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.     

Self-assembled 3D Co3O4-graphene frameworks with high lithium storage performance

Three-dimensional Co₃O₄-graphene frameworks (3D-CGFs) are prepared with a one-pot hydrothermal method. Co₃O₄ particles are in situ anchored on graphene sheets, and the resulting composite self-assembles into 3D architecture during the hydrothermal treatment. Scanning electron microscope, transmission electron microscope, powder X-ray powder diffraction, and Raman spectroscopy are employed to characterize the sample. When tested as anode materials for lithium-ion batteries, 3D-CGFs demonstrate remarkable electrochemical lithium storage properties, such as large and stable reversible capacity (>530 mAh g^?1 at 500 mA g^?1 over 300 cycles), good capacity retention (88 % retention after 300 cycles at 500 mA g^?1 compared with the 4th cycle), excellent high-rate performance (515 mAh g^?1 at 1 A g^?1), making it a promising candidate for high-performance anode materials, especially for high-rate lithium-ion batteries.     

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).     

Fabrication of Ni-Co binary oxide/reduced graphene oxide composite with high capacitance and cyclicity as efficient electrode for supercapacitors

Nickel-cobalt binary oxide/reduced graphene oxide (G-NCO) composite with high capacitance is synthesized via a mild method for electrochemical capacitors. G-NCO takes advantages of reduced graphene oxide (RGO) and nickel-cobalt binary oxide. As an appropriate matrix, RGO is beneficial to form homogeneous structure and improve the electron transport ability. The binary oxide owns more active sites than those of nickel oxide and cobalt oxide to promote the redox reaction. Attributed to the well crystallinity, homogeneous structure, increased active sites, and improved charge transfer property, the G-NCO composite exhibits highly enhanced electrochemical performance compared with G-NiO and G-Co₃O₄ composites. The specific capacitance of the G-NCO composite is about 1750 F g^?1 at 1 A g^?1 together with capacitance retention of 79 % (900/1138 F g^?1) over 10,000 cycles at 4 A g^?1. To research its practical application, an asymmetric supercapacitor with G-NCO as positive electrode and activated carbon as negative electrode was fabricated. The asymmetric device exhibits a prominent energy density of 37.7 Wh kg^?1 at a power density of 800 W kg^?1. The modified G-NCO composite shows great potential for high-capacity energy storage.     

Preparation and electrochemical performances of nanoporous/cracked cobalt oxide layer for supercapacitors

Nanoporous/cracked structures of cobalt oxide (Co₃O₄) electrodes were successfully fabricated by electroplating of zinc–cobalt onto previously formed TiO₂ nanotubes by anodizing of titanium, leaching of zinc in a concentrated alkaline solution and followed by drying and annealing at 400 °C. The structure and morphology of the obtained Co₃O₄ electrodes were characterized by X-ray diffraction, EDX analysis and scanning electron microscopy. The results showed that the obtained Co₃O₄ electrodes were composed of the nanoporous/cracked structures with an average pore size of about 100 nm. The electrochemical capacitive behaviors of the nanoporous Co₃O₄ electrodes were investigated by cyclic voltammetry, galvanostatic charge–discharge studies and electrochemical impedance spectroscopy in 1 M NaOH solution. The electrochemical data demonstrated that the electrodes display good capacitive behavior with a specific capacitance of 430 F g^?1 at a current density of 1.0 A g^?1 and specific capacitance retention of ca. 80 % after 10 days of being used in electrochemical experiments, indicating to be promising electroactive materials for supercapacitors. Furthermore, in comparison with electrodes prepared by simple cathodic deposition of cobalt onto TiO₂ nanotubes(without dealloying procedure), the impedance studies showed improved performances likely due to nanoporous/cracked structures of electrodes fabricated by dealloying of zinc, which provide fast ion and electron transfer routes and large reaction surface area with the ensued fast reaction kinetics.     

Composites of olive-like manganese oxalate on graphene sheets for supercapacitor electrodes

MnC₂O₄/graphene composites are prepared by a facile hydrothermal reaction with KMnO₄ using ascorbic acid as a reducing agent. Olive-like MnC₂O₄ particles are distributed uniformly on the surface of graphene sheets. The composites are evaluated as supercapacitor electrodes, which show that the specific capacitance of MnC₂O₄/graphene composites is 122 F g^?1, more than twice as high as that of free MnC₂O₄ at a current density of 0.5 A g^?1. In addition, this composite material exhibits an excellent cycle stability with the capacitance retention of 94.3 % after 1,000 cycles.     

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.     

Synthesis of carbon-coated Fe3O4 nanorods as electrode material for supercapacitor

Carbon-coated Fe₃O₄ and pure Fe₃O₄ nanorods are synthesized via hydrothermal reaction and subsequent sintering procedure. The as-prepared products characterized by X-ray diffraction and scanning electron microscopy analysis indicate that carbon coating does not affect the structure and morphology of Fe₃O₄. Transmission electron microscope shows that Fe₃O₄ nanorods are homogeneously coated by carbon layer with a thickness of approximately 2 nm. The electrochemical properties measured by cyclic voltammetry, galvanostatic charge–discharge cycling and electrochemical impedance spectroscopy tests show that carbon-coated Fe₃O₄ (Fe₃O₄/C) nanorods present improved electrochemical performance due to the carbon layer. A specific capacitance of 275.9 F?g^?1 is achieved at a current density of 0.5 A g^?1 in 1 M Na₂SO₃ aqueous solution for the Fe₃O₄/C nanorods in comparison to that of 208.6 F?g^?1 for pure Fe₃O₄.     

Co<Subscript>3</Subscript>O<Subscript>4</Subscript>@MnO<Subscript>2</Subscript> core shell arrays on nickel foam with excellent electrochemical performance for aqueous asymmetric supercapacitor

At present, a lot of attention has been paid to the reasonable design and synthesis of materials with core shell structure for high-performance supercapacitors. Herein, the Co₃O₄@MnO₂ core shell arrays on nickel foam are successfully synthesized via a facile and effective hydrothermal method followed with annealing process. The sample was characterized by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Electrochemical performance of the Co₃O₄@MnO₂ material was studied using cyclic voltammetry, charge/discharge cycling, and electrochemical impedance measurements in 6 mol L^?1 KOH aqueous electrolyte. The results indicated that the Co₃O₄@MnO₂ material presented excellent electrochemical performance in terms of specific capacitance, cyclic stability, and charge/discharge stability.     

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.     

Decorating reduced graphene oxide with Co3O4 hollow spheres and their application in supercapacitor materials

In this paper, a composite of reduced graphene oxide decorated by Co₃O₄ hollow spheres (Co₃O₄/RGO composite) has been synthesized by a one-pot solvothermal method. The samples are characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FT-IR), Raman spectra and so on. The results demonstrate that the Co₃O₄ hollow spheres with good purity and homogenous size are absorbed onto the reduced graphene oxide sheets as spacers to prevent the aggregation of the graphene oxide sheets. Furthermore, the well electrochemical properties demonstrate that the Co₃O₄/RGO composite might have potential applications as electrode materials for supercapacitors.     

Li4Mn5O12 prepared using l-lysine as additive and its electrochemical performance

l-Lysine was employed as additive to prepare face-centered cubic spinel Li₄Mn₅O₁₂. During the process, l-lysine played important roles such as complexing agent as well as combusting agent and adjusting the pH values of solution. The physical characteristics of Li₄Mn₅O₁₂ were characterized by X-ray diffraction and scanning electron microscopy. The electrochemical capacitance performance of Li₄Mn₅O₁₂ electrode was characterized by cyclic voltammetry, galvanostatic charge/discharge, and electrochemical impedance spectroscopy. These analyses indicated that Li₄Mn₅O₁₂ was able to deliver 168 F?g^?1 within the potential range of 0–1.4 V at a scan rate of 5 mV?s^?1 in 1 mol?L^?1 Li₂SO₄. Nine hundred cycles later, the capacitance faded to 165 F?g^?1 with cutting down by 0.003 F?g^?1 per cycling period and also can remain 98.2 % of original value, displaying a good cycling performance.     

Electrochemical performances of nano-Co3O4 with different morphologies as anode materials for Li-ion batteries

Three kinds of Co₃O₄ nanomaterials with different morphologies were synthesized controllably by a post-anneal-assisted hydrothermal method in this study. X-ray diffraction and scanning electron microscopy indicated that all three kinds of samples were pure cubic phase of Co₃O₄ with morphologies of nanorods, nanoclusters, and nanoplates. Moreover, the transmission electron microscopy (TEM) and high-resolution TEM showed that the Co₃O₄ nanorods were bamboo-like and highly crystalline structures. When these materials were applied to the lithium-ion batteries (LIBs) as anode materials, the Co₃O₄ of nanorods demonstrated the best performance. It has a stable reversible capacity of 954 mAh g^?1 as the anode of a LIB, much higher than the other two kinds of Co₃O₄ of rod-like nanoclusters and nanoplates, even after 35 cycles. All results showed that the morphology and microstructure take very important roles in the performance of Co₃O₄ as the anode materials in LIBs.     

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.     

Preparation and electrochemical properties of LiFePO<Subscript>4</Subscript>/graphene composites from tailoring graphene oxides

LiFePO₄/graphene composites have been prepared by using tailoring graphene oxide (GO) nanosheets as precursors. The structure and electrochemical properties of the composites were investigated by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), Raman microscopy, and a variety of electrochemical testing techniques. The decrease in graphene size reduces the contact resistance between activated materials, and enhances the lithium-ion transport in LiFePO₄/graphene composites. With low weight fractions of small-size graphene sheets, the composites show better electrochemical performance than those with large size graphene sheets.     

Fabrication of functionalized nitrogen-doped graphene for supercapacitor electrodes

A unique and convenient one-step hydrothermal process for synthesizing functionalized nitrogen-doped graphene (FGN) via ethylenediamine, hydroquinone, and graphene oxide (GO) is described. The graphene sheets of FGN provide a large surface area for hydroquinone molecules to be anchored on, which can greatly enhance the contribution of pseudocapacitance. X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Raman spectroscopy, and electrochemical workstation are used to characterize the materials. The nitrogen content exhibited in FGN can be up to 9.83 at.%, and the as-produced graphene material shows an impressive specific capacitance of 364.6 F g^?1 at a scan rate of 10 mV s^?1, almost triple that of the graphene (GN)-based one (127.5 F g^?1). Furthermore, the FGN electrodes show excellent electrochemical cycle stability with 94.4 % of its initial capacitance retained after 500 charge/discharge cycles at the current density of 3 A g^?1.     

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.     

Acacia gum-assisted co-precipitating synthesis of LiNi<Subscript>0.5</Subscript>Co<Subscript>0.2</Subscript>Mn<Subscript>0.3</Subscript>O<Subscript>2</Subscript> cathode material for lithium ion batteries

LiNi_0.5Co_0.2Mn_0.3O₂ particles of uniform size were prepared through carbonate co-precipitation method with acacia gum. The precursor of carbonate mixture was calcined at 800 °C, and a well-crystallized Ni-rich layered oxide was got. The phase structure and morphology were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The micro-sized particles delivered high initial discharge capacity of 164.3 mA h g^?1 at 0.5 C (1 C?=?200 mA g^?1) between 2.5 and 4.3 V with capacity retention of 87.5 % after 100 cycles. High reversible discharge capacities of 172.4 and 131.4 mA h g^?1 were obtained at current density of 0.1 and 5 C, respectively. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were performed to further study the LiNi_0.5Co_0.2Mn_0.3O₂ particles. Anyway, the excellent electrochemical performances of LiNi_0.5Co_0.2Mn_0.3O₂ sample should be attributed to the use of acacia gum.     

Addition of multiwalled carbon nanotube and graphene nanosheet in cobalt oxide film for enhancement of capacitance in electrochemical capacitors

In order to enhance the capacitance of electrochemical capacitors, multiwalled carbon nanotubes (MWCNTs) and graphene nanosheets (GNS) were added to cobalt oxide (Co₃O₄) paste. The composite film based on Co₃O₄/MWCNT/GNS (95:4:1 wt%) exhibited a capacitance of 294 F/g while the capacitance of Co₃O₄/MWCNT (95:5 wt%) and pure Co₃O₄ film is 205 and 163 F/g, respectively. The enhanced capacitance of Co₃O₄/MWCNT/GNS composite film was attributed to the electrochemical contributions of the Co₃O₄ nanoparticle attached to the GNS as well as their significantly increased specific surface areas by MWCNTs. Furthermore, the composite films showed faradaic redox capacitive behavior to double-layer capacitive behavior due to the different nanostructures of the composites.     

A novel synthesis of size-controllable mesoporous NiMoO<Subscript>4</Subscript> nanospheres for supercapacitor applications

A novel hydrothermal emulsion method is proposed to synthesize mesoporous NiMoO₄ nanosphere electrode material. The size of sphere-shaped NiMoO₄ nanostructure is controlled by the mass ratio of water and oil phases. Nickel acetate tetrahydrate and ammonium heptamolybdate were used as nickel and molybdate precursors, respectively. The resultant mesoporous NiMoO₄ nanospheres were characterized by X-ray diffraction, N₂ adsorption and desorption, scanning electron microscopy, and transmission electron microscopy. The electrochemical performances were evaluated by cyclic voltammetry (CV), cyclic chronopotentiometry (CP), and electrochemical impedance spectroscopy (EIS) in 6 M KOH solution. The typical mesoporous NiMoO₄ nanospheres exhibit the large specific surface area of 113 m² g^?1 and high specific capacitance of 1443 F g^?1 at 1 A g^?1, an outstanding cyclic stability with a capacitance retention of 90 % after 3000 cycles of charge-discharge at a current density of 10 A g^?1, and a low resistance.