200) plane of NiSe₂, and the (015) plane of NiCo-LDH (Fig. 4(d–g)). The corresponding STEM-EDS mapping images show that Ni, Co, Se, and O atoms are equally distributed in all samples (Fig. 4h).

Fig. 4 TEM images of (a) NCOSe-8 NAs and (b and c) NCOSe/Ni₂Co₁-LDH. (d–g) HRTEM images of NCOSe/Ni₂Co₁-LDH and (h) HAADF-STEM and respective EDS mapping of images of NCOSe/Ni₂Co₁-LDH.

The electrochemical performance of the electrodes was analysed in an electrolyte of 6 M (KOH) and a three-electrode set up, and a series of cyclic voltammetric (CV) and galvanostatic charge/discharge (GCD) curves were obtained to evaluate the capacity performance of the electrode materials. As shown in Fig. 5a, according to the CV plots for different selenization times at 10 mV s⁻¹, it can be demonstrated that NCOSe-8 has the largest integrated area and the largest redox peak, which may be due to the partial substitution of O in NiCo-CH by Se to generate active NCOSe-8 nanowires. This provides more redox active sites than complete selenization producing (Ni, Co)₂Se and other selenization times. While after 10 h of selenization, the material structure agglomerates and blocks the ion transport path and thus leads to a decrease in performance. The corresponding GCD also confirms the excellent electrochemical properties of NCOSe-8 (Fig. 5b). The relative CV and GCD curves of NCOSe-4, NCOSe-6, NCOSe-8, NCOSe-10, (Ni, Co)₂Se are shown in Fig. S7.† In addition, the effect of different NiCo ratios on selenization was also investigated, and Fig. S8† shows that NiCo = 1 : 1 has the largest integrated area, the strongest redox peak, and the smallest corresponding internal resistance.

Fig. 5 (a and b) CV and GCD curves of NiCo-CH, NCOSe-X, and (Ni, Co)₂Se at 10 mV s⁻¹ scan rate and 10 A g⁻¹ current density, respectively. (c) CV curves of the electrodes CF@Ni₂Co₁-LDH, NCOSe-8, and NCOSe/Ni_xCo_y-LDH at a 10 mV s⁻¹ scan rate. (d) The GCD curves of the CF@Ni₂Co₁-LDH, NCOSe-8, and NCOSe/Ni_xCo_y-LDH at a current density of 5 A g⁻¹. (e) Curves of cyclic voltammetry, (f) NCOSe/Ni₂Co₁-LDH specific capacity and capacity holding rates at current densities of 1–20 A g⁻¹. (g) NCOSe-X and (Ni, Co)₂Se electrode Nyquist plots. (h) Nyquist plots of the electrodes CF@Ni₂Co₁-LDH, NCOSe-8, and NCOSe-/Ni_xCo_y-LDH, respectively. (i) NCOSe-/Ni₂Co₁-LDH capacity retention at various current densities.

The CV curves of NCOSe-/Ni_xCo_y-LDH and CF@Ni₂Co₁-LDH are shown in Fig. 5c, where the CV closure curve area of NCOSe/Ni₂Co₁-LDH at 10 mV s⁻¹ is larger than those of NCOSe/Ni₁Co₁-LDH, NCOSe/Ni₁Co₂-LDH, NCOSe/Ni₃Co₁-LDH, CF@Ni₂Co₁-LDH, and the corresponding selenide NCOSe-8. Meanwhile, NCOSe/Ni₂Co₁-LDH is shown to have the longest discharge time at 5A g⁻¹, displaying superior specific capacity (Fig. 5d). Because when the molar mass ratio of Ni : Co = 1 : 1 or 1 : 2 or 3 : 1, as well as CF@Ni₂Co₁-LDH, the stacking of nanosheets is not favorable for electron transfer due to electrolyte penetration, resulting in less pronounced redox peaks and lower electrochemical capacity performance. Typical CV curves of NCOSe/Ni₂Co₁-LDH at scan rates ranging from 2 to 100 mV s⁻¹ are shown in Fig. 5e. There were distinct cathodic and anodic peaks at different scan rates indicating reversible redox reactions, and the GCD curves of NCOSe/Ni₂Co₁-LDH exhibited a relatively symmetrical shape indicating good coulometric efficiency. The electrode exhibited electrochemical capacities of 454.17, 433.33, 395.83, 377.78, and 363.89 mA h g⁻¹ at current densities of 1, 2, 5, 8, and 10 A g⁻¹ (Fig. 5f). Combined, it can be concluded that the redox rate increased due to the incomplete substitution of selenium. Together with the effective modulation of the nanocomponent, the active sites of the active nanowires also increase. In addition, the electrodeposition accelerates the rapid adsorption of nickel and cobalt ions in the electrolyte, thus modulating the electronic structure and increasing the electrical conductivity. The resulting nanocluster structure with a significant increase in the specific surface area shows excellent electrochemical properties.³¹ The possible Faraday reaction associated with this is described by the following equation:^3,32–35

NiSe₂ + OH⁻ ↔ NiSe₂OH + e⁻ (1)

NiSe₂ + H₂O + OH⁻ ↔ NiSe₂OH⁻ + e⁻ (2)

Ni(OH)₂ + OH⁻ ↔ NiOOH + e⁻ (3)

Co(OH)₂ + OH⁻ ↔ CoOOH + e⁻ (4)

CoOOH + OH⁻ ↔ CoO₂ + H₂O + e⁻ (5)

The CV and GCD curves for NCOSe/Ni₁Co₁-LDH, NCOSe/Ni₁Co₂-LDH and NCOSe/Ni₃Co₁-LDH CF@Ni₂Co₁-LDH are shown in Fig. S9.†

In addition, we compare the different selenization times and the different Ni/Co molar mass ratios as well as CF@Ni₂Co₁-LDH in the Nyquist plot Fig. 5(g and h). The Nyquist plot consists of a diagonal line in the low-frequency region, reflecting the diffusion resistance, and a quasi-semicircle in the high-frequency region, indicating the charge transfer kinetic properties.^36,37 Apparently, NCOSe/Ni₂Co₁-LDH has a smaller semicircle in the high frequency region, indicating a smaller charge equivalent series resistance (R_s) and charge transfer resistance (R_ct). The fit values of R_s and R_ct for the relevant samples (Table S1†) show that NCOSe/Ni₂Co₁-LDH has the smallest values of R_s (0.75 Ω) and R_ct (0.12 Ω), indicating that Ni₂Co₁-LDH can modulate the electronic structure and conductivity of the electrode material. The lower frequency region is steeper and exhibits lower ion diffusion resistance, which may be due to the deficiency of selenium atoms, lower content of oxygen atoms in NCOSe-8, enhanced electronegativity, increased conductivity, and a thin layer of LDH on the nanowire surface adhering to the Seide surface, alleviating layer buildup.^38,39 Then, Z versus frequency is shown in Fig. S10,† from which it can be seen that the resistance of NCOSe/Ni_xCo_y-LDH is smaller than that of NCOSe-8, where NCOSe/Ni₂Co₁-LDH has the lowest resistance and the best conductivity, which indicates that the synergistic effect can improve the conductivity of TMSe. Finally, the specific capacity Q_s (mA h g⁻¹) was calculated from the GCD curve:

Q_s = i × Δt/3.6Δm₊ (6)

The equations are as follows where i, Δt, and Δm₊ denote the discharge current (A) and discharge time (s), and load capacity. The specific capacities are 454.17 mA h g⁻¹ and 327.78 mA h g⁻¹ when the current density is 1 A g⁻¹ and 20 A g⁻¹. When the current density was increased from 1 to 20 A g⁻¹, there was excellent capacitance retention of 72.17% in Fig. 5i. Taken together, the synergistic effect of NCOSe-8 and NiCo-LDH has an excellent role in electrochemical energy storage.

According to the CV curves at different scan rates, the current density (i) obeys a post-exponential relationship:^4,40

i = av^b (7)

log(i) = log(a) + b log(v) (8)

where v denotes the scan rate and a and b denote the constants. The logarithmic plots of the peak cathode current density are shown in Fig. 6a. It can be clearly seen that the slope of the linear plot of NCOSe/Ni₂Co₁-LDH composites is high, indicating fast charge and ion transport rates, but the slope is less than 1, which is mainly due to the ohmic contribution.^41,42Fig. 6(b and c) show the capacity contribution analysis at 10 mV s⁻¹ for both the NCOSe-8 and NCOSe/Ni₂Co₁-LDH electrode materials. Remarkably, the percentage contribution of the capacitance of NCOSe/Ni₂Co₁-LDH increases from 59% to 67%. The excellent capacitance contribution reveals the dominance of redox processes at high scan rates, which mainly depends on the formation of NiCo-LDH with enhanced electrochemical properties.

Fig. 6 (a) Log curve of the peak cathodic density of NCOSe/Ni₂Co₁-LDH. (b and c) Contribution of capacitance and diffusion control of NCOSe-8 and NCOSe/Ni₂Co₁-LDH at the electrode at a scan rate of 10 mV s⁻¹. (d) Contribution ratio of NCOSe/Ni₂Co₁-LDH electrode at different scan rates.

In light of the above results, the excellent performance of NCOSe/Ni₂Co₁-LDH is mainly attributed to three reasons. First, the active nanowire NCOSe component not only serves as a nano-substrate for NiCo-LDH but also plays an important role in long-range electron transfer. Second, the NiCo-LDH shell layer can improve the stability of the electrode material and can provide buffer space for the electrolyte to further stimulate the redox reaction activity of the internal nanowires.⁴³ Third, since the NCOSe/Ni₂Co₁-LDH core–shell structure offers a large specific surface area, the permeable stacked nanosheets can not only increase the active sites but also provide ionization channels for the electrolyte ions to rapidly penetrate into the electrode material, and both the core–shell and the core can undergo redox reactions under continuous charge and discharge and the electrode material shows excellent multiplicative performance and electric capacity.

To further assess the energy storage principle and the electrochemical properties of the synthesized core–shell nanocluster NCOSe/Ni₂Co₁-LDH hybrid cathode, the hybrid supercapacitor was assembled with NCOSe/Ni₂Co₁-LDH as the anode and AC as the cathode. The assembly schematic of this HSC device was shown in Fig. 7a. To ascertain the voltage range of the HSC, the CV curves of AC from −1.0 V–0 V and NCOSe/Ni₂Co₁-LDH from 0–0.5 V were tested in Fig. 7b, respectively. The voltage values are determined based on the CV and GCD curves of the HSC at different voltage windows. Fig. S11(a)† showed the stable operating voltage of the HSC device for CF@NCOSe/Ni₂Co₁-LDH//AC at different potential windows extending up to 1.8 V. When the voltage window was increased to 1.8 V, the CV curve showed a typical rectangular shape which was well maintained without any distortion. Similarly, Fig. S11(b)† showed that the stable operating voltage under the GCD curve was also 1.8 V. Therefore, the voltage window for the CV curve and the GCD curve of the HSC was determined to be 0–1.6 V. As Fig. 7c shows, the different CV plots for a voltage of 1.6 V and a scan rate from 5 mV s⁻¹–100 mV s⁻¹. There were no obvious changes in the CV curves even at high scan rates, indicating that this HSC exhibited good rate performance. The GCD curves had symmetrical triangular characteristics, demonstrating that the hybrid supercapacitor had high Coulomb efficiency in Fig. 7d. The HSC achieves excellent capacitance capacities of 252.4, 238, 205.6, 171.0 and 158.1 F g⁻¹ at 1, 2, 5, 8 and 10 A g⁻¹, respectively, and energy densities up to 89.7 W h kg⁻¹ when the power density is 800 W kg⁻¹. Due to the incomplete substitution during selenization, the percentage of O atoms in NiCo-CH decreases. The excellent energy density is demonstrated by the enhanced electronegativity and increased conductivity, as well as the 2D-LDH nanosheets that promote ion transport and enhanced electronic conductivity. This high energy density HSC has great potential for application in electronic devices such as energy storage.

Fig. 7 (a) A diagrammatic representation of the HSC structure showing AC as the anode and NCOSe/Ni₂Co₁-LDH as the cathode. (b) CV curves of the cathode and anode of the NCOSe/Ni₂Co₁-LDH device under a 20 mV s⁻¹ scan rate. (c) The HSC device's CV curves at various scan speeds. (d) HSC device's GCD curves at different current densities.

A cycling stability test of NCOSe/Ni₂Co₁-LDH//AC cells was done at a 10 A g⁻¹ current density. The capacity was maintained at 106.7% at 5000 cycles and 95.6% after 10 000 cycles (Fig. 8a). With the penetration of the electrolyte, on the one hand, the conductive selenide containing the main element of the surface electrode material with good spark discharge properties increases the stability of the electrode in an alkaline electrolyte by allowing the effective diffusion of electrolyte ions. On the other hand, the core–shell nanoclusters have a huge surface area and hierarchical structure, which in turn promote the diffusion of hydrogen ions and suppress the hydrogen ions providing sufficient space for swelling during both the charging and discharging processes.^44,45 As shown in Fig. 8b, HSC has an outstanding specific capacity retention of 65.2% at a 10 A g⁻¹ current density. To investigate the energy density and power density of this HSC, Ragone plots are plotted and are shown in Fig. 8c. When the power density is 800 W kg⁻¹, the energy density of the HSC reaches 89.7 W h kg⁻¹ and when the power density is 1600 W kg⁻¹, the energy density is 84.6 W h kg⁻¹. The energy and power density values of this HSC device are better than the previously reported HSC values, such as (Ni,Co)Se₂/NiCo-LDH//PC (39 W h kg⁻¹ at 1650 W kg⁻¹),⁷ CNTs@NiCo-LDH//ZIF-8 (37.38 W h kg⁻¹ at 800 W kg⁻¹),²⁹ ZnO/C@(Ni,Co)Se₂//AC (65.67 W h kg⁻¹ at 800 W kg⁻¹),⁴⁶ (Ni,Co)Se₂@rGO//AC (52.6 W h kg⁻¹ at 803.4 W kg⁻¹),⁴⁷ P-(Ni,Co)Se₂ NAs//ZC (45 W h kg⁻¹ at 446.3 W kg⁻¹),⁴⁸ CC@NiCo-LDH/Co₉S₈ (38 W h kg⁻¹ at 800 W kg⁻¹),⁴⁹ Co(OH)₂/CoSe₂//CNTs (68.7 W h kg⁻¹ at 189 W kg⁻¹),⁹ CC/NiCoP @ NiCo-LDH//AC (57 W h kg⁻¹ at 850 W kg⁻¹),⁵⁰ and PANI@NiSe₂//AC (38.3 W h kg⁻¹ at 308 W kg⁻¹).⁵¹ To demonstrate the application potential of the device, we used solid-state HSCs in series. As seen in Fig. 8d, all the LEDs were lit, which provides a huge advantage for energy storage in electronic devices and as a potential power source (Fig. 8d).

Fig. 8 (a) Cycling stability at 10 A g⁻¹ of current density. (b) Capacity maintenance rate of NCOSe/Ni₂Co₁-LDH//AC. (c) Ragone plots of both our device and the previously discussed devices. (d) Two HSC lighting LED lamps.

4. Conclusions

In summary, we successfully synthesized core–shell nanocluster NCOSe/Ni₂Co₁-LDH structures on carbon fibers. The double anionic bonding of multivalent cations can improve the reaction kinetics and accelerate the redox reaction, while the active nanowires can promote the transport of core electrons along the edge of the core shell and increase the intrinsic conductivity of selenide. On the other hand, the use of electrodeposition promotes the diffusion of Ni and Co ions in the electrolyte, which accelerates the rapid attachment of NiCo-LDH nanosheets to the nanowire surface, thus exposing more active sites. The above factors enable NCOSe/Ni₂Co₁-LDH to exhibit ultra-high capacity (454.17 mA h g⁻¹ at 1 A g⁻¹) and excellent multiplicative performance. CF@NCOSe/Ni₂Co₁-LDH//AC demonstrates high energy density and good cycling stability when used as a supercapacitor. Therefore, the construction of such high-energy density nanocluster-like electrode materials based on synergistic effects has good energy storage advantages and application prospects.

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.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (22108107, 22075111, and 22225808), the China Postdoctoral Science Foundation (2021M691302), and the China Postdoctoral Science Special Fund (2021TQ0131).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2qi01739c

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