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Cotton is an inexpensive natural product, which is mainly composed of cellulose fibers. Here, based on carbonized cotton fibers as substrates, we report a simple and facile method for the fabrication of a modified porous graphene hydrogel electrode. The method involved addition of 1,5-diaminoanthraquinone (DAQ) onto graphene to obtain a decorated graphene hydrogel electrode. DAQ is uniformly anchored on graphene sheets by π–π interactions. A robust and flexible DAQ–CGH hydrogel electrode is prepared by a hydrothermal method and optimized through different mass ratios. The carbonized cotton fiber substrate guarantees high mechanical strength, meanwhile the conductive graphene sheets serve as an electron transport medium for DAQ to ensure its remarkable pseudocapacitance. The DAQ–CGH composite exhibits a high specific capacitance of 490.2 F g−1 at 1 A g−1 with a stable capacity retention of 89.93% after 10 000 cycles at 5 A g−1 and excellent mechanical flexibility (about 92.15% of its original capacitance after 1000 bending cycles). Moreover, the assembled DAQ-2-CGH SCs achieve a high energy density of 36.18 W h kg−1 and power density of 2145.53 W kg−1.

A flexible film electrode composed of sodium carboxymethyl cellulose (CMC), polypyrrole (PPy) and reduced graphene oxide (RGO) was designed by in situ polymerization and vacuum filtration. The obtained film displayed CMC–PPy nanospheres wrapped in an RGO framework, resulting in a free-standing and highly flexible supercapacitor electrode. The incorporation of hydrophilic CMC into the film enhanced the diffusion paths of electrolyte ions and prevented the volume deformation of PPy during the long-term charging/discharging. Furthermore, PPy provided a large pseudocapacitance and RGO endowed the film electrode with prominent flexibility. The remarkable interaction between CMC, PPy and RGO achieved a synergistic effect on the structural stability and electrochemical performance of the film electrode. When the mass ratio of CMC–PPy to RGO was 1 : 1, the highest specific capacitance of the sodium carboxymethyl cellulose–polypyrrol@reduced graphene oxide (S-CPR1 : 1) was 489 mF cm−2 (191 F g−1) with a mass of 2.56 mg cm−2 at a current density of 0.5 mA cm−2. And the area specific capacitance of the S-CPR1 : 1 electrode could remain as 101.6% of the original value after 1000 charge/discharge cycles, which was higher than the polypyrrol@reduced graphene oxide (S-PR) electrode (72.4%) prepared under the same conditions. Besides, the S-CPR1 : 1 electrode exhibited remarkable flexibility with 95.6% capacity retention after 500 fold cycles. These above results indicated that the S-CPR1 : 1 film electrode provided a simple way of developing free-standing and flexible electrode materials for next-generation electrochemical electrodes.

Designing flexible supercapacitors (FSCs) as energy storage devices for portable/wearable electronic products using the earth-abundant, renewable and environmentally friendly natural biomass has the prospect of building a green, sustainable society. Here, we report a new type of three-dimensional graphene hydrogel with lignin-based nitrogen-doped carbon dots as a decorative material. The nitrogen doped carbon dots are anchored in a graphene sheet through π–π bonds and electrostatic interactions, forming a stable three-dimensional network structure, which comprehensively improves the electrochemical performance of the electrode material. As an electrode, the specific capacitance reaches 387 F g−1 at 1 A g−1, and the rate performance reaches 75.7% when the current density increases to 7 A g−1. In addition, an all-lignin-based supercapacitor assembled with a lignin hydrogel as the electrolyte has superior cycle stability (cycle life after 5000 cycles is 92.3%) and rate performance. When the power density is 243 W kg−1, the energy density is 25.6 W h kg−1; moreover, the FSC has excellent electrochemical stability even at different bending angles. This all-lignin-based FSC with excellent electrochemical performance and flexibility opens up a new path for the application of lignin in flexible energy storage devices.

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.

A novel composite electrode has been fabricated by the direct deposition of MnO2 onto graphene networks surrounding a paper fiber (PFG). The paper fiber between graphene sheets could be used as a flexible substrate for MnO2 nanoparticles, and the microscopic morphologies and electrochemical performances of the MnO2@PFG electrodes were tuned via regulating the deposition current densities and deposition times. 3D graphene on PFG served as a highly conductive backbone with a high surface area for the deposition of the MnO2 nanoparticles, which provided high accessibility to electrolyte ions for shortening the diffusion paths. The MnO2-10-600 s@PFG composite electrode achieved a maximum specific capacitance of 878.6 mF cm−2 with an MnO2 loading mass of 3.62 mg cm−2 (specific capacitance of 187.7 F g−1) at a current density of 0.5 mA cm−2 in a 1 M NaSO4 aqueous solution. Additionally, the MnO2-10-600 s@PFG composite material with the most favorable composite ratio exhibited the highest energy density of 61.01 mW h cm−2, maximum power density of 1249.78 mW cm−2, excellent capacitance retention with no more than 7% capacitance loss after 10 000 cycles and good mechanical flexibility (about 91.06% of its original capacitance after 500 bending times). By combining the electric double layer capacitance of graphene networks with the pseudocapacitance of the MnO2 nanostructures, the flexible electrode showed much enhanced electrochemical capacitance behaviors with robust tolerance to mechanical deformation; thus, it is promising for being woven into textiles for wearable electronics.

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 ...

MnO2 nanoparticles with high theoretical capacitance were successfully synthesized on a reduced graphene oxide/lignin-based porous carbon (RGO/PC) composite membrane by a simple electrodeposition method, and a ternary RGO/PC/MnO2 composite electrode for flexible supercapacitors was prepared. The RGO/PC/MnO2-1200s composite electrode exhibited a maximum specific capacitance of 1136 mF cm−2 (MnO2 mass loading of 6.25 mg cm−2 and a mass specific capacitance of 135 F g−1) at a current density of 1 mA cm−2. This is attributed to the high surface area of the MnO2 nanoparticles and the conductive pathway provided by the RGO/PC framework. The assembled flexible solid-state symmetric supercapacitors have outstanding cycling stability (approximately 85.2% capacitance retention after 5000 cycles) and good mechanical flexibility (about 87.6% of the original capacitance after 500 bending cycles). Importantly, the device can possess a maximum energy density of 0.253 mW h cm−3 at 0.5 mA cm−2 and a maximum power density of 0.018 W cm−3 at 5 mA cm−2. The results above indicate that the RGO/PC/MnO2 composite has potential applications in flexible energy storage devices.

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.

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.

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.