Construction of hybrid Ni–Mn–Co–Ce oxide yolk-double shell hierarchical architectures for enhanced supercapacitors

Abstract

Hybrid metal oxides possess the favorable features of each component and are proven suitable candidates for supercapacitors. However, the construction of hybrid metal oxides with intricate hollow architectures is specially challenging. Herein, we exploit a simple strategy to synthesize NiO–MnO₂–Co₃O₄–CeO₂ yolk-double shell (Ni–Mn–Co–Ce oxide YDS) hierarchical architectures for enhanced supercapacitor performance. First, a Ni–Mn–Co–Ce glycerate-templated construction of yolk–shell hierarchical architecture intermediates was crafted using a solvothermal method. The yolk–shell hierarchical architecture intermediates were then transformed into Ni–Mn–Co–Ce oxide YDS hierarchical architectures via pyrolysis. Such designed hybrid Ni–Mn–Co–Ce oxide materials can be deemed attractive electrodes for supercapacitors with unique components and architectures. They demonstrated a high specific capacitance of 2126.7 F g⁻¹ at 4.0 A g⁻¹ and a mere 10.2% reduction in 10 000 cycles. The Ni–Mn–Co–Ce oxide YDS hierarchical architectures were further used to construct the cathode in an asymmetric supercapacitor, achieving an energy density of 100.4 Wh kg⁻¹ at 1200 W kg⁻¹. This work provides a simple avenue for fabricating hybrid metal oxides with complex hollow architectures toward high-performance supercapacitors.

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1. Introduction

Supercapacitors, which have attracted immense attention due to their high power densities and desirable cycle life, show great potential as sustainable energy storage devices.^1,2 Considering the energy storage strategy, supercapacitors primarily encompass double-layer capacitors and pseudocapacitors.^3,4 Particularly, pseudocapacitors are proven devices in garnering high specific capacitance and energy density due to the faradaic redox reactions of electrode materials.⁵ As such, intensified study efforts have been committed to exploring high-performance pseudocapacitors for energy storage.

Supercapacitors refer to electrode materials that play a giant role in device performance. Therefore, substantial efforts have been devoted to prospective electrode materials for supercapacitors. Transition metal oxides featuring large theoretical capacities and varied oxidation states are latent candidates for supercapacitors.⁶ To date, various transition metal oxides (MnO₂,⁷ Fe₃O₄,⁸ Co₃O₄,⁹ NiO,¹⁰ CeO₂,¹¹etc.) have been developed. Nevertheless, their low electrical conductivity and structural variation during cycles negatively affect rate performance and cycling capability.¹² In contrast, the combination of diverse metal oxides into one structure is a highly significant route for boosting electrochemical behaviors. This not only endows preferable conductivity but also maximizes electroactive sites owing to the synergistic effect between each metal oxide.^13–16 In recent years, substantial efforts have been pursued to develop high-performance hybrid metal oxides. Bi et al. prepared a hybrid NiO-CuO nanowire array via a hydrothermal method, followed by annealing. The specific capacitance reached 1450.8 F g⁻¹ at 2.0 mA cm⁻² and was significantly enhanced compared to pure NiO or CuO.¹⁷ Lim et al. reported a ternary Zn–Cu–Co oxide. It considerably increased the specific capacitance to 1776 F g⁻¹ at 1.0 A g⁻¹, which is superior to those of single metal oxides (ZnO, CuO and Co₃O₄) and binary metal oxides (Zn–Co oxides and Cu–Co oxides).¹⁸ Moreover, other hybrid metal oxides (e.g., NiO/ZnO,¹⁹ NiO/In₂O₃,²⁰ CoO/MnO₂, and Cu–Ni–Co oxide^21,22) have been prepared for high-performance supercapacitors. Thus, developing hybrid metal oxides is essential to supercapacitors.

Structural engineering has been proven to be a practical avenue for electrode materials in supercapacitors with enhanced properties.²³ Hierarchical hollow structures consisting of various assembling subunits are highly prospective candidates for improved supercapacitor performance because they inherit the advantages of subunits and hollow structures.^24,25 In particular, hierarchical hollow structures possess more active sites and a convenient diffusion route, leading to advanced rate behavior. Furthermore, hierarchical hollow structures can significantly relieve structural variations during charge–discharge cycles, resulting in boosted cycle durability.^26,27 Among the constructed subunits, two-dimensional (2D) structures (such as nanosheets and nanoplates) have demonstrated significant superiority for supercapacitors due to their large surface areas and short diffusion routes.^28,29 Therefore, hierarchical hollow structures consisting of 2D subunits are recognized as advanced electrode materials for realizing desired performances. In recent years, the explorations have been concentrating on the hollow structures of hybrid metal oxides. Chen et al. exploited Zn[Ni(CN)₄] as the precursor to prepare yolk-shelled ZnO–NiO microspheres. The resultant ZnO–NiO showed a specific capacitance of 1451 F g⁻¹ at 2.0 A g⁻¹, which is much higher than other reported NiO-based materials.³⁰ Tu et al. synthesized Fe₃O₄@MnO₂ hollow nanospheres via a hard template, which exhibited a specific capacitance of 375.14 F g⁻¹ at 0.5 A g⁻¹ and an energy density of 15.84 Wh kg⁻¹.³¹ Even if some achievements have been gained, these hollow structures are commonly constructed by zero-dimensional (0D) nanoparticles. Thus, constructing 2D subunits into hierarchical hollow hybrid metal oxides with complex interior structures is extremely challenging.

Herein, we have developed a simple route for synthesizing Ni–Mn–Co–Ce oxide YDS hierarchical architectures. As shown in Scheme 1, the Ni–Mn–Co–Ce glycerate solid spheres as templates were prepared using a one-pot solvothermal process. Afterwards, the Ni–Mn–Co–Ce glycerate was converted into yolk–shell hierarchical architectures by treatment with NMP and H₂O solutions at 160 °C for 6 h. Finally, the Ni–Mn–Co–Ce oxide YDS hierarchical architectures were synthesized via calcination of the yolk–shell hierarchical architectures. When the Ni–Mn–Co–Ce oxide YDS hierarchical architectures were evaluated as electrode material for supercapacitors, the attractive hierarchical hollow architectures and compositions endowed them with remarkable supercapacitor performance, showing a high specific capacitance of 2126.7 F g⁻¹ at 4.0 A g⁻¹ and excellent cycling durability with a mere 10.2% decrease for 10 000 cycles at 15 A g⁻¹. Furthermore, the Ni–Mn–Co–Ce oxide YDS hierarchical architectures enabled a solid-state asymmetrical device to exhibit a large energy density of 100.4 Wh kg⁻¹ at a power density of 1200 W kg⁻¹, demonstrating that the Ni–Mn–Co–Ce oxide YDS hierarchical architectures are advanced electrode materials for supercapacitors.

Scheme 1 Schematic of the synthesis of Ni–Mn–Co–Ce oxide YDS hierarchical architectures.

2. Experimental

2.1 Preparation of Ni–Mn–Co–Ce glycerate

Ni–Mn–Co–Ce glycerate was prepared by a simple solvothermal method. Typically, 0.5 mmol (0.1454 g) Ni(NO₃)₂·6H₂O, 0.5 mmol (0.1255 g) Mn(NO₃)₂·4H₂O, 0.5 mmol (0.1452 g) Co(NO₃)₂·6H₂O and 0.5 mmol (0.2171 g) Ce(NO₃)₃·6H₂O were dissolved in a mixed solvent containing 7.5 mL glycerol and 52.5 mL isopropanol. The resulting clear pink mixture was transferred into an autoclave and then heated at 200 °C for 24 h. The resultant product was washed with ethanol by centrifugation and dried at 60 °C to achieve Ni–Mn–Co–Ce glycerate.

2.2 Preparation of Ni–Mn–Co–Ce oxide YDS hierarchical architectures

Here, 0.10 g of Ni–Mn–Co–Ce glycerate powders were dispersed in an aqueous solution containing 5.0 mL of H₂O and 15 mL of N-methylpyrrolidone (NMP) under sonication for 30 min. The suspension was then sealed in an autoclave and heated at 160 °C for 6 h. After cooling naturally, the resulting product was centrifuged with ethanol, followed by drying at 60 °C. Subsequently, the product was heated up 450 °C at a rate of 2 °C min⁻¹ in air for 2 h to obtain the Ni–Mn–Co–Ce oxide YDS hierarchical architectures.

2.3 Material characterization

Structural characterizations were conducted with a Hitachi SU-8010 scanning electron microscope (SEM) and a JEOL JEM-2100 transmission electron microscope (TEM). The phase composition analysis was performed on a Rigaku Ultima-IV powder X-ray diffraction (XRD) instrument. The surface element composition was determined by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB250). A Micromeritics ASAP 2020 analyzer was used for N₂ adsorption–desorption tests under liquid N₂ conditions. Thermogravimetric (TG) analysis was carried out on a Q5000IR TA Instrument at a rate of 10 °C min⁻¹ in air. All electrochemical measurements, including cyclic voltammetry (CV), galvanostatic charge–discharge, and electrochemical impedance spectroscopy (EIS), were conducted using the CHI 660E electrochemical workstation.

2.4 Electrochemical measurements

We performed three-electrode measurements in 3 M KOH electrolyte with a Hg/HgO electrode and Pt wire as the reference and counter electrode. The working electrode was prepared by dispersing 80 wt% Ni–Mn–Co–Ce oxide YDS hierarchical architectures, 15 wt% acetylene black, and 5 wt% polyvinylidene difluoride (PVDF) binder into around 5.0 mL of isopropanol under milling for 30 min. The freshly formed uniform slurry was evenly deposited onto a Ni foam (1 × 1 cm²) and dried at room temperature. Subsequently, the Ni foam loaded with active material was pressed under 10 MPa and acted as the working electrode (mass loading of about 5 mg).

The solid-state asymmetrical supercapacitor measurements were conducted in KOH/PVA gel electrolyte with Ni–Mn–Co–Ce oxide YDS hierarchical architectures and activated carbon (AC) as the cathode and anode. To prepare the KOH/PVA gel electrolyte, a 15 mL aqueous solution containing 2.13 g KOH and 1.52 g polyvinyl alcohol (PVA) was strongly stirred at around 75 °C for 30 min and used for the electrolyte. Ni foam loaded with 80 wt% Ni–Mn–Co–Ce oxide YDS hierarchical architectures, 15 wt% acetylene black, and 5 wt% polyvinylidene difluoride (PVDF) binder served as the cathode. A similar cathode preparation process was also used for the anode. Both cathode and anode were immersed in the prepared KOH/PVA gel electrolyte for 10 min and packaged together like a sandwich. It was then dried at 60 °C for 12 h to remove excess moisture, resulting in the solid-state asymmetrical supercapacitor device.

The energy density (Wh kg⁻¹) and power density (W kg⁻¹) of the assembled solid-state asymmetrical supercapacitor device were calculated according to eqn (1) and (2):

where V is the working potential (V) of the asymmetrical supercapacitors. C is the specific capacitance (F g⁻¹) calculated from GCD curves recorded under varied current densities (1.5, 3.0, 6.0, 10, and 15 A g⁻¹). t refers to the discharge time in seconds (s).

3. Results and discussion

Ni–Mn–Co–Ce glycerate was exploited as a template to prepare Ni–Mn–Co–Ce oxide YDS hierarchical architectures. As shown in Fig. 1a–c, close structural characterization unveiled that the Ni–Mn–Co–Ce glycerate has a well-uniform sphere (∼2 μm) morphology with a distinct solid state and smooth surface. The prepared Ni–Mn–Co–Ce glycerate spheres were dispersed in NMP/H₂O solvent for further synthesis of the yolk–shell hierarchical architecture. Under treatment at 160 °C for 12 h, the sample retained a well-spherical structure (Fig. 1d). Detailed observation revealed that the sample has a yolk–shell hierarchical architecture, where the shell is constructed by ultrathin nanosheets as building units (Fig. 1e and f). To further elucidate the structural evolution process, the products were obtained at selected hydrothermal times and monitored by TEM, as exhibited in Fig. 1g–i. Fig. 1g displays the TEM image of Ni–Mn–Co–Ce glycerate spheres after treatment for 0.5 h; a rough sphere surface is evident. Ultrathin nanosheets emerged at the sphere surface after 2.0 h (Fig. 1h), which slowly converted into a yolk–shell hierarchical hollow architecture after 4.0 h. The shell consists of randomly assembled ultrathin nanosheets (Fig. 1i). Considering the above TEM characterizations, we proposed a templating strategy to form the yolk–shell hierarchical hollow architecture, as schematically shown in Fig. 1j. The solvothermally prepared Ni–Mn–Co–Ce glycerate spheres were used as the template. Metal-glycerate is soluble in water and serves as the template for the fabrication of hollow structures.^32,33 In NMP-H₂O mixed solution, NMP can produce alkaline conditions, enabling the released metal ions species to react with OH⁻ to generate ultrathin nanosheets on the surfaces of the spheres.³⁴ With time, more and more metal ions were released from the template, and a hierarchical shell was constructed with ultrathin nanosheet building units. Finally, the yolk–shell hierarchical hollow architecture was successfully obtained at the cost of the gradual expenditure of the template.

Fig. 1 (a and b) FESEM and (c) TEM images of Ni–Mn–Co–Ce glycerate. (d and e) FESEM and (f) TEM images of the yolk–shell hierarchical architecture. TEM images of the products obtained at selected hydrothermal times: (g) 0.5 h, (h) 2.0 h, and (i) 4.0 h. (j) Schematic of the formation process of the yolk–shell hierarchical architecture.

A carbonization procedure was subsequently conducted to transform the yolk–shell hierarchical hollow architecture intermediates into Ni–Mn–Co–Ce oxide. The XRD pattern (Fig. S1) verified that the mixed oxides consist of NiO (JCPDS No. 75-0197), MnO₂ (JCPDS No. 72-1982), Co₃O₄ (JCPDS No. 80-1540), and CeO₂ (JCPDS No. 75-0120). SEM images of the Ni–Mn–Co–Ce oxide exhibited the retained yolk–shell hierarchical hollow architecture morphology (Fig. 2a–c). The inner architecture of the Ni–Mn–Co–Ce oxide was further observed by TEM as displayed in Fig. 2d and e. Remarkably, the YDS hierarchical architecture configuration of the Ni–Mn–Co–Ce oxide is clearly visualized. High-resolution TEM characterization of the hierarchical shell demonstrated four evident lattice fringes with spacings of 0.20, 0.12, 0.28 and 0.32 nm, corresponding to the (200) facet of NiO, (200) facet of MnO₂, (220) facet of Co₃O₄ and (111) facet of CeO₂, respectively (Fig. 2f). These results align well with the selected area electron diffraction (SAED, inset of Fig. 2f) analysis. Fig. 3g exhibits the energy-dispersive X-ray spectroscopy (EDS) mapping images of the Ni–Mn–Co–Ce oxide, in which a homogeneous distribution of the elements Ni, Mn, Co, and Ce is observable. Further carbonization treatment allowed structural transformation to produce the unique YDS hierarchical architectures, which could be attributed to the thermally driven contraction procedure.³⁵ The formation process for the Ni–Mn–Co–Ce oxide YDS hierarchical architectures is schematically elucidated in Fig. 3h. In the yolk–shell hierarchical hollow architectures, the inner yolk of Ni–Mn–Co–Ce glycerate contains large amounts of organics, including polymerized isopropanol and glycerol. In particular, the TG analysis represented the mass loss up to about 47.25%, which further unveiled that a large carbon matrix composition exists in the yolk (Fig. S2). During the carbonization process, there are two forces in opposing directions. One is the contraction force (F_c) arising from the thermal decomposition of organics, resulting in inward shrinkage of the yolk. The other is the adhesion force (F_a) that results from the yolk surface, which inhibits the inward shrinkage of the yolk.^36,37 The yolk surface is first oxidized and decomposed to form a Ni–Mn–Co–Ce oxide crystal nucleus, which grows slowly to generate a Ni–Mn–Co–Ce oxide shell. Meanwhile, compared with F_a, the F_c is dominant during carbonization treatment. The undecomposed yolk shrinks inward and separates from the Ni–Mn–Co–Ce oxide shell. The unique Ni–Mn–Co–Ce oxide YDS hierarchical architectures are finally obtained after carbonization treatment.

Fig. 2 (a–c) FESEM images, (d and e) TEM images, and (f) high-resolution TEM image of Ni–Mn–Co–Ce oxide YDS hierarchical architectures. (g) Scanning TEM elemental mapping images of a Ni–Mn–Co–Ce oxide YDS hierarchical architecture. (h) Schematic of the formation process of the Ni–Mn–Co–Ce oxide YDS hierarchical architecture.

Fig. 3 XPS spectra of the Ni–Mn–Co–Ce oxide YDS hierarchical architecture: (a) Ni 2p, (b) Mn 2p, (c) Co 2p, and (d) Ce 3d.

XPS analysis was further utilized to characterize the surface element composition of the Ni–Mn–Co–Ce oxide. The Ni 2p spectra in Fig. 3a show two characteristic binding energies at 855.5 and 873.2 eV, attributed to Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺.³⁸ In the Mn 2p spectra (Fig. 3b), the peaks at 642.8 and 653.9 eV are attributed to Mn⁴⁺ 2p_3/2 and Mn⁴⁺ 2p_1/2.³⁹ As displayed in Fig. 3c, the Co XPS is deconvoluted into the Co³⁺ (779.7 eV for Co³⁺ 2p_3/2 and 794.7 eV for Co³⁺ 2p_1/2) and Co²⁺ (781.4 eV for Co²⁺ 2p_3/2 and 796.7 eV for Co²⁺ 2p_1/2), respectively.⁴⁰ The Ce 3d XPS exhibits six peaks (Fig. 3d), in which the first three peaks located at 882.4, 888.6 and 898.1 eV correspond to Ce 3d_5/2, while the last three peaks at 900.7, 906.8 and 916.4 eV correspond to Ce 3d_3/2.⁴¹ Based on the above XPS characterizations, we deduced that the mixed metal oxides, including NiO, MnO₂, Co₃O₄ and CeO₂, were successfully prepared.

Further analysis by N₂ adsorption–desorption demonstrated that the unique Ni–Mn–Co–Ce oxide YDS hierarchical architectures have a specific surface area of 276 m² g⁻¹ and a mesoporous structure (Fig. S3). Such a Ni–Mn–Co–Ce oxide with specific construction and chemical composition may possess abundant electrochemically active sites, which are beneficial for electrochemical energy storage. As shown in Fig. 4a, the electrochemical properties were estimated in a three-electrode glass cell by pairing the Ni–Mn–Co–Ce oxide YDS hierarchical architecture working electrode with a Hg–HgO electrode and Pt wire acting as the reference and counter electrodes, respectively. Assessment by CV analysis indicated that the Ni–Mn–Co–Ce oxide electrode exhibited one characteristic redox peak pair in the 0–0.6 V range, indicating that the energy storage is mainly pseudocapacitive, which is due to the reversible redox reactions between Ni–Mn–Co–Ce oxides and the KOH electrolyte. The correlative redox reactions can be explained as follows:^42–45

NiO + OH⁻ ↔ NiOOH + e⁻

MnO₂ + K⁺ ↔ MnOOK + e⁻

Co₃O₄ + OH⁻ + H₂O ↔ CoOOH + e⁻

CeO₂ + K⁺⁻ ↔ CeOOK + e⁻

Fig. 4 (a) Configuration of a three-electrode glass cell. Electrochemical performance of the Ni–Mn–Co–Ce oxide YDS hierarchical architecture: (b) CV profiles at different scan rates, (c) the relationship of peak current with scan rate, (d) the contribution ratio of the capacitive and ionic diffusion-controlled charge at different scan rates, (e) galvanostatic charge–discharge profiles under varied current densities, (f) specific capacitance under varied current densities, (g) cycling performance at 15 A g⁻¹ for over 10 000 cycles.

Well-defined CV curve outlines remained even at 100 mV s⁻¹, validating that the Ni–Mn–Co–Ce oxide electrode possesses a robustly reversible redox reaction (Fig. 4b). To further understand the electrochemical behaviors of the Ni–Mn–Co–Ce oxide electrode, the correlation of peak current with scan rate was analyzed by applying the following equation: i = av^b.⁴⁶ Here, i represents the current values of oxidation and reduction peaks, respectively. The value of b was applied to evaluate the capacitive and diffusion behavior of the electrode. If the b value is close to 0.5 or as high as 1, this is indicative of diffusion or a capacitive-dominated process. Fig. 3c shows that the b values of the oxidation peak (Peak 1) and reduction peak (Peak 2) are 0.716 and 0.654, respectively, unveiling a pseudocapacitive-dominated process in the electrochemical reaction of the electrode. Moreover, a quantitative evaluation of the percentage of the pseudocapacitive contribution was further performed according to the equation i(v) = k₁v + k₂v^1/2.⁴⁷ Here, k₁v and k₂v^1/2 demonstrate the distribution ratio of the capacitive and diffusion behaviors of the electrode, respectively. As displayed in Fig. 3d and Fig. S4, the pseudocapacitive contribution is 61% at 2.0 mV s⁻¹. On further increasing the scan rate to 100 mV s⁻¹, the pseudocapacitive contribution can reach 98.9%. Such a high pseudocapacitive contribution is largely due to its structural advantages and chemical compositions, enabling the Ni–Mn–Co–Ce oxide electrode to have remarkable electrochemical performance.⁴⁸ Galvanostatic charge–discharge measurements were conducted in the 0–0.5 V range under varied current densities to achieve the specific capacitance (Fig. 3e). The resulting charge–discharge curves showed evident plateaus, further demonstrating the faradaic charge–discharge process.⁴⁹ Impressively, the specific capacitances of Ni–Mn–Co–Ce oxide electrodes were as high as 2126.7, 1827, 1612, 1506, and 1317 F g⁻¹ at 4.0, 6.0, 8.0, 10 and 15 A g⁻¹, respectively. To increase the current density to 25 A g⁻¹, the specific capacitance can reach up to 821 F g⁻¹ (Fig. 3f). The cycling performance of the Ni–Mn–Co–Ce oxide electrode was also evaluated (Fig. 3g). At 15 A g⁻¹, a specific capacitance of 1183 F g⁻¹ was maintained for over 10 000 cycles. Moreover, around 100% coulombic efficiency was achieved during the durability test, revealing an excellent reversible redox reaction between the Ni–Mn–Co–Ce oxide electrode and KOH electrolyte. EIS analysis exhibited that the R_s of the Ni–Mn–Co–Ce oxide electrode had no visible changes after cycling (Fig. S5a). In addition, the maintained YDS hierarchical architecture of Ni–Mn–Co–Ce oxide electrode could be directly visualized after the 10 000-cycle test (Fig. S5b). These results prove that the Ni–Mn–Co–Ce oxide electrode, benefiting from its YDS hierarchical architecture and highly reversible redox reaction, exhibits exceptional stability. Of note, the electrochemical performance of the Ni–Mn–Co–Ce oxide electrode compares favorably with some reported single metal oxides or mixed metal oxides,^50–59 suggesting that the Ni–Mn–Co–Ce oxide YDS hierarchical architecture is favorable for supercapacitors (Table 1).

Table 1 Comparison of the electrochemical performance of the Ni–Mn–Co–Ce oxide YDS hierarchical architecture with other reported counterparts

Electrode materials	Morphology	Capacitance@low current density	Capacitance@high current density	Capacitance retention	Ref.
NiO		182 F g^−1, 1.0 A g^−1	118 F g^−1, 10 A g^−1	1.0 A g^−1, 5000, 83%	50
MnO2		137.4 F g^−1, 0.2 A g^−1	111.4 F g^−1, 5.0 A g^−1	—	51
Co3O4		1266 F g^−1, 1.0 A g^−1	534 F g^−1, 10 A g^−1	10 A g^−1, 5000, 87.5%	52
CeO2		492 F g^−1, 2.0 A g^−1	426 F g^−1, 15 A g^−1	10 A g^−1, 5000, 92.9%	53
Zn–Ni oxide		497 F g^−1, 1.3 A g^−1	298 F g^−1, 13.3 A g^−1	5.2 A g^−1, 2000, 117%	54
Co–Mn oxide		860 F g^−1, 2.0 A g^−1	624 F g^−1, 20 A g^−1	10 A g^−1, 10 000, 90.8%	55
Ni–Co oxide		1361 F g^−1, 1.0 A g^−1	753 F g^−1, 30 A g^−1	10 A g^−1, 3000, 76.4%	56
Ce–Mn oxide		178.5 F g^−1, 0.25 A g^−1	112.3 F g^−1, 32 A g^−1	2.0 A g^−1, 3000, 90.1%	57
Cu–Ni–Co oxide		262.5 F g^−1, 1.0 A g^−1	158.3 F g^−1, 10 A g^−1	10 mV s^−1, 3000, 107.9%	58
Ni–Mn–Ce oxide		1027.8 F g^−1, 3.1 A g^−1	520.8 F g^−1, 31.3 A g^−1	12.5 A g^−1, 5000, 97.82%	59
Ni–Mn–Co–Ce oxide		2126.7 F g ^−1 , 4.0 A g ^−1	1317 F g ^−1 , 15 A g ^−1	15 A g ^−1 , 10 000, 89.8%	This work

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As shown in Fig. 5a, we designed a solid-state asymmetrical supercapacitor device for practical applications, with a cathode of Ni–Mn–Co–Ce oxide with YDS hierarchical architecture, an anode of AC, and a PVA/KOH gel electrolyte. Further electrochemical tests revealed that the AC exhibits a typical double-layer capacitor feature during the charge–discharge process, as shown by CV curves (Fig. S6a). The specific capacitances were 167.6, 160.8, 156.8, 150.2, 148.5, and 132.5 F g⁻¹ at 4.0, 6.0, 8.0, 10, 15, and 25 A g⁻¹ (Fig. S6b and c). Therefore, the mass ratio between the cathode and anode was determined to be about 0.16 by balancing the charges in each electrode. Remarkably, an evident potential difference between the Ni–Mn–Co–Ce oxide cathode and the AC anode was clearly visualized, as presented in Fig. 5b. The Ni–Mn–Co–Ce oxide cathode behavior shows notable redox peaks, unveiling a typical battery characteristic. In contrast, the AC anode behavior showed a broader quasi-rectangular profile, demonstrating a capacitive feature. As seen from CV curves at varied scan rates (Fig. 5c), our assembled device possesses distinct capacitive and battery electrochemical behaviors in the 0–1.6 V test range. The profiles of all CV curves exhibited no significant changes, thus indicating good reversibility. Measurements of galvanostatic charge–discharge revealed that the assembled device showed the specific capacitances of 282.5, 256.8, 192.5, 140.6, and 123.8 F g⁻¹ at 1.5, 3.0, 6.0, 10, and 15 A g⁻¹, respectively (Fig. 5d and e). To further explore the flexibility and mechanical stability of the designed solid-state asymmetrical supercapacitor device, the device was subjected to different bending conditions for the charge–discharge tests. Notably, the specific capacitance remained at the initial value under various bending modes for over 200 cycles (Fig. 5f), confirming that the device shows great flexibility and mechanical stability. Moreover, the device displayed a robust long-term cycling life at 10 A g⁻¹, retaining 97.1% of the initial capacitance for 5000 cycles (Fig. 5g). The device with high specific capacitance and acceptable working voltage is expected to possess high energy density. The Ragone plot of Fig. 5h exhibits the energy density of our device up to 100.4 Wh kg⁻¹ at a power density of 1200 W kg⁻¹, and the energy density value at 12 005 W kg⁻¹ remained at 44 Wh kg⁻¹, suggesting its remarkable energy storage properties in comparison with reported counterparts, such as MnO₂/C nanoboxes//N-doped carbon,⁶⁰ Ni–Co oxide hollow microspheres//RGO@Fe₃O₄,⁶¹ Co₃O₄/N-doped carbon hollow spheres//AC,⁶² NiO hollow spheres//Fe₂O₃,⁶³ Co₉S₈@CoNiO₂ hollow cubes//AC,⁶⁴ Co₃O₄@NiMoO₄/CoMoO₄ hierarchical nanotubes//AC,⁶⁵ CuCo-LDH hollow polyhedron//CuCo-LDH hollow polyhedron,⁶⁶ ZnCo₂O₄/C yolk–shell//AC and NiCo-LDH hollow spheres//AC.^67,68

Fig. 5 (a) Schematic of the charge–discharge mechanism of the device. (b) CV profiles of the Ni–Mn–Co–Ce oxide YDS hierarchical architecture and AC at 20 mV s⁻¹. (c) CV profiles at different scan rates. (d) Galvanostatic charge–discharge profiles at various current densities. (e) Specific capacitance of the device. (f) Cycling performance at 10 A g⁻¹ for 200 cycles under different bending conditions. (g) Cycling performance at 10 A g⁻¹ for over 5000 cycles. (h) Ragone plots of the device.

Overall, the prepared Ni–Mn–Co–Ce oxide YDS hierarchical architectures show remarkable promise for applications in energy storage, which can be attributed to their architecture and component merits. First, the hierarchical shells configured by ultrathin nanosheets are beneficial for infiltrating the electrolyte and shortening the electron/ion transport distance, boosting the energy storage efficiency.^69,70 Second, the double shell architectures enable sufficient electrode/electrolyte contact area and active sites for redox reactions, ensuring superior electrochemical activities.^71,72 Finally, the gap between different shells offer enough cavity to prevent large volume expansion during the charge–discharge process, endowing a highly boosted cycling life.⁷³ Furthermore, the component complexity in Ni–Mn–Co–Ce oxide may further facilitate conductivity and enrich the redox centers.

4. Conclusions

We have presented a simple synthetic strategy for the construction of Ni–Mn–Co–Ce oxide YDS hierarchical architectures. Uniform Ni–Mn–Co–Ce glycerate solid spheres are first prepared as the templates, which are treated with NMP/H₂O and converted into yolk–shell hierarchical architectures by a hydrothermal route. A subsequent carbonization process results in the Ni–Mn–Co–Ce oxide YDS hierarchical architectures. With the unique architecture and component complexity, the prepared Ni–Mn–Co–Ce oxide hierarchical architectures show excellent performance as electrode materials in terms of high specific capacitance (2126.7 F g⁻¹ at 4.0 A g⁻¹) and robust cycling life (89.8% capacitance retention over 10 000 cycles). When we further evaluated the Ni–Mn–Co–Ce oxide YDS hierarchical architecture as the cathode and AC as the anode for a solid-state asymmetrical supercapacitor device, the device possessed a high energy density of 100.4 Wh kg⁻¹ at a power density of 1200 W kg⁻¹. This work not only presents an inspirational route for the construction of complex hollow architectures, but it also provides a prospective electrode material for supercapacitors.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the SI. Details about the XRD pattern of the Ni–Mn–Co–Ce oxide YDS hierarchical architectures; TGA curve for the yolk-shell hierarchical architectures intermediates; N₂ adsorption-desorption and pore size distribution curve of the Ni–Mn–Co–Ce oxide YDS hierarchical architectures; The capacitive contribution ratio at different scan rates; Nyquist plot of the Ni–Mn–Co–Ce oxide YDS hierarchical architectures electrode before and after 10 000 cycles and TEM image of Ni-Mn-Co-Ce oxide YDS hierarchical architectures after 10 000 cycles; Electrochemical performance of AC. See DOI: https://doi.org/10.1039/D5QI01324K.

Acknowledgements

This work was financed by the Major Science and Technology Special Project of Anyang City (2023A02GX009).

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