Scalable synthesis and superior performance of TiO<Subscript>2</Subscript>-reduced graphene oxide composite anode for sodium-ion batteries

TiO₂-reduced graphene oxide (RGO) composite was synthesized via a sol-gel process and investigated as an anode material for sodium-ion batteries (SIBs). A remarkable improvement in sodium ion storage with a reversible capacity of 227 mAh g^?1 after 50 cycles at 50 mA g^?1 is achieved, compared to that (33 mAh g^?1) for TiO₂. The enhanced electrochemical performance of TiO₂-RGO composite is attributed to the larger specific surface area and better electrical conductivity of TiO₂-RGO composite. The excellent performance of TiO₂-RGO composite enables it a potential electrode material for SIBs.     

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.     

Synthesis of graphene/nickel oxide composite with improved electrochemical performance in capacitors

A novel reduced graphene oxide/NiO nanosheet composite (r-GO/NiO) (ca. 75 % NiO in weight) was synthesized by a facile two-step method, where the NiO nanosheets were decorated with some voids. The composite was characterized by using X-ray diffraction, transmission electron microscopy, thermal gravimetric analysis, and Raman spectroscopy. The electrochemical properties of the composite were investigated by cyclic voltammetry, galvanostatic charge, and discharge measurements. The results show that the r-GO/NiO composite exhibits a stable average specific capacitance of ca. 1,139 F g^?1 (at 0.5 A g^?1) during 1,000 charge–discharge cycles, suggesting that the r-GO/NiO composite is a potential supercapacitor material. The main correlation between the electrochemical performance and the structure of the materials was studied, and the formation process of the composite was also discussed.     

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.     

Construction of 3D polypyrrole/CoS/graphene composite electrode with enhanced pseudocapacitive performance

Herein, 3D graphene/nickel foam (GE/NF) composite matrix was successfully fabricated by using NF as template through a self-catalytic thermal chemical vapor deposition process. By using the prepared GE/NF as substrate, CoS nanosheets were deposited via a facial one-step electrochemical deposition method. Owing to the advantage of GE in boosting the electrical contact between the electroactive host material and current collector, the as-prepared 3D CoS/GE/NF electrode demonstrated a superior capacitance value of 2308 F g^?1 at 1 A g^?1 and a high rate capability of 70.49% at 20 A g^?1. After depositing the polypyrrole (PPY) film on 3D CoS/GE/NF electrode, the electrochemical performance of CoS was further greatly improved and delivered an extremely high capacitance value of 3450 F g^?1 at 1 A g^?1, with good rate capability (62.61% at 20 A g^?1) and improved cycling stability. The enhanced electrochemical performance of PPY/CoS/GE/NF electrode is closely related to the advantage of PPY film in increasing the electrical conductivity and reinforcing the integrity of electrode.     

Simple microwave synthesis and improved electrochemical performance of Nb-doped MnO<Subscript>2</Subscript>/reduced graphene oxide composite as anode material for lithium-ion batteries

Niobium-doped MnO₂/reduced graphene oxide (Nb-MnO₂/RGO) composite has been successfully synthesized via a simple microwave radiation method. The samples were systematically studied by X-ray diffraction (XRD), thermogravimetric analysis (TG), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), transmission electron microscope (TEM), and electrochemical measurements. As the anode material for lithium-ion batteries, the Nb-MnO₂/RGO (molar ratio of Mn/Nb?=?50:1) (NMG50) showed an outstanding reversible discharge capacity of 556.6 mAh g^?1 after 50 cycles with a capacity retention of 77% at a charge-discharge rate of 0.1 A g^?1 and the reversible discharge capacity can still retain 223.3 mAh g^?1 at a current of 1 A g^?1, which is much higher than those for Nb-MnO₂/RGO (molar ratio of Mn/Nb?=?10:1) (NMG10) and undoped MnO₂/RGO (MG). The improved electrochemical performance could be attributed to the proper amount of Nb doping, which could enhance both the conductivity and the structure stability of MnO₂.     

MnO2/graphite electrodeposited under supergravity field for supercapacitors and its electrochemical properties

MnO₂/graphite electrode material is successfully synthesized by electrodeposition under supergravity field from manganese acetate and graphite suspending solution. X-ray diffraction and field emission scanning electron microscopy show that the obtained composite is γ-MnO₂/graphite. The process of depositing the MnO₂/graphite was shown by the schematic illustration. Galvanostatic charge/discharge and cyclic voltammograms tests are applied to investigate electrochemical performances of the composite electrodes prepared under supergravity fields. MnO₂/graphite synthesized under supergravity field exhibits good discharge capacitance and the specific capacitance is 367.77 F g^?1 at current density of 0.5 A g^?1. It is found that supergravity field has effects on the electrochemical performances of MnO₂/graphite material.     

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.     

Synthesis of δ-MnO<Subscript>2</Subscript> film on FTO glass with high electrochemical performance

δ-Manganese dioxide (MnO₂) has been proved to own the excellent electrochemical performances for a long time. But few of studies report the electrochemical performances of δ-MnO₂ film. Here, we synthesize δ-MnO₂ film on fluorine-doped tin oxide (FTO) glass via a simple redox reaction at the room temperature. The X-ray diffraction (XRD) and Raman spectroscopy are used to confirm the physical structure, whilst cyclic voltammetry and galvanostatic charge-discharge measurements are performed to investigate the electrochemical performances. Encouragingly, δ-MnO₂ film delivers a high specific capacitance (C _s) of 350.5 F g^?1 at 100 mV s^?1 and 275.0 F g^?1 at 5 A g^?1. The capacitance retention of δ-MnO₂ film can be up to 100 % after being charge/discharge at 2 A g^?1 with 1000 cycles. This research might further indicate that δ-MnO₂ film is a promising electrode material for supercapacitors.     

Enhanced electrochemical properties of Al-doped bulk manganese oxides synthesized by a facile liquid-phase method

Aluminum doped MnO₂ nanoparticles were synthesized by a simple liquid-phase process using potassium permanganate as oxidation agent, glycol as reducing agent. Specific capacitance of the optimal sample electrode can reach 290 F g^?1 after 10 cycles. The electrode also exhibits excellent cycle stability, retaining 86.6 % after 1,000 cycles. The infrared absorption bands of aluminum doped manganese oxide shift to high wave number for the reason that aluminum ion has smaller nuclear charge. The doping of aluminum strengthens the Mn–O bond and decreases the aggregation degree, thus the electrochemical properties are enhanced.     

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 electrochemical performance of porous activated carbon by forming composite with graphene as high-performance supercapacitor electrode material

In this work, a novel activated carbon containing graphene composite was developed using a fast, simple, and green ultrasonic-assisted method. Graphene is more likely a framework which provides support for activated carbon (AC) particles to form hierarchical microstructure of carbon composite. Scanning electron microscope (SEM), transmission electron microscope (TEM), Brunauer–Emmett–Teller (BET) surface area measurement, thermogravimetric analysis (TGA), Raman spectra analysis, XRD, and XPS were used to analyze the morphology and surface structure of the composite. The electrochemical properties of the supercapacitor electrode based on the as-prepared carbon composite were investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), charge/discharge, and cycling performance measurements. It exhibited better electrochemical performance including higher specific capacitance (284 F g^?1 at a current density of 0.5 A g^?1), better rate behavior (70.7% retention), and more stable cycling performance (no capacitance fading even after 2000 cycles). It is easier for us to find that the composite produced by our method was superior to pristine AC in terms of electrochemical performance due to the unique conductive network between graphene and AC.     

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.     

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.     

Supercapacitor performances of rich nitrogen-doped mesoporous graphene fabricated by a facile template-free copyrolysis process

Rich nitrogen-doped mesoporous graphene (NDMG) with a large specific surface area of 496.8 m² g^?1 and high electrical conductivity of 327.2 S cm^?1, and suitable pore size was synthesized by a facile co-thermal annealing of pre-prepared phenolic polymer and dicyandiamide. The NDMG has a high nitrogen content (7.9 wt%) and can act as promising electroactive materials for two-electrode symmetric supercapacitors. The NDMG cells displayed a high specific capacitance of ca. 316 F g^?1 at 0.5 A g^?1, which is much higher than that of the pristine graphene devices (ca. 123 F g^?1). Moreover, compared with the capacitance drop rate of pristine graphene devices (8.9 %), the specific capacitance of NDMG cells was decreased by only 3.2 % after 2000 cycles, exhibiting a good cycling performance and reversibility. In addition, the specific capacitance of the NDMG cells can reach 251 F g^?1 at 5.0 A g^?1, revealing an excellent rate capability and implying the ability to deliver a high energy density at a high power density. The good electrochemical performances of NDMG can be attributed to its high surface area, suitable mesopore size, and high electrical conductivity.     

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.     

New insight on nanostructure assembling of high-performance electrode materials: synthesis of surface-modified hexagonal β-Ni(OH)<Subscript>2</Subscript> nanosheets as an example

Hexagonal β-Ni(OH)₂ nanosheets with thickness of ～12 nm were synthesized by a hydrothermal method at 150 °C using nickel chloride as nickel source and morpholine as alkaline. Electrodes for application in pseudocapacitor were assembled through a traditional technique: pressing a mixture of β-Ni(OH)₂ nanosheets and acetylene black onto nickel foam. Due to the hexagonal shape of rigid β-Ni(OH)₂ nanosheet and the mediation of surface-modified glycerol during electrochemical charge–discharge cycles, a nanostructure of electrode material with facile interior pathway for the transfer of electrolyte was formed. As a result, the as-formed electrodes presented high specific capacitance of 1,917 F g^?1 at current density of 1.6 A g^?1 in 3 mol L^?1 KOH solution. At high charge and discharge current density of 31.3 A g^?1, the electrodes still remained a high specific capacitance of 1,289 F g^?1. The interesting results obtained from this investigation may provide a new insight for the synthesis of electrode materials with high electrochemical performance.     

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.     

Honeycomb-like manganese oxide nanospheres for redox supercapacitors

Honeycomb-like MnO₂ nanospheres were synthesized using stainless steel substrates by a facile chemical bath deposition method. The obtained nanospheres were about 200–400 nm in diameter and consisted of porous ultrathin nanosheets. Honeycomb-like MnO₂ nanospheres exhibited a high specific capacitance of 240 F g^?1 and 87.1% capacitance retention after 1000 cycles at a current density of 0.5 A g^?1. These remarkable electrochemical results imply great potential for applications of the honeycomb-like MnO₂ nanospheres in supercapacitors.     

Mesoporous MnCo2O4 spinel oxide nanostructure synthesized by solvothermal technique for supercapacitor

A manganese cobaltite spinel oxide was synthesized successfully via d-glucose-assisted solvothermal process. The structure and morphology of the sample heat treated at 300 and 400 °C for 6 h has been studied with X-ray diffraction, scanning electron microscope, and transmission electron microscope. Cyclic voltammograms at different scan rate have demonstrated that an excellent capacitance feature of MnCo₂O₄ spinel oxide electrode. Pseudotype-capacitive behavior of the sample was further corroborated by the charge–discharge measurements at various current densities. The estimated specific capacitance of spinel oxides with two calcination temperature was found to be 189 and 346 F g^?1 at a constant current density of 1 A g^?1. Observed specific capacitance and excellent cyclic stability of MnCo₂O₄ spinel oxide has ascribed to their high surface area and mesoporous microstructure. This facilitates to easy electrolyte ion intercalation and deintercalation at electrode/electrolyte interface. In this study, we suggest that the MnCo₂O₄ spinel nanostructure with high surface area and desired cation distribution could be a promising electrode material for next-generation high-performance supercapacitor.