Solid-phase electrochemical reduction of graphene oxide films in alkaline solution

Graphene oxide (GO) film was evaporated onto graphite and used as an electrode to produce electrochemically reduced graphene oxide (ERGO) films by electrochemical reduction in 6 M KOH solution through voltammetric cycling. Fourier transformed infrared and Raman spectroscopy confirmed the presence of ERGO. Electrochemical impedance spectroscopy characterization of ERGO and GO films in ferrocyanide/ferricyanide redox couple with 0.1 M KCl supporting electrolyte gave results that are in accordance with previous reports. Based on the EIS results, ERGO shows higher capacitance and lower charge transfer resistance compared to GO.     

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.     

Electrochemically reduced graphene oxide sheets for use in high performance supercapacitors

Graphene oxide (GO) was reduced by a rapid, effective and eco-friendly electrochemical method of repetitive cathodic cyclic potential cycling, without using any reducing reagents. The electrochemically reduced graphene oxide (ERGO) was characterized by UV–vis, EIS and zeta-potential measurements. Most of the oxygen functional groups in ERGO were successfully removed resulting in smaller charge transfer resistance. However, some electrochemically stable residuals still remained, enabling ERGO to facilitate electrolyte penetration and pseudocapacitance. Since ERGO was readily stabilized by cathodic potential cycling, it exhibited an outstanding stability in cycle life, nearly with no capacitive loss from the second cycle on. A specific capacitance of 223.6 F g⁻¹ was achieved at 5 mV s⁻¹, which makes the ERGO a competitive material for electrochemical energy storage.     

A flexible graphene/multiwalled carbon nanotube film as a high performance electrode material for supercapacitors

A flexible graphene/multiwalled carbon nanotube (GN/MWCNT) film has been fabricated by flow-directed assembly from a complex dispersion of graphite oxide (GO) and pristine MWCNTs followed by the use of gas-based hydrazine to reduce the GO into GN sheets. The GN/MWCNT (16 wt.% MWCNTs) film characterized by Fourier transformation infrared spectra, X-ray diffraction and scanning electron microscope has a layered structure with MWCNTs uniformly sandwiched between the GN sheets. The MWCNTs in the obtained composite film not only efficiently increase the basal spacing but also bridge the defects for electron transfer between GN sheets, increasing electrolyte/electrode contact area and facilitating transportation of electrolyte ion and electron into the inner region of electrode. Electrochemical data demonstrate that the GN/MWCNT film possesses a specific capacitance of 265 F g⁻¹ at 0.1 A g⁻¹ and a good rate capability (49% capacity retention at 50 A g⁻¹), and displays an excellent specific capacitance retention of 97% after 2000 continuous charge/discharge cycles. The results of electrochemical measurements indicate that the freestanding GN/MWCNT film has a potential application in flexible energy storage devices.     

EIS study of potentiostatically formed passive film on 304 stainless steel

Passive film was potentiostatically grown on Type 304 stainless steel at potentials between −0.4 and 0.3 V vs. Hg/HgSO₄ in 50 mV intervals. Electrochemical impedance spectroscopy was used to study properties of the grown passive films. Relevant circuit analogs were selected to fit the electrochemical impedance data obtained at each potential. The resultant parameters were used to calculate film thickness through both film capacitance and resistance. The calculated thicknesses were compared with ellipsometry thickness measurement results. An overestimation occurred when the film resistance was used to calculate the film thickness. On the other hand, when the CPE parameter Q was used, the film thickness was underestimated. Available approaches were employed to calculate the effective capacitance of the film. A comparison between the film thickness calculated from the effective capacitance and ellipsometry measurements suggested a surface distribution of time constants on the surface.     

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.     

Improved electrochemical performance of graphene oxide supported vanadomanganate (IV) nanohybrid electrode material for supercapacitors

Graphene oxide (GO)-supported polyoxometalates (POMs) have been considered as promising electrode materials for energy storage applications due to their ability to undergo fast and reversible redox reactions. Herein, vanadomanganate-GO composites (K₇Mn^IVV₁₃O₃₈.18H₂O-GO with 2:1 and 4:1 ratio) were investigated for use as potential electrode materials in supercapacitors (SCs). The K₇Mn^IVV₁₃O₃₈.18H₂O (MnV₁₃) was synthesized and anchored on GO through electron transfer interaction and electrostatic interaction to make the composite electrodes for the present study. All synthesized electrode materials were fully characterized by various techniques, e.g., Fourier Transform Infrared (FTIR) Spectroscopy, Powder X-ray Diffraction (XRD), Scanning Electron Microscopy/Energy Dispersive X-ray Spectroscopy (SEM/EDS) and High Resolution-Transmission Electron Microscopy (HR-TEM). The electrochemical properties of MnV₁₃/GO composites with different MnV₁₃/GO ratios were investigated by two-electrode cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) in different electrolytes. The MnV₁₃/GO composite of ratio 2:1 in 1 M LiCl electrolyte and that of ratio 4:1 in 1 M Na₂SO₄ electrolyte showed significant specific capacitance values of 269.15 F/g and 387.02 F/g, respectively and energy density of 37.38 Wh/kg and 53.75 Wh/kg, respectively for a scan rate of 5 mV/s. Interestingly, the 1:1 (MnV₁₃/GO) composite in 1 M Na₂SO₄ and 1 M LiCl electrolytes showed very low specific capacitance values as the deposition of MnV₁₃ on GO was not sufficient, as indicated by FTIR and SEM. Thus, it is evident that the specific capacitance value of these composite materials depends on the amount of MnV₁₃ deposited on GO and these composite materials exhibit the potential to improve the performance of GO-based SCs.     

A simple two-step electrochemical synthesis of graphene sheets film on the ITO electrode as supercapacitors

In this study, a simple and controllable two-step electrochemical process is described for the synthesis of graphene sheets (GS) film on a cleaned indium tin oxide (ITO) sheet electrode. Namely, the main procedures involve the electrophoretic deposition (EPD) of graphene oxide (GO) film onto ITO electrode and the subsequent in situ electrochemical reduction (ECR) of GO to generate GS film. X-ray photoelectron spectroscopy (XPS) measurement demonstrates that most of the oxygen-containing functional groups in GO film have been removed after ECR. By electrochemical measurements, the maximum specific capacitance of the prepared GS film electrode was calculated to be 156 F g⁻¹, besides, the capacitance retention of the material remained 78% after 400 times of cycling, showing a promising prospect as supercapacitor materials.     

Photoresist-derived porous carbon for on-chip micro-supercapacitors

Photoresist, which is frequently used in existing microelectronics processing, can be pyrolyzed to form a conductive carbon film. We demonstrate that a pyrolysis technique of SPR-220 photoresist, consisting of heating in Ar ambient to 900 °C followed by further annealing in an H₂/Ar mixture, results in a high surface area porous carbon, applicable to supercapacitor electrode fabrication. Electrochemical testing of the pyrolyzed photoresist film yields a specific areal capacitance of 1.5–3.5 mF/cm² and a specific volumetric capacitance of 15–35 F/cm³. These results are obtained on the as-pyrolyzed films, without additional activation or deposition of electroactive species. The cycling stability of the films is shown to be robust over 10,000 cycles. This photoresist pyrolysis process could be readily integrated into microelectromechanical systems or microelectronics technology for on-chip energy storage.     

Thin film graphene oxide membrane: Challenges and gas separation potential

Graphene oxide membranes were prepared by vacuum and pressurized ultrafiltration methods on the 12% modified Polyacrylonitrile (12mPAN) substrate to specify challenges, salient features, future directions, and potential of GO membrane for separation fields using characterization techniques and gas separation test (studied gases are CO₂, He and N2), which is an efficient tool for better understanding of GO membrane behavior. GO membrane structure was examined over a wide range of parameters, such as pore size range of substrate and its surface properties, pH of GO dispersion, GO content, synthesis pressure, operating pressure and temperature. The results show that the GO content does not hold a linear relationship with the permeance and selectivity. Film thickness, aggregates, synthesis pressure defects and interlayer spacing have significant effects on the gas separation performance of GO membranes which originate from the synthesis method and its conditions.     

Co3O4 thin film prepared by a chemical bath deposition for electrochemical capacitors

Co₃O₄ thin film is synthesized on ITO by a chemical bath deposition. The prepared Co₃O₄ thin film is characterized by X-ray diffraction, and scanning electron microscopy. Electrochemical capacitive behavior of synthesized Co₃O₄ thin film is investigated by cyclic voltammetry, constant current charge/discharge and electrochemical impedance spectroscopy. Scanning electron microscopy images show that Co₃O₄ thin film is composed of spherical-like coarse particles, together with some pores among particles. Electrochemical studies reveal that capacitive characteristic of Co₃O₄ thin film mainly results from pseudocapacitance. Co₃O₄ thin film exhibits a maximum specific capacitance of 227 F g⁻¹ at the specific current of 0.2 A g⁻¹. The specific capacitance reduces to 152 F g⁻¹ when the specific current increases to 1.4 A g⁻¹. The specific capacitance retention ratio is 67% at the specific current range from 0.2 to 1.4 A g⁻¹.     

Potentiostatically deposited polypyrrole/graphene decorated nano-manganese oxide ternary film for supercapacitors

A simple method based on potentiostatic polymerization was developed for the preparation of ternary manganese oxide-based nanocomposite films. The ternary nanocomposites, which were characterized using x-ray diffraction spectroscopy and x-ray photoelectron spectroscopy, showed that the manganese oxide within the film consisted of MnO₂ and Mn₂O₃. Electrochemical measurements showed that the ternary nanocomposite electrode exhibited high specific capacitance (up to 320.6 F/g), which was attributed to the morphology of a polypyrrole/graphene/manganese-oxide (PPy/GR/MnO_x) ternary nanocomposite. The experimental approach maximized the pseudocapacitive contribution from redox-active manganese oxide (MnO_x) and polypyrrole (PPy), as well as the electrochemical double layer capacitive (EDLC) characteristic from graphene (GR) sheets. Long cyclic measurements indicated that the specific capacitance of the ternary nanocomposite film could retain 93% of its initial value over 1000 charge/discharge cycles, in the potential range of −0.2 to 0.7 V versus silver/silver chloride electrode (Ag/AgCl).     

Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery

Electrochemically reduced graphene oxides (ERGO) are obtained under various reducing potentials in the phosphate buffer solution (PBS). Different characterization methods are used to analyse the changes of structure and surface chemical condition for graphene oxide (GO). The results show that GO could be reduced controllably to certain degree and its electrochemical activity towards VO₂⁺/VO²⁺ and V³⁺/V²⁺ redox couples is also tunable using this environmentally friendly method. The catalytic mechanism of the ERGO is discussed in detail, the CO functional groups other than the C–O functional groups on the surface of ERGO more likely provide reactive sites for those redox couples, leading to a more comprehensive understanding about the catalytic process than previous relevant researches. This controllable modification method and the ERGO as electrode reaction catalyst with enhanced battery performance are supposed to have promising applications in the all vanadium redox flow battery.     

Template‐free electrochemical synthesis and electrochemical supercapacitors application of polyaniline nanobuds

One‐step and template‐free synthesis of polyaniline nanobuds (PANI NBs), synthesized using anodic electrochemical polymerization method, is reported at room temperature onto a conducting indium‐tin‐oxide glass substrate. The PANI NBs are characterized for their structural, morphological, and electrochromic properties. Scanning electron microscopy image reveals NBs type architecture, perpendicular to the substrate surface. These NBs are broader at their bottoms and narrower at the tops. Individual NB has confirmed 30 nm top diameter and 100–200 nm length. Electrochemical supercapacitive behavior of the PANI NBs electrode is investigated by measuring cyclic voltammetry curves. Moderate specific capacitance of 128 F/g is achieved in the potential range of ?0.2 to 1.0 V at 10 mV/s sweep rate in 0.5 M H₂SO₄ electrolyte. Influence of inner and outer charges on a resultant specific capacitance is also investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

One-step electrochemical deposition of nickel sulfide/graphene and its use for supercapacitors

In this current work, the electrochemical co-deposition of nickel sulfide/electrochemically reduced graphene oxide(ERGO) nanocomposites is presented. During the electrochemical process, the graphene oxide nanosheets loose their hydrophilicity and precipitate onto the electrode. In the meantime, nickel sulfide is also electrochemically deposited on the electrode. The porous structure with ERGO covered by nickel sulfide, which facilitates the charge and ion transport in the electrode, has been observed by a scanning electron microscope. The cycle voltammetry curves as well as the galvanostatic charge/discharge curves of the nickel sulfide/ERGO nanocomposites exhibit distinct pseudocapacitive characteristic. The nanocomposites maintain 66.8% of the initial specific capacitance for the first 500 cycles, and only 4.6% loss of the specific capacitance is experienced for the further 1500 cycles, evidently showing a relatively high cycling stability. The results suggest that the nickel sulfide/ERGO is a promising electrode material for supercapacitors.     

Electrochemical synthesis of Pd particles on poly(vinylferrocenium)

Electrochemical synthesis of Pd particles on poly(vinylferrocenium) (PVF⁺) support was described. K₂PdCl₄ was used as the metal particle precursor. Pd particles were incorporated into the polymer matrix electrochemically either by cyclic voltammetric scans (between +1.0 V and −0.8 V vs. SCE) or by reduction at constant potential (at −0.8 V vs. SCE) from aqueous solution of K₂PdCl₄. Scanning electron microscopy (SEM) studies showed that the Pd particles were well dispersed in the polymer matrix. Experimental parameters such as polymer film thickness, number of cycles during cyclic voltammetric scans in K₂PdCl₄ and electrolysis time in K₂PdCl₄ were studied. The Pd/PVF⁺ system showed catalytic activity towards hydrazine oxidation and appreciable results were obtained when compared with the related studies.     

Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth electrode

We report a simple and cost-effective synthesis of hierarchically porous structure composed of Birnessite-type manganese dioxide (MnO₂) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition technique. Petal-shaped MnO₂, having sheet thickness of a few nm and typical width of 100 nm, with a strong adhesion on CC is observed. This hierarchically porous MnO₂–CC hybrid structure dose exhibit not only excellent capacitance properties, such as up to 425 F g⁻¹ in specific capacitance, but also high crack resistance owing to its efficient release of bending stress, as observed by cyclic voltammetry and galvanostatic charge/discharge measurements under different curvature of bending configurations. Furthermore, flexible supercapacitors based on this kind of MnO₂ nanosheet/CC electrode showed significantly improved stability in capacitive performance over 3000 cycles under the bending test, which is highly promising for future applications in flexible energy storage device.     

Self-assembled flower-like FeS2/graphene aerogel composite with enhanced electrochemical properties

Graphene aerogel (GA) supported flower-like ferrous disulfide (FeS₂) composite was synthesis by a two-step self-assembly method using eco-friendly and low-cost precursors. The formation of well-crystallized pyrite FeS₂ and the reduction of graphene oxide (GO) was demonstrated by X-ray diffraction, Fourier transform infrared spectroscopy and Raman spectroscopy. According to the scanning electron microscopy images, the flower-like FeS₂ distributes uniformly on the inter-linking GA networks. The electrochemical tests indicate that the as-prepared GA-FeS₂ exhibits enhanced specific capacitance (313.6 F/g at the current density of 0.5 A/g), which is almost twice as high as that of bare FeS₂ (163.5 F/g). It is noticed that this composite also has excellent cyclability (88.2% retention after 2000 cycles at 10 A/g) and low transfer resistance. A symmetric supercapacitor device with wide potential range was assembled using GA-FeS₂, while its energy density could reach 22.86 Wh/kg. The excellent specific capacitance, good rate capability, and high energy density make it a promising candidate for next generation supercapacitors.     

Investigation of thin sputtered Mn films for electrochemical capacitors

Pseudocapacitive manganese oxide films have been synthesized by anodic oxidation of metallic films deposited by sputtering. Results are presented from an electrochemical investigation into properties of these thin sputtered manganese films. Mn films with thickness ranging from 20 to 200 nm have been sputtered onto Pt coated Si wafers in an Argon atmosphere. Electrochemical oxidation converts the metal film into a porous, dendritic structure which displays significant pseudocapacitance. We have observed a specific capacitance (C_s) of 700 F/g when cycled very slowly at a constant current density of 160 μA/cm². The same films probed by cyclic voltammetry (CV) at a rate of 5 mV/s yielded a lower specific capacitance of 400-450 F/g. Post-oxidation material loading was measured to be in the range of 25-75 μg/cm².     

Effects of specific surface area of electrode and different electrolyte on capacitance properties in nano porous-structure CrN thin film electrode for supercapacitor

Thickness and specific surface area of the film electrode are critical parameters for supercapacitors. The relationship between the thickness and the specific surface area of the film directly affects the capacitance and electrochemical stability performance of super supercapacitors, which virtually affects the contact chance of ion in the electrolyte on the surface of electrode and the ion transport path of electrode. In this paper, the CrN thin films with a thickness of 200–3500 nm are prepared using direct current magnetron sputtering. Atomic force microscopy (AFM) technique is introduced to investigate the relationship between thickness and the specific surface area of the CrN films. The electrochemical performances of CrN electrode with the nanoporousper structure is analyzed in different electrolytes H₂SO₄, Na₂SO₄ and NaCl aquous solutions. The specific surface area of the film increases linearly with the film thickness increases. The areal capacitance is also linearly related to the specific surface area. The spurtted CrN film with a thickness of 3370 nm has a specific surface of up to 43.59 cm² per cm² footprint area. Its areal and volume capacitances reache to 53.92 mF cm^?2 and 650 F cm^?3 at 5 mV s^?1, respectively. In addition, the areal capacitance of CrN film electrode with 655 nm possesses reaches to 40.53 mF cm^?2 for 0.5 M H₂SO₄ solution, 32.69 mF cm^?2 for 0.5 M Na₂SO₄ solution and 9.17 mF cm^?2 for NaCl solution at a scan rate of 5 mV s^?1. Furthermore, the CrN film electrode exhibits excellent capacitance retention of 95.3%, 93.8% and 89.9% in H₂SO₄, Na₂SO₄ and NaCl electrolytes, respectively, after 2000 cycles. Therefore, the sputtered CrN thin film is an potential electrode material for electrochemical supercapacitors.