Hydrothermal synthesis of reduced graphene oxide-Mn3O4 nanocomposite as an efficient electrode materials for supercapacitors

Mn₃O₄ nanoparticles (NPs) are decorated with reduced graphene oxide nanosheets (rGO-Mn₃O₄) through a facile and eco-friendly hydrothermal method. The as-synthesized composite was characterized by XRD, SEM, TEM and Raman spectroscopy. The electrochemical properties of (rGO-Mn₃O₄) nanocomposite were studied as electrode materials for supercapacitors. The rGO-Mn₃O₄ nanocomposite exhibit high specific capacitance of 457 Fg^?1 at 1.0 A/g in 1 M Na₂SO₄ aqueous electrolyte. The rGO-Mn₃O₄ exhibits good capacitance retention by achieving 91.6% of its initial capacitance after 5000 cycles. The excellent electrochemical performance is attributed to the increased electrode conductivity in the presence of graphene network.     

Hydrothermally synthesized FeCo2O4 nanostructures: Structural manipulation for high-performance all solid-state supercapacitors

The exploration of high performance supercapacitors has received emerging the worldwide research interests in satisfying the gradually increased energy consumption. In this paper, we adopt a facile hydrothermal strategy to synthesize ternary FeCo₂O₄ directly on nickel foam. A series of structure such as nanowires, nanoflake@nanowire hetero-structure and hierarchical nanospheres have been achieved via modulating the synthetic time. The morphology and structure of the as-prepared samples are characterized by using scanning electron microscopy and X-ray diffraction spectroscopy. The relationship between the detail processing parameters and electrochemical performance are also revealed by cyclic voltammetry, galvanostatic charge-discharge measurements, cycle stability tests and electrochemical impedance spectroscopy. Notably, the as-prepared nanoflake@nanowire hetero-structure exhibits a high specific capacitance of about 969 F g^?1 at 2 A g^?1 in alkaline aqueous solution and a remarkable cycling stability (91% capacity retention after 2000 cycles). The excellent supercapacitors performance of nanoflake@nanowire hetero-structure can be attributed to the high conductivity, large active area as well as robust architectures that derive from structural synergetic effects. Furthermore, a symmetric all solid-state supercapacitor has been fabricated by using nanoflake@nanowire hetero-structure as both the anode and cathode electrodes. The as-fabricated supercapacitor delivers excellent electrochemical performance. It's anticipated that FeCo₂O₄ would be a promising material for electrochemical energy storage applications.     

Flexible supercapacitor electrodes based on TiO2/rGO/TiO2 sandwich type hybrids

The flexible nanostructured supercapacitors have gained vast majority of interests during recent years. In this article, flexible supercapacitor electrode based on TiO₂/rGO/TiO₂ sandwich is fabricated through a facile low cost solution process method based on pre-synthesized vapor assisted GO paper and titania sol. The XRD and FTIR spectroscopy analyses confirm the in-situ reduction of GO paper when faced with the titania sol. The Raman spectroscopy shows the coexistence of titania anatase phase beside rGO layers. Moreover, FESEM analysis demonstrates that the sandwich electrodes are composed of titania and rGO layers with thickness of about 660 nm and 15 µm, respectively. The optimum parameter for film deposition is 0.17 M concentration, water to Ti precursor ratio of 4, acid catalyst to Ti precursor ratio of 0.5, and solvent of 1-propanol. The supercapacitor electrode based on this optimum deposited sandwich illustrates capacitance of 83.7 F/g at scan rate of 5 mV/s and appreciable charge-discharge behavior. These hybrid pseudo- and electric double layer capacitance behavior in this supercapacitor not only can dramatically improve the performance of the future energy storage devices but also can be applicable in cost-effective wearable electronics.     

Design and synthesis of three-dimensional needle-like CoNi2S4/CNT/graphene nanocomposite with improved electrochemical properties

In this paper, we described a simple two–step method for preparing needle-like CoNi₂S₄/CNT/graphene nanocomposite with robust connection among its ternary components. The prepared CoNi₂S₄/CNT/graphene nanocomposite has been thoroughly characterized by spectroscopic (Fourier-transform infrared spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy), X-ray diffraction and thermogravimetric analysis. Microscopy techniques (scanning electron microscopy–energy dispersive spectroscopy and transmission electron microscopy) were employed to probe the morphological structures. The electrochemical properties of the as-prepared 3D architectures were investigated with three and two-electrode systems. In addition to its high specific capacitance (710 F g⁻¹ at 20 A g⁻¹), after charging–discharging for 2000 cycles, the electrode still maintained the capacity retention of about 82%. When used as the active electrode material for supercapacitors, the fabricated CoNi₂S₄–g–CNT nanostructure exhibited excellent specific capacitance and good rate capability, making it a promising candidate for next-generation supercapacitors.     

Mesoporous nitrogen-doped graphene aerogels with enhanced rate capability towards high performance supercapacitors

In this work, a novel preparation method and architecture of mesoporous nitrogen-doped graphene aerogels (GAs) using electrostatic attraction are reported. The sacrificial template, positively charged SiO₂ nanoparticles by using (3-Animopropyl) triethoxysilane (3-APTS), can significantly decrease the spacing between graphene oxide sheets, due to the strong electrostatic attraction between the graphene oxide and SiO₂. After the etching of templates, the ratio of mesopores is greatly increased, and the electrochemical performances of electrodes are enhanced. The mesoporous GAs yield an enhanced specific capacitance of 203 F/g at a current density of 1 A/g, and a capacitance fade of only 8.95% at a high current density of 20 A/g, indicating improved ion transport in mesoporous architecture. The controllable synthesized method can be further applied to prepare other mesoporous materials and such mesoporous nitrogen-doped GAs have great potential in high rate performance supercapacitors.     

Vertically aligned epitaxial graphene nanowalls with dominated nitrogen doping for superior supercapacitors

For graphene-based electrode materials, N doping is one of the leading approaches for enhancing the performance of supercapacitors. However, such an outstanding performance is suppressed by the agglomeration of graphene and unspecified N incorporation. Here, we demonstrate a direct growth of vertically epitaxial graphene nanowalls (GNWs) on flexible carbon cloths (CCs) via microwave plasma-enhanced chemical vapor deposition, whereby predominantly N doping was sequentially achieved by introducing in situ NH₃ plasma, to form N-doped GNWs (NGNWs). The vertically aligned three-dimensional (3D) architecture of epitaxial NGNWs and their unique selectivity to the specific N dopants make such electrodes an ideal platform, not only for enhancing the capacitive performance but also for studying the role of the CN bonding configuration in its performance. Remarkably, NGNW supercapacitors exhibit an excellent specific capacitance of 991.6 F/g (estimation based on the actively contributing component) and an apparent area-normalized capacitance of 1488.9 mF/cm², at a specific current of 14.8 A/g. This approach allows us to achieve an energy density of 275.4 Wh/kg at a power density of 14.8 kW/kg (specific current of 14.8 A/g), and a power density of 74.1 kW/kg at an energy density of 232.6 Wh/kg (specific current of 74.1 A/g) in 1 M H₂SO₄.     

A facile approach to produce holey graphene and its application in supercapacitors

We demonstrate a low-cost and simple method to prepare holey graphene nanosheets (HGNSs) by ultra-rapid heating during the process of thermal reduction/exfoliation of graphite oxide. The number density of the holes increased with the heating rate, and the size ranged between 10 and 250 nm. In addition, supercapacitors (SCs) using HGNSs as the electrode material were fabricated and their performances were evaluated. Compared to common graphene, HGNSs offered much higher capacitance values and better capability at high rates due to the much shorter cross-plane ion transport paths in the graphene stack through the large amount of holes on graphene sheets. The SC with HGNS electrodes exhibited an excellent high-rate capacitance of 170 F/g at 50 A/g in a 6 M KOH aqueous electrolyte. In the low-rate limit, the HGNS SC showed a very large specific capacitance of 350 F/g at 0.1 A/g.     

One-pot hydrothermal synthesis and supercapacitive performance of nitrogen and MnO co-doped hierarchical porous carbon monoliths

Nitrogen and MnO co-doped hierarchical porous carbon monolith (N-MnO-HPCM) materials were synthesized through a facile one-pot hydrothermal method. The resulting N-MnO-HPCM materials had hierarchical porous structure, high BET surface area (606 m²/g), large pore volume (0.33 cm³/g), and contained evenly dispersed MnO nanoparticles of about 6 nm in the carbon matrix. Their electrochemical performances as electrodes for supercapacitors were investigated. N-MnO-HPCM material exhibited an excellent electrochemical performance with a specific capacitance of 261.7 F/g at a current density of 1 A/g. It also showed a good rate capability with 74% capacity retention at high current density (5 A/g), indicating its potential applications in supercapacitors.     

Far-infrared reduced graphene oxide as high performance electrodes for supercapacitors

We report a novel far-infrared (FIR) thermal reduction process to effectively reduce graphene oxide films for supercapacitor electrode applications. The binder-free graphene oxide films used in this study were produced by electro-spray deposition of a graphene oxide colloidal solution onto stainless steel current collectors. The reduction of graphene oxide was performed using a commercial FIR convection oven that is ubiquitous in homes for cooking and heating food. The reduction process incorporated a simple, one-step FIR irradiation carried out in ambient air. Further, the FIR irradiation process was completed in ∼3 min, wherein neither special atmosphere nor high temperature was employed, resulting in an economic, efficient and simplified processing technique. The as-produced FIR graphene electrode gave a specific capacitance of ∼320 F/g at a current density of ∼0.2 A/g with less than 94% loss in specific capacitance over 10,000 charge/discharge cycles. This is one of the best specific capacitances reported for all-carbon electrodes without any additives. Even at ultrafast charge/discharge rates (current densities as high as ∼100 A/g), the FIR graphene electrode still delivered specific capacitances in excess of 90 F/g. The measured energy and power densities of the FIR supercapacitors were found to be ∼3–6 times higher than commercial (activated carbon) supercapacitor devices. This excellent electrochemical performance of the FIR graphene coupled with its ease of production (in air at low temperatures) using a commercial home-use FIR convection oven indicates the significant potential of this concept for large-scale commercial electrochemical supercapacitor applications.     

Hierarchical Co3O4@ZnWO4 core/shell nanostructures on nickel foam: Synthesis and electrochemical performance for supercapacitors

To improve the electrochemical properties of Co₃O₄ for supercapacitors application, a hierarchical Co₃O₄@ZnWO₄ core/shell nanowire arrays (NWAs) material is designed and synthesized successfully via a facile two-step hydrothermal method followed by the heat treatment. Co₃O₄@ZnWO₄ NWAs exhibits excellent electrochemical performances with areal capacitance of 4.1 F cm⁻² (1020.1 F g⁻¹) at a current density of 2 mA cm⁻² and extremely good cycling stability (99.7% of the initial capacitance remained even after 3000 cycles). Compared with pure Co₃O₄ electrodes, the results prove that this unique hierarchical hybrid nanostructure and reasonable assembling of two electrochemical pseudocapacitor materials are more advantageous to enhance the electrochemical performance. Considering these remarkable capacitive behaviors, the hierarchical Co₃O₄@ZnWO₄ core/shell NWAs nanostructure electrode can be revealed promising for high-performance supercapacitors.     

Influence of the electrochemical reduction process on the performance of graphene-based capacitors

Electrochemically reduced graphene was used as the key element in the preparation of electric double-layer capacitors where the thickness of the electrode was only a few hundred nano-meters. The resultant electrodes showed different specific capacitances after pre-reduction with scanning potential windows of −1.0 to 1.6 V, −1.5 to 0 V and −1.0 to 1.0 V. Also, a specific capacitance of 246 F/g was obtained as the graphene oxide electrode was reduced with an applied potential of −1.0 to 1.0 V for 4000 s. The influence of the residual oxygen functional groups and sp² domains in electrochemically reduced graphene were investigated for capacitance performance.     

Morphology-controlled porous Bi0.9La0.1FeO3 microspheres for applications in supercapacitors

Morphology-controlled porous Bi_0.9La_0.1FeO₃ (BLFO) microspheres with high specific surface area and pore volume have been synthesized by a novel one-step etching approach, in which BLFO particles were used as precursors, hydrazine and methyl mercaptoacetate function as reducing and complexation agent respectively. As the etching time goes in order from 30 to 90 min, different morphologies of porous BLFO microspheres can be obtained, such as dandelion, corolla, and acanthosphere-like architectures respectively, which corresponds to different surface area and pore volume of porous BLFO microspheres. Particularly, porous BLFO microspheres etched for 60 min has the largest specific surface area of 75.09 m² g^-1 m²g, much larger than 1.44 m² g^-1m², that of unetched BLFO particles. The enhanced surface area and pore volume brings about a great number of active sites that boost the intercalation and de-intercalation of electrolyte ions, and compared with unetched BLFO particles, porous BLFO microspheres exhibit the good conductivity and ion diffusion behavior, which are both conducive to excellent performance as supercapacitors. The largest specific capacitance of 561.48 F g^-1 Fg^?1 at a scan rate of 2 mV s^-1mV/s can be obtained when the etching time is 60 min, and it exhibits a good capacitance retention of 85.76% after 1500 cycles with the current density of 5 A g^-1A/g, much superior to 75.66%, that of BLFO particles. This research may offer a facile method to fabricate stable, flexible and high performance energy storage devices.     

Hydrous ruthenium dioxide/multi-walled carbon-nanotube/titanium electrodes for supercapacitors

Composite electrodes prepared by vertically aligned multi-walled carbon nanotubes (MWCNTs) coated with hydrous ruthenium dioxide (RuO₂·nH₂O) have previously been used in various supercapacitors. The specific capacitance when using RuO₂·nH₂O/MWCNT/Ti as electrodes in 1.0 M H₂SO₄ aqueous solution can reach up to 1652 F/g at a scan rate of 10 mV/s, which is larger than that of RuO₂·nH₂O/Ti or MWCNT/Ti. In this study, a RuO₂·nH₂O/MWCNT/Ti composite electrode was examined by X-ray photoelectron spectroscopy, which revealed the existence of hydrous ruthenium dioxide in the Ti current collector. The capacitive behavior of the electrode was analyzed by cyclic voltammetry and the galvanostatic charge–discharge method, and the morphology of the composite electrode was examined by scanning electron microscopy.     

Cobalt oxide nanocubes as electrode material for the performance evaluation of electrochemical supercapacitor

Porous cobalt oxide (Co₃O₄) nanocubes (NCs) were synthesized by a simple and cost-effective hydrothermal technique for the potential application of electrochemical supercapacitors. The hydrothermally synthesized materials exhibited the small cube like morphology with the average size of ~ 50 to 60 nm. The surface analysis revealed a good surface area, and high pore volume of the synthesized porous Co₃O₄ NCs. The capacitive properties of porous Co₃O₄ NCs electrode were investigated by cyclic voltammetry (CV) in 6 M KOH electrolyte and a high specific capacitance of ~ 430.6 F/g at a scan rate of ~ 10 mV s^?1 was observed. The capacity retention of up to ~ 85% after 1000 cycles was shown by the fabricated porous Co₃O₄ NCs electrode. The porous Co₃O₄ NCs showed excellent structural stability through cycling with promising capacity retention which suggested a good quality of porous Co₃O₄ NCs as electrochemical supercapacitor electrode.     

Restacking-inhibited nitrogen-incorporated mesoporous reduced graphene oxides for high energy supercapacitors

Graphene is considered a promising active electrode material due to a large surface area, high electronic conductivity, and chemical and mechanical stabilities for supercapacitor (SC) applications. However, the current bottleneck is the fabrication of restacking-inhibited graphene on an electrode level which otherwise loses the capability to achieve the aforementioned properties. Herein, we demonstrate the synthesis of restacking-inhibited nitrogen (N)-incorporated mesoporous graphene for high energy SCs. The melamine-formaldehyde acts as a restacking inhibitor by forming a bonding with reduced graphene oxide (RGO) through a condensation reaction and as an N precursor to be decomposed to create open pores and N sources upon heat treatment. The d-spacing increases up to 0.352 nm and the surface area is as high as 698 m² g^?1 with high mesoporosity, confirming restacking inhibition by N incorporation decomposed by melamine-formaldehyde. The restacking-inhibited RGO-based SC cells in organic electrolyte show the specific capacitance of 25.8 F g^?1, the energy density of 21.8 kW kg^?1 and 85% of capacitance retention for 5000 cycles, which are better than those of pristine RGO-based cells. These improved SC performances are attributed to the fast ion transport through a mesoporous channel in crumpled structure and the doping effect of N incorporation. This work provides a simple yet effective chemical approach to fabricate restacking-inhibited RGO electrodes for improved SC performances.     

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.     

Enhanced rate capability of nanostructured three-dimensional graphene/Ni3S2 composite for supercapacitor electrode

Three-dimensional graphene/Ni₃S₂ (3DG/Ni₃S₂) composite electrodes were produced by a facile two-step synthesis route involving chemical vapor deposition (CVD) growth of graphene foam and in situ hydrothermal synthesis of Ni₃S₂. The porous structure of the prepared 3DG is ideal for use as a scaffold for fabricating monolithic composite electrodes. The relative content of Ni₃S₂ initially increased and then decreased with increasing hydrothermal reaction time. The basal surface of the electrode was completely covered after 6 h of hydrothermal reaction. The size of the Ni₃S₂ microspheres also increased with increasing hydrothermal reaction time. The composite electrodes exhibited good specific capacitance (11.529 F cm⁻² at 2 mA cm⁻², i.e., 2611.9 F g⁻¹ at 5 mV s⁻¹) and cyclability (retention of 88.97% capacitance after 1000 charge/discharge cycles at 20 mA cm⁻²). These results are attributed to the fact that the uniform distribution of the Ni₃S₂ microspheres increased the specific surface area of the electrode and facilitated electron transfer and ion diffusion. The 3D multiplexed and highly conductive pathways provided by the defect-free graphene foam also ensured rapid charge transfer and conduction to improve the rate capability of the supercapacitors.     

Hydrothermal synthesis of 3D NixCo1−xS2 particles/graphene composite hydrogels for high performance supercapacitors

In this work, three dimensional (3D) Ni_xCo_1−xS₂/graphene composite hydrogels with different Ni contents (denoted as Ni_xCo_1−xS₂/GH (x = 0, 0.31, 0.56, 0.66, 1)) have been synthesized by a simple one-step hydrothermal method and utilized as the active materials of supercapacitors. The as-prepared samples present a 3D interconnected porous network with the pore sizes in the range of several to tens micrometers. Interestingly, the Ni_xCo_1−xS₂ particles are uniformly located on the graphene network and the particle size is evolved from ∼50 nm to ∼1.5 μm with the increase of Ni content. The electrochemical measurements revealed that the specific capacitance, rate capability and cyclability of different Ni_xCo_1−xS₂/GH electrodes are strongly affected by their different Ni content. Among these, the 3D Ni_0.31Co_0.69S₂/GH composite has the highest specific capacitance of 1166 F/g at a current density of 1 A/g. Furthermore, a specific capacitance of 559 F/g can be still maintained at high current density of 20 A/g. After 1000 charge–discharge cycles at 5 A/g, the specific capacitance remains a high value of 755 F/g.     

Synthesis of cross-linked graphene aerogel/Fe2O3 nanocomposite with enhanced supercapacitive performance

In this work, cross-linked graphene aerogel (CL-GA) and its composite with Fe₂O₃ nanoparticles (NPs) were synthesized through a one-step hydrothermal procedure by using p-phenylenediamine (PPD). Structural characterizations revealed that in the preparation of the composite PPD acts as a cross-liker and provides high surface area by decreasing restacking of graphene sheets and functions as nitrogen source simultaneously. The electrochemical characteristics of the nanocomposite were investigated by cyclic voltammetry (CV), galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS) and Fast Fourier transform continues cyclic voltammetry (FFTCCV). The results show that cross-linked graphene aerogel/Fe₂O₃ (CL-GA/Fe₂O₃) nanocomposite displays enhanced supercapacitive performance, where it has capacitance of 445 at 1 A g⁻¹, high energy density of 63 W h Kg⁻¹, and 89% capacitance retention after 5000 cycles in 3 M KOH. Presence of PPD considerably improved supercapacitive performance of nanocomposite as a result it could be promising material in synthesis of efficient graphene/metal oxide-based electrode material for high performance supercapacitors.     

Syntheses of nano-sized Co-based powders by carbothermal reduction for anode materials of lithium ion batteries

Co oxide powders were synthesized by spray drying, calcining, and then ball milling. Nano-sized Co-based powders were then prepared by carbothermal reduction at 873 K, 1073 K, and 1173 K of the synthesized Co oxide powders. Then, the electrochemical properties of the electrodes made with the Co-based powders were examined to evaluate their suitability as anode materials for Li-ion batteries. It was reported that among Co, CoO, and Co₃O₄, Co₃O₄ had the best cycling performance. However, in this work, Co showed the best cycling performance. This means that the mechanisms of the cycling performance of CoO and Co which were synthesized by different heat treatment methods are different from each other. The initial discharge capacities of three electrodes made with the powders reduction-treated at 873 K, 1073 K, and 1173 K were similar and about 1100 mA h/g, respectively. However, the electrodes made with the powders reduction-treated at 873 K and 1073 K had the discharge capacities at the second cycle which were less than 50% of the discharge capacity of the electrode made with the powder reduction-treated at 1173 K. The electrode made with the powder reduction-treated at 1173 K had a discharge capacity of 750 mA h/g at the 20th cycle, demonstrating that this electrode had good cycling performance.