Hydrothermal and soft-templating synthesis of mesoporous NiCo2O4 nanomaterials for high-performance electrochemical capacitors

Mesoporous nickel cobaltite (NiCo₂O₄) nanoparticles were synthesized via a hydrothermal and soft-templating method through quasi-reverse-micelle mechanism. The physicochemical properties of the NiCo₂O₄ materials were characterized via X-ray diffraction, scanning electron microscopy, X-ray photoelectron spectra, and nitrogen sorption isotherms measurements. The electrochemical performances of the NiCo₂O₄ electrode were investigated by cyclic voltammetry, chronopotentiometry, and electrochemical impedance spectroscopy tests. The obtained NiCo₂O₄ materials exhibit typical mesoporous structures, with an average particle size of about 200 nm, a specific surface area of 88.63 m² g^?1, and a total pore volume of 0.337 cm³ g^?1. The facile electrolytes penetration for the mesoporous structures favors high-performance of the NiCo₂O₄ electrode. The NiCo₂O₄ electrode shows a high specific capacitance (591 F g^?1 at 1 A g^?1), high-rate capability (248 F g^?1 at 20 A g^?1), and a good cycling behavior for tested 3,000 cycles, indicating a promising application for electrochemical capacitors.     

Reduced graphene oxide/hierarchal spinel nickel cobaltite nanoflowers composites for supercapacitor electrodes with ultrahigh capacitive and excellent rate performance

Researchers are extensively investigating transition metal oxides due to their unique porous architectural structure and remarkable electrochemical properties, which are suitable to boost the energy storage capabilities. In present work, facile chemical route was used to synthesize hierarchal spinel nickel cobaltite nanoflowers anchored reduced graphene oxide (NiCo₂O₄-rGO) as high performance electrode material. NiCo₂O₄ anchored rGO demonstrated specific capacitance of 2695 Fg^-1 at 1 Ag^-1, which is greater than pristine NiCo₂O₄ nanoflowers specific capacitance. NiCo₂O₄-rGO showed excellent stability and retention capability of 96% after 2500 cycles at 5 Ag^-1. Furthermore, NiCo₂O₄–rGO exhibited maximum energy density of 93.57 WhKg^?1 at power density of 250 WKg^-1. We have achieved specific capacitance and retention capability which is higher than previously reported results. This enhancement is mainly attributed to the spinel structure of NiCo₂O₄ and its robust structural affinity with rGO. Moreover, rGO possesses extended surface area provided ample of active sites and exceptional synergetic effect which helped to enhance the induction and consequently transportation of e^?/h⁺. More importantly due to its special morphological effects, in future NiCo₂O₄ anchored rGO nanoflowers may open new avenue in research but also used as an efficient electrode material for the construction of high performance supercapacitors.     

Decorating carbon nanosheets with copper oxide nanoparticles for boosting the electrochemical performance of symmetric coin cell supercapacitor with different electrolytes

The synergetic combination of double-layer capacitor carbon nanosheets and pseudocapacitive CuO particles with enhanced electrochemical properties had been proposed. Herein, CuO/carbon nanosheets electrode material with outstanding electrochemistry performance was successfully synthesized via a low-cost and controllable strategy. Such rational architecture integrates high-conductivity carbon nanosheets with rich-chemical-activity CuO particles. The surface-functional carbon nanosheets serve as a conductive substrate, provide an efficient pathway and accelerate the fast diffusion of electrons. This electrode material depicts high specific capacitance up to 183.9 and 371.1 F g⁻¹ at 1 A g⁻¹ in Na₂SO₄ and KOH electrolyte using three-electrode tests, respectively. Moreover, two symmetric devices using this CuO/carbon nanosheets electrode material were assembled with different electrolytes. The as-fabricated device with KOH electrolyte delivers remarkable energy density of 19.36 W h kg⁻¹ at power density of 355.6 W kg⁻¹ and still maintains 12.06 W h kg⁻¹ at 1750.7 W kg⁻¹. The as-fabricated device with Na₂SO₄ electrolyte achieves the maximum energy density of 12.46 W h kg⁻¹ at 355.6 W kg⁻¹. The capacitance retention rate is maintained at 94.4% after 2000 cycles in the as-fabricated coin cell supercapacitor with Na₂SO₄ electrolyte, showing outstanding long-cycling life. Herein, the strategic integration of CuO particles with two-dimensional functional carbon nanosheets as the electrode material provides superior electrochemical performance for supercapacitors.     

In situ synthesis of integrated dodecahedron NiO/NiCo2O4 coupled with N-doped porous hollow carbon capsule for high-performance supercapacitors

NiO and NiCo₂O₄ exhibit excellent synergistic effects and broad application prospects in electrochemical applications. However, the apparent interfacial instability between NiO and NiCo₂O₄ limits ion transport kinetics, charge/ion transfer, and electrochemical stability. In response, we developed and designed an integrated dodecahedron NiO/NiCo₂O₄ by a facile in-situ calcination method. Moreover, by utilizing the porous hollow structure of nitrogen-doped carbon capsules (N-Cc) as a conductive network, the N-Cc_x@NiO/NiCo₂O₄ heterostructures with stable interface structure, excellent electrolyte adsorption, and electron transfer pathways were carefully designed. The N-Cc_1.0@NiO/NiCo₂O₄ heterostructures are found to deliver an outstanding specific capacitance of 658.8 F g⁻¹, and a high energy density of 101.40 Wh kg⁻¹ at a power density of 775.03 W kg⁻¹, along with capacitance retention of more than 93.5% after 8000 cycles. Based on the DFT calculations and electrochemical experimental results, this work provides an effective in situ route for the construction of high-performance metal oxide heterostructure electrode materials for new energy storage devices.     

Polypyrrole-wrapped NiCo2S4 nanoneedles as an electrode material for supercapacitor applications

In this study, dicobalt tetrasulfide (NiCo₂S₄) nanoneedles were successfully synthesized by a two-step hydrothermal method on nickel foam. A layer of polypyrrole (PPy) was further wrapped on the surface of the NiCo₂S₄ nanoneedles by in-situ polymerization. The obtained NiCo₂S₄@PPy composite was investigated for supercapacitor applications, which exhibited a capacitance of 1842.8 F g^?1 at 1 A g^?1. An asymmetric supercapacitor device fabricated with an activated carbon negative electrode and NiCo₂S₄@PPy positive electrodes exhibited an energy density of 41.2 Wh kg^?1 at 402.2 W kg^?1 with a high charge–discharge cycling stability (92.8% after 5000 cycles). These results demonstrate that NiCo₂S₄@PPy electrodes have broad application prospects as energy storage electrode materials.     

Hierarchical mesoporous nickel cobaltite nanoneedle/carbon cloth arrays as superior flexible electrodes for supercapacitors

Hierarchical mesoporous NiCo₂O₄ nanoneedle arrays on carbon cloth have been fabricated by a simple hydrothermal approach combined with a post-annealing treatment. Such unique array nanoarchitectures exhibit remarkable electrochemical performance with high capacitance and desirable cycle life at high rates. When evaluated as an electrode material for supercapacitors, the NiCo₂O₄ nanoneedle arrays supported on carbon cloth was able to deliver high specific capacitance of 660 F g^-1 at current densities of 2 A g^-1 in 2 M KOH aqueous solution. In addition, the composite electrode shows excellent mechanical behavior and long-term cyclic stability (91.8% capacitance retention after 3,000 cycles). The fabrication method presented here is facile, cost-effective, and scalable, which may open a new pathway for real device applications.     

A new strategy for porous carbon synthesis: Starch hydrogel as a carbon source for Co3O4@C supercapacitor electrodes

A novel Co₃O₄@C composite with a three-dimensional (3D) interconnected network morphology was successfully fabricated by anchoring cobalt oxide nanocrystals onto porous carbon originating from starch hydrogels via freeze drying, precarbonization and thermal treatment in an aqueous system. Benefiting from unique structural features, the optimized electrode delivers an excellent capacitance of 1314.0 F g^?1 (1 A g^?1) and outstanding durability in terms of capacity preservation (93.5% over 10,000 cycles). In addition, an asymmetric supercapacitor consisting of DF-2 and active carbon exhibits an energy density of 149.1 Wh?kg^?1 at 800 W kg^?1 while maintaining great stability. The observed excellent performance is attributed to the unique 3D network, good conductivity and high surface-to-volumetric ratio of the carbon skeleton derived from the starch gel, which has wide scope for applications.     

Ultra-high specific capacitance of self-doped 3D hierarchical porous turtle shell-derived activated carbon for high-performance supercapacitors

The turtle shell of biomass waste is used as raw material, and the natural inorganic salt contained in it is used as a salt template in combination with a chemical activation method to successfully prepare a high-performance activated carbon with hierarchical porous structure. The role of hydroxyapatite (HAP) and KOH in different stages of preparation was investigated. The prepared turtle shell-derived activated carbon (TSHC-5) has a well-developed honeycomb pore structure, which gives it a high specific surface area (SSA) of 2828 m² g^?1 with a pore volume of 1.91 cm³ g^?1. The excellent hierarchical porous structure and high heteroatom content (O 6.88%, N 5.64%) allow it to have an ultra-high specific capacitance of 727.9 F g^?1 at 0.5 A g^?1 with 92.27% of capacitance retention even after 10,000 cycles. Excitingly, the symmetric supercapacitor assembled from TSHC-5 activated carbon exhibits excellent energy density and cycling stability in a 1 M Na₂SO₄ aqueous solution. The energy density is 45.1 Wh·kg^?1 at a power density of 450 W kg^?1, with 92.05% capacitance retention after 10,000 cycles. Therefore, turtle shell-derived activated carbon is extremely competitive in sustainable new green supercapacitor electrode materials.     

Preparation of NiCo2O4 Nanosheet Arrays and its High Catalytic Performance for H2O2 Electroreduction

Ni foam supporting‐NiCo₂O₄ nanosheet arrays (NiCo₂O₄/Ni foam) are successfully prepared by electrodeposition of the hydroxide precursor, followed by a thermal treatment in air without any template and surfactant. The morphology of NiCo₂O₄ nanosheet arrays is characterized by scanning electron microscope and transmission electron microscopy. The structure is analyzed using X‐ray diffractometer, X‐ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The catalytic performance of NiCo₂O₄/Ni foam electrode for H₂O₂ electroreduction in KOH solution is evaluated by means of cycle voltammetry and chronoamperometry, which exhibits an excellent catalytic performance and superior stability for H₂O₂ electroreduction. The reduction current density of H₂O₂ on NiCo₂O₄/Ni foam electrode is 0.378 A cm⁻² at –0.4 V in the solution containing 3.0 mol L⁻¹ KOH + 0.4 mol L⁻¹ H₂O₂, which is much larger than that on other metal oxides reported previously.     

Enhancing the performance and stability of NiCo2O4 nanoneedle coated on Ni foam electrodes with Ni seed layer for supercapacitor applications

We introduce a facile way to improve the performance of NiCo₂O₄ electrode by including a Ni seed layer. The seed layer deposited on Ni foam electrode (NiCo₂O₄/Ni@NF) shows the superior specific capacity of 1142 C g⁻¹ at 1 A g⁻¹ with the excellent cycle stability of ∼96% even after 5000 cycles at a higher current density of 5 A g⁻¹. These values are about 3.7 times higher than that of the electrode (NiCo₂O₄@NF) without a seed layer, which shows the specific capacity of 305 C g⁻¹@1 A g⁻¹ with cycle stability of 84% even at a lower current density of 1 A g⁻¹. The enhanced performance of the NiCo₂O₄/Ni@NF electrode may be attributed to lower interface resistance, fast redox reversible reaction, and improved surface active sites. Further, the asymmetric solid-state supercapacitor device is fabricated by using the NiCo₂O₄/Ni@NF electrode as a positive and reduced graphene oxide (rGO)-Fe₂O₃ nanograin as a negative electrode with PVA-KOH gel electrolyte, and the NiCo₂O₄/Ni20@NF//rGO-Fe₂O₃@NF asymmetric solid state device delivers an areal capacitance of 446 mF cm⁻² with a low capacitance loss of 18% even after 10000 cycles. Further, the fabricated asymmetric solid state device shows a maximum energy density of 124.3 Wh cm⁻² (at 3.58 kW cm⁻²) and power density of 14.88 kW cm⁻² (at 31.41 Wh cm⁻²).     

Easy synthesis of porous graphene nanosheets and their use in supercapacitors

We report the easy synthesis of porous graphene nanosheets (PGNs) using the etching of graphene sheets by MnO₂. An electrode made from PGNs exhibits a specific capacitance of 154 F g^?1 at 500 mV s^?1 in 6 M KOH compared to a value of 67 F g^?1 for graphene nanosheets, and a low capacitance loss of 12% after 5000 cycles. Interestingly, PGN electrode material shows an excellent rate capability due to its open layered and mesopore structures that facilitate the efficient access of electrolytes to the electrode material and shorten the ion diffusion pathway through the porous sheets. This approach offers the potential for cost-effective, environmentally friendly and large-scale production of PGNs.     

Sol–gel approach for controllable synthesis and electrochemical properties of NiCo2O4 crystals as electrode materials for application in supercapacitors

In this work, NiCo₂O₄ coral-like porous crystals, nanoparticles and submicron-sized particles have been systematically prepared via a facile sol–gel method, using citric acid as the chelating ligand and H₂O (H₂O-DMF) as solvent. The experimental results reveal that the initial molar concentration of reactants, reaction time and solvent species involved are crucial for preparing the target products. The as-obtained samples were characterized by means of XRPD, FESEM, HRTEM, EDS and SAED techniques. Finally, the electrochemical performances of NiCo₂O₄ crystals with distinct morphologies were evaluated by cyclic voltammetry, and galvanostatic charge–discharge cycling techniques. The results show that submicron-sized NiCo₂O₄ particles exhibit the best capacitive properties with high specific capacitance and excellent cycle stability. At a high mass loading (5.6 mg cm⁻²), specific capacitance value of submicron-sized NiCo₂O₄/Ni electrode reaches as much as 217 F g⁻¹, and 96.3% of which can be still maintained after 600 charge–discharge cycles. Therefore, NiCo₂O₄ crystal is very promising for real application in supercapacitors.     

Honeycomb-like NiCo2O4@Ni(OH)2 supported on 3D N−doped graphene/carbon nanotubes sponge as an high performance electrode for Supercapacitor

In order to increase the energy density of supercapacitor, a new kind electrode material with excellent structure and outstanding electrochemical performance is highly desired. In this article, a new type of three-dimensional (3D) nitrogen-doped single-wall carbon nanotubes (SWNTs)/graphene elastic sponge (TRGN?CNTs?S) with low density of 0.8 mg cm^?3 has been successfully prepared by pyrolyzing SWNTs and GO coated commercial polyurethane (PU) sponge. In addition, high performance electrode of the honeycomb-like NiCo₂O₄@Ni(OH)₂/TRGN-CNTs-S with core-shell structure has been successfully fabricated through hydrothermal method and then by annealing treatment and electrochemical deposition method, respectively. Benefited from 3D structural feature, the compressed NiCo₂O₄@Ni(OH)₂/TRGN-CNTs-S electrode exhibits high gravimetric and volumetric capacitance of 1810 F g^?1, 847.7 F cm^?3 at 1 A g^?1. The high rate performance and long-term stability was also obtained. Furthermore, an asymmetric supercapacitor using NiCo₂O₄@Ni(OH)₂/TRGN-CNTs-S cathode and NGN/CNTs anode delivered high gravimetric and volumetric energy density of 54 W h kg^?1 at 799.9 W kg^?1 and 37 W h L^?1 at 561.5 W L^?1. In summary, an excellent electrochemical electrode with new elastic 3D SWNTs/graphene supports and binder free pseudocapacitive materials was introduced.     

Materials and electrode designs of high-performance NiCo2S4/Reduced graphene oxide for supercapacitors

NiCo₂S₄ is one of the most promising bimetallic sulfides for use in energy-storage systems, but more studies are needed to endow NiCo₂S₄ with a high electrochemical reaction capability and reversibility. In this work, we present rationally materials design of an optimal NiCo₂S₄ nanoparticle in a reduced graphene oxide (RGO) matrix as a NiCo₂S₄/RGO nanocomposite. Furthermore, we report the improvements in the materials technology, demonstrating the NiCo₂S₄/RGO nanocomposite electrode with an excellent specific capacitance of 963–700 F g⁻¹ at 1–15 A g⁻¹, high capacitance retention of 70%, and long cycle life of 3000 cycles. The practical application is showcased in an asymmetric supercapacitor with a high active-material loading. The NiCo₂S₄/RGO nanocomposite shows a high energy density of 31 Wh kg⁻¹ at a power density of 987 W kg⁻¹ and maintains an excellent density of 23 Wh kg⁻¹ at a high power density of 7418 W kg⁻¹. The outstanding electrochemical utilization and stability of the NiCo₂S₄/RGO nanocomposite confirm that our systematic optimization in the materials science and technology in terms of the active-material synthesis, the electrode development, and the device design/fabrication would benefit the future development of high-performance supercapacitors.     

Conformal coatings of NiCo2O4 nanoparticles and nanosheets on carbon nanotubes for supercapacitor electrodes

NiCo₂O₄ is a promising electrode material for supercapacitors and it has been widely investigated. However, its low conductivity restricts the reaction kinetics. Combining it with carbon materials can efficiently overcome the issue. But, very limited research about the homogenous coatings of NiCo₂O₄ nanocrystals on carbon nanotubes (CNTs) is reported. In this work, thin nanosheets and small nanoparticles of NiCo₂O₄ densely coated on CNTs are synthesized by tuning the annealing time with a hybrid of metal hydroxide@CNTs as a precursor. In the precursor, core−shell structures are formed by conformally coating 2D metal hydroxides on CNTs. After annealing it at 300 °C for different time, NiCo₂O₄ nanosheets or nanoparticles are then obtained and the core−shell structure is remained. Due to the reduced crystal size of NiCo₂O₄ and the high conductivity of CNTs, the composites have large specific capacitances, excellent rate performances, and good stability. The composite of NiCo₂O₄ nanoparticles on CNTs has a higher specific capacitance, about 1786 F g⁻¹ at 0.5 A g⁻¹, than the hybrid of NiCo₂O₄ nanosheets on CNTs due to their different morphologies. Using the composite as positive electrode and activated carbon as negative electrode, a hybrid capacitor cell can work in a voltage of 1.6 V, delivering an energy density of 32.5 Wh kg⁻¹ at 800 W kg⁻¹, showing a large potential for supercapacitors.     

Impact of electrolyte additives (alkali metal salts) on the capacitive behavior of NiO-based capacitors

To improve the specific capacitance and energy density of electrochemical capacitor, nanostructured NiO was prepared by high temperature solid-state method as electrode material. The crystal structure and morphology of as-parepared NiO samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Cyclic voltammetry (CV) measurement was applied to investigate the specific capacitance of the NiO electrode. Furthermore, a novel mixed electrolyte consisting of NaOH, KOH, LiOH and Li₂CO₃ was prepared for the NiO capacitor, and the component and concentration of the four different electrolytes was examined by orthogonal test. The results showed that the NiO sample has cubic structure with nano-size particles, and the optimal composition of the electrolyte was: NaOH 2 mol L⁻¹, KOH 3 mol L⁻¹, LiOH 0.05 mol L⁻¹, and Li₂CO₃ 0.05 mol L⁻¹. At a scan rate of 10 mV s⁻¹, the fabricated capacitor exhibits excellent electrochemical capacitive performance, while the specific capacitance and the energy density were 239 F g⁻¹ and 85 Wh kg⁻¹, which was higher than one-component electrolyte.     

Uniform and porous Mn-doped Co3O4 microspheres: Solvothermal synthesis and their superior supercapacitor performances

Uniform and porous Mn-doped Co₃O₄ microspheres (Mn@Co₃O₄ MSs) assembled with many nanoparticles (NPs) were prepared through an initially solvothermal reaction and subsequent annealing treatment at 550 ^oC in air. These Mn@Co₃O₄ MSs had an average diameter of about 9?μm and possessed a BET specific surface area of 70.4?m²/g. The pore diameter was mainly centered at 12.3?nm and the mean pore size was measured to be 15?nm. When the Mn@Co₃O₄ MSs were used as electrode material for supercapacitors, the electrochemical performances were assessed in 2?M KOH aqueous solution using a typical three-electrode configuration. Such Mn@Co₃O₄-MSs-modified electrode exhibited a highly specific capacitance of 773?F/g at 1 A/g, 62.7% rate capability at 16 A/g, and 73.9% capacitance retention of its original value after 5000 cycles at 5 A/g. The excellently electrochemical behaviors indicate that such Mn-doped Co₃O₄ MSs can be used as a superior electrode material for advanced supercapacitors in the future. The current synthesis strategy is facile and can be further employed to prepare other electrode materials with outstanding electrochemical performances.     

Preparation of Three-Dimensional Co3O4/graphene Composite for High-Performance Supercapacitors

Here, we developed a simple and efficient route for the preparation of three-dimensional (3D) Co₃O₄-anchored graphene composites using the sacrificial template-assisted method and the subsequent deposition process of Co₃O₄ nanoparticles. As structural guiding materials, polystyrene (PS) spheres provide 3D porous architectures with a high surface area. 3D porous graphene materials serve as conductive supporters for the deposition of Co₃O₄ nanoparticles through precipitation growth. The 3D porous composite structures of Co₃O₄/graphene composites were intensively investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction. The 3D Co₃O₄/graphene composites show a high specific capacitance of 328?F?g^?1 with efficient and fast charge–discharge process in aqueous 6?M KOH electrolyte. In addition, the composites provide a good cycle lifetime, which retained 98% capacitance retention over 2000 cycles.     

Hexagonal boron nitride nanosheet/carbon nanocomposite as a high-performance cathode material towards aqueous asymmetric supercapacitors

The two-dimensional hexagonal boron nitride (h-BN) has garnered tremendous interest due to its unique mechanical, thermal and electronic properties. However, the application of h-BN has been restricted as electrode materials for supercapacitors because of its wide band gap and rather low conductivity. Herein, a carbon-modified hexagonal boron nitride nanosheet (h-BN/C) nanocomposite is prepared through a facile and scalable solid-state reaction. Interestingly, the h-BN/C nanocomposite as cathode material exhibits a pair of distinct and reversible redox peaks in 2?M KOH aqueous electrolyte. Because of the enhanced electrical conductivity derived from the modified carbon and the increased specific surface area, the h-BN/C nanocomposite presents a high specific capacitance of 250?F?g^?1 at the current density of 0.5?A?g^?1. More importantly, the fabricated aqueous asymmetric supercapacitor with the h-BN/C as cathode and activated carbon as anode displays an operating voltage of 1.45?V, an energy density of 17?Wh?kg^?1 at a power density of 245?W?kg^?1, and high stability up to 1000 cycles. Therefore, h-BN/C nanocomposite would promisingly be a cathode material for aqueous asymmetric supercapacitors.     

Ni@NiCo2O4 core/shells composite as electrode material for supercapacitor

A novel Ni@NiCo₂O₄ core/shells structure consisting of the Ni microspheres skeletons and nanosheet-like NiCo₂O₄ skins was designed and investigated as the electrochemical electrode for supercapacitor. Due to the unique architecture with Ni microspheres as the highly conductive cores improving the electrical conductivity of electrode and external nanosheet-like NiCo₂O₄ shells as the efficient electrochemical active materials facilitating the contact between the electrode and electrolyte, the as-prepared Ni@NiCo₂O₄ exhibited excellent electrochemical performance with high specific capacity of 597 F g⁻¹ (1 A g⁻¹) as well as remarkable capacitance retention of 96% (3000 cycles). These impressive results pave the way to design high-performance electrode materials for energy storage.