Materials Design and System Construction for Conventional and New-Concept Supercapacitors

Key issues facing electrospun carbon nanofibers in energy applications: on-going approaches and challenges

Electrospun carbon nanofibers (CNFs), with one-dimensional (1D) morphology, tunable size, mechanical flexibility, and functionalities by themselves and those that can be added onto them, have witnessed the intensive development and extensive applications in energy storage and conversion, such as supercapacitors, batteries, and fuel cells. However, conventional solid CNFs often suffer from a rather poor electrical conductivity and low specific surface area, compared with the graphene and carbon nanotube counterparts. A well-engineered porous structure in CNFs increases their surface areas and reactivity, but there is a delicate balance between the level and type of pores and mechanical robustness. In addition, CNFs by themselves often show unsatisfactory electrochemical performance in energy storage and conversion, where, to endow them with high and durable activity, one effective approach is to dope CNFs with certain heteroatoms. Up to now, various activation strategies have been proposed and some of them have demonstrated great success in addressing these key issues. In this review, we focus on the recent advances in the issue-oriented schemes for activating the electrospun CNFs in terms of enhancing the conductivity, modulating pore configuration, doping with heteroatoms, and reinforcing mechanical strength, in close reference to their applications in supercapacitors. The basic scientific principles involved in these activation processes and their effectiveness in boosting the electrochemical performance of CNFs are examined. Finally, some of the on-going challenges and future perspectives in engineering CNFs for better performance are highlighted.

Lithium-Ion-Based Electrochemical Energy Storage in a Layered Vanadium Formate Coordination Polymer

A vanadium formate (VF) coordination polymer and its composite with partially reduced graphene oxide (prGO), namely VF-prGO, can be applied as anode materials for Li-ion based electrochemical energy storage (EcES) systems in the potential range of 0-3 V (vs Li⁺ /Li). This study shows that a reversible capacity of 329 mAh g^-1 at a current density of 50 mA g^-1 after 50 cycles can be realized for VF along with a high rate capability. The composite exhibits even a higher capacity of 504 mAh g^-1 at 50 mA g^-1 . A good capacity retention is observed even after 140 cycles for both VF and the composite. An ex-situ X-ray photoelectron spectroscopy study indicates the involvement of V³⁺ /V⁴⁺ redox couple in the charge storage mechanism. A significant contribution of this reversible capacity is attributed to the pseudocapacitive behavior of the system.

Electrochemically Reduced Titania Nanotube Synthesized from Glycerol-Based Electrolyte as Supercapacitor Electrode

In this paper the synthesis of self-organized Titania nanotubes (TNTs) by a facile potentiostatic anodization in a glycerol-based electrolyte is reported. The optimized TNTs were subsequently reduced through a cathodic reduction process to enhance its capacitive performance. FESEM and XRD were used to characterize the morphology and crystal structure of the synthesized samples. XPS analysis confirmed the reduction of Ti4+ to Ti3+ ions in the reduced Titania nanotubes (R-TNTs). The tube diameter and separation between the tubes were greatly influenced by the applied voltage. TNTs synthesized at voltage of 30 V for 60 min exhibited 86 nm and 1.1 µm of tube diameter and length, respectively and showed high specific capacitance of 0.33 mF cm−2 at current density of 0.02 mA cm−2. After reduction at 5 V for 30 s, the specific capacitance increased by about seven times (2.28 mF cm−2) at 0.5 mA cm−2 and recorded about 86% capacitance retention after 1000 continuous cycling at 0.2 mA cm−2, as compared to TNTs, retained about 61% at 0.01 mA cm−2. The charge transfer resistance drastically reduced from 6.2 Ω for TNTs to 0.55 Ω for R-TNTs, indicating an improvement in the transfer of electrons and ions across the electrode–electrolyte interface.

Binder Free and Flexible Asymmetric Supercapacitor Exploiting Mn3O4 and MoS2 Nanoflakes on Carbon Fibers

Emerging technologies, such as portable electronics, have had a huge impact on societal norms, such as access to real time information. To perform these tasks, portable electronic devices need more and more accessories for the processing and dispensation of the data, resulting in higher demand for energy and power. To overcome this problem, a low cost high-performing flexible fiber shaped asymmetric supercapacitor was fabricated, exploiting 3D-spinel manganese oxide Mn₃O₄ as cathode and 2D molybdenum disulfide MoS₂ as anode. These asymmetric supercapacitors with stretched operating voltage window of 1.8 V exhibit high specific capacitance and energy density, good rate capability and cyclic stability after 3000 cycles, with a capacitance retention of more than 80%. This device has also shown an excellent bending stability at different bending conditions.

An electrochromic supercapacitor based on an MOF derived hierarchical-porous NiO film

Nickel oxide (NiO) is a promising candidate for future electrochromic supercapacitors due to its pronounced electrical properties and low cost. Unfortunately, the weak interaction between NiO films and conductive substrates results in poor cycling stability. In addition, the long color-switching time and low capacitance by the small lattice spacing in dense NiO impede its practical applications seriously. Herein, a hierarchical porous NiO film/ITO glass bifunctional electrode has been prepared via the solvothermal and subsequent calcination process of growing MOF-74 in situ on ITO, which shows outstanding cycle reversibility, excellent capacitance, high coloration efficiency and short color-switching time. Because of the strong binding force between the NiO film and substrate, and large surface areas with a hierarchical porous structure which are beneficial to the ion transport, the NiO film demonstrates perfect capacitive and electrochromic properties. As a bifunctional electrode, the NiO film shows a specific capacitance of 2.08 F cm-2 at 1 mA cm-2, large optical modulation of 41.08% and about 86% of optical modulation retention after 10 000 cycles. Furthermore, we assembled a bifunctional device whose energy condition can be roughly estimated according to the color state of the device. This finding can provide us with a new application of MOFs in the dual device of electrochromic supercapacitors.

Porous carbon derived from herbal plant waste for supercapacitor electrodes with ultrahigh specific capacitance and excellent energy density

Here in this work, porous carbon is prepared from waste of a traditional Chinese medicine Salvia miltiorrhiza flowers. Structures of the porous carbons are regulated by simply regulating of activation temperatures and dosages of activator. The optimized porous carbon owns a high specific surface area of 1715.3 m² g^-1 and total pore volume of 0.6392 cm³ g^-1, together with a unique hierarchical architecture and ultrahigh content of 45.97 at% self-doped O and 0.49 at% of N. When used as electrode materials for supercapacitors, the prepared porous carbon exhibited excellent specific capacitance and energy density as well as fantastic cycle stability. Under a current density of 0.5 A/g, the electrode based on this material showed high specific capacitance of 530 F/g, with fantastic rate performance of 258 F/g at 20 A/g and excellent cycle stability of 91% capacitance retention for 10,000 cycles at 10 A/g in a three-electrode system in 6 M KOH. In assembled supercapacitors, the SF-PC_700-3 based electrode worked under potential of 1 V and exhibited 222 F/g of specific capacitance at a current density of 0.5 A/g, and even when the current density was increased up to 30 A/g, the specific capacitance can still as high as 168 F/g, verified the excellent performance of SF-PC_700-3. Symmetric supercapacitors in Na₂SO₄ and TEABF₄/AN electrolyte showed voltage ranges of 1.8 V and 3 V respectively, and high energy density of 22.2 Wh Kg^-1 at 448. W Kg^-1 and 40.6 Wh Kg^-1 at 755.8 W Kg^-1 are obtained.

Polypyrrole/Carbon Nanotube Freestanding Electrode with Excellent Electrochemical Properties for High-Performance All-Solid-State Supercapacitors

In this study, a facile and environmentally friendly method was used to prepare a freestanding supercapacitor electrode displaying excellent areal capacitance and good cycle life performance. First, we prepared polypyrrole nanoparticles (PPyNP) through a simple in situ chemical polymerization using the plant-derived material curcumin as a bioavailable template. A PPyNP/f-CNT freestanding composite electrode of high mass loading (ca. 14 mg cm^-2) was prepared after blending the mixtures of the prepared PPyNP and functionalized CNTs (f-CNTs). The performance of the as-prepared material as a supercapacitor electrode was evaluated in a three-electrode setup using aqueous 1 M H₂SO₄ as the electrolyte. The PPyNP/f-CNT freestanding composite electrode exhibited a high areal capacitance of 4585 mF cm^-2 and a corresponding volumetric capacitance of 176.35 F cm^-3 at a current density of 2 mA cm^-2. A symmetric all-solid-state supercapacitor assembled using two identical pieces of PPyNP/f-CNT composite electrodes exhibited maximum areal energy and power density of 129.24 μW h cm^-2 and 12.5 mW cm^-2, respectively. Besides, this supercapacitor device exhibited good cycle life performance, with 79.03% capacitance retention after 10,000 charge/discharge cycles. These results suggest practical applications for these PPyNP/f-CNT freestanding composite electrode-based symmetric all-solid-state supercapacitors.

3D hierarchical porous nitrogen-doped carbon/Ni@NiO nanocomposites self-templated by cross-linked polyacrylamide gel for high performance supercapacitor electrode

Three-dimensional nitrogen-doped carbon network incorporated with nickel@nickel oxide core-shell nanoparticles composite (3D NC/Ni@NiO) has been facilely prepared, self-templated by the cross-linked polyacrylamide aerogel precursor containing NiCl₂. Characterizations reveal that the Ni@NiO nanoparticles distribute homogeneously in the 3D nitrogen-doped carbon matrix and the composite is of hierarchical porous structure. When used as supercapacitor electrode in a three-electrode system, the 3D NC/Ni@NiO exhibits enhanced electrical conductivity and excellent electrochemical performance, presenting a high specific capacitance (389F g^-1 at 5 mV s^-1), good rate capability (276 F g^-1 at 100 mV s^-1) and outstanding cycling performance (with the capacitance retention of 70.2% after 5000 charge-discharge cycles). This is due to the synergistic effects of conductive metallic nickel, pseudocapacitive nickel oxide as well as in situ nitrogen doping of carbon network. Moreover, an asymmetric supercapacitor (ASC) was fabricated with NC/Ni@NiO as positive electrode and active carbon as negative electrode. The ASC device exhibits a maximum energy density of 19.4 W h kg^-1 at a power density of 700 W kg^-1 and shows good cycling stability (73.8% capacity retention after 3000 cycles), indicating that it has great promise for practical energy storage and conversion application.

Role of Additives in Electrochemical Deposition of Ternary Metal Oxide Microspheres for Supercapacitor Applications

A simple two-step approach has been employed to synthesize a cobalt-nickel-copper ternary metal oxide, involving electrochemical precipitation/deposition followed by calcination. The ternary metal hydroxide gets precipitated/deposited from a nitrate bath at the cathode in the catholyte chamber of a two-compartment diaphragm cell at room temperature having a pH ≈ 3. The microstructure of the ternary hydroxides was modified in situ by two different surfactants such as cetyltrimethylammonium bromide and dodecyltrimethylammonium bromide in the bath aiming for enhanced storage performance in the electrochemical devices. The effect of the surfactant produces a transition from microspheres to nanosheets, and the effect of micelle concentration produces nanospheres at a higher ion concentration. The ternary hydroxides were calcined at 300 °C to obtain the desired ternary mixed oxide materials as the electrode for hybrid supercapacitors. X-ray diffraction analysis confirmed the formation of the ternary metal oxide product. The scanning electron microscopy images associated with energy-dispersive analysis suggest the formation of a nanostructured porous composite. Ternary metal oxide in the absence and presence of a surfactant served as the cathode and activated carbon served as the anode for supercapacitor application. DTAB-added metal oxide showed 95.1% capacitance retention after 1000 cycles, achieving 188 F/g at a current density of 0.1 A/g, and thereafter stable until 5000 cycles, inferring that more transition metals in the oxide along with suitable surfactants at an appropriate micellar concentration may be better for redox reactions and achieving higher electrical conductivity and smaller charge transfer resistance. The role of various metal cations and surfactants as additives in the electrolytic bath has been discussed.

Systematic Design of Polypyrrole/Carbon Fiber Electrodes for Efficient Flexible Fiber-Type Solid-State Supercapacitors

Fiber-type supercapacitors (FSC) have attracted much attention as efficient energy storage devices for soft electronics. This study proposes the synthesis of polypyrrole (PPy) on carbon fiber (CF) using electropolymerization as the energy storage electrode for FSC. Effects of the electrolyte, applied current, and time of electropolymerization for synthesizing PPy on CF are investigated. The configuration of the electrochemical system is also studied to better understand the electropolymerization of PPy. The highest specific capacitance (C_M) of 308.2 F/g are obtained for the PPy electrode prepared using 0.5 M pyrrole and 0.3 M NaClO₄ as the electrolyte at 40 mA for 20 min. The FSC assembled with PPy electrodes and the polyvinyl alcohol/H₃PO₄ gel electrolyte shows a C_M value of 30 F/g and the energy density of 5.87 Wh/kg at the power density of 60.0 W/kg. Excellent cycling stability with C_M retention of 70% and Coulombic efficiency higher than 98% in 3000 times charge/discharge process, and the good bending capability with C_M retention of 153% and 148%, respectively, under the bending angle of 180° and the bending times of 600 are achieved. This work gives deeper understanding of electropolymerization and provides recipes for fabricating an efficient PPy electrode for soft energy storage devices.

An environmentally friendly strategy to prepare nitrogen-rich hierarchical porous carbon for high-performance supercapacitors

A green route modulated by the addition of CaCl₂ during the potassium compound-assisted synthesis is developed for the first time for the synthesis of NRHPC.

MnO1.88/R-MnO2/Ti3C2(OH/F)x composite electrodes for high-performance pseudo-supercapacitors prepared from reduced MXenes

Mixed-valency MnO_x (MnO_1.88 and MnO₂)/Ti₃C₂(OH/F)_x composites electrode materials were in situ growth of MnO_x on the surface of Ti₃C₂(OH/F)_x, where the Ti₃C₂(OH/F)_x acts as both template and reducing agent.

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.

An Assembly and Interfacial Templating Route to Carbon Supercapacitors with Simultaneously Tailored Meso- and Microstructures

The development of facile strategies for simultaneously tailoring robust pore hierarchy and integrated microstructures in carbonaceous materials is critical for the efficient multiscale control of fluid, molecular/ionic, and charge transport in applications spanning separations, catalysis, and energy storage. Here, we synthesize three-dimensionally ordered hierarchically porous carbon powders by the assembly of glucose with silica nanoparticle building blocks of sacrificial NP-crystalline templates. Such template-replica coassembly offers an attractive alternative to conventional nanocasting by circumventing the need for sequential template preformation and infiltration-based replication. In addition, interfacial templating leads to hierarchically structured carbons with tunable mesopore volumes (as high as 5.8 cm³/g). Beyond mesostructuring, we identify the template-replica interface as a potentially versatile but generally unexploited handle for tailoring the sp² hybridized carbon content in the porous replicas under mild carbonization conditions and without specific chemical activation or catalytic graphitization. This multiscale (meso-micro) templating offered by a single template expands the potential versatility of nanocasting for the hierarchical structuring of replica materials. Application of the resulting carbons as electrochemical double layer capacitors demonstrates the combined benefit of simultaneously tailored pore hierarchy and tuned microstructures upon ion and charge transport, respectively, yielding supercapacitors achieving specific capacitance as high as 275 F/g in the aqueous electrolyte (H₂SO₄) and retention of 90% up to a current density of 10 A/g.

Ti-Doped Tunnel-Type Na4Mn9O18 Nanoparticles as Novel Anode Materials for High-Performance Supercapacitors

Nanomaterials with tunnel structures are extremely attractive to be used for electrode materials in electrochemical energy storage devices. Tunnel-structured Ti-doped Na₄Mn₉O₁₈ nanoparticles (TNMO-NPs) were synthesized by a facile and high-production method of the solid-state reaction with a high-energy ball-milling process. As electrode materials in the supercapacitor cell, the as-synthesized TNMO-NPs exhibit a high specific capacity of 284.93 mA h g^-1 (0.57 mA h cm^-2/1025.75 F g^-1). A superior rate capability with a decay of 36% is achieved by increasing the scan rates from 2 to 25 mV s^-1. To further explore the storage mechanism of Ti-doped Na₄Mn₉O₁₈ materials, density functional theory (DFT) calculations were used to calculate the activation energy for the ion immigration in the electrode, and the results show that the minimum ion diffusion barrier energy is 0.272 eV, indicating that the sodium ions could insert into the system easily. Through the scan-rate-dependent cyclic voltammetry analysis, the capacity value indicates a mixed charge storage of capacitive behavior and Na⁺ intercalation progress. A maximum energy density of 77.81 W h kg^-1 at a power density of 125 W kg^-1 is achieved, and a high energy density of 54.79 W h kg^-1 is maintained even at an ultrahigh power density of 3750 W kg^-1. The TNMO-NP supercapacitors show excellent flexibility at various bent (0-180°) states. The capacitive performance of the TNMO-NPs makes them promising cathode materials for flexible supercapacitors with high specific capacities and high energy densities.

Formation of needle-like porous CoNi2S4-MnOOH for high performance hybrid supercapacitors with high energy density

Seeking for suitable electrode materials and designing rational porous structures are great challenges for developing high performance supercapacitors. Herein, needle-like porous CoNi₂S₄-MnOOH (denoted as NCS-MO) were prepared via a simple two steps solvothermal method and used as battery-type electrode of supercapacitor for the first time. Owing to the multiple oxidation states of needle-like porous NCS-MO and the inherent porous structure, the electrode delivers outstanding electrochemical capacitive properties with a high gravimetric specific capacitance of 1267.7 F g^-1 at the scan rate of 1 mV s^-1. To further assess the practical electrochemical performances, we assembled a hybrid supercapacitor using the as-synthesized porous NCS-MO as cathode and active carbon as anode. The device exhibits excellent performance with a high energy density of 47.1 Wh kg^-1 at the power density of 998 W kg^-1 in an extended voltage range of 1.6 V and outstanding cycling stability. These results demonstrate that the needle-like porous NCS-MO could be promising potential electrode material for high performance supercapacitor.

Preparation of Core-Shell CQD@PANI Nanoparticles and Their Electrochemical Properties

In recent years, carbon quantum dots (CQDs) have been extensively investigated in many fields because of their incomparable and unique properties. However, the application of CQDs in the electrochemical field meets a big challenge due to their low specific capacitance. It is very important to improve the electrochemical performance of CQDs. In this study, a facile synthesis method was developed to synthesize CQD@PANI nanoparticles. The CQD@PANI nanoparticles were prepared via a precise quantitative adsorption polymerization method. TEM results showed that the PANI shell thickness could be adjusted by controlling the additive amount of aniline. Cyclic voltammetry and galvanostatic charge/discharge results showed that the electrochemical properties of CQDs have been significantly improved by coating PANI.

Electrochemical Energy Storage Properties of Ni-Mn-Oxide Electrodes for Advance Asymmetric Supercapacitor Application

In this work, we report a facile one-spot synthesis process and the influence of compositional variation on the electrochemical performance of Ni-Mn-oxides (Ni:Mn = 1:1, 1:2, 1:3, and 1:4) for high-performance advanced energy storage applications. The crystalline structure and the morphology of these synthesized nanocomposites have been demonstrated using X-ray diffraction, field emission scanning electron microscopy, and transmission electron Microscopy. Among these materials, Ni-Mn-oxide with Ni:Mn = 1:3 possesses a large Brunauer?Emmett?Teller specific surface area (127 m² g^?1) with pore size 8.2 nm and exhibits the highest specific capacitance of 1215.5 F g^?1 at a scan rate 2 mV s^?1 with an excellent long-term cycling stability (?87.2% capacitance retention at 10 A g^?1 over 5000 cycles). This work also gives a comparison and explains the influence of different compositional ratios on the electrochemical properties of Ni-Mn-oxides. To demonstrate the possibility of commercial application, an asymmetric supercapacitor device has been constructed by using Ni-Mn-oxide (Ni:Mn = 1:3) as a positive electrode and activated carbon (AC) as a negative electrode. This battery-like device achieves a maximum energy density of 132.3 W h kg^?1 at a power density of 1651 W kg^?1 and excellent coulombic efficiency of 97% over 3000 cycles at 10 A g^?1.

Mechanisms and reutilization of modified biochar used for removal of heavy metals from wastewater: A review

Heavy metals (HMs) pose serious threat to both human and environmental health and therefore, effective and low-cost techniques to remove HMs are urgently required. Because HMs are difficult to be biodegraded and transformed, adsorption is a most promising treatment method in recent times. Biochar (BC), a low-cost and sustainable adsorbent material, has recently attracted much research attention due to its broad application prospects. While BC has many merits, it has a lower HMs adsorption efficiency than traditional activated carbon, limiting its practical applications. Furthermore, the HMs retained by BC are difficult to be desorbed, making the used sorbent material hazardous wastes if not well disposed of under natural conditions. Therefore, it is critical to seek effective surface modifications for BC, to improve its ability to HMs removal ability and the recyclability of BC loaded with HMs. This review represents and evaluates the reported modification methods for BC, the corresponding HMs removal mechanisms and the potential for reutilization of BC loaded with HMs. This review provides a basis for the effective practical application of BC in the treatment of HMs containing wastewater.

Specifically Designed Ionic Liquids—Formulations, Physicochemical Properties, and Electrochemical Double Layer Storage Behavior

Two key features—non-volatility and non-flammability—make ionic liquids (ILs) very attractive for use as electrolyte solvents in advanced energy storage systems, such as supercapacitors and Li-ion batteries. Since most ILs possess high viscosity and are less prone to dissolving common electrolytic salts when compared to traditional electrolytic solvents, they must be formulated with low viscosity thinner solvents to achieve desired ionic conductivity and dissolution of electrolyte salts in excess of 0.5 M concentration. In the past few years, our research group has synthesized several specifically designed ILs (mono-cationic, di-cationic, and zwitterionic) with bis(trifluoromethylsulfonyl)imide (TFSI) and dicyanamide (DCA) as counter anions. This article describes several electrolyte formulations to achieve superior electrolytic properties. The performance of a few representative IL-based electrolytes in supercapacitor coin cells is presented.

Iron oxide-based nanomaterials for supercapacitors

As highly efficient and clean electrochemical energy storage devices, supercapacitors (SCs) have drawn widespread attention as promising alternatives to batteries in recent years. Among various electrode materials, iron oxide materials have been widely studied as negative SC electrode materials due to their broad working window in negative potential, ideal theoretical specific capacitance, good redox activity, abundant availability, and eco-friendliness. However, iron oxides still suffer from the problems of low stability and poor conductivity. In this review, recent progress in iron oxide-based nanomaterials, including Fe₂O₃, Fe₃O₄, Fe_xO_y, and FeOOH, as electrode materials of SCs, is discussed. The nanostructure design and various synergistic effects of nanocomposites for improving the electrochemical performance of iron oxides are emphasized. Research on iron oxide-based symmetric/asymmetric SCs is also discussed. Future outlooks regarding iron oxides for SCs are likewise proposed.

Facile synthesis of a nanoporous sea sponge architecture in a binary metal oxide

A novel galvanostatic electrochemical technique has been employed to synthesize a cobalt-nickel mixed oxide, a binary metal oxide, via a two-step route involving electrodeposition followed by calcination. A diaphragm cell was used for the electro-deposition of the binary hydroxide at room temperature in which the electrolyte comprises a nitrate and/or sulphate bath of the corresponding metal ions at pH 4. The electrodeposited product was calcined at 300 °C to obtain the desired oxide material. The formation of the binary metal oxide has been confirmed by X-ray diffraction analysis. The scanning electron microscopy images associated with energy dispersive analysis (EDS) suggest the formation of a nanoporous sea sponge architecture consisting of an interconnected array of nanosheets aligned perpendicular to each other. The elemental mapping analysis of the binary oxide illustrated the uniformity in the distribution of Co and Ni in the composite material. The TEM image shows that binary oxides are nanocrystalline materials. A nitrogen adsorption-desorption study supports the pore size distribution behaviour of the synthesized material. The hybrid capacitor based on the binary metal oxide cathode and activated carbon anode displayed a capacitive behaviour with a capacitance of 76 F g^-1 at a current rate of 2 mA with 98% efficiency after 1000 cycles. Due to the unique interconnected porous network and the role of binary cations, Co-Ni oxide exhibits superior electrochemical behaviour. The synthesis of binary oxides forming various morphologies, such as hexagonal, flower-shape, and sea sponge has been discussed.

Recent Advances and Perspectives of Carbon-Based Nanostructures as Anode Materials for Li-ion Batteries

Rechargeable batteries are attractive power storage equipment for a broad diversity of applications. Lithium-ion (Li-ion) batteries are widely used the superior rechargeable battery in portable electronics. The increasing needs in portable electronic devices require improved Li-ion batteries with excellent results over many discharge-recharge cycles. One important approach to ensure the electrodes' integrity is by increasing the storage capacity of cathode and anode materials. This could be achieved using nanoscale-sized electrode materials. In the article, we review the recent advances and perspectives of carbon nanomaterials as anode material for Lithium-ion battery applications. The first section of the review presents the general introduction, industrial use, and working principles of Li-ion batteries. It also demonstrates the advantages and disadvantages of nanomaterials and challenges to utilize nanomaterials for Li-ion battery applications. The second section of the review describes the utilization of various carbon-based nanomaterials as anode materials for Li-ion battery applications. The last section presents the conclusion and future directions.

Graphene-Graphite Polyurethane Composite Based High-Energy Density Flexible Supercapacitors

Energy autonomy is critical for wearable and portable systems and to this end storage devices with high-energy density are needed. This work presents high-energy density flexible supercapacitors (SCs), showing three times the energy density than similar type of SCs reported in the literature. The graphene-graphite polyurethane (GPU) composite based SCs have maximum energy and power densities of 10.22 µWh cm-2 and 11.15 mW cm-2, respectively, at a current density of 10 mA cm-2 and operating voltage of 2.25 V (considering the IR drop). The significant gain in the performance of SCs is due to excellent electroactive surface per unit area (surface roughness 97.6 nm) of GPU composite and high electrical conductivity (0.318 S cm-1). The fabricated SCs show stable response for more than 15 000 charging/discharging cycles at current densities of 10 mA cm-2 and operating voltage of 2.5 V (without considering the IR drop). The developed SCs are tested as energy storage devices for wide applications, namely: a) solar-powered energy-packs to operate 84 light-emitting diodes (LEDs) for more than a minute and to drive the actuators of a prosthetic limb; b) powering high-torque motors; and c) wristband for wearable sensors.

Size-dependent stability of ultra-small α-/β-phase tin nanocrystals synthesized by microplasma

Nanocrystals sometimes adopt unusual crystal structure configurations in order to maintain structural stability with increasingly large surface-to-volume ratios. The understanding of these transformations is of great scientific interest and represents an opportunity to achieve beneficial materials properties resulting from different crystal arrangements. Here, the phase transformation from α to β phases of tin (Sn) nanocrystals is investigated in nanocrystals with diameters ranging from 6.1 to 1.6 nm. Ultra-small Sn nanocrystals are achieved through our highly non-equilibrium plasma process operated at atmospheric pressures. Larger nanocrystals adopt the β-Sn tetragonal structure, while smaller nanocrystals show stability with the α-Sn diamond cubic structure. Synthesis at other conditions produce nanocrystals with mean diameters within the range 2–3 nm, which exhibit mixed phases. This work represents an important contribution to understand structural stability at the nanoscale and the possibility of achieving phases of relevance for many applications.

Key features of tin, including electronic band structure and opto-electronic properties, are influenced by the crystal structure. Here the authors report a microplasma process for the synthesis of ultra-small tin nanocrystals in which the crystal structure is dependent on crystallite size.

High-Performance Asymmetric Electrochromic-Supercapacitor Device Based on Poly(indole-6-carboxylicacid)/TiO2 Nanocomposites

A difunctional porous network of poly(indole-6-carboxylicacid) (PICA)/TiO₂ nanocomposites is first prepared using TiO₂ nanorod arrays as the scaffold. Because of the synergistic effect of PICA and TiO₂, the nanocomposites show good electrochemical performance, a high specific capacitance value (23.34 mF cm^-2), and excellent galvanostatic charge-discharge stability. Meanwhile, this nanocomposite can be reversibly switched (yellow, green, brown) with a high coloration efficiency (124 cm² C^-1). An asymmetric electrochromic-supercapacitor device (ESD) is also constructed using the PICA/TiO₂ nanocomposites as the anode material and poly(3,4-ethylenedioxythiophene) as the cathode material. This ESD has robust cycle stability and a high specific capacitance value (9.65 mF cm^-2), which can be switched from light green to dark blue. After charging, the device can light up a single LED for 108 s, and the energy storage level can also be monitored by the corresponding color changes. This constructed ESD will have great potential applications in intelligent energy storage and other smart electronic fields.

3D flower-like binary nickel cobalt oxide decorated coiled carbon nanotubes directly grown on nickel nanocones and binder-free hydrothermal carbons for advanced asymmetric supercapacitors

The development of high performance supercapacitors with high energy densities without sacrificing power densities has always been at the leading edge of the emerging field of renewable energy. Herein, the design and fabrication of innovative high performance binder-free electrodes consisting of coiled carbon nanotubes (CNTs) and biomass-derived hydrothermal carbon spheres (HTCSs) as, respectively, positive and negative electrodes is reported. High performance asymmetric supercapacitors (ASCs) were developed using novel 3D core/shell-like binary Ni-Co oxide (NCO) decorated coiled CNTs directly grown on Ni nano-cone arrays (NCAs) and HTCSs directly deposited on NCAs. Novel 3D structures of NCAs were synthesized via a facile and scalable cathodic electrodeposition route and coiled CNTs were directly grown on them by catalytic chemical vapour deposition (CVD) followed by a facile hydrothermal method to integrally decorate the coiled CNTs/NCAs by 3D flower-like NCO. A one-pot hydrothermal method is also used to direct the synthesis of biomass-derived HTCSs on NCAs to fabricate a novel binder-free negative electrode. The ASC based on NCO@coiled CNTs/NCAs//HTCSs/NCAs not only exhibits superior energy density (72.5 W h kg-1) at a reasonable power density of 1.4 kW kg-1, but also represents remarkable cycling durability (retaining almost over 85% of its initial capacitance after 5000 charge-discharge cycles). The fabricated ASC, therefore, seems to be a potent candidate for practical applications in future high performance energy storage systems.

Scalable nanohybrids of graphitic carbon nitride and layered NiCo hydroxide for high supercapacitive performance

The limited number of edge nitrogen atoms and low intrinsic electrical conductivity hinder the supercapacitive energy storage applications of the nitrogen-rich graphitic carbon nitride (g-C₃N₄). In this study, a novel graphitic carbon nitride/NiCo-layered double hydroxide (CNLDH), a two-dimensional nanohybrid, is prepared by a simple hydrothermal synthesis. The homogeneous interpolation of g-C₃N₄ nanosheets into NiCo LDH stacked nanosheets effectively increases the overall performances of the g-C₃N₄/NiCo LDH nanohybrid. The improved morphology of the nanohybrid electrode upon the addition of g-C₃N₄ to the NiCo LDH yields a specific capacity of 183.43 mA h g⁻¹ in 6 M KOH at 1 A g⁻¹, higher than those of bare g-C₃N₄ (20.89 mA h g⁻¹) and NiCo LDH (95.92 mA h g⁻¹) electrodes. The excellent supercapacitive performance of the CNLDH nanohybrid is complemented by its low internal resistance, excellent rate capability, and large cycling lifetime. Furthermore, the hybrid supercapacitor is assembled using CNLDH 0.1 as a positive electrode and activated carbon (AC) as a negative electrode. The hybrid supercapacitor device of CNLDH 0.1//AC shows the maximum specific capacity of 37.44 mA h g⁻¹ at 1 A g⁻¹ with remarkable energy density, power density and good cycling performance. This confirms that the CNLDH 0.1 nanohybrid is an excellent electrode material for supercapacitor applications.

A two dimensional CNLDH 0.1 nanohybrid supercapacitor electrode prepared by simple hydrothermal hybridization of g-C₃N₄ and NiCO LDH shows the maximum specific capacity of 183.43 mA h g⁻¹ with remarkable electrochemical performance.

Towards establishing standard performance metrics for batteries, supercapacitors and beyond

Electrochemical energy storage (EES) materials and devices should be evaluated against clear and rigorous metrics to realize the true promises as well as the limitations of these fast-moving technologies.

Controllable synthesis and electrochemical capacitor performance of MOF-derived MnOx/N-doped carbon/MnO2 composites

Different amount of carbon and nitrogen, for MOF-derived nitrogen-doped carbon/Mn₃O₄ composites, can result in the discrepancies of synergistic effect which plays an important role in final electrochemical performance.

In situ K2S activated electrospun carbon nanofibers with hierarchical meso/microporous structures for supercapacitors

Porous electrospun carbon nanofibers (CNFs) can be produced by a more advantageous ‘in situ activation’ method by electrospinning polyacrylonitrile (PAN) with an activation agent. However, most in situ activated electrospinning processes yield porous CNFs with rather limited surface area and less porosity due to the inappropriately selected activation agents. Here we found K₂S could perfectly meet both compatibility and reactivity requirements of PAN electrospinning to generate hierarchical meso/micropores inside electrospun CNFs. During the whole fabrication process, K₂S experiences a phase evolution loop and the hierarchical pore structures are formed by the reaction between K₂S oxidative derivatives and the as-formed carbon during heat treatment. The hierarchical meso/microporous CNFs not only showed a large surface area (835.0 m² g⁻¹) but also exhibited a high PAN carbonization yield (84.0 wt%) due to improved cyclization of PAN's nitrile group during the pre-oxidation stage. As an electrode material for supercapacitors, the corresponding electrodes have a capacitance of 210.7 F g⁻¹ at the current density of 0.2 A g⁻¹ with excellent cycling durability. The hierarchically porous CNFs produced via in situ activation by K₂S combine the advantages of interconnected meso/micropores and are a promising candidate for electrochemical energy conversion and storage devices.

K₂S was found to be an excellent in situ activation agent for the fabrication of electrospun carbon nanofibers with large surface area (835.0 m² g⁻¹) and hierarchical meso/microporous structures.

Silicon quantum dot-assisted synthesis of MoS2/rGO sandwich structures with excellent supercapacitive performance

A MoS₂/rGO nanocomposite with a unique sandwich structure is synthesized by using silicon quantum dots (SiQDs), exhibiting excellent electrochemical performance for supercapacitors.

Calcined chicken eggshell electrode for battery and supercapacitor applications

Biowaste eggshell can be used as a cathode while in its calcined form and it is found to be suitable as an anode in an electrochemical cell.