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Surface modification of graphene oxide (GO) is a powerful strategy to develop its energy density for electrochemical energy storage. However, pre-modified GO always exhibits unsatisfactory hydrophilia and its ink-relevant utilization is extremely limited. Although GO ink is widely utilized in fabricating micro energy storage devices via extrusion-based 3D-printing, simultaneously obtaining satisfactory hydrophilia and high energy density still remains a challenge. In this work, an in-situ surface engineering strategy was employed to enhance the performance of GO micro-supercapacitor chips. Three dimensionally printed GO micro-supercapacitor chips were treated with pyrrole monomer to achieve selective and spontaneous anchoring of polypyrrole on the microelectrodes without affecting interspaces between the finger electrodes. The interface-reinforced graphene scaffolds were edge-welded and exhibited a considerably improved specific capacitance, from 13.6 to 128.4 mF·cm−2. These results are expected to provide a new method for improving the performance of micro-supercapacitors derived from GO inks and further strengthen the practicability of 3D printing techniques in fabricating energy storage devices.

As one of the most important micro energy storage devices (MESDs), graphene-based micro-supercapacitors (G-MSCs) possess the advantages of excellent flexibility, long cycle life, affordability and high reliability. In most cases, constructing three-dimensional (3D) graphene networks is widely utilized to promote the permeation of electrolyte and enhance the utilization of active materials. In this work, conventional freeze-drying process is utilized in the fabrication of G-MSCs to constitute 3D interconnected networks micro-electrodes, and further by regulating the composition of inks, carbon spheres (CSs) at different mass loadings are introduced into the graphene scaffolds to further increase the active sites of the micro-electrodes. The fabricated all carbon-based MSC with the optimal mass loading of CSs (0.406 mg cm−2) exhibits a high specific areal capacitance of 17.01 mF cm−2 at the scan rate of 10 mV s−1 and a capacitance retention of 93.14% after 10000 cycles at the scan rate of 500 mV s−1. The proposed microfabrication process is facile and fully compatible with modern microtechnologies and will be highly suitable for large-scale production and integration. Keywords: Three-dimensional, Injecting, Freeze-drying, Mass loading, Supercapacitor

Direct ink writing is an interesting and attractive method to replace traditional fabrication techniques such as etching, milling, and lithography for flexible electronics because of its simple process, low cost, green nature, and good prospects for development and application. In this work, graphene/polypyrrole (G/P) binary conductive material was prepared by in situ polymerization of pyrrole monomer and graphene oxide with vitamin C (VC) as green reducing agent. Then graphene/polypyrrole/carbon black (G/P/CB) conductive ink was fabricated using a mixture of alcohol, ethylene glycol, glycerol, and deionized water as solvent. Electronic circuits were obtained by directly writing the prepared conductive ink on flexible substrate, high-gloss photographic paper. When the loading of G/P binary conductive material, carbon black, ethanol, ethylene glycol, glycerol, sodium carboxymethyl cellulose, and deionized water was 54 mg, 546 mg, 12 mL, 30 mL, 30 mL, 480 mg, and 51 mL, the electrical conductivity, solid content, contact angle, and viscosity was 146.3 μS/cm, 4.3%, 34.6°, and 35.8 mPa s, respectively. The ink exhibited excellent water and acid resistance, and the electronic circuit written using the ink showed good uniformity and mechanical flexibility. Scanning electron microscopy evaluation of cross-sections of G/P/CB ink lines on photographic paper and light-emitting diode experiments confirmed that the three-dimensional flexible paper-based conductive circuits containing 9 wt.% G/P formed a complete electrical network.

A three-dimensional Li 4 Ti 5 O 12 /carbon nanotubes/graphene composite (LTO-CNT-G) was prepared by ball-milling method, followed by microwave heating. The as-prepared LTO-CNT-G composite as anode material in lithium-ion battery exhibited superior rate capability and cycle performance under relative high current density compared with that of Li 4 Ti 5 O 12 /CNTs (LTO-CNT) and Li 4 Ti 5 O 12 /graphene (LTO-G) composites. Graphene nanosheets and CNTs were used to construct 3D conducting networks, leading to faster electron transfer and lower resistance during the lithium ion reversible reaction, which significantly enhanced the electrochemical activity of LTO-CNT-G composite. The synergistic effect of graphene and CNTs can greatly improve the rate capability and cycling stability of Li 4 Ti 5 O 12 -based anodes. The LTO-CNT-G composite exhibited a high initial discharge capacity of 172 mAh g −1 at 0.2 C and 132 mAh g −1 at 20 C, as well as an excellent cycling stability. The electrochemical impedance spectroscopy demonstrated that the LTO-CNT-G composite has the smallest charge-transfer resistance compared with the LTO-CNT and LTO-G composites, indicating that the fast electron transfer from the electrolyte to the LTO-CNT-G active materials during the lithium ion intercalation/deintercalation owing to the three-dimensional networks of graphene and CNTs.

Incompletely reduced graphene (RG) was prepared by reduction of graphite oxides via microwave irradiation at the power of 800 W and 1200 W, respectively. Fourier transform infrared spectra and X-ray photoelectron spectroscopy analyses showed that the RG contains flourish oxygen functional groups such as carboxyl, phenol, carbonyl, and quinone groups. X-ray diffraction, scanning electron microscopy and transmission electron microscopy were employed to investigate the structure, morphology of the RG samples. The electrochemical performance of the RG electrodes were measured in 5 M NaOH, 1 M H2SO4 and 1 M Na2SO4 electrolytes respectively. The supercapacitor performance of oxygen-containing functional group behavior was investigated. The specific pseudocapacitance per unit atomic percentage for either carboxyl or phenol group in 1 M H2SO4 and either carbonyl and quinone group in 5 M NaOH were obtained by galvano-statically charge/discharge measurements

High specific-capacity pseudocapacitive transition-metal-hydroxide (TMH) materials are desirable for future high performance transparent supercapacitors, but have been rarely reported previously. The successful synthesis of TMH materials with desired nanostructures is a key factor for their transparency. Here, Ni(OH)2 nanosheet transparent film (NNS-TF) was developed simply through a gas-liquid diffusion method. The nanostructures were enwrapped in graphene shells (NNS@Gr-TF) for using as transparent electrodes. The unique encapsulation structures build up rapid three-dimensional electron and ion transport pathways together with the underlying ITO layer. The specific areal capacitance (18.9 mF/cm2 at 0.1 mA/cm2) was greatly improved, at least a thousand times higher than the reported value for transparent devices based on planer CVD graphene, and ten times as that for 3D micro-structured graphene membrane.

Microbial electrosynthesis is a bioprocess where microbes reduce CO 2 into multicarbon chemicals with electrons derived from the cathode of a bioelectrochemical reactor. Developing a highly productive microbial electrosynthesis reactor requires excellent electrical connection between the electrochemical setup, the cathode, and the microbes. Copper is a highly conductive cathode material widely employed in electrochemical apparatuses. However, the antimicrobial properties of copper limit its usage for bioelectrochemistry. Here, biocompatible reduced graphene oxide coated on copper foam is synthesized as a cathode material for the microbial electrosynthesis of acetate from CO 2 . Dense and electroactive Sporomusa ovata biofilms form on the surface of reduced graphene oxide-coated copper foam electrodes while only scattered and damaged cells cover uncoated copper electrodes. Besides the formation of metabolically-active biofilms, acetate production rate from CO 2 is 21.3 and 43.5-fold higher with this novel composite cathode compared with an uncoated copper foam cathode and a reversed cathode made of reduced graphene oxide foam coated with copper, respectively. The results demonstrate that reduced graphene oxide can be employed as a biocompatible and conductive buffer between microbes and bactericidal electrode materials with excellent electrochemical property to enable highly performant microbial electrosynthesis.

Ultrasmall Fe2O3 nanoparticles/three-dimensional hierarchical porous graphene-like (Fe2O3/3D HPG) composite has been synthesized by a facile hydrothermal method, which is investigated as high performance electrode materials for supercapacitors. In the composite, the ultra-small-sized Fe2O3 nanoparticles with the average size of ~5 nm are uniformly distributed on the 3D HPG substrate. The as-prepared Fe2O3/3D HPG composite electrode exhibits good electrochemical performance with a maximum specific capacitance of 967 F g-1 at 1.0 A g-1 in the three-electrode system in 2.0 mol L-1 KOH aqueous solution, and an outstanding rate capability (74% capacitance retention at 30 A g-1 ). In addition, the composite electrode also exhibits an excellent cycling performance with 92% capacitance retention of the initial capacity after 5000 cycles at 1.0 A g-1.

In order to ease the agglomeration and huge volume change of SnO 2 particles, SnO 2 nanoparticles were usually anchored on reduced graphene oxide (rGO) and used as anode materials for lithium ion batteries. Unfortunately, graphene sheets tended to overlap with adjacent ones and SnO 2 nanoparticles still suffered from agglomeration and huge volume changes to some extent. In this paper, a composite SiO 2 @SnO 2 /rGO with coating and hierarchical structure was synthesized by a facile hydrothermal method. SnO 2 nanoparticles mono-dispersed on the surface of rGO sheets and SiO 2 spheres, while the SiO 2 @SnO 2 spheres were imbedded in the layers of rGO, which was in favor of alleviating the overlapping of graphene sheets and could make large spacious room to accommodate the huge volume changes of SnO 2 nanoparticles. SiO 2 @SnO 2 /rGO composite also displayed good electrochemical performance. In the first charge/discharge cycle, the SiO 2 @SnO 2 /rGO electrode exhibited a large discharge capacity of 1548 mA h g −1 at a current density of 100 mA g −1 and it still retained a discharge capacity of about 600 mA h g −1 after 100 cycles.

A composite consisting of well-dispersed and ultrafine hematite quantum-dots (∼2.7 nm) anchored on a three-dimensional ultra-porous graphene-like framework (denoted as Fe2O3-QDs–3D GF) has been designed by a facile and scalable strategy. In the composite, the ultra-porous 3D GF with high conductivity and high surface area was used as a conductive matrix with surface defective sites for the controllable growth of uniformly dispersed, ultra-small Fe2O3-QDs. The graphene framework can tightly hold a great amount of Fe2O3-QDs, thereby ensuring high utilization of active materials and the required conductivity to individual Fe2O3-QDs. The ultra-small-sized Fe2O3-QDs anchored on the 3D GF can endow the composite with a superior high surface area and enough active sites for electrochemical reactions, thus giving the composite a large specific capacitance. As expected, the as-prepared Fe2O3-QDs–3D GF electrode exhibited a high specific capacitance of 945 F g−1 at 1.0 A g−1 in a three-electrode system in 2.0 mol L−1 KOH aqueous solution. In addition, high-performance asymmetric supercapacitors have been fabricated with Fe2O3-QDs–3D GF as the anode and 3D hierarchical porous graphene (HPG) as the cathode, and they showed a very high energy density of 77.7 W h kg−1 at a power density of 0.40 kW kg−1 and maximum power density of 492.3 kW kg−1, as well as excellent cycling stability.

A peculiar nanostructure consisting of nitrogen-doped, carbon-encapsulated (N-C) SnO2@Sn nanoparticles grafted on three-dimensional (3D) graphene-like networks (designated as N-C@SnO2@Sn/3D-GNs) has been fabricated via a low-cost and scalable method, namely an in situ hydrolysis of Sn salts and immobilization of SnO2 nanoparticles on the surface of 3D-GNs, followed by an in situ polymerization of dopamine on the surface of the SnO2/3D-GNs, and finally a carbonization. In the composites, three-layer core-shell N-C@SnO2@Sn nanoparticles were uniformly grafted onto the surfaces of 3D-GNs, which promotes highly efficient insertion/extraction of Li(+). In addition, the outermost N-C layer with graphene-like structure of the N-C@SnO2@Sn nanoparticles can effectively buffer the large volume changes, enhance electronic conductivity, and prevent SnO2/Sn aggregation and pulverization during discharge/charge. The middle SnO2 layer can be changed into active Sn and nano-Li2O during discharge, as described by SnO2 + Li(+) → Sn + Li2O, whereas the thus-formed nano-Li2O can provide a facile environment for the alloying process and facilitate good cycling behavior, so as to further improve the cycling performance of the composite. The inner Sn layer with large theoretical capacity can guarantee high lithium storage in the composite. The 3D-GNs, with high electrical conductivity (1.50 × 10(3) S m(-1)), large surface area (1143 m(2) g(-1)), and high mechanical flexibility, tightly pin the core-shell structure of the N-C@SnO2@Sn nanoparticles and thus lead to remarkably enhanced electrical conductivity and structural integrity of the overall electrode. Consequently, this novel hybrid anode exhibits highly stable capacity of up to 901 mAh g(-1), with ∼89.3% capacity retention after 200 cycles at 0.1 A g(-1) and superior high rate performance, as well as a long lifetime of 500 cycles with 84.0% retention at 1.0 A g(-1). Importantly, this unique hybrid design is expected to be extended to other alloy-type anode materials such as silicon, germanium, etc.

Graphene-based nanocomposites have been one of the most attractive electrode materials of supercapacitors. But in most cases, graphene-based nanocomposites always use single nanoparticle as active material, which cannot meet the increasing demand for next generation of energy storage devices. In the present paper, we prepared Fe2O3–Ni(OH)2/graphene nanocomposite by simple one-step hydrothermal method. The dual-particle system reveals a synergistic effect between Fe2O3 and Ni(OH)2, i.e., Ni(OH)2 can enhance the electrical conductivity of Fe2O3, and then Fe2O3 can act as electrical conductivity bridges between Ni(OH)2 nanoparticles and RG. As a result, the Fe2O3–Ni(OH)2/graphene nanocomposite presents impressing electrochemical performance, including high specific capacitance (~857 F/g), good rate capability, and outstanding cycling stability (after 5000 cycles, 100 % retention).

Graphene/nickel oxide composite (G/NiO) was synthesized through a facile hydrothermal method and subsequently microwave thermal treatment by using alcohols-reduced graphene as substrate. The as-prepared G/NiO was characterized by X-ray diffraction, Raman spectra, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscope and transmission electron microscope. The results indicate that the graphene oxide has been successfully reduced to graphene, and NiO nanoparticles are homogeneous anchored on the surface of graphene, forming a globule-on-sheet structure. The loading content of NiO nanoparticles anchoring on the surface of graphene nanosheets can be controlled by adjusting the hydrothermal temperature. The G/NiO displays superior electrochemical performance with a specific capacitance of 530 F g −1 at 1 A g −1 in 2 M of NaOH. After 5000 cycles, the supercapacitor still maintains a specific capacitance of 490 F g −1 (92% retention of the initial capacity), exhibiting excellent cycling stability.

A three-dimensional lithium iron phosphate (LiFePO4)/carbon nanotubes (CNTs)/graphene composite was successfully synthesized via solid-state reaction. The LiFePO4/carbon nanotubes/graphene (LFP–CNT–G) composite used as Li-ions battery cathode material exhibits superior high-rate capability and favorable charge–discharge cycle performance under relative high current density compared with that of LiFePO4/carbon nanotubes (LFP–CNT) composite and LiFePO4/graphene (LFP–G) composite. Graphene nanosheets and CNTs construct 3D conducting networks are favor for faster electron transfer, higher Li-ions diffusion coefficient and lower resistance during the Li-ions reversible reaction. The synergistic effect of graphene nanosheets and CNTs improves the rate capability and cycling stability of LiFePO4-based cathodes. The LFP–CNT–G electrode shows reversible capacity of 168.9 mA h g−1 at 0.2 C and 115.8 mA h g−1 at 20 C. The electrochemical impedance spectroscopy demonstrate that the LFP–CNT–G electrode has the smallest charge-transfer resistance, indicating that the fast electron transfer from the electrolyte to the LFP–CNT–G active materials in the Li-ions intercalation/deintercalation reactions owing to the three-dimensional networks of graphene and carbon nanotubes.

A hybrid material composed of high density graphene/nickel oxide (RG/NiO) has been prepared by microwave-assisted in situ method of NiO at the defects of graphene. Graphene (RG) is obtained by thermal expansion of graphite oxide (GO). Graphene/nickel oxide composite is prepared by hydrothermal synthesis of aqueous mixture of RG and Ni(NO 3 ) 2 , followed by microwave irradiation of the hydrothermal synthesis product. SEM and TEM images show that NiO with size of 20–50 nm are uniformly distributed on the RG substrate. RG/NiO composite electrodes exhibit good electrochemical performance with a maximum specific capacitance of 617 F g −1 at the charge/discharge current density of 1 A g −1 with 5 M NaOH electrolyte. After 5000 cycling DC tests, a high level specific capacitance of 613 F g −1 is obtained (99.4% retention of the initial capacity), suggesting excellent long-term cycling stability.

Supercapacitor is an effective energy storage device that store energy using porous conducting materials. With its great performance, graphene has potential applications in supercapacitors. However, graphene prepared by thermal expansion of graphite oxide always has very limited storage capacity, so we treat the thermal exfoliated graphite oxide (TEGO) in mixed acid to enhance the hydrophily of TEGO and achieve higher capacitance. The electrochemical properties of supercapacitor electrodes are studied by cyclic voltammetry and galvanostatic charge/discharge methods. Supercapacitor prepared with acid treated graphene has good electrochemical performance, with a maximum specific capacitance of 196 F g−1 at the charge/discharge current density of 100 mA g−1 using 2 M KOH electrolyte, yielding an improvement of ∼40% compared with that of the pristine TEGO.

LiFePO4/graphene composites were prepared via a simple hydrothermal method. The as-prepared LiFePO4/graphene composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), galvanostatic charge-discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) tests. The lithium-ion batteries using LiFePO4/graphene composites as cathode material exhibited a discharge capacity of 165 mAh/g, which was 97% of the theoretical capacity of 170 mAh/g.

Dye-sensitized solar cells based on TiO 2 films with different printing layers (6-10) were fabricated by screen printing method. The prepared samples were characterized by scanning electron microscopy, X-ray diffraction and UV–vis absorption spectroscopy. The effects of thickness on the photoelectric conversion performance of the as-fabricated DSSCs were investigated. An optimum photoelectric conversion efficiency of 5.52% was obtained in a DSSC with 8 printing layers. Furthermore, after a moderate amount of graphene was combined with TiO 2 , the photoelectric conversion efficiency of the DSSC based on graphene/TiO 2 composite film rose from 5.52% to 6.49%, with an increase of η by 17.6%. The results indicated that graphene not only enhances the transport of electrons from the film to the fluorine doped tin oxide substrates and reduces the charge recombination rate, but also reduces the electrolyte–electrode interfacial resistance, clearly increasing the photoelectric conversion efficiency.