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A novel tannic acid–reduced graphene oxide (TAG) composite absorbent was prepared by using a simple one-step method. The prepared TAG was characterized by various analytical techniques including FTIR, Brunauer–Emmett–Teller (BET), scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Tannic acid (TA) not only can prevent graphene sheets from agglomerating by suppressing the hydrophobic stacking interactions but also can introduce a large number of phenol hydroxyls. The adsorption studies indicate that the TAG composite showed an excellent uptake capacity (179.22 mg/g) for Cr(VI) under the optimized conditions (viz., adsorbent dosages of 0.75 g/L, solution pH of 2.5, contact time of 360 min, and temperature of 60 °C), which is higher than that of most graphene oxide– and TA-based adsorbents. The BET specific surface area of TAG was calculated to be 118.3 m2/g; the pore diameter of Tannin acid-reduced graphene hydrogel (TAGH) was mainly 3–6 nm, with pore size peak at 5.0 nm. The kinetics of Cr(VI) adsorption on TAG followed the pseudo-second-order model. The isotherm studies confirmed that the Cr(VI) chemically adsorbed on TAG in a monolayer fashion, which was attributed to the electrostatic interaction between chromium ions and oxygen negative ions. The process of Cr(VI) adsorption was found to be thermodynamically spontaneous and endothermic.

Flexible supercapacitors (FSCs) have attracted widespread attention of many researchers as a type of portable energy storage devices. However, there are still challenges in preparing renewable and ...

A simple and template-free method for the fabrication of modified pulp fiber (PF)–polyacrylamide (PAM)–graphene (RGO) composite electrodes was developed. The pulp fiber–graphene electrodes decorated with polyacrylamide were synthesized via a facile vacuum-filtration process, and further optimized through different mass ratios. Benefiting from the porous structure and large amounts of hydrophilic groups in the paper fiber, the PF–PAM–RGO composite electrode demonstrated good hydrophilic property and ion transport capacity within the electrode. After adding the polyacrylamide molecule (PF–RGO), the PF–PAM–RGO composite electrode exhibited good mechanical properties and capacitance performance due to PF–RGO hydrogen bonding. Moreover, the PF–PAM–RGO composite electrode displayed a high area specific capacitance of 498.4 mF cm−2 with a mass of 2.32 mg cm−2 (specific capacitance of 214.8 F g−1) at 0.2 mA cm−2. The assembled PF–PAM–RGO composite electrode showed a good cycling stability (92.36% capacitance retention after 8000 charging/discharging cycles) and mechanical flexibility (87.65% capacitance retention after 500 bending/unbending cycles). Remarkably, the flexible device could achieve superior energy density of 80.59 μWh cm−2 and high power density of 2549.83 μW cm−2.

Poplar wood pulp was adopted as both frame and precursor for the synthesis of pulp fiber (PF)/reduced graphene oxide composite via a simple and low-cost method. In this method, the PF based on graphene (PFG) composite film electrode was prepared by a simple vacuum filtration process with various ratios (PF: reduced graphene oxide (RGO) = 5:1, PF:RGO = 5:2, PF:RGO = 5:3, PF:RGO = 5:4, PF:RGO = 5:5). In terms of special structures, the PFG can be used as electrodes without metal-collector, adhesives, and additives. The optimal ratio (PF:RGO = 5:4) film electrode displayed a high areal-specific capacitance of 683 mF/cm² at 1 mA/cm² with a mass of 5.3 mg/cm² (specific capacitance of 129 F/g) and good cycling stability (87.5% capacitance retention after 10,000 cycles at 5 mA/cm²) as well as excellent rate capability and high flexibility (suitable for any angle, even 180°). Moreover, the device could possess a maximum energy density of 47.71 μWh/cm² and a maximum power density of 1251 μW/cm². These results suggest that the composite PGF film is a promising electrode material.

Pectin, natural polysaccharide biopolymer, was chelated with cations (Mg2+/Ca2+) to form an interwoven framework. Herein, the graphene hydrogel electrodes were self-assembled by the synergistic effects of pectin-cations. The optimum combination proportion was determined, the Mg2+/Ca2+-pectin matrix cross-linked graphene hydrogel (Mg2+/Ca2+-PGH) electrodes exhibited a large specific capacitance of about 839.2 F g−1 with high coulombic efficiency of 191.8% at a current density of 1 A g−1. The assembled flexible supercapacitor displayed excellent stability (capacitance retention of 98.5% after 2000 charge/discharge cycles) and flexibility (the specific capacitance remained 98.4% of its original value after 500 folding/unfolding cycles). Such flexible and high-performance Mg2+/Ca2+-PGH electrodes are attractive in the field of lightweight, miniature and wearable energy storage devices.

A simple and low-cost approach toward flexible and free-standing electrodes is developed. Ternary composites of graphene/carbonized paper/tannic acid (GN/CP/TA) with various ratios were fabricated via a facile vacuum-filtration process. Through carbonization of widely used paper, the obtained carbonized paper possessed a good pore structure and an ion transport channel. Tannic acid (TA) can enter into the layers of graphene sheets and prevent graphene sheets from agglomerating by suppressing the π–π stacking interactions. Different amounts of tannic acid increased the distance between graphene sheets, facilitating the formation of GN/CP/TA composite electrodes without metallic current collectors, binders and additives. The optimal ratio (GN/CP/TA2) was determined and the composite electrodes displayed a high area specific capacitance of 530.2 mF cm−2 with a mass of 2.85 mg cm−2 (specific capacitance of 186 F g−1) at 0.2 mA cm−2. The composite electrode had good mechanical flexibility (about 82.94% retention in capacitance after 500 bending times) and prominent cycling stability (86.62% retention in capacitance over 10 000 cycles). Moreover, the device can achieve a superior energy density of 36.82 μW h cm−2 and a maximum power density of 1372.73 μW cm−2. These results demonstrated that the composite film was a promising electrode material and may be useful for the development of flexible high-performance and wearable energy storage devices. More importantly, this strategy provided a facile method for the fabrication of other types of composite film electrodes.

An automated multi-filtration cleanup (Auto m-FC) method with nitrogen-enriched activated carbon material based on modified QuEChERS (quick, easy, cheap, effective, rugged, and safe) extracts was developed. It was applied to pesticide multi-residue analysis in six representative crop commodities. The automatic device was aimed to improve the cleanup efficiency and reduce manual operation workload in cleanup step. By controlling extracts volume, flow rate and Auto m-FC cycles, the device could finish cleanup process accurately. In this work, nitrogen-enriched activated carbon mixed with alternative sorbents and anhydrous magnesium sulfate (MgSO4) was packed in a column for Auto m-FC and followed by liquid chromatography with tandem mass spectrometric (LC-MS/MS) detection. This newly developed carbon material showed excellent cleanup performance. It was validated by analyzing 23 pesticides in six representative matrices spiked at two concentration levels of 10 and 100μg/kg. Water addition volume, salts, sorbents, Auto m-FC procedure including the flow rate and the Auto m-FC cycles for each matrix were optimized. Then, three general Auto m-FC methods were introduced to high water content, high oil and starch content, difficult commodities. Spike recoveries were within 82 and 106% and 1-14% RSD for all analytes in the tested matrices. Matrix-matched calibrations were performed with the coefficients of determination over 0.997 between concentration levels of 10 and 1000μg/kg. The developed method was successfully applied to the determination of pesticide residues in market samples.

A graphene/activated carbon/MnO2 (GAM) composite was synthesized by reacting in a hydrothermal synthesis reactor and maintaining at 140 °C for 2 h. MnO2 anchored on graphene/activated carbon (GR/AC) sheets and the activated carbon (AC) particles distributed on the graphene (GR) surface provided numerous meso/micropores that act as active sites for the discharge reaction. The GR forms a three-dimensional network with excellent electrical conductivity for application. In this study, we focus on the mass ratio of GR/AC and MnO2. The test outcome indicated that the GAM electrodes displayed exceptionable electrochemical performances especially when the mass ratio of GR/AC and MnO2 is 2 : 3. The specific capacitance of GAM has achieved 378 F g−1 at a constant current density of 50 mA g−1 in the 7 M KOH electrolytic solution. Beyond that, the GAM 2 : 3 electrodes have also shown splendid cyclic ability with a 91.58% capacitance retention over 3000 circles. The three dimensional network structure is expected to represent a thrilling orientation for heightening the electrochemical property of MnO2 and could be generalized for projecting next-generation superior supercapacitors.

Free-standing, binder-free and flexible activated carbon/reduced graphene oxide (AC/rGO) composite films with various ratios were fabricated via a facile vacuum-filtration process. It was proved that two-dimensional rGO sheets were an ideal adhesive support for AC. The optimal ratio (AC : rGO = 2 : 1) was determined and the film electrodes displayed a high area specific capacitance of 486 mF cm−2 with a mass of 2.35 mg cm−2 (specific capacitance of 207 F g−1) at 0.2 mA cm−2. The assembled supercapacitor had the advantages of being flexible, lightweight, cheap and environmentally friendly, and can achieve a superior energy density of 16.2 μW h cm−2 at a power density of 100 μW cm−2 and 85% capacitance retention after 10 000 charging/discharging cycles. Moreover, 90% of the capacitance was retained after 1000 bending cycles and there was almost no performance change under different bending angles. These results well demonstrated a great potential for application of AC/rGO film electrodes in wearable and portable electronics.

In this work, a lignosulfonate has been used for synthesizing the graphene quantum dots (GQDs) with many functional groups by a two-step method. Then a graphene-based composite hydrogel (GH-GQD) was obtained through the modification of the functionalized GQDs. Results showed that the composite hydrogel had a highly porous structure and numerous active sites. Accordingly, the assembled GH-GQD electrode exhibited an impressive specific capacitance of 451.7 F g−1 at a current density of 0.5 A g−1, a good cycle stability (89.0% capacitance retention after 10000 charge/discharge cycles) and mechanical flexibility (93.3% capacitance retention bent to 180°). The present findings indicate a great potential of the graphene hydrogels modified with the functionalized GQDs in fabricating flexible supercapacitor electrodes.

High theoretical capacitance of MnO2 nanoparticles were successfully electrodeposited on the conductive graphene/activated carbon (GN/AC) composite film, and the urchin type MnO2 microspheres were controlled by adjusting the electro-deposition reaction times. The GN/AC/MnO2-1200s composite electrodes exhibited a maximum specific capacitance of 1231 mF/cm2 (MnO2 loading mass of 7.65 mg/cm2 and the mass specific capacitance of 123 F/g) at a current density of 0.5 mA/cm2. The assembled flexible solid-state symmetric supercapacitor had a good mechanical flexibility (about 88.6% of its original capacitance after 500 bending times) and prominent cycling stability (about 82.8% retention in capacitance over 10000 cycles). More importantly, the device could possess a maximum energy density of 0.27 mW h/cm3 and a maximum power density of 0.02 W/cm3. These results well demonstrate a great potential for applications of GN/AC/MnO2 composite electrodes in flexible energy storage devices.

In this work, flexible graphene supercapacitor electrodes were prepared from biomass-derived carbon dots/graphene composite film (CDGF) and hydrogel (CDGH) with/without an electrodeposited NiCo2S4 layer. Results showed that the dispersive CDs with abundant heteroatoms effectively inhibited the aggregation of graphene sheets and increased the number of active sites. By combining CDs modification and NiCo2S4 electrodeposition, the electrochemical performance of the prepared ternary composite electrodes is significantly enhanced. In particular, the CDGF-NiCo2S4 electrode has a specific capacitance of up to 1348 F g−1 at a current density of 0.5 A g−1. The resultant CDGF-NiCo2S4 flexible symmetric supercapacitor using 1.0 M H2SO4 as aqueous electrolyte shows a large specific capacitance (313 F g−1 at 0.5 A g−1) and a high energy density of 85.1 Wh kg−1 at a power density of 353 W kg−1. By contrast, the CDGH-NiCo2S4 flexible solid-state supercapacitor using polyvinyl alcohol (PVA)/H2SO4 as gel electrolyte has improved cycling stability (83.1% capacitance retention after 10000 charging/discharging cycles) and a better mechanical flexibility (96.8% capacitance retention after 1000 bending cycles). This work demonstrates the importance of CDs and metal sulfides in modifying graphene-based composite electrodes for flexible supercapacitor.

Due to the urgent demand for portable electronics, enormous research efforts are devoted for the development of flexible, wearable and lightweight supercapacitors. Herein, free-standing, binder-free and high-performance metal @ carbon sphere/graphene film electrodes (Fe@C/G and Co@C/G), consisting of β-cyclodextrins (β-CDs) as carbon sources, iron and cobalt as metal materials, and graphene sheets as “binders”, have been synthesized and characterized. The resultant Fe@C/G and Co@C/G film electrodes exhibit maximum areal specific capacitances of 2.19 F cm−2 and 0.80 F cm−2 at 0.5 mA cm−2 (438 F g−1 and 160 F g−1), respectively. Furthermore, a flexible asymmetric all-solid-state supercapacitor (Fe@C/G//Co@C/G) is assembled and it displays a series of intriguing features including high volumetric specific capacitance of about 13.3 F cm−3 at 0.5 mA cm−2, excellent volumetric energy density of up to 5.99 mWh cm−3 at a volumetric power density of 0.013 W cm−3, and good cycle stability of 87.3% capacitance retention after 10,000 charge/discharge cycles. Even after 500 bending/unbending cycles, still about 80.5% of its initial capacitance is retained. Such results well demonstrate its strong potential for the practical applications of flexible energy storage devices.