V2O5·0.6H2O nanoribbons as cathode material for asymmetric supercapacitor in K2SO4 solution

Recent advances in rational design for high-performance potassium-ion batteries

The growing global energy demand necessitates the development of renewable energy solutions to mitigate greenhouse gas emissions and air pollution. To efficiently utilize renewable yet intermittent energy sources such as solar and wind power, there is a critical need for large-scale energy storage systems (EES) with high electrochemical performance. While lithium-ion batteries (LIBs) have been successfully used for EES, the surging demand and price, coupled with limited supply of crucial metals like lithium and cobalt, raised concerns about future sustainability. In this context, potassium-ion batteries (PIBs) have emerged as promising alternatives to commercial LIBs. Leveraging the low cost of potassium resources, abundant natural reserves, and the similar chemical properties of lithium and potassium, PIBs exhibit excellent potassium ion transport kinetics in electrolytes. This review starts from the fundamental principles and structural regulation of PIBs, offering a comprehensive overview of their current research status. It covers cathode materials, anode materials, electrolytes, binders, and separators, combining insights from full battery performance, degradation mechanisms, in situ/ex situ characterization, and theoretical calculations. We anticipate that this review will inspire greater interest in the development of high-efficiency PIBs and pave the way for their future commercial applications.

Hierarchical NiMn-LDH Hollow Spheres as a Promising Pseudocapacitive Electrode for Supercapacitor Application

Layered double hydroxides (LDH) are regarded as attractive pseudocapacitive materials due to their impressive capacitive qualities that may be adjustable to their morphological features. However, the layered structure of LDH renders them susceptible to structural aggregation, which inhibits effective electrolyte transport and limits their practical applicability after limited exposure to active areas. Herein, we propose a simple template-free strategy to synthesize hierarchical hollow sphere NiMn-LDH material with high surface area and exposed active as anode material for supercapacitor application. The template-free approach enables the natural nucleation of Ni-Mn ions resulting in thin sheets that self-assemble into a hollow sphere, offering expended interlayer spaces and abundant redox-active active sites. The optimal NiMn-LDH-12 achieved a specific capacitance of 1010.4 F g-1 at a current density of 0.2 A g-1 with capacitance retention of 70% at 5 A g-1 after 5000 cycles with lower charge transfer impedance. When configured into an asymmetric supercapacitors (ASC) device as NiMn-LDH//AC, the material realized a specific capacitance of 192.4 F g-1 at a current density of 0.2 A g-1 with a good energy density of 47.9 Wh kg-1 and a power density of 196.8 W kg-1. The proposed morphological-tuning route is promising for designing template-free NiMn-LDHs spheres with practical pseudocapacitive characteristics.

Graphene: A Path-Breaking Discovery for Energy Storage and Sustainability

The global energy situation requires the efficient use of resources and the development of new materials and processes for meeting current energy demand. Traditional materials have been explored to large extent for use in energy saving and storage devices. Graphene, being a path-breaking discovery of the present era, has become one of the most-researched materials due to its fascinating properties, such as high tensile strength, half-integer quantum Hall effect and excellent electrical/thermal conductivity. This paper presents an in-depth review on the exploration of deploying diverse derivatives and morphologies of graphene in various energy-saving and environmentally friendly applications. Use of graphene in lubricants has resulted in improvements to anti-wear characteristics and reduced frictional losses. This comprehensive survey facilitates the researchers in selecting the appropriate graphene derivative(s) and their compatibility with various materials to fabricate high-performance composites for usage in solar cells, fuel cells, supercapacitor applications, rechargeable batteries and automotive sectors.

Effects of Valence States of Working Cations on the Electrochemical Performance of Sodium Vanadate

Supercapacitors have received much attention as large-scale energy storage devices for high power density and ultralong cycling life. In this work, sodium vanadate Na_0.76V₆O₁₅/poly(3,4-ethylenedioxythiophene) (PEDOT) nanocables with deficient bridge oxygen at the interface (denoted Vo^••-PNVO) have been tailored for supercapacitors through the in situ polymerization of 3,4-ethylenedioxythiophene and studied using three different electrolytes. Experiments and theoretical calculations reveal that all Na⁺, Zn²⁺, and Al³⁺ ions appear as hydrates in aqueous solutions but insert into the crystal structure as Na⁺ ions and Zn²⁺-H₂O and Al³⁺-H₂O hydrates, respectively. In comparison with the Zn²⁺-H₂O and Al³⁺-H₂O hydrates, Na⁺ ions with a smaller radius diffuse more quickly in Vo^••-PNVO. Thus, Vo^••-PNVO delivers better charge storage capability and stability when an electrolyte with Na⁺ ions is used. The results strongly suggest that an electrostatic interaction is significant in determining transport properties and storage capacities, rather than hydrate radii or valence states.

Binary vanadium pentoxide carbon-graphene foam composites derived from dark red hibiscus sabdariffa for advanced asymmetric supercapacitor

The development of advanced electrode materials derived from biomass for the next generation of energy storage devices, such as supercapacitors with high specific energy and specific power coupled with a good cycle stability, is required to meet the high demand for electric vehicles and portable devices. In this study, sustainable binary vanadium pentoxide carbon-graphene foam composites (V₂O₅@C-R₂HS/GF) were synthesized using a solvothermal method. The X-ray diffraction, Raman and FTIR techniques were used to study the structural properties of the composites (V₂O₅@C-R₂HS/20 mg GF and V₂O₅@C-R₂HS/40 mg GF). The SEM micrographs displayed an accordion-like morphology resulting from the graphene foam-modified V₂O₅@C-R₂HS composite. The V₂O₅@C-R₂HS, V₂O₅@C-R₂HS/20 mg GF and V₂O₅@C-R₂HS/40 mg GF composites were evaluated in a three-electrode configuration using 6 M potassium hydroxide (KOH) as an aqueous electrolyte. Furthermore, a two-electrode device was carried out by fabricating an asymmetric device (V₂O₅@C-R₂HS/GF//AC) where V₂O₅@C-R₂HS/20 mg GF was used as a positive electrode and activated carbon (AC) as a negative electrode at a cell voltage of 1.6 V in 6 M KOH. The V₂O₅@C-R₂HS/GF//AC showed a high specific energy and specific power values of 55 W h kg^-1 and 707 W kg^-1, respectively, at a specific current of 1 A g^-1. The asymmetric device presented a good stability test showing 99% capacity retention up to 10 000 cycles and was confirmed by the floating time up to 150 h with specific energy increasing 23.6% after the first 10 h. This article is part of the theme issue 'Bio-derived and bioinspired sustainable advanced materials for emerging technologies (part 2)'.

Silver Nanoparticles Embedded on Reduced Graphene Oxide@Copper Oxide Nanocomposite for High Performance Supercapacitor Applications

In this work, silver (Ag) decorated reduced graphene oxide (rGO) coated with ultrafine CuO nanosheets (Ag-rGO@CuO) was prepared by the combination of a microwave-assisted hydrothermal route and a chemical methodology. The prepared Ag-rGO@CuO was characterized for its morphological features by field emission scanning electron microscopy and transmission electron microscopy while the structural characterization was performed by X-ray diffraction and Raman spectroscopy. Energy-dispersive X-ray analysis was undertaken to confirm the elemental composition. The electrochemical performance of prepared samples was studied by cyclic voltammetry and galvanostatic charge-discharge in a 2M KOH electrolyte solution. The CuO nanosheets provided excellent electrical conductivity and the rGO sheets provided a large surface area with good mesoporosity that increases electron and ion mobility during the redox process. Furthermore, the highly conductive Ag nanoparticles upon the rGO@CuO surface further enhanced electrochemical performance by providing extra channels for charge conduction. The ternary Ag-rGO@CuO nanocomposite shows a very high specific capacitance of 612.5 to 210 Fg-1 compared against rGO@CuO which has a specific capacitance of 375 to 87.5 Fg-1 and the CuO nanosheets with a specific capacitance of 113.75 to 87.5 Fg-1 at current densities 0.5 and 7 Ag-1, respectively.

NH4V4O10 nanobelts vertically grown on 3D TiN nanotube arrays as high-performance electrode materials of supercapacitors

High performance supercapacitor without binders has attracted wide attention as an energy storage device. In this work, novel NH4V4O10 nanobelts were successfully synthesized and decorated into TiN nanotube arrays by a simple hydrothermal method. The as-prepared no-binder electrode hybrids exhibited excellent electrochemical performances with a specific capacitance of 749.0 F g-1 at 5 mV s-1 and a capacity retention of 85.7% after 200 cycles, which makes it an appealing candidate for electrode materials of supercapacitors.

The synergistic effect of the RuO2 nanoparticle-decorated V2O5 heterostructure for high-performance asymmetric supercapacitors

Schematic illustration of a 3 wt% RuO₂–V₂O₅//AC asymmetric supercapacitor device

Revealing the Intrinsic Origin for Performance-Enhancing V2O5 Electrode Materials

Revealing the intrinsic origin is critical for developing performance-enhancing V₂O₅ battery-type electrode materials. In this work, ultralong single-crystal V₂O₅ wires (W-V₂O₅) and V₂O₅ plate particles (P-V₂O₅) with similar physicochemical properties were compared to investigate the possible stimulative factors for pseudocapacitive enhancement. Our results indicate that besides the most-discussed specific surface area (or nanostructure), the enhanced electronic conductivity, the controllable interlamellar spacing distance, and the ion-transporting route as intrinsic origin also greatly affect their pseudocapacitive enhancement. First, the ultralong single-crystal wire structure can apparently enhance the electrons transport; second, the unique [001] facet orientation along the wire direction enlarges the interlamellar spacing distance and shortens the Li⁺ inserting route, thus facilitating the redox reactions by providing fast channels for charge carrier intercalation. Thus W-V₂O₅ showed much higher capacitance, better rate, and cycling capability than those of P-V₂O₅. This new insight presented here provides guidance for the design of V₂O₅ electrode materials and opens new opportunities in the development of high-performance battery-type electrode materials.

Facile Preparation of Ni-Co Bimetallic Oxide/Activated Carbon Composites Using the Plasma in Liquid Process for Supercapacitor Electrode Applications

In this study, a plasma in a liquid process (PiLP) was used to facilely precipitate bimetallic nanoparticles composed of Ni and Co elements on the surface of activated carbon. The physicochemical and electrochemical properties of the fabricated composites were evaluated to examine the potential of supercapacitors as electrode materials. Nickel and cobalt ions in the aqueous reactant solution were uniformly precipitated on the AC surface as spherical nanoparticles with a size of about 100 nm by PiLP reaction. The composition of nanoparticles was determined by the molar ratio of nickel and cobalt precursors and precipitated in the form of bimetallic oxide. The electrical conductivity and specific capacitance were increased by Ni-Co bimetallic oxide nanoparticles precipitated on the AC surface. In addition, the electrochemical performance was improved by stable cycling stability and resistance reduction and showed the best performance when the molar ratios of Ni and Co precursors were the same.

Enhanced Photoelectrocatalytical Performance of Inorganic-Inorganic Hybrid Consisting BiVO4, V2O5, and Cobalt Hexacyanocobaltate as a Perspective Photoanode for Water Splitting

Thin layers of BiVO4/V2O5 were prepared on FTO substrates using pulsed laser deposition technique. The method of cobalt hexacyanocobaltate (Cohcc) synthesis on the BiVO4/V2O5 photoanodes consists of cobalt deposition followed by electrochemical oxidation of metallic Co in K3[Co(CN)6] aqueous electrolyte. The modified electrodes were tested as photoanodes for water oxidation under simulated sunlight irradiation. Deposited films were characterized using UV-Vis spectroscopy, Raman spectroscopy, and scanning electron microscopy. Since the V2O5 is characterized by a narrower energy bandgap than BiVO4, the presence of V2O5 shifts absorption edge (ΔE = ~0.25 eV) of modified films towards lower energies enabling the conversion of a wider range of solar radiation. The formation of heterojunction increases photocurrent of water oxidation measured at 1.2 V vs Ag/AgCl (3 M KCl) to over 1 mA cm-2, while bare BiVO4 and V2O5 exhibit 0.37 and 0.08 mA cm-2, respectively. On the other hand, the modification of obtained layers with Cohcc shifts onset potential of photocurrent generation into a cathodic direction. As a result, the photocurrent enhancement at a wide range of applied potential was achieved.

A facile strategy for the synthesis of graphene/V2O5 nanospheres and graphene/VN nanospheres derived from a single graphene oxide-wrapped VO x nanosphere precursor for hybrid supercapacitors

It remains a challenge to develop a facile approach to prepare positive and negative electrode materials with good electrochemical performance for application in hybrid supercapacitors. In this study, based on a facile strategy, a single graphene oxide-wrapped VO x nanosphere precursor is transformed into both electrodes through different thermal treatments (i.e., graphene/VN nanospheres negative electrode materials and graphene/V2O5 nanospheres positive electrode materials) for hybrid supercapacitors. The conformally wrapped graphene has a significant influence on the electrochemical performance of VN and V2O5, deriving from the simultaneous improvements in electronic conductivity, structural stability, and electrolyte transport. Benefitting from these merits, the as-prepared graphene/VN nanospheres and graphene/V2O5 nanospheres exhibit excellent electrochemical performance for HSCs with high specific capacitance (83 F g-1) and good long cycle life (90% specific capacitance retained after 7000 cycles). Furthermore, graphene/VN nanospheres//graphene/V2O5 nanosphere HSCs can deliver a high energy density of 35.2 W h kg-1 at 0.4 kW kg-1 and maintain about 70% high energy density even at a high power density of 8 kW kg-1. Such impressive results of the hybrid supercapacitors show great potential in vanadium-based electrode materials for promising applications in high performance energy storage systems.

A Novel Hierarchical Porous 3D Structured Vanadium Nitride/Carbon Membranes for High-performance Supercapacitor Negative Electrodes

Transition-metal nitrides exhibit wide potential windows and good electrochemical performance, but usually experience imbalanced practical applications in the energy storage field due to aggregation, poor circulation stability, and complicated syntheses. In this study, a novel and simple multi-phase polymeric strategy was developed to fabricate hybrid vanadium nitride/carbon (VN/C) membranes for supercapacitor negative electrodes, in which VN nanoparticles were uniformly distributed in the hierarchical porous carbon 3D networks. The supercapacitor negative electrode based on VN/C membranes exhibited a high specific capacitance of 392.0 F g^-1 at 0.5 A g^-1 and an excellent rate capability with capacitance retention of 50.5% at 30 A g^-1. For the asymmetric device fabricated using Ni(OH)₂//VN/C membranes, a high energy density of 43.0 Wh kg^-1 at a power density of 800 W kg^-1 was observed. Moreover, the device also showed good cycling stability of 82.9% at a current density of 1.0 A g^-1 after 8000 cycles. This work may throw a light on simply the fabrication of other high-performance transition-metal nitride-based supercapacitor or other energy storage devices.

Interfacial Constructing Flexible V2O5@Polypyrrole Core-Shell Nanowire Membrane with Superior Supercapacitive Performance

Flexible membrane consisting of ultralong V₂O₅@conducting polypyrrole (V₂O₅@PPy) core-shell nanowires is prepared by a facile in situ interfacial synthesis approach. The V₂O₅ is for the first time demonstrated to show versatile function of reactive template to initiate the uniform and conformal polymerization of PPy nanocoating without the need for extra oxidants. The freestanding PPy-encapsulated V₂O₅ nanowire membrane is of great benefit in achieving strong electrochemical harvest by increasing electrical conductivity, shortening ion/electron transport distance, and enlarging electrode/electrolyte contact area. When evaluated as binder- and additive-free supercapacitor electrodes, the V₂O₅@PPy core-shell hybrid delivers a significantly enhanced specific capacitance of 334 F g^-1 along with superior rate capability and improved cycling stability. The present work would provide a simple yet powerful interfacial strategy for elaborate constructing V₂O₅/conducting polymers toward various energy-storage technologies.

Polycrystalline and Mesoporous 3-D Bi2O3 Nanostructured Negatrodes for High-Energy and Power-Asymmetric Supercapacitors: Superfast Room-Temperature Direct Wet Chemical Growth

Superfast (≤10 min) room-temperature (300 K) chemical synthesis of three-dimensional (3-D) polycrystalline and mesoporous bismuth(III) oxide (Bi₂O₃) nanostructured negatrode (as an abbreviation of negative electrode) materials, viz., coconut shell, marigold, honey nest cross section and rose with different surface areas, charge transfer resistances, and electrochemical performances essential for energy storage, harvesting, and even catalysis devices, are directly grown onto Ni foam without and with poly(ethylene glycol), ethylene glycol, and ammonium fluoride surfactants, respectively. Smaller diffusion lengths, caused by the involvement of irregular crevices, allow electrolyte ions to infiltrate deeply, increasing the utility of inner active sites for the following electrochemical performance. A marigold 3-D Bi₂O₃ electrode of 58 m²·g^-1 surface area has demonstrated a specific capacitance of 447 F·g^-1 at 2 A·g^-1 and chemical stability of 85% even after 5000 redox cycles at 10 A·g^-1 in a 6 M KOH electrolyte solution, which were higher than those of other morphology negatrode materials. An asymmetric supercapacitor (AS) device assembled with marigold Bi₂O₃ negatrode and manganese(II) carbonate quantum dots/nickel hydrogen-manganese(II)-carbonate (MnCO₃QDs/NiH-Mn-CO₃) positrode corroborates as high as 51 Wh·kg^-1 energy at 1500 W·kg^-1 power and nearly 81% cycling stability even after 5000 cycles. The obtained results were comparable or superior to the values reported previously for other Bi₂O₃ morphologies. This AS assembly glowed a red-light-emitting diode for 20 min, demonstrating the scientific and industrial credentials of the developed superfast Bi₂O₃ nanostructured negatrodes in assembling various energy storage devices.

Reticular V2O5·0.6H2O Xerogel as Cathode for Rechargeable Potassium Ion Batteries

Potassium ion batteries (KIBs), because of their low price, may exhibit advantages over lithium ion batteries as potential candidates for large-scale energy storage systems. However, owing to the large ionic radii of K-ions, it is challenging to find a suitable intercalation host for KIBs and thus the rechargeable KIB electrode materials are still largely unexplored. In this work, a reticular V₂O₅·0.6H₂O xerogel was synthesized via a hydrothermal process as a cathode material for rechargeable KIBs. Compared with the orthorhombic crystalline V₂O₅, the hydrated vanadium pentoxide (V₂O₅·0.6H₂O) exhibits the ability of accommodating larger alkali metal ions of K⁺ because of the enlarged layer space by hosting structural H₂O molecules in the interlayer. By intercalation of H₂O into the V₂O₅ layers, its potassium electrochemical activity is significantly improved. It exhibits an initial discharge capacity of ∼224.4 mA h g^-1 and a discharge capacity of ∼103.5 mA h g^-1 even after 500 discharge/charge cycles at a current density of 50 mA g^-1, which is much higher than that of the V₂O₅ electrode without structural water. Meanwhile, X-ray diffraction and X-ray photoelectron spectroscopy combined with energy dispersive spectroscopy techniques are carried out to investigate the potassiation/depotassiation process of the V₂O₅·0.6H₂O electrodes, which confirmed the potassium intercalation storage mechanisms of this hydrated material. The results demonstrate that the interlayer-spacing-enlarged V₂O₅·0.6H₂O is a promising cathode candidate for KIBs.

V2O5 nanorod electrode material for enhanced electrochemical properties by a facile hydrothermal method for supercapacitor applications

Metal oxides have attracted considerable interest due to their distinguished electrochemical properties and applications in multiple fields such as supercapacitors and solar cells.

Disodium citrate-assisted hydrothermal synthesis of V2O5 nanowires for high performance supercapacitors

Orthorhombic vanadium pentoxide (V₂O₅) nanowires with uniform morphology were successfully fabricated via a facile hydrothermal process. The effect of disodium citrate dosage on the crystallinity, morphology and electrochemical properties of the products was analyzed. Experimental results indicate that orthorhombic V₂O₅ nanowires with high crystallinity and diameter of about 20 nm can be obtained at 180 °C for 24 h when the dosage of disodium citrate is 0.236 g. Furthermore, the prepared V₂O₅ nanowires demonstrate a high specific capacitance of 528.2 F g⁻¹ at 0.5 A g⁻¹ and capacitance retention of 85% after 1000 galvanostatic charge/discharge cycles at 1 A g⁻¹ when used as supercapacitors electrode in 0.5 M K₂SO₄.

Orthorhombic vanadium pentoxide (V₂O₅) nanowires with uniform morphology were successfully fabricated via a facile hydrothermal process.

Assembling a supercapacitor electrode with dual metal oxides and activated carbon using a liquid phase plasma

Developing supercapacitor electrodes at an affordable cost while improving their energy and/or power density values is still a challenging task. This study introduced a recipe which assembled a novel electrode composite using a liquid phase plasma that was applied to a reactant solution containing an activated carbon (AC) powder with dual metal precursors of iron and manganese. A comparison was made between the composites doped with single and dual metal components as well as among those synthesized under different precursor concentrations and plasma durations. The results showed that increasing the precursor concentration and plasma duration raised the content of both metal oxides in the composites, whereas the deposition conditions were more favorable to iron oxide than manganese oxide, due to its higher standard potential. The composite treated with the longest plasma duration and highest manganese concentration was superior to the others in terms of cyclic stability and equivalent series resistance. In addition, the new composite selected out of them showed better electrochemical performance than the raw AC material only and even two types of single metal-based composites, owing largely to the synergistic effect of the two metal oxides. Therefore, the proposed methodology can be used to modify existing and future composite electrodes to improve their performance with relatively cheap host and guest materials.

Asymmetric Supercapacitor Electrodes and Devices

The world is recently witnessing an explosive development of novel electronic and optoelectronic devices that demand more-reliable power sources that combine higher energy density and longer-term durability. Supercapacitors have become one of the most promising energy-storage systems, as they present multifold advantages of high power density, fast charging-discharging, and long cyclic stability. However, the intrinsically low energy density inherent to traditional supercapacitors severely limits their widespread applications, triggering researchers to explore new types of supercapacitors with improved performance. Asymmetric supercapacitors (ASCs) assembled using two dissimilar electrode materials offer a distinct advantage of wide operational voltage window, and thereby significantly enhance the energy density. Recent progress made in the field of ASCs is critically reviewed, with the main focus on an extensive survey of the materials developed for ASC electrodes, as well as covering the progress made in the fabrication of ASC devices over the last few decades. Current challenges and a future outlook of the field of ASCs are also discussed.

Stimulus-Responsive Micro-Supercapacitors with Ultrahigh Energy Density and Reversible Electrochromic Window

Stimulus-responsive micro-supercapacitors (SR-MSCs) with ultrahigh volumetric energy density and reversible electrochromic effect are successfully fabricated by employing a vanadium pentoxide and electrochemical exfoliated graphene-based hybrid nanopaper and viologen as electrode and stimulus-responsive material, respectively. The fabricated high-performance SR-MSCs offer new opportunities for intuitively observing the working state of energy devices without the aid of extra equipment and techniques.

A Cr2O3/MWCNTs composite as a superior electrode material for supercapacitor

A Cr₂O₃/MWCNTs composite exhibited high specific capacitance and good cycling stability a in 1 M KOH aqueous solution. This suggests that Cr₂O₃ can be promising for practical applications in supercapacitors.

One step synthesis of Ni/Ni(OH)2 nano sheets (NSs) and their application in asymmetric supercapacitors

Ni/Ni(OH)₂ NSs were prepared by a facile hydrothermal method using EtOH as reducing agent. Asymmetric device is fabricated using AC and Ni/Ni(OH)₂ NSs as electrodes, with optimized specific capacitance of 62 F g⁻¹ and a maximum energy density of 23.45 W h kg⁻¹.

High Energy Density Hybrid Supercapacitor: In-Situ Functionalization of Vanadium-Based Colloidal Cathode

A novel and creative in-situ electrochemical activation method to transform vanadium ions to highly electroactive colloidal cathode in KOH solution under electric field has been designed. After undergoing electrochemical reaction, the in-situ-functionalized vanadium-based colloidal cathode can adapt their geometrical structure to the high pseudocapacitive activity. The vanadium-based colloids//activated carbon asymmetric supercapcitor displays a high energy density of 50.4 Wh/kg at a power density of 250 W/kg, which is higher than most reported vanadium-based supercapacitors. The main advantage of this system is that the materials synthesis and the device operation are performed in the same reactive environment. The obtained vanadium-based colloids can display high V³⁺ cation utilization ratios of about 100% for one-electron redox reactions. The present results highlight a new area of research on in-situ formation of reactive electrode materials under realistic environments, which can bring new chemistry and new structures of materials that are only present under the current in-situ reactive conditions.