Preparation of functionalized graphene sheets by a low-temperature thermal exfoliation approach and their electrochemical supercapacitive behaviors

Two kinds of functionalized graphene sheets were produced by thermal exfoliation of graphite oxide. The first kind of functionalized graphene sheets was obtained by thermal exfoliation of graphite oxide at low temperature in air. The second kind was prepared by carbonization of the first kind of functionalized graphene sheets at higher temperature in N₂. Scanning electron microscopy images show that both two kinds of samples possess nanoporous structures. The results of N₂ adsorption-desorption analysis indicate that both of two kinds of samples have high BET surface areas. Moreover, the second kind of functionalized graphene sheets has a relatively higher BET surface area. The results of electrochemical tests is as follows: the specific capacitance values of the first kind of functionalized graphene sheets in aqueous KOH electrolyte are about 230 F g⁻¹; the specific capacitance values of the second kind of functionalized graphene sheets with higher BET surface areas are only about 100 F g⁻¹; however, compared with the first kind of functionalized graphene sheets, the second kind has a higher capacitance retention at large current density because of its good conductive behaviors; furthermore, in non-aqueous EC/DEC electrolyte, the specific capacitance values of the first kind sample and the second kind sample are about 73 F g⁻¹ and 36 F g⁻¹, respectively.     

Influence of graphite surface properties on the first electrochemical lithium intercalation

The electrochemical insertion of lithium ions into graphite materials having different surface chemistry and defect concentration was studied during the first cycle in half-cell containing 1 M LiPF₆ in an electrolytic solvent mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC). The graphite surface properties were varied by thermal treatments in either hydrogen, oxygen, or nitrogen oxide or chemical treatment in boiling nitric acid. The influence of the surface modifications on the course of the first electrolyte reduction was investigated. The surface group chemistry was analyzed by temperature-programmed desorption coupled with mass spectrometry. The surface defect concentration was determined in terms of the active surface area (ASA) measured by oxygen chemisorption and a subsequent temperature-programmed desorption. The experimental results showed that the ASA parameter governs the exfoliation tendency of the graphite negative electrode material with the existence of a critical value below which the graphite systematically exfoliates. The specific charge loss during the first electrochemical insertion of lithium and the exfoliation behavior of the graphite negative electrode material are not influenced by the type and amount of oxygen surface groups. But hydrogen present on the graphite surface increased the graphite exfoliation tendency even for graphite materials with an ASA above the critical value.     

Graphene nanosheets as electrode material for electric double-layer capacitors

Graphene nanosheets (GNSs) with narrow mesopore distribution around 4 nm were mass-produced from natural graphite via the oxidation and rapid heating processes. The effects of oxidant addition on the morphology, structure and electrochemical performance of GNSs as electrode materials for electric double-layer capacitor (EDLC) were systematically investigated. The electrochemical properties of EDLC were influenced by the specific surface area, pore characteristics, layer stacking and oxygen-containing functional group contents of electrode materials. Deeper oxidation makes graphite possess both higher specific surface area and more graphene edges, which are favorable for the enhancement of capacitive performance of EDLC. The electrodes with freestanding graphene nanosheets prepared by coating method exhibited good rate capability and reversibility at high scan rates (to 250 mV s⁻¹) in electrochemical performances. GNS electrode with specific surface area of 524 m² g⁻¹ maintained a stable specific capacitance of 150 F g⁻¹ under specific current of 0.1 A g⁻¹ for 500 cycles of charge/discharge.     

Cobalt-boron amorphous alloy prepared in water/cetyl-trimethyl-ammonium bromide/n-hexanol microemulsion as anode for alkaline secondary batteries

Amorphous cobalt-boron (Co-B) with uniform nanoparticles was prepared for the first time via reduction of cobalt acetate by potassium borohydride in the water/cetyl-trimethyl-ammonium bromide/n-hexanol microemulsion system. The sample was characterized by X-ray diffraction, transmission electron microscopy, nitrogen adsorption-desorption, X-ray photoelectron spectroscopy, inductively coupled plasma, cyclic voltammetry, differential scanning calorimetry, temperature-programmed desorption, scanning electron microscopy, charge-discharge test and electrochemical impedance spectra. The results demonstrate that electrochemical activity of the as-synthesized Co-B was higher than that of the regular Co-B prepared in aqueous solution. It indicates that the homogeneous distribution and large specific surface area helped the electrochemical hydrogen storage of the as-synthesized Co-B. Furthermore, the as-synthesized Co-B even had 347 mAh g⁻¹ after 50 cycles, while the regular Co-B prepared in aqueous solution only had 254 mAh g⁻¹ after 30 cycles at a current of 100 mA g⁻¹. The better cycling performance can be ascribed to its smaller interfacial impedance between electrode and electrolyte.     

Electrochemical properties of heat-treated polymer-derived SiCN anode for lithium ion batteries

Silicon-carbon-nitrogen material (SiCN) is pyrolyzed from polysilylethylenediamine (PSEDA) derivation, followed by a heat-treating process at 1000 °C in Ar atmosphere. This heat-treated SiCN material has an excellent electrochemical performance as an anode for lithium ion batteries. Charge-discharge cycle measurements show that the heat-treated SiCN material exhibits a high first cycle discharge capacity of 829.0 mAh g⁻¹ and stays between 400 and 370 mAh g⁻¹ after 30 cycles. The discharge capacity remains above 300 mAh g⁻¹ at the high current density of 80 and 160 mA g⁻¹. These values are higher than untreated SiCN and commercial graphite anodes, which indicates that the heat-treating process improves the charge-discharge capacity, cycle stability and high-rate ability of SiCN anode. It is seemed that changes of SiCN structure, the formation of loose nano-holes on material surface and the formation of graphitic carbon phase in heat-treating process contribute to the improvement of electrochemical properties for SiCN anode.     

Novel tin-graphite composites as negative electrodes of Li-ion batteries

For the purpose of obtaining an improved performance of the graphite negative electrode of Li-ion batteries, a novel graphite-tin composite has been synthesized by reduction of tin chloride (SnCl₂) with KC₈ in THF medium. This composite contains nano-sized tin particles dispersed on the graphite surface and free tin aggregates. Lithium electrochemical insertion occurs both in graphite and in tin. An experimental reversible specific charge of 489 mA h g⁻¹ is found stable upon cycling. Such a value is lower than the maximum theoretical one of 609 mA h g⁻¹ suggesting that only a part of tin is involved in the lithium insertion/extraction process. This part of active tin responsible for the stable capacity could be that bound to graphite. To the contrary, free tin aggregates could contribute to an extra capacity that decreases upon cycling in relation with the volume changes that occurs during alloying/dealloying.     

EDLC performance of carbide-derived carbons in aprotic and acidic electrolytes

This study shows that carbide-derived carbons (CDCs) with average pore size distributions around 0.9-1 nm and effective surface areas of 1300-1400 m² g⁻¹ provide electrochemical double-layer capacitors with high performances in both aqueous (2M H₂SO₄) and aprotic (1M (C₂H₅)₄NBF₄ in acetonitrile) electrolytes.In the acidic electrolytic solution, the gravimetric capacitance at low current density (1 mA cm⁻²) can exceed 200 F g⁻¹, whereas the volumetric capacitance reaches 90 F cm⁻³. In the aprotic electrolyte they reach 150 F g⁻¹ and 60 F cm⁻³.A detailed comparison of the capacitive behaviour of CDCs at high current density (up to 100 mA cm⁻²) with other microporous and mesoporous carbons indicates better rate capabilities for the present materials in both electrolytes. This is due to the high surface area, the accessible porosity and the relatively low oxygen content.It also appears that the surface-related capacitances of the present CDCs in the aprotic electrolyte are in line with other carbons and show no anomalous behaviour.     

Capacitance behavior of activated carbon fibers with oxygen-plasma treatment

Modified activated carbon fibers (ACFs) were used as the electrodes of an electric double-layer capacitor and showed an enhanced capacitance effect after a RF-plasma treatment. The capacitance and the surface functional groups of the ACFs were studied. For the plasma-treated ACFs having a specific surface area of 1500 m² g⁻¹, the capacitance increased by 28% compared to the untreated sample and the highest electric capacitance value of 142 F g⁻¹ was achieved with an oxygen feed concentration of 10 vol.%. The Brunauer-Emmett-Teller (BET) surface area was 2103 m² g⁻¹, which was 34% higher than that of the untreated sample. The pore volume was similarly increased to 483.1 cm³ g⁻¹ STP, and from the pore distribution plot, quantities of mesopores of 10 nm or less and micropores also increased. However, in order to enhance the capacitance, the quinone functional group had a significant influence in addition to the BET surface area. The correlation between the capacitance and the number of quinone functional groups was confirmed because quinone is an electron acceptor.     

Nano/micro-hybrid NiS cathodes for lithium ion batteries

The present study highlights a low temperature process by which 1D stacked 3D microstructures of nickel sulfide comprised of nanospikes have been synthesized and assembled as cathodes for lithium chalcogenide batteries. These micro/nano-clusters were synthesized hydrothermally under different conditions. These clusters exhibited a surface area of 15 m² g⁻¹. The present study also provides the first reports on the electrochemical performance of these NiS microclusters as cathode materials in lithium fluoro-Tris-sulfonimide electrolyte for lithium ion batteries. A detailed study has been performed to elucidate how surface morphology and redox reaction behaviors underlying these electrodes impact the cyclic behavior and specific capacity. This electrode−electrolyte combination showed minimal dissolution of the electrode in the electrolyte which was confirmed by inductively coupled plasma atomic emission spectroscopy. From the electrochemical analysis performed an intrinsic correlation between the capacity, self-discharge property and the surface morphology has been deduced and explained on the basis of relative contributions from the redox reactions of nickel sulfide in lithium fluoro-Tris-sulfonimide electrolyte. A working model of lithium battery in a coin cell form is also shown exhibiting a specific capacity of 550 mAh g⁻¹.     

Non-catalyzed cathodic oxygen reduction at graphite granules in microbial fuel cells

Oxygen is the most sustainable electron acceptor currently available for microbial fuel cell (MFC) cathodes. However, its high overpotential for reduction to water limits the current that can be produced. Several materials and catalysts have previously been investigated in order to facilitate oxygen reduction at the cathode surface. This study shows that significant stable currents can be delivered by using a non-catalyzed cathode made of granular graphite. Power outputs up to 21 W m⁻³ (cathode total volume) or 50 W m⁻³ (cathode liquid volume) were attained in a continuous MFC fed with acetate. These values are higher than those obtained in several other studies using catalyzed graphite in various forms. The presence of nanoscale pores on granular graphite provides a high surface area for oxygen reduction. The current generated with this cathode can sustain an anodic volume specific COD removal rate of 1.46 kg_COD m⁻³ d⁻¹, which is higher than that of a conventional aerobic process. This study demonstrates that microbial fuel cells can be operated efficiently using high surface graphite as cathode material. This implies that research on microbial fuel cell cathodes should not only focus on catalysts, but also on high surface area materials.     

Physical and electrochemical characterization of CuO-doped activated carbon in ionic liquid

A series of CuO-doped activated carbons (CDACs) were prepared by chemical deposition. The electrochemical behavior of CDACs was investigated in electrochemical capacitors based on ionic liquid 1-ethyl-3-methylimidazolium thiocyanate ([EMIm]SCN) as electrolyte. The results indicated that a diffusion-controlling, reversible redox reaction of CuO particles happened in ionic liquid and porous carbon. When the amount of CuO-doped activated carbon with a specific surface area of 2460 m² g⁻¹ reached 2%, the single electrode average specific capacitance can reach the maximal value of 210 F g⁻¹, about 20% higher than the one used pure activated carbon as electrode material.     

Facile solvothermal synthesis of a graphene nanosheet-bismuth oxide composite and its electrochemical characteristics

This work demonstrates a novel and facile route for preparing graphene-based composites comprising of metal oxide nanoparticles and graphene. A graphene nanosheet-bismuth oxide composite as electrode materials of supercapacitors was firstly synthesized by thermally treating the graphene-bismuth composite, which was obtained through simultaneous solvothermal reduction of the colloidal dispersions of negatively charged graphene oxide sheets in N,N-dimethyl formamide (DMF) solution of bismuth cations at 180 °C. The morphology, composition, and microstructure of the composites together with pure graphite oxide, and graphene were characterized using powder X-ray diffraction (XRD), FT-IR, field emission scanning electron microscopy (FESEM), transmission electron microscope (TEM), thermogravimetry and differential thermogravimetry (TG-DTG). The electrochemical behaviors were measured by cyclic voltammogram (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS). The specific capacitance of 255 F g⁻¹ (based on composite) is obtained at a specific current of 1 A g⁻¹ as compared with 71 F g⁻¹ for pure graphene. The loaded-bismuth oxide achieves a specific capacitance as high as 757 F g⁻¹ even at 10 A g⁻¹. In addition, the graphene nanosheet-bismuth oxide composite electrode exhibits the excellent rate capability and well reversibility.     

Fast and reversible surface redox reaction of graphene-MnO2 composites as supercapacitor electrodes

We present a quick and easy method to synthesize graphene-MnO₂ composites through the self-limiting deposition of nanoscale MnO₂ on the surface of graphene under microwave irradiation. These nanostructured graphene-MnO₂ hybrid materials are used for investigation of electrochemical behaviors. Graphene-MnO₂ composite (78 wt.% MnO₂) displays the specific capacitance as high as 310 F g⁻¹ at 2 mV s⁻¹ (even 228 F g⁻¹ at 500 mV s⁻¹), which is almost three times higher than that of pure graphene (104 F g⁻¹) and birnessite-type MnO₂ (103 F g⁻¹). Interestingly, the capacitance retention ratio is highly kept over a wide range of scan rates (88% at 100 mV s⁻¹ and 74% at 500 mV s⁻¹). The improved high-rate electrochemical performance may be attributed to the increased electrode conductivity in the presence of graphene network, the increased effective interfacial area between MnO₂ and the electrolyte, as well as the contact area between MnO₂ and graphene.     

Electrochemical regeneration of a carbon-based adsorbent loaded with crystal violet dye

The removal of colour from a crystal violet dye solution using a non-porous, electrically conducting carbon-based adsorbent was systematically investigated under different operating conditions. Whilst the adsorptive process was very quick (up to 88% of equilibrium capacity could be achieved within 2 min), the adsorptive capacity of the adsorbent was very low (2 mg g⁻¹) compared with activated carbons. This was due to its low surface area. The conductivity of the adsorbent/electrolyte mixture within the anodic compartment of the electrochemical cell was found to be over 13 times greater with the new adsorbent compared with powdered activated carbon. One hundred percent could be achieved in a simple divided electrochemical cell using treatment times as low as 10 min by passing a charge of 25 C g⁻¹ at a current density of 20 mA cm⁻². The efficiency of electrochemical regeneration depends on a range of variables including charge passed, current density, treatment time, electrolyte type and concentration and the adsorbent bed thickness. Multiple adsorption and regeneration cycles indicate that there is little or no loss in adsorbent capacity on regeneration.     

Alkali carbonate-coated graphite electrode for lithium-ion batteries

Charge and discharge behavior of a graphite electrode for rechargeable lithium-ion batteries was successfully improved by pretreatment of graphite powders with A₂CO₃ (A = Li, Na, and K) aqueous solutions. In the process of the pretreatment, graphite powders were simply dispersed in the aqueous solutions, and then filtered and dried to modify the surface of graphite powder with solid alkali carbonate. With the optimum concentration of each carbonate, 1 wt.% Li₂CO₃, 5 wt.% Na₂CO₃, and 1 wt.% K₂CO₃, the irreversible reaction at the initial cycle was suppressed by the pretreatment which was capable of modifying the solid electrolyte interphase formed on the graphite electrode surface. Furthermore, the rate capability was improved by the surface modification, that is, the reversible discharge capacities at 175 mA g⁻¹ increased with adequate capacity retention in a 1 mol dm⁻³ LiClO₄ ethylene carbonate:diethyl carbonate electrolyte solution because of the kinetics enhancement of lithium-ion transfer at the interface.     

Enhanced electrochemical properties of LiFePO4/C synthesized with two kinds of carbon sources,PEG-4000 (organic) and Super p (inorganic)

Olivine structured LiFePO₄/C cathode was synthesized via a freeze-drying route and followed by microwave heating with two kinds of carbon sources: PEG-4000 (organic) and Super p (inorganic). XRD patterns indicate that the as-prepared sample has an olivine structure and carbon modification does not affect the structure of the sample. Image of SEM shows a uniform and optimized particles size, which greatly improves the electrochemical properties. TEM result reveals the amorphous carbon around the surface of the particles. At a low rate of 0.1 C, the LiFePO₄/C sample presents a high discharge capacity of 157.8 mAh g⁻¹ which is near the theoretical capacity (170 mAh g⁻¹), and it still attains to 149.1 mAh g⁻¹ after 200 cycles. It also exhibits an excellent rate capacity with high discharge capacities of 143.2 mAh g⁻¹, 137.5 mAh g⁻¹, 123.7 mAh g⁻¹ and 101.6 mAh g⁻¹ at 0.5 C, 1.0 C, 2.0 C and 5.0 C, respectively. EIS results indicate that the charge transfer resistance of LiFePO₄ decreases greatly after carbon coating.     

Electrochemical performance of flowerlike CaSnO3 as high capacity anode material for lithium-ion batteries

Nanosized CaSnO₃ is synthesized by a hydrothermal process and characterized by X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM). The SEM observation shows the sample has a porous flowerlike morphology. The electrochemical results exhibit that the stable and reversible capacity of 547 mAh g⁻¹ is obtained after 50 cycles at 60 mA g⁻¹ (0.1 C) and the corresponding charge capacity is determined to be 316 mAh g⁻¹ at the current density of 2.5 C. Cyclic voltammetry and electrochemical impedance spectroscopy data are analyzed to complement the galvanostatic results. The observed excellent performance is attributed to the porous structure and large surface area of flowerlike CaSnO₃.     

Performance of templated mesoporous carbons in supercapacitors

By analogy with other types of carbons, templated mesoporous carbons (TMCs) can be used as supercapacitors. Their contribution arises essentially from the double layer capacity formed on their surface, which corresponds to 0.14 F m⁻² in aqueous electrolytes such as H₂SO₄ and KOH and 0.06 F m⁻² for the aprotic medium (C₂H₅)₄NBF₄ in CH₃CN. In the case of a series of 27 TMCs, it appears that the effective surface area determined by independent techniques can be as high as 1500-1600 m² g⁻¹, and therefore exceeds the value obtained for many activated carbons (typically 900-1300 m² g⁻¹). On the other hand, the relatively low amount of surface oxygen in the present TMCs, as opposed to activated carbons, reduces the contribution of pseudo-capacitance effects and limits the gravimetric capacitance to 200-220 F g⁻¹ for aqueous electrolytes. In the case of non-aqueous electrolyte, it rarely exceeds 100 F g⁻¹.It is also shown that the average mesopore diameter of these TMCs does not improve significantly the ionic mobility compared with typical activated carbons of pore-widths above 1.0-1.3 nm.This study suggests that activated carbons remain the more promising candidates for supercapacitors with high performances.     

Effect of specific surface area on capacitance in asymmetric carbon/α-MnO2 supercapacitors

α-MnO₂ has been made using a solid state synthesis and the specific surface area then modified through milling. The formation of α-MnO₂ (specific surface area 96 m² g⁻¹) has been studied by SEM and powder XRD prior to milling. Electrode films (cast using MnO₂, graphite and PVDF) have been investigated using N₂ sorption at 77 K and show a more complex relationship than their parent oxides. Specific capacitances of 235 F g⁻¹ were observed in cyclic voltammetry studies in (NH₄)₂SO₄ (aq.) electrolyte. Good cyclability was observed in hybrid C/MnO₂ cells investigated through both galvanostatic and electrochemical impedance techniques. The specific capacitances of the cells were found to correlate with S_BET of the electrode films and not that of the parent MnO₂ powders.     

Large reversible capacity of high quality graphene sheets as an anode material for lithium-ion batteries

High quality graphene sheets were prepared from graphite powder through oxidation followed by rapid thermal expansion in nitrogen atmosphere. The preparation process was systematically investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy and Brunauer-Emmett-Teller (BET) measurements. The morphology and structure of graphene sheets were characterized by scanning electron microscope (SEM) and high-resolution transmission electron microscopy (HRTEM). The electrochemical performances were evaluated in coin-type cells versus metallic lithium. It is found that the graphene sheets possess a curled morphology consisting of a thin wrinkled paper-like structure, fewer layers (∼4 layers) and large specific surface area (492.5 m² g⁻¹). The first reversible specific capacity of the prepared graphene sheets was as high as 1264 mA h g⁻¹ at a current density of 100 mA g⁻¹. Even at a high current density of 500 mA g⁻¹, the reversible specific capacity remained at 718 mA h g⁻¹. After 40 cycles, the reversible capacity was still kept at 848 mA h g⁻¹ at the current density of 100 mA g⁻¹. These results indicate that the prepared high quality graphene sheets possess excellent electrochemical performances for lithium storage.