Sucrose-templated interconnected meso/macro-porous 2D symmetric graphitic carbon networks as supports for α-Fe2O3 towards improved supercapacitive behavior

In this study, ultrahigh electrochemical performance for interconnected meso/macro-porous 2D C@α-Fe₂O₃ synthesized via sucrose-assisted microwave combustion is demonstrated. Hematite (α-Fe₂O₃) synthesized via the same approach gave an encouraging electrochemical performance close to its theoretical value, justifying its consideration as a potential supercapacitor electrode material; nonetheless, its specific capacitance was still low. The pore size distribution as well as the specific surface of bare α-Fe₂O₃ improved from 145 m² g⁻¹ to 297.3 m² g⁻¹ after it was coated with sucrose, which was endowed with ordered symmetric single-layer graphene (2D graphene). Accordingly, the optimized hematite material (2D C@α-Fe₂O₃) showed a specific capacitance of 1876.7 F g⁻¹ at a current density of 1 A g⁻¹ and capacity retention of 95.9% after 4000 cycles. Moreover, the material exhibited an ultrahigh energy density of 93.8 W h kg⁻¹ at a power density of 150 W kg⁻¹. The synergistic effect created by carbon-coating α-Fe₂O₃ resulted in modest electrochemical performance owing to extremely low charge transfer resistance at the electrode–electrolyte interface with many active sites for ionic reactions and efficient diffusion process. This 2D C@α-Fe₂O₃ electrode material has the capacity to develop into a cost-effective and stable electrode for future high-energy-capacity supercapacitors.

In this study, ultrahigh electrochemical performance for interconnected meso/macro-porous 2D C@α-Fe₂O₃ synthesized via sucrose-assisted microwave combustion is demonstrated.     

Hexadecyl trimethyl ammonium bromide assisted growth of NiCo2O4@reduced graphene oxide/ nickel foam nanoneedle arrays with enhanced performance for supercapacitor electrodes

NiCo₂O₄@reduced graphene oxide (rGO)/nickel foam (NF) composites were prepared via a hydrothermal method followed by annealing assisted by hexadecyl trimethyl ammonium bromide (CTAB). NiCo₂O₄@rGO/NF nanoneedle arrays grew directly on Ni foam (NF) without using a binder. The effect of graphene oxide (GO) concentration on the electrochemical properties of the composite was studied. When the GO concentration was 5 mg L⁻¹, the as-prepared NiCo₂O₄@rGO/NF reaches the highest specific capacitance of 1644 F g⁻¹ at a current density of 1 A g⁻¹. Even at 15 A g⁻¹, the specific capacitance is still 1167 F g⁻¹ and the capacitance retention rate is 89% after 10 000 cycles at 10 A g⁻¹. Furthermore, a NiCo₂O₄@rGO/NF//graphene hydrogel (GH) asymmetric supercapacitor cell (ASC) device was assembled and exhibits a high specific capacitance of 84.13 F g⁻¹ at 1 A g⁻¹ and excellent cycle stability (113% capacitance retention) after 10 000 charge/discharge cycles at 10 A g⁻¹. This provides potential for application in the field of supercapacitors due to the outstanding specific capacitance, rate performance and cycle stability of NiCo₂O₄@rGO/NF.

Anisotropic NiCo₂O₄ nanoneedle arrays grew directly on Ni foam in the presence of rGO via the hydrothermal method followed by annealing assisted by hexadecyl trimethyl ammonium bromide (CTAB).     

One-pot synthesis of CoFe2O4/rGO hybrid hydrogels with 3D networks for high capacity electrochemical energy storage devices

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach. The three-dimensional (3D) network structure consists of well-dispersed CoFe₂O₄ nanoparticles on the surfaces of graphene sheets. As a binder-free electrode material for supercapacitors, the electrochemical properties of the CoFe₂O₄/rGO hybrid hydrogel can be easily adjusted by changing the concentration of the graphene oxide (GO) precursor solution. The results indicate that the hybrid material made using 3.5 mg mL⁻¹ GO solution exhibits an outstanding specific capacitance of 356 F g⁻¹ at 0.5 A g⁻¹, 68% higher than the pure CoFe₂O₄ counterpart (111 F g⁻¹ at 0.5 A g⁻¹), owing to the large specific surface area and good electric conductivity. Additionally, an electrochemical energy storage device based on CoFe₂O₄/rGO and rGO was assembled, which exhibits a high energy density of 17.84 W h kg⁻¹ at a power density of 650 W kg⁻¹ and an excellent cycling stability with 87% capacitance retention at 5 A g⁻¹ after 4000 cycles. This work takes one step further towards the development of 3D hybrid hydrogel supercapacitors and highlights their potential application in energy storage devices.

CoFe₂O₄/reduced graphene oxide (CoFe₂O₄/rGO) hydrogel was synthesized in situ via a facile one-pot solvothermal approach.     

MOF-derived (MoS2, γ-Fe2O3)/graphene Z-scheme photocatalysts with excellent activity for oxygen evolution under visible light irradiation

Constructing Z-scheme heterojunctions is considered as an effective strategy to obtain catalysts of high efficiency in electron–hole separation in photocatalysis. Unfortunately, suitable heterojunctions are difficult to fabricate because the direct interaction between two semiconductors may lead to unpredictable negative effects such as electron scattering or electron trapping due to the existence of defects which causes the formation of new substances. Furthermore, the van der Waals contact between two semiconductors also results in bad electron diffusion. In this work, a MOF-derived carbon material as a Z-scheme photocatalyst was synthesized via one-step thermal treatment of MoS₂ dots @Fe-MOF (MIL-101). Under visible light irradiation, the well-constructed Z-scheme (MoS₂, γ-Fe₂O₃)/graphene photocatalyst shows 2-fold photocatalytic oxygen evolution activity (4400 μmol g⁻¹ h⁻¹) compared to that of γ-Fe₂O₃/graphene (2053 μmol g⁻¹ h⁻¹). Based on ultraviolet photoelectron spectrometry (UPS), Mott–Schottky plot, photocurrent and photoluminescence spectroscopy (PL) results, the photo-induced electrons from the conduction band of γ-Fe₂O₃ could transport quickly to the valence band of MoS₂via highly conductive graphene as an electron transport channel, which could significantly enhance the electron–hole separation efficiency as well as photocatalytic performance.

The heterojunction between MoS₂ and γ-Fe₂O₃ was constructed via linking by in situ formed graphene, which resulted in a good photocatalyst for the oxygen evolution reaction, showing O₂ evolution activity of 4400 μmol g⁻¹ h⁻¹.     

Facile one-step hydrothermal synthesis of α-Fe2O3/g-C3N4 composites for the synergistic adsorption and photodegradation of dyes

With the expansion of industrialization, dye pollution has become a significant hazard to humans and aquatic ecosystems. In this study, α-Fe₂O₃/g-C₃N₄-R (where R is the relative percentage of α-Fe₂O₃) composites were fabricated by a one-step method. The as-prepared α-Fe₂O₃/g-C₃N₄-0.5 composites showed excellent adsorption capacities for methyl orange (MO, 69.91 mg g⁻¹) and methylene blue (MB, 29.46 mg g⁻¹), surpassing those of g-C₃N₄ and many other materials. Moreover, the ionic strength and initial pH influenced the adsorption process. Relatively, the adsorption isotherms best fitted the Freundlich model, and the pseudo-second-order kinetic model could accurately describe the kinetics for the adsorption of MO and MB by α-Fe₂O₃/g-C₃N₄-0.5. Electrostatic interaction and π–π electron donor–acceptor interaction were the major mechanisms for MO/MB adsorption. In addition, the photocatalytic experiment results showed that more than 79% of the added MO/MB was removed within 150 min. The experimental results of free-radical capture revealed that holes (h⁺) were the major reaction species for the photodegradation of MO, whereas MB was reduced by the synergistic effect of hydroxyl radicals (·OH) and holes (h⁺). This study suggests that the α-Fe₂O₃/g-C₃N₄ composites have an application potential for the removal of dyes from wastewater.

Simple one-step hydrothermal synthesis of α-Fe₂O₃/g-C₃N₄ composites for the synergistic adsorption and photodegradation of dyes     

Optimized NiCo2O4/rGO hybrid nanostructures on carbon fiber as an electrode for asymmetric supercapacitors

The NiCo₂O₄ nanowires and reduced graphene oxide (rGO) hybrid nanostructure has been constructed on carbon fibers (NiCo₂O₄/rGO/CF) via a hydrothermal method. The effects of graphene oxide (GO) concentration on the structure and performance of the NiCo₂O₄/rGO/CF were investigated in detail to obtain the optimized electrode. When the GO concentration was 0.4 mg ml⁻¹, the rGO/NiCo₂O₄/CF composite exhibited a maximum specific capacitance of 931.7 F g⁻¹ at 1 A g⁻¹, while that of NiCo₂O₄/CF was 704.9 F g⁻¹. Furthermore, the NiCo₂O₄/rGO/CF//AC asymmetric supercapacitor with a maximum specific capacitance of 61.2 F g⁻¹ at 1 A g⁻¹ was fabricated, which delivered a maximum energy density (24.6 W h kg⁻¹) and a maximum power density (8477.7 W kg⁻¹). Results suggested that the NiCo₂O₄/rGO/CF composite would be a desirable electrode for flexible supercapacitors.

The NiCo₂O₄ nanowires and rGO hybrid nanostructure was constructed on carbon fibers (NiCo₂O₄/rGO/CF) via a hydrothermal method.     

N and S co-doped graphene enfolded Ni–Co-layered double hydroxides: an excellent electrode material for high-performance energy storage devices

The performance of hybrid supercapacitors (HSCs) can be increased via the selection of higher capacitive electrode materials. Thus, layered double hydroxides (LDHs) have received extensive consideration in HSCs owing to their good ion-exchange properties, structural flexibility, and large specific surface area. Ni–Co-based LDHs show better specific capacitance, good synergy, and high-rate capability in aqueous electrolytes. However, LDHs suffer from low conductivity, which curbs the charge transfer and mass diffusion throughout the electrochemical process. Thus, the high performance of LDH-based supercapacitors is impeded. Hence, composites of LDH and conducting materials are used. Owing to its extraordinary conducting property, huge surface area, and cost-effectiveness, reduced graphene oxide (rGO) is used as conducting material for LDH-based composite electrodes. Moreover, via the incorporation of heteroatoms (N, S, etc.) into rGO, its electrochemical properties are further enhanced. Here, a novel composite electrode is prepared by wrapping Ni–Co-LDH with N and S-co-doped rGO (LDH-rGO-NS) via a hydrothermal process. The XPS C 1s spectra established the existence of N and S doping in the rGO. The electrochemical performance is enhanced due to an excellent ion/charge transfer rate because of N and S co-doping. The LDH-rGO-NS electrode offers a good charge transfer resistance of 0.24 Ω. The obtained anodic and cathodic b-values are 0.73 and 0.72, respectively. An admirable specific capacitance of 1388 F g⁻¹ is accomplished at a sweep rate of 100 mV s⁻¹. Furthermore, the obtained retention capacity is ∼71% after 2000 cycles. Moreover, the achieved specific capacitance is 2193 F g⁻¹ at the discharge current density of 5 A g⁻¹. The excellent electrochemical properties reveal the LDH-rGO-NS composites as encouraging electrode materials for HSCs.

A novel graphene embedded Ni–Co-LDH electrode was developed. The charge transportation rate was enhanced via N and S heteroatom doping, which results in an excellent discharge capacitance of 2193 F g⁻¹ at 5 A g⁻¹.     

Enhanced electrochemical performance of α-MoO3/graphene nanocomposites prepared by an in situ microwave irradiation technique for energy storage applications

Nanoparticles of α-molybdenum oxide (α-MoO₃) are directly grown on graphene sheets using a surfactant-free facile one step ultrafast in situ microwave irradiation method. The prepared α-MoO₃ and α-MoO₃/G nanocomposites are analysed by different characterization techniques to study their structural, morphological and optical properties. Transmission electron microscope images reveal the intercalation of three dimensional (3D) α-MoO₃ nanoparticles into 2D graphene sheets without any agglomeration. The electrochemical results exhibit improved performance for the α-MoO₃/G composite electrode compared to pristine α-MoO₃ owing to its structural superiority. The specific capacitance (C_s) values of the α-MoO₃/G composite and pristine α-MoO₃ are measured to be 483 and 142 F g⁻¹ respectively at a current density of 1 A g⁻¹. The α-MoO₃/G composite maintains a very strong cyclic performance after 5000 cycles. The capacitance retention of the composite electrode shows stable behavior without any degradation confirming its suitability as an enduring electrode material for high-performance supercapacitor applications.

Nanoparticles of α-molybdenum oxide (α-MoO₃) are directly grown on graphene sheets using a surfactant-free facile one step ultrafast in situ microwave irradiation method.     

Enhanced capacitive performance of cathodically reduced titania nanotubes pulsed deposited with Mn2O3 as supercapacitor electrode

A facile and simple pulse electrodeposition method was employed to deposit Mn₂O₃ nanoparticles on cathodically reduced titania nanotubes (R-TNTs) at different deposition time in the range of 3–15 min to investigate the influence of mass loading of Mn₂O₃ on the electrochemical performance of Mn₂O₃/R-TNTs nanocomposite for supercapacitor application. Mn₂O₃ nanoparticles were deposited on circumference of R-TNTs as well as in the nanotubes as revealed by FESEM images for all the deposited time. XPS result confirmed the presence of MnO₂ (Mn⁴⁺) and MnO (Mn²⁺) on the Mn₂O₃/R-TNTs composite which provide pseudocapacitive behaviour for the electrode. Mass loading of Mn₂O₃ increased linearly with deposition time as confirmed by EDX analysis. The sample deposited for 12 min exhibits the highest areal capacitance of 51 mF cm⁻² (which is 22 times enhancement over R-TNTs) at a current density of 0.1 mA cm⁻² and specific capacitance of 325 F g⁻¹ at 6 A g⁻¹. The sample also show a high-rate capability by retaining 80% of its capacitance even at higher current density of 30 A g⁻¹. Interestingly, it retained 98% of the capacitance over 5000 charge discharge cycles at 10 A g⁻¹ after initial drop to 95% at 200th cycles suggesting an excellent long-term chemical stability. A considerably low equivalent series resistance (ESR) and charge transfer resistance (R_ct) of 9.6 Ω and 0.4 Ω respectively was deduced from electrochemical impedance spectroscopy (EIS) analysis indicating good conductivity and improved charge transfer efficiency of Mn₂O₃/R-TNTs nanocomposite.

The mass loading of Mn₂O₃ by pulse electrodeposition (PED) onto reduced titania nanotubes (R-TNTs) greatly influences the electrochemical performance of the composite.     

Hematite iron oxide nanoparticles: apoptosis of myoblast cancer cells and their arithmetical assessment

Hematite (α-Fe₂O₃) forms iron oxide nanoparticles (NPs) which are thermally stable and have various electrochemical and optochemical applications. Due to their wide applicability, the present work was designed to form the hematite phase of iron oxide (αFe₂O₃NPs) NPs prepared via a solution process. Their cytological performance was checked with C2C12 cells. The crystalline property of the NPs was examined with X-ray diffraction patterns (XRD) and it was found that the size of the particles formed ranged from 12 to 15 nm. Structural information was also identified via field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM), which again confirmed that the size of each NP is about 12–15 nm. Surface topographical analysis was done via atomic force microscopy (AFM), which reveals that the size of the distance between two particles is in the range of 12 ± 3 nm. The C2C12 cells were cultured in a humidified environment with 5% CO₂ and were checked via a microscope. The αFe₂O₃NPs were used for cytotoxic evaluation against C2C12 cells. A MTT (3-(4,5-dimethyl thiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay was utilized to check the viability of cells in a dose-dependent (100 ng mL⁻¹, 500 ng mL⁻¹ or 1000 ng mL⁻¹) manner. The morphology of the cells under the influence of αFe₂O₃NPs for live and dead cells in a wet environment was confirmed via confocal laser scanning microscopy (CLSM). The apoptosis caused due to the αFe₂O₃NPs was evaluated in presence of caspases 3/7 with GAPDH genes, which confirmed the upregulation that is responsible in caspase 3/7 genes, with treatment of C2C12 at low (500 ng mL⁻¹) and high (1000 ng mL⁻¹) doses of αFe₂O₃NPs. Analytical studies were also performed to authenticate the obtained data for αFe₂O₃NPs using parameters such as precision, accuracy, linearity, limits of detection (LOD) and limit of quantitation (LOQ), quantitative recoveries and relative standard deviation (RSD). The analyses play a significant role in investigating the large effect of αFe₂O₃NPs on C2C12 cells.

Hematite (α-Fe₂O₃) forms iron oxide nanoparticles (NPs) which are thermally stable and have various electrochemical and optochemical applications.     

Synergies of co-doping in ultra-thin hematite photoanodes for solar water oxidation: In and Ti as representative case

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. The challenge is to develop corrosion resistant and chemically stable semiconductors that absorb sunlight in the visible region and, at the same time, have the band edges matching with the redox level of water. In this work, hematite (α-Fe₂O₃) thin films were prepared onto an indium-doped tin oxide (ITO; In:SnO₂) substrate by e-beam evaporation of Fe, followed by air annealing at two different temperatures: 350 and 500 °C. The samples annealed at 500 °C show an in situ diffusion of indium from the ITO substrate to the surface of α-Fe₂O₃, where it acts as a dopant and enhances the photoelectrochemical properties of hematite. Structural, optical, chemical and photoelectrochemical analysis reveal that the diffusion of In at 500 °C enhances the optical absorption, increases the electrode–electrolyte contact area by changing the surface topology, improves the carrier concentration and shifts the flat band potential in the cathodic direction. Further enhancement in photocurrent density was observed by ex situ diffusion of Ti, deposited in the form of nanodisks, from the top surface to the bulk. The in situ In diffused α-Fe₂O₃ photoanode exhibits an improved photoelectrochemical performance, with a photocurrent density of 145 μA cm⁻² at 1.23 V_RHE, compared to 37 μA cm⁻² for the photoanode prepared at 350 °C; it also decreases the photocurrent onset potential from 1.13 V to 1.09 V. However, the In/Ti co-doped sample exhibits an even higher photocurrent density of 290 μA cm⁻² at 1.23 V_RHE and the photocurrent onset potential decreases to 0.93 V_RHE, which is attributed to the additional doping and to the surface becoming more favorable to charge separation.

Solar energy induced water splitting in photoelectrochemical (PEC) cells is one of the most sustainable ways of hydrogen production. In this work, hematite (α-Fe₂O₃) thin film were modified by In³⁺ and Ti⁴⁺ co-doping for enhanced PEC performance.     

Enhanced electron transfer mediated detection of hydrogen peroxide using a silver nanoparticle–reduced graphene oxide–polyaniline fabricated electrochemical sensor

The current study aims at the development of an electrochemical sensor based on a silver nanoparticle–reduced graphene oxide–polyaniline (AgNPs–rGO–PANI) nanocomposite for the sensitive and selective detection of hydrogen peroxide (H₂O₂). The nanocomposite was fabricated by simple in situ synthesis of PANI at the surface of rGO sheet which was followed by stirring with AEC biosynthesized AgNPs to form a nanocomposite. The AgNPs, GO, rGO, PANI, rGO–PANI, and AgNPs–rGO–PANI nanocomposite and their interaction were studied by UV-vis, FTIR, XRD, SEM, EDX and XPS analysis. AgNPs–rGO–PANI nanocomposite was loaded (0.5 mg cm⁻²) on a glassy carbon electrode (GCE) where the active surface area was maintained at 0.2 cm² for investigation of the electrochemical properties. It was found that AgNPs–rGO–PANI–GCE had high sensitivity towards the reduction of H₂O₂ than AgNPs–rGO which occurred at −0.4 V vs. SCE due to the presence of PANI (AgNPs have direct electronic interaction with N atom of the PANI backbone) which enhanced the rate of transfer of electron during the electrochemical reduction of H₂O₂. The calibration plots of H₂O₂ electrochemical detection was established in the range of 0.01 μM to 1000 μM (R² = 0.99) with a detection limit of 50 nM, the response time of about 5 s at a signal-to-noise ratio (S/N = 3). The sensitivity was calculated as 14.7 μA mM⁻¹ cm⁻² which indicated a significant potential as a non-enzymatic H₂O₂ sensor.

The current study aims at the development of an electrochemical sensor based on a silver nanoparticle–reduced graphene oxide–polyaniline (AgNPs–rGO–PANI) nanocomposite for the sensitive and selective detection of hydrogen peroxide (H₂O₂).     

Template-assisted synthesis of NiCoO2 nanocages/reduced graphene oxide composites as high-performance electrodes for supercapacitors

Here we reported a coordinating etching and precipitating method to synthesize a complex binary metal oxides hollow cubic structure. A novel NiCoO₂/rGO composite with a structure of NiCoO₂ nanocages anchored on layers of reduced graphene oxide (rGO) were synthesized via a simple template-assisted method and the electrochemical performance was investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy tests as a faradaic electrode for supercapacitors at a graphene weight ratio of 1 wt% (1%). When used as electrode materials for electrochemical capacitors, the NiCoO₂/rGO composites achieved a specific capacity of 1375 F g⁻¹ at the current density of 1 A g⁻¹ and maintained 742 F g⁻¹ at 10 A g⁻¹. After 3000 cycles, the supercapacitor based on these nanocage structures shows long-term cycling performance with a high capacity of 778 F g⁻¹ at a current density of 1 A g⁻¹. These outstanding electrochemical performances are primarily attributed to the special morphological structure and the combination of mixed transition metal oxides and rGO, which not only maintains a high electrical conductivity for the overall electrode but also prevents the aggregation and volume expansion of electrochemical materials during the cycling processes.

Here we reported a coordinating etching and precipitating method to synthesize a complex binary metal oxides hollow cubic structure.     

Morphology control of α-Fe2O3 towards super electrochemistry performance

α-Fe₂O₃ with various morphologies including spindle, rod, tube, disk, and ring were synthesized through controlling the H₂PO₄⁻ etching process. The concentrations of H₂PO₄⁻ plays an important role in controlling the morphology change of the samples. Selected adsorption of H₂PO₄⁻ ions resulted in anisotropic growth. In addition, the etching of H₂PO₄⁻ occurred in the center of rods which resulted in tubal α-Fe₂O₃. Nanodiscs were created once the etching process occurred on the wall of the tube. The electrochemical test shows that disklike samples revealed excellent specific capacitance, rate capacity and cycling stability because of relative higher surface area and pore structure. For the CO catalytic oxidation properties, spindle samples exhibited super catalytic activity.

α-Fe₂O₃ with various morphologies including spindle, rod, tube, disk, and ring were synthesized through controlling the H₂PO₄⁻ etching process.     

Development of high-utilization honeycomb-like α-Ni(OH)2 for asymmetric supercapacitors with excellent capacitance

The low utilization rate of active materials has been a critical obstacle for the industrialization of ultracapacitors. In this study, a thin layer of cross-structured ultrathin α-Ni(OH)₂ nanosheets was successfully grown in situ on the surface of a nickel foam as a high-conductivity framework by a vibratory water bath route under a low temperature (80 °C) and mild conditions. Combining the ultrathin α-Ni(OH)₂ nanosheets and ultrashort electron transport, the strategy of a perfect intercalation structure of α-Ni(OH)₂ and a thin layer of active material on a continuous conductive framework resulted in a high utilization rate of active material, which further achieved high specific capacitance of 213.55 F g⁻¹ at 1 A g⁻¹ in a two-electrode system and high capacitance retention from three to two electrode system (753.79 F g⁻¹ at 1 A g⁻¹ in the three-electrode system). Meanwhile, the device also achieved high energy density of 74.94 W h kg⁻¹ at power density of 197.4 W kg⁻¹ and still retained 24.87 W h kg⁻¹ at power density of 3642 W kg⁻¹.

The low utilization rate of active materials has been a critical obstacle for the industrialization of ultracapacitors.     

In situ self-assembly of Ni3S2/MnS/CuS/reduced graphene composite on nickel foam for high power supercapacitors

Transition metal sulfides (TMS), as promising electroactive materials for asymmetric supercapacitors, have been limited due to their relatively poor conductivity and cycle stability. Here ternary Ni₃S₂/MnS/CuS composites were assembled in situ on nickel foam (NF) using a hydrothermal method via electrostatic adsorption of Ni⁺, Mn²⁺ and Cu²⁺ ions on a reduced graphene (rGO) nanosheet template. The chemical structure was characterized by various analytic methods. Ni₃S₂/MnS/CuS has spherical morphology assembled from closely packed nanosheets, while Ni₃S₂/MnS/CuS@rGO has a three-dimensional porous spherical structure with much lower diameter because rGO nanosheets can play the role of a template to induce the growth of Ni₃S₂/MnS/CuS. At a current density of 1 A g⁻¹, the specific capacitance was obtained to be 1028 F g⁻¹ for Ni₃S₂/MnS/CuS, 628.6 F g⁻¹ for Ni₃S₂/MnS@rGO, and 2042 F g⁻¹ for Ni₃S₂/MnS/CuS@rGO, respectively. Charge transfer resistance (R_ct) of Ni₃S₂/MnS/CuS@rGO (0.001 Ω) was much lower than that of Ni₃S₂/MnS@rGO by 0.02 Ω, and lower than that of Ni₃S₂/MnS/CuS by 0.017 Ω. After 5000 cycles, the Ni₃S₂–MnS–CuS@RGO electrode maintains 78.3% of the initial capacity at 20 A g⁻¹. An asymmetric supercapacitor was subsequently assembled using Ni₃S₂/MnS/CuS@rGO as the positive electrode and rGO as the negative electrode. The specific capacitance of asymmetric batteries was maintained at 90.8% of the initial state after 5000 GCD.

Transition metal sulfides (TMS), as promising electroactive materials for asymmetric supercapacitors, have been limited due to their relatively poor conductivity and cycle stability.     

Magnetite ultrafine particles/porous reduced graphene oxide in situ grown onto Ni foam as a binder-free electrode for supercapacitors

Here, we report a simple and green electrochemical route to fabricate a porous network of a Fe₃O₄ nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite supported on a nickel-foam substrate, which is directly used as a binder-free charge storage electrode. Through this method, pristine Fe₃O₄ NPs/Ni, p-rGO/Ni and Fe₃O₄ NPs@p-rGO/Ni electrodes are fabricated and compared. In the fabricated Fe₃O₄ NPs@p-rGO/Ni electrode, the porous rGO sheets served as a conductive network to facilitate the collection and transportation of electrons during the charge/discharge cycles, improving the conductivity of magnetite NPs and providing a larger specific surface area. As a result, the Fe₃O₄ NPs@p-rGO/Ni exhibited a specific capacitance of 1323 F g⁻¹ at 0.5 A g⁻¹ and 79% capacitance retention when the current density is increased 20 times, where the Fe₃O₄ NPs/Ni electrode showed low specific capacitance of 357 F g⁻¹ and 43% capacity retention. Furthermore, the composite electrode kept 95.1% and 86.7% of its initial capacitances at the current densities of 1 and 4 A g⁻¹, respectively, which were higher than those of a Fe₃O₄/NF electrode at similar loads (i.e. 80.4% and 65.9% capacitance retentions at 1 and 4 A g⁻¹, respectively). These beneficial effects proved the synergistic contribution between p-rGO and Fe₃O₄. Hence, such ultrafine magnetite particles grown onto a porous reduced GO network directly imprinted onto a Ni substrate could be a promising candidate for high performance energy storage aims.

Here, we report a simple and green electrochemical route to deposition of Fe₃O₄ nanoparticle-porous reduced graphene oxide (p-rGO) nanocomposite onto nickel foam substrate, which is directly used as a binder-free charge storage electrode.     

Performance enhancement of asymmetric supercapacitors with bud-like Cu-doped Mn3O4 hollow and porous structures on nickel foam as positive electrodes

Cu-doped Mn₃O₄ hollow nanostructures supported on Ni foams as high-performance electrode materials for supercapacitors were successfully synthesized through a facile hydrothermal method and subsequent calcination. The morphology, structure, and electrochemical performance of the as-prepared Mn₃O₄ nanostructures can be tuned just by varying the Cu doping content. Benefiting from the unique bud-like hollow structure, the 1.5 at% Cu-doped Mn₃O₄ sample has a high specific capacitance of 257.6 F g⁻¹ at 1 A g⁻¹ and remarkable stability (about 90.6% retention of its initial capacitance after 6000 electrochemical cycles). Besides, an asymmetric supercapacitor (ASC) cell based on the 1.5 at% Cu-doped Mn₃O₄ exhibits a high specific capacitance of 305.6 F g⁻¹ at 1 A g⁻¹ and an energy density of 108.6 W h kg⁻¹ at a power density of 799.9 W kg⁻¹. More importantly, the ASC shows good long-term stability with 86.9% capacity retention after charging/discharging for 6000 cycles at a high current density of 5 A g⁻¹.

The effect of Cu doping on the electrochemical performance of bud-like Mn₃O₄ nanostructures for supercapacitor application was comparatively investigated.     

Diaminopyrene modified reduced graphene oxide as a novel electrode material for excellent performance supercapacitors

In this paper, novel reduced graphene oxide (rGO) composites (DAPrGOs) modified by diaminopyrene (DAP) were successfully synthesized via a facile solvothermal reaction method and used for supercapacitors. Compared with the pristine rGO, the DAPrGO1 electrode showed distinctly better performance (397.63 F g⁻¹vs. 80.29 F g⁻¹ of pristine rGO at 0.5 A g⁻¹) with small charge transfer resistance. When a symmetric device was fabricated using DAPrGO1 as the active material, it also exhibited a high capacitance of 82.70 F g⁻¹ at 0.5 A g⁻¹ with an energy density of 25.84 W h kg⁻¹ at a power density of 375 W kg⁻¹, and even offered a high power density of 7500 W kg⁻¹ (18.71 W h kg⁻¹) at 10 A g⁻¹. Moreover, the device possessed good electrochemical stability up to 20 000 cycles, implying promising applications in energy storage fields.

Schematic illustration of the facile synthesis process of DAPrGOs nanocomposites, Ragone plots and the superior cyclic stability of the assembled DAPrGO1//DAPrGO1 SSS.     

Disodium citrate-assisted hydrothermal synthesis of V2O5 nanowires for high performance supercapacitors

Orthorhombic vanadium pentoxide (V₂O₅) nanowires with uniform morphology were successfully fabricated via a facile hydrothermal process. The effect of disodium citrate dosage on the crystallinity, morphology and electrochemical properties of the products was analyzed. Experimental results indicate that orthorhombic V₂O₅ nanowires with high crystallinity and diameter of about 20 nm can be obtained at 180 °C for 24 h when the dosage of disodium citrate is 0.236 g. Furthermore, the prepared V₂O₅ nanowires demonstrate a high specific capacitance of 528.2 F g⁻¹ at 0.5 A g⁻¹ and capacitance retention of 85% after 1000 galvanostatic charge/discharge cycles at 1 A g⁻¹ when used as supercapacitors electrode in 0.5 M K₂SO₄.

Orthorhombic vanadium pentoxide (V₂O₅) nanowires with uniform morphology were successfully fabricated via a facile hydrothermal process.