Sulfur-incorporated,porous graphene films for high performance flexible electrochemical capacitors

Graphene and its derivatives are considered potential electrode materials for flexible electrochemical capacitors (f-ECs), but their capacitive performances have to be improved for practical applications. Herein, we demonstrate fabrication of flexible sulfur (S)-incorporated reduced graphene oxide (SRGO) electrodes obtained by pyrolyzing free-standing film consisting of benzyl disulfide-functionalized graphene oxides at 900 °C. The effect of S incorporation on morphology and chemical structure of SRGO were investigated by various microscopic and spectroscopic methods. Incorporation of S and the crumpled and porous morphology of SRGO electrodes improve capacitive performance of f-ECs; SRGO f-ECs show a specific capacitance of 140.8 F/g at 1 A/g, rate capability of 91.5% retention, and cyclic performance of 93.4% after 1000 charge/discharge cycles at 4 A/g. Impressively, SRGO f-ECs exhibit excellent electrochemical and mechanical durability after 1000 charge/discharge cycles at a bending angle of 120° with values that greatly exceed those of conventional RGO-based f-ECs. This study provides a fundamental foundation of the correlation between S composition of carbon nanomaterials and their electrochemical (or surface) properties.     

Highly porous carbon spheres for electrochemical capacitors and capacitive flowable suspension electrodes

In flowable and conventional electrochemical capacitors, the energy capacity is largely determined by the electrode material. Spherical active material, with high specific surface area (SSA) represents a promising material candidate for film and flow capacitors. In this study, we synthesized highly porous carbon spheres (CSs) of submicrometer size to investigate their performance in film and suspension electrodes. In particular, we studied the effects of carbonization and activation temperatures on the electrochemical performance of the CSs. The CSs activated at optimum conditions demonstrated narrow pore size distribution (<3 nm) with high SSA (2900 m²/g) and high pore volume (1.3 cc/g), which represent significant improvement as compared to similar materials reported in literature. Electrochemical tests of CSs in 1 M H₂SO₄ solution showed a specific capacitance of 154 F/g for suspension electrode and 168 F/g for film electrode with excellent rate performance (capacitive behaviors up to 100 mV/s) and cycling performance (95% of initial capacitance after 5000 cycles). Moreover, in the film electrode configuration, CSs exhibited high rate performance (78 F/g at 1000 mV/s) and volumetric power density (9000 W/L) in organic electrolytes, along with high energy density (21.4 Wh/L) in ionic liquids.     

Effect of dodecyl benzene sulfonic acid on the preparation of polyaniline/activated carbon composites by in situ emulsion polymerization

Polyaniline (PANI)/activated carbon (AC) composites are prepared by in situ emulsion polymerization using dodecyl benzenesulfonic acid (DBSA). DBSA can play a role as both surfactant and dopant in the process of PANI synthesis. The effect of DBSA on the preparation of PANI/AC composites is investigated in this research. For this purpose, the composites are prepared in micellar solutions with various concentrations of DBSA. It is confirmed using X-ray diffraction (XRD) analysis and Fourier-transform IR (FT-IR) spectroscopy that DBSA actually participates in the PANI doping process. The PANI doped with DBSA (DBSA-PANI) covering the AC surfaces is observed with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrochemical properties of the composites are studied by cyclic voltammetry (CV). The composites show different values of specific capacitance, which was found to be a function of DBSA concentration. The composite prepared in 0.045 M DBSA solution shows the highest specific capacitance (115.2 F g^?1) among the prepared composites. The doping level of DBSA-PANI is increased with the concentration of DBSA in solution.     

Carbon nanotube/graphene composite for enhanced capacitive deionization performance

In this study, single-walled carbon nanotubes were combined with graphene oxide nanosheets in aqueous dispersion and then chemically reduced to form the carbon nanotube/graphene (CNT/G) composite as electrodes for capacitive deionization (CDI). The structure of the CNT/G composite was highly porous, with single-walled carbon nanotubes (SWCNTs) sandwiched between graphene sheets that functioned as spacers and provided diffusion paths for smooth and rapid ion conduction. The associated increase in the electrical double-layer capacitance enhanced capacitive deionization performance. The CNT/G composite achieved a specific capacitance of 220 F/g and an electrosorption capacity of 26.42 mg/g with 100% regeneration, showing great potential as a high performance electrode material in CDI applications.     

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.     

Nano graphene porous/conductive polymer as a composite material for energy storage in supercapacitors

This research studies the improving effects of graphene porous (GP) on the supercapacitive performance of a polyaniline/graphene porous (PANI/GP) nanocomposite. GP nanosheets were synthesized via chemical vapor deposition, and PANI/GP was electrochemically composited through successive cyclic voltammetry. The samples were characterized by fast Fourier transform infrared (FTIR), x-ray diffraction (XRD), and scanning electron microscopy (SEM), and energy-dispersive x-ray spectrometry (EDS) techniques. Porous GP nanosheets were uniformly dispersed in the composite structure. Furthermore, the electrochemical performances of the synthesized samples were compared using galvanostatic charge/discharge, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). Incorporating GP into the PANI significantly increased specific capacitance from 276 (in PANI) to 577 F/g (in PANI/GP). The electrochemical stability of electrodes was compared during 1000 successive charge/discharge cycles. After 1000 cycles, PANI/GP kept 90% of its initial capacitance, and only 25% of the charge storage capacitance of bare PANI remained.     

Preparation and characterization of manganese oxide/CNT composites as supercapacitive materials

Manganese oxide was synthesized and dispersed on carbon nanotube (CNT) matrix by thermally decomposing manganese nitrates. CNTs used in this paper were grown directly on graphite disk by chemical vapor deposition technique. The capacitive behavior of manganese oxide/CNT composites was investigated by cyclic voltammetry and galvanostatic charge–discharge method in 1 M Na₂SO₄ aqueous solutions. When the loading mass of MnO₂ is 36.9 μg cm^− 2, the specific capacitance of manganese oxide/CNT composite (based on MnO₂) at the charge–discharge current density of 1 mA cm^− 2 equals 568 F g^− 1. Additionally, excellent charge–discharge cycle stability (ca. 88% value of specific capacitance remained after 2500 charge–discharge cycles) and power characteristics of the manganese oxide/CNT composite electrode can be observed. The effect of loading mass of MnO₂ on specific capacitance of the electrode has also been investigated.     

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.     

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.     

High-performance hierarchical homologous scale-like CuCl/Cu foam anode for lithium ion battery

In situ growth of hierarchical scale-like CuCl on homologous Cu foam substrate for lithium ion battery (LIB) was achieved via facile anodization of Cu and the following rapid deposition of insoluble CuCl. The obtained hierarchical scale-like CuCl/Cu foam electrode were investigated in terms of scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), galvanostatic charge/discharge, rate performance, cycle stability and AC impedance. When used as the anode in LIB, the as-synthesized electrode with multi-scaled pores delivered satisfactory electrochemical performances. Particularly, the reversible discharge capacity was still maintained at about 197.2 mA h/g even after 1000 cycles at high rate of 10 C, indicating excellent endurance and structural stability during the fast cyclic charge/discharge. It can be concluded that the developed hierarchical scale-like CuCl/Cu foam electrode can not only improve the electronic conductivity, but also buffer the structure strain during long term cyclic lithiation/delithiation and maintain the structural stability. Seemingly, hierarchical CuCl with high Li-storage activity and highly conductive three dimensional (3D) porous homologous Cu foam substrate jointly contributed to high electrochemical performances.     

Kilohertz ultrafast electrochemical supercapacitors based on perpendicularly-oriented graphene grown inside of nickel foam

Ultrafast electrochemical supercapacitors (EC) that can work at or above kilohertz (kHz) frequency, 3–4 orders higher than traditional EC, call for a structure with extremely low equivalent serial resistance (ESR) and a reasonably large surface area. Three-dimensional perpendicularly-oriented graphene (POG) network, grown inside of Ni foam (NF) by microwave plasma chemical vapor deposition, is reported as electrode to fabricate such ultrafast EC. The folded POG inside NF provides a large surface area, while the straight-forward and wide-open porous structure of POG ensures fast ion migration. In conjunction with the intrinsic high electronic conductivity of graphene and Ni, POG/NF electrode based ultrafast EC was demonstrated with a specific cell capacitance of 0.32 mF/cm² at 1 kHz, a relaxation time constant of 0.248 ms, and an ESR of 70 mΩ. A charge–discharge rate as high as 500 V/s was also measured, at which the cyclic voltammogram maintained a rectangular shape, corresponding to a single electrode capacitance of 0.83 mF/cm².     

Effect of graphene thickness on the morphology evolution of hierarchical NiCoO2 architectures and their superior supercapacitance performance

The rational design of electrode materials with special hierarchical architectures which possess both high surface area and conductivity is significant to enhance the performance of supercapacitors. Herein, hierarchical NiCoO₂ architectures assembled by ultrathin mesoporous nanosheets are in-situ grown on graphene@Ni foam (G@NF) by a template-free solvothermal route and subsequent annealing process, which is used as self-supported and binder-free supercapacitor electrodes. The effect of graphene thickness on morphology evolution of NiCoO₂ is investigated. Benefiting from the synergistic effect between graphene with remarkable conductivity and hierarchical NiCoO₂ architectures with high specific capacity, the NiCoO₂/G@NF electrodes show greatly improved electrochemical performance compared to NiCoO₂@NF and G@NF. The optimized NiCoO₂/G@NF has a specific capacitance of 1220 F/g at 1 A/g. While the NiCoO₂@NF and G@NF are only 565 and 151 F/g, respectively. The optimized NiCoO₂/G@NF remains 840 F/g at 20 A/g, revealing a remarkable rate performance (69% capacity retention from 1 to 20 A/g). Moreover, an outstanding cyclic stability of 80% capacitance retention can be obtained after 5000 charge/discharge cycles at 10 A/g, whereas the NiCoO₂@NF is only 46.5%. These results suggest that the hierarchical NiCoO₂ architectures/graphene hybrids are good candidates as effective electrode materials for supercapacitors.     

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.     

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.     

A carbon electrode fabricated using a poly(vinylidene fluoride) binder controlled the Faradaic reaction of carbon powder

A carbon electrode for capacitive deionization (CDI) was fabricated by casting a slurry that was a mixture of activated carbon powder (ACP) and poly(vinylidene fluoride) (PVdF) dissolved in di-methylacetamide (DMAc) on the current collector. Electrochemical properties and adsorption/desorption behaviors of the carbon electrodes prepared with different PVdF contents (9–18 wt%) were characterized using cyclic voltammetry, chronoamperometry, and impedance spectroscopy methods. From the SEM images, carbon powders were coated with the PVdF binder and bound together. Capacitances of carbon electrodes were estimated in the range of 75.3–69.6 F/g, decreasing in tandem with PVdF contents, but the decrease was not significant. From cyclic voltammetric and chronoamperometric measurements, the electrochemical behaviors of the carbon electrodes were dependent not only on the electric double layer capacitance, but also on Faradaic reactions. However, Faradaic currents resulted from an electrochemical redox reaction of carbon surface controlled by the polymer binder. These results indicate that the electrochemical reaction on the carbon surface was suppressed due to the PVdF binder.     

An easy one-step electrosynthesis of graphene/polyaniline composites and electrochemical capacitor

An easy electrochemical technique is proposed to prepare electrochemically reduced graphene oxide (ERGO)/polyaniline (PANI) composites in a single step. The technique uses a two-electrode cell in which a separator soaked with an acid solution is sandwiched between graphene oxide (GO)/aniline films deposited on conductive substrates and an alternating voltage was applied to the electrodes. Successful preparations of ERGO/PANI composites were evidenced by characterizations due to UV–vis-NIR, FT-IR, XPS, XRD, and SEM measurements with free-standing films of ERGO/PANI obtained easily by disassembling the two-electrode cells. The ERGO/PANI films exhibited a high mechanical stability, flexibility, and conductivity (68 S cm⁻¹ for the composite film containing 80% ERGO) with nanostructured PANI particles (smaller than 20 nm) embedded homogeneously between the ERGO layers. The two-electrode cells acted as electrochemical capacitors (ECs) after a sufficient voltage cycling and exhibited relatively large specific capacitances (195–243 F g⁻¹ at a scan rate of 100 mV s⁻¹) with an excellent cycle life (retention of 83% capacitance after 20,000 charge–discharge cycles). Influences of the GO/aniline ratio, the sort of electrolytes, and the weight of the composite on the energy storage characteristics of ECs comprising the ERGO/PANI composites were also studied.     

Preparation of activated graphene and effect of activation parameters on electrochemical capacitance

Activation parameters such as temperature and the amount of potassium hydroxide (KOH) were varied during the synthesis of activated microwave-exfoliated graphite oxide (a-MEGO) and the effects of these parameters on the specific surface area of a-MEGO and electrochemical capacitance of a-MEGO electrodes were investigated. At 800 °C and a KOH/MEGO mass ratio of 6.5, a maximum specific surface area of 3100 m²/g was obtained and a high specific capacitance of 172 F/g (at 1 A/g constant current and 3.5 V maximal voltage) was measured in a two-electrode cell with a-MEGO electrodes in an organic electrolyte.     

High-performance supercapacitor of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper

Although supercapacitors have higher power density than batteries, they are still limited by low energy density and low capacity retention. Here we report a high-performance supercapacitor electrode of manganese oxide/reduced graphene oxide nanocomposite coated on flexible carbon fiber paper (MnO₂–rGO/CFP). MnO₂–rGO nanocomposite was produced using a colloidal mixing of rGO nanosheets and 1.8 ± 0.2 nm MnO₂ nanoparticles. MnO₂–rGO nanocomposite was coated on CFP using a spray-coating technique. MnO₂–rGO/CFP exhibited ultrahigh specific capacitance and stability. The specific capacitance of MnO₂–rGO/CFP determined by a galvanostatic charge–discharge method at 0.1 A g⁻¹ is about 393 F g⁻¹, which is 1.6-, 2.2-, 2.5-, and 7.4-fold higher than those of MnO₂–GO/CFP, MnO₂/CFP, rGO/CFP, and GO/CFP, respectively. The capacity retention of MnO₂–rGO/CFP is over 98.5% of the original capacitance after 2000 cycles. This electrode has comparatively 6%, 11%, 13%, and 18% higher stability than MnO₂–GO/CFP, MnO₂/CFP, rGO/CFP, and GO/CFP, respectively. It is believed that the ultrahigh performance of MnO₂–rGO/CFP is possibly due to high conductivity of rGO, high active surface area of tiny MnO₂, and high porosity between each MnO₂–rGO nanosheet coated on porous CFP. An as-fabricated all-solid-state prototype MnO₂–rGO/CFP supercapacitor (2 × 14 cm) can spin up a 3 V motor for about 6 min.     

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.     

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.