One-step hydrothermal synthesis of vanadium dioxide/carbon core-shell composite with improved ammonium ion storage for aqueous ammonium-ion battery

Aqueous nonmetallic ion batteries have garnered significant interest due to their cost-effectiveness, environmental sustainability, and inherent safety features. Specifically, ammonium ion (NH4+) as a charge carrier has garnered more and more attention recently. However, one of the persistent challenges is enhancing the electrochemical properties of vanadium dioxide (VO2) with a tunnel structure, which serves as a highly efficient NH4+ (de)intercalation host material. Herein, a novel architecture, wherein carbon-coated VO2 nanobelts (VO2@C) with a core-shell structure are engineered to augment NH4+ storage capabilities of VO2. In detail, VO2@C is synthesized via the glucose reduction of vanadium pentoxide under hydrothermal conditions. Experimental results manifest that the introduction of the carbon layer on VO2 nanobelts can enhance mass transfer, ion transport and electrochemical kinetics, thereby culminating in the improved NH4+ storage efficiency. VO2@C core-shell composite exhibits a remarkable specific capacity of ∼300 mAh/g at 0.1 A/g, which is superior to that of VO2 (∼238 mAh/g) and various other electrode materials used for NH4+ storage. The NH4+ storage mechanism can be elucidated by the reversible NH4+ (de)intercalation within the tunnel of VO2, facilitated by the dynamic formation and dissociation of hydrogen bonds. Furthermore, when integrated into a full battery with polyaniline (PANI) cathode, the VO2@C//PANI full battery demonstrates robust electrochemical performances, including a specific capacity of ∼185 mAh·g-1 at 0.2 A·g-1, remarkable durability of 93 % retention after 1500 cycles, as well as high energy density of 58 Wh·kg-1 at 5354 W·kg-1. This work provides a pioneering approach to design and explore composite materials for efficient NH4+ storage, offering significant implications for future battery technology enhancements.

Flexible quasi-solid-state zinc-ion hybrid supercapacitor based on carbon cloths displays ultrahigh areal capacitance

Over the past few years, the flexible quasi-solid-state zinc-ion hybrid supercapacitors (FQSS ZHSCs) have been found to be ideal for wearable electronics applications due to their high areal capacitance and energy density. The assembly of desirable ZHSCs devices that have promising practical applications is of high importance, whereas it is still challenging to assemble ZHSCs devices. In this study, a ZHSC that exhibited ultrahigh areal capacitance and high stability was developed by using an active carbon cloth (ACC) cathode, which could improve ionic adsorption. The as-obtained ACC cathode had an energy storage mechanism due to the electrical double-layer capacitive behavior of Zn2+, which was accompanied by the dissolution/deposition of Zn4SO4(OH)6·5H2O. The ACC//Zn@ACC ZHSC device exhibited an areal capacitance of 2437 mF cm-2 (81 F cm-3, 203 F g-1 under the mass of ACC with ∼12 mg cm-2) at 1 mA cm-2, an areal energy density of 1.354 mWh cm-2 at 1 mW cm-2, as well as high stability (with an insignificant capacitance decline after 20000 cycles), which was demonstrated to outperform the existing ZHSCs. Furthermore, the assembled flexible device still had competitive capacitance, energy density and service life when integrated into a FQSS ZHSC. When applied in practice, the device could achieve high mechanical flexibility, wearable stability and output. This study can inspire the development of the FQSS ZHSC device to satisfy the demands for wearable energy storage devices with high performance.

Controllable Transformation of Metal-Organic Framework Nanosheets into Oxygen Vacancy NixCo3-xO4 Arrays for Ultrahigh-Capacitance Supercapacitors with Long Lifespan

The amino-functionalized bimetal NH₂-NiCo-MOF nanosheet array is first fabricated on Ni foam substrates and then controllably transformed into oxygen vacancy bimetal oxide arrays by simply thermal annealing in air. This NiCo-based oxide array (Ni_xCo_3-xO₄/NF) achieves high capacitance (2484 F g^-1 at 1 A g^-1), excellent rate performance (91.4%), and long cycling life when assessed as promising electrode material for supercapacitors. Notably, the existing oxygen vacancy in Ni_xCo_3-xO₄ promotes the electrochemical performance of Ni_xCo_3-xO₄/NF due to the enhancement of electrical conductivity and capture capability for OH^-. In addition, the assembled asymmetric supercapacitor (ASC) device exhibits an excellent energy density of 39.3 W h kg^-1 at a power density of 800.2 W kg^-1, which still remains 32.2 W h kg^-1 even at a high power density of 7994.5 W kg^-1. Furthermore, a light-emitting diode can be lightened for more than 6 min, demonstrating a great potential for practical application of ASC devices. This work knocks on the door of a feasible strategy for designing and synthesizing 2D metal oxide nanosheet arrays with excellent electrochemical properties.

A dual-polymer strategy boosts hydrated vanadium oxide for ammonium-ion storage

Recently, aqueous rechargeable batteries employing ammonium-ions (NH₄⁺) as charge carriers have received increasing interest because of their merits of eco-friendly, low cost and sustainability. However, the supercapacitor based on NH₄⁺ charge carriers has rarely been reported probably owing to the lack of a suitable system to achieve acceptable capacitance and cycle performance for NH₄⁺ storage. Herein, we develop a dual-polymer strategy to boost the electrochemical properties of hydrated vanadium oxide (HVO) for outstanding NH₄⁺ storages based on a supercapacitor. One polymer polyaniline (PANI) is intercalated into the interlayer space of HVO (11.0 Å) to synthesize PANI-intercalation-HVO (PVO) with the expanded interlamellar spacing of 13.9 Å, which enhances the kinetics and stabilizes the structure during the NH₄⁺ (de)intercalation. The capacitance at 1 A·g^-1 is significantly improved from 156F·g^-1 (HVO) to 351F·g^-1 (PVO). The other polymer polyvinyl alcohol (PVA) is used to get the quasi-solid-state (QSS) PVA/NH₄Cl electrolyte, in which the cycle stability of PVO electrode is effectively improved. The PVO exhibits the capacitance retentions of 82% after 2000 cycles and 56% after 10,000 cycles, whereas this value is only 29% after 3000 cycles in NH₄Cl electrolyte. The findings reveal that this strategy can effectively reduce the diffusion resistance of ammonium ions and improve the energy storage efficiency of PVO. The flexible QSS PVO//active carbon hybrid supercapacitor (FQSS PVO//AC HSC) device is assembled and exhibits outstanding capacitance, long cycle stability, good mechanical stability and potential practical applications. This work may open up a new window for the study on the improved electrochemical properties of electrode materials for NH₄⁺ storage.

Fabrication of Phosphorus-Doped Cobalt Silicate with Improved Electrochemical Properties

The development of electrode materials for supercapacitors (SCs) is greatly desired, and this still poses an immense challenge for researchers. Cobalt silicate (Co₂SiO₄, denoted as CoSi) with a high theoretical capacity is deemed to be one of the sustainable electrode materials for SCs. However, its achieved electrochemical properties are still not satisfying. Herein, the phosphorus (P)-doped cobalt silicate, denoted as PCoSi, is synthesized by a calcining strategy. The PCoSi exhibits 1D nanobelts with a specific surface area of 46 m²∙g⁻¹, and it can significantly improve the electrochemical properties of CoSi. As a supercapacitor’s (SC’s) electrode, the specific capacitance of PCoSi attains 434 F∙g⁻¹ at 0.5 A∙g⁻¹, which is much higher than the value of CoSi (244 F∙g⁻¹ at 0.5 A∙g⁻¹). The synergy between the composition and structure endows PCoSi with attractive electrochemical properties. This work provides a novel strategy to improve the electrochemical performances of transition metal silicates.

Carbon coating on metal oxide materials for electrochemical energy storage

During the past decades, nano-structured metal oxide electrode materials have received growing attention due to their low development cost and high theoretical specific capacity, accordingly, quite a lot of metal oxide electrode materials are being used in electrochemical energy storage devices. However, the further development was limited by the relatively low electrical conductivity and the volume expansion during electrochemical reactions. Thus, many approaches have been proposed to obtain high-efficiency metal oxide electrode materials, such as designing nanomaterials with ideal morphology and high specific surface area, optimizing with carbon-based materials (such as graphene and glucose) to prepare nanocomposites, combining with conductive substrates to enhance the conductivity of electrodes, etc. Owning to the advantages of low cost and high chemical stability of carbon materials, core-shell structure formed by carbon-coated metal oxides is considered to be a promising solution to solve these problems. Therefore, this review mainly focuses on recent research advances in the field of carbon-coated metal oxides for energy storage, summarizing the advantages and disadvantages of common metal oxides and different types of carbon sources, and proposing methods to optimize the material properties in terms of structure and morphology, carbon layer thickness, coating method, specific surface area and pore size distribution, as well as improving electrical conductivity. In addition, the double or multi-layer coating strategy is also a reflection of the continuous development of carbon coating method. Hopefully, this rereview may provide a new direction for the renewal and development of future energy storage electrode materials.

Rice-like and rose-like zinc silicates anchored on amorphous carbon derived from natural reed leaves for high-performance supercapacitors

3D N, S, P-doped rice-like C-Zn4Si2O7(OH)2·H2O (C-ZnSi-N2) and rose-like C-Zn2SiO4 (C-ZnSi-CO2) are derived from reed leaves and used for application in supercapacitors. The as-prepared C-ZnSi architectures with a large number of hierarchical pores and high specific surface area from reed leaves have outstanding electrochemical performance. The obtained C-ZnSi-N2 shows 341 F g-1 at the current density of 0.5 A g-1, while the C-ZnSi-CO2 exhibits 498 F g-1, and both of the C-ZnSi materials significantly retain above 99% of their capacitance after 10 000 cycles. Furthermore, the flexible solid-state asymmetric supercapacitors (ASCs) synthesized from C-ZnSi and activated carbon (denoted as C-ZnSi-N2//AC and C-ZnSi-CO2//AC) achieve a high capacitance (405 and 194 mF cm-2 at the current density of 2 mA cm-2, respectively). Besides, the ASC devices show good cycling stability for 7300 cycles with 73% and 77% capacitance retention. The results presented in this study indicate that the N, S, P-doped C-ZnSi architectures from natural reed leaves are promising and efficient materials for manufacturing high performance supercapacitors.

Polyaniline-expanded the interlayer spacing of hydrated vanadium pentoxide by the interface-intercalation for aqueous rechargeable Zn-ion batteries

The metal ions or conductive macromolecules intercalated hydrated vanadium oxides for aqueous Zn-ion batteries (AZIBs) have received increasing attention in recent years. The strategy for the preparation of the intercalated hydrated vanadium oxides has been achieved great advances but is still a huge challenge. In this contribution, we develop an interface-intercalation method to synthesize the polyaniline-intercalated hydrated vanadium pentoxide (V₂O₅·nH₂O), denoted as PANI-VOH, as the cathode materials for AZIBs. The prepared PANI-VOH exhibits a 3D sponge-like morphology and the surface area of 190 m²·g^-1. The interlayer spacing of VOH is expanded to be 14.1 Å, which provides a lot of channels for the rapidly reversible (de)intercalation of Zn²⁺ ions. The coin-typed Zn//PANI-VOH battery shows the specific discharge capacity of 363 mAh·g^-1 at 0.1 A·g^-1 and stable cycling performance. Furthermore, the specific capacity remains 131 mAh·g^-1 after 2000 cycles at 5 A·g^-1, and the energy density is calculated to be 275 Wh·kg^-1 at 78 W·kg^-1 on the mass of PANI-VOH. The achieved values are comparable to or even much higher than that of the most state-of-the-art V-based cathode materials for AZIBs. The PANI intercalation can shorten the pathways and facilitate the transports for the migration of ions and electrons. Our finding guides a novel strategy for the intercalation of PANI into the layered materials to adjust their interlayer spacing, which exhibits super ions migration efficiency, as the cathode materials for AZIBs and even other multivalent ions batteries.

Lactone radical transformed methyl mercaptan-adsorbed activated carbon into graphene oxide modified activated carbon

Activated carbon was a widely-used adsorbent. However, it was usually classified as a hazardous waste after saturation adsorption for one pollution. For the first time, this article reported a regeneration method for the activated carbon saturated with methyl mercaptan. The regenerated carbon was partially transformed into graphene-oxide fragment with a thickness of 0.9-1.0 nm after a hydrothermal treatment at 180 °C. Electron paramagnetic resonance revealed that lactone group was transformed into lactone radical under the hydrothermal condition. The spins were increased from 4.54E+17-1.24E+18. The formed radical effectively reacted with the adsorbed methyl mercaptan and re-distributed the amorphous activated carbon to form lamellar graphene oxide. As a result, the spins were decreased from 1.24E+18-8.73E+17. At the same time, the amount of lactone group was decreased from 0.71 to 0.42 mmol/g. The regenerated activated carbon thus regained ability to adsorb methyl mercaptan. The main result of this paper puts forward a simple and low-cost method to obtain graphene oxide modified activated carbon from the regeneration of hazardous waste carbon. This conclusion makes contribution to the development of "zero-waste" conception.

The synthesis and electrochemical properties of low-crystallinity iron silicate derived from reed leaves as a supercapacitor electrode material

The design and preparation of electrode materials with excellent performance is particularly important due to the current global scarcity of energy supplies, especially those using sustainable and renewable materials. In this work, it is first proposed to apply iron silicate (FeSi), which is synthesized using environmentally friendly biomass as a raw material, as an electrode material for supercapacitors (SCs). FeSi is derived from the calcination of reed leaves (RLs) in combination with a hydrothermal method, and spherical FeSi retains the porosity of the RL precursors and shows remarkable electrochemical performance. The specific capacitance of FeSi as a SC electrode can reach 575 F g-1 at 0.5 A g-1 in the voltage window from -1 to -0.5 V. Simultaneously, the FeSi electrode exhibits favorable cycling stability with 76% capacitance retention after 10 000 cycles and outstanding electrical conductivity. This finding provides a novel method of preparing a kind of untapped electrode material, porous FeSi nanoparticles derived from RLs, and the resulting FeSi material shows enormous potential for energy storage via high-performance SCs.

Hierarchical core-shell 2D MOF nanosheet hybrid arrays for high-performance hybrid supercapacitors

Two-dimensional (2D) metal-organic frameworks (MOFs) with large surface area, ordered pores and ultrathin thickness have recently emerged as ideal electrode materials for supercapacitors (SCs). However, their straightforward applications are restricted by the drawbacks of self-stacking and unsatisfactory electrical conductivity. Herein, ultrathin Ni-MOF nanosheets have been grafted on zeolite imidazolate framework (ZIF-L)-derived porous Co3O4 nanosheets to form hierarchical core-shell Co3O4@Ni-MOF 2D nanosheet hybrid arrays. The porous Co3O4 "core" acts as a conductive skeleton for anchoring Ni-MOF and provides shortened ion diffusion paths. The Ni-MOF "shell" can expose large active sites. Benefiting from these merits and the synergic effects of the "core and "shell", the Co3O4@Ni-MOF/NF shows a high specific capacity (capacitance) of 225.6 mA h g-1 (1980.7 F g-1) at 1 A g-1 with decent capacitance retention (82.2% after 2000 cycles). The asymmetric two-electrode cell assembled from Co3O4@Ni-MOF/NF exhibits an energy density of 37.05 W h kg-1 at a power density of 800 W kg-1 with good long-term durability (75% capacitance retention after 10 000 cycles). Moreover, two charged cells can power a red light-emitting diode (LED) for up to 16 min, manifesting the great promise of Co3O4@Ni-MOF/NF for real energy storage devices.

“Double guarantee mechanism” of Ca2+-intercalation and rGO-integration ensures hydrated vanadium oxide with high performance for aqueous zinc-ion batteries

Ca²⁺-Intercalated hydrated V₂O₅/rGO (CaVOH/rGO) is synthesized via a facile hydrothermal process and applied as a cathode for ARZIBs with an admirable specific capacity (409 mA h g⁻¹ at 0.05 A g⁻¹) and excellent energy density (381 W h kg⁻¹).

VO2 Nanostructures for Batteries and Supercapacitors: A Review

Vanadium dioxide (VO₂ ) received tremendous interest lately due to its unique structural, electronic, and optoelectronic properties. VO₂ has been extensively used in electrochromic displays and memristors and its VO₂ (B) polymorph is extensively utilized as electrode material in energy storage applications. More studies are focused on VO₂ (B) nanostructures which displayed different energy storage behavior than the bulk VO₂ . The present review provides a systematic overview of the progress in VO₂ nanostructures syntheses and its application in energy storage devices. Herein, a general introduction, discussion about crystal structure, and syntheses of a variety of nanostructures such as nanowires, nanorods, nanobelts, nanotubes, carambola shaped, etc. are summarized. The energy storage application of VO₂ nanostructure and its composites are also described in detail and categorically, e.g. Li-ion battery, Na-ion battery, and supercapacitors. The current status and challenges associated with VO₂ nanostructures are reported. Finally, light has been shed for the overall performance improvement of VO₂ nanostructure as potential electrode material for future application.

Phase transformation in tungsten oxide nanoplates as a function of post-annealing temperature and its electrochemical influence on energy storage

The morphology and crystal structure of electrode materials have an enormous impact on their electrochemical properties for employment in supercapacitors for various applications. In this study, the transformations of the crystal structure of WO3·H2O nanoplates were conducted by post-annealing at 200 °C and 400 °C. The morphological and structural evolution of the electrodes was studied via FEG-SEM, HRTEM, FTIR, XRD, and Raman spectroscopy. The phase transition and enhanced degree of crystallinity were observed with increasing temperature. The orthorhombic structures of the hydrate WO3·H2O (W80), the mixed-phase with mesoporous structure (W200), and finally the monoclinic phase of WO3 structures (W400) were achieved at annealing temperatures of 80 °C, 200 °C, and 400 °C respectively. The electrochemical performance of electrode W200 showed the highest specific capacitance of 606 F g-1 as compared to electrode W80 (361 F g-1), and was two-fold greater than electrode W400 (302 F g-1) at a current density of 1 A g-1. Moreover, electrode W200 exhibited excellent cyclic stability of 89% at an ultrahigh scan rate of 100 mV s-1 after 4000 cycles. The results highlight that the mixed-phase WO3 nanoplates would make a suitable electrode material for supercapacitors with desired electrochemical features.

Fabrication of vanadium sulfide (VS4) wrapped with carbonaceous materials as an enhanced electrode for symmetric supercapacitors

Exploring electrode materials with excellent electrochemical performance is the key to the development of applications in energy storage and conversion. Herein, three-dimensional (3D) vanadium sulfide/carbon nanotubes/reduced graphene oxide (VS₄/CNTs/rGO) composite is synthesized by a simple one-step hydrothermal method. VS₄ short nanorods cover the both sides of the rGO sheets, and CNTs distribute at the edge of the composite to form a sandwich-like structure, which effectively prevents the accumulation of rGO. Due to the special 3D hierarchical structure, VS₄/CNTs/rGO exhibits a large specific surface area and a rich pore structure, and the addition of CNTs and rGO also improves the electrochemical properties of VS₄. At 1 A·g^-1, VS₄/CNTs/rGO exhibits a capacitance of 497 F·g^-1 (1374.0 C·g^-1) in the voltage range of -1.4 to 1.4 V, which is much higher than those binary materials including CNTs/rGO, VS₄/CNTs and VS₄/rGO. The VS₄/CNTs/rGO symmetric supercapacitor (SSC) device shows a remarkable electrochemical performance in a large potential window up to 2.2 V. The capacitance of VS₄/CNTs/rGO SSC device can reach 1003.5 mF·cm^-2 (2207.6 mC·cm^-2) at 0.5 mA·cm^-2, and it exhibits an energy density of 6.75 Wh·m^-2 (72.07 Wh·kg^-1) at a power density of 1.38 W·m^-2 (14.69 W·kg^-1). The high capacitance and energy density of the VS₄/CNTs/rGO composite in the high voltage interval make it as the potential energy storage material.

Electrospun Core-Shell Nanofibrous Membranes with Nanocellulose-Stabilized Carbon Nanotubes for Use as High-Performance Flexible Supercapacitor Electrodes with Enhanced Water Resistance, Thermal Stability, and Mechanical Toughness

A high-performance flexible supercapacitor electrode with a core-shell structure is successfully developed from cellulose nanocrystal (CNC)-stabilized carbon nanotubes (CNTs). By incorporating poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA), a cross-linked nanofibrous membrane (CNT-CNC/PVA-PAA) is prepared as the core material via directional electrospinning, followed by a thermal treatment. The flexible supercapacitor electrodes are eventually fabricated via the in situ polymerization of polyaniline (PANI), which was used as the coating shell material, on the aligned electrospun nanofibers. By taking advantage of the thermally induced esterification cross-linking that occurs among PVA, PAA, and the CNT-CNC nanohybrids, the membranes present with enhanced water resistance, mechanical strength, and thermal stability. After the surface coating of the PANI shell, the optimized PANI@CNT-CNC/PVA-PAA nanofibrous membranes exhibit a large porosity, an enhanced specific surface area, a superior tensile strength of ∼54.8 MPa, and a favorable electroconductivity of ∼0.44 S m^-1. As expected, the nanofibrous electrodes with a specific capacitance of 164.6 F g^-1 can maintain 91% of the original capacitance after 2000 cycles. The symmetrical solid-state supercapacitor assembled by the nanofibrous electrodes shows an excellent capacitance of 155.5 F g^-1 and a remarkable capacitance retention of 92, 90, and 89% after 2000 cycles under flat, bending, and twisting deformations, respectively.

High-Yield Continuous-Flow Synthesis of Spheroidal C60@Graphene Composites as Supercapacitors

Graphene spheres confining fullerene C60 are quantitatively formed under high-shear and continuous-flow processing using a vortex fluidic device (VFD). This involves intense micromixing a colloidal suspension of graphite in DMF and an o-xylene solution of C60 at room temperature in the absence of surfactants and other auxiliary substances. The diameters of the composite spheres, C60@graphene, can be controlled with size distributions ranging from 1.5 to 3.5 μm, depending on the VFD operating parameters, including rotational speed, flow rate, relative ratio of C60 to graphite, and the concentration of fullerene. An electrode of the composite material has high cycle stability, with a high areal capacitance of 103.4 mF cm-2, maintaining its capacitances to 24.7 F g-1 and 86.4 mF cm-2 (83.5%) at a high scan rate of 100 mV s-1.

Co(OH)F nanorods@K x MnO2 nanosheet core-shell structured arrays for pseudocapacitor application

In this work, Co(OH)F nanorods@K x MnO2 nanosheet core-shell nanostructure was assembled on Ni foam by a facile hydrothermal method and incorporated with an electrodeposition process. Benefiting from their core-shell nanostructure and heterogeneous nanocomposites, the arrays present high areal capacitance up to 1046 mF cm-2 at 1 mA cm-2 and display a remarkable specific capacitance retention of 118% after 3000 cycles. When the current density increases to 10 mA cm-2, the capacitance is 821 mF cm-2 displaying a good rate capability. The excellent electrochemical properties allow them to be used as a promising electrode material for pseudocapacitors and display wide application potential in the field of electrochemical capacitors.

Engineering Na-Mo-O/Graphene Oxide Composites with Enhanced Electrochemical Performance for Lithium Ion Batteries

Sodium molybdate (Na−Mo−O) wrapped by graphene oxide (GO) composites have been prepared via a simple in‐situ precipitation method at room temperature. The composites are mainly constructed with one dimension (1D) ultra‐long sodium molybdate nanorods, which are wrapped by the flexible GO. The introduction of GO is expected to not merely provide more active sites for lithium‐ions storage, but also improve the charge transfer rate of the electrode. The testing electrochemical performances corroborated the standpoint: The Na−Mo−O/GO composites delivers specific capacities of 718 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹, and 570 mAh g⁻¹ after 500 cycles at a high rate of 500 mA g⁻¹; for comparison, the bare Na−Mo−O nanorod shows a severe capacity decay, which deliver only 332 mAh g⁻¹ after 100 cycles at 100 mA g⁻¹. In view of the cost‐efficient and less time‐consuming in synthesis, and one‐step preparation without further treatment, these Na−Mo−O nanorods/GO composites present potential and prospective anodes for LIBs.