A hexaazatriphenylene-based porous organic polymer for high performance supercapacitor

Porous organic polymers (POPs) with high physiochemical stability and pseudocapacitive activity are crucial for supercapacitors with high specific capacitance and long cycle life. We report herein a hexaazatrinaphthylene-based POP (HPOP-1) for high-performance supercapacitor by introducing redox-active hexaazatrinaphthylene (HATN) moiety through Sonogashira–Hagihara coupling reaction. HATN moiety can undergo a proton-induced electron transfer redox reaction, which endows HPOP-1 with high pseudocapacitive activity. As electrode materials for supercapacitor application, HPOP-1 exhibits high specific capacitance (667 F g⁻¹ at 0.5 A g⁻¹) and long-term cyclic stability (90% capacitance retention after 10,000 cycles at 5 A g⁻¹) in a three-electrode system with 1 M H₂SO₄ as the electrolyte. In addition, HPOP-1 also exhibits a specific capacitance of 376 F g⁻¹ at 0.5 A g⁻¹ in 1 M KOH electrolyte. An asymmetric supercapacitor was further fabricated with HPOP-1 as negative electrode and rGO as positive electrode, respectively. The device delivers a specific capacitance of 63 F g⁻¹ at 0.5 A g⁻¹ and a rate performance of 37 F g⁻¹ at 5 A g⁻¹. Our work provides a facile approach for the design and preparation of pseudocapacitive POPs with high specific capacitance and long cycle life.     

High-performance Flexible Symmetric Supercapacitor Based on Heterostructured PANI@MoS2 Nanocomposite Electrode

Three-dimensional (3D) heterostructured molybdenum disulfide (MoS₂) is used as base materials for aniline monomer in situ polymerization on its surface. It is found that the aniline addition has a remarkable effect on the energy storage of the final compounds due to the improvement of the conductivity and structure stability combined with the synergistic effect between the two types of species. The optimal compound of PANI@MoS₂-150 not only shows a high capacitance value of 801.4 F ⋅ g⁻¹ at a current density of 0.5 A ⋅ g⁻¹ but also provides a high retention rate of 77.4 % after 10,000 cycles. The capacitance fading may be due to the increase of the internal resistance analyzed by EIS. Furthermore, a flexible symmetric supercapacitor based on PANI@MoS₂-150 has also been fabricated and the specific capacitance reaches 105 F ⋅ g⁻¹ at a current density of 1 A ⋅ g⁻¹. Impressively, the capacitance retention is larger than 100 % undergoing 10,000 cycles. Besides, the highest energy density of 21 Wh ⋅ kg⁻¹ was obtained. Additionally, the fabricated symmetric supercapacitor demonstrates excellent flexibility.     

Perovskite SrCo0.9Nb0.1O3−δ as an Anion‐Intercalated Electrode Material for Supercapacitors with Ultrahigh Volumetric Energy Density

We have synthesized and characterized perovskite‐type SrCo_0.9Nb_0.1O_3−δ (SCN) as a novel anion‐intercalated electrode material for supercapacitors in an aqueous KOH electrolyte, demonstrating a very high volumetric capacitance of about 2034.6 F cm⁻³ (and gravimetric capacitance of ca. 773.6 F g⁻¹) at a current density of 0.5 A g⁻¹ while maintaining excellent cycling stability with a capacity retention of 95.7 % after 3000 cycles. When coupled with an activated carbon (AC) electrode, the SCN/AC asymmetric supercapacitor delivered a specific energy density as high as 37.6 Wh kg⁻¹ with robust long‐term stability.     

Highly Dispersible Hexagonal Carbon–MoS2–Carbon Nanoplates with Hollow Sandwich Structures for Supercapacitors

MoS₂, a typical layered transition-metal dichalcogenide, is promising as an electrode material in supercapacitors. However, its low electrical conductivity could lead to limited capacitance if applied in electrochemical devices. Herein, a new nanostructure composed of hollow carbon–MoS₂–carbon was successfully synthesized through an l -cysteine-assisted hydrothermal method by using gibbsite as a template and polydopamine as a carbon precursor. After calcination and etching of the gibbsite template, uniform hollow platelets, which were made of a sandwich-like assembly of partial graphitic carbon and two-dimensional layered MoS₂ flakes, were obtained. The platelets showed excellent dispersibility and stability in water, and good electrical conductivity due to carbon provided by the calcination of polydopamine coatings. The hollow nanoplate morphology of the material provided a high specific surface area of 543 m² g⁻¹, a total pore volume of 0.677 cm³ g⁻¹, and fairly small mesopores (≈5.3 nm). The material was applied in a symmetric supercapacitor and exhibited a specific capacitance of 248 F g⁻¹ (0.12 F cm⁻²) at a constant current density of 0.1 A g⁻¹; thus suggesting that hollow carbon–MoS₂–carbon nanoplates are promising candidate materials for supercapacitors.     

Hierarchical Carbon Nanowire/Ni@MnO2 Nanocomposites for High-Performance Asymmetric Supercapacitors

A 3D hierarchical carbon cloth/nitrogen-doped carbon nanowires/Ni@MnO₂ (CC/N-CNWs/Ni@MnO₂) nanocomposite electrode was rationally designed and prepared by electrodeposition. The N-CNWs derived from polypyrrole (PPy) nanowires on the carbon cloth have an open framework structure, which greatly increases the contact area between the electrode and electrolyte and provides short diffusion paths. The incorporation of the Ni layer between the N-CNWs and MnO₂ is beneficial for significantly enhancing the electrical conductivity and boosting fast charge transfer as well as improving the charge-collection capacity. Thus, the as-prepared 3D hierarchical CC/N-CNWs/Ni@MnO₂ electrode exhibits a higher specific capacitance of 571.4 F g⁻¹ compared with those of CC/N-CNWs@MnO₂ (311 F g⁻¹), CC/Ni@MnO₂ (196.6 F g⁻¹), and CC@MnO₂ (186.1 F g⁻¹) at 1 A g⁻¹ and remarkable rate capability (367.5 F g⁻¹ at 10 A g⁻¹). Moreover, asymmetric supercapacitors constructed with CC/N-CNWs/Ni@MnO₂ as cathode material and activated carbon as anode material deliver an impressive energy density of 36.4 W h kg⁻¹ at a power density of 900 W kg⁻¹ and a good cycling life (72.8 % capacitance retention after 3500 cycles). This study paves a low-cost and simple way to design a hierarchical nanocomposite electrode with large surface area and superior electrical conductivity, which has wide application prospects in high-performance supercapacitors.     

Conjugated Copper–Catecholate Framework Electrodes for Efficient Energy Storage

A conjugated copper(II) catecholate based metal–organic framework (namely Cu-DBC) was prepared using a D₂-symmetric redox-active ligand in a copper bis(dihydroxy) coordination geometry. The π-d conjugated framework exhibits typical semiconducting behavior with a high electrical conductivity of ca. 1.0 S m⁻¹ at room temperature. Benefiting from the good electrical conductivity and the excellent redox reversibility of both ligand and copper centers, Cu-DBC electrode features superior capacitor performances with gravimetric capacitance up to 479 F g⁻¹ at a discharge rate of 0.2 A g⁻¹. Moreover, the symmetric solid-state supercapacitor of Cu-DBC exhibits high areal (879 mF cm⁻²) and volumetric (22 F cm⁻³) capacitances, as well as good rate capability. These metrics are superior to most reported MOF-based supercapacitors, demonstrating promising applications in energy-storage devices.     

High Energy Density Heteroatom (O,N and S) Enriched Activated Carbon for Rational Design of Symmetric Supercapacitors

Carbon-based symmetric supercapacitors (SCs) are known for their high power density and long cyclability, making them an ideal candidate for power sources in new-generation electronic devices. To boost their electrochemical performances, deriving activated carbon doped with heteroatoms such as N, O, and S are highly desirable for increasing the specific capacitance. In this regard, activated carbon (AC) self-doped with heteroatoms is directly derived from bio-waste (lima-bean shell) using different KOH activation processes. The heteroatom-enriched AC synthesized using a pretreated carbon-to-KOH ratio of 1:2 (ONS@AC-2) shows excellent surface morphology with a large surface area of 1508 m² g⁻¹. As an SC electrode material, the presence of heteroatoms (N and S) reduces the interfacial charge-transfer resistance and increases the ion-accessible surface area, which inherently provides additional pseudocapacitance. The ONS@AC-2 electrode attains a maximum specific capacitance (C_sp) of 342 F g⁻¹ at a specific current of 1 Ag⁻¹ in 1 m NaClO₄ electrolyte at the wide potential window of 1.8 V. Moreover, as symmetric SCs the ONS@AC-2 electrode delivers a maximum specific capacitance (C_sc) of 191 F g⁻¹ with a maximum specific energy of 21.48 Wh kg⁻¹ and high specific power of 14 000 W kg⁻¹ and excellent retention of its initial capacitance (98 %) even after 10000 charge/discharge cycles. In addition, a flexible supercapacitor fabricated utilizing ONS@AC-2 electrodes and a LiCl/polyvinyl alcohol (PVA)-based polymer electrolyte shows a maximum C_sc of 119 F g⁻¹ with considerable specific energy and power.     

Mixed Cu2Se Hexagonal Nanosheets@Co3Se4 Nanospheres for High-Performance Asymmetric Supercapacitors

Rational designing and constructing multiphase hybrid electrode materials is an effective method to compensate for the performance defects of the single component. Based on this strategy, Cu₂Se hexagonal nanosheets@Co₃Se₄ nanospheres mixed structures have been fabricated by a facile two-step hydrothermal method. Under the synergistic effect of the high ionic conductivity of Cu₂Se and the remarkable cycling stability of Co₃Se₄, Cu₂Se@Co₃Se₄ can exhibit outstanding electrochemical performance as a novel electrode material. The as-prepared Cu₂Se@Co₃Se₄ electrode displays high specific capacitance of 1005 F g⁻¹ at 1 A g⁻¹ with enhanced rate capability (56 % capacitance retention at 10 A g⁻¹), and ultralong lifespan (94.2 % after 10 000 cycles at 20 A g⁻¹). An asymmetric supercapacitor is assembled applying the Cu₂Se@Co₃Se₄ as anode and graphene as cathode, which delivers a wide work potential window of 1.6 V, high energy density (30.9 Wh kg⁻¹ at 0.74 kW kg⁻¹), high power density (21.0 Wh kg⁻¹ at 7.50 kW kg⁻¹), and excellent cycling stability (85.8 % after 10 000 cycles at 10 A g⁻¹).     

Intertwined Titanium Carbide MXene within a 3 D Tangled Polypyrrole Nanowires Matrix for Enhanced Supercapacitor Performances

The exploration of the rational design and synthesis of unique and robust architectured electrodes for the high capacitance, rate capability, and stability of supercapacitors is crucial to the future of energy storage technology. Herein, an in situ synthesis of multilayered titanium carbide MXene tightly caging within a 3 D conducting tangled polypyrrole (PPy) nanowire (NW) network is proposed as an effective strategy to prevent the aggregation of MXene, profoundly enhancing the electrochemical performance of the supercapacitor. Owing to the beneficial effects of an ideal 3 D interconnected porous structure and high electrical conductivity, the obtained electrode exhibits fast charge and ion transport kinetics as well as full usage of active material. As expected, the 3 D Ti₃C₂T_x@PPY NW exhibits a specific capacitance five times higher than that of pristine MXene (610 F g⁻¹), a good rate capability up to a current density of 25 A g⁻¹, and excellent stability with 100 % retention after 14 000 cycles at 4 A g⁻¹, outperforming the known state-of-the-art MXene-based supercapacitor. Our work provides a facile method for enhancing the performance of MXene-based energy storage devices.     

Synthesis of Co3O4/Carbon Heteroaerogels with Ultrahigh Capacitance via Polyethyleneimine Intercalation of Co2BIM4 Nanosheets

The development of metal–organic frameworks (MOFs)-based supercapacitors have attracted intense concentration in recent years due to their regularly arranged porous and tunable pore sizes. However, the performance of the MOFs-derived supercapacitors is also low because of their poor electrical conductivity and rarely accessible active sites. In the present work, we developed a Co-MOF (namely Co₂BIM₄, BIM=benzimidazole) nanosheets derived Co₃O₄/nitrogen-doped carbon (Co₂BIM₄-Co₃O₄/NC) heteroaerogel as a novel supercapacitor electrode. The 3D Co₂BIM₄-Co₃O₄/NC heteroaerogels were obtained by directly intercalating polyethyleneimine (PEI) into the interlayers of Co₂BIM₄ nanosheets and following by carbonizing the resulting Co₂BIM₄/PEI composite. The Co₂BIM₄-Co₃O₄/NC electrode possessed 3D conductive framework with an overlapped hetero-interface and expanded interlayers, leading to fast and stable charge transfer/diffusion and an enhanced pseudocapacitance performance. Therefore, the Co₂BIM₄-Co₃O₄/NC electrode showed ultrahigh capacitance of 2568 F g⁻¹ at 1 A g⁻¹, 1747 F g⁻¹ at 10 A g⁻¹, and excellent long cycling time with a capacitance preservation of 92.7 % following 10000 cycles at 10 A g⁻¹, which is very promising for applications in supercapacitors and other energy storage devices.     

Hierarchical FeCo2S4@FeNi2S4 Core/Shell Nanostructures on Ni Foam for High-Performance Supercapacitors

The design of electrode materials with rational core/shell structures is promising for improving the electrochemical properties of supercapacitors. Hence, hierarchical FeCo₂S₄@FeNi₂S₄ core/shell nanostructures on Ni foam were fabricated by a simple hydrothermal method. Owing to their structure and synergistic effect, they deliver an excellent specific capacitance of 2393 F g⁻¹ at 1 A g⁻¹ and long cycle lifespan as positive electrode materials. An asymmetric supercapacitor device with FeCo₂S₄@FeNi₂S₄ as positive electrode and graphene as negative electrode exhibited a specific capacitance of 133.2 F g⁻¹ at 1 A g⁻¹ and a high energy density of 47.37 W h kg⁻¹ at a power density of 800 W kg⁻¹. Moreover, the device showed remarkable cycling stability with 87.0 % specific-capacitance retention after 5000 cycles at 2 A g⁻¹. These results demonstrate that the hierarchical FeCo₂S₄@FeNi₂S₄ core/shell structures have great potential in the field of electrochemical energy storage.     

High performance pliable supercapacitor fabricated using activated carbon nanospheres intercalated into boron nitride nanoplates by pulsed laser ablation technique

Pliable supercapacitor, yielding specific capacitance (C_s) and energy density as high as 348 F g⁻¹ and 48.3 Wh Kg⁻¹ respectively was fabricated using modified activated carbon electrodes. The nanospheres of activated carbon (AC) were anchored on the nanoplates of boron nitride (BN) by employing the facile technique of pulsed laser ablation in liquid (PLAL) using 532 nm focused laser beam. Four different variants of electrode materials were synthesized by varying the weight percentage (1%, 3%, 5% and 10%) of BN in AC in the PLAL precursor solution. The morphological characteristics, the elemental composition and the structural analysis of the synthesized electrode materials were studied respectively by FESEM, XPS and XRD. The morphological studies indicated that the PLAL synthesis of the electrode materials resulted in proper intercalation of carbon nanospheres into BN nanoplates, which resulted in the observed enhanced performance of the fabricated supercapacitor. Four supercapacitors in this work were fabricated using the four variants of synthesized electrode materials in conjunction with gel polymer electrolyte (GPE). GPE are well known for their non-corrosive nature and best sealing ability to avoid any leakage that results in increasing the cycle life of the device. The performance of the fabricated supercapacitors was evaluated using cyclic voltammetry (CV), galvanostatic charge discharge (GCD) measurement and electrochemical impedance spectroscopy (EIS). The results indicate that the supercapacitor fabricated using 3% BN in AC as electrode material manifested the best specific capacitance and energy density. Also it was found that the supercapacitor maintained 85% of its initial capacitance even after 5000 charge/discharge cycles.     

Enhanced Supercapacitor Performance Using a Co3O4@Co3S4 Nanocomposite on Reduced Graphene Oxide/Ni Foam Electrodes

To avoid an enormous energy crisis in the not-too-distant future, it be emergent to establish high-performance energy storage devices such as supercapacitors. For this purpose, a three-dimensional (3D) heterostructure of Co₃O₄ and Co₃S₄ on nickel foam (NF) that is covered by reduced graphene oxide (rGO) has been prepared by following a facile multistep method. At first, rGO nanosheets are deposited on NF under mild hydrothermal conditions to increase the surface area. Subsequently, nanowalls of cobalt oxide are electro-deposited on rGO/Ni foam by applying cyclic-voltammetry (CV) under optimized conditions. Finally, for the synthesis of Co₃O₄@Co₃S₄ nanocomposite, the nanostructure of Co₃S₄ was fabricated from Co₃O₄ nanowalls on rGO/NF by following an ordinary hydrothermal process through the sulfurization for the electrochemical application. The samples are characterized by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The obtained sample delivers a high capacitance of 13.34 F cm⁻² (5651.24 F g⁻¹) at a current density of 6 mA cm⁻² compared to the Co₃O₄/rGO/NF electrode with a capacitance of 3.06 F cm⁻² (1230.77 F g⁻¹) at the same current density. The proposed electrode illustrates the superior electrochemical performance such as excellent specific energy density of 85.68 W h Kg⁻¹, specific power density of 6048.03 W kg⁻¹ and a superior cycling performance (86% after 1000 charge/discharge cycles at a scan rate of 5 mV s⁻¹). Finally, by using Co₃O₄ @Co₃S₄/rGO/NF and the activated carbon-based electrode as positive and negative electrodes, respectively, an asymmetric supercapacitor (ASC) device was assembled. The fabricated ASC provides an appropriate specific capacitance of 79.15 mF cm⁻² at the applied current density of 1 mA cm⁻², and delivered an energy density of 0.143 Wh kg⁻¹ at the power density of 5.42 W kg⁻¹.     

Application of a novel redox-active electrolyte in MnO2-based supercapacitors

This paper reports a novel strategy for preparing redox-active electrolyte through introducing a redox-mediator(p-phenylenediamine,PPD) into KOH electrolyte for the application of ball-milled MnO 2-based supercapacitors.The morphology and compositions of ball-milled MnO 2 were characterized using scanning electron microscopy(SEM) and X-ray diffraction(XRD).The electrochemical properties of the supercapacitor were evaluated by cyclic voltammetry(CV),galvanostatic charge-discharge(GCD),and electrochemical impedance spectroscopy(EIS) techniques.The introduction of p-phenylenediamine significantly improves the performance of the supercapacitor.The electrode specific capacitance of the supercapacitor is 325.24 F g-1,increased by 6.25 folds compared with that of the unmodified system(44.87 F g-1) at the same current density,and the energy density has nearly a 10-fold increase,reaching 10.12 Wh Kg-1.In addition,the supercapacitor exhibits good cycle-life stability.     

A High-Performance Asymmetric Supercapacitor Based on Tungsten Oxide Nanoplates and Highly Reduced Graphene Oxide Electrodes

Tungsten oxide/graphene hybrid materials are attractive semiconductors for energy-related applications. Herein, we report an asymmetric supercapacitor (ASC, HRG//m-WO₃ ASC), fabricated from monoclinic tungsten oxide (m-WO₃) nanoplates as a negative electrode and highly reduced graphene oxide (HRG) as a positive electrode material. The supercapacitor performance of the prepared electrodes was evaluated in an aqueous electrolyte (1 m H₂SO₄) using three- and two-electrode systems. The HRG//m-WO₃ ASC exhibits a maximum specific capacitance of 389 F g⁻¹ at a current density of 0.5 A g⁻¹, with an associated high energy density of 93 Wh kg⁻¹ at a power density of 500 W kg⁻¹ in a wide 1.6 V operating potential window. In addition, the HRG//m-WO₃ ASC displays long-term cycling stability, maintaining 92 % of the original specific capacitance after 5000 galvanostatic charge–discharge cycles. The m-WO₃ nanoplates were prepared hydrothermally while HRG was synthesized by a modified Hummers method.     

Zn–Co Sulfide Microflowers Anchored on Three-Dimensional Graphene: A High-Capacitance and Long-Cycle-Life Electrode for Asymmetric Supercapacitors

Zinc–cobalt double-metal sulfides (ZCS) as Faradic electrode materials with high energy density have great potential for supercapacitors, but their poor transfer efficiency for electrons and ions hinders their electrochemical response. Herein, ZnCo₂(CO₃)_1.5(OH)₃@ZCS microflower hybrid arrays consisting of thin nanolayer petals were anchored on three-dimensional graphene (ZnCo₂(CO₃)_1.5(OH)₃@ZCS/3DG) by a simple hydrothermal method and additional ion-exchange process. A ZnCo₂(CO₃)_1.5(OH)₃@ZCS/3DG electrode delivered high capacitance (2228 F g⁻¹ at 1 A g⁻¹) and long cycling life (85.7 % retention after 17 000 cycles), which are ascribed to the multicomponent structural design. The 3DG conductive substrate improves the electron-transfer dynamics of the electrode material. Meanwhile, the microflowers consisting of thin nanolayer petals could not only provide many active sites for ions to improve the capacitance, but also alleviate the volume expansion to ensure the structural stability. Furthermore, an all-solid-state asymmetric supercapacitor based on a ZnCo₂(CO₃)_1.5(OH)₃@ZCS/3DG electrode achieved a high energy density of 27 W h kg⁻¹ at 528.3 W kg⁻¹ and exhibits exceptional cyclic stability for 23 000 cycles. Its ability to light a blue LED for 9 min verified the feasibility of its application for energy storage devices.     

A Facile Strategy for the Preparation of MoS3 and its Application as a Negative Electrode for Supercapacitors

Owing to their graphene‐like structure and available oxidation valence states, transition metal sulfides are promising candidates for supercapacitors. Herein, we report the application of MoS₃ as a new negative electrode for supercapacitors. MoS₃ was fabricated by the facile thermal decomposition of a (NH₄)₂MoS₄ precursor. For comparison, samples of MoS₃&MoS₂ and MoS₂ were also synthesized by using the same method. Moreover, this is the first report of the application of MoS₃ as a negative electrode for supercapacitors. MoS₃ displayed a high specific capacitance of 455.6 F g^?1 at a current density of 0.5 A g^?1. The capacitance retention of the MoS₃ electrode was 92 % after 1500 cycles, and even 71 % after 5000 cycles. In addition, an asymmetric supercapacitor assembly of MoS₃ as the negative electrode demonstrated a high energy density at a high potential of 2.0 V in aqueous electrolyte. These notable results show that MoS₃ has significant potential in energy‐storage devices.     

Nickel/Cobalt Molybdate Hollow Rods Induced by Structure and Defect Engineering as Exceptional Electrode Materials for Hybrid Supercapacitor

Oxygen defects and hollow structures positively impact pseudocapacitive properties of diffusion/surface-controlled processes, a component of critical importance when building high-performance supercapacitors. Hence, we fabricated hollow nickel/cobalt molybdate rods with O-defects (D−H−NiMoO₄@CoMoO₄) through a soft-template and partial reduction method, enhancing D−H−NiMoO₄@CoMoO₄’s electrochemical performance, yielding a specific capacitance of 1329 F g⁻¹, and demonstrating excellent durability with 95.8 % capacity retention after 3000 cycles. D−H−NiMoO₄@CoMoO₄ was used as the positive electrode to construct an asymmetric supercapacitor, displaying an energy density of up to 34.13 Wh kg⁻¹ and demonstrating good predisposition towards practical applications. This work presents an effective approach to fabricate and use hollow nickel/cobalt molybdate rods with O-defects as pseudocapacitor material for high-performance capacitive energy storage devices.     

One-Pot Hydrothermal Synthesis of Ternary 1T-MoS2/Hexa-WO3/Graphene Composites for High-Performance Supercapacitors

A new ternary composite of 1T-molybdenum disulfide, hexagonal tungsten trioxide, and reduced graphene oxide (M-W-rGO) is synthesized by using a one-pot hydrothermal process. The synergetic effect of 1T-MoS₂ and hexa-WO₃ nanoflowers improves the electrochemical performance for supercapacitors by inducing additional active sites and hexagonal tunnels, respectively, which lead to high storage capacity and easy transfer of electrolyte ions. The ternary M-W-rGO composite has a high specific capacitance of 836 F g⁻¹ at 1 A g⁻¹, which is nearly twice that of binary composites of M-rGO and W-rGO with high capacitance retention of 86.35 % after 3000 cycles at a high current density of 5 A g⁻¹. This study provides a new ternary composite that can be used as an electrode material for high-performance supercapacitors.     

Mn3−xFexO4 Hollow Nanostructures for High-Performance Asymmetric Supercapacitor Applications

Design of hollow nanostructure and controllable phase of mixed metal oxides for improving performance in supercapacitor applications is highly desirable. Here we demonstrate the rational design and synthesis of Mn_3−xFe_xO₄ hollow nanostructures for supercapacitor applications. Owing to high porosity and the specific surface area that provides more active sites for electrochemical reactions, the electrochemical performance of Mn_3−xFe_xO₄ hollow nanostructure substantially enhanced comparing with pristine Mn₃O₄. Particularly, in 1.0 M KOH electrolyte, Mn_0.16Fe_2.84O₄ with a typical diameter of 20 nm exhibits excellent specific capacitance of 2675, 2320, 1662, 987 F g⁻¹ at current densities of 1, 2, 5, 10 A g⁻¹, respectively, which is significantly superior to those of other transition metal oxides. Besides, an asymmetric supercapacitor is assembled by using Mn_0.16Fe_2.84O₄ and activated carbon as a positive and a negative electrode, respectively. Electrochemical results indicate a high energy density of 42 Wh kg⁻¹ at a power density of 0.75 kW kg⁻¹, which makes this hollow nanostructure a highly promising electrode for achieving high-performance next-generation supercapacitors.