Junwei Lang

Flexible and conductive nanocomposite electrode based on graphene sheets and cotton cloth for supercapacitor

There is currently a strong demand for energy storage devices which are cheap, light weight, flexible, and possess high power and energy densities to meet the various requirements of modern gadgets. Herein, we prepare a flexible and easily processed electrode via a simple “brush-coating and drying” process using everyday cotton cloth as the platform and a stable graphene oxide (GO) suspension as the ink. After such a simple manufacturing operation followed by annealing at 300 °C in argon atmosphere, the as-obtained graphene sheets (GNSs)–cotton cloth (CC) composite fabric exhibits good electrical conductivity, outstanding flexibility, and strong adhesion between GNSs and cotton fibers. Using this GNSs–CC composite fabric as the electrode material and pure CC as the separator, a home-made supercapacitor was fabricated. The supercapacitor shows the specific capacitance of 81.7 F g−1 (two-electrode system) in aqueous electrolyte, which is one of the highest values for GNSs-based supercapacitors. Moreover, the supercapacitor also exhibits satisfactory capacitance in ionic-liquid/organic electrolyte. An all-fabric supercapacitor was also fabricated using pure CC as separator and GNSs–CC composite fabric as electrode and current collector. Such a conductive GNSs–CC composite fabric may provide new design opportunities for wearable electronics and energy storage applications.

Promising Porous Carbon Derived from Celtuce Leaves with Outstanding Supercapacitance and CO2 Capture Performance

Business costs and energy/environmental concerns have increased interested in biomass materials for production of activated carbons, especially as electrode materials for supercapacitors or as solid-state adsorbents in CO₂ adsorption area. In this paper, waste celtuce leaves were used to prepare porous carbon by air-drying, pyrolysis at 600 °C in argon, followed by KOH activation. The as-prepared porous carbon have a very high specific surface area of 3404 m²/g and a large pore volume of 1.88 cm³/g. As an electroactive material, the porous carbon exhibits good capacitive performance in KOH aqueous electrolyte, with the specific capacitances of 421 and 273 F/g in three and two-electrode systems, respectively. As a solid-state adsorbent, the porous carbon has an excellent CO₂ adsorption capacity at ambient pressures of up to 6.04 and 4.36 mmol/g at 0 and 25 °C, respectively. With simple production process, excellent recyclability and regeneration stability, the porous carbon that was derived from celtuce leaves is among the most promising materials for high-performance supercapacitors and CO₂ capture.

A hybrid supercapacitor based on flower-like Co(OH)2 and urchin-like VN electrode materials

A series of hybrid electrochemical capacitors were fabricated by using the flower-like cobalt hydroxide (Co(OH)2) and urchin-like vanadium nitride (VN) as the positive and negative electrode materials, respectively. Both Co(OH)2 and VN electrode materials showed excellent electrochemical performance due to their unique structure and fast reversible Faradic reaction characteristics. With different operation voltage window (OVW) and negative/positive mass ratios, the impact on capacitance performance of the hybrid supercapacitor was investigated thoroughly, which demonstrated that both mass ratio and OVW played an important role in their capacitance performance. Furthermore, theoretical modeling was performed and the simulation result was found to be in agreement with the experimental result for the influence of the negative/positive mass ratio on capacitance performance of the hybrid supercapacitor. When an optimized negative/positive mass ratio was located, the Co(OH)2//VN hybrid supercapacitor could be cycled reversibly in the high-voltage region of 0–1.6 V and delivered a high energy density of 22 W h kg−1. Even at a large power density of 15.9 kW kg−1, the hybrid supercapacitor still possessed a desirable specific energy density of 9 W h kg−1. Such an impressive hybrid supercapacitor was expected to be a highly promising candidate for application in high-performance energy storage systems.

Asymmetric Supercapacitor Based on Loose-Packed Cobalt Hydroxide Nanoflake Materials and Activated Carbon

Cobalt hydroxide nanoflakes with a maximum specific capacitance of 735 F/g are successfully synthesized by a facile chemical precipitation method. To enhance energy density, an asymmetric-type pseudo/electric double-layer capacitor is considered where Co(OH) 2 nanoflakes and activated carbon act as the positive and negative electrodes, respectively. The electrochemical properties of the two electrodes and the asymmetric supercapacitor are investigated in 2 M KOH aqueous electrolyte. Values for the maximum specific capacitance of 72.4 F/g and specific energy of 92.7 Wh/kg are demonstrated for a cell voltage between 0 and 1.6 V. By using the nanoflake Co(OH) 2 electrode, the asymmetric supercapacitor exhibits high energy density and stable power characteristics. The hybrid supercapacitor also exhibited a good electrochemical stability with 93.2% of the initial capacitance over consecutive 1000 cycle numbers.

Facile Synthesis of Fe2O3 Nano-Dots@Nitrogen-Doped Graphene for Supercapacitor Electrode with Ultralong Cycle Life in KOH Electrolyte.

Fe2O3 nanodots supported on nitrogen-doped graphene sheets (denoted as Fe2O3 NDs@NG) with different loading masses are prepared through a facile one-pot solvothermal method. The resulting Fe2O3 NDs@NG composites exhibit outstanding electrochemical properties in aqueous KOH electrolyte. Among them, with the optimal loading mass of Fe2O3 NDs, the corresponding Fe2O3 NDs@NG-0.75 sample is able to deliver a high specific capacitance of 274 F g(-1) at 1 A g(-1) and the capacitance is still as high as 140 F g(-1) even at a ultrahigh current density of 50 A g(-1), indicating excellent rate capability. More remarkably, it displays superior capacitance retention after 100,000 cycles (about 75.3% at 5 A g(-1)), providing the best reported long-term cycling stability for iron oxides in alkaline electrolytes to date. Such excellent electrochemical performance is attributed to the right combination of highly dispersed Fe2O3 NDs and appropriately nitrogen-doped graphene sheets, which enable the Fe2O3 NDs@NG-0.75 to offer plenty of accessible redox active sites, facilitate the electron transfer and electrolyte diffusion, as well as effectively alleviate the volume change of Fe2O3 NDs during the charge-discharge process.

Oxygen-enriched activated carbons from pomelo peel in high energy density supercapacitors

We used low-cost pomelo peel (PP) as a biomass-derived porous activated carbon to fabricate a high energy density symmetric supercapacitor. The porous activated carbon was prepared from the pomelo peel (PP) via pyrolysis and a KOH activation process, followed by heat treatment under an argon atmosphere. The resulting porous carbon possesses a very high specific surface area (2105 m2 g−1) and abundant oxygen functionalities, which lead to a wide voltage window of 1.7 V, a specific capacitance of 43.5 F g−1 and a high energy density of 17.1 W h kg−1 for the as-assembled AC//AC symmetric supercapacitor. These impressive electrochemical characteristics may indicate the PP to act as a new biomass source of carbonaceous materials for low-cost and high performance electrical energy storage devices.

Effects of concentration and temperature of EMIMBF4/acetonitrile electrolyte on the supercapacitive behavior of graphene nanosheets

Graphene nanosheets (GNSs)–ionic liquids (ILs) electrochemical system is of great interest as it shows excellent electrochemical properties for high performance supercapacitors. In this paper, the effects of concentration and temperature of ILs electrolyte on the electrochemical properties of a GNSs electrode are characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy measurements (EIS) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)/acetonitrile electrolyte. The results show that the internal resistance and the specific capacitance are strongly dependent on the variation of molar concentration of EMIMBF4, and the GNSs electrode exhibits high specific capacitance (128.2 F g−1) and a wide potential window (2.3 V) in 2.0 M EMIMBF4/acetonitrile electrolyte, indicating the excellent electrochemical performance. Moreover, the GNSs electrode has wide operating temperatures ranging from −20 °C to 60 °C with a potential window from −0.6 V to 1.5 V in the EMIMBF4/acetonitrile electrolyte. The result also reveals a weak dependence of the supercapacitive performance of the GNSs electrode on the temperature of the EMIMBF4/acetonitrile electrolyte. In addition, the specific capacitances have almost no decay after 1500 charge/discharge cycles in the above mentioned temperature region, demonstrating the good stability of the GNSs–ILs system in high-temperature and low-temperature environments.

Identifying pseudocapacitance of Fe2O3 in an ionic liquid and its application in asymmetric supercapacitors

Pseudocapacitance is commonly associated with surface or near-surface reversible redox reactions, as observed with transition metal oxides in alkaline aqueous electrolytes. Here, we demonstrate that pseudocapacitive behavior of Fe2O3 can occur in a 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4) ionic liquid (IL), and it is closely related to the chemical state variation between Fe3+ and Fe2+ on the surface of a Fe2O3 electrode during the charging/discharging process. By taking advantage of such pseudocapacitance, we prepared a promising electrode material, i.e., graphene nanosheet-supported Fe2O3 nanoparticles (denoted as Fe2O3@GNS), and then built high-performance asymmetric supercapacitors (ASs) using Fe2O3@GNS as the battery-type electrode material, commercial activated carbon (AC)/or activated polyaniline-derived carbon nanorods (denoted as APDC) as the capacitor-type electrode material, and EMIMBF4 IL as the electrolyte. The as-made ASs are able to work reversibly in a full operation voltage region of 0–4 V and exhibit very high energy density. Especially, the AS of Fe2O3@GNS//APDC achieves an extremely high energy density of 177 W h kg−1 and shows a superior combination of high energy and power density (the energy density still remains 62.4 W h kg−1 even at a high power density of 8 kW kg−1).

Electrochemical behavior of graphene nanosheets in alkylimidazolium tetrafluoroborate ionic liquid electrolytes: influences of organic solvents and the alkyl chains

In this study, the electrochemical properties of graphene nanosheets (GNSs) in alkylimidazolium tetrafluoroborate ionic liquids/organic solvent electrolytes are investigated by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS). The organic solvents with different functional groups exhibit a significant influence on the electrochemical properties of the GNSs. From series of organic solvents, in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIMBF4)/N,N-dimethylformamide (DMF, C3H7NO) electrolyte the GNS electrode shows the best electrochemical performance. Furthermore, the effect of the alkyl chains of ionic liquids on the electrochemical properties of GNSs was also evaluated through the electrochemical tests. The electrochemical properties of GNS electrode in 1-methyl-3-methylimidazolium tetrafluoroborate (MMIMBF4)/DMF electrolyte are better than those in EMIMBF4/DMF and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIMBF4)/DMF electrolytes. This may be attributed to the difference in the length of the alkyl chain on the imidazole ring, which results in the structural change of the electrode/ionic liquid interface and thus affects the electrochemical performance of the GNS electrode.

Engineering the Electrochemical Capacitive Properties of Microsupercapacitors Based on Graphene Quantum Dots/MnO2 Using Ionic Liquid Gel Electrolytes

All-solid-state microsupercapacitors (MSCs) have been receiving intense interest due to their potential as micro/nanoscale energy storage devices, but their low energy density has limited practical applications. It has been reported that gel electrolytes based on ionic liquids (ionogels) with large potential windows can be used as solid electrolytes to enhance the energy density of MSCs, but a systematic study on how to select and evaluate such ionogels for MSCs is rare. In this study, we construct a series of all-solid-state asymmetric MSCs on the interdigital finger electrodes, using graphene quantum dots (GQDs) as the negative electrode, MnO2 nanosheets as the positive electrode, and different ionogels as the solid electrolytes. Among them, the MSC using 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][NTF2]) with 4 wt % fumed SiO2 ionogel exhibited the best electrochemical performance, having excellent rate capability with the scan rate up to 2000 V s(-1), ultrafast frequency response (τ0 = 206.9 μs) and high energy density. The outstanding performance of this device mainly results from fast ion diffusion, high ion conductivity of the ionogel, and ionic liquid-matrix interactions. The results presented here provide guidance for picking out appropriate ionogels for use in high-performance all-solid-state MSCs to meet the growing requirement of micronanoscale energy storage devices. Additionally, the ultrafast frequency response of our MSCs suggests potential applications in ac line-filters.