Nitrogen-doped Reduced Graphene Oxide Sheets Research Articles

Mechanistic insights into the roles of precursor content, synthesis time, and dispersive solvent in maximizing supercapacitance of N-rGO sheets

Graphene oxide (GO) sheets were prepared using the improved Hummers’ method, followed by simultaneous reduction and nitrogen doping using an easy solvothermal route to obtain Nitrogen-doped reduced graphene oxide (N-rGO) sheets. The amount of N-doping was controlled using the different amounts of GO and Urea, keeping the ratio of the two (1:3) fixed. Here, we examined several variations in synthesis parameters, viz. change in precursor content, variation in synthesis time, and volume of dispersive solvent. The N-rGO sample prepared with 250 mg GO and 750 mg urea at 24-hour synthesis time in 80 ml dispersive solvent exhibits a superlative specific capacitance of 1244 F/g at 0.5 A/g using 3-electrode measurements in an acidic medium. The high value can be understood via the synergistic roles of optimum defects, degree of reduction, and types of N-environments. The symmetric 2-electrode device prepared by depositing the synthesized material onto carbon cloth exhibits supercapacitance of 635 F/g at 1 A/g current density and high cyclic constancy with capacitive retention of ∼ 96% after 10000 charge-discharge cycles. Our work provides critical mechanistic insights on the complex intermingling of an optimum N-content, the relative amount of the different N-environments, degree of reduction, and disorder to generate a high specific capacitance, remarkably better than previously reported works on similar materials. Notably, a relatively high graphitic-N, with moderate surface N-content (4.64%) and a moderate degree of reduction, leads to this higher specific capacitance.

Nitrogen-doped reduced graphene oxide/MoS2 ‘nanoflower’ composites for high-performance supercapacitors

Mixed-phase (MP) metal disulfides have interesting electrochemical properties which originate from the generation of the abundance of electrochemically active sites and a higher structural stability as compared with crystalline materials. However, there is less exploration in the design and performance of the MP materials for supercapacitors application. Herein, nitrogen-doped reduced graphene oxide (N-rGO) with MP molybdenum disulfide (MoS2) nanoflower (NGM) nanocomposite was self-assembled in a one-pot hydrothermal synthesis. The NGM nanocomposites featured a high surface area and electrical conductivity governed by the uniform growth of nanoflowers on the conductive N-rGO sheets. Coupled with an interconnected network of charge transport channels, the robust ion transport and lowered charge transfer resistance at the electrode-electrolyte interphase significantly boost the electrochemical activity enabling the electrodes to deliver a high specific capacitance (539.5 F g−1), exceptional energy and power densities (Pmax = 25.4 kW kg−1, and Emax = 71.5 Wh kg−1) and excellent capacitance retention of 95.3 % during long-term cycling.

Exploring supercapacitance of solvothermally synthesized N-rGO sheet: role of N-doping and the insight mechanism.

We demonstrate the method of achieving excellent supercapacitance in nitrogen-doped reduced graphene oxide (N-rGO) sheets by controlling the amount of N-content through the use of different ratios of GO and urea during solvothermal synthesis. Here, urea plays a dual role in reducing GO and simultaneously doping nitrogen into the GO flakes forming exfoliated N-rGO sheets. The nitrogen content in N-rGO samples rises with an increase in the amounts of urea and saturates at a value of ∼14% for the GO : urea ratios beyond 1 : 8. The obtained N-rGO sheets with ∼ 5% N-content (obtained for GO : urea ratio of 1 : 3) were demonstrated as excellent supercapacitor materials. Using a 3-electrode setup, the maximum specific capacitance obtained for this sample was 514 F g-1 at a current density of 0.5 A g-1 (mass normalized current). The insights into the origin of this excellent supercapacitive behavior are explained through our results on optimum N-content, the relative amount of different N-environments, defects/disorders, and the degree of reduction of GO. Importantly, a proper stacking of rGO sheets with moderate N-content (∼5-6%) and a moderate amount of defects is the key to achieve high specific-capacitance. Furthermore, our 2-electrode device demonstrates the excellence of our samples with a Csp of 237 F g-1, a power density of 225 W kg-1, and an energy density of 6.7 W h kg-1 at 0.5 A g-1, exhibiting a high cyclic constancy with high capacitive retention of ∼ 82% even after 8000 cycles. Hence, our work provides a way to control the properties of N-rGO in achieving excellent supercapacitive performance.

Fabrication of graphene-based electrochemical capacitors through reactive inverse matrix assisted pulsed laser evaporation

Electrodes constituted by nitrogen-doped reduced graphene oxide (NrGO) in combination with NiO nanostructures were fabricated by means of reactive inverse matrix assisted pulsed laser evaporation technique. The structure-composition of the electrode composites was tailored by laser-inducing chemical reactions of graphene oxide (GO) flakes with different precursor molecules (citric acid, ascorbic acid and imidazole) during GO deposition. Structural characterizations reveal the formation of wrinkles and nanoholes in the NrGO sheets, besides their coating with NiO nanostructures. Compositional studies disclose that imidazole precursor promotes the synthesis of NrGO with the largest degree of reduction and nitrogen doping (mainly with graphitic and pyridinic N). Electrochemical analyses of the obtained electrodes reveal that NiO nanostructures increase surface charge storage processes (double layer – pseudocapacitive) over diffusive ones, being the imidazole-based electrodes the ones exhibiting the best performance (up to 114 F cm−3 at 10 mV s−1). Symmetric and asymmetric electrochemical capacitors were also fabricated showing excellent robustness over 10,000 charge-discharge cycles at high specific currents.

Tris(2-benzimidazolylmethyl)amine-Directed Synthesis of Single-Atom Nickel Catalysts for Electrochemical CO Production from CO2.

The electrochemical reduction of carbon dioxide (CO2 ) to value-added products is a promising approach to reducing excess CO2 in the atmosphere. However, the development of electrocatalysts for highly selective and efficient electrochemical CO2 reduction has been challenging because protons are usually easier to reduce than CO2 in an aqueous electrolyte. Recently, single-atom catalysts (SACs) have been suggested as candidate CO2 reduction catalysts due to their unique catalytic properties. To prepare single-atom metal active sites, the stabilization of metal atoms over conductive supports such as graphene sheets to prevent metal aggregation is crucial. To address this issue, a facile method was developed to prepare single-atom nickel active sites on reduced graphene oxide (RGO) sheets for the selective production of carbon monoxide (CO) from CO2 . The tris(2-benzimidazolylmethyl)amine (NTB) ligand was introduced as a linker that can homogeneously disperse nickel atoms on the graphene oxide (GO) sheets. Because the NTB ligands form strong interactions with the GO sheets by π-π interactions and with nickel ions by ligation, they can effectively stabilize nickel ions on GO sheets by forming Ni(NTB)-GO complexes. High-temperature annealing of Ni(NTB)-GO under inert atmosphere produces nickel- and nitrogen-doped reduced graphene oxide sheets (Ni-N-RGO) with single-atom Ni-N4 active sites. Ni-N-RGO shows high CO2 reduction selectivity in the reduction of CO2 to CO with 97 % faradaic efficiency at -0.8 V vs. RHE (reversible hydrogen electrode).

Preparation and Characterization of Rice Husks-Derived Silicon-Tin/Nitrogen-Doped Reduced Graphene Oxide Nanocomposites as Anode Materials for Lithium-Ion Batteries

Silicon (Si) and Tin (Sn) are promising materials for anodes in lithium-ion batteries due to their high theoretical capacity and abundance of Si on earth. Si can be derived from rice husk which is the main agricultural byproduct in Thailand. However, the challenge of using these materials in lithium-ion batteries is the large volume expansion during charge-discharge process which leads to pulverization of electrodes. The effective solution is to combine these metals as composite with carbon supporter. Nitrogen-doped reduced graphene oxide (NrGO) has been used as carbon supporter in this research because of its high surface area, electrical conductivity and rate of electron transfer. To confirm phases of products, X-rays diffraction techniques (XRD) was measured. The results show that there were peaks of Si, Sn and carbon in XRD patterns. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to illustrate the morphology of prepared composites. From SEM and TEM results, there were small-sized particles of Si and Sn dispersed randomly on NrGO sheets. Furthermore, electrochemical properties of these products were measured to confirm their efficiency as anode materials in lithium-ion batteries by coin cell assembly. The prepared composite can deliver the highest initial capacity of 1600 mA h g-1 and expected to use as anode materials in the next generation lithium-ion batteries.

Homogeneous growth of TiO2-based nanotubes on nitrogen-doped reduced graphene oxide and its enhanced performance as a Li-ion battery anode

The pursuit of a promising replacement candidate for graphite as a Li-ion battery anode, which can satisfy both engineering criteria and market needs has been the target of researchers for more than two decades. In this work, we have investigated the synergistic effect of nitrogen-doped reduced graphene oxide (NrGO) and nanotubular TiO2 to achieve high rate capabilities with high discharge capacities through a simple, one-step and scalable method. First, nanotubes of hydrogen titanate were hydrothermally grown on the surface of NrGO sheets, and then converted to a mixed phase of TiO2-B and anatase (TB) by thermal annealing. Specific surface area, thermal gravimetric, structural and morphological characterizations were performed on the synthesized product. Electrochemical properties were investigated by cyclic voltammetry and cyclic charge/discharge tests. The prepared anode showed high discharge capacity of 150 mAh g−1 at 1 C current rate after 50 cycles. The promising capacity of synthesized NrGO-TB was attributed to the unique and novel microstructure of NrGO-TB in which long nanotubes of TiO2 have been grown on the surface of NrGO sheets. Such architecture synergistically reduces the solid-state diffusion distance of Li+ and increases the electronic conductivity of the anode.

Three-dimensional nitrogen-doped graphene wrapped LaMnO3 nanocomposites as high-performance supercapacitor electrodes

In order to improve the electrochemical properties of LaMnO3 perovskite (LMO) materials for supercapacitor applications, different amount of nitrogen-doped reduced graphene oxide (N-rGO) from 25 to 75 wt % has been incorporated with LMO to form nanocomposite materials. These three-dimensional structures are assembled through electrostatic attraction of the positive charged N-rGO and the negative charged LMO. The amount of N-rGO has a significant effect on the properties of the composite materials. Microscopic investigations confirm that LMO particles are wrapped by N-rGO sheets, and with the increase of N-rGO content, the dispersion of LMO particles in graphene matrix is enhanced. Electrochemical characterizations reveal that the specific capacitance of the nanocomposites has been dramatically improved. Compared with pristine graphene and LMO, the sample with a lower amount of N-rGO (25%) exhibited the highest specific capacitance (687 Fg−1 at 5 mVs−1) and the longest stability (79% retention after 2000 cycle at 10 Ag−1) among other composite materials. The superior capacitive properties make LMO/N-rGO nanocomposites stand out as a promising candidate for supercapacitor applications.

Synthesis of anatase TiO2 with exposed (001) facets grown on N-doped reduced graphene oxide for enhanced hydrogen storage

Homogeneously distributed TiO2 nanoparticles with (001) reactive facets were grown over nitrogen-doped reduced graphene oxide sheets (N-rGO) under solvothermal conditions. Hydrogen storage capacity of the system was significantly improved to 0.91 wt% at room temperature and pressure of 0.8 MPa that is the highest hydrogen storage ever reported for graphene-based nanocomposites at room temperature and low pressures. Importantly, this nanocomposite exhibits ∼91% capacity retention through 5 cycles with more than 88% release of the stored hydrogen at ambient conditions. Enhanced hydrogen uptake and capacity retention were attributed to the synergistic effect of i) reactive facets of TiO2, ii) high dispersion of nanoparticles with the average size of 6 nm, and iii) strong interaction between substrate and TiO2 nanoparticles through polarized C–N bonding of N-rGO sheets.

Electrospun porous nanorod perovskite oxide/nitrogen-doped graphene composite as a bi-functional catalyst for metal air batteries

The current generation is not only facing the shortage of fossil fuels in the near future, but also is responsible for preserving the environment for future generations. As a result, the development of clean energy systems is becoming an urgent focus for the research community. Metal air batteries have attracted much attention due to their extremely high energy density, and the rechargeability which is directly governed by the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, we present a new class of hybrid bi-functional catalyst consisting of porous nanorod perovskite La0.5Sr0.5Co0.8Fe0.2O3 (LSCF-PR) combined with nitrogen-doped reduced graphene oxide (NRGO) active towards both ORR and OER. The novel morphology of LSCF-PR is prepared by an electrospinning method, and incorporated into NRGO sheets. Electron microscopy reveals interesting composite morphology in which LSCF-PR is embedded between the sheets of NRGO, forming an efficient LSCF-PR/NRGO composite morphology for the electrochemical oxygen reactions. Electrochemical testing of the LSCF-PR/NRGO composite in alkaline medium results in excellent ORR and OER catalytic activities, verifying the effective combination for bi-functionality. LSCF-PR/NRGO presents not only a comparable or superior performance to state-of-the-art Pt/C catalyst for ORR or OER, respectively, but also better durability. These results highlight the applicability of LSCF-PR/NRGO composite having unique and efficient morphology as a promising bi-functional catalyst for metal air battery applications.

Scalable synthesis of hierarchically structured carbon nanotube-graphene fibres for capacitive energy storage.

Micro-supercapacitors are promising energy storage devices that can complement or even replace batteries in miniaturized portable electronics and microelectromechanical systems. Their main limitation, however, is the low volumetric energy density when compared with batteries. Here, we describe a hierarchically structured carbon microfibre made of an interconnected network of aligned single-walled carbon nanotubes with interposed nitrogen-doped reduced graphene oxide sheets. The nanomaterials form mesoporous structures of large specific surface area (396 m(2) g(-1)) and high electrical conductivity (102 S cm(-1)). We develop a scalable method to continuously produce the fibres using a silica capillary column functioning as a hydrothermal microreactor. The resultant fibres show a specific volumetric capacity as high as 305 F cm(-3) in sulphuric acid (measured at 73.5 mA cm(-3) in a three-electrode cell) or 300 F cm(-3) in polyvinyl alcohol (PVA)/H(3)PO(4) electrolyte (measured at 26.7 mA cm(-3) in a two-electrode cell). A full micro-supercapacitor with PVA/H(3)PO(4) gel electrolyte, free from binder, current collector and separator, has a volumetric energy density of ∼6.3 mWh cm(-3) (a value comparable to that of 4 V-500 µAh thin-film lithium batteries) while maintaining a power density more than two orders of magnitude higher than that of batteries, as well as a long cycle life. To demonstrate that our fibre-based, all-solid-state micro-supercapacitors can be easily integrated into miniaturized flexible devices, we use them to power an ultraviolet photodetector and a light-emitting diode.

Facile Single-Step Synthesis of Nitrogen-Doped Reduced Graphene Oxide-Mn3O4 Hybrid Functional Material for the Electrocatalytic Reduction of Oxygen

Development of efficient electrocatalyst based on non-precious metal that favors the four-electron pathway for the reduction of oxygen in alkaline fuel cell is a challenging task. Herein, we demonstrate a new facile route for the synthesis of hybrid functional electrocatalyst based on nitrogen-doped reduced graphene oxide (N-rGO) and Mn3O4 with pronounced electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution. The synthesis involves one-step in situ reduction of both graphene oxide (GO) and Mn(VII), growth of Mn3O4 nanocrystals and nitrogen doping onto the carbon framework using a single reducing agent, hydrazine. The X-ray photoelectron (XPS), Raman and FTIR spectral, and X-ray diffraction measurements confirm the reduction of GO and growth of nanosized Mn3O4. The XPS profile reveals that N-rGO has pyridinic (40%), pyrrolic (53%), and pyridine N oxide (7%) types of nitrogen. The Mn3O4 nanoparticles are single crystalline and randomly distributed over the wrinkled N-rGO sheets. The hybrid material has excellent ORR activity and it favors the 4-electron pathway for the reduction of oxygen. The electrocatalytic performance of the hybrid catalyst is superior to the N-rGO, free Mn3O4 and their physical mixture. The hybrid material shows an onset potential of -0.075 V, which is 60-225 mV less negative than that of the other catalyst tested. It has excellent methanol tolerance and high durability. The catalytic current density achieved with the hybrid material at 0.1 mg cm(-2) is almost equivalent to that of the commercial Pt/C (10%). The synergistic effect of N-rGO and Mn3O4 enhances the overall performance of the hybrid catalyst. The nitrogen in N-rGO is considered to be at the interface to bridge the rGO framework and Mn3O4 nanoparticles and facilitates the electron transfer.

Formation of nitrogen-doped graphene nanoscrolls by adsorption of magnetic γ-Fe2O3 nanoparticles

Graphene nanoscrolls are Archimedean-type spirals formed by rolling single-layer graphene sheets. Their unique structure makes them conceptually interesting and understanding their formation gives important information on the manipulation and characteristics of various carbon nanostructures. Here we report a 100% efficient process to transform nitrogen-doped reduced graphene oxide sheets into homogeneous nanoscrolls by decoration with magnetic γ-Fe2O3 nanoparticles. Through a large number of control experiments, magnetic characterization of the decorated nanoparticles, and ab initio calculations, we conclude that the rolling is initiated by the strong adsorption of maghemite nanoparticles at nitrogen defects in the graphene lattice and their mutual magnetic interaction. The nanoscroll formation is fully reversible and upon removal of the maghemite nanoparticles, the nanoscrolls return to open sheets. Besides supplying information on the rolling mechanism of graphene nanoscrolls, our results also provide important information on the stabilization of iron oxide nanoparticles.

Simultaneous N-doping of reduced graphene oxide and TiO2 in the composite for visible light photodegradation of methylene blue with enhanced performance

The nitrogen-doped P90 TiO2 (N-P90), nitrogen-doped reduced graphene oxide (N-RGO) and their composite were synthesized via a one-step annealing treatment process under NH3 atmosphere using commercial P90 TiO2 and GO as starting materials. The as-prepared N-P90, N-RGO and N-P90/N-RGO composite were characterized by field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and ultraviolet–visible diffuse reflectance spectroscopy (DRS). The results indicated that both the reduction of graphene oxide and the incorporation of nitrogen into both RGO and TiO2 matrices were accomplished simultaneously in the facile process. The photocatalytic activity of the as-prepared samples was evaluated using the degradation of methylene blue (MB) under visible light irradiation. N-P90/N-RGO composites showed a significantly enhanced photocatalytic performance compared with P90 TiO2, N-P90 and N-P90/RGO composites. The higher photocatalytic activity of N-P90/N-RGO composites can be ascribed to the more efficient separation of the photogenerated charges resulting from the improved electrical conductivity of the N-RGO sheets, as well as the enhanced absorption in the visible light region. Overall, this work demonstrated a facile approach of incorporating nitrogen into commercial TiO2 and RGO simultaneously and a novel strategy of fabricating a visible light-active photocatalyst with improved efficiency for mass application.