Flexible Ti‐Doped FeOOH Quantum Dots/Graphene/Bacterial Cellulose Anode for High‐Energy Asymmetric Supercapacitors

Ti‐doped FeOOH quantum dots (QD) decorated on graphene (GN) sheets are designed and fabricated by a facile and scalable synthesis route. Importantly, the Ti‐doped FeOOH QD/GN are successfully dispersed within bacterial cellulose (BC) substrate as bending anode with large loading mass for flexible supercapacitor. By virtue of its favorable architecture, this composite electrode exhibits a remarkable areal capacitance of 3322 mF cm^?2 at 2 mA cm^?2, outstanding cycle performance (94.7% capacitance retention after 6000 cycles), and excellent mechanical strength (68.7 MPa). To push the energy density of flexible supercapacitors, the optimized asymmetric supercapacitor using Mn₃O₄/GN/BC as positive electrode and Ti‐doped FeOOH QD/GN/BC as negative electrode can be cycled reversibly in the operating voltage range of 0–1.8 V and displays ultrahigh areal energy density of 0.541 mWh cm^?2, ultrahigh volumetric energy density of 9.02 mWh cm^?3, reasonable cycling performance (9.4% decay in specific capacitance after 5000 cycles), and good capacitive retention at bending state.     

Co3O4 Hollow Polyhedrons as Bifunctional Electrocatalysts for Reduction and Evolution Reactions of Oxygen

It is very important to exploit low‐cost and efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) electrocatalysts for the development of renewable‐energy conversion and storage techniques. Although much attention has been made to develop efficient catalysts for ORR and OER, it is still highly desired to create new bifunctional catalysts. In this study, Co₃O₄ hollow polyhedrons are synthesized as efficient bifunctional electrocatalysts for ORR and OER by simple one‐step annealing Co‐centered metal–organic frameworks (ZIF‐67). Due to the large specific surface areas and high porosity, the as‐prepared Co₃O₄ hollow polyhedrons exhibit excellent electrocatalytic activities for ORR and OER in alkaline media. Co₃O₄ hollow polyhedrons show higher peak current density (0.61 mA cm^?2) with four‐electron pathway than Co₃O₄ particles (0.39 mA cm^?2), better methanol tolerance and superior durability (82.6%) than commercial Pt/C electrocatalyst (58.6%) for ORR after 25 000 s. In addition, Co₃O₄ hollow polyhedrons also display excellent OER performances with smaller overpotential (536 mV) for 10 mA cm^?2 than Co₃O₄ particles (593 mV) and superior stability (86.5%) after 25 000 s. This facile one‐step strategy based on metal–organic frameworks self‐sacrificed templates can be used to develop the promising well‐defined porous hollow metal oxides electrode materials for energy conversion and storage technologies.     

Hierarchical porous ZnMn<Subscript>2</Subscript>O<Subscript>4</Subscript> synthesized by the sucrose-assisted combustion method for high-rate supercapacitors

A simple sucrose-assisted combustion and subsequent high-temperature calcination route have been employed to prepare hierarchical porous ZnMn₂O₄ nanostructure. When used as an electrode for supercapacitor, the ZnMn₂O₄ electrode displays a high specific capacitance of 411.75 F g^?1 at a current density of 1 A g^?1, remarkable capacitance retention rate of 64.28 % at current density of 32 A g^?1 compared with 1 A g^?1, as well as excellent cycle stability (reversible capacity retention of 88.32 % after 4000 cycles). The outstanding electrochemical performances are mainly attributed to its hierarchical porous architecture, which provides large reaction surface area, fast ion and electron transfer, and good structure stability. All these impressive results demonstrate that ZnMn₂O₄ shows promise for its application in supercapacitors.     

Hierarchical Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@C hollow microspheres with high capacity as an anode material for lithium-ion batteries

Three-dimensional hierarchical Co₃O₄@C hollow microspheres (Co₃O₄@C HSs) are successfully fabricated by a facile and scalable method. The Co₃O₄@C HSs are composed of numerous Co₃O₄ nanoparticles uniformly coated by a thin layer of carbon. Due to its stable 3D hierarchical hollow structure and uniform carbon coating, the Co₃O₄@C HSs exhibit excellent electrochemical performance as an anode material for lithium-ion batteries (LIBs). The Co₃O₄@C HSs electrode delivers a high reversible specific capacity, excellent cycling stability (1672 mAh g^?1 after 100 cycles at 0.2 A g^?1 and 842.7 mAh g^?1 after 600 cycles at 1 A g^?1), and prominent rate performance (580.9 mAh g^?1 at 5 A g^?1). The excellent electrochemical performance makes this 3D hierarchical Co₃O₄@C HS a potential candidate for the anode materials of the next-generation LIBs. In addition, this simple synthetic strategy should also be applicable for synthesizing other 3D hierarchical metal oxide/C composites for energy storage and conversion.     

Preparation of Co<Subscript>3</Subscript>O<Subscript>4</Subscript> hollow microsphere/graphene/carbon nanotube flexible film as a binder-free anode material for lithium-ion batteries

A flexible Co₃O₄ hollow microsphere/graphene/carbon nanotube hybrid film is successfully prepared through a facile filtration strategy and a subsequent thermally treated process. The composition, morphology, and structure of the as-prepared film are characterized by X-ray diffraction, X-ray photoelectron spectrometer, scanning electron microscopy, and transmission electron microscopy. Based on the morphology characterizations on the hybrid film, the Co₃O₄ hollow microspheres are uniformly and closely attached on three-dimensional (3D) graphene/carbon nanotubes (GR/CNTs) network, which decreases the agglomeration of Co₃O₄ microspheres effectively. In this hybrid film, the 3D GR/CNT network which enhances conductance as well as prevents aggregation is a benefit to help Co₃O₄ to exert its lithium storage capabilities sufficiently. When used as a binder-free anode material for lithium-ion batteries, the hybrid film delivers excellent electrochemical properties involving reversible capacity (863 mAh g^?1 at a current density of 100 mA g^?1) and rate performance (185 mAh g^?1 at a current density of 1600 mA g^?1).     

Free-standing 3D Co3O4@NF micro-flowers composed of porous ultra-long nanowires as an advanced cathode material for supercapacitor

The development of smart structured cathode materials for supercapacitors (SCs) has sparked tremendous interest. However, the appropriate design to achieve high capacitance and energy density-based cathode materials remains a major problem for energy storage systems. This article describes the effective synthesis of self-supported 3D micro-flowers composed of ultrathin nanowires array of Co₃O₄ on Ni foam (NF) using hydrothermal conditions (Co₃O₄@NF). The mesoporous Co₃O₄@NF with a high surface area, providing a rich active state for the Faraday redox reaction and increasing the diffusion rate of the electrolyte ions. The optimized Co₃O₄@NF-16h electrode exhibited supreme electrochemical performance by delivering a high specific capacitance of 1878, (1127) and 1200 (720 C g⁻¹) F g⁻¹ at 1.0 and 20 A g⁻¹, respectively. The Co₃O₄@NF electrode retained good capacitance stability of 91% over 10000 cycles at 20 A g⁻¹ with excellent rate-performance of 67% at 20 folded high current values. The obtained results for the Co₃O₄@NF electrode are presented the enhanced pseudocapacitive performance, indicating the substantial potential for high-performance supercapacitor applications.     

Hexagonal-like NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanostructure based high-performance supercapacitor electrodes

A novel approach of double hydroxide-mediated synthesis of nickel cobaltite (NiCo₂O₄) electro-active material by the hydrothermal method is reported. The obtained NiCo₂O₄ electro-active material displays the spinel cubic phase and hexagonal-like morphology. Thermogravimetry analysis confirms the thermal stability of the electrode material. The functional groups and phase formation of NiCo₂O₄ have been confirmed by FT-IR and Raman spectral analysis. The modified NiCo₂O₄ electrode exhibits the highest specific capacitance of 767.5 F g^?1 at a current density of 0.5 A g^?1 in 3 M KOH electrolyte and excellent cyclic stability (94 % capacitance retention after 1000 cycles at a high current density of 5 A g^?1). The excellent electrochemical performance of the electrode is attributed to the hexagonal-like morphology, which contributes to the rich surface electro-active sites and easy transport pathway for the ions during the electrochemical reaction. The attractive Faradic behavior of NiCo₂O₄ electrode has been ascribed to the redox contribution of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ metal species in the alkaline medium. The symmetrical two-electrode cell has been fabricated using the NiCo₂O₄ electro-active material with excellent electrochemical properties for supercapacitor applications.     

Facile Construction of 3D Reduced Graphene Oxide Wrapped Ni3S2 Nanoparticles on Ni Foam for High‐Performance Asymmetric Supercapacitor Electrodes

3D reduced graphene oxide (rGO)‐wrapped Ni₃S₂ nanoparticles on Ni foam with porous structure is successfully synthesized via a facile one‐step solvothermal method. This unique structure and the positive synergistic effect between Ni₃S₂ nanoparticles and graphene can greatly improve the electrochemical performance of the NF@rGO/Ni₃S₂ composite. Detailed electrochemical measurements show that the NF@rGO/Ni₃S₂ composite exhibits excellent supercapacitor performance with a high specific capacitance of 4048 mF cm^?2 (816.8 F g^?1) at a current density of 5 mA cm^?2 (0.98 A g^?1), as well as long cycling ability (93.8% capacitance retention after 6000 cycles at a current density of 25 mA cm^?2). A novel aqueous asymmetric supercapacitor is designed using the NF@rGO/Ni₃S₂ composite as positive electrode and nitrogen‐doped graphene as negative electrode. The assembled device displays an energy density of 32.6 W h kg^?1 at a power density of 399.8 W kg^?1, and maintains 16.7 W h kg^?1 at 8000.2 W kg^?1. This outstanding performance promotes the as‐prepared NF@rGO/Ni₃S₂ composite to be ideal electrode materials for supercapacitors.     

A Facile Method for Synthesis of Porous NiCo2O4 Nanorods as a High‐Performance Anode Material for Li‐Ion Batteries

Porous electrode materials with large specific surface area, relatively short diffusion path, and higher electrical conductivity, which display both better rate capabilities and good cycle lives, have huge benefits for practical applications in lithium‐ion batteries. Here, uniform porous NiCo₂O₄ nanorods (PNNs) with pore‐size distribution in the range of 10–30 nm and lengths of up to several micrometers are synthesized through a convenient oxalate co‐precipitation method followed by a calcining process. The PNN electrode exhibits high reversible capacity and outstanding cycling stability (after 150 cycles still maintain about 650 mA h g^?1 at a current density of 100 mA g^?1), as well as high Coulombic efficiency (>98%). Moreover, the PNNs also exhibit an excellent rate performance, and deliver a stable reversible specific capacity of 450 mA h g^?1 even at 2000 mA g^?1. These results demonstrate that the PNNs are promising anode materials for high‐performance Li‐ion batteries.     

Transition of hexagonal to square sheets of Co3O4 in a triple heterostructure of Co3O4/MnO2/GO for high performance supercapacitor electrode

Cobalt oxide and manganese oxides are promising electrode materials amongst the transition metal oxides (TMOs) for pseudocapacitors. The lack of reversibility and deterioration of capacitance at higher current densities is major flaw in Co₃O₄ as an electrode for supercapacitor while MnO₂ suffers from low electrical conductivity and poor cycling stability. It is inevitable to bridge the performance gap between these two TMOs to obtain a high performance supercapacitor based on environmental benign and earth abundant materials. Herein, we fabricated a hybrid triple heterostructure high-performing supercapacitor based on hexagonal sheets of Co₃O₄, MnO₂ nanowires and graphene oxide (GO) to form a composite structure of Co₃O₄/MnO₂/GO by all hydrothermal synthesis route. The Co₃O₄ square sheets serves as an excellent backbone with good mechanical adhesion with the current collector providing a rapid electronic transfer channel while the integrated nanostructure of MnO₂ NW/GO permits more electrolyte ions to penetrate capably into the hybrid structure and allows effective utilization of more active surface areas. A triple heterostructured device exhibits a high areal capacitance of 3087 mF cm⁻² at 10 mV s⁻¹ scan rate along with the exceptional rate capability and cycling stability having capacitance retention of ∼170% after 5000 charge/discharge cycles. The TMOs based pseudocapacitor with the conducting scaffolds anchoring based on graphene derivatives like this will pave an encouraging alternatives for next generation energy storage devices.     

Ultrahigh supercapacitance in cobalt oxide nanorod film grown by oblique angle deposition technique

Nanorod films of cobalt oxide (Co₃O₄) have been grown by a unique oblique angle deposition (OAD) technique in an e-beam evaporator for supercapacitor electrode applications. This technique offers a non-chemical route to achieve large aspect ratio nanorods. The fabricated electrodes at OAD 80° exhibited a specific capacitance of 2875 F/g. The electrochemically active surface area was 1397 cm⁻², estimated from the non-Faradaic capacitive current region. Peak energy and power densities obtained for Co₃O₄ nanorods were 57.7 Wh/Kg and 9.5 kW/kg, respectively. The Co₃O₄ nanorod electrode showed a good endurance of 2000 charge-discharge cycles with 62% retention. The OAD approach for fabricating supercapacitor nanostructured electrodes can be exploited for the fabrication of a broad range of metal oxide materials.     

Co3O4 Hollow Nanoparticles and Co Organic Complexes Highly Dispersed on N‐Doped Graphene: An Efficient Cathode Catalyst for Li‐O2 Batteries

Rechargeable Li‐O₂ batteries are promising candidates for electric vehicles due to their high energy density. However, the current development of Li‐O₂ batteries demands highly efficient air cathode catalysts for high capacity, good rate capability, and long cycle life. In this work, a hydrothermal‐calcination method is presented to prepare a composite of Co₃O₄ hollow nanoparticles and Co organic complexes highly dispersed on N‐doped graphene (Co–NG), which acts as a bifunctional air cathode catalyst to optimize the electrochemical performances of Li‐O₂ batteries. Co–NG exhibits an outstanding initial discharge capacity up to 19 133 mAh g^?1 at a current density of 200 mA g^?1. In addition, the batteries could sustain 71 cycles at a cutoff capacity of 1000 mAh g^?1 with low overpotentials at the current density of 200 mA g^?1. Co–NG composites are attractive as air cathode catalysts for rechargeable Li‐O₂ batteries.     

Urea-assisted solvothermal synthesis of monodisperse multiporous hierarchical micro/nanostructured ZnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> microspheres and their lithium storage properties

High-quality monodisperse multiporous hierarchical micro/nanostructured ZnCo₂O₄ microspheres have been fabricated by calcinating the Zn_1/3Co_2/3CO₃ precursor prepared by urea-assisted solvothermal method. The as-prepared products are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), and Brunauer-Emmett-Teller (BET) measurement to study the crystal phase and morphology. When tested as anode material for lithium ion batteries, the multiporous ZnCo₂O₄ microspheres exhibit an initial discharge capacity of 1,369 mAh g^?1 (3,244.5 F cm^?3) and retain stable capacity of 800 mAh g^?1 (1,896 F cm^?3) after 30 cycles. It should be noted that the good electrochemical performances can be attributed to the porous structure composed of interconnected nanoscale particles, which can promote electrolyte diffusion and reduce volume change during discharge/charge processes. More importantly, this ZnCo₂O₄ 3D hierarchical structures provide a large number of active surface position for Li⁺ diffusion, which may contribute to the improved electrochemical performance towards lithium storage.     

Dandelion-like mesoporous Co<Subscript>3</Subscript>O<Subscript>4</Subscript> as anode materials for lithium ion batteries

A dandelion-like mesoporous Co₃O₄ was fabricated and employed as anode materials of lithium ion batteries (LIBs). The architecture and electrochemical performance of dandelion-like mesoporous Co₃O₄ were investigated through structure characterization and galvanostatic charge/discharge test. The as-prepared dandelion-like mesoporous Co₃O₄ consisted of well-distributed nanoneedles (about 40 nm in width and about 5 μm in length) with rich micropores. Electrochemical experiments illustrated that the as-prepared dandelion-like mesoporous Co₃O₄ as anode materials of LIBs exhibited high reversible specific capacity of 1430.0 mA h g^?1 and 1013.4 mA h g^?1 at the current density of 0.2 A g^?1 for the first and 100th cycle, respectively. The outstanding lithium storage properties of the as-prepared dandelion-like mesoporous Co₃O₄ might be attributed to its dandelion-like mesoporous nanostructure together with an open space between adjacent nanoneedle networks promoting the intercalation/deintercalation of lithium ions and the charge transfer on the electrode. The enhanced capacity as well as its high-rate capability made the as-prepared dandelion-like mesoporous Co₃O₄ to be a good candidate as a high-performance anode material for LIBs.     

Liquid precipitation synthesis of Co3O4 for high-performance electrochemical capacitors

The amorphous Co₃O₄ nanostructure, which adopted sodium hexametaphosphate as structure-directing agent, has been successfully synthesized in large scale via two steps: preparation of the precursor and the calcination process. The results of X-ray diffraction indicate that the prepared materials are mainly composed of Co₃O₄; the formless Co₃O₄ nanoplate with loose structures is observed by scanning electron microscopy. Cyclic voltammetry, chronopotentiometry, and electrochemical impedance measurements are applied in a mild aqueous electrolyte (2 mol L^?1 KOH) to investigate the performance of the Co₃O₄, which show a high specific capacitance (SC) of 482.61 F g^?1 at 5 mA cm^?2. Besides, the SC degradation is only 10.05 % after 250 continuous charge–discharge cycles at 5 mA cm^?2, indicating an excellent electrochemical stability. The improved performance is reasonably ascribed to their irregular structure for ionic transport during the electrochemical reaction, which presents as promising candidates for supercapacitors.     

Enhanced lithium storage performance of a self-assembled hierarchical porous Co<Subscript>3</Subscript>O<Subscript>4</Subscript>/VGCF hybrid high-capacity anode material for lithium-ion batteries

A Co₃O₄/vapor-grown carbon fiber (VGCF) hybrid material is prepared by a facile approach, namely, via liquid-phase carbonate precipitation followed by thermal decomposition of the precipitate at 380 °C for 2 h in argon gas flow. The material is characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Brunauer-Emmett-Teller specific surface area analysis, and carbon elemental analysis. The Co₃O₄ in the hybrid material exhibits the morphology of porous submicron secondary particles which are self assembled from enormous cubic-phase crystalline Co₃O₄ nanograins. The electrochemical performance of the hybrid as a high-capacity conversion-type anode material for lithium-ion batteries is investigated by cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic discharge/charge methods. The hybrid material demonstrates high specific capacity, good rate capability, and good long-term cyclability, which are far superior to those of the pristine Co₃O₄ material prepared under similar conditions. For example, the reversible charge capacities of the hybrid can reach 1100–1150 mAh g^?1 at a lower current density of 0.1 or 0.2 A g^?1 and remain 600 mAh g^?1 at the high current density of 5 A g^?1. After 300 cycles at 0.5 A g^?1, a high charge capacity of 850 mAh g^?1 is retained. The enhanced electrochemical performance is attributed to the incorporated VGCFs as well as the porous structure and the smaller nanograins of the Co₃O₄ active material.     

Preparation of Co3O4 nanoplate/graphene sheet composites and their synergistic electrochemical performance

Co₃O₄ nanoplate/graphene sheet composites were prepared through a two-step synthetic method. The composite material as prepared was characterized using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and energy-dispersive X-ray spectroscopy. The platelet-like morphology of Co₃O₄ leads to a layer-by-layer-assembled structure of the composites and a good dispersion of Co₃O₄ nanoplates on the surface of graphene sheets. The electrochemical characteristics indicate that the specific capacitance of the composites is 337.8 F?g^?1 in comparison with the specific capacitance of 204.4 F?g^?1 without graphene sheets. Meanwhile, the composites have an excellent rate capability and cycle performance. The results show that the unique microstructure of the composites enhances the electrochemical capacitive performance of Co₃O₄ nanoplates due to the three-dimensional network of graphene sheets for electron transport increasing electric conductivity of the electrode and providing unobstructed pathways for ionic transport during the electrochemical reaction.     

Novel synthesis and characterization of ZnCo<Subscript>2</Subscript>O<Subscript>4</Subscript> nanoflakes grown on nickel foam as efficient electrode materials for electrochemical supercapacitors

ZnCo₂O₄ nanoflakes were directly grown on Ni foam via a two-step facile strategy, involving cathodic electrolytic electrodeposition (ELD) method and followed by a thermal annealing treatment step. The results of physical characterizations exhibit that the mesoporous ZnCo₂O₄ nanoflakes have large electroactive surface areas (138.8 m² g^?1) and acceptable physical stability with the Ni foam, providing fast electron and ion transport sites. The ZnCo₂O₄ nanoflakes on Ni foam were directly used as integrated electrodes for supercapacitors and their electrochemical properties were measured in 2 M KOH aqueous solution. The ZnCo₂O₄ nanoflake electrode exhibits a high capacitance of 1781.7 F g^?1 at a current density of 5 A g^?1 and good rate capability (62% capacity retention at 50 A g^?1). Also, an excellent cycling ability at various current densities from 5 to 50 A g^?1 was obtained and 92% of the initial capacitance maintained after 4000 cycles. The results demonstrate that the proposed synthesis route is cost-effective and facile and can be developed for preparation of electrode materials in other electrochemical supercapacitors.     

Facile Fabrication of Bicomponent CoO/CoFe2O4‐N‐Doped Graphene Hybrids with Ultrahigh Lithium Storage Capacity

A facile strategy is developed to fabricate bicomponent CoO/CoFe₂O₄‐N‐doped graphene hybrids (CoO/CoFe₂O₄‐NG). These hybrids are demonstrated to be potential high‐performance anodes for lithium‐ion batteries (LIBs). The CoO/CoFe₂O₄ nanoplatelets are finely dispersed on the surface of N‐doped graphene nanosheets (CoO/CoFe₂O₄‐NG). The CoO/CoFe₂O₄‐NG electrode exhibits ultrahigh specific capacity with 1172 mA h g^?1 at 500 mA g^?1 and 970 mA h g^?1 at 1000 mA g^?1 as well as excellent cycle stability due to the synergetic effects of N‐doped graphene and CoO/CoFe₂O₄ nanoplatelets. The well‐dispersed bicomponent CoO/CoFe₂O₄ is responsible for the high specific capacity. The N‐doped graphene with high specific surface area has dual roles: to provide active sites for dispersing the CoO/CoFe₂O₄ species and to function as an electrical conducting matrix for fast charge transfer. This method provides a simple and efficient way to configure the hybridized electrode materials with high lithium storage capacity.     

Hierarchical Mn<Subscript>1.5</Subscript>Co<Subscript>1.5</Subscript>O<Subscript>4</Subscript> microspheres constructed from one-dimensional nanorods as high-performance anode material for lithium-ion battery

Mn_1.5Co_1.5O₄ hierarchical microspheres have been successfully synthesized via a solvothermal method and an annealing procedure. Mn_1.5Co_1.5O₄ exhibits advanced cycling performance, and it retains a reversible capacity of 633 mA h g^?1 at a current density of 400 mA g^?1 with a coulombic efficiency of 99.0% after 220 cycles. Its remarkable performance is attributed to the hierarchical structure assembled with nanorods, which increases the contact area between each nanorod and electrolyte. More significantly, the open space between neighboring nanorods and the pores on the surface of nanorods can improve Li⁺ ion diffusion rate. Furthermore, the nanorods have rapid one-dimensional Li⁺ diffusion channels, which not only possess a large specific surface area for high activity but accommodate the volume change during lithiation–delithiation processes. Therefore, Mn_1.5Co_1.5O₄ hierarchical microspheres can act as a promising alternative anode material for lithium-ion battery.