An urchin-like MgCo2O4@PPy core-shell composite grown on Ni foam for a high-performance all-solid-state asymmetric supercapacitor

Insights on Prussian Blue Analogue Cathode Material Engineered with Polypyrrole Surface Protection Layer for Aqueous Rechargeable Zinc Metal Battery

One of the key intricacies against using Prussian blue analogues (PBAs) in aqueous batteries is their gradual dissolution in aqueous electrolytes, resulting in inadequate cycling stability. Besides, the rate capability of PBAs is limited due to their poor electrical conductivity. To overcome these challenges, it is essential to tune the physical and chemical properties of PBAs at the nano regime without affecting the inherent charge storage properties, especially at high-voltage operating conditions. Through this work, a strategy is demonstrated to enhance the electrochemical performance of vanadium-based PBA (V-PBA) by surface engineering using a conducting polymer nano-skin (V-PBA/PPy) for aqueous zinc metal batteries. The polypyrrole (PPy) nano-skin over the V-PBA nanoparticles acts as an electron percolation path to ameliorate the poor electronic conductivity of the otherwise pristine V-PBA. Interestingly, the V-PBA with an optimized polypyrrole coating (V-PBA/PPy-2) exhibits an enhanced specific capacity (173 mAh g-1 at 0.10 A g-1) than the pristine V-PBA counterpart (80 mAh g-1) and 85% capacity retention up to 500 cycles. The DFT calculation confirms the synergistic interaction between PPy and V-PBA and the presence of PPy favors the adsorption of Zn.

Machine Learning-Based Prediction of Supercapacitor Capacitance for MgCo2O4 Electrodes

Electrode materials are essential in the electrochemical process of storing charge in supercapacitors and have a significant impact on the cost and capacitive performance of the final product. Hence, it is imperative to make precise predictions regarding the capacitance of electrode materials in order to further the development of supercapacitors. MgCo2O4, with a theoretical capacitance of up to 3122 F g-1, holds immense research value as an electrode material. The objective of this study is to predict the capacitance of MgCo2O4 with high accuracy. This will be achieved by extracting numerous data from published papers and using some parameters as input features. The Recursive Feature Elimination (RFE) method was employed, using Random Forest (RF), Extreme Gradient Boosting (XGBoost) and Regression Tree (RT) as selectors to identify the optimal feature subset. Then, combining them with these three regression models to construct nine machine learning (ML) models. After performance evaluation and outlier analysis, the XGB-RFE-XGB model achieved R-squared (R2), root mean squared error (RMSE), and mean absolute error (MAE) of 0.95, 111.83 F g-1 and 68.25 F g-1, respectively, demonstrating its stability and reliability. Therefore, the XGB-RFE-XGB model can be used as a reliable predictive tool in subsequent experimental designs.

Flexible All-Solid-State Asymmetric Supercapacitors Based on PPy-Decorated SrFeO3-δ Perovskites on Carbon Cloth

The defective structure and high oxygen vacancy concentration of SrFeO3-δ perovskite enable fast ion-electron transport, but its low conductivity still hinders the high electrochemical performance. Herein, to enhance the conductivity of SrFeO3-δ-based electrodes, polypyrrole-modified SrFeO3-δ perovskite on carbon cloth (PPy@SFO@CC) has been successfully fabricated by electrodeposition of polypyrrole (PPy) on the surface of SFO@CC. The optimal PPy700@SFO@CC electrode exhibits a specific capacitance of 421 F g-1 at 1 A g-1. It was found that the outside PPy layer not only accelerates the electron transport and ion diffusion but also creates more oxygen vacancies in SrFeO3-δ, enhancing the charge storage performance significantly. Moreover, the NiCo2O4@CC//PPy700@SFO@CC device maintains a specific capacitance of 63.6% after 3000 cycles, which is ascribed to the weak adhesion forces between the active materials and carbon cloth. Finally, the all-solid-state flexible supercapacitor NiCo2O4@CC//PPy700@SFO@CC is constructed with PVA-KOH as the solid electrolyte, delivering an energy density of 16.9 W h kg-1 at a power density of 984 W kg-1. The flexible supercapacitor retains 69% of its specific capacitance after 1000 bending and folding times, demonstrating a certain degree of foldability. The present study opens new avenues for perovskite oxide-based flexible all-solid-state supercapacitors.

Hierarchically Developed Ni(OH)2@MgCo2O4 Nanosheet Composites for Boosting Supercapacitor Performance

MgCo₂O₄ nanomaterial is thought to be a promising candidate for renewable energy storage and conversions. Nevertheless, the poor stability performances and small specific areas of transition-metal oxides remain a challenge for supercapacitor (SC) device applications. In this study, sheet-like Ni(OH)₂@MgCo₂O₄ composites were hierarchically developed on nickel foam (NF) using the facile hydrothermal process with calcination technology, under carbonization reactions. The combination of the carbon-amorphous layer and porous Ni(OH)₂ nanoparticles was anticipated to enhance the stability performances and energy kinetics. The Ni(OH)₂@MgCo₂O₄ nanosheet composite achieved a superior specific capacitance of 1287 F g^-1 at a current value of 1 A g^-1, which is higher than that of pure Ni(OH)₂ nanoparticles and MgCo₂O₄ nanoflake samples. At a current density of 5 A g^-1, the Ni(OH)₂@MgCo₂O₄ nanosheet composite delivered an outstanding cycling stability of 85.6%, which it retained over 3500 long cycles with an excellent rate of capacity of 74.5% at 20 A g^-1. These outcomes indicate that such a Ni(OH)₂@MgCo₂O₄ nanosheet composite is a good contender as a novel battery-type electrode material for high-performance SCs.

Covalently Interlayer-Confined Organic-Inorganic Heterostructures for Aqueous Potassium Ion Supercapacitors

Artificial assembly of organic-inorganic heterostructures for electrochemical energy storage at the molecular level is promising, but remains a great challenge. Here, a covalently interlayer-confined organic (polyaniline [PANI])-inorganic (MoS₂ ) hybrid with a dual charge-storage mechanism is developed for boosting the reaction kinetics of supercapacitors. Systematic characterizations reveal that PANI induces a partial phase transition from the 2H to 1T phases of MoS₂ , expands the interlayer spacing of MoS₂ , and increases the hydrophilicity. More in-depth insights from the synchrotron radiation-based X-ray technique illustrate that the covalent grafting of PANI to MoS₂  induces the formation of MoN bonds and unsaturated Mo sites, leading to increased active sites. Theoretical analysis reveals that the covalent assembly facilitates cross-layer electron transfer and decreases the diffusion barrier of K⁺ ions, which favors reaction kinetics. The resultant hybrid material exhibits high specific capacitance and good rate capability. This design provides an effective strategy to develop organic-inorganic heterostructures for superior K-ion storage. The K-ion storage mechanism concerning the reversible insertion/extraction upon charge/discharge is revealed through ex situ X-ray photoelectron spectroscopy.

Enhanced catalytic reduction of p-nitrophenol and azo dyes on copper hexacyanoferrate nanospheres decorated copper foams

Catalytic reduction of nitroaromatic compounds using low-cost non-precious metal containing catalyst remains an essential topic in wastewater treatment. Herein, copper hexacyanoferrate nanospheres decorated copper foams (CF) were prepared by a facile method, and it was used as structured catalysts for the reduction of p-nitrophenol (p-NP) and azo dyes. The catalyst obtained by calcination at 200 °C shows the highest catalytic activity, with an almost complete reduction of p-NP within 3 min with a rate of 2.057 min^-1 at room temperature, and it exhibited excellent reusability in successive 6 cycles. The effects of temperature, initial concentration, pH, and flow rate on p-NP reduction were investigated. Moreover, the mechanistic investigation revealed that fast electron transfer ability and enhanced adsorption for p-NP contributed to its enhanced catalytic performances. This work put forward an efficient approach for the construction of structured catalysts with enhanced performance in catalytic reduction applications.

Enhanced Electrodes for Supercapacitor Applications Prepared by Hydrothermal-Assisted Nano Sheet-Shaped MgCo2O4@ZnS

In this paper, we report on nanodisc-shaped MgCo2O4 wrapped with ZnS, achieved using the sol–gel-assisted hydrothermal method. This enhances the electrochemical performance, with the electrode delivering superior supercapacitive performance compared to MgCo2O4. Moreover, the nanodisc provides more active sites and allows smooth charge transfer during faradaic reactions. The nanodisc-shaped MgCo2O4 with ZnS delivers a capacitance of approximately 910 F/g at 1 A/g. The fabricated asymmetric capacitor is composed of MgCo2O4@ZnS and activated carbon (AC). The nanodisc-shaped MgCo2O4@ZnS provides more active sites and allows the smooth transport of electrons during long-term cycling. In addition, the electrode side reactions and electrolyte decomposition are significantly reduced due to the ZnS coating on the surface of the MgCo2O4, allowing this asymmetric capacitor to deliver an energy density of 43 Wh·kg−1 at 1454 W·kg−1. The performance of the asymmetric capacitor exhibits enhanced supercapacitive performance and opens a new way to investigate asymmetric supercapacitor devices.

Progress in Preparation of Sea Urchin-like Micro-/Nanoparticles

Urchin-like microparticles/nanoparticles assembled from radial nanorods have a good appearance and high specific surface area, providing more exposed active sites and shortening the diffusion path of photoexcited carriers from the interior to the surface. The interfacial interaction and physical and chemical properties of the materials can be improved by the interfacial porous network induced by interlacing nano-branches. In addition, multiple reflections of the layered microstructure can absorb more incident light and improve the photocatalytic performance. Therefore, the synthesis and functionalization of three-dimensional urchin-like nanostructures with controllable size, shape, and hierarchy have attracted extensive attention. This review aims to provide an overview to summarize the structures, mechanism, and application of urchin-like microparticles/nanoparticles derived from diverse synthesis methods and decoration types. Firstly, the synthesis methods of solid urchin-like micro-/nanoparticles are listed, with emphasis on the hydrothermal/solvothermal method and the reaction mechanism of several typical examples. Subsequently, the preparation method of composite urchin-like micro-/nanoparticles is described from the perspective of coating and doping. Then, the research progress of urchin-like hollow microspheres is reviewed from the perspective of the step-by-step method and synchronous method, and the formation mechanism of forming urchin-like hollow microspheres is discussed. Finally, the application progress of sea urchin-like particles in the fields of photocatalysis, electrochemistry, electromagnetic wave absorption, electrorheological, and gas sensors is summarized.

Electrochemically induced surface reconstruction of Ni-Co oxide nanosheet arrays for hybrid supercapacitors

Transition metal oxides (TMOs) are promising materials for supercapacitors (SCs) because of their high theoretical capacity. However, their finite active sites and poor electrical conductivity lead to reluctant electrochemical performance. Herein, we report a facile electrochemical activation (ECA) method to boost the electrochemical activity of Ni-Co oxide nanosheet arrays (NiCoO NSA) for SCs. Specifically, honeycomb-like NiCoO NSA that was made through a solvothermal method followed by air annealing was activated by simply exerting certain cyclic voltammetry scans (the activated sample is named ac-NiCoO NSA). We have found this treatment results in dramatic surface structure change, forming numerous sub-nanostructures (nanoparticles and nano-leaves) on the NiCoO nanosheets. Rich antisite defects and oxygen vacancies in the NiCoO spinel phase were also created by the ECA treatment. Consequently, the ac-NiCoO NSA delivered a maximum capacity of 206.5 mAh g-1 (0.5 A g-1), which is about three times of the NiCoO NSA without treatment. A hybrid SC based on the ac-NiCoO NSA demonstrated excellent energy storage capacity (power density of 17.3 kW kg-1 and energy density of 45.4 Wh kg-1) and outstanding cyclability (>20,000 cycles, 77.4% retention rate). Our method provides a simple strategy for fabricating high performance TMOs for electrical energy storage devices like SCs.

Enhanced Pseudocapacitive Performance of Symmetric Polypyrrole-MnO2 Electrode and Polymer Gel Electrolyte

Herein, the nanostructured polypyrrole-coated MnO₂ nanofibers growth on carbon cloth (PPy-MnO₂-CC) to serve as the electrodes used in conjunction with a quasi-ionic liquid-based polymer gel electrolyte (urea-LiClO₄-PVA) for solid-state symmetric supercapacitors (SSCs). The resultant PPy-MnO₂-CC solid-state SSCs exhibited a high specific capacitance of 270 F/g at 1.0 A/g in a stable and wide potential window of 2.1 V with a high energy/power density (165.3 Wh/kg at 1.0 kW/kg and 21.0 kW/kg at 86.4 Wh/kg) along with great cycling stability (capacitance retention of 92.1% retention after 3000 cycles) and rate capability (141 F/g at 20 A/g), exceeding most of the previously reported SSCs. The outstanding performance of the studied 2.1 V PPy-MnO₂-CC flexible SSCs could be attributed to the nanostructured PPy-coated MnO₂ composite electrode and the urea-LiClO₄-PVA polymer gel electrolyte design. In addition, the PPy-MnO₂-CC solid-state SSCs could effectively retain their electrochemical performance at various bending angles, demonstrating their huge potential as power sources for flexible and lightweight electronic devices. This work offers an easy way to design and achieve light weight and high-performance SSCs with enhanced energy/power density.

Core-shell shaped Ni2CoHCF@PPy microspheres from prussian blue analogues for high performance asymmetric supercapacitors

Recently, prussian blue analogues (PBAs), as the most classical class of metal-organic frameworks, have been widely studied by scientists. Nevertheless, the inferior conductivity of PBAs restricts the application in supercapacitors. In this work, nickel cobalt hexacyanoferrate (Ni₂CoHCF) had been produced via a simple co-precipitation approach and coated with polypyrrole on its surface. The conductivity of PBAs was improved by the polypyrrole coating. The Ni₂CoHCF@PPy-400 microspheres were demonstrated to the outstanding specific capacity of 82 mAh g^-1at 1 A g^-1. After 3000 cycles, the Ni₂CoHCF@PPy-400 microspheres had a long cycle life and 86% specific capacity retention rate at 5 A g^-1. Additionally, it was coupled with activated carbon to build high performance asymmetric supercapacitor (Ni₂CoHCF@PPy-400//AC), which displayed a high energy density of 21.7 Wh kg^-1at the power density of 888 W kg^-1and good cycle stability after 5000 cycles (a capacity retention rate of 85.2%). What is more, the results reveal that the Ni₂CoHCF@PPy-400 microspheresare a prospective candidate for exceptional energy storage devices.

Polypyrrole-Based Metal Nanocomposite Electrode Materials for High-Performance Supercapacitors

Metallic nanostructures (MNs) and metal-organic frameworks (MOFs) play a pivotal role by articulating their significance in high-performance supercapacitors along with conducting polymers (CPs). The interaction and synergistic pseudocapacitive effect of MNs with CPs have contributed to enhance the specific capacitance and cyclic stability. Among various conjugated heterocyclic CPs, polypyrrole (PPy) (prevalently knows as “synthetic metal”) is exclusively studied because of its excellent physicochemical properties, ease of preparation, flexibility in surface modifications, and unique molecular structure–property relationships. Numerous researchers attempted to improve the low electronic conductivity of MNs and MOFs, by incorporating conducting PPy and/or used decoration strategy. This was succeeded by fine-tuning this objective, which managed to get outstanding supercapacitive performances. This brief technical note epitomizes various PPy-based metallic hybrid materials with different nano-architectures, emphasizing its technical implications in fabricating high-performance electrode material for supercapacitor applications.

Transfunctionalization of graphite fluoride engineered polyaniline grafting to graphene for High-Performance flexible supercapacitors

Low energy density is the major obstacle for the practical all-solid-state supercapacitors, which may be raised by the combination of the pseudocapacitance with the electrochemical double-layer capacitance. Although graphene and polyaniline have been demonstrated two effective materials, the synthetic route of graphene and their hybrid mode largely dictated the capacitive performances and cyclability of graphene/polyaniline nanocomposites. Herein, we employed commercial graphite fluoride as the precursor to obtain graphene with a well-preserved carbon lattice. After graphite fluoride functionalization by p-phenylenediamine (pPDA) and in situ oxidative polymerization of anilines, polyaniline (PANI) chains were covalently attached to graphene framework through pPDA bridges. Multiple characterizations were performed to confirm the covalent binding mode between graphene scaffolds and PANI partners, and electrochemical tests unraveled the as-prepared G-pPDA-PANI triads delivered a gravimetric capacitance as high as 638F g^-1 and a further amplified volumetric capacitance (up to 759F cm^-3). The bendable all-solid-state supercapacitors yielded an encouraging energy density of over 18 W   h L^-1 at a power density high to 5,950 W L^-1, while exhibiting an exceptional rate capability, cycling stability and mechanical flexibility.

Construction of hierarchical sea urchin-like manganese substituted nickel cobaltite@tricobalt tetraoxide core-shell microspheres on nickel foam as binder-free electrodes for high performance supercapacitors

Construction of binder-free electrodes with hierarchical core-shell nanostructures is considered to be an effective route to promote the electrochemical performance of supercapacitors. In this work, the porous Ni_0.5Mn_0.5Co₂O₄ nanoflowers anchored on nickel foam are utilized as framework for further growing Co₃O₄ nanowires, resulting in the hierarchical sea urchin-like Ni_0.5Mn_0.5Co₂O₄@Co₃O₄ core-shell microspheres on nickel foam. Owing to the advantages brought by unique porous architecture and synergistic effect of the multi-component composites, the as-prepared electrode exhibits a high specific capacitance (931 F/g at 1 A/g), excellent rate performance (77% capacitance retention at 20 A/g) and outstanding cycle stability (92% retention over 5000 cycles at 5 A/g). Additionally, the assembled Ni_0.5Mn_0.5Co₂O₄@Co₃O₄//AC (activated carbon) asymmetric supercapacitor achieves a high energy density (50 Wh/kg at 750 W/kg) and long durability (88% retention after 5000 cycles).

PANI/MoO3−x shell–core composites with enhanced rate and cycling performance for flexible solid-state supercapacitors and electrochromic applications

PANI/MoO_3−x shell–core composites show enhanced electrochemical and electrochromic performance as a bi-functional electrode material for flexible solid-state supercapacitors, attributed to a synergistic effect from PANI nanorods and MoO_3−x nanobelts.

Flexible supercapacitor electrodes using metal-organic frameworks

Advancements in the field of flexible and wearable devices require flexible energy storage devices to cater their power demands. Metal-ion batteries (such as lithium-ion batteries, sodium-ion batteries, etc.) and electrochemical capacitors (also called supercapacitors or ultracapacitors) have achieved great interest in the recent past due to their superior energy storage characteristics like high power density and long cycle life. A major bottleneck of using metal-ion batteries in wearable devices is their lack of flexibility. Low power density, toxicity and flammability due to organic electrolytes inhibit them from safe on-body device applications. On the other hand, supercapacitors can be made with aqueous electrolytes, making them a safer alternative for wearable applications. Metal-organic frameworks (MOFs) are novel candidates as electrode materials due to their salient features such as large surface area, three-dimensional porous architecture, permeability to foreign entities, structural tailorability, etc. Though pristine MOFs suffer from poor intrinsic conductivity, this can be rectified by preparing composites with other electronically conducting materials. MOF-based electrodes are highly promising for flexible and wearable supercapacitors since they exhibit good energy and power densities. This review focuses on the new developments in the field of MOF-based composite electrodes for developing flexible supercapacitors.

Facile hydrothermal synthesis of porous MgCo2O4 nanoflakes as an electrode material for high-performance asymmetric supercapacitors

In this work, porous MgCo₂O₄ nanoflakes (MgCo₂O₄ NFs) and MgCo₂O₄ nanocubes (MgCo₂O₄ NCs) have been successfully synthesized through a simple hydrothermal method combined with a post calcination process of the precursor in air. The morphology of the MgCo₂O₄ samples can be easily tuned by changing the hydrothermal temperature and reaction time, respectively. The porous MgCo₂O₄ NFs with an average pore size of 12.5 nm had a BET specific surface area up to 64.9 m² g^-1, which was larger than that of MgCo₂O₄ NCs (19.8 m² g^-1). The MgCo₂O₄ NFs delivered a specific capacitance of 734.1 F g^-1 at 1 A g^-1 and exhibited a considerable rate performance with 74.0% capacitance retention at 12 A g^-1. About 94.2% of its original capacitance could be retained after 5000 charge-discharge cycles at a constant current density of 5 A g^-1. An asymmetric supercapacitor (ASC) was assembled by using MgCo₂O₄ NFs as the positive electrode and AC as the negative electrode, and the ASC had a wide operation voltage of 1.7 V and a high energy density of 33.0 W h kg^-1 at a power density of 859.6 W kg^-1. Such outstanding electrochemical performances make the MgCo₂O₄ NFs a promising candidate for supercapacitor applications. In addition, the simple and scalable synthesis method can be extended to the preparation of other metal oxide-based electrode materials.

Polypyrrole-coated Fe2O3 nanotubes constructed from nanoneedles as high-performance anodes for aqueous asymmetric supercapacitors

Asymmetric supercapacitors (ASCs) show promising potential for electrochemical energy storage applications. However, the energy density of ASCs is limited by the poor electrochemical performance of anodes. To achieve high-performance ASCs, herein, Fe₂O₃ nanotubes constructed from Fe₂O₃ nanoneedles were fabricated by employing MnO₂ nanotubes as a self-sacrificing template, and then a layer of polypyrrole (PPy) was coated through an in situ chemical oxidative polymerization method to enhance their performance. The electrochemical tests indicate that the resultant PPy-coated Fe₂O₃ nanotubes (Fe₂O₃@PPy) exhibit a high areal capacitance of 530 mF cm^-2 at 1 mA cm^-2 and good cycling stability, which are superior to those of the Fe₂O₃ nanotubes. The superior performance of the Fe₂O₃@PPy nanotubes can be attributed to the synergistic effect between the PPy shell and Fe₂O₃ core, in which the conducting PPy shell not only works as a superhighway for charge transport, but also stabilizes the Fe₂O₃ nanotubes during charge-discharge processes. When the Fe₂O₃@PPy nanotubes were assembled with MnO₂ nanotubes, the as-assembled ASCs possess a high cell voltage of 2.0 V and deliver a high energy density of up to 51.2 Wh kg^-1 at a power density of 285.4 W kg^-1 with an excellent cycling stability (83.5% capacitance retention over 5000 cycles).

Cu-MOF derived Cu-C nanocomposites towards high performance electrochemical supercapacitors

For the development of asymmetric supercapacitors with higher energy density, the study of new electrode materials with high capacitance is a priority. Herein, the electrochemical behavior of nano copper in alkaline electrolyte is first discovered. It is found that there are two obvious reversible redox symmetric peaks in the range of -0.8-0.2 V in the alkaline electrolyte, corresponding to the conversion of copper into cuprous ions, and then converting cuprous ions into copper ions, indicating that the nanocomposite electrode has the characteristics of a pseudocapacitive reaction. It has a specific capacitance of up to 318 F g-1 at a current density of 1 A g-1, which remains at nearly 100% after 10 000 cycles at the same current density. When assembled with a Ni(OH)2-based electrode into an asymmetric supercapacitor, the device shows excellent capacitive behavior and good reaction reversibility. At 0.4 A g-1, the supercapacitor delivers a reversible capacity of 8.33 F g-1 with an energy density of 13.5 mW h g-1. This study first discovers the electrochemical behavior of nano copper, which can provide a new research idea for further expanding the negative electrodes of supercapacitors with higher energy density.

Facile preparation of hierarchical MgCo2O4/MgCo2O4 nanochain array composites on Ni foam as advanced electrode materials for supercapacitors

Schematic illustration of the two-step synthesis of MgCo₂O₄/MgCo₂O₄ directly grown on Ni foam.

g-C3N4/CuO and g-C3N4/Co3O4 nanohybrid structures as efficient electrode materials in symmetric supercapacitors

Metal oxide dispersed graphitic carbon nitride hybrid nanocomposites (g-C₃N₄/CuO and g-C₃N₄/Co₃O₄) were prepared via a direct precipitation method. The materials were used as an electrode material in symmetric supercapacitors. The g-C₃N₄/Co₃O₄ electrode based device exhibited a specific capacitance of 201 F g⁻¹ which is substantially higher than those using g-C₃N₄/CuO (95 F g⁻¹) and bare g-C₃N₄ electrodes (72 F g⁻¹). At a constant power density of 1 kW kg⁻¹, the energy density given by g-C₃N₄/Co₃O₄ and g-C₃N₄/CuO devices is 27.9 W h kg⁻¹ and 13.2 W h kg⁻¹ respectively. The enhancement of the electrochemical performance in the hybrid material is attributed to the pseudo capacitive nature of the metal oxide nanoparticles incorporated in the g-C₃N₄ matrix.

Comparison of electrochemical performance of symmetric supercapacitors based on g-C₃N₄/CuO and g-C₃N₄/Co₃O₄ electrodes.

Fabrication of hierarchical core/shell MgCo2O4@MnO2 nanowall arrays on Ni-foam as high-rate electrodes for asymmetric supercapacitors

Design and fabrication of a hierarchical core/shell MgCo₂O₄@MnO₂ nanowall arrays on Ni-foam by a facile two-step hydrothermal method. The electrochemical measurements prove these composites with MnO₂ definitely offer better supercapacitive performance of the MgCo₂O₄ electrode material. The nanowall structure provides more active sites and charge transfer during the Faradic reaction. The MgCo₂O₄@MnO₂ nanowall shows an excellent electrochemical performance (852.5 F g^-1 at 1 A g^-1). The asymmetric supercapacitor is composed of the MgCo₂O₄@MnO₂ nanowall and the activated carbon (AC). The energy densities of the asymmetric supercapacitor device can keep up 67.2 Wh·kg^-1 at 5760.0 W·kg^-1. The MgCo₂O₄@MnO₂ nanowall shows excellent supercapacitive performance and has a great potential for more research and application in the asymmetric supercapacitor devices field.

Asymmetric supercapacitors with high energy densities

The low energy densities of supercapacitors (SCs) are generally limited by the used anodes. To develop SCs with high energy densities, Fe3+ modified V2O5@GQDs (m-V2O5@GQDs) and ZIF-67-derived nanoporous carbon loaded with Mn3O4 (C/N-Mn3O4) were synthesized. After their detailed characterization using electron microscopy, X-ray methods and electrochemical techniques, they were further utilized as the anode and the cathode, respectively, to construct asymmetric supercapacitors (ASCs). The as-synthesized m-V2O5@GQDs improve the poor conductivity of V2O5, contributing greatly to a specific capacitance of 761 F g-1 at a current density of 2 A g-1. With application of a cell voltage of 2 V, an energy density of up to 99.4 W h kg-1 is achieved at a power density of 1000 W kg-1. Such ASCs also exhibit outstanding cycling performance (95% of initial capacitance even after 10 000 charging/discharging cycles). This study thus provides a new way to design and construct ASCs with high energy densities.

In situ growth of ZIF-8-derived ternary ZnO/ZnCo2O4/NiO for high performance asymmetric supercapacitors

In this paper, the rational design and synthesis of ZIF-8-derived ternary ZnO/ZnCo₂O₄/NiO wrapped by nanosheets is introduced. Polyhedral ternary ZnO/ZnCo₂O₄/NiO composites surrounded by nanosheets with different compositions are successfully fabricated through in situ growth on ZIF-8 templates and subsequent thermal annealing in air. Electrochemical investigation reveals that when the molar ratio of nickel nitrate to cobalt nitrate is 1, the composite material is more outstanding, which shows a high specific capacitance of 1136.4 F g^-1 at 1 A g^-1 and excellent cycling stability of 86.54% after 5000 cycles. Moreover, the excellent performance of this material is also confirmed by assembling an asymmetric supercapacitor. The assembled hybrid device can reach a large potential range of 0-1.6 V and deliver a high energy density of 46.04 W h kg^-1 as well as the maximum power density of 7987.5 W kg^-1.

Pseudocapacitance electrode and asymmetric supercapacitor based on biomass juglone/activated carbon composites

A novel electrode material incorporating renewable biomass-derived juglone biomolecules with commercial activated carbon (AC) granules has been through simple ultrasonic dispersion and dissolution–recrystallization and was found to exhibit good electrochemical performance. The juglone biomolecules are prepared by an ultrasound-assisted extraction method from abandoned walnut peel, which decreases pollution and increases economic efficiency. Through the dissolution–recrystallization process with AC, a hierarchical structure with nanosized juglone particles was obtained, and the AC particles worked as scaffolding to strengthen the slight biomolecules, thus expanding the active sites and effectively reducing the dissolution of the active materials. The pseudocapacitance fading mechanism was investigated by ex situ FTIR measurement and the porous structure ensures that the composite electrode has an enhanced specific capacitance of 248 F g⁻¹ compared to 172.8 and 62.5 F g⁻¹ for the respective AC and juglone samples. Besides, the excellent cyclic stability (retained 75% after 3000 charge–discharge cycles) was demonstrated. The highest area-specific capacitance of the composites was 1300 mF cm⁻². An asymmetric supercapacitor based on this composite electrode was assembled with an AC electrode as the counter electrode and exhibited good cyclic performance at a voltage of 1.2 V (retained 77% after 3000 charge–discharge cycles), which provides a high energy density of 12 W h kg⁻¹ at a power density of 0.18 kW kg⁻¹ and a high power density of 2 kW kg⁻¹ at an energy density of 9 W h kg⁻¹. This work explores the application of biomolecule-based composites in energy storage devices and provides a potential strategy for constructing environmentally friendly electrodes.

A strategy for transforming abandoned walnut peel to excellent pseudocapacitance material. The activated carbon reshapes and anchors the juglone, which combined the EDLC and pseudocapacitance to achieve high electrochemical performance.

NiCo-layered double-hydroxide and carbon nanosheets microarray derived from MOFs for high performance hybrid supercapacitors

Exploring porous nano-structured materials has great significance for energy storage equipment. The metal-organic frameworks (MOFs) can be used as the outstanding sacrificial templates for electrode material of high performance supercapacitors due to their superior features that high specific surface area and tunable pore size distribution. However, the poor conductivity of MOFs is one of the biggest barriers to achieve high rate capacity and stable cycling performance. Herein, MOFs derived NiCo-layered double-hydroxide (NiCo-LDH) and nitrogen-doped carbon nanosheets (NC) on the flexible carbon nanotubes (CNTs) film are rationally designed, both of which as the binder-free electrodes can greatly improve the specific surface area and reaction sites. An asymmetric supercapacitor based on porous NiCo-LDH nanosheets on CNTs (CNT@NiCo-LDH) as the positive electrode and the NC nanosheets on carbon nanotubes film (CNT@NC) as the negative electrode exhibits the maximum energy density of 37.4 W h/kg at the power density of 750 W/kg, as well as a long-term cycling stability (94.5% capacity retention after 5000 cycles). Rationally design such combination is a meaningful process for energy storage equipment with excellent electrochemical performance.

Designed formation of Co3O4/ZnCo2O4/CuO hollow polyhedral nanocages derived from zeolitic imidazolate framework-67 for high-performance supercapacitors

Zeolitic imidazolate frameworks have stimulated great attention due to their potential applications in energy storage, catalysis, gas sensing, drug delivery etc. In this paper, the three-dimensional porous nanomaterial Co3O4/ZnCo2O4/CuO with hollow polyhedral nanocage structures and highly enhanced electrochemical performances was synthesized successfully by a zeolitic imidazolate framework-67 route. The composites hold the shape of the ZIF-67 templates well and the shell has multiple compositions. In the process, we first synthesized the nanostructure hydroxide precursors and then transformed them into the corresponding metal oxide composites by thermal annealing in air. In addition, the mass ratio of Zn to Cu in this material is discussed and optimized. We found that when the mass ratio is 3, the composite material has better electrochemical properties. When applied as an electrode material, Co3O4/ZnCo2O4/CuO-1 shows enhanced pseudocapacitive properties and good cycling stability compared with Co3O4/ZnCo2O4, Co3O4/CuO and Co3O4/ZnCo2O4/CuO-2, and Co3O4/ZnCo2O4/CuO-3. The assembled Co3O4/ZnCo2O4/CuO-1//AC hybrid device can be reversibly cycled in a large potential range of 0-1.6 V and can deliver a high energy density of 35.82 W h kg-1 as well as the maximum power density of 4799.25 W kg-1.