Recent advances in metal oxide-based electrode architecture design for electrochemical energy storage

α(β)-PbO2 doped with Co3O4 and CNT porous composite materials with enhanced electrocatalytic activity for zinc electrowinning

The high energy consumption during zinc electrowinning is mainly caused by the high overpotential of the oxygen evolution for Pb–Ag alloys with strong polarization. The preparation of new active energy-saving materials has become a very active research field, depending on the synergistic effects of active particles and active oxides. In this research, a composite material, α(β)-PbO₂, doped with Co₃O₄ and CNTs on the porous Ti substrate was prepared via one-step electrochemical deposition and the corresponding electrochemical performance was investigated in simulated zinc electrowinning solution. The composite material showed a porous structure, finer grain size and larger electrochemical surface area (ECSA), which indicated excellent electrocatalytic activity. Compared with the Pb–0.76 wt% Ag alloy, the overpotential of oxygen evolution for the 3D-Ti/PbO₂/Co₃O₄–CNTs composite material was decreased by about 452 mV under the current density of 500 A m⁻² in the simulated zinc electrowinning solution. The decrease in the overpotential of oxygen evolution was mainly ascribed to the higher ECSA and lower charger transfer resistance. Moreover, it showed the lowest self-corrosion current density of 1.156 × 10⁻⁴ A cm⁻² and may be an ideal material for use in zinc electrowinning.

3D-Ti/PbO₂–Co₃O₄–CNTs composite electrode was fabricated through galvanostatic electrodepositon, which shows outstanding electrocatalytic activity to OER in harsh media (50 g L⁻¹ Zn²⁺ + 150 g L⁻¹ H₂SO₄).

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.

Hierarchical core-shell electrode with NiWO4 nanoparticles wrapped MnCo2O4 nanowire arrays on Ni foam for high-performance asymmetric supercapacitors

Rational construction of MnCo₂O₄-based core-shell nanomaterials with distinctive and desirable architectures possesses great potential in the advanced electrode material of high-performance supercapacitors. Here, a new class of hierarchical core-shell nanowire arrays (NWAs) with a shell of NiWO₄ nanoparticles and a core of MnCo₂O₄ nanowires is reported, which can significantly improve the electrochemical energy storage properties of supercapacitors. The unique core-shell structure endows the MnCo₂O₄@NiWO₄ NWAs electrode with a high areal specific capacitance of 5.09 F cm^-2 at a current density of 1 mA cm^-2 and a superior cyclic retention of 96% after 5000 charge-discharge cycles, which are more preferable than those of MnCo₂O₄ NWAs electrode. More importantly, an aqueous electrochemical energy storage device (core-shell MnCo₂O₄@NiWO₄ NWAs as the positive electrode and active carbon as the negative electrode, MnCo₂O₄@NiWO₄//AC ASC) was assembled and shows a high energy density of 0.23 mWh cm^-2 at a power density of 2.66 mW cm^-2, and 0.09 mWh cm^-2 at 16.00 mW cm^-2, indicating hopeful potential for practical applications. This work highlights the significance of NiWO₄ as a shell for hierarchical core-shell nanostructures, which can further improve the electron transport characteristic of the electrode material, thereby achieving performance breakthroughs in energy storage devices.

A novel synthesis towards a vanadium pentoxide porous nanodisk film as a cathode material for advanced Li-ion hybrid capacitors

A V2O5 porous nanodisk thin film is synthesized through a simple hydrothermal and subsequent VO2 template oxidation strategy. For the first time, V2O5 is employed as a cathode rather than an anode to construct lithium-ion hybrid capacitors. This design effectively utilizes the intrinsic layered structure of V2O5 for facile Li+ intercalation and facilitates the charge balance with the capacitive electrode, enabling superior performance of the device.

Photopatterning and Electrochemical Energy Storage Properties of an On-Chip Organic Radical Microbattery

One potential way to fabricate battery-on-chip is photopatterning electrochemical energy storage materials directly on electronics through lithography, but applicable materials are primarily limited to transparent photocurable resins. The transparency of the photoresist would be sacrificed after extra addition of insoluble inorganic battery materials and conductors. Given the importance of radical polymers for their appropriate solubility, optical transparency, and radical robustness, they may have potential application in on-chip energy storage, transport, and conversion devices. Herein, an anodic photoresist is proposed by modifying the MicroChem SU8 resist with a radical polymer poly(2,2,6,6-tetramethyl-4-piperidinyl-N-oxyl methacrylate) and an ionic conductor lithium perchlorate. It can be photopatterned on silicon wafer with 10 μm scale resolution, and it exhibits charge/discharge potentials at ca. 0.68 V versus silver chloride electrode; the coulomb efficiency is regarded as nearly equaling 100%. Although the specific capacity of the photopatterned film electrode is found to be modest, 1 × 10^-5 mA h·cm^-2, it presents 1/8 of its theoretical electron storage ability. All-solid-state half-cells with circular features 30 μm in diameter are prepared by means of overlay exposure using the as-prepared photoresist and lithium perchlorate-modified SU8 as the anodic electrode and solid electrolyte, respectively. These results suggest a promising way of using radical polymers for the integration of electrochemical energy in microelectronics.

Hybrid polyoxometalates as post-functionalization platforms: from fundamentals to emerging applications

Polyoxometalates (POMs) represent an important group of metal-oxo nanoclusters, typically comprised of early transition metals in high oxidation states (mainly V, Mo and W). Many plenary POMs exhibit good pH, solvent, thermal and redox stability, which makes them attractive components for the design of covalently integrated hybrid organic-inorganic molecules, herein referred to as hybrid-POMs. Until now, thousands of organic hybrid-POMs have been reported; however, only a small fraction can be further functionalized using other organic molecules or metal cations. This emerging class of 'post-functionalizable' hybrid-POMs constitute a valuable modular platform that permits coupling of POM properties with different organic and metal cation functionalities, thereby expanding the key physicochemical properties that are relevant for application in (photo)catalysis, bioinorganic chemistry and materials science. The post-functionalizable hybrid-POM platforms offer an opportunity to covalently link multi-electron redox responsive POM cores with virtually any (bio)organic molecule or metal cation, generating a wide range of materials with tailored properties. Over the past few years, these materials have been showcased in the preparation of framework materials, functional surfaces, surfactants, homogeneous and heterogeneous catalysts and light harvesting materials, among others. This review article provides an overview on the state of the art in POM post-functionalization and highlights the key design and structural features that permit the discovery of new hybrid-POM platforms. In doing so, we aim to make the subject more comprehensible, both for chemists and for scientists with different materials science backgrounds interested in the applications of hybrid (POM) materials. The review article goes beyond the realms of polyoxometalate chemistry and encompasses emerging research domains such as reticular materials, surfactants, surface functionalization, light harvesting materials, non-linear optics, charge storing materials, and homogeneous acid-base catalysis among others.

Thermally reduced fluorographenes as efficient electrode materials for supercapacitors

There is an urgent need for a simple and up-scalable method for the preparation of supercapacitor electrode materials due to increasing global energy consumption worldwide. We have discovered that fluorographene exhibits great potential for the development of new kinds of supercapacitors aimed at practical applications. We have shown that time control of isothermal reduction of fluorographite at 450 °C under a hydrogen atmosphere led to the fine-tuning of fluorine content and electronic properties of the resulting fluorographene derivatives. Charge transfer resistances (R_ct) of the thermally reduced fluorographenes (TRFGs) were decreased with respect to the pristine fluorographene; however, the R_ctvs. time-of-reduction plot showed a v-shaped profile. The specific capacitance vs. time-of-reduction of TRFG followed the v-shaped trend, which could be the result of the decreasing content of sp³ carbons and increasing content of structural defects. An optimized material exhibited values of specific capacitance up to 539 F g^-1 recorded at a current density of 0.25 A g^-1 and excellent cycling durability with 100% specific capacitance retention after 1500 cycles in a three-electrode configuration and 96.7% of specific capacitance after 30 000 cycles in a two-electrode setup.

A hierarchical NiCo2S4 honeycomb/NiCo2S4 nanosheet core-shell structure for supercapacitor applications

Transition metal sulphides are becoming one of the promising materials for energy storage applications. Particularly, an advanced electrode material architecture, which gives favourable electronic and ionic conductivity, is highly in demand. Herein, a hierarchical NiCo₂S₄ honeycomb/NiCo₂S₄ nanosheet core-shell structure is reported for supercapacitor applications. The core-shell structure was in situ grown on a nickel foam via two consecutive hydrothermal processes, followed by an electrochemical deposition process. Moreover, we tuned the deposition cycle to get abundant active sites with gaps of suitable sizes between the walls of the honeycomb structure for efficient electrolyte diffusion routes. The 3D honeycomb core structure was used as superhighway for electron transport to the current collector, while the ultrathin shell structure offered a large surface area with short electron and ion diffusion paths, thus leading to the faster kinetics and higher utilization of active materials. Thus, using the synergistic advantages of the core material and the shell material, the as-synthesized optimized electrode material came up with an excellent specific capacitance of 17.56 F cm^-2 at a current density of 5 mA cm^-2 and the highest cycling stability of 88.2% after 5000 cycles of charge-discharge process. Such advanced electrode architectures are highly promising for the future electrode materials.

A Rapid Synthesis of Mesoporous Mn2O3 Nanoparticles for Supercapacitor Applications

Mn2O3 nanomaterials have been recently composing a variety of electrochemical systems like fuel cells, supercapacitors, etc., due to their high specific capacitance, low cost, abundance and environmentally benign nature. In this work, mesoporous Mn2O3 nanoparticles (NPs) were synthesized by manganese acetate, citric acid and sodium hydroxide through a hydrothermal process at 150 °C for 3 h. The synthesized mesoporous Mn2O3 NPs were thoroughly characterized in terms of their morphology, surfaces, as well as their crystalline, electrochemical and electrochemical properties. For supercapacitor applications, the synthesized mesoporous Mn2O3 NP-based electrode accomplished an excellent specific capacitance (Csp) of 460 F·g−1 at 10 mV·s−1 with a good electrocatalytic activity by observing good electrochemical properties in a 6 M KOH electrolyte. The excellent Csp might be explained by the improvement of the surface area, porous surface and uniformity, which might favor the generation of large active sites and a fast ionic transport over the good electrocatalytic surface of the Mn2O3 electrode. The fabricated supercapacitors exhibited a good cycling stability after 5000 cycles by maintaining ~83% of Csp.

Facile Synthesis of Novel V0.13Mo0.87O2.935 Nanowires With High-Rate Supercapacitive Performance

Binary metal oxides composed of molybdenum–vanadium oxides are promising candidates for supercapacitors. Here, we report the synthesis of one-dimensional V_0.13Mo_0.87O_2.935 nanowires through a facile one-step hydrothermal method. This nanowire presented a high specific capacitance of 394.6 F g⁻¹ (1 mV s⁻¹) as an electrode applied to the supercapacitor. Importantly, this electrode showed a perfect rate capability of 91.5% (2 to 10 A g⁻¹) and a continuous verified outstanding cyclic voltammetry of 97.6% after 10,000 cycles. These superior electrochemical properties make the synthesized V_0.13Mo_0.87O_2.935 nanowires a prospective candidate for high-performance supercapacitors.

Inception of molybdate as a "pore forming additive" to enhance the bifunctional electrocatalytic activity of nickel and cobalt based mixed hydroxides for overall water splitting

Development of low-cost transition metal based electrocatalysts on inexpensive substrates for overall water splitting is essential to meet the future energy storage demand. In this article, we have synthesized a molybdate incorporated nickel cobalt hydroxide material on Cu mesh with nickel : cobalt : molybdenum in a 13.25 : 21.42 : 1 ratio and the electrode has shown excellent bifunctional electrocatalytic activity as it demonstrates overpotentials as low as 290 mV and 125 mV to reach 10 mA cm-2geo for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), respectively (after both iR and capacitance correction). Control studies with fourteen other nickel-cobalt based hydroxides and rigorous post-catalytic analysis suggested that though molybdate was not the active catalytic centre, it played a pivotal role in enhancing the activity of the material as - (i) it significantly improved the surface area and porosity of the as-synthesized material and (ii) owing to its continuous etching during electrochemical testing, it was found to increase the accessibility of electrochemically active catalytic sites lying in the bulk. Thus, molybdate acts as a "pore forming additive" during both synthesis and electrochemical treatment. Furthermore, the combination of nickel and molybdate helped in the formation of a 2D-sheet like morphology which in turn improves accessibility to catalytically active centres. In addition, the Cu mesh substrate notably lowers the charge transfer resistance. To the best of our knowledge, this is the first ever report of molybdate as a "pore forming additive" and will enthuse the designing of electrocatalytic materials with enhanced performance based on this strategy.

A highly alkaline-stable metal oxide@metal–organic framework composite for high-performance electrochemical energy storage

Most metal–organic frameworks (MOFs) hardly maintain their physical and chemical properties after exposure to alkaline aqueous solutions, thus precluding their use as potential electrode materials for electrochemical energy storage devices. Here, we present the design and synthesis of a highly alkaline-stable metal oxide@MOF composite, Co3O4 nanocube@Co-MOF (Co3O4@Co-MOF), via a controllable and facile one-pot hydrothermal method under highly alkaline conditions. The obtained composite possesses exceptional alkaline stability, retaining its original structure in 3.0 M KOH for at least 15 days. Benefitting from the exceptional alkaline stability, unique structure, and larger surface area, the Co3O4@Co-MOF composite shows a specific capacitance as high as 1020 F g−1 at 0.5 A  g−1 and a high cycling stability with only 3.3% decay after 5000 cycles at 5 A g−1. The as-constructed solid-state flexible device exhibits a maximum energy density of 21.6 mWh cm−3.

Encapsulating NiCo2O4 inside metal-organic framework sandwiched graphene oxide 2D composite nanosheets for high-performance lithium-ion batteries

Ternary transition metal oxides are promising candidates for developing high-performance lithium-ion batteries. In the present investigation, we explored sandwiched composite nanosheets by encapsulating NiCo2O4 nanoparticles inside the pores of ZIF-67 crystals that were in situ grown on both surfaces of graphene oxide (GO). SEM and TEM observations confirmed the successful construction of the sophisticated architecture. For the designed electrode structure, the scaffold of GO provided a fast conductive highway for the encapsulated NiCo2O4 nanoparticles, while the porous and elastic framework of ZIF-67 together with the flexible GO guaranteed efficient accommodation to the volumetric change of NiCo2O4. Moreover, the highly porous composite nanosheets are suitable for electrolyte infiltration, with enhanced ionic transportation kinetics. Accordingly, the reversible capacity of NiCo2O4@ZIF-67/GO was high up to 1025 mA h g-1 and 740 mA h g-1 after 80 cycles at 0.5 and 2.0 A g-1, respectively. At the current density of 4.0 and 8.0 A g-1, the capacity was still retained at 500 and 320 mA h g-1, respectively. Other analyses further manifested that the distinctive structure of NiCo2O4@ZIF-67/GO enhanced the charge transportation kinetics in comparison with the control sample of NiCo2O4@ZIF-67. Our strategy provided a new concept for developing high-performance electrode materials of lithium-ion batteries.

In situ construction of dual-morphology ZnCo2O4 for high-performance asymmetric supercapacitors

In this study, the controllable preparation of ZnCo₂O₄ with different morphologies in a reaction system and the orderly weaving of these morphologies into special structures was demonstrated, which might be impossible to achieve using other methods; herein, we successfully prepared a dual-morphology ZnCo₂O₄/N-doped reduced graphene oxide/Ni foam substrate (ZNGN) electrode by ultrasonic processing, a one-step hydrothermal method and a subsequent annealing process for high-performance supercapacitors. At first, ZnCo₂O₄ nanosheet orderly formed a honeycomb structure on the surface of Ni foam (NF); this improved the redox surface area of the electrode; then, feather-like ZnCo₂O₄ was evenly distributed over the honeycomb structure, playing the role of containment and fixation to provide space for material volume expansion during charging and discharging. The electrochemical test showed that the maximum capacitance of the ZNGN electrode was 1600 F g^-1 (960C g^-1) at the current density of 1 A g^-1 in a 6 M KOH solution. Moreover, the asymmetric supercapacitor ZNGN//activated carbon (ZNGN//AC) displayed the excellent energy density of 66.1 W h kg^-1 at the power density of 701 W kg^-1. Compared with the capacitance (233.3 F g^-1 and 326.6C g^-1) when ZNGN//AC was fully activated at 4 A g^-1, there was almost no loss in capacitance after 2000 charge-discharge cycles, and a 94% capacitance retention was achieved after 5000 cycles. Thus, this excellent electrochemical property highlights the potential application of the dual-morphology ZnCo₂O₄ electrode in supercapacitors.

Synthesis of Cubic Ni(OH)2 Nanocages Through Coordinating Etching and Precipitating Route for High-Performance Supercapacitors

Rational design of cage-like structure is an effective method for the improvement of the capacitive performance of transition metal hydroxides. In this work, cubic Ni(OH)2 nanocages (Ni(OH)2 NCs) were constructed through a coordinating etching and precipitating (CEP) route. Ni(OH)2 NCs possess abundant active sites, sufficient diffusion channels, and accelerated electron transfer rate, which are beneficial for electrochemical kinetics. As a positive electrode for supercapacitors, the Ni(OH)2 NCs/Ni foam (NF) electrode presents a high specific capacitance of 539.8 F g^-1 at 1 A g^-1, which is much larger than that of broken Ni(OH)2 NCs/NF (Ni(OH)2 BNCs/NF, 87.3 F g^-1 at 1 A g^-1). In addition, the Ni(OH)2 NCs/NF electrode still retains 96.9% of its initial specific capacitance after 2000 cycles. The asymmetric supercapacitor (ASC) devices were assembled using Ni(OH)2 NCs/NF and activated carbon (AC)/NF as positive and negative electrodes, respectively. The ASC exhibits a higher energy density of 23.3 Wh kg^-1 at a power density of 800 W kg^-1 compared to Ni(OH)2 BNCs/NF (3 Wh kg^-1 at 880 W kg^-1). These results demonstrate that the Ni(OH)2 NCs/NF electrode presents potential applications in the field of energy storage. The design of cage-like structure paves an effective way to achieve high-performance electrode materials.

Highly Ordered Mesoporous NiCo2O4 as a High Performance Anode Material for Li-Ion Batteries

The controlled synthesis of highly ordered mesoporous structure has attracted considerable attention in the field of electrochemistry because of its high specific surface area which can contribute the transportation of ions. Herein, a general nano-casting approach is proposed for synthesizing highly ordered mesoporous NiCo₂O₄ microspheres. The as-synthesized mesoporous NiCo₂O₄ microsphere materials with high Brunner-Emmett-Teller (BET) surface area (~97.77 m²g⁻¹) and uniform pore size distribution around 4 nm exhibited a high initial discharge capacity of ~1,467 mAhg⁻¹, a good rate capability as well as cycling stability. The superior electrochemical performance was mainly because of the highly porous nature of NiCo₂O₄, which rendered volume expansion during the process of cycling and shortened lithium-ions transport pathways. These properties showcase the inherent potential for use of highly ordered mesoporous NiCo₂O₄ microspheres as a potential anode material for lithium-ion batteries in the future.

Homogeneous Core/Shell NiMoO4@NiMoO4 and Activated Carbon for High Performance Asymmetric Supercapacitor

Here, we report the extraordinary electrochemical energy storage capability of NiMoO₄@NiMoO₄ homogeneous hierarchical nanosheet-on-nanowire arrays (SOWAs), synthesized on nickel substrate by a two-stage hydrothermal process. Comparatively speaking, the SOWAs electrode displays superior electrochemical performances over the pure NiMoO₄ nanowire arrays. Such improvements can be ascribed to the characteristic homogeneous hierarchical structure, which not only effectively increases the active surface areas for fast charge transfer, but also reduces the electrode resistance significantly by eliminating the potential barrier at the nanowire/nanosheet junction, an issue usually seen in other reported heterogeneous architectures. We further evaluate the performances of the SOWAs by constructing an asymmetric hybrid supercapacitor (ASC) with the SOWAs and activated carbon (AC). The optimized ASC shows excellent electrochemical performances with 47.2 Wh/kg in energy density of 1.38 kW/kg at 0–1.2 V. Moreover, the specific capacity retention can be as high as 91.4% after 4000 cycles, illustrating the remarkable cycling stability of the NiMoO₄@NiMoO₄//AC ASC device. Our results show that this unique NiMoO₄@NiMoO₄ SOWA has great prospects for future energy storage applications.

Recent Progress of Electrochemical Energy Devices: Metal Oxide–Carbon Nanocomposites as Materials for Next-Generation Chemical Storage for Renewable Energy

With the importance of sustainable energy, resources, and environmental issues, interest in metal oxides increased significantly during the past several years owing to their high theoretical capacity and promising use as electrode materials for electrochemical energy devices. However, the low electrical conductivity of metal oxides and their structural instability during cycling can degrade the battery performance. To solve this problem, studies on carbon/metal-oxide composites were carried out. In this review, we comprehensively discuss the characteristics (chemical, physical, electrical, and structural properties) of such composites by categorizing the structure of carbon in different dimensions and discuss their application toward electrochemical energy devices. In particular, one-, two-, and three-dimensional (1D, 2D, and 3D) carbon bring about numerous advantages to a carbon/metal-oxide composite owing to the unique characteristics of each dimension.

Facile Chemical Synthesis and Potential Supercapattery Energy Storage Application of Hydrangea-type Bi2MoO6

Soft chemical synthesis is used to obtain a hydrangea-type bismuth molybdate (Bi2MoO6) supercapattery electrode that demonstrates considerable energy/power density and cycling life. Structure and morphology studies, initially, reveal a phase-pure polycrystalline and hydrangea-type surface appearance for Bi2MoO6, which upon testing in an electrochemical energy storage system displays supercapattery behavior, a combination of a supercapacitor and a battery. From the power law, an applied-potential-dependent charge storage mechanism is established for the Bi2MoO6 electrode material. A Trasatti plot evidences the presence of inner and outer surface charges. The hydrangea-type Bi2MoO6 electrode demonstrates a specific capacitance of 485 F g-1 at 5 A g-1 and a stability of 82% over 5000 cycles. An assembled symmetric supercapattery with a Bi2MoO6//Bi2MoO6 configuration demonstrates energy and power densities of 45.6 W h kg-1 and 989 W kg-1, respectively. A demonstration elucidating the lighting up of three light-emitting diodes, connected in series, by the symmetric supercapattery signifies the practical potentiality of the as-synthesized hydrangea-type Bi2MoO6 electrode in energy storage devices.

Dehydration of Alginic Acid Cryogel by TiCl4 vapor: Direct Access to Mesoporous TiO2 @C Nanocomposites and Their Performance in Lithium-Ion Batteries

A new strategy for the synthesis of mesoporous TiO₂ @C nanocomposites through the direct mineralization of seaweed-derived alginic acid cryogel by TiCl₄ through a solid/vapor reaction pathway is presented. In this synthesis, alginic acid cryogel can have multiple roles; i) mesoporous template, ii) carbon source, and iii) oxygen source for the TiO₂ precursor, TiCl₄ . The resulting TiO₂ @alginic acid composite was transformed either into pure mesoporous TiO₂ by calcination or into mesoporous TiO₂ @C nanocomposites by pyrolysis. By comparing with a nonporous TiO₂ @C composite, the importance of the mesopores on the performance of electrodes for lithium-ion batteries based on mesoporous TiO₂ @C composite was clearly evidenced. In addition, the carbon matrix in the mesoporous TiO₂ @C nanocomposite also showed electrochemical activity versus lithium ions, providing twice the capacity of pure mesoporous TiO₂ or alginic acid-derived mesoporous carbon (A600). Given the simplicity and environmental friendliness of the process, the mesoporous TiO₂ @C nanocomposite could satisfy the main prerequisites of green and sustainable chemistry while showing improved electrochemical performance as a negative electrode for lithium-ion batteries.

A binder-free electrode architecture design for lithium-sulfur batteries: a review

Lithium-sulfur batteries (LSBs) are considered to be one of the most promising next-generation electrochemical power sources to replace commercial lithium-ion batteries because of their high energy density. However, practical application of LSBs is hindered by two critical drawbacks: "redox shuttle reactions" of dissolved polysulfides at the cathode side and Li dendrites at the Li anode side. Therefore, various approaches have been proposed to break down technical barriers in LSB systems. The overall device performance of LSBs depends on not only the development of host materials but also the superior architecture design of electrodes. Among these architectures, binder-free electrodes are verified to be one of the most effective structural designs for high-performance LSBs. Therefore, it is urgent to review recent advances in binder-free electrodes for promoting the fundamental and technical advancements of LSBs. Herein, recently emergent studies using various binder-free architectures in sulfur cathodes and lithium metal anodes are reviewed. These binder-free electrodes, with well-interconnected structures and abundant structural space, can provide a continuous pathway for fast/uniform electron transport/distribution, load sufficient active materials for ensuring high energy density, and afford large electrochemically active surface areas where electrons and Li ions can come into contact with the active materials for fast conversion reactions, thus leading to suitable applications for LSBs. Subsequently, the advantages and challenges of binder-free architectures are discussed from several recently emergent studies using binder-free structured sulfur cathodes or Li metal anodes. The future prospects of LSBs with binder-free electrode structure designs are also discussed.

Unusual formation of NiCo2O4@MnO2/nickel foam/MnO2 sandwich as advanced electrodes for hybrid supercapacitors

A facile three-step method is designed for large-scale preparation of a NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwich architecture with robust adhesion as an advanced electrode for high-performance supercapacitors. The synthesis contains the hydrothermal reaction of a cobalt-nickel hydroxide precursor on a nickel foam (NF) support and subsequent thermal conversion into spinel mesoporous NiCo₂O₄ nanowire arrays, followed by a hydrothermal oxidation reaction to synthesize NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches. Moreover, the tactics reported in this study enable easy control of the growth of NiCo₂O₄ on one side of the NF and MnO₂ nanosheets on both sides of the NF to obtain novel NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches. Because of the unusual structural and compositional features, the obtained NiCo₂O₄@MnO₂/nickel foam/MnO₂ sandwiches manifest excellent performance with high specific capacitance (1.70 C cm^-2 at 2 mA cm^-2), exceptional rate capability (78.5% retention at 20 mA cm^-2) and ultralong cycling stability (91% retention over 30 000 cycles at 20 mA cm^-2) as a battery-type electrode material for supercapacitors. When further assembled into an aqueous hybrid supercapacitor, it can deliver an energy density of 53.5 W h kg^-1 at a power density of 80 W kg^-1 and 20.7 W h kg^-1 at 8 kW kg^-1. This novel sandwich electrode provides a new idea for improving the electrochemical performance of hybrid supercapacitors.

Oxygen-Vacancy-Tunable Electrochemical Properties of Electrodeposited Molybdenum Oxide Films

Molybdenum oxides have been widely studied in recent years, owing to their electrochromic properties, electrocatalytic activities for hydrogen evolution reactions (HERs) and excellent energy storage performance. These characteristics strongly depend on the valence states of Mo in the oxides such as IV, V, and VI, which can be efficiently altered through oxygen deficiencies within the oxides. Here, we present a colloidal electrodeposition method to introduce oxygen vacancies in such Mo oxide films. We prepared uniform MoO _x films and investigated their electrochemical characteristics under different valence states IV, V, and VI. In this paper, we demonstrate that MoO_{2+ x} films, where Mo in valence states IV and V, can be used for high-performance supercapacitor electrodes. Due to their high conductivity, they exhibit an areal capacitance of 89 mF cm^-2 at 1 mA cm^-2 and negligible capacitance loss within 600 cycles. Additionally, we demonstrate that, in a complementary electrochromic device configuration, the introduction of an MoO_{2+ x} counter electrode remarkably lowers the activation potential of WO₃ from -2 to -0.5 V and achieves a fully bleached state at 0.5 V. These properties make the MoO_{2+ x} film an ideal counter electrode to store ions for an electrochromic device. Furthermore, MoO_{3- y} films, where Mo in the valence states V and VI, are obtained by annealing the electrodeposited MoO_{2+ x} film under 200 °C for 24 h. Such films exhibit an excellent catalytic for the HER with an overpotential of 201 mV. Furthermore, we show that MoO₃ films, where Mo at its highest oxidation state (VI), can be obtained via annealing the MoO_{2+ x} film at 300 °C for 6 h, and the resulting films exhibit battery characteristics. Our research provides a new and facile strategy to fabricate substoichiometric molybdenum oxide nanofilms and reveals the effect of different valences on the electrochemical performance of molybdenum oxide films, which opens new doorways for future research in the electrochemical applications of transition metal oxides.

Steam Selective Etching: A Strategy to Effectively Enhance the Flexibility and Suppress the Volume Change of Carbonized Paper-Supported Electrodes

Paper-supported electrodes with high flexibility have attracted much attention in flexible Li-ion batteries. However, they are restricted by the heavy inactive paper substrate and large volume change during the lithiation-delithiation process, which will lead to low capacity and poor rate capability and cyclability. Converting the paper substrate to carbon fiber by carbonization can substantially eliminate the "dead mass", but it becomes very brittle. This study reports a water-steam selective etching strategy that successfully addresses these problems. With the help of steam etching, pores are created, and transition-metal oxides are embedded into the fiber. These effectively accommodate the volume change and enhances the kinetics of ion and electron transport. The pores release the mechanical stress from bending, ensuring the sufficient bendability of carbonized paper. Benefiting from these merits, the steam-etched samples show high flexibility and possess outstanding electrochemical performance, including ultra-high capacity and superior cycling stability with capacity retention over 100% after 1500 cycles at 2 A g^-1. Furthermore, a flexible Li-ion full battery using the steam-etched Fe₂O₃@CNF anode and LiFePO₄/steam-etched CNF cathode delivers a high capacity of 623 mAh g^-1 at 100 mA g^-1 and stable electrochemical performances under the bent state, holding great promise for next-generation wearable devices.

Scalable Wire-Type Asymmetric Pseudocapacitor Achieving High Volumetric Energy/Power Densities and Ultralong Cycling Stability of 100 000 Times

Wire-shaped asymmetric pseudocapacitors with both pseudocapacitive cathode and anode are promising in facilitating device assembly and provide highly efficient power sources for wearable electronics. However, it is a great challenge to simultaneously obtain high energy and power as well as ultralong cycling life for practical demands of such devices. Herein, a device design with new cathode/anode coupling is proposed to achieve excellent comprehensive performance in a wire-type quasi-solid-state asymmetric pseudocapacitor (WQAP). The hierarchical α-MnO2 nanorod@δ-MnO2 nanosheet array cathode and MoO2@C nanofilm anode are directly grown on flexible tiny Ti wires by well-established hydrothermal and electrodeposition techniques, which ensures rapid charge/mass transport kinetics and the sufficient utilization of pseudocapacitance. The nanoarray/film electrode also facilitates integration with gel electrolyte of polyvinyl alcohol-LiCl, guaranteeing the durability. The resulting WQAP with 2.0 V voltage delivers high volumetric energy and power densities (9.53 mWh cm-3 and 22720 mW cm-3, respectively) as well as outstanding cycling stability over 100 000 times, surpassing all the previously reported WQAPs. In addition, the device can be facilely connected in parallel or in series with minimal internal resistance, and be fabricated at the 1 m scale with excellent flexibility. This work opens the way to develop high-performance integrated wire supercapacitors.

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.