Solvothermal synthesis of mesoporous NiCo2O4 spinel oxide nanostructure for high-performance electrochemical capacitor electrode

NiCo₂O₄ nanostructure was successfully synthesized via a d-glucose-assisted solvothermal process. Spinel-type cubic phase and mesoporous microstructure of the sample for different calcination temperatures were confirmed by X-ray diffraction and transmission electron microscopy. Typical pseudocapacitance feature of the NiCo₂O₄ treated at different temperatures was then evaluated in aqueous 6 M KOH electrolyte solution. Electrochemical measurements showed that the spinel nickel cobaltite nanostructure heated at 300 °C exhibits maximum specific capacitances of 524 F g^?1 at 0.5 A g^?1 and 419 F g^?1 at 10 A g^?1 with excellent cycle stability and only ~9 % of capacitance loss after 2,500 cycles. This demonstrates the potential application of the material for supercapacitors. The attractive pseudocapacitive performance of NiCo₂O₄ is mainly attributed to the redox contribution of the Ni and Co metal species, high surface area, and their desired mesoporous nanostructure.     

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.     

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.     

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.     

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.     

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.     

Hydrothermal synthesis of carbon- and reduced graphene oxide-supported CoMoO4 nanorods for supercapacitor

Hybrid CoMoO₄ nanorods with carbon (C) and graphene oxide (rGO) are successfully synthesized via one-step hydrothermal process. Hybrid α-CoMoO₄ nanorods have shown excellent electrochemical performances compared to pristine CoMoO₄ in alkaline electrolyte. Specifically, CoMoO₄/C nanorod exhibits a maximum specific capacitance of 451.6 F g^?1 at the current density of 1 A g^?1, whereas CoMoO₄/rGO shows high specific capacitance of 336.1 F g^?1 at the same current density. Both the hybrid nanorods show good rate capability even at high current density of 20 A g^?1 and long-term cyclic stability. The observed electrochemical features of the hybrid CoMoO₄ nanostructure could be attributed to the presence of highly conductive carbonaceous material on unique one-dimensional nanorod microstructure which enhances the electrical conductivity of the nanorods thereby allowing faster electrolyte ion diffusion during the redox process.     

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.     

Pulsed electrodeposition of mesoporous cobalt-doped manganese dioxide as supercapacitor electrode material

Cobalt-doped MnO₂, as electrode material for supercapacitor, was synthesized by pulse electrodeposition method. The morphology and structure of the products were characterized by X-ray diffraction (XRD) and field-emission scanning electron microscope (FE-SEM). The results show that the crystal structure of the products is γ-type, and the samples reveals a porous texture composed of manganese oxide nanosheets. Cyclic voltammetry (CV), electrochemical impedance spectrometry (EIS), and galvanostatic charge–discharge tests indicate that doping cobalt has great effect on the electrochemical performance of manganese dioxide material. A specific capacitance of 354 F g^?1 is obtained when the molar ratio of Mn to Co is 200:10. After 100 charge–discharge cycles in 6 M KOH solution, the specific capacitance stabilized at 333.6 F g^?1, exhibiting excellent capacitance retention ability.     

Synthesis and performance of Li2NiTi3O8 as a new anode material for lithium-ion batteries

The titanate spinel Li₂NiTi₃O₈ is proposed for the first time as a new anode for lithium-ion batteries and successfully synthesized via a facile ball-milling assisted solid-state reaction method. The sample is characterized by X-ray diffraction patterns (XRD), scanning electron microscope (SEM), transmission electron microscopy (TEM), galvanostatic charge–discharge tests, cyclic voltammetry (CV) tests, and electrochemical impedance spectroscopy (EIS). The results reveal that the Li₂NiTi₃O₈ nanoparticles have well-distributed morphology, and the particle size ranges between 100 and 300 nm. Although the initial coulombic efficiency is only 56.3 %, the Li₂NiTi₃O₈ electrode still exhibits a high rate capability and excellent cycling stability. The Li₂NiTi₃O₈ anode provides a large capacity of 212.3 mAh g^?1 at 0.1 A g^?1 after 10 cycle, which is close to its theoretical capacity (223.6 mAh g^?1). Even after 100 cycles, it still delivers a quite high capacity of 203.98 mAh g^?1, with no significant capacity fading. This indicates that the as-synthesized Li₂NiTi₃O₈ material is a promising anode material for lithium-ion batteries.     

NiCo<Subscript>2</Subscript>O<Subscript>4</Subscript> polyhedra with controllable particle size as high-performance anode for lithium-ion battery

Different particle sizes of dodecahedron precursors are synthesized by controlling the polarity of the solution. Through the results of scanning electron microscope (SEM) images, it can be found that different particle sizes of precursors present obvious edge angles and their morphology can be well retained after annealing. X-ray diffraction (XRD) measurements suggest that the annealed polyhedral products are pure single-phase NiCo₂O₄. When tested as lithium-ion battery anode, 0.5 μm NiCo₂O₄ polyhedra exhibits a specific capacity of 1050 mAh g^?1 at 0.1 C at the 60th cycle, which was higher than theoretical capacity of single metal oxide (NiO 718 mAh g^?1 and Co₃O₄ 890 mAh g^?1). It also exhibits the highest rate capability with an average discharge capacity of 890, 700, 490, 330, and 300 mAh g^?1 at 0.5, 2, 4, 8, and 10 C, respectively. Those advantages are attributed to that small-sized particle with great surface areas decrease the actual current density at the surface and inner of the prepared electrode.     

Promising nitrogen-doped porous nanosheets carbon derived from pomegranate husk as advanced electrode materials for supercapacitors

Nitrogen-doped porous activated carbons (N-PHACs) have been successfully synthesized using pomegranate husk as carbon precursor via ZnCl₂-activation carbonization and subsequent urea-assisted hydrothermal nitrogen-doping method. The obtained N-PHACs possesses abundant mesoporous structure, high specific surface area (up to 1754.8 m² g^?1), pore volume (1.05 cm³ g^?1), and nitrogen-doping content (4.51 wt%). Besides, the N-PHACs-based material showed a high specific capacitance of 254 F g^?1 at a current density of 0.5 A g^?1 and excellent rate performance (73% capacitance retention ratio even at 20 A g^?1) in 2 M KOH aqueous electrolyte, which is attributed to the contribution of double-layer capacitance and pseudocapacitance. The assembled N-PHACs-based symmetric capacitor with a wide operating voltage range of 0–1.8 V exhibits a maximum energy density of 15.3 Wh kg^?1 at a power density of 225 W kg^?1 and superior cycle stability (only 6% loss after 5000 cycles) in 0.5 M Na₂SO₄ aqueous electrolyte. These exciting results suggest that the novel N-doping porous carbon material prepared by a green and low-cost design strategy has a potential application as high-performance electrode materials for supercapacitors.     

Preparation and electrochemical performance of spherical MnOx with high capacitance

A promising electrode material, MnO_x, was prepared by chemical precipitation method using MnSO₄?·?H₂O as manganese source and (NH₄)₂CO₃ as precipitant. The as-prepared manganese oxide was characterized by field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), and X-ray diffraction (XRD), respectively. The effects of reaction temperature and calcination temperature on the electrochemical properties of the electrode materials were investigated. The results show that MnO_x with a spherical morphology exhibits an excellent performance in the energy storage field. Under the optimal experimental conditions, the specific capacitance of pure MnO_x reaches up to 592 F g^?2 in the alkaline media when the current density is 5 mA cm^?2 over the potential window from ?0.3 to +0.4 V. The combination resistance and transfer resistance of MnO_x are 0.86 and 0.08 Ω, respectively. These findings make MnO_x a promising electrode material for electrochemical capacitors.     

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 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.     

Zn-Co oxide electrodes with excellent capacitive behavior for using supercapacitor application

In the present study, super-capacitive behavior of spinel Zn-Co oxides (with different Zn⁺²/Co⁺² mol ratio) has been thoroughly investigated. The spinel of transition metal oxides with different morphologies has been synthesized with hydrothermal method on Ni foam as substrate layer. The specific capacitance of the Zn-Co oxide electrode prepared at 180 °C for 5 h with different Zn⁺²/Co⁺² mol ratios of 1:0, 2:1, 1:1, 1:2, 0:1 were investigated and measured 405, 842, 726, 1237, 705 F g^?1, respectively at 50 mV s^?1 scan rate. Zn-Co oxide with Zn⁺²/Co⁺² mol ratio of 1:2 was also synthesized at two different temperatures of 120 and 150 °C for 5 h with the specific capacitance of 1147, 917 F g^?1 at 50 mV s^?1 scan rate, respectively. Among the obtained data, the sample with Zn⁺²/Co⁺² mol ratio of 1:2 prepared at 180 °C for 5 h possessed highest specific capacitance. The cyclic life of this electrode showed 92% capacitance retention after 1000 cycle of charge-discharge. All results revealed that Zn-Co oxides had excellent supercapacitive properties due to multiple oxidation states and fast ion/electron transfer at the surface of electrode which could be offered as suitable devices for energy storage applications.     

Reduced Graphene Oxide decorated with Manganese Cobalt Oxide as Multifunctional Material for Mechanically Rechargeable and Hybrid Zinc–Air Batteries

Spinel MnCo₂O₄ nanoparticles on nitrogen‐doped reduced graphene oxide (MnCo₂O₄/NGr) are synthesized for advanced zinc–air batteries with remarkable cyclic efficiency and stability. The synthesized MnCo₂O₄/NGr exhibits good oxygen‐reduction reaction (ORR) activity with half‐wave potential E _1/2 of 0.85 V (vs reversible hydrogen electrode (RHE)), comparable to commercial Pt/C with E _1/2 of 0.88 V (vs RHE) along with superior oxygen electrode activity ΔE = 0.91 V for the ORR/OER (oxygen‐evolution reaction) in alkaline media. Durability tests confirm that MnCo₂O₄/NGr is more stable than Pt/C in alkaline environment. MnCo₂O₄/NGr functions with stable discharge profile of 1.2 V at 20 mA cm^?2, large discharge capacity of 707 mAh g^?1_Zn at 40 mA cm^?2 and a high energy density of 813 Wh kg^?1_Zn in a mechanically rechargeable zinc–air battery. The electrically rechargeable MnCo₂O₄/NGr zinc–air battery displays hybrid behavior with both Faradaic and oxygen redox charge–discharge characteristics, operating at higher voltage and providing higher power density and excellent cyclic efficiency of 86% for over 100 cycles compared to Pt/C with efficiency of around 60%. Moreover, hybrid zinc–air battery operates with a stable and energy efficient profile at different current densities.     

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.     

Morphology-controlled PANI nanowire electrode by using deposition scan rate in H<Subscript>2</Subscript>SO<Subscript>4</Subscript>/PVA polymer electrolyte for electrochemical capacitor

Polyaniline (PANI) nanowire electrode was successfully prepared using electrodeposition method. The morphology, thickness, and electrochemical performance of PANI electrode can be controlled by varying the deposition scan rates. Lower deposition scan rate results in compact and aggregates of PANI nanowire morphology. The uniform nanowire of PANI was obtained at the applied scan rate of 100 mV s^?1, and it was used as symmetric electrode coupled with H₂SO₄/polyvinyl alcohol (PVA) gel electrolyte. The different concentrations of H₂SO₄ acid in polymer electrolyte have influenced the electrochemical performance as well. The optimum specific capacitance and energy density of P100 PANI electrode in 3 M H₂SO₄/PVA gel polymer electrolyte was 377 F g^?1 and 95.4 Wh kg^?1 at the scan rate of 1 mV s^?1. The good stability of the electrode in this system is applicable to many wearable electronics applications.     

The lithiation studies of nanostructured tungsten oxide film prepared via electrochemical precipitation

In this work, nanostructured tungsten oxide was electrodeposited by cyclic voltammetric technique onto a stainless steel surface. The structure and surface morphology of the resulting oxide film were characterized by means of X-ray diffraction, scanning probe microscopy, and scanning electron microscopy. The electrochemical intercalation of lithium into the nanostructured tungsten oxide was studied using cyclic voltammetry, galvanostatic charge–discharge curves, and electrochemical impedance spectroscopy in a liquid electrolyte consisting of 1 M LiClO₄ in propylene carbonate. The as-deposited tungsten oxide indicated the capacity for electrochemical lithium insertion. The specific capacitance of 108.05 F?g^?1 was obtained at the constant discharge density of 0.07 mA?cm^?2.